CN116528963A

CN116528963A - Method for producing calcium carbonate and calcium carbonate

Info

Publication number: CN116528963A
Application number: CN202180078622.4A
Authority: CN
Inventors: 阪口裕允; 大国咲也夏; 江口健一郎; 织田彻; 南里泰德
Original assignee: Shiraishi Kogyo Kaisha Ltd
Current assignee: Shiraishi Kogyo Kaisha Ltd
Priority date: 2020-12-28
Filing date: 2021-12-24
Publication date: 2023-08-01
Also published as: JP2023069225A; JP7089311B1

Abstract

The present invention provides a method for producing calcium carbonate, which efficiently uses carbon dioxide and has a controlled production form, comprising the following steps: a carbon dioxide absorption step of absorbing carbon dioxide with a sodium hydroxide aqueous solution having a concentration of 5 to 21% to obtain a sodium carbonate aqueous solution having a concentration of 4 to 24%; a hydration step of reacting calcium oxide with an aqueous sodium hydroxide solution having a concentration of less than 6% to obtain a BET specific surface area of 5 to 40m ² A suspension of calcium hydroxide/g, namely milk of lime; and a carbonation step of adding and reacting the aqueous sodium carbonate solution to the lime milk.

Description

Method for producing calcium carbonate and calcium carbonate

Technical Field

The present invention relates to a method for synthesizing calcium carbonate by using flue gas of a combustion furnace or the like. In particular, the present invention relates to a method for producing calcite crystalline calcium carbonate having a fine particle shape. The present invention further relates to a method for producing a spindle-shaped calcite-crystal-type or acicular aragonite-crystal-type calcium carbonate. That is, the present invention relates to a method for separately producing calcium carbonate having various shapes by changing production conditions. The present invention further relates to calcium carbonate produced by these production methods.

Background

As a method for industrially synthesizing calcium carbonate, a carbon dioxide synthesis method is known in which carbon dioxide is blown into lime milk to carbonate the lime milk. As carbon dioxide used in the carbon dioxide synthesis method, flue gas of a lime kiln disposed close to a calcium carbonate production facility is often used. In addition, exhaust gas from a boiler, a garbage incinerator, or the like may be used as a supply source of carbon dioxide. However, in this case, the calcium carbonate production facility may not be installed in the vicinity of the firing furnace, and a flue gas pipe that communicates from a facility that is a supply source of carbon dioxide to the calcium carbonate production facility may be required to be laid. In the case of using flue gas, there is a problem that the carbon dioxide concentration of the flue gas in which the supply amount of carbon dioxide is not constant is also generally uneven, and carbonation cannot be performed efficiently. Further, since the temperature of the flue gas cannot be controlled, there is a problem that the properties of the produced calcium carbonate are easily affected by the flue gas temperature, and calcium carbonate of a desired shape cannot be produced. On the other hand, in the synthesis reaction of calcium carbonate by the carbon dioxide synthesis method, carbon dioxide is required to be temporarily dissolved in water, and thus the reaction time is long and the reaction efficiency is not high. In order to improve the carbon dioxide absorption efficiency, the reaction is often carried out at a low temperature, and is not suitable for a reaction carried out at a high temperature. There is also a problem in that not all of the carbon dioxide is used for the reaction, and unused carbon dioxide is released into the atmosphere.

Patent document 1 discloses a method in which an aqueous caustic soda (sodium hydroxide) solution is absorbed with carbon dioxide to form sodium carbonate (Na ₂ CO ₃ ) A method for producing calcium carbonate by reacting sodium carbonate with milk of lime (aqueous suspension of calcium hydroxide). In the method of patent document 1, even if the carbon dioxide concentration is not uniform, absorption in the caustic soda aqueous solution can be performed, and carbon dioxide can be stored in advance. Therefore, the calcium carbonate production facility can be provided at a location remote from the carbon dioxide production location. Sodium carbonate has a much higher solubility in water than carbon dioxide and also does not lower its solubility even at high temperatures, so that calcium carbonate can be produced at high temperatures and high concentrations. If carbon dioxide that has not been used for the reaction in the carbon dioxide chemical combination method described above can be recovered by the caustic soda aqueous solution, it is also possible to expect reduction in the amount of carbon dioxide released to the atmosphere.

Prior art literature

Patent literature

Patent document 1: japanese patent laid-open No. 2002-293537

Disclosure of Invention

In the production method of patent document 1, if caustic soda having a low concentration is used to absorb carbon dioxide, the absorption efficiency of carbon dioxide is lowered, and thus carbon dioxide cannot be utilized to the maximum extent. In this case, the concentration of the obtained sodium carbonate also becomes low, and thus the production efficiency of calcium carbonate also decreases. The particle size, crystal shape, and shape of the calcium carbonate obtained by the production method of patent document 1 are not clear. Accordingly, an object of the present invention is to produce calcium carbonate with a controlled morphology while efficiently utilizing carbon dioxide. Specifically, an object of the present invention is to efficiently produce calcite-crystal-form calcium carbonate having a fine particle shape by using exhaust gas or the like according to circumstances. Further, an object of the present invention is to efficiently produce calcite-crystal-type and aragonite-crystal-type calcium carbonate by using, for example, exhaust gas according to circumstances.

A second object of the present invention is to reuse carbon dioxide which is not used in the carbon dioxide absorption step, and to recycle sodium hydroxide contained in the filtrate or used cleaning liquid obtained in the calcium carbonate production step in the carbon dioxide absorption step or the hydration step for reuse.

A third object of the present invention is to enable carbon dioxide in exhaust gas of a combustion furnace or the like provided outside a calcium carbonate production site to be absorbed into an aqueous sodium hydroxide solution and be used as sodium carbonate to be transported to the production site, and further to suppress the amount of carbon dioxide released to the environment as a whole by implementing the present invention, thereby contributing to improvement of global warming.

A first aspect of the present invention relates to a method for producing calcium carbonate, comprising the steps of: a carbon dioxide absorption step of absorbing carbon dioxide with a sodium hydroxide aqueous solution having a concentration of 5 to 21% to obtain a sodium carbonate aqueous solution having a concentration of 4 to 24%; a hydration step of reacting calcium oxide with an aqueous sodium hydroxide solution having a concentration of less than 6% to obtain a BET specific surface area of 5 to 40m ² A suspension of calcium hydroxide/g, namely milk of lime; and a carbonation step of adding and reacting the aqueous sodium carbonate solution to the lime milk.

In the carbonation step, the solid content concentration of the lime milk may be adjusted to 1 to 24%, and an aqueous sodium carbonate solution may be added to the lime milk with the solid content concentration adjusted, and the reaction may be performed at a temperature of 9 to 80 ℃.

Further, the following method may be adopted, and the method further includes: and a solid-liquid separation step of separating the filtrate containing sodium hydroxide and calcium carbonate after the carbonation step.

In the above production method, the concentration of sodium hydroxide in the filtrate containing sodium hydroxide may be adjusted to 5 to 21%, and the filtrate may be used in the carbon dioxide absorption step.

A second aspect of the present invention is calcium carbonate produced by the method for producing calcium carbonate according to the first aspect.

A third aspect of the present invention relates to a method for producing calcium carbonate, comprising the steps of: a carbon dioxide absorption step of absorbing carbon dioxide with a sodium hydroxide aqueous solution having a concentration of 5 to 21% to obtain a sodium carbonate aqueous solution having a concentration of 4 to 24% or less;

a hydration step of reacting calcium oxide with an aqueous sodium hydroxide solution having a concentration of less than 6% to obtain a BET specific surface area of 5 to 40m ² A suspension of calcium hydroxide/g, namely milk of lime; and

And a carbonation step of adding the sodium carbonate aqueous solution obtained in the carbon dioxide absorption step to the lime milk and reacting the added solution.

The method for producing calcium carbonate is characterized in that in the carbonation step, the initial concentration of the lime milk is 1-6%, the concentration of the sodium carbonate aqueous solution is 4-22%, and the reaction is carried out at 9-25 ℃ to obtain a calcium carbonate having a BET specific surface area of 30-90 m ² Calcite crystalline calcium carbonate in the form of fine particles/g.

Here, the carbon dioxide that is not used in the carbon dioxide absorbing step may be reused in the carbon dioxide absorbing step as follows.

The following modes can also be adopted, and the method further comprises: a solid-liquid separation step of separating the filtrate containing sodium hydroxide and calcium carbonate after the carbonation step; and

and a cleaning step of cleaning the calcium carbonate obtained in the solid-liquid separation step with a cleaning liquid.

Alternatively, an aqueous solution of sodium hydroxide having a high concentration may be added to the filtrate obtained in the solid-liquid separation step and the used cleaning liquid obtained in the cleaning step, or the filtrate and the used cleaning liquid may be heated and concentrated to obtain an aqueous solution containing sodium hydroxide having a concentration of 5 to 21%, and the aqueous solution may be used in the carbon dioxide absorption step.

It is also preferable that the filtrate obtained in the solid-liquid separation step and the used cleaning liquid obtained in the cleaning step be adjusted to an aqueous solution having a sodium hydroxide concentration of less than 6%, and that the aqueous solution be used in the hydration step.

