Preparation of BF Burden from Titanomagnetite Concentrate by Composite Agglomeration Process (CAP)

Abstract

Titanomagnetite concentrate with high-TiO₂ content, is endowed with poor sintering property due to the coarse granularity and poor hydrophilicity compared to the conventional TiO₂-free iron concentrates. In this study, Composite agglomeration process (CAP), an innovative route for preparing blast furnace burden, was introduced to overcome the difficulties in titanomagnetite concentrate sintering. CAP results showed that the sinter yield of 75.52%, tumbler index of 62.87% and productivity of 1.633 t·(m²·h)⁻¹ were achieved in a laboratorial sinter pot. They are 17.38%, 13.08% and 17.82% higher than those of traditional sintering process (TSP) respectively. Moreover, CAP lowered solid fuel consumption by 3.52 kg_coke/t_product. In addition, the reduction disintegration index of the product (RDI_+3.15) was improved by 20.61%, while reduction index (RI) was decreased by 4.34%. In the CAP process of preparing BF burdens from titanomagntite concentrate, very few perovskite was generated in the pelletized part of CAP product for low CaO content. In the matrix part of CAP product, a large number of SFCA generated after perovskite fully crystallized from sinter melting for ultra-high CaO containing, thus the negative effects of perovskite on sinter quality were offset by SFCA for its excellent bonding property. Some melting phases in matrix part permeated through the outer layer of pellets and formed an interweaving structure, which bonded the pelletized part and matrix part together. As a results of this kind of mineralization mechanism, the high strength and excellent RDI property of CAP product were obtained, which proves that CAP is an effective agglomeration measure treatment for titanomagnetite concentrate.

1. Introduction

Vanadium-bearing titanomagnetite ore is an important multi-metal resource which attracts increasing attentions recently.^1,2) In China, more than 90 wt.% vanadium-bearing titanomagnetite ore reserves are located in Panzhihua-Xichang area of Sichuan province.^3,4) Titanomagnetite concentrate is one of the main dressing products of vanadium-bearing titanomagnetite ore, and is usually sintered to prepare blast furnace burden for ironmaking.^3,5) However, the sinter yield and quality of titanomagnetite concentrate are usually worse than those of conventional TiO₂-free iron ores.^5,6,7) On one hand, the deteriorated sintering performance is attributed to the crystallization of perovskite from the titanomagnetite concentrate fluxed sintering melt, as the fact that perovskite characterized by high hardness and brittleness could decrease sinter tumbler strength and reduction property.^8,9,10) On the other hand, titanomagnetite concentrate has coarse granularity and poor hydrophilicity, resulting in undesirable granulation, poor permeability of sintering bed and low productivity.^3,6)

Some methods have been proposed to solve the problems in titanomagnetite concentrate sintering. Hydrated lime was used to improve the sinter quality by improving the granulation and thermal stability of sintering mixture.^11,12) The granulation was further improved by prolonging granulation period from 4 min to 6 min and by controlling the moisture content fluctuating range of sinter mixture within ±0.1%.^13,14) Although several pretreatments, such as ball-milling, wet grinding or high pressure roller grinding, were used to improve titanomagnetite concentrate granulation,^13,14,15) the maximum utilization of the concentrate was limited to about 60%. Pre-granulation of titanomagnetite concentrate by adding binders or return fines has been proven to be effective to improve the permeability of the sinter bed.¹⁴⁾ However, the additional binder cost and difficulty in the process modifications limited the development of those measurements. Besides, sinter strength was still low due to the wide distribution of perovskite in the sinter of titanomagnetite concentrate.^9,13)

Ultra-high basicity (CaO/SiO₂) sintering technology (basicity > 2.2) facilitated the formation of silicoferrite of calcium and aluminum (SFCA), which was a superior bonding phase in sinter partially offsetting the negative impact of perovskite on sinter strength.¹⁶⁾ However, the lack of acidic burden to balance the slag basicity in the blast furnace gave a longstanding challenge in practice, and the ultra-high basicity sinter production is constricted largely in China nowadays. Therefore, there are still difficulties in titanomagnetite concentrate sintering.

