Method for absorbing carbon dioxide by ardealite

Abstract

The invention relates to a method for the absorption of carbon dioxide by ardealite, comprising the step of injecting carbon dioxide into a reaction medium comprising an alkaline solution and ardealite and forming calcium carbonate, the alkaline solution having a pH of between 7.5 and 9, characterized in that the alkaline solution is seawater.

Description

Technical Field

[0001] The present invention relates to a method for capturing carbon dioxide by means of phosphogypsum minerals. Background Art

[0002] Carbon dioxide (CO2) is a greenhouse gas that contributes to global warming. Its concentration in the atmosphere has increased significantly due to the increasing use of fossil fuels.

[0003] Sequestration of CO2 by mineral carbonation in aqueous media is an interesting alternative to reduce the concentration of this greenhouse gas. It is a simple chemical process that absorbs CO2 by calcium-rich compounds, resulting in the formation of calcium carbonate (a thermodynamically stable product) and has no harmful effects on the environment.

[0004] The phosphate chemical industry produces large amounts of the calcium-rich compound phosphogypsum, which consists mainly of calcium sulfate dihydrate (CaSO4, 2H2O). Characterization of this calcium- and sulfate-rich material has shown that it has good carbonation capacity. The preparation of calcium carbonate from phosphogypsum has been extensively studied. Therefore, many methods for preparing calcium carbonate from phosphogypsum have been proposed.

[0005] For example, WO 2010 / 142859 proposes a method for preparing calcium carbonate, which involves the use of calcium oxide to capture carbon dioxide and requires multiple reaction media with different pH values.

[0006] EP1513771A1 provides a method for producing a calcium carbonate precipitate, which uses calcium hydroxide and carbon dioxide to form calcium carbonate.

[0007] US 8846562 discloses a method for preparing calcium carbonate nanoparticles by reacting phosphogypsum as a calcium source with ammonia (NH3) and carbon dioxide (CO2). The process requires ammonia and a catalyst (such as a nickel salt). CN102627309A also uses ammonia treatment.

[0008] CN101337685A describes a process for preparing calcium carbonate using phosphogypsum decomposition slag at high temperature (950° C. to 1200° C.), which produces highly toxic hydrogen sulfide (H2S).

[0009] US20180022614 describes a method for converting phosphogypsum into calcium carbonate precipitate in the presence of an inorganic acid and a carbonate source (which may be CO2).

[0010] However, these processes are expensive, time-consuming and complicated to implement. For example, some of these processes require one or more intermediate carbonation steps, requiring specific reaction media. In addition, these processes often produce solid or liquid wastes that are harmful to the environment.

[0011] Therefore, there remains a need to provide a process for preparing calcium carbonate from phosphogypsum waste and CO2 which is simple to implement, transferable on a large scale, rapid, economically attractive and has a reduced impact on the environment. Summary of the invention

[0012] The present invention relates to a method for absorbing carbon dioxide by phosphogypsum, which comprises the steps of injecting carbon dioxide into a reaction medium comprising an alkaline solution (whose pH is between 7.5 and 9) and phosphogypsum and forming calcium carbonate, characterized in that the alkaline solution is seawater.

[0013] Preferably, the method comprises the step of preparing the reaction medium by dispersing phosphogypsum in an alkaline solution, preferably until the pH stabilizes at a value between 8.5 and 9.5.

[0014] Advantageously, the reaction medium is stirred during the carbon dioxide injection step.

[0015] Preferably, the method comprises a filtration step to collect the calcium carbonate formed.

[0016] Advantageously, carbon dioxide is injected at a flow rate of 0.7 to 1.3 l / min. Advantageously, phosphogypsum is present in the reaction medium in a concentration ranging from 0.1 kg / l to 0.5 kg / l. Preferably, the purity of the calcium carbonate obtained ranges from 30% to 65%, preferably from 45% to 65%. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Other features and advantages of the present invention will become apparent from the following detailed description with reference to the accompanying drawings, in which:

[0018] Figure 1 is a block diagram of the method of the present invention.

[0019] Figure 2 is a graph showing the change in pH with reaction time of carbonation of phosphogypsum.

[0020] Figure 3 It is a thermogravimetric analysis diagram of calcium carbonate obtained by the method of the present invention.

[0021] Figure 4 is the X-ray diffraction pattern of calcium carbonate obtained by the method of the present invention.

[0022] Figure 5 It is the infrared analysis spectrum of calcium carbonate obtained by the method of the present invention.

[0023] Figure 6 is a scanning electron microscope (SEM) image of calcium carbonate obtained by the method of the present invention.

