A method for batching highly reducing pellets based on secondary iron-bearing resources

Abstract

This invention relates to the field of metallurgical pelletizing technology, and discloses a method for batching highly reducible pellets based on secondary iron-bearing resources. The method includes the following steps: 80.0 to 84.77 parts by weight of secondary iron-bearing resources, 13.0 to 16.25 parts by weight of iron concentrate, and 1.5 to 2.5 parts by weight of light-calcined magnesia are mixed at high shear to obtain a mixed dry powder; the mixed dry powder is fed into a pelletizer, and a calcium-boron double-crosslinked lignin hydrosol containing 0.73 to 1.25 parts by weight of dry basis components is sprayed to obtain green pellets; the green pellets are then sequentially subjected to hot air flow drying, preheating treatment, and solid-phase roasting to obtain pellets, which are then cooled. In this invention, the hydrosol forms a physical barrier layer at the interface of the light-calcined magnesia particles, slowing down the penetration of free water molecules into the interior and inhibiting the volume expansion caused by magnesia hydration; the residual boron ions from solid-phase roasting decompose into the iron mineral crystals penetrate into the interior, causing lattice distortion and reducing the reduction activation energy.

Description

Technical Field

[0001] This invention relates to the field of metallurgical pelleting technology, specifically to a method for pelleting highly reducible ore pellets based on secondary iron-bearing resources. Background Technology

[0002] In the iron and steel smelting process, pelletizing and roasting various iron-containing secondary resources such as iron-containing dust, sludge, and waste residue is an important way to realize the recycling of iron resources. To improve the metallurgical properties of the finished pellets, light-burned magnesium oxide is usually added during the pelletizing process to adjust the composition. However, in actual production, the secondary iron-containing resources have complex compositions and poor hydrophilicity, resulting in a low pelletizing rate. Simultaneously, the added light-burned magnesium oxide easily undergoes a hydration reaction with free water during the pelletizing and preheating drying stages to form magnesium hydroxide, causing internal volume expansion, leading to cracks or even structural damage inside the green pellets, reducing their compressive strength and thermal stability.

[0003] On the other hand, secondary iron-bearing resources contain a high amount of gangue, which easily generates a low-melting-point liquid phase during the high-temperature solid-phase roasting stage. This low-melting-point liquid phase flows at high temperatures, filling and blocking the micropores inside the pellets, hindering the diffusion of reducing gases. Furthermore, the dense and stable crystal structure of iron minerals after conventional roasting and consolidation leads to a higher activation energy required for subsequent reduction reactions. These factors result in a decrease in the porosity and reduced permeability of the final pellets, failing to meet the technological requirements of modern blast furnace smelting for highly reducible pellets. In addition, traditional pelletizing processes often use sodium-based bentonite or sodium-based organic polymers as binders, introducing large amounts of alkali metal elements such as sodium and potassium into the pellets. This can easily lead to furnace lining erosion and coke deterioration during subsequent blast furnace smelting, increasing the alkali metal load on the blast furnace.

[0004] Therefore, the purpose of this invention is to provide a method for batching highly reducible pellets based on secondary iron-bearing resources, in order to overcome the shortcomings of the prior art. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a high-reducibility pelleting method based on secondary iron-bearing resources. This method solves the problems of easy hydration and volume expansion of lightly burned magnesium oxide during pelleting, which leads to the destruction of the green pellet structure, and the low reduction performance of the finished pellets due to the blockage of micropores by the low-melting-point liquid phase and the dense iron mineral crystals during the roasting stage.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A method for batching highly reducible pellets based on secondary iron-bearing resources includes the following steps: By weight, 80.0 to 84.77 parts of secondary iron-bearing resources, 13.0 to 16.25 parts of high-grade iron concentrate, and 1.5 to 2.5 parts of lightly calcined magnesia are mixed under high shear in a dry state to obtain a mixed dry powder. The mixed dry powder is continuously fed into a pelletizing machine, and a calcium boron double crosslinked lignin hydrosol containing 0.73 to 1.25 parts of dry base components is dynamically and uniformly atomized and sprayed onto the surface of the mixed dry powder to form pellets, thus obtaining green pellets. The raw balls are sequentially laid out for hot air flow drying and preheating treatment; Preheated green pellets are subjected to solid-phase roasting to obtain pellet ore; The pellet ore is cooled and discharged into the warehouse.

[0007] By adopting the above technical solution, and using the above-mentioned proportions of secondary iron-bearing resources, iron concentrate, and lightly calcined magnesia, along with calcium-boron double-crosslinked lignin hydrosol as a pelletizing binder and modifier, the thermal stability of green pellets and the reducibility of finished pellets are improved. The innovative reaction mechanism and process of this invention are described in the following steps: In the first step, during the pelletizing stage, the calcium-boron double-crosslinked lignin hydrosol, possessing a three-dimensional network structure, is evenly coated onto the surface of the mixed dry powder particles through atomized spraying. The hydrosol forms a physical barrier layer at the interface of the light-burned magnesium oxide particles. This physical barrier layer reduces the penetration rate of external free water molecules into the interior of the light-burned magnesium oxide, thereby inhibiting the volume expansion caused by the rapid hydration of magnesium oxide to form magnesium hydroxide, ensuring the structural integrity and compressive strength of the green pellets during the drying and preheating stages.

[0008] In the second step, during the solid-state roasting stage, as the temperature rises, the organic components in the hydrosol decompose and volatilize, and the remaining boron ions penetrate and dissolve into the iron mineral crystals. The introduction of boron ions causes lattice distortion in the iron minerals, altering the original crystal structure and thus reducing the reduction activation energy required for the subsequent reduction reaction.

[0009] The third step involves lightly calcined magnesium oxide participating in solid-phase reactions and reconstructing the micropores within the pellets during high-temperature roasting. Magnesium oxide can increase the melting point of the material system, inhibit the flow of low-melting-point liquid phases generated from gangue components in secondary iron-bearing resources, prevent low-melting-point liquid phases from filling and blocking reducing gas diffusion pores, and ensure that the pellets have good porosity and reducing gas permeability.

