Carbon-Containing Pellets with Laterite Nickel Ore（2）
Source: internal company
1. Spherical properties of laterite nickel ore
For the pelletizing performance of mineral powder, the greater the plasticity of mineral powder, the smaller the resistance during pelletizing, and the easier the particles are to produce plastic deformation. It can exert greater molecular adhesion, thus improving the ball pressing performance of the material. The plasticity of ball pressing materials mainly depends on the structural composition of the materials. Combined with the X-ray diffraction pattern and chemical analysis of laterite nickel ore, it can be concluded that there is a high content of kaolinite in the ore, and kaolinite belongs to clay minerals, so the plasticity of ore powder can be greatly improved. If a certain amount of limestone or CaO is added to laterite nickel ore, the plasticity of the mixture will be reduced.
The particle size of the material will also have a great influence on the briquette quality. According to the particle size distribution of laterite ore powder, it can be seen that the ore powder has a certain particle size composition. Experiments show that mineral powder with a certain particle size composition has better ball pressing performance than pure fine or coarse materials. The main reason may be that the bulk density and poor fluidity of fine materials lead to poor relative plastic deformation of materials, which affects the pellet forming rate and pellet strength. However, the volume of a single particle of coarse material is large, it is difficult to move and deform during pressing, and it is not easy to press. Moreover, the bulk density of coarse material is small, the voids between particles are relatively large, and the molecular adhesion is weak, which will eventually lead to the decrease of the strength of the agglomeration. Small particles of mineral powder with a certain particle size composition will fill into the middle area of the void between large particles during ball pressing, i.e. Coarse particles will act as aggregates. Fine particles play the role of base material, increasing the contact surface between material particles, shortening the distance between particle molecules, and improving the strength of pellets.
The microscopic morphology of laterite nickel ore powder was analyzed by scanning electron microscope, and the results are shown in the figure. As can be seen from the figure, the particle shape of the laterite ore powder is very irregular, and the particle surface is uneven and the specific surface area is large. In the process of pressing the ball, the particles can be fully fitted, which is very beneficial to improving the strength of the pellets.
2. Influence of coal blending rate on briquette strength
The carbonaceous reducing agents used in the direct reduction process of laterite nickel ore are anthracite and bituminous coal, which have the functions of reduction and heat supply. The type, particle shape and particle size of pulverized coal reductant have certain influence on the quality of pellets. Laterite nickel ore is added with 10% CaO by mass, anthracite and bituminous coal with particle size of 75-150 µ m, and 18% water by mass is added without binder to prepare carbon-containing pellets. In the subsequent reduction process, the carbon in the reducing agent will react with the oxygen in the laterite nickel ore, so that the laterite ore can be reduced.
Fig 1: SEM image of the laterite nickel ore Fig 2: Effect of atoms ratio of C to O on the strength of briquette with different reductants and coal sizes.
From the X-ray diffraction and chemical analysis of laterite nickel ore, we can conclude that iron in laterite nickel ore mainly exists in the form of Fe2O3 and nickel in the form of NiO. Therefore, when calculating the carbon-oxygen atomic ratio, only the oxygen atom number contained in Fe2O3 and NiO is considered, and the carbon atom number can be calculated by the fixed carbon content of pulverized coal. The influence of carbon-oxygen atom ratio in the briquette on the strength of the pellet under different reducing agents is shown in the figure. Both bituminous coal and anthracite have adverse effects on the strength of laterite nickel ore pellets. The compressive strength and falling strength of the pellets decrease with the increase of pulverized coal addition, and the strength of the pellets mixed with bituminous coal is lower than that of anthracite. For materials, when the particles and molecules of the materials are very close, there is mutual attraction between the particles of adjacent parts in the material, which is called cohesion. The order of cohesion is laterite powder > anthracite > bituminous coal. Cohesion can make the material easier to form balls, so it has a great influence on the strength of the pellets, so the addition of pulverized coal will reduce the strength of the pellets.
The particle size of pulverized coal will also have certain influence on the performance of laterite nickel ore pellets. When 10% CaO is added to the laterite nickel ore, In addition, three kinds of anthracite with different particle sizes are added, The carbon-containing pellets were prepared by adding 18% water under different binder conditions, The influence of different pulverized coal particle sizes on the pellet strength is shown in the figure. We can see that for pulverized coal with particle sizes of 37.5-75 µ m and 75-150 µ m, due to the low plasticity and poor hydrophilicity of pulverized coal at this time, the pellet strength gradually decreases with the increase of pulverized coal addition, and the degree of reduction of pellet strength by pulverized coal with particle sizes of 37.5-75 µ m is greater than that of 75-150 µ m. However, for pulverized coal with particle size of 150-300 µ m, when the carbon-oxygen atomic ratio in the pellet increases from 1.1 to 1.3, the strength of the pellet increases slightly, while when the carbon-oxygen atomic ratio exceeds 1.3, the strength of the pellet tends to decrease. Under normal circumstances, the strength of pellets decreases with the increase of pulverized coal addition. The reason for the above results may be that the particle size of 150-300 µ m pulverized coal is larger than that of laterite nickel ore powder, which can play a skeleton role in the mixture. With the increase of coal blending, the favorable effect of pulverized coal skeleton on pellet strength is greater than the unfavorable effect, resulting in the gradual increase of pellet strength. However, when the blending amount of coal exceeds a certain value, the influence degree of pulverized coal skeleton weakens, resulting in a decrease in pellet strength.