Method for manufacturing an ammonium phosphate fertilizer having low cadmium content

Abstract

The invention relates to a method for manufacturing an ammonium phosphate fertilizer from a phosphoric acid aqueous solution that has less than 50% P.sub.2O.sub.5 concentration and is obtained by wet phosphate ore treatment, said phosphoric acid containing traces of cadmium, comprising the following steps: (a) neutralizing said phosphoric acid solution (1) with ammonia (3) up to a molar ratio N/P of between 0.1 and 0.8, (b) reacting said partially neutralized solution (4) with a sulfide source (6) so as to form a cadmium sulfide precipitate (9), (c) separating said precipitate (9) so as to obtain a refined ammoniated phosphoric acid solution (10), (d) ammoniating and granulating said refined solution (10) so as to form said fertilizer (12).

Claims

1. A method for manufacturing an ammonium phosphate fertilizer from a phosphoric acid aqueous solution that has from 25 to 45% P.sub.2O.sub.5 concentration and is obtained by wet phosphate ore treatment by sulfuric acid, said phosphoric acid containing traces of cadmium, comprising the following steps: (a) neutralizing said phosphoric acid solution with gaseous ammonia up to a molar ratio N/P of between 0.1 and 0.8, (b) reacting said partially neutralized solution with a sulfide source so as to form a cadmium sulfide precipitate, (c) separating said precipitate so as to obtain a refined ammoniated phosphoric acid solution, (d) concentrating the refined ammoniated phosphoric acid solution, (e) ammoniating and granulating said concentrated refined ammoniated phosphoric acid solution so as to form said fertilizer, all steps (a) to (e) being carried out in an ammonium phosphate fertilizer manufacturing line.

2. The method of claim 1, wherein before the ammoniating-granulating step a complementary ammoniation of the refined solution is implemented so as to obtain a molar ratio N/P determined to obtain a NP or NPK type fertilizer.

3. The method of claim 1, wherein a molar ratio N/P during the step (b) of precipitation of cadmium sulfide is between 0.2 and 0.6.

4. The method of claim 1, wherein the sulfide source is used in excess with respect to the quantity of cadmium present in the phosphoric acid solution.

5. The method of claim 1, wherein the sulfide source introduced implements between 3 and 15 kg equivalent solid NaHS per ton of P.sub.2O.sub.5.

6. The method of claim 1, wherein the sulfide source is an aqueous NaHS solution or any other sulfide source having sufficient solubility in phosphoric acid to reach the stoichiometric ratio necessary for decadmiation.

7. The method of claim 1, wherein the step (b) of reaction with the sulfide source is carried out in a pressurized stirred reactor.

8. The method of claim 1, wherein the step (b) of reaction with the sulfide source is carried out in a pressurized tubular reactor.

9. The method of claim 1, wherein the step (a) of partial neutralization with ammonia and the step (b) of reaction with the sulfide source are implemented separately.

10. The method of claim 1, wherein the step (a) of partial neutralization with ammonia and the step (b) of reaction with the sulfide source are implemented simultaneously.

11. The method of claim 1, wherein a molar ratio N/P during the step (b) of precipitation of cadmium sulfide is between 0.3 and 0.5.

12. A method for manufacturing an ammonium phosphate fertilizer from a phosphoric acid aqueous solution that has from 25 to 45% P.sub.2O.sub.5 concentration and is obtained by wet phosphate ore treatment by sulfuric acid, said phosphoric acid containing traces of cadmium, comprising the following steps: (a) neutralizing said phosphoric acid solution with ammonia up to a molar ratio N/P of between 0.1 and 0.8, (b) reacting said partially neutralized solution with a sulfide source so as to form a cadmium sulfide precipitate, (c) separating said precipitate so as to obtain a refined ammoniated phosphoric acid solution, (d) ammoniating and granulating said refined ammoniated phosphoric acid solution so as to form said fertilizer, all steps (a) to (d) being carried out in an ammonium phosphate fertilizer manufacturing line.

13. The method of claim 12, wherein a molar ratio N/P during the step (b) of precipitation of cadmium sulfide is between 0.2 and 0.6.

14. The method of claim 12, wherein the sulfide source is used in excess with respect to the quantity of cadmium present in the phosphoric acid solution.

