METHOD OF FIXING CARBON DIOXIDE COMPRISING A STEP OF RECYCLING THE USED EXTRACTION SOLVENT

Abstract

A method of fixing carbon dioxide is provided. After metal ion components are extracted from natural mineral or steel slag through acid treatment, carbon dioxide is injected to fix carbon dioxide by carbonating the same. Since the procedure of pH adjustment is unnecessary, the reaction is carried out effectively, and a continuous process is enabled. Further, since the used extraction solvent is recycled, the cost of fixing carbon dioxide is reduced. The disclosed method of fixing carbon dioxide enables effective removal of carbon dioxide produced from the steelmaking industry, thereby significantly reducing greenhouse gas emission and allowing recycling of the conventionally discarded steel slag.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2010-0117700, filed on Nov. 24, 2010, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

(a) Technical Field

The present invention relates to a method of fixing carbon dioxide. More particularly, the present invention relates to a method for fixing carbon dioxide utilizing natural mineral or steel slag, thereby reducing emission of carbon dioxide into the atmosphere.

(b) Background Art

Global carbon dioxide emissions have been rapidly increasing due to increased fossil fuel consumption. Carbon dioxide emissions have been recently recognized as the main cause of global warming, and thus, countries around the world are making efforts to provide appropriate regulations to reduce such emissions.

The reduction in carbon dioxide emissions can be accomplished by reducing the use of fossil fuel itself, or by a method wherein the carbon dioxide produced by the use of fossil fuel is separated, collected and fixed. The separated and collected carbon dioxide can, for example, be used as a source for methanol synthesis. The separated and collected carbon dioxide can also be fixed by dumping it into the ocean or by using carbonate minerals.

According to one method, carbon dioxide emitted from plants is fixed using carbonate minerals. In particular, the carbon dioxide is reacted with alkaline substances (e.g. CaO, MgO, K₂O, Na₂O, etc.) existing in minerals to produce carbonates (e.g. CaCO₃, MgCO₃, Na₂CO₃, K₂CO₃, etc.).

Slags produced from steelmaking processes include molten iron pretreatment slag, converter slag, stainless steel slag, electric furnace slag, and the like. These steel slags are mostly buried, with a limited amount being utilized as cement or aggregates for road or building construction. Various other methods for utilizing the discarded steel slag are desired and have been proposed, particularly as it becomes more difficult to find landfills in which to bury the slag.

Korean Patent Application Publication No. 2002-0050429 proposes a method of fixing carbon dioxide on the surface of steel slag for use in harbor construction or artificial fish banks for fish. Korean Patent Application Publication No. 2006-0023206 proposes fixing carbon dioxide on the surface of slag containing a certain level of water by reacting it with carbon dioxide. However, these two methods require an excessive reaction time since the efficiency of the reaction between carbon dioxide and slag is very low. Thus, the methods are economically impractical.

Korean Patent No. 0891551 (Application No. 2008-0025573) proposes a method of solidifying carbon dioxide by carbonating alkaline components extracted from steel slag with gaseous carbon dioxide through either a pressurized or a normal-pressure hydrothermal reaction. However, the hydrothermal reaction consumes a great deal of energy, and it is unclear how the carbon dioxide can be fixed following the carbonation of the alkaline components.

Korean Patent No. 0801542 (Application No. 2006-0105753) proposes a method of fixing carbon dioxide using natural mineral talc. However, according to this method the particles must be 125 μm or less, thus requiring excessive energy for the pulverization of mineral. Further, after the alkaline components are treated with a weak acid solvent, such as acetic acid, for extraction, the pH is increased to fix the carbon dioxide through carbonation. This increase in pH causes some metal ions (e.g., Ca²⁺) to react with hydroxide ions (OH⁻) to form milky lime (Ca(OH)₂), resulting in suspension. Thus, an additional precipitation or filtration procedure is required. Since the precipitated carbonate is suspended in the solution, the separation is difficult and it is difficult to establish an industrially applicable continuous process.

