JP6977791B2 - Method for producing reducing agent and method for producing cement composition - Google Patents

Description

The present disclosure relates to a reducing agent and a method for producing the same, and a method for producing a cement composition.

Hexavalent chromium contained in wastewater and soil is harmful to the human body such as causing allergic symptoms, so the content is specified as 0.05 mg / L or less in water quality standards. For example, heavy metals such as chromium, which are derived from various raw materials and fuels, may be mixed in the cement clinker in a very small amount, which may lead to the elution of hexavalent chromium. It is known that hexavalent chromium is reduced by a reducing agent and converted into harmless trivalent chromium. Therefore, a method of reducing the elution of hexavalent chromium from solidified soil by adding a reducing agent such as sulfite to cement has been studied (see, for example, Patent Documents 1 and 2).

An example of sulfite is calcium sulfite produced by reacting sodium sulfite with slaked lime. However, commercially available sulfites are generally expensive.

Japanese Unexamined Patent Publication No. 2010-22795 Japanese Unexamined Patent Publication No. 2016-121047

When an expensive sulfite is used as a reducing agent, a method of atomizing it to increase the specific surface area and increasing the reactivity can be considered as a means for reducing the amount used. However, there is a concern that the manufacturing cost will be further increased if a new pulverization treatment is added. Therefore, the present disclosure provides a reducing agent capable of inexpensively producing a reducing agent having excellent reactivity and a method for producing the reducing agent. The present disclosure also provides a method for producing a cement composition capable of inexpensively producing a cement composition capable of sufficiently reducing the elution of hexavalent chromium by using such a reducing agent.

The method for producing a reducing material according to one aspect of the present disclosure includes a step of introducing an alkaline slurry containing an alkaline compound and water and a raw material gas containing sulfur dioxide into a slurry preparation unit, and the alkaline slurry and the raw material gas. The slurry preparation unit has a step of producing a reducing material slurry containing a sulfite and a step of deriving a reducing material slurry, and the SO ₂ concentration of the raw material gas is 200 to 100,000 ppm, and the slurry is prepared. The temperature and the concentration of the solid content of the reducing material slurry staying in the portion are 70 ° C. or lower and 0.1 to 50% by mass, respectively, and the content of sulfurous acid in the solid content is 1% by mass or more.

In the above production method, _{a raw material gas having an SO 2} concentration of 200 to 100,000 ppm is brought into contact with an alkaline slurry to generate a slurry containing a sulfite. Since the raw material gas having such an SO ₂ concentration can be prepared by using the exhaust gas generated in a factory or the like, it can be obtained at low cost. Further, since the temperature of the slurry is 70 ° C. or lower, the sulfite can be stably maintained. Further, the reducing agent slurry having a solid content concentration of 0.1 to 50% by mass is excellent in handleability. Further, since the content of sulfite in the solid content is 1% by mass or more, the reducing agent produced by the above production method has excellent reactivity. Therefore, the above-mentioned production method can inexpensively produce a reducing agent having excellent reactivity.

The slurry preparation unit has an absorption tower in which the raw material gas and the alkaline slurry come into contact with each other and the reducing material slurry stays, a circulation line for circulating the reducing material slurry staying in the absorption tower to the upper part of the absorption tower, and a circulation line. It may have a nozzle for spraying the circulated reducing material slurry into the absorption tower, and a lead-out line for deriving the reducing material slurry staying in the absorption tower. _{As a result, SO 2} contained in the raw material gas can be sufficiently converted into a sulfite.

The ratio (A / B) of the retention amount A of the reducing material slurry retained in the slurry preparation unit in terms of solid content and the extraction amount B of the reducing material slurry derived from the slurry preparation unit in terms of solid content per day. ) May be 0.2 to 7.0 days. This makes it possible to suppress oxidation while promoting crystal growth of sulfites. Therefore, it is possible to obtain a reducing agent having more excellent reactivity of the reducing agent and easily dehydrated.

In the above manufacturing method, an alkaline slurry tank that mixes an alkaline compound and water and stores an alkaline slurry to be supplied to a slurry preparation unit and a reducing material slurry tank that stores a reducing material slurry derived from the slurry preparation unit are provided. You may use it. As a result, the reducing agent can be continuously produced, and the reducing agent can be produced at a lower cost.

The alkaline compound may contain at least one selected from the group consisting of carbonates of alkaline earth metals, hydroxides of alkaline earth metals, carbonates of alkali metals, and hydroxides of alkali metals. As a result, sulfites can be efficiently produced.

The reducing agent slurry may contain at least one sulfite of an alkaline earth metal and an alkali metal. Since such sulfites are sufficiently reactive, they can sufficiently suppress the elution of hexavalent chromium when used as a raw material for soil conditioners, for example. The reducing agent slurry may contain at least one of an alkaline earth metal and an alkali metal bisulfite. By containing hydrogen sulfite, the elution of hexavalent chromium can be further suppressed.

Sulfites may include calcium sulfite. Since such sulfites are sufficiently reactive, the elution of hexavalent chromium can be sufficiently suppressed.

The above-mentioned production method may include a step of dehydrating the reducing agent slurry and drying it under reduced pressure to obtain a powdery reducing agent. The powdered reducing agent has excellent handleability. For example, cement composition can be produced by mixing with Portland cement.

The sulfites contained in the powdered reducing agent obtained by drying the reducing agent slurry have the lengths of the major axis and the minor axis of the main surface of 1 to 500 μm and 0.5 to 300 μm, respectively, and the thickness is long. It may contain flaky crystals that are 50% or less of the length of the shaft. Such sulfites are excellent in reactivity because of their small crystal size. Further, since it contains flaky crystals, it is possible to easily produce a reducing agent having excellent pulverizability and high reactivity.

The reducing agent according to one aspect of the present disclosure can be obtained by any of the above-mentioned production methods. The average particle size of the reducing agent is 10 μm or less. The reducing agent may be in the form of a powder obtained by drying the reducing agent slurry, or the solid content having an average particle size of 10 μm or less may be dispersed in a solvent to form a slurry. Such a reducing agent can be produced at low cost while having excellent reactivity.

