I. Introduction
At present, China is still in a magnetic pellet feed iron ore, but with the rapid development of pellet production of magnetite concentrate production can not meet the needs of pellet production. First of all, domestic concentrate powder has low iron content, coarse grain size, unreasonable particle size distribution and high water content, which has an adverse impact on the production and development of pellets in China. In addition, with the expansion of the production scale of pellets in China, the production of domestic concentrate powder is insufficient. Therefore, many domestic enterprises are planning or using imported concentrate to produce pellets. However, the imported ore is mainly hematite, so this article makes a brief summary and introduction of the production and use of Brazilian hematite concentrate in our factory.
Second, 5 million tons / year pellet process introduction
The design scale of Ezhou Pellet Plant is 5 million tons/year. The product structure is acidic pellets, which adopts the grate-rotary kiln production process; the iron raw material is 80% imported Brazilian hematite, and 20% self-produced magnetite ( Effects due to practical use in the market 15 to 40% fluctuations); use of pulverized coal and natural gas fuel. The ball is made into a cylinder pelletizer. A pelletizer, a high pressure roller mill, a vertical mixer, etc. are used to pretreat the pelletizing material. The process diagram is as follows.
Figure 1 5 million tons / year pellet production process diagram
Due to the design consideration of the high hematite ratio, there is a difference in the selection of thermal equipment compared to the process based on magnet fine powder. The main process equipment is as follows: chain grate: 5.664 × 69.4m, rotary kiln: φ6.858 × 45.72m, ring cooler: φ21.94m × 3.65m.
Third, the analysis of raw materials in Brazil
The iron raw materials produced by our pellets are mainly the Karagas hematite concentrate in northern Brazil and the self-produced magnet concentrate. The chemical composition of the pellets is shown in Table 1 below. It can be seen from Table 1 that the Brazilian mineral grade is higher and the silicon content is less. Sulfur, phosphorus and other harmful impurities are small, and the particle size is fine, especially the content of less than 10 μm is 27.57%. In the process of pellet production, such fine particles are advantageous for improving the ball-forming effect. However, the aluminum content is high and the water content is too large.
Table 1 Main physical and chemical properties of iron concentrate
Mineral | TFe % | SiO 2 % | FeO % | Al 2 O 3 % | P % | S % | Moisture % | Burnt out % | Specific surface area Cm2/g | -200 mesh % | -10μm % |
Brazil | 66.54 | 1.20 | 0.8 | 1.50 | 0.040 | 0.01 | 11.4 | 1.88 | 1100-1300 | 86 | 27.57 |
Self-produced | 65.34 | 2.96 | 23.93 | 0.87 | 0.014 | 0.1 | 9-10 | 1.64 | 860-1000 | 72 | 13.42 |
Fourth, the situation of Brazilian mines treated by high pressure roller mill
The roller mill is a device for processing iron concentrate, and the specific surface area of ​​the material is increased by rolling, thereby improving the ball forming performance and the ball strength. Our factory adopts the process configuration of one roll pressing and edge returning. The amount of trim returned is about 10%. The processing capacity of the device is 760t/h. The high pressure roll mill consists mainly of two grinding rolls. One roller is fixed to the equipment frame, the other is mounted on the rail to move freely, and the rail is fixed to the frame. The spacing between the two grinding rolls is not fixed and can be automatically adjusted according to the kneading characteristics of the ground material. The higher the kneading characteristics of the material, the greater the spacing between the rolls. The grinding pressure is applied to the bed between the rolls by moving the grinding rolls. The production practice of our factory shows that the Brazilian mine belongs to the easy-grinding mine. The high-pressure roller mill has obvious effect on the treatment of Brazilian mine. The specific surface area is increased by 700-900cm2/g. After high-pressure roller mill treatment, the ball-making effect is obviously improved.
