Basic flotation testwork and locked cycle testwork on a nominal grind of 140 µm K80 on the sulphide composites shows that about 85% of the copper and between 60% and 65% of the gold was recovered in the concentrate. Work in 2010 continued to focus on optimizing the grind and flotation conditions to improve metallurgical performance. This work culminated in a pilot plant run in December 2010 which successfully produced 75kg of gold-silver bearing copper concentrate from approximately 10 metric tons of diamond drill core representative of Caspiche sulfide mineralization. The resulting concentrates assayed on average 23.5% copper, 35 g/t gold, 75 g/t silver and 2.4% arsenic.
This 75kg of concentrate is sufficient to allow a number of studies into potential concentrate treatment options in early 2011. Caspiche, like many of the Chilean and the global copper porphyry projects has elevated arsenic which was identified early by the company. In early 2010, Exeter, through a study by SNC Lavalin Australia, identified a range of processes with the capability of addressing the arsenic issue. The Company selected two of these processes for detailed testwork on the basis that both are commercially proven and readily available. The two processes are Reduction Roasting and High Temperature/Pressure Oxidation. Reduction Roasting was recently selected for Codelco’s new Ministro Hales copper mine in northern Chile. Testwork on this concentrate confirmed the successful application of both reduction roasting and pressure oxidation technologies to reduce or eliminate arsenic from the sulfide concentrates produced by flotation. Highlights of testwork conducted by independent laboratories include:
Gravity concentration from the rougher tailings stream, the use of high pressure grinding rolls (HPGR) and optimized flotation reagent schemes are also being investigated to investigate its potential to increase gold and/or copper recoveries.
The mineral processing routes selected were based on metallurgical testing of samples considered to be representative of the main Caspiche mineralization types.
The oxide mineralization is treated in a conventional valley-fill heap leach operation, involving two stage crushing, leaching of crushed mineralization (after placing it on impermeable plastic liners), the collection of the leach solution in lined ponds and lastly, the recovery of the gold and some silver in an activated carbon recovery circuit (ADR Circuit)B. Following the recovery of the gold the solutions are recirculated.
Sulphide mineralization follows conventional porphyry copper flotation practice. Gold copper sulphide mineralization is crushed and milled to a relatively coarse size to then pass through a flotation circuit where the first gold-rich copper “rougher concentrate” is separated. This concentrate is milled to a much finer size to then pass through three further flotation stages to raise the copper grade to commercial copper concentrate levels.
The concentrate is treated via reductive roasting to reduce the arsenic content to levels acceptable to copper smelters. The arsenic is collected and treated to form a stable, safe, arsenic compound called scorodite which will be permanently stored in a lined impoundment.
During the treatment steps used to upgrade the copper concentrate, part of the first rougher concentrate which has been separated contains a significant amount of gold and a small quantity of copper. A separate flotation circuitE recovers most of these values in a “scavenger concentrate” which is then treated in a carbon-in-leach (CIL) plant. Gold and silver are recovered from the activated carbon in a second ADR circuit. Cyanide and a small quantity of copper are recovered from solution by a combination of precipitation and pH adjustment in a SARTF circuit.
Tailings from the main flotation circuit are thickened and placed in a tailings storage facility. Tailings from the CIL circuit are placed in a separate and smaller, plastic-lined tailings storage facility close to the concentrator.
The heapleach process route can be summarised as below: