Smelting Furnace

— Smelting Furnace: Furnace Family Selection by Feed and Duty

A smelting furnace reduces a compound feed — an oxide, a sulfide, a carbonate or a sulfate — with a reductant and a flux until the metal leaves its chemical combination and collects into one dense phase, leaving the gangue as a slag. A melting furnace only changes the phase of metal that is already metal, which is why it produces no slag at all, and that boundary runs through the whole page. The family is then selected from the feed arithmetic rather than from a catalogue. YuSai Machinery builds that family as a working station: working bands run from about 1,000 °C to 1,600 °C and above, throughput and reagent rates are engineered to your own assay and tonnage rather than taken from a model list, and the metal left in the slag is measured at every tap. What leaves the furnace is bullion, matte, speiss and slag plus the flue dust returned to the burden — a separated and documented stream, not a refined metal.

Product Overview

A smelting furnace is best separated from the machine it is most often confused with before anything else is discussed. Melting is a physical change: a solid metal becomes a liquid metal of the same composition, and the only variables are temperature, atmosphere and mould. Smelting is a chemical change: the valuable metal is held in a compound and a reductant has to be supplied before it can leave that compound, which is why a smelting furnace consumes reagents rather than only heat and why it produces a slag. The distinction is not academic. A bench kiln that reaches 1,150 °C and holds a one to three kilogram crucible — the specification of the electric units sold today under the words “smelting furnace” — cannot reduce an oxide feed at all, because iron oxide only begins to reduce to metallic iron from about 1,250 °C, some 300 °C below iron’s own melting point of 1,538 °C, and because aluminium oxide melts at about 2,072 °C and cannot be reduced by carbon at any temperature. Those units melt already-metallic scrap, which is a real and useful duty; the trade’s habit of calling it smelting is how a buyer ends up with a furnace that is correct for one job and wrong for the one described.

The families that do reduce a compound feed divide by where the reaction heat comes from and how the bath is arranged, and the division is commercial rather than pedantic. In a shaft or blast furnace the charge of ore, fuel and flux descends through a firebrick-lined shell against a blast of preheated air delivered through tuyeres, with the metal sinking to the crucible and the slag floating above it. In a reverberatory furnace the charge lies on a hearth and the flame passes over it in a separate combustion chamber, so the charge never touches the fuel; the classic vessel of that type is around 40 metres long, 3 metres high and 10 metres wide, and its slag still carries enough metal to need a downstream reduction step. In a short rotary furnace the vessel turns while a burner plays on the charge, bringing fresh surface to the reaction continuously, which is why the family is the workhorse of secondary lead. In a bath reactor the oxidant or the reductant is injected into the molten bath and the reaction itself carries the process, which is why the modern copper industry has largely moved that way: published figures put flash smelters at over half the world’s copper smelters and a single flash furnace in nickel service at around 2,000 tonnes of dried concentrate a day.

What the family does not decide is the furnace’s most important operating number, which is the temperature. The bath temperature is set by the slag it has to keep fluid, not by the melting point of the metal it is making, so the slag system is chosen before the vessel is drawn. A soda-iron slag melts below 800 °C and runs at 850–900 °C; a calcium-iron-silicate slag needs above 1,100 °C before it will flow; published flash practice sits at about 1,300 °C with high-magnesium slags at that ceiling. Everything the plant will spend follows from that number: the refractory grade and therefore the campaign length, the fuel or power rate, the volume of off-gas and the size of the boiler, the dust collector and the acid plant or scrubber behind it, and the split of the sulfur between the slag and the gas. Around the reduction sit the two decisions that turn a reaction into a plant. The collector phase — a matte, a lead bullion, an iron alloy or a deliberately formulated precious-metal collector matte — is where the value is dissolved so that it can be tapped and moved, and its grade is bought with the metal left in the slag. The slag is the phase everything unwanted is thrown into, and it has to be liquid at the working temperature, fluid enough to settle the droplets, compatible with the lining that contains it and low enough in the value to be released or sold.

What leaves the furnace is therefore a set of streams rather than a product, and the plant should be read as a design for all of them. A bullion, a matte or a collector alloy goes hot to the refining step, because solidifying and remelting it costs the heat twice and risks oxidising it. A slag is tapped, sampled and either released against an assay or sent to a cleaning step that recovers its value and returns it to the burden. A dust is captured, weighed and assayed and returned to the charge by agglomeration, because on most smelting feeds it is one of the richest streams in the plant. A gas is cooled while its heat is recovered as steam, cleaned of its dust, and either fed to an acid plant where its sulfur dioxide content supports one or scrubbed where it does not. And a heat record closes the balance: charge weight and assay in against product, slag, dust and residual weight and assay out, with the difference reported as the loss for that heat. What does not come out of a smelting furnace is a refined metal, and that boundary is deliberate rather than a limitation — the smelting stage exists to turn a dilute, variable and often unassayable feed into one concentrated, separated and documented stream that a refining step can handle at a sane volume.

Key Facts

Two duties answer to one name, and separating them comes first. A smelting furnace reduces a compound feed to metal and produces a slag; a melting furnace changes only the physical state of metal that is already metal, which is why it produces no slag at all. Both are real equipment and both are sold under the same words, and every selection figure on this page follows from telling them apart.
Five figures from the feed select the family. The state of the valuable metal. The sulfur and carbon that release or consume the reaction heat. The tonnage and the way the plant is fed. The value concentration, which sets the collector phase the plant has to make. And the sulfur the permit allows together with the fuel, power and water the site can supply.
A reduction furnace is a reagent user, and the reagents are weighed separately. Carbon reduces an oxide and decomposes a carbonate. Carbon alone converts a sulfate to a sulfide rather than to metal, so iron is added to release the metal from that sulfide. Flux is a third addition, and it is what builds the slag out of the gangue.
Four molten phases and one dust leave the furnace. Bullion, the dense metal phase carrying the value. Matte, a molten sulfide and the collector in most copper and nickel routes. Speiss, a molten arsenide or antimonide that forms when arsenic or antimony is in the feed and is tapped, weighed and assayed like any other product. Slag, the silicate carrying the gangue. And the flue dust captured from the gas, which returns to the burden.
Smelting does not refine. A reduction furnace puts the value into one dense phase that a refining step can handle; it does not remove the last fractions of base metal from it. Published copper practice runs an anode at 98.5 to 99.8 per cent copper and a cathode at 99.99 per cent, and the step between them is electrorefining.
The working band is set by the slag, not by the metal. Bath temperatures run about 1,200 to 1,600 °C across the families in industrial service, and a flash furnace runs at about 1,300 °C with high-magnesium slags pressing against that limit. A soda-iron slag melts below 800 °C and runs at 850 to 900 °C; a calcium-iron-silicate slag needs above 1,100 °C.
Collector grade and slag metal content are one decision. A higher-grade matte is worth more per tonne and carries less iron into the converting step, and it is bought with more metal in the slag and therefore a heavier slag-cleaning duty. A quotation that names only the collector has named half of it.
Slag metal content is the recovery figure that can be checked. The metal is present dissolved, on which the slag composition decides, and entrained as droplets, on which settling time and viscosity decide, and it can be sampled at every tap. Published discards across the furnace types in industrial service run from 0.5 to 55 per cent, which is a statement about slag control rather than about furnace brands.
A sulfide feed pays for part of its own heat, an oxide feed does not. Oxidising the sulfur and the iron releases enough heat for a bath reactor to hold its own temperature, which is why published flash practice uses very little hydrocarbon fuel. Reducing an oxide feed consumes heat, and no part of that bill is paid by the chemistry.
Capacity has three definitions and they are not interchangeable. The nameplate feed rate while the furnace is running, the batch size or bath inventory, and the on-stream factor set by the tapping cycle, the campaign, the reline and the maintenance pattern. Every figure on this page states which one it is.
The campaign is set at the slag line. Wear is concentrated there by the slag and, in a lead-bearing bath, by lead oxide, with the bath floor and any injection area close behind. The line is detailed as a replaceable element and surveyed against the original drawing at every shutdown; a published secondary-lead figure for a properly zoned lining is 18 to 24 months.
The feed gate is published rather than implied. Unassayable material, feed that would exceed the permit’s sulfur allowance, chloride-bearing feed that attacks the lining, sealed or hollow containers, mercury-bearing residues, wet charge and fines that would be carried out of the vessel are refused, each with its mechanism attached.

