San Antonio Properties — 2013 Technical Report

This report describes the conventional sulphuric acid heap leach, solvent extraction and electrowinning facilities at Luz del Cobre that have produced copper metal since May 2012.

Report context

The Luz del Cobre process facilities were designed and engineered by M3 Engineering and Technology Corp of Tucson, Arizona, in 2008. The facilities have been successfully producing copper metal since May 2012 and the process metallurgy has been subject to continuous improvement since that time. P&E visited the facilities on May 27 and 28, 2013. The Luz del Cobre component facilities were observed to be clean, neat and in safe operating condition.

Processing route

Crushing and materials handling

Two types of copper ores are mined , oxidized and mixed, where mixed contains copper sulphides as well as oxides. The combined ores are drilled and blasted then hauled about 1 km to a crusher. The run-of-mine ore size is generally quite fine. Ore is fed from a 70 t capacity hopper with a vibrating grizzly feeder to a 30" x 54" Cedar Rapids jaw crusher set at 4". The original M3 Engineering crushing process design had included 2-stage crushing , jaw followed by cone. This would have been preferred from a materials handling perspective to ensure more uniformity in the heap leach facility and capacity for reasonable sampling of stacked ore.

Adequate crushing capacity is available to process at least 3,000 tpd of ore. Crushing operations are not continuous and are operated during the day, or at night under lights. The crushing stage is quite dusty but dust is suppressed on the conveyors following the addition of barren solution. A significant amount of wood, originating from old underground mine workings, reports to the crushers and must be removed manually. Platforms for workers to safely perform this task were not identified by P&E.

Acid conditioning (curing) of crushed ore

The crushed ore is wetted with barren acidic solution followed by concentrated sulphuric acid during transfer by a series of moveable conveyor belt units (known as "grasshoppers") to the heap leach facility. Acid is added to the crushed ore on the first portable conveyor that is fed by the crushed ore reclaim conveyor. The purpose of these additions is two-fold: to promote agglomeration of the fines and to acid "cure" the ore to ensure wetting and initiation of the leaching process. The rate of acid addition is intended to be related to acid consumption measured in ore samples obtained at the open pit mine. Variations of the Luz del Cobre cure process are widely accepted in the copper heap leach industry. Where significant amounts of fine material are present in the crushed ore such as at Luz del Cobre, an agglomeration step is often included in the stacking process. Luz del Cobre crushing and stacking does not include specific agglomeration equipment.

Additional acid storage capacity has been indicated needed at this location to adequately cure the ore. Additional worker safety measures and protocols may be needed with respect to concentrated acid distribution , one emergency water shower was observed to be located several metres from the concentrated acid addition location.

Construction and operation of the heap leach facility

The leach pad is located adjacent to the crusher and down gradient in the narrow valley of the Arroyo El Salto. At this location, the valley walls are steep at 1.5H:1V and the valley floor has a 10% slope. The acidified ore is distributed with a small dozer in 4-metre lifts and the surface is ripped by the same dozer to promote uniformity for leaching.

Following placement, the ore is drip-irrigated on 0.45-metre grids with acidic barren solution for leaching the copper (producing copper sulphate) from the ore. Irrigation is achieved by the use of 16 mm O.D. drip pipes with drip points on a 0.45 by 0.45 metre grid. At the end of June 2013, 15,800 m² of leach pad were under irrigation. The irrigation rate is typically about 10–12 L/m²/hr. P&E noted a high degree of saturation and some ponded liquid on sections of the leach pad, consistent with zones of very fine, low hydraulic conductivity ore. While the high ferric/ferrous ratios in the pregnant leaching solution indicate good oxidation potential, probably enhanced by the placement of 6-inch aeration pipes below each 4-metre lift, zones of low hydraulic conductivity could be slowing the release of copper.

P&E recommends that a geotechnical field investigation be undertaken to determine the degree of saturation and physical stability of the Luz del Cobre leach pad, particularly in the down gradient embankment zones.

Leaching solution management

The copper-bearing solution, known as pregnant leaching solution (PLS) containing 3–5 g/L copper, is collected at the base of the heap leach facility for accumulation in the PLS pond immediately adjacent to the heap leach facility. The Luz del Cobre leach facility was constructed to include a 12,000 m³ PLS pond and a 26,000 m³ emergency overflow pond. The PLS pond is to provide surge capacity and allow suspended solids to settle out.

The use of these ponds was compromised by an extreme precipitation event on July 16, 2012, when a 3-hour, 128 mm rain event occurred. This precipitation amount is similar to the facility's design 24-hr 100-year storm design of 142 mm. The precipitation event filled the PLS pond with storm water and some of the leach pad was eroded into this pond. The inflow of solids damaged the inner PLS pond liner and rendered the pond inoperable for 11 months. The pond has finally been emptied and repairs are expected to be completed by end of July 2013.

