Method for preparing high-purity calcium carbonate from magnesium-containing limestone

Abstract

The application provides a method for preparing high-purity calcium carbonate from magnesium-containing limestone, and belongs to the field of mineral processing and chemical separation. The method comprises the following steps: crushing and screening the blocky limestone to obtain limestone particles; calcining the limestone particles at high temperature to obtain mixed clinker; adding water to the mixed clinker for stirring and digestion to obtain a mixed slurry; dispersing CO2 into micro-nano bubbles and introducing the bubbles into the mixed slurry for carbonization treatment to obtain a carbonized slurry; performing solid-liquid separation on the carbonized slurry to obtain calcium carbonate solids and magnesium-rich water; performing pyrolysis treatment on the magnesium-rich water to obtain a pyrolysis slurry; and performing solid-liquid separation on the pyrolysis slurry to obtain basic magnesium carbonate solids. The application realizes efficient and selective separation of calcium and magnesium by using the difference in carbonization reaction rates and product solubility of calcium and magnesium and by strengthening gas-liquid mass transfer with micro-bubbles. The residual magnesium content in the calcium carbonate product can be stably controlled below 0.5 wt%, and no strong acid or alkali is added throughout the process.

Description

Technical Field

[0001] This application relates to the fields of mineral processing and chemical separation technology, and in particular to a method for preparing high-purity calcium carbonate from magnesium-containing limestone. Background Technology

[0002] High-quality limestone, as a non-renewable non-metallic mineral resource, relies heavily on the purity of its calcium carbonate for its application value. However, natural limestone ore inevitably contains a certain amount of magnesium carbonate impurities. In downstream sectors such as building materials, metallurgy, chemicals, and flue gas desulfurization, various functional calcium carbonate products have strict limits on magnesium content; even trace amounts of residual magnesium can significantly degrade product performance.

[0003] Existing calcium-magnesium separation technologies each have limitations: high-temperature calcination relies on the temperature difference between calcium and magnesium carbonate decomposition, requiring temperatures above 900℃, resulting in high energy consumption, severe equipment corrosion, and incomplete separation; acid-base leaching, while achieving better separation, generates large amounts of waste liquid, leading to high environmental disposal costs; solvent extraction, while offering high separation precision, suffers from expensive and wasteful extractants, making it unsuitable for large-scale continuous production. Therefore, how to efficiently separate calcium and magnesium components from magnesium-containing limestone to prepare low-magnesium, high-purity calcium carbonate is a pressing technical problem that needs to be solved. Summary of the Invention

[0004] This application provides a method for preparing high-purity calcium carbonate from magnesium-containing limestone to solve the following technical problem: how to efficiently separate calcium and magnesium components from magnesium-containing limestone to prepare low-magnesium, high-purity calcium carbonate.

[0005] This application provides a method for preparing high-purity calcium carbonate from magnesium-containing limestone, the method comprising: The blocky limestone is crushed and screened to obtain limestone particles; The limestone particles are calcined at high temperature to obtain a mixed clinker of calcium oxide and magnesium oxide; Water is added to the mixed clinker and stirred to dissolve it, resulting in a mixed slurry of calcium hydroxide and magnesium hydroxide. CO2 is dispersed into micro / nano bubbles and then introduced into the mixed slurry for carbonization treatment to obtain a carbonized slurry. The carbonization treatment includes the following parameters: temperature ≤ 60℃, pressure 0.1MPa~0.5MPa, time 1h~5h, and CO2 gas flow rate 0.8m³ / h. 3 / h~2.0m 3 / h; The carbonized slurry was subjected to solid-liquid separation to obtain calcium carbonate solid and magnesium ion-enriched heavy magnesium water. The magnesium hydroxide solution is subjected to pyrolysis to obtain a pyrolysis slurry; The pyrolysis slurry was subjected to solid-liquid separation to obtain basic magnesium carbonate solid.

[0006] Optionally, the limestone particles have a particle size of 10 mm to 30 mm.

[0007] Optionally, the high-temperature calcination temperature is 850℃~950℃, and the calcination time is 1.5h~3h.

[0008] Optionally, the solid-liquid ratio of the mixed slurry is 1:4 to 1:8, in g / mL.

[0009] Optionally, the stirring speed for the stirring digestion is 30 r / min to 60 r / min, and the digestion time is 60 min to 120 min.

[0010] Optionally, the carbonization process uses a carbonization reactor; The carbonization reactor is equipped with a stirring component inside. The micro-nano bubbles are generated by at least one of jet shearing, ultrasonic cavitation, or microporous aeration.

[0011] Optionally, the pore size of the microporous aeration head is 0.4μm to 0.6μm, and the microporous aeration head is located at the bottom of the carbonization reactor.

