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Developing the next major nickel project in Brazil


The process selected for the Project is the production of nickel and cobalt sulphate product via High Pressure Acid Leaching (HPAL), mixed sulphide precipitate (MSP), pressure oxidation leaching (POX), cobalt solvent extraction (CoSX) and crystallization. Prior to the HPAL process barren free silica is removed from the ore via a beneficiation which involves crushing, scrubbing and washing, separation by screening and by hydrocyclones.

The plant will be constructed in two phases, with an initial capacity of 1 Mt per annum (Mt/a) autoclave feed, then after three years of production, a second process train will be constructed effectively doubling the autoclave feed rate to 2 Mt/a. The Stage 1 plant and project infrastructure will be constructed over a 31-month period, and stage 2, effectively duplicating the stage 1 capacity completed after year 3 of production.

The process plant has been designed to process 4.34 Mt/a of ROM ore at 1.07% Ni. Of this total feed, 2.34 Mt/a is rejected as coarse, low grade siliceous waste from the beneficiation plant. The beneficiated 2 Mt/a product at 1.85% Ni is fed to the HPAL processing plant as an upgraded feed (1 Mt/a per train). A common refining circuit treats the MSP produced from each train via POX, CoSX and crystallization. The crushing circuit was designed for 75% availability and all downstream process plant was designed for 85% availability. The proposed process plant has been designed to recover 94.4% and 94.9% of the nickel and cobalt from the HPAL feed at an acid consumption of 347 kg/t. The nickel and cobalt sulphate products are of high purity suitable for sale directly into the battery market.

Extensive metallurgical testwork and process design was undertaken on the Project by the former owner, Vale, at scoping, prefeasibility and feasibility stages, included drilling and pitting programs totalling 152,000 m, variability batch testwork, full-scale pilot testwork and detailed engineering studies. A five-year, exhaustive, metallurgical testwork and pilot plant program demonstrated that a high degree of mined ore upgrade using a simple beneficiation processes was possible. The resultant feed delivered 96% average leaching extraction for nickel and cobalt via HPAL technology.

Additional testwork has been completed by the current Project owner, HZM, during 2018 and 2019. This testwork on selected samples from Vermelho validated the potential to produce high-grade sulphate products using the HPAL process.

The 6,000 plus samples totalling over 160t used for PFS and Final Feasibility Study (FFS) piloting were large diameter drill core and were representative (geographically, of depth, ore type and by lithology). Additionally, 10% of the samples (1 m from every 10 m) was used for variability testing so piloting and variability were related.

Each of the steps in the process are described below:

  • Beneficiation
    The beneficiation process upgrades ROM nickel ore by rejection of free silica to produce a HPAL feed concentrate. There are two trains of beneficiation, each of which includes:

    • Crushing in toothed roll crushers
    • Scrubbing and washing to liberate nickel-bearing fines
    • Separation of the coarse siliceous waste by screening
    • Attritioning of undersize material
    • Separation of the attritioned fines with hydro-cyclones and high-capacity, high-frequency screens.

    The outcome of the overall beneficiation process is to separate the fine limonite and nickel-bearing clays from the coarser barren silica waste with a grain size greater than 0.15 mm. The coarse silica waste goes to a waste rejects stockpile and the concentrate is fed to the processing plant.

  • HPAL
    The leaching of nickel and cobalt is completed through two HPAL trains, each consisting of a slurry feed tank, medium and high-pressure direct contact heater vessels, autoclave, three stages of flash cooling, associated pumps, piping, reagent and utilities

  • Slurry neutralization and residue filtration
    Similar to HPAL, there are two trains of neutralization and pre-reduction. Each train consists of four neutralization tanks, two pre-reduction tanks, a large leach residue filtration surge tank and associated pumps, piping, reagent dosing and utilities

  • MSP (Mixed Sulphate Precipitation)
    As per the upstream areas, the MSP area is divided into two trains. Each MSP train consists of three agitated reactor vessels, a flash letdown vessel, flash recovery compressor, thickener, filter feed tank, pressure filter, overflow tank, polishing filter, filtrate and backwash tanks, recycles tank, vent gas scrubber, associated pumps, piping and utilities.

    The filtered PLS contains soluble nickel and cobalt sulphates plus some other gangue metal cations (zinc, magnesium, manganese, and minor iron, aluminium and calcium, etc.). The solution is pumped to a series of precipitation reactor vessels where nickel and cobalt is precipitated by direct sparging of hydrogen sulphide gas. Direct injection of low-pressure steam is used for heating and recycle surge tank and pumping is included to recover various refinery and scrubber bleed streams to the reactor vessels.

    The mixed base metal sulphide (nickel sulphide and cobalt sulphide) exits the pressurised reactor vessels via a flash vessel and is then thickened. Flash vapour is re-compressed and injected back into the reactor vessels to maximise hydrogen sulphide utilization. A large recycle of solids in the thickener underflow is used to act as a seed and promote particle growth in the reactor vessels.

