Precious Metal Smelting Machine

— Precious Metal Smelting Machine

A precious metal smelting machine is a crucible furnace that melts a weighed precious-metal charge under flux, drives the base metals out of the metal into a slag, and pours the cleaned metal into a preheated mould as a marked bar of declared weight. It is the one unit in a recovery train that does not raise fineness: what goes in as bullion, scrap, powder or sponge comes out as a homogeneous bar whose grade is set by the charge and confirmed by assay. YuSai Machinery builds it as a machine rather than a furnace body — an induction melting unit with a frequency-converted supply and a tilting body, or a resistance crucible furnace where the duty suits it; the crucible and its dressing schedule; the flux and oxidiser set; the mould bench with mould preheating; a sealed extraction hood with ducting, dust capture and scrubber. Working temperature is 1,600 °C in the standard design band with 1,800 °C available where the charge and the crucible call for it, batch weight is engineered to your charge weight and pour weight.

Product Overview

A precious metal smelting machine is a crucible furnace with a purpose narrower than its name suggests: it takes a charge that is already metallic, melts it, drives the base-metal fraction out of the metal into a slag, and casts the cleaned metal into a homogeneous bar. That single sentence fixes where the machine sits. It is not a refining unit — it does not dissolve, separate or upgrade a metal’s fineness. It is not a recovery circuit — it does not take metal out of a liquor. It is not a pyrometallurgical smelter — it does not take metal out of an ore or a concentrate. It takes metal, makes it uniform, cleans it of what flux can remove, and gives it a shape and a declared weight that can be assayed, traded and fed to the next step.

The machine exists in three families, and the choice between them is the first engineering decision on the page. A resistance crucible furnace heats a graphite or clay-graphite crucible from elements around it and works in a band around 1,100–1,200 °C; it is slow, it is cheap, it is quiet, and for sample preparation, hallmarking benches and melt shops running a few heats a day it is the honest answer. A medium-frequency induction melting furnace places the crucible inside a water-cooled copper coil and heats the metal by induced eddy currents, so the heat is generated inside the charge rather than conducted into it through a wall; it melts in minutes, it can hold a bath at temperature for sampling, and with a tilting body it pours a controlled stream into a mould. A vacuum or inert-gas unit covers the third case, where the alloy system cannot tolerate air or carbon and the bar’s surface and internal soundness matter more than throughput.

The induction family is then divided by how the metal leaves the crucible, because the pour is where a melting shop loses money and injures people. An integrated unit carries its own crucible and tilts when the charge is ready. A separately powered unit keeps the supply cabinet away from the furnace body so the cabinet is not cooked by radiated heat and the pour path is clear of cables. The largest bodies tilt hydraulically and require the floor, the headroom and the containment that a heavy pour implies. The choice is not a matter of taste: batch weight, pour weight, the number of heats per shift and the height at which a mould can be presented decide it, and the arithmetic is the same arithmetic whether the machine is in a jewellery workshop or a refinery.

What the furnace can and cannot do is decided by chemistry, and the page is explicit about both halves. What it can do is homogenise a parcel so that a sample taken from the bar represents the whole lot; remove base metals that report to an oxidising flux; absorb dissolved oxygen from a silver bath before casting so the bars do not spit as they freeze; take residual surface slag off the melt; and deliver a bar of known weight and known fineness. What it cannot do is raise fineness, remove silver from gold in bulk, or remove platinum group metals from anything — those are separation duties that belong to parting, to electrolysis, to solvent extraction and to the wet chemical routes, and a furnace that claims them is a furnace that will be followed by a dispute at the assay bench.

The furnace’s economics are dominated by two things, and neither is the purchase price. The first is the metal lost at every heat — to the crucible wall, to the dross, to the slag and to the filter dust — because that loss is precious metal at market price, recurring, and invisible unless the shop weighs and assays the fractions. The second is the crucible itself, which is a consumable with a service life measured in heats, whose failure mode is not a worn-out crucible but a cracked one that lets a full melt of metal run into the furnace lining. Both are addressed here as scheduled, measured items rather than as operating luck: the melt loss is closed on a balance sheet per heat, and the crucible carries a dressing procedure, an alloy history and a written retirement criterion.

The third cost, and the one most often discovered after installation, is the site itself. A melting station draws a heavy electrical supply at the machine terminals, needs circulating cooling water at a flow and an inlet temperature that the supply’s heat load sets, needs floor loading and headroom for a tilted body and for mould handling, needs a pour path with containment under it, and needs an extraction route sized on the crucible mouth rather than on the room. Melting clean bullion produces very little fume; melting coated, contaminated, damp or flux-dusted charge produces metal fume, flux dust and occasionally acid gas, and the extraction has to be sized for the second case because the shop will eventually run it. These five items — electrical, cooling water, floor and headroom, containment and drainage, and extraction — are issued as a schedule with a value against each line, so the builder can price the site before the furnace is shipped.

The machine also has limits, and they are stated here rather than discovered after purchase. A charge whose base-metal content is worth recovering in its own right should go to a refining route before it sees a furnace, because a furnace will only move those metals into a slag where they are harder to collect. A charge that arrives as a solution, a slurry or a plating rinse belongs in a recovery circuit. A charge that is too small in total volume to fill a crucible on a regular rhythm cannot carry the crucible, the flux, the power and the operator, and for that volume the honest answer is a shared melting service or a bench furnace rather than a production station.

What the customer receives is therefore bounded and documented: function-tested modules with material certificates; a site interface schedule with the five services itemised; installation and electrical, water and extraction checkout supervised; the first heat run on the customer’s own charge with the flux schedule and the power staging written against that charge; operators trained on the actual furnace and signed off against the standard operating procedure; and a performance trial that confirms the melt loss, the bar weight spread and the released fineness for the material tested. Every subsequent heat leaves with a charge weight, a bar weight, an assay and a closed balance.

Key Facts

One melt, defined. A melt is one weighed charge to one stored bar. Furnace time is 3–8 minutes on a hot induction unit and 20 minutes or more on a resistance unit, but the cycle that matters includes charging and fluxing, skimming, sampling, pouring, controlled cooling, weighing and reconciliation — which is what an operator is actually paid for.
A melting furnace does not refine. Melting homogenises a charge and separates base metals into a slag; it does not take 18-karat scrap to 999. A bar’s fineness comes from what was charged, and reaching 99.99% gold or silver is the job of the refining step — electrolytic or wet chemical — that follows the furnace, not of the furnace.
Working temperature. 1,600 °C is the standard design band for an induction melting unit on precious-metal duty; 1,800 °C is available where the alloy system and the crucible allow it. A resistance crucible furnace works in a lower band, at 1,100–1,200 °C, which is why it is a bench and sample furnace rather than a production one.
Melt loss is declared, not assumed. In a fire route the precious metal does not leave as vapour; it leaves as crucible dross, slag carry-over and bag-house dust. The charge is weighed, the bar is weighed, the dross, slag and filter dust are weighed and assayed, and the difference is reported as melt loss for that heat, with the collected fractions returned to the circuit.
The crucible is the consumable that decides the melt. Graphite below 1,600 °C, quartz above it or wherever carbon contamination is unacceptable, silicon carbide where thermal shock is the failure mode. Each alloy family gets its own crucible, because a crucible that has held a copper-rich melt will feed copper back into the next gold heat.
The melt gate is published. Sealed or hollow charge, damp or wet charge, mercury-bearing feeds, cadmium- and lead-coated scrap, beryllium-bearing alloys, halogen-bearing electronic scrap and cable, oxidiser-contaminated material and aluminium-bearing scrap are refused, each with the reason stated rather than with a warning to ask the sales desk.
Pour discipline is part of the machine. Moulds are preheated above 100 °C before the pour, silver melts are deoxidised before casting so the bars do not spit as they freeze, and the residual surface slag is taken off at the crucible spout rather than poured into the mould.
Extraction is engineered, and no zero-emission claim is made. A sealed hood, ducting on a calculated volume, a bag filter for flux dust and an alkaline scrubber where the charge chemistry demands one. Melting graded bullion needs little of it; melting coated, contaminated or flux-dusted charge needs all of it.
Batch weight and installed power are engineered. Crucible size, batch weight, melt-down time, supply rating, cooling-water duty, extraction volume, floor loading and the mould bench footprint are calculated from your charge mix and pour weight and stated as figures in the process design package, not selected from a fixed model list.
The product is a marked bar with a record. A cast bar stamped with its weight and melted under a numbered heat, released against assay with the slag and dust fractions declared, and accompanied by the melt record that closes the charge-to-bar balance.
Where the machine sits. Upstream of every refining route. The furnace produces the homogeneous bar, the anode plate or the grain that the electrolytic, wet chemical or pyrometallurgical step consumes, and it prepares the sample on which an outturn assay is based.

