Precipitation by pH (and Viable Alternatives)
To convert dissolved (ionic) metals into insoluble particles, the traditional method is adjust the pH of the solution using caustic, such as sodium hydroxide. The precipitating reaction forms metal hydroxides. The results obtained are effected by several conditions - one of which is the pH of the solution.
Every dissolved metal has a distinct pH at which the optimum hydroxide precipitation will occur. Here are some examples:
- Cadmium pH 11.0
- Copper pH 8.1
- Chromium pH 7.5
- Nickel pH 10.8
- Zinc pH 10.1
Metal hydroxides are amphoteric, i.e., they are increasingly soluble at both low and high pH, and the point of minimum solubility (optimum pH for precipitation) occurs at a different pH value for every metal. At a pH at which the solubility of one metal hydroxide may be minimized, the solubility of another may be relatively high. Since metal hydroxides are quite soluble, many such hydroxides will start to go back into solution if the pH changes even slightly.
Wastewaters from industrial processes usually contain several metals. For example. the typical process wastewaters from printed circuit board manufacturing contain copper, tin, lead and nickel. Such mixed metals create a problem when using hydroxide precipitation since the ideal pH for one metal may put another metal back in solution.
In addition, chelators, sequestering agents, bath additives, cleaners and electroless formulations will interfere with the reaction of hydroxide precipitation. From a practical point of view, it is impossible to eliminate such components from a waste stream. Good treatment practices, such as segregation and pretreatment of some process waste streams, will minimize the anti-precipitant effects of such ingredients. Otherwise, an alternative precipitant may be required in conjunction with pH adjustment.
VIABLE ALTERNATIVES
Water Specialists Environmental Technologies (WSET) has formulated precipitants which form metallic precipitates that dramatically reduce the solubility of metal hydroxides, thus allowing a margin for normal error in pH adjustment. For example, sulfide base precipitants form metal sulfides. The solubilities of metal sulfides are lower than those of corresponding metal hydroxides.
In addition, metal sulfides are not amphoteric. For example, lead can generally be precipitated as an hydroxide at a pH of 8.0-8.5. When the pH is raised to above 8.5, lead hydroxides become soluble. By using a WSET sulfide base precipitant, lead sulfides are formed which are not soluble at higher pH values.
The precipitants formulated by WSET are also "CHELATE BREAKERS" in that they break chelating or complexing rings which surround metal ions and which counteract hydroxide (pH adjustment) precipitation.
Mixed metal waste streams can be easily treated to within discharge limits when a "Chelate-Breaker" Chemistry is admixed after pH adjustment. Some of the metals precipitate upon pH adjustment and the balance precipitate upon addition of a "Chelate-Breaker".
Metal hydroxides are "water-loving" thus creating excessive and difficult to dewater sludge. "Chelate-Breaker" Chemistry creates a precipitate that is not "water-loving". As a direct result, the sludge volume is less and dewaters easily.
While iron salts may sometimes assist hydroxide precipitation by overpowering the chelators, the method creates enormous volumes of sludge that are difficult or impossible to dewater. One pound of iron salts generates EIGHT pounds of sludge! A very, very expensive methodology.
Each of these specially engineered chemistries allow the use of an electronic dosing control system which assures the minimum addition and consistent results within discharge compliance. Ask WSET for information on REAGENT CONTROL SYSTEMS.
NOTE:
The selection of a specific "Chelate-Breaker" precipitant and companion flocculants is dependent upon individual generator requirements. Contact the factory for technicians to assist you in developing the most effective combination. Free samples are available for doing jar tests and treatability studies.
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