Dewatering of aqueous suspensions

Details Dewatering of aqueous suspensions Dewatering of aqueous suspensions

2000-04-10

2002-04-23

Lovering, Richard D. (Department: 1712)

Colloid systems and wetting agents; subcombinations thereof; pro

Compositions containing an agent for breaking ; processes of...

C210S728000, C252S194000, C524S922000

Reexamination Certificate

active

06376561

ABSTRACT:

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. No. 4,342,953, U.S. Pat. No. 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) 976, 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 should generally be avoided because of precipitation during dissolution, and that dewatering of high solids mineral suspension is best performed using a solution of high molecular weight, water-soluble anionic bridging polymeric flocculant followed by a solution of low molecular weight water-soluble cationic flocculant or coagulant.
The object of the present invention is to improve the dewatering of high solids mineral suspensions, especially as regards the speed of drainage or dewatering. This is preferably achieved utilising a single flocculant material.
A process according4 to the invention for pressure dewatering an aqueous mineral suspension having a mineral solids content of at least 150 g/l comprises mixing into the suspension water-soluble anionic bridging polymeric flocculant having intrinsic viscosity at least 5 dl/g and water-soluble cationic polymeric flocculant, allowing the suspension to flocculate, and dewatering the flocculated suspension under pressure, and in this process the anionic and cationic flocculants are mixed into the suspension by blending one part by weight of the cationic polymeric flocculant with 2 to 20 parts by weight of the anionic polymeric bridging flocculant and sufficient water to give a polymer concentration of below 5% and under conditions whereby counterionic precipitation can occur and thereby forming an aqueous composition in which substantially all the anionic polymer which is not precipitatable by the cationic polymer is in solution, and mixing this aqueous composition into the suspension.
Thus in the invention we deliberately use materials which will undergo counterionic precipitation, we have an excess of the anionic flocculant so that a significant amount of anionic flocculant can be in solution irrespective of the amount of counterionic precipitation, and then we mix the resultant aqueous composition into the high solids suspension.
When activating polymer with water (i.e. dissolving the polymer) prior to adding it to a suspension, normal practice requires that the activated aqueous composition should be as homogeneous as possible and should contain substantially no visible evidence of precipitated or gelatinous material. In the invention, however, we find that improved performance is obtained even though the aqueous composition, on close examination, may be seen to be less homogeneous, often substantially less homogeneous, than would normally thought to be desirable.
We believe that what is happening is that the anionic polymer flocculant initially goes substantially wholly into solution but some of it is then precipitated onto or with the cationic flocculant to form a precipitate (which may be colloidal or larger). We believe that it is beneficial to add the aqueous composition to the mineral suspension while the aqueous composition contains both the dissolved anionic flocculant and the precipitate containing cationic flocculant and some of the anionic flocculant.
In order to mix the resultant aqueous composition into the high solids suspension, it is necessary to apply the conventional vigorous mixing which is always associated with distributing aqueous flocculant into a high solids suspension, such as screw mixing. This conventional vigorous mixing necessarily involves the application of high turbulence, agitation and shear to the combination of suspension and aqueous flocculant composition. We believe that this conventional high shear mixing initially distributes the dissolved anionic flocculant through the suspension and initiates flocculation of that but, then gradually degrades the precipitate and releases cationic flocculant into the suspension.
Whatever the mechanism, the process of the invention results in an unusual floc structure and in accelerated and improved dewatering of the suspension. Thus, the invention gives improved dewatering compared to the use of dissolved anionic polymer alone or dissolved anionic polymer followed by the conventional low molecular weight cationic polymer solution.
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