Removal of Magnesium and Aluminum Impurities From Wet Process Phosphoric Acid

One process for the preparation of phosphoric acid is known as the "wet process". This process involves the acidulation of phosphate rock with a mineral acid such as sulfuric, phosphoric or nitric to produce the phosphoric acid. During acidulation of the rock there will be formed, in addition to the phosphoric acid, a solid precipitate. If sulfuric acid is the digesting acid the precipitate will include gypsum as its major constituent. In the conventional dihydrate process the aqueous phosphoric acid may be separated from the precipitate by filtration to produce a dilute or unconcentrated filter grade phosphoric acid product usually containing from 25-33% P.sub.2 O.sub.5. The resulting filter cake is countercurrently washed and eventually discharged. The filtrate from the cake washings are used as future cake washing solutions or are returned to the reaction section.

Phosphate rock contains various amounts of impurities and the level of impurities found in many remaining phosphate deposits is increasing. Among the increasing impurities are magnesium and/or aluminum containing compounds. When the phosphate rock is digested with sulfuric acid, as discussed above, the metal compounds (usually salts and oxides of the metal) may readily go into solution. These impurities may then form precipitates and settle out as solids or sludge over a period of many days or even months. The filtration rate of liquids containing dissolved metal ions and finely divided solids is low and the filter media is easily clogged. The metal ion containing impurities are difficult and costly to remove from the phosphoric acid because when precipitated they may carry with them very appreciable quantities of valuable P.sub.2 O.sub.5 which is so combined that it cannot be effectively removed by leaching with water or dilute acid.

Further, when the phosphoric acid (25-33% P.sub.2 O.sub.5) is concentrated the solubility of many of the initially soluble impurities may be exceeded and further precipitation occurs. Also problems may exist when the phosphoric acid is neutralized, or otherwise treated, as for example in the manufacture of liquid plant food, wherein wet process phosphoric acid and ammonia are reacted to produce a substantially neutral solution for use as a liquid plant food base. During the neutralization of the acid which contains the dissolved impurities the precipitation of sludge, especially magnesium containing salts, can occur. These impurities act to form deposits which may clog the transfer and distribution equipment. These impurities also result in loss of P.sub.2 O.sub.5 values and these sludges may hinder the removal of other types of impurities.

Various methods have been suggested in the prior art to obtain a more pure form of phosphoric acid. According to U.S. Pat. No. 3,935,298 iron impurities can be removed by mixing an alkali metal halide with the starting phosphate rock and firing the mixture at a temperature of 300 to 1100.degree. F. of before acidulation. U.S. Pat. No. 3,562,769 teaches the use of alkali metal salts as disintegration-preventing agents which can be used to retain impurities in a solid state while phosphoric acid is extracted, while U.S. Pat. No. 3,408,162 discloses that by adding a relatively small amount of an alkaline salt of lignosulfonic acid to the wet process phosphoric acid solid impurities may be maintained in suspension and crystal growth may be inhibited. U.S. Pat. No. 3,554,694 discloses use of a sodium salt addition in order to remove the fluorine present and obtain a commercially pure sodium fluosilicate.

Other methods exist for the removal of aluminum. U.S. Pat. No. 3,843,767 shows treatment of the original ore with pure phosphoric acid to obtain a low concentration of aluminum and iron impurities. U.S. Pat. No. 2,954,287 shows the use of sulfuric acid in combination with alkali salts in order to keep free aluminum concentration low. In another process, shown in U.S. Pat. No. 2,494,736, silicon is first removed and then fluorine and sodium are added to form a crystalline sodim/aluminum/fluorine salt that is easily separable. One other process deals with the recovery of fluorine from phosphate ore which involves precipitating aluminum and fluorine in the form of fluoaluminate. See, for details, U.S. Pat. No. 3,512,927.

German Patent Application No. 2,046,295, based on U.S. patent application Ser. No. 866,752, recommends the addition of a source of soluble fluoride to wet process acid to precipitate magnesium impurities as magnesium-aluminum-fluoride salts. It proposes that the precipitation be induced in concentrated (45-53% P.sub.2 O.sub.5) wet process acid to avoid competition between silicon and aluminum for the added fluoride ion. However, the high viscosity of concentrated wet process acid makes phase separation difficult. Further, the process recommends adjustment of the aluminum to magnesium mole ratio to at least 1.11 and preferably 2.37 to 9.48. The fluoride to magnesium mole ratio is recommended to be at least 4.67 and preferably between 6.37 and 25.5.

