The Effect of Pre-Oxidation on the Reducibility of Chromite Using Hydrogen: A Preliminary Study

The majority of ferrochrome (FeCr) is produced through the carbothermic reduction of chromite ore. In recent years, FeCr producers have been pressured to curve carbon emissions, necessitating the exploration of alternative smelting methods. The use of hydrogen as a chromite reductant only yields water as a by-product, preventing the formation of carbon monoxide (CO)-rich off-gas. It is however understood that only the Fe-oxide constituency of chromite can be metalized by hydrogen, whereas the chromium (Cr)-oxide constituency requires significantly higher temperatures to be metalized. Considering the alternation of chromite’s spinel structure when oxidized before traditional smelting procedures, the effects on its reducibility using hydrogen were investigated. Firstly, the effect of hydrogen availability was considered and shown to have a significant effect on Fe metallization. Subsequently, spinel alternation induced by pre-oxidation promoted the hydrogen-based reducibly of the Fe-oxide constituency, and up to 88.4% of the Fe-oxide constituency was metallized. The Cr-oxide constituency showed little to no reduction. The increase in Fe-oxide reducibility was ascribed to the formation of an exsolved Fe2O3-enriched sesquioxide phase, which was more susceptible to reduction when compared to Fe-oxides present in the chromite spinel. The extent of Fe metallization of the pre-oxidized chromite was comparable to that of unoxidized chromite under significantly milder reduction conditions.publishedVersio

Source apportionment of ambient PM10−2.5 and PM2.5 for the Vaal Triangle, South Africa

The Vaal Triangle Airshed Priority Area (VTAPA), like other priority areas in South Africa, has an air pollution problem. Understanding the sources contributing to air pollution in this priority area will assist in the selection and implementation of appropriate control strategies. For this study, aerosol samples in the coarse (PM10-2.5) and fine (PM2.5) fraction were collected at four sites in the VTAPA during summer/autumn, winter, and spring. The contributing sources were identified and characterised based on the elemental and ionic compositions obtained through X-ray fluorescence and ion chromatography analysis. The highest seasonal median concentrations of PM10-2.5 (116 μg/m3) and PM2.5 (88 μg/m3) were observed in Sharpeville during the winter. The lowest median concentrations of PM10-2.5 (25 μg/m3) and PM2.5 (18 μg/m3) were detected in Zamdela during the summer/autumn period. At all sites, there was a high abundance of crustal elements in PM10-2.5 and a dominance of coal and biomass combustion-related elements in PM2.5. The Positive Matrix Factorisation receptor model identified dust-related and secondary aerosols as the major contributing sources of PM10-2.5. PM2.5 contributions were predominantly from coal burning for Sebokeng and Sharpeville and from industry, wood and biomass burning, and secondary aerosols for Kliprivier and Zamdela. The results of this study identify the main sources contributing to particulate air pollution in the VTAPA and provide local authorities with valuable information for decision-making.Significance: Dust, industry, domestic coal burning, vehicles, and wood and biomass combustion are the key sources of particulate air pollution in the VTAPA that need to be prioritised by decision-makers. Although Sebokeng and Sharpeville are located within the vicinity of industries, domestic coal burning has a greater contribution to particulate loading at these sites. Results from this study will assist in the design of local municipality air quality management plans for the VTAPA

Six-year observations of aerosol optical properties at a southern African grassland savannah site