[0012] Optionally, after the carbonization treatment and before solid-liquid separation, the method further includes: The carbonized slurry is kept at a constant temperature and allowed to stand for 20 to 40 minutes.

[0013] Optionally, the pyrolysis treatment is performed at a temperature of 70℃ to 95℃ for a time of 0.5h to 2h.

[0014] Optionally, the content of residual magnesium in the basic calcium carbonate solid is less than 0.5 wt%.

[0015] The technical solutions provided in this application have the following advantages compared with the prior art: This application provides a method for preparing high-purity calcium carbonate from magnesium-containing limestone. By dispersing CO2 into micro-nano bubbles and then passing it into a mixed slurry of calcium hydroxide and magnesium hydroxide for carbonation treatment, the gas-liquid interface area and mass transfer coefficient are greatly increased, significantly improving the dissolution rate and effective concentration of CO2 in the liquid phase, and greatly increasing the reaction probability. Through the synergistic enhancement of carbonation by pressurization and microbubbles, even low-concentration magnesium impurities in calcium carbonate will react. Under these highly efficient mass transfer conditions, the inherent difference in carbonation reaction rate and product solubility between Ca(OH)2 and Mg(OH)2 is utilized. Ca(OH)2 preferentially reacts with CO2 to form insoluble CaCO3 precipitate, while Mg(OH)2, with a sufficient supply of CO2, is more thoroughly converted into soluble Mg(HCO3)2 and remains stable in the liquid phase. Combined with precise control of the carbonation temperature below 60℃, this ensures that Mg(HCO3)2 does not prematurely pyrolyze and precipitate within this temperature range. Thus, after a single solid-liquid separation, calcium is extracted in the form of solid calcium carbonate, and magnesium is completely removed in the form of magnesium hydroxide, achieving highly selective separation of calcium and magnesium. The separated magnesium hydroxide is then pyrolyzed to decompose Mg(HCO3)2 into MgCO3 precipitate, and magnesium carbonate is recovered through a second solid-liquid separation. Ultimately, the residual magnesium content in the calcium carbonate product is stably controlled below 0.5 wt%. Attached Figure Description

[0016] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0017] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 A schematic flowchart illustrating a method for preparing high-purity calcium carbonate from magnesium-containing limestone, provided in an embodiment of this application; Figure 2 This is a schematic diagram of the carbonization reactor provided in the embodiments of this application; Figure label: 1-Inlet device, 2-Temperature measuring device, 3-Stirring device, 4-Feeding device, 5-Exhaust device, 6-Cafe body, 7-Microbubble generator, 8-Discharge device. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0020] The range descriptions used herein, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values ​​within that range. For example, the range descriptions of "1 to 6" or "1 to 6" cover all sub-ranges (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6) between 1 and 6. Unless otherwise specified, the terms "including" and "contains" as used herein mean "including but not limited to"; relational terms such as "first" and "second" are used only to distinguish different entities or operations and do not imply an actual order or relationship; "and / or" indicates that multiple situations can exist individually or simultaneously; expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships mentioned herein, such as mass ratios and molar ratios, should be understood as the correspondence between the antecedent and consequent terms of a proportional expression, according to the order of description. Unless otherwise specified, all contents, amounts added, etc., mentioned in this article are based on mass; for example, an addition of 1% means that the additive accounts for 1% of the total mass of the system; a content of 10% means that the component has a mass fraction of 10% in the composition. The raw materials, reagents, instruments, and equipment used in this article can all be obtained by purchasing them from the market or by preparing them using existing methods.

[0021] Figure 1 This is a schematic flowchart illustrating a method for preparing high-purity calcium carbonate from magnesium-containing limestone, as provided in an embodiment of this application.

[0022] like Figure 1 As shown in the embodiments of this application, a method for preparing high-purity calcium carbonate from magnesium-containing limestone is provided, the method comprising: The blocky limestone is crushed and screened to obtain limestone particles; Limestone particles are calcined at high temperature to obtain a mixed clinker of calcium oxide and magnesium oxide; Water is added to the mixed clinker and stirred to dissolve it, resulting in a mixed slurry of calcium hydroxide and magnesium hydroxide. CO2 is dispersed into micro / nano bubbles and then introduced into a mixed slurry for carbonization treatment to obtain a carbonized slurry. The carbonization treatment includes the following parameters: temperature ≤ 60℃, pressure 0.1MPa~0.5MPa, time 1h~5h, and CO2 gas flow rate 0.8m³ / h. 3 / h~2.0m 3 / h; The carbonized slurry was subjected to solid-liquid separation to obtain calcium carbonate solid and magnesium ion-enriched heavy magnesium water. The magnesium hydroxide solution was pyrolyzed to obtain a pyrolysis slurry. The pyrolysis slurry was subjected to solid-liquid separation to obtain basic magnesium carbonate solid.

