Selection Sizing and Developing the Optimum Gravity Gold Circuit for Your Project D Connelly1 ABSTRACT Gravity gold recovery has been practiced for thousands of years, making it one of the oldest forms of mineral processing. Gold is 19 times heavier than water although the barren rock is generally only 2.7 times heavier. Any potential gold ore needs to be characterised to assess its amenability to a gravity recoverable gold (GRG) process based on well planned and executed test work. Scale up considerations need to be considered in designing a gravity circuit. The economic impact of a gravity circuit should be justifi ed based on the installed cost and likely benefi ts. Maximising gravity gold recovery results in a faster cash fl ow, lower operating cost per ounce and higher overall gold recoveries compared to the several weeks required to recover gold adsorbed onto carbon. The advent of centrifugal concentrators such as the Knelson and Falcon, coupled with intensive leaching of concentrates (ILR), has revolutionised gravity gold circuit designs. Considerations associated with the sizing of the centrifugal machines and the percentage of the circulating load (bleed) that should be processed is discussed in this paper. The trends with gravity circuit layout, engineering, as well as optimum dumping cycles, are important areas which require close consideration. Key aspects of these are presented in this paper. There are a number of circuit confi gurations to be considered, including the integration of different gravity concentrators. Other gravity process design considerations such as fewer modules, the effect of equipment size and enhancement of process effi ciency using preconcentration, together with factors that affect OPEX and CAPEX are addressed. Practical considerations of maintenance, security from theft, minimisation of risk and water balance considerations are discussed using different equipment and circuit confi gurations. The trend towards using gravity recovery in fl otation plants is also reviewed. A number of examples are cited and good design aspects highlighted. INTRODUCTION The history of gold recovery is as old as mineral processing Operationally it was learnt that the cleaning cycle frequency itself. Gold is well suited to gravity treatment and early plants time was important, as was keeping the Knelson bowl holes used jigs, strakes, shaking tables and amalgam drums. The clean because the fl ow rate of water was more critical rather mineralogy of gold ores with respect to nature and occurrence, than the water pressure. particularly liberation, dictates the applicability of gravity Two examples of high gravity gold recovery process plants processing. were Callie (approximately 60 per cent) and Bronzewing Johnson drums were phased out (Central Norseman) and (70 to 80 per cent) of the gold recovered to a gravity the Knelson concentrator became the industry standard. This concentrate. These are exceptional high gravity gold has been signifi cantly improved over time and automated recoveries. It is believed a Kelsey Jig was specifi ed for the to be self-emptying. The Dutch State Mines (DSM) screen Wallaby project, one of the few gold applications. aperture ahead of the Knelson and the feed split is important Gravity gold has many advantages which should be seriously in maximising gravity gold recovery. considered in any fl ow sheet. The introduction of centrifugal The Knelson concentrator treated a portion of the cyclone concentrators such as the Knelson, Falcon and Kelsey Jig have underfl ow (20 per cent) stream, however at Bronzewing a revolutionised gravity gold recovery. Gold fl ow sheets usually vibrating screen was used to treat the cyclone underfl ow have gravity as the primary step followed by other downstream prior to feeding the Knelsons. During the 1980s there was a processes such as carbon-in-pulp (CIP). compelling technical argument concerning higher recovery, The processing of concentrates was fi rst carried out using faster cash fl ow, lower operating cost considerations and strakes and amalgam drums, this was discontinued in the CAPEX. Some companies used spirals (Ok Tedi, Granites) and 1970s due to occupational health and safety issues and gravity traps before the Knelson as a common design feature. environmental issues with mercury. Mercury was phased out 1. MAusIMM(CP), Director/Principal Consulting Engineering, Mineral Engineering Technical Services Pty Ltd (METS), PO Box 3211, Perth WA 6832. Email: [email protected] GRAVITY GOLD CONFERENCE / BALLARAT, VIC, 21 - 22 SEPTEMBER 2010 69 D CONNELLY after payouts of compensation to employees and it is now only and some companies postpone installing to a later stage of used as a laboratory tool. Operators who suffered the effects of project