Low-activity quicklime, soil improvement agent using the same, and method for producing low-activity quicklime

Abstract

To provide a method for producing low-activity quicklime suitable for a soil improvement agent or the like while suppressing energy cost during production.SOLUTION: An aqueous solution of an inorganic compound is sprayed onto powder or granular material of quicklime to generate hydrated lime on a surface of quicklime particles and to form sparingly soluble compounds by reaction between the hydrated lime and the inorganic compound. A supply amount of the aqueous solution is adjusted to obtain the low-activity quicklime in which a ratio of the hydrated lime and a proportion of the sparingly soluble compounds to be formed are at a prescribed proportion.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to low-reactivity quicklime, and more particularly to low-reactivity quicklime suitable as a soil conditioner and a method for producing the same.

Background Art

[0002] Generally, quicklime has high reactivity and is used in various applications that utilize its high activity. On the other hand, depending on the application, low-reactivity products with low initial reactivity may be preferred. An example of this is quicklime for soil improvement.

[0003] When powdered quicklime is sprayed for soil or ground improvement, there is a problem that dust is easily generated. Furthermore, in addition to the ease of generating dust itself, high-reactivity quicklime rapidly reacts with the water contained in the soil, generating fine slaked lime particles, and there is also a problem that dust scatters together with water vapor due to the heat generated at that time.

[0004] For this reason, many quicklimes for soil and ground improvement are subjected to dust prevention treatment. As a dust prevention treatment, generally, the mainstream method is to suppress dust generation by covering the surface of quicklime with Teflon (registered trademark) or the like. (Patent Document 1)

[0005] On the other hand, not only surface treatment but also using low-reactivity hard-burned quicklime as the quicklime raw material itself is also conceivable. Hard-burned quicklime is quicklime fired at a higher temperature than usual, and has the characteristic of low reactivity with water due to its small porosity, and is considered optimal as a raw material for soil improvement. In addition, as a manufacturing technique for low-reactivity quicklime other than hard-burned quicklime, Patent Documents 2 and 3 disclose a method for producing low-reactivity quicklime by supplying liquid saccharides or mist-like carbonated water or dilute sulfuric acid to lump quicklime.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, in the production of conventional hard-burned quicklime, a large amount of coke is required. Therefore, the energy cost during production is higher than that of ordinary quicklime, and due to the high firing temperature, there is a drawback that the damage to the furnace wall of the firing furnace is large. In addition, in the method of Patent Document 2, the viscosity of the saccharides described is very high, being 600 cp or more at normal temperature, and mechanical stirring and mist spraying cannot be used. Therefore, there are problems in terms of handling and the method during spraying. In the method of spraying carbonated water in the form of mist described in Patent Document 3, it is difficult to perform stable and effective spraying due to the generation of bubbles. Also, since the solubility of carbon dioxide is very low at normal temperature, low temperature and high pressure conditions are required to use carbonated water with a significant solubility, resulting in a problem of high equipment cost. In the case of dilute sulfuric acid, although there are no problems with spraying and solubility like carbonated water, since dilute sulfuric acid has strong acidity, there is a problem of high equipment cost such as changing to low-corrosive materials.

[0008] Furthermore, in recent years, against the backdrop of the significant acceleration of the trend of decarbonization due to the soaring prices of raw fuels and the reduction of CO2 in the supply chain, there is a demand for low-cost and easily manufacturable low-reactivity quicklime as an alternative to conventional hard-burned quicklime.

[0009] An object of the present invention is to provide a technique capable of easily producing low-reactivity quicklime while reducing the energy cost during production and the load on equipment.

Means for Solving the Problems

[0010] The low-activity quicklime of the present invention is low-activity quicklime produced by spraying an aqueous solution containing an inorganic compound that reacts with slaked lime to form an insoluble compound onto the powder or granules of quicklime. The low-activity slaked lime thus produced has a composite structure including slaked lime formed by the reaction of a part of the quicklime with the water in the aqueous solution within one particle, and an insoluble compound formed by the reaction of the slaked lime with the inorganic compound.

[0011] The soil conditioner of the present invention uses the above low-activity quicklime. Those containing materials other than low-activity lime are also included in the present invention.

