Description:FIELD OF INVENTION

This invention relates to the manufacturing process for granular single super phosphate fortified with nitrogen and to produce (5-15-0-10), (6-15-0-10) and (6-14-0-10) by fortifying Single Super Phosphate with nitrogen fertilizers.

BACKGROUND OF THE INVENTION

Application of inorganic fertilizers and/or amendments is becoming essential to increase the crop yields and to support the ever-increasing nutrient demands of growing population. Off all the applied inorganic fertilizers in India. Single Super phosphate is one among the fertilizer with growing demand in India.

Single super phosphate is the cheapest source of phosphorus, and it was the first inorganic fertilizer commercially produced. It contains 3 of the most important and critical nutrients for plant activity (i.e., Phosphorus, Sulphur, and Calcium) additionally it also contains minor quantities of micronutrients like Fe, Mg, Si etc. Production of Single Super Phosphate is easy and highly economical and does not result in generation of by-products like Phosphogypsum, like other P-Fertilizers. It can be produced from small scale to commercial scale with little difficulty.

In India SSP is the preferred Phosphorous fertilizer source for oilseeds and pulses, these crops require additionally sulphur, and sulphur requirement can be met by SSP as it contains 11% of sulphur.

Nowadays Indian soils are becoming deficient in sulphur nutrient, as most of the focus is given to fertilizers nutrients N, P and K. Very little amount of sulphur is applied, as a result sulphur is coming down in Indian soils. So, to address the sulphur deficiency in Indian soils SSP can be a better alternative.

Oils and pulses also require little amounts of nitrogen, as of now it is supplied through urea or AS, which increase the number of applications there by increased cost burden on the farmers. So, if we can add urea along with SSP to supply Nitrogen also, it will minimize the no of fertilizer applications and cost burden on the farmers. In India preferred source of Nitrogen is urea and interestingly Nitrogen fortified Single Super Phosphate can be a more effective fertilizer than DAP as it will have additional nutrients such as calcium and sulphur and therefore less than 3 bags may be enough to replace 1 bag of DAP. Therefore, Nitrogen fortified Single Super Phosphate can be more economical for the farmers and benefit the country by promoting self-reliance in fertilizers.

So, to minimize the financial burden on farmers by reducing the number of applications, dosage, labour & transportation cost and to substantially improve the Nitrogen & Phosphorus nutrient use efficiency and more importantly addressing the sulphur deficiency in Indian soils, there is a potential opportunity for supplying N and P together by fortifying SSP with Nitrogen fertilizers, preferably urea.

It clearly indicates that, there is need for a process for achieving urea addition to SSP in Indian SSP plants. We conceived an approach “to produce most affordable N&P fertilizer, by incorporating abundantly available low cost and economic source of Nitrogen in Single super phosphate.

To produce N-P fertilizers from SSP in a most economical way, there are three possible approaches that can be utilized using single super phosphate as the basic source for P nutrient:
Approach-1:
Ammoniation of SSP to produce ammoniated SP grades 6-15-0, 4-17-0.

In this approach, during the single super phosphate granulation, ammonia is introduced into the SSP bed inside the granulator.

Advantage of this approach is that during the ammoniation of SSP lot of reaction heat is released. And that heat will reduce the fuel consumption during drying by evaporating most of the water and reduces the drying cost. Major disadvantages of this process are, it requires handling of ammonia in SSP granulation plants where addressing safety hazards related to ammonia handling and storage is a challenge and calls for additional investment cost. Most of the Indian SSP plants are very basic in design. Ammoniation also leads to the P2O5 conversion from Water Soluble form to Citrate Soluble form, which is not favourable condition. Ammoniation of SSP is widely carried out in Brazil as it produces a low N grade which is highly suitable for soybean.

Approach-2:
Steam granulation of single super phosphate with ammonium sulphate (AS) or monoammonium phosphate (MAP) for producing grades 6-12-0, 10-10-10, 15-15-15.

Steam granulation of single super phosphate with AS or MAP is carried out in Brazil to produce NPK grades. Highest NPK grade with AS is 10-10-10 and with MAP 15-15-15. In India, most of the SSP plants depends on water granulation.
The major disadvantage with this approach is the limited availability of AS and MAP in India.

Approach-3:
Reaction of Single super phosphate with urea to produce Urea Super Phosphate 20-10-0.

Basically, urea and superphosphates are not compatible, when they are mixed, the mixture becomes wet due to release of water, can give rise to problems during granulation. which makes further processing difficult.

The present invention is to produce fertilizer grades NPS 5-15-0-10 & 6-15-0-10 from Single Super Phosphate and urea.

