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Pelletization
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Pelletization is of four different types of production
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Pelletizing:
Iron Ore pellets are spheres of typically 6–16 mm (0.24–0.63 in) to be used as raw material forblast furnaces or DRI Kilns They typically contain 62%-72% Fe and various additional material adjusting the chemical composition and the metallurgic properties of the pellets. Typically Limestone, Dolomite, and Olivine is added and Bentonite is used as binder. The process of pelletizing combines mixing of the raw material, forming the pellet and a thermal treatment baking the soft raw pellet to hard spheres. The raw material is rolled into a ball, then fired in a kiln to sinter the particles into a hard sphere. The configuration of iron ore pellets as packed spheres in the blast furnace allows air to flow between the pellets, decreasing the resistance to the air that flows up through the layers of material during the smelting. The configuration of iron ore powder in a blast furnace is more tightly-packed and restricts the air flow. This is the reason that iron ore is preferred in the form of pellets rather than in the form of finer particles. |
Main Technology
Pelletizing section: the main equipment is disc pelletizer with Dia.3.2-7.5m, 45-50° dip angle, 0.5~1m/s linear speed. Add water while pelletizing, but the amount can't be too much, or it will influence the strength of pellet, even make it broken. So it's an important factor to control material charging and watering.
Preheating section: The equipment is different according to the technology selection. Generally, the first step is to dry green pellet to avoid pellet cracking when it reaches to high temperature and the water mostly vaporises. The second is calcination, the calcination is separated into three sections preheating, calcination and soaking. Preheating is for removing residual moisture, calcination for consolidation by high temperature and increasing mechanical strength of pellet, at the same time, it can increase oxidizability and desulphurize the pellet.
Pelletizing section: the main equipment is disc pelletizer with Dia.3.2-7.5m, 45-50° dip angle, 0.5~1m/s linear speed. Add water while pelletizing, but the amount can't be too much, or it will influence the strength of pellet, even make it broken. So it's an important factor to control material charging and watering.
Preheating section: The equipment is different according to the technology selection. Generally, the first step is to dry green pellet to avoid pellet cracking when it reaches to high temperature and the water mostly vaporises. The second is calcination, the calcination is separated into three sections preheating, calcination and soaking. Preheating is for removing residual moisture, calcination for consolidation by high temperature and increasing mechanical strength of pellet, at the same time, it can increase oxidizability and desulphurize the pellet.
Vertical Kiln Type:
Vertical furnace is one heat-exchangeing equipment in the principle of back flow. The pellet goes down with even pacing in the vertical kiln. The hot gas is into furnace by nozzle rifile, exchanging heat bottom-up with the pellet top-down. After drying, preheating, the green pellet will be consolidated in roasting area, cooled at the bottom of kiln and then discharged. There sets ventilation wall in the furnace, drying bed in the furnace top. It improves the calcination condition, and increases the production capacity and pellet quality. The technical features are as following: 1) Pellet drying and preheating can be done at the upside by the rose hot gas. The top drying bed drys the pellet on the grate by the rising hot gas (the mixture of hot air from cooling zone and rising hot gas from roasting area. It is 550~750℃.). It not only accelerates the drying speed but also increases thermal efficiency by economizing the hot exhaust gas. At the same time, it reduces the ball-cracking in the process of drying and preheating because of reasonable air distribution, and dust decreased, air permeability enhanced, so it is useful to strengthen calcination. 2) It can produce 1100~1150℃ flue gas into furnace by burning the blast furnace of low hot value in the combustion room. With the ventilation wall seted, it almost solves the disturbance and mix of cooling air to the the flue gas, and ensure the calcinated temperature required by the magnetite ore. Besides that, it remains stable to the height of roasting area and calcination temperature for making the calcination consolidation smoothly. 3) Through ventilation wall, it can overcome the problem of dead stock column in the upper center of furnace caused by edge effect of airflow. The air distribution will be more uniform and the pellet quality will be increased. The heat consumption of magnetite ore by vertical kiln is about 350~600MJ/t. |
Belt type roasting machine : is similar to the belt type sintering machine. However, the production processes of the two machines are completely different. There are six sections in the whole belt type roasting machine, including drying, preheating, fuel ignition, roasting, soaking and cooling.
