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Method of Producing an Aluminium Boride

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Method of Producing an Aluminium Boride Europaisches Patentamt European Patent Office J) Publication number: D130 016B1 Dffice europeen des brevets © EUROPEAN PATENT SPECIFICATION © Date of publication of patent specification: 02.05.91 © Int. CI.5: C01B 35/04, C04B 35/58 © Application number: 84304044.5 © Date of filing: 15.06.84 © Method of producing an aluminium boride. ® Priority: 24.06.83 GB 8317243 © Proprietor: MOLTECH Invent S.A. 68-70 Blvd. de la Petrusse @ Date of publication of application: L-2320 Luxembourg(LU) 02.01.85 Bulletin 85/01 @ Inventor: Reeve, Martin Rebekoff © Publication of the grant of the patent: 543, Cedar Street 02.05.91 Bulletin 91/18 Beaconsfield Quebec(CA) © Designated Contracting States: BE DE FR GB IT NL SE © Representative: Pennant, Pyers et al Stevens, Hewlett & Perkins 5 Quality Court © References cited: Chancery Lane EP-A- 0 099 948 London, WC2A 1HZ(GB) GB-A- 1 192 677 CQ CO CO O Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition Uj shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid (Art. 99(1) European patent convention). Rank Xerox (UK) Business Services 1 EPO 130 016 B1 2 Description present invention there is provided a method of producing an aluminium boride in solid form, which This invention relates to a method of producing method comprises providing a body of molten alu- an aluminium boride. The term "aluminium boride" minium containing suspended particles of an alu- as used herein is to be understood as including 5 minium boride, passing the body of molten alumin- both aluminium diboride (AIB2) and aluminium ium through a filter which is pervious to the molten dodecaboride (AIB12), both of which are potentially aluminium but which retains the suspended par- useful precursors for production by reaction sinter- ticles, the filter medium being carbon in the form of ing of refractory composites containing titanium a plate or compact of pressed unbonded or lightly diboride (TiE^). Examples of reactions based on 10 sintered refractory aggregate, or a bed of loose AIB2 and AIB12 which have been reported include granular aggregate supported on a refractory alu- the following:- mina grill, and recovering the aluminium boride in 1. TiN + AIB2 = TiB2 + AIN for production of solid form from the filter by converting the carbon TiB2/AIN composite. filter medium to methane. 2. TiH2 + AIB2 = TiB2 + Al + H2 for produc- 15 Aluminium-boron alloys are commercially avail- tion of TiB2/AI composite. able, and can simply be heated to provide the 3. 3TiH2 + AIB2 + 1/3 AIB12 = 3TiB2 + 4/3 Al desired starting body of molten aluminium contain- for production of TiB2/AI composite. ing suspended particles of an aluminium boride. However, efforts to develop a commercially The industrial process for manufacturing viable method for manufacture of TiB2 - containing 20 aluminium-boron alloys uses a coreless induction materials based on reactions such as the above are furnace, mains frequency being preferred. A pre- handicapped by the restricted commercial availabil- determined quantity of potassium borofluoride ity and consequent high price of AIB2 and AIB12. KBF4. powder is fed via a screw feeder or other The proposed method of the present invention is suitable device on to the surface of the molten Al intended to produce these materials at lower cost 25 whilst supplying power to the induction coils to stir using commercially available raw materials and, as the melt. Boron (B) contents of commercial alloys far as possible, production processes in common are usually either nominally 3% or 4% and B industrial use. recovery from KBF4. is typically between 80 and GB-A-1, 192,677 describes a method of prepar- 90%. The temperature of the Al determines wheth- ing aluminium diboride flakes and which comprises 30 er the boride is produced as AIB2, which is the low adding up to 5% of a boron source to a bath of temperature phase, or AIB12. The transition tem- molten aluminium. The bath is maintained at an perature from AIB2 to AIB12 as reported in the ° elevated temperature, not exceeding 975 C, for a literature used to be somewhat uncertain, e.g. Mon- time sufficient to permit solubilization of the boron dolfo has reported approximately 1200°K (927° C) source and crystallization of the aluminium diboride 35 whereas Willey (ASTM Metals Handbook, 1973) flakes. The flakes may be recovered by, for exam- reported approximately 1350° C. The experience of ple, filtering the melt through a perforated graphite master alloy manufacturers indicates the latter to plate. This disclosure is concerned solely with the be too high. AIB2-containing alloys are produced at production of aluminium