Iron concentrate pellet sintering process principle and production experiment

1. Process characteristics The process characteristics are as follows: (1) From ingredients to primary mixing, it is the same as the traditional process. All or part of the mixture from the first mix enters the pelletizing workshop for pelletizing. The pellets with a particle size of 3 to 8 mm account for about 80% to 90%, and the pellets with a particle size of less than 3 mm are less than 15%. Because the mixture constitutes pellets The particle size composition of the mixture is improved to improve the air permeability of the material layer, thereby increasing production and improving sinter quality. ​

(2) The small pellets made in the pelletizing workshop enter the secondary mixer, and the externally rolled pulverized coal is added before the pellets enter the secondary mixing (generally, the amount of externally mixed pulverized coal accounts for 60% to 80% of the total amount), that is, The pellets are rolled with pulverized coal inside and outside the secondary mixer. The pellets that have been rolled with pulverized coal enter the sintering machine for distribution. Because the pulverized coal is wrapped on the surface of the pellets, it has sufficient contact with oxygen in the air and has good combustion effect, so it can be greatly Reduce solid fuel consumption. ​

(3) Quicklime is added externally, that is, before the pellets made in the pelletizing workshop are mixed into the second mix, quicklime is also added together with the pulverized coal. Its function is:

First, because quicklime is sticky, it helps coal powder adhere to the surface of small pellets;

Second, further granulation can be achieved in the second mix;

Third, quicklime is rolled on the outside of the pellet. The surface of the pellet is highly alkaline and the core is acidic (the alkalinity remains unchanged for the entire pellet). Because the surface of the pellet contains high CaO, the amount of calcium ferrite generated is increased. That is, calcium ferrite is bonded between the pellets, which can improve the strength of the sinter pellets on the one hand, and is also beneficial in improving the reducibility on the other. ​

(4) Fabric equipment. There are three ways to distribute the pellet sintering mixture onto the trolley, namely wide belt distribution, mud roller reflective plate distribution and multi-roller distribution. ​

(5) The particle size of the coated coal powder. The particle size of the pulverized coal rolled outside the pellet sintering process is the same as that of the pulverized coal used in ordinary sintering production, that is, the content of particles with a particle size less than 3 mm accounts for more than 85%. ​

(6) There is no need to set up a drying section before the sintering machine is burned. ​

(7) The pelletizing equipment can be either a disc pelletizing machine or a cylinder pelletizing machine. ​

2. Process flow The process flow is as follows:

(1) Pelletize all the mixture. The mixture from the batching room goes to the primary mixer. After mixing, it goes to the disc pelletizer or cylindrical mixer to form small balls of 3 to 8 ram. It then enters the secondary mixer to roll fuel and quicklime, and then enters the sintering machine for sintering. A kind of grape-like pellet sinter is produced. This process has both the characteristics of the pellet process and the advantages of the sintering process, so it is called “pellet sintering process”. The process flow is shown in Figure 1.
[next] (2) Part of the mixture is pelletized (a two-mixer plan). About 90% of the 1/2 mixture from the first mix enters the pelletizing workshop to form 3-8mm pellets. The mixture pellets then enter the secondary mixer, and the other 1/2 from the first mix is mixed. The material directly enters the secondary mixer. After the above two mixtures are rolled with pulverized coal inside and outside the secondary mixer (more than 85% of the pulverized coal with a particle size less than 3mm after being crushed by the existing coal pulverized crushing chamber), they enter the sintering machine for sintering, as shown in Figure 2.

