Aug. 28, 1973 E. WEISE ET AL 3,755,548 PRODUCTION OF ALUMINUM FLORIDE Filed Aug. ll, l97l

FIG 1

IRWENTORS: EBERHARD WEISE, NANFRED SCHULZE, 11ANFRED ROTHERT. "Amo?4 and%4. 4TTO2NES 3,755,548 United States Patent Office Patented Aug. 28, 1973 2 spontaneous liberation of water in the process of calcina 3,755,548 PRODUCTION OF ALUMINUM FLUORIDE tion and the temperature differences between the individ Eberhard Weise, Leverkusen, Manfred Schulze, Opladen, ual stages of the reactor. The sudden liberation of water and Manfred Rothert, Leverkusen, Germany, assigniors prevents the formation of a stable fluidised bed in the to Bayer Aktiengesellschaft, Leverkusen, Germany uppermost stage of the reactor. The temperature differ Filed Aug. 11, 1971, Ser. No. 170,733 ences between the individual stages produce thermal Claims priority, application Germany, Aug. 14, 1970, stresses in the material of the reaction vessel and there P 20 40 412.0 fore cause leakages at the flanges, which may be serious int, C. COf 7/50 when using such a highly corrosive material as hydro U.S. C. 423-489 7 Claims 10 fluoric acid. In the processes just described, additional energy must be supplied at the stage of calcination in an amount corresponding to the water content of the alu ABSTRACT OF THE DISCLOSURE minium hydroxide which has undergone reaction. Accord Reaction of aluminum oxide and gaseous hydrogen ing to German patent specification No. 815,343, this prob fluoride in a reaction zone in the presence of ammonia 5 lem is solved by arranging an oil burner underneath the at a temperature of 400 to 700° C., condensation of gase calcination stage. Another method of producing alumin ous products formed in said reaction Zone, separation of ium fluoride in a fluidised bed reactor has been described ammonia from resulting condensate, precipitation of am in U.S. patent specification No. 2,996,354. In this process, monium fluoride present in the condensate with reactive the individual steps of the process (calcination, reaction aluminum oxide and/or aluminum hydroxide to form 20 and cooling) are carried out in three separate apparatus. ammonium cryolite, introduction of ammonium cryolite The principle of the process is that the aluminium hy into said reaction zone and recovery of aluminum fluo droxide is substantially calcinated by the reaction gases in ride product. a pretreatment Zone, and the resulting aluminium oxide is then reacted with gaseous hydrofluoric acid to alu The invention relates to a process for the production 25 minium fluoride in a single stage fluidised bed reactor. of aluminium fluoride in a fluidised bed reactor. Although efficient heat economy is achieved by this proc The production of aluminium fluoride from aluminium ess, the expenditure required for the additional apparatus hydroxide and gaseous hydrogen fluoride has been de behind and in front of the main apparatus is very high. scribed in numerous patent specifications. As a rule, a It is known that very severe corrosion of the metal multi-stage fluidised bed reactor is employed in which 30 apparatus occurs in processes involving the use of hydro aluminium hydroxide is converted substantially com fluoric acid and fluorides under acid conditions. This is pletely into aluminium oxide in the uppermost stage by especially so when mixture of HF and H2O are formed an endothermic reaction (AHR2.98 k=-37 kcal/mol at a temperature below the dew point. Severe corrosion Al2O3) according to the equation: also occurs both in the main apparatus and in subsidiary 35 apparatus when aluminium hydroxide is reacted with gase ous hydrogen fluoride in a fluidised bed reactor unless the hot reaction gases from the lower stages supplying the highly corrosion resistant and therefore expensive mate necessary heat for calcination. The aluminium oxide then rials are used for the apparatus. In conventional methods passes through immersion pipes into the lower stages, of producing aluminium fluoride in a fluidised bed reactor, where the reaction between aluminium oxide and gaseous 40 it is not possible to prevent the waste gases from contain hydrogen fluoride takes place at a temperature in the range ing hydrogen fluoride. This hydrogen fluoride is utilised of from 400° C. to 700° C. by an exothermic reaction for the production of cryolite by washing the waste gases (AHRs K=-98 kcal./mol AlO3) in accordance with with sodium aluminate solution. the following equation: It is disadvantageous for economic reasons to combine 45 the production of AIF with that of Na3AFs, and a process which utilises the hydrogen fluoride in the