High Frequency Smelting Furnace

— High Frequency Smelting Furnace: Frequency, Penetration Depth and Melt-Rate Arithmetic

A high frequency smelting furnace melts metal that is already metal by inducing the current inside the charge, and the number that defines it is not a capacity in kilograms but a frequency in hertz. That frequency fixes the penetration depth of the induced current, the depth sets the smallest crucible bore the machine can couple to, and the bore sets the charge one heat can take; power and cycle time then follow from arithmetic rather than from a claim. The words high frequency cover 2 kHz to 500 kHz in commercial use, so this page compares hertz, bore and kilowatts instead of labels. Melting a kilogram of gold needs about 0.055 kWh of useful energy, published furnace data implies an overall efficiency of roughly one third, and the working figure is about 0.17 kWh of input energy per kilogram. YuSai Machinery engineers the station around your alloy, batch mass and cycle time, and states where melting stops and refining begins.

Product Overview

A high frequency smelting furnace is an induction machine, so the heat is generated inside the metal rather than delivered to its surface. An alternating current in a water-cooled copper coil builds an alternating magnetic field through the crucible, that field induces a current in the charge, and the resistance of the metal to that current raises its temperature. Nothing touches the metal to heat it, no fuel gas has to be kept out of the melt, and the same coil that heats the charge also stirs it, because the field exerts a force on the liquid. That single mechanism is why the family is used wherever a small, clean, fast and controllable melt of a valuable metal matters more than a large cheap one, and why its performance is decided by electrical and geometric figures rather than by the size of the shell.

The figure that decides the most is the frequency, because it sets how far into the metal the induced current flows. Current density in a conductor carrying alternating current is highest at the surface and falls away towards the centre, and the depth at which it has fallen to about 37 per cent of its surface value is called the reference depth. It is calculated as 503 times the square root of the resistivity in micro-ohm-metres divided by the product of relative permeability and frequency in hertz, so the depth falls as the frequency rises and grows as the metal becomes more resistive with temperature. The matching rule published by induction equipment makers is that the reference depth should not exceed about one eighth of the charge diameter, which is the same rule as a charge diameter of at least four to eight reference depths. Two consequences follow that the market rarely spells out: a smaller crucible requires a higher frequency, and a low-resistivity metal requires a higher frequency than a resistive one at the same bore.

The label itself has to be treated with suspicion before any of those figures can be compared, because the published bands disagree. One German induction reference puts low frequency at 50 to 500 Hz, medium at 500 Hz to 50 kHz and high at 50 kHz to 27 MHz; one laboratory furnace maker defines high as 100 to 450 kHz; one induction heating manufacturer places medium at 1 to 20 kHz and high at 30 to 80 kHz; one furnace builder classifies everything above 10 kHz as high. A Japanese furnace table sells an entire line under the heading of a high frequency induction furnace while listing 3 kHz for its smallest model and falling to 500 Hz at the top of the range, so the frequency drops as the capacity climbs from fifteen kilograms to five tonnes. The pattern hidden under those labels is the geometric rule being obeyed by every one of those models while the marketing word stays the same, and the only way to compare two machines called high frequency is to ask for the hertz, the bore and the kilowatts.

What follows from those three figures reaches all the way to the cost of a heat. The bore sets how much metal can couple to the coil and therefore the mass of one batch; the mass and the alloy set the energy required, which is the sensible heat needed to reach the melting point plus the latent heat needed to change the state, and that figure differs by a factor of nearly five across the metals a precious-metal shop handles. The efficiency with which electricity becomes molten metal is then bought from the machine: published data gives 790 kWh per tonne for cast iron at 1,500 °C, roughly a third of the thermodynamic minimum, and that ratio is the honest starting point for any discussion of running cost. Because a cold charge does not present a continuous current path, and because radiation and conduction losses are proportionally largest in a small vessel, the first cold heat of the day and the fifth successive heat do not produce the same cycle time. What leaves the crucible is an alloy of the composition that was charged, less the melt loss that went to the dross, the crucible skull and the dust. The value of the machine is that it delivers a homogeneous, correctly alloyed charge at a repeatable temperature, which is what the steps downstream of it require.

Application Scenarios

Precious-metal refineries casting their own anodes and bars. Upstream delivers reduced sponge and downstream is an electrolytic cell, so the melt has to be fast, clean and correctly dimensioned without picking up the base metals a fuel flame would put back in.
Gold-buying and second-hand jewellery counters closing a daily parcel. Mixed scrap taken in during the day has to become one homogeneous bar before it can be assayed representatively, valued and consigned.
Bullion traders and small bar producers. Collected gold melted into saleable bars of declared weight and composition, where cycle time, melt loss and repeatability decide the margin far more than tonnage does.
Jewellery manufacturers melting their own returns and alloying their own colours. Runners, sprues, filings and trade-ins remelted and alloyed to a specified carat, where a small crucible changeable between alloys matters more than throughput.
Precious-metal recyclers working from sweeps, dust and filter cake. Bench sweeps, polishing dust, incinerated electronic residue or plating sludge ash melted with a flux and a collector so the value gathers into one bar for the refining circuit.
Dental laboratories and dental scrap processors. Casting alloys and precious-metal dental scrap in batches measured in grams, where freedom from cross-contamination between alloys is the whole requirement.
Assay offices and precious-metal laboratories. A reliable melt of a small sample into a button or bead for fire assay, X-ray fluorescence or spectroscopy, where reproducibility outranks speed.
Catalyst processors concentrating platinum group metals. Spent automotive, petrochemical or chemical catalyst material after the carrier has been stripped, needing a small, hot, controllable melt to gather platinum, palladium and rhodium into a collector phase.
Printed circuit board and electronic-scrap downstream melting. A gold-, silver- and palladium-bearing concentrate, connector scrap or plated part melted into a collector bar before it can be traded or refined.
Platinum, rhodium and iridium laboratory melting. Metal measured in tens or hundreds of grams, with melting points from 1,555 °C for palladium to 1,964 °C for rhodium, in a vessel that has to tolerate a temperature no ordinary crucible grade holds indefinitely.
Silver and silver-alloy melting for brazing, contacts and sterling. Brazing alloys, electrical contact material and sterling stock, where silver’s melting point of about 962 °C sits comfortably inside the band and the cycle runs several batches an hour.
Coin, medallion and blank production in small batches. Minting and commemorative work where the alloy is melted and cast into blanks with composition and weight controlled closely enough to meet a declared fineness.
Sputtering targets, evaporation charges and master alloys. A small, homogeneous, low-inclusion melt of a specified composition, where the uniformity of the melt rather than its mass is what the customer is buying.
Copper and copper-alloy melting ahead of an anode or a wirebar. Copper, at a melting point of about 1,085 °C and an energy requirement of about 0.172 kWh per kilogram, melted for anode casting or alloying, with that higher energy per kilogram governing the running cost.
Research institutes, universities and pilot plants. A small instrumented induction melt whose frequency, power programme, atmosphere and thermal history can be recorded and repeated, and for which the data is the deliverable.
Filling the gap between a bench torch and a foundry furnace. Workshops that have outgrown hand melting but have neither the tonnage nor the electrical supply for a medium-frequency foundry furnace, and need a machine sized to their parcel rather than to a catalogue.