The method for producing a cement composition according to one aspect of the present disclosure includes a step of obtaining a cement composition containing a reducing material produced by the production method according to any one of the above and a cement clinker, and the cement clinker. The reducing material is contained in an amount of 0.1 to 20 parts by mass based on the solid content with respect to 100 parts by mass. Since this cement composition contains the reducing agent produced by the above-mentioned production method in a predetermined ratio, it is possible to sufficiently reduce the elution of hexavalent chromium and improve the strength of the solidified soil. Moreover, it can be manufactured at low cost.

In the step of obtaining the above-mentioned cement composition, a cement clinker, a reducing agent and gypsum may be mixed and pulverized to obtain a cement composition. This cement composition may contain 1 to 30 parts by mass of gypsum with respect to 100 parts by mass of cement clinker. This makes it possible to improve the strength of the solidified soil.

The method for producing a cement composition according to one aspect of the present disclosure includes a step of mixing a reducing material produced by any of the above-mentioned production methods with Portland cement and gypsum to obtain a cement composition, and the Portland cement. For 100 parts by mass, 0.1 to 20 parts by mass of the reducing material and 1 to 30 parts by mass of gypsum may be contained. Since this cement composition contains the reducing agent produced by the above-mentioned production method in a predetermined ratio, it is possible to sufficiently reduce the elution of hexavalent chromium and improve the strength of the solidified soil. Moreover, it can be manufactured at low cost.

The above-mentioned method for producing a cement composition may include a step of preparing Portland cement by mixing 1 to 30 parts by mass of gypsum with 100 parts by mass of cement clinker.

According to the present disclosure, it is possible to provide a reducing agent capable of inexpensively producing a reducing agent having excellent reactivity and a method for producing the reducing agent. Further, according to the present disclosure, it is provided a method for producing a cement composition capable of inexpensively producing a cement composition capable of sufficiently reducing the elution of hexavalent chromium by using such a reducing agent. Can be done.

FIG. 1 is a diagram showing an example of a reducing agent manufacturing apparatus. FIG. 2 is a schematic diagram showing an example of flaky crystals. FIG. 2A is a plan view of the flaky crystal, and FIG. 2B is a side view of the flaky crystal. FIG. 3 is a graph in which the results of Examples 1-1 to 1-9 are plotted with the horizontal axis representing the SO _{2 concentration in the raw material gas and the vertical axis representing the retention slurry temperature.} FIG. 4 is an SEM (scanning electron microscope) photograph of the reducing agent of Example 1-1. FIG. 4A is an SEM photograph enlarged from FIG. 4B. FIG. 5 is a graph plotting the results of Examples 1-1 to 1-9, where the horizontal axis is the SO ₂ concentration in the raw material gas and the vertical axis is the retention amount A / extraction amount B. FIG. 6 is a graph in which the experimental results are plotted with the horizontal axis representing the crushing time and the vertical axis representing the brain specific surface area. FIG. 7 is an SEM (scanning electron microscope) photograph of the powdered reducing agent a. FIG. 8 is a graph showing the results of the amount of Cr (VI) elution from the solidified soil for each content of calcium sulfite / hemihydrate. FIG. 9 is a graph showing the results of uniaxial compressive strength of solidified soil for each content of calcium sulfite / hemihydrate.

Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings as the case may be. However, the following embodiments are examples for explaining the present disclosure, and are not intended to limit the present disclosure to the following contents. In the description, the same reference numerals are used for the same elements or elements having the same function, and duplicate description may be omitted in some cases. In addition, the positional relationship such as up, down, left, and right shall be based on the positional relationship shown in the drawings unless otherwise specified. Further, the dimensional ratio of each element is not limited to the ratio shown in the figure.

The method for producing the reducing material according to one embodiment includes a step of introducing an alkaline slurry containing an alkaline compound and water and a raw material gas containing sulfur dioxide into a slurry preparation unit, and contacting the alkaline slurry with the raw material gas. The slurry preparation unit includes a step of producing a reducing material slurry containing a sulfurous acid salt and a step of deriving the reducing material slurry. The reducing agent produced by this production method may be in the form of a slurry, a dehydrated cake, or a powder. Therefore, unless otherwise specified, the term "reducing agent" in the present disclosure is not limited to any form thereof, and includes, for example, any form of slurry, dehydrated cake, and powder. Further, the "reducing agent slurry" in the present disclosure is a slurry-like reducing agent and contains sulfite as a solid content. For example, the “retained slurry” that stays in the absorption tower 11 and the “circulated slurry” that is circulated in the circulation line 12, which will be described later, are also a kind of reducing agent slurry.

The alkaline compound contained in the alkaline slurry contains at least one selected from the group consisting of carbonates of alkaline earth metals, hydroxides of alkaline earth metals, carbonates of alkali metals, and hydroxides of alkali metals. It's fine. This makes it possible to efficiently generate sulfites. Preferred alkaline earth metals include calcium and magnesium, and preferred alkali metals include sodium and potassium.

The sulfite contained in the reducing material slurry may contain at least one sulfite of an alkaline earth metal and an alkali metal. Since such sulfites are sufficiently reactive, for example, when used as a soil conditioner, they sufficiently suppress the elution of hexavalent chromium from the solidified soil and increase the strength of the solidified soil. be able to. From this point of view, the sulfite preferably contains calcium sulfite. The reducing agent slurry may contain at least one of an alkaline earth metal and an alkali metal, hydrogen sulfite, together with the sulfite. By containing hydrogen sulfite, the elution of hexavalent chromium can be further suppressed.

FIG. 1 is a diagram showing an example of a reducing agent manufacturing apparatus for carrying out the above-mentioned manufacturing method. The manufacturing apparatus 100 of FIG. 1 stores a slurry preparation unit 10 for preparing a reducing material slurry containing a sulfite and an alkaline slurry prepared by mixing an alkaline compound and water, and the alkaline slurry is stored in the slurry preparation unit 10. The alkaline slurry tank 30 to be supplied, the reducing material slurry tank 20 for storing the reducing material slurry derived from the slurry preparing unit 10, the water content adjusting unit 40 for reducing the water content contained in the reducing material slurry, and the reducing material slurry are cooled. A cooling unit 50 is provided.

