AU671248B2

AU671248B2 - Zirconium silicate grinding medium and method of milling

Info

Publication number: AU671248B2
Application number: AU16900/95A
Authority: AU
Inventors: Thomas Ian Brownbridge; Phillip M. Story
Original assignee: Kerr McGee Corp; Kerr McGee Chemical Corp
Current assignee: Tronox LLC
Priority date: 1994-01-25
Filing date: 1995-01-24
Publication date: 1996-08-15
Anticipated expiration: 2015-01-24
Also published as: EP0930098A1; DE69530132T2; ATE235318T1; EP0690749A1; ES2143616T3; EP0930098B1; SK117895A3; DE69530132D1; CN1042104C; PL310446A1; BR9506238A; CA2158969A1; WO1995019846A1; KR960700819A; FI954466A0; ZA95590B; CZ284563B6; CA2158969C; TW276208B; CZ235795A3

Abstract

A grinding medium including naturally occurring zirconium silicate sand characterized by a density in the range of from about 4g/cc absolute to about 6g/cc absolute is provided. Also provided is a method for milling a powder which includes steps of forming a milling slurry including a naturally occurring zirconium silicate sand grinding medium having a density in the range of from about 4g/cc absolute to about 6g/cc absolute.

Description

W095119846 PCTUS95/00963 ZIRCONIUM SILICATE GRINDING MEDIUM AND METHOD OF MILLING Backaround of the Invention 1. Field of the Invention The invention relates to grinding media and more particularly to zirconium silicate grinding media.

2. Description of the Prior Art Many applications such as the production of ceramic parts, production of magnetic media and manufacture of paints require that the ceramic, magnetic or pigment powder, be as completely dispersed within the particular binder appropriate for a given application as possible.

Highly dispersed ceramic powders result in ceramic parts of higher density and higher strength than those prepared from less completely dispersed solids. The data storage of magnetic media are limited by particle size and completely dispersed, finely divided powder magnetic media achieve maximum information storage. The optical properties of paints, such as hiding power, brightness, color and durability are strongly dependent on the degree of pigment achieved. Finely divided powders are required to achieve such complete powder dispersal. Typically, milling devices such as disc mills, cage mills, and/or attrition mills are used with a milling medium to produce such finely divided powders, ideally to reduce the powder to its ultimate state of such as, for example, to the size of a single powder crystallite.

Milling of some powders involves a de-agglomeration process according to which chemical bonds, such as hydrogen-bonded surface moisture, Van der Waals and forces, such as between particles, as well as any other bonds which are. keeping the particles together, must WO 95119846 PCT/US9510963 2 be broken and/or overcome in order to obtain particles in their state of ultimate division. One pigment powder which entails a de-agglomeration milling process to reduce it to a finely divided powder is titanium dioxide. Optimal dispersal of titanium dioxide pigment powder results in optimized performance properties, particularly improved gloss, durability and hiding power.

De-agglomeration processes are best performed using a grinding medium characterized by a small particle size which the smallest multiple of the actual size of the product particles being milled which can still be effectively separated from the product powder. In a continuous process, the grinding medium can be separated from the product particles using density separation techniques. In a typical or sand mill operated in a continuous process, separation of the grinding medium from the product can be effected on the basis of differences between settling rate, particle size or both parameters existing between the grinding medium and product powder particles.

Commercial milling applications typically use silica sand, glass beads, ceramic media or steel balls, for example, as grinding media. Among these, the low density of about 2.6g/cc, of sand and glass beads and the low hardness of glass beads restricts the materials which can be milled using sand glass beads. The use of steel shot is restricted only to those applications where iron contamination resulting from wear products of the steel shot during the milling process can be tolerated.

Thus, there exists a need for a relatively inexpensive, and non-toxic grinding medium which is characterized by a small particle size, a density sufficiently high for separation purposes to allow it to be used for the milling of WO 9519846 ICT/US9100963 3 a wide range of materials and which does not generate wear byproducts which result in contamination of the product powder.

Samytv of the Invyntion The invention provides a relatively inexpensive, dense and non-toxic, naturally occurring zirconium silicate sand grinding medium which has small particle size and a sufficiently high density to make it suitable for grinding a wide range of materials, while not contaminating the product with its wear byproducts as well as a method for milling a powder using this grinding medium.

