The role of aluminum fluoride in aluminum electrolysis

Pure cryolite has a melting point of approximately 1010℃. Adding an appropriate amount of AlF₃ can lower the primary crystallization temperature of the electrolyte to approximately 930–960℃ (usually controlled at around 950℃).

This directly brings two major economic benefits: Significantly reduced electrolysis temperature → Reduced heat loss and energy consumption
Allows the electrolytic cell to operate stably at lower temperatures.

Modern prebaked cells generally use acidic electrolytes (molecular ratio 2.2–2.5, or even lower as low as 2.1–2.3).

AlF₃ is the primary means of reducing the molecular ratio: An excessively high molecular ratio (alkaline) → Decreased alumina solubility, reduced current efficiency, and increased anodic effect. Regularly adding AlF₃ to control the molecular ratio within the required process range is the core of refined operation of the electrolytic cell.

In acidic electrolytes, increasing the AlF₃ concentration significantly improves conductivity (typically by 3%–8%), thereby:
Reducing cell voltage (especially the voltage drop across the electrode gap)
Increasing current efficiency (modern cells can achieve 92.5%–94%+)

During electrolysis, fluorine is lost through volatilization of HF, NaF, and AlF₃, formation of cryolite sludge, and adsorption by alumina.

Approximately 15–30 kg of aluminum fluoride is needed to compensate for these losses per ton of aluminum produced (depending on the molecular ratio control level and cell conditions).