Chemistry of inorganic compounds – Phosphorus or compound thereof – Oxygen containing
Patent
1989-11-03
1991-01-22
Langel, Wayne A.
Chemistry of inorganic compounds
Phosphorus or compound thereof
Oxygen containing
C01B 25234
Patent
active
049869701
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for removal of heavy metals, especially cadmium, from phosphoric acid containing solutions, primarily such solutions which are made by digestion of rock phosphate with nitric acid and subsequent removal of the main parts of calcium from the mother liquor. The method comprises pre-purification of the phosphoric acid solution/mother liquor for removal of sludge and insoluble components and subsequent partial neutralization with ammonia.
In past years it has become increasingly important to remove heavy metals, and then especially cadmium, partly because of more strict environmental regulations, but also because the supply of high grade rock phosphate having a low content of heavy metals has decreased.
Several methods for solving this problem have been proposed and at which stage during the manufacture of phosphate fertilizer the purification shall be performed. These proposals comprise calcination of the rock phosphate, leaching, precipitation or extraction of the heavy metals. It is just not adequate to find methods for separating the heavy metals from the valuable components as the content of heavy metals is relatively low, and methods must be developed which in practice are technically feasible and in addition economically acceptable.
Among the methods which comprise precipitation, that one described in DE No. 31 34 847 can be mentioned. This method relates to precipitation of Cd as sulfide. The most essential disadvantage of this method is addition of poisonous H.sub.2 S which creates large environmental problems in the production plant itself.
It is further known from the assignee's Norwegian application No. 86 4374 to remove Cd by co-precipitation with calcium at pH 2-7. The mother liquor is neutralized by ammonia at a temperature between 80 .degree. C. and the boiling point of the solution. Even though it is possible by this process to remove most of the Cd from the mother liquor, the high pH results in precipitation of valuable components too. This requires special ways of carrying out filtering and make-up stages in order to make the process economical.
There are also known several processes for removal of heavy metals like Cd by liquid--liquid extraction. It has been especially difficult to make such processes feasible when the phosphoric acid solution contains nitric acid. Up to now the extraction processes have not been found very suitable technically and economically for treatment of such large amounts of solutions which one had to deal with in fertilizer processes. One type of extraction process is described in patent application EP-91043. This process relates to removal of heavy metals like Cd, Hg and Pb from crude phosphoric acid (mother liquor) made by digestion of rock phosphate with nitric acid and subsequent removal of calcium nitrate. The crude phosphoric acid is purified and neutralized with ammonia to pH 0.5-1.5, whereupon it is contacted with a di-organyl-dithio phosphoric acid compound and an absorption agent, for instance active carbon. The heavy metals are thereby bound to the di-thio phosphoric acid compound which is adsorbed to the active carbon and removed therewith from the phosphoric acid solution. This process comprises high running costs, for instance because of costly chemicals and adsorbent. Regeneration of the adsorbent will, if possible, be expensive, and the alternative is to deposit large amount of refuse.
The object of the present invention was to remove heavy metals, especially Cd, in a technically and economically advantageous way from phosphoric acid containing solutions, especially solutions made by digestion of rock phosphate with nitric acid.
Having tested several of the known methods for removing heavy metals from phosphoric acid containing solutions, the inventors found that it was necessary to look for new methods. During this preliminary work one found that application of extraction agents would create several problems and therefore one decided to continue the study of precipitation processes. The problems related to
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Langel Wayne A.
Norsk Hydro a.s.
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