WET MILLING OF Mg(OH)2 SLURRY

Abstract: A method for producing a stabilized magnesium hydroxide slurry involving wet milling a starting magnesium hydroxide slurry. Also disclosed is a stabilized magnesium hydroxide slurry produced by the method and magnesia-based products produced from the wet milled magnesium hydroxide slurry. Washed magnesium hydroxide slurry is either (a) fed directly to a wet mill (1) at a controlled rate for particle size reduction, or (b) introduced to a disk filter (2) to increase percent solids to greater than about 60 %. If the disk filter (2) is used, the discharge is directed to a pug mill (3). Flow is recycled within the pug mill (3) through high-shear mixer (4). The pug mill discharge is collected in purge tank (5). A pump (6) is used to withdraw slurry from surge tank (5) and feed wet mill (1) at a controlled rate which rate dictates residence time in wet mill (1). This rate can be altered to control particles size reduction. The milled slurry is collected in storage tank (7).

Description: WET MILLING OF Mg(OH) 2 SLURRY This application claims priority to U.S. Provisional Patent Applications Nos. 60/056,094 and 60/071,748, the entire contents of which are incorporated herein by reference. Field of the Invention The present invention relates to a method for producing a stabilized magnesium hydroxide slurry that involves wet milling a starting magnesium hydroxide slurry. The present invention also relates to a stabilized magnesium hydroxide slurry produced by the wet milling method, dry magnesia-hydroxide and magnesia-based products produced using the wet milled magnesium hydroxide slurry of the invention. The invention is particularly useful for the production of particulate magnesium hydroxide suitable for use as a flame retardant additive. Background of the Invention Magnesium hydroxide, Mg(OH) 2 , is useful in various chemical processes, including, but not limited to, the following: pH adjustment; precipitation of heavy metal contaminants; scrubbing and neutralization of acidic vapors such as those associated with flue gases or process off-gases; and production of specialty magnesium compounds such as particulate Mg(OH) 2 , chemical grade magnesia, high reactivity MgO of various activities, periclase and so forth. It is desirable to obtain a stabilized magnesium hydroxide slurry that can be used for the uses described above. It is also desirable to develop economic routes to achieve such a magnesium hydroxide slurry. It is particularly desirable to obtain a particulate magnesium hydroxide with a median particle size of between about 0.5-5 um and surface area, as measured by gas absorption methods, of 4-25 m 2 /g. Particulate magnesium hydroxide is used, for example, as a flame retardant additive, due to its ability to endothermically decompose with the release of water and its environmentally attractive nature. It is also particularly desirable to develop economic routes to achieve such a particulate magnesium hydroxide. Prior art methods of producing particulate magnesium hydroxide include: (i) Adding lime to sea-water. This produces magnesium hydroxide with a BET surface area which is usually above 15 m 2 /g and is often over 30 πr/g. The BET surface area is generally in the range of 10-40 m 2 /g and the particle size (D 50 ) is generally in the range of 2-10 microns. This process, in general, results in magnesium hydroxide particles having a surface area that is too high to be used in a number of magnesium hydroxide products. (ii) Adding a base, such as ammonium, sodium or calcium hydroxide, to a solution of a magnesium salt. This usually produces magnesium hydroxide with a high BET surface area (>25 m 2 /g). Under special conditions, magnesium hydroxide powders with a particle size (D 50 ) of 0.5-1.5 microns can be obtained. These powders have a BET surface area in the range 3-10 m 2 /g and are effective flame retardant additives. However, such powders are relatively expensive to produce, since extensive washing of the precipitate is required to remove co-product (ammonium, sodium or calcium salt). (iii) Hydration of magnesium oxide produced by pyrolysis or calcination. Hydration of magnesium oxide produced by pyrolysis of a magnesium salt solution, for example, magnesium chloride, has been the favored process although this method is relatively expensive to carry out. Hydration of magnesium oxide, produced by pyrolysis of magnesium chloride, typically yields a magnesium hydroxide powder having a BET surface area of <12 m 2 /g and an average particle size (D