Manufacturing method of granular ammonium sulfate and manufacturing equipment of granular ammonium sulfate

Abstract

To manufacture granular ammonium sulfate with large grain diameter stably.SOLUTION: A manufacturing method of granular ammonium sulfate includes: reacting an ammonia-containing gas with a sulfuric acid aqueous solution to obtain an ammonium sulfate aqueous solution; introducing the ammonium sulfate aqueous solution to a crystal can; precipitating ammonium sulfate in the crystal can to obtain granular ammonium sulfate; and pulling a slurry containing the granular ammonium sulfate from a lower part of the crystal can. The manufacturing method of granular ammonium sulfate is characterized by that solid-liquid ratios at plural heights in the crystal can are measured and a pulling amount of the slurry is controlled based on the solid-liquid ratio.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method and equipment for producing granular ammonium sulfate. [Background technology]

[0002] A method using a crystallizer is widely used as an industrial method for producing granular ammonium sulfate. In this method, an aqueous solution of ammonium sulfate is introduced into a crystallizer, and granular ammonium sulfate is precipitated from the aqueous solution of ammonium sulfate in the crystallizer.

[0003] For example, in steelworks, since ammonia is contained in coke oven gas generated when coal is carbonized to produce coke, the coke oven gas is reacted with an aqueous sulfuric acid solution to remove ammonia. When ammonia is removed in this manner, an aqueous ammonium sulfate solution is obtained, and granular ammonium sulfate is produced in a crystallizer using the aqueous ammonium sulfate solution.

[0004] FIG. 1 is a schematic diagram showing the above-mentioned method for producing granular ammonium sulfate from coke oven gas.

[0005] First, coke oven gas 1 generated in the coke manufacturing process and an aqueous sulfuric acid solution 2 are reacted in an absorption tower 3 to produce an aqueous ammonium sulfate solution 4. The aqueous sulfuric acid solution 2 is prepared, for example, by reacting sulfuric acid 5 with water 6 in an intermediate tank 7, and is sent to the absorption tower 3 by a pump 8. Inside the absorption tower 3, for example, the aqueous sulfuric acid solution 2 is sprayed from above to cause the aqueous sulfuric acid solution 2 to absorb ammonia.

[0006] The obtained aqueous ammonium sulfate solution 4 is sent to a crystallizer 9. An evaporator 10 for concentrating the aqueous ammonium sulfate solution is provided on the upper part of the crystallizer 9. The aqueous ammonium sulfate solution 4 is circulated between the crystallizer 9 and the evaporator 10 via a pump 11 and a heater 12.

[0007] A vacuum device 13 is connected to the evaporator 10, and the aqueous ammonium sulfate solution 4 is vacuum concentrated. That is, the degree of supersaturation of the aqueous ammonium sulfate solution 4 is increased by evaporating and removing water under reduced pressure. The saturated aqueous ammonium sulfate solution 4 is discharged near the bottom of the crystallizer 9 through a downcomer 14 connected to the bottom of the evaporator 10. As a result, the ammonium sulfate crystals already present inside the crystallizer 9 are rolled up, and at the same time, new ammonium sulfate precipitates on the surface of the existing crystals, causing the crystals to grow. As the crystals grow in this manner, granular ammonium sulfate is obtained.

[0008] At this time, the grown crystals sink to the bottom of the crystallizer 9 due to their weight, while the relatively small crystals remain in the middle of the crystallizer 9. In this way, due to the classification effect at work inside the crystallizer 9, fully grown granular ammonium sulfate accumulates at the bottom of the crystallizer 9. The grown granular ammonium sulfate is then extracted from the bottom of the crystallizer 9 using a withdrawal pump 15 in a slurry state mixed with the granular ammonium sulfate aqueous solution. The extracted granular ammonium sulfate is dehydrated and dried to obtain the final granular ammonium sulfate.

[0009] The main use of the granular ammonium sulfate thus obtained is as an agricultural fertilizer, but from the viewpoint of dispersibility, a large particle size (for example, about 1.4 to 4.75 mm) is preferred. Therefore, a method for stably producing granular ammonium sulfate with a large particle size is required. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Publication No. 01-176214 [Patent Document 2] Japanese Patent Application Publication No. 05-213615 Summary of the Invention [Problem to be solved by the invention]

[0011] However, it has been difficult to stably produce granular ammonium sulfate having a large particle size using conventional methods. The reasons for this are explained below.

