THE DIRECT CONVERSION OF CELESTITE TO STRONTIUM CARBONATE IN SODIUM-CARBONATE AQUEOUS-MEDIA

The leaching of celestite (SrSO4) particulate samples with Na2CO3 solu tions 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 re action is topochemical, obeying a shrinking core model, incorporating diffusion of CO32- ions, through the solution contained in the pores o f the product layer, as the rate-limiting step. The model takes into a ccount the change in Na2CO3 concentration that occurred under the cond itions studied. In agreement with the model, the reaction rate increas es as the first power of the Na2CO3 concentration and is inversely pro portional to the square of particle size. The rate is insensitive to t he presence of Na2SO4 up to a total concentration of around 1 M, but d ecreases for higher concentrations. The rate of reaction decreases sig nificantly when the pH is lowered to values where a large proportion o f CO32- ions transforms to HCO3-; for example, at pH 9. The conversion rate of celestite increases systematically with increasing temperatur e; 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 s olution), can be completely transformed under mild Na2CO3 concentratio ns and temperatures (e.g., at 10% in excess of the stoichiometric amou nt required and 75 degrees C, respectively).