GRINDING AID MATERIAL, METHOD OF PRODUCING IT, USE OF IT, AND ABRASIVE MATERIAL CONTAINING IT

Abstract: A grinding aid for use in an abrasive material comprising abrasive grains and a grinding aid consists of particles obtainable by crushing a cold bath from the manufacture of metallic aluminium by reducing Al2O3. This grinding aid is produced by a method wherein the cold bath is crushed, optionally in several steps, whereby particles of an optional free metallic aluminium residue are rolled to flakes, following which the crushed material is screened to optionally sort out the formed aluminium flakes, and then the residual material is recovered as a product or is optionally subjected to further grinding and optionally screening to provide a material which preferably has a grain size distribution corresponding to 100 % by weight less than 100 micrometres. This grinding aid lends itself for use in the production of abrasive materials.

Description: Grinding aid material, method of producing it. use of it. and abrasive material containing it BACKGROUND OF THE INVENTION The present invention concerns a grinding aid consisting of fluoride-containing inorganic compounds for use in an abrasive material comprising abrasive grains and a grind¬ ing aid, a method of producing it, use of it as well as an abrasive material containing it. It is known to ιι«ip as a grinding aid in the abrasive composition in abrasive materials, such as sand¬ paper, emery cloth and grinding wheels, cf, e.g. KIRK- OTHMER: ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, Third Edi¬ tion, Vol. 10, 1980, line 672, and the mention of the prior art in WO 94/02562. WO 94/02562 concerns abrasive grains having an outer sur- face to which a grinding aid material is bonded via in- terparticle attraction. A large number of materials, i.a. cryolite, including synthetic cryolite, are mentioned as a grinding aid material. US Patent Specification No. 5 078 753 concerns an abra¬ sive material having on the abrading face, in addition to abrasive grains, erodable aggregates consisting of a finely ground filler and a resinous binder. The filler may be calcium carbonate, talc, glass, sodium sulfate, cryolite, fluoroborates, etc. Example V uses i.a. sodium cryolite. EP Patent Application No. 8 868 describes abrasive mate¬ rials consisting of brittle agglomerates of abrasive grains bonded in an inorganic matrix. The matrix may be based on natural cryolite, e.g. type S from KRYOLITSEL- SKABET 0RESUND A/S Danmark. These abrasive materials may be produced by melting natural cryolite, adding the abra¬ sive grains, cooling and crushing the cooled material. Grinding aids are used, because they have a positive in¬ fluence on the utility properties of the abrasive materi¬ als, i.a. grinding rate and product life. No specific knowledge is available about the mechanism of the product improvement obtained. However, it is assumed that the ef- feet of using cryolite as a grinding aid may be due to the circumstance that the chipping created by grinding locally takes place at very high temperatures, and that the cryolite acts as a flux in a melt produced by the chipping, whereby the viscosity of the melt is reduced. However, the effect of the cryolite may also be due to the circumstance that any tendency to block the gaps be¬ tween the abrasive grains is diminished. For many years KRYOLITSELSKABET 0RESUND A/S Danmark has supplied a ground natural cryolite for this purpose; thus, the company has supplied natural cryolite ground to -325 mesh, i.e. less than 45 micrometres, to the grinding disc industry. After the termination of the mining of cryolite in Green¬ land, natural cryolite is today no longer available in the market. A few producers (from Russia) sell grades which they call natural cryolite, but tests have shown that it is precipitated cryolite types. There are depos- its of natural cryolite in Russia, but, as far as is known, they are not suitable for working by flotation. Possibly, the Russian "natural cryolite" is made by ex¬ traction of such a deposit with subsequent precipitation. Thus, in the mid-1990s the cryolite types available in the market are generally synthetic, i.e. produced by a wet precipitation process, and it has been found that none of these commercially available cryolite types are suitable as a grinding aid in abrasive compositions. The difficulties involved by the use of synthetic cry¬ olite appear to be of two types: Firstly, there is an aesthetic problem, since the use of synthetic cryolite as a grinding aid results in undesir- able stripes in the coating on sandpaper, especially on the finer types. Further, it has been observed that the synthetic cryolite absorbs more binder than the natural cryolite on both fine and coarse sandpaper types. Secondly, there are productionally unacceptable varia¬ tions in the course of a production of e.g. sandpaper, there being thus an