Electrodialytic removal of Cd from straw ash in a pilot plant

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Abstract
Ashes from biomass combustion contain many macro and micro nutrients and its reuse in agricultural fields could be beneficial. In the case of straw ash, however, the content of Cd is often too high for the ash to be utilized this way. In Denmark the limiting concentration for spreading the ash in agricultural land is 5.0 mg Cd/kg and in order to utilize the nutrients in the straw ash it is necessary to decrease the Cd content to meet this limiting concentration. It has been previously shown, at a laboratory scale, that the Cd concentration can successfully be decreased by an electrodialytic method. Results from scale-up of the electrodialytic method to pilot scale are the focus. The capacity of the pilot plant was approximately 40 L ash suspension (2 kg ash). During the treatment, 40% of the Cd was removed from the straw ash, and the final obtained concentration was 6.8 mg Cd/kg. Although this concentration still exceeds the limiting concentration, valuable experience concerning the practical operation through scale-up was obtained. In addition, the experiments are a good basis for further scale-up and optimization of the process for improved Cd removal to meet the requirements. A very important finding consisted in the control of the osmotic water flux that emerged from the electrode compartments into the ash suspension during treatment.

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