[0012] In order to control the particle size of the granular ammonium sulfate finally obtained, it is necessary to control the crystal growth that proceeds inside the crystallizer, and for that purpose, it is important to control the number of crystals present in the crystallizer.

[0013] Here, the number N of crystals in the crystallizer is expressed by the following formula (1). N=N0+N in -N out …(1) N0: Number of microcrystals N in : Number of crystals added as seed crystals N out : Number of crystals drawn from the crystallizer

[0014] As can be seen from the above formula (1), the number of crystals N in the crystallizer is determined by three factors, among which the number of microcrystals N0 is extremely difficult to control. Therefore, the average particle size in the crystallizer often becomes small due to the mass generation of unwanted microcrystals.

[0015] When the number of crystals in the crystallizer increases suddenly, the usual solution is to increase the amount of crystals drawn, but this method does not allow stable control of the particle size. The reasons are as follows.

[0016] First, in the crystallizer, one crystal generally grows over about 3 to 10 hours and is extracted as granular ammonium sulfate. Since the crystals remain in the crystallizer for a long time, even if the amount of extraction is adjusted, it takes time for the effect to be reflected in the state inside the crystallizer. During that time, crystals with small particle sizes continue to be produced. However, if the amount of extraction is excessively increased in anticipation of the above-mentioned delayed response, it will result in overaction and will instead lead to the generation of a large amount of microcrystals. This is because if the amount of extraction is increased too much, the amount of crystals in the crystallizer will decrease, and the total surface area of ​​the crystals will also decrease accordingly. As a result, ammonium sulfate that cannot be precipitated on the crystal surface will precipitate in large amounts as microcrystals.

[0017] Such a sudden change in the amount of microcrystals generated or an overaction causes a large change in the number of crystals in the crystallizer, making it difficult to stably produce granular ammonium sulfate of the desired size.

[0018] Therefore, Patent Document 1 proposes a method of predicting the number of crystals in a crystallizer from the average particle size of the product ammonium sulfate and the total volume of crystal particles measured from the liquid depth pressure in the crystallizer, and controlling the amount of product ammonium sulfate withdrawn and the amount of seed crystals added.

[0019] In this method, the average particle size of the product ammonium sulfate, i.e., the final product, granular ammonium sulfate, is used for control. However, there is a time lag between when the granular ammonium sulfate is drawn off from the crystallizer and when it finally becomes product ammonium sulfate. In addition, in the process of turning the drawn off granular ammonium sulfate into product ammonium sulfate, the particle size may become smaller due to reasons such as the crystals being scraped off by the centrifuge in the drying process. Therefore, the particle size of the product ammonium sulfate does not necessarily indicate the particle size of the granular ammonium sulfate in the crystallizer. Furthermore, since this method does not know the number of crystals generated, if a large amount of microcrystals are generated, the particle size of the resulting granular ammonium sulfate will be small.

[0020] In addition, Patent Document 2 proposes a method in which the crystal concentration in the mother liquor extracted from the elevated crystallizer is measured, and when the crystal concentration becomes too high, the supply of the mother liquor from the elevated crystallizer to the elevated crystallizer is stopped, and a recycled liquid is supplied to the elevated crystallizer instead of the mother liquor. In this method, the concentration and liquid level in the elevated crystallizer are kept constant, so that the inclusion of impurities such as sulfuric acid liquid and tar in the ammonium sulfate crystals sent to the subsequent process can be suppressed. However, the above method focuses on the phenomenon that occurs in the elevated crystallizer after the slurry is extracted from the crystallizer, and does not focus on the crystal growth in the crystallizer where the crystal growth actually occurs. Therefore, the above method cannot control the crystal growth that proceeds inside the crystallizer.

[0021] As described above, it has been difficult to stably produce granular ammonium sulfate having a large particle size using conventional methods.

[0022] The present invention has been made to solve the above problems, and an object of the present invention is to stably produce granular ammonium sulfate having a large particle size. [Means for solving the problem]

[0023] As a result of intensive research, the present inventors have found that granular ammonium sulfate having a large particle size can be stably produced by measuring the solid-liquid ratio at multiple heights in a crystallizer and controlling the amount of the slurry withdrawn based on the solid-liquid ratio.

[0024] The present invention has been completed based on the above findings, and the gist of the present invention is as follows.

