OLIVINE DISSOLUTION KINETICS AT NEAR-SURFACE CONDITIONS

Fluidized bed dissolution experiments have been conducted as a function of pH at 25-degrees-C with fayalitic olivine (Fo6) in HCl solutions and with forsteritic olivine (Fo91) in solutions containing the organic ligand potassium hydrogen phthalate (KHP). At 25-degrees-C the dissolution rate (R) of fayalite as a function of pH is: R (mol cm-2s-1)=1.1.10(-10)a(H+)0.69+3.2.10(-14)+1.2.10(-16)a(H+)-0.31 (1) where a(H+) is the activity of H+ in solution. The dissolution rate at 25-degrees-C of Fo6 at a given pH is a factor of 6 greater than that of forsteritic olivine. The assumption that the rates increase on a molar basis with Fe content allows calculation of dissolution rates of Fe-Mg solid solution olivines of intermediate compositions. Batch-type dissolution experiments were completed with Fo91 at 65-degrees-C in solutions at pH 1.8, 6.0 and 9.8. The rate equation obtained from these experiments is: R (mol cm-2s-1)=3.5.10(-10)a(H+)0.5+1.0.10(-13)+6.3.10(-17)a(H+)-0.5 (2) When combined with previously published data for Fo91 at 25-degrees-C, the 65-degrees-C experiments indicate that the activation energy of the olivine dissolution reaction in acidic, organic-free solutions is approximately 19+/-2.5 kcal. mol-1. Dissolution experiments with forsteritic olivine in solutions containing KHP at 25-degrees-C demonstrate that the rate of dissolution is increased in these solutions relative to rates measured in KHP-absent HCI-H2O solutions. Apparently, the increase in rate is caused by Mg complexation at the olivine surface. Ligand-promoted dissolution is thought to occur in parallel with proton-promoted dissolution. Therefore, the net rate, R(net), is the sum of the two rates: R(net) (mol cm-2s-1)=0.8.10(-12)[L(P)]0.45+R(H+) (3) where [L(p)] denotes the concentration of KHP; and R(H+) denotes the proton-promoted dissolution rate.