A method for producing direct reduced pellets by a grate-kiln

Abstract

The application discloses a method for preparing direct reduction pellets by a chain grate-rotary kiln, and the method steps are as follows: 1) mixing uniformly according to the weight ratio of iron concentrate >=98.8% and bentonite <=1.2% to obtain a mixture; the iron concentrate has TFe >=68.5% and a particle size of -0.045 mm >=75%; the bentonite is sodium-based bentonite with a water absorption rate >=420%; 2) high-pressure roller grinding the mixture until the specific surface area of the mixture reaches >=1900cm 2 / g; and 3) pelletizing the mixture after high-pressure roller grinding, and the obtained green pellets have a moisture content of 7.0-8.0%, a compression strength of 10-15 N / piece. The method reduces the bentonite ratio, improves the falling strength of the green pellets, increases the qualified particle size proportion of the green pellets, improves the uniformity and air permeability of the material layer, improves the oxidation atmosphere of the rotary kiln, fully oxidizes the direct reduction pellets, reduces the FeO content of the pellets, and improves the strength of the direct reduction pellets.

Description

Technical Field

[0001] This invention belongs to the field of direct reduction technology in iron and steel metallurgy, and in particular to a method for preparing direct reduction pellets using a chain grate machine-rotary kiln. Background Technology

[0002] In the iron and steel metallurgical industry, traditional long-process steelmaking suffers from high energy consumption and large carbon emissions. The ironmaking process accounts for approximately 60%–70% of the total energy consumption in the steel industry, while the blast furnace process accounts for about 75% of the industry's carbon emissions. However, the carbon emissions from the "DRI (30-50%) + scrap steel (50-70%) + electric arc furnace" short-process steelmaking can be reduced by 40-65% compared to the "blast furnace + converter" long-process. Therefore, developing the "DRI (30-50%) + scrap steel (50-70%) + electric arc furnace" short-process is a key direction for promoting emission reduction in the steel industry.

[0003] The mainstream gas-based direct reduction technologies worldwide are Midrex and HYL, and the main raw material required for direct reduction technology is direct reduction pellets. The main difference between direct reduction pellets and those used in ordinary blast furnaces is that direct reduction pellets have a higher grade, requiring TFe ≥ 67%, SiO2 ≤ 3.0%, better reducibility, and an expansion index ≤ 15%. Currently, the main producer of direct reduction pellets is Brazil; there are currently no domestic companies producing direct reduction pellets, limited by iron ore resources and a lack of market demand. The requirements for TFe, SiO2, as well as physical and metallurgical properties, of direct reduction pellets differ significantly from those of ordinary blast furnace pellets. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a method for preparing direct reduction pellets using a chain grate-rotary kiln to meet the requirements of gas-based direct reduction.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: The method steps are as follows: 1) After mixing the iron concentrate ≥98.8% and bentonite ≤1.2% by weight, a mixture is obtained; the iron concentrate has TFe ≥68.5wt% and -0.045mm particle size ≥75%; the bentonite is sodium-based bentonite with a water absorption rate ≥420%;

[0006] 2) The mixture is subjected to high-pressure roller milling until the specific surface area of ​​the mixture reaches ≥1900 cm². 2 / g;

[0007] 3) The mixture after high-pressure roller milling is pelletized, and the resulting green pellets have a moisture content of 7.0-8.0 wt% and a compressive strength of 10-15 N / pellet.

[0008] 4) The green balls are spread on a chain grate machine for two drying and two preheating processes; the first drying temperature is 210℃~420℃ and the first drying time is 4min~6min, and the second drying temperature is 520℃~620℃ and the second drying time is 3min~5min; the first preheating temperature is 720℃~900℃ and the first preheating time is 3min~5min, and the second preheating temperature is 1000℃~1080℃ and the second preheating time is 9min~12min.

[0009] 5) The preheated green pellets are sent to a rotary kiln for roasting. The heat source is natural gas. The roasting temperature is 1200℃~1300℃ and the roasting time is 27min~35min.

[0010] 6) The roasted pellets are subjected to secondary cooling; the first cooling temperature is 950℃~1080℃ and the first cooling time is 9min~15min, and the second cooling temperature is 500℃~800℃ and the second cooling time is 9min~15min.

[0011] Furthermore, in step 1), the specific surface area of ​​the iron concentrate is ≥1700 cm². 2 / g, specific surface area of ​​bentonite ≥3500cm² 2 / g.

[0012] Furthermore, in step 1), the iron concentrate is first dried to a moisture content of 6.0–7.0 wt%.

[0013] Furthermore, in step 6), the pellets are discharged from the rotary kiln after the temperature reaches ≤150℃.

