GB2026999A

GB2026999A - Preparing Ammonium Polyphosphates

Info

Publication number: GB2026999A
Application number: GB7926796A
Authority: GB
Original assignee: Anic SpA
Current assignee: Anic SpA
Priority date: 1978-08-03
Filing date: 1979-08-01
Publication date: 1980-02-13
Also published as: IT7826429A0; ES483772A1; MA18548A1; IN151585B; IT1099588B; GB2026999B

Abstract

For preparing ammonium polyphosphates in solid or liquid form, a process is disclosed in which phosphoric acid in an atomized state and ammonia stream are introduced into, and reacted in, a reaction chamber under a pressure from 1.1 to 6 atmospheres and at a temperature of from 200 DEG C to 340 DEG C. The reaction products are discharged, with expansion through a restricted passage. A reactor is also described, which is particularly, but not exclusively, adapted to the performance of the ammoniation reaction, and important dimensional parameters of the structural members of the reactor are also indicated.

Description

SPECIFICATION Process For producing Ammonium Polyphosphates and Apparatus Suitable for Use In The Process This apparatus relates to a process for producing ammonium polyphosphates and to an apparatus suitable for use in the process.

Generally, in practice, the ammonium polyphosphate produced is a mixture of the ammonium salts of orthophosphoric acid and of a number of polyphosphoric acids. Thus the compounds which are present in the mixtures usually include ammonium pyrophosphate, tripolyphosphate and tetra-polyphosphate: small amounts of polyphosphate having a longer chain are often also present, i.e.

polyphosphates having 5 or more phosphorus atoms per molecule.

Ammonium polyphosphate is useful as a highly nutrient fertilizer. In addition, it is very soluble and possess a high sequestering power towards metallic cations which are present in phosphoric acid as produced by the wet process and which are thus maintained in solution, so that polyphosphates are particularly useful in the production of liquid fertilizers, that is, fertilizing solutions.

Ammonium polyphosphate is obtained, as a rule, by reacting superphosphoric (or polyphosphoric) acid with ammonia, or by molecularly dehydrating ammonium orthophosphate as produced by ammoniation of phosphoric acid.

Both the conventional techniques for producing ammonium polyphosphates are impaired by difficulties and shortcomings which must be offset in order that the process may be economically acceptable, and the quality of the product also acceptable.

The ammoniation of superphosphoric acid requires the use of special apparatus and particular materials, because of the characteristics of superphosphoric acid, such as its corrosive power, viscosity and low point of crystallization, whereas the ammoniation of phosphoric acid with molecular dehydration requires either the use of a pure acid (such as that obtained from kiln phosphorus), or, when for reasons of economy it is desired to use phosphoric acid obtained with the wet process, i.e. so-called wet phosphoric acid, particular conditions and apparatus must be used in order to ensure that the formation equilibrium of ammonium orthophosphate is such as not to cause any precipitation of that compound because it must remain in a condition which permits its immediate dehydration to polyphosphate.An accurate control is thus necessary of the conditions of temperature and pressure, in order to ensure, immediateiy after the formation of the orthophosphate, the shift of the reaction equilibrium towards the formation of polyphosphates, that is, towards the controlled molecular dehydration. It is thus required that the reaction times are short, that the temperature and pressure increases are controlled during the ammoniation reaction, and that water is immediately removed from the reaction system without, however, attaining such conditions as to encourage corrosion or undesired precipitation of polyphosphates.

A number of processes have been envisaged, the general feature of which is a system within which the ammoniation and dehydration reactions are caused to occur.