The slurry preparation unit 10 has an absorption tower 11 in which the raw material gas containing sulfur dioxide and the alkaline slurry are in contact with each other and the reducing material slurry is retained, a gas introduction line 16 for introducing the raw material gas into the absorption tower 11, and an alkaline slurry tank 30. A slurry introduction line 13 for introducing an alkaline slurry into the absorption tower 11, a circulation line 12 for circulating the reducing material slurry staying at the bottom of the absorption tower 11 to the upper part of the absorption tower 11, and a reducing material circulated in the circulation line 12. It has a nozzle 14 that sprays the slurry into the absorption tower 11, a lead-out line 15 that derives the reducing material slurry that stays at the bottom of the absorption tower 11, and a gas discharge line 17 that discharges the exhaust gas from the upper part of the absorption tower 11. .. In the present disclosure, the reducing agent slurry that stays at the bottom of the absorption tower 11 may be referred to as "retained slurry", and the reducing agent slurry that is circulated in the circulation line 12 may be referred to as "circulating slurry".

The alkaline slurry stored in the alkaline slurry tank 30 is prepared by stirring and mixing an alkaline compound and water. The concentration of the alkaline compound in the alkaline slurry is, for example, 5 to 25% by mass, preferably 10 to 20% by mass, and more preferably 15 to 20% by mass. The amount of the alkaline slurry supplied from the alkaline slurry tank 30 to the absorption tower 11 may be adjusted according to the pH of the reducing material slurry (circulating slurry or stagnant slurry). When the pH value of the reducing agent slurry (circulating slurry or stagnant slurry) in the absorption tower 11 or the circulation line 12 becomes equal to or less than the lower limit, control is performed to start or increase the supply amount of the alkaline slurry to the absorption tower 11. .. On the other hand, when the pH value of the reducing agent slurry (circulating slurry or stagnant slurry) becomes equal to or higher than the upper limit value, control is performed to stop or reduce the supply amount of the alkaline slurry to the absorption tower 11. Thereby, the pH of the reducing agent slurry can be adjusted to a predetermined range.

The upper limit of the pH of the reducing agent slurry is, for example, 6.5 or less, preferably 6.3 or less, more preferably 6.0 or less, and further preferably 5.8 or less. The lower limit of the pH of the reducing agent slurry is 5.0 or more, preferably 5.3 or more, more preferably 5.4 or more, and further preferably 5.5 or more.

_{The SO 2} concentration in the raw material gas is 200 to 100,000 ppm. _{The lower limit of the SO 2} concentration in the raw material gas is preferably 500 ppm, more preferably 1000 ppm, still more preferably 2000 ppm, and most preferably 3000 ppm. _{The upper limit of the SO 2} concentration in the raw material gas is preferably 50,000 ppm, more preferably 30,000 ppm, and most preferably 20,000 ppm. When the SO ₂ concentration is lower than the lower limit, the efficiency of producing a reducing agent (sulfite, etc.) decreases, and the proportion of gypsum having no reducing effect tends to increase. On the other hand, when the SO ₂ concentration exceeds the upper limit, _{the absorption efficiency of SO 2 contained in the raw material gas tends to decrease, and the amount of SO 2} discharged from the gas discharge line 17 tends to increase. The concentration of sulfur dioxide in the present disclosure is a volume-based (standard temperature) concentration, and can be measured using a commercially available measuring device.

As the raw material gas, various gases including sulfurous acid gas can be used. The raw material gas is, for example, exhaust gas generated from a cement clinker manufacturing facility, exhaust gas generated from a thermal power generation facility, gas from a sulfur dioxide bomb, combustion gas obtained by burning sulfur, and sulfide obtained by burning. It may contain at least one selected from the group consisting of a combustion gas and a heat-decomposed gas obtained by heat-decomposition of a sulfur compound (such as gypsum). For example, two or more kinds of gases having different sulfur dioxide concentrations may be mixed to adjust the sulfur dioxide concentration of the raw material gas. The temperature of the raw material gas introduced from the gas introduction line 16 to the absorption tower 11 may be, for example, 20 to 500 ° C. When the temperature of the raw material gas is low, it is not necessary to provide the cooling unit 50. Further, when the temperature fluctuation of the raw material gas is large, both the cooling unit 50 and the heating unit may be provided to control the temperature of the reducing agent slurry (retained slurry) in the absorption tower 11.

The cooling unit 50 is connected to a heat exchanger 52 that exchanges heat with the reducing agent slurry that stays at the bottom of the absorption tower 11. The cooling unit 50 supplies the refrigerant to the heat exchanger 52. The stagnant slurry 22 staying at the bottom of the absorption tower 11 is adjusted to a temperature of 70 ° C. or lower by heat exchange with the refrigerant. The temperature of the stagnant slurry 22 in the absorption column 11 is preferably adjusted to 60 ° C. or lower, more preferably 50 ° C. or lower, still more preferably 45 ° C. or lower, particularly preferably 40 ° C. or lower, and most preferably 35 ° C. or lower. As a result, the reaction temperature can be reduced even when the external temperature is high (such as in summer) or when the introduction temperature of the raw material gas into the absorption tower 11 is high. Therefore, it is possible to prevent oxidation of the produced sulfite and produce a reducing agent with higher efficiency. The temperature of the stagnant slurry 22 in the absorption tower 11 may be 5 ° C. or higher and may be 10 ° C. or higher from the viewpoint of preventing freezing.

