US3425546A - Method for recovering and reusing flotation product for use as froth flotation reagent - Google Patents

Description

United States Patent 3,425,546 METHOD FOR RECOVERING AND REUSING FLOTATION PRODUCT FOR USE AS FROTH FLOTATION REAGENT James B. Duke, Metuchen, N.J., assignor to Engelhard Minerals & Chemical Corporation, Edison, N.J., a corporation of Delaware No Drawing. Filed July 27, 1966, Ser. No. 568,119 US. Cl. 209 10 Claims Int. Cl. B03d 1/14 ABSTRACT OF THE DISCLOSURE In a flotation process for beneficiating a slimed ore pulp wherein added floatable particles such as particles of oiled calcite are employed to carry slimed mineral into the froth, the froth product is filtered and dried without removing or destroying flotation oils. The dried material is pulverized and then recycled along with fresh added floatable particles in subsequent carrier flotation operations.

This invention is concerned generally with the froth flotation concentration of very finely divided mineral ore pulps by the procedure described in US. 2,990,958, of which I am a coinventor. This invention is especially directed to recovering and reusing the auxiliary mineral (carrier) flotation reagent employed in carrying out this particular type of froth flotation process.

The froth flotation procedure of the above-mentioned patent is applicable to the selective concentration of ore pulps, such as pulps of discolored clay, which are too fine to respond in the desired manner to conventional froth flotation. The essence of the procedure resides in the addition to the dispersed ore pulp of a substantial quantity of very finely divided floatable particles, especially collector-coated (oiled) mineral particles such as fatty acid-reagentized calcite. These particles enhance or promote the selective flotation of a finely divided collectorcoated constituent of the dispersed feed and permit the beneficiation of ore pulps which are too fine to respond to conventional flotation.

In applying the process of US. 2,990,958 to the beneficiation of discolored kaolin clay, an aqueous pulp of the clay is treated with a dispersant including sodium silicate which also depresses the clay. The dispersed pulp is reagentized for the selective flotation of the colored impurities from the clay by adding fine mineral matter such as calcite and also a higher fatty acid which has an aflinity for the impurities in the clay and for the calcite. The pulp is aerated and the reagentized impurities (principally a fine-grained yellow titaniferous mineral) are carried into the froth by the air bubbles along with the reagentized calcite which appears to carry the impurities into the froth. (Hence the designation carrier particles in referring to the auxiliary mineral particles.) Small amounts of clay also report in the froth as a result of mechanical entrainment. The machine discharge product is a clay having substantially enhanced brightness as a result of the removal of the discolored titania impurity. (Small amounts of ferruginous impurities also are floated to some extent.) The clay is usually further brightened -by hydrosulfite bleaching which acts on the 3,425,546 Patented Feb. 4, 1969 ferruginous impurities still in the clay, reducing the ferruginous matter to soluble ferrous compounds which are removed by washing and filtration.

In carrying out carrier flotation processes, the quantity of auxiliary (carrier) particles is appreciable. In clay flotation, for example, the carrier is usually employed in amount of 10% to 30% of the weight of the'solids in the ore pulp. The carrier particles plus the collector reagents required to float the carrier therefore represent a substantial proportion of the overall reagent expense in the commercial operation of the process unless the carrier is recovered and reused. Attempts to recycle part of whole of the froth product for use as the carrier mate rial in subsequent flotation operations have been lacking in success. Although the used froth is composed of reagentized floatable mineral particles, the reuse of the product results in a drastic reduction of concentration efficiency. In other words, the untreated froth does not produce the desired carrier effect. This is true even if entrained clay is separated out of the froth. Prior to my discovery, it was not possible to reuse the original auxiliary mineral particles in the froth product without first removing the flotation reagents and separating the floated portion of the feed. In the case of clay flotation with a calcite carrier, this involved the removal of the flotation reagents and the separation of the titania and adherent clay from the calcite.

Accordingly, an object of this invention is to provide a simple, inexpensive method for recovering and reusing some of the froth product of a carrier flotation operation as a carrier material in a subsequent carrier flotation .operation without removing the oiling reagents and separating reagentized species in the froth.

Stated in another way, an object of the invention is to improve upon the economics of carrier flotation as a procedure for beneficiating slimed ore pulps by providing a method for treating the flotation froth product in a manner such that the original carrier material in the froth can be reused without purifying the froth to isolate the original carrier and separating it from the floated feed and oiling reagents (collectors).

