It's All in the Sludge: Elements That Are Always By-Products - ACS

Chapter 6

It’s All in the Sludge: Elements That Are Always By-Products Downloaded by UNIV OF FLORIDA on November 26, 2017 | http://pubs.acs.org Publication Date (Web): October 30, 2017 | doi: 10.1021/bk-2017-1263.ch006

Justin Pothoof, Grace Nguyen, and Mark A. Benvenuto* Department of Chemistry and Biochemistry, University of Detroit Mercy, 4001 W. McNichols Road, Detroit, Michigan 48221-3038, United States *E-mail: [email protected].

The periodic table has been taught predominantly as an exercise in memorization for decades, with connections to reactivity, but seldom to abundance of elements. As society changes and technological developments are brought to the market and general public, the need for formerly unused or under-used elements may increase drastically, and in some cases already has. An understanding of the origins and sources of the elements thus becomes an important component in educating students – the next generation of the adult work force – about the periodic table.

Introduction Is every element on the Periodic Table of the Elements of equal importance, and present in or on the Earth in equal abundance? Are the elements all extracted from their natural sources in the same manner, or are some simply part of the “sludge” of the production of other elements? Unfortunately, the table we use and depend on so heavily gives no indication. To any student new to the study of chemistry, the idea of the Periodic Table of the Elements is that this is a large document (often a wall chart) that simply lists all the elements known, in an apparently random order, using one or two apparently random letters, and with the implied idea that since all of them are given equal billing, all of them are of roughly equal importance. Students are eventually taught the correlation between such ideas as atomic number and number of protons and electrons, but at first glance, the table seems to have no order. Element names certainly have no system to them, and even the shape of the table does not appear © 2017 American Chemical Society

Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

Downloaded by UNIV OF FLORIDA on November 26, 2017 | http://pubs.acs.org Publication Date (Web): October 30, 2017 | doi: 10.1021/bk-2017-1263.ch006

to have any obvious reason. “Learning” the Periodic Table is essentially a large exercise in memorization. Our current image of the Periodic Table of the Elements is roughly 70 years old, and its shape is connected with the work of Professor Glenn Seaborg and others, who were interested in completing the transuranic elements and the seventh row of the Table through the f-block electrons. There are certainly similarities to this now established work and the much more recent work in which elements up to and including Element 118 – Oganneson – have been synthesized and verified, thus completing the seventh row of the Table through the p-block elements (1). And while such work extends our knowledge of the periodic table, it does not imbue any deeper understanding of either the prevalence of any elements, or their usefulness in society.

Abundance, Prevalence, and Uses of the Elements A thought exercise that can be conducted even in a freshmen-level chemistry class is a series of questions concerning the abundance of all matter. Table 1 lists the questions, as well as general answers, that focus the experiment on the abundance of a few, heavier elements.

Table 1. The Distribution of Matter Question

General Answer

Of what is almost all of the universe composed?

>>>99% is empty space, or nothing.

Of what is almost all of the remaining part of the universe, the matter of the universe, made?

>>>99% are the stars.

What are stars made of?

Hydrogen and helium

Where is almost all matter that is not stellar?

The planets.

Of the planets, where is most of the matter, and most of the light elements?

The gas giant planets.

How many planets do we know of that are composed of elements from hydrogen to uranium?

One, Earth.

What heavier, metallic elements are thus actually present in relatively small amounts, but are heavily used by humanity?

Iron, copper, aluminum (there are others as well).

The exercise makes it apparent how little of the matter in the known universe is actually any element other than hydrogen or helium. While this is admittedly an exercise whose scope is greater than what is usually undertaken in most chemistry 112 Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


classes, it does give a person some feel for how vast the scale is with which we can consider all matter, from hydrogen to the heaviest of elements. These questions can be stated in a number of different ways, and could go even further than what is presented here. But the large idea behind this is to focus student thinking on how elemental abundances exist, and how much or little of any of them exist in forms that are available to us. A series of questions such as those in Table 1 provide some focus and understanding of the Periodic Table of the Elements beyond mere proton counts. But a look at the chemical industry and what elements are used in large amounts represents another view of the table. For example, Table 2 shows several elements that are used on a very large scale, to produce relatively simple chemical compounds that in turn enable the quality of life enjoyed by much of the world’s people today.

