Ramu NiCo Project — 2019 Technical Report

The 2019 technical report describes a multi-stage processing operation that treats lateritic nickel-cobalt ore through washing, beneficiation, and high-pressure acid leaching to produce a mixed nickel-cobalt hydroxide product.

Report context

This technical report, dated July 2019, documents the Ramu NiCo Project, which incorporates two processing plants at the KBK site and a third plant, the Basamuk refinery, located about 135km from KBK on the coast about 75km southeast of Madang. The report presents the design basis, historical operating data from 2012 through 2018, and planned upgrades for progressively increasing plant production.

Processing route

Washing Plant

The washing plant is the first stage in treating mined ore to remove coarse, barren rocks. The plant comprises four identical trains in parallel. Each train begins with an ore bin that receives ‘as mined’ material from trucks. Ore is moved from the bin using an apron feeder into a rotary drum washer which has internal weirs to retard the flow and water sprays to wash the ore. At the discharge end of each drum, a screen removes material smaller than 50mm. The coarse +50mm material discharges onto a common conveyor and out onto a stockpile, from where it is transferred by front-end-loaders and trucks to be used as road metal and backfill.

The -50mm material drops into two parallel logwashers for further scrubbing. The logwasher is an inclined trough fitted with a rotating screw that moves the coarse material up to a discharge point. Water is sprayed into the logwasher to create a slurry that overflows the lower end into pumps that send the -3mm material to the beneficiation plant. The coarse material discharges to a common conveyor and then to a bank of three vibrating screens which separate the -3mm to the undersize, which joins the other -3mm material to be pumped to the beneficiation plant. The +3mm material is conveyed to a stockpile to be removed as waste. Hydraulically mined material is pumped from the mine to the bank of vibrating screens to also separate the -3mm material for beneficiation.

Process water is provided from the nearby river and is also recycled from the beneficiation plant. Electrical power for the wash plant is reticulated from the six-unit diesel power generation plant located adjacent to the beneficiation plant.

Beneficiation Plant

The purpose of the beneficiation plant is to remove chromite from the ore feed to be pumped to the Basamuk plant and to produce a chromite concentrate for sale. The beneficiation plant receives the -3mm slurry from the wash plant in two large 10m diameter tanks. The plant comprises two identical trains, each with a 10m diameter feed storage tank from which the slurry is pumped through a bank of hydrocyclones.

The hydrocyclone overflow, or “slimes” at a size distribution of -53µm, is pumped over a trash screen and then to the product thickener. The coarser +53µm to -3mm hydrocyclone underflow is fed to a bank of 13 triple-start spiral concentrators. The tailings, or overflow from the spirals containing the lighter material, is fed to desliming hydrocyclones. The heavier product is forwarded to a bank of 26 shaking tables that further concentrate the heavier chromite particles as feed to a spiral classifier to remove fugitive “slimes”, with the spiral underflow, or coarse material, sent to one of two common magnetic separators. The magnetic separators remove the free chromite particles from the lower quality material which will be discharged as a tailing product backfill into the pits. The chromite product is stockpiled and then trucked to a loading area about 5km downhill from the processing plant, then reclaimed by a front-end-loader into highway haulage trucks which transport the material to Lae for shipment to market.

The lighter material from the spirals and from the shaking tables is pumped to a bank of hydrocyclones set to separate the slurry to be pumped to the Basamuk refinery. The hydrocyclone underflow which is the coarse material is ground in a standard ball mill to reduce all material to suitable size. The ground and separated material is screened to remove any “trash” and then thickened in a large settling thickener. The thickener underflow at 15% to 19% solids w/w is stored in one of four 30m diameter storage tanks, and is then pumped over 135km to the Basamuk plant at about 1,600 cubic metres per hour.

Basamuk Refinery: Slurry Receipt

The slurry pumped from the KBK plant to Basamuk refinery arrives at grades of approximately 1.08% Ni and 0.12% Co. The slurry can be discharged into receiving storage tanks, from which it is pumped to a thickener to allow solids density to increase to greater than 30% solids w/w.

