Process for the preparation of high content of light calcium carbonate

Abstract

The present application relates to the industrial production of light calcium carbonate, and discloses a method for preparing high-content light calcium carbonate, which is aimed at the current situation that high-grade raw materials are increasingly scarce, and the requirement that the calcium carbonate content in raw materials (limestone or calcite) should be greater than 98.5% for producing high-content light calcium carbonate (calcium carbonate content greater than or equal to 98.5%). The method for preparing high-content light calcium carbonate comprises the following steps: controlling the carbonation reaction to the point where the conductivity of the system is at its lowest, and then continuing the reaction until the conductivity is increased to 2500-3000 muS / cm, thereby maintaining an appropriate degree of overcarbonation; centrifugally dewatering the light calcium carbonate slurry, and then drying it to a moisture content of 2-3.5%; and then microwave-assisted heating at 250-300 DEG C to obtain the finished product of high-content light calcium carbonate. The method for preparing high-content light calcium carbonate can meet the practical needs of using lower-grade raw materials (calcium carbonate content greater than or equal to 97.5%) to produce high-content light calcium carbonate (calcium carbonate content greater than or equal to 98.5%) at a low cost.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of industrial production of light calcium carbonate, and particularly relates to a preparation method of high-content light calcium carbonate. BACKGROUND

[0002] Light calcium carbonate is widely used in paper, paint, rubber and plastic, building material, feed, food, medicine and other production fields due to its low raw material requirement, simple production process, flexible product index control, excellent safety and environmental protection performance, etc. For example, it is used as a white pigment or filler in paper and paint to improve product gloss and aesthetic feeling, adjust rheological property, coatability, adhesion or hiding power; it is used as a "molecular skeleton" in rubber and plastic and building material to endow the material with higher mechanical properties such as hardness, rigidity, toughness and weather resistance; it is used as a calcium source in feed, food and medicine to directly or indirectly increase the content of calcium compounds in the products, improve the nutritional value or physiological properties.

[0003] The content of calcium carbonate in light calcium carbonate has a significant influence on the application performance and safety performance of downstream products, and a low content of calcium carbonate means a high content of impurities or unstable physical and chemical properties of the product. The feed, food and medicine production fields generally require that the content of CaCO3 is not less than 98.5%, and many enterprise standards require more than 99.0%. At present, the calcination-digestion-carbonization production mode is mainly used to produce the above high-content light calcium carbonate, in which high-grade limestone or calcite with a calcium carbonate content of more than 98.5% is used as raw material. However, the high-grade raw material is limited and shrinking in China, and the above production mode is difficult to continue. Other production methods, such as repeatedly calcining-digesting-carbonizing calcium carbonate as raw material to remove impurities and increase the content of calcium carbonate, are complicated, have serious energy waste and poor economic and environmental performance; adding calcium extraction agent, extracting and enriching calcium ions first, and then converting them into calcium carbonate again, has high production cost and is difficult to overcome the problem of reagent residue. Therefore, under the circumstances that the exploitation of high-grade calcium resources is shrinking, and the downstream application requires higher and higher purity of light calcium carbonate, it is urgent to develop a process for preparing high-content light calcium carbonate from low-grade limestone as raw material.

[0004] Chinese patent CN 201010190178.9 discloses a method for preparing light calcium carbonate and magnesium hydroxide from magnesium tailings, specifically: the mass percentage of calcium oxide in the magnesium tailings is 18% to 36%, the magnesium tailings are crushed and calcined to obtain calcined powder and CO2; the calcined powder is digested, carbonized and chlorinated with calcium chloride solution under CO2 atmosphere and stirring conditions to obtain a solution containing MgCl2 and light CaCO3 precipitate; the purity of the prepared light CaCO3 precipitate is above 98%, and can reach 99.5% under the optimal preparation conditions. However, the patent scheme is aimed at high-value utilization of magnesium tailings with calcium oxide content of 18% to 36%, and the process and application scenarios have significant particularity and limitation, and it is difficult to be applied to the production of ordinary light calcium carbonate. Therefore, it is still a subject with great practical significance to develop a process method for preparing high-content light calcium carbonate from low-grade raw materials at low cost. SUMMARY

[0005] In view of the problems that high-quality raw materials are in short supply, the process engineering is complex, the production cost is high, and the product index flexibility is low in the current production of high-content light calcium carbonate (calcium carbonate content ≥98.5%), the present application discloses a preparation method of high-content light calcium carbonate. Compared with the existing process, the method of the present application not only significantly improves the content of light calcium carbonate, meets the demand of preparing high-content light calcium carbonate from low-grade raw materials, but also saves non-renewable resources, reduces waste and carbon emissions, and reduces the comprehensive production cost, thereby playing a positive role in cost reduction and efficiency improvement and emission reduction.

[0006] The present application provides the following technical solutions:

[0007] A preparation method of light calcium carbonate, comprising the following steps:

[0008] 1) calcining limestone or calcite to obtain raw lime, and then adding a digestion solution to wet digestion to obtain a crude calcium hydroxide slurry;

[0009] 2) sending the crude calcium hydroxide slurry into a curing process for curing treatment after grading and sieving;

[0010] 3) removing impurity particles after curing of the calcium hydroxide slurry, adjusting the concentration and temperature, adding a crystal form control agent, and sending into a carbonation process;

[0011] 4) introducing carbon dioxide atmosphere into the calcium hydroxide slurry for carbonation reaction until the conductivity of the reaction system decreases to the lowest point, and continuing the reaction until the conductivity increases to 2500-3000 μS / cm to end, to obtain a light calcium carbonate slurry;

[0012] 5) centrifuging the light calcium carbonate slurry, drying to a water content of 2.0% to 3.5%, then crushing into particles, and heating to 250 to 300°C while applying microwave radiation, the frequency of the microwave being 2350 to 2550 MHZ, the power density being 1.20 to 2 W / g of calcium carbonate, and the microwave radiation being applied for 5 to 15 min, to obtain high-content light calcium carbonate.

