Lead Refining Furnace

— Lead Refining Furnace: Kettle Refining to 99.99% Lead Ingot

A lead refining furnace is a kettle-based fire-refining line in which lead bullion that already carries its impurities as dissolved metals is heated, held at a sequence of set temperatures and treated with reagents — sulfur, caustic soda with sodium nitrate, zinc, calcium and magnesium — so that copper, arsenic, tin, antimony, silver, gold, zinc and bismuth leave the bath as a dross, a slag or a crust that is skimmed off and sold, and the lead leaves the line as a refined ingot at the grade the customer’s own specification names. YuSai Machinery engineers the kettle with its combustion and agitation system, the lead transfer and skimming equipment, the hood and extraction that keep lead fume inside the plant, the casting line and the refining sequence itself — from your bullion assay, the impurities you have to remove and the grade you have to reach, so that the metal leaving the line matches the assay certificate issued with it.

Product Overview

A lead refining furnace sits at the end of a lead recovery flow, not at the beginning of it. Everything upstream — battery breaking, paste desulfurisation, reduction smelting in a rotary or blast furnace — is trying to deliver metal, and what it delivers is bullion: lead that carries copper, arsenic, tin, antimony, silver, gold, zinc and bismuth in solution, often with a tenth of a percent of antimony or more intentional in the smelting step because antimony makes the oxide easy to reduce and the slag fluid. That bullion is not a product. Battery grids will not tolerate it, oxide makers will not buy it and rolling mills cannot roll it, because each of those impurity elements does a specific kind of damage — antimony accelerates grid corrosion, bismuth embrittles and ruins the oxide’s electrochemical behaviour, copper and zinc attack the casting and the paste, and arsenic and antimony both harden the metal in ways the customer’s specification forbids. Refining exists to take those elements back out.

The reason this is a kettle line rather than a furnace in the ordinary sense is that almost nothing here happens at one temperature. Lead melts at 327.5 °C, which is low, and lead’s own chemistry is slow and close to equilibrium, which means the useful separation mechanisms are the ones that depend on a temperature being held precisely and a reagent being stirred in properly. Copper and copper arsenide are much less soluble in solid lead than in liquid lead, so cooling the bath towards its freezing point crystallises them out and floats them, and that is the whole of the copper drossing step — a liquation, with a little sulfur added only to harden the dross so it skims instead of smearing. Arsenic, tin and antimony are removed by oxidation, and the practical oxidant is molten caustic soda with sodium nitrate, the Harris process, which converts them into sodium salts that float as a slag and can be poured off; the same bath is where the later removal of residual calcium, magnesium and zinc is finished, because those three metals are also taken out by an alkaline oxidation.

Silver and gold are removed for a different reason and by a different mechanism. Zinc has almost no solubility in solid lead but a real one in liquid lead, and it takes silver and gold with it far more strongly than it takes lead, so when zinc is stirred into the bath a zinc-rich liquid separates and floats, carrying the silver and the gold into it. That crust is pressed off the surface, and it is the richest single stream in the plant: the silver and gold recovered downstream come out of it. What the step leaves behind is lead that is now carrying zinc, and since zinc is as unwelcome in the finished metal as the elements removed before it, the next stage has to take it back out — either by distilling it off under reduced pressure, which returns it to the circuit as metal, or by oxidising it into the caustic dross alongside the residual antimony.

Bismuth is the element that decides how far fire refining can go, and it is worth being direct about the mechanism. Bismuth and lead form a solution that no amount of oxidation will disturb, because bismuth is less reactive towards oxygen than lead is, so it survives every slagging step that removes antimony and tin. The route that works is to add calcium and magnesium into the melt, where they combine with bismuth to form a solid intermetallic, CaMg₂Bi₂, which is insoluble in lead and floats as a dross that is skimmed and then chlorinated to free the bismuth. The step is run at a specific temperature window, because the calcium and magnesium have to be dissolved into the melt at one temperature and the compound has to crystallise at another. It is also the step that sets the floor on the whole plant: below a certain bismuth content the calcium-magnesium addition stops being efficient and starts costing more than it recovers, and that is the boundary at which the four-nines grade of fire refining ends and electrorefining begins.

Around that sequence of chemical steps sits the plant itself, and its engineering is dominated by two things that are not chemistry at all. The first is heat and agitation. Lead’s thermal conductivity is about 35 W/m·K, roughly a tenth of aluminium’s, so a kettle heated from below and left alone stratifies: a hot layer sits over the burners, a cooler layer sits under the surface, and the reagent added at the top meets metal at the wrong temperature and reacts incompletely. The pump or the impeller is therefore a process device and not a convenience, and the same pump is what transfers the bath between kettles so that a multi-step sequence does not lose half a shift to ladling. The second is containment. Lead and its oxide are toxic, the kettle mouth and the skimming station are open sources, and the figures that govern the design are the 50 µg/m³ permissible exposure limit and the 30 µg/m³ action level for airborne lead. Hood geometry, the extraction volume, the velocity held across the hood opening, the baghouse and the housekeeping regime are all specified against those numbers, and the exhaust from the kettle and the casting line is a design output rather than an accessory.

Finally, the kettle itself has to be understood as a wear item rather than as a vessel. It is a welded steel pot, its wall thickness is set by the metal temperature and the head of lead above it, and it loses thickness from the inside for as long as it works. Its life is therefore tracked as a measured wall thickness against a retirement floor, taken at a stated interval, and the record of those surveys is part of the technical file. A plant that treats a kettle as a permanent asset and one that treats it as a measured consumable are two different designs, and only the second one can be permitted and insured in a regulated market.

Because bullion assays vary so widely between a battery recycler, a primary smelter and a plant refining scrap lead bought on the open market, the line is engineered from your numbers rather than selected from a catalogue. The impurities to be removed and the order they come out in follow from the assay; the number of slagging passes follows from the antimony and antimony-plus-arsenic load and the grade to be reached; the kettle count and volume follow from the number of steps in series, the batch size and the heel each kettle has to carry; the burner and agitation duty follow from the mass and the number of temperature set points per batch; the hood and extraction follow from the bath surface area and the number of open points; and the casting rate follows from the batch size and the cycle. Those figures are issued as a process design package, and we would rather revise the sequence than supply a kettle whose wall thickness cannot hold the metal it is asked to hold.

Key Facts

Refining is not melting. A lead refining furnace does not turn scrap into lead; it takes bullion that is already metal and takes the elements dissolved in it back out. Melting a solid charge and casting it into ingots is a different machine with a different duty, and which one you need is decided by the assay of the bullion, not by its tonnage.
The temperature is a sequence, not a range. Almost every published figure quotes one number for the whole plant, and hides the fact that the kettle is cycled between four or five set points. Copper drossing runs near the freezing point of lead, softening runs hot enough for oxidation, desilverising runs inside a zinc-partition window, debismuthising runs cold enough for a compound to crystallise, and casting runs just above the melting point. It is the set points, not the range, that decide the assay.
Copper drossing is a liquation, not a fluxing reaction. Copper and copper arsenide are far less soluble in solid lead than in liquid lead, so cooling the bath towards the freezing point makes them crystallise out and float; a small sulfur addition only hardens that dross so it can be skimmed instead of smeared back into the metal. The copper leaves as a solid phase, not as a sulfide that had to be manufactured.
Arsenic goes into an alkaline slag, not into a gas. Arsenic, tin and antimony are removed by the Harris process — molten caustic soda with sodium nitrate as the oxidant — and report to a sodium slag that is poured off. No commercial lead refinery removes arsenic with hydrogen to make arsine; arsine is a poison and not a refining route, and a page that describes one is describing a plant that does not exist.
Antimony removal ends on an equilibrium, not on a target. The antimony left in the lead follows the antimony content of the slag it sits in contact with, so no single pass reaches zero and the number of slagging passes is a commercial decision. The antimonial by-product is a co-product with its own market, and a refinery that treats it as waste throws away both the metal and the margin.
Desilverising is a zinc-partition step and the crust is the product. Zinc stirred into molten lead takes silver and gold into a zinc-rich phase that floats as a crust and is pressed off; that crust is the richest stream in the plant and it is where the precious-metal revenue on the metal balance is actually booked. The lead that leaves the step carries zinc, which becomes the problem the next step has to solve.
Dezincing can be done by vacuum instead of by chemistry. The zinc left behind after desilverising can be distilled out under reduced pressure, which returns zinc to the circuit as metal rather than as another slag, or it can be oxidised into a caustic dross. That choice is a plant-economics decision rather than a technical impossibility, and the two routes lead to different kettle counts.
Bismuth is removed with calcium and magnesium and freezes out as an intermetallic. On the Betterton–Kroll route, calcium and magnesium are dissolved into the melt and the bismuth leaves as a solid CaMg₂Bi₂ compound that floats and is skimmed; the dross is then chlorinated to free the bismuth. Bismuth is the element that decides whether fire refining alone can reach the grade the customer is asking for.
Fire refining has an honest ceiling and it is about 99.99%. A kettle route reaches the four-nines grade that battery makers, oxide makers and rolling mills buy against the standard specifications, and it does not reach five or six nines. Grades above that are produced by electrorefining in a fluosilicate or sulfamate electrolyte, which is a different plant with a different anode, a different cell and a different chemistry.
The kettle is a consumable, and its failure mode is a release of molten lead. The pot body is a steel vessel that is thickest when new and thinnest the day it is scrapped, and its life is measured in wall thickness against a floor set by the metal temperature and the head of lead above it — not in years. Every kettle needs a documented thickness survey and a written inspection interval, because loss of containment at four hundred degrees is not a maintenance event.
Lead transmits heat badly, which is why agitation is not optional. Lead’s thermal conductivity is around 35 W/m·K, roughly a tenth that of aluminium, so a still kettle stratifies into a hot layer at the burners and a cooler layer under the surface. Nothing dissolves, oxidises or partitions properly in a stratified bath, which is why the pump or the stirrer is what turns a heated pot into a reactor.
The lead exposure limits set the hood, not the walls of the building. The permissible exposure limit for lead is 50 µg/m³ as an eight-hour average with an action level of 30 µg/m³, and the kettle mouth, the skimming station and the ingot line are all fugitive sources. Hood geometry, extraction volume, the negative pressure held at the opening and the baghouse behind it are specified against those two figures, and a lead refining plant that has not been specified this way cannot be sold into a regulated market.
Configuration is engineered to your bullion, not selected from a model list. Kettle volume and kettle count, burner capacity, agitation and transfer, hood and extraction, dross and slag handling, casting rate and the refining sequence itself are engineered from your bullion assay, the impurities you must remove to reach your declared grade, your batch size and your site.

