FI83863C - Process for removal of heavy metals from phosphoric acid - Google Patents

Description

5 1 83863

Method for removing heavy metals from phosphoric acid -Förfarande för avlägsning av tungmetaller frän fosforsyra

The present invention relates to the removal of heavy metals, and in particular cadmium, from phosphoric acid using an oil agglomeration separation process.

10 Phosphoric acid is made from phosphorus-containing minerals extracted from nature. In manufacturing, two fundamentally different methods are distinguished, the thermal process and the wet process.

15 The thermal process produces elemental phosphorus and further obtains high purity phosphoric acid. Due to the high process costs of the process, its use is limited to the preparation of pure phosphates.

20 The wet process produces most of the world's phosphoric acid. The product contains most of the impurities in the crude phosphate, including heavy metals, without a separate purification process. In the wet process, the crude phosphate is dissolved in a mineral acid, usually sulfuric acid.

Phosphoric acid is separated from the resulting calcium sulfate slurry by filtration. Phosphoric acid produced by this method is used e.g. for the manufacture of fertilizers. Because phosphoric acid contains heavy metals as impurities, they should be removed from the phosphoric acid to reduce the heavy metal load on the environment. In particular, cadmium must be removed from phosphoric acid, which ends up in fertilizers and thus in food chains.

The object of the present invention is to remove heavy metals and in particular cadmium from phosphoric acid obtained by a wet process. It is a further object of the present invention to separate heavy metals, and in particular cadmium, from gypsum obtained as a by-product. Gypsum does not have enough 2 83863 uses in industry, so most of it should be disposed of as waste. One way of disposing of the waste is to discharge it into the sea slurried in water or to store it in piles on the surface, in which case the heavy metal content of the gypsum must comply with the prevailing regulations.

Several different procedures are known for removing heavy metals such as cadmium from phosphoric acid.

10 One method is the calcination of phosphate, which is usually intended to reduce the amount of phosphate organic matter and carbonate. A high temperature of about 1150 ° C must be used in the calcination, so the energy requirement of the process is high and, in addition, the removal of cadmium by calcination is poor, 15 especially if the Al, Fe and Si contents are high. EP 154554 discloses a two-step calcination process which can remove about 75% of phosphate cadmium on a small scale.

20 Cadmium sulphide precipitation from phosphoric acid gives a yield of more than 90% on the basis of small-scale experiments.

The disadvantages of the method are its high cost, and in addition it is considered to be corrosive, occupationally hazardous and explosive. FI patent 66162 describes a sulfide precipitation process in which an alkali is added before precipitation to neutralize free sulfuric acid. In small-scale experiments, the Cd content in the acid has been reduced from 60 ppms to 0.2 ppm.

30 Cadmium can also be removed by extraction or foaming. U.S. Patent 4,634,580 discloses a method for removing cadmium using a flotation chemical. DE 3 342 211 discloses an extraction process in which Cd is selectively extracted into the HCl salt of an alkylamine.

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For example, FI patent application 871867 discloses an ion exchange process by which cadmium can be removed from 40-50% phosphoric acid at 90 ° C using an anion exchanger complexed with iodide or bromide. The disadvantages of the method are the large waste streams, i.e. the large amount of filtrate required and the large volume of the regeneration solution. In addition, large amounts of ion exchange resins are also required.

FI patent application 892178 discloses a method in which heavy metals, and in particular cadmium, are removed from phosphoric acid using an electrolytic cell. However, the disadvantages of method 10 are the abundant hydrogen evolution and the high cost of complete cadmium removal.

It has now been found that the process according to the present patent application removes heavy metals from phosphoric acid very efficiently and inexpensively. The method according to the invention is based on the so-called an oil agglomeration separation method in which small particles are agglomerated with oil and thus transferred from the aqueous phase to the oil phase. In the process of the invention, a heavy metal chelating precipitating reagent and an organic solvent are added to the heavy metal-containing phosphoric acid, the mixture is aerated with stirring, the phases are allowed to separate and pure phosphoric acid is recovered.

25 In Indian Journal of Technology, Vol. 22, 1984, (bulk oil flotation) pp. 194-196 describes an oil agglomeration separation process for enriching graphite from ore using kerosene and water.

In the process according to the invention, a precipitating reagent and a solvent are added at different stages of the phosphoric acid production process or to phosphoric acid of different strengths. The mixture is aerated from below (subaeration), whereby the partially solid compound formed with cadmium, copper and other heavy metal precipitating chemicals is agglomerated as a solvent and transferred to the organic liquid phase by aeration and stirring. Aeration is not necessary, but it achieves an enhanced effect. The mixture is separated into two 4 83863 phases, of which the lower or pure phosphoric acid is recovered and the upper organic phase is regenerated. This makes it possible to recycle the solvent.