[0013] In the method for preparing high-content light calcium carbonate according to the present application, the end point of the carbonization reaction is controlled to be the point at which the conductivity of the reaction system drops to the lowest point, and then the reaction is continued until the conductivity rises to 2500 to 3000 μS / cm, to achieve appropriate over-carbonization, to create a weakly acidic environment, to reduce the pH and free alkali level of the slurry, to reduce the amount of magnesium carbonate and basic magnesium carbonate generated in the calcium carbonate slurry, to promote the conversion of the magnesium carbonate and basic magnesium carbonate into low-temperature hydrogen carbonate magnesium that is easy to decompose, and to ensure the purity of the calcium carbonate slurry. When the conductivity is lower than or higher than this level, the carbonization is insufficient or too deep, which is not conducive to the preparation of high-content light calcium carbonate. Then the obtained calcium carbonate slurry is centrifuged, dried to a water content of 2.0% to 3.5%, and then subjected to microwave-assisted heat treatment, to in-situ decompose the multi-hydrated magnesium carbonate and basic magnesium carbonate and the internal water in the light calcium carbonate product, to effectively remove about 0.1% to 0.3% of the internal water through microwave radiation heating, and to convert the high-molecular-weight multi-hydrated basic magnesium carbonate into low-molecular-weight magnesium oxide through thermal decomposition, to release more than 0.5% of the purification space. The preparation method according to the present application can effectively improve the content of light calcium carbonate, to achieve the preparation of high-content light calcium carbonate with a calcium carbonate content of ≥98.5% from limestone or calcite with a calcium carbonate content of ≥98.5%, and the cost is low, which can save non-renewable resources.

[0014] As a preferred method according to the present application, in step 1):

[0015] The particle size of the limestone or calcite used is 15 to 20 cm; the use of large-size limestone reduces the surface area / mass ratio, and reduces the adsorption, accumulation and enrichment of ash powder in the surface and pores of the quicklime after calcination;

[0016] And / or, the calcium carbonate content of the limestone or calcite is ≥97.5% or the calcium oxide content is ≥54.5%, and the magnesium oxide content is ≤1.0%; and / or, the calcination temperature is 900 to 950°C, and the calcination time is 20 to 30 h; the low-temperature calcination reduces the sintering, surface carbonation and pulverization of the lime, to ensure the purity of the lime;

[0017] And / or, the calcination fuel is natural gas or anthracite; the present application can use anthracite to produce high-content light calcium carbonate;

[0018] And / or, the mass ratio of the limestone or calcite to the calcination fuel is 100:8 to 10;

[0019] And / or, the digestive juice is water;

[0020] And / or, the mass ratio of quicklime and digestive juice is 1:5-7.

[0021] As a preferred method of the present application, in step 2):

[0022] The classification sieving is in turn 60-80 mesh, 100-120 mesh, 180-200 mesh, and 325-350 mesh sieving; the dust and other impurity particles in the calcium hydroxide slurry are removed by classification sieving to ensure the purity of the calcium hydroxide milk;

[0023] And / or, the curing time is 12-36 h; the calcium oxide is fully digested to convert it into calcium hydroxide, avoiding the "back to alkali" of light calcium carbonate.

[0024] As a preferred method of the present application,

[0025] The classification sieving is in turn 80 mesh, 120 mesh, 200 mesh, and 325 mesh sieving.

[0026] As a preferred method of the present application, in step 3):

[0027] The concentration is adjusted to 10%-15% and the temperature is adjusted to 35-45°C;

[0028] And / or, the crystal form control agent is hydrogen peroxide; the light calcium carbonate product has a regular small spindle shape and good dispersion performance;

[0029] And / or, the addition amount of the crystal form control agent is 0.5%-1% of the dry basis mass of calcium hydroxide.

[0030] As a preferred method of the present application, in step 4):

[0031] The volume flow rate of the carbon dioxide atmosphere is 6000-7000 m 3 / h, and the carbon dioxide volume concentration is 35.5%-37.5%.

[0032] As a preferred method of the present application, in step 4), the carbonization reaction process is:

[0033] The calcium hydroxide slurry is transported into the carbonization reactor and extracted from the bottom of the carbonization reactor by a circulating pump into a Venturi jet pipe, carbon dioxide atmosphere is introduced into the Venturi jet pipe to fully mix with the calcium hydroxide slurry, and then the mixture is again injected into the carbonization reactor for reaction, and the cycle is repeated until the conductivity of the reaction system reaches a minimum point, and then the reaction continues until the conductivity reaches 2500-3000 μS / cm, and the carbonization reaction is completed to obtain a light calcium carbonate slurry. The calcium hydroxide slurry and CO2 are subjected to violent shaking in the jet pipe, and after sufficient micro-mixing of the gas-liquid-solid three phases, the mixture is again injected into the carbonization reactor through the jet pipe for reaction.

[0034] As a preferred method of the present application, in step 5):

[0035] Drying to a moisture content of 2.5% to 3.5%;

[0036] And / or, the particle size of the broken particles is ≤75 μm;

[0037] And / or, the heating time is 20 to 30 min.

[0038] As a preferred method of the present application, in step 5):

[0039] Drying to a moisture content of 2.5% to 3.0%.

