2021 Volume 61 Issue 5 Pages 1469-1478
In the iron ore sintering process, the resistance to air flow is a major factor in deciding the flame front speed, which influences the sinter productivity and quality. In this work, pressure drop during sintering and the resistance to air flow was investigated in milli-pot sintering for different coke rates. The sintering experiments were conducted in a milli-pot (diameter 53 mm, height 400 mm) and pressure and temperature were measured at the same locations in the bed by four taps located equidistant to each other. The yield of sinter product was measured following a modified drop test and the mineralogy of the sinter product was analysed. The results from milli-pot sintering were then compared to the reported results from standard pilot-scale sintering, and it was found that the lower half of the milli-pot bed gave a reasonable representation of the pilot-scale sintering process. The results of sinter mineralogy, yield and productivity of the lower half of milli-pot at 5.5–8.0% coke rate were found to be similar to pilot-scale sintering tests at a corresponding coke rate from 3.5 to 5.5%. The maximum resistance to air flow in the bed was found to be in the region between the leading edge of the flame front at ~100°C and the trailing edge of the flame front at ~1200°C. This suggests that the maximum resistance to air flow includes the effect of de-humidification and combustion in addition to the high temperature “flame front” region usually defined at temperatures above 1100°C or 1200°C.
Iron ore sintering is widely used in the iron making industry to agglomerate iron ore fines for blast furnace ironmaking, which accounts for about 70% of the steel production around the globe.1,2,3) High demand for iron needs high production of sinter, which makes it important to enhance sinter productivity while keeping rigorous control of the sinter quality. Along with the raw mix composition, sintering time, maximum temperature achieved during sintering, and residence time at high temperature are the main factors controlling the quality of produced sinter. These factors are very much dependent on the resistance to air flow through the bed during sintering, which propagates the flame front down the bed. Flame front propagation is a complex process which involves heat transfer, a number of chemical reactions, phase change through melt formation and associated structural changes in the bed during sintering.4,5,6,7,8,9)
Resistance to air flow during sintering is an active research area which has been previously investigated by many authors and several theoretical and numerical models have been developed.9,10,11,12,13) These models typically suggest that the maximum resistance to air flow occurs in the flame front.1) Loo and Hutchens11) introduced a semi-empirical equation to quantify the resistance to air flow during sintering and showed that sintering air flow is a strong function of air flow in the green bed before sintering. In the model of Loo and Hutchens, the greater resistance to air flow during sintering was explained in terms of the structure of the flame front. The author’s previous work8) and Zhou et al.9) have shown that the pressure drop in the humidified bed and sintered bed account for ~30% to ~40% of total pressure drop during the initial period of sintering. The model of Loo and Hutchens can therefore be extended by considering the resistance of a range of zones occurring during sintering i.e. humidified bed zone, flame front zone, sintered bed zone. To quantify the resistance to air flow, the flame front is identified by the temperature progress in the sinter bed caused by the combustion of coke during sintering experiments and many definitions have been reported in the literature.6,14,15,16,17,18) Mostly these definitions are oriented around the coke combustion temperature or reaction zone in sintering. Loo et al.19) proposed that the flame front has a leading edge, where coke started to burn and trailing edge, where coke burned out. A starting temperature for the flame front of 700°C was proposed by Long16) whereas a higher temperature of 800°C was proposed by Fu et al.15) and both reported the end of the flame front at maximum temperature. Zhou et al.9) measured the pressure drop at multiple points in their pilot scale sinter pot, however the temperature was only measured at one or two locations. Zhou et al.9) concluded that the maximum resistance occurred in a high temperature zone which included the combustion zone (>727°C) and melting zone (>1100°C). The presence of different definitions of flame front in the literature6,9,14,15,16,17) shows the necessity to develop a better understanding of the specific region where the maximum resistance to air flow occurs.
