POTENTIAL CHANGES IN THE PHYSICAL BENEFICIATION PROCESSES THAT CAN IMPROVE THE RECOVERY GRADE OR COSTS FOR THE PLATINUM GROUP METALS

Transcription

1 J. Engelbrecht POTENTIAL CHANGES IN THE PHYSICAL BENEFICIATION PROCESSES THAT CAN IMPROVE THE RECOVERY GRADE OR COSTS FOR THE PLATINUM GROUP METALS J. Engelbrecht Multotec (Pty) Ltd Abstract Understanding the deportment of minerals in size fractions has already led to improvements in the flow sheets for platinum minerals beneficiation. There are, however, more potential changes in the flow sheets that may improve the recovery of platinum group metals or reduce the costs of beneficiation before refining. Based on known technology of pre-concentration, milling, gravity concentration, flotation, and magnetic separation a number of potential alterations or additions can be considered and motivated based on the mineral deportment in size fractions. Introduction The concept of using the deportment of minerals at different sizes to assist in the understanding of the behavior of the liberation process and beneficiation responses of the platinum group metal (PGM) minerals is not new. Fine milling of the tailings or the concentrate to increase recovery and/or grade has been successfully implemented at a number of platinum plants. There may, however, be other technologies that can lead to cost reductions or improvements in grade and/or recovery. Costs and revenue Table I shows the value chain of PGM beneficiation and the associated costs. It is clear that mining can contribute substantially from a cost reductions point of view while the liberation or milling and beneficiation processes can contribute with an increase in revenue, particularly if the recoveries can be improved. Cost reduction will also be significant if grades can be improved through preconcentration or better concentration prior to further refining, due to the high energy costs of refining. 343

2 Table I- PGM processing Costs Milling Smelting Base Precious Parameter Mining and and metal metal Total flotation converting refining refining Percent of total cost PGE grade 4 6 g/t g/t g/t 30 6 % >99.8% - PGE recovery (%) Concentration ratio > Lonmin 3 rd Quarter 200 Production Report Table II shows the value of the PGM minerals as well as copper and nickel in various ore types. A one percentage point increase in the recovery of platinum is estimated to result in U$8.6 million additional revenue per annum, and nearly double that value if all the associated minerals are taken into consideration for the South African mines. There is therefore a good financial incentive to improve the recoveries of the PGM minerals. Current operating costs are between US$0 to US$4 per ton of fresh feed, and it is a simple calculation to show that the increase in revenue per percentage recovery varies between US$.7 and US$3 per ton treated, depending on which reef is mined. This implies that a reasonable expenditure is justified in order to improve recovery. 344

3 Table II-Metal values in various ore types Price as on 0 May 202 ($) Merensky Ore UG2 ore Platreef ore g/t $/t of ore mass % g/t $/t of ore mass % g/t $/t of ore mass % Pt Pd Rh Ru Ir Au Total PGM+Au Average grades of the individual precious metals in Merensky, UG2 and Platreef ores and their current potential value 2 Market prices are as on May 202 Price as on 0 May 202 ($) Merensky Ore UG2 ore Platreef ore % in ore $/t of ore mass % % in ore $/t of ore mass % % in ore $/t of ore mass % Ni Cu Total Base Metals Total B Metals + PGM +Au Average content of metals in Merensky, UG2 and Platreef ores and their current potential value. Market prices are as on May

4 Mineralogy Owing to the low concentration and small particle sizes of PGM minerals and the mineralogical complexity of PGM ores, advances in equipment and processes were required before meaningful mineralogical observations could be made. The main characteristics of the three PGM ore types in South Africa are summarized in Table III. Table III-Main characteristics of the Bushveld Complex PGM ore types 3 Characteristics Merensky Reef Platreef UG2 Reef Thickness (m) PGM (g/t) Grade Ni (%) Cu (%) Gangue minerals PGM grain size (μm) PGM minerals PGM analysis 0 80 % pyroxene % plagioclase 3 % chromite 0. % talc % pyroxene 0 20 % plagioclase 3 % chromite 0. 3 % talc % chromite 2 % pyroxene % plagioclase % talc % Cooperite (PtS) + Braggite (Pt,Pd)S 0 30% Kotulskite (PdTe) + Michenerite (PdBiTe) 0 % Ru phases 8% Sperrylite (PtAs 2 ) 3 6% Isoferroplatinum (Pt 3 Fe) 3% Au/Ag phases Moncheite [(Pt,Pd)(Bi,Te) 2 PtTe 2 ] + Merenskylte [(Pd,Pt)(Bi,Te) 2 PdTe 2 ]>>Sperrylite (PtAs 2 )> Isoferroplatinum (Pt 3 Fe)> Braggite (Pt,Pd)S Cooperite (PtS) > Laurite (RuS 2 ) > Braggite (Pt,Pd)S> Malanite (CuPt. lr 0. S4)> Isoferroplatinum (Pt 3 Fe)> Sperrylite (PtAs 2 ) Pt (%) Pd (%) Rh (%) Ru (%) Ir (%) Os (%) Au (%)

