In galena flotation, lead ethyl xanthate, Pb(EtX)2, is precipitated and is physically adsorbed in the form of multilayers on the chemically adsorbed first layer of Pb(EtX). The solid particles are coated with layers of Pb(EtX)2 in heterogeneous manner over surface areas possessing both anodic and cathodic character. It is anticipated that the mode of formation of Pb(EtX)2 and the number of ions present, together with stable or unstable by-product species, have great influence on the surface characteristics of galena. The information obtained from a study of these parameters may have important practical applications in flotation
The precipitation study of Pb(EtX)2 was done utilizing the quick experimental techniques reported by Tezak and his associates and by Matijevic and his collaborators. A series of aqueous solutions of Pb(NO3)2 and of KEtX at 6.5 pH values and of various known concentratlons were prepared. The reacting solutions were mixed rapidly by putting 5.0 ml of one solution into the same volume of the other reactant in well-cleaned test tubes. Each test tube was shaken for one minute and was left in a constant temperature bath at 25°C.
The study concerning dissolution and decomposition of Pb(EtX)2 was carried out in a double-wall thermostatic reaction vessel. For each experiment, 0.5 to 1.0 gram of Pb(EtX)2 was treated with 300 to 500 ml. of distilled water. The temperature of the contents was kept constant by circulating water from a thermostat through the jacket of the vessel. Lead ethyl xanthate was kept in suspension by magnetic stirrer and by passing nitrogen gas, which helped in agitation of the mixture.
The precipitation regions which develop rapidly, moderately and slowly indicate that the course of precipitation is governed by the concentration of ions at the moment of mixing. Theoretically, the whole area to the right side of the solubility product line KSp should give precipitation. However, it was found that a coarse precipitation appeared immediately upon mixing in the region 1, and a finely dispersed colloidal precipitate in the region 2.
The rates of dissolution of Pb(EtX)2 from pH 4.5 to 10.5 have been studied at 25°, 35°, 45° and 60°C. The results of the two experimental series, one at 25°C (Figure 2) and the other at 45°C (Figure 3) have been chosen to discuss dissolution and decomposition. The ionic concentrations of xanthate at various pH values and the corresponding variations in the initial pH values as a function of time have been recorded.
At the acidic pH values like 5.5 and 6.5 when the rate of hydrolysis is smaller than that in more acid pH, a slight increase in pH is observed, while the rate of dissolution steadily increases with the increase in pH values. The continuous increase in the pH values in the dissolution study of Pb(EtX)2 in acidic solution indicates that hydrolysis of xanthate ions continues as the dissolution of Pb(EtX)2 proceeds.
In the dissolution study of Pb(EtX)2, a remarkable increase in xanthate ion concentration has been observed above the neutral pH. The concentration of xanthate ions is much higher at 9.5 pH than at 8.5 pH and still higher at 10.5 pH. At 7.5 and 8.5 pH values, as the process of dissolution proceeds, the metal ions become hydrolysed and the resulting Pb(OH)2 starts covering the surface of Pb(EtX)2 and the rate of dissolution declines.
In alkaline solutions, the xanthate ions decompose through many unstable products and end up with stable species like carbonate and sulphide ions.
