FIELD OF THE INVENTION

The present invention relates to a method of manufacturing alumina based sintered alumina abrasives. In particular, the present invention relates to a simplified process for making sintered alumina abrasives from an economical precursor such as bauxite. The product derived from the process has high true density and superior abrasive properties. The sintered alumina abrasives produced can be employed as both coated and bonded abrasives.

BACKGROUND OF THE INVENTION

Alumina abrasive grains have been utilized in abrasive applications for close to one hundred years. The two most common techniques for production of aluminous abrasive materials are the arc furnace technique and the sintering technique. In the arc furnace technique, the final product is generally known as a fused abrasive. In the sintering technique, the final product is generally known as a sintered abrasive. Both the fused abrasive materials and the sintered abrasive materials may be utilized in similar types of products. In the arc furnace technique, the final abrasive material product is called fused because it results from a melting operation. Typically, such operations are costly, particularly because of the energy-intensive nature of the process. Such operations may also be dangerous due to the extremely high temperatures of fabrication involved.

The alumina based fused abrasives such as brown fused alumina, white fused alumina etc. are often used for rough grinding as well as precision grinding. The well-known process of arc fusion followed by crushing and grading are used for the manufacturing of fused abrasives. This top down process is very energy intensive. The energy security of twenty first century raises question on the viability of this process. In case of brown fused alumina (~ 95% alumina), bauxite is fused at around 2200°C with a reducing agent such as coke/carbon. This reducing agent helps to reduce the impurities present in bauxite to A desirable level and separate the excess impurities as a metallic by-product. This process works well when the alumina content is moderately higher level (greater than 82%). However, due to the continuous depletion of bauxite quality across the globe i.e., decrease of the alumina content in bauxite, viability of this process is under scanner.

Sintered abrasives have been manufactured in the past in an attempt to reduce not only the operating temperatures required, but also the attendant potential hazards and the expense of making the abrasive grain particles of unwanted sizes. One example of such a sintered abrasive is given in US 3079243. In order to achieve a high density in the unsintered grain, said patent specifies that milled bauxite is compacted under affirmative pressure of the order of five tons per square inch, followed by formation of grain size agglomerates which are sintered. The use of aflirmative pressures requires the purchase and maintenance of presses or machinery which will exert said affirmative pressure to cause agglomeration and compaction.

US3491492 describes a process for making sintered abrasive grains in which calcined bauxite, or bauxite and Bayer process alumina mixtures are provided in a milled aqueous slurry having a high concentration of the solid by weight by virtue of the presence of ferric ammonium citrate, or ferric ammonium citrate and citric acid, thereafter drying this slurry to a coherent plate of controlled thickness, breaking this plate to yield cuboidal grains, screening the grains to obtain the sizes wanted, optionally rounding the edges and corners of the grains by air milling, screening the grains for size, sintering, cooling, and finally screening for size.

US4574003 discloses a sol-gel process for forming a dense, solid, alumina-based ceramic, comprising preparing an aqueous dispersion of aluminum oxide monohydrate containing a precursor of a modifying additive which is a metal-containing compound in the form of a strongly oxidizing, soluble salt, gelling the dispersion, drying the gelled dispersion to form a solid, calcining the solid, and sintering the calcined solid, wherein the process comprises adding to the aqueous dispersion at least about 10 weight percent, based upon the oxide equivalent of the soluble salt precursor, and at least one densification aid.

US4568363 discloses a method of making sintered aluminous abrasives from uncalcined bauxite. The uncalcined bauxite is mixed with sulphuric acid and dissolved. This forms aluminum sulfate hydrate as a major product. The aluminum sulphate hydrate is then dried to form aluminum sulphate which is then calcined to form gamma alumina and sulphur trioxide. A slurry of the gamma alumina is prepared, poured into a container and then dried to form an unfired body. The unfired body is then broken into particles and screened to size. Following this, the particles are sintered.

