Aggregation studies on sphalerite systems

Mirnezami, Mitra; Mirnezami, Mitra

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Aggregation studies on sphalerite systems

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Mirnezami, Mitra

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The aggregation behaviour of sphalerite suspension and the role of zinc and magnesium ions are investigated. Aggregation is monitored by suspension analysis (turbidity) and optical microscopy and, in particular, a conductivity-settling technique. To probe the mechanisms, electrophoretic mobility (zeta potential) measurements and field emission scanning electron microscopy (FE-SEM) imaging are used. In the case of sphalerite alone, for samples from a variety of sources, aggregation occurred at pH 8--10, well above the range in iso-electric point (pH 2--6). The aggregation is attributed to the presence of Zn(OH) 2, the dominant species over this pH range. To test whether zinc hydrolysis products promote aggregation silica and chalcopyrite suspensions were doped with Zn2+ ions; aggregation over the same pH range was found. This observation is similar to that of Healy and Jellet (1967) for zinc oxide, ZnO. They suggested aggregation was due to release of Zn2+ ions to form Zn(OH)2 which polymerizes and flocculates the particles. The same mechanism is proposed for sphalerite. Aggregation due to magnesium ions was determined using the settling rate of sphalerite and silica suspensions (individually) as a function of Mg 2+ concentration, pH and suspension density (%v/v solids). Aggregation at pH > 10 was found for both minerals corresponding to magnesium hydroxide. However, the mineral's response to the three variables suggests the mechanism for each is different. The proposed mechanism of aggregation by Mg(OH) 2 for sphalerite is chemical bridging and for silica, electrostatic bridging. Electrostatic bridging is revealed by aggregation passing through a maximum as a function of both coagulant concentration and pH. For sphalerite, while there is a maximum with [Mg2+] (Mg2+ concentration) there is none with pH (after allowing for the self-aggregation of sphalerite). Further, electrostatic bridging requires surface patches of the bridging material (Mg(OH)2) and the FE-SEM images showed no such evidence. The interpretation for silica aggregating with Mg2+ follows that proposed by Krishnan and Iwasaki (1986). The pH, [Mg2+] and solid concentration effects are compatible with electrostatic bridging, as is the morphology as hydroxide patches were identified by FE-SEM. In certain cases the conductivity-settling data suggested the particles were more conductive than the liquid. The conductivity-settling technique was adapted to measure the electrical conductivity of particles dispersed in water. The conductivity was estimated at the iso-conductivity point where the solution and the particles have the same conductivity. The technique was tested on chalcocite, chalcopyrite, galena, pyrite and sphalerite. The order of mineral conductivity followed that of their electrochemical rest potential, as expected. It is observed that the adsorption of xanthate significantly reduced the conductivity of chalcopyrite and copper activation increased the conductivity of sphalerite but treatment with lead had no effect.

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