Method of Producing an Aluminium Boride

Europaisches Patentamt European Patent Office J) Publication number: D130 016B1 Dffice europeen des brevets

® 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

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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. Hence, materials which are not attacked by the Al-B alloy. treatment with either HCI or HNO3 offers a means Possible materials include the borides and nitrides for separating AIB2 from AIB12 if this is desired. of groups lllb, IVb, Vb and VIb metals, particularly Aluminium borides can be separated from a TiB2, AIB2, AIB12, BN, TiN and AIN; and these granular carbon filter by converting the latter to materials may have advantages in some circum- 35 methane. This may be effected by exposing the stances as described below. Preferred filter media carbon to hydrogen gas at elevated temperatures are alumina and carbon. Either can be in the form in the presence of a finely divided nickel catalyst. of a plate or disc-shaped compact of porous or Such a process can conveniently be effected in a foamed material pervious to molten Al. However, it controlled atmosphere horizontal tube furnace. The is preferred to provide the filter media in the form 40 finely-divided metallic nickel can be subsequently of a bed or compact of granular material. The most separated from the aluminium borides by exposure preferred filter medium is a plate or disc-shaped to a stream of carbon monoxide gas which con- compact of pressed unbonded or lightly sintered verts the former to gaseous nickel carbonyl, from refractory aggregate. Alternatively, the filter may which the nickel can subsequently be recovered comprise a bed of loose granular aggregate sup- 45 and re-used. ported on a refractory alumina grill. The granular Alternatively, it may be possible to separate aggregate is preferably of a particle size to pass the greater part of the AIB2 from the filter medium through a 3 mesh sieve. If the filter medium is not by selective screening as the filter medium would wetted by Al, as is the case for alumina and normally be considerably coarser. carbon, then the filter openings may need to be 50 somewhat coarser, and/or a higher pressure dif- ference may need to be applied across the filter to cause the Al to flow through it. Claims On completion of the pour, the filter and resi- due are allowed to cool and removed from the 55 1. A method of producing an aluminium boride vessel. (which term includes aluminium diboride and The third stage of the method involves recov- aluminium dodecaboride) in solid form, which ery of the aluminium boride in solid form. When method comprises providing a body of molten

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aluminium containing suspended particles of molten metal, adding any further titanium or an aluminium boride, passing the body of mol- aluminium compound required for reaction, ten aluminium through a filter which is pervious and heating the resulting mixture of titanium to the molten aluminium but which retains the and aluminium compounds under reaction sin- suspended particles, the filter medium being 5 tering conditions to form a refractory compos- alumina in the form of a plate or compact of ite containing titanium diboride. pressed unbonded or lightly sintered refractory aggregate, or a bed of loose granular aggregate supported on a refractory alumina grill, Revendications and recovering the aluminium boride in solid w form from the filter by dissolving the alumina 1. Procede de production de borure d'aluminium filter medium in acid under conditions in which (ce terme qualifiant le diborure d'aluminium et the aluminium boride is insoluble. le dodecaborure d'aluminium) sous forme solide, selon lequel on prepare une masse d'alu- A method of producing an aluminium boride 75 minium fondu contenant des particules en sus- (which term includes aluminium diboride and pension d'un borure d'aluminium, on fait pas- aluminium dodecaboride) in solid form, which ser la masse d'aluminium fondu a travers un method comprises providing a body of molten filtre permeable a I'aluminium fondu mais rete- aluminium containing suspended particles of nant les particules en suspension, le materiau an aluminium boride, passing the body of mol- 20 de filtration etant en alumine sous la forme ten aluminium through a filter which is pervious d'une plaque ou d'un materiau compacts reali- to the molten aluminium but which retains the se a partir d'agregats refractaires non lies ou suspended particles, the filter medium being legerement frittes et compactes, ou d'un lit carbon in the form of a plate or compact of d'agregats granulaires libres supportes par une pressed unbonded or lightly sintered refractory 25 grille d'alumine refractaire, et on recupere le aggregate, or a bed of loose granular aggre- borure d'aluminium sous forme solide a partir gate supported on a refractory alumina grill , du filtre, par dissolution du materiau de filtra- and recovering the aluminium boride in solid tion a base d'alumine dans un acide dans des form from the filter by converting the carbon conditions dans lesquelles le borure d'alumi- filter medium to methane. 30 nium est insoluble.

3. A method of forming a solid mixture of par- 2. Procede de production d'un borure d'alumi- ticles of an aluminium boride with a refractory nium (ce terme qualifiant le diborure d'alumi- material selected from borides and nitrides of nium et le dodecaborure d'aluminium) sous groups lllb, IVb, Vb and Vlb metals, which 35 forme solide, selon lequel on prepare une method comprises providing a body of molten masse d'aluminium fondu contenant des parti- aluminium containing suspended particles of cules en suspension d'un borure d'aluminium, an aluminium boride, passing the body con- on fait passer la masse d'aluminium fondu a taining the suspended particles through a filter travers un filtre permeable a I'aluminium fondu medium comprising a refractory material se- 40 mais retenant les particules en suspension, le lected from borides and nitrides of groups lllb, materiau de filtration etant en carbone sous la IVb, Vb and Vlb metals, and separating the forme d'une plaque ou d'un materiau compac- filter medium containing the aluminium boride te realise a partir d'agregats refractaires non particles from the bulk of the molten metal. lies ou legerement frittes et compactes, ou 45 d'un lit d'agregats granulaires libres supportes 4. A method as claimed in claim 3, wherein the par une grille d'alumine refractaire, et on re- filter medium is selected from TiB2, AIB2, cupere le borure d'aluminium sous forme soli- AIB12, BN, TiN and AIN. de, a partir du filtre, par conversion du materiau de filtration a base de carbone, en metha- 5. A method of producing a refractory composite 50 ne. containing titanium diboride, which method comprises providing a body of molten alumin- 3. Procede de preparation d'un melange solide ium containing suspended particles of an alu- de particules d'un borure d'aluminium avec un minium boride, passing the body containing materiau refractaire choisi parmi les borures et the suspended particles through a filter me- 55 les nitrures des metaux des groupes lllb, IVb, dium comprising a nitride or boride of titanium, Vb et Vlb, selon lequel on prepare une masse separating the filter medium containing the alu- d'aluminium fondu contenant des particules en minium boride particles from the bulk of the suspension d'un borure d'aluminium, on fait