[0010] Preferably, the amount of secondary iron-containing resources is 82.01 parts, the amount of high-grade iron concentrate is 15.0 parts, the amount of light-burned magnesium oxide is 2.0 parts, and the amount of dry basis components of calcium-boron double-crosslinked lignin hydrosol is 0.99 parts.

[0011] By adopting the above technical solution, under the above specific proportions, the bulk density of the material and the interfacial bonding force reach a relatively balanced state, the pelletizing rate of green pellets is the highest, and the roasting consolidation strength and the micro porosity of the finished pellet ore are taken into account.

[0012] Preferably, the calcium-boron double-crosslinked lignin hydrosol is prepared in advance, and the preparation steps include: By weight, 80.0 to 90.0 parts of deionized water are injected into the reactor and heated and kept at a constant temperature; Turn on the mechanical stirrer and add 0.8 to 1.5 parts of boric acid to fully dissolve and form a solution system; Slowly add 0.5 to 1.0 parts of calcium hydroxide powder to adjust and stabilize the pH of the solution system; add 6.0 to 10.0 parts of calcium lignosulfonate at a uniform rate for continuous reaction to obtain calcium-boron double crosslinked lignin hydrosol.

[0013] By adopting the above technical solution, the preliminary preparation path of the adhesive was clarified. Inorganic ions were used to crosslink and modify organic polymers, and a composite hydrosol with both adhesive properties and modified ions was prepared.

[0014] Preferably, when preparing calcium-boron double-crosslinked lignin hydrosol, the process parameters are controlled as follows: the heating and constant temperature is 60 to 75°C; the mechanical stirring speed is set to 150 to 200 rpm; the pH value of the stable solution system is controlled to be 9.0 to 10.0; and the continuous reaction time after adding calcium lignin sulfonate is 30 to 45 minutes.

[0015] By adopting the above technical solution, the thermodynamic temperature, shear kinetic rotation speed, and pH are controlled within the set parameter window, which can ensure that each reaction group is fully decomposed and prevent local high concentrations from causing polymer agglomeration and precipitation.

[0016] Preferably, during the preparation of calcium-boron double-crosslinked lignin hydrosol, calcium lignin sulfonate is transformed into a network structure.

[0017] The transformation mechanism of the internal microstructure of the hydrosol using the above technical solution is explained as follows: Boric acid dissociates into tetrahydroxyborate ions in a weakly alkaline aqueous solution. Upon addition of calcium lignosulfonate, the active functional groups in the calcium lignosulfonate molecule undergo esterification condensation with the tetrahydroxyborate ions, constructing a covalent cross-linked network. Simultaneously, calcium ions released from the dissolution of calcium hydroxide and anionic groups in calcium lignosulfonate undergo charge neutralization and coordination, forming ionic cross-links. Under the dual cross-linking action of covalent and ionic bonds, the linear polymer of calcium lignosulfonate, originally free in the aqueous phase, undergoes conformational transformation and physical interweaving, transforming into a three-dimensional network structure exhibiting pseudoplastic fluid shear-thinning characteristics. This three-dimensional network structure exhibits high viscosity and maintains suspension stability under static conditions, while its viscosity decreases under the shear force of atomized spraying, facilitating uniform spreading on the material surface.

[0018] Preferably, the high-shear mixing time is 2 to 3 minutes.

[0019] By adopting the above technical solution, it is possible to ensure that secondary iron-bearing resources, iron concentrate and lightly calcined magnesia are uniformly dispersed at the microscale, avoiding fluctuations in pellet moisture and uneven roasting intensity caused by local component enrichment.

[0020] Preferably, in the pelletizing step, the overall moisture content of the green pellets is controlled to be 8.0% to 9.0%; the particle size of the green pellets is controlled to be between 10 and 16 mm.

[0021] By adopting the above technical solution, the moisture content and particle size can be controlled within the specified range, ensuring that the green pellets have sufficient capillary force and compressive strength to meet the mechanical load requirements of subsequent multi-layer fabric and hot air flow drying process.

[0022] Preferably, in the step of sequentially feeding the green pellets into a hot air flow drying and preheating treatment, the green pellets sequentially pass through a hot air flow drying section with a temperature of 150 to 300°C and a preheating section with a temperature controlled at 300 to 600°C; the heating rate of the preheating section is controlled at 15 to 20°C / minute.

[0023] By adopting the above technical solution, the gradient heating mode promotes the gradual removal of free water and bound water inside the green pellets, avoiding the rapid crusting on the surface that leads to violent expansion of internal water vapor and causes bursting, thereby improving the pelleting rate during the preheating stage.

[0024] Preferably, the process parameters for solid-phase roasting are: the maximum temperature of the rotary kiln roasting zone is controlled at 1180 to 1210°C, and the roasting time is 15 to 25 minutes.

[0025] By adopting the above technical solution, under the aforementioned temperature and time conditions, the iron mineral grains can grow sufficiently and become linked, and the lightly sintered magnesium oxide can be completely dissolved or form a high-temperature stable phase, thus retaining sufficient reduction pores while completing densification sintering.

[0026] Preferably, the cooling step includes: feeding the pellets into an annular cooler and cooling them to no higher than 150°C by forced air blowing from the bottom.

[0027] By adopting the above technical solution, forced convection heat exchange using air is used to rapidly cool the high-temperature pellets to a safe temperature that meets the requirements for subsequent belt conveyor transportation, thus completing the closed loop of the entire batching and consolidation process.

[0028] This invention provides a method for batching highly reducing pellets based on secondary iron-bearing resources. It has the following beneficial effects: 1. This invention establishes a pelletizing step by combining calcium-boron double-crosslinked lignin hydrosol with lightly calcined magnesium oxide. The hydrosol forms a physical barrier layer at the interface of the lightly calcined magnesium oxide particles, which slows down the penetration of external free water molecules into the interior of the lightly calcined magnesium oxide and inhibits the volume expansion caused by the rapid hydration of magnesium oxide to form magnesium hydroxide. This ensures the structural integrity and compressive strength of the green pellets during the drying and preheating stages.

[0029] 2. This invention utilizes calcium-boron double-crosslinked lignin hydrosol to decompose residual boron ions during the solid-phase roasting stage, promoting the penetration and solidification of boron ions into the interior of iron mineral crystals, causing lattice distortion in iron minerals, changing the original crystal structure, reducing the reduction activation energy required for subsequent reduction reactions, and improving the reduction performance of finished pellets.