Product Core Selling Points

The furnace family is chosen by arithmetic, not by catalogue position. Charge weight per heat, the melting point and melting range of the alloy system, the pour weight the mould has to fill, whether the metal must be protected from carbon or from the atmosphere, and how many heats a shift has to run — those five figures decide between a resistance crucible furnace, an induction unit with a tilting body, and a vacuum or inert-gas unit. This page states the arithmetic and then sells the family that fits it.
Melt loss is reported per heat. Crucible dross, slag carry-over and bag-house dust are weighed and assayed, the collected fractions go back to the circuit, and the difference between the charge and the bar is declared as the melt 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 machine states what it cannot do. Melting does not raise fineness. A page that implies a furnace turns scrap into 999 metal is selling a bar that will fail the buyer’s assay, so this page states the boundary and routes the fineness work to the refining step.
The melt gate is written down. Nine classes of charge are refused, each with its mechanism — steam explosion, metal-fume toxicity, crucible attack, violent oxidiser reaction, thermite-type reduction, acid-gas generation — instead of a note asking the customer to confirm suitability with a sales contact after the order.
The crucible has a life, a log and a replacement rule. Crucible type is selected against temperature and chemistry, a new crucible is dressed and sintered before its first heat, every crucible carries its alloy history, and inspection is on a scheduled interval with a written retirement criterion. Crucible consumption is declared per heat so the recurring cost is visible before it is spent.
Pour and mould discipline is engineered, not improvised. Mould preheating above 100 °C, a mould dress that releases the bar cleanly, a pour rate matched to the mould section, deoxidation for silver, and stock cast-iron moulds for the common gold and silver bar weights with high-purity graphite and bespoke moulds for the higher-purity and platinum-group routes.
The extraction is sized on a calculation. Sealed extraction at the crucible mouth rather than a canopy over the whole room, ducting on a stated volume, a bag filter for the flux dust that a canopy hood lets into the shop, and an alkaline scrubber added where the charge chemistry produces acid gas. The page does not claim zero emissions, because two of the three common charge classes do emit.
Sampling is built into the cycle. The melt is sampled from the bath through a probe or vacuum pin tube at the point where the grade can still be changed, and the result decides pour, re-flux or divert — rather than the grade being discovered at the buyer’s assay office after the bar has been cast and shipped.
The bar leaves with its numbers attached. Weight, fineness by assay, heat number and the declared slag and dust fractions are recorded and issued with the bar, which is what lets a lot be traded, consigned or audited without re-melting it.
Slag and dust are product, not waste. The slag is crushed, milled and returned as recycle feed, the filter dust is collected as a declared precious-metal-bearing stream, and the spent crucible is processed rather than discarded, so the melt loss figure is a real measurement rather than a write-off.
The furnace is the assay enabler. A heterogeneous parcel of scrap, sweeps or catalytic material cannot be assayed representatively until it has been melted and homogenised; the melting step is what turns an unassayable lot into a saleable one, and this page treats that as a primary function rather than a by-product.
The heat is one programme on one panel. Power staging, temperature hold, timed flux and oxidiser addition, sampling prompt, pour temperature and the melt record run as a staged sequence with a manual mode reserved for fluxing and skimming, so heats are reproducible across operators rather than dependent on which shift is on duty.
The machine is delivered as a working melting station. Furnace, supply, crucible set, flux and oxidiser schedule, mould bench, extraction, sampling tools and the melt record arrive as function-tested modules; installation, the first heat on the customer’s own charge, operator training and the performance trial are supervised, and the training is on the actual furnace rather than on a manual.
The melting station is priced as a structure you can compute. The recurring cost sits in the crucible, the flux, the power drawn during melt-down and holding, the mould dress and the extraction consumables, in that order. This page names the levers and prices them per project against your own tariff and wage rate rather than publishing a per-kilogram figure borrowed from somebody else’s cost base.
A furnace that should not have been sold is a furnace that will not be reordered. Where the volume is too small to carry a melting station, or the charge belongs in a chemical route rather than a furnace, this page says so and names the alternative.

Application Scenarios

Jewellery workshops and casting houses. Bench and shop-floor melting of sprues, rejected castings, bench sweeps after incineration, and returned product, consolidated into a grain or a bar that can be re-alloyed to the workshop’s own specification.
Bullion dealers, refineries and vaulting operations. Buyers of mixed karat scrap and second-hand bullion who need a homogeneous bar before the lot can be assayed representatively and quoted honestly, rather than an XRF reading of an uncertain surface.
Assay offices and hallmarking centres. Sample-scale melting where the charge is small, the value per heat is high and the purpose is to produce the homogeneous sample on which the reference determination is made.
Precious metal refineries in the pre-melt and anode-casting position. Crude metal and reduced powder melted and poured into anode moulds ahead of the electrolytic step, and cathode product melted into a saleable bar at the end of it.
Gold mines and alluvial operations. Doré melted and cast on site into a form the receiving refinery will accept, with the weight and the heat record attached to the parcel before it leaves the mine.
Electronic scrap and printed circuit board recyclers. Cemented, reduced or ion-exchange-derived gold powder, stripped coating foil and connector concentrate, dried and consolidated into an assayed bar instead of being sold as a powder at a discount to its contained metal.
Precious metal catalyst processors. Spent automotive, petrochemical and chemical catalyst material whose recovered platinum group metal values are melted and cast into a form that a purchaser will accept and assay.
Dental, medical and laboratory alloy processors. Dental casting alloys, laboratory crucibles, electrodes and precious metal ware, melted and cast in house where the lot is small and the material is frequently irreplaceable.
Electroplating and surface finishing companies. Gold and silver recovered from plating baths, strip liquors and rinse waters as a powder or a cemented precipitate, melted into a bar before sale or before return to the plating circuit.
Watch, eyewear and luxury goods manufacture. High-value, low-weight manufacturing scrap and returned product where the melting station sits inside the factory’s own compliance envelope rather than consigning metal to a third party.
Mints, bars and semi-finished product makers. Melting of verified stock into grain or bar of a declared weight and fineness, with the marking and the heat record produced at the furnace rather than applied afterwards.
Industrial operations holding an in-house precious metal unit. Chemical, glass, fibre and electronics manufacturers that accumulate precious metal residues and want the consolidation step inside their own site boundary.
Operations that must assay a heterogeneous parcel before selling it. Mixed scrap, sweeps, slags, residues and off-spec stock that cannot be sampled representatively in the solid state and can be sampled once, after melting, from the cast bar.
Secondary refiners running toll campaigns. Multi-customer melting where the crucible, the flux schedule and the mould have to change between lots with a documented clean-out between them, so one customer’s metal does not report into the next customer’s bar.
Sites whose constraint is the building rather than the budget. Workshops with a limited electrical supply, no cooling-water circuit, restricted floor loading or no extraction route, where the furnace has to be sized against the services the building can actually provide — which is why the site interface schedule exists as a document rather than as an assumption.
Pilot, laboratory and teaching metallurgy facilities. Small-batch melting used to establish how a particular charge behaves on a few kilograms before a production station is committed, and to train operators on crucible handling, fluxing and pour discipline.

Applicable Raw Materials

The furnace is fed metallic charge, and what decides whether a lot is admitted is its composition and its physical form rather than its commercial description. Each feed below enters the melt at the point its condition allows: material that is dry, correctly sized and inside the base-metal window is charged directly; material that is wet, oversized, oxidised, coated or above the base-metal limit is dried, sorted, blended, pretreated or routed to a refining step first. Output is stated for each feed.

Mine doré and smelted bullion. Button and bar doré from a gravity or elution circuit, carrying gold with silver and a modest base-metal load, melted under an oxidising flux to a homogeneous bar. Output: doré bar of declared weight and fineness, with the slag retained for return.
Crude gold and crude silver bars. Partially refined metal bought or produced on site, re-melted so that a single sample represents the whole parcel before it is valued or consigned. Output: homogeneous bar or anode plate with a representative sample.
Karat jewellery scrap. 18, 14, 9 and 22 karat scrap, old gold, broken chains and mixed jewellery lots, melted and cast as a homogeneous bar whose assay then decides whether the lot goes to parting, to refining or back to alloying. Output: homogeneous karat bar with a declared assay; base metal retained as slag where an oxidising flux is used.
Sprues, runners, gates and rejected castings. Re-melted in house, with the alloy family kept separate so that the returned metal matches the specification it came from. Output: clean casting grain or bar returned to the workshop’s own alloy stock.
Bench sweeps, polishing dust and workshop residues. Incinerated to remove organics, milled, blended and sampled, then melted under a heavier oxidising flux because the base-metal load is high. Output: bar of declared contained-metal weight; slag returned for re-milling.
Gold and silver powder from solution. Cemented, reduced, retorted or ion-exchange-derived powder, washed free of residual reagents, dried to the point where it cannot spit in the crucible, and consolidated into a bar. Output: cast bar replacing a discounted powder product.
Stripped coating foil, plating sludge and deplated ware. Foil, flake and sludge collected from stripping and plating recovery, dried, blended and melted into a bar of known contained-metal weight. Output: bar with a declared weight and fineness.
Cathode product, sponge and crystal from refining. Electrolytic cathode gold or silver, hydrometallurgical sponge and crystal, washed and dried, melted and cast to the product form the buyer takes. Output: saleable bar, grain or button released against assay.
Anode slime, leached residues and filter cakes. Dried, blended and melted under an oxidising flux where the precious metal content justifies it, or routed to wet chemical treatment where the base-metal matrix does not. Output: precious-metal-bearing bar or a concentrate routed to refining.
Crucible residue, floor sweepings and bag-house dust. The machine’s own collected fractions, re-melted as a low-grade blended charge under a heavy slag regime rather than written off. Output: recovered metal returned to the circuit; barren slag to disposal.
Spent crucibles and refractory ware. Crushed and added to the residue melt, because a crucible that has held precious metal always holds some of it in its walls. Output: metal recovered from the crucible body; refractory residue to the slag.
Precious-metal-bearing alloy scrap and brazing alloys. Au–Ag, Au–Pt, Au–Pd and brazing alloy scrap with a platinum group metal load, melted and cast with the platinum group metal content declared in the assay rather than assumed to have left with the slag. Output: cast bar with the platinum group metal content declared.
Dental, laboratory, medical and instrument alloy scrap. Small, high-value lots handled on their own crucible and their own heat record. Output: cast bar with a lot-specific assay.
Recovered catalyst concentrates and PGM residues. Concentrates, ashes and the output of a capture smelt, melted and cast into a form a purchaser will sample and buy. Output: PGM-bearing bar or button with a declared content.
Consigned and toll lots. Customer-owned metal run as a discrete heat with its own crucible history, its own mass balance and its own retained sample. Output: bar with a lot-specific mass balance and certificate.
Verified bullion, coin and bar stock. Old bars, coin and stock whose provenance requires re-melting before resale, cast into fresh bar of declared weight and fineness. Output: re-cast bar with a fresh assay and a new mark.
Feeds admitted only after preparation. Wet or damp charge is dried; oversize charge is cut or granulated; oxidised or corroded charge is blended to a stable composition; coated and plated charge is stripped or pretreated; powder is briquetted or fed under a cover to stop it being carried out of the crucible on the draught. Output: a charge the crucible can accept without a loss of metal to the extraction duct.
Feeds routed to refining before melting. Material whose base-metal content is worth recovering in its own right, or whose precious metal content is bound up with a matrix flux cannot remove, goes to the refining route first and returns as metal. Output: a bar produced from a cleaner charge, with the base metals recovered separately instead of diluted into a slag.
Charges that are not admitted — sealed or hollow. Sealed cans, closed vessels, capped tubes, gas-cylinder fragments, hollow or voided assemblies and any charge containing trapped air or liquid. A sealed volume in a crucible becomes a pressure vessel inside a bath of molten metal and ejects its contents when it fails. Output: not admitted; opened, cut or drilled and inspected first.
Charges that are not admitted — damp or wet. Wet charge, ice-bearing material, sludge that has not been dried, and wet powder. Water flashes to steam at a volume ratio that turns a crucible into a mortar. Output: not admitted; dried to the dryness criterion in the procedure before charging.
Charges that are not admitted — mercury bearing. Amalgam, mercury-contaminated bullion, dental amalgam scrap, mercury-cell residues and any material on which mercury has been used. Mercury boils far below the melt temperature of precious metals and leaves as a toxic vapour that the extraction must not be asked to carry. Output: not admitted; retorted first, with the mercury recovered under its own abatement.
Charges that are not admitted — cadmium, lead and zinc coated. Cadmium-plated and galvanised ware, lead-coated scrap, soldered assemblies with a high lead fraction, and bearing metals. These metals boil or fume at precious-metal melt temperatures and the fume is a health hazard rather than a housekeeping nuisance. Output: not admitted as a bulk fraction; stripped or diluted under a dedicated extraction regime.
Charges that are not admitted — beryllium bearing and toxic alloy. Beryllium-copper and beryllium-bearing instrument alloys, and any alloy whose oxide or fume is acutely toxic. Output: not admitted to a general melt station; handled only under a dedicated regime with its own extraction and monitoring.
Charges that are not admitted — halogen bearing and organic. Printed circuit board scrap, PVC-insulated cable, halogenated plastics, solvent-wet material and oily charge. Halogens attack the crucible, the metal and the ducting, and the organic load produces a fume the extraction was not sized for. Output: not admitted; pyrolysed or incinerated first, with the off-gas treated at that step.
Charges that are not admitted — oxidiser contaminated. Nitrate, nitrite, chlorate, perchlorate, permanganate and peroxide-bearing material, including dried salt residues and oxidiser-contaminated dust. An oxidiser meeting molten metal reacts violently rather than melting. Output: not admitted; the oxidiser is destroyed or the material is routed to a wet chemical route.
Charges that are not admitted — reactive light metals. Aluminium, magnesium and their alloys, including thin foil and swarf mixed into a precious-metal charge. These metals reduce the metal oxide in the bath in a strongly exothermic reaction that raises the bath temperature beyond the crucible’s limit. Output: not admitted; separated by sorting before the charge is made up.
Charges that are not admitted — cyanide bearing. Free-cyanide residues, cyanide-leach sludges that have not been destroyed, and plated parts still carrying cyanide complex. Cyanide meets molten metal as a toxic and, under some conditions, energetic system. Output: not admitted; the cyanide is destroyed at the treatment step before the residual solid is melted.
Charges that are not admitted — unassayed sealed consignments. Any parcel that cannot be opened, inspected and identified before charging, including drums of mixed unknown material and sealed containers of undeclared scrap. Output: not admitted; opened, identified and assayed first, because the crucible is the last place to discover what a charge contains.

