US3313492A

US3313492A - Grinding method

Info

Publication number: US3313492A
Application number: US421108A
Authority: US
Inventors: Daniel A Jacobs; James B Duke
Original assignee: Minerals and Chemicals Philipp Corp
Current assignee: Minerals and Chemicals Philipp Corp; Phibro Corp
Priority date: 1964-12-24
Filing date: 1964-12-24
Publication date: 1967-04-11
Anticipated expiration: 1984-04-11

Description

United States Patent 3,313,492 GRINDING METHOD Daniel A. Jacobs and James B. Duke, Metuchen, N.J., assignors to Minerals & Chemicals Philipp Corporation, Woodbridge, N.J., a corporation of Maryland N0 Drawing. Filed Dec. 24, 1964, Ser. No. 421,188 17 Claims. (Cl. 241-21) This invention relates generally to the grinding of finely divided solid material and is directed specifically to the grinding of finely divided solid material which ofiers resistance to being ground to an irnpalpable powder.

Finely divided material, especially mineral matter, is usually micronized in mills having high energy input. In one type of micronizer, usually referred to as a fluid energy mill, the material to be ground is suspended in a stream of a gaseous material, such as air or steam. Subdivision of the solid results from high speed particleto-particle impact. In another type of micronizer, which also utilizes a very high energy input, the material is suspended in a liquid along with solid grinding media and the mixture is subjected to intensive churning or agitation. Various forms of the latter type of micronizer have been suggested, one example being the process described in a publication of the US. Department of the Interior, Bureau of Mines, by I. L. Feld et al., entitled, Paper-Coating Clay From Coarse Georgia Kaolin by a New Attrition Grinding Process.

In contrast to the aforementioned processes, which require high energy input and intensive agitation of the charge material, is the grinding process described in U.S. 3,097,801 to James B. Duke, entitled, Method for Comminuting Kaolin Clay. This process constitutes a fundamental departure from the prior art micronizing processes in that it utilizes very mild agitation with relatively low energy input. In accordance with this process, finely divided clay material is formed into a fiuid aqueous slip and mixed with sand that is fine enough to pass through a mesh screen and is coarse enough to be retained on a 35 mesh screen (i.e., minus 10 plus 35 mesh sand). A smooth cylinder is partially filled with the mixture and is rotated at a relatively low peripheral speed, e.g., 200 to 300 feet per minute. The results have been remarkable in that the micron size clay platelets, which are very hard and resistant to grinding, have been eliectively comminuted.

An object of this invention is to improve the efiiciency of the aforementioned grinding process in which a fluid slip of material to be ground and finely divided grinding medium is rotated at relatively low speed in a drum or cylinder.

A specific object is to improve upon the rate of grinding obtainable by rotating a slip of finely divided charge material with particulate grinding medium.

This invention is a result of the unexpected discovery that by employing a finely divided grinding medium of substantially greater specific gravity than the sand grinding medium in the aforementioned process of J. B. Duke, and utilizing the heavy grinding medium in the form of round particles, the rate of grinding that can be realized :by controlled rotation of a slip of the charge material with the grinding medium is increased to a remarkable extent. This result was surprising since it was expected that a finely divided, high density grinding medium would have too much inertia to provide the required grinding action. This was especially true because a general characteristic of grinding processes using fine grained grinding media (e.g., the processes described in U.S. 1,956,293 to P. Klein et al., and U8. 2,581,414 to Hochberg) is that these processes utilize lightweight grinding media, such as sand or ceramics. These processes do not utilize heavy metals such as steel, which are generally restricted to use in processes employing grinding medium in the form of large particles, e.g., the Az-inch balls used in a ball mill. The latter mills are used for making relatively coarse grinds and function in a different manner from any of the aforementioned m-icronizing systems which grind in the minus 10 micron size range. Furthermore, it was very surprising that the heavy grinding medium was effective when it was in the form of round particles and was comparatively ineffectual when in the form of angular particles.