Product Core Selling Points

The page separates the two duties before it sells anything. A furnace that reduces a compound feed and a furnace that melts metal are different machines, and the trade uses one phrase for both. We start by putting your feed on the correct side of that line, because the answer decides the refractory, the gas train, the reagent schedule and the operating cost, and a project that starts on the wrong side of it cannot be rescued by a bigger furnace. If the material you hold is already metal, this page will say so and route you to the melting duty; if the value is combined, the reduction families below are the ones that apply.
The family is selected by arithmetic and the arithmetic is published. Feed state, sulfur and carbon, iron-to-sulfur ratio, tonnage and feeding pattern, value concentration and collector phase, the permit’s sulfur allowance and the site’s services — those are the inputs, and the family falls out of them. We show the reasoning instead of asserting that one vessel is best for every material, and we will tell you when a feed is better served by a wet route than by any furnace at all.
The reductant, the iron and the flux are three separate additions. Carbon reduces an oxide; carbon alone turns a sulfate into a sulfide; iron releases the metal from that sulfide; and the flux sets the slag’s melting behaviour and viscosity. Weighing them as one “additives” charge is how a plant buys the same reduction twice and still reports a high slag loss, so they are weighed and recorded separately against the assay.
The collector and the slag are quoted as one decision. Whether you make a copper matte, a lead bullion, a nickel matte or a precious-metal collector matte sets what the refining plant downstream receives; the matte grade or the bullion composition sets what the slag is allowed to carry; and the slag metal content sets the duty of the slag-cleaning circuit. We present the collector grade, the slag metal content and the slag-cleaning duty as three numbers from one calculation, because a high-grade collector quoted without its slag penalty attached is half a specification.
Slag loss is designed, measured and reported per heat. The slag is sampled at every tap and assayed, the dust is weighed and assayed and returned to the burden, and the difference between what was charged and what left as product, slag and dust is reported as the loss for that heat. A furnace whose loss is never measured is a furnace whose loss grows quietly in the direction of whoever benefits from not measuring it.
The heat balance is issued with the burden sheet, and it says whether the burner has to run. On a sulfidic feed the oxidation of the iron and the sulfur supplies the reaction heat and the furnace can hold its bath autothermally; on an oxidic or a sulfate feed the heat has to be bought, and the balance says how much. That calculation also sets the off-gas volume, the waste-heat recovery duty and the size of everything behind the furnace, so it is a design input rather than an operating discovery.
The temperature is set by the slag system, and the slag system is named before the furnace body is drawn. A soda-iron slag melts below 800 °C against over 1,100 °C for a calcium-iron-silicate slag, and the practical bath temperatures that follow are 850–900 °C and above 1,100 °C respectively. That choice moves the refractory grade, the fuel rate, the off-gas volume, the sulfur split between slag and gas and the whole shape of the gas train, which is why it is settled first.
Emissions and the off-gas train are engineered against the sulfur and the organic load, and no zero-emission claim is made. Post-combustion or an afterburner for the organics a scrap or electronic feed carries into the gas, a cooler that brings the gas into the filter media’s band, reagent injection or an acid plant interface where the sulfur balance asks for one, and a dust collector sized on a calculated volume with its catch returned to the burden. Two of the three common feed classes do emit, so the page states what the train is sized for instead of claiming that nothing leaves the stack.
The campaign is managed by measurement, not by a calendar. The slag line and the bath floor are surveyed against the original drawing at every shutdown, the wear rate is read from the difference, and the next campaign’s length and refractory specification come from that reading. The reline is planned against production rather than announced by a failure.
The furnace is delivered as a station, not as a shell. The vessel with its refractory zoning, the heat source, the reagent and feed handling, the tap and forehearth arrangement, the slag handling, the gas train, the control system with its alarm and trip schedule, the utilities schedule and the heat record all arrive as function-tested modules, with installation, refractory supervision, first-heat commissioning, operator training and a performance test that closes a metal balance.
The plant is priced as a structure you can compute. The recurring cost sits in the fuel or the power, the refractory campaign, the reductant, the flux, the labour on the tapping cycle and the gas train consumables, in an order that changes with the family. We name the levers and price them against your own tariff and wage rate rather than publishing a per-tonne figure borrowed from somebody else’s cost base.
The page states what the furnace cannot do. Reduction smelting puts the value into one dense phase; it does not raise the fineness of that phase to a saleable refined grade. Where the fineness work belongs, the page says so and hands the reader to it, because a plant that buys a smelting furnace expecting a refined product has bought the first of two stages and will discover the difference at the assay bench.
The gate is written down rather than implied. Unassayable material, feed that would exceed the permit’s sulfur allowance, chloride-bearing feed that attacks the lining, sealed containers, mercury-bearing residues, wet charge and un-agglomerated fines are refused, each with the mechanism attached, instead of being left for a sales conversation after the order.
The furnace is the assay enabler as well as the smelter. A mixed parcel of residues, dusts, slags or sweeps cannot be assayed representatively until it has been reduced and homogenised into one phase; the smelting step is what turns an unassayable lot into a tradeable one, and this page treats that as a primary function rather than a by-product.

Application Scenarios

Primary copper smelters treating sulfide concentrate. Plants whose feed is chalcopyrite or another sulfide concentrate and whose objective is a matte of a controlled grade for converting, with the sulfur recovered from the off-gas rather than released, and with the smelting step sized to the concentrator’s output rather than to a fixed model.
Secondary copper and brass smelters consuming scrap, cable and swarf. Plants whose feed is a variable mixture of metallic copper, oxide and organic-bearing material, where the metallic fraction is simply melted and the oxide fraction still needs a reductant, and where the furnace has to be tolerant of a copper content that changes from load to load.
Secondary lead smelters on battery paste and lead-bearing residue. Plants whose feed is largely lead sulfate and lead dioxide and which therefore need a reduction step with carbon for the oxide and iron for the sulfate, plus an enclosed charging and tapping arrangement because lead fume is regulated as an airborne exposure rather than as a nuisance.
Precious-metal recovery lines that need a collector step. Operations feeding printed circuit board concentrates, catalytic material, electronic residues, bench sweeps or plating sludges whose objective is to gather gold, silver and platinum group metals into one dense phase so that the refining circuit downstream sees a concentrated, consistent feed rather than a dilute and variable one.
Zinc and lead-zinc residue projects. Operations holding zinc leaching residue, lead-zinc bulk concentrate or zinc-bearing fume that need the lead and the copper taken into a phase that can be sold, with the zinc left in a slag that a fuming step can recover rather than lost to a dump.
Tin, antimony and bismuth smelters. Plants on non-ferrous feeds outside the copper-lead-zinc mainstream, where the collector is a metal rather than a matte, the speiss phase has to be anticipated because arsenic or antimony is present, and the slag system is chosen as much for its tolerance of those elements as for its melting point.
Precious-metal fire-refining benches working from crucible melts. Operations whose feed is already-metallic bullion, doré, sponge or bench material but which still need a slag-forming step — a flux and an oxidiser, a borax cover, a litharge addition — to pull base metal out of the metal before it is cast, which is a smelting duty carried out in a crucible rather than in a reducing bath.
Foundries and cast houses with a return-scrap balance to close. Plants that generate their own runners, gates, skimmings and machining swarf and want them brought back to a specified composition in house instead of sold at a discount, with the dross and the skimmings weighed and assayed rather than written off.
Anode slime and tankhouse residue concentration. Plants where electrolytic refinery slime is taken through a thermal concentration step before any wet treatment, so that the selenium, tellurium and base metals are reduced in one hot step and the precious metals are collected into a phase rather than carried through a wet circuit at volume.
Flue dust, filter cake and metallurgical sludge recovery. Operations holding baghouse dust, scrubber sludge, hydroxide and carbonate filter cakes and mixed plant residues that carry a metal value and have no clean route through a concentrate smelter, and which need a vessel tolerant of a genuinely mixed feed.
Smelters replacing a sinter and blast-furnace route. Plants whose present flowsheet burns coke, generates sinter, handles a large volume of off-gas and carries an acid plant sized on that volume, and which want a shorter line with a smaller gas train and a lower fuel bill on the same tonnage.
Waste-to-metal and end-of-life equipment programmes. Facilities with a collection obligation whose printed circuit board, component and metal fractions need a thermal concentration step to produce a saleable concentrate, a slag that meets its release criteria and an off-gas that can be cleaned, with the collected metals going on to refining.
Operations with a sulfur balance, a permit and a gas route to close. Plants whose feed is sulfidic and whose permit requires the sulfur to be recovered, and which therefore need a process whose off-gas is rich and steady enough for an acid plant or scrubbing train to accept rather than a variable stream that has to be diluted.
Metallurgical test houses and pilot plants. Laboratories and pilot facilities that need a small, instrumented reduction vessel to establish the reduction behaviour, the slag chemistry and the distribution of the values between the phases before a production line is committed, and for which the value of the furnace is the sample it produces rather than its tonnage.
Projects where the feed is the constraint and the flowsheet has to be built around it. Operations holding a residue, a by-product or a stock of material for which no standard route exists, and where the smelting step exists to make that specific material saleable rather than to run a conventional flowsheet well.
Refractory, campaign-life and availability-driven refurbishments. Smelters whose constraint has become the lining rather than the vessel, and which need a furnace whose slag line is detailed as a replaceable element and surveyed on a schedule rather than discovered at a failure.

Applicable Raw Materials

Copper sulfide concentrate. Chalcopyrite, bornite and chalcocite concentrates and the flotation products of copper ores, charged with a silica and lime flux so that the iron reports to a silicate slag and the copper and precious metals to a matte. The sulfur in the feed supplies part of the reaction heat, which is why this family can run with a low fuel rate. Output: copper matte at a controlled grade, an iron-silicate slag and a sulfur-dioxide-bearing off-gas.
Copper oxide, carbonate and mixed oxide-sulfide material. Malachite, azurite, chrysocolla, cuprite and oxidised surface material, plus mixed ores and oxide-sulfide blends that a clean sulfide smelter will not take on their own. The oxide fraction needs a carbon reductant and supplies no reaction heat of its own. Output: copper matte or black copper, with the blend ratio held so the bath stays at temperature.
Lead sulfide concentrate and lead-zinc bulk concentrate. Galena concentrates and lead-zinc bulk flotation products, including material at grades low enough that a clean concentrate trade will not take it, charged with iron and a flux to release the metal from the sulfide and to build a slag the plant can discard. Output: lead bullion carrying the silver, a zinc-bearing slag for fuming and a sulfur-bearing off-gas.
Lead-acid battery paste, raw or desulphurised. The fine sulfate, dioxide and oxide fraction from battery breaking, with the metallic grid fraction handled as its own stream or blended back into the charge. Where the paste is charged raw, the carbon and the iron are weighed as separate additions because carbon alone converts the sulfate to a sulfide. Output: lead bullion, a lead-bearing slag and a sulfur-bearing off-gas.
Lead, copper and tin drosses, skimmings and spurious oxide. Oxidised and metal-bearing dross from casting and refining, foundry skimmings and the oxide skins from melting pots, which carry a saleable metal value in a form that is not usable directly and which need a reduction pass. Output: recovered bullion returned to the melting circuit and a discardable slag.
Printed circuit board concentrate and electronic residue. Mechanically concentrated board material, component and connector residue, contact scrap and the fine metal-bearing fraction of an e-waste line, charged as a collector feed where the objective is to gather the gold, silver and platinum group metals rather than to make a base-metal matte. Output: a precious-metal-bearing collector matte or bullion and a discardable slag.
Spent catalysts and catalytic residue. Automotive, petrochemical and chemical catalyst streams that have been pre-treated to strip the carrier and now carry platinum group metals at a concentration worth smelting, together with the fines that a washing step will not recover. Output: a platinum-group-bearing collector phase and a slag carrying the carrier material.
Anode slime and tankhouse residue. Copper and lead electrolytic refinery slimes, selenium- and tellurium-bearing tankhouse residue and the associated filter cakes, taken through a thermal concentration stage before any wet treatment. Output: a precious-metal-rich bullion or matte with the base metals slagged off.
Flue dust, baghouse catch, filter cake and metallurgical sludge. Collected dusts, scrubber sludge, hydroxide and carbonate filter cakes, pickling and plating residues and mixed plant sludges that carry copper, lead, tin or precious metals and have no clean route through a concentrate smelter. Output: recovered values into the collector phase, with the balance returned to the burden.
Bench sweeps, polishing dust and precious-metal workshop residue. Jeweller’s and refiner’s sweeps, polishing and buffing dust, floor sweepings, spent plating and stripping solutions dried to a solid, and mixed workshop material that arrives without a reliable analysis. Output: a collector phase carrying the gold, silver and platinum group metals, and an assayable lot in place of an unassayable one.
Already-metallic bullion, doré and sponge requiring a slag-forming pass. Material in which the metal is present as metal but which carries base metals that must be removed before casting or refining, so that the furnace duty is a flux and oxidiser addition rather than a reduction. Output: a cleaner bullion or doré with the base metals in the slag.
Low-grade, off-specification and internally recycled matte and speiss. Matte that is below the grade the downstream step needs, speiss generated elsewhere in the plant, and accumulated intermediate material that has to be upgraded or consumed. Output: matte of the grade the next step specifies, or values returned to the collector phase.
Foundry returns, runners, gates and machining swarf. In-house scrap of known or nominally known composition that has to be brought back to a specification, with the skimmings and the dross from the remelt handled as their own stream. Output: specified alloy or ingot, with the dross weighed and assayed.
Blends deliberately formulated for the furnace. Copper, lead, tin and precious-metal bearing residues, dusts, slags and concentrates mixed to a designed metal content, a designed sulfur and carbon content, a designed iron-to-sulfur ratio and a designed moisture, so that the heat balance, the collector grade and the slag composition all land inside the window the furnace was built for. Output: a stable collector grade and a stable slag condition from a feed that is not itself stable.
Feed refused at the gate. Material that cannot be identified and assayed, feed whose sulfur would exceed the permit’s sulfur allowance, feed whose chloride load would attack the refractory, sealed or hollow containers, mercury-bearing residues, feed wet enough to flash on contact with the bath, and fines fine enough to be carried out of the vessel into the gas train rather than reacted in the bath. Output: a refusal with the mechanism stated, instead of a furnace run outside its window.

