Title of Invention

A METHOD AND SYSTEM FOR THE MANUFACTURE OF ORTHOPHOSPHATES

Abstract

The invention relates to a method and system of neutralizing trona soda with phosphoric acid comprising at least two stages, a first stage comprising neutralizing of trona soda with phosphoric acid to form an alkaline solution with pH in the range of 9.5 to 10.5; and a second stage comprising neutralizing the alkaline solution of the first stage with phosphoric acid to a pH in the range of 7 to 7. 2.

Full Text

FORM 2 THE PATENTS ACT, 1970 (39 of 1970) & THE PATENTS RULES, 2003 COMPLETE SPECIFICATION(See section 10, rule 13)1. Title of the inventionA METHOD AND SYSTEM FOR THE MANUFACTURE OF ORTHOPHOSPHATES.2. Applicant(s)Name Nationality AddressTATA CHEMICALS LIMITED INDIA BOMBAY HOUSE 24 HOMI MODI STREET, MUMBAI-4000013. Preamble to the description COMPLETE SPECIFICATIONThe following specification particularly describes the invention and the manner in which it isto be performed. The invention relates to a method and system for the manufacture of orthophosphates. More particularly, the invention relates to a method and system for the manufacture of orthophosphates using naturally available trona soda or sodium sesquicarbonate deposits. DESCRIPTION OF RELATED ART Polyphosphates, such as sodium tripolyphosphate, find widespread use in industry including detergent, ceramic, food, paints and fertilizers. Sodium orthophosphates, comprising of disodium hydrogen orthophosphate and sodium di-hydrogen phosphate, are an essential requirement for the production of sodium tripolyphosphate (STTP) and the established process for the preparation of sodium orthophosphates is by the reaction of soda ash or aqueous sodium hydroxide with phosphoric acid to obtain a liquor of sodium orthophosphates. The chemical reaction involved in the conventional process, as illustrated in figure 1, where five moles of sodium carbonate [soda ash] reacts with six moles of phosphoric acid to produce four moles of di-sodium hydrogen phosphate, two moles of mono sodium di-hydrogen phosphate, five moles of carbon dioxide and five moles of water is as follows: 5 Na2C03 + 6 H3PO4 = 4Na2HP04 + 2NaH2P04 + 5C02 + 5H20 (1) The availability of soda ash and the efficiency of using soda ash for the manufacture of sodium orthophosphates has been the subject matter of debate in recent years and there has been felt a need to identify alternate processes for the manufacture of orthophosphate mixtures. One such approach has been to use an alternative source of alkali such as natural soda ash for neutralization of phosphoric acid. Natural soda ash, obtained from chemical treatment of the mineral deposits of natural sodium sesquicarbonate is denser, more granular and less reactive and hence requires fine milling. Examples of such approaches have been discussed in US 4224294 and US 4661331 and the same have been incorporated herein by entirety. However, the use of natural soda ash as an alternative for the production of sodium orthophosphates suffers from certain limitations including the cost of milling natural soda ash, cost of obtaining natural soda, special handling and equipments required and requirement of high purity phosphoric acid. Moreover, these processes require separate storage facilities and the poor flow properties of the fine milled soda ash causes difficulty in achieving uniform and consistent feed for the processing equipment. In view of the above, and towards achieving cost efficiency while utilizing naturally available raw material, the applicant conducted experiments to ascertain the feasibility of using trona soda or natural sodium sesquicarbonate deposits for the production of orthophosphate mixtures. These experiments of utilizing naturally available trona soda and phosphoric acid for the production of orthophosphates resulted in the formation of a gelatinous precipitate in the orthophosphate liquor that could not be easily filtered out thereby making the process ineffective. SUMMARY OF THE INVENTION The application provides for a reverse neutralization process of phosphoric acid and alkali. The application relates to a method and system of preparing sodium orthophosphates using naturally occurring trona soda (Sodium Sesquicarbonate). The method and system in accordance with the teachings of this document overcome some of the limitations referred above and provide for the reverse neutralization of trona soda with phosphoric acid in two stages. The application provides for a method and system to overcome the limitation of the conventional neutralization of phosphoric acid with trona soda in a single step that produces a large amount of gelatinous precipitate that brings down the filtration rate of the slurry. The application relates to a method and system of neutralizing trona soda with phosphoric acid comprising at least two stages, a first stage comprising neutralizing of trona soda with phosphoric acid to form an alkaline solution with pH in the range of 9.5 to 10.5; and a second stage comprising neutralizing the alkaline solution of the first stage with phosphoric acid to a pH in the range of 7 to 7. 2. BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS The accompanying drawing illustrates the preferred embodiments of the invention and together with the following detailed description serves to explain the principles of the invention. Figure 1 illustrates the conventional process for the manufacture of orthophosphate mixtures using soda ash. Figure 2 illustrates the process for the manufacture of orthophosphate mixtures using trona soda in accordance with an embodiment of the invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated system, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. It will be understood by those skilled in the art that the foregoing general description and the following detailed description are exemplary and explanatory of the invention and are not intended to be restrictive thereof. The invention has been described for trona soda, chemically known as sodium sesquicarbonate that is a mixture of sodium carbonate and sodium bicarbonate, and available as a natural deposit in Lake Magadi, in the form of a mix of lake liquor, mud and large quantities of organic matter. It will however be obvious to a person in the art that the teachings of the document may be equally applied to other sources of soda. At the outset, with reference to the following table, a quality comparison between soda ash and naturally available trona soda is provided. As may be seen, synthetically manufactured soda ash is a purer form of soda with 99 percent sodium carbonate, no sodium bicarbonate and with little or no impurities. Trona soda on the other hand is a mixture of sodium carbonate and sodium bicarbonate with significant amounts of impurities and moisture. Soda ash is more than twice the cost of naturally available trona soda and thus use of trona soda for the manufacture of sodium orthophosphates would provide for significant cost savings, if limitations of impurities and composition can be economically and efficiently overcome. Table 1: Quality Comparison SODA ASH TRONA SODA Synthetic Natural deposit Pure Highly contaminated % Na2C03 99 42 % NaHC03 Nil 38 % WI 0.05 0.7 % NaF Nil 2 % Organic material 0 0.3 Moisture 0.3 16 Unit Price USD 220 100 Trials conducted at the applicants plant using trona soda in place of synthetic soda employing the conventional process of single stage neutralization at pH 7.0 failed to achieve plant production throughput on account of gelatinous precipitates formed during neutralization that could not be filtered out. Plant production was found to be only 30% of the rated capacity. Gelatinous precipitates essentially form on account of presence of high levels of fluorine, impurities and muddy material in the trona soda. Impurities are also substantially contributed by the phosphoric acid particularly when fertilizer or chemical grade of the same is used. Fertilizer or chemical grade phosphoric acid, as opposed to concentrated phosphoric acid, contains various impurities such as H2SiF6, H2S04, Ca, Mg, Fe, Al and large amounts of heavy metal impurities. These impurities are to be removed for the manufacture of technical grade orthophosphates. Fluorine present as impurity is required to be removed from the phosphoric acid before the phosphoric acid is used for neutralization with soda to make orthophosphate solution. During neutralization, the metals and heavy metal impurities present in phosphoric acid produce gelatinous precipitates in the orthophosphate liquor. The efficient filtration of gelatinous precipitates is the key performance measure of the plant production rate. In accordance with the teachings of this document, phosphoric acid and trona soda are reacted in two stages in lieu of conventional single stage. The process essentially includes a two stage reverse neutralization of trona soda with phosphoric acid. In the first stage, at a high pH of 9.5 to 10.5 sodium carbonate reacts with phosphoric acid. In the second stage sodium bicarbonate reacts with phosphoric acid, the resultant alkaline slurry of the first stage is further neutralized with phosphoric acid to bring down the pH to a neutral range of 7.0 to 7.1. In accordance with the preferred embodiment de-fluorinated fertilizer or chemical grade phosphoric acid and trona soda are reacted in two stages, in a manner such that in the first stage pH of the neutralization reaction is kept in the range 9.5 to 10.5 at Na:P mole ratio of 2:2.2 and at a temperature of 90 to 102 degree centigrade. At this first stage, 30 to 50 % trona soda is kept in excess in the neutralizer. The de-fluorinated phosphoric acid as used in accordance with an embodiment is typically a dilute acid with a 35 to 40 percent concentration. China clay or ball clay is also preferably dosed at this stage as a crystal modifier while the neutralization solution is being agitated by agitators. The first stage neutralization solution is kept for between one to three hours of residence time to allow for particle modification of precipitates. The china clay also assists in eliminating variability in the rate of filtration due to variability in impurities content in the phosphoric acid. In the second stage of neutralization, pH of the neutralization solution is brought down to pH 7.0-7.2, preferably in the range of 7.0 to 7.1, along with controlled dosing of de-fluorinated phosphoric acid at temperature in the range of 90 to 102 degree centigrade while being agitated and is preferably kept for between one to two hours of residence . time. The two stage reverse neutralization process addresses the issue of gelatinous precipitate formed in the reaction and helps achieve a filtration rate comparable to the conventional neutralization process of synthetic soda ash. The use of china clay as a crystal modifier also assists in achieving the desired filtration