Micon's October 2013 preliminary economic assessment for the Rojo Grande deposit in San Juan Province plans three-stage crushing to 12.5 mm, cement agglomeration, a 202-day primary leach and a five-column carbon plant making doré.
The Rojo Grande Deposit forms part of the Del Carmen Property in the El Indio Gold Belt of San Juan Province, Argentina, a belt that contains the El Indio, Tambo, Veladero and Pascua-Lama deposits. Malbex Resources Inc. acquired three blocks of mining concessions in the belt in 2008. Micon International prepared An Updated Mineral Resource Estimate and Preliminary Economic Assessment for the Rojo Grande Deposit, dated October 9, 2013. The selected process is a conventional three stage crushing circuit, with a valley fill heap leach. Gold and silver are recovered by carbon adsorption columns, stripping and regeneration, then electrowinning and smelting to produce doré bars for shipment to third party refiners. The design capacity is 5 Mt/y of leach feed.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Design throughput | 5,000,000 | t/y | 14,286 t/d at 350 days |
| Crushing plant rate | 6,600 | dry tons/day | As stated in the process description |
| Crushed product | 80% passing 12.5 | mm | Three stages |
| Life-of-mine feed grade | 0.87 g/t Au | Not applicable | 9.97 g/t Ag |
| Design feed grade | 1.07 g/t Au | Not applicable | 10.9 g/t Ag, 90th percentile |
| Gold extraction | 80 | % | Primary leach 70%, secondary 10% |
| Silver extraction | 10 | % | Plant design |
| Primary leach cycle | 202 | days | 4.2 m3/t solution |
| Application rate | 12 | L/h/m2 | Drip emitters |
| Primary leach area | 218,000 | m2 | About 2.9 Mt of ore |
| Lift height | 10 | m | Ultimate height 80 m |
| Leach pad area | About 600,000 | m2 | 80-mil HDPE liner |
| Carbon columns | 5 | up-flow columns | 54.0 m3/h/m2 |
| Elution batch | 8.0 | t | Loaded carbon |
| Average gold recovered | 112,000 | oz/y | Silver 521,000 oz/y |
Overview
Three-stage crushing and screening produces leach feed that is agglomerated with barren solution, cement and lime. Trucks carry the agglomerate to a valley fill leach pad, built with an embankment at the downstream limit to form a closed-end basin for ore and solution storage. Drip emitters apply cyanide solution, and about half the pregnant solution is recycled to the heap to raise its tenor. The rest goes to the adsorption, desorption and recovery (ADR) plant.
Key Process Stages
Crushing, screening and agglomeration
Run-of-mine leach feed is hauled from the pits and either stockpiled for blending, reclaimed by front-end loader, or dumped directly into the primary crusher hopper. The primary crusher and ROM stockpile sit between the mine and the leach pad. Tertiary screen undersize goes to an agglomeration drum where barren solution, cement at 3.0 kg/t and lime are added. Agglomerates go to a loadout bin with 1,500 t of live capacity. Crushing, pad loading, leaching and gold recovery operate 24 hours a day, seven days a week. Utilization is 70% for the primary crusher, 75% for the secondary and tertiary crushers, and 95% for leach and plant.
Leach pad loading and the valley fill facility
A remotely operated clamshell dump gate reclaims agglomerate onto a high-speed loadout conveyor built to fill a 100 t CAT 777 truck in two minutes. The pad is built in phases to about 600,000 m2 on a base of low-permeability soil beneath an 80-mil HDPE geomembrane, covered with crushed overliner and solution collection piping. Ore is placed in 10 m lifts to 80 m, with catch benches giving an operating slope of 2.5:1. The eastern embankment is zoned earthfill and rockfill with an internal drainage system, and a closure drain allows gravity drainage at closure. An under-drain reduces pore water pressure beneath the liners, and diversion channels north and south of the pad route surface water around it.
Heap leaching
Low rainfall makes water balance control critical. Leach solution goes onto the heap through buried drippers, which cut evaporative loss and winter freezing. Primary leaching delivers 4.2 m3 of solution per tonne over about 202 days at 12 L/h/m2, on 218,000 m2 holding about 2.9 Mt of agglomerate. Secondary leaching occurs when cells above are leached and adds about 10% gold recovery, at 2.5 m3 of solution per tonne. Heap solution retention is 9.2%, and average pregnant solution flow is 1,310 m3/h.
ADR plant and refining
Solution goes to a single train of five up-flow carbon columns. Cyanide is added to barren solution from the columns before it returns to the barren tank. If a major storm occurs, the pregnant pond overflows to an event pond that retains pad runoff until it can replace make-up water. Loaded carbon is acid washed with dilute hydrochloric acid, rinsed with raw water and eluted by pressure stripping. Spent acid wash and rinse are neutralized with caustic soda and discharged to the barren pond. Gold and silver are electrowon onto stainless steel wool cathodes, and the sludge is dried and smelted. Stripped carbon is regenerated at 700 °C in a gas-fired rotary kiln.
Additional Interesting Data and Summary
The design is based on McClelland metallurgical testwork, Micon's conceptual mine plan and its in-house process engineering experience. Reagent use averages 1.3 kg/t of lime and 0.54 kg/t of cyanide, set at 50% of the column test average. Bulk systems handle sodium hydroxide, sodium cyanide, hydrochloric acid, antiscalant, activated carbon and propane, and boreholes supply fresh water through a surge tank and booster pumps to a raw water tank.
Average recovery is about 9,992 g/d of gold and 46,000 g/d of silver, with design values of 12,229 g/d and 51,000 g/d. Average doré output is 661,000 oz/y. The ADR plant and event pond sit at the eastern edge of the pad, and an operator's station in the ADR building monitors leaching and recovery.
Key Processes
- Three-stage crushing and screening
- Agglomeration with cement and lime
- Valley fill heap leach with drip application
- Carbon adsorption in five up-flow columns
- Acid washing and pressure stripping
- Electrowinning and smelting
- Carbon regeneration in a rotary kiln
Source: An Updated Mineral Resource Estimate and Preliminary Economic Assessment for the Rojo Grande Deposit at the Del Carmen Property, El Indio Gold Belt, San Juan Province, Argentina, October 9, 2013.
