General Principles Of Metallurgy

[Charlesetta Blare]

Seriousphysical metallurgy of titanium started about 1948. Viewed some six years later, there is no question but that the progress made has been tremendous. Because a short review cannot hope to summarize all of the excellent work accomplished, stress has been placed on the general principles of titanium physical metallurgy developed so far.

Hydrometallurgy uses aqueous solutions to extract and refine metals [1, 2]. Although a typical metallurgical flowsheet tends to synergistically combine both pyrometallurgical and hydrometallurgical unit operations, the final steps in the purification and recovery of metals are nearly always hydrometallurgical. The transition to a climate-neutral society by 2050 relies heavily on the use of hydrometallurgy to extract critical raw materials, like cobalt, nickel, and lithium for batteries and rare-earth elements (REEs) for permanent magnets in electric motors and wind turbines. However, we must be careful to ensure that the processes applied to refine these metals needed for clean-energy production do not undermine our efforts by having an adverse environmental impact.

The past 10 years have been blessed with a much greater awareness of environmental issues in the extractive-metallurgy sector. This period has seen the launch of the Journal of Sustainable Metallurgy in 2015 [6] and an acknowledgement that the total environmental impact of a metallurgical process can only be properly evaluated via a lifecycle assessment (LCA) or, better still, via a multicriteria assessment (MCA). Unfortunately, such studies do not provide the metallurgist with practical guidelines as to how to improve the sustainability of a metallurgical flowsheet. In the area of environmentally friendly chemistry, the Twelve Principles of Green Chemistry have proven their worth in making chemical syntheses safer and greener (Table S1) [7, 8]. However, most of these principles are more closely associated with the synthesis of organic compounds and are not relevant to extractive metallurgy. The Twelve Principles of Green Chemistry were reformulated for engineering practice as the Twelve Principles of Green Engineering (Table S2) [9], and the Nine Principles of Green Engineering of the Sandestin Declaration (Table S3) [10]. Although some of these principles can be applied to extractive metallurgy, many others are less relevant because they were devised for manufacturing. At present, it is fair to say that such guidelines for extractive metallurgy are lacking. For this reason, we decided to formulate a set of design principles adapted to the field of hydrometallurgy that we hope will spur the development of more sustainable hydrometallurgical processes [11, 12]. These guidelines contribute to the targets of Goal 12 of the United Nations Sustainable Development Goals: Ensure sustainable consumption and production patterns [13].

Comparison of a simplified linear (left) and a circular (right) hydrometallurgical flowsheet for battery-grade cobalt and nickel production. For the circular process, a conceptual flowsheet is shown, with the re-introduction of protons to the system via hydrogen gas or via SX-assisted carbonation with CO2. The acid is regenerated, and the base-metal impurities are removed as metal carbonates. The different parts of the circular flowsheet will become more evident from the discussions further in the text

It is always possible to design a circular flowsheet by regenerating the acids and bases that are consumed in the process. However, every regeneration process requires energy. In general, it is energetically more favorable to reduce the consumption of chemicals rather than regenerate them, just as it is better to prevent waste (as much as possible) rather than to treat it or clean it up after it has been created. To do this requires knowledge of the chemical reactions involved and careful control of the process.

As guidelines for the design of circular hydrometallurgical flowsheets, we are proposing an interrelated set of principles,: i.e., the 12 Principles of Circular Hydrometallurgy (Table 1). These principles are to help metallurgical engineers achieve the goal of circularity in hydrometallurgy. As such, they are practical guidelines, presented in the form of imperatives, with each principle elaborated in more detail below. The Principles of Circular Hydrometallurgy have been numbered from 1 to 12. Although one could argue that some principles are more important than others, their order does not reflect a strict hierarchy. Some principles are more general (e.g., Principle 1: Regenerate reagents), whereas others are more specific (e.g., Principle 8: Electrify processes wherever possible). The principles are not independent and can often be combined to even more powerful principles, as explained in a separate section.

Regenerating all the waste-producing reagents is the main requirement when converting a linear flowsheet into a circular alternative. The regeneration must involve the minimum energy input and the smallest consumption of auxiliary reagents, with these reagents being regenerated whenever feasible. Furthermore, if we do not regenerate a reagent, we have to cover the costs of introducing fresh reagents (additional costs for logistics and warehousing) and waste treatment/management. The most common operations in hydrometallurgy that require the regeneration of reagents are leaching, solvent extraction (SX) and ion exchange (IX). Reagents to be generated are acids, bases, oxidizing and reducing agents, as well as other auxiliary products.