[0030] 3. This invention increases the melting point of the material system by adding lightly calcined magnesium oxide to participate in the solid-phase reaction and reconstruct the micropores inside the pellets. This inhibits the flow of low-melting-point liquid phase generated by gangue components in secondary iron-bearing resources, prevents the low-melting-point liquid phase from filling and blocking the reducing gas diffusion pores, and ensures the porosity and reducing gas permeability of the finished pellets.

[0031] 4. This invention adopts a fully calcium-based pre-complexed organic pelletizing formula, which cuts off the introduction of exogenous alkali metal elements such as sodium and potassium from the source, reduces the alkali metal engineering load of the finished pellets, and is conducive to the long-term operation of blast furnaces. Attached Figure Description

[0032] Figure 1 This is a rheological curve of the apparent viscosity of each test sample in Test Example 1 of the present invention as a function of shear rate. Figure 2 The volume expansion rate of each group of green pellets in Test Example 2 of this invention changes over time under constant temperature and high humidity conditions; Figure 3The figures show a comparison of the physical and mechanical strength of each group of green pellets and finished pellets in Test Example 3 of the present invention. (a) is a comparison of the drop strength of green pellets, (b) is a comparison of the compressive strength of green pellets, and (c) is a comparison of the compressive strength of finished pellets. Figure 4 This is a comparison chart of the metallurgical reduction properties of the pellets from each component in Test Example 4 of this invention; Figure 5 This is a comparison chart of the bursting temperature of green pellets and the alkali metal load of the finished product in each group of test examples 5 of the present invention. Detailed Implementation

[0033] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the preparation examples, examples, comparative examples, and test examples. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0034] Preparation Examples 1-3: Preparation Example 1: This preparation example provides a method for batching highly reducing pellets based on secondary iron-bearing resources, including the following steps: 80.0 parts of deionized water were injected into a reactor equipped with a heating jacket and a mechanical stirrer, and heated and kept constant at 60°C. The mechanical stirrer was turned on at 150 rpm, and 0.8 parts of boric acid were added and fully dissolved. 0.5 parts of calcium hydroxide powder were slowly added. Since calcium hydroxide is slightly soluble in water and will continuously participate in the neutralization reaction, the actual consumption and dissociation state of calcium hydroxide were monitored in real time using an online pH meter to adjust and stabilize the pH of the solution at 9.0. Under the thermodynamic conditions of maintaining 60°C and a pH of 9.0, 6.0 parts of calcium lignin sulfonate were added at a uniform rate, and the reaction was continuously carried out at a shear stirring speed of 150 rpm for 30 minutes. After the reaction, the linear polymer was transformed into a network structure, yielding a high-viscosity calcium-boron double-crosslinked lignin hydrosol. The generated hydrosol was pumped into an insulated storage tank for constant temperature storage.

[0035] Preparation Example 2: This preparation example provides a method for batching highly reducing pellets based on secondary iron-bearing resources, including the following steps: 85.0 parts of deionized water were injected into a reactor equipped with a heating jacket and a mechanical stirrer, and heated and kept at a constant temperature of 68°C. The mechanical stirrer was turned on at 175 rpm, and 1.15 parts of boric acid were added and fully dissolved. 0.75 parts of calcium hydroxide powder were slowly added, and the pH of the solution was adjusted and stabilized at 9.5 using an online pH meter for real-time monitoring. Under the thermodynamic conditions of maintaining 68°C and a pH of 9.5, 8.0 parts of calcium lignosulfonate were added at a uniform rate, and the reaction was continuously carried out at a shear stirring speed of 175 rpm for 38 minutes. After the reaction, a high-viscosity calcium-boron double-crosslinked lignin hydrosol was obtained. The generated hydrosol was pumped into an insulated storage tank for constant temperature storage and later use.

[0036] Preparation Example 3: This preparation example provides a method for batching highly reducing pellets based on secondary iron-bearing resources, including the following steps: 90.0 parts of deionized water were injected into a reactor equipped with a heating jacket and a mechanical stirrer, and heated and kept constant at 75°C. The mechanical stirrer was turned on at 200 rpm, and 1.5 parts of boric acid were added and fully dissolved. 1.0 part of calcium hydroxide powder was slowly added, and the pH of the solution was adjusted and stabilized at 10.0 using an online pH meter for real-time monitoring. Under the thermodynamic conditions of maintaining 75°C and a pH of 10.0, 10.0 parts of calcium lignosulfonate were added at a uniform rate, and the reaction was continuously carried out at a shear stirring speed of 200 rpm for 45 minutes. After the reaction, a high-viscosity calcium-boron double-crosslinked lignin hydrosol was obtained. The generated hydrosol was pumped into an insulated storage tank for constant temperature storage and later use.

[0037] Examples 1-3: Example 1: This embodiment provides a method for batching highly reducible pellets based on secondary iron-bearing resources, including the following steps: 84.77 parts of secondary iron-containing resources, 13.0 parts of high-grade iron concentrate, and 1.5 parts of lightly calcined magnesia were fed into a high-intensity mixer and subjected to high-shear mixing for 2 minutes under dry conditions to obtain a mixed dry powder. The mixed dry powder was continuously fed into a disc pelletizer, and 8.73 parts of the calcium-boron double-crosslinked lignin hydrosol obtained in Preparation Example 1 (0.73 parts on a dry basis, containing 8.0 parts of moisture) were dynamically and uniformly sprayed onto the surface of the tumbling material through a high-pressure atomizing nozzle. The moisture content of the green pellets was controlled at 8.0%, and qualified green pellets with a particle size of 10 to 16 mm were obtained. The qualified green pellets were placed on a chain grate machine trolley and passed sequentially through a 150°C hot air through-flow drying section and a preheating section. The temperature of the preheating section was controlled at 300°C, and the heating rate was controlled at 15°C / min. The preheated pellets are fed into a rotary kiln for solid-phase roasting in an oxidizing atmosphere. The maximum temperature of the roasting zone in the rotary kiln is controlled at 1180℃, and the roasting is maintained at this temperature for 15 minutes. The finished roasted pellets are then fed into an annular cooler and cooled to below 150℃ by forced air blowing from the bottom before being discharged into a storage bin.