According to one aspect of the invention, a naturally occurring zirconium silicate sand characterized by a density in the range of from about 4g/cc absolute to about 6g/cc more preferably in the range of from about 4.6g/cc absolute to about 4.9g/cc absolute and most preferably in the range of from about 4.75g/cc absolute to about 4.85g/cc absolute is provided.

Another aspect of the invention provides a method for a powder comprising steps of providing a starting powder characterized by a starting powder particle size and a d naturally occurring zirconium silicate sand grinding medium characterized by a grinding medium density in the range of from about 4.0g/ce absolute to about 6.0g/cc absolute and the starting powder and the grinding medium with a liquid medium to form a milling slurry; milling the milling slurry for a time sufficient to produce a product slurry including a product powder having a desired product powder particle size and having substantially the same composition as starting powder and separating the product slurry from the milling slurry.

WO 95119846 4 PCTUS95/00963 An object of this invention is to provide a naturally occurring zirconium silicate sand grinding medium.

Another object of this invention is to provide a method for milling a powder using a naturally occurring zirconium silicate sand grinding medium.

Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon reading the description of preferred embodiments which follows.

Description of Preferred bodfl nt i As used herein in the specification and in the claims which follow, the term "naturally occurring" indicates that the zirconium silicate sand is mined in the form of zirconium silicate sand of a particular particle size and is distinguished from zirconium silicate materials which are synthesized, manufactured or otherwise artificially produced by man. The zirconium silicate sand grinding medium of the invention occurs in nature in the appropriate size and shape which can be sorted to obtain the appropriate fraction for use a particular'grinding operation. The mined zirconium silicate sand is sorted to isolate the appropriate fraction of zirconium silicate sand, based on particle size considerations, to be used as a grinding medium. The term "grinding medium" as used herein in the specification and in the claims which follow refers to a material which is placed in a milling device, such as a disc mill, cage mill or attrition mill, along with the powder to be ground more finely or de-agglomerated to transmit shearing action of the milling device to the powder being processed to break apart particles the powder. L i i, j i i WO 95/19846 5 'CTUS9S/00963 The invention provides a grinding medium including naturally occurring zirconium silicate sand characterized by a density in the range of from about 4g/cc to about 6g/cc, more preferably in the range of from about 4.6g/cc to about 4.9g/cc Sand most preferably in the range of from about 4.75g/cc to about 4.85g/cc.

The naturally occurring zirconium silicate sand tends to be single phase, while synthetic zirconium silicate ceramic beads are typically multiphase materials. Surface such as aluminum, iron, uranium, thorium and other heavy metals as well as TiO, can be present on the surfaces of the naturally occurring zirconium silicate sand particles. Once the surface contaminants are removed by any surface preconditioning process known to one skilled in the such as, for example, washing and classifying, chemical analyses indicate that any remaining contaminants are within the crystal structure of the zirconium silicate and do not adversely affect the powder being milled.

Since the density of the naturally occurring zirconium silicate sand as described above exceeds the 3.8g/cc density typically characteristic of manufactured zirconium silicate beads, naturally occurring zirconium silicate sand grinding medium of a smaller particle size than that of manufactured zirconium silicate beads can be used, without the zirconium sand floating out of the milling slurry, thus ceasing to be effective as a grinding medium.

The zirconium silicate sand grinding medium can be characterized by a particle size which is the smallest multiple of the particle size of the finished product particle size, the milled product powder particle size, which can be effectively separated from the milled product powder.

Typically, the naturally occurring zirconium silicate sand i, i I W0 95/19846 PCTUS95100963 particle size is greater than 10microns and can be in the range of from about 100microns to about 1S00microns, more preferably in the range of from about 100microns to about Soomicrons and most preferably in the range of from about 5150microns to about 250microns. The mined, naturally occurring zirconium silicate sand can be screened using techniques well known to one skilled in the art to isolate a coarse fraction of sand having particles of an appropriate size to function as an effective grinding medium.

The grinding medium can be any liquid medium compatible with the product being milled and the milling process and can include water, oil, any other organic compound or a mixture thereof, and can be combined with the naturally occurring zirconium silicate sand to form a slurry. The liquid medium selected depending upon the product being milled. The milled product powder may or may not be separated from the liquid medium after the milling process is complete; however, the grinding medium is usually separated from the liquid medium after the milling process is complete.

