Recent Advances in Hydro- and Biohydrometallurgy Kostas A. Komnitsas www.mdpi.com/journal/minerals Edited by Printed Edition of the Special Issue Published in Minerals Recent Advances in Hydro- and Biohydrometallurgy Recent Advances in Hydro- and Biohydrometallurgy Special Issue Editor Kostas A. Komnitsas MDPI • Basel • Beijing • Wuhan • Barcelona • Belgrade Special Issue Editor Kostas A. Komnitsas Technical University of Crete (TUC) Greece Editorial Office MDPI St. Alban-Anlage 66 4052 Basel, Switzerland This is a reprint of articles from the Special Issue published online in the open access journal Minerals (ISSN 2075-163X) from 2018 to 2019 (available at: https://www.mdpi.com/journal/minerals/ special issues/Hydro Biohydrometallurgy) For citation purposes, cite each article independently as indicated on the article page online and as indicated below: LastName, A.A.; LastName, B.B.; LastName, C.C. Article Title. Journal Name Year , Article Number , Page Range. ISBN 978-3-03921-299-6 (Pbk) ISBN 978-3-03921-300-9 (PDF) c © 2019 by the authors. Articles in this book are Open Access and distributed under the Creative Commons Attribution (CC BY) license, which allows users to download, copy and build upon published articles, as long as the author and publisher are properly credited, which ensures maximum dissemination and a wider impact of our publications. The book as a whole is distributed by MDPI under the terms and conditions of the Creative Commons license CC BY-NC-ND. Contents About the Special Issue Editor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii Preface to ”Recent Advances in Hydro- and Biohydrometallurgy” . . . . . . . . . . . . . . . . . ix Kostas A. Komnitsas Editorial for Special Issue “Recent Advances in Hydro- and Biohydrometallurgy” Reprinted from: Minerals 2019 , 9 , 424, doi:10.3390/min9070424 . . . . . . . . . . . . . . . . . . . . 1 Christiana Mystrioti, Nymphodora Papassiopi, Anthimos Xenidis and Konstantinos Komnitsas Counter-Current Leaching of Low-Grade Laterites with Hydrochloric Acid and Proposed Purification Options of Pregnant Solution Reprinted from: Minerals 2018 , 8 , 599, doi:10.3390/min8120599 . . . . . . . . . . . . . . . . . . . . 5 Kostas Komnitsas, Evangelos Petrakis, Olga Pantelaki and Anna Kritikaki Column Leaching of Greek Low-Grade Limonitic Laterites Reprinted from: Minerals 2018 , 8 , 377, doi:10.3390/min8090377 . . . . . . . . . . . . . . . . . . . . 26 Ville Miettinen, Jarno M ̈ akinen, Eero Kolehmainen, Tero Kravtsov and Lotta Rintala Iron Control in Atmospheric Acid Laterite Leaching Reprinted from: Minerals 2019 , 9 , 404, doi:10.3390/min9070404 . . . . . . . . . . . . . . . . . . . . 40 Karina E. Salinas, Osvaldo Herreros and Cynthia M. Torres Leaching of Primary Copper Sulfide Ore in Chloride-Ferrous Media Reprinted from: Minerals 2018 , 8 , 312, doi:10.3390/min8080312 . . . . . . . . . . . . . . . . . . . . 53 P ́ ıa C. Hern ́ andez, Mar ́ ıa E. Taboada, Osvaldo O. Herreros, Te ́ ofilo A. Graber and Yousef Ghorbani Leaching of Chalcopyrite in Acidified Nitrate Using Seawater-Based Media Reprinted from: Minerals 2018 , 8 , 238, doi:10.3390/min8060238 . . . . . . . . . . . . . . . . . . . . 65 P ́ ıa C. Hern ́ andez, Junior Dupont, Osvaldo O. Herreros, Yecid P. Jimenez and Cynthia M. Torres Accelerating Copper Leaching from Sulfide Ores in Acid-Nitrate-Chloride Media Using Agglomeration and Curing as Pretreatment Reprinted from: Minerals 2019 , 9 , 250, doi:10.3390/min9040250 . . . . . . . . . . . . . . . . . . . . 81 Jonathan Castillo, Rossana Sep ́ ulveda, Giselle Araya, Danny Guzm ́ an, Norman Toro, Kevin P ́ erez, Marcelo Rodr ́ ıguez and Alessandro Navarra Leaching of White Metal in a NaCl-H 2 SO 4 System under Environmental Conditions Reprinted from: Minerals 2019 , 9 , 319, doi:10.3390/min9050319 . . . . . . . . . . . . . . . . . . . . 94 Jia-Ning Xu, Wen-Ge Shi, Peng-Cheng Ma, Liang-Shan Lu, Gui-Min Chen and Hong-Ying Yang Corrosion Behavior of a Pyrite and Arsenopyrite Galvanic Pair in the Presence of Sulfuric Acid, Ferric Ions and HQ0211 Bacterial Strain Reprinted from: Minerals 2019 , 9 , 169, doi:10.3390/min9030169 . . . . . . . . . . . . . . . . . . . . 106 Norman Toro, Nelson Herrera, Jonathan Castillo, Cynthia M. Torres and Rossana Sep ́ ulveda Initial Investigation into the Leaching of Manganese from Nodules at Room Temperature with the Use of Sulfuric Acid and the Addition of Foundry Slag—Part I Reprinted from: Minerals 2018 , 8 , 565, doi:10.3390/min8120565 . . . . . . . . . . . . . . . . . . . . 