Description:FIELD OF THE INVENTION

This invention relates to the manufacturing process to produce water-soluble mono ammonium phosphate from weak phosphoric acid containing high metal impurities. The present invention relates to a method of mono ammonium phosphate and mixed fertilisers containing them, through the process of acid ammoniation and physical separation technique with recovery efficiency up to 85%.

BACKGROUND OF THE INVENTION

Phosphoric acid usually is not used directly as a fertilizer because it is corrosive, but can be used to manufacture other fertilizers.

Phosphorus is one of the three major nutrients that is critical for plant growth. It is supplied to plant in the form of fertilizers like SSP, TSP, MAP, DAP, NPK’s, most of these products requires concentrated phosphoric acid as raw material for processing, which is again produced from Rock phosphate. To produce phosphoric acid from the rock phosphate there are 2 major processes available, i) Wet process, ii) Thermal process. Wet process involves digestion of finely ground rock phosphate in sulphuric acid followed by separation of phosphoric acid from calcium sulphate hydrates/gypsum through filtration. Based on the hydrate form present in the calcium sulfate, wet process is further classified into Di-Hydrate process and Hemi-Hydrate Process. Whereas thermal process involves combustion of rock phosphate in the presence of Silica to produce elemental phosphorus in the form of gas followed by oxidation to P2O5 and hydration/absorption in to diluted H3PO4 or water. This process requires temperatures up to 2000 oC, so it is energy intensive and highly expensive as it requires lot of energy and hemi and hemi-dihydrate processes are implemented by very few in the world due to investment and process limitations. So, most of the phosphoric acid production will be done through Di-hydrate production route generating weak phosphoric acid from the filter outlet. Phosphoric acid produced through this process contains 24 to 28% P2O5 with soluble and insoluble impurities of gypsum and metal salts. This weak acid is treated further in clarification system to separate the impurities from the acid or processed as it is through evaporator section to concentrate the acid to about 50-54% P2O5 content.

This treated or untreated phosphoric acid with P2O5 concentration of 24-28% as weak acid or with P2O5 concentration of 50-54% as concentrated acid will be utilized to produce different grades of bulk fertilizers or water-soluble fertilizers. The quality of the acid used for the production, depending upon the extent of the suspended and dissolved impurities present in it decides the type and the quality of the fertilizer produced. Quality of the acid used is particularly very important to produce water soluble fertilizer grades as it dictates the plant economics and operational efficiency while meeting the product specifications.

The concentration of the impurities in the acid and its quality is highly dependent upon the composition and the type of rock phosphate used to produce the acid. With depletion of the rock phosphate reserves worldwide over a period, usage of rock phosphate sources with varied rock composition and varied level of impurities has become order of the day. Hence, in a di-hydrate manufacturing process, weak acid produced typically contains gypsum suspended solids in the range of 0.5% to 1.5% and dissolved metal solids up to 5%. This acid, if concentrated without any solids separation, will result in the concentration of the solids up to 10% and sometimes even up to 15%.

In the present context to explain, water soluble MONO-AMMONIUM PHOSPHATE is one of the widely used source for phosphorous nutrient. This water-soluble MONO-AMMONIUM PHOSPHATE can be directly used as fertilizer, or it can be used as a raw material to produce different other water-soluble NP and NPK grades. Water soluble MONO-AMMONIUM PHOSPHATE grade is produced using different methods using both treated and untreated phosphoric acid, either weak or in concentrated form as the base raw material. Treatment and purification of the acid to produce MONO-AMMONIUM PHOSPHATE is highly cost intensive and is adopted by very few worldwide. Usage of concentrated acid involves additional cost investment on evaporation with added operational cost. Hence, it is always advantageous, if weak acid can be utilized to produce water-soluble MONO-AMMONIUM PHOSPHATE.

Weak phosphoric acid majorly contains gypsum solids, fluoro silicates and metallic salts as impurities in it. Suspended solids can be removed by clarification method and dissolved metal impurities can be removed by chemical treatment followed by physical separation. Pre- treatment of the acid following above steps before using it to produce MONO-AMMONIUM PHOSPHATE is a costly process and hence there is a need to develop a process where in weak acid can be used directly without any pre-treatment step. The major challenge involved in the usage of weak acid directly without pre-treatment is that all the dissolved impurities in acid precipitates out into slurry in the form of a fine suspended colloidal matter during ammoniation reaction. Separation of these fine suspended colloidal impurities along with the other suspended solids already present in the acid by adopting different techniques require detailed study, as it poses challenges to produce water soluble grade, particularly when the dissolved impurities concentration in acid is beyond 3%.