[0038] Example 2: This embodiment provides a method for batching highly reducible pellets based on secondary iron-bearing resources, including the following steps: 82.01 parts of secondary iron-containing resources, 15.0 parts of high-grade iron concentrate, and 2.0 parts of lightly calcined magnesia were fed into a high-intensity mixer and subjected to high-shear mixing for 2.5 minutes under dry conditions to obtain a mixed dry powder. The mixed dry powder was continuously fed into a disc pelletizer, and 9.49 parts of the calcium-boron double-crosslinked lignin hydrosol obtained in Preparation Example 2 (0.99 parts on a dry basis, containing 8.5 parts of moisture) were dynamically and uniformly sprayed onto the surface of the tumbling material through a high-pressure atomizing nozzle. The moisture content of the green pellets was controlled at 8.5%, and qualified green pellets with a particle size of 10 to 16 mm were obtained. The qualified green pellets were placed on a chain grate machine trolley and passed sequentially through a 220°C hot air through-flow drying section and a preheating section. The temperature of the preheating section was controlled at 450°C, and the heating rate was controlled at 18°C / min. The preheated pellets are fed into a rotary kiln for solid-phase roasting in an oxidizing atmosphere. The maximum temperature of the roasting zone in the rotary kiln is controlled at 1195℃, and the roasting is maintained at this temperature for 20 minutes. The finished roasted pellets are then fed into an annular cooler and cooled to below 150℃ by forced air blowing from the bottom before being discharged into a storage bin.

[0039] Example 3: This embodiment provides a method for batching highly reducible pellets based on secondary iron-bearing resources, including the following steps: 80.0 parts of secondary iron-containing resources, 16.25 parts of high-grade iron concentrate, and 2.5 parts of lightly calcined magnesia were fed into a high-intensity mixer and subjected to high-shear mixing for 3 minutes under dry conditions to obtain a mixed dry powder. The mixed dry powder was continuously fed into a disc pelletizer, and 10.25 parts of the calcium-boron double-crosslinked lignin hydrosol obtained in Preparation Example 3 (equivalent to 1.25 parts on a dry basis, containing 9.0 parts of moisture) were dynamically and uniformly sprayed onto the surface of the tumbling material through a high-pressure atomizing nozzle, controlling the moisture content of the green pellets to 9.0%, and obtaining qualified green pellets with a particle size of 10 to 16 mm. The qualified green pellets were placed on a chain grate machine trolley and passed sequentially through a 300°C hot air through-flow drying section and a preheating section. The temperature of the preheating section was controlled at 600°C, and the heating rate was controlled at 20°C / min. The preheated pellets are fed into a rotary kiln for solid-phase roasting in an oxidizing atmosphere. The maximum temperature of the roasting zone in the rotary kiln is controlled at 1210℃, and the roasting is maintained at this temperature for 25 minutes. The finished roasted pellets are then fed into an annular cooler and cooled to below 150℃ by forced air blowing from the bottom before being discharged into a storage bin.

[0040] Comparative Examples 1-5: Comparative Example 1: Compared with Example 2, the difference is that: no lightly calcined magnesium oxide and calcium boron double crosslinked lignin hydrosol are added; 2.0 parts of sodium-based bentonite are mixed with secondary iron-containing resources and high-grade iron concentrate; the amount of secondary iron-containing resources added is increased by 0.99 parts to make up the total dry material content; only 8.5 parts of clean water are sprayed during pelleting; and the rest of the operation remains the same.

[0041] Comparative Example 2: Compared with Example 2, the difference is that: instead of preparing calcium-boron double-crosslinked lignin hydrosol in advance, 0.8 parts of calcium lignin sulfonate, 0.115 parts of boric acid, and 0.075 parts of calcium hydroxide (based on dry weight) corresponding to the hydrosol in Example 2 are directly mixed in dry powder form with secondary iron-containing resources, high-grade iron concentrate, and lightly calcined magnesium oxide. During pelletizing, only 8.5 parts of water are sprayed, and the rest of the operation remains the same.

[0042] Comparative Example 3: Compared with Example 2, the difference is that lightly calcined magnesium oxide is not added to the mixed dry powder ingredients, and the amount of secondary iron-containing resources added is increased by 2.0 parts to make up the total number of dry materials, while the rest of the operations remain the same.

[0043] Comparative Example 4: Compared with Example 2, the difference is that when preparing the hydrosol in advance, only calcium lignosulfonate and calcium hydroxide are used, and boric acid is not added, while the rest of the operation remains the same.

[0044] Comparative Example 5: Compared with Example 2, the difference is that: calcium lignosulfonate in the raw materials for preparing the hydrosol is replaced with sodium lignosulfonate in equal amounts, boric acid is replaced with sodium tetraborate decahydrate in equal amounts, calcium hydroxide is not added, and the rest of the operations remain the same.

[0045] Test Examples 1-5: Test Example 1: Turn on the coaxial cylindrical rotational viscometer with a constant temperature water bath circulation system, and set and stabilize the temperature of the test cylinder and its jacket at 25.0±0.1℃.

[0046] Take 45 ml of each of the solutions prepared in Preparation Examples 1 to 3 and Comparative Example 4 and allowed to cool naturally to room temperature. After standing to remove bubbles, slowly inject the solutions into the test tube of the rheometer along the wall of the apparatus.

[0047] The rotor is lowered to the specified scale position, and the test system is kept at a constant temperature for 5 minutes to eliminate the shear history and internal residual stress generated during the sampling and loading process.

[0048] Set the viscometer's test program, run in rotation mode, and set the shear rate to 10 (s). -1 ), 50 (s) -1 ), 100 (s) -1 The gradients increase sequentially, and each set shear rate platform is maintained for 30 seconds.

[0049] The apparent viscosity data output by the instrument in the last 5 seconds of each shear rate range were collected and the arithmetic mean was taken. Each group of samples was tested in parallel three times, and the average of the three test results was taken as the final recorded data.