120 v Norman Toro, Manuel Salda ̃ na, Jonathan Castillo, Freddy Higuera and Roxana Acosta Leaching of Manganese from Marine Nodules at Room Temperature with the Use of Sulfuric Acid and the Addition of Tailings Reprinted from: Minerals 2019 , 9 , 289, doi:10.3390/min9050289 . . . . . . . . . . . . . . . . . . . . 133 Norman Toro, Manuel Salda ̃ na, Edelmira G ́ alvez, Manuel C ́ anovas, Emilio Trigueros, Jonathan Castillo and P ́ ıa C. Hern ́ andez Optimization of Parameters for the Dissolution of Mn from Manganese Nodules with the Use of Tailings in An Acid Medium Reprinted from: Minerals 2019 , 9 , 387, doi:10.3390/min9070387 . . . . . . . . . . . . . . . . . . . . 146 San Yee Khaing, Yuichi Sugai, Kyuro Sasaki and Myo Min Tun Consideration of Influential Factors on Bioleaching of Gold Ore Using Iodide-Oxidizing Bacteria Reprinted from: Minerals 2019 , 9 , 274, doi:10.3390/min9050274 . . . . . . . . . . . . . . . . . . . . 157 Jarno M ̈ akinen, Laura Wendling, Tiina Lavonen and P ̈ aivi Kinnunen Sequential Bioleaching of Phosphorus and Uranium Reprinted from: Minerals 2019 , 9 , 331, doi:10.3390/min9060331 . . . . . . . . . . . . . . . . . . . . 169 Jarno M ̈ akinen, Marja Salo, Jaakko Soini and P ̈ aivi Kinnunen Laboratory Scale Investigations on Heap (Bio)leaching of Municipal Solid Waste Incineration Bottom Ash Reprinted from: Minerals 2019 , 9 , 290, doi:10.3390/min9050290 . . . . . . . . . . . . . . . . . . . . 180 Ivan Nancucheo, Guilherme Oliveira, Manoel Lopes and David Barrie Johnson Bioreductive Dissolution as a Pretreatment for Recalcitrant Rare-Earth Phosphate Minerals Associated with Lateritic Ores Reprinted from: Minerals 2019 , 9 , 136, doi:10.3390/min9030136 . . . . . . . . . . . . . . . . . . . . 193 vi About the Special Issue Editor Kostas A. Komnitsas , Prof., holds a PhD degree in hydrometallurgy and is the director of the Laboratories of (i) Waste Management and Soil Decontamination, (ii) Ceramics and Glass Technology, and (iii) Ore Beneficiation in the School of Mineral Resources Engineering of Technical University Crete, Greece. He is an expert in the fields of hydro- and biohydrometallurgy, waste management and valorization, new materials, soil decontamination, environmental risk assessment, and LCA studies. Prof. Komnitsas has been appointed as a national representative/expert by the Greek General Secretariat of Research and Technology for research issues to the European Commission during the 6th Framework Programme (FP6) for the Action Integration and Strengthening of the European Research Area (2002–2004), for International Cooperation (2006–2009), and for the Action Societal Challenge ‘Climate action, environment, resource efficiency and raw materials’ (HORIZON 2020) (2013–today). Prof. Komnitsas has been appointed, so far, more than 80 times as evaluator of research proposals for the European Commission and National Research Foundations of several countries, including the US, Russia, Switzerland, Italy, Romania, Kazakhstan, Cyprus, Poland, Armenia, Serbia, etc. He has conducted more than 50 research projects (funded by national bodies and the EC, including life projects, previous FPs, and H2020). He has published more than 100 papers in peer-reviewed journals that have been cited over 2400 times, and his Scopus h-index is 28. vii Preface to ”Recent Advances in Hydro- and Biohydrometallurgy” Securing reliable and continuous access to raw materials and extraction of metals are important priorities in almost all countries in order to meet industrial needs, enable high-tech applications, maintain quality of life, and guarantee millions of jobs. Today hydro- and biohydrometallurgical processes are intensely investigated to solve bottlenecks in the raw materials supply; recover critical, base, and precious metals from low-grade ores and various types of wastes; and provide environmental solutions for various industrial problems. The roots of hydrometallurgy can be traced back to the era of alchemists, while modern hydrometallurgy dates back to 1887, when two important processes were invented, namely the cyanidation process for the treatment of gold ores and the Bayer process for the treatment of bauxites and the production of alumina. On