Utilization of Waste Clay from Coal Preparation Tailings for Brick Production

ABSTRACT: Most recently concentrated efforts were made by many researchers for alternative building structure materials such as bricks using different industrial and mine wastes. In this study, possibility of using clay rich coal preparation fine tailings for the production of structural bricks was systematically investigated. The effect of waste clay generated from a coal preparation tailing material addition on the durability and mechanical properties of the bricks was elucidated. Different ratios of the waste clay (0, 2.5, 5, 7.5, 10, 15 and 50% by mass) were mixed with the raw brick clay and results compared with that of bricks made from fresh clay raw materials. Test brick specimens were produced by the extrusion method. The samples were tested using the standard test methods and compared against the Turkish Standards (TS) of structural brick specifications. Results clearly indicate the possibility of successfully utilizing waste clay at low additive ratios. The bricks made with waste clay were demonstrated to have acceptable mechanical and chemical properties such as compressive strength, durability, frost resistance, ultrasound conductance, and water absorption. However, firing shrinkage was found to increase at this low mass waste clay ratio.

Keywords: Waste clay, coal preparation, tailing, brick

1. INTRODUCTION

In recent years researches regarding the needs to explore alternative raw materials and energy efficient technologies have been noticeable increased. Now, one of the main objectives of materials scientist is to find alternative construction materials which have low cost, proper mechanical properties, and at the same time are environmentally friendly. Mine tailings have reportedly been used as mine backfill, soil additive, mineral filler, and block aggregate. Regarding the brick production related application using mine wastes however; only a handful of literature has been cited. Among these, a few of specific alternative waste material used for brick production includes the use of red-mud wastes generated during aluminum production, and marble dusts. Furthermore, massive coal preparation plant tailing which are known to contain appreciable amount of clay and are known for their adverse impact on environment is also becoming a potential target material to be used in brick production. Bayat (1997) was studied the possibility of using red-mud in brick making. In another study same researcher was reported to explore the use of waste mud from Adana Cement Companies water treatment plant. As far as the fine coal cleaning plant tailings concern, it seems the best strategy is to first regain the fine coal fraction and then explore the use of the remaining clay containing minerals in a proper industry such as brick making. For example, -0.18 mm waste generated by the Tunçbilek Coal Preparation plant in western Turkey is reported to contain invaluable clay minerals including kaolinite, simectite, and illite along

with fine coal. Therefore the aim of this study was to explore the utilization of these clay wastes for the hollow structural brick production by extrusion method. This piece of research work is both expected to contribute to our basic understanding of construction material science while addressing these environmentally harmful coal cleaning wastes in an economical way.

2. MATERIALS AND METHODS

2.1 Materials

The coal slurry sample used in the experiments was taken from the discharge of fine tailings in Tunçbilek Coal Preparation Plant of G.L.I of Turkish Coal Enterprises, Kütahya-Turkey (Fig. 1). The representative sampling of coal tailings (400 L) was taken according to standards of handling and sampling of sludge and water waste (Turkish Standards, 2002). It should be noted that clay containing tailings (CCT) was taken from an overflow of a 44 mm hydro cyclone and the same sample used throughout the experimental work of brick production (Fig. 2).

The constituent material used in the present study was a local clay (brick clay: BC) from Çobanlar region located near East of Afyonkarahisar.

UTILIZATION OF WASTE CLAY FROM COAL PREPARATION TAILINGS FOR BRICK PRODUCTION

Erdinç Abi, Filiz Oruç and Eyüp Sabah

Department of Mining Engineering, Engineering Faculty, Afyon Kocatepe University, 03200 Afyon-Turkey

Erdinç Abi, Filiz Oruç and Eyüp Sabah / The Journal of ORE DRESSING ® 2011 23

Figure 1. A schematic illustration of point of sample taken from Tunçbilek coal preparation plant

Figure 2. The classification of coal preparation plant tailings by one-stage hydrocyclone system

2.2 Methods

2.2.1 Characterization tests

The characterization studies were carried out using a number of qualitative and quantitative analysis techniques. The chemical composition of the CCT and BC was made by X-Ray Fluorescence. Thermal analysis (DTA-TGA) was performed by LINSEIS L 81. The particle size distribution was determined by Fritsch-Analysette 22 Particle Size Analyzer. The mineral composition was determined by X-ray diffraction (XRD) using a Rigaku-Giger Flex. The density of CCT and BC was determined by a pycnometer.

