OA10640A - Method of depressing non-sulfide silicate gangue minerals - Google Patents

Abstract

A method for the depression of non-sulfide, silicate gangue minerals is provided wherein the depressant is a graft polymer of polyvinyl alcohol and an acrylamide or a mixture thereof with a polysaccharide.

Description

010640 1

METHOD OF DEPRESSING NON-SULFIDE SILICATE GANGUE MINERALS

BACKGROUND OF INVENTION 5 The présent invention relates to froth flotation processes for recovery of value sulfide minerais from base métal sulfide ores. More particularly, it relates to a method for thedépréssion of non-sulfide silicate gangue minerais in the beneficiation of value sulfideminerais by froth flotation procedures.

Certain theory and practice States that the success of a sulfide flotation process 10 dépends to a great degree on reagents called collectons that impart sélective hydrophobicityto the minerai value which has to be separated from other minerais.

Certain other important reagents, such as the modifiers, are also responsible for thesuccessful flotation séparation of the value sulfide and other minerais. Modifiers include,but are not necessarily limited to, ali reagents whose principal function is neither collecting 15 nor frothing, but usually one of modifying the surface of the minerai so that it does not float.

In addition to attempts at making sulfide collectons more sélective for value sulfideminerais, other approaches to the problem of improving the flotation séparation of valuesulfide minerais hâve induded the use of modifiers, more particularly depressants, todepress the non-sulfide gangue minerais so that they do not float along with sulfides 20 thereby reducing the levels of non-sulfide gangue, minerais reporting to the concentrâtes. A depressant is a modifier reagent which acts selectively on certain unwanted minerais andprevents or inhibits their flotation.

In sulfide value minerai flotation, certain non-sulfide silicate gangue minerais présenta unique problem In that they exhibit naturel floatability, Le. they float independent of the 25 sulfide value minerai collectais used. Even if very sélective sulfide value minerai collectorsare used, these silicate minerais report to the sulfide concentrâtes. Talc and pyrophyllite,both belonging to the class of magnésium silicates, are particularly troublesome in that theyare naturally highly hydrophobie. Other magnésium silicate minerais belonging to theclasses of divines, pyroxenes, and serpentine exhibit various degrees of floatability that 30 seems to vary from one ore deposit to the other. The presence of these unwanted mineraisin sulfide value minerai concentrâtes causes many problème Le. a) they increase the massof the concentrâtes thus adding to the cost of handling and transportation of theconcentrate, b) they compete for space in the froth phase during the flotation stage therebyreducing the overall sulfide value minerai recovery, and c) they dilute the sulfide concentrate 35 with respect to the value sulfide minerai content which makes them less suitable, and in 2 010640 some cases unsuitable, for the smelting thereof because they interfère with the smeltingoperation.

The depressants commonly used in sulfide flotation include such materials asinorganic salts (NaCN, NaHS, SO2, sodium metabisulfite etc) and small amounts of organiccompounds such as sodium thioglycolate, mercaptoethanol etc. These depressants areknown to be capable of depressing sulfide minerais but are not known to be depressantsfor non-sulfide minerais, just as known value sulfide collectons are usually not goodcollectons for non-sulfide value minerais. Sulfide and non-sulfide minerais hâve vastlydifferent bulk and surface Chemical properfies. Their response to various Chemicals is alsovastly different. At présent, certain polysaccharides such as guar gum and carboxy methylcellulose, are used to depress non-sulfide silicate gangue minerais during sulfide flotation.Their performance, however, is very variable and on some ores they show unacceptabledepressant activity and the effective dosage per ton of ore is usually very high (as muçhas 1 to 10 Ibs/ton). Their depressant activity is also influenced by their source and is notconsistent from batch to batch. Furthermore, these polysaccharides are also valuable r sources of food Le. their use as depressants reduces their usage as food and, storagethereof présents particular problems with regard to their attractiveness as food for vermin.Lastly, they are not readily miscible or soluble in water and even where water solutionsthereof can be made, they are not stable. U.S. Patent 4,902,764 (Rothenberg et al.)describes the use of polyacrylamide-based synthetic copolymers and terpolymers for useas sulfide minerai depressants in the recovery of value sulfide minerais. U.S. Patent4,720,339 (Nagaraj et al) describes the use of polyacrylamide-based synthetic copolymersand terpolymers as depressants for silicious gangue minerais in the flotation beneficiationof non*suifide value minerais, but not as depressants in the benefication of sulfide valueminerais. The '339 patent teaches that such polymers are effective for silica dépréssionduring phosphate flotation which also in the flotation stage uses fatty acids and non-sulfidecollectors. The patentées do not teach that such polymers are effective depressants fornon-sulfide silicate gangue minerais In the recovery of value sulfide minerais. In fact, suchdepressants do not exhibit adéquats depressant activity for non-sulfide silicate mineraisduring the beneficiation of sulfide value minerais. U.S. Patent 4,220,525 (Petrovich)teaches that polyhydroxyamines are useful as depressants for gangue minerais includingsilica, silicates, carbonates, sulfates and phosphates in the recovery of non-sulfide mineraivalues. Illustrative examples of the polyhydroxyamines disclosed include aminobutanetriols,aminopartitols, aminohexitols, aminoheptitols, aminooctitols, pentose-amines, hexoseamines, amino-tetrols etc. U.S. Patent 4,360,425 (Lim et al) describes a method for 3 010640 improving the results of a froth Notation process for the recovery of non-sulfide mineraivalues wherein a synthetic depressant is added which contains hydroxy and carboxyfunctionalities. Such depressants are added to the second or amine stage flotation of adouble float process for the purpose of depressing non-sulfide value minerais such asphosphate minerais during amine flotation of the siliceous gangue from the second stageconcentrate. This patent relates to the use of synthetic depressant during amine flotationsonly.

