Method for preparing ferrochrome pre-reduced pellets through hydrogen-solid carbon coupling reduction

Abstract

The invention discloses a method for preparing ferrochrome pre-reduced pellets through hydrogen-solid carbon coupling reduction, and belongs to the technical field of ferrochrome alloy metallurgy. The method comprises the steps that chromite, solid carbon, a binder and water are mixed for pelletizing, obtained wet pellets are sequentially subjected to drying, preheating and reduction roasting, and ferrochrome pre-reduced pellets are obtained; the reduction roasting comprises two stages of reduction processes: the first stage of reduction process is as follows: hydrogen is introduced from the outside at the temperature of 900-1100 DEG C, and the hydrogen and solid carbon are used for reduction; and the second-stage reduction process comprises the following steps: stopping introducing hydrogen at the temperature of 1200-1350 DEG C, and reducing by using solid carbon. According to the method, by designing a hydrogen-solid carbon coupling reduction system, the reduction efficiency and the alloying rate of chromite can be improved, meanwhile, the consumption of solid carbon is reduced, the purposes of energy conservation and emission reduction are achieved, and the method has the beneficial effects of being simple in process, low in energy consumption, small in carbon emission and the like.

Description

Technical Field

[0001] This invention relates to a method for producing pre-reduced chromite pellets, and more particularly to a method for preparing pre-reduced chromite pellets by hydrogen-solid carbon coupling reduction, belonging to the field of chromite alloy metallurgical technology. Background Technology

[0002] In existing technologies, ferrochrome alloys are mainly produced through a process of "ferrochrome powder agglomeration - submerged arc furnace smelting." The agglomeration and refining stage of ferrochrome powder is crucial to the combined energy consumption of both the agglomeration and submerged arc furnace smelting processes. It is a key step in improving the quality and efficiency of ferrochrome production, as well as saving energy and reducing carbon emissions, and is of great significance to the green and low-carbon development of the ferrochrome alloy industry. The pre-reduction pelletizing method has advantages over other agglomeration methods such as sintering and oxidation pelletizing. While achieving the agglomeration function, it also undertakes most of the reduction function in the submerged arc furnace smelting, reducing the reduction load on the furnace and significantly lowering the overall energy consumption of agglomeration and smelting. Currently, there are two main processes for producing pre-reduced ferrochrome pellets: the rotary kiln method and the rotary hearth furnace method. Compared to the rotary hearth furnace method, the rotary kiln process has advantages such as smaller equipment footprint, lower investment, and lower energy consumption, making it the mainstream process for pre-reduced ferrochrome pellet production, and it has experienced rapid development in recent years.

[0003] However, the difficulty of reducing chromite and the narrow suitable reduction temperature range are the main bottlenecks restricting its further development. Chromite has a complex mineral structure; chromium and iron are multi-element spinel minerals. Due to limitations imposed by interfacial reactions, the reduction of chromium oxides is far more difficult than that of iron oxides. At 1000℃~1300℃, reduction is mainly based on iron oxides, producing metallic iron; only above 1300℃ does chromite achieve a relatively fast reduction rate. In actual industrial production, to improve reduction efficiency, the reduction temperature must be increased, reaching a maximum of 1400℃~1450℃. However, at high temperatures, chromium briquettes are prone to melting, exacerbating ring formation in rotary kilns, thus narrowing its reduction temperature range. Simultaneously, at 1200℃~1500℃, chromium oxides are difficult to react with gaseous reducing agents; at this temperature, chromium reduction mainly relies on the reaction of solid carbon. Therefore, the high reduction temperature and reliance on coal for chromite reduction remain the main bottlenecks restricting energy consumption reduction.

