At present, there are three main treatment processes for nickel oxide ore globally: the pyrometallurgical process, the hydrometallurgical process, and the combined pyro-hydro process. The pyrometallurgical method can be further divided into two types based on the final product: one that produces ferronickel through reduction smelting, and another that produces nickel sulfide by sulfurizing smelting. On the other hand, the hydrometallurgical process is typically classified into ammonia leaching and acid leaching methods, depending on the leaching reagent used. The combined pyro-hydro process involves a pre-reduction roasting step followed by physical or chemical beneficiation to extract valuable components from the ore.
**Pyrometallurgical Process**
**(1) Reduction Smelting for Ferronickel Production**
The most widely used fire process globally is the production of ferronickel via reduction smelting. At least 14 plants around the world use this method, with an annual output of approximately 250,000 tons of nickel-iron (including nickel). Most of these operations take place in electric furnaces, while a few smaller facilities use blast furnaces.
Electric furnace smelting is highly flexible, capable of processing various types of nickel oxide ores, regardless of their particle size. This makes it suitable for both large-scale and small-scale operations. However, its major drawback is high energy consumption. In contrast, blast furnace smelting offers lower investment and energy costs, making it ideal for regions with limited power supply or low-grade ores. However, it has stricter requirements regarding magnesium content and cannot handle fine ores effectively.
**(2) Sulfurizing Smelting for Nickel Sulfide Production**
This method was among the earliest techniques used to treat nickel oxide ore, particularly in the 1920s and 1930s using blast furnaces. Although it shares similar disadvantages with ferronickel production, such as high energy demand, many modern facilities have shifted to electric furnace technology. Several large-scale plants in Indonesia and New Caledonia now produce over 40,000 tons of nickel annually from oxide ores.
Sulfur is commonly used as a vulcanizing agent, though it can be expensive and requires specific infrastructure for melting and spraying. In some Indonesian and New Caledonian plants, natural sulfur from volcanic craters is utilized, which helps reduce costs. The resulting nickel sulfide is versatile, allowing for further processing into different forms like nickel powder, pellets, or even anode plates for electrolytic refining.
**Hydrometallurgical Process**
**(1) Ammonia Leaching (Caron Method)**
Ammonia leaching was first introduced in the 1940s and became prominent in the Cuban Nicaro plant. However, it has limitations, such as low recovery rates for nickel (75–80%) and cobalt (40–50%), and is not suitable for ores containing copper, cobalt, or high levels of silicon and magnesium. As a result, only four plants worldwide still use this method, mostly built before the 1970s.
**(2) Acid Leaching**
Acid leaching involves high-pressure and high-temperature conditions (250–270°C, 4–5 MPa) where dilute sulfuric acid dissolves valuable metals like nickel and cobalt. Impurities such as iron and aluminum are removed through hydrolysis, and the remaining nickel and cobalt are recovered via solvent extraction or precipitation. This method is more efficient than ammonia leaching, especially when cobalt content is high. However, it is still not widely adopted due to complex equipment and operational challenges. Only three plants currently use this method, and its development remains in early stages.
**Combined Pyro-Hydro Process**
The Oyama Smelter in Japan is the only facility using a combined fire-wet process. It involves mixing the ore with coal, drying, calcining, and then beneficiating to produce a nickel-iron alloy. This process is cost-effective, with 85% of energy coming from coal, significantly reducing operating costs compared to electric furnace smelting. Despite its advantages, the process is still not fully stable, and production remains relatively low.
In summary, while the pyrometallurgical process is efficient, it suffers from high energy consumption, limiting its use to high-grade ores. The hydrometallurgical approach, although more complex, is better suited for low-grade ores. Future developments will likely focus on reducing energy use in pyrometallurgy and improving the efficiency of wet processes to meet growing demand for nickel.
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