the other hand, there is evidence that bioleaching was used in the Rio Tinto area in Spain prior to Roman occupation for the recovery of copper, as well as in China some 2000 years ago. Modern commercial biohydrometallurgical applications for the processing of ores commenced in the 1950s, focusing on the bioleaching of copper. Since then, biohydrometallurgy has been used for the treatment of various primary and secondary raw materials and the recovery of several metals, including good, copper, and rare-earth elements (REEs). It must be underlined that the critical role of bio- and hydrometallurgy in achieving sustainable development in various industrial sectors was identified more than 30 years ago. Kostas A. Komnitsas Special Issue Editor ix minerals Editorial Editorial for Special Issue “Recent Advances in Hydro- and Biohydrometallurgy” Kostas A. Komnitsas Technical University of Crete, School of Mineral Resources Engineering, 73100 Chania, Greece; komni@mred.tuc.gr; Tel.: + 302821037686 Received: 5 July 2019; Accepted: 10 July 2019; Published: 11 July 2019 Securing reliable and continuous access to raw materials and extraction of metals are important priorities in almost all countries in order to meet industrial needs, enable high-tech applications, maintain quality of life, and guarantee millions of jobs. Today, hydro- and biohydrometallurgical processes are intensely investigated to solve bottlenecks in the raw materials supply, recover critical base and precious metals from low-grade ores and various types of wastes, and also provide environmental solutions for various industrial problems [1–4]. The roots of hydrometallurgy are traced back to the period of the alchemists, while modern hydrometallurgy dates back to 1887, when two important processes were invented, namely the cyanidation process for the treatment of gold ores and the Bayer process for the treatment of bauxites and the production of alumina [ 5 ]. On the other hand, there is evidence that bioleaching has been used in the Rio Tinto area in Spain prior to Roman occupation for the recovery of copper, as well as in China some two thousand years ago [ 6 ]. Modem commercial biohydrometallurgical applications for the processing of ores commenced in the 1950s, focusing on bioleaching of copper [ 7 ]. Since then, biohydrometallurgy has been used for the treatment of various primary and secondary raw materials and the recovery of several metals, including gold, copper, and rare earth elements (REEs) [ 8 – 11 ]. It has to be underlined that the critical role of bio- and hydrometallurgy in achieving sustainable development in various industrial sectors has been identified more than 30 years ago [12]. This Special Issue of Minerals presents recent selective studies, carried out in di ff erent countries, that highlight advances in the fields of hydro- and biohydrometallurgy. It aims to attract the interest of the readers, and especially of young scientists and students in this fascinating scientific discipline. Three of the studies investigated leaching of laterites. Mystrioti et al. [ 13 ] investigated the e ffi ciency of stirred reactor hydrochloric acid leaching for the treatment of a low-grade saprolitic laterite. The leaching was carried out at 30% pulp density, by applying a counter-current mode of operation in order to better simulate industrial-scale operations and maintain Fe dissolution at low levels. This mode of operation was very e ffi cient in terms of minimizing Fe dissolution, which was maintained at 0.6%, but had a negative e ff ect on Ni and Co extraction, which was 55% and 63%, respectively, probably due to the passivation of ore grain surfaces by secondary iron precipitation products. The treatment of PLS (pregnant leach solution) involved a precipitation step for the removal of trivalent metals, Fe, Al, and Cr with the use of Mg(OH) 2 . Komnitsas et al. [ 14 ] investigated the e ffi ciency of column leaching of low-grade limonitic laterites with the use of H 2 SO 4 for the extraction of Ni and Co. Parameters studied were acid concentration (0.5 M or 1.5 M) and addition of 20 or 30 g / L of sodium sulfite (Na 2 SO 3 ). The experimental results showed that (i) Ni and Co extractions increased with the increase of H 2 SO 4 concentration and reached 60.2% and 59.0%, respectively, after 33 days of leaching with the use of 1.5 M H 2 SO 4 , and (ii) addition of 20 g / L Na 2 SO 3 in the leaching solution resulted in higher extractions for both metals (73.5% for Ni and 84.1% for Co, respectively). Finally, the extractions of Fe, Mg, Al, and Ca were quite low, namely, 7.9, 40.2, 23.3, and 51.0%, respectively. Miettinen et al. [ 15 ] investigated iron control during atmospheric acid leaching of two laterite types, a limonite and a silicate, in order to decrease acid consumption and iron dissolution. The process Minerals 2019 , 9 , 424; doi:10.3390 / min9070424 www.mdpi.com / journal / minerals 1 Minerals 2019 , 9 , 424 involved direct acid leaching of the limonitic laterite followed by simultaneous iron precipitation as jarosite after the addition of the silicate laterite for pH neutralization. The combined leaching and precipitation process reduced acid consumption and iron concentration in the pregnant leach solution (PLS). The acid consumption, which during the direct atmospheric leaching was approximately 0.7 kg H 2 SO 4 per kg of laterite was reduced during the combined process to 0.42 kg H 2 SO 4 per kg of laterite. In addition, Fe concentration in the PLS decreased from 10 g / L to approximately 2–3 g / L, resulting in significant savings compared with the conventional process. Salinas et al. [ 16 ] investigated copper extraction from a typical porphyry copper sulfide deposit from Antofagasta, Chile, using chloride–ferrous leaching. They carried out large-scale column leaching tests using 50 kg of agglomerated ore that was first cured for 14 days and then leached for 90 days. The highest Cu extraction, 50.23%, was achieved at 32.9 ◦ C with the addition of 0.6 kg of H 2 SO 4 , 0.525 kg of NaCl, and 0.5 kg of FeSO 4 per ton of ore. The e ff ect of agglomeration, curing, and temperature on the leaching kinetics of Cu was also assessed. Hernandez et al. [ 17 ] studied leaching of chalcopyrite ore containing 1.6 wt% Cu in a nitrate-acid–seawater system. The parameters studied were water quality (pure water and seawater), temperature (25–70 ◦ C), reagent concentration, nitrate type (NaNO 3 or KNO 3 ), and leaching duration. Results showed that up to 80 wt% of Cu can be extracted during leaching at 45 ◦ C in 7 days. In the absence of nitrates, under the same leaching conditions, only 28 wt% of Cu was extracted. The Cu extraction increased to 97.2 wt% with the use of 1 M H 2 SO 4 and 1 M NaNO 3 when the temperature increased to 70 ◦ C. The main disadvantage of this approach was the production of NOx gases that should be controlled in industrial operations. In another study, Hernandez et al. [ 18 ] investigated leaching of chalcopyrite in acid-nitrate–chloride media using mini-columns. The e ff ect of ore pretreatment, involving agglomeration and curing, as well as of several factors, namely addition of nitrate as NaNO 3 (11.7 and 23.3 kg / ton), chloride as NaCl (2.1 and 19.8 kg / ton), curing time (20 and 30 days), and temperature (25 and 45 ◦ C) was also evaluated. The maximum Cu extraction, 58.6%, after 30 days of curing at 45 ◦ C, was obtained during leaching with the addition of 23.3 kg of NaNO 3 / ton and 19.8 kg of NaCl per ton of ore. Copper extraction from the pretreated ore reached 63% during leaching at pH 1 and 25 ◦ C with the use of a solution containing 6.3 g / L of NaNO 3 and 20 g / L of NaCl. Castillo et al. [ 19 ] investigated the e ff ect of NaCl on the leaching of white metal from a Teniente converter in NaCl-H 2 SO 4 media and proposed a simplified two-stage mechanism. Parameters studied involved the concentration of ferric ion (1–10 g / L), NaCl (30–210 g / L), and H 2 SO 4 (10–50 g / L). The results showed that the addition of NaCl increased the dissolution of Cu from 55% to nearly 90%, whereas the e ff ect of sulfuric acid was only minor. The positive e ff ect of NaCl is mainly related to the action of chloro-complexes oxidizing agents in relation to the Cu + 2 / Cu + couple. Leaching of Cu takes place in two stages involving (i) transformation of chalcocite into covellite and production of Cu 2 + ions and (ii) reaction of covellite for the generation of Cu 2 + ions and elemental sulfur. Xu et al. [ 20 ] investigated the galvanic e ff ect of pyrite and arsenopyrite during leaching of gold ores in sulfuric acid, ferric ion, and HQ0211 bacterial strain solutions with the use of electrochemical testing (open-circuit potential, linear sweep