Research and Application of Improving the Ball Formation Rate of Kunming Steel Pellet Production Line

Acidic pellets with high alkalinity sinter are recognized as reasonable blast furnace charge structures. To further optimize the structure of blast furnace, into the furnace to improve the quality and reduce the cost of smelting, replace expensive imported pellets, Kunming Iron Steel Co., Ltd. (hereinafter referred to as Kunming Iron and Steel) 2004 July 22 put into operation a 1.2 million t / a chain machine - rotary kiln acid oxidation pellet production line. Affected by raw material conditions, design defects, equipment failures, lack of experience, etc., the ball formation rate at the initial stage of production is only about 30%, which seriously restricts the production level. To this end, Kunming Steel and Central South University have carried out a large number of experimental research and on-site research work, and have organized four stages of technological transformation and technical research, and achieved remarkable results. The ball-forming rate of Kunming Steel's pellet production line has been achieved. It has increased to more than 60% and has successfully reached production.

First, the analysis of the reasons for the low rate of formation

(1) Study on the ball-making experiment of single ore

The design structure of the Kunming Steel Pellet Production Line is “30% Dahongshan Iron Concentrate + 70% Brazilian MBR Pellet Fine Powder”, but due to external resources and transportation conditions, the actual material structure for most of the time after commissioning It is “25%-33% Brazilian MBR pellet fine powder +67%-75% provincial mixed concentrate”, in which the composition of mixed concentrate in the province is more complicated, mainly from Dahongshan concentrate, Mannankan concentrate, Yimen copper iron ore flotation plant tailings, and other coarse particles concentrate after the grinding by the second configuration of several raw materials and other products. The by-products of the flotation copper mine in Yimen Concentrator are affected by flotation reagents, and the sphericity is poor. The fineness of the fine powder of the MBR pellets in Brazil is relatively uniform, and the content of fine fraction is small, which is also difficult to become a ball material; other provinces The inner concentrate size is coarser, less than 0.074mm, the grain content is only about 50%, and the ball forming performance is not ideal. According to the ball test conducted by the Sintering Pellet Research Institute of Central South University, the static sphericity index of several materials used in the Kungang Steel Pellet Production Line is low, belonging to weak sphericity or non-spherical materials. See Table 1 for details. .

Table 1 Static sphericity performance of iron concentrate in Kunming Steel Pellet Production Line

(II) Experimental study on ball mixing in the field

The ball-making test was carried out under laboratory conditions to study the ball-making performance under different production conditions and the technical measures to increase the production rate. The main concentrate samples are pre-matched concentrates (iron concentrates without high-pressure roller mill in production, ie pre-dosed concentrates), roller mill concentrates (iron concentrates after high-pressure roller mill treatment in production), strong mixing Concentrate (the iron concentrate after treatment by the intensive mixer has been mixed with a certain amount of bentonite ). The results of the pelleting test are shown in Table 2.

Table 2 Mixing ball test results

Studies have shown that after a reasonable combination of different types of mixed iron concentrates, the inadequacies in the performance of a single iron concentrate can be compensated to a certain extent. When the amount of bentonite used in preconcentrated concentrate and strong mixed concentrate must reach 2.5% or more, the falling strength of the raw ball can reach 3.0 times/0.5m or more; when the amount of bentonite used in the roller mill concentrate must reach 2.0% or more, the falling strength of the raw ball can reach 3.0. Times / 0.5m or more.

(III) Pelletization test study on different concentrate pretreatment methods

Under the same raw material conditions, the pelletizing test results of the pre-concentrated concentrate after high-pressure roll grinding, strong mixing and grinding are shown in Table 3. The amount of bentonite used in the test was 2.0%, and the pelleting time was 10 min.

Table 3 Comparison of pelleting tests for different concentrate pretreatment methods

After the iron concentrate is subjected to high-pressure roll grinding, strong mixing and grinding, the falling strength of the raw balls will be improved to different extents. In comparison, the use of the grinding pretreatment method has a better effect on improving the falling strength of the green ball, but it has a certain influence on the bursting temperature of the raw ball.