Product Core Selling Points

The refining sequence is issued as a temperature and reagent schedule, not as a temperature range. Copper drossing, softening, desilverising, dezincing, debismuthising, final refining and casting each get their own set point, their own reagent addition and their own skim, on one sheet. Because the schedule is written down, a buyer can check it against their own bullion assay and against the grade they have contracted to deliver, instead of accepting a single number that describes none of the steps.
Copper is taken out by cooling, and the page says why. Copper and copper arsenide are much less soluble in solid lead than in liquid lead, so the bath is cooled towards the freezing point, the copper compounds crystallise and float, and a small sulfur addition hardens the dross so it can be skimmed cleanly. That is a liquation operation, and describing it as it actually works is what lets an operator set the temperature correctly rather than adding sulfur and hoping.
The softening step is named for what it is and what it removes. Arsenic, tin and antimony are oxidised out of the bath by the Harris process — molten caustic soda with sodium nitrate — and leave as a sodium slag. The reagent ratio, the addition rate and the slag condition are specified, and the step is described as the oxidation it is, because that is the only description an operator can act on.
The antimony equilibrium is stated rather than hidden. Residual antimony follows the antimony in the slag, so the number of passes and the destination of the antimonial by-product are design decisions taken with the customer, not afterthoughts. A refinery that knows its antimony ends up as a saleable co-product plans its metal balance differently from one that plans to landfill it.
Desilverising is designed around the crust, because the crust is the revenue. The zinc addition rate, the stirring time, the bath temperature and the pressing of the silver-zinc crust are all specified as a recovery problem, and the crust is routed to its own treatment rather than back into the lead. That single design choice is the difference between a refinery that recovers its precious metals and one that reports them as a loss.
Dezincing is offered as a choice with the consequences stated. Vacuum dezincing returns zinc as metal and removes a reagent from the circuit; alkaline dezincing is simpler and produces another slag. Both are quoted, with the kettle count, the reagent consumption and the by-product stream that each one implies, because the cheaper step to buy is not always the cheaper line to run.
Bismuth removal is specified down to the compound formed. Calcium and magnesium are dissolved into the melt and the bismuth is taken out as a solid CaMg₂Bi₂ intermetallic that floats and is skimmed, with the dross then chlorinated to free the bismuth. Bismuth is the element that decides whether an all-fire route can hit the customer’s specification, and the page says so before the quotation is issued.
The purity ceiling of fire refining is stated plainly. A kettle line reaches the four-nines grade that the market specifications call for. It does not reach five or six nines, and no kettle will. Saying where the ceiling is lets a buyer who needs a higher grade be routed to electrorefining instead of being sold a plant that cannot meet the certificate.
The kettle is treated as a wear part with a measurement regime. The pot body is a consumable, its life is tracked as wall thickness against a stated floor, and the survey interval and the retirement criterion are issued with the drawing. Loss of containment at refining temperature is a release of molten lead, so the thickness record is part of the operating envelope rather than a maintenance preference.
Agitation and transfer are designed as process equipment, not as conveniences. Lead’s thermal conductivity is about 35 W/m·K, so a still bath stratifies and the reactions stop. The pump or the impeller is what keeps every litre of the bath at the same temperature and in contact with the reagent, and the same pump is what moves the metal between kettles so the line does not lose hours to ladling.
The hood, the extraction and the baghouse are specified against the lead exposure limits. The kettle mouth, the skimming station, the dross handling point and the ingot line are each hooded, the extraction volume and the opening velocity are stated, and the baghouse behind them is sized for the dust load the plant actually generates. Every figure is traceable to the 50 µg/m³ permissible exposure limit and the 30 µg/m³ action level, which is the only basis on which a lead plant can be permitted.
The dross, the slag and the crust are each given a destination. Copper dross, Harris slag, silver-zinc crust and bismuth dross are four different streams with four different values, and each one is weighed, sampled and routed rather than tipped together. Separating them is what turns the by-products from a disposal cost into a recovery credit and stops a valuable stream being diluted into waste.
The line is specified around the grade, not around the tonnage. Kettle count, kettle volume, reagent schedule, number of slagging passes and casting rate all follow from the bullion assay and the contracted grade, so a plant asked to make four-nines battery lead and a plant asked to make a low-antimony alloy are not the same line with a different nameplate.
The whole configuration is written down before manufacture. Kettle volume and count, burner capacity, agitation and transfer, hood and extraction, dross and slag handling, casting rate, the temperature and reagent schedule and the site utilities are issued as a process design package, so the plant the customer receives is the plant that was designed against their assay.

Application Scenarios

Secondary lead smelters refining their own bullion. Battery recycling plants whose rotary or blast furnace produces hard lead carrying antimony, and which need to turn that bullion into refined ingots at a sellable grade rather than sell the bullion at a discount.
Lead-acid battery recycling lines selling battery-grade lead. Plants whose customers are battery makers, where the specification is the four-nines grade and the antimony content is written into the purchase order, and where the ingot has to arrive with a certificate rather than a claim.
Primary lead smelters finishing blast-furnace bullion. Operations whose bullion arrives with the full impurity load — copper, arsenic, tin, antimony, silver, gold, zinc and bismuth — and which need the complete fire-refining sequence rather than a single softening step.
Refineries recovering silver and gold from lead bullion. Sites where the precious-metal content of the bullion pays for the whole plant, and where the design priority is the desilverising step, the pressing of the zinc crust and the treatment of that crust.
Lead alloy houses making battery alloys to specification. Producers making calcium-tin, antimony or low-antimony grid alloys, where the refining sequence runs first and the alloying addition is made afterwards against a written composition.
Lead oxide and red lead producers. Plants where bismuth and antimony in the feed change the electrochemical behaviour of the oxide, and where the refining step exists to hold bismuth below the level the oxide specification allows.
Rolling mills and extrusion plants remelting their own scrap. Operations that generate clean internal scrap and need to remelt and lightly refine it back to the working grade without sending it out of the plant.
Cable, sheet and pipe producers. Manufacturers whose feed has to be soft, ductile and free of antimony and bismuth, and whose rejection rate is driven by hardness rather than by assay on paper.
Radiation shielding and ballast producers. Applications where the requirement is density and homogeneity rather than electrical behaviour, and where the refining target is a clean, oxide-free, gas-free ingot rather than a fine impurity specification.
Refineries handling high-bismuth bullion. Sites where bismuth in the bullion is high enough that fire refining alone cannot reach the target grade, and where the honest answer is a debismuthising step followed by a decision about electrorefining the remainder.
Plants processing purchased scrap lead. Operations refining scrap lead, cable sheath, pipe, sheet and dross bought on the open market, where the bullion assay changes lot by lot and the refining sequence has to be re-decided for each campaign.
Antimonial lead and antimony by-product producers. Plants whose antimony-bearing dross and slag are a saleable product rather than a waste, and which need the dross streams separated and quantified from the start.
Plants replacing open-pot refining with a controlled line. Operations still refining in an open pot with manual reagent addition and hand skimming, where the problems are inconsistent assay, high metal loss, uncontrolled temperature and an operator breathing lead fume.
Plants whose ingots are rejected on assay or on surface. Producers whose rejection rate comes from a drifting antimony or bismuth figure, or from an ingot with an oxide skin and shrinkage porosity, where the fix is a controlled temperature and reagent schedule and a proper casting machine rather than tighter inspection.
Small and pilot refining cells. Reduced-scale lines built from a single kettle upward, where the same temperature and reagent reasoning scales down instead of being replaced by a different concept.

Applicable Raw Materials

The line accepts lead that is already in metallic form and already melted or meltable: bullion from a smelting furnace, refined lead that has to be brought back to grade, clean scrap lead in a metallic form, and the internal returns of the plant itself. What it does not accept is a solid charge that has to be reduced — lead oxide, lead sulfate or battery paste — because reduction is a smelting duty carried out in a reducing furnace and not a refining one, and the reagents, the atmosphere and the off-gas are all different. Material that is wet, oily or contaminated with an element that would report to the slag or to the fume is dried, degreased or routed to smelting first. Output is stated for every feed.

Hard lead bullion from a blast or rotary furnace. The classical feed: bullion tapped from a reduction furnace carrying copper, arsenic, tin, antimony, silver, gold, zinc and bismuth, and often deliberately antimonial because antimony makes the smelting slag fluid. Output: refined lead ingot at the grade the refining sequence is designed to reach.
Soft lead bullion from a low-antimony smelting route. Bullion from a plant that has smelted a low-antimony feed, where the softening step has little to remove and the sequence shortens to copper drossing, desilverising and final refining. Output: refined lead ingot with the shortened sequence recorded against the lot.
Copper-drossed bullion transferred from an upstream kettle. Bullion that has already had its copper taken out and is arriving as a molten transfer rather than as cold bars, which removes one melting step from the cycle and one temperature set point from the day. Output: bullion entering the softening step at temperature.
Lead bullion with a high precious-metal content. Feed where the silver and gold in the bullion are the reason the plant exists, and where the desilverising step and the treatment of the zinc crust are designed first and the lead grade second. Output: refined lead ingot plus a silver-zinc crust routed to precious-metal recovery.
Scrap lead in clean metallic form. Cable sheath, sheet lead, pipe, roofing lead, wheel weights and battery lugs, melted and refined without having to be smelted first, which is the shortest route from scrap to a saleable ingot. Output: refined lead ingot from a melted and refined metallic scrap charge.
Refined lead returned for re-refining. Ingots or bars that failed a bismuth, antimony or surface check, melted back down and put through the part of the sequence that fixes the failure rather than through the whole line again. Output: re-refined lead ingot at the declared grade.
Plant internal returns. Kettle heel, splash and spill, ingot croppings, casting runner metal, and metal recovered from the dross and slag streams, all of which circulate inside the plant rather than leaving it. Output: recovered metal returned to the next charge with its weight recorded.
Copper dross and its liquated metal. The copper-bearing dross skimmed in the first step, treated in a small liquating furnace so that the metal entrained in it returns to the line and the copper concentrate leaves it. Output: liquated metal returned to the kettle and a copper dross sent for copper recovery.
Harris slag and the metal carried in it. The sodium slag from the softening step, which carries arsenic, tin and antimony as sodium salts along with entrained lead, and which is either sold as a by-product or worked up for its metal content. Output: a sodium slag by-product with entrained lead recovered and returned.
Parkes crust and the desilverised lead beneath it. The zinc-silver-gold crust pressed off the desilverising kettle, which is the richest stream in the plant and goes to its own treatment rather than back into the lead circuit. Output: a silver-zinc crust for precious-metal recovery and a dezincing feed for the next step.
Bismuth dross from the calcium-magnesium addition. The CaMg₂Bi₂-bearing dross skimmed from the debismuthising step, chlorinated to free the bismuth and to return the calcium and magnesium carrier to use. Output: a bismuth-bearing dross for bismuth recovery.
Lead alloys and specification feeds. Calcium-tin, antimony and other grid alloys charged against a written composition, where the refining sequence runs first and the alloying addition is made last so that the declared composition is not disturbed by a removal step. Output: alloy ingot at the declared composition and grade.
Feed that must be routed to smelting rather than to refining. Lead oxide, lead sulfate, battery paste, desulfurised paste and any lead-bearing residue in which the lead is chemically combined, because these need reduction and not refining. Output: a documented re-route to the smelting step with the reason recorded.
Feed that must be dried, degreased or segregated first. Wet scrap, oil-contaminated scrap, sealed or hollow components that will burst in a molten bath, and material carrying mercury, cadmium or a chloride load that would report to the fume — all of which are treated or rejected before they reach the kettle. Output: a documented rejection or a re-route with the reason recorded.
Feed whose assay is unknown. Because refining is designed from the assay, an unassayed lot is sampled and analysed before it is charged rather than after the ingot is rejected. Output: an assay report and a refining sequence decided against it.

