Structural issues of rod mills

The rod mill is driven to rotate the cylinder by an electric motor via a reducer and surrounding large gears, or directly by a low-speed synchronous motor through the surrounding large gears. The cylinder is filled with appropriate grinding media – steel rods. Under the action of centrifugal force and friction, the grinding media are lifted to a certain height and then fall in a throwing or dropping manner. The material to be ground enters the cylinder continuously through the feed opening, is crushed by the moving grinding media, and the product is discharged from the machine under the force of overflow and continuous feeding in order to proceed to the next processing stage. When the product particle size is required to be such that 80% of the particles are between 0.5 mm and 2.0 mm, a rod mill is generally used. The maximum particle size of the product produced by a rod mill is usually 4.7 mm, while the minimum value is 0.4 mm. The feed particle size for a rod mill is such that 80% of the particles are between 20 mm and 4 mm; the maximum feed particle size can even reach 50 mm. Due to factors such as the material of the grinding rods, their length cannot exceed 6.1 meters; otherwise, problems such as disordered rods can occur. As a result, the specifications for the rod sorting machine remain at 4.6×6.3 meters. When the ore is fed into the rod mill, the coarse particles are at the feed end while the fine particles are at the discharge end; therefore, the rods cannot remain completely parallel during operation and are instead inclined, which is also one of the reasons why the rods cannot be too long. The crushing ratio of rod mills ranges from 15:1 to 20:1. In most foreign processing plants, the conventional crushing and grinding process consists of crushers, rod mills, and ball mills. Rod mills are not widely used in Chinese processing plants; the feed size to ball mills is relatively large, which results in higher energy consumption, greater consumption of grinding media, and increased wear on the liner plates. In a rod mill, since the grinding rods are in line contact, larger particles are ground first; this results in selective grinding. As a result, the particle size distribution of the product is narrow, the particle sizes are uniform, over-grinding occurs less, and the grinding efficiency is high.