Mytilineos SA-Metallurgy Business Unit, Alumina and Aluminium plant (former Aluminium of Greece), is the only alumina refinery in Greece, processing each year about 1.8 Mt of bauxite ore (originating mainly from Greek bauxite mines), for the production of more than 800 kt of alumina and 750 ktons of BR. In 2006, the plant installed its first high-pressure filter press in Europe which produced a relatively dry BR. During the period 2006-2011 a total of 4 filter presses were installed and since 2012 all BR produced is filter-pressed and stored as a “dry” (moisture < 26%) by-product in an appropriate industrial landfill (Figure 1).

The proposed bauxite residue treatment comprises four stages as shown schematically in Figure 2. The first stage is the residue drying stage, necessitated by the high remaining moisture content of residues (up 25% w/w), even when the best available upstream technology (filter press drying) is used for bauxite residue handling. This stage could occur in a double skin rotary kiln, using the heat content of the EAF hot off-gases or using waste heat from the alumina refinery itself (i.e. spent steam form the Bayer process). The second stage of the process focuses on EAF feed material preparation: dried bauxite residue, coke fines and appropriate fluxes are mixed in order to adjust the properties of the produced slag. This mixture is fed into the EAF, where raw materials undergo reductive smelting and are transformed into three distinct fluid phases: liquid slag, liquid pig iron and off-gases. The off-gases after heat exchange in the bauxite residue dryer are sent to a bag-house unit to remove dust particles prior to releasing them to the atmosphere. The dust collected is recycled in the feed material. The liquid pig iron and slag phases are separated by sequential pouring (or by continuous tapping) and the liquid slag is left to cool.

(a)

(b)

Figure 1. Filter press in operation and BR discharge (a); site of BR disposal in the plant (b).

Iron recovery from BR has been thoroughly investigated, and the developed technologies have been reviewed in several publications [9] [10] [11] [12] . The pyrometallurgical recovery of iron from the BR valorises approximately half of the Greek BR stream and at the same time alters the properties of the remaining material (slag) in such a way that it can be further processed, resulting in a zero solid/liquid waste process. Solid state reduction roasting [13] [14] [15] and reductive smelting [11] [16] are perhaps the most promising ones for producing either high-grade magnetite (Fe₃O₄) or metallic Fe. Through carbothermic reduction smelting using an electric arc furnace (EAF), the iron content of BR can be extracted as pig iron and sold as steel scrap substitute (price 300-EUR/t) to the secondary steel industry. BR trace elements like V, Cr, and Ni, which are of great value to the iron/steel industry, are collected in the produced melting iron phase. The EAF carbothermic smelting technology was developed in the EC-funded ENEXAL project [17] (Figure 3). The pig iron produced was suitable for the secondary steel industry as a 15 wt% scrap substitute in EAF processing. Larger BR pig iron utilization as scrap substitute would be disruptive to steel mill operations, as the minor elements of the BR pig iron (Cr, V, S, P, …) would produce steel outside of the required specifications for typical applications like steel rebars. However, given the size of the iron and steel market, the secondary steel industry could absorb all the BR pig iron even at low-scrap substitute rates. The overall exergy efficiency of the new bauxite exploitation schema increases from 3% in the conventional Bayer Process to 6 to 9% depending on the method used to produce the electricity needed to power the Electric Arc Furnace. Additionally, as the novel process enables the single step co-production of two highly valuable by-products (pig iron and pyrometallurgical slag), it has the potential to significantly increase the versatility and profit margin of the alumina producing industry.

As red mud is generally characterized by high iron content many attempts have been made to produce pig iron from red mud, but so far no economically viable solution has been found. The extremely fine particles of red mud require agglomeration prior to feeding in conventional reactors; their high alkaline nature is unsuitable for blast furnace reductive smelting and the low, when compared to iron ores, content in iron oxides makes the production of pig iron a cost ineffective process [18] .

The treatment has the capability of processing finely sized materials, notably below 1 mm in particle size (dust like), without any dusty material loss in the off-gas stream. As seen in Figure 4, the feeder can deliver the dusty raw materials directly into the “arc zones” of each of its three electrodes, where flash smelting takes place. This feeder is below the off-gas suction tube thereby preventing loss of material, and it feeds the reactor with small batches at appropriate time intervals in order to allow enough time for ventilation of the off-gases produced from the previous batch. The operation of the is controlled by a PLC control system, which, based on system-dependent programmable parameters predicted accurately by a proprietary thermodynamic model, can control impedance of the electric arc produced by the electrodes through the control of the positioning of the electrodes within the furnace influencing the energy input, the arc stability, the solid charge melting pattern and the electrode consumption. Therefore, this EAF technology is ideal for processing the dust-like red mud without any pretreatment or substantial energy losses, thus providing the proposed process with a significant industrial advantage.

