I. Overview China's first 8m 2 pellet shaft furnace built in 1968 in Jinan steel production plant. By the early 1970s, a number of 8m 2 shaft furnaces in Chengde, Hangzhou, Anyang, Laiwu, Tangshan and other steel plants were put into production. At that time, the shaft furnace pellet was an emerging technology. There were many inexperiences in process determination, equipment design selection, and technical operation management. The daily output of the furnace was only maintained at 250-300 tons. In order to further increase the output of the shaft furnace, various improvements have been proposed by the plants. Through the production, the transformation, and the summary, Jinan Iron and Steel's "in-furnace short circuit" program has achieved success. The essence of "short-circuit in the furnace" is the initial "hood-winding wall", which eliminates the raw material layer in the center of the furnace, the airflow distribution tends to be reasonable, the process in the furnace is uniform and accelerated, and the daily output of the furnace is rapidly increased to 400 to 500t level. After continuous improvement, the "short circuit in the furnace" evolved into a modern "dry bed - wind guide wall". At that time, the shaft furnace pellets used slaked lime as a binder, which resulted in poor green ball strength, low bursting temperature, and severe pulverization on the dry bed, which became a major factor limiting the further increase in shaft furnace output. In the late 1970s, Hangzhou Iron and Steel first used sodium bentonite instead of slaked lime as a pellet binder, which greatly improved the strength and bursting temperature of the green ball, significantly improved the drying effect, accelerated the discharge rate, and doubled the daily output of the shaft furnace. It reaches the level of 800~1000t. Since the 1980s, China's shaft furnace pellet production has embarked on a stable development path.
By the end of 1998, China has (ranging in size from 5 ~ 16m 2, but mostly 8m 2) being produced pellet shaft furnace 25 Eagle, in 1998 the production of pellets over 630 million t, the ancient of the national pellets About 60% of the total output. In addition, several shaft furnaces are under construction.
In the past two decades, China's shaft furnace pellet production has made great progress, and the process and equipment have been basically finalized, and the operation and management have become more and more perfect. Summarizing these aspects of experience, it is of great significance to further improve the production technology level of the shaft furnace pellets.
Second, China's shaft furnace pellet production experience (a) to strengthen the preparation of raw materials 1. Improve the fineness of concentrate powder China's shaft furnace pellets used in iron-containing raw materials mainly magnet concentrate powder. Since most of the magnetite concentrate powder has a relatively coarse particle size (the content of the particle size less than 0.074 mm generally accounts for only 50% to 70%), the pelletizing performance is poor. Increasing the fineness of concentrate powder has been difficult to solve in pellet plants for many years. In recent years, the Nangang pelletizing workshop changed the original chlorinated pellet roasting equipment (chain kiln rotary kiln) into a shaft furnace, roasting acidic pellets, and using the original grinding machine to treat iron concentrate powder (mixed Material) achieved good results. The concentrate powder used by Nansteel is coarser and more versatile; the average content of particle size less than 0.074mm is only 41%, although the bentonite blending amount is as high as 5% in production, and the pelleting performance is still poor. After the treatment by the grinding machine, the content of the particle size less than 0.074mm reaches 68%, which greatly improves the pelletizing performance of the mixture. When the amount of bentonite falls below 2%, the output of the ф5.5m pelletizer reaches 50t/ Above h, the quality of the raw ball is also greatly improved: the average compressive pressure is above 2N/single ball, and the drop strength is more than 10 times/single ball on average. In 1998, the output of 8m 2 shaft furnace reached 409,000 tons, ranking the national advanced level, and the quality of finished balls also meets the requirements of national first-class products. After the concentrate powder is processed by the grinding machine, it not only improves the fineness. It also changes the shape of the particles, increases the surface area and surface activity, is conducive to the ball making operation, and more importantly reduces the amount of bentonite and improves the grade of the pellets. It must be pointed out that after using the grinding machine, the power consumption has increased, but as the output increases, the actual power consumption does not increase much. In 1998, Nangang's electricity consumption was 41.34kw · h / t, ranking the middle of the country. If you take into account the benefits of increased pellet grade iron ore caused by reduced bentonite use, the economic benefits will be even more significant. Therefore, concentrate powders having a particle size of less than 0.074 mm and less than 60% should be treated with a grinder (or similar grinding equipment).
2. Add concentrate powder (or mixture) and dried beneficiation plant facilities due to restrictions by the dewatering equipment capabilities, the water content of the plant concentrate powder is generally from 1% to 3% of a suitable amount of water required when comparing pelletizing. Although the excessively concentrated concentrate powder has a faster ball formation speed but is less strong in ball making, it is easy to produce powder during transportation and production. Therefore, it is necessary to control the water content of the concentrate powder (or mixture) to be slightly lower than 0.5% to 1.0% of the suitable moisture at the time of pelleting, and then add water to make the ball. For the control of concentrate water problems, each plant has paid enough attention. As early as the early 1970s, in order to overcome the difficulty of making the ball due to the excessive water content of the concentrate, the use of a ф2.2m×12m tumble dryer to dry the mixture was effective. In the subsequent reformation of the new shaft furnace or the old shaft furnace, the concentrate (or mixture) drying facilities have been added, and good results have been achieved. The commonly used drying equipment is a tumble dryer. At present, although the manufacturer has a series of stereotypes, most of them are designed for the cement industry. Due to the different material density of the treated materials, the pellet plant must pay attention to check the transmission power when selecting.
3. Increasing Bentonite Metering Equipment In the current pellet plant operation, the concentrate has been automatically metered, and the bentonite is used because of its small amount, difficult to measure, and still uses artificial ingredients, often affecting the pellet grade due to operational errors. Fluctuation. In order to control the amount of bentonite and increase the accuracy of the ingredients, it is necessary to increase the amount of small flow metering equipment. Practice has proved that the impulse flowmeter is a suitable bentonite metering device, which has the advantages of small volume, high precision and low price than the tape scale of the same specification. [next]
4. Using high-efficiency mixing equipment Mixing is an important part of the pellet production process and one of the key measures to determine the quality of pellets. At present, most shaft furnace manufacturers use a cylinder mixing (or drying) machine for (or instead of) mixing, the effect is not very satisfactory. The new shaft furnace adopts a powerful mixer imported or copied from abroad, and the mixing effect is improved. According to the use of Benxi Steel's 16m 2 shaft furnace: the effect of a strong mixer is equivalent to the mixing effect of the two-stage cylinder mixer; after the mixture is caused by the ball, the amount of powder entering the furnace is reduced from 3.84% to 2.51%. The various indicators of the furnace have been greatly improved. However, there are some problems in the use of the intensive mixer: first, the power consumption is high; second, the caries wear is large, the maintenance is frequent, and one spare is needed. It is recommended that in the old factory retrofit, a multi-wheeled belt mixer can be added after the cylinder mixing (or drying) machine. In this way, on the one hand, the mixing effect can be improved, and on the other hand, the small balls generated in the mixing process of the preceding stage can be pulverized. The grinding machine and other grinding machines have a mixing effect. If such equipment is used to treat the mixture, it can be used instead of the mixing equipment.