diboride in flake form and temperatures within the range 750 - 850° C, typi- there is no suggestion to remove the filter medium 40 cally 800 - 850° C. AIBi2-containing alloys are pro- by selective chemical attack. duced at 950 - 1200° C, typically 1000 - 1100° C. According to one embodiment of the present Microstructural examination of the cast materials invention there is provided a method of producing indicates that such alloys do not contain borides an aluminium boride in solid form, which method exclusively as AIB2 or AIB12 but predominantly so, comprises providing a body of molten aluminium 45 i.e. 90% or more of borides are in one or other containing suspended particles of an aluminium form. Particle size is typically within the range 5-50 boride, passing the body of molten aluminium microns, with some particles up to 100 microns. through a filter which is pervious to the molten The higher the B content the more difficult it aluminium but which retains the suspended par- becomes to produce a molten castable alloy at the ticles, the filter medium being alumina in the form 50 low temperatures suitable for AIB2 production, be- of a plate or compact of pressed unbonded or cause of the rapid increase in melt viscosity. Until lightly sintered refractory aggregate, or a bed of relatively recently no alloy containing 4% B was loose granular aggregate supported on a refractory produced with a predominantly AIB2 structure, but alumina grill, and recovering the aluminium boride such an alloy is now commercially available. The in solid form from the filter by dissolving the alu- 55 maximum B content which could be introduced into mina filter medium in acid under conditions in aluminium, whilst still maintaining adequate fluidity which the aluminium boride is insoluble. even at the high temperatures quoted above with a According to a second embodiment of the predominantly AIB12 structure is probably no more 2 5 EP 0 130 016 B1 than 5 or 6%. the filter medium is of T1B2, AIB2, AIB12, BN, UN, After alloying is completed the furnace is AIN or similar materials, it may be possible to use slightly tilted to pour off a molten potassium alu- the precipitated aluminium boride without separa- minium fluoride reaction product. tion. Thus for example a TiB2 filter carrying European Patent Specification 99948 A de- 5 precipitated AIB2 or AIB12 can be used for reaction scribes a method of separating high-purity alumin- with TiN or TiH2 for production of a TiB2/AIN or ium, by deposition on a rotating cooling body, from TiB2/AI composite. Alternatively, a TiN filter carry- a melt of aluminium containing aluminium boride. ing precipitated AIB2 can be used directly in reac- The remaining melt contains an enhanced con- tion (1) above. Filter media carrying precipitated centration of aluminium boride, and may thus be 10 aluminium borides may also contain residual Al particularly suitable for use in the second stage of metal. The latter can if necessary be removed, e.g. the method of this invention. by leaching with a dilute mineral acid, in order to The second stage of the method involves sepa- enable the filter medium to be mixed with the other rating the suspended aluminium boride particles powdered reactants. from the bulk of the molten metal. In theory, it is When the filter medium is alumina or carbon, it might be possible to effect separation by allowing will normally be desired to recover the aluminium the particles to settle and then decanting the super- boride from the filter. This may be effected by natant liquid, but this is likely to be time consum- dissolving the filter medium in a solvent in which ing. The separation therefore involves a filtration the aluminium boride is insoluble. step in which the body of molten metal is poured 20 Thus, for example, smelter grade alumina is into a heated filtration vessel. This may comprise readily attacked by mineral acids such as HCI, an upright tapered cast iron or steel shell lined with HNO3 and H2S04. AIB2 is reported to be insoluble alumina or other suitable refractory resistant to in HNO3 and H2SO4, and these acids therefore molten Al. If desired, reduced pressure can be offer a means of separating AIB2 from an alumina applied on the downstream side of the filter using a 25 filter medium. AIB1 2 is reported to be insoluble in vacuum pump, to draw the metal through the filter HCI and H2SO4, and these acids therefore offer a and into a receiving vessel. The metal can then, if means of separating AIBi2 from an alumina filter desired, be recycled to the induction furnace for medium. preparation of a fresh batch of the alloy. AIB2 is reported to be soluble in HCI, while As filter media, there may be used refractory 30 AIB12 is reported to be soluble in HNO3.
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