Red Mud is met with during the production of Aluminium (Al)
Bauxite, named after the French town "Baux de Provence", is a rather superficial ore, from which Alumine, precursor of Aluminium, is extracted
Bauxite extraction
A standard process for obtaining alumine (Al2O3), the precursor of Aluminium ( the metal we all use) is:
1/ Mechanically crush Bauxite ore

2/ Mix with caustic soda (NaOH)

3/ Heat in tanks for hours (this is what must have exploded)

4/ Leave to settle: the residue that accumulates in the bottom of the tanks is "red mud".
what does Red Mud contain;

- Fe2O3: 40 to 45%

- Al2O3: 10 to 15%

- SiO2: 10 to 15%

- NaOH: 25 to 40%

- traces of 238 Uranium decay series
The danger is linked to the caustic soda, and pH levels up to 13 have been reported.(this is the pH of a 1% solution of caustic soda).
It is reported that 1 million cubic metres of red mud "erupted". That represents 250 000 to 400 000 cubic metres of pure  NaOH, and would explain, due to its dilution 100 times in environment, the pH reached
To understand the safety hazards, go to Google and type "Caustic soda safety data sheet"