Method for fixing carbon dioxide and method for producing magnesium carbonate

Abstract

To provide a method for removing Cr(VI), a harmful substance eluted into a solution by heat treatment performed to enhance leachability of Mg from "Cr- and Mg-containing ore" containing chromium and magnesium and having a magnesium concentration of 5 mass% or more, in a "carbon dioxide fixation process" using the "Cr- and Mg-containing ore".SOLUTION: A method for fixing carbon dioxide comprises: a heat treatment St1 for heating "Cr- and Mg-containing ore" at a temperature of 500°C or higher and 800°C or lower; a solution purification treatment St2 for charging the ore after the heat treatment St1 into a solution and adding a divalent iron compound to the solution; and a carbon dioxide fixing treatment St3 for reacting magnesium eluted from the ore into the solution with carbon dioxide in the solution after the solution purification treatment St2.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for fixing carbon dioxide and a method for producing magnesium carbonate. More specifically, the present invention relates to a method for fixing carbon dioxide by fixing carbon dioxide to an ore containing magnesium and chromium, and a method for producing magnesium carbonate from an ore containing magnesium and chromium and carbon dioxide.

Background Art

[0002] With the rapid progress of global warming in recent years, along with the reduction of the emission amount of carbon dioxide (CO2) emitted, the practical application of carbon capture, storage, and utilization technology (CCSU) by fixing the emitted CO2 to minerals or the like has been demanded. As specific realization means of carbon capture, storage, and utilization technology (CCSU), various methods for fixing CO2, such as a method of injecting CO2 deep into the sea water or a method of injecting CO2 into the ground for storage, have been studied. However, all of these methods require a lot of costs for CO2 treatment and are not economical, and the subsequent impact on the environment is unclear, and there are still many problems in practical application.

[0003] On the one hand, as one of the above carbon capture and storage utilization technologies (CCSU), a technology that reacts a silicate mineral containing magnesium (Mg), calcium (Ca), etc. with CO2 to immobilize it as a carbonate (mineral carbonation technology) has attracted attention (see Patent Document 1). Carbonates such as magnesium carbonate (MgCO3) and calcium carbonate (CaCO3) are very stable at normal temperature and pressure and are not easily affected by environmental changes, so CO2 can be continuously and stably fixed in minerals over a long period of time. And moreover, MgCO3 and CaCO3 generated with the immobilization of CO2 by the above method can all be used in a wide range of industrial fields such as neutralizing agents, rubber, plastic reinforcing agents, pesticides, fertilizers, feed materials, etc., and since they are carbon-neutral materials, an increase in future demand is expected.

[0004] Here, as an example of a manufacturing process that can also contribute to the fixation of CO2 by using the above mineral carbonation technology, "a manufacturing process of magnesium carbonate (MgCO3) using an ore containing Mg as a silicate mineral" can be cited. In this manufacturing process, in order to react with CO2, it is necessary to sufficiently elute Mg from the above ore into the solution. However, in the above ore, Mg is contained as a silicate mineral having a complex crystal structure (for example, compounds such as Mg3Si2O5(OH)4 and Mg3Si4O 10 (OH)2, etc.), so the elution of Mg 2+ does not proceed easily. Therefore, when implementing the above manufacturing process, it has been necessary to perform some pretreatment to promote the elution of Mg 2+ into the solution for the above ore.

[0005] As specific means of the above pretreatment required when implementing Mineral Carbonation Technology, conventionally, a heat treatment technology has been proposed in which ores containing Mg as a silicate mineral are heated to promote the elution of Mg contained in the ores into a solution (see Patent Documents 2 and 3).

[0006] Here, in the above "process for producing magnesium carbonate (MgCO3) carried out using ores containing Mg as a silicate mineral", ores containing Mg as a silicate mineral at a concentration of 5% by mass or more can preferably be used. However, many of these ores often contain chromium (Cr) as well. For example, in addition to limonite ore used as a raw material ore for nickel smelting, oxide ores such as saprolite ore, and ores such as riebeckite, chrysotile, enstatite, forsterite, talc, magnesite, serpentine, olivine, and smectite correspond to such ores. Hereinafter, in this specification, ores containing Cr and Mg and having a magnesium concentration of 5% by mass or more, including these various ores, are referred to as "Cr- and Mg-containing ores".