If the powder bLing milled is a pigment for use in an oil based paint or ink, the liquid medium can be an oil such as a naturally derived oil like tung oil, linseed oil, soybean oil or tall oil or mixtures thereof. These naturally occurring oils can be mixed with solvents such as mineral spirits, or toluol or mixtures thereof which can further include substances such as gums, resins, dispersants and/or drying agents. The liquid medium can also include other materials used in the manufacture of oil based paints and inks such as alkyd resins, epoxy resins, nitrocellulose, melamines, and silicones.

If the powder being milled is a pigment for use in a water based paint, such as a latex paint, the liquid medium

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WO 95/19846 PCT/US95/00963 can be water, optionally including antifoaming agents and/or dispersants. Also, if the powder is a ceramic or magnetic powder, the medium can be water and can also include dispersants.

The naturally occurring zirconium silicate sand and the liquid medium can be combined to form a grinding slurry which is further characterized by a grinding slurry viscosity which can be in the range of from "'iout l.0cps to about 10,000cps, more preferably in the range of from about l.Ocps to about 10500cps and most preferably in the range of from about to about 100cps. In general, the grinding slurry viscosity is determined by the concentration of solids in the grinding slurry and, thus, the higher the concentration of solids in the grinding slurry, the higher will be the grinding slurry and density. There is no absolute upper limit to grinding slurry viscosity; however, at some viscosity, a point is reached where no grinding medium is needed, as is the case for plastics compounded in extruders, roll mills, etc. without a grinding medium.

The invention also provides a method for milling a powder including steps of providing a starting powder characterized by a starting powder particle size; providing a grinding medium including naturally occurring zirconium silicate sand characterized by a grinding medium density in the range of about 4.0g/cc absolute to about 6.0 g/cc absolute; providing a liquid medium; mixing the starting powder with the liquid medium to form a milling slurry; milling the milling slurry for a sufficient time to produce a product slurry including a product powder characterized by a desired product particle size and having substantially the same composition as the starting powder; and separating the product slurry including the product powder from the milling slurry.

~l ~l l- WO 95/19846 PCTIUS95/00963 8 The starting powder used in the method of the invention can be an agglomerated and/or aggregated powder. The agglomerated powder can be characterized by an agglomerated powder particle size less than about 500microns and more can be in the range of from about 0.01micron to about 200microns. For titanium dioxide pigment powders, the agglomerated powder has a particle size of in the range of from about 0.05micron to about 100microns which can be milled to approach the particle size of an individual titanium dioxide crystallite.

The starting powder can also be characterized by a starting powder density in the range of from about 0.8g/cc absolute to about 5.0g/cc absolute. The method of the invention is suitable for organic powders which typically have densities on the lower end of the above range as well as for inorganic powders such as titanium dioxide, calcium carbonate, bentonite or kaolin or mixtures thereof. The titanium dioxide starting powder can be an agglomerated titanium dioxide pigment which has a density in the range of from about 3.7g/cc to about 4.2g/cc.

The naturally occurring zirconium silicate sand used in the method of the invention can also be characterized by a zirconium silicate sand particle size greater than about 100microns and can be in the range of from about 100microns to 1500microns, more preferably in the range of from about 100microns to about 500microns and most preferably in the range of from about 150microns to about 250miicrons.

The liquid medium used in the method of the invention can be oil or water selected according to the criteria already described, O S. &ieStp S- e A(Mtk\.g can be carried out in any suitable milling device which employs a grinding medium, such as, but frI~ r~ i c~ WO 95/19846 PCT/US95/00963 not limited to, a bead mill, cage mill, disc mill or pin mill designed to support a vertical flow or horizontal flow. The milling process can be a batch or continuous process.

St-p Separating the product slurry from the milling slurry can be accomplished by distinguishing the product slurry, which contains the product powder along with liquid medium from the milling slurry on the basis of a difference between starting powder and grinding medium physical properties and product powder particle physical properties such as particle size, particle density and particle settling rate. As already described, the product powder may or may not be separated from the liquid medium after the milling process is complete; however, the grinding medium is usually separated from the liquid medium after the milling process is complete. The product powder can be separated from the product slurry and subjected to further processing such as dispersing the powder in a dispersing medium to form a dispersion. Depending upon whether the dispersion is an oil based paint or ink or a water based paint ink or a ceramic or magnetic powder dispersion, the dispersing medium ian be selected according to the same criteria as already described for the selection of the liquid medium. If the product powder is to be used in the product slurry, no further dispersing steps are needed.