Table 2. Products Made from the Elements (2) Element

Processing

Product

Example uses

Sulfur

Addition to O2 and water

Sulfuric acid

Fertilizer production

Nitrogen

Addition of elemental hydrogen

Ammonia

Fertilizer

Nitrogen

Addition to oxygen

Nitric acid

Oxidizers

Copper

Refined from ores

Elemental copper

Wiring (among many)

Iron

Refined from ores

Elemental iron

Iron and steel objects

Aluminum

Refined from bauxite

Elemental aluminum

Lightweight alloys

Chlorine

Electrolytic, of salt water

Sodium hydroxide, elemental chlorine, elemental hydrogen

Chlorine disinfectant, polyvinylchloride plastics

These six elements are produced and isolated at a level of millions of tons per year, and Table 2’s listing of example uses is always a very small fraction of a much large whole. Some, such as iron and copper, have been used for millennia, and have seen incredible improvements in production over that time. Others, such as aluminum, have a much shorter history from discovery to the present day, and thus have seen relatively rapid improvements in their large scale production. But approximately twenty elements are never mined, extracted, or in some way isolated as the main product of an operation, or are produced largely as secondary products to some other refining process. This was not necessarily a problem in the past, but with the changing needs of society, and the expanded uses of novel materials, their recovery, extraction and isolation becomes very important today. 113 Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

As well, increased needs for certain elements means that even if they were a major product in some past method of production, they are now valuable enough that they are pursued even as secondary products. Silver, gold, and the platinum group metals (PGM) are all examples of this. Table 3 gives a listing of these elements.


Table 3. Elements Co-Produced with Other Materials (2, 3) Element

Source

Co-produced with:

Example use

Bromine

Brines in Arkansas, and the Dead Sea

Common salt, magnesium

Flame retardants, pesticides

Cerium

Monazite and bastnäsite

Lanthanides

Polishing compounds

Gadolinium

Monazite ores

Other rare earth elements

High resistance alloys

Gallium

Bauxite

Aluminum

Semi-conductors

Germanium

Copper, lead, zinc ores

Copper, lead, or zinc

Fibre-optics

Gold

Copper ores

Copper, as part of anode muds

Electrical connections, store of wealth

Lithium

Brine pools

Salt

Ceramics, batteries

Neodymium

Monazite and bastnäsite

Other rare earth elements

Magnets

Platinum group metals

Anode mud or anode slime

Copper

Catalysts

Scandium

Apatite

Phosphorus

Lightweight alloys

Selenium

Copper, nickel, lead ores

Copper, nickel, or lead

Magnesium production

Silver

Lead ores

Lead, via the Parkes Process

Electronics, coins

Thorium

Monazite ores

Rare earth elements

Mag-thor alloys

It can be seen that several of the elements in Table 3 are ultimately extracted from monazite or bastnäsite ores. They are routinely co-located because of the similarities in chemical reactivity of many of the lanthanides and of thorium. But prior to the Second World War, there were few needs for these elements, and thus the isolation and refining of them – a tedious process requiring that steps be repeated many times – was not considered important, or worth improving. The current profile of needs and applications for elements such as cerium, neodymium and gadolinium are such that some greater understanding of these materials is necessary; logically this can be first taught to new, undergraduate students. 114 Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


In similar vein, the production of gallium is always a subordinate operation in the production of aluminum from bauxite. Yet the rise of a large, now mature semiconductor industry means that there is a steady need for this by-product element. As a third broad example, importantly, the electro-refining of copper on a large scale has meant the co-production of what are called anode muds or anode slimes. These waste materials, never refined out of copper before the advent of electrochemical methods of purification, are valuable enough co-products that they can sometimes become the driving force for the larger production of copper. If, for instance, copper sells on the world’s markets at $2.50 per pound, a 300-lb refined copper ingot has a base price of $750. If one ounce of gold is an anode mud co-product of a particular copper batch, and sells at $1,200 per troy ounce (a price at which is hovered for much of the year 2016), the co-product now generates more income than the main product. This becomes even more apparent if some amount of silver or the platinum group metals are also present in the anode mud.

Mendeleev’s “Eka” Elements and Their Uses Professor Dimitri Mendeleev is often credited as being the father of the Periodic Table of the Elements because of his ability to predict elements that had not yet been found, inclusive of what some of their chemical and physical properties should be, as well as the formulas of those compounds. Four undiscovered elements that he labeled with the term “eka” include eka-boron (scandium), eka-aluminum (gallium), eka-manganese (technetium), and eka silicon (germanium). At the time of Mendeleev’s publication of “The Principles of Chemistry,” technetium had still not been discovered and isolated, and in his table he notes that manganese forms compounds that include a formula of MnO3Z, and that an element he calls an “unknown metal” designated as “Em=99” should combine to form compounds of the formula EmO3Z (4). While there are applications for all four of these eka elements today, and thus easy connections to them to be made when teaching, technetium is arguably the most applicable now, because of the use of metastable technetium-99 (99mTc) in medical isotopes. The connection is that numerous undergraduate students who declare a major course of study in chemistry or biochemistry do so because they wish to go into the health professions in one career or another. The dependence upon this isotope of a rare element that the modern medical community has developed has been a cause of concern to some medical doctors, and a discussion of this has even reached the popular press (5). Clearly, an understanding of where elements are sourced from, and how abundant each is, becomes relevant for a variety of reasons.