Applicable Raw Materials

Gold scrap, old jewellery and carat gold. Mixed jewellery in 9, 14, 18 and 22 carat grades, charged with a flux cover so that base-metal oxides and surface contamination leave the metal. Output: a homogeneous bar or ingot at a declared carat and weight.
Doré, bullion and parted-gold material. Gold-bearing bullion from gravity or amalgamation circuits and the gold-rich metal from a parting operation, charged to be homogenised and cast. Output: a uniform bullion bar for assay and consignment.
Reduced gold and silver sponge, powder and cement. The finely divided metal product of a reduction or cementation step, carrying residual moisture, salts and filter aid, dried, fluxed and melted into a dense bar. Output: dense, dry bullion from a wet-process intermediate.
Silver scrap, sterling, coin silver and contact material. Silver in fabricated form and in electrical contact assemblies, melted for reuse or anode feed with the base-metal fraction controlled by fluxing. Output: silver bullion or anode plate at a declared composition.
Copper, copper alloy and anode trimmings. Copper cathode pieces, wire scrap, anode butts and alloy scrap, melted for anode casting, for alloying or for a wirebar route. Output: copper anode plate, wirebar or alloy ingot.
Platinum, palladium, rhodium, iridium and ruthenium sponge and scrap. Platinum group metal in wire, gauze, sheet, electrode, thermocouple and reduced sponge form, melted where the metal is already metallic, from about 1,555 °C for palladium to about 1,964 °C for rhodium. Output: a homogeneous platinum group metal ingot or alloy button.
Platinum-rhodium and platinum-iridium alloy scrap. Thermocouple wire, catalyst gauze, laboratory ware and electrodes, needing a melt temperature above the base-metal band and an atmosphere that excludes oxygen and carbon. Output: alloy ingot at a declared rhodium or iridium content.
Bench sweeps, polishing dust and floor sweepings. The fine metal-bearing waste of a jewellery or finishing workshop, melted with a flux and a collector so the value reports into one bar and the abrasive and organic load into the slag and fume. Output: an assayable collector bar in place of an unassayable lot.
Incinerated electronic residue, ash and filter cake. The solid product of an incineration, ashing or filtration step on board material, plating sludge or photographic residue, melted with a flux so the non-metallic fraction leaves as slag. Output: a precious-metal-bearing bar with the gangue rejected as slag.
Electronic-scrap concentrate, plated parts and connector scrap. Mechanically or hydrometallurgically concentrated board material, plated connectors and contact scrap, charged to collect gold, silver and palladium into one dense phase. Output: a concentrated collector bar or ingot for refining.
Spent catalyst concentrate and catalyst residue. Automotive, petrochemical and chemical catalyst material after carrier removal, or the residue of a leaching step, carrying platinum group metals at a concentration worth melting. Output: a platinum-group-bearing collector phase.
Dental alloys, dental scrap and casting alloys. Crown and bridge alloys, partial denture frameworks and casting residues, melted in small batches with the crucible changed between alloy families. Output: dental alloy ingot or a recovered precious-metal bar.
Anode slime and tankhouse residue, dried and fluxed. Electrolytic refinery slime and tankhouse residue taken through a small thermal concentration step, so selenium, tellurium and the base metals are reduced in one hot stage. Output: a precious-metal-rich button with the base metals slagged off.
Brazing alloys, master alloys and grain refiners. Alloy compositions melted to a specification for a downstream user, with a clean crucible, adequate stirring and the composition verified before the pour. Output: a certified master alloy ingot or cast bar.
Sputtering targets, evaporation charges and cast-to-shape melts. Metal melted and cast into the form the next process needs, where inclusions, gas content and segregation are the measures of success. Output: a homogeneous target blank or cast shape for machining.
Deliberately blended charges. Residues, dusts, sweeps and concentrates mixed to a designed composition and collector content, so the melt, the slag and the pour land inside the window the crucible grade and power programme were set for. Output: a stable bar composition from a feed that is not itself stable.
Feed refused at the gate. Wet or damp charge that will flash against a molten pool, sealed or hollow containers, mercury-bearing material, nitrate- or chlorate-bearing material that can react violently with molten metal, halide-bearing wet salt, oversized pieces, and material that cannot be identified and assayed. Output: a refusal with the mechanism stated, instead of a melt run outside its window.

Finished Products

Bars and ingots at a declared weight and composition. The primary product, poured from a crucible into a mould and released against its own analysis, with the weight and composition recorded as the record of that heat. Output: weighed and assayed bullion bar, ingot or cast block.
Granules and shot for downstream dissolution. Where the next step dissolves the metal rather than rolls it, the melt is poured through an orifice into a quench to give a graded granule with a predictable dissolution rate. Output: graded granules, produced on a precious metal granulator where that duty is separate.
Anode plates for an electrolytic refining step. The melt is cast into an anode at the thickness, lug geometry and surface condition the cellhouse requires, because an anode cast badly slows the cell for its whole cycle. Output: anode plates released to gold electrolysis equipment or the corresponding silver or copper circuit.
Vacuum or inert-gas cast ingots where gas content matters. Solidification under vacuum or an inert-gas overpressure removes dissolved gas before the metal is cast, where the melt is prone to gas pickup or the alloy is sensitive to it. Output: a low-gas ingot from a vacuum ingot casting machine.
Moulds and cast geometries for the next operation. Ingot moulds, plate moulds and shaped cavities at the geometry the receiving process needs, cast in iron, graphite or a coated grade so the released surface is clean. Output: cast forms released against a gold and silver ingot casting mold specification.
Assay buttons and samples from the same heat. A small cast button taken from the melt itself, so the analysis released with the bar belongs to the metal that was poured rather than to the charge that was weighed. Output: a retained assay sample tied to the heat record.
Homogenised melt for rolling, drawing or striking. Composition and temperature equalised before the pour, so the ingot goes straight to a rolling mill without a segregation-driven rejection. Output: a melt released for casting at a verified superheat and composition.
Certified master alloys and hardeners. Alloys prepared to a specification for a downstream user, where the certificate of analysis is part of the product and the composition is verified before release. Output: a certified alloy ingot or cast bar.
Platinum group metal ingots and alloy buttons. The dense metallic product of a small platinum, palladium, rhodium, iridium or ruthenium melt, released for the refining or fabrication step that follows. Output: a platinum group metal ingot or alloy button at a declared composition.
Dross, crucible skull and slag, weighed and assayed. Material deliberately thrown out of the metal, collected from the crucible, the mould and the floor and returned to the charge or to a recovery step, because in a precious-metal shop it is a value stream rather than a waste. Output: a weighed and assayed by-product returned to the burden.
Flue dust and fume from the extraction. The particulate captured from the extraction over the crucible and the pour, which carries a real value and is collected in sealed containers, weighed, assayed and returned. Output: captured dust, closed back into the heat balance.
A closed heat balance per melt. Charge weight and assay in, product, dross, skull and dust weight and assay out, with the difference reported as the melt loss for that heat rather than as an annual estimate. Output: a heat record that can be audited.
A documented handover to the refining step. What leaves the furnace is a separated, concentrated and documented metallic stream with its assay attached, and not a refined metal. Output: a collector bar or ingot consigned to a platinum palladium rhodium purification system equipment or a hydrometallurgical route.
A repeatable recipe per alloy. The coil, crucible grade, frequency, power programme, superheat ceiling and pour temperature that produced the accepted bar, written down so the next heat reproduces it. Output: a written operating recipe for every alloy the shop runs.

High Frequency Smelting Furnace Process Flow

The nine stages below are the working sequence of a high frequency induction melting station, written at the level of the electrical and geometric decisions rather than at the level of the pour. What has to be established about the alloy and the batch mass before a frequency can be chosen, which bore and which frequency satisfy the published matching rule between reference depth and charge diameter, how the coil, the gap and the lining thickness are set around that pair, how the power supply is rated and matched to a load that changes from a cold pile to a full pool, what the cooling circuit has to be, how the first hard melt of a session is started, how the melt-down and the superheat are run, and how the station is shut down and reset. The frequency, the bore, the coil geometry, the power rating, the cooling specification, the atmosphere and the superheat ceiling are all fixed during design against your own alloy, batch mass and cycle time.

High frequency smelting furnace working sequence — nine stages from the melt arithmetic that fixes the batch mass through the frequency and bore that satisfy the reference-depth rule, the coil and the matching of the power supply to a changing load, the cooling circuit and the cold start, the melt-down and superheat themselves, to a shutdown and a closed heat record, with the frequency, the bore and the energy per kilogram stated as figures rather than as labels.

