Dissolution kinetics of colemanite in citric acid solutions assisted by mechanical agitation and microwaves

The aims of this study are to investigate the dissolution kinetic models of colemanite in citric acid with both mechanical agitation (ME) and microwave (MW) experimental systems, to declare an alternative process to produce boric acid, and to expose the existence of a "MW effect", if it exists, particularly on activation energy. The parameters are particle size, acid concentration, temperature, stirring speed, and solid/liquid ratio. The dissolution rate of colemanite increased with increase in temperature, and decreased with particle size and solid/liquid ratio. In addition, calcium citrate was obtained by the reaction as a byproduct together with boric acid. The dissolution is controlled by diffusion through the product layer. The activation energy was calculated as 28.65 and 21.08 kJ/mol for ME and MW experimental systems, respectively. As a result, the rate expressions depending on the parameters chosen may be summarized as follows: for the ME system, 1 - 3(1 - X)(2/3) + 2(1 - X) = 8123D-(CW0.206)-C-1.156-W-0.219(S/L)(-0.929)[exp(-6854/RT)]t; for the MW system, 1 - 3(1 - X)2/3 + 2(1 - X) = 4332D(-1.264)C(0.886)W(0.219)(S/L)(-1.058) x [exp(-5043/RT)]t.