Adding an appropriate type and amount of binder to the material not only facilitates the pelletizing process and improves the density and strength of the pellets but also enhances the thermal stability and reducibility of the pellets, which is beneficial for smelting.
Binders used for cold consolidation molding mainly include water glass, cement, asphalt, humate, carboxymethyl cellulose, starch, etc.
However, all inorganic binders inevitably have the side effect of increasing harmful elements in iron concentrate, and they may also deteriorate the metallurgical properties of pellets or even cause environmental pollution. A inherent advantage of binders is that they are volatilized or burned off during the pellet roasting process, and most of them can significantly improve the mechanical strength of the pellets. Some can even enhance the metallurgical properties of the pellets, exhibiting strong viscosity, activity, and adsorption. Due to their characteristics, they can partially or completely replace bentonite in pellet production without causing a decline in pellet grade. The main issue is their high cost. Their features include being water-soluble, significantly increasing water viscosity, thereby improving the pelletizing process of fine concentrates, reducing the evaporation rate of water during drying, increasing the burst temperature of green pellets, and forming thin-film solid-phase bridges at many contact points between ore particles after drying, enhancing the strength of dry pellets. However, **binders share common problems in their use.
Due to the small amount used, it is difficult to control and mix uniformly during batching.** The decomposition temperature of the material is relatively low. After high-temperature drying and water evaporation, the binder will mostly decompose and volatilize, naturally reducing its consolidation effect. They are generally too expensive. These issues limit the widespread application of **binders. Both inorganic binders and **binders have advantages and disadvantages in the ore agglomeration process, leading some to consider combining the two to develop composite binders that leverage their strengths and mitigate their weaknesses, which has been successful. They have already been successfully applied in coal powder molding and cold consolidation molding of iron concentrate.
Composite binders not only act as adhesives to improve pellet strength but also serve as fluxes to adjust pellet basicity and enhance pellet reducibility. Therefore, composite binders are an important direction in binder research and development. In terms of pellet molding technology, extensive comparative tests have been conducted on three processes used domestically and internationally: the disc (drum) pelletizing method, the roller press briquetting method, and the high-frequency vibration compaction method. The quality of the finished pellets serves as the primary scientific basis for selecting the appropriate pelletizing method and equipment.
Pellets formed by the disc (drum) pelletizing method are dense, with high compressive strength and drop strength. The operation is simple, and it is easy to control the particle size range of the pellets. However, if the raw material density is low, the disc method is generally not suitable for pelletizing. The strength of pellets produced by the roller press briquetting method is relatively low, making them less suitable for large smelting furnaces where pellet strength is critical. The high-frequency vibration compaction method can avoid the drawbacks of the above two methods, producing high-strength pellets, and is the main production method for pelletizing in large smelting furnaces.