5 Table IV gives the mineral associations for the main streams for a UG2 beneficiation process. The major losses of PGM minerals in the tails were through PGM minerals locked in or attached to silica. This problem was addressed through the introduction of the fine grinding of the silicarich fraction through MIG IsaMill TM technology, predominantly by Anglo American Platinum. The second major loss is fine liberated PGM minerals. Table IV- Mineral associations for typical Amandelbult UG2 process samples 4 Association Feed Concentrate Tailings Tailings <0 μm Tailings >0 μm Tailings >3 μm Liberated Enclosed in BMS* Attached to BMS PGM/BMS/Silicate Enclosed in Silicate Attached to Silicate Enclosed in Oxide Attached to Oxide TOTAL Middlings Locked *BMS base metal sulphides Liberation Most of the modern flow sheets for UG2 ores incorporate a multiple liberation / beneficiation principle and are referred to as MF2 or MF3 flow sheets, as shown in Figures and 2. Excellent reviews and summaries of applications are available in the literature. The principle is also used for all sources of PGMs, i.e. Merensky, UG2, and Platreef. 347

6 Figure -MF2 Circuit Figure 2-MF3 circuit 348

7 Figure 3 shows a complex four-stage circuit for a UG2 plant. The pre-concentration stage was most probably successful due to the mining method, whereby excess material had to be mined beyond the reef itself as dictated by the mining method, and economic and safety considerations. Figure 3-Four-stage UG2 processing flow sheet The question is whether there is additional potential for improvement with significant economic benefits. Pre-concentration Sorting technologies are currently under investigation in addition to dense media separation (DMS) to remove barren and/or uneconomic ore as early as possible, and should be implemented if economically justified. Regardless of the process used for pre-concentration, it may be economically attractive to carry out the process underground to reduce the energy costs of hoisting the ore to the surface, and use the reject stream as backfill material. Milling Besides assessing the capital and operating costs of fully autogenous (FAG), semi-autogenous (SAG), ball, and stirred milling, the industry has already begun implementing inter-particle breakage technology (IPBT) through the use of high-pressure grinding roll (HPGR) technology. The major potential advantages of IPBT technology are: ) Reduction in energy consumption 2) Preferential breakage on grain boundaries. 349

8 The preferential breakage on grain boundaries is of particular interest to the platinum industry. Mineralogical investigations (Table IV) of all the sources showed a preference of the PGM minerals to occur on grain boundaries or within base metal sulphides. In the case of the UG2 Reef, the chromite spinel crystals are mostly barren of valuable PGMs and the grain sizes are coarser than 20 μm. If the chromite can be removed at a coarser size, a substantial saving in milling energy can be realized, provided the losses of PGMs are minimized. If the primary breakage is done through IPBT equipment and the secondary stage uses conventional indiscriminate crushing, most of the advantages can be lost. It will be better to continue to use the compression technology and use the advantages as far as possible. The only equipment currently available that can use IPBT to grind fine is the Loesch Mill and Horomill. Both utilize dry processing and can be used to grind down to 80% -20 μm, as used in the cement Industry. IPBT can be used for all three ore types. Platreef ore is hard and tough and is suitable for IPBT. In the case of the UG2 ore, IPBT will most probably be used to liberate the chromite spinel crystals, which can then be removed effectively as a secondary product through a wet process. Such a flow sheet is shown in Figure 4, where the chromite spinel crystals are removed through spiral concentrators. This is not a new technique, and the major criticism in the past was that spiraling led to the loss of PGM minerals to the concentrate, as well as circuit balance problems. This can be addressed in a number of ways: a) Flotation prior to gravity concentration b) Use of thickeners to balance the water circuit c) Use of a special spiral that captures the liberated heavy PGM minerals in a high-grade concentrate which can be returned to the main circuit. 30

