2nd International Conference on Trends in Industrial and Mechanical Engineering (ICTIME'2013) Sept 17-18, 2013 Hong Kong Feasibility Study on Physical Beneficiation of Low-Grade PGM Flotation Tailings using Spiral Classifiers and Enhanced Gravity Separators J. Siame, and H. Kasaini and chemical processes are used to extract the desired product Abstract— A study on physical beneficiation of platinum group from the run of the mine ore and produce a waste stream metals (PGMs) flotation tailings was carried out by means of spiral known as tailings. This process of product extraction is never classifiers and a gravity separator (Knelson Concentrator – KC-CVD 100% efficient, nor is it possible to reclaim all reusable and 6 unit). The economic mineral reserves for PGMs are found in the expended processing reagents and chemicals. The Busheveld Complex geological formation of South Africa unrecoverable and uneconomic metals, minerals, chemicals, particularly in the narrow strata of Merensky Reef, (Platreef) and the organics and process water are discharged, normally as slurry, UG2 chromitite layer. In addition, there are about 450 million tonnes to a final storage area commonly known as a Tailings of mine waste material (tailings and smelter slags) across the platinum industry which contains sufficient PGMs for economic Management Facility (TMF). exploitation. The major minerals in UG2 flotation tailings are Tailings are generally stored on the surface either within chromite {FeCr2O4, S.G = 4.5 – 4.8}, orthopyroxene {(Mg, retaining structures or in the form of piles (dry stacks) but can Fe)2Si2O6, S.G = 3.2 – 3.9} and plagioclase {Na0.5Ca0.5Si3AlO8, also be stored underground in mined out voids by a process S.G = 2.6 – 2.8} with small amounts of talc {Mg3Si4O10(OH)2, S.G commonly referred to as backfill [1]. = 2.7 – 2.8} and clay minerals (chlorites/phlogopite, S.G = 2.2 – The challenges associated with tailings storage are ever 2.75). PGMs in flotation tailings are either associated with silicates increasing. Advances in technology allow lower grade ores to (60%) or are free PGMs particles (10%) while 30% of them are be exploited, generating higher volumes of waste that require associated with sulphide minerals (pentlandite, chalcopyrite, safe storage. Environmental regulations are also advancing, pyrrhotite pyrite and millerite). On the basis of mineralogical data, about 70% of total PGMs associated with silicates and native alloys placing more stringent requirements on the mining industry, can be separated from oxide and sulphide minerals using gravity particularly with regard to tailings storage practices. This separators. This was confirmed by results from the spiral classifier ultimately places added pressure on the operators of a tailings and Knelson Concentrator where 71% of total PGMs were recovered facility who carry out the day to day roles of tailings discharge to the tailings (lighter fraction) and the overall grade of PGMs and water management [2]. increased from 0.75 g/t to 1.07 g/t. The highest amount of palladium The economic benefits of physical beneficiation of ores to be transferred from the feed solids (10 kg) to tailings (5 kg) was include the reduction of the impact on the environment, capital about 84%. The optimum conditions for operating a Knelson and operational expenditures which are often associated with Concentrator (KC-CVD 6) and spiral with regards to PGMs extractive metallurgical operations (concentrators, roasting, beneficiation are discussed further in this study. Even though the feed solids to the KC-CVD 6 gravity separator were deslimed, its capacity smelting, leaching and purification of solutions). Furthermore, to upgrade PGMs from 0.7 g/t to 1.11 g/t at an average recovery of physical beneficiation can be applied to reduce the carbon 71% compared well with the results from a spiral classifier where the print of a mining firm by scaling down its dependence on fines remained part of the feed. The recovery of PGMs by gravity chemicals, energy and water for extracting minerals. In separation was about 75% at an upgrade ratio of 1.8. general, physical pre-treatment of low grade ores is necessary for reducing the total mass of reagent-consuming minerals and Keywords— Beneficiation, Classifier, Gravity Separation, other gangue minerals in order to improve metallurgical plant Knelson Concentrator, Platinum, Spiral Classifier, UG2 Tailings. efficiency and productivity. There are over 450 million tonnes of flotation tailings and I. INTRODUCTION smelter slags containing appreciable amounts of PGMs in AILINGS consist of ground rock and process effluents South Africa. This is a huge resource for secondary PGMs. that are generated in a mine processing plant. Mechanical Several researchers have carried out feasibility studies of T recovering PGMs (often associated with gold) by recycling the tailings back to the flotation banks or by lixiviation with J. Siame is with the Copperbelt University, School of Mines and Mineral Sciences, Department of Chemical Engineering, P.O. Box 21692, Kitwe, HCl/H2SO4 acid [3]. The average grade of total PGMs in Zambia. (Phone: +260 969 206022; Fax: +260 212 228212; tailings vary from one mining firm to another. The average E-mail: [email protected]). PGM recoveries on concentrator feed solids vary between 80 H. Kasaini is with Rare Element Resources Corporation, Denver, USA. and 88% across the industry and depending on the feed grade (E-mail: [email protected]). 