US5782940 discloses a method of converting the preferred hydrated alumina oxide, boehmite, into a green particle which is then sintered to form an abrasive alumina particle. The process involves dispersing boehmite in water at a slightly elevated temperature with an acidic pH, then seeding it with alumina seed particles and drying it, then crushing to desired shapes and sizes, calcining the dried precipitate and sintering the calcined precipitate by microwave.

US6814917 discloses a process for producing a polycrystalline alumina sintered body which includes the steps of: subjecting alumina powder to ultrasonic irradiation, mechanical stirring not using a grinding medium, or ultrasonic irradiation resulting in slurry dispersed in a solvent; drying and forming the slurry to produce a green body; and then sintering the green body in an air atmosphere at a temperature in the range of 1400° C. to 1800° C; wherein the raw material is alumina powder which has a purity of 99.99 wt % or more and includes a alumina particles having polyhedral shape, having substantially no red surface and a D/H ratio of from 0.5 or more to 3.0 or less; the number-average particle size of from 0.1 urn or more to 1.0 urn or less; and a D90/D10 ratio of 7 or less.

Most of the processes described in the prior art utilize sol-gel process, or involve use of metal oxide precursors. In general, the viscosity of the dispersion or sol in the sol-gel process increases as its solids content increases, and a common optional step in conventional sol gel processes for making abrasive grain is crushing the dried gel. Typically, the crushed material does not provide a single grade, or size range, of dried particles, but rather a large distribution of particle sizes. Further, the addition of metal oxide precursors tends to increase the viscosity of the dispersion even beyond their effect on the solid content of the dispersion. Such increase in viscosity tends to increase, for example, the difficulty in providing a homogenous mixture of components in the dispersions. Further, the compaction processes such as pressing, extrusion which is used is an essential step which is again highly energy intensive. Also, most of the processes involve the use of expensive, very high quality alumina monohydrates and use of less expensive materials requires special processing steps.

Hence, there still exists a need for an alternate process to produce product from the lower alumina bearing bauxite as well as less energy intensive process is the biggest demand of the century. Improvement with respect to the density achieved of the resulting product also needs to be addressed.

The present invention discloses a simplified process of manufacturing sintered alumina abrasives starting from an economical precursor such as bauxite with consumption of considerably lower energy. The process does not require compaction and the abrasives particles are made at green stage which needs much lower energy. It also has highest tolerability of accommodating wide range of bauxite of different alumina content. The product made out of this process has adequate density which results in superior abrasive properties and small crystal size, and high hardness.

SUMMARY OF THE INVENTION

One aspect of the present invention provides a method for manufacturing alumina based sintered alumina abrasives comprising the steps of:
providing a mixture of calcined bauxite and reactive alumina; wet milling of the mixture;
adjusting the pH of the mixture to an acidic range; drying the slurry obtained in step c) followed by crushing and screening of particles;
calcining the screened particles; impregnating the calcined particles; and sintering the calcined particles.

Another aspect of the present invention relates to grinding wheels, coated abrasive products, bonded abrasive products comprising the alumina abrasive grains manufactured by the process of the present invention.

BREIF DESCRIPTION OF FIGURES

Further characteristics and advantages of the invention shall be evident from the following description of several preferred embodiments, given as indicative and not limiting, with reference to the attached figures, wherein:

Figure 1 demonstrates the flow diagram of sintered alumina manufacturing process representing one of the embodiments of the present invention.

Figure 2 is a comparison of abrasiveness of the sintered alumina abrasive grains prepared according to the process of the present invention with standard (Standard BFA which is the regular brown fused alumina, alumina ~ 95.32%).

DETAILED DESCRIPTION OF THE INVENTION

For the purposes of the following detailed description, it is to be understood that the invention may assume various alternative variations and step sequences, except where expressly specified to the contrary. Moreover, other than in any operating examples, or where otherwise indicated, all numbers expressing, for example, quantities of ingredients used in the specification are to be understood as being modified in all instances by the term "about". It is noted that, unless otherwise stated, all percentages given in this specification and appended claims refer to percentages by weight of the total composition.

