CS245829B1 - Preparation method of cyclo-tetraphosphate dizincate - Google Patents

Abstract

Process for preparing cyclo-tetraphosphate the calcined calcination of the starting oxide mixture, hydroxide or zinc carbonate a phosphoric acid in the phosphate molar ratio anions to zinc cations 1.9 to 2.25: 1, preferably 2.0 to 2.05: 1 and treating the zinc compound s phosphoric acid mass concentration 30 to 90%, preferably 85 to 80% wt. concentration; the obtained mixture containing zinc and phosphate ions are left before calcination, rest for at least 30 minutes and then heats up less than 20 ° C / min, preferably at 2 to 5 ° C / min to 220-800 ° C, preferably at a temperature of 300 to 500 ° C, while in space the calcined mixture is a vapor pressure higher than 20 kPa, preferably 60 to 100 kPa. The solution can be applied in phosphate technology and due to possible use product also in the pigment industry in the preparation of anticorrosive coatings mass and in agrochemistry in the preparation of microelements fertilizers.

Description

(54) A process for the preparation of disodium cyclotrophosphate

A process for the preparation of disodium cyclotrophosphate by calcining the starting mixture from zinc oxide, hydroxide or carbonate and phosphoric acid in a molar ratio of phosphate anions to zinc cations of 1.9 to 2.25: 1, preferably 2.0 to 2.05: 1 and treating the zinc compound with phosphoric acid with a concentration of 30 to 90% by weight, preferably 85 to 80% by weight. concentration; the resulting mixture containing zinc and phosphate ions is allowed to stand for at least 30 minutes prior to calcination and is then heated at a rate of less than 20 ° C / min, preferably 2 to 5 ° C / min to 220 to 800 ° C, preferably to a temperature of 300 to 500 ° C, wherein the water vapor pressure in the space of the calcined mixture is higher than 20 kPa, preferably 60 to 100 kPa.

The solution can be applied in the phosphate technology and due to the possible use of the product also in the pigment industry in the preparation of anticorrosive paints and in agrochemistry in the preparation of microelement fertilizers.

The present invention relates to the preparation of disodium cyclotrophosphate.

Disodium Cyclo-Phosphate (c— —ΖΠ2Ρ4 je12) is a thermally and chemically very stable substance with possible use as anticorrosive pigment in paints or as a micro-element fertilizer. The present data on its preparation are based mainly on the method of calcination of zinc dihydrogen phosphate dihydrate, where it is possible to obtain almost pure product under certain calcination conditions, but not yet more precisely specified. However, this process for the preparation of disodium cyclotrophosphate requires the use of the starting phosphate in pure form. However, obtaining zinc dihydrogen phosphate is a very demanding operation for the temperature control of its reaction, it is time consuming, requires high-quality raw materials, the utilization of which is relatively small. Technological use of zinc dihydrogen phosphate for the preparation of disodium cyclo-tetrafosphate is therefore not foreseeable. However, there is a more technologically feasible possibility of preparing cyclotrophosphate from a mixture of zinc oxide or hydroxide and phosphoric acid or ammonium phosphates. The existing data on the preparation conditions are again incomplete, but a mixture of condensed phosphates is always reported to be produced, one of the products being disodium cyclotrophosphate. In addition, pyrophosphates are formed, higher linear zinc phosphates and polyphosphoric acids are also formed by the independent condensation of the phosphorous component. These acids cause a reduction in the content of phosphorus anions that can condense to the zinc products, and this reduction is then compensated by the balance of unreacted zinc oxide from the zinc feedstock as other impurities. The preparation of pure disodium cyclotrophosphate, or a product with a high content thereof, is therefore not possible according to prior art methods.

