Whitening method for preparing precipitated calcium carbonate from high-content iron ore

Abstract

The invention belongs to the field of calcium carbonate, and discloses a whitening method for preparing precipitated calcium carbonate from high-content iron ore, calcium hydroxide slurry obtained by calcining a limestone raw material and carrying out a digestion reaction with water is used as a raw material, and the method comprises the steps of strengthened physical pre-iron removal, complexing conversion iron removal and adsorption separation. According to the method, the whitening effect is remarkable, the whiteness improving amplitude is large, and iron impurities in the high-content iron ore can be efficiently removed. The whiteness of a calcium carbonate product produced by adopting the technology can be improved by 3.0-4.5 units and is far higher than 0.5-1.5 units of a conventional technology, the whiteness of calcium carbonate prepared from the high-content iron ore can be improved to 97% or above, the requirements of the high-end field are met, and the problems of yellowing and greenness caused by Fe < 2 + > residues in a conventional reduction technology are thoroughly avoided; the nano Fe3O4 seed crystal is completely discharged along with magnetic field adsorption slag, no residue exists, the color phase and the color sense of the product are normal, and the appearance texture is remarkably improved.

Description

Technical Field

[0001] This invention belongs to the field of calcium carbonate, and specifically relates to a whitening method for preparing precipitated calcium carbonate from high-iron ore. Background Technology

[0002] For limestone ore with high iron content, after high-temperature calcination, the iron impurities in the ore are mainly converted into ferric oxide (Fe2O3), which appears brownish-red. However, after subsequent digestion and carbonization processes, some iron impurities are not effectively removed and remain as Fe. 3+ The form remains in the calcium carbonate product, resulting in low whiteness (usually below 92%), which cannot meet the needs of high-end applications.

[0003] Currently, the conventional calcium carbonate whitening technology in the industry mainly adopts a "reduction-complexation-filtration" process: sodium dithionite or thiourea dioxide is added to the digestion or carbonation liquid as a reducing agent to remove residual ferric ions (Fe3+). 3+ ) is reduced to ferrous ions (Fe) 2+ Then add tetrasodium EDTA as a complexing agent to react with Fe. 2+ A water-soluble complex is formed, and finally the complex is discharged with the filtrate through a pressure filtration process, thereby removing iron impurities.

[0004] However, this conventional technique has obvious limitations: first, the whitening effect is limited, with a whiteness increase of only 0.5-1.5 units, making it difficult to increase the whiteness of calcium carbonate prepared from high-iron ore to over 93%; second, the whiteness improvement is unnatural, and residual Fe remains after reduction. 2+ Its natural light green color will cause calcium carbonate products to have a slight greenish tinge, affecting the product's appearance and texture; thirdly, its complexation efficiency is not high, and EDTA tetrasodium's complexation with Fe... 2+ The complexation stability is limited, and some complexes are easily decomposed, resulting in iron impurities remaining. After long-term storage, the whiteness of the product is prone to decline. Fourth, the utilization rate of the reducing agent is low. Sodium hydrosulfite and thiourea dioxide are easily decomposed and ineffective in aqueous solution, requiring excessive addition, which increases production costs and easily introduces new impurities.

[0005] CN121181020A discloses a method for efficiently removing iron impurities from raw ore to prepare nano-calcium carbonate. In this method, during the slurry preparation process, Na2SO3 is used as a whitening agent to remove iron from the raw ore, and Na2SO3 acts as a reducing agent for Fe. 3+ Reduced to Fe 2+ Appropriate amounts of whitening agents, such as DTPA or triethanolamine, form colorless, hydrous iron chelates with iron ions, thereby achieving efficient iron removal. However, in this method, the residual ferrous chelates are easily oxidized during product drying, reverting to ferric ions.

[0006] CN119929859A discloses a method for preparing high-whiteness nano-calcium carbonate through rapid calcination at high temperature. This method uses one or more of MgSO4, Mg(NO3)2, MgCl2, and MgAc2 as a whitening agent, with an addition amount of approximately 2%. However, the introduction of magnesium ions not only leads to a decrease in product purity but also poses challenges to the control of crystal morphology and subsequent applications. Furthermore, this technology cannot fundamentally remove iron ions, and therefore cannot truly whiten ores with high iron content.

[0007] CN114604883A proposes a method for purifying and whitening calcium carbonate using a core-shell structure. This technique uses a slurry of calcium carbonate obtained by carbonation of low-whiteness calcium hydroxide slurry as raw material. First, an alkaline reagent is added to dissolve the silica present in the slurry. After aging at a constant temperature, the dissolved silicates coat the surface of the calcium carbonate, masking colored impurities and whitening the surface. Then, an acidic reagent is added to adjust the acidity and create a weakly alkaline environment. Aluminum salts are then slowly added, generating aluminum hydroxide under these weakly alkaline conditions. After drying, aluminum oxide is formed and coated onto the calcium carbonate surface a second time, resulting in high-whiteness calcium carbonate. However, this technique relies solely on silicate and alumina coatings to mask colored impurities and cannot truly remove iron ions. The whitening effect on high-iron ore is limited, and the coating layer is prone to yellowing after damage, and it can also introduce new impurities such as sulfates.