In carrying out the present invention, the grinding medium that is used is an abrasive or burnishing material that has a specific gravity substantially greater than, preferably at least double, the specific gravity of sand (silica), which is about 2.6. A preferred grinding medium is steel, which has a specific gravity of about 7.0 to 7.9, depending upon the composition. An essential characteristic of the heavy finely divided grinding medium is that it is generally round in form. Thus, we can use steel grinding medium in the form of small steel balls about -inch to /a-inch in diameter (about 6 to 20 mesh, Tyler). The desired results, however, are not obtained with an angular heavy grinding medium, such as steel grit or steel shavings. The abrasive medium does not need to be truly spherical in form since steel shot, which is generally ovoid in shape, has produced excellent results. Other grinding media having high specific gravity include: ferrosilicon, especially 15% Fe ferrosilicon which has a gravity of 6.8; tungsten carbide, a very heavy material having a specific gravity of 15.6; and magnetite which has a specific gravity of 5.2. Another essential characteristic of the grinding media is that it is composed of particles predominantly within a specific mesh size range; namely, minus 4 plus 35 mesh (Tyler). In other words, the major weight percentage, and preferably substantially all, of the grinding media should be fine enough to pass a 4 mesh (Tyler) screen and be coarse enough to be retained on a 35 mesh (Tyler) screen. The desired results are not obtained with grinding media that are too fine' or too coarse.

A wide variety of minus 325 mesh (about 40 microns) material, natural and synthetic, can be reduced in particle size by the process or" this invention. The process works well with crystalline material that is very hard and difiicult to grind in the minus 10 micron size range, e.g., kaolin clay, calcite and tale. The process is also very effective with non-crystalline (amorphous) material, such as, for example, calcined kaolin clay. The process is especially amenable to comminuting material that is relatively low in specific gravity, i.e., material which has a specific gravity less than about 3.0. Excellent results have been obtained in grinding material having a specific gravity within the range of about 2 to about 3. Thus, a characteristic of the process is that the density of the material to be ground is appreciably less than the density of the grinding medium that is used.

In putting the invention into practice, the minus 325 mesh grindable material is slipped with water, or other liquid, using a dispersing agent, if necessary, to impart fluidity to the slip. With most grindable material, the proportion of solid grindable material to liquid will be within the range of about 10 to 50 parts by weight of the solid grindable material to 50 to parts 'by weight of liquid, i.e., the charge of material to be ground is in the form of a slip containing 10% to 50% solids, weight basis, exclusive of the grinding medium. The solids of the slip Will vary with the nature of the liquid and solid and is limited by the necessity for forming a slip which is distinctly fluid. In the case of clay or calcined clay,

solids slips are preferred.

The slip is charged to a horizontal drum that is free from flights, bafiles or agitators, with the slip only partially filling the drum. It is with a volume of slip such that the slip with added grindpreferable to charge the drum ing medium fills about half the drum or is somewhat below the midpoint of the drum when the drum is at rest.

The finely divided grinding medium is added to the fluid slip (or vice versa) in amount such that the particles of grinding medium are completely immersed in the slip when the drum with slip and grinding medium is at rest. Especially good results have been realized when the volume of the particles of grinding medium was about onethird the volume capacity of the grinding drum and the entire contents of the drum, including the grinding medium, occupied about half the volume of the drum under quiescent conditions. In other words, the particles of steel grinding medium occupied about two-thirds of the total volume of the charge to the drum.

The horizontal drum and contents is rotated about the horizontal axis of the drum at a specific speed. This speed is such that the grinding medium and material to be ground remain suspended in the slip during the grinding and the slip with suspended medium tends to rise in the drum in the direction of rotation of the drum. The slip with suspended grinding medium remains substantially as a unitary or integral body (as opposed to a strongly agitated body in which extensive splashing of liquid occurs or in which a substantial part of the grinding medium rises out of the liquid and then descends back into the liquid). Within the unitary body, the grinding medium moves in a generally continuous, substantially elliptical closed path.

The slip containing grinding medium is tumbled until the micron-size particles in the slip have been comminuted to the desired size. The time to effect the desired degree of comminution is typically within the range of minutes to 10 hours, and is usually within the range of /2 hour to 2 hours.

