Engineering & Mining Journal

JUN 2013

Engineering and Mining Journal - Whether the market is copper, gold, nickel, iron ore, lead/zinc, PGM, diamonds or other commodities, E&MJ takes the lead in projecting trends, following development and reporting on the most efficient operating pr

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GETTING MORE GOLD oxide, based on the oxidation reduction potential (ORP). This process solved the foaming issue and the reaction time was reduced. With the Six Sigma continuous improvement methodology, a wet chlorination process control chart was drawn and the process was implemented (See Figure 2). This process has increased the gold bar/byproducts ratio significantly, and resulted in additional revenue and an increase in the chlorination process capacity. Fundamentals of ORP Figure 2: Control chart of wet chlorination process. slurry of slimes suspended in an aqueous solution of hydrochloric acid. The process of wet chlorination oxidizes and solubilizes the gold and selenium in the slimes. Hydrogen peroxide is used as the oxidant and hydrochloric acid provides the chloride for gold complex solubility. Side reactions that occur during this leach include solubilization of some slime impurities and conversion of lead sulfate to lead chloride. Process chemistry occurs via the following reactions: PbSO4 + 2HCl PbCl2 + H2SO4 bars produced, but it could also leave selenium in the leach residue, creating quality issues. One of the main problems that can occur during chlorination is rapid foaming of the tank content. When foam rises to a certain level in the leach tank, addition of reagents must stop. This delays the chlorination cycle resulting in a bottleneck in PM operations. To optimize this process, a Six Sigma process was carried out with efforts that focused on temperature control and dosage or reagents such as hydrogen per- As mentioned above, other than temperature, a major control of wet chlorination is through the monitoring of the ORP (or redox) potential. An ORP reading provides an indication of the ability of a solution to support oxidation or reduction. As the name implies, this is a potentiometric (voltage/millivolt) method. ORP electrodes contain both a reference electrode and a measuring electrode. The sensor used is in fact a combined pH/ORP sensor (See Figure 3). The reference electrode is identical to that used for pH, and as its name implies, serves as a stable reference voltage to compare the measurement electrode reading. The measurement electrode for ORP is platinum metal and for pH it is a glass electrode. The inert platinum electrode does not chemically react with the solution, but senses the electron "pull" of oxidizing solutions or the electron "push" of reducing solutions. The presence of oxidizing chemicals causes the millivolt reading to be (1) Ag2Se + 2HCl + 3H2O2 H2SeO3 + 2AgCl + 3H2O (2) 2Au + 8HCl + 3H2O2 2HCl + H2O2 H2O2 1/2O2 2HAuCl4 + 6H2O Cl2 + 2H2O + H2O (3) (4) (5) Although chlorine is a considerably cheaper oxidant than hydrogen peroxide, it was not chosen to be used in the wet chlorination for safety reasons as chlorine is an extremely dangerous material to store, transfer and deliver to the chlorination tank. Continuous Improvement The chlorination step is the heart of the precious metals recovery process. It is of such importance in the PM plant that it could affect the enterprise performance of KUC, not only in the number of gold Figure 3: Illustration of a pH/ORP electrode. JUNE 2013 • E&MJ 111

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