[0008] Therefore, developing an innovative whitening technology that achieves significant whitening effect, natural whiteness, thorough removal of iron impurities, and controllable cost has become a pressing technical challenge for the calcium carbonate industry, addressing the issue of residual iron impurities in the process of preparing calcium carbonate from high-iron ore. Summary of the Invention

[0009] The purpose of this invention is to overcome at least one deficiency of the prior art and to provide a whitening method for preparing precipitated calcium carbonate from high-iron ore.

[0010] The technical solution adopted in this invention is:

[0011] In a first aspect, the present invention provides a whitening method for preparing precipitated calcium carbonate from high-iron ore, using a calcium hydroxide slurry obtained by calcining limestone raw material and reacting it with water as the raw material, comprising the following steps:

[0012] Enhanced physical pre-iron removal: 0.03-0.08 wt% of nano-iron oxide seeds by dry weight of calcium hydroxide are added to the calcium hydroxide slurry for full adsorption and binding, and then magnetic field capture and pre-separation are carried out by a magnetic field device with a magnetic field strength of 0.3-0.6 T;

[0013] Iron removal by complexation conversion: Adjust the concentration of the calcium hydroxide slurry obtained in step 1) to 8-12 wt%, add 0.6-1.2 wt% of calcium hydroxide dry weight deferroamine B at 30-45℃ and continue the reaction for 40-60 min;

[0014] Adsorption separation: The calcium hydroxide slurry obtained in step 2) is fed into a chelating ion exchange resin fluidized bed and adsorbed in a countercurrent fluidization mode. The whitening process is completed after the adsorption is finished.

[0015] Specifically, the limestone raw material contains 0.3 to 1.0 wt% iron.

[0016] Specifically, in step 1), the average particle size of the nano-iron oxide seed crystals is 20–50 nm.

[0017] Specifically, in step 3), the chelating ion exchange resin is selected from aminophosphonic acid type or iminodiacetic acid type macroporous chelating resin.

[0018] Specifically, in step 3), the chelating ion exchange resin is selected from Lanxess Lewatit TP 207, Rohm and Haas Amberlite IRC 748, and Purolite Lewatit CNP 80.

[0019] Specifically, in step 3), the chelating ion exchange resin has a particle size of 0.3–1.2 mm and a specific surface area of ​​40–60 m². 2 / g.

[0020] Specifically, during the adsorption process in step 3), the fluidization rate is 1.2–2.0 m / h and the bed expansion rate is 30–50%.

[0021] Specifically, in step 3), the adsorption temperature is 30–45 °C and the residence time is 40–50 min.

[0022] Specifically, the magnetic field device in step 1) is selected from a pipeline magnetic field device.

[0023] Secondly, the whitening method described in the first aspect of this invention is applied in the preparation of lightweight micron-sized or nano-sized calcium carbonate from limestone raw materials.

[0024] The beneficial effects of this invention are:

[0025] 1. Significant whitening effect and substantial increase in whiteness: This invention abandons the traditional reduction process and uses iron-removing amine B as the core. Through a three-stage process of "nano-Fe3O4 seed modification and enhanced magnetic field pre-iron removal + specific complexation dissolution + fluidized bed deep adsorption," it efficiently removes iron impurities from high-iron-content ores. Calcium carbonate products produced using this technology can achieve a whiteness increase of 3.0–4.5 units, far exceeding the 0.5–1.5 units achieved by conventional technologies. It can increase the whiteness of calcium carbonate prepared from high-iron-content ores to over 97%, meeting the demands of high-end applications.

[0026] 2. Natural whiteness, no risk of yellowing or greening: Completely avoids the Fe... 2+ The residue caused by yellowing and greening is eliminated; the nano Fe3O4 seed crystals are completely removed by the magnetic field adsorbing the slag, leaving no residue. The product has normal color and texture, and the appearance and texture are significantly improved.

[0027] 3. Simple process and high controllability: Each process is easy to operate and control, and is suitable for the needs of continuous industrial production. Attached Figure Description

[0028] Figure 1 The images show a color comparison of the lightweight micron-sized calcium carbonate obtained in Example 2, Comparative Example 1, and Comparative Example 2. Detailed Implementation

[0029] The present invention provides a method for whitening calcium carbonate from high-iron ore, which specifically includes the following steps:

[0030] S1: Ore pretreatment and calcination

[0031] Limestone ore with an iron content of 0.3–1.0 wt% is selected as raw material, crushed to a particle size of 40–80 mm, and after removing surface soil and impurities, it is fed into a vertical kiln and calcined at 900–1100 ℃ to obtain calcium oxide.

