Environmental Considerations in Hydrometallurgy

Any effluent discharged to the environment must meet strict regulations. Nickel must be <0.5 mg/L and copper <0.1 mg/L. (The exact specification depends on locale.) Discharge pH usually must be in the range 6-8, practically neutral. Aqueous discharges must also be typically free of suspended solids. Sometimes the discharge limits are so strict that the natural feed water entering the plant does not meet the requirements! Hence in hydrometallurgy people strive to design flowsheets that have minimal discharges to the environment. Thus the water balance is a critical parameter in design. Water balance problems can cause failures of hydrometallurgical processes. The ideal process would be a closed loop, in which there is no discharge. In practice this is very difficult. In a closed loop everything recirculates, except the products. Inevitably this causes build-up of deleterious impurities that eventually cause unacceptable problems in the various aspects of the process. Examples are corrosion, decreases in product quality, inefficiency in energy consumption or reagent consumption, etc. Often the effects are unpredictable. These impurities must be removed, and this requires treatment and disposal of part of a process stream.

There are two significant benefits to recycling of process reagents and chemicals. It reduces costs, which makes a big difference for expensive reagents, such as sodium cyanide. Secondly it reduces environmental impact and thence the costs associated with having to treat effluents for discharge, which may be expensive. While this is desirable, it does not always work out well. Many processes have been devised for recovery and reuse of cyanide, again, but with little commercial success. Often they are technically and/or economically impractical. Hence cyanide is commonly destroyed prior to discharge.

Solid wastes are, of course also produced and usually there is no recourse but to dump them in a suitable, environmentally acceptable form. It is important that the solids not leach significant levels of toxic chemical species under the action of rain water, ground water or sea water. Finding suitable chemical forms for solid waste that meet environmental regulations can be very difficult. For instance , iron is virtually ubiquitous in hydrometallurgy. (Recall that it comprises >6% of the earth’s crust.) Iron will typically have to be rejected from hydrometallurgical processes as some suitable solid waste. What was once acceptable for disposal may with time become unacceptable as regulations evolve and tighten. Ammonium jarosite precipitate, NH4Fe3(SO4)2(OH)6, was once felt to be a very good way to dispose of iron. But, it has become less acceptable of late. Hematite, Fe2O3, is still an excellent form for disposal. It is formed in autoclaves at high pressure and temperature (>100°C).

An important environmental diagnostic procedure is the bioassay test. Species such as salmon or trout fry are placed in an aquarium with the effluent. The test requires that 50% or more of the organisms survive for 96 hours. Then the effluent passes the test.