Thermochemical destruction of asbestos-containing roofing slate and the feasibility of using recycled waste sulfuric acid
Introduction
Asbestos and asbestos-containing materials have been widely used in many applications, such as insulators, asbestos cement and fireproof construction materials, because of their low thermal conductivity and high mechanical strength. However, asbestos is known to be extremely carcinogenic, especially in causing a severe asbestosis, lung cancer and pleural mesothelioma when the respiratory system is exposed to it. As a result, nowadays, in most countries the mining, refinement and use of asbestos have been banned, apart from some exceptional applications.
From a toxicological point of view on asbestos, although there have been many studies by toxicologists and clinical research scientists to elucidate the clear mechanisms that cause severe toxicities, little is known about the crucial processes at the cellular/molecular levels, and still such studies leave unanswered which chemical or physical properties of asbestos are key factors in disease causation. Chemical reasons are explained with mineralogical compositions of asbestos. For instance, magnesium from chrysotile or iron from iron-containing asbestos (e.g., amosite or crocidolite) may be leached intracellularly, thereby inducing toxicity of the fibers [1], [2], or causing cytotoxicity through generating a highly reactive species (e.g., a hydroxyl radical or reactive oxygen) [3]. Pathogenic mechanisms by the physical properties of asbestos are related to the size and length of the fibers, morphology (i.e., asbestiform or non-asbestiform), etc. [4]. Thus, if any treatment technique can achieve the “detoxification” of asbestos through both chemical (i.e., decomposition) and physical destruction, it should be the target goal of treatment, regardless of which property would play an important role in inducing toxicity to life.
In South Korea, the most common use of asbestos occurred in the 1960–70s by the government-led project “New Village Movement”. As a part of the project, straw roofs in rural areas had been replaced with asbestos-cement roofing slates, and most of them are now obsolete and are being removing according to the Waste Asbestos Management Plan (2012–2021). According to the Korea Waste Statistics 2009, it was reported that 16 million tonnes of ACWs would be cumulatively generated by 2016. Currently, the Waste Management Law of South Korea classifies ACWs containing greater than 1 wt.% of asbestos as “Specific Hazardous Waste”, which must be disposed of only by controlled landfill in accordance with safety regulations. Regarding interim treatments before landfill, few thermal or mechanochemical techniques such as high-temperature incineration, or solidification with cement are legally permitted [5]. However, the confining and displacement of ACWs in landfill do not essentially destroy asbestos, and yet provide a load to landfill sites for several decades or for a permanent period, especially in a small country like South Korea. Thermal treatment involves the conversion of asbestos into non-asbestiform materials by melting the compositional elements at temperatures ranging from ca. 800 to 1400 °C or above [6], [7]. Similarly, microwave heating applies high power to inert asbestos materials [8]. Despite the efforts to lower operating costs, thermal treatments or microwave application are still energy-intensive and cost-demanding. Therefore, new or different treatment techniques need to be considered for significant amounts of ACWs. One approach, which was studied, is a chemical method using caustic acids [9], [10] or alkalis to transform the crystalline structures to noncrystalline forms or to decompose the constituents of asbestos. In fact, such chemical treatment is not a discovery, but a novel technology which has been applied a great deal in various media [10], [11], [12], [13], [14], [15], [16]. Of the acidic agents, sulfuric acid is known to be the most effective attacking chemical and this was also experimentally observed in our previous tests [17]. Therefore, in this research, sulfuric acid (H2SO4)-based chemical dissolution was used and in order to enhance or complete the breakage of chemical bonds thermal process (i.e., heating) was incorporated to the process of chemical digestion. Furthermore, sulfuric acid, one of the most widely applied chemicals in many industrial processes, accounts for about 20% (89,000 tonnes discharged in 2009) of the entire waste acids discharged annually in South Korea, so the reuse or recycling of waste sulfuric acid is strongly desirable [9].
Therefore, the main objective of this research is to evaluate the treatability of asbestos-containing roofing slate waste using low-temperature thermochemical treatment and to demonstrate the feasibility of reusing waste sulfuric acid as a replacement for a commercial acid. Concentration of acid and heating temperature applied in this study were chosen mainly for practical reasons such as estimated costs per tonne of ACWs, comparable to other treatment methods [18]. The authors expect that the results shown in this research would give an insight to future guidelines with regard to the disposal of asbestos-containing wastes.