[0024] Figure 7An energy dispersive X-ray (EDX) spectrum of calcium carbonate obtained by the method of the present invention is shown. DETAILED DESCRIPTION

[0025] The present inventors have developed a process for preparing calcium carbonate from phosphogypsum which meets the expressed need.

[0026] Thus, the method of the present invention comprises the step of injecting carbon dioxide into a reaction medium comprising an alkaline solution (whose pH varies between 7.5 and 9) and phosphogypsum, resulting in the formation of calcium carbonate.

[0027] The reaction medium into which the carbon dioxide is injected may be a dispersion of phosphogypsum in an alkaline solution, typically at a concentration of between 0.1 and 0.5 kg / litre. The dispersion may be prepared by stirring the phosphogypsum in an alkaline solution. Typically, stirring is performed using a magnetic stirrer. Stirring is typically maintained until the pH stabilizes, typically until the pH stabilizes in the range of 8.5-9.5. The resulting dispersion is homogeneous, with a high concentration of free calcium ions in solution.

[0028] Thus, in certain embodiments, the methods of the present invention comprise the following steps:

[0029] (a) dispersing phosphogypsum in an alkaline solution having a pH in the range of 7.5 to 9;

[0030] (b) injecting carbon dioxide into the dispersion obtained in step (a) to cause the formation of calcium carbonate.

[0031] The dispersion of phosphogypsum in the alkaline solution allows the release of calcium ions in the solution, thus promoting the carbonation reaction that occurs during the injection of carbon dioxide into the dispersion obtained at the end of step a). In fact, once carbon dioxide is injected into the dispersion obtained at the end of step a), it is rapidly converted into carbonate ions (CO3 2- ), which reacts with calcium ions (Ca 2+ ) reacts to form calcium carbonate (CaCO3).

[0032] In some embodiments, calcium carbonate seeds may be added to the dispersion obtained in step a). In other embodiments, carbon dioxide is injected into the reaction medium without prior dispersion, for example by stirring until the pH of the phosphogypsum in the alkaline solution is stable, typically until the pH is stable in the range 8.5-9.5.

[0033] The alkaline solution having a pH range of 7.5 to 9 is typically seawater. The term "seawater" as used herein refers to the salty water of the Earth's oceans, i.e. water containing dissolved salts. In the process of the present invention, when the alkaline solution is seawater, the seawater serves a triple role. On the one hand, it constitutes a source of alkalinity for the reaction medium. It avoids the use of strong bases, which can be expensive and toxic. On the other hand, it avoids the addition of ionized water, thereby avoiding deionization and additional mixing steps. Finally, it avoids the intermediate reaction step of converting phosphogypsum into an intermediate product, which is carbonated to obtain calcium carbonate. Seawater is particularly advantageous because it is an abundant and inexpensive resource.

[0034] The injection of carbon dioxide into the reaction medium, in particular into the dispersion, may be carried out at a flow rate of from 0.7 or 0.8 to 1 or 1.3 liters per minute. Advantageously, the carbon dioxide has a purity greater than 99%.

[0035] During the injection of carbon dioxide, the reaction medium, in particular the dispersion, is usually stirred using a magnetic stirrer or a mechanical stirring device suitable for industrial processes.Thus, the method of the invention may comprise a step of stirring the reaction medium, in particular the dispersion.

[0036] When all the phosphogypsum has reacted and the pH is therefore stable, the reaction medium, in particular the dispersion, is filtered, thereby obtaining calcium carbonate in solid form and an aqueous filtrate. Thus, the process of the present invention may comprise a step of filtering the reaction medium, in particular the dispersion, a drying step and a step of collecting the filtered and dried solid calcium carbonate.

[0037] The process of the invention allows the preparation of calcium carbonate with a yield ranging from 45% to 65%. The purity of the calcium carbonate obtained generally ranges from 30% to 65%.

[0038] The process of the present invention has the following advantages: since the pH value of the reaction medium is suitable for the release of calcium ions in solution, it allows the conversion of phosphogypsum directly into calcium carbonate in the reaction medium. Therefore, the process of the present invention does not require an additional step of converting phosphogypsum into an intermediate carbonation product and does not require changing the reaction medium. In addition, the process of the present invention does not produce any solid or liquid waste and thus proves to be environmentally friendly.

[0039] The method of the present invention can be Figure 1 As shown. First, phosphogypsum 1 is brought into contact with an alkaline solution 2. Carbon dioxide 3 is then injected into the reaction medium containing phosphogypsum 1 and alkaline solution 2. At the end of the carbonation reaction (when the phosphogypsum has reacted), calcium carbonate 4 and water 5 are collected.

