Characterization and abatement of SOx, NOx and PCDD/Fs in iron ore sinter machine wind legs


 It was observed that SOx and NOx, in large concentrations, are getting released from certain wind boxes below the sinter machine. The particulates released from specific wind legs were characterized using Quantitative Evaluation of Materials by Scanning Electron Microscopy (QEMSCAN). Particulates with spherical, cubical, needle and bar-like morphologies containing K, Na, Cl were found. Nitrogen-based solids were found in clutter-like morphology. Some particles had a mixture of the above, SOx and NOx. A method of dissolving SOx, NOx and breaking them down into harmless substances was explored in this research. The deposits in the wind legs were dissolved in demineralized water and solutions of sodium bicarbonate, urea, and di-sodium borate deca-hydrate (borax) to estimate the absorbance of K, Na, Cl, Ca, Mg, S, and N based compounds present. Demineralized water and sodium bicarbonate were found to be the most effective sorbents of SOx and NOx. The filtrates were examined under QEMSCAN and found that SOx and NOx are not present. Based on the above finding, a solution of sodium bicarbonate and water 0.01% v/v was sprayed into a wind box and found that SOx and NOx have got reduced by about 55%. To maximize the capture of SOx and NOx, the solution was optimized at 0.02% v/v. With this novel technique, capital intensive Desulphurization (De-SOx) and Denitrification (De-NOx) installation can be avoided. Additionally, an economical solution to the Polychlorinated dibenzo para-dioxins and polychlorinated dibenzofurans (PCDD/Fs) emission was explored in this research. Various physicochemical mechanisms of forming harmful substances are described in this paper.