It is believed that the foregoing processes have not obtained a high efficiency and that they tend to be heavily burdened with capital expense necessary for production equipment. The techniques involving precipitation of impurities have advantages over other potential purification techniques due to the simplicity and compatibility with existing phosphoric acid facilities. However, precipitated impurities within the acid can settle out to damage equipment and clog filters and they are responsible for a substantial loss of P.sub.2 O.sub.5 values.

It is therefore desired to obtain better quality phosphoric acid, and it is also desired to be able to remove the impurities in as large a quantity and as early in the process as is possible, without encountering substantial processing problems.

The invention generally relates to a method of removing magnesium and aluminum impurities from wet process phosphoric acid by adding to unconcentrated wet process acid a fluoride ion donating compound to create a fluoride ion to aluminum ion ratio between about 3.5/1 and 7/1 to permit formation of insoluble crystalline compounds having a stoichiometry ranging between MgAl.sub.2 F.sub.8 and MgAlF.sub.5. In a preferred embodiment the fluoride ion donating compound is added after separating the wet process acid from the gypsum slurry.

In some cases an aluminum ion donating compound is also added to the unconcentrated acid in an amount sufficient to bring the aluminum ion to magnesium ion ratio to between about 1.1/1 and 2.0/1. The unconcentrated acid may then be directed to a crystallizer to permit growth of crystals which will readily precipitate and permit removal from a wet process phosphoric acid having controlled amounts of magnesium and aluminum impurities.

Those skilled in the art of producing phosphoric acid by the "wet process" know that the composition of "wet process" acid is dependent upon the nature of the phosphate rock used in the process of producing the acid, but that most acids will contain unwanted impurities including magnesium and aluminum containing impurities which, if not remove, can slowly precipitate and settle out of the acid to form a sludge which can cause the acid to have a gelatinous property.

In accordance with the present invention it is possible to reduce the magnesium content of filter grade wet process acid (of magnesium salts which by conventional wet chemical analysis are expressed as MgO) to about 200 p.p.m. or lower, as well as reducing the aluminum content to low levels.

This reduction of magnesium and aluminum impurities from wet process acid is preferably accomplished after the phosphate rock has been acidulated by a mineral acid, such as sulfuric acid, and the solids, predominately gypsum, removed by filtration or settling. To the "filter grade" wet process acid (that acid which passes through the filter used to remove gypsum) which normally contains between 25 and 33% P.sub.2 O.sub.5, but may contain up to about 40% P.sub.2 O.sub.5 is added a fluoride ion donating compound such as hydrofluoric acid, sodium fluoride, sodium bifluoride, ammonium fluoride or ammonium bifluoride. While hydrogen fluoride gas may be used, it is not preferred for safety reasons. H.sub.2 SiF.sub.6, Na.sub.2 SiF.sub.6 and Na.sub.3 AlF.sub.6 were tested as possible sources of fluoride ion, but did not yield the preferred effects.

In order to demonstrate the relative effectiveness of the fluoride donating compounds the following Tables 1-5 are provided. To obtain the data set out in the tables, the following test procedures were employed:

1. MgO and MgCO.sub.3 were added to "filter grade" wet process acid to produce acids having different MgO content which might be representative of acids expected to be produced from phosphate ores which might be mined at different locations throughout the world.

2. Weighed amounts of the fluoride ion donating compound were added to weighed amounts of the above "filter grade" wet process acid and the solutions were vigorously shaken and stored overnight (16-20 hours) at 60.degree. C.

3. The liquid phase was separated by centrifugation and submitted for analysis of major components by conventional techniques.

The results of these tests were as follows:

Additional testing was performed to determine the effect of increasing quantities of fluoride ion donating compounds on filter grade wet process acid. In obtaining the results listed in Table IV, the same test procedure described above was followed except 100 gram samples of filter grade wet process acid were used: no foreign compounds were added to sample 1; 3.0 grams of 49% HF solution (hydrofluoric acid) and 1.4 grams of NaF were added to sample 2 in each instance; and 4.0 grams of 49% HF solution and 2.0 grams NaF were added to sample 3 in each instance.

