Chenmei Tang, Zhengqi Guo, Jian Pan, Deqing Zhu, Siwei Li, Congcong Yang, and Hongyu Tian, Current situation of carbon emissions and countermeasures in China’s ironmaking industry, Int. J. Miner. Metall. Mater., 30(2023), No. 9, pp.1633-1650. https://dx.doi.org/10.1007/s12613-023-2632-7
Cite this article as: Chenmei Tang, Zhengqi Guo, Jian Pan, Deqing Zhu, Siwei Li, Congcong Yang, and Hongyu Tian, Current situation of carbon emissions and countermeasures in China’s ironmaking industry, Int. J. Miner. Metall. Mater., 30(2023), No. 9, pp.1633-1650. https://dx.doi.org/10.1007/s12613-023-2632-7
Invited Review

Current situation of carbon emissions and countermeasures in China’s ironmaking industry

Author Affilications
  • Corresponding author:

    Zhengqi Guo E-mail: guozqcsu@csu.edu.cn

  • The iron and steel industry (ISI) involves high energy consumption and high pollution. ISI in China, a leading country in the ISI, consumed 15% of the country’s total energy and produced more than 50% of the global ISI’s carbon emissions. Therefore, in the context of global low-carbon economy and emission reduction requirements, low-carbon smelting technology in the ISI has attracted increasingly more attention in China. This review summarizes the current status of carbon emissions and energy consumption in China’s ISI and discusses the development status and prospects of low-carbon ironmaking technology. The main route to effectively reducing carbon emissions is to develop a gas-based direct reduction process and replace sintering with pelletizing, both of which focus on developing pelletizing technology. However, the challenge of pelletizing process development is to obtain high-quality iron concentrates. Consequently, the present paper also summarizes the development status of China’s mineral processing technology, including fine-grained mineral processing technology, magnetization roasting technology, and flotation collector application. This paper aims to provide a theoretical basis for the low-carbon development of China’s ISI in terms of a dressing–smelting combination.
  • With the aggravation of global warming, the issue of CO2 emissions has become the focus of social attention [12]. Global energy-related CO2 emissions reached 36.3 billion tons in 2021, with an annual increase of 6%. CO2 emissions from coal hit an all-time high of 15.3 billion tons in 2021, which is more than 40% of the annual increase in total global CO2 emissions [3]. The iron and steel industry (ISI) plays an indispensable role in the global basic industry and has contributed greatly to global economic growth [4]. In 2021, China’s crude steel output reached 1032.8 million tons, more than 90% of which was prepared by the blast furnace–converter long process. However, the conventional coal-based energy structure results in a large amount of CO2 emissions, sharing approximately 4%–7% of the total emissions and 16% of the industrial energy consumption in China [5]. Compared with developed countries, China has undergone economic development accompanied by serious environmental problems. Recently, starting from energy conservation and emission reduction, China has introduced relevant policies to accelerate the transition to a green and low-carbon economy and achieve green recovery and development. Hopefully, at the same time, the goal of peaking CO2 emissions by 2030 and carbon neutrality by 2060 can be achieved [6]. It will take more effort for China to achieve carbon neutrality by 2060 than for developed countries to achieve carbon neutrality by 2050. Remarkably, the main process of carbon emissions from steel production is blast furnace ironmaking [7], as shown in Fig. 1. The ironmaking process accounts for approximately 70% and 90% of the energy consumption and carbon emissions, respectively, of the entire steel production process [8]. Therefore, energy conservation and pollution reduction in the ironmaking systems are remarkably important for the long-term development of the ISI.

    Fig. 1.  CO2 emissions per ton of steel in different processes (Data source from Ref. [7]).

    Recently, many ultra-low carbon technologies for the ISI have been investigated, such as hydrogen-based direct reduction and carbon capture utilization and storage. Many scholars have studied the feasibility of reducing carbon emissions, and some groups have also evaluated the potential of carbon emissions in China’s ISI [911]. Hydrogen-based shaft furnace direct reduction of iron (DRI) is a relatively mature and environmentally friendly production process in the non-blast furnace ironmaking field. Its products are substitutes for scrap steel and diluents for residual elements in scrap steel. The production of DRI does not require coke, which saves energy consumption and exhaust emissions for converting coal and coke, saving energy, and protecting the environment, and can meet the growing environmental protection needs of modern iron and steel production enterprises [12]. Compared with blast furnace–basic oxygen furnace (BF–BOF) long process, DRI + electric arc furnace (EAF) short process is widely regarded as a great way to curb carbon emission and achieve a carbon peak on schedule [13]. However, the production of DRI requires high-quality iron concentrates. There is a serious shortage of high-grade iron ore concentrates in China, the dependence of iron ore on foreign countries is gradually increasing, and the production cost is also increasing. Therefore, the utilization of complex refractory iron ore in China is high of concern to various enterprises. Based on the introduction of carbon emissions and the status quo of the ISI, some theoretical support, and suggestions for the development of China’s ISI low-carbon transformation path in the future are put forward, and the development of ore dressing technology is also reviewed.

    In recent years, China’s ISI has been growing rapidly, and iron and steel production has continued to increase. Fig. 2 presents the production of crude steel in the world and China (mainland) from 2011 to 2021 [14]. Annual crude steel production in China has been increasing basically over the past decade. China produced 683 million tons of crude steel in 2011, which accounted for about 46% of world steel production in 2011. Crude steel production in China reached up to 1033 million tons in 2021, representing over 53% of global crude steel production. In addition, fossil energy resources, including coal, coke, and natural gas, as the main energy sources, have been used for ISI, resulting in the largest carbon emission behind the power system [15]. Therefore, a rapid increase in crude steel production is accompanied by rising carbon emissions. Global CO2 emissions hit a record 36.3 billion tons, and China’s CO2 emissions contributed nearly one-third in 2021.

    Fig. 2.  Production of crude steel in the world and China (mainland) from 2011 to 2021(Data source from Ref. [14]).

    The issue of climate change has become the focus of people’s attention, and carbon emission, as the main cause of climate change, has become a very serious problem [16]. China’s ISI still has high carbon characteristics and the steel produced is mainly based on the BF–BOF long flow process so far [17]. Although enterprises have taken many measures to save energy and reduce emissions in recent years, their pollutants and carbon emissions have not been effectively controlled. Compared with 2000, the comprehensive energy consumption of China’s ISI in 2020 has decreased by 20% to 545 kg coal equivalent per ton crude steel, but it has not yet reached the world’s advanced level. In traditional carbon reduction metallurgy, the BF ironmaking process is the main source of CO2 and gas pollutants in the ISI [18]. In terms of the energy structure of iron and steel enterprises, CO2 emissions from coal consumption account for 94%. The CO2 emissions before ironmaking account for 88.75%. The carbon dioxide emitted includes not only the carbon produced to reduce the iron ore to iron in the blast furnace, but also the energy consumption during the operation of other equipment such as coke ovens, sintering machines, and hot blast stoves in the ironmaking process. Specifically, the main sources of carbon emissions are the thermal efficiency of coke ovens and hot ovens, and the sintering process [19]. Therefore, the iron industry still has a long way to go. It is significant to vigorously promote low carbon reduction before ironmaking.

