GB2099012A

GB2099012A - Superior high sodium and calcium sol gel abrasive

Info

Publication number: GB2099012A
Application number: GB8215424A
Authority: GB
Original assignee: Kennecott Corp
Current assignee: Kennecott Corp
Priority date: 1981-05-27
Filing date: 1982-05-26
Publication date: 1982-12-01
Also published as: GB2099012B; BR8203073A; MX156659A; FR2512828A1; FR2512828B1; DE3219607C2; CA1195848A; DE3219607A1

Abstract

A process for forming an abrasive grain from a sol gel which contains aluminium oxide monohydrate, a dissolved metal containing sintering aid and from above about .05 to about 1.8 weight percent sodium plus calcium, provided that the weight percent calcium is from 0 to about 1.8 and the weight percent sodium is from 0 to about 4. The high sodium and calcium is permitted due to rapid heating of the sol gel after drying through a temperature range of from below about 800 DEG C to above about 1200 DEG C, prior to sintering the dried gel at a temperature above 1200 DEG C. A grain made by the process and coated, bonded and non-woven abrasive articles comprising the grain.

Description

SPECIFICATION Superior high sodium and calcium sol gel abrasive and process for its production This invention relates to abrasive grains and more particularly relates to a sintered type of abrasive grain.

In the prior art, abrasive grains, especially abrasive grains comprising metal oxides such as alumina, were traditionally made by fusion of the oxide followed by crushing the cooled fused oxide to form the abrasive grain.

More recently, abrasive grains have been manufactured by sintering the metal oxide, such as alumina. Such grains, while wearing reasonably well, still do not wear as long as desirable and in addition, do not cut as rapidly as desirable.

It is known that ceramic materials which may be in the form of abrasive products can be prepared by binding ceramic oxide particles together by using a mineral colloidal gel (see U.S.-A-2,455,358).

It is also known that colloidal dispersions can be gelled, dried and calcined to form porous ceramic materials (see e.g., U.S.-A-4,1 81,532).

It has also been known that free flowing spheres of pure or mixed oxides could be made by dispersing an oxide hydrate followed by forming a gel in the shape of spheres followed by sintering (see "Application of Sol-Gel Processes to Industrial Oxides", January 13, 1968, Chemistry and lndustry).

It has also been known by others in the United States, at least as early as 1 971, that such particles in spherical or even angular shapes could be used as abrasives. Such prior knowledge by others in the United States is evidenced by a report from the United Kingdom Atomic Energy Authority at Harwell, England, to Carborundum Company, Ltd., and subsequently to several personnel at The Carborundum Company in the United States.

Recently, U.S.--AA-4,314,827 issued for a non-fused aluminium oxide-based abrasive mineral, a process for its production and abrasive products comprising the abrasive mineral.

U.S.--AA-4,314,827 generally discloses that an abrasive mineral having randomly oriented crystallites with diameters on the order of 3000 Angstroms or less can be made by gelling a colloidal dispersion or hydrosol of alumina and a modifying component, followed by firing the gel. The disclosure makes it clear that the mineral must be free (less than about 0.05 total weight percent) of calcium and alkali metal. Due to this requirement, complex purification processes must be used when calcium and alkali metal are present, as they often are, in the alumina or modifying component. Calcium, for example, is usually present in commercial water supplies and in magnesium containing modifying components unless costly and complicated purification steps are used to remove the calcium.Similarly, sodium is commonly present in many aluminas unless removed by additional purification steps. There is no suggestion in the U.S. patent of any process which would permit the presence of calcium or alkali metal and no process was actually used with high calcium or alkali metal which inherently permitted the presence of high calcium or alkali metal.

In accordance with the present invention, a process is provided for forming ceramic particles from a gelled dispersion (sol gel), by drying and sintering the gel. The particles of the invention may be used for any suitable purpose. Such particles may be used in any application where temperature resistance, strength, hardness, wear resistance and inertness are desirable. Such particles may, for example, be used as fillers, in aggregates, in distillation columns or due to the surface porosity of some of such particles, may be used as catalyst supports. The most common use for such particles is as abrasives.

Such particles will therefore be referred to herein as abrasives, although it is to be understood that the term abrasive particle or abrasive grain is intended to include such particles regardless of their intended end use.