[0050] It should be noted that the core of this test example is to verify the rheological network structure of the double crosslinked hydrosol itself. Since Comparative Examples 1 and 2 only sprayed water in the pelletizing process and did not synthesize hydrosol in advance; the hydrosol used in Comparative Example 3 was exactly the same as that in Example 2, with the only variable being the lack of magnesium oxide in the dry powder formulation; the core purpose of Comparative Example 5 was to verify the alkali metal engineering load of the final product pellets. Therefore, Comparative Examples 1, 2, 3, and 5 did not have the prerequisites or necessity to participate in this rheological characterization test. This test only retains Comparative Example 4 (pelletizing solution lacking boric acid crosslinking agent) as the basic liquid phase control group.

[0051] Table 1. Apparent viscosity test results for each sample

[0052] Experimental conclusion: According to Table 1 and Figure 1 The data showed that the apparent viscosity of the hydrosols obtained in Preparation Examples 1 to 3 was significantly higher than that of the solution obtained in Comparative Example 4 at all tested shear rates. At 10 (s)-1 Under low shear rate conditions, the apparent viscosity of Preparation Example 3 reached 293.1 mPa·s, while that of Comparative Example 4 was only 18.6 mPa·s. Comparative Example 4, without the addition of boric acid, exhibited a lower apparent viscosity, and the viscosity value changed only slightly with increasing shear rate, displaying rheological characteristics close to those of a Newtonian fluid. This indicates that only conventional hydration between calcium lignin sulfonate and water molecules existed in the system, and a long-range cross-linked network structure was not formed.

[0053] In contrast, the viscosity of the systems increased by orders of magnitude after the introduction of boric acid in Preparation Examples 1 to 3. The tetrahydroxyborate ions released from boric acid undergo esterification condensation with the ortho-hydroxyl groups on the lignin molecular chains, forming covalent cross-links. Simultaneously, calcium ions in the formulation form ionic cross-links between molecular chains through electrostatic attraction. This dual cross-linking pre-complexation promotes the self-assembly of lignin macromolecules in the aqueous phase, forming a three-dimensional network structure, which manifests as a sharp increase in the apparent viscosity of the system on a macroscopic rheological level.

[0054] Combination Figure 1 The rheological curves show that the apparent viscosity of the hydrosols prepared in Examples 1 to 3 decreases systematically with increasing shear rate, exhibiting typical shear-thinning characteristics of pseudoplastic fluids. In the pelletizing process, the rheological properties enable the hydrosols to overcome internal network constraints and achieve atomization and dispersion under the high shear force generated by the high-pressure nozzle. When the atomized droplets contact the surface of the dry magnesium oxide particles, the shear stress disappears, and the system viscosity rapidly recovers. A high-viscosity physical barrier is constructed in situ at the solid-liquid interface of the particles. This barrier effectively blocks the penetration of free water into the magnesium oxide, inhibiting early malignant hydration expansion of magnesium oxide and verifying the effectiveness of the interfacial steric hindrance mechanism.

[0055] Test Example 2: From the materials produced by the disc pelletizers in Examples 1 to 3 and Comparative Example 2, 50 qualified green pellets with a particle size in the range of 12.0 ± 0.2 mm were randomly sampled and screened as test samples.

[0056] Using a digital vernier caliper with an accuracy of 0.02 mm, the initial diameter of each green ball was measured in three orthogonal directions, and the average value was used to calculate the initial volume.

[0057] After measurement, each group of green pellets was laid flat in a single layer on a perforated tray and placed in a constant temperature and humidity test chamber. The temperature inside the chamber was set to 35.0±0.5℃ and the relative humidity was set to 95±2%.

[0058] At the 4-hour, 8-hour, and 24-hour mark of the test, the raw pellets of each group were removed, and the diameter in the orthogonal direction was measured again. The volume expansion rate at the corresponding time point was calculated, and the volume expansion rate of samples in the same group was taken as the arithmetic mean. The raw pellets that were removed for measurement were not put back into the box.

[0059] During the constant temperature and humidity static test cycle, the surface morphology of the green balls in the tray was continuously monitored by a high-definition timed camera, and the time point when macroscopically visible cracks first appeared on the surface of each group of green balls was recorded.

[0060] Since Comparative Example 1 uses sodium-based bentonite and its formulation does not contain light-burned magnesium oxide, and Comparative Example 3 also does not contain light-burned magnesium oxide, neither of them presents the premise of magnesium oxide hydration expansion based on thermodynamics and material principles. The core purpose of Comparative Example 5 is to verify the amount of alkali metal introduced, and the sodium-based system used has a similar film-forming basis. Although Comparative Example 4 has a low liquid phase viscosity, it still provides a certain initial liquid phase distribution as a pelletizing liquid spray. Comparative Example 2 directly mixes the dry powder and then sprays it with water, completely destroying the prerequisite of the pre-complexed liquid phase encapsulating the solid phase, constituting the most direct and rigorous reverse verification of the hydration expansion mechanism. Therefore, Comparative Example 2 is selected as the core control group in this test, and the data of Comparative Examples 1, 3, 4, and 5 are not listed.

[0061] Table 2. Test results of volume expansion rate and cracking of green bulbs in each group under constant temperature and high humidity environment.

[0062] Experimental conclusion: According to Table 2 and Figure 2 According to the data, after the green balls prepared in Examples 1 to 3 were left to stand in a constant temperature and high humidity environment for 24 hours, no macroscopic cracks were observed on the surface. Figure 2 In Examples 1 to 3, the lines almost overlap. The green pellets obtained in Comparative Example 2 developed macroscopic cracks after standing for 3.2 hours, with a 24-hour volume expansion rate of 16.31%, accompanied by severe structural pulverization. Comparative Example 2 used a conventional dry-mixing process, mixing calcium lignosulfonate, boric acid, calcium hydroxide, and lightly calcined magnesium oxide powder together before spraying with water to form pellets. Free water directly contacted the surface of the active magnesium oxide particles. Magnesium oxide underwent an irreversible hydration reaction in the aqueous phase to generate magnesium hydroxide. This reaction was accompanied by a decrease in crystal density and an increase in molar volume. Internal stress accumulated rapidly, exceeding capillary force and initial interparticle friction, leading to the disintegration of the green pellet matrix.

