MXPA02005500A - Production of two alkali metal salts by a combined ion exchange and crystallisation process - Google Patents

Abstract

The invention relates a process for producing an alkali metal nitrate and an alkali metal phosphate in the same process from a phosphate raw material and a nitrate raw material comprising the steps of:a) reacting the phosphate raw material with the nitrate raw material to provide an aqueous nitrophosphate feed, optionally followed by the separation of solid material, b) introducing the aqueous nitrophophate feed into a first ion exchange step comprising an alkali metal loaded cationic exchange resin for exchanging cations present in the feed with alkali metal ions present on the resin to obtain a stream enriched in alkali metal ions, c) subjecting the stream from step (b) to a first crystallisation under such conditions that an alkali metal nitrate is crystallised and separating the crystallised alkali metal nitrate from the mother liquor, d) introducing the mother liquor from step (c) into a second ion exchange step comprising an alkali metal loaded cationic exchange resin for exchanging cations present in the mother liquor with alkali metal ions present on the resin to obtain a phosphate containing stream enriched in alkali metal ions, and e) subjecting the stream from step (d) to a second crystallisation under such conditions that an alkali metal phosphate is crystallised and separating the crystallised alkali metal phosphate from the mother liquor.

Description

PRODUCTION OF TWO METAL ALKALINE SALTS THROUGH A CRYSTALLIZATION AND COMBINED EXCHANGE PROCESS IONS Field of the Invention The present invention relates to a production process for a metal alkali nitrate and a phosphate such as a potassium nitrate and phosphate in the same process, which comprises ion exchange and crystallization steps. The salts produced are especially useful in horticulture as fertilizers, where fertilizers are frequently used through irrigation. The inventive process comprises the following unit operations: digestion, ion exchange, neutralization, solids separation, concentration and crystallization.

BACKGROUND OF THE INVENTION High purity metal nitrates or alkaline phosphates, completely soluble in water, are particularly used in horticulture and have a wide application in various industries, such as in the manufacture of pharmaceuticals, food or food. While in the past several methods have been proposed for their production, only a few have been commercialized. Potassium nitrate is the third potassium salt mostly used in agriculture and is traditionally produced from a mineral that contains sodium nitrate, potassium nitrate, some chlorides and sulfates.

The application of this technology is, however, limited by the availability of nitrate ore. Potassium nitrate can also be produced synthetically by a reaction of potassium chloride with nitric acid at low temperature followed by the extraction of the hydrochloric acid co-product with an organic solvent. Putting a volatile organic substance in contact with a nitrate can be dangerous, and recovering the solvent will have an impact on the execution and economy of the process. Hydrochloric acid, especially with nitric acid, is highly corrosive and causes serious limitations to equipment construction materials. In addition, due to the lack of a local need, hydrochloric acid will be considered as waste. The ion exchange technology has also been proposed for the production of potassium nitrate. In this process, the hydrogen ions of nitric acid are exchanged with the potassium ions of potassium chloride, resulting in a solution of potassium nitrate and a solution of hydrochloric acid, see US Patent No. 5 1 10 578. A disadvantage with This process of "direct" ion exchange is the risk of mixing potassium chloride and nitric acid, resulting in the formation of a highly corrosive fluid (regia water). Currently, most potassium phosphate salts used in industry and agriculture are produced based on pure raw materials, potassium hydroxide or carbonate and purified phosphoric acid. Potassium phosphates are excellent fertilizers and several investigations are carried out in order to find a process of economic production based on cheap raw materials in order to obtain a product of acceptable quality. The production of monopotassium phosphate from lower quality raw materials, potassium chloride and wet process phosphoric acid, has been investigated intensively during the last years, US Pat. Nos. 4,836,995; 4 885 148; and 5 114 460. In all the processes described in these three patents, the real challenge is the separation of chlorine from potassium. In these processes, this is done either by evaporation or by solvent extraction of the hydrochloric acid by-product. The direct evaporation of hydrochloric acid is problematic due to the formation of insoluble potassium phosphate compounds which will reduce the overall yield. The recovery of the solvent in the extraction process of the organic solvent is essential for the general economy and also to avoid organic material in the waste stream. In US Pat. No. 4,678,649, a process for the production of pure monopotassium phosphates without the use of solvents to remove hydrochloric acid is described. According to the process, the monopotassium sulfate is reacted with a phosphate component selected from a phosphate rock, dicalcium phosphate or mixtures of both in the presence of phosphoric acid. The results of the process are: gypsum, calcium phosphate, hydrochloric acid and monopotassium phosphate. When sulfuric acid is mixed at a high temperature with potassium chloride, monopotassium sulphate is produced, resulting in the evaporation of hydrochloric acid, which will limit the selection of Construction materials. Hydrochloric acid and significant amounts of gypsum generated in the process can be considered as waste. Ion exchange technology has also been considered in the production of fertilizers and especially in relation to the production of potassium salts without chlorine, see US Patent No. 3 993 466, 4 008 307 and 4 704 263. In the process of Potassium phosphate production described in U.S. Patent No. 4,008,307, the raw materials are phosphoric acid and potassium sulfate. The ion exchange process can be cationic or anionic. In both cases, the result will be a solution of potassium phosphate and a solution of sulfuric acid. An organic solvent is needed to extract potassium phosphate from a sulfate contained in a solution. In US Pat. No. 4,704,263, which relates to a cationic process for producing potassium phosphate, the feed streams for ion exchange are a solution of metal phosphate salt and a solution of potassium chloride. The metal phosphate salt can be calcium phosphate, magnesium phosphate or iron phosphate, and even more particularly, a monocalcium phosphate. A disadvantage of using monocalcium phosphate is the need for phosphoric acid and the low concentration of calcium ions in the solution, necessitating a step of calcium enrichment in the carousel system of continuous ion exchange (ISEP).

