DE1229073B - Process for the recovery of selenium as selenium dioxide from the reaction gases containing it - Google Patents

Description

Process for the recovery of selenium as> elene dioxide from that containing reactionases From the German patents 831837, 852 992, 863 493, 880134, 937 287 and 941 428, the Swiss patent 306 049, the British patent 727 318 and the USA patent 2 716 664 several methods for the recovery of selenium from the gas mixtures are already known, the z. B. arise in the production of unsaturated aldehydes by oxidation of olefins. Here, the hot gas mixtures are cooled and / or brought into contact with a surface that is partially cooled to a certain temperature above 50 ° C., preferably between 100 and 160 ° C., in order to deposit selenium and selenium compounds. In these cases, the separated selenium and its compounds are in the form of solid substances that have to be removed from the separators in some way.

In the method of the invention, these are more or less troublesome No operations are required because the selenium is dissolved in an aqueous solution Compound or contained in the gas mixture selenium dioxide is obtained. The selenium-containing Solutions or gas mixtures can be used directly to implement additional propylene to be used to acrylonitrile.

The process of the invention for the recovery of selenium as selenium dioxide from that containing reaction gases, which have been obtained by oxidizing propylene with oxygen or air in the presence of ammonia and selenium as a catalyst to acrylonitrile at a high temperature, is carried out by washing with weakly alkaline water at a pH from 7 to 9 and a temperature of 20 to 50 ° C and consists in the fact that the aqueous solution of ammonium selenium cyanide obtained by washing, which can also be used several times or concentrated to wash out fresh reaction gases, in a fine atomization in molecular Oxygen or gases containing it via copper oxide (Cu0), ferric oxide (Fe203) or nickel oxide (Ni0) as a catalyst, which can be applied to a carrier such as silica or diatomaceous earth and has a temperature of 500 to 700 ° C.

The result of the conversion of propylene to acrylonitrile Reaction gas is fed to a cooling device in which water with a pH value from 7 to 9 and with a temperature below 50 ° C is sprayed to an immediate To cause contact of gas and water. This gives an aqueous solution, which contains ammonium selenium cyanide (NH, SeCN). The aqueous solution and the reaction gas can then be separated from one another in the usual way. The aqueous solution can again after adjustment to the above-mentioned temperature and pH values be sprayed into the cooling device to reduce the content of ammonium selenium cyanide to increase.

The enriched wash water, which usually contains some acrylonitrile contains, is used together with oxygen or an oxygen-containing gas such as air, over or through a catalyst bed of the oxides of copper, iron and / or Nickel headed.

The temperature of the catalyst bed must be 500 ° C or above, because below 500 ° C a reaction of the selenium or the selenium compounds with the Catalyst can take place, while above 700 ° C a prevention of the catalyst effect entry.

The catalyst consists of copper oxide (Cu0), ferric oxide (Fe203) or Nickel oxide (MO). It can be used in the form of a fixed bed and on a support, such as silica or diatomaceous earth, be applied.

The enriched wash water is put into the oxidizer together sprayed with oxygen or an oxygen-containing gas such as air. The atomization takes place in a stream of air or oxygen, whereby the water evaporates and the Oxidation to selenium dioxide takes place. The received Gas mixture can the catalyst bed with a throughput rate of 10,000 to 30,000 cm3 per hour per cubic centimeter of catalyst volume are fed. The dwell time is 0.12 to 0.36 seconds.

The gas mixture leaving the oxidation device contains selenium as selenium dioxide. The selenium dioxide-containing gas mixture can immediately be reassembled with propylene be mixed to produce additional acrylic acid nitrite, or cooled, in order to obtain an aqueous solution of selenium dioxide, which also works directly with further Propylene can be mixed. The selenium dioxide is in the implementation of propylene to acrylonitrile in the same way as metallic selenium.

To demonstrate the formation of ammonium selenium cyanide during oxidation of propylene in the presence of ammonia, the following two experiments were carried out: A. A certain amount of more than 99.9% pure metallic selenium was found in one Porcelain boats heated to 300 ° C in an electric oven. Then there was air which contained 2 volume percent ammonia and 0.6 volume percent hydrogen cyanide passed through the oven and then through wash bottles containing pure water.

After a certain period of time only air was passed through, the heating of the furnace ceased, and ammonia and hydrogen cyanide were blown out. When normal aqueous sulfuric acid was added to the washing liquid up to pH 4, a red precipitate formed which was filtered off. The weight and selenium content of the precipitate and the content of ammonium sulfate and hydrogen cyanide in the filtrate were determined with the following results: Quantity Weight Selenium content Developed quantity to ver des Nieder- im Nieder steamed - cyan water - to ammo - Selenium impact blow substance niumsuffat g g ° / ogg 6.50 I 6.39 I 99.2 I 2.095 I 5.41 Accordingly, a molar ratio of Se: HCN: NH3 = 1.00: 0.96: 1.03 resulted in the washing water. The wash water contained 97.5% of the evaporated selenium and was precipitated by sulfuric acid. Before the addition of the sulfuric acid, no elemental selenium could be found in the wash water. Furthermore, no selenium was found in the filtrate of the selenium precipitation brought about by the addition of sulfuric acid in a colorimetric examination with 3,3'-diaminobenzidine.