Commonly, the pilot sinter pot with diameter around 300 mm is used to investigate the resistance to air flow during sintering, and this is believed to closely represent the sintering on industrial scale.2,3,10,20,21,22,23) These experiments can be costly, time consuming and labour intensive. A much smaller scale (~50 mm diameter), milli-pot configuration was introduced by Li et al.,3) to study the sintering process with some recommendation to change the experimental conditions to match pilot and industrial scale sintering. Li et al.3) reported temperature data in a milli-pot system for different coke rates but did not report any pressure drop or flow rate data. Whilst there are some limitations to the utilisation of the small diameter milli-pot, it has been shown to give a reasonable representation of the sintering conditions that occur in pilot and industrial sintering, and provides the opportunity to study the fundamental mechanisms of the sintering process at a small laboratory scale with the advantages of needing less material and being less expensive and less time consuming.
In the present work, the capability of the milli-pot configuration to represent the large scale sintering i.e. pilot and industrial scale was tested. The sinter material produced from the milli-pot sintering experiment was separated in two parts i.e. upper half and lower half, and characterised by mineralogy, strength and productivity parameters and compared with available pilot scale results. The milli-pot experiments were then used to investigate the pressure drop during sintering in different zones i.e. humidified bed zone, flame front zone, sintered bed zone. Pressure and temperatures were measured throughout the sintering process and analysed to identify the region where the maximum resistance to air flow occurs.
A blend of five types of iron ore fines (Australian and Brazilian) mixed with coke for fuel, dolomite, limestone and return fines was prepared maintaining a basicity (CaO to SiO2 ratio) of 1.9. Relatively higher coke rate was used in the blend as recommended by Li et al.3) to compensate for the high heat losses and wall effects in the milli-pot sintering. The detailed composition of raw mixture used in the pilot scale and milli-pot scale sintering is given in Table 1, and the approximate chemical composition of the blends is given in Table 2.
| Final Raw Mixture Component | Content, mass% (total dry mass basis) | |
|---|---|---|
| Pilot scale | Milli-pot scale | |
| Ore blend | 57.1 to 63.2 | 58.8 to 61.0 |
| Coke | 3.5, 4.05, 4.5, 5.0, 5.5 | 5.0, 5.5, 7.0, 7.5, 8.0 |
| Limestone and dolomite | 13.8 to 14.2 | 13.7 to 14.2 |
| Return fines | 20 | 20 |
| Blend Chemical Components | Content, mass% (total dry mass basis) | |
|---|---|---|
| Pilot scale | Milli-pot scale | |
| Total Fe | 56.6 | 56.7 |
| MgO | 1.8 | 2.0 |
| Al2O3 | 1.8 | 1.9 |
| CaO | 9.5 | 9.5 |
| SiO2 | 5.0 | 5.0 |
The mixture was granulated at an aim moisture of 6.5% (total wet mass basis) in a granulation drum (diameter 500 mm × depth 310 mm) at a speed of 20 rev/min. The mixture was first pre-mixed for 2 minutes followed by addition of the desired amount of water and then the granulation was continued for 3 minutes. The particle size distribution (retained mass%) and apparent density of the green feed were measured according to the procedure described by Ellis et al.24) and Li et al.25) These measured values are presented in Tables 3 and 4 respectively.
