The DeGrussa Copper Mine (DCM) is 100 per cent owned by Sandfire Resources NL and located 150 km north of Meekatharra in the Doolgunna region.
The greenfield deposit was discovered in 2009 with the first direct shipping ore (DSO) mined February 2012, with prompt development to an operating sulphide concentrator in September 2012. Figure 1 shows an aerial view of the current concentrator layout.
The DCM processing flowsheet (Figure 2) includes a primary crusher, SABC circuit, sulphide flotation with regrind, and pressure filtration of the concentrate stream. Paste is produced for underground backfill using a portion of the flotation tailings stream. While copper is the primary valuable element at 4.5-5.0 per cent Cu, there is also payable gold (1.7 g/t) and silver (13.6 g/t). The plant was designed to treat 1.5 Mt per year at 4.6 per cent Cu. Debottlenecking works have been completed since commissioning to increase plant capacity to 1.75 Mt per year.
The transitional ore contained chalcocite and native copper and was crushed and sold as DSO. The DSO grades averaged at 24.4 per cent Cu, 3.24 g/t Au, and 21.6 g/t Ag with some shipments in excess of 35 per cent Cu. The oxide ore was stockpiled for processing in the future.
The primary ore at DeGrussa is a volcanic-hosted massive sulphide (VHMS) deposit, which is a high grade copper deposit with payable gold and silver. The dominant sulphide minerals in the primary ore body are pyrite (35 per cent), chalcopyrite (14 per cent), pyrrhotite (4 per cent) and sphalerite (2.5 per cent). Non-sulphide gangue (NSG) minerals present in the ore include magnetite (2.5 per cent), stilpnomelane (2.5 per cent), minnesotaite (1 per cent) and talc (1 per cent) amongst others. Quite uniquely, the copper is only present as chalcopyrite in the primary ore (+99 per cent). The gold and silver are largely present as an electrum and are predominantly associated with the pyrite. Figure 3 shows typical ROM stocks.
There are two main ore types at DCM which behave differently in the flotation circuit: pyrite dominant and pyrrhotite dominant ore. The pyrite dominant ore type is characterised by the presence of fine grained interstitial chalcopyrite intergrowths within pyrite. The fine grained ore microtextures limit the chalcopyrite liberation, and therefore the treatment of this ore achieves a lower copper recovery and lower concentrate grade. Finer grinding in the primary mills, and regrind circuit often improves flotation performance. For pyrrhotite dominant ore types, the chalcopyrite grain size is typically coarser, with less intergrowths of gangue minerals. The chalcopyrite has a higher liberation level in comparison to the pyrite ore types, and the copper recovery is higher, and a higher concentrate grade is more easily achieved.
Blending is critical for the operation with ROM stockpile grades varying from 1 per cent Cu to >15 per cent Cu. Ore is stockpiled separately primarily based on stope, which can be further separated based on different mineralogy of particular ore parcels within a stope. Blending aims to provide a consistent copper feed grade to the concentrator, as well as considering other drivers of metallurgical performance, penalty components that may report to the concentrate stream, ore hardness, magnetics content, etc.
The primary crusher is a single stage Metso C140 jaw crusher, fed from the ROM pad with a front end loader. The crusher product feeds into a Crushed Ore Bin with a live capacity of approximately 12 hrs. The ore is reclaimed from the bin using three vibrating feeders.
The primary SAG mill is 7.3 m x 3.35 m with a 3.4 MW variable speed motor. During commissioning, the SAG mill was run in closed circuit with two 500 mm cyclones. The transfer size from the SAG mill was <100 µm which was much finer than design, and the target throughput rate of 187 tph was not always achievable. The SAG mill generated much more fines than the modelling indicated and this had detrimental effects on copper recovery in the flotation circuit. With the overgrinding in the SAG mill, and the circuit being SAG mill constrained for throughput rate, options to coarsen transfer size were investigated. Throughput rate was improved by increasing the transfer size from the SAG mill to utilise excess power in the ball mill, and the installation of a Sandvik CH440 pebble crusher. A single deck screen with 2 mm apertures was installed in place of the primary cyclone cluster with success. The SAG transfer size is still relatively fine at a P80 of 180 µm. These developments, in addition to mill liner design, steel charge and operational parameter optimisation, have seen the throughput rate increase from the design capacity of 187 tph to 208 tph, with the highest mill throughput rate achieved over a month being 225 tph.
The secondary ball mill is 4.7 m x 7.5 m with a 2.6 MW fixed speed motor and grated discharge. The ball mill is in closed circuit with twelve 250 mm cyclones to produce a flotation circuit feed with a target P80 of 45 µm. Both mills were commissioned with Turbo Pulp Lifters rather than the conventional radial pulp dischargers for improved energy efficiency.
