The news has been showing pictures of an environmental catastrophe in Hungary, where a huge spill of toxic sludge from an alumina processing plant has killed several people and lots of fish. I am not an expert on alumina refining, but I’m going to take an educated guess at what happened here using geochemical first principles.
Aluminum is mined from bauxite, which contains lots of aluminum hydroxide plus other contaminant elements. These need to be removed when the ore is processed into pure aluminum oxide.
Aluminum is very insoluble under moderate pH (pdf), dissolving only in very strong acids or bases. So any purification procedure based on aqueous chemistry is probably going to use either very low or very high pH. So their bauxite ore was treated with sodium hydroxide to increase the pH to over 13.
High pH solutions are extremely caustic, and will chemically burn people fish and other organisms just like strong acids do. That is what makes this dangerous.
In addition, most toxic metals are generally more soluble than aluminum is, so any dangerous elements originally present in the ore sill stay in solution. Their concentration will depend in part on whether they accumulate during mineral processing, and how much was in the original ore. That is why they probably don’t have a good heavy metals estimate yet. The same reasoning applies to radioactive U and Th.
As this sludge reacts with things and gets less basic, the solubility of aluminum and iron (a major contaminant in bauxite) will drop, and gibbsite and goethite can be expected to precipitate. The goethite (a.k.a. “rust”) and related iron minerals give the sludge its red hue.
This is all conjecture, however, so I’d love to hear from anyone with actual knowledge.
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