Effect of CaO and Ca(OH)₂ doses in the precipitation of Cr (III) and sludge formed from characteristic solution used in Mexican tanneries

We conducted a series of experiments to determine the effect of doses of CaO and Ca(OH)₂ added to a synthetic solution, prepared with the same salt of Cr as that found in Mexican tanneries, to precipitate the chromium and determine the amount of sludge. The stirring was kept at 150 rpm during 10 min of time of contact between reagents, followed by measurements of Cr in the supernatant and determination of the amount of sludge. The study was carried out in a wide range of dose of alkalis, including the stoichiometric value, and also we vary the initial concentration of Cr at fixed alkali doses, and duplicate both the concentration and alkali dose maintaining the stoichiometric ratio. To remove almost 100% of Cr³⁺, a stoichiometric or higher dose of alkali was necessary; nonlinear behavior was found between the excess of alkali and pH. Also, the addition of an excess of Cr in the initial solution was not reflected directly in remaining chromium in the supernatant. Finally, it was found that some excess of alkali contribute to increase the sludge because the impossibility to be dissolved at operation conditions.