waste gasses of In the lowermost stage of the reactor, the hot aluminium the reactor for the production of aluminium fluoride is fluoride is cooled by gaseous hydrogen fluoride which therefore superior to the one which utilises it for the in turn is heated almost to the reaction temperature. The production of NaAlF6. production of aluminium fluoride from aluminium hy 50 It is also known to produce ammonium cryolite by droxide and gaseous hydrogen fluoride in a three stage reacting ammonium fluoride with aluminium hydroxide in reactor is carried out by a process described in British aqueous solution (e.g. Z. anorg. allg. Chem. 3, 127 patent specification No. 656,374 which the process de (1893)): scribed in French patent specification No. 1,309,300 em ploys a four-state reactor and the process described in Ca 55 nadian patent specification No. 537,403 employs a seven It is advantageous in this process to use a large excess of Stage reactor. aluminium hydroxide (about 100%) because otherwise In German Auslegeschrift No. 1,020,608, a three-stage the removal of ammonium fluoride will not be quantita fluidised bed reactor is described in which the overflow tive. If aluminium fluoride is required as the end product, pipes run outside the reactor between the separate stages. 60 the excess of aluminium hydroxide is not a disadvantage This has the advantage that the overflow pipes can be because aluminium hydroxide must in any case be added cleaned from outside without the apparatus having to be subsequently during the course of the following reaction dismantled. These multistage fluidised bed reactors are with heat treatment: delicate apparatus which are liable to give trouble. Never theless, multistage fluidised bed reactors are used indus 65 trially because of the known advantage of cascade proc This method of producing aluminium fluoride has been esses. The simultaneous carrying out of the three process described e.g. in Austrian patent specification No. 269,071. steps (calcination, chemical reaction and cooling of the Ammonium cryolite can also be thermally decomposed end product) for the production of aluminium fluoride without the addition of aluminium hydroxide (Naturwis in an apparatus comprising several stages entails consider 70 senschaften, 26, 529, 1938) (NH4)3AlF6 350° C. able process technical difficulties which are due to the Hu-3 3,755,548 3 4 s ammonium fluoride formed is then returned to the via the intermediate stages of ammonium fluoroaluminate Cycle. complexes: Ammonium cryolite may also be obtained by a dry reaction of ammonium fluoride with aluminum oxides 12NHs -- 24HF - 12NHHF and hydroxides, and the ammonium cryolite formed may then be decomposed by heat or in the presence of alumi 6NH4AlF4 + Al2O3 - 8AlF -- 3Ho -- 6NH num oxides and/or hydroxides, e.g. according to the foll Tri------lowing equation: 24HF -- 4AO -> 8AlF3 - 12H2O 6HF -- Al2O3 --> 2AF 3 - 3EIO Other courses of the reaction, however, are also pOS 10 sible, and the process of the invention should in no way be limited by these theoretical considerations. (British patent specification Nos. 423,601 and 635,553). In the production of aluminium fluoride in a fluidised Lastly, ammonium cryolite may also be prepared by bed reactor, it is impossible to prevent hydrogen fluoride reacting B-aluminum fluoride with ammonium fluoride in from entering the waste gas, especially if a high AIF, aqueous solution (German Auslegeschrift No. 1,811,178). 5 content is required in the end product. For reasons of All the methods described above for the production of health and also for economical reasons it is not permis aluminum fluoride via the intermediate stage of ammoni sible to discharge the hydrogen fluoride into the at um cryolite necessitate heat treatment of (NH)AF at mosphere. In the conventional fluidised bed plants used the end of the process (with or without Al2O3 or for the production of aluminum fluoride, the reactor Al(OH)3). Owing to the problems of corrosion and 20 waste gas which contains HF is washed e.g. with sodium heat supply, no economically justifiable method has so aluminate solution, cryolite (Na3AEF) being formed. far been recorded for carrying out this heat treatment on This coupling of aluminium fluoride with cryolite is pre an industrial scale. vented when ammonia is used as an auxiliary component This invention relates to a process which may be car because the waste gas, which contains NH HO and HF, ried out continuously for the production of aluminum condenses and is worked up by distillation. An ammonia fluoride from aluminum oxide and gaseous