rate. Filtration is the critical operation for the production of dilute orthophosphate liquor and the production of precipitates is an essential precursor for the filtration. Filtration is typically not required in conventional processes of orthophosphate manufacture on account of the use of superior quality raw material, including concentrated phosphoric acid. The chemical reactions involved in the process as described above are: Stage 1: One mole of sodium carbonate reacts with one mole of phosphoric acid to produce one mole of di-sodium hydrogen phosphate, one mole of carbon di oxide and one mole of water. Na2C03 + H3PO4 = Na2HP04 + C02 + H20 (2) - Stage 2: One moles of sodium bicarbonate reacts with one mole of phosphoric acid to produce one mole of mono sodium di-hydrogen phosphate, one mole of carbon dioxide and one mole of water. NaHC03 + H3PO4 = NaH2P04 + C02 + H20 (3) Part of the di-sodium hydrogen phosphate gets converted to mono sodium di-hydrogen phosphate reacting with phosphoric acid to achieve molar ratio 2:1 of di sodium hydrogen phosphate to mono sodium di-hydrogen phosphate in the liquor. Na2HP04 + H3PO4 = 2NaH2P04 (4) The two stage neutralization process does not leave any or insubstantial amounts of un-reacted sodium carbonate or sodium bicarbonate in the resultant neutral orthophosphate liquor. In addition, to the principal reactions above, there are side reactions taking place with metals and heavy metal impurities in phosphoric acid. These reactions essentially involve the reaction of mono calcium phosphate with sodium carbonate and sodium bicarbonate. The sodium fluoride content in the trona soda is mostly precipitated in the alkaline medium as CaF2 in the reverse neutralization process which does not happen in the conventional single step process with trona soda. During neutralization, reactions (5) to ' (9) above, take place simultaneously and thereby lead to formation of crystalline precipitate of apatite which is essentially phosphate bearing compound of varied composition of CaC03,CaF2, Ca(OH)2 and CaHP04. Crystallography and crystallinity tests conducted showed that the major constituents of the precipitate are Carbonate hydroxyl apatite and Hydroxyapatite. More the crystalline precipitate better is the filtration rate. Amorphous precipitates are relatively gelatinous and filtration rate is poor. Reverse neutralization process produces more crystalline precipitates and thereby improves filtration rate even with large amount of fluorine in it. - Calcium and magnesium salts of PO4, F, CO3 and phosphates of iron, aluminum and other heavy metals constitute precipitates produced on neutralization of phosphoric acid. The above reactions lead to formation of precipitates of phosphates, and are the major factor for conversion efficiency of the process. Hence lesser is the P2O5 content in the neutral filter cake which is a waste, better is the conversion efficiency of the process to make orthophosphate . This has been achieved by substitution of phosphate from apatite crystals present in neutral filter cake with higher amount of fluorine through reverse neutralization process of Trona Soda. A study on CO2 content in the filter cake reveals that CO2 content is higher in the - cake produced by reverse neutralization of trona soda 2500ppm as compare to l000ppm for conventional process of light soda ash and trona soda. This indicates that there is substantial change in composition of filter cake achieved which results in more crystalline precipitates of apatite and thus manifested comparable filtration rate of orthophosphate liquor while Trona soda is used. With reference to figure 2 of the accompanying drawings, a two stage neutralizer system 10 for the two stage reverse neutralization of trona soda with phosphoric acid is illustrated. Phosphoric acid 16, wash water 18 and trona soda 20 are added to the first neutralizer 12 that has an agitator 24. Overflow from the first stage 22 is sent to the second neutralizer 14 that has agitator 26. Phosphoric acid is also added to the second neutralizer directly. Overflow from the second neutralizer is then sent for the further filtering. Lab Trials A series of lab scale trials were conducted where it was found that reverse neutralization of trona soda with de-fluorinated phosphoric acid while keeping 50% and 34% excess alkali in the 1st stage of the neutralization were found to exhibit better rate of filtration of neutral liquor and lower P2O5 loss through neutral filter cake as compared to liquor made from light soda ash. The average filtration rate in Kg P205 per Hr per M" for "neutral liquors made by 50% excess trona, 34% excess trona and light soda ash were found to be in the range of 150 to 250, 200 to 275 and 100 to 150 respectively. Accordingly, the total P2O5 give away was found to be 33 to 36%, 35 to 40% and 40 to 47% in the neutral filter cakes respectively for 50% excess Trona Soda ,34% excess trona soda and light soda ash. Plant Trials The teachings of this document were also applied to a plant scale trial where similar results were achieved. Production of orthophosphate liquor was comparable with the throughput currently achieved using light soda ash. The average throughput was found to be 7.58 to 8.7 ton P205 per hr by the trona soda process of reverse neutralization as described by this document, compared to the 7.33 to 9.79 Ton P2O5 per hr by conventional process of soda ash. Lesser water soluble P2O5 give away