PLS is piped to the solvent extraction (SX) plant by gravity in a single HDPE pipe capable of handling maximum PLS flows. The SX plant selectively extracts the copper from the PLS and a concentrated 50 g/L, relatively pure, copper solution is fed to the electrowinning (EW) plant. Sulphuric acid and water are added to the barren solution from SX and the regenerated barren solution is returned back up the valley to the heap leach using a single stage high pressure pump. The acidified barren solution is pumped from the SX plant area back to the leach pad via an 8-inch stainless steel pipe.

Solvent extraction and electrowinning

The SX plant uses two extraction units for extracting copper from the PLS solution and two stripping units to strip copper from the organic to produce a clean concentrated solution for feed to the EW facility. Only one stripping unit mixer is being used and the second stripping unit is used only as a settling unit. "Crud" (a precipitate-like substance) formation is at normal levels but is expected to increase with the presence of suspended solids in the PLS during the rainy season. Facilities are being commissioned to process this crud material and recover entrained organic and copper values. While the settlers are not covered, lightning arrestors have been installed. A fire suppressing foam system is currently being installed. The engineering contractor had erred in placement of the SX feed pond , the pond elevation is too low to allow the pond to feed SX by gravity while providing significant surge capacity.

P&E observations indicate, and available data confirm, that the SX facility is operating well and within normal operating parameters for a facility of this type.

The EW tank house facility was observed to be operating satisfactorily. Operational data indicate that current density is within normal operating ranges but current efficiency had been less than ideal. Current efficiency is improving as iron concentration in the electrolyte is being reduced (recently 4 g/L, now 2.6 g/L) and with short circuiting incidences being minimized.

Early in the operation, the EW tank house was troubled by the evolution of acid mists. The cladding on the building had darkened from exposure to this mist but principal structures have not been adversely affected. The acid mist levels in the EW plant were observed to be very low (the surface of the electrolyte is covered with floating plastic balls).

Removal of the copper sheets from the cathodes and polishing the stainless steel cathode blanks are performed manually. Example cathode blanks were observed to be in good condition, but a few were reported to have been damaged.

Process metallurgy and performance

Between April 2012 and the end of June 2013, 1.30 million tonnes of ore assaying about 1.0% copper was stacked on the leach pad. The leach pad irrigation commenced on April 12, 2012 following the pre-production stacking of 240,000 tonnes of ore assaying 1.05% CuT. The leaching solution initially contained 10–15 g/L H₂SO₄ applied at the rate of 7.20 L/m²/h.

For 2013 budget and planning purposes, Luz del Cobre assumes the following leaching profile: maximum recoverable copper is determined by the ratio of oxide to mixed ore being stacked and leached (80% for mixed ore and 75% for oxide). In 2013 the ratio of mixed to oxide ore is expected to be 4.7:1, resulting in the allocated overall recovery of 79%. Predicted copper extractions are conservative relative to earlier column test results at various laboratories.

Metallurgical recoveries in leaching at Luz del Cobre have been influenced by several factors: operation disruptions including the extreme rainfall in July 2012 and close to one month's facility shut down beginning in mid-March 2013 as a result of local civil actions (negative); refinements in materials handling, particularly curing of the ore in advance of stacking (positive); and lower than expected ore grade, particularly February to April 2013 (negative).

Copper production is currently increasing and the gap between actual and predicted is expected to diminish with time.

Key reported parameters

Technical qualifications

The sampling method used for ore fed to the leach pad is a simple one where samples of crushed ore are grabbed at the rate of one (approximately 10 kg) sample per 300 tonnes of crushed ore using a shovel. Obtaining representative samples of such heterogeneous material using this method is challenging and a bias of the copper content of the ore being placed on the leach pad may have been occurring. Crushed ore is drawn from a crusher discharge pile which naturally segregates the ore by size (coarse falls to the margins of the conical pile). This segregation increases difficulties in sampling and in the addition of the appropriate amount of barren solution and acid to the ore on the conveyors.

Moisture content is not normally measured at Luz del Cobre due to the prominent dry conditions, although this changes in the short wet season.

The column leach tests have been run from 100 to 240 days. Significant salt precipitation was noted in the columns possibly as a result of the columns being exposed to the hot sun.

The sample preparation section was observed to be a confined work space with very dusty conditions. The installation of dust collectors in sample preparation is considered by P&E to be a priority for worker health and safety and to minimise the potential for sample cross contamination.

Source: Red Tiger Mining Inc. San Antonio Properties Technical Report, 2013, Sections 17.0–17.5.