The Broken Hill Associated Smelters Proprietary Limited (“BHAS”) lead smelter, now owned by Nyrstar NV, has been the world’s largest lead smelter. Its staff was responsible for many significant technical developments in the lead smelting industry, including the updraft sinter plant and continuous lead refining.
Until 1990, BHAS recovered silver in a two-stage reverberatory cupellation process. 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. 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.
The change to the lance-based furnace increased oxygen utilization to 95% and the cycle time was reduced to a little less than eight hours, “but the grade of the doré which could be economically produced was poor.” The doré from the new furnace still contained about 0.8% lead and 0.4% copper. It was also found impractical to cast anode plates of doré directly from the Sirosmelt furnace, so the Sirosmelt doré had to undergo a further refining step in a reverberatory furnace, together with a sodium nitrate flux.
Then, in 1996, BHAS decided to modernize the refining circuit and replaced the Sirosmelt silver refining furnace with a BBOC furnace. Commissioning of the modernized refining circuit was completed in 1999, and the lead throughput was increased by 11%, with the silver refining capacity increasing to over 400 t/y.
The BBOC process proved to be “generally successful”, although it did suffer some problems with the lance jamming that were attributed to higher than expected levels of zinc in the feed, due to problems removing the zinc in earlier stages of the refinery circuit. The higher levels of zinc also caused higher than expected refractory wear and excessive lance consumption, because the heat generated by oxidizing the zinc was greater than that of oxidizing lead.
The BBOC furnace proved to be capable of producing doré containing as little as 0.01% lead and less than 0.1% copper at a temperature around 1050 °C, but BHAS wanted to cast the doré directly into anode plates using an existing doré casting conveyor. 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. Consequently, BHAS decided to increase the operating temperature to 1100–1150 °C so that the silver remained liquid until cast into the anode molds. A side effect of this is that the lead and copper content of the product doré are higher than if the furnace is operated at 1050 °C, at 0.2% lead and 0.6% copper. Thermodynamic calculations have shown that this is unavoidable at this higher operating temperature.