Classifying – separating – and assorting solids – Fluid suspension – Liquid
Patent
1996-08-14
1999-01-05
Lithgow, Thomas M.
Classifying, separating, and assorting solids
Fluid suspension
Liquid
209168, 209170, 209730, 209732, 210703, 210704, 210708, 210788, 2102212, 2105121, B03D 124, B03D 102, C02F 124, B01D 17035
Patent
active
058557695
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an apparatus and a method for the selective separation of hydrophobic material by froth flotation utilising centrifugal separation and to a centrifugal separator. The material which is selectively separated may be either hydrophobic per se or may be rendered hydrophobic or have its hydrophobicity enhanced by the use of appropriate reagent(s). The material may be separated from a suspension, slurry, emulsion or the like and includes material recovered in waste treatment processing (e.g. ink removal from paper during recycling processes) The material may be in the form of particulate material (eg. mineral or coal fines) or may be a liquid (eg. oil).
BACKGROUND ART
The recovery of mineral and coal fines presents considerable difficulties with the result that large losses of valuable resources are incurred in many operations. The separation of water contaminated with oil or other organic liquids is also difficult. Froth flotation is currently the most widely used method for fine particle processing but the cost of the flotation cells which are used is high. In a paper entitled "Study of Coal Flotation Practice--RD & D Requirement" published by BHP Central Research Laboratories in 1988, it was estimated that the base cell structure of mechanical flotation cells, including the rotor and stator accounts for almost 80% of the cost of a bank of cells, with the drive motors representing a further 6.5% of the cost. Accordingly, a significant reduction in the residence time (which dictates cell volume) would lead to a dramatic reduction in the capital costs of the cell and support structure.
Flotation is typically a process in which product particles in suspension are separated from reject particles on the basis of differences in their surface chemistry characteristics. The component to be floated is usually naturally hydrophobic or rendered hydrophobic by addition of suitable reagents. When air is introduced into the slurry system, the hydrophobic particles adhere to the air bubbles forming particle/bubble aggregates which rise to the surface of the flotation cell where they are removed as froth. Being hydrophilic, the remaining constituents of the slurry stay in suspension and are discharged as waste material which is generally referred to as tailings.
It has been identified that in the design of a flotation cell, four functions need to be accomplished, namely: zone, and flotation systems, particularly those which utilise mechanical agitation by impellers, the optimum levels of these four functions are not coincident. Additionally, the zone in which separation is performed is typically large due to slow transfer rates with the result that large process tanks are required.
Flotation has traditionally been carried out in mechanical flotation cells which are basically continuously stirred tank reactors in series. In these cells, particles are kept in suspension by impellers which also generate the bubbles and promote particle-bubble collision and attachment thus fulfilling the last three of the functions mentioned above. The turbulence which promotes particle bubble attachment also causes disruption of particle/bubble aggregates and mechanical entrainment of hydrophilic particles into the resultant froth. Additionally, the separation of the negatively buoyant hydrophobic particle/air bubble aggregates from the pulp containing the hydrophilic particles is driven by the force of gravity which makes it necessary for the slurry being processed to have a residence time within the flotation cell of the order of 3-5 minutes for coal and significantly longer for mineral applications. The design features of these cells therefore limit the overall efficiency of the flotation treatment of fine particles.
In recent years, the utilisation of countercurrent flotation columns to increase the efficiency of the flotation process has received significant attention. In these systems, air bubbles are generated continuously at the bottom of a column using spargers. The bubbles rise throu
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Firth Bruce Andrew
Ofori Philip Kwadwo
Commonwealth Scientific and Industrial Research Organisation
Lithgow Thomas M.
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