Thus, before describing the present invention in detail, it is to be understood that this invention is not limited to particularly exemplified systems or process parameters that may of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments of the invention only, and is not intended to limit the scope of the invention in any manner.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. In the case of conflict, the present document, including definitions will control.

It must be noted that, as used in this specification and the appended claims, the singular forms "a," "an" and "the" include plural referents unless the content clearly dictates otherwise. Thus, for example, reference to a "solvent" may include two or more such solvents.

The terms "preferred" and "preferably" refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the invention.

As used herein, the terms "comprising" "including," "having," "containing," "involving," and the like are to be understood to be open-ended, i.e., to mean including but not limited to.

The following definitions are used in connection with the present application unless the context indicates otherwise. As used herein,

The term "abrasive' refers to a substance used for abrading, grinding, polishing, lapping.

The term "bauxite" refers to a soft, whitish to reddish-brown rock consisting mainly of hydrous aluminum oxides and aluminum hydroxides along with silica, silt, iron hydroxides, and clay minerals. Bauxite forms from the breakdown of clays and is a major source of aluminum.
The term "Reactive
alumina" refers to a relatively high purity and small crystal size (<1 um) alumina which sinters to a fully dense body at lower temperatures than low soda, medium-soda or ordinary-soda aluminas. These powders are normally supplied after intensive ball-milling which breaks up the agglomerates produced after calcination. They are utilised where exceptional strength, wear resistance, temperature resistance, surface finish or chemical inertness are required.

The term "ball mill" refers a grinding mill which includes a shell having an axis, means for rotating this shell about its axis and a charge of material located within the shell. Such a charge of material normally consists of a plurality of particles serving to facilitate grinding as well as a charge of material to beg round. Commonly balls, pebbles, rods or the like are utilized as the grinding media in a mill of this type. The shell in a mill of this type is normally of a cylindrical configuration, however, various conically shaped shells and shells of other configurations are used in grinding mills falling within the broad classification of ball mills.

The term 'Wet grinding" refers to a milling process used to produce powder or paste from a solid using a liquid such as water. It can also be used in an abrasive process to reform hard objects.

The term "sinter" refers to coalesce into a single mass without actually melting.

The term "friability index" is an inverse measure of toughness. More specifically, this term indicates cross sectional reduction in size of grain which has been subjected to carefully controlled, uniform milling conditions. The comparative friability of conventional abrasives is usually evaluated by a standard comminution test wherein a sample of relatively coarse (for instance #16 or # 24 grit) material is ball milled under prescribed conditions. The friability index of the abrasive indicates the degree of fragmentation caused, which is defined as the percentage of milled material passing through a #16 mesh sieve.

Further, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention pertains.

The present invention relates to a method for manufacturing alumina based sintered alumina abrasives.

providing a mixture of calcined bauxite and reactive alumina; wet milling of the mixture;
adjusting the pH of the mixture to an acidic range; drying the slurry obtained in step c) followed by crushing and screening of particles;
calcining the screened particles; impregnating the calcined particles; and sintering the calcined particles.

The process of the present invention starts with crushing and screening of calcined bauxite to a particle size range of from about 100 to 200 micron, more preferably 120-160 micron. Desired amount of reactive alumina is added to the batch to obtain the desired level of alumina and titania content in the final product. The bauxite content of the mix can be varied from 10 to 90 % and balance is reactive alumina. The alumina content of the final product may range from about 75 to 98%, more preferably 85-95%. The alumina content of this milled bauxite may be determined by chemical analysis. Chemical analysis of milled bauxite of a representative sample employed for the purposes of the present invention has been provided in Table 1.

This mixture of milled bauxite and reactive alumina batch is subjected to wet milling in a ball mill and the desired amount water is also added to the ball mill for wet grinding and homogeneous mixing of ingredients. The water and powder ratio may be maintained in the range of 1:3 to 3:1. This mixture is milled for certain duration to bring down the particle size to less than about 5 microns.

