Electrolytic lead refining

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Lead nodules, electrolytically refined, 99.989%

The electrolytic lead refining is an industrially applied electrolysis process , which for recovering pure lead from lead bullion (prepurified crude lead or hut lead is used). This electrolysis is one of the processes used to refine metals. The principle of the process corresponds to that of copper refining , although simple aqueous acids such as sulfuric or hydrochloric acid cannot be used because of the poor solubility of many lead salts such as lead sulfate or lead chloride . Therefore, in the most important electrolysis process used to obtain pure lead, the Betts process , an aqueous solution of silica fluorine (PbSiF 6 ) and silicofluoric acid (H 2 SiF 6 ) with the addition of 0.1% gelatin is used as the electrolyte. The lead obtained has a high purity, usually> 99.99%, sometimes 99.999%. In 1990 the annual amount of lead represented by the Betts method was estimated at 1,000,000 tons, which was about 20% of the total annual production of lead at that time.

Procedure

The impure lead is poured into anode sheets, e.g. B. by means of casting wheels , and then brought into the electrolytic cell. There, the base lead is then dissolved by anodic oxidation . The metals that are more noble than lead, namely copper , arsenic , antimony and bismuth , do not dissolve, but remain as so-called anode sludge . In the practical Betts process, this remains as a porous adhesive layer that is scraped off when it is thick enough (> 1 cm); only a small part of the anode sludge falls to the bottom and is occasionally removed from there. Pure lead is deposited again on cathodes made of lead or steel. Additives such as gelatine or glue are necessary to ensure a smooth, completely tight deposit, since otherwise porous layers and dendrites will grow. There are current densities in the range of 100-260 A / m 2 is used.

On a smaller scale, as an alternative to the Betts process, perchlorate electrolytes with 5% perchloric acid or tetrafluoroboric acid (borofluoric acid) were used for the electrolytic production of pure lead . Tetrafluoroboric acid is chemically stable and has good electrical conductivity. Above all, their lead salt has a high solubility. However, it is more expensive and is therefore used less often.

Historical

The process is named after its inventor, Anson Gardner Betts , who received several patents for it from 1901 and also wrote a book about it, which was also translated into German. Betts' first patent on lead refining describes in particular the salt content of the electrolyte, which has the lead salt of hexafluorosilicic acid as an essential component. The second patent mentions the addition of gelatine or other substances that serve to smooth the precipitate. He called gelatine inexpensive and particularly suitable; he recommended one gram of gelatin for five kilograms of electrolyte solution. Another patent dealt with the processing of the anode sludge. Fritz Haber , who had made a study trip to North America in 1902, also made the process known in Germany.

Individual evidence

  1. ↑ a b c d e f J. A. González-Domínguez, Ernest Peters, David B. Dreisinger: The refining of lead by the Betts process . In: Journal of Applied Electrochemistry . tape 21 , no. 3 , March 1991, pp. 189-202 , doi : 10.1007 / BF01052570 .
  2. ↑ Gerhart Hantke: Gmelin's Handbook of Inorganic Chemistry Lead: Part C - Delivery 1. Metallurgy of lead · Compounds up to lead and chlorine . 8th edition. Springer, 2013, ISBN 978-3-662-12483-3 , pp. 47 (reprint of the 1969 edition).
  3. ↑ a b Hans Senn: On the knowledge of the electrolytic refining of lead in silicofluoric acid solution . In: Journal of Electrochemistry and Applied Physical Chemistry . tape 11 , no. 15 , April 14, 1905, p. 229-245 , doi : 10.1002 / bbpc.19050111502 .
  4. ↑ G. Brecka, K. Hein, H. -J. Lange, P. Paschen: A pyrometallurgical alternative: The refining electrolysis of lead and solder . In: JOM - The Journal of The Minerals, Metals & Materials Society (TMS) . tape 49 , no. 4 , April 1997, p. 62-64 , doi : 10.1007 / BF02914881 .
  5. ^ AE van Arkel, P. Assmann, G. Borelius, G. Chaudron, EJ Daniels, R. Gadeau, W. Geibel, W. Graßmann, CR Hayward, G. Jantsch, W. Kroll, K. Lins: Reine Metals: Manufacture · Properties · Use . Springer, Berlin 1939, ISBN 978-3-642-98880-6 , pp. 503 .
  6. ^ Anson Gardner Betts: Lead refining by electrolysis . 1st edition. John Wiley & Sons, Chapman & Hall, New York, London 1908 ( online on the Internet Archive pages ).
  7. ↑ Anson Gardner Betts, Viktor Engelhardt: Lead refining through electrolysis . by Anson Gardner Betts. Translated from English by Viktor Engelhardt (=  monographs on applied electrochemistry . Volume XXV. ). Publisher by Wilhelm Knapp, Halle a. P. 1910, OCLC 458522284 .
  8. ↑ Patent US679824 : Art or Process of Refining Lead by Electrolysis. Registered October 12, 1900 , published August 6, 1901 , inventor: Anson Gardner Betts. ‌
  9. ↑ Patent US713278 : Electrodeposited Lead. Registered October 9, 1902 , published November 11, 1902 , inventor: Anson Gardner Betts. ‌
  10. ↑ Patent US918647 : Process of Electrolytically Refining Lead. Applied April 22, 1903 , published January 3, 1905 , inventor: Anson Gardner Betts. ‌
  11. ↑ Patent US918647 : Treating anode slime from the electrolytic refining of lead. Registered January 23, 1907 , published April 20, 1909 , inventor: Anson Gardner Betts. ‌
  12. ↑ Process for processing anode sludge from electrolytic lead refining. In: Annual Report on the Achievements of Chemical Technology . with special consideration of electrochemistry and trade statistics for the year 1907. In: Dr. Ferdinand Fischer (Ed.): Annual report on the achievements of chemical technology . LIII. Year or New Series XXXVIII. Year 1. Department Inorganic Part. Otto Wigand, Leipzig 1908, II. Group metal extraction, silver and lead, p. 271 ( online from Internet Archive [accessed June 27, 2015]).