Effects of ore characteristics on product shape properties and breakage mechanisms in industrial SAG mills

Abstract

The effect of ore characteristics on the particle shape were studied using two detailed industrial surveys to monitor breakage events taking place inside industrial Semi-Autogenous (SAG) mills as a function of ore hardness. In order to validate the ore hardness effect, the SAG mill was operated in a controlled condition as much as possible. Different methods, including point load index, drop weight test, Bond mill work index and abrasion index, were employed to appropriately characterize the entering feed strength to the circuit. In addition, the shape of SAG mill products was quantified using an image analyzer system, for which elongation and angularity descriptors were calculated.Experimental studies have indicated that an increase in ore hardness is associated with an increase in point load index and Bond work index and a decrease in impact breakage parameter (A×b). Moreover, an increase in ore hardness corresponds to an increase in feed size. Hard ore grinding in the SAG mill resulted in finer products compared to the soft ore milling. In addition, it was found that the products of the soft ore milling were more elongated, angular and less rounded than those of the hard ore milling. This was attributed to changes in breakage mechanisms as ore strength changes. The weaker ore is ground mainly by the impact breakage while the stronger ore is subjected to abrasion and attrition events where the media impact could not sufficiently break the stronger particles. It was also found that the particle angularity and elongation distributions were fitted to the 3-parameter lognormal distribution, providing a convenient way to quantify changes in particle shape properties as a function of the ore characteristics. It was concluded that the main reason underlying the decrease of circuit throughput from 939 to 498t/h was due to changes in breakage mechanisms.