Finished Products

The line is configured around the grade the customer sells and around the point at which the customer takes delivery of it. Where the customer already operates a battery breaking line, a smelting furnace, a laboratory and an ingot warehouse, the refining line is supplied as the kettles with their combustion and agitation, the transfer and skimming equipment, the hood and extraction, the casting machine and the control system; where the customer buys metal on the open market and sells refined ingots, the dross and slag handling and the assay and marking equipment are included with it.

Four-nines refined lead ingots. The grade the battery, oxide and rolling markets buy against, produced by the full fire-refining sequence with the copper, arsenic, tin, antimony, silver, zinc and bismuth figures declared on the certificate. Output: refined lead ingot at 99.99% with the impurity figures reported individually.
Standard-specification refined lead ingots. Lead refined to the grade that the traded specifications recognise, where the minimum purity and the permitted maximum for each impurity are stated as a table rather than as one number. Output: refined lead ingot at the contracted minimum purity with an impurity table.
Battery-grade lead. Lead refined for grid and paste use, where antimony and bismuth are the two figures the customer looks at first because antimony accelerates grid corrosion and bismuth changes the electrochemical behaviour of the paste. Output: battery-grade ingot with the antimony and bismuth figures declared.
Low-antimony and antimony-free lead. Metal taken through enough softening and final refining passes to leave a residual antimony below the customer’s limit, with the number of passes and the slag destination recorded against the lot. Output: refined lead with a declared residual antimony and the pass count recorded.
Low-bismuth lead. Metal taken through the calcium-magnesium debismuthising step to the bismuth floor that the step can economically reach, with the residual bismuth declared rather than described. Output: refined lead with a declared residual bismuth, and a bismuth-bearing dross.
Battery grid alloys. Calcium-tin, calcium-tin-aluminium and antimony alloys made by refining first and alloying last, so that the removal steps do not strip the alloying element back out of the metal. Output: alloy ingot at the declared composition with a casting certificate.
Ingots in the weights the market trades. Standard ingot weights, characteristically 25 kg and 45 kg classes, cast mechanically so that the weights are uniform and the stack is stable; the weight range is engineered to your order book. Output: weighed, counted and bundled ingots at the declared weight.
Marked and traced ingots. Every ingot carries the grade, the weight, the manufacturer’s mark and a lot number, and the lot record carries the bullion assay, the refining schedule used, the reagent additions, the skims and the final assay. Output: marked ingots and a lot record that stands behind them.
As-cast ingot surface free of oxide skin. The metal is tapped into the casting machine from under the bath surface and the moulds are cooled on a controlled profile, so the ingot arrives with a clean surface rather than with a heavy oxide and dross skin that has to be dressed off later. Output: a clean as-cast surface released against a standard agreed before manufacture.
Ingots free of gas porosity and shrinkage pipe. Residual oxygen and hydrogen are removed by the final refining step and the mould cooling is controlled directionally, so the section is dense rather than porous at the top and loose in the middle. Output: a dense ingot with the shrinkage taken in the feed head rather than inside the ingot.
Silver-zinc crust for precious-metal recovery. The desilverising crust, pressed and weighed, which is the plant’s precious-metal revenue stream and is sold or treated rather than returned to the lead. Output: a weighed, sampled silver-zinc crust with its silver and gold content declared.
Copper dross for copper recovery. The copper-bearing dross from the first step, liquated to recover its entrained lead and then sold or treated for copper. Output: a liquated copper dross with its copper content declared.
Harris slag with its arsenic, tin and antimony declared. The sodium slag from the softening step, sampled and weighed, and either sold as a by-product or worked up for its metal values. Output: a sodium slag by-product with an assay and a destination.
Bismuth-bearing dross for bismuth recovery. The CaMg₂Bi₂ dross from the debismuthising step, chlorinated and worked up, where the bismuth recovered is frequently worth more than the treatment costs. Output: a bismuth-bearing dross worked up to a bismuth product.
A closed metal balance for every campaign. Bullion in, ingots out, each dross, slag and crust weighed and assayed, and the difference declared as the loss for that campaign, so that the irrecoverable fraction is visible and any drift is caught on the next campaign rather than at the month end. Output: a closed campaign metal balance with every stream accounted for.
An assay certificate for every lot. Grade and impurity figures declared by the method and the laboratory agreed in the contract, run on a sample taken from the bath and from the cast ingot, so acceptance does not depend on how the figure was produced. Output: an assay certificate for the lot.

Lead Refining Furnace Process Flow

The nine stages below are the working sequence of one lead refining line, from the bullion assay that decides which steps the plant will run, through the four chemical steps that take copper, arsenic-tin-antimony, silver-gold and bismuth out of the bath, to the final refining pass, the tap and the ingot. It is written at machine level rather than at plant level on purpose: this is the sequence a refining supervisor works through when the question is which impurity comes out first, at what temperature, with which reagent, and what has to happen to the dross afterwards. Kettle volume and kettle count, burner capacity, agitation and transfer, hood and extraction, dross and slag handling, casting rate and the temperature and reagent schedule are fixed during process design against your own bullion assay, the grade you have to reach and your site.

Lead refining line working sequence — nine stages from the bullion assay that decides the route through copper drossing, softening, desilverising, dezincing and debismuthising to a refined, assayed and marked ingot, with the copper dross, the Harris slag, the silver-zinc crust and the bismuth dross each leaving as a separate weighed and sampled stream.

Steps 1 — Bullion Assay and Refining Route Selection. Nothing about this plant is decided before the bullion has been analysed, because the impurity load is what selects the sequence. Copper is taken out by cooling, arsenic and tin and antimony by alkaline oxidation, silver and gold by zinc partitioning, zinc by distillation or oxidation, and bismuth by a calcium-magnesium addition, and each of those steps is only built if the assay says it is needed. A bullion with 0.05% copper and 0.3% antimony runs three steps; a bullion with two per cent antimony and a silver content that pays for the plant runs all of them, twice. The assay also fixes the number of passes, because residual antimony and residual bismuth each sit on an equilibrium with the stream they are removed into, and the target grade therefore has to be translated into a number of slagging and drossing operations rather than assumed to follow from one. This stage ends with a written temperature and reagent schedule, a kettle count and a metal balance, and it is the stage at which a bismuth content too high for fire refining is identified rather than discovered at the ingot. Reagent and utility input: a representative bullion sample and its full elemental assay, the grade and impurity limits to be delivered, the batch size and the campaign pattern, the dross and slag destinations available, existing furnace and laboratory capability. Output: a refining route and step sequence, a temperature and reagent schedule, a kettle count and volume, a metal balance between bullion in and ingots plus by-products out, and the utility and extraction duties that follow from it.

Steps 2 — Kettle Charging, Melt-Down and Copper Drossing. Cold bullion bars are charged into the copper-drossing kettle and melted down, or molten bullion is transferred in from the smelting furnace by pump or launder, and the kettle is filled to its working level rather than to its rim because agitation, reagent addition and skimming all need freeboard above the bath. The step that follows is a liquation and not a chemical reaction: copper and copper arsenide are far less soluble in solid lead than in liquid lead, so the bath is cooled back towards its freezing point and those compounds crystallise, float and thicken into a dross. A small sulfur addition is made only to harden that dross so that it skims as a coherent crust instead of smearing back into the metal, which is the opposite of what a sulfide-forming description implies. The dross is skimmed, weighed and routed to a small liquating furnace, where the lead entrained in it drains back and returns to the kettle while the copper leaves as a concentrate. The temperature matters more here than anywhere else in the sequence, because the bath has to be cold enough for the compounds to crystallise and hot enough to remain fluid enough to stir and skim, and a kettle held too hot simply retains its copper. Reagent and utility input: weighed bullion charge or a molten transfer, kettle preheat and combustion control, agitation or recirculation, a measured temperature set point near the freezing point of lead, a small sulfur addition, a skimming tool or machine and a liquating furnace for the dross. Output: copper-drossed bullion ready for softening, a weighed and sampled copper dross, and liquated metal returned to the kettle with its weight recorded.

Steps 3 — Softening by Oxygen Blowing and Harris Slagging. Arsenic, tin and antimony are removed by oxidation, and the industrial version of that is the Harris process: molten caustic soda is added to the bath with sodium nitrate as the oxidant, and the three elements are converted into sodium salts that float as a slag and are poured or skimmed off. Air or oxygen can be blown through the bath as well, which oxidises the same elements to their oxides and lifts them into the slag or into the fume, and the choice between blowing and purely alkaline oxidation is a function of how much arsenic and antimony there is and what the extraction system can carry. The step is run hot, because the reactions only proceed at a useful rate in a fluid bath and because the caustic has to stay molten on top of the metal rather than freezing into a crust, and it is run more than once, because antimony in particular leaves the metal only until the slag sitting on top of it reaches the same antimony activity — so the residual antimony after a pass is a function of the slag the pass produces, and the grade the customer wants is reached by deciding how many slags to make and what to do with each of them. Photographing the slag during the pass is how an operator judges the endpoint, and the bath is sampled and analysed rather than eyeballed for the final one. Reagent and utility input: molten caustic soda, sodium nitrate as oxidant, air or oxygen for blowing, agitation or recirculation to bring the bath into contact with the reagent, a measured softening temperature set point, a slag-skimming tool or machine, and an extraction system able to carry the oxide fume. Output: softened lead with arsenic, tin and antimony reduced, a weighed and sampled Harris slag carrying those elements as sodium salts, and a bath sampled and analysed between passes.

Steps 4 — Desilverising by Zinc Addition and Crust Removal. Silver and gold are taken out with zinc, and the mechanism is partitioning rather than reaction: zinc has a large solubility in liquid lead and almost none in solid lead, it combines strongly with silver and gold, and it is much lighter, so when it is stirred into the bath a zinc-rich liquid separates and floats, carrying the silver and gold with it. The zinc is added as cakes or ingots that are melted on the surface and stirred in mechanically, and the resulting crust is pressed while it is still pasty so that the entrained lead is squeezed back out of it and the crust goes to the retort or the vacuum unit as rich as it can be made. The addition is normally made in two stages, a heavy first zincing that takes the bulk of the silver and is pressed off, and a lighter second zincing that takes what is left and is skimmed by hand, because the marginal silver recovered in a second pass is worth the extra zinc and the extra handling. The bath temperature is held inside a window rather than at a single best number: too cold and the crust does not separate into a coherent layer, too hot and the zinc stays dissolved in the lead and takes nothing out. This is the step where the plant’s precious-metal revenue is actually captured, which is why the pressing, the weighing and the sampling of the crust are treated as production operations rather than as housekeeping. Reagent and utility input: zinc in cakes or ingots, mechanical stirring, a bath temperature held inside a stated partition window, a press or a squeezing arrangement for the pasty crust, a second lighter zincing and a hand skim, and a weighed and sampled crust route. Output: a pressed silver-zinc crust carrying the silver and gold, weighed and sampled, and desilverised lead that now carries residual zinc and is ready for the dezincing step.