Red mud chemical analysis used in the present work is given in Table 1.

Using carbon as a reducing agent, the red mud can undergo reductive smelting in the EAF producing pig iron and a slag. The pig iron produced in this way would amount to approximately 35% of the initial red mud charge, therefore pig iron production alone would not solve the industry’s waste disposal, as 65% of the red mud would still have to be disposed as EAF slag, while the expected turnover from selling such amounts of pig iron would not suffice to make for an economically viable process.

In order to design the process, the following operational parameters have to be established: melt temperature, required amount of coke and required amount of fluxes. As shown in the Ellingham diagram presented in Figure 5, the oxidation of carbon to CO_(g) can reduce H₂O, Fe₂O₃, Na₂O, K₂O at temperatures higher than 1000˚C, it can reduce SiO₂ and TiO₂ at temperatures higher than 1700˚C, MgO at temperatures higher than 1800˚C, while Al₂O₃ and CaO cannot be reduced at temperatures lower than 2000˚C. As the melting point of pure iron is 1537˚C the furnace operational temperature should be in the range of 1500˚C < T < 1700˚C in order to effectively reduce the iron in the red mud while avoiding silicon and titanium reductions. The furnace operational temperature for this study is therefore set at 1600˚C - 1700˚C and the reactions that are thermodynamically expected to take place and consume carbon are presented below:

Fe 2 O 3 + 3C = 2Fe ( l ) + CO ( g )         Δ G ( 16 00 ˚ C ) = − 483.493   kJ (1)

H 2 O ( g ) + C = H 2 ( g ) + CO ( g )         Δ G ( 16 00 ˚ C ) = − 197.551   kJ (2)

Na 2 O + C = 2Na ( g ) + CO ( g )         Δ G ( 16 00 ˚ C ) = − 86.499   kJ (3)

K 2 O + C = 2K ( g ) + CO ( g )         Δ G ( 16 00 ˚ C ) = − 260.111   kJ (4)

The evolution of Na_(g) and K_(g) according to Equations (3) and (4) are highly unlikely as these oxides are in reality part of aluminosilicates complexes which are not as easily reduced. For instance, if sodium in red mud is found as nepheline (a logical assumption as nepheline is a part of cancrinite), then its reduction is thermodynamically unlikely.

2NaAlSiO 4 + C = 2Na ( g ) + Al 2 SiO 5 + SiO 2 + CO ( g )         Δ G ( 16 00 ˚ C ) = 38.671   kJ (5)

Therefore, according to Equations (1) and (2) and the red mud chemical analysis given in Table 1 the amount of carbon required to reduce all iron and hydrogen content found in 1000 kg of red mud is 172.24 kg. To account for all possible side reactions an excess of 10% carbon will be used in the process (189.46 kg/1000kg red mud).

MYTILINEOS carried out the first bauxite residue smelting campaign in its pilot pyrometallurgical plant equipped with a 1 MVA electric arc furnace (EAF). The smelting tests utilized Bauxite Residue (BR), metallurgical coke and lime as raw materials, with the aim of producing cast iron and slag suitable for the creation of the “MudFire” construction materials. The recipe used adds relative to BR 30%wt and 18%wt Coke. At this loading, the C:Fe molar ratio is set at 2.2 (the stoichiometric demand is 1.5 for the reduction of Fe₂O₃ only, the excess being added to account for the other reduction taking place, including Na, Si, V, H₂, Cr, Ni, etc.) and the molar ratio of CaO to Al₂O₃ is set to 3.4. In each test 300 kg of dry KB, 90 kg of CaO, 80 kg of starter slag (from previous test) and 54 kg of coke are melted at 1600˚C - 1700˚C and 90 kg of pig iron and 300 kg of slag are produced. Five tests were performed during this first campaign and the test results are detailed in the tables and graphs below. Average furnace energy consumption was 2233 kWh/t BR. A total of 487 kg of cast iron and 1259 kg of slag were produced. (Table 2 and Table 3)