Red mud is a toxic byproduct of the industrial process that refines bauxite, raw aluminum ore, into aluminum oxide, or alumina.

(Alumina is put through a separate process, electrolysis, to make aluminum metal.)

Toxic red mud flooded thousands of homes this week near the town of Devecser, Hungary. (Bela Szandelszky/Associated Press)

Bauxite is a mixture of minerals. In addition to aluminum compounds it contains iron oxides, sand, clay and small amounts of a form of titanium oxide called anatase; it can also hold traces of radioactive minerals, such as uranium or thorium compounds.

This raw material is bathed in a solution of a strong base — sodium hydroxide (lye or caustic soda) — at a high temperature and pressure. The aluminum compounds in the bauxite dissolve in the hot caustic solution while the other components remain behind.

Everything that doesn't get dissolved in the process is called red mud, its rusty colour deriving from the iron compounds.

Depending on the origin, quality and composition of the bauxite, the amount of red mud left over from the alumina refining can vary widely. For every tonne of alumina produced, the process can leave behind a third of a tonne to more than two tonnes of red mud.

The mud is a complex chemical soup, a watery slurry of fine rock particles and salts, containing toxic heavy metals. It can be slightly radioactive if the original bauxite contained radioactive minerals.

The mud also has a high pH because of the sodium hydroxide solution used in the refining process. The base is strong enough to kill plant and animal life, and to cause burns and damage to airways if the fumes are breathed.

Other than some limited use as a pigment in the manufacture of bricks and concrete, the red mud is waste. Although it contains useful elements, such as iron, titanium and residual aluminum, there is no economically viable way to extract them from the mud.

Most refineries collect the mud in open ponds to allow some of the water to evaporate. Once it's dry, after several years, the red mud is buried or mixed with soil.

The red mud is kept in reservoirs, so there is the possibility of leaks and floods, as seen in western Hungary.

Because of this, some refiners, including the Rio Tinto Alcan refinery in Jonquière, Que., use powerful presses to squeeze the water out of the mud and evaporators to dry most of the rest. The company says up to 90 per cent of the Quebec site's waste is dried before being stored.