A fourth aspect of the present invention is calcium carbonate produced by the method for producing calcium carbonate according to the third aspect.

A fifth aspect of the present invention relates to a method for producing calcium carbonate, comprising the steps of: a carbon dioxide absorption step of absorbing carbon dioxide with a sodium hydroxide aqueous solution having a concentration of 13 to 21% to obtain a sodium carbonate aqueous solution having a concentration of 15 to 24% or less;

The method for producing calcium carbonate is characterized in that in the carbonation step, the initial concentration of the lime milk is 11-19%, the concentration of the sodium carbonate aqueous solution is 15-24%, and the reaction is carried out at 20-40 ℃ to obtain a calcium carbonate having a BET specific surface area of 4-20 m ² /g of fusiform calcite crystalline calcium carbonate.

Alternatively, an aqueous solution of sodium hydroxide having a high concentration may be added to the filtrate obtained in the solid-liquid separation step and the used cleaning liquid obtained in the cleaning step, or the filtrate and the used cleaning liquid may be heated and concentrated to obtain an aqueous solution containing sodium hydroxide having a concentration of 13 to 21%, and the aqueous solution may be used in the carbon dioxide absorption step.

A sixth aspect of the present invention is calcium carbonate produced by the method for producing calcium carbonate according to the fifth aspect.

A seventh aspect of the present invention relates to a method for producing calcium carbonate, comprising the steps of: a carbon dioxide absorption step of absorbing carbon dioxide with a sodium hydroxide aqueous solution having a concentration of 13 to 21% to obtain a sodium carbonate aqueous solution having a concentration of 15 to 24% or less;

The method for producing calcium carbonate is characterized in that in the carbonation step, the initial concentration of the lime milk is 11-24%, the concentration of the sodium carbonate aqueous solution is 15-24%, and the reaction is carried out at 40-80 ℃ to obtain a calcium carbonate having a BET specific surface area of 3-10 m ² Needle-shaped aragonite crystalline calcium carbonate per gram.

Alternatively, an aqueous solution containing sodium hydroxide at a concentration of 13 to 21% may be obtained by adding a high-concentration aqueous solution of sodium hydroxide to the filtrate obtained in the solid-liquid separation step and the used cleaning liquid obtained in the cleaning step, or by heating the filtrate and the used cleaning liquid and concentrating the mixture, and the aqueous solution may be used in the carbon dioxide absorption step.

An eighth aspect of the present invention is calcium carbonate produced by the method for producing calcium carbonate according to the seventh aspect.

Effects of the invention

According to the present invention, the aqueous sodium carbonate solution and sodium hydroxide in the production process can be produced at a higher concentration than in the conventional method, and calcium carbonate having a desired particle shape can be efficiently and continuously obtained by using a calcium hydroxide slurry at a higher concentration than in the conventional method. In addition, by returning the flue gas and the unreacted carbon dioxide in the process to the process for use and by increasing the concentration of the reaction liquid, the amount of the alkaline waste liquid containing carbon dioxide and sodium hydroxide generated during the production can be suppressed, and the load on the environment can be reduced. Further, since carbon dioxide in exhaust gas of a combustion furnace or the like provided outside a manufacturing site can be used, it has been conventionally useful as a method for fixing carbon dioxide released into the environment.

In the production method of the present invention, calcium carbonate having a desired shape and particle size can be obtained. Calcium carbonate suitable for various uses such as paper making, pigment coating, plastics, sealants, rubber, and foods can be continuously controlled and produced.

Drawings

Fig. 1 is a flowchart illustrating a manufacturing method of the present invention.

FIG. 2 is an electron micrograph (magnification: 30000) of calcium carbonate in which fine particles of the shape of particles obtained in example 1 were connected in a chain.

FIG. 3 is an electron micrograph (magnification: 20000 times) of spindle-shaped calcium carbonate obtained in example 2.

FIG. 4 is an electron micrograph (magnification: 10000 times) of needle-shaped calcium carbonate obtained in example 3.

FIG. 5 is an electron micrograph (magnification: 10000 times) of needle-shaped calcium carbonate obtained in example 4.

FIG. 6 is an electron micrograph (magnification: 10000 times) of needle-shaped calcium carbonate obtained in example 5.

Detailed Description

The embodiments of the present invention will be described in further detail, but the present invention is not limited to the following embodiments.

A first embodiment of the present invention is a method for producing calcium carbonate, comprising the steps of: a carbon dioxide absorption step of absorbing carbon dioxide with a sodium hydroxide aqueous solution having a concentration of 5 to 21% to obtain a sodium carbonate aqueous solution having a concentration of 6 to 24%; a hydration step of reacting calcium oxide with an aqueous sodium hydroxide solution having a concentration of less than 6% to obtain a BET specific surface area of 5 to 40m ² A suspension of calcium hydroxide/g, namely milk of lime; and a carbonation step of adding and reacting the aqueous sodium carbonate solution to the lime milk. The present embodiment is a method for producing calcium carbonate, which includes at least a carbon dioxide absorption step, a hydration step, and a carbonation step. The carbon dioxide absorbing step is a step of absorbing carbon dioxide with an aqueous sodium hydroxide solution to obtain an aqueous sodium carbonate solution. Sodium hydroxide is also commonly referred to as caustic sodaCommercially available products can be suitably used. The aqueous sodium hydroxide solution may be obtained by dissolving sodium hydroxide in water, or a liquid containing sodium hydroxide (so-called "white liquid") obtained in the paper-making step may be used. The concentration of sodium hydroxide in the aqueous sodium hydroxide solution used in the present step may be 5 to 21%, preferably 8 to 19%, and more preferably 13 to 18%. By setting the concentration of the aqueous sodium hydroxide solution to a maximum of 21% in this step, the carbon dioxide absorption efficiency can be improved. In the present embodiment, carbon dioxide absorbed by the aqueous sodium hydroxide solution may be a mixed gas containing carbon dioxide and other gases, in addition to carbon dioxide alone. As the carbon dioxide used in the present embodiment, exhaust gas containing carbon dioxide can be used. Examples of such exhaust gas include exhaust gas from lime kilns, boilers, garbage incinerators, cement kilns, refractory heating furnaces, steelmaking converters, steelmaking melters, cupola, coke gas generators, petroleum decomposition furnaces, glass manufacturing reverberatory furnaces, oil and gas generators, and acetylene generators. When the aqueous sodium hydroxide solution is allowed to absorb carbon dioxide, sodium carbonate is produced. Carbon dioxide can be absorbed until the concentration of sodium carbonate is 4 to 24%, preferably 10.2 to 22.8%, and more preferably 16.1 to 21.6%. In this specification,% is% by weight unless otherwise specified.

In the first embodiment, the hydration step is a step of reacting calcium oxide with an aqueous sodium hydroxide solution having a concentration of less than 6% to obtain lime milk. Milk of lime refers to an aqueous suspension of calcium hydroxide (aqueous calcium hydroxide slurry). The calcium oxide used in the hydration step is an oxide of calcium, which is generally called quicklime. The calcium oxide may be a commercially available calcium oxide. The concentration of the aqueous sodium hydroxide solution reacted with calcium oxide in this step is less than 6%. The calcium hydroxide obtained in this step is a calcium hydroxide commonly referred to as slaked lime. In the hydration step, a BET specific surface area of 5 to 40m is preferably obtained ² Suspension of calcium hydroxide/g. BET specific surface area can be based on gas adsorption of powder (solid) according to Japanese Industrial Standard JISZ8830) Is a specific surface area measurement method of (1) (ISO 9277:2010 A) to perform the measurement. By adjusting the amounts of calcium oxide and water to be reacted, a BET specific surface area of 5 to 40m can be obtained ² Calcium hydroxide/g. When the amount of water is increased relative to the amount of calcium oxide, calcium hydroxide having a large BET specific surface area can be obtained. Conversely, when the amount of water is reduced relative to the amount of calcium oxide, calcium hydroxide having a smaller BET specific surface area can be obtained. The hydration step and the carbon dioxide absorption step may be performed simultaneously, or the hydration step may be continued after the carbon dioxide absorption step, or the carbon dioxide absorption step may be continued after the hydration step. In order to finally obtain calcium carbonate in a desired form in the present embodiment, it is important to obtain calcium hydroxide having a BET specific surface area in an appropriate range in the hydration step.