Trace amounts of flocculating agent such as alum, ferric chloride, ferric sulfate, ferric ammonium citrate etc. may be added to the mix before discharging from the ball mill. Concentration of the flocculating agent may be in the range of 1 to 5%. pH of the slurry is suitably controlled between 2-5 in order to control the flocculation process by adding dilute organic or inorganic acids such as acetic acid, oxalic acid, citric acid, nitric acid, hydrochloric acid etc, or their mixtures. These additives can be added at any time during ball milling, more preferably at the end of milling before discharge. The output particle size of the particles obtained after milling is in the range of 1 to 5 microns.

The milled slurry is then filled into a stainless steel tray. The fine particles are allowed to settle and form a thick layer at the bottom of the tray. The water gets separated from solid and stays at the top of the solid layer. This water is then evaporated out in a hot oven where temperature is maintained at about 50 to 90 °C or more preferably 60 to 75 °C. Higher temperature may cause boiling of water which will also agitate the solid layer. Alternatively, filter press can be used to remove excess water and balance can be driven out by drying.

The dried solid layer forms a strong cake which is then crushed into different particle size range. The moisture of the dry cake plays a very important role of generating sharp edges of the crushed particles. The moisture may be suitably controlled between 1 to 5 %.

Crushed particles are then calcined at a temperature ranging from 950 to 1250 °C with duration of about 30 minutes to 2 hours. At this stage all volatiles get removed and particles acquire sufficient strength. Calcined particles are then impregnated with various additives such as magnesium chloride, manganese chloride, copper chloride, nickel chloride, lanthanum chloride, neodymium chloride etc. These additives can also be used as metal nitrates or sulfates. Dilute solution of at least one metal chloride or combination of them may be used. The salt concentration may be from about 5 to 15 wt %. The calcined material is soaked into the salt solution for a duration of about 5 to 30 minutes and then excess solution is separated out. This salt solution gets absorbed in the calcined materials especially sits in the porous space created during calcination. Impregnated grain is then dried in hot oven at about 110 to 150 °C for about 2 to 10 hours.

Dry impregnated grain is then sintered at high temperature in suitable furnace. The sintering temperature can be varied from about 1300 to 1600°C, more preferably from 1350 to 1500 °C and the soaking time can range from about 30 minutes to about 10 hours. The sintering furnace can be a muffle furnace, tunnel kiln, rotary furnace etc. Sintering step is very important to determine the final product properties. Depending upon the comical compositing, sintering schedule has to be altered to obtain the desired density i.e., 3.65g/cc to 3.90g/cc. The metal salts convert into their corresponding oxides upon heating and react with the alumina and other impurities to form low melting phases which helps in faster densification of the product.

By the use of the process of the present invention, it is possible to prepare a new variety of alumina abrasive grains at considerably lower energy with small crystal size, high hardness and density. The alumina particles are used for both coated and bonded abrasives. The sintered alumina abrasive produced using the process of the present invention has a true density in the range of about 3.60 to 4.98 g/cc, more preferably, 3.70 to 3.95 g/cc, micro hardness in the range of about 17 to 23 GPa, preferably, 18 to 21 GPa; and a friability index in the range of about 10 to 30, more preferably in the range of 14 to 25.

The sintered alumina grains produced according to the process of the present invention may be employed in the manufacture of products such as coated abrasive disks and grinding wheels. Alternatively, the abrasive particles of the invention may be used in coated abrasive products, bonded abrasive products, such as segments, and sharpening stones, which are comprised of a bond and sintered abrasives. The abrasive products are suitable for grinding all types of metal such as various steels like stainless steel, cast steel, hardened tool steel, cast irons, for example ductile iron, malleable iron, spheroidal graphite iron, chilled iron and modular iron, as well as metals like chromium, titanium and aluminum. As is the case with all abrasives and the bonded or coated products containing them, the abrasive and bonded products of the invention will be more effective grinding some metals than others and will be more efficient in some grinding applications than in others.