Mining and Mining By-Products While several chemicals are obtained primarily through mining and drilling, such as coal, oil, and common salt, currently three metallic elements are produced on an extremely large scale through the mining of a variety of their ores: iron, copper, and aluminum. For example, 29 million metric tons of pig iron, 1.3 million 115 Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


metric tons of copper, and 1.7 million metric tons of aluminum were produced from primary sources – mining – in 2015 alone (2, 3). These numbers do not include recycled metal, which means the amounts actually used are even higher than the just-mentioned numbers. These figures equate to roughly two thirds of aluminum and copper used in the United States being produced domestically, while over 80% of iron in the United States is produced from domestic sources (2). In Table 2 we have already seen a short list of elements refined and used on massive scales, several of which are mined, and in Table 3 we have seen several elements that are not produced as the primary product in some mining operation. Yet the field of metallurgy, with its constantly expanding number of alloys and materials, continues to see an increase in the number of alloys that are finding specific uses, even if they are niche uses. Even those that have been used for decades are being refined to higher purities, or being re-examined through the addition of trace elements (6). Perhaps the most ubiquitous example of a relatively new alloy, at least in terms of the needs of end users, are the magnets in cellular phones (7). Had no neodymium-iron-boron alloy with strong magnetic properties been discovered by General Motors and Sumitomo Special Metals, and subsequently utilized, it is conceivable that cell phones would still be as large as those first marketed in the 1980’s! It is not a great leap of imagination to believe that the future will see still more new, useful alloys and materials, some of which will utilize elements currently not produced on a large scale.

Conclusions First, what can be called an understanding of the origins, the industrial aspects and the end-user aspects of elemental and chemical production and refining can easily be incorporated into any discussion and teaching of the Periodic Table of the Elements. A greater understanding of the disparities of the presence of elements on the Periodic Table can be incorporated into chemistry classes as early as the freshmen year, when students are still engaged in memorizing element names, and can perhaps obviously be reintroduced in upper-level classes (8, 9). Such an approach brings relevance to our teaching, and deepens student understanding, making the table more than just a large group of symbols that students must memorize. Second and importantly, how the uses of various elements and the societal requirements for them change with time can become a means by which the table comes alive for students. The rise of aluminum from an elemental curiosity in the nineteenth century to a common, work horse metal in the early twentieth is one obvious example. The continuing rise in the need for rare earth elements in various applications, such as miniaturized electronics, is another such example. The rise and decline of bromine as a component of flame retardants, and of arsenic in pesticides, serve as still further examples of how the need for a specific element changes over the course of time. Third, the title of this paper points to the fact that some elements exist in widely dispersed deposits, in relatively small amounts, and that they will always be by-products or secondary products in any large chemical extraction 116

Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

and refining process. But these “secondary” elements become vitally important as the needs of society, and the products that enable the modern quality of life, change as we advance. Bringing such facts and this level of understanding to the teaching of secondary school and college-level chemistry classes adds relevance to the discussion, and brings to the fore a realization that the periodic Table of the Elements is a living, changing document in the classification of matter.

References


1. 2. 3. 4. 5.

6. 7. 8. 9.

What it takes to make a new element. Chemistry World 2017, 14 (1), 22–32. USGS. Mineral Commodity Summaries 2015. https://minerals.usgs.gov/ minerals/pubs/mcs/2015/mcs2015.pdf (accessed Jul 7, 2017). American Iron & Steel Institute. http://www.steel.org/ (accessed Feb 26, 2017). Mendeleéff, D. The Principles of Chemistry; P.F. Collier & Son: New York, 1902. Visualizing the Medical Isotope Crisis. Scientific American. https:// blogs.scientificamerican.com/sa-visual/visualizing-the-medical-isotopecrisis/ (accessed Feb 26, 2017). Read, W. T. Industrial Chemistry; John Wiley & Sons, Inc.: New York, 1943. Rare Earth Magnetics. http://www.rareearth.org/ magnets_patents_history.htm (accessed Feb 13, 2017). Benvenuto, M. A. Metals and Alloys: Industrial Applications; Walter DeGruyter GmbH: Berlin, Germany, 2016. Benvenuto, M. A. Industrial Chemistry; Walter de DeGruyter GmbH: Berlin, Germany, 2014.

117 Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

It's All in the Sludge: Elements That Are Always By-Products - ACS

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