KR20090026087A - A method and an apparatus for purification of phosphoric acid by fractional crystallization - Google Patents

Abstract

A method for purification of a phosphoric acid is provided to improve a recovery rate of the phosphoric acid by mixing and crystallizing waste matter and gain the phosphoric acid having high degree of purity. A method for purification of a phosphoric acid comprises the following steps of: mixing a first supplying compound including impurities of a first group and a second supplying compound including impurities of a second group; crystallizing the compound with a multi-stage method for crystallization; and extracting purified phosphoric acid by heating and discriminating a crystalline layer. The first supplying compound has the phosphoric acid as a main component. The impurities included in the first supplying compound and the second supplying compound, are different mutually.

Description

Purification method and purification apparatus of phosphoric acid by fractional crystallization {A METHOD AND AN APPARATUS FOR PURIFICATION OF PHOSPHORIC ACID BY FRACTIONAL CRYSTALLIZATION}

The present invention relates to a method and apparatus for purifying phosphoric acid contaminated with cations, anions, acids and / or organic elements by fractional crystallization according to the preambles of claims 1, 19 and 22.

For example, in etching methods used in the electronics industry, acids such as phosphoric acid, nitric acid and acetic acid are commonly used. Combinations of the foregoing acids are used, for example, to dissolve aluminum molybdenum alloys applied onto glass. Such aluminum molybdenum alloys are widely used in the manufacture of TFT (Thin Film Transistor) display devices.

In general, the acid waste remaining after the etching process comprises, depending on the reaction conditions, 60-95% by weight phosphoric acid, 1-10% by weight nitric acid, 2-30% by weight acetic acid and the remaining amount of water. The acid waste also contains 100-2000 ppm aluminum molybdenum metal ions. These acid wastes are diluted and converted into fertilizers.

As a method for purifying phosphoric acid contaminated with metal ions, various methods such as a membrane separation method, an ion exchange method, or a liquid extraction method have already been proposed.

The membrane separation method has advantages in terms of yield and purity of recovered phosphoric acid. However, the cost of the membrane separation plant is extremely high and the operation is very complicated. Problems may also arise due to the corrosiveness of nitric acid on the membranes used.

The ion exchange method uses an ion exchange resin or calcium zeolite to remove acid. However, these ion exchangers have disadvantages in that they can treat only low concentrations of acid because their ion exchange capacity is generally low.

Liquid extraction has the advantage that the process can be run continuously and the equipment is inexpensive. However, this method has a disadvantage in that phosphoric acid cannot be obtained at the high quality required by the electronics industry.

A method and apparatus for recovering phosphoric acid from an acid aqueous solution contaminated with metal ions and other acids is disclosed in JP-A-2006-069845. In this method, other acids and water are first removed by distillation. The phosphoric acid is then crystallized from the aqueous phosphoric acid residue and then separated by distillation. The mother liquor enriched with metal ions is removed as a waste after crystallization. Then, nitric acid, hydrochloric acid or acetic acid is added again to the purified phosphoric acid by distillation. This acid mixture serves as the product used in the new process.

The above purification method is very complicated because the phosphate waste is purified twice by distillation and once by crystallization.

KR-A-20050103570 discloses a method for separating and obtaining ultrapure phosphoric acid, nitric acid and acetic acid from contaminated etching solutions using layered crystallization and vacuum distillation. In the first step, nitric acid and acetic acid are separated by vacuum distillation. The distillation residue is poured into a layered crystallizer and maintained at an initial temperature of -20 to 30 ° C. This solution is then seeded with seed crystals and cooled to 60-20 ° C. The formed crystals are separated using the density difference between the crystal phase and the mother liquor. The obtained crystallization is heated to 0 to 40 ° C. and the partially molten crystal phase is filtered to obtain purified phosphoric acid. This method is carried out without the addition of additives or solvents.

It is an object of the present invention to provide a method and apparatus in which acid waste can be significantly reduced. In particular, it is an object of the present invention to provide a method which requires less energy and can be carried out on a commercial scale. Another object of the present invention is to use as few different method steps as possible.