[0025] 1. A method for producing granular ammonium sulfate, comprising reacting an ammonia-containing gas with an aqueous sulfuric acid solution to produce an aqueous ammonium sulfate solution, introducing the aqueous ammonium sulfate solution into a crystallizer, precipitating ammonium sulfate in the crystallizer to produce granular ammonium sulfate, and withdrawing a slurry containing the granular ammonium sulfate from the bottom of the crystallizer, measuring a solid-liquid ratio at a plurality of heights in the crystallizer, and controlling an amount of the slurry to be withdrawn based on the solid-liquid ratio.

[0026] 2. An absorption tower in which the ammonia-containing gas is reacted with an aqueous sulfuric acid solution to produce an aqueous ammonium sulfate solution; a crystallizer for precipitating ammonium sulfate from the aqueous ammonium sulfate solution to produce granular ammonium sulfate; an extracting means for extracting the slurry containing the granular ammonium sulfate from below the crystallizer; a solid-liquid ratio measuring means for measuring a solid-liquid ratio at a plurality of heights in the crystallizer; and a withdrawal amount control means for controlling an amount of the slurry withdrawn based on the solid-liquid ratio. Effect of the Invention

[0027] According to the present invention, granular ammonium sulfate having a large particle size can be stably produced. That is, by measuring the solid-liquid ratio at multiple heights in the crystallizer, the change in the amount of microcrystals produced in the crystallizer can be quickly observed. Then, by controlling the amount of slurry withdrawn based on the obtained solid-liquid ratio, it becomes possible to quickly reflect the increase or decrease in the number of crystals in the crystallizer in operational adjustments. As a result, the fluctuation in the number of crystals in the crystallizer can be suppressed, and ammonium sulfate crystals having a large particle size can be stably obtained. [Brief description of the drawings]

[0028] [Figure 1] FIG. 1 is a schematic diagram showing a method for producing granular ammonium sulfate from coke oven gas. [Diagram 2] FIG. 2 is a schematic diagram showing the growth process of granular ammonium sulfate in a crystallizer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] Next, a method for carrying out the present invention will be specifically described. Note that the following description shows an example of a preferred embodiment of the present invention, and the present invention is not limited to the following description in any way.

[0030] In a method for producing granular ammonium sulfate according to one embodiment of the present invention, an ammonia-containing gas is reacted with an aqueous sulfuric acid solution to produce an aqueous ammonium sulfate solution, the aqueous ammonium sulfate solution is introduced into a crystallizer, ammonium sulfate is precipitated in the crystallizer to produce granular ammonium sulfate, and a slurry containing the granular ammonium sulfate is withdrawn from the bottom of the crystallizer to produce granular ammonium sulfate.

[0031] In another embodiment of the present invention, a granular ammonium sulfate manufacturing facility includes an absorption tower in which an ammonia-containing gas is reacted with an aqueous sulfuric acid solution to produce an aqueous ammonium sulfate solution, a crystallizer in which ammonium sulfate is precipitated from the aqueous ammonium sulfate solution to produce granular ammonium sulfate, an extraction means for extracting a slurry containing the granular ammonium sulfate from below the crystallizer, a solid-liquid ratio measuring means for measuring a solid-liquid ratio at a plurality of heights in the crystallizer, and an extraction amount control means for controlling the amount of the slurry extracted based on the solid-liquid ratio.

[0032] In the present invention, a process similar to that of a conventional method for producing granular ammonium sulfate can be adopted, except that the solid-liquid ratio is measured and controlled based on the measured solid-liquid ratio, as described below. Therefore, the present invention can also use the production process and production equipment shown in Figure 1, unless otherwise specified. Figure 1 will also be referred to as necessary in the following description.

[0033] [Ammonia-containing gas] The ammonia-containing gas is not particularly limited, and any gas containing ammonia can be used. For example, coke oven gas can be used as the ammonia-containing gas.

[0034] [Ammonium sulfate aqueous solution] The above-mentioned ammonia-containing gas is reacted with an aqueous sulfuric acid solution to obtain an aqueous ammonium sulfate solution. The method for carrying out the reaction is not particularly limited, but it is preferable to carry out the reaction using an absorption tower 3 as shown in FIG. 1. In the absorption tower 3, for example, an aqueous sulfuric acid solution 2 is sprayed from above, and an ammonia-containing gas 1 is introduced from below, and the two are brought into countercurrent contact. As a result, the ammonia contained in the ammonia-containing gas is absorbed into the aqueous sulfuric acid solution to obtain an aqueous ammonium sulfate solution 4.

