Combustion Reaction of Pulverized Coal on the Deposit Formation in


Jun 23, 2016 - ABSTRACT: The serious deposits on the refractory bricks are found in the grate kiln in iron ore pellet plants, which significantly infl...
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Combustion Reaction of Pulverized Coal on the Deposit Formation in Kiln for Iron-ore Pellet Production Shuai Wang, Yufeng Guo, Feng Chen, Yu He, Tao Jiang, and Fuqiang Zheng Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01001 • Publication Date (Web): 23 Jun 2016 Downloaded from http://pubs.acs.org on June 24, 2016

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Combustion Reaction of Pulverized Coal on the Deposit Formation in Kiln for Iron-ore Pellet Production

Shuai Wang, Yu-Feng Guo*, Feng Chen, Yu He, Tao Jiang, Fu-Qiang Zheng School of Minerals Processing and Bioengineering, Central South University, 410083 Changsha, China *Author to whom correspondence should be addressed. E-mail: [email protected]

ABSTRACT: The serious deposits on the refractory bricks are found in the grate-kiln in ironore pellet plants, which significantly influences the pellet production. The effect of combustion reaction of pulverized coal on the deposit formation in kiln during iron-ore pellet production was investigated in this work. Hematite iron ore was used as raw material to be pelletized, and the pulverized coal, in general, was used as the fuel. The chemical compositions, microstructures of the deposit samples were detected through the chemical 1

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analysis methods, SEM and EDS. Effects of different residual carbon contents in coal ash and roasting temperature on the deposit formation were investigated by simulated experiments. And then the results indicated that the combustion reaction of coal had a significant influence on the deposits formation, under which condtions, the hematite grains in pre-heated pellet were reduced by surrounding residual carbon particles in unburned coal to wustite. Low melting point minerals were formed as wustite reacted with silicate, aluminum and calcium oxides, which adhered on the refractory bricks, especially the brick with cracks. The deposit was generated and became thicker when time went away.

1. INTRODUCTION Iron and steel play an important role in the development of economy and society, and most of iron is produced using blast furnace process in China. For higher blast furnace utilization coefficient, energy saving consumption and cost reduction, the ideal blast furnace burden composition is 70%-80% of high basicity sinter plus around 20%-30% of acid pellet in China.1,2 Pellet is an indispensable composition for the blast furnace burden due to its large production and good quality. The three common processes used for the pelletizing are the travelling grate process, the grate-kiln process and the shaft furnace process.3 The grate-kiln process is widely used around the world, especially in China. However, as pulverized coal is commonly used as the primary fuel during the grate-kiln process in China, the deposit formation upon the grate-kiln walls becomes a tough problem. 2

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The accumulated slag may cause unplanned stops, damage of machinery and wearing out of refractory wall linings in the rotary kiln. It causes production disturbances that affect the production capacity of the pelletizing plant.4 Several research works were reported and some measures were used to solve the problem in grate-kiln process.4-10 Technologic measures such as adjusting the fire flame orapplying the mechanical methods were taken to break the deposit down.11 Moreover, some methods were adopted to prevent the deposit produced by analyzing the theoretical reasons of deposit formation. However, the fundamental theory of the deposit mechanism have not been studied completely. Actually, many previous research works studied about the deposit in grate-kiln plants which were used gas as the fuel.4,12 Furthermore, the mainly concerned point about the deposit formation mechanism was that the interaction between pulverized pellet powder, quartz and alkali metal. In addition, it is commonly accepted that the ash deposition appeared frequently during the coal combustion.13-18 Previous work showed that ash deposit was found in coal fired boilers and the deposit layer was submitted to higher temperature and the presence of additional silicon oxide concentration.19 Chen et al.

