A lab-scale precipitator of alumina trihydroxide (gibbsite) is simulated for three extreme cases of micromixing, these being : the familiar cases of Maximum Mixedness and Complete Segregation, and a third case in which the fluid is mixed but the solids are segregated. Through this novel third case, the effect of segregated fluid on the particle size distribution is distinguished from the effect of segregated solids. It is found that these two effects are sometimes in opposition to each other, and the latter is dominant in a system undergoing aggregation but not nucleation. Few other simulations have considered this. Conclusions are drawn on the relevance of micromixing to simulating crystallisation, especially when aggregation is significant.