In order to illustrate the invention more clearly, the following examples are given explaining the preferred modes of carrying it into effect and the advantageous results obtained thereby. The use of examples in no way limits the scope and meaning of the invention or of any exemplified term. Likewise, the invention is not limited to various embodiments given in this specification.

EXAMPLES

Example 1
500g of bauxite powder ( size ~150 micron) and 750 g of reactive alumina (size -10 micron) are added to the ball mill. 400 ml of water was added to the mixture. Ball milling was continued for 46 hours. 25 g of ferric chloride was dissolved in water separately and added to the slurry. 60 ml of dilute oxalic acid (concentration 50wt. %) was added to the slurry to control the pH at 4. Ball milling was continued for another 2 hours for homogeneous mixing. The slurry was then discharged from ball mill and filled into stainless steel tray where solid particles get settled at the bottom and form a strong cake. Excess water was removed by drying at 70 °C for 12 hours. Dry cake was crushed and screened; particle between 1mm to 0.3 mm were taken up for calcination and fines were recycled. Calcined particles were then impregnated by soaking in 10 wt% lanthanum chloride solution for 30 minutes. Impregnated particles were then dried at 70 °C and followed by sintering at 1500 °C for 3 hours.
Sintered grains were characterized for density and abrasive properties. The true density was achieved 3.71 g/cc, hardness of 18.5 GPa and the friability index was 16% for grit F16. The density and micro-hardness variation with respect to sintering temperature is given in Table 2.

Example 2:

500g of bauxite powder (size -150 micron) and 750 g of reactive alumina (size -lOmicron) were added to the ball mill. 400 ml of water was added to the mixture. Ball milling was continued for 46 hours. 25 g of ferric ammonium citrate was dissolved in water separately and added to the slurry. 60 ml of dilute citric acid (concentration 50 wt. %) was added to the slurry to control the pH at 4. Ball milling was continued for another 2 hours for homogeneous mixing. The slurry was then discharged from ball mill and filled into stainless steel tray where solid particles get settled at the bottom and form a strong cake. Excess water was removed by drying at 70 °C for 12 hours. Dry cake was crushed and screened; particles between 1mm to 0.3 mm were taken up for calcination and fines were recycled. Calcined particles were then impregnated by soaking in 10 wt% lanthanum nitrate solution for 30 minutes. Impregnated particles were then dried at 70 °C and followed by sintering at 1550 °C for 3 hours.

Sintered grains were characterized for density and abrasive properties. The true density was achieved 3.78 g/cc, hardness of 19.2 GPa and the friability index was 14.5% for grit F16.

Example 3:

500g of bauxite powder (size -150 micron) and 750 g of reactive alumina (size -10 micron) were added to the ball mill. 40 ml of water was added to the mixture. Ball milling was continued for 70 hours. 25 g of ferric ammonium citrate was dissolve in
water separately and added to the slurry. 60 ml of dilute citric acid (concentration 50 wt
%) was added to the slurry to control the PH at 4. Ball milling was continued for another 2 hou f0r h0 ogeneous m.x.ng The shny ^ ^ discha^ ^ ^
filed into stainless steel tray where solid particles get settled at the bottom and fcnn a
strong cake. Excess water _ remoyed ^ ^ ^ 7qoc om ancf m a crushed and screened; particle between 1mm to 0 * *S
- were rm calcined pm:;zz :™7 *.-—-
ucu impregnated by soaking in 10 wt%
manganese chloride solution for 30 minutes. Impregnated particles were then dried at 70°C and followed by sintering at 1550°C for 3 hours.

Sintered grains were characterized for density and abrasive properties. The true density was achieved 3.76 g/cc, hardness of 19.55 GPa and the friability index was 13.70% for grit F24.