[0012] The present invention also provides a method for producing low-activity quicklime with suppressed reactivity. This method includes a step of spraying an aqueous solution of an inorganic compound that reacts with slaked lime to form an insoluble compound onto the powder or granules of quicklime to produce slaked lime by the reaction of a part of the quicklime with water, and a step of forming an insoluble compound on the surface of the quicklime by the reaction of the slaked lime with the inorganic compound in the above step.

Advantages of the Invention

[0013] The low-activity quicklime of the present invention, as a result of the almost simultaneous progress of the formation of slaked lime by the reaction of quicklime with water and the formation of an insoluble compound by the reaction of slaked lime with an inorganic compound due to the spraying of the aqueous solution of the inorganic compound, becomes low-activity quicklime in which the insoluble compound covers the surface of the powder or granules of quicklime as the raw material.

[0014] This low-activity quicklime does not require the energy cost and large-scale equipment for production such as low-activity hard-burned quicklime, and moreover, when used as a soil conditioner, it exhibits low activity and dust-proof properties equivalent to or higher than those of hard-burned quicklime.

Brief Description of the Drawings

[0015]

Figure 1

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Figure 3

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Figure 8

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Figure 10

Figure 11

Figure 12

Modes for Carrying Out the Invention

[0016] Hereinafter, the specific details of the low-reactivity quicklime of the present invention and its manufacturing method will be described. The low-reactivity quicklime of the present invention is produced by spraying an aqueous solution containing an inorganic compound that reacts with slaked lime to form an insoluble compound onto the powder or granular form of quicklime. A part (near the surface) of the quicklime particles is slaked lime produced by the reaction of quicklime with water, and at least a part of the surface has a structure in which an insoluble compound produced by the reaction of the inorganic compound and slaked lime is present.

[0017] The composition of the low-reactivity quicklime of the present invention varies depending on factors such as the size of the quicklime used as a raw material, the type of inorganic compound, the amount of inorganic compound contained in the aqueous solution, and the amount of aqueous solution sprayed onto the quicklime, and can take various forms. Hereinafter, the factors that determine the structure and composition will be described in detail.

[0018] First, as the quicklime, which is the raw material of the low-reactivity quicklime of the present invention, that produced by firing limestone can be used. The size is not limited, but the larger the particle diameter, the less scattering occurs due to wind or the like. Therefore, in order to suppress scattering, it is preferable that the particle diameter is relatively large to some extent. That is, it is preferably granular. On the other hand, in order for the sprayed aqueous solution to evenly cover the surface of the quicklime and promote the reaction between slaked lime and the inorganic compound, it is necessary for the particle diameter to be relatively small to some extent. From such a viewpoint, the raw material quicklime preferably has a particle diameter of 5 mm or less, more preferably 3 mm or less.

[0019] Also, it is desirable to use a material that does not substantially contain powder with a particle diameter of preferably 1 mm or less, more preferably 0.1 mm or less. Generally available quicklime with a particle diameter of 3 mm or less contains a large number of particles with a particle diameter of 1 mm or less. Such particles with a small particle diameter are likely to scatter by themselves, and when an aqueous solution of an inorganic compound is sprayed, it is considered that almost the entire particle changes to slaked lime and it is difficult to form a state covered with an insoluble compound.

[0020] By setting the particle size of the raw quicklime, for example, to 3 mm or less and 1 mm or more, a state where the inorganic compound aqueous solution is uniformly sprayed onto each particle can be easily obtained, and the production of slaked lime and the production of an insoluble compound by the reaction between the produced slaked lime and the inorganic compound can proceed rapidly and uniformly.

[0021] The inorganic compound is, for example, a carbonate, silicate, sulfate, etc. of an alkali metal, and can be used alone or as a mixture. The insoluble compound is a compound produced by the reaction of these inorganic compounds with slaked lime. For example, when the inorganic compound is sodium carbonate (Na2CO3) or potassium carbonate (K2CO3), it is calcium carbonate (CaCO3) obtained by the reaction of the following formula. Ca(OH)2 + Na2CO3 → CaCO3 Ca(OH)2 + K2CO3 → CaCO3

[0022] When the inorganic compound is sodium silicate (Na2SiO3) or sodium sulfate (Na2SO4), it is calcium silicate (CaSiO3) or calcium sulfate (CaSO4) obtained by the reaction of the following formula. Ca(OH)2 + Na2SiO3 → CaSiO3 Ca(OH)2 + Na2SO4 → CaSO4

[0023] In the low-activity quicklime particles of the present invention, the proportion of the insoluble compound produced by the reaction of slaked lime, that is, the inorganic compound covering the surface of the quicklime, is determined by the amount of the aqueous solution of the inorganic compound to be sprayed.