In order to further illustrate the present invention, the following examples are provided. The particular compounds, processes and conditions utilized in the examples are meant to be illustrative of the present invention and are not limited thereto.

(0 L i -L WO 95/19846 PCTUS95/00963 Example 1 The following example is provided to compare the performance as a grinding medium of conventional, commercially available synthetic zirconium silicate ceramic beads with the Sperformance of standard 10-40 mesh (U.S.)silica sand.

Sand mills having nominal grinding chamber capacities of 275 gallons and overall capacities of 500 gallons were loaded seprately with 3000 pounds of synthetic zirconium silicate ceramic beads of nominal 300micron and 210micron size and with 101200 pounds of standard 10-40 mesh silica sand, the highest mill loading feasible with silica sand. The mills loaded with 3000 pounds of synthetic zirconium silicate ceramic beads as well as the mill loaded with 1200 pounds of 10-40 mesh silica sand were operated at 16, 23 and per minute flow rates. The feed slurries fed through all mills had a density of 1.35g/cc and contained titanium dioxide, approximately 40% of which was less than 0.5micron in size in water. The size of the titanium dioxide particlee in the product slurry was measured using a Leeds and Northrupp 209200 series Microtrac" particle size analyzer in water with 0.2% sodium hexametaphosphate surfactant at ambient temperature. The results are summarized in Table 1 and indicate that the grinding efficiency of the synthetic zirconium silicate ceramic beads as indicated by the of product powder less than or equal to in size compares favorably with the grinding efficiency of 10-40 mesh silica sand.

W095/19046 WO 9519846PCIUS9$100963 Mill P.low Rxte(#aVmin) Grinding Medium A 30 300micron synthetic 66.57 zirconium silicate ceramic beads B 30 300micron synthetic 64.42 zirconium silicate ceramic beads A 23 300znicron syntheticzirconium silicate ceramic beads B23 300mlcron synthetic 70.41 zirconium silicate ___ceremic beads A '16 300micron synthetic 79.96 zirconium silicate ceramic beads B 16 300micron synthetic 71.26 zirconium silicate ceramic beads A 30 210micron synthetic 85.29 zirconium silicate ceramic beads B3 30 21 Omicron synthetic 7 4.7 zirconium silicate ceramic beads A 23 2 1Omicron synthetic 91.51 zirconium silicate ceramic beads B 23 210micron synthetic 83.11 zirconium silicate ceramic beads A 16 2l1 micron synthetic 95.22 zirconium silicate ceramic beads B16 2l0micron synthetic 95.22 zirconium silicate cermmic beads A 30 10-40 mesh 65.17 ______silica sand_ B 30 10-40 mesh 54.28 silica sand WO095119846 12 PCTIUS95W63 Mill Flow Rate(gaVm/in) Grinnd i.dium -9 %Produc50.S5nicron 61.96 A 23 10-40 mesh silic sand B 23 10-40 mesh 57.76 silica sand A 16 10-40 mesh 67.09 silica sand B 16 10-40 mesh 59.48 silica sand Furthermore, when the properties of finished pigments processed with the 210micron synthetic zirconium silicate ceramic beads were compared with those of pigments processed silica sand, several improvements with respect to the properties of finished pigments processed with silica sand were observed. The improvements included an approximately 57% reduction in break time, which is defined as time to incorporate the pigment into an alkyd resin, an approximately 1042% lowering in consistency, which is defined as the torque required to mix an alkyd resin paint system once the pigment is incorporated therein, an approximate 6 unit increase in B235 semi-gloss which iJ defined as a 60 degree gloss measurement in a latex paint system, a lowering by 12 units of B202H haze, which is defined as the relative depth an image can be perceived on a paint surface and an increase of approximately 2 units in B202 gloss, which is defined as a measurement at 20 degrees of reflected light from a paint system made in an acrylic resin.

It is noted that the naturally occurring zirconium silicate sand grinding medium, because of its higher density and single phase microstructure, can produce a pigment powder having superior properties to those obtained using the synthetic zirconium silicate ceramic beads as described above.

WO 95/19846 PCT/US95100963 13 £amn2iJ.

Example 2 is provided to compare the performance of synthetic zirconium silicate ceramic beads with the performance of the naturally occurring zirconium silicate sand Sgrinding medium of the invention. It is noted that the naturally occurring zirconium silicate sand has a higher density than the 3.8g/cc density of synthetic zirconium silicate products which allows use of smaller naturally occurring zirconium silicate sand particles by comparison with synthetic zirconium silicate product particle sizes, thereby providing greater grinding efficiency.

