The physical and chemical properties such as particle size, montmorillonite content, swelling degree, water absorption, and blue absorption of A, B, and C bentonites were studied under laboratory conditions. The effects of adding different quality and different proportion of bentonite on falling strength, compression strength, and shock temperature of green pellet were investigated. The experimental results show that the montmorillonite content, water absorption, and methylene blue absorption of bentonite-B are the highest. And the quality of bentonite-B is the best, followed by bentonite-C and bentonite-A poor quality. When the amount of bentonite-B reduced from 1.5% to 1.0%, the strength of green pellets and the shock temperature both decrease. As the same proportion of A, B, and C bentonites, the green-ball strength and shock temperature are as follows: bentonite-A > bentonite-B > bentonite-C.
Pellets should have the properties of high grade, good intensity, and uniform granularity as beneficial furnace burden in the ironmaking process. To increase yield and reduce coke consumption and to improve economic benefits, acidic pellet can be combined with highly basic pellet to form appropriate burden structure [
Bentonite is a traditional metallurgical pellet binder, which ensures that the dry and roasted pellets have certain strength properties that meet transportation requirements [
Bentonite has strong hygroscopicity and expansibility. It can absorb water by as much as 8–15 times its dry mass. When wet, it can expand by even up to 30-fold of its dry mass. Bentonite can be dispersed in aqueous medium as a viscous and suspended material. Bentonite solutions have viscous, variable, and lubricant properties. Different types of bentonites influence pellet properties, as well as the interaction among heterogeneous or homogeneous bentonites and iron concentrate [
Iron concentrates and bentonites produced by an iron and steel company in southwest China were used. The main chemical constituents of the iron concentrate and the bentonites were analyzed by X-ray Fluorescence (XRF). Both the granularity and moisture content of the iron concentrate were also determined. The results are presented in Tables
Physical and chemical analysis of the iron concentrate (%).
Constituents | TFe | SiO2 | Al2O3 | CaO | TiO2 | MgO | FeO | S | −0.074 mm | Moisture |
---|---|---|---|---|---|---|---|---|---|---|
Content | 57.57 | 5.93 | 1.20 | 1.10 | 1.80 | 4.80 | 19.78 | 0.413 | 73.28 | 4.5 |
Chemical analysis of the bentonites (%).
Constituents | MgO | CaO | Al2O3 | P2O5 | SiO2 | Fe2O3 | K2O | Na2O | MnO2 |
---|---|---|---|---|---|---|---|---|---|
Bentonite-A | 4.10 | 2.83 | 13.80 | 0.034 | 61.68 | 2.41 | 0.830 | 0.156 | 0.014 |
Bentonite-B | 3.58 | 3.34 | 15.93 | 0.040 | 71.12 | 1.97 | 0.527 | 1.070 | 0.020 |
Bentonite-C | 3.49 | 1.96 | 14.66 | 0.010 | 63.64 | 1.38 | 0.751 | 0.465 | 0.050 |
Size distribution of bentonite.
As indicated in Figure
The full size distribution of the iron ore concentrate.
The parameters that represent bentonite properties include colloid value, degree of swelling, water absorption, methylene blue absorption, and montmorillonite content. (1) Colloid value, or colloid content or colloid degree, indicates the montmorillonite content of bentonite. Colloid value is expressed by the total volume percentage of colloidal mixture in a sample to a certain proportion of water after standing for 24 hours. (2) Expansion factor, or degree of expansion, is related to the dispersion ability of bentonite in water. (3) Methylene blue absorption refers to the grams of methylene blue absorbed by 100 g bentonite in water. (4) Montmorillonite content is measured on the basis of the absorption of methylene blue by montmorillonite. Metallurgical bentonite quality standard of China (GB/T20973-2007) is adopted in the process of experiment to test and determine the main property of the bentonite.
