GB2186588A - Granular abrasive material - Google Patents

Abstract

A granular abrasive material is produced from a dispersion consisting of raw materials containing alumina, compounds containing silicic acid and other additives. The dispersion is ground to a sinterable slip with a grain size of less than 1 micrometer. The slip is dried and may be pressed before being subjected to a multi-stage heating process to produce sintered corundum crystals.

Description

SPECIFICATION Granular abrasive material The present invention relates to granular abrasive material and, more specifically, to a method of producing it.

Fused alpha-aluminium oxide corundum produced in electric smelting processes, for example in electric are furnaces, is the major material used for the production of abrasive tools by the entire abrasives industry.

The raw materials used to produce the normal fused forms of corundum are bauxite, prepared either directly from the natural state or chemically into calcined alumina, as well as additives, e.g.

reduction coke and scrap iron. The calcined alumina is extracted by thermal processing from aluminium oxide primarily obtained by the Bayer process. The calcined alumina contains varying amounts of alpha-aluminium oxide and representatives of gamma-aluminium oxides depending on calcining temperature and time.

Grinding tools, made with the bauxite or alumina formsofcorundum obtained from melts,when tested under established test conditions, give a specific time-related degree of abrasion and are found to have a specific service life, which can be measured in terms of the volume or weight of material ground away. Improvements ofthe grinding performance of normal forms of fused corundum have been achieved by thermal after-treatment processes (blue calcination of bauxite forms of corundum) or by alloying with other metal oxides, e.g. chromium oxide orzirconium oxide. Fed.Rep.German Patent Specification No. 22 27 642, describes fused corundum consisting of aluminium oxide and zirconium oxide in a eutectic composition (approximately 57 weight%AI203 and 43 weight % ZrO2).The material has a two-phase, microcrystalline solidification structure, obtained by the spontaneous cooling ofthe melt. This material, which will be referred to as "Zirkonkorund", has a superior grinding performance (time-related wear and service life) relative to the normal fused forms of corundum. The high cost of zirconium oxide and the expensive process for the necessary fast cooling, make granular "zirkonkorund" from fiveto sixtimes more expensive than normal fused corundum.

The increased performance of "Zirkonkorund" abrasive material as compared with normal corundum falls abruptly when grinding metal materials, such as steel, as the grain size is reduced and substantially levels off at a grain size of P 80. This result is also observed with other high performance abrasives.

A highly effective granular, abrasive corundum based on fused aluminium oxide is described in Fed.Rep.German Patent Application No. 3219 607.

This high grade granularfused corundum abrasive material is made by dispersing very fine crystalline aluminium oxide monohydrate in nitric acid and water containing other diss'olved metal-containing sintering aids to produce a gel which, after careful drying, is pre-reduced down to abrasive grain size.

The material is then calcined at a temperature between 250"C and 800"C to remove the chemically bound waterandthe acidic residues, primarily nitrogen oxides, which are extremely toxic and damaging to the environment.

The granular material is then heated to sintering temperatures of up to 1 6500C until a density of at least 85% of the theoretical density is achieved.

Similar methods of production of granular abrasive, corundum materal are described in European Patent Application No.0 024 099A and U.S. Patent Specification No.4518397. These methods require that the dispersed aluminium oxide monohydrate used as the raw material can only be contaminated with a maximum of 0.05% by weight of alkali or alkaline earth metal ions.

In the method described in European Patent Application No.0 152 768A the colloidal solution or gel is additionally ground in a vibration grinding mill, as a result ofwhich a sintered product of increased density and with no large surface areas on the particles is produced. The material is formed of homogeneously orientated alpha-aluminium oxide crystallites. It is common to all the four methods mentioned above thatthey can be carried out only by a colloidal solution/gel process with a very finely dispersed aluminium oxide monohydrate of boehmite type. These relatively expensive raw materials can only be obtained by means ofthe hydrolysis of aluminium organic compounds.This together with the costly process technology results in such colloidal solution/gel abrasives costing several times more than normal sintered forms of corundum.

Although sintered corundum material, e.g. tabular alumina, costs less it has a definitelyworse abrasion performance relative to fused forms of corundum and is therefore quite unsuitableforgeneral use in grinding tools.

The present invention provides a method of producing granular abrasive material comprising the steps of: forming a dispersion of material containing alumina, compounds containing silicic acid and other metal-containing additives, grinding the dispersion to a particle size of less than 1 micrometer to produce a sinterable slip, drying the dispersion, and subjecting said slip to a multi-stage heat treatment.

Preferably, the dried ground slip is compacted in a press.