There are few processes developed to produce Urea based superphosphates.

US5409516 (AZF GRAND PAROISSE) discloses a process for the manufacture of Urea superphosphates. AZF claims a 2- stage process to produce urea superphosphate, in first stage, sulphuric acid-urea reagent will be prepared by reacting urea with Sulphuric acid at 3.6:1 or 1.3:1 mole ratio, this reaction is highly exothermic and forms complex compounds [CO(NH2)2.H2SO4, (CO(NH2)2)2.H2SO4], and it requires a controlled cooling of the reagent. In the second stage, reagent mixes with finely ground rock/tri calcium phosphate in a paddle mixer to produce the Urea Super phosphate.

Advantage of AZF process is that it offers USP with highest N (20%). And major disadvantage of this process is reagent preparation, a highly exothermic reaction, if not controlled properly leads to safety concerns and additionally hydrolysis of urea followed by reaction with H2SO4 & H3PO4 leads to the formation of amide compounds and particularly biuret, which will deteriorate the quality of fertilizer and harmful to plants. This process is not suitable to the Indian SSP plants considering the economics of the expensive process and safety concerns.
WO2001042172A1 (Rotem Amfert) discloses a process, which is an improvised version of AZF Grand Paroisse process. In this process reaction of ground urea, sulphuric acid and rock phosphate will takes place in single stage in the presence of the gypsum, Phosphoric acid can also be added to the reactor if required. This process requires ground urea, handling of ground urea at commercial scale is highly difficult as it absorbs moisture quickly. In this process also there is a possibility for the formation of amide compounds that are harmful to the plants and reduction of water soluble P2O5 as urea is introduced through reaction route along with rock phosphate and sulfuric acid.

EP2774907 A2 (Zaklady Poland) discloses a process also claims the similar single stage process of preparing urea and sulphuric acid reagent in reaction along with Rock decomposition by adding sulphuric acid more than stoichiometric ratio and preparing process pulp by adding water to reactor in the range of 16-25%. This pulp will be directly fed to the granulator for further processing. In this complex approach, there is possibility for the formation of amide compounds and partial decomposition of rock phosphate, and it results in reduction of water soluble P2O5 in the product, which is again not favourable.

Product developed from above approach i.e., NP 20-10-0 contains N to P2O5 ratio of 2:1. whereas in India particularly crops like Oils and Pulses requires low Nitrogen along with moderate to high P2O5 fertilizers i.e., N: P2O5 ratio of 1:2.5-3.0.

US3936501 discloses the process for production of Urea – Phosphate fertilizer by reaction of solid urea with ortho-phosphoric acid in a substantially anhydrous form at constant feed of molar (1:1) ratios of ortho-phosphoric acid and the solid urea to an apparatus in which the reactants are mixed and at the same time conveyed onward e.g., by means of a pugmill.

All the above-mentioned processes are in one way or other involves reaction of sulphuric acid and Urea, which is highly exothermic reaction, required very precise control and cooling to control the heat generated, otherwise results in safety hazards, and importantly forms so many unwanted side reactions and end up in de graded quality of the product. And other processes involve production urea phosphate which is highly hygroscopic and difficult for further processing. Most of the processes claim urea calcium phosphate product with N up to 20%, but the total available phosphorus is below 10% P2O5, against 16% P2O5 minimum in the regular SSP products. All the above processes having their own limitations with respect to specific nutrient requirements of Indian soils, moisture control, additional cost, formation of hazardous amide compounds, handling of ammonia, constraints in the availability of MAP and AS at economical price.

OBJECTIVES OF THE INVENTION

The present process of SSP fortified with Nitrogen gives Products with P2O5 content up to 15% wt./wt. and Nitrogen up to 6.0% and sulphur and calcium up to 10.0% and 15-20% wt./wt. respectively and process is very safe and easily implementable, and it doesn’t involve urea-Sulphuric acid reactions and eliminates the formation of hazardous by-products.

So, the present invention addresses the above problems in an economically feasible way and minimizes the import burden of DAP to India to the possible extent.

