Ball Mill design technology
In order to design a ball mill and to calculate the specific energy of grinding, it is necessary to have equation(s) law which relates mill power and mill size and mill operating conditions.
Power draw varies as a function of ball loading and rotational speed. Lifter design is another parameter for the power draw of the mills. Although the cement mills sampled in this study have got similar ball loading, rotational speed and lifter desing, different mill power draws were recorded. The data give the opportunity to set the exact relationship between the mill diameter and power draw for the cement mills. As mentioned in the context, there are some several ways of determining the load in the mill.
Ball milling operations start with a design charge and under normal operating conditions. It is necessary to add the makeup charge. This will cause loosing the mill load data at the specific circuit. Therefore, it is the best way to calculate the mill load is to get the measurements. In order to achieve the most efficient operation, mill conditions should be optimized and the choice of mill conditions are dependent on the economics of ball loading and wear.
In this exercise the success of the calculation methods are compared with the ball tonnage recorded during the plant survey. As given in Figure 4 the predictions give very good fit with the measured data.
The mill load that is the volume of charge in the mill is the principal determinant of power draw. Estimation of the ball load that is mixed with the cement charge is difficult and can be highly erroneous. So direct measurement must be taken for calculation of mill load. A direct measurement of the load entails the crash stopping of the ball mill under load whilst the mill is running under steady state conditions.
Before taking measurement steady state conditions were verified by the plant staff, then the mills were crash stopped so that required measurements could be taken from the both compartments along the grinding path inside the mills. The load within the mills was determined by measuring the width and length of the charge and perpendicular distance between the charge and liner surface at various points in each compartment as seen in Figure 2. At the same time, all variables measured were recorded in the control room during these operations.
From these measurements the load volume can be calculated with using simple geometry and different equation proposed by Morrell and Allis Chalmers Company. Mill load volume can be also calculated with using Equation 9 if total ball tonnage value is known.
Ball mill with excellent structure
A new approach based on Morrell’s C model is used to calculate the power draw of dry multi-compartment ball mills. Calculat ed power draws were in good agreement with the measured values. Volumetric mill load calculated using different equations gave similar results. Increasing the number of measurements taken along the width and length of the mill would improve the accuracy of the calculation. It was found that the power draw of dry multi-compartment ball mills used in cement grinding could successfully be predic ted using this approach. However, this method needs to be validated with more data sets including variation of other operating parameters such as critical speed and lifter design.