SUMMARY

The present invention provides to a method for effectively fixing carbon dioxide that overcomes the problems with conventional methods. The present method is applicable to actual processes, whereby carbon dioxide can be fixed via liquid-phase reaction. The present method further provides improved reaction efficiency, at normal temperature under normal pressure, thus reducing energy consumption. Further, according to the present methods, it is unnecessary to adjust pH for carbon dioxide fixation, thus preventing the formation of suspended milky lime and allowing for a continuous process.

According to further embodiments, the present invention provides a method capable of lowering the cost of fixing carbon dioxide by recycling the used extraction solution after the separation of carbonates.

In a general aspect, the present invention provides a method of fixing carbon dioxide comprising: (a) treating natural mineral or steel slag with an acid to extract metal ion components; (b) injecting carbon dioxide into an extraction solution containing the metal ion components obtained in step (a) to carbonate the same; (c) transferring a solution in which carbonates resulting from step (b) are dissolved to a dissolved carbonate storage tank and storing the same; (d) transferring the stored dissolved carbonates to a carbonate separation/purification tank and adjusting pH to 7 or above to separate the carbonates; and (e) treating the solution from which the carbonates have been separated with an acid, and recycling the treated solution as the extraction solvent (acid) in step (a).

The above and other aspects and features of the present invention will be described infra.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will now be described in detail with reference to certain exemplary embodiments thereof illustrated in the accompanying drawings which are given hereinbelow by way of illustration only, and thus are not limitative of the disclosure, and wherein:

FIG. 1 shows an existing process of fixing carbon dioxide using slag; and

FIG. 2 shows a method of fixing carbon dioxide using natural mineral or steel slag according to the present invention.

It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of the disclosure. The specific design features of the disclosure as disclosed herein, including, for example, specific dimensions, orientations, locations and shapes, will be determined in part by the particular intended application and use environment.

DETAILED DESCRIPTION

Hereinafter, reference will now be made in detail to various embodiments of the present invention, examples of which are illustrated in the accompanying drawings and described below. While the disclosure will be described in conjunction with exemplary embodiments, it will be understood that the present description is not intended to limit the disclosure to those exemplary embodiments. On the contrary, the disclosure is intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the disclosure as defined by the appended claims.

The present invention provides a method of fixing carbon dioxide comprising:

(a) treating natural mineral or steel slag with an acid to extract metal ion components;

(b) injecting carbon dioxide into an extraction solution containing the metal ion components obtained in step (a) to carbonate the same;

(c) transferring the solution of step (b), in which carbonates resulting from step (b) are dissolved, to a dissolved carbonate storage tank and storing the same;

(d) transferring the stored dissolved carbonates to a carbonate separation/purification tank and adjusting pH to 7 or above to separate the carbonates; and

(e) treating the solution from which the carbonates have been separated (in from step (d)) with an acid, and recycling the acid treated solution for use as the extraction solvent (acid) in step (a).

In step (a), the natural mineral is not particularly limited. For example, peridotite, basalt, talc, serpentinite, wollastonite, etc., which contain a large amount of metal oxides, such as calcium oxide and/or magnesium oxide, may be used. The contents of calcium oxide and magnesium oxide in serpentinite and wollastonite are shown, for example, in Table 1.

In step (a), the steel slag may be selected from, but is not limited to, slag, electric furnace slag or converter slag produced from, for example, an ironworks during blast furnace, converter or oxygen blowing processes. Currently, steel slag is mostly utilized as cement or aggregates for road or building construction. The chemical composition of various steel slag is also shown in Table 1.

	TABLE 1
	CaO (wt %)	MgO (wt %)
Natural mineral	Serpentinite	0	40
	Wollastonite	48	0
Steel slag	Blast furnace slag	41	10
	Converter slag	46	2
	Electric furnace slag	20	5
	Ladle furnace slag	54	10

Since the natural mineral and the steel slag contain a large amount of metal oxides, including calcium oxide and magnesium oxide, they exhibit high basicity. As such, upon acid treatment, the metal components of the metal oxides are released into the solution, thus exhibiting alkaline property. Generally, these metal ion components are mostly Ca² and Mg²⁺, and the remainder may be, for example, K⁺, Na⁺, etc.

