Characterizing the unknown contribution of iron industries in atmospheric iron emissions using sensitivity analysis

Highlights

• Coarse iron particles are frequent in process samples and dust falls of the plant.

• Iron PM_2.5 is the main constitution in released particles from the stacks.

• Single/agglomerated sharp-/soft-edged iron particles were found common in micrographs.

• Under-prediction by the model proved the contribution of other iron industries.

• Sensitivity analysis showed the contribution of other industries in atmospheric pollution.

Abstract

Particulate matter (PM) released from industrial emissions could cause health problems, which are strongly associated with the chemical composition of PMs. This study aimed to estimate the unknown contribution of two iron industries using a known iron plant via sensitivity analysis. For this purpose, a comprehensive analysis was carried out on the characteristics of known iron plant, accessible sampling locations between the known plant and other two iron industries (unknown contributors), and in the heart of affected residential areas. A validated dispersion/deposition model was developed based on the gathered pieces of evidence of the known source. Analysis of the known processes and dustfalls classified the shape of known iron particles into seven main categories, including single and agglomerated of both sharp- and soft-edged fine (PM_2.5 – less than 2.5 μm in aerodynamic diameter) and coarse (greater than PM_2.5) iron particles. The high contribution of fine iron particles in the main stacks (more than 78.4%), very low contribution in the process samples (less than 10% in raw feed and around one-third of iron particles before the furnace) and almost insignificant contribution in the in-site dustfalls proved the transfer of fine iron particles to residential areas. However, elevated temperature after rotary Kiln resulted in the formation of agglomerated soft-edged fine/coarse particles. Therefore, PM_2.5 was considered as the main atmospheric iron size fraction during the model development. The highest iron concentration (1.34 μg m⁻³) was observed in the area close to the two iron industries. Although the model showed under-prediction considering only plant sources, sensitivity analysis revealed the contribution of other iron industries in under debate areas. The daily concentration of atmospheric particles measured through this study was within the guidelines proposed by the regulatory bodies, however, the iron contents were found high in some locations highlighting the need for detailed regulatory control.

Introduction

Iron ores, mainly contain iron-rich minerals, such as hematite (Fe₂O₃) and magnetite (Fe₃O₄), are mined and beneficiated to make concentrates. Concentrated minerals need to be further processed to be used directly in most iron-making furnaces, thereby, they are pelletized with binders and additives in iron processing plants. Iron industries just like oil industries (Amoatey et al., 2019) and sewage treatment plants (Baawain et al., 2017 and 2019) are recognized for their atmospheric emission potential (Mohiuddin et al., 2014; Omidvarborna, Baawain, Al-Mamun, et al., 2018). Fine (aerodynamic diameter less than 2.5 μm; PM_2.5) and coarse (aerodynamic diameter between 2.5 and 10 μm; PM_2.5-10) iron particles, contained abundant iron element, are released from the main operations, such as induration, blending, and mechanical (transportation, loading/unloading activities, etc.) processes. For example, a rotary Kiln, as the main part of the induration process could be as dusty as a fine crushing process, because the iron pellets are relatively weak and they abrade as they tumble (Jonsson et al., 2013).

The generation of these fine and coarse particles has significant effects on human health and the environment (Amoatey et al., 2018 and 2020). Iron particles could be contaminated with the iron ore impurities, such as Al, Si, Ca, and Mg and priority toxic heavy metals, such as Cr (VI), Cd, Ni and Pb (Bollati et al., 2010; Hu et al., 2006; Lau et al., 2016; Machemer, 2004). Due to high density of coarse iron particles (5.3 g cm⁻³), only fine iron particles can be transferred and routed to long distances, generally where residential areas are located (Omidvarborna, Baawain, Al-Mamun, et al., 2018). Hence, proper management plans have to be taken into consideration for sustainable development and to protect the surrounding environment near iron industries.

Literature review revealed that the physicochemical properties of atmospheric iron articles are related to feeding and processes of nearby iron industries (Galvão et al., 2018; Garimella & Deo, 2008; Querol et al., 2004). Previous studies reported that iron particles in the surrounding atmosphere of iron industries are among the major fractions of PM_2.5 and PM₁₀ emissions (Chang et al., 2005; Dai et al., 2015; Prati et al., 2000). Pope (1996) described how the closure of a steel mill resulted in an overall decrease in the concentration of associated Fe, Cu and Zn content particles. In addition, the Fe-bearing particles in total suspended particles and PM₁₀ were found in higher concentrations nearby steel plants (Choël et al., 2007; Ledoux et al., 2006). Mohiuddin et al. (2014) found that PM_2.5 and PM₁ fractions in the PM₁₀ particles, ranged from 35 to 62% and 20–46%, respectively, indicating the contribution of fine iron particles at the sampling sites. The iron emission of iron-based industries is complex because (Almeida et al., 2015): i) the major iron industries are located in very close proximity to each other making it difficult to distinguish between the iron emission sources; ii) there are different iron emission sources that produce a high amount of particles with various characteristics; and iii) the situation to distinguish the right source is complicated because some processes operate continuously, while others are not. Additionally, the influence of other external sources like road traffic, ships in case of ports, and resuspension of previously deposited particles make the case even more complicated.

Some studies predicted the dispersion of iron particles via air quality models due to the importance of iron concentration levels in the nearby iron industries. Singh and Perwez (2015) modeled total suspended PMs and PM₁₀ concentration levels using the Industrial Source Complex and U.S. Environmental Protection Agency (EPA) Regulatory Model (ISC-AERMOD) for mining complexes. The study revealed that with the increasing distance from the site, the concentration decreases exponentially. This finding might be attributed to the high specific gravity of the ore, meteorological status and undulating terrain, which assists the dry deposition process. Omidvarborna, Baawain, Al-Mamun, et al. (2018) studied dispersion and deposition of atmospheric iron particles released from the processing plant in both industrial and residential areas. This is the first study in the region aimed to develop a simple model based on the available dataset to identify the possible sources of atmospheric iron particles in some residential areas. To do so, the study included PM₁₀ as the only size of all particles from the plant, however, the contribution of other iron industries were remained untouched.

With reference to Almeida et al. (2015), where major iron industries are located in very close proximity to other iron industries, it is very difficult to distinguish among the contributions of each specific source on the formation of fine iron particles. Additionally, identifying real emission sources is always a challenging question to be answered in case of any legal litigations, especially when the industry sectors did not reveal their emissions data for investigations. The study aims to address the unknown contribution of iron sources in atmospheric iron emissions in the industrial port and nearby residential areas. For this purpose, various analytical and modeling approaches were followed to utilize the known contribution of an iron plant (hereafter named “plant”) to understand unknown contribution of two iron plants (hereafter named “other iron industries”) in atmospheric iron emissions. In order to characterize fine iron particles, physicochemical characteristics of iron particles generated by the plant (process, stacks, in the plant atmospheric environment, and dustfall) were studied and categorized according to their specifications. In comparison, atmospheric iron particles close to other iron industries and dust particles in nearby residential areas were collected and analyzed. The gathered data were used to develop and validate the dispersion/deposition model. Different scenarios were defined and sensitivity analysis was implemented to estimate the contribution of other iron industries in the atmospheric iron emissions.