Technical Report on the Dikulushi Underground Project

This technical report describes the processing plant, tailings storage facilities, and historical processing statistics for the Dikulushi Underground Project in the Democratic Republic of Congo, dated 12 December 2013.

Report context

This report is dated 12 December 2013 and covers the Dikulushi Underground Project located in the Democratic Republic of Congo. The processing methods described herein include the plant flowsheet design, tailings storage facilities, and processing statistics from both Anvil Mining operations and MWL operations.

Processing route

Plant Flowsheet

The processing plant and associated infrastructure was refurbished prior to start up in June 2010 and has been in continuous operation since.

The crushing plant consists of three stages: primary jaw crushing, followed by two stages of cone crushing in a closed circuit with a double deck vibrating screen, producing a minus 20 mm product for the grinding circuit feed. The grinding circuit consists of two overflow ball mills in parallel configuration in closed circuit with a 250 mm hydrocyclone. Each ball mill is powered by a 750 kW motor. The grind sizing parameter is 70% passing 106 microns. The mill is capable of treating in excess of 520,000 tonnes of ore per annum.

Both ball mills discharge to a common sump, and the slurry is pumped to a single 250 mm diameter cyclone. The cyclone underflow gravitates to an Outokumpu SK240 Unit Flotation Cell to recover coarse liberated copper sulphides, which report directly to the final concentrate. The cyclone overflow reports to conditioning and conventional flotation at 35% solids.

A relatively simple flotation circuit is in place; the circuit consists of two sections: a primary sulphide flotation and a secondary sulphide/oxide flotation.

Collector and frother addition is conventional when processing low grade ore. The splitting of the circuit is due to the presence of oxide minerals in some of the ore blends which require activation using sodium hydrosulphide (Na₂S) to enable them to be recovered. As sodium hydrosulphide can depress some sulphide minerals, the majority of the sulphide minerals are recovered in the primary sulphide flotation circuit.

The tailings from the primary sulphide flotation circuit are sulphidised and the liberated oxides and additional sulphides are recovered. In the event of the ore blend containing little or no oxides and thus not requiring sulphidising, the secondary sulphide circuit acts as a sulphide scavenger. The primary rougher circuit has provision for bypassing initial rougher concentrates directly to final concentrate. Lower grade rougher concentrates report to the cleaning flotation cells for upgrading.

Final tailings from the secondary rougher circuit are pumped to the tailings storage facility. Supernatant water is recovered from the tailings dam and recycled to the processing plant. The circuit is based on a nominal flotation time of 20 minutes in each of the rougher flotation stages and a minimum 15 minutes in each of the cleaner stages.

Final concentrate is pumped to a thickener and the underflow is pumped to a concentrate storage tank. The storage tank has sufficient capacity for 8 hours of concentrate production. A filter press with a capacity of 194 tonnes per day is operated in batch mode. Filter cake discharges directly onto a concrete floor below the filter where it is recovered and transported to a simple hopper/bagging arrangement with a skid steer loader. Concentrate is loaded into two tonne capacity bulk bags. Moisture content is near 10%. Each bag is weighed ready for despatch by truck to the Kilwa port.

Tailings Storage Facilities (TSF)

The first TSF for HMS tailings, designed by Knight Piésold Consulting, South Africa, covers 1.8 hectares and has been dormant since September 2004. A particularly coarse portion of the HMS tailings was recovered and processed through the flotation plant by Anvil. MWL has recovered and processed approximately 15,000 tonnes of coarse sand and slime material from this dump.

A second TSF (TD2), designed by D.E. Cooper and Associates, Australia, was built during the third quarter of 2004 to receive flotation tailings. This facility is located approximately 100 metres north of the HMS TSF, covers about 12 hectares and is 12 metres high on the eastern embankment. This facility has also reached capacity.

A third TSF (TD3), designed by Knight Piésold, is located adjacent to and north of the second TSF and covers a 21 hectare area. This dam is a typical hillside impoundment and provides the needed area to limit the rise rate of tailings at acceptable norms.

Supernatant tailings water is reclaimed via penstock arrangements for use in the processing plant.

The third TSF (TD3) was utilized until December 2008 and lay dormant until it was recommissioned in July 2010. At this juncture Knight Piésold was employed to carry out a volumetric assessment study to determine the storage capacity of the dam to accommodate 840,000 tonnes of tailings resulting from the processing of the low grade ore stockpile. The study concluded that a 2 metre embankment raise would be required during 2011.

Deposition continued until October 2010 when Knight Piésold was commissioned to further assess TD3 expansion capabilities to make provision for an additional 1,500,000 tonnes of deposition. The study concluded that the walls would require to be raised by 6 metres to accommodate this quantity of tailings. The raise will be carried out in two stages of 3 metres each, with the first raise completed in mid-2012.

Sub aerial tailings deposition to TD3 has continued since completion of this first stage 3 metre raise and as of June 2013, the TD3 survey pick up indicated a residual storage volume capacity of 425,000 cubic metres. Based on operational beach densities and freeboard management allowance, the residual storage capacity allowance is ensured until the TD3 stage 2 raise, which is planned for during the dry season in 2014.

In preparation for uninterrupted TD3 works during this construction period, an interim storage capacity lift of 2 metres is proposed in TD2 before TD3 works commence. This will also allow for ongoing back-up operational capacity.

This will provide a tailings storage facility capable of supporting the underground mining operation.

The information on the upgrading of TD3 has been supplied by MWL and Sedgman has not reviewed this data.

Processing Statistics

Anvil Processing

Anvil processed 137,256 tonnes of low grade between May 2008 and December 2008 when the open cut run of mine ore ran out prior to full production from underground. Some production results from February 2007 to April 2008 can be seen in the report's Table 17.1.

MWL Processing

MWL blended material from surface stockpiles and the HMS Tails through the plant to maximise copper output between June 2010 and February 2012. This was followed by the treatment of ore from satellite orebodies such as Boom Gate. The recoveries from this activity were much lower than from the fresh ore material from either the open pit or underground. It is reasonable to say that process recoveries and values are associated more with those from the previous open pit and underground mining operations carried out by Anvil. Processing statistics for the LG material completed by MWL are shown in the report's Table 17.2.

Commercial production from mining the Dikulushi open pit cutback commenced in November 2012 and was completed in July 2013. Processing of the cut back material is incomplete and open pit cut back ROM feed material will be processed till the end of 2013. Underground ore processing will commence for approximately 18 months to Mineral Reserve Completion.

Key reported parameters

Technical qualifications

The information on the upgrading of TD3 has been supplied by MWL and Sedgman has not reviewed this data. As stated in Section 13.3 of the report, the production data for the period June 2011 to July 2013 was supplied by MWL and has not been reviewed by Sedgman.

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*Source: Technical Report on the Dikulushi Underground Project, Democratic Republic of Congo, 12 December 2013, Sections 17.1–17.3.*