*3.2. Softening Behaviour of Sinters*

As the temperature increased, the sample bed initially expanded during the softening tests, and then there was a continuous contraction. The present study defines the temperature at 0% displacement contraction as the softening starting temperature (Ts). At 40% displacement contraction, the temperature is defined as the softening ending temperature (Te). Finally, the temperature at 100% displacement contraction is defined as the melting temperature (Tm) [15]. The softening and melting temperatures are represented in Graph A in Figure 7 by considering the S1 curve. The three sinters' and coke's displacement contraction curves are shown in Graph B in Figure 7.

**Figure 7.** Graph (**A**) indicates the softening and melting temperature at the respective displacement percentage for sinter S1. Graph (**B**) shows the sinter softening and melting behaviour with initial expansion followed by continuous contraction for the three sinters, S1, S2 and S3.

The softening temperature range is the difference between the softening ending temperature and softening starting temperature (Te-Ts). The cohesive zone range is the difference between the melting and softening starting temperatures (Tm-Ts). Therefore, as the sample expands and then contracts back to the initial position, then the temperatures of Ts followed by Te and Tm are calculated at the corresponding displacement percentage. The softening and cohesive range for the three different sinters are detailed in Table 5.

**Table 5.** Softening and melting temperatures of the sinters S1, S2, and S3 along with softening and cohesive range.


The sinters should have a higher softening starting temperature and a lower melting temperature, resulting in a narrow cohesive zone for the efficient functioning of the blast furnace. Although the three sinters are currently used as blast furnace feed, on conducting the softening and melting experiments and measuring the cohesive range among these sinters, S1 has the narrowest cohesive range, while S3 has the broadest. In addition, S3, the sinter with the highest FeO content and silica content among the three sinters, has the lowest softening start temperature and the highest melting temperature.

## *3.3. Correlation of Sinter Liquid Properties to Sinter Softening Properties*

Understanding sinter bulk compositions on sinter softening behaviour is essential in preparing sinters with higher softening start temperature, low-FeO containing slags on reduction, and a narrow cohesive zone. In addition, the estimation of phases present and phase compositions at peak sinter bed conditions is vital, as the sintering liquid phase properties play a crucial role in the final sinter morphology. Once the sintering liquid properties were determined by reheating and quenching the sinters, it was followed by the sinters' softening and melting behaviour analyses. Finally, the impact of sinter bulk

compositions and gangue content ((CaO + MgO + SiO<sup>2</sup> + Al2O3)/TFe) [3] on sinter cohesive zone behaviour was analysed and correlated to the sinter liquid behaviour in Figure 8.

**Figure 8.** Correlation of sintering liquid proportion and sinters gangue content with the sinters cohesive range.

The liquid proportion, gangue content, and cohesive range have increased from S1 to S3. Though the rise in gangue content from S1 to S2 was lower compared to the increase from S2 to S3, as shown in Figure 8, the liquid phase proportion more than doubled from S1 to S2, and the increased silica, calcia, and FeO contents along with decreased alumina in the sintered bulk, as detailed in Table 1, contributed to this. Despite an increase in the softening starting point from S1 to S2, as shown in Table 5, the significant rise in liquid proportion might have led to an increased glass phase in the sinters, resulting in the generation of viscous slags containing higher FeO, causing coagulation, and therefore leading to an increase in the melting point of S2. Additional increases in silica, calcia, and FeO contents in S3 showed a further rise in the liquid phase proportion. In S3, the liquid phase proportion was nearly 50% at the peak temperature of 1300 ◦C. It is far beyond the optimal liquid phase required; this resulted in a lower softening starting temperature and further broadening of the cohesive range from S2 to S3. This correlation shows the necessity of controlling the sintering liquid behaviour to achieve better sinter properties.

#### **4. Conclusions**

The sinters' reheating and quenching experiments determined the phases and phase compositions present at the peak sinter bed conditions. The liquid basicity and the liquid proportion increased with an increase in silica content in sinter bulk. Apart from that, the liquid phase proportion increased on increasing the peak operational temperature.

As the gangue content increases in the sinters, broadening/widening of the cohesive range is observed, detrimental to blast furnace efficacy. This suggests that though an optimal liquid phase is needed to bind the blend, the excess liquid raises the gangue content in sinters, resulting in high-FeO containing slags on reduction, apart from increasing the glass phase and increased sinter returns generation.

This research is a primary step in exploring the relationship between sintering liquid properties, sinter bulk properties, and softening properties. This study helps better understand the sintering liquid behaviour at peak temperature, which impacts the final sinter morphology, thereby the reduction behaviour of the sinters.

**Author Contributions:** Conceptualization, V.K., T.E., F.T., B.Z. and X.M.; methodology, B.Z. and X.M.; validation, V.K., B.Z., F.T. and X.M.; formal analysis, V.K.; investigation, V.K.; resources, D.W., W.P., S.C., T.E., B.Z. and X.M.; data curation, V.K. and F.T.; writing—original draft preparation, V.K.; writing—review and editing, V.K., F.T. and X.M.; visualization, V.K., D.W., W.P., S.C., F.T. and X.M.; supervision, X.M.; project administration, F.T. and X.M.; funding acquisition, F.T., B.Z. and X.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Shougang Group and Rio Tinto Iron Ore.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors would like to thank Shougang Group and Rio Tinto for providing the sinter samples and their financial support. The authors acknowledge the facilities, and the scientific and technical assistance, of the Australian Microscopy and Microanalysis Research Facility at the Centre for Microscopy and Microanalysis, the University of Queensland.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