[0007] And in a process that is technically closely related to the above Mineral Carbonation Technology, namely, in the "process for carbon dioxide fixation" and the "process for producing magnesium carbonate (MgCO3)" carried out using "Cr- and Mg-containing ores", when the above heat treatment is performed to promote the elution of Mg contained in the ore as a silicate mineral into a solution, the oxidation of Cr in the ore progresses, and it has come to be recognized as a new problem that even hexavalent chromium (Cr(VI)), which is a harmful substance, is eluted into the solution.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

[0009] The present invention has been proposed in view of the above circumstances, and in a "carbon dioxide fixation process" and a "magnesium carbonate production process" carried out using "Cr and Mg-containing ore" containing chromium and magnesium and having a magnesium concentration of 5% by mass or more, the object is to remove Cr(VI), which is a harmful substance eluted into the solution by a heat treatment performed to enhance the elution property of Mg from the "Cr and Mg-containing ore". [Means for Solving the Problems]

[0010] The inventors of the present invention have found that the above problems can be solved by removing Cr(VI) eluted into the solution by a heat treatment performed to enhance the elution property of Mg from the "Cr and Mg-containing ore" using a divalent iron compound (referred to as "divalent iron compound" in this specification) capable of eluting divalent iron ions, and have completed the present invention. Specifically, the present invention provides the following.

[0011] (1) A method for fixing carbon dioxide, comprising: a heat treatment of heating an ore containing chromium and magnesium and having a magnesium concentration of 5% by mass or more at a temperature of 500°C or higher and 800°C or lower; a solution purification treatment of charging the ore after the heat treatment into a solution and adding a divalent iron compound to the solution; and a carbon dioxide fixation treatment of reacting magnesium eluted from the ore into the solution with carbon dioxide in the solution after the solution purification treatment.

[0012] According to the method for fixing carbon dioxide in (1), when implementing the technology of reacting Mg contained in "Cr and Mg-containing ore" with CO2 to fix CO2 to MgCO3, that is, the mineral carbonation technology (Mineral Carbonation Technology), Cr(VI), which is a harmful substance eluted together with Mg by the heat treatment carried out to enhance the elution property of Mg from "Cr and Mg-containing ore", can be precipitated by the addition of a divalent iron compound, and Cr(VI), which is a harmful substance eluted into the solution, can be removed.

[0013] (2) In the solution purification treatment, after charging the ore after the heat treatment into the solution and adding a divalent iron compound to the solution, further perform pH adjustment to make the pH of the solution 7 or more and 10 or less. The method for fixing carbon dioxide according to (1).

[0014] (2) According to the method for fixing carbon dioxide in (2), in the process of implementing the method for fixing carbon dioxide described in (1), iron dissolved in the solution can be efficiently removed. Also, since more Mg for fixing CO2 can be left in the solution, the CO2 recovery efficiency by the method for fixing carbon dioxide described in (1) can also be improved.

[0015] (3) A method for producing magnesium carbonate, comprising: a heat treatment of heating an ore containing chromium and magnesium and having a magnesium concentration of 5% by mass or more at a temperature of 500°C or more and 800°C or less; a solution purification treatment of charging the ore after the heat treatment into a solution and adding a divalent iron compound to the solution; and a carbonate formation treatment of reacting magnesium eluted from the ore into the solution with carbon dioxide in the solution after the solution purification treatment.

[0016] According to the method for producing magnesium carbonate in (3), in the process of producing MgCO3 from "ore containing Cr and Mg", Cr(VI), which is a harmful substance eluted together with Mg by the heat treatment performed to enhance the elution property of Mg from the "ore containing Cr and Mg", can be precipitated by the addition of a divalent iron compound, and Cr(VI), which is a harmful substance eluted into the solution, can be removed.

[0017] (4) In the solution purification treatment, after charging the ore after the heat treatment into the solution and adding a divalent iron compound to the solution, further, a pH adjustment is performed to make the pH of the solution 7 or more and 10 or less. The method for producing magnesium carbonate according to (3).

[0018] (4) According to the method for producing magnesium carbonate in (4), in the process of carrying out the method for producing magnesium carbonate described in (3), iron dissolved in the solution can be efficiently removed. Also, since more Mg for fixing CO2 can be left in the solution, the production efficiency of magnesium carbonate by the method for producing magnesium carbonate described in (3) can also be improved. [Effect of the Invention]

[0019] According to the present invention, in the "process for carbon dioxide fixation" and the "process for producing magnesium carbonate" carried out using "ore containing Cr and Mg" containing chromium and magnesium and having a magnesium concentration of 5% by mass or more, Cr(VI), which is a harmful substance eluted into the solution by the heat treatment performed to enhance the elution property of Mg from the "ore containing Cr and Mg", can be removed. [Brief Description of the Drawings]

[0020]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0021] Hereinafter, embodiments of the present invention will be described in detail. It should be noted that the present invention is not limited to the following embodiments, and various modifications are possible without departing from the gist of the present invention.