(3) Part of the mixture is pelletized (two secondary mixers plan). Figure 3 shows a plan to build a new Ф3m×12m secondary mixer in a factory where the mixing capacity of one secondary mixer is insufficient, and to be placed side by side with the original secondary mixer. The front part of the pelletizing room is the same as the previous plan. The 3~8mm pellets coming out of the pelletizing room directly enter the new second mixer to roll coal powder (do not enter the existing second mixer); while the other 1/2 coming out of the first mixer The mixture directly enters the original secondary mixer and rolls pulverized coal (the content of pulverized coal after being crushed by the existing pulverized coal crushing chamber is less than 3mm, accounting for more than 85%). Then the mixture from the new secondary mixer and the original secondary mixer enters the sintering machine for sintering, as shown in Figure 3.
[next] 3. The influence of various factors on the sintering index We used Handan Xing concentrate powder (concentrate rate 100%, R=2.0) to conduct sintering cup tests and semi-industrial tests for the pellet sintering process, and analyzed the main factors that affect the sintering index. A discussion was held. ​

(1) Influence of pulverized coal particle size. Under the condition that the internal coal blending amount is 0.5% and the external coal blending amount is 2.8% (other conditions are the same as No. 15 and No. 16 in Table 1), the particle size of the pulverized coal is changed to less than 1mm, 2mm, and 3mm respectively. A series of experiments. ​

It can be seen from the test results that the three particle sizes of externally rolled pulverized coal are less than 1mm, less than 2mm, and less than 3mm. Require. ​

(2) The influence of internal coal blending amount. The internal coal blending amounts of the pellets in this test were 0%, 0.5%, and 0.8% respectively. From the test results, it can be seen that the yield, utilization coefficient and The strength of the drum is lower than the corresponding target for sinter pellets containing 0.5% coal. It can be seen that it is more suitable to add 0.5% coal powder inside the pellets.

(3) The influence of material layer thickness. There are three conditions for the thickness of the material layer in this test, namely 500mm, 550mm, and 600mm. When the material layer thickness is 500mm or 550mm, the effect is better, the utilization coefficient reaches above 1.5t/(m2·h), and other indicators are also good. When the material layer thickness is 600mm, only the utilization coefficient decreases, and other sintering indicators are also better. ​

4. Granulation of green balls

(1) Particle size composition before pelletizing. Taking a certain factory as an example, when a mixer is produced, the content of particles with a particle size less than 3mm accounts for more than 45%. ​

(2) Particle size composition after pelletizing. After primary mixing, the mixture enters the pelletizing tray for pelletizing. After exiting the pelletizing tray, the content of particles with a particle size greater than 3mm reaches 89.09%.

(3) The particle size composition after the fuel is rolled out of the secondary mixer. The content of the mixed particles with a particle size greater than 3mm that comes out of the pelletizing room reaches more than 87%. After this mixture enters the second-mix external rolling fuel, the content of the mixed particles with a particle size greater than 3mm increases slightly, reaching 89.42%.

5. Distribution technology of pellet sintering process There are three distribution methods of pellet sintering process. The first is mud roller reflective plate cloth, the second is multi-roller cloth, and the third is wide belt cloth. ​

(1) Mud roller reflective plate fabric. This distribution method is suitable for small sintering machines. The key is to adjust the length and inclination of the reflective plate. When the mixture falls on the reflective plate, the large ball has strong rolling force and falls on the lower part of the material layer, and the second largest ball is distributed on the material layer. In the middle, fine powder is spread on the surface of the material layer, forming natural segregation. ​

(2)Multi-roller fabric. After the mixture comes down from the mud roller, it falls on the multi-roller distributor and segregates. The big balls fall on the bottom of the material layer, the second largest balls are clothed in the middle of the material layer, and the fine powder is clothed on the surface of the material layer, forming natural segregation. ​

(3) Wide belt fabric. This kind of fabric is suitable for large-scale sintering machines. There is an ore trough above the incinerator of the sintering machine. Because large-scale sintering machines have high output values, if mud roller reflective plate fabrics are used, there must be an ore trough above the incinerator of the sintering machine. The ore trough is too small for the fabric. It cannot play a buffering role. If the ore tank is large and the mixture in the tank is tens of tons, it is easy to destroy the made balls, so wide belt fabrics are used. Some suppliers now use it very well. ​

6. Improvement of ball making equipment

(1) Types of pelletizing discs There are three types of pelletizing discs currently used in China (including pelletizing discs used in oxidation pellet plants and cement plants), namely Ф=4200mm, Ф=5500mm and Ф=6000mm disc pelletizing machines. , this is a domestic stereotyped product. From the current point of view, the Ф=4200mm disc pelletizing machine has a small ball-making capacity due to its small equipment, large number of installations, and large daily maintenance. In comparison, the Ф=6000mm disc pelletizing machine has a large ball-making capacity (it is the former 3 times), the maintenance amount is small, therefore, Ф=4200mm pelletizing disc has a tendency to be gradually replaced by Ф=6000mm pelletizing disc. ​

