This invention relates to the recovery of valuable metals and mineral salts from mineral bearing host rocks and to the purification of mineral salts formed by evaporation of natural brines.
Valuable metals are found in nature in mineral-bearing host rocks. Ores must be processed physically and chemically to produce commercial products. Normally the ores are mined underground, and then refined in factories on the surface. In order to economize on the mining cost and to minimize the environmental impacts, in-situ mining methods have been devised.
In recovery by in-situ mining of valuable metals such as the Carlin (Nevada) type of organic-rich metalliferous rocks containing appreciable amounts of disseminated gold and associated metals such as mercury, arsenic, antimony etc., or metalliferous black-shale deposits, containing uranium, gold, silver, copper, lead, zinc, and nickel etc., the problem is two-fold: the host rock has to be oxidized and it has to be made permeable for leaching solution.
Processes are known for recovery of disseminated metals in organic-rich shale, such as gold in deposits of the Carlin type, where ores are mined and milled before they are subjected to oxidation at high temperature are known. These process are commonly referred to as "shale-burning". Co-pending patent application Ser. No. 08/403,364 discloses a method to oxidize organic-rich shale by currently available methods of shale burning, before injecting leaching fluid into the burnt rock.
The currently available methods of in-situ shale burning have, however, two disadvantages:
(1) Porosity and permeability for shale to be burnt originate from the collapse of a roof above an excavated cavity. The cost of such excavation renders the process uneconomical.
(2) The fracturing of rubblized and burnt rock causes the leaching fluid to flow through fractures rather than pervasively through the whole rock. Such a flow pattern does not permit the leaching of metals from the largely intact sections of host rock which are not pulverized or not very minutely fractured.
The currently available methods of in-situ leaching metals from host rock have a disadvantage:
(1) The leaching solution must flow laterally through a porous and permeable host rock, and economic values cannot be leached out from a relatively impermeable host rock by such lateral flow of leaching solution.
It is an object of the present invention to improve control of chemical reactions in leaching metals and to recover valuable constituents at greater depths than now possible.
Humid regions with high groundwater table or arid regions with low groundwater table can be dealt with, because the hydrologic cell is designed to minimize lateral losses to natural groundwaters. Vertical flow through relatively impermeable bed is induced to flow from a lower aquifer upward to an upper aquifer or other collecting devices for reacted solution.