    ISI faces an imperious demand for developing new technologies to solve the issue of increasing carbon emissions. Reducing the coke ratio has always been a key issue in the development of BF technology [20]. A lot of scholars have studied new technologies that can be applied to various aspects of steel production, which are shown in Table 1.

    Table  1.  Energy-saving and emission reduction technologies to ironmaking for blast furnace [2122]
    No.New technology
    1BF top gas circulation technology
    2Efficient pulverized coal injection technology for BF
    3New burden of iron–carbon agglomerates (ICA)
    4Hydrogen-rich fuels using in BF
    5Gas injection technology
    6Purification of BF gas by pressure swing absorption
    7BF top iso-pressure blow off` optimization
    8BF power recovery turbine technology
    9Carbon capture, utilization, and storage technology
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    Recently, a new charge of iron–carbon agglomerates (ICA) has also been proposed, which is prepared by mixing iron ore and coal. It has the advantage of high reactivity under the catalysis of metallic iron. The preparation of this new charge usually uses the conventional chamber coke oven method and the briquette shaker method [23]. The new burden has a wide range and low requirements on the grade of iron ore raw materials, which reduces the production cost to a certain extent. And, the lower proportion of coke results in a reduction in carbon emissions. Besides, in the case of metallic iron catalysis, the new charge has high reducibility. Therefore, the decrease in the temperature of the thermal reserve zone has been realized, and the gas permeability and droplet performance of the charge layer in the BF can be improved, and thus the smelting efficiency can be enhanced. The preparation and application of iron coke are shown in Fig. 3 [2426]. Due to several advantages, this new ingredient has been developed in many countries around the world as a low-carbon ironmaking technology. In China, energy saving and emission reduction are imminent, this technology has also received extensive attention [23]. Wang et al. [27] at Northeastern University in China have studied the principle of low-carbon BF smelting with charging ICA. Meanwhile, the preparation process, the optimization of the metallurgical properties of ICA, as well as the reduction process of iron-bearing burden were also systematically studied. It was demonstrated that it will reduce CO2 emissions by about 25 kg per ton iron and reduce production costs by approximately 12 ¥ per ton iron. This technology is extended to the national blast furnace ironmaking production, and mitigation of CO2 emissions and economic benefit will be improved. At present, due to the continuous rise of coke prices, the application of the new burden of ICA technology will bring more significant low-carbon and low-cost effects. He et al. [28] explored new technologies such as biomass blast furnace air blowing, semi-coke, coke oven gas, etc., and found that they can replace a small amount of pulverized coal, which not only effectively alleviated coal shortage and decreased the emission of waste gas, but also seek an effective way for its resource utilization. However, the low metallurgical strength of iron coke is still the bottleneck restricting its popularization and application. The synergistic optimization of the reactivity and post-reaction strength of ICA, the gradual change behavior of ICA in the blast furnace, and the influence of ICA on the energy consumption of the blast furnace are key research directions in the future.

    Fig. 3.  Preparation and application of iron coke: (a) iron coke preparation process [24]; (b) preparation and application of iron coke by cold press molding [25] (Reprinted from Powder Technol., 328, H.T. Wang, W. Zhao, M.S. Chu, Z.G. Liu, J. Tang, and Z.W. Ying, Effects of coal and iron ore blending on metallurgical properties of iron coke hot briquette, 318-328, Copyright 2018, with permission from Elsevier); (c) effect of amount of hot pressed iron coke on the droplet property of comprehensive charge [24]; (d) schematic of low-carbon smelting in blast furnace using iron-coke [26]; (e) effect of using iron coke in blast furnace on carbon saving of whole ironmaking system [26].

    Generally, hydrogen-rich gas fuels in the ISI mainly include natural gas and coke oven gas, etc. Compared with blast furnace pulverized coal injection, natural gas, as clean energy, has the characteristics of reducing the environmental pollution. Moreover, due to lower greenhouse gas emissions and energy consumption, natural gas can replace a small amount of blast furnace pulverized coal injection [29]. However, because natural gas resources are scarce, natural gas injection can only be carried out in the United States and Russia, and others with rich natural gas reserves. In addition, as another hydrogen-rich gas fuel, the main components of coke oven gas are H2 (50vol%–60vol%), CH4 (20vol%–30vol%), and CO (5vol%–8vol%), which are one of the main by-products of the coking process. It has the characteristics of high calorific value, high flammable composition (90%), and fast burning speed, and can be used as a high-quality reducing agent. Liu et al. [30] have carried out the industrial experiment of adding coke oven gas for No. 4 BF of Jinan Iron and Steel Co., Ltd., China and found that for producing per ton iron, the coke ratio and coal ratio reduced by 5.28 kg and 40.63 kg, respectively, and CO2 emissions reduced by 75 kg when the gas volume of injection coke oven was 62.51 m3. However, there are still some problems in coke oven gas injection technology, which are worth exploring. For example, coke oven gas contains high tar and naphthalene. If it is not treated in time, it will block the pipeline and make it difficult to carry out the injection process.

    Oxygen blast furnaces were originally proposed to replace coke with coal and improve blast furnace production efficiency, but now focus on their potential to reduce carbon emissions from ironmaking processes. Oxygen instead of hot air can improve the coal injection ratio, reduce CO2 emissions, and maintain the theoretical combustion temperature [31]. After the CO2 is removed from the top gas, most of it is used for gas circulation, and a small part can be used for chemical synthesis. Captured CO2 is sequestered or recycled to push for net-zero emissions from blast furnace ironmaking. Oxygen blast furnaces with top gas circulation have the characteristics of high smelting efficiency, large coal injection ratio, low energy consumption, and low CO2 emissions. Therefore, the development of oxygen blast furnaces is of great significance to energy saving and consumption reduction and CO2 gas emission reduction in the ironmaking process. However, due to the immaturity of carbon dioxide separation technology and major changes to the blast furnace body, the research on oxygen blast furnaces has been in the stage of industrial experimentation.

