Phosphorus is an associated element with iron ore, which always dissolves in molten iron after iron making. The existence of phosphorus in steel significantly decreases the toughness of the final product. Consequently, phosphorus has been normally considered as an impurity and its content in steel product must be controlled below certain level.

Following this requirement, dephosphorization process for ferrous metallurgy has been developed. Thermodynamically, proper low temperature, high basicity and high oxygen partial pressure benefit the removal of phosphorus from hot metal. Therefore, lime-silicate-wustite slag has been widely used for achieving high phosphate capacity due to the consideration of producing cost.

After the addition of lime into the molten slag, it is expected that the slag composition is controlled within the dicalcium silicate primary region. However, due to the slow dissolution speed of lime at low refining temperature, proportion of solid phase increases and the kinetic condition for dephosphorization becomes worse, which leads to low dephosphorization efficiency. Normally, fluorite acts as an effective melting agent to accelerating the dissolution of lime, but the utilization of it has been restricted because of its harmfulness on the environment. Although larger FeO content provide better melting conditions, the FeO content in the slag cannot be increased too much, for the sake of preventing slag foaming and protecting the refractories. As a result, in order to achieve high basicity of the slag, the remained option is to add excessive amount of lime, which brings serious issues such as huge slag amount and low utilization efficiency of lime.

With the principle of effective utilization of the solid phase, an innovative development of refining process by using multiphase flux for dephosphorization has been investigated.

The research on the reaction behavior and mechanism of CaO with molten slag, mass transfer of phosphorus from liquid phase to solid solution, reaction behavior of phosphorus, phosphorus partition ratio between solid solution and liquid slag for various slag systems and the series of kinetic studies have been proceeded in last few decades.

However, on pursuing the reaction mechanism of phosphorus between solid solution and liquid slag, proper phase diagrams are still unavailable. Therefore, current research is aimed to focus on the equilibrium phase relationship for the multiphase flux system.

By using chemical equilibration technique the equilibrium phase relationship for the CaO-SiO2-FeO-P2O5(-Al2O3) system with oxygen partial pressure of 10(-10) atm or 10(-8) atm at 1673K and 1623K have been investigated.

In the case of the CaO-SiO2-FeO-P2O5 quaternary system with oxygen partial pressure of 10-10 atm at 1673K, liquidus has been proposed for the projection on the CaO-SiO2-FeO ternary section after the discussion of phase relationship. Comparing with the isothermal for the CaO-SiO2-FeOx ternary system equilibrated with iron at the same temperature, the liquid phase area remains similar at moderate SiO2 region, and shrinks towards the FeO apex with increasing T.Fe content. As equilibrated with liquid phase, the composition of solid solution locates along the tie line between 2CaO・SiO2 and 3CaO・P2O5.

For the same slag system equilibrated with oxygen partial pressure of 10(-8) atm at 1673K and 1623K, the phase relationship has also been clarified. Since higher oxygen partial pressure has been used, the liquidus at 1673K shifts against the CaO apex due to the increase of Fe(3+)/Fe(2+) ratio. Also, the liquid phase shrinks with decreasing temperature. By comparing with the liquidus for CaO-SiO2-FeO ternary system with oxygen partial pressure of 10(-8) atm at 1573K, the liquid phase area with same temperature at 1623K even shrinks. This is because the condensation of 2CaO・SiO2-3CaO・P2O5 solid solution consumes more CaO than 2CaO・SiO2 or2CaO・SiO2 .

Since Al2O3 normally exists in the slag system, the phase relationship for the CaO-SiO2-FeO-P2O5-Al2O3 system has been observed with oxygen partial pressure of 10(-10) atm at 1673K. It has been found that the liquidus shifts towards the CaO apex which means the liquid area enlarges, comparing with both the liquidus for the CaO-SiO2-FeOx ternary system equilibrated with metallic iron and the liquidus for the CaO-SiO2-FeO ternary system with oxygen partial pressure of 10(-10) atm at 1573K. The effect of Al2O3 addition on the enlargement of the liquid area is significant.

Besides, the solid solution equilibrated with the liquid phase in all slag systems investigated by current research, which has been confirmed to be 2CaO・SiO2-3CaO・P2O5 with different ratio between both, is homogeneous and other possible solid solutions such as 3CaO・SiO2 have not been observed.

By comparison of the phase relationship for various slag systems in current study, comprehensive discussions on the liquid phase, solid solution and the combination of both have been conducted. The the conclusions for the liquid phase have been drawn as above. While for the solid solution, it has been found that the effect of oxygen partial pressure, temperature and the Al2O3 addition on the solid solution is negligible. After discussion on each of the liquid phase and solid solution, the combination between both by using phosphorus partition has also been studied. It has been found that the phosphorus partition ratio between solid solution and liquid phase increases linearly with increasing T.Fe content in liquid phase and decrease with increasing CaO content in liquid phase.

For further investigating the effect of slag composition on the phosphorus partition ratio, regular solution model has been adopted for calculating the activity coefficients of components in liquid phase. The calculated activity coefficients and activities of P2O5 are very low which indicates a high phosphate capacity for multiphase flux when reacts with carbon saturated iron. Then the relationship between the calculated thermodynamic properties and the slag compositions such as T.Fe and CaO content in liquid phase has been discussed.