[0063] In Examples 1 to 3, calcium-boron double-crosslinked lignin hydrosols were pre-synthesized. During the pelletizing stage, the hydrosols were dispersed as atomized droplets onto the material surface. Combined with the rheological characterization results of Example 1, the high-viscosity hydrosols lost shear stress at the solid-liquid interface of magnesium oxide particles, rapidly recovering their high-viscosity state and constructing a semi-permeable steric hindrance membrane in situ. This membrane forms a physical barrier, increasing the mass transfer resistance of free water molecules diffusing into the magnesium oxide particles, thus transforming the intense hydration kinetics into a slow interfacial permeation process. The test results confirm that the interfacial steric hindrance mechanism can inhibit abnormal hydration expansion of magnesium oxide during the green pellet preparation and placement stages, ensuring the integrity of the green pellet structure and meeting the physicochemical strength indicators for industrial green pellet storage and transportation.

[0064] Test Example 3: In Examples 1 to 3, 100 green pellet samples with a particle size ranging from 10.0 to 12.5 mm were randomly selected from the pelletizing and roasting processes of Comparative Examples 1, 2, and 4, respectively, according to the standard sampling method. Additionally, 60 finished roasted pellets cooled to room temperature were also selected from each sample.

[0065] Drop strength of green balls was determined: 50 green balls from each group were dropped one by one from a height of 0.5 meters onto a flat steel plate with a thickness of 10 mm. The number of drops before macroscopic fracture of each green ball was recorded. The arithmetic mean of the number of drops of the 50 green balls was taken as the green ball drop strength index of the corresponding sample.

[0066] Determining the compressive strength of green pellets: Take the remaining 50 green pellet samples from each group and place them on the test platform of a computer-controlled intelligent compressive strength testing machine for pellets. Apply a vertical load at a constant loading rate of 15 mm / min and record the maximum load value at the moment of green pellet rupture. Take the arithmetic mean of the 50 test results as the green pellet compressive strength index of the corresponding sample.

[0067] Determining the compressive strength of finished pellets: Referring to the ISO 4700 international standard, 60 finished roasted pellets from each group were placed one by one in the center of the pressure plate of a compressive strength testing machine, and pressure was applied uniformly at a rate of 15 mm / min until the pellets were completely crushed. Outliers were removed, and the arithmetic mean was taken as the compressive strength index of the corresponding sample.

[0068] This test case compares and verifies the performance advantages of the preparation process of this invention in terms of the physical and mechanical strength of green pellets and finished pellets. Comparative Example 3 did not add lightly calcined magnesium oxide to its dry powder formulation; the core variable lies in the phase reconstruction and thermodynamic anti-blocking mechanism during the high-temperature reduction stage, not in the basic adhesive mechanical properties. Comparative Example 5 uses sodium lignosulfonate to replace calcium-based raw materials, mainly to verify the alkali metal engineering load of the finished pellets; the difference in room-temperature mechanical strength between sodium-based and calcium-based materials does not explain the key verification of the core mechanism. Therefore, to highlight the direct influence of the crosslinking pelletizing solution and interfacial steric hindrance mechanism on strength, Comparative Examples 3 and 5 were excluded from this test case.

[0069] Table 3. Physical and mechanical strength test results of green pellets and finished pellets in each group

[0070] Experimental conclusion: According to Table 3, Figure 3 According to the data, Examples 1 to 3 are superior to Comparative Examples 2 and 4 in all three strength dimensions (i.e., green pellet drop strength, green pellet compressive strength and finished pellet compressive strength). Figure 3 (a) Analysis of drop strength data for raw balls: the example group achieved 5.2 to 7.1 drops / 0.5m; Figure 3 (b) Analysis of the compressive strength data of green pellets showed that the values ​​in the example group ranged from 13.4 to 16.7 N / pellet; Figure 3 (c) Analysis of the compressive strength data of the finished pellets showed that the values ​​in the example group were as high as 2684 to 3158 N / piece, meeting the mechanical strength requirements of large blast furnace burdens. Comparative Example 1 served as the traditional benchmark group using sodium-based bentonite. Figure 3 (a) Same Figure 3 (c) shows that the corresponding strengths are all at the edge of the passing line, proving that the pre-complexed hydrosol of the present invention provides bonding performance that surpasses that of inorganic bentonite with a lower addition amount.

[0071] Comparative Example 2 used a dry mixing process without pre-synthesizing the pelletizing solution. Figure 3 (a) shows that the drop intensity of the raw ball is only 2.1 times / 0.5m. Figure 3 (c) shows that the compressive strength of the finished product decreased to 1534 N / piece. The lignin and boric acid in the dry powder state could not be uniformly dispersed at the molecular level during the pelletizing cycle, resulting in the loss of the liquid phase bridging network. At the same time, the microcracks generated by the early hydration expansion of magnesium oxide further expanded during the calcination process, causing the solid phase sintering network to break and the macroscopic mechanical properties to decrease.

[0072] Comparative Example 4 lacked boric acid in its pelleting solution formulation. Figure 3(b) The compressive strength of the pellets decreased to 8.6 N / pellet. The lack of borate covalent cross-linking prevented the formation of a high-viscosity three-dimensional network by lignin macromolecules, weakening the capillary force and film-forming pull of the pelletizing solution between material particles. During the roasting stage, the lack of transient catalytic action from calcium borate micro-liquid phase hindered lattice diffusion between iron mineral particles, resulting in incomplete development of solid-state bonding necks. Figure 3 (c) The compressive strength of the finished pellets was significantly lower than that of the example. The test results confirm that the double cross-linking pre-complexation mechanism can improve the liquid phase bridging force of the green pellets.

[0073] Test Example 4: From the finished roasted pellets of Examples 1 to 3 and Comparative Examples 1, 3, and 4, 500 grams of pellets with a particle size between 10.0 and 12.5 mm and no visible damage were selected as test samples.

[0074] The initial volume of 10 representative pellets in each sample group was measured and recorded using the volume displacement method. All 500-gram samples were then loaded into a heat-resistant steel reduction reaction tube.

[0075] The reduction reaction tube was placed in an intelligent temperature-controlled tube furnace, and nitrogen gas with a purity of 99.9% was introduced as a protective gas. The tube was heated at a rate of 10℃ / minute until the temperature inside the tube stabilized at 900±5℃.