This study utilized a disc pelletizer with 45° incline and 1 m diameter to conduct the pelletizing experiment. The pelletizing process consists of three stages: (1) mixing of the raw material to produce the raw pellet, (2) pellet enlargement, and (3) pellet hardening. During the pelletizing, the atomized water was added in the area of disc pelletizer. The water was deionized to avoid the effects of water chemistry on pelletization performance. The total weight of raw material was 4 kg. During the first stage, which occurred for 3 min, raw pellets were produced from 1 kg of raw material. After 9 min, more raw material was added to enlarge the raw pellets. During the last stage, the enlarged pellet was hardened for 3 min. The total time of pelletizing process was 15 min. Afterwards, the green pellets were removed and continually sprayed with water to maintain moisture content at
Green pellet properties include compression strength, moisture content, falling strength, and shock temperature. (1) To test compression strength, the green pellet was placed on an electronic scale and pressed at constant speed until the green pellet broke. The pressure value was recorded when the pellet broke. The same process was repeated 10 times, and the recorded pressure values were averaged over the 10 repeats. (2) To test the moisture content, the green pellet was pulverized. Then, a 20–25 g sample was placed in a rapid moisture measuring instrument to measure and record the value of moisture content. The same process was repeated thrice. The moisture content value was averaged over the three attempts. (3) To test falling strength, the green pellet was dropped from a height of 0.5 m until the ball broke for certain number of times. The test was repeated 10 times. Falling strength values were averaged over the 10 attempts. (4) Shock temperature was assessed with the Dynamic Method developed by AC Company, USA (Figure
Burst temperature measuring device.
Figure
Results of bentonite pellet test.
Table
Effect of bentonite type on the pellet index.
Exp. number | Moisture content/% | Falling strength/times·pellet−1 | Compression strength/N·pellet−1 | Shock temperature/°C | Bentonite content/% | Weight of qualified pellets (>10 mm)/kg |
---|---|---|---|---|---|---|
1 | 7.28 |
|
|
|
1.5 (A) | 0.94 |
2 | 6.79 |
|
|
|
1.5 (B) | 1.43 |
3 | 6.64 |
|
|
|
1.3 (B) | 0.50 |
4 | 7.46 |
|
|
|
1.0 (B) | 0.67 |
5 | 7.13 |
|
|
|
1.0 (C) | 1.41 |
Figure
Effect of different types and proportions of bentonite on the falling strength of the ball.
Figure
Effect of different types and proportions of bentonite on the compression strength of the ball.
Figure
Effect of different types and proportions of bentonite on the shock temperature of the ball.
The results indicate that the quality of the three different types of bentonites decreases in the following order: type B > type C > type A. Therefore, at a constant content of heterogeneous bentonites, falling strength and compression strength are dependent on bentonite quality because bentonite particles are fine with a specific surface area of 100 m2/g. Hence, the dispersion property of bentonite is not only acceptable, but its water absorption and expansion properties also enable fine particles to fill and absorb between the ore particles. The nature of the ore surface changes accordingly to form microcapillaries when a solid bridge and liquid bridge are added. When capillary force increases, the compression strength of the green pellet also improves [
This study investigated the influence of type and proportion of bentonite on falling strength, compression strength, and shock temperature of green pellets by adding bentonite of different types and proportions to iron concentrates. The following conclusions were made: Montmorillonite content is directly correlated with the degree of swelling, water absorption, and methylene blue absorption of bentonite. The degree of swelling, water absorption, and ethylene blue absorption increase as montmorillonite content increases. These characteristics indicate the good quality of bentonite. Adding bentonite improves the strength of the green pellet. The falling strength and compression strength of green pellet increase as bentonite content increases. For homogeneous bentonites, the strength of the green pellet increases as bentonite content increases. For heterogeneous bentonites, the strength of green pellet depends on the quality of bentonite at a constant bentonite content. Adding a higher proportion of high-quality bentonite to the iron concentrates increases the bursting temperature resistance of the green pellet.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The authors would like to acknowledge the National Natural Science Foundation of China (no. 51604054), Scientific and Technological Research Program of Chongqing Municipal Education Commission (nos. KJ1501307 and KJ1601331), and Chongqing Research Program of Basic Research and Frontier Technology (no. cstc2016jcyjA0647).