Steps 5 — Dezincing by Vacuum or Alkaline Treatment. The lead leaving the desilverising kettle carries zinc, and there are two accepted ways to take it out, with different consequences for the rest of the plant. The first is vacuum dezincing: zinc has a much higher vapour pressure than lead, so holding the bath under reduced pressure distils the zinc off and it can be condensed and returned to the circuit as metal, which means the reagent is recycled rather than consumed and the plant buys less zinc per tonne. The second is alkaline treatment: the bath is oxidised with caustic soda and sodium nitrate in exactly the way the softening step works, and the zinc leaves as an oxide in the dross, which is simpler, needs no vacuum vessel, and produces another stream that has to be sold or worked up. Which one is right is a plant-level decision that depends on the zinc price, the vacuum equipment already in the plant and how many kettles the sequence can afford to occupy, and the two routes do not give the same kettle count or the same cycle time. In both cases the endpoint is the residual zinc in the bath, and it is measured rather than inferred, because residual zinc is the element most likely to appear in the finished ingot as a bright hazy surface and a hardness figure the customer will reject. Reagent and utility input: a vacuum vessel and pump set where the distillation route is used, or molten caustic soda and sodium nitrate where the alkaline route is used, agitation or recirculation, a measured bath temperature, an assay method for residual zinc, and a route for the recovered zinc metal or the zinc-bearing dross. Output: dezinced lead at a declared residual zinc, and either condensed zinc metal returned to the desilverising step or a weighed and sampled zinc-bearing dross.

Steps 6 — Debismuthising with Calcium and Magnesium. Bismuth is the element that makes fire refining hard, because it is less reactive towards oxygen than lead is and therefore survives every slagging step that removes antimony and tin. The route that works is the calcium-magnesium addition, the Betterton–Kroll reaction: calcium and magnesium metals are submerged into the bath so that they dissolve, and they combine with the bismuth to form a solid intermetallic, CaMg₂Bi₂, which is insoluble in lead, has a lower density than lead and floats as a dross that is skimmed off. The step depends on two temperatures rather than one, because the calcium and magnesium have to be dissolved into the metal hot and the compound has to crystallise and float cold, and it needs real agitation because the reagents dissolve at the bottom of the bath and the compound separates at the top. The dross is then chlorinated, which converts the compound and frees the bismuth as a recoverable product and returns the calcium and magnesium carrier to use. The step is also where the plant reaches its economic floor: as the bismuth content falls, the fraction of the added calcium and magnesium that finds a bismuth atom to combine with falls with it, so the reagent cost per kilogram removed rises steeply, and beyond the floor the honest answer is to stop and electrolyse the remainder rather than chase it in the kettle. Reagent and utility input: calcium and magnesium metal added below the bath surface, an agitation system able to dissolve and mix them through the whole bath, a hot dissolution temperature and a colder crystallisation temperature held in sequence, a skimming tool or machine, and a chlorination step for the dross. Output: debismuthised lead at a declared residual bismuth, and a weighed and sampled CaMg₂Bi₂-bearing dross worked up to a bismuth product.

Steps 7 — Final Refining and Composition Adjustment. The bath now carries the residue of everything that was added to it, and the last chemical step takes those reagents back out. Residual calcium, magnesium and zinc are removed by the same alkaline oxidation that the softening step uses — caustic soda with sodium nitrate — which converts them into oxides that report to a final dross, and the residual antimony that the slagging passes left on the equilibrium can be trimmed in the same operation. This is also the stage where the metal is finally assayed against the contract rather than against an expectation: a sample is cast, prepared and analysed for the full impurity list, and the figure either releases the bath to casting or triggers one more pass. Where the customer’s product is an alloy rather than pure lead, the alloying addition is made after this step and not before, because every removal step in front of it would strip the alloying element out of the metal again — calcium and tin for a grid alloy, antimony for an antimonial grade, and the addition is made against a written composition and verified by analysis before the bath is tapped. The kettle is held at a temperature just above the freezing point of the finished metal through this stage, because the alloying elements dissolve slowly at low superheat and oxidise at high superheat. Reagent and utility input: molten caustic soda and sodium nitrate for the final oxidation, agitation or recirculation, a bath temperature near the freezing point of the finished metal, a casting and preparation bench for the sample, an assay laboratory and the agreed analytical method, and the alloying additions where an alloy is being made. Output: refined lead at the declared grade with the full impurity list analysed, or an alloy at the declared composition, released to casting with the analysis recorded against the lot.

Steps 8 — Refined Lead Tapping and Ingot Casting. The bath is tapped from under the surface rather than poured off the top, so that the last of the final dross and any floating oxide is left behind in the kettle instead of entering the mould, and the metal is delivered to the casting machine at a temperature just above its freezing point so that it fills the mould without flashing or freezing short. The casting machine runs the moulds on a chain, fills each one to a set weight, cools them on a controlled water or air profile, strips the ingot, marks it and stacks it; the weight is checked against the declared ingot weight and the count is reconciled against the bath weight, because that comparison is the check that the tap and the fill are still doing what they were designed to do. The ingot cools on the machine and not in the air of the shop, and the moulds are dressed and kept at a controlled temperature so that the ingot surface is clean and free of the oxide and dross skin that an uncontrolled pour produces, and so that the section is dense instead of porous and piped. The casting rate, the mould count and the cooling profile are sized from the batch size and the cycle, and the whole line is arranged so that the tap, the casting and the stacking do not become the bottleneck that the kettles were sized to avoid. Reagent and utility input: a tapping arrangement drawing from below the bath surface, a mould-dressing regime, a linear or carousel casting machine with controlled mould cooling, an ingot marking or stamping unit, a cooling-water circuit and a calibrated ingot scale. Output: weighed, marked and stacked refined lead ingots at the declared weight, with the count reconciled against the bath weight and the ingot samples taken for the release assay.

Steps 9 — Dross Handling, Assay Release and Kettle Turnaround. Every stream that left the bath during the sequence is weighed, sampled, cooled and labelled, because the copper dross, the Harris slag, the silver-zinc crust and the bismuth dross are four different products with four different values and a plant that tips them together has thrown away both the metal and the evidence. The crust is pressed and stored so that its entrained lead drains back, the slag is sized and sampled for arsenic, tin and antimony, and the bismuth dross goes to chlorination. At the same time the ingots are released against the agreed assay, the campaign metal balance is closed — bullion in, ingots out, every by-product stream weighed and assayed, and the difference declared as the loss for that campaign — and the difference between the metal that circulated inside the plant and the metal that has actually gone is stated separately. The kettle is then emptied of its heel or refilled for the next campaign, the burner, the agitator or the pump and the hood are inspected, and the wall thickness of every pot is checked against the survey interval and the retirement floor in the technical file, because the pot body is a consumable whose life is measured and not assumed. A kettle that has reached its thickness floor is retired with the record that retired it. Reagent and utility input: weighing and sampling equipment for each by-product stream, a press and a drainage arrangement for the crust, a chlorination unit for the bismuth dross, the agreed assay method and laboratory, an ultrasonic thickness gauge and the kettle survey record, and the campaign metal-balance sheets. Output: four weighed and sampled by-product streams with declared destinations, an assay certificate releasing the ingot lot, a closed campaign metal balance with the loss declared, and an inspected, surveyed kettle released for the next campaign.

Product Structure (Main Equipment)

The line is assembled from the three equipment groups below. Materials are selected item by item: the pot body in carbon steel plate formed and welded to a stated wall thickness, with the plate grade and the fabricated thickness chosen against the metal temperature and the head of lead above it; the combustion system in a burner train sized to the melt and the number of set points per cycle, with a refractory or a castable lining at the flame side; wetted parts in the pump, the agitator and the transfer launder in a heat-resistant steel or a coated iron; hoods and ducting in steel with a heat-resistant coating; and the baghouse, the fan and the stack selected against the dust load and the temperature of the gas they carry.

Kettle, hearth and combustion system

Pot body. A form-and-weld carbon steel pot, supplied to a stated plate thickness and a working bath level, with a reinforcement band at the rim and a base designed for the burner arrangement beneath it, and issued with the thickness survey points marked on the drawing.
Kettle setting and foundation. The pot sits in a setting with a combustion chamber beneath or around it, a refractory or castable lining at the flame side, and a foundation and plinth designed for the loaded mass of the pot, the bath and the heel.
Burner train. A gas, oil or dual-fuel burner arranged to distribute heat evenly around the pot rather than to impinge on one side of it, with a combustion air train and a modulating control valve that can hold each of the several temperature set points the sequence requires.
Combustion control. A temperature controller reading the metal bath rather than the flame, an air-to-fuel ratio control, and the interlocks and alarms that trip the burner on flame failure, on low combustion air or on an over-temperature bath.
Kettle cover and charging arrangement. A removable cover that limits radiation from the bath at the surface and reduces the open area the extraction has to handle, plus the charging route for cold bars, a molten transfer launder or pump line, and the reagent addition points for sulfur, caustic, sodium nitrate, zinc, calcium and magnesium.
Refractory and insulation. The flame-side lining, the insulation that keeps the pot shell cool enough to work beside, and the access for inspection and replacement of the lining at the end of a campaign.

Agitation, transfer and dross handling

Agitator or recirculation pump. A high-temperature pump or an immersed impeller that keeps the whole bath at one temperature and in contact with the reagent, sized for the viscosity and the density of molten lead and for the solids load during copper drossing and debismuthising, because lead’s thermal conductivity of about 35 W/m·K means a still bath stratifies and stops reacting.
Transfer equipment. Pump, launder and transfer launder arrangement between kettles, so that a multi-step sequence moves molten metal between vessels in minutes rather than by ladling, with the launder heated or drained so that it does not freeze between transfers.
Dross and slag skimming equipment. A skimming machine or an air-driven skimming tool sized to the bath surface, with the reach and the control to take a crust off a wide bath without dragging metal with it.
Crust press. A press or a squeezing arrangement for the pasty silver-zinc crust, arranged so that the entrained lead drains back into the kettle and the crust leaves the plant as rich as it can be made.
Liquating furnace for copper dross. A small hearth vessel in which the copper dross is held so that the metal entrained in it drains back to the line and the copper-bearing residue leaves as a concentrate.
Chlorination arrangement for bismuth dross. A vessel and gas handling set in which the calcium-magnesium-bismuth dross is treated with chlorine to free the bismuth, with the gas handling and scrubbing that the reagent requires.
Vacuum dezincing vessel. Where the distillation route is selected, a vacuum vessel, pump set and condenser arranged so that zinc distilled out of the bath is condensed and returned to the desilverising step as metal.
Dross and slag containment. Cooled, weighed and labelled pans and bins for each of the four by-product streams, kept separate from each other so that the copper dross, the Harris slag, the crust and the bismuth dross are never tipped together.