[0004] Currently, new technologies for energy conservation and emission reduction in the reduction of chromite are being developed. For example, Chinese patent (authorization publication number: CN113549726B) discloses the oxidative roasting of chromite pellets at 1230℃~1320℃, first converting chromite spinel into hematite and chromium oxide, and then reducing it at 800℃~1000℃ using reducing gases such as H2 and CO. In addition, Chinese patent application (authorization publication number: CN117051230B) discloses the direct reduction using a mixture of H2 and CO at 1200℃~1400℃. Although the above technology uses all H2 to replace traditional coal energy and reduces CO2 emissions, it still has shortcomings: 1) The addition of the oxidation roasting process will increase additional energy consumption; 2) Although the direct reduction of H2 is excellent for iron, it is poor for chromium; 3) Hydrogen reduction is an endothermic reaction, and the gaseous H2O product will take away some heat energy when it leaves the reaction system. Therefore, all-hydrogen reduction requires additional heat energy to maintain the reaction temperature, which increases energy consumption.

[0005] Therefore, there is an urgent need for a new pre-reduction pellet production process that can further reduce the reduction temperature of chromite, reduce solid carbon consumption, and achieve carbon reduction and energy saving. Summary of the Invention

[0006] To address the shortcomings of existing chromite pre-reduction processes, such as high reduction temperatures, excessive reliance on solid carbon, and high CO2 emissions, as well as the energy consumption required for all-hydrogen reduction, this invention aims to provide a method for preparing chromite pre-reduced pellets using hydrogen-solid carbon coupled reduction. This method employs a two-stage reduction roasting process, utilizing different reducing agents at different temperatures to continuously reduce chromite. At low and high temperatures, "hydrogen + solid carbon" and "solid carbon" are used as reducing agents, respectively, enabling the reduction of most iron oxides to metallic iron and a suitable amount of chromium oxides to metallic chromium, ultimately yielding chromite pre-reduced pellets with a high metallization rate. This method utilizes hydrogen-solid carbon coupled reduction. On one hand, hydrogen replaces some of the solid carbon in the reduction process, reducing the consumption of traditional fossil fuels and lowering carbon emissions. On the other hand, some solid carbon is used as a heat source to compensate for the drastic temperature drop during hydrogen reduction and the inability of hydrogen to reduce chromium oxides. Furthermore, this method introduces a low-temperature reduction stage to regulate the reduction reaction, reducing energy consumption while maintaining a high metallization rate in the pellets compared to conventional pre-reduction methods.

[0007] To achieve the above-mentioned technical objectives, the present invention provides a method for preparing ferrochrome pre-reduced pellets by hydrogen-solid carbon coupling reduction. The method involves mixing ferrochrome ore, solid carbon, binder and water to form pellets, and then subjecting the resulting wet pellets to drying, preheating and reduction roasting in sequence to obtain ferrochrome pre-reduced pellets.

[0008] The reduction roasting includes two reduction processes:

[0009] The first reduction process is as follows: at a temperature of 900℃~1100℃, hydrogen gas is introduced from the outside, and reduction is carried out using hydrogen gas and solid carbon.

[0010] The second reduction process is as follows: at a temperature of 1200℃~1350℃, the hydrogen gas is stopped, and reduction is carried out using solid carbon.

[0011] The key to this invention lies in its two-stage reduction process. The first stage is a low-temperature reduction stage. By controlling the temperature to limit the reaction between the introduced solid carbon and oxides, the reduction reaction of H2 dominates in this stage. The introduced hydrogen gas preferentially reacts with easily reducible iron oxides, thereby reducing the consumption of solid carbon. However, since hydrogen reduction is a strongly endothermic reaction, the thermal energy of the reduction reaction system decreases, leading to a slower reaction rate. Simultaneously, some solid carbon reacts, such as with iron oxides, ferrochrome spinel, and H2O. The released CO can further react with ferrochrome oxides (or burn exothermically in the oxidizing atmosphere section of the reactor), thus compensating for the thermal energy deficit in the hydrogen reduction process and preventing a decrease in the reaction rate due to temperature drop during reduction. The second stage is a high-temperature reduction stage, primarily utilizing the remaining solid carbon to deeply react with unreacted ferrochrome and Cr2O3, increasing the metallization rate of chromium.