Composite agglomeration process (CAP) of iron ore fines, developed by Central South University in China, is a new route for preparing blast furnace (BF) burden.¹⁷⁾ In CAP, part or all of fine-grained iron raw materials suitable for balling are made into acidic pelletized feed; the rest of fine grained and all coarser fine ores, fluxes, fuels and return fines are mixed, granulated and made into matrix feed with ultra-high basicity. The pelletized feed and matrix feed are mixed and fed into the sintering machine. After ignition and down draft firing, the feed is made into the composite agglomeration product, in which acidic pellets are embedded in the ultra-high basicity matrix. CAP has been put into practice firstly at Baotou Iron and Steel Company since 2008, which proves CAP is capable of improving sinter bed permeability, decreasing fuel consumption, and remarkably increasing productivity of sintering machine. Moreover, good quality BF burden can be obtained.^18,19,20) Similar process named mosaic embedding iron ore sintering (MEBIOS) displays its predominance on efficiently using low-grade iron ores and limonite, ensuring high productivity and reducibility and enhancing sinter strength.^21,22,23)

Our previous study²⁴⁾ showed that acidic sinter (basicity < 0.5) made from titanomagntite concentrate mainly solidifies in the form of titanohematite and titanomagnetite and very few perovskite was generated in this sinter. Ultra-high basicity (basicity > 2.5) facilitated the formation of SFCA in sinter, which can offset the adverse effect of perovskite to sinter strength due to its excellent bonding strength. Acidic sintering (basicity < 0.5) and ultra-high basicity (basicity > 2.5) sintering are considered to be better choices for titanomagnetite concentrate sintering.

Considering the descriptions above, CAP is believed to be a potential efficient method to achieve acidic and ultra high basicity sintering of titanomagnetite concentrate at the same process. Therefore, agglomeration of titanomagnetite concentrate by CAP was investigated in this study.

2. Experimental

2.1. Materials

Titanomagnetite concentrate from Panzhihua, Sichuan province, China was used as iron-bearing raw material in this study. The chemical composition of titanomagnetite concentrate, quick lime, coke breeze and bentonite are presented in Table 1. The size distribution of quick lime and coke breeze is presented in Table 2.

Table 1. Chemical composition of the raw materials (wt.%).

Materials	TFe	SiO2	Al2O3	CaO	MgO	P	S	V2O5	TiO2	LOI
Titanomagnetite	53.5	3.7	3.5	1.0	2.9	0.01	0.7	0.5	12.2	−1.4
Bentonite	1.9	63.4	14.6	2.4	3.6	0.05	0.1	–	–	10.0
Coke breeze	1.1	6.3	4.3	0.8	0.5	0.01	0.7	–	–	86.5
Quick lime	0.2	1.1	0.6	77.2	2.4	0.01	0.02	–	–	16.4

*  LOI: loss on ignition in air atmosphere at 900°C for 2 h.

Table 2. Particle size distribution of the raw materials (wt.%).

Particle size (mm)	+3	1–3	0.5–1	−0.5
Coke breeze	9.0	33.6	18.7	38.6
Quick lime	3.8	6.4	15.3	74.5

Particle size distribution and Blaine fineness of titanomagnetite concentrate are presented in Table 3. These results indicated that titanomagnetite concentrate is not suitable for balling due to its coarse-grained and low Blaine fineness.¹⁴⁾ High-pressure grinding rolls (HPGR) is a high energy efficient comminuting equipment. Comminution in HPGR is the result of the high interparticle stresses generated when a bed of solids is compressed and moved down in the gap between two pressurized rolls. Such high interparticle stresses result in a much greater proportion of fines in comparison to conventional crushing.^25,26) Pretreatment by HPGR had been proved to be useful for improving the ballability of iron ore concentrate.^13,14) For preparing pelletized feed, a part of titanomagnetite concentrate was pretreated by a laboratory-scale (Cylinder shape of 200 mm diameter and 300 mm length) HPGR at 1.17 MPa. Particle size distribution and Blaine fineness of ground titanomagnetite concentrate are presented in Table 3.

Table 3. Particle size characteristics of titanomagnetite concentrate before and after HPGR.

