Reduced-pressure leaching

It will be interesting to record some of the evaluatory remarks as regards the process. There is no doubt that the direct acid pressure leach route is an attractive alternative to the conventionally followed process for zinc sulfide processing. Incentives for its adoption on a commercial scale are the reduced capital cost and production of sulfur in its most favored form, the elemental form. Nevertheless, no process is perfect and acid pressure leach process is not an exception. There are at least two aspects of the process which pose some problems. The first is the behavior of impurities, and the second is the disposal of iron. 

The metallic arsenic is obtained primarily from its mineral, arsenopyrite. The mineral is smelted at 650 to 700°C in the absence of air. However, the most common method of production of the metal involves reduction of arsenic trioxide, AsOs with charcoal. Arsenic trioxide is produced by oxidation of arsenic present in the lead and copper concentrates during smelting of such concentrates. The trioxide so formed, readily volatilizes and is collected in a dust flue system where further treatment and roasting can upgrade the trioxide content. The trioxide vapors are then condensed and further purified by pressure leaching and recrystallization techniques. It is then reduced with charcoal to give metallic arsenic. 

Initially the coarse ore is mechanically reduced to a finely divided form and stirred with the requisite concentration of aqueous base prior to pressure leaching. The alumina from trihydrate bauxite (A1203 -3H20 natural bayerite or gibbsite, AllOHlj) is relatively easy to dissolve using 15-20% aqueous sodium hydroxide, under pressure, at temperatures of 120-140°C (Eq. 12.2). 

HydrometallurgicalProcesses. HydrometaHurgical refining also is used to extract nickel from sulfide ores. Sulfide concentrates can be leached with ammonia (qv) to dissolve the nickel, copper, and cobalt sulfides as amines. The solution is heated to precipitate copper, and the nickel and cobalt solution is oxidized to sulfate and reduced, using hydrogen at a high temperature and pressure to precipitate the nickel and cobalt. The nickel is deposited as a 99 wt % pure powder. 

Batch Percolators The batch tank is not unlike a big nutsche filter it is a large circiilar or rectangiilar tank with a false bottom. The solids to be leached are dumped into the tank to a uniform depth. They are sprayed with solvent until their solute content is reduced to an economic minimum and are then excavated. Countercurrent flow of the solvent through a series of tanks is common, with fresh solvent entering the tank containing most nearly exhausted material. In a typical ore-dressing operation the tanks are 53 by 20 by 5.5 m (175 by 67 by 18 ft) and extract about 8200 Mg (9000 U.S. tons) of ore on a 13-day cycle. Some tanks operate under pressure, to contain volatile solvents or increase the percolation rate. A series of pressure tanks operating with countercurrent solvent flow is called a diffusion battery. 

Acid leaching is usually carried out using sulfuric acid under pressure to dissolve the majority of the iron minerals and to release the cobalt and nickel. If this is carried out at 150-250°C, then the iron(III) is precipitated as haematite or jarosite, reducing the amount of iron in the leachate. The... 

Two other refining processes are also frequently employed. One involves hydrometallurgical refining in which sulfide concentrates are leached with ammonia solution to convert the copper, nickel, and cobalt sulfides into their complex amines. Copper is precipitated from this solution upon heating. Under such conditions, the sulfide-amine mixture of nickel and cobalt are oxidized to their sulfates. The sulfates then are reduced to metalhc nickel and cobalt by heating with hydrogen at elevated temperatures under pressure. The metals are obtained in their powder form. 

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