Analysis on the current status of research on red mud iron separation technology

Red mud is the largest waste generated during the alumina production process and the largest source of pollution in alumina plants. Due to different production methods and bauxite grades, approximately 0.5 to 2.0 tons of red mud are produced for every 1 ton of alumina produced. For sintering plants using nepheline as raw material, the amount of red mud produced for every 1 ton of alumina produced is as much as 5.5~7.5t, each ton of red mud also comes with 3~4m3 of alkali-containing waste liquid. With the development of the aluminum industry and the reduction of aluminum ore grades, the amount of red mud will become larger and larger. Red mud must be reprocessed and utilized in order to turn waste into treasure and reduce pollution [1].
It is estimated that the annual red mud production in the world is about 40 million tons, and my country’s annual red mud production is about 1 million to 1.5 million tons. At present, most alumina plants at home and abroad transport red mud to stockpiles, build dams for wet storage, and rely on natural sedimentation and separation to return the solution for reuse.

Such a large amount of red mud cannot be effectively and fully utilized, and the social and economic problems caused by it are quite complex:

① The construction of a red mud dump will occupy a large area of land, which will increase investment in infrastructure. According to Russian information, this alone increases the cost of alumina production by 2 to 3 rubles per ton;

② Red mud contains alkali and a small amount of radioactive substances, which are stored for a long time and cause dust to fly after drying, seriously polluting the atmosphere and Environment; ③ Due to wind and rain, red mud flows into rivers and lakes, causing siltation, poisoning water quality-assuranc, and directly affecting agricultural and fishery production. As society attaches great importance to environmental protection, there is an urgent need for the alumina industry to achieve harmless or zero emissions, to recycle red mud, and to study the comprehensive recycling of valuable components, which is of great practical significance. Topic[2].

1. Properties of red mud and occurrence status of iron

(1) Basic properties of red mud

Red mud is an insoluble residue, mainly composed of fine-grained mud and coarse-grained sand. Its chemical composition varies depending on the bauxite origin and alumina production method. Most of the red mud discharged from factory equipment is a slurry with a solid weight concentration of about 20% to 30%. The mother liquor is an alkaline liquid mainly composed of sodium aluminate (Na2O·3A12O3·5SiO2·nH2O) caustic soda, with a pH of 12 to 13. The solid part in red mud is hematite (32% ~ 48%), sodium aluminosilicate (32% ~ 50%), rutile (5% ~ 8%), emery (about 5%), quartz (about 4%) %) and a mixture of fine particles such as 3d-printing titanium magnetite (about 2%).
After testing, the phase composition of a certain Bayer process red mud is: sodalite-type hydrous sodium aluminosilicate: Na2O·Al2O3·1.7SiO·2.4H20; goethite: FeOOH; hematite: Fe2O3; quartz: SiO2 . Its chemical composition is shown in Table 1.

(2) Storage status of iron in red mud

Iron in red mud is mainly Fe2O3, with a small amount of FeO. The ratio of the former to the latter is almost 9:1. This is a colloidal Fe(OH)3 precipitate formed after the oxidation and hydrolysis of pyrite (FeS2) associated with natural bauxite; Fe(OH)2 colloid is unstable under strong alkalinity and heating conditions and has transformation This is the trend of goethite FeOOH. Goethite and colloidal Fe(OH)3 may coexist in fresh red mud, while Fe is mainly dispersed in the red mud as hematite. After stacking and drying, part of Fe2O3 will be converted into Iron complex silicates [3].

