A kinetic and mechanistic investigation of the formation of calcium sulphate in reactions that may be of use in flue-gas desulphurization II. Promotion of the CaSO 3 + 1/2 O 2 reaction by alkali metal salts

Abstract

The role of alkali cations, potassium (K + ) in particular, in promoting the reaction of calcium sulphite with oxygen between 823 and 923 K has been investigated. This extends our previous (part I) study of the kinetics and mechanism of this reaction in the absence of additives. Almost all the solids mixed with the reactant CaSO 3 (including K 2 SO 3 , K 2 SO 4 , Na 2 SO 4 , Rb 2 SO 4 , KI, KIO 3 , KBr and others added as 5% mass) increased both rates and extents of oxidation. The pattern of kinetic behaviour remained unchanged. As with CaSO 3 alone, the reaction was strongly deceleratory and ceased before conversion of all the reactant to CaSO 4 . It is concluded that the additive does not change the reaction mechanism but, during the oxidation process, increases the permeability to oxygen of the adherent product layer that forms a barrier between CaSO 3 and O 2 . Later recrystallization of the oxidized material makes this barrier impermeable to oxygen and reaction ceases. This increase in mobility of constituents of the product layer (particularly trans-barrier oxygen diffusion), inferred from the kinetic data, is consistent with microscopic observations which give evidence of superficial sintering of the solid. The promotional ability of each additive is, therefore, determined by a balance between enhanced ease of transport across the barrier and the diminution of rate that results from crystallite growth through sintering. This qualitatively accounts for the observed pattern of reaction rates and the relatively large variations in the activation energies calculated for similar rate processes. It is concluded that the inefficient use of CaCO 3 in flue gas desulphurization, whereby a proportion of the calcite remains unreacted, is a consequence of the strongly adherent barrier layer of product formed. This layer is not easily removed or suitably modified by additives to remove the inhibition. The most useful route to effective employment of the solid is to diminish particle sizes sufficiently (probably to crystallite edge lengths of less than 0.3 μm) to avoid limitation of the extent: of reaction by the product CaSO 4 barrier.