2.2.2 Brick production process

The standard brick production by extrusion method is often made with clay mixed with 20 to 25% water. This mixture is then forced through a die to create a long cable of material with proper width and depth.

Finally, it is then cut into desired length by a wall of wires. The introduction of holes on the bricks reduces the needed volume with the consequent reduction in cost.

In order to investigate the feasibility of CCT reuse in brick making, the CCT content in the BC-CCT mixture was varied from 2.5% to 50% by weight. The mixture was blended at a water-to-solid ratio of 1:5 and then extrusion molded in a 40x70x100 mm. The molded specimens were air-dried at room temperature for 24h, oven dried at 105°C for another 24h for the removal of water content, and then fired.

The reference brick clay was crushed to -1 mm followed by grinding process to further reduce the size. Calculations to determine the mixture ratio and preparation of suitable recipes were done by stochiometry (5). In order to achieve a good homogenization, reference brick clay and waste were mixed by hand following grinding process. Then, by adding 20% of water, the mixture was converted into a plastic form.

The plastic mixture was then passed through the extrusion machine 6-7 times followed by homogenization and shaping process. The dimensions of the shaped samples were 40x70x100 mm (Fig. 3). The samples were dried for 1 day at room temperature and after that an additional drying at 105°C was carried out in stove for 4 h.

Figure 3. The photos of samples produced by extrusion machine

The heating process in this work was closely simulated similar to a real brick manufacturing process. The oven temperature was ramped up to 300°C in 3h and then samples were kept at 300°C for an additional 2h. The temperature was ramped up to 600°C in 1h and samples kept at 600°C for an additional 1h. Later, the oven temperature was ramped up to designated temperatures (850, 950 and 1050°C) in 2h and maintained for additional 2h for baking. Finally, the samples were left to cool down to room temperature in the oven.

Sintering process was carried out according to the following temperature recipes, at 850ºC, 950ºC and 1050ºC, respectively (14):

24 Volume 13 - Issue 25

• From 0ºC to 100ºC for 1 hour • From 100ºC to 200ºC for 1 hour • From 200ºC to 300ºC for 1 hour • Kept 2 h at 300ºC (For removing the fly agents completely) • From 300ºC to 600ºC for 1 hour

Following the sintering process the bricks were kept in the kiln until cooling down to prevent the deformations that can occur due to thermal shocks. Various quality and performance tests were made on the sintered brick samples and the flow chart of the brick production process is shown in Fig. 4.

Figure 4. The flow chart of the brick specimen manufacturing and test process

2.2.3 Physical and mechanical tests

Linear drying shrinkage test was carried out according to TS 4790 (10). For this, 10 samples were measured for each additive dosage (9). Linear drying

shrinkage was calculated by Equation 1 as the percentage of plastic length of the sample as follows;

Sd = (Lp – Ld) / Lp x 100 (1)

where Sd represents linear drying shrinkage percent in %, Lp the plastic length of the test sample (mm), Ld the dry length of the test sample (mm).

Total linear firing shrinkage was calculated by Equation 2 as the percentage of plastic length (9, 10).

St = (Lp – Lf) / Lp x 100 (2)

where St represents total linear shrinkage (%), Lp plastic length of the test sample (mm), Lf fired length of the test sample (mm).

Loss of ignition (LOI) test was carried out according to TS 705 (9) and 10 samples were tested for each additive dosage. The loss of ignition percentage for each sintering temperature was determined by Equation 3.

LOI = (Pk – Pf) / Pk x 100 (3)

where LOI represents loss of ignition in %, Pk dry mass of the sample and Pf fired mass of the sample. Water absorption test was carried out according to the TS 705 and TS 771-1 (9, 12). An average value was is taken for the 10 measurements.

Compressive strength tests were carried according to the TS 771-1 and TS 705. For this test, 10 samples of each additive amount were used and the mean of the results were taken (9, 12). Computer supported hydraulic compression press with capacity of 100 kN (10 ton) was used for the compressive strength tests (Fig. 5).