After tumbling is completed, the slip is separated from the grinding medium by sedimentation and decantation, or by screening. To obtain the solids in the slip in dry form, the slip can be fiocced, filtered, dried and ground. Employing a metallic grinding medium such as steel, some metal-staining of the charge material may result from the continuous impact of the mineral charge with the metallic medium. If necessary or desirable, the metal stain, such as iron-stain, can be removed from the mineral particles by means of bleaching reagents such as, for example, zinc hydrosulfite.

While the suspension in the grinding drum consists of minus 325 mesh material to be ground, liquid (usually water) and finely divided grinding medium, it will be distinctly understood that other material can be present in the suspension. For example, a dispersing agent can be incorporated into the mixture to obtain suspensions of adequate fluidity or mobility. Small quantities of alkali or chelating agents can be added to the suspensions to prevent or minimize metal-staining. Also, mixtures of high specific gravity grinding medium can be used.

In putting this invention into practice, the grinding mill can be any horizontal vessel having a smooth cylindrical interior, such as a drum. The interior surfaces of the mill must be smooth and wettable by the liquid medium. The desired results are not realized when the liquid in the charge does not wet the inner surface of the drum. As mentioned hereinabove, the drum must be free from baffles or other agitator means. The grinding vessel is provided with means for continuously rotating the vessel about its horizontal axis, as for example, by rollers on which the vessel rests. For small scale experimental runs, an open-neck glass jar having a cylindrical body will suflice. Commercial mills may be adapted for closed circuit grinding 'by connecting the mill in series with a classification system to remove fines as they are formed and to recycle the oversize.

The following examples are given to illustrate further the present invention and to show some of its advantages.

In these examples, unless otherwise indicated, all proportions represent weight proportions and all mesh sizes refer to values measured on Tyler screens. All particle size of charge material refers to the size of the ultimate particles and is reported as microns (e.s.d.). Particle size in the micron size range was determined by the sedientation procedure described in TAPPI Standards, T649 SM54; particle size data in ranges below 0.5 micron were determined by a simple modification of the TAPPI method which provided for the use of a long arm centrifuge, as described in a publication by F. H. Norton and S. Speil in I. Am. Ceramic Soc., 21, 89 (1938).

The onegallon experimental rotating mill used in the examples was an open mouth cylindrical glass jar resting horizontally on a pair of horizontal rollers connected to a variable speed drive was to rotate continuously the jar about its horizontal axis without vibrating the jar. The speed of 100 r.p.m. used in the experiments corresponds to a peripheral velocity of about 300 ft./ min. and was such that the contents of the jar had the required action for grinding, as described hereinabove.

Example 1 Calcined kaolin clay was subjected to various grinding tests to obtain a minus 2.0 micron product.

In accordance with this invention, an aqueous slip of the minus 325 mesh calcined kaolin clay was tumbled in a rotating drum with steel balls (about 20 mesh). The results were then compared to the results obtained when an equal volume of minus 10, plus 35 mesh angular silica sand (such as was used in illustrative examples in U.S. 3,097,801 to James B. Duke) was substituted for the steel balls.

The starting clay used in the tests was obtained by calcining a fine size fraction of Georgia kaolin clay in a Nichols Herreshofi furnace. The fine size fraction of clay was 100% minus 325 mesh and about 90% minus 2.0 microns before calcination and about 90% minus 3.5 microns and about 75% minus 2.0 microns after calcination. The average particle size of the calcined clay feed was 1.3 microns e.s.d. (The term average particle size is described in U.S. 3,097,801.)

The calcined clay was slipped and then comminuted in the one-gallon jar as follows. Two hundred grams of the clay was added to the jar together with 600 ml. of water, 0.7 gram of tetrasodium pyrophosphate and 1400 ml. of the & steel balls (Abbott). The jar with contents was placed horizontally on a pair of horizontal rollers connected to a variable speed drive and the rollers were rotated in opposite directions at the same speed. As a result, the jar was continuously rotated about its horizontal axis. The charge was rotated at about 98 rpm. (300 ft./min.) for about two hours. After tumbling, the slip was decanted from the steel balls and then screened on a 325 mesh screen to separate the slip from the steel balls. The slip was flocced by addition of sulfuric acid, filtered and dried. A particle size analysis of the calcined clay product was then made. The test was then repeated with an equivalent volume of the sand (Whitehead #1) substituted for the steel balls. In both cases, a two-hour grinding period was used.

