Nickel-iron smelting process and principle
Uses of nickel and ferronickel
Nickel does not easily rust in the atmosphere and is resistant to corrosion from fluorine, alkalis, salts, and many organic substances. Nickel is a magnetic metal with good toughness and sufficient mechanical strength, allowing it to withstand various types of machining processes such as rolling, grinding, and welding.
Nickel is mainly used in the production of metallic materials, accounting for more than 70% of the total; it is used in electroplating, accounting for about 15% of the total nickel consumption; it is used as a catalyst in the hydrogenation process of petrochemicals; it is used as a chemical power source; it is used to make pigments and dyes; and it is used to make ceramics and ferrites.
Ferronickel is an alloy of nickel and iron, containing carbon, silicon, phosphorus, and other elements. Ferronickel is primarily used as an alloying agent in the smelting of stainless steel.
The chemical composition of ferronickel produced from laterite nickel ore is generally as follows: Ni 10%～15%, Si≤7%, C≤4.5%, P≤0.06%, S≤0.04%~0.35%.
Laterite nickel ore is a raw material for producing nickel-iron ore.
The raw materials for smelting nickel-iron using laterite nickel ore include laterite nickel ore, coke, and lime.
The mineral and chemical composition of lateritic nickel ore varies greatly, especially in nickel content and the MgO/SiO₂ mass ratio. Typical lateritic nickel ore composition is: Ni 1.3%–1.9%, Fe 10%–15%, SiO₂ 35%–45%, MgO 17%–25%, P 0.001%–0.007%, H₂O 25%–33%. Lateritic ore contains a large amount of bound water and requires roasting and dehydration before smelting.
The lime should have a CaO content of ≥82%.
The requirements for coke are: fixed carbon greater than 82%, ash content less than 15%, sulfur content ≤ 0.7%, moisture content less than 6%, and particle size 10-25mm.
Principle of nickel-iron smelting of laterite nickel ore
Lateritic nickel ore mainly contains various oxides such as NiO, Cr₂O₃, Fe₂O₃, Al₂O₃, MgO, and SiO₂. Based on the free energy data of oxide reactions, within the melting point range of lateritic nickel ore (1600–1700 K), the order of reduction of each oxide in a reducing atmosphere, from easiest to hardest, is: NiO > FeO > SiO₂ > Fe₂O₃ > MgO > Cr₂O₃ > Al₂O₃. NiO is reduced first, and its reduction temperature is lower than that of FeO. Utilizing this selective reduction principle, a carbon-deficient operation can be adopted to preferentially reduce almost all nickel oxides in the lateritic nickel ore to metal, while a suitable amount of the high-valence Fe₂O₃ is reduced to metal, and the remainder is reduced to FeO, which enters the slag. This achieves the purpose of enriching nickel. The degree of iron reduction is adjusted by adding the amount of reducing agent coking coal.
The arc temperature zone inside the electric arc furnace reaches over 2500℃, and the molten pool temperature can reach over 1800℃. The main reactions occurring at this temperature are:
NiO + C = Ni + CO
Fe₂O₃ + 3C == 2Fe + 3CO
NiFe₂O₄ + 4C = 2Fe + Ni + 4CO
Fe₂O₃ + C = 2Fe₂O + CO
FeO + C = Fe + CO
Cr₂O₃ + 3C = 2Cr + 3CO
MgO + C = Mg + CO
SiO₂ + 2C = Si + 2CO
The reaction produces a nickel-chromium-iron alloy containing elements such as silicon and magnesium. The actual chemical reaction inside the furnace is much more complex than described above.
Laterite nickel ore nickel-iron smelting process
There are three main methods for producing ferronickel from laterite nickel ore: blast furnace method, rotary kiln direct reduction method, and rotary kiln-submerged arc furnace combined method.
4.1 Blast Furnace Smelting
The main process for producing ferronickel in a blast furnace is as follows: ore drying and screening (large-scale crushing) – batching – sintering – sintered ore with coke blocks and flux added to the blast furnace for smelting – ferronickel ingot casting and slag water quenching – producing ferronickel ingots and water-quenched slag. Blast furnaces are primarily used for producing low-nickel pig iron, using laterite nickel ore containing approximately 50% iron and 1% nickel to produce low-nickel pig iron containing approximately 5% nickel. Typically, the coke-to-nickel ratio is around 800 kg coke/ton of ferronickel.
4.2 Direct reduction smelting in rotary kiln
Rotary kiln direct reduction smelting is widely recognized as the production method with the lowest energy consumption and cost. The basic process is: raw ore drying (large-scale crushing and grinding) – addition of reducing coal and flux – reduction and smelting in a rotary kiln – water quenching of the molten ore – crushing, grinding, and magnetic separation of the water-quenched slag and nickel-iron particles – production of nickel-iron granules and fine molten slag. This process does not require coke or large amounts of electricity, has a short process flow with few steps, and possesses strong competitiveness and viability. Currently, this process is not yet in large-scale production in China, mainly because issues such as batching, refractory materials, and ring formation have not yet been fully resolved.
