US3462013A - Method for beneficiating clay by flotation of colored impurities - Google Patents

Description

United States Patent "ice U.S. Cl. 209-5 6 Claims ABSTRACT OF THE DISCLOSURE Colored titaniferous impurities in clay are removed from the clay by froth flotation of a sodium silicate dispersed pulp of the clay in the presence of flotation reagents selective to the flotation of the colored impurities and a small amount of a water-soluble aluminum salt or manganous salt.

RELATED APPLICATIONS This application is a continuation-in-part of applicants copending application, Ser. No. 415,503, filed Dec. 2, 1964, now Patent No. 3,337,048, entitled Method for Beneficiating Clay by Flotation.

BACKGROUND OF THE INVENTION Clay, especially kaolin clay, is widely used as a pigment for coating paper. For such use, the value of the clay is dependent upon its whiteness or brightness since this property affects the appearance of clay-coated sheet material. High-grade coating clays have brightness values of at least about 86% to 87% when the brightness is determined by a light reflectance method hereinafter described. Small differences in brightness values of clays represent significant differences in the utility of the clays. Thus, a clay that has a brightness of 91.0% is markedly superior to a clay having a brightness of 89.0% or even 90.0%.

Some clays, especially certain foreign primary kaolin clays, possess adequate brightness for paper coating use after being beneficiated only to the extent that coarse agglomerates and grit are removed. Other clays, such as the sedimentary kaolin clays, as exemplified by Georgia kaolin clay, normally contain an appreciable quantity of finely divided colored impurities which detract significantly from the usefulness of the clays. These sedimentary clays, after degritting and fractionation to remove oversize, have a brightness which is far below the brightness values of high-grade coating clays.

The colored impurities that account for the low brightness of sedimentary kaolin clays include ferruginous matter. This constituent can be removed at least in part from the clay by chemical bleaching methods, such as with hydrosulfurous compounds which solublize the ferruginous matter so that it can be removed from the clay. Sedimentary kaolin clays usually also contain anatase, which is a yellow titaniferous impurity that has a very detrimental effect on the clay brightness. This type of impurity is substantially unaffected by the chemical bleaches used by the clay industry. To remove colored titaniferous impurities, it has been necessary to employ froth flotation in the presence of flotation reagents which bring about the selective flotation of the titaniferous matter from the clay. The flotation beneficiated clay is then treated with chemical bleaching reagents to solubilize and eliminate undesirable ferruginous matter. The flotation of the titaniferous matter from the clay can be achieved by dispersing the clay in water, incorporating 3,462,013 Patented Aug. 19, 1969 a higher fatty acid, such as tall oil acids, aerating the system and withdrawing a froth which is a concentrate of the yellow titaniferous impurities originally associated with the clay. Especially good results have been obtained with ultraflotation, which is a particular type of flotation process. In this type of flotation process, described in U.S. 2,990,958 to Ernest W. Greene et al., a substantial amount of finely divided insoluble particulate matter, usually finely divided calcite, is incorporated into the aqueous clay dispersion along with the fatty acid collector reagent. The added particulate matter, which is also reagentized by the fatty collector reagent, enhances the flotation of the finely divided colored impurities and reports in the froth along with the colored impurities. Ultraflotation in conjunction with chemical bleaching has resulted in the commercial availability of 90% to 91% brightness Georgia kaolin clay from clay crudes having brightness values less than To brighten clay by flotation and chemical bleaching to optimum brightness values, it has been found that it is essential to achieve complete dispersion of the clay by means of sodium silicate or a mixture of soluble carbonate and sodium silicate before subjecting the clay to flotation. It has been necessary, however, to carefully control the quantity of sodium silicate used. Even a small excess of the sodium silicate has been found to affect adversely the results of the flotation operation. In many cases, adequate control of dispersant dosage has been very difficult because of individual differences in the dispersant demand of different crudes, even crudes from different localities within the same mine.

In the case of gray (hard) kaoline crudes, the problem of controlling dispersant dosage so as to disperse the clay for flotation without depressing colored impurities can be especially troublesome. The dispersant requirements of gray kaolin crudes vary to a great extent with the age of the mined crudes and a crude which has been stored for several months after being mined may require substantially more sodium silicate than the same crude would have required if it had been dispersed immediately after being mined.