so ) of 0.8-1.3 microns. Hydration of magnesium oxide produced by calcination of magnesium hydroxide or magnesium carbonate is potentially attractive as there is no by-product to remove. The degree of calcination used determines the surface area of magnesium hydroxide powder. Mild calcination gives magnesium oxide with a BET surface area of at least 25 m 2 /g (often >50 m 2 /g). Such magnesium oxide hydrates easily but gives a magnesium hydroxide powder with a similarly high BET surface area. Hard calcination gives a magnesium oxide with a large particle size (D 50 ) and a low BET surface area (often <1 m 2 /g). This magnesium oxide is extremely difficult to hydrate unless a catalyst such as magnesium chloride is used. When a catalyst is used, the product usually comprises particles of relatively large particle size (D 50 ) and high BET surface area. Representative of prior art processes for the production of magnesium hydroxide are those processes disclosed in U.S. Pat. No. 3,739,058; U.S. Pat. No. 3,508,869; U.S. Pat. No. 2,940,831; and U.S. Pat. No. 3,080,215. A commercial magnesium hydroxide process such as disclosed in these patents will produce a particulate material having a size range of about 0.5 to 15 microns, with a median particle size of about 5-6 microns, 20% of the material being smaller than about 3 microns, and 20% being larger than about 9 microns. So long as they are sufficiently stable, magnesium hydroxide slurries represent an effective and convenient form by which magnesium hydroxide can be furnished. For example, stabilized slurries of magnesium hydroxide have many advantages over other forms of magnesium hydroxide, including the ability to be easily handled, transferred and stored, and the ability to be reliably dosed to chemical processes as desired. Magnesium hydroxide slurry may typically be derived from three basic sources: seawater, well brine and magnesite ore. In a preferred process, a magnesium hydroxide slurry is produced from the chemical reaction of dolime (CaO-MgO) and well brine. The well brine comprises primarily calcium chloride but also includes magnesium chloride. The chemical reaction of dolime and well brine produces a slurry of magnesium hydroxide in a chloride-containing liquor. The slurry is then further processed to increase solids content, typically to between about 30% and 60%. Unless indicated otherwise, all percentages in this application are weight percentages on an MgO basis. Richmond et al., U.S. Patent No. 5,514,357, discloses a method for producing a stabilized magnesium hydroxide slurry produced by conventional methods such as from well brine consisting of physically deflocculating the magnesium hydroxide solids in a starting slurry and optionally adding a cationic polymer and a thickening agent. Richmond et al., U.S. Patent No. 5,762,901, discloses a method for producing a stabilized magnesium hydroxide slurry consisting of physically deflocculating the magnesium hydroxide solids in a starting slurry and controlling the chloride ion content in the slurry. Witkowski et al., U.S. Patent No. 5,487,879, discloses a process for producing a stabilized slurry of magnesium hydroxide from burnt natural magnesite that invok es pressure hydrating a mixture containing burnt natural magnesite and water in the presence of chloride ions and cationic polymer. Although the above processes achieve a stabilized magnesium hydroxide slurry, there remains a need to further control the production of stabilized magnesium hydroxide slurry in order to control the characteristics of desired Mg(OH ), and MgO products, and, in particular, of particulate Mg(OH) 2 . The present invention permits the production of Mg(OH) 2 and MgO products that could not readily and economically be produced with conventional methods. These products include those that require submicron Mg(OH) 2 and MgO particle sizes, specified (e.g., high) surface area of Mg(OH) 2 and MgO, and specified (e.g., very high) density of Mg(OH) 2 and MgO. Summary of the Invention It is, therefore, an object of the present invention to provide a method for producing a stabilized magnesium hydroxide slurry having specified properties pursuant to the needs of the characteristics of the final slurry and/or of the final Mg(OH) 2 /MgO product. The present invention is thus directed to a method for producing a stabilized magnesium hydroxide slurry, the stabilized magnesium hydroxide slurry produced by the method and magnesia-based products produced using the stabilized magnesium hydroxide slurry of the invention. According