Ferrochrome waste management : addressing current gaps

Abstract

Various chromium (Cr) compounds, Cr metal and/or Cr-containing alloys are used in modern society. By volume the largest application for Cr is in the production of stainless steel, which owes its corrosion resistance mainly to the inclusion of Cr. Stainless steel is mostly produced from recycled scrap and ferrochrome (FeCr), a relatively crude alloy between Cr and iron (Fe). FeCr is mainly produced by the carbothermic reduction of chromite in submerged arc furnaces (SAFs) and direct current (DC) arc furnaces. Various wastes are generated during FeCr production, depending on the production route used. By reviewing the production routes, three currently applied wastes handling strategies were identified as requiring improvement, which were subsequently investigated. The first waste handling strategy investigated was the leaching of Cr(VI) from bag filter dust (BFD), originating from semi-closed SAF off-gas cleaning (results presented in Chapter 3). Small amounts of Cr(VI) are unintentionally formed during FeCr production. BFD contains the highest concentration of Cr(VI) of all FeCr wastes. Currently, BFD is contacted with water and treated to chemically reduce Cr(VI) before it is disposed in fit-for-purpose slimes dams. A major concern for FeCr producers is the presence of relatively high Cr(VI) concentrations in slimes dams, notwithstanding the treatment prior to disposal. The results presented in this study proved that the currently applied Cr(VI) treatment strategies of FeCr producer (with process water pH ≤ 9) only effectively extract and treat the water-soluble Cr(VI) compounds, which merely represent approximately 31% of the total Cr(VI) present in BFD. Extended extraction time, within the afore-mentioned pH range (pH ≤ 9), proved futile in extracting sparingly and water-insoluble Cr(VI) species, which represented approximately 34 and 35% of the total Cr(VI), respectively. Due to the deficiencies of the current treatment strategies, it is highly likely that sparingly water-soluble Cr(VI) compounds will leach from waste storage facilities (e.g. slimes dams) over time. Therefore, it is critical that improved Cr(VI) treatment strategies be formulated, which should be an important future perspective for FeCr producers and researchers alike. The second waste handling strategy investigated was the flaring of CO-rich off-gas (results presented in Chapter 4). The majority of cleaned CO-rich off-gas (after most of the particles have been removed) generated by closed SAF and DC furnace is flared on stacks. This is done, since the storing of large volumes thereof is problematic due to the toxic and explosive risks associated with it. However, flaring CO-rich off-gas wastes massive quantities of energy. In this study an alternative approach to the use of closed SAF CO-rich off-gas was explored. It is suggested that the thermal energy associated with the combustion of such offgas can at least partially be stored in the form of chemical energy, i.e. production of silicon carbide (SiC) from quartz and anthracite fines (partially classified as waste materials, which are generated on-site). SiC can partially replace conventional reductants used during FeCr production. The influences of quartz and anthracite particle size, treatment temperature and gaseous atmosphere (nitrogen or air) on SiC formation were investigated. A quartzanthracite mixture with 90% of the particles <350.9 μm carbothermically treated at 1600°C resulted in almost complete conversion of quartz to SiC in both nitrogen and air atmospheres. These results indicated significant potential for industrial application of the process. The third waste handling strategy investigated was the recycling of pre-oxidised chromite fines in the oxidative sintered pellet production process (Outotec steel belt sintering) (results presented in Chapter 5). Currently, recycling of such pre-oxidised chromite fines, collected from the pellet sintering off-gas and fines screened out from the sintered pellets, are limited to a maximum of 4 wt% of the total pellet composition since it is believed to adversely affect pellet quality. This limitation has resulted in the accumulation of pre-oxidised fine chromite stockpiles at some FeCr producers. According to literature, pre-oxidized chromite ore requires less energy to metallize if compared to normal chromite. Additionally, pre-oxidized chromite fines significantly improve chromite pre-reduction (solid state reduction). Considering these energy related benefits, the recycling of pre-oxidized fines beyond the current limitation of 4 wt% pellet composition was investigated. The results presented in this study proved that re-cycling of such fines, up to a limit of 32 wt% of the total pellet composition, improved cured pellet compressive and abrasions strengths. In addition, electron microprobe and quantitative X-ray diffraction (XRD) analyses demonstrated that chromite grains present in the pre-oxidized chromite fines at least partially consist of crystalline phases/compounds that will improve the metallurgical efficiency and specific electricity consumption (i.e. MWh/ton FeCr produced) of the smelting process