Advances in Understanding of Unit Operations in Non-ferrous Extractive Metallurgy 2021

A special issue of Metals (ISSN 2075-4701). This special issue belongs to the section "Extractive Metallurgy".

Deadline for manuscript submissions: closed (31 December 2021) | Viewed by 54805

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IME Process Metallurgy and Metal Recycling Department, RWTH Aachen University, 52056 Aachen, Germany
Interests: environmental protection; recycling; metallurgy; mineral processing; nanotechnology; waste water treatment
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Dear Colleagues,

The high demand for critical materials, such as rare earth elements, indium, gallium, and scandium, raises the need for an advance in understanding of the unit operations in non-ferrous extractive metallurgy. Unit metallurgical operations processes are usually separated into three categories: 1) hydrometallurgy (leaching, mixing, neutralization, precipitation, cementation, crystallization), 2) pyrometallurgy (roasting, smelting), and 3) electrometallurgy (aqueous electrolysis and molten salt electrolysis). In hydrometallurgy, the aimed metal is first transferred from ores and concentrates to a solution using a selective dissolution (leaching; dry digestion) under an atmospheric pressure below 100 °C and under a high pressure (40-50 bar) and high temperature (below 270°C) in an autoclave and tube reactor. The purification of the obtained solution was performed using neutralization agents such as sodium hydroxide and calcium carbonate or more selective precipitation agents such as sodium carbonate and oxalic acid. The separation of metals is possible using liquid/Liquid process (solvent extraction in mixer-settler) and solid-liquid (filtration in filter-press under high pressure). Crystallization is the process by which a metallic compound is converted from a liquid into a solid crystalline state via a supersaturated solution. The final step is metal production using electrochemical methods (aqueous electrolysis for basic metals such as copper, zinc, silver and molten salt electrolysis for rare earth elements and aluminum). Advanced processes, such as ultrasonic spray pyrolysis and microwave-assisted leaching, can be combined with reduction processes in order to produce metallic powders. Some preparation for the leaching process is performed via a roasting process in a rotary furnace, where the sulphidic ore was first oxidized in an oxidic form which is a suitable for the metal transfer to water solution. During the smelting process, the target metal is further refined at high temperatures and reduced to its pure form. The pyrometallurgical treatment of the ore was performed in an electric furnace and combined with a refining during distillation. Unit Operations in Non-ferrous Extractive metallurgy can be successfully used for the recovery of non-ferrous metals from secondary materials.

Prof. Dr. Bernd Friedrich
Dr. Srecko Stopic
Guest Editors

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Keywords

  • hydrometallurgy
  • pyrometallurgy
  • electrometallurgy
  • leaching
  • digestion
  • solvent extraction
  • precipitation
  • electrolysis
  • molten salt electrolysis
  • cementation
  • roasting
  • smelting
  • refining
  • ultrasonic spray pyrolysis
  • microwave leaching
  • recycling

Published Papers (18 papers)

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Editorial

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6 pages, 184 KiB  
Editorial
Advances in Understanding of Unit Operations in Non-Ferrous Extractive Metallurgy 2021
by Srecko Stopic and Bernd Friedrich
Metals 2022, 12(4), 554; https://doi.org/10.3390/met12040554 - 25 Mar 2022
Viewed by 1633
Abstract
The high demand for critical materials, such as rare earth elements, indium, gallium, and scandium, raises the need for an advance in understanding the unit operations in non-ferrous extractive metallurgy [...] Full article

Research

Jump to: Editorial, Review

11 pages, 2430 KiB  
Article
Electrochemical Investigation of Lateritic Ore Leaching Solutions for Ni and Co Ions Extraction
by Jonas Mitterecker, Milica Košević, Srecko Stopic, Bernd Friedrich, Vladimir Panić, Jasmina Stevanović and Marija Mihailović
Metals 2022, 12(2), 325; https://doi.org/10.3390/met12020325 - 12 Feb 2022
Cited by 2 | Viewed by 1722
Abstract
Determination of the extractive behavior of ionic species from lateritic ore leachates is complex, since the leachates are pregnant with tens of different ions in, as a rule, multiple oxide states. To examine the possible pathways of intrinsic electrochemical extraction of the crucial [...] Read more.
