Conversion kinetics of SrSO4 to SrCO3 in solutions obtained by dissolving/hydrolyzing of equimolar amounts of NH4HCO3 and NH4COONH2

Abstract

Celestite concentrate (SrSO4) is one of the most important raw materials used for the industrial production of strontium compounds. In this study, the effects of stirring speed, particle size, CO32– ion concentration and temperature on the conversion reaction rate of SrSO4 to SrCO3 in solutions obtained by dissolving/hydrolyzing a mixture of equimolar amounts of NH4HCO3 and NH4COONH2(AC) were investigated. The solution obtained after total dissolution/hydrolysis of AC consisted of NH4+, CO32–, HCO3–, H2CO3* and NH3. The conversion reaction proceeds according to dissolution and precipitation mechanism. Sr2+ ions formed during the dissolution of SrSO4 precipitates with CO32– ions as SrCO3 pseudomorphically and as a result porous SrCO3 layer is produced in the form of clusters. The rate determining step is the ion exchange reaction at the interface between dense SrSO4 and porous SrCO3 layers. The kinetic parameters for the ion exchange reaction were determined by applying the Shrinking Core Model. While the conversion reaction rate was found to be zero order up to a certain CO32– ion concentration, above this CO32– ion concentration, it was −0.7th order. The apparent activation energies for the zero and −0.7th order reactions were calculated as 64.84 and 47.79kJmol−1, respectively. The amount of S passed to the solution as SO42− ions was determined quantitatively by ICP-OES. The structural and morphological characterization of the celestite concentrate and solid reaction residues were carried out by XRD and SEM.