It is known that when phosphoric acid is made by treating phosphoric rock with sulfuric acid, a substantial portion of the impurities in the rock are dissolved and are present in the resultant phosphoric acid. The principal impurities which are difficult to remove are complexes containing calcium, aluminum, silicon, fluorine, magnesium and sulfate. As is well known, the removal of such impurities is most difficult and troublesome for phosphate producers.
In many prior art phosphoric acid purification processes the impure phosphoric acid has been concentrated by evaporation at elevated temperatures. During the concentration step compounds of fluorine, principally in the form of silicon tetrafluoride is evolved. This heating in the concentration step, however, does not remove compounds such as aluminum and magnesium. Futhermore, the fluoride fumes driven off by the heating process are harmful to the environment if not recovered from the vent gases. Such recovery systems for fluoride fumes are expensive and troublesome to maintain. Furthermore, during the concentration step there is considerable scale formation and objectionable corrosion of equipment by the fluorine compounds.
It has also been proposed to recover the fluorine component of phosphoric acid and sodium fluosilicate, by adding sodium chloride to the acid and precipitating sodium fluosilicate. This purification procedure suffers from the disadvantage of introducing chloride into the acid which can cause corrosion of metals.
British Pat. No. 1,024,924 discloses a process for removing impurities from wet-process phosphoric acid at temperatures of about 130.degree. C. at P.sub.2 O.sub.5 concentrations greater than 30%. This latter process is difficult to run due to severe corrosion at the higher temperatures and the instability of the anyhydrite formed during processing.
A detailed description of a method for producing phosphoric acid by the dihydrate process can be found in U.S. Pat. No. 4,277,448 to Ore et al. Processes for preparing phosphoric acid by hemihydrate processes are described in U.S. Pat. No. 4,196,172 to Ore et al and U.S. Pat. No. 4,220,630 to Ore.
Processes for producing phosphoric acid from high alumina pebble rock and/or from unbeneficiated or partially beneficiated phosphate rock matrix are described in U.S. Pat. No. 4,284,614 to Ore and copending applications Ser. No. 378,456 filed May 17, 1982, by Eli Chemtob and Gary L. Beer and in Ser. No. 282,466 filed July 13, 1981 by Fernando Ore, the entire disclosure of both of said applications being hereby incorporated herein. The present invention can be especially useful as a means of controlling the magnesium and/or aluminum content of the various streams containing impure phosphoric acid described in said patent and applications.
When phosphoric acid is made by any of these processes, impurities in the rock are dissolved and are present in the resulting phosphoric acid. The principal impurities which are difficult to remove are compounds and complexes containing calcium, aluminum, silicon, fluorine, sulfate, and magnesium. Removal of these impurities is important because the phosphoric acid is usually concentrated up to about 48 to 54 weight percent P.sub.2 O.sub.5 and, during storage, the impurities can precipitate to form a solid which is found objectionable by most consumers.
Many attempts have been made to remove impurities from wet process phosphoric acid. Exemplary of the techniques used are those described in U.S. Pat. Nos. 3,124,419; 3,206,282; 3,273,713; 3,379,501; 3,442,609; 3,481,700, and 3,642,439; and British Pat. Nos. 467,843 and 1,337,669. Methods described in these patents generally are complicated and difficult to use.
For example, British Pat. No. 1,337,669 discloses a process for precipitation of ionic metal impurities from phosphoric acid by adding an organic water soluble and alkali metal or ammonium ions thereto, filtering off the precipitate, then passing the filtered solvent/phosphoric acid phase through a strongly acid cation exchange resin and, finally, removing the solvent from the purified acid by distillation.
U.S. Pat. No. 3,642,439 to Moore et al, discloses a process for removing magnesium impurities from weak, wet process phosphoric acid by evaporating the acid until the SiO.sub.2 content of the acid is reduced to less than 0.2%, maintaining the evaporated acid at 50.degree.-100.degree. C. for at least 15 hours to form a precipitate comprising a magnesium-aluminum-fluoride-phosphate complex compound and separating the precipitate from the purified, concentrated phosphoric acid. Further disclosed is the addition of a "seeding" compound having a weight ratio of magnesium oxide to aluminum oxide of 1 to 1.4 and a ratio of magnesium oxide to fluorine of 1 to 2.2.
U.S. Pat. No. 4,229,804 to Parks describes a means of removing magnesium from impure phosphoric acid which has not been concentrated by evaporation and which can be relatively dilute with respect to P.sub.2 O.sub.5 analysis.
U.S. Pat. Nos. 4,136,199 and 4,243,643 to Mills discloses a process for removing metal ion impurities from concentrated or unconcentrated phosphoric acid by adding a precipitant, or reagent, which comprises ions of calcium and fluorine to cause precipitation of a fluoride solid which contains ions of magnesium and of other metals such as aluminum and sodium. The precipitant can be a solid comprising calcium fluoride, such as the sludge obtained by treating pond water from a phosphoric acid plant with lime or limestone (sometimes hereinafter called "Synspar") as described in U.S. Pat. No. 4,171,342 to Hirko et al. Other reagents disclosed are calcium fluoride and various synthetic fluorspars (e.g., U.S. Pat. No. 3,907,978; U.S. Pat. No. 4,043,803; U.S. Pat. No. 2,914,474; U.S. Pat. No. 2,780,523; U.S. Pat. No. 2,780,521; U.S. Pat. No. 3,379,501 and U.S. Pat. No. 3,800,029).
U.S. Pat. No. 4,264,563 to Sikdar describes a process for the preparation of a calcium fluoride-containing solid which does not contain significant amounts of free silica, e.g. silica gel or a silica sol. It is further disclosed that free silica in calcium fluoride solids used for treating impure phosphoric acid could interfere with the removal of contaminants from the phosphoric acid. The calcium fluoride-containing material can be used in the present invention.
There is a need for a process which is relatively simple and economical and which does not require evaporation to remove impurities from phosphoric acid. Evaporative processes to remove, for example, fluorine, typically a silicon tetrafluoride or fluosilicic acid, are environmentally undesirable, and capital and operating costs are intensive because of the requirement for expensive and troublesome pollution abatement systems.
Furthermore, the process should be such that there is little phosphorous, or P.sub.2 O.sub.5 lost as a result of the impurity removal process. For example, if the impurities are removed as a complex precipitate, such precipitate should not have substantial amounts of phosphorous (or P.sub.2 O.sub.5 values) in the form of a solid substance as part of the structure of the complex precipitate.
As used herein, the word "values" means a material which upon analysis contains the indicated element in chemical combination or, in the case of "P.sub.2 O.sub.5," contains the element phosphorous, in some chemical combination.
The present invention provides a process which does not have many of the above-described problems and disadvantages.