Experiments to leach potassium salts from fly ash from straw combustion were performed. The fly ash was produced in the bio mass boiler at the Avedøre power plant in south western Copenhagen, Denmark. The fly ash contained approximately 90 mass percent water soluble material. When the fly ash was dissolved at low pH, a slightly higher solubility was found. 100 gram fly ash consisted typically of 9 gram insoluble material, 9 gram calcium phosphate, 29 gram potassium sulfate and 53 gram of potassium chloride. In addition, 100 gram of fly ash contained approximately 1 mg of cadmium, corresponding to a concentration of cadmium of 10 ppm in the fly ash. Fly ash from the bio mass boiler at the Avedøre power plant apparently has a significantly larger content of potassium salts than fly ash from other boilers1.
The Extended UNIQUAC thermodynamic model was used for calculating relevant phase diagrams and calculations of the necessary amount of water required for dissolving all the KCl and all KCl + K2SO4 of the fly ash. This theoretical minimum amount of water was calculated at a range of temperatures between 10 and 100°C. The amount of water required at 100°C was less than half of that required at 10°C.
Experiments were performed in order to find a feasible method for separating the potassium salts of the fly ash from the ash residue and especially from the soluble cadmium salts found in the fly ash.
Experiments with counter current leaching of fly ash in a fluid bed gave unsatisfactory results. Apparently there was a lack of contact between the wash water and the ash. In addition, sedimentation was very slow resulting in an incomplete separation of wash water and ash residue.
Experiments with ion exchange by adding CaCl2 to the wash water and successive precipitation of gypsum or anhydrite gave unsatisfactory results. Process simulation had shown that by this method the necessary amount of washing water could be decreased. This is due to the higher solubility of potassium chloride compared to potassium sulfate. By using this reduced amount of washing water with CaCl2 a viscous, muddy slurry was obtained. It was not possible to separate this slurry into a liquid phase and a solid phase.
It was found that a good separation was obtained by intense stirring followed by centrifuging or by filtering the slurry in a filter press. The lab experiments were performed as a counter current leaching process in four stages. Depending on the applied amount of water relative to the theoretical minimum amount of water a complete separation could be obtained in two or more stages. More stages are required if less water is used. If the leaching is performed in less stages, a larger amount of water is required.
If the washing process was performed at pH 1, the phosphate fraction could successively be obtained as a fine, white powder of almost pure calcium phosphate by regulating pH to 12.
If the washing process was conducted at a pH between 1 and 5 and pH was successively adjusted to 12, cadmium could not be detected in the brine by current analysis methods. All cadmium was precipitated as phosphate.
A part of this work was reported at the 14th European Conference and Technology Exhibition on Biomass for Energy, Industry and Climate Protection
|Publication status||Published - 2006|