Sustainable Electrochem Extraction of Metal Resources from Waste Streams: From Removal to Recovery
The paper I'll discuss in this post is this one: Sustainable Electrochemical Extraction of Metal Resources from Waste Streams: From Removal to Recovery (Wei Jin and Yi Zhang, ACS Sustainable Chemistry & Engineering 2020 8 (12), 4693-4707)
This weekend, CSPAN history presented two academic lectures on the 1918 Flu Pandemic, where the strategies for containment, for better and for worse, were similar to what we are seeing 102 years later, in 2020.
They are here:
https://www.c-span.org/video/?465724-7/influenza-pandemic-world-war
https://www.c-span.org/video/?469755-1/1918-influenza-pandemic-public-information
Unless you are 102 years old or older, your entire life took place after that pandemic, which was very, very, very serious and which actually ended up killing more people than died in combat in World War I, which ended the same year as the pandemic began. Until now, what was extremely and overwhelming important probably garnered very little attention except in obscure academic treatises. I am not minimizing what we are seeing now - it's very possible Covid will kill me, as I am high risk, although if it doesn't something else will - but it behooves us to remember that there will be life on this planet when the pandemic is over, and major problems that were exigent before the pandemic will remain exigent afterwards as well.
The paper here is about one such exigent problem, the problem of both contamination by and depletion of various critical elements of the periodic table; many of which, while quite toxic, are also of high technological and economic importance. Our technology is structured so that these elements are isolate from dilute sources, usually ores, refined to high states of purity, and then re-diluted in the devices in which we use them. Not only are they diluted in these devices, but they are usually so diluted in a complex array of other elements, as well as a complex array of organic molecules, such as polymers, and insulating material and flame retardants of toxicological import. This nature represents chemical problems in separation and recovery.
For about five or ten years now, whenever I think of this problem, my mind turns to electrochemistry since the electron, which, because its energy is adjustable, is not only a flexible tool to reduce salts to their metals, but also is a tool for chemical separations. Much of my attention in this area has focused on the element uranium, which is an element that can save the world, and it's presence in various dilute matrices, notably seawater, natural formations and of course, as a side product of mining, not only uranium mining, but the mining of dangerous fossil fuels such as coal, oil and gas, all three of which, particularly in "fracking" settings, can result in uranium mobilization. Other means of distributing uranium in dilute but still potentially problematic forms include agriculture, since uranium is a constituent of many phosphate ores, and especially the mining of fossil and recharging ground water in regions where uranium is a natural constituent of soils and/or bedrock.
The removal of elements like uranium, and as we shall see, many other elements, results in the concentration in some device or substance. In the age of the waste mentality, sometimes these devices and substances are regarded as waste. Arguably however they are low grade (or even high grade) ores. As my generation has selfishly consumed almost all of the best ores of many elements in the periodic table, these kinds of ores may ultimately become very important sources of elements: I have argued this point about uranium many places on the internet.
This is why it is such a pleasure to see this paper in the most recent issue of ACS Sustainable Chemistry & Engineering.
This paper is a "perspective," essentially a review article, and it is not possible in this space to cover very much of it.
Nevertheless from the introductory text:
...Many technologies have been applied for efficient metal removal, such as chemical precipitation, adsorption/biosorption, and ion exchange.(11,12) However, they usually suffer from the drawbacks of secondary pollution generation, large reagent consumption, and high operating cost. Recently, electrochemical methods have attracted considerable attention for the remediation of metal-polluted water mainly owing to their advantages of environmental compatibility, high efficiency and versatility, operation feasibility, and cost effectiveness, via the employment of the green redox reagent “electron”...
The authors remark that many mine tailings contain valuable materials, noting that the Bayer process - the process that extracts alumina from bauxite - leaves between one and two and a half tons of "red mud," a highly caustic mixture of sodium hydroxide, iron oxide, titanium oxide and a host of other metals that might be considered as ores for iron, titanium and other compounds. The composition is given as "20?40 wt % Fe2O3, 10?25 wt % Al2O3, 3?10 wt % TiO2, and other metallic compounds." In theory, and sometimes in practice, all of these metals can be electrochemically recovered, iron and titanium in the FFC process, aluminum, enriched by the removal of the other elements, in the Hall process.
Historically, according to the authors, electrochemistry was utilized to purify water by removing heavy metals as spongey flocculents, largely consisting of metal oxides. The difference between historical approaches and the approach that the authors discuss is that these flocculents, particularly when used on a large scale, can be ores to recover metals for use.
Because the energy associated with an electron per unit of charge - the voltage - it is possible to make electrochemical adustments that not only remove and or collect metals but also separate them.
Here from the paper is a table of electromotive force (electrochemical or electrode potential) of a few elements in the periodic table:
In this table, those elements whose standard electrode potentials are negative require an input of energy to reduce them to metallic form, those with electrode potentials are positive require energy to oxidize them. Since these potentials all differ they can be separated by adjusting electrode voltages. While this a significant simplification, since other factors are involved, this is a well known strategy both on a lab scale and an industrial scale as well.