Determination of the extractive behavior of ionic species from lateritic ore leachates is complex, since the leachates are pregnant with tens of different ions in, as a rule, multiple oxide states. To examine the possible pathways of intrinsic electrochemical extraction of the crucial elements Ni and Co, it was necessary to make model solutions of these elements and to subject them to electrochemical examination techniques in order to obtain a benchmark. Beside Ni and Co, the model system for Fe had to be evaluated. Iron, as a dominant ore component by far, is the main interfering factor in the extraction processes of Ni and Co in rather low amounts from leaching solution. The leachate examination results were compared to separate model solutions, as well as to their combinations in concentrations and to pH values comparable to those of the leachate. The separation of the leachate components was initially performed by continuous increase in pH upon leaching with NaOH solution, and afterwards the pH-adjusted solutions were subjected to electrochemical investigation. With the purpose of connecting and quantifying the visual changes in leachate upon increase in pH, conductometric measurements were performed. Reactions of oxidation/precipitations were indicated, which led to the essential Fe removal by precipitation. Resulting solutions were found suitable for Ni and Co electrochemical extraction. Full article
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12 pages, 4746 KiB  
Article
Characterization of Defined Pt Particles Prepared by Ultrasonic Spray Pyrolysis for One-Step Synthesis of Supported ORR Composite Catalysts
by Gözde Alkan, Milica Košević, Marija Mihailović, Srecko Stopic, Bernd Friedrich, Jasmina Stevanović and Vladimir Panić
Metals 2022, 12(2), 290; https://doi.org/10.3390/met12020290 - 8 Feb 2022
Cited by 7 | Viewed by 1792
Abstract
Polygonal Pt nanoparticles were synthesized using ultrasonic spray pyrolysis (USP) at different precursor concentrations. Physicochemical analysis of the synthesized Pt particles involved thermogravimetric, microscopic, electron diffractive, and light absorptive/refractive characteristics. Electrochemical properties and activity in the oxygen reduction reaction (ORR) of the prepared [...] Read more.
Polygonal Pt nanoparticles were synthesized using ultrasonic spray pyrolysis (USP) at different precursor concentrations. Physicochemical analysis of the synthesized Pt particles involved thermogravimetric, microscopic, electron diffractive, and light absorptive/refractive characteristics. Electrochemical properties and activity in the oxygen reduction reaction (ORR) of the prepared material were compared to commercial Pt black. Registered electrochemical behavior is correlated to the structural properties of synthesized powders by impedance characteristics in ORR. The reported results confirmed that Pt nanoparticles of a characteristic and uniform size and shape, suitable for incorporation on the surfaces of interactive hosts as catalyst supports, were synthesized. It is found that USP-synthesized Pt involves larger particles than Pt black, with the size being slightly dependent on precursor concentration. Among ORR-active planes, the least active (111) structurally defined the synthesized particles. These two morphological and structural characteristics caused the USP-Pt to be made of lower Pt-intrinsic capacitive and redox currents, as well as of lower ORR activity. Although being of lower activity, USP-Pt is less sensitive to the rate of ORR current perturbations at higher overpotentials. This issue is assigned to less-compact catalyst layers and uniform particle size distribution, and consequently, of activity throughout the catalyst layer with respect to Pt black. These features are considered to positively affect catalyst stability and thus promote USP synthesis for improved properties of host-supported Pt catalysts. Full article
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11 pages, 2733 KiB  
Article
Mixed Oxides NiO/ZnO/Al2O3 Synthesized in a Single Step via Ultrasonic Spray Pyrolysis (USP) Method
by Duygu Yeşiltepe Özcelik, Burçak Ebin, Srecko Stopic, Sebahattin Gürmen and Bernd Friedrich
Metals 2022, 12(1), 73; https://doi.org/10.3390/met12010073 - 2 Jan 2022
Cited by 8 | Viewed by 2475
Abstract
Mixed oxides have received remarkable attention due to the many opportunities to adjust their interesting structural, electrical, catalytic properties, leading to a better, more useful performance compared to the basic metal oxides. In this study, mixed oxides NiO/ZnO/Al2O3 were synthesized [...] Read more.
Mixed oxides have received remarkable attention due to the many opportunities to adjust their interesting structural, electrical, catalytic properties, leading to a better, more useful performance compared to the basic metal oxides. In this study, mixed oxides NiO/ZnO/Al2O3 were synthesized in a single step via the ultrasonic spray pyrolysis method using nitrate salts, and the temperature effects of the process were investigated (400, 600, 800 °C). The synthesized samples were characterized by means of scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction and Raman spectroscopy analyses. The results showed Al2O3, NiO–Al2O3 and ZnO–Al2O3 systems with spinel phases. Furthermore, the Raman peaks supported the coexistence of oxide phases, which strongly impact the overall properties of nanocomposite. Full article
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16 pages, 3038 KiB  
Article
Spray-Pyrolytic Tunable Structures of Mn Oxides-Based Composites for Electrocatalytic Activity Improvement in Oxygen Reduction
by Miroslava Varničić, Miroslav M. Pavlović, Sanja Eraković Pantović, Marija Mihailović, Marijana R. Pantović Pavlović, Srećko Stopić and Bernd Friedrich
Metals 2022, 12(1), 22; https://doi.org/10.3390/met12010022 - 23 Dec 2021
Cited by 5 | Viewed by 2519
Abstract
Hybrid nanomaterials based on manganese, cobalt, and lanthanum oxides of different morphology and phase compositions were prepared using a facile single-step ultrasonic spray pyrolysis (USP) process and tested as electrocatalysts for oxygen reduction reaction (ORR). The structural and morphological characterizations were completed by [...] Read more.
