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 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 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 ) 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 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 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, , beneficiation are discussed further in this study. Even though the feed solids to the KC-CVD 6 gravity separator were deslimed, its capacity , 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 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 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). These units have capacities up to 23 000 x = 0 - 0.17 ton/day (Knelson CVD) and can take a closely sized feed.

Pyrite FeS 2 5.0 Shaking tables such as Mozley Multi Gravity Separators (MGS), with high throughput capacities (45 ton/day) could Millerite NiS 5.5 handle fine particles down to 1 µm from a closely sized feed. However, mineral jigs and spirals are heavy duty equipment Free or PGMs Free (27%) Native which can handle feed solids with wide particle size Minerals distribution but are generally better for large particle sizes N.B.: X = Mn, Mg, Li, Fe, Ni, Zn (>100 µm for spirals and > 1 mm for mineral jigs) [10], [11], Y = Al, Si [12]. The concentration criterion (Cc) is a mathematical expression which predicts the separation of minerals by

47 2nd International Conference on Trends in Industrial and Mechanical Engineering (ICTIME'2013) Sept 17-18, 2013 Hong Kong gravity taking into account the density (Á) of heavy and light were subjected to treatment by spirals in order to recover minerals which are settling at different rates in a fluid of a chromium. Chromium assay of the rougher flotation tailings known density. The value of Cc should be equal or greater varied between 25-29%. After gravity separation, a than 2.5 for easier separation irrespective of the sign [13]. concentrate with about 40% chromium was produced. The tailings from the spiral classifier were discarded. = ±2.5 (1) 휌ℎ푒푎푣푦−휌푓푙푢푖푑

푐 휌푙푖푔ℎ푡−휌푓푙푢푖푑 where,퐶 Á is the density≥ of the fluid or minerals (kg/m3). The specific gravities of minerals are shown in Table I. In this study, 30% of the total feed solids were above 106 µm. The solids in the fine fraction (-45 µm) accounted for 43%. The middle size ranges (-106 +75µm and -75 +45µm) contained about 14% and 13% of total solids, respectively. The PGMs distribution and particle size distribution (PDS) of feed solids are shown in Table II and III, respectively. The objective of the present study was to investigate and establish the extent to which PGM assays in UG2 flotation tailings can be upgraded using gravity separation methods (spiral classifier) and enhanced gravity separation process (Knelson concentrator continuous variable-discharge – KC- CVD). The grade of upgraded PGM tailings should make the PGM tailings amenable to effective leaching processes.

II. EXPERIMENTAL PROCEDURE Fig. 1 Schematic diagram of heavy metal spiral classifier test rig. A. Materials Solid samples of UG2 tailings were drawn from the rougher The operating variables such fluidization water flow rate, flotation circuit. The feed was used without further grinding. and G-force were selected using Knelson proprietary Table III shows the results of assay to particle size distribution “Independent Control System (ICS)”. A series of on-site test (PSD) in the feed sample. runs were conducted over a period of one and half months in batch mode for an 8-hour shift. It is worth noting here that the TABLE III four (4) test runs presented in this study were done at constant PSD OF FEED SOLIDS TO LABORATORY SPIRAL AND PGMS operating conditions (Table IV). Furthermore, the test runs DISTRIBUTION. Size fraction (µm) Wt. %, solids PGMs Distribution (%) were not performed consecutively due to the shutdown of the +106 33 30 plant to allow for the maintenance of equipment to be carried -106 +75 15 14 out. The percent recoveries were calculated using the two- -75 +45 17 13 product formula and the average percent recoveries are -45* 35 43 presented in this study for the period of one and half months. *The finer solids were not deslimed. B. Laboratory Apparatus: Spiral Classifier Test Rig TABLE IV OPERATING PARAMETERS FOR THE FOUR (4) KC - CVD TESTS. Figure 1 shows the experimental set up of a spiral classifier Feed capacity (tons/hr.) 1.0 in the laboratory. The flow rate (1.69 m3/hr.) and feed Fluidization water flow rate (m3/hr.) 2.7 densities (10–45% solids) were optimized. The feed size Feed density (% solids/weight) 60 G - Force (G) 30 (Table III) remained constant. In this set up, there was a Sampling time (s) 5.0 provision to recycle the tailings and middlings back to the spiral classifier. In a KC-CVD process, the heavier particles settle out of C. Pilot Scale: Spiral Classifier and Knelson Concentrator the fluid and report to the cone ring(s) of the device from (KC – CVD) where they are discharged continuously into a concentrate The purpose of the Knelson pilot tests was to recover launder. The finer particles (tailings) flow out to the top of the PGMs from spiral’s cleaner tailings and upgrade them. The cone into the tailings launder. The KC-CVD specifications are spiral tailings were treated as received from Northam Chrome shown in Table V. The particle size distribution (PSD) of the Producers (NCP) plant at Northam Platinum Ltd., South spiral classifier tailings which were the feed solids are given in Africa. Figure 2 shows the connection between a Knelson Table VI. concentrator and spiral classifier. The spiral’s cleaner tailings D. Sample analysis were tapped off using a 4 inch pipe and routed to the Knelson Samples of different masses for the concentrate and tailing device at a flow rate of 1 ton/hr. Initially, the rougher tailings solids were collected. The samples were filtered and then (Table III) from the rougher banks were deslimed and the dried at 50 ºC for 3 hours. All test runs were repeated in slimes were discarded. Subsequently, the deslimed fine solids triplicates. Solid samples (Pt, Pd, Rh, Cr and Fe) were

