Liquid purification or separation – Processes – Separating
Patent
1993-03-15
1994-09-13
Hruskoci, Peter A.
Liquid purification or separation
Processes
Separating
44564, 210728, 210729, 210778, B01D 3362
Patent
active
053466306
DESCRIPTION:
BRIEF SUMMARY
This invention relates to improved dewatering methods and agents for use in vacuum filtration operations in coal processing.
BACKGROUND OF THE INVENTION
Coal processing, particularly the treatment of metallurgical coal, usually incorporates washing steps as part of the beneficiation scheme. Coal washing is primarily to remove undesirable gangue minerals from the coal, usually by froth flotation. The water used in the coal cleaning process must be removed for a number of reasons. Firstly, there are usually moisture-limit specifications on coal sold to industry by the coal producers. Moisture in coal results in increased freight costs and reduced thermal-energy value. Secondly, in many parts of the world, shipment of wet coal in winter results in problems in freezing of the coal in rail cars. Thirdly, refuse coal must be dewatered prior to disposal.
The coal-cleaning process produces coal with a broad range of particle sizes. Coarse coal (i.e., >600 micron) and fine coal are treated differently to dewater effectively. Water is usually removed from coarse coal by screen drainage or centrifugation and from fine coal (i.e., flotation concentrates or refuse) by centrifugation or vacuum filtration. In some cases, thermal drying of fine coal is employed as a necessary step to achieve target moistures. Depending upon the mining operation, the type of coal treated and normal swings in process operations in the plant, the moisture levels in the cleaned coal can vary quite widely. Typical moisture levels in coarse coal range from 2-12%, while fine coal moistures can range from 15-30%.
Coal processing plants often have considerable trouble meeting target moisture levels in the product coal they sell. As a result of new mining methods employed in long-wall operations, the proportion of fine coal reporting to the prep plants has increased significantly. Fine coal is much more difficult and costly to dewater than coarse coal. One method often employed by coal prep plants to achieve moisture specifications is to blend high-moisture fine coal with low-moisture coarse coal in proportions necessary to just meet target moisture. While, in many cases, it would be much more cost effective for a coal prep plant to simply discard the fines and mine more coarse coal, for reasons of resource management/utilisation/conservation it is more prudent to treat the fines.
Vacuum filtration is the most commonly used means of mechanical treatment to dewater fine coal. Fine coal, in slurry form, reports to the filtration operation where the water is removed. Vacuum disc filters are the principal type used by the coal industry to filter fine coal, although vacuum belt filters are being utilised on an increasing basis. To assist in the efficient operation of vacuum filters, reagents are often added to the feed slurry.
The coal industry has been using anionic flocculants and cationic coagulants in the vacuum filtration of fine coal for a long time. These reagents are necessary to `thicken` the slurry as it is fed to the vacuum filter to ensure the proper formation of a filter cake. It is thought that the reagents function by binding the very fine coal particles to larger coal particles in a typical flocculation/coagulation process, thus producing a more uniform particle size distribution in the filter cake. This results in better permeability of the filter cake and less `blinding` of the filter cloth by the very fine coal particles.
Both anionic flocculants (usually high molecular weight acrylamide/acrylate co-polymers) and cationic coagulants (usually low molecular weight polyamines) are used, individually or in combination, to control filter cake formation. These reagents are always added to the slurry feeding the vacuum filters (i.e., slurry pre-treatment) in a manner typical of flocculant/coagulant addition in the minerals industry.
There has also been considerable interest by the coal industry in the use of surfactants, in combination with flocculants/coagulants, to enhance the dewatering of fine coal. Although many studies have been
REFERENCES:
patent: 1512499 (1924-10-01), Stenning et al.
patent: 4210531 (1980-07-01), Wang et al.
patent: 4410431 (1983-10-01), Roe
patent: 4892663 (1990-01-01), Keys
patent: 5167831 (1992-12-01), Dimas
Hruskoci Peter A.
Unilever Patent Holdings B.V.
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