Emerging SOM technology for the green synthesis of metals from oxides

This article is intended to demonstrate that the environmentally sound solid-oxide-membrane (SOM) technology is an emerging process that can efficiently synthesize metals and alloys directly from their oxide ores with minimum feed-material preparation and produce oxygen gas or water vapor as the major byproduct. To demonstrate the proof-of-concept and economic viability, this article will focus on the synthesis of magnesium metal. The current production methods for magnesium are either metallothermic reduction (magnetherm process) at thig temperatures (1,600°C) involving expensive metal reductant (FeSi) or electrolysis from a halide electrolyte bath that requires extensive and expensive feed-material preparation. Both these techniques are also energy intensive, have low yield and generate large quantities of waste reaction products harmful to the environment. In the SOM process, the oxide reduction is electrochemical and has efficiencies close to 100%. It will be shown that unlike the current metallothermic and the electrolytic processes, the SOM process has the potential to be more economic and less energy intensive, and its process products are environmentally benign. The results reported in this article for magnesium synthesis are also applicable for environmentally sound production of other high-energy-content metals that are produced by less-efficient techniques that result in environmentally harmful reaction products. To date, in addition to magnesium, the SOM process has been used to produce silicon, chromium, and iron, along with alloys from their respective oxides dissolved in appropriate solvents. For more information, contact U.B. Pal, Boston University, Manufacturing Engineering Department, 15 St. Mary’s Street, Boston, MA 02446; (617) 353-7708; e-mail upal@bu.edu.     

An Environmentally Friendly Process Involving Refining and Membrane-Based Electrolysis for Magnesium Recovery from Partially Oxidized Scrap Alloy

Magnesium is recovered from partially oxidized scrap alloy by combining refining and solid oxide membrane (SOM) electrolysis. In this combined process, a molten salt eutectic flux (45 wt.% MgF₂–55 wt.% CaF₂) containing 10 wt.% MgO and 2 wt.% YF₃ was used as the medium for magnesium recovery. During refining, magnesium and its oxide are dissolved from the scrap into the molten flux. Forming gas is bubbled through the flux and the dissolved magnesium is removed via the gas phase and condensed in a separate condenser at a lower temperature. The molten flux has a finite solubility for magnesium and acts as a selective medium for magnesium dissolution, but not aluminum or iron, and therefore the magnesium recovered has high purity. After refining, SOM electrolysis is performed in the same reactor to enable electrolysis of the dissolved magnesium oxide in the molten flux producing magnesium at the cathode and oxygen at the SOM anode. During SOM electrolysis, it is necessary to decrease the concentration of the dissolved magnesium in the flux to improve the faradaic current efficiency and prevent degradation of the SOM. Thus, for both refining and SOM electrolysis, it is very important to measure and control the magnesium solubility in the molten flux. High magnesium solubility facilitates refining whereas lower solubility benefits the SOM electrolysis process. Computational fluid dynamics modeling was employed to simulate the flow behavior of the flux stirred by the forming gas. Based on the modeling results, an optimized design of the stirring tubes and its placement in the flux are determined for efficiently removing the dissolved magnesium and also increasing the efficiency of the SOM electrolysis process.     

熔盐电解制备难熔金属的现状与展望

介绍了传统的熔盐电解法用于难熔金属制备的应用现状,分析了其存在的弊病：主要是传统熔盐电解法对电解质要求严格,难以找到合适的熔盐.而新开发的熔盐电解法--FFC法和SOM法,都从一定程度上降低了电解过程对电解质的要求,成为熔盐电解制备难熔金属发展的新方向,是低成本、连续化、无污染生产难熔金属的可行之路.上海大学综合分析了FFC法和SOM法的优缺点,提出了一种改良的SOM法,该方法有效避免了FFC法中的缺点,并在制备Ti,Ta,Cr上取得成功.     

The use of solid-oxide-membrane technology for electrometallurgy

Solid oxide membrane (SOM) technology has been employed in developing several new metal reduction technologies. This overview describes the SOM process for copper deoxidation and SOM technology for metal smelting, as well as applications to magnesium, titanium, and tantalum. The examples illustrate various configurations of the SOM, anode, and cathode that are best suited to the needs of each metal.     

Mechanisms governing tribochemical reduction of metals and non-metals from their oxides

The paper describes the physical and chemical mechanisms governing the tribochemical reduction of metals and non-metals from their oxides. The reduction was started by a tribochemical activation of initial constituents in a vibrating mill used as a reactor. Powders of oxides of copper, iron, molybdenum, tungsten, vanadium, titanium and boron and also such chemically pure metals as magnesium, aluminium and titanium were used as reaction constituents. The triboactivation energy contribution needed to start the explosive mechanism reduction is found to increase as the heat of reaction decreases and the heat of formation of the associated product rises. Introducing oxygen into the reactor during the tribochemical activation of powders of oxides together with magnesium causes the intense oxidation of magnesium and stimulates the reduction process. To increase the yield of tungsten monocarbide from tungsten concentrate, the reduction process should be implemented in an atmosphere of hydrogen or hydrocarbons.     