hydrogen fluo cal ammonium fluoride solution is formed from which ride at an elevated temperature in a fluidised bed reac the ammonia may be removed by means of a rectification tor, which process is characterised in that an aluminum column and returned to the cycle. Ammonium cryolite is oxide is reacted with hydrogen fluoride in the presence precipitated from the aqueous ammonium fluoride solu of ammonia at a temperature in the range of from 400 30 tion by the addition of a reactive aluminum oxide or hy C. to 700 C., the aluminum fluoride thus produced is drated oxide, and after filtering and drying it is intro removed from the reactor, the gases produced in the re duced into the fluidised bed reactor together with alumin action are condensed, ammonia is driven from the con ium oxide. In this way, hydrogen fluoride which has not densate, and ammonium fluoride in the condensate is pre been reacted in the fluidised bed reactor is indirectly re cipitated in the form of ammonium cryolite by means of turned for the production of aluminum fluoride. reactive aluminum oxide and/or hydroxide, and the thus Ammonium fluoride from other processes may also be ly precipitated ammonium cryolite is introduced into the used as described above, e.g. NHF formed by ammoni acal hydrolysis of H2SiF6. reactor. The possibility of returning hydrofluoric acid from the It has surprisingly been found that the production of 40 waste gas into the fluidised bed reactor via the intermedi aluminum fuoride in a fluidised bed reactor can be con ate stages of ammonium fluoride and ammonium cryolite siderably improved by carrying out the reaction of alumi in the process according to the invention enables HF to be nium oxide with gaseous hydrogen fluoride in the pres used in excess, based on the quantity of Al-O used, with ence of ammonia. the result that a higher yield of AEF is obtained. The addition of ammonia has advantageous effects on 45 In addition to its anti-corrision and catalytic functions, the technical and chemical course of the process and ammonia also carries out a purely physical function. For hence opens up the possibility of obviating the disadvan obtaining a stable fluidised bed in which the chemical re tages of the earlier processes. action takes place, it is best if the number of mols of the As already mentioned above, processes carried out un gaseous compound formed in the reaction is exactly equal der acidic conditions in apparatus carrying hydrofluoric 50 to the number put into the reaction. In the reaction acid and fluorides are accompanied by severe corrosion. According to this invention, therefore, a basic atmosphere AlO-6HF-22AF-I-3H2O (2) is produced in the reaction chamber and in the attached in which the ratio of gas put into the reaction and gas apparaus used for the production of aluminum fluoride in 55 formed is 2:1 per molar reaction, it is therefore advisable the fluidised bed reactor by adding ammonia to the reac to bring this ratio close to 1 by the addition of an inert tion gases so that the medium used for the process is gas. If reaction (2) is carried out in such a manner much less corrosive. The addition of ammonia virtually that 1 mol of ammonia is available for 1 mol of hydro eliminates corrosion, especially in the apparatus attached gen fluoride, then the ratio of gas volumes before and to the fluidised bed reactor, in which the gas mixtures are 60 after the reaction is already 12:9-1.33 and therefore not far removed from 1. cooled below the dew point, but in the fluidised bed re Aluminium oxides are understood in the context of actor itself the addition of ammonia also substantially this invention to mean dehydrated or at least partly de reduces the corrosive action of hydrogen fluoride, thereby hydrated aluminium hydroxides (hydrated oxides). Par enabling types of steel to be used which have a substan 65 ticularly reactive products are obtained by spray drying tially lower nickel content than the fluidised bed reac or impact calcination. Processes of this type have been tors conventionally used for the production of aluminum described e.g. in German patent specification No. 1,241 fluoride. 