Overview of Ferronickel Smelting Process in Submerged Arc Furnace

A ferronickel furnaces are specialized submerged arc furnace used for smelting nickel-iron alloys. Its main function is to add raw materials such as nickel ore, carbonaceous reducing agents (e.g., coke), and limestone in a specific ratio into the electric furnace, and then heat them through electrodes, causing a chemical reaction at high temperatures to smelt the nickel-iron alloy.

Overview of Ferronickel Smelting

In ferroalloy production, nickel-iron is one of the more refractory metals. SAF smelting of nickel-iron is currently the mainstream pyrometallurgical process for laterite nickel ore.

The submerged electric furnace smelting process for nickel-iron is a pyrometallurgical process that uses electricity as a heat source to reduce nickel and iron oxides in laterite nickel ore into metals under a high-temperature reducing atmosphere, which are then enriched in the nickel-iron alloy.

This process is mainly suitable for processing saprophytic laterite nickel ore with low water content. Due to its high silicon and magnesium content and relatively low iron content, it is suitable for producing ferronickel alloys with a nickel content of 10%-40%, a key raw material for the production of stainless steel (200 series, 300 series).

The smelting temperature of ferronickel furnaces typically between 1600-1700℃, making it a high-temperature nickel-iron SAF smelting furnace.

Submerged Arc Furnace Ferronickel Smelting Process

The SAF, as the core smelting equipment, is combined with upstream raw material pretreatment processes to form a complete process flow. Currently, the most mainstream and mature combination is the RKEF (Rotary Kiln-Submerged Arc Furnace) process.

Stage 1: Raw Material Preparation and Pretreatment

• Lawrence Nickel Ore: Primarily uses saprophytic clay type ore, requiring relatively high nickel grade (usually >1.8%), high MgO and SiO₂ content, and low Fe content. The ore needs to be dried to control moisture content.

• Reducing Agent: Commonly used are metallurgical coke, semi-coke, or anthracite. It requires high fixed carbon, low ash content, good reactivity, and particle size matching the ore.

• Fluoride: Mainly limestone or dolomite, used to adjust the basicity (CaO/SiO₂ ratio) of the final slag, lowering the melting point and viscosity, facilitating metal particle aggregation and slag-iron separation.

• Pre-treatment: Raw materials are precisely proportioned and mixed to ensure uniform and stable composition.

Stage 2: Rotary Kiln Pre-reduction

This is a key step in the RKEF process, directly impacting the electric arc furnace's energy consumption and efficiency.

Process: 

The mixed charge is fed into a long rotary kiln from the kiln tail, while pulverized coal or natural gas is injected from the kiln head as fuel. The charge rotates and moves forward within the kiln.

Functions:

1. Drying and Dehydration: Removes physical and crystalline water from the ore.

2. Preheating: Heats the material to 750-900°C.

3. Pre-reduction: Nickel and iron oxides in the ore undergo partial reduction with carbon in the reducing agent, producing metallic nickel, iron, or low-valence oxides. The degree of pre-reduction (especially nickel pre-reduction) is a core indicator of the quality of this process.

Output:

The high-temperature, partially pre-reduced hot material discharged from the kiln head is directly fed into the submerged arc furnace via a hot material conveying system. This significantly saves on the physical heat and reduction reaction heat of the electric furnace.

Stage 3: Submerged Arc Furnace Smelting

Equipment: 

Large, enclosed or semi-enclosed circular submerged electric furnace, typically with three electrodes (arranged in a triangle). Pre-treated hot charge is directly added to the furnace.

The furnace interior can be divided into the following sections from top to bottom:

1. Charge Layer: Newly added hot charge is preheated here.

2. Reaction Layer/Coke Layer: Highest temperature (1500-1600°C), where the main reduction reactions occur: NiO + C → Ni + CO, FeO + C → Fe + CO. Some SiO₂, MgO, etc., react with CaO to form low-melting-point slag.

3. Molten Pool Layer: The nickel-iron droplets generated during reduction sink and aggregate due to their high density, forming a nickel-iron molten pool; oxides (SiO₂, MgO, CaO, etc.) form a molten slag layer, floating on top.

Operational Characteristics: 

Employing a submerged arc operation, the electrode is deeply inserted into the material layer, and the electric arc burns stably within the furnace charge, resulting in high thermal energy utilization, a strong reducing atmosphere, and high metal recovery (nickel recovery typically >90%). [1]

Stage 4: Tapping, Refining, and Casting

The molten nickel and slag separate within the furnace due to their different densities and are discharged from different tapping ports. The lower portion of the molten nickel flows out first, entering the ladle. Then, the slag outlet is opened, and the upper molten slag is discharged and water-quenched.

Refining: Molten nickel from the submerged arc furnace typically contains high levels of impurities such as silicon, phosphorus, and carbon. It needs to be transported to a converter or AOD furnace for refining to further remove impurities such as Si, P, and C, adjusting the composition to meet the requirements of stainless steel plants.

Casting: The refined, qualified molten nickel is cast into molds and cooled into nickel-iron ingots (containing 10%-40% nickel) for sale. 

RKEF Process

The most mainstream and mature ferronickel furnaces smelting process in the world is still the "rotary kiln-submerged arc furnace (RKEF)" process combination. It is adopted by almost all newly built large-scale projects.

• RK (rotary kiln) stage: 

Responsible for drying, preheating and pre-reduction. The ore and some reducing agent are heated in the rotary kiln, and nickel-iron oxides are partially reduced to the metallic state, which greatly reduces the reduction load of the submerged arc furnace.

• EF (submerged arc furnaces) stage: 

Responsible for the final high-temperature smelting, reduction and slag-iron separation.

• RKEF advantages: 

Significantly reduced energy consumption (electricity consumption can be reduced by 20%-30%), high production efficiency, high nickel recovery rate, stable operation, suitable for large-scale production. [2]

The ferronickel submerged arc furnace RKEF process is a highly integrated system engineering project. Its core ideas are "pre-reduction energy saving" and "electric furnace smelting enrichment". 

By pre-treating raw materials to a high-temperature pre-reduced state using a rotary kiln, the energy efficiency of smelting furnaces such as SAF is greatly optimized, ultimately enabling the economical and large-scale production of nickel-iron alloy, a key raw material required by the stainless steel industry.

Sources: 

[1] Core journals in the field of metallurgical engineering, such as *Journal of Metallurgical Engineering* (JOM) and *Canadian Quarterly Journal of Metallurgy*.

[2] Handbook of Extractive Metallurgy of Nonferrous Metals*, published by the American Society of Mining, Metallurgical and Petroleum Engineers.