Hood, extraction, casting and control

Kettle hood and extraction. A hood over the bath, the reagent addition point and the skimming station, sized so that a stated capture velocity is held across the hood opening with the cover in place and with it open for skimming, and ducted to the baghouse rather than discharged into the shop.
Casting and tap hood. Extraction over the tap, the moulds, the ingot stripping point and the stacking area, because the pouring and cooling metal is a continuous low-level source rather than an intermittent one.
Baghouse and dust handling. A fabric filter baghouse sized for the dust load and the gas temperature, with a pre-cooler or a quench where needed, plus the sealed dust collection and the weighing of the collected dust as part of the campaign metal balance.
Extraction fan and stack. A fan selected against the system resistance with the hoods open and closed, and a stack arranged to discharge clear of the working areas, with the monitoring points the site permit requires.
Ingot casting machine. A linear or carousel casting machine with a set fill weight, controlled mould cooling, an ingot stripping arrangement, a marking or stamping unit and a take-away conveyor, sized so that the casting rate matches the kettle cycle rather than lagging behind it.
Mould set and dressing. Moulds in a geometry that produces the declared ingot weight, kept at a controlled temperature and dressed on a schedule so that the ingot surface is clean and the ingot releases without being pulled.
Weighing, sampling and marking. A calibrated scale for the bullion charge and the ingot, a sample casting and preparation bench, and the marking equipment that puts the grade, the weight, the maker’s mark and the lot number on the ingot.
Control and recording system. A PLC or DCS carrying the temperature and reagent schedule for each step in the sequence, the per-step set points and hold times, the reagent addition quantities and the agitator and burner states, with the alarms, interlocks and the event log that records each campaign against the bullion and ingot lot numbers.

Product Features

The refining sequence is issued as a temperature and reagent schedule. Every step carries its own set point, its own reagent addition, its own hold time and its own skim, on one sheet, because a kettle line that is operated to a single quoted temperature range cannot reproduce an assay.
Copper is removed by liquation, and the copper step is designed as one. The bath is cooled towards the freezing point of lead so that copper and copper arsenide crystallise and float, and the sulfur addition is specified as a dross hardener rather than as the reactive agent it is often described as.
Softening is described as an oxidation, with the reagent ratios stated. Caustic soda with sodium nitrate oxidises arsenic, tin and antimony into a sodium slag; the addition quantities, the bathing temperature and the slag condition are specified, and the number of passes is set by the target grade.
The antimony equilibrium is treated as a design constraint. Residual antimony follows the antimony in the slag it sits in contact with, so the pass count and the destination of the antimonial by-product are decided with the customer instead of being discovered at the ingot.
Desilverising is engineered around the crust as a revenue stream. The zinc addition is made in two stages, the bath is held inside a stated partition window, and the pasty crust is pressed so that entrained lead drains back before the crust leaves the plant.
Dezincing is offered as two routes with their consequences stated. Vacuum dezincing returns zinc as metal and removes a reagent from the circuit; alkaline dezincing needs no vacuum vessel and produces another dross. Both are quoted with their kettle count, reagent consumption and by-product stream.
Debismuthising is specified down to the compound formed. Calcium and magnesium are dissolved into the melt hot and the bismuth is taken out as solid CaMg₂Bi₂ cold, and the dross is chlorinated to free the bismuth and return the carrier to use.
The purity ceiling of fire refining is stated rather than implied. A kettle line reaches the four-nines grade the market buys against and does not reach five or six nines; a customer who needs more is routed to electrorefining rather than sold a plant that cannot meet the certificate.
Agitation is a process device and is sized as one. With a thermal conductivity of about 35 W/m·K, molten lead stratifies and stops reacting unless it is pumped; the same pump is what moves metal between kettles, so the transfer problem and the mixing problem are solved once.
Every temperature set point is measured on the bath. The controller reads the metal, not the flame or the shell, because all five separations in the sequence depend on the metal temperature and none of them can be run off a pyrometer pointed at the burner.
The kettle is supplied as a measured consumable. The pot body is a carbon steel vessel with the plate thickness, the survey points, the survey interval and the retirement floor written on the drawing, and the thickness record is part of the technical file rather than a maintenance note.
Hood, extraction and baghouse are specified against the lead exposure limits. The kettle mouth, the reagent addition point, the skimming station, the tap and the ingot line are each hooded, the capture velocity at the opening is stated, and the baghouse is sized for the dust the plant actually makes.
Four by-product streams leave the line separately. Copper dross, Harris slag, silver-zinc crust and bismuth dross are weighed, sampled and routed individually, which is what turns the by-products from a disposal cost into four credits on the metal balance.
The metal balance closes on every campaign. Bullion in, ingots out, every stream weighed and assayed, and the difference declared as the loss for that campaign, with the metal that circulates inside the plant stated separately from the metal that has left it.
Alloying is made after refining and not before. Every removal step would strip an alloying element back out of the metal, so the calcium, tin or antimony addition is the last operation before the tap and is verified by analysis against a written composition.
The line is specified around the grade, not around the tonnage. Kettle count, kettle volume, reagent schedule, pass count and casting rate all follow from the bullion assay and the contracted grade, so a four-nines battery-lead plant and a low-antimony alloy plant are designed as different lines.
Casting is a controlled operation rather than a pour. The tap draws from below the bath surface so the final dross stays in the kettle, the fill weight is set, the mould cooling is profiled, and every ingot is weighed and counted against the bath weight.
Configuration is issued as a process design package. Kettle volume and count, burner duty, agitation and transfer, hood and extraction, dross and slag handling, casting rate, the temperature and reagent schedule and the site utilities are written down and approved before manufacture.

Technical Parameters And Specifications

Lead refining furnace technical parameters. Kettle volume and kettle count, burner duty, agitation and transfer, hood and extraction, dross and slag handling, casting rate and the temperature and reagent schedule are engineered to your bullion assay and your contracted grade; the figures below are the design values the line is specified against.