48 2nd International Conference on Trends in Industrial and Mechanical Engineering (ICTIME'2013) Sept 17-18, 2013 Hong Kong analyzed by Northam Platinum Ltd. and SGS Lakefield averaged 0.75 g/t, with a platinum fraction of 62%. Chromium Research Africa (Pty) Ltd., Johannesburg, South Africa using and iron assays averaged 15.18 and 19.2% in the feed. a combination of analytical procedures: Fire Assay (FA), About 10 kg of the PGM tailings were then subjected to inductively coupled plasma (ICP-AES and ICP-OES,) and X- spiral separation. After a few test runs, the optimum feed ray fluorescence (XRF). density was found to be 30% solids at a flow rate of 1.69 3 m /hr. The assays of PGMs in the spiral tailings (lighter TABLE V fraction) and concentrates (heavier fraction) are shown in KC-CVD specifications. Tables VIII and IX. But, the final partition of solids and PGMs Feed capacity (tons/hr. solids) 0.5 - 2.0 are shown in Table X. Fluidization water flow rate (m3/hr.) 1.1 2.7 Feed density (% solids/weight) 0 - 75 According to Table VIII, the PGMs were upgraded from Maximum total volumetric throughput (m3/hr.) 4.0 0.75 to 1.07 g/t which represent an upgrade ratio of 1.4. The Feed size (mm) (Recommended) 1.7 recovery of PGMs to tailings was about 71% according to the Concentrate volume (% of feed rate) 1 - 50 1 - 50 (Variable) minerals distribution data (Table X). This result was achieved Concentrate weight (% of feed rate) 1 - 50 1 - 50 (Variable) Concentrator net weight (kg) 230 by lowering the grade of chromium in the feed by about 5% Motor Horse Power (HP) 1.5 which implied that there was great potential to beneficiate the Independent Control System (ICS) Standard with machine tailings further. About 29% of PGMs reported to the Variable Gravity Variable G-Force concentrate or heavier mineral fraction due to inclusions in metal sulphides which implies that PGMs should be liberated further by milling. TABLE VIII UG2 SPIRAL TAILINGS (LIGHTER FRACTION). +106 -106 +75 -75 +45 -45 Composite Pt (g/t) 0.95 0.38 0.57 0.42 0.60 Pd (g/t) 0.40 0.27 0.40 0.33 0.36 Rh (g/t) 0.18 0.12 0.18 0.16 0.11 Total PGMs (g/t) 1.53 0.77 1.15 0.91 1.07 % Fe 16.75 17.40 17.85 17.22 18.27 % Cr 13.82 13.96 13.56 13.46 14.40 Solids (g) 650 1290 2415 645 5000

