- Title
- Influence of mineralogy and pore structure on the reducibility and strength of iron ore sinter
- Creator
- Harvey, Tobin
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2020
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- This study investigated the influence of the mineralogy and the pore structure on the reducibility and strength of iron ore sinter. It was important to develop a mechanistic understanding of iron ore sintering with consistency across a range of typical size scales. Therefore, sintering was studied on a range of scales from industrial sinter samples to pilot scale pot grate sinters and laboratory scale analogue tests. Characterisation of the pore structure of sinters was achieved using a combination of X-ray computed tomography (CT), mercury intrusion porosimetry, nitrogen pycnometry and optical microscopy. The mineralogy of sinters was measured primarily by optical point counts with comparison made to both SEM and XRD based classification methods. Multilinear regression analysis was applied to understand the influence of mineralogy and pore structure on the reducibility and strength of pot grate and industrial sinter samples. Statistically significant differences between pot grate and plant sinters were observed with higher strength but lower reducibility measured in plant sinters. Increased primary hematite, decreased glass and lower porosity were correlated with improved tumble index. Increased glass content was correlated with decreased reducibility. Differences between pot grate vs industrial sinters were not accounted for by mineralogy and pore structure measurements alone, indicating either key parameters were not measured or the random variability in the samples was too high for adequate correlations to be obtained. Variability between particles was caused by local variations in raw material distribution, flame front temperature profile and gas atmosphere during the sintering process. Assimilation type analogues and bonding phase type analogue tests were created in an infra-red rapid heating furnace. By tightly controlling the sintering process conditions the variability between samples was controlled. Using analogue sinters, the maximum temperature, holding time, cooling rate, atmosphere and ore type were varied independently. The mechanisms of mineralogy and pore structure development were interpreted by focusing on the solid, liquid and gas reactions, formation and rearrangement during sintering. A full factorial experimental design was used to investigate the effect of maximum sintering temperature, atmosphere pO₂ and ore type on assimilation type iron ore sinter analogues. These analogues were created from 0.24 g of a chemical reagent adhering fines mixture (73.2% Fe₂O₃, 4.5% SiO₂, 1.9% Al₂O₃, 18.6% CaCO₃ and 1.8% MgCO₃) and 0.36 g of -1.0+0.71 mm ore. The fired basicity of the analogues was around 1.0, and the firing pattern followed a typical pot grate mid-point temperature profile. The structure of these analogues was characterised using high resolution X-ray CT with voxel size 4.5 μm. This determined the size, shape and connections of the pores within the samples. Most of the open pore volume was contained in a single continuous interconnected pore network. Analysis showed maximum temperature, ore type and the interaction between ore type and maximum temperature to have a statistically significant impact on tablet volume. A similar analysis showed maximum temperature, ore type and the interaction between maximum temperature and partial pressure of oxygen (pO₂) to have a statistically significant impact on total porosity. Greater melt volumes increased the size, sphericity and total volume of pores. However, the mineralogy of the assimilation type analogues was identified as being significantly different from a typical plant or pot grate sinter. To create analogues with similar mineralogy to a typical plant or pot grate sinter, the bonding phase analogue methodology was developed. These analogues were created from the -1.0 mm fraction of natural ore to which chemical reagent fluxes were added to give a fired basicity of 2.0, SiO₂ = 5.4 % and MgO = 1.8 %. These analogues were fired in the rapid heating furnace varying the maximum temperature (TMAX = 1250 - 1320 °C), holding time (tHOLD = 0.17 – 4 min) and cooling rate (RCOOL = 1 – 5 °C/s). Optical point counting was the main method used to classify the resulting mineral phases. Maximum temperature, was identified as having the strongest impact on mineralogy followed by holding time and cooling rate respectively. Oxidising atmosphere during cooling re-oxidised magnetite and promoted the formation of silico-ferrite of silicon and aluminium (SFCA) from hematite and silicate melt. Higher temperature and longer hold time promoted the transformation of SFCA- I to SFCA. The results of optical microscope mineral (OM) quantification were compared against SEM based (TIMA) and XRD quantification methods. The comparison showed general agreement between the methods, however, SFCA-I was not identified in analogues using XRD despite being identified in significant quantities