What is a Bayer Process Plant?
A Bayer process plant is principally a device for heating and cooling a large recirculating stream of sodium hydroxide solution through a temperature range of up to 500oF per cycle. Liquor flow rates of 5,000 to 10,000 gpm are common in the newer plants. Bauxite is added at the high temperature point, red mud is separated at an intermediate temperature, and alumina is precipitated at the low temperature point in the cycle.
To an important extent, the capital and operating costs of the Bayer plant are a function of the volume of caustic liquor circulated; therefore, both in the design and in the operation of the plant, a primary objective is to maximize the production of alumina per unit volume of caustic liquor circulated (yield).
Refining Bauxite to Alumina:
Of equal importance is the cost-related need to minimize consumption of bauxite, lime, caustic soda, and flocculant -- the raw materials for alumina production. Meeting this need requires digester conditions capable of extracting 98%+ of the available alumina in the bauxite and clarification systems designed to separate most of the soluble components (chemical values) from the red mud prior to disposing of it.
Fuel costs, which are a major operating cost, are minimized through the use of elaborate heat exchange systems at key points in the process. Because of the cyclic nature of the process, this schematic is often referred to as the Bayer "Wheel".
Bauxite is mixed with caustic soda solution and heated in digestion to dissolve and extract the alumina hydrate plus some undesirable impurities from the bauxite.
The components of bauxite which react with caustic and lime to form insoluble compounds and those components that do not dissolve, are called "red mud", and are separated from the alumina rich liquor in clarification by a settling/decantation process followed by countercurrent washing to recover the caustic and alumina values.
After filtration to remove the remaining trace solids, the "pregnant" liquor is cooled and treated with alumina trihydrate seed in the precipitation process. The alumina trihydrate crystals produced are separated and classified into size fractions. The coarsest fraction is washed free of caustic liquor and calcined to product. The finer fractions are treated and reused for "seeding" in the precipitation process.
The "spent" liquor after hydrate precipitation is fortified with new caustic to replace losses, concentrated in evaporators, and returned to digestion to close the liquor circuit.
Advances in understanding of basic chemistry and engineering of equipment have produced:
– Floury to Sandy Alumina
– Lime Soda Sinter Process
– Bx. Deposit Discoveries and Bx. Beneficiation
– Single Stream Digestion
– Tube Digestion
– Double Digestion
– Red Mud Separation and Washing – Flocculants
– Red Mud - Bx. Residue - Disposal
– Continuous Precipitation and Liquor Productivity Increases
– Liquor Purification – Causticization, Salting Out Evaporation, Liquor Calcination, Impurity Precipitation,
– Fluid Bed and Flash Calcination,
– “Chemical Products” Development and Use Basic Bayer Process still remains the same