DD231944A3 - PROCESS FOR PREPARING GROB CRYSTALLINES, IRON-ARMY POTASSIUM DIHYDROGEN PHOSPHATE BY COOLING CRYSTALLIZATION - Google Patents

Abstract

Process for the preparation of coarsely crystalline, low-iron potassium hydrogen phosphate by cooling crystallization. As starting materials Phophorsaeure and potassium hydroxide are used. The aim of the invention is to produce a coarsely crystalline low-iron KH2PO4, which is particularly suitable as a laboratory and fine chemical or for pharmaceutical purposes. It has now been found that coarsely crystalline, low-iron KH2PO4 can be prepared by Kuehlungskristallisation by a Fe2O5 concentration of not more than 0.01% is maintained in the reaction mixture by selecting appropriate raw materials, the P2O5 concentration to 20 to 28% at a KOH excess is adjusted from 8-13 mol% based on the stoichiometric conversion and at least 6% of the theoretically required amount of ethylenediaminetetraacetate is used to bind the iron.

Description

example 1

23 m ^{3 of} a 70 ° C hot KH ₂ PO ₄ -LoSung, prepared by equimolar reaction of potassium hydroxide and phosphoric acid, in an aqueous medium are cooled in a crystallizer within 16 hours at 35 ⁰ C, the salt separated from the mother liquor and in a rotary kiln dried.

The following table gives an overview of the dependence of the iron concentration in the crystals and the appearance of the crystals on the composition of the starting solution under otherwise identical conditions of preparation.

approach Composition of the 3 P ₂ O ₅ gene Fe ₂ O ₃ -Ge crystal No. gangslösunc stop haltdes structure Mass Ever (%) Kristal relationship 23.2 sats Fe ₂ O ₃ : K (Ppm) 1 1: 1830 25.2 45 fine needles 2 1: 1988 28.0 51 fine needles 3 1: 2216 67 fine

needles

The table shows that, if all the process parameters according to the invention are not observed, products with a relatively high Fe ₂ O ₃ concentration and fine-needle crystalline form are formed.

Example 2

According to Example 1, a solution with 24.8% P ₂ O ₅ , a potassium excess of 9.5 mol% and a mass ratio of Fe ₂ O ₃ : K of 1: 2175, prepared and worked up analogously. The resulting crystals are zwarbalkenförmig, but very finely crystalline and has a Fe ₂ O ₃ content of 43 ppm.

Example 3

In a crystallizer of potassium hydroxide solution and phosphoric acid in the presence of water, a potassium phosphate solution having a P ₂ 0 ₅ concentration of 25.8%, a potassium excess of 11.7 mol .-%, based on the stoichiometric compound KH ₂ PO ₄ , and a mass ratio Fe ₂ O ₃ : K of 1: 2273 produced. Prot crystals of ethylenediaminetetraacetate 100 g. Are added to the saturated solution and the procedure is continued as in Example 1.

The salt contains 10 ppm Fe ₂ O ₃ and has a coarsely crystalline, bar-shaped crystal structure.

The purity of the KH ₂ PO ₄ is further characterized by the following technical requirements:

Chloride (Cl) <0.0006% Sulfate (SO ₄ ) <0.005% Total nitrogen (N) <0.001% Heavy metals (as Pb) <0.001% Sodium (Na) <0.025% Asse <0.0002%

Claims (1)