    In addition, to get rid of over-reliance on fossil energy, ironmaking technology based on hydrogen energy has gradually attracted increasing attention. As the vanguard of hydrogen metallurgy in China’s steel industry, China Baowu Steel Group Co., Ltd. (Baowu) began planning hydrogen metallurgy green and low-carbon ironmaking projects in 2016. It has established hydrogen metallurgy innovation bases in Xinjiang Uyghur Autonomous Region and Guangdong Province of China. Subsequently, it jointly developed nuclear hydrogen production technology with China National Nuclear Corporation and Tsinghua University in China [32]. HBIS Group Co., Ltd., China (HBIS) started the construction of a hydrogen metallurgy demonstration project with a scale of 1.2 million tons, using zero-reforming technology to replace traditional carbon metallurgy. Zhongjin Taihang Mining Co., Ltd. in China (Taihang) has started to use hydrogen-based direct reduced iron technology and has formed gas-based shaft furnace reduced iron technology (CSDRI) [32]. In addition, Anshan Iron and Steel Group Co., Ltd. (Ansteel), Baotou Iron and Steel Group Co. Ltd. (Baosteel), Jianlong Group (Jianlong), and some other scientific research institutes in China have also carried out research on hydrogen metallurgy technology [33], all of which have achieved ideal results and reduced carbon emissions. The development status of hydrogen metallurgy in China is shown in Table 2 [34].

    Table  2.  Development of hydrogen metallurgy in China [34]
    CompanyProgressRemark
    BaowuBlast furnace hydrogen–carbon cycle; hydrogen reduction carbon replacement; nuclear hydrogen production technology.Reduce CO2 emissions by 30%.
    HBISEstablishment of “Hydrogen Technology and Industrial Innovation Centre.”Construction of 1.2 million tons of hydrogen metallurgy project. Hydrogen energy development and utilization project. Achieve annual carbon reduction of 60%, etc.
    JISCOEstablishment of “Hydrogen Metallurgy Research Institute.”Founded “Coal Based Hydrogen Metallurgy Theory”, “Shallow Hydrogen Metallurgy and
    Magnetization Roasting Theory”, etc.
    BaosteelDeveloping low-carbon metallurgical technology research and industrial application.Photovoltaic hydrogen production–pipeline hydrogen–green hydrogen metallurgy.
    AnsteelWind power + photovoltaic–electrolytic water hydrogen production–hydrogen metallurgy technology.Realized green hydrogen metallurgy.
    TaiHangCoke oven gas dry reforming + reduction + PERED shaft furnace process.Formed CSDRI gas-based shaft furnace iron reduction technology with independent intellectual property rights.
    JianlongStart high purity pig iron project.Realized hydrogen fusion reduction.
    Rizhao SteelLaunch hydrogen metallurgy projects.Plan to produce 50 tons of direct reduced iron with hydrogen.
    Note: JISCO—Jiuquan Iron & Steel (Group) Co., Ltd., China; Rizhao Steel—Rizhao Steel Holding Group Co., Ltd., China; PERED—Persian Reduction’ technology.
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    Under the condition that China’s iron and steel production still needs to be dominated by long processes for a long time, changing the blast furnace burden is an inevitable trend in the future to explore further reductions in energy consumption and carbon emissions [35]. The burden structures of BF in China, Europe, the United States, and others are shown in Table 3 [3637]. It is clear that the fuel ratio for produce steel in northern Europe where the burden structure is dominated by pellets has reached the level of 450 kg per ton steel, and the coal ratio is about 150 kg per ton steel. However, in China, where the charge structure is mainly sintered ore, the fuel ratio exceeds 500 kg per ton steel, and the productivity and ore grade are relatively low. In view of that, the burden structure with a high pellet proportion for BF is an effective way to fulfil carbon discharge reduction.

    Table  3.  Burden structure and production index of blast furnace [3637]
    Steel plantBurden structure / %Productivity /
    (t·m−3·d−1)
    TFe /
    %
    Coke ratio /
    (kg·t−1*)
    Coal ratio /
    (kg·t−1*)
    Fule ratio /
    (kg·t−1*)
    PelletSinterLump
    China157782.20057.21361144535
    Arcelor Mitta802002.36663.33335120455
    American Steel Alliance 14#802002.37761.89300160460
    SSAB392082.60065.69300150450
    SSAB497032.91065.26352 90442
    Note: TFe—total iron; *—per ton crude steel; SSAB—Svenskt Stål AB; SSAB3—Swedish SSAB Blast Furnace No. 3; SSAB4—Swedish SSAB Blast Furnace No. 4.
     | Show Table
    DownLoad: CSV

    An enhancement in the dosage of pellets in the BF requires the expansion of the production of pellets and the supply of iron concentrates used to produce the pellets. The production of pellets in China and the world in recent years is shown in Fig. 4. It can be found that China’s pellet production is much lower than the world average. Pellet production cannot meet the marketing requirements, and the number of pellets imported increased to 22.5 million tons in 2021 [38]. It is necessary to import many pellets or import concentrates to produce pellets. However, the production process of pellets and concentrates determines their higher costs, resulting in higher premiums for pellets and concentrates in the international market. At the same time, the increased demand for high-quality pellets and concentrates with the development of direct reduction abroad has also exacerbated the increase in the prices of pellets and concentrates. Increasing the proportion of pellets fed into the furnace will result in a higher cost of raw materials, and it will be difficult to be effectively compensated by the improvement in the BF output or the reduction of fuel ratio. Therefore, it is necessary to urgently develop beneficiation technology, develop and utilize refractory iron ore, and solve the problems of shortage of iron concentrate resources and the high cost of ironmaking.

    Fig. 4.  Pellet production and proportion in burdens [3941].

    The key to realizing ultra-low carbon and zero carbon green production in the ISI is to replace fossil energy with green hydrogen and green electric energy and develop the gas-based DR–EAF short process [42]. The CO2 emissions through the BF–BOF process reach 1.82 per ton of cure steel, while the CO2 emissions of gas-based DRI–scrap EAF short process and all scrap electric furnace smelting process is only 1.10 t per ton cure steel and 0.40 t per ton cure steel, respectively. Therefore, converting the blast furnace to an electric furnace is the main direction in the carbon reduction technology path of iron and steel production [43]. The raw materials of the global mainstream electric furnace steelmaking process are 50%–70% scrap + 30%–50% DRI.

    DRI is distinguished by its high purity and low carbon content, and continuous feeding can maximize the capacity of electric furnace transformers. It is widely considered as the most optimal iron source material for electric furnace steelmaking process worldwide. Fig. 5 illustrates the distribution of global DRI production in 2021 and the production of DRI in the world and in China over the years [44]. Since 2015, the global DRI output has increased by 50%, however, China’s DRI production remains relatively low and is growing at a slower pace. The international market price for DRI is comparatively high, and the supply is severely limited, with import quantities falling short of meeting actual production needs in China. Currently, China’s DRI technology is still in the developmental stage, with limited production and suboptimal quality that falls short of actual production requirements. Therefore, there is significant potential for growth in the production of DRI in the future, creating opportunities for further development in this sector.

    Fig. 5.  (a) Distribution of global DRI production by process in 2021 [44] and (b) the production in the world over the years.