[0076] Switch the gas input valve to introduce a mixed reducing gas consisting of 30% carbon monoxide and 70% nitrogen by volume, control the total gas flow rate at 15 liters / minute, and continue the isothermal reduction reaction for 180 minutes.

[0077] After the reduction reaction is complete, the mixed reducing gas is cut off, nitrogen is reintroduced, and the mixture is cooled to room temperature. The difference in total mass of the sample before and after the reaction is weighed, and the reduction index is calculated. At the same time, the volumes of 10 pre-labeled pellets are remeasured, and the percentage change in volume is calculated to obtain the reduction expansion index.

[0078] This test case compares and verifies the metallurgical properties of the finished pellets under high-temperature reducing conditions. Comparative Example 2 already exhibited structural fragmentation during the preliminary physical and mechanical tests, lacking the physical framework structure necessary to sustain the high-temperature gas-solid reaction test. The core purpose of Comparative Example 5 is to verify the final alkali metal engineering load. The high-temperature phase transition law generated by the sodium-based system used is common metallurgical knowledge and not a key focus for verifying thermodynamic blockage and lattice distortion mechanisms. Therefore, data from Comparative Examples 2 and 5 are not listed in this section.

[0079] Table 4. High-temperature reduction performance test results of pellets of each component

[0080] Experimental conclusion: According to Table 4 and Figure 4 The data shows that the reduction index of Examples 1 to 3 ranged from 68.45% to 73.92%, and the reduction expansion index was controlled between 8.76% and 12.15%. (Refer to...) Figure 4 The bar chart on the left vertical axis shows that the values ​​of the example group are significantly higher than those of Comparative Example 1 (58.21%). Comparative Example 1 uses traditional sodium-based bentonite, and the high-temperature calcination process generates a large amount of low-melting-point silicate liquid phase, which fills and blocks the micropores inside the pellets, hindering the diffusion of carbon monoxide reducing gas inward.

[0081] Reference Figure 4 The line chart on the right-hand vertical axis shows that the reflation index of Comparative Example 3 reaches 19.84%, while... Figure 4 The left-hand bar chart shows that the reduction degree of Comparative Example 3 decreased to 55.17%. The formulation of Comparative Example 3 lacked lightly calcined magnesium oxide, leading to excessive aggregation of the calcium ferrite micro-liquid phase generated during the calcination process, causing severe thermodynamic blockage. In this example, lightly calcined magnesium oxide was introduced. Magnesium ions preferentially react with iron oxide at high temperatures to form a high-melting-point magnesium ferrite phase, consuming excess low-melting-point liquid phase and reconstructing microporous channels, maintaining unobstructed diffusion paths for reducing gases.

[0082] Comparative Example 4: The pelleting solution lacked boric acid. Figure 4 The reduction rate was only 61.38%. The lack of boric acid to provide effective boron ions prevented lattice distortion within the iron mineral crystal lattice, thus hindering the reduction reaction activation energy and resulting in a slow high-temperature reduction process. The test results confirm that targeted in-situ release and thermodynamic phase reconstruction mechanisms can accelerate the pellet reduction reaction and inhibit abnormal expansion behavior.

[0083] Test Example 5: In the pelletizing and roasting processes of Examples 1 to 3, Comparative Examples 1 and 5, 200 green pellet samples with a particle size ranging from 10.0 to 12.5 mm were randomly selected each according to the standard sampling method, and 100 grams of finished roasted pellets cooled to room temperature were also selected.

[0084] Determining the green ball bursting temperature: Preheat the intelligent muffle furnace to the set reference temperature of 400℃, place 50 green ball samples from the corresponding group inside, and bake at a constant temperature for 5 minutes before removing them. Observe and count the number of green balls that peel off or burst on the surface. If the breakage rate is less than 20%, increase the furnace temperature by 25℃ and replace with a new batch of 50 green balls, repeating the above operation until the breakage rate reaches or exceeds 20%. Record the corresponding furnace temperature and denote it as the green ball bursting temperature index.

[0085] Determination of alkali metal loading in finished pellets: 100 grams of finished roasted pellets from each group were pulverized in a vibratory mill and passed through a 200-mesh standard sieve. 1.000 grams of powder sample was accurately weighed and placed in a polytetrafluoroethylene digestion vessel. A mixed solution of nitric acid and hydrofluoric acid was added for microwave high-temperature digestion until the powder sample was completely dissolved.

[0086] The digested solution was transferred and diluted to a 100 mL volumetric flask. The mass concentrations of sodium and potassium in the solution were determined using inductively coupled plasma optical emission spectrometry (ICP-OES). The mass percentages of sodium oxide and potassium oxide in the finished pellet were then calculated.

[0087] This test compares and verifies the thermal stability of green pellets and the alkali metal load of the finished pellets. Comparative Example 2 uses a dry-mixing process, resulting in uneven moisture distribution and a loose initial structure in the green pellets, which does not meet the physical integrity requirements for conducting burst temperature testing. Comparative Example 3's formulation does not include lightly calcined magnesium oxide, and Comparative Example 4's pelletizing solution lacks boric acid. The alkali metal introduction amounts in Comparative Examples 3 and 4 are consistent with the examples, and their verification focuses on high-temperature reduction phase transition and macroscopic mechanical strength, failing to provide a comparative analysis of differences in alkali metal load in conventional sodium-based systems. Therefore, the data from Comparative Examples 2, 3, and 4 are excluded from this test, while Comparative Example 1 (using sodium-based bentonite) and Comparative Example 5 (using sodium lignosulfonate) are retained as control groups.

[0088] Table 5. Test results of bursting temperature of green pellets and alkali metal content of finished pellets for each group.

[0089] Experimental conclusion: According to Table 5 and Figure 5 Based on the data, the bursting temperatures of the green pellets in Examples 1 to 3 ranged from 582℃ to 641℃, and the total alkali metal mass fraction of the finished pellets ranged from 0.027% to 0.038%. Combined with... Figure 5 The bar chart corresponding to the left vertical axis shows that the bursting temperature of the green pellets in the example is significantly higher than that of Comparative Example 1 (458℃) and Comparative Example 5 (526℃). Comparative Example 1 uses traditional inorganic bentonite, which is dense and has low porosity. During the drying and preheating stage, the internal moisture of the green pellets rapidly vaporizes, and the resulting steam pressure cannot be released in time, causing the matrix to burst at a lower temperature. The example introduces calcium-boron double-crosslinked lignin hydrosol. After the moisture evaporates, the organic polymer chains form microscopic capillary pores and exhaust channels inside the pellets, reducing the peak value of the accumulated internal steam pressure and improving the thermal shock resistance of the green pellets.

