Arsenic Removal from Geothermal Water Using High Gradient Magnetic Separation

抄録

On-site experimentation of high gradient magnetic separation (HGMS) to remove arsenic from geothermal water was conducted at the Kakkonda geothermal power plant in Shizukuishi, Iwate, Japan, from 2000 to 2005. By this HGMS method, the arsenic concentration in the geothermal water was reduced to less than the effluent standard of 0.1 mg/L but slightly higher than the environmental standard of 0.01 mg/L in Japan. To enhance the magnetic properties of arsenic-containing particles in geothermal water, we tried several pretreatment methods and found that the co-precipitation method using Fe(III) hydroxide generates suitable magnetized flocks for magnetic separation. Such treated geothermal water had a pH between 4.0 and 4.5 and a temperature higher than 90°C. The magnetic susceptibility of the resulting flocks was around 10⁻³, which is a typical value for paramagnetic materials. In the experiments we used a superconducting magnet to apply a magnetic field up to 2 T to a magnetic filter fabricated by packing 100 m-diameter ferromagnetic thin wires in a 50 mm ID paramagnetic stainless tube. With this HGMS system, the arsenic concentration was reduced from 3.25 mg/L to less than 0.07 mg/L at a flow velocity through the HGMS filter, 4.2 and 8.5 cm/s.