International Journal of Minerals, Metallurgy and Materials Volume 16, Number 5, October 2009, Page 494 Minerals Occurrence mechanism of silicate and aluminosilicate minerals in Sarcheshmeh copper flotation concentrate H.R. Barkhordari1), E. Jorjani1), A. Eslami2), and M. Noaparast3) 1) Mining Engineering Department, Science and Research Branch, Islamic Azad University, Poonak, Hesarak, Tehran, Iran 2) R&D Division––Sarcheshmeh Copper Complex, Rafsanjan, Iran 3) School of Mining Engineering, University College of Engineering, University of Tehran, Tehran, Iran (Received 2008-10-21) Abstract: The Sarcheshmeh copper flotation circuit is producing 5u104 t copper concentrate per month with an averaging grade of 28% Cu in rougher, cleaner and recleaner stages. In recent years, with the increase in the open pit depth, the content of aluminosilicate minerals increased in plant feed and subsequently in flotation concentrate. It can motivate some problems, such as unwanted consumption of reagents, decreasing of the copper concentrate grade, increasing of Al2O3 and SiO2 in the copper concentrate, and needing a higher temperature in the smelting process. The evaluation of the composite samples related to the most critical working period of the plant shows that quartz, illite, biotite, chlorite, orthoclase, albeit, muscovite, and kaolinite are the major Al2O3 and SiO2 bearing minerals that accompany chalcopyrite, chalcocite, and covellite minerals in the plant feed. The severe alteration to clay minerals was a general rule in all thin sections that were prepared from the plant feed. Sieve analysis of the flotation concentrate shows that Al2O3 and SiO2 bearing minerals in the flotation concentrate can be decreased by promoting the size reduction from 53 to 38 Pm. Interlocking of the Al2O3 and SiO2 bearing minerals with chalcopyrite and chalcocite is the occurrence mechanism of silicate and aluminosilicate minerals in the flotation concentrate. The dispersed form of interlocking is predominant. Key words: copper; aluminosilicate; silicate; flotation 1. Introduction Copper is a non-ferrous base metal with about 50 ppm average concentration in the earth’s crust [1]. Because of its properties such as thermal and electrical conductivity, high ductility, and malleability, copper has become a major industrial metal, ranking third after iron and aluminum in terms of quantities consumed [2]. Common copper minerals found in economic deposits are native copper (Cu), chalcocite (Cu2S), cuprite (Cu2O), covellite (CuS), bornite (Cu5FeS4), malachite (Cu2CO3(OH)2), azurite (2CuCO3˜Cu(OH)2), antlerite (Cu3SO4(OH)4), enargite (Cu3AsS4), chrysocolla (CuSiO3˜2H2O), and chalcopyrite (CuFeS2) [1, 3-5]. Porphyry copper deposits are the world’s main source of copper, molybdenum, rhenium, and significant sources of gold, silver, and tin and a range of by-product metals [1]. Sarcheshmeh is a major porphyry copper deposit, Corresponding author: E. Jorjani, E-mail: esjorjani@yahoo.com © 2009 University of Science and Technology Beijing. All rights reserved. which is located in Kerman Province in the southeastern part of Iran. Geologically, it divides into three zones: oxide, supergene, and hypogene [6]. The oval-shaped ore body has a dimension of 2000 mu900 m and centered on the Sarcheshmeh porphyry stock. Within the mentioned area, and according to the drilling depth of about 150 m, the ore body contains 450 million tones, averaging 1.13% Cu and 0.03% Mo at a cutoff grade of 0.4% Cu [7]. The hypogene zone that is being currently exploited consists of potassic alteration zone with potassium feldspar, secondary biotite, sericite, chlorite, and epidote minerals. In this zone hornblende, biotite, and plagioclase altered to amphibole, magnetite, and sericite, respectively [8]. The clay minerals resulted from alteration along with the other aluminosilicate and copper sulfide minerals are the major mineral phases that are fed to a copper concentrator plant [8]. In the concentrator plant, after three stages of crushing, the ore feeds to ball mills in a closed circuit with cyclones to produce 70% of the product finer Also available online at www.sciencedirect.com H.R. Barkhordari et al., Occurrence mechanism of silicate and aluminosilicate minerals in… than 75 μm. The concentrate of the first stage of flotation reground and the tailings are discarded to the final tails. The cleaning and re-cleaning stages produce copper concentrate (Fig. 1). The collectors of Nascol 1451 (dithiophosphate+mercaptobenzothiazole), Z11 (sodium isopropyl xanthate), frothers of Dow 250 (polypropylene glycol methyl ether), and methyl isobutyl carbonyl (MIBC) are the reagents