A method for preparing chromium-iron pre-reduced pellets by hydrogen-solid carbon coupling reduction

Abstract

The application discloses a method for preparing chrome-iron pre-reduced pellets by hydrogen-solid carbon coupling reduction, and belongs to the technical field of chrome-iron alloy metallurgy. The method is to mix chromite, solid carbon, a binder and water to form balls, and the obtained wet pellets are sequentially subjected to drying, preheating and reduction roasting to obtain chrome-iron pre-reduced pellets; the reduction roasting comprises two-stage reduction processes, the first-stage reduction process is that hydrogen is externally introduced, and hydrogen and solid carbon are used for reduction at a temperature of 900 DEG C to 1100 DEG C; and the second-stage reduction process is that hydrogen is stopped to be introduced, and solid carbon is used for reduction at a temperature of 1200 DEG C to 1350 DEG C. The method can improve the reduction efficiency and alloying rate of the chromite, and simultaneously reduce the consumption of the solid carbon, so that the energy-saving and emission-reducing purposes are achieved. The method has the characteristics of simple process, low energy consumption, low carbon emission and the like.

Description

TECHNICAL FIELD

[0001] The application relates to a production method of chromite pre-reduced pellets, in particular to a method for preparing chromite pre-reduced pellets by hydrogen-solid carbon coupling reduction, and belongs to the technical field of chromite alloy metallurgy. BACKGROUND

[0002] In the prior art, chromite alloy is mainly produced by the method of "chromite ore briquetting-ore smelting", wherein the briquetting finishing process is related to the comprehensive energy consumption of the briquetting process and the ore smelting process, is a key link for improving the quality and efficiency of chromite production, saving energy and reducing carbon, and is of great significance to the green and low-carbon development of the chromite alloy industry. The pre-reduced pellet method has more advantages than the sintering and oxidized pellet methods, which not only realizes the function of briquetting, but also undertakes most of the reduction function for ore smelting, reduces the reduction load of the ore smelting furnace, and significantly reduces the comprehensive energy consumption of briquetting-smelting. At present, the production of chromite pre-reduced pellets mainly has two processes of rotary kiln and rotary hearth furnace, wherein the rotary kiln process has the advantages of smaller equipment area, lower investment and lower energy consumption compared with the rotary hearth furnace process, and is the mainstream process for the production of chromite pre-reduced pellets, which has developed rapidly 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. The mineral structure of chromite is complex, and chromium and iron are spinel minerals combined with multiple elements. Due to the limitation of interface reaction, the reduction of chromium oxide is much more difficult than that of iron oxide. When the temperature is 1000 DEG C to 1300 DEG C, the reduction of iron oxide is mainly carried out, and metallic iron is generated; when the reduction temperature is higher than 1300 DEG C, the chromite reaches a faster reduction rate. In the actual industrial production process, in order to improve the reduction efficiency, the reduction temperature has to be increased, and the highest temperature can reach 1400 DEG C to 1450 DEG C. However, the chromite pellets are easy to melt at high temperature, which aggravates the rotary kiln ring, and the reduction temperature range is narrow. At the same time, the chromium oxide is difficult to react with the gas reducing agent at 1200 DEG C to 1500 DEG C, and the reduction of chromium mainly depends on the reaction of solid carbon at this temperature. Therefore, the high reduction temperature of chromite and the dependence on coal are still the main bottlenecks restricting the reduction of energy consumption level.

[0004] Currently, new technologies are developed for reducing chromite energy-saving and emission reduction, for example, Chinese patent (authorized publication number: CN113549726B) discloses that chromite pellets are oxidized and roasted at 1230℃~1320℃, and chromium iron spinel is first converted into hematite and chromium oxide, and then reduced at 800℃~1000℃ by using H2, CO and other reducing gases. In addition, Chinese patent application (authorized publication number: CN117051230B) discloses that H2 and CO mixed gas is used for direct reduction at 1200℃~1400℃. Although the above-mentioned technologies use full H2 instead of traditional coal energy and reduce CO2 emissions, there are still some deficiencies: 1) the additional energy consumption is increased due to the addition of the oxidation roasting process; 2) although the use of H2 direct reduction is excellent for iron reduction, the reduction effect on chromium is poor; 3) hydrogen reduction is an endothermic reaction, and the gaseous H2O produced will take away part of the heat energy, so full hydrogen reduction needs additional heat input to maintain the reaction temperature, which in turn increases the energy consumption.