[0032] S2: Digestion and Aging

[0033] The calcined calcium oxide is fed into a continuous digester, and digestion water is added at a mass ratio of 1:6 to 8 to carry out a continuous digestion reaction. The digested slurry is filtered through three vibrating screens of 60, 120, and 200 mesh to remove underburned particles and impurities. The slurry after sieving is sent to an aging tank and aged for 8 to 12 hours to fully mature the calcium hydroxide particles and make them uniform in size.

[0034] Furthermore, in step S2, the temperature of the digested water is controlled at 60–80 °C.

[0035] S3: Nano Fe3O4 seed modification + magnetic field pre-iron removal

[0036] Add 0.03–0.08 wt% of hydrophilic nano-Fe3O4 seed crystals to the aged calcium hydroxide slurry, based on the dry weight of calcium hydroxide. Turn on the stirring device and adjust the stirring speed to 200–300 r / min. Disperse at high speed for 15–30 min to allow the nano-Fe3O4 seed crystals to fully adsorb and combine with the weakly magnetic Fe2O3 particles in the slurry to form strongly magnetic Fe3O4-Fe2O3 composite particles.

[0037] The calcium hydroxide slurry with seed crystals is passed through a weak magnetic field device with a magnetic field strength of 0.3 to 0.6 T. The strongly magnetic Fe3O4-Fe2O3 composite particles in the slurry are efficiently captured and pre-separated by the magnetic field, thereby achieving enhanced physical pre-iron removal.

[0038] Furthermore, in step S3, the average particle size of the nano-iron oxide (Fe3O4) seed crystals is 20-50 nm;

[0039] Furthermore, in step S3, the pipeline-type weak magnetic field device uses a permanent magnet or an electromagnet, with a magnetic field gradient of 0.1 to 0.3 T / cm, to enhance the capture efficiency of Fe3O4-Fe2O3 composite particles.

[0040] S4: Concentration Adjustment and In-situ Complexation and Transformation of Iron Oxides

[0041] After magnetic field treatment, the calcium hydroxide slurry was adjusted to a concentration of 8–12 wt% and cooled to 30–45°C. Under stirring conditions, 0.6–1.2 wt% of deferoxamine B was added based on the dry weight of calcium hydroxide, and stirring was continued for 40–60 min to convert the solid ferric oxide in the slurry into a stable soluble ferric complex in situ.

[0042] Furthermore, in step S4, deferoxamine B is added in batches, in three separate additions of equal amount, with an interval of 10 minutes between each addition, and the stirring speed is controlled at 60–120 r / min.

[0043] S5: Chelating ion exchange resin fluidized bed deep adsorption separation

[0044] The calcium hydroxide slurry complexed in step S4 is fed into a chelating ion exchange resin fluidized bed and deep adsorption is performed using a countercurrent fluidization mode. The feed pump frequency is adjusted to control the fluidization rate at 1.2–2.0 m / h, so that the bed expansion rate is stabilized at 30–50%. The adsorption temperature is controlled at 30–45℃ and the residence time at 40–50 min. A porous distribution plate (pore size 0.5–1.0 mm, porosity 15–20%) is installed at the bottom of the fluidized bed, and a resin interception net (pore size 0.2 mm) is installed at the top to prevent resin loss.

[0045] Furthermore, the saturated resin is regenerated by countercurrent regeneration with 0.5–1.0 mol / L dilute hydrochloric acid at 40–50 °C for 60 min, rinsed until neutral, and then recycled.

[0046] Furthermore, in step S5, the chelating ion exchange resin, for Fe... 3+ It exhibits high selectivity and virtually does not adsorb Ca. 2+ To prevent calcium ion loss, aminophosphonic acid or iminodiacetic acid type macroporous chelating resins (such as Lanxess Lewatit TP 207, Rohm and Haas Amberlite IRC 748, and Purolite Lewatit CNP 80) should be preferred. The resin particle size should be 0.3–1.2 mm, and the specific surface area should be 40–60 m². 2 / g.

[0047] S6: Post-processing

[0048] The low-iron calcium hydroxide slurry obtained in step S5 is fed into a carbonization reactor. Depending on the target product type, the subsequent carbonization, post-treatment, and finished product preparation are completed using existing conventional processes.

[0049] Optionally, if light micron-sized calcium carbonate is to be prepared, the following method can be used: carbonation is carried out by directly introducing carbon dioxide until the pH of the slurry is 7.0 to 7.5, without surface modification. After carbonation, the slurry is dehydrated by pressure filtration, dried and pulverized in sequence to obtain a light micron-sized calcium carbonate product with a whiteness ≥98%.

[0050] Optionally, to prepare nano-calcium carbonate, the following method can be used: first, cool the slurry to 20-30℃, then use an existing carbonization process (such as adding an appropriate amount of crystal morphology control agent) to complete the carbonization until the pH of the slurry is 7.0-7.5, and add a suitable surface modifier (such as sodium stearate) during or after the carbonization process for modification. After the modification is completed, the product is sequentially dehydrated by pressure filtration, dried and pulverized to obtain nano-grade calcium carbonate with a whiteness ≥97%.