Finished Products

Bullion at a declared composition. The dense metal phase that carries the valuable metal out of the furnace, tapped hot and transferred molten to the next step, with its weight and its assay recorded as the primary product of the heat. In lead practice it is lead bullion carrying the silver and the gold that were dissolved into it; in precious-metal practice it is the collector metal into which the gold, silver and platinum group metals have been gathered. Output: weighed and assayed bullion, transferred molten.
Matte at a controlled grade. A molten sulfide phase tapped separately from the slag, at a grade chosen to suit the converting, leaching or refining step that consumes it, with the copper, nickel or precious metals of the feed reporting to it. Output: matte at the specified grade, tapped hot or granulated.
Precious-metal collector matte or collector alloy. Where the feed is electronic, catalytic or mixed precious-metal residue, the collector phase is formulated rather than incidental: its composition is chosen so that gold, silver and the platinum group metals dissolve into it instead of dividing between the phases or volatilising into the gas. Output: a collector phase carrying the precious metals, released to refining.
Black copper or blister copper. Where the feed is largely metallic and the oxidation is taken further than the formation of a matte, a crude copper phase is produced directly with the iron and the more reactive base metals removed to the slag. Output: black or blister copper for anode casting and electrorefining.
Speiss, declared as a product rather than hidden. Where arsenic or antimony is in the feed, the nickel-iron arsenide or antimonide phase forms of its own accord and carries those elements, together with a fraction of the valuable metal, into a stream of its own. It is tapped, weighed and assayed like any other product, and its treatment is planned, because a speiss that is not declared is a value stream that leaves the plant unaccounted for. Output: speiss, weighed and assayed, routed to its own treatment.
Anode plates or cast bars for the refining step. Where the next step is electrolytic, the molten collector phase is cast into anode plates at the thickness, lug geometry and surface condition that cellhouse requires. Output: anode plates for copper electrolysis equipment or the corresponding precious-metal circuit.
Granulated or shot intermediate. Where the plant’s next step is a leaching or dissolving operation rather than a converter, the hot phase is granulated or shot to a controlled size so that it dissolves at a predictable rate. Output: granulated intermediate sized for the leach, the corresponding equipment being a precious metal granulator where the duty is precious-metal specific.
Zinc oxide fume and zinc-bearing slag. Where the feed carries zinc, the reduction stage puts the zinc into a slag that a fuming step converts into a crude zinc oxide powder, so that the zinc is recovered rather than discarded with the gangue. Output: crude zinc oxide fume for the zinc circuit and a spent slag.
Released slag against an assay. The slag is a product in the sense that it has to meet a specification before it can leave the site: a metal content low enough for the slag to be saleable as aggregate or acceptable at a licensed disposal, with the metallurgical work done in the bath rather than sorted out afterwards. Output: finished slag, checked by assay against the release limit.
Slag concentrate returned to the burden. Where the collector grade is high, the slag necessarily carries more metal, and it is tapped as a stream for cleaning rather than as a discard, with the recovered values returned to the charge. Output: a slag concentrate returned to the burden and a final discard slag.
Dust and flue catch, weighed and assayed. The dust captured from the off-gas carries metal values and is one of the richer streams in a smelting plant. It is collected in sealed containers, weighed, assayed and returned to the burden by agglomeration or briquetting. Output: weighed and assayed dust, closed into the heat balance.
Sulfur dioxide, and sulfuric acid where the gas supports it. On a sulfidic feed the off-gas carries sulfur dioxide at a content high enough to make acid from, which converts what would otherwise be a disposal problem into a saleable product; where the sulfur balance does not support an acid plant, the gas goes to a scrubbing train instead and the sulfur leaves as a by-product of that train. Output: saleable sulfuric acid, or a desulfurised off-gas.
Steam from the waste-heat recovery. The heat taken out of the off-gas to bring it into the temperature band the gas train can accept is recovered as steam rather than thrown away to a cooling circuit. Output: process or power steam from the waste-heat boiler.
A closed heat balance and an assayable lot. Everything that enters and everything that leaves is weighed and assayed, the difference is reported as the loss for that heat, and the mixed or unassayable input has been converted into a homogeneous, tradeable product with a defensible analysis attached to it. Output: a heat balance that can be audited, and a lot that can be sold or consigned without re-melting it.

Smelting Furnace Process Flow

The nine stages below are the working sequence of a smelting circuit, written at the level where the furnace family is still open — what has to be established about the feed before a family can be chosen at all, which slag the chemistry allows, which collector the plant has to make, where the reaction happens in the vessel, how the molten phases are drawn apart, and what has to be done to the slag and the gas before the site can release them. Slag system, flux, reductant and iron additions, collector phase, bath temperature, refractory zoning, tapping arrangement, off-gas volume and gas-cleaning route are all fixed during process design against your own feed assay, your tonnage and the permit your site has to hold.

Smelting furnace working sequence — nine stages from the feed qualification that fixes the family through pretreatment, slag and collector selection, the reduction itself and the separation of the molten phases to a released slag, a cleaned gas train and a measured metal balance, with every stream weighed and assayed rather than estimated.

Steps 1 — Feed Qualification and Blend Design. Nothing about the furnace is decided before the feed has been characterised, because the feed fixes the duty, the slag and the collector at once. The valuable metal is determined element by element and, critically, its state is established — metal, oxide, sulfide, sulfate or a mixture — because that answer alone decides whether the job is melting or reduction and which families are even eligible. Sulfur, carbon and iron are determined next, because they release or consume the reaction heat, and moisture, chloride and fluorine because each of them attacks the lining, the gas train or the campaign. A representative sample is then blended to a designed composition so that the heat balance and the collector grade land inside the window the furnace will be built for. Reagent and utility input: sample preparation and full elemental assay, state-of-metal determination, sulfur, carbon, iron, chloride and moisture analysis. Output: a qualified feed specification, a blend design and a duty statement that reads either melting or reduction.

Steps 2 — Pretreatment Route and Moisture Control. Most feeds are not charged as they arrive, and the pretreatment a feed needs is a property of the feed rather than a preference. A sulfidic concentrate is given a partial roast or a sinter so that part of its sulfur leaves and the remainder is left in a usable form and the material handles without dusting. A wet paste or sludge is dried or agglomerated, because water meeting a molten bath flashes at a volume ratio that turns a charge into a mortar, and fines are agglomerated because otherwise they are carried out of the vessel into the gas train instead of reacting in the bath. Pretreatment adds fuel, maintenance and a dust source, so the flowsheet leaves it out wherever the feed allows. Reagent and utility input: drying or calcining fuel, sintering or roasting air, desulphurisation reagents, agglomeration binder and water. Output: a feed in the moisture, size and chemical form the furnace was designed to accept.

Steps 3 — Slag System and Flux Specification. This is the step that decides most of what the plant will cost, and it is taken before the furnace body is drawn. The slag has to be liquid at a temperature the refractory can hold, fluid enough for the collector droplets to settle through it, compatible with the lining that contains it, and low enough in valuable metal to be released or sold — four requirements that the flux recipe satisfies at once. The slag system is a choice between chemistries, and a hotter slag means a higher fuel rate, a larger off-gas volume, a heavier dust load and a different refractory grade, so the slag and the gas train are chosen together. Reagent and utility input: silica, lime or limestone, soda ash, iron-bearing additions and dolomite. Output: a flux recipe with a target slag composition and a stated slag melting and working band.

Steps 4 — Collector Phase and Burden Proportioning. The collector phase is chosen from the value concentration and the refining route downstream — a copper or nickel matte, a lead bullion, a formulated precious-metal collector matte, or an iron alloy — and it is where the commercial decision and the metallurgical one are the same decision. The burden calculation then fixes everything else: carbon weighed as the reductant for the oxides and the carbonates, iron weighed separately against the sulfur, because carbon alone converts a sulfate to a sulfide and the metal only leaves that sulfide when iron is present, and flux proportioned against the gangue. The collector grade and the slag metal content are read as one number with two sides. Reagent and utility input: weighed carbon, weighed iron, proportioned flux, collector additions and the assay the burden is calculated against. Output: a burden sheet per tonne of feed, a target collector grade and a target slag composition.

