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Qipeng Bao, Lei Guo, Hong Yong Sohn, Haibin Zuo, Feng Liu, Yongliang Gao, and Zhancheng Guo, New process for treating boron-bearing iron ore by flash reduction coupled with magnetic separation, Int. J. Miner. Metall. Mater., 31(2024), No. 3, pp. 473-484. https://doi.org/10.1007/s12613-023-2756-9
Cite this article as:
Qipeng Bao, Lei Guo, Hong Yong Sohn, Haibin Zuo, Feng Liu, Yongliang Gao, and Zhancheng Guo, New process for treating boron-bearing iron ore by flash reduction coupled with magnetic separation, Int. J. Miner. Metall. Mater., 31(2024), No. 3, pp. 473-484. https://doi.org/10.1007/s12613-023-2756-9
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研究论文

闪速还原熔分–磁选处理硼铁精矿的新方法



  • 通讯作者:

    郭磊    E-mail: leiguo@ustb.edu.cn

    郭占成    E-mail: zcguo@ustb.edu.cn

文章亮点

  • (1) 提出了一种闪速还原熔分-磁选处理硼铁精矿的新方法。
  • (2) 研究了一步、两步处理方法的还原和渣铁熔分效果。
  • (3) 解析了矿粉颗粒闪速还原熔分过程中的矿相转变和形貌演变规律。
  • 硼是一种重要的工业原料,硼资源常常与铁、镁等矿相伴生,而且矿相之间以细密的形式掺杂在一起,很难通过常规的选矿工艺实现不同矿相的解离和分离。本文提出一种闪速还原熔分–磁选处理硼铁精矿的新方法,该方法可以实现矿粉在颗粒尺度的渣铁熔分,结合破碎–磁选操作获得金属铁粉和富硼渣。分离出的渣相中B2O3含量可达18%以上,而金属铁中B含量低于0.03%。本文分别对一步法和两步法闪速还原熔分开展了研究,探究了矿粉粒度、温度等因素对还原和熔分过程的影响规律。并对闪速还原熔分过程中矿粉颗粒的矿相转变和形貌演变规律进行了深度解析。
  • Research Article

    New process for treating boron-bearing iron ore by flash reduction coupled with magnetic separation

    + Author Affiliations
    • Boron is an important industrial raw material often sourced from minerals containing different compounds that cocrystallize, which makes it difficult to separate the mineral phases through conventional beneficiation. This study proposed a new treatment called flash reduction–melting separation (FRMS) for boron-bearing iron concentrates. In this method, the concentrates were first flash-reduced at the temperature under which the particles melt, and the slag and the reduced iron phases disengaged at the particle scale. Good reduction and melting effects were achieved above 1550°C. The B2O3 content in the separated slag was over 18wt%, and the B content in the iron was less than 0.03wt%. The proposed FRMS method was tested to investigate the effects of factors such as ore particle size and temperature on the reduction and melting steps with and without pre-reducing the raw concentrate. The mineral phase transformation and morphology evolution in the ore particles during FRMS were also comprehensively analyzed.
    • Boron is an important industrial resource widely used in metallurgy, medicine, military, aerospace, nuclear energy, and agriculture [12]. Boron resources are unevenly distributed worldwide and are mainly in Turkey, the United States, Russia, Chile, and China. Boron-bearing minerals are generally of low grades and contain multiple types of accompanying minerals. For example, more than 90% of boron-bearing ores in China contain less than 12wt% B2O3 (the lowest economic grade for boron ore processing), making it difficult to directly utilize these ores. Boron-containing iron ores, which are mainly composed of iron, boron, magnesium, and uranium, are a potential resource for boron. For industrial applications, raw boron-iron ores need to be first processed and sorted to produce boron concentrates, iron concentrates, and tailings. Boron concentrates can be used for the production of borax, and iron concentrates can be used for iron production by blast furnaces [34] or direct reduction in a rotary hearth furnace [5]. The grade of boron-iron ores in China is relatively low (B2O3: 6wt%–8wt%, TFe: 27wt%–30wt%), and their mineral composition is complex. Fine-grained gangue minerals such as magnetite and szaibelyite (Mg2(OH)B2O4(OH)) are distributed in boron-iron ores. Boron and iron are difficult to completely separate by traditional beneficiation [67]. In addition, simultaneously improving the grade of the boron concentrate and the yield of boron is challenging [8].