voltammetry, Tafel, and electrochemical impedance spectroscopy, EIS) and frontier orbital calculations. The results indicated that (i) the linear sweep voltammetry curve and Tafel curve of the galvanic pair were similar to those of arsenopyrite, (ii) the corrosion behavior of the galvanic pair was consistent with that of arsenopyrite, and (iii) the galvanic e ff ect promoted the corrosion of arsenopyrite by simultaneously increasing the cathode and anode currents and reducing oxidation resistance. The frontier orbital calculation explained the principle of the galvanic e ff ect of pyrite and arsenopyrite from the view of quantum mechanics. Three papers studied leaching of marine nodules in the presence of reducing agents to extract Mn. In the first study [ 21 ], the surface optimization methodology was used to assess the e ff ect of three independent variables, namely time, particle size, and sulfuric acid concentration, on Mn extraction during leaching with H 2 SO 4 in the presence of foundry slag. In a second study [ 22 ], the e ff ect of 2 Minerals 2019 , 9 , 424 magnetite-rich tailings produced from slag flotation during leaching at room temperature (25 ◦ C) was explored. Other factors studied included MnO 2 / Fe 2 O 3 ratio in solution and agitation speed. The highest Mn extraction, 77%, was obtained at MnO 2 / Fe 2 O 3 ratio 0.5, 1 mol / L H 2 SO 4 , particle size − 47 + 38 μ m , and leaching time 40 min. Finally, in their third study [ 23 ], the authors optimized the main operating parameters through factorial experimental design. It is mentioned that the generation of Fe 2 + and Fe 3 + improved Mn extraction that reached 73% within 5 to 20 min. Khaing et al. [ 24 ] explored the factors that a ff ect bioleaching of gold ores in the presence of iodide-oxidizing bacteria. The factors studied, in order to maximize gold dissolution, included concentration of nutrients and iodide, initial cell number, incubation temperature, and shaking speed. The culture medium contained marine broth, potassium iodide, and gold ore. The main findings of the study were (i) gold contained in the ore was almost completely dissolved in the culture solution, incubated at 30 ◦ C and 35 ◦ C, (ii) the pH and redox potential of the culture solution were 7.7–8.4 and 472–547 mV, (iii) gold leaching rate in iodine–iodide solution was much faster compared with that of the conventional cyanidation process, and (iv) iodine can be recovered after leaching. Makinen et al. [ 25 ] investigated the e ffi ciency of a two-step sequential leaching process, involving bioleaching and chemical leaching, to treat apatite ores containing P and U impurities. The first leaching step, at pH ≥ 2, Eh + 650 mV and Fe 3 + concentration ≥ 1.0 g / L, enabled 89% extraction of U in 3 days. After solid–liquid separation, the second leaching step at pH ≤ 1.5 enabled the recovery of phosphorus from the solid leach residue. It is mentioned that despite the high leaching degree for P (98%), the duration of the process was quite long (28 days). In Brazil, lateritic deposits are often associated with rare earth element enriched phosphate minerals such as monazite. Given that monazite is highly refractory, rare earth elements (REEs) extraction is di ffi cult and normally involves high-temperature digestion with concentrated NaOH and / or H 2 SO 4 Nancucheo et al. [ 26 ] assessed the e ff ect of bioreductive dissolution of ferric iron minerals associated with monazite in stirred reactors using Acidithiobacillus (A.) species in pH and temperature-controlled tests. The results indicated that under aerobic conditions, A. thiooxidans at extremely low pH can enhance substantially the solubilization of iron from ferric iron minerals. Finally, Makinen et al. [ 27 ] investigated a robust and simple heap leaching approach for the recovery of Zn and Cu from municipal solid waste incineration bottom ash (MSWI BA). Also, they studied the e ff ect of autotrophic and acidophilic bioleaching microorganisms. Leaching yields for Zn and Cu varied between 18–53% and 6–44%, respectively. The main contaminants present in MSWI BA, namely Fe and Al, were easily liberated by sulfuric acid leaching, lowering the quality of PLS and imposing limitations for the industrial utilization of the process. Author Contributions: K.K. wrote this editorial. Acknowledgments: The guest editor would like to sincerely thank all authors, reviewers and the editorial sta ff of Minerals for their e ff orts and devotion to successfully complete this Special Issue. Conflicts of Interest: The author declares no conflict of interest. References 1. Panda, S.; Akcil, A.; Pradhan, N.; Deveci, H. Current scenario of chalcopyrite bioleaching: A review on the recent advances to its heap-leach technology. Bioresour. Technol. 