to the method of the present invention, a wet milled magnesium hydroxide slurry, produced by conventional methods such as from well brine, having a desired solids content, i.e., generally between about 30-80% solids, by weight (MgO basis), and viscosity of about 50-1000 cps; median particle size ranging from about 0.5-7 um; range of particle sizes from about < 0.1-30 um; and specific surface area of from about 5-25 m 2 /g, is subjected to wet milling to produce a stabilized magnesium hydroxide slurry having specified characteristics, such as viscosity of Mg(OH) 2 slurry, as well as median particle size, particle size range and surface area of Mg(OH) 2 solids. Wet milling, which may be carried out by a variety of procedures and equipment, refers to a process wherein the solid magnesium hydroxide particles in the slurry are ground using energy developed by particle to particle interactions, media to particle interactions, particle to grinding chamber interactions and shear forces. The method of the present invention advantageously produces a stabilized magnesium hydroxide slurry having specified characteristics that can be further processed to various magnesia-based products. The stabilized magnesium hydroxide slurry of the invention is, in particular, characterized by its controlled viscosity of the Mg(OH) 2 slurry, as well as by the controlled median particle size, controlled particle size range, and controlled surface area (which is indirectly controlled based on degree of particle size reduction), of Mg(OH) 2 solids. The stabilized magnesium hydroxide slurry of the invention has the aforementioned advantages as well as other advantages that will be apparent to those of ordinary skill in the art from the following more detailed description, taken in conjunction with the accompanying drawings. Brief Description of the Drawings FIG. 1 is a schematic showing the method for preparing a stabilized magnesium hydroxide slurry involving wet milling a starting magnesium hydroxide slurry of the present invention. The media, the mill speed and the throughput may be controlled to yield a desired product. FIG. 2 is a graph showing viscosity v. particle size for the (a) unmilled magnesium hydroxide slurry and (b) wet milled magnesium hydroxide slurry of Example 1 , showing milling to various particle sizes. FIGS. 3 and 4 are charts showing cumulative mass percent v. finer for the wet milled magnesium hydroxide slurries of Example 9. In the following description, like parts are designated by like reference numbers throughout the figures. Detailed Description of the Invention All patents, patent applications and literatures that may be cited in this description are incorporated herein by reference in their entirety. As a further aid to understanding the invention, without being limited thereby, it is believed that the wet milling process of the invention, i.e., wherein a magnesium hydroxide slurry is wet milled, advances over the conventional processes for making Mg(OH) 2 slurry. The process of the invention permits more rapid, less complicated processing of Mg(OH) 2 slurry. The process of the invention also advances over the conventional processes for making Mg(OH) 2 and MgO products and, in particular, for making particulate Mg(OH) 2 . The process of the invention permits more rapid, less complicated production of particulate Mg(OH) 2 . The method of the present invention for producing a stabilized magnesium hydroxide slurry using wet milling comprises passing a magnesium hydroxide slurry through a wet milling apparatus one time. The rate (residence time) is controlled, the media type and quality are selected and the media circulation rate is controlled. All of these variables impact the degree of milling of the Mg(OH) 2 particles in the stabilized magnesium hydroxide slurry. By controlling the process parameters of the wet milling process, a stabilized magnesium hydroxide slurry having specified characteristics, such as a specified viscosity of Mg(OH) 2 slurry, specified median particle size, specified particle size range, specified particle size distribution and a specified surface area of Mg(OH) 2 solids, can be produced. The method of the invention thus allows the production of a stabilized magnesium hydroxide slurry, as well as of Mg(OH) 2 and MgO products, having specified characteristics. Referring to FIG. 1, the method of the invention is described below: 1. Washed magnesium hydroxide slurry is either (a) fed directly to a wet mill 1 at a controlled rate for particle size reduction, or (b) introduced to a disk filter 2 to increase percent solids to greater than about 60%. 