| Size range, mm | Coke rate, mass% (total dry mass basis) | ||||
|---|---|---|---|---|---|
| 5.0 | 5.5 | 7.0 | 7.5 | 8.0 | |
| +11.2 | 1.0 | 0.3 | 2.7 | 0.9 | 1.0 |
| −11.2+8 | 4.1 | 4.3 | 3.3 | 3.1 | 4.2 |
| −8+6.3 | 5.4 | 4.2 | 4.2 | 2.7 | 2.9 |
| −6.3+4 | 19.2 | 19.9 | 14.3 | 15.9 | 16.7 |
| −4+2.8 | 15.9 | 16.2 | 15.1 | 15.5 | 16.3 |
| −2.8+2 | 13.2 | 12.3 | 12.2 | 12.0 | 12.9 |
| −2+1 | 20.4 | 20.4 | 19.1 | 20.5 | 19.4 |
| −1+0.5 | 14.6 | 15.7 | 16.3 | 17.2 | 15.6 |
| −0.5+0.25 | 4.9 | 5.0 | 9.3 | 10.2 | 8.6 |
| −0.25 | 1.3 | 1.7 | 3.5 | 2.0 | 2.4 |
| Granule property | Coke rate, mass% (total dry mass basis) | ||||
|---|---|---|---|---|---|
| 5.0 | 5.5 | 7.0 | 7.5 | 8.0 | |
| SMD, mm | 1.44 | 1.38 | 1.11 | 1.12 | 1.18 |
| Apparent density, g/cm3 | 3.287 | 3.156 | 3.066 | 3.098 | 3.155 |
A well tested sintering procedure3,8,11,12) was followed for conducting the sintering experiment on both pilot and milli-pot scales. The detailed procedure and equipment used for pilot scale (314 mm diameter × 600 mm height) was described in previous work.8)
The milli-pot sintering equipment is shown in Fig. 1(a) which consists of a steel tube (diameter 53 mm, height 400 mm, insulated by 100 mm thick Kaowool), vacuum pump, suction controlling valve, flow measuring tube, waste gas scrubbing tank, pressure and temperature probes and data logger. To create suction in the wind-box, a water ring vacuum pump was used with an automatic controlled bleed valve to control the suction. Pressure and temperature measurement locations are shown in Fig. 1(b). The pot was fitted with four taps (T1 to T4 and P1 to P4) with T joint at the end. Two separate taps (T5 and P5) for measuring the wind-box suction and temperature of the off gas were provided in the wind-box, below the burden bed. An LPG burner was used to initiate the ignition of the green bed. The pressure, temperature and flow measurements were recorded by an in-house built data logging system. Type K thermocouples insulated with a 3 mm Inconel sheath were used to measure the temperature at the centre of the bed, and differential pressure transmitters (Dwyer 616D) were used with one port connected to a pressure tap and the other to atmosphere to measure the differential pressure. A hot wire anemometer (Dwyer 641) air velocity transmitter was fitted in a 53 mm diameter flow measurement tube, with 400 mm of length upstream to minimise entrance effects, and was used to measure the inlet air flow rate at the top of the bed.
Schematic of milli-pot set up. (a) Overall milli-pot configuration; (b) Milli-pot steel tube. (Online version in color.)
The sintering process in milli-pot was designed to operate under similar process conditions used on pilot and industrial scale. For milli-pot sintering, the steel tube was filled with ~1.6 kg of ore granules using a hopper. The thermocouples were inserted into the centre of the bed at each tap location and the pressure tubes were attached to the taps. A flow measuring tube was place on the top of the bed to measure air flow rate in the green bed. Sintering was initiated by replacing the flow measuring tube by an LPG burner to ignite the top of the bed and setting suction pressure to 4 kPa in the wind-box for 90 seconds. The burner was then replaced with the flow measurement tube and the suction pressure was increased to 8 kPa (i.e. 20 kPa/m). The suction pressure was kept constant throughout the sintering process and the inlet air flow rate was measured during sintering. After recording maximum temperature in the wind box, which indicated end of sintering, air flow was continued for cooling the sintered bed until the off-gas temperature reached 40°C. Once the bed cooled, air flow rates were measured in the sintered bed to determine its permeability at different suction pressure values i.e. 4 kPa, 8 kPa, 10 kPa and 12 kPa. Sinter product was then removed gently from the steel tube retaining its original structure and used for further quality tests. The wet green bed bulk density achieved in all of the milli-pot sintering experiments was in the range of 1850 to 1950 kg/m3 which is similar to the reported values of pilot and strand sintering.3,8,26,27)
2.3.2. Sinter YieldA repeated drop test procedure was conducted on the sintered plug material from the milli-pot followed by screening at 5 mm to produce sinter product and return fines fractions. The sintered material was divided into two parts: sinter from the upper 200 mm (upper half) and lower 200 mm (lower half). Due to relatively low mass of sintered product in milli-pot (~0.3 kg) compared with a pilot scale (~15 kg) for drop test, the cushioning effect during dropping of the sinter material onto a steel plate was expected to be lower. Low sample weight in drop tests is known to lead to a high degree of particle breakage and a low yield value.28,29) To compensate for the low sample mass, the standard height for the drop test procedure was reduced from 4 by 2 m to 4 by 1.5 m. The percentage of sinter product (remaining +5 mm particle size) was quantified as yield. The productivity of the upper half and lower half of the milli-pot was calculated separately assuming that the flame front speed remained unchanged during sintering, and the sintering time was divided evenly.