[0051] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below with reference to specific embodiments.

[0052] Example 1

[0053] S1: Ore pretreatment and calcination

[0054] Limestone ore with an iron content of 0.32wt% was selected as raw material, crushed to a particle size of 40-80 mm, and after removing surface soil and impurities, it was fed into a vertical kiln and continuously calcined at 950 ℃ to obtain calcium oxide.

[0055] S2: Digestion and Aging

[0056] The calcined calcium oxide is fed into a continuous digester, and digestion water at 70 ℃ is added at a mass ratio of 1:6 to ash water for continuous digestion. The digested slurry is filtered through three vibrating screens of 60, 120 and 200 mesh to remove under-burned particles and impurities. The slurry after sieving is sent to an aging tank for aging for 10 hours to fully mature the calcium hydroxide particles and make them uniform in size.

[0057] S3: Nano Fe3O4 seed modification + pipeline magnetic field pre-iron removal

[0058] Add 0.03 wt% hydrophilic nano-Fe3O4 (average particle size 30 nm) seed crystals to the aged calcium hydroxide slurry based on the dry weight of calcium hydroxide. Adjust the stirring speed to 250 r / min for high-speed dispersion for 15 min to allow the nano-Fe3O4 seed crystals to fully adsorb and combine with the weakly magnetic Fe2O3 particles in the slurry to form strongly magnetic Fe3O4-Fe2O3 composite particles. Then, pass the slurry through a pipeline weak magnetic field device with a magnetic field strength of 0.4 T and a magnetic field gradient of 0.2 T / cm to complete the enhanced physical pre-iron removal.

[0059] S4: Concentration Adjustment and In-situ Complexation and Transformation of Iron Oxides

[0060] The concentration of the magnetically treated calcium hydroxide slurry was adjusted to 8 wt%, cooled to 35 ℃, and 0.65 wt% of deferoxamine B was added (in three equal portions, 10 min apart) based on the dry weight of calcium hydroxide, under stirring at 60 r / min. The mixture was stirred continuously for 40 min to convert the solid ferric oxide in the slurry into a stable soluble ferric complex in situ.

[0061] S5: Chelating ion exchange resin fluidized bed deep adsorption separation

[0062] The complexed slurry was fed into a fluidized bed filled with Lanxess Lewatit TP 207 aminophosphonic acid macroporous chelating resin. Deep adsorption was carried out in countercurrent fluidization mode. The fluidization rate was controlled at 1.5 m / h by adjusting the feed pump frequency to keep the bed expansion rate stable at 35%. The adsorption temperature was controlled at 35 ℃ and the residence time was controlled at 45 min.

[0063] S6: Post-processing

[0064] After adsorption, the slurry temperature is reduced to 32 ℃, and carbon dioxide is directly introduced. Carbonation is carried out at 32 ℃ until the pH of the slurry is 7.2. After carbonation, the slurry is dehydrated by pressure filtration, dried and pulverized to obtain a light micron-sized calcium carbonate product.

[0065] Example 2

[0066] S1: Ore pretreatment and calcination

[0067] Limestone ore with an iron content of 0.61 wt% was selected as raw material, crushed to a particle size of 40-80 mm, and after impurities were removed, it was sent to a vertical kiln for continuous calcination at 1000 ℃ to obtain calcium oxide.

[0068] S2: Digestion and Aging

[0069] Calcium oxide was added to digestion water at a mass ratio of 1:7 (ash to water) at 75 °C, and digestion was completed in a continuous digester. The slurry was filtered through a three-stage vibrating screen and then sent to an aging tank for aging for 10 h.

[0070] S3: Nano Fe3O4 seed modification + pipeline magnetic field pre-iron removal

[0071] Add 0.05 wt% of hydrophilic nano-Fe3O4 (average particle size 40 nm) seed crystals to the aged calcium hydroxide slurry based on the dry weight of calcium hydroxide. Disperse the mixture at a high speed of 280 r / min for 20 min to form strongly magnetic Fe3O4-Fe2O3 composite particles. Then, pass the slurry through a pipeline weak magnetic field device with a magnetic field strength of 0.5 T and a magnetic field gradient of 0.25 T / cm to complete the enhanced physical pre-iron removal.

[0072] S4: Concentration Adjustment and In-situ Complexation and Transformation of Iron Oxides

[0073] The slurry concentration was then adjusted to 10 wt%, cooled to 40 ℃, and 0.92 wt% of deferroamine B was added (in three portions, 10 min apart) on a dry basis of calcium hydroxide while stirring at 90 r / min. The mixture was stirred continuously for 50 min.

