Calcined colemanite as an additive for iron ore sintering process

Abstract

During sintering process, iron ores, fluxes and coke are agglomerated into a desirable blast furnace feed. The degrading iron ore quality and environmental norms has forced sinter producers to look for additives which can improve the sintering process efficiency. For the current work, use of calcined colemanite as an additive was studied by adding it from 0% to 4% within the sinter raw blend through lab-scale pot trials. During the sintering process, calcined colemanite forms a Calcium Borosilicate (Ca₁₁B₂Si₄O₂₂) phase at temperature above 400 °C which latter gets dissociated within the liquid melt thus increasing the liquid slag formation. Increasing calcined colemanite up to 2% increases tumbler index from 56.27% to 61.67%, decreasing thereafter to 55.33% at 4% of calcined colemanite. The sinter yield (+5 mm) is also found to be maximum of 89.39% at 2% of calcined colemanite. An increase in calcined colemanite from 0% to 4% increases sintering time from 20 to 26 min, decreasing the overall sinter productivity from 2.35 to 1.99 t/m²/h. Before integrating calcined colemanite within the sintering process at plant scale, its cost and efficacy must be considered. An alternate approach can be combining it with an organic binder where the two can complement each other in providing the required sinter properties.

Keywords

Iron ore sintering calcined colemanite sinter additive sinter strength sinter time

Introduction

In the year 2021, within Asia-Pacific region, China, Japan, India and Korea collectively produced 65% of the world's crude steel. Notably, about 50–55% of India's steel is produced through the blast furnace (BF) – basic oxygen furnace route, deviating from the global average of approximately 70%.¹ Sinter, a key raw material for BF, constitutes 50–70% of the total ferrous burden. This agglomerated product, comprising iron ores, fluxes and coke, plays a crucial role in BF operations.²

The granulation efficiency of the raw mix, a significant parameter in sinter production, directly impacts bed permeability, air flow, flame speed and bed temperature profiles. Effective granulation is vital for optimising sinter productivity, final quality and yield.^3,4 Binders play a pivotal role in creating particle contacts during granulation and promoting cohesion during sintering.^5,6 With the decline in iron ore quality and the imposition of stringent environmental norms, sinter producers seek affordable and high-quality binders to enhance physical and metallurgical properties, thereby improving efficiency and reducing overall costs.⁶ While organic binders have been extensively studied for pelletisation, their application in the sintering process remains limited due to challenges in providing sufficient strength in pre-heated and fired conditions.

There have been numerous study showing the use of organic binder for pelletisation process but not many have been utilised in the sintering process. The organic binder imparts good binding properties and quality to the green mix, but, it fails to provide sufficient strength in the pre-heated and fired condition resulting in poor slag bonding.^7,8

One possible way of using a binder can be to introduce a slag forming constituent to the sinter mix so that the strength of the sinter would be reinstated as a result of slag bonding. Boron compounds could be used as potential binders since they are known to produce low melting points glassy silicate phases. Some of the boron compounds being tested as binder or additive for an iron ore based mix are: colemanite, tincal, borax pentahydrate and boric acid.⁷

Among these, colemanite, a natural boron mineral found in alkaline lacustrine deposits, exhibits a melting point of 986 °C and a chemical composition of 43% B₂O₃, 26% CaO, 6.50% SiO₂.^7,9 Colemanite's ability to form boro-silicate glasses at relatively low temperatures in the presence of silica is a key factor contributing to improved sinter strength.¹⁰ The absence of acidic impurities and alkalis, coupled with its low melting temperature and favourable thermal and mechanical properties of boro-silicate glasses, make colemanite a promising choice as a slag-forming constituent.¹¹

Boron-based compounds have been employed to enhance the bonding characteristics of fired pellets. In a study conducted by O. Sivrikaya et al., an investigation into the bonding mechanism of pellets containing colemanite revealed that the augmented preheated and fired compressive strength of these pellets can be attributed to the physical melting of colemanite at the contact points of iron ore grains during the firing process.¹² Additionally, A. A. Akberdin et al. reported that the incorporation of borate ore as a fluxing agent in pellet production resulted in improved strength of the iron ore pellets.¹³ However, limited instances exist where colemanite has been utilised in the sintering process. Given the potential of colemanite to enhance slag bonding, there is a promising prospect for its application in improving the overall quality of sintered materials.