According to the present invention there is provided a process for forming abrasive grains which comprises: a) preparing a dispersion comprising from about 2 to about 60 weight percent aluminium oxide monohydrate; a dissolved metal containing sintering aid in an atomic ratio of metal in the sintering aid to aluminium in the aluminium oxide monohydrate of from 1:2 to 1::35, and from above about 0.5 to about 1.8 weight percent sodium plus calcium by weight of dispersed and dissolved metal containing solids in the dispersion, provided that the weight percent calcium is from 0 to about 1.8 and the weight percent sodium is from 0 to about 0.4; b) gelling said dispersion; c) drying the gelled dispersion at a temperature below the frothing temperature of the gel to vaporize free water; d) crushing the dried solid to form grains; e) rapidly heating the grains to above about 1 2000C in less than 10 minutes; and f) continuing to heat the grains at a sintering temperature between about 1 2000C and about 1 6500C for a sufficient sintering time to sinter the grains to a density above about 85% of theoretical density.

The invention also extends to a process for forming abrasives having alpha-alumina grains with a diameter from about 5,000 Angstroms to about 200,000 Angstroms which comprises: a) preparing a dispersion comprising from about 2 to about 60 weight percent aluminium oxide monohydrate; a dissolved metal containing sintering aid in an atomic ratio of metal in the sintering aid to aluminium in the aluminium oxide monohydrate of from 1:2 to 1 ::35, and up to about 1.8 weight percent sodium plus calcium by weight of dispersed and dissolved metal containing solids in the dispersion, provided that the weight percent calcium is from 0 to about 1.8 and the weight percent sodium is from 0 to about 0.4; b) gelling said dispersion; c) drying the gelled dispersion at a temperature below the frothing temperature of the gel to vaporize free water; d) crushing the dried solid to form grains; e) rapidly heating the grains to above about 1 2000C in less than 10 minutes; and f) continuing to heat the grains at a sintering temperature between about 1 2000C and about 1 6500C for a sufficient sintering time to sinter the grains to a density above about 85% of theoretical density.

The invention further extends to a process for forming abrasives having non-randomly oriented alpha-alumina grains which comprises: a) preparing a dispersion comprising from about 2 to about 60 weight percent aluminium oxide monohydrate; a dissolved metal containing sintering aid in an atomic ratio of metal in the sintering aid to aluminium in the aluminium oxide monohydrate of from 1:2 to 1::35, and up to about 1.8 weight percent sodium plus calcium by weight of dispersed and dissolved metal containing solids in the dispersion, provided that the weight percent calcium is from 0 to about 1.8 and the weight percent sodium is from 0 to about 0.4; b) gelling said dispersion; c) drying the gelled dispersion at a temperature below the frothing temperature of the gel to vaporize free water; d) crushing the dried solid to form grains; e) rapidly heating the grains to above about 1 2000C in less than 10 minutes; and f) continuing to heat the grains at a sintering temperature between about 1 2000C and about 1 6500C for a sufficient sintering time to sinter the grains to a density above about 85% of theoretical density.

Even though the abrasive grain product obtained by the process of this invention usually contains greater than about 0.05 weight percent sodium plus calcium, it is greatly superior to traditional fused alumina abrasive. High sodium and calcium in the grain is permitted due to rapid heating of the dried sol gel through a critical temperature range of from below about 8000C to above about 1200"C. The grain is then sintered at a temperature above about 12000 C.

It is preferred to calcine the grains prior to the rapidly heating step.

The preferred process comprises preparing a dispersion comprising from about 2 to about 60 weight percent aluminium oxide monohydrate; a dissolved or dispersed metal containing sintering aid in an atomic ratio of metal to aluminium of from 1:2 to 1:35, preferably from 1:7 to 1:25; and from above about 0.05 to about 1.8 percent sodium plus calcium by weight of dispersed and dissolved metal containing solids in the dispersion.

The weight percent calcium plus the weight percent sodium is preferably less than 0.6, and more preferably less than 0.15. The weight percent calcium can range from 0 to about 1.8 and the weight percent sodium can range from about 0 to about 0.4. The weight percent calcium is preferably from 0 to about 0.6 and more preferably from 0 to about 0.15 and the weight percent sodium is preferably from 0 to about 0.25 and more preferably from 0. to about 0.1.

After the dispersion is prepared, it is gelled and dried at a temperature below the frothing temperature of the gel to remove free water. Any suitable drying method known to those skilled in the art may be used. "Drying" as used herein means dewatering by any method including solvent extraction.

The dried solid is then crushed to form grains. The grains are then usually heated to between about 5000 and 8000C. The grains are then rapidly heated to above about 1 2000C in less than 10 and preferably less than 5 minutes. The grains then continue to be heated at a sintering temperature between about 1 2000C and about 1 6500C for a sufficient sintering time to sinter the grains to a density above about 85% of theoretical density, at least a portion of the heating at sintering temperature preferably occurring above 1 3000 C.

The resulting grain is a sintered sol gel abrasive which has a combined sustained cutting rate and wear resistance against carbon steel which is substantially better than prior art fused alumina grain, and which is comparable to prior art sintered sol gel abrasive grains which require low sodium and low calcium (less than about 0.05 weight percent) as described in U.S.--AA-4,314,827. "Substantially better" means a relative cutting performance at least 1.5 times better and usually at least two times better.