Acids are the most common lixiviants in leaching operations. A comparison of the most-often-used acids can be found in Table 2. In addition to sulfuric, hydrochloric, and nitric acid, also methanesulfonic acid (MSA) has been added to the list. Methanesulfonic acid is an emerging acid in hydrometallurgy [18, 19]. Protons are consumed either by reaction of the acid with the ore minerals during the leaching reaction or by reaction with a base. The latter neutralizes the excess acid after leaching and increases the pH for solution purification (e.g., removal of co-dissolved iron by precipitation as a hydroxide). If the anions of the acids are kept in the system, the acid can be regenerated by re-introducing the consumed protons to the system. This re-introduction of protons can be via (1) the direct reduction of dissolved metal ions by hydrogen gas [20], (2) the oxidation of water to oxygen gas at the anode during electrowinning (with release of protons) [21], (3) the hydrolysis of highly valent metal ions such as iron(III) [22], and (4) the formation of metal carbonates through the introduction of CO2 to the solution [23]. Hence, protons originate either from hydrogen gas or from water. CO2 only provides protons indirectly via its reaction with water.

Sulfuric acid (H2SO4) is the main acid used in hydrometallurgical flowsheets. However, it is not so easy to regenerate it from its sulfate salts by a low-temperature process. Na2SO4 is a problematic by-product that is formed by neutralization of excess H2SO4 by NaOH. Na2SO4 has a high solubility in water, so inhibiting its recovery from aqueous streams and its subsequent storage in a stable solid form. Impure Na2SO4 also has a low commercial value, and the discharge of wastewater containing dissolved Na2SO4 into the environment is not always allowed. The salt splitting of Na2SO4 into H2SO4 and NaOH is an attractive option because it solves the waste issue, while simultaneously regenerating the acid and base consumed in the process. Different electrochemical membrane processes have been developed to salt split Na2SO4. These are often based on electrodialysis, such as Bipolar Membrane Electrodialysis (BMED) [24, 25, 26]. The disadvantages of these methods are the slow kinetics and that the expensive membranes are easily clogged and fouled, requiring regular replacement.

Hydrochloric acid (HCl) is much easier to regenerate than H2SO4, which is driving the development of chloride hydrometallurgy. Pyrohydrolysis is applied in industry for the regeneration of HCl [4, 5]. This process is typically carried out in a spray roaster or a fluidized-bed reactor, where the metal chlorides react with steam at high temperatures to form the corresponding metal oxide and HCl gas. In theory, pyrohydrolysis can be used to hydrolyze the chloride of any multivalent metal ion to the corresponding oxide, but so far it has been restricted in industry to MgCl2, FeCl2, and NiCl2. Although pyrohydrolysis is an effective and well-proven technology, the process is capital and energy intensive. Nevertheless, there are major efforts to develop more energy-efficient versions of the classic pyrohydrolysis process. At the same time, low-temperature alternatives to pyrohydrolysis, which are more compatible with circular hydrometallurgy, are also being developed [4, 27]. Examples include sulfate crystallization and hydrolytic distillation. In electrowinning processes for the recovery of metals from chloride electrolytes, HCl is regenerated indirectly by collecting and burning chlorine gas evolved at the anode with hydrogen gas. However, collecting the Cl2 gas at the anode during electrowinning is not straightforward, and the additional step of chlorine burning complicates the process. Therefore, the direct electrolytic regeneration of HCl without an intermediate step to form Cl2 gas could be very beneficial to the development of circular hydrometallurgical flowsheets [28, 29]. If water is oxidized at the anode with release of oxygen gas, protons are formed simultaneously. These protons migrate to the cathode where they combine with chloride ions in solution to form hydrochloric acid. Recent advances in the direct electrolytic splitting of seawater, with O2 gas being produced at the anode, could initiate research into the direct regeneration of HCl from hydrometallurgical solutions [30]. The oxygen evolution reaction (OER) could successfully compete with the chlorine evolution reaction (CER) at reasonably high current densities, if suitable anodes are used [31]. Bipolar membrane electrodialysis (BMED) has been successfully used to generate concentrated HCl and NaOH solutions from concentrated brines [32], and this method could be extended to concentrated chloride solutions, provided that membranes with sufficiently long lifetimes can be developed.

3a8082e126