Colemanite contains enclosed water molecules within its structure, and as temperature rises, there is a rapid increase in internal pressure, leading to an explosive loss of water. This sudden release within the micropores causes a disruption of the framework.^11,12,14 Due to this characteristic, the direct utilisation of colemanite mineral is restricted. Consequently, to mitigate such issues, it is imperative to calcine the colemanite mineral before employing it as a sintering aid.^15,16

The present research endeavours to explore the efficacy of calcined colemanite as an additive within the iron ore sintering process. To achieve this objective, six sets of laboratory-scale sinter pot trials will be conducted, wherein the proportion of calcined colemanite in the sinter raw mix will be systematically varied from 0% to 4%. The impact of different additive dosages on the sintering process will be meticulously assessed through the analysis of various process parameters, as well as the evaluation of the resultant sinter's physical and metallurgical properties.

Materials and methods

Material selection for sintering process

For the current work, six sets of sinter pot trials were planned wherein the calcined colemanite was varied from 0% to 4% as shown in Table 1. Table 2 shows the mix proportion of different raw materials used for the trials. Raw materials consisted iron ore from five different sources, limestone, BF return fines, sinter return fines. The different proportions of iron ores were finalised based on the targeted chemistry for the final sinter to be produced. To ensure good repeatability of experiments, all the raw materials were divided by rotary sample divider first to ensure homogeneity for every test.

Table 1.

Calcined colemanite proportion for sinter pot trials.

Iron ore blend	Exp-1	Exp-2	Exp-3	Exp-4	Exp-5	Exp-6
Calcined colemanite (additive), %	0.00	0.50	1.00	2.00	3.00	4.00

Table 2.

Raw material mix proportion.

Raw material, %	Exp-1	Exp-2	Exp-3	Exp-4	Exp-5	Exp-6
Calcined colemanite (additive)	0.00	0.50	1.00	2.00	3.00	4.00
Iron ore 1	8.29	7.79	8.42	9.62	10.82	10.92
Iron ore 2	7.82	7.82	7.82	7.82	7.82	7.82
Iron ore 3	4.05	4.05	4.05	4.05	4.05	4.05
Iron ore 4	11.11	11.11	11.11	11.11	11.11	11.11
Iron ore 5	13.33	13.33	12.20	10.00	7.80	6.70
Pellet fines	4.50	4.50	4.50	4.50	4.50	4.50
BF return fines	12.50	12.50	12.50	12.50	12.50	12.50
Limestone	4.90	4.90	4.90	4.90	4.90	4.90
Dolomite	5.30	5.30	5.30	5.30	5.30	5.30
Micro pellets	3.70	3.70	3.70	3.70	3.70	3.70
Anthracite coal	1.90	1.90	1.90	1.90	1.90	1.90
Coke breeze	2.00	2.00	2.00	2.00	2.00	2.00
Sinter return fines	16.00	16.00	16.00	16.00	16.00	16.00
Calcined lime	2.60	2.60	2.60	2.60	2.60	2.60
Steelmaking slag	2.00	2.00	2.00	2.00	2.00	2.00
Total	100.00	100.00	100.00	100.00	100.00	100.00

Calcined colemanite used for the current study was procured from a third party vendor and was added in powder form to the base mix before the granulation process. Chemical analysis for the iron ores and calcined colemanite used for the sinter pot trials is shown in Tables 3 and 4 respectively. Calcined colemanite used for the trials had 44.50% CaO and 27% B₂O₃. Table 5 shows the particle size distribution for calcined colemanite, it has a mean particle size of 2.15 mm. Bulk density for the calcined colemanite is found to be 1.10 t/m³. Table 6 shows base mix analysis for all the six experiments.

Table 3.

Chemical analysis of the iron ores used in the raw mix.