Introduction
The raw materials used in the iron ore sintering process are iron ore nes (-10 mm size), uxes such as limestone and dolomite, solid fuel such as coke nes and anthracite (-3.15 mm size), and calcined lime (-1 mm size). These materials are mixed thoroughly, granulated by spraying the water, and charged over the continuously moving sinter machine. The top layer of the sinter bed is ignited in the Ignition Furnace at about 1200 o C. As the atmospheric air is sucked through the sinter bed by the waste gas fan, the combustion zone gradually travels down into the bed, until it reaches the bottom. The iron ore particles are bonded together with the slag (containing silicates) formed by fusing limestone and dolomite. The solid fuel is the source of heat for the fusion process.
A sinter machine of 204 m 2 sintering area with 17 wind boxes and another sinter machine of 496 m 2 sintering area with 31 wind boxes were considered in this study. The temperatures of the gases in the 17 wind boxes, below the moving sinter machine, are continuously monitored for process control, as illustrated in Fig. 1. The direction of sinter machine travel is indicated with an arrow mark. The temperature of the gases in the wind box gradually increases from 100 o C to 120 o C due to preheating, up to wind box number 3, and then drops to about 70 o C due to the re-condensation phenomenon up to wind box number 10. It gradually starts to rise from wind box number 11-12 (153  Iron ore sinter plants contribute to most of the emissions in Steel Plants Han et al. 2014Han et al. , 2019Gan et al. 2016). It is important to understand the underlying reasons from the sustainability point of view and avoid closure. The conspicuous trail of smoke from the stack gives a bad impression of pollution in the onlooker's mind. This research work was taken up to reduce the emissions from the sinter plant stack.
The plume visibility is related to the concentration of SOx, condensing into a mist; water vapor, and particulates (Kasama et al. 2009;Aries 2011). Iron ore, limestone, and dolomite contain chlorine, copper, zinc; heavy metals such as lead, mercury, zirconium, molybdenum, cadmium; apart from alkalis such as Na, K, F in traces (Wang and Zhang 2020). While the sintering process proceeds, their compounds are released through the wind boxes. Some of these particles, while traveling, stick to the structural members in the wind tract. Electrostatic Precipitator (ESP) installed before the stack of the sinter plants lters out the coarse dust particles containing iron ore, Ca, Mg, silicates, alkalis, chlorides, uorides, carbonates, sulfates, and oxides of a range of elements (Cavaliere and Perrone 2016;Ji et al. 2017b;Lv et al. 2018b). The ner particles that fail to settle down in ESP (particulates) exit into the atmosphere through the stack and pose an environmental issue. The stack emission contains particulates of SOx, NOx, KCl, VOCs, PCDDs, and PCDFs (commonly referred to as PCDD/Fs) which pose healthrelated hazards (KAWAGUCHI et al. 2002a, b;Yu et al. 2019). The emission pro le of SOx, NOx, and VOCs in sinter pot apparatus was investigated by Juexiu Li . The effect of fuel type on NOx emission was indicated through the mathematical model by Wenjie Ni (Ni et al. 2019). The major source of NOx was reported as coke breeze containing fuel N (Hu et al. 2017a). The particulates of 20-micron size can be trapped in the activated carbon, 10-micron size in membrane lters but PM 2.5 may pass through most of the lters commercially available (Lv et al. 2018a). Therefore, the size and shape of these particulates are important in the sense that the spherical particulates may slip through and bar-like morphology may get easily struck (Lanzerstorfer and Steiner 2016). Hence in this research, characterization and morphological study were taken up.
The particulates released from the bottom of the sinter machine are by and large light in weight. Some of them are native, such as micro nes present in the coke, calcined lime, limestone, dolomite, and iron ore nes. Some of them are recycled through ESP dust.
Due to the drying and re-drying phenomenon (as illustrated in Fig. 2), the loose and light particles get disconnected from the sinter mix granules and travel along with the stream of gas. The products of the reactions travel from the combustion zone down into colder sinter mix layers. Some of them are condensed back and some escape through the voids (Kasai et al. 2001a;KAWAGUCHI et al. 2002a;Kasama et al. 2009). This phenomenon is observed until the colder sinter mix is present below the combustion zone. The chemical analysis of the deposit in the entire wind tract starting from under-grate, wind boxes, wind legs, wind main, ESP electrodes and its internal structural members, the inlet of the waste gas fan, impeller, and the exit duct is characteristic of the various physical or chemical phenomenon occurring in that particular location.
Worldwide, various systems such as high-temperature bag lters; de-sulphurization, and De-NOx systems are being installed after the ESP, to capture the particulates. The latter technologies are cost-intensive and increase the complexity in terms of recycling the by-products from such systems, regeneration of activated carbon, and so on.
The emission of compounds of K, Na, Ca, Cl, N and S are being studied by researchers either by proactively inhibiting their formation or by trapping them before getting released into the atmosphere. A variety of De-SOx and De-NOx systems is devised and the issue of SOx and NOx is being addressed successfully (Cores et al. 2015). Wet ne system was indicatively preferred to Air ne (Smit et al. 2000) and Catalytic Oxidation (Boscolo et al. 2008a). Activated carbon has been found effective in trapping particulates up to PM 10 . Particles below 10 microns were found escaping through its pores (Lv et al. 2018a). Scrubbers were found to be effective but treating the polluted water was increasing the complexity (Rotatori et al. 2006;Copeland and Kawatra 2011). However, the above technologies are cost-intensive (CPCB 2004). They require a waste gas reconditioning tower to lower the temperature and De-SOx, De-NOx chambers that contribute to the cost. In our research on the recycling of sinter waste gas into the sinter bed, a reduction in particulate emissions was noticed (Angalakuditi et al. 2021). Selective Catalytic Reduction of NOx is one of the successful methods, wherein 95% reduction is reported with the help of NH 3 gas and vanadium and tungsten catalysts. NOx emission pro le was investigated by Chin-lu Mo and Kenji Tiara (Mo 1997;Taira 2019Taira , 2020. It was identi ed that the NOx conversion factor in the combustion zone was related to the amount of NOx released. Calcium hypochlorite was tried, to dissolve SO 2 and NOx, and about 85 to 100% removal was reported (Zhou et al. 2015b).
They discussed chemical -thermodynamic aspects of dissolution and reactions. However, an engineered solution is much needed in commercial-scale sinter plants. Slack lime, gypsum, and sodium sulfate are the other recommended sorbents of SOx and NOx. Hydrated lime was coated on the coke nes used for making sinter and a drop in NOx emission was reported by Katayama (Katayama et al. 2015). Ammonium sulfate was tried as an inhibitor of dioxins (Łechtańska and Wielgosiński 2014). Inhibition of the formation of harmful compounds was effective by adding urea along with the raw materials (Aries 2011). Adding anthracite coal was reported to reduce SOx, NOx emissions. Due to the absence of aliphatic compounds in anthracite coal, harmful VOCs are not formed. Bio-fuels are tried by various researchers to suppress the formation of harmful substances Fan et al. 2015). Charcoal was also administered in-lieu of coke nes (Lovel et al. 2007;Lu et al. 2015;Cheng et al. 2016;Fan et al. 2016). Biomass and biogas were suggested by Jose Adilson de Castro (De Castro et al. 2013;de Castro et al. 2018) through mathematical models, as an alternative to reduce harmful emissions. Consumption of solid fuel has been substituted by the injection of LNG and Oxygen into the sinter bed and the CO 2 , particulate emissions have come down (Oyama et al. 2011). In our research work, injecting hydrogen-rich gas into the sinter bed was tried to reduce the solid fuel consumption and thereby reduce the emissions (Acharyulu et al. 2021).
Evacuation of ESP dust and abstinence from direct recycling was found to be effective in JSW Steel, Vijayanagar Works.
Use of up to 40% anthracite coal containing 2.5-2.75% volatile matter, as alternate solid fuel was being practiced and lower emissions are observed since the year 2011. About 3400 Nm 3 of waste gas per ton of sinter is released into the atmosphere at a temperature of about 140 o C (or 1,100,000 Nm3/hr per machine of 2.5 MTPA capacity). The De-SOx and De-NOx systems installed before the stack, around the world do handle waste gas of this magnitude and hence are large in size and capital intensive. Handling the by-products of such systems increases the complexity further. JSW Steel, Vijayanagar Works produces about 13 million metric tons of iron ore sinter per annum and the volume of waste gas to be handled is very high (to the tune of 44,200 million Nm 3 in a year).
The maximum SOx and NOx are in the concentration range of 380 and 350 ppm; while the concentration in the stack was in the range of 50 to 170 ppm. The velocity of the gas in these wind legs was in the range of 100 m/s and a ow rate of 300,000 Nm 3 /hr (83 Nm 3 /s) was observed. This ow rate is much smaller compared to the ow through the stack of 1,100,000 Nm 3 /hr (305 Nm 3 /s). The volume of waste gas per wind leg varied from 75 to 300 Nm 3 per ton of sinter, depending upon its location from the feed end to the discharge end of the sinter machine. The typical velocity of the waste gas was in the tune of 60 to 100 m/sec; while the velocity in the stack was about 16 to 20 m/sec. It will be easier to handle the smaller quantity of waste gas and e ciently while the particulate concentration is high. It has been envisaged in this study to dissolve and break down the particulates in the wind boxes by spraying the solution of a variety of sorbents.
In this research, an attempt has also been made to provide a simple and cost-effective solution to abate the generic SOx and NOx in sinter machine wind legs. Various speci c compounds such as SO 2 , SO 3 , NO, N 2 O, and NO 2 separately were considered beyond the scope of work.
Additionally, an attempt has been made to address the generic PCDD/Fs issue. There are several compounds in the family of PCDD/Fs, the study of which was considered as beyond the scope of this work. PCDD/Fs are formed at a temperature above 1200 o C in the combustion zone of the sinter bed and are released when the reducing conditions are changed to oxidizing conditions (Salgueiro et al. 2003(Salgueiro et al. , 2004Kasama et al. 2006a). A deposit in the ESP was spotted containing KCl and PCDD/Fs (Fig. 3). It was very light, looked like eece, and has a density of 0.45 g/cm 3 . It has been dissolved in acetone, hexane, and toluene successfully. The PCDD/Fs are released from wind boxes 14, 15, and 16 where the gas temperature is in the range of 400-450 o C. The autoignition temperatures of acetone (C 3 H 6 O), hexane (C 6 H 14 ), and toluene (C 6 H 5 CH 3 ) are 465, 223, and 480 o C respectively. Spraying these organic substances may not be safe in such a hot zone. A method to suppress the formation of PCDD/Fs by injecting inert gas such as nitrogen has been discussed.

