Flow characteristics of slurry with rare-earth rich mud are investigated to examine lifting systems for mining rare-earth elements from deep seabed. Twelve samples, extracted from different locations and depths from seabed around the Minamitorishima with several concentration of rare-earth elements are mixed up with sea water to make slurry with specified volume concentration of mud 1.0% to 10.0%. Cone-plate type rotary viscometers are used to examine relations between shear stress and shear rate of slurry in each volume concentration of mud. As a result, it was found that shape of graph; the shear stress in the vertical axis and the shear rate in the horizontal axis was concave down and increasing during whole range of the shear rate. The shear stress increased gradually as the shear rate increased in case of low volume concentration of mud up to 3.0%. On the other hand, the shear stress changed significantly at small shear rate, and then gradually increased, then lineally in the end in case of larger volume concentration. In addition, similar characteristics under the same volume concentration, even though tested samples were extracted from different locations, depths and concentration of rare-earth elements. Further, three types of fluid model; the Power low model, the Bingham-Papanastasiou model and the Herchel-Bulkley-Papanastasiou model were fitted on the data using the least square techniques, then compared with each other. The last two models, i.e., the Bingham-Papanastasiou model and the Herchel-Bulkley-Papanastasiou model corrects deviations from the data when using “original” the Bingham model and the Herchel-Bulkley model, especially in the range of small shear rate under high volume concentration of mud. The Herchel-Bulkley-Papanastasiou model was the most appropriate model within the three models. Furthermore, correlation equations for parameters of the HerchelBulkley-Papanastasiou model were derived related to volume concentration of mud.

Removal of borate in mine drainages by co-precipitation with hydroxyapatite (HAp) was developed from a laboratory scale to a pilot scale. Weakly acidic initial pH facilitated to enhance dissolution of Ca(OH)₂ and decrease the dissolved carbonate concentration, leading to efficient immobilization of borate and arsenate. The NH₄H₂PO₄ lowered best the equilibrium B concentration among different phosphate sources, avoiding the lattice strain of HAp. The added molar ratio of P/Ca significantly influenced the decreasing behavior of the B concentration, showing the optimal value of 0.3. In case of P/Ca larger than 0.30, the excess concentration of PO₄^3- was probably adsorbed on Ca(OH)₂ particles to prevent the dissolution, resulting in inhibiting the formation of HAp. In case of P/Ca smaller than 0.30, the production of HAp was limited, leading to less immobilized borate. All the optimized conditions as above were applied to the pilot scale with a 250 L reactor, where borate concentration was effectively reduced in also both batch and continuous tests.