WHERE DOES ZINC COME FROM?
Zinc comes from natural mineral deposits of ore in the earth’s crust. The deposits may be close to the surface, in which case they will be mined from an open pit. Alternatively, they may be some distance underground and cannot be mined directly from the surface. Approximately two thirds of the world’s zinc production is derived from purely underground mining. Sometimes a surface deposit is mined and, once exploited, underground mining follows, or surface and underground mining may be carried out in a combined way. Ore comprises minerals (see below) together with waste materials, which are known as gangue. Common gangue minerals are silica (SiO2) as quartz or various silicates, aluminium oxide (Al2O3), calcite (CaCO3), dolomite (a mixture of calcite and magnesium carbonate, MgCO3), barytes (BaSO4), fluorspar (CaF2) and various iron compounds.
The largest zinc mining regions in the world are Latin America, China, Australia and North America (mainly Canada), each with a production of more than 1.5 million tonnes per year of zinc in concentrates. The predominant regions for zinc metal production, however, are China, Western Europe and Asia. It follows that a major world trade exists in zinc concentrates, mainly out of Australia, Latin America and Canada and into Asia (including China, which produces more zinc than it can mine) and Western Europe.
The principal mineral (defined as a naturally-occurring compound) from which zinc is produced is sphalerite, often called zinc blende or zinc sulphide (ZnS). But sphalerite rarely occurs alone and, in practice, zinc is found in the presence of minerals of lead, usually galena (PbS), copper, silver, gold and cadmium, as well as traces of many other species. However early production of zinc almost certainly used natural deposits of the less common calamine or zinc carbonate (ZnCO3) as the starting point because, when heated in small retorts in the presence of carbon, zinc vapour is produced and this can be condensed to zinc metal. The first person in Europe to describe the production of zinc from calamine was the German, Marggraf (1746) but the first full scale plant in Europe was built in 1743 by William Champion near Bristol, England.
The problem of the most common zinc mineral, sphalerite (ZnS), is that it cannot be directly reduced to zinc metal. Although William Champion was granted a patent in 1738, his patent was expressed in strange language, and a later patent by his brother, John, in 1758 provides more specific detail of the need to separate the zinc blende from other minerals and stone and to calcine (i.e. roast) it before reducing the calcine to metallic zinc. The roasting or calcining process was used to convert the zinc sulphide (ZnS) to zinc oxide (ZnO), this liberating copious quantities of the gas sulphur dioxide (SO2). Many early roasting plants made no attempt to capture the sulphur dioxide and the emission of this acid gas caused the destruction of the local vegetation.
The separation of the zinc mineral, sphalerite, from the other minerals and from the gangue was another significant problem, since the concentration of zinc in the ore itself might be typically only 3-10%, a level so low that it would cost too much to smelt directly and too much would be lost with the large quantity of waste. Whilst crushing and grinding the ore could assist in releasing the different minerals present, methods of separating them were, for many years, limited to exploiting their different densities, and these methods are not very effective when the zinc and lead minerals are intimately mixed. The discovery of the vast Broken Hill lead-zinc deposits in New South Wales, Australia, spurred on the development of the flotation process for separating the minerals.