Elsevier

Minerals Engineering

Volume 71, February 2015, Pages 205-215
Minerals Engineering

Use of oxidation roasting to control NiO reduction in Ni-bearing limonitic laterite

https://doi.org/10.1016/j.mineng.2014.11.011Get rights and content

Highlights

  • The formation of FeNi was decreased by oxidation roasting of limonitic laterite.

  • The reducibility of laterite was decreased with increasing roasting temperature.

  • Forsterite-type flux is more effective than pyroxene in suppressing NiO reduction.

  • Diffusion of Mg through the Ni source is critical in Ni diffusion in olivine-NiO.

  • Two Ni-olivine formation mechanisms during roasting were proposed.

Abstract

Use of limonitic laterite as an iron source in conventional ironmaking is restricted due to its gangue composition and small particle size. Even direct reduction cannot effectively produce direct reduced iron (DRI) because NiO would be reduced together with iron oxide to form Fe–Ni. A small amount of Ni (about 2 wt.%) in DRI degrades the physical properties of final steel products. The current study investigated how oxidation roasting of limonitic laterite ores affected NiO reduction, with the goal of producing Ni-free DRI and Ni-bearing slag. Ni-bearing slag can be a good secondary Ni resource. Oxidation roasting made NiO inert under H2 reduction at 900 °C by forming Ni-olivine. Optimum roasting temperature was proposed by examining phase transformation of limonitic laterite ores during heating and by FactSage calculation of the equilibrium Ni fraction in Ni-bearing phases. Furthermore, the effect of Mg–silicate forming additives on the control of NiO reducibility was clarified to maximize the suppression of NiO reduction. Among various additives such as MgSiO3, Mg2SiO4 and Fe–Ni smelting slag, Ni-free olivine-typed flux was found to be the most effective form of Ni-olivine because Ni–Mg ion exchange between Ni-bearing phase and Ni-free olivine occurs more readily than other Ni-olivine formation schemes. Finally, the mechanism of Ni-olivine formation during roasting was studied using a diffusion couple test. Calculated diffusivity values of Ni in Mg2SiO4 indicated that the two major routes of Ni-olivine formation while roasting limonitic laterite ore are (1) Ni partitioning within Mg–Ni silicate before crystallization and (2) Ni diffusion from spinel to Ni free olivine after crystallization.

Introduction

Due to world-wide depletion of high-grade lump ores, great efforts have been made to utilize high Al2O3 iron ores. Unfortunately, use of these ores causes critical problems in the blast furnace (BF) ironmaking process. During the sintering process, the low fluidity of primary melt resulted from high Al2O3 content produces a sinter ore with uneven properties, and it also tends to form coarse calcium ferrite phase which degrades bonding strength and causes low reduction disintegration index (Core et al., 2010, Lu et al., 2007). In BF operation, high Al2O3 content in the slag reduces the gas permeability in the upper part of BF, and increased slag volume results in high coke consumption (Lu et al., 2007).

Because laterite ores contain a high proportion of Al2O3, they have been used for Ni recovery to produce FeNi alloy or matte by either a smelting process, a Caron process or a high pressure acid leaching (HPAL) process (Dalvi et al., 2004, King, 2005, Warner et al., 2006). Generally, laterite ore can be classified into several layers depending on the depth of mining. Limonite laterite is the uppermost layer of laterite reserves; it is characterized by high Fe content (40–50 wt.%) and relatively low Ni content (∼1.5 wt.%). Saprolite laterite is in a lower layer, and is rich in Ni (∼3 wt.%), MgO and SiO2 but has a relatively low amount of Fe and Al (Brand et al., 1998, Burkin, 1987, Dalvi et al., 2004). Although limonitic laterite is not suitable for BF operation due to its fine particle size and high Al2O3 content, it might be a good iron resource because it has a high total iron. In addition, due to its relatively low Ni content, limonitic ore is cheaper than saprolitic laterite ore. The problem with the use of limonite laterite is that the slag generated during the conventional process of producing FeNi from Fe-enriched laterite contains high amounts of Fe and Ni, which cannot be upgraded. In addition, <2 mass% of Ni in Fe can degrade the mechanical properties of the final steel product. Thus, selective reduction of Fe from laterite to produce Ni-free iron and Ni-concentrated slag can be a good solution because both Fe and Ni can be extracted from the original ore. Ni-bearing slag can be used as a secondary Ni source because the composition of Fe extracted from limonitic laterite is similar to that extracted from saprolitic laterite. The technology to extract Ni using saprolitic ore has already been established (Burkin, 1987).