Parameter	Specification
Primary function	Fire refining of lead bullion in kettles, producing refined lead ingots at a declared grade by removing copper, arsenic, tin, antimony, silver, gold, zinc and bismuth in a sequence of temperature- and reagent-controlled steps
Line type	Kettle-based batch refining line: charge or molten transfer, copper drossing, softening, desilverising, dezincing, debismuthising, final refining, tapping, ingot casting, dross and slag handling
Working principle	Each impurity is removed by the mechanism that suits it: cooling for copper because it is far less soluble in solid lead than in liquid lead, alkaline oxidation for arsenic, tin and antimony because they form sodium salts that float, zinc partitioning for silver and gold because zinc takes them into a lighter phase, vacuum distillation or oxidation for residual zinc, and a calcium-magnesium addition for bismuth because it forms an insoluble intermetallic that floats
Refining route	Full fire-refining sequence where the bullion carries the full impurity load; a shortened sequence — copper drossing, desilverising and final refining — where the softening load is small; and a debismuthising decision taken against the bismuth content and the target grade
Impurity removal order	Copper, then arsenic, tin and antimony, then silver and gold, then zinc, then bismuth, then the residual calcium, magnesium and zinc from the reagents, with alloying last
Feed material	Lead in metallic form only: bullion from a reduction furnace, molten bullion transferred from an upstream kettle, clean scrap lead, internally returned metal and lead alloys charged against a written composition
Feed that is not accepted	Lead oxide, lead sulfate, battery paste and desulfurised paste, because the lead in them is chemically combined and needs reduction rather than refining; wet or oily scrap, sealed or hollow components, and material carrying mercury, cadmium or a chloride load
Bullion assay basis	The full elemental assay of a representative sample, taken before the refining route is fixed, because the impurity load and not the tonnage decides which steps the plant runs and how many passes each one takes
Copper drossing temperature basis	Held near the freezing point of lead, hot enough for the bath to stay fluid enough to stir and skim and cold enough for copper and copper arsenide to crystallise and float
Copper drossing reagent	A small sulfur addition, used to harden the dross so that it skims as a coherent crust instead of smearing back into the metal; the copper leaves as a solid phase and not as a sulfide that has to be formed
Copper drossing target	Copper reduced to the level the softening and final refining steps can carry, with the dross liquated so that entrained lead returns to the kettle and the copper leaves as a concentrate
Softening method	Oxidation of arsenic, tin and antimony into a floating sodium slag, by the Harris process with molten caustic soda and sodium nitrate, and by air or oxygen blowing where the load and the extraction system suit it
Softening reagent	Molten caustic soda with sodium nitrate as the oxidant; the reagent ratio, the addition rate and the slag condition are specified per pass
Softening temperature basis	Hot enough for the oxidation reactions to proceed at a useful rate and for the caustic to stay molten on top of the metal rather than freezing into a crust, and below the temperature at which lead itself oxidises into the slag in quantity
Softening slag	A sodium slag carrying arsenic, tin and antimony as sodium salts, poured or skimmed off, weighed, sampled and routed as a by-product rather than returned to the lead
Softening pass count	Set by the target grade and not by a fixed number, because residual antimony follows the antimony content of the slag the pass produces and there is no single pass that reaches zero
Arsenic removal mechanism	Oxidation into the alkaline slag as a sodium arsenate; arsenic is not removed with hydrogen, because arsine is a poison and no commercial lead refinery uses it as a refining route
Tin removal mechanism	Oxidation into the same alkaline slag as a sodium stannate, in the same pass as the arsenic
Antimony removal mechanism	Oxidation into the alkaline slag, ending on an equilibrium with the slag rather than on a fixed target, with the antimonial by-product treated as a co-product with its own market
Desilverising method	Zinc partitioning: zinc is stirred into the bath and takes silver and gold into a zinc-rich liquid that is lighter than lead and floats as a crust, which is pressed and removed
Desilverising temperature basis	Held inside a stated partition window rather than at one best figure; too cold and the crust does not separate into a coherent layer, too hot and the zinc stays dissolved in the lead and removes nothing
Zinc addition basis	Made in two stages: a heavier first zincing that takes the bulk of the silver and is pressed off, and a lighter second zincing that takes the remainder and is skimmed; the addition is sized against the silver and gold in the assay
Silver crust handling	Pressed while pasty so that entrained lead drains back to the kettle, then weighed, sampled and routed to precious-metal recovery as the richest stream in the plant
Dezincing options	Vacuum dezincing, which distils zinc off under reduced pressure and returns it to the circuit as metal, or alkaline dezincing, which oxidises it into a dross; both are quoted with their kettle count, reagent consumption and by-product stream
Vacuum dezincing basis	Reduced pressure applied to the bath, exploiting the much higher vapour pressure of zinc than of lead, with the distilled zinc condensed and returned to the desilverising step
Residual zinc measurement	Measured on a bath sample rather than inferred, because residual zinc shows up in the finished ingot as a hazy surface and a hardness figure the customer rejects
Debismuthising reagents	Calcium and magnesium metals, added below the bath surface so that they dissolve into the melt and combine with the bismuth
Debismuthising temperature basis	Two temperatures held in sequence: a hot dissolution temperature at which the calcium and magnesium enter the melt, and a colder crystallisation temperature at which the compound separates and floats
Bismuth compound formed	Solid CaMg₂Bi₂, insoluble in lead and lower in density than lead, which floats as a dross and is skimmed off rather than staying in solution
Bismuth dross treatment	Chlorination of the dross, which frees the bismuth as a recoverable product and returns the calcium and magnesium carrier to use; the bismuth recovered frequently exceeds the treatment cost
Debismuthising economic floor	Below a stated residual bismuth the fraction of added calcium and magnesium that finds a bismuth atom falls steeply and the reagent cost per kilogram removed rises with it, which is the boundary at which fire refining stops and electrorefining begins
Final refining reagent	The same alkaline oxidation used for softening — molten caustic soda with sodium nitrate — applied to remove residual calcium, magnesium and zinc and to trim residual antimony
Alloying sequence	Performed after final refining and immediately before the tap, because every removal step in front of it would strip an alloying element out of the metal; the addition is made against a written composition and verified by analysis
Purity reachable by fire refining	The four-nines grade — about 99.99% lead — which is the class the traded specifications and the battery, oxide and rolling markets buy against
Purity not reachable by fire refining	Five and six nines; grades above the four-nines class are made by electrorefining in a fluosilicate or sulfamate electrolyte, which is a separate plant with its own anode, cell and chemistry, and no kettle line reaches them
Lead melting point	327.5 °C, used as the datum the copper drossing set point and the casting temperature are taken from
Lead boiling point	1,749 °C, which is why vacuum dezincing can distil zinc out of a lead bath without evaporating lead
Lead density	About 11.34 g/cm³, or about 11,340 kg/m³, which sets the structural load on the kettle, the setting and the foundation for every litre of bath
Lead thermal conductivity	About 35 W/m·K, roughly a tenth that of aluminium, which is the reason a still bath stratifies and agitation is a process requirement rather than a convenience
Lead latent heat of fusion	About 23 to 24.5 J/g, which with the specific heat sets the burner duty needed to melt a charge inside the cycle the sequence allows
Kettle body material	Carbon steel plate, formed and welded to a stated wall thickness, with the plate grade and thickness selected against the metal temperature and the head of lead above it
Kettle setting	A pot set in a combustion chamber with a refractory or castable lining at the flame side, on a foundation and plinth designed for the loaded mass of the pot, the bath and the heel
Kettle wall thickness survey	Written into the drawing and the technical file, with the survey points marked, the survey interval stated and a retirement floor defined, because the pot body is a consumable and loss of containment at refining temperature is a release of molten lead
Kettle capacity basis	Engineered to your batch size, the number of steps in the sequence and the heel each kettle has to carry, with the working bath level set below the rim to leave freeboard for reagent addition and skimming
Working bath level	Below the rim, leaving the freeboard that agitation, reagent addition and skimming all require, and stated on the drawing rather than left to the operator
Agitation	A high-temperature pump or an immersed impeller, sized for the density and viscosity of molten lead and for the solids load during copper drossing and debismuthising, keeping the whole bath at one temperature and in contact with the reagent
Metal transfer	Pump and heated launder between kettles, so that a multi-step sequence moves molten metal in minutes instead of by ladling, with the launder drained or heated so that it does not freeze between transfers
Dross and slag skimming	A skimming machine or an air-driven skimming tool sized to the bath surface, with the reach and control to lift a crust without dragging metal out with it
Combustion and heating	A gas, oil or dual-fuel burner train arranged to distribute heat around the pot rather than impinge on one side of it, with modulating control able to hold each of the temperature set points in the schedule
Temperature measurement	Taken on the metal bath rather than on the flame or the shell, because every separation in the sequence depends on the metal temperature and none of them can be controlled off a pyrometer aimed at the burner
Hood and extraction	Individual hooding of the kettle mouth, the reagent addition point, the skimming station, the tap and the ingot line, each sized so that a stated capture velocity is held across the opening in both the covered and the open condition
Lead fume control basis	Specified against the permissible exposure limit for airborne lead of 50 µg/m³ as an eight-hour average and the action level of 30 µg/m³, which are the figures that set the hood geometry, the extraction volume and the housekeeping regime
Emission control	A fabric filter baghouse sized for the dust load and the gas temperature, with pre-cooling or quenching where needed, sealed dust collection, and the collected dust weighed and assayed into the campaign metal balance
Extraction fan and stack	A fan selected against the system resistance with the hoods open and closed, and a stack arranged to discharge clear of working areas with the monitoring points the site permit requires
Ingot casting	A linear or carousel casting machine with a set fill weight, controlled mould cooling, an ingot stripping arrangement, a marking or stamping unit and a take-away conveyor, sized so that the casting rate matches the kettle cycle
Tapping arrangement	The bath is tapped from below the surface so that the final dross and any floating oxide stay in the kettle instead of entering the mould
Ingot weight	Standard traded weights, characteristically 25 kg and 45 kg classes, cast mechanically so that the weights are uniform and the stack is stable; the weight range is engineered to your order book
Ingot marking and traceability	Grade, weight, maker’s mark and a lot number on the ingot, with the lot record carrying the bullion assay, the refining schedule used, the reagent additions, the by-product weights and the release assay
Refined lead grade	Declared against the contract as a minimum purity with a permitted maximum for each impurity, released by an agreed analytical method on a sample from the bath and from the cast ingot
By-product streams	Four separate streams — copper dross, Harris slag, silver-zinc crust and bismuth-bearing dross — each weighed, sampled and routed individually rather than tipped together
Metal balance	Closed for every campaign: bullion in, ingots out, every by-product stream weighed and assayed, and the difference declared as the loss for that campaign, with internal circulating metal stated separately
Control system	A PLC or DCS carrying the temperature and reagent schedule for each step, with the per-step set points and hold times, the reagent addition quantities, the agitator and burner states, and an event log recording each campaign against the bullion and ingot lot numbers
Interlocks and alarms	The burner interlocked against flame failure, low combustion air and over-temperature bath; the agitator interlocked against operation with the bath above its working level; and alarms for an out-of-window desilverising temperature, a burner trip, an extraction fan failure and a reagent addition outside the schedule
Operator requirement	A trained refining crew working to the written temperature and reagent schedule, with the kettle, the agitation, the reagent addition and the skimming treated as documented operations rather than as operator judgement
Materials of construction	Carbon steel for the pot bodies and the hoods, refractory or castable lining at the flame side, heat-resistant steel or coated iron for the pump, agitator and launder wetted parts, and steel ducting with a heat-resistant coating
Capacity, batch size, power, footprint and utilities	Engineered to your bullion assay, the impurities you must remove, your target grade, your batch size and campaign pattern, and stated in the process design package rather than taken from a fixed model list
Installation	Indoor on a level floor with the kettle settings, the transfer route, the casting line and the extraction ductwork shown on the layout drawing, with the baghouse, the fan and the stack located against the site permit rather than against convenience
Verification	Bullion assay before the route is fixed, bath samples between passes and before the tap, released against an agreed analytical method on the cast ingot, with every by-product stream weighed and assayed into a closed campaign metal balance

Technical Advantages

The refining sequence is written down as set points and reagent additions, so it can be reproduced. A kettle line operated to a single quoted temperature range cannot repeat an assay; a line operated to a per-step schedule can, and the schedule is issued as part of the design package rather than reconstructed from memory on the shop floor.
Copper is taken out by the mechanism that actually works. Copper and copper arsenide are far less soluble in solid lead than in liquid lead, so the drossing step is a controlled cooling operation in which those compounds crystallise and float, and the sulfur addition hardens the dross so that it skims cleanly. Describing it as a sulfide-forming reaction produces an operator who adds the wrong amount of the wrong reagent.
Arsenic, tin and antimony are removed into a slag that can be sold. The Harris process converts them to sodium salts that float and are poured off, and the slag leaves the plant as a weighed, sampled by-product with a declared content rather than as a waste stream. The route is also stated as what it is, so no operator is told to expect an arsine off-gas that does not exist.
The antimony equilibrium is designed around instead of denied. Residual antimony follows the antimony in the slag it sits in contact with, so the pass count, the reagent additions and the destination of the antimonial by-product are decided together with the customer. A plant that plans its antimony as a co-product plans a different number of kettles from one that plans to landfill it.
Desilverising is designed as a recovery operation, because that is where the money is. Zinc is added in two stages, the bath is held inside a partition window rather than at one temperature, and the pasty crust is pressed so that entrained lead drains back. The crust is the richest stream in the plant, and designing it as a production operation rather than as housekeeping is what makes the precious-metal balance close.
Dezincing is quoted as two routes with their real consequences. Vacuum dezincing returns zinc as metal and takes a reagent out of the circuit but adds a vacuum vessel; alkaline dezincing needs no vacuum but produces another dross. The customer sees both the capital and the reagent side, which is the only honest way to choose between them.
Debismuthising is specified to the compound that forms. Calcium and magnesium are dissolved into the melt hot and the bismuth leaves as solid CaMg₂Bi₂ cold, and the dross is chlorinated so that the bismuth is recovered and the carrier returns to use. Bismuth is the element that decides whether an all-fire route can meet the specification, and it is treated as a design question rather than a surprise.
The purity ceiling is stated so that the right plant is sold. A kettle line reaches the four-nines grade the market buys against and no kettle reaches five or six nines. A customer who needs the higher grade is told that the answer is electrorefining, which is a different plant — and a customer whose specification is within four nines is told that fire refining will meet it.
Agitation solves the mixing problem and the transfer problem at once. With a thermal conductivity of about 35 W/m·K, a still lead bath stratifies into a hot layer and a cooler layer and the chemistry stops at the boundary; the pump that fixes that is the same pump that moves metal between kettles, so a multi-step sequence does not lose half a shift to ladling.
Every set point is read on the metal. The controller measures the bath and not the flame or the shell, because all five separations in the sequence are functions of metal temperature and none of them can be run off a pyrometer aimed at a burner.
The kettle is delivered as a measured consumable with a retirement criterion. The plate thickness, the survey points, the survey interval and the thickness floor are on the drawing, and the record of those surveys is part of the technical file. Loss of containment at refining temperature is a release of molten lead, so the thickness record and the operating envelope are the same document.
The hoods and the baghouse are sized against the exposure limits, not against appearances. Capture velocity at the kettle mouth, at the reagent addition point, at the skimming station, at the tap and along the ingot line is stated for the covered and the open condition, and the baghouse is sized for the dust the plant actually makes. That is the only basis on which a lead plant is permitted, and it is also the only basis on which it can be operated without over-exposing the crew.
By-products leave as four separate, weighed and assayed streams. Copper dross, Harris slag, silver-zinc crust and bismuth-bearing dross have four different values and four different destinations, and keeping them apart is what turns the refining residue from a disposal cost into credits on the metal balance.
The campaign metal balance closes and the loss is declared. Bullion in, ingots out, every stream weighed and assayed, with the difference stated as the loss for that campaign and the metal circulating inside the plant reported separately. That distinction is what stops a shop-floor loss reading being mistaken for metal that has gone, and it makes any drift visible on the next campaign rather than at the month end.
Alloying is put where it cannot be undone. The calcium, tin or antimony addition is made after final refining and immediately before the tap, against a written composition and verified by analysis, because every removal step in front of it would take the alloying element back out of the metal.
Configuration is issued in writing before manufacture. Kettle volume and count, burner duty, agitation and transfer, hood and extraction, dross and slag handling, casting rate, the temperature and reagent schedule and the site utilities are engineered from your bullion assay and your contracted grade and issued as a process design package rather than discovered at commissioning.