      The iron concentrates (hereinafter boron-bearing iron concentrates) obtained from the beneficiation of boron-iron ores still contain 3wt%–6wt% B2O3. The clean separation of boron and iron in boron-bearing iron concentrates is critical for the comprehensive utilization of both elements. Since the 1950s, many scholars have conducted extensive research on this topic and proposed numerous treatment processes, including beneficiation method [911], wet leaching method [1215], direct-reduction magnetic-separation method [1,1618], blast furnace method [19], direct-reduction electric-furnace-melting method [20], and rotary hearth furnace method [2122]. When boron-containing iron concentrates are used in the blast furnace, the contents of B2O3 and MgO in the slag are increased, resulting in the poor fluidity of the slag and an increase in the coke ratio, thereby increasing the operating cost of the blast furnace. The high-magnesium slag causes serious erosion of the refractory materials, affecting the lifespan of the blast furnace. In addition, the partial reduction of boron oxide and the dilution by the added slag-forming agents lower the B2O3 content in the slag, affecting the secondary recovery of boron. Coal-based direct reduction in a rotary hearth furnace can selectively reduce boron-bearing iron concentrates to achieve the secondary separation of boron and iron. However, the pellet raw materials for this process require complex preparation, and the reducing agent used in the pellets inevitably introduces impurities, such as sulfur, resulting in an increase in the impurity content in the molten iron. In addition, ash and other additives dilute B2O3 in the slag, affecting the further recovery and extraction of boron. Among the numerous processing methods for boron-bearing iron concentrates, melting or magnetic separation based on pyroreduction can achieve the complete separation of boron and iron [23].

      In the flash reduction of iron ore powder, the ore particles come into contact with the reducing gas above the melting temperature and complete the reduction within a few seconds [24]. This technology has received widespread attention from the steel industry in recent years [2529]. In this process, the reduction is carried out at temperatures close to or exceeding the melting points of the metallic iron and slag phase, and the ore particles are completely reduced and simultaneously melted in the diluted gas phase. Under the Marangoni effect [3031], a particle morphology of spherical metallic iron wrapped by a slag phase is obtained. For some iron ores with complex compositions, the ore particles complete the flash reduction and flash melting in a very short time. Under this extreme condition, some impurity elements between the slag and iron exhibit nonequilibrium distribution. For example, the phosphorus content in the metallic iron phase after flash reduction–melting separation (FRMS) is lower than that after conventional direct reduction [32].

      This study introduces one-step and two-step FRMS for the treatment of boron-bearing iron concentrates. The former uses the as-received concentrate, and the latter involves the pre-reduction of the concentrate. Both methods were tested. Through flash reduction/melting, the reduction of boron-bearing iron concentrates with H2 and the separation of the slag/iron melts are achieved at the particle scale. The metallic iron phase and the slag phase containing boron can be efficiently separated through simple crushing and magnetic separation.

      The boron-iron ores used in this study were from Liaoning province, China, and their main components are shown in Table 1. The B content (in B2O3 equivalent) was 5.49wt%, and the iron grade was 51.41wt%; the ores also contained 0.8wt% sulfur. The results of comprehensive characterization are shown in Fig. 1. Macroscopically, the boron-iron ore powder appears dark gray. Microscopically, the surface of the particles has distinct edges and corners, and the interior is dense. As shown in Fig. 1(b), multiple minerals exhibit complex intercalation relationships, making it difficult to separate each mineral phase through simple beneficiation. XRD phase analysis shows that the particles are mainly composed of magnetite (Fe3O4), chrysotile (Mg3Si2O5(OH)4), szaibelyite (Mg2(OH)B2O4(OH)), ludwigite ((Mg,Fe)2FeBO5), and quartz (SiO2). The contents of S, Al, Ca, and other elements are low, and their spectral peaks are not shown. As a light element, boron is difficult to detect by EDS analysis. EPMA was used to characterize the main elements of the minerals to accurately determine the mineral phase distribution of the boron-iron ores, as shown in Fig. S1.

