Liquid purification or separation – Processes – Making an insoluble substance or accreting suspended...
Reexamination Certificate
1999-01-20
2001-01-09
Hruskoci, Peter A. (Department: 1724)
Liquid purification or separation
Processes
Making an insoluble substance or accreting suspended...
C209S005000, C210S734000, C210S735000, C210S738000
Reexamination Certificate
active
06171506
ABSTRACT:
The invention relates to processes for thickening aqueous mineral suspensions by sedimentation using a specified aqueous polymeric composition.
It is often desired to increase the solids content of an aqueous mineral slurry by a thickening operation. In a thickening operation the aqueous slurry of mineral solids is usually placed in a vessel such as a sedimentation tank (or thickener) which controls the movement of the aqueous slurry. The mineral solids settle under gravity and concentrate at the bottom of the tank to form a lower fraction, generally called the underflow solids. The clarified aqueous portion forms an upper supernatant fraction, generally called the overflow.
In thickening operations it is desired to obtain high overflow clarity, ie the content of mineral solids in the upper fraction should be low. This allows recycling of the aqueous fraction if desired. It also ensures minimum loss of solids (in cases where these are of value) and minimum discharge of solids (where these are waste products).
It is also desired to obtain at the same time high underflow solids density, ie the amount of water entrapped in the underflow solids fraction is low. This provides a thickened solids fraction of suitable consistency for further dewatering.
At the same time it is desired to have a high settlement rate, so that clarification and thickening are achieved as rapidly as possible. It is also desirable to obtain reliable and consistent quality of the underflow solids and the overflow.
In order to improve overflow clarity and settlement rate it is known to add polymeric flocculants and/or coagulants to the aqueous suspension in the thickener. In particular high molecular weight anionic polymers, for instance copolymers of acrylamide and sodium acrylate, having very high intrinsic viscosity, for instance 20 dl/g or greater, can be used.
It is also known, in order to increase overflow clarity in cases where the anionic flocculant alone does not give acceptable results, to add low molecular weight cationic coagulant polymers to the aqueous suspension in the thickener. The low molecular weight cationic coagulant, when used, is not added simultaneously with the high molecular weight anionic flocculant, since it is well known that polymers of this type tend to interact physically or chemically with each other, for instance to form a gel or a precipitate. This has hitherto generally been believed to reduce their effectiveness in carrying out their intended purposes of improving overflow clarity. Further, sequential addition is conventionally used because it is believed to provide a desirable floc structure.
In WO92/00248 we describe a process in which suspended solids are separated from an aqueous suspension by a sedimentation process. The invention described in this publication overcomes the well-known problem of co-precipitation of counterionic flocculants, by adding them directly to the suspension, usually as a flowing stream before it reaches the sedimentation tank, in the form of solid polymer. This is said to minimise incompatibility problems.
Other publications also concern sedimentation processes. CA 2,041,627 discloses clarifying slurries and muds by sedimentation with the addition of a high molecular weight polyacrylamide (ie a non-ionic polymer) and a minor amount of a low molecular weight ionic material which can be anionic or cationic. M. Kaiser in Comm. Eur. Communities, [Rep] EUR (1993), EUR 14621, 81 pp discloses studies which test the effect of combined flocculation with anionic and cationic polymers on sedimentation. The anionic and cationic agents are not stated to be added simultaneously and it is assumed that if they are added simultaneously the addition is carried out under conditions such that counterionic precipitation does not occur (as is conventional due to the accepted problems with counterionic precipitation). Alexsandrova et al in Khim. Tverd. Topl. (Moscow) (1976), (3), 57 to 64 disclose comparisons between sedimentation of waste anthracite and waste slurries from coal flotation. The flocculation of the anthracite slurries is tested using cationic flocculant alone or both cationic and anionic flocculants. It appears that when a combination is used an excess of cationic material is included. It is not stated whether the materials are added simultaneously and again it is assumed that if they are added simultaneously it is under conditions in which, as is conventional, counterionic precipitation is not permitted to occur.
Thus none of these disclosures is incompatible with the general belief that conventional blends of quaternary ammonium cationic and sodium anionic high molecular weight polymers should generally be avoided because of precipitation during make-up and dissolution and that thickening of suspensions of mineral solids is best performed using a solution of very high molecular weight water-soluble anionic bridging polymeric flocculant, usually alone but if necessary followed by a solution of low molecular weight water-soluble cationic coagulant to obtain acceptable results.
It would be desirable to be able to improve thickening operations so as to obtain, as well as good overflow clarity and rapid settlement, improved underflow solids density. It is generally found that although the standard application of high molecular weight anionic flocculant improves underflow solids density in comparison with no flocculent treatment, further improvement would be desirable. It is difficult to add polymer directly to the underflow solids in order to attempt to improve underflow solids density.
In the invention we provide a process of thickening an aqueous suspension of mineral solids by sedimentation comprising adding to the suspension water-soluble anionic bridging polymeric flocculant having intrinsic viscosity at least 5 dl/g and water-soluble cationic polymeric flocculant, allowing the solids to settle under gravity so as to form a lower underflow fraction containing a concentration of solids greater than that of the suspension and an upper overflow fraction containing a concentration of solids less than that of the suspension and removing the underflow fraction, characterised in that the anionic and cationic flocculants are added to the suspension as an aqueous composition formed by blending 1 part by weight of the cationic polymeric flocculant with 2 to 99 parts by weight, preferably 2 to 20 parts by weight of the anionic polymeric flocculant and sufficient water to give a total polymer concentration of below 5wt % and under conditions whereby counter-ionic precipitation can occur and in the aqueous composition substantially all of the anionic polymeric flocculant which is not precipitated by the cationic polymeric flocculant is in solution.
Thus the aqueous composition preferably comprises 1 part by weight of the cationic polymeric flocculant with 2 to 20 parts by weight of the anionic polymeric flocculant. More preferably the aqueous composition comprises anionic flocculant to cationic floculant in the range 85:15 to 95:5, preferably around 90:10.
In the invention therefore we deliberately use materials which do undergo counterionic precipitation, we have an excess of the anionic polymer so that a significant amount of anionic polymer can be in solution even in the presence of some counterionic precipitation and we mix the resultant aqueous composition into the aqueous suspension of mineral solids.
We have found that it is unexpectedly possible to obtain improved underflow solids density at the same time as good overflow clarity and settlement rate by a single addition to the aqueous suspension of the defined aqueous polymer composition containing anionic and cationic polymeric flocculent. Addition of a single composition containing anionic and cationic polymer can give better results than has previously been obtainable in conventional art processes using addition of very high molecular weight anionic flocculant. In particular we can improve underflow solids density without the need to attempt to add flocculant or coagulant to the underflow s
Allen Anthony Peter
Sedders Peter Albert
Ciba Specialty Chemicals Water Treatments Limited
Crichton David R.
Hruskoci Peter A.
LandOfFree
Thickening of aqueous mineral suspensions does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Thickening of aqueous mineral suspensions, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Thickening of aqueous mineral suspensions will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2456666