These drawbacks are overcome by the process for the preparation of disodium cyclotaphosphate according to the invention, which comprises calcining zinc oxide or zinc hydroxide or carbonate or a mixture thereof together with phosphoric acid, characterized in that the molar ratio of phosphate anions to zinc cations is 1.9 to 2 25: 1, preferably 2.0 to 2.05: 1, wherein the zinc compound is treated with phosphoric acid at a concentration of 30 to 90%, preferably 65 to 80% by weight. The resulting mixture is allowed to stand for at least 30 minutes before calcination and is then heated at a rate of less than 20 ° C / min, preferably 2 to 5 ° C / min to 220 to 800 ° C, preferably 300 to 500 And the water vapor pressure is maintained above 20 kPa, preferably 60 to 100 kPa, in the space of the calcined mixture. For the preparation of disodium cyclotrophosphate from completely pure raw materials, the molar ratio of phosphate and zinc ions would be exactly 2: 1 by exact stoichiometry. Depending on the purity of the phosphoric acid used (or the metal ion content) and the purity of the zinc raw material (metal admixtures), the ratio should be adjusted in the range of 1.9 to 2.25: 1, more preferably a small excess of phosphate anions ( 2 to 2.05: 1) than their lack of stoichiometry. A lower concentration of phosphoric acid would be more advantageous in terms of controlling the condensation reactions and hence the yield and purity of the product, but in turn would increase the energy requirement for evaporation of excess water and increase the volume of the starting mixture, moreover having the character of a slurry. Its calcination would then present technical problems, as opposed to calcination of a smaller volume mixture in the form of a solid or pasty, which arises when a phosphoric acid concentration is used in the upper part of said range. Preferably, therefore, an acid having a concentration of 65 to 80 wt. ° / o. The resulting mixture containing zinc and phosphate ions is allowed to stand for at least 30 minutes before calcination and is then heated at a rate of less than 20 ° C / min, preferably at 2 to 5 ° C / min to a temperature of 220 to 800 ° C, preferably to a temperature of 300 to 500 ° C, wherein the vapor pressure in the space of the calcined mixture is higher than 20 kPa, preferably 60 to 100 kPa. Leaving the mixture containing phosphate and zinc ions at least 30 minutes prior to calcination is necessary to allow the initial reaction of the zinc compound with phosphoric acid to be sufficiently carried out, i.e. a neutralization reaction using zinc hydroxide or, in particular, the decomposition reaction of zinc carbonate. since the released carbon dioxide and the resulting loosening of the mixture would complicate the calcination itself. The heating of the mixture at a rate of less than 20 ° C / min is selected so that the individual dehydration and condensation reactions successively taking place in the mixture can take place at or near the temperatures at which they occur at the slowest possible yield. Otherwise, unwanted cleavage of the intermediates would result in the release of the phosphorous component, which would then condense separately into polyphosphoric acids and thereby contaminate the main product and reduce its yield. When selecting a very low heating rate, a disproportionate increase in the product preparation time and thus a certain increase in material and energy requirements must be expected. In addition, it will be more difficult in this case to maintain sufficient water vapor pressure in the space of the calcined mixture and thereby prevent the possible breakdown of the intermediates. The lower limit of calcination temperature (220 ° C) is given by the temperature of the gradual formation of the first disodium cyclotrophosphate particles. The upper limit of the calcination temperature region (800 ° Cj) is related to the thermal stability of the cyclo-tetraphosphate, which melts at this temperature during the decomposition of the tetraphosphate cycles and their transition to higher linear phosphates. The preferred calcination temperature is 300-500 ° C. at a temperature above 300 [deg.] C., the pyrophosphate is formed at a sufficient rate and the amorphous product initially formed is converted into the microcrystals which are the most suitable form for the intended use. Keeping the water vapor pressure in the area of the calcined mixture higher than 20 kPa is necessary to prevent the formation of undesirable condensation by-products, possibly the transition from cyclo-tetraphosphate microcrystals from the so-called low-temperature modification, which is more advantageous for the intended practical use. The presence of water vapor slows down some condensation reactions somewhat, allows them to be more quantitative and also prevents the formation of undesired non-porous crust on the surface of the calcined mixture particles, which would prevent the quantitative course of dehydration reactions. It is advantageous to maintain the water vapor pressure above 60 kPa, when its action is sufficient in said direction. The upper limit for the advantageous vapor pressure in the area of the calcined mixture - 100 kPa is given mainly by the necessity of increased constructional, material and energy demands on the calcining device and the calcination line, when using higher pressures than atmospheric pressure. Moreover, increased braking of the condensation and dehydration reactions due to a water vapor pressure above 100 kPa would again disproportionately prolong the product preparation time.