increasing particle separation and particle agglomeration tendency during the course of the production. Finally, it has been reported that users of the abrasive materials have experienced unspecified performance prob¬ lems. Through detailed studies the applicants have been able to demonstrate both chemical and morphological differences between natural cryolite and synthetic cryolite, i.e. cryolite produced by a wet precipitation process. The chemical differences have the result that melting point and melt composition will be different for the two types. This may be of importance e.g. to the function as a flux during the grinding process. The chemical composition of various cryolite types is ap- proximately as follows: F ( % ) Al ( % ) Na ( % ) Materials from Kryolitselskabet øresund: Natural cryolite E 54.3 12.9 32.9 Natural cryolite S 54.0 12.3 31.5 Russian materials: Synthetic neutral 53.5-54.5 14-15 28-29 Synthetic acid 55.5 16.7 24.8 Hungarian material: Synthetic 53.2 13.3 where "%" means % by weight. Pure, neutral cryolite has the composition 3 NaF . 1 AIF3 , i.e. an NaF/AlF molar ratio of 3. A cryolite having an excess of AIF3, i.e. having an NaF/AlFg molar ratio below 3, is called acid cryolite, while a cryolite having an excess of NaF, i.e. having an NaF/AlF molar ratio above 3, is called a basic cryolite. The natural cryolite is neutral, while all the synthetic cryolites are more or less acid. The so-called "natural cryolite" from Russia has the same composition as syn¬ thetic cryolite and a particle shape and size which can just have been obtained by precipitation from an aqueous solution. Microscopic studies of various cryolite types show con¬ siderable morphological differences, Generally, all the studied synthetic cryolites consist of extremely fine crystals of a quite regular size, typical particle size: 1-2 micrometres, which have agglomerated to particles of a size less than about 25 micrometres, while the natural cryolites are very coarse-grained individual crystals or crystal fragments of widely different particle sizes and with irregularly shaped particles. In other respects, i.a. flowability and dusting and lumping ability, there are just minor differences between the various cryolite types. Thus, it will be seen that the cryolite types suitable as grinding aids are chemically characterized by being neu¬ tral, while the unsuitable grades have been more or less acid. The problem underlying the invention is to provide a grinding aid which is suitable as substitute for the finely ground, naturally occurring, neutral cryolite used previously, and which does not exhibit the above-men¬ tioned drawbacks associated with the use of synthetic cryolite, a method of producing it, use of it for produc¬ ing abrasive materials, and abrasive materials containing it. SUMMARY OF THE INVENTION It has now surprisingly been found that this problem is solved by a grinding aid consisting of fluoride-contain¬ ing inorganic compounds for use in an abrasive material comprising abrasive grains and a grinding aid, character- ized in that it consists of particles obtainable by crushing a cold bath from the manufacture of metallic aluminium by reducing AI2O3. As will be known, metallic aluminium is manufactured on a large industrial scale by reduction in electrolysis cells of AI2O3, which has been dissolved in a melt produced by fusing cryolite together with minor amounts of calcium fluoride and other fluorides. Additional aluminium fluo¬ ride is added to adjust the chemical composition of the melt bath, and 5-10 % by weight of aluminium oxide, which is the starting material proper in the aluminium manufac¬ ture, is continuously dissolved in the melt. Such electrolysis baths occur as waste in the closedown of the electrolysis cells, e.g. because of repairs or be- cause of the build-up of impurities in the bath after an extended period of use. The solidified electrolysis bath mass is usually called "cold hath", and it constitutes a recycling problem, because the mass balance of the alu¬ minium works means that the bath amount increases in the course of time. The cold bath contains an acid mixture of various alumin¬ ium fluorides, other metal fluorides, minor amounts of metallic aluminium, optionally coal from the lining of the electrolysis cell and ceramic insulation material. An important feature of the cold bath is that it contains considerable amounts of chiolite, which has the formula 5 NaF . 