Thermodynamic studies of xanthate adsorption on pyrite

Abstract

Electrochemical and thermochemical studies have been conducted to determine the thermodynamic functions, i.e., the standard free energy (ΔG°); entropy (ΔS°), and enthalpy (ΔH°) changes, for the adsorption of xanthates on pyrite. The electrochemical technique is based on the measurement of the open-circuit potentials of a pyrite electrode immersed in xanthate solutions at various temperatures. The thermochemical technique involves the determination of the heats of adsorption of xanthates on pyrite using a microcalorimeter. The electrochemical studies have shown that the ΔG° of the proposed half-cell reaction, i.e., 2X- X2 + 2e, is negative for all the xanthate homologues, i.e., potassium ethyl xanthate (KEX), potassium amyl xanthate (KAX), and potassium octyl xanthate (KOX), investigated. The major contribution to the free energy of adsorption comes from the -TΔS° term, while ΔH° is working adversely toward the negative free energy change. It has also been found that ΔG° increases with increasing hydrocarbon chain length and temperature. The calorimetric heats of adsorption of xanthate on pyrite increase significantly with decreasing surface coverage below the monolayer point, suggesting that xanthate adsorption on pyrite involves a heterogeneous surface reaction. Above the monolayer point, the calorimetric heats of adsorption remain constant, the magnitudes increasing in the order of KEX < KAX < KOX. The calorimetric heats of adsorption in the multilayer region are comparable to the standard enthalpy of adsorption determined from the electrochemical data by considering the cathodic reaction involving oxygen reduction.