Plant trials using the naturally occurring zirconium silicate sand grinding medium having a particle size in the range of from about 180-210microns in a cage mill showed that occurring zirconium silicate sand can be used successfully at production flowrates to effect removal of coarse particles, having a particle size greater than 0.Smicron in a titanium dioxide pigment. No appreciable loss of media from the mill was observed.

Example 2 was conducted by changing flowrates in mill B, operating with conventional silica sand, and of mill C, operating with naturally occurring zirconium silicate sand.

Sand loadings in mill B and mill C were similar to those used in Example 1, 1200 pounds of silica sand in mill B and 253000 pounds of naturally occurring zirconium silicate sand in mill C. Samples were obtained concurrently from both sand mills. Mill feed was also sampled to measure any particle size variability in feed particle size.

Particle size data, as provided in Table 2, shows that at a low flowrate (approximately 13 gallons/minute) or at a high flowrate (approximately 35 gallons/minute) the naturally occurring zirconium silicate sand is much more W0 95119846 14 PCTIUS9500963 efficient in reducing particle size, compared with the performance of the conventional silica sand.

After a period of continuous operation, both mill overflows were sampled for pigment optical quality and contamination.

Contamination of the pigment product from the naturally occurring zirconium silicate sand grinding medium was minimal as measured by x-ray fluorescence examination of the pigment solids found in the mill overflow. Metal contaminant levels also measured by x-ray fluorescence were similar to those observed in pigments milled using a conventional silica sand grinding medium. The optical quality of the pigment milled with the naturally occurring zirconium silicate sand as measured by the B381 dry color and brightness test which is defined as the total light reflected from a powder compact surface and the spectrum of reflected light i.e. color, was comparable to that obtained for samples milled using conventional silica sand. Results of these tests are summarized in Table 3.

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W0195/19846 15 PTU9106 MIUS95/00963 Tale Pigment Particle Size Data Parameter Mill B mill C Flowrate 13.2 13.2 (gal/mmn) Median Particle Diameter 0.37 0.24 Fraction ofParticles< 86.94 99.55 0.Smicron f1lowrate 35.2 35.2 (gal/min)__ Median Particle Diameter 0.38 0.37 Fraction of Particles 75.64 87.55 Table 3 Pigment Chemtical Composition and Optical Properties Property MilliB miniC A10 3 0.71 0.72 %ZrO 2 0.01 0.01 Calgon 0.06 0.06 Fe ppm 35 34 Ni ppm 10 8 B381 Brightness 97.87 97.94 B381 Color 1.14 1.09 After nineteen days of operation with the naturally occurring zirconium silicate sand, mill C was inspected for signs of wear on the rubber lining using a fiber optic probe inserted through a flange in the underside of the mill.

Essentially no signs of wear on the rubber lining were observed as indicated by the condition of the weavelike pattern on the rubber mill lining which is normally present on the surface of freshly lined mills. By contrast, in a mill

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WNO 9s 19846CTuSN/f which had been operated for only one week using a conventional silica sand grinding medium, the mill lining ahowed considerable wear, eapecially to the leading edges of the mill rotor bars where the weavelike pattern had been almost worn away.

anals.l The following example is provided to show the differences in particle size, impurity content and grinding perforiance among naturally occurring zirconium silicate sands obtained different natural sources.

Three naturally occurring zirconium silicate sand samples, hereinafter referred to as Sample 1, Sample 2 and Sample 3 were evaluated for particle size using a screen analysis conducted for thirty minutes on a RotapM Based on the data presented in Table 4, Sample 2 and Sample 3 are similar with respect to particle size, while Sample I is smaller, which can make it difficult to retain Sample 1 sand in a cage mill during a continuous process.

Particle Sizes of Zirconium Silicate Sand Samples

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Sample Origin Sample 1 Sample 2 Sample %180 microns 0.61 75.1 67.2 %150microns 5.73 16 32,1 less than 93,66 8.9 0.7 150microns The three naturally occurring zirconium silicate sand samples were also subjected to elemental analysis using x-ray fluorescence techniques. The results of the elemental analysis are given in Table S.

