CA2936519A1

CA2936519A1 - Method for treating suspensions of mineral particles in water with a thickener using a dual chemical treatment

Info

Publication number: CA2936519A1
Application number: CA2936519A
Authority: CA
Inventors: Trong Dang-Vu; Colin Klein; Scott Ramey
Original assignee: SPCM SA
Current assignee: SNF Group
Priority date: 2016-07-19
Filing date: 2016-07-19
Publication date: 2018-01-19

Abstract

Method for treating an aqueous suspension of mineral particles, said method comprising the step of feeding said suspension into a thickener, wherein the suspension is first flocculated by addition of an anionic flocculant, and then flocculation is enhanced by a subsequent addition of a cationic compound, wherein the two addition steps are separated by a mixing step.

Description

METHOD FOR TREATING SUSPENSIONS OF MINERAL PARTICLES IN
WATER WITH A THICKENER USING A DUAL CHEMICAL TREATMENT
FIELD OF THE INVENTION
The invention relates to a method for treating a suspension of mineral particles in water with a thickener using the following sequence: addition an anionic flocculant/
mixing step/addition of a cationic compound.
Suspensions of mineral particles in water include all types of tailings, or waste materials. The suspensions result from mineral ore processes. They are for instance industrial tailings and all mine wash and waste products resulting from exploiting mines, such as coal mines, diamonds mines, phosphate mines, metal mines (alumina, platinum, iron, gold, copper, silver, etc...). Suspensions can also result from drilling mud or tailings derived from the extraction of bitumen from oil sand. These suspensions generally comprise mineral particles such as clays, sediments, sand, metal oxides, and may contain oil mixed with water.
The invention is particularly dedicated to the treatment of oil sand tailings when said tailings are treated with a thickener.
BACKGROUND OF THE INVENTION
The treatment of tailings has become a technical, environmental and public policy issue.
It is common practice to use synthetic or natural polymers such as coagulants and flocculants to separate the solids from the liquid.
For a long time, and even nowadays, mineral tailings produced by physical or chemical ore treatment methods have been stored above ground in retention lagoons, ponds, dam or embankments in semi-liquid form. These large volumes of stored tailings therefore create a real hazard, notably if the dikes break.
The improvement of chemical and mechanical treatments of tailings is therefore a great challenge that needs to be addressed.
Various attempts were made in the past decades to improve the treatment of tailings in

2 order to efficiently recycle water and reduce the volume of tailings ponds.
Basically two types of method have been developed to treat tailings and separate solids from water: physical treatment and chemical treatment.
The main physical treatments include centrifugation, filtration, electrophoresis and electro-coagulation.
On the other hand, chemical methods are emerging. They include processes involving the addition of chemicals such as sodium silicate, organic flocculants, inorganic coagulants, oxidizing and reducing agents and carbon dioxide.
Thickeners.have been utilized extensively for numerous mining operations.
Thickeners are large, round tanks used to separate by sedimentation solids from liquids;
clear fluid overflows from the tank and particles sink to the bottom, the underflow. The main objective for a thickener is to obtain a high solid concentration in the underflow while maintaining a reasonably low solids concentration in the overflow.
Full scale thickener operations have shown that in general with a single anionic polymer treatment, tailings can be well-thickened. However, a single anionic polymer treatment cannot deliver satisfactory results when the fine/clay content of the thickener feed increases. Dual polymer treatments have been used to improve the thickener performance. In these dual polymer treatments, a cationic polymer is added to neutralize the charges of solid particles prior to the addition of the anionic polymer.
In all cases of dual polymer treatment for thickeners, the cationic polymer is always added before the anionic polymer addition.
Despite great advances in research over the last 10 years, there is still a need to develop new solutions to increase quality of overflow by decreasing solids content, as well as to improve the compaction of underflow thickener. There is a great need to recycle the overflow in order to recycle it immediately back to the process.
SUMMARY OF THE INVENTION
The present invention addresses the above needs by providing a method for improving the treatment of suspensions of solid particles in water thanks to the use of an anionic flocculant followed by a mixing step and the use of a cationic compound