The concentration of the solid content of the retained slurry 22 in the absorption column 11 is 0.1 to 50% by mass. The concentration is preferably 0.1 to 50% by mass, more preferably 1 to 30% by mass, still more preferably 3 to 28% by mass, from the viewpoint of improving handleability and facilitating dehydration and dry powdering. It is by mass, particularly preferably 10 to 25% by mass. The content of sulfite in the solid content is 1% by mass or more. The content of each sulfite can be determined by thermogravimetric analysis as shown in Examples or the XRD Rietveld method. The content of each bisulfite can be determined by the XRD Rietveld method.

A part of the stagnant slurry 22 staying at the bottom of the absorption tower 11 of the slurry preparation unit 10 is derived from the bottom and circulated to the upper part of the absorption tower 11 via the circulation line 12. The circulating slurry that has flowed through the circulation line 12 is sprayed into the absorption tower 11 from the nozzle 14 attached to the tip of the circulation line 12. The circulating slurry descending in the absorption tower 11 comes into countercurrent contact with the raw material gas introduced into the absorption tower 11 from the gas introduction line 16 connected below the nozzle 14. As a result, sulfur dioxide contained in the raw material gas is sufficiently incorporated into the circulating slurry, and the production of sulfites is promoted.

The reducing agent slurry generated in the absorption tower 11 is led out from the absorption tower 11 by the lead-out line 15 and stored in the reducing agent slurry tank 20. The temperature range of the derived reducing agent slurry, the concentration range of the solid content, and the range of the sulfite content in the solid content may be the same as those of the retained slurry 22. The temperature range of the circulating slurry, the concentration range of the solid content, and the range of the content of sulfite in the solid content may be the same as those of the retained slurry 22.

The content of the sulfite in the solid content is, for example, 40% by mass or more, preferably 60% by mass or more, more preferably 70% by mass or more, and particularly preferably 70% by mass or more, from the viewpoint of improving the reactivity of the reducing agent. Is 80% by mass or more, and most preferably 90% by mass or more.

The solid content conversion of the retained slurry 22 staying at the bottom of the absorption tower 11 of the slurry preparation unit 10 and the solid content conversion of the reducing material slurry derived from the lead-out line 15 of the slurry preparation unit 10 per day. The ratio (A / B) to the extraction amount B is 0.2 to 7.0 days. The ratio (A / B) is preferably 0.5 to 6.0 days, more preferably 1.0 to 5.0 days, and even more preferably 2.0 to 4.0 days. Most preferably 2.0 to 3.0 days. If the A / B is too small, the crystal growth of the reducing agent becomes insufficient and the dehydration performance tends to deteriorate. On the other hand, if the A / B is too large, the oxidation of the reducing agent tends to proceed.

The sulfurous acid content in the reducing agent slurry can be adjusted by _{the temperature of the stagnant slurry 22 in the absorption tower 11 and / or the SO 2} concentration contained in the raw material gas introduced into the absorption tower 11 from the gas introduction line 16. .. Specifically, the content of sulfurous acid in the reducing agent slurry can be increased by lowering the temperature of the retained slurry 22 and / or _{increasing the concentration of SO 2 introduced into the absorption tower 11.}

The reducing agent manufacturing apparatus shown in FIG. 1 may have an air bubbling facility (oxidation facility) attached to a normal flue gas desulfurization device. Such oxidation equipment is not essential and may not be necessary. The oxygen concentration contained in the raw material gas introduced into the absorption tower 11 is, for example, 20% by volume or less, preferably 18% by volume or less, more preferably 15% by volume or less, and most preferably 10% by volume or less. Is.

The reducing agent slurry led out to the reducing agent slurry tank 20 may be made into a concentrated slurry by performing a concentration step in the water content adjusting unit 40, may be made into a dehydrated cake by performing a dehydration step, and may be further subjected to a dry powdering step. It may be in the form of powder. The sulfites contained in such a reducing agent have the lengths of the major axis and the minor axis of the main surface of 1 to 500 μm and 0.5 to 300 μm, respectively, and the thickness is 50% or less of the length of the major axis. It may contain certain flaky crystals. Such sulfites are excellent in reactivity because of their small crystal size. Since the flaky crystals are excellent in pulverizability, a highly reactive reducing agent can be easily produced. The length and thickness of the major axis and the minor axis of the main surface of the flaky crystal can be measured based on the observation image by SEM.

FIG. 2 is a schematic diagram showing an example of flaky crystals included in the observation image by SEM. FIG. 2A is a plan view of the flaky crystal 60, and FIG. 2B is a side view of the flaky crystal 60. The length of the major axis 64 of the main surface 62 of the flaky crystal 60 is two different positions selected on the outer edge of the main surface of the plate-like crystal (the surface having the plane having the largest area). It is the maximum distance among the distances connecting points. The length of the minor axis 66 of the main surface 62 is the maximum distance among the distances connecting two points at different positions selected on the outer edge of the main surface 62 in the direction orthogonal to the major axis 64 of the main surface 62. Is. For example, if the main surface is square, the lengths of the main axis and the minor axis are both equal to the length of the diagonal line. The thickness of the flaky crystal is the maximum length along the direction orthogonal to the main plane. In FIG. 2B, the thickness of the flaky crystal 60 is T.

The length of the major axis 64 of the main surface 62 is preferably 1 to 500 μm, more preferably 3 to 300 μm, still more preferably 5 to 150 μm, and most preferably 10 to 100 μm. The length of the minor axis 66 of the main surface 62 is preferably 0.5 to 300 μm, more preferably 1 to 150 μm, and most preferably 5 to 50 μm. The thickness T of the flaky crystal 60 is preferably 50% or less, more preferably 30% or less, still more preferably 10% or less, and particularly preferably 10% or less, with respect to the length of the major axis 64 of the main surface. It is preferably 5% or less, and most preferably 1% or less. Specifically, the thickness T may be, for example, 30 μm or less, 10 μm or less, or 1 μm or less.