[0012] This invention significantly improves the reduction efficiency of chromite by designing a two-stage reduction process: in the first stage, hydrogen is used as the main reducing agent, and solid carbon is used as the heat source. The two work synergistically to improve the reduction efficiency of metal oxides and ensure a high metallization rate. During the low-temperature reduction process, hydrogen can undergo a gas-solid reaction with iron oxides within the temperature range of 900℃ to 1100℃. Compared with solid carbon reduction, the gaseous reducing agent has a significant kinetic advantage. Compared with carbon monoxide, hydrogen molecules are smaller and have a higher molecular diffusion rate, resulting in a faster and more complete hydrogen reduction rate. The heat loss and temperature drop in the reaction layer caused by hydrogen reduction are compensated by a small portion of the solid carbon. Simultaneously, chromium oxides that are difficult to reduce with hydrogen are initially reduced by solid carbon during the low-temperature reduction process. The remaining chromium oxides are further reduced by solid carbon at high temperatures, efficiently converting them into metallic chromium. This ensures a high reaction rate and metallization rate for chromite.

[0013] This invention, through its two-stage reduction process, also offers significant advantages in terms of low carbon emissions: In traditional pre-reduction processes, solid carbon performs all the reduction functions of chromite, releasing a large amount of CO2 during the reduction process. However, this invention uses hydrogen in the low-temperature reduction process to replace some of the solid carbon's function as a reducing agent, reducing solid carbon emissions by approximately 20%–30%. As a green energy source, hydrogen only produces H2O vapor after the reduction reaction, correspondingly reducing CO2 emissions by 20%–30%.

[0014] This invention achieves energy reduction by designing a two-stage reduction process: Currently, commonly used pre-reduction processes require kiln temperatures of up to 1400℃~1450℃ and long reduction times to increase the reduction rate. In contrast, the pre-reduction of most chromite ore in this invention is carried out at a low temperature of 900℃~1100℃, requiring significantly less energy than traditional pre-reduction processes.

[0015] In summary, the technical solution of this invention uses a hydrogen-solid carbon coupled reduction system to produce pre-reduced ferrochrome pellets. By setting low-temperature and high-temperature reduction sections to regulate the reaction, the production efficiency is improved, the production temperature is reduced, and the consumption of traditional coal reducing agents is reduced while maintaining a high metallization rate of the pre-reduced ferrochrome pellets. This reduces carbon emissions and energy consumption in the production process.

[0016] The first reduction process of this invention is carried out at a temperature of 900℃~1100℃, which is a low-temperature reduction process, and the second reduction process is carried out at a temperature of 1200℃~1350℃, which is a high-temperature reduction process. More preferably, the temperature of the first reduction process is 950℃~1000℃, and the temperature of the second reduction process is more preferably 1200℃~1250℃. During the first reduction process, the temperature is controlled at a relatively low level. Hydrogen is used to reduce the easily reducible iron oxides in the ferrochrome spinel structure to metallic iron. Simultaneously, the reaction between solid carbon and iron oxides is controlled to reduce solid carbon consumption. At the same time, some solid carbon is ensured to react and generate a small amount of carbon monoxide, which reacts exothermically with oxides or oxygen to replenish the heat energy absorbed by the hydrogen reaction and prevent a decrease in the reaction rate. If the temperature of the first reduction process is too high, the consumption of solid carbon will be too high, affecting the subsequent reduction of chromium oxides; if the temperature of the first reduction process is too low, the reduction effect of hydrogen on iron oxides will be poor, resulting in a low metallization rate. The second-stage reduction process is controlled at a higher temperature to accelerate the reaction between solid carbon and the difficult-to-react chromium oxides, and to accelerate the formation of a high-temperature liquid-phase cementite from solid carbon and metallic iron, which acts as a carbon carrier to accelerate the reduction of chromium oxides. If the second-stage reduction temperature is too high, the process energy consumption will increase; if the second-stage reduction temperature is too low, the reduction effect of solid carbon on chromium oxides will be poor, reducing the metallization rate.