Hybrid nanomaterials based on manganese, cobalt, and lanthanum oxides of different morphology and phase compositions were prepared using a facile single-step ultrasonic spray pyrolysis (USP) process and tested as electrocatalysts for oxygen reduction reaction (ORR). The structural and morphological characterizations were completed by XRD and SEM-EDS. Electrochemical performance was characterized by cyclic voltammetry and linear sweep voltammetry in a rotating disk electrode assembly. All synthesized materials were found electrocatalytically active for ORR in alkaline media. Two different manganese oxide states were incorporated into a Co3O4 matrix, δ-MnO2 at 500 and 600 °C and manganese (II,III) oxide-Mn3O4 at 800 °C. The difference in crystalline structure revealed flower-like nanosheets for birnessite-MnO2 and well-defined spherical nanoparticles for material based on Mn3O4. Electrochemical responses indicate that the ORR mechanism follows a preceding step of MnO2 reduction to MnOOH. The calculated number of electrons exchanged for the hybrid materials demonstrate a four-electron oxygen reduction pathway and high electrocatalytic activity towards ORR. The comparison of molar catalytic activities points out the importance of the composition and that the synergy of Co and Mn is superior to Co3O4/La2O3 and pristine Mn oxide. The results reveal that synthesized hybrid materials are promising electrocatalysts for ORR. Full article
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12 pages, 2808 KiB  
Article
One Step Production of Silver-Copper (AgCu) Nanoparticles
by Münevver Köroğlu, Burçak Ebin, Srecko Stopic, Sebahattin Gürmen and Bernd Friedrich
Metals 2021, 11(9), 1466; https://doi.org/10.3390/met11091466 - 16 Sep 2021
Cited by 9 | Viewed by 3066
Abstract
AgCu nanoparticles were prepared through hydrogen-reduction-assisted Ultrasonic Spray Pyrolysis (USP) and the Hydrogen Reduction (HR) method. The changes in the morphology and crystal structure of nanoparticles were studied using different concentrated precursors. The structure and morphology of the mixed crystalline particles were characterized [...] Read more.
AgCu nanoparticles were prepared through hydrogen-reduction-assisted Ultrasonic Spray Pyrolysis (USP) and the Hydrogen Reduction (HR) method. The changes in the morphology and crystal structure of nanoparticles were studied using different concentrated precursors. The structure and morphology of the mixed crystalline particles were characterized through X-ray diffraction analysis (XRD), scanning electron microscopy (FEG-SEM), transmission electron microscopy (TEM) and Energy-dispersive X-ray spectroscopy (EDS). The average particle size decreased from 364 nm to 224 nm by reducing the initial solution concentration from 0.05 M to 0.4 M. These results indicate that the increase in concentration also increases the grain size. Antibacterial properties of nanoparticles against Escherichia coli were investigated. The obtained results indicate that produced particles show antibacterial activity (100%). The AgCu nanoparticles have the usage potential in different areas of the industry. Full article
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14 pages, 2242 KiB  
Article
Thermal Decomposition and Kinetics of Pentlandite-Bearing Ore Oxidation in the Air Atmosphere
by Kristina Božinović, Nada Štrbac, Aleksandra Mitovski, Miroslav Sokić, Duško Minić, Branislav Marković and Jovica Stojanović
Metals 2021, 11(9), 1364; https://doi.org/10.3390/met11091364 - 30 Aug 2021
Cited by 6 | Viewed by 2216
Abstract
The roasting of sulfide ores and concentrates is one of the most important steps in pyrometallurgical metal production from primary raw materials, due to the necessity of excess sulfur removal, present in the virgin material. Pentlandite is one of the main sources for [...] Read more.
