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Latices as flocculants in selective flocculation of mineral suspensions Zhan, Yihong

Abstract

In contrast to polyelectrolyte flocculants, latices possess a certain degree of hydrophobicity, are compatible with flotation collectors and can be considered for use in a flotation circuit. In this study, three latices, polystyrene (PS), poly(2- ethylhexylmethacrylate) [P(EHMA)] and copoly(methyl acrylate - acrylic acid) [P(MAAA)] were prepared, characterized and tested in the flocculation and flotation of fine salt-type minerals (e.g., fluorite, apatite and calcite). PS and P(EHMA) latices were produced by emulsion polymerization with potassium oleate as an emulsifier. P(MAAA) latex was produced by emulsifier-free emulsion polymerization of methyl acrylate and acrylic acid in a molar ratio of 1.4 : 1. The surfaces of all latex particles contained carboxylic groups. While carboxylic groups were part of the adsorbed oleic acid on the surfaces of PS and P(EHMA) latices, carboxylic groups that were brought by one of the monomers (acrylic acid) were chemically bound to the surface of P(MAAA) latex. The particles of PS latex are rigid as characterized by the high glass transition temperature of the polymer while those of the P(EHMA) latex are relatively soft as characterized by the low glass transition temperature of the polymer. The affinity of the latices to the mineral surfaces is mainly due to the interaction between the carboxylic groups on the surface of latex particles and calcium sites on the mineral surfaces. The softness of the P(EHMA) latex probably facilitates the adhesion between the latex particles and mineral surfaces. The flocculation results with all the tested latices reveal that, at a same dosage of latices and their natural pulp pH, the P(EHMA) latex produced a higher flocculation recovery of fluorite, apatite and calcite than the other two latices, polystyrene and P(MAAA). The P(EHMA) and P(MAAA) latices all exhibit a higher flocculation recovery of fluorite and apatite than calcite. The difference of degree of flocculation of fluorite, apatite and calcite is even greater when the P(MAAA) latex was used. The chemically bound carboxylic groups on the surface of the P(MAAA) latex particles seem to play a major role in the interaction with the minerals. The carboxylic groups on the latex surface completely ionize at pH 9.2-9.5. At this pH and at a latex dosage of 2.2 kg/t, 90% of the apatite and only 5% of the calcite flocculated. At a lower dosage of 1.1 kg/t, 60% of apatite flocculated while calcite did not flocculate at all. While with the P(EHMA) latex, at a dosage of 1 kg/t, the flocculation recovery is about 90% for apatite and 45% for calcite. The addition of small amounts of dispersants, sodium tripolyphosphate and sodium silicate increases the apatite recovery and the P2O5 grade in the selective flocculation with the P(MAAA) latex from apatite-calcite and apatite-silica mixtures. At a P(MAAA) dosage of 1.1 kg/t, pH around 9.1-9.4 and 4 ppm of sodium tripolyphosphate, 60% of the apatite can be recovered from a 1:1 apatite-calcite mixture, with a 33% P2O5 grade. At a higher dosage of P(MAAA) latex and the same other conditions the apatite recovery can be increased to 90% while the P2O5 grade was not significantly changed and remained at around 33%. However with sodium silicate utilized in the apatite-calcite mixture, at a P(MAAA) dosage of 1.1 kt/t, pH around 9.1-9.4 and 20 ppm of sodium silicate, 70% of apatite can be recovered, with a 29% of P2O5 grade. Further increasing the sodium silicate concentration to 30 ppm did not significantly change the apatite recovery and the P2O5 grade. For a 1:1 apatite-silica mixture, at a P(MAAA) dosage of 1.1 kg/t, pH around 9.1- 9.3 and 20 ppm of sodium silicate, 80% of apatite can be recovered, with a 33% P2O5 grade. However with sodium tripolyphosphate utilized in the apatite-silica mixture, at a P(MAAA) dosage of 1.1 kt/t, pH around 9.1-9.3 and 2 ppm of sodium tripolyphosphate, 70% of apatite can be recovered at a 34% P2O5 grade. Further increase in the sodium tripolyphosphate concentration caused decrease of both grade and recovery. Deposition tests with P(MAAA) latex revealed that the latex has the higher deposition density and deposition rate on fluorite and apatite surfaces than on calcite surfaces. This corroborates the flocculation tests. The use of the P(MAAA) latex in the flotation of fine (100% -38 pm) apatite and phosphate ore was evaluated via Hallimond tube and batch flotation tests. The latex enhances the flotation of the fine apatite at pH around 9. This has been attributed to the formation of floes that increase the particle-bubble collision probability. The floes produced by the P(MAAA) latex are apparently strong enough to withstand the shearing force in a flotation cell.

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