[0017] As a preferred embodiment, the chromium-iron ratio (Cr / Fe) of the chromite is 1.3 to 1.9.

[0018] As a preferred embodiment, the solid carbon includes at least one of coke, anthracite, bituminous coal, and semi-coke. These solid carbons are common fossil fuels with high fixed carbon content in the prior art, with an industrial analysis fixed carbon content ≥85%. As a more preferred embodiment, the amount of solid carbon used is 5%~12% of the mass of chromite. The amount of solid carbon used is further 7%~10% of the mass of chromite. As a traditional fossil energy source, solid carbon releases CO2 during the reduction process, and its large-scale addition is detrimental to achieving emission reduction targets. However, the reduction of chromite is difficult, and gaseous reducing agents H2 and CO are difficult to reduce, requiring solid carbon for deep reduction; at the same time, H2 reduction is a strongly endothermic reaction, and the addition of some solid carbon to react and generate CO provides additional heat, forming a hydrogen-solid carbon coupling effect. Therefore, when the amount of solid carbon added is too high, CO2 emissions are too large, while when the amount of solid carbon added is too low, the reduction of chromite is insufficient and cannot meet the pre-reduction metallization rate requirements.

[0019] As a preferred embodiment, the particle size of both the chromite and the solid carbon is <0.15 mm. Smaller particle sizes of chromite and solid carbon facilitate more uniform mixing, increase the contact surface, and better promote solid-phase reactions.

[0020] As a preferred embodiment, the binder includes at least one of bentonite, quicklime, and organic binders. These binders are common types of binders in the prior art. As a preferred embodiment, the amount of binder used is no more than 5% of the mass of the chromite ore. The specific amount of binder used depends on the type of binder selected. For example, when inorganic binders such as bentonite and quicklime are selected, their amount is 1% to 5% of the mass of the chromite ore. When organic binders are used, because organic binders have obvious effects but are more expensive, the amount added is 0.06% to 0.2% of the mass of the chromite ore. The amount of binder added is optimized based on a comprehensive consideration of cost, bonding effect, impurity content, etc. Preferred inorganic binders such as bentonite and quicklime are inexpensive and have good high-temperature strength, but have a high impurity content. Therefore, if the amount used is too high, the iron and chromium grades of the pellets will decrease; if the amount used is too low, the strength of the pellets will decrease.

[0021] As a preferred embodiment, the total moisture content of the wet pellets is controlled at 5% to 8%.

[0022] As a preferred embodiment, the drying process ensures that the pellet moisture content is <0.5%. By drying the wet pellets under a drying device until the moisture content is at a low level, cracking of the pellets during subsequent heat treatment is avoided. Because the high moisture content of the wet pellets may cause them to burst and affect the normal operation of reduction equipment such as rotary kilns, drying is necessary to remove excess moisture.

[0023] As a preferred embodiment, the preheating conditions are: preheating at 800℃~1000℃ for 10min~30min in an air atmosphere. The preheating treatment is mainly to improve the pellet strength and meet the mechanical property requirements of the pellets during subsequent reduction roasting. The preheating temperature and time need to be controlled within a specified range. Too high a temperature or too long a time will cause carbon consumption within the preheated pellets; too low a temperature or too short a time will result in the pellet strength not meeting the equipment requirements.

[0024] As a preferred embodiment, the total reduction time of the two reduction processes is controlled within 2 to 4 hours. More preferably, the total reduction time is controlled within 2.5 to 3.5 hours. As another preferred embodiment, the time ratio of the first reduction process to the second reduction process is 3:1 to 1:1. The total reduction time is related to reaction kinetics. The first reduction time determines the reaction time between chromite and hydrogen, while the second reduction time determines the reduction time between chromite and solid carbon. The appropriate total reduction time and the time ratio of the two reduction stages need to be determined based on the reaction rates of chromite with hydrogen and solid carbon. If the total reduction time is too long, the process energy consumption increases; if the total reduction time is too short, the reduction is insufficient. If the first reduction process accounts for too large a proportion of the total time, the metallization rate of chromium is low; if the first reduction process accounts for too small a proportion, the reduction effect of hydrogen on iron oxides is weak, and excessive solid carbon participates in the reduction of iron oxides, also leading to a low metallization rate of chromium.