2. If the disk filter 2 is used, the discharge is directed to a pug mill 3. 3. Flow is recycled within the pug mill 3 through a Silverson (East Longmeadow, MA) high-shear mixer 4. Cationic polymer is injected at levels disclosed in U.S. Patents Nos. 5,514,357 and 5,762,901, based on chloride concentration in the liquor. The pug mill discharge is collected in a surge tank 5. 4. A pump 6 is used to withdraw slurry from the surge tank 5 and feed the wet mill 1 at a controlled rate which dictates residence time in the wet mill 1. This rate can be altered to control particle size reduction. 5. Media type (zirconium oxide, glass, steel, zirconium silicate, etc.) media size (1mm, 1.5mm, 2mm, etc.), quantity of media (50%, 60%, 70%, etc. of milling chamber filled) and mill media recirculation rate (600 RPM, 800 RPM, 1000 RPM, etc.) also dictate the degree of particle size reduction and surface area increase. The easiest ways to control particle size is throughput control and recirculation rate control. 6. The milled slurry is collected in a storage tank 6. The milled slurry is suitable as a stabilized slurry, and for drying to particulate Mg(OH) 2 , calcining to MgO, or calcining and sintering for production of periclase. The different final applications require particle size control to varying levels. For example, stabilized slurry, generally requires median particle size of about 1 - 4.5 um, dry (particulate) Mg(OH) 2 generally requires median particle size of about 0.5 - 1 um, 1.5 - 2.0 um, wherein unmilled at 5 - 8 um, high activity MgO generally requires a median particle size of about 0.7 - 1.0 um or unmilled, periclase pressing enhancement of about <1 um - 4 um, and periclase BSG improvement of about < 1.5 um. The stabilized magnesium hydroxide slurry of the present invention comprises median particle sizes of Mg(OH) 2 solids not easily achieved by conventional methods. For example: 1) A magnesium hydroxide slurry containing Mg(OH) 2 with median particle size of Mg(OH) 2 solids greater than 3 microns and less than 10 microns produced from dolime and well brine can quite easily be wet milled to achieve a stabilized magnesium hydroxide slurry containing Mg(OH) 2 with a median particle size of less than 0.5-6 microns, as desired. Wet milling conditions can successfully be altered to produce any median particle size desired that is less than the starting median particle size. 2) Reduction of the median particle size beyond 2 microns can result in excessively high viscosity. However, controlling other variables such as surface area, chloride ion content and percent solids or using additives such as cationic polymer can counter any negative effects. 3) Particulate Mg(OH) 2 with a median particle size less than 1 micron can successfully and economically be produced by wet milling the magnesium hydroxide slurry and then drying. In addition to flame retardant additives, particulate Mg(OH) 2 is used in, for example, plastics and rubber compounds, and as filler in, e.g., wire and cable coatings, etc. 4) Wet milling a stabilized magnesium hydroxide slurry to decrease particle size of Mg(OH) 2 particles prior to calcination to create MgO has multiple benefits. One benefit realized is that the loose bulk density of the calcined powder can be increased or reduced depending on required particle size distribution. Increasing the loose bulk density permits the addition of more product into trucks and railcars, thus reducing shipping costs on a 'per ton' basis. 5) The stabilized magnesium hydroxide slurry of the invention can be used to prepare lightly burnt MgO (i.e., Chemical Grade Magnesia) - Applications exist where customers desire sub-micron particle size magnesia. This product is currently produced by jet milling the MgO powder, which is energy intensive and very costly. Wet milling the Mg(OH) 2 slurry prior to calcination to sub-micron size enables production of sub-micron chemical grade magnesia directly off the calcination furnace. 6) In the production of periclase (Very Hard-Burnt MgO), the magnesium hydroxide slurry of the invention is first calcined to MgO powder similar to that described above. Secondly, the MgO powder is pressed into almond shaped briquettes (green briquettes). These briquettes are then fed into a high temperature shaft kiln for final heat treatment and production of high density MgO. It has been discovered that wet milling the Mg(OH) 2 slurry prior to calcination to less than 4 microns improves the pressing characteristics of the powder. The benefits include: a) the green briquette breaking strength is greatly increased, b) the density of the green briquette is higher, c) costs to produce green briquettes are less because of higher yield of good briquettes during pressing (less breakage), and d) the MgO powder can be calcined to a lesser degree, reducing operating costs, while still producing high quality green briquettes. 