Materials	Particle size distribution (wt.%)			Blaine fineness (cm2·g−1)
	+0.074 mm	0.074–0.045 mm	−0.045 mm
Before grinding	44.8	17.1	38.1	896
After grinding	32.1	27.8	40.1	1117

2.2. Methods

2.2.1. Experimental Procedure

Both the CAP and TSP experiments were performed in a laboratorial sinter pot with dimension of 170 mm diameter and 700 mm height. The flowsheet of the experiment is shown in Fig. 1.

Fig. 1.

Experimental flowsheet.

The ground titanomagnetite concentrate was fully mixed with 1.5 wt.% bentonite, and then balled into pelletized feed of a certain diameter (5–8 mm, 8–10 mm, 10–12 mm and 12–15 mm) in a 1000 mm diameter disc pelletizer. The moisture content of pelletized feed was controlled within 7.0–7.2 wt.%. The other part of titanomagnetite concentrate was mixed with return fines, coke breeze and slacked lime, and granulated to matrix feed with a size of −8 mm in a drum mixer with dimension of 600 mm diameter and 1000 mm length. The pelletized feed and matrix feed manually mixed according to a certain proportion for preparing agglomerate feed. The ratio of iron ore concentrate in pelletized feed to the iron-bearing materials was defined as pelletized feed proportion (PFP), and TSP test was performed without pelletized feed (PFP = 0%). The ratio of coke breeze to agglomerate feed was defined as coke breeze dosage (CBD). The composite basicity was determined by the weighted average value of pelletized feed basicity and matrix feed basicity. According to principle of CAP and definition of CAP parameters, the basicity and coke breeze content in pelletized feed and matrix feed are shown in Table 4. The blending conditions of pelletized feed raw materials and matrix feed raw materials in agglomeration tests are shown in Table 5.

Table 4. The basicity and coke breeze content of matrix feed.

PFP (%)	Composite basicity (–)	CBD (%)	Matrix feed
			Basicity (–)	Coke breeze content(%)
0	2.0	3.23	–	–
10	2.0	3.23	2.12	3.46
20	2.0	3.23	2.27	3.73
30	2.0	3.23	2.44	4.05
40	2.0	3.23	2.63	4.42
50	2.0	3.23	2.87	4.87
60	2.0	3.23	3.17	5.42
40	1.85	3.23	2.43	4.44
40	1.70	3.23	2.22	4.46
40	1.55	3.23	2.02	4.47
40	2.0	2.77	2.64	3.80
40	2.0	3.0	2.64	4.11
40	2.0	3.46	2.63	4.73
40	2.0	3.69	2.63	5.04

* The basicity of pelletized feed is 0.284 and pelletized feed contains no carbon.

** PFP-Pelletized feed proportion, CBD-Coke breeze dosage.

Table 5. The blending conditions of agglomeration tests.

Experimental conditions			Pelletized feed raw materials (%)		Matrix feed raw materials (%)
PFP (%)	Composite basicity (–)	CBD (%)	GTC	Bentonite	TC	Coke breeze	Quick lime	Return fines
0	2.0	3.23	0	0	67.51	3.23	6.18	23.08
10	2.0	3.23	6.75	0.10	60.76	3.23	6.18	23.08
20	2.0	3.23	13.50	0.20	54.01	3.23	6.18	23.08
30	2.0	3.23	20.25	0.30	47.26	3.23	6.18	23.08
40	2.0	3.23	27.00	0.41	40.51	3.23	6.18	23.08
50	2.0	3.23	33.76	0.51	33.75	3.23	6.18	23.08
60	2.0	3.23	40.51	0.61	27.00	3.23	6.18	23.08
40	1.85	3.23	27.30	0.41	40.94	3.23	5.45	23.08
40	1.70	3.23	27.58	0.41	41.38	3.23	4.73	23.08
40	1.55	3.23	27.88	0.42	41.82	3.23	3.99	23.08
40	2.0	2.77	27.20	0.41	40.80	2.77	6.15	23.08
40	2.0	3.00	27.10	0.41	40.66	3.00	6.16	23.08
40	2.0	3.46	26.91	0.40	40.36	3.46	6.19	23.08
40	2.0	3.69	26.81	0.40	40.21	3.69	6.21	23.08

* GTC- Ground titanomagnetite concentrate; TC- Titanomagnetite concentrate; Bentonite in pelletized feed is additional.

