1. Introduction
Red mud is discharged as waste during the production of alumina from bauxite. In the alumina process, bauxite ore dissolves in concentrated sodium hydroxide solution under heat and pressure. The resulting sodium aluminate liquor undergoes further processing to produce alumina with a large quantity of un-dissolved solid waste casted away. Since the solid waste contains iron oxides that contribute the characteristic brick red color, it is called ‘red mud’. Generally, the content of Fe2O3 in red mud is about 10–30%, even up to 50–60% in some cases [1]. Averagely, 0.3–2.5 tons of red mud may be generated after producing 1 ton of alumina depending on the grade of ore used [2]. Over 200 Mt of red mud have been stored up in China in the past ten years [3], still about 30 Mt of red mud are discharged annually [4].
Many approaches were reported to utilize red mud as a resource for environment friendly application. In most of these approaches, the separation of Fe2O3 from red mud is the key technology. Both of the separated parts have useful applications. For example, the iron-riched part could be used as iron source [5], and the other part might be used as refractory materials [6] but the content of Fe2O3 in refractory products should be less than 3%. Several methods have been reported to remove Fe2O3 from red mud, but most of them are problematic in actual utilization. It was reported that red mud was reduced by coal to get nuggets at temperature of 1400 °C [7]. High gradient superconducting magnet has been used to directly separate iron mineral from red mud, however, the magnetic intensity for separation needed to be more than 4 T due to the weakly magnetic substance of Fe2O3[8].
Transforming hematite in red mud into magnetite before separation is a good idea. It was reported that the transformation of hematite into magnetite could be rapidly realized under H2-rich hydrothermal conditions [9], or by roasting under reductive gas atmosphere, such as H2[10] and [11], methane or natural gas [12]. When Mayoral's group tried to clarify the mechanism of slagging in coal combustion, they found that pyrite could react with hematite (α-Fe2O3) under anaerobic conditions and magnetite (Fe3O4) was found in the products [13]. Pyrite (iron-pyrite, FeS2) is the most common substance in sulfide minerals and commercially used for the production of sulfur dioxide providing applications in the paper industry and in the manufacture of sulfuric acid. It is worthy to note that ferrous ion Fe2+ and the disulfide S22− moieties occur in pyrite simultaneously, which makes pyrite possess reductive property.
Based on these reports, a novel method to remove hematite from red mud has been proposed in this study. The flow-sheet diagram is shown in Fig. 1, hematite in red mud being transformed into magnetite with the help of pyrite, and then the resulting magnetite could be collected by magnetic separation. The treatment conditions, such as the mole ratio of pyrite to hematite, the roasting temperature and time, as well as the environmental concern of SO2 have been investigated in this study.