In recent decades, platinum group metals (ruthenium, rhodium, palladium, osmium, iridium, and platinum) have been the key materials in a variety of specific applications including catalysts, conductive layers in electronic devices, and high corrosion resistant equipments. The scarcity, high demands and high prices of platinum group metals resources have made recycling of platinum group metals a worldwide significant issue.

Most of platinum group metals have high stability as a metal at high temperature. Therefore, the recovery of platinum group metals by a pyrometallurgical method is an outstanding option. In this study, the recovery of platinum group metals by using liquid copper or liquid iron as a collector metal has been targeted. The process of interest consists of two main steps: (1) the melting of scraps containing platinum group metals with fluxes and the collector metal in a reducing atmosphere where platinum group metals from scraps are collected in liquid metal phase; and (2) the removal of the collector metal by oxidation to enrich platinum group metals in the metal phase.

Theoretically, platinum group metals are more stable as metals at a high temperature; therefore, the high recovery in a reducing atmosphere is expected. However, for the enrichment process, the loss of platinum group metals to slag due to oxidation in the highly oxidizing atmosphere can be critical. In order to improve the recovery of platinum group metals in an oxidizing atmosphere by minimization of the loss of platinum group metals into slag, the behavior of platinum group metals in molten slag and in the collector metal has been studied.

In fact, there is only a little information on properties of platinum group metals in molten slag and liquid collector metals. Thus, more investigations for most of metals in the platinum group, especially rhodium, ruthenium, iridium and osmium, are needed. Due to the critical situations from the scarcity, high demands, high prices, difficulties of separation due to chemical inertness, and availability in the same type of scrap of rhodium and platinum, a great concern has been put on investigations for rhodium and platinum with extended discussions in comparison to ruthenium.

With the objective to obtain knowledge for the improvement of the recovery process of platinum group metals, three topics were examined as follow.

Firstly, in order to understand the behavior of platinum group metals in molten slag, the dissolution behavior of platinum group metals at various conditions was studied. The dissolution mechanism of platinum and ruthenium was previously reported. In the present study, the dissolution mechanism of rhodium in Na2O-SiO2 slag at 1423 K to 1623 K and in CaO-SiO2 slag at 1773 K to 1873 K in an oxidizing atmosphere were investigated.

The solubility of rhodium in the slags was measured by equilibrating the pure metallic Rh with the Na2O-SiO2 slag at 1423-1623 K and oxygen partial pressure between 0.0042 and 1 atm, and with the CaO-SiO2 slag at 1773-1873 K and oxygen partial pressure between 0.100 and 1 atm.

The solubility of rhodium in both slag systems increased with increasing oxygen partial pressure, temperature, and basic oxide content in slags. The correlation between the solubility of rhodium and oxygen partial pressure suggested that rhodium dissolved in the slag as RhO(1.5). The dissolution of rhodium was slightly endothermic: the enthalpy change of the dissolution of solid rhodium was determined to be +50 ± 10 kJ/mol for the 50(mass pct)Na2O-50SiO2 and +188 ± 94 kJ/mol for the 56(mass pct)CaO-44SiO2 slag systems. In the 50(mass pct)Na2O-50SiO2 melt, the enthalpy change of dissolution of rhodium was less positive than that of platinum (i.e. +37 kJ/mol) while that of ruthenium was reported to be a large negative value of -130 ± 20 kJ/mol. The increase in the solubility of rhodium with basic oxide content indicated that rhodium exhibits acidic behavior in slag. The correlation between the solubility of rhodium and sulfide capacity of the slag, a representative of slag basicity, suggested that the ionic species of rhodium in slag is rhodate ion, RhO2-. Consequently, the following dissolution reaction of rhodium into slag was proposed.

Rh (s) + 3/4O2 (g) + 1/2O(2-) (in slag) = RhO2- (in slag)

From the dissolution reaction, rhodate capacity, the capacity of the slag to absorb rhodium as the rhodate ion, was defined. Rhodate capacity showed a linear correlation with theoretical optical basicity of the binary slags which was calculated from slag composition. The temperature dependence of rhodate capacity at a particular optical basicity was found to be very small. Since optical basicity of slags is considered as a generic measure of slag basicity, the use of the correlation between rhodate capacity and optical basicity to estimate the possible rhodium content in various slag systems was proposed.

Secondly, since it was known that the dissolution of platinum group metals is apparently influenced by slag composition, the loss minimization of platinum group metals in an oxidizing atmosphere by controlling slag composition can be an option. Since there are a number of materials coexisting in scraps, the proximate identification of the real slag system is difficult. In this study, effect of oxides addition to slag on the dissolution behavior of rhodium and platinum was determined.