BRIEF DESCRIPTION OF THE INVENTION It is an object of the present invention to provide a combined continuous process for producing in the same process both a high purity, water-soluble alkali metal nitrate and a high purity metal alkaline phosphate, soluble in water from cheap raw materials. It is another object of the present invention to provide a process, which in addition to producing the desired products, produces only products that are not harmful waste or can be upgraded to useful products. Therefore, the present invention provides a process for producing in the same process an alkali metal nitrate and a metal alkaline phosphate, a phosphate raw material and a nitrate raw material comprising the steps of: a) reacting the raw material of phosphate with the nitrate raw material to provide an aqueous nitrophosphate feed, optionally followed by separation of the solid material, b) introducing the aqueous feed of the nitrophosphate in a first ion exchange step, comprising a metallized alkaline cation resin for the exchange of cations present in the feed with the alkali metal ions present in the resin to obtain an enriched stream of alkali metal ions c) subject the stream from step (b) to a first crystallization under conditions such as alkali nitrate metal is crystallized, and the crystallized alkali metal nitrate is separated from the mother solution; d) introducing the stock solution from step (c) into a second ion exchange step, which consists of an alkaline cation exchange metalized resin for the exchange of the cations present in the mother solution with the alkali metal ions present in the resin, in order to obtain a phosphate containing a stream enriched with alkali metal ions, and e) subject the current from step (d) to a second low crystallization conditions such that the alkaline metal phosphate is crystallized, and that once crystallized it is separated from the mother solution. In a preferred embodiment of the invention, the process includes the step of: f) introducing the stock solution from step (e) to the first crystallization step (c). According to the above, the process of the present invention is a crystallization and ion exchange combined process comprising two ion exchange steps and two crystallization steps. Preferably, this process is a continuous process. Preferably, the cation exchange resins of both the first step and the second ion exchange step are part of the same ion exchange system, which comprises a multiple column system that operates as a bed of constant motion simulated, where the column is filled with said cation exchange resins. The alkali metal is potassium or sodium, preferably potassium. In addition, the two resulting products are preferably potassium nitrate and potassium phosphate such as monopotassium phosphate. The ion exchange resins can be regenerated with an alkali metal salt solution such as potassium chloride. Said cations to be exchanged and present in the feeds introduced in the first and second steps of ion exchange contain at least calcium, hydronium and optionally, depending on the phosphate feedstock, minor amounts of magnesium ions. The phosphate feedstock preferably comprises phosphate rock but other suitable phosphate feedstocks can be used, for example, monocalcium or dicalcium phosphate or phosphoric acid or mixtures of both. The nitrate raw material preferably comprises nitric acid but also other suitable nitrate raw materials can be used, for example, calcium nitrate or a mixture of nitric acid and calcium nitrate. The most preferred raw materials are phosphate rock and nitric acid. The first crystallization is preferably carried out by concentration at a temperature between -30 ° C and 80 ° C, preferably between -10 ° C and 10 ° C.

