Inorganic oxide particle process with improved energy and water efficiency

Abstract

The present invention relates to a treated inorganic oxide slurry and process of making including: a. treating an inorganic oxide particle with aqueous inorganic metal salt; b. washing the treated inorganic oxide particle with water and removing aqueous inorganic metal salt to form a washed slurry having 30-60% by weight treated inorganic oxide particles in water; c. milling the washed slurry to form a milled slurry; d. removing water from the milled slurry to form a dewatered filter cake having 60-85% by weight treated inorganic oxide particles in water; and e. contacting the dewatered filter cake with water, dispersant, or a mixture of water and dispersant to form a product slurry. The process uses less energy and water compared with prior art processes that include jet milling and drying steps, while maintaining performance. The slurries can be used in applications including coatings and paper.

Description

TITLE OF THE INVENTIONINORGANIC OXIDE PARTICLE PROCESS WITH IMPROVED ENERGY AND WATER EFFICIENCYCROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority of U.S. Provisional Application No. 63 / 652,770 filed May 29, 2024, the disclosure of which is incorporated herein by reference in its entirety.FIELD OF THE INVENTION

[0002] The present invention relates to a process for producing an inorganic oxide particle slurry including milling the slurry and then removing water to form a final slurry. The process uses less energy and water compared with prior art processes that include jet milling and drying steps, while maintaining performance.BACKGROUND OF THE INVENTION

[0003] Current processes for producing inorganic oxide particles employ jet milling and drying techniques between washing the particles and milling the particles, such as in US 9,505,022, US 6,395,081 , or US 5,730,795. However, such processes use a large amount of energy in heating the particles to dryness. Jet milling requires a large amount of energy due to heating and use of high-pressure gases. Additionally, slurries produced by this process would require additional water to combine with the dry particles.

[0004] Previous methods to avoid drying and jet milling the particles have been demonstrated; however, the processes did not allow for an aqueous treatment step, which presents unique issues. Salts generated in an aqueous treatment step harm particle properties and must be removed from the finished product.SUMMARY OF THE INVENTION

[0005] The process described herein avoids the drying and jet milling steps while maintaining the performance integrity of the particles. The present invention relates to a process of producing a treated inorganic oxide slurry comprising:a. treating an inorganic oxide particle with at least one aqueous inorganic metal salt to form a treated inorganic oxide particle; b. washing the treated inorganic oxide particle with water and removing aqueous inorganic metal salt to form a washed slurry having 30-60% by weight treated inorganic oxide particles in water; c. milling the washed slurry to reduce the particle size of the treated inorganic oxide particles and form a milled slurry; d. removing water from the milled slurry to form a dewatered filter cake having 60-85% by weight treated inorganic oxide particles in water; and e. contacting the dewatered filter cake with water, dispersant, or a mixture of water and dispersant to form a product slurry.The present invention further relates to a treated aqueous inorganic oxide particle slurry made by the process. The invention may include the addition of dry inorganic particles such as discharge from a dryer to raise solids for the product slurry.BRIEF DESCRIPTION OF THE DRAWINGS

[0006] FIG. 1 shows a flow diagram of the inventive process, including optional steps.

[0007] FIG. 2 shows comparative process flow diagram, including drying and jet milling that are not part of the present invention.DETAILED DESCRIPTION OF THE INVENTION

[0008] The present invention relates to a process of producing a treated inorganic oxide slurry comprising: a. treating an inorganic oxide particle with at least one aqueous inorganic metal salt to form a treated inorganic oxide particle; b. washing the treated inorganic oxide particle with water and removing aqueous inorganic metal salt to form a washed slurry having 30-60% by weight treated inorganic oxide particles in water; c. milling the washed slurry to reduce the particle size of the treated inorganic oxide particles and form a milled slurry; d. removing water from the milled slurry to form a dewatered filter cake having 60-85% by weight treated inorganic oxide particles in water; ande. contacting the dewatered filter cake with water, dispersant, or a mixture of water and dispersant to form a product slurry.

