CS257739B1 - Process for preparing condensed calcium phosphate with a molar ratio of P2O5 / CaO equal to one - Google Patents

Abstract

The method consists in calcining a mixture of calcium compound with phosphoric acid at a rate of less than 30 °C/min to a temperature of 300 to 900 °C, while maintaining a water vapor tension higher than 20 kPa in the calcined mixture. The solution can be applied in phosphate technology and, due to the possible use of the product, also in the pigment industry in the preparation of anti-corrosion coatings and in agrochemistry in the preparation of fertilizers.

Description

The invention relates to a method for preparing a condensed calcium phosphate having a molar ratio of P 2 O ₅ / CaO is equal to one. The compound is characterized by thermal and chemical stability and can be used as an anticorrosive pigment. No. 247 844 for paints or various high-temperature materials or as fertilizers with long-term effectiveness. The hitherto data on its thermal preparation are based primarily on the method of calcination of calcium dihydrogen phosphate hydrate, whereby under certain calcination conditions, but not yet more precisely specified, said product can be obtained.

However, such a process requires the use of the starting phosphate in pure form, which is a disadvantage since the starting dihydrogen phosphate needs to be prepared in advance.

Technological use of calcium dihydrogen phosphate for the preparation of said condensed calcium phosphate therefore cannot be envisaged for the above reasons. However, there is a technologically feasible possibility of its thermal preparation - from a mixture of a calcium compound with a volatile anion and phosphoric acid. The existing data on the conditions of this preparation are again incomplete, but is it always said that a mixture of condensed phosphates? in fact, pyrophosphates, higher linear calcium phosphates are formed as by-products and polyphosphoric acids are also formed by separate condensation of the phosphorus component.

These acids reduce the content of phosphorus anions that could otherwise condense to the desired calcium product, which is then compensated by the residual unreacted calcium oxide from the starting calcium raw material and the increased formation of dicalcium phosphate as an impurity. The preparation of the pure product is therefore not yet possible according to the methods given to date.

These drawbacks are eliminated by a process for preparing a condensed calcium phosphate having a molar ratio ^{Ρ 2θ5} ^ ³ θ equal to one according to the invention whose principle consists in calcining the oxide or hydroxide or calcium carbonate or mixtures thereof, together with phosphoric acid, wherein the ratio of phosphate anions to Ca The cations are expressed as a molar ratio of p ₂ O ₅ / CaO of 0.99 to 1.1: 1, preferably 1 to 1.02: 1, wherein the calcium compound is treated with phosphoric acid, preferably a weight concentration of 60 to 85%.

Preferably, the obtained mixture is allowed to stand for at least 30 minutes before calcination and is then heated at a rate of less than 30 ° C / min, preferably at a rate of 2 to 5 ° C / min to a temperature of 300 to 900 ° C, preferably to a temperature. 375 to 600 ° C, the water vapor pressure in the space of the calcined mixture being maintained above 20 kPa, preferably 80 to 100 kPa.

When preparing the product from completely pure raw materials, the molar ratio of phosphate and calcium ions would be most appropriate to the stoichiometry (ie ^P 2 ° 5 ^{? / CaO} = 1).

Depending on the purity of the phosphoric acid used (the content of the metal ions therein) and the purity of the calcium raw material (the metal admixtures), the ratio should be adjusted in the range of 0.99 to 1.1: 1, most preferably with a small excess of phosphate anions (P2O4 / CaO1 to 1.02: 1). The concentration of phosphoric acid is lower in terms of control of the condensation reactions and hence the yield and purity of the product; however, this lower concentration causes in turn an increase in the energy requirement for the evaporation of excess water and increases the volume of the starting mixture, which moreover has the character of a slurry.

Its calcination then presents technical problems, as opposed 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 60 to 85 wt.% Is used. %. Leaving the mixture containing phosphate and calcium ions to stand still for at least 30 minutes before calcination is advantageous for sufficient initial reaction of the starting calcium compound with phosphoric acid; this is a neutralization reaction using calcium hydroxide, or especially decomposition reactions of calcium carbonate, since the released carbon dioxide and the resulting boiling of the mixture would complicate the calcination itself.

The heating of the mixture at a rate of less than 30 ° C / min is selected so that the individual dehydration and condensation reactions successively taking place in the mixture can occur at or near the temperatures at which they occur at the slowest possible yield. .

Otherwise, unwanted cleavage of the intermediates would occur, releasing 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 product preparation time and thus a certain increase in material and energy requirements is to be expected. In addition, it will be more difficult in this case to maintain sufficient water vapor pressure in the calcined mixture space, thereby preventing possible cleavage of the intermediates.

