La India Project — 2012 Technical Report

The La India Project Technical Report details the metallurgical test work and processing design for a planned open-pit, heap-leach gold and silver operation in Nicaragua.

The La India Project Technical Report, dated 31 August 2012, presents the process design and metallurgical testing program for the proposed gold and silver operation. Extensive metallurgical testing was conducted for the project beginning in 2007, using representative samples from different ore zones and varying lithologies. This test work was used to select the process flowsheet and to develop the design criteria for the processing facilities. The selected process flowsheet consists of a three-stage crushing plant that feeds a heap leach operation, with gold and silver recovered from the pregnant leach solution using a carbon-based adsorption, desorption, and refining (ADR) plant.

Metallurgical testing included evaluation of alternative recovery methods. Gravity concentration of gold and silver was tested on higher-grade material, but poor results led to the rejection of this method. Similarly, flotation tests on sulphide-bearing samples produced concentrates that were too low in grade for a saleable product. An agglomeration and percolation test determined that a crush size of 100% passing 25 millimetres would be satisfactory, and that cement should be added to the crushed rock for pH control and additional stability of the heap. Sample composites were generated for column and bottle-roll leach tests; different geological domains gave varying silver and gold recovery results. An overall project average gold recovery of 80% was selected for the project.

The process flowsheet is based on an ore processing rate of 6.0 million dry tonnes per year, equivalent to 17,250 dry tonnes per day. Design criteria for the processing system are detailed in an appendix to the report. The process is designed to treat the ore through primary, secondary, and tertiary crushing stages before agglomeration and stacking on the leach pad. The heap leach pad has a total capacity of 50 million dry tonnes. Gold and silver are recovered from the pregnant solution in a carbon adsorption circuit, and the loaded carbon is treated in an acid wash and elution circuit before electrowinning and smelting to produce a gold-silver doré. Operating costs for the processing operations were estimated, and total average processing cost is stated at $4.01 per tonne of ore placed on the heap leach pad.

Critical Data

Overview

The processing design for the La India Project is based on a conventional heap leach flowsheet with three-stage crushing. The design followed significant metallurgical testing conducted since 2007 across different ore zones and lithologies. The test work program rejected both gravity concentration and flotation due to poor results, and instead validated the heap leach approach with an overall average gold recovery of 80%. The operation is designed to process 6.0 million dry tonnes per year, with ore agglomerated using cement before being stacked on a large leach pad. Process water is to be predominantly recycled, with additional makeup water estimated at 12 to 24 litres per second depending on the season. All electrical power is supplied by diesel generators at the site.

Key Process Stages

Run-of-mine ore is delivered to a primary crushing area, where a vibrating grizzly separates fines. Oversize material is crushed in a primary jaw crusher and rejoins the grizzly undersize. The primary crushed ore is then stockpiled in a canyon below the crushing area. The crushed ore stockpile has a live capacity of 18,600 tonnes and a total capacity of 58,000 tonnes.

From the stockpile, ore is conveyed to a screen tower. Screen undersize material bypasses the secondary cone crusher and reports to the screen tower feed conveyor, while oversize feeds the secondary crusher, which discharges back onto the same conveyor. The secondary crushing stage is followed by a twin-screen tertiary screen tower. Screen undersize material is directed to a crushed ore product stockpile, while oversize is conveyed to a surge bin that feeds three tertiary cone crushers. A belt sampler is positioned over the conveyor that handles the final crushed product.

The final crushed ore, at 100% passing 25 millimetres, is belt-agglomerated with cement. Cement is added at a rate of 2.5 kg/tonne from two 100-tonne cement silos for pH control and added heap stability. The agglomerated ore is transported by a series of overland conveyors and a transfer grasshopper conveyor to the heap-leach stacking line. The final overland conveyor feeds a series of up to 23 grasshopper conveyors that feed a radial stacker for heap construction.

The heap leach pad is designed with a total capacity of 50 million dry tonnes and is built in two phases. Pregnant solution from the heap is collected and processed through a carbon adsorption circuit using cascade-type columns. Solution flows through the carbon columns, with barren solution returned to the barren solution tank for reuse in leaching or for process and agglomeration duties. The solution overflow from the final column discharges onto a carbon safety screen to catch entrained carbon.

Once the carbon in the first column is sufficiently loaded with gold, that carbon is advanced to the acid wash and desorption circuits, and carbon in each column is advanced one column. The acid wash cycle continues until pH stabilises between 1.0 and 2.0 without acid addition for a minimum of one hour of circulation. After acid washing, loaded carbon is transferred to a strip vessel for elution. Gold and silver are eluted at an operating temperature of 135°C and pressure of 340 kPa, using heat exchangers to preheat strip solution. Gold and silver are continuously extracted from the pregnant eluate by electrowinning. The gold-bearing sludge from the electrowinning cells is filtered, dried, and smelted in a tilting crucible fuel-oil-fired furnace to produce gold-silver doré and slag.

A site laboratory is equipped for sample preparation and assays using atomic absorption, fire assay, and cyanide-soluble analyses, along with a metallurgical test work area for process optimisation.

Additional Interesting Data and Summary

The process operating cost estimate was developed for all ore types and domains, estimated by specific operating area. The costs include crushing and conveying, leaching, plant, refinery, and laboratory, including major consumables. Process maintenance and personnel costs, including process supervision, are included. The total average operating cost estimate for processing is $4.01 per tonne of ore placed on the heap leach pad, for 44.6 million tonnes of ore over the mine life. The process materials required include lime, sodium cyanide for heap leaching, antiscalant, caustic soda, carbon granules, hydrochloric acid, and fluxes such as silica, borax, fluorspar, niter, and soda ash.

Main electrical power is supplied by four 1,230-kW diesel generators, with a fifth 1,230-kW generator as backup, providing 6.6 MW of total installed capacity against an estimated demand of 4.8 MW. Process water is largely sourced by reusing and recycling process water, with additional makeup water required at an estimated 12 to 24 litres per second depending on the time of year. The use of impacted water from the mine and waste dumps as process makeup is described as a means to reduce the volume of impacted water requiring management and to conserve water in the region.

Key Processes

• Heap leaching with agglomeration using cement for pH control and stability
• Three-stage crushing (primary jaw, secondary cone, tertiary cone)
• Carbon adsorption (cascade-type columns) for gold and silver recovery from pregnant solution
• Acid wash and Zadra pressure elution for carbon regeneration and gold recovery
• Electrowinning and smelting to produce gold-silver doré

Source: La India Project , 2012 Technical Report, 2012-08-31. Project website: La India Project Overview

Project website: La India Project, 2012 Technical Report