A particle size distribution curve of the minus 325 mesh portion of the feed was obtained from the particle size analysis of the feed and this curve was compared to curves for the products obtained by tumbling the calcined clay with the steel balls or sand for the same grinding periods. The results, summarized in Table I, show that using the small steel balls, in accordance with this invention, the average particle size of the calcined clay was reduced to a value of only 0.58 micron. This represented a 50% reduction in average particle size since the feed had an average particle size of 1.3 microns. Using the sand grinding medium of the prior art, however, the average size of the ground material was 0.78 micron. Thus, when the heavy steel grinding medium was employed, the average particle size of the ground product was 40% less than when the sand grinding medium of the prior art was used.

Data in Table I also indicate that the steel balls were especially effective in grinding the finer size fractions of the feed, particularly the minus 1.0 micron fraction. Thus, using the steel balls, there was about twice as much new minus 1.0 micron calcined clay produced by grinding as compared to sand grinding.

TABLE I.EFFECT OF GRINDING MEDIA ON GRINDING CALCINED CLAY A series of grinding tests, similar to those of Example I, was carried out with a coarse size fraction of uncalcined Georgia kaolin clay (NoKarb) that had been obtained by hydraulic classification of a Georgia kaolin clay crude. The average particle size of the feed clay was 4.8 microns and the objective of the grinding operation was to obtain a maximum content of minus 2.0 micron clay particles in the product. In the tests, the minus plus 35 mesh sand was compared to the steel balls in grinding runs carried out for various grinding periods. It was found that the 3& steel balls produced about the same size product in 30 minutes as an equal volume of the sand produced in about 120 minutes. By this comparison, the steel balls were about four times as effective as the sand in micronizing the coarse size fraction of naturally-occurring clay.

Example 111 Tests were made to illustrate the use of steel grinding medium in the form of steel shot to grind the NoKarb clay with maximum production of minus 2.0 micron clay particles. As in the previous examples, the results of the grinding with heavy grinding media were compared to results with the Whitehead #1 silica sand.

Two different samples or steel shot, each minus 10 plus 35 mesh, were used. The shot samples, which were supplied under the trade names S280 and S390 (Wheelabrator Corporation), had been produced commercially by melting premium steel scrape, shotting, heat reating and sizing. The density of the steel shot was reported as being a minimum of 7.3 g./cc., as determined by the displacement of alcohol. Screen analyses of the two shot samples are given below, along with a screen analysis of the Whitehead #1 sand. These analyses show that all grinding media was minus 10 plus 35 mesh and, therefore, generally similar in size. The analyses show also that the S280 shot was somewhat finer than the sand, while the S390 shot was somewhat coarser than the #1 sand.

SCREEN ANALYSES OF GRINDING MEDIA Weight Percent Mesh Size Steel Shot #1 Sand Composite 100. 0 100. 0 100.0

1 Trace.

The NoKarb clay was slipped in water, using 200 grams clay and 600 grams water. The slip was placed in the oneagallon glass jar together with 1400 ml. of grinding medium and 0.3 gram 0 brand sodium silicate (containing about 9% Na O, 29% SiO and 62% water, weight basis). The jar was continuously rolled at r.'p.m. for 30 minutes. The grinding medium was then separated from the clay slip by screening through a 325 mesh screen. The slip was flocced with sulfuric acid, filtered, dried and analyzed for particle size distribution by the sedimentation method. The average particle size of products obtained with the different minus 10 plus 35 mesh grinding media is tabulated in Table II.

