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 Getting More Gold from Slimes: Higher Productivity in Wet Chlorination Continuous monitoring of in-line oxidation reduction potential assisted a major copper producer in developing a sustainable, efficient approach to slimes processing that required less costly reagents, increased yield and improved efficiency. By Daniel Kim and Shijie Wang, Rio Tinto Kennecott Utah Copper; and Doug Brees, Mettler-Toledo Ingold Inc. Sustainable development as a concept and as a goal has spread rapidly and is now an integral facet of the operations of most major mining enterprises. With an emphasis on values and goals, mining companies focus on maximizing resources and positioning themselves to take advantage of technological development, while still considering sustainable development in regard to profitability, safety and operating efficiency. At Rio Tinto Group's Kennecott Utah Copper (KUC), a focus on economic prosperity, social well-being, environmental stewardship, and strong governance systems has been practiced within its business model since 2005. Understanding these practices and being able to identify projects that will contribute to sustainable development is key to the company's long term success and ability to continue operations. KUC's intention is to get to a point where everything the company does is driven by sustainable development. A refinery modernization program undertaken in the mid-1990s at KUC led to an increase in tank house production capacity from 200,000 metric tons per year (mt/y) to 310,000 mt/y (850 mt per day average). Cathode quality and appearance improved even though the anode impurity levels increased, unit operating costs were reduced through increased productivity and safety, and environmental protection also improved. More recently, through implementing multiple-step Six Sigma processes, the refinery has further enhanced its productivity, produces higher quality copper, increased its profitability, and met stringent customer demands. This article describes KUC's slimes process improvements, in particular wet chlorination for gold leaching. Kennecott Copper Anode Slimes Process Figure 1: Material flow diagram of slimes processing in the precious metal plant. 110 E&MJ • JUNE 2013 The KUC refinery is located at Magna, approximately 16 miles west of Salt Lake City, Utah, USA. In the refinery's tank house, commercial anodes from the KUC smelter are electrorefined. When a copper anode dissolves during electrorefining in the refinery tank house, slimes (insoluble material) are continuously released from the anodes and deposit at the bottom of the cells. These insoluble metals and compounds comprise various amounts of copper, gold, silver, selenium, tellurium, lead, bismuth and barite, together with traces from the platinum group metals. The slimes are then pumped to the precious metals (PM) plant as a slurry in a mixture of drain electrolyte, cell and floor wash water, and anode / cathode wash water. In the PM plant, the anode slimes are initially thickened in three settling tanks, each of 40,000 gallon (151,000 liter) capacity. The liquid phase is decanted and filtered through two plate and frame filters. When the filtered solution is clear enough it is returned to the tank house as electrolyte make-up solution. Next, the electrolytic refinery slimes are leached with sulphuric acid and oxygen in two autoclaves to achieve rapid and complete oxidation of the copper present in the slimes. The decopperized slimes then go through a wet chlorination process. In the following step, the leach solution is fed to a solvent extraction process that includes two-stage countercurrent extraction with dibutyl carbitol (diethylene glycol dibutyl ether) and multi-stage countercurrent scrubbing with 1–3 molar hydrochloric acid. Gold is reduced directly from the organic phase by contact with an aqueous solution of the reducing agent, and crude selenium is recovered from the gold-free raffinate by reduction to its elemental form. Finally, wet chlorination residue is repulped in a dilute solution of sodium carbonate and leached with a solution of aqueous ammonia for lead and silver recovery (See Figure 1). Wet Chlorination Slimes at Kennecott have a relatively high gold to silver ratio of 1:10, hence hydrometallurgical processing through chlorination was chosen to allow for production of gold. Wet chlorination of decopperized slimes is achieved through the metered addition of hydrogen peroxide to agitated

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