808 and British patent specification No. 896,966. Not only It was also surprising to find that the presence of am these highly reactive forms of aluminium oxide or monia in the reaction of aluminum oxide with gaseous 70 partially dehydrated aluminium hydroxides (cz-AlO3, hydrogen fluoride in the fluidised bed reactor exerts a AlOOH and intermediate forms) may be used but also catalytic effect which results in increased volume/time so-called N-Al2O3 (cz-Al2O3) which is used for fusion yields. It is postulated that the reaction of aluminium electrolysis. Reactive aluminium oxide and/or alumin oxide with gaseous hydrogen fluoride in the presence of ium hydroxides (hydrated oxides) are understood in the ammonia takes place substantially as represented below, 75 context of this invention to mean oxides and hydrated 3,755,548 5 6 oxides of aluminium in general which have no corundum ture of ammonium bifluoride (NHHF) before they are structure, e.g. y-AlO3, Al(OH)3, AlOOH and mixtures of introduced into the fluidised bed reactor. The two gases these forms. are therefore separately heated to at least 300 C. and FIG. 1 is a flow diagram of the process according to preferably to 400° C. before being introduced into the the invention. The letters in the diagram have the fol mixing chamber underneath the lowermost inflow base. lowing meaning: An apparatus suitable for carrying out the process ac cording to the invention is illustrated in FIG. 2 in which C=calcination; the reference numerals have the following meaning: F-fluidised bed reactor; K=condensation; O 1- Fluidised bed reactor; D=distillation: 2-Cyclone; K=vessel with stirrer; 3-Rectification column; =preheater; 4-Ammonia preheater; WA=heat exchanger 5-Hydrogen fluoride preheater; Tr=drier; and 6-Drier; Fis-filtration. 7-Drying drum; 8-Filter; and For the recovery of aluminium fluoride in the process 9-Cascade of stirrer vessels. described here, the quantity of gas added (consisting of hydrogen fluoride and ammonia) should be so calculated From the drier 6 e.g., an impact heating chamber ac in relation to the diameter of the reactor and the loosening 20 cording the British patent specification No. 896,966, up point of the solid material that a stable fluidised bed aluminium oxide and from the drying drum 7 ammonium is obtained. The proportion of ammonia must be at least cryolite are conveyed into the fluidised bed reactor 1, each sufficient to enable the hydrogen fluoride which has es through a coil. The gases (ammonia and hydrogen fluo caped to be bound in the form of ammonia fluoride and 25 ride) flow through the inflow bases shown in broken it should not exceed the volumetric throughput of hy lines in the drawing into the reaction chamber (single drogen fluoride, in order that a sufficient volume/time stage or two-stage) where the conversion to aluminium yield may be obtained. Although the volumetric ratio of fluoride takes place. The waste gases from the fluidised ammonia: hydrogen fluoride may range between 1:20 and bed reactor (HO, NH and HF) are condensed after 1:1, a mixture of 1 part by volume of ammonia to 2 parts 30 the dust has been separated (in the cyclone group 2) and by volume of hydrogen fluoride has been found to be the are then distilled in a column 3. Ammonia distills over most suitable gaseous starting mixture. According to the top of the separating column and returns to the fluid Equation 2, the quantity of aluminium oxide should be ised bed reactor through the heat exchanger 4 while am calculated to provide a maximum of 1 mole of reactive monium fluoride solution having an ammonium fluoride aluminium oxide to 6 mols of hydrogen fluoride. If de 35 content of 10 to 20% by weight is discharged from the sired, an excess of hydrogen fluoride may be used, up to sump of the column. The reaction of ammonium fluoride 10% more than the stoichiometrically required quantity with aluminium oxide to ammonium cryolite takes place in the cascade of vessels 9. The ammonium cryolite enters for a 90%. AlF, but preferably an excess of 5% is used. the fluidised bed reactor 1 after having been separated on The ammonium fluoride solution which has been freed 40 the filter 8 and dried in the drum 7. The ammoniacal from ammonia and is discharged from the sump of the vapours from the cascade of vessels 9 are introduced into distillation column is treated with aluminium oxide to column 3 to remove ammonia. The heat exchanger 5 in precipitate ammonium cryolite. A temperature of between which hydrogen fluoride is preheated with hot aluminium about 80 and 110° C., preferably 95 C., is most satis fluoride is constructed as a fluidised bed reactor. The factory for this precipitation. Surprisingly, precipitation crude gases from hydrofluoric acid production are used of ammonium fluoride is almost quantitative (about 97%) 45 as source of hydrogen fluoride. Silicon tetrafluoride which when a reactive aluminium oxide which has been prepared is frequently present in the crude gas does not enter into e.g. by impact heating of