Effect of ball size and powder loading on the milling efficiency of a laboratory-scale wet ball mill
Introduction
Wet ball mill is one of the most predominantly used method for the purpose of mixing and grinding of raw materials in laboratories and industry [1], [2], [3]. The ball mill process is very complicated process governed by many parameters, such as ball size, ball shape, ball filling, slurry loading (with respect to ball amount), powder loading with respect to the amount of total slurry (slurry viscosity), and rotation speed. There has been a high industrial interest in optimizing such ball mill parameters from the viewpoint of the comminution of ores [1]. In ceramic laboratories, ball mill is often carried out to mainly achieve a thoroughly mixed state of starting powders with initial average particle size (d50) of 1–20 μm. In laboratories, a polyethylene-based small bottle container (nominal volume of approximately 250 ml) and zirconia balls with nominal diameter of approximately 1–10 mm are frequently employed. However, reports on the efficient combinations of ball size—rotation speed have been sparse in the literature. Here we report that there exists an optimal ball size for efficient milling at a given rotation speed, based on a laboratory-scale wet ball mill. Also, the effect of powder loading on the particle size reduction has been investigated at given conditions of ball size and rotation speed.
There have been investigations on the influences of parameters associated with grinding balls such as ball size distribution [4], [5] and ball shape [6] on the particle comminution. Salili et al. [7] reported the importance of the small ball size and small mass ratio of powder to ball for the efficient ball mill, while only two sets of experimental data for these variables were provided. Thus, the optimization of ball size could not be pursued. In the experimental work of Shinozaki and Sennai [8], the stored energy per gram of milled powder (gamma Fe2O3), ΔHs, was given by 24.2N(d/D)2.57, where N is the number of steel balls, d is the diameter of ball, D is the diameter of the milling pot. In the modeling work of Kurlov and Gusev [9], it was shown that a fraction of the energy deposited in the processed material is consumed for the creation of microstresses, which slows down the comminution process.
As for the influence of the characteristics of wet medium on the milling efficiency, several groups [10] investigated the influence of liquid physical properties such as density, surface tension and viscosity on the rate of grinding, while some other groups [11] investigated the effect of bulk slurry rheology on the efficiency of grinding. There also have been efforts to investigate the kinetics of particle breakage [12] and modeling of the grinding process [13]. The particle breakage rate increases with milling time [12].
Section snippets
Experimental procedure
10 g of alumina powder (99.6%, CA-5M, KC Corp., Youngam, Jeonnam, Korea) with average diameter of 6.0 μm (d50) was loaded to a polyethylene-based bottle (approximately 60 mm in inner diameter and 250 ml in nominal volume) with 500 g of zirconia balls and 70 ml of distilled water. The 500 g of balls reached approximately 50% of the bottle height regardless of the ball size due to the same packing factor of the randomly packed different-sized spherical balls [14]. The 50% ball filling of mill container
Effect of ball size
The effect of ball size on the particle size reduction has been investigated first for varying rotation speed of the container. Percent passing and size distributions of the milled Al2O3 powder are shown in Fig. 1, Fig. 2, respectively, as a function of particle size for varying ball size. The average particle sizes (d50) of the milled Al2O3 powder are shown in Fig. 3 as a function of ball size for varying rotation speeds.
In Fig. 3, as anticipated, at a given ball size, a higher rotation speed
Conclusions
10 g of alumina powder (average diameter of 6.0 μm) was ball milled in aqueous medium (70 ml of distilled water) by 500 g of zirconia balls with varying diameter (1, 2, 3, 5, and 10 mm) in polyethylene bottles (500 ml) for 12 h at varying rpm, and the resultant particle size of the milled powder was analyzed. At a given rpm, there exists an optimum ball size to yield minimum particle size. The optimum ball diameter decreases from 5 to 2 mm as the rpm increases from 50 to 153. This result has been
Acknowledgments
This work was financially supported by the Geo Advanced Innovative Action (GAIA) Project no. (RE201202040), funded by the Ministry of Environment of Korea through the Soil Environment Center at Korea Environmental Industry & Technology Institute (KEITI). The authors thank Mr. Shafqat Ullah for technical assistance.
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