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Zero-conditioning time concept in flotation

University of British Columbia

In the particle-to-bubble attachment process collector species are needed at the attachment point to render the mineral surface hydrophobic. It is common to assume that such species are transported toward mineral particles via diffusion/mixing and render the mineral surface hydrophobic by adsorption. However, collector species can also be brought to the point of attachment by bubbles carrying the species on their surfaces. That ensures that the collector is at the right time at the point where it is needed to facilitate the attachment of the particle to the bubble. It is generally accepted that flotation with amines is very rapid and that only a short conditioning time is required in such a cationic flotation. This indicates that the transportation of the collector species toward mineral particles is very favorable when amines are applied. This implies that in such systems the major portion of amine is brought to the mineral surface by bubbles, and that the conventional introduction of such flotation reagents to the pulp via dissolution in the pulp does not constitute the best way of utilizing such reagents. These ideas were tested in a reverse coal flotation process. In this process a quaternary aliphatic amine, dodecyltrimethyl ammonium chloride (DTAC), was used as a collector to float gangue while depressing coal particles. The process is interesting because low rank/oxidized coals float poorly, and because of the possible utilization of such clean coal products in the form of coal-water slurries. However, to float gangue efficiently the process requires very high dosages of the quaternary amine since adsorption of the amine onto coal is very high. However, if amine is transported with bubbles then it should be possible to carry out the process at "zero conditioning time" that would (i) reduce the amount of the amine adsorbed by coal thus reducing its overall consumption and (ii) improve gangue flotation. The process can be carried out either without any conditioning step prior to flotation, or, more efficiently, with the amine introduced to the flotation system with the stream of bubbles. The experiments confirmed entirely the hypothesized mechanism in which amine is mainly transported by bubbles, and showed that, with the zero conditioning tests, coal reverse flotation could be successfully carried out at reduced amine consumption. However, in order to become a viable option the coal reverse flotation process requires a very substantial reduction of the quaternary amine consumption. In search for the ways of reducing the collector dosage in this process the attention was turned to polymers which in potash flotation are utilized as blinders. The batch flotation tests carried out using a mechanical cell confirmed that some polyacrylamides worked very well as blinders in the coal reverse flotation process. At the same time the standard flocculation tests showed that the polyacrylamides acted as total non-selective flocculants. This obvious discrepancy revealed the importance of conditioning in selective flocculation. Reverse flotation with a simultaneous use of polyacrylamide, which significantly reduced consumption of amine collector, can be selective only after sufficient conditioning. It was found that the polyacrylamides with a different degree of anionicity responded differently to conditioning. Only the addition of the polymers with a lower degree of anionicity promoted the flotation of gangue. The flocculation turned out to be significantly more selective after more intense conditioning with polyacrylamide. Since in reverse flotation coal is not recovered as a hydrophobic froth product but as a hydrophilic concentrate, it is much easier to utilize such a product to produce a coalwater slurry. The rheological measurements indicate that the viscosity reducing chemical additives are not needed when the slurries are prepared from the reverse flotation clean coal product. Additionally, since coal reverse flotation increases the column carrying capacity because of the reduced yield of froth product, it could minimize the associated problems in the use of flotation columns in coal flotation. While this thesis deals specifically with coal reverse flotation, these results have much broader implications since amines and polyacrylamides are widely applied in flotation and flocculation.

Text

2011-01-24

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http://hdl.handle.net/2429/30776

Mining Engineering

University of British Columbia

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