Hydrometallurgical wastes

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. 

In future, the developments in hydrometallurgy will need to be concerned more with environmental problems associated with pertinent processes. Hydrometallurgical processes produce a variety of waste liquors and unwanted solid products which must be treated before their final disposal. There are two main objectives in such waste treatments the first is to recover valuable impurities and unused reagents from the solutions and the second is to ensure that the release of associated materials does not pollute the environment to an unacceptable extent. 

Hydrometallurgical process, for multimetal waste separation, 21 400 Hydrometallurgical refining, 17 92-93 Hydrometallurgical recycling,... 

To discuss the processes for the recovery of metals from waste, more than one hydrometallurgical procedure has to be considered, to establish general processing concepts for a broad range of feed materials, such as ... 

Some years ago, the process, shown in Fig. 14.20, was suggested as a centrally located plant for treatment of metal-containing waste in Sweden. The operation is based on the use of a pyrometallurgical induction converter and hydrometallurgical AmMAR process technology. 

The incentive to build a pyro-hydrometallurgical plant is often based on political opinions and somewhat diffuse environmental public demands and therefore the decision process is complicated. The direct investment of such a plant is high the return on assets is relatively low, because of the unstable value and uncertain situation of feed materials (raw material or waste) and the fluctuating prices of end products. 

Although the problem of disposal of large amounts of metal waste is faced by most industrialized countries, relatively few centrally located operations for waste recovery have, to date, been started. Sweden still deposits its dewatered metal waste in a simple landfill, although Swedish industry has been in the forefront of developing both hydrometallurgical and pyrometallurgical recovery techniques. The same applies to most European countries however, interest in environmentally safe recovery has increased in recent years and recovery plants are now being considered. 

The economic outcome of the operation of a centrally located facility is comparable with all other hydrometallurgical operations and is highly dependent on price and quality of the metals produced. All products have to conform to commercial specifications. Adequate quality and quantity of cheap feed materials are also essential. It is very important to consider the fact that the transformation of waste and raw material usually involves a dramatic increase in the value of the feed material. The economic result cannot be isolated to an auxiliary process, but is the result of the combined effects on the total operation. 

Efilox A process for destroying cyanide wastes in hydrometallurgical effluents by the use of Caro s acid. The cyanide ion is oxidized to cyanate ion. 

Rosenbaum et al. (1971) have reported that centrifugal extractors can be used for the recovery of vanadium by a hydrometallurgical route. Centrifugal extractors have also been considered for the treatment of waste liquors from nuclear energy systems. 

Because all ores contain more than one metallic element of value, it is quite possible that in the very near future, hydrometallurgical processes will be developed to extract a whole line of products which at present are being discarded with the waste material. The dwindling supply of naturally occurring ore deposits, the increased demand due to an expanding technological age, and the pressure for maintaining a clean environment will help accelerate the development of such ideal processes. 

The majority of the ore tonnage treated by hydrometallurgical means is processed at atmospheric pressure because of the obvious economic advantage. In the case of the secondary recovery of additional copper values from mine waste dumps, mined-out ore bodies, and marginal deposits of ores with complex mineralization, the only economic process has to be at ambient pressure. 

In the hydrometallurgical industry, Fe " solutions are often produced as waste products and hydrothermal methods are being investigated as a possible route for their disposal by production of hematite of saleable quality (Riveros and Dutrizac, 1997). 

As mentioned previously, most of the wastes from hydrometallurgical operations are simply treated and then sequestered in tailings impoundments. For those operations that do not have tailings impoundments, more complete process scenarios have had to be designed. For nearly every operation, pretreatment wastewaters, rinse waters, or wash waters must include neutralization and metal removal before the solutions can be discharged, impounded, or reused. The most widespread method for removing dissolved metals from solution is to precipitate them as solids for separation. 

---