The multi-stage heat treatment preferably includes three stages. Inthefirststagethe dried ground slip is preheated to about 250 - 60000. In the second stage the preheated slip is further heated to about 1100 140000 and held at this temperature for 10 to 30 minutes. lnthethirdstagetheslipisfurtherheatedto about 1400 to 170000 and sintered to a density of more than 85% ofthe theoretical density of corundum such asto produce corundum crystals including both alpha-aluminium oxide and a silicatic phase, and the diameter ofthe corundum crystals is less than 5 micrometers.

Advantageously, the diameter ofthe corundum crystals is less than 1 micrometer.

In an alternative embodiment the multi-stage heat treatment consists of two stages. The dried group slip is preheated in the first stage to about 250 - 600 C, and in the second stage, to about 1400 1700"C.

The present method uses raw materials that are advantageous in cost term and which are subjected to a definite ceramic preparation, so that a definite calcining curve is kept to, in order to produce a sintered corundum of very finely crystallized form which has at least 85% of the theoretical density of corundum. The resulting granular abrasive material contains as its principal constituent alpha-aluminium oxide, and as accessory constituents, a silicatic phase as well as at least one crystalline compound of bivalent, trivalent or quadrivalent metais or a combination thereof. The term "accessoryconstituents" meansthattheirsum does not exceed 45% by weight. The crystalline compounds may be simple or compound oxides, such as spinels.They can either be distributed as separate phases in the matrix e.g. zirconium oxide, or also be fully or partly dissolved in the corundum lattice, e.g. chromium oxide. The silicatic phase may be present fully or partly as glass.

Thecorundumcrystalsofthegranularabrasive material should have a diametersmallerthan 5 micrometers, preferably smalierthan 2 micrometers, and advantageouslysmallerthan 1 micrometer. The crystallographic axes of the material are randomly distributed.

In a sintered colloidal solution/gel abrasive, for example as described in Fed.Rep.German Patent Application No.32 19 607, the crystallites over areas of 0.5 - 20 micrometers are homogenously orientated. This restriction disappears when the present method is employed since a colloidal solution and subsequent gelling are not used.

Moreover, the raw materials in the present method do not have to be capable of forming such a solution.

The material produced by the present method is differentiated from other sintered aluminium oxides by its regular, very fine crystalline structure and the particular multiphase composition which gives the abrasive grain its increased toughness and outstanding abrasive properties, and thus makes it a high performance abrasive grain with superior grinding properties. Forthe production ofthe abrasive material it is possible to use simple and economic raw materials, e.g. aluminium hydroxide orcalcined alumina obtained from it, either alone, or in a mixture. There are no restrictions as regards purity, as demanded in the method described in European Patent Application No.0 024 099A or fineness orspecificsurface, as demanded in the references given above and also in Fed.Rep.German PatentApplication No.32 19607.The calcined alumina can contain alpha-aluminium oxide in amounts of O to 98%.

The alumina-containing raw materials are used togetherwith 0.3 to 8weight%, preferably 1 weight %ofSiO2, as well as with 0.2 to 1 2weight%, preferably 1 weight% of a spinel-binding bivalent metal oxide, or another compound ofthe corresponding metal, and possibly further additives.

The dispersion produced from these raw materials is subjected to wet grinding. The indications given are calculated as percentages by weight of the corresponding oxides and refer to the amount of finished abrasive.

The grinding process can take place in an aqueous suspension or in a suspension in an organic liquid and is carried out until substantially all the particles of raw material employed have sizes smaller than 1 micrometer, preferably, however, smaller than 0.1 micrometer. "Substantially all" means in this specification more than 95% relative to the overall volume of solid material. Any grinding process may be used which supplies the required degree of fineness.

The ground material, dried orfreed from organic solvents is then subjected to the actual sintering process either directly or at the end offurther mixing and compacting processes. Preferably the material is compacted to dry pressing which is preferably carried out isostatically. Drying may be effected at temperatures between 50 and 600"C, preferably between 100 and 1 60"C. The reduction oftheformed or unformed material to abrasive grain size may be effected both before and after the completion ofthe sintering process.

The ceramic calcining ofthe lumpy or comminuted,formed or unformed material to sintered abrasive grain is effected in several steps. In the first heating stage the material is carefully brought to a temperature between 250 and 600"C, and held at the latter temperature for some minutes.

This step is used for the expulsion of the chemicaliy bound water orforthe burning out of any organic components. Thereafter, the material is quickly brought to a temperature between 1100 and 1 400"C, again held atthis level for lotto 30 minutes, and then heated quickly to a temperature between 1400 and 1700 C, preferably 1450 - 1 550"C and sintered to a density of over 85% of the theoretical density. If the starting raw materials contain no aluminium hydroxideAl(OH)3,the second step may be omitted and the material may be heated directlyfromthefirst calcination stage to a final sintering temperature.

Calcination temperatures hig her than suggested, lengthy sintering periods and slow rates of heating up reduce the abrasion performance ofthefinished material. The superiority of the sintered abrasive grain produced by this method over conventional fused corundum will be illustrated inthefollowing exampleswhich are not intendedto showthe entire range of methods falling within the scope ofthe invention.

