RELATIONSHIP BETWEEN ZINC AND LEAD
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. There is also a much lower grade of 98.5% zinc, the main impurity being lead, and this used to be the standard grade, called GOB (Good Ordinary Brand) in Europe or PW (Prime Western) in North America. The predominance of this grade as the one used in applications when the production of zinc first became established came about because it was a very suitable quality for general galvanising and because it was the natural grade produced by thermal smelting processes. The complete separation of lead from zinc was not easy.
Lead usually occurs naturally with zinc i.e. the zinc mineral, sphalerite (ZnS) is usually found in combined deposits with the lead mineral, galena (PbS), and the thermal methods of producing zinc do not easily make a complete separation of lead from zinc. Lead is in many ways very different from zinc – density, reactivity (lead is not particularly reactive, requires much less energy to produce and was therefore produced much earlier than zinc). Nonetheless zinc and lead have a certain affinity for one another. This affinity, whilst being a nuisance in some respects, is exploited in a number of metal smelting processes. Zinc metal is quite soluble in lead metal, as is lead in zinc, both to a degree that depends on temperature. Complete solution of one in the other occurs only at high temperatures. In one method of smelting and recovering zinc, zinc vapour is condensed into a large quantity of molten lead, and this rapid solution into lead prevents the zinc vapour from oxidising to zinc oxide, something that the highly reactive zinc has a strong desire to do. Subsequently the lead is cooled and the zinc separates out from it and is, as a result, recovered. Further cooling of the zinc allows the lead content to be reduced to about 1%, i.e. to GOB quality. Without this affinity between zinc and lead this process (the Imperial Smelting Process) would not function.
A complete separation of zinc from lead can only be achieved by distillation; without distillation a small amount of lead will always remain with the zinc and a small amount of zinc will always remain with the lead.