(2) Strengthening the ball making process and reducing the powder in the raw ball 1. Selecting the large ball making machine Ñ„5.0m, Ñ„5.5m, Ñ„6.0m disc pelletizing machine with advanced structure, complete functions, high output per unit area, is The preferred equipment for the specification of the shaft furnace in the new design, while considering a certain reserve factor.
2. Increasing the looseness of the stream In order to increase the looseness of the stream during the fabric, the mixture is dispersed by a loosening device before being fed into the pelletizer.
3. Increasing the temperature of the mixture In the absence of drying measures for the mixture, the water can be used to raise the yield and quality of the green ball. Bengang's shaft furnace production experience proves that when the hot water temperature is 70-80 °C, the yield of ф5.5m pelletizing plate is increased by 5~7t/(tai · h), reaching 30~35t/(tai · h); The ball drop strength is increased by 3 times/single ball on average, reaching 7-8 times/single ball; the content of the ball in the raw ball is 10~15mm is increased from 80% to 93%.
4. Screening out the powder in the raw ball In order to ensure the gas permeability of the furnace, it is not caused by the powder entering the furnace, and it is necessary to screen out the powder of 6-8 mm in the raw ball. Since the proportion of super-globules larger than 20 mm in actual production is not large, in order to simplify the process, the raw ball screening machine may not have a sieve function. The commonly used raw ball screening equipment is a roller sieving machine. The roll skin is made of ceramic and is wear-resistant, but it is easy to be broken during manufacturing or production, and is now gradually changed to urethane rubber roll skin.
The raw material return material is usually crushed and returned to the former tank of the pelletizer.
(III) Improvement of the distribution equipment The shaft furnace distribution equipment is essentially a belt conveyor that performs reciprocating linear motion on the top of the furnace. 8～10m 2 shaft furnace distribution machine, there is no track in the furnace, this part is cantilever structure. The belt drive of most cloth machines is fixed on the body frame and moves with the body. Practice has proved that this type of cloth machine is prone to fabric segregation. When the car body moves forward or backward, the amount of raw balls given to the tape of the cloth car changes, and the more the car body moves backwards, the more raw material is on the tape. The material surface thus laid into the furnace is not horizontal but inclined, which affects the normal operation of the furnace. In order to correct this segregation, the machine has to operate intermittently: stay in the thin area of ​​the material for a while. The result also increases the labor intensity of the fabric workers. In order to completely overcome fabric segregation and improve the level of automation, the newly designed distribution machine should fix the tape transmission device on the ground.
According to the transmission mode of the traveling mechanism of the cloth machine, the cloth machine has two kinds of steel wire transmission and gear transmission. Regardless of the type of transmission, the body part of the cloth machine is required to be light in weight, and the inertia generated during the commutation is small, which is convenient for control and operation.
When the stroke of the cloth machine exceeds 6.4m (that is, the length of the furnace mouth of the 12m 2 shaft furnace), the height of the rod is increased due to the excessive cantilever at the front of the cloth machine, resulting in an increase in the ball drop and an increase in the ball breaking rate. In this case, the cantilever structure is no longer used and the track can be laid in the furnace. In order not to affect the operation and maintenance, the furnace track can be erected along the two side wall panels of the dust cover of the cloth machine.
(Iv) improved discharge apparatus Group 1. Roll Crusher Roll Crusher unit rolls by the teeth 7 to 11 work independently of the composition, roll spacing is 80 ~ 100mm, and therefore can not control the gear unit discharge roller, which has a main function Two: one is the active material column; the second is the broken large piece. The toothed roller bears the weight of the column in the whole furnace in the furnace, and is also subjected to the scouring of the high-temperature dust-containing airflow, and the working conditions are bad. Except at the roller body hard surfacing alloy, the intermediate shaft need water cooling. In view of the large torque and low speed of the toothed roller, hydraulic transmission should be adopted. According to the form of its transmission cylinder, it is divided into two types: one is a piston cylinder (also divided into a single cylinder and a double cylinder). Because the piston cylinder has simple structure, convenient maintenance and easy to solve spare parts, it has been widely used. The other is a swing cylinder, which has the advantages of advanced structure and convenient operation. The disadvantage is that the cylinder has high processing precision and high cost, and the spare parts are not easy to solve. Wear in a dusty environment. After many years of practice, it is best to seal the shaft end of the toothed roller with a grease packing. Regularly add high temperature grease during use.
2. Electric vibration feeder The electric vibration feeder below the shaft discharge funnel is the only equipment for controlling the discharge of the shaft furnace. The feeding amount of the electric vibrating feeder is adjustable, and the operation switch is mastered by the top cloth distributor. Under the principle of “dry ball into the furnaceâ€, the operation is started according to the working condition of the drying bed. Since the environment in which the vibrating feeder is located is high temperature and dusty, designing the equipment requires special consideration.
(V) Selection of cooling equipment Although the pellet ore is partially cooled in the furnace, it is not sufficient. Due to the limitation of the furnace process conditions, the cooling air volume of the pellet ore unit is only 600~1000m 3 /t. There are about 400 to 600 °C. If transported with a tape conveyor, additional cooling outside the furnace must be added. At present, there are roughly the following types of furnace cooling equipment used by each factory:
(1) After pulling out with a chain plate machine or a bucket winch, unload it on the ground and allow it to cool naturally;
(2) After pulling out with the chain plate machine, the steel mesh belt conveyor is connected, and it is naturally cooled during transportation;
(3) cooling with a vertical cooler;
(4) Cool with a light blast belt cooler.
The above several cooling methods can be adopted by each factory according to local conditions. [next]
(6) Dust removal improvement of the shaft top of the shaft furnace Because the shaft furnace is operated under positive pressure, a large amount of dusty exhaust gas emerges from the furnace mouth, which not only pollutes the environment, but also is unsafe for the operator. The furnace mouth must be closed to remove the exhaust gas. deal with. The electrostatic precipitator is an ideal dust removal device . The dust concentration in the exhaust gas is less than 100mg/m 3 , which is in line with national emission standards. Some early-stage shaft furnaces still use cyclone dust collectors, and the dust concentration in the exhaust gas is far from the standard, and should be changed to an electrostatic precipitator as soon as possible.
The test results show that the physical properties of the electric dust removal ash are close to the concentrate powder, which can be matched with the mixture for reuse. However, it must be noted that water must be moistened before transportation to prevent secondary dust.