SUMMARY THE INVENTION An object of this invention is to improve the effectiveness of froth flotation as a method for removing colored impurities from clay.

One specific object is to buffer the effect of sodium silicate dispersant in clay flotation so that the clay is not so sensitive to the quantity of alkali silicate dispersant and small amounts of excess dispersant will not adversely affect flotation results.

Another object is to improve the brightness of flotation beneficiated clays.

Still another object is to provide a process for brightening gray kaolin clays to brightness values comparable to those of high-grade white coating clays.

I have discovered that difficulties resulting from the possible use of excessive quantities of sodium silicate dispersant may be obviated by adding a small amount of a water-soluble salt of aluminum or a water-soluble manganous salt to a sodium silicate dispersed pulp of kaolin clay, preferably before the clay pulp is conditioned for froth flotation with reagents selective to the flotation of colored impurities, especially titaniferous impurities.

Thus, in accordance with the present invention, the selective froth flotation of titaniferous impurities from a sodium silicate dispersed pulp of discolored kaolin clay is carried out in the presence of an added watersoluble aluminum or manganous salt. As a result of the presence of the polyvalent metallic salt additive, the beneficiated clay will contain no more Ti0 and be at least as bright as the beneficiated clay would be if optimum sodium silicate dosage were used, this being a rare possibility. If, by chance, the pulp is formulated with an opti mum quantity of dispersant, the salt additive will have no deleterious effect. In fact, in some cases, a further improvement in titania flotation may be realized.

DESCRIPTION OF THE INVENTION The preferred aluminum salts that are used in putting the invention into practice are aluminum sulfate, nitrate, chloride, acetate and mixtures thereof. The use of hydrated salts is within the scope of the invention. Ammonium and potassium alums are also suitable. Soluble manganous salts include the nitrate, chloride, sulfate and acetate salts. Mixtures of manganese salts and aluminum salts are useful in carrying out the invention.

The quantity of salt that is employed is usually within the range of about 0.1 to 2.0 pounds per ton of clay, with especially good results being obtained with 0.4 to 0.8 pound per ton. When too little salt is used, the effect of its use may not be appreciable. When too much salt is used, clay recovery may be reduced appreciably as a result of the activation of the clay by the additive. With very large excesses of salt, fluocculation of the pulp will occur. This, of course, is undesirable since the pulp must be well dispersed during conditioning and flotation.

The preferred method for adding the salt involves the incorporation of an aqueous solution of the salt to a solution of the sodium silicate dispersant before the clay is dispersed with the sodium silicate. The concentration of the salt solution is not critical. Concentrations within the range of 1% to 20% by weight are suggested. In carrying out the form of the invention wherein the salt is incorporated in the form of a hydrosol, the solution of salt is added to an aqueous solution of the sodium silicate until a stable hydrosol is formed by the addition of the salt. The hydrosol contains a colloidally dispersed precipitate, probably a complex sodium aluminum silicate or similar manganese compound. The hydrosol has a turbid appearance, exhibits the Tyndall phenomenon and is free from fiocs. The use of such a hydrosol in clay flotation is described and claimed in my copending application, Ser. No. 415,503, filed Dec. 2, 1964, now Patent No. 3,337,048. When using the salt additive in the form of a hydrosol, the clay product may be brighter and appreciably lower in titania content than the beneficiated clay would be using an optimum quantity of dispersant.

The following table contains data illustrating the effect of addition of various quantities of aluminum sulfate to a solution of brand sodium silicate (the registered trademark of a solution reported to contain 9.16% Na O, 29.5% SiO and about 62% H O, weight basis). The data represent results obtained by slowly adding a 1% (weight basis) aqueous solution of alum with agitation into a aqueous solution of 0 brand sodium silicate (5 parts by weight 0 brand sodium silicate to 100 parts by weight water) and allowing the composition to stand for the periods of time indicated.

