Use and Furnace Capacity

The maximum blast air input volume for the standard furnace of 17.2 m upper shaft cross-section is close to 34 000 Nm /h. The typical furnace blast maximum intensity is close to 2000 Nm /h/m of shaft area. On this basis such a standard furnace has the capacity to bum  

For coke with a fixed carbon content of 85 per cent this represents close to 240 tonnes per day of coke as the maximum burning rate. Practical rates will be below this figure due to plant intensity and availability issues. 

The manner in which carbon is utilised for zinc production is affected by heat balance issues as well as chemical requirements for metal reduction. The heat balance is influenced by the amount of slag produced, the blast preheat and the preheat achieved in coke and sinter. Apart from the quantity of zinc oxide reduced, the chemical factors include the lead and iron contents of sinter and the moisture entering the furnace in blast air and in sinter or additives. Based on typical operating conditions as indicated above and a lead production of close to 50 per cent of the zinc production, the empirical formula used to relate carbon consumption to zinc production is given by Equation 6.1  

Carbon consumed = 0.655 x zinc produced + 0.152 x slag produced 

For a typical slag production of 0.9 tonne per tonne of zinc produced, Equation 6.1 indicates a carbon to zinc ratio of 0.8 or a zinc to carbon ratio of 1.25. Hence, the maximum zinc production from a standard furnace would be 205 x 1.25 = 256 tonnes per day. This represents the zinc which can be produced in furnace gases to the condenser and is reduced in practice by inefficiencies, loss of plant availability for cleaning and breakdown, and by reduced intensity of operations for various reasons. For 92 per cent recovery of vaporised zinc to product metal and 90 per cent plant utilisation this will equate to an annual capacity of 77 400 tonnes. 

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