Steps 1 — Melt Duty and Charge Mass Arithmetic. The alloy, its melting point, its specific heat and its latent heat at working temperature are taken, the energy to raise one kilogram to fully liquid at the required superheat is added up, and the batch mass and cycle time give the energy per heat. That energy divided by the cycle time gives the average power the machine has to deliver, and that number is the first real specification of the project. The figures differ by a factor of nearly five across the metals a precious-metal shop handles, from about 0.055 kWh per kilogram for gold to about 0.262 for cast iron. Setting and utility input: charge analysis and weight, the alloy and carat to be produced, melting point, specific heat and latent heat at working temperature, the superheat allowance and the cycle time required. Output: the energy per heat and per kilogram, the average power the machine must deliver and the batch mass it has to melt.

Steps 2 — Frequency and Crucible Bore Matching. The reference depth is calculated for the alloy at its working temperature as 503 times the square root of the resistivity in micro-ohm-metres divided by the product of relative permeability and frequency in hertz, and the published matching rule requires that depth to stay below about one eighth of the charge diameter. The bore is fixed first, by the volume the batch mass occupies in a crucible of practical proportions, and the frequency that satisfies the rule at that bore follows from the calculation. Because the ratio of diameter to reference depth rises with the diameter and falls as the metal becomes more conductive, a small bore and a low-resistivity metal both push the frequency up. The rule is a floor rather than an operating point, and the design runs above it because a cold charge couples poorly and the melt rate is bought with power density. Setting and utility input: the alloy’s resistivity at working temperature, candidate crucible bores from the batch mass, the published diameter-to-reference-depth rule and the frequency options the available power supplies offer. Output: a matched frequency and bore pair, with the reference depth stated at working temperature and the margin above the rule recorded.

Steps 3 — Coil Gap, Turns and Lining Thickness Setting. The coil is wound to an inside diameter larger than the crucible by an annular gap that trades magnetic coupling against the room the refractory lining and crucible wall need, and that gap is the largest single determinant of how much of the inverter’s output reaches the metal. Turn count, pitch and coil height are set so that the field covers the charge rather than the empty part of the crucible, and the lining thickness between coil and crucible is held as thin as its dielectric and thermal duty allows, because every millimetre of refractory is a millimetre of gap paid for in reactive power. The copper section and coolant passage are sized for the coil current at the design frequency, and the winding record is kept because a re-wound coil that differs from the original changes the machine. Setting and utility input: the crucible’s outside diameter and height, coil copper section and coolant passage, the annular gap and its coupling trade-off, refractory cement and lining thickness, and the coil’s insulation class. Output: a wound coil with its inside diameter, turns, pitch, gap and lining thickness recorded as the design basis.

Steps 4 — Inverter Rating and Frequency Tracking Test. The average power from the first stage is divided by the plant-wide efficiency the station is expected to reach, which published furnace data puts at roughly one third on cast iron, to give the input rating, and the inverter is selected with that rating and a frequency tracking band wide enough to hold the load as the charge passes from a loose cold pile to a full liquid pool. The tank circuit, the capacitor bank and the tap or transformer ratio are set to match the coil to the supply at the design frequency, and because the reactive current in a small coil is large the capacitor duty and the coil coolant duty are checked together. The protection schedule covers overcurrent, overvoltage, phase loss, cooling failure, over-temperature and crucible leak detection, and it is proved by test rather than asserted. Setting and utility input: the required input rating from the energy per heat and the expected efficiency, inverter rating and topology, capacitor bank and tank circuit, tap or transformer ratio, and the protection settings and frequency tracking band. Output: a matched power supply with its tracking band, its tank circuit and its trip schedule verified against the coil.

Steps 5 — Cooling Circuit Commissioning and Water Chemistry. Published limits for induction furnace cooling circuits run to a pH band of 6.5 to 7.5, total hardness below 100 ppm, chloride below 30 ppm, sulfate below 20 ppm, ammonium below 0.1 ppm, total ion content below 0.5 ppm and an evaporated residue below 120 ppm, with the water returned below about 45 °C at a flow and pressure that scale with the power drawn. Filtration or softening, a conductivity or chloride check on the make-up, a closed loop with its own heat rejection, and drainage and freeze protection are all part of the design. Because a restricted passage in a coil shows up first as a hot spot and then as a failure, the flow and temperature interlocks are proved here rather than discovered in production. Setting and utility input: treated water and its make-up, filtration or softening, flow and pressure for the coil and the inverter circuits, the return temperature limit, conductivity and chloride monitoring, and drainage or antifreeze protection. Output: a cooling circuit verified against published quality limits, with its flow, temperature and leak interlocks tested and recorded.

Steps 6 — Cold-Charge Coupling and Tap Ratio Set-Up. A loose pile of cold solid pieces touches its neighbours at points rather than over surfaces, so the induced current has a discontinuous path and the load presented to the coil is poor until a pool forms and couples the pile into one conductor; published operating figures for induction melting are quoted for a hot lining with the cover closed precisely because that is the good case. The remedies are designed in: a starting programme that tolerates a badly matched load in the first minutes, an optional liquid heel or a graphite or silicon-carbide susceptor, a charge packed to increase contact area, and a tap or ratio setting chosen for the cold load rather than only the hot one. The minimum charge on which the machine starts reliably is measured and written down, because a shop that starts a heat below it will conclude the machine is faulty. Setting and utility input: charge packing and piece size, optional liquid heel or susceptor, tap or transformer ratio for the cold load, the starting power programme and the crucible condition. Output: a verified cold-start procedure with the minimum reliable starting charge and the ratio settings for it recorded.

Steps 7 — Full-Power Ramp and Melt-Down Execution. Melt-down is run at the highest power the coil and the crucible will accept rather than at a comfortable intermediate setting, because the melt rate is bought with power density and a machine run at half its rating takes more than twice as long per heat. The power programme is written per alloy as a time against power curve with the points at which the power is pulled back, and the melt rate observed in production is checked against the arithmetic of the first stage, so that a machine running slower than its own energy budget is investigated rather than accepted. The same field that melts the charge also stirs it. A charge that bridges or sinters above a molten pool is the classic failure of this stage, and the charging practice and the stirring are what prevent it. Setting and utility input: the power programme per alloy, the melt-rate target from the energy arithmetic, charge additions during melt-down, field intensity and stirring, and the crucible’s temperature limit. Output: a fully molten and homogenised pool at the set temperature, with the cycle time recorded against the arithmetic.

Steps 8 — Superheat Hold and Temperature Release. Superheat is where a small precious-metal melt loses value fastest, because a small bath has the worst surface-to-volume ratio in the shop and the metal is most reactive at the moment it is hottest. The superheat ceiling is set per alloy and held rather than exceeded, the cover, the flux and the gas blanket keep oxygen and nitrogen away from the surface, and where the alloy is sensitive to gas pickup the melt is taken under vacuum or an inert atmosphere. Temperature is read on the metal rather than on the shell, by an immersion couple where the alloy permits one and by an optical instrument where it does not, and the composition is verified from a sample taken from the melt before the pour rather than from the charge that was weighed. Setting and utility input: the superheat ceiling per alloy, cover, flux or gas blanket, immersion or optical temperature measurement, a sample for composition verification and the release temperature. Output: a melt released at a verified temperature and composition, with the reading and the sample recorded against the heat.

Steps 9 — Shutdown, Water Purge and Furnace Reset. Power is removed on the programmed sequence with the lining allowed to cool under control rather than shocked, the cooling circuit is kept running to take the stored heat out of the coil and the capacitors and is then purged or protected against freezing, and the crucible is inspected for cracking, glazing, wall thinning and metal penetration before it is used again. The dross, the crucible skull and the spillage are collected, weighed and assayed so that the heat’s melt loss can be closed, the extraction filter is checked because a precious-metal melt loads it with real value, and the coil insulation, capacitor bank, tap connections and earth bonding are inspected on a schedule. The heat log closes with the charge in, the product out and the loss attributed, and the station is reset to the condition the next heat expects to start from. Setting and utility input: the power-down sequence, controlled cooling, cooling-circuit purge or antifreeze protection, crucible inspection, dross and skull collection and assay, consumable inspections and the heat log. Output: a safe and reset furnace, a closed heat record with its melt loss attributed, and the starting condition for the next heat.

