1. Field of the Invention
The present invention relates to a purification of a wet process phosphoric acid by a solvent extraction with butanol as a solvent.
In the specification, the following terms are defined as follows. "Wet process phosphoric acid" means wet process phosphoric acid prepared by treating a phosphate rock with sulfuric acid. "Butanol" means n-butanol and 1 or isobutanol. "NAKMG" means sodium, potassium or magnesium. "NAKMG compound" means "one or more oxides, hydroxides, carbonates or phosphates of NAKMG. "Solvent extraction" means a treatment comprising an extraction battery, a purification battery and a stripping battery. "Extraction battery" means a battery for extracting phosphoric acid into a solvent phase by contacting a solvent with wet phosphoric acid and an aqueous phase discharged from the purification battery by a counter current extraction method. "Purification battery" means a battery for extracting impurities into an aqueous phase by contacting a high purity phosphoric acid aqueous solution with an extracted solution obtained by an extraction battery by a counter current extraction method. "Stripping battery" means a battery for extracting phosphoric acid into an aqueous phase by contacting pure water with the solvent phase obtained from the purification battery.
The present invention is characterized by purifying the wet process phosphoric acid having high concentration to obtain high purity phosphoric acid in high yield even though butanol is used as a solvent.
2. Description of the Prior Arts
Most of cation impurities in the wet process phosphoric acid are in the form of phosphates which are not easily extracted with a solvent and whose solubilities are decreased depending upon decrease in the concentration of phosphoric acid. Therefore, scaling is easily caused in the stage of low concentration of phosphoric acid in the extraction battery near the stage for discharging the raffinate. In accordance with the simple process for contacting butanol with the wet process phosphoric acid, the yield of phosphoric acid can not be high enough and is about 50 to 70%. Among the known process for improving the yield, the process for incorporation of hydrochloric acid in the extraction battery is the practically applicable process. In accordance with this process, it is easy to increase the yield, however, the hydrochloric process suffers from the following disadvantages.
(1) It is not easy to separate impurities of Zn, Cd and Sb from phosphoric acid. A separate complicated treatment for the separation is required. (U.S. Pat. No. 3,920,797 and Japanese Unexamined Patent Publication No. 17399/1974). PA0 (2) Efficiency for separating the other impurities is inferior so as to increase the load for the purification battery. PA0 (3) Hydrochloric acid is expensive. PA0 (4)Corrosion resistance of the apparatus should be considered. PA0 (1) To prevent scaling and eliminate treatment for removing sulfate (SO.sub.4) from the purified phosphoric acid and the wet process phosphoric acid even though sulfuric acid is incorporated in the extraction battery; and PA0 (2) satisfactorily removing impurities even though butanol is used as a solvent and to increase concentration of the wet process phosphoric acid to higher than 35 wt.% as P.sub.2 O.sub.5 and the concentration of the extracted solution is increased higher than 130 g./liter as H.sub.3 PO.sub.4. PA0 (a) feeding a wet process phosphoric acid having a concentration of P.sub.2 O.sub.5 of higher than 35 wt.% into an extraction battery; PA0 (b) adding sulfuric acid into a stage in fifth or higher stage from the step of discharging an extracted solution and in second or higher stage from the step of discharging raffinate in the extraction battery at a gram equivalent ratio given by subtracting the amount of sulfuric acid in the wet process phosphoric acid fed into the extraction battery from 1.2 to 2.0 times of the sum of total cation impurities in the wet process phosphoric acid fed into the extraction battery and total components of NAKMG added to the purification battery; PA0 (c) discharging said extracted solution having a concentration of H.sub.3 PO.sub.4 of 130 to 180 g./liter from said extraction battery; PA0 (d) adding at least one of NAKMG compound in second or higher stage from the step of discharging the solvent phase and in second or higher stage from the step of discharging the aqueous phase in the purification battery to give more than 0.4 g. equivalent per liter of the concentration of NAKMG compound in the aqueous phase in the stage where NAKMG is added in the same battery.
When sulfuric acid is used instead of hydrochloric acid, the yield is increased, however, the following disadvantages are found.
The wet process phosphoric acid usually contains 1 to 10 wt. % of sulfuric acid. It has been difficult to effectively remove sulfuric acid by the solvent extraction method. Therefore, the wet process phosphoric acid is treated with a calcium compound such as phosphate rock, calcium oxide and calcium hydroxide to remove sulfate (SO.sub.4) as calcium sulfate before using it in the solvent extraction method. The resulting wet process phosphoric acid has high calcium content and accordingly, if sulfuric acid is incorporated in extraction battery, calcium sulfate is produced to cause the scaling. Therefore, it has been difficult to increase the yield with sulfuric acid as an industrial process. Consideration has been given to remove sulfuric acid group by treating purified phosphoric acid with a barium compound after the solvent extraction of the wet process phosphoric acid. However, the content of sulfate (SO.sub.4) in the purified phosphoric acid is high to require large amount of expensive barium compound. Therefore, this process could not be practically used.
In order to increase the yield of phosphoric acid, it is preferable to use higher concentration of the wet process phosphoric acid. When the concentration of the wet process phosphoric acid is low, large amount of a solvent and many numbers of stages are required to increase the yield.
When the concentration of the wet process phosphoric acid is high, only small amount of solvent is required and the concentration of phosphoric acid in the extracted solution fed from the extraction battery to the purification battery is high. Usually, when the concentration of phosphoric acid in the extracted solution is high, it is not easy to separate impurities from phosphoric acid in the purification battery. However when the butanol is used as solvent, this tendency remarkable results are obtained. Therefore, in the conventional process using butanol, the wet process phosphoric acid having a concentration of P.sub.2 O.sub.5 of 30 to 35 wt.% has been used to give a concentration of H.sub.3 PO.sub.4 in the extracted solution of lower than 130 g./liter, usually about 100 g./liter even though there are disadvantages of (1) the requirement of a large amount of solvent and many numbers of stages (2) low concentration of purified phosphoric acid obtained from the washing battery, and (3) large load for concentration and large load for treatment of the raffinate obtained from the extraction battery. In the process for improving the yield by using hydrochloric acid, the separation of the impurities from phosphoric acid is inferior and accordingly, it has been further difficult to increase the concentration.
On the other hand, as described, even though the concentration of the wet process phosphoric acid is high, the concentration of phosphoric acid is low near the stage for discharging the raffinate in the extraction battery. In order to increase the yield, it is necessary to use a solvent having high extraction efficiency even though the concentration of phosphoric acid is low. Butanol can substantially completely extract phosphoric acid and is ideal from this viewpoint.