(7) The shaft of the cooked ball facility must be filled with a cooked ball before the furnace is opened. Usually, the condition of the furnace (such as handling the collapse accident) also needs to use part of the cooked ball, and the facility that transports the cooked ball to the top of the furnace is called "Upper ball facilities." The common upper ball facilities are as follows:
(1) Using a forklift or a tilting portable tape machine to directly feed the cooked ball to the raw ball conveyor of the upper shaft furnace, and enter the furnace through the cloth machine;
(2) Set up a special transport ball conveyor belt system, which is transported from the finished mine tank to the top of the furnace and enters the furnace through the distribution machine;
(3) Set the cooked ball hoist, transport the cooked ball to the top of the furnace, and enter the furnace through the cloth machine.
Note: The cooked ball can't pass the raw ball sieving machine, so as not to wear the sieve roll.
(VIII) Improvement of shaft furnace body 1. Basic characteristics of Chinese pellet shaft furnace The Chinese pellet shaft furnace has the following characteristics:
(1) Positive pressure operation requires that all parts of the furnace have good air tightness.
Low quality coal gas blast furnace (2) with a plurality of fuel supplied metallurgical plant, rational energy use;
(3) Adopting the technology of “dry bed and air guiding wallâ€, the airflow distribution is reasonable, and the fan pressure is lower than half of the same type of shaft furnace in foreign countries.
2. Combustion Chamber In theory, the shaft furnace combustion chamber is a space for generating hot exhaust gas. As long as the generated hot exhaust gas satisfies the shaft furnace roasting requirements (such as temperature, pressure, oxygen content, etc.), it is independent of the shape and position of the combustion chamber. Of course, the most economical and rational configuration is that the combustion chamber is placed close to the sides of the furnace. The shape may be rectangular or circular (either horizontally or vertically); and the horizontal circular combustion chamber is divided into two types which are parallel to the furnace and perpendicular to the furnace. The key is that the structure is reasonable and meets the requirements of positive pressure operation. The shape of the combustion chamber determines the number of burners. The number of burners on the side of the rectangular combustion chamber should not be too small, preferably 3 to 5, too much operation is inconvenient, too little affects the uniformity of the atmosphere. The number of burners on the side of the horizontal circular combustion chamber is 2 to 4.
3. Drying belt Drying is the key to shaft furnace roasting. "Dry ball into the furnace" is an important principle of shaft furnace operation, which is the premise of ensuring that the furnace is antegrade, not agglomerating, high yield and high quality. The factors affecting the drying process are mainly the airflow temperature and the airflow velocity. At present, the drying temperature of most shaft furnace tops is low, and the air flow speed is too small, which affects the performance of the furnace. Under the condition of ensuring a certain baking time, shortening the height of the preheating roasting belt is beneficial to increase the temperature of the drying airflow, and appropriately increasing the cooling air flow rate is beneficial to increase the drying airflow speed. When the shaft furnace is newly designed, the drying bed area can be appropriately increased, the thickness of the drying bed layer can be reduced, and the drying process can be accelerated. The ratio of the total area of ​​the drying bed to the cross-sectional area of ​​the calcined zone is about 1.8 to 2.0. The ball stays on the drying bed for about 10 to 12 minutes.
The angle of the drying bed should not be too large, generally around 41 ° ~ 42 °. In the operation, the drying bed inclination angle can be adjusted at any time according to the specific production conditions. The seamless bed water beam is generally made of seamless steel pipe, and the cooling method may be water cooling or vaporization cooling. Since the vaporization cooling pipe takes up a large space, it will increase the height of the upper part of the furnace. From the perspective of domestic production practice, it is more suitable to select water cooling.
Dry tweezers are available in single rows and in multiple rows. The materials are cast steel. The scorpion seams are straight and horizontal, so it is better to not block the pupils. In order to improve the antioxidant capacity of the scorpion, the material is preferably a low alloy cast steel.
4. Preheating the calcining belt The distance from the center line of the water beam along the center line of the water beam to the center line of the exit of the fire tunnel is called the preheating roasting belt. The height should meet the minimum firing time requirement of the pellet in the belt, and the general residence time is 50-60 min. The cross-sectional area of ​​the straight section of the narrowest portion of the calcining belt is equal to 1/2 of the nominal area of ​​the shaft furnace. China's 8 ~ 16m 2 shaft furnace roasting belt has a single side width of 1.5 ~ 2.1m.
5. Shape of the fire tunnel The model demonstrates that the flow of hot exhaust gas from the burner tunnel is in the upward parabola. Thus, at the same level on the exit of the fire channel, the temperature and atmosphere of the gas stream are not uniform, which will affect the quality of the roasting. If the blasting port is inclined downward by an angle (to prevent the pellet from clogging the fire channel, the horizontal angle of the slant is greater than 30°), the gas flows toward the trajectory—the parabola is also inclined downward by an angle, and the uniformity of the airflow on the same horizontal surface is greatly improve.
The blasting port is in the shape of a hole, and the ejection speed is in the range of 10 to 15 m/s.
6. The wind guide wall and the uniform wind guide wall are a grid-shaped hollow wall that is placed in the center of the upper part of the shaft furnace. The function is to ensure that there is sufficient cooling air (also called wind outside the wall) to pass through the roasting belt to supplement the exhaust gas of the combustion chamber. The shortage of oxygen in the middle makes it possible to use the blast furnace gas for roasting in the shaft furnace; the other rising cooling air is directly guided to the drying bed of the furnace through the air guiding wall to participate in the drying without interfering with the roasting belt. Due to the existence of the wind guide wall, the various processes in the furnace (drying, preheating roasting, soaking, cooling) are more obvious and more balanced, which provides conditions for the high yield and quality of the shaft furnace. Different raw materials (different amounts of FeO), different fuels (different oxygen content in the exhaust gas of the combustion chamber) require different amounts of wind outside the wall to pass through the calcining zone, and the parameters (height and width) of the wind deflecting wall are also different.
The distance from the center line of the fire passage to the lower edge of the wind guide wall is called the tropics, and its role is to make the pellets more fully oxidized and more uniform in quality. In order to prevent the bulk material from being stuck, the wall of the furnace on the side of the furnace should be kept vertical.
The low temperature life of the wind guide wall and its water beam (consisting of two rows of seamless steel tube composite beams) (generally only about 1a) is a major problem in the current shaft furnace. Analysis of the reasons, mainly because the structure is not reasonable:
(1) The wind guide wall is thin, only half a brick thick (116mm). This is the old-fashioned 8m2 shaft furnace renovation. It is necessary to increase the wind guide wall without excessively reducing the cross-sectional area of ​​the roasting belt. The air guiding wall of the newly designed shaft furnace should be properly thickened to maintain at least one brick thickness (232mm);
(2) The water beam section stiffness is not enough. The water guide wall water beams are parallel in two rows, and the strength of the connecting members in the middle is critical. At a high temperature of about 1000 ° C, if it is not cooled by water, any heat-resistant steel components will be burnt out, and the two drainage beams that have lost contact will lose stability under the side pressure of the charge on both sides. It is recommended to change the connecting plate to a water-cooled steel pipe. The overall rigidity of the water beam is greatly increased, the life of the water beam is improved, and the overall life of the wind guiding wall is also increased. The structure of the wind guide wall can last for 3 to 4 years.