Product Structure (Main Equipment)

Electrical supply, power conversion and matching

Incoming supply, rectifier and its harmonics. A supply connection sized on the input rating the energy per heat produces rather than on the nameplate of an inverter, with the rectifier, the direct-current link and any harmonic filtering or power-factor correction the supply authority requires, because a high-frequency inverter is a non-linear load.
Inverter sized for a load that moves. A transistor or thyristor inverter rated for the average power the cycle needs, with a frequency tracking band wide enough to hold the load as the charge passes from a loose cold pile through a partially molten mass to a full liquid pool.
Capacitor bank and tank circuit matched to the coil. The bank that tunes the coil to resonance at the design frequency, with its reactive duty, coolant duty and derating for ambient temperature stated, because a bank chosen for the average load is the component that fails when the load swings cold.
Tap, ratio or transformer matching. The tap or transformer ratio that presents the coil’s impedance to the supply at the design frequency, set for the cold load as well as the hot one and recorded, because a heat that stalls at melt-down is usually a matching problem rather than a power shortage.
Protection, interlocking and the alarm schedule. Overcurrent, overvoltage, phase loss, coolant flow and temperature, converter over-temperature, crucible leak detection and earth-leakage protection, wired to trip rather than to warn where the consequence is a molten-metal leak, and proved by test at commissioning.

Coil, crucible, lining and containment

Water-cooled copper induction coil. A coil wound to the inside diameter, turn count and pitch the coupling design calls for, in the copper section and coolant passage the design current requires, with its insulation specified for the voltage the coil sees and its surfaces protected against metal splash.
Crucible in the grade the alloy and the temperature require. A graphite, clay-graphite, silicon-carbide, alumina, magnesia or zirconia crucible selected against the alloy’s melting point, its reactivity and the flux in use, because graphite is itself a susceptor that couples to the field and a crucible right for gold is not automatically right for a fluoride-bearing flux.
Optional susceptor or auxiliary heating for a poorly coupled charge. A graphite or silicon-carbide sleeve, or the coil geometry that puts the field through a conducting susceptor, used where the charge is a loose powder or a low-resistivity mass that couples badly cold.
Lining, cement and crucible support between coil and metal. The refractory cement and consumable lining that hold the crucible in position and insulate the coil electrically and thermally, held as thin as the dielectric and thermal duty permits because every millimetre is a millimetre of magnetic gap.
Molten-metal leak detection and containment. A sensing element set where the metal would arrive at the coil if a crucible failed, a hearth or bed that gives the escaping metal somewhere to go, and a layout that keeps liquid metal away from the coolant circuit, because the credible serious accident in a small high-frequency furnace is a crucible failure rather than an electrical one.
Shell, frame and magnetic shielding. A shell and frame stiff enough to hold the coil concentric with the crucible through thermal cycling, with the yoke or shielding arrangement that reduces stray flux and keeps it out of the frame, out of the floor reinforcement and away from nearby steel.

Cooling, atmosphere, extraction and control

Closed cooling loop with its own heat rejection. A closed circuit feeding the coil, the capacitor bank and the converter, with filtration or softening, a heat exchanger or chiller, a conductivity or chloride check on the make-up, and drainage and freeze protection where the station stands idle in a cold workshop.
Cover, gas blanket and vacuum interface. A closed lid for a small crucible, a reducing or inert gas admission with its flow control, and, where the alloy requires it, the interface that lets the melt be taken under vacuum or an inert atmosphere instead of under air.
Extraction over the crucible, the pour and the crucible change. Local extraction sized on the fume the melt and the flux produce, arranged so that the dust, which in a precious-metal shop carries a real value, is captured, weighed and returned to the charge rather than lost to the roof.
Temperature measurement and composition verification. An immersion couple for the alloys that tolerate one, an optical pyrometer where they do not, and the sample-taking arrangement that lets the melt be verified before the pour, because composition is released on evidence and not on the recipe having been followed.
Control system, recipes and the heat record. Control of the power programme, frequency tracking, superheat ceiling, gas flow and cooling set points, with a recipe stored per alloy, an alarm and trip schedule written against the failure modes of an induction melt, and a heat record that closes the charge-to-product balance.
Utilities, floor loading and layout. Connected electrical load, treated cooling water with its heat rejection, compressed air, the floor loading of a station whose vessel is small but whose power cabinet and cooling skid are not, the lifting arrangement for a crucible of molten metal, and the physical separation of water, electricity and liquid metal.

Product Features

The frequency is stated as a figure and defended as arithmetic. Every proposal carries the frequency, the crucible bore and the reference depth at working temperature in the same line, so that the published rule between them can be checked rather than trusted.
The energy per kilogram is published before the melt rate. The sensible and latent heat of your alloy are added up per kilogram, the cycle time is then applied to give the power the machine has to deliver, and the melt rate is presented as the consequence rather than as the claim.
The efficiency of the station is stated as a ratio with its basis. Published data of 790 kWh per tonne for cast iron at 1,500 °C against a theoretical minimum of about 257 kWh per tonne gives a ratio of roughly one third, which is the honest bridge between a thermodynamic calculation and an electricity bill.
The cold charge is treated as the design case. The starting programme, the minimum reliable starting charge, the optional heel or susceptor and the tap setting for the cold load are worked out at commissioning, because a station commissioned only on hot heats will be blamed for a fault it does not have on the first melt of a Monday morning.
Coupling is a design variable, not a by-product of the casting. The annular gap between coil and crucible, the coil height against the charge, the lining thickness and the frequency are set together, because coupling is where a correctly rated machine loses its power before the metal ever sees it.
The frequency is matched to the metal at temperature. Because reference depth rises with the square root of resistivity and resistivity rises steeply with temperature, the machine is set for the alloy at working temperature, and the record shows what depth was assumed when the coil was wound.
Precious-metal work gets its own electrical design. A one-kilogram gold melt and a one-tonne iron melt are different electrical machines, because the low-resistivity metals and the small bore both demand the higher frequency, which is why this station is specified around precious-metal batches rather than upsized from a foundry furnace.
The consumables are named, costed and scheduled. The crucible grade and its campaign, the lining and cement, the coil and its insulation, the leak detection elements and the filter media are listed with expected lives against your alloy and cycle count.
The atmosphere is specified with the melt. Cover, flux, gas blanket, vacuum or inert interface and the extraction over the crucible and the pour are designed together, since superheat in air is where a small precious-metal bath loses value and where a light alloy loses its composition.
The superheat ceiling is held per alloy and the pour is repeatable. Superheat is set to what the pour and the mould require rather than to what the machine can reach, and release temperature, mould preheat, pour rate and the aim of the stream are written as a recipe, so that a second shift produces a bar the first shift would accept.
The station is handed over with its record. Coil geometry, crucible grade, frequency, the reference depth assumed, power programme, cooling set points, superheat ceiling, pour temperature and the heat log are issued as a technical file, which is what lets a shop prove its melt loss and its composition to a customer or an auditor.

Technical Parameters And Specifications

Parameters below are the design basis for the range. Charge mass, bore, frequency, coil geometry, power rating, cycle time, atmosphere, cooling specification and every site service are engineered to your own alloy, batch mass, cycle time, crucible grade and site rather than selected from a model list. Figures quoted from published practice carry their basis so that they can be recomputed.