with the neutral filter cake, a measure of good filterability and washability of filter cake, was found much to be much lower in the case of the trona soda process of reverse neutralization as compared to the conventional soda ash process. The average water soluble P2O5 give away in the neutral filter cake was found to be 4.7 to 13% in case of the trona soda process of reverse neutralization as compared to 8.6 to 18% in case of the conventional soda ash process. The resultant 5 liquid/slurry has got good filterability and improved P2O5 recovery in the neutral filter cake. We claim: 1. A method of neutralizing trona soda with phosphoric acid comprising at least two stages, a first stage comprising neutralizing of trona soda with phosphoric acid to form an alkaline solution with pH in the range of 9.5 to 10.5; and a second stage comprising neutralizing the alkaline solution of the first stage with phosphoric acid to a pH in the range of 7 to 7. 2. 2. A method of neutralizing as claimed in claim 1 wherein a residence period of between one to three hours is provided for the alkaline solution between the first and second stage. 3. A method of neutralizing as claimed in claim 1 wherein a residence period of at least one hour is provided to the neutralized solution after the second stage. 4. A method of neutralizing as claimed in claim 1 wherein china clay is added during the first stage neutralization. 5. A method of neutralizing as claimed in claim 4 wherein the addition of china clay during the first stage neutralization is accompanied by agitation by agitators. 6. A method of neutralizing as claimed in claim 1 wherein the first stage neutralization is accompanied by agitation by agitators 7. A method of neutralizing as claimed in claim 1 wherein excess trona soda is kept in the first stage neutralization. 8. A method of neutralizing as claimed in claim 7 wherein 30 to 50 percent excess trona soda is kept during the first stage neutralization. 9. A method of neutralizing as claimed in claim 1 wherein the Na: P mole ratio in first stage is in the range of 2 to 2.2. 10. A method of neutralizing as claimed in claim 1 wherein the Na: P mole ratio in the second stage is in the range 1.55 to 1.75. 11. A method of neutralizing as claimed in claim 1 wherein the temperature of the first stage neutralization is in the range of 90 degrees centigrade to 102 degrees centigrade. 12. A method of neutralizing as claimed in claim 1 wherein the temperature of the second stage neutralization is in the range of 90 degrees centigrade to 102 degrees centigrade. 13. A method of neutralizing as claimed in claim 1 wherein the second stage neutralization is accompanied by agitation by agitators. 14. A method of neutralizing as claimed in claim 1 wherein the phosphoric acid is chemical or fertilizer grade. 15. A method of neutralizing as claimed in claim 1 wherein sodium carbonate of the trona soda reacts with the phosphoric acid to form di sodium hydrogen phosphate in the first stage neutralization. 16. A method of neutralizing as claimed in claim 1 wherein sodium bicarbonate of the trona soda reacts with the phosphoric acid to form mono sodium di hydrogen phosphate in the second stage neutralization. 17. A method of neutralizing as claimed in claim 1 wherein a part of the di sodium hydrogen phosphate of the first stage is converted to mono sodium di hydrogen phosphate in the second stage neutralization. 18. A method of neutralizing as claimed in claim 1 wherein the molar ratio of di sodium hydrogen phosphate to mono sodium di hydrogen phosphate is 2:1 at the end of the second stage neutralization. 19. A method of neutralizing trona soda with phosphoric acid for the manufacture of orthophosphates comprising at least two stages, a first stage neutralizing the trona soda with phosphoric acid to a pH in the range of 9.5 to 10.5 such that an alkaline solution is formed; and a second stage comprising neutralizing the alkaline solution of the first stage with phosphoric acid to a pH in the range of 7 to 7. 2 to form orthophosphate liquor. 20. A method of neutralizing trona soda with phosphoric acid for the manufacture of orthophosphates as claimed in claim 19 wherein the orthophosphate liquor is filtered to remove precipitates formed during the first and second stage neutralization. 21. A system for neutralizing trona soda with phosphoric acid comprising a first neutralizer and a second neutralizer; the first neutralizer neutralizing trona soda with phosphoric acid to form an alkaline solution with pH in the range of 9.5 to 10.5; and the second neutralizer neutralizing the alkaline solution of the first neutralizer with phosphoric acid to a pH in the range of 7 to 7. 2 22. A method of neutralizing substantially as herein described and as illustrated by figure 2. 23. A system for neutralizing substantially as herein described and as illustrated by figure 2. ABSTRACT The invention relates to a method and system of neutralizing trona soda with phosphoric acid comprising at least two stages, a first stage comprising neutralizing of trona soda with phosphoric acid to form an alkaline solution with pH in the range of 9.5 to 10.5; and a second stage comprising neutralizing the alkaline solution of the first stage with phosphoric acid to a pH in the range of 7 to 7. 2. Figure 2

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2005-MUM-2007-CLAIMS(AMENDED)-(16-5-2013).pdf

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