Case Study: Indium and Tin

Please begrudge me this series of questions:

Where does any raw material come from? Meaning, where on the surface of this planet can we find an ore body (a mineral reserve) to mine? Reality check: as long as you enjoy your refrigerators, your laptops, your light bulbs, and your transportation vehicles, we will need to mine our mineral reserves. Once you know where an ore body is…

...how much can you take out of the earth and still have the product be economically viable?

Once you get an ore out of the ground, think about how it is processed and refined, think about the clean-up necessary following the mining venture. What are the environmental impacts of the purified substance once it has been liberated and refined from the raw ore? What happens to the the material/chemical when you don’t want it any more? How much energy is required to recycle it? What is the fate and toxicology of the material when you release it back into the environment?

These questions are intimately involved in environmentally aware materials science They can be part of a cradle-to-grave study, describing the impact of a material on our society and our environment. I’m going to portray the first section of this inquiry: the raw materials, and where to get them. I will compare two elemental materials for my case study, indium and tin metal.

A sub-discipline in geology is economic geology. An economic geologist is responsible to find an ore body (using scientific principles of ore formation); then 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 finally report the findings and estimates to the public or to an interested private company. However, we don’t need to be economic geologists to determine the global ore reserves of a mineral or element, the information resources are on-line via the USGS (United States Geological Survey). In our case, the USGS has done this sort of work for the public and reported annual estimates of all 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.

The element indium is used in ITO (Indium Tin Oxide), and tin used in both ITO and fluorine-doped tin oxide (FTO, or SnO₂:F). Both are used with Transparent Conductive Oxides (TCOs) that are popular right for the flat screen display industry, and for new solar cell designs. You see, we’re still talking about materials for photovotaics—it’s everywhere!

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 2007 Mineral Commodities Summary we get access to a full PDF of data. What do we find out?

Well, indium use grew by 10% over 2006 (and 15% over 2005), the USA doesn’t have any of our own on reserve indium (a 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. Restated: 85% of all indium is wasted and discarded in processing of reel-to-reel polymer coatings for plasma screens and LCD laptop screens.

And at the top of the charts: consider the USGS estimate for the global primary reserve base of indium (that’s all the indium in the world available to mine: about 6,000 metric tons) and our immediate rate of consumption of indium for technology (global refining of 480-500 metric tons per annum). Now, if we assume we can mine all the indium in the world and refine it at our current rate, we are facing a worst case scenario of only 12-13 years of indium left before we have spread it out over all the surfaces we can. Then we’re done with indium as a primary resource and recycling will be the only option to access indium. Wow, that didn’t take long to burn through!

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 2007 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 (indium is right up there with silver). While tin is not an abundant element (a mere 11 million tons), we do have tin stockpiled, and tin recycling is around 61% as of 2005 (data from the Minerals Yearbook for Tin. 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 very small amount of information at hand regarding toxicity, one could guess that if you don’t recycle either of these, you are unlikely to significantly alter the environment at the waste repository. More study is needed, of course. So at the moment, it’s more of a choice base on economics and reserve materials.

Note: Image by JRSB (2007)