EFFECT OF ADDITION OF ALUM TO A DILUTE SODIUM SILICATE SOLUTION Ml. of 1% alum Percent solution per alum, based 100 ml. 5% on anhysodium silidrous weight; pH at;

catc solution of sodium Tiuic end of silicate Rtsult (hr.) time 100 50 Appreciablc fl0ccu 1 9. 5

lation.

While optimum removal of the finely divided titania impurities from the clay is usually realized by incorporating the polyvalent metal salt with the sodium in the form of a hydrosol, as described above, substantial improvements are also obtained by adding the salt after the clay ha been dispersed with sodium silicate but before the dispersed pulp is aerated and a froth produced. Thus, when the discolored clay has already been dispersed in water with a quantity of sodium silicate somewhat is excess of the sodium silicate that is used for optimum flotation results, the TiO content of the clay after flotation will be lower, frequently significantly lower, if a small amount of polyvalent metallic salt such as alum is added to the sodium silicate dispersed pulp at any time before the pulp is subjected to froth flotation. However, it is very desirable to incorporate the salt before oiling reagents (e.g., fatty acids) are added to the dispersed pulp and to agitate the pulp after the aluminum salt is added. When the flotation reagents include a solid particulate carrier, such as calcite, as described in U.S. 2,990,958, or finely divided polymeric particles, as described in U.S. 3,224,- 582, the salt may be incorporated after addition of the carrier and before addition of buffering salts such as ammonium sulfate and flotation oils (fatty acid, etc.) since the effect of the aluminum or manganese salt may be more pronounced when added at such point. However, even when the aluminum salt is added after the carrier, pH buffer and flotation oils, the use of the aluminum or manganese salt will be of appreciable benefit. The salt may also be incorporated into a pulp of the clay before addition of all or a portion of the dispersant.

As mentioned, the polyvalent metallic salt is employed as a flotation reagent in a pulp of discolored clay which is dispersed with water-soluble sodium silicate, or preferably, a mixture of water-soluble carbonate and sodium silicate. Sodium silicates having Na O to SiO weight ratios of 1:160 to 123.75 are water-soluble and useful in effecting the dispersion of an aqueous clay pulp. The quantity of sodium silicate required to disperse any clay pulp depends upon the nature of the clay and the possible use of soluble carbonate salts. An optimum quantity may vary very substantially with crudes from the same portion of a mine, as mentioned above. With most clays, sodium silicate is employed in amount within the range of 0.5 to 4.0 pounds anhydrous sodium silicate per ton of clay. Most usually, 1.0 to 3.0 pounds per ton anhydrous sodium silicate is used. Using the commercially available 0 brand sodium silicate (which contains about 62% water), about 2.0 to 10.0, and preferably 3.0 to 6.0 pounds, of the solution is used for every ton of clay. Preferably, a water-soluble carbonate salt, especially sodium carbonate or ammonium carbonate, is added to the clay pulp in amount of 2.0 to 10.0 pounds per ton of clay before addition of the sodium silicate (or hydrosol made by addition of metallic salt to a dilute solution of the sodium silicate). With white clays, ammonium carbonate is the preferred carbonate. With gray clays (clays similar in composition to the white clays but having a distinctly gray tinge), sodium carbonate is preferred.

The flotation pulps should be prepared with about 10% to 30% clay solids.

The flotation reagents used in carrying out the process of this invention include fatty acid reagents which are selective to the titania impurities in the clay. The flotation reagents used in ultraflotation are preferred. As mentioned in U.S. 2,990,958 to Ernest W. Greene et al., reagents for ultraflotation concentration include, in addition to higher fatty acid, especially a mixture of resin acid and fatty acid (tall oil acids), a substantial quantity of finely divided auxiliary particles different from the clay and rendered floatable by the oils used to float selectively the titania impurities. Minus 325 mesh calcite is preferred although other finely divided minerals 01' polymeric particles may be employed. Additional quantities of sodium silicate can be incorporated into the pulp after addition of the auxiliary particles if the addition of the carrier particles results in flocculation of the pulp. Am-

5 monium hydroxide can be used to adjust the pH to adesired level of about 8.0 to 8.5. The pH can be buffered, as by the addition of ammonium sulfate when ammonium hydroxide is used to adjust pH. Flotation is carried out in an alkaline pulp, producing a froth which is a concentrate of impurities in the clay intimately mixed with the finely divided additive. The beneficiated clay, in the form of a dispersed pulp, reports in the machine discharge product. The recovery of beneficiated clay can be. improved by refloating the froth product one ormore times.