3 According to the invention, it was found that the addition into a suspension, such as tailings, of an anionic flocculant, followed by the addition of a cationic compound significantly improves the quality of overflow and the compaction of the underflow of a thickener when these additions are separated by a mixing step.
DETAILED DESCRIPTION OF THE INVENTION
The invention relates to a method for treating an aqueous suspension of mineral particles comprising feeding said suspension into a thickener, wherein the suspension of mineral particles is first flocculated by addition of an anionic flocculant, and then flocculation is enhanced by a subsequent addition of a cationic compound, wherein the two addition steps are separated by a mixing step.
In a particular embodiment of the invention, an anionic flocculant is added into the pipe transporting the suspension of mineral particles to the thickener, and the subsequent addition of a cationic compound is made either into the pipe transporting the suspension of mineral particles, and/or either into the thickener.
In an alternative form of the invention, an anionic flocculant is added into the thickener and the subsequent addition of a cationic compound is made into the thickener.
In another alternative form of the invention, an anionic flocculant and a portion of the cationic compound are added into the pipe transporting the suspension of mineral particles to the thickener, and the remaining of the cationic compound is added into the thickener.
Within the thickener, the flocculated solids settle to the bottom of the thickener and are preferably mechanically raked to increase compaction of the solids and push the solids to the discharge at the bottom of the thickener, referred to as the underflow.
Recovered water, called overflow, flows over a weir at the top of the thickener.
According to a preferred embodiment, the overflow of the thickener can be used to prepare a solution' of the anionic flocculant or the cationic compound, said solution being used into the method of the invention.
According to the invention, the underflow of the thickener may be pumped to a deposition area for dewatering. In a preferred embodiment; the underflow of the thickener is flocculated during transport from the thickener to the deposition area. This

4 secondary flocculation is made with the addition of at least an anionic flocculant.
In general, aqueous suspensions of mineral particles are concentrated and contains preferably between 2% and 30% solids, more preferably between 5% and 30%
solids by weight, and even more preferably between 10 and 15% solids by weight of the suspension .of mineral particles. They are also called mineral tailings. But suspensions having lower solids content may be efficiently treated with the method of the invention.
It has been discovered that the method according to the invention is especially efficient when the suspension of mineral particles is a tailings resulting from oil sand extraction.
The treatment of oil sand tailings has recently become an increasing issue in Canada.
The oil sands tailings are alkaline aqueous suspensions which contain un-recovered residual bitumen, salts, soluble organic compounds, sands and clays. The tailings are discharged to tailings ponds for storage.
The dual chemical treatment of the invention outperforms the known processes.
It increases underflow solids content which leads to significant reduction of tailings volume and faster reclamation of the tailings. It increases recovered water quality.
Recovered 'water can be recycled immediately back to a process, for example for the bitumen extraction process or for making down polymer solution. This considerably reduces operation cost.
It also increases robustness of the process (widens operation windows). The dual chemical treatment of the invention is less sensitive to thickener feed variation, maintaining good performance with wide dosage windows, mixing.
Anionic flocculant In the present invention, the anionic flocculant is preferably a synthetic water soluble polymer and is advantageously obtained by polymerization of at least one non-ionic monomer and at least one anionic monomer.
The anionic flocculant is preferably a synthetic water soluble polymer having an anionic,ity ranging from between 10 to 70 mol%, preferably from 10 to 90 mol%, more preferably from 20 to 40 mol %. The molecular weight of the anionic flocculant is preferably comprised between 5 and 40 million daltons, and more preferably between 10 and 20 million.