At the beginning of each test, a 20 mm-height hearth layers were loaded on the bottom grate of the sinter pot. Then, the agglomerate feed was manually charged into the sinter pot and ignited at 1150°C for 2 min under a suction pressure of 5 kPa. After ignition, the suction pressure was increased to 10 kPa for sintering. Sintering time (t) was defined as the period from the start of ignition to the moment when off-gas reached the highest temperature. At last, the sintered cake was cooled in air flow for 5 min under suction pressure of 5 kPa.

Main evaluation indexes of CAP include permeability before ignition (PBI), sintering speed (SS), yield, productivity and solid fuel consumption (SFC). PBI was tested in a device as shown in our previous work²⁷⁾ and calculated according to the following Eq. (1):   

$$\text{PBI}=\frac{\text{Q}}{\text{S}}{\left(\frac{\text{h}}{\mathrm{\Delta }\text{p}}\right)}^{0.6}\mathrm{\hspace{0.17em} },\mathrm{\hspace{0.17em} }\text{JPU}$$(1)

where PBI is the permeability before ignition (J.P.U); Q is the air volume passing the agglomerate feed canister in unit time (m³/s); S is the cross-sectional area of agglomerate feed canister (m²); H is the height of agglomerate feed canister (m), and Δp is the pressure drop through the bed (Pa).

Sintering speed (SS), Yield, Productivity (P) and solid fuel consumption (SFC) were calculated according to the following Eqs. (2), (3), (4), (5), respectively:   

$$\text{SS}=\frac{\text{H}}{\text{t}}\mathrm{\hspace{0.17em} },\mathrm{\hspace{0.17em} }\text{mm}/\text{min}$$(2)

  

$$\text{Yield}=\frac{\text{M}-{\text{M}}_0}{\text{M}}\times 100\mathrm{\hspace{0.17em} },\mathrm{\hspace{0.17em} }\text{\%}$$(3)

  

$$\text{P}=0.06\times \left(\frac{\text{M}-{\text{M}}_0}{\text{S}\times \text{t}}\right)\mathrm{\hspace{0.17em} },\mathrm{\hspace{0.17em} }\text{t}/\left({\text{m}}^2\cdot \text{h}\right)$$(4)

  

$$\text{SFC}=1\mathrm{\hspace{0.17em} }000\times \frac{{\text{M}}_1\times \text{CBD}}{\text{M}-{\text{M}}_0}\mathrm{\hspace{0.17em} },\mathrm{\hspace{0.17em} }{\text{kg}}_{\text{coke}}/{\text{t}}_{\text{product}}$$(5)

where H is the height of agglomerate feed bed (mm); t is the sintering time (min); M is the mass of agglomerate cake (kg); M₀ is the mass of return fines (kg); M₁ is the mass of agglomerate feed (air dry basis, kg); S is the cross-sectional area of agglomerate feed bed (m²); and CBD is the coke breeze dosage (%).

2.2.2. Characterization of Agglomeration Products

The cooled sinter cake was crushed for free-falling tests (free-falling 3 times from 2 m height), and then screened with the International Standards Organization (ISO) sieves (5 mm, 10 mm, 16 mm, 25 mm and 40 mm). The +5 mm fraction of agglomerate is CAP products and the −5 mm fraction is return fines. Tumbler index (TI) was determined by using the ISO tumble test referred to ISO 3271.

Representative products were crushed into particles with the size of 10–12.5 mm to determine reduction disintegration index (RDI) referred to GB T13242-91 (China) and reduction index (RI) referred to GB T13241-91 (China). Some representative samples of products were ground for FeO analysis.

Some additional samples were heat mounted in resin and polished to mirror finish and examined by an optical microscope (DM-REX, Leica, Germany), and a scanning electron microscope-energy dispersive spectrometer (SEM-EDS, FEI, America) for minerals identification and microstructure analysis.