In accordance with some embodiments, the acid treatment may be performed at about pH 3-5. To provide the desired pH, an acidic substance such as acetic acid, sodium acetate, hydrochloric acid, etc., may be used at a suitable concentration.

Upon acid treatment, the solution containing the metal ions, e.g., Ca²⁺, Mg²⁺, etc., is provided with an acidic pH.

According to the conventional methods for fixing by carbonation, the pH of the extraction solution is adjusted to about 12, followed by injection of carbon dioxide to form a carbonate precipitate. During this procedure, some of the dissolved metal ions (e.g., Ca²⁺, Mg²⁺, etc.) react with hydroxide ions (OH⁻) to form milky lime, thus resulting in suspension. Unless the suspending solid is effectively separated, it becomes difficult to fix carbon dioxide through a continuous process. As a result, treatment of carbon dioxide on a large scale is very difficult, if not impossible.

In accordance with the present invention, the procedure of adjusting the pH to about 12 is omitted and, thus, this limitation is overcome. In particular, by omitting the pH adjustment procedure, the present invention avoids the formation of milky lime and, consequently, the formation of suspending solids. Thus, the present invention allows for a continuous process, and treatment of carbon dioxide on a large scale is possible. According to the present invention, the procedure for adjusting pH to an alkaline range (pH 7 or above) for separation of the carbonates is carried out after the CO2 reaction with metal ion components is completed. Since the metal ions have participated in reactions to form the carbonates, there is very little possibility that they will form suspending solids through reaction with hydroxide ions. The solution in which the carbonates (i.e. the carbonates resulting from the step (b) carbon dioxide fixing) are dissolved is then transferred to a dissolved carbonate storage tank and for storage. Subsequently, the stored dissolved carbonate solution is transferred to a carbonate separation/purification tank, and the pH of the solution is adjusted to 7 or above to separate the carbonates. Consequently, the procedures of carbonation and carbonate separation can be performed continuously.

In embodiments of the present invention, the carbonation in step (b) may be performed at normal temperature under normal pressure. As referred to herein, “normal temperature” and “normal pressure” mean that no additional heat or pressure needs to be applied. For example, the temperature may be about 0-40° C., and in accordance with some embodiments about 10-25° C., and the pressure may be about 0.1-5 atm, and in accordance with some embodiments about 0.5-2 atm. In accordance with the present invention, the carbonation can be completed within 5 minutes without requiring an energy-consuming hydrothermal reaction. According to some embodiments, the rate of injection of the carbon dioxide in step (b) can be controlled so as to provide a desired time for completion of carbonation. In particular, according to various embodiments, the carbonation reaction of step (b) can be completed within 5 minutes when the injection rate of carbon dioxide is about 2 L/min. Further, since the release of the metal ion components in step (a) can be accomplished within 2 hours, the overall process can be completed in about 2 hours, making it a very economical process. Thus, carbon dioxide can be effectively fixed using natural mineral or steel slag (e.g., slag, electric furnace slag or converter slag produced from an ironworks during blast furnace, converter or oxygen blowing processes), thereby remarkably reducing the emission of greenhouse gas and allowing the utilization of the previously discarded steel slag.

The carbon dioxide gas treated by the method according to the present invention may be any carbon dioxide gas, including those produced as by-products from industrial furnaces such as blast furnaces, lime burning furnaces, coking furnaces, etc., sintering or hot-rolling processes, power generation, waste heat boilers, and the like. Accordingly, if an ironworks is equipped with a facility capable of fixing carbon dioxide generated in the steelmaking process, the steel slag produced during the process may be utilized to fix the global warming-causing gas on site. Through this, environmental regulations can be met, and by-products such as metal oxides can be utilized as a new source of revenue, rather than waste.