[0074] S5: Chelating ion exchange resin fluidized bed deep adsorption separation

[0075] The slurry was fed into a fluidized bed filled with Rohm and Haas Amberlite IRC 748 iminodiacetic acid type macroporous chelating resin. The fluidization rate was controlled at 1.8 m / h, the bed expansion rate at 40%, the adsorption temperature at 40 ℃, and the residence time at 45 min to complete the deep adsorption.

[0076] S6: Post-processing

[0077] After adsorption, the slurry temperature drops to 36 ℃, and carbon dioxide is introduced at 36 ℃ to carbonize to pH 7.3. Subsequently, it is filtered, dried and pulverized to obtain a lightweight micron-sized calcium carbonate product.

[0078] Example 3

[0079] S1: Ore pretreatment and calcination

[0080] Limestone ore with an iron content of 0.98wt% was selected as raw material, crushed to a particle size of 40-80 mm, and after impurities were removed, it was sent to a vertical kiln for continuous calcination at 1050 ℃ to obtain calcium oxide.

[0081] S2: Digestion and Aging

[0082] Calcium oxide was added to 80 °C digestion water at a ratio of 1:8 (by mass of ash water) to complete continuous digestion. The slurry was filtered through a three-stage vibrating screen and then aged for 12 h.

[0083] S3: Nano Fe3O4 seed modification + pipeline magnetic field pre-iron removal

[0084] Add 0.08 wt% of hydrophilic nano-Fe3O4 (average particle size 50 nm) seed crystals to the aged calcium hydroxide slurry based on the dry weight of calcium hydroxide. Disperse the mixture at a high speed of 300 r / min for 30 min to form strongly magnetic Fe3O4-Fe2O3 composite particles. Then, pass the slurry through a pipeline weak magnetic field device with a magnetic field strength of 0.6 T and a magnetic field gradient of 0.3 T / cm to complete the enhanced physical pre-iron removal.

[0085] S4: Concentration Adjustment and In-situ Complexation and Transformation of Iron Oxides

[0086] The slurry concentration was then adjusted to 12 wt%, cooled to 45 ℃, and 1.18 wt% of deferoxamine B (added in 3 portions, 10 min apart) was added while stirring at 120 r / min, based on the dry weight of calcium hydroxide, and stirring was continued for 60 min.

[0087] S5: Chelating ion exchange resin fluidized bed deep adsorption separation

[0088] The slurry was fed into a fluidized bed filled with Lewatit CNP 80 aminophosphonic acid macroporous chelating resin. The fluidization rate was controlled at 2.0 m / h, the bed expansion rate at 50%, the adsorption temperature at 45 ℃, and the residence time at 50 min to complete the deep adsorption.

[0089] S6: Post-processing

[0090] After adsorption, the slurry temperature drops to 40 ℃, and carbon dioxide is introduced at 40 ℃ to carbonize to pH 7.4. Subsequently, it is filtered, dried and pulverized to obtain a lightweight micron-sized calcium carbonate product.

[0091] Example 4

[0092] S1: Ore pretreatment and calcination

[0093] Limestone ore with an iron content of 0.61 wt% was selected as raw material, crushed to a particle size of 40-80 mm, and after impurity removal, it was continuously calcined at 1000℃ to obtain calcium oxide.

[0094] S2: Digestion and Aging

[0095] Add 75℃ digestion water at a ash-to-water mass ratio of 1:7 to complete continuous digestion. After filtering through a three-stage sieve, the slurry is aged for 10 hours.

[0096] S3: Nano Fe3O4 seed modification + pipeline magnetic field pre-iron removal

[0097] Add 0.05 wt% of hydrophilic nano-Fe3O4 (average particle size 40 nm) seed crystals to the aged calcium hydroxide slurry based on the dry weight of calcium hydroxide. Disperse the slurry at high speed for 20 min at a stirring speed of 280 r / min to form strongly magnetic Fe3O4-Fe2O3 composite particles. Then, pass the slurry through a pipeline weak magnetic field device with a magnetic field gradient of 0.5 T and a magnetic field gradient of 0.25 T / cm to complete the enhanced physical pre-iron removal.

[0098] S4: Concentration Adjustment and In-situ Complexation and Transformation of Iron Oxides

[0099] The concentration was then adjusted to 10 wt%, cooled to 40 °C, and 0.92 wt% of deferoxamine B was added (in three portions, 10 min apart) while stirring at 90 r / min, and stirred for 50 min.

[0100] S5: Chelating ion exchange resin fluidized bed deep adsorption separation

[0101] The sample was then fed into a fluidized bed filled with Rohm and Haas Amberlite IRC 748 iminodiacetic acid type macroporous chelating resin. The fluidization rate was controlled at 1.8 m / h, the bed expansion rate at 40%, the adsorption temperature at 40 ℃, and the residence time at 45 min to complete the deep adsorption.

