Gas Cleaning System - Electrostatic Precipitators - Operation
December 12, 2008

Introduction Gas Temperature Gas Composition Gas Impurities         Zinc         Lead         Arsenic         Mercury         Fluorine and Chlorine         Selenium         Acid Mist/SO3

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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:

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.