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Gas Cleaning System
April 11, 2009
Introduction
There are many types of equipment for the removal of dust,
fumes, mists and fine liquid droplets from gases. These devices fall into five basic
groups. Each group has its own particular fields of application.
Special expertise is required to determine the best technical
and economical solution to a gas cleaning problem. Some of the equipment options
available to the designers are:
| Gravity Settling Chambers |
The principle of this method of
collection is to pass the gases through a large chamber at a very low velocity.
Cleaning of the gas is achieved by dust particles falling out of the gas stream under the
influence of gravity. |
| Cyclone Collectors |
These devices force the gas to
develop a spinning motion. Dust particles are removed by centrifugal forces. |
| Wet Scrubbers |
In these devices, dust or fume
particles in the gas stream are contacted or wetted with a liquid to increase their
mass. These larger particles can then be more easily removed in either gravity or
centrifugal separators. |
| Fibre and Fabric Filters |
The principle of this method of
collection is to pass the gas stream through a semi-porous medium of woven cloth or felt
which retains the dust particles. |
| Electrostatic
Precipitators |
These devices use electrical charge
solid and/or liquid particles in the gas stream causing them to migrate to a collection
surface. Once the particle contacts the collection surface is removed from the gas.
Gas velocities are kept low to eliminate the chance of re-entrainment.
Electrostatic precipitators are effective in removing sub-micron particles. |
Gas Impurities
Metallurgical processes and spent acid decomposition produce a variety of
impurities that end up in the gas going to the acid plant. The method chosen to
treat and clean the gas will depend on the impurities present, quantity in the gas,
interactions between other components, operating conditions, weak acid concentration, etc.
The following table lists only a few impurities that may be present in the gas.
|
Smelter |
Gas |
Weak Acid |
Weak Acid
Treatment |
| Sulphur Dioxide – SO2 |
|
|
Dissolved in weak acid.
Reducing agent |
Stripping with air |
| Sulphur Dioxide – SO3 |
|
Present as submicron particles |
Contributes to the acid
concentration |
Neutralization |
| Arsenic – As |
Majority of arsenic reports to the
slag with minor amount entering the melt and a smaller fraction being volatilized. |
Reducing Conditions – As4 or As2S3
Oxidizing Conditions – As2O3
More info ... |
As2O3 dissolves slowly to give HAsO2
(arsenious acid). HAsO4 (arsenic acid) is not present
since it is oxidized by SO2 to HAsO2.
As4, As2S3 and As2O3
(undissolved) can be easily filtered. |
Unlike most metal species, arsenic will not form insoluble sulphate salts
when the acid is concentrated.
Neutralize acid to pH 2.5 to 3.0 (< 0.5% H2SO4).
Add ferric flocculating agent and chemical oxidant. Ferric arsenate (FeAsO4)
will precipitate. More info ... |
| Selenium – Se |
Greater proportion of selenium is
volatilized relative to arsenic |
Reducing Conditions – H2Se (but is rapidly
oxidized to H2SeO3)
Oxidizing Conditions – H2SeO3 |
H2SeO3
is rapidly reduced to elemental selenium |
Filterable |
| Lead - Pb |
|
Present as PbO or PbO2 |
Present as PbSO4 |
Precipitation |
| Mercury – Hg |
|
Present as elemental mercury vapour. Upon cooling HgO will form. Mercury Removal |
Reduced from mercuric ion to
mercurous ion by SO2. Will react with selenium to form
HgSe. |
HgSe is filterable |
| Fluorine - F |
|
Present as HF
More info ... |
Present as HF |
Addition of sodium silicate will form H2SiF6
(fluosilicic acid) |
| Chlorine – Cl |
|
Present as HCl |
Present as HCl |
Presence of HCl tends to increase
the solubility of some metals in weak sulphuric acid (i.e. Pb, Sn) |
Typical Gas
Cleaning Systems
Metallurgical off-gases will be treated in a number of
different ways prior to entering the acid plant gas cleaning system. Examples of how
the gas is treated are given in the Metallurgical
Processes section. Acid regeneration plant also require a gas cleaning system
downstream of the decomposition furnace. Both metallurgical and regeneration gases
require cleaning prior to entering the contact section of an acid plant. The gas
cleaning method used will depend on the specifics of the gas.
| Gas Source |
Gas Cleaning System |
| Noranda Converter - Custom Copper
Smelting |
Low pressure drop venturi quench followed
by retention tower |
High pressure drop variable throat
venturi followed by a cyclonic separator |
Primary condenser - vertical alloy shell
and tube |
Primary WESP |
Secondary condenser - vertical alloy
shell and tube |
Secondary WESP |
| Molybdenum Smelter |
Dynawave RJS quench system |
Packed gas cooling tower |
Primary WESP |
Secondary WESP |
Karbate condensers |
Mesh Pad |
| El Teniente smelter and PS converters |
Quench tower |
Fixed throat venturi scrubber |
Packed gas cooling tower |
Primary WESP |
Secondary WESP |
|
| Outokumpu Flash Furnace |
Quench tower |
Fixed throat venturi |
Packed gas cooling tower |
Primary WESP |
Secondary WESP |
|
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