136
A.B. Mukherjee
considerably when 502 is added to the system. The authors believe that 502
reduces HgO to elemental (Hg")g, which has a stronger tendency to escape
through activated carbon. However, it has been suggested that the reaction of 502
with HgO(s.g) will not have any effect on the adsorption of Hg" by activated
carbon (Zacharwski et a1. 1987). The overall reaction is given below:
There is a chance that H2S0 4 will form due to reaction of SO J with the moisture
in the gases, causing corrosion problems inside the reactor (Expert Panel on
Mercury Atmospheric Processes 1994; O. Lindqvist 1996, pers. comm.).
In this process, the charcoal bed contains many tons of Hg in the form of
mercury (I) sulphate. Generally, after every 5 years, the filters are inspected at
Boliden mineral in Sweden and the charcoal in the filter is replaced. The used
charcoal is stored under controlled conditions for future disposal together with
other hazardous waste materials (A. Svedberg 1997, pers. comm. F. Habashi
1997).
It is feasible to remove Hg from the Hg-Ioaded charcoal by leaching with acid,
NaOel solution or cyanide solution and the Hg can be recovered from the
solution by cementation-precipitation process in the presence of iron scrap (F.
Habashi 1997).
3.4
Selenium Filter
This process is based on the reaction between Hg", Hgg and Se forming mercury
selenite (HgSe) which is a stable compound. The operating temperature is kept to
about llO "c. At higher temperatures, Se will be lost from the filter due to its high
vapour pressure (Y. Lundgren 1996). The filter material is porous, inert, and
soaked in a solution of selenium dioxide (SeO,) which is then dried. This
compound is converted into activated Se when the 502 comes into contact with
the filter. The filter element absorbs Hg until its content reaches 10-15%. The Se
filter will be inactive in the presence of moist gas. It can be reactivated, but Se
loss is inevitable. For this reason, moist gases must pass through mist eliminators
to remove liquid droplets and the relative humidity should be lowered before the
gas enters into filter units. The Hg collection efficiency varies between 95 and
99·5%.
3.5
Selenium Scrubber
In this process, amorphous solid elemental Se is maintained in the wash liquid
containing H2S0 4 which accelerates the removal of Hg" from the 502 , The acid
concentration in the unit must be between 10-60%. At lower concentrations,
soluble selenium sulphur compounds are formed whereas at higher concentrations, the elementary selenium is quickly oxidized to selenium dioxide or
A.B. Mukherjee
considerably when 502 is added to the system. The authors believe that 502
reduces HgO to elemental (Hg")g, which has a stronger tendency to escape
through activated carbon. However, it has been suggested that the reaction of 502
with HgO(s.g) will not have any effect on the adsorption of Hg" by activated
carbon (Zacharwski et a1. 1987). The overall reaction is given below:
There is a chance that H2S0 4 will form due to reaction of SO J with the moisture
in the gases, causing corrosion problems inside the reactor (Expert Panel on
Mercury Atmospheric Processes 1994; O. Lindqvist 1996, pers. comm.).
In this process, the charcoal bed contains many tons of Hg in the form of
mercury (I) sulphate. Generally, after every 5 years, the filters are inspected at
Boliden mineral in Sweden and the charcoal in the filter is replaced. The used
charcoal is stored under controlled conditions for future disposal together with
other hazardous waste materials (A. Svedberg 1997, pers. comm. F. Habashi
1997).
It is feasible to remove Hg from the Hg-Ioaded charcoal by leaching with acid,
NaOel solution or cyanide solution and the Hg can be recovered from the
solution by cementation-precipitation process in the presence of iron scrap (F.
Habashi 1997).
3.4
Selenium Filter
This process is based on the reaction between Hg", Hgg and Se forming mercury
selenite (HgSe) which is a stable compound. The operating temperature is kept to
about llO "c. At higher temperatures, Se will be lost from the filter due to its high
vapour pressure (Y. Lundgren 1996). The filter material is porous, inert, and
soaked in a solution of selenium dioxide (SeO,) which is then dried. This
compound is converted into activated Se when the 502 comes into contact with
the filter. The filter element absorbs Hg until its content reaches 10-15%. The Se
filter will be inactive in the presence of moist gas. It can be reactivated, but Se
loss is inevitable. For this reason, moist gases must pass through mist eliminators
to remove liquid droplets and the relative humidity should be lowered before the
gas enters into filter units. The Hg collection efficiency varies between 95 and
99·5%.
3.5
Selenium Scrubber
In this process, amorphous solid elemental Se is maintained in the wash liquid
containing H2S0 4 which accelerates the removal of Hg" from the 502 , The acid
concentration in the unit must be between 10-60%. At lower concentrations,
soluble selenium sulphur compounds are formed whereas at higher concentrations, the elementary selenium is quickly oxidized to selenium dioxide or