The dry powdering step may be carried out by drying the dehydrated cake obtained in the dehydration step under reduced pressure while applying shear. By going through such a dry powdering step, the flaky crystals are crushed while being dried. The average particle size of the powdered reducing agent is preferably 10 μm or less, more preferably 5 μm or less, further preferably 3 μm or less, particularly preferably 2 μm or less, and most preferably 1 μm or less. The average particle size referred to here is a median diameter (D50: particle size when the cumulative frequency is 50%) measured using a laser diffraction type particle size distribution measuring device. As the laser diffraction type particle size distribution measuring device, SALD-2200 (device name) manufactured by Shimadzu Corporation can be used. Since flaky crystals are easily pulverized, a reducing agent having a high degree of powderiness and high reactivity can be obtained without using a pulverizer such as a mill. However, it does not exclude crushers such as mills.

The method for producing a cement composition according to an embodiment includes a step of blending the reducing agent obtained by the above-mentioned production method with a cement clinker to obtain a cement composition. The reducing agent may be in the form of a slurry, a powder, or a dehydrated cake. Gypsum may also be added. The cement composition may contain, for example, 0.1 to 20 parts by mass of a reducing agent with respect to 100 parts by mass of cement clinker, or 2 to 20 parts by mass based on the solid content.

In the above step of obtaining a cement composition, the reducing agent and the cement clinker may be sequentially or simultaneously introduced into a crusher (for example, a mill) and mixed while pulverizing to prepare a cement composition. This makes it possible to adjust the particle size of the reducing agent while mixing. If a high-temperature cement clinker and a reducing agent slurry or a dehydrated cake-like reducing agent are introduced into a pulverizer, drying, mixing and pulverization can be performed in parallel to produce a cement composition.

The pulverizability of the cement composition can be improved by blending the reducing agent and the cement clinker, further blending gypsum as needed, and pulverizing the cement composition together. When gypsum is blended, the blending amount of gypsum is 1 to 30 parts by mass with respect to 100 parts by mass of cement clinker. The degree of powderiness of the cement composition is, for example, 2500 to 10000 cm ² / g, preferably 3000 to 8000 cm ² / g, more preferably 3500 to 7000 cm ² / g, and further preferably 4000 to 4000 in terms of the specific surface area of the brain. 6000cm was ² / g, most preferably 5000~6000cm ² / g. The higher the specific surface area of the brain, the easier it is to obtain the effect of improving the pulverizability of the reducing agent.

The method for producing a cement composition according to another embodiment is produced by a step of mixing cement with, for example, a powdery reducing material obtained in a dry powdering step or a dehydrated cake-like reducing material obtained in a dehydrating step. You may. In addition, powdered gypsum may be mixed together. The types of cement are ordinary Portland cement, early-strength Portland cement, moderate heat Portland cement, low heat Portland cement, sulfate-resistant Portland cement, blast furnace cement (A, B, C), fly ash cement (A, B). Either species or C species) may be used. The blending amount of the reducing agent with respect to 100 parts by mass of the cement is, for example, 0.1 to 20 parts by mass, preferably 2 to 15 parts by mass, and more preferably 4 to 10 parts by mass based on the solid content. When gypsum is added, the amount of gypsum added is 1 to 30 parts by mass with respect to 100 parts by mass of cement.

The cement composition may be used as it is as a soil conditioner, or may be mixed with gypsum to be a soil conditioner. The soil-improved soil may be prepared by blending the soil to be improved, the cement composition and gypsum.

Although one embodiment of the present disclosure has been described above, the present disclosure is not limited to the above embodiment. For example, when the exhaust gas discharged from the gas discharge line 17 of the manufacturing device 100 of FIG. 1 contains sulfur dioxide, a device similar to the manufacturing device 100 is provided on the downstream side of the gas discharge line 17, and reduction is performed using the device. A material slurry may be produced. Further, in the manufacturing apparatus 100 of FIG. 1, a part of the circulation line 12 and the lead-out line 15 is shared, but the present invention is not limited to this, and the circulation line 12 and the lead-out line 15 are individually connected to the bottom of the absorption tower 11. May be done.

The contents of the present disclosure will be described in more detail with reference to Examples and Comparative Examples, but the present disclosure is not limited to the following examples.

(Examples 1-1 to 1-9)
[Preparation of reducing agent slurry]
In the manufacturing apparatus as shown in FIG. 1, a raw material gas at 200 ° C. containing sulfur dioxide was introduced from the gas introduction section. As shown in Table 1, the reducing agent slurries of Examples 1-1 to 1-9 having different contents of calcium sulfite / hemihydrate were produced by changing _{the SO 2 concentration in the raw material gas.} In each of the examples, the pH and temperature of the reducing agent slurry at the bottom of the absorption tower were 5.50 to 5.80 and 30 to 40 ° C., respectively.

[Analysis of solid content concentration in reducing agent slurry]
The reducing agent slurry was dried in air at 100 ° C., and the solid content concentration was measured from the amount of mass loss. The specific calculation formula is as follows. The solid content concentration of each example was as shown in Table 1.
-Solid content concentration (mass%) = (mass of reducing material slurry-mass reduction amount) / mass of reducing material slurry x 100

[Solid composition analysis]
The reducing agent slurry was filtered and dehydrated to obtain a cake. This cake was dried in air at 60 ° C. to obtain a solid content. The powder XRD analysis of the obtained solid content was performed, and it was confirmed that only calcium sulfite / hemihydrate, calcium carbonate, and dihydrate gypsum were detected. Then, thermogravimetric analysis was performed to determine the content (% by mass) of calcium sulfite hemihydrate, calcium carbonate, and dihydrate gypsum in the solid content. The results are as shown in Table 1. The analysis conditions are as follows.

<Powder XRD analysis>
An X-ray diffractometer (manufactured by Bruker AXS Co., Ltd., acceleration voltage: 30 kV, current: 10 mA, tube: Cu) was used.

<Thermogravimetric analysis>
Using a commercially available thermogravimetric analyzer, the change in thermogravimetric analysis when heated to 200 ° C., 600 ° C. and 900 ° C. was measured. At 200 ° C, the dihydrate gypsum dehydrates. Calcium sulfite oxidizes at 600 ° C. Calcium carbonate decomposes at 900 ° C. Using the change in mass at each temperature, the contents of dihydrate gypsum, calcium sulfite and calcium carbonate were calculated based on the following formulas. The sample mass is the mass of the solid content used in the thermogravimetric analysis. It should be noted that the holding at each heating temperature was set until the weight change disappeared. The holding time at each heating temperature was 1 hour. In addition, the atmosphere at the time of heating was air.