According to the invention, this object is achieved by the method according to claim 1. This way,

Mixing a first feed mixture comprising phosphoric acid as a main component and predominantly containing a first group of impurities with a second feed mixture comprising phosphoric acid as a main component and predominantly containing a second group of impurities, wherein The impurities present at the highest concentrations in the feed mixture of the species are different from each other, and the mixture of the first feed mixture and the second feed mixture is crystallized by a multi-stage crystallization method and obtained for extraction of purified phosphoric acid. It is characterized by heating and fractionating the crystal layer.

Purification of phosphoric acid by crystallization can be more economically achieved when raw phosphoric acid used in other industrial processes or raw phosphoric acid derived from other manufacturing processes are mixed and then purified by crystallization. Purification using crystallization has the advantage that the depletion of different impurities can occur independently of each other. The present inventors have found that, for example, phosphoric acid (pickling acid) used for large-scale pickling has other impurities as compared to the phosphoric acid used for LCD manufacturing, and for this reason it is treated with contaminated phosphoric acid in the LCD manufacturing process. It has been found that it can be purified particularly advantageously in the case.

The impurities of the first and second groups preferably comprise different acids, anions, cations or other compounds, respectively. Under such circumstances, through the dilution effect, impurities in the mixed mixture are reduced to some extent. This allows the dilution effect to be reduced in one or two steps of crystallization, so-called crystallization, by using different mixtures of advantageous composition (ie different main impurities). According to a particularly preferred variant, the phosphoric acid used in the etching step of the semiconductor manufacturing process, in particular in the LCD manufacturing process, is mixed with the phosphoric acid used for pickling. In this way, the cost can be greatly reduced because the amount of phosphoric acid to be disposed of or fertilized is considerably reduced. It is also possible to mix the crude acid with other acids mentioned above, which are obtained in a so-called wet process and which do not have a suitable purity for use in the LCD manufacturing process.

It is preferable that the first group contains molybdenum and / or aluminum ions, and the second group contains iron and / or sodium ions and / or phosphorus containing acids as main impurities. These impurities can be removed mainly by crystallization. The process according to the invention is not limited to the purification of two different feed mixtures, but can also be used for the purification of three or more different feed mixtures whose main impurities are different.

The subject matter of the invention is also a method according to claim 7, which method comprises

Crystallization is carried out with a plurality of crystallization stages,

If necessary, the phosphoric acid content (weight) in the mixture used is adjusted to less than 91.5% by weight by addition of water,

The mixture used is cooled, phosphoric acid hemihydrate is deposited as a crystal layer on the crystal surface, and then the crystal layer is heated and fractionated to obtain purified phosphoric acid. This method has the advantage that the phosphoric acid can be obtained in the absence of cations, anions, acids and / or organic compounds, so that the phosphoric acid can be reused. The addition of water is advantageous in that heavy metal ions are washed away more quickly. By the addition of water the process is controlled in such a way that phosphate-hemihydrate is obtained. This occurs when the initial content of phosphoric acid is about 63-92% by weight. Implementation in this range has the advantage that the concentration of phosphoric acid in the crystallization residue can be significantly lower than if the initial mixture for crystallization already has a higher purity (eg by pre-distillation). In each case it is preferred to add water to the initial mixture so that the content of phosphoric acid is 80-91% by weight. This has the advantage that, on the one hand, extremely low temperature carriers are not required to obtain the desired yield, and on the other hand, clear accumulation of water in the residue results in favorable separation of undesirable ions. . In addition, the thermal insulation requirements of the crystallization plant are lower. In each case it is preferred to add water so that the content of phosphoric acid is 89-90.5% by weight. Energy efficiency is particularly good when operating in this range. If the phosphate hemihydrate is crystallized instead of the phosphoric acid itself under the above-mentioned conditions, there are no disadvantages because the purified phosphoric acid is in any case diluted with water for subsequent use.