[0035] As the aqueous sulfuric acid solution 2, it is preferable to use dilute sulfuric acid whose concentration is adjusted by mixing sulfuric acid 5 and water 6 in a separate tank (intermediate tank 7) as shown in FIG.

[0036] [Crystallizer] Next, the ammonium sulfate aqueous solution is introduced into a crystallizer, and ammonium sulfate is precipitated in the crystallizer to produce granular ammonium sulfate. The crystallizer is not particularly limited, and any crystallizer can be used as long as it can precipitate ammonium sulfate from the ammonium sulfate aqueous solution inside. Typically, as shown in FIG. 1, a crystallizer 9 equipped with an evaporator 10 on top can be used. In this case, the ammonium sulfate aqueous solution 4 is circulated between the crystallizer 9 and the evaporator 10.

[0037] [Pull-out] The granular ammonium sulfate grown in the crystallizer is finally withdrawn from the bottom of the crystallizer. At that time, the granular ammonium sulfate is withdrawn in a slurry state together with a part of the aqueous ammonium sulfate solution. Therefore, the granular ammonium sulfate manufacturing equipment of the present invention is provided with an extraction means for extracting the slurry containing the granular ammonium sulfate from the bottom of the crystallizer. The extraction means is not particularly limited and any means can be used, but typically, an extraction pump 15 can be used as shown in FIG. 1.

[0038] The treatment after the extraction is not particularly limited, but in general, it is preferable to perform solid-liquid separation using a centrifuge and then dry the solid, i.e., granular ammonium sulfate. After drying, the granular ammonium sulfate can be optionally separated according to particle size using a sieve or the like.

[0039] Here, the process in which granular ammonium sulfate grows inside the crystallizer and is finally pulled out will be described.

[0040] Figure 2 is a schematic diagram showing the growth process of granular ammonium sulfate inside the crystallizer 9. First, the aqueous ammonium sulfate solution 4 vacuum-concentrated in the evaporator 10 is discharged near the bottom of the crystallizer 9 through a downcomer 14 connected to the bottom of the evaporator 10. As a result, new fine crystals are generated inside the crystallizer 9, and are lifted up (mainly to the middle region) together with the ammonium sulfate crystals already present (Figure 2(a)).

[0041] Inside the crystallizer 9, the crystals gradually grow and their particle size increases. The grown crystals sink to the bottom of the crystallizer 9 due to their weight, while the relatively small crystals fly up and stay in the middle of the crystallizer 9. In this way, due to the classification effect at work inside the crystallizer 9, fully grown granular ammonium sulfate accumulates at the bottom of the crystallizer 9 (Figure 2(b)).

[0042] The grown granular ammonium sulfate is extracted from the bottom of the crystallizer 9 in a slurry state mixed with the granular ammonium sulfate aqueous solution using a withdrawal pump 15 (Figure 2(c)). The extracted granular ammonium sulfate is dehydrated and dried to obtain the final granular ammonium sulfate.

[0043] If the withdrawal speed is insufficient compared to the growth speed of the crystals, the large grains remaining in the crystallizer 9 will grow further and exceed the desired grain size range (FIG. 2(d)). Such excessively grown coarse granular ammonium sulfate may be treated as a defective product.

[0044] In this way, a classification effect is at work inside the crystallizer, with fine crystals rising mainly to the middle level of the crystallizer, while larger crystallized particles sink to the lower level.

[0045] [Solid-liquid ratio measurement] In the present invention, in order to accurately grasp the state of the crystals in the crystallizer as described above, it is important to measure the solid-liquid ratio at multiple heights in the crystallizer. By measuring the solid-liquid ratio, the amount of crystals at the position where the measurement is performed can be quantitatively evaluated. Furthermore, by measuring the solid-liquid ratio at multiple heights, the distribution of crystals in the crystallizer can be known. Therefore, the granular ammonium sulfate manufacturing equipment in one embodiment of the present invention is equipped with a solid-liquid ratio measuring means for measuring the solid-liquid ratio at multiple heights in the crystallizer.

[0046] Here, the "solid-liquid ratio" is the ratio of the weight of solids to the total weight (the total weight of solids and liquid), that is, "weight of solids / (weight of solids+weight of liquid)".