[0023] It should be noted that the crushing and screening step, through mechanical crushing and screening, regularizes the lumpy limestone into granules, which can increase the specific surface area of ​​the ore, provide uniform heat conduction conditions for the subsequent high-temperature calcination process, and ensure that the calcination reaction is sufficient and consistent; it removes surface impurities, reducing the interference of ineffective components on the subsequent reaction system; and controlling the particle size can ensure that the material packing density in the calcination furnace is moderate, avoiding incomplete calcination due to excessively large particle size or dust entrainment loss due to excessively small particle size.

[0024] The high-temperature calcination step utilizes the thermal decomposition chemical properties of carbonates to decompose CaCO3 and MgCO3 in limestone into CaO and MgO, respectively, releasing CO2. The core function of this step is to convert the sparingly soluble carbonates into more chemically reactive oxides, providing a reaction basis for subsequent hydration digestion to generate hydroxides; at the same time, the high-temperature treatment can further remove organic matter and volatile impurities from the ore.

[0025] The stirring and digestion step involves reacting the CaO and MgO clinker with water to produce suspended Ca(OH)₂ and Mg(OH)₂ particles, forming a homogeneous slurry. Thorough digestion ensures that oxides in the clinker are converted to hydroxides as completely as possible, preventing incomplete calcium-magnesium separation due to differences in reactivity of undigested particles during subsequent carbonization. Stirring maintains the uniformity of particle suspension, preventing sedimentation and stratification that could lead to uneven slurry concentration.

[0026] The carbonization process is the core step in achieving selective separation of calcium and magnesium. CO2 is dispersed into micro / nano bubbles and then introduced into the mixed slurry. Utilizing the significant difference in carbonization reaction rates and product solubility between Ca(OH)2 and Mg(OH)2, Ca(OH)2 preferentially carbonizes to form the insoluble CaCO3 precipitate, while Mg(OH)2, with a sufficient CO2 supply, is converted into soluble Mg(HCO3)2 and stably exists in the liquid phase. Thus, efficient extraction of calcium components can be achieved through a single solid-liquid separation step.

[0027] The carbonization temperature is limited to ≤60℃. At this temperature, the carbonization rate of Ca(OH)2 is sufficient to reach complete carbonization within 4 hours, while Mg(HCO3)2 remains in a well-dissolved state and is not prone to premature pyrolysis. If the temperature is too high (>60℃), it will cause Mg(HCO3)2 to decompose prematurely into MgCO3, which co-precipitates contaminating products. For example, the carbonization temperature can be 25℃, 30℃, 35℃, 40℃, 50℃, 55℃, 60℃, etc.

[0028] The pressure is limited to 0.1 MPa to 0.5 MPa. This slight positive pressure can moderately increase the driving force for CO2 dissolution and the concentration of reactants, accelerating the carbonization process. If the pressure is too low (<0.1 MPa), the gas-liquid mass transfer driving force will be insufficient, the dissolution rate will be low, and the microbubbles will easily collapse. If the pressure is too high (>0.5 MPa), the equipment investment and maintenance costs will increase significantly, and the marginal benefits will decrease. For example, the pressure for carbonization treatment can be 0.10 MPa, 0.2 MPa, 0.25 MPa, 0.3 MPa, 0.4 MPa, 0.5 MPa, etc.

[0029] The carbonization time is limited to 1 to 5 hours. This window ensures that Ca(OH)₂ is fully carbonized to the equilibrium conversion rate and avoids over-carbonization. If the time is too short (<1 hour), calcium will not precipitate sufficiently, and residual calcium will co-decompose with magnesium during the pyrolysis stage, reducing the purity of the by-product. If the time is too long (>5 hours), CaCO₃ will be over-carbonized, generating soluble Ca(HCO₃)₂, which will then dissolve and be lost, increasing energy consumption. For example, the carbonization time can be 1.0 hour, 2.0 hour, 3.0 hour, 3.5 hour, 4.0 hour, 4.2 hour, 4.5 hour, 4.8 hour, 5.0 hour, etc.

