General process information
Alumina recovery begins by passing the bauxite ore through screens to sort it by size. It is then crushed to produce relatively uniformly sized material. The ore is then fed into large grinding mills and mixed with a caustic soda solution (sodium hydroxide) at high temperature and pressure. The material finally discharged from the mill is called slurry.
The resulting liquor contains a solution of sodium aluminate and undissolved bauxite residues containing iron, silicon, and titanium. These residues – commonly referred to as “red mud” – gradually sink to the bottom of the tank and are removed.
The slurry is pumped to a digester where the chemical reaction to dissolve the alumina takes place. In the digester the slurry is heated to 300° Fahrenheit (145° Celsius). It remains in the digester under those conditions from 30 minutes to several hours.
More caustic soda is added to dissolve aluminum containing compounds in the slurry.
Undesirable compounds either don’t dissolve in the caustic soda, or combine with other compounds to create a scale on equipment which must be periodically cleaned. The digestion process produces a sodium aluminate solution. Because all of this takes place in a pressure cooker, the slurry is pumped into a series of “flash tanks” to reduce the pressure and heat before it is transferred into “settling tanks.”
Settling is achieved primarily by using gravity, although some chemicals are added to aid the process. Just as a glass of sugar water with fine sand suspended in it will separate out over time, the impurities in the slurry – things like sand and iron and other trace elements that do not dissolve – will eventually settle to the bottom.
The liquor at the top of the tank (which looks like coffee) is now directed through a series of filters. After washing to recover alumina and caustic soda, the remaining red mud is pumped into large storage ponds where it is dried by evaporation.
The alumina in the still warm liquor consists of tiny, suspended crystals. However there are still some very fine, solid impurities that must be removed.
The giant-sized filters consist of a series of “leaves” – big cloth filters over steel frames – and remove much of the remaining solids in the liquor. The material caught by the filters is known as a “filter cake” and is washed to remove alumina and caustic soda. The filtered liquor – a sodium aluminate solution – is then cooled and pumped to the “precipitators.”
The clear sodium aluminate from the settling and filtering operation is pumped into precipitators. Fine particles of alumina – called “seed crystals” (alumina hydrate) – are added to start the precipitation of pure alumina particles as the liquor cools. Alumina crystals begin to grow around the seeds, then settle to the bottom of the tank where they are removed and transferred to “thickening tanks.” Finally, it is filtered again then transferred by conveyor to the “calcination kilns.”
From precipitation, the hydrate is filtered and washed to rinse away impurities and remove moisture. A continuous conveyor system delivers the hydrate into the calcining kiln. The calcining kiln is brick-lined inside and gas-fired to a temperature of 2,000 °F or 1,100 °C. It slowly rotates (to make sure the alumina dries evenly) and is mounted on a tilted foundation which allows the alumina to move through it to cooling equipment.
The result is a white powder: pure alumina.