NI 43-101 ITR for the TRX Buckreef Project PEA

This article presents the processing design, ore characterisation parameters, and testwork results for the Buckreef Project as reported in the May 2012 NI 43-101 Independent Technical Report.

Report context

The process plant design and costing for the large conventional plant for the Buckreef Project was undertaken by K'Enyuka and reported in a document entitled "Preliminary Economic Evaluation – Processing" Dec 2011. In addition, a high level study of smaller 30,000tpm modular plants was conducted by SMS for Case 3, the results of which are reported in a document entitled "Mining Study for a PEA for Buckreef Gold Project, Tanzania" (SMS-058-12, March 2012). The report forms part of the NI 43-101 Independent Technical Review for the TRX Buckreef Project PEA dated May 2012.

Processing route

Ore Characterisation

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 (Section 13). The fresh sulphide orebody contains pyrite and arsenopyrite and some improvement in recovery could be obtained by flotation of the sulphides and subjecting them to fine grinding to liberate the gold. However the incremental recovery would be unlikely to justify the added expense and complexity of the plant.

The oxide zone of the deposits 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 and therefore the comminution circuits of the conventional plants are designed to accommodate the higher quantities of clay rich, oxide zone RoM at BZMA.

The presence of arsenic has impact on the environmental management and the possibility of flotation and separate processing and disposal of the concentrated sulphide stream could be considered. However, there is also arsenic in the oxide ore and therefore the separate flotation of the sulphides has not formed part of the PEA.

Large Conventional Processing Plant Design

Comminution Circuits

The oxide zone of the BRMA and BZMA deposits is clay rich saprolitic material and potentially poses a significant problem in terms of material handling and secondary crushing. A semi-autogenous grinding (SAG) circuit, followed by a ball mill, would minimise this impact. However the rod mill to ball mill index ratios of the various ore bodies indicate that there is a likelihood of a build up of critical size material in the SAG mill, which would indicate the need for a pebble crusher in the circuit.

The large tonnage of oxide material in the BZMA favours the use of this SAG mill, ball mill and pebble crushing (SABC) circuit for this material, 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 the BZMA and although clay rich oxide zone material it may still represent a handling and crushing problem, this will be of shorter duration. Furthermore, there is potential to blend fresh RoM material with oxide material as there are a number of deposits to be processed in this area.

As the design strategy adopted is to relocate the initial plant to the BZMA after the BRMA ore is depleted, it would be preferable to have smaller modular units. 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, which would need to be relocated. It may be possible to defer installation of the tertiary crusher if only oxide material is treated in the early stage of the plant.

Gravity Circuit

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

Carbon-in-Leach

The metallurgical testwork indicates that there is limited of a "preg robbing" effect and hence CIL will be used in preference to Leach/carbon-in-pulp (CIP).

BRMA Plant Description

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

The crushed ore from the crushing and screening circuit stored in the mill feed bin is fed to the ball mill. The 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. The tramp screen oversize and concentrator tailings are returned to the mill feed, along with the balance of the cyclone underflow. The classifying cyclone overflow feeds via a tramp/woodchip screen to the pre-leach thickener.

BZMA Plant Phase 1 Description

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

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

Pre-Leach, Leach and Recovery Circuits

These circuits are duplicated in each of the BRMA and BZMA plants. The overflow from the mill circuit classifying cyclones is fed via a tramp/woodchip screen to the pre-leach thickener. Flocculant is added to the thickener feed to aid settling and the thickener overflow is discharged into a process water tank.

The thickener underflow is pumped to a pre-leach tank where a final pH adjustment with lime and pre-aeration of the pulp takes place. The slurry flows through a series of leach/adsorption tanks where cyanide is added, the gold dissolved and adsorbed onto activated carbon. The activated carbon is retained in tanks by means of inter-stage screens and transferred in a counter-current flow by pumping slurry containing the carbon from each tank to the preceding tank. The gold loading increases progressively up the train and the loaded carbon from the first CIL tank will be transferred to the elution plant. Barren regenerated carbon is added to the tail end of the circuit.

The loaded carbon is screened from the slurry stream and washed on a vibratory screen, and transferred via a holding bin into a column for acid washing to remove calcium build up on the carbon. Thereafter, the carbon is transferred to an elution column where gold is removed from the carbon by a caustic-cyanide soak and washing with hot water. The eluted carbon is transferred to a bin prior to regeneration in a rotary kiln, where adsorbed impurities will be removed from the carbon.

The gold in the eluate from the elution process is recovered by plating onto stainless steel wool in the electrowinning cells and then stripped by high pressure spray. Gold sludge from the cells and stripping process is filtered and dried in a calcine oven and smelted with fluxes to produce gold bullion for dispatch to a refinery.

The gravity concentrate from the milling circuit is subjected to intensive cyanidation in a reactor with the addition of high concentrations of cyanide and possibly other additives including oxygen. The solid residue from this process is returned to the milling circuit and the gold bearing solution is treated in the electrowinning cells.

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

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 the capital and operational costs in comparison to the large conventional plant design. 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. The site preparation would be approximately USD0.5m per unit and the cost of relocation to BZMA would be USD1.0m per unit.

The SMS review highlighted that the installed power requirement per 30,000tpm module is 1.9MW, which equates to approximately 10MW for the BRMA plant alone. The equipment capital for the five modular plants for BRMA is USD100.0m compared to the equipment capital of USD78.7m (installed equipment cost only) for the large plant.

Alternative Processing and Heap Leach

SMS metallurgical consultants suggested alternative comminution strategies that could improve project economics and efficiencies and which should be interrogated in the PFS, including skid mounted comminution units, Vertical Shaft Impactors or High Pressure Grinding Rolls, and RoM single stage milling.

Based on the metallurgical testwork, heap leaching is not recommended due to the sulphide component of the ore being unlikely to be amenable to heap leach, and the oxide component containing significant quantities of clay which creates channelling and coating of the particles rendering them impervious to cyanide leaching.

Key reported parameters

Processing Plant Availability and Throughput

Source: K'Enyuka 2011

Buckreef Project Ore Characterisation for the Processing Plant Design

Source: K'Enyuka 2011

*Values assumed based on available information

Capital Expenditure of the Buckreef Project Large Conventional Process Plant