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

[0006] In view of the defects of the existing chromite pre-reduction process, such as high reduction temperature, excessive dependence on solid carbon and large CO2 emission, and the shortcomings of full hydrogen reduction requiring energy compensation, the purpose of the present application is to provide a method for preparing chromium-iron pre-reduced pellets by hydrogen-solid carbon coupled reduction. The method uses a two-stage reduction roasting process to continuously reduce chromite at different temperatures using different reducing agents. At low and high temperatures, “hydrogen + solid carbon” and “solid carbon” are used as reducing agents, respectively, to reduce most of the iron oxides to metallic iron and an appropriate amount of chromium oxides to metallic chromium, and finally obtain chromium-iron pre-reduced pellets with high metallization rate. This method uses hydrogen-solid carbon coupled reduction. On the one hand, hydrogen replaces part of the solid carbon to undertake the reduction function, reducing the consumption of traditional fossil energy in the pre-reduction process and reducing carbon emissions. On the other hand, part of the solid carbon is used as a heat supply to compensate for the sharp reduction in temperature during hydrogen reduction and the technical problem that hydrogen cannot reduce chromium oxides. In addition, the method introduces a low-temperature reduction stage to regulate the reduction reaction, which reduces the energy consumption while maintaining a high metallization rate of the pellets compared with conventional pre-reduction.

[0007] To achieve the above technical purpose, the present application provides a method for preparing chromium-iron pre-reduced pellets by hydrogen-solid carbon coupled reduction. The method is to mix chromite, solid carbon, binder and water to form pellets, and the obtained wet pellets are sequentially dried, preheated and reduced, to obtain chromium-iron pre-reduced pellets.

[0008] The reduction roasting includes two-stage reduction process:

[0009] The first-stage reduction process is that hydrogen is externally introduced and solid carbon is used for reduction at a temperature of 900 DEG C to 1100 DEG C;

[0010] The second-stage reduction process is that solid carbon is used for reduction at a temperature of 1200 DEG C to 1350 DEG C.

[0011] The key of the technical scheme of the present application is that two-stage reduction processes are designed, the first-stage reduction process is a low-temperature reduction stage, the solid carbon is limited to react with the oxides by controlling a lower temperature, the reduction reaction of H2 is dominant in the stage, the hydrogen gas introduced is preferentially reacted with the easily reducible iron oxides, thereby the consumption of the solid carbon is reduced. However, the hydrogen reduction is a strong endothermic reaction, the heat energy of the reduction reaction system is reduced, thereby the reaction rate is reduced. At the same time, part of the solid carbon is reacted, such as the reaction with the iron oxides, the reaction with the chromite, the reaction with H2O, the CO released can further react with the iron-chromium oxides (or is combusted in the oxidation atmosphere stage of the reactor) to release heat, thereby the heat energy gap of the hydrogen reduction process is made up, and the reduction process is avoided to reduce the reaction rate due to the temperature reduction. The second-stage reduction process is a high-temperature reduction stage, mainly through the residual solid carbon to deeply react with the unreacted chromite and Cr2O3, thereby the metallization rate of chromium is improved.

[0012] The present application has obvious improvement in improving the reduction efficiency of the chromite by designing two-stage reduction processes: in the first-stage reduction process, the hydrogen gas is used as the main reducing agent, and the solid carbon is used as the heat energy supplement source, both of which are used to improve the reduction efficiency of the metal oxides and ensure a higher metallization rate. In the low-temperature reduction process, the hydrogen gas can react with the iron oxides in the gas-solid reaction at a temperature of 900 DEG C to 1100 DEG C, compared with the solid carbon reduction, the gas reducing agent has obvious advantages in kinetics, compared with the carbon monoxide, the hydrogen gas has smaller molecules and higher molecular diffusion rate, the hydrogen reduction rate is faster and more complete, the heat loss and the temperature reduction of the reaction layer caused by the hydrogen reduction process are supplemented by a small part of the solid carbon, and the chromium oxides difficult to be reduced by the hydrogen gas are preliminarily reduced by the solid carbon in the low-temperature reduction process, the residual chromium oxides are deeply reduced by the solid carbon at a high temperature, thereby the chromium oxides are efficiently converted into the metallic chromium, thereby the high reaction rate and the high metallization rate of the chromite can be ensured.