Materials And Methods
The materials used in this research were deposits in wind legs, ESP hoppers and the electrostatic elds of the iron ore sinter plant of JSW Steel, Vijayanagar Works. Demineralized water, sodium bicarbonate, urea, and borax were used in the dissolution study of the particulates containing SOx, NOx. Acetone, Hexane and Toluene were used to see the dissolution of the deposit over the ESP elds containing PCDD/Fs. Pulse-jet metering pump of Milton Roy make was used to spray the sodium bicarbonate solution into the wind legs.
Sympatec GmbH make RODOS Micron Size Particle Analyzer was used for the size distribution of ESP dust particles. Wet chemical analytical method and Thermo Fisher make X-Ray Fluorescence Analyzer (XRF model 101/367) Optiquant method was used for assessing the contents of the ESP dust and wind leg deposits. FeO was evaluated by the wet chemical analytical method. LECO make Carbon Sulphur Analyzer SC832HT was used to nd out the carbon and sulfur present in the wind leg deposits. Bruker make QEMSCAN (Quanta 650 FEG) was used to map the contents of the deposits in the wind legs and ESP 4A, 4B elds. The ltrates after dissolution were mapped using QEMSCAN and EDAX make TEAM Energy Dispersive Spectroscope EDS XFlash Detector 5030. Various APHA standards, IS-3025 series for assessing water quality were adopted to measure the dissolution of SOx, NOx, and chlorides. IMR make portable ue gas analyzer IMR-2800P was used to measure CO, CO 2 , O 2 , SOx, NOx and Horiba make Portable Gas Analyzer PG300 were used to measure SOx, NOx levels in the wind legs, following US EPA air quality monitoring guidelines and IS-11255. The assessment of PCDD/Fs was done according to USEPA Method 23A @ 11% oxygen.