[0040] As a preferred method of the present application, in step 5):

[0041] The heating and microwave irradiation are carried out in a relatively closed space and under a negative pressure formed by air extraction outside. Compared with the prior art using costly nitrogen protection or replacement technology measures, the present application eliminates the negative influence of CO2 in air on the calcium carbonate content by a relatively closed working environment and a slight negative pressure, and the vacuum degree of the negative pressure is preferably 50 to 1000 Pa, and more preferably 100 to 200 Pa.

[0042] The beneficial effects of the present application are as follows:

[0043] Compared with the prior art method for preparing light calcium carbonate using quicklime as a raw material, the preparation method of the present application can effectively improve the calcium carbonate content in the light calcium carbonate product, and can prepare high-content light calcium carbonate with a content of not less than 98.5% from low-grade quicklime with a calcium carbonate content of 97.5%, thereby meeting the practical demand for producing high-purity light calcium carbonate with a main content of more than 98.5% using ordinary raw materials. Meanwhile, the preparation method of the present application has low cost, less chemical addition compared with other process methods, simple and easy production, strong process compatibility, flexible and adjustable indexes, and can be grafted to most light calcium carbonate production processes in China. BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1 is the SEM picture of the light calcium carbonate prepared in Example 1.

[0045] Figure 2 is the SEM picture of the light calcium carbonate prepared in Comparative Example 1.

[0046] Figure 3 is the thermal gravimetric curve of the light calcium carbonate prepared in Example 1 and Comparative Example 1.

[0047] Figure 4 is the SEM picture of the high-purity light calcium carbonate prepared by the double decomposition method in Comparative Example 8.

[0048] Figure 5 is a SEM picture of high purity light calcium carbonate prepared by carbonization method in Comparative Example 9. DETAILED DESCRIPTION

[0049] The specific embodiments of the present application are further described below.

[0050] The raw materials used in the present application are commercially available or commonly used in the art, unless otherwise specified; the methods in the following examples are conventional methods in the art, unless otherwise specified.

[0051] Example 1

[0052] A method for preparing light calcium carbonate, comprising the following steps:

[0053] 1) Mix limestone (CaCO3 content 98.1%, MgO content 0.57%) with anthracite in a mass ratio of 100:8.5 into a kiln, and obtain quicklime after calcining at 900°C for 30h, then add the obtained quicklime and water into a roller digestion machine in a mass ratio of 1:6 for wet digestion to obtain a crude calcium hydroxide slurry;

[0054] 2) Pass the calcium hydroxide slurry through 80 mesh and 120 mesh sieves in sequence, then flow into a transfer slurry tank, and then pump into a slurry refining process, further remove impurities through 200 mesh and 325 mesh sieves, and then pump into a slaking tank for slaking for 24h;

[0055] 3) After further removing impurity particles through a suspension separator, pump the slaked calcium hydroxide slurry into a calcium hydroxide fine slurry tank, adjust the concentration (10.0%) and temperature (35.0°C), add 0.7% hydrogen peroxide (based on the mass of calcium hydroxide dry basis), and pump into a carbonization reactor;

[0056] 4) Start the bottom circulating pump of the carbonization reactor, extract and transport the calcium hydroxide slurry to a Venturi jet pipe, and mix with the purified CO2 kiln gas (concentration 37.54%, flow rate 6000m 3 / h) transported by a high-pressure Roots blower to form a gas-liquid-solid three-phase micro-mixture, and then spray the mixture into the carbonization reactor again through the jet pipe, and repeat the process until the electrical conductivity of the reaction system reaches the minimum point, and then continue over-carbonization until the electrical conductivity reaches 2500μS / cm, and the carbonization reaction is completed to obtain a light calcium carbonate slurry;

[0057] 5) The light calcium carbonate slurry is dewatered to a powder with a moisture content of 34.3% by a high-speed centrifuge, then heated and dried to a moisture content of 3.0%, and then refined to less than 75 μm (200 mesh sieve) by an ultrafine pulverizer to break up calcium carbonate lumps, transported by a closed pipeline to a closed microwave radiation-assisted heating device (electric heating), heated to 250°C for deep heat treatment for 30 min, the microwave frequency is 2450 MHZ, the power density is 1.25 W / g of calcium carbonate, microwave irradiation for 10 min, and a one-way valve is used to extract air outside to maintain a vacuum degree of 100 Pa during the treatment process, to obtain a high-content light calcium carbonate product.

[0058] Example 2

[0059] A method for preparing light calcium carbonate, the steps are as follows:

[0060] 1) Limestone with a lump size of 15-20 cm (CaCO3 content 98.1%, MgO content 0.57%) is mixed with anthracite in a mass ratio of 100:8 into a kiln, and after calcining at 900°C for 25 h, raw lime is obtained, then the obtained raw lime is added into a roller digestion machine with water in a mass ratio of 1:7 for wet digestion to obtain a coarse calcium hydroxide slurry;

[0061] 2) The calcium hydroxide slurry is sequentially screened through 80 mesh and 120 mesh screens and then flows into a transfer slurry tank, and then is pumped into a slurry refining process, and after further impurity removal through 200 mesh and 325 mesh screens, is punched into a curing tank for curing for 30 h;

[0062] 3) After the cured calcium hydroxide slurry is further removed from impurity particles by a suspension separator, is pumped into a calcium hydroxide fine slurry tank, after adjusting the concentration (15.0%) and temperature (40.1°C), 0.5% of hydrogen peroxide (dry basis of calcium hydroxide mass) is added, and is pumped into a carbonization reactor;

