TRX Buckreef Project PEA – Recovery Methods

This section details the proposed processing routes for the Buckreef Project, drawing on the May 2012 PEA, with separate large conventional and small modular plant designs for oxide and sulphide ores.

Report context

The recovery methods described in this section form part of the NI 43-101 Independent Technical Report for the TRX Buckreef Project Preliminary Economic Assessment, dated May 2012. The process plant design and costing for the large conventional plant was undertaken by K'Enyuka and reported in a document entitled "Preliminary Economic Evaluation – Processing" Dec 2011. A high level study of smaller 30,000tmp modular plants was conducted by SMS for Case 3, reported in "Mining Study for a PEA for Buckreef Gold Project, Tanzania" (SMS-058-12, March 2012). The PEA process design concept is modular in configuration, so that the process plant at BRMA can be relocated to the BZMA to increase capacity in the latter area.

Processing route

Ore types and mineralogy

The BRMA and BZMA orebodies consist of an upper, weathered, oxidised zone overlying a variable transition zone and lower, primary, fresh sulphide orebodies with depth. The ores are free milling with direct leach recoveries in the lower 90% range and are amenable to gravity recovery. The fresh sulphide orebody contains pyrite and arsenopyrite. The oxide zone is clay rich saprolitic material, which potentially poses a significant problem in terms of material handling and secondary crushing. The oxide zone is more developed at BZMA.

Large Conventional Processing Plant

Comminution Circuits

A semi-autogenous grinding (SAG) circuit, followed by a ball mill, would minimise the impact of clay rich saprolitic material. However, the rod mill to ball mill index ratios of the various orebodies indicate a likelihood of a build up of critical size material in the SAG mill, indicating the need for a pebble crusher in the circuit. The large tonnage of oxide material at BZMA favours the use of this SAG mill, ball mill and pebble crushing (SABC) circuit, although the pebble crushing might be delayed until harder transition or sulphide ore is treated.

The oxide zone at BRMA is significantly less developed than at BZMA. As the design strategy is to relocate the initial plant to BZMA after BRMA ore is depleted, smaller modular units are preferable. Secondary and tertiary crushers with their associated conveyors and screens would be easier to relocate than a large SAG mill. The secondary and tertiary crushers would feed a ball mill. It may be possible to defer installation of the tertiary crusher if only oxide material is treated in the early stage.

BRMA Plant – Crushing and Screening

The 150,000tpm RoM is tipped onto a static grizzly above the RoM bin, from where ore is extracted via an apron feeder to a vibrating grizzly. Oversize from the grizzly is fed to a jaw crusher. Grizzly undersize, together with jaw crusher product, is fed to the secondary crusher screen. Secondary screen oversize is fed to the secondary crusher via a feed bin. Screen undersize, together with secondary crusher product, is conveyed to the tertiary crusher screen. Tertiary screen oversize is fed to the tertiary crusher via a feed bin and tertiary crusher product joins the feed to the tertiary screen. Tertiary screen undersize is conveyed to the ball mill feed bin.

BZMA Plant Phase 1 – Primary Crushing

The 300,000tpm BZMA RoM ore is tipped onto a static grizzly above the RoM bin, from where ore is extracted via an apron feeder to a vibrating grizzly. Oversize from the grizzly is fed to a jaw crusher. Grizzly undersize, together with jaw crusher product, is fed to the SAG mill.

Milling and Gravity Concentration – BRMA

Crushed ore from the mill feed bin is fed to the ball mill. Mill discharge flows through a vibrating screen into the mill discharge sump and is pumped to the classifying cyclone. A portion of the cyclone underflow is fed via a tramp screen to a centrifugal gravity concentrator. Tramp screen oversize and concentrator tailings are returned to the mill feed, along with the balance of the cyclone underflow. Classifying cyclone overflow feeds via a tramp/woodchip screen to the pre-leach thickener. Flocculant is added to the thickener feed. Thickener overflow discharges into a process water tank.

Milling, Pebble Crushing and Gravity Concentration – BZMA Phase 1

SAG mill discharge flows through a mill discharge vibrating screen into the mill discharge sump and is pumped to the classifying cyclones. Oversize material from the SAG mill discharge screen is either discarded or, if necessary, conveyed to a pebble crusher with product returned to the SAG mill feed. A portion of the cyclone underflow is fed via a tramp screen to a centrifugal gravity concentrator. Tramp screen oversize and concentrator tailings are returned to the ball mill feed, together with the balance of the cyclone underflow. Ball mill discharge passes through a vibrating screen and is returned to the mill discharge sump.