[0013] The present application has obvious advantages in low carbon emission by designing two-stage reduction process: in the traditional pre-reduction process, solid carbon bears the entire reduction function of chromite, and a large amount of CO2 is released in the reduction process. In the present application, hydrogen gas replaces part of the solid carbon to bear the function of reducing agent in the low-temperature reduction process, reducing the solid carbon by about 20% to 30%. As a green energy, hydrogen gas only produces H2O vapor after the reduction reaction, which reduces the CO2 emission by 20% to 30%.

[0014] The present application can reduce energy consumption by designing two-stage reduction process: in order to improve the reduction rate, the maximum temperature in the kiln can reach 1400℃ to 1450℃, and the reduction time is long. In the present application, the pre-reduction of most of the chromite is carried out at a low temperature of 900℃ to 1100℃, and the energy consumption is much lower than that of the traditional pre-reduction process.

[0015] In summary, the technical scheme of the present application uses hydrogen-solids carbon coupled reduction system to produce pre-reduced chromite pellets, and regulates the reaction by setting low-temperature and high-temperature reduction stages, which maintains the high metallization rate of the pre-reduced chromite pellets, improves the production efficiency, reduces the production temperature, reduces the consumption of traditional coal reducing agent, and further reduces the carbon emission and energy consumption in the production process.

[0016] The temperature of the first-stage reduction process of the present application is 900℃ to 1100℃, which is a low-temperature reduction process, and the temperature of the second-stage reduction process is 1200℃ to 1350℃, which is a high-temperature reduction process. The temperature of the first-stage reduction process is further preferably 950℃ to 1000℃, and the temperature of the second-stage reduction process is further preferably 1200℃ to 1250℃. In the first-stage reduction process, the temperature is controlled at a lower temperature, hydrogen gas is used to reduce the easily reduced iron oxides in the chromite spinel structure to metallic iron, and the reaction between solid carbon and iron oxides is controlled to reduce the consumption of solid carbon, while ensuring that part of the solid carbon reacts to generate a small amount of carbon monoxide, which reacts with oxides or oxygen to release heat, supplementing the heat energy absorbed by the hydrogen reaction to prevent the reaction rate from decreasing. If the temperature of the first-stage 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-stage reduction process is too low, the reduction effect of hydrogen gas on iron oxides will be poor, and the metallization rate will be low. The second-stage reduction process is controlled at a higher temperature, which can accelerate the reaction between solid carbon and difficult-to-react chromium oxides, accelerate the formation of carburized body of solid carbon and metallic iron into high-temperature liquid phase, and accelerate the reduction of chromium oxides as a carbon carrier. If the temperature of the second-stage reduction process is too high, the energy consumption of the process will increase; if the temperature of the second-stage reduction process is too low, the reduction effect of solid carbon on chromium oxides will be poor, and the metallization rate will decrease.

[0017] As a preferred scheme, the Cr / Fe ratio of the chromite is 1.3 to 1.9.

[0018] As a preferred scheme, the solid carbon includes at least one of coke, anthracite, bituminous coal, and semicoke. These solid carbons are fossil fuels with high fixed carbon content commonly used in the prior art, and the fixed carbon content of industrial analysis is ≥85%. As a more preferred scheme, the amount of the solid carbon is 5% to 12% of the mass of the chromite. The amount of the solid carbon is further 7% to 10% of the mass of the chromite. As a traditional fossil energy, the solid carbon releases CO2 in the reduction process, and a large amount of addition is not conducive to the realization of the emission reduction target. However, the reduction of the chromite is difficult, and the gaseous reducing agent H2 and CO are difficult to reduce, and the solid carbon is needed for deep reduction. At the same time, the H2 reduction is a strong endothermic reaction, and the additional heat is provided by the reaction of the partial solid carbon to generate CO to form the coupling of hydrogen and solid carbon. Therefore, when the solid carbon is added too much, the CO2 emission is too large, and when the solid carbon is added too little, the reduction of the chromite is insufficient and cannot meet the pre-reduction metallization rate requirement.

