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Sulphuric Acid on the WebTM Technical Manual DKL Engineering, Inc.

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Sulphuric Acid on the Web

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Gas Cleaning System
April 11, 2009

Introduction
Gas Impurities
Associated Links

Quench System
Venturi Scrubbers
Gas Cooling
Weak Acid Cooling
Electrostatic Precipitators
Mercury Removal
Fluoride Removal
Arsenic Removal
Selenium Removal
Weak Acid Valves


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