The Ramu deposit is a typical laterite nickel and cobalt deposit formed by weathering and leaching of the original ultramafic intrusive rocks, mostly dunite with small amount of harzburgite and pyroxenite in a tropical climate with large amounts of rainwater, saturated with atmospheric carbon dioxide, and a local monsoonal rainfall pattern. When the ultramafic rocks are subject to strong weathering, olivine, augite and other ferromagnesian silicate minerals rich in nickel, cobalt and other elements will decompose; the released SiO2 is progressively removed by ground and/or surface water in the form of colloid or siliceous acid, and the ferrous iron is oxidised to ferric iron and converted into hydroxides and oxides, such as lepidocrocite, goethite and hydrohematite, and left in situ. Nickel, cobalt and other elements are absorbed by the clays in the saprolite, or directly precipitated from the colloidal solution, or enriched in secondary nickel silicate minerals, consequently forming a lateritic nickel-cobalt deposit within the weathering crust. At Ramu, nickel and cobalt have been enriched from a background of around 0.3% Ni and 0.01% Co in the ultramafic bedrock up to grades averaging 0.9% Ni and 0.1% Co in the laterite profile.

The Ramu laterite nickel and cobalt deposit occurs in the weathering crust of ultramafic intrusive rocks, mostly dunite. The deposit above the dunite bedrock is divided into six laterite layers, from top to bottom, the humic layer (Q), the red limonite (O), the yellow limonite (L), the saprolite (S), the upper rocky saprolite (R1) and the lower rocky saprolite (R2).

Nickel grade is less than 0.5% in the humic layer, and is generally less than 0.5%, but occasionally above 0.5%, in the red limonite. These two layers are generally considered as overburden for mining and are stripped off before mining the lower mineralised laterite layers, including yellow limonite, saprolite, upper rocky saprolite and lower rocky saprolite. The nickel grade in the lower mineralised layers averages approximately 1.0%, and the cobalt grade averages approximately 0.1%.

The distribution of each relevant element in the laterite profile shows different patterns. The nickel and magnesium grades generally increase from top to bottom; but at the bottom of the lower rock saprolite, nickel grades reduce but magnesium grades still increase. The aluminium grade reduces with the increasing depth as it is a residual component in the weathering and leaching process. Nickel is apparently enriched in both saprolite and rocky saprolite, but cobalt is only enriched in the saprolite.

The principal ore minerals identified in the Ramu deposit include goethite, asbolan and garnierite.

Goethite is found as ochre-coloured, porous, cryptocrystalline, needle-like matrix in the limonite and saprolite zones of the laterite. The highest concentrations of goethite occur in the yellow limonite. Goethite-silica, goethitesmectite and other goethite-clay mixtures dominate the matrix of these zones. The average nickel grade contained within the goethite structure has been measured by electron microprobe analyses at 1.6% Ni in the limonite zone and 2.9% Ni in the saprolite zone.

Asbolan occurs as bluish black dendrites and fracture coatings throughout the laterite profile. It has a range of compositions containing elemental mixtures of cobalt, nickel, manganese and aluminium. In the limonite zone, asbolan assays by electron microprobe analysis at 8.4% Co and 5.2% Ni, and in the saprolite zone, it assays 5.6% Co and 15.1% Ni.

Garnierite, or nickeliferous serpentine, is an apple green mineral found at deeper levels in the deposit in the alkaline weathering zone, generally at the base of the limonite horizon and in the saprolite and rock saprolite zones. The most common occurrence of this mineral is as one to ten centimetre wide veins, as fracture infillings and in the weathered rind of bedrock boulders. Garnierite may also occur as infill with serpentine in calcic magnesite breccia. Garnierite displays a range of compositions based on the proportions of serpentine, talc and lizardite minerals. From analysis of 14 samples, lizardite contains an average of 1.2% Ni.

The distribution of the laterite layers is generally controlled by topography. The laterite layers dip at angles generally between 10º to 35º, consistent with the topography dip angles. All laterite layers in the deposit vary significantly in thickness because of the topographic control and erosion.