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passer la masse contenant les particules en die Schritte: Bereitstellung einer Masse ge- suspension, a travers un materiau de filtration schmolzenen Aluminiums, enthaltend suspen- comprenant un materiau refractaire choisi par- dierte Teilchen von Aluminiumborid, Passieren mi les borures et les nitrures des metaux des der Masse von geschmolzenem Aluminium groupes lllb, IVb, Vb et Vlb, et on separe le 5 durch ein Filter, welches durchlassig fur das materiau de filtration contenant les particules geschmolzene Aluminium ist, aber die suspen- de borure d'aluminium, de la masse de metal dierten Teilchen zuruckhalt, wobei das Filtrier- fondu. medium Kohlenstoff ist, in Form einer Platte oder eines kompakten, lose gepressten oder 4. Procede selon la revendication 3, dans lequel 70 leicht gesinterten hitzebestandigen Aggrega- le materiau de filtration est choisi parmi TiB2, tes, oder in Form eines Bettes aus einem lose AIB2, AIB12, BN, TiN et AIN. kornigen Aggregat auf einem hitzebestandigen Aluminiumoxidgitter, und Wiedergewinnen des 5. Procede de production d'une composition re- Aluminiumborids in fester Form vom Filter fractaire contenant du diborure de titane, selon 75 durch Umwandlung des Kohlenstoff-Filtrierme- lequel on prepare une masse d'aluminium fon- diums in Methan. du contenant des particules en suspension d'un borure d'aluminium, on fait passer la mas- 3. Verfahren zur Ausbildung einer festen Mi- se contenant les particules en suspension a schung von Teilchen eines Aluminiumborids travers un materiau de filtration comprenant un 20 mit einem hitzebestandigen Material, ausge- nitrure ou un borure de titane, on separe le wahlt aus Boriden und Nitriden von Metallen materiau de filtration contenant les particules der Gruppen lllb, IVb, Vb und Vlb, umfassend de borure d'aluminium, de la masse de metal die Schritte: Bereitstellung einer Masse ge- fondu, on ajoute un autre compose derive du schmolzenen Aluminiums, enthaltend Teilchen titane ou de I'aluminium, necessaire a la reac- 25 von Aluminiumborid, Passieren der Masse, tion, et on chauffe le melange resultant de welche die suspendierten Teilchen enthalt, composes derives du titane et de I'aluminium, durch ein Filtriermedium, enthaltend ein hitze- dans des conditions reactionnelles de frittage bestandiges Material, ausgewahlt aus Boriden appropriees pour former une composition re- und Nitriden von Metallen der Gruppen lllb, frataire contenant du diborure de titane. 30 IVb, Vb und Vlb, Abtrennen des die Alumini- umboridteilchen enthaltenden Filtriermediums aus der Masse des geschmolzenen Metalls. Anspruche 4. Verfahren nach Anspruch 3, worin das Filtrier- 1. Verfahren zur Herstellung eines Aluminiumbo- 35 medium ausgewahlt ist aus TiB2, AIB2, AIB12, rids (einschliesslich Aluminiumdiborid und Alu- BN, TiN und AIN. miniumdodecaborid) in fester Form, umfassend die Schritte: Bereitstellung einer Masse aus 5. Verfahren zur Herstellung eines hitzebestandi- geschmolzenem Aluminium, enthaltend Teil- gen Verbunds, enthaltend Titandiborid, umfas- chen eines Aluminiumborids, Passieren der 40 send die Schritte: Bereitstellung einer Masse Masse an geschmolzenem Aluminium durch geschmolzenen Aluminiums, enthaltend su- ein Filter, welches fur das geschmolzene Alu- spendierte Teilchen eines Aluminiumborids, minium durchlassig ist, aber die suspendierten Passieren der die suspendierten Teilchen ent- Teilchen zuruckhalt, wobei das Filtriermedium haltenden Masse durch ein Filtriermedium, ent- Aluminiumdioxid ist in Form einer Platte oder 45 haltend ein Titannitrid oder Titanborid, Abtren- eines kompakten, lose gepressten oder leicht nen des die Aluminiumboridteilchen enthalten- gesinterten, hitzebestandigen Aggregates oder den Filtriermediums von der Masse des ge- in Form eines Bettes aus einem losen korni- schmolzenen Metalls, Zufugen weiterer fur die gen Aggregat, getragen von einem feuerfesten Reaktion erforderlicher Titan- oder Aluminium- Aluminiumoxidgitter, und Wiedergewinnen des 50 verbindungen, und Erhitzen der erhaltenen Mi- Aluminiumborids in fester Form vom Filter schung von Titan- und Aluminiumverbindungen durch Auflosen des Aluminiumoxid-Filtrierme- unter Reaktionssinterbedingungen, wobei ein diums in Saure bei Bedingungen, unter denen hitzebestandiger Verbund, enthaltend Titandi- das Aluminiumborid unloslich ist. borid, gebildet wird. 55 2. Verfahren zur Herstellung eines Aluminiumbo- rids (einschliesslich Aluminiumdiborid und Alu- miniumdodecaborid) in fester Form, umfassend