• Primary objective of the invention is to add value to SSP fertilizer by fortifying with urea.
• Another objective is to produce a nitrogen-phosphorus-sulfur fertilizer with Single Super Phosphate as a base source.
• Another objective of the process is to develop an easily implementable process by eliminating process challenges associated with SSP-urea in compatibility.
• Another objective of the invention is that product contains available phosphorous (in the form of P2O5) from 12% to 16%, of which minimum 80% will be in the water-soluble form.
• Other objective of this invention is to produce a product which contains 4% to 6% of Nitrogen in the form of urea and 0% to 1% of nitrogen in the form of ammonical form.
• One more objective of the invention is the product contains Sulphur (in the form of sulphate sulfur) from 9% to 12% wt./wt.
• One more objective of the invention is that product contains Calcium in the form of (MCP, DCP, CaSO4.2H2O) from 15% to 20% wt./wt.
• One more objective of the product is that it contains free moisture in the range of 0.5% to 5%.
• One more objective of the invention is to produce a product with good granule integrity, thereby ensuring easy of handling & storage on par with regular NP counterparts.
• Another objective of the process is ease of scalability and ease of operation by ensuring process safety.

DESCRIPTION OF THE INVENTION

Direct mixing of Single Super Phosphate with urea results in reaction of urea with components of Single Super Phosphate and releases moisture, which makes the product wet and difficult to process further. To address above issues, we have developed a process in which, cured powder single super phosphate is granulated using hot urea slurry prepared by mixing urea with phosphogypsum that can prevent incompatible reactions during granulation process.

The present invention involves development of a 2-step procedure, i.e. to prepare hot urea slurry mixed with phosphogypsum in the first step and use it as a granulation medium in the second step to granulate powder single super phosphate. Phosphogypsum is calcium sulphate hydrate formed as by-product of fertilizer from phosphate rock. Phosphogypsum is a waste obtained in process of phosphoric acid production. It consists of calcium sulfate with different hydration ratios and traces of sulfuric, phosphoric and fluorosilicic acids as well as their salts and rare earth elements and radioactive elements. It is a good soil conditioner.

The process is as follows:

Stage-1: Preparation of Urea-phosphogypsum slurry
Stage-2: Production of NPS (5-15-0-10), (6-15-0-10) and (6-14-0-10) grade fertilizers by granulating the powder SSP with Urea-phosphogypsum slurry.

Preparation of Urea – Phosphogypsum Slurry:

As per desired formulation, raw materials i.e., Prilled urea and phosphogypsum are fed through weigh feeders to the steam jacketed reactor with agitator. Required quantity of water is added to maintain slurry consistency with a specific gravity in the range of 1.2 to 1.6 and to make sure that there is no accumulation of gypsum at the bottom of the reactor.

Steam to be supplied to the jacket to maintain slurry temperature at 60-90 oC. Periodically Slurry temperature, UN% in slurry, and specific gravity to be noted. For better mixing of urea & gypsum, slurry temperature to maintained at 80oC and specific gravity in between 1.3-1.4 (refer fig. 1).
Fig. 1 Urea-Phospogypsum Slurry preparation scheme

Part no. Part names
11 Phospogypsum
12 Prilled Urea
13 Process water
14 Steam in
15 Reactor
16 Steam out
17 Urea-gypsum slurry to GSSP Granulator

Production of NPS 5-15-0-10, 6-15-0-10 and 6-14-0-10 Fertilizers:

The production method is illustrated in fig. 2.

Part no. Part names
201 Phospogypsum
202 Prilled Urea
203 Process water
204 Steam in
205 Reactor
206 Steam out
207 Powder SSP
208 Rotary Granulator
209 HAG
210 Atmospheric Air
211 Hot Air
212 Rotary Drier
213 Hot Air out
214 Rotary Cooler
215 Primary Screens
216 Secondary Screens
217 Over size
218 Fines
219 Pulverizer
220 Product

Single super phosphate produced in Powder SSP Plant is kept for curing without adding any additional filler, this cured SSP Powder is fed to the feeding hopper using a payloader. From Feed hopper, powder SSP is conveyed to granulator drum. To the granulator drum, hot urea -Phosphogypsum slurry prepared in the previous step is pumped and sprayed on to this powder SSP material. Due to slurry moistening and rolling action of the material in the rotary drum, granule formation will take place. Additionally, to control and optimize the granulation moisture, water/steam can be added.

Granulator discharge wet granular material is fed into the rotary drier drum. Hot air from Coal furnace is fed to the dryer drum in co-current to the material flow to aid the drying of the moist material from the granulator. Dryer discharge material temperature to be maintained below 75 Deg.C to avoid the conversion of urea to other nitrogen forms. Dryer discharge material is then fed to cooler drum for cooling of the hot granules before further processing.

After cooling, cooler discharge material then goes through two screening sections to remove the undersize and oversize material. While the undersize material is directly going to the recycle belt conveyor, the oversize material will go through an oversize pulverizer to crush the oversize material so that only undersize material is recycled. The recycled material is again fed to granulator. The total recycle to product ratio is in the range of 0.5-1.5:1.0

Fig. 2 Scheme for production of granular Nitrogen Phosphate Sulphate fertilizers.

