(113e) Estimation of Specific Rate of Grinding to Optimize the Industrial Batch Ball Mill

Abstract

Grinding forms one of the major unit operations in various industries such as pharmaceutical, cement, ceramics, mineral processing and beneficiation [1]. Ball mills are widely used and less carefully studied. It has been observed that in most cases these ball mills are operated inefficiently and much of the energy is lost. This results in longer than expected grinding times. The present study is taken to control the particle size distribution and reduce the specific energy consumption in an industrial ball mill.

A batch mill of 2.21m diameter and 2.43m is used for grinding silica from average size of 3mm to 100% below 80 microns. Silica pebbles are used as grinding media. Samples are collected at one hour interval time and properties of slurry such pH and product size distribution are measured. The particle size distribution is measured using laser size analyzer (Ankersmid?). The erosion of the grinding media is monitored by measuring the decrease in the depth of ball bed.

The breakage mechanism is studied and the selection function and breakage functions are estimated. These functions are used to solve mass balance equation given in equation (1) which describes the comminution in batch ball mill [2].

                                   (1)

The functions S_iis related to particle size x_i and is given by equation (2)

               
                                  (2)

and equation (3) gives relation between breakage distribution function for production of  particle size ?i' from size ?j'.

                                                                                                                                           (3)

The specific rate of breakage is estimated from equation 2 at various times of grinding. The results of the calculation are given in Fig.1.

\s

Fig.1 Specific rate of breakage of silica as a function of time of grinding on 63µm

It is seen from the Fig.1 that there is an optimum at a particular time of grinding. The optimum is approximately at 5.5 hours indicates that selectivity of particles to the grinding media has reached a maximum and the rate of grinding reduces beyond this time.

            It is observed that to increase the selectivity of particles it is necessary to reduce the grinding media size. The experiments are carried in stirred ball mill (SBM) with ball size of 1.5mm and tip velocity 7.6m/s. The product obtained from industrial ball mill at six hours of grinding was taken and further milled in SBM. The desired product was obtained within 15 minutes. The results of the size analysis are shown in Fig.2.

It is observed from the Fig.2 that the time of grinding is reduced from 16 hrs to 6 hrs 15 minutes by taking the product at sixth hour of grinding from ball mill and grinding in Stirred Ball mill

            Fig .2 has been plot between product size distribution of particle size with time of different kind of mill such as ball mill and Stirred ball mill (SBM)

\s

Fig.2 Particle size cumulative distribution functions Q3 (x) for Silica material

The methodology to optimize grinding mill performance by estimating specific rate grinding has been successfully demonstrated to optimise industrial batch mill.

  Nomenclature

References

1. K. Viswanathan, B. Pitchumani, Distribution function of comminution kinetics - modeling and experimental study, Int. J. Miner. Proc., 1982, v9, pp. 385-392.
2. M.H. Ramian, O.P. Amar, B. Pitchumani, An approach to study comminution in an attrition mill, Powder handling & processing. 1992, v4, n1, pp. 27.