According to yet another embodiment of the method of the invention, a second heat exchanger surface is included, which is arranged slightly away from the first heat exchanger surface or arranged to be in direct contact with the first heat exchanger surface and is kept at low temperature. The second heat exchanger surface keeps the phosphate crystals available to the crystallizer and provides crystal seed for uniform and rapid nucleation on the first heat exchanger surface during the initial phase of crystallization. For example, preferably a convoluted pipe can be arranged between the first heat exchanger surfaces, for example acting as a second heat exchanger surface.

Crystallization is preferably carried out in a plurality of crystallization stages, in which the purity of the feed mixture increases from low to high crystallization stages. Crystallization is preferably carried out in at least three stages, preferably four or more stages of crystallization stage. Basically, although contaminated phosphoric acid can be pre-distilled, it is preferred that contaminated phosphoric acid is fed directly for the implementation of the crystallization process. Pre distillation, if carried out, gives an acid content of greater than 92-93% by weight. This means that the distillation product is preferably diluted with water to obtain H ₃ PO ₄ 1 / 2H ₂ O during the subsequent crystallization process. Direct crystallization of the undiluted distillation residue will cause the phosphoric acid content in the residue to be reduced to about 93%, depending on the phase diagram of the phosphoric acid / water system.

The particular stage of crystallization is preferably used as the feed stage of the next higher stage. As a result, substantially any purity can be obtained using the above-described crystallization method. The remaining fraction of the particular stage is preferably collected and fed to the next lower stage as a feed fraction. This allows for maximum recovery of phosphoric acid. The sweating fraction of a particular stage can be collected and then added as a feed fraction of the same stage.

The subject of the invention is also a phosphoric acid purification device according to claim 19. The apparatus is equipped with a water supply mechanism to adjust the weight ratio of phosphoric acid in the feed mixture to a specific value. The advantages of such mechanisms have already been discussed in detail in the foregoing description of the method. It is preferable to have a mechanism for measuring the amount of phosphoric acid and the amount of water added. Organizations of this kind are well known to those of ordinary skill in the art. According to a preferred embodiment, in operation the second heat exchanger surface, through which the second heat carrier flows, is provided in the crystallizer in direct contact with or slightly spaced from the first heat exchanger surface. In the case of having a second heat exchanger surface, it has been shown to be particularly effective for classes of compounds in which nucleation is delayed by the tendency to subcool.

Another subject of the invention is a purification plant according to claim 21. The purification plant is provided with a crystallizer, in direct contact with or slightly spaced from the surface of the second heat exchanger, in which the second heat carrier flows during operation. This means that two heating / cooling circuits are provided to allow heat carriers of different temperatures to flow through the first and second heat exchanger surfaces. Such purification plants can generally be used for products that are prone to overcooling, such as phosphoric acid, which inhibits and retards nucleation and further causes initial crystallization to occur in an uncontrolled manner without a second heat exchanger surface. Examples of preferred crystallization plants according to the invention have already been discussed above.

According to the present invention, purification of phosphoric acid contaminated with cations, anionic acids and / or organic elements can be made energy efficient. In addition, high-purity phosphoric acid can be obtained through multi-stage crystallization, and the phosphoric acid can be recovered and reused in a higher yield by mixing and crystallizing different acid wastes from the individual crystallization.

The invention will be explained by way of example with reference to the drawings. In each embodiment the same reference numbers will be used for the same parts.

FIG. 1 shows a crystallization apparatus with four tanks 13, 15, 17, 19 for storing different phosphoric acid fractions from the stationary crystallizer 11. The feed fraction is conveyed to tank 13 via line 21, which may be interrupted by valve 23. Then, the negative feed fraction is transferred to the crystallizer 11 through the first feed line 25 and the pump 33. The outlet 29 is located at the base 27 of the crystallizer, and a line 31 having a shutoff valve 32 is connected thereto. The line 31 is connected to the header 41, and the header 41 communicates with the tanks 13, 15, 17, 19 via lines 43, 45, 47, 49. Lines 43, 45, 47 and 49 are equipped with valves 44, 46, 48 and 50 which shut off respective lines. The used material can be conveyed in lines 25, 52, 53, 51 by pumps 33, 35, 37, 39.

