||Selektiv krystallisation af kaliumsalte fra biomasseflyveaske
||Thomsen, Kaj (Center for Phase Equilibria and Separation Processes, Department of Chemical Engineering, Technical University of Denmark, DTU, DK-2800 Kgs. Lyngby, Denmark)
||Technical University of Denmark, DTU, DK-2800 Kgs. Lyngby, Denmark
||The combustion of biomass makes an ash which is rich in nutrient salts. Unfortunately, it also contains some
heavy metals such as cadmium, which means that the ash is not allowed to be used in the agricultural sector,
according to the danish announcement about using ash from gasification and combustion of biomass and biomass waste for agricultural purposes (also called: The biomass announcement). Therefore the ash has been deposited until now, even though the announcement has as a principle that the nutrient salts from the straw ash should go back to the fields, and the nutrient salts from the tree ash should be sent back to the forest.
Since the ash is partially soluble in water, there could be a possibility to leach some of the nutrient salts
from the fly ash in such a way that the cadmium would remain insoluble. It will be investigated if potassium
salts like K2SO4 and KCl can be fractionally crystallized from the solution with the dissolved nutrient salts.
There were several purposes with this project. First it was investigated which kind of equipment could be used
for the crystallization of the potassium salts K2SO4 and KCl from a solution with the dissolved potassium
salts. Then some of the theory about crystallization was examined. Equipment for crystallization experiments
was designed, and at last these experiments were carried out. The project was divided into four parts:
• 1st part: A literature study, where it was investigated which kind of crystallizers that were used on
industrial scale, and the theoretical basis for crystallization was partly described.
• 2nd part: Design of equipment with which the crystallization experiments would be made.
• 3rd part: An investigation of relevant phase diagrams for crystallization, and a simulation of the process
with aqsol .
• 4th part: Crystallization experiments in order to find the flow velocities which gave a sufficient crystal
size distribution. The simulated process was compared with the results from the experiments, and the
purity of the precipitated crystals was determined by analysis.
It was shown that it is possible to fractionally crystallize K2SO4 with a content of chloride lower than 0,18
wt %, but none of crystals achieved the desired size of 2-4 mm.
Creation date: 2006-06-22
Update date: 2007-02-24