-Content rate of dihydrate gypsum (mass%) = mass reduction amount until the mass change disappears after heating from room temperature to 200 ° C. x ([Molecular weight of dihydrate gypsum] / [Crystal water contained in dihydrate gypsum] Molecular weight]) / sample mass x 100
-Calcium carbonate content (mass%) = (mass of sample after heating to 900 ° C and no change in mass) x [mass of sample after heating to -600 ° C and no change in mass) x [CaCO ₃ Molecular weight] / [Molecular weight of CO ₂ ] / Mass of sample x 100
-Calcium sulfite content (mass%) = 100- (dihydrate gypsum content + calcium carbonate content)

As shown in Table 1, all of Examples 1-1 to 1-9 contain calcium sulfite / hemihydrate, calcium carbonate and dihydrate gypsum in the solid content, and calcium sulfite / hemihydrate. The content of the substance was 30% by mass or more. In particular, in _{Examples 1-1 and 1-2 in which the SO 2} concentrations in the introduced gas are 3078 ppm and 3140 ppm, respectively, the contents of calcium sulfite and hemihydrate are 91.4% by mass and 90.2, respectively. It showed a high value of mass%.

FIG. 3 is a graph plotting the results of Examples 1-1 to 1-9, with the horizontal axis representing the SO ₂ concentration in the raw material gas and the vertical axis representing the temperature of the stagnant slurry staying at the bottom of the absorption tower. .. Further, the numerical value in the mass% unit in FIG. 3 indicates the content of calcium sulfite / hemihydrate in the solid content in the retained slurry (reducing agent slurry).

4 (A) and 4 (B) are SEM (scanning electron microscope) photographs of the reducing agent of Example 1-1. FIG. 4A is an SEM photograph enlarged from FIG. 4B. The crystals of calcium sulfite / hemihydrate contained in the reducing agent are thin with a minor axis length of 10 to 30 μm on the main surface, a major axis length of 15 to 100 μm on the main surface, and a thickness of 1 μm. It was confirmed that it was a plate-like crystal. Such crystals are easily crushed and can be easily made into fine particles of less than 1 μm without using a crusher.

In each embodiment, the amount of the retained slurry staying at the bottom of the absorption tower, the flow rate of the reducing material slurry extracted from the bottom of the absorption tower, and the solid content concentration of each slurry indicate that the slurry stays at the bottom of the absorption tower. The solid content (retention amount A) of the stagnant slurry (reducing material slurry) and the solid content (extraction amount B) of the reducing material slurry derived from the bottom of the absorption tower per day were determined. In addition, the ratio (A / B) of the retention amount A and the extraction amount B was determined. These results are as shown in Table 2.

FIG. 5 is a graph plotting the results of Examples 1-1 to 1-9, with the horizontal axis representing _{the SO 2} concentration in the raw material gas and the vertical axis representing A / B in Table 2. Further, the numerical value in the mass% unit in FIG. 5 indicates the content of calcium sulfite / hemihydrate contained in the solid content in the retained slurry (reducing agent slurry). As shown in FIG. 5, _{it was confirmed that the higher the SO 2} concentration and the lower the A / B value, the higher the calcium sulfite content tends to be.

[Crushability of reducing agent]
The slurry of Example 1-1 was dried in air at 60 ° C. to prepare a reducing agent (solid content). In addition, flue gas desulfurization gypsum was prepared for comparison. This flue gas desulfurization gypsum is a dihydrate gypsum produced in a thermal power generation facility that is desulfurized by the lime-gypsum method. (Hereinafter referred to as drained dihydrate gypsum), each of these was pulverized with a ball mill for 10 minutes, and the specific surface areas of the brains were compared. The brain specific surface area was measured according to JIS R5201: 2015 “Physical test method for cement”. The specific surface area of the brain of the reducing agent was 9360 cm ² / g, whereas the specific surface area of the brain of the flue gas desulfurized gypsum was 2370 cm ² / g. As described above, since the reducing agent tends to have a high brain specific surface area, it was confirmed that the reducing agent has good pulverizability.

(Example 2-1, Comparative Example 2-1 and Reference Example 2-1 to 2-6)
[Preparation of cement composition]
The reducing agent slurry of Example 1-1 was dried in air at 60 ° C. to prepare a reducing agent (solid content, water content: <1% by mass). The reducing agent and / or drainage dihydrate gypsum and ordinary Portland cement clinker were put into a ball mill with the formulations shown in Table 2 and pulverized for a predetermined time to obtain a cement composition. The degree of powderiness of the cement composition thus obtained was evaluated. The evaluation method is as follows.

[Evaluation of powderiness of cement composition]
The degree of powderiness of the cement composition was evaluated by the specific surface area of the brain and the residue of 45 μm. The brain specific surface area was measured according to JIS R5201: 2015 “Physical test method for cement”. The 45 μm residue was measured in accordance with JCAS K-02-2004 “Cement Powder Degree Test Method Using 45 μm Net Sieve”. The results of these measurements are shown in Table 3.

[Evaluation result of crushability of cement composition containing reducing agent]
The cement compositions of Example 2-1 and Example 2-2 containing the reducing agent have a shorter brain specific surface area than the cement composition of Comparative Example 2-1 containing no reducing agent. It became 4420-4470 cm ² / g.

FIG. 6 is a graph in which the results of Table 3 are plotted with the horizontal axis representing the crushing time and the vertical axis representing the brain specific surface area. In FIG. 6, the black circles are the data when the mixing ratio of the reducing material is 15.5% by mass, and the black squares are the mixing ratio of the reducing material is 4% by mass and the mixing ratio of the drained dihydrate gypsum is 11. It is the data of 5% by mass. On the other hand, the white triangle is the data in which the reducing agent is not blended and the blending ratio of the drained dihydrate gypsum is 15.5% by mass. As shown in FIG. 6, it was confirmed that the specific surface area of the brain can be increased in a short time by blending the reducing agent. As described above, the pulverization efficiency of the cement composition can be improved by blending the reducing agent containing calcium sulfite and hemihydrate.

