CS264247B1 - Process for preparing zinc-magnesium double cyclo-tetrafluorophosphates - Google Patents

Abstract

The method of synthesizing double zinc-magnesium cyclotetraphosphates c-Zn2_xMgxP4O12, where x E (0.2), consists in calcining a mixture of oxide, hydroxide, carbonate or hydroxide-carbonate of zinc and magnesium and phosphoric acid, with the heating rate being less than 60°C/min, to a temperature of 280 to 800°C and the vapor pressure in the calcination space being higher than 10 kPa.

Description

The invention relates to a process for the preparation of zinc-magnesium double cyclotetaphosphates.

Zinc-magnesium _double cyclo-tetraphosphates are represented by the formula c-Zn _{2 x x} Mg _x P _{40 12} , where x can range from near zero to close to two. These compounds are characterized by high thermal and chemical stability, they are white to colorless and their use can be as non-color special hygienic and ecologically harmless pigments or in various high-temperature materials or for agrochemical purposes. The only method so far disclosed is to crystallize them from a cooling melt containing phosphate anions in the necessary amounts together with zinc and magnesium cations. However, this process for the preparation of double zinc magnesium cyclophosphates is particularly suitable for preparative laboratory purposes. Thus, very pure products can be prepared, but this method is of little use for a wider technological application to produce products for pigmentary or agrochemical purposes that do not require completely pure products. Indeed, this method requires high energy, since it is necessary to transfer the starting mixture to the melt at high temperatures and then, in addition, it must be allowed to cool in a controlled manner (under continuous heating). Technological implementation of this method is also complicated by the necessity of providing a dry atmosphere or vacuum in the melt space.

Said drawbacks are eliminated by a process for the preparation of zinc-magnesium double cyclotetrophosphates of the formula c-Zn _{2 x x} Mg _x P ₄ O ₁₂ , where x can range from values close to zero to values close to two, that the zinc oxide, hydroxide, zinc carbonate together with the magnesium oxide, hydroxide, carbonate or hydroxide in a Zn / Mg ratio corresponding to a molar ratio of (2-x) / x is calcined with phosphoric acid in the starting mixture, where the amount of phosphate anions a molar ratio of P ₂ O ₅ / (Zn + Mg) of 0.95 to 1.4, preferably 1 to 1.05, the starting mixture being prepared by treating the zinc and magnesium compounds with phosphoric acid, preferably a concentration of 45 to 90 wt. % H ₃ PO _4, and the starting mixture is preferably allowed to continue for at least 2 h freely react, preferably with stirring and then begins to heat up rate of less than 60 ^DEG C / min, preferably at 1 to 5 ° C / min to a temperature 280 to 800 ° C, preferably 400 to 650 ° C, wherein the water vapor pressure in the calcination space is higher than 10 kPa, preferably 60 to 100 kPa.

The ratio of the zinc and magnesium compounds in the starting mixture is chosen according to the requirement for the zinc and magnesium cations in the product. The content of magnesium cations (x) may range from values close to zero to values close to two. A value of zero or two cannot be used, since then it would not be a double product, but a pure disodium cyclotrophosphate c-Zn ₂ P ₄ O ₁₂ resp. o pure magnesium di-cyclotetrophosphate c-Mg ₂ P ₄ 0i ₂ . The amount of phosphoric acid in the starting mixture, expressed as the molar ratio P ₂ O ₅ / (Zn + Mg), would be most suitable to use exactly stoichiometry (i.e., said ratio equal to one) when processing completely pure raw materials. However, according to the purity of the phosphoric acid used (the content of the metal ions therein) and the purity of the zinc and magnesium raw materials (metal admixtures), the ratio should be adjusted within the range of 0.95 to 1.4, more preferably the phosphate anions (1 to 1.05) than their lack of stoichiometry. The concentration of phosphoric acid is more advantageous in terms of controlling the condensation reactions and thus the yield and purity of the product, but this lower concentration in turn increases the energy requirement for evaporation of excess water and increases the volume of the starting mixture, moreover having the character of a slurry. Its calcination then presents technical problems as compared to the calcination of a smaller volume mixture, which would be in the form of a solid or pasty, which results when a phosphoric acid concentration in the preferred range of 45 to 90 wt.% Is used. %. Leaving the mixture containing phosphate, zinc and magnesium ions to react spontaneously for at least 2 hours is advantageous for the initial reaction of the starting zinc and magnesium compounds with phosphoric acid, i.e. the neutralization reaction using hydroxides, or especially the decomposition reaction of carbonates, carbon dioxide and the resulting leavening of the mixture would complicate the calcination itself. For this reason, stirring of the reacting starting mixture is also advantageous. The heating of the mixture at a rate of less than 60 ° C / min is chosen so that the individual dehydration and condensation reactions taking place in all stages at or near the temperatures at which they occur can occur as slowly as possible with the highest 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 energy requirements must be expected. In addition, it will be more difficult in this case to maintain sufficient water vapor pressure in the area of the mixture to be treated, thereby preventing possible breakdown of the intermediates. The lower limit of calcination temperature (280 ° C) required is given by the temperature of the gradual formation of the first zinc-magnesium double cyclotetaphosphate particles. The upper limit of the calcination temperature region (800 ° C) is related to the thermal stability of zinc-magnesium double cyclotetaphosphates, since some of them melt congruently at this temperature. The preferred calcination temperature in the range of 400 to 650 ° C is due to the fact that double cyclotetaphosphates are formed at a temperature above 400 ° C at a sufficient rate and initially forming

