Circulation of heavy metal thallium in co-processing cement kilns

Thallium (Tl) poses a significant pollution risk due to its high volatility during clinker calcination. Thermodynamic calculations and multi-stage monitoring have revealed that, despite sulfur suppression, over 90% of Tl volatilised as thallium chloride at 900°C due to chlorine. Volatilised Tl migrated with gas, condensing in cooler zones, enriching Tl in mill-outlet raw meal. The recycling of Tl-enriched dust from the suspension preheater boiler and bag filter further intensified the Tl enrichment in the kiln-feed raw meal. As this enriched material reached the two-stage cyclone, it re-adsorbed and condensed Tl from the gas and reintroduced Tl back into the high-temperature zone again, forming an internal ‘volatilisation-condensation’ cycle. After calcination, only 0–27.85% of input Tl resided in the clinker, while the bypass system released merely 1.31–6.66%. Crucially, the bag filter efficiently intercepted volatile Tl, preventing detectable Tl emissions from kiln gas. Therefore, despite high volatility, the internal cycle, effective dust capture and dust recycling led to closed-loop circulation and enrichment of most Tl within the system, preventing environmental release. In addition, co-processing Tl-containing solid waste amplified the imbalance between Tl input and output, leading to greater internal circulation and enrichment within the kiln system, without significantly altering clinker Tl content or emissions.