Figure 2 shows the positions of the BBOC at various stages of the operating cycle.
The BBOC is held in an upright position during the charging stage. A solid or liquid charge is added using an overhead crane. The furnace is then tilted forward so that the lance is above the charge, and the charge is melted using an oil or natural gas burner that is inserted near the top of the furnace. Once the charge has been melted, the furnace is tilted back into the blowing position and oxygen is blown into the bath. Slag formed from the oxidation of lead and zinc is removed periodically by tilting the furnace forward again and pouring it off.
The oxygen flow rate during blowing for a three tonne capacity furnace is 20–30 Nm/h. Zinc is initially oxidized to form a zinc oxide dross on the surface of the charge, but as lead oxide subsequently forms, a fluid slag of zinc and lead oxides is created. Most of the copper is removed at the same time as the lead. The final removal of copper to a level of 0.04% is undertaken at the end of the process by further additions of lead to collect the copper.
If the lance needs to be replaced at any time during operation, this is done by tilting the furnace forward until the lance is above the surface of the bath, where it can be removed and replaced without the contents of the furnace draining through the hole in the furnace lining.
The cupellation process finishes when the silver is about 99.7% pure. At this point, the silver is poured from the furnace and transferred to another furnace, where a flux is added to upgrade and remove the oxygen from the silver to produce market bullion of 99.9% purity.