EXAMPLES The following examples are given to contribute to a better understanding of the present invention and to illustrate its benefits.

Examples I and II, which follow, illustrate the application of my process to the flotation beneficiation of white sedimentary Georgia kaolin clay (Washington County clay). Representative samples of the minus 325 mesh portions of this crude contained about 2.6% TiO and had a brightness of about 80% before being beneficiated by flotation and bleaching.

In beneficiating the white clay, the following general dispersion and flotation procedures were used. In some cases, these procedures were modified as described in the examples. In describing the procedures, all reagents are reported on a weight basis and represent pounds per ton of dry clay unless otherwise indicated. The TiO analyses of products are reported on a volatile-free weight basis and were based upon the weight of products obtained after heating to constant weight at 1800 F.

Dispersion-white clay Twelve hundred and fifty grams of dry'clay was diluted with soft water to 30% solids, weight basis, and transferred to a Fagergren flotation cell. When used, sodium carbonate or ammonium carbonate was added as a 5% (weight basis) aqueous solution and in amount to provide a desired quantity, typically 2.0 to 6.0 pounds sodium or ammonium carbonate, per ton of clay. The pulp was agitated without aeration for 1 minute. A 5% aqueous solution of brand sodium silicate was prepared by adding parts by weight 0 brand to 100 parts by weight water. The 5% solution was added to provide the desired quantity of dispersant, typically 3.0 to 8.0 pounds 0 brand per ton of clay. When sodium or ammonium carbonate was used, the dilute sodium silicate solution was incorporated immediately after agitating the pulp for a minute with the carbonate. After addition of the sodium silicate solution, thepulp was agitated for 30 minutes in the Fagergren flotation cell and the pH recorded. The slip was then degritted by passing it through a 325 mesh screen.

Ultraflotation-white clay A portion of the pulp containing 750 grams of the degritted clay (dry clay basis) was diluted with 250 ml. soft water, 112.5 grams of minus 325 mesh calcite having an average particle size of about 5 microns Drikalite) was added and agitated without aeration for 1 minute. The pulp in the conditioner was at about 25% solids, weight basis. Forty-five ml. of a 5% ammonium sulfate solution was added and the pulp was conditioned for /2 minute. An emulsion of the following composition was then added to the pulp and conditioned for 17 minutes: 250 ml. of soft water, 30 ml. of a 2 /2% aqueous solution of ammonium hydroxide, 137 drops of a mixture of equal parts of crude tall oil acids and a 50% solution of neutral calcium petroleum sulfonate in mineral oil supplied under the registered trademark Calcium Petronate. After 5 minutes conditioning time had elapsed, 111 drops of lubricating oil (Eureka M) were added. The reagent quantities represented by the reagent additions are as follows:

Reagent- Lbs./ ton Calcite ...4- 300 (NH SO 6.0 NH.,,OH 2.0 Tall Oil Acids 4.5 Calcium Petronate 4.5 Eureka M Oil 8.0

1 Aqueous emulsion.

The pH of the pulp at the end of conditioning was typically 8.5-8.8.

The conditioned pulps were subjected to aeration and froth flotation in a 1000 cc. Air Flow flotation cell, removing a froth product for 10 minutes. The froth products were refloated three times without addition of reagents, producing three products in each case: a froth product (F.P.)'; a first machine discharge product (MD-1); and a combined machine discharge product (-MD2,3,4). MD-l typically contained about 6.5% solids and MD-2,3,4 about 2.5% solids. The machine discharge products were analyzed by a standard chemical method for TiO Brightness values of products were determined by TAPPI Standard Method T-646 m-54, as described on pages 159A and 160A of the October 1954 issue of TAPPI (a monthly publication of the Technical Association of the Pulp and Paper Industry). The method measures the light reflectance of a clay sample and thus gives a quantitative indication of its brightness or whiteness.