20

found that the coal

combustion has a significant influence on the deposit formation in rotary kiln for lime, they inferred that the liquid phases were formed at high temperature due to the residual unburned coal. What is more, coal combustion that caused deposit formation in a grate-kiln plant was mentioned previously, and the deposit formation process is complicated by the addition of particles from disintegrated iron ore pellets and the coal ash in the gas flow.21,22 The research 3

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of Baosteel showed that the reasons of deposit formation were much powder including pulverized pellet and coal ash, adding the function of low smelting point materials such as olivine, calcium olivine and aluminum melilite.23 Stjernberg et al4,22 showed that the reactions with the lining material and nucleating in the brick were enhanced by potassium and sodium, which lowered the eutectic temperatures. However, the coal combustion on the deposit formation of refractory brick in grate-kiln is not fully understood. Thus, in order to reveal the formation mechanisms of deposit in coal-base grate-kiln plant and find the preventive and suitable methods, the relationship between the combustion reaction of pulverized coal and deposit formation in iron ore pellet plant was investigated in the present work. Simulated examinations with different conditions of coal combustion were taken. Finally, the effects of pulverized coal combustion on deposit formation in the gratekiln were summarized and a process of deposit formation was presented. This study will assists us in understanding the deposit formation mechanism from perspective of coal combustion, and to take preventive actions by controlling coal combustion.

2. MATERIALS AND METHODS 2.1. Description of the Grate-Kiln Plant The grate-kiln plant studied was an iron ore pellet plant in China. The length of the grate furnace is 56 m and the width is 4.5 m. The length of the kiln is 40 m, and the diameter is 6.1 m. The kiln is lined with refractory brick which is based on aluminum oxide (Al2O3) and its 4

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chemical composition was shown in Table 1. The average temperature in the kiln is about 1280 ℃, while the temperature may be higher in the burner zone. 2.2. Materials The deposits samples, pre-heated pellet powder and pulverized coal were provided by a Pellet Plant in China. The chemical compositions of the refractory brick, iron ore concentrate, pre-heated pellet powder, and coal ash were shown in Table 1, Table 2 and Table 3, respectively. Table 2 shows that the total iron grade of the iron ore concentrate accounts for 65.47% and the FeO content is only 4.58%, and the iron ore belonged to hematite. The total iron of pre-heated pellet was decreased to 64.17% due to addition of bentonite. The industrial analysis of coal is shown in Table 4. The pulverized coal was used as fuel, with fixed carbon of 65.48% on an air dry basis (FCad), volatile matter of 17.95% on a dry ash free (Vad) basis, ash of 13.43% on an air dry basis (Aad). Coal ash possesses high silica, aluminum and calcium oxides, assaying 50.64% SiO2, 26.31% Al2O3 and 7.2% CaO, respectively.

Table 1. Chemical Composition of Refractory Brick (wt %) TFe

SiO2

CaO

Al2O3

MgO

K2O

TiO2

2.76

13.79

1.46

63.55

0.13

0.53

3.27

Table 2. Chemical Compositions of Iron Ore Concentrate and Pre-heated Pellet (wt %) 5

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kinds

TFe

FeO

SiO2

CaO

Al2O3

MgO

K2O

Na2O

S

Iron ore concentrate

65.47

4.58

2.59

1.15

0.93

0.56

0.12

0.051

0.032

Pre-heated pellet

64.17

trace

3.89

1.76

1.50

0.62

0.11

0.14

0.075

Table 3. Chemical Composition of Coal Ash (wt %) TFe

SiO2

CaO

Al2O3

MgO

K2O

Na2O

S

3.80

50.64

7.20

26.31

1.10

1.00

1.06

1.22

Table 4. Proximate Analysis of Pulverized Coal Mad,%

Aad,%

Vad,%

Fcad,%

3.14

13.43

17.95

65.48

Figure 1. Schematic picture of the deposits in the kiln Deposits samples were removed from the kiln when the plant had to be shut down because of the thicker deposit layer. Table 5 shows the chemical compositions of different deposit 6

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samples which were removed from the different locations in the kiln (Figure 1), and their locations were from the outlet to inlet by per 5 meters. The thickness of deposits in the kiln became thicker from outlet to inlet (Figure 1).

