||Rensning af imprægneret træ for kobber, krom og arsen
||Hansen, Bjørn B. (Department of Chemical and Biochemical Engineering, Technical University of Denmark, DTU, DK-2800 Kgs. Lyngby, Denmark)
||Thomsen, Kaj (CERE, Department of Chemical and Biochemical Engineering, Technical University of Denmark, DTU, DK-2800 Kgs. Lyngby, Denmark)
||Technical University of Denmark, DTU, DK-2800 Kgs. Lyngby, Denmark
||Today newly impregnated wood contains no arsenic and only few heavy metals. It hasn't always
been like this. Since January first of 1997 Denmark prohibited import, export, sale and use of
arsenic impregnated wood [MST.dk]. Before 1997 arsenic impregnated wood was heavily used in
outdoor wood constructions. In Denmark it is estimated that about 4 million tons of CCA
impregnated wood is still in use today. CCA is an acronym for copper, chromium and arsenic which
is the impregnation chemicals used to conserve the wood. The CCA treated wood will need to be
disposed of in the future. Arsenic is the most problematic of these chemicals and only 0,1 wt% is
enough to classify the wood as hazardous [MST.dk]. In this report it has been attempted to remove
the CCA chemicals from impregnated wood, using oxalic acid and NaOH. It has also been
attempted to add NaCl to the oxalic acid.
Previously the reports [Rasmussen, 2008] and [Vegger, 2009] have described how CCA chemicals
can be removed from impregnated wood. Their work has been used to guide this project in the right
direction with respect to cleaning chemicals. The experiments have shown that it is possible to clean
more than 80% arsenic and approximately 75% chromium from CCA impregnated wood using
oxalic acid. Copper showed to be more difficult than expected and only about 25% could be
removed. The experiments clearly showed that the highest oxalic acid concentrations gave the best
results. The oxalic acid experiments were performed over 1, 3 and 6 hours and it was shown that
longer extraction times may improve the results even further.
It was also attempted to add NaCl to the oxalic acid, but this affected the result in a negative way.
NaOH was also attempted as a chemical for the removal of CCA chemicals from wood. This was
done in the hope that NaOH could selectively remove arsenic while leaving copper and chromium
in the wood. This was found to be the case, but as only 20% of the arsenic was removed NaOH was
shown to be infective.
After the CCA chemicals had been removed from the wood it was attempted to recover the cleaning
liquid. To do this could make the extraction more profitable. The cleaning liquid was attempted
cleaned by am electrochemical method developed by RecoMeta. Unfortunately it was not possible
to show results that indicated that the method would be viable. An experiment using a synthetic
arsenic solution did, however, show that >99% of the arsenic was removed. This result indicate that
further experiments may have more success than the ones performed here.
||Technical University of Denmark (DTU) : Kgs. Lyngby, Denmark
Creation date: 2010-07-06
Update date: 2010-07-06