There have been suggested, for example, a horizontal frusto conical reactor having a stirrer equipped with blades coaxial with respect to the reactor, as disclosed in United States Patent Specification No. 3,387,036, which is adapted for the continuous ammoniation of phosphoric acid having a high concentration and a high viscosity; a two-stage system as disclosed in United States Patent Specification No. 3,382,059, involving ammoniation to orthophosphate and subsequent superammoniation and dehydration; tubular reactors having two consecutive stages, with counterflow circulation of ammonia and phosphoric acid, as disclosed in United States Patent Specification No. 3,420,624; packed columns, for reducing the hydrolysis of the reaction products, as disclosed in the United States Patent Specification No. 3,788,817; and tubular reaction vessels with equicurrent flow of the acid in the interior of the ammonia stream in order to overcome the problems of the corrosion caused by the acid, under controlled conditions of temperature and pressure, as disclosed in the United States Patent Specification No.

3,950,495.

From time to time research workers have solved the specific problems of the reaction and have suggested procedures which were deemed the most suitable at the time.

The conventional technique, however, did not remove a few shortcomings, such as, for example, the rather bulky size of the reactor on account of the low output, and the need to use a rather concentrated phosphoric acid in the feed stream. According to the present invention, there is provided a process for the production of "ammonium polyphosphase" (as hereinbefore defined), which process comprises feeding ammonia and atomized phosphoric acid to a reaction chamber, reacting the phosphoric acid and ammonia in the reaction chamber under a pressure of from 1.1 to 6 atmospheres and at a temperature of from 200"C to 3400C, and discharging the reaction product from the chamber by expansion through a restricted passage.

It has surprisingly been found that the disadvantages associated with the conventional techniques can be overcome or reduced if the ammoniation of phosphoric acid takes place within a reaction chamber of a comparatively reduced volume, by feeding one of the reactants in a particular manner and by discharging the reaction products with a concurrent expansion.

In the method of the present invention, the atomized phosphoric acid and ammonia may be fed separately to the reaction chamber; alternatively the atomized phosphoric acid can be entrained in the influent ammonia.

Preferably phosphoric acid and ammonia are reacted in the reaction chamber for a time of less than one second.

The restricted passage is preferably provided in a wall of the reaction chamber opposite to the port or ports through which ammonia and phosphoric acid are introduced into the chamber.

The phosphoric acid which is fed to the reaction chamber can have a concentration which is as low as 50% of P2Os; it may be fed after preheating to a temperature of at least 900C.

Ammonia is preferably fed after preheating to at least 600C. Phosphoric acid can either be atomized with any conventional atomizing system, or it can be entrained by the gaseous ammonia stream and be drawn into the chamber by that stream, by means of an ejector or otherwise, in which the motive fluid is ammonia, and it can be atomized during the suction and discharge step in the reaction chamber.

The present invention also provides a reactor, by which the process according to the present invention can be performed both simply and cheaply.

The reactor according to the present invention is provided with a chamber defined by two opposite end walls and a side wall which is preferably cylindrical, one end wall having one or more ports for the introduction of the atomized phosphoric acid and the ammonia, and the other end wall being provided with an interchangeable diaphragm having at least one bore, there being downstream of the diaphragm a body having internally a shape selected from among those of a prism, a cylinder, and a diverging frustum of cone or a pyramid, with the smaller base of the frustum abutting the diaphragm, the body leading at its downstream end into a terminal tube.

The aforementioned other end wall may have the shape of a converging frustum of a cone or of a pyramid, the interchangeable diaphragm being fastened to the smaller base of the converging frustum of the cone or of the pyramid.

If the diaphragm has a single bore, preferably it is at the geometrical centre of the diaphragm. The diaphragm may rest on and be fastened to the reactor bottom end wall in an appropriate conventional manner.

As regards the shape of the body, cylindrical and frustoconical outlines are preferred. The terminal tube is generally cylindrical and has an appropriate length.

The diaphragm can have that face confronting the chamber recessed, preferably with a conical shape with the cone apex lying on the bore axis; as an alternative, the same face can be planar and have a bore so shaped as to correspond to a de Laval nozzle.

Preferably the body and at least part of the terminal tube are cooled, conveniently with a fluid flowing in a jacket placed externally of the body and tube.

The diaphragm can also be equipped with an appropriate cooling system connected or unconnected with the cooling system for the body and tube, the cooling system for the diaphragm preferably being provided in the interior of the diaphragm.