Example 4:

500g of bauxite powder (size -150 micron) and 750 g of reactive alumina (size -10 micron) were added to the ball mill. 400 ml of water was added to the mixture. Ball milling was continued for 46 hours. 25 g of ferric ammonium citrate was dissolved in water separately and added to the slurry. 60 ml of dilute citric acid (concentration 50 wt. %) was added to the slurry to control the pH at 4. Ball milling was continued for another 2 hours for homogeneous mixing. The slurry was then discharged from ball mill and filled into stainless steel tray where solid particles get settled at the bottom and form a strong cake. Excess water was removed by drying at 70 °C for 12 hours. Dry cake was crushed and screened; particle between 1mm to 0.3 mm was taken up for calcination and fines were recycled. Calcined particles were then impregnated by soaking in a solution containing 5 wt% lanthanum chloride and 5 wt% manganese chloride for 30 minutes. Impregnated particles were then dried at 70 °C and followed by sintering at 1550 °C for 3 hours.

Sintered grains were characterized for density and abrasive properties. The true density was achieved 3.81 g/cc, hardness of 20.88 GPa and the friability index was 14% for grit F24. Abrasiveness of the grains is evaluated using pulverization test. The detail description of test procedure is as described in US patent No. 8043392 B2. The pulverization test results of the 4 samples of these examples are given in table 3.

WE CLAIM:

1. A method for manufacturing alumina based sintered alumina abrasives comprising the steps of:
providing a mixture of calcined bauxite and reactive alumina;
wet milling of the mixture;
adjusting the pH of the mixture to an acidic range;
drying the slurry obtained in step c) followed by crushing and screening of particles;
calcining the screened particles;
impregnating the calcined particles; and
sintering the calcined particles.

The method as claimed in claim 1, wherein the bauxite content of the mix is from 10 to 90%.

The method as claimed in claim 1, wherein bauxite has a particle size of 100 to 200 microns.

The method as claimed in claim 1, wherein the alumina content of the mixture of calcined bauxite and reactive alumina is in the range of 85 to 95%.

The method as claimed in claim 1, wherein the wet milling is carried out in a ball mill in the presence of water.

The method as claimed in claim 5, wherein the ratio of water to the mixture of calcined bauxite and reactive alumina is in the range of 1:3 to 3:1.

The method as claimed in claim 5, wherein trace amount of a flocculating agent is added to the mix before discharging from the mill.

The method as claimed in claim 7, wherein the flocculating agent may be selected from alum, ferric chloride, ferric sulfate, or ferric ammonium citrate.

The method as claimed in claim 7, wherein concentration of the flocculating agent may be in the range of 1 to 5%.

The method as claimed in claim 1, wherein output particle size of the particles obtained after wet milling is in the range of 1 to 5 microns.

The method as claimed in claim 1, wherein pH of the mixture is adjusted to acidic range in step c) with dilute organic or inorganic acids selected from acetic acid, oxalic acid, citric acid, nitric acid, hydrochloric acid.

The method as claimed in claim 1, wherein drying of the slurry in step d) is carried out at 50 to 90 °C.

The method as claimed in claim 1, wherein moisture content of the screened particles obtained in step d) ranges between 1 to 5%.

A method as claimed in claim 1, wherein calcination in step e) is carried out at a temperature of 950 to 1250 °C.

A method as claimed in claim 1, wherein the impregnating agent for impregnation in step f) may be selected from metal chlorides, nitriates, sulfates or their mixtures.

A method as claimed in claim 14, wherein the impregnating agent is selected from magnesium chloride, manganese chloride, copper chloride, nickel chloride, lanthanum chloride, or neodymium chloride.

A method as claimed in claim 1, wherein concentration of the impregnating agent may range from about 5 to 15%.

A method as claimed in claim 1, wherein sintering is carried out in a muffle furnace, tunnel kiln, or a rotary furnace.

A method as claimed in claim 1, wherein the sintering temperature is in the range ofl350°Ctol500°C.

Grinding wheels, coated abrasive products, and bonded abrasive products comprising the alumina abrasive grains manufactured according to the process as claimed in claim 1.