Comparison of grinding kinetics between a typical ball mill and a ball mill fitted with a breaker plate
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I.
INTRODUCTION
THE purpose of this study was to investigate and compare the breakage parameters, specific rate of breakage, and breakage function of a typical laboratory batch ball mill with a similarly sized ball mill fitted with a breaker plate (Figure 1) and to find out if the breaker plate can improve the grinding efficiency of ball mills. The breaker plate divides the mill into two semicylindrical zones, so that material to be broken and grinding media from one zone do not mix with the other zone. Besides dividing the mill into two equal halves, the main purpose of the breaker plate is to act both as a lifter and as a hammer. It is well known[1– 4] that in tumbling mills, the lifters eliminate the slip against an otherwise smooth liner and increase the dynamic pressure and the intensity of abrasive interactions in the bulk of the grinding charge. They also increase the intensity of the impactive interactions in a mill as a result of their lifting action, which promotes cataracting of the grinding charge. The material to be studied was limestone, because, in practice, limestone is finely ground using ball mills.[5] II.
DESCRIPTION OF THE METHOD
A. Apparatus The ball mill with the breaker plate (Figure 1) was constructed of steel. It is a circular cylinder 18-cm long with a diameter of 19.1 cm. There were no lifters. A breaker plate made of steel with a thickness of 6 mm divides the ball mill into two equal semicylinders, each semicylinder having a volume of 2476 cm3. The typical ball mill without breaker plate was 16.4-cm long with a diameter of 20 cm. There were no lifters. The total volume was 5149.6 cm3. Steel balls of a single size were used in the study, with a diameter of 25.6 mm and density of 7.89 g/cm3. For size analysis, standard sieves of Endecotts series were used on a vibratory Endecotts shaker. The sieve ratio used was the square root of 2, and sieves from 4 to 0.1254 mm were used.
C. GOTSIS, Professor of Chemistry, and K. SOFOKLEOUS, Research Assistant, are with the Chemistry Department, Aristotelio University, GR 54006, Thessaloniki, Greece. Manuscript submitted April 29, 1996.
METALLURGICAL AND MATERIALS TRANSACTIONS B
B. Material The material studied was limestone. It is naturally occurring in the region of Asvestochori, Thessaloniki, Greece. It has an apparent density of about 2 g/cm3. A chemical analysis of the material is given in Table I. The material studied is of the grade used for building construction.
C. Procedures A mill is operating at its critical speed, or critical frequency of rotation (fc), when a particle with zero radius located on the mill shell remains in a centrifugal condition. Given the mill diameter D, fc can be calculated from fc 5 42.3/=D rpm. Applying this formula for the ball mill without the breaker plate gives fc 5 94.58 rpm, and for the ball mill with the breaker plate, fc 5 96.79 rpm. For the ball mill with the breaker plate, 15 steel balls with a diameter of 25.6 mm were used in each semicylindrical zone. The grinding charge weight of 100 g was used for
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