Iron ore blend	Fe(t), %	SiO₂, %	Al₂O₃, %	CaO, %	MgO, %	MnO, %	LOI, %
Iron ore 1	63.71	4.41	1.95	0.011	0.009	0.012	2.21
Iron ore 2	58.68	4.50	2.00	0.17	0.10	4.50	4.58
Iron ore 3	62.73	2.39	3.18	0.15	0.04	0.07	4.00
Iron ore 4	59.65	8.00	3.63	0.13	0.06	0.09	3.04
Iron ore 5	56.55	7.51	4.32	1.26	0.14	0.57	4.90

Table 4.

Chemical analysis of calcined colemanite.

Composition	wt.%
Fe₂O₃	1.08
SiO₂	14.29
Al₂O₃	1.59
CaO	44.50
MgO	6.30
MnO	0.038
P	0.036
Na₂O	0.17
K₂O	1.02
B₂O₃	27.00

Table 5.

Particle size distribution of calcined colemanite.

Size fractions	Calcined colemanite, %
+6.3 mm	0
+5.0 mm	2.36
+4.0 mm	9.54
+3.15 mm	8.06
+1. mm	56.05
+0.5 mm	14.95
−0.5 mm	9.05
Total	100.00
MPS, mm	2.15

MPS: mean particle size.

Table 6.

Base mix analysis for the sintering process.

Base mix	Fe(t), %	SiO₂, %	Al₂O₃, %	CaO, %	MgO, %	MnO, %	Na₂O, %	P, %	S, %	LOI, %	B₂O₃, %	C, %	Basicity (B2)
Exp-1	48.93	6.30	3.14	11.69	1.68	0.91	0.04	0.07	0.12	7.34	0.00	4.03	1.85
Exp-2	48.58	6.35	3.14	11.92	1.71	0.91	0.04	0.06	0.07	0.12	0.14	4.03	1.88
Exp-3	48.31	6.37	3.11	12.13	1.74	0.91	0.04	0.07	0.12	7.28	0.28	4.03	1.91
Exp-4	47.76	6.39	3.04	12.55	1.80	0.89	0.04	0.07	0.12	7.19	0.57	4.02	1.96
Exp-5	47.21	6.41	2.98	12.98	1.86	0.88	0.04	0.07	0.12	7.09	0.85	4.02	2.02
Exp-6	46.59	6.48	2.95	13.41	1.93	0.87	0.04	0.07	0.11	7.03	1.13	4.02	2.07

Sintering conditions

Sinter pot trials consist of two steps as shown in Figure 1, first step is preparation of green mix with an optimum amount of moisture and second step is to conduct the pot trials using the green mix.

Preparation of green mix

First, an iron ore blend was prepared by mixing the different iron ore as shown in Table 3 in required proportion. Within the iron ore blend, coke breeze, flux, return fines along with required proportion of calcined colemanite, are added and mixed properly and a base mix is prepared. This base mix is transferred to the granulation drum. Initially dry mixing is carried out for 4 min followed by water addition and then wet mixing for another 5 min. The amount of water to be added depends on the moisture levels within the wet mix which are to be maintained at around 8%. Fine particles adhere during the granulation process to form grains of appropriate size.¹⁷ Table 7 shows the granulation drum specification.

Table 7.

Specification for the granulation drum.

Drum diameter, m	1.00
Drum length, m	0.50
Rotation speed, rpm	15

Sintering pot trials

Experiments were conducted using a lab-scale sintering machine. It consist of a cylindrical bed, a burning hood, suction fan with a cooling mechanism and an exhaust pipe. Lab-scale sinter pot setup shown in Figure 2 was used for the current studies. Arrangements were made to insert thermocouples at three different heights of the bed and one in the wind box. The final granulated mix obtained from the granulation drum was placed in a cylindrical drum ‘sinter pot’. Allowing the suction fans to run, the top surface was ignited for 2 min and the flame front was allowed to travel downward. There are thermocouples placed throughout the sinter bed to monitor and control the movement of flame front through the sinter bed. The thermocouple helps to monitor the movement of flame front through the sinter bed to understand the pace at which sintering is being performed. Thermocouple helps to monitor the temperature differential across different zones of the sinter bed. One more thermocouple is placed along the wind box to monitor the burn through point, that is, the point at which wind box attends the maximum temperature, which signifies the conclusion of the sintering process. At the end of the experiments, fused mass of iron ore, limestone and coke in the form of sinter is produced. Table 8 shows the test conditions that were applied for all the trials.