Finished Products

The machine is configured around the product form the customer sells or feeds forward, from a cast bar through to anode plate, grain and the declared side streams. Where the customer already operates a mould bench, an assay bench and a refining route, the melting station is delivered as the furnace, supply, crucible set, flux schedule and extraction; where it does not, the mould bench, sampling tools and the melt record are inside the scope.

Precious metal bar and ingot. The primary output: metal melted, fluxed, skimmed, sampled and poured into a preheated mould, cooled under control, de-moulded, weighed and marked. Output: a cast bar with a declared weight, a heat number and a fineness confirmed by assay.
Anode plate for the electrolytic step. Metal melted and poured into an anode mould of the geometry and thickness the refining cell requires, with the hanger positions and the immersion envelope respected. Output: a cast anode plate at the geometry the refining step specifies.
Casting grain and shot. Metal melted and poured through a grain die or into water so the workshop receives free-flowing grain it can weigh and melt again without re-cutting a bar. Output: clean grain of declared fineness for the jewellery and alloying trade.
Homogeneous sample and the retained sample. Metal sampled from the bath through a probe or vacuum pin tube, and a reference portion of the cast product retained under the heat number. Output: a sample on which an outturn assay can be agreed, and a retained reference for later dispute.
Button and bead for the assay bench. Small melts prepared for a reference determination, kept separate from production heats so that a bench result is never confused with a production record. Output: assay-quality button or bead with its own heat record.
Slag and dross. The oxide and flux layer that carried the base metals out of the metal, tapped or skimmed, cooled, crushed and milled. Output: milled slag returned to the melt as recycle feed, with the barren residue declared for disposal.
Bag-house and filter dust. The fine fraction captured by the extraction, collected as a separately declared precious-metal-bearing stream rather than discharged. Output: filter dust returned to the circuit as a blended charge.
Spent crucible and refractory fraction. The crucible that has held a precious-metal melt still holds metal in its wall and base, so it is crushed and processed rather than discarded. Output: metal recovered from the crucible body; refractory residue to the slag.
Melt record and balance sheet. Charge weight, alloy identification, flux and oxidiser additions, bath temperature and hold time, sample result, pour temperature, bar weight, and the weighed and assayed slag, dust and crucible fractions. Output: a closed charge-to-bar mass balance for that heat, with the melt loss stated.
Certificated bar for trade or consignment. The bar, its mark, its weight, its assay and its heat record issued together, so the metal can be traded, consigned or audited without being re-melted. Output: a bar released against assay with its documentation attached.
Metal returned to the customer’s own alloy specification. Melting is also a formulation step: verified metal is re-alloyed to a specified composition and cast, with the composition confirmed by analysis rather than by the weight of the additions. Output: bar or grain of declared alloy composition.
Metal prepared for an outturn assay on a mixed parcel. A heterogeneous lot that could not be sampled representatively in the solid state is melted, mixed and cast, and the sample is then taken from the bar. Output: a parcel converted from unassayable to assayable, which is often the whole commercial purpose of the melt.

Precious Metal Smelting Machine Process Flow

The nine stages below are one full melt cycle, from the charge survey that decides whether the lot is admitted through to a loaded furnace reset for the next heat. Stages 4 and 5 are where the melt is decided and where it can still be changed: the flux and oxidiser regime set at Stage 4 is what removes the base metals and protects the crucible, and the sample taken at Stage 5 is the last point at which the heat can be re-fluxed, adjusted or diverted instead of poured. The downstream separation of whatever remains in the metal — parting, electrolysis, solvent extraction, wet chemical treatment — is not part of this cycle and is dealt with on the gold refining machine and precious metal hydrometallurgical refining plant pages. Engineering setpoints are fixed during process design against your own charge and confirmed on the first production heat.

Precious metal smelting machine flow — one full melt cycle, from a surveyed charge to a marked bar and a closed mass balance, with slag, dust and crucible fractions returned on the same machine.

Steps 1 — Charge Survey and Furnace Setting. The lot is surveyed before anything is charged, because every decision downstream is taken from that survey. The charge is weighed, its form recorded, and its composition established by sorting, by XRF on a prepared surface and by a reference determination where the lot is heterogeneous; the melt gate is applied and any refused fraction is set aside with the reason written against it. From the survey the operator confirms the crucible type and size, the furnace and power setting, the flux and oxidiser recipe, the pour weight, the mould to be used and the heat record that will be raised. Where the charge is wet, oversized, oxidised, coated or above the base-metal window, the route sheet directs it to drying, cutting, blending or a refining step instead of the crucible, and that decision is recorded rather than improvised at the furnace mouth. Reagent and utility input: charge weighing, sorting and sample preparation, XRF and reference analysis, route sheet and heat record, melt-gate checklist. Output: a surveyed charge with its furnace setting, crucible, flux recipe and mould confirmed, or a documented diversion to preparation or refining.

Steps 2 — Crucible Preparation and Mould Preheating. A new crucible is dressed and sintered before its first heat, because an undressed crucible sheds its wall into the first melt and is the most common cause of a contaminated bar; a returning crucible is inspected, its alloy history checked against the charge, and its wall thickness compared with the retirement criterion. The crucible is seated in the coil or the furnace chamber so that it is centred, at the correct height and free to expand, and the coil, the insulation, the water circuit and the temperature measurement are checked before power is applied. The mould is prepared in parallel: cleaned, dressed so the bar releases without tearing, preheated above 100 °C so that the pour does not freeze against a cold face, and positioned so the pour stream enters without turbulence and the operator is not standing in the path of a splash. The pour path and the containment under it are cleared, and the extraction is started before the furnace is energised rather than when the crucible is already in the bath. Reagent and utility input: crucible and dressing compound, crucible history log, coil and insulation check, circulating cooling water, mould bench with mould dress, mould preheating, containment tray, extraction start-up. Output: a seated and inspected crucible, a preheated dressed mould at the pour position, and a cleared containment and extraction path.

Steps 3 — Charging, Melt-Down and Power Staging. The charge is built into the crucible rather than thrown at it: heavier pieces to the bottom against the crucible wall, fine and powdery material covered or briquetted so the draught does not carry it out, and the charge completed in two or three increments so that the bath never bridges and the crucible is never loaded above its working level. Power is applied on a staged programme — a controlled ramp while the charge is cold and the crucible is thermally shocked, a high stage while the bulk of the metal is still solid, and a reducing stage as the last solid disappears — rather than at full power from a cold start, which is what cracks crucibles and drives metal into the wall. Once the bath is liquid it is brought to the melting-and-holding band for the alloy, agitated so that the composition equalises, and held there long enough for the melt to become uniform rather than merely liquid, because a melt that is poured early is a bar with a gradient in it. Reagent and utility input: staged power programme, charge increments, briquetted and covered fines, circulating cooling water, temperature measurement, cover flux on the bath surface. Output: a completed, uniform, liquid bath at the working temperature, with the crucible and the charge level inside their limits.

