The direct conversion of celestite to strontium carbonate in sodium carbonate aqueous media

The leaching of celestite (SrSO4) particulate samples with Na2CO3 solutions was investigated. The effects of the variables: stirring speed; particle size; Na2CO3 and Na2SO4 concentrations; temperature; solution pH; and solid/liquid ratio were studied. It was concluded that the reaction is topochemical, obeying a shrinking core model, incorporating diffusion of CO32- ions, through the solution contained in the pores of the product layer, as the rate-limiting step. The model takes into account the change in Na2CO3 concentration that occurred under the conditions studied. In agreement with the model, the reaction rate increases as the first power of the Na2CO3 concentration and is inversely proportional to the square of particle size. The rate is insensitive to the presence of Na2SO4 up to a total concentration of around 1 M, but decreases for higher concentrations. The rate of reaction decreases significantly when the pH is lowered to values where a large proportion of CO32- ions transforms to HCO3-; for example, at pH 9. The conversion rate of celestite increases systematically with increasing temperature; the apparent activation energy being 70.05 kJ/mol. Under well mixed conditions, even relatively dense pulps (e.g., 400 g mineral in 1 l solution), can be completely transformed under mild Na2CO3 concentrations and temperatures (e.g., at 10% in excess of the stoichiometric amount required and 75 degrees C, respectively).