Table V sets forth the results of tests run to determine the effect of employing a mixture of fluoride ion donating compounds. In obtaining the data reported in Table V the testing procedure used in obtaining the data reported in Table IV was used, except with respect to sample 2, in each instance was added 3.0 grams 49% HF soluton and to sample 3, 3.0 grams HF solution and 1.4 grams NaF.

It is possible, according to one embodiment of the present invention, to add the fluoride ion donating compound to the wet process acid before filtering of by-product gypsum therefrom. However, this approach is not the preferred embodiment because silica in the acid competes with the aluminum ions for the fluoride ion and, therefore, more fluoride ions are required to precipitate impurities from the wet process acid. Also, the filtration rates are decreased when the acid is treated with hydrofluoric acid prior to filtration.

Generally, from the tests performed, it can be concluded that impurities will precipitate only when the ratio of fluoride ions to aluminum ions exceeds 3/1 and that when the ratio exceeds 6/1 the precipitation of impurities is less efficient.

Further, aluminum precipitation is limited by the amount of magnesium present and magnesium precipitation is limited by the amount of aluminum present. The ratio of aluminum ions to magnesium ions precipitated can generally vary between 1.1 and 2.0. It may be necessary, in some cases, to add alum or another aluminum ion donating compound (such as scrap aluminum, for example) to the wet process acid in order to attain the desired ion ratio set forth above. This addition can be effected either before or after filtration of the acid. Thus, for example, the alum can be added during the digestion of the phosphate rock by mineral acids and before filtration of the resulting wet process phosphoric acid. Conversely, in some cases magnesium ion donating compounds may need to be added to achieve the desired ratio.

To demonstrate the embodiment of the present invention wherein aluminum ion donating compounds are added for this purpose, the following experiment was performed. To 100 gm of base sample of wet process phosphoric acid (having the composition set forth in Table VI) was added 10.24 g of 49% HF solution. After storage for 24 hours at about 60.degree. C. this acid was centrifuged to yield solids free acid, designated "No Alum Added" in Table VI. Another 100 gm of this base sample acid was admixed with the proper amount of alum, Al.sub.2 (SO.sub.4).sub.3.18H.sub.2 O to adjust the Al.sub.2 O.sub.3 :MgO mole ratio to 2. To this acid 10.24 gm of 49% HF solution was added and after storage for 24 hours at about 60.degree. C. the acid was centrifuged to obtain the solids free acid designated "Alum Added" in Table VI.

As can be seen from a study of Table VI the addition of the alum to obtain the preferred mole ratio of Al.sub.2 O.sub.3 :MgO provided greatly increased reduction of the MgO:P.sub.2 O.sub.5 ratio in the acids treated with the fluoride ion donating compound.

The crystalline compounds formed by the process of the subject invention, while difficult to analyze, have a stoichiometry ranging between MgAl.sub.2 F.sub.8 and MgAlF.sub.5 and it is believed that one of the dominant compounds can be represented by the formula MgAl.sub.2 F.sub.8. This belief is supported by infrared spectral data obtained from samples of the precipitated solid phase. In order to obtain a sample of the solid phase for spectral analyses, the solids were washed with reagent grade phosphoric acid and then the phosphoric acid was extracted from the solids with .eta.-butanol and then dried under a vacuum. The spectral data, including that the Al to F ratio is approximately 1:4, is set forth in Table VII.

Separation of the crystals containing unwanted Mg and Al ions can best be effected if the crystals are permitted to grow to a size at which they will readily settle. Those skilled in the art are familiar with techniques for growing crystals from solutions and may adapt those principles to effect either batch or continuous crystallization of the Mg and Al containing impurities so the purified wet process acid may be separated for further use. See, e.g. A. Van Hook, Crystallization: Theory and Practice, Reinhld, New York, 1961.

The wet process acid produced by the above described techniques is a higher quality acid than produced by conventional wet process acid processes, admits of easier concentration by evaporator techniques, produces a higher P.sub.2 O.sub.5 acid in the evaporator with less sludge formed, and provides an acid which can be more easily retracted with ammonia and other compounds to produce useful products.