The deposit lies within the late Miocene aged El Boleo Formation within fine to coarse clastic sedimentary rocks that were deposited in the late Miocene in deltas and near-shore shallow marine basins.

The deposit, which occurs in Boleo formation, consists of several mineralized groups called “Mantos” (manto is a specific Spanish term used in mining parlance for mineralized layer or stratum). In general, each mantos is distinct in terms of copper-cobalt ratio and relative zinc, manganese and carbonate concentrations. Mineralization is finely disseminated in the slump breccias over up to 20m thick intervals.

The project is an underground and surface mining operation to provide run-of-mine ore to a fully integrated hydrometallurgical facility. Integrated underground mining which comprise short wall and room and pillar method using continuous miners along with small cuts open mining techniques are to be employed. The mine plan will be continuously optimized on the basis of resource update by additional drilling and operational condition with the aim of maximizing economics.

The Boleo ores are clayey and generally fine grained and easily broken. The milling circuit design takes material from the ROM pad into a static grizzly with the oversize to a primary crusher. The combined product is then scrubbed in copper solvent extraction raffinate. The scrubber product is screened with the oversize sent to a secondary crusher. The combined product goes to the ball mill sump before cycloning. The coarse cyclone product is directed to the ball mill. Milling is conducted in raffinate.

The milled product is heated to a target temperature of 80ºC for atmospheric leaching. Leaching proceeds in two steps: acid, oxidative leaching with manganese dioxide oxidation for leaching of Cu, Zn and Co sulphides and: acid reductive leaching with addition of sulphur dioxide gas to reduce residual manganese dioxides. The reductive leaching of surplus manganese dioxide is essential for maximizing the extraction of cobalt.

The slurry from oxidation and reduction leaching is then partially neutralized using local limestone that is available on the Boleo mine lease. The purity of the limestone is about 65–70% and it is highly reactive for acid neutralization. A small amount of air is added during partial neutralization to ensure that there is no residual cuprous ion (that may form if reductive leaching goes to too low a redox value) in solution. Cuprous ion is not extracted by conventional oxime solvent extraction reagents.

Solid-liquid separation and CCD washing
High rate thickening involves the dilution of the incoming slurry (by recycle of overflow from the same thickener) so as to create a dilute slurry (3–5% solids) for flocculation and settling into the thickener bed. The wash solution to be used at Boleo consists of a combination of barren solution (after cobalt, zinc and manganese removal) and fresh brine solution.

Copper solvent extraction and electrowinning The recovery of copper from complex chloride containing solutions is feasible using modern selective copper solvent extractants combined with a ‘wash stage’ during SX recovery of copper to prevent transfer of chloride from leaching through to electrowinning. This approach has been widely reported at plants in Chile with saline PLS solutions and has been adopted for Boleo as well. High levels of copper extraction are expected due to the low free acid level in solution (after partial neutralization) and the use of a strong oxime formulation.

Copper is electrowon conventionally using permanent cathode blank technology.

The copper solvent extraction raffinate contains free acid (from copper solvent extraction) and so it is advantageous to return a portion of the raffinate to the milling and leaching circuit, to reduce overall use of fresh acid and to concentrate Co, Zn and Mn in solution. The balance of the raffinate is directed to the Co, Zn, Mn recovery circuit.

Cobalt and zinc solvent extraction CSIRO (Australia) has developed a range of synergistic extractants, tailored to specific metal separations. In the case of Boleo, the major challenge is the extraction of cobalt and zinc without extraction of manganese. The particular synergistic system selected for Boleo is the Versatic 10 –LIX 63 mixture. Cobalt is loaded in preference to zinc, which in turn is loaded in preference to manganese with this system.

The cobalt and zinc circuit is designed for bulk extraction and bulk stripping of zinc and cobalt. Some of the zinc strip solution is also employed as a manganese scrub solution make-up. Zinc and cobalt loading must be done with pH control. The most convenient alkali for this duty is sodium carbonate. Bulk stripping of zinc and cobalt is performed using sulphuric acid and pH control.

Cobalt metal electrowinning The zinc/cobalt (bulk) strip solution from DSX will be treated sequentially by zinc solvent extraction and stripping and then cobalt solvent extraction, stripping and electrowinning. The zinc and cobalt solvent extraction circuits will both use Cyanex 272 extractant. The zinc sulphate strip solution will be sent to zinc sulphate crystallization. The cobalt strip solution is purified and electrowon.

There is provision in the cobalt electrolysis circuit for ion exchange polishing of minor elements prior to sulphate electrolysis. The cobalt cathode deposit is harvested, crushed and marketed.

Zinc sulphate crystallization The recovery of zinc as a metal product by electrolysis is technically challenging and probably not cost-effective at a low production rate (notwithstanding the current prices of zinc metal). Hence, the zinc product selected for the Boleo process is zinc sulphate crystals. Zinc sulphate is used in a variety of agricultural formulations (fertilizers and feeds).