Tabornoe and Gross Gold Project — 2012 Technical Report

This technical report describes the heap leach processing operations at the Tabornoe Project and presents the process design criteria for the proposed Gross Project, based on testwork and operational experience.

Report context

This report, dated September 2012, documents the processing methods and design parameters for the Tabornoe and Gross Gold Project in Russia. The Tabornoe Project was an operating heap leach operation at the time of the site visit in August 2011. The Gross Project was at the design stage, with process design criteria prepared by IRGIREDMET and issued in March 2012. The report presents historical operating data for Tabornoe for the period 2008 to 2011 and design specifications for the proposed Gross operation.

Processing route

Tabornoe , Crushing

At the time of the site visit, two parallel crushing circuits were operating, each capable of 250–300 tph depending on feed size distribution. Ore was fed into a feed hopper by bulldozer and screened at 30 mm on an inclined vibrating screen, with undersize reporting directly to the heap feed stockpile. Oversize material was crushed in a primary jaw crusher with a reported capacity of 120–150 tph, then fed to a pair of secondary jaw crushers operating in parallel with reported capacities of 60–70 tph. The secondary crusher product was screened on a second 30 mm angled vibrating screen, with undersize directed to the heap feed stockpile and oversize directed to a separate stockpile of coarse material used for leach pad foundations. At the time of the visit, only one crushing circuit was in operation.

Two crushing circuits had been removed to make way for an additional heap. Metso mobile crushing units had been purchased and were due for delivery in June 2012. One crushing circuit had been re-installed to maintain crushing capability until the new mobile units arrived.

Tabornoe , Heap Leaching

The Tabornoe Project used heap leach technology to recover gold from crushed ore. Gold was hosted in a sequence of fine grained sandstones with uniform gold distribution. The material was naturally porous with a distinct lack of clays, providing excellent percolation properties.

Heap bases were designed with five layers: compacted soil sculpted to form the outline and gradient; 15–20 cm of concrete-asphalt mixture; a plastic impermeable membrane; geotextile placed on top of the plastic liner in areas of elevation and on joints; and collection pipes wrapped in geotextile laid in sump positions and covered in sand, with coarse +30 mm material laid over the sand to enhance permeability. Bunding surrounded each leach pad.

Ore was piled onto the coarse material layer to a height of 12–20 m. Each lift was irrigated until an arbitrarily designated barren gold grade of 0.08 mg/L was achieved in the leachate, after which a second lift was started. Intermediate liners were not used between lifts. At the time of the visit, five heaps existed at the site, with heights of approximately 30 m (Heaps 1, 2, 3) and 40 m (Heap 5). Proposed future developments included further stacking of Heap 5b, preparation of Heap 7, preparation of Heap 6, and further stacking of Heaps 1a and 1b.

Fresh cyanide solution was pumped from the mixing area to a series of pump stations for distribution. The target cyanide concentration was 0.5 g/L. In 2010, the flowrate of fresh leach solution supplied to the heaps was reported to be slightly over 9,000 m³/day. Irrigation was conducted using wobblers during summer and buried pipelines during winter. Sprinkler distribution was reported to be 10 m apart. In 2010, irrigation density was reported to be 140 L/m²/day. During winter, leach solution was heated to 10°C before being pumped to the heaps.

Each heap had one or more sump positions into which pregnant solution was channelled. Solutions drained by gravity to the hydrometallurgical plant where they were combined in three feed solution tanks. In 2010, the volume of pregnant solution was approximately 9,000 m³/day with an average gold content of 0.62 mg/L.

Tabornoe , Gold Recovery

Pregnant solution was pumped through one of nine parallel pairs of up-flow absorption columns, each containing 6 t of carbon. Each pair of columns was taken offline when barren solution grade increased above 0.8 mg/L Au. Adsorption to target levels of around 2 kg Au/t of carbon took approximately seven days.

Elution was conducted within the absorption columns using an ethylene glycol process. The strip solution consisted of 10% NaOH and 20% ethylene glycol, heated to 92–95°C and circulated through the pair of columns. Each elution cycle was reported to be no longer than 72 hours. The eluate was collected in a separate electrolyte tank prior to electrowinning.