[0024] It is preferable to use the inorganic compound in an amount such that the insoluble compound produced by the reaction with slaked lime can cover at least a part of the surface of the raw quicklime powder or granules. Specifically, it is preferably 0.01 to 0.03 equivalents based on the slaked lime produced by the reaction of quicklime and water in the aqueous solution. As the amount relative to the raw material powder / granules, it is preferably 0.5% or more, more preferably 1% or more on a weight basis.

[0025] By adjusting the concentration of the inorganic compound in the aqueous solution and the spraying amount according to the concentration, it is possible to ensure the supply amount of water for making the ratio of quicklime and slaked lime as described above, and a suitable amount of the inorganic compound supplied by spraying. The concentration of the inorganic compound in the aqueous solution is not limited, but is preferably 15% to 25% by weight, more preferably 20% to 25% by weight. The spraying amount can be adjusted, for example, by the number of spraying times.

[0026] The low-reactivity quicklime of the present invention has a structure in which slaked lime is formed around the quicklime particles and a hardly soluble compound is further formed around the slaked lime by spraying an aqueous solution containing an inorganic compound. By having this structure, the activity t in the digestion heat generation test u is 500 seconds or more, and particularly preferably 1000 seconds or more under particularly suitable conditions, resulting in low-reactivity quicklime with low initial reactivity. The amount of the hardly soluble compound in the produced quicklime varies depending on the addition rate of the above-described inorganic additive, the amount of water supplied, etc., but is preferably 0.5% by weight or more, more preferably 0.8% by weight or more. The upper limit is not limited, but is 4.0% by weight or less, preferably 3.5% by weight or less.

[0027] As described above, the low-reactivity quicklime of the present invention has low and suppressed initial reactivity. Therefore, when used as a treatment agent for treating acidic soil or acidic objects to be treated, there are no problems such as rapidly reacting with environmental moisture to generate heat or the particles being easily scattered due to this heat generation, and it is a highly safe treatment agent, particularly suitable for use as a soil conditioner. In addition, since the surface of the particles has a structure covered with a relatively hard hardly soluble compound, it is difficult to scatter during handling and spraying, easy to handle, and has no burden on the environment.

[0028] The soil conditioner of the present invention contains the above-described low-reactivity quicklime, but may appropriately contain other materials such as slag and gypsum.

[0029] Next, a method for producing the low-reactivity quicklime of the present invention will be described. The low-activity quicklime of the present invention is produced by spraying an aqueous solution of the above-described inorganic compound onto a powder or granule of quicklime as a raw material. In this production process, for example, when the inorganic compound is sodium carbonate (Na2CO3), by spraying its aqueous solution onto quicklime, first, the digestion reaction shown in (1) occurs on the surface of each particle that comes into contact with the aqueous solution, and then it is considered that an insoluble compound (for example, CaCO3) is generated by the reaction (2) between the slaked lime generated on the particle surface and the inorganic compound.

[0030] CaO + H2O → Ca(OH)2(1) Ca(OH)2+ Na2CO3→ CaCO3(2)

[0031] As a result, particles are produced in which at least a part of the surface of the quicklime particles is covered with slaked lime and an inorganic compound. As a method of supplying the aqueous solution, by adopting spraying, an amount of the aqueous solution that can digest only a part of the quicklime can be supplied to the entire particles. In FIG. 1, the outline of the production method of the present invention is schematically shown. As shown in the lower part of FIG. 1, preferably, by stirring during spraying, a desired amount of the aqueous solution can be brought into contact with the entire particles. Alternatively, as shown in the lower right of FIG. 1, the raw material may be sprayed in a state where the aqueous solution sufficiently penetrates.

[0032] Note that the above-described reaction proceeds with a delay after stirring (for example, the reaction is delayed by about 10 to 15 minutes after the reaction), so a stirring time of about 30 minutes after spraying is sufficient.