As applied to the continuous carrier froth flotation of kaolin clay, an object of this invention is the provision of a method for processing the froth product without separating out the titania impurity and without removing the flotation reagents, whereby processed froth product can be continuously recycled in the flotation cells without substantially reducing the effectiveness of the removal of colored impurities during flotation.

Briefly stated, in accordance with the present inven tion, the froth product obtained by subjecting a very finely divided or slimed dispersed ore pulp to selective froth flotation in the presence of oiling reagents and carrier particles (mineral or organic particles different in composition from the slimed ore) is treated to place the froth in a state or condition suitable for use as a portion of the carrier particles in a subsequent flotation operation by the following combination of steps.

(1) Nonoiled sl-imes, present in the froth as a result of mechanical entrainment, are removed from the froth. l referably, this is accomplished by hydraulic sedimentation whereby the nonoiled slime is removed as an aqueous suspension from the oiled constituents which form a sediment. The froth is thickened simultaneously with the liberation and removal of the water-avid particles);

(2) The resulting washed froth, which includes oiled carrier particles and an oiled constituent of the feed is filtered and dried at a temperature and for a time sufficient to place the froth residuum in a solid, pulverulent condition but insuflicient to decompose or destroy either the oiling reagents or the carrier mineral;

(3) The dried froth is pulverized to a finely divided state, producing a product suitable for recycling.

A portion of the recycle product obtained by these steps is employed in combination with fresh, makeup carrier and oiling reagents (for collector coating the makeup carrier and recycle product) to condition a new charge of finely divided ore pulp for carrier froth flotation.

The procedure is repeated substantially ad infinitum, with the washed, heat-treated recycle product being blended with fresh carrier particles and the mixture used as the carrier in subsequent flotation operations.

From this brief description of the invention, it can be seen that the essence of the process for treating the froth product of carrier flotation for use as a portion of the carrier in subsequent carrier flotation is the removal of nonoiled gangue or tailings from the froth, followed by the drying of the froth residuum at controlled temperature and the pulverization of the dried froth material.

An essential feature of the process is that the washed froth is dried to a pulverulent condition and that the drying is carried out at a tempera-ture below which the flotation reagents in the froth are destroyed. Unless the cake is dried to pulverulent, substantially bone-dry condition, the recycled product produces poor flotation results. On the other hand, overdrying with resulting destruction of oiling reagents and/or mineral particles leads to poor flotation results.

The procedure, as applied to the flotation beneficiation of dsicolored kaolin clay with the preferred minus 325 mesh calcite as the carrier, can be briefly described as follows. In the startup, a dispersed aqueous slip of the discolored clay is conditioned for flotation beneficiation with a substantial amount of minus 325 mesh calcite and anionic (fatty acid) collector reagent. The pulp is subjected to froth flotation in an alkaline flotation circuit, producing a froth product which is a concentrate of titania impurity in intimate association with oiled calcite particles and leaving beneficia-ted kaolin in the tailings (machine discharge). The froth is subjected to gravity concentration, whereby the clay slime is removed as an overflow eflluent and the underflow contains the titania, calcite and adherent oiling reagents. The underflow is filtered and dried to a pulverulent condition at a temperature not exceeding about 250 F. (the maximum temperature for the particular reagents used). The dried froth is pulverized and a portion of the pulverized material is blended with fresh calcite, preferably using a smaller quantity of calcite than recycle froth for economic reasons. The mixture is used with supplementary oiling reagents to condition a new dispersed slip of discolored c ay.

The economic advantage resulting from the practice of the process of the invention varies with the quantity of recycle product that can be blended with fresh carrier without substantially affecting the efiiciency of flotation concentration. This quantity will obviously vary with the mineral pulp being treated and with the species of carrier. However, the magnitude of the benefit may be illustrated by an operation of the process with a Georgia kaolin clay in which 75 parts of recycle carrier was used with 25 parts of fresh carrier to effect a very substantial reduction in flotation reagent cost without impairing substantially product quality or quantity.

In putting the invention into practice, the froth from the flotation cells, or a precalculated portion of the froth product, is agitated without aeration in order to remove adherent (nonoiled) gangue. In the case of the froth flotation beneficiation of kaolin clay, the adherent gangue will be clay and the process will be described with especial reference to the beneficiation of clay. Details of the carrier flotation in general are found in US. 2,990,958, and a description of a plant-scale operation of the flotation beneficiation of kaolin with a calcite carrier appears in Ultraflotation at Minerals & Chemicals Philipp Corporation, Frank A. Seeton, Bulletin No. M4-Bl17, Denver Equipment Company. Reference is made herein to the details contained in these publications.