PRIOR ARTS

US Patent No. 3894143 claims a process for production of crystalline MONO-AMMONIUM PHOSPHATE from wet process acid and ammonia, which involves mixing of acetone with phosphoric acid to form aqueous solution followed by partial neutralization with Ammonia and precipitating out the impurities to form pure mixture of the acetone and partially neutralized acid which is again neutralized with ammonia further to form MONO-AMMONIUM PHOSPHATE + acetone solution followed by separation of MONO-AMMONIUM PHOSPHATE.

US Patent No. 4269814 claims a process for preparing Mono ammonium phosphate, which consists of somany process steps i.e., soda ash addition to weak phosphoric acid to precipitate the fluoride compounds followed by vacuum filtration. Ammoniation, centrifugal separation of hydroxide precipitates, addition of sequestering agent for metals chelation followed by activated carbon treatment and filtration followed by crystallization of MONO-AMMONIUM PHOSPHATE. This approach highlights mainly i) addition of sequestering agents (Ethylenediamine tetra acetic acid and Diethylenetriamine pentaacetate) to MONO-AMMONIUM PHOSPHATE solution, which reacts/chelates metallic ions to form water soluble compounds. ii) heat-treating the aqueous solution at a temperature of 70° C. or more. iii) crystallizing MONO-AMMONIUM PHOSPHATE out of the aqueous solution. This process is highly cost intensive as it involves so many operations and dosage of sequestering agent at 0.2% to 2% is economically not viable and end up at higher production costs.

US Patent No. 4325927 claims a process of method of producing MONO-AMMONIUM PHOSPHATE from concentrated Phosphoric acid (54% P2O5). Initially 54% P2O5 Phosphoric acid diluted with water to a concentration of about 26% P205. Followed by controlled ammoniation to pH of between about 1.5 to 2.5 over a period of between about 60 and 75 minutes. And then aging the slurry to promote crystal growth for between about 10 and 15 minutes. Again, ammoniation to a pH of 4.0 to 5.0 over a period of 45 minutes to 1 hour and filtration of insoluble matter followed by crystallization of MONO-AMMONIUM PHOSPHATE.

European Patent No. EP 0161 704 A2 claims a process for producing water soluble MONO-AMMONIUM PHOSPHATE. This process describes that Wet process acid of 30% to 54% P2O5 acid neutralized with ammonia in presence of fossil meal to a PH range of 4.0 to 4.5 and slurry temperature at 80 to 95 oC and with residence time of 10 to 30 minutes, followed high temperature filtration where sludge is removed, and the clear liquor treated in crystallizer followed by centrifugation. Then MONO-AMMONIUM PHOSPHATE crystals dried in in a dryer. Whereas mother liquor is partially drained and cooled and then recycle back to the reactor. In this process P2O5 lost through drain and P2O5 yield efficiency is 70% which is low. From the pilot trials data, further dropped.

US Patent No.4610853A claims a process for producing purified monoammonium phosphate directly from wet process phosphoric acid with a high yield. The process comprises reacting wet process phosphoric acid with NH3 at 50-80 C. at a N/P molar ratio of 0.9-1.35. Separating the resulting slurry containing MONO-AMMONIUM PHOSPHATE crystals and an insoluble sludge by means of a thickener into a slurry containing the portion of MONO-AMMONIUM PHOSPHATE crystals and a slurry containing the portion of insoluble. Former slurry treated in centrifugation to recover MONO-AMMONIUM PHOSPHATE; while treating the latter slurry by heating it to 60°-95 C. to dissolve MONO-AMMONIUM PHOSPHATE contained therein, removing the sludge by centrifugal separation, and concentrating the filtrate to recover MONO-AMMONIUM PHOSPHATE; and combining the two portions of MONO-AMMONIUM PHOSPHATE. This process claims up to 80% of P2O5 recovery but didn’t discuss about the purity of MONO-AMMONIUM PHOSPHATE produced.