Parameter	Specification
Capacity (charge basis)	Engineered to your alloy, the batch mass one heat has to take, the cycle time your shift pattern needs and the crucible grade your melt permits; reported as a batch mass per heat, with the number of heats per shift stated separately
Duty definition	Melting of a charge that is already metallic, with the crucible as the vessel and the induced current as the heat source; the machine does not reduce a compound feed and does not raise the fineness of the metal
Machine family	Coreless induction crucible furnace, high frequency, with the frequency selected from the alloy and the bore rather than from a catalogue
Working frequency	Engineered from the charge diameter and the alloy and stated in hertz; the range in commercial use under the high frequency label runs from about 2 kHz to about 500 kHz, so the figure is quoted per project rather than as a house band
Reference depth	503 times the square root of the resistivity in micro-ohm-metres divided by the product of relative permeability and frequency in hertz, stated at the alloy’s working temperature because resistivity rises with temperature
Published matching rule	The reference depth should not exceed about one eighth of the charge diameter, equivalently a charge diameter of at least four to eight reference depths; treated as a floor under the frequency, with the margin above it recorded per project
Reference depth reference points	At 20 °C and a relative permeability of one: gold about 11.1 mm at 50 Hz, 2.48 mm at 1 kHz, 0.79 mm at 10 kHz and 0.35 mm at 50 kHz; iron about 22.2 mm, 4.96 mm, 1.57 mm and 0.70 mm
Frequency in published furnace practice	3 kHz at 15 kg capacity, 2 kHz at 30 kg and 50 kg, 1 kHz at 100 kg and 200 kg and 500 Hz above 300 kg, in one manufacturer’s table sold as a high frequency induction furnace line
Frequency band definitions in use	German induction reference: low 50 to 500 Hz, medium 500 Hz to 50 kHz, high 50 kHz to 27 MHz. Heating equipment maker: medium 1 to 20 kHz, high 30 to 80 kHz. Laboratory furnace maker: high 100 to 450 kHz. One furnace builder: high above 10 kHz
Crucible bore	Fixed by the batch mass and the practical proportions of a crucible, then used as the input to the frequency match; quoted with every proposal because it is what makes a frequency right or wrong
Charge mass per heat	Engineered from the crucible, the alloy and the pour required; published practice allows an overcharge of up to about 10 per cent above rated capacity, which is a margin for the charge and not a second capacity figure
Crucible grades	Graphite, clay-graphite, silicon carbide, alumina, magnesia and zirconia, selected against the alloy’s melting point and reactivity, the flux and the thermal cycles per shift; graphite is itself a susceptor
Coil specification	Water-cooled copper wound to the inside diameter, turn count, pitch and copper section the coupling design requires, with the annular gap, lining thickness and insulation class quoted as a set
Power supply	Transistor or thyristor inverter with a frequency tracking band wide enough to hold the load from a cold solid pile to a full liquid pool, quoted with its capacitor bank, tap or ratio and harmonic treatment
Input rating	The energy per heat divided by the cycle time divided by the plant-wide efficiency the station is expected to reach, stated as a connected load with the rectifier and harmonic duty alongside it
Energy to melt one kilogram	About 0.055 kWh for gold, 0.089 for silver, 0.092 for platinum, 0.148 for palladium, 0.172 for copper and 0.262 for cast iron, computed as sensible heat from 25 °C to the melting point plus latent heat of fusion
Latent heat of fusion, reference values	About 63 kJ per kilogram for gold, 105 for silver, 206 for copper, 103 for platinum, 165 for palladium and 247 for cast iron
Specific heat, reference values	About 0.13 kJ per kilogram per kelvin for gold, 0.23 for silver, 0.39 for copper, 0.13 for platinum, 0.24 for palladium and 0.46 for cast iron
Melting points in scope	About 962 °C for silver, 1,064 °C for gold, 1,085 °C for copper, 1,555 °C for palladium, 1,768 °C for platinum and 1,964 °C for rhodium
Efficiency basis	790 kWh per tonne for cast iron at 1,500 °C against a theoretical minimum of about 257 kWh per tonne, a ratio of roughly one third quoted as the basis for running-cost arithmetic rather than as a guarantee
Working energy per kilogram	About 0.17 kWh of input energy per kilogram of gold at that efficiency basis; the equivalent for each alloy in your shop is computed from its own energy per kilogram
Cycle time	Engineered per alloy and per crucible; published figures assume a hot lining with the cover closed and no residual melt, exclude charging, slagging, composition adjustment and temperature measurement, and carry a variation of about five per cent
Cold-start behaviour	The minimum charge on which the machine starts reliably, the starting power programme, the tap setting for the cold load and any heel or susceptor arrangement are established at commissioning and written into the recipe
Superheat	Set per alloy against the pour and the mould rather than against the machine’s ceiling, which is itself quoted as a figure, because excess superheat is paid for in oxidation, gas pickup and crucible life
Atmosphere	Air under a cover or a flux, a reducing or inert gas blanket, or a vacuum or inert-gas interface where the alloy is sensitive to gas pickup; the working ceiling is stated against the atmosphere given
Stirring	Inherent in the induced field, requiring no separate mechanism; intensity falls as the frequency rises, which is read as milder mixing and slower crucible and lining erosion at the high-frequency end
Cooling water chemistry	pH 6.5 to 7.5, total hardness below 100 ppm, chloride below 30 ppm, sulfate below 20 ppm, ammonium below 0.1 ppm, total ion content below 0.5 ppm and evaporated residue below 120 ppm
Cooling water conditions	Return temperature below about 45 °C, supply pressure about 0.15 to 0.2 MPa and a flow that rises with the power drawn; filtration or softening and a conductivity or chloride check are part of the specification
Crucible failure protection	A leak detection element set where the metal would reach the coil, a hearth or bed that gives the escaping metal somewhere to go, and a layout that keeps liquid metal away from the coolant circuit
Extraction and dust recovery	Local extraction over the crucible, the pour and the crucible change, sized on the fume produced, with the captured dust weighed, assayed and returned to the charge
Melt loss	Measured per heat as the difference between the weighed and assayed charge and the weighed and assayed product, dross, skull and captured dust, and reported per alloy so that a drift is visible as a trend
Consumables	Crucible, lining and cement, coil insulation, leak detection elements, moulds and filter media, each with an expected life against your alloy and cycle count and separated from the warranty
Instrumentation	Power, frequency, coil current and voltage, coolant flow and temperature, leak detection state, bath temperature by immersion couple or optical instrument, and charge and product weights
Control and recipes	Power programme, frequency tracking, superheat ceiling, gas flow and cooling set points under one control system, with a stored recipe per alloy and a heat record per melt
Utilities	Connected electrical load with its rectifier and harmonic duty, treated cooling water with its heat rejection, compressed air, and the drainage and freeze protection a station needs when idle
Site services and layout	Engineered to your plot: floor loading for a station whose vessel is small but whose power cabinet and cooling skid are not, the lifting arrangement for a crucible of molten metal, and the separation of water, electricity and liquid metal
Handover document	Coil geometry, crucible grade and bore, frequency, the reference depth assumed, power programme, cooling specification, superheat ceiling, pour temperature, heat log and consumables schedule

Melt Energy per Kilogram

Computed as the sensible heat from 25 °C to the melting point at the given specific heat plus the latent heat of fusion. These are thermodynamic minima; the electricity the meter records is this figure divided by the plant-wide efficiency, which published data puts at roughly one third.

Metal	Melting point	Specific heat (kJ/kg·K)	Latent heat (kJ/kg)	Minimum energy (kJ/kg)	Minimum energy (kWh/kg)
Gold	1,064 °C	0.13	63	198	0.055
Silver	962 °C	0.23	105	321	0.089
Platinum	1,768 °C	0.13	103	330	0.092
Palladium	1,555 °C	0.24	165	532	0.148
Copper	1,085 °C	0.39	206	619	0.172
Cast iron	1,538 °C	0.46	247	943	0.262

Melt-Rate Check for Gold

The mass, the cycle time and the energy are three sides of one triangle. The table applies the working figure of about 0.17 kWh of input energy per kilogram of gold, so the required average input is the energy column divided by the cycle time. A claim that cannot be placed in this table has left out the figure that would make it checkable.

