Reduction behavior of SnO2 in the tin-bearing iron concentrates under CO–CO2 atmosphere. Part I: Effect of magnetite
Graphical abstract
Introduction
Tin-bearing iron ore, with the total reserve more than 0.5 billion tons, is recognized as one of typically complex iron ore resources in China [1]. The tin content remained in the iron concentrates after multistage beneficiation is still in excess of 0.08 wt.%, so they can't be used as iron-making burdens directly [2], [3]. A selective reduction volatilization (SRV) process of tin recovery and pellet preparation for blast furnace iron-making from the tin-bearing iron concentrates, namely a coal-based rotary kiln direct reduction process, has been developed and successfully performed in the pilot-scale and semi-industrialization tests by the authors' group. It is especially noteworthy that the volatilization ratio of Sn has obvious fluctuation as the variation of roasting parameters during the SRV process. It is found that the residual tin content in the roasted products varies between 0.06–0.30 wt.%. In addition, previous studies have suggested that the tin-bearing iron concentrates are mainly composed of magnetite, quartz and calcite besides cassiterite [2], [3]. Therefore, it is necessary to determine the effect of main mineral components on the reduction behavior of cassiterite (SnO2) in order to more effectively utilize these tin-bearing concentrates.
Cassiterite is the only economic value tin-bearing mineral in the earth crust [4], [5]. The predominant impurity elements in cassiterite concentrates include Si, Ca and Fe for the skarn type and quartz vein type tin ores. Associated gangue minerals include quartz, calcite and magnetite, which can't be separated perfectly by beneficiation combined methods, such as gravity concentration and froth flotation [6]. Pyrometallurgical reduction smelting process is an effective route to separate tin from the gangue minerals. In the past few decades, numerous researches on the reduction of stannic oxide (cassiterite) by carbonaceous reductants were carried out [4], [5]. The results indicate that SnO2 is mainly reduced by the gaseous intermediates of CO and H2. And the overall reduction rate of SnO2 is controlled by the gasification of carbon (C + CO2 = CO), that is Boudouard reaction [7], [8]. Admixtures of K2CO3, SiO2, Al2O3, CaO or metallic tin can accelerate the reduction of cassiterite by graphite. And all these admixtures decrease the activation energy for the reduction reaction due to their catalytic effects on the Boudouard reaction [6], [9].
It is reported that Fe2O3 and SnO2 is reduced synchronously according to Fe2O3 → Fe3O4 → FeO → Fe and SnO2 → SnO → Sn [10], respectively. The reduction of iron oxides, as reported, consumes more reducing agent, and the formation of tin-iron alloy, called “hardhead”, results in more than 10% tin-losing during the reduction smelting process [4], [5].
Previous studies suggested that Fe–Sn spinel (Fe3-xSnxO4) was found in tin-smelting slag under reduction conditions at temperatures above 1000 °C [11]. However, it is uncertain whether Fe–Sn spinel is generated and affects the volatilization of Sn or not during the SRV process of the tin-bearing concentrates. In addition, Fe3-xSnxO4 has attracted much interest as a kind of functional materials widely used as electrical transformer cores, magnetic memory devices, ferrimagnetic materials and heterogeneous catalysts [12], [13], [14], [15]. In the laboratory researches, Sn-doped ferrite samples have been prepared by means of precipitation exchange method (solid phase reaction in the solution) [15], [16], [17].
Due to the lack of fundamental information on the possible solid state reactions between SnO2 and iron oxides, the major objectives of this research was 1) to investigate the effect of magnetite on the volatilization behavior of SnO2, 2) to determine the phase transformation of SnO2 and Fe3O4 and 3) to ascertain whether Fe–Sn spinel is produced or not under CO–CO2 atmosphere by using XRF, XRD, SEM–EDS, etc.
Section snippets
Tin-bearing iron concentrates
The tin-bearing iron concentrates used in this study were taken from Inner Mongolia Autonomous Region, China. The mass percentage of the samples with a granularity below 0.075 mm was found as 91.2 wt.%. The main chemical compositions given in Table 1 were determined by X-ray fluoroscopy (XRF, Axios mAX, Holand PANalytical Co., Ltd). The XRD (XRD, D/max 2550PC, Japan Rigaku Co., Ltd) pattern of the sample is presented in Fig. 1.
It was seen from Table 1 that the total iron grade (TFe) of the sample
Effect of the main parameters on the volatilization ratio of Sn
The effects of CO content, reduction temperature, reduction time and the tin content of the sample on the Sn volatilization ratio were examined. CO content refers to the CO volume concentration in the CO–CO2 mixed gas (i.e., CO / (CO + CO2)).
Conclusions
- (1)
The tin volatilization ratio was obviously affected not only by reduction temperature, time and CO content, but also by the tin content in the tin-bearing iron concentrates. The volatilization of Sn and the formation of Fe–Sn spinel happened simultaneously during the reduction roasting process. At higher CO content, the decrease of tin volatilization ratio was mainly attributed to the generation of metallic tin.
- (2)
Fe–Sn spinel was easily formed even if the CO content was low, which was the main
Acknowledgments
The authors would express their heartful thanks to National Natural Science Foundation of China (No.51574283), Co-Innovation Center for Clean and Efficient Utilization of Strategic Metal Mineral Resources, Hunan Provincial Innovation Foundation for Postgraduate (CX2015B054) and the Teachers' Research Fund of Central South University (2013JSJJ028).
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Both corresponding authors contribute equally to this paper.