[0102] S6: Post-processing

[0103] The adsorbed slurry was cooled to 25 °C, and 0.3% (based on dry calcium carbonate) of citric acid was added as a crystal morphology control agent. Carbon dioxide was introduced at 25 °C to carbonize the slurry to pH 7.2. After carbonization, 3.5 wt% of sodium stearate was added for surface modification. After modification, the slurry was dehydrated by pressure filtration, dried, and pulverized to obtain the finished nano-calcium carbonate product.

[0104] Example 5

[0105] S1: Ore pretreatment and calcination

[0106] Limestone ore with an iron content of 0.82wt% was selected as raw material, crushed to a particle size of 40-80 mm, and after impurity removal, it was continuously calcined at 1020℃ to obtain calcium oxide.

[0107] S2: Digestion and Aging

[0108] Add 78 °C digestion water at a ash-to-water mass ratio of 1:8 to complete continuous digestion. The slurry is then filtered through a three-stage sieve and aged for 11 hours.

[0109] S3: Nano Fe3O4 seed modification + pipeline magnetic field pre-iron removal

[0110] Add 0.07 wt% of hydrophilic nano-Fe3O4 (average particle size 45 nm) seed crystals to the aged calcium hydroxide slurry based on the dry weight of calcium hydroxide. Disperse the mixture at a high speed of 300 r / min for 25 min to form strongly magnetic Fe3O4-Fe2O3 composite particles. Then, pass the slurry through a pipeline weak magnetic field device with a magnetic field gradient of 0.55 T and a magnetic field gradient of 0.28 T / cm to complete the enhanced physical pre-iron removal.

[0111] S4: Concentration Adjustment and In-situ Complexation and Transformation of Iron Oxides

[0112] The concentration was then adjusted to 11 wt%, cooled to 42 °C, and 1.05 wt% of deferoxamine B was added (in three portions, 10 min apart) while stirring at 100 r / min, and stirred for 55 min.

[0113] S5: Chelating ion exchange resin fluidized bed deep adsorption separation

[0114] The sample was then fed into a fluidized bed filled with Lanxess Lewatit TP 207 aminophosphonic acid macroporous chelating resin. The fluidization rate was controlled at 1.8 m / h, the bed expansion rate at 45%, the adsorption temperature at 40 ℃, and the residence time at 50 min to complete the deep adsorption.

[0115] S6: Post-processing

[0116] The adsorbed slurry was cooled to 28 ℃, and 0.5% (based on dry calcium carbonate) of sucrose was added as a crystal morphology control agent. Carbon dioxide was introduced at 28 ℃ to carbonize to pH 7.3. In the later stage of carbonization, 3.5wt% of sodium stearate was added for modification. After modification, the product was successively dehydrated by pressure filtration, dried and pulverized to obtain nano calcium carbonate product.

[0117] Comparative Example 1

[0118] Compared with Example 2, this comparative example did not undergo any whitening treatment and is a calcium carbonate product prepared from raw ore.

[0119] Comparative Example 2

[0120] Compared with Example 2, this comparative example does not perform step S3, nano-Fe3O4 seed modification + pipeline magnetic field pre-iron removal process; otherwise, it is the same as Example 2.

[0121] Comparative Example 3

[0122] Compared with Example 2, this comparative example does not perform the concentration adjustment and in-situ complexation conversion process of iron oxide in step S4, but is otherwise the same as Example 2.

[0123] Comparative Example 4

[0124] Compared with Example 2, this comparative example does not perform the chelating ion exchange resin fluidized bed deep adsorption separation process in step S5, but is otherwise the same as Example 2.

[0125] Comparative Example 5

[0126] This comparative example uses conventional whitening techniques, as detailed below:

[0127] Limestone ore with an iron content of 0.61 wt% was selected as raw material, crushed to a particle size of 40-80 mm, and after impurities were removed, it was sent to a vertical kiln for continuous calcination at 1000℃ to obtain calcium oxide.

[0128] Calcium oxide was added to digestion water at a mass ratio of 1:7 (ash to water) at 75 °C, and digestion was completed in a continuous digester. The slurry was filtered through a three-stage vibrating screen and then sent to an aging tank for aging for 10 h.

[0129] Then add 0.2% (based on dry calcium carbonate, the same below) of sodium dithionite, stir for 60 minutes, then add 0.2% of triethanolamine and stir for 30 minutes. After cooling to 36°C, carbon dioxide is introduced to carbonize to pH 7.3. Subsequently, the product is obtained by pressure filtration, drying and pulverizing.

[0130] Comparative Example 6

[0131] Compared with Example 4, the iminodiacetic acid type macroporous chelating resin in the fluidized bed in step S5 was replaced with ordinary cation exchange resin (Zhengguang 001×7 strong acid styrene cation exchange resin), and the rest was the same as in Example 4.

[0132] Comparative Example 7

[0133] The difference between this comparative example and Example 2 lies in the order of complexation transformation and magnetic field iron removal. The specific steps are as follows:

[0134] S1: Ore pretreatment and calcination

[0135] Limestone ore with an iron content of 0.61 wt% was selected as raw material, crushed to a particle size of 40-80 mm, and after impurities were removed, it was sent to a vertical kiln for continuous calcination at 1000 ℃ to obtain calcium oxide.