(2) Scraper method Based on the scraper method, the pelletizing disc can be divided into a fixed scraper pelletizing disc and a rotating scraper pelletizing disc. ​

1. Fixed scraper ball making disk

(1) The installation position of the pelletizing disc scraper. The scraper is fixed on the horizontal truss above the pelletizing disk. The angle between the edge scraper and the edge of the pelletizing disk is no more than 30° in the first quadrant of the pelletizing disk. The center scraper is installed at the junction of the third and fourth quadrants.

(2) Scraper raw materials. The scraper material of the ball-making disc is a long-standing problem. The main reason is that the scraper made of ordinary materials has poor wear resistance and will be worn away after a few days of continuous use, causing the adhesive material on the disc surface to rise. This not only speeds up the scraper speed The wear and tear also increases the transmission load of the pelletizing disc. Through research and production practice, it has been proved that the use of high chromium-molybdenum wear-resistant cast iron scrapers is more effective. The original carbon is mainly dispersed in the metal matrix in the form of carbides, and has excellent wear and tear resistance. Practice has proved that the service life of high chromium) molybdenum wear-resistant cast iron is much longer than that of medium manganese steel plates, about 6 to 8 times longer. Although the manufacturing cost of high chromium) molybdenum wear-resistant cast iron is relatively high, in terms of cost performance, the cost actually decreases. Moreover, due to the extension of service life, the number of replacements of the scraper is reduced, which reduces the operator’s work in cleaning up the material on the disk, thereby improving the efficiency of the ball making disk. [next]

(3) Raw materials for the pelletizing disk. Because the surface of the pelletizing disk has to withstand the violent erosion of the mixture, the disk surface made of ordinary steel plates will soon be worn out. In order to prevent the disk surface from being worn through, it is necessary to select wear-resistant disk surface materials. People have used a variety of materials in the past. Among them, rubber wear-resistant ceramic linings are expensive but have good wear resistance. The disadvantage is that they cannot withstand slightly higher temperatures. For example, in When repairing and welding the ball making disk close to the lining plate, it will cause the lining plate to fall off. When reinstalling, it will be difficult to reinstall and strengthen the place, and it will also cause the surrounding lining plates to fall off. ​

Later, ultra-polymer lining boards, ceramic tile lining boards, gray greenstone cast stone, etc. were successively used. After using a variety of lining materials, we believe that limestone cast stone is better because, firstly, it has lower cost and is easy to install, and secondly, it has good wear resistance and moderate friction, which is conducive to the tumbling growth of the mixture. , the general life span is more than 3 months. However, it should be noted that this kind of material is relatively brittle. When inspecting the ball making disk or cleaning the material on the disk, do not hit it with too much force with metal objects to avoid damaging the limestone cast stone lining plate. ​

2. Rotating scraper pelletizing disc This type of pelletizing disc mechanical transmission structure is imported from abroad. The scraper is a disc rotating scraper (rotating disc diameter Ф1100mm, speed 6r/min), which can effectively scrape the entire disc surface. , to prevent the material surface from rising, keep the disk surface flat, and improve the working efficiency of the pelletizing disk. ​

(1) Raw materials for the ball making plate. Now, there are two more suitable materials that can be used: 1) Steel mesh bottom lining. A 5mm thick steel mesh is paved and welded on the entire surface of the pelletizing disk. The mesh size of the steel mesh is 40mm × 60mm. Because of these meshes, the mixture is bonded and fixed with a layer of mixture on the entire steel mesh, forming a mixture. The bottom lining composed of raw materials will conflict with each other during the operation of the pelletizing disc. It can be said that the surface of the pelletizing disc will not be worn. As the bottom lining of the pelletizing disk, the expanded steel mesh is inexpensive and simple to install. It is also easy to repair if damaged. 2) Limestone cast stone lining. As mentioned earlier in the fixed pelletizing disc bottom lining, it is more suitable to use limestone cast stone lining. This type of bottom lining can also be used in the rotating scraper pelletizing disc, which can reduce the wear of the scraper. ​

