Red mud is the biggest waste generated during the production of aluminum oxide, alumina plant is the biggest source of pollution. Due to the difference between the production method and the grade of bauxite, about 0.5 to 2.0 tons of red mud is produced for each 1t of alumina produced. The sintering process plant with nepheline as raw material produces as much red mud per ton of alumina. 5.5 to 7.5 tons, 3 to 4 m3 of alkali-containing waste liquid per ton of red mud. With the development of the aluminum industry and the reduction of the grade of aluminum ore, the amount of red mud will become larger and larger, and the red mud must be reused in order to reduce waste and reduce pollution [1] .
It is estimated that the annual output of red mud in the world is about 40 million tons, and the annual output of red mud in China is about 1 million to 1.5 million tons. At present, most of the alumina plants at home and abroad will transfer the red mud to the yard, damming the wet storage method, and returning to the solution by natural sedimentation. Such a large amount of red mud has not been effectively and fully utilized, and the social and economic problems it brings are quite complicated: 1 The construction of a red mud yard will occupy a large area of ​​land and increase infrastructure investment. According to Russian data, this alone increases the production cost of alumina by 2 to 3 rubles per ton; 2 the red mud contains alkali and a small amount of radioactive material, which is stored for a long time. After drying, it causes dust to fly, seriously polluting the atmosphere and Environment; 3 due to wind and rain, causing red mud to flow to rivers and lakes, causing siltation, poisoning water quality, directly affecting agricultural and fishery production. With the emphasis on environmental protection work, the urgent need for the alumina industry to achieve harmless emissions or zero emissions, the red mud resources, and the comprehensive recycling of valuable components, is an important practical significance. Question [2] .
First, the nature of red mud and the occurrence of iron
(1) Basic nature of red mud
Red mud is an insoluble residue consisting mainly of fine-grained mud and coarse-grained sand. Its chemical composition varies depending on the bauxite origin and alumina production methods. Most of the red mud discharged from the plant equipment, such as solid weight concentration, is a slurry of about 20% to 30%. The mother liquor is an alkaline liquid mainly composed of sodium aluminate (Na 2 O·3A1 2 O 3 ·5SiO 2 ·nH 2 O) caustic soda, and has a pH of 12-13. The solid portion of the red mud is hematite (32 to 48%), aluminum silicate (32% -50%), rutile (5% to 8%), silicon carbide (about 5%), quartz ( about 4%) and titanium magnetite (about 2%) mixture of fine particles and the like.
After testing, the phase composition of a Bayer red mud is: sodalite-type aqueous sodium aluminosilicate: Na 2 O·Al 2 O 3 ·1.7SiO·2.4H 2 0; goethite: FeOOH; Mine: Fe 2 O 3 ; Quartz: SiO 2 . Its chemical composition is shown in Table 1.
(2) Storage status of iron in red mud
Iron is mainly composed of Fe 2 O 3 in red mud and contains 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 by oxidative hydrolysis of pyrite (FeS 2 ) associated with natural bauxite; the Fe(OH) 2 colloid is unstable under strong alkalinity and heating conditions, The trend of conversion to goethite FeOOH, in the fresh red mud, goethite and colloidal Fe(OH) 3 may coexist, while Fe is mainly dispersed in hematite by hematite. After stacking and drying, part of Fe 2 O 3 will be converted into iron composite silicate [3] .
Second, the research status of the process of selecting iron in red mud
Regarding the research on the iron selection process from red mud, foreign countries such as Japan, the United States, and Germany have already started in the 1970s. In view of the high content of iron oxide in Bayer red mud, the United States has long proposed a method of producing iron using red mud and applied for a patent. The patent proposes to treat red mud by reduction roasting, control the water content of red mud to below 30%, and then naturally evaporate. The dry red mud is fluidized and calcined in a reducing atmosphere, and the iron oxide is converted into magnetite. Pure metallurgical mass. In addition, McDowell Wellman Engineering Company of the United States has developed a method for producing iron by treating a red mud with a disc sintering machine. The method is to sinter the red mud and coal pellets and then smelt them in an electric furnace. The recovery rate of iron is as high as 98% to 99%, 1t. Pig iron consumes 5 to 8 tons of red mud [4] .