Results And Discussion
The opacity at the exit of the Process ESP is a metric of visible emissions. The data of an industrial-scale sinter plant, for the last two years, was statistically analyzed to understand the effect of process parameters on opacity. The dependency of opacity (mg/Nm 3 ) on calcined lime, VM in solid fuel, and Loss on Ignition (LOI) of iron ore nes are depicted in Fig. 4. To minimize the emissions, it is recommended to minimize the VM in the solid fuel, the micro nes in the raw materials, LOI in the iron ore nes, and usage of calcined lime (Zhang et al. 2015).
During the combustion of solid carbonaceous fuel such as coke breeze containing fuel N, S, P, and volatile matter, in the presence of other raw materials of the iron ore sintering process, harmful compounds are unintentionally produced. SOx and NOx are the most commonly experienced particulate emissions. Oxidation of organic sulfur, volatile matter (VM), and fuel N contribute to the formation of SOx and NOx. Typical content of sulfur, nitrogen, and VM in coke nes varieties A, B, C, D, and anthracite A, B, C are measured and given in Table 1. To control the SOx, NOx emissions, it is recommended to use solid fuel varieties containing lower sulfur and nitrogen.  Nitrogen could be easily released in pyrolysis (Hu et al. 2017b). The organic substances liberated from the pyrolysis (tar as an example) join the gas stream, causing these particulates to stick to the structural members in the gas tract. As the particulates continue to impinge on the structural members, accretion, diffusion, and crystallization occur. Vapors of organic sulfur and phosphorous are formed at 280 and 444 o C respectively. The possible product compounds are SO 2 , SO 3 , SO 4 , P 2 O 5 depending upon the thermodynamic conditions. Fuel nitrogen, volatile matter, and other organic substances (regarded as Loss on Ignition) are also subjected to pyrolysis. The sticky deposits are observed in the wind legs 5 to 10 only. Beyond these wind legs, the sinter mix below the combustion zone gets su ciently hot and the condensation of organic substances is forgone.
SOx and NOx content in the wind legs was measured with the help of IMR Gas Analyzer. The variation of SOx and NOx in wind legs is illustrated in Fig. 5 (a) for a sinter machine containing 17 wind legs and in Fig. 5 (b) for a sinter machine with 31 wind legs respectively. The sinter machine with 17 wind legs was of particular interest in this research work. The SOx increased gradually to a maximum at wind leg 6 and then reduced beyond wind leg 10. NOx gradually increased until wind leg 7, reduced at wind leg 8, and then increased to a maximum in wind leg 10. It persisted up to wind leg 13. It was noticed that corrosion was signi cant in wind legs 5 to 10. However, the distribution of SOx, NOx over wind legs varied from sinter machine to Machine and the variations in the sintering process. The trend of SOx has got shifted to the farther wind legs 20 to 29 in sinter machine with 31 wind boxes and that of NOx was seen between wind legs 4 and 25.
The CO 2 and O 2 content in the waste gas in the wind legs was measured with the help of IMR Gas Analyzer and is graphically represented in Fig. 5 (c) and (d). As the sintering process proceeds, due to the combustion of solid fuel, CO and CO 2 are released and the O 2 content gradually drops. After the combustion process is ended in the later stages of the process, O 2 content increases. In other words, reducing conditions change to oxidizing conditions after the BTP. Until then, the partial pressure of oxygen is low due to the prevailing reducing conditions. This phenomenon is very important in the formation of PCDD/Fs (Ooi and Lu 2011  . Silicates and carbonates combine with alkalis to form alkali silicates (Na 2 SiO 3 and K 2 SiO 3 ) and alkali carbonates (Na 2 CO 3 , K 2 CO 3 ). Reaction with CO gas can also form K 2 CO 3 . The oxidation of alkalis Na and K takes place between 300 and 800 o C respectively and their vapors such as Na 2 O and K 2 O are released. The other stoichiometric compounds are NaO 2 , Na 2 O 2 , K 2 O 2 , KO 2, and K 2 SO 4 (Ji et al. 2017a). With the presence of chlorine, NaCl and KCl are formed (Ji et al. 2017b). They vaporize at high temperatures (1074 and 1400 o C respectively) and hence are released from the last wind boxes of the sinter machine (Kasama et al. 2006b;Ji et al. 2017a).
H 2 S, H 2 SO 4, and HCl are some of the acids formed owing to the hydrogen present in the fuel gas used for ignition and through the usage of water for granulation. Through water vapors, volatiles, and hydrated phases present in iron ore, hydrogen is available to form these acids. The CO and CO 2 gases are the other sources of reactions with SO 2 . These acids are the major source of corrosion of wind legs and ESP collecting electrodes. Mist of acids and SO 3 also add to the visibility of the plume (Kasama et al. 2009;Aries 2011).
The chemical analysis of the deposit in wind leg 10 was assessed as given in  The release of PCDD/Fs varies as the sintering progresses (Kasama et al. 2006b;Qian et al. 2018). The rst release of the point where the temperature of the gas is maximum) in the sinter machine (Kasama et al. 2006b). They suggested that the gas temperature is critical to the release of PCDD/Fs and hence BTT has to be optimized to control the PCDD/Fs emissions. However, observing the O 2 content in the waste gas in the wind legs, it is evident that PCDD/Fs release is a function of oxygen content. Furans formation was noticed when oxygen was in the tune of 16% under the ignition furnace, where the temperature was in the tune of 100 o C only. Further, PCDD/Fs were not present until the O 2 was less than 10%. As  Table 3.
NOx was considered as a major area to be focussed on. PCDD/Fs were in the range of 0.0014 to 0.0021 ng ITEQ/Nm 3 .  Kasama et al. 2006b). Recirculation of ESP dust along with the raw materials has indicated aggression in the formation of PCCDs (Boscolo et al. 2008b). Mixing rice husk and sugar into sinter mix reduced NOx emission. . The deliberate addition of PVC and NaCl into the sinter mix has resulted in increased emission of PCDD/Fs, ascertaining that chlorine is the major source of PCDD/Fs formation (Kasai et al. 2001b). Hence sources of chlorine are to be regulated in the inputs. The absorbance of PCDDs in acetone, hexane, and toluene was indicated by Wan Ying Shiu (Shiu et al. 1988a). At the BTP, the sintering process is completed and the cooling effect of the sinter is evidenced in the remaining two wind boxes. The voidage increases soon after the sintering is completed and a huge amount of atmospheric air gushes through the last wind boxes. Therefore, it becomes di cult to arrest the formation of PCDD/Fs. Their formation can be minimized essentially by restricting access to oxygen.

