A method for low-temperature reduction roasting pretreatment of chromite pellets under a CH4-containing reducing atmosphere

Abstract

This invention provides a method for low-temperature reduction roasting pretreatment of chromite pellets under a CH4-containing reducing atmosphere, belonging to the field of metallurgical technology. The method includes the following steps: mixing chromite powder, binder, and water to form pellets, obtaining green chromite pellets; mixing CH4 and a reducing gas to obtain a CH4-containing reducing atmosphere, wherein the reducing gas includes H2 and / or CO; and reducing roasting the green chromite pellets in the CH4-containing reducing atmosphere. This invention introduces CH4 gas into the reducing atmosphere, without using solid carbon. CH4 can decompose and release carbon at relatively low temperatures, which is beneficial for the smooth operation of common industrial equipment. The released carbon has high activity and can effectively dissociate the spinel phase in chromite, and rapidly achieve effective reduction of Fe and Cr oxides therein, obtaining chromite pellets with a certain degree of pre-reduction of metallic iron and chromium and a certain strength.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, and in particular to a method for low-temperature reduction roasting pretreatment of chromite pellets under a reducing atmosphere containing CH4. Background Technology

[0002] my country is a major producer of stainless steel, accounting for more than half of the global output. The main raw material for stainless steel smelting is ferrochrome alloy, which is currently mainly produced by melting and reducing ferrochrome ore in submerged arc furnaces. This smelting process generates a large amount of energy consumption and emissions.

[0003] Studies have shown that reasonable and effective pretreatment of chromite pellets is a key way to reduce the overall energy consumption of submerged arc furnaces. Currently, methods for processing chromite powder include traditional cold briquetting, advanced Outotec, and SRC pretreatment processes. Compared to traditional cold briquetting, advanced Outotec and SRC processes reduce the overall energy consumption of chromite alloys by more than one-third. However, even with relatively advanced pretreatment processes like the latter, which mainly rely on adding carbon (coke powder or coal powder) to the green chromite pellets and using the combustion of solid carbon to achieve strength and metal pre-reduction, the potential for further reduction in overall energy consumption and emissions is limited. Reduction roasting of chromite pellets under H2-CO atmosphere can achieve a certain level of strength and metal pre-reduction, but the required temperature is high, typically around 1300℃, making smooth operation of industrial plants challenging. More importantly, the spinel phase in chromite cannot be effectively dissociated under H2 or CO-based reduction conditions without the use of solid carbon, resulting in the inability to reduce Cr oxides and relatively limited pre-reduction of Fe metal. Summary of the Invention

[0004] In view of this, the purpose of this invention is to provide a method for low-temperature reduction roasting pretreatment of chromite pellets under a reducing atmosphere containing CH4. This invention introduces CH4 gas into the reducing atmosphere. CH4 can decompose and release carbon at a relatively low temperature. The released carbon is highly reactive and can effectively dissociate the spinel phase in chromite, and rapidly achieve the effective reduction of Fe and Cr oxides therein.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0006] This invention provides a method for low-temperature reduction roasting pretreatment of chromite pellets under a reducing atmosphere containing CH4, comprising the following steps:

[0007] Chromite powder, binder and water are mixed and pelletized to obtain chromite green pellets;

[0008] CH4 is mixed with a reducing gas to obtain a reducing atmosphere containing CH4;

[0009] The green chromite pellets are subjected to reduction roasting in the CH4-containing reducing atmosphere.

[0010] Preferably, the volume percentage of CH4 in the CH4-containing reducing atmosphere is 5% to 60%, and the reducing gas includes H2 and / or CO.

[0011] Preferably, the source of CH4 includes one or more of the following: natural gas, coke oven gas, coalbed methane, fossil fuel extraction, decomposition of organic waste, anaerobic combustion of biomass, and landfill gas well collection.

[0012] Preferably, the excess coefficient of the CH4-containing reducing atmosphere is 1.00 to 3.00.

[0013] Preferably, the reduction calcination temperature is 800–1100°C, and the holding time is 0.5–3 hours.

[0014] Preferably, the particle size of the chromite powder is <74μm.

[0015] Preferably, the chromite powder contains 35-50 wt% Cr2O3, 15-35 wt% Fe2O3, 1-10 wt% FeO, 2-10 wt% MgO, 3-15 wt% Al2O3, and 2-10% SiO2.

[0016] Preferably, the mass of the binder is 1 to 8% of the mass of the chromite powder.

