Circular economy meets the drawdown economy: Enhanced weathering of industrial solid waste as a win-win solution

Negative emission technologies (NETs) can contribute to mitigating climate change. Enhanced weathering (EW) is a NET with considerable carbon sequestration potential. It is based on accelerated chemical reactions between alkaline minerals and water and carbon dioxide to generate bicarbonates in runoff that flows into the ocean to achieve long-term carbon sequestration. EW also presents a potential valorisation pathway for the use of industrial residues. Therefore, we selected non-hazardous industrial wastes (NHIWs) as the alkaline mineral of EW. The processes for implementing EW (crushing, transporting, and spreading NHIWs) will produce additional CO₂ emissions and require energy input, and these values may vary in the life cycle of the specific facilities. To address this gap, we developed a multi-period mixed integer nonlinear programming (MINLP) model to optimize NHIWs flows between sources and sinks over the planning horizon, while considering the annual CO₂ sequestration target for achieving regional carbon drawdown. Our results show that the unit net CO₂ sequestration cost for removing 1% and 1.5% annual carbon emissions of Shandong province is $273 and $378 /t CO₂, respectively. It is estimated that from 2020 to 2030, the maximum CO₂ sequestration of NHIWs produced by Shandong province will be 305.78 Mt CO₂. The developed model can also be applied to other regions and countries as guidance for EW deployment and support for waste utilization towards circular economy.

High-silicon iron ore tailing is typically regarded as abundant mining waste of little use; it occupies a vast area of land and is harmful to human health and the ecological environment. Nevertheless, high-silicon iron ore tailing contains abundant quartz resource. In this study, a superconducting high gradient magnetic separation (S-HGMS) coupling fluorine-free mixed acid leaching technology was used to prepare high-purity quartz from high-silicon iron ore tailing. The S-HGMS technology was applied to separate and extract quartz from high-silicon iron ore tailing. Subsequently, the obtained quartz concentrate was subjected to an acid mixture leaching procedure to extract its impurities and obtain high-purity quartz. The optimal conditions determined for the magnetic separation process were a solid concentration of 4% and slurry flow velocity of 0.12 m/s. Under these conditions, the SiO₂ grade of the quartz concentrate reached 98.56 ± 0.13%, and the SiO₂ recovery was 60.59 ± 0.13%. The optimal conditions of the leaching process were a solid–liquid ratio of 1:4, leaching temperature of 80 °C, reaction time of 10 h, and acid mixture of HNO₃, HCl, and H₂SO₄ (molarity ratio of 1:4:1). Under these conditions, the SiO₂ purity reached a maximum value of 99.92 ± 0.01% in the high-purity quartz. Furthermore, we proposed a process for evaluating the recovery potential of Fe from iron-rich substances and recycling the leaching solution. The applied technology achieved a comprehensive utilization of high-silicon iron ore tailing and no waste discharge, meanwhile provides a theoretical basis and data support for its industrialization.

Enhanced weathering of alkaline rocks and minerals is a negative emissions technology (NET) that is potentially scalable to deliver gigaton-level carbon dioxide removal (CDR) for climate change mitigation. This technique relies on the acceleration of naturally occurring weathering reactions with water and carbon dioxide by reducing these substances into a fine powder with high specific surface area. The ex situ enhanced weathering process chain consists of the acquisition of suitable natural or synthetic materials, grinding to a fine particle size, transportation, and application on suitable sites. Future enhanced weathering systems can be envisioned as supply chain-like networks which have to be optimized to deliver maximum CDR given physical constraints. There is a notable research gap in the literature on decision support for such systems. To address this gap, a mixed integer linear programming (MILP) model is developed in this work to optimize enhanced weathering networks for CDR. The model also incorporates the availability of multiple transportation options and constraints on network topology. Two test cases are used to demonstrate the model capability to determine optimal and near-optimal networks. The top ten solutions in these two scenarios yield total CDR levels of 3.4316–3.4363 Mt and 15.27017–15.27960 Mt.