[0047] The method for measuring the solid-liquid ratio is not particularly limited, and the solid-liquid ratio can be measured by any method.

[0048] In one embodiment of the present invention, a sample of a mixture of solids and liquids is taken from inside the crystallizer, the weights of the solids and liquids contained in the sample are measured, and the weight of the solids is divided by the total weight (weight of the solids + weight of the liquid). The sample can be obtained, for example, by submerging a sampling container to a predetermined height (depth) inside the crystallizer, collecting the sample in the container, and then pulling up the sampling container. The sample can be collected manually or automatically by machine.

[0049] In another embodiment of the present invention, the solid-liquid ratio can be measured indirectly. For example, a differential pressure gauge can be used to measure the differential pressure between the atmosphere and a predetermined height (depth) in the crystallizer, and the solid-liquid ratio can be calculated from the differential pressure. In other words, the solid-liquid ratio measuring means can include a differential pressure measuring means and a calculation means for calculating the solid-liquid ratio from the differential pressure measured by the differential pressure measuring means. Hereinafter, a method for calculating the solid-liquid ratio from the differential pressure will be described.

[0050] First, a differential pressure ΔP between a predetermined measurement position in the crystallizer and the atmosphere is measured using a differential pressure measuring means. When the depth of the measurement position from the liquid surface is Δh, the specific gravity of the sulfuric acid slurry at the measurement position is ρ, and the gravitational acceleration is g, the differential pressure ΔP is expressed by the following formula (2). ΔP = ρΔh g …(2)

[0051] By modifying the above formula (2), the specific gravity ρ of the slurry can be expressed by the following formula (3). ρ = ΔP / (Δh g) …(3)

[0052] Furthermore, when the solid-liquid ratio is V, the specific gravity of the ammonium sulfate crystals is d, and the specific gravity of the granular ammonium sulfate solution is D, the specific gravity ρ of the sulfuric acid slurry can be expressed as the weighted average of the specific gravity d of the ammonium sulfate crystals and the specific gravity D of the ammonium sulfate solution, as shown in the following formula (4). ρ=d·V + D(1-V) …(4)

[0053] The following formula (5) is derived from the above formulas (3) and (4). That is, the solid-liquid ratio V can be calculated from the differential pressure ΔP using the following formula (5). V = ΔP / {Δh g(dD)}- D / (dD) …(5)

[0054] In this way, the method of determining the solid-liquid ratio from the differential pressure does not require sampling of the solution in the crystallizer, and therefore continuous monitoring of the solid-liquid ratio can be easily performed. Therefore, the granular ammonium sulfate manufacturing equipment in one embodiment of the present invention can include, as the solid-liquid ratio measuring means, a differential pressure measuring means and a calculation means for calculating the solid-liquid ratio from the differential pressure measured by the differential pressure measuring means.

[0055] It is preferable to use a bubbler tube for measuring the differential pressure. A bubbler tube is a tube that is inserted inside a crystallizer and bubbles gas from its tip. By using a bubbler tube, it is possible to measure the differential pressure between the position of the tip of the bubbler tube and the atmosphere. Therefore, the facility for manufacturing granular ammonium sulfate in one embodiment of the present invention can be equipped with a bubbler tube as the differential pressure measuring means.

[0056] The number of points at which the solid-liquid ratio is measured is not particularly limited as long as it is 2 or more, and may be any number. As described later, the desired effect can be obtained even with measurement at two points, but from the viewpoint of further enhancing the effect, it is preferable to perform the measurement at three or more points. On the other hand, even if the measurement is performed at an excessively large number of heights, the effect will be saturated and the cost required for the equipment and operation will increase. Therefore, the number of points at which the solid-liquid ratio is measured is preferably 8 or less, more preferably 6 or less, and even more preferably 4 or less.

[0057] For example, when calculating the solid-liquid ratio from the differential pressure measured by a bubbler tube, a bubbler tube may be installed at each measurement point so that the tip (gas outlet) of the bubbler tube is located at the measurement position of the differential pressure (i.e., the measurement position of the solid-liquid ratio).

[0058] The height at which the solid-liquid ratio is measured is not particularly limited and can be any height, but it is preferable to measure at least two positions, the lower and middle levels of the crystallizer, and more preferably to measure at three positions, the lower, middle and upper levels. The reason for this is explained below.