      Table 1.  Chemical compositions of the boron-bearing iron concentrates wt%
      TFe FeO MgO SiO2 B2O3 S Al2O3 CaO TiO2 MnO
      51.41 21.24 16.88 5.77 5.49 0.80 0.36 0.31 0.14 0.10
       | Show Table
      DownLoad: CSV
      Fig. 1.  Morphologies and phases of the raw material: (a) macrograph, (b) XRD pattern, (c, d) surface morphology, and (e) cross-section morphology.

      FRMS experiments were performed on the boron-iron ore particles combined with pre-reduction according to the requirements. The slag and iron phases were then separated by grinding the solidified particles, followed by magnetic separation. The configurations of the flash reduction/melting reactor and the fixed bed pre-reduction device used in the experiment are shown in Figs. S2 and S3, respectively. Both of these instruments were heated with silicon molybdenum rods, and the tubes were made of corundum (the tube sizes of the flash reduction/melting reactor and the fixed bed pre-reduction device were ϕ80 mm × 1200 mm and ϕ60 mm × 1000 mm, respectively).

      The specific experimental conditions and parameters are listed in Table 2. First, the effects of temperature and particle size on the reduction and melting and the morphology evolution in the particles during the one-step FRMS were studied. Owing to the complex intercalation relationships of multiple minerals, no significant difference in composition was observed among the particles with different sizes. For the two-step method, pre-reduction was first conducted at different temperatures to determine the appropriate conditions. The ore powder was then pre-reduced under these conditions and used as the raw material for FRMS to investigate the effects of variables such as particle size and reduction temperature. The pre-reduction gas was pure H2 with a flow rate of 500 mL·min−1 and a reduction time of 4 h. The reduction time for the FRMS experiment was around 1–2 s [33]. The feeding rate of the concentrate in the FRMS experiment was set to 0.1128 g·min−1, and the reducing atmosphere was H2 with a flow rate of 200 mL·min−1.

      Table 2.  Experimental parameters
      Experiments Pre-reduction temperature / °C FRMS temperature / °C Particle size / μm
      One-step method 800/900/1000/1100/1200/
      1300/1400/1500/1550
      <30/30–50/50–100/
      100–150/150–300
      Pre-reduction 700/800/900 50–100
      Two-step method 900 1480/1500/1520/1540 50–100
      Two-step method 900 1550 30–50/50–100/
      100–150/150–300
       | Show Table
      DownLoad: CSV

      For the experiments, the boron-bearing iron concentrates of different particle sizes (<30, 30–50, 50–100, 100–150, 150–300 μm) were reduced under different temperatures (800−1500°C at 100°C intervals, and 1550°C). The cross-sectional morphology of the processed sample particles is shown in Fig. 2. At temperatures below 1100°C, the metalization ratio was low, and the cross-sectional morphology was not significantly different from that of the raw material. Therefore, only the particle morphologies at 1100°C and above are presented.

      Fig. 2.  Cross-sectional morphologies of the particles of different sizes after the one-step FRMS at different temperatures.

      Fig. 2 indicates that with the increase in temperature, the morphology of the particles gradually changed, and the amount of metallic iron increased. At the same temperature, the degrees of particle melting and reduction increased with the decrease in particle size. At 1200°C, the large particles (>100 μm) showed no significant changes, and some small iron-bearing particles were reduced. In addition, the reduced metallic iron particles softened and aggregated into a dense metallic iron shell, enveloping the mineral phase with a low internal metalization ratio. The internal mineral phase composition of the particles was relatively complex (Fig. 2(c2)) and clearly divided into two different regions. EDS detection results show that the lighter-colored region was mainly the wustite, and the darker-colored region was the slag phase containing SiO2 and MgO. At the flash reduction temperature of 1300°C, most of the iron-containing mineral particles smaller than 30 μm were still present in the form of particles wrapped in metallic iron shells. Meanwhile, the metallic iron layer noticeably thickened. A small number of the particles were present in the form of flocculent metallic iron embedded in the slag, as shown in Fig. 2(a3). Some of the 30–100 μm particles melted into spherical shapes, and those larger than 100 μm showed no significant morphology changes.

