Gold-Paraffin Process

A process called Gold-paraffin is under development to recover gold from gravity concentrates or fresh ores. The process should be simple enough to be used as an alternative to amalgamation. The process is based on the selective adhesion of liberated gold particles to hydrophobic materials and their separation from a pulp of the ore (like mercury in amalgamation). As a main advantage, this process is not deleterious to the environment, since hydrophobic material used is paraffin-wax, extremely used in the food and pharmaceutics industries.

In bench scale, the process is carried out heating a 25-30% solids pulp of the ore to a temperature 2 oC higher than the melting point of the paraffin-wax used (approx. 68 oC), at this temperature a certain amount of paraffin is added and let melt. After melting, a stirrer motor is turned on to disperse the paraffin and allow it to be in contact with the ore. The gold selectively adhered to the paraffin-wax is transferred to the paraffin phase after the agitation is stopped. After cooling, at room temperature, the paraffin (S.G. 0.8) is a floating solid and then can be removed and treated to recover the gold contained in it.

The theoretic viability of the process was analyzed based on the work of Jacques et.al. (1), these authors studied the feasibility of a solid particle (phase 1) be transferred from one liquid (phase 3) to other one (phase 2), in a system of a solid and had two immiscible liquids, where one liquid (phase 2) wets preferently the solids particles (phase 1). The two states, initial (I) and final (II) are presented in the figure.

By analyzing the free energy and the forces taking part in the interface, Jacques obtained an expression for the difference of free energy of states II-I as a function of the three phase contact angle, θ (measured in phase 2, paraffin), and the ratio of the dispersed liquid droplet radius to the particle radius (n = r₂ / r₁). The most stable state is the one with the lowest free energy, which implies a three phase contact angle θ < ¶/2 and n → ∞, for the separation to occur (state II).

Figure shows Process of particle (phase 1) distribution from continuous liquid (phase 3) into dispersed liquid (phase 2), showing phase relationship between initial state I and final state II configurations (1).

The experiments were carried out with a gravity concentrate from the region of Itabira (State of Minas Gerais) containing 11 g/t gold. The effect of some variables of the system is discussed below. It was evaluated the influence of the amount of paraffin, and found in increase in the gold recovery as the paraffin concentration increases in the medium, in the range from 25 to 75% paraffin in relation to the sample. In some experiments the ph of the pulp was adjusted to try to obtain an electrostatic attraction between the gold particles and the paraffin droplets. The literature (2) gives an isoelectric point at pH = 2.0 for gold and in the range from pH = 3.0 to 5.0 for paraffin (3), so a condition of pH = 3.0 was chosen where the gold particles would be negative and the paraffin droplets positive. Indeed the gold recovery was improved by this condition. The use of xanthate as collector for gold had a good effect on the metal recovery.

The results of the gold recoveries by this process are in the range 40 to 50%, although lower than the results usually obtained by amalgamation (80-90%), they can be considered promising. Using a gold plate it was found θ = 80^o for the gold paraffin water, additional studies are being undertaken in order to improve the adhesion gold-paraffin, i.e. decreasing θ; for comparison purpose the gold-mercury-water is θ ≈ 20^o. Also, the dynamic conditions of the process to permit the gold particles to float joined to (solid) paraffin, after cooling, appear to be rather critic. Considering the environmental origin of the study, the process deserves better investigation.

References.
1. M.T. Jacques, A.D. Hovarongkura, and J.D. Henry Jr. Feasibility of Separation processes in Liquid-Liquid Solid Systems: Free Energy and Stability Analysis. AIChE Journal, 25 (1) (1979), 160-170.

2. D.W. Thompson, and I.R. Collins, Electrical properties of the Gold Aqueous Solution Interface. Journal of Colloids and Interface Science. 152 (1) (1992), 197-204.

3. S. Chander, J.M. Wie, and D.W. Fuerstenau. On the Native Floatability and Surface Properties of Naturally Hydrophobic Solids. AIChE Symposium Series. 150 (71) (1975), 183-188.