The most commonly available ore used in the extraction of zinc is called zinc blende also known as zinc sulphide (ZnS). Historically extraction of zinc has been more difficult than the extraction of other metals because, at temperatures where metals such as iron typically begin to melt, zinc is a gas. Due to its very low boiling point, at such temperatures, zinc will escape with exhaust gases. Zinc blende does not contain a very high percentage of zinc and hence it needs to be concentrated. The best concentration method for zinc ore is known as froth flotation.
Concentration of Ore: Froth Flotation Process
The Froth Flotation process is mainly used to remove gangue from sulphide ores. The ore is powdered and a suspension is created in the water. The main ingredients of froth flotation are the Collectors and Froth Stabilizers. Collectors (pine oils, fatty acids etc) increase the wettability of the metal part of the ore and allow it to form a froth and froth stabilisers (cresols, aniline etc) sustain the froth. The oil wets the metal and the water wets the gangue. Paddles and air constantly stir up the suspension to create the froth. This frothy metal is skimmed off the top and dried to recover the metal.
Metal from Concentrated Ore
The next step in the extraction of zinc is called roasting. The concentrated ore is finely ground into small pieces and then suspended in a rising stream of air. The sulphur content can be reduced hugely with this process. This also converts the zinc blende to ZnO which is way more reactive. The reaction occurring here is
Zinc Oxide is then mixed with coke and ground into fine pieces and heated at 1673 K in fire clay containers in a process which is also known as the Belgian process. Here the zinc oxide gets reduced to metallic zinc. The carbon from coke acts as the reducing agent in this reaction. The reaction of the final step of extraction of zinc is
Since the boiling point of zinc is way back at 1180 K, the zinc formed immediately vaporises and distils over leaving behind non-volatile impurities such as cadmium, lead and iron. This gas containing zinc vapours is collected and cooled to yield solid zinc. This crude metal is called Spelter. It can be further purified by fractional distillation.
Zinc alloys with many other metals: Nickel silver, bronze, brass and soft electrical solder all contain zinc. A lot of the zinc manufactured is used to galvanise iron to prevent corrosion. Learn about metals that have shaped the course of humanity only at BYJU’s.