The push towards reducing our global carbon footprint and employing circular material use systems calls for innovative recycling solutions to process varying input materials and taking advantage of new processing routes. Recycling waste electrical and electronic equipment (WEEE) often involves a combination of pyrometallurgical and hydrometallurgical processes. The correct translation of different input materials and metals into the right metallurgical process and equipment can substantially impact how they are recovered as metals rather than being lost in off-gas or slag.
WEEE fume treatment challenges
Using recycled material in a metallurgical process results in a much lower quality of input materials and a wide range of harmful substances in the off-gas. Typical impurities that need to be treated in a fume treatment plant during the WEEE recycling process include sulfur compounds, chlorine compounds, halogen gas compounds, dioxins, CO, NOx, mercury, water, dust, and many more.
In contrast to traditional fume treatment systems comprising only one or two fume treatment steps, fume treatment for WEEE recycling entails combining different cleaning technologies. Each of the furnace process steps and the fumes and pollutants produced is fundamentally different and requires specific technologies for removal. However, the challenge is not only to reduce harmful emissions, but also to protect the fume treatment equipment from the impurities that may damage it, shorten its lifetime, or affect its fume treatment efficiency. Consequently, we must examine different fume treatment combinations to find the most cost-effective and environmentally efficient method. All our cleaning systems are designed to be flexible, consisting of different modules that can be exchanged or defined depending on the planned charge mix.
The result is an all-encompassing fume treatment plant that combines various technologies to minimize environmental impact and run the plant as efficiently as possible. Energy recovery is also a part of fume treatment systems and is becoming more and more relevant due to rising energy costs.
Fume treatment and furnace technologies are interdependent
Our fume treatment plants are designed in tandem with the connected furnace technologies so that specific fume treatment steps can already be performed via efficient furnace control. The furnace process is also partly controlled by the fume treatment plant. For example, the NOx emissions can be reduced through gas cleaning. But they can also be reduced in a prior step by running the furnace in a way that minimizes NOx forming without compromising optimal metallurgical conditions.Removing the limitations between WEEE and the fume treatment plant leads to smoother operation and optimized results in terms of both the product and the related emissions.
Typical gas cleaning steps for fume treatment plants in metal recycling
- Post-combustion chamber: The post-combustion chamber ensures the complete combustion of the off-gas generated in the furnace. The gas temperature in the POC is controlled by injecting combustion air.
- Energy recovery system: Instead of a post-combustion chamber, the fume treatment plant can be combined with an energy recovery system, which generates steam and produces relatively low off-gas volumes.
- Selective non-catalytic reduction unit (SNCR): Selective non-catalytic reduction (SNCR) has been used extensively to attain high NOx emission reductions by injecting a urea solution into the post-combustion chamber.
- Selective catalytic reduction unit (SCR): In some cases, a lower level of NOx output is required, depending on local regulations and the NOx production process already in place. That can be achieved with an SCR unit that works at lower temperatures and reduces NOx emissions to a lower level than the SNCR.
- Quench: The quench works like an evaporative cooler system, where a fine water spray is injected into the off-gas inside the quenching tower and cools down the off-gas.
- Bag filter unit: The filter house is one of the last fume treatment steps, separating any remaining dust. The filter area is chosen according to the required emission limits. A dust content down to 1 mg/Nm³ is achievable with SMS group’s baghouse filter technology.
- Scrubber unit: The scrubber unit absorbs gaseous inorganic components such as sulfur dioxide, hydrogen chloride, chlorine, and hydrogen fluoride.
- Activated carbon / sodium bicarbonate injection: Depending on the local resources and existing facilities, a wet gas treatment solution can sometimes be avoided by injecting materials such as activated carbon or sodium bicarbonate.
- X-Pact® Fume Treatment Assist: Our range of fume treatment technologies is rounded off by the X-Pact® Fume Treatment Assist. It is an intelligent calculation module that optimizes fume treatment quality, thereby reducing energy needs. The module is easy to retrofit, and offers significant advantages for revamp or upgrade projects. Energy cost savings of 21 % for an existing plant were demonstrated in a before-and-after comparison.