[0059] As mentioned above, the classification effect is at work in the crystallizer, and while the grown granular ammonium sulfate accumulates in the lower part of the crystallizer, many of the fine crystals rise to the middle part. Therefore, by measuring the solid-liquid ratio in both the lower and middle parts, the state of the crystals in the crystallizer can be grasped more accurately. In addition, the upper part of the crystallizer is usually an area where no crystals exist, so by measuring the solid-liquid ratio in this upper part, the state of the crystals in the crystallizer can be grasped more accurately.

[0060] In one embodiment of the present invention, the lower, middle and upper stages are defined as follows. Upper row: Region where the solid-liquid ratio is less than 1% Middle: Solid-liquid ratio is 1% or more and less than 65% Lower: Region where the solid-liquid ratio is 65% or more

[0061] When the solid-liquid ratio is measured at the above-mentioned position, sampling and pressure difference measurement may be performed at a position where the solid-liquid ratio satisfies the above-mentioned condition under normal operating conditions.

[0062] In another embodiment of the present invention, the lower stage, the middle stage, and the upper stage can also be defined as follows based on the relative height from the bottom surface of the crystallizer. Upper row: Areas where the relative height is between 70% and 100% Middle row: Area where the relative height is 50% or more and less than 70% Lower row: Area where the relative height is greater than 0% and less than 50% Here, the relative height is the height from the bottom surface of the crystallizer when the height from the bottom surface of the crystallizer to the liquid surface is taken as 100%.

[0063] In another embodiment of the present invention, the lower stage, the middle stage, and the upper stage can be defined as follows based on the relative volumes from the bottom surface of the crystallizer. Upper row: Areas with relative volume between 65% and 100% Middle row: Area with relative volume 40% or more and less than 65% Lower row: Areas with relative volume between 0% and less than 40% Here, the relative volume is the volume from the bottom surface when the volume from the bottom surface of the crystallizer to the liquid surface is taken as 100%.

[0064] [Control of withdrawal amount] In the method for producing granular ammonium sulfate of the present invention, the solid-liquid ratios at multiple heights in the crystallizer are measured as described above, and the amount of slurry withdrawn is controlled based on the measured solid-liquid ratios. The granular ammonium sulfate production equipment of the present invention also includes a withdrawal amount control means for controlling the amount of slurry withdrawn based on the measured solid-liquid ratios. Specific examples of suitable methods for controlling the withdrawal amount are described below in several cases.

[0065] First, a preferred method for controlling the extraction amount will be described using an example in which the solid-liquid ratio is measured at three locations, the upper, middle, and lower levels. In this embodiment, the extraction amount W is determined from the solid-liquid ratio based on the following formula (6). The extraction amount W can be expressed, for example, as the weight of solids extracted per unit time (t / h). As the weight of solids, for example, the dry weight of solids (crystals) contained in the slurry can be used. W=α1×V1+α2×V2+α3×V3 …(6) Where: V1: Solid-liquid ratio in the upper stage V2: Solid-liquid ratio in the middle stage V3: Solid-liquid ratio in the lower stage α1~α3: Coefficients

[0066] The method of determining the coefficients α1, α2, and α3 is not particularly limited, and they may be appropriately adjusted so that granular ammonium sulfate of the desired particle size is obtained. The above coefficients may be adjusted using the particle size of the finally obtained granular ammonium sulfate as an index, but the coefficients may also be adjusted using the lower-stage solid-liquid ratio as an index. That is, among the solid-liquid ratios at the upper, intermediate, and lower stages, the lower-stage solid-liquid ratio has the greatest effect on the particle size of the granular ammonium sulfate. Therefore, the correlation between the particle size of the granular ammonium sulfate and the lower-stage solid-liquid ratio is obtained in advance, and the lower-stage solid-liquid ratio at which the desired particle size is obtained based on the correlation may be set as the target solid-liquid ratio.

[0067] The correlation between the particle size of granular ammonium sulfate and the lower stage solid-liquid ratio can be obtained, for example, by the following procedure. First, a target granular ammonium sulfate manufacturing facility is operated to manufacture granular ammonium sulfate. At that time, the lower stage solid-liquid ratio is measured. Meanwhile, the granular ammonium sulfate extracted from the crystallizer is subjected to solid-liquid separation, dried, and then the particle size is measured. For example, the median value (median diameter) in the mass-based particle size distribution can be used as the particle size. The particle size distribution can be measured by using a plurality of sieves with different mesh sizes.

