Process for producing potassium sulfate from wood pulping operations

Abstract

The present invention relates to a process for recovering potassium sulfate from ash taken from a recovery boiler in a wood pulping operation. The ash is dissolved in water, and the resulting solution is evaporated to produce a concentrated dissolved ash solution, which is then processed in a glaserite crystallizer. The glaserite crystallizer produces glaserite crystals by contacting the ash with water or condensate cooled to a temperature of 0°C to 25°C in a chiller, or by cooling the crystallizer brine to a temperature of -10°C to 25°C. The glaserite crystals and cooling water are sent to an SOP crystallizer, where the sodium sulfate and potassium sulfate from the glaserite crystals dissolve in the cooling water, forming a solution containing both sodium sulfate and potassium sulfate. A portion of the potassium sulfate, exceeding its solubility limit, recrystallizes as SOP. The SOP is recovered by a solid-liquid separation process.

Description

[Technical Field]

[0001] Related Applications This application claims priority to U.S. Provisional Patent Application No. 63 / 431,221, filed December 8, 2022, the disclosure of which is incorporated herein by reference in its entirety.

[0002] Technical Field The present invention relates to a process for producing potassium sulfate (also known as KSO or SOP), and more particularly to a process for producing potassium sulfate during the treatment of ash produced in wood pulping operations. [Background technology]

[0003] Potassium sulfate is an excellent fertilizer, high in potassium and low in chloride. It provides essential nutrients that help plants resist disease and pests, and is generally considered a high-quality, valuable fertilizer that produces high-quality crops. While there are numerous methods for producing potassium sulfate, in recent years, interest has grown in producing it as a by-product of industrial waste stream treatment, as many industrial waste streams contain potassium and sulfate. The problem with these processes for SOP recovery is that they are inefficient and susceptible to contamination by sodium salts. Excess sodium entering the SOP crystallizer can result in relatively low SOP yields and make the process uneconomical. While the solubility of sodium sulfate is relatively independent of temperature, the solubility of SOP is highly temperature-dependent, so operating the system at lower temperatures can significantly improve yield and operability. Summary of the Invention

[0004] The present invention relates to a process for producing SOP during the treatment of ash generated in wood pulping operations. The process involves the production of glaserite (3KSO 4 ). 4.A glaserite slurry containing (Na2SO4) crystals is produced. The glaserite crystals are separated from the slurry. In one embodiment, the water or condensate is sent to a chiller, where the water or condensate is cooled to a temperature between 0°C and 25°C to form chilled water. The chilled water and glaserite crystals are sent to an SOP crystallizer, where the chilled water contacts the glaserite crystals. In the SOP crystallizer, sodium sulfate and potassium sulfate dissolve from the glaserite crystals into the chilled water. A large portion of the potassium sulfate then recrystallizes as SOP. Because the amount of SOP crystallized is inversely proportional to temperature, more SOP precipitates at lower temperatures. This produces a sodium sulfate-rich solution containing recrystallized SOP in the SOP crystallizer. The sodium sulfate-rich solution is sent to a solid-liquid separator, where the recrystallized SOP is separated from the sodium sulfate-rich solution. This SOP recovery process is less susceptible to yield loss due to sodium contamination, which may result from upstream operations or chemical instability. Additionally, this SOP recovery process tends to minimize or reduce process water requirements.

[0005] More specifically, in one embodiment, the present invention includes the following:

[0006] 1. A process for recovering potassium sulfate (SOP) from ash produced in a wood pulping operation, comprising: A. Recovering ash containing potassium, chlorides, sulfates carbonates and sodium from the recovery boiler of a wood pulp operation; B. Mixing the ash with an aqueous solution to form a dissolved ash solution; C. concentrating the dissolved ash solution in an evaporator to form a concentrated dissolved ash solution; D. passing the concentrated dissolved ash solution to a glaserite crystallizer to produce a glaserite slurry containing glaserite crystals; E. Separating glaserite crystals from the glaserite slurry; F. Sending water to the chiller, G. Cooling the water in the chiller to a temperature of 0℃ to 25℃ to form cooling water; H. Sending glaserite crystals and cold water to the SOP crystallizer; I. Dissolving glaserite crystals in cold water in an SOP crystallizer to produce a solution containing sodium sulfate and potassium sulfate; J. Recrystallizing potassium sulfate as SOP in an SOP crystallizer to produce a sodium sulfate-rich solution in an SOP crystallizer; and K. A process comprising: passing the sodium sulfate-rich solution containing the recrystallized SOP to a solid-liquid separator; and separating the recrystallized SOP from the sodium sulfate-rich solution.