Atmospheric aerosols have a significant effect on earth's radiative budget, particularly on regional scales. This paper presents a similar to 6 year, in situ, ground level aerosol scattering and absorption dataset, measured at a background site strategically positioned to enable differentiation of the effect of anthropogenic, population density and open biomass burning activities on a regional scale. Relatively well-defined seasonal and diurnal patterns were observed for all the aerosol optical properties, i.e. scattering coefficient (sigma(SP)), absorption coefficient (sigma(AP)), single scattering albedo (omega(0)) and Angstrom exponent of scattering (sigma(SP)). These patterns were explained by considering southern African specific sources and metrological conditions. Using a receptor modelling method (auto-generated source maps) it was found that air masses that had higher sigma(SP), sigma(AP) and omega(0), and lower alpha(SP), if compared with the relatively clean background, passed over source regions with significant industrial or other anthropogenic activities, higher population density, re-circulation of polluted air masses and higher open biomass burning frequency. To quantify differences, four source regions were defined, i.e. Karoo, Kalahari, anti-cyclonic recirculation pattern and the industrial hub of South Africa. Air masses that had passed over the Karoo source region represented the cleanest regional background conditions, while air masses that had passed either over the industrial hub and/or the anti-cyclonic recirculation pattern represented the most significant anthropogenically impacted, as indicated by the aerosol optical properties. The omega(0) medians of air masses that had passed over the Karoo (0.80-0.86) were 9, 12 and 7% lower than in air masses that had passed over source regions with the highest omega(0) median, in the warmest/wettest, coldest, and driest, peak open biomass burning periods, respectively.Peer reviewe

Size-resolved characteristics of inorganic ionic species in atmospheric aerosols at a regional background site on the South African Highveld

Aerosols consist of organic and inorganic species, and the composition and concentration of these species depends on their sources, chemical transformation and sinks. In this study an assessment of major inorganic ions determined in three aerosol particle size ranges collected for 1year at Welgegund in South Africa was conducted. SO42- and ammonium (NH4+) dominated the PM1 size fraction, while SO42- and nitrate (NO3) dominated the PM1-2.5 and PM2.5-10 size fractions. SO42- had the highest contribution in the two smaller size fractions, while NO3- had the highest contribution in the PM2.5-10 size fraction. SO42- and NO3- levels were attributed to the impacts of aged air masses passing over major anthropogenic source regions. Comparison of inorganic ion concentrations to levels thereof within a source region influencing Welgegund, indicated higher levels of most species within the source region. However, the comparative ratio of SO42- was significantly lower due to SO42- being formed distant from SO2 emissions and submicron SO42- having longer atmospheric residencies. The PM at Welgegund was determined to be acidic, mainly due to high concentrations of SO42-. PM1 and PM1-2.5 fractions revealed a seasonal pattern, with higher inorganic ion concentrations measured from May to September. Higher concentrations were attributed to decreased wet removal, more pronounced inversion layers trapping pollutants, and increases in household combustion and wild fires during winter. Back trajectory analysis also revealed higher concentrations of inorganic ionic species corresponding to air mass movements over anthropogenic source regions.Peer reviewe

Size-resolved characteristics of inorganic ionic species in atmospheric aerosols at a regional background site on the South African Highveld

Aerosols consist of organic and inorganic species, and the composition and concentration of these species depends on their sources, chemical transformation and sinks. In this study an assessment of major inorganic ions determined in three aerosol particle size ranges collected for 1year at Welgegund in South Africa was conducted. SO42- and ammonium (NH4+) dominated the PM1 size fraction, while SO42- and nitrate (NO3) dominated the PM1-2.5 and PM2.5-10 size fractions. SO42- had the highest contribution in the two smaller size fractions, while NO3- had the highest contribution in the PM2.5-10 size fraction. SO42- and NO3- levels were attributed to the impacts of aged air masses passing over major anthropogenic source regions. Comparison of inorganic ion concentrations to levels thereof within a source region influencing Welgegund, indicated higher levels of most species within the source region. However, the comparative ratio of SO42- was significantly lower due to SO42- being formed distant from SO2 emissions and submicron SO42- having longer atmospheric residencies. The PM at Welgegund was determined to be acidic, mainly due to high concentrations of SO42-. PM1 and PM1-2.5 fractions revealed a seasonal pattern, with higher inorganic ion concentrations measured from May to September. Higher concentrations were attributed to decreased wet removal, more pronounced inversion layers trapping pollutants, and increases in household combustion and wild fires during winter. Back trajectory analysis also revealed higher concentrations of inorganic ionic species corresponding to air mass movements over anthropogenic source regions.Peer reviewe