The circuit was originally commissioned with flash flotation cells which were fed via a split of the underflow from both primary and secondary cyclones, and a flash flotation cleaner which reported to the final concentrate stream. The water balance and recirculating loads created by the flash flotation circuit promoted poor stability in the grinding circuit and ultimately the concentrate from these cells included composite chalcopyrite particles which would have benefitted from further particle size reduction, so the flash circuit was decommissioned. Figure 4 shows the grinding circuit where the redundant primary cyclone cluster can still be seen.
Flotation and regrind
The product from the ball mill cyclones is the feed to the flotation circuit. The flotation circuit includes two OK50 roughers, six OK50 scavengers, one Eriez column cell in a cleaner-scalper duty, six OK30 cleaners, four OK30 cleaner scavengers, and six OK16U Recleaners. A M5000 IsaMill is used for regrinding and is critical to the performance of the flotation circuit. The IsaMill improves chalcopyrite liberation by grinding to a P80 of 15-20 µm. The column cell was installed in January 2015 to increase cleaning capacity and improve the recovery of fine chalcopyrite. (Figure 5).
The flotation circuit configuration sees the rougher concentrate reporting to the column for cleaning, as the chalcopyrite particles in this stream are fast floating and well liberated. The scavenger concentrate reports to the regrind circuit, and the IsaMill discharge then reports to the column cell. The column cell concentrate reports to the final concentrate stream, and the column tails becomes part of the cleaner feed in addition to the recleaner tails. The cleaner concentrate is further upgraded in the recleaner, with the recleaner tails reporting back to the cleaner feed as a circulating load, and the recleaner concentrate reporting to the final concentrate stream. The cleaner scavenger cells are the final stage of the cleaner circuit, where lower grade composite particles are typically recovered. The cleaner scavenger concentrate, in addition to the rougher and scavenger concentrates, are sent to the IsaMill for re-grinding. The cleaner scavenger and scavenger tails combined make up the final plant tailings stream. The original circuit design saw the cleaner scavenger tails stream recycling to the scavenger circuit, however this created a recirculating load of zinc (sphalerite activated by Cu and Pb ions present in solution) and also reduced the residence time in the primary scavenging circuit.
The circuit control incorporates a Courier Online Stream Analyser (OSA) to indicate real time copper grades. A higher level control system can then be used to target a concentrate grade on selected streams by manipulating mass pull rates with air and level. The operations team are responsible for reagent dose rate changes while monitoring the plant visually and with the aid of the OSA assays. Lime is used as an iron sulphide depressant, and non-sulphide gangues are generally controlled by ore blending. Guar and SMBS can be dosed as required in addition to those two primary controls, however the preferred strategy for managing concentrate diluents is via ROM pad blending. The flotation circuit produces a 24.5 per cent Cu concentrate at 91 per cent recovery.
The plant concentrate produced in the flotation circuit is deaerated using froth busters and thickened in a high rate thickener. A Larox pressure filter is used to produce a cake moistures suitable for transportation to either Geraldton or Port Hedland ports via half height shipping containers.
Tailings and paste
The final flotation tails stream is classified (deslimed) via sixteen 150 mm cyclones. The overflow (COF) reports to the high rate tailings thickener, and the underflow is used as paste plant feed material. A small portion of thickened COF is bled back into the paste plant feed as required to achieve optimum particle size distribution and slurry density. A horizontal belt filter produces a 17 per cent moisture cake which is conveyed to a paste mixer. Filter cake is mixed with unfiltered slurry to achieve the target paste density specification, along with the binder agent (typically 4-8 per cent) which consists of blast furnace slag (85 per cent) and lime (15 per cent), to produce a suitable paste material for backfill.
The thickened COF from the cyclone cluster reports to the tailings storage facility as the final tail from the concentrator. Tailings slurry is deposited sub-aerially from the perimeter embankment resulting in tailings beaching to promote formation of the supernatant pond at the decant tower. (Figure 6).
The tailings storage facility comprises of a single circular cell covering a base footprint of 15.0 hectares and a total footprint of 25.3 hectares at full height. The facility comprises a fully lined floor and starter embankment placed over compacted clay, thus effectively providing a double lining.
Sandfire, in JV with Talisman Mining Ltd, have recently discovered the Monty deposit located 10 km from DCM. This deposit has similar mineralogy and studies are being undertaken to determine the best treatment options for this. This will provide DCM with a new high grade ore source which will be blended with current LOM stocks.
A pre-rougher column cell will be commissioned in 2017 to improve flotation residence time and recovery from the rougher scavenger bank. This allows for higher grade feed materials to be treated, and increases capacity for further increases in mill throughput rate.
Increased paste backfilling requirements have borne a project investigating the use of centrifuging for increasing the mass yield of tailings usable for paste production. This is predicted to increase paste production by 20-30 per cent. This will be the first time this arrangement has been used on sulphide tailings for producing a paste product.