GB2039867A - Defluorinated calcium phosphate - Google Patents

Abstract

Calcium phosphate for use in animal feedstuffs having the following characteristics: total P 15 to 21% by weight F < 0.2% Ca >/= 25% Na 6 - 10% As < 10 ppm crystalline Na3Ca6(PO4)5 > 70% crystalline apatite < 1% crystalline beta -Ca3(PO4)5 < 5% citric solubility > 90%, is made by (a) adding H3PO4 to phosphate ore and after at least 5 minutes, adding Na compound and recycled fines from (c) with ratios controlled to give CaO:Na2O:P2O5 molar ratio in product of 7.6 - 3.6: 1 : 2.9 - 1.5. (b) granulating while drying at above 100 DEG C with hot gases from (d); (c) grading granules to 0.5 - 5mm with recycle of fines to (a); (d) calcining selected granules at 1100-1300 DEG C using hot air from (e) to support combustion and (e) tempering in air at less than 800 DEG C for less than 2 minutes.

Description

SPECIFICATION Natural defluorinated calcium phosphate This invention relates to a new grade of defluorinated calcium phosphate and to a process for its preparation.

Defluorinated calcium phosphates are important industrial products which are generally used as supplements for animal feeds or as important constituents of these supplements in addition to nitrogen derivatives, sugars, oligo-elements and vitamins.

One method for preparing defluorinated calcium phosphate which we have used in the past is based on the neutralisation of a defluorinated phosphoric acid, i.e. either a phosphoric acid retained thermally or a purified acid obtained by the wet method, with a calcium compound such as finely ground lime, calcium carbonate, calcium chloride. The product obtained, which is a mixture of dicalcium and monocalcium phosphate, is practically colourless and is highly suitable for animal feeds but has the serious disadvantage of being expensive to buy.

Another method of preparing a defluorinated calcium phosphate comprises thermally treating a natural phosphate, generally in the presence of adjuvants required to promote defluorination, so as to obtain a product which animals can assimilate.

The phosphates obtained by this method have the commercial disadvantage of being highly coloured and consequently manufacturers of supplements for animal feed reject them in favour of phosphates prepared by the wet method.

Moreover, the thermal defluorinated phosphates which are on the market at present do not have all the characteristics of phosphates prepared by the wet method. It would be useful to be able to obtain a defluorinated natural calcium phosphate which was as suitable an animal feedstuff as calcium phosphates prepared by the wet method.

According to one aspect of the present invention there is provided a calcium phosphate having the following characteristics: Total P 15 to 21% by weight F < 0.2% Ca 25% Na 6-10% As < 10 ppm Crystalline Na3 Cae (PO4)5 > 70% crystalline apatite < 1% crystalline p-Ca3 (PO4)2 < 5% citric solubility > 90% This grade of calcium phosphate meets the requirements for an animal feedstuff particularly well.

According to a further aspect of the present invention there is provided a process for the preparation of a calcium phosphate which process comprises (a) adding phosphoric acid to a natural phosphate ore, and then, after at least 5 minutes, adding a sodium compound together with recycled fine particles of the product of step (c), the quantities of the various reactants being controlled so that the molar ratio of CaO/na2O/P205 in the final product is from 7.611/2.9 to 3.6/1/1.5, (b) forming the product into granules while drying the product at a temperature of more than 100"C using hot gases issuing from step (d) as a source of heat, (c) grading the resultant granules so that those particles having dimensions of from 0.5 to 5 mm are selected for further treatment while smaller particles are recycled to step (a), (d) calcining the particles of the selected size range at a temperature of from 1 1 OO"C to 1300 C using hot air emanating from step (e) to support combustion, and (e) tempering the calcined product in air at a temperature of less than 800"C for a period of less than two minutes.

One advantage of the present process over known processes is that it is possible to use any of the natural calcium phosphates which are available at present as raw materials, even those phosphates which are relatively low in P205 such as Algerian phosphates which contain 28.5% of P2O. The present process is also relatively low in its consumption of energy. Thus in general it has a no. 2 heavy fuel consumption of the order of 80 to 100 kg per tonne of defluorinated phosphate obtained, whereas in the known methods used by us commonly 2 to 4 times as much fuel is used.

A further advantage of the present process is that it is equally suitable for phosphates which are poor or rich in silica, whereas in the previously proposed processes it was usually essential to use an ore having an SiO2 content of more than 2.% by weight (see British Patent No. 902 361), or to use an ore with an SiO2 content of less than 2% by weight (see French Patent No. 2 026 878).

