Factors affecting material mixing and pelleting

The degree of mixing of the materials in the mixer and the quality of the pellets are related to the nature of the frit itself, the method of wetting the water, and the operating factors.
(1) The influence of the nature of the raw materials. Cementitious material is easy to make big ball, red iron ore and limonite ore pelletizing than magnetite easy, but the impact of the mix is just the opposite. The particle size of the material is too large, it is easy to cause segregation, it is difficult to mix, and it is also disadvantageous for pelletizing. In the case of the same particle size, polygonal and irregular materials are easier to spheron than smooth materials. The difference in density of materials is also too great for mixing and pelletizing.
Nippon Steel samples from the sintered mixture to study the state of the mixture. It was found that the pellets were generally composed of core particles and adherent particles, and were named as quasi-particles. The conditions for quasiparticle formation are closely related to the particle size composition. The smaller the intermediate size particles, the more advantageous it is to form the quasiparticles. The ratio of core particles to adhering particles should be appropriate.
Figure 1 shows the relationship between the adhesion ratio and the particle size.

The spheroidal condition of the sinter is affected by both the original properties of the raw materials (particle size composition, strength of the core particles, degree of particle adhesion, etc.), as well as the raw material preparation conditions (such as the pelletizing characteristics of the mixer, the amount of water added, and whether or not the binder is added). )Impact. Therefore, if the raw material conditions are prepared exactly the same, the spheronization of the material depends first on the original properties of the raw materials, mainly on their fractional distribution.
The returning mine generally has a large grain size. The combination of returning ore and concentrate is unfavorable for pelletizing and mixing. In foreign countries, the returning mine was added to the secondary mixer and mixed with other sintered pellets for a short time. In order to avoid the gas permeability of the sintered material during the long-time mixing process. Industrial production tests have shown that this is better than the effect of the first mixing, and the material layer can maintain good gas permeability when the moisture content of the sintering material fluctuates greatly. According to statistics, this method can increase productivity by 16%. Another method is to pre-cast the fine powder and sulfuric acid slag in a disc pelletizer, and mix the other raw materials in another one, mix them, and then send them together twice. The cylinder ball handler is also designed to improve the breathability of the material layer.
(2) Control the amount of water added and the method of adding water. The moisture of the mixture has an important influence on the sintering process. 1) Through the surface tension of water, the small particles of the mixture are spheroidized to improve the gas permeability of the column; 2) the surface of the wet ore has less resistance to air friction and is also beneficial to improve the gas permeability.
As the moisture increases, the gas permeability of the mixture increases, thereby increasing productivity. Generally, the optimum value is reached and then lowered until the mud limit, so that the air can no longer pass through the mixture. Figure 2 shows the effect of moisture on the sintering productivity of the rich ore and limonite mixture when sintered.

It can be seen from the figure that the use of rich ore sintering has a humidity of 9-10%, and the productivity has a significant maximum value. The use of braided limonite is used for sintering, preferably with a humidity of 12-14%. The returning mineral is added with 30%. The ratio of iron-containing materials is the best, and the excessive productivity is decreased. In addition, the amount of return minerals increases, and the moisture required for optimum gas permeability is reduced. [next]
In the laboratory test, the cold state is often used to determine the moisture permeability of the mixture. However, in the sintering process, in order to make the mixture fully wet, the hot returning mine should be watered, and then water is added to the mixing machine to fully wet the mixture, and the secondary mixture is added with water to strengthen the ball forming effect. The final moisture of the sinter is measured at the exit of the secondary mixer by manual inspection or by using a neutron moisture measuring device.
The manual inspection uses the drying method and the observation method. Drying method takes out the sinter material, weigh it in #)), put it in an oven and heat it to 110 °C to dry, then weigh it, and the amount of water lost is set.

The observation method is to use the eye to test the external characteristics of the material by experience. When the water is appropriate, the following phenomena are observed:
1) After holding the material tightly, it can keep the lumps, and it can be dispersed with a slight agitation;
2) Feel soft after holding the material, a small amount of powder sticks to the hand;
3) There are 1 to 3 mm small balls;
4) No special luster on the material surface.
When the water is insufficient, the hand can't be a ball, no ball. When the water is too large, the material is shiny, and after the hand is held into a group, it is not easy to spread after stirring, and the mud sticks to the hand.
The water addition method is a measure to strengthen the mixing operation of the mixture. Generally, when the magnet concentrate is sintered, the moisture content of the mixture is controlled to be about 8 to 9%, wherein 7 to 8% of the mixed water is added, and 1 to 2% is added by the second mixing. The sintering plant of the Southern Jiangsu Mining and Selection Company adopts the second mixing. Sectional water addition method, the secondary mixer (ф2.8×6 m) is divided into several sections along the length direction: the preparation section (0.5 m) is the receiving place, no water is added; the main wetting section (0.5 m) is added with water to Forming the core of the pellet; the pelletizing section (2 meters) does not add water, so that the pellet is formed into a hard solid; the supplementary watering section (0.5 m) wets the un-ball portion; the small sphere hardening section (2.5 m). It is reported that After adopting this method, the <1.6 mm portion of the mixture is reduced by 17% and the gas permeability is increased by 15%.
(3) Operational factors. The inclination of the mixer determines the residence time of the material in the mixer. The larger the angle of inclination, the shorter the mixing time of the materials, so the effect of mixing and pelletizing is also poor. The general cylinder mixer is used for one mixing and its inclination angle is not more than 4°. For the second mixing, the inclination angle should not be more than 2.5°. The rotation speed of the mixer determines the movement condition of the material in the cylinder, and the rotation speed is too small. The centrifugal force is small, and the material rises in a certain height at a certain height in the cylinder to form a stacked state, and the mixing and pelleting efficiency are low. If the rotation speed is too large, the centrifugal force is too large, so that the material is tightly attached to the cylinder wall, so that the mixing and spheroidation are completely lost. After calculation, the critical speed of the mixer is (Dc is the diameter of the mixer minus 0.1 m), and the speed of the cylinder mixer should be 25 to 35% of the critical speed. The length of the mixer determines the time of the material in the mixer. The mixing time of the iron concentrate is generally not less than 2.5 to 3.0 minutes, that is, mixing for about 1 minute at a time and mixing for 1.5 to 2.0 minutes for a second time. Current foreign trends have extended the mixing time to 5 minutes. For example, the Nagoya No. 1 sintering machine in Japan has increased the mixing time to three times due to insufficient mixing and pelletizing time, and the mixing time is 7.25 minutes. The original one-time mixing and the second mixing of the Oita No. 1 machine were respectively 1.7 minutes, and the mixing time was increased by three times and the mixing time was 8.1 minutes. At present, many factories in China have already achieved certain effects by extending the secondary mixer from 6 meters to 9 meters. Shougang Yizhu will change the secondary mixer from ф2.5×6 meters to ф3×10 meters, which will be extended from 1 minute to 3 minutes and 10 seconds.
The filling factor of the mixer has a good effect on increasing the yield, but the filling coefficient is too high to limit and destroy the movement of the material, which is unfavorable for mixing and pelletizing. It is generally considered that the primary mixing filling factor should not be greater than 15%, and the secondary mixing filling coefficient is smaller than the former.

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