JPH0425205B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for removing sulfate ions from phosphoric acid by extraction method.

In contrast to "dry method phosphoric acid," which is produced using yellow phosphorus as a starting material, phosphoric acid produced by refining crude phosphoric acid obtained by decomposing phosphate rock with sulfuric acid is called "wet method phosphoric acid." Among these wet method phosphoric acids, those produced by purification by solvent extraction are referred to as "extraction method phosphoric acids."

Methods for purifying crude phosphoric acid are broadly classified into chemical precipitation, solvent extraction, ion exchange, and crystallization, but the solvent extraction method is currently the most industrialized process. In the solvent extraction method, orthophosphoric acid is selectively extracted using various organic solvents such as alcohols, ketones, ethers, and phosphoric acid esters, but the extractability and selectivity with respect to impurities are greatly influenced by the solvent. Ru. A general feature of the solvent extraction method is that it is excellent in removing cationic impurities such as Fe, Al, and Ca, but is not sufficient in removing anionic impurities such as SO ₄ and F. For this reason, in order to remove these impurities, the current situation is to combine chemical precipitation methods and the like as pre- and post-treatments to the solvent extraction method. In recent years, a method has been proposed in which anionic impurities such as SO ₄ and F are extracted into the solvent phase using an amine solvent as in Japanese Patent Publication No. 54-31478, but in terms of quality, it is not suitable for industrial use. The technology has not yet been perfected to purify it for commercial, food, or pharmaceutical use.

In addition, the crystallization method is known to be effective for both cationic and anionic impurities, and is theoretically considered the best method, but the content may vary depending on the type of impurity in the crude phosphoric acid. Because of the large difference, it has not been adopted industrially because it requires repeated recrystallization and purification of the mother liquor.

As mentioned above, the production of wet phosphoric acid combines various purification methods, resulting in a quality comparable to that of dry phosphoric acid, which has traditionally been used for industrial, food, and pharmaceutical purposes. In terms of manufacturing costs, due to the recent energy situation, the wet method is becoming more advantageous than the dry method, except in regions where electricity prices are extremely low. Under such circumstances, SO ₄ is an item that must be targeted for the quality of dry process phosphoric acid, and the SO ₄ of dry process phosphoric acid is SO ₄ /P ₂ O ₅ = 10 weight ppm or less.

Many methods have been proposed for removing sulfate ions, but in order to virtually completely remove sulfate ions, a barium compound is added to phosphoric acid to generate barium sulfate, which is a very sparingly soluble salt. Removal in a furnace is the most reliable and generally known method.

However, this method also causes various practical problems, such as the following, because barium sulfate crystals are extremely fine crystals and residual barium ions cannot be ignored due to the solubility product.

(1) The more sulfate ions in phosphoric acid are substantially completely removed, the more barium ions dissolve in phosphoric acid and contaminate the product phosphoric acid.

(2) Products with different sulfate ions in phosphoric acid When phosphoric acid is mixed in a storage tank and left for a long period of time, ultrafine crystals of barium sulfate are formed, creating turbidity that can be seen with the naked eye.

(3) Therefore, operational quality control is required in which the concentration of sulfate ions in the product must be strictly checked.

(4) Furthermore, if unreacted barium ions remain, the above-mentioned clouding phenomenon becomes noticeable, so it is necessary to carry out aging for 24 hours or more, preferably 36 hours or more.

(5) When the product phosphoric acid obtained by the above method is used for aluminum chemical polishing liquid or secondary salts, if sulfate ions are mixed in from the auxiliary raw materials, ultrafine crystals of barium sulfate will form after being left for several days. This produces cloudiness that can be seen with the naked eye, lowering the product's value and causing problems in the manufacturing process. In particular, when producing a chemical polishing solution for aluminum, copper sulfate, nickel sulfate, or sulfuric acid is added, which causes the above-mentioned clouding phenomenon to occur significantly, resulting in poor functional evaluation.

(6) In addition, the barium compounds used in the above method are generally limited to barium carbonate, barium hydroxide, etc. in consideration of the impact on product quality, but these industrial chemicals include Since strontium, which is similar to barium, exists in an amount of about 0.5 to 1.0%, 1 to 20 ppm by weight of strontium is mixed into the product phosphoric acid.

(The solubility product of strontium sulfate is about 1000 times that of barium sulfate, but depending on the reaction conditions, about 60 to 80% of the mixed strontium co-precipitates with barium sulfate.) As a result of intensive research to solve the above technical issues, we have found that the sulfate ion concentration can be reduced by crystallization method.
SO ₄ /P ₂ O ₅ = 10 weight ppm or less can be removed, and trace amounts of barium ions and strontium ions can also be removed virtually completely. Also, the operation for removing sulfate ions using barium compounds can be reduced, and the above-mentioned The present invention was completed based on the knowledge that cloudiness can also be prevented.

