Nickel mining with the CNA Pentos

Introduction

Nickel laterite in the Sorowako and Petea area of South Sulawesi, Indonesia, is classified into one of two main types, depending on the bedrock and silica-to-magnesia ratio in the mineralization: West Block and East Block. The East Block mine is a high-recovery, low-olivine (∼60¬–65%) parent rock, whereas West Block is low-recovery, high-olivine (∼80–90%) parent rock. Petea is a mainly lateritic landform with below-average nickel content.

Figure 1. Map showing the mine area (Image source: Vale Inco Limited, 2010).

Nickel ore processing 

The flowchart in Figure 2 shows that processing the nickel ore at the PT Vale plant is a complex operation, involving numerous different phases. The pyrometallurgy process begins with an initial mineralization drying stage, followed by reduction, smelting, and conversion to produce the final dried product. 

Figure 2.  Schematic  process flow for nickel laterite mining and processing (Image source: Zhang, Tsang and Brandon, 2022).

Drying 

Wet mineralization from the wet ore stockpile (WOS) is fed to three rotary kiln dryers. This process partially dries the feed mineralization, reducing the moisture content from 30–40% to 19–21% and resulting in dry kiln product (DKP). East- and West-type mineralizations are screened separately, each producing DKP of ∼10mm. The East- and West-type DKPs are then placed in separate piles in dried ore storage (DOS) buildings. 

Reduction 

From the DOS, mineralization is fed to the counter-current fired reduction kiln. A blend of East- and West-type mineralization is used. The reduction kiln comprises three zones with increasing temperatures and reduction potentials. 

The first zone pre-heats the DKP feed (with its 19–21% moisture level), completely drying it. The second zone, referred to as the calcination zone, removes all water bound in the crystal structures of the nickel laterite minerals. The third zone makes the biggest contribution to reduction by injecting high sulfur fuel oil, and is therefore referred to as the reduction zone. When introduced, the sulfur combines with the iron and nickel to form iron-nickel sulfides. The resulting product, calcine, flows out of the reduction kiln into a refractory-lined surge bin and is transported to the furnace feed bins. 

Smelting 

The calcine is smelted in an open electric arc furnace. Here, the oxide (slag) phase separates from the sulfide (matte) phase. The slag is skimmed from the furnace almost continuously and is disposed of; the matte is tapped periodically as required by the converters. 

The assay of the furnace matte is in the range of 25–28% Ni. Molten furnace matte is transferred to the converters through ladles. Air/oxygen is blown in to oxidize the remaining iron, before silica is added as flux to slag away the oxidized iron. During converting, the lower-grade electric furnace matte is converted to Bessemer matte with a nominal analysis of 78% Ni, 20% S, and 2% Co. 

Finally, the converter product is granulated, dried, screened, and packed in bags for shipment. 

The CNA Pentos for kiln feed monitoring 

PT Vale has installed a CNA Pentos at the kiln feed to track the blend of West Block and East Block dry ore. It monitors the kiln feed to ensure that the level of nickel in the dry kiln product is maintained at ∼1.91%. The CNA Pentos also includes moisture calibration to enable PT Vale to track moisture content in the kiln feed.

Figure 3. Placement of the current CNA Pentos at the site (Image source: Zhang, Tsang and Brandon, 2022).

The goal is to deploy a second CNA at the mines, to monitor the quality of the ore being fed into the WOS. In this way, PT Vale intends not only to accurately track the grade arriving from the East and West blocks but also to optimize its mining operations in both blocks.

Basicity control in lateritic nickel processing 

As well as ore-grade control and monitoring at various stages of nickel processing, the CNA has another unique application in New Caledonia, where SLN Eramet has installed a system at the smelter of its Doniambo plant to enable basicity control. 

Due to the laterite’s high MgO and SiO2 content, high temperatures (∼1600°C, i.e. ∼14 GJ/t-ferronickel) are needed during the electric arc furnace smelting. When basicity was increased from 0.58 to 0.61 and the FeO content from 5.0 to 12.5 wt% in the final slag, viscosity decreased and the required smelting temperature dropped by 86°C. This led to a decrease in energy consumption, from 14 to 11 GJ/t-ferronickel. 

With an annual output of 0.24 million tons of ferronickel, this means a yearly decrease in the plant’s total energy consumption of 0.16 million GJ/year – equivalent to reducing CO2 emissions by 45 kilotons/year.

Key take away

Real-time monitoring of nickel laterites enables huge energy savings during mining and processing. Reducing the energy consumption makes the processing of more sustainable and reduced drastically the environmental impact by less waste handling and reduced CO2 emissions. 

CNA Nickel helps at the following processing steps:

1. WOS (wet ore stockpile)
   optimized and mining operations, ore sorting, blending and stockpiling,
   early detection of not processable waste or low-grade ores
   selective mining of ore domains 
2. DOS (dry ore storage)
   kiln feed monitoring to ensure constant nickel content
   optimal energy use during further reduction 
3. EF (electric furnace)
   Basicity control to reduce CO₂ emission in the electric furnace 

Figure 4. Key benefits from CNA Pentos at the site (Image source: Zhang, Tsang and Brandon, 2022).

References

1. Vale Inco Limited, 2010. External Audit of Mineral Reserves PT. Indo Operations Sorowako. Volume 2, Section 4.
   
2. Zhang, Y., Tsang, B. and Brandon, P., 2022. Nickel Smelter Production Improvement Study Using Discrete Event Simulation. In: IFAC Workshop on Automation in the Mining, Mineral and Metal Industries.