Potassium nitrate production method and system for treating waste discharged from cement plant

Abstract

Provided is a potassium nitrate production method that makes it possible to effectively utilize waste which has been discharged from a cement plant, so as to obtain potassium nitrate easily and highly efficiently, said method comprising: a step for extracting potassium chloride from chlorine bypass dust discharged from the cement plant by bringing the chlorine bypass dust into contact with a saturated aqueous solution of potassium chloride at room temperature; a step for bringing the potassium chloride obtained in the step for extracting the potassium chloride into contact with nitric acid obtained from nitrogen oxides contained in combustion exhaust gas discharged from the cement plant; and a step for obtaining potassium nitrate by heating, at 75-175°C, a fluid obtained in the contacting step. Also provided is a system for treating waste which has been discharged from the cement plant.

Description

Potassium nitrate manufacturing method and cement factory waste treatment system

[0001] The present invention relates to a method for producing potassium nitrate and a system for treating waste discharged from a cement factory.

[0002] In cement manufacturing, combustion exhaust gas generated by burning coal, heavy oil, and recycled fuel is used to dry the raw materials, and then discharged outside the system after passing through a dust collection process. The components of combustion exhaust gas include carbon dioxide and trace amounts of chlorides, as well as a large amount of nitrogen oxides (hereinafter referred to as NOx) resulting from fuel combustion, etc., and nitrogen dioxide (hereinafter referred to as NO) contained in NOx is ２ These emissions (sometimes written as "NOx") are causing environmental impacts such as acid rain and adversely affecting the human body. In light of these issues, cement plants are making ongoing efforts to reduce NOx emissions, for example by selecting fuels with low nitrogen content and removing NOx using denitrifiers.

[0003] Currently, catalytic reduction (SNCR) methods using urea or alcohol are used as NOx removal technologies in cement plants (see, for example, Patent Document 1). Furthermore, it is known that excess chlorides generated during the burning of recycled fuels at cement plants have adverse effects on cement properties. Therefore, cement manufacturers are working to control cement quality by recovering chlorides as chlorine bypass dust using cooled probes. The generated chlorine bypass dust is washed with water and reused as a cement raw material. Meanwhile, the water after washing is adjusted to a composition that complies with municipal wastewater standards and is sufficiently diluted before being discharged, but it still contains a large amount of potassium chloride. Furthermore, a method for producing potassium chloride salt from chlorine bypass dust has been proposed (see, for example, Patent Document 2).

[0004] JP 2009-189989 A JP 2019-26521 A

[0005] In the SNCR method described above, the optimum temperature for the denitration reaction is 800°C or higher, and operation in an extremely high-temperature environment is required, so it is difficult to say that it is a simple method. In addition, it can be assumed that the hot gas that has passed through the urea spray area is not denitrated, and that approximately 400 ppm of NOx is emitted into the atmosphere. Therefore, denitration in cement plants can only be performed within a limited temperature range, and the recovered NOx ２ It is difficult to reuse it effectively.

[0006] As described above, the water obtained after washing the chlorine bypass dust contains a large amount of potassium chloride. However, since the water also contains heavy metals, it cannot be used for food, and since the water also contains chlorine, its use as a fertilizer is limited. In addition, in the above-described method for producing potassium chloride salt, flue gas desulfurization effluent is added to the chlorine bypass dust to obtain a slurry, and then the slurry is subjected to solid-liquid separation to obtain a filtrate containing potassium chloride and selenium. However, a reducing agent (heavy metal remover) must be added to remove selenium from the filtrate, which increases the number of production steps.

[0007] For these reasons, NOx and chlorine bypass dust emitted from cement plants are not currently being used effectively.

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for producing potassium nitrate that can effectively utilize waste discharged from cement factories and can produce potassium nitrate highly efficiently and easily, and a system for treating waste discharged from cement factories.

[0009] As a result of extensive research into solving the above problems, the present inventors have found that the above problems can be solved by the following invention.