Chinese Patent CN 105293459B titled “The method of the complete water-soluble MONO-AMMONIUM PHOSPHATE of Wet-process Phosphoric Acid Production and coproduction ammonium magnesium phosphate” claims a process for production of both MONO-AMMONIUM PHOSPHATE and ammonium magnesium phosphate”. It describes addition of sodium salt to phosphoric acid and react for 1 to 2 hr and conduct defluorination followed by filtration. Defluorinated phosphoric acid ammoniated with gas ammonia to pH be 2.5] followed by aging the liquor for 20 minutes. Aged liquor again ammoniated to pH be 4.5 then through gravitational settling separated supernatant liquor and sludge. Supernatant liquor concentrated followed by crystallisation by cooling and filtered the final MONO-AMMONIUM PHOSPHATE.

Sludge again ammoniated to pH 5.8 again aged for about 20 min and dried the slag to obtain ammonium magnesium phosphate. In process product MONO-AMMONIUM PHOSPHATE purity is at 42%-45% which is extremely low.

As detailed above, several processes are known for production of ammonium phosphate, but all such methods eithers require complex processes for the treatment and purification. The present invention tries to overcome the sufferings of prior art and the present invention relates to novel process for production of water-soluble mono-ammonium phosphate by using weak phosphoric acid.

It was considered that phosphoric acid of concentration higher than 28% involve an extremely hardly separable precipitate. It is also known that phosphoric acids from wet processes, contain large amounts of anionic and cationic impurities, and that the product from the reaction of such a type of acids with ammonia is constituted by a mud-like suspension of meal phosphates in a lattice of ammonium phosphates. This invention it is surprisingly identified that it is possible to produce water soluble ammonium phosphate by using weak phosphoric acid and reacting it with ammonia under determined process conditions, without having the sufferings/drawbacks hereinabove suffered in mentioned prior art.

OBJECTIVE OF THE INVENTION

A general object of the invention is to produce a fully soluble, pure and well-defined mono ammonium phosphate avoiding problems with the prior art.

• Primary objective of the invention is to develop a manufacturing process for water-soluble mono-ammonium phosphate fertilizer from weak phosphoric acid (24%-28% P2O5) with high content of impurities of both suspended and soluble nature by adopting only physical separation techniques and without adopting any chemical treatment methods which are cost intensive.
• Other objective of the invention is to produce water-soluble mono-ammonium phosphate product with 11% to 14% ammonia nitrogen as mono-ammonium phosphate (MAP) is a widely used source of Phosphorous (P) and Nitrogen (N).
• Other objective of the invention is to produce water-soluble mono-ammonium phosphate product with P2O5 in the range of 52% to 58%.
• Other objective of the invention is to produce a product which is 100% soluble in water. Mono-ammonium phosphate is an important granular fertilizer. A water soluble and dissolves rapidly in adequately moist soil and upon dissolution two basic component of the fertilizer separate again to release ammonium and phosphate both of which plants rely for growth.
• The above objects and methods and devices are achieved according to the invention. One major advantage with the present invention is that well defined fully soluble pure and well-defined mono-ammonium phosphate are possible to produce in an industrially applicable process in an efficient and ecommerce manner.

Scheme for manufacturing of water-soluble MAP from weak phosphoric acid block flow diagram of the invention is illustrated in fig. 1.

Legend used in the invention is given in below :

1 Neutralizer
2 Flocculant preparation tank
3 Dilution tank 1
4 Dilution tank 2
5 Filter Press
6 Intermediate feed tank
7 Pressure leaf filter
8 Spray drier feed tank
9 Spray drier
10 Wet Scrubber
11 Scrubber recirculation tank
12 Pre-Coat tank

DESCRIPTION OF THE INVENTION

Almost all water-soluble phosphoric salts such as soluble fertilizer are derived from phosphoric acid. Phosphoric acid is produced either by wet and thermal process phosphoric acid for fertilizer production specifically is solely based on wet-digestion of rock phosphate. The neutralization of phosphoric acid with ammonia is done in a vessel, wherein the mole ratio of ammonia to phosphoric acid in the reactor needs to be at a level which gives the max solubility for the slurry and for mono-ammonium phosphate it is preferred to be less than 1. For operation control, the ammonia to phosphoric acid mole ratio is determined by monitoring the pH of slurry.

