Zirconium silicate grinding medium

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: The invention relates to a method of deagglomerating and reducing the particle size of titanium dioxide pigment powder using a zirconium silicate grinding medium. 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, respectively, 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 capabilities 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 dispersal 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 division 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 electrostatic forces, such as between particles, as well as any other bonds which are keeping the particles together, must 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 is 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 bead 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 2.6 g/cm 3 , of sand and glass beads and the low hardness of glass beads restricts the materials which can be milled using sand or 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. JP 60-211637 discloses production of a magnetic recording medium in a high speed stirring type mill using the mill to mix and disperse a magnetic paint using balls or beads of pebble-shaped pieces of natural or synthetic zirconium consisting essentially of ZrO 2 and SiO 2 having an average grain size of about 0.3 to 3.0 mm. The particles are said to have a specific gravity of 3.6 to 4.2 and to disperse well into the magnetic paint. GB 679552 relates to a grinding medium comprising zircon sand, preferably having a granule size of from 50 to 150 µm. Titanium dioxide may be milled in the form of a slurry or a dispersion in water, and specific examples refer to filling glass jars with a mixture of the dispersion and zircon sand, putting them on to a trundler and milling for 16 hours at 164 rpm. Thus, there exists a need for a method using a relatively inexpensive, dense and non-toxic grinding medium which is characterised by a small particle size, a density sufficiently high for separation purposes to allow it to be used for the milling of a wide range of materials and which does not generate wear byproducts which results in contamination of the product powder. A relatively inexpensive, dense and non-toxic, naturally occurring zirconium silicate sand grinding medium which has small particle size and a sufficiently high density is suitable for grinding a wide range of materials, while not contaminating the product powder with wear byproducts as well as a method for milling a powder using this grinding medium. The invention provides a method of de-agglomerating and reducing the particle size of agglomerated titanium dioxide pigment powder having a particle size in the range of from 0.05 to 100 microns comprising: (1) mixing the titanium dioxide pigment powder with a grinding medium and a liquid medium to form a milling slurry, the grinding medium comprising naturally occurring zirconium silicate sand having a density of from 4.6 to 4.9 g/cm 3 absolute and a particle size from 100 to 500 µm; (2) milling the milling slurry in a mill chosen from disk mills, cage mills and attrition mills for a time sufficient to produce a product slurry including a titanium dioxide product powder having a desired particle size, the powder being deagglomerated and reduced in particle size and having substantially the same composition as the starting powder; and (3) separating the product slurry including the titanium dioxide pigment powder from the milling slurry so that the grinding medium remains in the slurry. 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. 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 used in the method of the invention occurs in nature in the appropriate size and shape which can be sorted to obtain the appropriate fraction for use in 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 of the powder. The method of the invention employs a grinding medium including naturally occurring zirconium silicate sand characterized by a density in the range of from 4.6 g/cm 3 to 4.9 g/cm 3 and preferably in the range of from about 4.75 g/cm 3 to about 4.85 g/cm 3 and a particle size of from 100 to 250 µm. The naturally occurring zirconium silicate sand tends to be single phase, while synthetic zirconium silicate ceramic beads are typically multiphase materials. Surface contaminants such as aluminum, iron, uranium, thorium and other heavy metals as well as TiO 2 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 art, 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 3.8 g/cm 3 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 silicate 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 particle size is greater than 100µm and can be in the range of from about 100µm to about 1500µm, more preferably in the range of from about 100µm to about 500µm and most preferably in the range of from about 150µm to about 250µm. 