[0030] The CO2 gas flow rate is limited to 0.8 m³ / s. 3 / h~2.0m 3 / h, this range meets the stoichiometric requirements for complete carbonization of Ca(OH)2 and matches the microbubble dispersion efficiency; too low a flow rate (<0.8m) 3 The low dissolved CO2 concentration ( / h) and the limitation of gas-phase mass transfer make calcium precipitation difficult to complete; excessively high flow rates (>2.0 m³ / h) further exacerbate the problem. 3 The flow rate ( / h) exacerbates turbulent erosion, damaging the CaCO3 crystal structure, inhibiting sedimentation, and drastically reducing gas utilization. For example, the CO2 gas flow rate during carbonization treatment can be 0.8 m³ / h. 3 / h, 1.0m 3 / h, 1.2m 3 / h, 1.4m 3 / h, 1.6m 3 / h, 1.8m 3 / h, 2.0m 3 / h etc.

[0031] The first solid-liquid separation separates the CaCO3 precipitate generated by the carbonation reaction from the liquid phase containing Mg(HCO3)2, directly obtaining high-purity calcium carbonate solid product, while collecting magnesium-rich hydrate as raw material for the next process.

[0032] The pyrolysis step applies high temperature to the magnesium hydroxide solution, causing the soluble Mg(HCO3)2 to undergo a thermal decomposition reaction, transforming it into insoluble MgCO3 precipitate, thereby separating and recovering the magnesium component from the liquid phase. This step achieves secondary recycling of magnesium resources, avoiding resource waste and environmental burden caused by the direct discharge of magnesium-containing wastewater.

[0033] The second solid-liquid separation separates the MgCO3 precipitate generated by pyrolysis from the pyrolysis mother liquor, ultimately obtaining magnesium carbonate as a solid byproduct. The separated mother liquor (mainly containing a small amount of undecomposed magnesium and calcium salts) can be recycled back to the digestion process, realizing the closed-loop utilization of water resources.

[0034] In some embodiments, the limestone particles have a particle size of 10 mm to 30 mm.

[0035] Limestone particle size (10mm~30mm): This particle size range ensures the uniformity of heat conduction from the particle surface to the interior during the calcination process, resulting in a complete and consistent carbonate decomposition reaction. It also avoids the loss of excessively fine powder due to entrainment in the high-temperature airflow. Furthermore, this particle size matches the contact efficiency between clinker and water in the subsequent slaking process, which is beneficial for the complete conversion of oxides during hydration. For example, the limestone particle size can be 10mm, 12mm, 15mm, 18mm, 20mm, 22mm, 25mm, 28mm, 30mm, etc.

[0036] In some embodiments, the high-temperature calcination temperature is 850℃~950℃, and the calcination time is 1.5h~3h.

[0037] High-temperature calcination temperature (850℃~950℃) and time (1.5h~3h): This temperature range ensures that both CaCO3 and MgCO3 in the limestone reach their respective decomposition temperature thresholds, achieving complete decomposition into CaO and MgO. Combined with a holding time of 1.5h~3h, this allows the decomposition reaction to proceed fully, avoiding insufficient activity in subsequent hydration reactions due to incomplete decomposition. This calcination process balances decomposition efficiency and energy economy, yielding highly active mixed clinker. For example, the high-temperature calcination temperature can be 850℃, 870℃, 890℃, 900℃, 910℃, 930℃, 950℃, etc., and the high-temperature calcination time can be 1.5h, 1.8h, 2.0h, 2.2h, 2.5h, 2.8h, 3.0h, etc.

[0038] In some embodiments, the solid-liquid ratio of the mixed slurry is 1:4 to 1:8, in g / mL.

[0039] Solid-liquid ratio of the mixed slurry (1:4 to 1:8, g / mL): This ratio provides sufficient water to accommodate the stoichiometric amount of water required for the complete hydration of CaO and MgO, while ensuring suitable fluidity and pumpability of the slurry, facilitating the transport and mixing of materials between various processes. A reasonable water volume also provides a sufficient medium for the dissolution and ionization of Ca(OH)2 in the subsequent carbonization reaction, which is beneficial for the efficient conduct of the carbonization reaction. For example, the solid-liquid ratio of the mixed slurry can be 1:4, 1:4.5, 1:5, 1:5.5, 1:6, 1:6.5, 1:7, 1:7.5, 1:8, etc.

[0040] In some embodiments, the stirring speed for stirring digestion is 30 r / min to 60 r / min, and the digestion time is 60 min to 120 min.

[0041] Stirring speed (30 r / min ~ 60 r / min) and digestion time (60 min ~ 120 min): This stirring speed range is sufficient to maintain uniform suspension of solid particles in the slurry, preventing Ca(OH)2 and Mg(OH)2 particles from settling and stratifying. Simultaneously, it ensures timely peeling and exposure of fresh reaction interfaces on the surface of newly formed hydroxides, accelerating the hydration process. Combined with a sufficient digestion time of 60 min ~ 120 min, it ensures that oxides in the clinker are converted into hydroxides as completely as possible. For example, the stirring speed for stirring and digestion can be 30 r / min, 35 r / min, 40 r / min, 45 r / min, 50 r / min, 55 r / min, 60 r / min, etc., and the digestion time can be 60 min, 70 min, 80 min, 90 min, 100 min, 110 min, 120 min, etc.

