The use of induction heating as an alternative to gas or oil fired heating for the fire, refining of precious metals is attractive for several reasons:
- (1) compactness,
- (2) less waste heat, therefore cooler room temperatures,
- (3) cleaner operation,
- (4) no off-gas discharge,
- (5) energy efficient as less material heated and
- (6) less time to reach temperature.
The particular application of interest at the Bulldog operation involved the smelting of a steel wool/precious metal and flux charge. Since this particular charge is not an efficient susceptor, the crucible must be of a material that is a susceptor (plumbago or silicon carbide). It is also possible to use a furnace with an independent susceptor. The testwork was conducted to assess the technical potential of induction heating for the Bulldog charges.
The tests were conducted using steel-wool/silver and or silver sludge from the electrolytic cells to which fluxes were added. The objective was to fuse the charge so as to obtain a slag-metal melt which could be evaluated in terms off dissolution of steel-wool, silver reporting to the slag and fineness of silver (after a suitable contact period). Although silver was used in this evaluation, the results as they pertain to induction heating would be applicable to the refining of gold.
During a recent visit to the Rand Refinery in South Africa, the world’s largest precious metal refinery, it was observed that they rely solely on induction heating for the refining, melting and alloying of gold and silver bullion. The area in which several large induction furnaces were operating and containing several hundred kilograms of molten bullion was pleasantly cool and immaculately clean. The furnaces were all mounted with hydraulic rams so that they could be tilted for pouring with minimum difficulty and excellent control. This large scale bullion handling was very impressive and demonstrates the utility of induction heating in this application.
Two induction units were used in the testwork:
- (1) Westinghouse Corp. 10 kva RF (400 kHz) generation and
- (2) Inductotherm Corp. 30 kva, 4200 Hz generation.
The Westinghouse unit was used in preliminary testwork with small uninsulated plumbago crucibles (IV diameter x 3″) (0.04 m x 0.08 m). Later work used fire-clay lined plumbago crucibles of the same size but smaller capacity. A single test was made with a cut down fire clay crucible to determine if adequate coupling could be achieved using a cylindrical pellet of steel wool/silver and fluxes as the susceptor.
The Inductotherm unit was used for larger charges with a Model T-30 furnace which contains a graphite susceptor. A plumbago crucible (3″ diameter x 4″)(0.08 m x 0.10 m) and fire clay crucibles (2½” diameter x 3½”)(0.06 m x 0.09 m) were used with this furnace. The unit was also run with an induction coil and a fire clay lined silicon carbide crucible.
The first six tests were conducted to observe the behavior of a charge when subjected to induction heating. Small samples (12 gm (0.12 kg) of steel wool/silver sludge) were heated in a plumbago crucible until molten. The charge including fluxes occupied a volume of about 100 cm³ (10 -4 m³), the charge was kept molten from 15 to 30 minutes after which it was poured into a conical mold. The silver button was then separated from the slag. The silver button weight was typically 43 to 45% of the weight of the silver sludge. The flux composition used was: 1 to 5 parts Borax, 1 to 5 parts Nitre, and 1 part silica by weight to the weight of the steel wool.
The results of these tests indicated that the plumbago crucibles were effective susceptors but drastically reduced the oxygen concent of the slag by formation of carbon monoxide. There was insufficient oxidation to completely oxidize the steel wool. In one test an iron button was formed on top of the silver button and the silver exhibited magnetic susceptibility. The temperature of these tests was estimated to be 1400°C (1673.15°K).
The addition of Fluorspar to the flux charge (0 to 3 parts by weight to weight of steel wool) and the use of a fire clay lined crucible increased the oxidizing conditions of the slag sufficiently to completely oxidize the steel wool and produce a refined silver button.
In one large scale test a silica carbide crucible, lined with fire clay, was used to smelt 2 kg of steel wool/silver material together with 1 kg of fluxes and produced a 0.950 kg refined silver ingot. The silicon carbide crucible acted as a susceptor and the volume of the lined crucible was approximately 4l (0.004 m³). The total dry charge occupied about half of this volume and when molten was again reduced by half to 950 cm³ (9.5 x 10-4 m³).
A single test on a pressed cylindrical pellet consisting of steel wool/silver plus fluxes indicated that satisfactory agglomeration, and thus contacting, of steel wool and flux components could be achieved. However, the pellet was a poor susceptor and direct heating of the charge was unsatisfactory. This may be a useful technique, nevertheless, to reduce the dry charge volume.
The results of these tests indicate that induction heating can be successfully used to fire refine silver/steel wool and flux composites. Fire clay lined silicon carbide crucibles appear to be the most satisfactory. While specific flux charges will have to be determined for differing steel wool/precious metal charges in order to provide the correct amount of oxidation, a guideline suitable flux charge appears to be: 1-5 parts Borax, 1-5 parts Nitre, 1 part silica, and 0-3 parts Fluorspar by weight to 1 part steel wool. The required temperature and contact time between the slag and precious metal will need to be determined for a specific application. As a guide, temperatures in the range 1100°C to 1250°C (1373°K to 1523°K) and contact times of 1 to 1.5 hours (after the charge is molten) appears to be satisfactory.
An estimation of the volumetric requirements of a crucible to treat 1200 Troy ozs (37.32 kg) of precious metals in a single batch would be 20,000 to 25,000 cm³ (0.020 to 0.025 m³). The major portion of this volume is required for the fluxes and allows for a 20% freeboard. A crucible with internal dimensions of 11″ diameter x 17″ (0.28 m x 0.43 m) would be suitable. As an alternative to single large batch treatment, several smaller batches and a small crucible could be used which would reduce the size of furnace required but increase the amount of processing time. This would be an economic decision based on the amount of material to be refined.