Dissolution of Wollastonite in a Packed Bed Contactor

In a packed-bed reactor grains of the calcium silicate mineral wollastonite dissolved incongruently. A silicon-rich residue layer appeared to form on the grain surfaces and this quickly limited the overall rate of mineral dissolution. Approximately 54% of the silicon associated with the dissolved calcium was retained on the grains and about 46% went into solution with the calcium. As the amount of Ca dissolved per unit area of grain surface approached approximately 0.5 mmol/cm2 the effluent concentration of Ca decreased to a value that was significantly less than the equilibrium concentration (approximately 300 mmol/L) and the effluent pH decreased from approximately 10 to slightly greater than the influent value of 6.8. The effluent Ca and Si concentrations were effectively predicted by a finite difference model that used layer-by-layer calculation of a residue layer mass transfer resistance (for the Ca ion) acting in series with an extraparticle mass transfer resistance. The effluent pH was predicted using chemical equilibrium calculations and the assumption that the dissolved Si controls the proton balance across the column.