[0068] The above measurement is carried out under a plurality of conditions, and a correlation between the particle size of granular ammonium sulfate and the lower solid-liquid ratio can be obtained from the relationship between the measured lower solid-liquid ratio and the median diameter. In this case, it is preferable to approximate the median diameter of granular ammonium sulfate with a function (e.g., a linear function) of the lower solid-liquid ratio. Based on the correlation thus obtained, a target lower solid-liquid ratio for obtaining granular ammonium sulfate of a desired particle size can be determined.

[0069] Once the target lower stage solid-liquid ratio is determined, the coefficients α1, α2, and α3 are then determined using the target lower stage solid-liquid ratio as an index. Although the method is not particularly limited, typically, the lower stage solid-liquid ratio is measured by operating for a certain period of time using a tentative coefficient, and if the measured lower stage solid-liquid ratio is lower than the target lower stage solid-liquid ratio, the coefficient is adjusted so as to reduce the withdrawal amount W, and conversely, if the measured lower stage solid-liquid ratio is higher than the target lower stage solid-liquid ratio, the coefficient is adjusted so as to increase the withdrawal amount W. By repeating this procedure, the coefficients can be obtained at which the lower stage solid-liquid ratio is stabilized at a value close to the target lower stage solid-liquid ratio.

[0070] In this case, it is preferable to adjust each coefficient one by one in the order of α3, α2, and α1, rather than adjusting multiple coefficients at the same time. This is because the lower solid-liquid ratio has the greatest effect on particle size, and the upper solid-liquid ratio has the least effect on particle size. For example, with α2 and α1 kept constant, α3 is adjusted so that the lower solid-liquid ratio is closest to the target lower solid-liquid ratio. Next, with α3 and α1 kept constant, α2 is adjusted so that the lower solid-liquid ratio is closest to the target lower solid-liquid ratio. Finally, with α3 and α2 kept constant, α1 is adjusted so that the lower solid-liquid ratio is closest to the target lower solid-liquid ratio. Note that when using the formula (7) described later, the last step may be omitted.

[0071] When granular ammonium sulfate is produced using the method of the present invention, the actual withdrawal amount is adjusted so as to be the withdrawal amount W calculated by the above formula (6). The withdrawal amount can be adjusted, for example, by adjusting the aperture of a control valve provided in a pipe for withdrawing the slurry from the crystallizer.

[0072] The frequency of adjusting the amount of extraction is not particularly limited, and may be continuous or intermittent. However, there is generally a time lag of several tens of minutes to several hours between the time when the slurry is extracted from the crystallizer and the time when the crystals are dried and the amount of extraction is measured, although this depends on the equipment. Therefore, if the amount of extraction is adjusted continuously or frequently, overaction may occur. Therefore, it is preferable to adjust the amount of extraction intermittently, and the time interval at that time is preferably 10 minutes or more, more preferably 20 minutes or more, and even more preferably 30 minutes or more. On the other hand, if the time interval is too long, it becomes difficult to control the particle size with sufficient accuracy. Therefore, the time interval is preferably 5 hours or less, more preferably 4 hours or less, even more preferably 3 hours or less, and most preferably 2 hours or less.

[0073] Furthermore, when the solid-liquid ratio is measured at two locations, the middle and lower levels, the withdrawal amount W can be calculated using the following formula (7). W = α2 × V2 + α3 × V3 … (7)

[0074] As described above, the microcrystals rise to the middle stage, but barely reach the upper stage, so the solid-liquid ratio in the upper stage is usually less than 1%. Therefore, even when a three-stage solid-liquid ratio is used, the effect of the solid-liquid ratio in the upper stage on controlling the withdrawal amount is smaller than that of the middle and lower stages. Therefore, it is possible to control the withdrawal amount using only the solid-liquid ratios in the lower and middle stages as described above.

[0075] Conversely, when the solid-liquid ratio is measured in four or more stages, a term expressed by the product of the solid-liquid ratio Vn in the fourth or subsequent stage and the coefficient αn should be added to the above formula (6).