7) A significant increase is seen in the Bulk Specific Gravity (BSG), of fired briquettes (periclase) after high temperature heat treatment when the starting slurry is wet milled to less than 1.5 microns. BSG is one of the most important properties desired for periclase used in refractory brick. An increase in BSG results in a minimization of brick failure. Voids are thus sintered out during heat treatment. Smaller voids are easier to eliminate. Wet milling results in smaller voids between particles in pressed briquettes. 8) Rotary kiln operations which produce MgO from Mg(OH) 2 slurry, can be improved by employing wet milling of a starting slurry. Product sizing control and enhanced BSG's may be obtained. In terms of heat treatment, the rotary kiln produces MgO burnt harder than the calcining furnace and lighter than that burnt on the shaft kilns. Operating costs may thus be reduced. 9) Production of stabilized slurry using wet milling requires only 1 pass through the equipment versus multiple passes with the conventional methods, such as the use of a Gaulin or a Silverson. Both the Gaulin and the Silverson deflocculate through shear. By wet milling, the Mg(OH) 2 particles are deflocculated and broken into smaller particle sizes. 10) Experimental data shows that upon freezing and thawing the slurries of the invention have acceptable stability properties. It is a fact that the properties of the starting Mg(OH) 2 slurry dictate to a large extent the final properties of Mg(OH) 2 and MgO products produced from it. Wet milling is a tool that enables enhanced "custom tailoring" of the starting slurry beyond conventional methods. Wet milling can enable significant advances in product quality while at the same time reduce operating costs. The present invention will be further illustrated by the following non-limiting Examples. The Examples are illustrative and do not limit the claimed invention to the particular materials, conditions, process parameters and the like recited herein. Example 1 A starting magnesium hydroxide slurry "(98 HP)" was taken from a raw feed surge tank. The chemistry, particle size, specific surface area, percent solids and viscosity are indicated in Table 1 (below). This slurry contained 500 ppm of Betz 1195 (cationic coagulant) and was initially deflocculated with a disk filter pug mill and Silverson high shear mixer prior to storage in the tank and wet milling. The typical process sequence in stabilized magnesium hydroxide production is similar to that disclosed in U.S. Patents Nos. 5,514,357 and 5,762,901 and U.S. Patent Applications Serial Nos. 08/853,412 and 08/968,135, the entire contents of which are incorporated herein by reference. Alternately, in this trial, one pass was completed with a model LV-40, EMCO Zinger Mill (Epworth Mfg. Co., South Haven, MI). The mill chamber was loaded 70% full (190 lbs.) with zirconium oxide grinding media. Flow through the mill was controlled to 14-17 gallons per minute (gpm). A total of 16,500 gallons of stabilized slurry was produced. Several differences were noted in the properties of stabilized slurry from this trial versus stabilized slurry prepared according to conventional processes: 1) apparent specific surface area increased after wet milling; and 2) particle size reduction with wet milling was 2 um greater than with conventional processes (median particle size fell at least 2 microns versus 1 micron or less, breakage of particles was determined by x-ray monitored sedimentation (Micromeritics Co., GA), wherein a sample is dispersed in liquid; x-ray beams are radiated through the liquid; and as the particles settle, a measurement of how readily the particles settle is determined). Similarities were also observed: 1) 7-day drainage = 94.7%. The % drainage is an adequate measure for assessing the stability of a slurry. This is determined based on the pourability of a sample after a given settling period. As an example, a pre-weighed 5 W high x 2" diameter high density polyethylene sample bottle with cap, available from Cole Parmer, is filled with slurry to capacity. After a given period of unagitated storage at