Table 5. Chemical Compositions of the Deposits Samples in the Kiln (wt %) Samples

TFe

FeO

SiO2

1#(5m)

66.04

Trace

2.51

2#(10m)

66.40

Trace

3#(15m)

64.22

4#(20m)

CaO

Al2O3

MgO

K2O

Na2O

S

1.74

1.53

0.81

0.043

0.058

0.0050

2.66

1.95

1.3

0.88

0.042

0.070

0.0050

Trace

4.05

1.91

1.91

0.93

0.062

0.095

0.0055

62.98

Trace

5.19

2.45

2.46

0.85

0.088

0.12

0.0050

5#(25m)

61.93

Trace

5.91

2.41

2.25

1.37

0.089

0.16

0.0070

6#(30m)

62.11

Trace

5.72

1.91

2.36

0.79

0.11

0.64

0.0023

7#(35m)

62.60

Trace

5.72

2.02

2.11

0.9

0.082

0.19

0.0020

2.3. Methods 2.3.1 Simulated samples From Table 5, the elements and main compositions distribution curves were shown in Figure 2. The total iron contents decreased firstly, then increased from outlet to inlet, while, the contents of SiO2, CaO and Al2O3 increased firstly then gradually decreased. What is more, contents of FeO in deposit samples were of small quantity due to the whole oxidative 7

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atmosphere in the kiln. In the grate-kiln plant scene, it was observed that from outlet to inlet the deposits layer in kiln became thicker. Compared the chemical compositions of deposits samples with raw materials, it was inferred that the deposits were the mixtures of pulverized pre-heated pellet and coal ash.

Figure 2. Elements distributions of the deposits in kiln In this study, in order to simulate the deposit formation process, 5# (25m) deposit sample was chose due to its typical composition. The elemental balance of the chemical compositions of deposits was calculated by analyzing the chemical compositions of coal ash, pre-heated pellet and deposit sample. The elemental balance calculation depended on the balances of three elements of iron, silicon and aluminum, respectively. The mass ratio of pre-heated pellet and coal ash was calculated by the Equations (1) and (2). 𝑤(𝑇𝐹𝑒/𝑆𝑖/𝐴𝑙)𝑃𝑒𝑙𝑙𝑒𝑡 ∙ 𝑥(𝑃𝑒𝑙𝑙𝑒𝑡) + 𝑤(𝑇𝐹𝑒/𝑆𝑖/𝐴𝑙)𝐴𝑠ℎ ∙ 𝑦(𝐴𝑠ℎ) = 𝑤(𝑇𝐹𝑒/𝑆𝑖/𝐴𝑙)𝐷𝑒𝑝𝑜𝑠𝑖𝑡 Equation (1) 8

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𝑥(𝑃𝑒𝑙𝑙𝑒𝑡) + 𝑦(𝐴𝑠ℎ) = 100 Where, 𝑤(𝑇𝐹𝑒/𝑆𝑖/𝐴𝑙)𝑃𝑒𝑙𝑙𝑒𝑡 ,

𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 (2)

𝑤(𝑇𝐹𝑒/𝑆𝑖/𝐴𝑙)𝐴𝑠ℎ and 𝑤(𝑇𝐹𝑒/𝑆𝑖/𝐴𝑙)𝐷𝑒𝑝𝑜𝑠𝑖𝑡

are

represented the contents of total iron, SiO2 and Al2O3 in pre-heated pellet, coal ash and deposit, respectively. 𝑥(𝑃𝑒𝑙𝑙𝑒𝑡) and 𝑦(𝐴𝑠ℎ) are represented the mass fractions of coal ash and preheated pellet powder, respectively. The mass ratio of coal ash and pre-heated pellet powder was estimated which is based on the calculation results. The mass fraction of pre-heated pellet powder was 96.32%, and coal ash was 3.68%. Finally, the estimated chemical compositions of the deposit (25m) are shown in Table 6. It can be seen that the estimated composition of the mixture is very close to the real composition of the 5# (25m) deposit. Therefore, it is reasonable to use the mixture which represented real deposit to investigate the deposit formation.

Table 6.