The angle between the side wall of the reaction chamber and the other end wall, which may be a frustoconical orfrustopyramidal end wall, is preferably from 900 to 1600C, more preferably from 1200 to 1450, the angle of 900 corresponding to the case in the other end wall is plantar.

The diameter of the diaphragm bore is preferably such that the critical velocity can be attained therethrough.

Preferably when the body is a cylindrical or frusto-conical body, the diameter of which at that end contacting the diaphragm, is from 5 to 20 times the diameter of the diaphragm bore.

The apical angle of the cone or the pyramid of the diverging frustoconical or frustopyramidal body is preferably up to 250, the angular magnitude of zero corresponding to the case in which the frustum of the cone or of the pyramid is replaced by a prism or a cylinder.

The terminal tube can lead to a second conventional reactor intended for diluting and cooling the material and for collecting the melt.

In practice, the reactor of, and employed in the process of, the present invention fulfils the following requirements: simplicity of construction, reduced size, no problem with foaming (this problem generally is to be faced in reactors with a stirrer, in which phosphoric acid is contained), short stay times (less than one second), quick polycondensation of the acid owing to an instantaneous withdrawal of water in the form of superheated steam, high polyphosphate yield, great processing versatility, and reduced occurrence of corrosion and erosion phenomena.

In practice, diaphragms with bores and nozzles of various sizes are adopted, consistent with the pressural conditions obtaining in the reaction chamber.

The process of the present invention may be used for obtaining either solid polyphosphate, or polyphosphate in solution.

It is desired to produce polyphosphate in solution, the ammoniation in the reaction chamber is best carried out only to the attainment of pH of 2.5-3.3, and the melted product thus obtained subjected to a further ammoniation up to a pH of 6.1 to 6.5, to dilution with water, or dissolution, and to cooling to about 500C in second conventional reactor into which the terminal tube from the reaction chamber leads.

From the second reactor the solution of liquid fertilizer can be collected (10--344-0; 11-33- 011-37-0) with a concentration of polyphosphates ranging from 40% to 60%. As a rule, a content of about 50% of polyphosphates is enough to ensure stability at low temperatures and for a long storage life.

If it is desired to produce solid polyphosphate, it will suffice to carry out the ammoniation in the reaction chamber to a pH of about 4, the fused material being then discharged into a second reactor, which is maintained at a temperature of 2000C, to keep the polyphosphate in the molten condition, the latter reactor being intended only to act as a first reservoir and mixer four the melt. The molten ammonium polyphosphate can then be sent from the second reactor to pelletization and other processing stages.

It is possible to add in the second reactor other ingredients for the production of finished liquid or solid fertilizers.

During the reaction in a first reaction chamber associated with a second reactor, on account of the existing pressure and temperature conditions, the evolution of steam is experienced during the reaction, and of ammonia, which can be used for preheating and preliminary concentration of the phosphoric acid to be reacted. This recovery of heat could reduce the cost of the acid by 3%-4% approximateiy.

In the case in which the end portion of the terminal tube of the main reactor dips into a second conventional reactor, the reaction gases are absorbed by the solution in the second reactor. Ammonia will serve for the adjustment of pH and thus the quantity of ammonia to be added to the second reactor will consequently be reduced, and condensation will cause the amount of water, to be added for obtaining the expected product, also to be reduced.

By operating with the main reactor not merged with the second reactor, the gases escaping from the melt would be sent to a third reaction chamber, or to a scrubber, where they would contact phosphoric acid with 54% of P205, the ammonia being thus recovered and other heat being thus evolved, which, added to that contributed by the gases, would succeed in raising the concentration of the acid.

An acid would thus be obtained with a content of P205 of 57%59% and a nitrogen content of 2%4% which could be fed to the main reactor for ammoniation and polycondensation purposes.

The following non-limiting illustrative example described the production of ammonium polyphosphate in solution according to the present invention.