9 Figure 4-Flow sheet incorporating fine grinding and spirals for chromite removal Milling and classification The effect of classification efficiency on mill capacity and the particle size distribution of the product is well documented 6. The mineral industry in general was slow to adapt this principle, but it has been successfully applied in the industrial minerals sector 7. In simple terms, the advantages of efficient classification and circulating load are based on the principle of break and remove as soon as possible. This implies that both the residence time in the mill, or more correctly - the residence time distribution and the efficiency of the classifier, will determine the overall effect. Figure summarizes the effect of classification efficiency and mill capacity (constant P 80 ) as a function of circulating load. The potential increase in mill capacity is approximately 20% if one compares current operating conditions with potential operating conditions. Figure 6 shows the effect on the particle size distribution for gold ore 8. 3

10 Cumulative % Passing % % 0% CL CL % C 0 L L CL R C RO Cla 0 SIN ass 00 N - sifi RA ca AM tio ML on e LER eff R fici en 000 ncy 0 y = 0 00% %

11 Hydrocyclones have been the preferred classifier in the mineral industry, but the hydraulic effect may be a disadvantage for the process. A good example is in the processing of UG2 ore, where the over-grinding of the chromite spinel crystals is not productive but will be the result of the hydraulic effect in the hydrocyclone, whereby the heavier particles will be classified finer. The disadvantage of the hydraulic effect can be overcome by a physical classifier such as the vibrating screen. Unfortunately, the screening area required and the associated capital and running cost has been an implementation barrier, as was the case with the micro-screens at the Crocodile River concentrator. This can be addressed by a combination of a hydraulic and a physical classifier as shown in Figure 7. Instead of screening the complete circulating load, only the cyclone overflow is screened to produce the correct particle-size distribution for the lighter fractions. Figure 7-Combination of physical and hydraulic classification Flotation Flotation is the preferred beneficiation process for PGM minerals. Flotation is a complex process, and the recovery of a specific mineral will depend mainly on the following characteristics: Hydrophobicity Particle size Degree of liberation. 33

12 Table IV shows that the losses of PGMs to the tailings for UG2 ores are due mainly to locked particles in silicates and liberated PGM particles. Losses in locked particles have been addressed by finer grinding. The detail of the liberated PGM minerals shows that the major loss occurs in the -0 μm fraction (+80%) 4. The mineralogical analyses also confirmed that the PGM minerals are predominantly in the 2 6 μm fraction. It is well known that the flotation rate of fine particles is slow, based on particle-bubble collision models. An excellent review has been published by Dai et al. 9. Experimental results and the model developed by Dukhin et al. 0 showed clearly that fine particles do float and that smaller bubbles increase the rate of flotation. In order to improve the recovery and/or the rate of flotation, the effects of bubble size and the intensity of shear forces have to be considered, as shown in Figure 8 Newcombe et al.. Detachment Impeller speed Detachment No lift Coarse Sedimentation Low rec No lift Fine Low rec Sedimentation Bubble size Figure 8-The effect of turbulence and bubble size on the particle size in flotation 34

13 In order to float fine particles, a different reagent mix may be required because the mechanism of adhesion and the surface area are different compared to coarse particles. Similarly, the flotation cell conditions will be different, with lower-intensity agitation required for the recovery of fine particles, which is not conducive for coarse particles. Although a split flotation circuit was tried and abandoned (), the concept of fine particle flotation after coarse flotation or after classification may prove to be a viable economic option. The advantage of froth washing to improve the grade in column flotation has been proven and has also been introduced in conventional flotation cells. This is certainly a proven technique to improve the grade of the concentrate in flotation by removing entrained and entrapped particles, and should be used. Preferential shedding occurs whereby fast-floating particles crowd slow-floating gangue particles off the surfaces of bubbles, and can be used to recover high-grade froth products if applicable. Preferential shedding and froth washing can therefore be used to recover higher grade products and limit contamination by entrained and entrapped gangue particles, as well as slow floating-gangue components in the froth. Magnetic concentration Pyrrhotite and pentlandite, as well as certain PGM minerals, are magnetic 2,3 and equipment is available to recover the fine platinum minerals. Pyrrhotite is normally slow floating and can account for significant losses in the final tails if insufficient residence time is available in flotation. High-intensity wet magnetic separation techniques like WHIMS and SLON can be used to recover some of the magnetic minerals lost to the tailings. Typical recovery-size relationship for hematite is shown in Figure 9 4. This implies that both the very fine and the oversize magnetic particles that were not recovered through the flotation process may be recovered by magnetic separation. 3