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 is 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 being 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, naphtha 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, urethanes 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 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 are combined to form a grinding slurry which is further characterized by a grinding slurry viscosity which can be in the range of from about 1 mPas to 10 kPas (1.0cps to about 10,000cps), more preferably in the range of from about 1 to 500 mPas (1.0cps to about 500cps) and most preferably in the range of from about 1 to 100 mPas (1.0cps 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 viscosity 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 starting powder used in the method of the invention is an agglomerated titanium dioxide pigment powders, the agglomerated powder has a particle size of in the range of from 0.05 µm to 100 µm 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/cm 3 absolute to about 5.0 g/cm 3 absolute. The titanium dioxide starting powder can be an agglomerated titanium dioxide pigment which has a density in the range of from about 3.7 g/cm 3 to about 4.2g/cm 3 . The liquid medium used in the method of the invention can be oil or water selected according to the criteria already described. Milling is carried out in a disk mill, cage mill or attrition mill, designed to support a vertical flow or horizontal flow. The milling process can be a batch or continuous process. The step of 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 or ink or a ceramic or magnetic powder dispersion, the dispersing medium can 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. 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 performance of standard 10-40 mesh (U.S.) silica sand. Sand mills having nominal grinding chamber capacities of 1041 litres (275 gallons) and overall capacities of 1893 litres (500 gallons) were loaded separately with 1361 kg (3000 pounds) of synthetic zirconium silicate ceramic beads of nominal 300µm and 210µm size and with 544 kg (1200 pounds) of standard 10-40 mesh (U.S.) silica sand, the highest mill loading feasible with silica sand. The mills loaded with 1361 kg (3000 pounds) of synthetic zirconium silicate ceramic beads as well as the mill loaded with 544 kg (1200 pounds) of 10-40 mesh (U.S.) silica sand were operated at 61, 87 and 14 litres per minute (16, 23 and 30 gallon per minute) flow rates. The feed slurries fed through all mills had a density of 1.35g/cm 3 and contained titanium dioxide, approximately 40% of which was less than 0.5µm in size in water. The size of the titanium dioxide particles in the product slurry was measured using a Leeds and Northrupp 9200 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 percentage of product powder less than or equal to 0.5 µm in size compares favorably with the grinding efficiency of 10-40 mesh (U.S.) silica sand. Mill Flow Rate l/min (gal/min) Grinding Medium %Product≤0.5µm A 114 (30) 300micron synthetic zirconium silicate ceramic beads 66.57 B 114 (30) 300micron synthetic zirconium silicate ceramic beads 64.42 A 87 (23) 300micron synthetic zirconium silicate ceramic beads ― B 87 (23) 300micron synthetic zirconium silicate ceramic beads 70.41 A 68 (16) 300micron synthetic zirconium silicate ceramic beads 79.96 B 68 (16) 300micron synthetic zirconium silicate ceramic beads 71.26 A 114 (30) 210micron synthetic zirconium silicate ceramic beads 85.29 B 114 (30) 210micron synthetic zirconium silicate ceramic beads 74.72 A 87 (23) 210micron synthetic zirconium silicate ceramic beads 91.51 B 87 (23) 210micron synthetic zirconium silicate ceramic beads 83.11 A 68 (16) 210micron synthetic zirconium silicate ceramic beads 95.22 B 68 (16) 210micron synthetic zirconium silicate ceramic beads 95.22 A 114 (30) 10-40 mesh (U.S.) silica sand 65.17 B 114 (30) 10-40 mesh (U.S.) silica sand 54.28 A 87 (23) 10-40 mesh (U.S.) silica sand 61.96 B 87 (23) 10-40 mesh (U.S.) silica sand 57.76 A 68 (16) 10-40 mesh (U.S.) silica sand 67.09 B 68 (16) 10-40 mesh (U.S.) silica sand 59.48 Furthermore, when the properties of finished pigments processed with the 210µm synthetic zirconium silicate ceramic beads were compared with those of pigments processed with 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 42% 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 is defined as a 60 degree gloss measurement in a latex paint system, a lowering by approximately 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. Example 2 Example 2 is provided to compare the performance of synthetic zirconium silicate ceramic beads with the performance of the naturally occurring zirconium silicate sand grinding medium of the invention. It is noted that the naturally occurring zirconium silicate sand has a higher density than the 3.8g/cm 3 density of synthetic zirconium silicate products which allows use of smaller naturally occurring zirconium silicate sand particles by comparison with the 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 