'4A Y9 WO 9.119845 17 PCTIUS95/00963 Chemical Analysis of Zirconium Elemental Silicate Sands Sample Origin Sample 1 Sample 2 Sample 3 Element %Na 0.38 0.41 0.2 %A 0.16 0.16 0.73 %Si 15.15 15.43 14.5 %CI 0.2 0.24 0.1 %TI 0.13 0.13 0.21 %Y 0.2 0.19 0.19 %Zr 48.16 47,69 48,88 %Il 0.92 0.99 0.93 %0 34.49 35 34.07 Trace Analysis P (ppm) 659 K (ppm) 134 Ca (ppm) 327 614 689 Cr (ppm) 177 Mn (ppm) 201 Fe (ppm) 7'129 714 711 Sr (ppm) 81 Pb (ppm) sO Th (ppm) 90 200 180 U (ppm) 180 200 220 A laboratory scale grinding study was also performed with the three naturally occurring zirconium silicate sands. The study was conducted in a cage mill under a steandard laboratory sand load of 1.8:1 zirconium sand to pigment load. Table 6 shows the percent of particles passing 0.5micron, i.e., particles having sizes smaller than O.Smicron after 2, 4 and 108 minutes of grinding, as well as the median particle diameter at these times. The pigment was an untreated interior enamel grade titanium dioxide pigment. Particle sizes were p a L r i I i I frj~ 1' W0 9SI19846 PCTIUS95/00963 determined using a microtrac" particle size analyzer as has been described before.

Pigment Grinding Performance Sample Sample I Sample 2 Sample 3 Origi Particle Size Particle size Particle Size Time Median %/Passing Median %/Pasmng Median %/Passing ____Diameter 0.5n'icron Diameter 0.Smicron Diaeer Feed (0 1 21.09 1 21.09 1 21.09 minutes)______ 2 minutes 0.45 61.93 0.48 53.45 0.48 53.66 4 minutes 0.38 80.96 0.42 69.84 0.42 71.53 8 minutes 0.33 194.02 0.35 87.97 0.36 88.66

Claims

19- The claims defining the invention are as follows: 1. A grinding medium including naturally occurring zirconium silicate sand characterized by a density in the range of from 4g/cc absolute to 6g/cc absolute. s 2. A grinding medium according to claim 1 characterised by a density in the range of from 4.6g/cc absolute to 4.9g/cc absolute, 3. A grinding medium according to claim 1 or 2 characterised by a density In the range of from 4.75g/cc absolute to 4.85g/cc absolute to 4.85g/cc absolute. 4. The grinding medium according to any one of the preceding claims wherein said 0o naturally occurring zirconium silicate sand is characterized by a zirconium silicate sand particle size and wherein said zirconium silicate sand particle size is the smallest multiple of a milled product powder particle size which can be separated from a milled product powder. A grinding medium according to claim 4 wherein said zirconium silicate sand is particle size is greater than about 100 microns. 6. A grinding medium according to claim 5 wherein said zirconium silicate sand particle size is in the rnage of from 100 microns to 1500 microns. 7. A grinding medium according to claim 6 wherein the zirconium silicate sand particle size is in the range of from 100 microns to 500 microns. 20 8. A grinding medium according to claim 6 or 7 wherein the zirconium silicate particle size is in the range of from 150 micron: to 250 microns. 9. A grinding medium according to any one of the preceding claims further including a liquid medium. A grinding medium according to claim 9 wherein said liquid medium is a liquid medium selected from the group consisting of water, oil, organic compounds and mixtures thereof. 11. A grinding medium according to claim 9 or 10 wherein said naturally occurring zirconium silicate sand and said liquid medium are combined so that they form a grinding slurry. 12. A grinding medium according to claim 11 wherein said grinding slurry is further characterized by a grinding slurry viscosity and said grinding slurry viscosity is in the range of form about 1.0cps to about 10,000cps. ii :r NRA L 13. A grinding medium according to claim 12 wherein the grinding slurry viscosity Is in the range of from about 1 .Ocps to about S00cps, 14. A grinding medium according to claim 12 or 13 wherein the grinding slurry viscosity Is in the range of from about IOcps to about 100cp3. 15, A method for milling a powder including the steps of: 1) providing a starting powder characterized by a starting powder particle size; 2) providing a grinding medium including naturally occurring zirconium silicate sand characterized by a grinding medium density In the range of from absolute to 6.0g/cc absolute and a particle size in tho range of from 100 microns to 500 microns; 3) providing a liquid medium; 4) mixing said starting powder, said grinding medium and said liquid medium to form a milling slurry; 15) milling said milling slurry in a high energy mill for a time sufficient to produce a product slurry including a product powder characterized by a desired product *1 powder particle size and having substantially the same composition as said starting powder; and 6) separating said product slurry Including said product powder from said milling slurry so that said grinding medium remains in said milling slurry. 16. A method according to claim 15 wherein the high energy mill has a nominal shear rate of from 6000 to 14000 reciprocal minutes and an agitator peripheral speed of from 1000 to 2500 feet per minute. 17. A method according to claim 16 wherein the high energy mill Is selected from the group consisting of disc mills and cage mills. 18. A method according to claim 15 wherein said starting powder Is an agglomerated powder, 19. A method according to claim 18 wherein said agglomerated powder is further characterized by an agglomerated powder particle size and said agglomerate powder particle size is In the range of from 0.01 micron to 500 microns. A method according to claim 18 or 19 wherein the agglomerated powder has a particle size in the range of from 0.01 micron to 200 microns. I KNitwi. o -21-