Bleachingwhite clay The machine discharge products were flocced by addition of sulfuric acid in amount to reduce the pH to 2.5. The flocced clay was bleached with zinc hydrosulfite solution, following typical practice of the industry. The clay was then filtered. The brightness of the clay after being bleached was measured and compared to the brightness of the clay after flotation but before bleaching for the purpose of determining the further improvement in brightness resulting from the chemical bleach.

EXAMPLE I To illustrate the effect of the quantity of dispersant on the flotation of titania impurities from the Washington County kaolin clay, flotation tests were carried out by the procedure above described with combinations of sodium or ammonium carbonate and sodium silicate dispersant, using from 3.0 to 8.0 lbs/ton 0 brand sodium silicate. The results, summarized in Table I, show that when the pulp contained too much sodium silicate (8.0 lbs/ton with this particular clay), the beneficiated clay product was more discolored and contained more Ti0 than when smaller quantities (3.0 to 5.0 lbs./ ton) of the sodium silicate were used. These results therefore indicate that more titania will be removed by flotation of this particular clay when less than 8.0 lbs./ ton of sodium silicate is used. However, as mentioned above, it is not always practical or possible to disperse a clay pulp for flotation with fair assurance that the particular dispersant dosage is such as to assure optimum flotation with the reagents that are used.

0 brand Unbleached Properties of MD-l sodium brightness, Wt. percent NaZCOQ (NHmCOa silicate percent TiO 2 7 EXAMPLE 11 This example illustrates the desirability of adding an aluminum salt to the sodium silicate dispersed clay pulp before the pulp is subjected to flotation. The clay that was used contained 2.60% TiO and was the same crude used in the previous example. In this example tests were carried out with 8.0 pounds per ton of brand sodium silicate dispersant, a quantity which, as shown in the previous example, did not result in optimum titania removal by froth flotation. In all cases, 25 ml. of a ammonium carbonate solution was added to 1250 grams of the Washington County crude previously blunged in water to 30% solids and the dispersant composition was then added to the carbonate-treated slip and conditioned for 1 minute.

In one experimental test, the alum was incorporated by mixing 100 ml. of 5% 0 brand sodium silicate solution (5 parts by weight 0 brand to 100 parts by weight in the first machine discharge products contained 0.19% to 0.20% TiO when the alum was added after the clay pulp was dispersed with 8.0 lbs./ ton sodium silicate. These results compare favorably to the 0.18% to 0.19% TiO products produced by using smaller quantities of sodium silicate (see Table I) and were distinctly superior to the results obtained with 8.0 lbs/ton sodium silicate without any alum addition. Similar benefits were observed in studies of the properties of the combined machine discharge products. I

Thus, it was found that by adding alum to the pulp at any time before aeration and flotation, the titania removal was always better with a pulp containing a slight excess of sodium silicate dispersant.

Data in Table II for the recovery of clay in the combined machine discharge products indicate that the use of alum did not substantially impair recovery unless the TABLE II.EFFEC'I ON FLOIA'IION OF METHOD OF ALUM ADDITION TO CLAY PULP Slip treatment, lbs/ton MD-1 MD-I4 0 brand sodium Percent Percent Percent Percent (NHihCO; silicate 3 Alg(SO4)3.l-8H3O Method of alum addition wt. T10 Wt. TiO

2.0 8.0 0.4 Alum and sodium silicate added as mixture 47.9 0.15 91,0 0,17

(hydrosol).

2,0 8.0 0.4 Alum and sodium silicate added as mixture 41.7 0.15 37.7 0.1

(hydrosol) 2.0 0.4 Sodium silicate added first followed by alum 48.5 0.19 91.3 0,

after one minute.

