Ores mined for their base metal content often contain precious metals, usually gold and silver. These have to be removed from the base metals as part of the refining processes used to purify the metals. In the case of copper electrolytic refining, the gold and silver fall to the bottom of the electrolytic refining cell as “slimes” that are subsequently treated to recover gold and silver as byproducts. In the case of lead refining, silver, gold, and other precious metals are typically removed using the Parkes process, in which zinc is added to the impure lead bullion to collect the silver, gold and other precious metals.
The BRM lead refinery at Northfleet in England uses the Parkes process followed by liquation and a vacuum induction retort to recover precious metals. 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. Prior to the development of the BBOC, BRM used cupellation in a 15 tonne (“t”) reverberatory cupellation furnace to recover the precious metals from this mixture. Three of these furnaces were used to produce 450 t of silver per year.
Cupellation works by exposing the mixture at high temperature to air or oxygen. The base metals, being less noble than silver and gold, react with the oxygen to form their oxides, which separate from the noble metals to form a slag that floats on the top of the residual metals (or “doré”). At BRM, the doré contains 99.7% silver.
To maximize the oxygen transfer from the blast air in the reverberatory furnace, a shallow bath is used, thus increasing the surface-area-to-volume ratio of the furnace.
A problem with using reverberatory furnaces for cupellation is that the zinc oxidizes first, forming a crust across the top of the molten material. 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. This is both labor-intensive and also results in the loss of some of the silver. Similarly, the oxidized lead slag has to be removed when it forms to maintain the operation, and its removal also results in loss of silver.
The BBOC was developed by BRM personnel as a way of reducing these and other problems, such as low energy efficiency and low oxygen utilization, associated with the reverberatory cupellation process.