Steps 5 — Reduction or Oxidation Control in the Bath. The stage the furnace is named for is controlled by one variable, the oxygen potential of the bath. Reducing conditions drive the metal out of its compound and into the collector phase; oxidising conditions drive the iron and the more reactive base metals into the slag and the sulfur into the gas. The bath temperature is held on the value the slag system requires and read on the bath rather than on the flame. On a sulfidic feed the oxidation of the iron and the sulfur releases enough heat for the vessel to hold itself at temperature with little or no supplementary fuel, and the mode is switched to match the enthalpy of the material actually charged; on an oxidic or a sulfate feed the reaction consumes heat and the burner or the power supply carries the whole duty. Reagent and utility input: reductant, oxidant or oxygen-enriched gas, fuel or electric power, process air and any inerting gas the chemistry requires. Output: the valuable metal concentrated into one dense phase, with the gangue leaving as a slag and the sulfur leaving in the gas or fixed in the slag.

Steps 6 — Phase Settling and Separate Draw. A smelting bath is not one liquid but two or three liquids of different density that have to be given the time and the stillness to separate and then drawn apart without being mixed again. The separation is designed rather than hoped for: the vessel provides a settling region distinct from the reaction region, the slag and bath depths are set so that the droplets get a residence time long enough to fall out, and the tapholes sit at different heights so that slag, matte and bullion can be drawn in sequence — slag over the higher notch, the denser phases lower down, with a forehearth that gives any of the wrong phase the chance to separate again and drain back. The order of the draw matters as much as the heights. Reagent and utility input: tapping tools and launders, forehearth and settling arrangement, sampling and assay. Output: separated slag, matte, bullion and speiss streams, each drawn, weighed and sampled on the way out.

Steps 7 — Slag Cleaning Route and Discard Criteria. The slag still carries metal, present dissolved and entrained as droplets, and much of the plant’s recovery is won or lost here. It is attacked on both fronts: chemically, by holding the slag on the composition the flux recipe was built for, because a slag off its target holds more metal in solution; and mechanically, by slow cooling so that the droplets grow and can be liberated, by settling or flotation after grinding, by an electric cleaning furnace, or by a fuming pass where the slag carries zinc. The decision taken at every tap is whether the slag is released as it stands against its assay or routed to cleaning and returned to the burden, and it is taken on a sample rather than on a look. Reagent and utility input: cooling and granulation water, grinding and flotation or settling duty, cleaning-furnace energy, reductant for a fuming pass. Output: a slag concentrate returned to the burden and a final slag released against an assayed discard criterion.

Steps 8 — Fume Volume, Gas Cleaning and Sulfur Balance. The gas carries dust, heat, sulfur dioxide and whatever volatile metals the bath has thrown off, and all four have to be dealt with before it can be released or sold. A waste-heat boiler or a quench brings the gas into a temperature band the downstream equipment and the filter media can survive while recovering the heat as steam; where the feed carries organics, the gas is also held in the high-temperature zone long enough for the carbon monoxide and the organic fraction to combust completely, which is the control on dioxin formation. The dust is then captured, weighed, assayed and returned to the burden, where it is one of the richer streams in the plant. The sulfur route is the last and largest decision, and it is taken on the calculated sulfur balance. Reagent and utility input: cooling water and boiler feedwater, baghouse or precipitator media, acid plant or scrubber reagents, induced draught power. Output: recovered steam, weighed and assayed dust returned to the burden, and saleable acid or a cleaned and scrubbed off-gas.

Steps 9 — Product Release to Refining and Balance Closure. The heat ends when the streams have been drawn, weighed and assayed and the balance between them has been closed. The collector phase is released in the form the next step needs — an anode plate for an electrolytic route, a cast bar or a granulated intermediate where the next step dissolves or converts it — and released hot wherever the flowsheet allows, because solidifying and remelting it costs the heat twice and risks oxidising it. What leaves is a separated, concentrated and documented stream, not a refined metal. The heat closes with the charge weight and assay against the weight and assay of the product, the slag, the dust and the residue left in the vessel, with the difference reported as the loss for that heat. Reagent and utility input: casting or granulation arrangements, mould or anode preparation, weighing and sampling, and the assay laboratory. Output: a released collector phase with its weight and assay, a released slag, a closed heat balance and a documented handover to refining.

Product Structure (Main Equipment)

Feed preparation, reagent handling and charging

Feed reception, blending and weighing. Receiving and stock bays arranged so that a blended charge can be built rather than tipped in, with weigh hoppers, proportioning belts or a batching system that holds the burden sheet’s ratio between concentrate, flux, reductant and iron. The blending arrangement is part of the machine because the heat balance and the collector grade are only as stable as the ratio the charging system can actually hold.
Drying, calcining or roasting equipment where the feed requires it. A dryer for wet paste and sludge, a calciner or roaster for a sulfidic feed that needs part of its sulfur removed ahead of the furnace, and a sinter or briquetting arrangement where the feed has to be agglomerated. Each is sized on the feed’s moisture or sulfur load rather than added later when the dust generation or the sulfur balance has become a problem.
Desulphurisation circuit where the sulfate load justifies it. Reagent preparation, reaction and separation for a paste or residue whose sulfate would otherwise carry the sulfur into the furnace and out through the gas train, with the by-product handled as its own stream rather than as a waste.
Charging arrangement matched to the vessel. A sealed and enclosed charging system for a rotary, shaft or bath vessel where gas escape has to be prevented at the charge point, and a weighed and proportioned feed system for a continuously charged vessel where the charging rate is trimmed against the bath temperature and the gas analysis rather than run to a fixed set point.
Fume and dust control at the charge and transfer points. Local extraction on the receiving, blending, drying and charging stations, because a smelting feed raises dust at every transfer and the dust is both a value stream and an exposure.

Vessel, refractory and heat source

Furnace shell and containment. A welded steel shell in the geometry the duty requires — a vertical shaft, a horizontal barrel, a hearth with a separate combustion chamber, or a bath vessel — with the shell insulated rather than water-jacketed wherever the molten material should be held away from water by a refractory freeze line rather than by a cooling circuit.
Refractory lining, zoned by location rather than specified as one brick. The working lining is chosen per zone because the wear mechanisms differ: chemical attack by the slag and, in a lead-bearing bath, by lead oxide at the slag line; mechanical and thermal load at the bath floor and, in an injection vessel, at the reaction area; gas-side wear in the freeboard and the uptake. The slag line is detailed as a replaceable element with its own dimensions on the drawing, because the survey at the end of every campaign measures it and its wear rate sets the campaign length.
Heat source matched to the feed and the local energy price. A fuel burner on gas, oil or a liquid fuel; a solid-fuel or coke charge where the process depends on it; an induction or resistance supply where the feed is already metallic and the duty is melting; a graphite or carbon electrode arrangement where the duty is an electric reduction or a slag-cleaning furnace; and the plasma and arc options where the temperature a special feed requires exceeds what a fuel can deliver in the atmosphere available. The heat source is a selection output, not a house standard.
Oxidant or reductant injection where the family uses it. Tuyeres, lances, submerged injectors or a burner-and-oxygen arrangement, with the injection position, the gas purity and, where more than one injector is used, the distribution between them, all held as design variables because the injection pattern sets the reaction distribution in the bath.
Bath and shell instrumentation. Bath temperature read on the bath rather than on the flame or the shell, level measurement through the wall or by probe, shell temperature monitoring to watch the freeze line, and gas temperature and analysis at the vessel outlet, so that the set points the burden sheet was calculated against can actually be read and held.

Tapping, slag handling, gas train and control

Tapholes, notches and forehearths. Separate draws set at different heights for the slag and the denser phases, with a forehearth, launder or settling arrangement long enough for any of the wrong phase to separate again and drain back, and with the tapping positions accessible while the vessel is hot rather than only after it has cooled.
Slag handling: granulation, casting or slow cooling. Water granulation into a stockpile, casting into beds or blocks for a slow-controlled cool that grows the entrained droplets so a later grinding and flotation pass can liberate them, or a direct route to a cleaning or fuming furnace — the choice being set by the slag’s metal content and by the cleaning route the plant has.
Dust capture and the dust return circuit. A baghouse or an electrostatic precipitator sized on the calculated off-gas volume and dust load, with sealed collection, weighing and assay, and an agglomeration or briquetting arrangement to put the catch back into the charge, because the dust rate has to be designed rather than discovered.
Off-gas cooling, waste-heat recovery and sulfur route. An uptake and waste-heat boiler or quench that brings the gas into the temperature band the train can accept while recovering the heat as steam, a post-combustion or afterburner arrangement where the feed carries organics, and then either an acid plant interface or a reagent-based scrubbing train sized on the sulfur balance the feed and the permit produce.
Control system, interlocks and the heat record. Measurement and control of feed and reagent rates, bath temperature, injection flows, tapping weight and gas-side conditions, carried by a control system that holds the burden recipe and the set points for the chosen family, with an alarm and trip schedule and a record that closes the charge-to-product balance for every heat rather than for the month.
Utilities, site services and layout. Electric power, fuel, instrument air, cooling and process water with its treatment, dust and slag handling space, the floor loadings and the lifting arrangements the vessel and its refractories impose, and the plot arrangement that connects the furnace to the feed bay, the gas train and the product route — all calculated from the mass and energy balance rather than taken from a template.