[0042] Figure 2 This is a schematic diagram of the carbonization reactor provided in an embodiment of this application.

[0043] In some implementations, such as Figure 2 As shown, the carbonization process uses a carbonization reactor; The carbonization reactor is equipped with a stirring component inside; The micro-nano bubbles are generated by at least one of jet shearing, ultrasonic cavitation, or microporous aeration.

[0044] The internal stirring components of the carbonization reactor provide continuous and gentle mechanical stirring during the carbonization process, which promotes the uniform dispersion of CO2 microbubbles in the slurry, enhances the contact and mass transfer between the gas, liquid and solid phases, and avoids local gas aggregation or short-circuit escape; at the same time, it ensures that the temperature and concentration field distribution in the reaction system is uniform, and prevents local over-carbonization or uneven reaction.

[0045] The jet shearing method utilizes the strong shearing force and turbulent pulsation generated by high-speed jets to cut and tear the introduced gas into micro- and nano-sized bubbles. It has the advantages of simple structure and easy scale-up, but the bubble size distribution is relatively wide.

[0046] Ultrasonic cavitation utilizes the periodic compression and rarefaction effects generated when ultrasound propagates in a liquid, causing bubbles to grow, oscillate, and implode and collapse in the sound field. The cavitation effect generates local high temperature and pressure and strong shock waves, which further break the bubbles down to the submicron or even nanometer scale. The bubbles are small and uniform in size, but the energy consumption is high and the equipment cost is relatively expensive.

[0047] Micropore aeration uses a medium with uniform micropores (such as micropore air distribution head, micropore aeration disc, etc.) to cut the gas into tiny bubbles after it passes through the micropores under pressure. The bubble size can be effectively controlled by controlling the pore size. It is easy to operate, runs stably, and is suitable for large-scale industrial applications.

[0048] In some embodiments, the pore size of the microporous aeration head is 0.4μm to 0.6μm, and the microporous aeration head is located at the bottom of the carbonization reactor.

[0049] Microporous gas distributor at the bottom of the vessel (pore size 0.4μm~0.6μm): This microporous structure can cut the introduced CO2 gas into a large number of micro- and nano-bubbles, greatly increasing the gas-liquid interface area, significantly improving the CO2 dissolution rate and effective concentration, and providing a sufficient source of reactants for the carbonization reaction; the microbubbles rise slowly in the slurry, prolonging the gas-liquid contact time and improving CO2 utilization. For example, the pore size of the microporous gas distributor can be 0.4μm, 0.45μm, 0.5μm, 0.55μm, 0.6μm, etc.

[0050] In some embodiments, after carbonization and before solid-liquid separation, the method further includes: Keep the carbonized slurry warm and let it stand for 20 to 40 minutes.

[0051] After carbonization, the mixture is kept at a constant temperature for 20 to 40 minutes: This provides sufficient time for the fine CaCO3 particles generated during the carbonization reaction to settle, allowing the solid particles to spontaneously aggregate and settle, facilitating efficient subsequent solid-liquid separation processes and reducing the suspended solids content in the filtrate. For example, the holding time for the carbonized slurry can be 20 minutes, 25 minutes, 30 minutes, 35 minutes, or 40 minutes.

[0052] In some embodiments, the pyrolysis treatment is carried out at a temperature of 70°C to 95°C for a time of 0.5 h to 2 h.

[0053] Pyrolysis temperature (70℃~95℃) and time (0.5h~2h): This temperature range meets the thermodynamic requirements for the thermal decomposition of Mg(HCO3)2 into MgCO3, accelerating the pyrolysis reaction. The 0.5h~2h holding time ensures sufficient precipitation of magnesium components, improving magnesium recovery rate. For example, the pyrolysis temperature can be 70℃, 75℃, 80℃, 85℃, 90℃, 95℃, etc., and the pyrolysis time can be 0.5h, 0.8h, 1.0h, 1.2h, 1.5h, 1.8h, 2.0h, etc.

[0054] In some embodiments, the content of residual magnesium in the basic calcium carbonate solid is less than 0.5 wt%.