(2) Scraper raw materials. Because the surface of the pelletizing plate uses expanded steel mesh as the backing, the mixture will easily stick to each other and rise, which requires a scraper to scrape off the mixture to prevent it from rising. Originally, the scraper was made of ordinary materials and had a diameter of only 28mm. It was seriously worn. As the scraper rod wore away, the material surface of the mixture gradually increased. After a few days of continuous operation, the material surface would rise to the surface of the rotating cutterhead. Not only is the work of cleaning up the disk surface very arduous, but it also greatly reduces the efficiency of the ball-making disk. For this reason, the scraper rod of the rotary scraper is made of YG6 and YG8 carbide. This material is usually a machined cutting tool with high hardness and good wear resistance. The application practice in blast furnace coal injection has proven that the service life of cemented carbide is much longer than that of other materials. For example, the bushing made of 40Cr quenched steel on the top of the pulverized coal transportation pipeline for blast furnace coal injection was used in less than 10 days. Wear through, and the bushing made of YG8 carbide can be used for more than two years. The service life of the scraper rod made of YG6 or YG8 cemented carbide can reach more than two months. In order to extend the service life of the scraper rod as much as possible and at the same time reduce the manufacturing cost as much as possible, according to the degree of wear of the scraper rod during mixing, brazing is used to weld the lower part of the scraper rod embedded in the mixture layer with wall thicknesses of 6mm, 5mm, 4mm three sections of carbide casing of different thicknesses, with a total length of 105mm. The scraper rod is welded to the rotating cutterhead. After all the carbide brass parts are ground away, the material on the disk is cleaned up, and then a new scraper rod is welded. The structure and dimensions of the scraper bar are sketched in the figure. ​

(3) Atomized water nozzle: The mechanism of pelletizing the mixture in the pelletizing tray is to add water to the surface of the mixture. The surface of the mixture is fully and evenly moistened by the water. Through the capillary bonding force of the water, the pelletizing tray is formed. The tumbling mixture is bonded together to form small balls of different particle sizes. In order to increase the balling rate of the pelletizing disk, each pelletizing disk is equipped with two atomized water nozzles. Because the loading points of the new and old pelletizing disks are different, the installation directions are different. Generally, one is placed in the unloading point area and one is placed in the ball forming area. ​

The materials of the atomized water nozzle are all made of 1CRl8Ni9Ti stainless steel, which has very good anti-oxidation and rust resistance. Its structural principle is a type of spiral nozzle, which is mainly composed of a tangential nozzle shell, a plug and a needle. The water enters from the tangential direction of the circular nozzle shell, undergoes pressurization and a period of swirling flow, and then enters the tapering section. The swirling speed gradually accelerates, and then sprays mist water through the nozzle. The atomization scattering angle of the atomized water is 50°～60°. Now the water spray volume of the atomized water nozzle used by Ф=4200 pelletizing plate is when the water pressure is about 0.2MPa, the water spray volume of each atomized water nozzle is 150～180kg/h. This The characteristic of this kind of atomized water nozzle is that it is equipped with a needle, which can frequently clear the nozzle to avoid clogging when the water is not stopped. After one year of production practice, we are still in normal use. [next]

(4) External coal blending system Secondary blending and fueling is to add a part of the fuel in the batching room into the primary mixer for mixing and then pelletizing. The pelletized mixture is blended with fuel externally before entering the secondary mixer. The proportion of external fueling is (30% ~ 50%): (70% ~ 50%) Nuclear scales or electronic scales can be used to weigh external coal blending. A certain factory uses an activity value of 137 for external coal blending weighing. After more than a year of production and use, it has been shown that the nuclear scale is relatively durable if properly maintained. The measurement of external coal blending is done by inputting the weighing signal of the mixture amount into the computer through the nuclear scale installed on the mixing belt, and the computer assigns the coal blending disc feeder according to the set coal blending proportion. The coal is fed to the coal blending belt at the corresponding speed, and then passes through the device

The nuclear scale on the coal blending belt approves the coal blending amount and feeds it back to the computer for adjustment. Production practice in recent years has shown that after adopting external coal blending technology, the solid fuel consumption per ton of sinter is compared with that without external coal blending, which can generally save 10kg/t of coal consumption.