used in the flotation circuit. To separate and recover the molybdenite mineral from the copper concentrate, copper and iron sulfide minerals are depressed and the molybdenite floats. The molybdenite concentrate with the grade between 53% and 55% Mo and the copper sulfide concentrate with the grade of 31% Cu (chalcopyrite, chalcocite, and covellite) and recovery of 83%-87% are obtained as final concentrates, depending on the ore type and operating conditions [9-10]. In recent months, the presence of aluminosilicate minerals in the plant feed has resulted in several problems; these problems can be categorized as follows [11]: x Increasing of the collector and frother consumption in the process; x Instability of the forth zone; x Increasing of fine particles in crushing and grinding circuits; Fig. 2. 495 x Flocculation phenomena in the froth zone; x Decreasing of the copper grade in the flotation concentrate; x Increasing of Al2O3 and SiO2 in the copper concentrate; subsequently, the smelting should be performed at a higher temperature and the slag discharging is more difficult. Fig. 1. plant. Flotation circuit of the Sarcheshmeh concentrator The sum of Al2O3 and SiO2 in the copper concentrate should be limited to 6% [12]. The evaluation of the Sarcheshmeh copper concentrate indicated that in recent months the sum of Al2O3 and SiO2 is higher than the permitted limit (Fig. 2) based on the monthly composite samples. To solve the mentioned problems, the characterization of silicate and aluminosilicate minerals and the mechanism of the presence in flotation concentrate are necessary, which is the subject of the current work. Variations of Al2O3 and SiO2 in the copper concentrate from Mar 20, 2007 to Mar 20, 2008. 2. Experimental results 2.1. Sampling The sampling and characterization studies were performed on the feed, final concentrate, and the tail of the plant in the most critical period, Jan 20 to Feb 20 2008, as shown in Fig. 2. The X-ray diffraction analysis, thin and polished sections, and sieve analysis were used to perform the characterization studies. 2.2. X-ray diffraction and florescence studies The Philips (PW17C) diffraction and Philips (PW1480) florescence units were used to identify the mineral constituents and chemical composition of the samples, respectively. 2.3. Microscopic studies: thin and polished sections The thin section studies were used to determine the grain size, shape, mineral intergrowth, and alteration of nonmetallic minerals. The polished sections were prepared by mixing 1 g of the samples with 10 g of the molding powder. The mixture was placed in the sample making equipment under a pressure of 4200 Pa at 140qC for 20 min. The microscope Leica (DM-LP) 496 International Journal of Minerals, Metallurgy and Materials, Vol.16, No.5, Oct 2009 model was used to study the sections. 3. Results and discussion 3(c)), plagioclase with an intensive alteration to sericite (Fig. 3(d)), the quartz particle with aluminosilicates inclusion (Fig. 3(e)), and the quartz-sericite alteration around quartz particles (Fig. 3(f)) are shown in the figure. The presence of altered minerals as sericite and chlorite, which are the indexes of the clay mineral generation, can be seen in the mentioned sections; it can increase the possibility of the clay mineral occurrence in the flotation concentrate. 3.1. X-ray analysis 3.3. Sieve analysis of the flotation concentrate The XRD and major element analyses for the feed, concentrate, and tail are shown in Table 1. The results show that quartz, illite, biotite, and muscovite are Al2O3 and SiO2 bearing minerals in the flotation concentrate. 3.2. Feed thin section studies The size distribution of the concentrate was determined in size classes of +53, 53+45, 45+38, and 38 —m. The analysis of Cu, Mo, Fe, Al2O3, and SiO2 was determined in the mentioned fractions. The results are shown in Table 2 and Fig. 4. 2.4. Size distribution studies Size distribution studies were performed to determine the aluminosilicate mineral distribution and contribution in different size fractions of the flotation concentrate. The thin sections from the flotation circuit feed and the corresponding samples in mining stops were studied by optical microscopy (Fig. 3). The intensive alteration is a general rule in all sections. The hornblende mineral, which alters to amphibole (Fig. 3(a)), the hornblende mineral, which is going to alter to amphibole (Fig. 3(b)), unaltered plagioclase (Fig. Table. 