Estimated Chemical Composition of the Mixture (wt %)

Component

TFe

FeO

SiO2

CaO

Al2O3

MgO

K2O

Na2O

S

Mixture

61.95

0.00

5.61

1.96

2.41

0.64

0.14

0.17

0.12

5#(25m)

61.93

Trace

5.91

2.41

2.25

1.37

0.089

0.16

0.0070

2.3.2 Evaluation methods The important step of deposit formation is that mixtures start to adhere to the refractory bricks. Thus, the effects of pulverized coal combustion and roasting temperature on adhesion 9

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were investigated. Pulverized coal was milled to 80% passing at 0.074mm. Coal ashes which contained different residual carbon contents were measured by the following method. Pulverized coal (2 grams) was put into the porcelain boat, then burned at 815℃ in an electric tube furnace with different time. The adhesion between the mixtures which consist of pulverized pre-heated pellet, coal ashes and refractory bricks were evaluated by the following method. Firstly, refractory bricks were cut into small pieces and roasted in a muffle furnace at 1280℃ for 3h in order to remove water from bricks and keep the weight of bricks constant in the following experiments. Secondly, the constant proportion of pulverized pre-heated pellet and coal ash mixtures were placed on the surface of the refractory bricks, then roasted in a muffle furnace at different temperature for 5 hours. At last, the refractory bricks were taken out and cooled in the air. The mass fraction of mixtures was 3 grams and the contact area between the mixtures and refractory bricks was constant (Figure 3). Furthermore, as the Figure 3 illustrated, roasted mixtures were dealt with a method of simulating the roll function of pellets in the rotary kiln. As shown in Figure 3, after the roasted mixtures cooling, a roller (3 kg) was used to roll on the samples twice with constant speed (about 10cm/s). The roller moved along the horizontal line, that is, the move direction was parallel to the refractory brick surface. Thus, there was only the vertical force (the gravity force of roller) on the deposit. Then the refractory bricks which adhered with the mixtures were overturned, after loose parts of the mixtures falled, the weight of last parts of mixtures 10

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and bricks was weighed. Finally, bonding effects were evaluated by amounts of mixtures (Equation [3]) which adhered to the refractory brick.

Wadhesion  Wbrick +mixture  Wbrick Where,

Equation [3]

Wadhesion is represented to the weight of adhesion on the refractory brick, Wbrick mixture

is represented to the weight of refractory brick and adhered part of mixture and

Wbrick is

represented to the weight of refractory brick after roasted.

Figure 3. Schematic picture of evaluated method for the adhesion between the mixtures and refractory brick 2.3.3 Thermodynamic analysis FactSage 7.0 thermodynamic software was used to analyze the related reactions, draw the phase diagrams and calculate the amount of liquid phases in this study. Moreover, the “Reaction”, “Equlib” and “Phase Diagram” modules were adopted and used databases were “FToxid” and “FactPS”.24 11

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2.3.4 Analysis of samples The chemical composition of refractory brick was measured by X-ray fluorescence spectroscopy (XRF) (PANalytical Axios mAX, PANalytical B.V., Almelo, The Netherlands). The chemical compositions of iron ore concentrate, pre-heated pellet powder and deposits samples were determined by methods for chemical analysis with Chinese standards which were shown in Table 7.25 The chemical compositions of coal ash was measured by methods for chemical analysis with Chinese standards GB/T 1574-2007.26 Proximate analysis of coal was conducted by the Chinese standard GB/T 212-2008, and the Chinese standard is corresponding to ISO 11722:1999, ISO1171:1997 and ISO 562:1998.27 Table 7. Chinese Standards Used to Analyze Samples 25 Items