According to the process of the present invention, it is possible to increase the pH of the current from the second ion exchange step (d) to a value between 3 and 6 to precipitate impurities such as calcium and magnesium phosphates which can be recycled to the step (a) as part of the phosphate feedstock. To increase the pH to the desired value, a base, preferably potassium hydroxide, is used. The second crystallization is preferably carried out by adjusting the pH to a value between 4 and 5 and by means of concentration at a temperature between 0 ° C and 100 ° C, preferably between 30 ° C and 80 ° C. To increase the pH to the desired value, a base, preferably potassium hydroxide, is used. If the phosphate raw material contains fluorides, it is possible to remove them either before the first crystallization step or before the second crystallization by increasing the pH, thus obtaining the precipitation of the calcium fluoride that is subsequently separated.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram illustrating the process of the present invention, and Fig. 2 is a schematic representation of the ion exchange operation.

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a multi-step process as defined above. n the later, this process of Crystallization and combined ions exchange adapted to produce pure potassium nitrate and potassium phosphate will be described in more detail. The ion exchange can be carried out in a multiple column system such as a commercially available sham bed available. Referring to Fig. 1, the unit operations of the present multi-stage process are: Preparation of a solution of nitrophosphate and if necessary, separation of solids, Stage I ion exchange, Crystallization of potassium nitrate, Stage II of ion exchange, monopotassium phosphate crystallization, and ion exchange regeneration. Preparation of nitrophosphate solution Phosphate rock or other appropriate phosphate feedstock is reacted with a nitric acid in a conventional rock digestion process to obtain a nitrophosphate mixture. Part of the phosphate rock can be replaced by a phosphate salt or by phosphoric acid, also replacing a part of the nitric acid. By varying the relative amounts of the raw materials, the proportion of the two products can be adjusted within a fairly wide range. If the mixture is obtained, water is added to it in an established countercurrent system, and the solid fraction is separated. Stage I of ion exchange. The nitrofosphate solution rich in Calcium TSI is fed to a column system filled with an ion exchange resin, in this case a strong macroporous cationic resin with a high oxidizing capacity. The ion exchange columns are operated in countercurrent, which can be described as a simulated motion bed unit. The calcium in the nitrophosphate solution is exchanged with potassium from the potassium-laden resin. The affluent enriched with potassium FS1 can be related as a mixture of calcium nitrate and potassium nitrate in phosphoric acid. Crystallization of potassium nitrate. Potassium nitrate is precipitated after the concentration of the tributary of the first ion exchange step. The crystallization can be effected by any conventional crystallization technique. The crystallization of potassium nitrate can be carried out at temperatures from 80 ° C to -30 ° C and preferably from -10 ° C to 10 ° C. Stage II of ion exchange. The TS2 mother solution of the crystallization of potassium nitrate becomes richer in calcium due to the elimination of potassium in the crystallization of potassium nitrate. In relation to anions, the mother solution has become richer in phosphate compared to the nitrate that was eliminated during the crystallization of potassium nitrate. This mother solution TS2 is again recycled to the system of ion exchange columns to the second step just where the tributary of I ion exchange is extracted. The introduction of the second step of ion exchange allows to improve the efficiency in the elimination of calcium as well as the subsequent crystallization of Potassium Phosphate. Regeneration of ion exchange. The calcium, hydronium and magnesium ions on the resin are exchanged with potassium by adding a solution of potassium chloride, TK. The affluent is an acid solution of calcium chloride, KK. Crystallization of monopotassium phosphate. The FS2 affluent with a low calcium content of the second ion exchange step is concentrated, and for example: potassium hydroxide is added to adjust the pH value to between 3 and 6. When increasing the pH, the phosphates will precipitate of calcium and magnesium, since they are separated from the fluid and discharged or recycled as a source of phosphate to the digestion reactor. The liquid phase can be recognized as a mixture of potassium phosphate and potassium nitrate. By controlling the temperature of the crystallization, the water content and the ratio between the nitrate and phosphate ions in the crystallization solution a monopotassium phosphate product can be produced. The monopotassium phosphate can be crystallized at temperatures between 0 and 100 ° C, preferably between 30 and 80 ° C, at a pH range of 4 to 5. The RCI mother solution, which now consists mainly of potassium nitrate, is again recycled to the crystallization of potassium nitrate. Elimination of fluorides. The fluorides that originate from the phosphate rock can be easily removed either before the crystallization step by increasing the pH and by subsequent separation of the precipitated calcium fluoride.