[0022] Here, the "method for fixing carbon dioxide" of the present invention is a method for fixing CO2 to a mineral, and is a fixing method in which CO2 is reacted with Mg in a solution and fixed to the mineral as MgCO3. This method is a suitable process for specifically implementing the "Mineral Carbonation Technology", which is one of the "Carbon Capture and Storage Utilization Technologies (CCSU)". Further, this process can also be carried out as a "method for producing magnesium carbonate" for producing MgCO3 from the "ore containing Cr and Mg" defined above, that is, "ore containing Cr and Mg and having a magnesium concentration of 5% by mass or more" and CO2. Hereinafter, first, embodiments when the present invention is carried out as a "method for producing magnesium carbonate" will be described in detail. Embodiments when the present invention is carried out as a "method for fixing carbon dioxide" will be described later. It should be noted that as the above solution used when carrying out the present invention, in any of the above embodiments, an aqueous solution using water as a solvent is preferably used.

[0023] <Method for Producing Magnesium Carbonate> The "method for producing magnesium carbonate" of the present invention is a production method for producing MgCO3 from the "ore containing Cr and Mg" defined above, that is, "ore containing Cr and Mg and having a magnesium concentration of 5% by mass or more" and CO2.

[0024] FIG. 1 is a flowchart showing the process flow of the “method for producing magnesium carbonate” (and “method for fixing carbon dioxide”) of the present invention. As shown in the figure, in the “method for producing magnesium carbonate” of the present invention, a heat treatment St1 for heating “Cr and Mg-containing ore”, charging the ore after the heat treatment St1 into a solution (preferably an aqueous solution), and adding a divalent iron compound to the solution are performed in a solution purification treatment St2. In the above solution after the solution purification treatment St2, a carbon dioxide fixation treatment St3 for producing MgCO3 by reacting Mg eluted from the ore after the heat treatment St1 into the above solution with CO2 is sequentially performed. Regarding the recovery of the produced MgCO3, it is preferably further performed by a carbonate recovery treatment St4 for recovering MgCO3 from the solution by solid-liquid separation treatment.

[0025] [Cr and Mg-containing ore] The “Cr and Mg-containing ore” used in the “method for producing magnesium carbonate” of the present invention only needs to satisfy the above requirements of containing Cr and Mg and having a magnesium concentration of 5% by mass or more, and is not limited to a specific ore, but is preferably an ore containing Mg as magnesium silicate. As an example, as described above, in addition to limonite ore, nickel oxide ores such as saprolite ore, and lizardite, chrysotile, enstatite, forsterite, talc, magnesite, serpentine, olivine, smectite, etc. can be mentioned. The magnesium concentration (% by mass) in the “Cr and Mg-containing ore” can be specified by a method of leaching 0.5 g of the ore in hydrochloric acid and measuring it by ICP emission spectrometry.

[0026] Still, in the "ore containing Cr and Mg", as described above, most of Mg is contained as a silicate mineral (such as magnesium silicate (Mg3Si2O5(OH)4)). Magnesium silicate has a three-dimensional and complex crystal structure with Mg existing at its center, so it is difficult for Mg to elute from magnesium silicate into the solution. Therefore, in the above carbon dioxide fixation treatment St3, a heat treatment of the "ore containing Cr and Mg" is effective as a pretreatment for sufficiently promoting the elution of Mg into the solution.

[0027] Among the above "ores containing Cr and Mg", ores containing Mg as magnesium silicate and having a mass ratio of Mg to Si (Mg / Si) in the ore of 0.5 or more can be particularly preferably used. Moreover, those having a ratio of 1.0 or more can be more preferably used. When the Mg / Si ratio is 0.5 or more, more Mg can be eluted into the solution after the heat treatment. Still, a compound with a large amount of Si relative to Mg has a tendency to have a more complex crystal structure and make it difficult for Mg to elute. Also, the concentration (mass) of Si in the "alkaline earth metal-containing ore" can be specified by a method of measuring a sample obtained by mixing 2.5 g each of sodium peroxide and sodium carbonate with 0.5 g of the ore, heating and melting it to 800 °C in an electric furnace, leaching with hydrochloric acid after cooling, and performing ICP emission spectroscopic analysis. Similarly, the magnesium concentration (mass) in the "alkaline earth metal-containing ore" can be specified by a method of leaching 0.5 g of the ore in hydrochloric acid and measuring it by ICP emission spectroscopic analysis.