Chinses standards

Methods

Total Fe

GB/T6730.5-2007

Titanium(Ⅲ) chloride reduction methods

FeO

GB/T6730.8-1986

The potassium dichromate volumetric method

Si

GB/T6730.10-1986

The gralimetric method

Al

GB/T6730.11-2007

EDTA titrimetric method

S

GB/T6730.61-2005

High frequency combustion with infrared absorption method 12

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K, Na

GB/T6730.49-1986

The flame atomic absorbtion spectrophotometric method

Mg, Ca

GB/T6730.13-2007

EGTA-CyDTA titrimetric method

The combustion characteristic of pulverized coal was measured by TG-DSC with a Thermal Analyzer (NETZSCH STA 449 C, Germany) at a heating rate of 10 °C/min from 20 °C to 1100 °C in air, and the gas flow rate was 150 mL/min. Approximately 10 mg sample was put into a calcined alumina crucible. In order to analyze adhesion on the refractory bricks, the roasted samples with refractory brick were mounted with epoxy resin and polished for the following analysis. The microstructures of interface of mixture and refractory brick were observed by scanning electron microscope (SEM, FEI Quanta-200, FEI Company, GG Eindhoven, The Netherlands), and the compositional analyses were carried out using an energy dispersion system (EDS, Ametek Inc., Paoli, CO, USA) within the SEM.

3. RESULTS 3.1. Combustion of Pulverized Coal As shown in Figure 4a, TG-DSC curves indicated that the combustion speed of coal was slow. In general, the completely combustion of coal needed enough time and residue of coal 13

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would be found if the coal combustion have not finished. Besides, as the Figure 4b illustrated, it was clear to show that the residual carbon content of coal ash with different combustion time, and the residual carbon content decreased when the combustion time increased. The combustion of pulverized coal was close to burn out after 45 minutes. Coal ashes which contained different residual carbon contents were used in the following experiments.

(a)

(b)

Figure 4. The TG-DSC curves of pulverized coal (a); Residual carbon content of coal ash with different combustion time (b) 3.2. Effects of the Residual Carbon Content In this section, the effects of residual carbon contents on the amounts of mixtures which adhered upon the refractory bricks were investigated. The mixtures with different residual carbon contents were placed on the surface of refractory bricks with constantly circular contact area, then roasted in a muffle furnace at 1280℃ for 5 hours. According to Table 7 14

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and Figure 5, the adhesion weights increase as the residual carbon contents increase. The adhesion was invalid upon the refractory bricks if no carbon in coal ash. It can be inferred that whether the coal combustions burn out or not had an important influence on the formation of deposits upon the refractory bricks.

Table 7. Adhesion Weight on the Refractory Bricks with Different Residual Carbon Kinds

Refractory bricks (g)

Roasted (g)

Adhesion weight (g)

5M(0%)

499.28

499.36

0.08

5M(40%)

211.98

214.67

2.69

5M(59%)

215.28

218.04

2.76

“5M is represent to the 5# mixtures, the percentages in brackets following 5M are the residual carbon content of coal ashes”

Figure 5.

Deposits on the refractory bricks with different high carbon content in coal ash 15

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3.3. Effects of the roasting Temperature As we know, the inside temperatures are different in different locations of kiln. The aim of this section was to study the effects of roasting temperatures on the amounts of mixtures adhered upon the refractory bricks. The mixtures with constant residual carbon content (wt, 40%) were placed on the refractory bricks with constant contact area, then roasted in a muffle furnace at 1150℃, 1200℃ and 1280℃for 5 hours, respectively. Results were given in Table 8 and Figure 6. As listed in Table 8 and shown in Figure 6, little amount of deposit was found on the refractory bricks when the temperature below 1200℃. Apparently, the amounts of mixtures adhered upon the refractory bricks increased when the roasting temperature above 1200℃. It indicated that the deposits were easier to form at higher roasting temperatures. Therefore, with the presence of residual carbon in coal ash, the formation of slagging deposits were inevitable at the high roasting temperature (1280℃) in the kiln of grate-kiln plant Table 8. Adhesion Weight on the Refractory Bricks at Different Temperature Temperature(℃)

Refractory bricks(g)

Roasted(g)

Adhesion weight(g)