In order to evaluate the utilization of multiphase flux for the dephosphorization process, simulation for the application of current observed phase relationships on the practical dephosphorization process with the perspective of mass balance has been conducted. The total slag amount and the CaO consumption of dephosphorization process by using multiphase flux have been obtained. Both values are much lower comparing with the condition of practical refining, which indicates the advantage of using multiphase flux for dephosphorization.

鉄鋼中不純物元素のりんを除去する鉄鋼精錬の脱りんプロセスでは、脱りん処理に用いられるフラックスは固相CaOを含む固液共存融体であるCaO系マルチフェーズフラックスとなっており、マルチフェーズフラックスへのりんの濃化反応機構の研究が進められている。本研究は、反応機構を検討するための基礎となるCaO-SiO2-FeO-P2O5系フラックスの平衡相関係を明らかにした研究であり7章からなる。

第1章は序論であり、研究の背景、鉄鋼製錬における脱りん反応に関するこれまでの研究、マルチフェーズフラックスによるりんの除去反応機構に関するこれまでの研究について述べ、本研究の位置づけ、重要性を明らかにし、本研究を行う背景、目的について述べている。

第2章では、本研究で用いた化学平衡法による実験方法について述べ、実験条件の決定法について説明している。

第3章では、実際の脱りんプロセスにおける操業条件に近い酸素分圧10(-10) atm、温度1673KでのCaO-SiO2-FeO-P2O5系フラックスの固溶体相と液相の相関係を明らかにした結果について述べている。CaO-SiO2-FeO-5mass%P2O5系フラックスを1923Kで溶融し、酸素分圧10(-10) atmで1673Kまで10K/min で冷却し、固液共存状態を保持し平衡させたのち試料を急冷した。冷却後の試料をSEM観察し、またEDSにより組成分析することにより、固溶体相、液相の相の同定、組成の分析を行い、相関係を明らかにし、その結果をCaO-SiO2-FeO、およびCaO-SiO2-P2O5系三元系状態図にまとめた。液相線はCaO-SiO2-FeO系の液相線とほぼ同様であり5mass%P2O5の影響は小さく、固溶体相の組成は2CaO・SiO2-3CaO・P2O5の組成となっていることを明らかにした。

第4章では、相関係に及ぼす酸素分圧、温度の影響を調べるため、第3章と同様の実験を、酸素分圧10(-8) atm、温度1673Kおよび1623Kで行い、CaO-SiO2-FeO-P2O5系フラックスの相関係を測定した結果について、CaO-SiO2-FeO、およびCaO-SiO2-P2O5系三元系状態図にまとめている。液相線は高FeO側に移動し、液相領域は小さくなる。固溶体相の組成は2CaO・SiO2-3CaO・P2O5の組成となっていることを明らかにした。

第5章では、精錬プロセス中でフラックス中に含まれるAl2O3 の相関係に及ぼす影響を明らかにするため、酸素分圧10(-10) atm、温度1673KでのCaO-SiO2-FeO-5mass%P2O5-5mass%Al2O3系フラックスの相関係を測定した結果について述べている。得られたCaO-SiO2-FeO、およびCaO-SiO2-P2O5系三元系状態図から、5mass%Al2O3の添加により液相線は高CaO側に移動し、液相領域が拡大することを報告している。これはCaO-SiO2-FeO系で報告されているAl2O3 の添加効果と同様であり、Al2O3 添加による液相領域の拡大効果を明らかにした。また、固溶体相の組成は2CaO・SiO2-3CaO・P2O5の組成であり、Al2O3 添加による影響はないことを報告している。

第6章では、第3章から第5章までで得られた固溶体相-液相の相関係の実験結果を基に、CaO-SiO2-FeO-P2O5-Al2O3系マルチフェーズフラックスの相関係を考察し、また、マルチフェーズフラックスを用いた脱りんプロセスの検討結果について述べている。実験結果から推定したCaO-SiO2-FeO-P2O5四元系状態図での固溶体相-液相の相関係に基づいて、脱りんプロセスにおいて、固溶体相へ鋼中りんがりん酸化合物として除去される機構を新しく提案している。また、マルチフェーズフラックスを活用したりんの除去プロセスでの操業条件を検討し、CaO原単位を大きく減らすことができるという結果を示している。

第7章では本論文の統括である。

以上のように、本論文ではCaO-SiO2-FeO-P2O5系フラックスの固溶体相-液相の平衡相関係を明らかにし、鉄鋼精錬プロセスでのマルチフェーズフラックスによる脱りん反応を相関係に基づいて考察して、精錬プロセスに関する重要な知見を得ており、本研究の成果はマテリアルプロセス工学への寄与が大きい。

なお、本論文第3章、第4章、第5章、第6章は松浦宏行、月橋文孝との共同研究であるが、論文提出者が主体となって分析及び検証を行ったもので、論文提出者の寄与が十分であると判断する。

したがって、博士(科学)の学位を授与できると認める。