pH-Dependent Leaching Characteristics of Major and Toxic Elements from Red Mud

Area of Science:

• Environmental Science
• Geochemistry
• Materials Science

Background:

• Red mud, a byproduct of alumina production, poses environmental challenges due to elemental leaching.
• Understanding red mud's leaching behavior is crucial for effective waste management and environmental risk assessment.

Purpose of the Study:

• To investigate the pH-dependent leaching behavior of elements from red mud.
• To compare leaching characteristics between red mud produced by Bayer and combined processes.
• To identify the primary mechanisms controlling metal leaching in red mud.

Main Methods:

• Batch leaching tests were performed on five red mud samples across a pH range of 2-13, following US Environmental Protection Agency (USEPA) Method 1313.
• Geochemical modeling using MINTEQ 3.1 was employed to elucidate metal leaching mechanisms.
• Elemental concentrations and anion leaching were analyzed.

Main Results:

• Cationic leaching patterns were observed for Ca, Mg, and Ba, while Al, As, and Cr exhibited amphoteric leaching.
• Chloride (Cl-) leaching was pH-independent; fluoride (F-) leaching reached 1.4-27.0 mg/L at natural pH.
• Red mud from the combined process showed higher leaching concentrations for Pb, As, Cr, and Cu compared to the Bayer process at equivalent pH.
• Leaching concentrations were not strongly correlated with total elemental content.
• Geochemical modeling confirmed that metal leaching (Al, Ca, Fe, Cr, Cu, Pb, Mg, Ba, Mn) is primarily solubility-controlled, influenced by the dissolution/precipitation of (hydr)oxides, carbonates, and sulfates.

Conclusions:

• Elemental leaching from red mud is significantly influenced by pH, with distinct patterns for different elements.
• Process type (Bayer vs. combined) affects the leaching of critical contaminants.
• Solubility of mineral phases dictates metal leaching, underscoring the importance of understanding red mud mineralogy for environmental management.