In the first embodiment, the carbonation step is a step of reacting the lime milk obtained in the hydration step with the sodium carbonate aqueous solution obtained in the carbon dioxide absorption step to obtain calcium carbonate. This process is also commonly referred to as a causticizing process. In this step, it is very preferable to use the lime milk with a solid content concentration of 1 to 24%. Preferably, when the aqueous sodium carbonate solution is added to the lime milk having the solid content concentration prepared in the above-described range, the aqueous sodium carbonate solution is preferably added so that the molar ratio of the amount of sodium carbonate present in the aqueous sodium carbonate solution is 0.9 to 1.5 relative to the amount of calcium hydroxide present in the lime milk. Particularly preferably, an aqueous sodium carbonate solution having a concentration of 4 to 24% is added. In this case, it is very preferable to add an aqueous sodium carbonate solution to the lime milk for 60 to 180 minutes or 100 to 150 minutes. Preferably, an aqueous sodium carbonate solution, the concentration of which is adjusted as needed, obtained in the carbon dioxide absorbing step is added to the lime milk having a solid content concentration, and the mixture is reacted at a temperature of 9 to 80 ℃ or 10 to 55 ℃. Too high or too low a reaction temperature in the carbonation step increases the cost required for heating or cooling, for example, energy. In addition, when the BET specific surface area of calcium hydroxide is adjusted in the above manner in order to produce calcium carbonate containing a large amount of aragonite crystals, the aragonite crystals (needles) tend to have a thicker shape when the reaction temperature in the carbonation step is increased. Preferably, the reaction in the carbonation step is performed by stirring the reaction liquid. Preferably, the stirrer can be adjusted so that the time required for slowly adding the aqueous sodium carbonate solution to the lime cream until the lime cream is completely mixed (complete mixing time) is 3 to 25 seconds or 5 to 22 seconds. As means for stirring the reaction vessel, conventionally used propeller mixers, paddle mixers, ribbon mixers, turbine blade mixers, horseshoe blade mixers, yarn winding blade mixers, mixer mixers, magnetic mixers, and the like can be used. In the reaction in this step, calcium carbonate and sodium hydroxide are produced. The water-soluble sodium hydroxide is dissolved in the reaction solution, and the calcium carbonate having low water solubility is precipitated as a solid.

In the first embodiment, the method may further include a solid-liquid separation step of separating calcium carbonate produced by the reaction in the carbonation step from the reaction liquid and taking the calcium carbonate out in a solid state. The reaction solution (filtrate) remaining after separating the solid calcium carbonate is an aqueous sodium hydroxide solution, and can be reused in the carbon dioxide absorption step. When the filtrate is reused in the carbon dioxide absorption step, the concentration of sodium hydroxide is preferably adjusted to 5 to 21%, more preferably 8 to 19%, and still more preferably 13 to 18%.

The calcium carbonate according to the second embodiment of the present invention obtained by the production method according to the first embodiment may have a crystal form such as calcite crystal, aragonite crystal, or spheronite crystal. In each of the above steps, various crystalline calcium carbonates can be produced by changing the concentration, temperature, and the like. The obtained calcium carbonate particles may have various shapes such as a substantially cubic, spindle-like, needle-like, and micro-spherical crystal-connected shape, in addition to the spherical shape.

Next, a flow of a first embodiment of the present invention will be described with reference to fig. 1. Fig. 1 shows a flow of a method for producing calcium carbonate according to an embodiment of the present invention using carbon dioxide in exhaust gas discharged from a combustion furnace or the like. In the figure, 1: carbon dioxide absorption step, 2: hydration process, 3: carbonation process, 4: solid-liquid separation step, 5: and (3) a cleaning procedure. The exhaust gas discharged from the combustion furnace or the like is appropriately subjected to a dust removal treatment to obtain a purified gas containing carbon dioxide. On the other hand, a method of properly mixing process water and a high-concentration sodium hydroxide aqueous solution to adjust the concentration of the process water to 5 to 21% sodium hydroxide aqueous solution is prepared, and the process water and the high-concentration sodium hydroxide aqueous solution are used to absorb purified gas (carbon dioxide absorbing step 1). Thus, an aqueous sodium carbonate solution having a concentration of 4 to 24% was obtained. The unreacted gas that has not been absorbed in the carbon dioxide absorbing step 1 is returned as indicated by arrow 10 and reused in the carbon dioxide absorbing step 1.

On the other hand, calcium oxide and water for hydration (which means an aqueous sodium hydroxide solution having a concentration of less than 6%) are prepared, and reacted (hydration step 2), and if necessary, classification operation or the like is performed to obtain a BET specific surface area of 5 to 40m ² The calcium hydroxide suspension per gram is the refined lime milk.

When the purified lime milk thus obtained is reacted with an aqueous sodium carbonate solution (carbonation step 3), calcium carbonate is produced. The produced calcium carbonate is filtered (solid-liquid separation step 4), and the obtained solid calcium carbonate is washed with a washing liquid (washing step 5). The filtrate obtained in the solid-liquid separation step 4 and the used cleaning liquid obtained in the cleaning step 5 are recovered and reused as the aqueous sodium hydroxide solution in the carbon dioxide absorption step 1 or the hydration water in the hydration step 2 (arrows 20 and 30).

In the first embodiment, in the carbonation step 3 of fig. 1, the initial concentration of lime milk is set to 1 to 24% and the concentration of the sodium carbonate aqueous solution is preferably set to 4 to 24%, and the reaction is performed at 9 to 80 ℃.

According to the method for producing calcium carbonate of the first embodiment, carbon dioxide can be efficiently absorbed by a relatively high-concentration aqueous sodium hydroxide solution. In this case, an aqueous sodium carbonate solution having a desired concentration can be obtained regardless of the concentration of carbon dioxide. As described in the following embodiments, a predetermined time is allowed to react between a predetermined concentration of sodium carbonate aqueous solution and a predetermined concentration of lime milk as a solid component at a predetermined temperature in the carbonation step, whereby calcium carbonate having a desired shape can be produced.

In the method for producing calcium carbonate according to the first embodiment, since the aqueous sodium hydroxide solution as the absorbent for carbon dioxide can be reused repeatedly, the waste liquid is small, and the load on the environment can be reduced.

A third embodiment of the present invention is a method for producing calcium carbonate, comprising the steps of: a carbon dioxide absorption step of absorbing carbon dioxide with a sodium hydroxide aqueous solution having a concentration of 5 to 21% to obtain a sodium carbonate aqueous solution having a concentration of 4 to 24%; a hydration step of reacting calcium oxide with an aqueous sodium hydroxide solution having a concentration of less than 6% to obtain a BET specific surface area of 5 to 40m ² A suspension of calcium hydroxide/g, namely milk of lime; and a carbonation step of adding the sodium carbonate aqueous solution obtained in the carbon dioxide absorption step to the lime milk and reacting the same.

The production method is characterized in that in the carbonation step, the initial concentration of the lime milk is 1-6%, the concentration of the sodium carbonate aqueous solution is 4-22%, and the reaction is carried out within the range of 9-25 ℃ to obtain the product having a BET specific surface area of 30-90 m ² Calcite crystalline calcium carbonate in the form of fine particles/g. The present embodiment is a method for producing calcium carbonate, which includes at least a carbon dioxide absorption step, a hydration step, and a carbonation step. The carbon dioxide absorption step is to absorb carbon dioxide (CO) by an aqueous sodium hydroxide solution ₂ ) And obtaining an aqueous sodium carbonate solution. Sodium hydroxide is also commonly referred to as caustic soda, and commercially available products can be suitably used. The aqueous sodium hydroxide solution may be obtained by dissolving sodium hydroxide in water, or a liquid containing sodium hydroxide (so-called "white liquid") obtained in the paper-making step may be used. The concentration of sodium hydroxide in the aqueous sodium hydroxide solution used in the present step may be 5 to 21%, preferably 8 to 15%. By setting the concentration of the aqueous sodium hydroxide solution to a maximum of 21% in this step, the carbon dioxide absorption efficiency can be improved. In the present embodiment, carbon dioxide absorbed by the aqueous sodium hydroxide solution may contain, in addition to carbon dioxide alone, carbon dioxideThere are mixed gases of carbon dioxide and other gases. As the carbon dioxide used in the present embodiment, exhaust gas containing carbon dioxide can be used. Examples of such exhaust gas include exhaust gas from lime kilns, boilers, garbage incinerators, cement kilns, refractory heating furnaces, steelmaking converters, steelmaking melters, cupola, coke gas generators, petroleum decomposition furnaces, glass manufacturing reverberatory furnaces, oil and gas generators, and acetylene generators. When the aqueous sodium hydroxide solution is allowed to absorb carbon dioxide, sodium carbonate is produced. Can absorb carbon dioxide until the concentration of sodium carbonate is 4-24%. In this specification,% is% by weight unless otherwise specified.

In addition, carbon dioxide which is not used in the carbon dioxide absorbing step is not directly released to the atmosphere, and is highly preferably reused so as to be absorbed into the aqueous sodium hydroxide solution in the carbon dioxide absorbing step from the viewpoint of environmental protection.