in some of the analogues using OM. Clearly, classification of SFCA by texture yields different results to chemical and crystallographic techniques. The pore structure of replicate samples created under the same conditions was measured using a combination of MIP, nitrogen pycnometry, and optical microscopy. Solid-state sintered samples were also created using a tube furnace, sintering for long times (3 h) at 1100 °C. The pre-liquid phase (≤ 1100 °C) had only minor influence on the structure by solid state sintering. Liquid formation significantly increased the rate of pore structure change by increasing the diffusion rate, viscous flow and rearrangement of materials driven by capillary pressure of the wetting melts. Calculations showed finer particle sizes to promote densification due to higher Laplace pressures in the capillaries. Experimentally densification rate was increased with increasing maximum temperature. Holding time and cooling rate were identified as having less impact compared to maximum temperature. In TMAX = 1320 °C over-sintering was observed. This was identified by the swelling of the total analogue volume and was caused by a swelling of the pores within the analogues. The mechanisms contributing the majority of pore swelling was proposed to be a combination of: Bubble coalescence reducing the Laplace pressure inside the closed pores; and Gas generation inside the sinter from the reduction of hematite to magnetite. The strength of bonding phase analogues was measured using an axial compression test. Strength of sinter analogues had a strong dependence on the maximum temperature. It was hypothesised that the enclosed area metric (EA) would be a useful predictor of sinter strength, however, EA was not to be a good predictor of sinter strength in this study, with similar EA values shown to have significantly different strengths and pore structures. Maximum strength was achieved at TMAX = 1320 °C, tHOLD = 1 min and RCOOL = 5 °C/s and corresponded to minimum porosity. Longer sintering time at TMAX = 1320 °C decreased strength due to over-sintering caused by pore swelling. Mineralogy was not identified as significantly influencing strength. Extrapolating the results from this work has implications for industrial iron ore sintering. Sinter in the lower portion of the bed is exposed to higher temperatures for longer times. If the temperature is too high for too long, pore swelling may occur, degrading the sinter strength. This analysis suggests that manipulation of the sintering process to provide a short time at high temperature may maximize sinter strength. The effect of ore type on strength was also investigated. For different natural ores the same trends of strength vs sintering conditions was observed. This gave confidence that the sintering mechanisms including the mechanisms of over-sintering were not specific to one type of ore alone. Statistically significant differences between the ores were identified and variation was seen depending on the sintering conditions. The reducibility of analogues was measured isothermally at 900 °C in an atmosphere of 30 % CO and 70 % N₂. Firing analogues at TMAX= 1320 °C for 10 s resulted in an improvement in the degree of reduction after 60 minutes (R60) over the unfired analogues, however, a significant decrease in reducibility occurred with longer holding times. The highest reducibility was obtained at low temperature for a longer time, suggesting that there is a trade-off between the conditions required for maximum strength and maximum reducibility in iron ore sinters. Reducibility in analogue sinters was best predicted by structural parameters. Analogues with higher pre-reduction porosity and larger pore size were found to be more reducible. Mineralogy was not identified as having a major impact on the reducibility. By separating the variables TMAX and tHOLD that are normally linked in iron ore sintering, analogues with similar reducibility but different mineralogy were produced. In addition, analogues with similar mineralogy but different reducibility were also produced. This result together with the good correlation between structure and reducibility suggests the structure of iron ore sinters is more important to reducibility than their mineralogy. Where structure, refers to the spatial distribution of solid and void (pores) of which the sinter is composed. Bonding phase sinter analogues were compared to pot and plant sinters based on their structure, mineralogy, strength and reducibility. Analogues fired to TMAX = 1320 °C, tHOLD = 1 min and RCOOL = 5 °C/s had both mineralogy and open pore structure most similar to a typical pot or plant sinter. Extrapolation of the analogue reducibility to 20 mm diameter particles using a kinetic equation predicted an analogue reducibility that fit within the normal range measured for pot and plant sinters.
- Subject
- iron ore; sinter; strength; reducibility; mineralogy; pore; structure
- Identifier
- http://hdl.handle.net/1959.13/1412610
- Identifier
- uon:36504
- Rights
- Copyright 2020 Tobin Harvey
- Language
- eng
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