[0010] That is, the present invention relates to the following: [1] A method for producing potassium nitrate, comprising: a step of contacting chlorine bypass dust discharged from a cement plant with a saturated aqueous potassium chloride solution at room temperature to extract potassium chloride from the chlorine bypass dust; a step of contacting the potassium chloride obtained in the potassium chloride extraction step with nitric acid obtained from nitrogen oxides contained in combustion exhaust gas discharged from the cement plant; and a step of heating the fluid obtained in the contacting step at 75°C or higher and 175°C or lower to obtain potassium nitrate. [2] The method for producing potassium nitrate according to [1] above, wherein the chlorine bypass dust is contacted with the saturated aqueous potassium chloride solution by reflux. [3] The method for producing potassium nitrate according to [1] above, wherein the fluid obtained by contacting the chlorine bypass dust with the saturated aqueous potassium chloride solution is subjected to a first solid-liquid separation at 80°C or higher and 150°C or lower. [4] The method for producing potassium nitrate according to [3], wherein the saturated aqueous potassium chloride solution recovered by the first solid-liquid separation is cooled to room temperature and then subjected to a second solid-liquid separation to obtain potassium chloride as a solid content. [5] The method for producing potassium nitrate according to [4], wherein the potassium chloride obtained by the second solid-liquid separation is used in the contacting step. [6] The method for producing potassium nitrate according to [4], wherein the saturated aqueous potassium chloride solution recovered by the second solid-liquid separation is reused for contacting with the chlorine bypass dust. [7] The method for producing potassium nitrate according to any of [1] to [6], wherein the nitric acid content is more than 1.0 molar equivalent and not more than 2.0 molar equivalents relative to 1.0 molar equivalent of the potassium chloride. [8] The method for producing potassium nitrate according to any of [1] to [7], wherein the nitric acid concentration is 10% by mass or more and 60% by mass or less. [9] The method for producing potassium nitrate according to any of [1] to [8], wherein the step of obtaining potassium nitrate further comprises heating to dryness at 100°C or more.

[10] The method for producing potassium nitrate according to [9], wherein nitric oxide and chlorine produced by the heating and drying are recovered.

[11] A treatment system for waste discharged from a cement factory, comprising: a chlorine bypass dust receiving facility that receives chlorine bypass dust discharged from a cement factory; a combustion exhaust gas receiving facility that receives combustion exhaust gas discharged from the cement factory; a potassium chloride extraction facility that brings the chlorine bypass dust into contact with a saturated aqueous potassium chloride solution at room temperature to extract potassium chloride from the chlorine bypass dust; a facility that brings the potassium chloride obtained in the potassium chloride extraction facility into contact with nitric acid obtained from nitrogen oxides contained in the combustion exhaust gas; and a potassium nitrate production facility that heats the fluid obtained in the contacting facility at a temperature of 75°C or higher and 175°C or lower to obtain potassium nitrate.

[0011] According to the present invention, it is possible to provide a method for producing potassium nitrate that can effectively utilize waste discharged from cement factories and easily obtain potassium nitrate, and a system for treating waste discharged from cement factories.

[0012] 1 is a schematic diagram showing an example of a method for producing potassium nitrate according to the present embodiment; 2 is a diagram showing an example of a treatment system for waste discharged from a cement factory according to the present embodiment; 3 is a schematic diagram showing a potassium chloride extraction device used in Reference Example 1;

[0013] Hereinafter, an embodiment of the present invention (hereinafter, sometimes referred to as "the present embodiment") will be described. The present invention is not limited to the following embodiment, and can be implemented with any modifications within the scope that does not impair the effects of the invention. Note that the notation of a numerical range as "AA to BB" in this specification means "at least AA and at most BB." Furthermore, in this specification, the numerical values ​​associated with "at least," "at most," and "to" in describing a numerical range are values ​​that can be arbitrarily combined. For example, when a certain numerical range is described as "CC to DD" and "EE to FF," the numerical ranges "CC to FF" and "EE to DD" are also included.

[0014] [Method for Producing Potassium Nitrate] The method for producing potassium nitrate of the present embodiment includes: a step of bringing chlorine bypass dust discharged from a cement factory into contact with a saturated aqueous solution of potassium chloride at room temperature to extract potassium chloride from the chlorine bypass dust; a step of bringing the potassium chloride obtained in the potassium chloride extraction step into contact with nitric acid obtained from nitrogen oxides contained in combustion exhaust gas discharged from the cement factory; and a step of heating the fluid obtained in the contacting step at a temperature of 75°C or higher and 175°C or lower to obtain potassium nitrate.

[0015] In the method for producing potassium nitrate of the present embodiment, chlorine bypass dust and combustion exhaust gas are used as waste materials discharged from a cement factory, potassium chloride contained in the chlorine bypass dust is brought into contact with nitric acid obtained from nitrogen oxides contained in the combustion exhaust gas, and the resulting fluid is heated within a temperature range of 75°C or higher and 175°C or lower. This simple method makes it possible to obtain potassium nitrate with high purity and high yield.

[0016] Fig. 1 is a schematic diagram showing an example of a method for producing potassium nitrate according to the present embodiment. As shown in Fig. 1, the method for producing potassium nitrate according to the present embodiment includes a step of extracting potassium chloride, a step of contacting potassium chloride with nitric acid, and a step of obtaining potassium nitrate.