The process of the invention is a thermal process which in practice can only be carried out continuously.

The phosphate ore used in step (a) should have a particle size such that the phosphoric acid reacts sufficiently rapidly with the ore. Generally, an ore with an average particle diameter of 150 microns is used.

The reaction of acid and ore can be carried out in any suitable mixer, such as conventional phosphoric acid reaction tanks placed in series and mixers with horizontal arms. The phosphoric acid is preferably a wet acid containing from 25 to 55% P2O5, and this acid may be used cold or at 50 - 60"C if it is prepared in an adjacent workshop, thus reducing the reaction time.

The sodium compound is added after the acid and ore have reacted for a period of at least 5 minutes. This time may very according to the ore and the apparatus used. Preferably, sodium carbonate or sodium hydroxide are used, but it is possible to use any other sodium salt, such as sodium sulphate, nitrate, chloride or phosphate; mixing is continued until the treated mass is satisfactorily homogenised. The conditioning apparatus used to perform step (b) differs according to whether a solid sodium reagent such as sodium carbonate, or a liquid reagent such as a sodium hydroxide solution, is used.

If a solid sodium reagent is used, the homogenised mixture of step (a) is placed in a lump-remover followed by a screen which separates the large particles, which are recycled to the lump-remover, while the remaining particles are passed into a drier. It is possible to use any drying means which does not destroy the granules. For example, preferably a rotary oven or a tunnel oven can be used. These can be heated by the gases emanating from the clacination step (d), so that the temperature of the product is raised above 1 OO"C.

The dried product is screened to separate out the fractions smaller than 0.5 mm which are recycled to step (a) and added to the mixture of acid and ore along with the sodium derivative, and the particles larger than 5 mm are recycled to the lump-remover, whilst the particles of product having sizes ranging from 0.5 to 5 mm are passed on to be preheated in the calcination step (d).

If a liquid sodium reagent is added, the slurry leaving the preliminary attack is passed directly to a granulator-drier. The granules formed, heated to a temperature of at least 100 C, are screened, the fines are recycled into the inlet of the granulator, the coarse particles are returned to step (a) as described above and the particles having sizes of from 0.5to 5 mm are passed on to be preheated in the calcination step (d).

The calcination of step (d) should be carried out at from 1100 to 1300"C, preferably from 1180 to 1250"C.

This step is generally preceded by a preheating step in which the particles of product are heated to about 700"C by the hot gases leaving the calciner. Calcination may be carried out in a fluidised bed or in a rotary furnace. The heated gases resulting from cooling of the product of step (e) are used as a source of heat in this step of the process.

If a fluidised bed is used, the preheating and the calcinating at 115000 1250"C are effected in two separate apparatus, whereas if a rotary furnace is used only one apparatus is required, the preheating being carried out in the cooler part of the rotary furnace.

In order to conserve energy, the gases issuing from the preheater are used for the conditioning in step (b) of the process.

It is generally preferable to effect calcination in known manner in the presence of water vapour. If calcination is carried out using a fuel oil burner, 10 to 20% by volume water vapour is added to the gaseous mixture obtained; if calcination is effected using a gas burner, the water vapour obtained by the combustion of the gas is sufficient.

The tempering of step (e) must be such as to enable the calcined product to be cooled as rapidly as possible and in any case in under 2 minutes. Preferably, tempering is carried out with cold air in conditions such that in less than 2 minutes the temperature of the product leaving the calciner is reduced from 11 500C 1250"C to less than 800"C. Fluidised bed or vibrating bed techniques are particularly suitable for carrying out this tempering. A cold air belt-type or fluidised bed cooler, for example, brings the defluorinated phosphate obtained to ambient temperature, and the heat recovered at this stage can be used, for example, to supply a boiler in step (d) or be used for the drying in step (b).

The defluorinated phosphates prepared by this method have all the properties described hereinbefore, which makes them capable of meeting the requirements of different users. In particular, the specified contents of the various crystalline phases are essential in order to obtain products which are fully satisfactory as animal feedstuffs.

A variety of different apparatus can be used to obtain the defluorinated phosphate of the present process.

An embodiment of the process of the present invention will now be described, by way of example only, with reference to the sole figure of the accompanying drawings, which shows a flow diagram of a plant suitable for performing the process of the present invention.

