METHOD AND APPARATUS FOR PRODUCING NITROPHOSPHATE FERTILIZERS

Abstract: A method of producing nitrophosphate fertilizers includes the steps of treating a phosphate bearing rock with nitric acid, preferably in the ratio of 2 parts by mass of acid to one part of mass of rock, to form an acidic slurry containing nitrogen compounds, water or citric acid soluble phosphate compounds and inert solid materials; feeding the slurry to a mixing chamber; ammoniating the slurry with an ammoniat­ ing agent such as ammonia in the mixing chamber and using the heat evolved in the ammoniation reaction to evaporate off any liquid present; and adding into the mixing chamber dur­ ing the ammoniation step an amount of sulphuric acid suffi­ cient to inhibit the reversion of the soluble phosphate com­ pounds into insoluble phosphate compounds, and preferably also an amount of phosphoric acid to adjust the N:P ratio of the final product and to stabilize the nitrophosphates formed.

Description: FIELD OF THE INVENTION THIS invention relates to a method of producing nitrophosphate fertilizers, and to apparatus for use in the production of nitrophosphate fertilizers. BACKGROUND OF THE INVENTION In phosphate based fertilizers, the solubility of the phosphate compounds is of particular concern because this represents the availability of the phosphorous to the plants. Phosphate bearing rock from which most phosphate based fertilizer is produced contains essentially tricalcium phosphate (Ca3 (P04)2) which is quite insoluble in water or acid soils. However, monocalcium phosphate ( Ca (H2 P04)2 ) and dicalcium phosphate ( CAHP04) are soluble in water and in acids such as citric acid. In order to convert the tricalcium phosphate in the phosphate bearing rock into mono-and dicalcium phosphates, the rock is generally treated with sulphuric acid. Depending on the ratio of acid to rock, varying proportions of mono-and dicalcium phosphate are produced along with various inert solid materials such as gypsum (CaSO4). The water and acid soluble salts may then be treated with ammonia to produce ammonium phosphates such as monoammonium phosphate (MAP) (NH4 H2 PO4) and diammonium phosphate (DAP) (NH4)2 HYPO, which form the basic raw material for fertilizers. In an alternative method, the phosphate bearing rock may be treated with nitric acid to yield a mixture of calcium nitrate, phosphoric acid and monocalcium phosphate as well as inert solid compounds. Amnoniation of this mixture produces a mixture of ammonium phosphate, dicalcium phosphate and ammonium nitrate which is termed generally nitrophosphates. Although treatment with nitric acid has certain advantages over treatment with sulphuric acid, there are various problems associated with this process. For, example, the nitrophosphate feedstocks have high moisture contents and give rise to high recycle rations in the plant. SUMMARY OF THE INVENTION According to the invention there is provided a method of producing nitrophosphate fertilizers including the steps of: treating a phosphate bearing rock with nitric acid to form an acidic slurry containing nitrogen compounds and water or citric acid soluble phosphate compounds; feeding the slurry to a mixing chamber; ammoniating the slurry with an ammoniating agent in the mixing chamber and using the heat evolved in the ammoniation reaction to evaporate off any liquid present; and during the ammoniation step, adding into the mixing -chamber an amount of sulphuric acid sufficient to inhibit reversion of the soluble phosphate compounds into insoluble phosphate compounds. During the ammoniation step an amount of phosphoric acid may be added into the mixing chamber to adjust the N:P (nitrogen:phosphorus) ratio of the.final product and to stabilize the nitrophosphates formed. In particular the phosphoric acid prevents decomposition of the ammonium nitrate formed. The amount of phosphoric acid added will depend on the desired fertilizer formulation. The temperature during the ammoniation reaction is approximatelyl300C and can be as high as 17OOC, However the temperature should be controlled so that it does not reach a level at which the nitrophosphates would decompose. Water may be added into the mixing chamber during the ammoniation step to control the temperature. The ratio of nitric acid to rock used is important and depending on the type of phosphate rock used, would be in the order of 2:1 by mass. Generally a 60% ni:tric acid solution is used. The ammoniating agent is preferably ammonia. The acidic slurry formed may be filtered before being fed into the mixing chamber. to remove the inert:sol id materials present therein. However, for low-grade fertilizers, which are preferred in some cases, it is possible to omit this filtering step, with its associated expense and to feed the slurry still containing the inert solid materials directly from the acdification step to the mixing chamber. The slurry is preferably fed directly into the mixing chamber in order to conserve the heat evolved in the acidification step. The reaction between the nitric acid and the rock is an exothermic reaction and the temperature of the slurry will rise to 60-700C. The heat of reaction is useful in the next step to evaporate off the liquid present. The method may be carried out batchwise but preferably it is carried out continuously, i.e. with continuous feed of the slurry into the mixing chamber. The slurry is preferably fed tangentially into the mixing chamber. The ammoniating agent, sulphuric acid and phosphoric acid are preferably fed into the mixing chamber through a sparger. From the mixing chamber the ammoniated slurry may be fed into a spray drying chamber and dried to a powder. Alternatively, the ammoniated slurry may be fed into a granulator and granulated. The invention also provides apparatus for use in the production of nitrophosphate fertilizers comprising a tank reactor for reacting nitric acid with phosphate bearing rock and a mixing chamber connected to the tank reactor for redeiving acidic slurry from the reactor, the mixing chamber having a preferably tangential inlet for the acidic slury, at least one other inlet for an ammoniating agent and a nozzle pipe along which the mixture can pass to a chamber outlet. The other inlet or inlets are preferably connected to a sparger inside the mixing chamber. The mixing chamber may discharge into a spray drying chamber. Alternatively, the mixing chamber may discharge into a granulator. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagram showing apparatus used for the production of nitrophosphate fertilizers according to the invention; Figure 2 shows a mixing chamber according to the invention, for use in the apparatus of Figure 1; and Figure 3 is an end view of the mixing chamber of Figure 2. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Raw phosphate rock is fed along a conduit 1 into a stirred tank reacter 2. 60% nitric acid is stored in a tank 3 and is fed by a pump 4 along 0 a pipe 5 into the reactor 2. The nitric acid will be at about 30 C and under a pressure of l5kPa. The nitric acid and rock are stirred by a stirrer 6 and react to form a mixture of nitrogen compounds, water and citric acid soluble phosphate compounds and inert solid materials such as gypsum. Approximately 20 minutes retention time in the reactor is sufficient to convert substantially all the phosphates into a water soluble -form. The reaction is exothermic and under adiabatic conditions the temperature will rise to 60-7OOC. Some nitrous fumes will be evolved during the reaction. These fumes are extracted and scrubbed by means not shown. The acidic slurry is pumped by a pump 7 along pipes 8,9 to a pipe reactor with a mixing chamber indicated schematically at 11. The slurry passes to the mixing chamber at a temperature of approximately 606C and-a pressure of 750 kPa. Ammonia is fed into the mixing chamber along pipes 12 and 13. The amazonia will be at a pressure of approximately 900 kPa. The ammoniation reaction occurs at a pressure of approximately 600 kPa and at a temperature of approximately 1300C. Water may be introduced into the mixing chamber along pipes 16 and 17 to control the temperature in the mixing chamber. The neutralization reaction between the ammonia and the acidic slurry is virtually instantaneous and the heat evolved evaporates the water introduced. Sulphuric acid can be added directly into the reactor 2 as indicated by arrow 14. However, it is preferred to add the sulphuric acid directly into the mixing chamber along pipes 18 and 19. Normally,when the acidic slurry is ammoniated and dried the soluble phosphates will revert into insoluble phosphates. This reaction is inhibited by the addition of a small amount of sulphuric acid, in the order of 5%. Phosphoric acid can optionally be added directly into the reactor 2 but again it is preferred to add it along pipes 20 and 21 directly into the mixing chamber. The phosphoric acid is added to adjust the N:P ratio of the final product in order to optimise the downstream formulation of complex fertilizers. The phosphoric acid is equally desirable to stabilize the process and to prevent decomposition of ammonium nitrate formed in the mixing chamber. The amount of phosphoric acid added will depend on the final desired fertilizer formulation. The mixing chamber 11 discharges into a spray drying chamber 22 where the steam is flashed off and the solid product particles are dried and cooled by a current of air. The exhaust fumes are extracted by an extractor 23 and passed to a scrubbing plant (not shown). The dry powdery product which collects at the bottom of the chamber at a temperature of approximately 350C and with a moisture content of about 5% can then be used as a dry feed material in a NPK granulation plant. The dry powder-product can be considered to be a mixture of monoammonium phosphate, ammonium nitrate, ammonium sulphate and some calcium phosphate. The overall process conversion of phosphate bearing rock to water soluble phosphate is in the order of 78% and the overall conversion to citric acid soluble phosphates is in the order of 97% Alternatively the product from the mixing chamber 11 can be obtained as a melt and introduced directly into a drum granulator (not shown) to produce granular NPK fertilizers. The pipe reacter with the mixing chamber 11 is shown in Figures 2 and 3. A nozzle pipe 30 extends either into a drum granulator or into a spray drying tower. There are four inlets 31, 32 and 33, 38 to the nozzle. The acidic slurry enters through one tangential inlet 33, ammonia is introduced via inlet 32, water is introduced through inlet 31, sulphuric acid through the pipe 38a and phosphoric acid through pipe 38b to the second tangential inlet 38. The inlets 31 and 32 lead to a sparger section 34 which has a number of outlets into an auxiliary chamber 35 to which the tangential inlets 33 and 38 are connected. Mixing takes place in this auxiliary chamber 35 and in the nozzle 30 and the mixture then exits via outlet 36. The mixing occurs at a pressure of approximately 600 kPa and at a temperature of approximately 1300C. A considerable amount of moisture evaporates from the mixture leaving the outlet 36, and the mixing chamber and nozzle can be designed and operated in such a way as to produce either a powder or a melt. The remainder of the moisture is driven off either in the spray drying tower or in the drum granulator.

