Quarry dust an essential component in the pyrometallurgical recovery of valuable materials from mixed fine-grained waste

Within the FINEST project (https://finest-project.de/) we tackle the use and management of finest particulate anthropogenic material flows in a sustainable circular economy. Subproject 3 "FINEST Disperse Metals" has set itself the goal of safely mixing various fine and ultra-fine-grained material flows that accumulate as waste during various industrial processes and extracting metallic raw materials from them in a subsequent multi-stage pyrometallurgical process, as well as generating a safe slag that can be disposed of without monitoring.

The idea of mixing different fine-grained waste streams arose from the experience that the search for economically and ecologically sustainable forms of recycling for the individual types of waste often failed despite promising approaches. This is usually due to individual parameters of the chemical or phase composition. The mixing we are aiming for allows us to select the technological parameters much more freely and to react to the rapid changes that are typical of waste streams.

We will implement this concept using three different material streams:

• So-called shredder fines from plastics, car and WEEE recycling
• Metallurgical fly ash from non-ferrous metallurgy
• Quarry dust and dust from concrete recycling.

 

The experimental work is preceded by a comprehensive characterization of the material flows in terms of chemical and phase composition, general physical parameters such as particle size distribution, particle shapes and flowability as well as any potential hazards. Based on this, both the mixing behavior and the pyrometallurgical parameters are modeled.

We use quarry dust for two main purposes. Their main contribution is to act as slag formers in the pyrometallurgical processing of the mixtures in the plasma furnace. This should be combined with the lowest possible liquidus temperatures and viscosities of the slag system. We are currently using four different quarry dusts from active quarries in the Free State of Saxony for our investigations (1 granite, 2 granodiorites and 1 amphibolite) as well as material from the concrete recycling of an extensive demolition project in Dresden.

Optimized mixing ratios between the individual quarry dusts and various metallurgical flue dusts were determined as part of the modeling using FactSage^TM.  The behavior during mixing of the material flows was initially investigated theoretically using model materials. The mixing steps were visualized using X-ray computed tomography.