The invention also includes bonded, coated and non-woven abrasive articles comprising the novel grain. For example, not only do coated abrasives manufactured from the preferred abrasive grain of the invention continue to have a high cutting rate until the grain is worn from the backing, but the total quantity of material cut by the abrasive article is superior to most prior art grains.

In accordance with the process of the invention, the grain is prepared from a liquid (preferably water) dispersion comprising from about 2 to about 60, usually from about 10 to about 40 weight percent aluminium oxide mono-hydrate (Al OOH); a dissolved metal containing sintering aid at an atomic ratio of metal in the sintering aid to aluminium of from 1:2 to 1 :35; and up to about 1.8 and usually from above about 0.05 to about 1.8 percent combined alkali metal (especially sodium) and calcium by weight of dispersed and dissolved solids in the dispersion.

The weight percent calcium plus the weight percent sodium is preferably less than 0.6, and more preferably less than 0.15. The weight percent calcium can range from 0 to about 1.8 and the weight percent sodium can range from about 0 to about 0.4. The weight percent calcium is preferably from 0 to about 0.6 and more preferably from 0 to about 0.1 5 and the weight percent sodium is preferably from 0 to about 0.25 and more preferably from 0 to about 0.1.

The quantity of aluminium oxide monohydrate in the dispersion is usually from about 10 to about 40 and preferably from about 1 5 to about 35 percent by weight of the dispersion. Aluminium oxide monohydrate as used herein is intended to include aluminium oxide hydrates having the stochiometric formula 2(AI203.XH20) where x is 0.5 to 3. Aluminium oxide monohydrate is also known as boehmite.

Aluminium oxide monohydrate, as used herein, is also intended to include, without limitation, pseudo boehmite.

In general, all solids in the dispersion are preferably dissolved or in colloidal form. The dispersion is formed by any suitable means which may simply be mixing of the aluminium oxide monohydrate with water. The liquid is almost always water but may be another liquid such as low molecular weight alcohol. Any suitable mixing apparatus may be used including both high and low shear mixers.

Dispersing aids such as acid are frequently employed. For example, from about 0.02 to 0.25 and, preferably, 0.05 to about 0.15 mole of HNO3 or other voiatile mineral acid per mole of aluminum oxide monohydrate greatly aids dispersion. "Volatile mineral acid" means an acid which will vaporize from the dispersion, gel or dried gel at a temperature below sintering temperature or all of whose residues will either so vaporize or from a part of an oxide within the finished grain. Examples of such acids are nitric, hydrochloric, acetic and formic acids.

The solids in the finished dispersion may comprise up to 50 weight percent of additional ingredients other than aluminum oxide monohydrate, preferably compounds such as silica, magnesia, chromia and titanium dioxide in colloidai or dissolved form, or precursors of such compounds in colloidal or dissolved form.

The solids in the finished dispersion may comprise up to 50 weight percent of additional ingredients other than aluminum oxide monohydrate, preferably compounds such as silica, magnesia, chromia and titanium dioxide in colloidal or dissolved form, or precursors of such compounds in colloidal or dissolved form.

The dissolved or dispersed metal containing sintering aid is added to the dispersion in an atomic ratio of metal to aluminium of from 1:2 to 1:35 and preferably at a ratio of from 1: 7 to 1: 25. The sintering aid is a dispersible or soluble metal oxide or metal oxide precursor, i.e., a compound which will form a metal oxide during drying, calcining or sintering. In general, the sintering aid is usually a precursor of magnesium oxide, zinc oxide, cobalt oxide or nickel oxide and is therefore, when the liquid is water, a water soluble or dispersible compound of magnesium, zinc, nickel or cobalt. Specific examples of such precursors are the nitrates and chlorides of those metals. The nitrates of those metals and especially of magnesium are particularly preferred.The sintering aid may be prepared in situ, for example by adding magnesium oxide or hydroxide to an aqueous solution of inorganic acid such as hydrochloric or nitric acids to form a water soluble salt of magnesium. The sintering aid is usually a water soluble salt but may be a water soluble base.

The sodium and calcium in the dispersion usually results naturally from calcium impurities in other components of the dispersion. The usual source of sodium is from alumina. The usual source of calcium is from impurities in magnesium containing sintering aid or water. "Calcium" and "sodium", as used herein, mean chemically bound calcium and sodium, which due to the electropositive nature of calcium and sodium, are almost always calcium or sodium ion, i.e., free calcium or sodium ions or ionically bound sodium or calcium.

