This study investigates the thermodynamic properties of the ferroboron alloys and boron oxide containing slags, which coexist in a carbothermic ferroboron process. This study is composed of 6 chapters.

In the chapter 1, a detailed introduction was given for ferroboron alloys. Most common production methods were paid some special attention. Basic points about carbothermic reduction and metallothermic reduction processes, their initial process design parameters were mentioned. After introducing the common practices, a deeper look into the ferroboron production were given and most important problems of the current production trends were discussed.

In addition, previous studies on these problems were mentioned in detail. It was seen that the literature lacks the required reliable and complete data for ferroboron processing. Therefore, construction of a thermodynamic database for ferroboron production processes was taken as the main objective of this study. In order to achieve this goal, this study was divided into two major parts: the thermodynamics of Fe-B based liquid ferroboron alloys and the thermodynamics of B2O3-bearing slag systems.

In chapter 2, the thermodynamic properties of the Fe-B alloy systems were investigated experimentally. As the experimental method, chemical equilibration with liquid Ag was selected. However, because of a lack of reliable data on the Ag-B system in the 1773-1873 K range, the properties of B in liquid Ag were examined as preliminary experiments. The solubility of B at 1273 K, 1773 K and 1873 K was measured as 0.055 ± 0.003, 0.149 and 0.305 ± 0.0075, respectively. The activity coefficient of B at infinite dilution in Ag,γ0(B in Ag) , with respect to solid standard state was determined to be 194 and 172 at 1773 K and 1873 K respectively, for solutions of B concentration less than 0.01 at. %.

After finding the properties of B in the Ag-B system, Fe-B binary alloys were examined for the activities of the components at 1773 K and 1873 K. Activity vs. composition relations were clarified and it was seen that the binary Fe-B system deviates considerably in the negative direction for both components at these temperatures. The excess Gibbs energy of the alloys at 1873 K was also calculated to compare the degree of deviation with other binary Fe-X alloys. At the same time, the activity coefficient of B at infinite dilution,γ0(B in Ag) , and self interaction parameter of B in Fe,εB(B in Fe) , were calculated at 1773 K and 1873 K.

After clarifying the properties of the Fe-B binary system, investigations were expanded to the Fe-B-C ternary system to determine the effect of C on the properties of the B at 1873 K. Initially, the C solubility was determined as a function of B and a strong inverse relation was found. Following this, the activities of B and Fe were measured and isoactivity curves of B and Fe were drawn for the liquid Fe-B-C alloys at 1873 K. Following the activity calculations, the effect of C on B was mathematically expressed by the interaction parameter of C on B at C saturation as εC(B Csatd)=11.8+0.4 This value was considered to be in excellent agreement with the solubility and reported data.

Before finalizing the experiments on the Fe-B based alloys, the solubility of C in the Fe-B-Si-C quaternary alloys was determined at 1873 K. It was found that the addition of Si into Fe-B-C(satd). alloys results in a further decrease in the C solubility. Therefore, it was concluded that elemental Si can be used to reduce the C content of the melts without any refining process.

In chapter 3, B2O3-containing binary slag systems were investigated at 1873 K by chemical equilibration method using Cu as the reference metal. The thermodynamic properties of B2O3-bearing slags were of primary importance in ferroboron processing; therefore, three binary systems i.e., MgO-BO(1.5), CaO-BO(1.5) and SiO2-BO(1.5), were evaluated in terms of thermodynamic properties, for their potentials as alternative raw materials.

Experiments were performed using Fe-B-Csatd. alloys and Cu as reference metals under 1 atm CO atmosphere in graphite crucibles at 1873 K. Even though initial experiments utilized Fe-B-C(satd). alloys as the reference metal phase for activity measurements in the MgO-BO(1.5) and CaO-BO(1.5) binary systems, because of the limitations arising from the use of these reference alloys, the reference melt was replaced by Cu to perform experiments within complete BO(1.5) range.

To measure the properties of slags by this method, the properties of B in the liquid Cu-B system became necessary at 1873 K. Therefore, the properties of B in Cu phase were measured and compared with the published data and they exhibited quite good agreement. The self interaction parameter of B in Cu phase and the activity coefficient of B at infinite dilution were calculated and the temperature dependences of these properties were shown in mathematical expressions.

After investigating the Cu-B binary alloys, the measurements of the BO(1.5) activities were continued in the MgO-BO(1.5) and CaO-BO(1.5) systems at 1873 K under 1 atm CO atmosphere. From the measured data of BO(1.5) activities, the activities of the other components were calculated by Gibbs-Duhem integration.

The alkaline-earth borate systems, MgO-BO(1.5) and CaO-BO(1.5), showed similar complex behaviors. At low BO(1.5) concentrations, both systems deviated in negative direction indicating the alkaline-earth oxides had higher affinity for BO(1.5) and isolate the species, (BO3)(3-), whereas with an increase in BO(1.5) composition beyond X(BO1.5) > 0.5, the deviation drastically shifted from negative to positive. This significant change was attributed to the formation of more complex ions and finally the beginning of the formation of the polymeric borate network.

