(BAT) Reference Document for the Production of Chlor-alkali ...

Chapter 4
Technical description
A summary description is provided in Table 4.24. Operating temperatures for thermal
desorption typically range from 320 °C to more than 800 °C. Thermal desorption can be carried
out with the help of vacuum or vibrating equipment [ 245, Euro Chlor 2009 ], [ 263, Khrapunov
et al. 2007 ], [ 266, US EPA 2007 ].
Achieved environmental benefits
The achieved environmental benefit of this technique is the decontamination of soil.
Environmental performance and operational data
The temperature and the residence time are the most important factors for the efficiency of
thermal desorption. In general, middle-range temperatures of 540 – 650 °C can decrease the
concentration of the residual mercury to levels below 2 mg/kg and the mercury can be reclaimed
with a purity of 99 % despite its different structures and forms [ 252, Chang and Hen 2006 ].
At a mercury cell chlor-alkali site in Bohus (Sweden), approximately 25 kt of soil contaminated
with mercury and PCDDs/PCDFs were planned to be treated in the period of 2000 – 2003.
Thermal desorption was planned to take place in a rotary kiln using vacuum at a capacity of
2 t/h. The mercury contained in the exhaust gases (10 mg/m 3 ) was then supposed to enter a wet
scrubber containing hydrogen peroxide which oxidises elemental mercury for its precipitation as
mercury sulphide [ 197, Götaverken Miljö 2000 ]. {Please TWG provide more information.}
At a mercury and diaphragm cell chlor-alkali site in Taipei (Taiwan), a full-scale test with a
total of 4174 m 3 soil was carried out in 2001 – 2003. Two sets of thermal desorption kilns were
used with batch masses of 2 t per kiln and a capacity of 8 t/d per kiln. Optimised performance
was obtained at a temperature of 750 °C and a residence time of 3 h. The post-treatment system
mainly consisted of condenser, condensation water tank, buffer tank and sulphur-impregnated
activated carbon tank (Figure 4.10) [ 252, Chang and Hen 2006 ].
NB: 1: Thermal desorption kiln;
2: Cooling water recycle system;
3: Condensation water separate tank;
4: Sulphur-impregnated activated carbon adsorption tank;
5: Coagulation tank;
6: Dewatering device;
7: Buffer tank;
8: Sulphur-impregnated activated carbon adsorption tank.
Source: [ 252, Chang and Hen 2006 ]
WORKING DRAFT IN PROGRESS
Figure 4.10: Flow diagram of the on-site thermal desorption system used at a chlor-alkali site in
Taipei (Taiwan)
262 December 2011 TB/EIPPCB/CAK_Draft_1