In view of the foregoing and especially in view of the teachings of U.S. 4,902,764which teaches the use of certain polyacrylamide-based copolymers and terpolymers forsulfide minerai dépréssion during the recovery of value sulfide minerais, we hâveunexpectedly found that certain polymers, alone or in conjunction with polysaccharide, areindeed excellent depressants for non-sulfide silicate gangue minerais (such as talc,pyroxenes, olivines, serpentine, pyrophyllite, chlorites, biotites, amphiboles, etc). Thesesynthetic polymer depressants and blends with polysaccharides hâve now been found tobe excellent alternatives to the polysaccharides used currently alone since they are readilymiscible or soluble in water, are non-hazardous and their water solutions are stable. Theuse thereof will increase the availability of polysaccharides as a valuable human foodsource and their performance is not variable. The polymers, moreover, can bemanufactured to adhère to stringent spécifications and, accordingly, batch-to-batchconsistency is guaranteed. The synthetic polymers lend themselves readily to modificationof their structure, thereby permitting tailor-making of depressants for a given application.

SUMMARYOFTHE INVENTION

In accordance with the présent invention there is provided a method whichcomprises beneficiating value sulfide minerais from ores with the sélective rejection of non-sulfide silicate gangue minerais by: a. providing an aqueous pulp slurry of finely-divided, liberation-sized oreparticles which contain said value sulfide minerais and said non-sulfidesilicate gangue minerais; b. conditioning said pulp slurry with an effective amount of non-sulfide silicategangue minerai depressant, a value sulfide minerai collecter and a frothingagent, said depressant comprising either (1) a polymer of polyvinylaicoholto which is grafted an acrylamide monomer and, optionally, a comonomercopolymerizable with said acrylamide monomer, or a mixiture of said 010640 polymers, or (2) a blend of said polymer or polymère and a polysaccharide,and c. collecting the value suifide minerai having a reduced content of non-sulfidesilicate gangue minerais by froth flotation.

DESCRIPTION QF THE INVENTION INCLUDING PREFERRED EMBODIMENTS

The polymer depressants used in the présent invention may comprise, as the graftedmonomers, such acrylamides as acrylamide per se, alkyl acrylamides such asmethacrylamide, ethacrylamide and the like.

The comonomers may comprise any monoethylenically unsaturated monomercopolymerizable with the acrylamide monomer such as hydroxyalkylacrylates andméthacrylates e.g. 1,2-dihydroxypropyl acrylate or méthacrylate; hydroxyethyl acrylate orméthacrylate; glycidyl méthacrylate, acrylamido glycolic acid; hydroxyalkylacrylamides such.as N-2’hydroxyethylacrylamide; N-1-hydroxypropylacrylamide; N-bis(1,2-dihydroxyethyl)acrylamide; N-bis(2-hydroxypropyl)acrylamide; and the like, acrylic acid;methacrylic acid; alkali métal or ammonium salts of acrylic and/or methacrylic acid; vinylsulfonate; vinyl phosphonate; 2-acrylamido-2-methyl propane sulfonic acid; styrene sulfonicacid; maleic acid; fumaric acid; crotonic acid; 2-sulfoethylmethacryiate; 2-acrylamido-2-methyl propane phosphonic acid acrylonitrile; vinyl alkyl ethers, such as vinyl butyl ether,and the like.