(Example 3-1)
[Manufacturing of cement composition]
Using a ball mill usually used for cement production, ordinary Portland cement clinker, dihydrate gypsum and a reducing material were mixed and pulverized to produce the cement composition of Example 3-1.

As the reducing agent, a reducing agent slurry prepared in the same manner as in Examples 1-1 to 1-9 was used. A dehydrated cake having a water content of 60% by mass was prepared by dehydrating the reducing agent slurry having a solid content concentration of 6% by mass, and then put into a ball mill. The solid content of the reducing material slurry was 78.1% by mass of calcium sulfite / hemihydrate, 16.3% by mass of dihydrate gypsum, and 5.6% by mass of calcium carbonate.

The content of calcium sulfite / hemihydrate in the cement composition calculated from the composition and compounding ratio of the reducing agent was 3.48% by mass. The brain specific surface area of the cement composition was 5050 cm ² / g. Compared with the case where the reducing agent slurry containing calcium sulfite / hemihydrate was not added, the milled amount of the cement composition per unit time was improved by up to 60%. Therefore, it was possible to increase the production of the cement composition.

(Examples 4-1 to 4-9, Comparative Examples 4-1 to 4-5)
[Manufacturing of cement composition]
The reducing agent slurry a was prepared in the same manner as in Examples 1-1 to 1-9. The solid content of the reducing material slurry a was 64.5% by mass of calcium sulfite / hemihydrate, 30.0% by mass of dihydrate gypsum, and 5.5% by mass of calcium carbonate. The reducing agent slurry a was filtered and dehydrated, and then dried using a high-speed mixer (manufactured by EarthTechnica Co., Ltd., trade name: FS10, capacity: 11 L). The dry powdering was carried out by stirring the inside of the kettle of the high-speed mixer at 60 ° C. under reduced pressure (60 kPa) while purging with nitrogen. As a result, a reducing agent a having a water content of less than 1% by mass was obtained.

FIG. 7 is an SEM photograph of the powdered reducing agent a. As shown in FIG. 7, the reducing agent a had a sufficiently small particle size.

Reducing agent slurries b and c were prepared in the same manner as in Examples 1-1 to 1-9. The composition of the solid content of the reducing agent slurries b and c is as shown in Table 4. The reducing agent slurries b and c were dried at 60 ° C. to obtain powdered reducing agents b and c (moisture content: less than 1% by mass). Separately from this, a commercially available reagent calcium sulfite / hemihydrate (manufactured by Wako Pure Chemical Industries, Ltd.) was prepared for comparison.

The average particle size of the reducing agents a, b, and c was measured by the following procedure. Water was used as the dispersion medium, and ultrasonic dispersion was performed to prepare a measurement sample. The particle size distribution was measured using a laser diffraction type particle size distribution measuring device (manufactured by Shimadzu Corporation, device name: SALD-2200). The median diameter (D50: particle size when the cumulative frequency is 50%) was obtained from the obtained particle size distribution and used as the average particle size. The results are as shown in Table 4.

Ordinary Portland cement, drained dihydrate gypsum and reducing agent (reducing agent a, b, c, calcium sulfite / hemihydrate as a reagent) are blended so as to have the blending ratio shown in Table 5, and a locking mixer is used. And mixed to prepare cement compositions of Examples 4-1 to 4-9 and Comparative Examples 4-1 to 4-5. No reducing agent was added in Comparative Example 4-1.

The content of calcium sulfite / hemihydrate in the cement compositions of Comparative Examples 4-1 to 4-5 shown in Table 5 is the same as the mixing ratio of calcium sulfite / hemihydrate as a reagent. _{The SO 3} conversion content of gypsum in the cement compositions of Comparative Examples 4-1 to 4-5 is the total value of _{the SO 3} conversion values of gypsum derived from ordinary Portland cement and drained dihydrate gypsum.

The content of calcium sulfite / hemihydrate in the cement compositions of Examples 4-1 to 4-9 is the mixing ratio of the reducing agent and the calcium sulfite / hemihydrate by thermogravimetric analysis of the solid content of the reducing agent. It was calculated by multiplying the content of. _{The SO 3} conversion content of gypsum in the cement composition of Examples 4-1 to 4-9 is the total value of _{the SO 3} conversion values of the gypsum derived from the reducing agent, ordinary Portland cement and the drained dihydrate gypsum. ..

[Evaluation of solidified soil]
Using the cement compositions of each example and each comparative example, soil ground improvement was carried out in the following manner, and the performance was evaluated. Kanto Loam (wet density: 1.36 cm ³ / g) was prepared as the soil to be treated. The cement compositions of each Example and each Comparative Example ^{were blended with the soil to be treated at a ratio of 300 kg / m 3} and mixed using a Hobart mixer to obtain a mixture.

The prepared mixture was packed in three layers using a rammer in a cylindrical frame having a diameter of 50 mm and a height of 100 mm, and then sealed and cured at 20 ° C. until the age of 7 days. Made. The solidified soil prepared was subjected to an elution test in accordance with Notification No. 46 of the Environment Agency (August 23, 1991), and the amount of hexavalent chromium [Cr (VI)] eluted was determined. The amount of hexavalent chromium eluted was determined by quantifying the concentration of hexavalent chromium in the filtrate after shaking by the diphenylcarbazide absorptiometry. In addition, the uniaxial compressive strength of the prepared solidified soil at ages 7, 28 and 91 days was measured according to JIS A 1216 “Soil uniaxial compression test method”. The results of these measurements are shown in Table 6.

In Table 6, when the amount of Cr (VI) elution is displayed as "<0.002", it indicates that it was less than the lower limit of detection. Example 4-9 using the cement composition containing the reducing agent a as the reducing agent showed the highest uniaxial compressive strength. In addition, the amount of Cr (VI) elution in Example 4-9 was also relatively low.

