3,189,446 United States Patent 0 "Ice Patented June 15, 71965

1 2 The resulting bismuth oxychloride can be used directly 3,189,440 PRGCESS FOR THE PREPARATION OF in the reduction process. PURE BISMUTH When the crude bismuth has a high content of impuri~ Heinz-Gunther Plust, Spreitenbach, Aargan, Switzerland, ties, many of these impurities may be removed by puri assignor to Aktiengesellschaft, Brown, Boveri & Cie, ?cation of the bismuth trichloride produced by the chlori Baden, Switzerland, a joint-stock company nation before the hydrolysis. This puri?cation of the No Drawing. Filed May 6, 1963, Ser. No. 278,431 bismuth trichloride may be suitably carried out by sub Claims priority, application /Sgvéitzerland, May 9, 1962, limation or zone re?ning. In addition to or instead of this 1 1 Claim. (Cl. 75-70) pre-puri?cation by sublimation, a further re?ning action 10 may be eifected by a partial hydrolysis of the bismuth In the preparation of compound semiconductors such chloride to bismuth oxychloride. as are used in thermo-electric applications it is essential A further explanation ‘of the process is given in the that the starting materials be of very high purity. An > following. example of this is bismuth which is a component of the For the chlorination of the bismuth a quartz tube is thermo-electric material bismuth telluride (BizTeg). ‘The 15 used which has a gas inlet at one end and a cooled collect better known methods for the puri?cation of bismuth are ing ?ask at the other end. The central section of the by aqueous electrolysis, molten electrolysis and, in par tube is enclosed by some form of heater such as a furnace. ticular, a process using bismuth nitrate as starting mate In the heated zone is placed a boat containing about 300 rial. In this latter method bismuth nitrate is converted grams of bismuth. The tube is ?rst heated to about 500° to sulphide, the sulphide is oxidized with nitric acid and 20 C. while passing pure nitrogen. Then a gas mixture the the resulting oxide is reduced to pure bismuth by potas composition of which is 10 litres per hour of nitrogen sium cyanide. and 20 litres per hour of chlorine is passed through the None of these processes, however, yields bismuth of tube and the temperature is raised to 700—800° C. The spectral purity. It is not possible by these known process-V bismuth and chlorine react to give bismuth trichloride es to remove such impurities as copper and silver to a 25 which is sublimed by the nitrogen into the cooled collect su?iciently low level that they no longer affect the prop ing ?ask. When the reaction has ceased, the apparatus is erties of the semiconductors produced therefrom. The allowed to cool in a nitrogen atmosphere. In this way particular disadvantage of these methods is that they it is possible to chlorinate 300 grams of bismuth in about necessitate the use of other materials which must also 2 hours with a yield of bismuth chloride of from 94 to be of high purity if such methods are to be effective. 30 98%. It is the purpose of the present invention to provide The ?nal hydrolysis is carried out by mixing the bismuth a process for the preparation of spectrographically pure chloride with distilled water and then decanting the bismuth wherein the aforesaid disadvantage is avoided resulting mixture. The precipitate of bismuth oxychloride and by means of which such pure bismuth may be pro is ?ltered off and dried at about 130° C. The yield for duced in considerable quantities. The method is char 35 this part of the process is about 97.5%. V acterized in that bismuth oxychloride is reduced in pure For the reduction of the bismuth oxychloride a quartz hydrogen at about 900° C. tube is used through which a stream of pure hydrogen The method depends on the fact that the reduction of ?ows and whose centre section is surrounded by a heating bismuth oxychloride with hydrogen does not proceed, as element. In the heated zone a boat containing about 50 expected, according to the equation: 40 grams of bismuth oxychloride is placed. The reduction is carried out at about 900° C. at which temperature the vapour pressure of bismuth oxychloride but that intermediate bismuth trichloride is produced. is still quite low. With a hydrogen ?ow of 30 liters per This bismuth trichloride sublimes immediately at the hour the reduction is completed in 15 minutes. The yield temperature of the reaction (about 900° C.) and con of bismuth according to the equation: denses on the cooler parts of the apparatus. The reaction proceeds thus according to the equation: is about 70%. The evaporating bismuth trichloride carries along me 50 The resulting bismuth chloride sublimes at the reaction tallic impurities as chlorides or complex compounds temperature of 900° C. and is redeposited ‘on the cold whereas the remaining metallic bismuth is present in a part of the quartz tube. This bismuth chloride may be very pure form. The pure bismuth thus obtained is used once again in the hydrolysis process to produce spectrographically free from such impurities as boron, bismuth oxychloride thereby materially increasing the 55 lead, copper, silver, titanium and aluminum. yield of pure bismuth to over 90% so that the overall This method has one important advantage in that be yield of pure bismuth from the raw bismuth is about cause the process is carried out in pure hydrogen, which 85-90%. can be easily obtained by purifying technical hydrogen, It is desirable to transform the approximately spherical no further pure material or additional matter is necessary. particles of bismuth produced during the reaction into The preparation of the starting material bismuth oxy 60 rods and this is accomplished quite simply by melting chloride is most advantageously carried out by chlorina these particles in a porcelain crucible and pouring the tion of raw bismuth to bismuth trichloride according to ?uid metal into a preheated hard-glass tube which is the equation: closed at its lower end. 65 Spectrographic analysis shows that the pure bismuth is with subsequent hydrolysis of the bismuth chloride: free from boron, copper, lead, silver, titanium and alumi BiCl3+H2O—->Bi0Cl+2HCl mum and is therefore suitable for the preparation of com 3,189,440 4% t pound semiconductors with favourable electrical prop Reierences Cited by the Examiner V V

Veri?es.‘ ' ' ' ' Mellor: Comprehensive Treatise on Inorganic and The i I claim: . oretical Chemistry, vol. IX,v Longmans, Green and Co., A process for the preparation of spectrally pure bismuth it, N.Y., 1929, pages 6604664. 7 . ‘which comprises the. following steps: ParrzpZone Re?ning and Applied Techniques, George " (a) chlorination of bismuth to give bismuth trichloride, i Newnes Limited, Strand, England, page 171. ' (b) hydrolysis of said bismuth trichloride to give bis muth oxychloride, t BENJAMIN HENKIN, Primary Examiner. ‘ (0) reduction of said bismuth 'oxychloride with pure hydrogen at about 900° C. DAVID RECK, Examiner.