Polymer-based investigations into the surface chemistry of some sulphide minerals and the beneficiation of a low grade siliceous copper ore

Abstract

Tanzania is endowed with rich mineral deposits containing gold, tanzanite, diamond, coal, uranium, iron ore, gemstones with low grade of the sulphide minerals namely chalcopyrite and pyrite. It is the third largest producer of gold in Africa after South Africa and Ghana. It is pertinent to note that the ores in Tanzania composed of mainly base metals, namely copper in the form of chalcopyrite, disseminated in silicates, in some locations of Mpanda Mineral Field (MMF) are rarely beneficiated. This is due to the presence of large proportions of silica slimes, present in the lean grade ores of copper in the form of chalcopyrite, thereby making it even more challenging for mining industries to process MMF ores. Taking into consideration the continuous depletion of the high grade mineral deposits, stricter environmental regulations and the necessity to utilize lean ores with complex and intricate mineralogy, it becomes germane to develop a beneficiation strategy to extract mineral values from the MMF ore of Tanzania. Towards this, polysaccharides have been evaluated as selective flocculants for chalcopyrite present in the MMF ore, after dispersion of the siliceous slimes. The chalcopyrite enriched pre-concentrate obtained after selective-dispersion-flocculation, was beneficiated by the froth flotation process. With this background, the present research investigation was taken up, with the following major objectives: 1) Evaluation of polysaccharides such as acacia gum (AG), xanthan gum (XG) and guar gum (GG) as well as polyethylene oxide (PEO) based on adsorption density measurements, on individual minerals such as chalcopyrite, pyrite and quartz as well as on the MMF ore. 2) Elucidation of the surface chemical changes brought about on the chosen minerals and MMF ore in the presence of the polymers and/or sodium trisilicate (STS) at the mineral-solution interface. 3) Delineation of the interaction mechanisms of the chosen polymers with the chosen minerals and MMF ore. 4) Assessment of the flocculation efficiency of the chosen polysaccharides based on selective-dispersion-flocculation studies on the MMF ore, as a pre-concentration step. 5) Beneficiation of the flocculated chalcopyrite enriched fraction of MMF ore by froth flotation. All the mineral samples used have been characterized using X-ray diffraction and FTIR spectroscopic techniques. Further, the characterization of the MMF ore has been carried out using petro-mineralogical analysis, X-ray diffraction and FTIR spectroscopy. The chemical analysis of the ore has also been performed. Extensive and detailed surface chemical studies, namely electrokinetic and adsorption-desorption measurements, have been carried out on the chosen mineral and ore samples as a function of pH, polymer and/or dispersant (STS) concentration, apart from adsorption kinetics. The dissolution tendencies of the minerals and ore have been ascertained as a function of time and pH, in the presence of the polymers. The complexation of the polymers with the metal ions of relevance has been examined in the bulk solution by co-precipitation studies as a function of pH. Based on results generated at the mineral/ore solution interface and that in the bulk solution, the mechanisms of adsorption of the polymers on the chosen minerals and ore have been delineated. The mechanisms proposed have been further confirmed by FTIR spectroscopic studies on the adsorbed polymeric reagents. In the light of the knowledge gained with the basic surface chemical studies, selective-dispersion-flocculation studies using AG, XG and GG as selective flocculants for the chalcopyrite component of the MMF ore, have been carried out in the presence of STS as a dispersant for the silica slimes present in the ore. The degree of flocculation has been determined as a function of desliming stages, to assess the flocculation efficiency of the chosen polysaccharides. The chalcopyrite enriched fraction of the MMF ore, obtained as a pre-concentrate, has been subjected to flotation using xanthate collector blends, in rougher and cleaner stages, to obtain chalcopyrite concentrate of optimum grade and recovery of copper, with a significantly reduced amount of silica. A novel approach has been the addition of PEO as a depressant of silica, in the flotation stage, which resulted in a higher grade and recovery of copper, obviating the need for the cleaner flotation stage. The salient findings of this research study are summarized below: The electrokinetic measurements indicate that the iso-electric points (IEPs) of chalcopyrite, pyrite, quartz mineral samples and the MMF ore are located at pH 5.6, pH 6.7, pH 2.3 and pH 2.7, respectively. All the mineral and ore samples are rendered more negative, proportionate to the concentration of STS added. Similarly, the mineral and ore samples become less positive below the IEP or more negative beyond the IEP, after the addition of AG or XG, and a shift in the IEP towards lower pH values is observed, especially discernable with respect to chalcopyrite and pyrite, indicative of