Dosage calculation model for humic acid-enhanced in-situ leaching of weathered crust elution-deposited rare earth ores

Humic acid (HA) is an environmentally friendly and efficient aid leaching agent, however, the mechanism of HA and ammonium sulfate ((NH 4 ) 2 SO 4 ) on rare earth (RE) in-situ leaching process and the effect of HA on ammonium reduction remain unclear. Herein, the potential of HA as an aid leaching agent to optimize the dosage of (NH 4 ) 2 SO 4 and reduce the residual ammonium salt was investigated, controlling the ammonia nitrogen pollution. Moreover, the calculation model of (NH 4 ) 2 SO 4 amount and unit consumption equation were established based on mass action law. The result indicated that the leaching efficiency of RE was increased by 3.96 % in the presence of 0.2 g·L −1 of HA, suggesting there was a synergistic effect between HA and (NH 4 ) 2 SO 4 during the RE leaching process. This effect was further confirmed by the leaching mechanism. Specifically, when the value of mass action quotient ( K t ) increased, the reaction molar ratio ( n ) was decreased as compared with that of pure ammonium sulfate, indicating HA has a promoting effect on the ion exchange reaction between rare earth ions and ammonium ions. Additionally, it was revealed that the (NH 4 ) 2 SO 4 dosage was reduced by 34 % based on the law of mass action. Notably, the optimal (NH 4 ) 2 SO 4 concentration was lowered from 0.1 mol·L −1 for the traditional technology to 0.08 mol·L −1 for this work. These findings provided new insights into strengthening the leaching process of weathered crust elution-deposited rare earth ores by HA, which were valuable for elucidating the interaction mechanisms and realizing the source control of ammonia nitrogen pollution.