2. Current research status of iron selection technology from red mud

Research on the process of selecting iron from red mud has been carried out in foreign countries such as Japan, the United States, and Germany in the 1970s. In view of the high iron oxide content of Bayer red mud, the United States has long proposed a method of producing iron from red mud and applied for a patent. This patent proposes to use reduction roasting to treat red mud, control the moisture content of the red mud below 30%, and then evaporate naturally. The dry red mud is fluidized and roasted in a reducing atmosphere, and the iron oxide is converted into magnetite, which is then magnetically separated to make high-quality Pure metallurgical lump. In addition, the American Mcdowell Wellman Engineering Company has developed a method of using a disc sintering machine to process red mud to produce iron. This method sinters the red mud and coal into briquettes and then smelts them in an electric furnace. The iron recovery rate is as high as 98% to 99%, 1 ton Pig iron consumes 5 to 8 tons of red mud [4].
The Japanese patent proposes reduction roasting treatment of red mud, converting iron oxide into magnetite, and recovering alumina from the rest [5]. The red mud is first filtered to a moisture content of 30%, then evaporated naturally, and then roasted in a fluidized bed. In the fluidized bed, the material is reduced with reducing gas to turn the iron oxide into magnetized iron. Magnetic substances are magnetically separated and concentrated into high-purity metallurgical briquettes. It was found in the test that under strictly controlled conditions, the reduction reaction of roasted red mud can proceed until the hematite in the red mud is completely converted into sponge iron, and then magnetic separation is performed. After the sponge iron briquettes are obtained, they can be directly used in electric furnaces to make steel, which is simpler and more economical than using magnetite.
Russia, Hungary, Canada, Spain and other countries have also conducted a lot of research on the properties of red mud and methods of selecting iron from it. Hungarian scholars proposed a process for treating red mud slag by chlorination roasting. This process uses a two-stage reaction of reduction and oxidation to obtain slag with high TiO2 content. Al2O3 and V2O5 are also enriched in the slag [6].
my country’s research on red mud iron separation started relatively late, starting in the late 1980s. The Guangxi Metallurgical Research Institute conducted research on direct reduction iron smelting using Bayer process red mud from Pingguo bauxite as raw material and Guangxi coal as the reducing agent [7]. This process is to mix Bayer process red mud and coal to make briquettes, dry them and perform reduction roasting. Finally, magnetic separation can produce high-grade sponge iron.
Liu Wanchao et al. [8] used Bayer process red mud as raw material, recovered iron through direct reduction roasting and magnetic separation, and the magnetic separation residue was used to produce building materials. The iron oxide content in the red mud is 27.93%, and hematite (brown) iron ore is the main presence. After discussing the effects of factors such as roasting temperature, roasting time, carbon powder and additive dosage on the experimental results, ideal roasting conditions were obtained. Under these conditions, the total iron content in the concentrate obtained after grinding and magnetic separation is 89.05%, the metallization rate is 96.98%, and the recovery rate is 81.40%. It can be used as sponge iron. The magnetic separation residue is mixed with saltpeter lime and subjected to pressure forming and steam curing. The compressive strength of the specimen can reach 24.10MPa, and can be used to produce steam-cured bricks and other building materials. The main mineral composition of the residue was transformed from nepheline to gehlenite before and after steam curing. Thermodynamic analysis proved the possibility of this reaction occurring under experimental conditions.
Gao Jianyang [9] used red mud, added self-made additives, and adopted a new process flow of coal-based direct reduction roasting-slag iron magnetic separation-cold solidification molding to study the metallic iron crystals in the Bayer process of red mud coal-based direct reduction. Grain growth characteristics, and focused on the impact of additive types and roasting conditions on the grain growth characteristics of metallic iron, to produce high-quality sponge iron. The metallization rate of the product is 92.9%, the iron grade is 93.7%, and the iron recovery The rate is 94.42%, which can be used as semi-steel raw material for electric furnace steelmaking, opening up a new way for the comprehensive utilization of red mud.
In view of the complex components and fine particle size of the Bayer process red mud of Pingguo Aluminum Company, Guan Jianhong used an SLon-type vertical ring pulsating high-gradient magnetic separator to recover iron from the red mud. After small-scale tests and semi-industrial tests , an iron concentrate containing TFe54.70% was obtained, with a recovery rate of 35.36%. The obtained qualified iron concentrate can be used as raw material for blast furnace ironmaking, which provides a way for industrial implementation of the recovery of iron from red mud.
Liao Chunfa et al. [11] used coke as the reducing agent and determined the optimal parameters of the roasting process: the ratio of red mud: coke is 80:15; the reduction roasting temperature is 1150°C; the roasting time is 1.5h; the soft magnetic mold core material field strength of the magnetic separation is 0.9kt. It can enrich 56.5% of the iron concentrate; its primary recovery rate reaches 63.3%, and the remaining iron is recovered after acid leaching.
Judging from the experimental results, the rare metals are further enriched in the separation slag, and the rare earths are effectively separated. Afterwards, acid leaching is used to separate rare metals from the separation slag, and the material processing volume will be greatly reduced.
Jiang Pingguo et al. [12] used wet pulsation high gradient magnetic separation to recover iron minerals in Bayer process red mud. The process involves roasting bauxite containing 13% ferric oxide at low temperature and then dissolving it through the Bayer method. The red mud is magnetically separated. The iron concentrate after magnetic separation can be used as a raw material for blast furnace ironmaking.
Gong Lianchun [13] Yang invented a method of directly using red mud to prepare iron oxide red, which specifically involves the field of pigment production. The process is shown in Figure 1.

Li Liangxing [14] et al. studied the impact on the recovery rate and grade of iron when red mud was reduced and roasted by adding sodium carbonate, using coke as the reducing agent. The optimal conditions obtained from the experiment are: the mass ratio of red mud, sodium carbonate and coke is 5:5:1; the reduction roasting temperature is 1000°C; the roasting time is 60 minutes.
After reduction roasting, the red mud is magnetically separated to obtain iron concentrate. The magnetically separated concentrate contains very few impurities, mainly elemental iron. The iron recovery rate can reach 80%, and the grade is above 70%.

3. Conclusion

Since the development of the alumina industry, the processing and comprehensive utilization of red mud has been one of the problems that the world urgently needs to solve. The recovery of iron in red mud is an important aspect of the comprehensive utilization of red mud.
(1) Studying the physical phases of red mud proves that iron mainly exists in the form of hematite and goethite in red mud, with the former accounting for more than 90%. At the same time, each mineral mostly exists in the form of Fe, Al, and Si cements, with fine grains and extremely incomplete crystallization, which causes great difficulties in the sorting and extraction of iron.
(2) From a thermodynamic and kinetic perspective, it is completely feasible to reduce iron in red mud. Reduction roasting is carried out at around 50 to 1250°C to complete the reforming of the crystal structure and separate the finely distributed iron and aluminum.
(3) At the same time, due to changes in the alumina raw materials and production process conditions of the iron-containing minerals in red mud, the proportions of the main iron-containing minerals goethite and hematite also change. In the phase composition of red mud, the hematite content can fluctuate from 19.0% to 33.5%, while the goethite content also fluctuates from 16.0% to 3.9%. Goethite is cryptocrystalline or microcrystalline, mostly related to Other minerals are cemented, so the degree of goethite conversion affects iron recovery.
(4) The technical difficulty of recovering iron from high-speed railway red mud is not high. The main problem is to consider the chemical composition of red mud, the composition of raw bauxite and the production process of alumina. According to the characteristics of different red mud, It is necessary to have a relatively optimized extraction process to reduce resource waste and energy consumption, reduce recycling costs, and truly achieve sustainable economic development. In terms of environmental protection and economy, the comprehensive utilization of red mud has achieved a win-win situation.