Kinetics of chromium removal from spent tanning liquors using acetylene production sludge

The removal of Cr(III) present in spent leather tanning baths by precipitation with acetylene production sludge (APS) is studied. The hydroxide and carbonate contents in APS were 1.05 and 0.10 g of CaCO3/g of dry sludge, respectively, and the Cr concentration in the tanning bath was 2467 mg/L. The experimental work was carried out at 15, 20, 25, 30, and 35 degreesC using APS samples of different grain size (89,178, and 356 mum). Percent removals after 180-min reaction time using 356-mum APS particles range from 99.0 to 99.9 as temperature increased from 15 to 35 degreesC. Similar or higher removal rates are obtained when using smaller APS particles. Chromium removal efficiencies using APS as precipitant are comparable with those obtained with NaOH, CaO, or MgO. The Cr conversion X increases with time t and is related to the particle radius and the absolute temperature T by the expression X/1 - X = r(0)(-0.22) exp((45.45-13401/T))t. The average activation energy for the reaction is 111.4 kJ mol(-1). Despite some deviations observed for the finer particles and the lower temperature (15 degreesC), the model describes adequately the results obtained for all APS particle sizes at 20 degreesC.