3 AIF3, i.e. a compound having an NaF/AlF3 molar ratio of 5/3, together with cryolite. For example, the cold bath may typically contain 40-44 % by weight of cry¬ olite and 40-44 % by weight of chiolite. It has surprisingly been found that cold bath material after suitable preparation is useful as a substitute for natural cryolite, even though this cold bath material has an NaF/AlF3 ratio corresponding to an extremely acid cry¬ olite. Possible abrasive grains are particles of all the mate- rials previously used for this purpose, in particular particles of materials having a Mohs hardness of at least 8, preferably at least 9, e.g. selected from the group consisting of sintered, alpha alumina-based ceramic par¬ ticles, fused alumina particles, fused alumina-zirconia particles, diamond particles, boron nitride particles, silicon nitride particles, boron carbide particles and garnet particles as well as combinations of these. According to a preferred embodiment, essentially all the metallic aluminium present in the cold bath is removed. The content of metallic aluminium in the grinding aid is preferably less than 0.5 % by weight, in particular less than 0.1 % by weight. According to another preferred embodiment, the present grinding aid contains cryolite and chiolite in a total amount of at least 75 % by weight, preferably at least 80 % by weight. Preferably, the grinding aid contains chiolite in an amount of 20-60 % by weight, and it preferably has an NaF/AlF molar ratio of between 2.5 and 1.875. According to another preferred embodiment, an optional content of metallic aluminium in the grinding aid is pre- sent as particles having a particle size distribution corresponding to 100 % by weight less than 50 micrometres and 50 % by weight less than 20 micrometres. The particles in the grinding aid of the invention pref- erably have a grain size distribution corresponding to 100 % by weight less than 100 micrometres, preferably 100 % by weight less than 75 micrometres, in particular 100 % by weight less than 50 micrometres, especially corre¬ sponding to 100 % by weight less than 50 micrometres and 50 % by weight less than 20 micrometres. The grinding aid of the invention is preferably produced by a method comprising the following steps: an electrolysis cell used in the manufacture of metal- lie aluminium by reducing AI2O3 in a bath of fused sodium aluminium fluoride is cooled and broken to pieces which are suitable for preliminary sorting, following which the available free aluminium and graphite are sorted out, and then - the residual material, i.e. the cold bath, is subjected to crushing, optionally in several steps, whereby par¬ ticles of an optional free metallic aluminium residue are rolled to flakes, following which the crushed material is screened to optionally sort out the formed aluminium flakes, and then - the residual material is recovered as a product or is optionally subjected to further grinding and optionally screening to provide a material having a grain size dis¬ tribution corresponding to 100 % by weight less than 50 micrometres and 50 % by weight less than 20 micrometres. The present invention moreover concerns a method of pro¬ ducing such a grinding aid, which is characterized by comprising the following steps: - an electrolysis cell used in the manufacture of metal¬ lic aluminium by reducing AI2O3 in a bath of fused sodium aluminium fluoride is cooled and broken to pieces suit¬ able for preliminary sorting, following which - the available free aluminium and graphite are sorted out, and then the residual material, i.e. the cold bath, is sub¬ jected to crushing, optionally in several steps, whereby particles of an optional free metallic aluminium residue are rolled to flakes, following which the crushed material is screened to optionally sort out the formed aluminium flakes, and then the residual material is recovered as a product or is optionally subjected to further grinding and optionally screening to provide a material which preferably has a grain size distribution corresponding to 100 % by weight less than 100 micrometres. The present invention also concerns the use of such a grinding aid for producing an abrasive material. The present invention moreover concerns an abrasive mate¬ rial comprising abrasive grains and a grinding aid con- sisting of fluoride-containing inorganic compounds, char¬ acterized in that the grinding aid consists of particles obtainable by crushing a cold bath from the production of metallic aluminium by reducing AI2O3. Preferred embodiments of the abrasive material of the in¬ vention correspond to the embodiments which are described above in connection with the description of the preferred grinding aids. An essential feature of the present method is that it may comprise a process step in which the metallic aluminium is provided in a form which permits all the metallic alu¬ minium to be removed by screening. This may take place by a number of crushing and grinding methods, which are de- scribed more fully in the following examples. After removal of metallic aluminium, the screened mate¬ rial may be subjected to further grinding or be used in the form it has, depending on the particle size which is advantageous for the contemplated application. For grinding discs and coarse sandpaper, use may prefer¬ ably be made of a material which has been crushed so