[0017] This invention provides a method for low-temperature reduction roasting pretreatment of chromite pellets under a reducing atmosphere containing CH4, comprising the following steps: mixing chromite powder, binder and water to form pellets to obtain green chromite pellets; mixing CH4 and reducing gas to obtain a reducing atmosphere containing CH4; and reducing roasting the green chromite pellets in the reducing atmosphere containing CH4.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0019] This invention introduces CH4 gas into a reducing atmosphere without using solid carbon. CH4 can decompose and release carbon at relatively low temperatures, which is beneficial to the smooth operation of common industrial equipment. The released carbon has high activity and can effectively dissociate the spinel phase in chromite and quickly achieve effective reduction of Fe and Cr oxides therein. In particular, the pre-reduction degree of Cr is significantly improved, resulting in chromite pellets with a certain degree of pre-reduction of metallic iron and chromium and a certain strength. The effect of reducing the overall energy consumption and emissions of ferrochrome alloys is more significant, and the application prospects are broad.

[0020] Furthermore, by adjusting the volume percentage of methane in the CH4-containing reducing atmosphere, the flow rate of the CH4-containing reducing atmosphere, the excess coefficient, and the reduction roasting temperature, this invention effectively controls the amount of low-melting-point binder phase generated, further improving the strength of the pellets after reduction roasting. The compressive strength of the reduced-roasted chromite pellets reaches over 1500N, and the green pellet drop strength is 3-8 times / 0.5m, fully meeting the requirements for charging into the submerged arc furnace. Moreover, the pre-reduction degree of Fe and Cr reaches 90% and 50%, respectively, and the total metal pre-reduction degree reaches 70%. Attached Figure Description

[0021] Figure 1 The bar chart shows the iron and chromium metallization rates of the chromite pellets prepared in Examples 1-5 and Comparative Examples 1-2.

[0022] Figure 2 The bar chart shows the compressive strength of the chromite pellets prepared in Examples 1-5 and Comparative Examples 1-2.

[0023] Figure 3 The image shows the microstructure and surface scan of the chromite pellets prepared in Example 1. Detailed Implementation

[0024] This invention provides a method for low-temperature reduction roasting pretreatment of chromite pellets under a reducing atmosphere containing CH4, comprising the following steps:

[0025] Chromite powder, binder and water are mixed and pelletized to obtain chromite green pellets;

[0026] CH4 is mixed with a reducing gas to obtain a reducing atmosphere containing CH4;

[0027] The green chromite pellets are subjected to reduction roasting in the CH4-containing reducing atmosphere.

[0028] This invention involves mixing chromite powder, binder, and water to form pellets, thereby obtaining green chromite pellets.

[0029] In this invention, the particle size of the chromite powder is preferably <74μm.

[0030] In this invention, the chromite powder preferably contains 35-50 wt% Cr2O3, more preferably 38-45 wt% Fe2O3, more preferably 20-30 wt% FeO, more preferably 1-10 wt% FeO, more preferably 3-5 wt% MgO, more preferably 2-10 wt% Al2O3, more preferably 8-12 wt% SiO2, and more preferably 2-10% SiO2, more preferably 5-9 wt%. This invention does not impose any special limitations on the source of the chromite powder; sources well known to those skilled in the art can be used.

[0031] In this invention, the mass of the binder is preferably 1 to 8% of the mass of chromite powder, more preferably 2 to 5%.

[0032] In this invention, the binder preferably comprises bentonite.

[0033] The present invention does not impose any special limitation on the specific method of ball formation; any method known to those skilled in the art can be used.

[0034] This invention mixes CH4 with a reducing gas to obtain a reducing atmosphere containing CH4.

[0035] In this invention, the volume percentage of CH4 in the CH4-containing reducing atmosphere is preferably 5% to 60%.

[0036] In this invention, the flow rate of the CH4-containing reducing atmosphere is preferably 0.1 to 1 L / min.

[0037] In this invention, the excess coefficient of the CH4-containing reducing atmosphere is preferably 1.00 to 3.00, more preferably 1.50 to 2.80.

[0038] In this invention, the reducing gas preferably includes H2 and / or CO. When the reducing gas is a mixture of H2 and CO, this invention does not have a particular limitation on the volume ratio of H2 to CO in the mixture, and any volume ratio of the mixture can be used.

[0039] In this invention, the source of CH4 preferably includes one or more of the following: natural gas, coke oven gas, coalbed methane, fossil fuel extraction, decomposition of organic waste, anaerobic combustion of biomass, and landfill gas well collection.