[0009] The inorganic oxide particle of step a may be any suitable particle, including but not limited to oxides of Ti, Al, Zn, Cu, Fe, Mn, or Si, such as TiCh, AI2O3, ZnO, CuO, Fe2C>3, MnO, MnsC , Mn2O3, MnC>2, MnOs, Mn2O?, or SiCh. Blends or mixtures of inorganic oxides may also be used. Such compounds may be in any particle form. For example, TiO2 particles may be in rutile or anatase form, and they may be made by the chloride process or sulfate process. Particles may be a pigment or nanoparticle. By “pigment” it is meant that the primary particles have an average particle size of less than 1 pm; in another aspect, the particles have an average particle size of about 0.020 to about 0.95 pm; in another aspect, about 0.050 to about 0.75 pm; and in another aspect, about 0.075 to about 0.50 pm. By “nanoparticle” it is meant that the primary particles typically have an average particle size diameter of less than about 100 nm as determined by dynamic light scattering that measures the particle size distribution of particles in liquid suspension. The nanoparticles are typically agglomerates that may range from about 3 nm to about 6000 nm. The inorganic oxide particle serves as a “core particle” for the further treatments described herein.

[0010] The inorganic oxide particle of step a may have inorganic oxide treatments on the surface of the core inorganic oxide particle prior to being introduced in step a. Typical oxide include oxides of Ti, Al, Zn, Cu, Fe, Mn, or Si, such as TiC , AI2O3, ZnO, CuO, Fe2Os, MnO, MnsO4, Mn2Os, MnO2, MnOs, Mn2O?, or SiO2. Mixtures of inorganic oxides may also be applied, and the treatment oxide may be the same or different than the oxide of the core inorganic oxide particle. Such oxides may be formed on the core inorganic oxide particle by any suitable method, including aqueous treatment, by oxidation of metal chloride, or by condensed phase aqueous oxide. Alumina may be present in the inorganic oxide particles and may be amorphous, boehmite, or a mixture thereof. In one aspect, the inorganic oxide particle comprises TiO2. In one aspect, the inorganic oxide particle is TiO2 having a surface treatment of SiO2 and / or AI2O3.

[0011] The inorganic oxide particles are treated in step a with at least one aqueous inorganic metal salt to form a treated inorganic oxide particle, such that theaqueous inorganic metal salt yields an inorganic oxide layer on the core inorganic oxide particle. Such a process may be performed by any suitable aqueous treatment process. For example, an inorganic metal salt may be combined with water and the inorganic oxide particle, in any order, to form a slurry. The slurry may comprise the inorganic oxide particle in any suitable amount, such as about 1-35% by weight, or about 25-35% by weight, or about 30% by weight, all based on the total slurry weight (the weight of the treated inorganic oxide particles in water). The mixture may be heated, such as to a temperature of 30-95 °C, 30-90 °C, 35-90 °C, 40-90 °C, or 45- 90 °C, and the pH may be adjusted to the appropriate conditions to precipitate the inorganic metal salt onto the inorganic oxide particle. For basic inorganic metal salts, the pH range may be about 3.5-7.5; or in another aspect, about 5.0-6.5. For acidic inorganic metal salts, the pH range may be about 7.0-10.0; or in another aspect, about 8.5-9.5.

[0012] The inorganic metal salt may be a compound containing Al, Si, Zr, Ba, or Zn. Such compounds can be used under certain conditions to precipitate inorganic oxides onto a particle. These inorganic metal salt compounds include, but are not limited to, alkali metal salts, such as sodium or potassium silicate, sodium or potassium aluminate; metal sulfates, such as aluminum sulfate; or metal halides, such as aluminum chloride. In one aspect, the treated inorganic oxide particle in step a comprises about 1 .0-20.0% by weight inorganic oxide treatment layer resulting from the treatment of step a, based on the total weight of the inorganic oxide particle; in another aspect, comprises about 4.0-8.0% inorganic oxide treatment layer resulting from the treatment of step a, based on the total weight of the inorganic oxide particle; and in a further aspect, comprises about 5.0-7.5% inorganic oxide treatment layer resulting from the treatment of step a, based on the total weight of the inorganic oxide particle.