In the third embodiment, the hydration step is a step of reacting calcium oxide with an aqueous sodium hydroxide solution having a concentration of less than 6% to obtain lime milk. Herein, milk of lime refers to an aqueous suspension of calcium hydroxide (calcium hydroxide aqueous slurry). The calcium oxide used in the hydration step is an oxide of calcium, which is generally called quicklime. The calcium oxide may be a commercially available calcium oxide. The concentration of the aqueous sodium hydroxide solution reacted with calcium oxide in this step is less than 6%. The calcium hydroxide obtained in this step is a calcium hydroxide commonly referred to as slaked lime. In the hydration step, a BET specific surface area of 5 to 40m is preferably obtained ² Suspension of calcium hydroxide/g. The BET specific surface area can be measured according to Japanese Industrial Standard JISZ8830 "method for measuring specific surface area of powder (solid) based on gas adsorption" (ISO 9277:2010). By adjusting the amount and concentration of the reacted calcium oxide and sodium hydroxide aqueous solution, a BET specific surface area of 5 to 40m can be obtained ² Calcium hydroxide/g. When the amount of the aqueous sodium hydroxide solution is increased relative to the amount of calcium oxide, calcium hydroxide having a large BET specific surface area can be obtained. Conversely, when reduced relative to the amount of calcium oxideWhen the amount of the aqueous sodium hydroxide solution is small, calcium hydroxide having a small BET specific surface area can be obtained. In addition, in the hydration step, when an aqueous sodium hydroxide solution having an excessively high concentration is used, the BET specific surface area of the obtained calcium hydroxide tends to be high, calcium hydroxide having a desired BET specific surface area cannot be obtained, and the viscosity of lime milk becomes high, and the treatment becomes difficult. In particular, the BET specific surface area of calcium hydroxide contained in the lime milk obtained in the step is set to 5 to 40m ² However, most of the crystal forms of calcium carbonate obtained in the carbonation step described below are preferably in the form of fine particles. The hydration step and the carbon dioxide absorption step may be performed simultaneously, or the hydration step may be continued after the carbon dioxide absorption step, or the carbon dioxide absorption step may be continued after the hydration step. In order to finally obtain calcium carbonate in a desired form in the present embodiment, it is important to obtain calcium hydroxide having a BET specific surface area in an appropriate range in the hydration step.

In the third embodiment, the carbonation step is a step of reacting the lime milk obtained in the hydration step with the sodium carbonate aqueous solution obtained in the carbon dioxide absorption step to obtain calcium carbonate. This process is also commonly referred to as a causticizing process. In this step, it is very preferable to use the lime milk with a solid content concentration of 1 to 6%. It is particularly preferable to perform the reaction so that the initial concentration of lime milk is 1 to 6% and the concentration of the sodium carbonate aqueous solution is 4 to 22%. The sodium carbonate aqueous solution may be used by appropriately adjusting the concentration of the sodium carbonate aqueous solution having a concentration of 4 to 24% obtained in the carbon dioxide absorption step. In this case, although the reaction scale for performing the carbonation step is also dependent on, it is very preferable to add the aqueous sodium carbonate solution slowly to the lime milk, for example, for a certain period of time, such as 60 to 180 minutes or 100 to 150 minutes. The reaction in the carbonation step is preferably performed by stirring the reaction liquid. Preferably, the stirrer can be adjusted so that the time required for slowly adding the aqueous sodium carbonate solution to the lime cream until the lime cream is completely mixed (complete mixing time) is 3 to 25 seconds or 5 to 22 seconds. As means for stirring the reaction vessel, conventionally used propeller mixers, paddle mixers, ribbon mixers, turbine blade mixers, horseshoe blade mixers, yarn winding blade mixers, mixer mixers, magnetic mixers, and the like can be used.

The concentration-adjusted aqueous solution of sodium carbonate obtained in the carbon dioxide absorption step is added to the lime milk having the initial solid content adjusted, and reacted at a temperature of 9 to 25 ℃ to obtain a lime milk having a BET specific surface area of 30 to 90m ² Calcite crystalline calcium carbonate in the form of fine particles/g. The reaction temperature in the carbonation step is too high or too low, and it is not possible to obtain calcium carbonate having a desired BET specific surface area and shape, in addition to increasing the cost required for heating or cooling, such as energy required for cooling. In the reaction in this step, calcium carbonate and sodium hydroxide are produced, and water-soluble sodium hydroxide is dissolved in the reaction solution, whereby calcium carbonate having low water solubility is precipitated as a solid. In the present specification, "fine particle-shaped crystals" means crystals having a BET specific surface area of 30m, for example ² And/g or more fine primary particles as a constituent. As crystals of calcium carbonate, structural polymorphisms such as calcite crystals, aragonite crystals, and spheronite crystals are known, but calcium carbonate produced by the production method of the present embodiment is calcite crystals. Examples of the fine particle-shaped crystals include crystals having a crystal structure in which a single crystal is hexahedral, and at least one pair of opposite faces of the hexahedral are rhombohedral (hexagonal rhombohedral lattice).

In the third embodiment, the method may further include a solid-liquid separation step of separating calcium carbonate produced by the reaction in the carbonation step from the reaction liquid and taking the calcium carbonate out in a solid state. The method may further include a step of washing the solid calcium carbonate obtained in the solid-liquid separation step with a washing liquid. The calcium carbonate obtained in the solid-liquid separation step is preferably washed with water. The liquid (filtrate) remaining after the separation of the solid calcium carbonate in the solid-liquid separation step and the used cleaning liquid obtained in the cleaning step are aqueous sodium hydroxide solutions. The aqueous sodium hydroxide solution may be reused in the carbon dioxide absorption step. When the filtrate and the used cleaning liquid are reused in the carbon dioxide absorption step, the concentration of sodium hydroxide is preferably adjusted to 5 to 21%. The concentration of sodium hydroxide is adjusted, for example, as follows: the concentration is performed by adding a high-concentration aqueous sodium hydroxide solution (an aqueous sodium hydroxide solution having a concentration of 5% or more), adding solid sodium hydroxide, or heating the filtrate and the used cleaning liquid.

The filtrate obtained in the solid-liquid separation step and the used cleaning liquid obtained in the cleaning step may be reused in the hydration step. When the filtrate and the used washing liquid are reused in the hydration step, the concentration of sodium hydroxide is preferably adjusted to less than 6%. The concentration of sodium hydroxide is adjusted, for example, by dilution with water.

Next, a flow of a third embodiment of the present invention will be described with reference to fig. 1. Fig. 1 shows a flow of a method for producing calcium carbonate according to an embodiment of the present invention using carbon dioxide in exhaust gas discharged from a combustion furnace or the like. In the figure, 1: carbon dioxide absorption step, 2: hydration process, 3: carbonation process, 4: solid-liquid separation step, 5: and (3) a cleaning procedure. The exhaust gas discharged from the combustion furnace or the like is appropriately subjected to a dust removal treatment to obtain a purified gas containing carbon dioxide. On the other hand, a method of properly mixing process water and a high-concentration sodium hydroxide aqueous solution to adjust the concentration of the process water to 5 to 21% sodium hydroxide aqueous solution is prepared, and the process water and the high-concentration sodium hydroxide aqueous solution are used to absorb purified gas (carbon dioxide absorbing step 1). Thus, an aqueous sodium carbonate solution having a concentration of 4 to 24% was obtained. The unreacted gas that has not been absorbed in the carbon dioxide absorbing step 1 is returned as indicated by arrow 10 and reused in the carbon dioxide absorbing step 1.

On the other hand, calcium oxide and water for hydration (aqueous sodium hydroxide solution having a concentration of less than 6%) are prepared, reacted (hydration step 2), and if necessary, classification is performed to obtain a BET specific surface area of 5 to 40m ² The calcium hydroxide suspension per gram is the refined lime milk.

In making this obtainWhen the refined lime milk is reacted with an aqueous sodium carbonate solution (carbonation step 3), calcium carbonate is produced. The produced calcium carbonate is filtered (solid-liquid separation step 4), and the obtained solid calcium carbonate is washed with a washing liquid (washing step 5). The filtrate obtained in the solid-liquid separation step 4 and the used cleaning liquid obtained in the cleaning step 5 are recovered and reused as the aqueous sodium hydroxide solution in the carbon dioxide absorption step 1 or the hydration water in the hydration step 2 (arrows 20 and 30). In the third embodiment, in the carbonation step 3 of fig. 1, the initial concentration of lime milk is set to 1 to 6% and the concentration of the sodium carbonate aqueous solution is set to 4 to 22%, and the reaction is performed at 9 to 25 ℃. The calcium carbonate according to the fourth embodiment of the present invention can be obtained by the production method according to the third embodiment. The calcium carbonate according to the fourth embodiment preferably has a BET specific surface area of 30 to 90m ² Calcite crystalline calcium carbonate in the form of fine particles/g.

According to the method for producing calcium carbonate of the third embodiment of the present invention, carbon dioxide can be efficiently absorbed into a sodium hydroxide aqueous solution having a relatively high concentration. In this case, an aqueous sodium carbonate solution having a desired concentration can be obtained regardless of the concentration of carbon dioxide. In addition, in the carbonation step, a predetermined initial concentration of an aqueous sodium carbonate solution and a predetermined initial solid content concentration of lime milk are reacted at a predetermined temperature for a predetermined time, whereby calcite-shaped calcium carbonate having a desired fine particle shape can be produced.

In the method for producing calcium carbonate according to the third embodiment of the present invention, carbon dioxide and an aqueous sodium hydroxide solution can be repeatedly reused, so that carbon dioxide and waste liquid discharged to the environment are small, and the load on the environment can be reduced.