[0040] The chemical reactions carried out in the method of the present invention are as follows.

[0041] After contact with alkaline solutions, especially seawater, phosphogypsum (CaSO4, 2H2O) releases calcium ions (Ca 2+ ):

[0042] [Mathematical formula 1]

[0043] CaSO4,2H2O+alkaline solution→Ca 2+ +SO4 2 -+H2O

[0044] Using only seawater as the alkaline solution without adding an additional alkalinity source makes it possible to generate a large amount of ions other than sulfate ions SO4. 2- In addition, calcium ions Ca 2+ Released into solution.

[0045] When carbon dioxide is injected into an alkaline solution, in particular into seawater, carbonic acid H2CO3 is formed according to equation (2):

[0046] [Mathematical formula 2]

[0047] CO2+H2O→H2CO3

[0048] According to equation (3), carbonic acid dissociates to release protons (H + ) and bicarbonate ions HCO3 - :

[0049] [Mathematical formula 3]

[0050]

[0051] The pH of alkaline solutions, especially seawater, controls this reaction. + As the concentration decreases (particularly corresponding to an increase in pH), the rebalancing of the equation results in a reaction to the right and more carbonic acid H2CO3 dissociates into H + and HCO3 - On the contrary, H + An increase in concentration (i.e. a decrease in pH) will cause a reaction to the left and consume HCO3 - H2CO3 is formed.

[0052] According to equation (4), bicarbonate HCO3 - formed and then dissociated, releasing a proton (H + ) and carbonate ions CO3 2- :

[0053] [Formula 4]

[0054]

[0055] For a given pH in an alkaline solution, especially in seawater, i.e. a given H + Concentration, adjust carbonate ion CO3 2- and bicarbonate ions HCO3 - The relative amount of the ions is increased until equilibrium is reached. A pH greater than 7 is conducive to the formation of carbonate ions CO3 2- , while pH less than 7 is conducive to the generation of bicarbonate ions HCO3 - Generally speaking, seawater is alkaline (pH>7), thus promoting the formation of carbonate CO3 2- formation.

[0056] According to equation (1), the calcium ions Ca released 2+ With carbonate ion CO3 2- The reaction forms calcium carbonate CaCO3, which precipitates according to equation (5):

[0057] [Formula 5]

[0058]

[0059] The method of the invention may be particularly useful for capturing carbon dioxide, for example thereby reducing greenhouse gas concentrations. Therefore, in other words, the invention also relates to a method for mineral capture of carbon dioxide, comprising the step of injecting carbon dioxide into a reaction medium comprising an alkaline solution (whose pH varies between 7.5 and 9) and phosphogypsum to cause the formation of calcium carbonate. The steps of the method may be as described above.

[0060] This process does not produce hydrogen sulfide (H2S).

[0061] Example

[0062] A purely illustrative and non-limiting example of the absorption of CO2 by phosphogypsum in an alkaline medium is described below.

[0063] Embodiment 1: By injecting pure gaseous CO2 directly into the reaction medium, phosphogypsum (not pre-dispersed in alkali absorbs CO2 in a liquid medium.

[0064] Carbon dioxide was injected at a flow rate of 0.7-0.8 l / min into a reaction medium containing phosphogypsum (mass = 17-25 g) and seawater (volume = 150-250 ml) without previously dispersing the phosphogypsum in the seawater. The whole was stirred.

[0065] The stirring is continued until all the phosphogypsum is converted into a solid phase and a supernatant. According to equation (6), the reaction mixture is filtered to obtain a solid phase and a liquid phase mainly composed of calcium carbonate:

[0066] CaSO4,2H2O+seawater+CO2→CaCO3,+SO4 2- +H2O

[0067] Embodiment 2: By injecting pure gas CO2 directly into the reaction medium, phosphogypsum (dispersed in alkaline medium) (middle) absorbs CO2.

[0068] The test was carried out under the same conditions as in Example 1. However, in this example, carbon dioxide was injected into the reaction medium comprising phosphogypsum and seawater after the phosphogypsum was pre-dispersed in seawater. The dispersion was carried out by stirring the phosphogypsum in seawater for a period of between 15 and 25 minutes until the pH stabilized between 8.5 and 9.5.

[0069] After the pH stabilized, carbon dioxide CO2 was injected at a flow rate of 0.7-0.8 liters / minute. In this example, the reaction medium became homogeneous and Ca 2+ The ions are more free in the solution, which further promotes the carbonation reaction.