The roasting of sulfide ores and concentrates is one of the most important steps in pyrometallurgical metal production from primary raw materials, due to the necessity of excess sulfur removal, present in the virgin material. Pentlandite is one of the main sources for nickel pyrometallurgical production. The knowledge of its reaction mechanism, products distribution during oxidation and reaction kinetics is important for optimizing the production process. Raw pentlandite-bearing ore from the Levack mine (Ontario, Canada) was subjected to oxidative roasting in the air atmosphere. A chemical analysis of the initial sample was conducted according to EDXRF (Energy-Dispersive X-ray Fluorescence) and AAS (Atomic Adsorption Spectrometry) results. The characterization of the initial sample and oxidation products was conducted by an XRD (X-ray Diffraction) and SEM/EDS (Scanning Electron Microscopy with Energy Dispersive Spectrometry) analysis. Thermodynamic calculations, a phase analysis and construction of Kellogg diagrams for Ni-S-O and Fe-S-O systems at 298 K, 773 K, 923 K and 1073 K were used for proposing the theoretical reaction mechanism. A thermal analysis (TG/DTA—Thermogravimetric and Differential Thermal Analyses) was conducted in temperature range 298–1273 K, under a heating rate of 15° min−1. A kinetic analysis was conducted according to the non-isothermal method of Daniels and Borchardt, under a heating rate of 15° min−1. Calculated activation energies of 113 kJ mol−1, 146 kJ mol−1 and 356 kJ mol−1 for three oxidation stages imply that in every examined stage of the oxidation process, temperature is a dominant factor determining the reaction rate. Full article
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17 pages, 2050 KiB  
Article
Electrorefining Process of the Non-Commercial Copper Anodes
by Radmila Markovic, Vesna Krstic, Bernd Friedrich, Srecko Stopic, Jasmina Stevanovic, Zoran Stevanovic and Vesna Marjanovic
Metals 2021, 11(8), 1187; https://doi.org/10.3390/met11081187 - 26 Jul 2021
Cited by 4 | Viewed by 4466
Abstract
The electrorefining process of the non-commercial Cu anodes was tested on the enlarged laboratory equipment over 72 h. Cu anodes with Ni content of 5 or 10 wt.% and total content of Pb, Sn, and Sb of about 1.5 wt.% were used for [...] Read more.
The electrorefining process of the non-commercial Cu anodes was tested on the enlarged laboratory equipment over 72 h. Cu anodes with Ni content of 5 or 10 wt.% and total content of Pb, Sn, and Sb of about 1.5 wt.% were used for the tests. The real waste solution of sulfuric acid character was a working electrolyte of different temperatures (T1 = 63 ± 2 °C and T2 = 73 ± 2 °C). The current density of 250 A/m2 was the same as in the commercial process. Tests were confirmed that those anodes can be used in the commercial copper electrorefining process based on the fact that the elements from anodes were dissolved, the total anode passivation did not occur, and copper is deposited onto cathodes. The masses of cathode deposits confirmed that the Cu ions from the electrolyte were also deposited onto cathodes. The concentration of Cu, As, and Sb ions in the electrolyte was decreased. At the same time, the concentration of Ni ions was increased by a maximum of up to 129.27 wt.%. The major crystalline phases in the obtained anode slime, detected by the X-ray diffraction analyses, were PbSO4, Cu3As, SbAsO4, Cu2O, As2O3, PbO, SnO, and Sb2O3. Full article
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9 pages, 2152 KiB  
Article
Aluminium Recycling in Single- and Multiple-Capillary Laboratory Electrolysis Cells
by Andrey Yasinskiy, Sai Krishna Padamata, Ilya Moiseenko, Srecko Stopic, Dominic Feldhaus, Bernd Friedrich and Peter Polyakov
Metals 2021, 11(7), 1053; https://doi.org/10.3390/met11071053 - 30 Jun 2021
Cited by 6 | Viewed by 2215
Abstract
This work is a contribution to the approach for Al purification and extraction from scrap using the thin-layer multiple-capillary molten salt electrochemical system. The single- and multiple-capillary cells were designed and used to study the kinetics of aluminium reduction in LiF–AlF3 and [...] Read more.