A fifth embodiment of the present invention is a method for producing calcium carbonate, comprising the steps of: a carbon dioxide absorption step of absorbing carbon dioxide with a sodium hydroxide aqueous solution having a concentration of 13 to 21% to obtain a sodium carbonate aqueous solution having a concentration of 15 to 24% or less; a hydration step of reacting calcium oxide with a sodium hydroxide aqueous solution having a concentration of less than 6%,the BET specific surface area is 5 to 40m ² A suspension of calcium hydroxide/g, namely milk of lime; and a carbonation step of adding the sodium carbonate aqueous solution obtained in the carbon dioxide absorption step to the lime milk and reacting the same.

The production method is characterized in that in the carbonation step, the initial concentration of the lime milk is 11-19%, the concentration of the sodium carbonate aqueous solution is 15-24%, and the reaction is carried out within the range of 20-40 ℃ to obtain the product having a BET specific surface area of 4-20 m ² /g of fusiform calcite crystalline calcium carbonate. The present embodiment is a method for producing calcium carbonate, which includes at least a carbon dioxide absorption step, a hydration step, and a carbonation step. The carbon dioxide absorption step is to absorb carbon dioxide (CO) by an aqueous sodium hydroxide solution ₂ ) And obtaining an aqueous sodium carbonate solution. Sodium hydroxide is also commonly referred to as caustic soda, and commercially available products can be suitably used. The aqueous sodium hydroxide solution may be obtained by dissolving sodium hydroxide in water, or a liquid containing sodium hydroxide (so-called "white liquid") obtained in the paper-making step may be used. The concentration of sodium hydroxide in the aqueous sodium hydroxide solution used in the present step may be 13 to 21%, preferably 15 to 20%. By setting the concentration of the aqueous sodium hydroxide solution to a maximum of 21% in this step, the carbon dioxide absorption efficiency can be improved. In the present embodiment, carbon dioxide absorbed by the aqueous sodium hydroxide solution may be a mixed gas containing carbon dioxide and other gases, in addition to carbon dioxide alone. As the carbon dioxide used in the present embodiment, exhaust gas containing carbon dioxide can be used. Examples of such exhaust gas include exhaust gas from lime kilns, boilers, garbage incinerators, cement kilns, refractory heating furnaces, steelmaking converters, steelmaking melters, cupola, coke gas generators, petroleum decomposition furnaces, glass manufacturing reverberatory furnaces, oil and gas generators, and acetylene generators. When the aqueous sodium hydroxide solution is allowed to absorb carbon dioxide, sodium carbonate is produced. Can absorb carbon dioxide until the concentration of sodium carbonate is 15-24%. In the present specification, unless otherwise specified Obviously,% is weight percent.

In addition, carbon dioxide which is not used in the carbon dioxide absorbing step is not directly released to the atmosphere, and is very preferably reused so as to be absorbed into the aqueous sodium hydroxide solution in the carbonation step from the viewpoint of environmental protection.

In the fifth embodiment, the hydration step is a step of reacting calcium oxide with an aqueous sodium hydroxide solution having a concentration of less than 6% to obtain lime milk. Herein, milk of lime refers to an aqueous suspension of calcium hydroxide (calcium hydroxide aqueous slurry). The calcium oxide used in the hydration step is an oxide of calcium, which is generally called quicklime. The calcium oxide may be a commercially available calcium oxide. The concentration of the aqueous sodium hydroxide solution reacted with calcium oxide in this step is less than 6%. The calcium hydroxide obtained in this step is a calcium hydroxide commonly referred to as slaked lime. In the hydration step, a BET specific surface area of 5 to 40m is preferably obtained ² Suspension of calcium hydroxide/g. The BET specific surface area can be measured according to Japanese Industrial Standard JISZ8830 "method for measuring specific surface area of powder (solid) based on gas adsorption" (ISO 9277:2010). By adjusting the amount and concentration of the reacted calcium oxide and sodium hydroxide aqueous solution, a BET specific surface area of 5 to 40m can be obtained ² Calcium hydroxide/g. When the amount of the aqueous sodium hydroxide solution is increased relative to the amount of calcium oxide, calcium hydroxide having a large BET specific surface area can be obtained. Conversely, when the amount of the aqueous sodium hydroxide solution is reduced relative to the amount of calcium oxide, calcium hydroxide having a smaller BET specific surface area can be obtained. In addition, in the hydration step, when an aqueous sodium hydroxide solution having an excessively high concentration is used, the BET specific surface area of the obtained calcium hydroxide tends to be high, calcium hydroxide having a desired BET specific surface area cannot be obtained, and the viscosity of lime milk becomes high, and the treatment becomes difficult. In particular, the BET specific surface area of calcium hydroxide contained in the lime milk obtained in the step is set to 15 to 40m ² However, most of the crystal forms of calcium carbonate obtained in the carbonation step described below are preferably spindle-shaped. The hydration step and the carbon dioxide absorption step may be performed simultaneously in parallel, or may be performed simultaneouslyThe hydration step is continued after the carbon dioxide absorption step, or the carbon dioxide absorption step is continued after the hydration step. In order to finally obtain calcium carbonate in a desired form in the present embodiment, it is important to obtain calcium hydroxide having a BET specific surface area in an appropriate range in the hydration step.

In the fifth embodiment, the carbonation step is a step of reacting the lime milk obtained in the hydration step with the sodium carbonate aqueous solution obtained in the carbon dioxide absorption step to obtain calcium carbonate. This process is also commonly referred to as a causticizing process. In this step, it is very preferable to use the lime milk with the solid content concentration adjusted to 11 to 24%. It is particularly preferable to perform the reaction so that the initial concentration of lime milk is 11 to 19% and the concentration of sodium carbonate aqueous solution is 15 to 24%. The sodium carbonate aqueous solution may be used by appropriately adjusting the concentration of 15 to 24% of the sodium carbonate aqueous solution obtained in the carbon dioxide absorption step. In this case, although the reaction scale for performing the carbonation step is also dependent on, it is very preferable to add the aqueous sodium carbonate solution slowly to the lime milk, for example, for a certain period of time, such as 60 to 180 minutes or 100 to 150 minutes. The reaction in the carbonation step is preferably performed by stirring the reaction liquid. Preferably, the stirrer can be adjusted so that the time required for slowly adding the aqueous sodium carbonate solution to the lime cream until the lime cream is completely mixed (complete mixing time) is 3 to 25 seconds or 5 to 22 seconds. As means for stirring the reaction vessel, conventionally used propeller mixers, paddle mixers, ribbon mixers, turbine blade mixers, horseshoe blade mixers, yarn winding blade mixers, mixer mixers, magnetic mixers, and the like can be used.

The concentration-adjusted aqueous sodium carbonate solution obtained in the carbon dioxide absorption step is added to the lime milk having the initial solid content concentration adjusted, and the reaction is performed at a temperature ranging from 20 to 40 ℃. The calcium carbonate according to the sixth embodiment of the present invention can be obtained by the manufacturing method according to the fifth embodiment. The calcium carbonate according to the sixth embodiment preferably has a BET specific surface area of 4 to 20m ² /g of fusiform calcite crystalline calcium carbonate. The reaction temperature in the carbonation step is too high or too low, and it is not possible to obtain calcium carbonate having a desired BET specific surface area and shape, in addition to increasing the cost required for heating or cooling, such as energy required for cooling. In the reaction in this step, calcium carbonate and sodium hydroxide are produced, and water-soluble sodium hydroxide is dissolved in the reaction solution, whereby calcium carbonate having low water solubility is precipitated as a solid.

In the fifth embodiment, the method may further include a solid-liquid separation step of separating calcium carbonate produced by the reaction in the carbonation step from the reaction liquid and taking the calcium carbonate out in a solid state. The method may further include a step of washing the solid calcium carbonate obtained in the solid-liquid separation step with a washing liquid. The calcium carbonate obtained in the solid-liquid separation step is preferably washed with water. The liquid (filtrate) remaining after the separation of the solid calcium carbonate in the solid-liquid separation step and the used cleaning liquid obtained in the cleaning step are aqueous sodium hydroxide solutions. The aqueous sodium hydroxide solution may be reused in the carbon dioxide absorption step. When the filtrate and the used cleaning liquid are reused in the carbon dioxide absorption step, the concentration of sodium hydroxide is preferably adjusted to 13 to 21%. The concentration of sodium hydroxide is adjusted, for example, as follows: the concentration is performed by adding a high-concentration aqueous sodium hydroxide solution (an aqueous sodium hydroxide solution having a concentration of 13% or more), adding solid sodium hydroxide, or heating the filtrate and the used cleaning liquid.

Next, a seventh embodiment of the present invention is a method for producing calcium carbonate, comprising the steps of: a carbon dioxide absorption step of absorbing carbon dioxide (CO) by a sodium hydroxide aqueous solution having a concentration of 13 to 21% ₂ ) Obtaining sodium carbonate aqueous solution with the concentration of 15-24%; a hydration step of reacting calcium oxide with an aqueous sodium hydroxide solution having a concentration of less than 6% to obtain a BET specific surface area of 5 to 40m ² A suspension of calcium hydroxide/g, namely milk of lime; and a carbonation step of adding the sodium carbonate aqueous solution obtained in the carbon dioxide absorption step to the lime milk and reacting the same.

