THE CONVERSION OF LEAD SULFATE TO LEAD CARBONATE IN SODIUM-CARBONATE MEDIA

The reaction of sized PbSO4 powders with Na2CO3 media has been investigated over a broad range of experimental conditions. The reaction obeys the shrinking core model incorporating diffusion through the product layer formed on the particles. The rate increases as the first power of the Na2CO3 concentration and decreases with increasing concentrations of the Na2SO4 reaction product. The rate is independent of NaNO3 additions, but decreases with increasing concentrations of NaCl or KCl. At a given Na2CO3 concentration, the rate decreases as the pH decreases because of the conversion of CO32- ions to HCO3-. The rate decreases significantly when the pH is raised from 11.5 to 12.0, and this decrease is attributed to changes in the reaction products/product morphologies. At pH values > 12, the PbSO4 dissolves in the solution. The PbSO4 conversion rate increases with increasing temperature, and the apparent activation energy is 15.8 kJ/mol. Hydrocerussite, Pb3(CO3)2(OH)2, is formed under mild reaction conditions, but the formation of NaPb2(CO3)2(OH) is favoured by long retention times, high Na2CO3 or NaCl concentrations and elevated temperatures. The experimental results support a mechanism based on rate control by the diffusion of carbonate ions through the solution trapped in the pores of the constantly thickening product layer formed on the agglomerated PbSO4 particles.