The amount of clay in the fr-oth will be typically of the order of about 10% by weight of the solids in the froth. Normally the solids in the froth will be adequate for clay removal without addition of water. However, water can be added to reduce the solids to a more suitable level. Since the slimed clay is not oiled it will remain suspended in the water while the oiled calcite and oiled titania will sink. The clay can therefore be removed as an overflow by decantation, siphoning or centrifugal separators. The underflow, which is a flocculated aqueous pulp of oiled particles, is filtered. The filter cake is then dried. Using a calcite carrier with fatty acid collector reagents, the drying should be carried out at a temperature within the range of 150 F. to 250 F., preferably 175 F., and until the filter cake is substantially water-free. The drying can be carried out in any suitable oven, for example, a rotary furnace, a mufile furnace or a spray drier. Drying time obviously will vary with the drying temperature and drying equipment that is employed. At temperatures appreciably above 175 F. undesirable charring of the oiling reagents may occur. At temperatures appreciably below 175 F. the drying rate maybe uneconomical.

The dried material is crushed and ground to minus 325 mesh in a hammer mill or other mill capable of producing a fine grind.

An ore pulp is then conditioned with a mixture of the dried froth product and fresh (unused) carrier. If desired, the froth product and fresh carrier can be dry blended before these materials are added to the conditioner or they can be added separately. For economic reasons, the highest proportion of dried froth to fresh carrier that is consistent with the realization of adequate flotation is employed. In representative clay flotation process, from 60 to parts by weight of the dried froth to 40 to 20 parts by weight of fresh carrier are used. The use of as much as parts of dried froth to 5 parts of fresh carrier may be adequate in some operations. Suitable proportions can he arrived at by simple experimentation.

The total quantity of fresh and used carrier should be similar to the quantity of carrier used if only fresh carrier were employed. For example, in a process operating with 15% calcite (based on the clay weight) during startup, each subsequent flotation might be carried out with 10% dried froth recycle (based on the clay weight) and 5% fresh carrier. However, the use of a somewhat larger quantity of carrier may be advisable when recycling dried froth, in accordance with this invention.

The quantity of organic oiling reagent employed when recycling dried froth product corresponds substantially to the amount that would be employed if fresh carrier were used. However, slight variations may be advisable in some cases. In clay flotation, a mixture of crude or refined tall oil and oil-soluble petroleum sulfonate is the recommended oiling reagent. The oiling reagents are added to the conditioners in the usual manner. In clay flotation, it is recommended to refloat the froth product several times without addition of reagents in order to improve the clay recovery. The froth, or a portion of the froth, is then washed to remove entrained clay, dried and pulverized for recycle, in accordance with this invention.

The invention and some of its advantages will be apparent from the examples which follow.

Example I This example illustrates the use of the process of the invention in the flotation beneficiation of gray Georgia kaolin clay.

A sample of Mattie Ivey gray kaolin from a deposit near McIntyre, 6a., was blunged in water, employing 6977 grams of the clay (6000 grams dry clay) and 12,700 grams softened water. A 5% aqueous solution of soda ash was added to the blunged clay in amount of 300 milliliters, corresponding to 5.0# Na CO per ton of clay. The slip was agitated for 20 minutes. The pH of the slip was 6.9 after addition of the soda ash. One hundred and eight milliliters of a 5% solution of O brand sodium silicate (3.0# O per ton) was then added and agitated for minutes. The pH was 8.0 after addition of the sodium silicate. The dispersed slip was screened on a 325 mesh Tyler screen to eliminate grit and coarse agglomerates and the slip was fractionated on a centrifuge to produce a slip of fine size clay calculated to contain at least 80% by Weight of particles finer than 2.0 microns, equivalent spherical diameter. A portion of the slip was careful-1y dried, pulverized and the brightness measured with a G. E. recording spectrophotometer.

Cycle 1 A 2381 portion of the minus 325 mesh slip of fine size gray clay containing 500 grams of dry clay was conditioned for flotation by adding 87.5 grams Drikalite (calcite classified to a mean particle size of 5 microns), followed by: 30 milliliters of a 5% aqueous solution of ammonium sulfate (6.0# ton of clay); an emulsion containing 25 milliliters of a 2.5% aqueous solution of ammonium hydroxide (2.5 lton), 250 milliliters of water and 80 drops of a 50-50 mixture of refined tall oil and neutral calcium petronate, a neutral oil-soluble petroleum sulfonate. (The quantities of oiling reagents employed correspond to 4.5#/ton of tall oil and 4.5'#/ton of petroleum sulfonate); and 71 drops of Eureka M oil, corresponding to 8.0# oil per ton of clay. The pH of the slip after conditioning was 8.6. The hydrocarbon oil was added after conditioning for 5 minutes and total conditioning time was 17 minutes.