    Fig. 6 exhibits the flow chart of MIDREX and HYL-III processes. MIDREX gas-based shaft furnaces are currently the most well-developed gas-based direct reduction process, which has taken the leading position in direct reduction [45]. This process (Fig. 6(a)) has the characteristics of mature production technology, high production rates, low energy consumption, and good product quality. As shown in Fig. 6(b), compared with MIDREX technology, the HYL-III process adopts high-pressure operation in the reaction zone. In addition, natural gas cracking uses steam as the cracking agent, which avoids the problem of catalyst sulfur poisoning caused by the cracking agent. Therefore, there is no special requirement for the sulfur content of the ore in the furnace. The use of the MIDREX process and the HYL-III process has greatly increased China’s steel production.

    Fig. 6.  Flow chart of (a) MIDREX and (b) HYL-III process [46].

    The coal-based direct reduction methods are classified according to the main equipment used, mainly including the tunnel kiln method, rotary kiln method, and rotary hearth furnace method. The rotary kiln method is currently the mainstream process of coal-based direct reduction. The rotary kiln process accounts for more than 95% of the total global coal-based DRI production. It has the advantages of cheap reducing agents and a wide range of processing products. However, it has problems such as limited production capacity, large investment, high technological level restrictions, and an easy “ring” of fine-grained materials and coal ash. However, it can use cheaper non-coking coal instead of coke to reduce real estate costs, operates at lower temperatures, requires less feedstock, and is more efficient.

    The iron ore of China is rich with reserves of about 20.76 billion tons, accounting for about 11% of the world’s total reserves [47]. The distribution of iron ore resources is shown in Table 4. A quarter of China’s iron ore is concentrated in the Anshan mining area in Liaoning province [48], which is currently the mining area with the largest reserves and mining in China, mainly including Anshan Iron Mine and Benxi Dataigou Iron Mine. The lean ore in the Anshan mining area accounts for 98% of iron ore reserves, and its average iron grade is about 30wt%. It must be processed by mineral processing, and the iron content can reach more than 60wt% after cleaning. The iron ore in North China is mainly distributed in Xuanhua, Qian’an, and Handan in Hebei province, as well as in Inner Mongolia and Shanxi, accounting for about 30% of the total iron ore reserves. The iron ore in the central and southern regions is mainly Daye iron ore in Hubei province, and other places such as Xiangtan in Hunan province, Anyang in Henan province, Jiangxi province, and Guangdong province have reserves of a certain scale. The ore in the Daye mining area is mainly iron–copper symbiotic ore with an iron content of 40wt%–50wt%.

    Table  4.  Representative mines in China and production status in 2020 (Data source: China International Trust Investment Corporation (CITIC) Securities, Toubao Research Institute)
    MineRegionProven iron
    ore reserves /
    billion tons
    Annual iron
    ore production /
    million tons
    AnshanLiaoningTotal 540
    BenxiLiaoning30
    PanzhihuaSichuan2.115
    Ma’anshanAnhui1.6
    Bayan OboInner Mongolia1.413
    JingtieshanGansu0.510
    ShiluHainan 0.46
    MeishanJiangsu 0.26 5
    Qian’anHebei 0.2511
    DayeHubei0.1 3
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    However, although China is rich in iron ore resources, there are few rich ores and more lean ore. The average iron grade of 34.5wt% is well below the global average. The amount of mining is large, but the yield is low. On average, only 1 kg of steel can be produced for every 5 kg of lean iron ore. The large-scale steel production demand leads China to import large quantities of high-grade iron ore, and the annual iron ore import volume is maintained at more than 1 billion tons. In 2021, China imported 1.126 billion tons of iron ore, with an external dependence of 82.3%. Besides, there are many co-associated components in Chinese iron ore, which can be divided into five types: one is single iron ore, which basically does not contain co-associated components, mainly magnetite. The second is iron polymetallic ore, often coexisting Cu, S, Co, Zn, Mo, Sn, and gypsum, and accompanying components such as Cu, S, Pb, Zn, Co, and Au. The third is vanadium titanomagnetite and the fourth is polymetallic-rare earth–iron ore. The fifth is uranium–boron–iron ore, in which ferroboron is symbiotic. In addition, compared with countries rich in iron ore resources, iron ore resources in China are characterized by more small and medium-sized deposits and fewer large and super-large deposits. At present, there are 2037 proven iron ore deposits, but only 10 of them are super-large iron deposits with reserves greater than 1 billion tons. In China’s iron and steel production, the long-process production process of iron ore-making with iron ore as raw material occupies a dominant position, and China’s future demand for iron ore will remain extremely high. Thence, it is also necessary to efficiently develop China’s mineral resources and increase resource reserves and self-sufficiency. Through technological innovation, the problems of difficult mining, difficult selection, and low utilization rate of iron ore resources is expected to be solved, and the scope of available resources can be expanded.

    In China, hematite and magnetite with particle sizes less than 0.045 mm and 0.038 mm, respectively, are generally known as fine-grained iron ore [49]. It is difficult to use conventional beneficiation technology to treat fine-grain iron ore. Yuanjiacun iron mine in the Lüliang area, Shanxi province, China is a typical mineral with many iron ore types, complex structure, mineral nesting, and fine grain size. The main minerals are hematite and other oxide minerals, and the useful minerals are embedded in fine particle sizes. Among them, the quartz-type primary iron ore, quartz-type iron oxide ore, specularite-type iron oxide ore, and amphibole-type iron oxide ore in Yuanjiacun are all fine-grained iron ore with grades ranging from 31.13wt% to 36.56wt% [50]. For different types of fine-grained iron ore, predecessors have carried out many beneficiation experiments and inferred that the same beneficiation process has different effects on various ore types [51]. The quartz-type primary iron ore could be separated by a stage grinding–weak magnetic–reverse flotation process to achieve a better separation effect. The quartz-type iron oxide ore and specularite-type iron oxide ore can be separated by the process of stage grinding–weak magnetic–strong magnetic–reverse flotation to gain satisfied separation results. High-quality iron concentrates above 65wt% can be achieved and the corresponding recovery was as high as 61.33%–78.85% by this process. For micro-fine magnetite, the iron concentrate with 66wt% TFe grade and 80% recovery was obtained by the two processes of magnetic separation process and magnetic separation–reverse flotation process. For the hematite and siderite, all of them can be converted into magnetite through coal-based reduction roasting experiments, and iron concentrate with iron content of 65.4wt% and iron recovery rate of 92.7% can be obtained. According to previous research results, the optimal beneficiation process for different types of fine-grained iron ore is summarized in Table 5 [5255].