3. Results and Discussion

3.1. Agglomeration of Titanomagnetite Concentrate by CAP

3.1.1. Effect of Pelletized Feed Proportion (PFP)

To investigate the effect of PFP on agglomeration indexes, experimental conditions were fixed as follows: matrix feed moisture content 8.0%, coke breeze dosage (CBD) 3.23%, composite basicity 2.0 and pelletized feed size 10–12 mm. When the PFP equals to 0, the agglomeration process refers to the traditional sintering process (TSP). As PFP increases from 0 to 60%, the basicity of matrix feed increases from 2.0 (sinter feed of TSP) to 3.17 and the coke breeze content of matrix feed increases from 3.23% to 5.42% accordingly. Figure 2 shows that the productivity, yield, and tumbler index increases with increasing PFP. However, increments of CAP indexes were little at when PFP was 20%. CAP indexes increased significantly as the PFP increased from 20% to 50%. By further increasing the PFP to 60%, the TI decreased sharply, while the increments of productivity and yield became smaller. It is indicated that low PFP (< 20%) in CAP had moderate effect on the agglomeration process, higher PFP (>50%) in CAP led to the poor TI and the optimal PFP is 30%–50%.

Fig. 2.

Effect of PFP on agglomeration indexes of CAP.

3.1.2. Effect of Pelletized Feed Size

Under the conditions of matrix feed moisture content 8.0%, CBD 3.23%, composite basicity 2.0 and PFP 40%, the effect of pelletized feed size on agglomeration indexes of CAP is presented in Fig. 3. The yield and productivity was improved, while the TI increased firstly and then decreased with increasing size of pelletized feed. Therefore, the optimal pelletized feed size is 8–12 mm when considering TI, the yield and the productivity of CAP.

Fig. 3.

Effect of pelletized feed size on agglomeration indexes of CAP.

3.1.3. Effect of Composite Basicity

By fixing the matrix feed moisture content at 8.0%, CBD at 3.23%, PFP at 40% and pelletized feed size at 10–12 mm, the effect of composite basicity on agglomeration indexes of CAP is shown in Fig. 4. When pelletized feed proportion and basicity are constant, the composite basicity is determined by the basicity of matrix feed. In this study, quick lime (CaO) was added to adjust the basicity of matrix feed, while the content of SiO₂ changed very little due to the dosage variation of quick lime and coke breeze. With the basicity of matrix feed increases from 2.02 to 2.63, the corresponding composite basicity increases from 1.55 to 2.0. The yield, the productivity and the tumbler index are improved with the composite basicity increasing from 1.55 to 2.0. Therefore, higher basicity is beneficial to the agglomeration indexes of CAP in the experiment range investigated. Moreover, agglomeration indexes of CAP at basicity 1.55 are even better than the sintering indexes of TSP at basicity 2.0 (see Fig. 2). It indicates that CAP is not only able to improve agglomeration indexes, but also to prepare low-basicity BF burden, which can partially solve the shortage of acidic BF burden in China.

Fig. 4.

Effect of composite basicity on agglomeration indexes of CAP.

3.1.4. Effect of Matrix Feed Moisture Content

Moisture content of matrix feed is important for granulation, and the permeability of the sinter bed. The effect of matrix feed moisture content on agglomeration indexes of CAP are presented in Fig. 5. In these tests, the PFP is 40%, pelletized feed size is 10–12 mm, CBD is 4.2%, and composite basicity is 2.0. The results show that the yield and tumbler index decrease with the moisture content increasing from 7.0% to 9.0%, and the productivity increases and then decreases significantly with a maximum value at moisture content of 8.5%. Therefore, suitable moisture content of matrix feed is 8.0% when considering TI, the yield and the productivity of CAP.

Fig. 5.

Effect of matrix feed moisture on agglomeration indexes of CAP.

3.1.5. Effect of Coke Breeze Dosage (CBD)

Under the conditions of matrix feed moisture content 8.5%, the PFP 40%, pelletized feed size 10–12 mm and composite basicity 2.0, the effect of CBD on agglomeration indexes of CAP is presented in Fig. 6. As the CBD increases from 2.77% to 3.69%, the productivity and tumbler index increase. With the coke breeze dosage further increases up to 3.69%, the productivity and tumbler index decreases subsequently. The optimal coke breeze dosage was 3.23%.

