![]() ![]() In the case of gravity recovered gold, faster cash flow, higher overall recovery and lower cost per ounce are all significant advantages. Metallic gold is common and the size can vary from colloidal gold through to nugget gold and as alloys with other metals or within sulphides. ![]() The Scanning Electron Microscope and gold analyses combined with diagnostic leaching are useful in understanding the nature and occurrence of the gold present. Sink Float analyses is useful because the major problem with gold is to locate sufficient particles and be confident these particles represent gold occurrence. Polished sections are useful, but old technology is being replaced by Qemscan. Characterising the feed is the first basic step in determining the most applicable separating equipment and developing the optimum flowsheet. Mineralogy determines the amenability to gravity processing particularly the particle size, degree of liberation, density differential, particle shape, composition and hydrophobicity. The process risks with gravity gold are high a number of projects have miscalculated in terms of the percentage of gravity gold recovery. The OPEX is rarely a problem because usually this is the lowest cost gold to produce. Generally companies go from bench scale to full scale but piloting has been used where upgrading or retrofitting was considered for an existing operation.ĬAPEX is a major consideration with fully installed gravity circuits adding some 2 to 5% additional CAPEX and some companies postpone installing to a later stage of project development. Piloting has rarely been used for Greenfield projects. Heavy Media Separation (HMS) has been looked at but not applied for some low-grade ore bodies.Īt New Celebration and Granny Smith gravity concentration of the sulphides in the CIP tailings stream was employed, including fine grinding of the gravity concentrate recovered from the tailings. The In Line Pressure Jig (Figure 2) has found application with some operations (Big Bell). The removal of tramp iron continues to be a problem at some operations. However, many operators returned to half size Wilfley tables, although some operators use gold wheels. Operators who suffered the effects of mercury poisoning failed to recover their health. Mercury was phased out after payouts of compensation to employees and it is now only used as a laboratory tool. The processing of concentrates was first carried out using strakes and amalgam drums, this was discontinued in the 1970s due to occupational health and safety issues and environmental issues with mercury. Gold flowsheets usually have gravity as the primary step followed by other downstream processes such as carbon in pulp (CIP). The introduction of centrifugal concentrators such as the Knelson, Falcon and Kelsey Jig have revolutionised gravity gold recovery. Gravity gold has many advantages which should be seriously considered in any flowsheet. It is believed a Kelsey Jig was specified for the Wallaby project, one of the few gold applications. These are exceptional high gravity gold recoveries. Two examples of high gravity gold recovery process plants were Callie (approximately 60%) and Bronzewing (70 to 80%) of the gold recovered to a gravity concentrate. Operationally it was learnt that the cleaning cycle frequency time was important, as was keeping the Knelson bowl holes clean because the flow rate of water was more critical rather than the water pressure. Some companies used spirals (Ok Tedi, Granites) and gravity traps before the Knelson as a common design feature. During the 1980s there was a compelling technical argument concerning higher recovery, faster cash flow, lower operating cost considerations and CAPEX. The Knelson concentrator treated a portion of the cyclone underflow (20%) stream, however at Bronzewing a vibrating screen was used to treat the cyclone underflow prior to feeding the Knelsons. The DSM screen aperture ahead of the Knelson and the feed split is important in maximising gravity gold recovery. This has been significantly improved over time and automated to be self-emptying. Johnson drums were phased out (Central Norseman), and the Knelson concentrator became the industry standard. The mineralogy of gold ores with respect to nature and occurrence, particularly liberation, dictates the applicability of gravity processing. Gold is well suited to gravity treatment and early plants used jigs, strakes, shaking tables and amalgam drums. The history of gold recovery is as old as mineral processing itself. ![]()
0 Comments
Leave a Reply. |