7. Cooling zone The distance from the cooling air inlet to the lower edge of the wind beam of the wind deflector is called the cooling zone. Considering the balance angle of each belt in the furnace, the amount of cold air blasted into the furnace is limited, generally around 800m 3 /t, the discharge temperature is about 500 °C, the calculated height of the cooling zone is 3 ~ 3.5m, and the residence time is 2 ~3h. The cold air vent should also be inclined downward at an angle, and the cold air vents on both sides are preferably staggered. [next]
(9) Main principles of shaft furnace operation 1. The dry ball enters the furnace, and the bottom of the furnace eliminates the agglomeration accident. The so-called “dry ball into the furnace†means that the cloth worker carefully observes the dry condition of the green ball on the drying bed. When the lower half of the drying bed (at least to 1/3) the green ball is fully dry (when the green ball is grayish white), the switch of the discharge electric vibration can be activated to start the discharge. Due to the limitations of the drying process, the discharge operation is discontinuous in actual production. After implementing the principle of “dry ball into the furnaceâ€, the stubborn disease in the operation of the shaft furnace—the “caking in the furnace†phenomenon was completely eliminated. With the improvement of production management level, the forbidden zone of the so-called "mine powder roasting temperature range greater than 200 °C to build a shaft furnace" no longer exists.
2. Control the temperature of the calcining zone to achieve high yield and quality. The magnetite pellets release a large amount of heat during the oxidative roasting process, so the combustion chamber temperature is usually lower than the annealing zone atmosphere temperature. Depending on the type of raw material, this difference is around 150 to 250 ° C. This temperature is almost constant when the raw materials and fuel conditions are constant. Since the center temperature of the calcining zone cannot be measured with a thermocouple, most shaft furnace operators control the combustion chamber temperature as an important parameter for shaft furnace operation. However, the production practice proves that it is easy to cause the pellet quality to fluctuate simply by operating according to the temperature of the combustion chamber. Here the concept of "baking zone temperature" (also known as side wall fire channel temperature) is introduced, which more accurately reflects the roasting of the furnace. This is because the combustion chamber temperature is the temperature in the space of the combustion chamber after combustion of the gas. It is only related to the fuel type, the ratio of the air gas flow rate and the structure of the combustion chamber; and the temperature of the calcination zone is in addition to the above factors. The types of raw materials, the change of the flow, and the distribution of the gas flow in the layer are related. It must be pointed out that in actual production, the temperature of the calcination zone is not the temperature of the gas in the real layer due to the limitation of the depth of the thermocouple insertion. It is the temperature of the gas in the edge layer, which has a certain error. There are often cases where the temperature of the calcination zone is higher than the temperature of the combustion chamber over a period of time, while in another period of time, the temperature of the calcination zone is lower than the temperature of the combustion chamber; if the quality of the pellets calcined by the former is acceptable, then the latter case It is obviously under-burned and the quality has dropped. The above facts show that it is not comprehensive to control the operation of the shaft furnace simply by the temperature of the combustion chamber, and controlling the temperature of the roasting belt and controlling the temperature of the combustion chamber is the correct measure for the operation of the shaft furnace.
III. Problems and development direction of China's shaft furnace pellet production (I) Problems (1) With the development of shaft furnace production technology and the improvement of operation management level, the output of pellet furnace has reached a new level. It is not difficult to produce 300,000 tons of pellets per year in an 8m 2 shaft furnace;
(2) The pellets are of good quality and basically meet the smelting requirements of the blast furnace of the factory;
(3) The main indicators of the shaft furnaces of each factory vary greatly. In addition to the raw materials, there are still many weak links in operation management.
1. The amount of bentonite is greatly exceeded. According to the convention, when producing acidic pellets, the amount of bentonite added is less than 2%, that is, the maximum consumption of bentonite per ton of pellets is about 20kg, and the average consumption of bentonite in the national shaft furnace in 1998 is about The amount is as high as 40kg/t, which is a big waste! Due to the increase in the amount of bentonite, the iron grade is directly reduced, causing serious consequences of “reduce iron and increase focus†in the blast furnace. The reasons for the excess of bentonite are: (1) poor quality of concentrate (grain size, high water content), in order to improve the yield and quality of raw balls, a large amount of bentonite is added; (2) the quality of bentonite is poor; (3) the equipment of bentonite is not accurate, Large error
(4) The mixing device is not effective. Most shaft furnace manufacturers use a cylinder mixing (or drying) machine for mixing, and the amount of bentonite is high. Nangang uses a grinding mill to treat the mixture. In addition to good grinding, the mixture is thoroughly mixed. The amount of bentonite is reduced (the average annual consumption is only 17.7 kg / t). In addition, the search for efficient and inexpensive organic binders or composite binders is also the direction of future efforts.
2. FeO content in a few shaft furnace pellets exceeds the standard. The FeO content in most shaft furnace pellets in the country is less than 2%, while the FeO content in individual shaft furnace products is as high as 2% to 4%, which should not be. There are two reasons for the analysis: one is that the calcination temperature is insufficient, the oxidation is insufficient, and the pellet strength is not high; the other is that the oxygen content in the combustion chamber exhaust gas is too low, or the wall is cooled outside the calcination zone. Less wind also causes insufficient oxidation.
3. The iron grade of the shaft furnace ore mine is low. The iron grade of the imported pellets is generally above 65%. The iron grade of the shaft furnace pellets in China is mostly around 60%, and the highest is only 64.30%. The lowest value is 56.16%, the gap is very large. The reason for the analysis is that the fine powder performance is too poor, the iron grade is low, the SiO 2 content is high, the grain size is coarse, and the water is large, which leads to high ratio of bentonite, which is also the reason for the low iron grade of the pellet.
(II) Development direction After more than 30 years of hard work, China's shaft furnace pellet production technology has reached the world's advanced level, but from the perspective of development, there is still much work to be done:
(1) Vigorously develop large-scale pellet furnaces. After years of continuous efforts, the annual output of China's largest 16m 2 pellet shaft furnace has exceeded the original design level. In 1998, it reached 559,000 tons, and it is expected to reach 600,000 tons in 1999. The product quality and consumption indicators have also remained at the domestic advanced level. The scientific research task of the large shaft furnace that was issued by the state in that year has been successfully completed. In the future, qualified enterprises can give priority to the 16m 2 shaft furnace during construction. Under the same construction conditions, the construction of a 16m 2 shaft furnace will save about 30% compared with the construction of two 8m 2 shaft furnaces.