Claims: CLAIMS 1. A method of producing nitrophosphate fertilizers by treating a phosphate bearing rock with nitric acid to form an acidic slurry containing nitrogen compounds and soluble phosphate compounds, characterized in that slurry is fed to a mixing chamber (11), and is ammoniated in the mixing chamber with an ammoniating agent in such a way that the heat evolved in the ammoniation reaction is used to evaporate off any liquid present and that during the ammoniation step there is added into the mixing chamber an amount of sulphuric acid (via 18,19) sufficient to inhibit reversion of the soluble phosphate compounds into insoluble phosphate compounds. 2. A method according to claim 1 characterised in that during the ammoniation step an amount of phosphoric acid is added (via 20,21) into the mixing chamber (11) to adjust the N:P ratio of the final product and to stabilize the nitrophosphates formed. 3. A method according to claim 1 or claim 2 characterised in that water (via 16,17) is added into the mixing chamber (11) during the ammoniation step to control the temperature in the mixing chamber. 4. A method according to any one of the preceding claims characterised in that the ratio of nitric acid to phosphate bearing rock is approximately 2:1 by mass. 5. A method according to any one of the preceding claims wherein an approximately 60% solution of nitric acid.is used. 6. A method according to any one of the preceding claims characterized in that the ammoniating agent is ammonia. 7. A method according to any one of the.preceding claims characterized in that the slurry fed into the mixing chamber (11) also contains inert solid compounds. 8. A method according to any one of the preceding claims characterized in that the slurry is fed directly (via 8,9) to the mixing chamber (11) to conserve the heat evolved in the acidification step. 9. A method according to any one of the preceding claims characterized in that the slurry is fed continuously to the mixing chamber (11). 10. A method according to any one of the preceding claims characterized in that the slurry is fed tangentially (via 33) into the mixing chamber (11). 11. A method according to any one of the preceding claims characterized in that the ammoniating agent is fed into the mixing chamber through a sparger. 12. A method according to any one of claims 2 to 10 characterized in that the phosphoric and/or sulphuric acids are fed-tangentially into the mixing chamber. 13. A method according to any one of the preceding claims characterized in that the ammoniated slurry is fed to a spray drying chamber (22) and is dried to a powder. 14. A method according to any one of claims 1 to 12 characterized in that the ammoniated slurry is fed to a granulator and is formed into granules. 15. Apparatus for use in the production of nitrophosphate fertilizers, the apparatus comprising a tank reactor (2) for reacting the nitric acid with phosphate bearing rock, and a mixing chamber (11) connected to the tank reactor, characterized in that the mixing chamber (11) is adapted for receiving and mixing at elevated temperature and pressure acidic slurry from the reactor (2) and an ammoniating agent, and that the mixing chamber includes a nozzle pipe (30) along which the mixture passes to an outlet (36). 16. Apparatus according to claim 15 characterized in that the mixing chamber (11) has a tangential inlet (33) for the acidic slurry. 17. Apparatus according to claim 15 or claim 16 characterized in that the mixing chamber (11) includes a sparger (34) forming an inlet for an ammoniating agent. 18. Apparatus according to any one of claims 15 to 17 chacterized -in that the mixing chamber (11) discharges into a granulator 19. Apparatus according to any one of claims 15 to 17 characterized in that the mixing chamber (11) discharges into a spray drying chamber (22).

Inventors: VAN SCHALKWYK, PIERRE LAFRAS
Applicants: OMNIA FERTILIZER LIMITED
Application year: 1981
Main classification: C05B11
IPC codes: C05B11