Gravity Circuit

Metallurgical testwork results indicate a recovery advantage to utilising gravity concentration of free gold. Some sulphide associated gold in the gravity concentrate can be subjected to more intensive cyanidation. The gravity circuit would be incorporated into the ball milling circuit with the concentrate processed in a secure area, possibly within the gold room area.

Carbon-in-Leach

Metallurgical testwork indicates limited "preg robbing" effect, hence CIL will be used in preference to leach/carbon-in-pulp (CIP).

Pre-leach, Leach and Recovery Circuits

These circuits are duplicated in each of the BRMA and BZMA plants. Thickener underflow is pumped to a pre-leach tank where final pH adjustment with lime and pre-aeration of the pulp takes place. Slurry flows through a series of leach/adsorption tanks where cyanide is added, gold dissolved and adsorbed onto activated carbon. Activated carbon is retained by inter-stage screens and transferred counter-currently. Loaded carbon from the first CIL tank is transferred to the elution plant. Barren regenerated carbon is added to the tail end of the circuit.

Loaded carbon is screened, washed, and transferred via a holding bin into a column for acid washing to remove calcium build up. Carbon is then transferred to an elution column where gold is removed by a caustic-cyanide soak and washing with hot water. Eluted carbon is transferred to a bin prior to regeneration in a rotary kiln. Regenerated carbon together with fresh make up carbon is screened and returned to the tail end of the CIL circuit.

Gold in the eluate is recovered by plating onto stainless steel wool in electrowinning cells and stripped by high pressure spray. Gold sludge is filtered and dried in a calcine oven and smelted with fluxes to produce gold bullion.

Gravity concentrate from the milling circuit is subjected to intensive cyanidation in a reactor with high concentrations of cyanide and possibly other additives including oxygen. Solid residue is returned to the milling circuit and gold-bearing solution is treated in electrowinning cells.

Tailings from the CIL circuit are screened to recover any carbon which may have escaped and are then processed in a series of tanks to reduce cyanide content prior to pumping to the tailings disposal facility.

Small Modular Processing Plants

As an alternative to the large conventional plants, a small modular plant configuration was interrogated for the PEA. SMS obtained quotations for 30,000tpm modular plants and reviewed capital and operational costs. Each modular plant is a self-contained unit comprising a 30,000tpm crushing/milling, CIL and electrowinning module. The initial plant at BRMA would require five such modules at an individual cost of USD20m each. Site preparation would be approximately USD0.5m per unit and relocation cost to BZMA would be USD1.0m per unit.

The SMS review highlighted several issues. The total capital requirement for the small modular BRMA plant would be USD129.0m and USD125.0m for BZMA. Installed power requirement per 30,000tpm module is 1.9MW, equating to approximately 10MW for the BRMA plant alone. Equipment capital for the five modular plants for BRMA is USD100.0m compared to equipment capital of USD78.7m for the large plant. Non-equipment based costs were not included in the modular plant quotation. The technical and economic review suggests that small capital savings in the modular plant option are off-set by higher power, labour, reagent and maintenance costs.

Alternative Processing

SMS metallurgical consultants suggested alternative process plant components that could improve project economics and efficiencies, to be interrogated in the PFS. These include alternative comminution strategies to reduce power and cost, skid mounted comminution units (jaw crusher/cone crusher), skid mounted Vertical Shaft Impactors or High Pressure Grinding Rolls, and RoM single stage milling.

Heap Leach

Heap leach technology was investigated on the BRMA and BZMA materials. Based on metallurgical testwork, heap leaching is not recommended. Heap leach of milled ore is not feasible and RoM material would require agglomeration using lime and cement. The sulphide component is unlikely to be amenable to heap leach. The oxide component contains significant quantities of clay, creating channelling and coating of particles, rendering them impervious to cyanide leaching.

Key reported parameters

*Values assumed based on available information per the source.

Technical qualifications

The process design and costing for the large conventional plant was undertaken by K'Enyuka and reported in Dec 2011. The small modular plant study was conducted by SMS and reported in March 2012. Process design criteria values marked with an asterisk were assumed based on available information. The PEA is a preliminary assessment and the technical and economic review of the modular plant option suggests further optimisation could be investigated in a PFS. The separate flotation of sulphides has not formed part of the PEA due to the presence of arsenic in both oxide and sulphide ore.

Source: NI 43-101 Independent Technical Report for the TRX Buckreef Project PEA, May 2012, Section 17 Recovery Methods.