Europâisches Patentamt ® European Patent Office ® Publication number: 0112310 Office européen des brevets B1 ® EUROPEAN PATENT SPECIFICATION (§) Date of publication of patent spécification: 04.02.87 (D Int. Cl.4: C 01 B 19/00 (D Application number: 83850307.6 ® Dateoffiling: 15.11.83 ® Process for preparing selenium salts. (§) Priority: 22.12.82 SE 8207344 (73) Proprietor: Boliden Aktiebolag Box 5508 S-1 14 85 Stockholm (SE) (§) Date of publication of application: 27.06.84 Bulletin 84/26 (72) Inventor: Bjornberg, Arne Blasaregatan 40 (§) Publication of the grant of the patent: S-932 00 Skelleftehamn (SE) 04.02.87 Bulletin 87/06 Inventor: Martensson, Ulf Sigvard Falmyrvagen 56 S-931 53 Skelleftea (SE) (§) Designated Contracting States: Inventor: Paulsson, Karin Marianne AT BE CH DE FR GB IT LI LU NL SE Smaltaregatan 1 1 D S-932 00 Skelleftehamn (SE) (58) References cited: EP-A-0 020 826 @ Representative: Inger, Lars Ulf Bosson et al DE-A-1 667 764 Boliden AB Patent & Trade Marks Box 5508 GB-A- 808 637 S-1 14 85 Stockholm (SE) GMELIN, HANDBUCH DER ANORG. CHEMIE, Suppl. vol. B1, System nr 10, 1981, pag. 227,297 C0 CM 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 shall CL be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been LU paid. (Art. 99(1) European patent convention). Courier Press, Leamington Spa, England. denitration and chlorination extraction, reducing the selenium by sulphur dioxide. Technical field Gmelin Handbook of Inorganic Chemistry, sup- The present invention relates to a method for plement, vol. B1, system nr 10, 1981, pages 227 preparing selenium salts, particularly alkalimetal and 297 describes the reaction of selenious acid selenate, from metallic selenium; and to the with alkali hydroxides in aqueous solutions to products produced. form selenites; as well as the formation of the The object of the invention is to provide a selenate ion by oxidizing Se032- with O2, 03 or possibility of rationally producing alkalimetal H202. However, any reaction in basic environment selenate, these salts being essential salts for the is not disclosed, and oxidation of Se032- in acidic administration of selenium to human beings and environment follows the route and drawbacks animals as trace elements. indicated above. Consequently there is a need for a method Background art which enables high-quality selenate to be manu- It is known to administer sodium selenite, factured in a simple and rational manner, Na2Se03, as a trace substance to human beings especially selenate having alkalimetal cations. and to animals, selenium constituting an impor- tant trace substance for both human beings and Description of the invention animals. In this respect, selenite is administered It has now surprisingly been found possible to either indirectly, in animal fodder or in foodstuffs, produce selenates in a simple and rational or directly, in the form of a spray which is manner by means of the present invention, which administered orally. is characterized by dissolving metallic selenium in It is known that in the case of human beings, nitric acid to form selenious acid; reacting the such illnesses as multiple sclerosis (MS), cardiac- selenious acid with an alkalimetal hydroxide, and/ vascular illnesses, and certain forms of cancer can or an alkalimetal carbonate to form an alkalimetal be related to a selenium deficiency. It is also selenite; oxidizing the selenite in a basic environ- known that selenium deficiency in animals can ment with an oxidant in the form of hydrogen lead to, among other things, muscular dystrophy peroxide to form a selenate, which is then spray- and reduced fertility. dried. It is also known that selenium can be adminis- Further, characterizing features of the invention tered through plants which are eaten by animals, are set forth in the following claims. the selenium being incorporated in the fertilizers By means of the invention it is possible to used, in plant top-dressing substances, or in the produce an extremely finely divided selenate seed. When concerning the amount of selenium having a small residual selenite content (<1%) absorbed by plants, selenate is more effective and a low residual water content. than selenite. Consequently, the administration of The particulate form in which the products are selenium through the fertilizers used constitutes a obtained enables them to be used readily in simple and rational method, especially in the case fertilizers, since it is readily simple to fix the of large herds of grazing animals, when com- products to the surfaces of granules, using known pared with administering selenium orally in the adhesion techniques. form of selenite. Both sodium selenate, and possium selenate The large herds of animals which roam and can be produced by means of the method accord- graze free in many