In accordance with the process of the invention, relatively high amounts of calcium and sodium can be tolerated in the dispersion, thus complicated and expensive purification steps are avoided. In accordance with the invention, up to about 1.8 percent combined sodium and calcium can be present in the dispersion and a finished abrasive grain superior to traditional fused alumina grain will still be obtained. Preferably no more than 0.6 percent combined sodium and calcium is present. The percent sodium and calcium as above described is by weight of dispersed and dissolved metal containing solids in the dispersion. The percentages are essentially the same as the percentages in the finished abrasives.

After the dispersion is prepared, it is gelled. The addition of the dissolved or dispersed metal containing sintering aid, preferably magnesium nitrate, to the dispersion usually serves to gel the dispersion. If the solids content of the dispersion is quite low, the liquid may either have to be vaporized in order for the dispersion to gel or another method of gelling may have to be employed, i.e., the addition of a gelling agent.

After the dispersion is gelled, it is dried at a temperature below the frothing temperature of the gel to vaporize free water. "Dried" or "drying", as used herein, means that at least 90% of free (unbound) water is removed to form a solid. The "frothing temperature" is the temperature at which the gel will foam or froth at the pressure applied to the gel. Drying may be accomplished by any means known to those skilled in the art. When heat is used, the drying temperature is usually from about 80 to about 1 200C. The drying time depends upon the quantity of water or other liquid present, upon drying pressure, and upon drying temperature. The drying time is usually from about 1 to about 72 hours at atmospheric pressure. The resulting dried gel is usually, but not necessarily, a translucent solid, i.e., a solid through which light will pass in diffused form.

After the solid is dry, it is crushed or broken by any suitable means such as a hammer or ball mill to form particles or grains. Any suitable method for comminuting the solid may be used and "crushing" is intended to include all such methods.

After crushing, the grains are usually heated to a temperature between about 5000C and about 8000C until essentially all water is removed and until all components of the grains are either in the form of ceramics (usually metal oxides) or else are vaporized. When the grains reach a temperature between about 2500C and about 3000C, the acid residues are driven off. Between about 2500C and about 8000C, and usually between about 3000C and 6000C, essentially all water is usually removed.

"Essentially all", as used in this context, means that all free water and over 90 percent of bound water is removed. The calcining time to remove essentially all water is usually from about 5 to about 20 minutes.

After drying and after calcining, the grains are rapidly heated to above about 12000 C. "Rapidly heated" means sufficiently fast to permit formation of an abrasive having a good cutting rate and good wear resistance. The rapid heating usually occurs in less than 10, preferably in less than 5 and most preferably in less than 1 minute. This rapid heating step permits the formation of a superior abrasive grain from a gelled dispersion (sol gel) containing relatively high amounts of calcium and sodium. Any suitable means or method for rapidly heating the grains may be used such as injection of the grains in other than bulk form, i.e., separately, into a furnace preheated to above 1 2000C and preferably to above 13000 C.

It is possible to eliminate the calcining step and instead rapidly heat the grains to above about 1 2000C after drying. This particular procedure is particularly desirable for producing certain type abrasives.

After the grains are rapidly heated to above 12000 C, they continue to be heated at a sintering temperature between about 1 2000C and about 1650"C and preferably between 12500 and 1 5000C for a sufficient sintering time to sinter the grains to a density above about 85% of theoretical density. In the case where the abrasive is primarily aluminium oxide with about 6% magnesium oxide by weight of aluminium oxide, the desired density is above about 3.3 grams per cubic centimeter. At least a portion of the heating usually occurs above 1 3000C. The sufficient time to sinter the grain depends on sintering temperature and is usually from about 5 to about 30 minutes but may be less than about 5 minutes, e.g., from about 1 to about 5 minutes.

When looking at polished thin sections of sintered grain manufactured in accordance with the process of the invention in a transmission optical microscope at about 500X or 700X with crossed polarizers, one observes the microstructure of material with a nominal composition of alumina6% magnesia to consist of areas of from about 5,000 up to about 200,000 Angstroms in nominal diameter which extinguish as a unit upon sample rotation. The extinction is believed to result, in this instance, from the birefringent alpha alumina phase which is predominant. In order for these areas to extinguish as a unit, it is believed that they have to be either continuous alpha alumina grains or areas of smaller non-randomly oriented alumina grains.If the alpha alumina grains either have a diameter ranging from about 5,000 Angstroms to about 200,000 Angstroms or have much smaller diameters but are non randomly oriented, the weight percent combined calcium plus sodium in the grains may be less than .05.

The microstructure of the alumina grain of the invention differs markedly from that of normal sintered or fused alumina grain of a similar composition in the degree of homogeneity of distribution of metal oxide sintering aid. On firing previously calcined material, it is believed that a transformation from an atomistically homnogeneous distribution of magnesia in gamma alumina transforms to a microscopically homogeneous intimate mixture of alumina and spinel.