2015 , 196 , 694–706. [CrossRef] [PubMed] 2. Musariri, B.; Akdogan, G.; Dorfling, C.; Bradshaw, S. 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Bioreductive dissolution as a pretreatment for recalcitrant rare-earth phosphate minerals associated with lateritic ores. Minerals 2019 , 9 , 136. [CrossRef] © 2019 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http: // creativecommons.org / licenses / by / 4.0 / ). 4 minerals Article Counter-Current Leaching of Low-Grade Laterites with Hydrochloric Acid and Proposed Purification Options of Pregnant Solution Christiana Mystrioti 1 , Nymphodora Papassiopi 2 , Anthimos Xenidis 2 and Konstantinos Komnitsas 1, * 1 Department Mineral Resources Engineering, Technical University Crete, 73100 Chania, Greece; chmistrioti@metal.ntua.gr 2 School of Mining and Metallurgical Engineering, National Technical University of Athens, 15780 Zografos, Greece; papasiop@metal.ntua.gr (N.P.); axen@metal.ntua.gr (A.X.) * Correspondence: komni@mred.tuc.gr; Tel.: +30-28210-37686 Received: 22 November 2018; Accepted: 14 December 2018; Published: 18 December 2018 Abstract: A hydrochloric acid hydrometallurgical process was evaluated for Ni and Co extraction from a low-grade saprolitic laterite. The main characteristics of the process were (i) the application of a counter-current mode of operation as the main leaching step (CCL), and (ii) the treatment of pregnant leach solution (PLS) with a series of simple precipitation steps. It was found that, during CCL, co-dissolution of Fe was maintained at very low levels, i.e., about 0.6%, which improved the effectiveness of the subsequent PLS purification step. The treatment of PLS involved an initial precipitation step for the removal of trivalent metals, Fe, Al, and Cr, using Mg(OH) 2 . The process steps that followed aimed at separating Ni and Co from Mn and the alkaline earths Mg and Ca, by a combination of repetitive oxidative precipitation and dissolution steps. Magnesium and calcium remained in the aqueous phase, Mn was removed as a solid residue of Mn(III)–Mn(IV) oxides, while Ni and Co were recovered as a separate aqueous stream. It was found that the overall Ni and Co recoveries were 40% and 38%, respectively. About 45% of Ni and 37% of Co remained in the leach residue, while 15% Ni and 20% Co were lost in the Mn oxides. Keywords: low-grade saprolitic laterite; counter-current leaching; pregnant leach solution; purification 1. Introduction Nickel is a metal in high demand for industrial applications such as the production of stainless steel, non-ferrous alloys, alloy steels, plating, foundry, and batteries due to its beneficial properties (strength, corrosion resistance, high ductility, good thermal and electric conductivity, magnetic characteristics, and catalytic properties) [ 1 ]. Nickel can be found naturally in lateritic ores which are formed by weathering of ultramafic rocks or in sulfide resources. Due to the decline of sulfide deposits and high-grade laterites, there is a need for adapting or optimizing technologies aiming at increasing metal recovery from low-grade laterites in order to make their treatment economically feasible [ 2 ]. It is well known that conventional mineral processing techniques, including flotation heavy-media separation and others, cannot be readily applied to oxide ores such as laterites, unlike sulfides, due to the complex nature of the ores and the fact that nickel is hosted in several mineral phases [3]. Hydrometallurgical processes for nickel and cobalt recovery from low-grade laterite ores can be classified as high-pressure acid leaching (HPAL), atmospheric acid leaching (AL), heap leaching (HL), and biological leaching (BL). Minerals 2018 , 8 , 599; doi:10.3390/min8120599 www.mdpi.com/journal/minerals 5 Minerals 2018 , 8 , 599 HPAL is often selected for the treatment of low-grade laterite ores, but presents drawbacks such as expensive equipment in order to stand the harsh leaching conditions (typical operating temperature