Service Process

Step 1 — Bullion Assay and Route Consultation. Send a representative sample of the bullion or scrap lead you intend to refine, together with the grade and the individual impurity limits you have contracted to deliver, the batch size and the campaign pattern, and the by-product destinations you have available. Our metallurgical team analyses the sample across the full impurity list, works out which separations the assay actually requires, decides how many softening passes the target grade implies and where the economic floor of the debismuthising step falls for your bismuth level, and states plainly if the grade you want is outside what a fire refining route can reach. Output: an assay report, a recommended refining route and step sequence, and a preliminary proposal with the kettle count the sequence implies.

Step 2 — Laboratory and Pilot Refining Trials. Bench and pilot work on your metal confirms the temperature and reagent schedule against the real bath: the copper drossing set point that crystallises the copper compounds without dragging silver into the dross, the caustic and sodium nitrate additions and the pass count for your antimony and arsenic load, the zinc addition and the partition window for the silver in your assay, the dezincing route, the calcium and magnesium addition and the two temperatures of the debismuthising step, and the alloying addition where an alloy is required. Output: a trial refining report with bath analyses between steps and ingot analyses at the end, and a temperature and reagent schedule for your metal.

Step 3 — Process Design Package and Layout. The confirmed schedule is converted into the plant: kettle volume and kettle count, the working bath level and the heel each pot carries, burner duty and the combustion control scheme, the agitation and transfer arrangement, the reagent storage and dosing, the hooding and extraction volumes for each open point, the baghouse and fan selection, the dross and slag handling and containment, the casting machine and the mould set, and a layout and utility schedule against your site. Output: a process design package with kettle and setting drawings, the temperature and reagent schedule, the extraction and baghouse specification, the layout and the utility schedule, and a firm quotation.

Step 4 — Fabrication, Assembly and Factory Testing. The pot bodies are formed and welded to the drawing and dimensionally checked with their survey points marked; the settings, burner trains and combustion controls are built and function tested; the pump or agitator and the transfer equipment are assembled; the hoods, ducting, baghouse, extraction fan and casting machine are produced and pre-assembled; and the control system is loaded with the temperature and reagent schedule and checked against a simulated sequence. Output: assembled, pre-piped, pre-wired and factory-tested modules, with the kettle thickness records and the control system sequence test released.

Step 5 — Shipping, Installation and Commissioning. We supervise the site services interface, the setting and levelling of the kettles, the installation of the transfer route, the casting line and the extraction ductwork, the balancing of the hoods so that the stated capture velocity is held at every open point in both the covered and the open condition, the tie-in of the fuel, electrical, cooling-water and compressed-air services, and then a full commissioning campaign on your own bullion until the ingot released at the end of the sequence meets the contracted grade by the agreed analytical method, with every by-product stream weighed and assayed. Output: a producing lead refining line at contracted performance, with the commissioning record, the balanced extraction record and a closed campaign metal balance.

Step 6 — Operator Training and Handover. On-site training covers the reading and use of the temperature and reagent schedule, kettle charging and molten transfer, each step’s set point, reagent addition and skim, the recognition of the slag and dross conditions that mark an endpoint, the pressing and sampling of the silver-zinc crust, the two temperatures of the debismuthising step, the final refining pass and the release analysis, tapping and casting, ingot weighing and marking, the by-product streams and the campaign metal balance, and the kettle thickness survey, supported by standard operating procedures and a complete handover dossier. Output: a trained crew and the complete documentation set.

Step 7 — Process Optimisation and Ongoing Support. Remote troubleshooting, kettle and agitator spares, burner and control service, mould and casting consumables, and periodic performance audits in which the bullion assays, the ingot assays, the reagent consumptions, the by-product weights and assays, the campaign metal balances and the kettle thickness surveys are reviewed against the design, with schedule revisions as your bullion source or your product mix changes. Output: sustained performance, a documented thickness history and spare-part continuity.

Warranty And After-Sales Service

Warranty period. Twelve months from commissioning or eighteen months from shipment, whichever falls first, covering design, materials and workmanship of the equipment supplied, with the terms and the excluded wear parts stated in the contract.
Wear part and consumable clarity. Pot bodies, refractory and castable linings, burners and burner parts, pump and agitator wear components, hood and duct coatings, moulds, and baghouse filter media are identified as consumables with their expected service intervals declared up front, so the operating cost of the line is visible before it is commissioned rather than after.
Kettle thickness survey service. Periodic ultrasonic thickness surveys of every pot body against the marked survey points and the retirement floor in the technical file, with the survey record kept as part of the plant history, so a kettle is retired on measurement rather than on appearance. Output: a thickness survey report against the drawing.
Kettle and setting refurbishment. Re-lining of the setting, burner-duct repair, rim and reinforcement-band repair where the drawing allows it, and the replacement of a pot body at the end of its measured life, planned so that a campaign is not interrupted by a kettle that has reached its floor. Output: a documented refurbishment with the kettle returned to drawing tolerance.
Refractory and lining supply. Flame-side refractory and castable materials, insulation and the consumables for a re-line, supplied as service items with lead times stated at order.
Burner, combustion control and instrument service. Burner servicing, flame-safety and ratio-control verification, thermocouple and controller calibration, and replacement parts, so that each of the temperature set points in the schedule can be held at the value the schedule names. Output: a calibration and verification record against the schedule.
Pump and agitator service. Impeller and shaft wear inspection, seal and bearing replacement, and clearance setting, because the agitator is what keeps the bath mixed and a worn impeller shows up first as a drifting assay rather than as a mechanical fault. Output: a service report with the clearances restored to specification.
Mould and casting service. Mould inspection, re-machining and replacement, fill-weight verification, cooling-profile checks and marking-unit service, so that the ingot weight and the as-cast surface are held against a maintained mould rather than a worn one. Output: an ingot weight and surface verification report against the lot records.
Extraction, baghouse and duct service. Inspection of hoods, ductwork, the fan and the baghouse, filter media replacement, capture-velocity re-measurement at every open point, and stack monitoring-point checks, because the extraction specification is what the plant’s permit rests on. Output: a capture-velocity and baghouse condition report.
Spare parts availability. A recommended commissioning and first-year spares set is quoted with the line, and spare parts remain available through the equipment’s service life with lead times stated at order.
Remote support. Telephone, email and messaging support for operating questions, alarm interpretation, an assay that has drifted, a dross or slag condition that has changed, a crust that will not separate and casting defects while your line is running. Output: same-day response on operating issues.
On-site service. Engineer attendance for warranty faults, process problems and periodical check-ups, with response and attendance times agreed in the contract according to your location. Output: on-site intervention and a service report.
Assay and metal balance verification service. Periodic review of the ingot assays, the by-product weights and assays and the campaign metal balances against your own laboratory and weighbridge records, so that the irrecoverable loss and the internal circulating metal are tracked over time rather than assumed from commissioning. Output: a verification report against the campaign records.
Operator retraining and refresher visits. Additional training, refresher sessions and process optimisation visits as your bullion source, your reagent supply, your product mix or your target grade changes. Output: updated training and revised procedures.
Documentation set. Operation and maintenance manuals, kettle, setting and burner drawings, the temperature and reagent schedule for each product, hood and extraction drawings, the baghouse and fan specification, casting machine and mould drawings, electrical and control drawings, layout, spare parts catalogue, safety data for the reagents and the gases used, and the emergency response plan covering a molten metal spill, a lead fume release, a chlorine release at the bismuth dross treatment step and an electrical isolation. Output: complete documentation handover.
Compliance support. Assistance with extraction and emission parameter declarations, reagent storage and handling requirements, and the handling, storage and consignment of the dross, slag and baghouse dust streams. Output: technical data for your permit applications.
Lifetime technical support. The refining schedule, the kettle drawings and the survey records remain yours, and our engineering team remains available for schedule modification and by-product route advice for the operating life of the line. Output: long-term engineering continuity.

Frequently Asked Questions

What is a lead refining furnace?

A lead refining furnace is a kettle-based line in which lead bullion that already contains its impurities as dissolved metals is heated, held at a sequence of set temperatures and treated with reagents so that copper, arsenic, tin, antimony, silver, gold, zinc and bismuth leave the bath as a dross, a slag or a crust that is skimmed off, and the lead leaves as a refined ingot at the grade the customer’s specification names. It is not a smelter: nothing is reduced here and no lead oxide or battery paste is charged. The line consists of one or more steel kettles set over combustion chambers, a pump or an impeller that keeps the whole bath at one temperature and in contact with the reagent, the reagent addition and skimming equipment, hoods and extraction over the kettle mouth, the skimming station and the ingot line, a baghouse, and a casting machine that produces weighed and marked ingots. The refining route itself is decided from the bullion assay, because the impurity load and not the tonnage determines which of the steps the plant has to run.

Is a lead refining furnace the same as a lead melting furnace?

No, and the two do different jobs. A lead melting furnace takes solid lead scrap – battery grids and lugs, cable sheath, sheet lead, pipe and clean metallic scrap – and melts it into a pool of metal, removes the floating dirt and sulfate crust, and pours ingots that are still carrying every impurity the scrap had. A lead refining furnace takes lead that is already molten or already metallic and takes the dissolved impurities back out of it by chemical means, because copper, arsenic, tin, antimony and bismuth will not leave a lead bath simply because it is hot. Melting for longer does not refine lead: the impurities are in solution and need a reagent and a temperature set point to be removed. In practice most plants run both, melting first and refining second, and some plants refine a molten transfer straight from the smelting furnace so that the melting step is skipped altogether.

What purity can a lead refining furnace reach?

Fire refining in a kettle reaches the four-nines grade, which is 99.99% lead, and that is the class the traded specifications and the battery, oxide and rolling markets actually buy against. It does not reach five or six nines, and no kettle line does, because the last impurities are removed by mechanisms that end on an equilibrium with the slag or the dross they are removed into rather than on a target. Where a process specification demands a higher grade – a fine-chemical or a speciality oxide application, for example – the route that reaches it is electrorefining in a fluosilicate or sulfamate electrolyte, which is a separate plant with its own anode casting, its own cells, its own electrolyte and its own spent-electrolyte treatment. A supplier who quotes six nines for a kettle line is quoting a figure that belongs to a different process, and it is worth asking which one.

Which impurities does a lead refining furnace remove, and in what order?

Copper first, then arsenic, tin and antimony together, then silver and gold, then zinc, then bismuth, and finally the residual calcium, magnesium and zinc left over from the reagents themselves. The order is fixed by the chemistry and not by preference. Copper has to go first because it is removed by cooling the bath towards the freezing point of lead, and a bath that still carries silver cannot be cooled that far without dragging silver into the copper dross. Arsenic, tin and antimony come next because they need a hot oxidising bath, which is the opposite condition to the copper step. Silver and gold need zinc and a lower temperature window again. Zinc has to be removed before bismuth, because the calcium and magnesium used for bismuth would otherwise be consumed by the zinc. And a final pass exists because every reagent added has left something behind in the metal.