Fig. 6.

Effect of CBD on agglomeration indexes of CAP.

3.1.6. Comparison between CAP and TSP

From the above experimental results, the optimized parameters of CAP were obtained as follows: the moisture content of matrix feed 8.0%, CBD 3.23%, composite basicity 2.0, PFP 40% and pelletized feed size 10–12 mm.

a) Agglomeration Performances

Under the recommended conditions, obtained CAP indexes and TSP (PFP = 0%) indexes are shown in Table 6. The results show that the yield and tumbler index of CAP are quite high and the solid fuel consumption (SFC) was only 37.35 kg_coke/t_product. Compared to those of TSP, the yield and productivity of CAP increased by 17.38% and 17.82%, respectively, and the solid fuel consumption reduced by 3.52 kg_coke/t_product. Consequently, it is proved that CAP is an effective and efficient for titanomagnetite concentrate agglomeration, in terms of productivity, energy consumption and CO₂ emission.²⁸⁾

Table 6. Main agglomeration performances of CAP and TSP.

Agglomeration Process	PFP (%)	PBI (JPU)	SS (mm/min)	Yield (%)	P (t/(m2·h))	SFC (kgcoke/tproduct)
CAP	40	0.21	21.2	75.52	1.633	37.35
TSP	0	0.17	19.7	64.34	1.386	40.87
Variation	40	0.04	1.5	11.18	0.247	3.52
Increment (%)	–	23.53	7.61	17.38	17.82	−8.62

b) Agglomeration Products Properties

Table 7 shows the main properties of products from CAP and TSP. The tumbler index increases by 13.08%. RDI_+3.15 of CAP product reaches 84.81%, which was higher than that of TSP sinter by 20.61%. The improvement of RDI indicates that a potential increase in the permeability of the upper zone of BF could be achieved by using CAP product. RI of CAP product is lower than that of TSP sinter by 4.34%, which is mainly attributed to the increasing FeO content of CAP product. The FeO content of CAP product is 11.84%, higher than that of TSP sinter by 3.92%. Generally, the increase of FeO content enhances the difficulty of reduction from iron oxide to Fe in BF. However, it was proved that higher FeO content in TiO₂-bearing BF burden was efficiently improved smelting intensity of BF by inhibiting the reduction from titanium oxide to Ti(C, N).³⁾

Table 7. Main agglomeration products properties of CAP product and TSP product.

Agglomerates	TI (wt.%)	RDI+3.15 (wt.%)	RI (wt.%)	FeO content (wt.%)
CAP product	62.87	84.81	72.60	11.84
TSP sinter	55.60	70.32	75.89	7.92
Variation	7.27	14.49	−3.29	3.92
Increment (%)	13.08	20.61	−4.34	49.49

3.2. Mineralization Characterization of Titanomagnetite Concentrate Agglomeration by CAP

SEM and EDS analyses were used to identify the mineral constituents in the CAP product and TSP sinter. Figure 7 shows the elemental analysis of the main minerals in the TSP sinter. Iron oxide contains Ti, which can be identified as titanomagnetite or titanohematite (Point 2 and 3). Perovskite were found in TSP sinter (Point 1). Complex silicate containing Ca, Fe, Al, Ti and Mg was also observed (Point 4). As shown in Fig. 8, the matrix part of CAP product is composed of titanomagnetite or titanohematite (Point 1), perovskite (Point 3), SFCA (Point 2) and complex silicate (Point 4).

Fig. 7.

SEM image and EDS analysis result of TSP sinter.

Fig. 8.

SEM image and EDS analysis result of matrix part in CAP product.