The developed process is more advantageous as we are using cured single super phosphate and granulating it with hot slurry prepared by mixing urea and phospho gypsum, thereby eliminating all unwanted reactions and incompatibility issues associated with other processes mentioned in the prior arts. The developed process also ensures ease of operation achieving good product characteristics and process safety as it doesn’t involve addition of urea during sulfuric acid and rock phosphate reaction or urea-sulphate reagent preparation or direct addition of urea to single super phosphate.

Pilot trials:

Raw materials:

Single Super Phosphate: SSP produced at Coromandel SSP Plant, Udaipur, Rajasthan. with 16.98% available P2O5 and 11.4% free moisture.

Phosphogypsum: Gypsum with 94% purity and 10% free moisture collected from Coromandel international limited, Visakhapatnam, Andhra Pradesh

Urea: Prilled Urea with 46% N and moisture less than 0.5%

Example: 1

• 15 Kg of Urea prills were taken in steam jacketed bench scale reactor and dissolved in 4.5 liters of water.
• To the above solution 10.5 Kg of wet Phosphogypsum (including 10% moisture) is added and mixed thoroughly.
• Heated the above mixture to 80 oC and kept under agitation for about 30 minutes.
• Above slurry is taken and sprayed on the bed of SSP powder in a bench scale rotary drum granulator and prepared granular product.
• Wet granules were dried in tray drier to below 3.0% moisture.
• Dried product samples were analyzed for physical properties and sent to QC-Lab for nutrient analysis.
• Same set of trials were conducted again granulated in pan granulator and analyzed.

Example: 2

• 20 Kg of Urea prills were taken in steam jacketed bench scale reactor and dissolved in 2.2 liters of water.
• To the above solution 8.5 Kg of wet Phosphogypsum (including 10% moisture) is added and mixed thoroughly.
• Heated the above mixture to 85-90 oC and kept under agitation for about 20 minutes.
• Above slurry is taken and sprayed on the bed of SSP powder in a bench scale rotary drum granulator and prepared granular product.
• Wet granules were dried in tray drier to below 3.0% moisture.
• Dried product samples were analyzed for physical properties and sent to QC-Lab for nutrient analysis.

Example: 3

• 20 Kg of Urea prills were taken in steam jacketed bench scale reactor and dissolved in 5 liters of water.
• To the above solution 13.86 Kg of wet Phosphogypsum (including 10% moisture) is added and mixed thoroughly.
• Heated the above mixture to 80 oC and kept under agitation for about 40 minutes.
• Above slurry is taken and sprayed on the bed of SSP powder in a bench scale rotary drum granulator and prepared granular product.
• Wet granules were dried in tray drier to below 3.0% moisture.
• Dried product samples were analyzed for physical properties and sent to QC-Lab for nutrient analysis.
• Same set of trials were conducted again by granulating in pan granulator and analyzed.

NPS (5-15-0-10) & NPS (6-15-0-10) and (6-14-0-10) nutrient analysis:

Trial No %UN %TN %CS. P2O5 %TP2O5 %Moisture
1 4.8 4.8 14.8 15.3 2.1
2 5.2 5.2 14.9 15.1 2.0
3 5.7 5.7 15.1 15.6 1.9
4 6.2 7.7 14.6 15.0 2.2
5 2.9 5.6 15.9 17.0 0.9
6 5.7 5.7 15.1 15.2 2.2
7 6.1 6.7 14.9 15.3 2.5
8 6.1 6.7 14.8 15.1 1.7

Product Moisture vs Crushing Strength:
Drum Granulation Product:
NPS 5-15-0-10 NPS 6-15-0-10
Avg. CS, Kgf/granule %Moisture Avg. CS, Kgf/granule %Moisture
1.78 2.44% 1.36 2.78%

Pan Granulation Product:
NPS 5-15-0-10 NPS 6-15-0-10
Avg. CS, Kgf/granule %Moisture Avg. CS, Kgf/granule %Moisture
2.06 1.92% 2.05 1.72%

 
Hygroscopicity study:

SSP fortified with nitrogen product samples kept for hygroscopicity study for analysing moisture absorption and moisture holding study. SSP fortified with Nitrogen were compared with Urea Ammonium Phosphate (NP 28-28-0), Regular GSSP.

Here we compared Nitrogen fortified SSP with Urea Ammonium Phosphate (NP 28-28-0) as UAP contains 16-18% UN and is one of the toughest fertilizers to handle. From the below data it is evident that SSP fortified with nitrogen is having better physical characteristics then commercial UAP product. And Nitrogen fortified SSP is having better moisture holding capacity than others and moisture holding capacities higher than UAP fertilizer.