Basamuk Refinery: High Pressure Acid Leach

The thickened underflow slurry is pumped to a slurry storage tank for each of the three HPAL circuits. Each HPAL circuit comprises a high-pressure autoclave which has three stages of pre-heat vessels and respective pumping and three stages of heat recovery (flash) vessels, all interconnected. Slurry is pumped from the storage vessel into the first pre-heat vessel along with heat that has “flashed” from the third or low temperature flash vessel. The heated slurry is then pumped using a conventional centrifugal slurry pump to the second stage of pre-heat where the second stage flash gases are directed to heat the slurry further. The slurry from second stage pre-heat is pumped to the third pre-heat stage using three centrifugal pumps in series along with gases from the first or high temperature flash stage. The slurry from the third stage preheat vessel at around 200°C is then pumped into the autoclave using a high-pressure positive displacement pump. Each autoclave is equipped with two pumps, each with its own discharge into the autoclave and capable of pumping full throughput but generally operating in parallel at about 50% capacity, to provide circuit redundancy.

The autoclaves are designed to operate at a temperature of about 250°C and pressures of about 43 bar or 4,300kPa. High-pressure steam is injected into the autoclave to raise the pressures to 4,300kPa. Each autoclave is lined with a high quality titanium and has multiple compartments, each equipped with agitators. Slurry residence time is about 60 minutes per autoclave. Sulphuric acid is injected into the autoclave at a rate required to achieve target extraction of greater than 95% for nickel and cobalt.

Basamuk Refinery: Partial Neutralisation

The slurry discharge from the third flash vessel has a high level of free acid which must be partially neutralised to allow further downstream processing to recover the nickel and cobalt values. Neutralisation is undertaken using locally sourced limestone and the free acid is effectively reduced to a low level. Compressed air is added to assist oxidisation of ferrous iron to ferric iron which will precipitate into the slurry. The air also removes minor CO₂ build-up in the slurry flow. The PN circuit is made up of multiple agitated vessels into which the limestone slurry is injected and tanks can be by-passed for maintenance if necessary. The neutralisation time should be greater than 90 minutes and is affected by other recycle streams from the second stage of Fe and Al removal as well as the second stage of Ni and Co precipitation. The reported neutralisation time ranges from 3 to 4 hours.

Basamuk Refinery: Counter-Current Decantation

The neutralised slurry is fed to a seven-stage series of thickeners where the slurry continues in a forward direction from CCD1 down to CCD7. The CCD thickener overflows are pumped counter-current to the slurry flows. The final solids product from CCD7 is sent to residue neutralisation before discharge as a final tailings product. Process control has been set up to ensure that the underflow solids density is controlled at about 45% solids w/w prior to discharge to tailings neutralisation to ensure the maximum recovery of Ni and Co into the overflow liquor. The overflow from CCD1 contains all the recoverable Ni and Co in solution, which is then fed to the Fe and Al precipitation circuit.

To aid settling, a flocculant or settling agent is added to the CCD thickener feed flows. To assist metal recovery, a high wash ratio is provided. The 36m diameter thickeners are equipped with an eduction system in the feed well to increase the feed solids dilution. Flocculant is added to each CCD thickener to assist settling. The extra demand for aqueous is satisfied by recycling the second-stage Ni/Co precipitation circuit thickener overflow.

Basamuk Refinery: Iron and Aluminium Removal

Prior to recovering Ni and Co, it is necessary to reduce the amount of iron and aluminium in the liquor. Ramu undertakes this in multi stages of precipitation to reduce the amount of Ni and Co co-precipitation. The first stage of Fe/Al precipitation entails addition of limestone to the liquor to raise the pH, allowing the reaction time to be about six hours using at least six tanks. The objective in the first stage is to remove up to 80% of the Al and 60% of the Fe while minimizing Ni/Co precipitation. The resultant slurry is thickened in a single thickener unit with the thickener underflow sent to a plate-and-frame filter. The filter cake is repulped using barren liquor and the slurry is pumped to tailings neutralisation. To maximise Fe precipitation, compressed air is injected into the reaction vessels to further oxidise Fe²⁺ to Fe³⁺. The thickener underflow density is controlled at about 35% to 40% solids w/w.