countries throughout the ing to the invention. world, have greatly increased the need of The chemical reactions involved in the method supplying selenate either through the fertilizers are: used or in the form of selenate prills. It is known to produce sodium selenate elec- trolytically, starting from a solution of sodium selenite. It is also known to produce selenic acid H2SeO4 by oxidizing selenious acid H2Se03 with hydrogen or alternatively: peroxide. The resultant selenic acid, however, contains 2-3% selenious acid. The selenic acid can then be reacted with sodium carbonate, to form sodium selenate, although the selenate be- comes contaminated with elementary selenium. When selenic acid is reacted with sodium The invention will now be described with ref- hydroxide, the end product is contaminated with erence to the accompanying drawing, the single sodium selenite. figure of which illustrates a plant for manufactur- EP-A1-0 020 826 describes a method for ing selenate. separating and recovering metals, including selenium, from anode slime by leaching, solvent Example 1 extraction, precipitation and reduction, whereby 790 grams of so-called standard selenium the anode slime is primarily leached with nitric (99.9% Se) were dissolved in 1300 ml of warm acid, separating the selenium with liquid-liquid to 65%-nitric acid. The selenious acid, H2Se03, obtained, 1290 grams, was then admixed with a stoichiometric amount of NaOH, 800 grams, as a 50%-so!ution of sodium hydroxide, there being 1. A method for producing metal selenate from obtained 1730 grams of sodium selenite and a metallic selenium, via selenious acid, charac- further 360 grams of water. The pH was adjusted terized by dissolving metallic selenium in nitric to 11, by adding a further, minor amount of acid, to obtain selenious acid; converting the NaOH. Hydrogen peroxide (50%) was then stirred selenious acid to an alkali metal selenite, by into the sodium-selenite solution in a reacting said selenious acid with an alkali metal stoichiometric excess, at a temperature of 70°C, hydroxide and/or an alkali metal carbonate, ox- the sodium selenite being oxidized to sodium idizing the alkalimetal selenite with hydrogen selenate. The hydrogen peroxide was added peroxide at elevated temperature in a basic slowly to the sodium-selenite solution. The resul- environment to pH 10-12, to form a selenate, tant solution, which, upon completion of the which is then dried in a spray dryer. oxidizing process, contained sodium selenate, 2. A method according to claim 1, characterized was passed through a buffer tank and a filter to a by oxidizing the selenite to selenate at a tempera- spray dryer, in which the product was dried. The ture of from 60 to 75°C, preferably at 70°C. temperature of the ingoing drying air was 360°C, 3. A method according to claim 1, characterized and the temperature of the outgoing air 110°C. in that oxidation of the selenite to selenate is The end product had the form of a white, solid, effected by means of hydrogen peroxide in powderous sodium selenate, containing 98% stoichiometric excess. Na2Se04, 0.9% Na2Se03 and a balancing amount 4. A method according to claim 1, characterized of water, partly in chemically bound form and in that drying is effected with hot air, which has an partly as free water. The yield was 98%. outgoing temperature of 100°C, preferably 110°C. Referring now to the accompanying drawing, 5. A method according to claim 3, characterized the above reactions were carried out in a plant by introducing the hydrogen peroxide used in the which comprised a first reaction vessel 1, in which oxidation process to the reaction mixture beneath pulvurent selenium was admixed with nitric acid. the surface thereof, preferably at a location adja- The resultant selenous acid was then transferred cent an agitator means arranged in said mixture. to a second reaction vessel 2, via a filter 3 and an 6. A method according to claim 1, characterized intermediate buffer tank 4. The reaction vessel 2 is in that the aforesaid reactions take place in the provided with a stirrer 5, a supply conduit 6 for presence of water. 50%-NaOH, and a supply conduit 7 for 50%-H202. The exit orifice of the conduit 7 is located adjacent the stirrer 5. In a first reaction stage in the second reaction vessel 2, NaOH is supplied until pH is 1. Verfahren zur Herstellung von Metallselenat reached, and the temperature is adjusted to 70°C, aus metallischem Selen über Selenige Säure, whereafter H202 was supplied through the con- dadurch gekennzeichnet, daß man metallisches duit 7. The oxidizing reaction by which selenate is Selen in Salpetersäure auflöst, um Selenige Säu- formed is exothermic, and hence the solution is re zu erhalten, die Selenige Säure in ein Alkalise- cooled so as to maintain a temperature of 70°C.