Paper

CHOICE OF PROCESS

As known, none of the different processes were chosen. This is due to the following:

The Boliden Norzinc process has its optimum process temperature on 30-40°C. The coldest point of the off-gas system at Tinfos is 48°C. A lower temperature would have a negative effect on the PAH-removal and the solubility of CO. A gas containing dust and tar may result in clogging of the filter mass. Due to the regeneration of calomel (Hg₂Cl₂), the process is more suited for gases with a higher Hg-concentration than that of Tinfos. When the above is taken into account, the B-N process was not considered as the optimum solution to Tinfos.

Selenium filter (Boliden-Contec), producing mercury-selenide is very well suited as a final deposit. The low solubility of this end product and the small amounts of material to be deposited, makes this process promising. The relatively small filter units make it easy to install them into the existing plant. All these factors taken into consideration it was concluded that the use of selenium filters would be a very good solution for Tinfos. However, when such filters were tried in a pilot plant at Tinfos, the problems with clogging of the filter became obvious. This is due to the amount and nature of the dust in the furnace off-gases. This was the main reason to the rejection of the selenium filter process.

Active carbon injection (ABB fläkt). Also this process was considered to give an adequate solution to Tinfos problem, with a well-proven technology. However, the investment cost and the operation costs was expected to be higher than an equivalent selenium filter. The deposit of considerable amounts of Hg containing filter dust could be a problem from this process. The process was thus considered to be the second choice.

The MEDISORBON^® process came in to discussion on a later stage than the three other processes, and was thus not considered in the same way. It is presented here to show the variety of Hg-removal processes. However, it is obvious that the demand for low dust- and humidity-concentrations in the off-gas, could be an obstacle for the use of this process at Tinfos, as was the case for the selenium filter.