Table 8.

Sintering parameters applied for all the trials.

Ignition of top surface	120 s
Air suction (during ignition)	400 mm of H₂O
Air suction (after ignition)	1200 mm of H₂O
Hearth layer	60 mm
Total bed height	700 mm
Ignition flame temp	1300 °C
Sinter pot diameter	300 mm

The final sinter product from all the experiments was tested for its metallurgical and physical properties including productivity, sintering time, product yield, strength and¹⁸ fine generation.

Theoretical calculation using FactSage

FactSage is a commercial software used for thermodynamic calculations in ironmaking and steelmaking. For the current work, before conducting the actual trials FactSage was utilised to understand the liquid melt formulation during the sintering process. The base mix chemistry shown in Table 6 was used as the input raw mix for the FactSage calculations. The reactions happening for all the planned experiments from 400 °C to 1300 °C in the step of 50 °C were calculated. Based on the base mix blend, FactSage calculations were performed to identify the intermediate phases of boron formed during the sintering process and also to track the boron's presence within the final liquid melt of product sinter.

It is crucial to note that there is no solution databases for SFCA (silico-ferrites of calcium and alumina) phase in Factsage up to now. During the sintering process, the primary reactions are liquid phase reaction, involving slag generation, and the solid reactions between FeO, CaO, SiO₂, MgO etc. For the current study, the database of FactPS and FToxid were selected along with FToxid-SLAGA and FToxidMeO_ phases to calculate the liquid melt generation and solid-state reactions. The details of these databases are present in the software or on the official site of the software.¹⁹

The amount of liquid phase generated during the sintering process at different temperatures is shown in Figure 3. It is seen that the amount of liquid phase increases with increasing the temperature. It is found that the amount of liquidus phase is found to be increasing with an increase in additive proportion and increasing temperature. The amount of liquid phase generated at 1300 °C increases from 63.45% to around 70.65% with an increase in additive percentage up to 4%. It is found that a significant portion of the boron within the additive ends up in the final slag melt in the form of B₂O₃. Along with B₂O₃, other phases such as NaBO₂, B₂S₃ and NaBS₂ are present in the final melt in a negligible proportions.

Additionally, from the FactSage calculation it was observed that there is a formation of an intermediate solid calcium borosilicate (Ca₁₁B₂Si₄O₂₂) phase at temperature above 400 °C for all the experiments with use of additive. Figure 4 shows the formation of Ca₁₁B₂Si₄O₂₂ at different temperatures. The amount of Ca₁₁B₂Si₄O₂₂ also increases with an increase in additive proportion. It can be seen from the figure that up to a temperature of 900 °C the amount of this calcium borosilicate phase increases whereas after that the phase starts to decrease.

In order to understand the movement of boron from this phase during the same time, the amount of B₂O₃ forming within the liquid melt phase was studied. Figure 5 shows the amount of B₂O₃ present within the liquid melt phase between 400 °C and 1300 °C. From the graph it can be seen that above 900 °C the amount of B₂O₃ phase within the liquid phase increases. Thus, it can be understood that above 900 °C the calcium borosilicate phases starts to dissociate within the melt and the boron is present in the melt in the form of B₂O₃. Majority of the boron within the additive ends up in the final liquid melt in the form of B₂O₃, a very small negligible proportion of boron is present in the form of NaBO₂, B₂S₃ and NaBS₂. Thus it can be understood that the intermediate calcium borosilicate phase contributes significantly to the increase in liquid melt formation which can be observed in Figure 3.

Based on these findings from the Factsage calculations, it was understood that with the use of additive there is an increase in the liquid melt formation that could benefit the sintering process and increase the slag bond phenomenon. Also during the sintering process, the additive initially forms a calcium borosilicate phase at a temperature above 400 °C that remains stable till 900 °C and later gets dissociated into liquid melt. These findings also provided crucial insights into the behaviour of boron and melt formation during the sintering process, guiding the optimisation of additive proportions to achieve desired process outcomes.