[0028] [Heat treatment] In the "method for producing magnesium carbonate" of the present invention, the heat treatment St1 is a treatment for heating "Cr- and Mg-containing ore" such as limonite ore by firing or the like. The temperature of the heat treatment for the heat treatment St1 is preferably such that the "Cr- and Mg-containing ore" is heated at a temperature of 500°C or higher and 800°C or lower (furnace atmosphere temperature), and more preferably heated at a temperature of 600°C or higher and 750°C or lower, as verified and confirmed in the following "magnesium elution test". The heating time is preferably 2 hours or longer, and more preferably 4 hours or longer.

[0029] Further, in the "method for producing magnesium carbonate" of the present invention, as a pretreatment before charging the "Cr- and Mg-containing ore" into the heat treatment St1, from the viewpoint of handleability, the particle size may be adjusted by pulverizing so as to be about 300 μm or less. The pulverization of the "Cr- and Mg-containing ore" may be by a general method, and specific examples include pulverization by a rod mill and a ball mill. However, the particle size of the "Cr- and Mg-containing ore" does not significantly affect the elution property of Mg.

[0030] Here, Table 1 below shows, as a specific example of the "Cr- and Mg-containing ore", the results of the "magnesium elution test" conducted to confirm how the elution rate of Mg into the solution changes depending on the difference in the heating temperature in the heat treatment St1 for a nickel oxide ore (saprolite ore: particle size 150 μm to 300 μm) having a magnesium concentration of 6% by mass or more. The heating time (firing time) in each test example was 4 hours, and the heating temperature was as shown in Table 1 below.

[0031] The "magnesium elution test" was carried out by charging 5 g of the above-mentioned "Cr and Mg-containing ore (nickel oxide ore)" with different heating conditions during the heat treatment St1 into 200 mL of a solution (pure water at a temperature of 60°C), and blowing CO2 gas (manufactured by Takamatsu Teisan Co., Ltd., concentration 99.995% by volume) into the solution at a blowing rate of 0.1 L / min while stirring at a rotation speed of 450 rpm for 2 hours. The magnesium elution rate (%) in Table 1 was measured and calculated by the following "method for measuring the elution amount of magnesium and method for calculating the elution rate" after the 2-hour stirring elapsed. (Method for measuring the elution amount of magnesium and method for calculating the elution rate) Magnesium elution rate = Magnesium concentration in solution (mg / L) × Volume of eluate (L) / (Magnesium concentration in ore (mass%) ÷ 100 × Ore mass (mg)) Each of the above magnesium concentrations was measured by ICP emission spectrometry.

[0032] From the test results of this "magnesium elution test", when the heat treatment St1 by firing was not performed, the magnesium elution rate was 1.4%. And it was confirmed that the magnesium elution rate was sufficiently improved to 32.0% by the heat treatment St1 at 600°C where the heating temperature was 500°C or higher. And it was confirmed that the magnesium elution rate decreased to 1.5% at 900°C where the heating temperature exceeded 800°C. It is considered that the decrease in the magnesium elution rate due to heating at 900°C is due to the magnesium silicate (Mg3Si2O5(OH)4) with a complex crystal structure destroyed by heating recrystallizing again in the form of Mg2SiO4.

[0033] [Table 1]

[0034] Regarding whether the complex crystal structure of the minerals contained in the "Cr- and Mg-containing ore" has actually been sufficiently destroyed by heating, it can be determined by XRD analysis, for example, by confirming the peak of the (002) plane of lizardite (Mg3SiO5(OH)4). Also, for minerals other than lizardite, for example, the peaks of forsterite or talc can also be used as alternatives. As an apparatus for performing XRD analysis, for example, an X-ray diffractometer "Empyrean (manufactured by Malvern)" can be preferably used.