Tabornoe , Electrowinning and Smelting

The eluate was pumped from the electrolyte tank and circulated through two electrowinning cells in parallel. After electrowinning, the sludge was washed off the cathodes and dried in a secured oven. A 40 kW induction furnace was used to smelt the dried cathode sludge into doré, typically grading 77–78% Au.

Tabornoe , Carbon Regeneration

After each strip cycle, carbon was regenerated. Carbon from one column was pumped to a carbon holding tank where it was washed with HCl and water, then sent through a kiln at approximately 70 kg/hr with heat treatment at temperatures up to 800°C. After regeneration, the carbon was pulped with water and pumped back to the columns.

Tabornoe , Mass Measurement and Sampling

Belt weightometers were not utilised for measuring tonnage of material being sent for stacking. Approximately 50% of ore being stacked was by-passing the crushing circuits and being delivered directly to the heaps. On-site, 25 t and 40 t trucks were used. A full 25 t truck could be weighed on the weighbridge; a full 40 t truck was too heavy and an average value was assigned. During winter, the weighbridge could freeze and a simple tally of trips was used. A new weighbridge capable of weighing both truck sizes was planned but no specific installation date was given.

The crushing circuit fines product was sampled manually once every 2 hours by directing approximately 100 kg of material off the conveyor onto a sampling platform, then coning and quartering twice, jaw crushing in-situ, riffled, and about 300 g sent to the laboratory. Solution samples of approximately 100 mL were taken from each heap sump at 3-hour intervals during the day and once during the night shift. Hydrometallurgical plant solution samples were taken of fresh leach solution (every 2 hours), pregnant solution (every 2 hours), barren solution (every 2 hours), strip solution (each batch), electrolyte solution (every hour during electrowinning), barren electrolyte (every hour during electrowinning), and fresh concentrated cyanide and caustic solutions (each batch). Samples were also taken of cathode sludge (each clean) and each doré bar.

Tabornoe , Assay Laboratory

The laboratory employed three staff, one technically qualified, operating 24 hours per day, seven days per week. Throughput was estimated at 50 solid samples per day. Solid samples underwent both express cyanidation/AAS analysis (one day) and comprehensive acid digest/AA analysis (2–3 days) for Au. Solution samples underwent AAS analysis. The laboratory was accredited to the national standard. Internal quality control included solid sample standards sourced from Rocklabs Ltd used in every batch and standard solutions for AAS calibration.

Tabornoe , Plant Performance

The target recovery was approximately 75%. Leach recoveries were 63.0% in 2010 and 79.0% in 2011 (year-end). Carbon losses in 2010 were estimated at approximately 26 t, attributed to fines generated by abrasion and pumping. The hydrometallurgical plant availability was 100% for 2010–2011. Leaching performance was reduced during winter due to low temperatures.

Tabornoe , Processing Costs and Labour

The total processing component of production cost in 2011 was given as US$292.68/oz, equating to US$5.89/t of ore treated. Labour consisted of 75 employees in crushing, 80 in heap leach and process plant, 3 in chemical storage, and 3 in the laboratory. All staff were Russian nationals working two months on-site followed by one month rest.

Gross , Process Design Criteria

A process design criteria document prepared by IRGIREDMET, issued in March 2012, was reviewed and approved by Snowden. The design was based on small scale heap leach testwork conducted at SGS Vostok Limited in Chita, Russia, as well as experience from the Tabornoe heap leach operation.

Gross , Ore Preparation

The ore preparation circuit consisted of a two-stage crusher circuit with an intermediate stockpile for primary crusher product. A rock breaker was included to break ROM ore to below 800 mm prior to the crusher circuit. ROM ore was to be transported by haul trucks to the ROM pad adjacent to the mining pit, then introduced to the primary crusher with a bulldozer. Belt conveyors were to transfer primary crusher product to secondary crushers located closer to the heap leach site. Primary crushed ore was to be stockpiled prior to introduction to the secondary crushers. The crushed ore from the secondary crushers was to be screened on vibrating screens before transfer by conveyors to the stacker for heap construction. The primary crusher was specified as a gyratory crusher, with secondary crushing performed by three identical cone crushers.

