Research on free settling of coarse mineral particles is very important and fundamental to design of the hydraulic lifting pipe system. Based on mechanical property of coarse particles in fluid, a formula for calculating the settling rate of the coarse mineral particles dropped in still water was obtained, which can be used to reveals general laws of the settling motion of the particles in still water. By considering real working conditions of the lifting pipe system which transport coarse mineral particles in deep sea, settling procedure of coarse mineral particles such as polymetallic nodules, cobalt-rich crusts and hydrothermal sulfide were captured by high speed photography, and diameters of these particles are 10, 20, 30, 40, and 50 mm, separately. By observing the procedure, we found the motion of the particles was transformed to near-uniform motion after a long time. The experimental results confirmed the maximum settling rate and settling distance at the maximum settling rate is related to the diameter of the particles, and got the correlation curves. According to the observational experimental, the relationship between the settling rate and settling time was derived and the relation curves between the settling time and settling distance at the acceleration stage of the settling procedure was also acquired. A comparison between theoretically analyzed results and the experimental data indicated that they were in agreement.

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