that the greater part of the material has a size which is less than 100 micrometers. For finer sandpaper, the grain size of the material must be less than 75 miπrnmetres, For the finest grades of sandpaper, it has been found that the grain size must be less than 50 micrometres. A particularly preferred grain size distribution is: 4-20 % by weight less than 5 micrometres; 30-70 % by weight less than 15 micrometres; 75-97 % by weight less than 30 micrometres. GENERAL PART The invention will be illustrated more fully below by a number of examples: EXAMPLE 1 A sample of a cold bath from the aluminium works ISAL in Iceland was crushed in a jaw crusher. Hereby, the par¬ ticles of metallic aluminium present in the cold bath were in the form of flat, rolled flakes which could eas¬ ily be sorted out by screening. The residual material was subjected to chemical analysis, which showed: 51.09 % by weight of NaOH-soluble F and 15.15 % by weight of total Al. Composition: Cryolite: 44.9 % by weight; Chiolite: 46.4 % by weight; CaF 2 : 4.1 % by weight; A1 2 0 3 : 2.3 % by weight; inactive: 2.3 % by weight; Free Al: 0 % by weight. The residual material was then subjected to grinding in a first grinding step in a swing mill of the Siebtechnik make, in which it was ground to a particle size of 100- 150 micrometres. After screening, the product was sub¬ jected to further grinding in a second grinding step in a ball mill, producing a product of 85 % by weight under 45 micrometres and nothing over 100 micrometres, and a spe- cific surface area (determined by BET) of 0.551 m* cm 3 . 18 kg of this product were used for pilot production of a plurality of sandpaper types. The results showed that this material did not exhibit the drawbacks which were characteristic of the synthetic cryolite, but, on the contrary, had the same excellent performance as the natu¬ ral cryolite used previously. EXAMPLE 2 The starting material desribed in example 1 was treated as described in example 1, but with the second grinding step performed in an AFG 400 Jet Mill, following which the ground material was screened on an Alpine ATP 100 air screen with a flow of material of 500-600 kg/hour. The resulting product, which exhibited just as good util¬ ity properties as the product described in example 1, had a specific surface area (determined by BET) of 0.694 m^ /cm 3 and the following grain size distribution: Nothing over 100 micrometres, 50 % by weight less than 15 mi¬ crometres. EXAMPLE 3 The starting material described in example 1 was treated as described in example 1, but with the second grinding step performed in a pilot ball mill of the Alpine make, following which the ground material was screened on an Alpine ATP 100 air screen with a flow of material of 30 kg/hour. The resulting product, which exhibited almost just as good utility properties as the products described in ex- amples 1 and 2, had a specific surface area (determined by BET) of 0.723 m 2 /cm 3 and the following grain size dis¬ tribution: Nothing over 100 micrometres, 50 % by weight less than 16 micrometres. EXAMPLE 4 A sample of a cold bath from an aluminium works in Bah¬ rain was crushed in a ring mill. Hereby, the metallic aluminium particles present in the cold bath were in the form of flat rolled flakes which could easily be sorted out by screening on a 103 micrometre screen. The material was subjected to chemical analysis, which showed: 46.32 % by weight of NaOH-soluble F and 15.61 % by weight of total Al before crushing. 46.76 % by weight of NaOH-soluble F and 14.28 % by weight of total Al after crushing and screening. Composition before crushing: Cryolite: 42.1 % by weight; Chiolite: 40.7 % by weight ; CaF 2 : 6.5 % by weight; AI2O3 : 2.9 % by weight; inactive: 9.2 % by weight; Free Al: 1.5 % by weight. Composition after crushing and screening: Cryolite: 42.5 % by weight; Chiolite 41.1 % by weight; CaF 2 : 6.5 % by weight; AI2O3: 2.9 % by weight; inactive: 7.0 % by weight; Free Al: 0 % by weight; NaF/AlF3 molar ratio: 2.22. This product was used for pilot production of a plurality of sandpaper types. The results showed that this material did not exhibit the drawbacks which are characteristic of the synthetic cryolite, but, on the contrary, had the same excellent performance as the ground natural cryolite used previously. EXAMPLE 5 The starting material described in example 1 was treated as described in example 1, but using in the second grind¬ ing step a Jet Mill AJ 100 with a flow of material of 48 kg/hour. In the subsequent air screening, the overfrac- tion contained some free metallic aluminium which was sorted out. The screened product had the following screen curve, ex¬ pressed in % by volume: 97 % less than 35 micrometres; 50 % less than 14 micrometres; 10 % less than 5 micrometres. This product showed just as good results in the produc¬ tion of fine-grained sandpaper as the product described in example 1.