Gross , Heap Leaching

Due to harsh winter conditions, heap leach solutions were to be preheated to at least 9°C prior to transfer to the heaps during winter. During winter, solution was to be stored in and transferred from tanks rather than ponds. Heap foundations were recommended to be constructed using two liners with a bitumen-concrete layer between the liners. Leach solutions were to be introduced via drippers buried in trenches approximately 500 mm deep to provide insulation. PLS was to be transferred to carbon adsorption columns for gold recovery. Barren solutions were to be returned to the barren solution pond and tank after cyanide concentration was adjusted to the initial design specification. The design specified an average heap size of 2,500,000 t with approximately 4.8 leach cycles possible per year based on a leach cycle of 75 days.

Gross , Gold Adsorption

PLS from the heap leaching circuit was to be pumped to one of five adsorption streams, each consisting of three columns filled with activated carbon. Carbon in the columns was to be fluidised, leading to bed expansion of 25% to 30%. After adsorption, barren solution was to be returned to the barren solution tank or pond after pH and cyanide concentration readjustment.

Gross , Gold Elution and Electrowinning

Loaded carbon was to be transferred from the adsorption columns to one of three elution columns on a daily basis. Elution was to be conducted in a closed circuit with electrowinning cells under pressure and elevated temperatures of 130°C to 150°C. After elution, carbon was to be regenerated prior to returning to the adsorption columns. An acid wash of carbon could be undertaken prior to or after elution, but was usually done prior to elution in the acid wash column or tank. Thermal regeneration was to be undertaken once loading capacity deteriorated, not after each elution.

Gross , Gold Bullion Production

During electrowinning, gold was to be recovered onto stainless wool cathodes, which were then manually removed along with any sludge. Cathodes and sludge were to be dried and roasted in a roasting oven, initially preheated to 150°C until dry, then roasted at approximately 650°C, before being transferred to one of two electric induction furnaces for smelting with fluxing agents. Slag was expected to contain approximately 200–300 g/t Au; IRGIREDMET proposed that slag be crushed and returned to the heap leach operation.

Gross , Spent Ore Neutralisation

After completion of the leach cycle, a final water rinse of spent ore was specified, followed by destruction of cyanides, thiocyanides, or cyanide complexes. The proposed method used hypochlorite solution (Ca(OCl)₂). Subsequent to issuing the initial design criteria, IRGIREDMET recommended removing operating costs associated with spent ore neutralisation from the operating cost estimate, justified on the basis of innovative technology for detoxification of heap leaching wastes patented by IRGIREDMET JSC (Russian Federation Patent № 2316400). Capital allowances for the neutralisation circuit were retained for emergency use.

Gross , Effluent Treatment

All solutions generated were to be treated and cyanide compounds broken down before final storage or discharge. An alkaline solution was to be prepared in a make-up tank with hypochlorite added manually. Effluent was to be treated in an agitated tank with hypochlorite solution, with pH controlled at 10.0 to 10.5 by caustic addition and reduction-oxidation potential controlled at +200 to +220 mV. Chlorinated solutions were to be transferred to a settling tank after a reaction time of approximately 1 hour. Clarified solutions were to be either discharged or recycled. Solids accumulating in the settling tank were to be stored in a disposal site. An allowance for capital costs for effluent neutralisation equipment was made for emergency use; no allowance was made for effluent neutralisation in the operating cost estimate.

Key reported parameters

Technical qualifications

The report notes that tonnage treated at Tabornoe for 2008–2011 was calculated using a tally/average weight method and is therefore only an approximation that cannot be used for construction of an accurate metallurgical balance. The average gold grade of heap feed was calculated from samples taken from the crushing circuit undersize product conveyor; little or no sampling was undertaken on loads by-passing the crushing circuit and no sampling was undertaken on crushing circuit oversize material used for pad foundations. The performance data for the leaching process can therefore only be seen as an approximation of the real situation.

No figures for 2011 pregnant solution volume or gold content were provided in the information from Nord Gold.

The report states that irrigation and construction of heaps appeared to be a continuous process, sometimes occurring simultaneously, which was likely to cause problems when calculating final recoveries due to excessive carry over of gold values between accounting periods.

At the time of the site visit, in some areas the seals between bund liners appeared split; these were reportedly to be dealt with in the near future.

Source: Tabornoe and Gross Gold Project, Russia, NI 43-101 Technical Report, September 2012, Sections 17.1 and 17.2.