(2) Research and development of new high-efficiency and low-cost binders, replacing or partially replacing bentonite, and striving to reduce the binder content of each shaft furnace to below 20kg/t;
(3) Research and development of pelletized shaft furnaces using coal as fuel to create conditions for the construction of shaft furnaces in mines;
(4) Improve the raw material conditions, improve the iron grade of the concentrate powder, and reduce the SiO 2 content. Develop and develop the Run (grinding) mill series to solve the problem of coarse grain size of the common shaft furnace raw materials and improve the production and quality of the shaft furnace;
(5) Develop new materials and new structures for the wind deflector and its water beams to ensure that their service life is above 'years';
(6) Strengthen the study of mathematical models, establish quantitative empirical formulas between the relevant parameters of the wind guide wall and its various influencing factors, and better serve the design and production.
By the end of 1998, China has (ranging in size from 5 ~ 16m 2, but mostly 8m 2) being produced pellet shaft furnace 25 Eagle, in 1998 the production of pellets over 630 million t, the ancient of the national pellets About 60% of the total output. In addition, several shaft furnaces are under construction.
In the past two decades, China's shaft furnace pellet production has made great progress, and the process and equipment have been basically finalized, and the operation and management have become more and more perfect. Summarizing these aspects of experience, it is of great significance to further improve the production technology level of the shaft furnace pellets.
Second, China's shaft furnace pellet production experience (a) to strengthen the preparation of raw materials 1. Improve the fineness of concentrate powder China's shaft furnace pellets used in iron-containing raw materials mainly magnet concentrate powder. Since most of the magnetite concentrate powder has a relatively coarse particle size (the content of the particle size less than 0.074 mm generally accounts for only 50% to 70%), the pelletizing performance is poor. Increasing the fineness of concentrate powder has been difficult to solve in pellet plants for many years. In recent years, the Nangang pelletizing workshop changed the original chlorinated pellet roasting equipment (chain kiln rotary kiln) into a shaft furnace, roasting acidic pellets, and using the original grinding machine to treat iron concentrate powder (mixed Material) achieved good results. The concentrate powder used by Nansteel is coarser and more versatile; the average content of particle size less than 0.074mm is only 41%, although the bentonite blending amount is as high as 5% in production, and the pelleting performance is still poor. After the treatment by the grinding machine, the content of the particle size less than 0.074mm reaches 68%, which greatly improves the pelletizing performance of the mixture. When the amount of bentonite falls below 2%, the output of the ф5.5m pelletizer reaches 50t/ Above h, the quality of the raw ball is also greatly improved: the average compressive pressure is above 2N/single ball, and the drop strength is more than 10 times/single ball on average. In 1998, the output of 8m 2 shaft furnace reached 409,000 tons, ranking the national advanced level, and the quality of finished balls also meets the requirements of national first-class products. After the concentrate powder is processed by the grinding machine, it not only improves the fineness. It also changes the shape of the particles, increases the surface area and surface activity, is conducive to the ball making operation, and more importantly reduces the amount of bentonite and improves the grade of the pellets. It must be pointed out that after using the grinding machine, the power consumption has increased, but as the output increases, the actual power consumption does not increase much. In 1998, Nangang's electricity consumption was 41.34kw · h / t, ranking the middle of the country. If you take into account the benefits of increased pellet grade iron ore caused by reduced bentonite use, the economic benefits will be even more significant. Therefore, concentrate powders having a particle size of less than 0.074 mm and less than 60% should be treated with a grinder (or similar grinding equipment).
2. Add concentrate powder (or mixture) and dried beneficiation plant facilities due to restrictions by the dewatering equipment capabilities, the water content of the plant concentrate powder is generally from 1% to 3% of a suitable amount of water required when comparing pelletizing. Although the excessively concentrated concentrate powder has a faster ball formation speed but is less strong in ball making, it is easy to produce powder during transportation and production. Therefore, it is necessary to control the water content of the concentrate powder (or mixture) to be slightly lower than 0.5% to 1.0% of the suitable moisture at the time of pelleting, and then add water to make the ball. For the control of concentrate water problems, each plant has paid enough attention. As early as the early 1970s, in order to overcome the difficulty of making the ball due to the excessive water content of the concentrate, the use of a ф2.2m×12m tumble dryer to dry the mixture was effective. In the subsequent reformation of the new shaft furnace or the old shaft furnace, the concentrate (or mixture) drying facilities have been added, and good results have been achieved. The commonly used drying equipment is a tumble dryer. At present, although the manufacturer has a series of stereotypes, most of them are designed for the cement industry. Due to the different material density of the treated materials, the pellet plant must pay attention to check the transmission power when selecting.
3. Increasing Bentonite Metering Equipment In the current pellet plant operation, the concentrate has been automatically metered, and the bentonite is used because of its small amount, difficult to measure, and still uses artificial ingredients, often affecting the pellet grade due to operational errors. Fluctuation. In order to control the amount of bentonite and increase the accuracy of the ingredients, it is necessary to increase the amount of small flow metering equipment. Practice has proved that the impulse flowmeter is a suitable bentonite metering device, which has the advantages of small volume, high precision and low price than the tape scale of the same specification. [next]
4. Using high-efficiency mixing equipment Mixing is an important part of the pellet production process and one of the key measures to determine the quality of pellets. At present, most shaft furnace manufacturers use a cylinder mixing (or drying) machine for (or instead of) mixing, the effect is not very satisfactory. The new shaft furnace adopts a powerful mixer imported or copied from abroad, and the mixing effect is improved. According to the use of Benxi Steel's 16m 2 shaft furnace: the effect of a strong mixer is equivalent to the mixing effect of the two-stage cylinder mixer; after the mixture is caused by the ball, the amount of powder entering the furnace is reduced from 3.84% to 2.51%. The various indicators of the furnace have been greatly improved. However, there are some problems in the use of the intensive mixer: first, the power consumption is high; second, the caries wear is large, the maintenance is frequent, and one spare is needed. It is recommended that in the old factory retrofit, a multi-wheeled belt mixer can be added after the cylinder mixing (or drying) machine. In this way, on the one hand, the mixing effect can be improved, and on the other hand, the small balls generated in the mixing process of the preceding stage can be pulverized. The grinding machine and other grinding machines have a mixing effect. If such equipment is used to treat the mixture, it can be used instead of the mixing equipment.
(2) Strengthening the ball making process and reducing the powder in the raw ball 1. Selecting the large ball making machine Ñ„5.0m, Ñ„5.5m, Ñ„6.0m disc pelletizing machine with advanced structure, complete functions, high output per unit area, is The preferred equipment for the specification of the shaft furnace in the new design, while considering a certain reserve factor.
2. Increasing the looseness of the stream In order to increase the looseness of the stream during the fabric, the mixture is dispersed by a loosening device before being fed into the pelletizer.