[0040] In this invention, the source of H2 preferably includes green hydrogen obtained from various renewable energy sources, as well as hydrogen obtained through other means, such as: hydrogen production by water electrolysis, hydrogen production from mineral fuels, hydrogen production by water gasification, syngas from petroleum thermal cracking, hydrogen production from natural gas, hydrogen production from coke oven gas refrigeration, hydrogen as a byproduct of electrolysis of brine, hydrogen produced as a byproduct of brewing industry, or hydrogen produced by the reaction of iron with steam.

[0041] In this invention, the source of CO preferably includes blast furnace gas, coke oven gas, electric arc furnace gas, or coal-fired industrial waste gas.

[0042] This invention fully utilizes CO from secondary energy sources such as secondary gas from electric arc furnaces, combined with green hydrogen obtained from renewable energy sources or other means, and CH4 from coke oven gas, natural gas, or coalbed methane. Through reduction roasting at a relatively low temperature, chromite pellets with strength meeting furnace requirements and high pre-reduction of Fe and Cr are obtained. This invention has great potential in reducing the overall energy consumption of ferrochrome alloys and has broad application prospects.

[0043] After obtaining green chromite pellets and a CH4-containing reducing atmosphere, the present invention performs reduction roasting on the green chromite pellets in the CH4-containing reducing atmosphere.

[0044] In this invention, the reduction calcination temperature is preferably 800-1100℃, and the holding time is preferably 0.5-3h.

[0045] In this invention, the heating rate from room temperature to the reduction calcination temperature is preferably 5 to 20 °C / min.

[0046] In this invention, the reduction roasting is preferably carried out in a reactor that can achieve sealing and precise control of gas composition, and more preferably includes a vertical furnace, a rotary kiln, a rotary hearth furnace or a belt roaster.

[0047] In this invention, an inert gas is preferably introduced into the reactor to remove impurities such as oxygen, nitrogen, and carbon dioxide. Then, the green chromite pellets are placed into the device, and when the temperature is raised to the reduction roasting temperature, the reducing atmosphere containing CH4 is introduced.

[0048] In this invention, the inert gas is preferably nitrogen or argon.

[0049] After the reduction roasting is completed, the present invention preferably loads the obtained ferrochrome pellets with a certain strength and metal pre-reduction degree into a submerged arc furnace for smelting, which significantly reduces the overall energy consumption and emissions of ferrochrome alloys.

[0050] In this invention, the process of loading the material into the electric arc furnace for smelting preferably includes cooling. This invention does not impose any special limitations on the specific cooling method, and any method known to those skilled in the art can be used.

[0051] To further illustrate the present invention, the following detailed description, in conjunction with examples, of the low-temperature reduction roasting pretreatment method for chromite pellets under a CH4-containing reducing atmosphere provided by the present invention, should not be construed as limiting the scope of protection of the present invention.

[0052] Example 1

[0053] The contents of each substance in chromite powder are as follows: Cr2O3 40wt%, Fe2O3 28wt%, FeO 4wt%, MgO 6wt%, Al2O3 10wt%, SiO2 7%, with the balance being MnO and TiO2.

[0054] Green pellets were prepared by adding bentonite (2% by mass of chromite powder) and an appropriate amount of water to chromite powder. The green pellets were placed in a corundum crucible and then placed in a horizontal tube furnace. Argon gas was introduced during the heating process, and the temperature was increased to 1100℃ at a rate of 20℃ / min. Then, the temperature was switched to a CH4-containing reducing atmosphere with a volume ratio of CH4:H2 of 1:1. The excess coefficient of the CH4-containing reducing atmosphere was 2, the flow rate was 0.2L / min, and the reduction roasting time was 1h. During the cooling process, argon gas was switched back. After cooling to room temperature, the pellets were removed to obtain reduced chromite pellets.

[0055] Example 2

[0056] The preparation method of Example 2 is basically the same as that of Example 1, except that the constant temperature "1100℃" is adjusted to "1300℃".

[0057] Example 3

[0058] The preparation method is basically the same as that in Example 1, except that the CH4-containing reducing atmosphere is adjusted to a volume ratio of CH4:H2:CO = 1:1:1.

[0059] Example 4

[0060] The preparation method is basically the same as that in Example 1, except that the flow rate of the CH4-containing reducing atmosphere is adjusted to "1L / min, and the excess coefficient of the CH4-containing reducing atmosphere is 2".