180-210µm in a cage mill showed that naturally occurring zirconium silicate sand can be used successfully at production flowrates to effect removal of coarse particles, having a particle size greater than 0.5µm 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, i.e., 544 kg (1200 pounds) of silica sand in mill B and 1361 kg (3000 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 either a low flowrate (approximately 49 litres/minute (13 gallons/minute)) or at a high flowrate (approximately 132 litres/minute (35 gallons/minute)) the naturally occurring zirconium silicate sand is much more 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. Pigment Particle Size Data Parameter Mill B Mill C Flowrate l/min (gal/min) 50 (13.2) 50 (13.2) Median Particle Diameter 0.37 0.24 Fraction of Particles <0.5 µm 86.94 99.55 Flowrate l/min (gal/min) 133 (35.2) 133 (35.2) Median Particle Diameter 0.38 0.37 Fraction of Particles <0.5 µm 75.64 87.55 Pigment Chemical Composition and Optical Properties Property Mill B Mill C %Al 2 O 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 which had been operated for only one week using a conventional silica sand grinding medium, the mill lining showed considerable wear, especially to the leading edges of the mill rotor bars where the weavelike pattern had been almost completely worn away. Example 3 The following example is provided to show the differences in particle size, impurity content and grinding performance among naturally occurring zirconium silicate sands obtained from 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 Rotap™. Based on the data presented in Table 4, Sample 2 and Sample 3 are similar with respect to particle size, while Sample 1 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 Sample Origin Sample 1 Sample 2 Sample 3 %180 µm 0.61 75.1 67.2 %150µm 5.73 16 32.1 % less than 150µm 93.66 8.9 0.7 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 5. Elemental Chemical Analysis of Zirconium Silicate Sands Sample Origin Sample 1 Sample 2 Sample 3 % Element %Na 0.38 0.41 0.2 %Al 0.16 0.16 0.73 %Si 15.15 15.43 14.5 %Cl 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 %Hf 0.92 0.99 0.93 %O 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) 729 714 711 Sr (ppm) 81 --- --- Pb (ppm) 50 --- --- 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 standard laboratory sand load of 1.8:1 zirconium sand to pigment load. Table 6 shows the percent of particles passing 0.5µm, i.e., particles having sizes smaller than 0.5µm, after 2, 4 and 8 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 determined using a Microtrac™ particle size analyzer as has been described before. Pigment Grinding Performance Sample Origin Sample 1 Sample 2 Sample 3 Particle Size Particle Size Particle Size Time Median Diameter %Passing 0.5µm Median Diameter %Passing 0.5µm Median Diameter %Passing 0.5µm Feed(0 minutes) 1 21.09 1 21.09 1 21.09 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 94.02 0.35 87.97 0.36 88.66

Claims: A method of de-agglomerating and reducing the particle size of agglomerated titanium dioxide pigment powder having a particle size in the range of from 0.05 to 100 microns comprising: (1) mixing the titanium dioxide pigment powder with a grinding medium and a liquid medium to form a milling slurry, the grinding medium comprising naturally occurring zirconium silicate sand having a density of from 4.6 to 4.9 g/cm 3 absolute and a particle size from 100 to 500 µm; (2) milling the milling slurry in a mill chosen from disk mills, cage mills and attrition mills for a time sufficient to produce a product slurry including a titanium dioxide product powder having a desired particle size, the powder being de-agglomerated and reduced in particle size and having substantially the same composition as the starting powder; and (3) separating the product slurry including the titanium dioxide pigment powder from the milling slurry so that the grinding medium remains in the slurry. A method according to claim 1, wherein the liquid medium is chosen from water, oil, organic compounds and mixtures thereof. A method according to claims 1 or 2, wherein the milling slurry has a viscosity of from 1 mPas to 10 kPas (1.0 to 10,000 cps). A method according to claim 3, wherein the milling slurry has a viscosity of from 1 to 500 mPas (1.0 to 500 cps). A method according to claim 4, wherein the milling slurry has a viscosity of from 1 to 100 mPas (1.0 to 100 cps). A method according to claim 1, wherein the naturally occurring zirconium silicate sand forming the grinding medium has a density of from 4.75 to 4.85 g/cm 3. A method according to claim 1, wherein the naturally occurring zirconium silicate sand forming the grinding medium has a particle size of from 150 to 250 µm.

Inventors: BROWNBRIDGE THOMAS IAN, STORY PHILLIP M, BROWNBRIDGE, THOMAS IAN, STORY, PHILLIP M.
Applicants: KERR MCGEE CHEMICAL LLC, KERR-MCGEE CHEMICAL LLC
Application year: 1995
Main classification: B02C23
IPC codes: B02C17, B02C23