The coaxial mixers consisting of an inner Rushton turbine or propeller and an outer anchor were combined first with the just off-bottom suspension in viscous and high solid loading systems. In this study, a new method based on RGB brightness analysis was proposed to determine the cloud height, and the measures to promote cloud height were also discussed. In addition, the effects of rotation modes, pumping modes and solid loading on the just-suspension power were investigated. The results showed that the co-rotation coaxial mixers with an up-pumping inner impeller could achieve the just suspension at lower power consumption and were recommended. The combination of anchor and up-pumping propeller was the most energy efficient among the investigated mixers, whereas its cloud height was relatively low. The high off-bottom clearance and the combined inner impeller could promote the cloud height with little power variation, but the enlargement of inner impeller had no obvious effect. (C) 2019 Elsevier Ltd. All rights reserved.