Application Of Quaternary Ammonium Salts As Collector In Flotation Of Scheelite And Research Of The Reaction Mechanism

The separation of scheelite from other calcium-contained gangue minerals has always been acknowledged as a great challenge in mineral processing. The traditional anionic collectors exihibit low selectivity against the calcium-contained gangue minerals because they react with scheelite through bonding with the calcium atoms of the surface, which means they can also react with the gangue minerals in the same way. In this sense, the cationic collectors are superior to the anionic ones in selectivity theoretically, because the zeta potential of scheelite is negative while the one of calcium-contained gangue minerals are positive in a certain pH range. In this thesis, the application of quaternary ammonium salts as collector in the flotation of scheelite was investigated. And the research of the adsorption mechanism, adsorption behavior and adsorption structure of quaternary ammonium salts on scheelite and calcite were also carried out. Meanwhile, as an entry point, the differences in surface structure between scheelite and calcite were revealed.(1) Flotation performance of quaternary ammonium salts as collector in the flotation of scheelite from calciteFlotation tests of pure minerals and artificial mixed minerals were carried out to investigate the flotation performance of quaternary ammonium salts as colletor for scheelite and calcite. In combination with adsorption and surface tension tests, the flotation results were analyzed.Compared with the traditional anionic collector sodium oleic acid, cationic collector quaternary ammonium salts DDAC and TOAC have stronger collectivity and selectivity for scheelite. The collectivity of DTAC for scheelite is much weaker than other two quaternary ammonium salts, DDAC and TOAC. And it can’t accomplish the flotation of scheelite from calcite.(2) Adsorption mechanism and adsorption model of quaternary ammonium salts as collector on the surface of scheelite and calcite Adsorption mechanism of quaternary ammonium salts as collector on the surface of scheelite and calcite was investigated by zeta potential tests, FTIR analysis and the analysis with molecule orbital theory. And the adsorption model was obtained in the research of adsorption process of quaternary ammonium salts on the surface of the two minerals by adsorption tests combined with zeta potential tests.In flotation quaternary ammonium salts adsorp on the surface of scheelite and calcite with physical interactions. It is indicated that quaternary ammonium salts as collector for scheelite can’t adsorp on the surface of scheelite and calcite through chemical interaction by the analysis of structures of quaternary ammonium salts with molecule orbital theory and the FTIR analysis of the adsorption of quaternary ammonium salts on the surface of scheelite and calcite. The results of zeta potential tests indicate that not only electrostatic interaction but also van de Wals forces exist in the adsorption of quaternary ammonium salts on the surface of scheelite and calcite.The adsorption of quaternary ammonium salts on the surface of scheelite and calcite can be divided into two stages:in the first stage, quaternary ammonium salts adsorp on the mineral surface through electrostatic interaction; after the concentration reach a certain value when it enters the second stage, the adsorption occurs through the association of quaternary ammonium salts which produces micelles on the surface. The results of zeta potential tests, surface tension tests and adsorption tests indicate that in both stages the interaction of DDAC with the surface and the association of DDAC molecules are the strongest, the second is TOAC and the last is DTAC. Therefore, at the same concentration, the adsorption of DDAC is most efficient, TOAC is the second and DTAC is the least. The relation between the flotation performance of the three quaternary ammonium salts and their adsorption on the surface of scheelite and calcite indicates that the interaction of quaternary ammonium salts with mineral surfaces and the association of the collectors are not recommended to be too stong like DDAC and not to be too weak like DTAC. That of TOAC is suitable and its flotation performance is the best. The weaker the association of quaternary ammonium salts is, the more obvious the transition from the first stage to the second stage is. That is why the fitting of the adsorption behavior of DDAC on the surface of scheelite and calcite to Langmuir adsorption isotherm is good while the one of DTAC is bad. While, the adsorption behavior of all the three quaternary ammonium salts fits well "two step" adsorption isotherm.(3) Microscopic adsorption structure of quaternary ammonium salts as collector on the surface of scheelite and calciteBy molecular dynamics simulation, the microscopic adsorption structure of quaternary ammonium salts as collector on the surface of scheelite and calcite in solution was investigated in three aspects:the interfacial water structures of scheelite and calcite; adsorption configuration of a single molecule of quaternary ammonium salts on the surface of scheelite and calcite; micelle structures of quaternary ammonium salts.There are two water layers on the interface of calcite (104), only one layer on the interface of scheelite (101)ca and two layers on the interface of scheelite (101)wo4but with lower adsorption density than calcite. The interfacial water layers on calcite (104) is close-packed and ordered, while the one on scheelite (101) is relatively disordered, which favors quaternary ammonium salts as collector to adsorp on the mineral surface in the place of the water originally adsorped.In solution the calcium atoms on the (101)wo4and (101)ca of scheelite have a further relaxation under the influence of water in comparison with the case in vaccum, while the tungsten atoms changed little. It means calcium atoms on the surface are solvated more easily than the tungtate. In the case of calcite (104), the relaxation of calcium atoms on the surface is almost the same with the one of the carbonate.Quaternary ammonium salts as collector are capable of replacing the water adsorped to interact directly with the scheelite (101) wo4, however their interaction with calcite (104) is not strong enough to replace the water adsorped. Instead, they might adsorped on the surface through electrostatic interaction with the mineral ions solvated.The micelle structure of quaternary ammonium salts depends on the structure of carbon chains of their hydrophobic group. The micelle structure formed on the surface of scheelite and calcite speculated from the one formed in solution is that:the degree of micellization on the surface is highest for DDAC with the shape of cylinder; the second highest is DTAC with shape of spherical; TOAC don’t form any micelles with a certain shape. In other words, the association of TOAC is not so strong as DDAC. As a result, the adsorption amount of TOAC on the surface of calcite through association is much less and the difference in adsorption between the surfaces of scheelite and calcite is largerthan DDAC and consequently TOAC is superior to DDAC in selectivity for scheelite.(4) Application of quaternary ammonium salts as collector in flotation of scheelite at room temperatureIn flotation of scheelite from Shizhuyuan, the roughing was first carried out with733and as collector by the "lime method". Then, the rough concentrate with acidified sodium silicate added was agitated with high intensity before the blank concentrating was carried out. Last, a concentrating with quaternary ammonium salts as collector was carried out. The grade and recovery of the concentrate are51.02%and54.64%when DDAC was used as collector, while the grade and recovery are52.01%and51.54%when TOAC was used as collector. It indicates that TOAC is superior to DDAC in flotation efficiency. Furthermore, the comparison with the results of twice blank concentrating revealed that quaternary ammonium salts as collector improved the flotation efficiency of scheelite from calcite in the concentrating significantly.