Characterization of Particulates
The assessment of characteristics of the dust collected in the ESP is an important step in designing the ESP for maximizing collection e ciency (Lanzerstorfer and Steiner 2016;Lanzerstorfer 2017). The other critical properties of the dust are basicity (varies from 1.3 to 2.0), speci c resistivity (3 X 10 11 to 2 X 10 12 Ω cm), and bulk density (0.68 to 1.2 g/cm 3 ). As the basicity and resistivity increase, the collection e ciency reduces (Khosa et al. 2003).
In the present study, samples were collected from each hopper of ESP 1A, 1B, 2A, 2B, 3A, 3B, 4A, 4B; wind legs no. 6, 7, 8, 9, 10 and ESP top chamber deposit near 4A, 4B elds. The dust samples collected from ESP hoppers were analyzed for size distribution and chemistry. The size analysis indicated that the particle size decreased from the inlet to the exit hopper of ESP; the nest particles being found towards the exit, as illustrated in Fig. 7.
The chemical analysis of the dust in the hoppers 1A, 1B, 2A, 2B, 3A, 3B, 4A, and 4B is given in Table 5. The dust contained relict particles from raw materials and the products of various reactions and condensates. The samples of deposits in wind legs and ESP elds 4A, 4B were exclusively studied with the help of QEMSCAN for morphology and elemental analysis. After characterization, they were dissolved in various solutions to see how much S, N, and Cl compounds got dissolved, speci cally. The ltrates left out thereby were again examined under QEMSCAN to ascertain that the above compounds were not present.
The samples of the deposits on the wind legs were assessed for the presence of Carbon and Sulphur in the Coal Characterization Laboratory using Carbon -Sulfur Analyzer. The result is shown in Table 6. The content of sulfur followed the pattern of SOx formation, along the length of the sinter machine. The carbon content increased initially, decreased later, and then shot up sporadically. After wind leg no. 10, no deposits were found; indicating that the unburnt ne solid fuel particles or carbonates were released in the drying section of the sinter bed in two phases. The presence of carbon and sulfur in large quantities is related to the drying phenomenon occurring in the sinter bed, above these wind legs. The rst phase corresponds to the primary drying, where the moisture in the green mix gets released due to the combustion of solid fuel. As the vapor travels down into the colder bed, it gets re-condensed, increasing the moisture. As the combustion zone travels further down, this layer also gets dried by releasing the excess moisture. At this point, another batch of ne solid fuel or carbonate particles is released (secondary phase, Fig. 2). It is also intriguing to note that most of the low-temperature particulates are released well before Burn-through Rise Point (BRP).
The chemical analysis (in %) of the deposits in the wind legs is given in Table 7.

