BACKGROUND TO ZINC PRODUCTION
World consumption of zinc is nearly 10 million tonnes per annum and is growing at 2-3% per annum. For a “mature” commodity this growth rate, whilst not exciting, is steady and upwards. The industry itself is, however, extremely fragmented, and over the past several decades it has been unable to break out of a situation of a regular surplus of supply over demand. Whilst production has been less than consumption for several periods, these have been short, and far longer periods have been experienced with a surplus of production. The result has been falling prices and a rather unprofitable industry.
Zinc metal is produced in all continents of the world and location of production is more closely in tune with zinc consumption than is the mining of zinc and the production of zinc concentrates – in other words zinc metal tends to be produced close to the market, not close to the original zinc mining source. Whilst it could be argued that production of zinc metal would be, for reasons of transport or for the easier disposal of waste, more appropriately carried out close to the mine, there are usually stronger reasons for locating smelters close to the metal market. A mine has a limited lifespan and is usually remotely located, and sourcing raw material for a smelter located at a (later closed) mine is difficult. In addition, smelters generally produce large quantities of sulphuric acid, which is costly to transport large distances, and a location close to other industries is desirable. Furthermore, large economic sources of energy are required for zinc production and these are less likely to be available in the more remote mining locations than where there is a developed infrastructure.
Whilst zinc smelters are, in general, located close to major zinc consumers, there are some exceptions. Australia produces surplus zinc because a coastal smelter location here can be a convenient location for growing South East Asian markets. And new smelters tend to be located in emerging-developing countries. This is partly because costs of production tend to be lower than in developed countries; also, even if production is initially surplus to current domestic requirements, the domestic market tends to be growing fast and is expected to “catch up” later; in addition emerging-developing countries base their development on achieving high revenues from exports and production surplus to domestic requirements is seen as desirable. China is the prime example of this, pushing large tonnages of surplus zinc into world markets ever since 1997, to the dismay of other producers around the world.
The standard zinc product is Special High Grade zinc, with an assay of 99.995% zinc, i.e. it can contain a maximum of 50 parts per million of impurities. Not all zinc smelters can meet this quality and there are lower grades. There is also a grade of 98.5% zinc, the main impurity being lead, and this used to be the standard grade, called GOB (Good Ordinary Brand) or PW (Prime Western).
As described under “Where Does Zinc Come From?” the raw material for zinc manufacture is a sphalerite or zinc sulphide (ZnS) concentrate containing 50-60% zinc, about 30% sulphur and other gangue components, impurities and minor metals. The process of making zinc has several clear steps in converting the impure ZnS to high purity zinc, but these steps may take place in a different order according to the process used. Firstly, because it is not possible to produce zinc directly from zinc sulphide, the zinc sulphide must be roasted to burn off the sulphur, leaving the zinc in the form of impure zinc oxide (ZnO). Secondly the zinc oxide must be reduced to metallic zinc, this always, because zinc is a highly reactive metal, requiring the use of large quantities of energy. Thirdly the zinc must be separated from the gangue and other impurities and from minor metals. And finally the zinc must be cast into a suitable shape for sale, generally ingots of 25kg weight, but increasingly “jumbo” ingots of several tonnes in weight.
The original methods of zinc production were thermal – this means with the use of heat. Zinc metal is produced at zinc smelters, the word smelter implying the use of heat and carbon, as was universally the case when the recovery of zinc first started. The word smelting now includes zinc production by hydrometallurgical means, i.e. production from solutions and suspensions in water. In practical terms the zinc is recovered by electrolysis, that is by applying an electric current through a solution of zinc sulphate (ZnSO4) whereby the zinc in solution is reduced to metallic zinc by the application of electrical energy.
The size of zinc smelters varies from a few thousand tonnes per year zinc capacity to more than 400,000 tonnes per year, the most common size, except in China, being in the range 100,000 to 150,000 tonnes per year. In the Western World there are approximately 50 zinc smelters but, because of the large number of small smelters in China, the total for the Former Eastern Bloc would be considerably in excess of 100.