[0063] 4) A bottom circulating pump of the carbonization reactor is started to extract and transport the calcium hydroxide slurry to a Venturi jet pipe, and after the purified CO2 kiln gas (concentration 37.2%, flow rate 6000 m 3 / h) transported by a high-pressure Roots blower is violently shaken in the jet pipe, after gas-liquid-solid three-phase micro-mixing is fully achieved, is sprayed again into the carbonization reactor through the jet pipe, and this process is cyclically performed until the electrical conductivity of the reaction system decreases to a minimum point, and then over-carbonization is continued until the electrical conductivity increases to 2700 μS / cm, the carbonization reaction is completed, and a light calcium carbonate slurry is obtained;

[0064] 5) The light calcium carbonate slurry is dewatered to a powder with a moisture content of 34.7% by a high-speed centrifuge, and then heated and dried to a moisture content of 3.5%, and the calcium carbonate lumps are disintegrated to a particle size of 75 μm or less (200 mesh screen) by a super-micro pulverizer, and then transported by a closed pipeline to a closed microwave radiation-assisted heating device (electric heating) and heated to 250°C for deep heat treatment for 30 min, the microwave frequency is 2350 MHZ, the power density is 1.20 W / g of calcium carbonate, the microwave irradiation is 12 min, and a vacuum degree of 100 Pa is maintained by external air extraction through a one-way valve during the treatment process, and a high-content light calcium carbonate product is obtained.

[0065] Example 3

[0066] A method for preparing light calcium carbonate, comprising the following steps:

[0067] 1) Limestone with a lump size of 15-20 cm (CaCO3 content 98.1%, MgO content 0.57%) is mixed with anthracite at a mass ratio of 100:9 into a kiln, and then calcined at a low temperature of 950°C for 22 h to obtain quicklime, and then the obtained quicklime is added into a roller digestion machine with water at a mass ratio of 1:5 for wet digestion to obtain a coarse calcium hydroxide slurry;

[0068] 2) The calcium hydroxide slurry is sequentially screened through 80 mesh and 120 mesh screens and then flows into a transfer slurry tank, and then is pumped into a slurry refining process, and then is further screened through 200 mesh and 325 mesh screens to remove impurities, and then is pumped into a slaking tank and is slaked for 12 h;

[0069] 3) The slaked calcium hydroxide slurry is further pumped into a calcium hydroxide fine slurry tank after removing impurity particles through a suspension separator, and then the concentration (14.9%) and temperature (44.8°C) are adjusted, and then 0.7% of hydrogen peroxide (dry basis) is added, and then the mixture is pumped into a carbonization reactor;

[0070] 4) A bottom circulating pump of the carbonization reactor is started to extract and transport the calcium hydroxide slurry to a Venturi jet pipe, and then the calcium hydroxide slurry is mixed with purified CO2 kiln gas (concentration 35.7%, flow rate 7000 m 3 / h) transported by a high-pressure Roots blower to produce violent oscillation in the jet pipe, and then the gas-liquid-solid three-phase microcosmic mixture is sprayed again into the carbonization reactor through the jet pipe, and then the process is cyclically performed until the electrical conductivity of the reaction system reaches a minimum point, and then the over-carbonization is continued until the electrical conductivity reaches 3000 μS / cm, and then the carbonization reaction is completed to obtain a light calcium carbonate slurry;

[0071] 5) The light calcium carbonate slurry is dewatered to a powder with a moisture content of 34.3% by a high-speed centrifuge, then heated and dried to a moisture content of 2.9%, and then the calcium carbonate agglomerates are disintegrated by a super-micro pulverizer to a particle size of 75 μm or less (200 mesh screen), and then transported by a closed pipeline to a closed microwave radiation-assisted heating device (electric heating) and heated to 270°C for deep heat treatment for 25 min, the microwave frequency is 2450 MHZ, the power density is 1.30 W / g of calcium carbonate, the microwave irradiation is 15 min, and a vacuum of 200 Pa is maintained by external air extraction through a one-way valve during the treatment process, to obtain a high-content light calcium carbonate product.

[0072] Example 4

[0073] A method for preparing light calcium carbonate, comprising the following steps:

[0074] 1) Limestone with a lump size of 15-20 cm (CaCO3 content 97.5%, MgO content 1.0%) is mixed with anthracite at a mass ratio of 100:10 into a kiln, and after calcination at a low temperature of 950°C for 20 h, raw lime is obtained, and then the obtained raw lime is added into a roller digestion machine with water at a mass ratio of 1:5 for wet digestion to obtain a coarse calcium hydroxide slurry;

[0075] 2) The calcium hydroxide slurry is sequentially filtered through 60 mesh and 100 mesh screens and then flows into a transfer slurry tank, and then is pumped into a slurry refining process, and is further filtered through 200 mesh and 350 mesh screens to remove impurities, and then is pumped into a slaking tank for slaking for 12 h;

[0076] 3) After the slaking of the calcium hydroxide slurry, the slurry is further pumped into a calcium hydroxide fine slurry tank through a suspension separator to remove impurity particles, and then the concentration (14.8%) and temperature (44.7°C) are adjusted, 1% of hydrogen peroxide (dry basis) is added, and then the slurry is pumped into a carbonization reactor;

[0077] 4) A bottom circulating pump of the carbonization reactor is started to extract and transport the calcium hydroxide slurry to a Venturi jet pipe, and then the calcium hydroxide slurry is jetted into the carbonization reactor again through the jet pipe after being violently shaken with purified CO2 kiln gas (concentration 35.8%, flow rate 7000 m 3 / h) delivered by a high-pressure Roots blower, and after the gas-liquid-solid three-phase micro-mixing is fully completed, the process is cycled until the electrical conductivity of the reaction system reaches the minimum point, and then the over-carbonization is continued until the electrical conductivity reaches 2500 μS / cm, and the carbonization reaction is completed to obtain a light calcium carbonate slurry;

[0078] 5) The light calcium carbonate slurry is dewatered to a powder with a moisture content of 34.6% by a high-speed centrifuge, then heated and dried to a moisture content of 2.0%, and then the calcium carbonate lumps or particles are refined to less than 75 μm (200 mesh screen) by a super-micro pulverizer, and then transported by a closed pipeline to a closed microwave radiation assisted heating device (electric heating) and heated to 300°C for deep heat treatment for 20 min, the microwave frequency is 2550 MHZ and the power density is 2 W / g of calcium carbonate, the microwave irradiation is 5 min, a one-way valve is used to extract air outside to maintain a vacuum degree of 200 Pa during the treatment process, and a high content of light calcium carbonate product is obtained.