Results and discussions

Microstructure and SEM analysis of sinter

Figure 6 shows the microstructure analysis of sinter produced at 0%, 2% and 4% calcined colemanite. From the microstructure analysis, phase analysis was performed to understand the variation of different phases within the sinter sample. Figure 7 shows the variation of different phases within the sinter as seen in the microstructure. It can be seen that with an increase in additive the amount of pores within the product sinter increases. Higher porosity implies a more porous structure with interconnected voids.²⁰ It can be seen from the microstructure that with an increase in additive the amount of SFCA phase also increases. This is in line with the increased liquidus phase observed within the FactSage calculations.

Figure 8 shows the SEM images for the sinter sample produced using 0%, 2% and 4% calcined colemanite. Point analysis with SEM was performed to understand the presence of different phases within the sinter sample. During the SEM analysis, a large number of samples were analysed and only few are shown below. Table 9 shows the various phases identified during the SEM analysis. SEM analysis showed the presence of B₂O₃ within the SFCA phase of sinter sample. Majority of the boron present within the calcined colemanite was found to be present within the SFCA of the sinter sample.

Table 9.

SEM point analysis of sinter with 0%, 2% and 4% calcined colemanite.

Sample point	FeO, %	MgO, %	Al₂O₃, %	SiO₂, %	CaO, %	B₂O₃, %
(a) Exp-1 (calcined colemanite – 0%)
1	81.19	3.02	3.62	3.8	81.19	8.38
2	71.11	0.00	6.22	6.97	71.11	15.71
3	12.09	0.00	5.92	35.33	12.09	46.66
(b) Exp-4 (calcined colemanite – 2%)
1	92.54	2.10	2.15	0.00	92.54	3.07
2	70.23	0.80	6.93	6.35	70.23	14.99
3	20.15	0.00	4.80	26.66	20.15	30.99
(c) Exp-6 (calcined colemanite – 4%)
1	55.00	0.00	4.50	17.66	55.00	22.66
2	93.75	1.75	2.05	0.00	93.75	2.18
3	10.70	0.00	2.61	24.52	10.70	29.43

Impact of calcined colemanite on wet and dry bulk density

Figure 9 shows the dry and wet bulk density variation of sinter raw mix at different levels of calcined colemanite addition. Bulk density represents the amount of material that can fit into a given space or volume. It is measured before and after water addition in dry and wet form. It plays a critical role during the iron ore sintering process as it can affect sinter bed compaction, thus affecting the sinter bed permeability, bed stability, gas flow and combustion along the bed.²¹ From the graph it is clear that there is not much variation in the bulk density parameter of sinter raw mix. The dry bulk density is found to be constant at around 1.90 kg/m³ and the wet bulk density is constant at around 1.65 kg/m³. This lack of improvement in dry and green mix bulk density signifies calcined colemanite's limited influence on green mix strength and quality. It improves the slag forming ability of the sinter with early and enhanced melt formation at elevated temperature thus it has no impact at the initial green mix stage.¹¹

Impact of calcined colemanite on sintering time and flame front speed

Figure 10 shows the impact of calcined colemanite on sintering time and flame front speed (FFS). The sintering time is the time between the start of ignition till the exhaust gas temperature reached maximum. The FFS can be defined as the speed at which the flame front travels through the sinter bed. It can be seen that with an increase in calcined colemanite from 0 to 4% the sintering time increases from 20 to 26 min and decreases the phase at which flame front moves through sinter bed. Calcined colemanite forms low melting point silicate phases leading to a dense and rigid sinter that hinders the diffusion of gases and liquids, which increases the sintering time. Also with an increase in calcined colemanite there is an increase in low melting point phases which takes more time to form as well as more time to cool down.¹⁰

Impact of calcined colemanite on sinter strength

Figure 11 shows the variation in Tumbler Index (TI) and Abrasive Index (AI) of the sinter with the use of calcined colemanite. The strength of the produced sinter is determined by the Tumbler test (TI) IS 6495:1984.¹⁸ The TI of sinter increases from 56.27% to 61.67% up to a calcined colemanite of 2%, whereas, beyond 2% calcined colemanite it decreases to 55.33%.