Product Features

The duty is established before the family is proposed. The feed is characterised element by element and its valuable metal is classified as chemically combined or already metallic, because that single answer decides whether the requirement is a reduction furnace or a melting furnace and rules out whole families before anything else is discussed.
The slag system is a design output with a temperature attached. The slag family, its target composition and its working band are stated as figures, because the slag’s melting behaviour is what the bath temperature is set to and what the refractory grade, the flux rate and the off-gas volume follow from. A plant that has not selected its slag system has not selected anything.
The reductant and the iron are separate weighed additions with a ratio between them. Carbon reduces oxides and carbonates; carbon alone converts a sulfate to a sulfide; iron releases the metal from that sulfide. The iron-to-sulfur ratio is a controlled figure, and the two additions are recorded separately so that the reduction can be adjusted on evidence.
The collector phase and the slag metal content are one calculation. The collector grade the plant asks for determines what the slag will carry, which determines the duty of the slag-cleaning circuit. All three numbers are issued together, so that a high-grade collector is never quoted without its slag penalty attached.
The heat balance is issued with the burden sheet and says whether the burner runs. On a sulfidic feed the oxidation of the sulfur and the iron supplies the reaction heat and the vessel can hold its bath autothermally or semi-autothermally; on an oxidic or a sulfate feed the heat has to be bought. The balance also sizes the fuel or power rating, the off-gas volume and everything behind the vessel.
The capacity basis is stated, not implied. Nameplate feed rate, batch size or bath inventory, and on-stream factor are three different numbers, and every quotation states which one is being given and how the other two were derived from the tonnage and the shift pattern, so that two quotations can be compared on the same basis.
Loss is measured per heat rather than claimed per year. The charge is weighed and assayed, the product, the slag and the dust are weighed and assayed, the difference is reported as the loss for that heat, and the fractions that carry value are returned to the burden. A recovery figure quoted without a slag assay behind it cannot be checked and is not offered here.
The slag is a designed product with a release criterion. Its composition is set so that it is liquid at the working temperature, fluid enough to settle the collector droplets, compatible with the lining, and low enough in the valuable metal to be released or sold against an assayed limit.
The gas train is sized on the sulfur and the organic load the feed actually produces. Post-combustion for the organics, cooling to the filter media’s band, calculated dust capture with the catch returned to the burden, and an acid plant interface or a scrubbing train selected on the sulfur balance. No zero-emission claim is made, because two of the three common feed classes do emit.
The refractory is zoned and the campaign is managed by survey. The working lining is specified per zone rather than as one brick, the slag line is detailed as a replaceable element, and the campaign length is set by repeated surveys against the original drawing rather than by a calendar interval.
The vessel is enclosed where the feed demands it. Enclosed charging, enclosed tapping and induced draught on the vessel, the charging points and the material handling wherever lead, arsenic or cadmium is in the feed, because an open furnace transfers its emission problem into the workforce and its compliance problem into the permit review.
The furnace is instrumented to the set points the design was built on. Bath temperature read on the bath, level measurement, shell temperature to watch the freeze line, per-injection flow where the family uses injection, and gas temperature and analysis at the outlet, with the control system carrying the burden recipe and an alarm and trip schedule written against the failure modes of the chosen family.
The station is delivered with the record that makes it auditable. Heat records, burden sheets, assay certificates for the products, the slag and the dust, refractory survey reports and the closed metal balance are issued as a technical file, which is what lets the plant demonstrate its recovery to its own management, to a toll customer and to a regulator asking about the metal balance of the site.
The refusals are published. Material that cannot be identified and assayed, feed that would exceed the permit’s sulfur allowance, chloride-bearing feed that attacks the lining, sealed or hollow containers, mercury-bearing residues, wet charge and fines that would be carried out of the vessel are refused, each with its mechanism stated rather than with a request to contact the sales desk.

Technical Parameters And Specifications

Parameter values below are the process design basis for the range. Throughput, vessel dimensions, refractory zoning, reagent rates, heat rating and every site service are engineered to your feed assay, your tonnage, your collector phase, your slag system and your permit rather than selected from a model list. Figures quoted from published operating practice are given with their source basis so that they can be checked.

Parameter	Specification
Capacity (feed basis)	Engineered to your feed assay, the value concentration, the collector phase you are making, the slag you have to discard and your shift pattern; reported as a nameplate feed rate with the batch size and the on-stream factor stated separately, because the three are not interchangeable
Duty definition	Stated explicitly as melting (physical change of already-metallic charge, no slag chemistry) or reduction smelting (chemical change of a compound feed, slag produced); the duty is fixed from the feed before a family is proposed
Furnace family	Selected from the feed arithmetic rather than offered as a house standard; the families in scope are the crucible and induction units, the short rotary, the reverberatory, the shaft or blast, the electric reduction and the bath-smelting reactors with injected oxidant or reductant
Feed state accepted	Metal, oxide, carbonate, sulfate, sulfide, hydroxide, mixed residue or blend; the state of the valuable metal is the first selection figure and it decides whether reduction is required at all
Working temperature band	About 1,000 °C for a short rotary reduction duty, 1,200 to 1,600 °C across the smelting families in industrial service, and 850 to 900 °C where a soda-iron slag is run; the set point is derived from the slag system, not from the metal’s melting point
Published reference points	Flash furnace operation at about 1,300 °C with high-magnesium slags at the upper limit; reduction of iron oxide to metallic iron from about 1,250 °C; iron melting at 1,538 °C; aluminium oxide melting at about 2,072 °C and not reducible by carbon
Bath temperature control	Read on the bath rather than on the flame or the shell; held on the value the slag system requires, with the tolerance stated per project and the control loop sized to the vessel’s thermal inertia
Slag system	Selected before the vessel is designed: a soda-iron slag melting below 800 °C, or a calcium-iron-silicate slag requiring above 1,100 °C, or the common iron oxide, silica and lime base; the choice sets the refractory grade, the fuel rate, the off-gas volume and the sulfur split
Slag requirements	Liquid at the working temperature, low enough in viscosity to settle the collector droplets, chemically compatible with the lining, and low enough in valuable metal to meet a release or sale criterion
Slag metal content	Designed as a target, sampled at every tap and reported per heat; published discards from furnace types in industrial service run from 0.5 to 2 per cent in an electric furnace reducing reverberatory slag through 2 to 4 per cent in a controlled short rotary slag to 30 to 55 per cent in an unreduced reverberatory slag
Flux	Silica, limestone or lime, soda ash, dolomite and iron-bearing additions, proportioned against the gangue assay to the target slag composition and recorded per heat
Reductant	Carbon in the form of coke, coal, charcoal or another carbon source, weighed against the oxide and carbonate load; carbon alone converts a sulfate to a sulfide and does not release the metal
Iron addition	Weighed separately from the carbon against the sulfur in the feed, with the iron-to-sulfur ratio held as a controlled figure, because iron is what releases the metal from a sulfide
Collector phase	Copper matte, nickel matte, lead bullion, a formulated precious-metal collector matte or an iron alloy, chosen from the value concentration and the refining route downstream; the collector grade and the slag metal content are issued as one calculation
Collector grade reference	Matte copper content in the 30 to 50 per cent range in published industrial practice, with the grade rising as the matte is upgraded and the slag metal content rising with it
Pretreatment	Drying or calcining for moisture, a partial roast or sinter for a sulfidic feed, desulphurisation where the sulfate load requires it, and agglomeration or briquetting for fines that would otherwise be carried out of the vessel
Charging	Batch or continuous according to the family; enclosed and sealed at the charge point wherever gas escape or fume has to be contained, with the charging rate trimmed against the bath temperature and the off-gas analysis on a continuously charged vessel
Heat source	Fuel burner on the locally priced gas or liquid fuel; solid fuel or coke where the process depends on it; induction or resistance heating for a melting duty; graphite electrodes for an electric reduction or slag-cleaning duty; arc or plasma where a special feed needs the temperature
Heat balance	Issued with the burden sheet: the heat required to raise the charge to the tapping temperature plus the endothermic reaction load, less the heat released by oxidising the sulfur and the iron in the feed, divided by the vessel’s thermal efficiency and stated per tonne of feed
Autothermal operation	Available on a sulfidic feed where the oxidation of the sulfur and the iron releases enough heat to hold the bath; switched to a semi-autothermal mode to match the enthalpy of the material actually charged, with the supplementary burner carrying charge heating and bath holding
Reductant and fuel consumption	Engineered from the heat balance and the assay and stated per tonne of feed against your own fuel and power tariff; a per-tonne figure borrowed from another plant’s feed is not offered
Off-gas volume and composition	Calculated from the burden and the injection rates; flash smelting practice produces 20 to 50 per cent sulfur dioxide by volume, and a weaker or more variable gas is routed to a scrubbing train instead of an acid plant
Off-gas high-temperature residence time	Designed so that carbon monoxide and the organic fraction combust completely in the high-temperature zone before the gas is cooled, which is the control on dioxin formation where the feed carries electronic scrap
Waste-heat recovery	Uptake and boiler or quench recovering the off-gas heat as process or power steam, sized on the calculated volume and bringing the gas into the temperature band the downstream equipment and filter media accept
Dust capture and return	Baghouse or electrostatic precipitator sized on the volume and dust load; the catch is collected in sealed containers, weighed, assayed and returned to the burden by agglomeration or briquetting, since on most smelting feeds it is one of the richest streams in the plant
Sulfur route	Acid plant interface where the sulfur dioxide content and stability support acid making, or a reagent-based scrubbing train with by-product handling where they do not; selected on the sulfur balance the feed and the permit produce
Refractory lining	Specified by zone rather than as one brick: the slag line against chemical attack, the bath floor and reaction area against mechanical and thermal load, the freeboard and uptake against the gas; the slag line is detailed as a replaceable element
Refractory campaign life	Set by the slag-line survey rather than by a calendar interval; a published secondary-lead figure for a properly zoned lining is 18 to 24 months of continuous operation, with the slag zone and the liquid-metal zone bricked differently
Containment and enclosure	Insulated shell with the freeze line held in the refractory rather than a water-cooled jacket wherever molten material must be kept away from a water circuit; enclosed charging, tapping and material handling with induced draught wherever lead, arsenic or cadmium is present
Tapping arrangement	Separate draws at different heights for the slag and the denser phases, with a forehearth, launder or settling arrangement that lets any of the wrong phase separate and drain back
Slag handling	Water granulation, casting into beds or blocks for a controlled slow cool, or a direct route to a cleaning or fuming furnace, selected on the slag metal content and the cleaning route available
Metal balance	Closed per heat on charge in, product out, slag, speiss, dust and the residue left in the vessel, all weighed and assayed, with the difference reported as the loss for that heat
Instrumentation	Bath temperature and level, shell temperature to monitor the freeze line, injection flows and pressures where the family uses injection, feed and reagent rates, off-gas temperature and analysis, tapping weight
Control system	Carries the burden recipe, the bath temperature target, the injection or burner set points, the charging rate and the tapping sequence, with the alarm and trip schedule written against the failure modes of the chosen family
Automation and remote access	Measurement, interlocking and recording under the control system; remote monitoring and diagnostic access provided where the site’s network policy permits it
Utilities	Electric power, fuel, instrument air, cooling and process water with its treatment, granulation water where used, and the compressed air and reagent preparation the flux and reduction schedule requires
Site services and layout	Engineered to your plot, your feed and slag handling, your gas route, your floor loadings, your lifting arrangement and your permit conditions, all taken from the mass and energy balance
Materials of construction	Carbon steel shell, refractory and insulation specified by zone, heat-resisting steel in the high-temperature gas path, and injection or burner components in the material the bath chemistry requires

Furnace Family Comparison

he families below are compared against the same duty window. The band figures are the working values across families in industrial service and are given as a selection aid rather than as a specification for a specific project.