46 2nd International Conference on Trends in Industrial and Mechanical Engineering (ICTIME'2013) Sept 17-18, 2013 Hong Kong (4–8 g/t); the final flotation tailings may contain significant Table II illustrates the mineralogical profile of UG2 amounts of PGMs (0.7–1.6 g/t). In the past three decades, flotation tailings which contain about 0.7 g/t, total PGMs PGM losses to flotation tailings were very high mainly (Northam Platinum Ltd.). The most floated PGMs are because there were no reliable analytical tools to verify the associated with metal oxides and sulphides while PGMs PGM assays in concentrator tailings until the mini enclosed or attached to silicates are lost to the tailings together laparoscopy-assisted (MLA) technique was introduced to the with significant amount of free PGM particles [7]. platinum industry. Therefore, stockpiles of flotation tailings On the basis of mineralogical data shown in Table II, about constitute an important secondary resource of PGMs for the 57% of total PGMs in flotation tailings are attached or global market. But, beneficiation of PGM tailings by further enclosed in silicates while 13% of total PGMs are associated flotation has limited benefits since only about 30% of PGMs with native minerals. Both silicates and native minerals are are associated with base-metal sulphides and the rest are either lighter minerals compared to metal oxides and sulphides and inclusions or attached to clay minerals, with a small fraction therefore physical separation of these minerals is possible being free or native minerals of PGMs. Fine grinding is under optimized granulometric conditions. Therefore, about required to liberate the PGMs which are locked in either 70% of total PGMs can be transferred to lighter minerals chromite matrix or silicates [4]. Owing to the mineral hardness resulting in PGMs upgrade. The extent of the upgrade ratio of silicates and chromite (5–6 Mohs scale) energy depends on the partition coefficient of solids. consumption in the milling process is expected to be high. Further milling of the heavier minerals from the gravity The mineralogical profile of PGM feed solids to the separators may be necessary to liberate 30% of PGMs which concentrators is illustrated in Table I along with PGMs which are associated or attached to metal sulphides or the heavy are associated with or attached / enclosed in 25% oxide, 12% minerals (almost silica or clay free minerals) may be subjected silicate, 36% sulphide or 27% native minerals. Mineralogical to froth flotation in order to recover PGMs [8], [9]. data on UG2 samples was generated by means of MLA technique. The UG2 samples are flotation tailings from the TABLE II Northam Platinum Limited plant operations in South Africa. MINERALOGICAL DATA ON UG2 TAILINGS. Complete recovery of PGMs which are locked in silicates, Minerals PGM Distribution Oxides Enclosed 2% native minerals and chromite matrix is difficult. Therefore, Silicates Association 25% most of the PGMs in flotation tailings are attached or enclosed Attached 21% in silicates [5], [6]. Enclosed 11% Sulphides Association 26% Attached 2% TABLE I Enclosed 2% MINERALOGICAL DATA ON UG2 FEED SOLIDS TO THE CONCENTRATOR. Free or Native Minerals Free 13% Chemical Formula S.G PGMs Oxide Chromite FeO.Cr2O3 4.7 Attached/Enclosed In this study, the KC-CVD was used to treat rougher Minerals (25%) flotation tailings that were pretreated by desliming (removal of Silicate Orthopyroxene (Mg, Fe) 2Si 2O6 3.5 Attached/Enclosed –45 µm fractions) using a spiral classifier. The final tailings Minerals (12%) from the spiral classifier (less heavy minerals) were transferred to the pilot enhanced gravity separator at a flow Plagioclase Na 0.5Ca0.5 Si 3AlO 8 2.7 rate of 1.69 m3/hr. Talc Mg3Si 4O10 (OH)2 2.7 The rationale of the concept of upgrading PGMs in flotation tailings lies in the fact that a decrease in chromite Chlorites X4-6Y4O10(OH,O)8 2.4 minerals (heavier and abundant) will almost invariably lead to an upgrade of PGMs. Chromites are almost not physically or phlogopite KMg AlSi O F(OH) 2.8 3 3 10 chemically associated with PGMs in tailings. Sulphide Pentlandite (Fe, Ni)9S8 4.8 Association In industry, gravity separation units are classified Minerals (3%) according to the average particle size of the feed; fine solids Chalcopyrite CuFeS 2 4.2 Attached/Enclosed (40 - 75 µm) require units that permit settling of particles in a (33%) fluid media under a centrifugal force (Knelson Concentrator, Pyrrhotite Fe(1 - x)S; 4.5 Kelsey, Falcon).