Depending on the composition of the feed mixture, the purification plate may have more than four or less than four tanks for the intermediate storage of the phosphoric acid fraction. In the illustrated embodiment, the tank 15 serves to collect and store the pure phosphate fraction rather than the phosphate fraction stored in the tank 13. Tank 15 is connected to crystallizer 11 via line 52 so that the mixture can be fed to the crystallizer for further purification stages.

Residue of the purification process is discarded via line 51. Purified phosphoric acid is transferred via line 53 for reuse.

A particular feature of the purification plant of FIG. 1 is that water can be added to tank 15 via line 54 to adjust the content of phosphoric acid to a predetermined value, preferably about 90% by weight. Line 54 may be interrupted by valve 56.

The first heat exchanger surface is indicated by reference numeral 55, which is connected to the first heat generator / cooler via lines 57, 59. A special feature of this crystallizer is that it has a second heat exchanger surface 60 through which the second heat carrier can flow through lines 64 and 66 in operation. In area, the second heat exchanger surface 61 is considerably smaller than the first heat exchanger surface, the sole purpose of which is that the phosphate crystal can be continuously used as seed crystal in the crystallizer. For this reason, the second heat exchanger surface 60 may be disposed slightly away from the first heat exchanger surface 55 or may contact the first heat exchanger surface at an appropriate location. Preferably, the second heat exchanger surface flows through the second heat exchanger surface, and the second heat exchanger surface is kept at a low temperature, so that upon filling of the crystallizer, crystal growth immediately occurs at the second heat exchanger surface 60. And then subsequently transferred to the first heat exchanger surface 55 to allow for even growth of the crystals in a controlled manner at a short time later on the first heat exchanger surface.

2 and 3 show two examples as a method of adding water to the mixture used. According to FIG. 2, water can be added to the mixture in a controlled manner by using a closed circuit 61 with a control valve 63. The connecting line 65 between the line 52 and the tank 15 enables circulation of the feed mixture. Water can be mixed simultaneously during circulation. Depending on the position of the valves 67 and 69, the feed mixture may be circulated or fed to the crystallizer by a pump.

The embodiment shown in FIG. 3 differs from the embodiment shown in FIG. 2 in that dilution of the feed mixture occurs directly during transfer to the crystallizer. To this end, a (stop) mixer 71 is provided, and the mixer 71 has a plurality of baffle plates 73. The baffle plate 73 is provided for turbulence, so that the added water and the feed mixture are thoroughly mixed.

Referring to the state diagram of phosphoric acid (see FIG. 4), the process according to the invention takes place in the region where phosphoric acid hemihydrate (H ₃ PO ₄ 1 / 2H ₂ O) is obtained. Preferably, in the state diagram, crystallization takes place in the region where H ₃ PO ₄ is present in the feed mixture at a ratio of 63-91.5% by weight, preferably 80-91% by weight. This has the advantage that crystallization of phosphate hemihydrate (H ₃ PO ₄ 1 / 2H ₂ O) can already occur at about 24 ° C.

Example

Example 1

Primary Crystallization (First Stage)

Raw acid with the following parameters was taken from a production plant for the manufacture of TFT displays.

Mo-ion, ppm 1,359

Al-ion, ppm 1,361

Density, d ₂₀₄ 1.7554

H ₃ PO ₄ Content (% by weight) Approx. 93.2

Stationary crystallizers were used as purification apparatus. This crystallizer has a plurality of heat exchanger surfaces, preferably arranged perpendicularly to the vessel, in which heat transfer flows through the surfaces.

H ₃ PO ₄ Water was admixed with the raw acid (waste acid) in an amount such that the content dropped to about 90% by weight. The feed mixture prepared in this way was fed to a pilot crystallizer to crystallize. The temperature of the heat carrier of the crystallizer was initially adjusted to 25 ° C. After filling the crystallizer with the feed mixture to be crystallized, the temperature of the heat carrier was lowered to 15 ° C. within 6 hours.