Example 1 A mixture of 2000 g calcined alumina,1000 lOOOgof aluminium hydroxide, 42 g quartz powder, 130 g magnesium oxide,5 litreswaterand250 m160% acetic acid, is subjected to intensive grinding in a ball mill to prepare a slip in which substantially all the particles have a size smallerthan 0.1 micrometers.

The slip is carefully dried in an electrically-heated drier. This slip thus dewatered is pulverised and calcined for 45 minutes at 500"C. Subsequently, from this powder, and with the aid of an isostatic press, at a pressure of 2 Kbar, lumps are produced which are then heated in an electrically-heated laboratory furnace. The furnace is brought up from ambient temperature to 600C, in about 60 minutes and then quickly heated upto 1300"C, in about 10 minutes and held at this temperature for 20 minutes. Then, the temperature is raised to 1 500"C in less than 5 minutes and the lumps are calcined for a further30 minutes.After cooling, the density is established at 93% ofthe theoretical density and the lumps are crushed in a jaw crusher. From the crushed material a granular material abrasive is sieved outto grain size P36 of the FEPA standard, and processed in the usual manner as an abrasive on a support. Forth is purpose a support of vulcanised fibre as available in the trade of 0.84 mm thickness is provided with a binding agent. The binder consists of some 50% of a liquid phenol-resol with a phenol to formaldehyde ratio of about 1:1.5 and a solids ratio of about 80%, as well as of some 50% ground chalk with an average particle size of about 20 micrometers.It is applied by means of a doctor blade coating method in an amountof about 230 g persq.m.Subsequently, according to a process usual forthe production of abrasives on a support, the abrasive grain of P36size is electrostatically deposited on the vulcanised fibre coated with resin, in an amount of approximately 900 grammes per square metre. Thecoated support is then dried and hardened according to a temperature pattern usual for this purpose.Subsequently, by means of roller coating, a second binding coating is applied in an amount of approx. 490 g/m2. Forthe second coating the same binder system is used as for the basic coating but some 50% by weight ofthe chalk is replaced by synthetic cryolite. The coated vulcanic fibre is subsequently heated for30 minutes to 90"C, 60 minutes to 1 OO"C, 30 minutes to 110 C,30 minutes to 1200C, andthenfinallyabout60 minutes to 130 C, so that the binder system is hardened.After drying, the abrasive on vulcanised fibre support is evenlyflexibilised and discs of 125 mm diameter are punched out, to be reclimatised in the usual manner to a moisture level of less than 8%.

Thevulcanisedfibregrinding discs thus obtained were tested on a commercially available high frequency plate grinding apparatus against cold-rolled fine metal plates of CK45-03 (DIN 17200) of size 500 x 100 x 2 mm. Forthis purpose the grinding disc is inclined with an angle of attach of 10 degrees and rotated at a speed of 6500 r.p.m.,five times per cycle for a duration of 9.5 seconds each time over the lone edge of the sttel plate.

Subsequently the amount of cuttest material is determined by weighing. The contact pressure is initiallyatthestartofthetest40 N and is increased with each new cycle by 5N to a constant loading of 60 N. The test is continued until less than lOg is cut in a cycle. The total of metal removed isthenthegrinding performance of the test disc in grammes. For comparison purposes, a vulcanised fibre grinding disc is made with normal fused corundum of P36 grain size. This disc is otherwise identical to the other disc and is made in the same way. The comparison disc ways tested under the same conditions. The grinding performance of this disc is taken to be 100% forthe relative comparison.

The disc manufactured with sintered abrasive grain according to the method of invention achieved agrinding performance of 350% compared with the grinding performance ofthe comparison disc coated with normal fused corundum.

Example 2 A starting mixture is prepared from 2500 g calcined alumina, 50 g quartz powder, 150 g magnesium oxide, 6 litres water and 240 ml 90% acetic acid. From this mixture a granular abrasive material according to the method of Example 1 is produced. The resulting material has a density of 94% of the theoretical density. The material is processed in a similar manner to Example I to produce vulcanised fibre discs, and tested. On testing,the abrasive performance achieved is 374% ofthe grinding performance ofthe comparison disc coated with normal corundum.

Example 3 The method of Examples 1 and 2 is (essentially) repeated but with a mixture of 2500 g calcined alumina, 35g quartz powder, 75 g zirconium silicate, 150 g magnesium oxide, 5 litreswaterand 240 ml 90% acetic acid. The isostatically compressed lumps are heated slowly to 6000C and then quickly to 1 2500C and kept there for 25 minutes. Then the temperature is raised quicklyto 1450 C, andthe lumpsare sintered for 30 minutes to a density of 93% ofthe theoretical density. The grinding test is carried out in the manner already described and yields a grinding performance of 384% of that of a vulcanised fibre grinding disc coated with normal corundum.