[0017] [Step of Extracting Potassium Chloride] This step involves bringing chlorine bypass dust discharged from a cement plant into contact with a saturated aqueous solution of potassium chloride at room temperature to extract potassium chloride from the chlorine bypass dust. In this specification, "chlorine bypass dust" refers to particulate matter with a high chlorine content contained in exhaust gas extracted from a probe located at the end of a cement kiln. Chlorine bypass dust includes oxides of metals such as calcium, potassium, and sodium, as well as halides such as chlorides.

[0018] The proportions of substances contained in chlorine bypass dust cannot be generalized because they vary depending on the cement plant, etc.; however, for example, calcium oxide is typically 15 to 45 mass%, preferably 20 to 40 mass%, and more preferably 25 to 35 mass%. Potassium oxide is typically 5 to 40 mass%, preferably 20 to 40 mass%, and more preferably 25 to 40 mass%. Sodium oxide is typically 0 to 40 mass%, preferably 0.5 to 40 mass%, and more preferably 1 to 40 mass%. Chlorides (e.g., calcium chloride, potassium chloride, sodium chloride, etc.) are typically 10 to 30 mass%, preferably 15 to 28 mass%, and more preferably 18 to 25 mass%. Other substances (e.g., metal oxides other than calcium, potassium, and sodium) are typically 10 to 25 mass%, preferably 12 to 22 mass%, and more preferably 15 to 20 mass%. The proportions of substances contained in the chlorine bypass dust can be measured, for example, using an energy dispersive X-ray fluorescence (XRF) analyzer. Specifically, it can be measured by the method described in the Examples.

[0019] The contact of the chlorine bypass dust with the saturated aqueous potassium chloride solution at room temperature (25° C.) is not particularly limited, but is preferably carried out by reflux from the viewpoint of further increasing the purity and yield of the extracted potassium chloride. The reflux is carried out, for example, by a method in which a contact tower into which the chlorine bypass dust and the saturated aqueous potassium chloride solution have been introduced is heated, and a part of the generated gas is cooled in a cooler from the top of the tower and returned to the contact tower.

[0020] From the viewpoint of further increasing the solubility of potassium chloride, the liquid temperature during reflux is preferably 100° C. or higher, more preferably 110° C. or higher, and even more preferably 120° C. or higher. There is no particular upper limit, but it is 350° C. or lower. The reflux time is preferably 0.1 to 5 hours, more preferably 0.3 to 2 hours, and even more preferably 0.4 to 1 hour.

[0021] For heating during reflux, it is preferable to use combustion exhaust gas discharged from a cement factory as a heat source, from the viewpoint of further demonstrating the effects of the present invention (see FIG. 1). The temperature of the combustion exhaust gas cannot be generalized because it varies depending on the source of the exhaust gas used, but it is usually 30°C or higher and 250°C or lower, preferably 50°C or higher and 200°C or lower, more preferably 70°C or higher and 180°C or lower, and even more preferably 80°C or higher and 150°C or lower. When the temperature of the combustion exhaust gas is within the above range, it is easy to adjust the liquid temperature to the desired temperature range.

[0022] From the viewpoint of further increasing the solubility of potassium chloride in the fluid obtained by contacting the chlorine bypass dust with the saturated aqueous potassium chloride solution and increasing the amount of potassium chloride extracted, it is preferable to perform the first solid-liquid separation at a temperature of 80° C. or higher and 150° C. or lower. From the above viewpoint, the temperature of the fluid when performing the first solid-liquid separation is preferably 90° C. or higher and 130° C. or lower, more preferably 90° C. or higher and 110° C. or lower.

[0023] The first solid-liquid separation method is not particularly limited, and examples thereof include a method using a pressure separation device that separates solids and liquids by applying pressure or squeezing using a filter, a method using a centrifugal separator that separates solids and liquids by the action of centrifugal force, and a method using a sedimentation separation device that allows the solids to settle by leaving the mixture to stand. Among these, the method using a centrifugal separator is preferred because it is necessary to separate the mixed liquid (fluid) at a high temperature in a short time.

[0024] The solid content (for example, potassium chloride) recovered by the first solid-liquid separation may be desalted and used in a cement factory.