The effective weight average molecular weight range of the polyvinylalcohols issurprisingly very wide, varying from at least about ten thousand, preferably from aboutthirty thousand to millions e.g. 2 million preferably to about 1 million..

The polysaccharides useful as a component in the depressant compositions usedin the process of the présent invention include guar gums; modified guar gums; cellulosicssuch as carboxymethyl cellulose; starches and the like. Guar gums are preferred.

The ratio of the polysaccharide to the grafted polymer in the depressant compositionshould range from about 9:1 to about 1:9 respectively, preferably from about 7:3 to about3:7, respectively, most preferably from about 3:2 to 2:3, respectively.

The dosage of depressant useful in the method of the présent invention ranges from about 0.01 to about 10 pounds of depressant per ton of ore, preferably from about 0.1 to about 5 Ib./ton, most preferably from about 0.1 to about 1.0 Ib./ton of ore.

When mixtures of the grafted polyvinyialcohol polymers discussed above are used as the depressant, they may be uSed in ratios of 9:1 to 1:9, preferably, 3:1 to 1:3, most 5 010640 preferably 3:2 to 2:3, respectively.

The weight ratio of the acrylamide to the polyvinyl alcohol in the depressants usedherein should range from about 99 to 1 to about 1 to 1, preferably from about 10 to 1 toabout 4 to 1 respectively. The concentration of the optional copolymerizable comonomers 5 should be less than about 50%, as a weight percent fraction, preferably from about 1 toabout 30% of the total monomers.

The acrylamide monomer grafted polyvinylalcohol may be prepared by any methodknown to those skilled in the art such as that taught in EPO-A-117978; Melnik et al; Dokl.Akad. Nauk Uter. SSR, Ser B; Geol. Khim. Brol. Nanki (6), 48-51, Russian 1987; Burrows 10 et al; J. Photochem. Photobiol. A,63(1), 67-73, English, 1992. Generally, the acrylamidemonomer, alone or in conjunction with the optional comonomer, may be grafted onto thepolyvinylalcohol in the presence of ceric ion catalyst, e.g. ceric ammonium nitrate, as acatalyst at a température ranging from about 10-50° with intermittent cooling for from about 2-6 hours. Termination of the reaction is effected after a constant solution viscosity is 15 reached by raising the pH with diluted caustic solution to neutrai or above. Generally, theamount of catalyst employed should range from about 0.3 to about 5.0%, by weight, basedon the combined weignt of monomers to be grafted, preferably from about 0.8 to about4.0%, same basis, the preferred range resulting in a grafted polymer having a moreeffective depressant activity. 20 The new method for beneficiating value sulfide minerais employing the synthetic depressants of the présent invention provides excellent metallurgical recovery with improvedgrade. A wide range of pH and depressant dosage are permissible and compatibility of thedepressants with frothers and sulfide value minerai collectors is a plus. 5 The présent invention is directed to the sélective removal of non-sulfide silicate gangue minerais that normally report to the value sulfide minerai flotation concentrate, eitherbecause of naturel floatability or hydrophobicity or otherwise. More particularly, the instantmethod effects the dépréssion of non-suifide magnésium silicate minerais while enablingthe enhanced recovery of sulfide value minerais. Thus, such materials may be treated as, 10 but not limited to, the following:

Talc

Pyrophyllite

Pyroxene group of MineraisDiopside 15 Augite

Homeblendes

Enstatite

Hypersthene 6 010640

Ferrosilite

Bronzite

Amphibole group of mineraisTremoliteActinoliteAnthophyllite

Biotite group of mineraisPhlogopiteBiotite

Chlorite group of mineraisSerpentine group of minerais

Serpentine

Chrysotile

Palygorskite

Lizardite

Anitgorite

Olivine group of mineraisOlivineForsteriteHortonoliteFayalite

The following examples are set forth for purposes of illustration only and are not tobe construed as limitations on the présent invention except as set forth in the appendeddaims. Ail parts and percentages are by weight unless otherwise specified. In theexamples, the following designate the monomers used: AMD = acrylamide PVA = poiyvinylalcohol AA - acrylic acid MAMD = methacrylamide AN = acrylonitrile VBE - vinylbutylether t-BAMD = t-butyiacryiamide HPM s 2-hydroxpropyi méthacrylate AMPP = 2-acrylamido-2-methylpropane phosphonic add CMC = cartxjxymethyl cellulose C = comparative

Background Example 1

Préparation of Ceric Ammonium Nitrate catalyst solution 54.82 parts of ceric ammonium nitrate (0.1 M) are dissolved in one liter of 1.0 N nitric acid.

