Hydrometallurgical processing wastes

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,... 

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. 

Although the hydrometallurgical process is more complicated than the thermal process, its principal virtue resides in the essential recovery of all of the materials in spent lead-acid batteries, including the plastic materials, and in the minimal generation of waste streams. As actufd data become available on the operational characteristics and the economics of full scale plants, it will be possible to make a meaningful assessment of this process and its merits relative to other processes for the recycling of lead-acid batteries. 

Recovery of valuable metals from secondary sources. At the present state of development the more promising metal recovery processes based on SIR systems appear to be in the following applications in terms of both process performance and economic considerations (a) Recovery of metals from dilute solutions, particularly where such solutions are available at low cost (e.g., waste solution from other processes, mine waters, or dump leaching solutions) (b) separation of metals from concentrated solutions obtained by hydrometallurgical processing of complex ores, concentrates, mattes, and scraps and purification of process solutions (such as electrolytes) which may contain a variety of metals that have been only partially recovered in the conventional processing steps (c) separation and purification of met-... 

The separation of metal ions is of fundamental importance in research and technology. It is widely used in analytical chemistry and radiochemistry and also has industrial applications in the chemical industry, in the nuclear industry, in the mineral and hydrometallurgical processes, and in environmentally related applications. The selective removal of toxic components from waste effluents and the separation and preconcentration of the different metal species from aqueous solutions can be achieved by solvent extraction (SX), ion exchange (IX), and solvent-impregnated resin technique. 

Selective separation of toxic heavy metal ions from waste solutions is frequently required in hydrometallurgical processing (7). Solvent extraction is known to be an useful method to separate such metal ions from solutions. To make a continuous process of such a system, liquid membrane separations have been developed (2, 5). However, the unstability of these systems often pose a problem to practical industrial applications. With respect to the system stability, polymeric membrane separations may be feasible. The problem for the polymeric membrane separations is their poor transport selectivity toward heavy metal ions. Thus a development of the polymeric membrane separation which can show a comparable metal separation ability with the liquid membrane separation is an important issue to perform the practical heavy metal ion separation (4). 

Hydrometallurgical processes involve many steps to achieve recovery of metal in the form and purity desired, starting from ores or other sources such as tailings or waste solutions/leachates. In the first step, an ore, which probably contains the metal at a low concentration (e.g. 1%), is ground in a grinding circuit and then sent to a leaching tank, where an acid, often sulphuric acid, is employed to leach the metal into solution (Figure 11.4.1). The slurry is then sent to a thickener, where flocculation is employed... 

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. 

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. 

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. 

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. 

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. 

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