Where does any raw material come from? Meaning, where on the surface of this planet can we find an ore body to mine? Once you know where a ore body is, how much can you take out of the earth and still have the product be economically viable (considering clean-as-you go mining)? Once you get a metal ore out of the ground (you must dig a mine), think about how it is processed and refined, and then think of the environmental impact of the purified substance once it has been liberated and refined from the raw ore. What do you do with the raw material when you have it? What happens to the the purified material/chemical when you don’t want it any more? How much energy is required to recycle it? What is the fate of the material when you release it back into the environment? This is a cradle-to-grave study, and is a core principle in environmentally aware materials science.
We don’t need to just think in an abstract way about the global ore reserves of a mineral or element, there are information resources on-line at our web fingertips via the USGS (United States Geological Survey). As an aside, I truly enjoy the transparency of operation, open access to information, and the hands-on attitude of the USGS. Sure, it may be a bloated government operation, but it’s one that probably does far more positive action for the improved quality of life of Americans than several other, more aggressive modern mega-gov-departments.
One of the sub-disciplines is in geology is economic geology. An economic geologist is responsible to find an ore body (using scientific principles of ore formation); estimate the size, purity (for valued minerals), and accessibility of the ore body via prospecting (drilling, satellite imagery, GIS global positioning information, and a battery of geophysical tests); and then report the findings and estimates to the public or to an interested private company. In our case, the USGS has done this sort of work for the public and reported annual estimates of major minerals of interest. If you visit the Commodity Statistics and Information page, you’ll find a virtual treasure trove (literally and figuratively) of information on mined materials on a global scale.
As two examples, I will use the elements “Indium” that is used in ITO (Indium Tin Oxide), and “Tin” used in ITO and fluorine-doped tin oxide (FTO, or SnO2:F). Both are used with Transparent Conductive Oxides (TCOs) that are popular right now for the LCD industry, and some use is being channelled to new solar cell designs.
If I look down the USGS list to find Indium, I see a whole host of information summarized just in the introduction. Wow, that’s great! We need to mine zinc sulfide ore to get indium, so in order to get more indium there needs to be more demand for zinc. But wait, there’s more! In the Indium 2006 Mineral Commodities Summary we get access to a full PDF of data. What do we find out? Well, indium use grew by 15% from 2004 to 2005, we don’t have any of our own on reserve (stockpile), and we import the majority of our indium from China, Canada, and Japan (from highest to lowest). Also, only 15% of all ITO goes into actual products. Due to difficulties, length of processing, and high costs, the rest of the unused ITO is scrapped and not recycled at all.
OK, now compare Indium to the element found in TCOs, Tin. Fluorine-doped tin oxide (SnO2:F) has many of the same uses as ITO, but costs less and is often more useful in solar cell research (because indium tends to diffuse into the light absorbing materials with thermal annealing). In the Tin 2006 Mineral Commodities Summary, we again get some juicy information. We see that, again, we don’t mine tin much at all inside the US, but the countries that we do import it from are much more diversified. The applications of tin itself are much more diversified, and it is not nearly the trace metal resource that indium is (right up there with silver). While tin is not an abundant element, we do have tin stockpiled, and tin recycling is around 61% as of 2005. No words about SnO2:F recycling, though–my suspicion is that it is just as difficult to recycle as ITO thin films.
Which of the two would you choose to work with?
I will make one additional note on the environmental impact of both of these materials in their oxide state as thin films. Both tin oxide and indium oxide are pretty stable at Earth’s surface, relatively inert (they don’t dissolve easily), and have not been found to cause dramatic toxicity problems as oxides. Given the small amount of information at hand regarding toxicity, if you don’t recycle either of these, you are unlikely to significantly alter the environment at the waste repository. So it’s more a choice of economics and reserve materials at the moment. Phew, at least something that we make for new technologies is useful and not horribly toxic!
But you know what? Sometimes even oxides are altered, and then there may be a toxicity risk in the dissolved species. This is the true weak spot in the case study, where we assume the small amount of research on tin oxide and indium oxide is sufficient to assure environmental fate. Once upon a time, not so long ago, in a county not far from your home, scientists perceived PCBs (polychlorinated biphenyls, used as electrical insulators, liquid coolants, and fire retardants) as “safe” (on the basis of a small amount of information). They are quite inert and not many people had researched their toxicity, until after they were widely distributed in our everyday technologies–and then it became apparent that things were not so safe with PCBs. In the organic chemistry laboratories, you also tended to wash your hands with benzene then, but that’s another story.
In environmental materials science, we need to know the extremes at which our materials break down, and if those conditions exist in the outside environment. Research on the chemical stability of these films, and the fate of dissolved tin and indium species would be handy information. A classic euphemism in materials science was make it, then break it. This used to be used for building materials like cement, steel, aluminum alloys, and brass. We now need to adapt it to a new wave of materials science that also includes molecular compounds and nanoparticles and thin films.