Charge mass	Cycle time	Input energy required	Required average input
1 kg	2 minutes	0.17 kWh	about 5 kW
2 kg	3 minutes	0.34 kWh	about 7 kW
3 kg	3 minutes	0.51 kWh	about 10 kW
4 kg	3 minutes	0.68 kWh	about 14 kW
5 kg	3 minutes	0.85 kWh	about 17 kW
6 kg	3 minutes	1.02 kWh	about 20 kW
10 kg	8 minutes	1.70 kWh	about 13 kW
20 kg	15 minutes	3.40 kWh	about 14 kW

Published Frequency Band Definitions

The same two words are used for three orders of magnitude of frequency. The bands below are taken from published equipment references, and they are the reason a high frequency smelting furnace has to be specified in hertz rather than in adjectives.

Source of the definition	Low frequency	Medium frequency	High frequency
German induction heating reference	50 to 500 Hz	500 Hz to 50 kHz	50 kHz to 27 MHz
Induction heating equipment maker	not defined	1 to 20 kHz	30 to 80 kHz
Laboratory and vacuum furnace maker	50 Hz to 10 kHz	not defined	100 to 450 kHz
Another induction heating maker	not defined	about 300 Hz to 10 kHz	about 100 to 500 kHz
Furnace builder’s classification	below 150 Hz	150 Hz to 10,000 Hz	above 10,000 Hz
Furnace table sold as a high frequency line	not defined	500 Hz to 3 kHz across 15 kg to 5 t	not defined

Technical Advantages

The page answers the comparison question the market avoids. Two machines called a high frequency smelting furnace can differ by two orders of magnitude in frequency, and the industry’s own published tables use the term for anything from 500 Hz to 500 kHz, so quoting hertz, bore and kilowatts on one line turns a comparison into a decision.
The melt rate is derived rather than asserted. The sensible heat and the latent heat of your alloy are added up per kilogram, the cycle time and the plant-wide efficiency are applied, and the average input power that falls out is the rating the machine is built to, which is a number the buyer can check.
The efficiency of the station is published with its basis. Published data of 790 kWh per tonne for cast iron at 1,500 °C against a theoretical minimum of about 257 kWh per tonne gives a ratio of roughly one third, which is the bridge between a thermodynamic calculation and an electricity bill.
The cold charge is designed as the design case. The minimum reliable starting charge, the starting programme, the tap setting for a cold load and any heel or susceptor arrangement are worked out at commissioning, because the first heat of a session is the one most specifications quietly exclude.
Coupling is treated as a first-class variable. The annular gap between coil and crucible, the coil height against the charge, the lining thickness and the frequency are set together, because coupling is where a correctly rated machine quietly loses its power before the metal receives it.
The frequency is matched to the alloy at temperature. Resistivity rises steeply with temperature and the reference depth with its square root, so the assumed depth is stated on the drawing and can be rechecked when the alloy changes.
Precious-metal melting gets its own electrical design. Silver, copper, gold and aluminium have the lowest resistivities in industrial use, and a small crucible raises the frequency requirement further through the ratio of diameter to reference depth, which is a different calculation from upsizing a foundry furnace.
The consumables are named with their lives and their cost. Crucible grade and campaign, lining and cement, coil insulation, leak detection elements and filter media are listed against your alloy and cycle count, because a running-cost figure that excludes the parts which wear is one the shop floor will revise upward.
The cooling water specification leaves with the machine. A pH band of 6.5 to 7.5, hardness below 100 ppm, chloride below 30 ppm, sulfate below 20 ppm and an evaporated residue below 120 ppm, with a return temperature, flow and pressure, verified at commissioning.
Melt loss is reported per heat and per alloy. The charge is weighed and assayed in, the product, dross, skull and captured dust are weighed and assayed out, and the difference is the melt loss for that heat, which in a precious-metal shop is the largest single operating cost.
The page states where melting stops. A crucible furnace homogenises metal and delivers it at a controlled temperature; it does not raise its fineness and does not separate the platinum group metals from one another. Stating that boundary costs us sales where the customer needed a refining line, and it is why the customers who buy a melting station get what they specified.
The station leaves with the record that makes it auditable. Coil geometry, crucible grade, frequency, the reference depth assumed, power programme, cooling specification, superheat ceiling, pour temperature and the heat log are issued as one technical file.

Related equipment and processes: smelting furnace for the family selection, the slag chemistry and the feed arithmetic that decide whether the duty is a melt or a reduction, precious metal smelting machine for the flux, the casting discipline and the melt-loss budget of a precious-metal melt cycle, gold refining process and silver refining process for the fineness work that a melting furnace does not do, gold electrolysis machine and copper electrolysis equipment for the cellhouse that consumes a cast anode, and precious metal electrolysis equipment and precious metal recovery refining system where the bar this station casts enters a wider refining or recovery train.

Service Process

Alloy and duty review. You send the alloys, the carat or composition to be produced, the form the metal arrives in and the pour temperature, and we return the melting point, sensible heat, latent heat and superheat allowance for each, naming where the duty belongs to a different machine. Output: a written duty statement per alloy with its energy per kilogram.
Batch mass and cycle-time sizing. The batch mass one heat has to take and the cycle time your shift works to are converted into the energy per heat and the average input power, which is the step that fixes the size of everything behind the crucible. Output: the energy per heat, the required average input power and the batch mass the station is sized to.
Frequency and bore match, issued as a calculation. The bore is derived from the batch mass and the practical proportions of a crucible, the reference depth is calculated for your alloy at working temperature, and the frequency is fixed by the published matching rule with the margin above it stated. Output: a frequency and bore pair with the reference depth at working temperature and the margin recorded.
Coil, crucible and coupling design. Coil inside diameter, annular gap, turn count, pitch, copper section and lining thickness are set together against the coupling required, and the crucible grade is selected against the alloy, the temperature and the flux, with the trade-offs documented rather than resolved silently. Output: a coil and crucible specification with the coupling trade-off stated.
Power supply, matching and cooling specification. The inverter rating and topology, frequency tracking band, capacitor bank, tap or ratio and harmonic treatment are quoted as a matched set with the coil, and the cooling circuit is specified as a chemistry, a flow, a pressure and a return temperature. Output: a matched electrical specification and a cooling water specification.
Test melt on your material where the behaviour is uncertain. For an alloy with no history, a finely divided charge, a new flux or an unfamiliar crucible grade, a sample melt establishes the cold-start behaviour, the practical melt rate, the crucible condition afterward and the melt loss achieved. Output: a test melt report with the measured cycle time, energy and melt loss.
Manufacturing, coil winding and works inspection. The coil is wound and its geometry recorded, the crucible support and leak detection element are fitted and proved, the power cabinet is assembled and run against a matching load, and every part carrying a setting is checked before dispatch. Output: an assembled station with its works inspection records and its coil dimensional record.
Site installation, electrical and cooling commissioning. Installation of the power cabinet, furnace frame, cooling skid and extraction, with the electrical supply, earth bonding, harmonic treatment and cooling circuit commissioned to specification, including the water chemistry check and the interlock tests. Output: an installed station with its electrical, cooling and interlock commissioning records.
Cold-start commissioning and supervised first heats. The station is commissioned on cold charges rather than prepared hot ones, the minimum reliable starting charge is established, the tap setting for the cold load is fixed, and the first heats are run with our engineers on the floor. Output: a station running to its design figures on your own alloy, with the cold-start procedure written down.
Operator training on the recipes. Training covers what the energy arithmetic means, why the tap setting differs between a cold and a hot load, how the superheat ceiling is held, what the cooling alarms mean, and what to do when the alloy changes. Output: trained operators and a written operating and recipe manual.
Performance demonstration and handover. The station is run to demonstrate the designed cycle time and energy per kilogram, and the demonstration closes a melt balance: charge weighed and assayed in, product, dross, crucible skull and captured dust weighed and assayed out, the difference being the melt loss. Output: a performance test report with a closed melt balance and a handover certificate.
Pour, mould and granulation discipline. Where the station feeds a mould, a granulator or a casting machine, the release temperature, mould preheat, pour rate and the aim of the stream are agreed and recorded, so the pour is part of the recipe rather than the operator’s judgement. Output: a written pour specification per alloy and per mould.
Consumables and melt-loss review. At agreed intervals the crucible campaign, lining condition, coil insulation, coolant chemistry and melt-loss trend are reviewed against the design, and the consumables schedule is adjusted from the data rather than from a calendar. Output: a consumables and campaign plan with a melt-loss trend report.
Alloy change and re-rating. When you add an alloy, change a crucible grade or push a cycle time, the reference depth, power programme and superheat ceiling are recalculated and a revised recipe is issued before the first production heat. Output: a revised recipe and re-rating note for every alloy change.
Expansion and integration support. Where a shop outgrows one station, or the melt has to feed a granulator, an ingot caster, a vacuum caster or an electrolytic cell, the additional station and the interface to the next process are specified against the same arithmetic. Output: an expansion and integration proposal with the process interface defined.

