Primetals Technologies’ new By-Product Leaching plant has now begun full operation. It was designed to treat the dust from off-gas cleaning systems such as the sinter-plant-oriented MEROS system. By combining these two environmental solutions—MEROS and By-Product Leaching—Primetals Technologies has developed a method to recycle valuable materials for reuse in sintering and pelletizing plants. This innovative setup also reduces carbon emissions and landfill use without sacrificing efficiency.
Adapting to Current Challenges
Countries worldwide have continued to develop and implement various measures and legislation to reduce disposable waste and encourage the reuse of waste material. Following such guidelines, multiple industries, such as the iron and steel industry, use desulfurization (DeSOx) plants, such as MEROS (Maximized Emissions of Sintering), to reduce waste gas emissions. These emission-reduction plants effectively remove harmful substances from the off-gas, capturing sulfur dioxide, dust, and 90 to 99% of the dioxins, furans, and heavy metals, which are separated and collected as residue. However, this innovation produces another type of waste: residual dust, which contains high concentrations of heavy metals, naturally occurring harmful compounds, and other toxic substances. Manufacturers must then dispose of the dust from the desulfurization process.
One standard disposal method is sending the dust to a landfill site. However, according to environmental guidelines and based on the dust’s chemical composition, the dust is classified as “toxic waste.” Disposal in landfills is costly and limited to ever-shrinking hazardous-waste areas. Additionally, the dust must also meet specific requirements before being deposited in a landfill.
One such requirement is the stabilization and solidification of the dust with cement to render it immobile. Solidification ensures that rain or high air humidity cannot circulate harmful substances from the dust, polluting the surrounding soil and water. However, soluble salts in the dust can also affect the long-term stability of the cement block and compromise the containment of heavy metals, making this an ineffective long-term solution.
Concrete solidification also requires a high cement-to-dust ratio, which reduces the ability to reuse the dust in the future. Using cement also means additional CO2 emissions, producing between 200 and 550 kilograms of CO2 per ton of cement. Concrete solidification and disposal in landfills are the methods used for iron manufacturing to date.
From Theory to Innovation
Several scientific papers investigate the possibility of reusing dust from sintering or pelletizing plants. These papers conclude that direct use of dust is problematic due to the high alkaline concentration. The dust from a dry desulfurization plant contains approximately 44% sulfate, 13% chloride, 24% sodium, and 11% potassium. The composition of the dust from a dry desulfurization plant using sodium bicarbonate—i.e., MEROS—, depending on the desulfurization rate, is approximately 67 to 77% sodium sulfate, 3 to 8% sodium chloride, and 20 to 30% suspended solids.
The alkalis—sodium and potassium—have a particularly negative effect on blast furnace performance. These elements decrease the efficiency of the blast furnace’s reduction process, locking the coke’s incineration and building up scaffolds. Thus, the dust requires treatment based on its chemical composition before reuse in sinter or pelletizing plants. Additionally, the type of agent used in the desulfurization process determines how the residue, or dust, must be processed after collection. When using sodium bicarbonate, the agent successfully used by the MEROS system, it is important to note that the by-product from the MEROS plant cannot typically be recycled or reused in other applications, primarily due to its high salt and alkaline concentrations.
By implementing a leaching plant after the desulfurization plant, the leaching process separates harmful materials, such as heavy metals and naturally occurring compounds, from the sinter dust. This innovation allows for more than 90% of the dust to be treated in an environmentally beneficial way, reutilizing up to 30% of materials previously deposited as residuals, recycling up to 70% of the dust, which is dissolved in water, used in the treatment process, and reducing deposits in landfills to just 10% of the residual dust.
Dust discharge is reduced by as much as 90% with a By-Product Leaching plant implemented.
The Leaching Plant Step by Step
In the first step, the dust from a desulfurization plant is thoroughly mixed with water in the leaching tank. The resulting solution consists of suspended solids, including insoluble particles from the sinter dust—for example, activated carbon—and dissolved salts from the dust and the desulfurization agent, as well as particles already present in the water added during this step.
The flow chart shows the sinter and pelletizing plants, MEROS, and By-Product Leaching plant moving from left to right.
Once the mixing is complete, the solution is filtered. The main task of the filtering system is the separation of particulate matter from leaching water. The filtered particulate will be washed, dried, and discharged back to the sinter or pelletizing plants. Depending on the desulfurization rate and the efficiency of the electrostatic precipitators, 20 to 30 percent of the undissolved materials from the dust, for example, iron oxide (Fe2O3) and activated carbon, can be reutilized. Following the filtration process, chemical treatment steps, like heavy metal removal, extract harmful substances from the solution. Due to the high concentration of toxic substances, approximately 10 percent of the dust is deposited to a specific landfill in the form of sludge, which results from the chemical cleaning process.
Finally, and in accordance with environmental regulations, the chemically treated water is filtered again with a sand filter and then polished with an activated-carbon filter before being discharged to a wastewater treatment plant or the sea, depending on local conditions and regional emission standards.
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The End Results
Responding to a worldwide trend in waste material handling, Primetals Technologies has developed a unique leaching plant that reduces waste material deposits in landfills to just 10% of the residual dust and reduces additional carbon emissions by eliminating cement use. Used in conjunction with MEROS, Primetals Technologies provides a tailor-made treatment system to recover raw materials, provide environmentally friendly wastewater disposal, and reduce harmful waste material to a small fraction.
The compact layout and modular system enable a tailor-made design tailored to the customer's needs. By implementing a leaching plant after the desulfurization plant, customers will see a considerable reduction in long-term operational expenditure (OPEX), especially as landfill disposal costs increase as landfill space decreases year after year. Not only does the By-Product Leaching system satisfy the fulfillment of Best Available Techniques requirements, but it also decreases overall CO2 emissions and reduces a steel plant’s overall carbon footprint.
Introducing By-Product Leaching
Introducing a By-Product Leaching plant in addition to gas-cleaning systems such as MEROS yields several benefits:
- By-Product Leaching takes care of large quantities of dust containing heavy metals, naturally occurring harmful components, and other toxins, concentrated to a fraction of the original residue.
- Designed for the comprehensive recovery and utilization of by-products from dry-dedusting systems
- Tailor-made design and compact layout based on a modular system
- Recovery of up to 90 percent of the by-product, which can then be reused and recycled
- Fulfills “Best Available Technology” requirements
- Adjustable to meet individual legal requirements
- By-Product Leaching plants make a substantial contribution to the circular economy.