[0033] In addition, since an insoluble compound generated by the reaction between slaked lime and an inorganic compound can obtain a low-activation effect even in a small amount, the amount (lower limit value) of the inorganic compound that generates the insoluble compound is not particularly limited, but preferably, it is 0.005 equivalent or more, more preferably 0.01 equivalent or more, based on the slaked lime to be generated. However, it is preferably 0.03 equivalent or less. The concentration of the inorganic compound in the aqueous solution is not particularly limited as long as it satisfies the above-described supplied water amount and inorganic compound amount, but it is preferably 15% to 25% on a weight basis.

[0034] The low-reactivity quicklime of the present invention can be obtained by a simple process of spraying an aqueous solution of an inorganic compound at a predetermined concentration uniformly onto the powder or granular material of quicklime as the raw material, so that slaked lime and an insoluble compound are mixed or formed in layers on the surface of each particle. Such quicklime has a structure in which at least a part of the surface is covered with a substance different from quicklime, and thus is less likely to scatter during handling or spraying compared to quicklime powder. Further, even when it is sprayed onto an environment (soil) containing moisture, the initial reaction with the moisture in the environment is suppressed, so that the rapid heat generation due to the reaction of quicklime with moisture is suppressed, and the scattering of the powder accompanying the heat generation is suppressed.

Example

[0035] Hereinafter, examples of the low-reactivity quicklime of the present invention and its production method will be described. In the following description, “%” is based on weight unless otherwise specified.

[0036] <Example 1> As the raw material, quicklime with a particle size of 3 mm or less (obtained by firing and pulverizing limestone in a firing furnace or the like) was prepared, and 20 g of Na2CO3 was dissolved in 100 g of water to prepare a 16.7% Na2CO3 aqueous solution. This was put into a spray container, sprayed onto 200 g of quicklime, and reacted while stirring with a Hobart mixer for 30 minutes to obtain the low-reactivity quicklime of Example 1. At that time, the spray amount was adjusted by spraying the Na2CO3 aqueous solution in multiple portions, and five types of low-reactivity quicklime with different ratios of slaked lime generated by the reaction of the water supplied by the Na2CO3 aqueous solution and quicklime were produced. Table 1 shows the ratios (molar ratio: mol%) of quicklime and slaked lime and the theoretical production amount of calcium carbonate generated by the reaction with slaked lime (content in the low-reactivity quicklime: weight%) in the five types of low-reactivity quicklime.

[0037]

Table 1

[0038] In Table 1, "agent concentration" represents the concentration (wt%) of the inorganic compound (Na2CO3 in Example 1) in the aqueous solution of the inorganic compound, and "agent addition rate" represents the addition rate (wt%) of the inorganic compound to the raw quicklime (the same applies hereinafter).

[0039] Regarding the low-activity quicklime of Example 1, a digestion heat generation test was conducted to measure the activity (t u ), the maximum temperature (Tmax), and the temperature rise. Here, the digestion heat generation test is one of the methods for evaluating the reactivity of quicklime with water and was carried out according to the procedure of German Industrial Standard DIN EN 459-2. 150 g of quicklime was added to 600 ml of water (water temperature adjusted to 20 °C), and a temperature-time graph was created to calculate the maximum temperature Tmax and the time (t u = activity) when the temperature reached 80% of the maximum temperature. The measurement results of the activity are shown in Table 1, and the measurement results of the temperature rise are shown in Fig. 2(A). For reference, the results of the same test for the raw quicklime (blank) and the hard-burned quicklime (manufactured by Okutama Kogyo Co., Ltd.: hard-burned product) are shown in Table 1.

[0040] In Fig. 2(A) showing the temperature rise, the thick line represents the digestion heat generation test result of the raw quicklime, and the dotted line represents the digestion heat generation result of the hard-burned product. Also, 90:10, 80:20, and 70:30 represent the ratio of quicklime to slaked lime generated by agent spraying (the same hereinafter). From the results of Fig. 2(A), it can be seen that the low-activity quicklime of Example 1 has suppressed digestion heat generation compared to the raw quicklime, and in particular, the initial heat generation up to 200 seconds is significantly suppressed.

[0041] Also, when compared with the conventional hard-burned quicklime (hard-burned product), for example, when the agent addition rate is 1.6% or more (the molar ratio of quicklime to slaked lime is 80:20 or more), the initial activity is equivalent to that of the hard-burned product. Since a part is changed to slaked lime, the maximum temperature is slightly higher than that of the hard-burned product, but the temperature-time graph changes almost the same as that of the hard-burned product, and it was found that the initial activity can be easily reduced only by agent spraying.