The use of hydrogen as a potential reductant in the chromite smelting industry

The chromium (Cr) content of stainless steel originates from recycled scrap and/or ferrochrome (FeCr), which is mainly produced by the carbothermic reduction of chromite ore. Ever-increasing pressure on FeCr producers to curtail carbon emissions justifies migration from traditional FeCr production routes. The interaction between hydrogen and chromite only yields water, foregoing the generation of significant volumes of CO-rich off-gas during traditional smelting procedures. For this reason, the use of hydrogen as a chromite reductant is proposed. In addition to thermodynamic modelling, the influence of temperature, time, and particle size on the reduction of chromite by hydrogen was investigated. It was determined that, at the explored reduction parameters, the iron (Fe)-oxides presented in chromite could be metalized and subsequently removed by hot-acid leaching. The Cr-oxide constituency of chromite did not undergo appreciable metalization. However, the removal of Fe from the chromite spinel allowed the formation of eskolaite with the composition of (Cr1.4Al0.6)O3 in the form of an exsolved phase, which may adversely affect the reducibility of chromite. The study includes the limitations of incorporating hydrogen as a reductant into existing FeCr production infrastructure and proposes possible approaches and considerations.publishedVersio

Assessment of polar organic aerosols at a regional background site in southern Africa

A recent paper reported GCxGC-TOFMS analysis used for the first time in southern Africa to tentatively characterise and semi-quantify 1000 organic compounds in aerosols at Welgegund - a regional background atmospheric monitoring station. Ambient polar organic aerosols characterised are further explored in terms of temporal variations, as well as the influence of meteorology and sources. No distinct seasonal pattern was observed for the total number of polar organic compounds tentatively characterised and their corresponding semi-quantified concentrations (sum of the normalised response factors, Sigma NRFs). However, the total number of polar organic compounds and Sigma NRFs between late spring and early autumn seemed relatively lower compared to the period from mid-autumn to mid-winter, while there was a period during late winter and early spring with significantly lower total number of polar organic compounds and Sigma NRFs. Relatively lower total number of polar organic compounds and corresponding Sigma NRFs were associated with fresher plumes from a source region relatively close to Welgegund. Meteorological parameters indicated that wet removal during late spring to early autumn also contributed to lower total numbers of polar organics and associated Sigma NRFs. Increased anticyclonic recirculation and more pronounced inversion layers contributed to higher total numbers of polar organic species and Sigma NRFs from mid-autumn to mid-winter, while the influence of regional biomass burning during this period was also evident. The period with significantly lower total number of polar organic compounds and Sigma NRFs was attributed to fresh open biomass burning plumes occurring within proximity of Welgegund, consisting mainly of volatile organic compounds and non-polar hydrocarbons. Multiple linear regression substantiated that the temporal variations in polar organic compounds were related to a combination of the factors investigated in this study.Peer reviewe

Assessing SO2, NO2 and O3 in rural areas of the North West Province

Publisher Copyright: © 2021 The Author(s). Published under a Creative Commons Attribution Licence. All Rights Reserved.Air quality monitoring has been lacking in the rural and western North West Province. Here ambient sulphur dioxide (SO2), nitrogen dioxide (NO2) and ozone (O3) concentrations, monitored with passive samplers at 10 sites, are presented. Widespread SO2 and NO2 problems weren't observed. However, regular O3 standard limit exceedances are likely across the province. Increased SO2 and NO2 concentrations in the colder and drier months were evident. Inversion layer trapping of low-level emissions during the colder months and open biomass burning in the drier months increased ground level pollutant concentrations. Wet deposition of SO2 and NO2, and enhanced SO2 conversion to particulate sulphate, result in lower wet season concentrations. O3 concentrations were lower from May to July and higher from August to March. Three phenomena contributed to this. Firstly, shorter daylight hours (less photochemistry) and secondly, lower biogenic volatile organic compound (O3 precursors) concentrations during the colder months. Thirdly, the late winter/ early spring open biomass burning peak lead to elevated carbon monoxide (CO) concentration (also an O3 precursor). Spatial patterns indicated higher SO2 concentrations in the west, due mainly to industrial emissions. The NO2 spatial map indicated two areas of higher concentration, i.e. Bapong in the east due mainly to industrial emissions, and Taung with its higher population density. The O3 spatial map was almost the inverse of NO2. The lower O3 and higher NO2 around Taung indicated that O3 is likely titrated there. Additionally, the results indicate that non-point source emissions of NO2 are high enough to results in exceedances of the O3 standard limit. Overlay back trajectory maps showed that sites in the east are more frequently impacted by pollution transported from the Mpumalanga Highveld, Vaal Triangle and the Johannesburg-Pretoria megacity if compared to the west. Conversely, cleaner air masses impact the west more than sites in the east.Peer reviewe