[0055] The key technology for achieving a residual magnesium content of less than 0.5 wt% in basic calcium carbonate solids lies in dispersing CO2 into micro-nano bubbles and then introducing it into a mixed slurry for carbonation. These microbubbles significantly increase the gas-liquid interface area and mass transfer coefficient, resulting in a substantial increase in the dissolution rate and effective concentration of CO2 in the liquid phase. Under these highly efficient mass transfer conditions, utilizing the inherent differences in carbonation reaction rates and product solubility between Ca(OH)2 and Mg(OH)2, Ca(OH)2 preferentially reacts with CO2 to form insoluble CaCO3 precipitate; while Mg(OH)2, with a sufficient CO2 supply, is more thoroughly converted into soluble Mg(HCO3)2, which remains stably in the liquid phase. Combined with precise control of the carbonation temperature between 50℃ and 60℃, this ensures that Mg(HCO3)2 does not prematurely pyrolyze and precipitate within this temperature range. This allows for complete removal of the magnesium component with the filtrate (demagnesium hydroxide) during solid-liquid separation, achieving highly selective separation of calcium and magnesium, and ultimately maintaining the residual magnesium content in the calcium carbonate product stably below 0.5 wt%.

[0056] The present application is further illustrated below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards / industry standards / the disclosure herein; if there are no corresponding national standards / industry standards / the disclosure herein, they are performed according to generally accepted international standards, conventional conditions, or conditions recommended by the manufacturer.

[0057] Example 1 ① Raw material crushing and screening: The block limestone is fed into the crusher for crushing, and then screened by the screening machine to obtain limestone particles with a particle size of 10mm. After removing impurities, it is ready for use.

[0058] ② High-temperature calcination to prepare mixed clinker: The above limestone particles are fed into a calcining furnace, and the calcination temperature is set to 850℃ and the calcination time is 3h, so that the calcium carbonate and magnesium carbonate in the raw materials are fully decomposed to generate calcium oxide and magnesium oxide mixed clinker. After discharge, the material is naturally cooled to room temperature.

[0059] ③ Preparation of mixed slurry by closed digestion: The cooled mixed clinker is transported to a closed digestion tank, and water is added at a solid-liquid ratio of 1:4. The stirring device is turned on and the stirring speed is set to 30 r / min. The mixture is continuously digested for 120 min to obtain a uniform and stable calcium hydroxide-magnesium hydroxide mixed slurry.

[0060] ④ Carbonization separation of calcium components: Pump the mixed slurry into the carbonization reactor, start the stirring device inside the reactor, set the stirring speed to 140 r / min, control the reactor temperature at 50℃ and the reactor pressure at 0.2 MPa; continuously introduce carbon dioxide gas through the bottom 0.5 μm microporous gas diffuser, controlling the gas flow rate at 0.8 m³ / min. 3 The gas flow was stopped after 4 hours of continuous carbonation. The mixture was kept at a constant temperature and allowed to stand for 30 minutes to allow the calcium carbonate to settle completely. The material in the reactor was then sent to a filtration unit for solid-liquid separation to obtain high-purity calcium carbonate solid. The filtrate was magnesium ion-rich heavy magnesium water.

[0061] ⑤ Magnesium extraction by pyrolysis of magnesium hydroxide: The collected magnesium hydroxide is heated to 70°C and kept at that temperature for 2 hours to obtain a pyrolysis slurry; the pyrolysis slurry is subjected to solid-liquid separation to obtain the mother liquor and magnesium carbonate solid.

[0062] ⑥ Tail gas and mother liquor treatment: Carbon dioxide tail gas is recovered and recycled, and the separated mother liquor is reused in the digestion process.

[0063] The residual magnesium content in the high-purity calcium carbonate solid obtained in this embodiment is 0.42 wt%.

[0064] Example 2 ① Raw material crushing and screening: The block limestone is fed into the crusher for crushing, and then screened by the screening machine to obtain limestone particles with a particle size of 20mm. After removing impurities, it is ready for use.

[0065] ② High-temperature calcination to prepare mixed clinker: The above limestone particles are fed into a calcining furnace, and the calcination temperature is set to 900℃ and the calcination time is 2h, so that the calcium carbonate and magnesium carbonate in the raw materials are fully decomposed to generate calcium oxide and magnesium oxide mixed clinker. After discharge, the material is naturally cooled to room temperature.

[0066] ③ Preparation of mixed slurry by closed digestion: The cooled mixed clinker is transported to a closed digestion tank, and water is added at a solid-liquid ratio of 1:6. The stirring device is turned on and the stirring speed is set to 45 r / min. The mixture is continuously digested for 90 min to obtain a uniform and stable calcium hydroxide-magnesium hydroxide mixed slurry.