Microstructure is also the critical factor for the properties of agglomerate, which is shown in Fig. 9. In the TSP sinter, the interweaved structure between titanomagnetite and titanohematite are the main structures with good strength. Perovskite and herringbone-like titanohematite are observed in the silicates and the gaps between joined crystals of titanomagnetite or titanohematite as shown in Fig. 9(a). The lower sinter strength and poor RDI_+3.15 of TSP sinter are ascribed to the existence of perovskite and herringbone-like titanohematite, since perovskite embrittles the sinter and herringbone-like titanohematite enlarges the volume expansion of sinter during the procedure of reduction from hematite to magnetite at 500–650°C.^1,29,30) Moreover, there is little SFCA in the TSP sinter, which serves as an excellent bonding phase. It is regarded as the other reason for the poor strength and RDI.^1,30)

Fig. 9.

Microstructure of agglomerates. (a)-Perovskite in TSP sinter; (b)-Acid pelletized part in CAP product; (c)-Perovskite in matrix part in CAP product; (d)-SFCA in matrix part in CAP product; (e)-Interweaved structure in transitional zone in CAP product.

In the CAP product, much titanomagnetite is observed in Fig. 9(b). The titanomagnetite content increases significantly than that of TSP sinter, which can be proved by the increase of FeO content (Table 7). It is indicated that much titanomagnetite has not yet been completely oxidized in CAP product. It is the main reason for high RDI_+3.15 and low RI of CAP product. Figure 9(c) shows the perovskite distributed in matrix part of CAP. Compared to the TSP product, the perovskite content in CAP product is lower. In addition, the relative content of SFCA in the matrix part of CAP product increases as shown in Fig. 9(d). Figure 9(e) shows the transitional region between pelletized part and matrix part. In this region, titanomagnetite interweaving with acicular SFCA bonds the pelletized part and the matrix part together.

To explain the difference of minerals and microstructures between TSP sinter and CAP product, the CaO–TiO₂–Fe₂O₃ phase diagram was employed. Figure 10 shows a projection of the liquidus surface of the ternary phase diagram^31,32). In both TSP and CAP, titanomagnetite concentrate is the only raw material, in which Fe₂O₃/TiO₂ is fixed at about 86:14 if all Fe are oxidized to Fe₂O₃. AB in the figure shows mixtures changing as the CaO content increases. When CaO content is low, magnetite and hematite are produced from mixture. Therefore, titanomagnetite and titanohematite are generated from pelletized part of titanomagnetite CAP, without the generation of perovskite. As the CaO content increase, perovskite generates within a wide range. Owing to the melting point of perovskite (1970°C) is much higher than that of calcium ferrite (CF, 1220°C), perovskite precipitation happens in the earlier stage of crystallizing. The production of perovskite increases with the increasing CaO content with abundant TiO₂ content. As the consumption of TiO₂, the remained CaO is reaction with Fe₂O₃ for the formation of CF. In the actual system of titanomagnetite concentrate TSP in Panzhihua area, the CaO and SiO₂ contents of the sinter feed are generally fixed at 6.8%–11.0% and 4%–5% respectively (Basicity of sinter feed within 1.7 to 2.2) and the TiO₂ content is 6.0%–10.0%. In this range, most of CaO is consumed for the formation of perovskite. As a result, there is few CaO remained for the formation of SFCA. This is the reason why a large number for perovskite and little SFCA produced in the TSP sinter as shown in Fig. 9(a).

Fig. 10.

A projection of the liquidus surface of the CaO–Fe₂O₃–TiO₂ ternary phase diagram.^31,32)

In addition, the high melting point, hardness and brittleness of perovskite, and the decreased amount of melting phase, leading to the poor sinter yield and strength of TSP (PFP = 0%) sinter as shown in Fig. 2. As the further increase of CaO content in the system of CaO–TiO₂–Fe₂O₃, TiO₂ is consumed for formation of perovskite and the TiO₂ content in this system decreases. When the TiO₂ content in this system is lower than about 2%, the sufficient CaO remaining after the formation of perovskite results in the generation of some minerals with relatively low melting-temperature in conjunction with other phases, such as calcium diferrite (CF₂), calcium ferrite (CF) and dicalcium ferrite (C₂F) as shown in the shaded area on the bottom of Fig. 10. Large amount of these phases, especially CF with good bonding strength and reducibility may contribute to good strength and reducibility of agglomerate. The basicity of matrix part of titanomagnetite CAP changes in the range of 2.12 to 3.17. Although a large amount of perovskite generates, there is plenty of CaO to form SFCA with other compositions. Owing to the excellent strength, high-temperature fluidity and adhesivity of SFCA, the adverse effects of perovskite to agglomerate strength can be partially offset.