钛铁混合氧化物短流程直接制备钛铁合金

利用固体透氧膜(SOM)法研究钛铁混合氧化物短流程直接制备钛铁合金。在熔融CaCl2熔盐体系中,以二氧化钛和氧化铁的混合氧化物压片为阴极,氧化钇稳定氧化锆管(YSZ)内的碳饱和铜液为阳极, 在1100 ℃,槽电压3.5 V的条件下电解2~6 h。经SEM、EDS、XRD等分析表明：二氧化钛和氧化铁被电解还原为钛铁合金,直接从氧化物制备钛铁合金是可行且高效的     

Zum Aktiv-Passiv-Übergang von Metallen

Active-passive transition of metals The importance of the Flade potential on the active-passive transition of metals is critically reconsidered. In contrast to previous results on iron it will be shown that this transition is a continuous process. Based on a kinetic model for the iron dissolution and passivation, a new interpretation of the active passive transition is given by which the previous thermodynamic interpretations of the Flade potential and its inconsistencies with experimental results is replaced.     

Interaction between erosion and high-temperature corrosion of metals: The erosion-affected oxidation regime

The combined erosion-oxidation of nickel, cobalt, and the oxides of these metals have been studied at 780°C in air to examine two regimes of interactionnamely, a regime of erosion-enhanced oxidation during which an oxide scale of constant thickness covers metal specimens, and a regime of oxidation-affected erosion that is characterized by a composite surface layer of metal, oxide, and erodent. In the case of cobalt, these two regimes have been documented and the transition from one regime to another described. For the range of conditions examined, only the oxidation-affected erosion regime was observed for nickel due to its lower oxidation rate compared to cobalt.     

Passivity in metals and alloys

Attempted explanations of passivity in metals and alloys since the early proposals of Faraday in 1836 fall into the categories: (1) metal modification, (2) reaction velocity, (3) oxide film, and (4) adsorption. Historically the subject proved to be unusually complex; general concurrence on any one viewpoint was not achieved despite considerable effort and thought by generations of able investigators. Notwithstanding, progress is being made as the search continues.The present survey emphasizes that two basic mechanisms of passivity prevail depending on the metal and its environment. The first depends on a diffusion barrier stoichiometric oxide or other type of reaction product film; the second depends on reduced reaction kinetics introduced by a surface film of chemisorbed oxygen ranging from less than a monolayer to the dimensions of a thin non-stoichiometric metal oxide of variable hydrogen content. The latter film in part impedes rate of metal ion hydration; it is a source of passivity mostly in the transition metals and their alloys.The film of chemisorbed oxygen is more strongly bonded to the base metal than is a metal oxide. This accounts for an observed better resistance to erosion and cavitation damage of metals passive because of chemisorbed oxygen (e.g. 18-8 stainless steel) compared to metals protected by stoichiometric oxides (e.g. iron, copper, cupro-nickels, lead). Since chemisorbed oxygen can be mechanically activated to form the metal oxide by pronounced impingement, abrasion and rubbing of the metal surface, any factor that resists such activation is beneficial. Alloyed cobalt apparently acts in this way, explaining why Co-base passive alloys are appreciably more resistant to erosion, cavitation damage and fretting corrosion than the Ni- or Fe-base passive alloys.     

Passivity in metals and alloys

Attempted explanations of passivity in metals and alloys since the early proposals of Faraday in 1836 fall into the categories: (1) metal modification, (2) reaction velocity, (3) oxide film, and (4) adsorption. Historically the subject proved to be unusually complex; general concurrence on any one viewpoint was not achieved despite considerable effort and thought by generations of able investigators. Notwithstanding, progress is being made as the search continues.The present survey emphasizes that two basic mechanisms of passivity prevail depending on the metal and its environment. The first depends on a diffusion barrier stoichiometric oxide or other type of reaction product film; the second depends on reduced reaction kinetics introduced by a surface film of chemisorbed oxygen ranging from less than a monolayer to the dimensions of a thin non-stoichiometric metal oxide of variable hydrogen content. The latter film in part impedes rate of metal ion hydration; it is a source of passivity mostly in the transition metals and their alloys.The film of chemisorbed oxygen is more strongly bonded to the base metal than is a metal oxide. This accounts for an observed better resistance to erosion and cavitation damage of metals passive because of chemisorbed oxygen (e.g. 18-8 stainless steel) compared to metals protected by stoichiometric oxides (e.g. iron, copper, cupro-nickels, lead). Since chemisorbed oxygen can be mechanically activated to form the metal oxide by pronounced impingement, abrasion and rubbing of the metal surface, any factor that resists such activation is beneficial. Alloyed cobalt apparently acts in this way, explaining why Co-base passive alloys are appreciably more resistant to erosion, cavitation damage and fretting corrosion than the Ni- or Fe-base passive alloys.     