    Table  5.  Typical mineral processes for fine grain iron ores
    Raw materialMineral processingBeneficiation index
    Magnetite
    TFe: 31.18wt%
    −0.045 mm: 94%
    Stage grinding and stage separationTFe: 66.95wt%
    Recovery: 72.61%
    Hematite
    TFe: 28.36wt%
    −0.038 mm: 98%
    Selective flocculation–reverse flotationTFe: 62.50wt%
    Recovery: 68%
    Magnetite-hematite mixed iron ore
    TFe: 33.21wt%
    −0.038 mm: 95%
    Weak magnetic separation–strong magnetite separation–resurfacing–reverse flotationTFe: 65.36wt%
    Recovery: 82.03%
     | Show Table
    DownLoad: CSV

    The magnetization roasting technology is widely used in the beneficiation process of refractory iron ore. Hematite, limonite, siderite, and other weakly magnetic iron minerals are selectively reduced to ferromagnetic magnetite after magnetization roasting, and liberation pre-treatments to facilitate the magnetic separation and enrichment process [56]. The frequently-used magnetization roasting methods include the following techniques including shaft furnace roasting, rotary kiln roasting, fluidized bed roasting, and microwave-assisted roasting. Recently, magnetization roasting technology and equipment have been studied in detail in some factories of China. Many studies show that the shaft furnace process requires a narrow range of iron ore particle size, and can only process lump ore of 15–75 mm. Low iron recovery using the shaft furnace roasting is due to the chemical heterogeneity of the reduced ore. Thus, the shaft furnace is seldom used in magnetization roasting technology up to now.

    The rotary kiln can process iron ore with finer grain size (<25 mm). For different types of refractory iron ore, such as oolitic hematite, limonite, hematite, and siderite, predecessors have carried out some researches [5763]. Refractory ore can achieve a better beneficiation effect through the beneficiation process system of rotary kiln magnetization roasting–grinding–weak magnetic separation. For different iron ore types, the suitable roasting temperature and roasting time were different. The roasting temperature of siderite and limonite was about 750°C, while the roasting temperature of oolitic hematite was higher at 1000°C. The oolitic hematite with an iron grade of 39.38wt% can obtain the mineral processing indicators with an iron concentrate grade of more than 65wt% and an iron recovery rate of more than 78% after beneficiation. The recovery rate of limonite after beneficiation was higher, which can reach more than 90%. For the Daxigou siderite ore with a Fe content of 26.8wt%, a concentrate with an iron grade of 59.8wt% can be obtained, and the recovery rate was 86.4%. Previous studies have found that, compared to the shaft furnace, rotary kiln magnetization roasting technology has been relatively mature in the application of mineral processing and can achieve a better separation effect. However, rotary kiln magnetized roasting has the problems of high energy consumption and serious clinker formation, which still need to be explored and solved in the future.

    Apart from the shaft furnace and the rotary kiln, magnetization roasting was also carried out by fluidized bed furnaces to deal with fine particles with high heat transfer efficiency, mass transfer rate, and low heat consumption [64]. Furthermore, the fluidized bed roasting was easy to control and improved the efficiency of magnetization reaction and uniformity of products [65]. Yu and Chen [6667] put forward a new concept of flash magnetization roasting and developed a transport bed reaction device that can work together with multi-stage cyclone separators. Predecessors have used this technology to conduct beneficiation experiments on goethite, limonite, oolitic hematite, and siderite, etc. [6869]. Several typical refractory iron ores have high iron grades after flash magnetization roasting, and the results are shown in Fig. 7. For these ores, the iron grade can only be increased to 40wt%–50wt% by the traditional physical beneficiation including magnetic separation, gravity separation, or flotation. Furthermore, iron recovery was only able to reach 50%–60%. Zhu and Li [70] explored a multi-stage circulating fluidized bed magnetization roasting process for weak magnetic iron ore. Using multi-stage fluidized reduction roasting at about 450°C, an experiment was carried out on a 33% iron limonite in Yunnan province, and it was found that the iron grade and recovery rate could reach 57% and 95%, respectively. Li et al. [71] studied the phase transition during fluid reduction roasting of high-phosphorus oolitic hematite, and the results showed that the hematite was reduced to magnetite using H2 at 650°C. The increase in temperature would cause the magnetite to be over-reduced, forming a weakly magnetic-floating body. Therefore, under the calcination condition of 650°C, the iron content of magnetic separation was 58.7% Fe with a recovery of 84.0%.

    Fig. 7.  Flash magnetizing roaster–magnetic separation and its beneficiation effect [53,6465].

    Professor Han and his team from Northeastern University put forward the suspension roasting process of “pre-oxidation-heat-storage reduction-reoxidation” for complex refractory iron ores [72]. Kong et al. [73] used this technology to study the beneficiation of the mixed ore of hematite and limonite. In addition, beneficiation tests were carried out on Angang’s flotation tailings and oolitic hematite in theDong’anshan sintering plant. With the optimum conditions, the iron concentrates with 56wt%–61wt% Fe and a recovery of 78%–84% [74] can be prepared. Yuan et al. [75] found that high-quality iron concentrates with an iron grade of 66.55wt% with 77.01% recovery were gained fromDong’anshan lean hematite with an iron grade of 31.63% through pre-enrichment, suspension magnetization roasting–low intensity magnetic separation. Zhang et al. [76] conducted a pilot-scale beneficiation test of suspension magnetization roasting for Jingtieshan refractory iron ore. The results showed that the suitable roasting temperature, CO flow, N2 flow, and feed rate were 520°C, 4.0 m3/h, 2.0 m3/h, and 100 kg/h, respectively. Finally, the iron grade of product concentrates was 60.1wt% and the recovery was 90.1%.

    Nunna et al. [77] used microwave-assisted magnetization roasting technology to conduct beneficiation experiments on low-grade rich goethite, and the results showed that a pellet concentrate with a grade in excess of 62wt% could be produced with an iron recovery of 88%. Zhou et al. [78] used microwave-assisted technology to conduct beneficiation experiments on oolitic hematite, and finally a low-phosphorus and high-grade iron concentrate was obtained, which achieved the purpose of increasing iron and reducing phosphorus of oolitic hematite. Roy et al. [79] compared the performance of titanomagnetite and goethite in terms of iron grade and iron recovery by microwave roasting and weak magnetic separation experiments. Under the optimal experimental conditions, titanomagnetite can obtain iron concentrate with an iron grade of 62.57wt% and an iron recovery rate of 60.01%, and goethite concentrate with an iron grade of 64.4wt% and a low iron recovery rate of 33.3%. Microwave-assisted roasting is a potential alternative technology for iron ore magnetization. But it is still in the stage of laboratory research, and there are few industrial applications.

    In addition, the predecessors also proposed a coal-based reduction magnetic separation process that used coal as a reducing agent to reduce iron oxides to metallic iron below the melting point, which mainly included two processes: the reduction of iron oxides and the growth of metallic iron particles [80]. Li et al. [81] studied the recovery of iron from copper slag by using deep reduction magnetic separation and found that coke powder can be used to reduce iron efficiently from the copper slag. Under the optimal conditions, the iron recovery rate of copper slag was 91.82%, and the iron grade could reach 96.21%. Zhang et al. [82] reviewed the research on reductive magnetic separation of coal-based refractory ferrous resources and demonstrated that the coal-based reduction magnetic separation process could recover iron from refractory iron-containing resources such as copper slag, red mud, nickel slag, and oolitic hematite. The grade and recovery rate of iron concentrate were usually above 80wt%. However, coal-based emission reduction consumes a large amount of coal and energy, which will cause a large number of CO2 emissions. It is not conducive to the realization of the low-carbon emission goals of human society.

