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

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 498 t/h was due to changes in breakage mechanisms.

Figure shows the probability plots of shape descriptors with a 95% confidence level along with their distributions after fitting to the 3-parameter lognormal distribution. Distribution mean and median for Product 1 are higher than Product 2. It means that the comminution of Feed 1 produced more elongated and less rounded particles compared to the grinding of Feed 2. Changes in the shape properties of the SAG mill products are likely to be a direct result of the breakage mechanisms altered during the milling process due to some changes in the feed characteristics. The weaker ore is mainly ground by the impact breakage event while the stronger ore is subjected to abrasion and attrition events where the media impact couldn’t sufficiently break the strong particles. As a result, the comminution of soft ores leads to relatively coarser products with more elongated and less rounded particles, and rough surfaces. In contrast, during the comminution of hard ores, the attrition and abrasion events become dominant and particles are subjected to more surface attrition.Figure optionsDownload as PowerPoint slideHighlights
► Ore effects on the product shape were studied in order to monitor breakage events in SAG mill.
► The quantitative shape properties were fitted well to the 3-parameter lognormal distribution.
► An increase in ore strength corresponds to a decrease in product size, angularity and elongation.
► By increasing ore hardness, the main breakage event changes from impact to attrition and abrasion.
► A change in breakage mechanisms during hard ore milling dramatically decreases the feed rate.