Though he had risen to prominence as a geologist and mine operator, Hoover focused much of his attention on raising money, restructuring corporate organizations, and financing new ventures.[43] He specialized in rejuvenating troubled mining operations, taking a share of the profits in exchange for his technical and financial expertise.[44] Hoover thought of himself and his associates as "engineering doctors to sick concerns", and he earned a reputation as a "doctor of sick mines".[45] He made investments on every continent and had offices in San Francisco; London; New York City; Paris; Petrograd; and Mandalay, British Burma.[46] By 1914, Hoover was a very wealthy man, with an estimated personal fortune of $4 million (equivalent to $108.21 million in 2021).[47]
He co-founded the Zinc Corporation to extract zinc near the Australian city of Broken Hill, New South Wales.[48] The Zinc Corporation developed the froth flotation process to extract zinc from lead-silver ore[49] and operated the world's first selective ore differential flotation plant.[50] Hoover worked with the Burma Corporation, a British firm that produced silver, lead, and zinc in large quantities at the Namtu Bawdwin Mine.