[0019] As a preferred scheme, the particle size of the chromite and the solid carbon both satisfies <0.15 mm. The smaller particle size of the chromite and the solid carbon is more conducive to the uniform mixing of the particles of the two, the increase of the contact area, and the better promotion of the solid phase reaction.

[0020] As a preferred scheme, the binder includes at least one of bentonite, quicklime, and organic binder. These binders are binder types commonly used in the prior art. As a preferred scheme, the amount of the binder is not higher than 5% of the mass of the chromite. The specific amount of the binder is related to the type of the binder selected, for example, when the inorganic binder such as bentonite and quicklime is selected, the amount is 1% to 5% of the mass of the chromite. When the organic binder is used, the amount is 0.06% to 0.2% of the mass of the chromite because the effect of the organic binder is obvious but the cost is high. The addition amount of the binder is optimized based on the comprehensive consideration of the cost, binding effect, impurity content, and the like. The preferred inorganic binder such as bentonite and quicklime has low price and good high-temperature strength, but has high impurity content. Therefore, when the amount is too high, the grade of the pellet iron and the grade of the chromium decrease, and when the amount is too low, the strength of the pellet decreases.

[0021] As a preferred scheme, the total water content of the wet pellet is controlled to be 5% to 8%.

[0022] As a preferred scheme, the drying is performed to ensure that the water content of the pellet is <0.5%. Through the drying treatment of the wet pellet, the water content of the pellet is dried to a low level in the drying equipment to avoid the cracking of the pellet in the subsequent heat treatment process. Because the water content of the wet pellet is high, the direct entry into the reduction equipment such as the rotary kiln may cause the explosion and affect the normal operation of the equipment, and the drying is needed to remove the excess water.

[0023] As a preferred scheme, the preheating treatment is performed at 800-1000 DEG C for 10-30 min in an air atmosphere. The preheating treatment is mainly to improve the strength of the pellets to meet the requirement of the mechanical properties of the pellets in the subsequent reduction roasting process. The preheating temperature and time need to be controlled in the specified range, and the temperature is too high and the time is too long, which will cause the carbon consumption in the preheated pellets, and the temperature is too low and the time is too short, which will result in that the strength of the pellets does not meet the requirement of the equipment.

[0024] As a preferred scheme, the total reduction time of the two-stage reduction process is controlled to be 2-4 h. The total reduction time of the two-stage reduction process is further preferably controlled to be 2.5-3.5 h. As a preferred scheme, the time ratio of the first-stage reduction process to the second-stage reduction process is 3:1-1:1. The total reduction time is related to the reaction kinetics, the first-stage reduction time determines the reaction time of the chromite with hydrogen, and the second-stage reduction time determines the reduction time of the chromite with solid carbon, and the appropriate total reduction time and the time ratio of the two-stage reduction process need to be determined according to the reaction rate of the chromite with hydrogen and solid carbon. If the total reduction time is too long, the energy consumption of the process is increased, and if the total reduction time is too short, the reduction is insufficient. If the time ratio of the first-stage reduction process is too large, the metallization rate of chromium is low, and if the time ratio of the first-stage reduction process is too small, the reduction of the iron oxides by hydrogen is small, and too much solid carbon participates in the reduction of the iron oxides, which also leads to a low metallization rate of chromium.

[0025] As a preferred scheme, the partial pressure of H2 in the first-stage reduction process is 0.05-0.10 MPa. The flow rate of hydrogen in the first-stage reduction process affects the total amount of hydrogen in the reduction process, and the more the chromite in the pellets, the greater the required flow rate of hydrogen. Therefore, the flow rate of hydrogen needs to be controlled according to the composition of the pellets, if the flow rate of hydrogen is too large, the energy consumption is increased, and if the flow rate of hydrogen is too small, the reaction of the iron oxides is incomplete, and the metallization rate of the reduced pellets is low. In the second-stage reduction process, the reduction of the chromium oxides by solid carbon is the main reaction, and hydrogen has little effect on the reaction, and the hydrogen can be stopped to reduce the energy consumption.