room temperature, the cap is removed, a glass stirring rod is inserted to the bottom of the bottle, and the end is slowly rotated one turn around the inner periphery of the bottle. The bottle is then weighed, inverted 180° for a period of 15 seconds, reweighed, and percent drainage calculated as follows: % drainage =(fιlled bottle weight - drained bottle weight) x 100 (filled bottle weight - bottle weight) The solids remaining in the bottle are then probed with a stirring rod to determine whether they are soft, tacky (e.g., like bubblegum) or hard. A slurry is deemed to have "long term stability" if it has a percent drainage of at least 90%o after 7 days, at least 85%) after 14 days and at least 80% after 28 days. The present inventors have also surprisingly discovered that the stabilized Mg(OH) 2 slurry prepared according to the invention (as exhibited by Example 1) may be frozen and then thawed, while retaining stability. This was demonstrated by taking a sample of frozen slurry, allowing to thaw, and determining stability as above. 7-day drainage results showed stability to be 88%-92%, without tacky sedimentation. This is in contrast to the characteristics of conventional slurries, which lose stability upon freezing and thawing. Example 2 Referring to Table 2, different grades of Mg(OH) 2 slurry, produced from a dolime/well brine reaction, were wet milled to the particle size ranges indicated therein compared with unmilled slurries. Samples 3, 4, 7 and 8 were wet milled. In all cases, wet milling to a finer degree resulted in sticker material (higher compactions). The compaction test involves forming a cylindrical pellet with a die and press. 5.5 grams of Mg(OH) 2 powder (or MgO powder) is placed in a 1" diameter die. A pellet is then formed at 175 psi loading on a press. The pellet is removed from the die. The breaking strength is then determined with a load applied to the flat face of the pellet. A higher compaction value indicates that flow properties of the powder are inferior or alternately, that the compacting properties are improved. Example 3 Referring to Table 3, Table 3 shows results of stabilized slurry production. Particle size target was 3-4 um, which was 1.5-2 um less than the starting particle size of the slurry. Viscosity was improved versus the unmilled slurry. The unmilled slurry was prepared using disk filter, addition of cationic coagulant and Silverson. Results indicate that essentially all +100 mesh and +325 mesh particles were eliminated (i.e., 145 um and 44 um, respectively). Example 4 Referring to Table 4, Table 4 shows results comparing the conventional (i.e., use of a Gaulin) process for production of stabilized slurry to the wet milling method. The results indicate superior reduction of the coarse agglomerates (+100 mesh and +325 mesh), with wet milling. Referring to Table 5, Table 5 shows additional results for particle size and distribution for stabilized slurry produced via wet milling. A 270 mesh screen (55 um) was added in this evaluation. Example 5 Referring to Tables 6 and 7, Tables 6 and 7 show results of a trial to produce sub- micron high reactivity chemical grade MgO from a wet milled Mg(OH) 2 slurry. The starting slurry had a median particle size of 6.69 um. The slurry was wet milled to 1.00 um. The slurry was then calcined with a vertical multiple hearth furnace (Herreshoff). The resultant MgO had a surface area of about 45 m 2 /g and a median particle size less than 1 um. Typical results with unmilled product yield median particle size in excess of 3 um at the same activity (Surface Area = 45 m 2 /g). Example 6 Referring to Tables 8 and 9, Tables 8 and 9 show results of a trial to enhance the pressing characteristics of MgO powder by wet milling the starting Mg(OH) 2 prior to calcination to 1.56 um. Results were compared to unmilled Mg(OH) 2 . Both starting slurries were calcined identically, as indicated by activity test and surface area. The wet milled sample showed enhanced compaction strength, 66 psi, versus 26 psi for the unmilled sample. Example 7 Referring to Table 10, Table 10 show that wet milling breaks particles. This conclusion is drawn based on the fact that particle size is reduced and surface area is increased. Example 8 Referring to Tables 11 and 12, Tables 11 and 12 show another example of production of sub-micron high reactivity MgO by wet milling Mg(OH) 2 slurry prior to calcination. An unmilled slurry was used as a comparison. Tables 11 and 12 also demonstrate significant improvement in the pressing characteristics of MgO produced