Question

Complete precipitation of Al(OH)\(_3\) from sodium aluminate in settling tanks (in Bayer's process) takes about ___________ hours.

• A. 2
• B. 35
• C. 60
• D. 72

Hint:

Remember the contrast in timescales within the Bayer process:

• Digestion: Fast, high temperature, high pressure (a few hours).

• Precipitation: Slow, lower temperature, atmospheric pressure (many hours, e.g., 50-80 hrs).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The Bayer process is the principal method for producing alumina (Al\(_2\)O\(_3\)) from bauxite ore. A key step in this hydrometallurgical process is the precipitation of pure aluminum hydroxide, Al(OH)\(_3\), from a supersaturated sodium aluminate solution.

Step 2: Detailed Explanation:
The precipitation step involves the hydrolysis of the sodium aluminate solution: \[ \text{NaAl(OH)}_4(aq) \rightarrow \text{Al(OH)}_3(s) + \text{NaOH}(aq) \] This process is not spontaneous and needs to be carefully controlled to ensure a high yield of product with the correct crystal size. The key process parameters are:

• Seeding: The solution is "seeded" with fine crystals of previously produced Al(OH)\(_3\) to provide nucleation sites for crystal growth.

• Cooling: The solution is cooled from the digestion temperature to a lower temperature (around 50-70°C) to increase the supersaturation and drive the precipitation.

• Time: This is a crystal growth process, which is inherently slow. The solution is held in very large, agitated precipitation tanks for an extended period to allow the crystals to grow to a filterable size. The residence time is typically very long.
Industrial practice involves residence times ranging from 30 to as long as 90 hours. The value of

60 hours is a very typical and representative figure for this slow crystallization process. Options like 2 hours are far too short. 35 hours is on the low end, while 72 hours is also plausible but 60 is a common average.

Step 3: Final Answer:
The precipitation of aluminum hydroxide in the Bayer process is a slow process, with typical residence times in the settling/precipitation tanks being around 60 hours.