(4) Examination of production process

The pelleting rate of the Kunming Steel pellet production line is calculated by dividing the finished pellets of the pellets per unit time by the amount of pellets. The other domestic enterprises generally divide the ball output by the amount of the ball and divide it by the amount of the ball. Percentage points. Therefore, the pelletizing rate of the pellet is not only closely related to the physical and chemical properties of the raw materials, the preparation method, the surface properties and hydrophilicity of the material, the pelletizing equipment and process parameters, the quality of the green ball, but also the number of times of transfer with the raw ball. Transit height, grate-rotary kiln thermal system, etc. are also closely related. On the basis of the experimental research, Kunming Steel and Central South University also organized a special person to examine the changes in the particle size and strength of the pellets during the transfer process from the pelletizer to the grate, as well as the quality of the preheated balls and the calcined balls. According to the results of the examination, the main reasons for the low pelleting rate of the Kunming Steel pellet production line are as follows: 1 The iron concentrate has poor ball forming performance (such as Brazilian mine, copper tail fine, etc.), resulting in the mixture into a ball during the ball making process. It is difficult to grow up; 2 the water content of the ball is too high (10.5%), which leads to the merger and growth of the pellets during the ball making process, which makes the ball drop strength, compressive strength and burst temperature lower; 3 the position of the scraper in the pelletizer The location of the water addition and the improper watering method result in the grading of the pellets in the sphere is not obvious; 4 the number of transfer of the raw balls in the production process is large, and the drop in the transfer point is large, resulting in the defective ball of the original strength being broken during transportation; The operating parameters of the 5 chain boring machine are unreasonable. The wind temperature and wind speed of the air drying I and II are too high, and the poor permeability of the material layer causes the green ball bursting amount in the drying process.

Second, improve the production practice of raw ball formation rate

(1) Optimizing the structure of raw materials

Due to changes in the supply of raw materials, the material structure of the pellets has been adjusted several times, as shown in Table 4.

Table 4 Pellet material structure comparison%

The pelleting properties of a single iron concentrate are generally not optimal and must be ore blended to optimize the material structure. According to the research results of the ball-making performance and roasting performance of Kunming Iron and Steel Mine, the ratio of iron concentrate was adjusted in production practice. The material structure of the first stage basically adopts the material mode of one third of each of Brazil, Dahongshan and Youjing; the material structure of the second stage gradually increases the province with better test peening effect. Mannankan iron concentrate consumption, the total amount of materials used has reached four; the third stage further increased the amount of Mannankan iron concentrate in the province, appropriately reducing the ratio of Dahongshan and Youjing; The main feature is to increase the amount of self-produced Dahongshan mine and gradually stop the use of expensive Brazilian iron concentrate. The use of self-produced concentrate in the province has reached 90%-100%, and the total material content has reached a record. 5 kinds.

In addition to changes in the structure of the raw materials, the quality of iron concentrates in the province has also been gradually improved. The main physical and chemical properties of pellet concentrates are shown in Table 5.

Table 5 Physicochemical properties of concentrates for pellets%

(2) Reduce the impact of raw ball transport

1. The drop of the birth ball is transferred. Through the maintenance and shutdown, the ball transfer belt conveyor has been modified several times: the D101 belt head wheel is reduced by about 50mm, and the slats are increased to reduce the falling speed of the ball. The D102 belt head wheel is reduced by about 100mm; the D103 belt head wheel is lowered. About 100mm; by reducing the diameter of the pendulum fabric belt head wheel to reduce the drop difference of about 200mm; D102 to D103, wide belt to small ball roller screen, small ball roller screen to the chain machine and other drop points increase the slat.

2, reduce the speed of the tape machine. Decreasing the belt speed can change the ball throwing trajectory, reduce the throwing speed, and reduce the falling impact of the ball, thus protecting the ball from cracking. The belt motor (1450r/min) such as D101B~H was replaced with a low-speed motor (960r/min), and the belt speed was reduced from 1.2 m/s to 0.8m/s.

