The role of calcium in xanthate adsorption onto sphalerite

This paper presents results of a laboratory study designed to look into the effect of calcium on sphalerite hydrophobization with xanthate making use of zeta potential, contact angle, microflotation and UV/vis spectrometry techniques. The experimental results show that xanthate adsorbs onto the mineral surface at neutral and slightly acid pH as reflected by contact angle, floatability (by microflotation) and xanthate adsorption (by UV/vis spectrometry) measurements. Apparently, xanthate adsorption is due to both the less negative surface charge of the mineral and to the presence of Zn2+ in equilibrium with the mineral, rather than Zn(OH)2, which is the product of the mineral oxidation with dissolved oxygen at pH above 8. When calcium is present in the aqueous solution at concentrations similar to that of saturation for gypsum precipitation (e.g., about 0.017 mol/L), calcium apparently adsorbs on the sphalerite surface decreasing its negative charge (e.g., zeta potential) and promoting xanthate adsorption and hydrophobization, as reflected by contact angle and floatability measurements (ca. 30° and 33% recovery, respectively). Calcium adsorption appears to be favoured by the increase in pH, as reflected by the increased xanthate adsorption, thus suggesting that the calcium hydroxo-complex (CaOH+) is the adsorbing species. At pH 10 and 11, precipitation of calcium hydroxide (observed by SEM-EDS measurements at pH 11) appears to counteract xanthate adsorption, substantially decreasing mineral hydrophobicity. Calcium and xanthate appear to display some type of chemical interaction while calcium and dithiophosphinate do not.