[0026] The comprehensive metallization rate of the chromium-iron pre-reduced pellets prepared by the method is 60-70%.

[0027] The method for preparing chromium-iron pre-reduced pellets by hydrogen-solid carbon coupling reduction provided by the application comprises the following specific steps:

[0028] (1) A certain amount of solid carbon with a mass of 5wt.%~15wt.% of chromite and inorganic binder with a mass of 1wt.%~5wt.% of chromite (or organic binder with a mass of 0.06wt.%~0.2wt.% of chromite) are mixed with a certain amount of chromite, and water is additionally added to control the moisture content of the material at 5%~8%, and the mixture is obtained after being fully mixed, and wet pellets are prepared in a disc pelletizer; wherein the chromium-iron ratio Cr / Fe of the chromite is 1.3~1.9; the chromite and solid carbon reducing agent are ground before mixing, and the particle size meets <0.15mm; 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, etc.; the mixing equipment is one or more of a strong mixer, a horizontal mixer, and a wheel mill; wherein the diameter of the wet pellets is 8mm~10mm.

[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 at 800℃~1000℃ in an air atmosphere to obtain preheated pellets; wherein 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 first-stage reduction and 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 scheme of the present application has the following beneficial effects:

[0032] (1) The present application takes hydrogen-solid carbon coupling system as the reduction system of chromium iron pellet pre-reduction. In the low-temperature reduction process, hydrogen is used as the main reducing agent, and solid carbon is used as the supplement source. The two are used to reduce the chromium iron ore pellets. In the range of 900 DEG C to 1100 DEG C, hydrogen is used as the reducing agent to react with iron oxide in the gas-solid reaction, which has obvious advantages in kinetics compared with the solid-solid reaction of solid carbon as the reducing agent. Compared with carbon monoxide as the reducing agent, hydrogen has smaller molecules and higher molecular diffusion rate, so the reduction rate of hydrogen is faster and more complete. However, the reduction reaction of hydrogen is a strong endothermic reaction, which causes the temperature of the reaction layer to decrease during the reduction process, and the reaction rate will decrease sharply. At the same time, the reduction potential of hydrogen is not as good as that of solid carbon, so it is difficult to reduce chromium oxide. In the low-temperature reduction process, part of the solid carbon is used for reduction to form a hydrogen-solid carbon coupling reduction system, which can make up for the decrease of the reaction layer temperature in the hydrogen reduction process to maintain the fast and efficient hydrogen reduction, reduce most of the easily reducible iron oxides to metallic iron, and at the same time, make part of the chromium oxides also be preliminarily reduced in the low-temperature reduction process. Then, the solid carbon is used to reduce the chromium oxides at high temperature to make the difficult-to-reduce chromium oxides be efficiently converted into metallic chromium, further improve the metallization rate of the pellets, and make the comprehensive metallization rate of the pellets reach the level of 60% to 70%.

[0033] (2) The present application uses hydrogen as the main reducing agent in the first-stage 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 in the reduction process. As a green energy, hydrogen only produces H2O gas after the reduction reaction, which reduces the CO2 emission by 20% to 30%.

[0034] (3) The present application divides the reduction reaction into two stages, i.e. the first-stage reduction at low temperature and the second-stage reduction at high temperature. In the first-stage reduction process, the hydrogen-solid carbon coupling reduction system is designed. On the one hand, the use of hydrogen as the main reducing agent can reduce the reduction temperature, reduce the consumption of solid carbon, and fully play the reduction role of hydrogen on the easily reducible iron oxides. On the other hand, compared with the traditional solid carbon reduction process which needs to be reduced at 1400 DEG C to 1450 DEG C for 4 to 5 hours, the pre-reduction of most of the chromium iron ore in the present application is carried out at low temperature of 900 DEG C to 1100 DEG C, which requires much lower energy consumption than the current traditional pre-reduction process. DETAILED DESCRIPTION

[0035] In order to facilitate the understanding of the present application, the following will combine the preferred embodiments to describe the present application more fully and in detail, but the protection scope of the present application is not limited to the following specific embodiments.