That is, in the present invention, in order to remove sulfate ions in phosphoric acid by extraction method to SO ₄ /P ₂ O ₅ = 10 weight ppm or less, the P ₂ O ₅ concentration is maintained at 58 weight % or more, and barium is added to the phosphoric acid solution. After adding the compound and removing the generated barium sulfate, it is cooled and converted into H ₃ PO ₄・1/
An extraction method for producing 2H ₂ O crystals and a mother liquor, separating the crystals from the mother liquor, and washing the crystals. A method for removing sulfate ions in phosphoric acid is provided.

The present invention will be explained in more detail below. The wet process phosphoric acid used in the present invention uses crude phosphoric acid obtained by decomposing phosphate rock with sulfuric acid as a starting material. In addition to various metal impurities derived from phosphate rock, crude phosphoric acid usually contains 1 to 5% by weight of sulfate ions derived from excess sulfuric acid during decomposition as SO ₄ . As a method for removing sulfate ions from this crude phosphoric acid, there is a method of adding an inexpensive calcium compound such as phosphate rock or slaked lime to remove the sulfate ions as gypsum. However, since gypsum still has a considerable solubility, sulfate ions remain at about 1000 to 2000 ppm by weight, and it is difficult to reduce the solubility below that level.

In addition, extracted phosphoric acid can be obtained by extracting the phosphoric acid solution thus treated using various organic solvents such as alcohols, ketones, ethers, and phosphoric esters. This solvent extraction method is excellent in removing various metal impurities, but when the obtained dilute phosphoric acid is concentrated to a P ₂ O ₅ concentration of 62% and the sulfate ion concentration is examined, it is 500 ppm to 2000 ppm by weight. ppm
It is. Furthermore, this concentrated acid is cooled,
Even if crystals of H ₃ PO ₄ 1/2H ₂ O are obtained, crystal growth is performed slowly at SO ₄ /P ₂ O ₅ = 35 to 90 ppm by weight, although it depends on the crystallization rate. Even if the crystals are washed with twice the amount of pure phosphoric acid, SO ₄ /P ₂ O ₅ = 15 ~
The limit is 55ppm. As a result, we used the crystallization method to remove sulfate ions after solvent extraction.
To purify SO ₄ /P ₂ O ₅ to less than 10 ppm by weight,
Two to three recrystallizations have to be carried out.

Repeating recrystallization in this way means that H ₃ PO ₄
Since the heat of dissolution of 1/2 H ₂ O is as large as 40 Kcal/Kg, it consumes a lot of energy and also increases the purification process of the mother liquor. In order to carry out one-stage crystallization, the present invention combines a "reaction step" in which sulfate ions are removed with a barium compound before the "crystallization step", and the sulfate ion concentration of the final product phosphoric acid is reduced by SO ₄ / P ₂ O ₅ = 10 ppm by weight or less.

Next, the present invention will be explained in detail in order of steps.

(1) Reaction process In this process, a barium compound is added to the extraction method phosphoric acid, and through one-step crystallization, SO ₄ /P ₂ O ₅ = 10
Adjust the sulfate ion concentration to less than ppm by weight.
SO ₄ /P ₂ O ₅ = 500 ppm by weight or less, preferably 330
Remove to weight ppm or less.

Preferred barium compounds include barium carbonate, barium hydroxide, barium phosphate, barium sulfide, and the like. As for the form in which these substances are added, quality control can be easily carried out if these substances are dissolved in pure water or phosphoric acid.

The reaction method can be either a batch method or a continuous method, but even if some unreacted barium ions remain or are shot, they can be removed in the crystallization step described later, so even in a continuous method, the amount of barium compound added can be adjusted to match the amount of sulfate ions. There is no need for strict control near the equivalence point.

Further, since the reaction does not occur near the equivalence point, the reaction time may be maintained for 6 hours or more, preferably 12 hours or more as an average residence time.

Considering the solubility of barium sulfate and the industrial crystallization rate in the crystallization process described below, _{the P 2 O 5 concentration of} the _phosphoric acid solution is 58% by weight or more, preferably 62% by weight or more. 66% by weight is good. When the P ₂ O ₅ concentration reaches 67% by weight, the viscosity increases, making it difficult to remove fine barium sulfate crystals.

In addition, the temperature of the treatment liquid is preferably 30 to 60°C, preferably 40 to 60°C, considering the freezing point of phosphoric acid, viscosity, and material.
50℃ is good.

(2) Crystallization process The phosphoric acid obtained in the above reaction process is continuously supplied to a cooling crystallizer (e.g., stirred tank type), and is liquefied with cooling water or a refrigerant depending on the P ₂ O ₅ concentration and crystallization rate. Maintain temperature between 0 and 25°C. H ₃ PO ₄・1/
Since the crystal growth of 2H ₂ O is large, if the average residence time in the crystallizer is maintained for 3 to 6 hours, the particle size can be reduced to 5 to 5.
10 mm crystals are obtained.