1150

324.41

324.54

0.13

1200

248.30

250.62

2.32

1280

211.98

214.67

2.69

16

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Figure 6. Deposits on the refractory bricks with different roasting temperature 3.4. Effects of the low Carbon Content in Coal Ash In order to confirm the specific amounts of carbon contents in coal ash which significantly caused the adhesion on the refractory bricks, simulated experiments were presented. The coal ashes with special carbon contents were prepared by mixing the pulverized coal and coal ash which burned completely. The coal ashes with different carbon contents were mixed with preheated pellet powder, then they were placed on the refractory bricks and roasted in a muffle furnace at 1280℃ for 5 hours. In addition, it can be observed in Table 9 and Figure 8 that the amounts of adhesion mixtures increase as the carbon contents increase. Obviously, more deposit was found when the carbon content of coal ash increased to 5%. As can be seen from Figure 7, a few adhesion deposit appeared when residual carbon content is 3%. Thus, it displayed that the mixtures were easier to adhere upon the refractory bricks if the carbon 17

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content of coal ash above 5%. Furthermore, Figure 9 shows the SEM and EDS images of interface between the mixture and refractory brick, and it can be seen that part of deposit eroded the refractory bricks.

Table 9. Adhesion Weight on the Refractory Bricks with Low Carbon Content Carbon content (%)

Figure 7.

Refractory brick (g)

Roasted (g)

Adhesion weight (g)

1

442.41

442.52

0.11

2

414.75

414.98

0.23

3

370.77

371.13

0.36

5

313.23

315.74

2.51

Deposits on the refractory bricks with different low carbon content

18

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Figure 8. Effects of the Carbon Content on the Adhesion Situation

Figure 9. SEM and EDS images of interface of mixtures with refractory brick

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3.5 The Process of Deposit Formation The results of simulated experiments indicated that the amount of adhesion mixtures increased as the residual carbon content in coal ash increased. In general, liquid phases promoted the formation of deposit at high temperature. Thus, it can be inferred that hematite particles were reduced to FeO by residual carbon and the amount of FeO increased as the residual carbon content increased. In addition, Figure 10 shows that the effects of FeO on liquid areas of phase diagrams of Al2O3-SiO2-CaO-FeO at 1200℃ and 1300℃. The liquid phase lines at 1200℃ and 1300℃ with different FeO contents were draw by FactSage 7.0. The pink, green, blue and red lines in phase diagrams were represented to the liquid lines with 5%, 10%, 15% and 30% FeO in the quarternary systems phase diagram, respectively. It is clear that the liquid phase areas expanded significantly as FeO contents increased from 5% to 30%.

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Figure 10. Effects of FeO on liquid areas of phase diagrams of Al2O3-SiO2-CaO-FeO at 1200℃ and 1300℃ (FactSage 7.0)

Figure 11. Effect of FeO on amount of liquid phase in deposit (FactSage 7.0) The amounts of liquid phase in deposit with different contents of FeO were calculated by FactSage 7.0 according to chemical composition of 5# deposit sample. As shown in Figure 11, the amounts of liquid phase increased as temperatures increased, similarly, the amounts of liquid phase also increased as FeO content increased. In short, increasing of FeO content formed lots of low melting point minerals, and leaded to more liquid phase, then the appeared liquid phase caused to formation of deposits on refractory bricks in the kiln.

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Based on the results of simulated experiments and above analysis, the process of deposit formation in coal-based grate-kiln plant for iron ore pellet was summarized. The schematic picture of deposits formation was shown in Figure 12. In summary, the model of formation process was concluded as following steps. Apparently, the residual carbon was presented in coal ash if coal combustion is incomplete (Figure 12 [1]), then the hematite particles were reduced by surrounding residual carbon particles when they were mixed with each other in the kiln under high temperature (Figure 12 [2]), and then the FeO appeared and started to react with quartz, mullite, anorthite to form some low melting point minerals (Figure 12 [3]), in addition, the low melting point minerals adhered upon the refractory bricks especially when the bricks had some cracks and the deposits appeared and became thicker finally (Figure 12 [4]).