4.3 Rotary kiln-submerged arc furnace smelting
The rotary kiln-submerged arc furnace smelting process is widely used both domestically and internationally for producing nickel-iron alloys with high nickel content. The complete smelting process is as follows: raw ore drying and large-scale crushing → coal and flux mixing and thorough drying and pre-reduction in the rotary kiln → submerged arc furnace reduction smelting → nickel-iron molten iron casting and slag water quenching → production of nickel-iron ingots (or water-quenched nickel-iron granules) and water-quenched slag.
The main processes consist of the following:
(1) Drying: Moisture is removed from nickel ore using a rotary drying cylinder;
(2) Ingredients: The laterite ore, anthracite, and limestone are mixed and proportioned in a certain ratio;
(3) Pre-reduction: Reduction in a rotary kiln;
(4) Smelting: Hot materials are directly fed into the furnace for smelting;
(5) Refining: Desulfurization, desiliconization, and decarburization refining of the alloy;
(6) Casting or Granulation: Granulation, drying, packaging, or casting ingots in casting machines;
(7) Flue gas purification: exhaust gas is discharged after entering the dust collector and electric furnace gas is recovered.
Laterite nickel ore is transported from the port to the stockyard for storage and mixing. The laterite ore stored and premixed in the raw material yard is first dried in a drying kiln to remove most of the moisture, then crushed and screened. Limestone and reducing agent are screened and crushed in the raw material yard and preparation room, and then mixed with the dried laterite ore and sent to the rotary kiln.
In the rotary kiln, the raw materials are further dried, roasted, and pre-reduced to produce nickel slag (a partially reduced product) at approximately 900–1000°C. The rotary kiln flue gas is discharged after passing through a waste heat boiler, dust removal, and desulfurization. The dust is mixed with the raw materials and then reintroduced into the kiln.
Nickel slag is added to the submerged arc furnace silo (lined with refractory bricks) under a closed, insulated condition (elevated feeding trolley). According to process requirements, the hot reduction products are secondary-batched using an electronic light rail scale before being sent to the submerged arc furnace smelting system. The slag is distributed into the submerged arc furnace through feeding pipes at different locations. The submerged arc furnace is semi-enclosed (or fully enclosed), with self-baking electrodes, submerged arc smelting, reducing and separating crude ferronickel and slag, while simultaneously generating submerged arc furnace gas containing approximately 75% CO. This gas is purified and sent to the rotary kiln burner, where it is used as fuel along with pulverized coal. Dust from the dust collector is treated and returned to the raw material yard. The submerged arc furnace slag, after water quenching, can be used as building material for road construction and brick making.
Liquid nickel-iron alloy is periodically poured from the submerged arc furnace into a ladle, transported by ladle car to the casting plant for casting, and qualified nickel-iron ingots are stored and sold. The product of the submerged arc furnace is crude nickel-iron, and a desulfurizing agent can be added to the molten iron ladle before tapping, so that desulfurization occurs simultaneously with tapping.
Crude ferronickel contains impurities such as Si, C, and P, and requires further refining. After slag removal, it is fed into a converter for oxygen blowing to remove silicon. At the same time, nickel-containing scrap is added to prevent the molten iron temperature from being too high. After silicon removal, slag is removed (or slag is blocked to tap the iron), and it is fed into an alkaline converter for oxygen blowing to remove phosphorus and carbon. Limestone is added to create alkaline slag. The molten ferronickel refined in the alkaline converter is sent to the casting workshop to be cast into qualified commercial ferronickel blocks or sent directly to the steel plant while still hot.
The overall process flow is shown in Figure 1.
Energy conservation in nickel-iron production
Most of the raw ore used for smelting nickel-containing pig iron in electric arc furnaces comes from abroad. The raw ore is mostly fine ore, with an attached moisture content of around 30% and a crystal water content of around 10%. Therefore, this ore must be sintered before it can be smelted in the furnace. Sintering is the first step in the smelting process, which not only affects the normal operation of the smelting process but also influences the technical and economic indicators of the product. At the same time, the sintering process consumes a significant amount of energy, so the sintering equipment and process should be selected rationally. Vertical kiln sintering and rotary kiln sintering are the best choices.
Using hot materials in the furnace is a major direction for energy conservation, which will significantly reduce electricity consumption and should be actively tested. Utilizing hot materials sintered in a rotary kiln can not only save costs but also increase the market competitiveness of products.
Technical indicators and raw material consumption for nickel-iron production
The main raw materials and energy consumption for producing nickel-iron with Ni content of 10%~15%, Si≤7%, C≤4.5%, P≤0.06%, and S≤0.04%~0.35% are as follows: laterite nickel ore 7200 kg/t, lime 2300 kg/t, coke 470 kg/t (hot material), electrode paste 40 kg/t, and electricity consumption 6000 kW•h/t (hot material).