CS 264 247 B1 amorphous products are converted to microcrystals which are the most suitable form for the intended use. The upper limit of this preferred temperature range (650 ° C) is given by the fact that, at this temperature, double cyclo-tetraphosphate formation reactions have already taken place (if advantageous heating rates have been used), even at higher water vapor pressures in the calcination space. Maintaining a water vapor pressure in the space of the calcined mixture greater than 10 kPa is necessary to prevent the formation of undesirable by-products of condensation and to prevent cleavage and separate condensation of the phosphorus component. The water vapor present somewhat slows down the condensation reactions, allows them to be more quantitative, and also prevents the formation of undesired non-porous crusts on the surface of the calcined mixture particles, which would prevent the quantitative course of the 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 pressure of water vapor in the space of the calcined mixture - 100 kPa - is given mainly by the necessity of increased constructional, material and energetic demands on the calcining device and the calcination line, at 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 with a preferred weight concentration of 0.5 to 10%. This operation is performed ev. as a purification of the obtained product in its pure form. The action of said acids removes all unwanted by-products and ev. residues of the starting mixture which thus pass into solution. The zinc-magnesium double cyclo-tetraphosphates resist the action of these acids. When zinc-magnesium double cyclo-tetraphosphates are prepared for use for pigmentary or agrochemical purposes, and in particular if they are prepared under the preferred conditions of the invention, such purification is not required.

The process makes it possible to prepare zinc-magnesium double cyclotetraphosphates 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 - dilute and less pure phosphoric acid and less pure zinc compound. Under the conditions of preparation of the product according to the invention

Claims (2)

SUBJECT 1. A process for the preparation of zinc-magnesium double cyclo-tetraphosphates of the formula c-Zn _{2 x} Mg _x P ₄ O | ₂ , wherein x may range from zero to two, characterized in that the oxide, hydroxide, zinc carbonate together with the oxide, hydroxide, carbonate or hydroxide-carbonate of the present invention may be preferred. to prepare almost pure double zinc-magnesium cyclo-tetraphosphates. If less pure products are obtained, they can be purified by the process according to the invention. The following are examples of the preparation of zinc magnesium magnesium cyclophosphates according to the invention. Example 1 100 g of zinc carbonate (containing 51% Zn) and 65.4 g of magnesium carbonate (containing 29% Mg) were mixed with 480 g of phosphoric acid at a concentration of 65% H ₃ PO ₄ and allowed to react spontaneously with stirring for 3 hours . The mixture was then heated at 2 ° C / min to 480 ° C for 4 hours at this temperature. The water vapor pressure in the space of the calcined mixture was maintained at 85 to 95 kPa. Was obtained 329 g of product which contained 91% zinc double cyklotetrafosforečnanu magnesium formula _C-4 ZnMgP Oi _second Example 2 100 g of zinc oxide (containing 79% Zn) and 35.4 g of magnesium hyderoxide (containing 35% Mg) were carefully mixed with 722 g of 45 wt% phosphoric acid. % H ₃ PO ₄ and allowed to react spontaneously with occasional stirring for 5 h. The mixture was then heated at 1 ° C / min to 550 ° C for 2 hours at this temperature. The water vapor pressure in the space of the calcined mixture was maintained at 75 to 85 kPa. 352 g of product were obtained, which was further purified by leaching with 4% hydrochloric acid. % HCl. After washing with water and drying at 130 ° C afforded 344 g of product containing 97% cis-Zn ₅ Mg P _{4 W} Oi _second Example 3 50 g of zinc hydroxide slurry (containing 30.6% Zn) and 71.1 g of a magnesium hydroxide slurry (containing 24% Mg) were carefully mixed with 224 g of 85 wt% phosphoric acid. % H ₃ PO ₄ and allowed to react spontaneously with occasional stirring. The mixture was then heated at 5 ° C / min to 510 ° C and held at this temperature for 1 h. The water vapor pressure in the space of the calcined mixture was maintained at 80 to 90 kPa. 193 g of product were obtained, which contained 93.5% of zinc magnesium magnesium double cyclo-tetrophosphate of the formula c-Zn ₀₅ Mg ₁₅ P ₄ O ₁₂ . OF THE INVENTION The magnesium compounds in a Zn / Mg ratio relative to the molar relationship of (2-x) / x are calcined with phosphoric acid in the starting mixture wherein the amount of phosphate anions to bivalent cations corresponds to a molar ratio P ₂ O ₅ / (Zn + Mg) a value of 0.95 to 1.4, preferably a value of 1 to 1.05, at The starting mixture is prepared by treating the zinc and magnesium compounds with phosphoric acid, preferably at a concentration of 45 to 90% by weight. % H ₃ PO ₄ and the starting mixture is preferably allowed to react for at least 2 hours, preferably with stirring, and then heated at a rate of less than 60 ° C / min, preferably 1 to 5 ° C / min, to 280 to 800 ° C, preferably 400 to 650 ° C, wherein the water vapor pressure in the calcination space is higher than 10 kPa, preferably 60 to 100 kPa. Method according to claim 1, characterized in that the product after calcination is treated with hydrochloric, sulfuric, nitric or phosphoric acid, preferably a concentration of 0.5 to 10% by weight.