Normal sintering of abrasive grains includes consolidation by either a diffusional mechanism of preexisting alpha alumina crystals or a liquid phase densification mechanism. Sintering of sol gel abrasives may include a displasive polymorphic transformation of gamma alumina to alpha alumina and spinel involving minimum diffusion. This unique transformation normally results in a marked reduction in sintering temperature.

The invention further includes bonded, coated and non-woven abrasive articles comprising the abrasive grain of the invention. In general, the abrasive grain of the invention may be described as a sintered abrasive grain comprising alumina, a metal oxide which aids sintering, and up to about 1.8 and usually from above about 0.05 to about 1.8, preferably to about 0.6 and most preferably to about 0.1 5 weight percent combined calcium plus sodium.

As previously discussed, the preferred sintered grains manufactured in accordance with the process of the invention, have excellent wear characteristics and in addition, maintain a high cutting rate and remove larger quantities of carbon steel stock than most prior art grains. High sodium and calcium containing prior art grains do not have a combination of cutting rate and wear resistance as high as the best abrasive grains manufactured in accordance with the present invention.

As previously discussed, the sintered sol gel abrasive of the invention has a relative cutting performance on coated discs against carbon steel which is better than fused alumina abrasive. The preferred sol gel abrasives of the invention have such a cutting rate which is at least two and usually at least three to four times better than traditional fused alumina abrasive. Relative cutting performance is determined as defined and described in the following examples.

The following examples serve to illustrate and not limit the present invention.

EXAMPLE 1. Prior Art A high calcium sol gel abrasive grain was made essentially in accordance with a process similar to the prior art, wherein a dried high calcium sol gel was heated slowly and essentially uniformly from ambient temperature to 1 3700C. In particular, 10,199 grams of Condea Chemie Dispural boehmite was dispersed in 20.5 gallons of water and 573 ml of concentrated nitric acid was then added to form a sol (colloidal solution). 73.9 grams of titanium IV isopropoxide dispersed in 4150 ml of isopropyl alcohol and 36.5 grams of Nalco-Chemical NalcoageB 1034A colloidal silica dispersion were then mixed into the sol.

31 62 grams of magnesium nitrate was dissolved in 2 gallons of water and the resulting solution was added to the sol with stirring. Gelling occurred almost immediately. Stirring was continued for about 5 minutes.

The gel was then transferred to plastic trays at a depth of about 10 centimeters. The trays were placed in a steam heated dryer to dry the gel which took about 60 hours.

The dry gel was passed through a roll crusher to reduce it to -28 mesh granules. A size fraction -28 to +48 mesh was separated by sieving.

The dry granules were placed in aluminium oxide coated saggers and placed into a kiln. The kiln was heated to 1 3700C over a period of six hours and held at 1 3700C for 30 minutes. The kiln was turned off and allowed to cool. The resulting fired grains were porous and were analyzed as having a calcium content of 0.14 weight percent. The density was less than 85% of theoretical.

The sintered granules were size classified on a conventional sifter to meet the ANSI 74.18-1977 specification for 50 grit.

A single coated abrasive material was made by electrostatically coating the 50 grit grain on a vulcanized fiber backing.

The fiber selected was abrasive grade 0.030 inch vulcanized fiber, having a nominal weight of 67 pounds per ream (480-9 x 11 sheets).

A maker adhesive mix, consisting of a commercial one-stage, liquid phenolic resin with a formaidehyde to phenol ratio of about 1:1 and ground limestone with an average particle size of between 17 and 25 microns, was made using a 1:1 net weight mix proportion.

The maker mix was then heated to 900F and roll coated on the fiber backing. About 14 pounds of adhesive per ream was applied.

Using conventional sandpaper making equipment, the 50 grit abrasive was electrostatically projected onto the fiber carrying the maker mix with about 38 pounds per ream of grain being applied.

The abrasive adhesive coated bacl-ing was then heated to 1 750F for one hour and 2000 F for two hours in the maker rack. After drying, a size coat was then applied by standard roll coating methods with approximately 21 Ibs/ream being applied. The size mix consisted of the same 1:1 phenolic resin-filler ratio. However, a non-buffered synthetic cryolite with an average particle size of 25 microns was used as the filler. Drying and curing was then accomplished by heating the coated material for one hour at 1 500F, four hours at 1 750F, and 16 hours at 220F.

After curing the material was humidified in the conventional manner to a moisture content of less than 8% by weight. The material was then uniformly flexed and die cut into seven inch discs. These were then evaluated on a conventional pneumatic disc grinder using 101 8 cold rolled steel as a workpiece and compared to a control disc made and handled in exactly the same manner except the abrasive grain was fused alumina. In this test, the abrasive disc was placed on the disc grinder in the standard manner and a 1 x 2 x 11 inch workpiece was positioned so that it engaged the disc on the 1 inch flat side at a 10--150 angle. The disc was passed back and forth along the workpiece.

