Lixiviacion Bacteriana

II Taller Internacional de Procesos Hidrometalurgicos – HydroProcess 2008

Mayo 14-16, 2008. Hotel Sheraton, Santiago, Chile.

“LABORATORY AND DEMONSTRATION SCALE OPTIMISATION OF THE QUEBRADA BLANCA HEAP LEACH BACTERIAL REGIME USING GEOLEACH™”

G. Kelly, G. Ahlborn, E. Carretero, M. Gunn, & P. Harvey*

GeoBiotics, LLC

Suite 310, 12345 W. Alameda Parkway

Lakewood, Colorado 80228 USA

1-303-277-0300

1-303-277-1772 (fax)

www.GeoBiotics.com

*corresponding author

ABSTRACT

The Quebrada Blanca (Quebrada Blanca) operation produces copper by heap leaching secondary sulphide mineralization. As the proportion of chalcopyrite increases their copper extraction declines, and GEOLEACHTM testwork has been completed by GeoBiotics LLC (GBL) indicating that higher temperature thermophile oxidative leach regimes provide the greatest improvement in copper extraction and acid consumption.

The paper presents the test results and discusses the various initiatives being developed for a full scale demonstration trial in 2008. The objective of the trial is to define how closely the full scale heap can emulate the test performance. The challenge involved is to conduct the trial within the materials preparation, handling and pad systems that are in use at Quebrada Blanca, and to maintain the thermal conditions required by the thermophiles in a cold climate.

The key elements in the engineering of the demonstration heap are:

• the use of heat containment to make the transition from the initial acid leaching to autothermal oxidation, and

• the implementation of the heap thermal management with the HotHeap® algorithms to control aeration and irrigation rates and the ratio of their respective mass flows.

BACKGROUND

The Quebrada Blanca mine is located at 4,400 m altitude in the Andes Mountain Range in Region I of northern Chile. The mine is approximately 170 km due southeast of the port city of Iquique, 240 km by road, and 1,500 km north of Santiago. Teck Cominco has a 76.5% interest in the Quebrada Blanca Mine, with the remaining interests being held by Inversiones Mineras S.A., a Chilean private company, (13.5%) and Empresa Nacional de Mineria, a Chilean government entity, (10%). Teck Cominco acquired its interest in Quebrada Blanca through the acquisition of Aur Resources in August 2007.

The Quebrada Blanca project went on stream in 1994 and produces 75,000 mt/yr copper cathodes. The mine is an open-pit truck-and-shovel operation, employing standard drilling and blasting techniques. The mine processing facilities produce copper cathode by utilizing crushing, agglomerating, stacking, bacterial heap leaching and dump leaching of secondary copper sulphides and solvent extraction and electrowinning (SX-EW) processes. The copper cathode produced is trucked to the Chilean port of Iquique for shipment to purchasers. This is one of the few applications of leaching technology for predominantly copper sulphide mineralization and the first at high altitude and low ambient temperatures. The value of additional recovery has increased dramatically over the last few years with the increased copper price, and the savings attached to any decrease in acid consumption have increased as well with the recent escalations in acid costs.

The GEOLEACH™ technology developed by GeoBiotics LLC is applicable for whole ore systems where the recoverable metals occur as sulphide minerals. The heap based bio-oxidation process can generate high temperatures from oxidation of the sulphides in whole ores. Operating heat losses can prevent the temperature rise required to sustain target bio-oxidation rates and, without significant increase in temperatures beyond ambient within the heap, sulphide chemical leaching kinetics are extremely slow. In the case of chalcopyrite the oxidation and extraction of copper is limited by passivation of the mineral surface at temperatures typically below 55oC.

The GEOLEACH™ technology is designed to maximize heat conservation through careful control of aeration and irrigation rates. GEOLEACH™ has been built onto best industry practice in the operation of bio-leaching and heap leaching facilities, and added to this is the HotHeap® control and operating philosophy supported by advanced process modeling. GEOLEACH™ is very similar to conventional whole ore acid heap leaching systems but with the addition of a management regime to maintain biological activity and maximize heat conservation. The process is simple, robust, and ideally suited to operation in remote locations.

