Optimization of leaching conditions for removing sodium from sodium-rich coals by orthogonal experiments

- Leaching treatments were used to selectively remove sodium by some mild agents.
- Leaching temperature was the significant factor in the W-leaching treatment.
- RES showed a volcanic relationship with the concentration of leaching agents.
- The optimal conditions were verified to be efficient and time-saving.
- The optimal conditions had a good universality among sodium-rich coals.

The leaching pretreatment has been adopted to mitigate the ash-related problems encountered during gasification and combustion of sodium-rich coals. However, the wide-spread applications of this technology are limited to produce the ultra clean coal through remove all the ash by some aggressive agents. In the present study, some mild agents, such as deionized water, ammonium acetate solution and diluted hydrochloric acid, were employed to selectively remove troublesome sodium rather than all the ash. The primary leaching conditions such as particle size, temperature, residence time and concentration of agents were optimized by orthogonal experiments for the first time to achieve maximum removal efficiency of sodium (RES). Based on the range analysis, it was found that the significant factor was leaching temperature in the water-leaching (W-leaching) treatment and the RES showed a positive correlation with temperature. However, in both ammonium acetate-leaching (A-leaching) and diluted hydrochloric acid-leaching (H-leaching) treatments, the concentration of agents were found to play the dominant role and RES presented a volcanic relationship with it. Under optimal conditions, the leaching treatments were less time-consuming with improved RES (47.6% for W-leaching, 97.9% for A-leaching and 96.8% for H-leaching). Besides, the properties of coal samples after leaching were determined by a series of analysis methods. The results showed that the leaching treatments could significantly decrease the content of AAEMs and coal ash. Due to the removal of AAEMs, the ash fusion temperatures increased, while both gasification and combustion reactivity decreased obviously. The application to other three sodium-rich coals confirmed that the optimal leaching conditions had a good universality.