[0025] Furthermore, it is preferable to cool the saturated aqueous potassium chloride solution recovered as a liquid fraction by the first solid-liquid separation to room temperature (25°C) and then perform a second solid-liquid separation, from the viewpoint of further increasing the purity and yield of the extracted potassium chloride. The higher the purity and yield of the potassium chloride, the higher the purity and yield of potassium nitrate obtained in the step of obtaining potassium nitrate, which will be described later. The second solid-liquid separation can obtain potassium chloride as a solid fraction. This potassium chloride is used in the step of contacting potassium chloride with nitric acid, which will be described later, as shown in Figure 1. Examples of methods for the second solid-liquid separation include the methods described above for the first solid-liquid separation method. Among these, a method using a sedimentation separation device is preferred, as the particle size of the potassium chloride produced can become very small depending on the conditions.

[0026] As shown in FIG. 1, the saturated aqueous potassium chloride solution recovered as a liquid by the second solid-liquid separation is preferably reused for contact with the chlorine bypass dust.

[0027] [Step of contacting potassium chloride with nitric acid] This step is a step of contacting the potassium chloride obtained in the potassium chloride extraction step with nitric acid obtained from nitrogen oxides contained in combustion exhaust gas discharged from a cement factory. By contacting the potassium chloride with the nitric acid, potassium nitrate is produced, and a fluid containing the potassium nitrate is obtained. The temperature during the contact cannot be generally specified because it varies depending on the concentration of nitric acid, but is usually preferably less than 75°C, more preferably 70°C or less, and even more preferably 60°C or less. There is no particular lower limit, but it is preferably 20°C or more.

[0028] The method for producing the nitric acid is not particularly limited, but for example, a wet recovery method can be mentioned in which nitrogen oxides (NOx) contained in combustion exhaust gas are absorbed into water using a gas purification device described in JP 2017-51899 A to obtain nitric acid.

[0029] Examples of a method for contacting the potassium chloride obtained in the potassium chloride extraction step with the nitric acid include a method in which the potassium chloride and the nitric acid are introduced into a reaction device capable of mixing the potassium chloride and the nitric acid, and then mixed.

[0030] The reaction equipment is not particularly limited, but a stirring equipment is preferred from the viewpoint of more efficiently proceeding the reaction and obtaining potassium nitrate as a product. The stirring equipment is not particularly limited, but examples thereof include a Henschel mixer, a mixing shaker, a tumbler mixer, a V-type mixer, a double-cone type mixer, a ribbon type mixer, a Nauta mixer, and a Super Mixer. Among these, a Henschel mixer is preferred from the viewpoint of more efficiently proceeding with the reaction between the potassium chloride and the nitric acid.

[0031] The content of the nitric acid is preferably more than 1.0 molar equivalent and not more than 2.0 molar equivalents, more preferably more than 1.1 molar equivalents and not more than 1.8 molar equivalents, and even more preferably more than 1.2 molar equivalents and not more than 1.6 molar equivalents, relative to 1.0 molar equivalent of the potassium chloride. When the content of the nitric acid is within the above range, potassium nitrate can be obtained with higher purity and higher yield.

[0032] The concentration of the nitric acid is preferably 10% by mass or more and 60% by mass or less, more preferably 15% by mass or more and 50% by mass or less, even more preferably 15% by mass or more and 45% by mass or less, and even more preferably 18% by mass or more and 42% by mass or less. When the concentration of the nitric acid is within the above range, potassium nitrate can be obtained with higher purity and higher yield.

[0033] [Step of Obtaining Potassium Nitrate] This step is a step of obtaining potassium nitrate by heating the fluid obtained in the contacting step at 75°C or higher and 175°C or lower. When the heating temperature for heating the fluid is within the above range, potassium nitrate can be obtained with high purity and high yield. From this viewpoint, the heating temperature is preferably 75°C or higher and 170°C or lower, more preferably 80°C or higher and 160°C or lower, and even more preferably 80°C or higher and 150°C or lower.

[0034] From the viewpoint of further demonstrating the effects of the present invention, it is preferable to use, as a heat source for the heating, combustion exhaust gas discharged from a cement factory, as shown in Fig. 1. The temperature of the combustion exhaust gas is as described in the step of extracting potassium chloride.

[0035] The reaction equipment used when heating the fluid is not particularly limited, but from the viewpoint of more efficiently proceeding with the reaction, a stirring equipment is preferred. As the stirring equipment, the stirring equipment exemplified as the stirring equipment that can be used in the step of contacting potassium chloride with nitric acid, among which a Henschel mixer is preferred, can be mentioned. As the stirring equipment, the stirring equipment that can be used in the step of contacting potassium chloride with nitric acid can be used as it is, or a different stirring equipment can be used.