Backaround Example 2

Graft Copolymerization

To a solution of 5.0 parts of polyvinyl alcohol (mol. wt. approx. 10,000) in 150 parts 7 010640 of water, 30.9 parts of a 52% acrylamide monomer solution are added. With good agitation5 parts of the above ceric catalyst solution are introduced slowly. The reaction mixture iskept at 25- 30°C with intermittent cold water cooling. The graft polymerization is continuedfor 3 to 4 hours until a constant solution viscosity is obtained. The reaction is terminatedby raising the pH of the mixture with diluted caustic solution to a neutral or slightly alkalinepH.

Backoround Examoles 3 and 4

Following the above Example 2, graft copolymers of AMD and PVA of highermolecular weight, i.e., 20,000 and 50,000, are also prepared.

Backqround Examole 5 A graft terpolymer is prepared by adding 30.9 parts of a 52% acrylamide monomersolution and 7.2 parts of acrylic acid monomer to a solution of 5.0 parts of PVA (mol. wt.50,000) in 150 parts water. A total of 10 parts of ceric catalyst solution are used for thispréparation. Other copolymers are prepared similariy, e.g. using acrylonitrile and vinyl butylether. EXAMPLES 1-10

An ore containing approximately 3.3% Ni and 16.5% MgO (in the form of Mgsilicates) is ground in a rod mill for 5 min. to obtain a pulp at a size of 81% -200 mesh. Theground pulp is then transferred to a flotation cell and is conditioned at naturel pH (-8-8.5)with 150 parts/ton of copper sulfate for 2 min., 50 to 100 parts/ton sodium ethyl xanthatefor 2 min. and then with the desired amount of a depressant and an alcohol frother for 2min. First stage flotation is then conducted by passing air at approximately 3.5-5 l/min. anda concentrate is collected. In the second stage, the pulp is conditioned with 10 parts/tonof sodium ethyl xanthate, and specified amounts of the depressant and the frother for 2 min.and a concentrate is collected. The conditions used in the second stage are also used inthe third stage and a concentrate is collected. Ail of the flotation products are filtered, driedand assayed.

The resuits for the depressant activity of two AMD/PVA graft copolymers arecompared with that of guar gum and polyvinylalcohoi in Table 1. In the absence of anydepressant, the Ni recovery is 96.6% which is considered very high and désirable; the MgOrecovery is 61.4% which is also very high, but considered highly undesirable. The Ni gradeof 4.7% obtained is only slightly higher than that in the original feed. With guar gum at 420and 500 parts/ton, the MgO recovery is in the range of 28.3 to 33.5% which is considerabiylower than that obtained in the absence of a depressant, and Ni recovery is about 93%which is lower than that obtained in the absence of depressant. A réduction in Ni recovery 8 010640 is to be expected in the process of reducing MgO recovery since there is invariably somemineralogical association of Ni minerais with the Mg-silicates and, when the latter aredepressed, some Ni minerais are also depressed. When the graft copolymers of theprésent invention are used, there is a much greater réduction in the MgO recoveriescompared to that with guar gum. The Ni recoveries are aiso slightiy lowered compared withthat of guar gum, but the Ni grades in the concentrate are much higher than those obtainedwith guar gum. These findings indicate the very strong depressant activity of the graftcopolymers at ail of the dosages used. They also suggest that much lower dosages of thegraft copolymers can be used; in this case the Ni recoveries would improve whilemaintaining the low MgO recoveries.

The résulte also demonstrate that when a polyvinyl alcohol polymer is used as is,i.e., without grafting to the AMD monomer, the metallurgical performance is poor;depressant activity is quite non-selective. The Ni recovery is greatly reduced (82.9% vs.the recovery of 88% for the graft copolymer under identical conditions). Thus the graftcopolymer is much superior to as-is polyvinyl alcohol.