[0079] Example 5

[0080] A method for preparing light calcium carbonate, comprising the following steps:

[0081] 1) Limestone with a lump size of 15-20 cm (CaCO3 content 98.1%, MgO content 0.57%) is mixed with anthracite at a mass ratio of 100:9.5 into a kiln, and after calcination at a low temperature of 930°C for 25 h, raw lime is obtained, and then the obtained raw lime is added into a roller digestion machine with water at a mass ratio of 1:7 for wet digestion to obtain a coarse calcium hydroxide slurry;

[0082] 2) The calcium hydroxide slurry is sequentially screened through 80 mesh and 120 mesh screens and then flows into a transfer slurry tank, and then is pumped into a slurry refining process, and is further screened through 180 mesh and 350 mesh screens to remove impurities, and then is punched into a slaking tank and is slaked for 36 h;

[0083] 3) After the slaked calcium hydroxide slurry is further separated from impurity particles by a suspension separator, the slurry is pumped into a calcium hydroxide fine slurry tank, the concentration (15%) and temperature (45°C) are adjusted, 0.5% of hydrogen peroxide (dry basis) is added, and then the slurry is pumped into a carbonization reactor;

[0084] 4) A bottom circulating pump of the carbonization reactor is started to extract and transport the calcium hydroxide slurry to a Venturi jet pipe, and then the slurry is jetted into the carbonization reactor again after being violently shaken in the jet pipe with purified CO2 kiln gas (concentration 35.5%, flow rate 7000 m 3 / h) transported by a high-pressure Roots blower, the gas-liquid-solid three-phase micro-mixing is sufficient, and then the slurry is jetted into the carbonization reactor again through the jet pipe, the process is cyclically performed until the electrical conductivity of the reaction system reaches the minimum point, and then the over-carbonization is continued until the electrical conductivity rises to 2600 μS / cm, the carbonization reaction is completed, and a light calcium carbonate slurry is obtained;

[0085] 5) The light calcium carbonate slurry is dewatered by high-speed centrifuge to a powder with a moisture content of 34.1%, then heated and dried to a moisture content of 2.5%, and then the calcium carbonate lumps or particles are refined to less than 75 μm (200 mesh sieve) by an ultrafine pulverizer, and then transported by a closed pipeline to a closed microwave radiation assisted heating device (electric heating), heated to 260°C for deep heat treatment for 25 min, the microwave frequency is 2500 MHZ, the power density is 1.8 W / g of calcium carbonate, the microwave irradiation is 8 min, and a vacuum degree of 150 Pa is maintained by one-way valve during the treatment process to obtain a high content of light calcium carbonate product.

[0086] Example 6

[0087] A method for preparing light calcium carbonate, which is different from example 1,

[0088] In step 4), the bottom of the carbonization reactor is a jet circulating pump, which is started to pump the slurry out of the carbonization reactor and then recirculate back to the carbonization reactor to react with the purified CO2 kiln gas delivered by the high-pressure Roots blower. This process is repeated until the conductivity of the reaction system reaches the minimum point, and then over-carbonization is continued until the conductivity rises to 2500 μS / cm, the carbonization reaction is completed, and the light calcium carbonate slurry is obtained.

[0089] Example 7

[0090] A method for preparing light calcium carbonate, which is different from example 1, using natural gas instead of anthracite in step 1).

[0091] Control group (light calcium carbonate prepared by traditional method of limestone)

[0092] 1) Crush the limestone blocks (CaCO3 content 98.1%, MgO content 0.57%) to a particle size of 50-60 mm, then calcine at 1000°C for 30 h under a natural gas atmosphere, then add the obtained quicklime and water in a mass ratio of 1:6 into a roller digestion machine for wet digestion to obtain a crude calcium hydroxide slurry;

[0093] 2) The calcium hydroxide slurry is separated by a 120 mesh sieve and then flows into a transfer slurry tank, and then is pumped into a slurry refining process, and then is further impurity-removed by 200 mesh and 325 mesh sieves and then is punched into a curing tank for curing for 24 h;

[0094] 3) The calcium hydroxide slurry after curing is further impurity-removed by a suspension separator, then is pumped into a calcium hydroxide fine slurry tank, then the concentration (10.0%) and temperature (35.0°C) are adjusted, then 0.7% of hydrogen peroxide (dry basis of calcium hydroxide mass) is added, and then is pumped into a carbonization reactor;

[0095] 4) input purified CO2 kiln gas (concentration 37.54%, flow 6000 m 3 / h) into the carbonation reactor to react to pH < 7.5, carbonation reaction ends, to obtain light calcium carbonate slurry;

[0096] 5) dewater the light calcium carbonate slurry through a high-speed centrifuge to a water content of 34.3% in the powder, then heat dry to a water content of 0.5%, then pass through an ultrafine pulverizer to disperse calcium carbonate lumps to less than 75 μm (200 mesh screen full pass), to obtain light calcium carbonate finished product.

[0097] Comparative Example 1

[0098] The difference from Example 1 is that after dewatering through a centrifuge in step 5), directly heat dry (temperature 250°C) to a water content of less than 0.5%, then pass through an ultrafine pulverizer to disperse calcium carbonate lumps to less than 75 μm (200 mesh screen full pass), to obtain light calcium carbonate finished product.