Cationic Compound The cationic compound may be selected from the group consisting of a synthetic cationic flocculant, a synthetic cationic coagulant, a cationic inorganic coagulant, a

5 cationic natural polymer and semi-natural polymer.
In a preferred embodiment, the cationic compound is a synthetic cationic flocculant or a synthetic cationic coagulant.
Cationic Flocculant In the present invention, the cationic flocculant is preferably a synthetic water soluble polymer and is advantageously obtained by polymerization of at least one non-ionic monomer and at least one cationic monomer. The molecular weight of the cationic flocculant is preferably more than 2 million and less than 40 million dal-tons.
The cationic flocculant has a cationicity preferably ranging from between 20 to 100 mol%, more preferably from 40 to 80 mot %. The molecular weight of the cationic flocculant is preferably comprised between 5 and 40 million daltons, and more preferably between 7 and 20 million.
Cationic coagulant In the present invention, the cationic coagulant is preferably a synthetic water soluble polymer and is obtained advantageously by polymerization of at least one cationic monomer. The molecular weight of the cationic coagulant is preferably 1 million daltons or less.
The cationic coagulant has preferably a 100 mol% cationicity. The molecular weight of the cationic coagulant is preferably comprised between 50.000 and 1 million daltons, and more preferably between 200.000 and 1 million.
Cationic inorganic coagulant Cationic inorganic coagulant is preferably chosen , in the following group:
(poly)aluminium chloride, (poly)aluminium sulfate, (poly)aluminium chlorohydrtate, ferric chloride, ferric sulfate, ferrous chloride, ferrous sulfate, ferrous chlorosulfate, bentonite, gypsum, and calcium chloride, etc.
Cationic natural or semi-natural polymer The cationic natural polymer is preferably chosen in the following group:
cationic = CA 02936519 2016-07-19

6 cellulose, cationic dextrin, cationic dextran, chitosan, starch and starch derivates, polysaccharides, and alginates.
Monomers The monomers useful for the preparation of the water soluble polymer (anionic flocculent and cationic compound) according to the invention, are described hereafter.
Non-ionic monomers are preferably selected from the group comprising acrylamide;
methacrylamide; N-mono derivatives of acrylamide; N-mono derivatives of methacrylamide; N,N derivatives of acrylamide; N,N derivatives of methacrylamide;
acrylic 'esters; and methacrylic esters.
The most preferred non-ionic monomer is acrylamide.
Anionic monomers are preferably selected from the group comprising monomers having a carboxylic function and salts thereof; monomers having a sulfonic acid function and salts thereof; monomers having a phosphonic acid function and salts thereof, They include for instance acrylic acid, acrylamide tertio butyl sulfonic acid, methacryliC acid, maleic acid, itaconic acid; and hemi esters thereof.
The most preferred anionic monomers are acrylic acid, acrylamide tertio butyl sulfonic acid (ATBS), and salts thereof. Generally, salts are alkaline salts, alkaline earth salts or ammonium salts.
Cationic monomers are preferably selected from the group comprising dimethYlaminoethyl acrylate (DMAEA) quatemized or salified; dimethylaminoethyl methacrylate (DMAEMA) quaternized or salified; diallyldimethyl ammonium chloride (DADMAC); acrylamidopropyltrimethylammonium chloride (APTAC); and methacrylamidopropyltrimethylammonium chloride (MAPTAC).
Monomers having a hydrophobic character may also be used as comonomer for the preparation of the water soluble polymer (anionic or cationic) but at a concentration in weight. based on the total monomer content of less than 5%. They are preferably selected from the group comprising (meth)acrylic acid esters having an alkyl, arylalkyl or ethoxylated chain; derivatives of (meth)acrylamide having an alkyl, arylalkyl or dialkyl chain; cationic allyl derivatives; anionic or cationic hydrophobic (meth)acryloyl derivatives; and anionic or cationic monomers derivatives of (meth)acrylamide bearing a hydrophobic chain.