While cooling the feed mixture to 15 ° C., a crystal layer of pure H ₃ PO ₄ 1 / 2H ₂ O forms on the surface of the heat exchanger. After crystallization in this manner, the uncrystallized residue was released. Subsequently, the temperature of the heat carrier rose to 35 ° C. within 7 hours. In the meantime, the crystallized layer began to sweat. During this warming-up phase, the sweating fractions were collected.

After sweating, the temperature of the heat carrier was increased to 50 ° C. to dissolve the remaining crystallized mass. The measurement data is summarized in the table below.

Table 1: Data Summary Table for Primary Crystallization

Fraction Mass (g) Mo (ppm) Al (ppm) D ₂₀₄ H ₃ PO ₄ (% by weight) Feed fraction 48,390 1,359 1,361 1.7454 89.9 Residue 11,880 3,098 2,649 1.7094 86.8 Sweating Fraction 12,200 1,578 1,504 1,7434 89.7 Crystalline cargo 24,300 395 655 1,7635 91.5

Second Crystallization (Second Step)

The crystallized fraction produced in the primary crystallization stage is the (intermediate product) fraction, which fraction is further purified to act as feed fraction for the secondary crystallization stage. To the feed fraction used here, water is added in an amount such that the content of phosphoric acid is about 90% by weight. The secondary crystallization stage is carried out in a temperature / time pattern similar to that used in the primary crystallization stage. The results of secondary crystallization are summarized in Table 2 below.

Table 2: Data Summary Table for Secondary Crystallization

Fraction Mass (g) Mo (ppm) Al (ppm) D ₂₀₄ H ₃ PO ₄ (% by weight) Feed fraction 10,760 395 647 1.7478 90.1 Residue 1,560 1,078 1,507 1.7024 86.2 Sweating Fraction 3,552 1,078 1,507 1,7024 89.5 Crystalline cargo 5,648 188 387 1,7630 91.5

Tertiary Crystallization (Tier 3)

In the tertiary crystallization stage, a plurality of crystallized fractions from the secondary crystallization stage are further purified. Sufficient water is added again in an amount such that the initial content of phosphoric acid is about 90% by weight. Similar temperature-time profiles are used. The results are shown in Table 3.

Table 3: Data Summary Table for Tertiary Crystallization

Fraction Mass (g) Mo (ppm) Al (ppm) D ₂₀₄ H ₃ PO ₄ (% by weight) Feed fraction 10,745 172 370 1.7478 90.1 Residue 1,231 509 1,098 1.7004 86.0 Sweating Fraction 3,970 190 408 1,7420 89.6 Crystalline cargo 5,544 84 182 1,7625 91.4

4th crystallization (4th stage)

In the fourth crystallization stage, the plurality of crystallized fractions from the tertiary crystallization stage are further purified. Further processes are similar to those carried out in tertiary crystallization stages. The results in the fourth crystallization stage are shown in Table 4.

Table 4: Summary of Data for Quaternary Crystallization

Fraction Mass (g) Mo (ppm) Al (ppm) D ₂₀₄ H ₃ PO ₄ (% by weight) Feed fraction 9,681 85 184 1.7466 90.0 Residue 1,644 274 619 1.6956 85.6 Sweating Fraction 3,337 75 161 1,7478 90.1 Crystalline cargo 4,700 25 47 1,7636 91.5

As can be seen from Table 4, metal ions (eg Mo and Al ions here) were reduced from 1/25 to 1/50 in four successive crystallization stages. Basically it can be seen that adding a crystallization stage will further reduce metal ions. In other words,

The crystallized fraction of each stage can then be introduced as a feed fraction at the higher stage.

The residue fraction of each stage can be collected and then admixed to the bottom feed fraction.

The sweating fractions of each stage can be collected and admixed to the feed fractions of the same stage in the sequence of steps (so-called cycles).

Water is added to the feed fraction in an amount such that the content of phosphoric acid is 85-91% by weight, preferably 89-90.3% by weight.

