Frothers, Bubbles and Flotation National Park Service History .
as well as private mining and milling companies who own historic mills. . copper, lead, zinc, gold, silver) or non metallic ores (e.g. day, phosphate, coal) . size of flotation mills. . twentieth century hard rock milling operations; a brief history of the milling .. (San Francisco: Government Printing Office, 1990), Appendix B, p.Live Chat