Claims: PATENT CLAIMS: 1. A grinding aid consisting of fluoride-containing in¬ organic compounds for use in an abrasive material com- prising abrasive grains and a grinding aid, c h a r a c t e r i z e d in that it consists of par¬ ticles obtainable by crushing a cold bath from the manu¬ facture of metallic aluminium by reducing AI2O3. 2. A grinding aid according to claim 1, c h a r a c ¬ t e r i z e d in that essentially all the metallic alu¬ minium present in the cold bath is removed. 3. A grinding aid according to claims 1-2, c h a r a c - t e r i z e d in that the content of metallic aluminium in the grinding aid is less than 0.5 % by weight, pref¬ erably less than 0.1 % by weight. 4. A grinding aid according to claims 1-3, c h a r a c - t e r i z e d in that it contains cryolite and chiolite in a total amount of at least 75 % by weight, preferably of at least 80 % by weight. 5. A grinding aid according to claims 1-4, c h a r a c - t e r i z e d in that it contains chiolite in an amount of 20-60 % by weight. 6. A grinding aid according to claims 1-5, c h a r a c ¬ t e r i z e d in that it has an NaF/AlF molar ratio of between 2.5 and 1.875. 7. A grinding aid according to claims 1-6, c h a r a c ¬ t e r i z e d in that an optional content of metallic aluminium in the grinding aid is present as particles having a particle size distribution corresponding to 100 % by weight less than 50 micrometres and 50 % by weight less than 20 micrometres. 8. A grinding aid according to claims 1-7, c h a r a c - t e r i z e d in that its particles have a grain size distribution corresponding to 100 % by weight less than 100 micrometres, preferably 100 % by weight less than 75 micrometres, in particular 100 % by weight less than 50 micrometres. 9. A grinding aid according to claim 8, c h a r a c ¬ t e r i z e d in that its particles have a grain size distribution corresponding to 100 % by weight less than 50 micrometres and 50 % by weight less than 20 microme- tres. 10. A grinding aid according to claims 1-9, c h a r a c ¬ t e r i z e d in that it is produced by a method com¬ prising the following steps: an electrolysis cell used in the manufacture of metal¬ lic aluminium by reducing AI2O3 in a bath of fused sodium aluminium fluoride is cooled and broken to pieces which are suitable for preliminary sorting, following which the available free aluminium and graphite are sorted out, and then the residual material, i.e. the cold bath, is sub- jected to crushing, optionally in several steps, whereby particles of an optional free metallic aluminium residue are rolled to flakes, following which the crushed material is screened to optionally sort out the formed aluminium flakes, and then the residual material is recovered as a product or is optionally subjected to further grinding and optionally screening to provide a material having a grain size dis¬ tribution corresponding to 100 % by weight less than 50 micrometres and 50 % by weight less than 20 micrometres. 11. A method of producing a grinding aid according to claims 1-10, c h a r a c t e r i z e d by comprising the following steps: an electrolysis cell used in the manufacture of metal¬ lic aluminium by reducing I2O in a bath of fused sodium aluminium fluoride is cooled and broken to pieces suit¬ able for preliminary sorting, following which the available free aluminium and graphite are sorted out, and then the residual material, i.e. the cold bath, is sub- jected to crushing, optionally in several steps, whereby particles of an optional free metallic aluminium residue is rolled to flakes, following which the crushed material is screened to optionally sort out the formed aluminium flakes, and then the residual material is recovered as a product or is optionally subjected to further grinding and optionally screening to provide a material which preferably has a grain size distribution corresponding to 100 % by weight less than 100 micrometres. 12. Use of a grinding aid according to claims 1-10 for producing an abrasive material. 13. An abrasive material comprising abrasive grains and a grinding aid consisting of fluoride-containing inorganic compounds, c h a r a c t e r i z e d in that the grind¬ ing aid consists of particles obtainable by crushing a cold bath from the manufacture of metallic aluminium by reducing AI2O3. 