FI82672C - FOERFARANDE FOER ELEKTROLYTISK AVLAEGSNING AV TUNGMETALLER FRAON FOSFORSYRA. - Google Patents

Description

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The present invention relates to the electrolytic removal of heavy metals, and in particular cadmium, from phosphoric acid.

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.

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

The wet process produces most of the world's phosphoric acid. Without a separate purification process, the product contains most of the impurities contained in the crude phosphate, including heavy metals. In the wet process, the crude phosphate is dissolved in a mineral acid, usually sulfuric acid. A flow chart of a typical wet process phosphoric acid plant is shown in Figure 1. 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. As 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 the phosphoric acid obtained by the 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 2,82672 uses in industry, so most of it should be disposed of as waste. One way to dispose of the waste is to dispose of it in water, so that the heavy metal content of the gypsum must comply with the prevailing regulations.

5

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

One method is the calcination of phosphate, the purpose of which is generally to reduce the amount of phosphate organic matter and carbonate. A high temperature of about 1150 ° tulee should 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, especially if the Al, Fe and Si concentrations are high. EP-15 patent 154554 discloses a two-step calcination process which can remove about 75% of phosphate cadmium on a small scale.

Sulfide precipitation of cadmium 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 ppm to 0.2 ppm.

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 3342211 discloses an extraction process in which Cd is selectively extracted into the HCl salt of an alkylamine.

For example, FI patent application 871867 discloses an ion exchange process which can remove 40-50% of 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 resin are also required.

It has now been found that the process of the present invention efficiently separates cadmium from phosphoric acid. The method is performed using an electrolytic cell, a membrane or a diaphragm cell.

The method of the present invention does not have the disadvantages of the above methods. The method of the invention is effective; it removes cadmium almost completely. In addition, a significant advantage of the method according to the invention is the ease of regeneration of the electrolysis medium used at low cost.

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. In the process, the P2 ° 5-P; * - 'content of the return acid is about 15-20%. The P205 content of the crude phosphate is about 35% and that of the product acid about 25-30%.

The mixture ratio of reflux acid and crude phosphate fed to the reactor varies, for example, from 1: 1 to 3: 2 calculated as P 2 O 5.

In the following, the invention is described by way of example.

30 Example

In the construction of the electrolytic cell, it is essential that the cell has an anode chamber and a cathode chamber filled with granules or powder. Anode and cathode solutions are circulated through the chambers. Suitable electrode material is, for example, graffiti.

A certain volume of phosphoric acid was circulated as a function of time through electrolytic cells filled with graphite granules (0 1-3 mm). Powdered graphite 4,82672 or activated carbon was used in the comparative experiments. The volume of the cells was 48 ml or 96 ml, of which the filling accounted for 49%. Metallic cadmium or zinc can also be used as a filler.

Cd-contaminated phosphoric acid was recycled on the cathode side and one concentrated pure phosphoric acid was recycled on the anode side for PaO 2. Solutions with concentrations of: (A) P 2 O 5 20.2% and Cd 10.5% 10 (B) PaOs 15.8% and Cd 8.0% were used as acids.

Cadmium precipitated on the cathode side on the surface of the graphite granules. The anode side remained clean. The test results are shown in the following Table 1, where 15 1 = test number

2 = electric current A

3 = volume of recirculation solution in ml 4 = recirculation rate in ml / h

20 5 = acid temperature ° C

6 = voltage at 4h V

7 = thickness of the filler layer on the cathode side cm 8 = name of the acid 9 = quality of the filler (1 = granular Graf., 2 = powdered Graf., '25 3 = activated carbon) 10 = cadmium removal at 4h% li 5 82672

Table 1 1_2 3_4_5 6 7 8 9 10 14 1.8 109 218 25 4.8 1 A 1 78.1 5 18 1.8 109 109 25 4.9 1 A 1 88.6 15 1.2 109 218 25 4 .4 1 A 1 82.9 19 1.2 109 109 25 4.4 1 A 1 97.1 10 30 1.8 218 438 25 5.4 2 A 1 85.7 32 1.8 218 218 25 4, 8 2 A 1 93.3 31 1.2 218 436 25 4.2 2 A 1 91.4 33 1.2 218 218 25 4.1 2 A 1 91.4 15 34 1.2 218 218 30 4.1 2 A 1 84.8 35 1.2 218 218 30 4.3 2 B 1 81.3 36 1.2 109 109 25 4.6 1 A 3 43.8 20 37 1.8 109 109 25 6.5 1 A 3 34.3 38 1.2 109 109 25 3.8 1 A 2 74.3 39 1.2 109 109 25 3.8 1 B 2 80.0 25 40 0.8 109 109 25 2.8 1 A 1 94.3 41 0.8 109 109 25 2.9 1 B 1 97.5 42 0.5 109 109 25 3.2 1 A 1 93.3 43 0.3 109 109 25 2.9 1 A 1 41 , 0 V. 30

It can be seen from Table 1 that most of the cadmium was removed by the process of the invention using granular graphite as filler. Even with graphite powder, fairly good results were obtained, but only well below half of the cadmium could be removed with activated carbon.

Graphite regeneration was performed by changing the locations of the poles 40 to recover the cadmium. Either phosphoric acid or water was used as the regeneration solution.

Claims (3)

6,82672 1. A method for the production of tungsten metal, cadmium ash, for the phosphorus genome by electrolysis-cell filling with granules or powders, stem anodes and cathodes, and for separating the variables with diaphragms or 25 membranes, the cells of which are -circulating phosphorus contaminant in the tungsten metal and in the anodic section of the phosphorus oxide in the form of a mixture of P2O5, varvid tungmetallen utfälls pä fyllnaden. 30 2. For the purposes of claim 1, a reference is made to the inclusion of cadmium in the product as a whole in the production of phosphorus pyrene. A method for removing heavy metals, in particular cadmium, from phosphoric acid using a granular or powder-filled electrolytic cell with anode and cathode compartments separated by a diaphragm or a membrane, characterized in that phosphoric acid contaminated with heavy metal , whereupon the heavy metal precipitates on the surface of the filler. 10 Process according to Claim 1, characterized in that the removal of cadmium 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 is carried out from the acid returned to the reactor of the wet process for the production of phosphoric acid. 20 Patentkrav 3. A patent according to claim 1, kannneteck-35 n a t av att avlägsning av cadmium utförs frän den reactorn äterförda syran vid vätprocessen av phosphorsyras framställningsförfarande.