TABLE IX UG2 SPIRAL CONCENTRATE (HEAVIER FRACTION). Fig. 2 Schematic diagram showing the connection of the Knelson +106 -106 +75 -75 +45 -45 Composite concentrator- KC-CVD (1 ton/hr.) to spiral classifiers at Northam Pt (g/t) 0.47 0.21 0.19 0.27 0.41 Platinum Ltd [14]. Pd (g/t) 0.13 0.12 0.11 0.15 0.16 Rh (g/t) 0.09 0.11 0.08 0.09 0.13 Total PGMs (g/t) 0.69 0.44 0.38 0.51 0.70 TABLE VI PSD OF SPIRAL TAILINGS WHICH WERE THE FEED SOLIDS TO KC-CVD. % Fe 15.73 16.43 17.36 17.21 17.32 % Cr 21.21 21.39 20.81 21.78 21.52 Size Fraction % Wt. % Cum Wt. Solids (g) 658 1235 2318 789 5000 +212 2.2 2.2 -212 +150 13.3 15.5 B. Knelson concentrator continuous variable-discharge -150 +75 57.1 72.6 -75 +53 20.3 92.9 (KC-CVD -53 +45 3.7 96.6 The average feed rate was 1 t/hr. (0.15 kg/s) and the -45* 3.4 100 sampling frequency was every 5 seconds. Since the G-Force, *The finer solids were deslimed. fluidization fluid and feed rate were constant, composite samples of concentrates and tailings were created for each test III. RESULTS AND DISCUSSION run. Table XI shows the assays of solids in the feed, A. Laboratory Spiral Classifier Test Rig concentrates and tailings of the KC-CVD. The feed to the spiral classifier was sampled and PGMs in The result obtained in the first run of KC-CVD tests showed each size fraction was determined (Table VII). that 71% of PGMs can be recovered to 45% of solids (Table XI). In this case, the PGMs were upgraded from 0.70g/t in the TABLE VII feed to 1.11g/t in the tailings. This represents an upgrade ratio UG2 FEED TO THE SPIRAL. of 1.6 which is slightly higher than the value obtained in spiral +106 -106+75 -75+45 -45 Composite classifier (1.4). Pt (g/t) 0.93 0.38 0.31 0.70 0.47 Even though the feed solids to the KC-CVD device were Pd (g/t) 0.23 0.18 0.21 0.31 0.19 Rh (g/t) 0.09 0.06 0.08 0.09 0.09 deslimed (removal of the -45 µm fraction), the upgrade and Total PGMs (g/t) 1.25 0.62 0.60 1.10 0.75 recoveries achieved compared very well with similar results % Fe 15.00 16.57 16.65 16.05 15.18 from a laboratory spiral classifier. The G-force (gravitational % Cr 18.42 18.61 18.10 17.94 19.20 force) was not varied beyond 30. Therefore, further tests are Solids (g) 1298 2535 4863 1304 10000 required to study the effect of the G-Force (30 – 120) and

fluidization rate of water in the KC-CVD. The total PGMs in rougher tailings over a two-week period of sampling from the Northam Platinum Ltd., South Africa

49 2nd International Conference on Trends in Industrial and Mechanical Engineering (ICTIME'2013) Sept 17-18, 2013 Hong Kong

IV. CONCLUSIONS successfully to beneficiate the flotation tailings which were Even though the feed solids to the KC-CVD gravity first pre-treated by spiral classifiers. About 71% of PGMs separator were deslimed, its capacity to upgrade PGMs from were recovered from flotation tailings by using spiral 0.7g/t to 1.11 g/t at an average recovery of 71% compared classifiers and a further 70% of PGMs was recovered from the well with the results from a spiral classifier where the fines spiral classifier tailings using KC-CVD. Total recovery of remained part of the feed. The KC-CVD was applied PGMs to the lighter minerals or silicates was 75% at an upgrade ratio of 1.8.

APPENDIX TABLE X UG2 SPIRAL CONCENTRATE (HEAVIER FRACTION). Element(s) MASS DISTRIBUTION PERCENT DISTRIBUTION Feed (g) Tailings (g) Conc. (g) Feed (%) Tailings (%) Conc. (%) Pt (g/t) 0.00460 0.00300 0.00160 100.0 65.2 34.8 Pd (g/t) 0.00215 0.00180 0.00035 100.0 83.7 16.3 Rh (g/t) 0.00080 0.00055 0.00025 100.0 68.8 31.3 Total PGMs (g/t) 0.00755 0.00535 0.00220 100.0 70.9 29.1 % Fe 1819.5 913.5 906.0 100.0 50.2 49.8 % Cr 1930.0 720.0 1210.0 100.0 37.3 62.7 Solids (g) 10000.0 5000.0 5000.0 100.0 50.0 50.0

TABLE XI KC-CVD TEST RESULTS: PGM ASSAYS OF FEED, CONCENTRATES AND TAILINGS. SOLIDS AND PGMS DISTRIBUTION.

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