[0099] Comparative Example 2

[0100] The difference from Example 1 is that after heat drying to a water content of 1% in step 5), microwave-assisted heating treatment under the same conditions, to obtain light calcium carbonate finished product.

[0101] Comparative Example 3

[0102] The difference from Example 1 is that after dewatering to a water content of 34.3% in step 5), directly use microwave-assisted heating treatment, to obtain light calcium carbonate finished product.

[0103] Comparative Example 4

[0104] The difference from Example 1 is that after drying to a water content of 3.0% in step 5), under a nitrogen atmosphere (100 mL / min) use microwave irradiation under the same conditions, without electric heating, to obtain light calcium carbonate finished product.

[0105] Comparative Example 5

[0106] The difference from Example 1 is that after heat drying to a water content of 3.0% in step 5), only use microwave irradiation for the same time, microwave frequency 2450 MHz, power density 250 W / g, without electric heating, to obtain light calcium carbonate finished product.

[0107] Comparative Example 6

[0108] The difference from Example 1 is that in step 4), after the conductivity decreases to a minimum, then continue to increase to 1000 μS / cm, carbonation reaction ends, to obtain light calcium carbonate slurry.

[0109] Comparative Example 7

[0110] The difference from Example 1 is that in step 4), the carbonization reaction is continued to 4000 μS / cm after the conductivity is minimized, and the carbonization reaction is ended, to obtain a light calcium carbonate slurry.

[0111] 1. Performance test of light calcium carbonate

[0112] The physicochemical indexes of the light calcium carbonate products prepared in the above examples, the control group and the comparative example are shown in Table 1. The detection methods of the indexes are as follows:

[0113] Calcium carbonate content: tested according to 3.4 of GB / T19281-2014.

[0114] Specific surface area: tested according to the method specified in GB / T19587-2004.

[0115] Average particle size: tested according to 3.26.2 of GB / T19281-2014.

[0116] Settling volume: tested according to 3.22 of GB / T19281-2014.

[0117] Water absorption: tested according to 6.18 of GB / T23957-2021.

[0118] pH value: tested according to 3.18 of GB / T19281-2014

[0119] Table 1: Physicochemical indexes of light calcium carbonate products

[0120]

[0121]

[0122] As can be seen from the above table, the limestone with a calcium carbonate content of 97.5% can be used to prepare high-content light calcium carbonate with a content of 98.5% according to the present application, the obtained light calcium carbonate product has a low pH value, a wide adjustment range of particle size, settling volume and water absorption, a high main content, and is more Figure 1 And Figure 2 It can be seen that the light calcium carbonate product prepared by the present application has high crystal shape regularity, and therefore can meet the needs of more production fields, specifically:

[0123] As can be seen from the comparison of Example 1 and Comparative Example 1, using microwave-assisted heat treatment can significantly improve the calcium carbonate content of the light calcium carbonate product compared with direct heat drying. This is because the content of magnesium element (in terms of magnesium oxide, relative molecular weight 40) in the raw material limestone or calcite used in the domestic production of light calcium carbonate is generally above 0.5%. In the process of precipitating calcium carbonate by wet carbonation of lime, the magnesium element is converted into magnesium carbonate (MgCO3, 84 g / mol), basic magnesium carbonate (4MgCO3·Mg(OH)2·4H2O, 546 g / mol) and a small amount of hydrated magnesium sulfate (MgSO4·7H2O, 246 g / mol) and other forms, and is finally mixed into the light calcium carbonate product. In this way, with the change in the form of magnesium element, the multiplication of the molecular weight of the compound, the effect on the content of the light calcium carbonate product reaches 1.0% or more. At the same time, the surface hydroxyl content of the light calcium carbonate is high, and it is easy to agglomerate to form micron-sized inclusions, and the water molecules inside are also difficult to remove in the conventional drying process, which also affects the main content of calcium carbonate. If the magnesium salt in the light calcium carbonate product can be effectively decomposed and the wrapped water can be removed, the content of the light calcium carbonate can be improved. However, it is not easy to decompose the magnesium salt in the light calcium carbonate product. According to the literature, the decomposition temperature of basic magnesium carbonate is 200-300°C, the decomposition temperature of magnesium carbonate is about 560°C, and the decomposition of magnesium sulfate requires a high temperature of 1124°C or more. The drying temperature of industrial production of light calcium carbonate is usually below 250°C, the thermal conductivity coefficient of calcium carbonate is low, the temperature rises slowly, the bulk temperature does not exceed 150°C, and there is no theoretical basis and practical condition to decompose large molecular magnesium salt in the drying process. Greatly increasing the drying temperature (such as 400°C) and prolonging the drying time (such as 60 min) can decompose part of the magnesium salt, but it will bring problems such as high energy consumption, low efficiency and uneconomicalness. The inventors found that microwave-assisted heating treatment can effectively decompose the magnesium salt in the light calcium carbonate. As can be seen from the thermal weight loss curve shown in Figure 3 As can be seen from the comparison of Example 1 and Comparative Example 1, using microwave-assisted heat treatment can significantly improve the calcium carbonate content of the light calcium carbonate product compared with direct heat drying. This is because the content of magnesium element (in terms of magnesium oxide, relative molecular weight 40) in the raw material limestone or calcite used in the domestic production of light calcium carbonate is generally above 0.5%. In the process of precipitating calcium carbonate by wet carbonation of lime, the magnesium element is converted into magnesium carbonate (MgCO3, 84 g / mol), basic magnesium carbonate (4MgCO3·Mg(OH)2·4H2O, 546 g / mol) and a small amount of hydrated magnesium sulfate (MgSO4·7H2O, 246 g / mol) and other forms, and is finally mixed into the light calcium carbonate product. In this way, with the change in the form of magnesium element, the multiplication of the molecular weight of the compound, the effect on the content of the light calcium carbonate product reaches 1.0% or more. At the same time, the surface hydroxyl content of the light calcium carbonate is high, and it is easy to agglomerate to form micron-sized inclusions, and the water molecules inside are also difficult to remove in the conventional drying process, which also affects the main content of calcium carbonate. If the magnesium salt in the light calcium carbonate product can be effectively decomposed and the wrapped water can be removed, the content of the light calcium carbonate can be improved. However, it is not easy to decompose the magnesium salt in the light calcium carbonate product. According to the literature, the decomposition temperature of basic magnesium carbonate is 200-300°C, the decomposition temperature of magnesium carbonate is about 560°C, and the decomposition of magnesium sulfate requires a high temperature of 1124°C or more. The drying temperature of industrial production of light calcium carbonate is usually below 250°C, the thermal conductivity coefficient of calcium carbonate is low, the temperature rises slowly, the bulk temperature does not exceed 150°C, and there is no theoretical basis and practical condition to decompose large molecular magnesium salt in the drying process. Greatly increasing the drying temperature (such as 400°C) and prolonging the drying time (such as 60 min) can decompose part of the magnesium salt, but it will bring problems such as high energy consumption, low efficiency and uneconomicalness. The inventors found that microwave-assisted heating treatment can effectively decompose the magnesium salt in the light calcium carbonate. As can be seen from the thermal weight loss curve shown in