Warranty And After-Sales Service

Warranty on the station and its systems. The furnace frame and coil, power supply and its control, capacitor bank and tank circuit, cooling skid, leak detection system, extraction and instrumentation are warranted against defects in design, materials and workmanship for a stated period from commissioning, with the period, exclusions and wear-part list written into the contract.
Wear parts are named and separated from the warranty. The crucible, lining and cement, coil insulation, leak detection elements, moulds and filter media are consumables with defined lives against your alloy and cycle count, so the boundary between a warranty claim and a planned replacement is clear before the order.
Coil support through its life. The coil’s dimensional record is kept with the machine, so a re-insulated or rebuilt coil returns to the geometry the coupling design was based on, and a coil that has drifted dimensionally is identified as the cause of a lost melt rate rather than blamed on the power supply.
Crucible and lining support. Crucible grade selection against a new alloy, the campaign review, the inspection criteria for cracking, glazing, wall thinning and metal penetration, and the safe change-out procedure, because the crucible decides both the cycle and the risk in a small induction melt.
Cooling circuit condition monitoring. Coolant chemistry, flow, return temperature and the coil passage condition are reviewed against the published limits at agreed intervals, so scale or chloride attack is corrected in the water treatment rather than discovered as a burnt coil.
Power supply spares, repair and exchange. Inverter modules, capacitor units, control boards, temperature and flow sensors and leak detection elements are available as a strategic stock package sized to your shift pattern, with a converter repair or exchange route agreed at purchase.
Leak detection and crucible failure response. The sensing element and its trip path are tested on the same schedule as the crucible inspections, and the response to a detected leak, including shutdown sequence, metal containment and safe restart after a crucible change, is written into the operating manual rather than improvised.
Remote diagnostic and process support. Where the site permits remote access, the power, frequency, coil current, coolant and temperature data are available to us, so a melt rate that has fallen can be examined with the station’s own data in hand rather than described over a telephone.
On-site service and response. Field service engineers for mechanical, electrical, control and process work, with the response time and call-out arrangements stated in the contract and the service organised around the production windows you have.
Operator refresher and new-staff training. Training is repeated when the alloy changes, when a new form of charge is used, or when the operating team turns over, because a recipe only produces the same bar twice if the people holding it understand why each figure in it is there.
Melt-loss and energy review. Heat records are reviewed against the design at agreed intervals, both for melt loss per alloy and for the energy per kilogram actually achieved, so a drift in either is corrected from data before the year end.
Escalation and root-cause work. Where a melt fails to meet the design, a crucible fails early, a cycle time is not reached or a coil loses its match, the investigation is carried out jointly against the heat records, and the outcome is a correction to the recipe, the consumable or the hardware.
Process data, records and traceability. Recipes, power programmes, heat logs, melt balances, crucible and lining records, coolant logs and inspection reports are maintained as a technical file, so the shop’s own melt loss and composition records are auditable.
Upgrade and rebuild support. A second station, a larger crucible within the coil’s design limit, a vacuum or inert-gas interface, a granulation or ingot casting interface, a higher-frequency power supply and instrumentation upgrades are supported through the life of the installation.

Frequently Asked Questions

What is a high frequency smelting furnace, and what does the frequency actually change?

A high frequency smelting furnace melts metal that is already metal by inducing the current inside the charge, an alternating current in a water-cooled copper coil around the crucible building a magnetic field that drives a current through the metal, whose own resistance then heats it. The frequency decides how far into the metal that induced current penetrates. Current density is highest at the surface of a conductor and falls away toward the centre, and the depth at which it has fallen to about 37 per cent of its surface value is the reference depth, calculated as 503 times the square root of the resistivity in micro-ohm-metres divided by the product of relative permeability and frequency in hertz. A higher frequency gives a thinner heated layer, which is what a small crucible needs; a lower frequency gives a deeper layer, which is what a foundry furnace with a metre-wide bore uses.

Why do two furnaces sold as high frequency run at 2 kHz and at 500 kHz?

Because the words are used commercially across about two orders of magnitude and no authority enforces them. One German induction heating reference divides the bands at 500 Hz and 50 kHz, one induction heating manufacturer puts medium at 1 to 20 kHz and high at 30 to 80 kHz, one laboratory furnace maker calls anything from 100 to 450 kHz high frequency, and one furnace builder classifies everything above 10 kHz as high frequency. A published furnace table sells an entire line under that heading while listing 3 kHz for its 15 kg model and 500 Hz at the top of the range. The label therefore tells you nothing and the hertz tells you everything, so a quotation that does not state a frequency is a quotation for an unknown machine.

How is the frequency chosen for a particular alloy and crucible?

It is matched to the bore rather than chosen from a preference. The bore comes first, from the mass of metal a heat has to take and the practical proportions of a crucible. The reference depth of the alloy is then calculated at its working temperature, and the published matching rule requires that depth to stay below about one eighth of the charge diameter, which is the same statement as a charge diameter of at least four to eight reference depths. The frequency that satisfies the rule at that bore is the frequency the machine is built for. Because the ratio of diameter to reference depth rises with the diameter and falls as the metal becomes more conductive, a small bore pushes the frequency up, which is why a precious-metal crucible sits at the high-frequency end and a foundry furnace at the low end.

What is penetration depth, and why does it decide the crucible size?

Penetration depth, also called reference depth, is the depth below the surface of a conductor at which the density of the alternating current induced in it has fallen to about 37 per cent of its surface value, and it is the scale on which induction heating operates rather than an abstract property. It is calculated as 503 times the square root of the resistivity divided by the product of relative permeability and frequency, so it grows with resistivity and falls with frequency. It decides the crucible size because the published rule ties the two together: if the reference depth is large compared with the diameter of the charge, the field is mostly wasted in the space around the metal and in current paths that do not heat the whole mass efficiently. At 20 degrees Celsius a gold charge has a reference depth of about 11 millimetres at 50 Hz and about 0.35 millimetres at 50 kHz, and the value grows as the metal heats because resistivity rises with temperature.

Why do precious metals need a higher frequency than steel?

Because the reference depth is proportional to the square root of resistivity, and the precious metals have the lowest resistivities of any common metal. At 20 degrees Celsius silver is about 0.0159 micro-ohm-metres, copper about 0.0168, gold about 0.0244 and aluminium about 0.0265, against about 0.0971 for iron and several times that for stainless steel. At one and the same frequency the layer of a gold charge that actually carries the induced current is therefore much thinner than the same layer in a steel charge, and a thin layer in a small crucible is a small coupled volume. Two things then push the frequency up for a precious-metal melt, the low resistivity of the metal and the small bore of the crucible it needs, and both follow from the same formula rather than from preference.

How much energy does it take to melt a kilogram of gold?