[0036] For example, it is preferable to heat the fluid from the temperature in the contacting step to the heating temperature (hereinafter also referred to as "temperature after heating") and then stir the mixture. From the viewpoint of obtaining potassium nitrate with higher purity and higher yield, it is preferable that the temperature after heating is not lower than 75°C. The stirring time is preferably 0.1 to 5 hours, more preferably 0.1 to 2 hours, and even more preferably 0.2 to 1 hour.

[0037] This step preferably further comprises heating to dryness at 100°C or higher, from the viewpoint of further increasing the purity and yield of the potassium nitrate obtained. Furthermore, a portion of the nitric acid reacts with chloride ions in the solution to produce nitrosyl chloride (NOCl), which is difficult to separate from the solution. On the other hand, the nitrosyl chloride decomposes at 100°C or higher to produce nitric oxide (NO) and chlorine (Cl). ２ Therefore, by carrying out the heating and drying process at 100° C. or higher, the produced nitrosyl chloride is decomposed and can be recovered as nitric oxide and chlorine.

[0038] The generated gas containing nitric oxide and chlorine is subjected to removal of moisture using, for example, a heat exchanger, and then nitric oxide is adsorbed and recovered using zeolite (e.g., N114), after which chlorine gas is appropriately recovered. The moisture removed by the heat exchanger may be reused in the above reaction.

[0039] The recovered nitrogen monoxide can be used to produce nitric acid, as shown in Figure 1. The chlorine can also be used as a raw material for vinyl chloride, bleaching powder, pickling agents, pH adjusters, etc.

[0040] The heating to dryness is preferably carried out while ventilating. The heating temperature is preferably 110°C or higher and 160°C or lower, more preferably 115°C or higher and 155°C or lower, and even more preferably 120°C or higher and 150°C or lower. The heating time is preferably 0.1 to 5 hours, more preferably 0.2 to 2 hours, and even more preferably 0.4 to 1 hour. From the viewpoint of further demonstrating the effects of the present invention, it is preferable to use combustion exhaust gas discharged from a cement factory as a heat source for the heating (see FIG. 1). The temperature of the combustion exhaust gas is as described in the potassium chloride extraction step. According to the production method of this embodiment, combustion exhaust gas discharged from a cement factory can be used as a heat source in the potassium chloride extraction step, the contacting of potassium chloride with nitric acid, and the potassium nitrate extraction step, and as a NOx source for producing nitric acid.

[0041] The potassium nitrate thus obtained is recovered. Since the potassium nitrate may adhere to the wall surface of the reaction equipment, it may be recovered by appropriately spraying water on it and drying it. Alternatively, the reaction solution obtained by the above reaction may be naturally cooled, and the precipitated potassium nitrate may be recovered by solid-liquid separation and dried. Since the purity of the obtained potassium nitrate depends on the purity of potassium chloride, it is preferable to recrystallize the potassium nitrate as needed.

[0042] The purity of the potassium nitrate is preferably 80% or more, more preferably 85% or more, and even more preferably 95% or more. The yield of the potassium nitrate is preferably 75% or more, more preferably 85% or more, and even more preferably 95% or more.

[0043] [System for treating waste discharged from a cement factory] A system for treating waste discharged from a cement factory of this embodiment includes: a chlorine bypass dust receiving facility that receives chlorine bypass dust discharged from the cement factory; a combustion exhaust gas receiving facility that receives combustion exhaust gas discharged from the cement factory; a potassium chloride extraction facility that brings the chlorine bypass dust into contact with a saturated aqueous potassium chloride solution at room temperature to extract potassium chloride from the chlorine bypass dust; a facility that brings the potassium chloride obtained in the potassium chloride extraction facility into contact with nitric acid obtained from nitrogen oxides contained in the combustion exhaust gas; and a potassium nitrate production facility that heats the fluid obtained in the contacting facility at a temperature of 75°C or higher and 175°C or lower to obtain potassium nitrate.

[0044] Fig. 2 is a diagram showing an example of a treatment system for waste discharged from a cement factory according to this embodiment. As shown in Fig. 2, the treatment system 100 for waste discharged from a cement factory according to this embodiment includes a chlorine bypass dust receiving facility 10, a combustion exhaust gas receiving facility 20, a potassium chloride extraction facility 30, a facility 40 for contacting potassium chloride with nitric acid, and a potassium nitrate production facility 50. Between the combustion exhaust gas receiving facility 20 and the facility 40 for contacting potassium chloride with nitric acid, a nitric acid production facility 21 for obtaining nitric acid from nitrogen oxides contained in the combustion exhaust gas may be provided.