B. In this experiment, instead of metallic selenium, an aqueous solution of selenium dioxide was added to the boat. With the rest of the same procedure as in experiment A, the following results were obtained: Salary development the added weight selenium content amount benen solution, the lower in the lower cyan to ammo expressed impact impact water sulphate_ than se fabric gg ° / ogg 3.35 I 3.55 I 99.0 I 1.12 [2.91. Accordingly, the molar ratio of Se: HCN: NH3 = 1: 0.93: 0.99; the weight of the selenium precipitate corresponded to 105% of the amount of selenium contained in the added solution. As in experiment A, neither selenium dioxide nor metallic selenium were found in the wash water. From these experiments it can be concluded that in the production of acrylonitrile from propylene, the hydrogen cyanide and the unreacted ammonia react with selenium or selenium dioxide to form ammonium selenium cyanide, which dissolves in the water used to wash out the reaction gases. Example 1 Propylene was mixed with such an amount of air that a mixture containing 2.3 volume percent propylene was obtained. This gas mixture was heated to 310 ° C. and passed over metallic selenium, whereupon it contained 1.4 g selenium per cubic meter. The gas mixture was then passed together with ammonia at a temperature of 400 ° C. through a pipe containing copper oxide as a catalyst and then through a pipe containing molybdenum oxide as a catalyst; a reaction gas was obtained which contained 1.65 percent by volume of acrylonitrile. This reaction gas was fed to a washing device and water with a pH of 8.2 and a temperature of 20 ° C. was fed at the same time until the gas temperature had dropped to 50 ° C. The washing water and the reaction gas were then separated from one another in a cyclone.

The separated washing water was heated to a temperature of 20 ° C and adjusted to a pH of 8.2 with an aqueous caustic soda solution and again returned to the scrubber: contained the gas leaving the scrubber 0.02 g selenium per cubic meter and was repeated in the - washer in the circuit returned until the selenium content had risen to 10.2 g / l. The wash water was concentrated at normal pressure and 90 ° C. until the selenium content was 102 g / l. Afterward this washing water was washed at a rate of 11 / hour. and with a throughput speed of 20,000 cm3 per hour per cubic centimeter of catalyst volume together with 13 m3 Air is pumped through a tube every hour, which is supported on silica gel with 650 cms applied copper oxide was filled and kept at 500 ° C. The outflowing The gas mixture contained 8.77 g selenium per cubic meter. It was cooled down to 70 ° C, whereby 964 cms of an aqueous solution per hour with a selenium content of 105 g / l were obtained.

The yield was 99.20 / "based on the amount used at the beginning Selenium. The aqueous solution was a stream of air containing 2.3 percent by volume propylene, added in an amount that a gas mixture with a selenium content of 1.4 g / ms was present, which was converted into a reaction gas with 1.63 volume percent acrylonitrile as above became. Example 2 The procedure of Example 1 was repeated with the exception that the enriched wash water at a rate of 11 / hour. together Introduced with 13 m3 of air per hour into an oxidation device kept at 650 ° C was that with 650 cm3 of nickel oxide was filled. With a flow velocity of 11.5 m3 / h a gas mixture was obtained which contained 8.80 g of selenium per cubic meter. The selenium yield was 99.9%, based on the selenium content of the wash water. The outflowing gas mixture was mixed with air containing propylene, whereby obtained a gas mixture with 2.3 volume percent propylene and 1.4 g selenium per cubic meter was, which was implemented as in Example 1, creating a gas mixture with 1.66 percent by volume Acrylonitrile was created.

Claims (1)

Claim: Process for the recovery of selenium as selenium dioxide from those containing reaction gases which have been obtained by oxidation of propylene with oxygen or air in the presence of ammonia and selenium as a catalyst to acrylonitrile at high temperature, by washing with weakly alkaline water at a pH Value of 7 to 9 and a temperature of 20 to 50 ° C, characterized in that the aqueous solution of ammonium selenium cyanide obtained by washing, which can also have been used or concentrated several times to wash out fresh reaction gases, is finely atomized in molecular oxygen or this containing gases via copper oxide (Cu0), ferric oxide (Fe203) or nickel oxide (Ni0) as a catalyst, which can be applied to a carrier such as silica or diatomaceous earth and has a temperature of 500 to 700 ° C. Considered publications: German Patent Nos. 831837, 852 992, 863 493, 880134, 937 287, 941428; Swiss patent specification No. 306 049 (reported in Chemisches Zentralblatt, 1956, p. 7936); British Patent Nos. 719 635, 727 318; U.S. Patent No. 2,716,664.