Statistical analysis of factors driving surface ozone variability over continental South Africa

Statistical relationships between surface ozone (O-3) concentration, precursor species and meteorological conditions in continental South Africa were examined from data obtained from measurement stations in north-eastern South Africa. Three multivariate statistical methods were applied in the investigation, i.e. multiple linear regression (MLR), principal component analysis (PCA) and -regression (PCR), and generalised additive model (GAM) analysis. The daily maximum 8-h moving average O-3 concentrations were considered in these statistical models (dependent variable). MLR models indicated that meteorology and precursor species concentrations are able to explain similar to 50% of the variability in daily maximum O-3 levels. MLR analysis revealed that atmospheric carbon monoxide (CO), temperature and relative humidity were the strongest factors affecting the daily O-3 variability. In summer, daily O-3 variances were mostly associated with relative humidity, while winter O-3 levels were mostly linked to temperature and CO. PCA indicated that CO, temperature and relative humidity were not strongly collinear. GAM also identified CO, temperature and relative humidity as the strongest factors affecting the daily variation of O-3. Partial residual plots found that temperature, radiation and nitrogen oxides most likely have a non-linear relationship with O-3,while the relationship with relative humidity and CO is probably linear. An inter-comparison between O-3 levels modelled with the three statistical models compared to measured O-3 concentrations showed that the GAM model offered a slight improvement over the MLR model. These findings emphasise the critical role of regional-scale O-3 precursors coupled with meteorological conditions in daily variances of O-3 levels in continental South Africa.Peer reviewe

Seasonal influences on surface ozone variability in continental South Africa and implications for air quality

Although elevated surface ozone (O-3) concentrations are observed in many areas within southern Africa, few studies have investigated the regional atmospheric chemistry and dominant atmospheric processes driving surface O-3 formation in this region. Therefore, an assessment of comprehensive continuous surfaceO(3) measurements performed at four sites in continental South Africa was conducted. The regional O-3 problem was evident, with O-3 concentrations regularly exceeding the South African air quality standard limit, while O-3 levels were higher compared to other background sites in the Southern Hemisphere. The temporal O-3 patterns observed at the four sites resembled typical trends for O-3 in continental South Africa, with O-3 concentrations peaking in late winter and early spring. Increased O-3 concentrations in winter were indicative of increased emissions of O-3 precursors from household combustion and other low-level sources, while a spring maximum observed at all the sites was attributed to increased regional biomass burning. Source area maps of O-3 and CO indicated significantly higher O-3 and CO concentrations associated with air masses passing over a region with increased seasonal open biomass burning, which indicated CO associated with open biomass burning as a major source of O-3 in continental South Africa. A strong correlation between O-3 on CO was observed, while O-3 levels remained relatively constant or decreased with increasing NOx, which supports a VOC-limited regime. The instantaneous production rate of O-3 calculated at Welgegund indicated that similar to 40 % of O-3 production occurred in the VOC- limited regime. The relationship between O-3 and precursor species suggests that continental South Africa can be considered VOC limited, which can be attributed to high anthropogenic emissions of NOx in the interior of South Africa. The study indicated that the most effective emission control strategy to reduce 03 levels in continental South Africa should be CO and VOC reduction, mainly associated with household combustion and regional open biomass burning.Peer reviewe