Effect of Grinding Media Size on Ferronickel Slag Ball Milling Efficiency and Energy Requirements Using Kinetics and Attainable Region Approaches

Abstract

The aim of this study is to evaluate the effect that the size of grinding media exerts on ferronickel slag milling efficiency and energy savings. A series of tests were performed in a laboratory ball mill using (i) three loads of single size media, i.e., 40, 25.4, and 12.7 mm and (ii) a mixed load of balls with varying sizes. In order to simulate the industrial ball milling operation, the feed to the mill consisted of slag with natural size distribution less than 850 μm. Grinding kinetic modeling and the attainable region (AR) approach were used as tools to evaluate the data obtained during the ball milling of slag. Particular importance was given to the determination of the specific surface area of the grinding products, the identification of the grinding limit, and the maximum specific surface area which could be achieved when different grinding media sizes were used. The results showed that, in general, the breakage rates of particles obey non-first-order kinetics and coarse particles are ground more efficiently than fines. The AR approach proved that there is an optimal grinding time (or specific energy input) dependent on the ball size used for which the volume fraction of the desired size class is maximized. The use of either 25.4 mm balls or a mixed load of balls with varying sizes results in 31 and 24% decrease in energy requirements, compared to the use of balls with small size (12.7 mm).

Reliability of the Non-linear Modeling in Predicting the Size Distribution of the Grinding Products Under Different Operating Conditions

“…Figure 2a clearly shows that from the energy-saving point of view, the 25.4-mm diameter balls improve energy efficiency for the coarse feed fraction −3.35 + 2.36 mm, whereas the process becomes inefficient when smaller or larger balls are used. This finding is consistent with the results of previous studies which indicate that there is an optimum grinding media size that maximizes the ball milling efficiency [37,42,43]. The optimum ball diameter depends on, among others, the feed-to-product size ratio, the mill dimensions, and the breakage parameters.…”

“…As a result, the fraction of the space between the balls at rest that is filled with material (interstitial filling) U was 50%. The power of the mill was calculated using the formula presented in a previous paper [37].…”

Grinding Media in Ball Mills-A Review

“…However, it is not always when ball size distribution brings a positive result, since the efficiency of the mill is also affected by the feed size. This was shown in a recent experimental study by Petrakis and Komnitsas [47] done on ferronickel slag with particle size less than 0.85mm where a seasoned charge of balls had a lower grinding efficiency than mono-sized balls with a diameter of 25.4mm.…”

“…Also, as the ball filling ratio increased, the influence of the ball size distribution also increased with a maximum power draw at 35% ball filling volume. Petrakis and Komnitsas [47] also found that there is an optimal specific energy input for a given ball size distribution to get the required product size.…”

“…Another study conducted by Petrakis et al [47] on grinding ferronickel slag using different-sized balls (12.7mm, 25.4mm, 40mm) showed that for each ball size, the breakage rates obey non-first-order kinetics and coarser particles are ground more efficiently as the grinding time increases. This may be due to the energy transfer mechanisms in ball mill machines whereby coarser particles are preferentially ground or be cushioned by fines from the energy dissipation through inter-particle friction and heterogeneities produced in the feed particles [48].…”