利用固体透氧膜提取海绵钛的新技术

对克劳尔法、电解四氯化钛法等海绵钛传统生产方法以及在熔融CaCl2中直接电解还原二氧化钛制取钛的新工艺进行了总结和分析。针对这些方法中存在的不足,提出了一种利用固体透氧膜技术直接从含钛氧化物矿制备海绵钛的连续、低成本、无污染的绿色冶金新工艺。     

真空升华法从废镁合金中回收镁--镁合金无害化处理技术

表文介绍了双真空电炉升华法从废镁合金中切削屑中回收镁的原理、设备和工艺流程。认为此工艺是一项产量较大、生产成本较低、无环境污染的处理废弃镁舍金（包括废弃镁合金碎料）的较好方法。     

重金属强化含金矿石的氰化浸出

通过分析铊、铋、汞和铅等重金属强化金氰化溶解的电化学原理,对含金氧化物矿石和难浸硫化物金精矿进行了重金属强化浸金研究.结果表明:重金属对金氰化溶解的阳极过程有显著的强化作用,但在常规供氧条件下,金的溶解速率并未显著提高;只有同时采用阴极强化措施,才能使重金属起到显著提高金溶解速率的作用;对于含金氧化物矿石,单独采用重金属强化即可明显提高浸金速率,如果在过氧化氢助浸的基础上添加重金属,金的浸出速率会有更大幅度的提高;对硫化物金精矿而言,单独采用重金属无明显强化效果,只有在添加过氧化氢作为辅助氧化剂的基础上,重金属对金的浸出才能起到强化的作用,该体系中过氧化氢起到了强化阴极过程和氧化硫化物的双重作用.     

Microscopic Approach of Adhesion and Wetting of Liquid Metal on Solid Ionocovalent Oxide Surface

The adhesion and wetting of non-reactive liquid metals with solid ionocovalent oxides are studied on thebasis of the experimental work of adhesion W data obtained with the sessile drop method.An analysis of theexperimental W values of different liquid metals on various solid oxides is first performed to evidence the de-pendence of the work of adhesion of a metal/oxide system on the electron density of the metal and on thethermodynamic stability of the oxide.An electronic model is then proposed to describe the microscopic mech-anism of metal-oxide interactions.Based on the model,the contact angle and the work of adhesion of differentliquid metals on various solid oxides can be interpreted and estimated,and their correlations to the variousphysical quantities of the oxides can be easily deduced.The basic consideration of the model is that the adhe-sion between a metal and an oxide is assured by the electron transfer from the metal into the oxide valenceband which is not completely filled of electrons at high temperatures,and is enhanced when this electron trans-fer at the metal/oxide interface is intensified.The influence of interface defects on the wetting and adhesion issuggested and discussed.     

Self-Repairing Metal Oxides

The oxidation of Cu, Zr, and alloys forming chromia, alumina, and zirconia was studied in a closed reaction chamber in O₂ gas near 20 mbar. Information on the position of oxide growth has been gained from the ¹⁸O/SIMS technique. Rates of O₂ dissociation on metal oxides, Au, and Pt have been evaluated from measurements in labeled O₂. The experimental results indicate that hydrogen in the metal substrates induces increased metal-ion transport in internal oxide surfaces during oxidation, which leads to increased oxide growth at the oxide–gas interface. Experiments also show that oxides of rare-earth metals (REM) and Pt catalyze the dissociation of O₂. An increased rate of O₂ dissociation can lead to increased transport of oxygen ions in the oxides and increased oxide growth at the substrate–oxide interface. A balanced transport of metal and oxygen ions in metal oxides that leads to oxide growth at both the metal–oxide and at the oxide–gas interface is found to be favorable for the formation of protective oxides with good adherence to the metal substrate. Depending on the original proporation of metal–to–oxygen ion transport in the oxide, an addition of hydrogen will increase or decrease the oxidation kinetics. In analogy, an addition of REM will increase or decrease the oxidation kinetics, depending on the original proportion of metal-to-oxygen ion transport.     