The abrasive disc in this test was rotated at a nominal 5400 RPH's on a hard rubber type back-up pad 7" in diameter. Eight pounds of dead weight in-feed force was exerted on the workpiece. Testing was for 30 seconds after which stock removed from the bar was measured (weight before grind-weight after grind) and recorded. This sequence was continued until the measured stock removed was 5 grams or less per grinding interval. Total stock removed in this manner for the test disc was compared to the total stock removed for the control disc. (Relative cutting performance).

The high calcium (0.14 weight percent) slowly heated sol gel abrasive (substantially in accordance with the prior art) had only 97% of the cutting performance of standard and inexpensive fused alumina abrasive grain.

EXAMPLE 2.

Example 1 was substantially repeated except that instead of slowly heating the dried granules as in Example 1., the granules were calcined at 5500C for about 30 minutes and were then introduced into a rotary tube furnace at 1390"C to rapidly heat them to above 1 2000C in accordance with the present invention. The granules (grains) were heated to 1 3900C in about 10 minutes and retained at 1 3900C for about 30 minutes to sinter the grains. Any other differences from Example 1 were minor. The resulting sintered grains were analyzed as having a calcium content of 0.11 weight percent and a specific gravity over 3.3 (greater than 85% of theoretical).Upon testing as described in Example 1., the discs were found to have a relative cutting performance which was 395 percent of the cutting performance of fused alumina abrasive grain. In other words, the grain of the invention had a cutting performance 3.95 times the cutting performance of fused alumina abrasive grain.

EXAMPLE 3.

Example 2 was substantially repeated except that the dried grains were calcined at 6000C instead of 5500 C. The finished grain was found to have a calcium content of 0.12 weight percent and a density in excess of 85% of theoretical. The relative cutting performance was 4.39 times the performance of fused alumina abrasive grain.

EXAMPLE 4.

Example 2 was substantially repreated except that the magnesium nitrate used was prepared by dissolving 720 grams of magnesium hydroxide in 1.66 gallons of water containing 1660 ml of concentrated nitric acid. The resulting solution was added to the sol while stirring. As in Example 2, gelling occurred immediately and stirring was continued for 5 minutes.

The resulting grain had a calcium content of near 0.08 weight percent, a density in excess of 85% of theoretical, and cutting performance about 4.5 times the cutting performance of fused alumina. In addition, the resulting grain had a cutting performance about 2.4 times the cutting performance of a commercial rapidly cooled fused alumina-zirconia grain and a cutting performance comparable to a commerical low calcium sintered sol gel alumina containing grain precoated on a commercial disc, which grain is similar to the low calcium and sodium grain described in U.S. Patent No. 4,314,827.

EXAMPLE 5.

A high calcium and sodium sol gel abrasive grain was made essentially in accordance with Example 2. In particular, 20,559 grams of Conden Chemie Dispural(D boehmite was dispersed in 33.5 gallons of water and 1250 ml of concentrated technical grade nitric acid diluted with 2 gallons of water was then added to the dispersion to form a sol (colloidal solution).

6341 grams of magnesium nitrate was dissolved in 4 gallons of water and the resulting solution was added to the sol with stirring. Gelling occurred almost immediately. Stirring was continued for about 5 minutes.

The gel was then transferred to plastic trays at a depth of between 2.5 and 3.75 centimeters. The trays were placed in an electrically heated dryer to dry the gel which took about 48 hours.

The dry gel was passed through a roll crusher to reduce it to -20 mesh granules. A size fraction -20 to +48 mesh was separated by sieving.

The granules were calcined at 5500C for about 30 minutes and were then fast fired in a rotary tube furnace at 1.2 revolutions per minute at about 1 3950C to rapidly heat them to above 1 2000C in accordance with the invention. The granules (grains) were heated to 1 3900C for an additional 10 minutes. The resulting grain had a calcium content of about 0.07 weight percent and a sodium content of 0.01 5 weight percent. The density was in excess of 85% of theoretical.

The sintered granules were size classified on a conventional sifter to meet the ANSI 74.1 8-1977 specification for 36 grit.

A single coated abrasive material was made by electrostatically coating the 36 grit grain on a vulcanized fiber backing.

A maker adhesive mix, consisting of a commercial one-stage, liquid phenolic resin with a formaldehyde to phenol ratio of about 1:1 and ground limestone with an average particle size of between 17 and 25 microns, was made using a 1:1 net weight mix proportion.