Automobile catalysts have been the major scraps for the recycling of rhodium and platinum. Therefore, MgO, Al2O3, and SiO2, which are from a honeycomb structure (Cordierite, MgO-Al2O3-SiO2) in the catalysts, have been candidates of slag components. Moreover, by the removal of liquid copper or liquid iron by oxidation during the enrichment process, the content of either CuOx or Fe2O3 in slag can be relatively high. Thus, CuOx and Fe2O3 are considered as the major components of the recycling slag.

The solubility of rhodium and platinum was measured by equilibrating Na2O-SiO2-MgO, Na2O-SiO2-Al2O3, Na2O-SiO2-Fe2O3, and Na2O-SiO2-CuOx slags (at a constant Na2O/SiO2 ratio of 0.97) with pure solid rhodium or pure solid platinum in air (oxygen partial pressure at 0.21 atm) at 1473 K. The dissolution of copper or iron from slag into solid rhodium or solid platinum was observed. Hence, instead of the solubility, the activity coefficients of RhO1.5 and PtO (the species of platinum in slag which was reported by previous research) in slags were used to compare the effects of oxides addition.

From experiments, Al2O3 and Fe2O3 behaved acidic in the slag. Al2O3 was found to drastically suppress the dissolution of rhodium and platinum into slag, whereas the activity coefficients of RhO(1.5) and PtO in slag were only slightly increased with increasing content of Fe2O3. Since MgO is less basic than Na2O, it behaved as a diluent for the solubility of rhodium and platinum in slag. Activity coefficients of RhO(1.5) and PtO in slag were found to decrease with increasing content of CuOx in slag.

In the industrial view points, when the highly basic flux is used, Al2O3 and MgO can be the candidates of the recycling slag components which help suppressing the dissolution of rhodium and platinum into slag. Furthermore, when comparing the effect of Fe2O3 and CuOx addition on the dissolution behavior of rhodium and platinum in slag, it is deduced that the use of liquid iron as a collector metal for recovery of rhodium and platinum in an oxidizing atmosphere is more preferable than copper.

Referred to the dissolution reaction of platinum which was reported previously, platinate capacity of slag was derived in this study. From experimental results, the relations between rhodate capacity and platinate capacity of the ternary slags and the calculated theoretical optical basicity were obtained. Combined to the results for the binary slags, the linear regressions between rhodate capacity or platinate capacity and theoretical optical basicity were found to be the best fit relationship. With assumptions of slag composition, activities of rhodium or platinum in the collector metal, and oxygen partial pressure, the contents of rhodate ion and platinate ion in multi-component slags were estimated.

Thirdly, since thermodynamic properties of a platinum metal in the collector metal are the important factors which affect the effectiveness of the recovery process, with the focus on the use of copper as the collector metal, thermodynamic properties of the Cu-Rh system was examined.

The activity coefficients of copper in Cu-Rh solid alloys were determined by equilibrating the Cu-Rh alloys with pure solid Cu2O at 1473 K at different activities of copper in the alloys 1473 K. The calculation of the activity of copper was made by referring pure solid copper and pure solid Cu2O, which was assigned to unity, as standard states. The activity of rhodium in copper at 1473 K was then estimated by Gibbs-Duhem relation. The collectability of copper for rhodium was discussed in comparison to that of iron.

From experimental results, the activity of copper in Cu-Rh solid alloys showed the large positive deviation from ideality, in which the content of copper in the solid alloys varied from 0.05 to 0.48 for 0.27 < a(Cu)< 0.86. Previous study suggested that, at 1473 K, the activity of iron in Fe-Rh (Rh rich, XFe < 0.3) solid alloys at exhibited the negative deviation from ideality. Accordingly, it was considered that iron is a more preferable collector metal for rhodium than copper. In case of platinum, the activity of platinum in both the Cu-Pt system (1373 K, 1473 K, and 1542-1673 K) and the Fe-Pt system (1473-1673 K and 1823 K) exhibited a large negative deviation from ideality. Therefore, it was deduced that both copper and iron can be the collector metal candidates for platinum.

