The residue or bauxite tailings , which is mostly iron oxide , is highly caustic due to residual sodium hydroxide . It was historically stored in lagoons ; this led to the Ajka alumina plant accident in 2010 in Hungary , where a dam bursting led to the drowning of nine people . An additional 122 sought treatment for chemical burns . The mud contaminated of land and reached the Danube . While the mud was considered non-toxic due to low levels of heavy metals , the associated slurry had pH of 13
Ajka alumina plant accident
Ajka alumina plant accident
Aluminium hydroxide