Journal of MMIJ (Oct 2018)
Numerical Analysis on Air-Lift Pumping System for Mining Rare-Earth Rich Mud
Abstract
A numerical analysis is conducted to examine an air-lift pumping system for mining rare-earth rich mud under 6000m deep seabed around the Minami-Tori-Shima. The numerical scheme in the analysis adopts the one-dimensional drift-flux model, which solves pressure and velocities as well as volumetric fractions of each phase in the air-slurry two-phase flow along a lifting pipe. The empirical formulas derived from experiments using rare-earth rich mud around the Minami-Tori-Shima estimate the pseudoplastic viscosity and the rheology constant as a function of volumetric concentration of mud in slurry. They are devised in the scheme to evaluate the pressure drop in air-slurry two-phase flow as the power-law fluid. Parametric studies were conducted to examine steady characteristics of the air-lift pumping system by changing the air flow rate, the water depth where the air was injected and back pressure. As a result, the basic characteristics of the air-lift pumping system were clarified. For example, with the air flow rate of 6kg/s, depth of water where air was injected of 3000m, back pressure of 0.2MPa (G), 6040m lifting pipe out of which 6000m is submerged, and 40m is above sea level, pipe diameter below the depth where the air was injected of 0.15m and above that depth 0.2m, it was found that the maximum mud flow rate was 1.01x103t/d, volumetric concentration of mud was 6.2%, slurry velocity was 3.8m/s, mixture velocity at the exit of the lifting pipe was 52m/s and power required was 2.0x103kW. In addition it was found that the performance of the system decreases when using the scheme as the power-law fluid compared with the one using the Mori-Ototake formula as Newtonian fluid. Furthermore, specifying back pressure is useful because of increased efficiency of the system, as well as for keeping the mixture velocity down at the exit of the lifting pipe.
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