7 Polymers of the invention are linear or structured. As is known, a structured polymer is a polymer that can have the form of a star, a comb, or has pending groups of pending chains on the side of the main chain. The polymers of the invention, when structured, remain water soluble.
The polymerization can be carried out according to any polymerization techniques well known to= a person skilled in the art: solution polymerization, suspension polymerization, gel polymerization, precipitation polymerization, emulsion polymerization (aqueous or reverse) followed by an isolation step in order to obtain a powder, for example a spray drying step, or micellar polymerization followed by an isolation step for example a precipitation step in order to obtain a powder.
The polymerization is generally a free radical polymerization, preferably by inverse emulsion polymerization or gel polymerization. By free radical polymerization, we include free radical polymerization by means of U.V. azoic, redox or thermal initiators and also Controlled Radical Polymerization (CRP) techniques or template polymerization techniques.
According to particular embodiment of the invention, the anionic flocculant is a copolymer of acrylamide and acrylate, preferably a copolymer of 70mol%
acrylamide and 30mol% of sodium acrylate.
According to another particular embodiment of the invention, the cationic compound is a synthetic cationic flocculant of quaternized dimethylaminoethyl acrylate.
According to another particular embodiment of the invention, the cationic compound is a synthetic cationic coagulant of diallyldimethyl ammonium chloride.
Dosage In the method of the present invention, the anionic flocculant is added into the aqueous suspension of mineral particles to treat ill an amount preferably comprised between 25 and 500 g/ ton of solid mineral particles of the suspension of mineral particles, preferably between 75 and 150 g/ton.
The synthetic cationic flocculant is added into the aqueous suspension of mineral particles to treat in an amount comprised between 25 and 250 g/ ton of solid mineral particles of-the suspension of mineral particles, preferably between 25 and 75 g/ton.

8 The synthetic cationic coagulant is added into the aqueous suspension of mineral particles, to treat in an amount comprised between 25 and 250 g/ ton of solid mineral particles of the suspension of mineral particles, preferably between 25 and 75 g/ton.
The cationic inorganic coagulant is added into the aqueous suspension of mineral particles to treat in an amount preferably comprised between 50 and 2500 g/
ton of solids, preferably between 100 and 2000 g/ton of solids.
The cationic natural polymer is added into the aqueous suspension of mineral particles to treat in an amount comprised between 25 and 2500 g/ ton of solid mineral particles of the suspension of mineral particles, preferably between 100 and 2000 g/ton.
Anionic flocculant and cationic compound addition According to the invention, the water soluble polymers are preferably added in the form of an aqueous solution. Polymers are often available in powder form, sometimes in emulsion form. Prior to their addition, they are preferably prepared in concentrated solution, called "mother solution" at a concentration comprised between 0.1 and 2 weight ,%. A solution is prepared for each of the anionic and cationic water soluble polymer, generally by diluting the "mother solution". Then, said diluted solutions are added into the suspension of mineral particles in the appropriate amount and according the specific sequence of the method of the invention.
Mixing Step The mixing step may occur in the pipeline transporting the treated tailings to the thickener, in the thickener, or may be caused by inline static mixer or inline dynamic mixer.
Obviously, the following examples and figures are only given to illustrate the subject matter of the invention, which is in no way restricted to them.
Figure 1 is a diagram showing the underflow solids content (in weight %) and the overflow clarity (recovered water quality) (in clarity wedge) of thickener feed from an oil sand extraction site 1, treated by anionic and/or cationic polymer in using different addition sequences.
Figure 2 shows flocculation results for a thickener feed obtained from a different Oil Sands operator treated using varying addition sequences of anionic and cationic