  • POX (Pressure Oxidation)
    Unlike the upstream processes, the POX area consists of a single train that processes all mixed sulphide produced (i.e. from both MSP trains). The mixed nickel-cobalt sulphide filter cake is re-pulped with demineralised water in a re-pulp tank in each train, then pumped to the single MSP surge tank then autoclave feed tank. The mixed sulphide slurry is pumped by piston diaphragm pump to a five-compartment horizontal autoclave

    Oxygen is sparged along the length of the vessel to fully oxidise the sulphides to sulphate and solubilise nickel and cobalt. Slurry is withdrawn from compartment one or two and flash cooled to remove heat that is generated from the sulphide oxidation process before being recycled back to the autoclave feed tank

  • Impurity removal
    The purpose of the impurity removal area is to adjust the pH of the PLS from the POX discharge and to precipitate the remaining trace amounts of copper, iron, aluminium and chromium from solution. As per the POX area, the impurity removal circuit consists of a single train. The impurity removal circuit includes six reactor tanks followed by a clarifier, overflow tank, polishing filter and backwash tank and pumps, clarifier underflow recycle and advance pumps, filter feed tank and pressure filter. The impurity removal filter cake is recycled back to HPAL discharge (neutralization area) to leach any co-precipitated nickel and cobalt.

  • CoSX (Cobalt solvent extraction)
    Consistent with POX and impurity removal areas, the CoSX area consists of a single train. The purified PLS from the impurity removal polishing filter is pumped into the CoSX circuit which consists of extraction, scrubbing, sequential cobalt and zinc stripping and nickel pre-loading stages. The circuit uses industry standard Cyanex 272 extractant in a low aromatic content, high flashpoint diluent, to extract the cobalt, leaving the nickel in the raffinate

  • Nickel and cobalt sulphate crystallization
    The cleaned cobalt loaded strip liquor at pH 3 contains 80 g/ℓ cobalt as sulphate, plus only approximately 300 ppm free H2SO4. The liquor is evaporated in a single draft tube baffle crystalliser, operating under vacuum at approximately 50 mBar absolute pressure and less than 40°C. The vacuum is generated by steam-jet thermo vapour re-compression (TVR). A portion of the overhead vapours is re-compressed by the motive steam. This reheats the vapours and allows them to act as heating medium in the crystalliser heat exchanger, thus reducing the net steam demand. The remaining process vapours are re-compressed with a small vacuum booster pump, to raise the pressure and temperature and allow the vapours to be condensed by cooling water in a plate condenser, without needing chilled water.

    The crystalliser includes a forced circulation leg, which draws liquor from near the surface of the vapour liquid interface by a low head, high flow axial flow pump. The re-circulating liquor is heated in a shell and tube heat exchanger to just under the boiling temperature, under hydrostatic pressure only. The heated liquor returns to the crystalliser vessel where it flashes. The cobalt sulphate crystallises as the heptahydrate: CoSO4•7H2O, which is the stable phase at the operating temperature and pressure. Settled slurry is pumped to a pusher centrifuge, with a portion of the centrate bled to mixed nickel and cobalt hydroxide precipitation, and the remainder returned to the crystalliser. The cobalt sulphate is centrifuged, and dried, before being packaged in bulk bags for product storage and distribution.

  • Acid liquor neutralization
    The acid liquor neutralization area neutralises plant effluent and recycles it as process water. There are two trains of acid liquor neutralization, consistent with the upstream MSP area. Barren liquor from the MSP area is the main input to the acid liquor neutralization area but minor streams such as zinc strip liquor, POX scrubber bleed and the hydrogen electrolyser scrubber bleed also feed into the acid liquor neutralization area. The area consists of two agitated neutralization tanks, air blower and receiver, thickener, overflow tank, process water pond and associated pumps, piping, reagents and utilities.

  • Kieserite crystallization
    To avoid accumulation of magnesium sulphate in the recycled process water, a portion of the process water is sent to the kieserite crystallization area. Similar to the acid liquor neutralization area, there are two trains of kieserite crystallization. Waste steam from the HPAL area is used in the kieserite crystalliser.

    The feed liquor is first concentrated in a falling film pre-concentrator which is operated by means of mechanical vapour re-compression (MVR) units. Pre-concentrator operates close to the solubility limit before feeding to the crystalliser. The crystalliser is a forced circulation type of device, where the solution concentrates at the same time when the crystals are formed. The crystalliser is operated by TVR principle (i.e. vapor is compressed by means of ejector back to heater). Excess amount of vapor is condensed in surface condenser.

    Crystal slurry from the crystallization vessel is pumped to the thickener and thickened slurry is led to the centrifuge for solid-liquid separation. The separated crystals are then dried in a fluid bed dryer to final dryness. Drier off-gas is purified by means of cyclones and scrubber. Mother liquor from the thickener and from centrifuge is collected into the mother liquor tank and most of it is recycled back to crystallization.

  • Sulphuric acid plant

Dual trains of 1,200 t/d acid plants have been included in the plant design. This is consistent with the dual trains of HPAL and associated processing plant. Outotec sulphur burning, double absorption, acid plants equipped with HEROS™ heat recovery have been selected. The cost compares favourably with that of a water-cooled design, and the additional low-pressure steam and the extra power produced compared to conventional acid plant designs assists with the process plant heat balance and kieserite crystallization area steam requirements.

HPAL Process Flow Sheet for the Vermelho Nickel Project

HPAL Process Flow Sheet for the Vermelho Nickel Project
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