[0007] In another embodiment, the solution contained within the SOP crystallizer is circulated through a chiller to maintain the temperature of the solution within the SOP crystallizer between -10°C and 25°C. In one embodiment, the solution within the SOP crystallizer is a mixture of water fed to the SOP crystallizer and brine produced during the dissolution of glaserite crystals in the SOP crystallizer. This solution is sometimes simply referred to as aqueous solution. In either case, the process for recovering SOP from the glaserite crystals remains the same: glaserite crystals are dissolved in the SOP crystallizer to produce a solution of sodium sulfate and potassium sulfate. Once the potassium sulfate dissolves, it crystallizes as SOP and is recovered from the sodium sulfate-rich solution through a solid-liquid separator.

[0008] Other objects and advantages of the present invention will become apparent and obvious from a consideration of the following description and accompanying drawings which are merely illustrative of the invention. [Brief explanation of the drawings]

[0009] [Figure 1] Figure 1 is a schematic diagram showing the basic SOP retrieval process.

[0010] [Figure 2] FIG. 2 is a schematic diagram showing the treatment process of ash from a recovery boiler of a wood pulping operation, illustrating the treatment process of ash to recover SOP.

[0011] [Figure 3] FIG. 3 is a schematic diagram similar to FIG. 2, but showing a slightly different process for retrieving SOPs. DETAILED DESCRIPTION OF THE INVENTION

[0012] With further reference to the drawings, and particularly to FIG. 1, ash from the recovery boiler is sent to tank 20, where it is dissolved in water. In some cases, all or substantially all of the ash from the recovery boiler is sent to tank 20. In other cases, only a portion of the ash from the recovery boiler is sent to tank 20. In either case, the ash sent to tank 20 is dissolved to form a dissolved ash solution. The dissolved ash solution is sent to an evaporator or series of evaporators 22. The evaporators 22 concentrate the dissolved ash solution to form a concentrated dissolved ash solution (purge stream) and a concentrate containing a mixture of sodium sulfate and burkeite (2Na2SO4.Na2CO3) crystals. The concentrated dissolved ash solution is typically relatively rich in chloride and potassium. The concentrate produced by evaporator 22 is sent to solid-liquid separator 24, which separates the sodium sulfate crystals from the burkeite and mother liquor. The burkeite and sodium sulfate are typically returned to the wood pulping operation. The mother liquor produced by solid-liquid separator 24 may be returned via line 26 to the first stage evaporator or evaporators 22 .

[0013] As shown in FIG. 1, the purge stream in the form of a concentrated ash solution is sent to glaserite crystallizer 28 through line 25. Upon entering crystallizer 28, the concentrated ash solution is cooled, preferably adiabatically. Adiabatic cooling refers to lowering the temperature of a system without removing heat from the system. One common method of adiabatic cooling is to reduce the pressure within the crystallizer. Because temperature and pressure in a closed system are directly proportional, a reduction in one reduces the other. In one embodiment, the adiabatic cooling process in glaserite crystallizer 28 occurs until the crystallizer temperature reaches approximately 50°C. In crystallizer 28, glaserite crystallizes through the adiabatic cooling process. This forms a concentrated glaserite slurry, which is sent from crystallizer 28 to solid-liquid separator 30. During the adiabatic cooling of the concentrated ash solution in stream 25, crystallizer 28 produces purge stream 32. Purge stream 32 contains a relatively rich chloride concentration. Purge stream 32, with its relatively rich chloride concentration, can be further processed or disposed of by conventional means. 1, a portion of concentrated purge stream 32 may be recycled to evaporator 22 via line 34. The amount of purge stream 32 sent from the plant or recycled to evaporator 22 may vary depending on the chloride concentration in stream 32 and the chloride concentration in the dissolved ash solution.

