Evaporative concentration of clay slurries

Concentrating evaporators – Superheating and pressure release – With extruder

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159 171, 159 23, 159 44, 159 471, 159901, 159DIG8, 159DIG32, 202174, 202176, 202235, 203 78, 203 88, 203100, 203DIG8, B01D 126, B01D 306

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057308365

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

This invention relates to the evaporative concentration of clay slurries, and more particularly to an improved method and apparatus for concentrating beneficiated clay slurries by an evaporation process.


BACKGROUND OF THE INVENTION

Clay slurries, such as Kaolin, are beneficiated in an aqueous slurry having a low solids content. The most widely used method of dewatering such slurries after beneficiation involves initial processing in a vacuum or pressure filter to remove a first portion of the water from the slurry, typically to a level of about 50% to 70% solids. The resultant filter cake is then subjected to direct contact evaporation, usually by means of a spray dryer, to increase the solids content to about 95% to 99%.
As has already been recognised in the disclosure of U.S. Pat. No. 4,687,546 Willis, assigned to Georgia Kaolin Company Inc., the concentration of beneficiated clay slurries by spray drying is an inefficient method of evaporating the water from the slurry which requires the use of relatively clean hot gases to contact the slurry spray. This patent describes the use of non-contact or indirect evaporative heat exchangers to remove the water from the slurry. The specification of this patent claims that this method of processing the slurry avoids the problems of agglomeration which are prevalent with spray drying and results in a Kaolin product of improved brightness.
As a result of test work conducted by the present applicant to evaluate the concentration of Kaolin slurries by evaporation using standard techniques of free boiling and natural circulation, it is believed that the use of evaporative heat exchange techniques in the concentration of Kaolin slurries will result in the formation of a thick skin on the surface of the Kaolin slurry and a build up or accretion of high solids slurry on the heat exchange surfaces even at relatively low solids levels (40% to 45%). When this technique of free evaporation was tested in a long tube vertical evaporator, it was quickly found that plugging of the tubes rapidly occurred and concentration could not effectively be achieved as a continuous process even when low solids slurries were processed at high specific circulation rates.


SUMMARY OF INVENTION AND OBJECT

It is an object of the first aspect of the present invention to provide an improved method and apparatus for the concentration of clay slurries in which the problems associated with the use of indirect evaporative heat exchange are substantially overcome or ameliorated.
In a first aspect, the invention provides a method for the concentration of clay slurries comprising the steps of subjecting the slurry to indirect heat exchange in a heat exchanger to elevate the temperature of the liquid in the slurry while specifically suppressing boiling of the liquid within the heat exchanger, and subjecting the heated slurry to rapid pressure reduction to flash-evaporate part of the liquid from the slurry.
By suppressing boiling in the heat exchanger, the build up of high solids slurry at the heat exchange surfaces is significantly reduced thereby allowing continuous processing on a more efficient basis. The use of indirect heat exchange to heat the slurry allows the use of waste heat which is often available at processing plants, although not generally suitable for use in spray drying techniques. Thus, the combination of indirect heat exchange with the suppression of boiling to avoid fouling of the heat exchanger provides a particularly efficient and relatively inexpensive method of concentrating the clay slurry.
The rapid pressure reduction of the heated slurry is preferably achieved at the entrance to a separator vessel positioned downstream of the heat exchanger. The necessary concentration of a clay slurry, such as Kaolin, to the necessary solid level (for example, from about 34% solids to about 69% solids) is most suitably achieved by the use of multiple-effect evaporation, with each effect including an indirect heat exchanger, such as a plate heat exchanger, and a separ

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patent: 3609366 (1971-09-01), Schwartz
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patent: 4246039 (1981-01-01), Mixon, Jr.
patent: 4430156 (1984-02-01), Casper et al.
patent: 4640740 (1987-02-01), Moore et al.
patent: 4642904 (1987-02-01), Smith, Jr.
patent: 4687546 (1987-08-01), Willis
patent: 5036599 (1991-08-01), Thompson
William and Gardner, "Film Drum Dryers" Industrial Drying 1977.
Perry, "Chemical Engineers" Handbook 1941 pp. 1502-1504.
Howe, "Industrial and Engineering Chemistry", Rotary Steam-Tube Dryer (Bill), vol. 30, 1938 pp. 997-998.

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