Sulfide Precipitation

Other precipitation methods have also been shown to be very effective for silver recovery [84]. In particular, precipitation with sodium sulfide has been used for desilvering both fixers and washwaters because silver sulfide is one of the most insoluble metallic species known, having a solubility product in water of 10-51. This method involves first adjusting the pH of the normally acidic fixer with sodium hydroxide to an alkaline pH, to prevent the liberation of toxic hydrogen sulfide gas when the reactants are combined. Sodium sulfide is then added and the precipitated silver sulfide allowed to settle, after which it can be removed by decanting and/or filtration. This method has been used for many decades; however, although highly effective for removing silver, it is not very popular because of the "rotten egg" smell of sulfide and the potential hazard of forming hydrogen sulfide gas if personnel are not adequately trained. Silver levels as low as 0.01 mg/L have been attained in the laboratory.

As previously mentioned, sodium (or calcium) sulfide is cheap, readily available, quickacting, and very simple to use. Despite its safety hazards it is currently being used extensively as an instant "curbside precipitant" by sidewalk entrepreneurs in some Third World countries, such as China, who purchase used fixers for their silver content.

In practice, the efficiency of sulfide precipitation is highly dependent on the filtration step. To avoid manual handling hazards and to ensure reliability, the technique was automated in the early 1970s by LaPerle [113]. This process uses automatic pH and specific ion electrodes to control the pH and add specific reagents as needed. An enclosed reaction tank and filtration system with automatic pumping cycles, plus an emergency override actuated by a hydrogen sulfide gas detector, virtually eliminated the safety hazard and much of the bad odor usually associated with handling sodium sulfide. Effluent silver could be reduced down to very low levels easily able to comply with environmental regulations. However, owing to subsequent developments with other methods, including less hazardous precipitants such as TMT, this automated method is not being used today.



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