[0045] The chlorine bypass dust is supplied to a chlorine bypass dust receiving facility 10 from a probe located at the end of the cement kiln (not shown). The chlorine bypass dust supplied to the chlorine bypass dust receiving facility 10 is supplied to a potassium chloride extraction facility 30 through a supply line (1), where it is brought into contact with a saturated aqueous solution of potassium chloride at room temperature supplied from another line, thereby extracting potassium chloride (solid). The extracted potassium chloride is supplied to a facility 40 for contacting potassium chloride with nitric acid through a supply line (2).

[0046] From the viewpoint of further exerting the effects of the present invention, the potassium chloride extraction equipment 30 preferably includes a contact tank 31 for bringing the chlorine bypass dust into contact with the saturated aqueous potassium chloride solution, a first solid-liquid separation device 32 for performing a first solid-liquid separation of the fluid obtained in the contact tank 31 within a desired temperature range, a cooling tank 33 for cooling the saturated aqueous potassium chloride solution recovered by the first solid-liquid separation to room temperature, and a second solid-liquid separation device 34 for performing a second solid-liquid separation of the saturated aqueous potassium chloride solution cooled to room temperature (see FIG. 2 ).

[0047] The combustion exhaust gas is supplied from a cement plant to a combustion exhaust gas receiving facility 20 through a supply line (not shown). The combustion exhaust gas supplied to the combustion exhaust gas receiving facility 20 is then supplied through a supply line (3) to a nitric acid production facility 21, where nitric acid is produced from the nitrogen oxides contained in the combustion exhaust gas. The produced nitric acid is supplied through a supply line (4) to a facility 40 for contacting potassium chloride with nitric acid, where potassium nitrate is produced by contact with the potassium chloride, and a fluid containing the potassium nitrate is obtained. The fluid obtained is supplied through a supply line (5) to a potassium nitrate production facility 50, where it is heated at a temperature of 75°C to 175°C, thereby producing potassium nitrate as a solid. Note that in FIG. 2, the facility 40 for contacting potassium chloride with nitric acid and the potassium nitrate production facility 50 are separate facilities; however, the facility 40 for contacting potassium chloride with nitric acid and the potassium nitrate production facility 50 may be the same facility.

[0048] The reactions, raw materials, devices (equipment), etc. used in the potassium chloride extraction facility 30, nitric acid production facility 21, facility 40 for contacting potassium chloride with nitric acid, and potassium nitrate production facility 50 are the same as those described in the potassium nitrate production method. In addition, in the potassium chloride extraction facility 30, facility 40 for contacting potassium chloride with nitric acid, and potassium nitrate production facility 50, the combustion exhaust gas from the combustion exhaust gas receiving facility 20 may be used as a heat source for heating.

[0049] The present invention will now be described in detail with reference to examples, but the present invention is not limited to these examples in any way.

[0050] A laboratory-scale test simulating the present invention was carried out as follows. The raw materials used are as follows: [Chlorine bypass dust] Chlorine bypass dust recovered from a cement factory: calcium oxide (CaO): 31.5 mass%, potassium oxide (K ２ O): 28.4% by mass, sodium oxide (Na ２ O): 1.0%, chloride: 20.5 mass% (including chlorides such as calcium chloride), other substances: 18.6 mass%. The proportions of substances contained in the chlorine bypass dust were measured using an energy dispersive X-ray fluorescence (XRF) analyzer (Epsilon3 XLE, manufactured by Malvern PANalytical). [Nitric acid] Nitric acid was obtained by absorbing NOx contained in exhaust gas (carbon dioxide concentration: 10 to 30 mass%, water concentration: 5 to 20 mass%, NOx concentration: 50 to 800 mass ppm) generated at a cement factory into water using a gas purification device (MKN, manufactured by Pollution Prevention Equipment Laboratory Co., Ltd.). Nitric acid concentration: 60 to 61 mass%.

