Zinc Recovery from Slags

The most common approach uses the injection of coal with air and oxygen into a molten slag bath at around 1200°C. Zinc oxide is reduced to zinc vapour according to Equations 8.1 to 8.3  

The vapour pressure of zinc above a slag bath relates to the CO/CO, partial pressure ratio in the gas and the activity of ZnO in the slag according to Equation 8.4  

The activity of ZnO can be related to the mole fraction of ZnO in the slag by the activity coefficient as in Equation 8.5  

Liquid zinc cannot be formed in the slag unless the vapour pressure is above the saturation vapour pressure of liquid zinc at the operating temperature. The saturation vapour pressure is given by Equation 8.6, and under atmospheric pressure conditions at around 1200 to 1300°C the saturation vapour pressure well exceeds the vapour pressure derived from reduction. Hence liquid zinc cannot form in the slag bath. 

The equilibrium constant for zinc oxide reduction according to Equation 8.4 is given as a function of temperature in Table 8.1, together with the partial pressure of zinc vapour in equilibrium with a slag containing four per cent ZnO, as the end-point slag fumer composition, and a reduction gas ratio of C0 C02 of 2 1. Also shown is the saturation vapour pressure of liquid zinc metal according to Equation 8.6. 

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There is only one day of capacity between the zinc fume recovery process in the smelter and the fume leach process in the zinc plant. Therefore, if there is an upset in the leach circuit, it is necessary to stop the slag fuming process for zinc recovery and stockpile smelter slag for recovery in the future. The same is true for the flow of residue from the zinc plant circuits to the smelter. Residue slurry is transported from the zinc plant to the smelter, where it is... 

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. 

From Table 7.4 bullion with less than 0.5 per cent sulfur can be achieved as well as slags containing around one per cent lead. However, where slag fuming facilities are available for zinc recovery, low lead levels can be sacrificed for furnace throughput as in the case of the Teck-Cominco owned Trail plant. The lead content of slag will be almost fully recovered with the zinc oxide fume and will be recycled in the form of zinc plant leach residues containing lead as lead sulfate. 

Off-gas from the electric furnace can contain significant amounts of lead and zinc, which are oxidised to a fume by the addition of air in an after burner chamber. If high zinc recovery is required from the slag the electric furnace temperature must be raised, but this will cause a substantial increase in the amount of lead to fume. Bullion and slag are separately and continuously tapped from the electric furnace. 

The curve in Figure 8.8 is relatively flat with little difference in overall furnace capacity when operating between three and nine per cent residual zinc in slag from a feed of 17 per cent zinc. However, zinc recovery is maximised at the lower residual zinc level and it is common practice to operate at a tail slag composition of around 2.5 to three per cent zinc. Below this there will be a significant fall-off in furnace capacity. 

Ash from coal dissolves in the slag and dilutes the zinc content. This tends to reduce zinc recovery and hence low ash coals generally give higher zinc recovery. The use of high ash coal at the expense of zinc recovery may, however, be justified by the lower price of that coal. 

For the most part, the zinc materials recovered from secondary materials such as slab zinc, alloys, dusts, and compounds are comparable in quality to primary products. Zinc in brass is the principal form of secondary recovery, although secondary slab zinc has risen substantially over the last few years because it has been the principal zinc product of electric arc furnace (EAF) dust recycling. Impure zinc oxide products and zinc-bearing slags are sometimes used as trace element additives in fertilizers and animal feeds. About 10% of the domestic requirement for zinc is satisfied by old scrap. 

Indium may be recovered from zinc ores by several patented processes. Usually it is recovered from residues obtained from zinc extraction. The residues, slags, fume, or dusts from zinc smelting or lead-zinc smelting are treated with a mineral acid. Other steps involved in recovery often vary, but mostly use solvent extraction and precipitation steps. In some processes, treatment with caustic soda yields indium hydroxide. The hydroxide is calcined to obtain oxide, which then is reduced with hydrogen at elevated temperatures to obtain the metal. Distillation or electrolysis are the final steps to... 

The decomposition of the lower sulfides of the heavy metals and the recovery of the metal as soluble salts and of sulfur in the elemental form have been demonstrated for pyrite, pyrrhotite, chalcopyrite, sphalerite, galena, molybdenite, and associated metals such as nickel and cobalt. Pyrite and chalcopyrite are higher sulfides and to be amenable to this treatment have to be thermally decomposed at 600-650 C prior to leaching. The reactions with nitric acid are exothermic, and are carried out below 1 atm and at around 100°C. In addition to the sulfides, this technique has been applied successfully to the extraction of nonferrous metals from partly oxidized sulfide ores, fayalite slags, copper scrap, and other intermediate products, such as residue from electrolytic zinc plats. 

Most lead concentrates contain a portion of zinc and vice versa, so that both metals are impurities from the viewpoint of the prime metal contained in the concentrates. Therefore, each smelting activity inevitably produces disposal materials containing the other metal. Korea Zinc has been committed to maximize metal recoveries in lead and zinc production and to reject all non-valuable materials in the concentrate feed in a stable form such as slag. In this respect, Korea Zinc has developed a concept integrating lead and zinc production plants as shown in Figure 1. 

It was concluded that none of the known smelting routes could be adapted to smelt zinc rich sulphide feed with a high slag fall and that, therefore, they would not be suitable for raising the recoveries of value from polymetallic ores. 

Itoh, U, Yamakita, T and Yoneoka, Y, 1980. The recovery of PW zinc from lead blast furnace slags by electro-thermic process at Chigirishima smelter of Toho Zinc Co Ltd, in Proceedings Australia/Japan Extractive Metallurgy Symposium, pp 313-319 (The Australasian Institute of Mining and Metallurgy Melbourne). 

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