Process Flow of Ferronickel Smelting in a DC Submerged Arc Furnace (DC-SAF)

The ferronickel production process in a DC Submerged Arc Furnace (DC-SAF) covers the complete sequence from raw material preparation to final tapping of molten metal. DC-SAF is characterized by high thermal efficiency, strong stability, and excellent adaptability to diverse laterite ores. By utilizing a stable direct-current arc as the primary heat source, DC-SAF enables efficient smelting, high metal recovery, and continuous large-scale operation.

This section provides a detailed, step-by-step analysis of the full DC-SAF ferronickel smelting process, covering raw material preparation, furnace charging, arc heating, metallurgical reactions, slag and metal tapping, gas-handling systems, cooling infrastructure, automation, and overall energy performance.

1. Raw Material Preparation

DC-SAF technology accommodates a broad range of raw materials, making it especially suitable for modern laterite nickel projects. It can process:

• High-moisture ores (20–30% H₂O)

• High-Mg or high-Si laterite ores

• Fine ores and powders (no need for pelletizing)

• Low-grade laterite (1.0–1.6% Ni)

Proper raw material preparation is essential for maintaining furnace stability, improving energy efficiency, and ensuring consistent metallurgical performance.

(1) Drying

Moisture reduction is critical. A rotary dryer, rotary kiln, or fluidized hot-air dryer is used to lower moisture to 10–15%. Benefits include:

• Lower furnace energy consumption

• Better permeability and gas flow in the charge column

• Reduced risk of slumping or crust formation in the furnace

(2) Crushing & Screening

Particle size is typically controlled at 0–50 mm, with fines ideally limited to 30–40%. Stable particle size distribution ensures optimal:

• Heat transfer

• Reduction kinetics

• Charge bed porosity

• Arc penetration

(3) Blending (Ore–Coal–Flux Preparation)

Accurate blending directly determines smelting efficiency and metal recovery. A typical blend includes:

• Laterite ore

• Reducing agents: coke breeze or coal

• Fluxes: limestone, dolomite, quartz (depending on slag chemistry)

Target outcomes:

• Stable MgO/SiO₂ ratio

• Controlled slag viscosity

• Optimal metallization and carbon addition

2. Charging System

DC-SAF uses a sealed and automated charging system. It typically consists of:

• Top feed chutes

• Bell-less top or rotary distributor

• Enclosed charging hoppers

• Automated feed control PLC

Charging objectives:

• Maintain uniform charge bed distribution

• Ensure consistent arc coverage

• Control burden height over the arc

• Maintain a stable “submerged arc” operating state

Continuous and precise charging is essential for smooth furnace operation and high productivity.

3. DC Arc Heating & In-Furnace Metallurgical Reactions

(1) Arc Formation and Bath Development

When DC power is applied:

• The graphite electrode generates a stable, concentrated arc

• The arc penetrates deeply into the charge bed

• Intense localized heating initiates melting

• A molten bath of ferronickel and slag forms gradually

Distinct advantages of DC arc:

• Highly stable compared to three-phase AC arcs

• Minimal arc wandering

• Strong penetrating heat flux reaching deep ore layers

• Uniform and predictable thermal distribution

(2) Reduction Reactions

Key reduction reactions include:

• NiO + C → Ni + CO↑

• FeO + C → Fe + CO↑

As temperature rises, higher oxides (Fe₂O₃, Fe₃O₄) reduce progressively to FeO and finally metallic iron. Improvement effects:

• Metal droplets coalesce and settle

• Slag oxygen potential decreases

• Overall nickel and iron recoveries increase

(3) Metal–Slag Separation

DC-SAF naturally promotes clean separation due to:

• Stable arc → reduced turbulence

• Higher metal-to-slag density contrast

• Smooth settling of ferronickel droplets

• Less entrainment of slag into metal

• Lower nickel losses in slag (20–50% reduction compared with AC-SAF)

A well-maintained slag composition results in:

• Low viscosity

• Good fluidity

• Minimal metal entrainment

(4) Bath Circulation

The centralized heat input of DC arcs induces:

• Strong natural convection

• Uniform temperature distribution

• Better reduction kinetics

• Improved reaction completeness

Furnace stability is greatly enhanced during long-cycle continuous operation.

4. Slag Tapping

Slag is tapped periodically through a side slag notch. Objectives include:

• Maintain slag height

• Control slag chemistry

• Prevent over-accumulation above desired level

Tapped slag is often used for:

• Mineral wool production

• Granulated slag powder

• Construction aggregates

Optimized slag chemistry ensures:

• Smooth tapping

• Low metal content in slag

• High furnace productivity

5. Metal Tapping (Ferronickel)

Ferronickel metal is tapped through a dedicated metal tap hole.

(1) Tapping Operation

Molten ferronickel flows into:

• A refractory-lined launder

• Metal ladles

• Optional ladle-refining units

Tapping intervals depend on furnace design and production targets.

(2) Composition Adjustment

Ferronickel chemistry can be fine-tuned to meet customer specifications (e.g., 10–20% Ni). Adjustments may include:

• Addition of scrap or low-nickel feed

• Flux additions

• Metallic nickel recycling materials

(3) Casting / Granulation

Product forms:

• Ferronickel ingots

• Granulated ferronickel (water-granulated)

Granulation advantages:

• Rapid cooling

• Good size distribution

• Ease of transport

• Compatibility with stainless-steel EAF and AOD furnaces

6. Gas Handling & Environmental Control

DC-SAF employs a fully sealed furnace with an integrated gas-handling system consisting of:

• Primary hood and ducting

• Cyclone separator

• Baghouse filter (high-efficiency dust removal)

• Waste heat recovery (WHR) systems

Environmental benefits:

• 20–40% lower dust generation than AC-SAF

• Lower CO and particulate emissions

• Captured heat can be reused for ore drying

• Reduced overall carbon footprint

7. Cooling System

A multi-zone cooling architecture ensures long campaign life:

• Electrode cooling

• Furnace roof/crown cooling

• Copper bottom anode cooling

• Segmented shell cooling plates

Adequate cooling:

• Protects refractory lining

• Maintains structural integrity

• Enables 24/7 continuous operation

8. Automation & Digital Control

Modern DC-SAF installations integrate advanced automation:

• PLC + DCS control architecture

• Automatic electrode regulation

• Real-time power input control

• AI-assisted burden distribution and feed optimization

• Online slag composition monitoring

• Bath temperature and liquid-level sensors

Automation benefits:

• Increased furnace stability

• Higher metal recovery

• Lower electrode consumption

• Reduced operational variability

9. Energy Efficiency & Performance Metrics

DC-SAF provides industry-leading energy and process performance:

These advantages make DC-SAF the preferred technology for new-generation ferronickel plants.

Summary

The complete DC-SAF ferronickel smelting process integrates stable DC arc heating, optimized charge preparation, automated charging, efficient reduction reactions, clean metal–slag separation, and advanced environmental and digital control systems.

Thanks to its superior ore flexibility, high metal recovery, lower energy consumption, and strong compatibility with modern automation, DC-SAF has become a leading technology for contemporary ferronickel production — especially in regions processing low-grade laterite ores.