Method for producing quicklime

Abstract

To produce quicklime efficiently by shortening a production time and reducing carbon emissions.SOLUTION: After a crushed limestone is put into a kiln, superheated steam is sent into the kiln as a heat source and is fired continuously in contact with the limestone to produce quicklime. A firing temperature is 1000°C or less, preferably 900°C or less, and a firing time is 60 minutes or less, preferably 30 minutes or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for producing quicklime by heating from a calcium carbonate-containing raw material containing calcium carbonate as a main component, such as limestone, eggshells, and seashells.

Background Art

[0002] The main method for producing quicklime is to heat the mined limestone at a high temperature, and fossil fuels such as petroleum, coal, and natural gas are used as fuels for the firing furnace. In firing with fossil fuels, since firing is performed by heated air, there are problems such as low thermal conductivity and long firing time, and CO2 contained in combustion air and combustion exhaust gas exists as a partial pressure, which acts in the direction of suppressing the decarbonation reaction and has poor efficiency. In addition, there is a problem of an increase in CO2 emissions discharged with the combustion of fossil fuels.

[0003] Regarding the issue of CO2 emissions, in recent years, technologies for recovering and utilizing CO2 have been developed. However, in firing with fossil fuels, the concentration of CO2 gas in the exhaust gas is diluted to about 30% or less by the introduction of combustion air, so it is costly to selectively recover CO2 from the diluted gas.

[0004] As a firing method that does not use fossil fuels, heating by an electric furnace can be considered. However, in order to fire limestone to produce quicklime, it is necessary to heat at a high temperature of about 1000 ° C for a long time (for example, 1 hour or more), which requires a large amount of electricity consumption.

[0005] On the other hand, mainly in the food field, as a heat source for heat treatment, superheated steam, which has higher energy than saturated steam and is less likely to generate water droplets, is used. Patent Document 1 describes that in the production of ceramics, the ceramic material after primary firing is secondarily fired with superheated steam, and its application in fields other than food is also expected. However, there have been no reported examples regarding the application of superheated steam in the field of synthesis.

Prior Art Documents

Patent Documents

[0006] [Patent Document 1] Japanese Patent No. 5925025 [Disclosure of the Invention] [Problems to be Solved by the Invention]

[0007] An object of the present invention is to provide a method for producing quicklime with reduced CO2 emissions and efficiently in a relatively short time. [Means for Solving the Problems]

[0008] The method for producing quicklime of the present invention is characterized in that superheated steam is used as a heat source for firing when producing quicklime by firing a calcium carbonate-containing raw material. Specifically, after pulverizing the calcium carbonate-containing raw material and charging it into a firing furnace, a gas as a heat source is fed into the firing furnace, and it is a method of firing while continuously bringing it into contact with the calcium carbonate-containing raw material to produce quicklime, characterized in that superheated steam is used as a heat source.

[0009] The firing temperature is 1000°C or lower, preferably 900°C or lower, and the firing time is 60 minutes or shorter, preferably 30 minutes or shorter.

[0010] The present invention also provides quicklime produced by the above method. Since the quicklime of the present invention can be fired at a relatively low temperature in a short time, it can prevent shrinkage, so the digestion heat generation time (t u ) defined by European standard DIN EN459-2 is 10 seconds or shorter, and it has high activity. [Effects of the Invention]

[0011] According to the manufacturing method of the present invention, since fossil fuel is not used as the heat source for firing, the CO2 emissions throughout the manufacturing process can be reduced. Further, in firing with superheated steam, the exhaust gas consists only of the CO2 gas and steam of the decarboxylation reaction. Therefore, by bringing the exhaust gas to a temperature below the condensation temperature of the steam, a CO2 gas with a concentration close to 100% can be easily recovered.

[0012] Furthermore, since superheated steam is a gas composed only of water molecules, a reducing atmosphere is formed and the partial pressure of CO2 does not exist, so the decarboxylation reaction is efficiently carried out. In addition, since superheated steam has excellent heat transfer properties, firing can be performed in a shorter time compared to conventional methods using fossil fuels or electric furnaces. Since the firing time can be shortened, it can be applied not only to conventional firing furnaces but also to small-sized firing furnaces.

[0013] The quicklime produced by the manufacturing method of the present invention has a relatively small particle size and high activity, and can be used as a raw material for producing slaked lime and also as a raw material for various chemical products.

Brief Description of the Drawings

[0014]

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Mode for Carrying Out the Invention

[0015] Hereinafter, the method for producing quicklime of the present invention will be specifically described. First, the calcium carbonate-containing raw material (hereinafter, also simply referred to as the raw material) is a naturally-derived raw material mainly composed of calcium carbonate. In addition to limestone, waste materials mainly composed of calcium carbonate such as eggshells and shells can be used. Those pulverized to a predetermined particle size prior to firing are used. The particle size is not particularly limited, but those with an average particle diameter preferably of 30 mm or less, more preferably 20 mm or less, and still more preferably 10 mm or less are used. The pulverization of the raw material can be performed using, for example, a pulverizer. Also, if necessary, washing with water and classification are performed. By making the particle sizes uniform, the firing can be made uniform, and higher-quality quicklime can be obtained.