      At the FRMS temperature of 1400°C, the metalization ratio of particles smaller than 100 μm was high, and a large amount of metallic iron accumulated inside the particles. The small metallic iron particles dispersed in the slag phase had spherical shapes. The particles of 100–150 μm sizes completely melted into spherical shapes with low metalization ratio, and the slag phase was distributed as a network in the interstices of wustite. The particles larger than 150 μm underwent partial melting and exhibited ellipsoidal or irregular shapes. When the flash reduction temperature was 1500°C, the particles of all size ranges underwent melting and spheroidization, and their metalization ratio gradually decreased with the increase in particle size. The reduction, spheroidization, and slag–iron–melt separation of the particles smaller than 30 μm were relatively complete, and the main body was spherical metallic iron with a small number of flocculent slag particles adhering to the surface. At 1550°C, the particles smaller than 30 μm melted thoroughly, and the metallic iron and slag disengaged well. The slag surrounded the metallic iron ball, forming an outer layer.

      The metallic iron and total iron contents of the samples were measured by chemical titration analysis, and the metallization ratio was calculated, as shown in Fig. 3. The metalization ratio of the ore particles increased with temperature within the experimental range, but the impact of temperature varied on the particles of different sizes. For the particles larger than 50 μm, the metalization ratio was consistently low (less than 20%) within the experimental temperature range and did not change significantly with temperature. For the 30–50 μm particles, the metallization ratio increased slightly as the temperature increased under 1300°C. Above 1400°C, the metalization ratio rapidly increased with temperature, which was consistent with the changes in particle microstructure. Most particles had dense metallic iron shells at 1400°C. At 1550°C, the metalization ratio reached 97%. For the particles smaller than 30 μm, the reduction degree increased rapidly with temperature above 1000°C. Above 1500°C, the metalization ratio reached a relatively high level of 97%. At 1550°C, the increase in the metalization ratio was no longer significant, ultimately reaching 99%.

      Fig. 3.  Variation in the metalization ratio of the particles after FRMS treatment under different conditions.

      On the basis of the experimental results for the one-step FRMS, the fine-grained particles perform well in FRMS under high temperatures. The reduction and separation of slag and iron at the particle scale may be achieved simultaneously by applying appropriate experimental conditions. Under the experimental conditions in this work, the 30–50 μm particles need to reach 1550°C, and those smaller than 30 μm only need to reach 1500°C to achieve good reduction and melting separation.

      Fig. 2 shows that the morphology of the ore particles varied greatly at different temperatures during FRMS. Even at the same temperature, multiple types of particles with different morphologies appeared mainly due to the differences in particle composition and initial morphology. The different phase compositions and initial morphologies of the ore particles were indicative of their different reduction and melting characteristics. In addition, the metallic iron content and melting fraction of the ore particles reacting under the same conditions were also different. Understanding the morphology evolution in the particles of different sizes during the reduction is necessary to achieve a high metalization ratio and good slag/iron separation. The particles of <30, 50–100, and 150–300 μm size ranges were selected as representative samples, and the morphology changes of the iron-containing particles during FRMS were analyzed as shown in Fig. 4.

      Fig. 4.  Schematic of particle morphology evolution in FRMS.

      Fig. 4(a) shows the morphology evolution in the ore particles of <30 μm sizes. At 1100°C, the particles with good reduction characteristics (a2-1) were continuously reduced, and the reduced metallic iron was distributed inside and outside the particles in a morphology similar to sponge iron. Meanwhile, the morphology of the gangue phase remained basically unchanged. The particles with poor reduction characteristics (a2-2) were dense internally and only had a small amount of metallic iron on the surface. When the temperature was increased to 1200°C, the reduction rate accelerated, and the iron generation rate was greater than the iron diffusion rate into the particles. The newly generated metallic iron had a relatively high surface energy and softened on the surface of the particles, forming a dense metallic iron layer. The unreduced part inside a particle (a3-1) had a high content of iron oxides and a low content of gangue, resulting in a uniform wustite phase with a low melting point. During cooling and solidification, a small amount of the slag that was originally dissolved in the wustite phase continuously precipitated and finally distributed in the interstices of the wustite. Some other particles (a3-2) were mainly composed of the gangue phase that had a high melting point and thus did not melt. When the temperature was increased to 1300°C, the metalization ratio further increased, and the metallic iron began to soften. Some particles (a4-2) formed a surface metallic iron layer at low temperatures and began to migrate inward. However, the metallic iron layer aggregated on the surface due to the high MgO content, high viscosity, and high migration resistance of the internal slag and eventually formed dispersed small iron balls. Some particles (a4-1) had a thick metallic iron layer that did not melt completely and still maintained a dense layer. When the temperature was further increased to 1400°C, the metallic iron further softened, causing it to sag and migrate inward (a5-2) on some particles. Some completely melted particles (a5-1) almost achieved the complete separation of slag and iron. At 1500°C, most of the metallic iron phases (a6-1) in the particles achieved complete phase disengagement, and the slag formed an outer layer adhering to the surface of the metallic iron balls. A few particles with a high-viscosity slag (a6-2) formed multiple iron spheres dispersed in the slag. At 1550°C, the particles (a7) were completely melted and spheroidized, and the slag and iron were fully melted and separated. The slag phase formed an outer layer (whose size depends on the amount of slag in the particle) that adhered to the metallic iron ball. With the metalization ratio further increasing, the residual iron oxide content and the overall volume of the slag further decreased.