[0076] The solid-liquid ratio V3 in the lower stage may be multiplied by a coefficient α3 and used to control the withdrawal amount W as in the above equations (6) and (7), but it is preferable to use a constant term β as in the following equation (8). W = α1 × V1 + α2 × V2 + β … (8) Here, β is a constant determined by the solid-liquid ratio V3 in the lower stage.

[0077] For example, as shown in the following example, the value of the constant term β is determined in advance according to the range of the lower stage solid-liquid ratio V3. Then, the value of β used in calculating the withdrawal amount X is changed based on the measured lower stage solid-liquid ratio V3. Lower solid-liquid ratio V3 is 55% or less: β=1.5 Lower solid-liquid ratio V3 is over 55% and 60% or less: β=2.0 Lower solid-liquid ratio V3 is over 60%: β=2.5

[0078] When the extraction amount is determined using the constant term β as described above, even if the lower stage solid-liquid ratio changes, it does not affect the extraction amount W as long as the value is within a certain range. Therefore, the extraction amount W is changed less frequently than when the above formulas (6) and (7) are used. In the actual production of granular ammonium sulfate, after the slurry is extracted, the liquid is separated and dried, but if the extraction amount X is changed frequently, it becomes necessary to adjust the drying conditions each time. Therefore, by treating the lower stage solid-liquid ratio V3, which has a large effect on the extraction amount W, as a constant term as described above, drying can be performed smoothly. EXAMPLES

[0079] (Example) In order to confirm the effect of the present invention, the withdrawal amount was controlled using the above formula (8) and the operation was carried out for two days. During the operation, the solid-liquid ratio was measured and the withdrawal amount was adjusted at 4-hour intervals. The coefficients in formula (8) were α1 = 0.15, α2 = 30, and β was as exemplified above.

[0080] The ammonium sulfate crystals obtained by the above operation were dried and then sieved through a number of screens with different mesh sizes. Among the ammonium sulfate crystals sieved, large crystals with a particle size of 1.4 to 4.75 mm were classified as acceptable, and the rest were classified as unacceptable. The final average large crystal yield during the above operation period was 73%.

[0081] (Comparative Example) For comparison, the withdrawal amount W was controlled based only on the lower stage solid-liquid ratio V3, and operation was performed under the same conditions as in Example 1. That is, the withdrawal amount W was controlled using the above formula (9). W = β …(9)

[0082] As a result, the final average large grain yield during the operation period was 70%.

[0083] As described above, according to the method of the present invention, a high large grain yield of 73% was achieved. This yield was 3% better than the case where only the lower stage solid-liquid ratio was used for control (comparative example). It is common for the production of granular ammonium sulfate from an ammonia-containing gas as in the present invention to be carried out on an extremely large scale, such as when coke oven gas is used in a steelworks. Therefore, even a 1% difference in yield results in a difference of tens of thousands of tons in annual production volume, and therefore the effect of the present invention can be said to be extremely significant in industry. [Explanation of symbols]

[0084] 1. Coke oven gas (ammonia-containing gas) 2 Sulfuric acid aqueous solution 3. Absorption tower 4. Ammonium sulfate aqueous solution 5 Sulfuric acid 6 water 7 Intermediate tank 8. Pump 9. Crystallizer 10 Evaporator 11 Pump 12 Heater 13 Vacuum equipment 14 Downcomer 15 Extraction pump

Claims

1. A method for producing granular ammonium sulfate, comprising reacting an ammonia-containing gas with an aqueous sulfuric acid solution to produce an aqueous ammonium sulfate solution, introducing the aqueous ammonium sulfate solution into a crystallizer, precipitating ammonium sulfate in the crystallizer to produce granular ammonium sulfate, and withdrawing a slurry containing the granular ammonium sulfate from below the crystallizer, measuring a solid-liquid ratio at a plurality of heights in the crystallizer, and controlling an amount of the slurry to be withdrawn based on the solid-liquid ratio.

2. an absorption tower in which the ammonia-containing gas is reacted with an aqueous sulfuric acid solution to produce an aqueous ammonium sulfate solution; a crystallizer for precipitating ammonium sulfate from the aqueous ammonium sulfate solution to produce granular ammonium sulfate; an extracting means for extracting the slurry containing the granular ammonium sulfate from below the crystallizer; a solid-liquid ratio measuring means for measuring a solid-liquid ratio at a plurality of heights in the crystallizer; and a withdrawal amount control means for controlling an amount of the slurry withdrawn based on the solid-liquid ratio.

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