Alumina refining is the largest single use of caustic soda in the world. Almost all alumina, the raw material for aluminium, is made from bauxite by the Bayer process, which depends on caustic soda at every step. Here is how it works and why caustic consumption is such a focus for refineries.

The Bayer process in four steps

1. Digestion: crushed bauxite is mixed with hot, concentrated caustic soda solution under pressure. The aluminium minerals dissolve as sodium aluminate.
2. Clarification: the insoluble residue, mainly iron oxides and silica compounds, settles out as bauxite residue (red mud).
3. Precipitation: the clear liquor is cooled and seeded, and aluminium hydroxide crystallises out.
4. Calcination: the aluminium hydroxide is heated to drive off water, leaving alumina.

The caustic liquor is then concentrated and recycled back to digestion.

Where caustic is lost

Because the liquor is recycled, refineries only buy caustic to replace losses. The main losses are:

• Reactive silica in the bauxite, which forms insoluble sodium aluminium silicates and carries caustic away with the red mud.
• Liquor entrained in red mud, reduced by washing the residue counter-currently.
• Impurities such as carbonate and organics that build up in the liquor and must be removed.

Bauxite with high reactive silica therefore costs more to process, because it consumes more caustic.

Why it matters for every caustic buyer

Alumina demand is a major driver of the global caustic soda market. When new refineries start up or alumina output rises, caustic soda demand rises with it, which can push prices up for all users, from water treatment plants to soap makers.

Caustic quality

Large refineries mainly use liquid caustic delivered in bulk. Smaller users of caustic, such as laboratories, maintenance teams and process plants, usually buy flakes. Either way, purity, carbonate and chloride content affect performance.

Following caustic prices

Because caustic soda is a global commodity, local prices follow these trends. For an example of a daily published caustic soda price with a five-month chart, see our group's caustic soda page for Oman.