3, tape machine roller encryption. The roller on the D103 belt is encrypted, and a set of rollers is added between each two sets of rollers, which is equivalent to a one-fold increase in the density of the idler. After the roller is increased, the accumulation of the green ball on the belt is more stable, reducing the relative motion between the green balls, thereby reducing the amount of ball destruction and increasing the ball formation rate.

4. Regular cleaning system. In order to improve the screening efficiency of the roller sieving machine and ensure that the qualified raw balls do not enter the return ball, a related operation and maintenance system has been formulated. First, the pelletizer is required to clean the accumulated material between the screen rollers in a timely manner, keep the gap between the rollers unobstructed, and avoid the qualified raw balls from entering the large ball through the large ball roller screen or the powder from the small ball roller screen into the chain grate; secondly, regularly check the roller screen. The degree of change in the gap was found to be adjusted or replaced as appropriate due to the wear of the roller or the displacement of the roller bearing housing.

(3) Optimizing the ball making process system

1. Inclination and speed adjustment. The inclination and rotation speed of the pelletizing disc directly affect the running track and residence time of the mixture in the tray, and the suitable rotation speed and inclination angle of the raw materials of different properties are also different. The rotation speed of the ball plate of Kunming Steel Pellet is adjusted by changing the drive pulleys of different diameters. Only four kinds of rotation speeds can be selected at the initial stage of production: 7.0, 7.5, 8.0, 8.5r/min. Through production practice, 7.5r/min is determined to be an appropriate value. Then match the speed by adjusting the tilt angle. The production practice of the four stages proves that the grain size of the pellets in the pelletizing disk becomes larger and smaller periodically, and the inclination angle must be adjusted to change the running track and ball forming time of the mixture in the tray, so that the material is in the mother ball area and the long ball. The distribution of the area is more reasonable, and the particle size and the amount of the ball are stabilized. Although the raw materials change frequently and vary greatly, by adjusting the inclination angle, the powder can be prevented from being produced, the qualified raw balls can be produced, and the sphericity rate can be steadily increased.

2. Scraper structure and position adjustment. The electric bottom scraper is changed to a wear-resistant ceramic cutter head for the problem that the disk surface of the pelletizer is not stable, the bottom of the disc is rough, the sub-table, the electric scraper is seriously worn, and the position is not conducive to the rational division of the material in the ball disc. Increase the contact area with the disk surface. In addition, due to the unreasonable installation position of the rotary blade (the position of the clock is about 1:30), the function of splitting and diversion cannot be achieved. After the study, the rotary side scraper is removed, and the fixed side scraper is added directly above the disc. The angle of the disc edge is adjustable, and the trajectory and distribution of the material in the disc are more reasonable. The distribution of materials in the cue ball area and the long ball area is more suitable, the powder output of the ball disc is obviously reduced, the ball yield and the quality of the green ball are obviously improved.

3, the ball side height adjustment. At the initial stage of production of Kunming Steel's oxidized pellet production line, due to the influence of raw material conditions and the effect of high-pressure roller mill, the particle size and particle size composition of the mixture were poor. The content of the mixture was less than 0.074mm and the content of less than 0.045mm was only 70%. 40%. In order to extend the mixing time of the mixture and increase the ball formation rate, the edge height of the pelletizing ball was increased from 600mm to 700mm in 2005. After the transformation, the powder produced was significantly reduced, and the strength of the green ball was significantly improved.

4. Add water pipe shape and position adjustment. In actual production, the inclination angle, rotation speed, side height and scraper of the pelletizing disc are relatively fixed. Changing the shape of the water supply pipe and the watering position become the main means to improve the quality of the raw ball and improve the ball formation rate. Through investigation and study, test water pipes of different lengths, different pipe diameters, different water outlet diameters and different water outlet holes. The test uses a tee to mix compressed air and water to form atomized water and add it to the pelletizing disk. Putting in different combinations, etc., has gained some valuable practical experience. However, the results show that different mineralization schemes, different raw material conditions need to properly adjust the water supply pipe position or even replace the water pipes of different shapes. For the convenience of adjustment, there are currently more than three different shapes of water pipes, and the water supply pipe position is not fixed.

