Effect of chemical composition on the ζ-potential of chromite

Chromite samples from Africa, Brazil, Philippines and Russia were investigated through electrophoretic mobility and method of Mular and Roberts measurements. Chemical composition, lattice constant and specific gravity were determined. According to the chemical composition of the samples, empirical equations to predict the point of zero charge (PZC) of oxides and hydroxides were applied (Parks’ and Yoon’s treatments). The isoelectric point (IEP) of chromite is independent of the electrolyte concentration and confirmed that KNO3 is an indifferent electrolyte for this mineral. The PZC of the chromites investigated is around pH 6.5. The maximum difference found between measured and calculated values was 1.0 pH unit. The results obtained by the method of Mular and Roberts are close to the predicted PZC for three of the four chromite samples. The results obtained by microelectrophoresis point to a difference between measured and calculated values of 0.6 pH unit for samples from Russia and Africa, and 1.0 pH unit for the sample from the Philippines. The four investigated samples display a good atomic ordering in the spinel structure. The presence of lattice vacancies, as well as the existence of sites proner to oxidation and/or hydration may account for the difference between the calculated and measured values.

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► PZC of the chromites investigated is around pH 6.5.
► KNO3 is an indifferent electrolyte for chromite.
► The investigated samples display a good atomic ordering in the spinel structure.
► Lattice vacancies may affect the PZC of chromites.