Furthermore, the process according to the invention consists in treating the product after calcination with hydrochloric, sulfuric, nitric or phosphoric acid. This operation is performed as ev. purification of the obtained product in its pure form, for example for analytical or preparative purposes. The action of said acids removes all unwanted by-products and ev. residues of the starting mixture which thus pass into solution. Disodium cyclophosphate resists the action of these acids and is thus completely pure after leaching with acids and after washing with water and drying. If disodium cyclotrophosphate is prepared for use as a pigmentary fertilizer or as a micro-element fertilizer, and in particular if it has been prepared under the preferred conditions of the invention, such purification is not required.

The process makes it possible to prepare disodium cyclotrophosphate under technologically feasible conditions, with a sufficient yield and with a sufficient purity of the product. It also allows the use of lower quality raw materials - diluted and less pure phosphoric acid and less pure zinc compounds. Under the conditions of preparation of the product of the invention indicated to be preferred in the present invention, practically pure disodium cyclotrophosphate can be prepared. If less pure product is obtained, it can be purified according to the process of the invention.

Example 1

100 g of zinc oxide (containing 79% Znj) was mixed with 278.6 g of 85 wt% H3PO4 phosphoric acid, and after 1 hour of rest, the mixture was heated at 5 ° C / min to 500 ° C for 3 hours. The vapor pressure in the space of the calcined mixture was maintained at 70 to 85 kPa The product after calcination contained more than 95% disodium cyclotrophosphate, by leaching with hot hydrochloric acid at a concentration of 15% by weight HCl, washing with water and drying at 100 ° C 235.5 g of pure disodium cyclotrophosphate were obtained.

Example 2

100 g of zinc carbonate (containing 51% Znj) was mixed with 235.2 g of 65 wt.% H3PO4 phosphoric acid and left to stand for 2 hours. The mixture was then heated at 3 ° C / min to 450 ° C for 4 hours. The water vapor pressure in the space of the calcined mixture was maintained at 80 to 95 kPa and the product contained more than 97% disodium cyclotrophosphate.

Example 3

100 g of waste zinc hydroxide sludge containing 15% Zn were mixed with 120 g of waste phosphoric acid containing 40 wt. % H 3 PO 4 and after 30 minutes of standing, the mixture was heated at 1 ° C / min to 350 ° C for 2 hours at this temperature. The water vapor pressure was maintained at 30 to 40 kPa in the area of the calcined mixture. The product contained 85% disodium cyclotrophosphate and was further hot-leached with 10% sulfuric acid. concentration, washed with water and dried at 200 degrees Celsius. 41.3 g of pure disodium cyclotrophosphate were obtained.

Claims (2)

SUBJECT A process for the preparation of disodium cyclotrophosphate by calcination of an oxide or hydroxide or carbonate or a mixture thereof together with phosphoric acid, characterized in that the molar ratio of phosphate anions to zinc cations is maintained at 1.9 to 2.25: 1, preferably 2.0 % to 2.05: 1 and the zinc compound is treated with phosphoric acid, a concentration of 30 to 90% by weight, preferably 65 to 80% by weight. concentration, the resulting mixture containing zinc and phosphate ions is allowed to stand for at least 30 minutes before calvingnase and then heated at a rate of less than 20 ° C / min, preferably at 2 to 5 ° C / min to 220 to 800 ° C, preferably at a temperature of 300 to 500 ° C, wherein a water vapor pressure greater than 20 kPa, preferably 60 to 100 kPa, is maintained in the space of the calcined mixture. 2. A process according to claim 1, wherein the product is treated with hydrochloric, sulfuric, nitric or phosphoric acid after calcination.