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Gas Cleaning System - Electrostatic
Precipitators - Operation
December
12, 2008
Introduction
There are many process factors that can
affect the performance and maintenance of a WESP. The operation of the metallurgical
process and the upstream gas cleaning system all will affect the operation of the WESP.
Gas
Temperature
The temperature of the gas affects the
electrical characteristics of the gas. For a given applied voltage, the discharge
current increases with increasing temperature. The gas temperature entering the WESP
is controlled by the upstream gas cooling tower or condensers and is generally quite
constant. The design gas temperature is dictated by the plant water balance and
usually cannot be changed to improve WESP performance.
Gas
Composition
The composition of the gas affects the
electrical characteristics. If the gas composition is subject to large changes there
will be a noticeable affect on the operation of WESP. This is particularly evident
during the initial start-up of the equipment.
Gas
Impurities
It is important to know exactly what will
be in the gas when specifying and designing a WESP for a metallurgical acid plant.
Some impurities will easily removed while others will be more difficult so it is important
to know the type and concentration of all gaseous components.
The exact form in which various
contaminants appear in the gas is not generally known. The contaminant may appear in
the elemental form, as a particle, oxide, sulphate, arsenate, etc. The most probable
form of various components are as follows:
| Impurity |
Form of Impurity |
Chemical Formula |
| Fluoride |
Particulate |
HF |
| Chloride |
Particulate |
HCl |
| Arsenic |
Oxide, Elemental |
As2O3, As |
| Selenium |
Oxide, Elemental |
SeO, Se |
| Mercury |
Elemental |
Hg |
| Zinc |
Sulphate |
ZnSO4 |
| Lead |
Sulphate, Elemental |
PbSO4, Pb |
| Copper |
Sulphate |
CuSO4 |
Acid mist, copper, nickel and iron are
generally considered easy to collect. More difficult impurities include zinc, lead,
arsenic and antimony.
Zinc
Zinc enters the gas cleaning system as a
fume and dust. When the fume is condensed the resulting particles are very fine and
are not easily removed in the scrubbing stages. On entering the WESP the presence of
zinc causes severe depression of the electrical corona discharge, which results in poor
precipitation of all gas impurities. This problem is easily addressed during the
design phase by providing higher operating voltages and high intensity discharge
electrodes.
Lead
Lead is similar to zinc in that it is
difficult to remove in the wet gas scrubbing section of the gas cleaning system. The
problem can be overcome by design for a high intensity electric field in the WESP.
Arsenic
Arsenic enters the gas cleaning system as a
fume and tends to condense and deposit in the cold parts of the plant. The WESP are
the most common place to find deposits of arsenic since they are typically downstream of
the gas cooling stage. Arsenic tends to form crystal on the collection surfaces and
prevents effective precipitation of other impurities. Frequent and adequate flushing
of the precipitators is required to remove these deposits.
Mercury
Mercury enters the gas cleaning system as a
vapour and like arsenic will condense in the colder parts of the plant. Mercury will
attack the lead parts of the precipitator forming an amalgam. Modern WESP have
tended away from lead as a material of construction for the collecting surfaces but may
still be present in various components of the electrical discharge system.
Fluorine
and Chlorine
These halides cause corrosion of the lead
components of a WESP. The lead discharge electrodes are the most susceptible to
attack and subsequent breakage. Removal of fluorine and chlorine must be done in the
upstream wet scrubbing stages. Effective and proper operation of the upstream
equipment is required.
Usually the presence of fluorine and
chlorine are known at the design stage and the wet scrubbing process can be design to
properly remove these impurities. Situations often arise where the feed to the
upstream metallurgical process changes resulting in increased amounts of halides for which
the system was not design to handle. This when problems in the WESP will begin to
occur.
Selenium
Any selenium that passes through the wet
scrubbing stages is likely to deposit on the collecting surfaces of the WESP as red
amorphous selenium which is very difficult to flush out. The presence of selenium in
the WESP is quite obvious from the red coloured deposits.
Acid
Mist/SO3
The amount of SO3 entering the
gas cleaning system is a function of the operation of the upstream metallurgical process.
In general, the lower the oxygen content of the gas, the lower the SO3
content. On entering the gas cleaning system the SO3 comes in
contact with water and a fine acid mist is formed. The majority of the acid mist
formed will be less than 1 micron in size. The amount of acid mist removed prior to
the WESP varies depending on the design and operation of the wet scrubbing stages.
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