This work is a contribution to the approach for Al purification and extraction from scrap using the thin-layer multiple-capillary molten salt electrochemical system. The single- and multiple-capillary cells were designed and used to study the kinetics of aluminium reduction in LiF–AlF3 and equimolar NaCl–KCl with 10 wt.% AlF3 addition at 720–850 °C. The cathodic process on the vertical liquid aluminium electrode in NaCl–KCl (+10 wt.% AlF3) in the 2.5 mm length capillary had mixed kinetics with signs of both diffusion and chemical reaction control. The apparent mass transport coefficient changed from 5.6∙10−3 cm.s−1 to 13.1∙10−3 cm.s−1 in the mentioned temperature range. The dependence between the mass transport coefficient and temperature follows an Arrhenius-type behaviour with an activation energy equal to 60.5 kJ.mol−1. In the multiple-capillary laboratory electrolysis cell, galvanostatic electrolysis in a 64LiF–36AlF3 melt showed that the electrochemical refinery can be performed at a current density of 1 A.cm−2 or higher with a total voltage drop of around 2.0 V and specific energy consumption of about 6–7 kWh.kg−1. The resistance fluctuated between 0.9 and 1.4 Ω during the electrolysis depending on the current density. Thin-layer aluminium recycling and refinery seems to be a promising approach capable of producing high-purity aluminium with low specific energy consumption. Full article
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12 pages, 8592 KiB  
Article
Microstructural and Cavitation Erosion Behavior of the CuAlNi Shape Memory Alloy
by Tatjana Volkov-Husović, Ivana Ivanić, Stjepan Kožuh, Sanja Stevanović, Milica Vlahović, Sanja Martinović, Srecko Stopic and Mirko Gojić
Metals 2021, 11(7), 997; https://doi.org/10.3390/met11070997 - 22 Jun 2021
Cited by 8 | Viewed by 1906
Abstract
Microstructural and cavitation erosion testing was carried out on Cu-12.8Al-4.1Ni (wt. %) shape memory alloy (SMA) samples produced by continuous casting followed by heat treatment consisting of solution annealing at 885 °C for 60 min and, later, water quenching. Cavitation resistance testing was [...] Read more.
Microstructural and cavitation erosion testing was carried out on Cu-12.8Al-4.1Ni (wt. %) shape memory alloy (SMA) samples produced by continuous casting followed by heat treatment consisting of solution annealing at 885 °C for 60 min and, later, water quenching. Cavitation resistance testing was applied using a standard ultrasonic vibratory cavitation set up with stationary specimen. Surface changes during the cavitation were monitored by metallographic analysis using an optical microscope (OM), atomic force microscope (AFM), and scanning electron microscope (SEM) as well as by weight measurements. The results revealed a martensite microstructure after both casting and quenching. Microhardness value was higher after water quenching than in the as-cast state. After 420 min of cavitation exposure, a negligible mass loss was noticed for both samples. Based on the obtained results, both samples showed excellent cavitation resistance. Mass loss and morphological analysis of the formed pits indicated better cavitation resistance for the as-cast state (L). Full article
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13 pages, 4470 KiB  
Article
NdFeB Magnets Recycling Process: An Alternative Method to Produce Mixed Rare Earth Oxide from Scrap NdFeB Magnets
by Elif Emil Kaya, Ozan Kaya, Srecko Stopic, Sebahattin Gürmen and Bernd Friedrich
Metals 2021, 11(5), 716; https://doi.org/10.3390/met11050716 - 27 Apr 2021
Cited by 32 | Viewed by 4380
Abstract
Neodymium iron boron magnets (NdFeB) play a critical role in various technological applications due to their outstanding magnetic properties, such as high maximum energy product, high remanence and high coercivity. Production of NdFeB is expected to rise significantly in the coming years, for [...] Read more.
Neodymium iron boron magnets (NdFeB) play a critical role in various technological applications due to their outstanding magnetic properties, such as high maximum energy product, high remanence and high coercivity. Production of NdFeB is expected to rise significantly in the coming years, for this reason, demand for the rare earth elements (REE) will not only remain high but it also will increase even more. The recovery of rare earth elements has become essential to satisfy this demand in recent years. In the present study rare earth elements recovery from NdFeB magnets as new promising process flowsheet is proposed as follows; (1) acid baking process is performed to decompose the NdFeB magnet to increase in the extraction efficiency for Nd, Pr, and Dy. (2) Iron was removed from the leach liquor during hydrolysis. (3) The production of REE-oxide from leach liquor using ultrasonic spray pyrolysis method. Recovery of mixed REE-oxide from NdFeB magnets via ultrasonic spray pyrolysis method between 700 °C and 1000 °C is a new innovative step in comparison to traditional combination of precipitation with sodium carbonate and thermal decomposition of rare earth carbonate at 850 °C. The synthesized mixed REE- oxide powders were characterized by X-ray diffraction analysis (XRD). Morphological properties and phase content of mixed REE- oxide were revealed by scanning electron microscopy (SEM) and Energy-dispersive X-ray (EDX) analysis. To obtain the size and particle size distribution of REE-oxide, a search algorithm based on an image-processing technique was executed in MATLAB. The obtained particles are spherical with sizes between 362 and 540 nm. The experimental values of the particle sizes of REE- oxide were compared with theoretically predicted ones. Full article
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17 pages, 5307 KiB  
Article
Atomic Layer Deposition of aTiO2 Layer on Nitinol and Its Corrosion Resistance in a Simulated Body Fluid
by Rebeka Rudolf, Aleš Stambolić and Aleksandra Kocijan
Metals 2021, 11(4), 659; https://doi.org/10.3390/met11040659 - 18 Apr 2021
Cited by 11 | Viewed by 2464
Abstract
Nitinol is a group of nearly equiatomic alloys composed of nickel and titanium, which was developed in the 1970s. Its properties, such as superelasticity and Shape Memory Effect, have enabled its use, especially for biomedical purposes. Due to the fact that Nitinol exhibits [...] Read more.