1. Mineralogical composition and chemical analysis of the feed, final concentrate, and tail of the flotation circuit Product Feed Concentrate Tail It can be seen that the copper grade decreases, the Al2O3 and SiO2 grades increase with the size of the particles presented in the flotation concentrate increasing. It can be concluded that the interlocked aluminosilicate minerals with copper associated minerals can be decreased with a further size reduction from 53 to 38 Pm. Chemical composition / wt% Mineralogical composition Quartz, illite, biotite, chlorite, orthoclase, albeit, muscovite, pyrite, chalcopyrite, and chalcocite Chalcopyrite, chalcocite, pyrite, molybdenite, galena, quartz, illite, biotite, and muscovite Quartz, illite, biotite, chlorite, orthoclase, albeit, pyrite, and muscovite 3.4. Polished section studies of the flotation concentrate The polished section photomicrographs, which were prepared from the flotation concentrate, are shown in Fig. 5. The interlocked chalcopyrite with gangue minerals (Fig. 5(a)), the pyrite and chalcopyrite simple interlocking with gangue minerals (Fig. 5(b)), chalcopyrite vein interlocking and pyrite dispersed interlocking with gangue minerals (Fig. 5(c)), and interlocking of pyrite, chalcocite, and chalcopyrite also chalcopyrite with gangue minerals (Fig. 5(d)) are shown in the figure. It can be concluded that interlocking of chalcopyrite, pyrite, and chalcocite with gangue minerals, also the attachment of copper bearing minerals to pyrite (with gangue inclusion), can be a reason of the presence of aluminosilicates in the flotation concentrate. 3.5. Copper bearing mineral liberation studies The common point counting method using a polar- Cu Mo Al2O3 SiO2 0.78 0.020 15.21 63.80 26.60 0.840 2.49 6.15 0.09 0.008 15.85 64.23 ized microscope was used to determine the liberation degree, which defines as a ratio of free valuable particles to the total valuable particles [13-14]. Fig. 6 presents different types of interlocked minerals. To obtain the liberation degree of copper bearing minerals in the flotation concentrate, the line scanning was used. The total length of free valuable particles was divided by the total length of free valuable particles as well as interlocked with gangue minerals. The results are listed in Table 3. The results are evident that the liberation degrees of chalcopyrite, covellite, and chalcocite increase from 60.27% to 98.25%, 80% to 95.83%, and 82.44% to 99.17%, respectively, with the fraction sizes decreasing from +53 to 38 —m. It should be noticed that the dispersed interlocking is a predominant form, which liberating is very difficult and causes fine particles in the flotation circuit. H.R. Barkhordari et al., Occurrence mechanism of silicate and aluminosilicate minerals in… 497 Fig. 3. Photomicrographs of thin sections: (a) hornblende mineral altered to amphibole; (b) hornblende mineral that is going to alter to amphibole; (c) unaltered plagioclase; (d) plagioclase with an intensive alteration to sericite; (e) quartz particle with aluminosilicate inclusion; (f) quartz-sericite alteration around quartz particles. Table 2. Sieve analysis of the flotation concentrate Size fractions / —m Retained on screen / wt% Cu / wt% Mo / wt% Fe / wt% Al2O3 / wt% SiO2 / wt% +53 53+45 45+38 38 Total 15.82 10.38 5.98 67.82 100 19.89 23.56 24.77 29.27 26.92 0.486 0.914 1.22 0.865 0.83 24.25 25.22 25.43 25.73 25.42 4.56 2.92 2.19 1.69 2.23 11.45 7.19 6.14 4.15 5.74 4. Effect of feed size reduction on the cleaner circuit Fig. 4. Variations of Cu, Al2O3, and SiO2 in different size fractions of the concentrate. To evaluate the effects of feed size reduction on the cleaner circuit of Sarcheshmeh Plant, a sample was prepared from the overflow of cyclone in the regrinding circuit (Fig. 1). The sample was floated in a laboratory scale flotation cell for 5 min without any further reagent and in three different sizes: (a) without any 498 International Journal of Minerals, Metallurgy and Materials, Vol.16, No.5, Oct 2009 further size reduction; (b) reground in the ball mill for 4 min; (c) reground in the ball mill for 8 min. The size characteristics of the mentioned samples and the cleaner flotation results are shown in Tables 4 and 5, respectively. Fig. 5. Photomicrographs of the polished sections: (a) interlocked chalcopyrite with gangue minerals; (b) pyrite and chalcopyrite simple interlocking with gangue minerals; (c) chalcopyrite vein interlocking and pyrite dispersed interlocking with gangue minerals; (d) interlocking of pyrite with chalcocite and chalcopyrite as well as chalcopyrite with gangue minerals. The results presented in Table 5 show that further size reduction from 83% 44 —m to 89.9% 44 —m can decrease SiO2 and Al2O3, presented to the cleaner concentrate, from 7.56% and 3.14% to 4.91% and 1.83%, respectively. The mentioned condition improves