Steps 4 — Fluxing, Oxidative Slagging and Skimming. This is the stage that removes what melting alone cannot, and its regime is written per charge rather than poured by judgement. A cover flux protects the bath from the atmosphere and reduces metal loss at the surface; an oxidising component converts the base metals into oxides that a borax-based slag will take up; a fluidiser keeps the slag mobile enough to be skimmed instead of trapping droplets of metal in a viscous layer. The additions are made in stages with a rake or a graphite rod, the bath is worked so the slag rises, and the slag is skimmed off cleanly and into a slag pan rather than dragged over the crucible lip and back into the bath. For a crude-metal charge the additions are heavier and the melt is held longer, because crude metal carries more base metal than a finished ingot and the slag has more work to do. For a silver bath that carries base metals the oxidising blow is made with air or with an oxidising salt, and the deoxidation and slag removal that follow are what stop the bars spitting as they freeze. The end of the stage is judged from the behaviour of the bath surface, from the colour and viscosity of the slag, and from the design hold time rather than from a clock alone. Reagent and utility input: borax cover and slag former, oxidising additions, fluidiser, rake or graphite rod, slag pan, air or oxidiser lance where the charge calls for it. Output: a metal bath with the base-metal fraction taken into a mobile slag, a clean metal surface at the pour, and a separately weighed slag for assay.

Steps 5 — Melt Sampling and Grade Decision. The bath is sampled while it can still be changed, and the sample is taken in a way that represents the melt rather than the surface. A probe or a vacuum pin tube is drawn from below the surface and away from the crucible wall, the sample is cooled and prepared, and the composition is established by spectrometric analysis against the instrument’s calibration with a reference determination where the result sits near a commercial boundary. The operator compares the result with the specification and takes one of three decisions on the record: pour, because the melt is inside specification; re-flux and hold, because the base metals or the dissolved gas have not yet been taken out; or divert, because the melt belongs on a refining route rather than in the mould. Sampling at this point is what converts a melting shop from a process whose grade is discovered at the buyer’s assay office into one whose grade is known before the metal is cast, and it is the reason the furnace is also the equipment that prepares a parcel for an outturn assay. Reagent and utility input: probe or vacuum pin sampling tool, sample preparation, spectrometer with its calibration, reference determination where required, grade decision record. Output: a recorded composition for the heat and a decision to pour, re-flux or divert.

Steps 6 — Tapping, Pouring and Ingot Casting. The melt is poured at the temperature the mould and the section demand, which is above the liquidus of the alloy but as low as the pour will allow, because every degree above what the pour needs is paid for in gas pickup, in mould wear and in a coarser cast structure. The crucible is tilted or the furnace tapped at a controlled rate matched to the mould section, so the stream is compact and stays inside the mould gate rather than spraying and folding air into the bar. The residual surface slag is held back at the crucible spout — taken off with a rod or a wisp of straw rather than poured into the mould — and for a silver melt the bath is deoxidised immediately before the pour, because silver that has taken up oxygen releases it as the bar freezes and sprays metal out of the open mould. The mould is filled in one pass without stopping, the bar is left to freeze under control rather than quenched, and where the alloy system or the product standard requires it the pour is made into an inert-gas or vacuum envelope. The heat number is transferred to the bar by marking at the mould or immediately after de-moulding. Reagent and utility input: controlled tilting or tapping, pour temperature control, mould dress and preheated mould, slag retention at the spout, deoxidation addition for silver, marking punch and heat number, inert-gas or vacuum envelope where specified. Output: a cast bar or plate of the design geometry, marked with its heat number and freezing under control.

Steps 7 — Cooling, De-Moulding and Charge Reconciliation. The bar is cooled on a controlled curve rather than quenched, de-moulded without being levered against a hot face, and weighed on a calibrated balance before it is presented for assay. The charge weight and the bar weight are reconciled against the weighed slag, the weighed and assayed filter dust and the crucible and floor residues collected during the heat, and the difference is reported as the melt loss for that heat rather than being absorbed silently. The reconciliation is the machine’s own check on itself: a loss materially above the declared figure points to a draught that is carrying powder out of the crucible, to a slag that is trapping metal in a viscous layer, to a crucible that has begun to fail, or to a pour that has spilled, and each of those has a different remedy. Bar weight is also compared with the pour weight the mould was set for, because the spread across a shift is the figure a customer actually feels when the bars are sold by weight. Reagent and utility input: controlled cooling, de-moulding tools, calibrated balance, weighed slag and dust fractions, residue collection, heat record entry. Output: a weighed, marked bar; a closed charge-to-bar balance with the melt loss stated; and residues identified for return.

Steps 8 — Dross, Slag and Filter Dust Recovery. Nothing that holds precious metal leaves the station as waste. The slag is cooled, crushed and milled, sampled and returned to the melt as a blended recycle charge or, where its precious metal content is high, held as a declared concentrate for the refining route; the filter dust from the extraction is collected, weighed and assayed on the same basis, because it is the fine fraction of whatever the draught took off the bath; and the spent crucible, the furnace lining at the end of its campaign, the floor sweepings and the mould dress residue are processed rather than discarded. The returned fractions are blended into a low-grade charge run under a heavier slag regime, which is how the metal that would otherwise appear as an unexplained loss reappears as product. Slag that has been milled and re-melted to the point where it carries no recoverable value is declared barren and its disposal is documented, so the point at which recovery stops is a recorded decision rather than an unstated one. Reagent and utility input: slag cooling and crushing, mill and sample preparation, filter dust collection and assay, crucible and lining crushing, blending and return-to-charge schedule, barren slag disposal record. Output: milled slag and filter dust returned as declared recycle feed; barren residue documented for disposal.

Steps 9 — Fume Extraction, Crucible Change-Out and Reset. The heat closes with the station returned to a state the next heat can start from. The extraction is kept running through the pour and the cooling period and is shut down only after the bath is covered or empty, the ducting and the filter are checked and the collected dust is drawn off into the declared stream, and where the charge chemistry produced acid gas the scrubber liquor is checked and the abatement performance recorded. The crucible is inspected for cracking, oxidation, wall thinning and glazing against its retirement criterion, its alloy history is updated, and it is either returned to service, retired to the residue stream or replaced and re-sintered. The furnace chamber, the coil, the water circuit and the temperature instrumentation are checked, the containment tray is cleaned and its contents recovered, the mould is inspected and returned to the bench, and the heat record — charge survey, flux and oxidiser additions, temperatures and hold times, sample result, bar weight, slag, dust and crucible fractions, and the melt loss — is closed and filed. Reagent and utility input: extraction run-on and filter change, scrubber liquor check, crucible inspection and retirement criterion, spare crucible and dressing compound, coil and water circuit check, containment clean-out, heat record closure. Output: a documented, inspected station reset for the next heat and a closed heat record filed against the cast bar.

Product Structure (Main Equipment)

The station is assembled from the seven module groups below. Materials are selected for the two environments the machine actually lives in — a bath of molten precious metal on one side and a working shop on the other: furnace body and coil housing in a refractory-lined steel shell, crucibles in graphite, silicon carbide or quartz by temperature and chemistry, crucible tooling and slag rakes in graphite or refractory-coated steel, moulds in cast iron or high-purity graphite, ducting and hoods in stainless steel at the hot end, and containers and linings in mild steel with a refractory wear layer.

Melting furnace and heating source

Induction melting unit with a tilting body. A refractory-lined crucible chamber inside a water-cooled copper coil, mounted on a frame that tilts to pour, with the coil, the insulation and the crucible seat arranged so the crucible is centred, free to expand and replaceable without stripping the coil. Output: the melting vessel, with a controlled pour and a crucible that can be changed on schedule.
Resistance crucible furnace. A chamber of heating elements around a clay-graphite or graphite crucible with a lid, used where the duty is sample-scale, intermittent or needs a slow controlled hold rather than a fast melt. Output: a quiet bench-scale melting and holding furnace for sample, hallmarking and small-lot work.
Vacuum or inert-gas melting chamber. A sealed chamber with a gas or vacuum circuit for alloys that cannot tolerate air or carbon, and for bars whose internal soundness matters. Output: a cast bar produced under a controlled atmosphere.
Furnace body, lining and spout. A steel shell with a refractory lining, a working lining that is patched on a scheduled interval, and a pouring spout shaped so the stream is compact and the residual surface slag is held back rather than carried into the mould. Output: a furnace body that survives its own thermal cycling and pours a clean stream.
Tilting or tapping mechanism. Manual, geared, motorised or hydraulic tilt depending on the pour weight and the headroom, with a controlled rate and a positive stop so the pour can be held or stopped mid-stream. Output: a controlled pour rate matched to the mould section and to the operator’s position.
Furnace foundation, containment and pour path. A base designed for the static and dynamic load of a tilted body, a containment tray under the crucible and the pour, and a clear, guarded pour path. Output: a pour that is contained when something goes wrong rather than spread across the floor.

Crucible set and dressing station

Crucible, selected by temperature and chemistry. Graphite below 1,600 °C, quartz above it or where carbon contamination is unacceptable, silicon carbide where thermal shock is the failure mode; each crucible assigned to one alloy family and logged. Output: a crucible that does not carry one customer’s base metals into the next customer’s bar.
Crucible dressing and sintering station. A dedicated area with the dressing compound, the sintering schedule and the handling tools for a new crucible, plus storage that keeps a crucible dry between heats. Output: a crucible that survives its first heat instead of shedding its wall into it.
Crucible handling tools and stands. Tongs, lifting frames and stands sized to the crucible and to the metal weight, so a full crucible is never moved by hand or by a tool that was not made for it. Output: crucible handling that does not crack the vessel at the point of highest value.
Crucible history log and retirement criterion. A record of the alloys run in each crucible, the heats it has taken, and the wall thickness or crack criterion at which it is retired. Output: a scheduled crucible change instead of a failure discovered by a bath of metal on the floor.

