Effect of cations on unfired magnetite pellet strength

Bentonite clay is the most popular binder for iron ore concentrate pellets. To minimize bentonite cost and to reduce the amount of silica that it contributes to the finished pellet, the industry needs to minimize bentonite dosage by maximizing its effectiveness as a binder. However, the behavior of any given bentonite in iron ore pelletization has proven to be impossible to predict accurately. Therefore, each bentonite must be evaluated by trial-and-error tests. In this paper it is shown that the reason bentonite behavior was impossible to predict is that the moisture in the iron ore concentrate can have much higher levels of dissolved cations than had previously been realized. In one operating plant, the moisture in the magnetite concentrate had levels of Ca+2 and Mg+2 that were 500 times higher than had been expected from analyses of the plant recycle water. These cations are known to strongly impact the binding effectiveness of bentonites. Seven bentonites were studied to determine the effects of concentrate moisture chemistry on the ability of the bentonites to produce iron ore concentrate pellets with satisfactory wet knock and wet and dry compressive strengths. When iron ore concentrate from an operating plant was flushed with distilled water to remove the dissolved cations before it was mixed with each of the bentonites and pelletized, the pellet properties improved. When the cations were removed, the wet knock values increased an average of 92% and the dry compressive strength increased an average of 46%. The bentonites were not all equally affected by the cations in the water - some of the bentonites were strongly affected by the water chemistry, while others were only slightly affected