[0016] Superheated steam can be produced by a superheated steam generator (superheater) having a structure similar to that of a once-through boiler in which a pipe through which steam passes is arranged inside. In the superheated steam generator, by heating from the outside of the pipe, while maintaining the pressure of the steam, only the temperature can be raised above the saturation temperature, and superheated steam of up to about 1200 °C can be obtained. In the production method of the present invention, superheated steam of 1100 °C to 1200 °C is used as the superheated steam.

[0017] In the present invention, after filling the above-described calcium carbonate-containing raw material, preferably pulverized to a predetermined particle size, into a firing furnace, superheated steam is fed into the firing furnace through the supply port (supply path) of the heating gas of the firing furnace, and mixed with the above-described raw material in the firing furnace and heat treatment is performed while maintaining a temperature of preferably 1000 °C or less.

[0018] As the firing furnace, a vertical furnace such as a Beckenbach furnace, a Merts furnace, or a Komachi furnace, or a furnace having the same structure as a conventional limestone firing furnace such as a rotary kiln can be used. For example, as shown in FIG. 1, in a conventional firing furnace, by connecting a superheated steam generator to a supply path for supplying hot air (high-temperature air generated by burning fossil fuel), firing can be performed in the same manner as in the conventional case.

[0019] For firing, while sending superheated steam at about 1100 to 1200 °C coming out of the superheated steam generator into the furnace storing the pulverized raw material at a predetermined supply rate, the raw material and the superheated steam are continuously brought into contact to heat the raw material. Depending on the type of the firing furnace, stirring may be involved at this time. The firing time, that is, the time for continuously sending the superheated steam, also varies depending on the particle size of the raw material. For example, for a raw material with a particle size of 1 mm to 30 mm, it may be 60 minutes or less, and for limestone with a particle size of 10 mm or less, even 30 minutes or less can convert almost the entire amount of calcium carbonate derived from the raw material into calcium oxide.

[0020] After completion of firing, the cooling time until the fired product is taken out becomes the residence time. The quicklime after firing may be subjected to post-treatment such as classification according to its subsequent use and requirements.

[0021] According to the method of the present invention, by using superheated steam having excellent heat transfer properties, firing can be performed efficiently at a low temperature and in a short time as compared with the conventional method using combustion air. Further, since firing can be performed at a low temperature, quicklime having a digestion heat generation time (the digestion heat generation time (t u )) of 10 seconds or less can be obtained. The digestion heat generation time is an index of the activity of quicklime, and it is known that the lower the firing temperature, the shorter it becomes. For a fired product at 1000 °C using conventional fossil fuel, t u is about 30 to 300 seconds, whereas in the method of the present invention, since the firing temperature can be made less than 1000 °C, quicklime having a short digestion heat generation time can be obtained. Thereby, high-activity quicklime can be obtained.

[0022] Moreover, according to the method of the present invention, since superheated steam that can be produced only by electricity and water is used, by using renewable energy such as solar power generation and wind power generation as the power for producing superheated steam, the use of fossil fuels can be reduced to zero. Since the exhaust gas during firing becomes only CO2 by cooling, its recovery can be easily carried out.

[0023] The quicklime obtained by the method of the present invention can be used as it is for applications such as soil improvers and heat-generating materials, and can also be used for the production of slaked lime and light calcium carbonate, and as a raw material for other chemical syntheses.

Examples

[0024] <Example 1> (1000 °C, 60 minutes) After crushing limestone with a crusher, it was classified into particles with particle sizes (1) 1 - 3 mm, (2) 3 - 10 mm, and (2) 10 - 30 mm.

[0025] On the other hand, these three types of limestone with different particle sizes were put into a firing furnace (a small experimental furnace), and superheated steam was continuously supplied from the combustion gas inlet of the firing furnace by a superheated steam generator (Tokuden Co., Ltd. UPSS-W20H) for firing. At this time, the supply amount of superheated steam (supply amount per hour) was set to 20 kg / hour, and the firing temperature in the firing furnace was maintained at 1000 °C by adjusting the temperature of the superheated steam in the range of 1100 °C to 1200 °C, and firing was carried out for 60 minutes. Then, after natural cooling until the fired product reached a temperature at which it could be taken out, the fired product was taken out of the furnace, and qualitative analysis by XRD measurement was performed to confirm whether it had become quicklime (calcium oxide).

[0026] For each particle size, the XRD measurement results are shown in FIGS. 2A to 2C, and the SEM is shown in FIG. 3.

[0027] As shown in FIGS. 2A to 2C, for the fired products fired by the method of Example 1, at all particle sizes, the peaks derived from limestone (calcium carbonate) disappeared, and the peaks of a single-phase calcium oxide were confirmed. In FIG. 3, (1), (2), and (3) are fired products of limestone with particle sizes of 1-3 mm, 3-10 mm, and 10-30 mm, respectively.