Table 1

Feed Assay: 3.31% Ni and 17.58% MgO

Example Depressant Parts/Ton Ni Rec. Ni Grade MgO Rec. 1C None 0 96.6 4.7 61.4 2C Guar Gum 350+70+80 93.0 7.7 28.3 3C Guar Gum 300+60+60 92.9 6.7 33.5 4 AMD/PVA (23K) 75/25 300+70+80 91.6 9.2 18.7 5 AMD/PVA(23K) 75/25 350+85+100 90.1 9.6 14.2 6 AMD/PVA(23K) 75/25 350+70+80 90.0 8.3 20.7 7 AMD/PVA(23K) 75/25 280+56+64 90.6 7.5 23.0 8 AMD/PVA(50K) 75/25 350+70+80 88.0 9.5 16.7 9 AMD/PVA(50K) 75/25 280+56+64 84.8 7.8 17.3 10C PVA(50K) 350+70+80 82.9 6.4 38.1 9 010640 EXAMPLES 11-20

The gangue silicate minerais from the same ore as in Examples 1-10 are treated with a dosage of depressant of 1.0 Ib./ton unless otherwise specified in accordance with the flotation procedure thereof. The results are set forth in Table II, below, the lower the value under the column heading % Recovery (gangue silicate) the better the depressant.

Table II

Example Depressant % Recovery (Gangue Silicate) 11C None 85 12C . Polyvinyi alcohol 75 13C Guar 3.4 14 60/40 AMD/PVA 8.9 15 75/25 AMD/PVA 8.7 16 80/20 AMD/PVA 3.0 17 87/13 AMD/PVA 1.3 18 90/10 AMD/PVA 0 19 92.5/7.5 AMD/PVA 7.9 20 97.5/2.5 AMD/PVÀ 7.8 ΕΧΑΜΕΙΕ5-21:24 A PVA graft copolymer is prepared in accordance with Background Examples 1-5 above, with varying amounts of ceric iron catalyst. The results are shown in Table III, below, following the flotation procedure of Examples 11-20. 10 010640

Table III

Example Depressant % Catalyst (Ce) % Recovery (Gangue Silicate) 21 75/25 AMD/PVA 0.5 44.6 22 75/25 AMD/PVA 1.3 8.7 23 75/25 AMD/PVA 1.96 3.0 24 75/25 AMD/PVA 2.6 2.6 EXAMPLES 25-28

The ftotation procedure of Examples 11-20 is again followed except that differentgraft copolymers are employed. The results are set forth in Table IV below.

Table IV

Example Depressant % Recovery(Gangue Silicate) 25 AMD/AN/PVA 80/10/10 7.75 26 AMD/AN/PVA 85/5/10 3.28 27 AMD/AA/PVA 66/24/10 16.60 28 AMD/VBE/PVA 80/10/10 14.70 EXAMPLES 29-31

The flotation procedure of Examples 11-20 is again followed except that the molecularweight of the PVA is varied. The results are shown in Table 5, below.

Table V

Example Depressant Molecular Wt. (PVA) % Recovery (Gangue Silicate) 35 90/10 AMD/PVA 9-10K 7.1 36 90/10 AMD/PVA 13-23K 4.6 37 90/10 AMD/PVA 31-50K 3.3 11 010640 EXAMP.LE.3g

The flotation procedure of Examples 1-10 is again followed except that the depressant is a 1:1 blend of the depressants of Example 8 and Example 27. Similar results areachieved. EXAMP1ES.39-42

An ore containing approximately 3.3% Ni and 16.5% MgO (in the form of Mgsilicates) is ground in a rod mill for 5 min. to obtain a pulp at a size of 81% -200 mesh. Theground pulp is then transferred to a flotation cell and is conditioned at natural pH (-8-8.5)with 150 parts/ton of copper sulfate for 2 min., 50 to 100 parts/ton sodium ethyl xanthatefor 2 min. and then with the desired amount of depressant blend and an alcohol frother for2 min. First stage flotation is then conducted by passing air at approximately 3.5-5 l/min.and a concentrate is collected. In the second stage, the pulp is conditioned with 10parts/ton of sodium ethyl xanthate, and specified amounts of the depressant blend and thefrother for 2 min. and a concentrate is collected. The conditions used in the second stageare also used in the third stage and a concentrate is collected. Ail of the flotation productsare filtered, dried and assayed.