The conditioned slip was aerated and subjected to froth flotation in a 1000 gram minerals separation airflow flotation machine, producing a froth product for 10 minutes. The froth product was cleaned three times without addition of reagents, each time removing a froth product for 10 minutes. The machine discharge products were combined to produce a total machine discharge product.

The total machine discharge product was weighed to ascertain clay recovery and the brightness of the product was measured with a G. E. recording spectrophotometer. A portion of the machine discharge was bleached in accordance with a process described in my copending application, Ser. No. 330,634, filed Dec. 16, 1963, now US. 3,353,668, The bleaching procedure was as follows. A 1% aqueous solution of potassium permanganate was added to the machine discharge in amount of 5# per ton of clay. The mixture was agitated and then aged at room temperature for 48 hours. The resulting oxidized slip was then bleached with zinc hydrosulfite bleach liquor (5# zinc per ton of clay).

The results are as follows:

AF=Increase in brightness as result of flotatlon. AB =Increase in brightness as result of chemical bleaching. AT=AF+AB Overall increase in brightness.

Preparation of recycle carrier In accordance with the present invention, the final froth product from the first cycle was placed in a 3000 milliliter beaker and allowed to stand for three minutes without agitation. As a result, the reagentized calcite and reagentized titania settled and the water-avid, nonreagentized clay remained in suspension. The clay suspension was siphoned, leaving the mixture of calcite and titania impurity, which was filtered without addition of a flocculating agent and dried for 2 hours in a 175 F. despatch oven. The weight of the dried product was 113 grams.

The dried material was pulverized with a mortar and pestle and a 65.6 gram portion was separated out for subsequent reuse.

First recycle (Cycle 2) To another 2381 gram portion of the dispersed minus 325 mesh slip of fine size discolored gray clay containing 500 grams of dry clay, 21.9 grams of fresh Drikalite carrier was added. The slip was conditioned for 30 seconds. Ammonium sulfate solution (30 milliliters of 5% solution) was added, agitated for 30 seconds, followed by the addition of the 65.6 grams of dried used froth produce from Cycle 1. The pulp was agitated for one minute after addition of the recycle froth. It will be noted that the slip contained a 75:25 mixture of used and new carrier in amount of 87.5 grams, corresponding to the weight of the fresh calcite employed in the first cycle. The pulp was then conditioned with drops of the emulsion of tall oil and petroleum sulfonate and 71 drops of the hydrocarbon oil. Conditioning time, reagents and reagent quantities were the same used in the first cycle with the exception that dried, cleaned froth was substituted for 75% of the Drikalite, as described above. The clay was floated and bleached, also duplicating the procedure of Cycle 1.

The results for the second cycle are as follows:

Cycle 2 Percent Bright- The froth from the second flotation cycle was settled, clay removed by siphoning, the sediment filtered, dried at F. for 2 hours, pulverized and a 65.6 gram portion separated out and used to condition another 2381 gram portion of dispersed slip along with a 21.9 gram portion of fresh Drikalite and 6.0#/ ton ammonium sulfate.

An emulsion containing 2.0#/ton ammonium hydroxide and 4.5 ton each of the tall oil and calcium petronate was then added. The quantity of reagents, order of addition of reagents, reagent dilution and details of flotation, bleaching, etc., were the same employed in Cycle 2.

Cycle 2 was repeated until a total of 22 cycles had been TABLE I.EFFECT OF USING TREATED FROTH AS FLOTAIION REAGENT ON THE BRIGHTENING AND REMOVAL OF COLORED IMPURITIES IN GRAY KAOLIN CLAY Brightness, Percent Froth Product Feed Total MD Cycle Number Percent Unbleached Bleached Percent Percent Percent Percent Percent Percent Weight TMD* TiOz Fez 3 TiOz F8203 T: FezO;

Feed TMD* Average 92.5 80. 7 85. 6 90. 1 6. 61 0. 75 1. 26 1. 32 0. 38 1. 16

Total machine discharge product.

carried out. During the cycles, other batches of the sample of gray kaolin were employed. A summary of the results of the 22 cycles appears in Table I.

