Comparative study of acid mine drainage neutralization by calcium hydroxide and concrete sludge–derived material
Introduction
Acid mine drainage (AMD) is acidic effluent from operating and abandoned mines, and often includes toxic elements. AMD can be neutralized by limestone (CaCO3) or calcium hydroxide (Ca(OH)2), and toxic elements are removed as hydroxides or other chemical forms to avoid environmental impacts. However, the cost of treatment is often a heavy burden for mining companies or municipalities that operate the AMD treatment processes. Furthermore, generation of AMD can continue for tens of years or longer. Decreasing the cost of AMD treatment is an urgent issue, and one method to decrease the cost is by substituting neutralizing materials with alkaline wastes. Neutralization of AMD with a variety of wastes, such as paper mill waste (Pérez-López et al., 2010), red mud and its related materials (Couperthwaite et al., 2013, Douglas et al., 2010, Doye and Duchesne, 2005, Doye and Duchesne, 2003, Kaur et al., 2018, Paradis et al., 2006, Tuazon and Corder, 2008, Zijlstra et al., 2010), cement kiln dust (Doye and Duchesne, 2005, Doye and Duchesne, 2003, Mackie and Walsh, 2015, Paradis et al., 2006), and coal fly ash (Gitari et al., 2018, Gitari et al., 2013, Gitari et al., 2010, Gitari et al., 2008b, Gitari et al., 2008a, Madzivire et al., 2019, Madzivire et al., 2010, Madzivire et al., 2011, Madzivire et al., 2014) have been examined. The use of alkaline wastes can reduce carbon dioxide (CO2) emissions from AMD neutralization because conventional neutralization agents emit CO2 during their production or during AMD neutralization; i.e. Ca(OH)2 releases CO2 during production from CaCO3, and CaCO3 generates CO2 during AMD neutralization.
Concrete sludge is alkaline waste consisting of cement, aggregates, and water. The use of fresh concrete generates concrete sludge as a waste product. Approximately 1 %–2% of all fresh concrete prepared for construction is disposed of as concrete sludge (Iizuka et al., 2017). Based on worldwide cement production of 4.1 billion metric tonnes in 2020 (US Geological Survey, 2021), approximately, 4.1 million tonnes of produced cement is disposed of as concrete sludge. In Japan, concrete sludge is generally landfilled or reused as a mixing material for road-bed materials after solid/liquid separation. The separated alkaline solution is neutralized with an acid, such as sulfuric or hydrochloric acid. However, producers of concrete poles and piles indicate that this process is expensive, costing approximately 50–100 USD/t (5000–10,000 JPY/t) (Iizuka et al., 2017). Therefore, more effective and inexpensive processes for waste concrete recycling are required.
Several concrete sludge recycling methods have been proposed, including reusing sludge as a raw material for cement production (Chatveera et al., 2006), as fine aggregates for concrete production, and recycling of sludge water in concrete production. Besides these construction recycling methods, chemical use of concrete sludge is also promising (Ho et al., 2021), such as CO2 fixation and utilization (Iizuka et al., 2012a, Iizuka et al., 2012b, Iizuka et al., 2017) and as a water purification material (Hongo et al., 2014, Iizuka et al., 2012c, Nonaka et al., 2010, Sasaki et al., 2014a, Sasaki et al., 2014b, Sasaki et al., 2012, Sasaki et al., 2011, Tsunashima et al., 2012).
In this study, the performance of the concrete sludge–derived neutralization material was investigated and compared with the performance of calcium hydroxide with actual AMD generated from an abandoned Japanese mine.
A bench scale plant similar to an actual AMD treatment plant was built and operated in the mine site. The process could achieve effective and economic waste treatment and AMD neutralization simultaneously by using the waste as a neutralization material.
Section snippets
Materials
Actual AMD generated from an abandoned Japanese mine was used for neutralization experiments. Neutralization experiments were conducted at the mine site.
Concrete sludge–derived neutralization material (PAdeCS®) was produced using concrete sludge generated during centrifugal production of concrete poles and piles at the East Nippon Concrete Company’s Kawashima-Daini Plant, Chikusei, Ibaraki Prefecture, Japan. The detailed production method of the concrete sludge–derived material is reported
Acid mine drainage neutralization by a pilot-scale plant
Table 1 shows the amounts of the neutralization materials required for the AMD treatment. Final pH values of the treated AMD in the A series (CaCO3 + PAdeCS®) and B series (CaCO3 + Ca(OH)2) were 7.5 ± 0.05 and 7.9 ± 0.2, respectively. In the first neutralization step with CaCO3, 2.79 kg CaCO3 (2.44 kg on the first day and 3.13 kg on the second day) was needed to neutralize 1 m3 AMD to a pH range of 6.5–6.7. In the second neutralization step with PAdeCS (A series) or Ca(OH)2 (B series), 0.29 kg/m
Conclusions
A neutralizing agent was produced from concrete sludge generated during the production of secondary concrete products by centrifugal molding, and the effect on AMD neutralization was evaluated. The effect was compared with that of calcium hydroxide. Nine reaction tanks were temporarily installed and operated to imitate the actual treatment of AMD. The amount of discharged water and sediment generated were evaluated. The amount of PAdeCS required to neutralize the mine wastewater was 1.5 times
CRediT authorship contribution statement
Atsushi Iizuka: Conceptualization, Methodology, Formal analysis, Investigation, Resources, Writing – original draft, Visualization. Hsing-Jung Ho: Formal analysis, Writing – original draft, Visualization. Takeshi Sasaki: Formal analysis, Investigation, Resources, Writing – original draft, Visualization. Hiroyuki Yoshida: Investigation, Resources. Yasuyuki Hayakawa: Conceptualization, Methodology, Investigation, Project administration, Funding acquisition. Akihiro Yamasaki: Investigation,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was supported by the Ministry of Land, Infrastructure, Transport and Tourism of Japan. We thank Tara Penner, MSc, from Edanz (https://jp.edanz.com/ac) for editing a draft of this manuscript.
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