[0014] The beneficial effects of adopting the above technical solution are as follows: the bentonite used in this invention is a high-efficiency composite sodium-based bentonite, and the iron concentrate has high fineness and high grade; at the same time, the use of high-pressure roller mill increases the specific surface area of ​​the mixture, which not only reduces the bentonite ratio, but also improves the drop strength of green pellets, increases the proportion of qualified green pellets, and improves the uniformity and permeability of the material layer; the use of high-calorific-value fuel natural gas for roasting can reduce oxygen consumption, improve the oxidizing atmosphere of the rotary kiln, enable the direct reduction pellets to be fully oxidized, reduce the FeO content of the pellets, and improve the strength of the direct reduction pellets.

[0015] The performance indicators of the directly reduced pellets obtained by this invention are: TFe 67-69%, SiO2 2.0-3.0%, FeO 0.4-1.5%, compressive strength 2680-2910N, reducing index 3.9-5.1%, expansion index 14-15%, and metallization rate 92-95%. Attached Figure Description

[0016] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0017] Figure 1 This is a schematic diagram of the process of the present invention. Detailed Implementation

[0018] The method for preparing direct reduced pellets using a chain grate machine and rotary kiln employs the following steps:

[0019] 1) The raw materials are iron concentrate and bentonite; the iron concentrate has a TFe content of ≥68.5wt%, a SiO2 content of ≤2.2wt%, and a specific surface area of ​​≥1700 cm². 2 / g, -0.045mm particle size ≥75wt%; if TFe is low, it will affect the grade of direct reduction pellets, further affecting the performance of the electric furnace; if the specific surface area is low and the proportion of -0.045mm particle size is low, it will affect the amount of bentonite added, and thus affect the grade of direct reduction pellets. Excessive moisture content in the iron concentrate will affect the processing effect of the high-pressure roller mill, while insufficient moisture content will affect the stability of pelletizing. The iron concentrate is fed into a dryer and dried at a temperature of 800℃~1050℃, controlling the moisture content of the dried iron concentrate to 6.0%~7.0wt%.

[0020] The bentonite is sodium-based bentonite with a water absorption rate ≥420%, a colloidal value ≥75% / 3g, and a specific surface area ≥3500cm². 2 / g, -0.045mm particle size ≥60wt%, moisture ≤11wt%, Na2O ≥2.3wt%, SiO2 ≤55wt%. By controlling the above indicators, the drop strength of green pellets and the qualified particle size of green pellets can be guaranteed, the amount of bentonite added can be reduced, and the quality of direct reduced pellets can be further guaranteed.

[0021] The raw materials are mixed in a ratio of ≥98.8 wt% iron concentrate and ≤1.2 wt% bentonite, preferably 98.8–99.5 wt% iron concentrate and 0.5–1.2 wt% bentonite; after mixing, a mixture is obtained.

[0022] 2) The mixture is fed to a high-pressure roller mill for high-pressure roller milling to increase the specific surface area of ​​the mixture until the specific surface area of ​​the mixture reaches ≥1900 cm². 2 / g.

[0023] 3) The mixture after passing through the high-pressure roller mill is conveyed into a pelletizing disc for pelletizing. The pelletizing disc rotates at 6–8.5 rpm / min and has an inclination angle of 45°–51°. The moisture content of the resulting green pellets is controlled at 7.0%–8.0 wt%. If the moisture content is too high, the green pellets will not dry sufficiently, making them prone to cracking and affecting the roasting of the pellets and the quality of the final direct reduction pellets. The resulting green pellets should have a drop count of 5–7 times per 0.5 m and a compressive strength of 10–15 N / pellet. If the drop count is too high, the pellets will have good plasticity but will easily stick together; if the drop count is too low, the green pellets will break during transportation, affecting the qualified particle size of the green pellets and the air permeability during drying and preheating.

[0024] 4) The green pellets are screened with a large sieve gap of 15mm and a small sieve gap of 8mm to obtain qualified green pellet size, with 8-15mm particle size content ≥90%.

[0025] 5) The green pellets are evenly spread onto the chain grate machine and subjected to two drying cycles followed by two preheating cycles. Both drying cycles are exhaust drying. The first drying temperature is 210℃~420℃ and the first drying time is 4min~6min. The second drying temperature is 520℃~620℃ and the second drying time is 3min~5min. The first preheating temperature is 720℃~900℃ and the first preheating time is 3min~5min. The second preheating temperature is 1000℃~1080℃ and the second preheating time is 9min~12min. In the above process, the purpose of the first drying is to remove moisture from the green pellets, the second drying is to remove crystal water from the green pellets, the first preheating is for the initial oxidation of the green pellets, and the second preheating is to raise the temperature of the green pellets so that they do not crack when entering the rotary kiln.

