Zinc direct roasting process

The zinc sulfide in the concentrate is always converted to oxide by roasting. An exception is the direct leach process described below. The principal overall roasting reaction is strongly exothermic and provides excess heat which is recovered. 

Around 1850, S. Wetherill of the New Jersey Zinc Company perfected a roasting furnace in which a grate was charged with coal and then covered with a mixture of zinc ore and coal. The zinc was reduced by the partial combustion of the coal and reoxidized at the furnace exit (direct or American process). These furnaces were subsequently improved but are now no longer used. During the second half of the 19th century, the use of ZnO in rubber was introduced to reduce the time needed for vulcanization. The discovery of the first organic accelerators for vulcanization in 1906 added to the importance of ZnO, which acts as an activator in these materials. 

Two types of oxidation reactions are of interest in ceramics oxidation of metals and oxidation of sulfides. The oxidation of sulphides is a common extractive metallui cal process, generating an oxide ceramic powder. The oxide product is usually an intermediate product on the way to metal production but if sufficiently pure it can be used directly as a ceramic powder. A common example is the roasting of zinc sulphide to form zinc oxide,... 

Roasting. Copper and lead sulfides are directly smelted but not zinc sulfide. However, theoretical calculations are encouraging (20) and, if an efficient means of condensing zinc rapidly from 1600 K in the presence of carbon dioxide, sulfur dioxide, and steam can be devised, the process may be feasible. The reaction of zinc vapor to yield zinc oxide or zinc sulfide presents the main difficulty. 

The raw materials for the manufacture of zinc oxide by the American process are roasted ores, concentrates of zinc ores or more recently zinc hydroxide or zinc carbonate from the processing of scrap. These are reduced with coal and directly reoxidized to ZnO in the same reaction chamber. The purity of the zinc oxide is normally somewhat inferior to that from the French process. Grades of various purities can be produced by fractionation. 

Derivation (1) Dust or fume from roasting zinc ores is collected, mixed with coal or coke and sodium or zinc chloride, and sintered. The cadmium fume is collected in an electrostatic precipitator, leached, fractionally precipitated, and distilled. (2) By direct distillation from cadmium-bearing zinc. (3) By recovery from electrolytic zinc process (approximately 40%). 

Today, hydrometaUurgy is well established as the principal method for extraction of many important industrml metals. Hydrometallurgy for the direct treatment of base metal sulfide concentrates, as a widely used technology, must yet prove itself. The roast-leach electrowinning of zinc is a noteworthy exception and is evolving as standard practice hi the zinc industiy worldwide. Relatively recent developments by way of jarosite and iron oxide hydrolysis and precipitation processes have improved recovery and helped secure zinc hydromemllurgy as standard in the industiy.w... 

---