5. Adjustment of feeding mode. Through long-term observation and experiment, the loose material device is added to the head of the dragging scale which conveys the material to the ball disc, and at the same time, the pile height of the material on the dragging scale is reduced, so that the material is loosely distributed into the pelletizing disc, to a certain extent The adjustment of the line fabric to the surface fabric increases the contact area between the new material and the cue ball, further increasing the ball forming speed.

(4) Optimization of the thermal system

According to the production situation, the thermal system is optimized. The specific adjustments are shown in Table 6.

Table 6 Adjustment of the thermal system of pellet production

From the second stage, the temperature level of the I and ll sections of the exhaust drying was gradually reduced, and the temperature level of the preheating section and the kiln head was appropriately lowered in the fourth stage.

(5) Strengthening the pretreatment of raw materials

In order to give full play to the effect of high-pressure roller mill on raw material pretreatment, reduce the feed volume and feed moisture of high-pressure roller mill, and eliminate the influence of bentonite and preheating ball on high-pressure roller mill, the ball return system was modified in the second stage. . After the transformation, the return ball and dust no longer enter the high-pressure roller mill, the feed amount is reduced to about 200t/h, the feed moisture is reduced to 8.5% to 9.0%, and the influence of bentonite and preheating ball on the high-pressure roller mill is also Elimination. In the same year, German experts were asked to carry out on-site commissioning. The working pressure and working current of the roller mill were increased to about 60×10 ⁵ Pa and 400 A, respectively, to reach the rated parameters. After the transformation and commissioning, the roller grinding effect and the spheroidality of the mixture are obviously improved. The mass fraction of the concentrate less than 0.074mm and the fraction of less than 0.045mm can be increased by 5% to 8%, and the pelleting rate is increased by about 5%.

(6) Optimization of other process parameters

In addition to optimizing the raw material structure and thermal system, other process parameters are also adjusted. See Table 7 for details.

Table 7 Process parameter adjustment

It can be seen from Table 7 that due to the adjustment of the raw material structure and the improvement of the fineness of the concentrate in the province, the content of the fraction of less than 0.074 mm in the mixture is significantly increased. The moisture content of the raw balls is also gradually reduced, which provides conditions for improving the quality of the green balls and optimizing the subsequent processes. In addition, considering the fact that the raw water is too large, the blasting temperature of the grate is too high, and the gas permeability of the material layer is not ideal, the pellets were repaired in May 2005, and the height of the side plate of the grate was reduced from 200 mm to 160mm.

(7) Implementation effect

The variation of the ball formation rate in each stage of Kunming Steel's 1.2 million t/a oxidized pellet production line is shown in Figure 1. It can be seen from Figure 1 that due to the sufficient experimental research, accurate analysis of the causes, and strong rectification measures, the actual pelleting rate of the 1.2 million t/a oxidized pellet production line of Kunming Steel has increased from 34.32% at the initial stage of production to the fourth stage. 66.32%, an increase of 32 percentage points, an increase of 93.24%.

Third, the conclusion

The on-site process examination and experimental research results show that the main reason for the low ball formation rate at the initial stage of the production of the 1.2 million t/a oxidized pellet production line of Kunming Steel is the unreasonable raw material structure, the poor performance of single iron granulation, and the ruling process system. Not reasonable, the raw ball transfer gap is too large, and the preheating roasting system is underoptimized. Through four stages of technical transformation and production practices, most of these problems have been rectified and implemented. Due to sufficient experimental research, accurate analysis of causes and strong rectification measures, the actual pelleting rate of Kunming Steel's 1.2 million t/a oxidized pellet production line increased from 34.32% at the initial stage of production to 66.32%, an average increase of 32 percentage points. 93.24%.