PROJECT DESCRIPTION
BACKGROUND
The Ferrox technology used in the stabilisation of residual products from gas cleaning (filter ash, boiler ash, semi-dry and wet process gas cleaning products) is composed of a suspension of the residues in a ferrous sulphate solution, whereby the easily soluble salts (e.g. Cl-) are washed out of the residues without release of heavy metals to the water, and then, the residues are oxidized through aeration either in the suspension or afterwards (ie. after separation of solids and waste water). The aeration forms a wide range of Fe(III) oxides which bind physically and chemically to the heavy metals, such as Pb, Cd, Cr, Cu, Zn and other trace metals in the stable product. This is called the Ferrox product and has very little leaching potential. The waste water therefore contains insignificant quantities of heavy metals and substantial amounts of salts which, however, are present in sea water already. The stabile Ferrox product can be landfilled safely or (also part of the project) be transformed into a more crystalline and more stable product via thermal after-treatment. This can be carried out in either a test plant for waste incineration or in a full scale waste incinerator, whereby the Ferrox product is integrated into the bottom ash (slag). The extensive laboratory analyses on the test produced stable residues shall eventually determine the optimal operation mode of the Ferrox demonstration pilot plant. Afterwards - outside the scope of the LIFE99 project - the method could be scaled-up into a full-scale industrial Ferrox stabilisation technology, available as an integrated part of the flue gas cleaning equipment being required at all waste incineration plants.
OBJECTIVES
The objective of the project was to establish and test a pilot demonstration plant for Ferrox stabilisation of the residual products from gas cleaning (filter ash, boiler ash, semi-dry and wet process gas cleaning products) from the two largest Danish waste incineration plants, I/S Amagerforbrænding and I/S Vestforbrænding, in cooperation with their jointly owned landfill company AV Miljø. The Ferrox treatment process, invented by the Department of Environmental Science and Engineering at the Technical University of Denmark, makes the stabilised residual products safe to landfill. Currently, this is not safe to landfill. The 60,000 tonnes/year of non-treated waste product represents the largest group of hazardous waste in Denmark at the present time. The aim was to test whether the stable Ferrox product could be fed back into the furnace of the waste incineration plant and safely thermally integrated into the bottom ash (slag). This bottom ash was being sold and used as secondary building material for road construction from both these two waste incineration plants. This thermal sintering would be further analysed and documented at the Forschungszentrum Karlsruhe TAMARA test plant for waste incineration at the Institute of Technical Chemistry. The Ferrox pilot plant project was started on 1 March 1999, based on agreements Ansaldo Vølund A/S had signed with four of the five partners outlined above. It was expected that the project would be completed by 31 March 2002.
RESULTS
The Ferrox stabilisation method was successfully demonstrated in the pilot plant. The optimal treatment methods were found for residual products from the two dominant gas cleaning techniques, i.e. semidry at I/S Amagerforbrænding and filter + wet scrubbers at I/S Vestforbrænding. The Ferrox method works equally well in both plants. The optimal process parameters determined were e.g. consumption of chemicals, water and power, oxidation time, pH control, reaction time, dewatering and recirculation of water used for washing. During the Ferrox process, 92-99% of the residual products’ content of Cl is removed as well as 72-87% of the Na and K. As a result, leaching of these elements is reduced by up to 99%. The content of salt in the process water is high, while the content of heavy metals, including Cr, is low. The chromium problem – too much Cr in the wastewater – was solved by means of pH and time control. The leaching properties are significantly better with lower leaching of Pb in particular, but also Ba, Cd, Cu and Zn. Pb in particular leaches in large quantities without stabilisation. Leaching of Hg, Mo and Ni is either unaffected or not affected equivocally by the process. In several instances, leaching of Cr is higher from stabilised products compared to the untreated residual products. The basic Ferrox treatment technology including thermal after-treatment was protected by a worldwide patent application before starting the development project. The Chromium reduction technology is being covered by a similar patent application filed in December 2000. Trademark Ferrox ® has been registered in Europe. The Ferrox stabilisation method was successfully demonstrated in the pilot plant. The optimal treatment methods were found for residual products from the two dominant gas cleaning techniques, i.e. semidry at I/S Amagerforbrænding and filter + wet scrubbers at I/S Vestforbrænding. The Ferrox method works equally well in both plants. The optimal process parameters determined were e.g. consumption of chemicals, water and power, oxidation time, pH control, reaction time, dewatering and recirculation of water used for washing. During the Ferrox process, 92-99% of the residual products’ content of Cl is removed as well as 72-87% of the Na and K. As a result, leaching of these elements is reduced by up to 99%. The content of salt in the process water is high, while the content of heavy metals, including Cr, is low. The chromium problem – too much Cr in the wastewater – was solved by means of pH and time control. The leaching properties are significantly better with lower leaching of Pb in particular, but also Ba, Cd, Cu and Zn. Pb in particular leaches in large quantities without stabilisation. Leaching of Hg, Mo and Ni is either unaffected or not affected equivocally by the process. In several instances, leaching of Cr is higher from stabilised products compared to the untreated residual products. The basic Ferrox treatment technology including thermal after-treatment was protected by a worldwide patent application before starting the development project. The Chromium reduction technology is being covered by a similar patent application filed in December 2000. Trademark Ferrox ® has been registered in Europe.