 Table 5: Data Summary Table for 5th-Order Crystallization

Fraction Mass (g) Mo (ppm) Al (ppm) D ₂₀₄ H ₃ PO ₄ (% by weight) Feed fraction 4,719 25 47 1.7465 90.0 Residue 688 96 190 1.6954 85.6 Sweating Fraction 1,734 21 39 1,7477 90.1 Crystalline cargo 2,297 7 10 1,7609 91.2

Table 6: Data Summary Table for 6th-Order Crystallization

Fraction Mass (g) Mo (ppm) Al (ppm) D ₂₀₄ H ₃ PO ₄ (% by weight) Feed fraction 2,155 6 10 1.7465 90.0 Residue 320 33 59.5 1.6954 85.6 Sweating Fraction 667 2 3 1,7477 90.1 Crystalline cargo 1.168 0.8 1.0 1,7609 91.1

Example 2

Mixture of two acids of different origin

Originally used LCD acid and foreign acid were mixed at a ratio of 7: 3. All data relating to impurities in the table below are in ppm unless otherwise stated.

 Table 7: Data Summary for Two Mixed Acids

mountain Mo1 Al Fe Na Si SO ₄ H ₃ PO ₄ Used acid 1,359 1,361 - - - - - A foreign mountain - - 316 263 33 198 412 Mixed acid 941 953 96 80 10 60 125

Implementation with Mixed Acids

In crystallization experiments with mixed acids in the above manner, we will investigate the attainable separation of individual ions. The experiment includes two stages of purification and a recovery stage (striping stage).

First stage

The starting (or starting) mixture was crystallized in the first stage and a sufficient amount of material was produced in the subsequent second stage. The data is summarized in Tables 8 and 9.

Table 8: Summary of Data for the First Phase

Fraction Mass (g) Mo Al Fe Na Si SO ₄ H ₃ PO ₃ H ₃ PO ₄ (% by weight) Used amount 10,800 941 953 96 80 10 60 125 90.0 Residue 1.507 2,498 2,412 194 236 24 190 339 86.0 Sweating fraction 3,535 1,242 1,161 99 113 14 80 156 89.3 Crystalline cargo 5,758 349 443 69 19 4 14 50 91.5

Table 9: Summary of Data for the Second Stage

Fraction Mass (g) Mo Al Fe Na Si SO ₄ H ₃ PO ₃ H ₃ PO ₄ (% by weight) Used amount 10,840 1,039 1,021 96 91 11 66 135 89.9 Residue 1.636 2,795 2,573 195 255 25 214 361 85.9 Sweating fraction 3,163 1,381 1,240 99 138 17 87 171 89.1 Crystalline cargo 6,041 385 486 69 22 4 16 55 91.4

The feed fraction of the secondary first stage includes the sweating fraction from the primary first stage.

2nd stage

The molten crystallized fractions from the two first stages were mixed together to use the resulting fraction as feed material for the second stage.

 Table 10: Data Summary for Stage 2

Fraction Mass (g) Mo Al Fe Na Si SO ₄ H ₃ PO ₃ H ₃ PO ₄ (% by weight) Used amount 10,975 367 465 69 20 4 15 53 89.5 Residue 3,045 795 936 117 50 9 36 108 86.4 Sweating fraction 3,107 336 408 60 18 4 14 49 89.6 Crystalline cargo 4,823 117 205 44 2.8 1.5 2.0 20 91.4

Recovery

Two residue fractions of the first stage were mixed together to crystallize a portion of the composition in the recovery stage.

Table 11: Data Summary for Recovery Stage

Fraction Mass (g) Mo Al Fe Na Si SO ₄ H ₃ PO ₃ H ₃ PO ₄ (% by weight) amount 2.392 2,653 2,496 195 246 25 202 350 2,392 Residue 355 6,125 5,298 412 733 61 601 925 335 Sweating fraction 690 4,201 3,832 217 365 35 305 488 690 Crystalline cargo 1,367 1,020 1,134 30 66 10 53 140 1,367

In this test, only one recovery stage was used, but it is also possible to collect the residue from this recovery stage and reprocess it in one or more recovery stages.