FIG. 8 shows the results of the amount of Cr (VI) elution from the solidified soil shown in Table 6 for each content of “calcium sulfite / hemihydrate” shown in “composition of cement composition” shown in Table 5. It is a graph which shows. The amount of hexavalent chromium eluted from the solidified soil using the cement composition of each example was almost the same as that of the solidified soil using the reagent calcium sulfite / hemihydrate.

FIG. 9 is a graph showing the results of the uniaxial compressive strength of the solidified soil shown in Table 6 for each content of “calcium sulfite / hemihydrate” shown in the “composition of cement composition” shown in Table 5. be. In Comparative Examples 4-2 to 4-5 using the reagent calcium sulfite / hemihydrate, the uniaxial compressive strength tends to decrease when the content of calcium sulfite in the cement composition is 4% by mass or more. rice field. On the other hand, the cement composition of Examples 4-1 to 4-4 containing the reducing agent b and the cement composition of Examples 4-5 to 4-8 containing the reducing agent c are contained in the cement composition. Even if the content of calcium sulfite exceeded 4% by mass, the uniaxial compressive strength was maintained higher than that of the additive-free one (Comparative Example 4-1).

According to the present disclosure, it is possible to provide a reducing agent capable of inexpensively producing a reducing agent having excellent reactivity and a method for producing the reducing agent. Further, according to the present disclosure, a method for producing a cement composition capable of inexpensively producing a cement composition capable of sufficiently reducing the elution of hexavalent chromium by using such a reducing agent. Can be provided.

10 ... Slurry preparation unit, 11 ... Absorption tower, 12 ... Circulation line, 13 ... Slurry introduction line, 14 ... Nozzle, 15 ... Derivation line, 16 ... Gas introduction line, 17 ... Gas discharge line, 20 ... Reducing material slurry tank, 22 ... Retained slurry (reducing material slurry), 30 ... Alkaline slurry tank, 40 ... Moisture adjusting part, 50 ... Cooling part, 52 ... Heat exchanger, 60 ... Flake crystal, 62 ... Main surface, 64 ... Long axis, 66 ... short axis, 100 ... manufacturing equipment.

Claims (11)

A step of introducing an alkaline slurry containing an alkaline compound containing at least one selected from a carbonate of calcium and a hydroxide of calcium and water, and a raw material gas containing sulfur dioxide into a slurry preparation unit.
A step of bringing the alkaline slurry into contact with the raw material gas to generate a reducing agent slurry containing calcium sulfite in the slurry preparation section.
It has a step of deriving the reducing agent slurry from the slurry preparation unit.
_{The SO 2} concentration of the raw material gas is 200 to 100,000 ppm, and the SO 2 concentration is 200 to 100,000 ppm.
The ratio of the retention amount A of the reducing agent slurry retained in the slurry preparation unit in terms of solid content to the extraction amount B of the reducing agent slurry derived from the slurry preparation unit in terms of solid content per day ( A / B) is 1.0 to 5.0 (day),
The temperature and the concentration of the solid content of the reducing agent slurry staying in the slurry preparation unit are 70 ° C. or lower and 0.1 to 50% by mass, respectively.
A method for producing a reducing agent, wherein the content of calcium sulfite in the solid content is 1% by mass or more. The slurry preparation unit
An absorption tower in which the raw material gas and the alkaline slurry come into contact with each other and the reducing agent slurry stays,
A circulation line that circulates the reducing agent slurry staying in the absorption tower to the upper part of the absorption tower.
A nozzle that sprays the reducing agent slurry circulated in the circulation line into the absorption tower, and
The method for producing a reducing agent according to claim 1, further comprising a lead-out line for deriving the reducing agent slurry retained in the absorption tower. The method for producing a reducing agent according to claim 1 or 2, wherein the content of calcium sulfite in the solid content of the reducing agent slurry derived from the slurry preparation unit is 40% by mass or more. An alkaline slurry tank for storing the alkaline slurry, which is a mixture of the alkaline compound and the water and supplied to the slurry preparation unit.
The method for producing a reducing agent according to any one of claims 1 to 3, wherein a reducing agent slurry tank for storing the reducing agent slurry derived from the slurry preparing unit is used. The reducing agent slurry containing sulfite calcium hydrogen production method of reducing agent according to any one of claims 1-4. The method for producing a reducing agent according to any one of claims 1 to 5 , which comprises a step of dehydrating the reducing agent slurry and drying it under reduced pressure to obtain a powdered reducing agent. The calcium sulfite contained in the powdered reducing agent obtained by drying the reducing agent slurry has a length of 1 to 500 μm and a length of 0.5 to 300 μm on the major axis and the minor axis of the main surface, respectively, and has a thickness. The method for producing a reducing agent according to any one of claims 1 to 6 , wherein the reducing agent contains flaky crystals having a length of 50% or less of the length of the major axis. A step of obtaining a cement composition containing a reducing agent produced by the production method according to any one of claims 1 to 7 and a cement clinker.
A method for producing a cement composition, which comprises 0.1 to 20 parts by mass of the reducing agent based on the solid content with respect to 100 parts by mass of the cement clinker. In the step, the cement clinker, the reducing agent, and gypsum are mixed and pulverized.
The method for producing a cement composition according to claim 8 , wherein the cement composition containing 1 to 30 parts by mass of the gypsum with respect to 100 parts by mass of the cement clinker is obtained. A step of mixing a reducing agent produced by the production method according to any one of claims 1 to 7 with Portland cement and gypsum to obtain a cement composition.
A method for producing a cement composition, which comprises 0.1 to 20 parts by mass of the reducing agent and 1 to 30 parts by mass of the gypsum with respect to 100 parts by mass of the Portland cement. The method for producing a cement composition according to claim 10 , further comprising a step of preparing the Portland cement by mixing 1 to 30 parts by mass of gypsum with 100 parts by mass of cement clinker.

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