Moreover, in the process of mineralization, the matrix part with large amount of liquid phase wraps around the pelletized feed and some liquid phase such as SFCA, and permeates through the outer layer of pelletized feed. After cooling, a kind of composite agglomerate was produced, in which acidic pelletized part is embedded into the matrix part of sinter with ultra-high basicity. Therefore, there are no unbounded pellets in CAP product. This kind of mineralization mechanism leads to the CAP indexes improved significantly as shown in Section 3.1.

It is worthy to be mentioned, the investigators²⁸⁾ comment that some separate pellets in agglomeration product and the strength of those pellets is the key factor of sinter quality. In fact, their understanding is inaccurate for blinding to the pelletized feed is embedded into the matrix part in CAP product. The production practice in Baotou iron and steel company also proves that there is no separated pellet in CAP products.

PFP and pelletized feed size are the crucial factors in composite agglomeration bed structure and distribution of basicity and solid fuel. When PFP is less than 20% or the pelletized feed size is less than 8 mm, the bed structure has minor change, especially in PBI.²⁰⁾ In addition, the detrimental effect of perovskite cannot completely be offset by the limited generation of SFCA as the basicity in matrix feed is less than 2.5. When PFP is higher than 50 wt.%, the coke breeze content in matrix feed is more than 4.87 wt.%, which is too much for agglomeration. On the other hand, the reducing atmosphere in matrix feed is unfavorable for the generation of SFCA.

According to the mineralization analysis of agglomerates, the consolidation mechanism of CAP product from titanomagnetite concentrate can be concluded in terms of: 1) the pelletized feed is consolidated via solid phase consolidation in the form of crystallization and joined crystals of titanomagnetite or titanohematite and a very few of perovskite generated; 2) the matrix part is consolidated via liquid phase consolidation in the form of SFCA and silicates, as the detrimental effect of perovskite can be offset by SFCA; and 3) the transitional zone composed of interweaved titanomagnetite, titanohematite and SFCA from the matrix part bonds the pelletized part and matrix part.

4. Conclusions

In this study, CAP was applied to prepare BF burden from titanomagnetite concentrate. Process parameters of the CAP based on sinter pot experiments were optimized, and the properties of products and the mineralization mechanism in CAP were investigated. The major conclusions are as follows:

(1) The optimized process parameters for titanomagnetite concentrate agglomeration by CAP are as follows: the matrix feed with moisture content 8.0%, coke breeze dosage 3.23%, and composite basicity 2.0; the pelletized feed with size 8–12 mm and proportion 30%–40%. Under the optimized process parameters, yield 75.52%, tumbler index 62.87% and productivity 1.633 t·(m²·h)⁻¹ are achieved, which are 17.38%, 13.08% and 17.82% higher than those of TSP, respectively. Moreover, CAP lowers solid fuel consumption by 3.52 kg/t_product. In addition, the RDI_+3.15 increases by 20.61%, while RI decreases by 4.34% in CAP.

(2) Reducing the detrimental effect of perovskite is the key to improve the sintering performance of tiatnomagnetite concentrate. CAP is proved being feasible to achieve that aim by dividing two streams of feeds with different basicity. For the pelletized part of CAP product, in which a very few of perovskite are generated due to the low CaO content. In the matrix part of the CAP product, a large amount of SFCA was generated after perovskite fully crystallized from sinter melting for the ultra-high CaO content, and consequently the negative effect of perovskite on sinter quality can be offset by SFCA. As for the transitional zone between pelletized part and matrix part, some melting phase in matrix part permeates through the outer layer of pellets, and then an interweaved structure is formed to bond these two parts together.

Acknowledgements

The authors wish to thank the New Century Excellent Talents of the Ministry of Education (NCET-11-0515) for financial support of this work. This work was also finacially supported by Co-Innovation Center for Clean and Efficient Utilization of Strategic Metal Mineral Resources.

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