from wet milled slurry, which is evidenced in the higher MgO compaction values versus the unmilled. This is in contrast to existing Mg(OH) 2 slurry production, wherein, only deflocculation occurs, i.e., apparent particle size falls, but surface area does not change. Example 9 Tables 13 and 14 and FIGS. 3 and 4, respectively, show particle size analyses of sub-micron wet milled Mg(OH) 2 . Although the present invention has been fully described by way of examples with reference to the accompany drawings, it is to be noted various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the present invention, they should be construed as being included therein. TABLE 1 Analysis of wet milled 98HP slurry from Tank-IOO to Tank-2I0B. (Settling Test) c CD ω rn en X m > q 3 c * Properties of starting magnesium hydroxide slurry prior to wet milling. The starting slurry contained 50 ppm Betz 1 195 and was initially r m deflocculated with a disk filter pug mill and Silverson high shear mixer. r TABLE 2 DRY HYDROXIDE COMPΛCIIONS O c O O H c H m H CO X m m H " 53 c r- m t TABLE 3 Results of Wet-Milled FloMag Production Runs (981 IP) O C CD CO 5 rπ CO X m r~ m to en TΛΠI F Oril comparisons Tor Wei Milled and Ciulinircd rioMag CO c CD CO H H C σ> -I m O x m m π 3 c r- m ro en TABLE 5 Screen Analysis for Wei-Milled 98HP FloMag. O c CD CO H C H m oo CO x m m H c r- m M n TABLE 6 Wet Milled 98l.ib Slutry Lightly burnt MgO produced from wet milled slurry CO c CD CO H H C vθ H rπ CO x m m H Slurry properties before and after wet milling. c r- m κ> σ> Table 7 CO c CD CO H C H m O O X m m H 3 Calcined MgO results (calcined after milling) c t- m σ> Table 8 Calcinated Wet Milled Shirty MgO Powder MgO Powder Sample Activity Test Result Compaction Strength 97LB not milled 21.4 seconds 26 psi 97LB wet milled 21.4 seconds 66 psi CO c to 1.56 um D CO Conclusion: Pressing is greatly enhanced with wet milling. C H Is. rπ CO Table 9 x m Calcined MgO* Calcined MgO* m Sample Surface Area Particle Size 30 c 97LB not milled 19.6 m7g 3 32 um r- m r 97LB wet milled 19.8 m 2 /g 0.51 um en to 1.56 um Conclusion: Smaller Mg(OH), particle size results in smaller MgO particle size at same level of calcination. This is the reason that compaction strength increased. * Calcined after milling. Table 10 Wet Milled Slurry Properties Median Part. Size %<2um Surface Area 98HP Slurry 5.22 microns 26.4% 12.0 Vg (Unmilled) CO C CD Wet Milled 0.79 microns 67.4% 19.0 Vg CO H l- C to H 97LB Sluny 6.74 um 10.0% 14.5 m 2 /g m (Unmilled) CO x m Wet Milled 1.56 um 54.6% 21.6 Vg Ώ 3 c m Conclusion: Particles can be broken with wet mill. The proof is the increase experienced with surface area. σ> Table 11 98HP Lightburn Sluπy Calcination - Unmilled versus Wet Milled Activity MgO Powder MgO MgO Slurry 5 CMA Compaction Surface Area Part. : Size Unmilled 5.8 10 7 1 12 60 08 3 99 7 9 13 50.8 4 01 8 4 14 48.75 3.81 8.4 15 Mg(OH), to submicron levels. Compaction values are substantially enhanced Table 12 Periclase BSG Fnliancctncnt Typical (Unmilled) Slurry vs. Wet Milled Slurry. Grain Date Brig. Temp Time Firines BSG Misc Info CO c CD 98HP 3600 20 3 402 Unmilled CO H 98HP 5/6/97 lab 3600 20 3 451 Wet milled H C H to rπ 98LitB 8/1 1/97 lab 3600 20 3 402 Unmilled CO 98LilB 8/1 1/97 lab 3600 20 3 472 Wet milled x m m H 98LB 7/15/97 lab 3600 20 3 426 Unmilled 98LB 7/15/97 lab 3600 20 3 485 Wet milled c r- 97LB 05/15/97 lab 3600 20 3.426 Unmilled M 97LB 05/15/97 lab 3600 20 3 483 Wet milled 97LB 08/28/97 lab 3600 20 3 472 Unmilled, press at 20 ton 97LB 08/28/97 lab 3600 20 3.523 Wet milled, press at 20 ton AH wet milled slurry samples had median diameters less than 1.5 um. Table 13 98HP 8 th 6REF SV-4 SAMPLE ID: 98HP 8 1 H 6REP SV-4 UNIT NUMBER: 1 SAMPLE TYPE: HYDROXIDE START 13:50:57 LIQUID TYPE: SEDISPERSE A-11 REPORT 14:28:15 TOT RUN TIME 0:37:06 SAM DENS: 2.3800 g/cc LIQ DENS: 0.7450 g/cc ANALYSIS TEMP: 05.0 DEG C RUN TYPE: High Speed LIQ VISC: 1.1208 cp STARTING DIAMETER 100.00 μm REYNOLDS NUMBER: 0.53 ENDING DIAMETER: 0.18 μm FULL SCALE MASS % 100 24A Table 14 98HP SV-4 3GPH 1 PASS SEDIGRAPH 5100 02.08 UNIT NUMBER: 1 SAMPLE DIRECTORY NUMBER: 98-DRX-2/4 START 11:42:23 SAMPLE ID: 98HP SV-4 8GPH 1 PASS REPORT 12:35:16 TOT RUN TIME 0:52:44 SAMPLE TYPE: HYDROXIDE SAM DENS: 2.3800 g/cc LIQUID TYPE: 32DISPERSE A- 1 1 LIQ DENS: 0.7450 g/cc ANALYSIS TEMP: 35.0 DEG C RUN TYPE: High Speed LIQ VISC: 1.1204 cp STARTING DIAMETER 100.00 μm REYNOLDS NUMBER: 0.53 ENDING DIAMETER: 0.18 μm FULL SCALE MASS % 100 VISC. 220 CPS 24B