TABLE II.-EFFECT OF COMPOSITION OF GRINDING MEDIA ON THE GRINDING OF A COARSE SIZE FRAC- TION OF KAOLIN CLAY [4.8 microns average particle size] Average particle size of Grinding media: ground clay product, microns Steel Shot (S280) 1.40 Steel Shot 5390 1.40 #1 Sand 2.74

Data in Table II show that when minus 10 plus 35 mesh steel shot was used, the average particle of ground clay was twice as small as when sand of about the same size as the shot was used. A comparison of the average size of ground clay products with the average size of the clay feed (4.8 microns) shows that the steel shot was about 200% more effective than sand in reducing the average particle size of the clay.

Using the sand, it was found that about 37% of the product was finer than 2.0 microns, as compared to 16% for the feed. When the S280 shot was employed, 65% of the product was minus 2.0 microns. Thus, more than twice as much of the desired new minus 2.0 micron clay was obtained with the steel shot.

Example IV To study the effect of shape of a steel grinding medium on grinding efficiency in the process, samples of commercial steel grit similar in particle size distribution to the S280 and S390 steel shot were obtained. The grit (G12 and G16, products of Wheelabrator Corporation) had been produced by crushing steel shot and screening the crushed material. The density of the steel grit samples was reported as being a minimum of 7.6 grams per cc., as determined by the displacement of alcohol. The grit was generally in the form of quartered balls and was observed to be distinctly angular in shape when examined under a magnifying glass.

Following are screen analyses of the minus 10 plus 35 mesh grit samples, showing that the grit was generally similar in size to the steel shot grinding medium of the previous sample. One grit sample (G12) was a little coarser than the sand grinding medium of the prior art and the other sample of lgrit (G16) was a little finer than the sand grinding medium.

SCREEN ANALYSIS OF GRINDING MEDIA Weight Percent l lesh Size Steel Grit Composlte. 100.0 100.0

Using the steel grit samples as the grinding medium, the procedure of the preceding example was repeated in full. It was found that when each of the angular grits was used in place of the substantially round shot, the average particle size of the products was 2.4 microns, in comparison to the 1.4 average particle size obtained using shot of similar size. Using the G12 or G16 grit, only about 40% by weight of the products was finer than 2.0 microns, as compared to 16% minus 2.0 micron clay in the feed and 65% minus 2.0 micron clay roduct using the shot (S280). Thus, about 25% more of the desired minus 2.0 micron clay was produced using the steel shot instead of grit of similar size.

Example V In accordance with this invention, minus 325 mesh limestone having a density of 2.71 g./ cc. (Chemcarb #11) was micronized with the /s steel balls by placing 200 grams of the limestone, 600 ml. soft water and 6965 grams of the A3 steel balls in the one-gallon jar and rotating the jar with contents for 2 hours at about 100 r.p.m. The procedure was repeated substituting an equal volume of the Whitehead #1 sand for the /s" steel balls.

Following are particle size distribution of the limestone charge and ground products, illustrating the marked superiority of the small steel balls over the sand in micronizing the limestone.

TABLE III.EFFECT OF COMPOSITION OF GRINDING MEDIA ON THE GRINDING OF LIMESTONE A sample of talc ore was tabled and a minus 35 mesh aqueous table concenrate was obtained. In accordance with this invention, 426 grams of the aqueous table concentrate at 47.0% solids was diluted to 25% solids with soft water and placed in the one-gallon jar with 6965 grams of the /s" steel balls. The jar with contents was rolled for 2 hours at 100 r.p.m. and the contents screened over a 14 mesh screen to separate the grinding medium from the talc pulp. The minus 14 mesh talc pulp was thickened by sedimentation and bleached with zinc hydrosulfite, filtered, washed and dried at 175 F. In order to determine the size of individual talc particles rather than the size of talc agglomerates in the dried product, the aqueous pulp was screened by means of a wet screening process through a 325 mesh screen. It was found that when the ore pulp was ground in the manner described, the product contained 92.6% by weight of material that would pass the 325 mesh screen.

In contrast, when a similar sample of tabled talc ore concentrate at 50% solids was ground for 2 hours in a type A Abbe Single Assay Mill (8.75 dia.) containing 12.7 pounds of Alunclum cylinders and rolled at r.p.m. for 4 hours, screened, filtered and dried, the product contained only 64.1% of minus 325 mesh ore when tested by the wet screening procedure.