Figure 2 Schematic diagram of high-pressure roller mill processing raw iron concentrate
V. Balling performance of Brazilian mines
Raw balls are the basis for roasting and finished pellets. Without good quality of raw balls, no matter what kind of roasting process is used, the quality of finished pellets is not guaranteed. The factors that determine the quality of raw balls are mainly the two aspects of pelletizing raw materials and pelletizing process parameters. Our factory adopts the process of cylindrical pelletizer. The main features of the process are that the raw ball can not be automatically graded, and the internal circulation is larger. Due to the mixing of raw balls and returning minerals, the surface of the raw ball is stuck with mineral powder, and the surface is not smooth. Therefore, the requirements for pelletizing materials are more demanding. The production and quality of our factory are shown in Table 2.
Table 2 Quality of raw balls
Match % | Raw water | Raw ball falling | Raw ball compression | Production at the table | Bentonite addition | Production period | |
Brazil | Self-produced | % | Times/time | N/piece | T/h | % | |
100 | 0 | 11.6 | 3.5 | 6.8 | 100~150 | 1.0 to 1.2 | Initial production |
80 | 20 | 10.8 | 4.2 | 7.6 | 150~180 | 1.5 to 1.8 | 2006.8~12 |
70 | 30 | 10.5 | 4.0 | 7.5 | 150~220 | 1.8 to 2.0 | 2007～ |
At the initial stage of trial production, all of them were Brazilian mines. The dryers were not put into normal operation for various reasons. They were put into operation for a period of time in April and then stopped due to failure. Brazilian mines have large moisture (11% to l3.5%), which has an impact on the quality of the pellets and the stability of the production operations. At the same time of initial production, the high-pressure roller mill was not debugged, and the fineness of the iron fine powder could not be stabilized. Compared with the use of high-pressure roller mill, it is found that the influence on raw material fineness, sphericity and pelletizing quality is large. When the production is all Brazilian mines, the raw water is relatively high. Increased the load on the thermal system. Later, due to the adjustment of raw material structure, the proportion of self-production ore was gradually increased. After adding the self-produced ore, the water content of the raw ball is slightly decreased. At the same time, due to the improvement of the equipment and the improvement of the operation level, the drying, grinding and thermal operation are gradually stabilized, so the pellet production is gradually normal and stable. The output has increased steadily. However, the surface finish of the ball is reduced. The strength of the green ball is still not ideal, and the raw ball has a higher powder content. In short, 100% to 70% of the Brazilian red ore ratio can produce qualified raw balls, and the amount of bentonite added is small. Can basically meet the process of production.
Sixth, pellet baking and finished ball quality
The proportion of high hematite is considered in the design of the thermal system of the pellet production in our factory. A sophisticated airflow system is used. The low-temperature hot air in the blast drying section comes from the third cooling section of the ring cooler; the heat source in the draft drying section is the recovered hot exhaust gas from the preheating section II of the grate; the preheating section I heat source is the hot exhaust gas from the second cooling section of the ring cooler. The preheating section II heat source is the hot exhaust gas from the kiln tail of the rotary kiln; the high temperature hot air of the first cooling section of the ring cooler directly enters the kiln for secondary air. At the same time, a chain burner is added to the second stage of the chain boring machine and the regenerative pipe. Practice has proved that this airflow system is reasonable and reliable, and has strong adaptability to raw materials. The pellets produced with a high proportion of Brazilian red or even 100% full red mine can fully reach the quality index of magnetite pellets. High strength, less harmful impurities such as sulfur and phosphorus, high degree of reduction and stable quality. However, the amount of red ore is too much, and the reduction expansion and reduction pulverization are large. Adding about 30% of domestic magnetite is beneficial to reducing reduction expansion and reduction pulverization. The quality of the finished ball produced is shown in Table 3 below. The main thermal parameters are shown in Table 4 below.
