Evolution of ferronickel particles during the reduction of low-grade saprolitic laterite nickel ore by coal in the temperature range of 900–1250°C with the addition of CaO–CaF₂–H₃BO₃

The method of producing ferronickel at low temperature (1250–1400°C) has been applied since the 1950s at Nippon Yakin Kogyo, Oheyama Works, Japan. Limestone was used as an additive to adjust the slag composition for lowering the slag melting point. The ferronickel product was recovered by means of a magnetic separator from semi-molten slag and metal after water quenching. To increase the efficiency of magnetic separation, a large particle size of ferronickel is desired. Therefore, in this study, the influences of CaO, CaF₂, and H₃BO₃ additives on the evolution of ferronickel particle at ≤1250°C were investigated. The experiments were conducted at 900–1250°C with the addition of CaO, CaF₂, and H₃BO₃. The reduction processes were carried out in a horizontal tube furnace for 2 h under argon atmosphere. At 1250°C, with the CaO addition of 10wt% of the ore weight, ferronickel particles with size of 20 μm were obtained. The ferronickel particle size increased to 165 μm by adding 10wt% CaO and 10wt% CaF₂. The addition of boric acid further increased the ferronickel particle size to 376 μm, as shown by the experiments with the addition of 10wt% CaO, 10wt% CaF₂, and 10wt% H₃BO₃.