A growing awareness of the potential negative impact of harmful emissions on the environment and public health has put pressure on mineral processing and mining operators. Governments have been introducing increasingly strict legislation to regulate emission levels and reduce the impact of activities, such as mineral processing, on the environment.

One of the main areas of focus for this legislation is the regulation of harmful sulphurous emissions in off- gasses. Regulations have tightened, requiring companies to monitor and reduce their emissions of pollutants. In the United States, the Environmental Protection Agency (EPA) has introduced regulations such as the Clean Air Act, which sets limits on emissions of sulphur dioxide, nitrogen oxides, and particulate matter from industrial sources.

Similarly, in Europe, the Industrial Emissions Directive (IED) sets limits on emissions of pollutants from industrial processes, including flue gas emissions. These, and other similar directives requires companies to use best available techniques to reduce their emissions and to monitor and report their emissions data.

Solutions, however, are available, scalable and ready to implement, so mineral processing can comply with these increasingly strict regulations. For instance, WSA (Wet gas Sulphuric Acid) for sulphur abatement, catalytic filters removing dust, NOX and VOC, and oxidation catalyst DNO/CK are clean air technologies in Topsoe’s portfolio that are already helping in mineral processing.

Topsoe’s WSA technology has been widely adopted in the metallurgical industry and beyond with more than 170 global references from companies it has helped reduce their environmental impact and comply with stringent emissions regulations. Plants operating in the metallurgical industry include platinum, lead and copper sulphide smelters, molybdenum disulphide/zinc sulphide roasters, manganese leaching and lead(II) sulphide sinter plants.

Topsoe told IM: “Many new projects are under construction globally. Capacities of gas flows ranges from 9,000 to 1,000,000 Nm3/h with down to 0.5 vol.% SO2 content and sulphuric acid productions from 20 to 1,140 MTPD. The world’s largest platinum electric furnace Anglo American Platinum in South Africa is amongst these, where Topsoe was selected amongst 40 technology contenders evaluated.”

It adds: “One reason for this success is that implementation could not be easier. Topsoe is experienced in providing an end-to-end solution, including licensing of technology, basic engineering, proprietary equipment, technical services for construction, commissioning and start-up, training, catalyst produced in-house and a knowledgeable, dedicated team.”

Why WSA and not dry-gas units or scrubbers?

WSA is a highly efficient and cost-effective technology for SO2 abatement from industrial emissions utilising only air, cooling water and boiler-feed water with no emissions or effluents. By removing up to 99.99% of sulphur content, WSA ensures compliance with emission standards and legislations. “WSA technology is based on the principle of converting SO2 into commercial-grade sulphuric acid (H2SO4), which can then be used in a variety of industrial applications, opening commercial opportunities. Thus, the process not only reduces emissions of SO2, which is a major contributor to acid rain and air pollution, but also produces a valuable byproduct. Depending on the concentration of sulphur, high-pressure superheated steam is co-generated, furthermore reducing operating cost.”

WSA has several advantages over industrial alternatives for SO2 abatement in the mineral processing area of mining. Industrial alternatives on the market are dry-gas sulphuric acid units and scrubbers. Dry-gas technology includes a catalytic step for converting SO2 to SO3, and an absorption step for absorbing the SO3 in sulphuric acid. Scrubbers injects a liquid into the flue gas, commonly NaOH, H2O2 or Mg(OH)2 in metallurgical industry, which reacts with SO2 and removes it. However, both technologies face critical issues in their application. Dry-gas units have issues when running on lower SO2 contents, and scrubbers are very expensive as they use large amounts of increasingly more expensive chemicals and produce a lot of low/no value waste.

How does WSA work?

WSA is a wet gas sulphuric acid process, which unlike the dry gas process does not require drying of the SO2 off-gas before entering the conversion catalyst. The WSA plant comprises of a SO2 converter utilising two or three beds of VK-WSA catalyst, followed by the proprietary Topsoe WSA Condenser for condensation of sulphuric acid.

In the SO2 converter, SO2 is oxidised to SO3, followed by hydration to sulphuric acid. Both reactions are exothermic, which is utilised by well-designed heat exchange and process control strategies. This allows for the unit to be operated easily at low SO2 concentrations between 0.1-10%v, while maintaining optimal performance. The efficient heat exchange and extra heat recovered from the hydration and condensation processes enable the WSA process to operate at a very low autothermal point, all the way down to 1.8%v.

The autothermal point is the minimum concentration of SO2 required for the process to be self-sustaining, meaning that it can produce enough heat to maintain the necessary temperature without the need for external heating. This means that the WSA process can operate at much lower SO2 concentrations than other sulphuric acid production processes without external heating or fuel gas consumption at steady operation, thus reducing the environmental impact and improving efficiency.

Well-suited for managing the fluctuation of metallurgical SO2 gas

Topsoe’s WSA technology is designed to handle the fluctuations in metallurgical SO2 gas that may arise due to intermittent operation of upstream smelter/roaster units. The WSA unit achieves this by utilising a large amount of direct heat exchange and very fast temperature control. The WSA unit can handle large fluctuations in both the feed gas flow and SO2 concentration automatically, without the need for operator interference. This is because the unit can adjust its process parameters in real-time to maintain optimal conditions for the conversion of SO2 to sulphuric acid.

During start-up or dynamic operation, a support burner or electrical heater can be utilised to maintain converter temperature, ensuring safe operation at zero to full capacity. This feature also helps to reduce the time required for start-up and shutdown, thereby increasing the overall efficiency of the process. Overall, Topsoe’s WSA technology is exceptionally well-suited for managing the fluctuation of metallurgical SO2 gas, providing a reliable and efficient solution for the production of high-quality sulphuric acid.

The sulphuric acid formed in the SO2 converter’s final cooling step is kept above its dew point, to avert the corrosive liquid form. Its corrosive nature sets high demands for the air cooled Topsoe proprietary WSA condenser, which is designed as a falling film condenser with modules of glass tubes and heat and acid resistant bricks.

Key advantages of WSA technology:

• High efficiency – WSA removes up to 99.99% sulpur content for flue gas with contents between 0.1-10%v SO2.
• Generating value – WSA produces commercial grade sulphuric acid, the most utilised chemical in the world.
• Best-in-class energy economy – WSA has superior heat recovery by recovering the heat from hydrolysis and condensation, and with WSA, the wet flue gas is directly treated, handling up to 30% water in the feed, thus eliminating the large energy requirement of drying it for dry-gas units.
• Lower OPEX – WSA has a much lower OPEX than dry-gas units, with only 30% of the cooling water requirement for dry-gas units.
• Robust to fluctuating conditions – an electrical heater or support burner allows for safe, dynamic operation of the WSA plant at zero to full capacity.
• Clean technology – WSA consumes no chemicals and generates no waste streams.