As the precipitating reagent, a thiophosphate is used, such as, for example, Na-diethyldithiophosphate, the use of which as a foaming reagent is known per se, for example from Spec. Pub. - R. Soc. Chem., Vol. 61, 1986, pp. 109-112. This surfactant forms a complex with the resolving agent 10 in solution, i.e. in the case of the present application with heavy metals and in particular cadmium. Another thiophosphate surfactant reagent such as sodium diisobutyl dithiophosphate can also be used as the precipitating reagent. The complex is in the form of a fine, partially oily precipitate that cannot be filtered from solution. According to the invention, the complex precipitated or optionally in solution is agglomerated by means of a solvent with a solvent on the surface of phosphoric acid. As the solvent, hydrocarbons or other common organic solvents are used, for example, methyl isobutyl ketone or isononanol. The use of a solvent according to the invention has been found to be very advantageous because it allows the chelate to be completely separated from the acid without loss of phosphoric acid. In conventional flotation, the loss of phosphoric acid with the foam 25 and in the filtrations is significant.

Cadmium can be separated at different stages of the wet process of phosphoric acid production. The separation can be carried out either from the product acid or from the acid returned to the reactor, in which case the Cd content of the phosphoric acid in the reactor decreases and this in turn results in a decrease in the Cd content of the gypsum.

The process according to the invention makes it possible to easily separate cadmium and other heavy metals from phosphoric acid without filtration, so that no phosphoric acid loss occurs. In addition, the process has the advantage of being able to recycle the solvent and precipitation reagent, making it advantageous to carry out the process.

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In the following, the invention is described by way of examples. Example 1 used phosphoric acid with a P2Os content of about 30% and containing about 64 ppm cadmium. Examples 2 and 3 used phosphoric acid having a P 2 O 5 content of about 20%, a Cd content of about 10.9 ppm and a Cu content of about 27.8 ppm.

Example 1 1560 g of phosphoric acid (1.2 l), 3 g / l of Na-diethyldithiophosphate and 100 ml of methyl isobutyl ketone (MIBK) were placed in a vessel. Aeration at 730 rpm. During aeration, the MIBK was partially mixed with acid, but separated immediately after stirring. The following Table-15 shows the results of the experiment, which show that aeration time of 5 minutes was sufficient.

Table

Aeration time Acid Cd content MIBK Cd content 20 5 min 2 ppm 1530 ppm 10 min 2 ppm 1530 ppm 15 min 3 ppm 1530 ppm 25 Example 2 To a 2 liter vessel was added 1198 g (= 1 L) of phosphoric acid, 1 g Na diisobutyldithiophosphate and 100 ml of methyl isobutyl ketone as solvent. Aeration was performed for 15 minutes at 3000 rpm. The organic phase was then separated by decantation. After the experiment, the Cd concentration in the acid was 0.6 ppm and in the MIBK 174 ppm.

70 ml was recovered from MIBK.

Example 3 1198 g of phosphoric acid, 1 g of Na-diisobutyldithiophosphate and 100 ml of isononanol were placed in a 2-liter vessel. Aeration was performed for 15 minutes at 3000 rpm. After the experiment, the Cd content in the acid and isononanol was analyzed. When the 83863 acid content in the acid was 0.6 ppm, in isononanol 122 ppm and the copper content in the acid was <0.4 ppm.

Claims (6)

7 83863 A process for removing heavy metals from phosphoric acid using an oil agglomeration separation method, characterized in that a heavy metal-binding precipitating reagent and an organic solvent are added to the heavy metal-containing phosphoric acid 5, the mixture is aerated with stirring, the phases are separated and the phosphorus is recovered. Process according to Claim 1, characterized in that the heavy metal is cadmium and / or copper. Process according to Claim 1 or 2, characterized in that the precipitating reagent is thiophosphate. 15 Process according to Claim 3, characterized in that the thiophosphate is sodium diisobutyl dithiophosphate or sodium diethyl dithiophosphate. Process according to Claim 1, characterized in that the removal of cadmium and / or copper is carried out from the product acid of the wet process for the production of phosphoric acid. Process according to Claim 1, characterized in that the removal of cadmium and / or copper is carried out from the acid returned to the reactor of the wet process for the production of phosphoric acid. 8 83863