Bottom-blown oxygen converter
[2] The BRM lead refinery at Northfleet in England uses the Parkes process followed by liquation and a vacuum induction retort to recover precious metals.
[3] The product of this process is a feed for the BBOC consisting of a mixture of lead, silver (60–75%), zinc (2–3%) and copper (2–3%), with trace amounts of gold.
[6] A problem with using reverberatory furnaces for cupellation is that the zinc oxidizes first, forming a crust across the top of the molten material.
[3] This crust prevents the penetration of oxygen to the rest of the material, and so it has to be manually broken up and removed using a rabble bar.
[6] Refractory wear in the BBOC is largely confined to the slag line, at the top of the metal, where attack by litharge (lead oxide) is greatest.
[6] Slag formed from the oxidation of lead and zinc is removed periodically by tilting the furnace forward again and pouring it off.
"[6] The BRM staff subsequently tried to increase the oxygen transfer rate by using lances submerged in the bath of the reverberatory furnace and found that there was some benefit in doing this.
[6] However, the wear rate of the lances was excessive and it was realized that the basic design of the furnace, with its shallow bath, was not conducive to the development of a high-intensity reactor.
[6] Initial tests of the bottom injection of oxygen were carried out on a small scale at Imperial College, London, using a nitrogen-shrouded tuyere.
[3] These showed that under certain conditions a protective accretion would form at the tip of the injector, and that oxygen utilization was high, with the oxidation reactions generating sufficient heat to keep the furnace hot until the final stages of refining when the impurity levels were low.
[3] Based on the success of the small-scale tests, and with calculations indicating that the new design would have significant energy savings over the reverberatory furnace, the BRM staff built a 1.5 t pilot plant with a working volume of 150 liters (“L”).
[4] The oxygen injector was a fixed tuyere, located at corner of the base with the side wall, with an annular nitrogen shroud.
[4] The solution eventually developed was the concept of the moveable lance system in place of the fixed tuyere that had been used initially.
[4] The South African company Rand Refinery Limited rebuilt its smelter in 1986, incorporating two 1.5 t TBRCs and a small static reverberatory furnace for cupellation to produce doré bullion containing gold and silver.
[7] In January 1993, the management team of Rand Refinery decided to review alternate technologies to replace the TBRC–reverberatory furnace circuit, with the objective of having cupellation undertaken in a single stage.
[7] This included a 45% reduction in bulk oxygen costs and halving the number of operators required to run the plant.
[10] This process suffered from low recoveries (80–83%), a long cycle time (4–5 days) that caused large in-process inventories, inefficient use of labor and energy, and poor workplace hygiene.
[11] After a test work program undertaken at Ausmelt’s facilities in Melbourne, BHAS switched to using a process based on the Sirosmelt top-submerged lance in June 1990.
[12] Casting using the existing conveyor proved impossible at an operating temperature of 1050 °C, because the high thermal conductivity of the silver resulted in it freezing before it reached the molds.
[12] Consequently, BHAS decided to increase the operating temperature to 1100–1150 °C so that the silver remained liquid until cast into the anode molds.
[16][17] However, the reverberatory furnaces suffer from similar disadvantages in copper anode doré production as they do in lead refineries,[18] including resulting in a large inventory of gold in the system.
[17] The ASARCO Amarillo copper refinery switched in 1991 from reverberatory furnace treatment of anode slimes to a BBOC to reduce the gold inventory.
[6] A single 3 t capacity BBOC was installed, and it was found to increase rejection of selenium from the slimes, with a reduction in fluxing requirements of about 80%.
[19] It chose the BBOC over the TBRC technology because of the ease of control of the bath temperature, its high oxygen efficiency and its simple maintenance.
[19] This was done in February 1994 and was reported to be “giving very good results.”[19] The Takehara copper refinery of the Mitsui Mining & Smelting Company Limited of Japan commissioned a BBOC in its precious metals department in 1993.
