Pebbles discharged from an autogenous (AG) or semi-autogenous (SAG) mill are often considered to have unique properties as compared to the full feed stream to the primary mill. These particles that "survive" the mill have been assumed to be harder, more competent composition. The competence of the pebbles has a significant effect on the surrounding processes. If hardness of pebbles is known at the project phase, it could reduce/increase required installed power to manage pebble capacities and may impact the total quantity of pebble crushers required. However, many projects only have ore properties obtained from core samples, and often the pebble handling is designed with possibly assumed factors for increased hardness. This study used three approaches to compare pebble properties to raw ore properties. The first approach looked at a broad database for Impact Crushability (IC) Work Index of raw ore and the derived IC Work Index from pebble crusher operating data. The second approach compared operating data at plants where both secondary crushers and pebble crushers are utilized, and the similarity of these crushers allows for comparison of the energy utilized in crushing raw ore versus pebbles. The results of the operating IC Wi were 22% higher for pebbles compared to raw ore but varied drastically from site to site. After eliminating anomalies, the pebbles had on average a 9% higher IC Wi than the raw ore. The third approach was lab testing from mine sites with focus on the mechanical properties of sampled pebbles compared to samples of raw ore prior to the mill. Direct lab testing indicated that the pebbles on average had a 24% higher IC Wi. The results showed that pebbles generally are more competent and more consistent in hardness than the raw ore entering the primary mill.
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