RED MUD PROCESSING
The waste from the conversion of the bauxite into aluminium oxide, red mud, is one of the largest disposal problems in the world.
Pressure-cooking bauxite with sodium hydroxide solution to dissolve the alumina content should be a simple enough process, but silica in bauxite reacts to give sodium aluminium silicate. The iron oxide content of bauxite is a diluent and a materials-handling problem. Expensive sodium hydroxide is lost to the tailings because of the difficulty in washing the insoluble material.
As the ferric oxide, with its variable amount of combined water, is such a nuisance, there has been some interest in using reducing agents to convert the smallest particles and the surface of the larger particles into the non-hydrated magnetite.
I have suggested (GB patent 2 569 767) mixing ferrous sulphate (another waste) with red mud and heating, so that the reaction occurs with the alkali in the red mud to produce ferrous hydroxide, which would react with ferric oxide to give magnetite. Magnetite is not hydrated and so more readily filtered off.
The process would neutralise most of the alkali in the red mud and the magnetite might be separated magnetically for use as iron ore. Low-grade magnetite iron ore is upgraded by magnetic separation in the USA.
There have been many proposals for the recovery of the materials from the sodium aluminium silicate of red mud. The compound is readily soluble in dilute sulphuric acid. The solution contains sodium sulphate, aluminium sulphate and colloidal silica. The colloidal silica has hitherto been difficult to remove. Fortunately, an American company offers filter membranes within a vibrating module. Gelatinous hydrated silica can now be filtered off without clogging the filters.
I have proposed (GB patent application 2 576 266) a process, whereby the membrane-filtered sodium sulphate and aluminium sulphate solution is heated under pressure to give a precipitate of basic sodium aluminium sulphate and a solution of sodium hydrogen sulphate. This solution can be recycled to treat more red mud. There would have to be an input of sulphuric acid. Dilute sulphuric acid is readily available as a waste from many processes. The granular basic sodium aluminium sulphate can be heated with sodium carbonate solution to give insoluble basic sodium aluminium carbonate (dawsonite).
Dawsonite can be heated to give sodium aluminate. There is a large market for sodium aluminate. It is reacted with sodium silicate solution to give a precipitate of sodium aluminium silicate. This forms the bulk of commercial washing powders. It softens water and helps lift dirt off washing.