[0042] <Example 2> (Particle sorting) For the quicklime (particle size: 3 mm or less) used as the raw material in Example 1, a sieve with an opening of 1 mm was used to cut off particles with a particle size of 1 mm or less. The sample remaining on the sieve (3 - 1 mm sample) was sprayed with an aqueous Na2CO3 solution in the same manner as in Example 1. However, in Example 2, during the reaction with a Hobart mixer, stirring was not performed, and it was switched to static placement in a stainless-steel vat. In Example 2, similar to Example 1, three types of low-activity quicklime with chemical addition rates of 0.8%, 1.6%, and 2.4% were obtained.

[0043] The results of the activity evaluation are shown in Table 2 and Figure 2(B).

[0044]

Table 2

[0045] The low-activity quicklime of Example 2 from which particles with a particle size of 1 mm or less were removed was confirmed to be superior in activity suppression in both activity (t u ) and maximum temperature (Tmax) compared to Example 1. Also, from the graph in Figure 2(B), it can be seen that the temperature rise is gentler in Example 2 than in Example 1, that is, the initial activity is suppressed.

[0046] Also, for Example 1 and Example 2, t u with respect to the ratio of Ca(OH)2 obtained by thermal analysis is shown in Figure 3(A), and a graph plotting t u with respect to the addition rate of Na2CO3 is shown in Figure 3(B). As shown in Figures 3(A) and 3(B), it was confirmed that the low-activity quicklime of Example 2 has a higher activity suppression effect at the same spraying amount (Na2CO3 addition rate) compared to Example 1.

[0047] <Comparative Example 1> (Chemical mixing) To confirm the effect of spraying, the same activity evaluation as in Example 1 was also performed on a sample (hereinafter referred to as a chemical mixture) in which a powder of an inorganic compound chemical was directly added to and mixed with quicklime. As the chemical mixture, 10% of Na2CO3 reagent powder was added to 200 g of quicklime, and the mixture was stirred with a Hobart mixer for 30 minutes.

[0048] The results of the activity evaluation are shown in Table 3 and Figure 4 together with the results of Example 1 (drug addition rates: 0.8%, 1.6%, 2.4%). From these results, the drug mixture was inferior in the activity reduction effect to the sprayed samples. This factor is considered to be because the coating of quicklime by CaCO3 does not occur. From this result, it was confirmed that the spraying method with a solution is more optimal than the method of adding Na2CO3 in powder form.

[0049]

Table 3

[0050] <Comparative Example 2> (Water spraying) Using water instead of the Na2CO3 aqueous solution of Example 1, the same reaction of spraying the raw material quicklime to convert a part of the quicklime into slaked lime was carried out in the same way, and the quicklime-slaked lime particles of Comparative Example 2 were obtained. For Comparative Example 2 as well, by adjusting the number of sprayings, the amount of water supplied was changed, and five types of quicklime with different ratios of quicklime and slaked lime were obtained. A digestion heat generation test was carried out for each sample, and the activity t u and the temperature rise were measured. The measurement results of the activity t u are shown in Table 4, and the measurement results of the temperature rise are shown in Figure 5(B). The results of Example 1 are shown in Table 4 and Figure 5(A) for reference.

[0051]

Table 4

[0052] As shown in Table 4 and Figure 5(B), even with only water spraying, an effect of reducing the maximum temperature can be obtained compared to the blank (raw material quicklime). However, in the case of only water, for example, with quicklime containing 20 mol% of slaked lime, the activity t u is about twice that of the blank, but in Example 1 with the same slaked lime ratio, the activity is as low as about five times that of the blank, and it was found that a significantly excellent effect can be obtained by adding a small amount of Na2CO3.

[0053] Furthermore, from the comparison of FIGS. 5(A) and 5(B), it was confirmed that in the spraying of the inorganic compound aqueous solution, the temperature rise in the initial stage was slower and gentler than that in water spraying.