Invention claim: Process for the preparation of coarsely crystalline, low-iron potassium hydrogen phosphate by cooling crystallization from potassium hydroxide and phosphoric acid, by the addition of Aethylendiamintetraacetat, characterized in that in the reaction mixture by selecting appropriate raw materials Fe ₂ 0 ₃ concentration of 0.01% maximum is maintained, the P ₂ 0 ₅ concentration is adjusted to 20 to 28% at a KOH excess of 8-13 mol .-% based on the stoichiometric conversion, for binding the Fe ³⁺ at least 6% of the theoretically required amount of Aethylendiamintetraacetat is used. Field of application of the invention The present invention relates to a process for the preparation of coarsely crystalline, low-iron potassium dihydrogen phosphate by cooling crystallization when using potassium hydroxide solution and phosphoric acid as starting materials. Characteristic of the known technical solutions For use as laboratory and fine chemicals as well as in the pharmaceutical and film industry, products of the highest purity are required. For these applications, potassium dihydrogen phosphate is prepared by direct neutralization of phosphoric acid and potassium hydroxide or potassium hydroxide in a molar ratio of 1: 1 and subsequent cooling crystallization of the resulting solution. Finely crystalline needles are obtained from hot aqueous solutions (Z. Elektroch., 31 [1925]). Depending on the nature of the starting materials, products with different degrees of contamination are obtained. Specifically, the inclusion of iron ions in the form of iron-containing microcrystallites in Monokaliumdihydrogenphosphat during crystallization is problematic, since so iron-containing products arise that are no longer used for special users. Although, at the beginning of the cooling crystallization, iron ions initially form relatively stable complexes with the phosphate ions present in the solution (SJE Salmon, J., al., 1952, 2316/23, 1953, 2644/9), it is evident that the crystallization of KH ₂ PO ₄ concurrent formation of acidic iron phosphate (E. ANGELESCU, G. BALAESCU colloid journal 47 [1929] 217), which proceeds preferentially if during the crystallization process a part of KH ₂ PO ₄ crystallizing in tetragonal pyramids has already formed and the ratio of Fe ₂ O ₃ to P ₂ O ₅ in favor of Fe ₂ O ₃ shifts. It is known in the production of orthophosphates to prevent contamination by heavy metal salts in that complexing agents u. a. EDTA be added. (DE-PS 2,934,919) This measure also leads to the production of ammonium dihydrogen phosphate that bar-shaped crystals are obtained. According to AP 1.901.020 (1932) it is possible. Contain impurities of iron ions in phosphate solutions by the addition of Na ₄ P ₂ Oy prior to the onset of crystallization complex so that low-iron products are obtained. Especially for the production of KH ₂ PO ₄ , however, this compound is unsuitable because it contaminates the solution with Na ⁺ ions. Although other complexing agents are known from analytical or preparative chemistry, they show in the presence of phosphates, which themselves form extremely stable iron phosphate complexes with comparable complex stability constants. Deviations in the complex binding capacity. According to G. SCHWARZENBACH, The Complexometric Titration, Ferdinand Enke Verlag Stuttgart, 1960, Bd.45 bind 37.2 mg EDTA 5.585 mg Fe. To achieve low-iron phosphates, methods are also suitable in which iron is separated from the solution to be crystallized. The problem here is that the chosen method of separation must be selective and the existing, present in large excess cations or anions do not interfere. An example of this principal route of iron separation from alkali metal salts of such acids which form complexes which are particularly resistant to Fe ions, such as phosphoric acid, is WP 50571. Thereafter, after reduction of the iron to the bivalent stage and addition of O-phenanthroline or a ' a'-Dipyridyl removed the complex compounds formed by adsorption on cation exchange resins. Although in this method and other analogous methods, the Fe enrichment is avoided by reusing accumulating mother liquors, but it is generally only possible to make the Fe separation from dilute solutions, so that a subsequent focus on the starting solution is required. This additionally incurs energy and material costs for stainless steel containers, which make the entire process uneconomical. Object of the invention The aim of the invention is to produce a coarsely crystalline low-iron KH ₂ PO ₄ , which is particularly suitable as a laboratory and fine chemical or for pharmaceutical purposes. Explanation of the essence of the invention The object of the invention is to make the production conditions of potassium dihydrogen phosphate from potassium hydroxide and phosphoric acid so that the formation of iron-containing microcrystallites is prevented and a coarsely crystalline product of specific purity is obtained. It has been found that at an iron concentration of at most 0.01% as Fe ₂ O ₃ in a starting solution by setting a P ₂ O ₂ concentration of 20.0 to 28.0%, preferably 24.0 to 27.0%, and an addition of 50 to 150 mg, preferably 100 mg, of ethylenediaminetetraacetate per kg of KH ₂ PO ₄ avoids the formation of iron-containing microcrystallites and thus a coarsely crystalline low-iron KH ₂ PO _{4 is} obtained. Decisive for the process according to the invention, in addition to the parameters already mentioned, is the consistent maintenance of a potassium excess of 8.0 to 13.0 mol%, preferably 11.5-12.0 mol%, based on the stoichiometric compound KH ₂ PO ₄ , in the starting salt solution and the resulting mass ratio Fe ₂ O ₃ : K from 1: 1704 to 1: 2495, preferably from 1: 2110 to 2385. Surprisingly, it has been found in the process according to the invention that a complexone addition (ethylenediaminetetraacetate) of 6 %, which would be necessary for the quantitative binding of the total amount of iron present in the starting solution, is sufficient. The advantage of the procedure according to the invention further consists in the fact that a coarsely crystalline low-iron KH ₂ PO _{4 is} obtained by simple means - observance of said potassium ion excess. It eliminates material and energy intensive cleaning stages. The new process produces a product which is readily centrifugable and storable and which, unlike products after customary saturation to the so-called monopoint (without KOH excess), contains no fine needle-shaped crystals but rather coarse-crystalline, bar-shaped crystals. The KOH excess according to the invention increases the space-time yield. When more than 65% of the amount of ethylenediaminetetraacetate theoretically required for iron bonding is used, the crystal growth and the crystal image are undesirably affected. The result is the finest needle-shaped crystals that do not build up even after longer crystallization times.