1. 1. Asahi Shinbun, October 13th, 2012.
2. 2. Home page of the Ministry of Environment: http://www.env.go.jp/
3. 3. K. Tomita: Mineral processing of nonmetal minerals, Yogyo Kyokai, pp. 126–129 (1974)
4. 4. B.A. Wills: Mineral processing technology, 3rd edition, pp. 302 (1985)