[0025] As a preferred approach, the partial pressure of H2 introduced during the first reduction stage is 0.05 MPa to 0.10 MPa. The hydrogen flow rate during the first reduction stage affects the total amount of hydrogen required; the more chromium-iron oxides in the pellets, the greater the required hydrogen flow rate. Therefore, the hydrogen flow rate needs to be controlled based on the pellet composition. If the hydrogen flow rate is too high, energy consumption increases; if the hydrogen flow rate is too low, the iron oxides will not react completely, resulting in a low metallization rate of the reduced pellets. The second reduction stage primarily involves the reduction of chromium oxides by solid carbon. Hydrogen has a relatively small impact on this reaction and can be stopped to reduce energy consumption.

[0026] The overall metallization rate of the ferrochrome pre-reduced pellets prepared by this invention is 60%~70%.

[0027] The method for preparing ferrochrome pre-reduced pellets by hydrogen-solid carbon coupling reduction provided by the present invention includes the following specific steps:

[0028] (1) Mix 5 wt.% to 15 wt.% of solid carbon and 1 wt.% to 5 wt.% of inorganic binder (or 0.06 wt.% to 0.2 wt.% of organic binder) of chromite with a certain amount of chromite, add water to control the moisture content of the material at 5% to 8%, and mix thoroughly to obtain a mixture. Then, make wet pellets in a disc pelletizer. The chromium-iron ratio (Cr / Fe) of the chromite is 1.3 to 1.9. The chromite and solid carbon reducing agent are ground before mixing, and the particle size is <0.15 mm. The solid carbon includes at least one or more of coke, anthracite, bituminous coal, and semi-coke. The inorganic binder includes at least one or more of bentonite and quicklime, and the organic binder includes sodium carboxymethyl cellulose. The mixing equipment is one or more of a high-power mixer, a horizontal mixer, and a roller mill. The wet pellets are 8 mm to 10 mm in diameter.

[0029] (2) The wet pellets obtained in step (1) are dried in a drying device until the moisture content of the pellets is <0.5%, and then the dried pellets are preheated in an air atmosphere at 800℃~1000℃ to obtain preheated pellets; the preheating time is 10min~30min; the compressive strength of the preheated pellets is required to be greater than 200N.

[0030] (3) The preheated pellets obtained in step (2) are subjected to reduction roasting; wherein the reduction roasting is divided into a first stage reduction and a second stage reduction with different temperatures and reducing agents; wherein the first stage reduction temperature is 900℃~1100℃, and H2 with a partial pressure of 0.05MPa~0.10MPa is introduced; the second stage reduction temperature is 1200℃~1350℃, and H2 is stopped; the total reduction time is 2h~4h, and the ratio of the first stage reduction time to the second stage reduction time is 3:1~1:1.

[0031] Compared with the prior art, the technical solution of the present invention brings the following beneficial effects:

[0032] (1) This invention uses a hydrogen-solid carbon coupling system as the reduction system for the pre-reduction of chromite pellets. In the low-temperature reduction process, hydrogen is used as the main reducing agent and solid carbon is used as the supply source, and the two work together to reduce the chromite pellets. In the range of 900℃~1100℃, hydrogen, as a reducing agent, undergoes a gas-solid reaction with iron oxides. Compared with the solid-solid reaction carried out by solid carbon as a reducing agent, it has a significant advantage in kinetics. Compared with carbon monoxide as a reducing agent, hydrogen molecules are smaller and have a higher molecular diffusion rate, resulting in a faster and more complete hydrogen reduction rate. However, the reduction reaction of hydrogen is a strongly endothermic reaction. During the reduction process, the temperature of the reaction layer decreases, and the reaction rate drops sharply. At the same time, the reducing potential of hydrogen is not as high as that of solid carbon, making it difficult to reduce chromium oxides. In the low-temperature reduction process, some solid carbon is used to participate in the reduction, forming a hydrogen-solid carbon coupled reduction system. This system can compensate for the decrease in the temperature of the reaction layer during the hydrogen reduction process, so as to maintain the rapid and efficient hydrogen reduction. Most of the easily reducible iron oxides are reduced to metallic iron, and some chromium oxides are also initially reduced during the low-temperature reduction process. Then, solid carbon is used at high temperature to further reduce the chromium oxides, so that the difficult-to-reduce chromium oxides are efficiently converted into metallic chromium, further improving the metallization rate of the pellets and making the overall metallization rate of the pellets reach a level of 60% to 70%.

