This article was presented to the International Seminar "Review of the Agenda: New Initiatives in the Mining Sector", carried out on the 8-9 May, 2002, Santiago-Chile. This seminar was organized by the Chilean Copper Commission (COCHILCO), Chilean Mining Council and Universidad de Concepcion. The Seminar presentations are published in COCHILCO's web site.
Introduction - Methods of SO2 Emission Control
SO2 gas emissions are mainly generated in pyrometallurgical processing of sulphide ores, burning of sulphur, and combustion of fossil-fuel (charcoal and oil). Even in the operations that convert SO2 into sulphuric acid, it is found that in many cases, residual emissions of low concentration of SO2 still remain. And nowadays in many countries, these have to be controlled as a result of more stringent emission limits being enforced.
Various methods have been employed for SO2 emission control. Wet scrubbing with a chemical absorbent in water is generally the basis for these processes. In many older operations an alkali such as lime is used, but it carries the problem of generating large amounts of solid waste that have to be disposed, implying in a significant disposal cost.
The main SO2 absorption systems are: Ca(OH)2; NaOH; CaCO3; NH3; MgCO3; ZnO; and H2O2. These systems are classified as either throwaway or marketable depending whether the waste stream generated in the process will require disposal, or it contains a potentially valuable product such as sulphuric acid. Up to 1990 in the USA, throwaway systems were the most widely used techniques for large scale SOx control. These technologies included wet scrubbing using for example wet lime or limestone, sodium alkali or seawater, and dry scrubbing, where a lime slurry contacts the fluegas in a spray dryer (1).
Nowadays, the scenario has changed to favouring the adoption of techniques that produce recyclable products and do not generate solid wastes in view of the rising costs associated with waste disposal. In that respect, one alternative that complies very well with these conditions is scrubbing with Hydrogen Peroxide (H2O2).
A comparison recently published by Takayama et all (2) made for the selection of a SO2 scrubbing process for a zinc plant, on a first screening based on the criteria of recyclable product and no solid waste, indicated scrubbing towers with H2O2, NaOH, or ZnO calcine. The final evaluation between these alternatives indicated the selection of the Hydrogen Peroxide system, which was based on the analyses synthesized in Table 1:
Table 1: Comparison between SO2 scrubbing systems:
| H2O2 | NaOH | ZNO calcine |
| Capital Expenditure | high | medium/high | high |
| Operating costs | high | high | low |
| Technical risks | low | low/medium | low/medium |
| Capability of yielding final levels od SO2 very low | high | medium | medium |
Hydrogen Peroxide
Hydrogen Peroxide (H2O2) is a very powerful non-contaminating liquid oxidant employed in several mining and metallurgical plants around the world, for leaching, solution purification, treatment of effluents and residues. It is commercially available in 50 L drums, 1 m3 plastic-containers, and iso-tanks in concentrations up to 70% H2O2.
Hydrogen Peroxide can be employed directly in concentrated or diluted forms. It can also be used to generate highly oxidizing OH free radicals, by photo-activation with UV radiation, or by reaction with FeSUp>2+ / Fe3+ ions (Fenton system), and it can also be converted into Caro s Acid (H2SO5) - more efficient for oxidation in slurries.
Treatment of SO2 Emissions with H2O2 Scrubbing
Gases containing SO2 and SO3 are generated in metal sulphide roasting, smelting and conversion. And although most plants convert these gases into marketable sulphuric acid, it is possible to still have final emissions of SO2 above legal limits usually in the order of 100 ppm V.
Hydrogen peroxide is being used industrially to treat these emissions by oxidation, converting the gas into H2SO4, leaving no residues behind. The oxidative absorption reaction is very fast and highly efficient:
SO2 (g) + H2O2 (aq) -> H2SO4 (aq)
Figure 1 shows a tabulation of thermodynamic data for this reaction between 20 and 100 oC demonstrating its very high efficiency in this temperature range.
Go to Figure 1 (Thermodynamic data for SO2 scrubbing with H2O2.)
Advantages of the Hydrogen Peroxide Scrubbing Process
- The reaction product is diluted H2SO4 (in concentrations up to 50%), that may be recirculated to the absorption tower in the main sulphuric acid plant, and/or alternatively be recycled in the metal extraction circuit.
- The fact that the reaction is extremely efficient and fast allows the obtention of very low final levels of residual SO2 in the treated gas (2 ppm).
- Only a small excess (15%) of H2O2 is required for quantitative conversion of SO2 to H2SO4.
Case History
Several plants around the world employ H2O2 for SO2 scrubbing. The example reported below concerns a plant in Italy that had emissions of HCl and SO2 vapours in an air stream and had a H2O2 scrubbing system installed after design by P. V. Deo (of Solvay Chemicals) (3) who provided the following information transcribed from his report:
The contaminated air is first scrubbed with water to remove HCl vapours. It is then scrubbed in another scrubber (a packed column) using aqueous hydrogen peroxide solution, after which is finally discharged to the atmosphere. The use of H2O2 enables the removal of SO2 in a relatively small scrubber and the recovery of SO2 as H2SO4. The relevant data on the scrubber is given below:
| Gas rate | 5000 m3/h |
| Liquid rate | 30 m3/h |
| Inlet gas temperature | 50 °C /h |
| Exit liquid temperature | 40 °C |
| Packed column | 0.94 m dia. X 4.2 packed height (5.7 m tall) |
| Packing | 2 polypropylene Pall rings |
| Pressure drop across the column | 14 cm of water |
| SO2 inlet concentration | 0.08 to 2.1% (v/v) |
| SO2 exit concentration | 5 mg/m3 (~2 ppm v) |
| Concentration of H2 SO4 | 24% (w/w) |
| H2O2 usage | 0.609 kg of H2O2/kg of SO2O2 |
The scrubbing system was able to clean the contaminated air to a very low level of SO2 (2 ppm v) with only a small excess (15 %) of H2O2 above stoichiometry.
The simplified gas scrubbing flow sheet for this operation is shown in Figure 2.
Go to Figure 2 (Scrubbing system for removal of HCl and SO2 from a contaminated air stream)
Conclusion
Wet scrubbing of SO2 emissions with Hydrogen Peroxide is a clean, fast and highly efficient process, that can be operated in simple scrubbing columns.
The main chemical cost is limited by a just above stoichiometric consumption of hydrogen peroxide.
References
1) R. MCINNES, R. VAN ROYEN, Desulfurizing Fluegases, Chemical Engineering, September 1990, 124-127
2) T. TAKAYAMA et all, Preparacao para a expansao na refinaria de zinco eletrólitico da CPM, VI Southern Hemisphere Meeting of Mineral Technology, Rio de Janeiro, 2001
3) P.V. DEO, Case history Removal of SO2 from Gaseous Effluent Using Hydrogen Peroxide, Solvay Interox Technical Information, 1992
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