Referring to the drawing, natural phosphate ore and phosphoric acid are introduced through ducts 1 and 2, respectively, into the first part I of a horizontal double-screw mixer l-ll. The required sodium compound is introduced via line 3 into the second part II of the mixer, and the recycled fine particles obtained from the screens VI or XIII are introduced via line 4. If a solid sodium reagent is used, the mixture obtained is passed via line 5 to the lump-remover Ill, and then via line 6 to a screen IV where it is sifted. The sifted particles pass along a duct 8 to a drier V and then via line 9 to a second screen VI where it is again sifted. Fine particles from screen VI are passed to mixer II via line 4 and the course particles from screen VI are recycled to lump-remover Liy via lines 22 and 7.If a liquid sodium reagent is used the mixture obtained is passed via line 23 into a granulator-drier XII, and then passes along line 24 to a third screen XIII for sifting. Coarse and fine particles outside the desired range are passed back to mixer 11 via lines 26 and 4. The granules leaving screens VI or XIII via lines 10 and 25 supply the preheating before calcining. The preheater VII is followed by a calciner VIII which is supplied with hot air via line 12, with fuel via line 13, and optionally with water vapour via line 14. The calcined product passes along line 11 to a tempering apparatus IX where it is cooled by cold air entering via line 15. It then passes through line 16 into a cooler X where cold air is introduced via line 18 and leaves via line 27. Cooled, defluorinated calcium phosphate is obtained at line 17.

To avoid undesirable loss of heat, the air leaving the preheater via line 19 is used, for example, in the drier V as shown (or in the granulator-drier Ill). The cooled gases emanating via line 20 from the drier V are passed into a washer-purifier XI before being dicharged into the atmosphere via line 21.

The following Examples illustrate the present invention.

Example 1 Mixer l-ll of the plant described above is supplied at 1 with 100 parts by weight per hour of a particulate Togo phosphate having an average particle diameter of 150cm and containing 36.5% of P2O5, 4.15% of F and 51% of CaO. 52 parts per hour of H3PO4 containing 35% of P205 are added via line 2, then 25 parts per hour of sodium carbonate are added via line 3.The mixture leaving mixer l-ll via line 5 is broken up at Ill, screened at IV to a particle size of less than 5 mm, dried at V at 1 200C and screened again at VI to eliminate the fine and coarse particles outside the desired range formed during drying.tThe resultant granulate of the desired range of particle size is preheated, then calcined over 2 hours in a rotary furnace VII-VIII where its temperature increases from 1 20"C to 1 240"C. The temperature of the exhaust gases in line 19 is 550"C. The resulting product is tempered at 600"C in a fluidised bed IX, then cooled to ambient temperature at X.

Taking into account the recycling effected as described above. 127 parts per hour of a defluorinated calcium phosphate having the following characteristics are obtained: total P 18.45% F 0.02% Ca 29.5% Na 7.8% As 8 ppm crystalline Na3Ca6(PO4)5 95% crystalline apatite not detected crystalline ss-Ca3(PO4)2 not detected citric solubility 96% Example 2 Example 1 is repeated but mixer l-ll is supplied with 45.5 parts per hour of H3PO4 containing 35% of P205, and 20 parts per hour of Na2CO3.Treatment is carried out exactly as described in Example 1 and 122 parts of a defluorinated calcium phosphate suitable for animal feed having the following characteristics is obtained: total P 18.0% F 0.18% Ca 29.8% Na 6.2% As 8 ppm crystalline Na2Ca6(PO4)5 90% crystalline apatite not detected crystalline ss-Ca3(PO4)2 2% citric solubility 92% Example 3 This example is carried out with the same Togo phosphate and under the same conditions as in Example 1, except as regards the preheating and calcining. The preheating is carried out in a cyclone at 1 20"C to 700"C followed immediately by calcining in a fluidised bed at 1220 C. The defluorinated calcium phosphate obtained has substantially the same characteristics as the product prepared in Example 1.

Example 4 100 parts per hour of a Moroccan phosphate ore containing 32.3% of P2O5, 4% of F and 51.9% of CaO are treated with 63.5 parts per hour of H3PO4 containing 35% of P205 and 25 parts per hour of Na2CO3, under the same conditions as Example 1.

129 parts per hour of a defluorinated calcium phosphate having the following characteristics are obtained: total P 18.3% F 0.06% Ca 29.5% Na 6.9% As 6 pom crystalline Na3CaÔ(PO4)5 94% crystalline apatite not detected crystalline p-Ca3 (PO4)2 3% Citric solubility 94% Example This example is carried out with the same Moroccan phosphate and under the same conditions as Example 4, except as regards the preheating and calcining. Thus, the preheating is carried out in a cyclone from 120"C to 7000C instead of in a rotary furnace, the preheating being immediately followed by calcination at 1220 C.

