Alumina is a catalyst, an abrasive, a bright white pigment and the basis for beautiful gemstones.
Aluminium oxide, also known as alumina, is a pretty simple compound, just Al2O3. Aside from silicates, aluminas are the most abundant mineral of the earth crust. Although you might not realise, I bet you’ve probably used it and I think many of you have admired it at some point in your life.
Australia produces the largest amount of alumina because she has a large body of bauxite. Jamica, South Africa and some other countries also have a good reserve.
Alumina is produced from bauxite, the main aluminium ore, which is a mixture of different hydroxide minerals. The mineral is crushed and milled, treated with base, heated and filtered before the alumina is produced by heating to around a thousand degrees Celsius.
One of the waste products from this process – named after its inventor Carl Bayer – is the infamous ‘red mud’ that caused an environmental disaster in October 2010, when a million cubic metres of it were released in Hungary. The red mud itself contains iron oxides (hence the colour) as well as oxides of silicon, titanium and other metals. The main problem with the mud is its strongly alkaline nature, ranging from pH 10 to 13, and it was this that did the damage in the Hungarian spill.
Alumina is a bright white solid with a trigonal crystal structure, with oxygen atoms in a hexagonal-close-packed arrangement and the aluminium atoms occupying two-thirds of the octahedral holes.
Its bright whiteness is behind some of its uses, as a filler for plastics, in paint as a shiny pigment and to reflect sunlight in some sunscreens.
One of alumina’s other useful properties is its extreme hardness: it ranks below diamond, but not much else, on the Mohs scale. So if you’ve ever used sandpaper or a nail file, the chances are it had some alumina in the mix to help you with whatever abrasive task you were involved in.
More interesting from a chemical point of view is alumina’s power as a catalyst and a catalytic support. As a catalyst on its own, it can turn dangerous hydrogen sulfide gas into much safer pure sulfur in the Claus process. In a similar vein, as a catalyst support it helps out in the important reactions that remove sulfur from oil, helping to keep our fuel cleaner when we burn it.
Now on to the fun stuff!
The reason you might have admired some alumina is because corundum, its naturally occurring mineral form, occasionally forms rubies and sapphires. And they’re certainly worth admiring – or at least their prices are.
But if both rubies and sapphires are alumina, how come they’re such different colours? The answer, you’ll be glad to hear, is all down to chemistry. Both stones contain small impurities – without them, they’d be colourless and nowhere near as pretty. But the way the impurities work to give the colours is quite different.
Rubies contain chromium as well as the aluminium ions. Both metals are comfortable in the 3+ oxidation state, so chromium fits quite happily into the octahedral space and bonds to six oxygen atoms. But chromium 3+ ions each have three extra valence electrons compared to aluminium, and in that octahedral bonding, they all end up unpaired – each solely occupies one of three orbitals a little bit lower in energy than two empty ones.
Interested to know more? Click here for an article about Iron Ore