Table 3 Quality of finished balls
Red mine ratio% | TFe% | SiO 2 % | FeO% | Al 2 O 3 % | P% | S% | R times | Compressive strength N / ball |
100 | 66.18 | 1.98 | 0.11 | 1.46 | 0.02 | 0.015 | 0.15 | 2568 |
80 | 65.19 | 2.65 | 0.13 | 1.34 | 0.03 | 0.015 | 0.18 | 2680 |
70 | 64.85 | 3.21 | 0.25 | 0.89 | 0.03 | 0.015 | 0.19 | 2628 |
Red mine ratio% | Anti-wear index (-0.5mm)% | ISO drum +6.3mm% | Reduction degree% | Restore expansion RSI% | Reduced powdered RDI +6.3% | Production period |
100 | 3.02 | 91.60 | 23.03 | 70.5 | Initial production | |
80 | 3.25 | 93.23 | 75.97 | 20.25 | 72.9 | 2006.8~12 |
70 | 3.43 | 92.8 | 18.63 | 78.3 | 2007～ |
Table 4 Main thermal operation parameters
Red mine ratio% | Rotary kiln head temperature °C | Rotary kiln kiln temperature °C | Grate dry drum temperature °C | Grate dryer drying temperature °C | Chain grate preheating for a period of temperature °C | Chain grate preheating two-stage temperature °C | Production period |
100 | 1150 | 1100 | 180 | 280 | 650 | 1080 | Initial production |
80 | 1080 | 1060 | 200 | 300 | 650 | 1030 | 2006.8~12 |
70 | 980 | 1050 | 220 | 310 | 620 | 1000 | 2007～ |
At the initial stage of production, mainly for red mines, the temperature control is high, and the experience of raw ball production and quality fluctuation is insufficient, and the thermal operation adjustment is relatively poor, but still can produce high-quality, high-grade and high-strength pellet products. Through the production practice after several months of production, the process control parameters suitable for the factory were gradually explored, and according to the red ore ratio and production change, the process control standards were revised in time, and the thermal operation was standardized; The control of the high temperature of the machine, the control of the chain machine, the kiln head, the ring cooling I section, and the ring cooling II section negative pressure operation.
7. The impact of mines in central and southern Brazil on production
In actual production, the iron raw materials are mainly the Karagas hematite concentrate in northern Brazil. Due to the influence of market supply and demand, the central and southern mines in Brazil are also used in small quantities in our factory. The southern part of Brazil has a coarser grain size with a specific surface area of ​​only 400-700 cm2/g, as shown in Table 5 below. Appearance was slightly black, metallic shine. There are more 0.2-0.1 mm grain-level particles in the mine. This part of the particles not only reduces the specific surface area of ​​the concentrate, but also has poor pelleting properties. After blending, the yield of the raw ball is obviously affected, and at the same time, it mainly adheres to the surface of the green ball, so that the surface of the raw ball is rough, the quality is lowered, the anti-wear performance of the dry ball is lowered, and a powder is formed in the rotary kiln. It has a great impact on production. However, the southern mineral grade is higher, the Al 2 O 3 content is low, and it has a cost advantage. According to the experience of our factory: the ratio of 5 to 8% of mine control in southern Brazil is a relatively successful attempt.
Table 5 Main physical and chemical properties of iron concentrate
Mineral | TFe % | SiO 2 % | Al 2 O 3 % | P % | -1mm % | -0.5mm % | -0.25mm % | -0.15mm % | -0.106mm % | -0.075mm % | Specific surface area Cm2/g |
Central | 67.44 | 1.50 | 0.9 | 0.060 | 99.49 | 99.74 | 99.47 | 94 | 89.8 | 81 | 670 |
Southern | 65.34 | 2.96 | 0.87 | 0.014 | 99.2 | 97.0 | 94 | 86 | 78.6 | 64.1 | 400 |
Eight, conclusion
(1) The iron ore concentrate of Caragas, Brazil is high in iron grade, with low impurity content and good grindability. The balling effect is good.
(2) Producing pellets mainly from Brazilian hematite. In the production, the corresponding measures are taken to improve the high-pressure roller mill and thermal operation system from the process, and 100% of the red mine can produce high. Grade, high strength pellets. However, with a high proportion of red ore, attention should be paid to the high aluminum content and the reduction expansion and low-temperature reduction pulverization.
(3) Adding about 30% of domestic magnetite can not only improve the roasting performance, but also solve the problem of high temperature reduction and pulverization of pellets produced in Brazil.
(4) 5-8 percent of the minerals in southern Brazil are a suitable ratio for relative success.
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