Furnace family	Feed state	Working band	Heat source	Principal products	First figure that selects it
Crucible and induction melting unit	Metal, already reduced	1,000 to 1,600 °C	Induction, resistance or gas	Homogeneous bar or shot	Whether the value is already metallic
Short rotary reduction furnace	Oxide, sulfate, residue, scrap	1,000 to 1,400 °C	Fuel burner, oxygen enrichment optional	Bullion, slag, matte, dust	Feed form and the tonnage one batch must take
Reverberatory hearth furnace	Oxide, calcine, scrap	1,100 °C and above	Fuel burner in a separate chamber	Bullion or matte, and a slag needing reduction	Whether a large, gently handled bath is wanted
Shaft or blast furnace	Sinter, lump ore, lump scrap	1,200 °C and above	coke plus preheated air blast	Bullion, slag, matte	Whether the charge can be held as a permeable bed
Electric reduction furnace	Oxide, slag, calcine	1,300 °C and above	Graphite electrodes	Bullion, and a low-value slag	Whether a very low slag metal content is required
Bath smelting reactor	Sulfide and oxide concentrate, residue	About 1,300 °C	Reaction heat, supplementary burner	Matte or bullion, slag, strong sulfur dioxide gas	Whether the feed’s own sulfur can carry the heat
Crucible fire-refining furnace	Metal carrying base-metal impurities	Set by the flux and oxidiser used	Induction, resistance or gas	Cleaner bullion, doré or ingot	Whether the metal is already present as metal
Fuming furnace	Lead or zinc bearing slag	Set by the slag and the reductant	Fuel burner with a reductant	Crude zinc oxide fume	Whether the slag still carries recoverable zinc

Technical Advantages

The page refuses to answer the wrong question. Most of the market sells a kiln that melts already-metallic scrap under the name “smelting furnace”, with a working ceiling around 1,150 °C and a crucible of one to three kilograms; that is a real product, and it cannot reduce an oxide feed, because iron oxide only begins to reduce to metal from about 1,250 °C and aluminium oxide cannot be reduced by carbon at any temperature at all. Starting the selection with the state of the valuable metal is what keeps a project off a machine that is correct for a different job.
The family follows from the feed arithmetic, and the arithmetic is published. Five figures from the feed — the state of the value, the sulfur and carbon that release or consume heat, the tonnage and the feeding pattern, the value concentration and collector phase, and the permit’s sulfur allowance with the site’s services — narrow the field before a vessel is drawn. A buyer who follows the reasoning can check the recommendation instead of accepting it, and a feed that no furnace serves well is identified before the order rather than after commissioning.
The slag system, not the vessel, is treated as the first engineering decision. Because the slag’s melting behaviour sets the bath temperature, and the bath temperature sets the refractory grade, the fuel rate, the off-gas volume and the sulfur split, choosing the slag system first is the difference between a plant that was designed and a plant that was assembled. A soda-iron slag running at 850 to 900 °C against a calcium-iron-silicate slag above 1,100 °C is not a tuning choice; it is a different plant.
Reductant, iron and flux are three separate additions with a ratio between them. Carbon reduces oxides and carbonates, carbon alone converts a sulfate to a sulfide, iron releases the metal from that sulfide, and the flux sets the slag. Weighing them as one “additives” charge is how a plant pays for its reduction twice and still reports a high slag loss, so they are proportioned and recorded separately against the assay.
The collector grade is quoted with its slag penalty attached. A high-grade matte is worth more per tonne and carries less iron into the converting step, and it is bought with a higher metal content in the slag and a heavier slag-cleaning duty. Publishing the collector grade, the slag metal content and the cleaning duty as three numbers from one calculation is what lets the decision be taken commercially rather than discovered metallurgically.
The heat balance is a design deliverable, and it says whether the burner has to run. A sulfidic feed can pay for a large part of its own heat through the oxidation of its sulfur and iron, which is why flash smelting practice uses very little hydrocarbon fuel; an oxidic or sulfate feed consumes heat and pays for all of it. Stating that difference from the feed assay is a structural statement about operating cost, not a matter of tuning a burner.
Loss is reported as a measured figure per heat. Charge weight and assay against product, slag, speiss, dust and residual weight and assay, with the difference declared as the loss for that heat and the value-bearing fractions returned to the burden. A recovery percentage quoted without the slag assay behind it cannot be falsified, and this page does not offer one.
Slag is treated as a designed product with a release criterion. Its composition is set so that it is fluid at the working temperature, settles the collector droplets, survives contact with the lining and meets an assayed discard or sale limit. Published discards across the furnace types in industrial service span 0.5 to 55 per cent metal, and the spread is a statement about slag control rather than about brands.
The sulfur is closed by design rather than by dilution. A sulfidic feed produces an off-gas carrying tens of per cent sulfur dioxide in flash practice, which an acid plant converts into a saleable product; a weaker or more variable gas is scrubbed and its sulfur handled as a by-product. Selecting the route on the calculated sulfur balance is what turns a disposal gas into a by-product line, and it is decided with the slag system rather than added afterwards.
Dust is returned rather than disposed of. The catch from the gas train is collected in sealed containers, weighed, assayed and put back into the charge by agglomeration or briquetting, because on most smelting feeds it is one of the richer streams in the plant. The collection and return equipment is sized on the calculated dust rate instead of being added once the dust has become a problem.
The campaign is managed by survey and the lining is zoned. The slag line, the bath floor and the reaction area wear differently, so the working lining is specified per zone and the slag line is detailed as a replaceable element. The campaign length comes from measuring that line at every shutdown against the original drawing, so a reline is planned against production instead of announced by a failure.
Autothermal capability is presented as a balance to be held. On a sulfidic feed the vessel can hold its own bath temperature on the heat of oxidation, and the mode is switched to match the enthalpy of the material actually charged — which is why the balance is issued with the burden sheet and why the supplementary burner is sized for charge heating and bath holding rather than for the process heat. Autothermal operation sold as a property of the vessel rather than as a balance will be blamed for a control failure it did not cause.
The page states where smelting stops. A reduction furnace puts the valuable metal into one dense phase and separates the gangue into a slag; it does not remove the last fractions of base metal. Published copper practice runs an anode at about 98.5 to 99.8 per cent copper and a cathode at 99.99 per cent, and the step between them is refining. Stating that boundary costs us a sale in the projects where the customer actually needed a refining plant, and it is the reason the customers who do buy a furnace get what they specified.
The station leaves with the record that makes it auditable. Burden sheets, heat records, product, slag and dust assays, refractory survey reports and a closed metal balance are issued as a technical file, which is what lets a plant prove its recovery to its own management, to a toll customer and to a regulator asking about the metal balance of the site.

Related equipment and processes: precious metal smelting machine for the melting duty on already-metallic precious-metal charge, lead smelting furnace for the reduction duty on lead-bearing residue and paste, oxygen-enriched bottom-blowing furnace and oxygen-enriched side-blowing furnace for the bath-smelting families, lead refining furnace and precious metal refining purification system for the refining step the collector phase goes to, and precious metal granulator and gold and silver ingot casting mold where the collector phase is released as granules or cast bars.

Service Process

Feed assay review and duty statement. You send the analysis of the material you intend to smelt, element by element, with its sulfur, carbon, iron, chloride and moisture, and with the state of the valuable metal identified as metal, oxide, sulfide, sulfate or a mixture. We work through what that material actually is, what has to be blended with it, and whether the duty it requires is melting or reduction — because that is the answer that decides which furnace families remain eligible. Output: a written duty statement that reads either melting or reduction, and a view of what is feasible on your feed.
Bench and pilot test work on your material. Where the feed is unusual, mixed or of uncertain behaviour, the material is run through test work to establish its reduction behaviour, the slag chemistry it produces, what volatilises into the gas, how the dust behaves, and the distribution of the valuable metal between the collector phase, the slag and the dust. That distribution, not the brochure, is what decides the recovery. Output: a test report with the measured distributions and a recommended operating window.
Slag system selection. The slag family, its target composition and its working temperature are selected against four requirements at once: liquid at a temperature the refractory holds, fluid enough to settle the collector droplets, compatible with the lining, and low enough in metal to be released or sold. The flux recipe and the flux rate follow from it. Output: a slag specification with its melting band, a flux recipe and a stated discard criterion.
Collector phase selection and burden calculation. The collector phase is chosen from the value concentration and the refining route downstream, and the burden sheet is then calculated from the assay: carbon as the reductant for the oxides and carbonates, iron weighed separately against the sulfur, and flux proportioned against the gangue. The collector grade and the slag metal content are issued as one calculation with two sides. Output: a burden sheet per tonne of feed, a target collector grade and a target slag metal content.
Heat and mass balance, and the autothermal statement. The heat required to raise the charge to the tapping temperature and to drive the endothermic reactions is set against the heat released by oxidising the sulfur and the iron in the feed, the difference is divided by the vessel’s thermal efficiency, and the result is stated per tonne of feed together with the mode the furnace will run in. The flows of feed, flux, reductant, gas, product, slag and dust are calculated across the whole line from those figures. Output: a heat and mass balance, an autothermal or semi-autothermal statement and a reagent consumption schedule.
Furnace and refractory design. The vessel geometry, bath depth, working volume and charge arrangement are fixed from the balance; the refractory is specified by zone with the slag line detailed as a replaceable element; the tapping arrangement is set so that the phases can be drawn separately; and the enclosure and draught provisions are detailed where the feed requires them. Output: a furnace and refractory design package with the drawings.
Gas train, dust circuit and permit support. The off-gas volume, temperature, composition and dust load are issued as process data, and the cooling and waste-heat recovery, the dust capture and return, and the acid plant interface or scrubbing train are designed against them, with the emission points and the sulfur balance identified for your permit application. Output: gas-side process data and a gas train design basis.
Mass balance, layout and equipment scope. The equipment list, the plot layout, the floor loadings, the lifting arrangements and the utility requirements are issued from the calculated flows rather than from a standard configuration, so that the feed bay, the furnace, the gas train and the product route fit the plot you actually have. Output: a mass and energy balance, a layout drawing and an equipment scope list.
Manufacturing, assembly and works inspection. The shell, the refractory, the heat source, the injection or burner assembly, the feed and reagent system, the tapping and forehearth arrangement, the gas train and the control system are manufactured and assembled, with the parts that carry a setting — injection angles, burner geometry, taphole heights, ladle or launder falls — checked and recorded before dispatch. Output: an assembled station with works inspection records.
Site installation and refractory supervision. Installation of the shell and mechanical equipment with refractory installation supervised and the dry-out schedule specified, because the way the moisture is driven out of a new lining is the single largest determinant of how long the first campaign lasts. Output: an installed station with a dry-out record and a signed refractory installation report.
Commissioning on your own feed, with the first heat supervised. The furnace is heated on its commissioning curve, the feed and reagent system is calibrated against the burden sheet, the injection or burner settings are trimmed to the design values, and the first heats are run with our engineers on the floor until the bath temperature, the collector grade and the slag condition are where the design said they would be. Output: a furnace running to its design set points on your own material, with the first heat records.
Operator training on the burden sheet and the slag. Training is given on the operating recipe as it applies to this furnace: what the burden calculation is for, how the reductant and the iron are adjusted when the assay moves, what the slag assay at each tap is telling the operator, what the off-gas analysis says about the completeness of the reaction, and what to do when the feed changes. Output: trained operators and a written operating manual for the station.
Performance demonstration and handover. The line is run to demonstrate the designed throughput, collector grade and slag metal content, with the feed, the product, the slag, the speiss, the dust and the gas all weighed and assayed so that the demonstration is a closed metal balance rather than a duration. Output: a performance test report with a closed mass balance and a handover certificate.
Refractory and campaign planning. Before handover, the slag-line detail and the survey schedule are agreed, the wear measurement method is demonstrated on site, the spare and consumable list is issued with the expected campaign against your feed, and the reline plan is written against the production calendar you intend to run. Output: a campaign plan with a survey and consumable schedule.
Feed change review and ongoing optimisation. When you intend to change the feed, run a new blend for the first time or push the collector grade, we recalculate the burden sheet and the heat balance against the new assay and issue a revised recipe rather than leaving the furnace to be re-tuned by trial in production. Output: a revised recipe and heat balance for every feed change, with the change recorded in the technical file.