[0028] <Example 2> (900 °C, 60 minutes) Similar to Example 1, three types of limestone with different particle sizes were used, the temperature of the superheated steam was adjusted so that the firing temperature was 900 °C, and firing was carried out for 60 minutes. For the fired products obtained in Example 2, qualitative analysis by XRD measurement was also performed in the same manner as in Example 1. The results are shown in FIGS. 4A to 4C. Also, the SEM of the fired product using limestone with a particle size of 3-10 mm is shown in FIG. 5.

[0029] As shown in FIGS. 4A to 4C, it was also confirmed that when fired by the method of Example 2, it was converted into a single-phase calcium oxide in the same manner as in Example 1 where the temperature was 1000 °C. However, in the fired product using a particle size of 10-30 mm, a small amount of residual calcium carbonate was confirmed (FIG. 4C). Also, in accordance with the European standard DIN EN459-2, the digestion heat generation time t u of the fired product with a particle size of 3-10 mm was measured, and it was 6 seconds.

[0030] <Examples 3 and 4> (900 °C, 30 minutes, 15 minutes) The firing time was changed to 30 minutes (Example 3) and 15 minutes (Example 4), and firing was carried out at a firing temperature of 900 °C in the same manner as in Example 2 for the rest. The results of XRD measurement for the limestone with a particle size of 3-10 mm in each of Example 3 and Example 4 are shown in FIGS. 6 and 7. In Example 3 where the firing time was 30 minutes, in the result of using limestone with a particle size of 10-30 mm, a slight peak of calcium carbonate was confirmed and a single-phase calcium oxide could not be produced. Therefore, in Example 4, firing using a particle size of 10-30 mm was not carried out. The SEM of the fired products of limestone with particle sizes of 1-3 mm and 3-10 mm in Example 3 is shown in FIG. 8.

[0031] Table 1 shows the summary of the results of Examples 1 to 4 and the XRD results. In Table 1, "〇" indicates that a single-phase calcium oxide was obtained by XRD measurement, "△" indicates that a slight peak of calcium carbonate was observed together with calcium oxide in the XRD measurement, and "×" indicates that a large amount of calcium carbonate peaks were observed together with calcium oxide in the XRD measurement.

[0032]

Table 1

[0033] As is clear from the results shown in Table 1, it was confirmed that by setting the firing temperature to 1000 °C or lower, quicklime of a single-phase calcium oxide can be obtained from limestone. Also, when the firing temperature is 900 °C or lower and the firing time is 15 minutes even with a particle size of 10 mm or less, it was confirmed that quicklime of a single-phase calcium oxide can be obtained.

[0034] <Comparative Examples 1 and 2> Except for using an electric furnace (Asahi Rika Seisakusho ARF-40KC) instead of superheated steam, firing was carried out at a firing temperature of 1000 °C (Comparative Example 1) and a firing temperature of 900 °C (Comparative Example 2) for 60 minutes in the same manner as in Example 1, and qualitative analysis by XRD measurement was performed on the obtained fired products. These results are also shown in Table 1. Also, the XRD results and SEM of Comparative Example 1 (when using a particle size of 10 - 30 mm) and Comparative Example 2 (when using a particle size of 1 - 3 mm) are shown in FIGS. 9 and 10. Also, the SEM of the fired product (particle size 3 - 10 mm) obtained in Comparative Example 1 is shown in FIG. 11.

[0035] As is clear from the results in Table 1 and the results shown in FIGS. 9 and 10, when using an electric furnace, in Comparative Example 1 where the same firing temperature of 1000 °C as in Example 1 was set, it was confirmed that when the particle size is large, it cannot be completely converted to calcium oxide and a long firing time of 1 hour or more is required. Also, at a firing temperature of 900 °C, even when the particle size is small (1 - 3 mm), the firing is insufficient at a firing time of 60 minutes, and it was confirmed that it is necessary to increase the firing temperature or lengthen the firing time.

[0036] From the comparison of SEM images of the calcined products of Example 1 and Comparative Example 1 calcined at a firing temperature of 1000 °C and a firing time of 60 minutes using limestone with a particle size of 3 - 10 mm (Figs. 3(2) and 11), finer-grained quicklime was obtained when using superheated steam than when using an electric furnace.

Claims

1. A method for producing quicklime by pulverizing limestone as a calcium carbonate-containing raw material, charging it into a firing furnace, feeding a gas as a heat source into the firing furnace, and firing while continuously bringing it into contact with the raw material, characterized in that superheated steam not containing carbon dioxide is used as the gas of the heat source.

2. The method for producing quicklime according to Claim 1, wherein the firing temperature is 1000 °C or lower.

3. The method for producing quicklime according to Claim 1 or 2, wherein the particle size of the limestone is 30 mm or less.

4. The method for producing quicklime according to any one of Claims 1 to 3, wherein the firing time is 60 minutes or less.

5. The method for producing quicklime according to any one of Claims 1 to 4, wherein the temperature of the superheated steam is 1100 °C to 1200 °C.

6. The method for producing quicklime according to any one of Claims 1 to 5, wherein the firing furnace is any one of a Beckenbach furnace, a Merts furnace, a rotary kiln, and a vertical furnace.

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