[0033] (2) In this invention, hydrogen is used as the main reducing agent in the first stage of reduction, which partially replaces the reduction function of solid carbon in the traditional pre-reduction process, reduces the consumption of traditional chemical energy, and thus reduces the CO2 released during the reduction process. As a green energy source, hydrogen only produces H2O gas after the reduction reaction, which reduces CO2 emissions by 20% to 30%.

[0034] (3) The present invention divides the reduction reaction into two stages: the first stage of reduction is carried out at a low temperature and the second stage of reduction is carried out at a high temperature. In the first stage of reduction, a hydrogen-solid carbon coupled reduction system is designed. On the one hand, by using hydrogen as the main reducing agent, the reduction temperature can be reduced, the consumption of solid carbon can be reduced, and the reduction effect of hydrogen on easily reducible iron oxides can be fully utilized. On the other hand, compared with the traditional solid carbon reduction process, which requires reduction at 1400℃~1450℃ for 4~5 hours, most of the pre-reduction of chromite in the present invention is carried out at a low temperature of 900℃~1100℃, and the required energy consumption is much lower than that of the current traditional pre-reduction process. Detailed Implementation

[0035] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0036] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The patent terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of the invention.

[0037] Unless otherwise specified, the various reagents and raw materials used in this invention are commercially available products or products that can be prepared by known methods.

[0038] In the following examples, the test indicators are as follows: the compressive strength of the pellets is tested according to GB / T14201-2018; the total iron content (TFe) and total chromium content (TCr) are tested according to GB / T5687.13-2021; the metallic iron content (MFe) is tested according to GB / T6730.6-2016; there is currently no standard for the determination of metallic chromium content (MCr), so in this method, after leaching with oxalic acid, it is determined by X-ray fluorescence spectrometry in GB / T 5687.1; the total carbon content (TC) is tested according to GB / T 4699.4-2008. Iron metallization rate η Fe Chromium metallization rate η Cr and total metallization rate η T Calculate using the following formula:

[0039] ;

[0040] ;

[0041] ;

[0042] Example 1

[0043] Chromite (Cr / Fe ratio of 1.4) and coke were ground before mixing to ensure a particle size of <0.15mm. Then, 10.0wt% coke and 3.0wt% bentonite were mixed with the chromite, and water was added to bring the moisture content to 5%. After thorough mixing, a mixture was formed into wet pellets with a diameter of 10±1mm using a disc pelletizer. The wet pellets were dried to a moisture content of <0.5% using a drying device. The dried pellets were then preheated in air at 1000℃ for 20 minutes to obtain preheated pellets. The compressive strength was measured to be 311N, meeting the requirement of greater than 200N. The preheated pellets were subjected to reduction calcination. The reduction temperatures for the first and second stages were set at 950℃ and 1200℃, respectively. H2 was introduced at a partial pressure of 0.05 MPa in the first stage, and the H2 introduction was stopped in the second stage. The total reduction time was 3 hours, with a reduction time ratio of 2:1 between the first and second stages (2.0 hours for the first stage and 1.0 hour for the second stage). After calcination, the preheated pellets yielded pre-reduced pellets with a compressive strength of 2206 N, an iron metallization rate of 98.66%, a chromium metallization rate of 35.81%, and a total metallization rate of 62.95%.