The maker mix was then heated to 900F and roll coated on the fiber backing. About 23 pounds of adhesive per ream was applied.

Using conventional sandpaper making equipment, the 36 grit abrasive was electrostatically projected onto the fiber carrying the maker mix with about 62 pounds per ream of grain being applied.

The abrasive adhesive coated backing was then heated to 1750Fforone hour and 200 Ffortwo hours in the maker rack. After drying, a size coat was then applied by standard roll coating methods with approximately 23 Ibs/ream being applied. The size mix consisted of the same 1:1 phenolic resinfiller ratio. Drying and curing was then accomplished by heating the coated material for one hour at 150 F, four hours at 1 750F, and 16 hours at 2250 F.

After curing the material was humidified in the conventional manner to a moisture content of less than 8% by weight. The material was then uniformly flexed and die cut into seven inch discs. Five of these discs were then evaluated on a conventional pneumatic disc grinder using quenched and tempered 4140 steel (hardness 285-320 BHN) as a workpiece and compared to a control disc made and handled in exactly the same manner except the abrasive grain was fused alumina. In this test, the abrasive disc was placed on the disc grinder in the standard manner and a 1 x 2 x 11 inch workpiece was positioned so that it engaged the disc on the 1 inch flat side at a 10-1 5O angle. The disc was passed back and forth along the workpiece.

The abrasive disc in this test was rotated at a nominal 5400 RPM's on a hard rubber type back-up pad 7" in diameter. Eight pounds of dead weight in-feed force was exerted on the workpiece. Testing was for 30 seconds after which stock removed from the bar was measured (weight before grind-weight after grind) and recorded. This sequence was continued until the measured stock removed was 5 grams or less per grinding interval. Total stock removed in this manner for the test disc was compared to the total stock removed for the control disc. (Relative cutting performance.) The high calcium and sodium (0.07 combined weight percent) rapidly heated sol gel abrasive had a mean cut of 612 grams (almost 7 times the cut of standard fused alumina). The results are shown in Table 1.

EXAMPLES 6-19.

The procedure of Example 5 was followed except that various known quantities of calcium and sodium were added to the dispersion prior to the addition of magnesium nitrate. The sodium and calcium were added as sodium and calcium nitrate solutions. The sodium nitrate solution contained 0.54 grams per ml of NaNO3, equivalent to 0.2 grams/ml calculated as Na2O, and the calcium nitrate solution contained 0.58 grams per ml of Ca(NO3)2, equivalent to 0.2 grams/ml calculated as CaO. The results are set forth in Table 1. The calcium and sodium in the grain are analyzed by emission spectroscopy.The slight variation between added calcium and actual calcium in the grain is believed due to additional calcium present in magnesium nitrate and in water and the slight variation between added sodium and actual sodium in the grain is believed to be due to impurities in the components of the grain, especially water, and to some vaporization of sodium during calcining and sintering.

For comparison, the mean cut of traditional fused alumina grain is set forth in Table 1.

TABLE 1 % Ca in % Na in Mean Cut EXAMPLE mls Ca++ mis Na+ Grain Grain Grams 5 -0- -0- 0.06 0.01 612.0 6 24.9 -0- 0.08 0.02 644.8 7 -0- 83 0.06 0.06 504.8 8 83 -0- 0.13 0.01 494.6 9 O- 166 0.06 0.12 437.6 10 24.9 83 0.08 0.07 424.0 11 166 -0- 0.18 0.04 424.0 12 83 166 0.13 0.13 383.2 13 373.5 -0- 0.33 0.02 369.8 14 166 166 0.18 0.12 326.4 15 166 270 0.19 0.21 275.4 16 --00- 270 0.06 0.18 270.0 17 83 270 0.13 0.20 242.4 18 -0- 481.5 0.06 0.27 197.2 19 373.5 481.5 0.36 0.36 185.4 alumina 88.2 EXAMPLE 20 Examples 7, 11 and 1 5 were repeated except that the grain was slowly fired in a manner similar to prior art Example 1. In particular, the grain was slowly fired in a stationary kiln from ambient temperature to 1 5000C over a time period of 1 6 hours followed by 30 minutes at 15000 C. The resulting abrasives were so poor that no cutting data could be obtained.

The foregoing examples clearly demonstrate the superiority of the cutting performance of high calcium and sodium sol gel abrasive grains manufactured in accordance with the process of the invention and show that when the step of rapidly heating dried high calcium and sodium sol gel to sintering temperature is omitted, as in the prior art, the resulting grain is an inferior grain.

