Chemical Precipitation is the most common technology used to remove dissolved (ionic) metals from
solutions, such as process wastewaters containing toxic metals. The ionic metals are converted to an insoluble form (particle) by the chemical reaction between the soluble metal compounds
and the precipitating reagent. The particles formed by this reaction are removed from solution by settling and/or filtration. The unit operations typically required in this technology
include neutralization, precipitation, coagulation/ flocculation, solids/liquid separation and dewatering.
The effectiveness of a chemical precipitation process is dependent on several factors,
including the type and concentration of ionic metals present in solution, the precipitant used, the reaction conditions (especially the pH of the solution), and the presence of other constituents
that may inhibit the precipitation reaction.
The most widely used chemical precipitation process is hydroxide precipitation (also referred to as precipitation by pH), in which metal
hydroxides are formed by using calcium hydroxide (lime) or sodium hydroxide (caustic) as the precipitant. Each dissolved metal has a distinct pH value at which the optimum hydroxide
precipitation occurs - from 7.5 for chromium to 11.0 for cadmium. Metal hydroxides are amphoteric, which means they are increasingly soluble at both low and high pH values. Therefore, the
optimum pH for precipitation of one metal may cause another metal to solubilize, or start to go back into solution. Most process wastewaters contain mixed metals and so precipitating these
different metals as hydroxides can be a tricky process.
As an alternative to hydroxide precipitation, WATER SPECIALISTS TECHNOLOGIES (WST) has formulated precipitants that form
metallic compounds such as sulfides, carbonates, or carbamates. One of the primary advantages of using these precipitants in place of hydroxide precipitation is that the solubilities of the
metallic compounds formed are dramatically lower than those of corresponding metal hydroxides. In addition, these metallic compounds are not
amphoteric and remain insoluble over a wide pH range. There is no need to keep changing the pH value of the solution for each metal. These precipitants simultaneously precipitate a wide variety of metals at a single pH value.
The precipitants formulated by WST are also known as "chelate breakers". Process wastewaters often contain chelators and other types of complexing compounds that bond to,
or encapsulate, metallic ions and inhibit their precipitation. Hydroxide precipitation alone cannot precipitate such metals. WST precipitants chemically break chelating and complexing
bonds and expose the metallic ions to precipitation. In addition, the compounds formed by these precipitants are "non-water-loving". The results are less sludge and easier
dewatering. Metal hydroxides are "water-loving", and thus increase the sludge volume and are difficult to dewater.
Waste streams containing mixed metals can be easily treated to within
regulatory discharge limits by admixing WST precipitants after pH adjustment. While some of the metals precipitate as hydroxides during pH adjustment, those that are chelated or complexed
will precipitate upon addition of the WST precipitant.
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