As the third binary system, SiO2-BO(1.5) binary melts were investigated. However, after initial experiments, significant Si dissolution in the Cu phase was observed. Thus, to perform the measurements, the effect of Si on B in the ternary Cu-B-Si alloys was evaluated by controlled dissociation of B into various Cu-Si alloys equilibrated with BN under fixed N2 pressure. The first- and second-order interaction parameters of Si on B were calculated as ε(si)B=-0.61 and ρ(si)B=-14.53.

After determining the properties of B in the Cu-B-Si ternary system at 1873 K, activities of BO(1.5) and SiO2 were measured and calculated, respectively. This system showed a strong negative deviation, implying a strong interaction between the two components in liquid state.

In order to make a comparison between these three binary systems for ferroboron processing, the concept of B partition ratio was introduced. According to this pre-evaluation, the raw material with the highest γ(BO1.5) was expected to yield the highest B dissolution. The highest γ(BO1.5), i.e., the lowest B partition and thus the highest B recovery was expected from MgO-BO(1.5) slags, the next highest from CaO-BO(1.5) and the lowest from SiO2-BO(1.5).

In chapter 4, as the third part of the experiments, this study was extended to the ternary slag systems with the introduction of SiO2 to the MgO-BO(1.5) and CaO-BO(1.5) slags because the presence of Si in the metal phase, and hence, its resulting oxide SiO2 will lead the formation of ternary slags. Like in binary system, experiments were performed by chemical equilibration method under controlled C-CO equilibrium at 1873 K.

The results showed that both ternary systems exhibited considerable negative deviations within the investigated range. Although binary systems of alkali-earth borates exhibited similar behavior, the effect of SiO2 was noticeably different in each system. In the MgO-BO(1.5)-SiO2 system, the activity of BO(1.5) decreased to a minimum value for slags containing about 5-10 mass % SiO2. With an increase of the SiO2 beyond 30 mass %, the effect of SiO2 leveled off. On the other hand, in the CaO-BO(1.5)-SiO2 system, at low BO(1.5) compositions, the addition of SiO2 caused a slight increase in activity up to 25 mass % SiO2 indicating a preferential interaction between CaO and SiO2. Beyond this point, the activity of BO(1.5) reached a constant value and becomes virtually independent of SiO2 or CaO content for an almost constant BO(1.5) composition. For this system, the 15-25 mass % SiO2 composition range was considered to be the composition having the highest BO(1.5) activity.

Based on the activity measurements in two ternary slag systems, SiO2 was found to be highly effective in altering the BO(1.5) activity in both systems, even at small amounts because of its ability to change the behavior of BO(1.5); this was especially evident in the MgO-BO(1.5)-SiO2 system. In the light of the results, the addition of SiO2 was expected to be disadvantageous in yielding higher B recovery in ferroboron processing.

In chapter 5, a detailed evaluation of the possible applications of the measured data was performed. Based on the experimental findings and some processing assumptions, the initial criteria for alternative B2O3-bearing material selection were proposed. Furthermore, the alternative materials, selected among natural boron minerals, were evaluated in terms of process efficiency by considering the method of reduction, the production scale and the final product specifications.

Also, the conditions to obtain highest B recovery from these candidate minerals were evaluated. As effective process parameters, the raw material, i.e., the initial slag composition, the final product composition and the partial pressure of CO; hence, the final slag composition were selected. It was found that among these process parameters, the starting material had the highest impact on the process efficiency. Also, a decrease in the partial pressure of CO by applying vacuum had a significant positive effect on B recovery. Morevoer, as the last and least effective method, lowering the B content in metal phase resulted in a slight increase in the B recovery.

Eventually, in chapter 6, the results of the experiments of Fe-B based alloys and B2O3-containing slags were summarized. Based on the findings throughout this research, it was concluded that this study contains the reliable, fundamental thermodynamic research, scientific insight and its applicable data of a ferroboron production process. This research is expected to have a direct impact on the improvement of the current carbothermic ferroboron production.

本論文は、フェロボロンの新たな溶製プロセスの検討に必要な溶融Fe-B基合金およびB(2)O(3)含有スラグの熱力学的性質を測定した研究であり、全5章よりなる。

第1章では、緒言としてフェロボロン合金の溶製法をボロンの資源とともに示し、被還元性の低いボロン原料の問題点を指摘し、生産性の高いプロセス開発の必然性を述べている。また、既往の研究としてFe-B基合金およびB2O3含有スラグのこれまで報告された熱力学的性質について纏め、それを踏まえて明らかにすべきデータを示し、本研究の目的について述べている。