The thickener overflow and the filtrate from the filter are then fed to the second stage Fe/Al precipitation circuit. More limestone is added to allow the pH to rise, requiring about four hours and four reaction vessels. The second stage Fe/Al precipitation slurry is then thickened to remove the precipitated solids. Air is added as in the first stage precipitation and seeding is also used if necessary to maintain the thickener underflow density. The solids from the second stage thickener are pumped to the PN circuit for re-leach of the co-precipitated Ni and Co. The overflow from the thickener is fed to the Ni/Co precipitation circuits.

Basamuk Refinery: Nickel and Cobalt Precipitation

The liquor from the Fe/Al precipitation circuits is treated in a further precipitation circuit using sodium hydroxide to precipitate a mixed hydroxide product (MHP). The Ni/Co precipitation takes place in two stages. The reactor discharge is fed to a thickener, with the thickener overflow fed to the second stage circuit while the underflow is split as seed and as product. The product stream feeds a filter, with the filtrate also sent to the second stage circuit. The filter cake is discharged into a storage bin from which the material is packaged into one tonne bags ready for shipment to the market. The second stage of Ni/Co precipitation is a means to ensure that overall metal recovery is maximised.

Basamuk Refinery: Tailings Treatment

Tailings treatment requires neutralisation before the slurry is discharged to the DSTP. The circuit comprises a bank of five reactors and slaked lime is added to the slurry to bring pH to over 8.0. Once neutralised, the slurry is pumped to the DSTP station where it is then discharged.

Basamuk Refinery: Plant Reagent Supply

Sulphuric acid is the main reagent consumed in the HPAL process, at a consumption rate of about 900ktpa of acid. Ramu has installed two acid plants that burn elemental sulphur to produce greater than 98.5% sulphuric acid in double catalysis and double adsorption units. The sulphur is purchased on the open market. The production of acid produces significant high-pressure steam used for HPAL heating and low pressure steam used throughout the plant.

Limestone and lime are also reagents with high overall consumption rates. Limestone is mined at a quarry near the Basamuk plant and is transported to the plant with a large stockpile storage capacity to accommodate wet season quarrying delays. The limestone is reclaimed and crushed in a two-stage jaw and cone crushing circuit with the fines sent to a grinding mill circuit for further size reduction and slurrying.

Flocculants comprise a significant component of reagent requirements. Other reagents and consumables include grinding balls for the limestone mill, burned lime, platinum catalyst for the acid plants, and sodium hydroxide for Ni/Co first stage precipitation.

Basamuk Refinery: Plant Ancillary Services

Auxiliary steam production is necessary to augment steam required for the HPAL units if acid plant steam is insufficient. Two 245t steam boilers have been installed and operate burning mostly residual oils collected throughout the operation. Compressed air supply is conventional, utilising conventional air compressors producing high pressure air for use throughout the plant including the air injection in the neutralisation and precipitation steps for ferrous to ferric oxidation.

Key reported parameters

Technical qualifications

Ramu NiCo considers the detailed design of the three processing plants to be commercially sensitive and the description in the report is restricted to generalities. The report states that BDA has inspected the plants, has had access to recent production reports, and has been provided with information on the plant upgrades which are planned so that plant production can be progressively increased. BDA considers that the two KBK processing plants have the capability to produce the required tonnages of ore feed to the Basamuk hydrometallurgical plant and the Basamuk plant can treat the ore to produce the budgeted amount of MHP annually.

Production from the KBK plant is dependent upon the ability of the Basamuk plant to take feed. The report notes that the KBK plant took approximately four years to ramp up to the planned throughput, though the ramp-up was interrupted in 2016 because of an incident at the Basamuk plant which forced a three-month shutdown. Chromite production has slowly increased but remains slightly below target, related to ore grade and not material to plant performance.

The design operating time for the HPAL plant is 7,500 hours/year or 85.6% of the time. HPAL plants generally operate continuously until a circuit shutdown is required. Past practice has evolved from shutdowns every eight months to experiencing a shutdown at about every twelve months.

Source: Ramu Nickel Cobalt Operations, July 2019, Section 17 – Recovery Methods.