    Magnetite concentrate is the main raw material for the production of oxidizing pellets, but high-grade magnetite resources are increasingly scarce and expensive. The rich ore powder for sintering is generally hematite with coarse particle size, but it is rich in resources and low in cost. If the fine grinding of the rich ore powder for sintering is used for the production of pellets, it could not only reduce the cost of ironmaking but also expand the iron ore resources for pelletizing. Predecessors have done a lot of rese arches on the feasibility of finely ground Brazilian card powder for pellet production [8387]. It was found that the green pellets properties, compressive strength, and reducibility of the roasted pellets could meet the requirements of industrial production when the proportion of finely ground rich ore powder was controlled within 30%, and at the same time, the amount of bentonite was reduced. For flux pellets, it could still meet the requirements of blast furnace smelting when the proportion of fine grinding powder was increased to 45wt%, and the addition of finely ground smelted smelting powder could improve the reduction performance of the pellets as well as high-temperature reflow performance. Zhang et al. [88] studied sinter iron ores and titanium ores used in pelletizing and indicated that the falling strength and compression strength of green pellets would be increased when fine-grinded sinter iron ores were added. Ti-bearing pellets with high titanium content could be produced by using ground titanium ores, and the proportion of high titanium concentrate should not be larger than 20wt%.

    Cationic collectors are generally amines, and some amines have been often used in the iron ore flotation process [89]. However, the presence of slime in iron ore has a great influence on the collection effect of amine collectors, and the foam viscosity of amine collectors is high, so there is a problem of difficulty in defoaming. Therefore, it is very important to strengthen the exploration and development of new cationic collectors for improving the reverse flotation technology of cationic collectors in China.

    Given the defoaming difficult and high-cost problem, Zhu et al. [90] developed a cationic collector DYP for normal temperature flotation, which had many benefits of quick defoaming, high selectivity, low synthesis cost, and a relatively simple pharmaceutical system. In the flotation of artificially mixed ore, the quartz removal rate reached more than 97%. For a mixed magnetic concentrate from Dong’anshan sintering factory, Yang et al. [91] conducted a reverse flotation test using DYP. The resulting concentrate had an iron grade of 60.52% and an iron recovery of 73.17% under the conditions of pulp temperature of 25°C and DYP dosage of 180 g/t.

    The development of a new room temperature collector is one of the hotspots in mineral processing to improve beneficiation efficiency in recent years. Zhu et al. [92] developed a novel low-temperature resistant cationic collector called DBA-2 and the results showed that the flotation of −0.038 mm pure quartz can obtain a good index of the recovery rate of 98.0% when the dosage was 197.5 mg/L. As DBA-2 requires fine quartz grain size, reverse flotation of some coarse quartz grains was limited. Zhu et al. [93] also developed a cationic collector called DBA-1. Compared with DBA-2, the DBA-1 collector had a better recovery effect on quartz with a particle size of 0.038–0.074 mm and the flotation recovery rate of quartz can reach 97.3%.

    Lei et al. [94] synthesized a new type of cationic collector M-201 using sodium silicate, epichlorohydrin, and trimethylamine hydrochloride as raw materials. Compared with dodecylamine and ether amines, M-201 has the characteristics of higher collection capacity, good low-temperature resistance, high foam brittleness, and fast defoaming, which solved the problem of high foam viscosity of traditional cationic flotation reagents. Cheng et al. [95] reported a novel ether amine cationic collector butane-3-heptyloxy-1,2-diamin (BHLD) for quartz flotation and found that the recovery rate of quartz was 98.9%. They also invented an ether amine cation collector DCZ, and the flotation performance of the DCZ was studied with quartz, hematite, and magnetite as samples. The results showed that DCZ was extremely adaptable to temperature and pH values when the flotation temperature is in the range of 5–35°C and the pH value was 12. Under this condition, the recovery rates of quartz, hematite, and magnetite were 99.5%, 99.0%, and 84.0%, respectively. Liu et al. [96] synthesized a new type of Gemini ester-containing cationic surfactant M-302B from epichlorohydrin, dodecyl dimethyl amine, and succinic acid as raw materials, and the flotation experiments were carried out on quartz and magnetite. The results showed that M-302B could be used for the separation of magnetite and quartz by flotation. Compared with dodecylamine, M-302B had a strong collecting ability, low-temperature resistance, and easy elimination of foam.

    Ge et al. [97] designed and synthesized new ether polyamine GE series collectors (GE-601, GE-609, and GE-651C, etc.) and they were used in the research iron ore reverse flotation desilication. The foam performance of GE-609 and dodecylamine in the flotation of Taigang Jianshan iron ore was investigated, and it is found that the amount of foam produced by GE-609 is equivalent to 1/3 of the latter compared GE-609 with dodecylamine. Weng et al. [98] discovered a novel quaternary ammonium salt surfactant M-302 containing ester bonds and hydrocarbon tails. M-302 was widely applied as a cationic collector for magnetite flotation silicate. Compared with dodecylamine chloride, M-302 had a strong collection capacity, a wide effective service temperature range, and a high foam collapse rate. Huang et al. [99] investigated a gemini surfactant, ethane-1, 2-bis(dimethyldodecylammoniumbromide) (EBAB) synthesized from N,N,N’,N’-tetramethylethylenediamine with 1-bromododecane. The chemical constitution of the surfactant and the conventional monomeric surfactant dodecylammonium chloride (DAC) as well as its application are displayed in Fig. 8. Applying it to the cation reverse flotation test of silicon in a magnetic separation concentrate of Anshan iron and steel Gongchangling iron mine. It can be found that under the condition of silicon content of 1.79 %, the iron grade and iron recovery rate of the concentrate can be increased to 70.58% and 98.42%, respectively.

    Fig. 8.  Application effect of EBAB and DAC [99]. Reprinted from Chem. Eng. J., 257, Z.Q. Huang, H. Zhong, S. Wang, L.Y. Xia, W.B. Zou, and G.Y. Liu, Investigations on reverse cationic flotation of iron ore by using a Gemini surfactant: Ethane-1, 2-bis(dimethyl-dodecyl-ammonium bromide), 218-228, Copyright 2014, with permission from Elsevier. Cc: collector concentration.

    As can be seen from the above results that although many researchers have done a lot of work in the research and development of new cationic collectors, the types of cationic collectors are still relatively small and the ether amines still dominate in the industry even with lower selectivity. The development of new cationic collectors is still an important work in the future.