[0070] Embodiment 3: By directly injecting pure gas CO2 to accelerate the nucleation process, the phosphorus Gypsum (dispersed in an alkaline medium) absorbs CO2.

[0071] The test was carried out under the same conditions as in Example 2. However, in this example, calcium carbonate seeds (a few milligrams (3 to 6 mg) of calcium carbonate) were added to the reaction medium before the CO2 injection.

[0072] Once CO2 is injected, the first particles of calcium carbonate precipitate and multiply rapidly (nucleation and pegmatism), thus accelerating the carbonation process.

[0073] Embodiment 4: By injecting pure gas CO2 directly into the reaction medium at a high flow rate, phosphogypsum (dispersed in alkaline medium) absorbs CO2.

[0074] The test was carried out under the same conditions as in Example 2. However, in this example, carbon dioxide was injected at a flow rate of 1-1.3 liters / minute.

[0075] For Examples 1 and 2, the purity of the calcium carbonate obtained was between 45% and 65%, and for Examples 3 and 4, the purity of the calcium carbonate obtained was between 40% and 50%.

[0076] The kinetics of carbonation Figure 2 , which represents the variation of the pH of the reaction medium with the reaction time in Example 2. The pH is initially equal to 8.5 and drops to a value between 5.5 and 6 after a reaction time of 10 minutes. The drop in pH corresponds to the acidification of the aqueous phosphogypsum solution due to the generation of ions H according to equations (3) and (4). + At the same time, carbonate ions CO3 2 -The concentration decreases with the precipitation of solid calcium carbonate CaCO3.

[0077] The obtained calcium carbonate was analyzed by thermogravimetric analysis (TGA) in Example 2. Figure 3 A graph showing the mass m (%) of calcium carbonate as a function of temperature T (°C) is shown. When the temperature reaches 600°C, the mass of calcium carbonate decreases, with a mass loss of about 35%. This mass loss corresponds to the decomposition of calcium carbonate CaCO3 into calcium oxide CaO and carbon dioxide CO2 according to reaction (7):

[0078] [Formula 7]

[0079] CaCO3→CaO+CO2

[0080] This analysis allowed to estimate the purity of the obtained calcium carbonate to be greater than 60%.

[0081] Figure 4 and Figure 5 The X-ray diffraction pattern and infrared spectrum of the calcium carbonate obtained in Example 2 are shown respectively, and it can be estimated that the purity of the calcium carbonate is between 30% and 65%.

[0082] Figure 6 Shown is a scanning electron microscope (SEM) micrograph of the calcium carbonate obtained in Example 2. The calcium carbonate was in powder form with a particle size of less than about 300 microns.

[0083] Table 1 below shows the amounts of different chemical elements present in the calcium carbonate obtained in Example 2, measured by energy dispersive X-ray analysis (EDX), the spectrum of which is shown in Figure 7 This analysis allows to estimate the purity of the obtained calcium carbonate to be greater than 60%. Calcium and oxygen obviously make up the majority, while other elements present in seawater are present in small amounts or even trace amounts.

[0084] [Table 1]

[0085] element calcium oxygen magnesium sodium atom[%] 51.30 5.73 1.30 0.17

[0086] References

[0087] WO2010 / 142859

[0088] EP1513771A1

[0089] US 8846562

[0090] CN102627309A

[0091] CN101337685A

[0092] US 20180022614

Claims

1. A method for absorbing carbon dioxide by phosphogypsum, comprising the step of injecting carbon dioxide into a reaction medium comprising an alkaline solution and phosphogypsum and forming calcium carbonate, wherein the pH of the alkaline solution is between 7.5 and 9, characterized in that The alkaline solution is sea water.

2. The method according to claim 1, comprising the step of preparing the reaction medium by dispersing the phosphogypsum in the alkaline solution, preferably until the pH stabilizes at a value between 8.5 and 9.

5.

3. A method according to any one of the preceding claims, wherein: The reaction medium is agitated during the carbon dioxide injection step.

4. The method according to any one of the preceding claims, comprising a filtering step to collect the calcium carbonate formed.

5. A method according to any one of the preceding claims, wherein: The carbon dioxide was injected at a flow rate of 0.7 to 1.3 liters / minute.

6. A method according to any one of the preceding claims, wherein: The phosphogypsum is present in the reaction medium in a concentration range of 0.1 kg / L to 0.5 kg / L.

7. A method according to any one of the preceding claims, wherein: The purity of the calcium carbonate obtained ranges from 30% to 65%, preferably from 45% to 65%.

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