[0036] Unless otherwise defined, all terms used in the description, including technical and scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art. The description uses the specific terminology of the state of the art, solely for the purpose of a clear and complete description of the application.

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

[0038] In the following examples, the test indicators, the detection standard of the compressive strength of the pellets is GB / T 14201-2018; the detection standard of the total iron content TFe and the total chromium content TCr is GB / T 5687.13-2021; the detection standard of the metallic iron content MFe is GB / T 6730.6-2016; the metallic chromium content MCr has no determination standard, in the present method, it is determined by the X-ray fluorescence spectrometry method in GB / T 5687.1 after oxalic acid leaching; the detection standard of the total carbon content TC is GB / T 4699.4-2008. The iron metallization rate η Fe , the chromium metallization rate η Cr and the total metallization rate η T are calculated as follows:

[0039] ;

[0040] ;

[0041] ;

[0042] Example 1

[0043] The chromite (chromium / iron ratio Cr / Fe is 1.4) and the coke are ground before mixing, so that the particle size meets <0.15 mm. Then 10.0wt% of the coke by mass of the chromite, 3.0wt.% of the bentonite by mass of the chromite are mixed with the chromite, and water is additionally added to make the moisture content of the material 5%. After being fully mixed, the mixture is made into wet pellets with a diameter of 10±1 mm in a disc pelletizer. The wet pellets are dried in a drying device until the moisture content of the pellets is <0.5%, and then the dried pellets are preheated at 1000℃ in an air atmosphere for 20 min to obtain preheated pellets. The compressive strength is measured to be 311 N, which meets the requirement of being greater than 200 N. The preheated pellets are subjected to reduction roasting, and the first-stage and second-stage reduction temperatures are set to 950℃ and 1200℃, respectively. H2 with a partial pressure of 0.05 MPa is introduced in the first stage, and H2 is stopped in the second stage. The total reduction time is 3 h, and the ratio of the first-stage reduction time to the second-stage reduction time is 2:1, i.e. the first-stage reduction time is 2.0 h and the second-stage reduction time is 1.0 h. After roasting, the pre-reduced pellets are obtained, and the compressive strength is 2206 N, the iron metallization rate is 98.66%, the chromium metallization rate is 35.81%, and the total metallization rate is 62.95%.

[0044] Example 2

[0045] The chromite (chromium / iron ratio Cr / Fe is 1.6) and the reducing agent are ground before mixing, so that the particle size meets <0.15 mm. Then 11.0wt% of the coke by mass of the chromite, 3.0wt.% of the bentonite by mass of the chromite are mixed with the chromite, and water is additionally added to make the moisture content of the material 8%. After being fully mixed, the mixture is made into wet pellets with a diameter of 10.0±0.5 mm in a disc pelletizer. The wet pellets are dried in a drying device until the moisture content of the pellets is <0.5%, and then the dried pellets are preheated at 1000℃ in an air atmosphere for 25 min to obtain preheated pellets. The compressive strength is measured to be 357 N, which meets the requirement of being greater than 200 N. The preheated pellets are subjected to reduction roasting, and the first-stage and second-stage reduction temperatures are set to 1000℃ and 1300℃, respectively. H2 with a partial pressure of 0.1 MPa is introduced in the first stage, and H2 is stopped in the second stage. The total reduction time is 3 h, and the ratio of the first-stage reduction time to the second-stage reduction time is 3:2, i.e. the first-stage reduction time is 1.8 h and the second-stage reduction time is 1.2 h. After roasting, the pre-reduced pellets are obtained, and the compressive strength is 2133 N, the iron metallization rate is 97.98%, the chromium metallization rate is 34.28%, and the total metallization rate is 61.50%.