[0013] The treated inorganic oxide particles resulting from step a are then washed with water in step b, and the water is at least partially removed with aqueous inorganic metal salts dissolved therein, thus removing the residual inorganic metal salt. Aqueous inorganic metal salts that are removed in step b include those added in step a but may also include aqueous inorganic metal salts that are present from inorganic oxide particle production. Aqueous inorganic metal salts include, for example, alkali metal salts, metal halides or chlorides, metal sulfates, metal nitrates,or metal fluorides. In one aspect, the washed slurry resulting from step b has a resistance of at least about 2 kOhms-s; in another aspect, the washed slurry has a resistance of at least about 4 kOhms-s; and in another aspect, the washed slurry has a resistance of at least 6 kOhms-s.

[0014] Any suitable mechanical means of washing the treated inorganic oxide particles may be used, as long as the salts are removed from the mixture. Removal of water with aqueous inorganic metal salts can be performed by filtration, for example, by rotary vacuum filter, belt filter, or filter press. Although some temperature control may be used, it is less preferred to heat the slurry since energy efficiency is desired. After removal of aqueous inorganic metal salts, the treated inorganic oxide particles form a washed slurry having a solids content of 30-60% by weight treated inorganic oxide particles in water; in another aspect, the washed slurry has 30-50% by weight treated inorganic oxide particles in water; and in another aspect, the washed slurry has 35-45% by weight treated inorganic oxide particles in water, all based on the total washed slurry weight.

[0015] The washed slurry, having the solids content specified above, is then milled to reduce the particle size of the treated inorganic oxide particles and to form a milled slurry. The solids content and order of operations are important in this process. If milling is performed before or during the washing step, for example, the washing step is less efficient due to difficulties with removing water and dissolved solids from small particles. The removal process is slower, and the dissolved solids may remain on the particle surfaces. Any suitable wet milling method may be used, including but not limited to milling by media mill, sand mill, immersion mill, or basket mill. The desired milled particle size may vary based on particle composition and end use application.

[0016] In one aspect, the washed slurry contains a processing aid that may be added during the washing step or after washing but before the milling step. Such a processing aid may help with the grinding step, such as to lower viscosity or increase solids content, or it may be added at this stage to aid processing after the milling step. Processing aids often are used to impart stabilization of the small-sized particles making up the slurry. Dispersants, for example, may be added prior to milling to achieve milled slurries having higher solids contents. Examples ofprocessing aids include but are not limited to dispersants, surfactants, or pigment stabilizers; more specific examples include inorganic processing aids, including but not limited to silicates or alkali metal phosphates; small organic molecule processing aids; or polymeric processing aids, including but not limited to those having carboxylic acid groups for anchoring or those having molecular weights between 1 ,000 and 10,000. Specific examples of inorganic processing aids include but are not limited to sodium pyrophosphate or sodium hexametaphosphate; and specific examples of small organic processing aids include but are not limited to citric acid. Specific examples of polymeric processing aids include but are not limited to dispersants sold by Dow Chemical under the Tamol™ brand, such as Tamol-851 ™, Tamol-1124™, Tamol-731 A™, or Tamol-165A™; dispersants sold by Arkema under the Coadis™ brand, such as Coadis 144A™; and dispersants sold by Byk under the Disperbyk™ brand, such as Disperbyk 191 ™.

[0017] The milled slurry is subjected to a water removal step to form a dewatered filter cake having 60-85% by weight treated inorganic oxide particles in water. Water removal is performed by any suitable mechanical means, as opposed to drying by heat treatment, to optimize energy efficiency. For example, water may be removed by filtration or centrifuge methods, where filtration includes but is not limited to filtration by filter press, cross-flow filter, or belt filter. Water is partially but not completely removed in this step. In one aspect, the dewatered filter cake contains 65-85% by weight treated inorganic oxide particles in water; in another aspect, the dewatered filter cake has 70-85% by weight treated inorganic oxide particles in water; in another aspect, dewatered filter cake has 73-85% by weight treated inorganic oxide particles in water; in another aspect, dewatered filter cake has 75- 85% by weight treated inorganic oxide particles in water; in another aspect, dewatered filter cake has 70-81% by weight treated inorganic oxide particles in water; in another aspect, dewatered filter cake has 73-81 % by weight treated inorganic oxide particles in water; and in another aspect, the dewatered filter cake has 75-81% by weight treated inorganic oxide particles in water, all based on the total dewatered filter cake weight.