      Fig. 4(b) shows the morphology evolution of the ore particles of 50–100 μm sizes. Below 1200°C, the particle morphology was not significantly different from that of the raw material. When the temperature was increased to 1300°C, the main body of a particle melted to form a spherical shape. Some particles (b3-1) had a thin layer of metallic iron on the surface due to the low metalization ratio, and the slag was dispersed in the matrix of the wustite phase. When the temperature was increased to 1400°C, some particles with low iron contents (b4-1) showed fine metallic iron spheres on the surface. The metallic iron spheres did not further coalesce due to the high-viscosity slag acting as the main body of the particles. The particles with high iron contents (b4-2) formed a morphology of metallic iron migrating toward the interior of the particles, and the surface of the particle was fully covered by a layer of metallic iron. At 1500°C, the metallic iron further aggregated and formed various intermediate shapes. The majority of the metallic iron aggregated and became embedded in the wustite phase due to the higher wettability between metallic iron and wustite compared with slag. When the temperature was increased to 1550°C, the amount of the low-melting-point FeO component decreased in some high-magnesium slag due to the increase in the metalization ratio. The resulting increase in the melting point of the slag hindered the aggregation of metallic iron and caused the formation of particle morphology, as shown in (b6-2). Some other particles (b6-1) had good slag/iron separation, and the metallic iron aggregated into spherical shapes.

      Fig. 4(c) shows the morphology evolution in the ore particles of 150–300 μm sizes. The maximum temperature for the large particles under the same experimental conditions was relatively low due to their large size. Therefore, the morphology of the particles below 1300°C showed minimal difference from the raw material. At 1400°C, the particles began to soften. Although the slag viscosity in these particles was high due to the presence of components such as SiO2 and MgO, they still maintained the approximate contour of the raw material (c3). When the temperature was increased to 1550°C, the particles completely melted into a uniform liquid phase and became spherical. However, the solubility of MgO in the wustite was limited, and it continuously precipitated during the cooling, forming a network distribution as shown in (c4-1). Some particles with a high content of gangue (c4-1) did not form a uniform liquid phase, even at high temperatures. These particles were divided into two phases: an irregularly shaped wustite inside and a slag containing MgO outside.

      According to the analysis of particle morphology evolution, reducing the particle size of the raw materials can effectively improve the reduction and melting effects. The high magnesium content in the ores had a significant impact on the fluidity of the slag during spheroidization. Especially in the case of a high metalization ratio, the low content of the low-melting-point FeO in the slag and the high viscosity of the slag phase hindered the separation of the slag and metallic iron. Under the experimental conditions in this work, the ore particles smaller than 50 μm achieved good separation through the one-step FRMS. For the large ore particles, the main constraint was still the reduction. Therefore, the two-step process of combining low-temperature pre-reduction and FRMS was investigated and discussed in the following section.

      The boron-bearing iron concentrates of 50–100 μm size were pre-reduced at 700, 800, and 900°C using hydrogen, and the metalization ratios of the pre-reduced samples are shown in Fig. 5.

      Fig. 5.  Iron content and metallization ratio of ore particles before and after pre-reduction (50–100 μm).

      The total iron content of the boron-bearing raw iron concentrates was 49.4wt%. When the pre-reduction temperature was increased, the metalization ratio of ore powder also increased, and the metallization ratio reached 97% at 900°C. When the temperature was increased to 1000°C, the ore powder softened, the particles adhered to each other, and the metalization ratio did not significantly increase. The micromorphology and element distribution of the particles after pre-reduction at 900°C are shown in Fig. S4.

