Mechanistic Aspects of Metal Sulfate Decomposition Processes

Abstract

The decomposition reactions of the crystalline sulfate phases CaSO₄MgSO₄ZnSO₄ZnO • 2ZnSO₄CuSO₄and CuO • CuSO₄ were studied by the torsion-effusion method with simultaneous mass-loss measurement, and by mass spectrometry. Decomposition pressures and vapor compositions were derived from the results. In all instances, these phases decompose to the corresponding oxides or oxysulfates, plus a gas phase that would be predominantly SO₂ and O₂ at equilibrium. However, most of the processes are severely limited kinetically because of failure to attain S0₃—SO₂ equilibrium in the gas phase, and direct desorption of SO₃ is frequently observed. Certain noble metals and p-type semiconducting oxides are observed to catalyze the SO₃—SO₂ conversion and in some instances lead to dramatic increases in the observed effusion pressure. In a few cases, the catalytic additives convert the gas phase to the equilibrium SO₂ + O₂ composition, but have no detectable effect on decomposition pressure, suggesting that the SO₃ to SO₂ conversion may or may not be closely coupled to the initial sulfate ion decomposition step. An attempt is made to correlate this wide range of observed behavior. In addition, decomposition pressures derived by extrapolation to zero orifice size are compared with values calculated from thermochemical data to test the suitability of the effusion method for thermodynamic studies of these materials.

Article Note

^*Author to whom all correspondence and reprint orders should be addressed.