Investigation of the effect of cylpebs grinding media on ball mill dry grinding performance of a chromite ore

The purpose of this thesis is to investigate the ball mill dry grinding performance of a chromite ore using steel mono-size and multi-size cylindrical cylpebs distributions. Grinding media size and shape are important parameters which affect the grinding performance in a ball mill. In this context, effects of grinding media size and its distribution on dry grinding performance were investigated by conducting batch kinetic tests in a standard Bond ball mill using cylindrical media shape. Run-of mine ore sample was a low-grade type and provided from Muğla-Karacaören region. Ore sample was subjected to jaw crushing and rod milling to obtain The -3.35 mm material to be used in the ball mill batch grinding tests. The -3.35mm material was ground in a standard Bond ball mill in batch for 25sec, 1min, 3min, 5min and 10min retention time intervals using mono-size and multi-size charge distributions. 40x40mm, 30x30mm, 25x25mm, 19x19mm and 16x16mm cylpebs sizes were used in the applied charge distributions. Particle size distributions of the mill feed and product were determined by dry screening from the top size (-3.35mm) down to 0.038mm using a Ro-tap sieve shaker. Repeatability of the mono-size and multi-size grinding conditions were tested and statistically analysed. Specific energy consumption (kWh/t) of the batch tests were estimated from the Bond's energy equation (Napier Munn et al., 2005). Average specific breakage rates of particles were estimated from perfect mixing mathematical model (Whiten, 1976) and were used to evaluate grinding performance for the test conditions. Mill feed, discharge particle size distributions and Broadbent-Callcott theoretical breakage function were used in the back-calculation of specific breakage rate functions from the perfect mixing model. Model fitting module of the JKSimMet mineral processing software was used in the back-calculation of the specific breakage rates. Repeatable test results were obtained for mono-size and multi-size cylpebs applications. Cylpebs size and charge distribution effects could be successfully reflected on the average specific breakage rate functions with the applied methodology. Particle size distributions of the batch tests indicated systematic size reduction for each cylpebs charge application. Particle size distribution of the mill product got finer as the retention time was increased in the mill. Mono-size cylpebs test results indicated that, specific breakage rates of particles finer than 425µm could be increased by the 19x19mm charge application at 3min retention time interval. Multi-size cylpebs distributions indicated the necessity of using 40x40mm and 30x30mm cylpebs sizes in the distributions to break particles coarser than 0.212mm effectively. Breakage rates and thus, grinding performance of particles finer than 0.425mm could be increased by using a charge distribution with an weighed average size of 21x21mm at 5 min retention time. Exponential correlations between mill product fineness (P80) and retention time were established for mono-size and multi-size cylpebs applications. Logarithmic correlations were found between P80 and specific energy consumption for mono-size and multi-size cylpebs applications. Linear correlations were established between specific energy consumption and retention time for mono-size cylpebs grinding. Effect of mono-size silpeps charge on specific breakage rates was best reflected at 5min retention time condition. Size fractions coarser than 0.884mm were determined to be broken at a higher rate when cylpebs charge size distribution got coarser. Specific breakage rates of size fractions finer than 0.884mm were determined to increase significantly when the cylpebs charge size got finer. Relationship between cylpebs size and maximum specific breakage rate (rmax) for 25sec grinding condition of mono-size cylpebs applications was established and a linear correlation was obtained whereas exponential correlations for 1min, 3min and 5min and a power correlation for 10min were obtained. Relationships between retention time and specific breakage rates for 2.5mm, 0.442mm and 0.221mm particle sizes, which are critical for gravity concentration process, were established related to mono-size and multi-size cylpebs distribution applications. A power correlation for 25sec and exponentional correlations were obtained for the other retention time intervals between weighed cylpebs size and maximum specific breakage rate (rmax) for multi-size cylpebs applications. A power correlation was obtained for 25sec whereas, exponentional correlations were obtained for the other retention time intervals between maximum cylpebs size (c-max) and maximum breakage rate (rmax). Second degree polynomial emprical correlations were obtained between maximum cylpebs size (c-max) and maximum particle size (Xm) for multi-size cylpebs distributions. A power correlation was obtained between Xm size and retention time when 25x25mm maximum cylpebs size was used in the charge distributions. Cylpebs grinding media size and distribution effects could have been reflected on the specific breakage rates and thus, on the dry grinding performance successfully. Test results were thought to give a general idea about the dry grinding performance of industrial scale ball mills as full size distribution (-3.35mm) was used in the batch tests. Keywords: Grinding, Ball mill, Cylpebs, Breakage rate, Mathematical modelling, Chromite