New Technology Improves Oxidation of...
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Billy J. King & Charles M. West
Wet Process Specialists
New Technology Improves Oxidation of Sulfite
Presented to America’s Battery
Recyclers in October of 2004
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Case Study
• Existing lead-acid battery recycler utilizes forced air oxidation for sulfite removal from SO2 scrubber wastewater
• Increases in production and sulfur content of the feedstock stress the existing air oxidation system performance
• Facility researches root causes of sulfite oxidation issues and evaluates options for resolving problem
• Facility successfully installs new technology for sulfite oxidation
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Background Origins of Sulfite
• Wet caustic scrubbing is a common control technology for removal of SO2 from lead-acid battery recycling smelter flue gas
• This technology results in the formation of sulfite (SO3)-2 in the scrubber wastewater: SO2 + H2O H2SO3 H2SO3 + 2NaOH Na2SO3 + 2H2O
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Background Wet Scrubber Operations
Stack Exhaust
>95% SO2
Removal
Make-up WaterSO2-laden Gas
Caustic
Wastewater
Blowdown
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Background Need for Sulfite Removal
• Sulfite is a powerful oxygen scavenger
• The toxicity of sulfite to aquatic species at relatively low concentrations has been documented
• Sulfite can create oxygen-deficient zones in surface water and interfere with biological treatment processes in municipal sewers when discharged
• Most common method for removal is oxidation to sulfate with forced air:
Na2SO3 + ½ O2 Na2SO4
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Issues
• Facility SO2 scrubber operating conditions at upper margin of design capacity
• As a result, scrubber blowdown can contain elevated concentrations (near saturation) of sulfite and total dissolved solids (TDS)
– Concentrations of TDS near saturation, combined with high temperatures, severely limit transfer of oxygen in forced air oxidation systems
– Elevated levels of TDS can also lead to formation of other oxo-anions of sulfur that are more difficult to oxidize than sulfite
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Issues Facility Scrubber Wastewater
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Issues Sulfur Oxo-Anions
Disulfite Dithionite Sulfite
Thiosulfate
Sulfide
Sulfate
Dithionate Tetrathionate
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Issues Gibb’s Free Energy & Volt Equivalent
Volt equivalent diagram for sulfur-containing species in acid solution.
H2S
S2O3 S4O6
HS2O4
S3O6H2SO3
S2O6
HSO4
S8
-0.5
0
0.5
1
1.5
2
2.5
3
-2 0 2 2.5 3 3.5 4 5 6
(Average) Oxidation state of sulfur
Vo
lt e
qu
ivale
nt/
vo
lt
-50
0
50
100
150
200
250
300
-DG
/kJ m
ol-
1
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Issues Sulfite and Sulfate Solubility
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Issues Existing Forced Air Oxidation Tanks
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Options Analysis
• Chemical Oxidation with Hydrogen Peroxide – Effectively oxidizes sulfite and other sulfur oxo-anions – Low capital costs – Extremely high operating (chemical) costs
• Expand Forced Air Oxidation System – Predictable but slow oxidation rates – Excessive footprint requirements – Relatively high operating costs (energy)
• New Technology Utilizing Pure Oxygen and Pressure – Dramatically increases transfer of oxygen increasing the rate of
treatment – Minimal footprint requirements – Low capital and operating costs
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Options Analysis Available Footprint for the New System
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Options Analysis Footprint
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Sulfite Oxidation Reaction Rates vs. Pressure
0
5000
10000
15000
20000
25000
30000
35000
0 5 10 15 20 25 30 35
Time (min)
Su
lfit
e C
on
ce
ntr
ati
on (
mg
/L)
15 psi 30 psi 60 psi
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Options Analysis Reaction Rate
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Options Analysis Technology Cost Comparison
Technology Normalized Total Cost*
[$/ton (SO3-2)]
Chemical Oxidation
with H2O2
$600
Forced Air Oxidation $40
Pressure Oxidation $40
* Based on 10-year capital depreciation
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Pressure Oxidation System Schematic
Reactor
Vessel
Oxygen
Supply
Sample Port
Feed
Pump
Influent
Wastewater
Supply
Flow Pressure pH/TempEffluent
Wastewater
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Pressure Oxidation System
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Project Successes
• Pressure oxidation system provides complete oxidation of residual sulfite when operated at design conditions
• Average overall sulfite oxidation efficiency at facility increased from 80% to >97%
• Furnace production and utilization increases realized upon installation of pressure oxidation system