    Early iron ore flotation processes usually used anionic collectors directly, usually fatty acids, petroleum sulfonates, and later hydroxamic acid salts [100]. As well as resin acids, soaps, alkyl sulfonates, and alkyl sulfonates were often used to flotation iron ores, especially hematite [101]. Recently, the anionic reverse flotation process is more and more widely used in China’s mineral processing industry.

    Min et al. [102] introduced a complex anionic collector 915BM for use in the reverse flotation of high phosphorus oolitic hematite from western Hubei. The results showed that a concentrate with 55.90wt% Fe and iron recovery of 80.73% was obtained under the conditions of a pulp temperature of 25°C, coarse selection pH value of 11, and dosages of starch, calcium oxide, 915BM, and selected 915BM of 1000, 500, 600, and 200 g/t, respectively.

    Cui et al. [103] developed a new type of aliphatic carboxylic acid collector CM-5 was prepared by modifying the structure of the agent, and single-mineral flotation experiments of quartz, chlorite, and hematite were carried out. The results showed that CM-5 had a better selective collection effect on chlorite after the action of starch and CaCl2 under high alkaline conditions, but a weaker effect on hematite under the same conditions.

    Tang et al. [104] conducted a temperature adaptability experiment on the new low-temperature resistant anion collector CY-12# and found that a concentrate with 69.68wt% Fe and the iron recovery rate can reach 98.62% under the condition that the slurry temperature was 15°C. Compared with the flotation index obtained at the temperature of 30°C, the iron concentrate grade decreased by 0.27 percentage points, but the iron recovery increased by 0.44 percentage points.

    Luo et al. [105] found that the chlorinated oxidation of tar to form chlorinated and non-chlorinated fatty acids and oleic acid were mixed to obtain RA series anion collectors, which have the characteristics of simple production, low cost, relatively non-toxic, and common raw materials. RA-315 was initially used in Anshan Iron and Steel’s Qidashan concentrate, China, which increased its iron grade to 65.33wt%, and the iron recovery rate was 80.72%. Later, RA-515, RA-715, and RA-915 were developed, of which RA-715 and RA-915 had been used in industrial anion reverse flotation and showed good collection and selectivity [106].

    Fang et al. [107] conducted flotation experiments on single minerals using a new collector ((E)-8,11-dihydroperoxyoctadec-9-enoic acid, EDEA), and the results found that EDEA has a better flotation effect on quartz when pH > 11, and it has almost no flotation effect on hematite and magnetite when pH = 8~13. Under the conditions of pH = 12 and CaCl2 dosage of 60 mg/L, the recovery rate of quartz was as high as 92.35% when the dosage of EDEA was 120 mg/L, while the recovery rate of quartz was 62.44% when the dosage of oleic acid was 120 mg/L. The small-scale flotation test results found that EDEA has a better separation effect on quartz and iron ore when the flotation temperature is low.

    Luo et al. [108] synthesized a new type of anionic collector YRA-5 using cotton oil oleic acid as raw material. In the synthesis process, an appropriate amount of surfactant was added to improve collector activity. The reverse flotation test of 1 coarse, 1 fine, 2 sweeps closed-circuit process was carried out on the strong magnetic tailings of the second stage of Lilou Iron Mine, China, at a slurry temperature of 17°C. The result showed that YRA-5 has good selectivity and low-temperature resistance, and the obtained concentrate had an iron grade of 65.91% and a recovery rate of 91.61%.

    Shanxi Tengfei Mining Company, China, adopted a magnetic separation–reverse flotation process to process low-grade iron ore with an iron content of 15.01wt%–26.60wt%, which has the problems of unstable concentrate quality, low recovery rate, and high tailings grade. Given the technical problems of the mine, Ge et al. [109] studied a new type of normal temperature collector MG. In the production and application of Tengfei Mining Company, China, MG obtained excellent indicators of iron concentrate grade of 65.18wt% and recovery rate of 92.17%. Compared with the company’s original collector, the recovery rate has increased by 7.62%, and the tailings grade has decreased by 9.96%, which indicates that MG is a good collector for normal temperature reverse flotation.

    At present, collectors have higher requirements on temperature, which makes the selection and control of dosing in the flotation process more and more complicated, and the energy consumption is high. For anion reverse flotation, low-temperature anion collectors are gradually favoured, and its development is particularly important.

    Mixed collectors are usually mixtures of anionic and cationic collectors with higher selectivity compared to single collectors [110111]. Appropriate synergistic mixtures can reduce the consumption of the collector, thus reducing environmental pollution and production costs. Chen et al. [112] synthesized a new type of amine combined collector XK-28 with a mass ratio of XK-I : XK-II as 2:8 and conducted a roughing experimental study on the actual minerals fed in Gongchangling flotation. The results showed that a better flotation index can be obtained when the pH value of the pulp was 6.5 and the dosage of XK-28 was 90 g/t. The iron concentrate grade is 68wt%, and the concentrate iron recovery rate was 62%, which was about 4% higher than the dodecylamine grade. In addition, the results of defoaming experiments showed that XK-28 as a collector overcomes the disadvantages of the high viscosity of dodecylamine foam and difficulty in defoaming. With regard to ultrafine magnetite concentrates, Lu et al. [113] studied a reverse flotation collector mixture with sodium oleate (NaOl) and dodecylamine (DDA). During reverse flotation, the combined action of anionic and cationic collectors made most of the quartz and iron-bearing minerals (chlorite) be removed from the magnetic concentrate at the same time, and the optimal molar ratio of NaOl and DDA was 10, while the starch inhibits ultrafine magnetite. Although the particle size was less than 25 μm, a good separation effect was achieved. Finally, under the condition that the iron content was 52.98wt%, the effect of iron concentrate grade and recovery rate of 65.52% and 80.66% was achieved, respectively.

    Many scholars have also conducted comparative studies on the collection effects of anionic collectors, cationic collectors, and mixed collectors. Tian et al. [114] performed experiments on the separation of titanium pyroxene from ilmenite using the anionic collector NaOl, dodecylamine acetate (DAA), and a mixture of the two (NaOl–DAA). The results showed that the mixed solution can separate ilmenite and titanium pyroxene in the pH value of 5–7. Compared with DAA and NaOl, the mixed collector had excellent flotation performance and high selectivity, and reagent consumption using this mixture was greatly reduced compared to using NaOl alone. Yang et al. [115] showed that cetyltrimethylammonium bromide (CTAB) mixed with NaOl also had a better separation effect than the two alone. As shown in Fig. 9, under specific conditions, particularly at a pH value of 5.5, with a molar ratio of CTAB : NaOl as 2:1 and at a concentration of 0.2 mmol/L, the obtained iron grade was 57wt% with a recovery rate of 79%, while the recovery of the two collectors cannot exceed 65% when used at similar or even higher doses respectively. Using CTAB, Fe grade reached 53wt% at 0.12 mmol/L with a recovery of 42%, while Fe grade reached 46wt% grade at 3.0 mM with a recovery of 60% using NaOl.