INTRODUCTION

Quebrada Blanca pioneered large scale acid agglomeration for heap leaching and heap aeration to promote bacterial activity for the leaching of secondary copper minerals. However, at 4,400 m ASL in the Andes it is difficult to sustain the optimum conditions for bacterial activity.

The mineralogy of the ore is gradually changing toward a higher proportion of primary chalcopyrite, and the average head grade is declining as Quebrada Blanca mine the less enriched deeper transitional ore. This has created the incentive for investigation of leaching developments to maintain or improve copper extraction. The chalcopyrite is refractory to the standard mesophile bacterial leaching regimes that are currently being sustained in the heaps.

The ore type classification used by Quebrada Blanca consists of four major categories. The diagnostic mineralogy is shown in Table 1 including a typical ore blend (IMK-QZM), leach residue and the low grade dump material. The ore is crushed to nominally 80% passing 12.7mm.

Table 1: Typical Composition of Quebrada Blanca Ores.

The QZM and IMK material is blended 50:50 before being agglomerated at 5 kg/t sulphuric acid at 7% moisture using solvent extraction raffinate and water. The raffinate is heated to approximately 30oC before agglomeration to prevent freezing of the raffinate and to provide some heat to the stacked material. Dump material is dumped on a separate pad and 50 g/l sulphuric acid solution is added for 2 weeks to condition the material before raffinate is used to leach the dump material. The overall fresh acid consumption of the process in broad terms is as follows;

Ore Leach

• 5 kg/t to agglomeration

• 4 kg/t to raffinate reporting to the ore heap.

Dump Leach

• 2 kg/t to dump leach precondition

• 1.5 kg/t to raffinate reporting to the dump leach.

The ore is stacked on a ~3% sloping compacted base and slotted ‘drain-coil’ drainage lines spaced at 4 m are run out between the stacker and the ore batter to collection drains. Drilled air lines are inserted into the ore batter as it is stacked at approximately 1 m height above the drain lines. The air lines are connected to a sealed manifold down-slope that is bled occasionally to release any solution. The air lines are supplied upslope from centrifugal blowers with a system pressure of ~2kPa and flow of around 0.4 Am3/h/m2 of heap. The panels are nominally 40,000 tonnes and are stacked at approximately 80 m width in lines to fit their pad area. The ore is irrigated via an emitter network at 6 – 12 L/h/m2.

GENERAL PROGRAM OBJECTIVES

The GEOLEACH™ technology involves thermal control to both ramp up and then sustain the appropriate microbial regime for the target mineral. HotHeap® is the proprietary operating technique, supported by dynamic modeling, which delivers this outcome by the manipulation of irrigation and air rates through the kinetic cycle. In simplistic terms the system controls the mass-flow ratio of air injection to liquid irrigated expressed as Gair:Gliq.

Testwork and modeling indicates that improved performance can be achieved at Quebrada Blanca in:

• Copper extraction

• Leach kinetics

• Acid consumption

The increased copper extraction is achieved by optimizing the biological activity and regime. The laboratory testing indicates that performance improves as the leach regime temperature increases and the microbial population transforms from mesophiles through moderate thermophiles to extreme thermophiles.

The thermal environment imposed in testwork is difficult to sustain at 4,400 m altitude on exposed heaps, and Quebrada Blanca currently use two techniques to combat the prevailing temperatures. Agglomeration solutions are heated to temperatures in the region of 30oC. However this energy is rapidly lost on the conveyors feeding the stacker. The heaps are also covered in thermo-film and studies indicate a 6 – 7oC gain through this practice. Overall Quebrada Blanca operates a bacterial regime at the low end of the viable temperature range.

The increased kinetics can translate to less time on the pad but Quebrada Blanca are not limited by leach area so they do not place a high priority rating on the achievement of the 150 leach period versus their current 450 days. This gain in kinetics was only achieved in optimized test conditions and would also need to be confirmed at full scale. There is little current incentive to increase ore throughput as the SX/EW and crushing plants would require expansion. Reduced acid consumption is achieved through maximizing the generation of acid from oxidation of sulphide minerals, with the acid contribution primarily from pyrite.

In broad terms successful bio-oxidation

...