[0035] [Solution purification treatment] In the "method for producing magnesium carbonate" of the present invention, the solution purification treatment St2 is a treatment that includes charging a solution of the "Cr- and Mg-containing ore" in which the complex crystal structure has been destroyed in the heat treatment St1 and adding a divalent iron compound to the solution. In the solution purification treatment St2, by allowing an appropriate amount of a divalent iron compound to be present in a solution containing Cr(VI) eluted into the solution together with Mg from the above-mentioned "Cr- and Mg-containing ore" (hereinafter also referred to as "Cr(VI)-containing solution"), at least the reaction shown in the following (Formula 1) can be made to proceed in the solution to remove Cr(VI) in the solution. More specifically, in the solution purification treatment St2, first, by making the reaction shown in the following (Formula 1) proceed, Cr(VI), which is a harmful substance, can be sufficiently removed from the solution. Also, for Cr remaining in the solution at this stage 3+ by further making the reaction shown in (Formula 2) proceed by neutralization treatment, the chromium component can be completely removed from the solution in the form of a hydroxide. (Formula 1) Cr 6+ +3Fe 2+ →Cr 3+ +3Fe 3+ (Formula 2) Cr 3+ +3NaOH→Cr(OH)3+3Na +

[0036] Still, the addition of the divalent iron compound to the solution in the solution purification treatment St2 is preferably carried out after Mg has eluted into the solution, that is, in a state where Cr(VI) has already eluted into the solution. However, the purpose of the solution purification treatment St2 is "to cause the reactions shown in the above (Formula 1) and (Formula 2) to proceed in the solution by having an appropriate amount of divalent iron ions (Fe 2+ 2+) present in the solution." Therefore, as long as the above object can be achieved, the order of performing the above "two treatments" (loading of the Cr- and Mg-containing ore into the solution and addition of the divalent iron compound to the solution) in the solution purification treatment St2 is not necessarily limited to the above order in which the loading of the Cr- and Mg-containing ore into the solution is carried out first.

[0037] The divalent iron compound added to the solution in the solution purification treatment St2 may be any compound that can elute divalent iron ions, such as iron(II) sulfate, iron(II) chloride, ammonium iron(II) sulfate, etc., and these various divalent iron compounds can be used. Among these, iron(II) sulfate, which is the cheapest and most suitable for use under sulfuric acid acidity, can be particularly preferably used.

[0038] The specific method of adding the divalent iron compound to the solution may be a method of adding a solid of an iron compound (the "divalent iron compound") capable of eluting divalent iron ions in the "solution" or a "divalent iron compound solution" using the "divalent iron compound" as a solvent to the above "solution". The addition amount of the divalent iron compound can be appropriately set according to the amount of Cr in the solution. Specifically, it is preferable to adjust so that the addition amount of the "divalent iron compound" (when added as the "iron compound solution", the product of the mass of the solution and the concentration of the divalent iron compound) becomes about 6 equivalents as the chemical equivalent with respect to the amount of Cr in the solution. Still, the measurement of the Cr concentration in the solution can be carried out by ICP emission analysis. Also, the temperature of the above "solution" and "divalent iron compound solution" when adding the divalent iron compound is not a problem at normal temperature or the going temperature, but each solution may be heated as necessary.

[0039] After adding the "divalent iron compound", it is preferable to carry out a stirring reaction using a stirrer or the like. The solution after the reaction is separated and filtered through a membrane filter or the like, whereby the precipitated Cr compound (chromium hydroxide) can be recovered.

[0040] In addition, when performing the Cr removal treatment in the above aspect, the pH of the solution is preferably 5.0 or more and 7.0 or less. As the alkali used for pH control to allow the reactions shown in the above (Formula 1) and (Formula 2) to proceed in the solution, sodium hydroxide, magnesium hydroxide, calcium hydroxide, ammonia, etc. can be appropriately selected and used without particular limitation. Among these, from the viewpoint of ease of managing the precipitate, sodium hydroxide can be preferably used. Also, magnesium hydroxide and magnesium carbonate containing magnesium that contribute to the fixation of carbon dioxide can also be effectively utilized as the alkali agent used for this pH control.

[0041] Here, in the solution after the reduction treatment of Cr, the iron of the added divalent iron compound will be dissolved in excess. These dissolved irons can be precipitated and removed as iron hydroxide by adding an alkali such as NaOH to adjust the pH. The pH during this removal treatment is preferably 7.0 or more and 10.0 or less. By setting the pH of the solution to 7.0 or more, the removal of Fe can be efficiently performed, and by setting it to 10.0 or less, more Mg for fixing CO2 can be left in the solution. As the alkali used for this pH control, sodium hydroxide, magnesium hydroxide, calcium hydroxide, ammonia, etc. can be appropriately selected and used without particular limitation. Among these, from the viewpoint of ease of managing the precipitate, sodium hydroxide can be preferably used. Also, magnesium hydroxide and magnesium carbonate containing magnesium that contribute to the fixation of carbon dioxide can also be effectively utilized as the alkali used for this pH control.

[0042] [Carbon dioxide fixation treatment] In the "method for producing magnesium carbonate" of the present invention, the carbon dioxide fixation treatment St3 performed following the above-described solution purification treatment St2 is a treatment in which Mg is reacted with CO2 in a solution (hereinafter also referred to as "Cr(VI)-removed solution") in a state where chromium has been sufficiently removed through the solution purification treatment St2. In the carbon dioxide fixation treatment St3, CO2 gas is blown into the "Cr(VI)-removed solution", and within the solution, MgCO3 can be generated by reacting the eluted Mg with the CO2 blown in as a gas.

[0043] The method for blowing CO2 gas into the "Cr(VI)-removed solution" is not particularly limited. As an example, it can be performed by a method of blowing CO2 gas using a disk-shaped diffuser, a cylindrical diffuser, or the like. The CO2 concentration of the CO2 gas blown into the "Cr(VI)-removed solution" may be 10% or more. However, by using a higher concentration of CO2 gas exceeding 10%, MgCO3 can be produced more efficiently.

[0044] The ratio of "Cr and Mg-containing ore" to the "Cr(VI)-removed solution" is preferably 2% by mass or more and 30% by mass or less, and more preferably 5% by mass or more and 20% by mass or less. By setting the amount (mass ratio) of "Cr and Mg-containing ore" in the "Cr(VI)-removed solution" to 2% by mass or more, a high precipitation efficiency of MgCO3 can be achieved. Also, by setting the amount (mass ratio) of the alkaline earth metal-containing ore in the solution to 30% by mass or less, the viscosity of the "Cr(VI)-removed solution" in a slurry state can be appropriately maintained, and the handleability is improved.

[0045] In addition, the source of CO2 used in the carbon dioxide fixation treatment St3 is not particularly limited. As an example, by utilizing the CO2 gas discharged in various metal smelting processes, such as the CO2 gas from the neutralization step in a plant for metal smelting (for example, nickel smelting using laterite ore) that contains Mg as an impurity, and implementing the production method of the present invention in parallel within the same plant, it is also possible to contribute to reducing the total amount of CO2 gas emissions from these metal smelting processes.

[0046] [Carbonate recovery process] The carbonate recovery process St4 performed following the carbon dioxide fixation process St3 is a process of recovering MgCO3 generated in the carbon dioxide fixation process St3 from the solution. The specific method for recovering MgCO3 is not particularly limited, but as an example, it can be by a method of separating and recovering the liquid with a filter press or other filtration machine, or by a method of volatilizing and recovering the liquid component of the solution.

[0047] [Method for fixing carbon dioxide] The "method for fixing carbon dioxide" of the present invention is, as described above, a method for fixing CO2 to a mineral, and is a fixing method in which, in the same manner as the above-described "method for producing magnesium carbonate", CO2 is reacted with Mg in a solution and fixed in the mineral as MgCO3.

[0048] Also in the "method for fixing carbon dioxide" of the present invention, as the ore for obtaining Mg for reacting with CO2, the "Cr and Mg-containing ore" can be used in the same manner as the "method for producing magnesium carbonate" of the present invention described in detail above. Then, a heat treatment St1 for heating this "Cr and Mg-containing ore", charging the ore after the heat treatment St1 into a solution (preferably an aqueous solution), and adding a divalent iron compound to the solution are performed in a solution purification process St2. In the above solution after the solution purification process St2, by reacting Mg eluted from the ore after the heat treatment St1 into the above solution with CO2, a carbon dioxide fixation process St3 for generating MgCO3 is sequentially performed, whereby CO2 can be fixed in the mineral as MgCO3. Further, the MgCO3 in which CO2 is fixed can be recovered by a carbonate recovery process St4 that performs solid-liquid separation treatment from the solution in the same manner as the carbonate recovery process St4 in the "method for producing magnesium carbonate" of the present invention.

[0049] [Application to the wet smelting method of nickel oxide ore] The "method for fixing carbon dioxide" and the "method for producing magnesium carbonate" of the present invention can both be implemented as methods for performing a pretreatment step (demagnesium step S1) of removing Mg contained as an impurity in the form of silicate in the raw ore during the implementation of the "wet smelting method of nickel oxide ore" shown in FIG. 2. Such an embodiment of the production method of the present invention is particularly effective when using a nickel oxide ore with a magnesium concentration exceeding 5% by mass, such as "saprolite ore", as the raw ore during the implementation of the "wet smelting method of nickel oxide ore" (hereinafter also referred to as "high-Mg nickel oxide ore" in this specification).