Features: 1)There set bed charge and rim charge, the bed charge is for protecting grate and trolley from burning-out by high temperature, and air distribution balance. It can absorb part of waste heat when drying under layer, and the latent heat is recoveried in the blast cooling zone again to maintain the calcination temperature at substratum for the pellet quality. The rim charge is useful to protect the two sides board of trolley from burning-out, and air leakage. The two items can keep the bed of material to be fully calcinated and the trolley service for a long time. 2) It adopts the process of mixing air blast and draft to dry pellet, not only to intensify drying but also to increase the quality and production of pellet. 3) It takes air blast to cool pellet, after cooling, part heat will circulate directly, part will circulate with the help of blower. The circulating heat is usually used in air-drafting section. 4) The hot exhaust gas from air boxes in various drafting sections will circulate to the blast drying area or drafting preheat zone after adjusting temperature based on the demand. 5) Because of low temperature and much moisture of exhaust gas in drying section, it's emptied. The heat consumption lower because the hot exhaust gas from calcination and cooling zone is used for drying, preheating and combustion-supporting. When to calcinate the magnetite concentrate pellet, the advanced factory takes 380~400MJ/t heat , generally the max number is only 600MJ/t, while calcinating the hematite concentrate pellet, it need about 800~1000MJ/t heat. |
Chain Grate machine -Rotary kiln system is composed of chain grate machine, rotary kiln and cooller The drying, dewatering and preheating is completed in the chain grate machine, the high temperature done in rotary kiln and cooling finished in cooling machine. The process features are as following:
1) It can use the heat gas from rotary kiln to do blast drying, air draft drying and preheating for the pellet on the grate machine. And the length of each stage can be adjusted according to the feature of mineral type. Because it takes chain grate machine only for drying and preheating, the bed charge is not necessary to be set.
2) With the pellet rolling in kiln, it will uniformly heat each part and the contact between particle is closer so that the pellet is of high strength and uniform quality.
3) It can produce oxidized pellet or reduction (or metallation) pellet by controlling the kiln based on the technical requirement, and process multi-metallic mineral by chloridizing roasting.
4) It easily appears ring form etc when misoperation. The heat consumption of chain gratemachine - rotary kiln technology is of big difference with kinds of mineral. As roasting magnetite ore, it is 0.6GJ/t, and 1GJ/t for roasting hematite ore, In addition, it takes 1.35~1.5GJ/t to calcinate mixture of hematite ore and limonite.
1) It can use the heat gas from rotary kiln to do blast drying, air draft drying and preheating for the pellet on the grate machine. And the length of each stage can be adjusted according to the feature of mineral type. Because it takes chain grate machine only for drying and preheating, the bed charge is not necessary to be set.
2) With the pellet rolling in kiln, it will uniformly heat each part and the contact between particle is closer so that the pellet is of high strength and uniform quality.
3) It can produce oxidized pellet or reduction (or metallation) pellet by controlling the kiln based on the technical requirement, and process multi-metallic mineral by chloridizing roasting.
4) It easily appears ring form etc when misoperation. The heat consumption of chain gratemachine - rotary kiln technology is of big difference with kinds of mineral. As roasting magnetite ore, it is 0.6GJ/t, and 1GJ/t for roasting hematite ore, In addition, it takes 1.35~1.5GJ/t to calcinate mixture of hematite ore and limonite.
Plain Circular Straight Grate Type
Features of the Plane Circulation Straight Grate Technology (PCSG)
With this technique of pellet firing, the drying, preheating, roasting and cooling processes are done in the same equipment, which is simple, reliable and easy to maintain, and suitable for magnetite, hematite and other types of iron ores. The PCSG technique has limited heat loss, unlike the Grate-Kiln process in which the return grate bed of the travelling grate radiates a good amount of heat, and the heat loss from the rotary kiln is also considerable due to the wide area of the kiln and the return hot air pipeline.