aluminium hydroxides is used reactions in the fluidised bed reactor. Silicon tetrafluoride in an excess of 5 to 20%, preferably 10%, of the stoichio is hydrolysed in the condensed ammoniacal waste gases metrically required quantity without applied pressure. In and converted into ammonium fluoride and silicic acid. the process described here, this product is the preferred 50 The silicic acid must be removed, which requires a product used for the precipitation of ammonium cryolite filtration apparatus between the sump of the column 3 and the cascade of vessels 9. Sulphur dioxide which is from aqueous ammonium fluoride solution. The disad normally present in the crude gases of hydrofluoric acid vantage of first having to carry out a dehydration when furnaces also does not enter into the end product but using aluminium oxide instead of aluminium hydroxide 55 is removed from the condensed waste gas in the form of is more than compensatd by the improved reactivity of ammonium sulphite. the product and by the fact that the precipitate thus ob In an apparatus as shown in FIG. 2, 5.7 kg. of AlO tained contains a smaller quantity of chemically bound (95%), 2.5 kg. of (NH4)3AlF6 (95% finely ground), water (in the form of aluminium hydroxide). Reaction 5.64 kg. of HF and 3.95 kg. of ammonia are introduced (2) is exothermic, producing about 100 kcal. per mol of 60 hourly into the two stage fluidised bed reactor. The hy AlO at 500 C. The quantity of heat liberated heats up drogen fluoride and ammonia had been heated to a tem the reaction mixture and therefore heat must be removed; perature of 400° C. and the aluminium oxide to 450° C. the temperature in the reactor can be adjusted to the The upper stage of the reactor was operated at 450° C. required level by varying the quantity of cooling medium. and the lower stage at 550° C. A 90 to 92% AIF having In the process according to the invention, the lowest 65 an NH3 content of less than 0.01% was obtained. operating temperature in the reactor is determined by the What is claimed is: decomposition temperature of the ammonium fluoro alu 1. A process for producing aluminum fluoride which minate complexes which are formed as intermediate comprises reacting aluminum oxide and gaseous hydrogen products. It is in the region of 350° C. This temperature 70 fluoride at a temperature of 400 to 700° C. in the presence may be allowed to rise to 700 C. but is preferably kept of ammonia in a fluidized bed reaction zone, condensing within the range of from 450° C. to 550 C. gaseous water, ammonia and hydrogen fluoride product Owing to the formation of ammonium fluorides from obtained from said reaction zone to obtain water, ammo hydrogen fluoride and ammonia, these two gases must be nia and ammonium fluoride, removing ammonia from raised to a temperature above the decomposition tempera 75 resulting condensate and introducing it into said fluidized 3,755,548 8 bed reaction zone in an amount at least sufficient to con 7. The process of claim 1 wherein said precipitation of vert hydrogen fluoride in said gaseous product to ammo ammonium cryolite is carried out at a temperature be nium fluoride during said condensation, precipitating am tween about 80 and 10 C. monium fluoride present in said condensate with reactive aluminum oxide, aluminum hydroxide or a mixture there 5 References Cited of to form ammonium cryolite, introducing resulting am UNITED STATES PATENTS monium cryolite into said fluidized bed reaction zone 1,300,110 4/1919 Betts ------23-88 X and recovering resulting aluminum fluoride product. 2,981,597 4/1961 Tarbutton et al. ------23-88 2. The process of claim 1 wherein said temperature is 3,473,887 10/1969 Chu et al. ------23-88 between about 450 and 550 C. O 3. The process of claim 1 wherein the volume ratio 3,635,659 1/1972 Kidde ------23-88 of ammonia:hydrogen fluoride is between 1:20 and 1:1. 3,647,366 3/1972 Thoonen ------23-88 4. The process of claim 1 wherein the volume ratio of FOREIGN PATENTS ammonia: hydrogen fluoride is 1:2. 656,374 8/1951 Great Britain ------23--88 5. The process of claim 1 wherein 6 to 6.6 mols of 179,082 8/1935 Switzerland ------23-88 hydrogen fluoride are employed per mol of reacted alu 149,408 1962 U.S.S.R. ------23-88 minum oxide. 220,970 1968 U.S.S.R. ------23-88 6. The process of claim 1 wherein the ammonium fluoride formed on condensation is precipitated in the form EDWARD STERN, Primary Examiner of ammonium cryolite by the addition of a 5 to 20% stoichiometric excess of said reactive aluminum oxide, U.S. C. X.R. aluminum hydroxide or a mixture thereof. 423-135, 465, 470