[0018] Because mechanical means are used to remove water, treated inorganic oxide particles may be present in the water that is removed from the milled slurry in step d. In one aspect, the water removed from the milled slurry in step d may contain5-100 g / L of treated inorganic oxide particles in water; in another aspect, the water removed from the milled slurry has 5-90 g / L treated inorganic oxide particles in water; and in another aspect, the water removed from the milled slurry has 10-75 g / L treated inorganic oxide particles in water, all based on the total removed water weight. This water removed from the milled slurry may be reintroduced into the process at any step, including to the initial slurry make-up of inorganic oxide particles, the aqueous treatment step a, the washing step b, the milling step c, or the dewatering step d. This reintroduction step would serve to maximize yield of the treated inorganic oxide particles, reduce waste, and / or reduce total water usage. In one aspect, the water removed from the milled slurry in step d is recycled and combined with the treated inorganic oxide particles during the washing step b. In one aspect of this process, the water removed from the milled slurry may be combined with a separate water source to form the water for washing the treated inorganic oxide particles in washing step b.

[0019] In another aspect, treated inorganic oxide particles from other sources may be introduced into the process at any step, including to the aqueous treatment step a, the washing step b, the milling step c, or the dewatering step d. This reintroduction step would serve to maximize yield of treated inorganic oxide particles from separate sources, reduce waste, and / or serve to increase solids when needed. In one aspect, the treated inorganic particles from other sources may be in the form of dry particles; in another aspect the treated inorganic particles from other sources may be in the form of a particle slurry in water. By “other sources” is meant that the treated inorganic particles come from a separate process or production line. Dry particles may be sourced, for example, from a finished dry particle product or from an intermediate step of another process, such as the output of a dryer discharge. Dry inorganic particles may be added at up to about 50% of the weight of the milled slurry of step c, and more preferably, less than about 25% of the weight of the milled slurry.

[0020] Once the inorganic oxide particle dewatered filter cake is formed, the mixture is contacted with water, at least one dispersant, or a mixture of water and dispersant to form a product slurry. This can be done to reduce the viscosity of the mixture or otherwise allow for easier processing of the inorganic oxide particles in industrial applications. In one aspect, the product slurry contains 60-85% by weighttreated inorganic oxide particles; in another aspect, the product slurry has 65-85% by weight treated inorganic oxide particles; and in another aspect, the product slurry has 70-85% by weight treated inorganic oxide particles, all based on the total product slurry weight. In some cases, only dispersant is added to form the product slurry having a lower viscosity.

[0021] In one aspect, a processing aid is added to the dewatered filter cake or product slurry. Such a processing aid may be added during the dewatering or contacting step or after the dewatering or contacting step but before the milling step. In some cases, the processing aid may be the dispersant of step e. Such a processing aid may help with further milling or may help with general processing and handling of the slurry, such as to lower viscosity or increase solids content.Examples of processing aids include but are not limited to dispersants, surfactants, or pigment stabilizers; more specific examples include inorganic processing aids, including but not limited to silicates or alkali metal phosphates; small organic molecule processing aids; or polymeric processing aids, including but not limited to those having carboxylic acid groups for anchoring or those having molecular weights between 1 ,000 and 10,000. Specific examples of inorganic processing aids include but are not limited to sodium pyrophosphate or sodium hexametaphosphate; and specific examples of small organic processing aids include but are not limited to citric acid. Specific examples of polymeric processing aids include but are not limited to dispersants sold by Dow Chemical under the Tamol™ brand, such as Tamol-851 ™, Tamol-1124™, Tamol-731 A™, or Tamol-165A™; dispersants sold by Arkema under the Coadis™ brand, such as Coadis 144A™; and dispersants sold by Byk under the Disperbyk™ brand, such as Disperbyk 191 ™.