9 polymer.
EXAMPLE
Laboratory tests show that the dual polymer process according to the invention increases the overflow quality (decreased solids content) and compaction of the thickener underflow.
Methodology Undiluted thickener feed and process water samples are obtained from two Oil Sands operators. The solids content of the thickener feed tailings is10%.
Three polymers are used:
- anionic flocculant, AF: copolymer of acrylamide and sodium acrylate (7 /3 molar ratio) ¨ Molecular weight of 13-14 Million daltons;
- cationic flocculant, CF: polymer of DMAEA MeCI - Molecular weight of Millions daltons, and - cationic coagulant, CC: PolyDADMAC ¨ Molecular weight of 600-800 Thousand daltons.
Polymer solutions are prepared at a concentration of 0.1% by weight using process water. 500 mL samples of thickener feed are placed in graduated cylinders and are treated either with anionic polymer alone (single AF), with anionic flocculant and cationic flocculant or coagulant in different sequences. Each polymer solution (anionic and cationic) was added in stages and the cylinder was inverted a number of times between different stages.
The flocculated samples were allowed to settle for 10 min after which, the quality of the overflow, as well as solid content of the resulting underflow, were determined.
Results The properties of the thickener feed tested are shown in Table 1.
Figure 1 shows the effect of polymer addition sequence on the underflow solids content and overflow clarity of thickener feed obtained from Site 1. It can be seen that the addition of cationic flocculant (CF) before anionic flocculant (AF) improves clarity and underflow solids content slightly. However, if CF is added after AF, the clarity and solids content increase significantly.
=

Figure 2 shows similar results for thickener feed obtained from Site 2. As seen previously, adding cationic coagulant (CC) after AF increases both clarity and underflow compaction.

Sample Solids content SFR Clay content pH
[wt%] [ /0]
Site 1 10.0 0.8 34 8.2 Site 2 10.0 1.0 27 8.5 Table 1: Thickener feed properties SFR is the Sand / Fine Ratio: ratio of mass of sand (coarse solids, >44 urn) to fines (fine solids, <44um).
=

Claims

1/ Method for treating an aqueous suspension of mineral particles, said method comprising the step of feeding said suspension into a thickener, wherein the suspension is first flocculated by addition of an anionic flocculant, and then flocculation is enhanced by a subsequent addition of a cationic compound, wherein the two addition steps are separated by a mixing step.

2/ Method according to claim 1, wherein the anionic flocculant and the cationic compound are added into a pipe transporting the suspension to the thickener.

3/ Method according to claim 1, wherein the anionic flocculant and a portion of the cationic compound are added into a pipe transporting the suspension of mineral particles to the thickener, and the remaining of the cationic compound is added into the thickener

4/ Method according to claim 1, wherein the anionic flocculant is added into a pipe transporting the suspension of mineral particles to the thickener, and the cationic compound is added into the thickener.

5/ Method according to claim 1, wherein the anionic flocculant and the cationic compound are added to the suspension of mineral particles into the thickener.

6/ Method according to any one of claims 1 to 5, wherein the cationic compound is a synthetic cationic flocculant or a synthetic cationic coagulant.

7/ Method according to any one of claims 1 to 6, wherein the anionic flocculant is a synthetic water soluble polymer having an anionicity ranging from between 10 to 90 mol%.

8/ Method according to any one of claims 1 to 7, wherein the anionic flocculant is added in an amount comprised between 25 and 500 g/ ton of solid mineral particles of the suspension of mineral particles.

9/ Method according to any one of claims 6 to 8, wherein the synthetic cationic flocculant or the synthetic cationic coagulant is added in an amount comprised between 25 and 250 g/ ton of solid mineral particles of the suspension of mineral particles.

10/ Method according to any one of claims 1 to 9, wherein the aqueous suspension of mineral particles has a concentration of solid mineral particles comprised between 2%
and 30% solids by weight of the suspension of mineral particles.

11/ Method according to any one of claims 1 to 10, wherein the aqueous suspension of mineral particles is from oil sand extraction.

12/ Method according to claim 1, wherein the anionic flocculant is a copolymer of acrylamide and acrylate and the cationic compound is a synthetic cationic flocculant of quaternized dimethylaminoethyl acrylate.

13/ Method according to claim 1, wherein the anionic flocculant is a copolymer of acrylamide and acrylate and the cationic compound is a synthetic cationic coagulant of diallyldimethyl ammonium chloride.