      Under the experimental conditions in this work, the magnetite phase was reduced to form porous iron. However, ludwigite was relatively dense and difficult to completely reduce, and its reduction products were also difficult to coalesce and were dispersed in the gangue phase as very fine grains. Even when the metallization ratio of the reduced iron reaches 97% after pre-reduction, separating boron-rich slag and metallic iron by simple grinding and separation is still impossible. In addition, the metalization ratios of the large ore particles pre-reduced at 900°C were lower than those of the 50–100 μm particles. The large particles with low metalization ratios constituted the majority of the unreduced part of the ore. Thus, at high temperatures, the iron oxide must be separated from the boron/magnesium-containing phase to increase the reduction. In the following section, the results of the FRMS experiments using the ore particles with relatively high metalization ratios pre-reduced at 900°C will be discussed.

      Ore particles with different sizes (30–50, 50–100, 100–150, and 150–300 μm) were first pre-reduced at 900°C and then used as raw materials in subsequent FRMS under a hydrogen atmosphere at 1550°C. The particles melted as they traveled through the gas and were collected as a solid product. Their microscopic morphology is shown in Fig. 6.

      Fig. 6.  Morphologies of the powders with different particle sizes after the two-step FRMS at 1550°C.

      After the two-step FRMS, the particles smaller than 150 μm achieved a good separation of slag and iron, with the two phases attached together. The particles of 150–300 μm sizes were limited to a relatively short residence time in the high-temperature zone. Although the particles were not completely melted, the significant separation of the slag and metallic iron phases was observed, and the particles became denser, which is conducive to the subsequent crushing and magnetic separation. The metalization ratios of the samples after the pre-reduction and subsequent FRMS were determined by chemical titration, as shown in Fig. 7.

      Fig. 7.  Metallization ratios of samples with different particle sizes after pre-reduction and subsequent FRMS.

      The metallization ratios of the pre-reduced ore powder and the product from the two-step treatment were higher for the samples with small particle sizes. After pre-reduction, the ore particles of 150–300 μm size reached a metallization ratio of 79%, which increased to 89% after FRMS. The metallization ratio of the ore particles with 30–50 and 50–100 μm size reached over 99% after FRMS.

      The ore particles of 50–100 μm size pre-reduced at 900°C were used as the raw materials in the subsequent FRMS under a hydrogen atmosphere at different temperatures (1480, 1500, 1520, and 1540°C). The microscopic morphologies of the collected samples are shown in Fig. S5.

      Well-separated slag and spherical iron particles were obtained from the particles of 30–50 μm sizes subjected to the two-step processing at 1550°C and subsequent grinding and magnetic separation. As shown in Fig. 8, the proposed two-step method could effectively achieve the separation of the slag phase and metallic iron, and the slag content in the iron was very small. The main phases of the separated slag were suanite (Mg2B2O5), kotoite (Mg3BO6), and olivine (Mg2SiO₄). The obtained metallic iron powder maintained a spherical shape without significant deformation because iron could be separated from the slag phase at a low manual grinding intensity.

      Fig. 8.  Phase analysis of the spherical metallic iron and slag obtained after grinding and magnetic separation: (a) surface morphology of ore particles before grinding; (b) surface and (c) cross-section morphologies of the separated metallic iron; (d) surface and (e) cross-section morphologies of separated slag; XRD patterns of separated (f) iron and (g) slag.

      Grinding and magnetic separation were also performed on the ore powder treated by the two-step process under other conditions to study the effects of temperature and particle size on the distribution of boron in the separated slag and metallic iron. The B contents in the slag and iron phases were determined by ICP analysis, as shown in Fig. 9. The B content in the metallic iron was calculated as the mass fraction of elemental B, and that in the slag was calculated as the mass fraction of B2O3. Fig. 9(a) shows the B content in the slag and iron for different particle sizes. Boron was mainly distributed in the slag phase for the particles of 30–50 and 50 –100 μm sizes, with the B2O3 content in the slag reaching over 18wt% and the B content in the iron below 0.03wt%. In the large particles of 100–150 μm sizes, the B2O3 content in the separated slag was lower at 16.7wt%, and the B content in the iron remained essentially unchanged. This phenomenon occurs because the metallization ratio of the 100–150 μm particles after FRMS was 96%, and about 5% of the unreduced iron oxide was present in the form of FeO in the slag, causing the dilution of B2O3 in the slag. For the same reason, the B2O3 content in the separated slag was even lower at only 14.5wt% for the ore particles of 150–300 μm sizes, which had a low metalization ratio. Owing to the incomplete melting and aggregation of the slag and metallic iron in the ore particles of 150–300 μm sizes, the slag and metallic iron become difficult to separate. After magnetic separation, the metallic iron contained some slag, which increased the content of B in the iron.