Example Decanted commercial-grade phosphoric acid, which had a P205 rating of 59.5%, was heated with steam to a temperature of 1 260C by passage through a heat exchanger and was fed to the reaction chamber through an atomizer at a rate of 5 litres per hour. Simultaneously, there was fed by means of another atomizer gaseous ammonia which had been preheated to the temperature of 130 C in a heat-exchanger, and maintained under a pressure of 2.5 kg/cm2.

The pressure in the reaction chamber attained the value of 1.9 kg/cm2 with a diaphragm having a bore of 1.5 mm diameter. The resulting melt, which had a P205 content of 61.06% (51% of which was in the form of polyphosphates) and a N2 content of 11.15%, the pH of the 10% solution being 2.6, was immediately discharged into another reactor, which had a stirrer, with a static head equal to about 40% of its volume, such head being a solution of ammonium polyphosphates having a composition of 10-34-0 (percentages of each fertilizing element, viz.:N, P205, K20).

In the initial stage of the process, this solution could be replaced by water. The stirred mass was further ammoniated by the introduction of further ammonia until the pH was 6.3 (10% solution), and then cooled with water at 500C by recycling ammonia through an exchanger by the agency of a pump.

The liquor so obtained had a composition 10-34-0, more that 50% of the P205 being in the form of polyphosphates, and it had a dark colour, with a specific gravity of about 1.390 at a temperature between 50 C and 600C.

The resulting solution appeared to be very muddy. After a few days, it separated spontaneously and the solid phase which separated was evaluated to be about 8% by volume. Upon filtration through a precoated filter, is-was possible to separate all the solid particles in suspension.

A limpid liquor, having a brilliant green hue, was obtained and it was very stable both at low temperature and after long storage times.

A sample of a product which had the following specification: Total P205=34.3%; P205 as orthophosphates=1 6.4%; ammoniacal nitrogen=10.8%; specific gravity at 200C=1.406; conversion into polyphosphates=52.5%; and kinematic viscosity=32.3 centistokes; was of minus 1 70C. After one month, the solution was perfectly clear.

A process was also carried out for the concentration of the phosphoric acid produced by the wet method. Phosphoric acid which had a P205 rating of from 50% to 57% was preheated at about 130 C and sent from a reservoir, via a piping and a pump, to a first reactor, wherein it reacts with ammonia fed at a pressure of from 1 to 1.5 kg/cm2, water being driven off in the form of steam, the result being a concentration of the liquid phase.

In the first stage, a temperature of 1 500 C- 1 650C was attained and the initial formation of polyphosphates was experienced, of the order of 2% to 3%.

The reaction product was fed to a second reactor equipped with a stirrer and in which the product remained for about 30 minutes at 1 450C-1 550C. The terminal portion of the reactor was immersed in the liquid which had been produced, and the steam evolved contributed towards an additional concentration of the acid by being bubbled therethrough.

The acid of the second reactor, which had a P205 rating of 57%59% and a nitrogen rating of from 2% to 4%, was drawn off by the pump and sent to the reaction in a further reactor, wherein the ammoniation is carried out in a manner similar to that described above. The reactor was connected to the further conventional reactor which is equipped with a stirrer, to which ammonia was fed and from which the product was discharged. There can be obtained a melt having a composition of 12-60-0 intended for the production of a solids fertilizer.

Claims

1. A process for the production of "ammonium polyphosphate" (as hereinbefore defined), which process comprises feeding ammonia and atomized phosphoric acid to a reaction chamber, reacting the phosphoric acid and ammonia in the reaction chamber under a pressure of from 1.1 to 6 atmospheres and at a temperature of from 2000C to 3400C, and discharging the reaction product from the chamber by expansion through a restricted passage.

2. A process according to claim 1, wherein the restricted passage is provided in a region of the reaction chamber opposite to that at which the phosphoric acid and ammonia are fed to the reaction chamber.

3. A process according to claim 1 or 2, wherein the phosphoric acid is fed to the reaction chamber after having been preheated to a temperature of at least 900C and the ammonia is fed to the reaction chamber after having been preheated to at least 600C.