Similar to the previous stages using only LCD acid, separation of Mo and Al ions was effectively achieved in mixed acid experiments.

5 shows, with reference to examples of possible mixtures of LCD acid and other acids (eg pickling acid) used, which shows that the proportion of waste phosphoric acid is reduced to a minimum by using the method according to the invention. will be. The content of ions of each kind in the mixture in which the feed mixtures are mixed is reduced by the dilution effect described above. In this way, pure acids of the same purity and the same ionic content in the residue can be obtained in higher yields when they are purified by mixing them than by separately purifying the two kinds of acids without mixing them. In the embodiment shown in Fig. 5, one ton of pure acid and 400 kg of residue can be recovered from one ton of LCD acid and 400 kg of pickling acid from the pickling process, i.e. 1.4 tons of mixed acid can be recovered. have. However, in the case of separate purification, it is expected that 840 kg of pure acid and 560 kg of residue will be obtained.

By using pickling acid in the same amount as the yield of phosphoric acid waste, 100% of phosphoric acid can be recycled back to the LCD manufacturing process.

A process for purifying contaminated phosphoric acid, wherein the first feed mixture containing phosphoric acid as the main component and predominantly containing the first group of impurities comprises a second supply containing phosphoric acid as the main component and predominantly containing the second group of impurities. Mixed with the mixture. The impurities present at the highest concentrations in the two feed mixtures are different from each other. The feed mixture mixed together is crystallized through a multistage crystallization process, and the obtained crystal layer for extraction of purified phosphoric acid is heated and melted to fractionate. The apparatus for carrying out the invention is characterized in that it comprises a water supply mechanism for adjusting the weight ratio of phosphoric acid in the feed mixture to a specific value.

Explanation of symbols in the drawings

11: stop crystallizer

13, 15, 17, 19: tank for phosphoric acid fractionation

21: Supply Line for Raw Phosphoric Acid

23: Shut-off valve in supply line

25: first transfer line

27: base of crystallizer

29: exit

31: line

32: shut-off valve

33, 35, 37, 39: transfer pump

41: header

43, 45, 47, 49: connection line between header and tank

44, 46, 48, 50: shut-off valve

51: residue discharge line

53: purified phosphoric acid line

54: water supply line

55: first heat exchanger surface

56: shut-off valve in line 54

57, 59: line for heat transfer

60: second heat exchanger surface

61: control circuit

62: control valve

65: connecting line between line 52 and tank 15

67, 89: valve

71: mixer

73: baffle plate

1 is a schematic diagram of a crystallization plant with four tanks for storing different phosphoric acid fractions from a static cystallizer in accordance with the present invention;

FIG. 2 is a partial view of the crystallization plant shown in FIG. 1, showing a first embodiment of a water supply mechanism;

Figure 3 shows a second embodiment of a water supply mechanism,

4 is a state diagram of phosphoric acid,

5 shows an example of the mass balance in the purification method according to the present invention.

Claims (23)

