Method for processing ash, particularly fly ash
Disclosed is a method for processing ash, particularly fly ash, in which method several elements are separated from the ash. In the method both noble metals and rare earth elements are separated.
1. Method for processing ash, in which method several elements are separated from the ash, characterized in that the several elements include both noble metals and rare earth elements, the noble metals and the rare earth elements are separated from the ash in two extraction processes, wherein the noble metals are separated in the first extraction process and the rare earth elements are separated in the second extraction process, and solutions obtained in the extraction processes are precipitated in two steps, in the first step of which noble metals are precipitated and in the second step of which rare earth elements are precipitated.
2. Method according to claim 1 , characterized in that the extraction processes and the precipitation steps are linked to each other.
3. Method according to claim 1 , characterized in that in the first extraction process noble metals are dissolved using a water solution having an oxalate content.
4. Method according to claim 1 , characterized in that in the second extraction process rare earth elements are dissolved out of undissolved ash from the first extraction process, with a mixture of sulphuric acid and nitric acid.
5. Method according to claim 1 , characterized in that an oxalate-extraction solution obtained from the first extraction process is processed in at least two stages, in such a way that in a first precipitation stage a first precipitation solution containing sulphide and ammonium chloride is added to the oxalate-extraction solution, in order to separate iridium and copper, and in a second precipitation stage the pH of the oxalate-extraction solution remaining from the first stage is raised in order to precipitate the rest of the noble metals.
6. Method according to claim 5 , characterized in that in the first precipitation stage the pH of the oxalate-extraction solution is 1.5±0.3 and in the second precipitation stage the pH of the solution is 8.5±0.3.
7. Method according to claim 1 , characterized in that an oxalic acid solution is added to an extraction solution obtained from the second extraction process, in order to separate rare earth elements as a third precipitation stage.
8. Method according to claim 7 , characterized in that in the third precipitation stage the pH of the extraction solution is 1.5±0.3.
9. Method according to claim 7 , characterized in that an oxalic acid solution is added to the third precipitation stage.
10. Method according to claim 1 , characterized in that ash undissolved in the extraction processes is washed with water in wash stages before following treatment of the undissolved ash.
11. Method according to claim 10 , characterized in that the wash solution formed in the wash stages is returned to the extraction processes after the wash stages.
12. Method according to claim 1 , characterized in that the extraction processes are boosted by one or more of adjusting the temperature of the solutions, adjusting the pressure of the solutions, agitating the solutions, treating the solutions mechanically, and directing ultrasound to the solutions.
13. Method according to claim 6 , characterized in that in the second precipitation stage the pH is adjusted with ammonia and after the nitrogen remaining after the second precipitation stage is collected as fertilizer.
14. Method according to claim 7 , characterized in that the oxalic acid used in the third precipitation stage is recycled to the first extraction process.