[0046] Example 3

[0047] Chromite (chromium iron ratio Cr / Fe is 1.9) and reducing agent are ground before mixing, so that the particle size is all less than 0.15 mm. Then 12.0wt% anthracite and 0.2wt.% sodium carboxymethyl cellulose (CMC) are added to the chromite, and water is additionally added to make the moisture content of the material 8%. After mixing, wet pellets with a diameter of 10.0±0.5mm are prepared in a disc pelletizer. The wet pellets are dried in a drying device until the moisture content of the pellets is less than 0.5%. Then the dried pellets are preheated at 1000℃ in an air atmosphere for 10min to obtain preheated pellets, and the compressive strength of which is 320N, meeting the requirement of greater than 200N. The preheated pellets are reduced and roasted, and the first-stage and second-stage reduction temperatures are set to 1100℃ and 1350℃, respectively. H2 with a partial pressure of 0.1 MPa is introduced in the first stage, and H2 is stopped in the second stage. The total reduction time is 4h, and the ratio of the first-stage reduction time to the second-stage reduction time is 1:1, i.e. the first-stage reduction time is 2h and the second-stage reduction time is 2h. After roasting, pre-reduced pellets are obtained, and the compressive strength of which is 1656N, the iron metallization rate is 95.98%, the chromium metallization rate is 33.28%, and the total metallization rate is 60.35%.

[0048] Comparative Example 1

[0049] Compared with Example 1, the only difference is that H2 is not introduced in the first-stage and second-stage reduction processes.

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

[0051] Comparative Example 2

[0052] Compared with Example 1, the only difference is that no coke is added to the mixture, i.e. the chromite is only mixed with bentonite at a ratio of 97.0wt.%:3.0wt.%.

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

[0054] Comparative Example 3

[0055] Compared with Example 1, the only difference is that no low-temperature stage is set, i.e. the preheated pellets are reduced at 1200℃ for 3h, and H2 with a flow rate of 0.3L / min is introduced during the reduction.

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

[0057] Comparative Example 4

[0058] The only difference compared with Example 1 is that the high-temperature stage is not set, i.e. the preheated pellets are reduced at 1050℃ for 3h, during which H2with a flow rate of 0.3L / min is introduced.

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

[0060] Comparative Example 5

[0061] The difference compared with Example 1 is that the mixed material is mixed with 15.0% of coke, i.e. the chromite, coke and bentonite are mixed at 82.0wt.%: 15.0wt.%: 3.0wt.%; the reduction roasting is reduced at 1300℃ for 4h, during which no H2is introduced.

[0062] The compressive strength of the pre-reduced pellets is 2052N, the iron metallization rate is 98.33%, the chromium metallization rate is 35.03%, and the total metallization rate is 63.04%. The conditions of this comparative example are the commonly used conditions for the production process of pre-reduced pellets. Compared with Example 1, this comparative example does not set a low-temperature stage, and the reduction temperature is 100℃ higher than the high-temperature stage in Example 1, and the total reduction time is 1h longer, which is estimated to reduce energy consumption by 20%~25%; in addition, this comparative example is mixed with 15.0% of coke, and accordingly the CO2emission in the reducing agent increases by about 5%.

Claims

1. A method for preparing chromium-iron pre-reduced pellets by hydrogen-solid carbon coupled reduction, characterized in that: Mixing chromite, solid carbon, binder and water to make wet pellets, and the wet pellets are dried, preheated and reduced in turn, and the chromite pre-reduced pellets are obtained; The reduction roasting includes two-stage reduction process: The first-stage reduction process is that hydrogen is introduced externally, and hydrogen and solid carbon are used for reduction at 900-1100 DEG C; The second-stage reduction process is that hydrogen is stopped to be introduced, and solid carbon is used for reduction at 1200-1350 DEG C.

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

9.

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

5. The method for preparing chromium-iron 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. The method according to claim 1, 2, 3 or 5, characterized in that: The amount of the solid carbon is 5-12% of the mass of the chromite; The amount of the binder is not higher than 5% of the mass of the chromite.

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

8. The method according to claim 1, characterized in that: The drying is to ensure that the water content of the pellets is <0.5%; The preheating treatment is at 800-1000 DEG C for 10-30 min in air atmosphere.

9. The method according to claim 1, characterized in that: The total reduction time of the two-stage reduction process is controlled to be 2-4 h; The time ratio of the first-stage reduction process to the second-stage reduction process is 3:1-1:

1.

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