第2章ではFe-B基合金の熱力学的性質の測定を行っている。本研究では、溶融Agをレファレンスメタルとして溶融Fe-B基合金と共存させる化学平衡法を採用しているが、測定温度範囲におけるAgのBの熱力学的性質をまず明らかにしている。溶融Ag中への固体Bの溶解度を測定することにより、1773 K および1873 Kにおける Ag中のBの無限希薄状態での固体基準の活量係数γo(Bin Ag)を194および172と求めている。 その後、種々の組成の溶融Fe-B合金と溶融Agとを平衡させ、実験後のAg中B濃度と上記で得た活量係数の値から溶融Fe-B合金中のBの活量を求めることにより、1773 K および1873 KにおけるFe-B2元系溶融合金の熱力学的性質を全液相範囲に調査し、理想溶液より負に偏倚した活量線図であることを明らかにしている。これまで報告のあったFe活量測定からの計算値に較べて精度も高く、溶鉄中Bの活量係数やBの自己相互作用係数など重要な基本データも求めている。

引き続き、酸化物の熱炭素還元を考える際に必要なCを加えた3元系について1873 K.で検討を行い、Fe-B-C溶融合金中のBの活量をほぼ全組成範囲で求め等活量線図を示している。また、その熱力学的な図式積分によりFeの活量線図も求め、特に炭素飽和下での測定から同温度での炭素の飽和溶解度をBの濃度関数として示し、BとCの相互作用の大きさを示すパラメータとしてεC(B Csatd)=11.8+0.4を初めて明らかにしている。さらに、第4成分としてSiの添加の影響も調査し、Cの溶解度の変化から、Siの添加がフェロボロン溶製プロセスにおいてC濃度を低下させることを述べている。

第3章ではB2O3含有2元系スラグに着目し、BO(1.5)-MgO系、BO(1.5) - CaO系およびBO(1.5) - SiO2系スラグの熱力学的性質の測定を1873 Kで行っている。 当初はCO雰囲気下での溶融Fe-B-C合金との平衡により、実験後のそれぞれの組成と2章で明らかにした溶融Fe-B-C合金中のBの活量係数からスラグ中のBO(1.5)の活量を求めていたが、同方法では測定可能なスラグ組成に限界があるため、それぞれの系における高BO(1.5)濃度のスラグについてはCu-B合金と平衡させることにより測定を行っている。なお、Cu中Bの活量係数については、これまでの報告値もばらつきが大きく測定温度でのデータが不足していたため、グラファイトるつぼ中制御されたCO分圧下でCuをBO(1.5)と溶融平衡させ予め測定を行い、無限希薄状態でのCu中固体Bの活量係数 lnγ°(B)=-10706 T + 7.58および自己相互作用パラメータ ε(B B)=8485 T + 2.54 を得ている。

1873 K でのBO(1.5) - MgO系およびBO(1.5) - CaO 系の測定結果では、双方の系で BO(1.5) の活量がモル分率0.5近傍で急激に変化し、濃度上昇とともに理想からの偏倚が負から正に転じることが示されている。一方、BO(1.5) - SiO2系については全組成範囲を通して理想より負に偏倚し双方の酸化物の親和力が強いことが示されている。なお本系の測定では、平衡するCu合金中のSi濃度が非常に大きくなるため、制御された窒素分圧下でBN共存させて測定したSiとBの相互作用パラメータについても および と予め求めている。

これらの2元系スラグの測定結果から、BO(1.5) - MgO系がBO(1.5)の活量係数を大きくするという観点からフェロボロンの溶製に適していることを明らかにしている。

第4章では実際の原料に入る可能性の高いSiO2の影響を明らかにするため、BO(1.5) - MgO系およびBO(1.5) - CaO系にSiO2を加えた3元系スラグの熱力学的性質を1873 Kで測定している。測定方法は3章で採用した溶融CuとCO雰囲気下グラファイトるつぼ中で平衡させる方法で、実験後のB濃度からBO(1.5)の活量を求めている。また、両スラグ系は2液相分離領域が存在するため、その組成範囲や共役線もあわせて求めている。SiO2の両2元系への添加効果は非常に大きいがSiO2濃度が小さい場合、BO(1.5)-MgO系ではBO(1.5)の活量係数を大きくするのに対し、BO(1.5) - CaO系においては、特に高CaO領域では逆にBO(1.5)の活量係数を小さくしている。これはCaOがMgOよりも高塩基性であるためと考えられているが、詳細な検討は課題として残されている。

第5章では、これらのデータを元に炭素熱還元法によるフェロボロン溶製プロセスの解析を行い、新たなフェロボロン溶製プロセスについて評価を行っている。

第6章では、これらを総括し本論文で得られた成果をもとに、今後検討すべき課題を示している。

以上のように、本論文は溶融Fe-B基合金およびB2O3含有スラグの熱力学的性質を明らかにしたものであり、これらの成果はフェロボロンの新たな溶製プロセス開発のための学術的な基礎知見を与え、フェロアロイ製造技術の発展に大きく寄与するものである。

よって、本論文は博士(工学)の学位請求論文として合格と認められる。