In the process of fuming zinc, lead, tin and rare metals contained in molten slag are extracted into vogons. The process was first carried out in 1927 and has not changed significantly since then. There are currently about 10 fusion plants in operation around the world. All of them consist of a caissonized shaft furnace in which a suspension of coal dust in the air is blown through a layer of molten slag. The coal dust is the fuel by combustion of which the required temperature of the slag is maintained. As a rule, the excess air to coal dust ratio in the blast is about 0.7. Partial combustion of coal creates a reducing atmosphere in the furnace. By varying the air-to-coal ratio, the process is controlled, the amount of heat generated and the degree of slag recovery are varied.