[0050] The "wet smelting method of nickel oxide ore" that can be carried out using the "method for fixing carbon dioxide" or the "method for producing magnesium carbonate" of the present invention as a partial process is a known process as an overall process, and its flow is as shown in FIG. 2 as an example. In this overall process, a demagnesium step S1 carried out for the purpose of separating and recovering Mg from nickel oxide ore, a leaching step S2 of adding sulfuric acid to the nickel oxide ore with a reduced magnesium concentration and performing a leaching treatment under high temperature and high pressure to obtain a leaching slurry, a solid-liquid separation step S3 of separating the leaching slurry into a leaching solution and leaching residue, a neutralization step S4 of adjusting the pH of the leaching solution to separate a neutralization precipitate containing impurity elements to obtain a neutralization final solution, and a sulfidation step S5 of adding a sulfiding agent to the neutralization final solution to generate a sulfide containing nickel (Ni) are sequentially carried out.

[0051] Then, as described above, the "method for fixing carbon dioxide" and the "method for producing magnesium carbonate" of the present invention can be implemented as suitable technical means for performing the magnesium removal step S1 among the respective steps (S1 to S5) of the above "wet smelting method of nickel oxide ore". Even when the magnesium removal step S1 is carried out by the "method for fixing carbon dioxide" or the "method for producing magnesium carbonate" of the present invention, the mixing of Cr(VI) into the solution discharged from the magnesium removal step S1 can be prevented. Therefore, for the other respective steps, such as the leaching step S2, the solid-liquid separation step S3, the neutralization step S4, and the sulfidation step S5, they can be safely carried out in the same manner as the implementation mode of the conventionally known "wet smelting method of nickel oxide ore".

[0052] [Magnesium Removal Step] The magnesium removal step S1 is a step of eluting Mg from a "high-Mg nickel oxide ore" such as saprolite ore to obtain a "low-Mg nickel oxide ore". In this specification, a nickel oxide ore having a magnesium concentration of less than 5% by mass is referred to as a "low-Mg nickel oxide ore". However, in the magnesium removal step S1, regarding the magnesium concentration of the "low-Mg nickel oxide ore" obtained by reducing the magnesium concentration of the "high-Mg nickel oxide ore", as described above, it may be less than 5% by mass, preferably 2.0% by mass or less, and more preferably 0.5% by mass or less.

[0053] As described above, for example, in "high-Mg nickel oxide ores" such as saprolite ore, Mg is mainly contained as magnesium silicate (Mg3Si2O5(OH)4) having a stable phyllosilicate structure, so sufficient dissolution into the solution is not easy. However, by performing the magnesium removal step S1 by each of the above methods of the present invention, it becomes possible to dissolve Mg from the "high-Mg nickel oxide ore" and perform the treatment to obtain the above-mentioned "low-Mg nickel oxide ore" at low cost and efficiently, and together with the CO2 reduction treatment. Also, in this embodiment, the source of CO2 to be reacted with the dissolved Mg is not particularly limited. As an example, by utilizing the CO2 gas generated from fossil fuels in an autoclave or the like, the CO2 gas discharged from the neutralization step S4 which is a downstream process, etc., that is, the CO2 gas discharged within the process, it is also possible to contribute to the reduction of the total amount of CO2 gas emissions from the nickel smelting plant. Further, in the wet smelting method of nickel oxide ore, ores containing Mg at a high concentration such as saprolite ore, which have not been actively utilized conventionally, can be used as low-Mg nickel oxide ores without reducing the original productivity of the "wet smelting method (HPAL process).

Example

[0054] Hereinafter, the effects of the "carbon dioxide fixation method" and the "magnesium carbonate production method" of the present invention will be specifically described with reference to examples. However, the present invention is not limited to the following examples.

[0055] Table 2 below shows the "magnesium concentration and chromium concentration investigation test" in which the Mg concentration and Cr concentration in the solution in the "Cr(VI)-containing solution" and the "solution after Cr(VI) removal" were confirmed when the heat treatment St1 and the solution purification treatment St2 in the "carbon dioxide fixation method" of the present invention were sequentially performed using the "nickel oxide ore" used in the above "magnesium elution test" as the material ore.

