Continuous circuit coprecipitation of arsenic(V) with ferric iron by lime neutralization: Process parameter effects on arsenic removal and precipitate quality

Abstract

The coprecipitation of arsenic(V) with ferric iron was studied through the use of continuous circuit coprecipitation experiments that involved lime neutralization of acidic sulfate solutions (Fe/As molar ratio of 4) to pH 8. The influence of coprecipitation circuit design on arsenic removal was evaluated through one, two and three-stage experiments as well as the use of solids recycling in a two-stage circuit. The two-stage (operating respectively at pH 4 and 8) continuous circuit configuration produced the lowest residual arsenic concentration and the lowest specific surface area coprecipitate. Two-stage continuous experiments were also used to examine the influence of nickel and aluminum, co-ions that are common in industrial solutions. Nickel was not observed to significantly influence the residual dissolved arsenic under the conditions tested. Aluminum was found to be a suitable equimolar substitute for a portion of the ferric iron. X ray diffraction and Raman spectroscopic data indicated that the coprecipitates consisted of a mixture of gypsum, poorly crystalline ferric arsenate and (arsenic-bearing) ferrihydrite. Calculations based on the pH of point of zero charge (pH pzc) suggested that the content of ferric arsenate ranged from 24% to 57% and was influenced by the coprecipitation circuit design. The highest ferric arsenate content was observed with the two-stage coprecipitation circuit which also yielded the lowest residual arsenic concentration. The results indicated that subtle chemical differences induced in the coprecipitates by the process could be of significant influence to the geochemical stability of arsenic.