2.0 8. 0 0. 4 Alum added after calcite flotation reagent 51. 4 0, 20 92. 2 0, 20

2.0 8. 0 0. 4 Alum adtded after emulsified fatty acid flotation 44. 4 0. 19 89. 4 0. 20

reagen 2.0 8. 0 Control, no alum added 51. 3 0.23 91.8 0, 22

1 Conditioned for 30 minutes. 2 Conditioned for 60 minutes.

water) with 25 ml. of 1% alum solution for minutes, adding the resulting stable hydrosol to the clay pulp and agitating without aeration for 30 minutes. Another test was carried out in the same way but the conditioning time was 60 minutes. In still another test, the sodium silicate and alum were added separately. In that case after addition of ammonium carbonate solution to the 30% solids clay pulp and one minute conditioning, 100 ml. of the 5% solution of 0 brand sodium silicate was added to the pulp and conditioned for 1 minute. Twenty-five ml. of 1% aqueous alum solution was then added and conditioned for 29 minutes. In other tests, alum was added either after the calcite carrier and before addition of emulsified fatty acid reagent or after the emulsified fatty acid reagent. In all cases the pulp was degritted over a 325 mesh screen after the dispersant treatment and before being reagentized for ultraflotation concentration with calcite carrier and emulsified fatty acid reagent. The pH of the pulps were about 8.1 after dispersion and degritting. After addition of the emulsified flotation reagents, the pH of the pulps were 85:0.1.

Analyses were made on MD-l and on composites of MD-l and MD-2,3,4 (indicated as MD-l-4), with the results summarized in Table II.

The data in Table II show that, using 2.0 lbs/ton ammonium carbonate, 8.0 lbs/ton sodium silicate and no alum additive, the beneficiated clay in the first machine discharge product (MD1) contained 0.23% TiO and the beneficiated clay in the combined machine discharge products contained 0.22% TiO As noted in Table I, the MD- 1 products contained only 0.18% to 0.19% TiO when smaller quantities of sodium silicate had been employed to disperse this clay. Data in Table II show that when the alum was added as a hydrosol, the TiO content of the MD-l was only 0.15% in spite of the fact that 8.0 lbs./ ton sodium silicate had been used in the dispersion step. Thus, the titania removal was even better when the hydrosol was used in spite of the fact that an excessive amount of dispersant had been used in preparing the clay for the selective flotation of colored impurities.

Data in Table II also show that the beneficiated clay 8 Wet basis.

alum was used as a hydrosol and conditioning time was EXAMPLE III The following example illustrates the benefits of carrying out the flotation beneficiation of a gray Georgia kaolin clay in the presence of an added aluminum salt or manganese salt.

The same procedures employed in dispersing and floating the white Washington County clay were used with the gray kaolin clay with the following exceptions. Sodium carbonate was incorporated into the clay pulp as a 5% aqueous solution in amount of 8.0 lbs./ton of clay crude and conditioned for 30 minutes before addition of the solution of sodium silicate or hydrosols formed by adding alum or manganous sulfate to the sodium silicate solution. Addition of the sodium carbonate increased the pH of the clay pulp to about 5.8. In a control test, sodium silicate was used to disperse the sodium carbonate treated pulp in amount of 5.0 pound 0 brand per ton of clay. In another test, a 1% alum solu tion was mixed into the 5% sodium silicate solution to form a hydrosol and the hydrosol was conditioned with a portion of the gray clay pulp for 20 minutes. This addition corresponded to the use of 5 .0 pounds of 0 brand sodium silicate per ton of clay and 0.4 pound of Al (SO .18H O per ton of the clay. In still another test, a 1% aqueous solution of MnSO .H O was incorporated into 5% sodium silicate solution and the result ing hydrosol employed in amount to provide 0.4 pound MnSO .H O per ton of clay and 5.0 pounds of 0 brand sodium silicate per ton of clay.

The dispersed clay pulps were separately fractionated before ultraflotation by means of a centrifuge and a fine fraction recovered as an aqueous pulp. The quantity of flotation reagents used with each pulp of the fine fraction of gray clay varied somewhat from the quantity employed with the white clay because previous experience had shown that such a variation in reagent quantity would be necessary to achieve optimum results. The reagents used with the gray clay were as follows:

Reagent Lbs./ ton Calcite (minus 325 mesh) 600 (NH SO 6.0 NH OH 3.0 Tall Oil Acids 6.2 Calcium Petronate" 6.2 Eureka M Oil 8.0

1 Aqueous emulsion.

Conditioning time was 30 minutes and the flotation procedure was the same used with the white clay.