Example 4 Following the method of Example 1, using a starting mixture of 2500 g calcined alumina, 40 g quartz powder, 125 g magnesium oxide, 225 grammes citric acid and 4 litres of water, a ground slip is prepared with particle sizes of mainly less than 0.1 micrometers. The slip is carefully dried for 24 hours. During this time the suspension shrinks to a solid which is soft yet nonetheless brittle. The individual lumps are crushed in a jaw crusher, and the fraction ofthe crushed material comprised between 0.5 and 1 mm is separated. The sieved material is packed into an aluminium oxide crucible and heated slowly in an electrically-heated furnace from ambient temperature to 500 Cand held atthat level for 100 minutes. Then the temperature is quickly raised in 15 minutesto 1500 C and kept constantfor45 minutes. The sintered material is hard and resistant and has a density of 95% ofthe theoretical density of corundum. With the granular abrasive material of FEPA P 36 size obtained, vulcanised fibre abrasive discs are made according to the method of Example 1. The grinding testyields a result of 381% of the grinding performance ofthe comparison disc coated with normal corundum.

Example 5 Agranularabrasive material is made according to the method of Example 4 using a starting mixture of 2500 g calcined alumina, 45 g quartz powder, 125 g magnesium oxide, 225 g citric acid and 4 litres water.

On this occassion the sintering temperature is only 1450 C. The grinding test supplies a performance of 414% of that of the comparison disc coated with normal corundum, and 135% ofthe grinding performance of a vulcanised fibre abrasive disc coated with "Zirconkorund".

Example 6 The method of Example 5 is repeated but with 50 instead of 45 g quartz powder. The precrushed ground material is heated slowlyfor8 hours from ambient temperature to 1 500"C a nd th ere sintered for 12 hours. The finished granular abrasive material has a density of 97% ofthetheoretical density of corundum and a crystallite diameter of more than 1 micrometer. The grinding test yields a grinding performance of 289% ofthe grinding performance of a vulcanised fibre disc coated with normal corundum and still 95% ofthe grinding performance of a grinding disc coated with "Zirkonkorund".

The described method may also be used forthe production of ceramic lumps from al readysintered aluminium oxide. In this particular case the subsequent crushing of the lumpsto abrasive grain size is eliminated.

Claims (12)

1. Amethodofproducing granularabrasive material comprising the steps of: forming a dispersion of material containing alumina, compounds containing silicicacid and other metal-containing additives, grinding the dispersion to a particle size of less than 1 micrometerto produce a sinterableslip, drying the dispersion,and subjecting said slip to a multi-stage heattreatment.

2. A method as claimed in claim 1, wherein said sinterable slip has a particle size of less than 0.1 micrometer.

3. A method as claimed in claim 1 or2,wherein said slip is compacted in a press.

4. A method as claimed in any one ofthe preceding claims, wherein said materials containing alumina are selected from the group consisting of calcined alumina with a content in alpha-aluminium oxide of O to 98% by weight, a I u minium hydroxides, and mixtures thereof and further compounds of the metals silicon, zirconium, titanium, chromium, iron, magnesium, zinc, cobalt and nickel a lone or in combination.

5. A method as claimed in any one ofthe preceding claims, wherein said heattreatment is carried out in three stages, wherein said first stage includes preheating said slip to 250 - 600"C, the second stage includes further heating said slip to 1100 - 1400"Cand maintaining this temperature for a period of 10-30 minutes, and said third stage includes further heating said slip to 1400 - 1700into sinter itto a density of more than 85% ofthe theoretical densityofcorundum such asto produce corundum crystals including both alpha-aluminium oxide and also a silicatic phase, and the diameter of said corundum crystals is less than 5 micrometers.

6. A method as claimed in claim 5, wherein the diameterofsaid corundum crystals is less than 1 micrometer.

7. A method as claimed in claim 5 or 6, wherein in the finished granular abrasive material the proportion of silicatic phase is from 0.3to 10% by weight, which phase may be a glass.

8. A method as claimed in any one of claims 5to 7, wherein said abrasive granular material comprises furthersimple or complex metal oxides dissolved or dispersed in the corundum matrix in addition to said alpha-aluminium oxide and silicatic phase.

9. A method according to claim 8,wherein the proportion of said metal oxides is from 0.2 to 45% by weight.

10. A method as claimed in any one of claims 1 to 4, wherein said heat treatment is carried out in two stages, wherein said first stage includes preheating said slip to 250 - 600"C, and the second stage includes further heating said preheated slip to 1400 1700"C.

11. Amethodofproducing granularabrasive material substantially as herein described with reference to any ofthe accompanying examples.

12. A granular abrasive material produced by a method as claimed in any one ofthe preceding claims.