Polysaccharide-based investigations into the surface chemistry of some sulphide and hydrophobic minerals and processing of a complex sulphide ore

Abstract

The gradual depletion of high grade ores and the fine dissemination of values, together with the complex mineralogy of lean grade samples, has made it imperative to develop innovative technologies for their processing. The processing of complex sulphide ores containing copper–lead–zinc minerals poses a significant challenge. Some of the factors hampering the beneficiation of polymetallic sulphide ores in India are the fine intergrowth and intimate association of constituent minerals, along with the presence of deleterious hydrophobic gangue minerals such as talc, mica, and graphite. These problems are further compounded by stringent environmental regulations that must be complied with. Moreover, in order to produce cleaner final concentrates, the selectivity of reagents for a given mineral is of prime importance. Conventional inorganic modifiers such as dichromates, cyanides, ferrocyanides, hydrosulphides, and hypochlorites meet such requirements, but their use has raised concerns on environmental grounds. Natural biodegradable agents such as polysaccharides, which are nontoxic and relatively inexpensive, are expected to gain importance in the years to come. Although polysaccharides have been used in the mineral industry for several decades for various applications, the lack of understanding of their underlying mechanisms of interaction with mineral surfaces has restricted their use as selective modifiers. The main thrust of the present research investigation has been directed toward the following aspects: 1. Evaluation of polysaccharides such as dextrin and guar gum based on adsorption density measurements on individual sulphide and hydrophobic minerals. 2. Assessment of their depressant action based on flotation tests on individual sulphide and hydrophobic minerals, as well as differential flotation studies on typical synthetic mixtures. 3. Delineation of interaction mechanisms of polysaccharides with various sulphide and hydrophobic minerals. 4. Flotation tests on a typical Indian complex sulphide (Pb–Zn–Cu) ore using polysaccharide based depressants. Detailed adsorption, electrokinetic, and flotation studies have been carried out to investigate the interaction of dextrin and guar gum with the sulphides galena, sphalerite, chalcopyrite, pyrite, and inherently hydrophobic minerals: talc, biotite mica, and graphite. These minerals are significant constituents of complex sulphide ores and therefore pertinent to the present study. Kinetic studies indicate that adsorption equilibrium is attained within an hour for most sulphides, whereas adsorption density reaches saturation within 30 minutes in the case of talc and mica. The adsorption densities of dextrin and guar gum on sulphide minerals show distinct adsorption maxima at pH 11.5, 10, 9.5, and 7.5 for galena, pyrite, chalcopyrite, and sphalerite, respectively. In contrast, for hydrophobic solids, polymer adsorption is almost independent of pH. A common observation is that guar gum exhibits higher adsorption magnitudes compared with dextrin for all substrates. All adsorption isotherms exhibit Langmuirian behaviour, and in some cases a high affinity trend is observed. The adsorption sequence is: For dextrin: PbS » FeS > ZnS CuFeS > graphite > talc > mica For guar gum: PbS » ZnS > FeS CuFeS talc > graphite > mica The importance of metallic sites in the matrix for adsorption has been clearly demonstrated for hydrophobic solids, where a decrease in adsorption density was observed after interaction with a complexing agent. Electrokinetic experiments indicate that the isoelectric point (iep) of galena, sphalerite, chalcopyrite, talc, mica, and graphite lies below pH 3, whereas pyrite shows an iep at pH 7.5. The addition of increasing concentrations of dextrin/guar gum reduces the negative electrophoretic mobility values proportionally, without shifting the iep-similar to the effect of an indifferent electrolyte. The results suggest conformational rearrangements of macromolecules upon loading, shifting the shear plane further from the surface. To examine polymer interaction with metal ions in solution, dissolution and co precipitation tests were performed. Dissolution experiments indicate release of metal ions from minerals, while co precipitation tests confirm polymer–metal ion interaction in bulk solution. The pH of maximum metal ion precipitation coincides with that of polymer adsorption maxima for corresponding minerals. These interactions are further supported by solution conductance and EDAX studies. Flotation tests on individual minerals in the presence of polymeric depressants corroborate adsorption results. The depressant efficiency of dextrin and guar gum at pH 11 follows the order: PbS > FeS > CuFeS > ZnS > talc > graphite > mica Again, guar gum acts as a better depressant than dextrin for all minerals. Differential flotation tests on synthetic galena/mica and galena/sphalerite mixtures at pH 12 reveal that mica and sphalerite can be selectively floated while galena is effectively depressed by polysaccharides. Guar gum’s superior performance is attributed to its favourable cis conformation of hydroxyl groups and its higher molecular weight. Based on surface chemical studies at the solid–solution interface and in bulk solution, the interaction mechanisms of polysaccharides with sulphide minerals can be summarised as: • Mineral dissolution releases metal ions that form neutral or charged hydroxo complexes, which interact with the polymer both at the mineral surface and in solution via hydrogen bonding and chemical interactions. • Direct interaction with hydroxylated mineral surfaces through hydrogen bonding at alkaline pH. Metal hydroxide precipitates at these pH values can also bind polymer molecules. • Adsorption may also occur via chemical interaction between dissolved metal ions and polymer functional groups. For inherently floatable hydrophobic minerals, hydrophobic association also contributes, alongside chemical and hydrogen bonding forces. ESCA and FTIR spectral studies support these proposed mechanisms. Using insight gained from surface chemical investigations and tests on synthetic mixtures, batch flotation studies on a typical Indian complex sulphide ore were conducted using statistically designed experiments. Preliminary results indicate that polysaccharide based depressants can yield Pb and Zn grades and recoveries comparable to current practice.