3. Increasing the temperature of the mixture In the absence of drying measures for the mixture, the water can be used to raise the yield and quality of the green ball. Bengang's shaft furnace production experience proves that when the hot water temperature is 70-80 °C, the yield of ф5.5m pelletizing plate is increased by 5~7t/(tai · h), reaching 30~35t/(tai · h); The ball drop strength is increased by 3 times/single ball on average, reaching 7-8 times/single ball; the content of the ball in the raw ball is 10~15mm is increased from 80% to 93%.
4. Screening out the powder in the raw ball In order to ensure the gas permeability of the furnace, it is not caused by the powder entering the furnace, and it is necessary to screen out the powder of 6-8 mm in the raw ball. Since the proportion of super-globules larger than 20 mm in actual production is not large, in order to simplify the process, the raw ball screening machine may not have a sieve function. The commonly used raw ball screening equipment is a roller sieving machine. The roll skin is made of ceramic and is wear-resistant, but it is easy to be broken during manufacturing or production, and is now gradually changed to urethane rubber roll skin.
The raw material return material is usually crushed and returned to the former tank of the pelletizer.
(III) Improvement of the distribution equipment The shaft furnace distribution equipment is essentially a belt conveyor that performs reciprocating linear motion on the top of the furnace. 8～10m 2 shaft furnace distribution machine, there is no track in the furnace, this part is cantilever structure. The belt drive of most cloth machines is fixed on the body frame and moves with the body. Practice has proved that this type of cloth machine is prone to fabric segregation. When the car body moves forward or backward, the amount of raw balls given to the tape of the cloth car changes, and the more the car body moves backwards, the more raw material is on the tape. The material surface thus laid into the furnace is not horizontal but inclined, which affects the normal operation of the furnace. In order to correct this segregation, the machine has to operate intermittently: stay in the thin area of ​​the material for a while. The result also increases the labor intensity of the fabric workers. In order to completely overcome fabric segregation and improve the level of automation, the newly designed distribution machine should fix the tape transmission device on the ground.
According to the transmission mode of the traveling mechanism of the cloth machine, the cloth machine has two kinds of steel wire transmission and gear transmission. Regardless of the type of transmission, the body part of the cloth machine is required to be light in weight, and the inertia generated during the commutation is small, which is convenient for control and operation.
When the stroke of the cloth machine exceeds 6.4m (that is, the length of the furnace mouth of the 12m 2 shaft furnace), the height of the rod is increased due to the excessive cantilever at the front of the cloth machine, resulting in an increase in the ball drop and an increase in the ball breaking rate. In this case, the cantilever structure is no longer used and the track can be laid in the furnace. In order not to affect the operation and maintenance, the furnace track can be erected along the two side wall panels of the dust cover of the cloth machine.
(Iv) improved discharge apparatus Group 1. Roll Crusher Roll Crusher unit rolls by the teeth 7 to 11 work independently of the composition, roll spacing is 80 ~ 100mm, and therefore can not control the gear unit discharge roller, which has a main function Two: one is the active material column; the second is the broken large piece. The toothed roller bears the weight of the column in the whole furnace in the furnace, and is also subjected to the scouring of the high-temperature dust-containing airflow, and the working conditions are bad. Except at the roller body hard surfacing alloy, the intermediate shaft need water cooling. In view of the large torque and low speed of the toothed roller, hydraulic transmission should be adopted. According to the form of its transmission cylinder, it is divided into two types: one is a piston cylinder (also divided into a single cylinder and a double cylinder). Because the piston cylinder has simple structure, convenient maintenance and easy to solve spare parts, it has been widely used. The other is a swing cylinder, which has the advantages of advanced structure and convenient operation. The disadvantage is that the cylinder has high processing precision and high cost, and the spare parts are not easy to solve. Wear in a dusty environment. After many years of practice, it is best to seal the shaft end of the toothed roller with a grease packing. Regularly add high temperature grease during use.
2. Electric vibration feeder The electric vibration feeder below the shaft discharge funnel is the only equipment for controlling the discharge of the shaft furnace. The feeding amount of the electric vibrating feeder is adjustable, and the operation switch is mastered by the top cloth distributor. Under the principle of “dry ball into the furnaceâ€, the operation is started according to the working condition of the drying bed. Since the environment in which the vibrating feeder is located is high temperature and dusty, designing the equipment requires special consideration.
(V) Selection of cooling equipment Although the pellet ore is partially cooled in the furnace, it is not sufficient. Due to the limitation of the furnace process conditions, the cooling air volume of the pellet ore unit is only 600~1000m 3 /t. There are about 400 to 600 °C. If transported with a tape conveyor, additional cooling outside the furnace must be added. At present, there are roughly the following types of furnace cooling equipment used by each factory:
(1) After pulling out with a chain plate machine or a bucket winch, unload it on the ground and allow it to cool naturally;
(2) After pulling out with the chain plate machine, the steel mesh belt conveyor is connected, and it is naturally cooled during transportation;
(3) cooling with a vertical cooler;
(4) Cool with a light blast belt cooler.
The above several cooling methods can be adopted by each factory according to local conditions. [next]
(6) Dust removal improvement of the shaft top of the shaft furnace Because the shaft furnace is operated under positive pressure, a large amount of dusty exhaust gas emerges from the furnace mouth, which not only pollutes the environment, but also is unsafe for the operator. The furnace mouth must be closed to remove the exhaust gas. deal with. The electrostatic precipitator is an ideal dust removal device . The dust concentration in the exhaust gas is less than 100mg/m 3 , which is in line with national emission standards. Some early-stage shaft furnaces still use cyclone dust collectors, and the dust concentration in the exhaust gas is far from the standard, and should be changed to an electrostatic precipitator as soon as possible.
The test results show that the physical properties of the electric dust removal ash are close to the concentrate powder, which can be matched with the mixture for reuse. However, it must be noted that water must be moistened before transportation to prevent secondary dust.
(7) The shaft of the cooked ball facility must be filled with a cooked ball before the furnace is opened. Usually, the condition of the furnace (such as handling the collapse accident) also needs to use part of the cooked ball, and the facility that transports the cooked ball to the top of the furnace is called "Upper ball facilities." The common upper ball facilities are as follows:
(1) Using a forklift or a tilting portable tape machine to directly feed the cooked ball to the raw ball conveyor of the upper shaft furnace, and enter the furnace through the cloth machine;
(2) Set up a special transport ball conveyor belt system, which is transported from the finished mine tank to the top of the furnace and enters the furnace through the distribution machine;
(3) Set the cooked ball hoist, transport the cooked ball to the top of the furnace, and enter the furnace through the cloth machine.
Note: The cooked ball can't pass the raw ball sieving machine, so as not to wear the sieve roll.