Morphological Study using QEMSCAN
The deposits in the wind legs 6 to 10 had a variety of morphology as seen with QEMSCAN. The scanning speci cations were: 20 kV; Kα Series for N, O, F, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, Mn, Fe, Ni, and Zn; Lα series for Zr, Mo, Cd, Pb. Compounds containing Ca, Si, Al, Na, K, S, and N are found in varying morphology and size, as shown in Fig. 8 to Fig. 16. In this section, the scanned images are given followed by the spectrum and a table of elemental analysis. The bar/rod/cube/sphere/platelike morphologies observed are due to crystallization. The sticking of particulates to each other is due to diffusion, as noticed in Fig. 15. The non-crystalline substances are seen as clutter (without a prominent shape) in Fig. 8 and Fig. 14 (E).
The deposition on its inspection door revealed the mechanism of deposition. The needle-like particles were found to impinge continuously, assimilate and appear star-like particles. Wei LV et al (Lv et al. 2018a) had reported similar micrographs.
Chlorine was seen as globules: K & Cl were found on ball-like particles. Impingement of various particles on substrate indicated signs of inter-particle diffusion and assimilation (Ji et al. 2017b;Lv et al. 2018a). Hence bar-like particles appeared as stars in Fig. 15.

The Concept of Dissolving as a Means of Abatement
In the commercially available technologies, the harmful substances such as SOx, NOx are either dissolved in water spray (scrubbers), reacted with ammonia, urea (Aries 2011;Chun et al. 2017) or passed over catalysts or trapped in the pores of the activated carbon (Han et al. 2014;Lv et al. 2018a). It was understood that wet scrubbing was one of the most effective methods of capture (Copeland and Kawatra 2011). In this technique, the water dissolves and carries away sulfur and nitrogen compounds. The concentration of sulfur and nitrogen in the water increases after dissolution. This technique was adopted in this research and the trials were conducted to see if the concentration of sulfur, nitrogen compounds rise after dissolving.

Dissolution Study to Capture SOx, NOx, and Chlorides
Sodium bicarbonate (NaHCO 3 ) (Walawska et al. 2013;Lanzerstorfer 2017), urea (CON 2 H 4 ) (Aries 2011;Chun et al. 2017), and di-sodium borate deca-hydrate (Na 2 B 4 O 7 10(H 2 O) or Borax) were selected as they are known for high absorbance of SOx and NOx. In recent times, sodium bicarbonate is preferred to sodium carbonate or CaCO 3 as a means of treating sinter waste gas, especially for De-SOx. Borax is used in determining the ammonical nitrogen in the water and hence was selected. It was anticipated that solutions of borax, sodium bicarbonate, urea will be sprayed into wind legs.
The samples of wind leg deposits were dissolved for 24 hours in demineralized water, and 1% v/v solutions of sodium bicarbonate urea and di-sodium borate deca-hydrate to see how much SOx and NOx were dissolved. After dissolving, the solution was found to have increased levels of sulfates and nitrates. The ltrates were collected, dried, and examined under QEMSCAN. SOx and NOx were completely absent. Water and the solution of sodium bicarbonate were found to be the most effective sorbents. Hence it was planned to spray the solution of water and sodium bicarbonate into a wind box, wherein the concentration of NOx was highest. However, as the measured gas velocity in the wind box is about 105.3 m/s and ow rate of 82.77 Nm 3 /s, it was apprehended that the chances of the particulate population coming in contact with the droplets of the solution were limited.

Trial Work with Water
To see the dissolution rate of chlorides in tap water, a 3-gram sample of Wind Leg no. 7 was dissolved in 100 ml of water and left for 24 hours (to take care of the poor wettability of this dust). After ltering, the solution was found to have the properties indicated in Table 8. From the above, it is evident that SOx, NOx got dissolved, and hence the sulfate and ammonical nitrogen have increased in the solution. Chlorides have also gone up, meaning KCl, NaCl has also be captured. The ltrate was observed under QEMSCAN and found that no salts had remained. It has Ca, Fe, O majorly. The images of the ltrate, spectra, and elemental analyses are shown in Fig. 17 to Fig. 22.

Dissolving in De-mineralised Water
Another 10 gram of sample from Wind Leg no. 7 was dissolved in 1000 ml DM water and put on a shaker for one hour and 24 hours. After ltering, the properties of the solution were measured as given in Table 9. It is understood from the above that majority of the sulfate is dissolved within an hour. Chlorides and nitrates took more time to dissolve. The dissolution of nitrates and sulfates is far less than normal water.