[0044] Example 2

[0045] Chromite (Cr / Fe ratio of 1.6) and reducing agent were ground before mixing to ensure a particle size of <0.15mm. Then, 11.0wt% coke and 3.0wt% bentonite were mixed with the chromite, and water was added to bring the moisture content to 8%. After thorough mixing, a mixture was formed into wet pellets with a diameter of 10.0±0.5mm using a disc pelletizer. The wet pellets were dried to a moisture content of <0.5% using a drying device. The dried pellets were then preheated in air at 1000℃ for 25 minutes to obtain preheated pellets. The compressive strength was measured to be 357N, meeting the requirement of greater than 200N. The preheated pellets were subjected to reduction calcination. The reduction temperatures for the first and second stages were set at 1000℃ and 1300℃, respectively. H2 was introduced at a partial pressure of 0.1 MPa in the first stage, and the H2 introduction was stopped in the second stage. The total reduction time was 3 hours, with a reduction time ratio of 3:2 (1.8 hours for the first stage and 1.2 hours for the second stage). After calcination, the preheated pellets yielded pre-reduced pellets with a compressive strength of 2133 N, an iron metallization rate of 97.98%, a chromium metallization rate of 34.28%, and a total metallization rate of 61.50%.

[0046] Example 3

[0047] Chromite (Cr / Fe ratio of 1.9) and reducing agent were ground before mixing to ensure a particle size of <0.15mm. Then, 12.0wt% anthracite and 0.2wt% sodium carboxymethyl cellulose (CMC) were mixed with the chromite, and water was added to bring the moisture content to 8%. After thorough mixing, a mixture was formed into wet pellets with a diameter of 10.0±0.5mm using a disc pelletizer. The wet pellets were dried to a moisture content of <0.5% using a drying device. The dried pellets were then preheated in air at 1000℃ for 10 minutes to obtain preheated pellets. The compressive strength was measured to be 320N, meeting the requirement of greater than 200N. The preheated pellets were subjected to reduction calcination. The reduction temperatures for the first and second stages were set at 1100℃ and 1350℃, respectively. H2 was introduced at a partial pressure of 0.1 MPa in the first stage, and the H2 introduction was stopped in the second stage. The total reduction time was 4 hours, with a 1:1 ratio between the first and second stage reduction times, i.e., 2 hours for the first stage and 2 hours for the second stage. After calcination, the preheated pellets yielded pre-reduced pellets with a compressive strength of 1656 N, an iron metallization rate of 95.98%, a chromium metallization rate of 33.28%, and a total metallization rate of 60.35%.

[0048] Comparative Example 1

[0049] The only difference from Example 1 is that H2 is not introduced during either the first or second stage of reduction.

[0050] The pre-reduced pellets have a compressive strength of 1911 N, an iron metallization rate of 96.25%, a chromium metallization rate of 14.35%, and a total metallization rate of 51.96%.

[0051] Comparative Example 2

[0052] The only difference compared to Example 1 is that no coke was added to the mixture; that is, chromite was mixed with bentonite at a ratio of 97.0 wt.% to 3.0 wt.%.

[0053] The pre-reduced pellets have a compressive strength of 297 N, an iron metallization rate of 12.87%, a chromium metallization rate of 0.45%, and a total metallization rate of 5.82%.

[0054] Comparative Example 3

[0055] The only difference compared to Example 1 is that a low-temperature section is not set, that is, the preheated pellets are reduced at 1200°C for 3 hours, during which H2 is introduced at a flow rate of 0.3 L / min.

[0056] The pre-reduced pellets have a compressive strength of 1766 N, an iron metallization rate of 99.02%, a chromium metallization rate of 16.65%, and a total metallization rate of 52.22%.

[0057] Comparative Example 4

[0058] The only difference compared to Example 1 is that a high-temperature section is not set, that is, the preheated pellets are reduced at 1050°C for 3 hours, during which H2 is introduced at a flow rate of 0.3 L / min.