    Fig. 9.  Mixed collector of NaOl/CTAB and its application effect: (A) graphical illustration of adsorption model of mixed CTAB/NaOl on the surface of minerals; (B) mechanism of action; (C) flotation recoveries of magnetite and enstatite with mixed collectors as a function of pulp pH (Mole ratios of CTAB to NaOl = 2:1, CTAB and NaOl combined 4 × 10−4 mol/L); (D) the effect of mole ratio of CTAB to NaOl on flotation recoveries of magnetite and enstatite at pH 5.5 [115]. Reprinted from Colloids Surf. A., 570, Z.C. Yang, Q. Teng, J. Liu, W.P. Yang, D.H. Hu, and S.Y. Liu, Use of NaOl and CTAB mixture as collector in selective flotation separation of enstatite and magnetite, 481-486, Copyright 2019, with permission from Elsevier.

    The classification and comparison of different types of collectors and their advantages and disadvantages are shown in Table 6. Relatively speaking, mixed collectors are the most promising in the flotation industry, which is of great significance in improving the performance of existing reagents, increasing production indicators, reducing costs, and solving practical production problems. In mixed trap systems, the adsorption of cation and anion trap ions increases simultaneously, and the surface cation excess is comparable to that of the anion. The mechanism of synergistic effect after the combination of collectors can be summarized as follows: co-adsorption mechanism, hydrophobic end lengthening mechanism, adsorption promotion mechanism, and solution environment improvement. At present, combined collectors are mainly used in the flotation of refractory ores such as ilmenite, quartz, apatite [116118].

    Table  6.  Comparison of three kinds of the collectors
    TypeCollectorAdvantagesDisadvantages
    Cationic collectorMixed amine, aliphatic amine, dodecylamine, ether amine, PAMSimple operation and flotation reagent system; good separation effect by combing with magnetite separation process.Poor selectivity, low efficiency, and heavy pollution.
    Anionic collectorHalogenated fatty acid, hydroxy fatty acid, sulfonated fatty acid, amino fatty acid, amide fatty acidsStrong adaptability of raw materials, high separation effect, steady production, and used widely.Complex operation and flotation reagent system, used in high temperature environment and high energy consumption.
    New room temperature collectorDBA-2, DBA-1, DMP-1, DTX-1, DL-1, DZN-1, Fly-101, …Strong adaptability of raw materials, low energy consumption, high separation effect, steady production, and used widely.Being still in the laboratory research or semi-industrial test stage.
    Mixed collectorXK-28, NaOl–DDA, NaOl–DAA, CTAB–NaOl, …Higher selectivity, lower consumption of the collector, lower environmental pollution, and production costs.There are few studies on the mechanism of synergistic effect, which hinders its development to a certain extent.
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    In the context of global efforts to reduce CO2 emissions, low-carbon ironmaking technologies are being actively developed globally. Whether it is to increase the proportion of pellets in the BF or to develop hydrogen metallurgy technology, it is inseparable from the production of high-grade pellets. The improvement of the ore grade into the BF reduces the amount of slag and the coke ratio of the BF, thereby achieving the purpose of reducing CO2 emissions and realizing the low-carbon operation of the BF. It is generally believed that a 1 percent point increase in iron ore grade in the furnace reduces the BF coke ratio by about 1.5% [119]. Besides, if the proportion of pellets in the BF burdens is increased by 1%, CO2 emissions per ton of steel can be reduced by about 0.25%–0.3%. The core of producing high-grade pellets is the acquisition of high-grade iron ore concentrate. Therefore, in order to obtain high-grade concentrate, it is necessary to improve the beneficiation technology first. The long-term development of low-carbon ironmaking technology is inseparable from the development of mineral processing technology.

    (1) In the context of global decarbonization, there remain subjects regarding the reduction of CO2 emissions in the steel industry. From a production perspective, the main source of carbon emissions is the use of coke and coal, which is related to the energy consumption structure of China’s ISI. In traditional carbon reduction metallurgy, the BF ironmaking process is the main source of CO2 and gas pollutants, and the key to energy saving and emission reduction in the ISI lies in the ironmaking process.

    (2) Carbon peaking and carbon neutrality goals have become the future strategic directions in China. The coal-based energy structure and the BF–converter-based smelting process have led to huge pressure on carbon reduction. Under the current technical conditions, new technologies such as injecting hydrogen-rich fuel to replace solid carbon fuel, removing CO2 from furnace top gas and recycling it, replacing sintering with pellets, and gas-based shaft furnace direct reduction represented by green hydrogen, etc., will achieve engineering application and promotion in the future.

    (3) In China’s ISI, the long process of sintering–blast furnace–converter accounts for 90%. Therefore, increasing the proportion of pellets entering the furnace and developing gas-based reduction are important ways to reduce carbon emissions, which require more high-quality iron ore concentrates. However, due to the lack of high-quality iron resources in China, it is necessary to develop advanced beneficiation technology to produce and prepare pellet concentrates.

    (4) The main feature of fine-grained minerals is that the mass is small but the surface area is large, so the sorting of fine-grained minerals is relatively difficult. Previous research on fine-grained iron ore dressing showed that stage grinding–stage magnetic separation and magnetic separation–reverse flotation process are likely effective processes for processing fine-grained magnetite. For the fine particle hematite, the effective separation methods are strong magnetic–desliming–reverse flotation, selective flocculation–reverse flotation, and strong magnetic–centrifugal beneficiation. Magnetization roasting is usually used for the beneficiation of weak magnetic iron ores such as hematite, limonite, and siderite. Compared to the shaft furnace, rotary kiln magnetization roasting technology has been relatively mature in the application of mineral processing and can achieve a better separation effect. However, rotary kiln magnetized roasting has the problems of high energy consumption and serious clinker formation, which still need to be explored and solved in the future.

    (5) The flotation effect of reverse flotation is good, and it has a good development prospect. The choice of cationic and anionic flotation agents depends on the mineralogical structure of the iron ore itself. The cationic flotation agent will produce more foam during the flotation process, which is not conducive to continuous industrial production, so it is less used in China. Compared with the cationic reverse flotation process, the anionic reverse flotation process has the advantages of higher concentrate grade, lower sensitivity, and lower collector cost. However, reverse anionic flotation requires more activators and needs to be completed at higher temperature and alkalinity conditions. Relatively speaking, mixed collectors are the most promising in the flotation industry, and they are expected to use fewer flotation agents to achieve better flotation under the same conditions.

    This work was financially supported by the Natural Science Foundation China (No. 52274343), the Youth Natural Science Foundation China (No. 51904347), and the China Baowu Low Carbon Metallurgy Innovation Foundation (No. BWLCF202102).

    Zhengqi Guo is a youth editorial board member for this journal and was not involved in the editorial review or the decision to publish this article. All authors declare that there are no competing interests.

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