The BBOC furnace is a cylindrical steel vessel with a protective internal lining of refractory bricks. It is mounted on a tilting frame that allows it to be held at different angles at different stages of its operating cycle (see Figure 2). A hood is fixed over the top of the furnace, providing a seal that prevents lead and other fumes from escaping during the furnace’s operation (see Figure 1).
The key feature of the BBOC is the shrouded lance that passes through the refractory bricks at the bottom of the furnace. This lance allows oxygen to be injected directly into the molten metal contained in the furnace, away from the refractory lining. Doing so allows the region of high reaction rates to be removed from the vicinity of the lining, thus reducing its wear.
By injecting the oxygen directly into the bath, rather than blowing it on top (as in the case of the reverberatory cupellation furnace or top-blown rotary converters), the oxygen transfer efficiency is not impeded by the presence of the slag layer. It results in an oxygen utilization efficiency approaching 100%.
The lack of interference in the oxygen transfer by the slag layer has a couple of key benefits. The first is that the increased certainty in the estimation of oxygen utilization efficiency means that it is easier to calculate the endpoint of the process, making process control much easier. The second is that a thicker slag layer can be tolerated (because the oxygen does not have to pass through it), and this means that the losses of silver to the slag are reduced (because it is the silver at the interface between the metal and slag that becomes entrained during the removal of the slag and the thicker the slag layer, the smaller the silver content of the removed slag). BRM reported a decrease in the silver content of the BBOC slag compared to the reverberatory furnace slag of 50%.
BRM found that the reaction rate of the BBOC was 10–20 times that of its reverberatory cupellation furnace.
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. This is combated by using fused-grain, direct-bonded magnesite-chrome bricks to line the inside of the furnace shell.