The specific gravity values mentioned herein refer to values obtained by assigning the density of Water at 4 C. and normal atmospheric pressure the value of unity.

We claim:

1. A process for reducing the size of micron-sized particles which comprises:

partially filling a bathe-free, agitator-free, cylindrical drum having a smooth inner surface with a fluid suspension comprising said micron-sized particles, liquid and particulate grinding medium having a density appreciably greater than the density of said particles and being composed largely of substantially round particles fine enough to pass through a 4 mesh sieve and coarse enough to be retained on a 35 mesh sieve,

said liquid being employed in amount suflicient to form a fluid slip with said micron-sized particles and said particulate grinding medium being present in amount such that the particles thereof are distributed throughout a substantial amount of the total volume of said fluid suspension,

and continuously rotating said drum about its horizontal axis at a speed less than critical and such that said suspension within the drum is in the form of an in= tegral body occupying substantially only the lower portion of the drum, the inividual particles of grinding medium within the suspension traveling in continuous, generally elliptical paths within the suspension, said rotation being continued until said micronsized particles are reduced in size.

2. The method of claim 1 in which the grinding medium has a density at least twice as great as the density of the micron-sized material to be ground.

3. The method of claim 1 in which the grinding medium is steel.

4. The method of claim 1 in which the grinding medium is minus 10 plus 35 mesh steel shot.

5. The method of claim 1 in which the micron-sized material to be ground has a specific gravity within the range of about 2 to about 3 and the grinding medium has a specific gravity within the range of about 6 to about 8.

6. A process for reducing the size of micron-sized particles which, comprises:

partially filling a baflle-free, agitator-free cylindrical drum having a smooth inner surface with a fluid suspension comprising said micron-sized particles, water and substantially round steel particles fine enough to pass through a 4 mesh sieve and coarse enough to be retained on a 35 mesh sieve,

said water being employed in amount suflicient to form a fluid slip with said micron-sized particles and said steel particles being present in amount such that the particles thereof are covered completely by the liquid in said suspension and occupy a substantial amount of the total volume of said fluid suspension,

and continuously rotating said drum about its horizontal axis at a speed less than critical and such that said suspension is in the form of an integral body occupying substantially only the lower portion of the drum, the individual particles of steel within the suspension traveling in continuous, generally elliptical paths within the suspension, said rotation being continued until said micron-sized particles are reduced in size.

7. The method of claim 6 in which said steel particles are in the form of balls.

8. The method of claim 6 in which said steel particles are in the form of shot.

9. The method of claim 6 in which said micron-sized particles comprise naturally occurring kaolin clay and said steel is in the form of minus 10 plus 35 mesh shot.

10. The method of claim 6 in which the steel particles occupy about one-third the total volume of the drum.

11. The method of claim 6 in which the fluid suspension occupies about onehalf the volume of the drum, and the steel particles occupy about two-thirds of the total volume of said suspension.

12. A method for grinding minus 325 mesh mineral particles which comprises charging a cylindrical horizontal battle-free, agitator-free drum with a fluid suspension comprising minus 325 mesh mineral particles, Water and minus 10 mesh plus 35 mesh, generally round particles of steel shot, said suspension being used in amount to fill about half of said drum and said steel particles being employed in amount sufiicient to occupy about two-thirds of the total volume of the suspension, continuously rotating the drum at a speed less than critical with suspension for a time Within the range of about 10 minutes to about 2 hours, and separating the mineral particles and water from the steel shot.

13. A process for reducing the size of micron-sized particles which comprises:

partially filling a baffle-free, agitator-free cylindrical drum having a smooth inner surface with a fluid suspension comprising said micron-sized particles, liquid and 1/32" steel balls, said liquid being employed in amount sufficient to form a fluid slip with said micron-sized particles and said steel balls being present in amount such that the particles thereof are distributed throughout a substantial amount of the total volume of said fluid suspension,

and continuously rotating said drum about its horizontal axis at a speed less than critical and such that said suspension within the drum is in the form of an integral body occupying substantially only the lower portion of the drum, the individual steel balls within the suspension traveling in continuous, generally elliptical paths Within the suspension.