Samples of sinter product (+5 mm) from the upper half and lower half were crushed to < 2.0 mm size and mounted in epoxy resin blocks. The blocks were then cut and polished for microscopy (Zeiss Axioscope) analysis.30) Quantitative mineralogy data was collected using point counting method and the mineral classification scheme described in detail in Honeyands et al.31) The results of mineralogy of sinter product were reported on total volume basis and compared with the pilot scale sinter.
Figure 2 plots temperature profiles in the bed during sintering for coke rates of 5.5%, 7.0%, 7.5% and 8.0%. The temperature measured at each tap (T1 to T4) was likely to be the temperature of the gas rather than solid, however, this was affected by the local solid composition around the tip of the thermocouple, e.g., ore, coke or flux.3,7) The maximum temperature measured was limited for K- type thermocouple, so temperature readings >1370°C were not recorded in these experiments. The maximum temperature generally increased from T1 through to T4 down the bed, as expected, with higher maximum temperatures measured for the higher coke rate experiments, except for coke rate 5.5% where the temperature decreased from T3 to T4 which can be due to several reasons like incomplete sintering, non-uniform flame front or flux particles surrounding the thermocouple tip.
Temperature profiles in the bed at T1, T2, T3 and T4 during sintering for coke rate of 5.5%, 7.0%, 7.5% and 8.0%. (Online version in color.)
The temperature at tap T1 (80 mm from top) was low (~1000°C) in all cases which is attributed to higher heat loss from top of the milli-pot sintering system and the lack of heat convected from higher in the bed, which contributes to the temperature at T2–T4. Li et al.3) also reported such observation in the milli-pot system and similar observation was also reported in pilot scale sintering system.3,17,18,32)
The low temperature of T1 indicates a lower extent of agglomeration of granules in top part of the milli-pot and hence lower yield is expected in the top section of the bed. The yield calculated as per the method described earlier is plotted against coke rate for both upper and lower half in Fig. 3. The yield increases with increasing coke rate in each case, however, the upper half yield is systematically lower than the lower half. Hara et al.32) reported a low yield (~60%) in top ~10% part of the bed for pilot scale sintering. Cheng et al.33) also reported relatively lower yield in top part of the sinter bed. Yield and tumble index were related to a Melt Quantity Index (MQI, enclosed area above 1100°C in temperature-time profile) by Cheng et al.33) This is similar to the enclosed area reported by Loo34) and Harvey et al.35) It was found that yield increased with increasing melt quantity index down the bed, and that an MQI of greater than ~34000°C·s (~570°C·min) resulted in a tumble index and yield above 65% and 75% respectively.
Yield comparison of the different scale sintering at various coke rate.
Figure 4 shows the MQI calculated for milli-pot sintering as per the method described by Cheng et al.,33) at different tap locations for different coke rates. It was found that the MQI in the lower half of the milli-pot (T3 and T4) was greater than ~34000°C·s for coke rates greater than 5.5%. It can be seen from Fig. 3 that a higher coke rate is required in the milli-pot to achieve the same yield in the lower half, e.g. 7.0% coke is required to achieve a yield of ~80% in the milli-pot, compared to 5.0% at pilot scale. Corresponding coke rate from 3.5% to 5.5% in pilot scale is 5.5% to 8.0% in milli-pot. Also, a comparable yield for coke rate 5.5% in the lower half of milli-pot to its corresponding coke rate in pilot scale can be observed from Fig. 3, which indicates that the relatively low temperature at T4 in this case was due to the location of the thermocouple tip rather than being a true measurement of bed temperature. Based on the temperature profiles and yield analyses, it was noted that sintering in the lower half of the bed with coke rate adjustment was more representative of large-scale sintering. Therefore the mineralogy, productivity and analysis of pressure profile were specifically analysed for the lower half of the milli-pot in the remainder of this paper.
Melt Quantity Index at different tap location for different coke rate.