Boron compounds are known for their low melting temperatures and for their ability to lower the melting temperature of silicates. During the sintering process, formation of low melting compound along the grains of iron oxides leads to solid-state bonding by inter-diffusion of contacting grains. The presence of calcined colemanite causes the partial melting and growth of iron oxide grains via binding as a function of temperature, the higher the temperature more is the partial melting leading to a stronger binding.¹²

The increase in sinter strength during the initial addition occurs because of the increase in melt formation as visible from the FactSage calculations.⁹ Raghavan et al. computed B–Fe–Si (boron–iron–silicon) isothermal section at 1000 °C.²⁰ Many different phases such as FeB, FeSi, Fe₂B, Fe₅SiB₂, Fe_4.7Si₂B, Fe₂Si_0.4B_0.6 and combinations were depicted in this study.¹² The presence of all their elements in the sinter feed forms few of these phases during the sintering process. The increase in the strength of sinter is mainly due to the bonding mechanism of these phases.

On the other hand, the calcined colemanite has significant amount of boron in their compositions and also with the presence of SiO₂ in varying amount in iron ores leads to the formation of borosilicate glasses known for their low melting temperature. Their occurrence also contributes to the strength of sinter through slag bonding.⁷

Table 10 shows the chemical analysis of the sinter produced at 0%, 2% and 4% of calcined colemanite. From the chemical analysis it is found that with an increase in calcined colemanite there is an increase in CaO and SiO₂. These sinter compositions are reduced to a ternary phase diagram of CaO–FeO–SiO₂, and the normalised compositions are plotted in the corresponding phase diagram as shown in Figure 6.²²

Table 10.

Chemical analysis for sinter produced with 0%, 2% and 4% of calcined colemanite.

Sr. No.	Calcined colemanite, %	Fe (T)	FeO	SiO	Al₂O₃	CaO	MgO	MnO	CaO/SiO₂
Sr. No.	Calcined colemanite, %	%							CaO/SiO₂
1	0	52.80	9.70	6.22	3.42	12.11	2.10	1.06	1.95
2	2	52.07	9.10	6.31	3.39	12.95	2.12	1.01	2.05
3	4	52.27	9.27	6.36	3.30	13.30	2.14	1.02	2.30

From the ternary phase diagram (Figure 12), it can be seen that with an increase in calcined colemanite from 0% to 2%, there is formation of Ca₂SiO₄ (C₂S) phase whereas, above 2% there is a formation of Ca₃SiO₆ (C₃S) phase with a comparatively high melting point.

In order to understand the melting behaviour of product sinter and presence of high temperature melting phase within the sinter produced with the use of additive, fusion analysis of sinter was carried out using Ash fusion analysis apparatus (HESSE Instruments, Model No- EM201-17K in normal atmospheric condition). Using ash fusion analysis the softening characteristic of the sample can be analysed. It shows the temperature at which the initial melt formation takes place. For the current analysis a 50 gm of sinter sample was used and heated to a temperature of 1600 °C. The sample is mounted in the form of a cylinder and heated at a 20 °C/min. Based on the analysis, the deformation temperature, softening temperature and flow temperature of the sinter samples were obtained as shown in Table 11 to understand the softening melting behaviour of sinter samples.

Table 11.

Fusion analysis of sinter sample produced using calcined colemanite.

Fusion analysis	Exp 4 (calcined colemanite – 2%)	Exp 6 (calcined colemanite – 4%)
Deformation temperature, °C	1307	1310
Softening temperature, °C	1310	1330
Flow temperature, °C	1516	1560
Deformation range, °C	1307–1516	1310–1560
Flow range, °C	1375–1516	1400–1560

Figure 13 shows fusion analysis results for the sinter sample with 2% and 4% of calcined colemanite. From the analysis it was seen that sinter sample with 4% additive shows a slightly higher flow temperature (liquidus temperature) compared to that of sinter sample with 2% additive. Flow range for the sample with 4% additive lie in the range of 1400–1560 °C showing the presence of high melting point phases as indicated by the ternary diagram.

