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How does the sludge get produced and how could it escape?

When aluminium is extracted from bauxite via the so-called Bayer process, red sludge forms as a by-product. The sludge is normally kept in large reservoirs where its fluid and solid components separate into water and mud.

What caused the accident is yet unclear, but it is likely that heavy rain has caused the dam containing the reservoir to break.

It is also possible that the reservoir was just not large or strong enough to hold the sludge it was filled with. What is the chemical composition of the sludge?

It contains mainly fluoride, sulphate and aluminate, but also chrome, nickel, manganese and heavy metals such as lead. Its arsenic concentration is at least a hundred times above the allowed threshold for drinking water.

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