The results for the depressant activity of a 1:1 blend of AMD/PVA graft copolymerwith guar gum is compared with that of guar gum alone and the graft copolymer atone atthe same dosage in Table VI. In the absence of any depressant, the Ni recoveiy is 96.6%which is considered very high and désirable; the MgO recovery is 61.4% which is also veryhigh, but considered highly undesirable. The Ni grade of 4.7% obtained is only slightlyhigher than that in the original feed. With guar gum at 500 parts/ton, the MgO recovery is28.3%, which is considerabiy lower than that obtained in the absence of depressant, andNi recovery is about 93% which is also lower than that obtained in the absence of adepressant. A réduction in Ni recovery is to be expected in the process of reducing MgOrecovery since there is invariably some mineraiogical association of Ni minerais with theMg-silicates and, when the latter are depressed, some Ni minerais are also depressed.With the AMD/PVA graft copolymer at the same dosage, there is significant réduction inMgO recovery compared with that of guar gum. In the case of the blend of guar gum andsynthetic polymer at the same dosage, however, there is further increase in the depressantactivity compared with that of the two components individually. The grade of the Ni inconcentrate also increases. The results also suggest that much lower dosages of the blendcan be used; in this case the Ni recoveries would improve while maintaining the low MgOrecoveries. 12 010640

Table VI

Feed Assay: 3.31% Ni and 17.58% MgO

Example Depressant Parts/Ton Ni Rec. Ni Grade MgO Rec. 39C None 0 96.6 4.7 61.4 40C Guar Gum 350+70+80 93.0 7.7 28.3 41C AMD/PVA (23K) 75/25 350+70+80 90.0 8.3 20.7 42 Guar Gum and AMD/PVA (23K)75/25; 1:1 350+70+80 88.6 9.2 18.7

Examples 43-53

When the procedure of Examples39-42 are again followed except that the15 depressant components are varied, as are their concentrations, as set forth in Table VII, beiow, similar results are achieved. 13 010640

Table VII

Example Grafted Polymer (GP) Polysaccharide (PS) GP:PS Ratio 43 AMD/AN/PVA 80/10/10 Guar Gum 9:1 44 AMD/PVA (50K) 75/25 CMC 4:1 45 AMD/AA/PVA 66/24/10 Starch 1:1 46 AMD/PVA 97.5/2.5 Guar Gum 1:9 47 AMD/AN/PVA 85/5/10 Modified Guar 2:3 48 AMD/PVA 87/13 Starch 3:2 49 AMD/VBE/PVA 80/10/10 Guar Gum 2:1 50 AMD/PVA* CMC 1:1 51 AMD/PVA (9-10K) Guar Gum 3:2 52 AMD/PVA(13-23K) Guar Gum 3:2 53 AMD/PVA (31-50K) Guar Gum 3:1

Made with 2.6% of Ce catalyst.

Claims (11)

14 010640 WE CLAIM:

1. A method which comprises beneficiating value suifide minerais from oreswith sélective rejection of non-sulfide silicate gangue mineralswhich comprises: a. providing an aqueous pulp slurry of finely-divided, liberation-sized oreparticles which contain said value sulfide minerais and said non-sulfidesilicate gangue minerais; b. conditloning said pulp slurry with an effective amount of non-silicategangue minerai depressant, a value sulfide minerai collector and afrothing agent, said depressant comprising either (1) a polymer ofpolyvinylalcohol onto which is grafted an acrylamide and, optionally, acomonomer copolymerizable with said acrylamide, or (2) a mixture of saidpolymer and a polysaccharide and c. collecting the value sulfide minerai having a reduced content of non-sulfide silicate gangue minerais by froth flotation.

2. A method according to Claim 1 wherein the weight ratio of the acrylamideto the polyvinyi alcohol ranges from about 99 to 1 to about 1 to 1, respectively.

3. A method according to Claim 1 wherein the graft polymer contains lessthan about 50 weight percent of said comonomer.

4. A method according to Claim 1 wherein the molecular weight of thepolyvinyi alcohol is at least about 10,000.

5. A method according to Claim 1 wherein said comonomer, when présent,is selected from the group consisting of acrylonitrile, (meth)acrylic acid and a vinylalkylether.

6. A method according to Claim 1 wherein the weight ratio of the acrylamideto the polyvinyi alcohol ranges from about 10 to 1 to about 4 to 1.

7. A method according to Claim 1 wherein the graft polymer contains fromabout 1 to about 30 weight percent of said comonomer.

8. A method according to Claim 1 wherein the molecular weight of saidpolyvinyi alcohol is at least 30,000.

9. A method according to Claim 1 wherein the polysaccharide is guar gum.

10. A method according to Claim 1 wherein the polysaccharide iscarboxymethyl cellulose.

11. A method according to Claim 1 wherein the polysaccharide is starch.