The results from the processes described above are solid potassium nitrate and solid monopotassium phosphate, and a solution of calcium chloride, which can be improved to a salable product. The raw materials, which may be used in the process, include phosphate rock or other appropriate sources of phosphate, potassium chloride, potassium hydroxide or other alkaline substance, and optionally an alkaline calcium compound such as calcium oxide for neutralization and precipitation. An advantage of the process according to the invention is that pure products can be produced from basically inexpensive raw materials. Two valuable products are obtained in the same process. The designed process offers a flexible choice of salts produced as well as the relative amounts of them. The present process provides a high efficiency and high utilization of resins in the ion exchange unit and the use of raw materials are maximized in their entirety while the amount of waste generated is kept to a minimum. No corrosive gaseous substance or gypsum is formed, and no organic solvent is needed. Compared to conventional ion gap processes, the present invention is more energy efficient than in the production of two products in separate processes, where the total degree of dissolution is higher. EXAMPLES Example 1 The ion exchange process was operated according to the configuration, presented in Figure 2, which consists of 16 columns.

A nitrophosphate mixture was prepared by mixing phosphate rock with nitric acid. After the solids separation a solution was obtained that we call TS1. The other solution that is fed, TS2, was obtained from the crystallization of potassium nitrate after the first step of ion exchange. The resin was regenerated with a solution of potassium chloride, TK. The compositions of TS1, TS2 and TK are illustrated in Table 1. The feeds were ordered in a sequential mode: TS2 followed by TSI and washing in the production stage; TK followed by washing in the regeneration stage. The temperature was maintained between 30 and 60 ° C through the process of several stages of ion exchange. Table 1 Compositions of ion exchange-fed solutions TS1, TS2 and TK.

After a steady state was achieved, some samples were taken from product currents FS1, FS2 and KK. The compositions of FSI, FS2 and KK are presented in Table 2. The currents of the product are named after the fed streams, which means that after passing through the ion exchange column TS1 becomes FS1, TS2 becomes FS2 and TK becomes KK. Table 2 Compositions of the solutions resulting from the exchange of ions FS1, FS2 and KK It was possible to operate the ion exchange in two steps with the use of a resin significantly higher than 60%. In the first and second exchange step, 60 and 50% of the calcium was removed respectively in the feed streams TS1 and TS2, taking into account the dissolution of the fed streams. Example 2 In a second ion exchange experiment, the number of columns was increased to 18 by adding two more columns in the production step, one in each step, to investigate the possibilities for future improvement in calcium removal, ie , the efficiency in the ion exchange process. The process conditions were similar to those described in Example 1. The compositions of the inlet and outlet streams are presented in Tables 3 and 4. Table 3 Compositions of the ion exchange fed solutions TSI, TS2 and TK Table 4 Compositions of exit solutions in the ion exchange FS1, FS2 and KK By increasing the number of columns from 16 to 18, it was possible to improve the elimination of calcium ions in the ion exchange process in two stages: 64 and 71% of the calcium was removed in the two steps respectively. Example 3 In another experiment, the ion exchange was operated as in Example 1. The FSI output solution was concentrated by evaporation to provide a water content of 60% in the stock solution, TS2, after the crystallization of potassium nitrate. After the separation of insolubles, the potassium nitrate was crystallized at 5 ° C. The crystals were separated from the mother solution by means of filtration, washed with water and dried. The stock solution was re-introduced into the TS2 comp ion exchange system. The compositions of the inlet and outlet streams as well as that of the potassium nitrate crystals obtained are presented in Table 5. Table 5 Composition of potassium nitrate crystals and inlet crystallization solutions FS1 and RC1 and exit solution TS2 [% p / p] As can be seen from the results in Table 5, a pure product of potassium nitrate can be obtained. Example 4 In another experiment, the ion exchange was operated as in Example 1, The exit solution FS2 was neutralized with potassium hydroxide to a pH of 4.2 to precipitate impurities having calcium and magnesium. The precipitates were separated by means of filtration, and the monopotassium phosphate was crystallized from the CFS solution at 50 ° C by means of a vacuum crystallization. The crystals were separated from the mother solution by means of filtration, washed with water and dried. The crystallization of the stock solution RC1 was mixed with FS1 and concentrated as described in the crystallization process of potassium nitrate. The compositions of the inlet and outlet streams as well as that of the potassium phosphate crystals obtained are presented in Table 6. Table 6 Compositions of potassium phosphate crystals and of inlet and outlet crystallization solutions CFS and RC1 As can be seen from the results in Table 6, a pure potassium phosphate product with very little calcium, nitrogen or chloride residue can be obtained.