[0054] Regarding the sprayed quicklime with the agent of Example 1 and the water-sprayed quicklime of Comparative Example 2, t is plotted against the ratio of Ca(OH)2 obtained as a result of thermal analysis. u The graph thus obtained is shown in FIG. 6. For reference, t of a mixture of quicklime and slaked lime at the same ratio of Ca(OH)2 (mixture: Comparative Example 1) is also shown. In Example 1, the increase in the ratio of Ca(OH)2 is linked to the increase in the addition ratio of the inorganic compound. u

[0055] From the graph of FIG. 6, it can be seen that in both Example 1 and Comparative Example 2, as the ratio of Ca(OH)2 increases, t becomes higher, and an activity suppression effect can be obtained compared to the simple mixture (Comparative Example 1). However, in Example 1, a more significant activity suppression effect is observed compared to Comparative Example 2 (water spraying), and the effect of the inorganic compound (the poorly soluble compound formed thereby) is recognized. u

[0056] Also, from FIG. 6, in the sprayed quicklime with the agent of Example 1, it is recognized that as the addition rate of the inorganic compound increases, t increases linearly, indicating that the activity suppression depends on the agent addition rate. This is presumably because the more the agent (Na2CO3), the more the formation of poorly soluble Ca salts, and the activity suppression by coating is promoted. u

[0057] Furthermore, from the graph showing the relationship between the slaked lime ratio and the activity shown in FIG. 6, even when the slaked lime ratios are the same, the decrease in activity in Example 1 is greater than that in Comparative Example 2. In particular, on the high addition rate side where the slaked lime ratio is 20% or more, the difference from the comparative example becomes large, and it is clearly confirmed that this difference is caused by the poorly soluble Ca salt. That is, it was shown that the spraying of the Na2CO3 aqueous solution has an advantage and an activity suppression effect over water spraying.

[0058] <Comparative Example 3> (Carbonated water) ​​​Using carbonated water (commercial product) instead of the aqueous Na2CO3 solution in Example 1, the reaction was carried out in the same manner as in Example 1, spraying the raw quicklime to convert part of the quicklime into slaked lime, and the quicklime-slaked lime particles of Comparative Example 3 were obtained. For Comparative Example 3 as well, by adjusting the number of sprays in the same manner as in Comparative Example 1, the amount of water supplied was varied to obtain three types of quicklime with different ratios of quicklime to slaked lime. For each of them, a digestion heat generation test was conducted, and the active t u and the temperature rise were measured. The measurement results of the active t u are shown in Table 5, and the measurement results of the temperature rise are shown in Fig. 5(C).

[0059] As shown in Table 5, the effect of spraying carbonated water was almost the same as that of spraying only water, and the superiority of Example 1 using the additive became apparent. Also, even when the spray amount was increased, the maximum temperature did not decrease much, and the value of the active t u was lower than that of water spraying. This is considered to be because the spraying efficiency decreases due to foaming with carbonated water.

[0060] <Comparative Example 4> (Dilute sulfuric acid) Using dilute sulfuric acid (concentration 16.7%) instead of the aqueous Na2CO3 solution in Example 1, the reaction was carried out in the same manner as in Example 1, spraying the raw quicklime to convert part of the quicklime into slaked lime, and the quicklime-slaked lime particles of Comparative Example 4 were obtained. For Comparative Example 4 as well, by adjusting the number of sprays in the same manner as in Comparative Example 2, the amount of water supplied was varied to obtain three types of quicklime with different ratios of quicklime to slaked lime. For each of them, a digestion heat generation test was conducted, and the active t u and the temperature rise were measured. The measurement results of the temperature rise are shown in Fig. 5(D).

[0061] The results of measuring the active t u were almost the same as those in Example 1. However, as shown in Fig. 5(D), although a good result was obtained for the temperature rise suppression effect at the initial temperature of the digestion experiment when "quicklime: slaked lime" was 90:10, no increase in the suppression effect as in Example 1 was observed even when the number of sprays (drug addition rate) changed.

[0062] <Example 3> (Concentration change) To confirm the differences due to the concentration of Na2CO3 in the aqueous Na2CO3 solution, an aqueous Na2CO3 solution with a concentration of 25% by weight was prepared. Using a sample with a particle size of 1 - 3 mm similar to that in Example 2 as the raw quicklime, and otherwise in the same manner as in Example 1, the low-activity quicklime of Example 3 was obtained. At that time, the spraying amount was adjusted by spraying the aqueous Na2CO3 solution in multiple portions, and three types of low-activity quicklime with different ratios of slaked lime generated by the reaction of the water supplied by the aqueous Na2CO3 solution and the quicklime were each manufactured. Table 5 shows the ratios (mol%) of quicklime and slaked lime and the amount (theoretical value) of CaCO3 generated by the reaction with slaked lime in the three types of low-activity quicklime.