The Japanese patent proposes a reduction roasting treatment of red mud, conversion of iron oxide into magnetite, and the rest of the recovery of alumina [5] . The red mud is first filtered to a moisture content of 30%, then naturally evaporated, and then calcined in a fluidized bed. In the fluidized bed, the material is reduced with a reducing gas to convert the iron oxide into magnetized iron. The magnetic substance is magnetically separated and concentrated to form a high-purity metallurgical mass. It was found in the test that under the strict control conditions, the reduction reaction of the roasted red mud can be carried out until the hematite in the red mud is completely converted into sponge iron, and then magnetic separation is carried out. After obtaining the sponge iron group, it can be directly used for electric steelmaking, which is simpler and more economical than using magnetite.
Russia, Hungary, Canada, Spain and other countries have also carried out a lot of research work on the nature of red mud and the method of selecting iron from it. Hungarian scholars proposed a process of chlorination roasting to treat red mud slag. The process obtained a two-stage reaction of reduction-oxidation to obtain slag with high TiO 2 content, and Al 2 O 3 and V 2 O 5 were also enriched in slag [ 6] .
China's research on red mud iron selection started late and only began in the late 1980s. The Guangxi Metallurgical Research Institute used the Bayer method red mud as the raw material and the Guangxi coal as the reducing agent for the direct reduction of ironmaking [7] . The process comprises mixing Bayer red mud and coal into a dough, drying and then performing reduction and roasting, and finally magnetic separation can produce high-grade sponge iron.
Liu Wanchao et al. [8] used Bayer's red mud as raw material to recover iron by 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 the red (brown) iron ore is mainly present. Based on the effects of calcination temperature, calcination time, carbon powder and additive dosage on the experimental results, the ideal calcination conditions were obtained. Under this condition, the concentrate obtained after fine magnetic separation has a total iron content of 89.05%, a metallization rate of 96.98%, and a recovery rate of 81.40%, which can be used as sponge iron. The magnetic separation residue is mixed with lime and lime by pressure forming and steam curing. The compressive strength of the test piece can reach 24.10MPa, which can be used for the production of steamed bricks and other building materials. The main mineral composition of the residue before and after steaming was transformed from nepheline to calcium aluminum yellow feldspar . The thermodynamic analysis proved the possibility of the reaction under the experimental conditions.
Gao Jianyang [9] used red mud, blended with homemade additives, and used a new process of coal-based direct reduction roasting-slag ferromagnetic separation-cold solidification to study the metal iron crystals in the direct reduction process of Bayer process. The characteristics of grain growth, and the effects of additives and calcination conditions on the growth characteristics of metal iron grains were discussed. High-quality sponge iron was produced. The metallization rate of the product was 92.9%, and the iron content was 93.7%. Iron recovery The rate is 94.42%, which can be used as semi-steel raw material for electric steelmaking, which opens up a new way for the comprehensive utilization of red mud.
Guan Jianhong used the SLon vertical ring pulsation high gradient magnetic separator to recover iron from red mud, which was tested by small-scale test and semi-industrial test. The iron concentrate containing FFe 54.70% was obtained, and the recovery rate was 35.36%. The obtained qualified iron concentrate can be used as a raw material for blast furnace iron making, and for the recovery of iron in red mud, a way to be industrially implemented is found.
Liao Chunfa et al [11] used coke as a reducing agent to determine the best parameters of the roasting process: red mud: coke ratio of 80:15; reduction roasting temperature of 1150 ° C; roasting time of 1.5 h; magnetic field strength of magnetic separation is 0.9kt. It can enrich 56.5 % of iron concentrate; its recovery rate reaches 63.3%, and the remaining iron is recovered after acid leaching.
From the experimental results, the rare metal is further enriched in the separated slag, and the rare earth is effectively separated. The acid is then used to separate the rare metals from the separated slag, and the amount of material processed is greatly reduced.
Jiang Pingguo et al [12] used wet pulsation high gradient magnetic separation to recover iron minerals in Bayer red mud. The method comprises the steps of: calcining a bauxite containing 13% ferric oxide, firstly calcining at a low temperature, and then being dissolved by a Bayer process, and magnetically selecting the red mud, and the iron concentrate after magnetic separation can be used as a raw material for blast furnace iron making.
Gong Lianchun [13] Yang invented a method for directly preparing red iron oxide from red mud, specifically related to the field of pigment production, and the process is shown in Fig. 1.
Li Liangxing [14] and other research on the recovery rate and grade of iron in the case of coke as a reducing agent in the red mud by the addition of sodium carbonate reduction roasting. The optimum conditions obtained by the experiment are: the mass ratio of red mud, sodium carbonate and coke is 5:5:1; the reduction calcination temperature is 1000 ° C; the calcination time is 60 min.
