This area is equipped with induction tilting furnaces (e.g. 500 kg capacity, 2000 Hz) for refining and induction tilting holding furnaces (e.g. 1000 kg capacity) for casting, together with the requisite auxiliary equipment. The power required for melting and the refinery is processed in a central station and reticulated to the individual furnaces. For example, 350 kW frequency converters are used in parallel to generate power at 800-900 V which is in turn delivered to the central busbar system. From these bars it is fed to the individual furnaces as required. Each furnace is provided with extensive overload protection, voltage control, automatic power factor correction and metering devices. A system of load balancing between departments ensures that maximum demands are kept as steady as possible. And ice bank in circuit with water circulating pumps provides the necessary constant temperature cooling water. Provision is also made for emergency flushing of the water system with mains water in the event of a power supply failure.
A furnace charge is made by the selection of material from the assayed stock on hand. The ingots are loaded onto a pallet, their mass measured and transferred to the refining area. On arrival at the furnace is returned to the working position and power applied to melt the charge. In normal operations, 120 kW will melt 500 kg in 40-45 minutes. A flux consisting of 4 kg of borax, silica and salt is added to the furnace on top of the ingots prior to melting. This absorbs any extraneous impurities, protects the lining of the crucible and inhibits volatilization. Some crucibles used in these furnaces are alumina lined clay/graphite cylindrical types and have a life of 18 to 26 heats depending on the type of bullion refined. Bullion high in base metals, especially lead, has a deleterious effect on the lining. Fume extraction is facilitated by means of fixed and moveable hoods that are connected to the main duct leading to a scrubber and electrostatic precipitator.
Loading bullion bars into the crucible
When the bullion is fully melted, two ceramic tubes 25 mm o.d. x 12 mm i.d. x 1 m long are inserted into the molten bullion and clamped into position when they have reached the full depth of the crucible. Rubber hoses are connected to the ceramic tubes and chlorine from the main distribution network is fed to the pipes. To promote better distribution of the gas in the molten metal, 3 mm diameter holes are drilled in each pipe near the submerged end.