[0063]

Table 5

[0064] Also, for the low-activity quicklime of Example 3, a digestion heat generation test was conducted in the same manner as in Example 1, and the activity t u and the temperature rise were measured. The measurement results of the activity t u are also shown in Table 5. As can be seen from the results in Table 5, as the concentration of the sprayed aqueous Na2CO3 solution increased, a more excellent activity reduction effect was shown.

[0065] Also, the measurement results (temperature - time graph) of the temperature rise in Example 3 compared with Example 2 are shown in FIGS. 7(A) and (B). FIG. 7(A) shows the results of Example 2, and (B) shows the results of Example 3. Also, for Example 2 and Example 3, the change of t u with respect to the Na2CO3 addition amount is shown in FIG. 8(A), and the amount of Ca(OH)2 generated with respect to the Na2CO3 addition amount is shown in FIG. 8(B).

[0066] As shown in FIG. 7(B), in Example 3 (Na2CO3 concentration 25%), the temperature rise is gentler than in Example 2 (Na2CO3 concentration 16.7%) shown in FIG. 7(A), and it can be seen that a higher concentration of the sprayed aqueous Na2CO3 solution shows a more excellent activity reduction effect.

[0067] Also, as shown in FIG. 8(A), the activity t ushowed high values depending on the addition amount of Na2CO3, and activity inhibition was observed. Also, as shown in the graph of Fig. 8(B), as the addition amount of Na2CO3 increased, the production amount of Ca(OH)2 increased linearly. However, it was found that even when the addition amount of Na2CO3 was the same, the higher the Na2CO3 concentration, the less the amount of Ca(OH)2 produced. From this, it is considered that by using an aqueous solution of an inorganic compound with a high concentration, the ratio of Ca(OH)2 can be reduced and the proportion of poorly soluble Ca salts can be increased. As a result, it was found that the higher the concentration, the higher the activity inhibition effect obtained.

[0068] <<Examples 4 to 6>> (Change in type of chemical agent) <Example 4> (Potassium) Except for using K2CO3 instead of Na2CO3 as the aqueous solution of the inorganic compound to be sprayed, low-activity quicklime containing slaked lime and CaCO3, which is the reaction product of it and K2CO3, was produced in the same manner as in Example 1. In Example 4, three types of low-activity quicklime with chemical agent (K2CO3) addition rates of 0.8%, 1.6%, and 2.4% were obtained. For these low-activity limes, a digestion heat generation test and thermal analysis were also performed in the same manner as in Example 1. The results are shown in Table 6 and Fig. 9. In Fig. 9, the results of Example 1 are also shown for reference.

[0069]

Table 6

[0070] As can be seen from Fig. 9, even when K2CO3 was used as the inorganic compound, an effect equivalent to that when Na2CO3 was used was obtained.

[0071] <Examples 5 and 6> (Example 5: Sodium sulfate) (Example 6: Sodium silicate) Low activity quicklime containing slaked lime and sparingly soluble Ca salts, which are reaction products of slaked lime and inorganic compounds, was produced in the same manner as in Example 2, except that Na2SO4 (Example 5) and Na2SiO3 (Example 6) were used instead of Na2CO3 as the aqueous solution of inorganic compounds (chemicals) to be sprayed. The low activity limes of Examples 5 and 6 were also subjected to extinguishing heat tests and thermal analyses in the same manner as in Example 1. The results are shown in Table 7 and Figures 10(A) and (B). For reference, the results of Comparative Example 2 (water spray: no chemicals) are shown in Figure 10(C).

[0072] [Table 7]

[0073] As can be seen from the results shown in Table 7 and Figure 10, by using an agent that generates a poorly soluble compound as an inorganic compound, the initial activity suppression effect was obtained similar to that when Na2CO3 was used. In particular, compared to the "water spray" shown in Figure 10(C), it can be seen that the initial temperature rise is greatly suppressed. From these results, it was confirmed that activity suppression by poorly soluble Ca salts is possible by using an agent that generates poorly soluble Ca salts other than Na2CO3.

[0074] Figure 11 shows the relationship between the additive rate and activity of three types of chemicals (Na2CO3, Na2SO4, and Na2SiO3). u The relationship between the drug addition rate and t u From this relationship, it can be seen that the activity decreases as the addition rate increases, and that the activity decreases significantly especially when the Na2SiO3 solution is sprayed.