from 250 to 270 ◦ C) and high initial capital cost [4,5]. AL of low-grade laterites is considered as a promising method which efficiently recovers Ni and Co with low cost due to milder leaching conditions. The majority of studies were carried out using sulfuric acid as lixiviant and fewer tested nitric and hydrochloric acid [ 1 , 6 – 8 ]. More recent studies comparing the performance of HCl and H 2 SO 4 , under atmospheric conditions indicated that hydrochloric acid is more effective for both nickel and cobalt extraction [ 4 , 8 ]. However, AL using hydrochloric acid as a lixiviant results in high iron and magnesium dissolution which is related to high acid consumption and requires a complex purification treatment. HL is a well-known process due to the numerous applications for the treatment of copper, uranium, and gold ores, and requires low investment and operating costs. However, Ni and Co recoveries by HL are relatively low compared with leaching in stirred reactors [ 9 , 10 ]. Studies were carried out evaluating the bio-hydrometallurgical HL of laterites using organic acids, citric acid, and oxalic acid, or using heterotrophic micro-organisms which metabolize organic compounds and excrete organic acids, namely carboxylic acids. These microorganisms should be tolerant to metal mixtures [11,12]. The majority of studies aim at optimizing the conditions of atmospheric leaching, in order to maximize Ni and Co extraction, and fewer deal with the purification of pregnant leach solution (PLS), which has low concentrations of Ni (0.8–3.3 g/L) and Co (0.05–0.2 g/L) and high concentrations of the major elements, mainly Fe (13–37 g/L) and Mg (8–40 g/L) [ 13 , 14 ]. The main processes for PLS purification include precipitation, solvent extraction, and ion exchange. Precipitation takes place when the chemical species exceed their limit of solubility by changing the solution pH or temperature, or by adding a reagent [ 15 ]. Solvent extraction presents high selectivity and involves mixing of PLS with an organic phase; the resulting emulsion is allowed to separate, and the valuable metals are transferred to the organic phase [ 11 , 16 , 17 ]. The use of ion-exchange media, such as resins, zeolite, and active carbon, in order to exchange cations or anions from a solution, presents high recoveries and is environmentally friendly, but these media have limited efficiency in mixed stream solutions [15]. The high ferric iron content constitutes the major impurity in the PLS produced by the atmospheric acid leaching of laterites [ 18 ]. The removal of iron from PLS involves a difficult and costly step. Moreover, it often results in important Ni and Co losses (5–20%) [ 19 ]. Iron removal has been extensively studied in zinc industry in the early 1960s and several techniques have been developed for iron precipitation as jarosite, goethite and hematite [ 19 ]. The majority of studies for the purification of sulphate leach liquors involve the addition of limestone or calcium hydroxide, in order to increase the pH of the solution, so that metal hydroxides are formed and precipitate [ 16 , 20 , 21 ]. Ni and Fe hydroxides precipitate at different pH, however it is difficult to avoid co-precipitation of Ni during the removal of Fe. Nickel co-precipitation takes place due to its adsorption on the surface of iron hydroxides or substitution in the precipitates. The temperature increase affects the stability and the crystallization of the precipitates. For these reasons the process conditions such as pH, temperature, and duration of precipitation have serious impact on the properties of the precipitates and the potential Ni and Co losses [18,22]. Some steps of the purification treatment of the hydrochloric acid leach solutions are yet to be thoroughly investigated. Filippou and Choi [ 23 ] evaluated the step of iron precipitation by adding sodium hydroxide or urotropin at 100 ◦ C. However, the formation of various allotropes of FeOOH made the step of solid/liquid separation difficult and resulted in high losses of the other metals. Increasing the pH to 5.5 improved solid/liquid separation, but caused higher losses of Ni and Co [ 24 ]. Beukes et al. [ 24 ] suggested that nickel and cobalt adsorption on the surface of iron oxides (goethite and hematite) is dependent on the presence of precipitates and not on their quantity. The researchers focused