15. The method of claim 12, wherein the sulfide source introduced implements between 3 and 15 kg equivalent solid NaHS per ton of P.sub.2O.sub.5.

16. The method of claim 12, wherein the sulfide source is an aqueous NaHS solution or any other sulfide source having sufficient solubility in phosphoric acid to reach the stoichiometric ratio necessary for decadmiation.

17. The method of claim 12, wherein the step (b) of reaction with the sulfide source is carried out in a pressurized stirred reactor.

18. The method of claim 12, wherein the step (b) of reaction with the sulfide source is carried out in a pressurized tubular reactor.

19. The method of claim 12, wherein the step (a) of partial neutralization with ammonia and the step (b) of reaction with the sulfide source are implemented separately.

20. The method of claim 12, wherein the step (a) of partial neutralization with ammonia and the step (b) of reaction with the sulfide source are implemented simultaneously.

21. The method of claim 12, wherein the ammonium phosphate fertilizer is selected from the group consisting of MAP (Mono-Ammonium-Phosphate) and DAP (Di-Ammonium-Phosphate) fertilizers.

22. The method of claim 12, wherein a molar ratio N/P during the step (b) of precipitation of cadmium sulfide is between 0.3 and 0.5.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other characteristics and advantages of the invention will become clear from the detailed description that follows, with reference to the appended drawings, in which:

(2) FIG. 1 is a block diagram of the method for manufacturing fertilizer according to one embodiment of the invention,

(3) FIG. 2 is a block diagram of the method for manufacturing fertilizer according to another embodiment.

DETAILED DESCRIPTION OF THE INVENTION

(4) The decadmiation method implemented in the present invention is based on a precipitation of cadmium in the form of sulfides during the method for manufacturing ammonium phosphate fertilizers.

(5) Sulfides, like carbonates, oxides and hydroxides, are water insoluble salts. Like cadmium, other trace metal elements also give precipitates M.sub.nS.sub.m (M being the symbol of the metal in question, n and m represent the stoichiometric proportions of said metal and sulfur in the precipitate) of very low solubility. Precipitation is, however, greatly affected by the acidity of the medium.

(6) The method according to the invention is based on a readjustment of the steps of reaction between ammonia and phosphoric acid necessary for the manufacture of ammonium phosphate fertilizers, by integrating therein, when the reaction medium favors it, a step of precipitation of cadmium in the form of cadmium sulfide.

(7) The main steps of the method for manufacturing are: a first step of reaction between phosphoric acid and ammonia. The partial neutralization is conducted up to a molar ratio N/P so as to avoid the formation of ammonium phosphates in the operating conditions of the method. Depending on the P.sub.2O.sub.5 concentration of the phosphoric acid solution, which can reach up to 50% P.sub.2O.sub.5, a molar ratio N/P of between 0.1 and 0.8 may be used. adding an inorganic sulfide source, such as a NaHS solution for example, leading to the formation of a cadmium sulfide precipitate. separating said precipitate vis--vis the ammoniated acid, by a separating technique known per se, so as to obtain a refined ammoniated phosphoric acid solution. optionally, concentrating said refined solution, if need be, the implementation of a complementary ammoniation of the refined solution (optionally concentrated) to obtain the desired molar ratio N/P for the fertilizer, granulating the ammoniated acid so as to form the fertilizer.

(8) FIG. 1 illustrates an embodiment of said method.

(9) The starting product is a phosphoric acid aqueous solution 1, containing up to 50% P.sub.2O.sub.5, preferably between 20 and 50% P.sub.2O.sub.5, and in an even more preferred manner between 25 and 45% P.sub.2O.sub.5. Said solution has a cadmium content which can exceed 150 mg/kg P.sub.2O.sub.5.

(10) Said solution 1 is introduced into a gas-liquid reactor 2 into which gaseous ammonia 3 is also introduced. The reactor 2 may be of the stirred vessel or tubular reactor type. The nitrogen/phosphorous molar ratio of the ammoniated acid 4 formed in the reactor 2 is between 0.1 and 0.8, preferably between 0.2 and 0.6, and in an even more preferred manner, between 0.3 and 0.5.

