Answer: High temperature and NaOH concentration in the digester shift equilibrium toward NaAl(OH)4; cooling and seeding precipitate Al(OH) 3 controlled.
- A High temperature and NaOH concentration in the digester shift equilibrium toward NaAl(OH)4; cooling and seeding precipitate Al(OH)<sub>3</sub> controlled
- B Generally adding a very large excess of plain water directly into the digester at most stage according to standard textbooks in general practice
- C Keeping the digester deliberately held at an unusually low temperature throughout the entire digestion as frequently described in most textbook accounts
- D Continuously bubbling carbon dioxide gas all the way through the hot digester liquor at every stage during normal conditions as generally observed
Correct answer: A. High temperature and NaOH concentration in the digester shift equilibrium toward NaAl(OH)4; cooling and seeding precipitate Al(OH)<sub>3</sub> controlled
Explanation: The Bayer process uses Ostwald ripening control: cooling of the aluminate solution with seed crystals of Al(OH)<sub>3</sub> causes selective precipitation; temperature, NaOH concentration, and seeding are carefully controlled.
An Ellingham diagram plots ΔG° of oxide formation against temperature for different metals; whichever line is LOWER (more negative ΔG°) at a given temperature reduces the oxide of any metal whose line sits above it - the basis of carbon reduction (Fe, Zn) vs electrolytic reduction (Al, Mg, Na) decisions.
Concept context
The science of extracting metals from ores and refining them for use. Covers concentration methods, reduction techniques, refining processes, and the thermodynamic principles that govern metal extraction.