An eco-friendly approach for NaCl recovery from organic pollutants-containing waste salt by roasting together with low-grade pyrolusite
Graphical abstract
Introduction
Energy consumption has shown exponential growth in recent years, with the rapid development of the world, the increase of which is related to the economic development of developing countries and world population growth (Li et al., 2021b). A large amount of complex solid waste is inevitably produced during the rapid development of the world, of which waste salt is usually filled or deposited without treatment because of containing the large amount of organic matter, causing serious environmental pollution and a waste of salt resource (Alshehrei and Ameen, 2021, Meena et al., 2019, Ostendorf et al., 2006, Kang et al., 2021). Waste salt is generated in many chemical industries during production processes such as dyes, pharmaceuticals, fine chemicals, sewage treatment industries. In recent years, the amount of waste salt produced has increased. According to statistics, in 2015, the amount of waste salt produced in China’s chemical industry was about 3 million tons, and the exponentially increasing salt-containing hazardous solid waste would be the biggest bottleneck for the company’s next development (Fakoya, 2020, Wang et al., 2021, Zhao et al., 2021). As the country pays more and more attention to environmental protection, the harmless disposal and resource utilization of the organic pollutants-containing waste salt will be a trend in the future.
For the treatment of organic pollutants-containing waste salt, in addition to the landfilled method, torrefaction is a common method because it is efficient to decompose organic compounds under high-temperature (Giro-Paloma et al., 2019, Abdul Samad et al., 2017, Hu et al., 2021, Chen et al., 2020, Grosso et al., 2011, Ren et al., 2021, Kajiwara et al., 2021). However, the toxic and harmful gasses emitted during the torrefaction process pose a great threat to the environment (Liang et al., 2021, Li et al., 2021c, Shen et al., 2021, Di Maria et al., 2021, Ju et al., 2021). In a world with finite resources, waste or residues, including organic waste such as waste salt, must be considered as sources of secondary raw materials, and solid wastes containing organic matter are now regarded as readily available, widely distributed, and flexible renewable resources (Alibardi et al., 2020, Ma et al., 2018). So, based on the existing resource and environmental problems, there is no doubt that the effective and safe disposal of waste salt has become an urgent environmental problem to be solved. Fortunately, due to the high solubility of salt, this type of organic pollutants-containing waste salt can be dissolved in water, so other treatment methods such as coagulation and flocculation (Khatri et al., 2015), membrane separation technology (Zhao et al., 2019), electrodialysis (Pisarska et al., 2017), electrolysis (Lefebvre and Moletta, 2006) and other methods are developed to treat wastewater containing organic matter. However, these methods are usually subject to great development restrictions due to their high cost and poor performance. Consequently, it was imperative to develop a practical and environmentally friendly method that involves the efficient removal of organic pollutants as well as the recovery of salt resources from waste salt (Li et al., 2020d).
The core problem for recycling of raw waste salt is to remove the organic matter. High-temperature pyrolysis is considered to be a potential method for the treatment of solid waste containing organic matter, because it can quickly remove organic matter while generating a large amount of energy gasses such as CO, CH4 and (Moško et al., 2021, Luo et al., 2020). These gasses have strong reducibility, so they are widely used in industry. Research had shown that, with the increase of temperature, the smaller the gas molecules produced by the decomposition of organic matter, the higher the efficiency of the generated energy gas, and the content of and CO increased significantly (He et al., 2010). It is well known that pyrolusite is stable under acidic or alkaline conditions, but due to the formation of acid-soluble manganese oxides, manganese can be acidic extracted under reducing conditions (Zhao et al., 2010). At present, the main reduction technology for pyrolusite is roasting reduction, which can be achieved using coal-based reducer or organic materials such as sawdust, cellulose, walnut shell, sugarcane, banyan-leaves, and tealeaves (Sinha and Purcell, 2019, Li et al., 2020c, Li et al., 2019a, Li et al., 2020b, Li et al., 2019c). The core of these methods is to use the reducing gasses produced by the pyrolysis of organic matter at high-temperature (Li et al., 2021a, Li et al., 2019b, Li et al., 2020a). Therefore, based on the properties of waste salt and pyrolusite, they can be mixed and roasted together to realize the co-recovery of sodium chloride and manganese.
This paper taking the waste salt and low-grade pyrolusite ore as the research object. In order to purify and recycle the NaCl in waste salt and use the energy gasses produced by pyrolysis of organic matter, an eco-friendly approach for NaCl recovery from organic pollutants-containing waste salt by roasting together with low-grade pyrolusite ore was presented. Under different temperatures and time, the removal efficiency of organic matter in waste salt and the composition of pyrolysis gasses were explored. The dual effects of roasting to remove organic pollutants from the waste salt and increase manganese leaching efficiency were systematically investigated, and the roasting mechanism and process were explored. The roasting coupling process not only realizes the purification of the raw waste salt and decreases the emission of harmful gasses during the high-temperature pyrolysis of waste salt, but also recovers manganese from low-grade pyrolusite and bring good economic benefits. Thus, the cleaner and efficient production was achieved.
Section snippets
Materials
The waste salt and low-grade pyrolusite ore in Hunan were obtained as the research object in this study. Both of the samples were crushed, ground, dried, and characterized by X-ray fluorescence (XRF) and X-ray diffraction (XRD). The results of multi-element analysis and the XRD pattern of the waste salt are shown in Table 1 and Fig. 1, and the results of multi-element analysis and the XRD pattern of the low-grade pyrolusite ore are shown in Table 2 and Fig. 2.
It can be seen from Table 1 and
Characteristics of the raw waste salt
The raw waste salt, which is dark black with a moisture content of 1.29wt% and a total organic matter content of 16.45wt%, comes from a Pesticide Chemical Co., Ltd. in Hunan Province, China, which operates the production and sales of carbamate products, phosgene products, hydrochloric acid, pesticides and pesticide processing reagents, etc. For analyzing the existing state of the organic pollutants, SEM-EDS element mappings were conducted. As shown in Fig. 4-b, the shape of the raw waste salt
Conclusions
In this study, the feasibility of an eco-friendly approach method for NaCl recovery from organic pollutants-containing waste salt by roasting together with low-grade pyrolusite was demonstrated. The primary conclusions are summarized as follows
- (1)
Raw waste salt contained a large amount of organic matter, mainly long-chain hydrocarbons, aldehydes, benzene rings, heterocyclic compounds, pyridine, and aromatic nitro organics. In an oxygen-isolated and 700 °C environment, the organic matter in the
CRediT authorship contribution statement
Jinrong Ju: Investigation, Data curation, Writing - original draft, Writing - review & editing. Yali Feng: Conceptualization, Supervision. Haoran Li: Methodology, Supervision. Xin Li: Software. Qian Zhang: Software. Chenglong Xu: Validation. Shunliang Liu: Validation.
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.
Acknowledgments
We gratefully acknowledge the financial support from long term special project for National, China (Project No. JS-KTHT-2019-01 and DY135-B2-15) and the key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China . Help from the Analytical & Testing Center of University of Science and Technology Beijing is also greatly appreciated.
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