Mechanism of bentonite action in iron ore pellets
Source: internal company
The Pellet method represents the development direction of fine powdery iron concentrate, bentonite as a dry binder plays a decisive role in the process of caking, therefore, it is of great significance to study the bonding mechanism of bentonite to iron ore pellets.
Action mechanism of bentonite in pellet
The preparation of iron ore pellets goes through three stages: pelletization (wet/raw)→ drying and preheating (dry)→ roasting. Bentonite plays a different role in the three stages.
1. Granulation (wet pellets) stage
According to Rumpf theory, Iron ore pellets should belong to the low viscosity liquid bonding type. There are 10 kinds of forces in the pellet granulation process. The mechanism related to the action mechanism of bentonite was discussed in this paper.
1.1 Increase capillary force by regulating the water content
The water in the pellet and its type are crucial to the strength of the pellet. The strength of bentonite has a strong absorption ability and can rise to adjust and control the ball's free water content, and this kind of adjustment and control does not change the water amount, its essence is to particle-free water adsorption of montmorillonite interlayer water into layers and the flowing water, Reducing the proportion of excess free water content, making the free water liquid reach the saturation point, improving the capillary force adhesion of pellet, thus improving the strength of pellet.
1.2 Lubrication increases molecular forces
Bentonite absorbs water and expands. Due to the weak molecular force between the montmorillonite layers, under the action of mechanical force (agitating, rolling), the layers are very easy to slide, resulting in lubrication. This lubricating effect can play two roles in pellets: one is to reduce the friction of particles, increase the plasticity of green pellets, reduce the porosity, increase the density of the pellets, and increase the strength of the pellets. Stone flakes can be coated on the surface of iron powder particles to smooth the surface of the particles, reduce their surface roughness, reduce the contact distance between particles, and increase the attraction between particles.
1.3 Surface electronegativity increases electrical adhesion
since the tetrahedral Si4+ in the montmorillonite structure is replaced by Al3+ or Fe3+ or the Al3+ or Fe3+ in the octahedron is replaced by Mg2+, Fe2+, the balance of electricity price is destroyed, and there are metal cations in the interlayer to maintain its electrical balance. Cationic hydration, montmorillonite expands and disperses, and its layer has a permanent negative charge. The surface of metal oxides, magnetite, and hematite powders is positively charged. Bentonite binds the iron powder together through colloidal electrical properties and electro-molecular forces.
2. Drying and preheating (dry pellet) stage
2.1 Moisture conduction effect, improve the burst temperature
The main reason for pellet bursting is the poor diffusion of an internal wet fraction. The water absorption performance of bentonite can reduce the evaporation rate of water in the pellet, so that the water is released slowly, thus reducing the vapor pressure inside the pellet, which is beneficial to increase the pellet burst temperature.
2.2 Inhibit the migration of soluble salt and prevent the ball from uniting shell and bursting.
Bentonite has strong ion exchange and adsorption properties. In the process of pellet drying, bentonite can capture and fix these soluble ions by exchange adsorption, limiting their free migration, and preventing the burst caused by the crust of salt on the pellet surface.
3. Roasting Stage
Bentonite mainly acts as a flux during pellet roasting. The si, Ca, Mg, K, and NA in bentonite are a good flux. When the roasting temperature reaches the reaction temperature of these melting components and iron, a solid phase reaction will occur, and new compounds such as ferric silicate, calcium ferrite and calcium silicate will be generated, forming solid phase bonding, increasing the strength of the roasted pellets, so that the pellets can avoid dust and breakage during transportation.