Warranty And After-Sales Service

Warranty on the station and its systems. The furnace shell, the refractory installation, the heat source and its controls, the injection or burner assembly, the feed and reagent handling, the tapping and forehearth arrangement, the gas train and the control system are warranted against defects in design, materials and workmanship for a stated period from commissioning, with the period, the exclusions and the wear-part list written into the contract rather than left to interpretation.
Wear parts are named and separated from the warranty. The working lining and its slag-line element, the burner or injection components, the taphole refractories, the launder and forehearth refractories and the filter media are consumables with defined lives, listed as such with their expected campaign against your feed, so that the boundary between a warranty claim and a planned replacement is clear before the furnace is ordered.
Refractory campaign survey support. The slag-line, bath-floor and reaction-area surveys taken at every shutdown are reviewed against the original drawing and against the previous survey, and the wear trend is fed back into the refractory specification and the next campaign’s setting, so that campaign length is managed rather than discovered.
Slag-condition and loss review. The slag assays taken at each tap and the closed heat balances are reviewed against the design at agreed intervals, so that a drift in the reductant, the iron or the flux rate is corrected from the data rather than from a report that the plant is losing metal.
Spare parts and strategic stock. Slag-line and taphole refractories, burner or injection components, feed and reagent wearing parts, launder and forehearth refractories, gas train parts, instrumentation and control spares are available as a strategic stock package sized to your campaign pattern, so that a planned shutdown is not extended by a part that has to be made.
Remote diagnostic and process support. Where the site permits remote access, the control and instrument data are available to us for diagnostics, so that a bath temperature deviation or an off-gas anomaly can be examined with the plant data in hand rather than described over a telephone.
On-site service and response. Field service engineers for mechanical, refractory, control and process work, with the response time and the call-out arrangements stated in the contract and the service organised around the shutdown windows you have rather than around our calendar.
Operator refresher and new-staff training. Training is repeated when the feed changes, when the plant changes the product it is making, or when the operating team turns over, because a burden sheet and a slag specification only work if the people running the furnace understand what they are holding and why.
Feed change and blend support. A new feed, a new supplier or a new blend is put through the burden calculation before it is charged, so that the heat balance, the collector grade and the slag condition are known in advance rather than discovered on the tap floor.
Escalation and root-cause work. Where a heat or a campaign fails to meet the design — a slag that will not settle, a collector grade that drifts, a lining that wears faster than the survey predicted — the investigation is carried out jointly against the heat records and the assay data, and the outcome is a correction to the recipe or the design rather than a discussion about whose fault it was.
Process data, records and traceability. The burden sheets, the operating set points, the tapping records, the assays, the heat balances, the refractory surveys and the gas-side logs are maintained as a technical file for the plant, so that the operating history and the recovery figures are auditable and so that a new feed can be assessed against what the furnace has actually done.
Upgrade and rebuild support. Relining, refractory specification upgrades, heat source conversion where the local energy price has moved, tapping and slag-handling modification for a changed product, and gas train upgrades are supported through the life of the plant, so that a furnace that has run for several campaigns can be re-specified for what it is being asked to do next rather than replaced.

Frequently Asked Questions

What is a smelting furnace, and how is it different from a melting furnace?

A smelting furnace takes a feed in which the valuable metal is chemically combined, as an oxide, a sulfide, a carbonate or a sulfate, and heats it with a reductant and a flux until the metal leaves its chemical combination, collects into a molten phase and separates from the gangue, which leaves as a slag. A melting furnace does something categorically different: it changes a solid metal into a liquid metal of the same composition, and its variables are temperature, atmosphere and mould rather than chemistry. The practical test is whether a slag is produced by design, because a slag only exists when something has been deliberately thrown out of the metal or out of the feed. The trade uses one phrase for both machines, which is why the distinction matters commercially: a countertop electric unit sold as a smelting furnace reaches about 1,150 degrees and holds a crucible of a kilogram or two, and it melts scrap perfectly well, but it cannot reduce an oxide feed, because iron oxide only begins to reduce to metal from about 1,250 degrees and because aluminium oxide melts at about 2,072 degrees and cannot be reduced by carbon at any temperature at all.

Which furnace families are used for industrial smelting, and how is one selected?

The families in industrial service are the crucible and induction melting units, the short rotary reduction furnace, the reverberatory hearth furnace, the shaft or blast furnace, the electric reduction furnace, the bath-smelting reactors that inject an oxidant or a reductant into a molten bath, and the fuming furnace that recovers zinc from a lead-bearing slag. Selection is not a matter of preference; it follows from the feed. The first figure is the state of the valuable metal, which decides whether reduction is required at all. The second is the sulfur and the carbon in the feed, which is what releases or consumes the reaction heat and therefore decides whether the vessel can hold its own temperature. The third is the tonnage and the way the plant is fed, because a continuously fed vessel and a batch vessel are not substitutes. The fourth is the value concentration, which sets the collector phase the plant has to make. The fifth is the permit’s sulfur allowance together with the fuel, power, water and draught the site can supply. Those five answers usually leave one or two families standing.

What decides whether a job needs a smelting furnace or a melting furnace?

The state of the valuable metal in the feed, and nothing else. If the metal is already present as metal, the job is melting, and the equipment is a crucible, induction or resistance furnace whose duty is to homogenise the charge, remove what can be removed by a flux and an oxidiser, and cast it. If the metal is held in a compound, the job is reduction, and the equipment has to supply a reductant, form a slag and separate phases. The two duties can appear in the same plant on consecutive days, which is why the first question we ask about a feed is not its tonnage but its chemistry. A project that starts on the wrong side of this line cannot be rescued by buying a larger furnace of the wrong family, and it is the single most common error in enquiries that arrive described as a smelting furnace requirement.

Why does a smelting furnace need a reductant, a flux and sometimes a collector metal?

Because the three do different jobs and cannot substitute for one another. The reductant removes the oxygen, the sulfur or the carbon dioxide that is holding the metal in combination: carbon reduces an oxide and decomposes a carbonate, and where the feed is a sulfate, carbon alone converts the sulfate to a sulfide rather than to metal, so iron is added to release the metal from that sulfide. The flux is not a reductant at all; it is what builds a liquid slag out of the gangue and the oxidised iron, so that the slag can be tapped, so that it is fluid enough to let the metal droplets settle through it, and so that it does not attack the lining. The collector metal is a third function again: where the valuable metal is too dilute or too finely divided to be tapped as free metal, a phase is provided for it to dissolve into, whether that is a matte, a lead bullion, an iron alloy or a deliberately formulated collector matte. Reductant, iron and flux are therefore weighed as separate additions with a ratio between them, and treating them as one additives charge is a reliable way to pay for the reduction twice.

What does a smelting furnace actually produce?

It produces a set of streams rather than a single product, and the plant has to be designed for all of them. A bullion, which is the dense metal phase carrying the valuable metal and any dissolved precious metals. A matte, which is a molten sulfide phase and the collector in most copper and nickel routes. A speiss, which is an arsenide or antimonide phase that forms of its own accord when arsenic or antimony is in the feed and which carries those elements together with a fraction of the value. A slag, which is the silicate phase carrying the gangue and a residual amount of the metal. A dust captured from the off-gas, which on most smelting feeds is one of the richer streams in the plant and is returned to the burden. A gas, which carries sulfur dioxide, heat and whatever volatilised, and which goes to an acid plant or a scrubbing train. And a heat record that closes the balance between what was charged and what left. What does not come out of a smelting furnace is a refined metal, and that boundary is deliberate rather than a limitation.

How is the capacity of a smelting furnace defined?

By three numbers that are not interchangeable and are frequently confused. The nameplate feed rate is the tonnage the furnace is designed to consume per hour or per day while it is running. The batch size or the bath inventory is what one charge or one bath actually holds, which is a different number and is the one that decides the crane, the charging arrangement and the ladle. The on-stream factor is the fraction of calendar time the furnace is genuinely available, and it is set by the tap-to-tap cycle, the campaign length, the reline, the maintenance pattern and the availability of the feed bay behind it. A quotation that gives one figure without saying which of the three it is cannot be compared with another quotation, and dividing a nameplate rate by a calendar year is the most common way a smelting project is over-promised to its own board. We state the basis of every figure and engineer the three together from your tonnage and your shift pattern.