[0059] The pre-reduced pellets have a compressive strength of 1349 N, an iron metallization rate of 94.87%, a chromium metallization rate of 7.15%, and a total metallization rate of 45.97%.

[0060] Comparative Example 5

[0061] Compared with Example 1, the difference is that: 15.0% coke is added to the mixture, that is, chromite, coke and bentonite are mixed at 82.0wt.%:15.0wt.%:3.0wt.%; the reduction roasting is carried out at 1300℃ for 4 hours, during which H2 is not introduced.

[0062] The pre-reduced pellets had a compressive strength of 2052 N, an iron metallization rate of 98.33%, a chromium metallization rate of 35.03%, and a total metallization rate of 63.04%. The conditions in this comparative example are commonly used in the pre-reduced pellet production process. Compared to Example 1, this comparative example did not include a low-temperature section, and the reduction temperature was 100°C higher than the high-temperature section in Example 1. The total reduction time was 1 hour longer, and the estimated energy consumption was reduced by 20%~25%. Furthermore, this comparative example incorporated 15.0% coke, correspondingly increasing CO2 emissions from the reducing agent by approximately 5%.

Claims

1. A method for preparing ferrochrome pre-reduced pellets by hydrogen-solid carbon coupled reduction, characterized in that: Chromite, solid carbon, binder and water are mixed and pelletized. The resulting wet pellets are then dried, preheated and reduced roasted to obtain chromite pre-reduced pellets. The reduction roasting includes two reduction processes: The first reduction process is as follows: at a temperature of 900℃~1100℃, hydrogen gas is introduced from the outside, and reduction is carried out using hydrogen gas and solid carbon. The second reduction process is as follows: at a temperature of 1200℃~1350℃, the hydrogen gas is stopped, and reduction is carried out using solid carbon.

2. The method for preparing ferrochrome pre-reduced pellets by hydrogen-solid carbon coupling reduction according to claim 1, characterized in that: The chromite has a chromium-to-iron ratio (Cr / Fe) of 1.3 to 1.

9.

3. The method for preparing ferrochrome pre-reduced pellets by hydrogen-solid carbon coupling reduction according to claim 1, characterized in that: The solid carbon includes at least one of coke, anthracite, bituminous coal, and semi-coke.

4. The method for preparing ferrochrome pre-reduced pellets by hydrogen-solid carbon coupled reduction according to any one of claims 1 to 3, characterized in that: The particle size of both the chromite and the solid carbon is <0.15 mm.

5. The method for preparing ferrochrome pre-reduced pellets by hydrogen-solid carbon coupling reduction according to claim 1, characterized in that: The binder includes at least one of bentonite, quicklime, and organic binder.

6. A method for preparing ferrochrome pre-reduced pellets by hydrogen-solid carbon coupling reduction according to claim 1, 2, 3 or 5, characterized in that: The amount of solid carbon used is 5% to 12% of the mass of the chromite; The amount of the binder used shall not exceed 5% of the mass of the chromite.

7. The method for preparing ferrochrome pre-reduced pellets by hydrogen-solid carbon coupling reduction according to claim 1, characterized in that: The total moisture content of the wet pellets is controlled at 5% to 8%.

8. The method for preparing ferrochrome pre-reduced pellets by hydrogen-solid carbon coupling reduction according to claim 1, characterized in that: The drying process is carried out to ensure that the moisture content of the pellets is <0.5%; The preheating conditions are as follows: preheating at 800℃~1000℃ for 10min~30min in an air atmosphere.

9. The method for preparing ferrochrome pre-reduced pellets by hydrogen-solid carbon coupling reduction according to claim 1, characterized in that: The total reduction time for the two reduction processes is controlled within 2 to 4 hours. The time ratio of the first restoration process to the second restoration process is 3:1 to 1:

1.

10. The method for preparing ferrochrome pre-reduced pellets by hydrogen-solid carbon coupling reduction according to claim 1, characterized in that: The partial pressure of H2 introduced during the first reduction process is 0.05 MPa to 0.1 MPa.