on solvent extraction processes due to the difficulties which occurred upon the precipitation of iron oxides. Many neutral extractants such as tri- n -butyl phosphate (TBP), tri- n -octylphosphine oxide (TOPO), amines, and mixtures of them were tested, but effective iron removal requires many steps 6 Minerals 2018 , 8 , 599 due to the absence of an adequate “salting out agent”. In a solution with concentration of 200 g/L MgCl 2 , the removal of iron (40 g/L) was equal to 99.4% using TBP and methylisobutyl ketone (MIBK) with some losses of Co [ 11 ]. Demopoulos et al. [ 25 ] tested hydrolytic distillation for the precipitation of ferric iron as Fe 2 O 3 and the recovery of Cl − as HCl. Nickel and cobalt were precipitated as oxides at an operating temperature of ~170 ◦ C. Zhang et al. [ 2 ] applied hydrolytic distillation on synthetic laterite-leaching solutions and showed that 95.5% of iron was removed as hematite, 94% of Ni and Co remained in the aqueous phased as soluble chlorides, and excess HCl was distilled with a final recovery in the order of 77.9%. The removal of Mn and Mg constitutes another significant challenge for PLS purification treatment [15,17]. In this study, a counter-current leaching (CCL) scheme was evaluated for the HCl leaching of a saprolitic ore. The CCL mode of operation was adopted in order to maintain the co-dissolution of Fe at low levels and obtain satisfactory extraction of Ni and Co, while operating at high pulp densities close to the industrial practice. The proposed flowchart for the treatment of PLS consists of a series of simple precipitation steps. 2. Materials and Methods 2.1. Investigated Flowchart A conceptual flowchart illustrating the main steps of the HCl leaching process, which were investigated in the framework of this study, is shown in Figure 1. In this flowchart, the first step concerns the leaching of laterite ore. The next step (precipitation of trivalent metals) involves the removal of iron and other trivalent metals, e.g., Cr(III), Al(III), etc., in the form of hydroxides, using MgO as a neutralization agent and maintaining the pH at relatively acidic conditions, i.e., 3.5. The solution resulting from this treatment step contains all divalent metals, including Ni, Co, Mn, Mg, and Ca. The following step aims primarily at the separation of alkaline earths, Mg and Ca, from the other divalent metals by increasing the pH close to 9. This step is combined with the oxidation of Mn(II) to the tetravalent state, Mn(IV), and, for this reason, is denoted as alkaline oxidation (AO) in the flowchart of Figure 2. Mg and Ca remain in the aqueous phase, while Ni, Co, and Mn are recovered in the form of a solid residue containing Ni(OH) 2 , Co(OH) 2 , and MnO 2 . The next step, acidic dissolution (AD), involves the acidic treatment of the solid residue, in order to enable the selective dissolution of Ni and Co, given that the high-valence Mn is expected to remain in the solid state. The aqueous solution of the AO step contains mainly Mg, Ca, and Cl − anions. Calcium can be removed from these solutions via precipitation as CaSO 4 · xH 2 O [ 26 , 27 ]. The final stream containing Mg and Cl can be treated by pyrohydrolysis. By this process, it is possible to regenerate HCl, which is recycled in the leaching step, producing MgO. As seen in Figure 1, the investigated flowchart involves a series of successive precipitation steps. Initial thermodynamic calculations were carried out using Visual Minteq software [ 28 ] to identify the optimum pH ranges, which could allow the selective separation of impurities, through precipitation of metal hydroxides, from the valuable metals. For this reason, the model was run using fixed pH values in the whole pH range between 0 and 14. The following metal hydroxides were considered as possible precipitates: ferrihydrite (Fh), Al(OH) 3 , Cr(OH) 3 , Ni(OH) 2 , Co(OH) 2 , pyrochroite (Mn(OH) 2 ) or pyrolusite (MnO 2 ), brucite (Mg(OH) 2 ), and portlandite (Ca(OH) 2 ). 7 Minerals 2018 , 8 , 599 Figure 1. The main steps of the investigated HCl leaching process for the treatment of laterite ores. 2.2. Materials The saprolitic laterite used in this study was collected from Larco deposits in the area of Kastoria (northern Greece). The sample (50 kg) was homogenized, ground using vibration milling for 10 min, and the particle size of the working sample was under 74