The mineralogy of the lower half sinter product from milli-pot sintering was compared to the pilot scale sinter product. Silico-ferrites of calcium and aluminium (SFCA) are reported to have high reducibility and good mechanical strength and are desirable phases in the sinter product.36,37,38,39) Magnetite and secondary hematite are the other two major components in sinter which define the sinter strength and are formed in more reducing and oxidizing conditions respectively and crystallise from the melt. Figure 5 compares the contents of primary hematite, secondary hematite, magnetite and SFCA phases for milli-pot and pilot scale sintering.
Mineralogy of the sinter product from the lower half of milli-pot compared with pilot scale sinter product.
The primary hematite shows a decreasing trend with increasing coke rate, as primary hematite dissolution in the melt is favoured for high residence time at temperatures greater than 1200°C. The primary hematite content of the lower half product of milli-pot sintering was within the range of ±10% with the sinter product from pilot scale. For SFCA and magnetite the results were within the range of those from pilot scale except for the coke rate of 7.5%. High temperature favours magnetite formation40) and due to relatively longer time at high temperature (large value of MQI) with the coke rate of 7.5%, the vol% of magnetite is high and SFCA is low in comparison to 8.0% coke rate. The higher coke rate may also have decreased the oxygen partial pressure in the sinter bed, resulting in an increase in the magnetite content of the sinter product.41,42) The comparison of magnetite content with coke rate given in Fig. 5(c) suggests that this effect was minimal. Overall, the mineralogy of sinter product from the lower half of milli-pot was within ±10% range compared to pilot scale sintering.
3.2.2. ProductivityThe productivity of the lower half sinter product from milli-pot was measured and the results were compared with the available sinter pot and sinter strand results reported in the literature,43) Fig. 6. For the corresponding coke rates, the lower half productivity was found to be ~5% lower than pilot scale sintering. Productivity depends on sintering speed and yield of the sinter product, and Fig. 3 shows yield in milli-pot sintering is equivalent to pilot scale, so the reason of lower productivity must be sintering speed. This is due to the bed pressure drop per unit length (ΔP/L) which influences the sintering time and hence sintering speed; ΔP/L used in the milli-pot (20 kPa/m) was ~25% lower than that used in pilot scale sintering (26.7 kPa/m) which resulted in different sintering speed and pilot scale (300 × 600 mm) sintering speed was between 24 to 28 mm/min compared to 18 to 21 mm/min in milli-pot sintering for different coke rates. Although ΔP/L value used in milli-pot experiments was similar to that used in the sinter strand (~21 kPa/m), the productivity was ~10% lower in the lower half of the milli-pot as the coke rate was relatively high. The same effects of ΔP/L on yield and productivity were reported by Umadevi et al.44) From productivity results comparison, it was noted that the sinter product from the lower half of the milli-pot using an increased coke rate was similar to the larger scale sinter productivities.
Comparison of the productivity in different scale sintering at various coke rate.
Overall, the yield, mineralogy and productivity in the milli-pot at coke rate from 5.5% to 8.0% showed ±10% variation from the pilot scale sinter pot for corresponding coke rate ranging from 3.5% to 5.5%. The results indicate that the lower half part of milli-pot sintering with an increased coke rate gives a reasonable representation of pilot and industrial scale sintering. Following this, it is expected that a high coke rate of ~9% in milli-pot experiments would be well suitable to represent coke rate of ~6.5% (sinter strand case) on pilot scale or sinter strand scale. This hypothesis will be tested in future experiments.
3.3. Analysis of Milli-pot Sintering Pressure and Temperature MeasurementsIn previous studies,4,7,18) flame front speed was shown to be one of the most important parameters that critically control the sinter quality. A key parameter which affects the flame front speed is air flow rate during sintering. It has been shown in the previous section that the lower half of the milli-pot sintering with increased coke rate is more representative of pilot scale sintering, hence in following sections the measurements of temperature and pressure profiles in the lower half of milli-pot have mainly been analysed and used to define the region of maximum resistance to air flow during sintering.