Dissolving in Sodium Bicarbonate Solution
To verify the role of sodium bicarbonate, a 5-gram sample Wind Leg no. 8 was mixed with 5 grams of sodium bicarbonate and dissolved in 500 ml of DM water. It was allowed to get mixed thoroughly on a shaker for 24 hours. After ltering, the solution was characterized as given in Table 10. It is seen that most of the sulfates and some amount of chlorides got dissolved.

. Dissolving in Borax Solution
The dust was dissolved in Di-Sodium Borate Deca-Hydrate and ltrate was taken out. The properties of the solution after dissolution are given in Table 11. More amount of Ca, Mg, Chlorides got dissolved than nitrates and sulfates.

Dissolving in Urea Solution
A 15% reduction of SOx by adding 0.02% urea in the sinter mix was reported by Eric Aries and D Poole (Aries 2011). To establish the optimum concentration, the urea content was varied as 0.1%, 1%, 5% & 10%. The dissolution data of chloride, sulfate, and ammonical nitrogen are given in Table 12. The ltrate was examined in QEMSCAN and the observations are as given below.
From the data, it is evident that urea is not only very effective for nitrates and sulfates but also chlorides. The summary is illustrated in Fig. 23. The ltrates invariably indicated reduction/absence of potassium, sodium, chlorides, sulfates, and nitrates and hence they can be removed by dissolving in water, DM water, sodium-bi-carbonate solution, urea solution, and borax solution. The residues have Ca, Mg, Si, Al, and Fe; the constituents of sinter slag. All other harmful substances had got removed, inferring that the dissolution technique was successful. However, the properties of the solution after dissolution vary and show the possibility of corrosion of steel structure/ESP electrodes. Increased NH 3 emissions were reported due to the usage of urea (Aries 2011).
From the results summarized in Fig. 23, it is evident that water, demineralized water, and sodium bicarbonate are the most effective means of removing SOx and NOx. Hence, the solution of water and sodium bicarbonate was tried by spraying into the wind boxes.

The Technique of Spraying in the Wind Boxes
From the dissolution studies, it was evident that SOx, NOx can be dissolved successfully, and hence the solution of sodium bicarbonate and water could be sprayed into wind boxes of the running sinter machine to see the breakdown of SOx, NOx.
The arrangement of spraying the sodium bicarbonate solution into the wind box is illustrated in Fig. 24.
A solution of 0.01% v/v sodium bicarbonate in water was prepared and a pulse jet metering pump was used to spray the solution into the wind box at a rate of 2 liters per minute. A portable gas analyzer was used to monitor the NOx level continuously. The distance between the water spray and the gas analyzer port was 2 meters and the temperature inside the wind box was 71 o C during the spraying activity. The NOx level of 556 ppm came down to 306 ppm (about 46% reduction) rapidly in about 0.06 seconds. The SOx level reduced from 368 ppm to 20 ppm (about 94% reduction). The average SOx, NOx reduction rate was in the range of 55%. After increasing the concentration of sodium bicarbonate to 0.02% v/v, the reduction has increased to the 90% range.
Sodium bicarbonate was found to be an effective De-NOx agent. It breaks down into sodium carbonate above 80 o C. Above a temperature of 120 o C, it acts as a de-sulphurising agent. Sodium carbonate de-sulphurises above 343 o C (Keener 2016;Keener and Davis 2016). Therefore, it could be used for both De-NOx and De-SOx in various wind boxes depending upon the temperature. This advantage eliminated the need of using sodium carbonate. 46% reduction in NOx in 0.06 seconds and 94% reduction in SOx was considered as signi cant. The kinetics of the SOx and NOx reduction with varying temperature and stoichiometric ratio of sodium bicarbonate in the solid-state was discussed elaborately by Walawska (Walawska et al. 2013). The NOx removal e ciency was reported as dependant on the temperature. The highest value corresponds to 30 to 50 seconds of contact at 300 o C and the lowest at 850 o C. The same principle was adopted in the present study; and found that the kinetics were many folds higher (55% removal in 0.06 seconds).
A 500 gram of sodium bicarbonate in 50 liters of water corresponded to 0.01% v/v or 10,000 ppm. The typical NOx particulates present in the gas were in the range of 400 ppm. The probability of the droplets of the solution coming in contact with particulates was considered fair enough. The underlying principles of SOx, NOx removal are given below in the form of equations Ji et al. 2017a).
To increase the effectiveness of NOx removal especially, the concentration was increased from 0.01% v/v to 0.02% v/v. The removal of SOx essentially depends upon the transformation of sodium bicarbonate to sodium carbonate at temperatures above 170 o C. However, in the above experimental work, the removal of SOx was satisfactory at temperatures in the range of 110 to 140 o C also. This was attributed to the high static pressure (-120 mBar) and velocity of the particles (Lanzerstorfer and Steiner 2016;Lanzerstorfer 2017). The summary of SOx, NOx reduction is given in Fig. 25. The variation of SOx and NOx removal with temperature was noticed to be non-linear and is indicated in Fig. 26.