Power supply, control and temperature instrumentation

Frequency-converted induction supply with IGBT switching. A supply cabinet that converts the incoming three-phase mains to the working band and regulates power to the coil, with self-diagnostic protection rather than a fuse that fails with the furnace. Output: a supply that holds a programmed power profile instead of an on-off switch.
Supply separation and cooling. The supply cabinet sited away from the furnace body so it is not cooked by radiated heat, and a water circuit with a closed loop and a chiller where the duty is continuous. Output: a supply whose service life is set by its components rather than by its position in the shop.
Temperature measurement and control. A K-type or infrared measurement arrangement with a control accuracy of ±1 to ±2 °C in the working band, plus a bath thermocouple or probe for sampling and pour decisions. Output: a bath held at a set temperature rather than a bath judged by eye.
Staged power programme and manual mode. A programmed ramp, melt stage, hold stage and pour stage on the panel, with a manual mode reserved for fluxing, skimming and sampling. Output: heats that are reproducible across operators and shifts.
Alarm and interlock set. Over-temperature, water-flow loss, water over-temperature, coil over-current, supply over-temperature, crucible-overload indication and extraction-failure interlock, each with a trip action. Output: a fault that stops the heat instead of quietly ruining the crucible.

Flux, slagging and skimming tools

Flux and oxidiser set. Borax as the standard cover and slag former, soda ash and silica as the slag system, an oxidising addition where base metals have to be driven out, and a fluidiser where the slag has to stay mobile. Output: a slag that takes the base metals out of the metal and can be skimmed off cleanly.
Weighed addition station. Flux and oxidiser weighed per heat against the charge survey rather than poured by judgement, with the addition weights entered in the heat record. Output: a slag regime that is repeatable, and a melt loss that can be explained.
Rake, rod and skimming tooling. Graphite or refractory-coated tools long enough to work the bath without the operator leaning over it, plus a slag pan sized to the slag volume of the heat. Output: slag removed from the bath and from the pour, and not dragged back in over the crucible lip.
Deoxidation and surface treatment provisions. Wood blocks or an equivalent deoxidiser for silver baths, and a straw wisp or rod for taking the last of the surface slag off at the spout. Output: a silver bar that does not spit as it freezes, and a pour with a clean surface.

Mould bench, casting and marking

Mould set. Stock cast-iron moulds for the common gold and silver bar weights, high-purity graphite moulds where carbon pickup or surface finish matters, anode moulds at the geometry the refining step specifies, and bespoke moulds for the platinum group metals. Output: a cast product of the geometry and the weight the customer actually sells.
Mould preheating station. A controlled preheat above 100 °C before the pour, with the temperature on the record, so the pour does not freeze against a cold face or open a cold shut in the bar. Output: a bar with a sound surface and a repeatable weight.
Mould dress and release preparation. Mould dress applied and renewed on a schedule so the bar releases without tearing and the mould face is not progressively destroyed. Output: a bar that does not have to be cut out of the mould or dressed back to weight.
Marking, stamping and weight recording. Heat number and weight applied at the mould or immediately after de-moulding, together with the balance used for the bar weight. Output: a bar whose identity travels with it into trade, consignment or the next process step.
Grain die and granulation option. A pour-through die or water granulation arrangement where the customer’s product form is grain rather than bar. Output: free-flowing grain of declared fineness for the workshop or the alloying bench.

Extraction, dust capture and abatement

Sealed extraction hood at the crucible mouth. Extraction taken at the source, at the crucible and the pour, rather than a canopy over the room that lets the fine flux fraction spread through the shop first. Output: fume and flux dust captured where they are produced.
Ducting and air-volume calculation. Ducting sized to a stated volume for the hood, the pour position and the mould bench, with make-up air so the extraction does not fight the shop’s own ventilation. Output: an extraction that works with the furnace door open and during the pour, not only when the shop is idle.
Bag filter and dust collection. A filter stage that captures the flux dust as a collectable, weighable stream instead of discharging it, with access for changing the elements. Output: a dust fraction that is weighed and assayed and returned, rather than a loss out of the stack.
Alkaline scrubber where the charge demands it. An absorption stage for acid gas where the charge chemistry produces it, with the scrubbing liquor checked and the abatement performance recorded. Output: an abatement regime matched to the charge rather than a claim that no abatement is ever needed.
Monitoring and compliance support. Instrumentation and records for the parameters your permit requires, and the design data behind them, so the station can be declared rather than apologised for. Output: the technical data for an air and effluent permission application.

Sampling, weighing and the melt record

Bath sampling tools. A probe or vacuum pin tube that draws a sample from below the surface and away from the crucible wall, with the sample preparation bench and the reference determination arrangements that go with it. Output: a sample that represents the melt rather than its surface.
Calibrated charge and product balances. Two balances, one for the charge and one for the product and the residue fractions, on a calibration schedule. Output: a mass balance whose numbers are traceable.
Residue collection containers. Dedicated, labelled containers for slag, filter dust, crucible residue and floor sweepings, each weighed before it leaves the station. Output: the loss fractions accounted for one by one instead of a single unexplained difference.
Heat record and balance sheet. A single record per heat carrying the charge survey, the additions, the temperatures and hold times, the sample result, the bar weight and the residue weights, closed with the melt loss. Output: a heat whose behaviour can be reconstructed months later, with the numbers to do it.

Product Features

A published reason for every furnace family. Resistance, induction and vacuum units are separated on the page by the duty each one is for, so the buyer can see why one family is being offered rather than being told that it is the best one.
Melt loss closed on a balance sheet per heat. Charge, bar, slag, filter dust and crucible fractions are weighed and assayed and the difference is declared, which turns the largest recurring cost in a melting shop into a number that can be managed.
The limit of the machine stated before the sale. The page says plainly that melting does not raise fineness, so a buyer does not purchase a furnace expecting a refinery.
A melt gate with mechanisms rather than warnings. Nine classes of charge refused with the reason — steam explosion, metal-fume toxicity, crucible attack, oxidiser reaction, thermite-type reduction, acid-gas generation — so the rule can be applied by an operator at the furnace rather than by a sales contact after the order.
Crucible selection by temperature and chemistry. Graphite, silicon carbide or quartz with the choice rule stated, one alloy family per crucible, a dressing and sintering procedure for a new crucible, and a written retirement criterion instead of a failure discovered in service.
Flux and oxidiser regime written per charge. Cover, slag former, oxidiser and fluidiser specified against the charge survey, weighed per heat and recorded, so the slag does the work rather than the operator’s judgement.
Pour discipline engineered into the station. Pour temperature set above the liquidus but as low as the section allows, mould preheated above 100 °C, a controlled stream matched to the mould gate, residual slag held at the spout, and deoxidation for silver so the bars do not spit as they freeze.
Under-surface sampling with the grade decision taken at the bath. A probe or vacuum pin sample drawn below the surface and analysed against a calibration, with pour, re-flux or divert recorded while the metal is still liquid.
Extraction taken at the source. A sealed hood at the crucible and the pour rather than a canopy over the room, ducting on a stated volume, a bag filter that captures the flux dust as a weighable stream, and an alkaline scrubber where the charge chemistry produces acid gas.
No zero-emission claim. The page states that melting graded bullion produces very little fume and that melting coated, contaminated or flux-dusted charge produces metal fume, flux dust and sometimes acid gas, and that the extraction is sized for the second case because the shop will eventually run it.
Slag, dust and spent crucibles returned rather than written off. The fractions are crushed, milled, blended and re-melted under a heavier slag regime, and the point at which the residue is declared barren is a recorded decision.
A stated site interface schedule. Electrical supply and supply rating, circulating cooling-water flow and inlet temperature, floor loading and headroom for a tilted body, containment and drainage, and extraction route and volume, each listed with a value against the line.
Bar weight spread treated as a specification. The pour weight the mould is set for is compared with the weighed bar across a shift, because the spread is what a buyer feels when the metal is sold by weight.
Marking and identity at the furnace. Heat number and weight applied at the mould or immediately after de-moulding, so a bar carries its identity into trade, into consignment or into the next process step.
Toll-lot discipline. A documented clean-out between customer lots, a crucible assigned by alloy family, and a lot-specific mass balance, so one customer’s metal does not report into the next customer’s bar.
Formulation as a first-class function. Re-alloying to a specified composition with the result confirmed by analysis rather than by the weight of the additions, which is a melting duty that a refining page cannot describe.
Assay responsibility placed honestly. The machine prepares and homogenises the sample and hosts the sampling; the reference determination is made on the assay bench and the customer’s own laboratory is free to run the retained sample and disagree.
A stated economic floor. The volume below which a production melting station cannot carry its crucible, its power, its extraction and an operator is stated, together with the alternatives that suit that volume better.
Materials chosen for the two environments the machine lives in. Refractory-lined steel shell, copper coil with a closed water circuit, graphite, silicon carbide or quartz crucible, cast-iron or high-purity graphite moulds, and stainless ducting at the hot end.
The station arrives as modules and leaves as a working shop. Furnace, supply, crucible set, flux schedule, mould bench, extraction, sampling tools and heat record arrive function-tested, and installation, the first heat on the customer’s own charge and operator training are supervised.

Technical Parameters And Specifications

The figures below are the standard design basis and are confirmed or adjusted during process design once the charge survey is complete. Crucible size and type, batch weight, melting rate, supply rating, cooling-water duty, extraction volume, floor loading and the mould bench footprint are engineered to your charge mix, pour weight, shift rhythm and building services rather than taken from a catalogue of fixed models.

Precious metal smelting machine — technical parameters and specifications.