About 0.055 kilowatt-hours of useful energy, counted as the sensible heat needed to bring the kilogram from 25 degrees Celsius to its melting point of about 1,064 degrees plus the latent heat needed to change its state. The specific heat of gold is about 0.13 kilojoules per kilogram per kelvin and its latent heat of fusion about 63 kilojoules per kilogram, giving about 198 kilojoules or 0.055 kilowatt-hours per kilogram. The figure differs across a range of nearly five from metal to metal: silver about 0.089, platinum about 0.092, palladium about 0.148, copper about 0.172 and cast iron about 0.262 kilowatt-hours per kilogram. The electricity the meter records is that figure divided by the plant-wide efficiency, and published furnace data implies an efficiency of roughly one third, giving a working figure of about 0.17 kilowatt-hours of input energy per kilogram of gold.

How can I check a melt rate I am quoted?

Put three figures side by side: the mass, the cycle time and the power. The mass and the alloy give the energy required, the energy divided by the cycle time gives the average input power the machine has to deliver, and that number has to sit inside the rating offered. Six kilograms of gold in three minutes needs about 1.02 kilowatt-hours, which over three minutes is an average input of about 20 kilowatts; four kilograms in the same three minutes needs about 14 kilowatts. A quotation that gives a rate without a mass, a mass without a power or a power without a cycle time has removed the figure that would let you check it, and the missing figure is always the inconvenient one.

Why is the first melt of the day slower than the rest?

Because a loose pile of cold solid pieces does not present a continuous path to the induced current until a pool forms and couples the charge into one conductor. The pieces touch at points rather than over surfaces, the contact resistance between them is high, and until liquid metal bridges them the load presented to the coil is a poor one. A liquid heel from the previous heat, a graphite or silicon-carbide susceptor, a charge packed to increase contact area and a starting programme that tolerates a badly matched load all shorten the first heat, and published furnace figures are quoted for a hot lining with the cover closed and no residual melt precisely because that is the good case. A cycle time measured on a cold crucible is always longer than the same figure measured on the third heat of the day.

How long does a melt cycle take?

The cycle follows from the mass and the power rather than from a house figure, and it is calculated as the energy per heat divided by the average input power the machine can deliver into the load, both adjusted for the plant-wide efficiency. At the working figure of about 0.17 kilowatt-hours of input energy per kilogram of gold, a five kilowatt station needs about two minutes for a kilogram, a twenty kilowatt station about three minutes for six kilograms, and a fourteen kilowatt station about fifteen minutes for twenty kilograms. Published furnace figures assume a hot lining with the cover closed, exclude charging and discharging, slagging, composition adjustment and temperature measurement, and carry a stated variation of about five per cent between melts, so a cycle time quoted without those exclusions is not the cycle time the shop will experience.

Which crucible material should be used for which alloy?

The crucible is selected against the alloy, the working temperature, the flux and the number of thermal cycles. Graphite and clay-graphite are the workhorses for gold and silver work because they tolerate thermal shock, resist molten precious metals and couple directly to the field, but they are consumed by an oxidising atmosphere and by some fluxes. Silicon carbide sits between graphite and the ceramics in both temperature and cost. Alumina, magnesia and zirconia ceramics hold higher temperatures and resist more reactive fluxes, which is what a platinum group metal melt requires, and they are correspondingly less tolerant of thermal shock. The crucible is a consumable with a campaign, not a permanent part of the vessel.

Does a high frequency furnace stir the melt, and does that matter?

It does, and the stirring comes from the same field that heats the metal. The magnetic field exerts a force on the liquid charge and moves it around the crucible, which is what homogenises the composition and helps the flux and the slag do their work. The intensity of that stirring falls as the frequency rises, so the high-frequency end of the range gives a milder and less erosive mixing action and the lower-frequency end a stronger one, which is one reason a high-frequency crucible shows less lining and crucible wear than a mains-frequency furnace of the same size. Published furnace literature treats the mild mixing of a high-frequency machine as a benefit for lining consumption, and the stirring supplied by a medium-frequency machine is correspondingly stronger.

What cooling water quality does the furnace need?

Published limits for induction furnace cooling circuits are a pH of 6.5 to 7.5, total hardness below 100 parts per million, chloride below 30 parts per million, sulfate below 20 parts per million, ammonium below 0.1 parts per million, total ion content below 0.5 parts per million and an evaporated residue below 120 parts per million, with the water returned below about 45 degrees Celsius at a supply pressure of about 0.15 to 0.2 megapascals and a flow that rises with the power the machine draws. These are not recommendations about comfort; a restricted passage in a coil caused by scale, or chloride pitting through a coil wall, ends in a burnt coil and a coolant leak next to liquid metal. Filtration or softening and a routine conductivity or chloride check on the make-up water are part of the design.

What atmosphere does a precious-metal melt need?

Superheat is where a small precious-metal bath loses value fastest, because a small bath has the worst surface-to-volume ratio in the shop and the metal is at its most reactive at the moment it is hottest. A closed cover and a flux cover are the minimum, a reducing or inert gas blanket is the next step, and an alloy sensitive to gas pickup is taken under vacuum or a controlled inert atmosphere. The atmosphere also decides the crucible’s life, because a graphite crucible burns away in air at temperature and survives much longer under a blanket. Extraction over the crucible and over the pour is part of the same specification, since the dust and fume carry a real value and an exposure.

What melt loss should I expect?

The honest answer is the one your own heat records produce, which is why the charge is weighed and assayed in and the product, the dross, the crucible skull and the captured dust are weighed and assayed out, with the difference declared as the melt loss for that heat. The loss has four components: metal that leaves as fume and is captured by the extraction or lost past it, metal that goes into the dross and the flux cover, metal left as a skull in the crucible and as spillage around the mould, and metal lost to the crucible wall by penetration and reaction. It is driven by the alloy, the superheat held, the atmosphere, the flux and the pouring discipline, so a figure quoted for one shop’s alloy is not transferable to another’s. What is transferable is the method, and the method is to measure it every heat.

Can this furnace refine gold or separate platinum from palladium?

No, and it does not claim to. A crucible melting furnace homogenises metal, removes what a flux and an atmosphere can remove, and delivers the charge at a controlled temperature to a mould or a granulator. It does not raise the fineness of the metal: what leaves the crucible is an alloy of the composition that was charged, less the melt loss, and a longer or hotter melt does not change that. Raising fineness is a hydrometallurgical or an electrolytic duty, and separating the platinum group metals from one another is a sequence of chemical separations rather than a thermal one. A shop that buys a melting furnace expecting a refined product has bought the first stage of a two-stage process, and this page says so before the order rather than after it.

What charge materials can be melted?

Anything that is already metallic and fits the crucible: gold and silver scrap and reduced sponge, doré and bullion, copper and copper alloy, platinum group metals in wire, gauze, sheet, electrode and sponge form, platinum-rhodium and platinum-iridium alloy scrap, dental alloys, brazing and master alloys, sputtering targets and cast-to-shape melts. Sweeps, incinerated electronic residue, ash and filter cake are melted with a flux and a collector so the value gathers into one bar and the gangue leaves as slag. What is refused is feed that cannot be identified and assayed, wet or damp charge that will flash against a molten pool, sealed or hollow containers, mercury-bearing material, nitrate or chlorate bearing material that can react violently with molten metal, oversized pieces that will not fit the crucible, and any charge whose composition would damage the crucible grade being used.

What site services and floor space does the station need?

The station is a set of units rather than one box, and each has its own requirement. The power cabinet needs the connected electrical load the energy arithmetic produced, with the rectifier, harmonic treatment and power factor correction the local supply authority requires, because a high-frequency inverter is a non-linear load. The cooling skid needs treated water to the specification above with a route for its heat rejection, which in a small shop is a chiller or a closed loop to site water. The furnace frame needs a floor that will carry the machine and the crucible full of liquid metal, a lifting arrangement for changing crucibles, and drainage and freeze protection. Extraction is needed over the crucible, the pour and the crucible change. The most important layout rule is physical separation: liquid metal, water and electrical equipment are kept apart, and the pour is aimed away from the coolant circuit.