[0014] The glaserite slurry produced in the adiabatic cooling crystallizer 28 is sent to a solid-liquid separator 30. Various types of solid-liquid separators, such as filters, centrifuges, etc., can be used. In either case, the solid-liquid separator 30 separates the glaserite crystals from the glaserite slurry. The glaserite crystals are then sent to an SOP crystallizer 38. As explained in more detail below, the function of the SOP crystallizer is to recover SOP from the glaserite crystals. To accomplish this, a water source is routed to a chiller 50. The chiller 50 is used to cool water to a temperature ranging from 0°C to 25°C, thereby producing chilled water. The chilled water from the chiller 50 is sent to the SOP crystallizer 38, where the glaserite crystals come into contact with the chilled water. When the glaserite crystals come into contact with the chilled water in the SOP crystallizer, both the sodium sulfate and potassium sulfate in the glaserite crystals dissolve in the chilled water. After dissolution, the potassium sulfate recrystallizes as SOP in the SOP crystallizer. However, adding enough water to dissolve the sodium sulfate leaves excess SOP as crystals. What remains in the SOP crystallizer is sodium sulfate and SOP solution, including recrystallized SOP.

[0015] The recrystallized SOP is recovered through a process in which the sodium sulfate-rich solution is sent to a solid-liquid separator 40, where the recrystallized SOP is separated from the sodium sulfate-rich solution. As shown in Figure 1, the sodium sulfate solution will contain a large amount of potassium sulfate. This solution can be recycled to the glaserite crystallizer 28 via line 42.

[0016] Referring now to Figure 2, this illustrates another embodiment of the present invention that is similar in many respects to that shown in Figure 1 and described above, with some differences. Solid-liquid separator 30 in Figure 2 separates the glaserite slurry into glaserite crystals and a liquid recycle stream 36. In the embodiment illustrated in Figure 2, the liquid recycle stream is recycled to crystallizer 28.

[0017] Also shown in Figure 2, the glaserite crystallizer 28 produces condensate. The condensate is sent through a heat exchanger that cools the condensate. From the heat exchanger, the condensate is sent to the chiller 50, which further cools the condensate to about 0°C to 25°C, as described above, to form chilled water that is sent to the SOP crystallizer 38, as described above.

[0018] The embodiment of FIG. 3 is similar to the process shown in FIG. 2 and described above. However, in the process of FIG. 3, a chiller 50 is placed in the recirculation line around the SOP crystallizer 38. Therefore, the aqueous solution in the SOP crystallizer is circulated through the chiller 50. The chiller's function is to cool the solution in the SOP crystallizer 38 to a temperature range of −10° C. to 25° C. This temperature range is sufficient to dissolve the glaserite crystals in the sodium sulfate and potassium sulfate solution, allowing the dissolved potassium sulfate to be recrystallized as SOP in the SOP crystallizer. As explained above, this effectively leaves a sodium sulfate-rich solution in the SOP crystallizer 38 containing the recrystallized SOP. Typically, the solution in the SOP crystallizer 38 is a mixture of water or condensate and brine. In this case, condensate from the glaserite crystallizer is fed to the SOP crystallizer 38, while brine is the clarified effluent produced by the SOP crystallizer.

[0019] Several laboratory tests have verified the effectiveness of the SOP recovery process described above. Before describing the tests, it should be noted that glaserite crystals do not necessarily have the same ratio of sodium to potassium. In these laboratory tests, the glaserite used contained 73.8% K2SO4 by weight and 26.2% sodium sulfate by weight. The tests were conducted with enough water to dissolve substantially all of the glaserite. However, in commercial operations, it is possible to choose to add enough water to reduce the sodium concentration to less than 2% of the product SOP in order to have a reasonable reaction time. In the first test, 100 g of glaserite crystals was contacted with 492 g of water at 50°C. All of the sodium sulfate dissolved, and 71.1 g of potassium sulfate also dissolved. This left 2.7 g of potassium sulfate, for a yield of 2.7%. In the second test, 100 g of glaserite crystals was contacted with 490 g of water at 35°C. This dissolved all of the sodium sulfate and 60.4 g of potassium sulfate. This leaves 13.4g of potassium sulfate, a yield of 13.4%. In a final test, 100g of glaserite was contacted with 504g of water at 5°C. This dissolved all of the sodium sulfate and also dissolved 42.2g of potassium sulfate, leaving 31.6g of potassium sulfate, a yield of 31.6%.