[0022] For some applications, it may be desired to have even smaller particle sizes or a specific particle size distribution. In this case, it may be useful to further mill the product slurry. In one aspect, the process comprises step f: milling the product slurry to form a milled product slurry. The milling step f may be performed by the same processes used in step c. In one aspect, step f is performed by media mill, sand mill, immersion mill, or basket mill.

[0023] The process of the current invention results in a treated inorganic oxide particle slurry, where water is the carrier medium for the slurry. In one aspect, theprocess of the current invention excludes steps of jet milling and / or drying the inorganic particles. In one aspect, the process steps a-e, or steps a-f, are performed at a temperature below 100 °C; in another aspect, the process steps a-e, or steps a- f, are performed at a temperature below 95 °C; and in another aspect, the process steps a-e, or steps a-f, are performed at a temperature below 90 °C. Because of the lack of jet milling and drying steps used in this process, the process and resulting treated inorganic oxide particle slurry have a low carbon dioxide emission (CO2e) value. In one aspect, the process steps a-e have a CC e value of less than 0.2 tons CC e per ton inorganic oxide according to Equation A-1 in 40 CFR Part 98 of the US Environmental Protection Agency. In another aspect, the process steps a-e have a CC>2e value of less than 0.1 tons CC e per ton inorganic oxide according to Equation A-1 in 40 CFR Part 98 of the US Environmental Protection Agency.

[0024] The final treated inorganic oxide particle slurry may be used in several applications, including in coatings such as architectural coatings, where it is combined with a coating base to form a pigmented coating; or it may be used in paper or paper slurry products, where it is combined with a paper slurry to form a paper or pigmented paper slurry. Preferred architectural coatings include emulsion paints where water is the liquid carrier, including but not limited to exterior paint, interior paint, or specialty paints. Drying the final slurry may result in dry pigment that can be used in still further applications.EXAMPLES

[0025] Tamol™ 851 is a dispersant product and monoisopropanolamine (MIPA) is a pH modifier, both available from Dow, Midland, Ml.

[0026] ZirPro ER-120S are ceramic beads having a size of 0.8-1.0 mm, available from Saint-Gobain, Malvern, PA.Viscosity

[0027] Viscosity was measured using a Brookfield viscometer using spindle 3 at a shear rate of 100 rpm.Optical Density

[0028] The Optical Density test provides measurement of the scattering power of pigment in dilute aqueous slurries with high precision. The Optical Density test is an absolute test that does not need to be compared to any standard. Therefore, test results on a given sample on a given day can be compared to all samples in a database. This attribute makes it a useful screening tool for the development of new pigments.

[0029] The % solids of an inorganic oxide particle slurry were measured, and the mixture was diluted with distilled water to a concentration of 20 ppm inorganic oxide. A cuvette with cell pathlength of 1 cm was filled with the diluted inorganic oxide mixture and placed in a spectrophotometer, for example, an UltraScan PRO. The % transmission at 850 nm was recorded, and Optical Density was calculated as:Optical Density = -log(% Transmittance 1 100)Higher Optical Density values indicate more light scattering by the inorganic oxide particles.Relative Tint Strength (RTS)

[0030] Tint Strength is a function of the light scattering of a pigment. A paint base was mixed with 2.2 lbs inorganic oxide particles per gallon of paint base, and a colorant was added to give approximately 55% ± 5 reflectance. The samples were mixed using a Speedmixer® DAC 1100.1 FVZ-HV (Hauschild, Farmington Hills, Ml) and drawn down onto a 5.5-in x 11.25-in Leneta Form WM card using a drawdown blade with 0.006 in clearance. The paint films were allowed to dry for at least 2 hours, and the reflectance of the samples were recorded as R°° using a Labscan® XE optical instrument (HunterLab, Reston, VA). The reported R°° was an average of 3 readings from the top, middle, and bottom of each sample. The R°° readings were used to calculate K / S values according to the equation: (100 - 7?oo)2200 X Rco

[0031] A control sample is measured to use in the calculation. RTS is then calculated as:K / S of controlRTS x 100 K / S of sampleGloss

[0032] Gloss at 20° and 60° was measured according to ASTM D3928-00a (2018) “Standard Test Method for Evaluation of Gloss or Sheen Uniformity”.Resistance