Parameter	Specification
Machine type	Precious metal melting station: crucible furnace with its heating supply and control, crucible set and dressing station, flux and slagging tooling, mould bench with preheating and marking, sealed extraction with dust capture, sampling and weighing station, and the heat record
Process basis	Crucible melting of a metallic precious-metal charge under flux: the charge is melted, the base-metal fraction is taken into a slag, the bath is sampled and graded, and the metal is poured into a preheated mould as a marked bar of declared weight
Boundary against neighbouring equipment	Downstream of recovery and of reduction to metal, upstream of every refining route. A melting furnace does not raise fineness, does not take metal out of a liquor and does not take metal out of an ore; parting, electrolysis, solvent extraction and wet chemical treatment are separate duties on separate pages
Furnace family — resistance	Crucible furnace heated from elements around a clay-graphite or graphite crucible, working in a band around 1,100–1,200 °C with a lid and a slow controlled hold; the bench and sample-scale answer where the duty is intermittent rather than continuous
Furnace family — induction	Medium-frequency induction furnace: crucible inside a water-cooled copper coil, heat generated inside the charge, tilting body for a controlled pour, integrated or separately powered supply
Furnace family — vacuum or inert	Sealed chamber with a gas or vacuum circuit for alloy systems that cannot tolerate air or carbon and for bars whose internal soundness matters; selected on the alloy and the product standard rather than on throughput
Heating principle	Induction: an alternating field induces eddy currents in the charge so the metal is its own heating element and the crucible wall is not the heat path. Resistance: a heated element transfers heat through the crucible wall. The distinction sets melt-down time, temperature ceiling and crucible life
Working temperature, standard design	1,600 °C in the standard design band for an induction melting unit on precious-metal duty
Working temperature, upper band	1,800 °C available where the alloy system and the crucible allow it; above 1,600 °C the crucible changes from graphite to quartz, because graphite is consumed faster at the higher temperature and introduces carbon into the melt
Temperature control accuracy	±1 to ±2 °C in the working band, with bath measurement by thermocouple or probe for sampling and pour decisions
Capacity, batch weight	Engineered to your charge weight, alloy mix and pour weight and stated as a figure in the process design package rather than selected from a fixed model list
Capacity — what sets it	The pour weight the mould has to fill, the melting range of the alloy system, the number of heats a shift must run, and the headroom and floor loading available for a tilted body
Melting rate	3–8 minutes of furnace time for a hot induction unit from charge to liquid, against 20 minutes or more for a resistance crucible furnace on the same weight; the melting rate is a property of the heating principle, the charge form and the crucible, not of the supply rating alone
Power supply and frequency	Frequency-converted induction supply with IGBT switching, cabinet sited away from the furnace body; the supply rating and the working band are engineered to the crucible size, the pour weight and the shift rhythm and stated in the process design package
Electrical supply	Three-phase supply stepped down, converted and regulated at the machine, sized to the melting duty rather than to the furnace body alone
Cooling	Water-cooled coil and supply on a closed circuit with a chiller where the duty is continuous; flow, inlet temperature and pressure duty engineered to the supply heat load and stated in the site interface schedule
Crucible — graphite	Standard crucible below 1,600 °C, good thermal conductivity and the lowest cost per heat, at the price of carbon pickup in the melt and a finite life measured in heats
Crucible — silicon carbide	Selected where thermal shock is the failure mode and where the crucible has to survive a fast or irregular cycle, at lower thermal conductivity than graphite
Crucible — quartz	Selected above 1,600 °C and wherever carbon contamination of the melt is unacceptable, including platinum group metal work, at the price of a more fragile vessel and a more careful dressing procedure
Crucible — cast iron	Not used for precious-metal melting: iron contaminates the melt, adds a metal that has to be removed later, and slows the melt because it conducts heat away from the charge
Crucible discipline	One alloy family per crucible, a dressing and sintering procedure before first use, dry storage between heats, inspection against a written retirement criterion, alloy history logged, and crucible consumption declared per heat
Flux system	Borax as the standard cover flux and slag former, soda ash and silica as the slag system, an oxidising addition where base metals have to be driven out of the metal, and a fluidiser where the slag must stay mobile enough to be skimmed instead of trapping metal droplets
Slagging regime	Additions specified per charge from the charge survey, weighed per heat, added in stages and worked into the bath, with the slag skimmed into a slag pan rather than dragged over the crucible lip
Crude-metal melts	Heavier flux and oxidiser additions and a longer hold than a finished-ingot melt, because crude metal carries more base metal and the slag has more work to do; the additions and the hold are written against the charge rather than against a default recipe
Silver deoxidation	Silver melts are deoxidised immediately before the pour, because a silver bath that has taken up oxygen releases it as the bar freezes and sprays metal out of the open mould
Sampling	Probe or vacuum pin sample drawn from below the bath surface and away from the crucible wall, prepared and analysed against the instrument calibration, with a reference determination where the result sits near a commercial boundary
Grade decision	Pour, re-flux and hold, or divert to a refining route — taken and recorded while the metal is still liquid, before the bar is cast
Pour temperature	Above the liquidus of the alloy and as low as the section and the mould dress allow, because temperature above what the pour needs is paid for in gas pickup, mould wear and cast structure
Mould preheat	Above 100 °C before the pour, with the temperature recorded, so the pour does not freeze against a cold face or open a cold shut in the bar
Mould set	Stock cast-iron moulds for the common gold and silver bar weights, high-purity graphite moulds where carbon pickup or surface finish matters, anode moulds at the geometry the refining step specifies, bespoke moulds for the platinum group metals
Mould dress	Applied and renewed on a schedule so the bar releases without tearing and the mould face is not progressively destroyed; mould dress residue collected and returned with the other residue fractions
Product forms	Precious metal bar and ingot; anode plate for the electrolytic step; casting grain and shot; homogeneous sample and retained sample; assay button or bead; slag and dross; bag-house and filter dust; spent crucible and refractory fraction
Melt loss and accountability	Declared per heat against a weighed and assayed balance: charge in, bar out, slag, filter dust and crucible fractions weighed and assayed, with the collected fractions returned to the circuit and the point at which the residue is declared barren documented
Melt loss — what drives it	Crucible dross, slag carry-over, filter dust from the extraction draught, powder carried out of the crucible during charging, and pour spillage; not evaporation of the precious metal itself, which is why the loss is captured rather than ventilated away
Melt gate — refused charges	Sealed or hollow charge; damp or wet charge; mercury-bearing feed; cadmium-, lead- and zinc-coated scrap; beryllium-bearing and toxic alloys; halogen-bearing and organic charge; oxidiser-contaminated material; aluminium, magnesium and reactive light metals; free-cyanide residue; unassayed sealed consignments
Extraction and dust capture	Sealed extraction hood at the crucible and pour positions rather than a room canopy; ducting sized to a stated volume for the hood, the pour and the mould bench; bag filter capturing the flux dust as a collected, weighable stream; make-up air so the extraction does not fight the shop ventilation
Off-gas abatement	Alkaline scrubber stage where the charge chemistry produces acid gas, with the scrubbing liquor checked and the abatement performance recorded; the melting station does not claim zero emissions, because coated, contaminated and flux-dusted charges do emit
Emission and effluent control	Extraction at the source, dust captured rather than discharged, containment tray under the crucible and the pour, mould and floor washings collected and settled, and the residue fractions routed to recovery rather than to drain
Instrumentation and control	Staged power programme with ramp, melt, hold and pour stages and a manual mode for fluxing, skimming and sampling; temperature measurement and control in the working band; water-flow and water-temperature monitoring; heat record raised and closed per heat
Alarm and interlock schedule	Over-temperature of the bath or the supply, loss of cooling-water flow, cooling-water over-temperature, coil over-current, crucible-overload indication and extraction failure, each with a documented trip action and a log entry
Heat records	Charge survey and weight, alloy identification, crucible and its history, flux and oxidiser additions, bath temperature and hold time, sample result and grade decision, pour temperature, mould and marked bar weight, slag, filter dust and crucible fractions, and the closed melt loss
Site services and utilities	Electrical supply and supply rating, circulating cooling-water flow and inlet temperature, floor loading and headroom for a tilted body and mould handling, containment and drainage, extraction route and air volume, and compressed air where the station uses it, each listed with a value in the site interface schedule
Materials of construction, summary	Refractory-lined steel shell; copper coil with a closed treated-water circuit; graphite, silicon carbide or quartz crucible; graphite or refractory-coated tooling; cast-iron or high-purity graphite moulds; stainless ducting and hood at the hot end; mild-steel containers with a refractory wear layer
Footprint and layout	Furnace, supply cabinet, crucible and dressing area, mould bench with preheating, extraction route, residue storage and the operator’s pour position engineered to your building and its access, with the figure stated in the process design package
Acceptance testing	First heat run on the customer’s own charge, confirming the melting rate, the melt loss against the balance sheet, the bar weight spread and the released fineness for the material tested, with a performance certificate issued
Documentation	Process design package with the charge survey basis and the mass balance method, site interface schedule, operating procedures for charging, fluxing, skimming, sampling, pouring and crucible change-out, alarm schedule, electrical and water drawings, equipment layout, spare parts catalogue, heat record template and the safety and hazard assessment
Consumables and replacement	Crucibles, dressing compound, flux and oxidiser, mould dress, refractory patching material, filter elements, gaskets and seals, and the tools that take the wear, each with a stated replacement interval and a recommended on-site stock list issued at handover