[0075] Furthermore, in order to confirm the state in which sparingly soluble salts were formed on the surface of the low activity quicklime obtained in Examples 5 and 6, the surface of the sample was observed using a scanning electron microscope (SEM), and elemental mapping of the chemical spray sample was performed using energy dispersive X-ray spectroscopy (EDS) to observe the distribution tendency of elements. For reference, the same observation was also made on the raw quicklime (blank) and the sample sprayed with water only. The results are shown in Figure 12.

[0076] As shown in Fig. 12, only amorphous crystals of quicklime are observed in the blank sample (upper left in the figure). In the water spray sample (upper right in the figure), fine particulate crystals, which are thought to be slaked lime produced by the reaction of water and quicklime, are formed on the surface of the amorphous crystals of quicklime. In contrast, in the chemical spray samples (lower left and lower right in the figure), crystal formation clearly different from that of the water spray sample is observed. Particularly in the case of Na2SiO3 (lower right), the part that seems to be the surface of quicklime has changed to a shape completely covered with fine particles. It is considered that the reason why the Na2SiO3 aqueous solution showed an especially excellent activity reduction effect is that the surface of quicklime was almost completely covered with an insoluble compound.

[0077] Regarding Na2SO4, rather than covering the surface of quicklime with an insoluble compound, many needle-like crystals are formed on the surface of slaked lime, and it is considered that this contributes to the reduction of activity.

[0078] From the results of the above examples, it was proved that by spraying an aqueous solution of an inorganic compound that forms an insoluble compound by reaction with slaked lime, the activity of quicklime can be easily suppressed without the need for high-temperature heat treatment or the like.

[0079] In addition, when the low-activity quicklime thus obtained is used as a soil conditioner or a spraying agent for roads, while maintaining the function as quicklime, heat generation is suppressed, and an effect of preventing dust scattering associated with heat generation can be obtained.

Claims

1. Low-activity quicklime produced by spraying an aqueous solution containing an inorganic compound onto powder or granules of quicklime, wherein a part of the quicklime within one particle contains slaked lime produced by reaction of the quicklime with water in the aqueous solution, and a sparingly soluble compound produced by reaction of the slaked lime with the inorganic compound, and is characterized by the low-activity quicklime containing the same.

2. The inorganic compound is one or more selected from carbonates, silicates, and sulfates of alkali metals, and the sparingly soluble compound contains one or more of carbonates, silicates, and sulfates of calcium, and is characterized by the low-activity quicklime according to Claim 1.

3. The inorganic compound contains at least one of sodium carbonate and potassium carbonate, and is characterized by the low-activity quicklime according to Claim 2.

4. The powder or granules have a particle size of 3 mm or less, and are characterized by the low-activity quicklime according to Claim 1.

5. The powder or granules have a particle size of 1 mm or more, and are characterized by the low-activity quicklime according to Claim 1.

6. The sparingly soluble compound is contained in an amount of 0.5% or more by weight based on the quicklime, and is characterized by the low-activity quicklime according to Claim 1.

7. A soil conditioner comprising the low-activity quicklime according to any one of Claims 1 to 6.

8. A method for suppressing the reactivity of quicklime to produce low-activity quicklime, comprising spraying an aqueous solution of an inorganic compound that reacts with slaked lime to produce a sparingly soluble compound onto powder or granules of quicklime, and including a step of producing slaked lime by reaction of a part of the quicklime with water, In the step of producing slaked lime, a method for producing low-activity quicklime, characterized in that a sparingly soluble compound is produced on the surface of the quicklime by reaction of the slaked lime with the inorganic compound.

9. A method for producing low-activity quicklime according to Claim 8, wherein the concentration of the inorganic compound in the aqueous solution is 5% by weight or more, and is characterized by the method for producing low-activity quicklime.

10. A method for producing low-activity quicklime according to Claim 8, wherein the addition rate of the inorganic compound is 0.5% by weight or more based on the powder or granules of quicklime, and is characterized by the method for producing low-activity quicklime.

11. A method for producing low-activity quicklime according to Claim 8, further comprising a step of using powder or granules of quicklime as a raw material, sieving them through a sieve with a mesh size of 1 mm, and preparing powder or granules of quicklime having a particle size of 3 mm or less and 1 mm or more.

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