(11) The ammoniated acid 4 is next made to react with a sulfide source 6 such as an aqueous NaHS solution or Na.sub.2S in a reactor 5 in order to obtain a solution 7 containing a cadmium sulfide precipitate. Generally speaking, the sulfide source may be any sulfide source that has sufficient solubility in phosphoric acid to reach the stoichiometric ratio necessary for decadmiation. The reactor 5 may be of the pressurized stirred vessel or tubular reactor type. Since they involve rapid reactions, the steps of pre-neutralization and precipitation may be carried out simultaneously, reactors 2 and 5 then being merged.

(12) In a particularly advantageous manner, the sulfide source is used in excess with respect to the quantity of cadmium present in the phosphoric acid, in order to ensure precipitation of practically all the cadmium present. Given the cadmium contents normally encountered in phosphoric acid, the quantity of solid NaHS is preferentially between 3 and 15 kg per ton of P.sub.2O.sub.5.

(13) The precipitate 9 is separated in a separation device 8 by a known separating technique, such as filtration, decantation or centrifugation.

(14) The refined solution 10 obtained at the end of the separation step next undergoes a complementary ammoniation in one or more steps, during granulation in a granulator 11 and/or before, with ammonia in the gaseous and/or compressed liquid state 13 in order to reach the desired molar ratio N/P for the fertilizer 12.

(15) The granulation in the granulator 11 may be carried out in the presence of different materials: recycled fertilizer (circulating load), sources of nutritional elements and trace elements, ballast, additives, etc. according to the methods in force.

(16) The granulated fertilizer 12 may be a binary (of type: MAP, DAP, etc.) or ternary (NPK) fertilizer if a potassium source is used during the manufacture, said source being added during granulation.

(17) FIG. 2 illustrates an alternative embodiment of the invention. Elements that are identical to those of FIG. 1 are designated by the same reference signs and are thus not described again.

(18) This alternative comprises, after the separation step implemented in the device 8, a concentration of the refined solution 10 by evaporation in an evaporation device 14.

(19) It is thus possible to implement the prior steps with low P.sub.2O.sub.5 concentrations, (that is to say typically less than 30%), which are more favorable to the precipitation of cadmium sulfides. The granulating and, optionally, the complementary ammoniating, may be implemented with the refined solution 10 and the concentrated solution 15.

(20) Whatever the embodiment implemented, the fertilizer obtained has a cadmium content less than 40 mg/kg P.sub.2O.sub.5.

Example 1

(21) The starting product is a phosphoric acid aqueous solution containing 42% P.sub.2O.sub.5 and containing 61 ppm of cadmium (i.e. 145.2 mg Cd/kg P.sub.2O.sub.5), manufactured by wet process by sulfuric attack of phosphate ore.

(22) An ammoniation of said solution is implemented up to a molar ratio N/P=0.4 in a first reactor.

(23) The ammoniated acid is next introduced continuously at a rate of 110 g/min with 1.53 g/min of a 30% by weight NaHS solution prepared from solid NaHS of 70% purity (containing around 30% of crystallization water), in a second reactor, of tubular type.

(24) The residence time in said reactor is 3 minutes and the reaction temperature is 80 C.

(25) The reaction product is filtered so as to remove the cadmium sulfide precipitate, then the solution thereby refined is ammoniated to reach a N/P ratio of 1.85 and granulated in DAP form.

(26) The fertilizer obtained is characterized by a cadmium content of 19.2 mg Cd/kg P.sub.2O.sub.5.

Example 2

(27) The objective of this second example is to show the effect of the P.sub.2O.sub.5 concentration of the starting phosphoric acid on the cadmium content reduction performances, by comparing the test of example 1 with two other tests carried out according to the same protocol as in example 1 but with different concentrations of phosphoric acid.