In the previous examples, the chloride was added to simulate the accumulation of possible impurities where the chloride will be mostly impure because it can not be removed by precipitation. As you can see from the composition of the two products, the chloride has not caused any problems.

Claims (15)

1. CLAIMS 1. A process for the production of an alkali metal nitrate and an alkaline metal phosphate in the same process, using a phosphate raw material and a nitrate raw material, and comprising the following steps. a) reacting the phosphate feedstock with the nitrate feedstock to provide an aqueous nitrophosphate feedstock, optionally followed by separation of the solid material, b) introducing the nitrophosphate feedstock in a first ion exchange step, comprising a metallized alkaline cationic resin for the exchange of cations present in the feed with the alkali metal ions present in the resin to obtain an enriched stream of alkali metal ions c) subject the stream from step (b) to a first crystallization under conditions such that the alkali metal nitrate is crystallized, and the crystallized alkali metal nitrate is separated from the mother solution, d) introducing the stock solution from step (c) into a second ion exchange step, which consists of a metallized cationic alkaline resin of exchange for the exchange of the cations present in the mother solution with the alkaline metal ions present in the resin, in order to obtain a phosphate containing a current enriched with alkali metal ions; and e) subjecting the stream from step (d) to a second crystallization under conditions such that the alkali metal phosphate is crystallized and, once crystallized, is separated from the mother solution.
2. 2. The process of claim 1, further comprising the following step: f) introducing the stock solution from step (e) to the first crystallization step (c).
3. 3. The process of claim 1 or 2, wherein the cation exchange resins either from the first or second ion exchange step are part of the same ion exchange system.
4. 4. The process of claim 3, wherein the ion exchange system comprises a multiple column system operating as a simulated continuous motion bed.
5. The process of any of claims 1 to 4, wherein the ion exchange resins are regenerated with a solution of an alkali metal salt.
6. 6. The process of any of claims 1 to 5, wherein the alkali metal is potassium.
7. The process of any of claims 1 to 6, wherein the crystallized alkali metal nitrate contains potassium nitrate, and the crystallized alkali metal phosphate contains monopotassium phosphate.
8. 8. The process of any of claims 1 to 7, wherein said cations to be exchanged and present in the feeds introduced into the first and second step of ion exchange contain calcium, hydrogen ions and optionally magnesium ions.
9. 9. The process of any of claims 1 to 8, wherein the phosphate feedstock contains phosphate rock, monocalcium phosphate or dicalcium phosphate and / or phosphoric acid. The process of any of claims 1 to 9, wherein the nitrate raw material contains nitric acid and / or calcium nitrate. eleven . The process of any of claims 1 to 10, wherein the first crystallization is carried out by decreasing the temperature and by concentration. The process of any of claims 1 to 11, wherein the pH of the stream from step (d) is increased to a value between 3 and 6 to precipitate the impurities, which are separated. The process of claim 12, wherein the precipitated and separated impurities contain calcium and magnesium phosphates, which are recycled to step (a) as part of the phosphate feedstock. The process of any of claims 1 to 13, wherein the second crystallization is carried out by adjusting the pH to a value between 4 and 5 and by concentration. The process of claims 12 or 14, wherein the potassium hydroxide or other alkaline material is added to increase the pH to the desired value. RESU MEN The invention relates to a process for the production of an alkali metal nitrate and an alkali metal phosphate in the same process, using a phosphate raw material and a nitrate raw material, and comprising the following steps: a) reacting the phosphate raw material with the nitrate raw material to provide an aqueous nitrophosphate feed, optionally followed by separation of the solid material; b) introducing the aqueous nitrophosphate feed into a first ion exchange step, comprising an alkaline cationic resin metallized for the exchange of cations present in the feed with the alkali metal ions present in the resin to obtain an enriched stream of alkali metal ions; c) subjecting the stream from step (b) to a first crystallization under conditions such that the alkali metal nitrate is crystallized, and the crystallized metal alkali nitrate is separated from the stock solution, d) introducing the stock solution from step (c) into a second step of ion exchange, which consists of an alkaline cation exchange metalized resin for the exchange of the cations present in the mother solution with the alkali metal ions present in the resin, in order to obtain a phosphate containing an enriched stream of alkali metal ions, and e) subjecting the stream of step (d) to a second crystallization under conditions such that the alkali metal phosphate is crystallized, and that once crystallized is separated from the mother solution.