According to the present invention, the solution with the carbonates separated in step (d) is not discarded but can be recycled as the extraction solvent (also referred to as “acid”) in step (a). Since the solution resulting from step (d) has a pH of 7 or above, it is first treated with an acid to adjust pH to 3-5 for subsequent use as the extraction solvent in step (a). The acid treatment may be carried out using one or more acid(s) preferably selected from a group consisting of formic acid, acetic acid, propionic acid, phosphoric acid, phosphorous acid, citric acid and oxalic acid. The recycled extraction solvent can be mixed with a fresh extraction solvent. and the mixture supplied to the extracting device (as shown, for example, in FIG. 2). By recycling the extraction solution, the cost related to preparation, purification and disposal of the extraction solution is reduced, and the overall cost of carbon dioxide fixing can be reduced significantly.

In accordance with the present invention, a method is provided whereby carbon dioxide is fixed by carbonation without pH adjustment, and whereby high-purity carbonate can be prepared in about 2 hours. Further, since a continuous process is enabled by the present method, treatment of carbon dioxide in large scale is possible. In addition, since the extraction solution can be recycled, the cost related to preparation, purification and disposal of the extraction solution can be reduced and the overall cost of carbon dioxide fixing can be reduced significantly.

The method of fixing carbon dioxide according to the present invention, whereby metal ions are extracted through acid treatment of natural mineral or steel slag, is much more effective than the existing solid-gas reaction methods. Since the reaction is carried out at normal temperature under normal pressure, without requiring an energy-intensive procedure such as hydrothermal synthesis, energy consumption can be reduced. Further, since the pH adjustment for fixing carbon dioxide is omitted, formation of suspended milky lime can be prevented, and a continuous process is enabled. Further, since the extraction solution can be recycled, the cost related to preparation, purification and disposal of the extraction solution can be reduced and the overall cost of carbon dioxide fixing can be reduced significantly.

Since the method of fixing carbon dioxide according to the present invention allows reduction of carbon dioxide emissions using steel slag, which is currently used only for valueless applications, it can be an effective measure to cope with the greenhouse gas reduction requirement imposed on steelmaking companies, while allowing for recycling of the discarded steel slag as carbonate.

In addition, it is noted that the cost related to preparation, purification and disposal of the extraction solution used in a carbon dioxide fixing process accounts for more than 50% of the total cost of fixing carbon dioxide. Thus, according to the present invention, the overall cost of carbon dioxide fixing is reduced by reducing the cost related to preparation, purification and disposal of the extraction solution. Further, the net amount of fixed carbon dioxide can be enhanced as the amount of the used extraction solution is decreased.

The present invention has been described in detail with reference to specific embodiments thereof. However, it will be appreciated by those skilled in the art that various changes and modifications may be made in these embodiments without departing from the principles and spirit of the disclosure, the scope of which is defined in the appended claims and their equivalents.

Claims

1. A method of fixing carbon dioxide comprising the steps of:

(a) treating natural mineral or steel slag with an acid to extract metal ion components, thereby providing an extraction solution containing the metal ion components;

(b) injecting carbon dioxide into the extraction solution containing the resulting metal ion components to carbonate the same, thereby providing a solution in which resulting carbonates are dissolved;

(c) transferring the solution in which the resulting carbonates are dissolved to a dissolved carbonate storage tank;

(d) transferring the solution from the dissolved carbonate storage tank to a carbonate separation/purification tank, and adjusting pH to 7 or above to separate the carbonates;

(e) treating the solution from which the carbonates have been separated with an acid; and

(f) recycling the treated solution to step (a) as an extraction acid.

2. The method of fixing carbon dioxide according to claim 1, wherein said treating with the acid in step (e) is performed at about pH 3-5.

3. The method of fixing carbon dioxide according to claim 1, wherein said carbonating is performed at normal temperature under normal pressure.

4. The method of fixing carbon dioxide according to claim 1, wherein step (e) comprises adjusting pH to about 3-5 using one or more acid(s) selected from a group consisting of formic acid, acetic acid, propionic acid, phosphoric acid, phosphorous acid, citric acid and oxalic acid.