Eletrokemisk fjernelse af Cd fra bioasker i pilotskala og vurdering af mulighederne for nyttiggørelse af behandlet aske i beton

Electrochemical removal of cadmium from bio ashes has been demonstrated in pilot scale, and the remediated ashes have been evaluated for possible reuse as either fertilizers, or in concrete products. 5 remediation experiments have been completed, using straw combustion fly ash or fly ash from cocombustion of wood and fuel oil.

During these emediation experiments the process has been upscaled stepwise, from an initial distance between the electrodes of 35 cm and a tank volume of 300 L ash suspension, to a final electrode distance of 245 cm, a total tank volume of 2.1 m3, and inclusion of up to 6 "concentration-units". The ash volumes to be remediated made up to between 8.4 and 82.5 kg dry matter, prior to eventual pre-wash.

The first four remediation experiments were made with straw combustion fly ash, the fifth contained both straw combustion fly ash and cocombustion fly ash, in separate compartments. The demonstration experiments have in many ways confirmed the results obtained in smaller scale in the previous project PSO FU 3206.

It is demonstrated that electrochemical removal of cadmium from bio ashes is possible also in larger scale than laboratory scale and bench scale, as final concentrations of cadmium below the regulatory limits for recycling of straw ashes have been reached. Furthermore, new findings such as the importance of choosing more acid resistant materials for the plant have showed up.

The use of concentration units contributed positively to the separation of cadmium from the ash suspension, but when using concentration units the "natural" acidification of the ash during the remediation process is delayed, and thus it is recommended to add acid to the ash before and eventually during the remediation process to decrease pH more rapidly.

The ashes were analyzed before and after remediation for evaluation of the potential of reusing the ashes in concrete products, or recycle them as fertilizers.