Production of Zinc
|Zinc is recovered from Cu-Pb-Zn polymetallic ores with 1-4% zinc as sulphides with silver, gold, cadmium and bismuth in them, with subsequent concentration using the froth flotation method, during which the concentrates with 50-60% zinc are also enriched by lead, copper and, sometimes, by pyrites, and then usually roasted in boiling bed to oxidize the zinc sulfide to zinc oxide. Sulfur dioxide gas, SO2, evolved during the process is utilized for sulphuric acid production. There are two ways for recovering zinc from ZnO. The first way consists of several pyrometallurgical processes that reduce zinc oxide sintering of the roasted concentrate for granulity and gas permeabilty and then distilling the metallic zinc from the resulting mix reducing using carbon (coke) at 1200-1300°C:|
ZnO + C = Zn + CO
The distilled zinc is purified in segregation process during which the iron and lead impurities are separated, and 98.7% purity is achieved. More complex and expensive rectification methods may allow obtaining 99,995% of purity.
Electrolysis is the most common solution, during which the roasted concentrates are treated by sulphuric acid. Electrolysis of the obtained sulphates solution in tanks leaded or viniplasted inside is the following step. The purity of 99.95% is common for electrolytic method.
Preparation of Zinc
- Dry Process of Extraction. - The ore is concentrated by washing, roasted to the oxide, and heated to a white heat with carbon. The reduced metal distils over and is collected in receivers.
The roasting is usually performed on the sulphide (zinc blende) or the carbonate (calamine, zinc spar, smithsonite), but zinc silicates are sometimes dehydrated by heating and reduced by carbon. The roasting must be sufficient to convert the sulphide completely into oxide. If the temperature is too low some zinc sulphate is formed that is converted into sulphide during the reduction with carbon. This sulphide is very difficult to reduce.
Zinc blende is usually roasted in multiple-bedded reverberatory furnaces that are frequently heated by waste gases from the smelting furnaces. The sulphur dioxide evolved by the roasting blende may be used in the manufacture of sulphuric acid.
The reduction is effected by heating the roasted oxide with ground coal in fireclay retorts. The English and Carinthian methods of conducting the process have been abandoned. In the Belgian process the retorts have a circular or elliptical section, and are set in the furnace in tiers. In the Silesian process the retorts have a section resembling a narrow window with an arched top, and are usually arranged in a single row, though there may be two tiers. There are various other differences of detail between the two methods. It is now usual to fire by gas, and the regenerative principle is almost universal. Electric smelting has been successfully practised in British Columbia, Sweden, and other places. In the Rhenish process the retorts are somewhat larger than in the Silesian, and are muffle-shaped or elliptical.
The distillation of the volatilised zinc secures the metal. The condensers should be hot enough to keep the condensed metal in a liquid state. If the retorts are kept well filled with carbon monoxide, no zinc oxide is produced to distil over with the metal. If carbon dioxide is present, oxidation of the metal occurs. The formation of zinc oxide in an oxidising atmosphere has prevented the successful smelting of zinc in blast furnaces.
- Wet-extraction processes have been studied and recommended for commercial application. In these processes the zinc is converted into a soluble salt which is then dissolved out. Thus zinc blende has been calcined to zinc sulphate and extracted with dilute sulphuric acid, or roasted with sodium chloride and extracted with water. These and similar processes have been variously combined with the dry-distillation process. The Leadville ores are roasted to zinc sulphate and extracted with water. The extraction of the zinc is completed by chlorination.
- The electrolytic preparation of zinc is employed by Messrs. Brunner Mond, Ltd. The calcium-chloride liquors obtained in the ammonia-soda process are treated with crude zinc oxide and carbon dioxide. Zinc chloride and calcium carbonate are thus formed. The zinc-chloride liquors are electrolysed in a cell with carbon anodes and rotating disc-shaped iron cathodes. The chlorine produced is used for making bleach, and the zinc is stripped from the cathodes and melted down. The metal is guaranteed to contain 99.96 per cent, of zinc, and is valuable for making ductile brasses.
Zinc of 99.9 per cent, purity can be obtained by electrolysing zinc sulphate solution. The metal thus prepared is sometimes brittle, and the brittleness is probably due to occluded hydrogen.
Gallo proposes the electrolysis of a mixture of zinc fluoride and sodium chloride. The admixture of salt, by lowering the melting-point to 500° C., permits the electrolytic decomposition of the fused fluoride without risk of previous decomposition by water-vapour. Better results are obtained by this use of the fluoride than by electrolysing the fused chloride. The latter process is difficult to apply.
In an electrolytic method recently described, the solution obtained by treating the ore is purified and then freed from lead, copper, and cadmium by zinc dust. It is then electrolysed between an anode of lead and a cathode of aluminium. The current density at the cathode is 2.6 amp. per sq. dcm. The zinc deposit is easily removed.
Purification of Zinc
|Spelter, as commercial zinc is commonly called, is often refined by liquation when the crude material contains about 3 per cent, of lead. By keeping the metal molten for some days the heavier lead collects at the bottom. The top layers, in which the lead content has diminished to about 1 per cent., representing the solubility of lead in zinc at the temperature employed, are then removed. Cadmium and iron are also separated during this process. |
Spelter usually contains lead, iron, and cadmium, and traces of arsenic, antimony, tin, sulphur, and carbon. Copper and silver may be present, and occluded gases. Small quantities of various other elements, including phosphorus, may be present.
Zinc of 99.7-99.9 per cent, purity can be obtained by passing the volatilised products from the retorts through " fume filters." The zinc-vapour, being less dense than the "lead fume," etc., passes more readily through the incandescent carbon or other porous material, such as fireclay, constituting the filter, and condenses very free from impurities. If the zinc-vapour is led from the fume filters into the condensers through nozzles packed with carbon to exclude the action of air, the formation of "fume" is prevented. Zinc fume or "blue powder " is a mixture of finely divided zinc and zinc oxide.
Zinc can be refined electrolytically by using zinc sheet as cathode and impure zinc as anode. By distilling electrolytic zinc, obtained by repetitions of this process, metal of 99-999 per cent, purity can be obtained.
Zinc can be prepared free from arsenic by melting it with sodium, and spectroscopically pure by repeated distillation in vacuo.
Zinc of 99-95 per cent, purity is said to be obtained by treating a saturated solution of pure zinc sulphate with sodium amalgam at 81° C., washing the crystals, drying them, and removing the mercury by distillation in a vacuum slowly at 400° C.