For application of fertilizer, there is a need for fully soluble ammonium phosphates. But typically, wet process phosphoric acid contains a substantial amount of impurities such as iron, aluminum, calcium, magnesium, cadmium, etc. which form water-insoluble solids upon neutralization with ammonia and hence fertilizer grade ammonium phosphate typically are not completely water soluble.

Weak phosphoric acid (24-28% P2O5) manufactured through wet phosphoric acid process containing gypsum suspended solids and dissolved solids in the range of 0.5-1.5% and 3- 5% respectively is taken directly into the continuous stirred tank reactor-Neutralizer (1) and is partially neutralized with anhydrous ammonia to a mole ratio of 0.9 to 1.2 and simultaneously water is added to the reactor for diluting the slurry with water at a ratio of 1.0:1.0. Then this diluted mono-ammonium phosphate product slurry then pumped to dilution tank (3) to further dilute the mono-ammonium phosphate slurry to mono-ammonium phosphate:water ratio of 1:4.0 by adding water. During this process of weak phosphoric acid ammoniation followed by dilution with water, all the mono-ammonium phosphate formed is soluble in water and stay in soluble form and all the metal impurities will form water insoluble complexes and precipitate out as fine particles. Simultaneously 0.1% flocculant solution is be prepared in flocculation tank (2) and is pumped to tank (3)/ (4) at desired flow rate and mixed thoroughly. When this flocculant solution gets mixed with slurry, it will increase the affinity b/w insoluble particles and bring together all the insoluble metal complex particles to form flocs and slowly this floc size increases to a level which can be separated through filtration. Meanwhile pre-coat liquid from pre coat tank (2) is re-circulated to the filter press (5) and make sure the pre-coat formed evenly on the filter surface. Once clear formation of flocs in the dilution tank is ensured, diluted slurry from tank (3) is pumped to Filter press (5), initially filter press discharge filtrate is recycled back to the dilution tank till clear filtrate is achieved. Once clear filtrate is achieved at Filter press (5) discharge, then it is directed to steam jacketed intermediate feed tank (6). Activated carbon is added to the dilution tank (6) in a batch mode as per the requirement and tank will be under agitation. Parallelly Pressure leaf filter (7) will be taken in line and is coated with pre-coat. Then diluted slurry from intermediate tank (6) is fed to Pressure leaf filter (7). Until clear filtrate is observed, diluted slurry will be recycled back to the feed tank (6). Clear filtrate is collected in spray drier feed tank (8). Filtrate liquid from spray drier feed tank (8) is sprayed into the spray dryer (9). Powder Product is collected at the spray dryer cyclones and analyzed for grade and turbidity (NTU-nephelometric turbidity unit). Flue gasses passes through the scrubbing system (10). Where escaped fine mono-ammonium phosphate dust will be collected in scrubber tank (12) and recycled back to the neutralization reactor.

Through the above process of acid ammoniation and physical separation techniques, we achieved P2O5 recovery efficiency up to 85% with phosphoric acids containing high impurities, cost effective and easily scalable.

Examples:

Analysis of the phosphoric acid (Produced in coromandel international limited Phosphoric acid plant -II.

% P2O5 27.81
Sp.gr. 1.31
% Solids 0.3
% H2SO4 1.45
% F 1.5
Cl 646
% CaO 0.92
% MgO 0.38
% Fe2O3 0.68
% SiO2 0.87
% Al2O3 0.37

Five examples will be given to the purpose of better illustrating the invention, such examples however being not to be considered as being limitative of the invention.

Example-1:

1500 g of 27% P2O5 phosphoric acid was taken and neutralized with 100 g of anhydrous ammonia to mole ratio of 0.942 and then diluted with 1500 g of DM water. Above slurry was mixed thoroughly in 5000 ml beaker with lab scale agitator for about 15 minutes. To this slurry 100 ml of 0.1% flocculant solution was added. Slurry kept under mild agitation for a while to initiate the flocculation, the obtained slurry was filtered. Filtrate is again treated with 2.5 g of activated carbon and heated to 800C, this liquid is filtered in lab scale filter followed by spray drying and analyzed. 552.2 g of water-soluble mono-ammonium phosphate was achieved with P2O5 at 55%. which reflects to 75% of P2O5 recovery in yield.