[0056] (Magnesium concentration and chromium concentration investigation test) The "Investigation Test of Magnesium Concentration and Chromium Concentration" was conducted according to the procedures described in the following (a) to (h). (a) 200 mL of pure water was heated to 60°C, and 5 g of the above-mentioned "nickel oxide ore" calcined at 600°C for 4 hours was weighed and charged into the above pure water. (b) While blowing CO2 gas into the aqueous solution obtained in procedure (a), it was stirred for 120 minutes using a stirrer to elute Mg, and then the aqueous solution was filtered using filter paper. (c) The Mg concentration, Cr concentration, and Fe concentration of the solution ("Cr(VI)-containing solution") obtained in procedure (b) were measured by ICP emission spectrometry. (d) 100 mL of the above solution ("Cr(VI)-containing solution") obtained in procedure (b) was measured and placed in a beaker, and the pH was adjusted to 6 using 64% sulfuric acid. (e) 12.8 g of iron(II) sulfate heptahydrate was weighed, dissolved in pure water, and made up to 100 mL to prepare an iron(II) sulfate solution with a concentration of 70 g / L of FeSO4. (f) The above solution ("Cr(VI)-containing solution") obtained in procedure (b) was set on a stirrer, and while stirring at 700 rpm under normal temperature conditions, 1 mL of the iron(II) sulfate solution obtained in procedure (e) was added to the filtrate ("Cr(VI)-containing solution") with a pipette, and the mixture was stirred and reacted for 15 minutes. (g) A part of the solution ("Cr(VI)-removed solution") obtained in procedure (f) was taken out using a pipette, filtered through a membrane filter, and then the Mg concentration, Cr concentration, and Fe concentration of the "Cr(VI)-removed solution" were measured by ICP emission spectrometry. (h) The solution obtained in step (f) (the "solution with Cr(VI) removed") was adjusted stepwise to pH 7, pH 8, pH 9, pH 10, pH 10.5, and pH 11 by adding 8 mol of NaOH while continuing stirring. Each solution was separated, filtered through a membrane filter, and the Mg concentration, Cr concentration, and Fe concentration were measured by ICP emission spectrometry. In addition, in each of the solutions of the "solution with Cr(VI) removed obtained in step (f)" and the "solution obtained in step (h)" shown in Table 2 below, since a sufficient addition amount of iron sulfate (divalent iron compound) was added with respect to the amount of Cr in the solution, in each of the above solutions, Cr(VI) had sufficiently precipitated. Therefore, the "Cr concentration" of each of the above solutions shown in the table is considered to be substantially the concentration of Cr(III).

[0057]

Table 2

[0058] From the test results of the "Investigation Test of Magnesium Concentration and Chromium Concentration" shown in Table 2, according to the present invention, Cr(VI), which is a harmful substance eluted together with Mg by the heat treatment performed to enhance the elution property of Mg from the "ore containing Cr and Mg", can be precipitated by the addition of a divalent iron compound, and Cr(VI), which is a harmful substance eluted into the solution and removed from the solution, can be sufficiently removed. Also, by appropriately adjusting the pH of the solution after the removal treatment, it was confirmed that iron dissolved in the solution can be efficiently removed in the process of implementing the present invention.

Explanation of Symbols

[0059] St1 Heat treatment St2 Solution purification treatment St3 Carbon dioxide fixation treatment St4 Carbonate recovery treatment S1 Demagnesium process S2 Leaching process S3 Solid-liquid separation process S4 Neutralization process S5 Sulfidation process

Claims

1. A heat treatment of heating an ore containing chromium and magnesium and having a magnesium concentration of 5% by mass or more at a temperature of 500°C or higher and 800°C or lower, a solution purification treatment of charging the ore after the heat treatment into a solution and adding a divalent iron compound to the solution, and a carbon dioxide fixation treatment of reacting magnesium eluted from the ore into the solution with carbon dioxide in the solution after the solution purification treatment. A method for fixing carbon dioxide, comprising the above steps.

2. In the solution purification treatment, after charging the ore after the heat treatment into the solution and adding a divalent iron compound to the solution, further adjusting the pH of the solution to 7 or higher and 10 or lower. The method for fixing carbon dioxide according to Claim 1.

3. A heat treatment of heating an ore containing chromium and magnesium and having a magnesium concentration of 5% by mass or more at a temperature of 500°C or higher and 800°C or lower, a solution purification treatment of charging the ore after the heat treatment into a solution and adding a divalent iron compound to the solution, and a carbon dioxide fixation treatment of reacting magnesium eluted from the ore into the solution with carbon dioxide in the solution after the solution purification treatment. A method for producing magnesium carbonate, comprising the above steps.

4. In the solution purification treatment, after charging the ore after the heat treatment into the solution and adding a divalent iron compound to the solution, further adjusting the pH of the solution to 7 or higher and 10 or lower. The method for producing magnesium carbonate according to Claim 3.

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