Solid-liquid mixing is a critical step in many industrial processes, from battery electrode production to pharmaceutical formulations. These processes typically require a homogeneous suspension of particles to ensure consistent product quality. For decades, engineers have relied on the Zwietering correlation, an effective classical engineering model for estimating the "just-suspended speed" (N JS)—the minimum impeller rotational speed that prevents particles from settling to the bottom. Dr. FURUKAWA Haruki from the Department of Life Science and Applied Chemistry at Nagoya Institute of Technology (NITech) in Japan explains: "The Zwietering correlation predicts that N JS increases with larger particles, greater density differences, and smaller impellers. While this works well for dilute suspensions, it often falls short in predicting behaviour in dense industrial slurries." To develop more reliable mixing guidelines for high-density suspensions, a research team led by Dr. FURUKAWA and including Dr. KATO Yoshihito from the Department of Life Science and Applied Chemistry at NITech investigated how impeller placement affects mixing behaviour and energy efficiency in suspensions containing 20–70 wt% solid particles. The study findings were made available online on 25 March 2026 and will be published in volume 187 of the Journal of the Taiwan Institute of Chemical Engineers on 1 October 2026.
The researchers examined mixing performance of suspensions in a mixing vessel by varying impeller positions under both baffled and unbaffled conditions. In addition to visually determining N JS, they evaluated particle behaviour through torque and power measurements. Their experiments revealed that in baffled conditions, placing the impeller near the solid-liquid interface (the boundary between the settled particle bed and the overlying liquid) resulted in a decrease in N JS compared to lower impeller positions. Meanwhile, unbaffled conditions lowered the overall N JS value, offering a more energy-efficient approach for mixing dense slurries. These findings contrast with Zwietering experiments, which are typically conducted with baffled vessels and based on the common assumption that positioning the impeller lower in the vessel increases particle mobilization and reduces N JS.
Source
Haruki Furukawa et al, Effect of impeller placement on solid–liquid mixing at high particle loadings, Journal of the Taiwan Institute of Chemical Engineers (2026). DOI: 10.1016/j.jtice.2026.106738 / https://www.nitech.ac.jp/eng/news/2026/14005.html / https://phys.org/news/2026-05-guidelines-dense-suspensions-revealed.html
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