      Fig. 9.  Variation of B content in separated slag and iron with (a) particle size and (b) temperature.

      Fig. 9(b) shows the B content in the slag and metallic iron obtained from the 50–100 μm particles subjected to the two-step process with FRMS at different high temperatures, followed by the grinding and magnetic separation of the produced particles. The B2O3 content in the separated slag did not change significantly with temperature and remained at around 18.3wt%. The B content in the separated metallic iron decreased slightly with the treatment temperature. As shown in Fig. S5, the particles of 50–100 μm sizes exhibited poor separation at 1480 and 1500°C, and a large amount of sponge-structured iron was found in the particles. During the grinding, the broken brittle slag was embedded into the irregular sponge-structured iron, causing slag inclusion and increased B content in the final separated iron.

      Analysis of the effects of different particle sizes and temperatures on the B content in the slag and iron after magnetic separation reveals that the metalization ratio of the ore powder and the melting of the slag and iron had a significant impact on the recovery of boron in the slag. When the metalization ratio was low, the unreduced iron oxide diluted the B content in the slag. Poor slag/iron melting caused slag inclusion in the iron phase, affecting the quality of iron powder and decreasing the slag recovery.

      The previous raw material analysis shows that the main boron-bearing minerals in the boron-bearing iron concentrates were ludwigite and szaibelyite. The ludwigite itself is a boron-iron composite mineral phase and was closely embedded in magnetite and gangue. After the low-temperature pre-reduction, ludwigite was reduced to metallic iron and suanite, with szaibelyite being converted to suanite. Ludwigite was relatively dense and difficult to completely reduce, and its reduction products were also difficult to separate. Metallic iron dispersed in the gangue phase in the form of small grains. Under the pre-reduction conditions of this work, some unreduced ludwigite phases remained inside the ore particles. Owing to the low pre-reduction temperature, the boron in the suanite phase was not reduced and absorbed into the iron phase. EPMA elemental analysis also confirms that the boron was mainly distributed in the suanite phase and the residual ludwigite phase.

      The 30–50 μm ore particles obtained from FRMS at 1550°C in the two-step method were subjected to EPMA elemental analysis, and the results are shown in Fig. 10. Fig. 10(a) shows the typical details of the boron-containing slag/iron particles. The slag was in a glassy state and was mainly composed of B2O3, MgO, SiO2, and a small amount of Al2O3. A small amount of MgO precipitated at the outer edge of the slag because the particles were small and the slag was rapidly cooled down from being subjected at 1500°C to room temperature within a few seconds, causing it to remain in a glass phase. Fig. 10(b) shows that boron was distributed in the slag phase, but no obvious boron was observed in the metal iron. The B content varied greatly among the different particles of the slag, with some particles not even containing any boron.

      Fig. 10.  EPMA element analysis of particle cross-section after two step FRMS at 1550°C: cross section information of (a) a typical particle and (b) different particles; (c) element concentrations in different points.

      As shown in Fig. 10(c), the B2O3 content in some slag particles (point 1) reached 38.68wt%, and the metallic iron (point 2) did not contain boron. Therefore, even at 1550°C, boron oxide was difficult to reduce within a few seconds of FRMS, and most of it was enriched in the slag. Therefore, the B content in the metallic iron measured after grinding and magnetic separation was mainly brought by the inclusion of the slag rather than being chemically dissolved in the metal phase.