The results in Table I show that 'with due regard being made to slight variation in the feed clay, the flotation operation was substantially as effective in brightening the clay and reducing colored titania impurities after 21 recycles of the treated froth as the fresh carrier was in the first cycle. The data show also that the ultimate bleached brightness was not adversely affected by recycling the treated froth. Thus, throughout the entire operation, the increase in brightness resulting from the flotation step per se was 4.9% while the bleached beneficiated clay averaged 90.1% brightness. Of special interest is the fact that the TiO content of the flotation beneficiated clay averaged only 0.38%, corresponding to the TiO content of the clay beneficiated with 100% fresh calcite in Cycle 1, in spite of the enrichment of titania in the flotation cell as a result of the presence of titania in the reused carrier.

Example II The procedure of Example I was repeated, with appropriate variation, using a white discolored Georgia kaolin. White kaolin is similar to gray kaolin in chemical composition and differs from gray kaolin in color-cast and particle form. In this case, the total carrier quantity was increased from the (based on the clay weight) to In the first cycle the carrier was 100% fresh Drikalite, as in Example I. In the second through twentysecond cycles the carrier was a 60:40 mixture of dried, reused froth to new carrier instead of the 75:25 mixture of Example I.

The other variation in reagents was that the white clay was dispersed with 7.0#/ ton O brand sodium silicate as the sole dispersant.

The results showed that the increase in brightness as a result of flotation and the percent TiO in the flotation beneficiated clay remained very consistent during the twenty-two cycles. A moderate decrease in bleached brightness occurred from the first to the third cycle (90.7% to 89.5%) but after the third cycle the bleached brightness was fairly constant at 89.7%. Clay recovery averaged 93.7% during the process.

Example III The procedure of Example I was duplicated with the exception that the recycle filter cake was not dried before it was used in a 75:25 mixture with fresh Drikalite in carrier benefication of gray kaolin clay. Thus, the recycled material was obtained by permitting the froth product to settle, decanting supernatant clay, and filtering. Another variation was that the process was carried out for only six cycles. (The reasons will be apparent.)

In calculating the quantity of filter cake to be recycled, the moisture content of the cake was determined and an appropriate quantity was used to produce a 75 mixture, on a moisture-free weight basis.

The data for the first and sixth cycles are shown in Table II along with data for the first and sixth cycles with the dried filiQ gi ke of Example I. It will be noted that in each case a mixture of 75 parts by weight of used carrier was employed with 25 parts by weight of new carrier and that the starting clays were of substantially identical composition and color.

TABLE IL-EFFECT OF DRYING WASHED FILTER CAKE ON THE EFFECTIVENESS OF CAKE AS CLAY FLOTATION REAGENT Filter Cake Dried at Undried Cycle N 0. 1:

Machine discharge:

AF=Incrcase in unbleached brightness by flotation of impurities only An examination of the results tn Table II shows that the recycling of wet filter cake had a definite harmful elfect on AF, which was correlated with an increased titania content and reduced bleached brightness. From the first to the sixth cycle there was a two point loss in bleached brightness, and a two point loss in AF, with a. correlative increase in product TiO content from 0.41% to 0.64%. In contrast, by drying the washed froth, in accordance with this invention, there was a negligible decrease in bleached and unbleached brightness and AF, and the Ti0 content of the products were substantially the same from the first to the sixth cycles.

An evaluation of the difference in kind between the action of the wet and dried filter cake as a carrier flotation reagent has led to the hypothesis that the controlled drying of the washed froth product resulted in a cementation of oil and titania to the calcite, thereby preventing the separation of the oil and titania from the calcite during the subsequent conditioning step during which the recycled froth was agitated intensely with pulp. Aeration and flotation of the pulp in the presence of the cemented particles apparently resulted in the normal carrier effect of the added particles. Thus, the oiled cemented particles appeared to carry titania into the froth without dropping titania from previous flotation operations into the machine discharge product. On the other hand, conditioning of a clay pulp with the undried filter cake, outside the scope of the invention, appeared to result in the removal of the other reagents from the calcite with the subsequent formation of balls including discolored titania. These balls accumulated and dropped from the froth into the machine discharge product, minimizing the brightening and impairing the titania reduction of the clay product.

This explanation would account for the fact that fresh carrier and oiling reagents must be blended with the recycle froth products since the heat-cementation of oiling reagents and titania to the calcite would eventually result in excessive particle buildup if fresh carrier and oiling reagents were not blended with recycle carrier.

