For proper design of SAG mills and other types of grinding mills, the operating conditions have to be defined. Predicting the composition of the SAG mill charge is a difficult task since a number of significant phenomena associated with the charge motion must be taken into consideration.
These observations and assumptions led to the recognition of the need for better understanding of the behavior of the SAG charge during mill operation. In this context a plexiglas scale SAG mill was built and operated with different fillings and sizes of ceramic balls (simulating the ore) to allow visual observation of the charge motion and its implications. Some results of this study are described in this article.
The mill is a scaled-down replica of a commercial Koppers 6.1 m x 2.13 m (20 ft x 7 ft) SAG mill (see Fig. 1). The original design was developed by Vanderbeek and Herbst and was constructed by the Scale Model Makers of Cleveland, Tennessee. It is built of plexiglas and therefore allows the observation of the action inside the mill. The cylindrical part of the mill is 0.61 m (2 ft) in diameter and 0.305 m (1 ft) in length. The two end walls are conical (150 ).
The mill was operated at three different rotational speeds: 75%, 44% and 24% of the critical speed. The critical speed for this mill was 54 revolutions/minute.
For most steady-state conditions examined there is no creation of a water pool (which is typical of overflow mills) at the toe of the ball charge.
The immediate observation when a slurry is fed into the mill is a very significant reduction in the noise emitted from the mill. The second observation is that there is drastic change in the steady-state segregation patterns. In addition, limestone hold-up, among the ceramic balls, is higher near the feed wall than near the discharge wall.
35% Filling, 1.9 cm (¾ in) Ceramic Balls 44.3%. It is clear that because the limestone fills the spaces between the lifters of the end walls, there is no reason for small balls to concentrate near the end walls. Therefore, the segregation pattern, at both low and high rotation speeds, is such that large balls concentrate at the periphey, while small balls concentrate in the charge core. In addition, the balls are not “flying” at 75% of critical speed, because of the additional powder. The angle of repose could not be determined due to the fact that slurry covered the walls.
The conical shape and the lifters on the end walls cause different segregation patterns; these patterns can affect the grinding significantly. For example, at 35% filling (feed water only), 75% of critical speed, a high concentration of small balls is observed near the end walls. This makes the fine material easily available for exit.