The production method is characterized in that in the carbonation step, the initial concentration of the lime milk is 11-24%, the concentration of the sodium carbonate aqueous solution is 15-24%, and the reaction is carried out within the range of 40-80 ℃ to obtain the product having a BET specific surface area of 3-10 m ² Needle-shaped aragonite crystalline calcium carbonate per gram. The present embodiment is also a method for producing calcium carbonate, which includes at least a carbon dioxide absorption step, a hydration step, and a carbonation step, similarly to the fifth embodiment. The carbon dioxide absorbing step of the seventh embodiment is a step of absorbing carbon dioxide with an aqueous sodium hydroxide solution to obtain an aqueous sodium carbonate solution, and may be performed exactly in the same manner as in the carbon dioxide absorbing step of the fifth embodiment described above.

The carbon dioxide which is not used in the carbon dioxide absorbing step is not directly released to the atmosphere, and is very preferably reused so as to be absorbed into the aqueous sodium hydroxide solution in the carbonation step from the viewpoint of environmental protection. The same as the fifth embodiment described above.

In the seventh embodiment, the hydration step is a step of reacting calcium oxide with an aqueous sodium hydroxide solution having a concentration of less than 6% to obtain lime milk. The hydration step of the seventh embodiment may be performed exactly in the same manner as the carbon dioxide absorbing step of the fifth embodiment described above. In the seventh embodiment, the hydration step and the carbon dioxide absorbing step may be performed simultaneously in parallel, or the hydration step may be continued after the carbon dioxide absorbing step, or the carbon dioxide absorbing step may be continued after the hydration step. In particular, the BET specific surface area of calcium hydroxide contained in the lime milk obtained in the step is set to 5 to 20m ² Per g, but is preferably set forth belowMost of the crystal forms of calcium carbonate obtained in the carbonation step (c) are acicular aragonite. In order to finally obtain calcium carbonate in a desired form in the present embodiment, it is important to obtain calcium hydroxide having a BET specific surface area in an appropriate range in the hydration step.

In the seventh embodiment, the carbonation step is a step of reacting the lime milk obtained in the hydration step with the sodium carbonate aqueous solution obtained in the carbon dioxide absorption step to obtain calcium carbonate. This process is also commonly referred to as a causticizing process. In this step, it is very preferable to use the lime milk with the solid content concentration adjusted to 11 to 24%. It is particularly preferable to perform the reaction so that the initial concentration of lime milk is 11 to 24% and the concentration of sodium carbonate aqueous solution is 15 to 24%. The sodium carbonate aqueous solution may be used by appropriately adjusting the concentration of the sodium carbonate aqueous solution having a concentration of 15 to 24% or less obtained in the carbon dioxide absorption step. In this case, although the reaction scale for performing the carbonation step is also dependent on, it is very preferable to add the aqueous sodium carbonate solution slowly to the lime milk, for example, for a certain period of time, such as 60 to 180 minutes or 100 to 150 minutes. The reaction in the carbonation step is preferably performed by stirring the reaction liquid. Preferably, the stirrer can be adjusted so that the time required for slowly adding the aqueous sodium carbonate solution to the lime cream until the lime cream is completely mixed (complete mixing time) is 3 to 25 seconds or 5 to 22 seconds. As means for stirring the reaction vessel, conventionally used propeller mixers, paddle mixers, ribbon mixers, turbine blade mixers, horseshoe blade mixers, yarn winding blade mixers, mixer mixers, magnetic mixers, and the like can be used.

The concentration-adjusted aqueous sodium carbonate solution obtained in the carbon dioxide absorbing step is added to the lime milk having the initial solid content concentration adjusted, and the reaction is performed at a temperature ranging from 40 to 80 ℃. The calcium carbonate according to the eighth embodiment of the present invention can be obtained by the manufacturing method according to the seventh embodiment. The calcium carbonate according to the eighth embodiment preferably has a BET specific surface area of 3 to 10m ² Needle-shaped aragonite crystalline calcium carbonate per gram. Carbonic acidThe reaction temperature in the conversion step is too high or too low, which increases the cost required for heating or cooling, and also does not allow calcium carbonate to be obtained having a desired BET specific surface area and shape. In order to produce calcium carbonate containing a large amount of acicular aragonite crystals, when the BET specific surface area of calcium hydroxide is adjusted in the above manner, the shape of the aragonite crystals (needles) tends to become thicker when the reaction temperature in the carbonation step is increased. In the reaction in this step, calcium carbonate and sodium hydroxide are produced, and water-soluble sodium hydroxide is dissolved in the reaction solution, whereby calcium carbonate having low water solubility is precipitated as a solid.

In the seventh embodiment, the method may further include a solid-liquid separation step of separating calcium carbonate produced by the reaction in the carbonation step from the reaction liquid and taking the calcium carbonate out in a solid state. The method may further include a step of washing the solid calcium carbonate obtained in the solid-liquid separation step with a washing liquid. The calcium carbonate obtained in the solid-liquid separation step is preferably washed with water. The liquid (filtrate) remaining after the separation of the solid calcium carbonate in the solid-liquid separation step and the used cleaning liquid obtained in the cleaning step are aqueous sodium hydroxide solutions. The aqueous sodium hydroxide solution may be reused in the carbon dioxide absorption step. When the filtrate and the used cleaning liquid are reused in the carbon dioxide absorption step, the concentration of sodium hydroxide is preferably adjusted to 13 to 21%. The concentration of sodium hydroxide is adjusted, for example, as follows: the concentration is performed by adding a high-concentration aqueous sodium hydroxide solution (an aqueous sodium hydroxide solution having a concentration of 13% or more), adding solid sodium hydroxide, or heating the filtrate and the used cleaning liquid.

Next, a flow of the fifth and seventh embodiments of the present invention will be described with reference to fig. 1. Fig. 1 shows a flow of a method for producing calcium carbonate according to the first and second embodiments of the present invention using carbon dioxide in exhaust gas discharged from a combustion furnace or the like. In the figure, 1: carbon dioxide absorption step, 2: hydration process, 3: carbonation process, 4: solid-liquid separation step, 5: and (3) a cleaning procedure. The exhaust gas discharged from the combustion furnace or the like is appropriately subjected to a dust removal treatment to obtain a purified gas containing carbon dioxide. On the other hand, a method of properly mixing the process water and a high-concentration sodium hydroxide aqueous solution to adjust the concentration to a concentration of 13 to 21% sodium hydroxide aqueous solution is prepared, and the purified gas is absorbed (carbon dioxide absorbing step 1). Thus, an aqueous sodium carbonate solution having a concentration of 15 to 24% or less was obtained. The unreacted gas that has not been absorbed in the carbon dioxide absorbing step 1 is returned as indicated by arrow 10 and reused in the carbon dioxide absorbing step 1.

In the purified lime milk thus obtained, calcium carbonate is produced when an aqueous sodium carbonate solution is reacted (carbonation step 3). The produced calcium carbonate is filtered (solid-liquid separation step 4), and the obtained solid calcium carbonate is washed with a washing liquid (washing step 5). The filtrate obtained in the solid-liquid separation step 4 and the used cleaning liquid obtained in the cleaning step 5 are recovered and reused as the aqueous sodium hydroxide solution in the carbon dioxide absorption step 1 or the hydration water in the hydration step 2 (arrows 20 and 30).

In the fifth embodiment, in the carbonation step 3 of fig. 1, the initial concentration of lime milk is set to 11 to 19% and the concentration of the sodium carbonate aqueous solution is set to 15 to 24%, and the reaction is performed at 20 to 40 ℃. Thus, a BET specific surface area of 4 to 20m can be obtained ² /g of fusiform calcite crystalline calcium carbonate (sixth embodiment). On the other hand, in the seventh embodiment, in the carbonation step 3 in fig. 1, the following is performed The initial concentration of lime milk is 11-24%, the concentration of sodium carbonate aqueous solution is 15-24%, and the lime milk is reacted at 40-80 ℃. Thus, a BET specific surface area of 3 to 10m can be obtained ² Per gram of acicular aragonite crystalline calcium carbonate (eighth embodiment).

According to the method for producing calcium carbonate of the embodiment of the present invention, a relatively high concentration aqueous sodium hydroxide solution can efficiently absorb carbon dioxide. In this case, an aqueous sodium carbonate solution having a desired concentration can be obtained regardless of the concentration of carbon dioxide. In addition, in the carbonation step, a predetermined initial concentration of the aqueous sodium carbonate solution and a predetermined initial solid content concentration of the lime milk are reacted for a predetermined time in a predetermined temperature range, whereby calcium carbonate having a desired shape can be produced.

In the method for producing calcium carbonate according to the embodiment of the present invention, carbon dioxide and an aqueous sodium hydroxide solution can be repeatedly reused, so that carbon dioxide and waste liquid discharged to the environment are small, and the load on the environment can be reduced.