[0090] Combining the detailed data of alkali metal elements in Table 5 and Figure 5The vertical axis trend chart on the right shows that the total alkali metal content in Comparative Examples 1 and 5 is as high as 0.542% and 0.403%, respectively, exceeding the alkali metal load standard for blast furnace charge. Detailed analysis of the table data reveals that Comparative Example 1, using sodium-based bentonite, not only introduces a high sodium oxide content of 0.441%, but also introduces 0.101% potassium oxide from natural mineral impurities. Comparative Example 5, using sodium lignosulfonate, reduces the potassium oxide content to 0.018%, but the sodium oxide mass fraction still reaches 0.385%. During high-temperature roasting and subsequent blast furnace reduction processes, the introduced sodium and potassium ions will cause furnace lining erosion and deterioration of coke reactivity.

[0091] In comparison, the sodium oxide mass fraction in Examples 1 to 3 ranged from 0.016% to 0.023%, and the potassium oxide mass fraction ranged from 0.011% to 0.015%. These examples used calcium-based raw materials such as calcium lignosulfonate to replace sodium-based binders, cutting off the introduction of exogenous sodium and potassium elements at the source, resulting in a significant reduction in various alkali metal indicators of the finished pellets. Test results confirm that the all-calcium-based pre-complexed organic pelletizing formula effectively reduces the alkali metal load of the finished pellets while improving the thermal stability of the green pellets during preheating and drying, meeting the requirements for long-life blast furnace operation.

Claims

1. A method for batching highly reducing pellets based on secondary iron-bearing resources, characterized in that, Includes the following steps: By weight, 80.0–84.77 parts of secondary iron-bearing resources, 13.0–16.25 parts of high-grade iron concentrate, and 1.5–2.5 parts of lightly calcined magnesia are mixed under high shear in a dry state to obtain a mixed dry powder. The mixed dry powder is continuously fed into a pelletizing machine, and a calcium boron double crosslinked lignin hydrosol containing 0.73 to 1.25 parts of dry base components is dynamically and uniformly atomized and sprayed onto the surface of the mixed dry powder to form pellets, thus obtaining green pellets. The green balls are sequentially spread out and subjected to hot air flow drying and preheating treatment; The preheated green pellets are then subjected to solid-phase roasting to obtain pellet ore. The pellets are cooled and discharged into a storage bin.

2. The method for batching highly reducing pellets based on secondary iron-bearing resources according to claim 1, characterized in that, The amount of the secondary iron-bearing resource is 82.01 parts, the amount of the high-grade iron concentrate is 15.0 parts, the amount of the light-burned magnesium oxide is 2.0 parts, and the amount of the dry basis component of the calcium-boron double-crosslinked lignin hydrosol is 0.99 parts.

3. The method for batching highly reducing pellets based on secondary iron-bearing resources according to claim 1, characterized in that, The calcium-boron double-crosslinked lignin hydrosol is prepared in advance, and the preparation steps include: By weight, 80.0 to 90.0 parts of deionized water are injected into the reactor and heated and kept at a constant temperature. Turn on the mechanical stirrer and add 0.8 to 1.5 parts of boric acid to fully dissolve and form a solution system; Slowly add 0.5 to 1.0 parts of calcium hydroxide powder to adjust and stabilize the pH of the solution system; add 6.0 to 10.0 parts of calcium lignin sulfonate at a uniform rate for continuous reaction to obtain calcium-boron double crosslinked lignin hydrosol.

4. The method for batching highly reducing pellets based on secondary iron-bearing resources according to claim 3, characterized in that, The process parameters for preparing the calcium-boron double-crosslinked lignin hydrosol are controlled as follows: The heating and constant temperature is 60-75℃; the mechanical stirring speed is set to 150-200 rpm; the pH value of the solution system is controlled to stabilize the pH value at 9.0-10.0; and the continuous reaction time after adding the calcium lignosulfonate is 30-45 minutes.

5. The method for batching highly reducing pellets based on secondary iron-bearing resources according to claim 3, characterized in that, In the preparation of the calcium-boron double-crosslinked lignin hydrosol, under the conditions of maintaining a temperature of 60-75℃ and a pH of 9.0-10.0, the reaction is carried out continuously for 30-45 minutes at a mechanical stirring speed of 150-200 rpm, so that the linear polymer of the calcium lignin sulfonate is transformed into a network structure.

6. The method for batching highly reducing pellets based on secondary iron-bearing resources according to claim 1, characterized in that, The high-shear mixing time is 2 to 3 minutes.

7. The method for batching highly reducing pellets based on secondary iron-bearing resources according to claim 1, characterized in that, In the pelletizing process, the overall moisture content of the green pellets is controlled to be 8.0% to 9.0%; the particle size of the green pellets is controlled to be between 10 and 16 mm.

8. The method for batching highly reducing pellets based on secondary iron-bearing resources according to claim 1, characterized in that, In the step of sequentially feeding the green pellets into a hot air flow drying and preheating treatment, the green pellets sequentially pass through a hot air flow drying section with a temperature of 150-300℃ and a preheating section with a temperature controlled at 300-600℃; the heating rate of the preheating section is controlled at 15-20℃ / minute.

9. The method for batching highly reducing pellets based on secondary iron-bearing resources according to claim 1, characterized in that, The solid-phase roasting is carried out in an oxidizing atmosphere inside a rotary kiln; the process parameters for the solid-phase roasting are as follows: The maximum temperature of the rotary kiln firing zone is controlled at 1180–1210℃, and the firing time is 15–25 minutes.

10. A method for batching highly reducing pellets based on secondary iron-bearing resources according to claim 1, characterized in that, The cooling step includes: feeding the pellets into an annular cooler and cooling them to below 150°C by forced air blowing from the bottom.

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