Fertilizer type Product moisture% %Moisture Absorption %Moisture Holding Capacity
UAP 28:28:0 1.19% 2.34 9.34
SSP 1.21% 0.34 13.48
Nitrogen fortified SSP 1.69% 1.68 19.23

The Nitrogen fortified SSP prepared in this invention is thus a modified novel product with good yield in the process and ensures better availability of fertilizer content in the composition. Hence this invention relates to a novel method of a process for the preparation from Nitrogen fortified SSP containing sufficient Nitrogen and phosphorous and said process wherein following are specifically avoided in the process such as use of hazardous ammonia, rare ammonium sulphate (AS), rare mono ammonium phosphate (MAP), sulphuric acid, etc each of which are sufferings as identified in prior art. Since such materials are not added, there is no undue and unwarranted hazard identified in this novel process and hence the operation of each and all steps are simple, sage and investment cost will also be reduced accordingly.

The invention hereby discloses a method of preparing single super phosphate (SSP) fortified with nitrogen fertilizer comprising the steps of preparing a mixture in a steam jacketed reactor adapted with an agitator and feeding the reactor with a combination of prilled urea, phosphogypsum and water into the reactor which is maintained at a temperature in the range 60-90°C by feeding steam to the reactor jacket, wherein maintaining the specific gravity of the mixture in the range 1.2 to 1.6 in order to obtain slurry thereof. Then spraying the prepared slurry into a rotary granulator containing dry single super phosphate (SSP) powder therein within, in selective optional presence of water and/or steam for obtaining wet granules thereof. Then drying the wet granules prepared in a rotary drier at a temperature less than 75°C at the exit for obtaining dried granules thereof. Further, cooling the dried granules in a cooler for obtaining cooled granules thereof. Later in continuation screening the cooled granules on size basis. Transferring back the under-size cooled screened granules through a recycle belt conveyor into the rotating granulator for recycling. This is followed crushing the over-size cooled screened granules into under-size and transferring the crushed under-size cooled granules through a pulverizer into the rotary granulator for recycling. Finally collecting the screened collected granules for storage as end product.

The invention in another aspect teaches a method of preparing single super phosphate (SSP) fortified with nitrogen, wherein the most preferred temperature within the reactor is 80°C for the method adopted in the disclosure.

In another aspect, the invention also includes a disclosure of preparing single super phosphate (SSP) fortified with nitrogen, wherein the most preferred specific gravity of the slurry is in the range of 1.3-1.4.

In another aspect, the invention discloses a fertilizer composition prepared by the methods disclosed herein.

Although the invention has been described with reference to certain preferred embodiments, examples or experiments, it will be appreciated that many variations and modifications may be made within the provided scope of the invention. Hence it is intended that the preferred embodiments and all of such variations and modifications be included within the scope and spirit of the invention as defined by claims.
, Claims:WE CLAIM:

1. A method of preparing single super phosphate (SSP) fortified with nitrogen fertilizer comprising the steps of:

a. preparing a mixture in a steam jacketed reactor adapted with an agitator and feeding the reactor with a combination of prilled urea, phosphogypsum and water into the reactor which is maintained at a temperature in the range 60-90°C by feeding steam to the reactor jacket,
b. maintaining the specific gravity of the mixture of step (1a) in the range 1.2 to 1.6 in order to obtain slurry thereof,
c. spraying the prepared slurry of step (1b) into a rotary granulator containing dry single super phosphate (SSP) powder therein within, in selective optional presence of water and/or steam for obtaining wet granules thereof,
d. drying the wet granules prepared in step (1c) in a rotary drier at a temperature less than 75°C at the exit for obtaining dried granules thereof,
e. cooling the dried granules of step (1d) in a cooler for obtaining cooled granules thereof,
f. screening the cooled granules of step (1e) on size basis,
g. transferring back the under-size cooled granules of step (1e) through a recycle belt conveyor into the rotating granulator for recycling,
h. crushing the over-size cooled granules of step (1e) into under-size and transferring the crushed under-size cooled granules through a pulverizer into the rotary granulator for recycling, and
i. collecting the screened collected granules of step (1f) for storage as end product.

2. The method of preparing single super phosphate (SSP) fortified with nitrogen as claimed in claim 1(a), wherein the most preferred temperature within the reactor is 80°C.

3. The method of preparing single super phosphate (SSP) fortified with nitrogen as claimed in claim 1(b), wherein the most preferred specific gravity of the slurry is in the range of 1.3-1.4.

4. A fertilizer composition as prepared by any of the claims 1 to 3.