The defluorinated calcium phosphate obtained has the following characteristics: total P 18.5% F 0.12% Ca 29.5% Na 6.9% As 6 ppm crystalline Na3Cae(PO4)5 90% crystalline apatite not detected crystalline ss-Ca3 (PO4)2 4% citric solubility 90% Example 6 This example is carried out using the same Moroccan phosphate and under the same conditions as Example 4, but using 82.3 parts per hour of H3PO4 containing 27% of P205 and 38 parts per hour of a 50% aqueous sodium hydroxide solution.

129 parts per hour of a defluorinated calcium phosphate are obtained, which has the same characteristics as those of the product of Example 4.

Example 7 This example is carried out with the same Moroccan phosphate and under the same conditions as in Example 4, but using 57 parts of H3PO4 containing 35% of P2O5, and 22 parts per hour of NaCI. 123 parts per hour of a defluorinated calcium phosphate having the following characteristics are obtained: P 18.3% F 0.08% Ca 29.5% Na 6.4% As 6 ppm crystalline Na3Ca6(PO4)5 91% crystalline apatite not detected crystalline tricalcium phosphate 3% citric solubility 92% Example 8 This example is carried out with the same Moroccan phosphate and under the same conditions as Example 7 but using 25 parts per hour of anhydrous Na2SO4 instead of NaC1. 123 parts of a defluorinated calcium phosphate having the following characteristics are obtained: : P 18.3% F 0.09% Ca 29.5% Na 6.4% As 6 pom crystalline Na3Ca6(PO4)5 90% crystalline apatite not detected crystallinetricalcium phosphate 4% citric solubility 91.6.

Claims (14)

1. A calcium phosphate having the following characteristics: total P 15 to 21% by weight F < 0.2% Ca 25% Na 6-10% As < 10 ppm > 70% crystalline Na3Cas(PO4)5 > 70% crystalline apatite crystalline p-Ca3(PO4)2 < 5% citric solubility > 90%

2. A phosphate according to Claim 1 having a phosphorus content of from 18 to 19%.

3. A phosphate according to Claim 1 or 2, having a citric solubility greater than 94%.

4. Calcium phosphate according to Claim 1 substantially as described in any one of the foregoing Examples.

5. A process for the preparation of a calcium phosphate according to one of Claims 1 to 4, which process comprises (a) adding phosphoric acid to a natural phosphate ore, and then, after at least 5 minutes, adding a sodium compound together with recycled fine particles of the product of step (c), the quantities of the various reactants being controlled so that the molar ratio of CaO/Na2O/P205 in the final product is from 7.6/112.9 to 3.6/1/1.5, (b) forming the product into granules while drying the product at a temperature of more than 100"C using hot gases issuing from step (d) as a source of heat, (c) grading the resultant granules so that those particles having dimensions of from 0.5 to 5 mm are selected for further treatment while smaller particles are recycled to step (a), (d) calcining the particles of the selected size range at a temperature of from 1100"C to 1300 C using hot air emanating from step (e) to support combustion, and (e) tempering the calcined product in air at a temperature of less than 800"C for a period of less than two minutes.

6. A process according to Claim 5, wherein sodium carbonate is used as the sodium reagent in step (a).

7. A process according to Claim 5, wherein sodium hydroxide is used as the sodium reagent in step (a).

8. A process according to Claim 5, 6 or 7, wherein the calcination of step (d) is performed in a fluidised bed at a temperature of from 1150 to 1 2500C and is preceded by a preheating of the product to 700 - 800"C using the vapours issuing from the calcination.

9. A process according to Claim 8, wherein the preheating is effected by direct heat exchange between the vapours issuing from the calcination and the product passing through the preheater.

10. A process according to Claim 5,6 or 7, wherein the calcination of step (d) is effected in a rotary furnace the maximum temperature of which is from 1 1 50"C to 1 2500C.

11. A process according to Claim 5 substantially as hereinbefore described with reference to, and as iliustrated in, the accompanying drawing.

12. A process according to Claim 5 substantially as described in any one of the foregoing Examples.

13. A calcium phosphate according to any one of claims 1 to 4 when prepared by a process as claimed in any one of Claims 5 to 12.

14. An animal feedstuff incorporating a calcium phosphate as ciaimed in any one of Claims 1 to 4 and 13.