Analysis WS MAP
%AN 11.3
%TN 11.3
%WS P2O5 55.10
%TP2O5 55.26
%Moisture 1.88
MR 0.953
Dry Sludge
%AN 9.21
%TN 9.21
%WS P2O5 34.69
%TP2O5 (regular method) 49.4
%Moisture 1.09

Example-2:

Slurry prepared as in example-1 one was diluted with water at MAP: water ratio of 3.5:1 and prepared 577 g of WS MAP as above with P2O5 recovery at 78%.

Analysis MAP
%AN 11.27
%TN 11.27
%WS P2O5 55.0
%TP2O5 55.39
%Moisture 1.75
MR 0.945
Dry Sludge
%AN 8.76
%TN 8.76
%WS P2O5 32.76
%TP2O5 (regular method) 47.3
%Moisture 1.89

Example-3:

1500 g of 27% P2O5 phosphoric acid was taken and neutralized with 100 g of anhydrous ammonia to mole ratio of 0.935 and then diluted the above slurry with 2250 g of DM water, the slurry was mixed thoroughly in 5000 ml beaker with lab scale agitator for about 15 minutes. To this slurry 200 ml of 0.1% flocculant solution was added. Slurry kept under mild agitation for a while to initiate the flocculation. Then slurry was filtered. Filtrate was again treated with 3 g of activated carbon and heated to 80 oC. this filtrated was filtered in lab scale filter and clear filtrate is achieved. This clear filtrate was spray dried in a lab scale spray drier and analyzed for N & P2O5 in the product. 620 g of WS MAP product with P2O5 at 55.5% was achieved, which reflects to 85% of P2O5 recovery in yield.

Analysis WS MAP
%AN 11.32
%TN 11.32
%WS P2O5 55.21
%TP2O5 55.52
%Moisture 1.4
MR 0.942
Dry Sludge
%AN 8.76
%TN 8.76
%WS P2O5 34.57
%TP2O5 (regular method) 46.3
%Moisture 2.89

Example -4:

Phosphoric Acid Analysis: With high impurities

% P2O5 26.41
Sp.gr. 1.33
% Solids 2.40
% H2SO4 3.10
% F 0.76
Cl 35.50
% CaO 0.93
% MgO 1.06
% Fe2O3 1.26
% SiO2 0.60
% Al2O3 0.88

2000 g of 26.41% P2O5 phosphoric acid with high solid percentage was taken and neutralized with 115.20 g of anhydrous ammonia to mole ratio of 0.96 and then diluted the above slurry with 2000 g of DM water. Above slurry was mixed thoroughly in 5000 ml beaker with lab scale agitator for about 15 minutes. To this slurry 200 ml of 0.1% flocculant solution was added. Slurry kept under mild agitation for a while to initiate the flocculation. Then slurry was filtered. Filtrate was again treated with 3 g of activated carbon and heated to 80 oC. this filtrated was filtered in lab scale filter and clear filtrate is achieved. This clear filtrate was spray died in a lab scale spray drier and analyzed for N & P2O5 in the product. 728 g of WS MAP product with P2O5 at 56.9% was achieved, which reflects to 78% of P2O5 recovery in yield.

Example -5:

Phosphoric Acid Analysis: With high impurities

% P2O5 26.55
Sp.gr. 1.32
% Solids 1.00
% H2SO4 3.10
% F 0.76
Cl 35.50
% CaO 0.80
% MgO 1.76
% Fe2O3 1.37
% SiO2 0.60
% Al2O3 1.00

2000 g of 26.55% P2O5 phosphoric acid with high metal impurities percentage (6.0%) was taken and neutralized with 115 g of anhydrous ammonia to mole ratio of 0.95 and then diluted the above slurry with 2000 g of DM water. Above slurry was mixed thoroughly in 5000 ml beaker with lab scale agitator for about 15 minutes. To this slurry 150 ml of 0.1% flocculant solution was added. Slurry kept under mild agitation for a while to initiate the flocculation. Then slurry was filtered. Filtrate was again treated with 3 g of activated carbon and heated to 80 oC. this filtrated was filtered in lab scale filter and clear filtrate was achieved. This clear filtrate was spray died in a lab scale spray drier and analyzed for N & P2O5 in the product. 709 g of WS MAP product with AN @ 13.66% and WS P2O5 at 54.54% was achieved, P2O5 recovery was 73%.