[0124] As can be seen from the comparison of Example 1 and Comparative Examples 2 and 3, without controlling the moisture content of light calcium carbonate to a suitable range, microwave-assisted heat treatment cannot achieve the ideal effect of degrading impurity compounds such as high molecular weight magnesium salts and increasing the content of light calcium carbonate.

[0125] As can be seen from Examples 1 and 4, the increase in the content of light calcium carbonate by microwave irradiation alone is limited. This is because the heat generated by microwave irradiation alone is insufficient to heat the light calcium carbonate to the temperature at which magnesium salt impurities are fully decomposed. However, as can be seen from Comparative Example 5, even after significantly increasing the microwave power, satisfactory results cannot be achieved. This is because the total magnesium salt content in light calcium carbonate is not high, and the content of basic magnesium carbonate is even lower, so the effect of high-power microwaves is not obvious.

[0126] Compared to Examples 1 and Comparative Examples 6 and 7, in addition to microwave-assisted heating, precise control of the carbonation process is also crucial for increasing the content of light calcium carbonate. Excessive or insufficient carbonation cannot promote the full conversion of magnesium into basic magnesium carbonate with a lower decomposition temperature, or even water-soluble magnesium bicarbonate, and the effect of impurity removal is also difficult to guarantee.

[0127] Cost accounting

[0128] The calcium carbonate product prepared from the control group was further processed into high-purity calcium carbonate through metathesis and carbonation methods, respectively.

[0129] Comparative Example 8 (Multiple Decomposition Method)

[0130] 1) In this example, 35% hydrochloric acid was used to dissolve the calcium carbonate product (provided by the control group) to obtain Ca 2+ A CaCl2 solution with a concentration of 200 g / L;

[0131] 2) After filtering to remove acid-insoluble matter from the CaCl2 solution, ammonia water is added to adjust the pH value of the solution to 12. Impurities such as Mg, Fe, and Al are removed by precipitation. After filtration again, a clear CaCl2 mother liquor is obtained.

[0132] 3) Slowly add Na2CO3 solution (1.05 times the theoretical amount) to the CaCl2 mother liquor under stirring conditions. After precipitation is complete, extract the supernatant and dehydrate the bottom slurry to obtain calcium carbonate filter cake.

[0133] 4) Wash the calcium carbonate filter cake three times with tap water equal to twice the dry weight of the calcium carbonate, then send it to the drying process and dry it until the moisture content of the powder does not exceed 0.5%. After crushing and grading, light calcium carbonate is obtained.

[0134] Comparative Example 9 (Carbonization Method)

[0135] 1) Dissolve the calcium carbonate product (provided by the control group) in 35% hydrochloric acid to obtain Ca 2+ A CaCl2 solution with a concentration of 200 g / L;

[0136] 2) After removing the acid-insoluble substances in the CaCl2 solution by filtration, ammonia water is added to adjust the PH value of the solution to 12, and Mg, Fe, Al and other impurity elements are removed by precipitation method, and the clear CaCl2 mother liquor is obtained by filtration again;

[0137] 3) Ammonia water is added to the CaCl2 mother liquor to make the molar ratio of NH4 + / Ca 2+ maintained at 2-2.1, and then pumped into a carbonation tower, and the purified CO2 kiln gas is introduced to start carbonation, and the reaction is completed when the PH value of the system is reduced to below 7.5, and the calcium carbonate slurry is obtained;

[0138] 4) The calcium carbonate slurry is dehydrated to obtain calcium carbonate filter cake, and the calcium carbonate filter cake is washed with 2 times the mass of tap water based on the dry mass of the calcium carbonate for 3 times, and then sent to a drying process, dried to a powder moisture content of not more than 0.5%, and then crushed and classified to obtain light calcium carbonate.