Also, the hypothesis would account for the fact that overdrying is detrimental to the preparation of the recycle product since overdrying will destroy the organic oiling reagents (and the carrier particles if the temperature is sufficiently elevated). Destruction if the oiling agents would preclude cementation, as would destruction of the carrier particles.

I claim:

1. In the selective flotation concentration of mineral matter from a slimed ore pulp wherein said pulp is conditioned for selective flotation of said mineral matter with a substantial quantity of (1) minus 325 mesh floatable insoluble particles, different in chemical composition from said slimed ore pulp, and (2) collector reagents selective to the flotation of said mineral matter, and the ore pulp is subjected to froth flotation, producing a froth product which is a concentrate of collector-coated mineral matter in intimate association with collector-coated minus 325 mesh insoluble particles and a machine discharge product, the improvement which comprises treating said froth product to render it suitable for use as a portion of said particles (1) in a subsequent operation of said selective flotation by removing water-avid particles from said forth product, drying the remainder of the forth product to a pulverulent condition at a temperature below which said collector reagents are destroyed, and pulverizing the dried product, thereby producing a material adapted for use as a portion of said minus 325 mesh flotatable insoluble particles.

2. The method of claim 1 wherein said flotatable insoluble particles are mineral particles collector-coated with the same collector reagent employed to selectively coat said mineral matter in said pulp.

3. The method of claim 1 wherein said slimed ore pulp is a pulp of clay containing titaniferous impurities and said collector reagents include an anionic reagent capable of selectively floating said titaniferous impurities from said clay in the presence of said minus 325 mesh floatable particles, whereby said froth product is a mixture including said minus 325 mesh floatable particles collectorcoated with said anionic reagent, titaniferous impurities collector-coated with said anionic reagent and water-avid clay particles which are removed from said froth b -fore said froth is dried to pulverulent condition.

4. In the froth flotation treatment of clay to remove titaniferous colored impurities wherein a dispersed pulp of said clay is subjected to froth flotation in the presence of (1) a substantial quantity of added finely divided calcium carbonate and (2) an anionic collector reagent selective to the flotation of titaniferous colored impurities in said clay and to the flotation of said added calcium carbonate, thereby producing as a froth product a conoentrate of titaniferous impurity in intimate association with said added calcium carbonate and a machine discharge which is a clay of reduced colored titania content, the improvement whereby said froth product is rendered capable of being used in lieu of a portion of said added calcium carbonate in a subsequent flotation beneficiation of discolored clay which comprises removing entrained clay from said froth, drying the froth at a temperature within the range of 150 F. to 250 F. until it is pulverulent and pulverizing the dried froth.

5. The method of claim 4 wherein said clay is Georgia kaolin clay.

6. The method of claim 4 wherein said pulp of dispersed clay contains sodium silicate.

7. The method of claim 4 wherein a portion of said pulverized froth product is employed with fresh calcium carbonate and additional anionic collector reagent in a subsequent clay flotation operation, using to parts by weight of said pulverized froth product to 40 to 5 parts by weight of fresh calcium carbonate.

8. The method of claim 4 wherein adherent clay is removed from the froth and the froth is simultaneously thickened by subjecting the froth to sedimentation and removing an aqueous suspension of the clay as an overflow.

9. The method of claim 4 wherein said froth is dried at about F.

10. The method of claim 4 wherein adherent clay is removed from the froth and the froth is simultaneously thickened by subjecting the froth to sedimentation, and an aqueous suspension of the clay is removed as an overflow, and the underflow is filtered and the resulting filter cake is dried.

References Cited UNITED STATES PATENTS 2,936,887 5/1960 Wilson 209 166 3,151,062 9/1964 Duke 209-3 3,224,582 12/1965 Iannicelli 209166 FRANK W. LUTTER, Primary Examiner.

ROBERT HALPER, Assistant Examiner.

US. or. X.R. 209 166, 11

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,425 ,546 February 4 1969 James B. Duke It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

In the heading to the printed specification, lines 5 and 6, "Engelhard Minerals & Chemical Corporation, Edison, N. J. should read Englehard Minerals & Chemicals Corporation, Menlo Park, Edison, N. J. Column 3 line 37, "dsicolored" should read discolored Column 5, in the table second column, line 2 thereof, "90.4" should read 91.4 same table, fourth column line 2 thereof, "5. 0" should read 5.5 Column 6, in the table sixth column, line 3 thereof, "10.4 should read 10. l Column 8, line 36 "tn" should read in Column 9, line 30, "flotatable" should read floatable Signed and sealed this 24th day of March 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr. E. JR.

Attesting Officer Commissioner of Patents