(VIII) Improvement of shaft furnace body 1. Basic characteristics of Chinese pellet shaft furnace The Chinese pellet shaft furnace has the following characteristics:
(1) Positive pressure operation requires that all parts of the furnace have good air tightness.
Low quality coal gas blast furnace (2) with a plurality of fuel supplied metallurgical plant, rational energy use;
(3) Adopting the technology of “dry bed and air guiding wallâ€, the airflow distribution is reasonable, and the fan pressure is lower than half of the same type of shaft furnace in foreign countries.
2. Combustion Chamber In theory, the shaft furnace combustion chamber is a space for generating hot exhaust gas. As long as the generated hot exhaust gas satisfies the shaft furnace roasting requirements (such as temperature, pressure, oxygen content, etc.), it is independent of the shape and position of the combustion chamber. Of course, the most economical and rational configuration is that the combustion chamber is placed close to the sides of the furnace. The shape may be rectangular or circular (either horizontally or vertically); and the horizontal circular combustion chamber is divided into two types which are parallel to the furnace and perpendicular to the furnace. The key is that the structure is reasonable and meets the requirements of positive pressure operation. The shape of the combustion chamber determines the number of burners. The number of burners on the side of the rectangular combustion chamber should not be too small, preferably 3 to 5, too much operation is inconvenient, too little affects the uniformity of the atmosphere. The number of burners on the side of the horizontal circular combustion chamber is 2 to 4.
3. Drying belt Drying is the key to shaft furnace roasting. "Dry ball into the furnace" is an important principle of shaft furnace operation, which is the premise of ensuring that the furnace is antegrade, not agglomerating, high yield and high quality. The factors affecting the drying process are mainly the airflow temperature and the airflow velocity. At present, the drying temperature of most shaft furnace tops is low, and the air flow speed is too small, which affects the performance of the furnace. Under the condition of ensuring a certain baking time, shortening the height of the preheating roasting belt is beneficial to increase the temperature of the drying airflow, and appropriately increasing the cooling air flow rate is beneficial to increase the drying airflow speed. When the shaft furnace is newly designed, the drying bed area can be appropriately increased, the thickness of the drying bed layer can be reduced, and the drying process can be accelerated. The ratio of the total area of ​​the drying bed to the cross-sectional area of ​​the calcined zone is about 1.8 to 2.0. The ball stays on the drying bed for about 10 to 12 minutes.
The angle of the drying bed should not be too large, generally around 41 ° ~ 42 °. In the operation, the drying bed inclination angle can be adjusted at any time according to the specific production conditions. The seamless bed water beam is generally made of seamless steel pipe, and the cooling method may be water cooling or vaporization cooling. Since the vaporization cooling pipe takes up a large space, it will increase the height of the upper part of the furnace. From the perspective of domestic production practice, it is more suitable to select water cooling.
Dry tweezers are available in single rows and in multiple rows. The materials are cast steel. The scorpion seams are straight and horizontal, so it is better to not block the pupils. In order to improve the antioxidant capacity of the scorpion, the material is preferably a low alloy cast steel.
4. Preheating the calcining belt The distance from the center line of the water beam along the center line of the water beam to the center line of the exit of the fire tunnel is called the preheating roasting belt. The height should meet the minimum firing time requirement of the pellet in the belt, and the general residence time is 50-60 min. The cross-sectional area of ​​the straight section of the narrowest portion of the calcining belt is equal to 1/2 of the nominal area of ​​the shaft furnace. China's 8 ~ 16m 2 shaft furnace roasting belt has a single side width of 1.5 ~ 2.1m.
5. Shape of the fire tunnel The model demonstrates that the flow of hot exhaust gas from the burner tunnel is in the upward parabola. Thus, at the same level on the exit of the fire channel, the temperature and atmosphere of the gas stream are not uniform, which will affect the quality of the roasting. If the blasting port is inclined downward by an angle (to prevent the pellet from clogging the fire channel, the horizontal angle of the slant is greater than 30°), the gas flows toward the trajectory—the parabola is also inclined downward by an angle, and the uniformity of the airflow on the same horizontal surface is greatly improve.
The blasting port is in the shape of a hole, and the ejection speed is in the range of 10 to 15 m/s.
6. The wind guide wall and the uniform wind guide wall are a grid-shaped hollow wall that is placed in the center of the upper part of the shaft furnace. The function is to ensure that there is sufficient cooling air (also called wind outside the wall) to pass through the roasting belt to supplement the exhaust gas of the combustion chamber. The shortage of oxygen in the middle makes it possible to use the blast furnace gas for roasting in the shaft furnace; the other rising cooling air is directly guided to the drying bed of the furnace through the air guiding wall to participate in the drying without interfering with the roasting belt. Due to the existence of the wind guide wall, the various processes in the furnace (drying, preheating roasting, soaking, cooling) are more obvious and more balanced, which provides conditions for the high yield and quality of the shaft furnace. Different raw materials (different amounts of FeO), different fuels (different oxygen content in the exhaust gas of the combustion chamber) require different amounts of wind outside the wall to pass through the calcining zone, and the parameters (height and width) of the wind deflecting wall are also different.
The distance from the center line of the fire passage to the lower edge of the wind guide wall is called the tropics, and its role is to make the pellets more fully oxidized and more uniform in quality. In order to prevent the bulk material from being stuck, the wall of the furnace on the side of the furnace should be kept vertical.
The low temperature life of the wind guide wall and its water beam (consisting of two rows of seamless steel tube composite beams) (generally only about 1a) is a major problem in the current shaft furnace. Analysis of the reasons, mainly because the structure is not reasonable:
(1) The wind guide wall is thin, only half a brick thick (116mm). This is the old-fashioned 8m2 shaft furnace renovation. It is necessary to increase the wind guide wall without excessively reducing the cross-sectional area of ​​the roasting belt. The air guiding wall of the newly designed shaft furnace should be properly thickened to maintain at least one brick thickness (232mm);
(2) The water beam section stiffness is not enough. The water guide wall water beams are parallel in two rows, and the strength of the connecting members in the middle is critical. At a high temperature of about 1000 ° C, if it is not cooled by water, any heat-resistant steel components will be burnt out, and the two drainage beams that have lost contact will lose stability under the side pressure of the charge on both sides. It is recommended to change the connecting plate to a water-cooled steel pipe. The overall rigidity of the water beam is greatly increased, the life of the water beam is improved, and the overall life of the wind guiding wall is also increased. The structure of the wind guide wall can last for 3 to 4 years.