21. A method according to claim 15 wherein said starting powder Is an aggregated powder.

22. A method according to claim 15 wherein said starting powder and said product powder are further characterized by a powder density in the range of from 0.8g/cc absolute to 5g/cc absolute.

23. A method according to claim 15 wherein said starting powder is an organic powder.

24. A method according to claim 15 wherein said starting powder is an inorganic powder.

25. A method according to claim 15 wherein said starting powder is an agglomerated titanium dioxide pigment. 26, A method according to any one of claims 15 to 25 wherein the zirconium silicate sand particle size Is in the range of from 150 microns to 250 microns. 27, A method according to any one of claims 15 to 26 wherein said liquid medium is Is a liquid compatible with said method and with said powder. I

28. A method according to claim 16 wherein said high energy mill has a vertical flow design.

29. A method according to claim 16 wherein said milling device has a horizontal flow

30. A method according to claim 15 wherein said step of separating said product slurry from said milling slt!rry is accomplished by distinglishing said product slurry from said milling slurry on the basis of a difference between starting powder, grinding medium and product powder physical properties wherein the physical properties are selected from the group consisting of particle size, particle density and particle settling rate.

31. A method according to claim 15 wherein said steps are preformed continuously.

32. A method according to claim 15 wherein said steps are performed according to a batch process.

33. A method according to claim 15 further comprising steps of separating said product powder from said product slurry and dispersing said product powder in a dispersing medium to form a dispersion. 34, A method according to claim 33 wherein said dispersing medium is a liquid imedium compatible with said powder and said method, f ^A :1 i uS wi~wiwfee -22- A method according to claim 15 wherein said zirconium silicate sand particle size is the smallest multiple of a milled product powder parti:le size which can be separated from a milled product powder.

36. A method according to claim 15 further comprising a liquid medium.

37. A method according to claim 36 wherein said liquid medium is a liquid medium selected from the group consisting of water, oil, organic compounds and mixtures thereof.

38. A method according to claim 36 or 37 wherein said naturally occurring zirconium silicate sand and said liquid medium are combined so that they form a grinding slurry. to 39. A method according to claim 38 wherein said grinding slurry is further characterized by a grinding slurry viscosity and said grinding slurry viscosity is in the 4 range of from 1.0 cps to 10,000 cps.

40. A method according to claim 15 wherein the grinding medium has a density in the range of from 4.6 g/cc absolute to 4.9 g/cc absolute. s15 41. A method according to claim 40 wherein the grinding medium has a density in 8 the range of from 4.75 g/cc absolute to 4.85 g/cc absolute,

42. A method according to claim 15 wherein said zirconium silicate sand particle size is in the range of from 150 microns to 250 microns,

43. A method according to claim 39 wherein said grinding slurry is further j characterized by a grinding slurry viscosity in the range of from 1.0 cps to 500 cps.

44. A method according to claim 43 wherein said grinding slurry is further characterized by a grinding slurry viscosity in the range of from 1.0 cps to 100 cps. A grinding medium according to claim 1, substantially as hereinbefore described with reference to any one of the examples.

46. A method according to claim 15, substantially as hereinbefore described with reference to any one of the examples, i DATED: 12 June, 1996 PHILLIPS ORMONDE FITZPATRICK Attorneys for: KERR-MCGEE CHEMICAL CORPORATION .0 71