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<p>The direct reduction of high-phosphorus oolitic iron ore (HPOIO) faces the dual challenges of inefficient dephosphorization and restricted iron grain growth due to its elevated phosphorus content and ultrafine mineral dissemination. This study proposes a CaCO<sub>3</sub>–Na<sub>2</sub>SO<sub>4</sub> composite additive to synergistically address both issues. Process optimization demonstrated that adding 5% CaCO<sub>3</sub> and 5% Na<sub>2</sub>SO<sub>4</sub> increased the metallization rate of reduced briquettings from 66.40 to 88.56%, with the equivalent circle diameter (ECD) of metallic iron grains expanding from 12.16 to 19.40&#xa0;μm. The magnetic separation performance significantly improved: the iron grade increased from 82.84 to 92.57%, the iron recovery increased from 76.20 to 87.23%, the phosphorus content decreased from 0.14 to 0.063%, and the dephosphorization rate increased from 67.88 to 90.45%. Mineralogical characterization revealed that the composite additive facilitated iron particle coalescence via FeS liquid‒phase bridging, forming a continuous metallic iron network favorable for slag‒metal separation, whereas phosphorus-bearing minerals were immobilized in the CaO‒Al<sub>2</sub>O<sub>3</sub>‒SiO<sub>2</sub>‒P<sub>2</sub>O<sub>5</sub> multiphase slag. Mechanism investigations revealed that CaO derived from CaCO<sub>3</sub> decomposition reacts with metastable P<sub>2</sub>O<sub>5</sub> to form thermodynamically stable calcium phosphates, driving phosphorus enrichment in the slag phase. Concurrently, Na<sub>2</sub>SO<sub>4</sub> promoted low-melting FeS liquid formation, accelerating iron grain migration and driving metallic iron coarsening. This dual-function mechanism provides a technically and economically viable solution for the industrial utilization of refractory HPOIO.</p> Graphical Abstract <p></p>

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Efficient Dephosphorization and Enhanced Iron Grain Growth in the Direct Reduction of Refractory High-P Oolitic Iron Ore: Mechanisms Promoted by CaCO3–Na2SO4 Additives

  • 期刊论文

  • 发表日期：

    2025年7月15日

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  • Mengjie Hu,
  • Deqing Zhu,
  • Jian Pan,
  • Zhengqi Guo,
  • Congcong Yang,
  • Siwei Li,
  • Wen Cao

摘要

The direct reduction of high-phosphorus oolitic iron ore (HPOIO) faces the dual challenges of inefficient dephosphorization and restricted iron grain growth due to its elevated phosphorus content and ultrafine mineral dissemination. This study proposes a CaCO3–Na2SO4 composite additive to synergistically address both issues. Process optimization demonstrated that adding 5% CaCO3 and 5% Na2SO4 increased the metallization rate of reduced briquettings from 66.40 to 88.56%, with the equivalent circle diameter (ECD) of metallic iron grains expanding from 12.16 to 19.40 μm. The magnetic separation performance significantly improved: the iron grade increased from 82.84 to 92.57%, the iron recovery increased from 76.20 to 87.23%, the phosphorus content decreased from 0.14 to 0.063%, and the dephosphorization rate increased from 67.88 to 90.45%. Mineralogical characterization revealed that the composite additive facilitated iron particle coalescence via FeS liquid‒phase bridging, forming a continuous metallic iron network favorable for slag‒metal separation, whereas phosphorus-bearing minerals were immobilized in the CaO‒Al2O3‒SiO2‒P2O5 multiphase slag. Mechanism investigations revealed that CaO derived from CaCO3 decomposition reacts with metastable P2O5 to form thermodynamically stable calcium phosphates, driving phosphorus enrichment in the slag phase. Concurrently, Na2SO4 promoted low-melting FeS liquid formation, accelerating iron grain migration and driving metallic iron coarsening. This dual-function mechanism provides a technically and economically viable solution for the industrial utilization of refractory HPOIO.

Graphical Abstract