Technical Advantages

The furnace family is argued from the duty, not asserted. Resistance, induction and vacuum units are separated by the melting range of the alloy, the pour weight the mould must fill, the shift rhythm and the atmosphere the alloy needs, each stated on the page. Result: a buyer who can see why one family is right for one charge and wrong for the next, instead of a quotation for whichever unit is in stock.
Melt loss is a measured number rather than an operating assumption. Charge, bar, slag, filter dust and crucible fractions are weighed and assayed and the difference is declared per heat, and the collected fractions are returned to the melt. Result: the largest recurring cost in a melting shop becomes visible and manageable, and a rise in loss can be attributed to the draught, the slag viscosity, the crucible or the pour instead of being written off in aggregate.
The page states the limit of the equipment. Melting homogenises and cleans; it does not raise fineness, and the refining work is named and routed elsewhere. Result: a buyer who purchases a furnace for what it does, rather than a buyer who discovers the boundary in a dispute at the assay bench.
The melt gate is a published rule with the mechanism behind it. Ten classes of charge are refused with the physical reason, from steam explosion and metal-fume toxicity to crucible attack and thermite-type reduction. Result: an operator can refuse a charge at the furnace mouth without a phone call, and the shop does not discover the reason by having the accident the rule exists to prevent.
The crucible is treated as a scheduled consumable, not as an accessory. Selection rule by temperature and chemistry, a dressing and sintering procedure before first use, one alloy family per crucible, dry storage, a logged alloy history, a written retirement criterion and declared consumption per heat. Result: crucible failure becomes a planned change rather than a bath of metal on the floor, and cross-contamination between customer lots is designed out in the toolroom rather than policed by the operator.
Cast iron is ruled out explicitly. Iron crucibles contaminate a precious-metal melt, add a metal that has to be removed at the next step and slow the melt by conducting heat out of the charge. Result: one common false economy is closed before it is purchased, with the reason attached.
The flux regime is written against the charge and weighed per heat. Cover, slag former, oxidiser and fluidiser specified from the charge survey rather than poured by judgement, with the addition weights in the heat record. Result: a slag that takes the base metals out and stays mobile, a melt loss that can be explained, and a heat that another operator could repeat.
Pour and mould discipline is engineered into the station. Pour temperature set above the liquidus but as low as the section and the dress allow, mould preheated above 100 °C and recorded, a controlled stream inside the mould gate, residual slag held at the spout, and deoxidation for silver. Result: bars that cast without cold shuts, without spitting silver and without having to be dressed back to weight.
Sampling is a stage in the cycle, not an afterthought. A below-surface probe or vacuum pin sample is analysed while the bath is still liquid and the decision to pour, re-flux or divert is recorded. Result: grade is known before the bar is cast rather than after the buyer’s assay, which is the difference between a lot that can be traded at its value and a lot that has to be re-melted.
The melting step also produces the assay. A heterogeneous parcel cannot be sampled representatively in the solid state, and melting makes it uniform. Result: a lot that could not be valued honestly becomes a lot that can, which is frequently the entire commercial reason for owning the furnace.
Extraction is taken at the source and sized on a calculation. A sealed hood at the crucible and the pour rather than a canopy over the room, ducting on a stated volume, a bag filter capturing the flux dust as a weighable stream, and make-up air so the extraction does not fight the shop. Result: fume and dust captured where they are produced, and a dust fraction that is weighed and returned instead of being discharged up a stack.
The honest emission position. The page states that melting graded bullion produces very little fume and that coated, contaminated or flux-dusted charge does produce metal fume, flux dust and sometimes acid gas, and that the extraction is sized for the second case. Result: a station that is declared rather than apologised for, and a claim that survives the first visit from a regulator because it was never an absolute one.
The site interface schedule is a document with values in it. Electrical supply and rating, cooling-water flow and inlet temperature, floor loading and headroom, containment and drainage, extraction route and volume, and compressed air where it is used. Result: the builder can price and prepare the site before the furnace ships, instead of the plant discovering a missing service during commissioning.
Bar weight spread is specified rather than hoped for. The pour weight the mould was set for is compared with the weighed bar across a shift. Result: the spread the customer feels when the metal is sold by weight is a managed figure instead of a surprise.
Slag, dust and spent crucibles are returned, not written off. Fractions are crushed, milled, blended and re-melted under a heavier slag regime, and the point at which residue is declared barren is documented. Result: the metal that would otherwise appear as an unexplained loss reappears as product, and the disposal decision is recorded.
Toll-lot separation is designed rather than promised. A documented clean-out between customer lots, a crucible assigned by alloy family and a lot-specific mass balance. Result: one customer’s base metals do not report into the next customer’s bar, which is the failure a toll refiner cannot survive.
The station is modular. Furnace, supply, crucible set, flux schedule, mould bench, extraction, sampling tools and heat record can be extended or reconfigured as the charge mix changes, against services that were identified at the outset. Result: the entry station proves the route on the customer’s own material instead of becoming stranded capital.
The economic floor is stated openly. The volume below which a production melting station cannot carry its crucible, its power, its extraction and an operator is disclosed together with the alternatives that suit that volume. Result: qualified enquiries, and customers who are not sold a station that should not have been sold.
Running cost is given as a structure you can price locally. The recurring cost sits in the crucible, then the flux and oxidiser, then the power drawn during melt-down and holding, then the mould dress and the extraction consumables, with labour modest across a heat. Result: a total cost of ownership the customer can compute against their own tariff and wage rate rather than a per-kilogram figure lifted from somebody else’s cost base.

Service Process

Step 1 — Charge data and duty review. You provide the charge type, an assay or typical analysis, the form the charge arrives in, the pour weight the moulds must fill, the number of heats per shift, the product form required, and any site constraints on electrical supply, cooling water, floor loading, headroom or extraction. We state at this stage whether the charge can be melted as surveyed or needs drying, cutting, blending, stripping or a refining step first, and whether the volume is above the economic floor for a melting station. Output: preliminary route statement, boundary conditions and a clear scope of supply.
Step 2 — Furnace selection and station configuration. The furnace family, crucible type and size, supply rating, cooling duty, extraction volume, mould bench arrangement and sampling provisions are calculated from your charge survey and pour weight, with the melting rate, the working temperature band and the expected melt loss written down beside them. Output: a configuration proposal with the design basis stated, plus a draft site interface schedule.
Step 3 — Laboratory melt test on your material. A representative sample of your charge is melted at bench scale to establish the melting behaviour, the flux and oxidiser addition that removes the base metals, the working temperature band, the achievable bar weight, the cast surface, the fineness reached from that charge and the actual melt loss on real material rather than on assumption. Output: a test report with a cast sample returned for your own assay.
Step 4 — Module fabrication and factory function test. Furnace body, coil and lining, supply and control cabinet, crucible set and tooling, mould bench, extraction hood and ducting are fabricated, assembled and function-tested before shipment, with material certificates and test records retained and issued with the station. Output: a factory function test record and the shipping documentation set.
Step 5 — Site interface schedule and building preparation. The site services are confirmed against your building, together with the foundation and floor finish, the containment and drainage, the pour path and its guards, the extraction route to atmosphere and the residue storage area. Output: a signed interface schedule, layout drawing and utility connection drawing your builder can work from.
Step 6 — Installation, electrical checkout and extraction commissioning. Our engineers supervise setting the furnace and the supply cabinet, seating and sintering the first crucible, aligning the coil and the water circuit, pressure testing the cooling loop, checking out the electrical and control installation, and commissioning the extraction and the filter so the required volume is measured rather than assumed. Output: a mechanically complete melting station ready for its first heat.
Step 7 — First heat on your own charge. The first heat is run on your charge with the flux and oxidiser schedule, the power staging and the pour temperature written against that charge, and the melt loss, the bar weight and the released fineness measured against the process design basis. Output: a commissioned station with its setpoints recorded and a measured melt loss for your material.
Step 8 — Operator training and production qualification. Training covers charge inspection and the melt gate, crucible dressing and handling, charging and power staging, fluxing, slagging and skimming, sampling and grade decision, pour temperature and mould preparation, deoxidation for silver, marking and weighing, the residue fractions and the heat record, taught on the actual furnace. The production trial then confirms the melting rate, the bar weight spread, the fineness and the melt loss, and the operators sign off the standard operating procedures. Output: trained operators, a signed SOP set and a performance certificate for the first production heat.
Step 9 — Consumables, spares and continuing process support. The crucible and consumable stock list with its replacement intervals is handed over with the station, spare parts are supplied thereafter, and our engineers stay available for process troubleshooting and for re-specifying the flux schedule, the crucible and the power staging as your charge mix changes. Output: continuing production support with a stocked consumables position and revised procedures as the charge changes.

Warranty And After-Sales Service

Warranty period. Twelve months from successful commissioning or eighteen months from shipment, whichever falls earlier, covering design, manufacture and assembly defects of the supplied equipment. Excluded: consumables such as crucibles, dressing compound, flux and oxidiser, mould dress, refractory patching material and filter elements, and damage from misuse or from a charge outside the agreed specification.
Performance commitment. Where the charge matches the tested sample, we stand behind the melting rate, the bar weight spread, the released fineness and the melt loss established in the process design package, with the production trial as the reference point. Output: a documented performance acceptance attached to the station’s file.
Spare parts. Crucibles and crucible tooling, moulds, refractory patching material, cooling-circuit parts, supply components, filter elements, gaskets and seals are quoted at the time of order and held available afterwards, with a recommended on-site stock list issued at handover. Output: an on-site consumables position agreed before the station enters production.
Remote support. Telephone, email and messaging support on operating questions, alarm interpretation, temperature and power staging questions, slag behaviour and crucible diagnosis, whenever the station is running. Output: same-day response on operating issues.
On-site service. Engineer attendance for warranty faults, process problems and periodical check-ups, with response and attendance times agreed in the contract according to your location. Output: on-site intervention and a service report.
Crucible and refractory assessment visits. Periodical inspection of crucible condition, lining wear and mould faces against the retirement criteria, with a written recommendation on replacement intervals for your actual operating pattern rather than a generic schedule. Output: a replacement plan built from your own consumption record.
Operator retraining and refresher visits. Additional training, refresher sessions and process optimisation visits as the charge mix, the product standard or the alloy system changes, including re-establishing the flux regime and the power staging after a change of charge supplier. Output: updated training and revised procedure documents.
Charge-mix change support. Where a customer adds a new charge type, we assess it against the melt gate, establish the flux and oxidiser additions it needs, run a bench melt if the answer is not obvious, and update the route sheet and the crucible assignment accordingly. Output: a documented admission decision and a revised recipe for the new material.
Documentation set. Operation and maintenance manuals, the process design package with the charge survey basis and the mass balance method, the site interface schedule, equipment layout, electrical and water drawings, the alarm schedule, the crucible log template, the spare parts catalogue, the safety and hazard assessment for the reagents handled, and the emergency response plan. Output: complete documentation handover at commissioning.
Compliance support. Assistance with extraction and abatement parameter declarations, filter and scrubber performance verification, and the handling, storage and disposal requirements for the fluxes, the oxidisers and the residue streams. Output: technical data for your air, effluent and waste permit applications.
Lifetime technical support. The process design remains yours, and our engineering team remains available for modification and expansion advice for the operating life of the station. Output: long-term engineering continuity.