Section snippets
Asbestos materials
Asbestos-containing (roofing) slate wastes (hereafter, ACS) were taken from a government-registered hazardous waste treating company. Waste materials were shipped polyethylene double-packed to a HEPA filtration equipped laboratory, and were briefly rinsed with tap water in order to remove the attached dirt such as soil and moss. After drying them at room temperature, the slate samples were ground with a blade mill for 3 min and sieved to get a particle size below 0.5 mm. The presence of
Characterization of untreated asbestos samples
Table 1 shows the chemical compositions of natural chrysotile asbestos and an ACS sample measured by X-ray fluorescence. Raw chrysotile is mostly composed of about 42 wt.% MgO and 36 wt.% SiO2 together with 4 wt.% Fe2O3 and 1 wt.% Al2O3 which may be contained due to replacements of Mg and Si, respectively as reported in the literature [19]. Since ACS materials are mixtures of asbestos with cement, their chemical matrices are similar to ordinary Portland cement (OPC), showing about 40 wt.% CaO and 19
Conclusions
This study has demonstrated that chrysotile asbestos existing in both a pure form and a mixed form with cement components can be destroyed by chemical treatment incorporated with a low-temperature heating process (i.e., thermochemical treatment). By applying a 10–24-h thermochemical treatment using 5 N H2SO4 heated at 100 °C, raw chrysotile fibers lost their chemical compositions and asbestiform structure. Determination of an activation energy using a rate constant indicates that chemical
References (26)
- et al.
The health effects of chrysotile: current perspective based upon recent data
Regulatory Toxicology and Pharmacology
(2006) - et al.
Change of carcinogenic chrysotile fibers in the asbestos cement (eternit) to harmless waste by artificial carbonization: petrological and technological results
Journal of Hazardous Materials
(2013) - et al.
The destruction of chrysotile asbestos using waste acids
Waste Management and Research
(1986) - et al.
Hydrothermal conversion of chrysotile asbestos using near supercritical conditions
Journal of Hazardous Materials
(2010) - et al.
Microwave thermal inertisation of asbestos containing waste and its recycling in traditional ceramics
Journal of Hazardous Materials
(2006) - et al.
Evaluation of reaction variables in the dissolution of serpentine for mineral carbonation
Fuel
(2007) Olivine dissolution in sulphuric acid at elevated temperatures-implications for the olivine process, an alternative waste acid neutralizing process
Journal of Geochemical Exploration
(1998)An update on the detoxification processes for silica particles and asbestos fibers: successes and limitations
Journal of Toxicology and Environmental Health: Part B
(2005)- et al.
Role of iron in the reactivity of mineral fibers
Toxicology Letters
(1995) - et al.
Free radical activity of natural and heat treated amphibole asbestos
Journal of Inorganic Biochemistry
(2001)
Study on the thermal decomposition of chrysotile asbestos
Journal of Thermal Analytical Calorimetry
Microwave thermal inertisation of asbestos containing waste and its recycling in traditional ceramics
Journal of Hazardous Materials
Cited by (36)
Sustainable management of hazardous asbestos-containing materials: Containment, stabilization and inertization
2023, Science of the Total EnvironmentA combined system for asbestos-cement waste degradation by dark fermentation and resulting supernatant valorization in anaerobic digestion
2022, ChemosphereCitation Excerpt :Apart from some patented processes tested on real or pilot scale (Table 1S), such treatments are currently little-used due to the high consumption of energy and reagents (Spasiano and Pirozzi, 2017). As an example, a hydrothermal process based on the use of sulfuric acid at 100 °C for 24 h was successfully tested on laboratory scale for the denaturation of chrysotile (Mg3Si2O5(OH)4) contained in an asbestos-cement waste (Nam et al., 2014). However, this process required a large amount of sulfuric acid to dissolve the other constituents of the hardened cement paste, such as calcium-based compounds.
Biological treatment of asbestos cement wastes by Aspergillus niger and Acidithiobacillus thiooxidans
2022, Applied Clay ScienceAsbestos waste recycling using the microwave technique – Benefits and risks
2021, Environmental Nanotechnology, Monitoring and ManagementProduction of vitrified material from hazardous asbestos-cement waste and CRT glass cullet
2021, Journal of Cleaner ProductionChanges in concentrations and characteristics of asbestos fibers dispersed from corrugated asbestos cement sheets due to stabilizer treatment
2021, Journal of Environmental Management