development. mercury poisoning failed to recover their health. The OPEX is rarely a problem because usually this is the Gemini tables were common (Figure 1). However, many lowest cost gold to produce. operators returned to half size Wilfl ey tables, although The process risks with gravity gold are high; a number of some operators use gold wheels. The removal of tramp iron projects have miscalculated in terms of the percentage of continues to be a problem at some operations. gravity gold recovery. MINERALOGY Mineralogy determines the amenability to gravity processing particularly the particle size, degree of liberation, density differential, particle shape, composition and hydrophobicity. Characterising the feed is the fi rst basic step in determining the most applicable separating equipment and developing the optimum fl ow sheet. Polished sections are useful, but old technology is being replaced by Qemscan. Sink Float analyses is useful because the major problem with gold is to locate suffi cient particles and be confi dent these particles represent gold occurrence. The scanning electron microscope and gold analyses combined with diagnostic leaching are useful in understanding the nature and occurrence of the gold present. Metallic gold is common and the size can vary from colloidal gold through to FIG 1 - Gemini Table (Downer EDI Mining supplied image). nugget gold and as alloys with other metals or within sulfi des. (see Figure 3) The Inline Pressure Jig (Figure 2) has found application with some operations (Big Bell). Heavy media separation (HMS) has been looked at but not applied for some low-grade orebodies. ADVANTAGES OF GRAVITY At New Celebration and Granny Smith gravity concentration In the case of gravity recovered gold, faster cash fl ow, higher of the sulfi des in the CIP tailings stream was employed, overall recovery and lower cost per ounce are all signifi cant including fi ne grinding of the gravity concentrate recovered advantages. Gravity gold recovery is environmentally friendly from the tailings. as no reagents are required. Other benefi ts include reducing Piloting has rarely been used for greenfi eld projects. the CIP feed grade, recovering coarse gold that would Generally companies go from bench scale to full scale but otherwise be slow to leach, improving CIP leach kinetics, piloting has been used where upgrading or retrofi tting was reducing carbon loadings, reducing gold in circuit lock up and considered for an existing operation. reducing cyanide consumption. The trend over the last twenty CAPEX is a major consideration with fully installed gravity years has been to maximise gravity gold recovery for the above circuits adding some two to fi ve per cent additional CAPEX reasons. FIG 2 - InLine pressure Jig Cross-section (Gekko Systems image). 70 GRAVITY GOLD CONFERENCE / BALLARAT, VIC, 21 - 22 SEPTEMBER 2010 SELECTION SIZING AND DEVELOPING THE OPTIMUM GRAVITY GOLD CIRCUIT FOR YOUR PROJECT to evaluate various scenarios based on comminution and ore recovery. Test work should target samples widely dispersed throughout the oxide, transition and primary zones. Samples need to be obtained with greater defi nition such as laterite, coffee rock, pisolites, saprolite etc. The impact of recovery with depth would be useful to understand if this can be tested in the primary core. Applying geometallurgical modelling techniques can directly reduce the risks associated with meeting production targets. Geometallurgy has the potential to act on both the consequences and likelihood axes to decrease risk. Areas of key focus include: concentration of deleterious elements, hardness, grindability, FIG 3 - Locked Gold (METS - internal source). mineral species and ‘mineral grade’, SAMPLE SELECTION AND REPRESENTIVITY mineral liberation, metallurgical recovery, Proper sample selection and planned laboratory test work form the basis of process selection based on gravity recoverable mining recovery, gold (GRG). Interpretation of test results and translating to drillability, an operable fl ow sheet is critical. Test plans should include fragmentation, gravity test work on samples selected spatially throughout reagent consumption, and the orebody to confi rm the range of recoveries encountered smelter enabling characteristics. (see Figure 4). Composites are good at hiding variability. The There are a number of critical factors that need to be in place samples need designated coordinates so the recovery can be if a geometallurgical approach is to be successful. The fi rst and assigned to a location in the block model and be used in the mine schedule. This data will be used in conjunction with the foremost factor is to breakdown some of the traditional barriers variability data and support the recovery predictions used for between