[0136] S2: Digestion and Aging

[0137] Calcium oxide was added to digestion water at a mass ratio of 1:7 (ash to water) at 75 °C, and digestion was completed in a continuous digester. The slurry was filtered through a three-stage vibrating screen and then sent to an aging tank for aging for 10 h.

[0138] S3: In-situ complexation and transformation of iron oxides

[0139] Adjust the concentration of the aged calcium hydroxide slurry to 10 wt%, cool it to 40 ℃, and add 0.92 wt% of deferoxamine B (in 3 portions, 10 min apart) based on the dry weight of calcium hydroxide while stirring at 90 r / min, and continue stirring for 50 min.

[0140] S4: Pipeline magnetic field iron removal

[0141] To the calcium hydroxide slurry after the complexation reaction, 0.05 wt% of hydrophilic nano-iron oxide (Fe3O4) seed crystals (average particle size 40 nm) were added based on the dry weight of calcium hydroxide. The stirring speed was adjusted to 280 r / min for high-speed dispersion for 20 min to form composite particles. Then, the slurry was separated by magnetic field using a pipeline weak magnetic field device with a magnetic field strength of 0.5 T and a magnetic field gradient of 0.25 T / cm.

[0142] S5: Chelating ion exchange resin fluidized bed deep adsorption separation

[0143] The slurry was fed into a fluidized bed filled with Rohm and Haas Amberlite IRC 748 iminodiacetic acid type macroporous chelating resin. The fluidization rate was controlled at 1.8 m / h, the bed expansion rate at 40%, the adsorption temperature at 40 ℃, and the residence time at 45 min to complete the deep adsorption.

[0144] S6: Post-processing

[0145] After adsorption, the slurry temperature drops to 36 ℃, and carbon dioxide is introduced at 36 ℃ to carbonize to pH 7.3. Subsequently, it is filtered, dried and pulverized to obtain a lightweight micron-sized calcium carbonate product.

[0146] Analysis and Summary

[0147] To verify the effectiveness of the whitening method of the present invention, the calcium carbonate products prepared in Examples 1-5 and Comparative Examples 1-7 were subjected to performance testing. The testing methods strictly followed relevant industry standards. The specific testing methods and performance results are summarized below.

[0148] The testing method is as follows:

[0149] Iron content (Fe2O3 residue): determined by inductively coupled plasma optical emission spectrometry (ICP-OES) according to GB / T 19281-2014 "Analytical Methods for Calcium Carbonate", with a detection accuracy of 0.001wt%.

[0150] Blue light whiteness: The whiteness of the blue light was measured using a whiteness meter according to GB / T 23774-2009 "General Method for Determination of Whiteness of Inorganic Chemical Products" under the following conditions: D65 light source and 10° field of view.

[0151] LAB colorimetry: The colorimetry is determined using GB / T 1543-2005 "Determination of opacity (paper backing) of paper and paperboard (diffuse reflectance method)" and a colorimeter. L* represents brightness (the larger the value, the brighter the brightness), a* represents red-green deviation (negative value is green, positive value is red), and b* represents yellow-blue deviation (negative value is blue, positive value is yellow).

[0152] The specific results are shown in Table 1 below:

[0153] Table 1

[0154]

[0155] like Figure 1 The experimental results in Table 1 demonstrate that the three-stage combined process of "nano-Fe3O4 seed modification with enhanced magnetic field pre-removal + specific complexation dissolution + fluidized bed deep adsorption" proposed in this invention exhibits significant advantages in iron removal and whitening of calcium carbonate. As shown in Examples 1-5, the residual Fe2O3 content of the treated products was strictly controlled below 0.010 wt% (reaching a minimum of 0.006 wt%), and the blue light whiteness remained stable in the range of 97.6%–98.5%.

[0156] In contrast, Comparative Example 5, which uses conventional whitening technology with sodium dithionite, had a high Fe2O3 residue of 0.034 wt%, a blue light whiteness of only 94.0%, and a significant deviation in hue ab value, failing to achieve a "natural white" effect. This confirms that conventional chemical reduction whitening alone is insufficient to overcome the bottleneck of removing trace iron impurities, while the novel combined process achieves deep removal of iron impurities and a comprehensive leap in optical performance through the synergistic effect of physical and chemical processes.

[0157] The residual Fe2O3 content in Comparative Example 2 (omitting step S3) (0.021 wt%) was significantly higher than that in Example 2 (0.008 wt%), and the whiteness was significantly reduced. This indicates that after adding hydrophilic nano-Fe3O4 seeds, under high-speed dispersion, the seeds adsorb and encapsulate the surface of weakly magnetic Fe2O3 particles through surface energy and van der Waals forces, forming strongly magnetic composite aggregates (Fe3O4-Fe2O3) with a high specific magnetic susceptibility. This process transforms extremely fine weakly magnetic impurities into strongly magnetic targets that can be directly intercepted by a 0.4–0.6 T pipeline magnetic field, significantly reducing the absolute iron content in the matrix and alleviating the burden on subsequent chemical refining.

