Olivine dissolution in sulphuric acid at elevated temperatures - implications for the olivine process, an alternative waste acid neutralizing process

A kinetic model for the dissolution of olivine in sulphuric acid at elevated temperatures is presented. Such a model is essential for the design of an industrial process for the neutralization of waste acids by olivine, or for the production of precipitated silica from olivine. The effects of temperature, grainsize fraction and olivine amount on the neutralization rate have been quantified by carrying out a range of complete-neutralization experiments. Under natural weathering conditions the dissolution rate of olivine is controlled by rates of surface reactions, During neutralization of acids at elevated temperatures the reaction mixture must be stirred vigorously to prevent the precipitation of silica on the dissolving olivine grains. Under these conditions dissolution is kinetically controlled by surface reactions. For the interpretation of the experimental data a measure of the hydrogen ion activity is required. Since it is not possible to accurately measure pH in acids at high temperatures, the hydrogen ion activities were calculated using the Pitzer model. The rate equation coefficients and constants were obtained by fitting an expression for the rate equation to the experimental data. The following equation was obtained, that enables the calculation of the neutralization progress as a function of time: d[H+]/dt = -e(-) E-act/RXT x 10(8) x (1.92 +/- 0.12) x S-geom x A(H+)(0.33) [mol min(-1)] with E-act the activation energy of 66.5 +/- 2 kJ mol(-1), S-geom the geometrical surface area of the olivine grains, calculated from the grainsize distribution and A, the hydrogen ion activity calculated with the Fitter model, using the computer program PHRQPITZ. The equation was validated for the dissolution of Norwegian olivine, with grainsizes from 63 to 300 mu m, in sulphuric acid at temperatures ranging from 60 to 90 degrees C. (C) 1998 Elsevier Science B.V. All rights reserved.