14 00 90 Recovery of Hematite Particle size micron Figure 9-Recovery as a function of size for hematite with WHIMS Gravity concentration Batch centrifugal gravity concentrators have been developed for gold and were followed by continuous machines like the Knelson and Falcon concentrators. Sepro Mineral Systems Corp has developed an ultrafine (UF) separator, and the results for a tin recovery application are given in Figure 0. The recoveries for heavier PGM minerals should be higher and this could be an attractive option for the small liberated PGM minerals. According to the manufacturer the removal of coarse particles and dilute feed conditions before treatment are essential. 36

15 Recovery of Sn Particle size Figure 0-Recovery of Sn as a function of size Conclusion Despite the fact that advanced flow sheets are already used in the PGM beneficiation process, there seem to be additional technologies that can further improve the recoveries or grades of PGM mineral products or can reduce costs. The application of a specific technology will depend on the economic viability of the process. A summary is given in Table V of different concepts that can potentially be applied in the beneficiation of PGM minerals for different ore types. Table V Merensky Ore UG2 Ore Platreef Ore Pre-concentration Classification efficiency in milling Inter-particle breakage Fine particle flotation Magnetic separation Gravity concentration Fine gravity concentration 37

16 References. Rule, C.M. and Plint, N. What will the typical PGM concentrator look like? Keynote address: 8th International Comminution Symposium (Comminution 202), Vineyard Hotel, Cape Town, 6 20 April Von Gruenewaldt. The mineral resources of the Bushveld Complex. Mineral Science and Engineering, vol. 9, no. 2, Apr pp Newell, A.J.H. The processing of platinum group metals (PGM). Pincock, Allen & Holt Perspectives, no 89, Mar Rule, C., and De Waal, H. IsaMill TM Design improvements and operational performance at Anglo Platinum. Metplant 20. Plant Design & Operating Strategies World s Best Practice, Perth, Western Australia, 8 9 August 20. Australasian Institute of Mining and Metallurgy, Carlton, Australia. Pp Penberthy, C.J. The effect of mineralogical variation in the UG2 chromitite on recovery of platinum-group elements. PhD thesis, University of Pretoria. December Engelbrecht, J.A. The effect of classification efficiency on closed circuit grinding. The role of the Practical Metallurgist Symposium, 989. Mine Metallurgical Managers Association, South Africa. pp Roettle, J. Improving grinding performance with high efficiency classification. 8th International Comminution Symposium (Comminution 202), Vineyard Hotel, Cape Town, 6 20 April Guest, R.N. A laboratory investigation of the importance of the circulating load in the control of particle size distribution. SA Mechanical Engineer, Feb pp Dai, Z., Fornasiero, D., and Ralston, J. Particle-bubble collision models a review. Advances in Colloid and Interface Science, vol. 8, pp Dukhin, S.S., Kretzschmar, G., and Miller, R. Dynamics of Adsorption of Liquid Interface. Ch. 0. Elsevier, Amsterdam, 99.. Newcombe, B., Bradshaw, D., and Wightman, E. Flash flotation and the plight of the coarse particle. Minerals Engineering, vol. 34, Jul pp

17 2. Vermaak, M.K.G. Fundamentals of the flotation behavior of palladium bismuth tellurides. PhD thesis Faculty of Engineering, Built Environment and Information Technology, University of Pretoria. May De Villiers, J. The composition and crystal structures of pyrrhotite: A common but poorly understood mineral. Proceedings of Mintek 7, Randburg, South Africa, 4 June Forssberg, K.S.E. and Kostkevicius, N.R. Comparative pilot scale tests with wet high intensity magnetic separators. Erzmetal, vol. 3, no. 6, 982. pp McAlistar S. Private communication 202. The Author Johan Engelbrecht, Director International Business Division, Multotec (Pty) Ltd Johan Engelbrecht is currently the Director for the International Business Division for the Multotec Group of companies. He, as a qualified Metallurgical Engineer, was responsible for the formation of Multotec Process Equipment (Pty) Ltd, which has become well known worldwide for cyclones, spirals, samplers and magnetic separators. Multotec is regarded as a world leader in dense medium cyclone separation as well as spirals and sampling in the Coal Industry. It is therefore not surprising that Multotec Process Equipment received the award for leader in R & D in the Technology Top 00 awards in 2006 and the Group of Companies the Exporter of the Year for Gauteng award in Johan was Coalman of the year in 998 and has been awarded lifetime membership by the South African Coal Processing Society. He is well known worldwide and has presented numerous papers at various conferences. 39

18 360