A further variation was that after the flotation beneficiated gray clay was flocced with sulfuric acid, it was oxidized by treatment with a solution of potassium permanganate before being bleached with zinc hydrosulfite. The permanganate treatment was carried out by adding a 1% aqueous solution of potassium permanganate to the machine discharge product in amount of 5 pounds KMnO per ton of clay and agitating for 60 minutes. In some tests, the permanganate treatment and zinc hydrosulfite treatments were carried out at about 190 F. (The use of a permanganate treatment before reducing bleach in the brightening of clay is disclosed and claimed in US. 3,353,668 to James B. Duke patent application, Ser. No. 330,634, filed Dec. 16, 1963, by James B. Duke, which is a continuation-in-part of Ser. No. 236,685, filed Nov. 9, 1962, now abandoned.)

When metallic salt was not present (the control experiment), the brightness of the flotation beneficiated, bleached gray kaolin clay was 90.6%, as compared to a brightness of 79.8% for a similar size fraction of the crude. When the flotation was carried out in the presence of the alum, however, product brightness was increased to 91.2%, a value comparable to that of the best imported kaolin clays and the best beneficiated domestic white kaolin clays. When flotation was carried out in the presence of manganous sulfate, clay brightness after bleaching and flotation was 91.5%. These results show, therefore, that the flotation was more effective with the gray clay when it was carried out with an aluminum or manganese salt additive.

I claim:

1. In a process for removing colored titaniferous impurities from kaolin clay by froth flotation wherein an aqueous pulp of said clay is dispersed with a water-soluble sodium silicate dispersant consisting of oxides of sodium and silicon and water, the dispersed pulp is conditioned for the flotation of colored impurities with collector reagents selective to said colored impurities and the conditioned pulp is subjected to froth flotation producing a froth which is a concentrate of colored impurities and a machine discharge product containing a clay of reduced impurity content, the improvement which comprises adding a small amount of a water-soluble salt of a polyvalent metal selected from the group consisting of aluminum, manganese and mixtures thereof to said pulp after it has been dispersed with said sodium silicate but before the dispersed pulp has been subjected to froth flotation, said salt being added in amount insuflicient to flocculate said pulp, whereby the adverse effect of the possible use of an excessive quantity of sodium silicate dispersant is obviated.

2. The method of claim 1 wherein said salt is aluminum sulfate.

3. The method of claim 1 wherein said salt is employed in amount within the range of 0.1 to 2 1bs./ton of said clay.

4. The method of claim 1 wherein said collector reagent comprises an emulsified fatty acid and said salt of a polyvalent metal is added and mixed in said pulp containing sodium silicate dispersant before said emulsified fatty acid is added.

5. The method of claim 1 wherein said collector reagent comprises a higher fatty acid and finely divided particles different from said clay and capable of being collector-coated by said fatty acid.

6. The method of claim 1 wherein said sodium silicate dispersant is employed in amount that exceeds a quantity which would produce a machine discharge product of lower TiO analysis if said flotation were carried outwith a smaller quantity of sodium silicate and in the absence of said salt of a polyvalent metal.

References Cited UNITED STATES PATENTS 1,541,182 6/1925 Plowman 2095 1,847,212 3/ 1932 Feldenmeimer 2095 2,158,987 5/1939 Maloney 209-5 X 3,337,048 8/1967 Mercade 209-166 X 2,408,656 10/1946 Kirk 2523 13 2,569,680 10/1951 Leek 2095 X 2,657,183 10/1953 Bechtold 252-313 2,894,628 7/1959 Duke 209-166 2,990,958 7/ 1961 Greene 209-166 3,072,255 1/1963 Greene 209-l66 2,794,783 6/ 1957 Podschus 2523 13 HARRY B. THORNTON, Primary Examiner R. B. HALPER, Assistant Examiner US. Cl. X.R. 209-166