Examples

The following describes embodiments of the present invention.

Example 1: synthesis of calcite-crystalline calcium carbonate in the form of Fine particles

(1) Carbon dioxide absorption step

The mixed gas containing 30% by volume of carbon dioxide-air was introduced into an aqueous solution of sodium hydroxide having a concentration of 11.8% until the pH of the aqueous solution became 11.5. 423kg of an aqueous sodium carbonate solution having a concentration of 14.8% are obtained.

(2) Hydration step

Mixing calcium oxide in water to hydrate the calcium oxide to obtain calcium hydroxide suspension, namely lime milk. The BET specific surface area of the obtained calcium hydroxide was measured in accordance with Japanese Industrial Standard JISZ8830 (ISO 9277:2010), and found to be 15.9m ³ And/g. The concentration of lime milk was adjusted to obtain 553kg of lime milk having a solid content of 5.0%.

(3) Carbonation process

553kg of stone obtained in the hydration stepThe lime milk is introduced into a reaction tank equipped with a propeller stirrer. Here, 423kg of the sodium carbonate aqueous solution obtained in the carbonation step was added over 120 minutes, and the reaction solution was stirred. At this time, the propeller stirrer was operated so that the total mixing time in the reaction tank was 20 seconds, and the temperature in the reaction tank was adjusted to 15 ℃. The resulting calcium carbonate suspension was filtered, washed with water and dried in a constant temperature dryer at 105 ℃ for 1 hour. 49kg of calcium carbonate powder was obtained. As a result of observation of the obtained calcium carbonate by an electron microscope, fine particles of about 20nm were in a chain-like connected form. FIG. 2 is an electron micrograph (magnification: 30000) of calcium carbonate obtained in example 1. The BET specific surface area (measured according to JIS Z8830) of the calcium carbonate having the fine particle shape was 60.0m ² G (row at 15℃in the carbonation step reaction temperature, column at 15% sodium carbonate aqueous solution concentration in Table 1).

[ other Synthesis examples of calcium-titanium crystalline calcium carbonate in the form of fine particles ]

In example 1, various sodium carbonate aqueous solutions having different concentrations were prepared. The aqueous sodium carbonate solutions having different concentrations were slowly added to the lime milk having a solid content concentration of 5.0% obtained in the hydration step of example 1, and stirred by a propeller stirrer to set the temperature in the reaction tank to 10 ℃/15 ℃/20 ℃ and the carbonation step was performed. The results of each synthesis example are shown in table 1 below.

TABLE 1

Table 1 shows the crystal shape and BET specific surface area of the calcium carbonate produced in each synthesis example. For example, example 1 is described in "60" in a row at a reaction temperature of 15℃in the carbonation step and in a column at a sodium carbonate aqueous solution concentration of 15%. In the sense that a BET specific surface area of 60m was obtained by the method of example 1 ² Per g (BET specific surface area in the table is rounded to the actual value), of calcite-crystalline calcium carbonate in the form of fine particles. In table 1, "-" means that sodium carbonate is precipitated from an aqueous sodium carbonate solution under such conditions, and therefore it is difficult to perform a reaction under a predetermined condition.

As shown in table 1, in the method of the present invention, calcium carbonate having a desired crystal shape and BET specific surface area can be produced by adjusting the solid content concentration of the initial milk of lime, the concentration of the aqueous sodium carbonate solution, and the reaction temperature in the carbonation step.

Example 2: synthesis of crystalline calcium carbonate of spindle-shaped calcite (1)

(1) Carbon dioxide absorption step

The carbon dioxide-air mixture gas containing 30% by volume of carbon dioxide was introduced into an aqueous sodium hydroxide solution having a concentration of 12.9% until the pH of the aqueous solution became 11.5. 630kg of an aqueous sodium carbonate solution having a concentration of 16.0% was obtained.

(2) Hydration step

Mixing calcium oxide in water to hydrate the calcium oxide to obtain calcium hydroxide suspension, namely lime milk. The BET specific surface area of the obtained calcium hydroxide was measured in accordance with Japanese Industrial Standard JISZ8830 (ISO 9277:2010), and found to be 15.9m ² And/g. The concentration of lime milk was adjusted to obtain 389kg of lime milk having a solid content of 15.0%.

(3) Carbonation process

389kg of lime milk obtained in the hydration step was introduced into a reaction tank equipped with a propeller stirrer. Here, 630kg of the sodium carbonate aqueous solution obtained in the carbonation step was added over 120 minutes, and the reaction solution was stirred. At this time, the propeller stirrer was operated so that the total mixing time in the reaction tank was 21 seconds, and the temperature in the reaction tank was adjusted to 25 ℃. The resulting calcium carbonate suspension was filtered, washed with water and dried in a constant temperature dryer at 105 ℃ for 1 hour. 79kg of calcium carbonate powder was obtained. The obtained calcium carbonate was observed by an electron microscope, and as a result, it was in a spindle shape. FIG. 3 is an electron micrograph (magnification: 20000 times) of calcium carbonate obtained in example 2. The BET specific surface area (measured according to JIS Z8830) of the fusiform calcite crystalline calcium carbonate was 5.9m ² /g (in Table 2, carbonation)Row at 25 ℃ process reaction temperature, column at 16% sodium carbonate aqueous solution concentration).

Example 3: synthesis of acicular aragonite crystalline calcium carbonate (1)

(1) Carbon dioxide absorption step

(2) Hydration step

(3) Carbonation process

389kg of lime milk obtained in the hydration step was introduced into a reaction tank equipped with a propeller stirrer. Here, 630kg of the sodium carbonate aqueous solution obtained in the carbonation step was added over 120 minutes, and the reaction solution was stirred. At this time, the propeller stirrer was operated so that the total mixing time in the reaction tank was 21 seconds, and the temperature in the reaction tank was adjusted to 50 ℃. The resulting calcium carbonate suspension was filtered, washed with water and dried in a constant temperature dryer at 105 ℃ for 1 hour. 79kg of calcium carbonate powder was obtained. The obtained calcium carbonate was observed by an electron microscope, and as a result, an aragonite in a needle shape was obtained. FIG. 4 is an electron micrograph (magnification: 10000 times) of calcium carbonate obtained in example 3. The BET specific surface area (measured according to JIS Z8830) of the acicular aragonite crystalline calcium carbonate was 6.3m ² G (row at 50℃in the carbonation step, column at 16% sodium carbonate aqueous solution concentration) in Table 2.

Example 4: synthesis of acicular aragonite crystalline calcium carbonate (2)

(1) Carbon dioxide absorption step

The carbon dioxide-air mixture gas containing 30% by volume of carbon dioxide was introduced into an aqueous sodium hydroxide solution having a concentration of 15.6% until the pH of the aqueous solution became 11.5. 666kg of aqueous sodium carbonate solution having a concentration of 19.0% were obtained.

(2) Hydration step

Mixing calcium oxide in water to hydrate the calcium oxide to obtain calcium hydroxide suspension, namely lime milk. The BET specific surface area of the obtained calcium hydroxide was measured in accordance with Japanese Industrial Standard JISZ8830 (ISO 9277:2010), and found to be 15.9m ² And/g. The concentration of lime milk was adjusted to obtain 366kg of lime milk having a solid content of 20.0%.

(3) Carbonation process

366kg of lime milk obtained in the hydration step was introduced into a reaction tank equipped with a propeller stirrer. The 666kg of the aqueous sodium carbonate solution obtained in the carbonation step was added over 120 minutes, and the reaction solution was stirred. At this time, the propeller stirrer was operated so that the total mixing time in the reaction tank was 21 seconds, and the temperature in the reaction tank was adjusted to 50 ℃. The resulting calcium carbonate suspension was filtered, washed with water and dried in a constant temperature dryer at 105 ℃ for 1 hour. 99kg of calcium carbonate powder was obtained. The obtained calcium carbonate was observed by an electron microscope, and as a result, an aragonite in a needle shape was obtained. FIG. 5 is an electron micrograph (magnification: 10000 times) of calcium carbonate obtained in example 4. The BET specific surface area (measured according to JIS Z8830) of the acicular aragonite crystalline calcium carbonate was 8.5m ² G (row at 50℃in the carbonation step, column at 19% sodium carbonate aqueous solution concentration) in Table 3.

Example 5: synthesis of acicular aragonite crystalline calcium carbonate (3)

(1) Carbon dioxide absorption step

(2) Hydration step

Mixing calcium oxide in water to hydrate the calcium oxide to obtain calcium hydroxide suspension, namely lime milk. The BET specific surface area of the obtained calcium hydroxide was measured in accordance with Japanese Industrial Standard JISZ8830 #ISO9277:2010 Measurement was conducted, and the result was 15.9m ² And/g. The concentration of lime milk was adjusted to obtain 389kg of lime milk having a solid content of 15.0%.