Claims: What is claimed is: 1. A method for preparing a stabilized magnesium hydroxide slurry, comprising wet milling a starting magnesium hydroxide slurry. 2. The method of claim 1, wherein parameters of said wet milling are controlled to control properties of said stabilized magnesium hydroxide slurry. 3. The method of claim 2, wherein said properties include a median particle size of magnesium hydroxide solids in said magnesium hydroxide slurry. 4. The method of claim 1, wherein said wet milling reduces a median particle size of Mg(OH), solids in said starting magnesium hydroxide slurry to less than about 4 microns. 5. A stabilized magnesium hydroxide slurry, wherein said stabilized magnesium hydroxide slurry is prepared by wet milling a starting magnesium hydroxide slurry. 6. The stabilized magnesium hydroxide slurry of claim 5, wherein said stabilized magnesium hydroxide slurry has a median particle size of less than about 4 microns. 7. A method for preparing a dry Mg(OH) 2 powder, comprising: wet milling a starting magnesium hydroxide slurry; and 25 drying said wet milled magnesium hydroxide slurry to form said dry Mg(OH) 2 powder, wherein said dry Mg(OH) 2 powder has a median particle size of less than about 2 microns. 8. A method for preparing a dry MgO powder, comprising: wet milling a starting magnesium hydroxide slurry; drying said wet milled magnesium hydroxide slurry to form a dry Mg(OH) : powder; and calcining said dried Mg(OH) 2 powder to form said dry MgO powder, wherein said dry MgO powder has a median particle size of less than about 3 microns. 9. The method of claim 8, wherein said wet milling of said starting magnesium hydroxide slurry results in an increase in a loose bulk density of said dried MgO powder. 10. The method of claim 8, wherein said dried MgO powder is used to produce lightly burnt MgO. 11. The method of claim 8, wherein said dried MgO powder is used to produce very hard burnt MgO. 12. The method of claim 11, wherein said very hard burnt MgO is made by pressing MgO powder into briquettes and heat treated to produce periclase. 26 13. The method of claim 12, wherein said wet milling of said starting magnesium hydroxide slurry results in an increase in a bulk specific gravity of said periclase. 14. The method of claim 12, wherein said wet milling of said starting magnesium hydroxide slurry results in an increase in a breaking strength of said periclase. 15. A method for producing a stabilized magnesium hydroxide slurry, comprising: deflocculating a starting magnesium hydroxide slurry; and wet milling said deflocculated magnesium hydroxide slurry. 16. The method of claim 16, wherein said stabilized magnesium hydroxide slurry comprises Mg(OH) 2 particles having a surface area between about 12.0 and 15.0 mVg. 17. The method of claim 16, wherein said stabilized magnesium hydroxide slurry comprises Mg(OH) 2 particles having an average particle size of less than about 4 microns. 18. The method of claim 16, wherein said stabilized magnesium hydroxide slurry has a solids content of between about 50% and about 65%. 27 19. The method of claim 16, wherein said stabilized magnesium hydroxide slurry comprises between about 25% and about 45% Mg(OH) 2 particles less than 2 microns in size. 20. The method of claim 16, wherein said stabilized magnesium hydroxide slurry comprises between about 15% and about 35% Mg(OH) 2 particles less than 1 micron in size. 21. The method of claim 16, wherein said stabilized magnesium hydroxide slurry comprises Mg(OH) 2 particles having an average size between about 2.5 and 5 microns. 22. A stabilized magnesium hydroxide slurry prepared by the method of claim 16, wherein said stabilized magnesium hydroxide slurry is capable of maintaining stability upon being frozen and thawed. 23. The method of claim 1, wherein said starting magnesium hydroxide slurry comprises Mg(OH) 2 particles having an average particle size between about 4 microns and 8 microns. 24. A method for controlling the particle size of a magnesium hydroxide slurry, comprising controlling wet milling of a starting magnesium hydroxide slurry. 25. A method for preparing a dry Mg(OH) 2 powder, comprising: 28 wet milling a starting magnesium hydroxide slurry; and drying said wet milled magnesium hydroxide slurry to form a dry Mg(OH) 2 powder. 29

Inventors: GUTOWSKI ROBERT J, RICHMOND ALVIN, GUTOWSKI, ROBERT, J., RICHMOND, ALVIN
Applicants: GUTOWSKI ROBERT J, MARTIN MARIETTA MAGNESIA, RICHMOND ALVIN, GUTOWSKI, ROBERT, J., MARTIN MARIETTA MAGNESIA SPECIALTIES, INC., RICHMOND, ALVIN
Application year: 1998
Main classification: C01F5
IPC codes: C01F5