Effect of slag variations on ANSI/AWS A5.1–91 E6013 electrode properties: replacement of TiO2 in electrode coating with MnO,FeO, CaO,MgO, K2O,or Na2O

Abstract

The present work is part of a research programme to study the effect of variation of the coating composition of covered rutile manual electrodes on both operational and deposited weld metal properties. According to previous results obtained for rutile electrodes, an increase of slag basicity, achieved through modifications in the coating, produces an improvement of the impact properties of the deposited weld metal, as well as a decrease in the diffusible hydrogen, along with beneficial changes in the operational properties of the electrode. The aim of the present work is to study the influence of the replacement of titanium oxide with oxides of alkali metals (sodium and potassium), alkaline earth metals (calcium and magnesium), and iron and manganese oxides in the coating of an ANSI/AWS A5.1–91 E6013 type electrode on the operational characteristics of the electrode and the microstructure, mechanical properties, and diffusible hydrogen content of the electrode weld metal. Tensile strength and Charpy V notch impact properties were measured in the all weld metal samples produced using each of the seven electrodes designed for the present study. A cross-section from each coupon was used to determine the chemical composition of the deposit and to carry out a metallographic study using a light microscope. As a result of the addition of different oxides, changes in the operational properties were observed using both direct and alternating current, as well as a decrease in the diffusible hydrogen content in the weld metal. Also, electrodes in which rutile was substituted by sodium oxide in the coating showed an improvement in the impact properties of the deposit.     

Focussed ion beam sectioning for the 3D characterisation of cracking in oxide scales formed on commercial ZIRLO™ alloys during corrosion in high temperature pressurised water

Using FIB sectioning and reconstruction techniques we have performed a quantitative analysis on the microstructure of cracks and the topography of the metal–oxide interface in oxides formed on ZIRLO™ alloys in high-temperature water. The most significant observation is the continuous production of cracks both before and after the transition in kinetics, not a sudden burst of crack nucleation at transition as assumed in the literature. By concluding that cracks are not generated as a result of the transition and are not the primary cause, we suggest that a process by which cracks within the scale become connected to the oxidising environment through interconnected nanoporosity may be critical in controlling the overall rate of oxidation.     

The role of noncrystalline films in the oxidation and corrosion of metals

The role of oxide perfection is a major factor in controlling the rate of oxidation and corrosion of metals. Studies of the reactivity of tantalum, silicon, and amorphous metals are particularly revealing in this regard. Two states of oxide perfection are found: single crystal and vitreous. The former is seldom encountered in practice because it requires epitaxial growth of a single crystal oxide on the metal. However, bond flexibility in vitreous oxides allows them to adapt to various substrates, even polycrystalline ones. Degradation of both single crystal and vitreous oxide results in the formation of polycrystalline oxide. Here, grain boundaries provide paths for easy ion movement and resultant fast oxide growth. Stabilization of vitreous oxide involves bond strength, multiple configurations with equivalent energies, substrate effects, and purity. The control of these factors is the technological challenge to achieving more perfect, and hence more protective, oxides.Invited keynote paper, Session on Amorphous Metals and Films, Fall Meeting of the Electrochemical Society, Los Angeles, California, October 1979.     

Modern magnesium-base deformable alloys and composite materials (a review)

State-of-the-art deformable magnesium alloys and magnesium-base composite materials are analyzed. The significance of these materials in advanced fields of engineering is confirmed. It is shown that the main physical and mechanical characteristics of magnesium deformable alloys make them competitive to aluminum alloys used for the same purposes. Prospects of alloys of the Mg-Zn-Zr-REM system are considered. Some efficient processes of production of magnesium-base semiproducts, such as casting with elevated rates of crystallization, hydropressing, superplastic deformation, and direct (ingotless) rolling, are described. It is predicted that the direction of metals science dealing with improvement of traditional and development of novel magnesium alloys will be advanced and the range of their application will widen due to creation of new processes of production of deformable semiproducts from them.     

Pre-oxidation of Inconel Alloys for Inhibition of Carbon Deposition from Heated Jet Fuel

Exposure of superalloy surfaces to jet fuel at elevated temperatures leads to the formation of carbonaceous deposits and metal sulfides. The formation of stable oxide layers on alloy surfaces can reduce the activity of the constituent transition metals that catalyze the dehydrogenation of hydrocarbons and the subsequent carbon deposit growth. The metals Ni, Cr, Fe, Mn, Al, Ti and Nb + Ta form thermodynamically stable oxide layers after oxidation above 800°C under O₂ flow. In this study, we investigated the formation of oxides and spinels on three different Ni-base superalloys (Inconel 600, Inconel 718, and Inconel X750) and their activity towards carbon and sulfur deposit formation from jet fuel (JP-8) thermal stressing at 600°C and 34 atm (500 psig) for 5 hr. Metal oxide formation during pre-oxidation and the behavior of pre-oxidized samples in thermal stressing were found to depend strongly on the minor element composition of these superalloys.