The lower limit of calcination temperature (300 ° C) required is given by the temperature of the gradual formation of the first product particles. The upper limit of the calcination temperature range (900 ° C) is related to the thermal stability of condensed calcium phosphate, which is non-congruent at this temperature. The preferred calcination temperature in the range of 375 to 600 ° C is that the product is formed at a temperature above 375 ° C at a sufficient rate and the amorphous product initially formed is converted to the microcrystals which are the most suitable form for the intended use. The upper limit of this preferred temperature range (600 ° C) is given by the fact that at this temperature the product formation reactions have already taken place (if used - advantageous heating rates), even at higher water vapor pressures in the calcination space, so further calcination is useless.

Maintaining a water vapor pressure in the space of the calcined mixture greater than 20 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 crust 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 80 kPa, when its action is sufficient. The upper limit for the advantageous pressure of water vapor in the area of the calcined mixture - 100 kPa - is given above all by the fact that during its event. Exceeding increases the constructional, material and energy requirements for the calcination plant 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, preferably a concentration of 0.5 to 10% by weight. This operation is performed as ev. Purification of the obtained product in its pure form, eg for analytical or preparative purposes. The action of said acids removes all undesirable side products and ev. residues of the starting mixture which thus pass into solution.

Condensed calcium phosphate with mol. with a P 2 O 4 / CaO ratio equal to one, it resists these acids and is thus completely pure after leaching with acids and after washing with water and drying. If the product is prepared for use for pigmentary purposes or as a long-acting fertilizer and in particular if it has been prepared under the preferred conditions of the invention, this cleaning is not required.

The process allows the preparation of condensed calcium phosphate with mol. a ratio of ^Ρ 2θ5 ^ ^Η θ equal to one under technologically feasible conditions, with sufficient yield and with sufficient purity of the product. It also allows the use of low-quality diluted and less pure phosphoric acid; practically pure product can be prepared under the conditions of preparation of the product of the invention indicated to be preferred in the present invention. If less pure product is obtained, it can be purified according to the process of the invention.

Example 1

100 g of calcium carbonate (containing 40% Ca) were mixed with 398 g of 50 wt% phosphoric acid. % H ₃ PO ₄ and allowed to stand for 2 h. The mixture was then heated at 500 ° C at a rate of ° C / rt for 3 h at this temperature. The water vapor pressure in the space of the calibrated mixture was maintained at 70 to 85 kPa. 198 g of a product were obtained which contained 98% condensed calcium phosphate {mol. ratio ^P 2 ⁽ - ⁾ 5 ( ^CaO = 1).

Example 2

100 g of calcium hydroxide slurry containing 27% Ca was carefully mixed with 192 g of 70 wt% phosphoric acid. The mixture was heated at 5 ° C / min to 600 ° C for 3 h at this temperature. The water vapor pressure in the space of the calcined mixture was maintained at 85 to 98 kPa. The product after calcination contained more than 97% condensed calcium phosphate with mol. the ratio of P ₂ O / CaO equal to one. I leach hot with hydrochloric acid at a concentration of 5 wt. % HCl, washing with water and drying at 150 ° C gave 236 g of pure product.

Example 3

100 g of calcium oxide containing 71% Ca were mixed with 417 g of phosphoric acid containing 85 wt. % ^Η 3 ^Ρ θ4 ^{and the} mixture was heated at a rate of 2 ° C / min to 350 ° C for 3 hours at this temperature. The water vapor pressure was maintained at 60 70 kPa in the calcined mixture. Prodkut contained 98.5% condensed calcium phosphate (with a molar ratio of P 2 O 4 / CaO equal to one) and was obtained 356 g.

Claims (2)

A process for preparing condensed calcium phosphate with mol. a ratio of P 2 / CaO equal to one calcination of calcium oxide or hydroxide or carbonate or a mixture thereof together with phosphoric acid, characterized in that the ratio of phosphate anions to calcium cations expressed in moles. the ratio of P ₂ O - / Ca 0 is 0.99 to 1.1, preferably 1 to 1.02: 1, and the calcium compound is treated with phosphoric acid, preferably a concentration of 60 to 85% by weight, after which the calcium-containing mixture obtained is obtained and the phosphate ions are preferably allowed to stand for at least 30 min before calcination and then heated to a temperature of less than 30 ° C / min, preferably 2 to 5 ° C / min, to a temperature of 300 to 900 ° C, preferably to a temperature of 375 to 900 ° C, wherein the water vapor pressure in the space of the calcined mixture is higher than 20 kPa, preferably 80 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.