[0033] Resistance gives an indication of the amount of inorganic metal salts present in the mixture. A low resistance indicates higher concentration of inorganic metal salts, while a higher resistance indicates a lower concentration. Inorganic oxide particles (50 g) were mixed at room temperature with 250 ml_ of distilled water. A conductivity probe (Horiba DS-71 , Kyoto, Japan) was immersed in the mixture, and the measured resistance was recorded. The calculated resistance was calculated and reported according to the equation below.Measured ResistanceCalculated Resistance — > _ — - — - —1 + [0.02 X (30 — Temperature)]Metal or Metal Oxide Content

[0034] Amounts of metals and metal oxides were analyzed by X-ray fluorescence (XRF). In the case of metal salt treatments, an amount of metal salt was calculated and added to yield a certain metal oxide treatment content, based on the total weight of the particle. The amount, in a percentage of total particle weight, was then confirmed by XRF.Comparative Example A

[0035] TiO2 particles containing about 1 % by weight AI2O3 and 4% by weight NaCI were slurried with water to produce a mixture having about 25% solids. The slurry was treated with aqueous sodium silicate, then sodium aluminate, with HCI, to produce a TO2 core particle with about 3% by weight SiO2 and 2.7% by weight AI2O3 on the surface. The treated slurry was filtered on a rotary vacuum filter yielding a solids content of 40% by weight. It was centrifuged at 9000 rpm for 20 minutes to yield an inorganic oxide particle mixture having 72% by weight solids. The mixture(713.7 g) was charged into a vessel with Tamol 851 (2.06 g) and Ml PA (0.25 g) and placed on a Dispermat™ fitted with a 60-mm diameter cowles blade. The mixture was ground at 2000 rpm for 15 minutes and screened (No. 325, 45 pm) before an antimicrobial (0.5 g) was added, resulting in a final slurry having 72.35% solids by weight. The final slurry was tested according to the Test Methods above. This example represents a process, where no milling step was performed before water removal step d.Example 1

[0036] TiC particles containing about 1 % by weight AI2O3 and 4% by weight NaCI were slurried with water to produce a mixture having about 25% solids. The slurry was treated with aqueous sodium silicate, then sodium aluminate, with HCI, to produce a TiC>2 core particle with about 3% by weight SiC and 2.7% by weight AI2O3 on the surface. The treated slurry was filtered on a rotary vacuum filter yielding a solids content of 40% by weight. It was placed in a 500-mL plastic bottle with ZirPro ER-120S (1.15 x mass of filter cake) and shaken on a Red Devil paint shaker for 2.25 hours. The resulting milled slurry was double screened (No. 35, 500 pm, then No. 325, 45 pm) and centrifuged at 9000 rpm for 20 minutes to yield an inorganic oxide particle dewatered filter cake having 76.5% by weight solids. The mixture (629.5 g) was charged into a vessel with Tamol 851 (1.93 g), MIPA (0.25 g), and water (34.3 g) to form a product slurry and placed on a Dispermat™ fitted with a 60- mm diameter cowles blade. The mixture was ground at 2000 rpm for 15 minutes and screened (No. 325, 45 pm) before an antimicrobial (0.5 g) was added, resulting in a milled product slurry having 72.53% solids by weight. The milled product slurry was tested according to the Test Methods above.Example 2

[0037] Example 1 was repeated. The centrifuged dewatered filter cake (639.6 g, 76.5% solids) was charged into a vessel with Tamol 851 (1 .96 g), MIPA (0.25 g), and water to form a product slurry having approximately 72% solids. The mixture was placed on a Dispermat™ fitted with a 60-mm diameter cowles blade and ground at 2000 rpm for 15 minutes and screened (No. 325, 45 pm) before an antimicrobial (0.5g) was added, resulting in a milled product slurry having 72.83% solids by weight.The milled product slurry was tested according to the Test Methods above.Comparative Example B