After the red mud is subjected to reduction roasting, the iron concentrate is magnetically selected, and the magnetic separation concentrate contains very few impurities, mainly elemental iron. The recovery rate of iron can reach 80% and the grade is above 70%.
Third, the conclusion
Since the development of the alumina industry, the treatment and comprehensive utilization of red mud has been one of the most urgent problems in the world. The recovery of iron from red mud is an important part of the comprehensive utilization of red mud.
(1) The study of red mud phase proves that iron is mainly in the form of hematite and goethite in red mud, the former accounting for more than 90%. At the same time, the minerals are mostly in the form of Fe, Al and Si cements. The crystal grains are fine and the crystals are extremely incomplete, which makes great difficulty in the separation and extraction of iron.
(2) From the perspective of thermodynamics and kinetics, the reduction of iron in red mud is completely feasible. The reduction roasting is carried out at about 50 to 1250 ° C to complete the crystal structure reforming, and the fine-grained iron-aluminum can be separated.
(3) At the same time, the proportion of iron-bearing iron minerals in red mud due to changes in alumina raw materials and production process conditions, the proportion of iron-bearing ore and iron ore. In the composition of the red mud phase, the hematite content may fluctuate between 19.0% and 33.5%, while the goethite content also fluctuates between 16.0% and 3.9%. The goethite is cryptocrystalline or microcrystalline, and Other minerals are cemented, so the degree of goethite conversion affects the recovery of iron.
(4) The process of recovering iron from high-speed red mud is not difficult. The most important problem is to consider the chemical composition of red mud and the composition of raw bauxite and the production process of alumina. It is necessary to have a relatively optimized extraction process, reduce resource waste and energy consumption, reduce recycling costs, and truly achieve sustainable economic development. In terms of environmental protection and economy, we have achieved a win-win situation in the comprehensive utilization of red mud.
references
[1] Wang Wenzhong. Some thoughts on the comprehensive utilization of metallurgical resources [J]. China Metallurgy, 1996, (2): 35-37.
[2] Yang Zhimin. Comprehensive recovery and utilization of alumina production in China [J]. World Nonferrous Metals, 2002, (2): 35-38.
[3] Jing Yingren, Jing Yingqin, Yang Qi. Basic properties of red mud and its engineering characteristics [J]. Light Metal, 2001, (4): 20-23.
[4] Luige Piga, Pausto Pochettj, Luisa Stoppa. Recovering metals from red mud generate during A in a production [J] . Jom, 2004, 45 (11): 54-59.
[ 5 ] Xiang Qinfang, Liang Xiao hong, SchiesingerMarkE, etal. Low temperature reduction of ferriciron in red[J]. Light Metals: Proceeding of Session, TMS Annual Meeting Feb 11 Novl5 2000: 157-162.
[6] Agrawal, K. K. Sahu, B. D. Pandey. Solid waste manage ment in non-ferrous industries in India [J]. Resources, Conservation and Recycling, 2004, 42 (2): 400-403.
[7] Yu Qiming, Zhou Meihua, Li Maokang, et al. Comprehensive utilization of red mud and its environmental protection function [J]. Jiangxi Chemical Industry, 20007, (4): 125-127.
[8] Liu Wanchao, Yang Jiakuan, Xiao Bo. Bayer method red iron extraction and residue preparation building materials [J]. Chinese Journal of Nonferrous Metals, 2008, 18(1): 187-192.
[9] Gao Jianyang. Study on Direct Reduction of Sponge Iron by Bayer Red Mud[J], Jinan, 2007 Proceedings of the National Symposium on Nonferrous Metals, 2007, 210-214.
[10] Guan Jianhong. Experimental study on the recovery of iron in red mud by pulsating high gradient magnetic separator [J]. Jiangxi Nonferrous Metals, 2000, 14 (4): 1 5 ~ 18.
[11] Liao Chunfa, Jiang Pingguo, Jiao Yufen. Research on the process of recovering iron from red mud [J]. China Mining, 2007, 16 (2): 93-95.
[12] Jiang Pingguo, Wang Hongzhen. Research progress in the recovery of iron from red mud [J]. Sichuan Nonferrous Metals, 2005, (2): 23-25.
[13] Guan Lianchun. A method for preparing iron oxide red by using red mud [P]. CNl01077793, 2007-11-28.
[14] Li Liangxing, Huang Yulin, Luo Jun. Research on the process of recovering iron from red mud [J]. Shanghai non-ferrous metals. 2009.30(1). 19-21.
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