Frequently Asked Questions

Does a precious metal smelting machine refine the metal, or only melt it?

It melts, homogenises and fire-refines the surface chemistry, but it does not raise fineness. A furnace can melt a charge, take base metals into a slag, remove dissolved gas from a silver bath, and cast a homogeneous bar; it cannot separate silver from gold in bulk and it cannot take an 18-karat charge to 999. The fineness of the bar is set by what was charged and is confirmed by assay, and reaching 99.99% or better is the job of the refining step that follows, whether that is electrolytic, wet chemical or a combination. We state this boundary plainly because a buyer who expects a refinery to come out of a furnace will eventually have a bar that fails their customer’s assay, and the argument that follows is always more expensive than the sentence that would have prevented it.

Which furnace family should be used for a given precious metal charge?

Four figures decide it. The melting range of the alloy system sets the temperature ceiling and therefore the crucible material. The pour weight the mould has to fill sets the crucible size and the body. The number of heats a shift must run decides whether the shop needs the speed of induction or can live with the slower cycle of a resistance furnace. The atmosphere the alloy needs decides whether an open crucible is acceptable or whether the melt has to be made under inert gas or vacuum. A resistance crucible furnace in the 1,100-1,200 °C band is the right answer for sample preparation, hallmarking benches and occasional small melts; a medium-frequency induction unit with a tilting body is the answer for production melting in the 1,600 °C band and above; a vacuum or inert-gas unit is the answer where the alloy cannot tolerate air or carbon. Size follows from pour weight and shift rhythm, and we state the figures rather than quoting a model number.

What is melt loss, and how is it declared rather than assumed?

Melt loss is the difference between the precious metal in the charge and the precious metal in the cast bar, and in a properly run furnace it is not evaporation. The precious metal leaves the bath in three places: into the crucible wall and base as dross, into the slag where droplets are trapped in a viscous layer, and into the extraction duct as the fine fraction of the powder and flux dust. Our method is to weigh the charge, weigh the cast bar, weigh and assay the slag, the filter dust and the crucible and floor residues, return the collected fractions to the circuit, and report the difference as the melt loss for that heat. That converts the largest recurring cost in a melting shop from an assumption into a number that can be managed, and it means that a rise in loss points at the draught, the slag viscosity, the crucible or the pour instead of being written off in aggregate at the end of the month.

Which charges must not be put into a precious metal melting furnace?

Ten classes, and each one is refused for a physical reason rather than for caution. Sealed or hollow charge, because a sealed volume inside a bath of molten metal fails as a pressure vessel and ejects it. Damp or wet charge, because water flashes to steam at a volume ratio that turns the crucible into a mortar. Mercury-bearing feed, because mercury boils far below the melt temperature and leaves as a toxic vapour. Cadmium-, lead- and zinc-coated scrap, because those metals fume at precious-metal melt temperatures. Beryllium-bearing alloys, whose fume is acutely toxic. Halogen-bearing electronics, PVC cable and organic charge, because halogens attack the crucible, the metal and the ducting while the organic load produces a fume the extraction was not sized for. Oxidiser-contaminated material such as nitrate, chlorate or peroxide residues, because an oxidiser meeting molten metal reacts violently rather than melting. Aluminium, magnesium and reactive light metals, because they reduce the metal oxide in the bath in a strongly exothermic reaction that takes the bath past the crucible’s limit. Free-cyanide residue. And any sealed consignment that cannot be opened, identified and assayed first.

How is the crucible chosen, and when is it replaced?

Two variables decide the material: the working temperature and the chemistry of the melt. Graphite is the standard choice below 1,600 °C and the lowest cost per heat, at the price of some carbon pickup in the metal and a finite life measured in heats. Silicon carbide is chosen where thermal shock is the failure mode and the crucible has to survive a fast or irregular cycle. Quartz is chosen above 1,600 °C and wherever carbon contamination of the melt cannot be accepted, including platinum group metal work, at the price of a more fragile vessel and a more careful dressing procedure. Cast iron is not used for precious-metal melting at all, because it contaminates the melt, adds a metal that has to be removed downstream and slows the melt by conducting heat out of the charge. Replacement is on a written criterion rather than on failure: every crucible is dressed and sintered before its first heat, assigned to one alloy family, stored dry, logged with the heats and alloys it has taken, and inspected on a schedule for cracking, oxidation, wall thinning and glazing until it meets the retirement criterion.

Why is the mould preheated before pouring, and to what temperature?

Because a pour into a cold mould freezes against the mould face before it can flow and fill, which produces a cold shut, a lap, a torn surface and a bar that is short of its design weight. The mould is therefore preheated above 100 °C before the pour, and the temperature is recorded on the heat sheet. Overheating the mould is its own problem: it destroys the mould dress faster, it slows the freeze and it lets the bar pick up a coarser structure, so the preheat is a band rather than a target to exceed. For silver melts the pour is also deoxidised immediately before casting, because a silver bath that has taken up oxygen releases it as the bar freezes and sprays metal out of the open mould, which is a metal loss and a burn hazard at the same time.

Which fluxes are used, and what does each one do?

Four jobs have to be done, and the recipe is written per charge to do them. A cover flux, normally borax, protects the bath surface from the atmosphere and reduces the metal lost at the surface. An oxidising addition converts the base metals in the metal into oxides so that they can leave. The slag system itself, built on borax with soda ash and silica, has to dissolve those oxides and hold them. A fluidiser keeps the slag mobile enough to be skimmed rather than leaving it viscous and full of trapped metal droplets, which is where a badly built slag quietly steals the metal it was meant to remove. The additions are weighed per heat rather than poured by judgement, they are added in stages and worked into the bath, and the weights go into the heat record, which is what makes a slag regime repeatable and a melt loss explainable.

How is the melt sampled and graded before it is poured?

A probe or a vacuum pin tube is drawn from below the bath surface and away from the crucible wall, because a surface sample sees a different composition from the bulk, and the sample is cooled and prepared for analysis against the instrument’s calibration, with a reference determination where the result sits near a commercial boundary. The result is compared with the specification while the metal is still liquid and one of three decisions is recorded: pour, because the melt is inside specification; re-flux and hold, because the base metals or the dissolved gas have not been fully taken out; or divert, because the melt belongs on a refining route rather than in the mould. Sampling at this point is what changes a melting shop from a process whose grade is discovered at the buyer’s assay office into one whose grade is known before the metal is cast.

What extraction and abatement does a melting station require?

It depends entirely on the charge, and we size it for the harder case because the shop will eventually run it. Melting graded bullion in a dressed crucible produces very little fume, and a sealed extraction hood at the crucible and the pour, ducted on a stated volume, is normally sufficient. Melting coated, corroded, contaminated or flux-dusted charge produces metal fume and flux dust, and the extraction has to take those at the source rather than letting a room canopy spread them through the shop first, which means extraction at the crucible mouth, a ducting volume calculated for the hood, the pour and the mould bench, and a bag filter that captures the flux dust as a collected, weighed stream instead of discharging it. Where the charge chemistry produces acid gas, an alkaline scrubber stage is added and its performance is recorded. We do not claim zero emissions, because two of the three common charge classes do emit, and a claim that cannot survive a site inspection is worse than no claim at all.

What does one melt cycle look like from a weighed charge to a stored bar?

Nine stages. The lot is surveyed and its furnace setting, crucible, flux recipe and mould confirmed, or diverted. The crucible is inspected or dressed and seated, and the mould is cleaned, dressed and preheated above 100 °C. The charge is built into the crucible in increments under a cover flux and the power is staged through ramp, melt and hold rather than applied at full power from cold. The bath is fluxed, oxidised and skimmed until the base metals have left the metal. The melt is sampled below the surface and graded, and the decision to pour, re-flux or divert is recorded. The metal is poured at the design pour temperature into the preheated mould and the bar is marked with its heat number. The bar is cooled on a controlled curve, de-moulded, weighed and reconciled against the charge, with the melt loss declared. The slag is crushed, milled and returned, the filter dust is collected and assayed, and the spent crucible goes to the residue stream. The extraction is run on through the pour, the crucible is inspected or replaced, the station is reset and the heat record is closed.

What size of smelting machine fits a given workshop or refinery?

Size follows from the pour weight the mould must fill and the number of heats a shift has to run, not from a floor area target. The two practical limits are usually the electrical supply and the extraction, because a melting station draws a heavy supply at the machine terminals and needs an extraction route that a leased unit may not have. The economic floor is different in kind: below a certain annual volume a production melting station cannot carry its crucible consumption, its power and an operator across the small number of heats it runs, and the honest answers at that volume are a bench-scale resistance furnace for intermittent work, a shared melting service, or a chemical route if the metal is in a liquor rather than in a solid. We state where that boundary sits rather than selling a station into it.

Where does the melting furnace sit in the recovery train?

Immediately before every refining route and after every recovery route, and it is the step that makes the rest of the train work commercially. Upstream of it are mechanical liberation, pyrolytic treatment and the recovery circuits that bring precious metal back to the metallic state — cementation, reduction, ion exchange, stripping, electrowinning. Downstream of it is separation: parting, electrolysis and electrowinning or the hydrometallurgical routes that dissolve, separate and upgrade to a certified fineness. What the furnace contributes is a homogeneous bar, an anode plate or grain of known weight and known composition, and a retained sample, which is what allows a heterogeneous parcel to be valued honestly and what allows the refining step to be specified against a real feed rather than against an estimate. It is also the only step in the train that prepares the sample on which an outturn assay is agreed.