[0038] Comparative Example A was repeated. The centrifuged mixture (713.7 g, 72.5% solids) was charged into a vessel with 50% aqueous citric acid (1.12 g) and MIPA (1 .05 g), resulting in a mixture at 72.5% solids. The mixture was placed on a Dispermat™ fitted with a 60-mm diameter cowles blade and ground at 2000 rpm for 15 minutes and screened (No. 325, 45 pm) before an antimicrobial (0.5 g) was added, resulting in a final slurry having 72.94% solids by weight. The final slurry was tested according to the Test Methods above.Example 3

[0039] Example 1 was repeated. The centrifuged dewatered filter cake (574.1 g, 77.37% solids) was charged into a vessel with 50% aqueous citric acid (0.89 g), MIPA (0.86 g), and water (38.7 g) to form a product slurry having approximately 72.5% solids. The mixture was placed on a Dispermat™ fitted with a 60-mm diameter cowles blade and ground at 2000 rpm for 15 minutes and screened (No. 325, 45 pm) before an antimicrobial (0.5 g) was added, resulting in a milled product slurry having 73.07% solids by weight. The milled product slurry was tested according to the Test Methods above.Example 4

[0040] Example 1 was repeated. The centrifuged dewatered filter cake (577.3 g, 77.4% solids) was charged into a vessel with 50% aqueous citric acid (0.89 g), MIPA (0.86 g), and water (38.7 g) to form a product slurry having approximately 72% solids. The mixture was placed on a Dispermat™ fitted with a 60-mm diameter cowles blade and ground at 2000 rpm for 15 minutes and screened (No. 325, 45 pm) before an antimicrobial (0.5 g) was added, resulting in a milled product slurry having 72.95% solids by weight. The milled product slurry was tested according to the Test Methods above.Table 1. Performance Data for Examples 1-4 and Comparative Examples A-B

[0041] The performance data in Table 1 demonstrates that the milling step postwash is essential to reducing viscosity of the final slurry while also increasing optical density. Additionally, tint strength and gloss of a resulting paint are also increased.Preparation Example 1

[0042] Oxidation base TiO2 slurry containing 1 % alumina and 4% sodium chloride was treated with aqueous sodium silicate and HCI in an amount to add 3% by weight silica. It was subsequently treated with aqueous sodium aluminate and HCI to add an additional 1 .75% by weight alumina, resulting in a treated inorganic oxide particle mixture. This same treated inorganic oxide particle mixture was used for Preparation Examples 1-2 and Comparative Examples C-F. The sample, starting at around 25% by weight solids, was filtered through a leaf filter for 60 s to mimic a rotary vacuum filter, and the resulting filter cake was measured for weight, depth, and solids content.Comparative Example C

[0043] 50% by weight of the treated inorganic oxide particle mixture fromPreparation 1 was milled via media mill to reduce the particle size and recombined with 50% by weight unmilled treated inorganic oxide particle mixture. The blended sample was washed with water and filtered through a rotary vacuum filter having a leaf filter for 60 s, and the resulting filter cake was measured for weight, depth, and solids content.Comparative Example D

[0044] The treated inorganic oxide particle mixture from Preparation 1 was milled via media mill to reduce the particle size. The sample was washed with water andfiltered through a rotary vacuum filter having a leaf filter for 60 s, and the resulting filter cake was measured for weight, depth, and solids content.Table 2. Process Measurements for Preparation Example 1 and Comparative Examples C-D

[0045] As shown in Table 2, the amount of filter cake recovered from the milled slurry was significantly lower than for unmilled slurry during the same filtration time. This shows the washing rate of the milled slurry would be significantly lower in a rotary vacuum filter than unmilled slurry, and additional filters would need to be installed to recover product in a reasonable time. This supports the importance of milling after the washing step, rather than before the washing step.Preparation Example 2

[0046] The treated inorganic oxide particle mixture was washed with water and filtered through a horizontal filter press, and the resulting filter cake was measured for weight, depth, and solids content.Comparative Examples E-F

[0047] The treated inorganic oxide particle mixture from Preparation 1 was milled via media mill to reduce the particle size. Comparative Example E was milled once via media mill, and Comparative Example F was milled twice sequentially via media mill. The milled sample was washed with water and filtered through a horizontal filter press, and the resulting filter cake was measured for weight, solids content, wash time and rate, and resistance.Table 3. Process Measurements for Preparation Example 2 and Comparative Examples E-F

[0048] According to Table 3, the pre-milled particle slurries show a much lower wash rate compared to the unmilled particles. This would require significantly more filter presses to be installed to wash the same amount of milled slurry. The resistance indicates a similar or increased reduction of inorganic metal salts when using the inventive process, compared to washing pre-milled particle slurries. This again supports the importance of milling after the washing step, rather than before the washing step.