[0051] (Reference Example 1) The following operation was performed using the potassium chloride extraction apparatus shown in Figure 3. An excess amount of potassium chloride (Kanto Chemical Co., Ltd., 32326-00) was added to ultrapure water and mixed thoroughly. Because cooling occurred due to endothermic heat immediately after mixing, the mixture was allowed to return to room temperature (25°C) before being mixed again and left to stand for at least one day. 80 mL of the supernatant of the resulting saturated potassium chloride solution was collected and placed in a round-bottom flask equipped with a Liebig tube as shown in Figure 3, to which 20 g of chlorine bypass dust was added and stirred. The saturated potassium chloride solution containing chlorine bypass dust was heated to 120°C using a mantle heater and refluxed for 30 minutes. After this, the solution was placed in a centrifuge (Kokusan Co., Ltd., H-112 model) equipped with a filter cloth (Kokusan Co., Ltd., TR84385) at a high temperature and subjected to solid-liquid separation (first solid-liquid separation) at a peripheral speed of 800 m / s while maintaining the temperature at 80°C or higher. The separated dehydrated cake (solids) can be used as a cement raw material by desalination. The treated water used in the desalination treatment can be returned to the round-bottom flask and reused as a saturated aqueous potassium chloride solution. The filtrate obtained by the first solid-liquid separation was left to stand for 8 hours or more and returned to room temperature (25°C), causing potassium chloride crystals to precipitate. These crystals were recovered by solid-liquid separation (second solid-liquid separation) and then dried in a dryer at 105°C for 1 day or more. The dried potassium chloride (crystals) were analyzed using an X-ray diffractometer (manufactured by PANalytical, trade name: X'Pert Pro) and found to have a purity of 99% and a yield of 6.4 g (86 mmol).

[0052] The amounts of water, potassium chloride (KCl), and sodium chloride (NaCl) in the saturated aqueous potassium chloride solution before the first and second solid-liquid separations, the amounts of filtrate, crystals, and deposits on the equipment after the first and second solid-liquid separations, and the amounts of change are shown in Table 1. The amounts of change in Table 1 are (total amount after the first and second solid-liquid separations) - (total amount before the first and second solid-liquid separations) for each component in the saturated aqueous potassium chloride solution.

[0053]

[0054] From Table 1, it can be seen that although there are losses of each component in the saturated potassium chloride aqueous solution due to wall adhesion 1 and 2, there is almost no need to consider these losses during continuous operation of an actual plant. Furthermore, since the potassium chloride in the saturated potassium chloride aqueous solution (filtrate) reaches equilibrium with the potassium chloride in the raw material when the filtrate is recycled, it is presumed that a certain amount of potassium chloride is produced during continuous operation.

[0055] Reference Example 2 The filtrate (saturated aqueous potassium chloride solution) recovered by the second solid-liquid separation in Reference Example 1 was returned to the round-bottom flask for reuse, and the same operation as in Reference Example 1 was carried out to obtain potassium chloride crystals. This operation was repeated three times. The obtained potassium chloride crystals were analyzed using an energy dispersive X-ray fluorescence analyzer (Epsilon3 XLE, manufactured by Malvern PANalytical). The results are shown in Table 2.

[0056]

[0057] Table 2 shows that when the saturated aqueous potassium chloride solution (filtrate) recovered by the second solid-liquid separation was recycled, high-purity potassium chloride with few impurities was obtained in all of the first, second, and third runs.

[0058] Example 1 6.4 g of potassium chloride crystals (purity: 99%) obtained in the same manner as in Reference Example 1 and 24.3 mL of nitric acid diluted to a concentration of 20% by mass (1.0 molar equivalent of nitric acid per 1.0 molar equivalent of potassium chloride) were placed in a round-bottom flask and thoroughly stirred at room temperature (25°C) to dissolve. The mixture was then heated to 70°C while aerating at 0.5 m / s and stirred for 1 hour, then further heated to 120°C and heated to dryness for 0.5 hours. After heating to dryness, the solid matter obtained by further heating was recovered and dried in a dryer at 105°C for at least one day. Meanwhile, the gas in the round-bottom flask was subjected to water vapor removal using a heat exchanger, and nitric oxide was recovered using zeolite (N114). Analysis using a detector tube gas analyzer (Gastec Corporation, detector tube: 8HH) revealed that approximately 3-5% chlorine gas was constantly detected. The dried solid was analyzed using an X-ray diffractometer (manufactured by PANalytical, trade name: X'Pert Pro), and it was found that potassium nitrate (8.2 g, yield: 79%) had a purity of 83%. The results are shown in Table 3.

[0059] (Examples 2 to 4) Potassium nitrate was obtained in the same manner as in Example 1, except that the amount (molar equivalent) of nitric acid to 1.0 molar equivalent of potassium chloride and the heating temperature [temperature after heating (°C)] were changed to the values ​​shown in Table 3. The results are shown in Table 3.

[0060] Example 5: 6.4 g of potassium chloride crystals (purity: 99%) obtained in the same manner as in Reference Example 1 and 24.3 mL of nitric acid diluted to a concentration of 20% by mass (1.0 molar equivalent of nitric acid per 1.0 molar equivalent of potassium chloride) were placed in a round-bottom flask and thoroughly stirred at room temperature (25°C) to dissolve. The mixture was then heated to 110°C while aerating at 0.5 m / s, stirred for 0.2 hours, and then heated to dryness at 110°C for 1 hour to obtain a solid. Potassium nitrate was obtained in the same manner as in Example 1. The results are shown in Table 3.