[0067] ④ Carbonation separation of calcium components: The mixed slurry is pumped into the carbonation reactor, and the stirring device inside the reactor is started. The stirring speed is set to 140 r / min, and the temperature inside the reactor is controlled at 55℃ and the pressure inside the reactor is 0.25 MPa. Carbon dioxide gas is continuously introduced through the bottom 0.5 μm microporous gas diffuser, and the gas flow rate is controlled at 1.4 m³ / h. After 4 hours of continuous carbonation reaction, the gas supply is stopped. The mixture is kept at the temperature and allowed to stand for 30 minutes to allow the calcium carbonate to settle completely. The material in the reactor is then sent to the vacuum filtration unit for solid-liquid separation to obtain high-purity calcium carbonate solid. The filtrate is magnesium ion-enriched heavy magnesium water.

[0068] ⑤ Magnesium extraction by pyrolysis of magnesium hydroxide: The collected magnesium hydroxide is heated to 80℃ and kept at that temperature for 1 hour to obtain a pyrolysis slurry; the pyrolysis slurry is subjected to solid-liquid separation to obtain the mother liquor and magnesium carbonate solid.

[0069] ⑥ Tail gas and mother liquor treatment: Carbon dioxide tail gas is recovered and recycled, and the separated mother liquor is reused in the digestion process.

[0070] The residual magnesium content in the high-purity calcium carbonate solid obtained in this embodiment is 0.38 wt%.

[0071] Example 3 ① Raw material crushing and screening: The block limestone is fed into the crusher for crushing, and then screened by the screening machine to obtain limestone particles with a particle size of 30mm. After removing impurities, it is ready for use.

[0072] ② High-temperature calcination to prepare mixed clinker: The above limestone particles are fed into a calcining furnace, and the calcination temperature is set to 950℃ and the calcination time is 1.5h, so that the calcium carbonate and magnesium carbonate in the raw materials are fully decomposed to generate calcium oxide and magnesium oxide mixed clinker. After discharge, the material is naturally cooled to room temperature.

[0073] ③ Preparation of mixed slurry by closed digestion: The cooled mixed clinker is transported to a closed digestion tank, and water is added at a solid-liquid ratio of 1:8. The stirring device is turned on, and the stirring speed is set to 60 r / min. The mixture is continuously digested for 60 min to obtain a uniform and stable calcium hydroxide-magnesium hydroxide mixed slurry.

[0074] ④ Carbonation separation of calcium components: The mixed slurry is pumped into the carbonation reactor, and the stirring device inside the reactor is started. The stirring speed is set to 140 r / min, and the temperature inside the reactor is controlled at 60℃ and the pressure inside the reactor is controlled at 0.3 MPa. Carbon dioxide gas is continuously introduced through the bottom 0.5 μm microporous gas diffuser, and the gas flow rate is controlled at 2.0 m³ / h. After 4 hours of continuous carbonation reaction, the gas supply is stopped. The mixture is kept at the temperature and allowed to stand for 30 minutes to allow the calcium carbonate to settle completely. The material in the reactor is then sent to the vacuum filtration unit for solid-liquid separation to obtain high-purity calcium carbonate solid. The filtrate is magnesium ion-enriched heavy magnesium water.

[0075] ⑤ Magnesium extraction by pyrolysis of magnesium hydroxide: The collected magnesium hydroxide is heated to 95℃ and kept at that temperature for 0.5h to obtain a pyrolysis slurry; the pyrolysis slurry is subjected to solid-liquid separation to obtain the mother liquor and magnesium carbonate solid.

[0076] ⑥ Tail gas and mother liquor treatment: Carbon dioxide tail gas is recovered and recycled, and the separated mother liquor is reused in the digestion process.

[0077] The residual magnesium content in the high-purity calcium carbonate solid obtained in this embodiment is 0.45 wt%.

[0078] The residual magnesium content in the high-purity calcium carbonate solids obtained in Examples 1-3 is summarized in Table 1.

[0079] Table 1. Residual magnesium content (wt.%) in high-purity calcium carbonate solids from Examples 1-3

[0080] As shown in Table 1, the residual magnesium content in the high-purity calcium carbonate solid obtained in this embodiment is less than 0.5 wt%.

[0081] Furthermore, one or more technical solutions in the embodiments of this application have at least the following technical effects or advantages: (1) Outstanding green and environmentally friendly: This application does not add corrosive reagents such as strong acids and strong alkalis throughout the process, avoiding the environmental problems of generating a large amount of waste liquid in the traditional acid and alkali leaching method; the unreacted CO2 in the carbonization process can be collected and recycled for the carbonization process, and the pyrolysis mother liquor can be recycled for the digestion process, realizing the internal recycling of gas and liquid resources, with no harmful waste discharged, which meets the requirements of green chemical development.