3.3.1. Pressure and Temperature ProfilesFigure 7 presents the time varying temperature and pressure profiles at the same locations in the bed for a coke rate of 7.5%. Similar profiles were observed for coke rates of 5.5%, 7.0% and 8.0%. P2 to P5 and T2 to T5 are the pressure and temperature measurements respectively at a certain tap. In the upper half of the milli-pot the agglomeration of granules was much less as can be seen from the temperature profile itself in Fig. 2 and yield values in Fig. 3. The pressure profiles in milli-pot sintering were similar to those in pilot scale sintering reported in Zhou et al.9) A sharp decrease in the pressure profile was observed as shown in the Fig. 7 with a concurrent temperature rise measured at the same location. This has been further analysed to calculate the pressure drop in each zone with temperature. Along with the mineralogy and yield results, pressure and temperature profile also support the conclusion that the lower half part in milli-pot sintering is well representative of pilot scale sintering.
Pressure and temperature profiles during sintering with coke rate of 7.5%. (Online version in color.)
Figure 8 shows an illustrative diagram of the potential zones that may contribute to the overall pressure drop in sintering, including the humidified bed (zone-1, < ~60°C), de-humidification and goethite dehydroxylation zone (zone 2, ~100°C to ~420°C), calcination zone (zone-3, > ~700°C), combustion zone (zone-4, > ~727°C), melting zone (zone-5, > ~1200°C) and sintered bed zone (zone-6, < ~1200°C on cooling side).9,17,19,45,46,47)
Illustrative diagram of different zones during sintering.
Pressure drop during sintering with coke rate 7.5% in zones within the height ranges tap 1 to tap 2 (P2-P1), tap 2 to tap 3 (P3-P2), tap 3 to tap 4 (P4-P3) and tap 4 to wind-box (P5-P4) were analysed (Fig. 9). The purpose of this analysis was to identify the effect of various processes on the change in pressure drop during sintering and to identify the region of maximum resistance. Pressure-drop in the zone between tap 1 and 2 reached a value of ~4 kPa even though this was a region where less agglomeration of particles was noted during sintering. The pressure drop in different zones down the bed increased as the sintering proceeds which is consistent with the higher temperatures, melt formation and higher granule agglomeration as observed in improved yield and productivity in the lower half of the milli-pot. The extent of sintering or agglomeration of granules between tap 1 and 2 was significantly lower but the pressure drop in this zone was ~50% of the total wind-box suction pressure. This observation reflects that the significant resistance to air flow during sintering can be attributed to combined effect of several other concurrent physico-chemical processes including de-humidification and calcination of fluxes, dehydroxylation of goethite and combustion. Besides the air flow resistance incurred by the above-mentioned processes, increasing melt formation at high temperature adds extra resistance which is reflected by increased pressure drop between tap 3 and 4 further down the bed.
Pressure drop between different zones for different coke rate during sintering (AFR = air flow rate). (Online version in color.)
The contribution of various processes and the pressure profiles with the temperature along the bed height were analysed to identify the RMR in the bed during sintering. In this analysis the leading and trailing edges of RMR were determined as shown schematically in Fig. 10. Three different regions in the pressure profile marked by different slopes i.e. low, medium and high can be noticed which describe the sintered bed zone, humidified bed zone and region of maximum resistance, respectively. The leading edge of flame front was determined using method (a) (Fig. 10(a)), where the proposed temperature was ~100°C at which the first structure change by de-humidification can be observed. The pressure and temperature profiles were plotted along the bed height at the time when the measured temperature in the leading edge of the flame front at a certain tap reached a value equal or greater than ~100°C. Method (b) was used to determine the trailing edge (Fig. 10(b)), ~1200°C was proposed as below this temperature solidification commences45) and generates rigid structures. The profiles were plotted at the time when the measured temperature in the trailing edge of the flame front at a certain tap reached a value equal to or less than ~1200°C. Based on these pressure profiles, different zones in the bed during sintering were identified.
Illustrative description of pressure profiles plotted by methods (a) and (b) for coke rate 5% at different time identifying three zones i.e. sintered bed zone (SZ), region of maximum resistance (RMR) and humidified bed zone (HZ), FF is >1100°C.
