Liquid purification or separation – Processes – Making an insoluble substance or accreting suspended...
Patent
1998-02-05
2000-05-16
Simmons, David A.
Liquid purification or separation
Processes
Making an insoluble substance or accreting suspended...
210728, 210734, B01D 2101
Patent
active
060632917
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the dewatering of high solids mineral suspensions utilising anionic and cationic flocculant.
It is well known to dewater a high solids mineral suspension, for instance a suspension having a solids content of above 150 g/l, by mixing into the suspension polymeric flocculant, allowing the suspension to flocculate and then dewatering the flocculated suspension under pressure, for instance on a belt press. Conventionally, the flocculant is an anionic bridging polymeric flocculant having intrinsic velocity at least 5 dl/g. For instance, it is common to use high molecular weight copolymers of sodium acrylate and acrylamide. The use of sulphonate polymers is known from for instance U.S. Pat. Nos. 4,342,953, 4,704,209 and GB 2,268,422.
There is extensive literature, as discussed in more detail below, indicating that there are various situations when it is desirable to flocculate a suspension utilising both anionic polymer and cationic polymer. Often, the cationic polymer is the major component. In some instances both polymers are high molecular weight bridging flocculants while in others one of the polymers is a bridging flocculant and the other is a lower molecular weight flocculant, for instance of the type that would often be referred to as a coagulant.
In particular, when dewatering high solids mineral suspensions it is known to use high molecular weight anionic bridging flocculant followed by low molecular weight cationic flocculant, often referred to as a coagulant. Thus it is common to add a dilute aqueous solution of a high molecular weight anionic flocculant to the suspension, mix this flocculant into the high solids mineral suspension and then add a dilute aqueous solution of a low molecular weight cationic coagulant before dewatering on the belt press or otherwise.
In the various processes where counterionic flocculants are used, normal practice is to provide the counterionic flocculants as individual solutions and to keep these solutions separate from one another prior to addition to the suspension which is to be treated. This is because many combinations of counterionic flocculants, when mixed in solution, will tend to form a gelatinous precipitate due to counterionic precipitation occurring. In WO92/00248 counterionic flocculants of this type are added as a mixed powder direct into a suspension which is to be flocculated, so that they dissolve in the suspension. Unfortunately this necessitates prolonged mixing of the suspension because of the relatively slow rate of dissolution of high molecular weight bridging flocculants, and this prolonged mixing can be undesirable and wasteful of energy, especially when dealing with a high solids mineral suspension.
When dewatering other suspensions, it is known to formulate blends of anionic and cationic polymers under particular conditions which prevent counterionic precipitation occurring. For instance the presence of free acid and/or added inorganic electrolyte can reduce the risk of counterionic precipitation occurring, and careful selection of the proportions of the counterionic polymers can also minimise the risk of precipitation.
Unfortunately, this dictates that the polymers are selected for their solubility properties rather than, as is normally preferred, for their performance in the flocculation process. For instance in U.S. Pat. No. 3,539,510 the problem of counterionic precipitation is noted and is avoided by using, as the cationic polymer, a polymer which is substantially free of quaternary ammonium groups.
Disclosures of various other processes using both cationic and anionic polyelectroltyes are in DE-A-4421455, JP-A-05038404, JP-A-62129200, JP-A-62289300, JP-A-04300700, JP-A-63252600, CA-A-2041627, JP-A-02009500, JP-A-63012792, JP-A-62125893, JP-A-61234999, JP-A-61200897, JP-A-61054300 and JP-A-58215454 and GB-A-1549874, and Khim Tverd Topl (Moscow) 1976, 3, 57-64.
None of these alter the general situation which is that conventional blends of quaternary ammonium cationic and sodium anionic high molecular weight polymers
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International Search Report.
Allen Anthony Peter
McColl Philip
Staines George Thomas
Ciba Specialty Chemicals Water Treatments Limited
Crichton David R.
Lawrence Frank M.
Simmons David A.
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