本論文は、乾式プロセスによる廃棄物からの白金族元素の回収を検討するうえで重要な、スラグならびにコレクターメタル中での熱力学的性質を調査した研究であり、全6章からなる。

第1章では、まず白金族元素の需給動向をまとめており、各元素の用途拡大による近年の需要増加と価格高騰の状況を提示し、白金族元素を持続的に利用するうえでのリサイクルの必要性を示している。電子製品や触媒部品等から白金族元素を回収する手法として、各元素が幅広い温度域にて金属状態としての安定性を有することから、乾式プロセスによる分離回収法の有効性を述べている。その有力なプロセスとして、廃材を銅や鉄などのコレクターメタルとフラックスとともに溶解することで白金族元素をコレクターメタル中に抽出した後、コレクターメタルを酸化除去することで白金族元素を濃縮する手法を説明している。そのプロセスではフラックス、またはコレクターメタルが酸化して生成するスラグに、白金族元素の酸化損失が生じることが課題となっており、それを解決するためのスラグならびにコレクターメタル中の白金族元素の熱力学的性質の重要性を指摘し、本研究の目的を述べている。

第2章では、酸化性雰囲気においてNa2O-SiO2スラグとCaO-SiO2スラグ中へのロジウムの溶解挙動を調査し、スラグ中のロジウムの溶解形態について熱力学的に検討を行っている。

第一に、50(mass pct)Na2O-50SiO2 スラグと56(mass pct)CaO-44SiO2 スラグを、それぞれ大気下1423-1623 K、酸素1気圧下1773-1873 Kにおいて、ロジウム坩堝中で溶融させることで、スラグ中ロジウムの溶解度の温度依存性を調査している。それぞれのスラグ中で、溶解度が正の温度依存性を示すことから、スラグへのロジウムの溶解が吸熱性であることを示している。

次いで、スラグ中ロジウムの溶解度の酸素分圧依存性とスラグ組成依存性を調査しており、雰囲気中酸素分圧が高く、スラグが高塩基度であるほど溶解度が大きいことを明らかにしている。またスラグの塩基度の一指標であるサルファイドキャパシティとスラグ中ロジウムの溶解度との相関関係から、スラグ中へのロジウムの溶解反応を(Rh (s) + 3/4O2 (g) + 1/2O(2-) (in slag) = RhO2- (in slag))のように推定している。この溶解反応から、スラグ中のロジウムの溶解特性の指標としてロデイトキャパシティーを新たに定義しており、種々の組成のスラグ中のロジウムの溶解度を試算できるように同キャパシティーの光学的塩基度を用いた推算式を提示している。

第3章では、実際のプロセスでフラックスあるいはコレクターメタルの酸化滓に混入しうる酸化物成分の、スラグ中ロジウムならびに白金の溶解挙動への影響を調査している。混入成分として、自動車用触媒の担体材料のコーディエライト(MgO-Al2O3-SiO2)の構成成分のAl2O3とMgO、コレクターメタルの酸化成分のCuOxとFe2O3について調査を行っている。

Na2O/SiO2比を0.97としたNa2O-SiO2-MgO, Na2O-SiO2-Al2O3, Na2O-SiO2-Fe2O3, Na2O-SiO2-CuOx の各スラグを、大気中1473Kにてロジウムもしくは白金坩堝中で溶融させ、スラグ中のロジウムもしくは白金の濃度を測定している。Al2O3を添加した場合、その濃度の増加に伴いスラグ中ロジウムと白金の濃度が顕著に減少することを示している。MgO、Fe2O3の添加によっても両元素とも濃度が減少する傾向にあり、一方、CuOxを添加した場合には顕著な変化がないことを明らかにしている。以上の結果に基づき、フラックスへのロジウムや白金の酸化損失を抑制するためには、コーディエライトの構成成分のMgOとAl2O3をフラックスの主成分に用いること、またコレクターメタルの酸化滓への損失を抑制するためには、コレクターメタルに鉄を用いることが好適であることを述べている。

第4章では、コレクターメタルの銅中のロジウムの熱力学的性質を調査している。1473 Kにて酸素分圧制御下でCu2OとCu-Rh 合金を平衡させることにより、Cu-Rh合金中銅の活量を測定している。Cu-Rh合金中の成分の活量が理想溶液より正に偏倚することを示している。一方、既報にてFe-Rh合金のロジウムの活量は理想溶液より負に偏倚することが報告されていることから、コレクターメタルにより廃棄物からロジウムを抽出する際には、銅より鉄がコレクターメタルとして好適であることが述べられている。

第5章では前章までの結果を踏まえ、コレクターメタルを用いた乾式プロセス中のロジウムならびに白金のスラグへの酸化損失を抑制して収率良く回収するために最適なフラックス組成を示している。

第6章では本研究により得られた成果を総括している。

以上のように、本論文はロジウムと一部白金について、スラグならびにコレクターメタル中の熱力学的性質を調査したものであり、これらの成果は白金族元素の乾式プロセスによる効率的回収を達成するうえでの基礎的知見を与え、白金族元素のリサイクルシステムの最適化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。