Claims

CLAIMSWhat is claimed is:

1. A process of producing a treated inorganic oxide slurry comprising: a. treating an inorganic oxide particle with at least one aqueous inorganic metal salt to form a treated inorganic oxide particle; b. washing the treated inorganic oxide particle with water and removing aqueous inorganic metal salt to form a washed slurry having 30-60% by weight treated inorganic oxide particles in water; c. milling the washed slurry to reduce the particle size of the treated inorganic oxide particles and form a milled slurry; d. removing water from the milled slurry to form a dewatered filter cake having 60-85% by weight treated inorganic oxide particles in water; and e. contacting the dewatered filter cake with water, dispersant, or a mixture of water and dispersant to form a product slurry.

2. The process of claim 1 , where the inorganic oxide particle is selected from oxides of Ti, Al, Zn, Cu, Fe, Mn, or Si.

3. The process of claim 1 , where the inorganic oxide particle is TiC>2.

4. The process of claims 1-3, where the aqueous inorganic metal salt is a compound containing Al, Si, Zr, Ba, orZn.

5. The process of claims 1-4, where the removing aqueous inorganic metal salt step in washing step b includes using rotary vacuum filter, belt filter, or filter press.

6. The process of claims 1-5, where the milling step c is performed by media mill, sand mill, immersion mill, or basket mill.

7. The process of claims 1-6, where the washed slurry, dewatered filter cake, or product slurry further comprises a processing aid.

8. The process of claim 7, where the processing aid is selected from a dispersant, surfactant, or pigment stabilizer.

9. The process of claims 1-8, where the water removal step d is performed by filtration or centrifuge.

10. The process of claim 9, where the water removal step is performed by filter press, cross-flow filter, or belt filter.11 . The process of claims 1-10, where the water removed from the milled slurry in step d is recycled and combined with the treated inorganic oxide particles during the washing step b, and wherein the water removed from the milled slurry in step d contains treated inorganic oxide particles.

12. The process of claim 11 , where the water removed from the milled slurry is combined with a separate water source to form the water for washing the treated inorganic oxide particles in washing step b.

13. The process of claims 1-12, where treated inorganic oxide particles from a separate source are added prior to or during the water removal step d or contacting step e.

14. The process of claim 13, where the treated inorganic oxide particles from a separate source are dry treated inorganic oxide particles.

15. The process of claims 1-14, further comprising the step: f. milling the product slurry of step e to form a milled product slurry.

16. The process of claim 15, where the milling step f is performed by media mill, sand mill, immersion mill, or basket mill.

17. The process of claims 1-16, where the treated inorganic oxide particle of treatment step a is in an aqueous mixture having 1-35% by weight treated inorganic oxide particles in water.

18. The process of claims 1-17, where the process excludes drying inorganic oxide particles and excludes jet milling.

19. The process of claims 1-18, where the process steps a-e have a carbon dioxide equivalent (CChe) of less than 0.2 tons CC e per ton inorganic oxide according to Equation A-1 in 40 CFR Part 98 of the US Environmental Protection Agency.

20. The process of claims 1-19, where the process step d forms a dewatered filter cake having 70-85% by weight treated inorganic oxide particles in water.21 . A treated aqueous inorganic oxide particle slurry produced by the process of claims 1-20.

22. The process of claims 1-20, further comprising combining the product slurry or milled product slurry with a coating base to form a pigmented coating.

23. The process of claims 1-20, further comprising combining the product slurry or milled product slurry with a paper slurry to form a pigmented paper slurry.

24. A pigmented coating formed by the process of claim 22.

25. A paper slurry or paper formed by the process of claim 23.

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