The Salobo deposit, located in Carajás, southeastern of Pará, is one of the largest copper reserves in Brazil. Although severa! mineralogical studies have been developed for this ore, its origin is still controversial, with severa! interpretations, such as volcanogenic copper-bearing oxide and voicanogenic massive sulfide and iron oxide (Cu-U-Au-REE). In comparison with other well-known deposits, it is a rare example of mineralization. Particular characteristics such as disseminated mineralization, fine grain and its hardness impose serious difficulties to copper concentrates production. Due to ore complexity it is difficult the metallurgical treatment, reasons why it is constantly submitted to geological and technological reevaluations. The literature on Salobo deposit is expressive but detailed works about microchemistry and technological characterization in comminution are rare or restricted to Salobo Metais S.A. company. The objectives of this work dealt with these questions. Microchemical analyses using microprobe and SEM/EDS in samples of holes and ore piles (research gallery G3) of Salobo, allowed the identification of sulfide mineralization with bornite (4%), chalcocite (2%) and chalcopyrite (0.5%), and variable proportions of molybdenite, cobaltite, safflorite, niqueline, siegenite, gold, silver, graphite, ilmenite, hematite, Te-Ag, uraninite and REE minerais. These minerais occur in schist iron formations where the deposit es found: a) magnetite and massive fayalite, eventually banded and b) banded biotite and magnetite. These groups considered as gangue (magnetite 53% and silicates 40%) contain minor amounts of gamet, amphibole, quartz, plagioclase and subordinate amounts of fluorite, greenalite, minnesotaite, stilpnomelane, apatite, monazite, allanite and occasionally siderite, goethite and malachite. Sulfides are preferentially concentrated in magnetite rich iron formations. Copper sulfides occur as crystals less than 3.0 mm and as disseminated fine grains, with fine alternated banded and/or foliated silicates, veiniets and/or long/short stringers, tiny inclusions, bornite/chalcocite and bornite/chalcopyrite mirmekitic intergrowth and bornite-chalcocite and bornite-chalcopyrite substitutions. These minerais were formed by complex processes and are characterized by compositional controls, mainly for the presence of Fe in them. Solid solutions of bomite and chalcopyrite were formed at high temperatures and gave way to those iron excesses. Atomic radios Cu/Fe of bomite (4.3-4.9) and chalcopyrite (average of 0.9) at high temperatures allowed the co-existence of bornite-chalcopyrite equilibrium and therefore of
bornite/chalcopyrite. Iron contents (maximum 0.96%) in chalcocite have been incorporated at those temperatures when the structure is highly disordered. Chalcopyrite lamellaes following the { 111 } orientation in bornite as well as the bornite/chalcocite and bornite/chalcopyrite intergrowth suggest exsolution. Although those phases are associated with severa' minerais in different paragenesis, the ore features have been affected drastically by metamorphism difficulting the reconstruction of its pre-metamorphic evolution. Ore grinding produced physical changes in the grain size and according to time, long or short, of mineral comminution the pulp reologie is modified. That process originates a grain size - 270 # (53 µm), 80 % wt. passing, grounding time on 4 hours (dry) and 2 hours (humid) adapted to copper concentration. Different volumetric fractions of copper sulfides in particles were obtained through both processes: larger fraction (6 % volume) to grain sizes < 53 µm and with a prevailing fraction (7 to 15 % volume) ranging from 26.9 to 7.5 µm. Physical modification shows larger magnetite proportions than silicate ones with a clear incidence of magnetite density in the hydrocyclone classification. Mineralogically, in the comminuted products, occur the same minerals established in ROM but with chemical modifications in copper sulfides. Magnetite is the main host for sulfides and greenalite is more frequent among the silicates, fluorite being also common. Proportions of S, Fe and Cu in bornite, chalcocite and chalcopyrite are variable relative to ROM and stoichiometry, varying in function of the grain size (larger chemical variation in grain sizes of 26.9 to 7.5 pm than on the 2360 to 37µm fraction). Iron can reach up to 6.0% wt. in chalcocite. Chemical variations in S, Cu and Fe formed ternary sulfides: bornite, characterized as "complex mistures" rich in iron (Cu4.34-4.76Fe1.03-1.04S4.0) and chalcopyrite rich in Fe Cu0.93Fe1.08S2.0 (as a solid solution extension of chalcopyrite). Chalcocite oxidation and high values of Fe in its structure also contributed to the reaction of binary sulfides: djurleite and digenite Cu1.77-1.84Fe0.04-0.06S1.0. Those ternary (Cu-Fe-S) and binary (Cu-S) copper sulfides have been formed in the initial oxidation state with superficial alterations induced by temperature (25°C on) and comminution. These sulfides were formed and controlled by the phase relationships in the Cu-Fe-S system. Low copper content in sulfides leads to a slower chemical variation than there is an excess of iron. These variations favoured the appearance of oxidized surfaces on copper sulfides with different products of oxidation [M1-nS and nM(OH)2]. Chemical variations showed to be dependent on the grain size, with smaller oxidations in sizes > 53 µm and larger oxidations in
sizes <53 µm, caused by a combination of surface area and ability of chalcocite to be oxidized. Iron excess mainly as highly reactive colloidal particles could have been generated by: mill material, abrasive action of particles and probable magnetite oxidation, producing chemical variation in mill atmosphere and electrochemical corrosion processes. Comminuted ore conserves the lepidoblastic textures of the silicates biotite, fayalita and greenalite and granoblastics of magnetite or bornite, chalcocite and chalcopyrite grains. Crystals of copper sulfides, liberated and mixed with high percentage of magnetite and silicates are intensively fractured and eroded and sometimes fullfilling cracks and/or fractures of greenalite. They difficult the sulfide liberation. Copper sulfide liberations increase gradually when the grain size is finer (more than 50 % in grain sizes < 29.6 µm). Only in fractions < 37 µm (Cumulative liberation yield CLY90), the copper bearing particles begin to migrate and for higher degrees of liberation though such tendency can still be insufficient for the purposes of sulfide concentration. Besides the strong metamorphic recrystallization of the schists of ore formations, its high hardness, the extremely variable grain sizes of sulfides (5 to 300 µm) and the mineralogical ore complexity (mineralogical associations, disseminations, intergrowth complexes), this microchemical investigations, in ROM and in comminution products, revealed a significant chemical variation in copper sulfides. Iron present in sulfide mineral reticules is the main contaminant to chemical modifications (Cu/Fe ratio) influencing the quality of copper concentrate in mineral processing. It has been already established that between copper sulfides and other components of pulps during grinding and flotation (water, species collectors or modifiers) occur an interaction through electrochemical mechanisms producing oxidized species, where the chemical composition of the mineral in question is very important. The technological alternative adapted to treat the copper concentrate, with basis in mineralogical and microchemical studies in run-of-mine and comminution products, seems to be the hydrometallurgy because they can take advantage the production of fine grains and to use the reground for ultrafine grains production. These can be submitted to oxidation processes of sulfides to promote copper extraction. Finally the metallic copper extraction can follow the solvent extraction/electrowinning (SX/EW) process.
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