[0139] The physical and chemical indexes of the light calcium carbonate products prepared in the above-mentioned Comparative Example 8 and Comparative Example 9 are shown in Table 2, and the SEM images are shown in Figs. 2 and 3. Figure 4 、 5

[0140] Table 2: Physical and chemical indexes of each light calcium carbonate product

[0141]

[0142] From the above table and Figure 1 、 Figure 4 、 Figure 5 , it can be seen that the light calcium carbonate product obtained by the present application has high regularity of crystal shape, low pH value, wide adjustment range of particle size, sedimentation volume and water absorption, etc. The high-purity light calcium carbonate prepared by using the prior art has irregular crystal shape, coarse particles or serious agglomeration, single index and serious polarization, and obviously cannot meet the diversified requirements

[0143] 2.2 Cost comparison of high-purity light calcium carbonate prepared by different methods, as shown in Tables 3 and 4, respectively.

[0144] Table 3: Comparison of resource consumption, environmental protection index and cost of preferred scheme and comparative examples of the present application

[0145]

[0146] Note:

[0147] ① The hazardous / solid waste includes the mass of sodium chloride, ammonium chloride and other substances that can be collected during subsequent treatment of process water;

[0148] ​The direct cost includes raw materials, reagents, energy, and labor, manufacturing cost, does not include equipment, factory depreciation and hazardous waste disposal cost; the reagent cost is calculated according to the current purchase price;

[0149] The raw material and chemical reagent cost required for producing one ton of PCC by the process of the application and the mainstream high-purity light calcium carbonate process are calculated in Table 4.

[0150]

[0151] In summary, the process method provided by the application not only can improve the main content of light calcium carbonate, but also has significant comparative advantages in saving non-renewable resources, reducing three wastes and carbon emissions, and reducing production cost. At the same time, the good compatibility and graftability of the method of the application with the conventional light calcium carbonate process also provides a new technical path and practical choice for domestic light calcium carbonate enterprises to transform and upgrade, realize green and low-carbon and high-quality development.

Claims

1. A process for the preparation of high content light calcium carbonate, characterized in that, The method comprises the following steps: 1) calcining limestone or calcite to obtain quicklime, and then adding a digestion solution to digest the quicklime to obtain a crude calcium hydroxide slurry; 2) sending the crude calcium hydroxide slurry to a slaking process after grading and sieving, and performing slaking treatment; 3) removing impurity particles from the calcium hydroxide slurry after slaking, adjusting the concentration and temperature, adding a crystal form control agent, and sending the calcium hydroxide slurry to a carbonation process; 4) introducing carbon dioxide into the calcium hydroxide slurry to perform a carbonation reaction, until the conductivity of the reaction system reaches a minimum value, and then the reaction is continued until the conductivity reaches 2500-3000 µS / cm, to obtain a light calcium carbonate slurry; 5) centrifuging and dewatering the light calcium carbonate slurry, drying the light calcium carbonate slurry to a water content of 2.0%-3.5%, and then performing ultramicro-pulverization on the calcium carbonate lumps to a particle size of less than 75 µm, and then heating the calcium carbonate to 250-300°C and applying microwave irradiation, with a microwave frequency of 2350-2550 MHZ and a power density of 1.20-2 W / g of calcium carbonate, and after microwave irradiation for 5-15 min, a high-content light calcium carbonate is obtained.

2. The production method according to claim 1, characterized by, In step 1): The particle size of the limestone or calcite used is 15-20 cm; and / or, the calcium carbonate content of the limestone or calcite is ≥97.5% or the calcium oxide content is ≥54.5%, and the magnesium oxide content is ≤1.0%; and / or, the calcination temperature is 900-950°C, and the calcination time is 20-30 h; and / or, the calcination fuel is natural gas or anthracite; and / or, the mass ratio of the limestone or calcite to the calcination fuel is 100:8-10; and / or, the digestion solution is water; and / or, the mass ratio of the quicklime to the digestion solution is 1:5-7.

3. The preparation method according to claim 1, characterized in that, In step 2): The grading and sieving is performed in the following order: 60-80 mesh, 100-120 mesh, 180-200 mesh, and 325-350 mesh; and / or, the slaking time is 12-36 h.

4. The preparation method according to claim 3, characterized in that, The grading and sieving is performed in the following order: 80 mesh, 120 mesh, 200 mesh, and 325 mesh.

5. The preparation method according to claim 1, characterized in that, In step 3): The concentration is adjusted to 10%-15%, and the temperature is adjusted to 35-45°C; and / or, the crystal form control agent is hydrogen peroxide; and / or, the addition amount of the crystal form control agent is 0.5%-1% of the dry mass of the calcium hydroxide.

6. The method of claim 1, wherein, In step 4): The volumetric flow rate of the carbon dioxide atmosphere is 6000–7000 m³. 3 / h, carbon dioxide volume concentration 35.5%~37.5%.

7. The production method according to claim 1 or 6, characterized by, The carbonation reaction process in step 4) is as follows: The calcium hydroxide slurry is fed into a carbonation reactor, and is extracted from the bottom of the carbonation reactor by a circulating pump and introduced into a Venturi jet pipe, carbon dioxide is introduced into the Venturi jet pipe to mix with the calcium hydroxide slurry, and the mixture is injected into the carbonation reactor again to perform a reaction, and the process is repeated until the conductivity of the reaction system reaches a minimum value, and then the reaction is continued until the conductivity reaches 2500-3000 µS / cm, to end the carbonation reaction and obtain a light calcium carbonate slurry.

8. The method of claim 1, wherein, In step 5): The drying is performed until the water content is 2.5%-3.5%; and / or, the particle size of the broken particles is ≤75 µm; and / or, the heating time is 20-30 min.

9. The preparation method according to claim 8, characterized in that, In step 5): The drying is performed until the water content is 2.5%-3.0%.

10. The production method according to claim 1 or 8, characterized by, In step 5): The heating and microwave treatment are performed in a closed space and under negative pressure formed by external air extraction.

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