The increase in high melting point phase during the sintering process is also indicated by the highly porous sinter structure produced as seen in Figure 6(a) to (c), which can be a reason for the drop in the sinter strength above 2% of calcined colemanite. Phase analysis shown in Figure 7 highlights an increase in both the SFCA phases and the porous structure within the sinter. Higher porosity implies a more porous structure with interconnected voids, which can weaken the overall cohesion and load-bearing capacity of the sinter.²⁰ Also with increased sintering time, the glassy silicate phases inside the sinter tends to have a more crystalline structure with a comparatively lower strength which can be another reason for the drop in strength beyond 2% of calcined colemanite as seen in Figure 6(d) to (f). Increased glassy silicate phases further contribute to the formation of a more porous structure.⁶

Impact of calcined colemanite on sinter product yield (+5 mm) and fine generation (−5 mm)

Figures 14 and 15 show the impact of calcined colemanite on sinter product yield (+5 mm) and fine generation (−5 mm). The sinter product yield and fine generation are defined as the +5 mm and −5 mm size fraction of the produced sinter respectively. With an increase in calcined colemanite up to 2% it is observed that the sinter product yield initially increases to 89.39% but declines thereafter. While on the other hand, it is observed that the fine generation decreases with increase in calcined colemanite up to 2% reaching a minimum of 10.61% and increases thereafter. The variation in the sinter product yield can be attributed to changes in sinter strength which leads to the variation in sinter final product output.²³ At a calcined colemanite proportion of 2%, it has been observed that the sinter strength reaches its maximum level, resulting in the formation of more robust sinter and ultimately reducing the occurrence of breakages and dropping fine generations to a minimum level thus increasing the product yield to a maximum extend.

Impact of calcined colemanite on sinter productivity

Figure 16 shows the impact of calcined colemanite on sinter productivity. The sinter productivity is the sinter product yield (+5 mm) per unit area of the sinter pot per unit sintering time. With an increase in calcined colemanite, productivity of the sinter decreases from 2.35 to 1.99 t/hr/m². Even though there is an increase in sinter product yield up to 2% of calcined colemanite there is also an overall increase in sintering time thus, the productivity of the sinter is found to be decreasing.

Conclusion

Based on the FactSage analysis, it was understood that using calcined colemanite as an additive increases the overall liquid melt formation, thus enhancing the slag bonding. Microstructure and SEM analysis of the sinter sample showed that nearly all the boron from the calcined colemanite ends up in SFCA phase within the sinter as predicted by the Factsage calculations.

The utilisation of calcined colemanite has demonstrated negligible impact on both dry and green mix bulk density, indicating minimal contribution to the green mix strength in the sintering process. The introduction of calcined colemanite up to 2% results in an elevation of sinter TI from 56.27% to 61.67%. However, a further increase to 4% leads to a decline in sinter TI to 55.33%. The maximum sinter product yield (+5 mm) is achieved at 2% calcined colemanite, reaching 89.39%. As the calcined colemanite content rises from 0% to 4%, the sintering time increases from 20 to 26 min, consequently reducing the overall productivity of the sinter from 2.35 to 1.99 t/m²/h. Notably, the addition of 2% calcined colemanite yields favourable outcomes in terms of sinter strength and product yield, albeit with a decrease in sinter productivity.

In conclusion, the findings emphasise that the incorporation of calcined colemanite, or inorganic additives in general, offers minimal improvement in the cohesive and adhesive forces crucial for the establishment and maintenance of green pellet integrity. The observed lack of enhancement in dry and green mix bulk density indicates a limited impact on green mix strength and quality. Considering the associated cost of calcined colemanite, the marginal increase in sinter strength, accompanied by prolonged sintering time and reduced productivity, challenges its viability as a standalone effective additive for the sintering process.

To overcome these challenges, a promising approach can be combining calcined colemanite with an organic binder. This synergistic combination can leverage the granulation benefits of the organic binder, while simultaneously harnessing the boron compounds from calcined colemanite to impart the necessary properties to the sinter during its formation at elevated temperatures.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Dhanraj Patil

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