This cartoon from the paper shows an example of how it might work:
The caption:
It is as the authors make clear, not always the case that electrochemical approaches are appropriate for all situations. Much depends on concentrations, (which may be very low), economics, and the costs and supply of materials and/or reagents, as well as the cost of disposing (or, as the paper seeks to advance recovering) side products. This cartoon chart from the paper shows the various kinds of approaches:
The caption:
Note that the waste water, especially if it is concentrated in metals, can be further purified by electrochemical means with recovery of the materials.
The subsequent cartoon in the paper generalizes industrial material processing technologies, focusing on their relationship to the generation of wastewater:
The caption:
Taken together these options suggest many hybrid approaches. Consider the case of desalination. Worldwide, the main approach currently in use on an industrial scale is membrane driven (RO) approaches, as outlined in figure 3. To recover uranium - although the uranium already mined and isolated is sufficient to cover all of humanity's energy needs for centuries (in "breed and burn" reactors) if we pass the seawater being piped into a desalination plant with an appropriate resin (uranium capture resins are well known) we can recover uranium for future generations to utilize.
I personally believe that a better approach to desalination could and should be developed, albeit requiring some materials science advances. That process would involve heating seawater to supercritical water where two different phases would exist, one being relatively pure water, and the other brine. The energy required to this can be partially recovered by allowing the separated pure water phase to expand against a turbine, and allowing the saline phase to evaporate against a turbine. The advantage of this process would be to oxidize suspended micro and macro plastics, as well as algae and seaweeds and eutrophic biomass through a supercritical water oxidation procedure, producing hydrogen and carbon oxides, a mixture known as syn gas. Since seawater contains higher concentrations of carbon dioxide than does air, this will have the additional benefit of removing carbon dioxide from the environment for use. The resulting brines and dried salts might then be subjected to the electrochemical separations that the paper describes. An important element recovered in this process, depending on the location and nature of the seawater or water stream, is the element phosphorous, which has previously, in cleaner times, been cyclized from the sea to land by seabirds, with birds being organisms under chemical, mechanical, and climate threats. (As we build useless wind farms at sea in our misguided worship of so called "renewable energy" this pathway in the phosphorous land sea cycle may face threats. Birds matter.)
Another rich source of metals that ends up being pollution rather than being a resource is electronic waste.
Reference 24 in the review paper under discussion is this paper: A review of current progress of recycling technologies for metals from waste electrical and electronic equipment (Xu and Zhang, Journal of Cleaner Production,Volume 127, 20 July 2016, Pages 19-36). It too, is a review article.
That paper (Xu and Zhang) has a showing a graphic of a generalized composition of various types of electronic waste:
It also has some useful tables, again generalized:
A cartoon of one process to recover materials from electronic waste is given in the paper cited at the outset.
It would seem to me, off the top of my head, that melt processes are the best in those devices containing a lot of copper, for the recovery of precious metals like platinum, palladium, silver and gold. These elements are extractable into liquid copper which can then be solidified and dissolved in concentrated nitric acid. Under these conditions the platinum and gold will not dissolve, whereas the silver, copper, and some palladium, and the platinum and gold and residual palladium can be filtered. The silver can be removed by precipitation with hydrochloric acid; and the copper by electrolysis. The residual solids (Au, Pt, Pd) can then be dissolved in aqua regia and recovered by exploiting electrochemical means.
This is a slightly different approach than is described in figure 6 from the paper reproduced just now.
Although people like to prattle on and on about how "green" batteries are, this as a subtext to the mistaken idea that so called "renewable energy" is "green" these batteries can be and are recycled (albeit only partially) but this is not a risk free or necessarily clean process.
I have discussed this issue in other posts in this space:
Dealing with 11 Million Tons of Lithium Ion Battery Waste: Molten Salt Reprocessing.
Identity and Toxicity of Off Gases in Thermolysis Lithium Battery Recycling Schemes.
I won't therefore discuss the environmental issues nor the socio-political issues connected with the cobalt they contain, but will only note an interesting approach that is discussed in this paper, which is "slurry electrolysis" which is described in a graphic for lithium batteries that are cobalt free, utilizing lithium manganate electrodes.
The authors write:
A figure connected with this approach is found in the paper:
The caption:
I love that ruthenium plated titanium anode. I have reference 50 in my files and will check it out when I am done here.
This picture seems to show an ion selective membrane. These sorts of membranes are very useful for another area discussed in the paper, which is capacitive deionization. This type of deionization is utilized in Heather Willauer's scheme to electrolytically produce jet fuel from the carbon dioxide dissolved in seawater, a wonderful technology, but one that is assumed - since dumping dangerous fossil fuel waste is "free" - to be uneconomical while the price of the dangerous fossil fuel petroleum is low.
Here is a schematic for capacitive deionization, which is also discussed at length in this paper:
The caption:
Since I am not shy about expressing my enthusiasm for nuclear power as the world's last best hope to save the environment, it behooves me to post a graphic about nuclear fuel reprocessing from the paper, which is this one:
The caption:
I'm not sure about the precise details of this paper, but I muse often on electrochemical refining techniques for the recovery of valuable materials from used nuclear fuel, so called "nuclear waste."
I don't regard, as this graphic does, that fission products are "waste." In fact, I have convinced myself that they are all quite valuable.
In any case, this is an interesting paper, well worth going through. Someday our world will reopen, and when it does, this paper may be accessed in a good scientific library, or obtained now, via subscription.
Be safe, be well, and enjoy, as I am enjoying, the pleasure of being alive.
