The hot metal dephosphorization has been uniquely developed to meet the increasing demand for low phosphorus steel production in Japan. The dephosphorizing reagent is usually composed of lime (CaO), iron ore (FeOx) and some other additives. The generated CaO-SiO2-FeOx-P2O5 based slag after dephosphorization contains considerable amount of solid CaO, which causes problems such as increase of slag volume and difficulty of slag recycling. Though fluorspar (CaF2) had been used as an additive to enhance CaO dissolution into liquid slag, the using of CaF2 is restricted at present owing to its toxic property to human society. Consequently, the hot metal dephosphorization slag is a multi phase flux with solid and liquid phases coexisting. The reduction of CaO consumption and slag emission is the existing problem that the steel makers confront. Since the dephosphorizing slag contains both liquid and solid phases, it is considered that the problem can be solved by improving the transfer of phosphorus from liquid phase into solid phase in the slag. Therefore, the idea of developing an innovative refining process by using multi phase flux is proposed. In order to develop the new technology, the fundamental information including thermodynamic data, mass transfer, microscopic reaction mechanism, physical properties of the multi phase flux etc are indispensable. As a part of the whole research project, the present thesis is devoted to clarify the reaction behavior of phosphorus in the multi phase flux and the influence factors.

The experimental investigation is composed of three parts: reaction between solid 2CaO・SiO2 and homogeneous CaO-SiO2-FeOx-P2O5 slag at 1573 and 1673 K, reaction between 2CaO・SiO2 and multi phase CaO-SiO2-FeOx-P2O5 slag at 1573 K and dissolution behavior of silicocarnotite (5CaO・SiO2・P2O5) into the homogeneous CaO-SiO2-FeOx slag at 1573 and 1673 K.

Many researchers suggested that the precipitation of 2CaO・SiO2 phase is the necessary condition for the formation of solid 2CaO・SiO2-3CaO・P2O5 phases in the multi phase flux. Following this idea, solid 2CaO・SiO2 is firstly reacted with homogeneous CaO-SiO2-FeOx-P2O5 slag to examine the formation of solid P2O5 rich phases. The synthesized solid 2CaO・SiO2 disc is dipped into liquid slag with various composition for different reaction time at 1573 and 1673 K. After the reaction, the interface between 2CaO・SiO2 and slag is observed and analyzed by SEM/EDS. According to the analysis of the interfaces after reaction for different time, the reaction mechanism is clarified. It is understood that the P2O5 condensed phases are formed at the 2CaO・SiO2/slag interface as fast as less than 1 s. However, the solid P2O5 rich particle is not big enough to be detected separately by the present analysis method. Thus the P2O5 condensed phase is identified as the mixture of 2CaO・SiO2-3CaO・P2O5 solid solution or compound and the surrounding liquid slag. The occurrence of dissolution of the solid P2O5 rich phase soon after its formation is observed, indicating that the transfer of phosphorus in the multi phase flux from liquid phase to solid phase is reversible. Reaction temperature and CaO/SiO2 ratio of the initial slag influence the stability of solid P2O5 rich phases. Higher temperature induces the dissolution of P2O5 condensed phase while larger CaO/SiO2 ratio of the bulk slag has the opposite effect.

Upon the finding that the solid P2O5 condensed phase re-dissolves into the liquid slag, solid 2CaO・SiO2 is also reacted with the multi phase CaO-SiO2-FeOx-P2O5 slag. The liquid phase of the multi phase slag is saturated with solid 5CaO・SiO2・P2O5 at 1573 K before the reaction. The P2O5 condensed phases with FeO content less than 16.0 mass%, CaO/SiO2 ratio larger than 1.5 and P2O5 content over 1.0 mass% are observed at the interface. It is considered that the solid 2CaO・SiO2 reacts with the liquid phase in the multi phase slag to form the P2O5 condensed phase. Compared with the results of reaction with homogeneous slag, the P2O5 condensed phases can be observed in much wider region. Such difference implies that the re-dissolution of the P2O5 condensed phases is restrained in the case that the bulk liquid phase is saturated with solid P2O5 rich phase.

Since it is observed that the transfer of phosphorus in the multi phase flux from liquid phase to solid phase is reversible, the dissolution behavior of the 2CaO・SiO2-3CaO・P2O5 solid solution or compound in liquid slag is studied. The synthesized 5CaO・SiO2・P2O5 being the representative of solid P2O5 rich phase is dipped into the homogeneous CaO-SiO2-FeOx slag at 1573 and 1673 K. Since the initial slag is free of P2O5, the increase of P2O5 content in the slag is the indication of the dissolution behavior of solid 5CaO・SiO2・P2O5. The result shows that the dissolution of solid phosphate compound can be divided into the following stages: interaction, disintegration, reaction and diffusion. The diffusivity of P2O5 in the liquid slag is calculated according to the concentration profile of P2O5 across the interface. In accord with the results in the abovementioned study, higher temperature is favored for the dissolution in some cases, while larger CaO/SiO2 ratio of the bulk slag has the opposite effect.