Although several researchers have focused on increasing the Ni grade in FeNi alloys produced from laterite reduction, few studies of the recovery of iron from Fe-rich limonitic laterite have been conducted (Takagi and Furui, 1987). The most basic ore with the highest content of Ni and Fe can be metallized easily by gaseous reduction of saprolitic laterite at 500–1100 °C (Utigard and Bergman, 1992). Kawahara et al. (1988) examined the reducibility of both limonitic and saprolitic type laterite by H2 reduction. By bromine–methanol solution leaching method, they accurately measured reduced fraction of NiO and concluded that NiO in saprolitic ore is more difficult to reduce than that in limonitic ore. After Kawahara’s work, Valix and Cheung (2002) reported that reduction after calcination of saprolite makes Ni and Co inert. Recently, Li et al. (2012) confirmed that sodium sulfate effectively increases the reducibility of NiO during reduction roasting, and can be used to increase Ni grade in FeNi alloy. Finally, Rhamdhani et al. (2009) conducted several thermodynamic calculations to understand the phase transformation that occurs during reduction roasting of laterite. Most of the previous studies of laterite reduction attempted to increase the reducibility of NiO by using a pre-treatment like pre-roasting. NiO in the form of silicate is inert during reduction, so these studies attempted to suppress silicate formation by conventional pre-treatment of the ore. However, to suppress the reduction of NiO for the selective reduction of iron oxide, a new pre-treatment method is required because NiO is thermodynamically easier to reduce than is Fe2O3.

The present study considers pre-roasting of limonitic laterite in oxidizing conditions to make NiO-silicate that is free from FeO. Reduction of pre-roasted ore can produce Ni-free DRI and Ni-concentrated slag. Two types of limonitic ores were tested by oxidation roasting followed by gaseous reduction. Phase transformations during oxidation roasting of limonitic laterite were identified and pre-roasting was confirmed to suppress NiO reduction. In addition, a diffusion couple test was conducted to determine the mechanism of Ni-silicate formation during oxidation roasting.

Section snippets

Materials preparation

Two limonitic laterite ores (Table 1) were used in the present study. Both are typical limonitic laterite, but Ore B has some saprolitic characteristics. The raw ores were analyzed using X-ray diffraction (XRD) (Fig. 1). Ore A is mainly composed of hematite, goethite, spinel (trevorite) and SiO2; Ore B shows traces of goethite and hematite, spinel and SiO2. Due to the strong peak of hematite, the peak of Mg–silicate was difficult to identify in both cases. The ores were sieved to <1 mm, and 2 

Phase transformation of limonitic laterite during oxidation roasting – optimization of roasting temperature

Roasting temperature affected XRD patterns of Ore A and Ore B differently (Fig. 2). In Ore A, roasting at TR = 700 °C for 24 h fully decomposed the goethite to hematite; crystallized olivine (Mg2SiO4) started to form only when TR exceeded ∼800 °C. The intensity of the spinel (trevorite) peak increased with increasing TR. Ore B showed a similar pattern to Ore A except that much smaller amount of goethite in Ore A was observed before roasting, and some of pyroxene (MgSiO3) was found after roasting.

Evaluation of Ni diffusivity in olivine

Because fixation of Ni in Ni-free olivine flux is controlled by the diffusion of Ni in olivine, the diffusion coefficient of Ni in olivine should be quantified. At a fixed temperature, the diffusion coefficient of a certain element is generally a function of average concentration of the element in the matrix. The Boltzmann–Matano technique (Geiger and Poirier, 1973) was used to calculate the diffusivity of Ni in olivine based on the concentration profile of Ni (Fig. 12, Fig. 13) by assuming

Conclusions

The effect of oxidation roasting of limonitic laterite ores on the suppression of NiO reduction was investigated for the production of Ni-free Fe and Ni-bearing slag from these ores. The following conclusions were obtained:

  • (1)

    The formation of FeNi was decreased by oxidation roasting of limonitic laterite ores at roasting temperatures TR > 815 °C followed by gaseous reduction at 900 °C. The MgO/SiO2 ratio in the ore was the critical factor for formation of Ni-olivine formation because when this ratio

Acknowledgments

The authors are grateful for valuable advice and financial support provided by POSCO.

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