Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B02—CRUSHING, PULVERISING, OR DISINTEGRATING; PREPARATORY TREATMENT OF GRAIN FOR MILLING
  • B02C—CRUSHING, PULVERISING, OR DISINTEGRATING IN GENERAL; MILLING GRAIN
  • B02C17/00—Disintegrating by tumbling mills, i.e. mills having a container charged with the material to be disintegrated with or without special disintegrating members such as pebbles or balls
  • B02C17/18—Details
  • B02C17/20—Disintegrating members
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B02—CRUSHING, PULVERISING, OR DISINTEGRATING; PREPARATORY TREATMENT OF GRAIN FOR MILLING
  • B02C—CRUSHING, PULVERISING, OR DISINTEGRATING IN GENERAL; MILLING GRAIN
  • B02C23/00—Auxiliary methods or auxiliary devices or accessories specially adapted for crushing or disintegrating not provided for in preceding groups or not specially adapted to apparatus covered by a single preceding group
  • B02C23/18—Adding fluid, other than for crushing or disintegrating by fluid energy

Definitions

• the invention relates to a method for milling a powder, using a zirconium silicate grinding medium.
• 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.
• the grinding medium can be separated from the product particles using density separation techniques.
• 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.
• 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.
• the invention provides a method for milling a powder comprising steps of providing a starting powder and a grinding medium comprising naturally occurring zirconium silicate sand characterized by a grinding medium density in the range of from 4.75 to 4.85 g/cm 3 absolute and a particle size of from 150 to 250 ⁇ m and mixing the starting powder and the grinding medium with a liquid medium to form a milling slurry; milling the milling slurry in a high energy mill selected from disk mills, cage mills and attrition mills 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 the starting powder and separating the product slurry from the milling slurry so that the grinding medium remains in the milling slurry.
• 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 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.
• grinding medium 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 4.75 g/cm 3 to 4.85 g/cm 3 , and a particle size of from 150 to 250 ⁇ m.
• 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.
• 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.
• the naturally occurring zirconium silicate sand particle size is in the range of from 150 ⁇ m to 250 ⁇ m.
• 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.
• 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.
• the liquid medium can be water, optionally including antifoaming agents and/or dispersants.
• 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 1mPas (1.0cps) to about 10kPas (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).
• a grinding slurry viscosity which can be in the range of from about 1mPas (1.0cps) to about 10kPas (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).
• 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.
• concentration of solids in the grinding slurry 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 can be an agglomerated and/or aggregated powder.
• the agglomerated powder can be characterized by an agglomerated powder particle size less tnan about 500 ⁇ m and more preferably can be in the range of from about 0.01 ⁇ m to about 200 ⁇ m.
• the agglomerated powder has a particle size of in the range of from about 0.05 ⁇ m to about 100 ⁇ m 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 5.0g/cm 3 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/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.
• Step (5) of milling can be carried out in any suitable milling device which employs a grinding medium, such as, but not limited to, a bead mill, cage mill, disc mill or pin mill designed to support a vertical flow or horizontal flow.
• a grinding medium such as, but 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.
• Step (6) 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.
• 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.
• 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.
• 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 300 ⁇ m and 210 ⁇ m 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 0.5 ⁇ m in size in water.
• the size of the titanium dioxide particles in the product slurry was measured using a Leeds and Northrupp 9200 series MicrotracTM 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 ⁇ m in size compares favorably with the grinding efficiency of 10-40 mesh (U.S.) silica sand.
• 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 is provided to compare the performance of conventional silica sand 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.
• 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 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.
• 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 ⁇ m 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 ⁇ m 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
• 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.
• 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.
• 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.
• Sample 1 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 RotapTM. 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 microns 0.61 75.1 67.2 %150microns 5.73 16 32.1 % less than 150microns 93.66 8.9 0.7
• 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 0.5micron, i.e., particles having sizes smaller than 0.5micron, 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 MicrotracTM particle size analyzer as has been described before.

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• Engineering & Computer Science (AREA)
• Food Science & Technology (AREA)
• Silicates, Zeolites, And Molecular Sieves (AREA)
• Crushing And Grinding (AREA)
• Pigments, Carbon Blacks, Or Wood Stains (AREA)
• Disintegrating Or Milling (AREA)
• Compositions Of Oxide Ceramics (AREA)
• Finish Polishing, Edge Sharpening, And Grinding By Specific Grinding Devices (AREA)
• Silicon Compounds (AREA)
• Adhesives Or Adhesive Processes (AREA)
• Polishing Bodies And Polishing Tools (AREA)

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