In Fig. 11, both pressure and temperature profiles for 7.5% coke rate are shown according to method (a). The highest pressure gradient was observed to occur between the taps with leading flame front temperature of ~100°C and its preceding tap i.e. top to tap 1, tap 1 to 2, tap 2 to 3 and tap 3 to 4 for time 1, time 2, time 3 and time 4 respectively. Between these two tap locations, all physical and chemical changes i.e. de-humidification, calcination of fluxes, dehydroxylation of goethite, combustion and melting occur. The slopes of the pressure profile in the humidified bed zone and sintered bed zone were observed to be almost invariant during the sintering process. A sharp change in slope at the leading edge of flame front at ~100°C can be identified as the start of the region of the maximum resistance.
(a) Pressure profile and (b) Temperature profile at the time when measured tap temperature on leading edge of FF reaches value ~100°C during sintering for coke rate 7.5%.
To identify the end of RMR, both pressure and temperature profiles were plotted in Fig. 12 at different time instants as per method (b) described earlier. The “time 2” profile shows that pressure drop between tap 2 (1198°C) and tap 3 (60°C) is maximum, the “time 4” profile shows maximum pressure drop between tap 4 (1192°C) and wind-box. The “time 3” profile shows that tap 3 and tap 4 temperature was 1191°C (<1200°C) and 806°C (>100°C) respectively and de-humidification zone, calcination zone, dehydroxylation of goethite zone and part of coke combustion zone was between tap 4 and wind-box whereas part of coke combustion zone and melting zone was between tap 3 and 4, hence the maximum pressure drop at “time 3” was between tap 3 and wind box instead of tap 3 and 4.
(a) Pressure profile and (b) Temperature profile at the time when measured tap temperature on trailing edge of FF reaches value ~1200°C during sintering for coke rate 7.5%.
From method (b) it was found that maximum pressure drop was observed between the tap with temperature ~1200°C and its subsequent tap with less than 100°C temperature which shows that the RMR is in front of the trailing edge of FF with ~1200°C temperature.
Based on this analysis, the region of maximum resistance is identified as the combination of the de-humidification, dehydroxylation of goethite, calcination, coke combustion and melting zone in temperature range from ~100°C in leading flame front and to ~1200°C in trailing edge of flame front, zone-7 in Fig. 8. The contribution of each of these zones to the air flow resistance has not been analysed at this stage, due to the low spatial resolution available in temperature and pressure measurements. Increases in resolution will be considered for future work.
Iron ore sintering process in a milli-pot configuration was demonstrated over a range of coke rates varying from 5.5% to 8.0%. A range of operating and sinter quality parameters were obtained and compared with the pilot and industrial scale sintering.
Analysis of bed temperatures suggested that the sintering in the lower half of the milli-pot was a reasonable representation of the large scale sintering process. The coke rate had to be increased in the milli-pot to achieve a comparable lower half sinter quality. The yield and productivity of the lower half sinter product at 5.5–8.0% coke rate were within ±10% of pilot scale sinter product at a corresponding coke rate of 3.5–5.5%. The mineralogy of the lower half sinter product from the milli-pot product at 5.5–8.0% coke rate was within ±10% of pilot scale sinter product at a corresponding coke rate of 3.5–5.5%.
Simultaneous measurement of pressure drop and temperature profiles in each zone indicated that the region of maximum resistance to air flow involved bed sections of de-humidification, dehydroxylation of goethite, calcination of fluxes, combustion and melting of granules. Further analysis of pressure profiles allowed the definition of the region of maximum resistance to air flow to be the region between ~100°C at the leading edge of the flame front to ~1200°C at the trailing edge of the flame front.
The authors gratefully acknowledge the Australian Research Council in supporting the ARC Research Hub for Advanced Technologies for Australian Iron Ore. The authors are thankful to BHP for financial support and permission to publish the paper. The authors are also thankful to University of Wollongong for allowing to use their laboratory facilities for conducting milli-pot experiments, TUNRA workshop staff, University of Newcastle to build the milli-pot and Gareth Penny and Leanne Matthews, technical staff of CIMR, University of Newcastle for assistance.