A synthetic consideration is made on the reaction behavior of P2O5 by comparing the present findings of phosphorus behavior at 2CaO・SiO2/slag interface with the previously reported results of phosphorus behavior at CaO/slag interface. It is clarified that the 2CaO・SiO2-3CaO・P2O5 solid solution or compound formed at CaO/slag interface is not the product of the reaction between firstly formed 2CaO・SiO2 phase, and CaO plus P2O5 in the slag as proposed by the previous researchers, but directly formed by the reaction between solid CaO, and SiO2 plus P2O5 in the slag. In the phase diagram of the CaO-SiO2-FeOx-P2O5 quaternary system, the existence of primary phase regions of solid 5CaO・SiO2・P2O5 and 2CaO・SiO2-3CaO・P2O5 solid solution is assured according to the formation of 2CaO・SiO2-3CaO・P2O5 phases both at CaO/slag and 2CaO・SiO2/slag interfaces. The reaction behavior of P2O5 in the CaO-SiO2-FeOx-P2O5 multi phase flux can be well explained by considering the phase relationship in the CaO-SiO2-FeOx-P2O5 quaternary system.

According to the experimental findings and the synthetic consideration of the reaction mechanism, an innovative dephosphorization process is proposed to improve the refining efficiency while ease the environmental burden. In the new process, two types of lime with different function are added by different patterns. Lime powder with small particle size is mainly served as the reagent for slag formation and dephosphorization, while porous lime lump with large particle size is used for fixing the phosphorus from the liquid phase. The purpose of reducing the lime consumption and slag emission can be achieved. The innovative process also shows better recyclability than the present existing technology.

In brief, the reaction behavior of P2O5 in the multi phase flux is clarified in the present thesis. The mechanism, procedure and influence factors for the phosphorus transfer from liquid phase into solid phase are understood. This study has provided reliable fundamental information to develop the innovative refining technology in steel production.

本論文は、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、効率的に脱りんを行うために使用されるCaO系フラックスが固相CaOを含む固液共存融体であることから、このフラックスをマルチフェーズフラックスととらえて、固体CaO系化合物を利用したマルチフェーズフラックスへのりんの濃化反応、溶融銑鉄から除去するプロセスの反応機構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、鉄鋼製錬における脱りん反応に関するこれまでの研究、および固液共存のCaO系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、CaO-SiO2-FeOx-P2O5系フラックスと固体2CaO・SiO2の反応挙動を測定し反応機構を解析した結果を述べている。1523および1673Kで溶融したCaO-SiO2-FeOx-P2O5系フラックスへ固体2CaO・SiO2を1~600秒浸漬し、反応界面近傍の生成化合物、フラックスの状況をSEM観察し、また組成を分析することにより、反応機構について考察した。P2O5を含む化合物は2CaO・SiO2-3CaO・P2O5 固溶体であり、P2O5を含む化合物相は反応初期に短時間の間に固液界面近傍で生成し、固体2CaO・SiO2のフラックスへの溶解とともに、フラックス中へ溶解するという機構を明らかにした。また、反応温度、フラックスの塩基度にP2O5を含む化合物の生成速度が依存することを明らかにした。

第3章では、第2章で明らかにしたP2O5を含む化合物の反応機構に及ぼす影響を明らかにするため、固体5CaO・SiO2・P2O5を含むCaO-SiO2-FeOx-P2O5系マルチフェーズフラックスに固体2CaO・SiO2を1~600秒浸漬し、反応界面近傍の生成化合物、フラックスの状況をSEM観察し、また組成を分析することにより、反応機構について考察した結果を述べている。第2章での観察結果に比べて、固体5CaO・SiO2・P2O5を含むフラックスの場合は、P2O5を含む固体化合物が溶融フラックス中に広範囲にわたり観察されている。この観察結果から、固体5CaO・SiO2・P2O5を含む場合の、固液界面近傍でのP2O5を含む固体化合物の生成に及ぼす影響が大きいと結論し、P2O5を含む固体化合物の生成機構を提案している。

第2章で、フラックス中に生成する固溶体は2CaO・SiO2-3CaO・P2O5 であることを明らかにしたので、第4章では、2CaO・SiO2-3CaO・P2O5 のフラックス中への溶解挙動を明らかにするため、P2O5を含む固体化合物としてシリコカーノタイト5CaO・SiO2・P2O5を用いて、1523および1673KでCaO-SiO2-FeOx系フラックスへ浸漬し、固液界面近傍の反応について解析を行った結果を述べている。反応界面近傍におけるP2O5を含む固体化合物濃度の分布から、溶解反応過程と拡散過程からなる反応機構を提案し、反応温度、塩基度の影響について考察している。

第5章では、第2章から第4章までの実験で得られた結果を基にCaO系マルチフェーズフラックス中でのP2O5を含む固体化合物相の生成について、CaO-SiO2-FeOx-P2O5四元系状態図での相関係から考察した結果を述べている。これまでの測定結果から、固体CaOと溶融フラックス界面での反応について、5CaO・SiO2・P2O5もしくは2CaO・SiO2-3CaO・P2O5固溶体が初期に生成し、続いて2CaO・SiO2相、CaO-FeO相が生成するという新しい反応機構を提案している。

第6章では、実際の製鋼プロセスにおいて効率的に脱りん反応を行うため、第2章から第5章で得られた結果から脱りんプロセスを熱力学に基づいて考察し、マルチフェーズフラックスを活用した新しい精錬プロセスの提案、操業条件の検討結果を述べている。

第7章では本論文の統括である。

以上のように、本論文ではマルチフェーズフラックス中でのP2O5を含む化合物の反応機構を明らかにし、鉄鋼精錬プロセスにおける脱りん反応を熱力学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2章、第3章、第4章、第5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。