Vertical Coaxial Centrifugal Mill Grinding Theory And Experimental Research

Grinding is an important operation in the industries of mineral processing, metallurgy, chemical engineering and building materials and so on, which greatly affects the entire national economy, because over 10 billion tons of ores need to be comminuted annually in China, over 5 percent of eletricity power of the whole country and 1 million tons of steel are consumed in the grinding stage annually. Therefore, improving efficiency and reducing consumption in the grinding operation is of great significance. For this purpose, Kunming University of Science and Technology developed a new type of grinding equipment-Vertical Coaxial Centrifugal Mill (VCCM). VCCM is a kind of centrifugal mill, and is mainly made up of drum, frame, driving device, feed device, oriented device and discharge device. The operating principle of VCCM can describe as:the materiel makes the centrifugal movement drived by the high speed rotation drum, then its movement track is changed by the oriented plate in the drum, thus the materiel is ground by the combined action of impact, extrusion and shear.Some basic researches on the grinding theory of the VCCM are made in this Ph.D dissertation by the methods such as theoretical analysis, numerical simulation and laboratory experiment. The main contents are as follows:Detailed analysis of structural characteristics and operating principle of VCCM shows that VCCM has several main advantages:high production capacity of unit volume, almost no liner consumption, extensive using in either conventional grinding or autogenous grinding, either for coarse grinding or ultrafine grinding and uniform size of product, etc.. In VCCM, Material is comminuted by the joint role of impact force, extrusion force and shear stress from the grinding media and other materials. The calculation formula of centrifugal force of the materiel in the drum is C(?). The centrifugal acceleration made by drum rotation are 15-45 times than the gravity acceleration when the mill speed is inside the pale of design. The speed and pressure distributions of two-phase flow in the drum almost accord with the distribution law of speed and pressure of forced eddy, namely, in the same cross section of the drum, the farther away from the center, the larger the speed and pressure are, vice versa.The state of the force and movement of the flowfield in VCCM is very complex. After analyzing the state of the force and movement of the medium in VCCM, the trajectory equation of the impact movement of the steel ball is concluded:y-y0=(?)(x0-x). From the above equation, it can be founded that the impact trajectory is.only related to the position of the oriented plate in the mill drum. The formula used to calculate the whole speed of impact movement is obtained as:v=(?) The speed v, which is the steel ball impacting to the drum wall, is in a direct ratio with the rotating speed n and the drum radius R. The normal speed vn and the tangential speed v, can be regulated by regulating the angle a of the oriented plate in the mill, consequently, the impact stress and shear stress can be regulated. The formula used to calculate the impact stress F of the steel ball to the materials is concluded: F=(?). The impact stress F of the steel ball to the materials has the linear increasing relationship with the mass of steel ball m, the drum speed n and its radius R, and the cosine value of angle a of the oriented plate. The formula used to calculate the impact frequency K of steel ball in unit time is also concluded K is associated with the speed n and the position parameters a andβof the oriented plate, and is in no relationship with radius R. As a result, increasing the drum speed can increase of the impact frequency of steel ball to the materials in unit time, thereby the grinding efficiency is improved.The numerical simulation of the flowfield in VCCM by the software CFD shows that the changing principle of velocity gradient of the flowfield, namely, the farther the fluid is from the the center of the drum, the higher speed it has. The region where the pressure changes largely locates in the vicinity of the oriented plate, especially in the two ends of the oriented plate, which is the main grinding region. The pressure, however, is small in the region between the plate and drum, thus the grinding efficiency is low. Further study shows that the main grinding region is closed, to the liner of drum, in which the vicinity region at both ends of the oriented plate is the main region of impact comminution and other region is the main region of abrasion comminution.Mill speed, throughput and grinding concentration are the main operating parameters of grinding process, which have a direct effect on the results. Autogenous grinding parameter tests of VCCM with pyrite ore show that mill speed and throughput have great influence on the grinding performance. When the mill speed is constant, the average size of the production is reduced with the decreasing of throughput, but with gradual reducing of the throughput, the decreasing degree of the average size becomes gentle. With increasing of the mill speed, the capacity rises rapidly, while the average size of products quickly decreases, which confirms that increasing of the mill speed can greatly improve the grinding efficiency. Grinding concentration has a certain impact on the grinding performance. The grinding performance is relatively poor due to the coarse products when the grinding concentration is 55%. However, it changes gently when the grinding concentration is improved between 60% and 70%. When the grinding concentration is 70%, the pulp viscosity is too larger to discharge smoothly. Therefore, the appropriate grinding concentration is from 60% to 65%. The results of comparative tests show that the eletricity power consumptions per ton ore of the VCCM and the conventional ball mill (CBM) are almost same, but the production capacities of unit cubage of VCCM are 4.57 times than CBM, hence the VCCM will play an important role in increasing the grinding efficiency.The conventional grinding parameter tests are conducted using Vanadium-titanium magnetite tailings, and the results obtained show that the fineness of product decreases with the increaing of mill speed when the throughput is invariable, and the higher the speed, the higher efficiency of fine grinding obtain. When the mill speed is constant, the fineness of product increases with the increasing of throughput, and the changing gradient gradually increases.Grinding kinetics can reflect the grinding velocity. Experimental investigations on grinding kinetics of VCCM under the different mill speed reveal that the general grinding kinetics equation of product size distribution can be demonstrated as:Ri(t)= Ri(0)exp[-k(d)tn(d)]. The grinding kinetics equation of VCCM under different mill speed can also be demonstrated from the general equation through the curve fit method, thus the distribution rate of grinding products in any time and any size can be calculated. When the mill speed is constant, the grinding velocity decreases with the increasing of grinding time, and the decreasing trend is obvious in a short period of time. The longer the time, the gentler the decreasing trend, especially under a high mill speed, the materials will be ground to very fine in a very short period of time. When the grinding time is constant, the grinding velocity increases rapidly with increasing of the mill speed, and the higher the mill speed, the more obvious the trend is.