How much energy does smelting consume per tonne of feed?

There is no defensible single figure, and a page that quotes one without a basis is quoting somebody else’s feed. The honest answer is a calculation with five terms: the heat required to raise the charge from ambient to the tapping temperature, the heat consumed by the endothermic reduction reactions, the heat released by the exothermic oxidation of the sulfur and the iron already in the feed, the heat lost to the walls, the gas and the cooling circuits, and the thermal efficiency of the vessel. The first two are properties of the feed and the slag, the third is often the largest single term and it is why a sulfidic feed and an oxidic feed of the same tonnage have different fuel bills, and the last is a property of the furnace family. That is also why published figures for flash smelting show very little hydrocarbon fuel being used: the process is running largely on the oxidation of its own sulfur and iron. We issue the balance per tonne of feed as a design deliverable, priced against your own fuel and power tariff.

How is metal loss to the slag controlled and measured?

It is controlled in three places and then measured at every tap. Chemically, by holding the slag on the composition the flux recipe was calculated for, because a slag off its target composition holds more metal in solution. Mechanically, by giving the collector droplets the residence time and the stillness to settle out, which is what the vessel’s settling region, its bath depth and its slag depth are for. Operationally, by treating the collector grade and the slag metal content as one decision, since a higher-grade collector necessarily carries more metal into the slag and a higher slag metal content means a heavier duty on the slag-cleaning circuit. The measurement is the slag assay at every tap against a release or cleaning criterion, and the reason it is the number to insist on is that it can be checked. A recovery percentage quoted without a slag assay behind it, and without the dust closed into the balance, cannot be falsified and therefore cannot be relied on.

How long does a refractory campaign last, and what ends it?

The campaign ends when the slag line reaches its retirement thickness, and it is therefore managed by survey rather than by a calendar. Wear in a smelting furnace is not uniform: the slag line takes chemical attack from the slag and, in a lead-bearing bath, from lead oxide; the bath floor and the reaction area take mechanical and thermal load; the freeboard and the uptake take gas-side wear. The slag line is normally the limiting location and in an injection vessel the reaction area competes with it. Because the mechanism is known, the line is detailed as a replaceable element with its own dimensions on the drawing, and it is measured at every shutdown against that drawing so that the wear rate sets the next campaign’s length and specification. A published figure for a properly zoned secondary-lead lining is 18 to 24 months of continuous operation, with the slag zone and the liquid-metal zone bricked differently, but that number is a consequence of the feed, the slag system and the survey discipline rather than a specification that can be transferred.

What off-gas does a smelting furnace produce, and how is it cleaned?

The gas carries dust, heat, sulfur dioxide and whatever volatile metals the bath has thrown off, and on a feed containing electronic scrap or oil-bearing material it also carries carbon monoxide and organic species. The train begins with cooling, in a waste-heat boiler or a quench, which brings the gas into the temperature band the downstream equipment and the filter media can survive while recovering the heat as steam. Where the feed carries organics, the gas is also held in the high-temperature zone long enough for those species to combust completely, which is the control on dioxin formation. The dust is then captured, weighed, assayed and returned to the burden, because it is a value stream rather than a waste. The sulfur route is the largest decision and it is taken on the calculated sulfur balance: a sulfidic feed produces a gas rich enough for an acid plant, which converts the sulfur into a saleable product, while a weaker or more variable gas is scrubbed with a reagent and its sulfur handled as a by-product. No zero-emission claim is made, because two of the three common feed classes do emit.

Which fuel or energy source is used, and can it be changed?

The heat source is a selection output rather than a house standard, and it is chosen from the feed and from the local energy price. A fuel burner on gas, oil or a liquid fuel is the default across the rotary, reverberatory and fuming families. Coke or another solid fuel is used where the process depends on it, as a shaft furnace does. An induction or resistance supply is used where the feed is already metallic and the duty is melting. Graphite electrodes are used where the duty is an electric reduction or a slag-cleaning operation and a very low slag metal content is wanted. Plasma or arc heating appears where a special feed needs a temperature or an atmosphere that a fuel cannot deliver. On a sulfidic feed a large part of the heat comes from the reaction itself and the burner is sized only for heating the charge from cold and holding the bath during casting and shutdown, which is the real meaning of an autothermal furnace. Conversion between fuels is a rebuild of the heat source and the gas side rather than a switch, and it is supported through the life of the plant when the local energy price moves enough to justify it.

Why does a smelting feed sometimes have to be roasted or sintered first?

Because the form the feed arrives in is often not the form the vessel can use, and each pretreatment exists to fix a specific problem. A sulfidic concentrate is sometimes given a partial roast so that part of its sulfur is removed and the remainder is left in a state the furnace can handle, and a sinter pass makes a fine concentrate into a permeable lump that a shaft furnace can hold and that will not be carried out of the vessel as dust. A wet paste or sludge is dried or agglomerated, because water meeting a molten bath flashes at a volume ratio that turns a charge into a mortar, and because fines that will not agglomerate leave in the gas train instead of reacting in the bath. Where a sulfate load would otherwise carry the sulfur through the furnace and into the gas train, a desulphurisation step is inserted ahead of it and its reagent and by-product are handled as their own circuit. The reason we treat pretreatment as a decision rather than a given is that it adds fuel, maintenance and a dust source, so a vessel that can accept moist or fine material directly is worth more than its purchase price in the unit operations it removes.

What feed materials can be smelted, and which are refused?

The range is wide and it is defined by chemistry rather than by a list: copper, lead, nickel and tin concentrates; battery paste raw or desulphurised; drosses, skimmings and spurious oxide; printed circuit board concentrates and electronic residue; pre-treated spent catalysts; anode slime and tankhouse residue; flue dust, baghouse catch, filter cake and metallurgical sludge; bench sweeps and workshop residue; already-metallic bullion and doré that need a slag-forming pass; low-grade matte and speiss; and foundry returns, all of them either as they arrive or blended to a designed composition. What is refused is refused for a stated mechanism rather than for a policy: material that cannot be identified and assayed, because the burden sheet is built on the assay and an unknown feed cannot be charged safely; feed whose sulfur would exceed the sulfur balance the permit allows; feed whose chloride load would attack the lining; sealed or hollow containers, because they burst; mercury-bearing residues, because mercury goes to the gas and the gas goes to people; feed wet enough to flash on contact with the bath; and fines fine enough to be carried straight out of the vessel without reacting in it.

Is a smelting furnace the same thing as a refining furnace?

No, and the difference is the whole reason the two steps exist separately. A smelting furnace destroys chemical combination and separates phases: it puts the valuable metal into one dense stream and the gangue into a slag, and in doing so it takes a dilute, variable and often unassayable feed and turns it into a concentrated and documentable one. A refining furnace removes the residual impurities from metal that is already metal, working element by element, and its output is a declared purity rather than a separated phase. Published copper practice makes the gap visible in order of magnitude: an anode leaves the smelting side at roughly 98.5 to 99.8 per cent copper and a cathode leaves electrorefining at 99.99 per cent, and that last fraction is not achievable in a reduction vessel at all. Reading the two as one step is how a plant buys a smelting furnace expecting a refined product and discovers the difference at the assay bench.

What is the difference between a matte, a speiss, a bullion and a slag?

All four are molten phases that can coexist in the same bath, and they separate because they have different densities and different chemistries. A bullion is the dense metal phase: it carries the valuable metal and, in lead practice, the gold and silver that have dissolved into it. A matte is a molten sulfide phase, lighter than bullion and heavier than slag, and it is the collector in most copper and nickel routes because the valuable metals dissolve into it readily. A speiss is a molten arsenide or antimonide phase that forms when arsenic or antimony is present in the feed and is deliberately collected into a phase rather than left to distribute itself unpredictably; it is lighter than matte and carries a fraction of the valuable metal, which is why it is tapped, weighed and assayed like any other product instead of being treated as an accident. A slag is the silicate phase, and it is the one everything unwanted is thrown into. Each is tapped at its own height in the vessel and transferred separately, and a plant that cannot draw them apart has a mixed bath rather than four products.

How is a smelting furnace commissioned and accepted?

Commissioning runs through four stages that are each signed off separately. The refractory is dried on a written temperature curve with holds, because water trapped in a new lining turns to steam and spalls the brick from the inside, and that schedule is the largest single determinant of the first campaign’s length. The vessel is then heated to the temperature at which the heat source and, where the family uses them, the injection or burner settings can be commissioned and trimmed to their design values. The first heats are then run on the customer’s own feed with our engineers on the floor, with the burden sheet calibrated, the reductant and iron additions adjusted to the assay actually being charged, and the bath temperature, collector grade and slag condition brought to where the design said they would be. Acceptance is then the performance demonstration, and it is written as a closed mass balance rather than as a duration: the feed, the product, the slag, the speiss and the dust are weighed and assayed and the balance is closed against the design figures. Training on the burden sheet and the slag assay runs alongside the last two stages, so that the acceptance test is performed by the operators who will run the furnace rather than only by the people who built it.

What is the smallest project that makes an industrial smelting furnace worthwhile?

The threshold is not a tonnage; it is whether the materials can be made assayable and whether the plant can be made safe, and both have to be true before size is even discussed. If you hold mixed residues, dusts, slags or sweeps that cannot be assayed representatively as they stand, a smelting step pays for itself by turning an unassayable lot into a homogeneous and tradeable one, because nobody can buy or consign a parcel whose analysis depends on which part of the pile the sample came from. If you hold a by-product that a conventional smelter will not take, the threshold is set by whether the value in it justifies the reagents, the energy and the gas train. What does push the threshold up is the gas side, because a sulfidic or electronic feed brings a dust collector, a cooling train and either an acid plant interface or a scrubbing train with it, and that equipment has a minimum sensible size. Where a feed is dilute, high in impurity, or contaminated with mercury or chloride, the honest answer is often a wet route rather than a furnace, and this page will say so. Where the feed is genuinely metallic rather than combined, the right machine is a melting furnace and not a smelting furnace at all.