      In this test, the boron-bearing iron concentrate particles were pre-reduced at 900°C, placed in a graphite crucible, and melted under a protective atmosphere (Ar) in a tube furnace. After holding the temperature at 1550°C for 60 min, the crucible and its content were cooled to room temperature to obtain a slag and metallic iron. Fig. 11 shows the morphology of the melted content, and the results of the EPMA element analysis at various points in Fig. 11(b) and (d) are listed at the bottom. The metallic iron was blocky and very dense, with a thin layer of slag attached to the surface. The slag had the shape of a gray and black block with a brittle texture. The phases in the slag can be divided into two types according to different colors. The dark gray point 1 contained suanite and a small amount of SiO2 in a solid solution. The light gray points 2 and 3 were olivine in columnar and irregular granular forms mainly composed of MgO and SiO2 (the molar ratio of MgO/SiO2 is close to 2). Small amounts of Al2O3 and B2O3 were also found in the solid solution. According to the contrast and elemental analysis, the bright white matrix (point 4) was metallic iron containing 0.33wt% B and 0.75wt% Si, which originated from the reduction of the corresponding oxides by C in the graphite crucible at the high temperature. In addition, a small amount of gray ferrous sulfide (point 5) was present in the iron matrix. According to ICP analysis, the molten iron contained 0.35wt% B and 1.13wt% S, indicating that direct melting caused the valuable B and the harmful S to enter the metallic iron phase.

      Fig. 11.  Morphologies and phase analysis of the iron and slag after direct smelting at 1550°C: (a) macro and (b) micro morphologies of slag phase; (c) macro and (d) micro morphologies of iron phase; (e) quantitative analysis of elements with EPMA.

      On the basis of the above results, a technology roadmap for using boron-bearing iron concentrates in FRMS was proposed, as shown in Fig. 12. First, the low-grade ludwigite ore is subjected to a preliminary separation step to obtain a boron-containing iron concentrate, a boron concentrate, and gangue tailings through processes such as magnetic beneficiation or magnetic-levitation combined with beneficiation. The boron concentrate can be directly subjected to boron refining, and the boron-containing iron concentrate is treated using FRMS. Owing to the requirements of beneficiation, the ore is finely ground to a particle size of below 200 mesh (75 μm). The fine-grained (<30 μm) boron-bearing iron concentrate can be treated by the one-step FRMS, and the coarse particles (>30 μm) by the two-step FRMS. The fully reduced and melted slag/iron bound particles obtained from this step are separated into spherical metal iron powder and boron-rich slag through further grinding and magnetic separation. The obtained high-quality iron powder can be considered as feeds for powder metallurgy or steelmaking, and the boron-rich slag can be used for boron production after roasting and activation.

      Fig. 12.  Technology roadmap of using boron-bearing iron concentrates in FRMS.

      A process called FRMS was developed for utilizing boron-bearing iron concentrates to reduce the ore particles while achieving slag/iron separation at the particle scale. Spherical metal iron powder and boron-rich slag were obtained after grinding and magnetic separation, thereby achieving the separation of iron and boron components. The specific conclusions relevant to the development of this process are listed below.

      (1) Through the one-step FRMS for fine-grained boron-bearing iron concentrates, reduction and slag/iron separation could be achieved simultaneously, and good reduction and melting effects could be achieved at temperatures above 1550°C. For large ore particles, the two-step FRMS method is proposed. This approach includes a pre-reduction step that reduces the burden of FRMS and effectively lowers the processing temperature.

      (2) The treatment temperature has no significant impact on the B contents in the separated slag and iron. Boron is mostly distributed in the glassy slag phase. The B2O3 content in the slag can reach over 18wt%, and the B content in the iron is less than 0.03wt%.

      (3) Compared with traditional processes such as coal-based reduction and electric furnace melting or blast furnace ironmaking, FRMS does not require pelletizing and slagging. Slag/iron separation is completed in a very short time, and the migration of harmful elements to the metal iron phase is effectively inhibited. The final micro-alloyed spherical iron powder could be used as a good feed in various fields, such as powder metallurgy. This process is a promising new approach for the comprehensive utilization of complex iron ores.

      Acknowledgements: This study was financially supported by the Science and Technology Special Plan Project from China Minmetals Group (No. 2020ZXA01), the International Exchange and Growth Program for Young Teachers (No. QNXM20220061), and the National Key Research and Development Program of China (No. 2022YFC2906100).

      Zhancheng Guo is an editorial board member for this journal and was not involved in the editorial review or the decision to publish this article. The authors declare no conflict of interest.

      The online version contains supplementary material available at https://doi.org/10.1007/s12613-023-2756-9.

    • Supplementary Information-s12613-023-2756-9.docx
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