Nitinol is a group of nearly equiatomic alloys composed of nickel and titanium, which was developed in the 1970s. Its properties, such as superelasticity and Shape Memory Effect, have enabled its use, especially for biomedical purposes. Due to the fact that Nitinol exhibits good corrosion resistance in a chloride environment, an unusual combination of strength and ductility, a high tendency for self-passivation, high fatigue strength, low Young’s modulus and excellent biocompatibility, its use is still increasing. In this research, Atomic Layer Deposition (ALD) experiments were performed on a continuous vertical cast (CVC) NiTi rod (made in-house) and on commercial Nitinol as the control material, which was already in the rolled state. The ALD deposition of the TiO2 layer was accomplished in a Beneq TFS 200 system at 250 °C. The pulsing times for TiCl4 and H2O were 250 ms and 180 ms, followed by appropriate purge cycles with nitrogen (3 s after the TiCl4 and 2 s after the H2O pulses). After 1100 repeated cycles of ALD depositing, the average thickness of the TiO2 layer for the CVC NiTi rod was 52.2 nm and for the commercial Nitinol, it was 51.7 nm, which was confirmed by X-ray Photoelectron Spectroscopy (XPS) and Scanning Electron Microscope (SEM) using Energy-dispersive X-ray (EDX) spectroscopy. The behaviour of the CVC NiTi and commercial Nitinol with and without the TiO2 layer was investigated in a simulated body fluid at body temperature (37 °C) to explain their corrosion resistance. Potentiodynamic polarisation measurements showed that the lowest corrosion current density (0.16 μA/cm2) and the wider passive region were achieved by the commercial NiTi with TiO2. Electrochemical Impedance Spectroscopy measurements revealed that the CVC NiTi rod and the commercial Nitinol have, for the first 48 h of immersion, only resistance through the oxide layer, as a consequence of the thin and compact layer. On the other hand, the TiO2/CVC NiTi rod and TiO2/commercial Nitinol had resistances through the oxide and porous layers the entire immersion time since the TiO2 layer was formatted on the surfaces. Full article
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15 pages, 3862 KiB  
Article
Synergism Red Mud-Acid Mine Drainage as a Sustainable Solution for Neutralizing and Immobilizing Hazardous Elements
by Hugo Lucas, Srecko Stopic, Buhle Xakalashe, Sehliselo Ndlovu and Bernd Friedrich
Metals 2021, 11(4), 620; https://doi.org/10.3390/met11040620 - 12 Apr 2021
Cited by 6 | Viewed by 2113
Abstract
Acid mine drainage (AMD) and red mud (RM) are frequently available in the metallurgical and mining industry. Treating AMD solutions require the generation of enough alkalinity to neutralize the acidity excess. RM, recognized as a waste generating high alkalinity solution when it is [...] Read more.
Acid mine drainage (AMD) and red mud (RM) are frequently available in the metallurgical and mining industry. Treating AMD solutions require the generation of enough alkalinity to neutralize the acidity excess. RM, recognized as a waste generating high alkalinity solution when it is in contact with water, was chosen to treat AMD from South Africa at room temperature. A German and a Greek RM have been evaluated as a potential low-cost material to neutralize and immobilize harmful chemical ions from AMD. Results showed that heavy metals and other hazardous elements such as As, Se, Cd, and Zn had been immobilized in the mineral phase. According to European environmental standards, S and Cr, mainly present in RM, were the only two elements not immobilized below the concentration established for inert waste. Full article
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11 pages, 3742 KiB  
Article
Synthesis of Silica Particles Using Ultrasonic Spray Pyrolysis Method
by Srecko Stopic, Felix Wenz, Tatjana-Volkov Husovic and Bernd Friedrich
Metals 2021, 11(3), 463; https://doi.org/10.3390/met11030463 - 11 Mar 2021
Cited by 12 | Viewed by 3018
Abstract
Silica has sparked strong interest in hydrometallurgy, catalysis, the cement industry, and paper coating. The synthesis of silica particles was performed at 900 °C using the ultrasonic spray pyrolysis (USP) method. Ideally, spherical particles are obtained in one horizontal reactor from an aerosol. [...] Read more.