the copper grade in the concentrate from 26.6% to 27.6%. Fig. 6. Different types of interlocked minerals [15]. Table 3. Liberation degree and interlocking types of copper minerals in the flotation concentrate Minerals Chalcopyrite Covellite Chalcocite Size fraction / —m 38 +3845 +4553 +53 38 +3845 +4553 +53 38 +3845 +4553 +53 Liberation degree / % Interlocking type (with gangue) 98.25 94.03 82.01 60.27 Dispersed Dispersed Dispersed and central Dispersed, central and Simple 95.83 92.07 83.32 80 Dispersed Dispersed and central Dispersed and central Dispersed, central and Simple 99.17 93.85 85.29 82.44 Dispersed Dispersed Dispersed and central Central and simple H.R. Barkhordari et al., Occurrence mechanism of silicate and aluminosilicate minerals in… Table 4. Portion of 44 μm in the samples 499 % Without size reduction Reground for 4 min Reground for 8 min 83 84.5 89.8 Table 5. Composition of the cleaner flotation concentrate wt% Sample Cu SiO2 Al2O3 Mo Without size reduction Reground for 4 min Reground for 8 min 26.6 27.2 27.6 7.56 6.34 4.91 3.14 3.05 1.83 0.86 0.99 0.89 5. Conclusions (1) Jan 20, 2008 to Feb 20, 2008 was the most critical period for the occurrence of Al2O3 and SiO2 in the flotation concentrate; therefore, the characterization studies were performed on the related samples. (2) XRD studies show that quartz, illite, biotite, chlorite, orthoclase, albeit, and muscovite are the main Al2O3 and SiO2 bearing minerals in the plant feed from which quartz, illite, biotite, and muscovite present in the subsequent concentrate. (3) With the increase in size fraction presented in the flotation concentrate, the copper grade decreased and the Al2O3 and SiO2 grades increased. As a conclusion, interlocking of the Al2O3 and SiO2 bearing minerals with chalcopyrite and chalcocite could be the occurrence mechanism; further size reduction can improve the problem. (4) The polished section studies show that chalcopyrite and chalcocite have interlocking with pyrite. Gangue inclusions can be seen clearly in the chalcopyrite and pyrite particles present in the flotation concentrate. (5) The liberation studies of different size fractions in the flotation concentrate show that the liberation degrees of chalcopyrite, covellite, and chalcocite increase with decreasing the particle sizes from +53 to 38 —m. (6) The dispersed is the predominant form of minerals interlocking in the flotation concentrate and the liberating is very difficult. (7) Cleaner flotation tests on the sample were prepared from the overflow of the regrinding circuit of Sarcheshmeh Plant, showing that the size reduction to 89.9% 44 —m can decrease SiO2 and Al2O3, presented to the cleaner concentrate, from 7.56% and 3.14% to 4.91% and 1.83%, respectively. The re- cleaner circuit of Sarcheshmeh Plant could be the guarantee for the final criteria of SiO2 and Al2O3. References [1] British Geological Survey, Copper: Definition, Mineralogy and Deposits, 2007, p.1.http://www.mineralsuk.com. [2] USGS Minerals Information, Copper Statistics and Information, 2008, p.1. http://minerals.usgs.gov/minerals/ pubs/commodity/copper. [3] Mineral Information Institute, Copper, 2008, p.1. http:// www. mii.org/Minerals/photocopper.html. [4] Copper Minerals and Uses, 2009, p.1. http://www.mineralprospector.com/html/copper.html. [5] M. Jansen and A. Taylor, Overview of gangue mineralogy issues in oxide copper heap leaching, [in] Copper 2003 Conference, 2003, p.2. [6] National Iranian Copper Industries Company, Sarcheshmeh Operating Manual, Section 1: Introduction and Overall Plant Description, 1977, p.1. [7] G.C. Waterman and R.L. Hamilton, The Sarcheshmeh porphyry copper deposit, Econ. Geol., 70(1975), p.568. [8] J. Shahabpoor, Geology of Sarcheshmeh Copper Mine Report, 1982, p.17. [9] S. Banisi and J.A. Finch, Testing a flotation column at the Sarcheshmeh copper mine, Miner. Eng., 14(2001), p.785. [10] M. Poorkani and S. Banisi, Industrial use of nitrogen in flotation of molybdenite at the Sarcheshmeh copper complex, Miner. Eng., 18(2005), p.735. [11] S.M. Bulatovic, Handbook of Flotation Reagents, Chemistry, Theory and Practice: Flotation of Sulfide Ores, Elsevier, 2007, p.1. [12] A. Bagherian, Sarcheshmeh Copper Concentrator Plant Visit Report, 2004, p.6. [13] B.A. Wills, Mineral Processing Technology, 6th Ed., Musselburgh, 1997, p.15. [14] E.G. Kelly and D.J. Spottiswood, Introduction to Mineral Processing, Wiley-Interscience, 1989, p.21. [15] S. Banisi, H.R. Iranmanesh, M.R. Shayestefar, and H. Shekarchyan, Mineralogical tracing of metallurgical resultsthe Sarcheshmeh copper mine case, [in] 35th Annual Meeting of the Canadian Mineral Processors, Ottawa, 2003, p.418.