After separating the obtained crystals from the mother liquor in a centrifuge with a rotation speed of 3000 rpm, the weight ratio to the amount of crystals was determined.
Cleaned crystals are obtained by washing with pure phosphoric acid (phosphoric acid in which washed crystals are dissolved) of 0.1 or more, preferably 0.15 to 0.25. In particular, the cleaning effect is greater if the H ₃ PO ₄ concentration of pure phosphoric acid is 90% by weight or less to dissolve some of the crystals. Further, pure phosphoric acid used for washing and a part of the separated mother liquor are circulated to the crystallizer to maintain the crystal suspension density in the tank at 250 to 350 kg/m ³ . Depending on the barium ion concentration, the remainder of the separated mother liquor will become cloudy after being left for several days.
Sulfate ions, barium ions, and strontium ions are removed by circulation as it is or after concentration to a reaction process.

Next, examples of the present invention will be shown. Moreover, "%" and "ppm" in the examples represent weight basis.

Example 1 Phosphoric acid was produced by a wet method using crude phosphoric acid obtained by decomposing South African phosphate rock (BPL80) with sulfuric acid, separating gypsum, and concentrating it as a starting material. The composition of the crude phosphoric acid was as shown below.

P ₂ O ₅ 54.2% SO ₄ 1.21% Fe 0.41% Mg 1.12% Next, the above crude phosphoric acid was supplied to an extraction step and purified using the solvent isoamyl alcohol. The extraction process consists of 36
Consists of a stage mixer/settler type extractor,
The flow rate ratio (solvent phase flow rate/aqueous solution phase flow rate) of the extraction stage (2 stages) was 4.4, the flow rate ratio (solvent phase flow rate/reflux flow rate) of the washing stage (20 stages) was 10.3, and the back extraction stage (12 stage) flow rate ratio (solvent phase flow rate/pure water)
I did it in 5.8. As a result, the yield was 67%, and the raffinate was used for fertilizer by recovering the solvent.

Next, the dilute phosphoric acid (P ₂ O ₅ concentration 29%) obtained in the extraction process was recovered as a solvent, mixed with a portion of the mother liquor obtained in the crystallization process described later, and concentrated under reduced pressure of -680 mmHg. , I obtained concentrated phosphoric acid with the following composition.

P ₂ O ₅ 66.7% SO ₄ 0.23% The obtained concentrated phosphoric acid was supplied at 1000 g/hr to a complete mixing type reaction tank (capacity 8), and a phosphoric acid solution (Ba5.15 %,
Sr620ppm, _{P2O5 25.5} %) 59g/hr was added.
This slurry was passed through a pressurized leaf filter precoated with diatomaceous earth to obtain a clear phosphoric acid solution having the following composition.

P ₂ O ₅ 64.7% SO ₄ 170ppm Ba 5.4ppm Sr 8.7ppm This clear phosphoric acid solution is supplied to a stirred tank type crystallizer (effective volume 11), and the mother liquor and Pure phosphoric acid used for washing the crystals was also supplied, and the liquid temperature was maintained at 21-22°C with cooling water.

The resulting crystal (H ₃ PO ₄・1/2H ₂ O)
and mother liquor slurry in a centrifuge (rotation speed
After separating the mother liquor at 3000 rpm), the crystals were washed using pure phosphoric acid for washing (P ₂ O ₅ concentration 61.7%) of about 0.14 to 0.18 (by weight) based on the amount of crystals.

The washed crystals were dissolved in warm water, and 715 g of pure phosphoric acid (P ₂ O ₅ concentration 61.7%) was added per hour, of which 115 g per hour was used for washing the crystals. Further, the mother liquor (61.9% P ₂ O ₅ , 360 ppm SO ₄ ) and pure phosphoric acid used for washing were circulated to the crystallizer, but a portion of the mother liquor (500 g per hour) was circulated to the concentration step.

The composition of the pure phosphoric acid obtained in this example was as shown below.

P ₂ O ₅ 61.7% SO ₄ SO ₄ /P ₂ O ₅ = 2 ppm or less Ba 0.2 ppm or less Sr 0.2 ppm or less Add the reagent copper sulfate (special grade) to 1 kg of this pure phosphoric acid to adjust the sulfate ion concentration to 500 ppm. There was no turbidity visible to the naked eye even after leaving it for one day. For comparison, a similar test was conducted on the clear phosphoric acid obtained from the reaction process, and as a result, cloudiness was visible to the naked eye, and cloudiness remained even when left as it was at room temperature for one month.

[Brief explanation of drawings]

The drawing shows 1 of the extraction method phosphoric acid production method according to the present invention.
An example flow sheet is shown.

Claims (1)

[Claims] 1. Extraction method Sulfate ion in phosphoric acid SO ₄ /P ₂ O ₅ = 10
P ₂ O ₅ concentration 58 to remove to weight ppm or less
% by weight or more, and add a barium compound to the phosphoric acid solution to remove dissolved sulfate ions to SO ₄ /P ₂ O ₅ = 500.
After reducing the weight to less than ppm and filtering out the generated barium sulfate (hereinafter referred to as the "reaction step"), it is cooled to produce crystals of H ₃ PO ₄ 1/2H ₂ O and a mother liquor. An extraction method for removing sulfate ions from phosphoric acid, which is characterized by separation from the mother liquor and washing (hereinafter referred to as "crystallization step"). 2. A method for removing sulfate ions from phosphoric acid using the extraction method according to claim 1, wherein the mother liquor separated in the crystallization step is recycled to the reaction step.