Silica has sparked strong interest in hydrometallurgy, catalysis, the cement industry, and paper coating. The synthesis of silica particles was performed at 900 °C using the ultrasonic spray pyrolysis (USP) method. Ideally, spherical particles are obtained in one horizontal reactor from an aerosol. The controlled synthesis of submicron particles of silica was reached by changing the concentration of precursor solution. The experimentally obtained particles were compared with theoretically calculated values of silica particles. The characterization was performed using a scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS). X-ray diffraction, frequently abbreviated as XRD, was used to analyze the structure of obtained materials. The obtained silica by ultrasonic spray pyrolysis had an amorphous structure. In comparison to other methods such as sol–gel, acidic treatment, thermal decomposition, stirred bead milling, and high-pressure carbonation, the advantage of the ultrasonic spray method for preparation of nanosized silica controlled morphology is the simplicity of setting up individual process segments and changing their configuration, one-step continuous synthesis, and the possibility of synthesizing nanoparticles from various precursors. Full article
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21 pages, 4522 KiB  
Article
Processing Tests, Adjusted Cost Models and the Economies of Reprocessing Copper Mine Tailings in Chile
by Malte Drobe, Frank Haubrich, Mariano Gajardo and Herwig Marbler
Metals 2021, 11(1), 103; https://doi.org/10.3390/met11010103 - 6 Jan 2021
Cited by 20 | Viewed by 4594
Abstract
To increase resource efficiency, mining residues–especially tailings–have come into the focus of research, companies, and politics. Tailings still contain varying amounts of unextracted elements of value and minerals that were not of economic interest during production. As for primary mineral deposits, only a [...] Read more.
To increase resource efficiency, mining residues–especially tailings–have come into the focus of research, companies, and politics. Tailings still contain varying amounts of unextracted elements of value and minerals that were not of economic interest during production. As for primary mineral deposits, only a small share of tailings offers the possibility for an economic reprocessing. To minimize exploration expenditure, a stepwise process is followed during exploration, to estimate the likelihood of a project to become a mine or in this case a reprocessing facility. During this process, costs are continuously estimated at least in an order of magnitude. Reprocessing flowsheets for copper mine tailings in Chile were developed and costs and revenues of possible products from reprocessing were examined for a rough economic assessment. Standard cost models with capex and opex for flotation, leaching, and magnetic separation were adopted to the needs of tailings reprocessing. A copper tailing (around 2 M t) that also contains magnetite was chosen as a case study. A combination of magnetic separation and leaching gave the best economic results for copper and magnetite. The adopted cost models showed positive results at this early stage of investigation (semi-technical scale processing tests). Full article
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15 pages, 2737 KiB  
Article
Selenate Adsorption from Water Using the Hydrous Iron Oxide-Impregnated Hybrid Polymer
by Vesna Marjanovic, Aleksandra Peric-Grujic, Mirjana Ristic, Aleksandar Marinkovic, Radmila Markovic, Antonije Onjia and Marija Sljivic-Ivanovic
Metals 2020, 10(12), 1630; https://doi.org/10.3390/met10121630 - 4 Dec 2020
Cited by 8 | Viewed by 1720
Abstract
Hybrid adsorbent, based on the cross-linked copolymer impregnated with hydrous iron oxide, was applied for the first time for Se(VI) adsorption from water. The influence of the initial solution pH, selenate concentration and contact time to adsorption capacity was investigated. Adsorbent regeneration was [...] Read more.
Hybrid adsorbent, based on the cross-linked copolymer impregnated with hydrous iron oxide, was applied for the first time for Se(VI) adsorption from water. The influence of the initial solution pH, selenate concentration and contact time to adsorption capacity was investigated. Adsorbent regeneration was explored using a full factorial experimental design in order to optimize the volume, initial pH value and concentration of the applied NaCl solution as a reagent. Equilibrium state was described using the Langmuir model, while kinetics fitted the pseudo-first order. The maximum adsorption capacity was found to be 28.8 mg/g. Desorption efficiency increased up to 70%, and became statistically significant with the reagent concentration and pH increase, while the applied solution volume was found to be insignificant in the investigated range. Based on the results obtained, pH influence to the adsorption capacity, desorption efficiency, Fourier transform infrared (FTIR) and X-ray diffraction (XRD) analysis of loaded adsorbent, it was concluded that the outer- and inner-sphere complexation are mechanisms responsible for Se(VI) separation from water. In addition to the experiments with synthetic solutions, the adsorbent performances in drinking water samples were explored, showing the purification efficiency up to 25%, depending on the initial Se(VI) concentration and water pH. Determined sorption capacity of the cross-linked copolymer impregnated with hydrous iron oxide and its ability for regeneration, candidate this material for further research, as a promising anionic species sorbent. Full article
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14 pages, 4239 KiB  
Article
Basic Sulfate Precipitation of Zirconium from Sulfuric Acid Leach Solution
by Yiqian Ma, Srecko Stopic, Xuewen Wang, Kerstin Forsberg and Bernd Friedrich
Metals 2020, 10(8), 1099; https://doi.org/10.3390/met10081099 - 13 Aug 2020
Cited by 19 | Viewed by 4454
Abstract
H2SO4 was ensured to be the best candidate for Zr leaching from the eudialyte. The resulting sulfuric leach solution consisted of Zr(IV), Nb(V), Hf(IV), Al(III), and Fe(III). It was found that ordinary metal hydroxide precipitation was not feasible for obtaining [...] Read more.