The amount of heat resistant alloy steel, total weight of main and auxiliaries, and investment of main part of the PCSG is only50% that of the widely used Chain Circular Straight Grate Roasting Machine. And the investment is about 60% that of the whole “travelling grate – rotary kiln – annular cooler” system.
With less civil construction work needed such as foundation, plant buildings and steel structures, the construction cost and construction period of PCSG are remarkably reduced.
As pellets remain undisturbed throughout the whole induration process in this PCSG technique, such defects as ring-forming and caking are avoided, which are common in the Grate-Kiln process, thus reducing the difficulty of operation and making it suitable for nearly all types of iron ores.
As limited dust is generated from the process, dust-removal is not necessary for return hot air, thus reducing the cost of de-dusting equipment and operation.
As preheated pellets remain still with no transportation needed, requirements on strength of pellets are negligible, thus lowering the amount of binder and improving the grade of pellets.
Multiple burners are mounted in the preheating and roasting sections to control temperatures in better accuracy, bringing the following advantages:
1) Better flexibility of operation;
2) Easier adjustment of temperature profile;
3) Adaptable firing temperature and temperature gradient for different raw materials.
As the refractory of the top cover are stationary, contact with pellets is avoided; therefore it’s free from mechanical wear and abrasion, dramatic temperature changes, and mass dust generation, as a result the service life of refractory is extended.
The separation of return trolley and indurations production line makes overhaul of the trolley easier, as a result productivity is improved.
The recycling of hot roasting air substantially reduces energy consumption of pellet firing. The adoption of the PCSG technique makes it possible to control the composition and quality of pellets more flexibly to meet process requirements of blast furnace steel making or DRI.
Features of the Plane Circulation Straight Grate Technology (PCSG)
With this technique of pellet firing, the drying, preheating, roasting and cooling processes are done in the same equipment, which is simple, reliable and easy to maintain, and suitable for magnetite, hematite and other types of iron ores. The PCSG technique has limited heat loss, unlike the Grate-Kiln process in which the return grate bed of the travelling grate radiates a good amount of heat, and the heat loss from the rotary kiln is also considerable due to the wide area of the kiln and the return hot air pipeline.
The amount of heat resistant alloy steel, total weight of main and auxiliaries, and investment of main part of the PCSG is only50% that of the widely used Chain Circular Straight Grate Roasting Machine. And the investment is about 60% that of the whole “travelling grate – rotary kiln – annular cooler” system.
With less civil construction work needed such as foundation, plant buildings and steel structures, the construction cost and construction period of PCSG are remarkably reduced.
As pellets remain undisturbed throughout the whole induration process in this PCSG technique, such defects as ring-forming and caking are avoided, which are common in the Grate-Kiln process, thus reducing the difficulty of operation and making it suitable for nearly all types of iron ores.
As limited dust is generated from the process, dust-removal is not necessary for return hot air, thus reducing the cost of de-dusting equipment and operation.
As preheated pellets remain still with no transportation needed, requirements on strength of pellets are negligible, thus lowering the amount of binder and improving the grade of pellets.
Multiple burners are mounted in the preheating and roasting sections to control temperatures in better accuracy, bringing the following advantages:
1) Better flexibility of operation;
2) Easier adjustment of temperature profile;
3) Adaptable firing temperature and temperature gradient for different raw materials.
As the refractory of the top cover are stationary, contact with pellets is avoided; therefore it’s free from mechanical wear and abrasion, dramatic temperature changes, and mass dust generation, as a result the service life of refractory is extended.
The separation of return trolley and indurations production line makes overhaul of the trolley easier, as a result productivity is improved.
The recycling of hot roasting air substantially reduces energy consumption of pellet firing. The adoption of the PCSG technique makes it possible to control the composition and quality of pellets more flexibly to meet process requirements of blast furnace steel making or DRI.