14. A method for grinding kaolin clay which comprises partially filling a baffle-free, agitator-free, horizontal,

10 smooth inner-surfaced drum with a fluid aqueous kaolin clay slip containing 10% to clay solids and minus 10 mesh plus 35 mesh, generally round particles of steel shot, said steel shot being present in amount such that the particles thereof are distributed throughout a substantial amount of the total volume of said clay slip,

rotating said drum about its horizontal axis at a speed below critical and such that the steel shot is suspended in said clay slip and said suspension is in the form of an integral body occuping substantially only the lower portion of the drum, the individual particles of steel shot within the suspension traveling in continuous, generally elliptical paths Within the suspension,

and rotating said drum in said manner until said clay is reduced in size.

15. The method of claim 14 wherein said clay is a calcined clay.

16. A method for grinding kaolin clay which comprises filling a baffle-free, agitator-free, horizontal, smooth innersurfaced drum about half full With a mixture of a fluid aqueous kaolin clay slip containing 10% to 50% clay solids and minus 10 mesh plus 35 mesh, generally round particles of steel shot, said steel shot being present in amount such that the particles thereof occupy about tWothirds of the total volume of said mixture,

rotating the drum with said mixture at a speed less than critical and such that said mixture is a suspension in the form of an integral body occupying substantially only the lower portion of the drum, the individual particles of grinding medium Within the suspension traveling in continuous, generally elliptical paths within the suspension, said drum being rotated for a time sufiicient to reduce the particle size of said clay, and separating said steel shot from said clay slip.

17. The method of claim 16 wherein said clay is a calcined clay.

References Cited by the Examiner UNITED STATES PATENTS 2,991,017 7/1961 Hukki 241-l84 3,008,656 11/1961 Weston 241-184 3,008,657 11/1961 Szegvari 241-184 WILLIAM W. DYER, JR., Primary Examiner.

G. A. DOST, Assistant Examiner.

Claims

1. A PROCESS FOR REDUCING THE SIZE OF MICRON-SIZED PARTICLES WHICH COMPRISES: PARTIALLY FILLING A BAFFLE-FREE, AGITATOR-FREE, CYLINDRICAL DRUM HAVING A SMOOTH INNER SURFACE WITH A FLUID SUSPENSION COMPRISING SAID MICRON-SIZED PARTICLES, LIQUID AND PARTICULATE GRINDING MEDIUM HAVING A DENSITY APPRECIABLY GREATER THAN THE DENSITY OF SAID PARTICLES AND BEING COMPOSED LARGELY OF SUBSTANTIALLY ROUND PARTICLES FINE ENOUGH TO PASS THROUGH A 4 MESH SIEVE AND COARSE ENOUGH TO BE RETAINED ON A 35 MESH SIEVE, SAID LIQUID BEING EMPLOYED IN AMOUNT SUFFICIENT TO FORM A FLUID SLIP WITH SAID MICRON-SIZED PARTICLES AND SAID PARTICULATE GRINDING MEDIUM BEING PRESENT IN AMOUNT SUCH THAT THE PARTICLES THEREOF ARE DISTRIBUTED THROUGHOUT A SUBSTANTIAL AMOUNT OF THE TOTAL VOLUME OF SAID FLUID SUSPENSION, AND CONTINUOUSLY ROTATING SAID DRUM ABOUT ITS HORIZONTAL AXIS AT A SPEED LESS THAN CRITICAL AND SUCH THAT SAID SUSPENSION WITHIN THE DRUM IS IN THE FORM OF AN INTEGRAL BODY OCCUPYING SUBSTANTIALLY ONLY THE LOWER PORTION OF THE DRUM, THE INDIVIDUAL PARTICLES OF GRINDING MEDIUM WITHIN THE SUSPENSION TRAVELING IN CONTINUOUS, GENERALLY ELLIPTICAL PATHS WITHIN THE SUSPENSION, SAID ROTATION BEING CONTINUED UNTIL SAID MICRONSIZED PARTICLES ARE REDUCED IN SIZE.