[0020] Of course, the present invention may be practiced otherwise than as specifically described herein without departing from its essential characteristics. The present embodiments are to be considered in all respects as illustrative and not restrictive, and all changes that come within the meaning and range of equivalency of the appended claims are intended to be embraced therein.

Claims

1. 1. A process for recovering sulfate of potassium (SOP) from ash produced in a wood pulping operation, comprising: recovering ash containing potassium, chloride, sulfate carbonate, and sodium from a recovery boiler of said wood pulping operation; mixing the ash with an aqueous solution to form a dissolved ash solution; concentrating the dissolved ash solution in an evaporator to form a concentrated dissolved ash solution; passing the concentrated dissolved ash solution to a glaserite crystallizer to produce a glaserite slurry containing glaserite crystals; separating the glaserite crystals from the glaserite slurry; Sending water to the chiller, cooling the water in the chiller to a temperature of between 0°C and 25°C to form chilled water; passing the cold water and glaserite crystals to an SOP crystallizer and contacting the glaserite crystals with the cold water; dissolving the glaserite crystals in the cold water in the SOP crystallizer to obtain a solution containing sodium sulfate and potassium sulfate; recrystallizing a portion of the potassium sulfate as SOP in the SOP crystallizer to obtain a sodium sulfate-rich solution in the SOP crystallizer; and passing the sodium sulfate rich solution containing the recrystallized SOP to a solid-liquid separator and separating the recrystallized SOP from the sodium sulfate rich solution.

2. 10. The process of claim 1, wherein the water sent to the chiller originates as condensate produced by the glaserite crystallizer.

3. 3. The process of claim 2, wherein the condensate is sent through a heat exchanger before being sent to a chiller.

4. 10. The process of claim 1, further comprising recycling the sodium sulfate rich solution to the glaserite crystallizer.

5. 1. A process for recovering sulfate of potassium (SOP) from ash produced in a wood pulping operation, comprising: recovering ash containing potassium, chloride, sulfate carbonate, and sodium from a recovery boiler of said wood pulping operation; mixing the ash with an aqueous solution to form a dissolved ash solution; concentrating the dissolved ash solution in an evaporator to form a concentrated dissolved ash solution; passing the concentrated dissolved ash solution to a glaserite crystallizer to produce a glaserite slurry containing glaserite crystals; separating the glaserite crystals from the glaserite slurry; passing said glaserite crystals to an SOP crystallizer containing an aqueous solution; cooling the aqueous solution in the SOP crystallizer by passing the aqueous solution through a chiller to produce a cooled aqueous solution which is returned to the SOP crystallizer; cooling the aqueous solution in the chiller to a temperature sufficient to maintain the temperature of the aqueous solution in the SOP crystallizer at −10° C. to 25° C.; dissolving the glaserite crystals in the cooled aqueous solution in the SOP crystallizer to obtain an aqueous solution containing sodium sulfate and potassium sulfate; recrystallizing a portion of the potassium sulfate as SOP in the SOP crystallizer to obtain a sodium sulfate-rich aqueous solution in the SOP crystallizer; and passing the aqueous solution enriched in sodium sulfate and containing the recrystallized SOP to a solid-liquid separator to separate the recrystallized SOP from the aqueous solution enriched in sodium sulfate.

6. 6. The process of claim 5, further comprising recycling the aqueous solution rich in sodium sulfate to the glaserite crystallizer.

7. 6. The process of claim 5, comprising forming the aqueous solution in the SOP crystallizer by mixing water with brine produced by the SOP crystallizer.

8. 10. The process of claim 1, wherein a portion of the potassium sulfate remains dissolved after the portion of the potassium sulfate recrystallizes.

9. 6. The process of claim 5, wherein a portion of the potassium sulfate remains dissolved after the portion of the potassium sulfate recrystallizes.

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