(3) Carbonation process

389kg of lime milk obtained in the hydration step was introduced into a reaction tank equipped with a propeller stirrer. Here, 630kg of the sodium carbonate aqueous solution obtained in the carbonation step was added over 120 minutes, and the reaction solution was stirred. At this time, the propeller stirrer was operated so that the total mixing time in the reaction tank was 21 seconds, and the temperature in the reaction tank was adjusted to 80 ℃. The resulting calcium carbonate suspension was filtered, washed with water and dried in a constant temperature dryer at 105 ℃ for 1 hour. 79kg of calcium carbonate powder was obtained. The obtained calcium carbonate was observed by an electron microscope, and as a result, an aragonite in a needle shape was obtained. FIG. 6 is an electron micrograph (magnification: 10000 times) of calcium carbonate obtained in example 5. The acicular aragonite crystalline calcium carbonate had a BET specific surface area (measured according to JIS Z8830) of 3.4m ² G (row at reaction temperature 80℃in the carbonation step, column at concentration of 16% in sodium carbonate aqueous solution) in Table 2.

[ other Synthesis examples of calcium carbonate in the form of spindle-shaped calcite crystals ]

In example 1, various sodium carbonate aqueous solutions having different concentrations were prepared. The aqueous sodium carbonate solutions having different concentrations were slowly added to the lime milk having a solid content concentration of 15.0% obtained in the hydration step of example 1, and stirred by a propeller stirrer so that the temperature in the reaction tank was 20 ℃/25 ℃/40 ℃, followed by a carbonation step. The results of each synthesis example are shown in table 2 below.

[ other Synthesis examples of acicular aragonite crystalline calcium carbonate ]

In example 2, various sodium carbonate aqueous solutions having different concentrations were prepared. The aqueous sodium carbonate solutions having different concentrations were slowly added to the lime milk having a solid content concentration of 15.0% obtained in the hydration step of example 2, and stirred by a propeller stirrer so that the temperature in the reaction tank was 50 ℃/65 ℃/70 ℃/80 ℃, thereby performing the carbonation step. The results of each synthesis example are shown in table 2 below.

In example 4, various sodium carbonate aqueous solutions having different concentrations were prepared. The aqueous sodium carbonate solutions having different concentrations were slowly added to the lime milk having a solid content concentration of 20.0% obtained in the hydration step of example 3, and stirred by a propeller stirrer so that the temperature in the reaction tank was 40 ℃/50 ℃/60 ℃/70 ℃/80 ℃, followed by a carbonation step. The results of each synthesis example are shown in table 3 below.

TABLE 2

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TABLE 3

The crystal shape and BET specific surface area of the calcium carbonate produced in each synthesis example are shown in tables 2 and 3. For example, example 2 is described in "spindle 6" of a row at a carbonation process reaction temperature of 25℃and a column at a sodium carbonate aqueous solution concentration of 19%. In the sense that a material having a BET specific surface area of 6m was obtained by the method of example 2 ² Per g (BET specific surface area in the table is a value obtained by rounding off the actual measurement value). In tables 2 and 3, "-" means that sodium carbonate is precipitated from an aqueous sodium carbonate solution under these conditions, and therefore it is difficult to perform the reaction under the predetermined conditions.

As shown in tables 2 and 3, in the method of the present invention, calcium carbonate having a desired crystal shape and BET specific surface area can be produced separately by adjusting the solid content concentration of the initial lime milk, the sodium carbonate aqueous solution concentration, and the reaction temperature in the carbonation step.

The method of the present invention can efficiently use carbon dioxide because a relatively high concentration aqueous sodium hydroxide solution is allowed to absorb carbon dioxide. By the method of the present invention, needle-shaped crystals and spindle-shaped crystals of calcium carbonate can be produced. By changing the concentration, reaction temperature, reaction time, mixing time, and the like of the lime milk and the sodium carbonate aqueous solution in the carbonation step, calcium carbonate having a desired crystal form can be produced. The method of the present invention can reduce the load on the environment as a whole, because carbon dioxide, filtrate, and the like are reused.

Industrial applicability

The calcium carbonate produced by the method of the present invention can be widely used as a pigment for paper coating and a pigment for paint or ink, in addition to being particularly used as a filler for sealing materials, adhesives, rubber compositions, plastic compositions, paper and the like.

Claims

1. A method for producing calcium carbonate, comprising the steps of:

a carbon dioxide absorption step of absorbing carbon dioxide with a sodium hydroxide aqueous solution having a concentration of 5 to 21% to obtain a sodium carbonate aqueous solution having a concentration of 4 to 24%;

and a carbonation step of adding and reacting the aqueous sodium carbonate solution to the lime milk.

2. The method according to claim 1, wherein,

in the carbonation step, the solid content concentration of the lime milk is adjusted to 1 to 24%, and the sodium carbonate aqueous solution is added to the lime milk with the solid content concentration adjusted, and reacted at a temperature of 9 to 80 ℃.

3. The manufacturing method according to claim 1 or 2, characterized by further comprising:

After the carbonation step, the mixture is separated into a filtrate containing sodium hydroxide and calcium carbonate.

4. The method according to claim 3, wherein,

the concentration of sodium hydroxide in the filtrate containing the sodium hydroxide is adjusted to 5 to 21%, and the filtrate is used in the carbon dioxide absorption step.

5. A calcium carbonate, which is characterized in that,

manufactured by the manufacturing method according to any one of claims 1 to 4.

6. A method for producing calcium carbonate, comprising the steps of:

a carbonation step of adding the sodium carbonate aqueous solution obtained in the carbon dioxide absorption step to the lime milk and reacting the added solution,

the manufacturing method is characterized in that,

in the carbonation step, the initial concentration of the lime milk is set to 1 to 6%, the concentration of the aqueous sodium carbonate solution is set to 4 to 24%, and the aqueous sodium carbonate solution is reacted at a temperature ranging from 9 to 25 ℃ to obtain a powder having a BET specific surface area of 30 to 90m ² Calcite crystalline calcium carbonate in the form of fine particles/g.

7. The method according to claim 6, wherein,

the carbon dioxide that is not used in the carbon dioxide absorbing step is reused in the carbon dioxide absorbing step.

8. The manufacturing method according to claim 6 or 7, characterized by further comprising:

a solid-liquid separation step of separating the filtrate containing sodium hydroxide and calcium carbonate after the carbonation step; and

9. The method of manufacturing according to claim 8, wherein,

an aqueous solution of sodium hydroxide having a high concentration is added to the filtrate obtained in the solid-liquid separation step and the used cleaning liquid obtained in the cleaning step, or the filtrate and the used cleaning liquid are heated and concentrated to obtain an aqueous solution containing sodium hydroxide having a concentration of 5 to 21%, and the aqueous solution is used in the carbon dioxide absorption step.

10. The method of manufacturing according to claim 8 or 9, wherein,

the filtrate obtained in the solid-liquid separation step and the used cleaning liquid obtained in the cleaning step are adjusted to an aqueous solution having a sodium hydroxide concentration of less than 6%, and the aqueous solution is used in the hydration step.

11. A calcium carbonate, which is characterized in that,

manufactured by the manufacturing method according to any one of claims 6 to 10.

12. A method for producing calcium carbonate, comprising the steps of:

a carbon dioxide absorption step of absorbing carbon dioxide with a sodium hydroxide aqueous solution having a concentration of 13 to 21% to obtain a sodium carbonate aqueous solution having a concentration of 15 to 24% or less;

the manufacturing method is characterized in that,

in the carbonation step, the initial concentration of the lime milk is 11-19%, the concentration of the aqueous sodium carbonate solution is 15-24%, and the aqueous sodium carbonate solution is reacted at 20-40 ℃ to obtain a powder having a BET specific surface area of 4-20 m ² /g of fusiform calcite crystalline calcium carbonate.

13. The method of manufacturing according to claim 12, wherein,

14. The manufacturing method according to claim 12 or 13, characterized by further comprising:

15. The method of manufacturing according to claim 14, wherein,

an aqueous solution of sodium hydroxide having a high concentration is added to the filtrate obtained in the solid-liquid separation step and the used cleaning liquid obtained in the cleaning step, or the filtrate and the used cleaning liquid are heated and concentrated to obtain an aqueous solution containing sodium hydroxide having a concentration of 13 to 21%, and the aqueous solution is used in the carbon dioxide absorption step.

16. The method of manufacturing according to claim 14 or 15, wherein,

17. A calcium carbonate, which is characterized in that,

manufactured by the manufacturing method according to any one of claims 12 to 16.

18. A method for producing calcium carbonate, comprising the steps of:

the manufacturing method is characterized in that,

in the carbonation step, the initial concentration of the lime milk is 11-24%, the concentration of the aqueous sodium carbonate solution is 15-24%, and the aqueous sodium carbonate solution is reacted at 40-80 ℃ to obtain a powder having a BET specific surface area of 3-10 m ² Needle-shaped aragonite crystalline calcium carbonate per gram.

19. The method of manufacturing according to claim 18, wherein,

20. The manufacturing method according to claim 18 or 19, characterized by further comprising:

21. The method of manufacturing according to claim 20, wherein,

22. The method of manufacturing according to claim 20 or 21, wherein,

23. A calcium carbonate, which is characterized in that,

manufactured by the manufacturing method according to any one of claims 18 to 22.