In one aspect the invention relates to a process for producing purified water-soluble mono ammonium phosphate from wet process phosphoric acid (24%-28% P2O5) containing high impurities ranging from 3% to 15% comprising of ammoniation of weak acid to mole ratio 0.9 to 1.2, flocculation followed by pressure filtration of dilute ammoniated slurry. This is followed by hot Filtrate treating with activated carbon followed by filtration, and finally spray drying of the clean filtrate. The process is novel for the reason that no chemical treatment steps of the process and wherein mono-ammonium phosphate formed will be in soluble in water, and impurities are precipitated out, wherein in step (b) impurities further form flocs, and wherein flocs are filtered out, and wherein the clean filtration is sprayed into spray dryer.

In another aspect the invention relates to the concentration of said wet process phosphoric acid is maintained at about 24% -28%.

In another aspect the invention relates to phosphoric acid is ammoniated to obtain N/P2O5 mole ratio of 0.9 to 1.2, more preferably MR less than 1.0. and reaction temperature at 80 to 95 oC.

In another aspect the invention relates to Ammoniated liquor is diluted to 3:1 to 5:1 ratio of water to slurry, preferably at ratio of 4:1.

In another aspect the invention relates to 0.1% flocculant solution is mixed with diluted solution and kept under agitation for about 10 to 60 minutes, preferably 30 minutes.

In another aspect the invention relates to flocculated slurry will be filtered in hydro press.

In another aspect the invention relates to filtered slurry will be treated with activated carbon at a temperature of 50 to 70 oC, preferably 55 -65 oC.

In another aspect the invention relates to clear filtrate is spray dried.

In another aspect the invention relates to product contains N % from 10% to 14% and P2O5% from 45% to 62%.

It must also be understand that many changes and modification may be made in the devices/apparatus/equipments and the method within the scope of present invention. It is particularly to be understood that the details of the specific embodiment and example which have been employed to describe the invention are not intended to be limitative upon the scope of the invention, except in so far as included in the terms of the accompanying claims. All variations and modifications are within the scope of invention.
, Claims:WE CLAIM :

1. A process for producing purified water-soluble mono ammonium phosphate from wet process phosphoric acid (24%-28% P2O5) containing high impurities ranging from 3% to 15% comprising:
a) ammoniation of weak acid to mole ratio 0.9 to 1.2,
b) flocculation followed by pressure filtration of dilute ammoniated slurry,
c) hot Filtrate treating with activated carbon followed by filtration, and
d) spray drying of the clean filtrate.

the process characterised is that no chemical treatment is used in steps (b) to (d) wherein in step (a) mono-ammonium phosphate formed is soluble in water, and impurities are precipitated out, wherein in step (b) impurities further form flocs, wherein in step (c) flocs are filtered out, and wherein in step (d) clean filtration is sprayed into spray dryer.

2. The process as claimed in claim 1, wherein the concentration of said wet process phosphoric acid is about 24% -28%.

3. The process as claimed in claim 1, phosphoric acid is ammoniated to N/P2O5 mole ratio of 0.9 to 1.2, more preferably MR less than 1.0. and reaction temperature at 80 to 95 oC.

4. The process as claimed in claim 1, ammoniated liquor is diluted to 4:1 ratio of water to slurry.

5. The process as claimed in claim 1, wherein the 0.1% flocculant solution is mixed with diluted solution and kept under agitation for about 30 minutes.

6. The process as claimed in claim 1, wherein flocculated slurry will be filtered in hydro press.

7. The process as claimed in claim 1, filtered slurry will be treated with activated carbon at a temperature of 55 -65 oC.

8. The process as claimed in claim 1, clear filtrate is spray dried.

9. The process as claimed in claim 1, product contains N % from 10% to 14%.

10. The process as claimed in claim 1, product contains P2O5% from 45% to 62%.