(28) Table 1 summarizes the test conditions and the performances obtained:

(29) TABLE-US-00001 TABLE 1 P.sub.2O.sub.5 content of the phosphoric acid (%) 29 38 42 Average cadmium content (ppm) in the 45.7 55.9 61.0 starting product (phosphoric acid) Cd/P.sub.2O.sub.5 ratio (mg/kg) in the starting 152.3 147.0 145.2 product Ammoniating molar ratio (N/P) 0.4 0.4 0.4 Residence time in the reactor of the pre- 3 3 3 ammoniated acid/NaHS mixture (min) Input flow rate of the ammoniated 95.95 102.52 109.39 phosphoric acid (g/min) Input flow rate by weight of the NaHS 0.87 1.30 1.53 solution (g/min) Cadmium content (mg Cd/kg P.sub.2O.sub.5) in the 6.4 128 19.2 final product (fertilizer) Cadmium removal yield (%) 95.8 91.3 86.8

(30) The results presented in table 1 show that, for a same N/P ratio at the end of the pre-neutralization step, the cadmium removal yield is better with reduced P.sub.2O.sub.5 concentration.

Example 3

(31) The aim of example 3 is to study the effect of the molar ratio N/P of the ammoniated acid during the precipitation of cadmium sulfides on the decadmiation performances of the fertilizer.

(32) To this end, tests carried out with a concentration of the starting phosphoric acid of 29% P.sub.2O.sub.5 and partial neutralization molar ratios N/P respectively of 0.2, 0.4 and 0.6 are compared.

(33) Table 2 summarizes the test conditions and the performances obtained:

(34) TABLE-US-00002 TABLE 2 P.sub.2O.sub.5 content of the phosphoric acid (%) 29 29 29 Average cadmium content (ppm) in the 45.7 45.7 45.7 starting product (phosphoric acid) Cd/P.sub.2O.sub.5 ratio (mg/kg) in the starting 152.3 152.3 152.3 product Ammoniating molar ratio (N/P) 0.2 0.4 0.6 Residence time in the reactor of the pre- 3 3 3 ammoniated acid/NaHS mixture (min) Input flow rate of the ammoniated 95.95 95.95 95.95 phosphoric acid (g/min) Input flow rate by weight of the NaHS 0.87 0.87 0.87 solution (g/min) Cadmium content (mg Cd/kg P.sub.2O.sub.5) in the 21.2 6.4 24 final product (fertilizer) Cadmium removal yield (%) 86.1 95.8 98.4

(35) The results presented in table 2 show that, for a same concentration of starting phosphoric acid, the cadmium removal yield is better with a high ammoniating molar ratio (N/P).

Example 4

(36) This example takes advantage of the good performances of the decadmiation by the method using a diluted phosphoric acid solution, here with a concentration of 27% P.sub.2O.sub.5, and implementing a later concentration, according to the synoptic of FIG. 2.

(37) Table 3 presents the test conditions and the performances obtained with two ammoniating molar ratios (respectively 0.2 and 0.4):

(38) TABLE-US-00003 TABLE 3 Ref. of the example 4.1 4.2 P.sub.2O.sub.5 content (%) of the phosphoric acid 27 Average cadmium content (ppm) in the 41.2 starting product (phosphoric acid) Cd/P.sub.2O.sub.5 ratio (mg/kg) in the starting 152 product Ammoniating molar ratio (N/P) 0.2 0.4 Residence time in the reactor of the pre- 3 3 ammoniated acid/NaHS mixture (min) Flow rate of ammoniated phosphoric acid 96 96 (g/min) Flow rate by weight of NaHS solution 0.87 0.87 (g/min) Average Cd content at the output (ppm) 5.1 <l.d. Cadmium content (mg Cd/kg P.sub.2O.sub.5) of the 18.9 refined solution before the concentration step

(39) As seen in example 3, the decadmiation performance is better with the highest molar ratio N/P.

(40) The two pre-ammoniated acids, treated by NaHS and refined, were then concentrated under a vacuum of 0.2 bar.

(41) Table 4 presents the test conditions and the performances obtained with the two refined solutions:

(42) TABLE-US-00004 TABLE 4 Ref. of the example 4.1 4.2 Pre-ammoniated P.sub.2O.sub.5(%) 51.70 48.80 phosphoric acid, Average Cd content 9.7 1.0 treated by NaHS (ppm) and concentrated Cadmium content (mg Cd/kg P.sub.2O.sub.5) in the 18.80 211 final product (fertilizer) Cadmium removal yield (%) 85.2 98.6

REFERENCES

(43) U.S. Pat. No. 4,986,970 U.S. Pat. No. 4,378,340