A process for purifying by crystallization using contaminated aqueous phosphoric acid as a feed mixture, The feed mixture may contain various impurities such as cations, anions, acids, organic elements, etc., crystallizes and partially melts the feed mixture containing contaminated phosphoric acid to separate purified phosphoric acid, The purification method, Mixing a first feed mixture comprising phosphoric acid as a main component and predominantly containing a first group of impurities with a second feed mixture comprising phosphoric acid as a main component and predominantly containing a second group of impurities, Impurities present in the highest concentrations in the first and second feed mixtures are different from each other, Wherein the mixture of the first feed mixture and the second feed mixture is crystallized by a multi-stage crystallization method, and the obtained crystal layer is heated and fractionated for extraction of purified phosphoric acid. The method of claim 1, wherein the impurities of the first and second groups each comprise different acids, anions, cations or other compounds. The process according to claim 1 or 2, wherein the first group comprises molybdenum and / or aluminum ions as the main impurity. The process according to claim 1 or 2, wherein the second group comprises iron and / or sodium ions and / or phosphorus containing acids as main impurities. The process according to claim 1, wherein the first and second feed mixtures are each phosphoric acid used in an etching process, phosphoric acid used in a pickling process or crude phosphoric acid. . The process according to any one of claims 1 to 5, wherein more than two feed mixtures are used and each feed mixture is different from each other in major impurities. A process for purifying by crystallization using contaminated aqueous phosphoric acid as a feed mixture, The mixture may contain various impurities such as cations, anions, acids, organic elements, etc., crystallizes and partially melts the feed mixture containing contaminated phosphoric acid to separate purified phosphoric acid, The purification method, Crystallization is carried out with a plurality of crystallization stages, If necessary, the phosphoric acid content in the mixture used is adjusted to less than 91% by weight with the addition of water, The mixture used is cooled, phosphoric acid is deposited as a crystal layer on the crystal surface and the crystal layer is sweated and fractionated to obtain purified phosphoric acid. 8. A process according to claim 7, wherein the process is controlled by the addition of water in such a way that phosphate hemihydrate is obtained. The process according to claim 7 or 8, wherein water is added to the feed mixture so that the content of phosphoric acid is 63-91% by weight. 9. A process according to claim 7 or 8, wherein water is added to the feed mixture so that the content of phosphoric acid is between 80 and 91% by weight. The process according to claim 7 or 8, wherein water is added to the feed mixture so that the content of phosphoric acid is 89-90.5% by weight. The method of claim 7, wherein the second heat exchanger surface is arranged at a short distance from the first heat exchanger surface or in direct contact with the surface of the first heat exchanger and is kept at a low temperature. Purification method to use. The process according to any one of claims 7 to 12, wherein the crystallization is carried out in a plurality of crystallization stages, and the purity of the feed mixture increases from low to high crystallization stages. The process according to claim 13, wherein the crystallization is carried out in three or more stages, preferably four or more stages. 15. The process according to any one of claims 7 to 14, wherein contaminated phosphoric acid from the industrial production process is provided to the fractionation layer crystallization process directly without prior distillation. 16. The process according to any one of claims 7 to 15, wherein the crystallization of the particular crystallization stage is then used as the feed stage of the higher stage. 17. The process according to any one of claims 7 to 16, wherein the residue of the particular crystallization stage is collected and then admixed to the lower stage. 18. The process according to any one of claims 7 to 17, wherein each sweating fraction of a particular crystallization stage is collected and added to the feed mixture of the same crystallization stage. In a refining apparatus having a stationary crystallizer 11 for purifying phosphoric acid contaminated with cations, anions, acids, and / or organic compounds, the crystallizer 11 is a surface of a first heat exchanger through which a first heat carrier flows. Connected to tanks 13, 15, 17, 19 via lines 31, 41, 43, 45, 49 for (intermediate) storage of the mixture used, The apparatus comprises a water supply mechanism for adjusting the weight ratio of phosphoric acid in the feed mixture to a specific value. 20. The purification apparatus of claim 19, comprising a device for measuring the amount of phosphoric acid and the amount of water added. The second heat exchanger surface 60 according to claim 19 or 20, in which the second heat carrier flows in direct contact with the first heat exchanger surface 55 or at a short distance from the first heat exchanger surface 55. And is provided to the crystallizer (11). A device with a stationary crystallizer (11), wherein the crystallizer (11) has a first heat exchanger surface (55) through which a first heat carrier flows and lines (31, 41) for (intermediate) storage of the mixture used. Connected to the tanks 13, 15, 17, 19 via 43, 45, 49, In operation, the second heat exchanger surface 60, through which the second heat carrier flows, is provided in the crystallizer 11 in direct contact with the first heat exchanger surface 55 or short-distance from the first heat exchanger surface 55. Characterized in that the device. 23. The apparatus of claim 22, comprising first and second heat generators / coolers in communication with the first and second heat exchanger surfaces.

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