Zinc is a metal found in the earth’s crust and hold the 25th place in order of abundance among the elements, its main uses are in alloys, and on the coating for other metals, when iron and steel are coated with Zinc they are called galvanized.
In a Zinc processing plant the primary resource is constituted by Zinc sulphides coming usually, from different mines, this resource is transformed by different process that will be summarized in this article.
Roasting and purification of gases
The zinc concentrate is heated in furnaces, at temperatures between 700 and 800 Celsius degrees, where in the presence of air the sulfur oxide contained in the ore is burned and converted into zinc oxide, also called calcine, and another part is converted into zinc sulphate, which once cooled and purified in the acid plant, will be transformed into sulfuric acid.
Zinc and the other metals contained in the calcine are dissolved in a dilution of sulfuric acid, this process has two stages: neutral leaching and acid leaching.
In neutral leaching, most of the calcine is dissolved, with the exception of zinc ferrites (iron oxide and zinc). Thickeners are used, to separate this ferrites, the undissolved solids are separated from the zinc sulphate solution. The clarified solution is sent to the purification stage, while undissolved solids are subjected to acid leaching.
The acid leaching take place at a temperature close to the boiling. In this way, all metals dissolve, except those that form insoluble compounds in sulfuric media, such as lead, calcium and silica. The resulting solution is subjected to a hydrolysis process, after which a basic insoluble iron sulphate called jarosite is formed, jarosite and the undissolved metals conform the final residue of the process.
The dissolution of zinc sulphate from the neutral leaching stage is treated by a continuous process in two stages, to eliminate other dissolved metals, such as copper, cadmium or cobalt, which are recovered as sub products. Once the filtration is done, the zinc sulphate solution is cooled by cooling towers and pumped to a storage tank.
This phase of the process, occurs with the passage of an electric current through the purified solution of zinc sulfate, which causes the pure metallic zinc to be deposited on the cathode, the resulting zinc sheets are automatically detached and transported for fusion and casting.
Fusing and casting
Zinc sheets obtained from the electrolysis process are melted in electric induction furnaces. Once the zinc is melted, it is sent to the casting machines in order to produce the various commercial forms of Zinc bars required by the market.
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