Acidity metal-bearing waste

Several processes have been used to combat acid-rock drainage that has already been formed. Metal precipitation and acid neutralization with lime or limestone is the most common technique used to limit the environmental impact of an acidic, metal-bearing solution from waste material. Metal hydroxide formation is quite simply achieved with moderate changes in the pH of the solution. Metal sulfide formation is typically preferred because the sulfide precipitates are more stable, more quickly formed, and much easier to remove by filtration or sedimentation. However, the sulfide sources (such as hydrogen sulfide gas, sodium sulfide, and thiourea) are very expensive on a per pound of metal removed basis, and can produce a noxious hydrogen sulfide gas emission from the mixing tanks. The chemistry of precipitation is discussed in more detail later. 

Acidity is perhaps the most serious longterm threat from metal-bearing wastes. Water seeping from mine refuse has been passing increased metal concentrations into receiving waters for decades. The threat is especially great in waters with little buffer capacity - that is, in carbonate-poor areas, where dissolved-metal pollution can be spread over great distances. 

Concentrated waste solutions are obtained from spent metal plating baths and etchants. However, the majority of metal wastes are soflds or sludges obtained from the hydrolysis of metal-bearing solutions and industrial process effluents. Most of these water-insoluble wastes are composed of hydroxides or basic salts of the contained metals. Eor processing by hydrometallurgical routes the materials must be brought into solution usually by acid or ammoniacal or alkaline digestion. 

Similar recoveries and waste reductions can be expected for other metal-bearing spent acids, depending on specific compositions. Moreover, because the method uses standard equipment and available technology, it can readily meet immediate needs. 

There are two mechanism at work when Mg(OH)2 is being used to neutralize metal-bearing acidic waste. First, the supply of hydroxyl ion by dissolution for metal precipitation as the hydroxide see reactions (10.2) and (10.3) ... 

The jarosite process separates icon(III) from zinc in acid solution by precipitation of MFe2(0H)g(S0 2 where M is an alkali metal (usuaUy sodium) or ammonium (see Fig. 2) (40,41). Other monovalent and hydronium ions also form jarosites which are found in the precipitate to some degree. Properly seeded, the relatively coarse jarosite can be separated from the zinc-bearing solution efficiently. The reaction is usuaUy carried out at 95 0 by adding ammonia or sodium hydroxide after the pH has been adjusted with calcine and the iron oxidized. The neutral leach residue is leached in hot acid (spent + makeup) with final acidity >20 g/L and essentiaUy aU the zinc, including ferrite, is solubilized. Ammonium jarosite is then precipitated in the presence of the residue or after separating it. If the residue contains appreciable lead or silver, they are first separated to avoid loss to the jarosite waste solids. Minimum use of calcine in jarosite neutralization is required for TnaxiTniiTn recovery of lead and silver as weU as zinc and other metals. 

There are two other aspects that should be mentioned here that may directly affect the choice of the milling process. First, the uranium ore often contains other metals that have commercial value, like vanadium or niobium, for example, and their recovery may influence the process selected for uranium recuperation. Second, uranium itself may be a by-product of other processes like gold extraction, niobium, and tantalum production or phosphoric acid manufacture. Thus, recovery of low levels of uranium from phosphates, columbite, or gold-bearing minerals may not be economical in itself, but extracting uranium as a by-product from the waste streams of these operations could be commercially sensible. 

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