Crystallex's Las Cristinas gold project

Abstract The Las Cristinas deposit in Venezuela contains proven and probable reserves of 12.8 million ounces of gold at a grade of 1.20 g/t. The deposit comprises fully oxidized saprolite (SAPO) overlying a layer of sulphide-enriched saprolite (SAPS) which lies above carbonate- leached bedrock (CLB) and carbonate stable or un-leached bedrock (CSB). Gold occurs at a similar concentration in all lithological units.
Virtually all of the copper originally in SAPO has been leached out and deposited in the SAPS zone. SAPS, which makes up about 10% of the ore reserve, contains about 0.09% cyanide soluble copper. The bedrock units contain about 0.1% copper as chalcopyrite and
A previous operator began investigation of the Las Cristinas deposit in 1991. Over the next seven years, the company drilled 1,174 holes with a total length of 159 km, conducted extensive metallurgical testwork, including pilot plant operations, and completed feasibility studies and detailed design. Construction was started in 1997 and again in 1999, however, on each occasion it was suspended.
Crystallex has been producing gold in Venezuela since the early 1990s and continues production at its Revemin mill near El Callao. In 1997, Crystallex acquired Inversora Mael, which had held two of the claims to the Las Cristinas concessions. In September 2002, Crystallex entered into a definitive agreement with the Corporación Venezolana de Guayana (CVG) to develop the Las Cristinas deposit.
In late 2002, Crystallex started an extensive program of studies and testwork to determine an optimum development plan for Las Cristinas. Mine Development Associates, SNC–Lavalin Engineers and Constructors, SGS Lakefield Research Limited, J.R. Goode and Associates, and A. Laplante from McGill University acted as the main contractors.
The flowsheets considered in the 1990s included the use of carbon-in-leach (CIL) for SAPO. All other ore types would have been processed by flotation to produce a gold-copper concentrate that would be shipped to an offshore copper smelter for processing. Cyanide leaching of cleaner tailings would have been required to maximize gold recovery, however, the high copper content of the leach feed made it necessary to also consider cyanide recovery.
Initial studies by Crystallex indicated that direct leaching of all ore types would provide about 11% more effective gold recovery than the previously selected flotation route and additionally would offer lower capital and operating costs. The higher return was possible because the all-leach route avoided transportation and smelter charges, transport and smelter losses and deductions, and the capital and operating costs associated with the flotation plant, transportation, and dock facilities.
In 2003, Crystallex initiated metallurgical tests using samples representing the different rock types and cyanide- soluble copper content. Gravity concentration tests, including McGill gravity recoverable gold tests, established that centrifugal concentrators would recover about 25% of the gold and that concentrate was amenable to intensive cyanidation. Small-scale leach tests on gravity tailings established the relationships between rock type, grind, leach time, cyanide-soluble copper assay level, and gold extraction and cyanide demand. A grind P80 of 70 mm and a 36-hour leach time were shown to be optimum. Other bench tests confirmed the comminution characteristics and rheological properties of the different ore types and mixtures. Pilot-scale high-rate thickening tests provided design data across a range of ore mixtures.
Following the bench-scale work, Lakefield operated a pilot plant on two ore blends, one low in SAPS and the other high in SAPS. The pilot plant included batch grinding; semi-continuous gravity concentration with batch intensive cyanidation of the concentrate; continuous CIL with batch carbon elution; and natural degradation, cyanide destruction, and environmental testing on the tailings.
The pilot plant processed approximately 1 t of ore over 20 days of continuous operation. Gravity gold recovery over this period was 33% and 35% for the two blends. Overall gold recovery was 89% for both feed types. Cyanide consumption, carbon stripping, and other parameters were monitored and found to be as expected.
The bench- and pilot-scale testwork and other studies confirmed that a SAG-ball mill-gravity-CIL route is effective for all Las Cristinas ore types and will give about 89% gold recovery. This paper describes the testwork, discusses the 20,000 t/d plant design, and presents economic data.
Keywords: Las Cristinas, Gold, Saprolite, Gravity, Leaching, CIL, Thickening, Rheology, Natural degradation, Cyanide destruction, Plant design.
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