4. A process according to claim 1, 2 or 3, wherein the phosphoric acid and ammonia are fed separately to the reaction chamber.

5. A process according to claim 1,2 or-3, wherein the atomized phosphoric acid is entrained in the ammonia fed to the reaction chamber.

6. A process according to claim 5, wherein the phosphoric acid is entrained in an ejector by the stream of ammonia fed to the reaction chamber.

7. A process according to any preceding claim, wherein the velocity of flow of the reaction products out of the restricted passage is the critical velocity.

8. A process according to any preceding claim, wherein the reaction chamber is defined by two opposite end walls and a side wall, one of the end walls having one or more ports for introducing phosphoric acid and ammonia into the chamber, and the other end wall being provided with an interchangeable diaphragm having at least one bore, there being downstream of the diaphragm a body having internally a shape selected from among those of a prism, a cylinder, and a diverging frustum of cone or of a pyramid, with the smaller base of the frustum of cone or of a pyramid, with the smaller base of the frustum abutting the diaphragm, the body leading at its downstream end into a terminal tube.

9. A process according to claim 8, wherein said other end wall has the shape of a conveying frustum of a cone or of a pyramid, the interchangeable diaphragm being fastened to the smaller base of the converging frustum of the cone of the pyramid.

10. A process according to claim 8 or 9 wherein the diaphragm has a single bore at its geometrical centre.

11. A process according to claim 9 or to claim 10 when appendant to claim 9, wherein the diaphragm has that face confronting the reactor chamber recessed.

12. A process according to claim 11, wherein sain confronting face of the diaphragm is in the form of a cone with the cone apex lying on the axis of the bore.

13. A process according to claim 9 or to claim 10 when appendant to claim 9, wherein the diaphragm has that face confronting the reactor chamber solid, with the bore shaped as a de Laval nozzle.

14. A process according to any one of claims 8 to 1 3 wherein the terminal tube is cylindrical.

1 5. A process according to any one of claims 8 to 14 wherein the body and at least part of the terminal tube are cooled by a fluid flowing through a jacket externally to the body and the tube.

16. A process according to any one of claims 8 to 15, wherein the diaphragm is equipped with a cooling system.

i i. A process according to claim 16, when appendent to claim 15, wherein the cooling system for the diaphragm is connected to the cooling system for the body and terminal tube.

18. A process according to claim 16 when appendant to claim 1 5, wherein the cooling system for the diaphragm is independent of that for the body and terminal tube.

19. A process according to claim 16, 1 7 or 18, wherein the cooling system for the diaphragm is internal to the latter.

20. A process according to claim 9, or any one of claims 10 to 19 when appendant to claim 9, wherein the angle between the chamber side wall and the frusto-conical or frustopyramidal end wall is up to 1600.

21. A process according to claim 20, wherein said angle isfrom 1200 to 1450.

22. A process according to any one of claims 8 to 21, wherein said body is a cylindrical or frustoconical body, the diameter of which at that end contacting the diaphragm, is from 5 to 20 times the diameter of the diaphragm bore.

23. A process according to claim 9, or to any one of claims 10 to 22 when appendant to claim .9, wherein the apical angle of the cone or pyramid containing the diverging frustum of the cone or pyramid is 250 as a maximum.

24. A process according to claim 1, substantially as hereinbefore described.

25. A reactor having a reaction chamber defined by two opposite end walls and a side wall, one of the end walls having one or more ports for introducing phosphoric acid and ammonia into the chamber, and the other end wall being provided with an interchangeable diaphragm having at least one bore, there being downstream of the diaphragm a body having internally a shape selected from among those of a prism, a cylinder, and a diverging frustum of cone or of a pyramid, with the smaller base of the frustum of cone or of a pyramid, with the smaller base of the frustum abutting the diaphragm, the body leading at its downstream end into a terminal tube.

26. A reactor according to claim 25, and further having the feature or features specified in any one of claims 9 to 23.