7. Cooling zone The distance from the cooling air inlet to the lower edge of the wind beam of the wind deflector is called the cooling zone. Considering the balance angle of each belt in the furnace, the amount of cold air blasted into the furnace is limited, generally around 800m 3 /t, the discharge temperature is about 500 °C, the calculated height of the cooling zone is 3 ~ 3.5m, and the residence time is 2 ~3h. The cold air vent should also be inclined downward at an angle, and the cold air vents on both sides are preferably staggered. [next]
(9) Main principles of shaft furnace operation 1. The dry ball enters the furnace, and the bottom of the furnace eliminates the agglomeration accident. The so-called “dry ball into the furnace†means that the cloth worker carefully observes the dry condition of the green ball on the drying bed. When the lower half of the drying bed (at least to 1/3) the green ball is fully dry (when the green ball is grayish white), the switch of the discharge electric vibration can be activated to start the discharge. Due to the limitations of the drying process, the discharge operation is discontinuous in actual production. After implementing the principle of “dry ball into the furnaceâ€, the stubborn disease in the operation of the shaft furnace—the “caking in the furnace†phenomenon was completely eliminated. With the improvement of production management level, the forbidden zone of the so-called "mine powder roasting temperature range greater than 200 °C to build a shaft furnace" no longer exists.
2. Control the temperature of the calcining zone to achieve high yield and quality. The magnetite pellets release a large amount of heat during the oxidative roasting process, so the combustion chamber temperature is usually lower than the annealing zone atmosphere temperature. Depending on the type of raw material, this difference is around 150 to 250 ° C. This temperature is almost constant when the raw materials and fuel conditions are constant. Since the center temperature of the calcining zone cannot be measured with a thermocouple, most shaft furnace operators control the combustion chamber temperature as an important parameter for shaft furnace operation. However, the production practice proves that it is easy to cause the pellet quality to fluctuate simply by operating according to the temperature of the combustion chamber. Here the concept of "baking zone temperature" (also known as side wall fire channel temperature) is introduced, which more accurately reflects the roasting of the furnace. This is because the combustion chamber temperature is the temperature in the space of the combustion chamber after combustion of the gas. It is only related to the fuel type, the ratio of the air gas flow rate and the structure of the combustion chamber; and the temperature of the calcination zone is in addition to the above factors. The types of raw materials, the change of the flow, and the distribution of the gas flow in the layer are related. It must be pointed out that in actual production, the temperature of the calcination zone is not the temperature of the gas in the real layer due to the limitation of the depth of the thermocouple insertion. It is the temperature of the gas in the edge layer, which has a certain error. There are often cases where the temperature of the calcination zone is higher than the temperature of the combustion chamber over a period of time, while in another period of time, the temperature of the calcination zone is lower than the temperature of the combustion chamber; if the quality of the pellets calcined by the former is acceptable, then the latter case It is obviously under-burned and the quality has dropped. The above facts show that it is not comprehensive to control the operation of the shaft furnace simply by the temperature of the combustion chamber, and controlling the temperature of the roasting belt and controlling the temperature of the combustion chamber is the correct measure for the operation of the shaft furnace.
III. Problems and development direction of China's shaft furnace pellet production (I) Problems (1) With the development of shaft furnace production technology and the improvement of operation management level, the output of pellet furnace has reached a new level. It is not difficult to produce 300,000 tons of pellets per year in an 8m 2 shaft furnace;
(2) The pellets are of good quality and basically meet the smelting requirements of the blast furnace of the factory;
(3) The main indicators of the shaft furnaces of each factory vary greatly. In addition to the raw materials, there are still many weak links in operation management.
1. The amount of bentonite is greatly exceeded. According to the convention, when producing acidic pellets, the amount of bentonite added is less than 2%, that is, the maximum consumption of bentonite per ton of pellets is about 20kg, and the average consumption of bentonite in the national shaft furnace in 1998 is about The amount is as high as 40kg/t, which is a big waste! Due to the increase in the amount of bentonite, the iron grade is directly reduced, causing serious consequences of “reduce iron and increase focus†in the blast furnace. The reasons for the excess of bentonite are: (1) poor quality of concentrate (grain size, high water content), in order to improve the yield and quality of raw balls, a large amount of bentonite is added; (2) the quality of bentonite is poor; (3) the equipment of bentonite is not accurate, Large error
(4) The mixing device is not effective. Most shaft furnace manufacturers use a cylinder mixing (or drying) machine for mixing, and the amount of bentonite is high. Nangang uses a grinding mill to treat the mixture. In addition to good grinding, the mixture is thoroughly mixed. The amount of bentonite is reduced (the average annual consumption is only 17.7 kg / t). In addition, the search for efficient and inexpensive organic binders or composite binders is also the direction of future efforts.
2. FeO content in a few shaft furnace pellets exceeds the standard. The FeO content in most shaft furnace pellets in the country is less than 2%, while the FeO content in individual shaft furnace products is as high as 2% to 4%, which should not be. There are two reasons for the analysis: one is that the calcination temperature is insufficient, the oxidation is insufficient, and the pellet strength is not high; the other is that the oxygen content in the combustion chamber exhaust gas is too low, or the wall is cooled outside the calcination zone. Less wind also causes insufficient oxidation.
3. The iron grade of the shaft furnace ore mine is low. The iron grade of the imported pellets is generally above 65%. The iron grade of the shaft furnace pellets in China is mostly around 60%, and the highest is only 64.30%. The lowest value is 56.16%, the gap is very large. The reason for the analysis is that the fine powder performance is too poor, the iron grade is low, the SiO 2 content is high, the grain size is coarse, and the water is large, which leads to high ratio of bentonite, which is also the reason for the low iron grade of the pellet.
(II) Development direction After more than 30 years of hard work, China's shaft furnace pellet production technology has reached the world's advanced level, but from the perspective of development, there is still much work to be done:
(1) Vigorously develop large-scale pellet furnaces. After years of continuous efforts, the annual output of China's largest 16m 2 pellet shaft furnace has exceeded the original design level. In 1998, it reached 559,000 tons, and it is expected to reach 600,000 tons in 1999. The product quality and consumption indicators have also remained at the domestic advanced level. The scientific research task of the large shaft furnace that was issued by the state in that year has been successfully completed. In the future, qualified enterprises can give priority to the 16m 2 shaft furnace during construction. Under the same construction conditions, the construction of a 16m 2 shaft furnace will save about 30% compared with the construction of two 8m 2 shaft furnaces.
(2) Research and development of new high-efficiency and low-cost binders, replacing or partially replacing bentonite, and striving to reduce the binder content of each shaft furnace to below 20kg/t;
(3) Research and development of pelletized shaft furnaces using coal as fuel to create conditions for the construction of shaft furnaces in mines;
(4) Improve the raw material conditions, improve the iron grade of the concentrate powder, and reduce the SiO 2 content. Develop and develop the Run (grinding) mill series to solve the problem of coarse grain size of the common shaft furnace raw materials and improve the production and quality of the shaft furnace;
(5) Develop new materials and new structures for the wind deflector and its water beams to ensure that their service life is above 'years';
(6) Strengthen the study of mathematical models, establish quantitative empirical formulas between the relevant parameters of the wind guide wall and its various influencing factors, and better serve the design and production.
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