AT72604B - Process for the preparation of pure zinc oxide. - Google Patents

Description

  

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  Process for the preparation of pure zinc oxide.



   The present process intends to produce pure zinc oxide using zinc minerals and especially those minerals whose zinc content is too low to be processed by metallurgy or using any zinc salt as a starting material.



   So far, the zinc has been leached from the minerals in a known manner using a suitable acid such as sulfuric acid, hydrochloric acid or nitric acid; The acid used was expediently mixed with a sufficient amount of water in order to prevent the zinc salt from crystallizing out during the leaching process. As usual, the alkaline solutions obtained were freed from the foreign metals, some of which were also dissolved by the acid, and the zinc was then precipitated as zinc sulfite by means of ammonium sulfite, from which pure zinc oxide was obtained after filtering and washing by annealing , which is suitable for all known uses, namely as a white pigment for painting and as a starting material for the production of pure metallic zinc.

   The sulfur dioxide that escapes during the glow is used to regenerate the ammouium snint.



   In carrying out this known process, the procedure used to date was to add the ammonium sulfate to the zinc solution in an equivalent amount to the dissolved zinc salt.



   Under these conditions, however, some of the zinc used is not precipitated and remains dissolved in the mother liquor. This circumstance is not an obstacle to the preparation of zinc sulphite in the laboratory, but it becomes so in the case of the factory production of this product, for the amount of zinc which escapes the precipitation reduces the yield by 10 to 200%.



   In the above process, these losses are avoided and almost all of the zinc is precipitated by adding caustic ammonia or carbonate of ammonia to the reaction mixture or to the ammonium sulfite used for this purpose during or before the reaction. This effect of ammonia could not be foreseen, because it is well known that most zinc precipitates are dissolved again by an excess of ammonia or carbonate of ammonia. You can also do the caustic
Generate ammonia or the carbonic acid ammonia during or before the reaction by adding to the reaction mixture or the ammonium sulfite used for this purpose
Adds alkali or alkaline earth which, through double reaction with ammonium sulphite, produces ammonia or carbonate of ammonia, namely hydrated lime or carbonate of sodium.



   The sulfur dioxide that is produced during the glowing of the zinc sulphite becomes
Generation of Ammoniumsuiit used, so that this is regenerated again and again, as already mentioned.



   According to a known method, ammonium sulfate is produced by absorbing sulfur dioxide in ammonia water. This process would not be suitable for the present invention, because the sulfur dioxide which escapes during the glowing of the zinc sulphite or the zinc blende contains a certain amount of air, and it was found:

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 EMI2.1
 is oxidized, resulting in a significant loss of sulfur dioxide;
2. that the added air, when passing through the aqueous ammonia, carries with it a significant amount of ammonia, causing a loss of ammonia.



   It was also found that ammonium sulfite is significantly less oxidizable in the presence of free acid than in the presence of free alkali, and that the
 EMI2.2
 Ammonia absorbed, but probably in a solution of neutral or weakly acidic ammonium sulfate. One could also use weakly alkaline ammonium sulphite for this purpose, because a low alkalinity would quickly be rendered harmless by the acidic sulfur dioxide.



   Since ammonium sulfite and ammonium bisulfite in aqueous solution are not fruity at normal temperature, ammonia cannot be carried away by the air flow; In addition, the ammonium sulfite becomes immediately acidic due to the absorbed sulfur dioxide, so that the oxidation to sulfate is only slight under these conditions, as already mentioned. The sulfur dioxide dissolved in the ammonium sulfite as bisulfite is subsequently neutralized outside the absorption apparatus with caustic ammonia, and. in a closed container to avoid oxidation by air.



  The ammonia required for this purpose is obtained from the ammonia salt, which is produced in the precipitation of the zinc sulfite by double conversion according to the following equation
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Claims (1)

PATENT CLAIM: Process for the preparation of pure zinc oxide by precipitating the solutions of zinc salts with ammonium sulfite and expelling the sulfur dioxide from the precipitated zinc sulfite by annealing, characterized in that the zinc is precipitated from the solutions in the presence of caustic ammonia or carbonate of ammonia, and that this is done on heating the sulfur dioxide escaping from the zinc sulfite is introduced into a solution of ammonium sulfite, whereby the ammonium sulfite is converted into ammonium bisulfite, from which the ammonium sulfite required for zinc precipitation and for the absorption of sulfur dioxide is obtained by adding caustic ammonia, which the latter is produced from the ammonium salt solution formed during zinc precipitation.