[0026] 6) The preheated green pellets are sent to a rotary kiln for roasting, with natural gas as the heat source; the roasting temperature is 1200℃~1300℃ and the roasting time is 27min~35min.

[0027] 7) The roasted pellets undergo two cooling processes: the first cooling temperature is 1080℃~950℃, and the cooling time is 9min~15min; the second cooling temperature is 800℃~500℃, and the cooling time is 9min~15min; the pellets are discharged from the rotary kiln when the temperature is ≤150℃, thus obtaining the direct reduced pellets. The two-stage cooling process involves first cooling the roasted pellets at a higher ambient temperature to avoid a sudden temperature drop that could cause compressive imbalance in the pellet ore; then cooling the pellets at a lower ambient temperature to improve cooling efficiency. This results in better pellet strength and a lower ferrous oxide content in the pellet ore.

[0028] Examples 1-8: The specific method for preparing direct reduced pellets using this chain grate machine-rotary kiln is as follows.

[0029] 1) The parameters of the iron concentrate used in each embodiment are shown in Table 1; the parameters of the bentonite used in each embodiment are shown in Table 2;

[0030] Table 1: Parameters of Iron Concentrate

[0031]

[0032] Table 2: Parameters of Bentonite

[0033]

[0034] 2) The pelletizing parameters and green pellet parameters for each embodiment are shown in Table 3;

[0035] Table 3: Pelletizing and Green Pelletizing Parameters

[0036]

[0037] 3) The chain grate machine used is 41 meters long and 4 meters wide. The drying and preheating process parameters for each embodiment are shown in Table 4.

[0038] Table 4: Drying and Preheating Process Parameters

[0039]

[0040] 4) The rotary kiln used has a diameter of 5 meters and a length of 35 meters; the roasting parameters and two cooling parameters for each embodiment are shown in Table 5;

[0041] Table 5: Calcination parameters and two cooling parameters

[0042]

[0043] 5) The indicators of the directly reduced pellets obtained in each embodiment are shown in Table 5;

[0044] Table 6: Direct Reduction Pellet Indicators

[0045]

[0046] As shown in Table 6, the parameters of the obtained direct reduction pellets meet the requirements of gas-based direct reduction technology, and at the same time provide reference data for finding iron concentrate and bentonite raw materials required for direct reduction pellet ore.

Claims

1. A method for preparing direct reduction pellets using a chain grate machine-rotary kiln, characterized in that, The method steps are as follows: 1) Mix the iron concentrate ≥98.8% and bentonite ≤1.2% by weight to obtain a mixture; the iron concentrate has TFe ≥68.5wt% and -0.045mm particle size ≥75%; the bentonite is sodium-based bentonite with a water absorption rate ≥420%; 2) The mixture is subjected to high-pressure roller milling until the specific surface area of ​​the mixture reaches ≥1900 cm². 2 / g; 3) The mixture after high-pressure roller milling is pelletized, and the resulting green pellets have a moisture content of 7.0-8.0 wt% and a compressive strength of 10-15 N / pellet. 4) The green balls are spread on a chain grate machine for two drying and two preheating processes; the first drying temperature is 210℃~420℃ and the first drying time is 4min~6min, and the second drying temperature is 520℃~620℃ and the second drying time is 3min~5min; the first preheating temperature is 720℃~900℃ and the first preheating time is 3min~5min, and the second preheating temperature is 1000℃~1080℃ and the second preheating time is 9min~12min. 5) The preheated green pellets are sent to a rotary kiln for roasting. The heat source is natural gas. The roasting temperature is 1200℃~1300℃ and the roasting time is 27min~35min. 6) The roasted pellets are subjected to secondary cooling; the first cooling temperature is 950℃~1080℃ and the first cooling time is 9min~15min, and the second cooling temperature is 500℃~800℃ and the second cooling time is 9min~15min.

2. The method for preparing direct reduction pellets using a chain grate machine-rotary kiln according to claim 1, characterized in that: In step 1), the specific surface area of ​​the iron concentrate is ≥1700 cm². 2 / g, specific surface area of ​​bentonite ≥3500cm² 2 / g.

3. The method for preparing direct reduction pellets using a chain grate machine-rotary kiln according to claim 1, characterized in that: In step 1), the iron concentrate is first dried to a moisture content of 6.0-7.0 wt%.

4. A method for preparing direct reduction pellets using a chain grate machine-rotary kiln according to claim 1, 2, or 3, characterized in that: In step 6), the pellets are discharged from the cooling equipment after the temperature reaches ≤150℃.

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