Effect of dissolved metal sulphates on gas-liquid oxygen transfer in agitated quartz and pyrite slurries

A previous study on mass transfer in agitated three phase systems showed that for quartz slurries the volumetric oxygen transfer coefficient kLa [s−1] decreases with increasing solids fraction, whereas pyrite particles increase the kLa value. The present study was conducted in continuation of these results and attempts to explain why the addition of pyrite particles resulted in an increase in kLa. For this purpose, the effect of ionic strength, mineral density and oxygen consumption due to homogeneous reactions was examined. Neither the high density of pyrite nor the oxygen consumption could offer an explanation for the increased kLa values obtained for pyrite slurries. The variable mineral density did not affect kLa at all, while the consumption of oxygen was not large enough to cause an enhancement of kLa. However, by reducing bubble coalescence frequency and thus bubble size, the ionic strengths found for pyrite slurries could partially explain the increase in kLa. For 15 vol.% pyrite slurries, the ionic strengths proved to account for at least 60% of the enhancement. The variable(s) and/or mechanism(s) causing the remaining 40% of the enhancement could not be identified (yet). To demonstrate the effect of ionic strength as such, various electrolytes (viz. CuSO4, FeSO4, ZnSO4 and Al2(SO4)3) were added to a 15 vol.% quartz slurry. For all metal sulphates, increasing the ionic strength up to about 0.25 mol/l resulted in an increase of kLa by up to a factor 2.5.