Brownian Motion and Enhanced Kinetics
The rapid denitri cation can be explained with the principle of Brownian Motion. Brownian motion signi es the incessant movements of particles in random directions in a solution in which the particles are much larger than the solvent molecules. Analogically, if the gas laden with NOx particles is regarded as a continuum, the sodium bicarbonate spray can be considered as the solvent molecules. The velocity of the continuum is 20 to 100 m/s, in which the NOx particles are also traveling. The ionic solution of sodium bicarbonate enters the wind box, get sprinkled into tiny droplets and colloid with them. However, NOx particles have Brownian Motion. As they drift from the original path, the probability of colliding with sodium bicarbonate-laden water droplets increases. The time for the motion of sodium bicarbonate molecules is shorter than that for the motion of NOx particles. The particulates in the gas and the sodium bicarbonate particles move randomly following Brownian dynamics wherein collisions in the tune of 10 12 to 10 20 are possible.
Additionally, as the solution spreads in the wind box, the suspended particles move from higher concentration to lower concentration increasing the probability of collisions with suspended particulates in the continuum of gas. The denitri cation occurs predominantly by diffusion.

Dissolution Study of PCDD/Fs using QEMSCAN
The alkaline compounds were found in the ESP dust, stack and also found sticking to the internals of the ESP. A puffy and light material was spotted sticking in the top of the last elds 4A and 4B of ESP, in large quantity, as shown in the location marked in Fig. 27. Its physical appearance and chemistry are shown in Fig. 28. It had compounds containing K, Cl, F, and C.
The deposition found in the ESP eld 4A and 4B (Fig. 7) was examined under QEMSCAN. The chlorine present in the ESP deposit was essentially from KCl and PCDD/Fs. Based on the research of Shiu and Yu (Shiu et al. 1988b;Yu et al. 2019), 10 grams of ESP deposit was dissolved in 100 ml of acetone, hexane, and toluene. The ltrates were examined in QEMSCAN and EDAX and the results are shown in Fig. 30 to Fig. 34.

QEMSCAN of Acetone Filtrate
As seen in Fig. 31, the K and Cl are clustered and are seen as lumps, whereas the oxygen is scattered. This infers that oxygen was not present where chlorine was present or in other words, PCDD/Fs were not present after dissolving in acetone.

QEMSCAN of Toluene Filtrate
After dissolving the deposit in toluene (50 grams in 500 ml), the ltrate was analyzed in XRF. The oxide analysis is given in Table 13. As seen in Fig. 34, the K and Cl are ubiquitously distributed over the entire area of the image, whereas Ca and oxygen are scattered as clusters. This infers that chlorine was not present with oxygen or in other words, PCDD/Fs are not present after dissolving in toluene.

Dissolution study of PCDD/Fs using EDS
The ltrates obtained after dissolving in acetone, hexane, and toluene were examined under EDS and the results are given below in Fig. 35 to Fig. 39. The locations with a high amount of oxygen and chlorine were given prime consideration for detecting the presence of PCDD/Fs. The other locations had KCl and uoride. The increased content of carbon is attributed to have come from the solvents namely acetone, hexane, and toluene. Discernment of particulates in the ltrates and vicinity of oxygen and chlorine clusters was studied using the mapping method. The location-wise elemental analysis is provided in Table 14 and Table 15. Locations 1,2,3,7 and 8 have both chlorine and oxygen in    Illustration of wind box and wind leg below the sinter machine   Morphology in Wind Leg 10 The deposition on its inspection door revealed the mechanism of deposition. The needle-like particles were found to impinge continuously, assimilate and appear star-like particles. Wei LV et al (Lv et al. 2018a) had reported similar micrographs. Cluster of rod (NOx, SOx, P2O5), needle (CaS, PbN) and cube (NaCl) & spherical (KCl) morphologies; indicating impingement of particulates and inter-particle diffusion & assimilation Figure 16 Page 36/51 Cubical particulates containing NaCl, KCl, KCN, C in NOx background (wind leg 10) Mapping summary of the ltrate (hexane) Figure 37 Element-wise mapping of ltrate (hexane) Figure 38 Mapping summary of ltrate (toluene) Figure 39 Element-wise mapping of ltrate (toluene) Figure 40 Schematic of sodium bicarbonate spray and nitrogen injection