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Figure 12. Schematic picture of deposits formation in the kiln

4. DISCUSSIONS The deposit formation is a well-known phenomenon in grate-kiln process during the production of iron ore pellet. Especially, the deposit became more serious when the fuel is coal. Many prior works have documented the formation of deposit in rotary-kiln for iron ore pellet or lime. However, these studies have either been short-term studies or have not focused on the effects of coal combustion. In this study, we focused on the effect of coal combustion on the deposit formation and tested the simulated experiments. We found that the coal combustion had a significant influence on the deposit formation in refractory bricks. Increasing of residual carbon in coal ash and roasting temperature all could promote the mixture adhere upon the refractory brick. Theoretically, combustion of fuel can be initiated whenever oxygen comes in contact with fuel. However, the temperature and composition of the fuel and oxygen supply dictate the nature of the reaction.28 In addition, it is very probably that the coal combustion did not terminate with the low density of oxygen or the thick particle size of pulverized coal. Thus, with the presence of residual carbon in coal ash, reduction atmosphere appeared in the part of the kiln due to the residual carbon particles, even though the whole kiln was under the condition of oxidizing atmosphere. It is commonly accepted that hematite is easily reduced under reduction condition. Furthermore, it appeared that the hematite particles were first converted to magnetite, then to wustite, and ultimately 23

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to iron as Reaction (2), (3) and (4).29 The reduction of Fe2O3 to FeO is faster than that of FeO to Fe because its CO2/CO equilibrium ratio is higher and hence the rate of oxidation of carbon is faster.30 Moreover, Figure 13 shows that the iron oxides reduction proceeds through the series Fe2O3-Fe3O4-FeO-Fe because temperature in kiln was above 1200℃. Previous research of ash sintering under reducing condition shown that wustite, almandite and fayalite act as the key factors influencing the initial sintering behavior of ash in the temperature range from 1273 (1000℃) to 1373K (1100℃). With increasing temperature, wustite was formed at around 1173K (900℃) and started to react with quartz, mullite, anorthite to form some low melting point minerals, such as fayalite, almandite, hercynite, etc.31 Thus, the low melting point minerals was engendered due to incomplete combustion of pulverized coal, and then the liquid phase also was generated and obtained the development at high temperature, finally the formation of liquid phase leaded to deposit formation.

rGӨm=169457.8-172.75T J/mol

C(s)+CO2(g)=2CO(g)

rGӨm=147454.9-241.09T J/mol

3Fe2O3(s)+C(s)=2Fe3O4(s)+CO(g)

rGӨm=198103.8-204.55T J/mol

Fe3O4(s)+C(s)=3FeO(s)+CO(g)

rGӨm=147622.2-147.13T J/mol

FeO(s)+C(s)=Fe(s)+CO(g)

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(1) (2) (3) (4)

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Figure 13.

The reduction equilibria of iron oxides with carbon

In addition, the deposit formation was complicated which was generated by the reactions of the residues of K, Na, S and FeO together. However, the quantity of alkali metal and S is small and the presence of FeO lowered the melting point which resulted in more liquid phase. Furthermore, the presence of FeO in deposit formation process was only due to the residue of coal. This study therefore indicates that the coal combustion had an important effects on the deposit formation. An evaluated method was illustraed and phase diagrams were used to help analyze the formation process. Our study will promote a better understanding of deposit formation process in grate-kiln process for production of iron ore pellet.

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5. CONCLUSION It was clear that the residues of coal was presented during the deposit formation in kiln which coal was used as fuel and the combustion of coal has a significant effect on the formation of deposits on the refractory bricks in kiln. Hematite particles in pre-roasted pellet were reduced to FeO with the presence of residual carbon existed in coal ash due to the incomplete combustion of coal. The generated Fe2+ reacted with Si, Ca, Al, K, Na et. al to form some low melting point minerals, such as fayalite, almandite, hercynite and so on, which adhered upon the refractory bricks especially the bricks with cracks. The amount of liquid phases increased during deposit formation process with the function of alkalis metal. In order to decrease the trend of deposit, combustion of pulverized coal should be controlled accurately to make the coal combust completely.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

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Proc. Combust. Inst. 2011, 33, 2853-2861.

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