Claims

1. A process for forming abrasive grains which comprises: a) preparing a dispersion comprising from about 2 to about 60 weight percent aluminium oxide monohydrate; a dissolved metal containing sintering aid in an atomic ratio of metal in the sintering aid to aluminium in the aluminium oxide monohydrate of from 1:2 to 1::35, and from above about .05 to about 1.8 weight percent sodium plus calcium by weight of dispersed and dissolved metal containing solids in the dispersion. provided that the weight percent calcium is from 0 to about 1.8 and the weight percent sodium is from 0 to about 0.4; b) gelling said dispersion; c) drying the gelled dispersion at a temperature below the frothing temperature of the gel to vaporize free water; d) crushing the dried solid to form grains; e) rapidly heating the grains to above about 1 2000C in less than 10 minutes; and f) continuing to heat the grains at a sintering temperature between about 1 2000C and about 1650 C for a sufficient sintering time to sinter the grains to a density above about 85% of theoretical density.

2. A process for forming abrasives having alpha-alumina grains with a diameter from about 5,000 Angstroms to about 200,000 Angstroms which comprises: a) preparing a dispersion comprising from about 2 to about 60 weight percent aluminium oxide monohydrate; a dissolved metal containing sintering aid in an atomic ratio of metal in the sintering aid to aluminium in the aluminium oxide monohydrate of from 1:2 to 1 ::35, and up to about 1.8 weight percent sodium plus calcium by weight of dispersed and dissolved metal containing solids in the dispersion, provided that the weight percent calcium is from 0 to about 1.8 and the weight percent sodium is from 0 to about 0.4; b) gelling said dispersion; c) drying the gelled dispersion at a temperature below the frothing temperature of the gel to vaporize free water; d) crushing the dried solid to form grains; e) rapidly heating the grains to above about 1 2000C in less than 10 minutes; and f) continuing to heat the grains at a sintering temperature between about 1 2000C and about 1 6500C for a sufficient sintering time to sinter the grains to a density above about 85% of theoretical density.

3. A process for forming abrasives having non-randomly oriented alpha-alumina grains which comprises: a) preparing a dispersion comprising from about 2 to about 60 weight percent aluminium oxide monohydrate; a dissolved metal containing sintering aid in an atomic ratio of metal in the sintering aid to aluminium in the aluminium oxide monohydrate of from 1:2 to 1::35, and up to about 1.8 weight percent sodium plus calcium by weight of dispersed and dissolved metal containing solids in the dispersion, provided that the weight percent calcium is from 0 to about 1.8 and the weight percent sodium is from 0 to about 0.4; b) gelling said diseersion; c) drying the gelled dispersion at a temperature below the frothing temperature of the gel to vaporize free water; d) crushing the dried solid to form grains; e) rapidly heating the grains to above about 1 2000C in less than 10 minutes; and f) continuing to heat the grains at a sintering temperature between about 1 2000C and about 1 6500C for a sufficient sintering time to sinter the grains to a density above about 85% of theoreti density.

4. A process accoridng to claim 2 or 3 in which the dissolved metal containing sintering aid has from above about 0.05 to about 1.8 weight percent sodium plus calcium by weight of dispersed and dissolved metal containing solids in the dispersion.

5. A process according to any one of the preceding claims further including, prior to the rapidly heating step, the step of calcining the grains.

6. A process according to any one of the preceding claims in which the atomic ratio of metal in the sintering aid to aluminium in the aluminium oxide monohydrate is from 1:7 to 1:25.

7. A process according to any one of the preceding claims in which the weight percent sodium plus calcium is less than 0.6.

8. A process according to claim 7 in which the weight percent sodium plus calcium is less than 0.15.

9. A process according to any one of the preceding claims in which the weight percent calcium is from 0 to about 0.6.

10. A process according to claim 9 in which the weight percent calcium is from 0 to about 0.15.

11. A process according to any one of the preceding claims in which the weight percent sodium is from 0 to about 0.25.

12. A process according to claim 11 in which the weight percent sodium is from 0 to about 0.1.

13. A process according to any one of the preceding claims in which the grains are rapidly heated to above about 1 2000C in less than 5 minutes.

14. A process according to any one of the preceding claims wherein the sintering aid is a water soluble compound of magnesium, zinc, nickel or cobalt.

1 5. A process for forming abrasive grains substantially as herein described with reference to any one of examples 2 to 1 9 inclusive.

1 6. A sintered abrasive grain composition produced by the process according to any one of the preceding claims.

1 7. An abrasive grain comprising alumina, a metal oxide, and up to about 1.8 weight percent sodium plus calcium, provided that the weight percent calcium is from 0 to about 1.8 and the weight percent sodium is from 0 to about 0.4.

18. An abrasive grain according to claim 17 having alpha-alumina grains with a diameter from about 5,000 Angstroms to about 200,000 Angstroms.

1 9. An abrasive grain according to claim 1 7 having non-randomly oriented aipha-alumina grains.