(5) Steam preheating mixture The temperature of the sintering mixture has a great influence on the sintering production. If the temperature of the mixture is lower than the dew point, due to the condensation of water vapor on the surface of the mixture particles in the lower layer, the Excessive increase of moisture in the lower material layer affects the air permeability of the sintered material layer, which is not conducive to the thickening of the sintered material layer, thereby affecting the improvement of sintering output value. To this end, a steam nozzle is installed in the secondary mixer to heat the mixture. For the pelletized mixture, although hot return mineralization is added to the mixture to increase the temperature of the material, due to the open-air operation of the pelletizing plate section, The temperature drops, so steam is required to further increase the temperature of the mixture. The steam pipe is inserted from the inlet end of the mixer, and five steam nozzles are installed side by side along the axis of the mixer. The front end of the nozzle is 600 to 800mm away from the mixing surface of the mixer. The steam preheating can make the material pass through. The temperature of the ball mixture increases from 45℃ to nearly 60℃. The steam pressure of the steam preheated mixture is about 0.3kPa, and the steam consumption is 4~5kg per ton of sinter. 7. External quicklime technology (1) External quicklime The advantages are based on the pellet sintering process, that is, after the mixture from the primary mixer enters the pelletizing workshop and is pelletized, it is blended with coal and also with quicklime before entering the secondary mixing. The advantages are: First, the external blending of quicklime is conducive to improvement. Granulation function can increase output value;

Second, the surface of the pellets is coated with a layer of quicklime, which helps the coal powder to coat more on the surface of the pellets;

Third, the high CaO content on the surface of the pellets is conducive to the formation of calcium ferrite, that is, the surface of the pellets has high alkalinity and the calcium ferrite content is high (the interior is low alkalinity, and the total alkalinity of the mixture remains unchanged). The balls are bonded together by calcium ferrite. Because calcium ferrite has high strength and good reducing properties, it is beneficial to improve the strength and reducing properties of sinter pellets, and is beneficial to blast furnace smelting. ​

(2) The best conditions for adding quicklime to the outside. After testing the sintering cup of Baotou Steel Company’s sintering mixture (R=1.2) with quicklime added to the ball, it can be seen that the effect of adding quicklime to the outside is relatively significant. The optimal conditions are as follows:

(1) After the mixture is pelletized, the amount of external quicklime is fixed at 1.5% and the proportion of external coal is changed. After the Baotou Steel sintering mixture is pelletized, the proportion of pyridine is 3%, and the external proportion is 1.5% (the inside and outside account for 50% each), and the proportion of external pulverized coal is changed (the proportion of pulverized coal is 5%). When the external proportion of fixed quicklime is 1.5%, the sintering target is better when the external proportion of pulverized coal is 2.5% (accounting for 50% of the total). The sintering targets of only external quicklime but not coal and neither pulverized coal nor quicklime are suitable. ​

(2) After the mixture is pelletized, the proportion of external pulverized coal is fixed and the proportion of external quicklime is changed. After Baotou Steel’s sintering mixture is pelletized, the proportion of external coal blending is fixed at 2.5% (the total amount of external coal blending is 5%), and the proportion of quicklime is changed to 1%, 1.5%, and 2% (the total proportion of quicklime is 3%). When the external blending of pulverized coal is fixed at 2.5%, among the three conditions of 1.0%, 1.5% and 2.0% of external quicklime, the latter two have better sintering targets, but compared with 1.5% of external quicklime and 2.0% of external quicklime, the latter two Generally nearby. From the above two sets of test results, it can be seen that in the pellet sintering process under the condition of external coal blending, external quicklime can significantly improve the sintering target. The proportion of external quicklime can be about 50% of the total proportion.