Based on the MgO content, the weathering crust lateritic nickel and cobalt ore is divided into the iron ore type when MgO<10%, the ferromagnesian ore type when MgO content ranges from 10% to 20%, and magnesium ore type when MgO>20%. The average MgO content in the O, L and S layers is less than 10%, and therefore these zones belong to the iron ore type; the average MgO content in the R1 layer is approximately 18%, therefore it belongs to the ferromagnesian ore type; and the MgO content in the R2 layer is approximately 20.4%, and therefore it belongs to the magnesium ore type.

The Kurumbukari (KBK) owner-mining operation utilizes conventional open-pit mining methods. MCC carries out all mining operations with a fleet of excavators and trucks along with other ancillary equipment to support the mining fleet. After an initial trial period, hydro- sluicing, using hydraulic water jets, was introduced in 2016 as a second form of extraction where the geometry was suitable; hydro-sluicing has accounted for around 30% of production since its introduction.

The hydraulic water jets dislodge and transport fine material from the mining face, down an inclined drainage channel to a collection point, where it is pumped directly to the wash plant for transfer to the beneficiation plant.

After the initial logging of the trees by specialised teams, the humus/topsoil and overburden are generally removed by bulldozers; excavator and truck haulage are used if the quantities are significant. The topsoil is temporarily stockpiled on the mining area, and then excavated and hauled to the mining area boundary and stockpiled for later mining rehabilitation reclaim. Overburden is excavated with small-scale excavators and hauled in articulated sixwheel dump trucks. Overburden is either placed directly into pit voids, backfilling the old mining areas in preparation for mining rehabilitation, or stockpiled at the mining area boundaries for later reclaim.

Limonite, saprolite, and rocky saprolite ore are excavated and hauled to either one of the four ore bins at the washing plant or placed on the run of mine (ROM) pad for later reclaim. Excavation from multiple mining areas, being a combination of different pit locations and different stratigraphic layers within the pits, ensures that the ore feed is a blend of these different ore types; a key goal is consistency in the average grades of nickel, cobalt, magnesium, and aluminium in the plant feed.

Large rocks or boulders are identified at the working face in the pit and excluded from the ore to be loaded on trucks. Only the smaller- sized rocks (<0.35m) pass through the grizzly apertures at the washing plant so excavators operate on the ROM pad as a final step in the ore sorting process. The excavator operators scalp the larger rocks from the top of the grizzly screens to prevent blockages. Oversize rocks are stockpiled on the ROM pad for later reclaim and back-haulage to the mining area, either for disposal into the pit void or for road construction.

Hydro-sluicing was introduced as a low-cost innovative mining solution, focused on sluicing the -2mm limonite and saprolite ore. Mining areas are selected for hydro-sluicing based on taking advantage of the natural floor gradient. Hydro-sluicing with multiple water cannons located along the mining face is assisted by an excavator to maintain access to -2mm ore by removing rocks from the mining face or pit floor. The material from the hydrosluicing is pumped to the washing plant for screening and then pumped with the ore slurry from the washing plant to the beneficiation plant.

Ore fed through the grizzlies is wet screened to remove oversize material and any remnant tree root or other organic matter. The undersize material is generally fine clay, and this clay slurry is pumped to the beneficiation plant.

The mining fleet at KBK mine has multiple 20- 30t hydraulic excavators and Volvo 35-40t articulated haul trucks with ancillary equipment including dozers and front end loaders. There are two workshops on site to carry out routine maintenance checks and planned and breakdown maintenance.

The hydrocyclone underflow which is the coarse material is ground in a standard ball mill to reduce all material to suitable size.

The Ramu NiCo project incorporates two processing plants at the KBK site, the wash plant located adjacent to the open pits and the beneficiation plant located about 1.5km from the wash plant. A third processing plant, the Basamuk refinery, is located about 135km from KBK on the coast about 75km southeast of Madang.

The two KBK plants treat the mined ore to first remove coarse (+3mm) material as waste and then treat the -3mm material to remove chromite before sending the nickel-cobalt rich concentrate to the Basamuk plant. The Basamuk plant treats the concentrate to produce a mixed nickel-cobalt hydroxide product (MHP) which grades about 39% Ni and 3.8% Co.

Washing Plant
The first stage in treating the mined ore requires washing the feed material to remove coarse, barren rocks. The wash plant comprises four identical trains in parallel. The first stage in each train is the ore bin that receives the ‘as mined’ material from the trucks. Ore is moved from the b ........