[0061] (Examples 6 to 16 and Comparative Examples 1 and 2) Potassium nitrate was obtained in the same manner as in Example 5, except that the concentration of nitric acid (% by mass), the amount of nitric acid (molar equivalent) mixed relative to 1.0 molar equivalent of potassium chloride, and the heating temperature [temperature after heating (°C) and heating to dryness (°C)] were changed to the values ​​shown in Table 3. The results are shown in Table 3.

[0062]

[0063] Table 3 shows that the potassium nitrate obtained by the production method of this embodiment is all highly pure and in high yield (Examples 1 to 16).

[0064] The method for producing potassium nitrate according to the present embodiment effectively utilizes waste materials discharged from cement factories to produce potassium nitrate with high efficiency and ease. The potassium nitrate thus obtained is suitable for use as compost for agricultural cultivation, microalgae cultivation, and the like, and as explosives.

[0065] 100: Treatment system for waste discharged from cement factories 10: Chlorine bypass dust receiving equipment 20: Combustion exhaust gas receiving equipment 21: Nitric acid production equipment 30: Potassium chloride extraction equipment 31: Contact tank 32: First solid-liquid separation equipment 33: Cooling tank 34: Second solid-liquid separation equipment 40: Equipment for contacting potassium chloride with nitric acid 50: Potassium nitrate production equipment 1: Supply line (1) 2: Supply line (2) 3: Supply line (3) 4: Supply line (4) 5: Supply line (5)

Claims

1. A method for producing potassium nitrate, comprising the steps of: contacting chlorine bypass dust discharged from a cement plant with a saturated aqueous solution of potassium chloride at room temperature to extract potassium chloride from the chlorine bypass dust; contacting the potassium chloride obtained in the potassium chloride extraction step with nitric acid obtained from nitrogen oxides contained in combustion exhaust gas discharged from the cement plant; and heating the fluid obtained in the contacting step at a temperature of 75°C to 175°C to obtain potassium nitrate.

2. The method for producing potassium nitrate according to claim 1, wherein the contacting of the chlorine bypass dust with the saturated aqueous potassium chloride solution is carried out by reflux.

3. The method for producing potassium nitrate according to claim 1, wherein the fluid obtained by contacting the chlorine bypass dust with the saturated aqueous potassium chloride solution is subjected to first solid-liquid separation at a temperature of 80°C or higher and 150°C or lower.

4. The method for producing potassium nitrate according to claim 3, wherein the saturated aqueous potassium chloride solution recovered by the first solid-liquid separation is cooled to room temperature and then subjected to a second solid-liquid separation to obtain potassium chloride as a solid component.

5. The method for producing potassium nitrate according to claim 4, wherein the potassium chloride obtained by the second solid-liquid separation is used in the contacting step.

6. The method for producing potassium nitrate according to claim 4, wherein the saturated aqueous potassium chloride solution recovered by the second solid-liquid separation is reused for contact with the chlorine bypass dust.

7. The method for producing potassium nitrate according to claim 1, wherein the content of nitric acid is more than 1.0 molar equivalent and not more than 2.0 molar equivalents per 1.0 molar equivalent of potassium chloride.

8. The method for producing potassium nitrate according to claim 1, wherein the concentration of the nitric acid is 10% by mass or more and 60% by mass or less.

9. The method for producing potassium nitrate according to claim 1, wherein the step of obtaining potassium nitrate further comprises heating to dryness at 100°C or higher.

10. The method for producing potassium nitrate according to claim 9, wherein nitric oxide and chlorine produced by the heating and drying are recovered.

11. A treatment system for waste discharged from a cement factory, comprising: chlorine bypass dust receiving equipment for receiving chlorine bypass dust discharged from the cement factory; combustion exhaust gas receiving equipment for receiving combustion exhaust gas discharged from the cement factory; potassium chloride extraction equipment for extracting potassium chloride from the chlorine bypass dust by contacting the chlorine bypass dust with a saturated aqueous potassium chloride solution at room temperature; equipment for contacting the potassium chloride obtained in the potassium chloride extraction equipment with nitric acid obtained from nitrogen oxides contained in the combustion exhaust gas; and potassium nitrate production equipment for heating the fluid obtained in the contacting equipment at a temperature of 75°C or higher and 175°C or lower to obtain potassium nitrate.

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