specific adsorption. This can be attributed to the presence of carboxyl functional groups in AG and XG polymers. On the contrary, the addition of GG or PEO, reduces the magnitude of the negative zeta potential values of the mineral or ore samples, without altering their IEP values, due to the shift of the plane of shear, as a consequence of their high molecular weights. Adsorption kinetics studies reveal that the adsorption of all the chosen polymers takes place quite rapidly and equilibrium is attained within 0.5 h-1.5 h, for the chosen minerals and ore samples. However, it is found that no adsorption of AG and XG takes place on quartz. The adsorption densities of AG and XG continuously decrease with increase of pH, for chalcopyrite, pyrite and the MMF ore. Further, their adsorption densities are decreased in proportion with the concentration of STS added, with respect to the above – mentioned sulphide minerals and ore, though the trend with respect to pH is maintained. The decrease in the adsorption densities of AG and XG with increase of pH can be explained by the increase in the negative charge of the mineral and ore surfaces, with increase of pH and the anionic nature of the two polymers. It is interesting to note that the adsorption density of GG exhibits a maximum at pH 9.6 and pH 10.5 for chalcopyrite and pyrite, respectively, with the adsorption maxima coinciding with the pH of IEP of the copper or ferric hydroxide, respectively. The adsorption of GG for MMF ore is found to be independent of pH. Additionally, in the presence of STS, the adsorption density of GG, is reduced for the two sulphide minerals and the MMF ore, without altering the trend with respect to pH. The adsorption density of PEO for chalcopyrite and pyrite is found to continuously increase with increase of pH, while it is independent of pH for the MMF ore. Further, the adsorption of PEO remains unchanged in the presence of STS, for the sulphide minerals and the ore, maintaining the trend observed with respect to pH. As mentioned earlier, AG and XG do not adsorb on quartz, while the adsorption of GG and PEO on it, remains independent of pH. The adsorption density of GG for quartz is reduced with the proportional increase in the concentration of STS, while that of PEO on quartz is not affected by the addition of STS. The adsorption isotherms of the polymers for the chosen minerals and ore follow Langmuirian behavior, adhering to L1 or L2 type of the Giles classification. The Gibbs free energies of adsorption of the polymers are found to be in the range of -25 kJ/mol to -35 kJ/mole for the sulphide minerals and ore, attesting to chemisorption. However, the Gibbs free adsorption of GG and PEO for quartz is -8.8 kJ/mol and -14.4 kJ/mol, suggestive of a physisorption process. Desorption studies show only partial reversibility of adsorption for the sulphide minerals and ore, though a high degree of desorption could be achieved for the adsorption of GG and PEO on quartz and MMF ore. The results of the desorption tests lend support to the nature of the adsorption process observed for the polymers, with respect to the mineral and ore adsorbents investigated. Dissolution studies indicate release of the lattice metal ions from the sulphide minerals and MMF ore. Co-precipitation tests confirm complexation of copper, iron, aluminium and calcium ions with the polymeric reagents in the bulk solution. Consequent to the pre-treatment of the sulphide minerals and the ore with EDTA, a reduction in the adsorption density of the chosen polymers is observed, attesting to the importance of the lattice metal ions in facilitating adsorption. Based on the findings of the surface chemical studies carried out, both at the mineral/ore solution interface and in the bulk solution, the adsorption mechanisms of AG, XG and GG for the chosen sulphide minerals and ore have been elucidated, to be governed by hydrogen bonding, electrostatic and chemical forces of interaction. The mechanism of interaction of GG and PEO with quartz is found to be mainly by hydrogen bonding. FTIR spectroscopic studies provide evidence in support of the mechanisms proposed. As a follow up of the fundamental studies performed, the beneficiation of the MMF ore has been carried out. Initially, selective-dispersion-flocculation studies were undertaken in order to produce a pre-concentrate, enriched in chalcopyrite. The grade of chalcopyrite is enhanced to 19.8% using 30 mg/L concentration each of STS dispersant and XG as a flocculant. A separation efficiency (SE) of 84% has been achieved using XG as a flocculant. The SE for the flocculants can be arranged in the following sequence: XG > AG > GG. The pre-concentrate of chalcopyrite so obtained was used as the feed for the flotation process. The best results could be achieved with the chalcopyrite pre-concentrate obtained by using XG as a flocculant and PEO as a depressant for silica. These correspond to a copper grade of 29% with 90% recovery and containing silica of 0.9% grade with 1.1% recovery in the concentrate. The maximum selectivity index of 29.5 could be achieved, under the optimized conditions. A conceptual flowsheet has been proposed for the beneficiation of MMF ore.