[0082] (2) Mild reaction conditions and low threshold for industrialization: Compared with the traditional high-temperature calcination method which requires maintaining a high temperature of over 900°C for calcium and magnesium separation, the carbonization separation step of this application can be completed under mild conditions of 50°C to 60°C and 0.2MPa to 0.3MPa. It does not require extreme working conditions of high temperature and high pressure, and has moderate requirements for equipment materials and sealing levels, which significantly reduces equipment investment and operation and maintenance costs, and makes it easy to achieve large-scale continuous industrial production.

[0083] (3) High separation efficiency and high product purity: By enhancing gas-liquid mass transfer through microbubbles and combining multi-parameter coordinated control of temperature, pressure, time and gas flow rate, the calcium and magnesium components are separated efficiently and selectively. The residual magnesium content in the obtained calcium carbonate solid can be stably controlled below 0.5wt%, which meets the stringent quality requirements of high-end application fields for low magnesium calcium carbonate.

[0084] (4) The process is simple and the operation is highly controllable: This application adopts a four-step main process route of calcination → digestion → carbonization → pyrolysis. The process is closely connected and the operating parameters are clear. With the synergistic effect of stirring and microporous gas distribution in the carbonization reactor, the reaction process is highly controllable and reproducible, avoiding the problem of unstable separation effect caused by operation fluctuations in traditional processes.

[0085] (5) Strong adaptability of raw materials and high resource utilization rate: This application has broad requirements for the grade of limestone raw materials and is applicable to magnesium-containing limestone resources with different magnesium contents; at the same time, the magnesium components separated by the pyrolysis step are recovered in the form of magnesium carbonate, realizing the graded resource utilization of calcium and magnesium, two valuable elements, and improving the comprehensive added value of ore resources.

[0086] (6) High added value of products and broad application prospects: The magnesium impurity content of the low magnesium calcium carbonate product obtained in this application is less than 0.5wt%, which can meet the quality requirements of high-purity calcium carbonate in the fields of fine chemicals, electronic materials, high-end building materials and flue gas desulfurization; the by-product magnesium carbonate can also be sold as a chemical raw material. The parallel operation of the two product lines significantly improves the overall economic benefits of the project.

[0087] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for preparing high-purity calcium carbonate from magnesium-containing limestone, characterized in that, The method includes: The blocky limestone is crushed and screened to obtain limestone particles; The limestone particles are calcined at high temperature to obtain a mixed clinker of calcium oxide and magnesium oxide; Water is added to the mixed clinker and stirred to dissolve it, resulting in a mixed slurry of calcium hydroxide and magnesium hydroxide. CO2 is dispersed into micro / nano bubbles and then introduced into the mixed slurry for carbonization treatment to obtain a carbonized slurry. The carbonization treatment includes the following parameters: temperature ≤ 60℃, pressure 0.1MPa~0.5MPa, time 1h~5h, and CO2 gas flow rate 0.8m³ / h. 3 / h~2.0m 3 / h; The carbonized slurry was subjected to solid-liquid separation to obtain calcium carbonate solid and magnesium ion-enriched heavy magnesium water. The magnesium hydroxide solution is subjected to pyrolysis to obtain a pyrolysis slurry; The pyrolysis slurry was subjected to solid-liquid separation to obtain basic magnesium carbonate solid.

2. The method according to claim 1, characterized in that, The limestone particles have a particle size of 10mm to 30mm.

3. The method according to claim 1, characterized in that, The high-temperature calcination temperature is 850℃~950℃, and the calcination time is 1.5h~3h.

4. The method according to claim 1, characterized in that, The solid-liquid ratio of the mixed slurry is 1:4 to 1:8, and the unit is g / mL.

5. The method according to claim 1, characterized in that, The stirring speed for the stirring digestion is 30 r / min to 60 r / min, and the digestion time is 60 min to 120 min.

6. The method according to claim 1, characterized in that, The carbonization process uses a carbonization reactor; The carbonization reactor is equipped with a stirring component inside. The micro-nano bubbles are generated by at least one of jet shearing, ultrasonic cavitation, or microporous aeration.

7. The method according to claim 6, characterized in that, The pore size of the microporous aeration head is 0.4μm to 0.6μm, and the microporous aeration head is located at the bottom of the carbonization reactor.

8. The method according to claim 1, characterized in that, After the carbonization treatment and before solid-liquid separation, the method further includes: The carbonized slurry is kept at a constant temperature and allowed to stand for 20 to 40 minutes.

9. The method according to claim 1, characterized in that, The pyrolysis treatment is carried out at a temperature of 70℃ to 95℃ for a time of 0.5h to 2h.

10. The method according to claim 1, characterized in that, The content of residual magnesium in the basic calcium carbonate solid is less than 0.5 wt%.

Images