[0158] The iron removal effect of Comparative Example 3 was severely deteriorated, with the Fe2O3 residue rebounding to 0.030 wt%, indicating that physical methods alone cannot solve the problem of extremely fine iron impurities encapsulated in intergranular spaces or micropores. Desferrioxamine B, as a specific iron chelating agent, can convert insoluble iron oxide into water-soluble complexes in situ. This step successfully achieved the transfer of the target impurity from the "solid phase" to the "liquid phase."

[0159] Complete separation of free iron complexes in the liquid phase is crucial to ensuring product purity. By comparing Example 2 with Comparative Example 4, and Example 4 with Comparative Example 6, the specificity and irreplaceability of chelating resins in deep iron removal can be clearly demonstrated.

[0160] Colorimeter data (L*, a*, b*) provide intuitive optical evidence for the impurity removal mechanism. Iron oxides mainly contribute to the yellow and dark background phases in inorganic powders. The L* values ​​(98.4–99.2) of the example products are systematically higher than those of the comparative examples (96.2–97.0), verifying that absorbing impurities are largely eliminated. The b* values ​​of the examples drop sharply to the 0.25–0.33 range, while those of the comparative examples generally remain between 0.86 and 1.07. This abrupt change in b* values ​​directly confirms that Fe2O3 and its hydrates, which induce the yellow phase, have been deeply stripped away. The a* values ​​of the samples in each group of examples fluctuate very little, indicating that the purification system has good chemical inertness and does not introduce byproducts that induce other color deviations.

[0161] In summary, this study developed a novel combined refining process of "nano-Fe3O4 seed modification with enhanced magnetic field pre-removal of iron + specific complexation dissolution + fluidized bed deep adsorption". This process effectively overcomes the technical barrier of removing trace iron impurities in the preparation of high-purity calcium carbonate.

[0162] The above is a further detailed description of the present invention and should not be considered as a limitation on the specific implementation of the present invention. For those skilled in the art, simple deductions or substitutions without departing from the concept of the present invention are all within the protection scope of the present invention.

Claims

1. A whitening method for preparing precipitated calcium carbonate from high-iron ore, characterized in that, Using calcium hydroxide slurry obtained from calcined limestone and digestion reaction with water as raw material, the process includes the following steps: 1) Enhanced physical pre-iron removal: 0.03-0.08 wt% of nano-iron oxide seeds by dry weight of calcium hydroxide are added to the calcium hydroxide slurry for full adsorption and binding, and then magnetic field capture and pre-separation are carried out by a magnetic field device with a magnetic field strength of 0.3-0.6 T; 2) Complexation and iron removal: Adjust the concentration of the calcium hydroxide slurry obtained in step 1) to 8-12 wt%, add 0.6-1.2 wt% of calcium hydroxide dry weight iron-removing amine B at 30-45℃ and continue the reaction for 40-60 min; 3) Adsorption and separation: The calcium hydroxide slurry obtained in step 2) is fed into a chelating ion exchange resin fluidized bed and adsorbed in a countercurrent fluidization mode. The whitening process is completed after the adsorption is completed.

2. The whitening method according to claim 1, characterized in that, The limestone raw material contains 0.3 to 1.0 wt% iron.

3. The whitening method according to claim 1, characterized in that, The average particle size of the nano-iron oxide seed crystals in step 1) is 20–50 nm.

4. The whitening method according to claim 1, characterized in that, In step 3), the chelating ion exchange resin is selected from aminophosphonic acid type or iminodiacetic acid type macroporous chelating resin.

5. The whitening method according to claim 4, characterized in that, In step 3), the chelating ion exchange resin is selected from Lanxess Lewatit TP 207, Rohm and Haas Amberlite IRC 748, and Purolite Lewatit CNP 80.

6. The whitening method according to claim 1, characterized in that, In step 3), the chelating ion exchange resin has a particle size of 0.3–1.2 mm and a specific surface area of ​​40–60 m². 2 / g.

7. The whitening method according to claim 1, characterized in that, During the adsorption process in step 3), the fluidization rate is 1.2–2.0 m / h and the bed expansion rate is 30–50%.

8. The whitening method according to claim 1, characterized in that, During the adsorption process in step 3), the adsorption temperature is 30–45 °C and the residence time is 40–50 min.

9. The whitening method according to claim 1, characterized in that, In step 1), the magnetic field device is selected from a pipe-type magnetic field device.

10. The application of the whitening method according to claim 1 in the preparation of lightweight micron-sized calcium carbonate or nano-calcium carbonate from limestone raw materials.

Images