14. An abrasive material according to claim 13, c h a r a c t e r i z e d in that essentially all the metallic aluminium present in the cold bath is removed. 15. An abrasive material according to claims 13-14, c h a r a c t e r i z e d in that the content of metal¬ lic aluminium in the grinding aid is less than 0.5 % by weight, preferably less than 0.1 % by weight. 16. An abrasive material according to claims 13-15, c h a r a c t e r i z e d in that the grinding aid con¬ tains cryolite and chiolite in a total amount of at least 75 % by weight, preferably at lest 80 % by weight. 17. An abrasive material according to claims 13-16, c h a r a c t e r i z e d in that the grinding aid con¬ tains chiolite in an amount of 20-60 % by weight. 18. An abrasive material according to claims 13-17, c h a r a c t e r i z e d in that the grinding aid has an NaF/AlF3 molar ratio of between 2.5 and 1.875. 19. An abrasive material according to claims 13-18, c h a r a c t e r i z e d in that the content of metal¬ lic aluminium in the grinding aid is present as particles having a particle size distribution corresponding to 100 % by weight less than 50 micrometres and 50 % by weight less than 20 micrometres. 20. An abrasive material according to claims 13-19, c h a r a c t e r i z e d in that the particles in the grinding aid have a grain size distribution corresponding to 100 % by weight less than 100 micrometres, preferably 100 % by weight less than 75 micrometres, in particular 100 % by weight less than 50 micrometres. 21. An abrasive material according to claim 20, c h a r a c t e r i z e d in that the particles in the grinding aid have a grain size distribution corresponding to at least 100 % by weight less than 50 micrometres and 50 % by weight less than 20 micrometres. 22. An abrasive material according to claims 13-21, c h a r a c t e r i z e d in that the grinding aid is produced by a method comprising the following steps: an electrolysis cell used in the manufacture of metal¬ lic aluminium by reducing AI2O3 in a bath of fused sodium aluminium fluoride is cooled and broken to pieces suit¬ able for preliminary sorting, following which the available free aluminium and graphite are sorted out, and then the residual material, i.e. the cold bath, is sub¬ jected to crushing, optionally in several steps, whereby particles of an optional free metallic aluminium residue are rolled to flakes, following which the crushed material is screened to optionally sort out the formed aluminium flakes, and then the residual material is recovered as a product or is optionally subjected to further grinding and optionally screening to provide a material having a grain size dis- trinsvis 100 vaegt-% mindre end 75 micron, isaer 100 vaegt-% min¬ dre end 50 micron. 21. Slibemateriale ifølge krav 20, k e n d e t e g n e t ved, at partiklerne i slibehjaelpemidlet har en komstørrelsesforde- ling svarende til 100 vasgt-% mindre end 50 micron og 50 vaegt-% mindre end 20 micron. 22. Slibemateriale ifølge krav 13-21, k e n d e t e g n e t ved, at slibehjaelpemidlet er fremstillet ved en fremgangsmade omfattende følgende trin: - en elektrolysecelle anvendt ved fremstilling af metallisk aluminium ved reduktion af A1 2 0 3 i et bad af smeltet natriumalu- miniumfluorid afkøles og brydes i stykker, der er egnet til grovsortering, hvorefter det tilgsngelige fri aluminium og grafit frasorteres, hvorpa - det tilbagevaerende materiale, dvs. koldbadet, underkastes knusning, eventuelt i flere trin, hvorved partikler af eventu- elt resterende frit metallisk aluminium udvalses til flager, hvorefter - det knuste materiale sigtes, saledes at de dannede aluminium- flager eventuelt frasorteres, hvorpa - det tilbageblevne materiale udtages som produkt eller eventu¬ elt underkastes yderligere formaling og eventuelt sigtning til dannelse af et materiale med en kornstørrelsesfordeling svaren¬ de til 100 v3»gt-% mindre end 50 micron og 50 vaegt-% mindre end 20 micron.

Inventors: KAABER HENNING, KAABER, HENNING
Applicants: KAABER HENNING, WOLFF & KAABER, KAABER, HENNING, WOLFF & KAABER A/S
Application year: 1995
Main classification: C09K3
IPC codes: C01F7, C09K3