[0061] Example 5

[0062] The preparation method is basically the same as that in Example 1, except that the chromite powder is adjusted to "Cr2O3 43wt%, Fe2O3 30wt%, FeO 5wt%, MgO 7wt%, Al2O3 9wt%, SiO2 5%, with the balance being MnO and TiO2".

[0063] Comparative Example 1

[0064] The preparation method is basically the same as that in Example 1, except that the reducing atmosphere containing CH4 is adjusted to "volume ratio H2:CO = 2:8", the constant temperature is adjusted to "1100℃" to "1300℃", and the reduction calcination time is 3h.

[0065] Comparative Example 2

[0066] The preparation method is basically the same as that in Example 1, except that the reducing atmosphere containing CH4 is adjusted to "volume ratio H2:CO = 6:4", the constant temperature is adjusted to "1100℃" to "1300℃", and the reduction calcination time is 3h.

[0067] Test Example 1

[0068] The Fe and Cr metallization rates of chromite pellets prepared in Examples 1-5 and Comparative Examples 1-2 are as follows: Figure 1 As shown. By Figure 1 As can be seen, in Example 1, after reduction roasting at a constant temperature of 1100℃ for 1 hour in a CH4-containing atmosphere, the metallization rates of Fe and Cr in the pellets were significantly higher than those treated in an atmosphere without CH4. Comparative Examples 1 and 2, roasted at 1300℃ for 3 hours in an atmosphere without CH4, showed Fe metallization rates of 31.4% and 57.5%, respectively, with almost no Cr reduction in both cases. The introduction of CH4 resulted in a significantly higher Fe / Cr metallization rate in the pellets compared to those reduced under methane-free conditions (Comparative Examples 1 and 2), even with a 200℃ reduction in temperature (from 1300℃ to 1100℃) and a two-thirds reduction in time (from 3 hours to 1 hour). This data indicates that the cracking and desorption of carbon by CH4 at relatively low temperatures can effectively crack the spinel phase of chromite and promote the deep reduction of Cr and Fe oxides within it. This invention introduces CH4 into the reduction roasting process of chromite pellets, which can significantly improve the pre-reduction degree of Fe and Cr, while reducing the reduction roasting temperature, thereby effectively reducing the overall energy consumption of smelting in electric arc furnaces.

[0069] Test Example 2

[0070] The compressive strength of the chromite pellets prepared in Examples 1-5 and Comparative Examples 1-2 is as follows: Figure 2 As shown. By Figure 2 It can be seen that the strength of chromite pellets after reduction roasting under reducing atmospheres with or without methane is higher than 2000N, fully meeting the requirements for feeding into the submerged arc furnace. Furthermore, the chromite pellets obtained under a reducing atmosphere containing methane exhibit higher strength.

[0071] Test Example 3

[0072] The microstructure and surface scan of Example 1 are as follows: Figure 3 As shown. By Figure 3It can be seen that white iron metal particles and the binder phase are distributed on the surface of the unreduced chromite spinel in the pellets and are bonded together, which improves the strength of the pellets. Surface scanning results show that the binder phase is mainly composed of CaO-SiO2-FeO.

[0073] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for low-temperature reduction roasting pretreatment of chromite pellets under a reducing atmosphere containing CH4, characterized in that, The specific steps are as follows: Chromite powder, binder and water are mixed and pelletized to obtain chromite green pellets; CH4 is mixed with a reducing gas to obtain a reducing atmosphere containing CH4; The chromite green pellets are subjected to reduction roasting in the CH4-containing reducing atmosphere; The volume ratio of CH4 to H2 in the CH4-containing reducing atmosphere is 1:1, and the reducing gas includes H2. The excess coefficient of the CH4-containing reducing atmosphere is 2.00; The reduction calcination temperature is 1100℃, and the holding time is 1 hour; The flow rate of the CH4-containing reducing atmosphere is 1 L / min; The chromite powder contains 35-50 wt% Cr2O3, 15-35 wt% Fe2O3, 1-10 wt% FeO, 2-10 wt% MgO, 3-15 wt% Al2O3, and 2-10% SiO2.

2. The low-temperature reduction roasting pretreatment method according to claim 1, characterized in that, The sources of CH4 include one or more of the following: natural gas, coke oven gas, coalbed methane, fossil fuel extraction, decomposition of organic waste, anaerobic combustion of biomass, and landfill gas well collection.

3. The low-temperature reduction roasting pretreatment method according to claim 1, characterized in that, The particle size of the chromite powder is <74μm.

4. The low-temperature reduction roasting pretreatment method according to claim 1, characterized in that, The mass of the binder is 1-8% of the mass of the chromite powder.

Images