Why does copper drossing work by cooling rather than by adding a flux?

Because the separation is a solubility difference and not a chemical reaction. Copper and copper arsenide are far less soluble in solid lead than in liquid lead, so as the bath is cooled towards the freezing point of lead those compounds can no longer stay in solution, they crystallise, and they float to the surface. The bath is then held in the narrow band that keeps it fluid enough to stir and skim while the crystals have already formed, which is a genuine operating skill and the reason the step is specified as a temperature rather than as a reagent addition. A small quantity of sulfur is added, but its job is to harden the dross so that it skims as a coherent crust instead of smearing back into the metal; it is not the agent that removes the copper. A description that says sulfur forms copper sulfide and removes the copper gets the reagent right and the mechanism wrong, and an operator working from it will cool the bath to the wrong point.

What is the Harris process, and what does it remove?

The Harris process is the alkaline softening step, and it removes arsenic, tin and antimony. Molten caustic soda is added to the lead bath with sodium nitrate as the oxidant, and the three elements are oxidised into sodium salts that float on the metal as a slag and are poured or skimmed off. Air or oxygen can be blown through the bath as well, which oxidises the same elements to their oxides and lifts them into the slag or into the fume, and which of the two approaches suits a plant depends on the arsenic and antimony load and on what the extraction system can carry. The step is run hot enough for the caustic to stay molten on top of the metal and for the reactions to proceed at a useful rate, and it is normally run more than once, because residual antimony follows the antimony content of the slag it sits in contact with. The antimony and tin in that slag are recoverable, so the slag is weighed and sampled rather than discarded.

How is silver removed from lead, and where does it go?

Silver and gold are removed with zinc, by partitioning rather than by reaction. Zinc dissolves readily in liquid lead and hardly at all in solid lead, and it combines much more strongly with silver and gold than with lead, so zinc stirred into the bath takes the two precious metals into a zinc-rich liquid that is lighter than lead and floats on it as a crust. That crust is pressed while it is still pasty so that entrained lead is squeezed back into the kettle, and it then leaves the line as the richest stream in the plant: it goes to a retort or a vacuum distillation unit, where the zinc is distilled off and returned to the desilverising step and the silver-gold alloy is separated and refined. The addition is normally made in two stages, a heavier first zincing for the bulk of the silver and a lighter second zincing for the remainder, because the marginal silver that the second pass recovers is worth the extra zinc.

How is zinc removed after desilverising?

By one of two routes, and the choice is a plant-level decision rather than a technical one. Vacuum dezincing exploits the fact that zinc has a much higher vapour pressure than lead, so holding the bath under reduced pressure distils the zinc out and it can be condensed and returned to the desilverising step as metal, which means the plant buys less zinc per tonne and carries a vacuum vessel instead. Alkaline dezincing oxidises the zinc with molten caustic soda and sodium nitrate, exactly as the softening step does, and the zinc leaves as an oxide in a dross, which needs no vacuum equipment and produces one more stream to sell or work up. Both routes leave the same residual zinc in the metal, and the endpoint is measured on a bath sample rather than inferred, because residual zinc in the finished ingot shows up as a hazy surface and a hardness figure that the customer rejects.

How is bismuth removed from lead?

With calcium and magnesium, on what is generally called the Betterton-Kroll route. Bismuth cannot be oxidised out of lead because it is less reactive towards oxygen than lead is, so it survives every slagging step that removes antimony and tin. Instead, calcium and magnesium metals are submerged into the bath so that they dissolve, and they combine with the bismuth to form a solid intermetallic, CaMg2Bi2, which is insoluble in lead and lighter than it, so it floats as a dross and is skimmed off. The step needs two temperatures held in sequence, because the calcium and magnesium have to be dissolved into the melt hot and the compound has to crystallise and float cold, and it needs real agitation, because the reagents go in at the bottom of the bath and the compound separates at the top. The dross is then chlorinated, which frees the bismuth as a recoverable product and returns the carrier to use. There is also a floor on the step: as the bismuth content falls, the fraction of added calcium and magnesium that finds a bismuth atom falls with it, and beyond that point the reagent cost per kilogram removed rises steeply, which is where an all-fire route stops and electrorefining begins.

Why does a lead refining kettle need agitation?

Because lead does not conduct heat well enough to mix itself. Its thermal conductivity is about 35 W/m·K, roughly a tenth that of aluminium, so a kettle heated from below and left alone settles into a hot layer over the burners and a cooler layer under the surface, and the boundary between them is where the chemistry stops. Every step in the sequence depends on the whole bath being at the set temperature when the reagent is added, and on the reagent being brought into contact with all of it rather than only with the top. The same pump or impeller does a second job that matters just as much: it moves molten metal between kettles, so a sequence with five steps moves five batches of metal in minutes instead of losing half a shift to ladling. It also has to survive the solids load thrown at it during copper drossing and debismuthising, which is why it is specified as process equipment rather than as a stirrer.

What temperature does a lead refining kettle run at?

There is no single temperature, and a supplier who gives one number has not described the line. The kettle is cycled between four or five set points. Copper drossing runs near the freezing point of lead, because the copper compounds only crystallise as the bath cools. Softening runs hot, because the alkaline oxidation needs a fluid bath and the caustic has to stay molten on top of the metal. Desilverising runs inside a window that is cold enough for the zinc crust to separate and hot enough that the zinc does not simply stay dissolved in the lead. Dezincing runs at whatever the chosen route needs, with the vacuum route held hot enough to keep the zinc vapour pressure up. Debismuthising runs two temperatures in sequence, hot to dissolve the calcium and magnesium and cold to float the compound. Final refining and casting both run just above the freezing point of the finished metal. The schedule is written per step and per reagent, and it is issued with the plant.

What is the kettle made of, and how long does it last?

The pot body is carbon steel plate, formed and welded to a stated wall thickness, and its life is measured rather than estimated. The thickness is set by two things: the metal temperature and the head of lead standing above the point being considered, because both of them load the wall and both are constant for a given bath level. The pot loses thickness from the inside for as long as it works, so the drawn plate thickness, the marked survey points, the survey interval and the retirement floor are all written into the technical file, and the surveys are recorded. It is deliberately described as a consumable rather than as an asset, because loss of containment at refining temperature is a release of molten lead and not a maintenance event. What actually determines the service life is the combination of bath temperature, the number of temperature cycles per campaign, the solids load during drossing, and the quality of the combustion control.

How is lead fume controlled during refining and casting?

By hooding every open point and sizing the extraction against the lead exposure limits rather than against the size of the room. The kettle mouth is covered and hooded, and the reagent addition point, the skimming station, the tap and the whole length of the ingot line are hooded and extracted as well, because the pouring and cooling metal is a continuous low-level source rather than an intermittent one. The hoods are specified with a capture velocity held across the opening in both the covered and the open condition, the ducting is designed so that the capture does not collapse when a second hood is opened, and a fabric filter baghouse behind it is sized for the dust load and the gas temperature the plant actually produces. The numbers that drive the design are the permissible exposure limit for airborne lead of 50 µg/m³ as an eight-hour average and the action level of 30 µg/m³. The dust collected in the baghouse is weighed and assayed into the campaign metal balance, so the extraction system is also a metal recovery device.

What ingot weights and grades does the line cast?

Ingots are cast in the standard traded weights, characteristically the 25 kg and 45 kg classes, in a casting machine that sets the fill weight, cools the moulds on a controlled profile and strips, marks and stacks the ingot, so that the weights are uniform rather than approximate. The weight range and the count per bundle are engineered to your order book rather than fixed by the machine. The grade is whatever the contract names, declared as a minimum purity with a permitted maximum for each individual impurity rather than as a single number, because a customer buying battery-grade lead looks at the antimony and bismuth figures first. Alloy ingots – calcium-tin grids, antimonial grades and other written compositions – are cast on the same line, with the alloying addition made after final refining and verified by analysis before the tap. Every ingot carries its grade, its weight, the maker’s mark and a lot number, and the lot record carries the bullion assay, the refining schedule used and the release assay.

Can one kettle do the whole refining sequence?

It can be done in one kettle, and it is done that way in small plants, but the sequence is then serialised in time rather than in space and the consequences show up in three places. The first is throughput: each step needs its own heat-up, hold, skim and settle, so a single kettle spends most of the day waiting for the metal to reach the next set point, and the campaign is as slow as the sum of its steps. The second is contamination: a kettle that has just made a Harris slag and is then used for desilverising carries slag and caustic into a step that is supposed to be a clean zinc partition, and the cross-contamination shows up as a drifting silver figure rather than as an obvious fault. The third is thermal cycling: a kettle used alternately for the copper step and the bismuth step is cycled between the two extremes of the temperature range, which is exactly the cycling that consumes wall thickness. The design answer is normally two to five kettles depending on the sequence, with transfer by pump, and a single-kettle answer for a small plant where throughput is not the constraint.

What utilities and site services does a lead refining furnace need?

Fuel for the burners in the form of natural gas, LPG or oil, combustion air and the burner control train, electricity for the pump or agitator drives, the casting machine, the extraction fan and the control system, cooling water for the moulds and the casting machine’s cooling circuit, compressed air if the skimming or the reagent handling is air-actuated, and the reagents themselves stored and dosed in a way that suits their hazards: sulfur, caustic soda, sodium nitrate, zinc, calcium and magnesium. Beyond those sit the services that a lead plant cannot be built without, which are the hood and duct system, the baghouse, the extraction fan and the stack with the monitoring points the site permit requires, and a drainage and containment arrangement for metal, dross and any reagent spill. Utility quantities are a consequence of the kettle volume, the burner duty, the number of set points per campaign and the casting rate, which is why the utility schedule is issued with the layout drawing once the bullion assay and the target grade are fixed, rather than quoted as a fixed figure beforehand.

What is verified before a lead refining plant is accepted?

Three things are verified in the factory and four on site, and the methods are agreed in the contract before manufacture so that acceptance does not depend on how the figures are produced. In the factory the kettle bodies are checked against their wall thickness drawing and the survey points are marked, the burner train and its interlocks are function tested, and the control system is checked against the temperature and reagent schedule with a simulated sequence. On site the pot settings, the transfer route, the casting line and the extraction ductwork are installed and levelled, the hoods are balanced so that the stated capture velocity is held at each open point with all the others in their normal position, and then a campaign is run on the customer’s own bullion through the full sequence until the released ingot meets the contracted grade with the impurity list analysed by the agreed method. The by-product streams are weighed and assayed at the same time, so the acceptance package carries a closed campaign metal balance alongside the ingot assay certificate, and the kettle thickness survey record is handed over with the rest of the technical file.

Lead Refining Furnace

Product Overview

Key Facts

Product Core Selling Points

Application Scenarios

Applicable Raw Materials

Finished Products

Lead Refining Furnace Process Flow

Product Structure (Main Equipment)

Kettle, hearth and combustion system

Agitation, transfer and dross handling

Hood, extraction, casting and control

Product Features

Technical Parameters And Specifications

Technical Advantages

Service Process

Warranty And After-Sales Service

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