H2SO4 was ensured to be the best candidate for Zr leaching from the eudialyte. The resulting sulfuric leach solution consisted of Zr(IV), Nb(V), Hf(IV), Al(III), and Fe(III). It was found that ordinary metal hydroxide precipitation was not feasible for obtaining a relatively pure product due to the co-precipitation of Al(III) and Fe(III). In this reported study, a basic zirconium sulfate precipitation method was investigated to recover Zr from a sulfuric acid leach solution of a eudialyte residue after rare earth elements extraction. Nb precipitated preferentially by adjusting the pH of the solution to around 1.0. After partial removal of SO42− by adding 120 g of CaCl2 per 1L solution, a basic zirconium sulfate precipitate was obtained by adjusting the pH to ~1.6 and maintaining the solution at 75 °C for 60 min. Under the optimum conditions, the loss of Zr during the SO42 removal step was only 0.11%, and the yield in the basic zirconium sulfate precipitation step was 96.18%. The precipitate contained 33.77% Zr and 0.59% Hf with low concentrations of Fe and Al. It was found that a high-quality product of ZrO2 could be obtained from the basic sulfate precipitate. Full article
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Review

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20 pages, 5318 KiB  
Review
Advances in Understanding of the Application of Unit Operations in Metallurgy of Rare Earth Elements
by Srecko Stopic and Bernd Friedrich
Metals 2021, 11(6), 978; https://doi.org/10.3390/met11060978 - 18 Jun 2021
Cited by 16 | Viewed by 5027
Abstract
Unit operations (UO) are mostly used in non-ferrous extractive metallurgy (NFEM) and usually separated into three categories: (1) hydrometallurgy (leaching under atmospheric and high pressure conditions, mixing of solution with gas and mechanical parts, neutralization of solution, precipitation and cementation of metals from [...] Read more.
Unit operations (UO) are mostly used in non-ferrous extractive metallurgy (NFEM) and usually separated into three categories: (1) hydrometallurgy (leaching under atmospheric and high pressure conditions, mixing of solution with gas and mechanical parts, neutralization of solution, precipitation and cementation of metals from solution aiming purification, and compound productions during crystallization), (2) pyrometallurgy (roasting, smelting, refining), and (3) electrometallurgy (aqueous electrolysis and molten salt electrolysis). The high demand for critical metals, such as rare earth elements (REE), indium, scandium, and gallium raises the need for an advance in understanding of the UO in NFEM. The aimed metal is first transferred from ores and concentrates to a solution using a selective dissolution (leaching or dry digestion) under an atmospheric pressure below 1 bar at 100 °C in an agitating glass reactor and under a high pressure (40–50 bar) at high temperatures (below 270 °C) in an autoclave and tubular reactor. The purification of the obtained solution was performed using neutralization agents such as sodium hydroxide and calcium carbonate or more selective precipitation agents such as sodium carbonate and oxalic acid. The separation of metals is possible using liquid (water solution)/liquid (organic phase) extraction (solvent extraction (SX) in mixer-settler) and solid-liquid filtration in chamber filter-press under pressure until 5 bar. Crystallization is the process by which a metallic compound is converted from a liquid into a crystalline state via a supersaturated solution. The final step is metal production using different methods (aqueous electrolysis for basic metals such as copper, zinc, silver, and molten salt electrolysis for REE and aluminum). Advanced processes, such as ultrasonic spray pyrolysis, microwave assisted leaching, and can be combined with reduction processes in order to produce metallic powders. Some preparation for the leaching process is performed via a roasting process in a rotary furnace, where the sulfidic ore was first oxidized in an oxidic form which is a suitable for the metal transfer to water solution. UO in extractive metallurgy of REE can be successfully used not only for the metal wining from primary materials, but also for its recovery from secondary materials. Full article
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