Figure 5. Compression and tensile strength test machine

Ultrasound tests were carried out by MATES brand transducer of 55 kHz. These tests were performed to


all samples of bricks on which were previously used for the compressive strength tests.

Frost resistance tests were performed according to TS 705. For this, first the ¼ lengths of the brick samples were filled with water and the bricks were immersed vertically in the water case. Then, at the end of 1 h the 1/2 length, and at the end of 2 h ¾ length, and then finally at the end of 24 h the whole brick body was immersed in water. Then, The bricks were kept immersed for 48 h. Later, bricks were taken out of the water and kept in a deep freeze for 2 h at temperature set to -15°C and then removed from freezer and kept at room temperature for an additional 1 h. This process was repeated 25 times. Finally the compressive strengths measurements were performed and compared with that of normal conditions (9, 12). Harmful magnesia and lime test were performed according to TS 705. For this, 7 samples were tested for each group.

3. RESULTS AND DISCUSSION

3.1. Characterization studies

The chemical composition and density values of BC and CCT are presented in Table 1. As can be seen from the Table 1, the loss on ignition (LOI) values

were about 30% and 26% for BC and CCT, respectively. Furthermore, SiO2 was found to be concentrated in both of these raw materials.

Table 1. Chemical compositions of BC and CCT Composition (%) BC CCT Na2O 1.30 <0.11MgO 4.60 2.79 Al2O3 19.20 19.20 SiO2 52.10 44.73 P2O5 - 0.17 SO3 - 0.22 K2O 3.70 1.26CaO 3.40 0.94TiO2 0.80 0.57MnO - 0.19Fe2O3 5.60 4.03LOI (800°C) 9.20 25.81 Density (gr/cm3) 2.72 2.01

The DTA-TG analysis result for BC material is presented in Fig. 6.

XRD analysis indicated that the main minerals for CCT were quartz, calcium carbonate, feldspar, kaolinite, illite and montmorillonite (Fig. 7a). The mineral composition of BC on the other hand, was composed of quartz, kaolinite, smectite, illite, feldspar and dolomite (Fig. 7b).

Figure 6. DTA and TG analysis of experimental raw material


(a)

(b) Fig 7. XRD analysis of CCT (a) and BC (b)

3.2 Linear drying shrinkage test

Linear drying shrinkage test results as a function of ratio of waste clay added addition ratio and at different sintering temperatures are presented in Fig. 8.

As illustrated in Fig. 8, the drying shrinkage was increased with increasing the amount of waste clay added. This behavior is believed to be related to plasticity of water. The brick material which has high plasticity in water is expected to leave the structure at higher rates resulting in enhanced drying shrinkage rates (13). The drying shrinkage rate also increases with the amount of waste clay added resulting in

further addition of water in order to adjust its plasticity. It should be noted that drying shrinkage rates were within the TS standards (max 8%).

3.3 Firing shrinkage test

As shown in Fig. 9. The firing shrinkage values increased with increasing sintering temperature. This is an expected behavior. The solid phase reactions and the ratio of liquid phase can increase with increasing sintering temperature resulting in sample shrinkage due to the liquid phase filled in pores (1). At the same temperature, the shrinkage was found to be higher with increasing waste clay additive. This behavior may be explained by the existence of fine


coal constituent in these clay samples which are believed to be burned off during high temperature baking cycle. The quantity of shrinkage was higher since volatile matters in coal is expected to be much higher (especially in lignite type coals used in the current work) than that of clay samples. The changes in shrinkage at 1050°C were seen to be more pronounced on additive clays then that of normal

clays. The reason for this again can be explained by the complete removal of fine coal fraction from the brick body. According to the TS (Turkish Standards), the maximum desired shrinkage value is given as 10% (9). Therefore, it is clear from Fig. 9 that 10% max value exceeded the required value 50% additive samples sintered at 1050°C.

850 ºC 950 ºC 1050 ºC

0 % 3.10 3.07 3.292.5 % 3.71 3.27 3.405 % 4.13 3.94 3.847.5 % 5.23 4.69 4.2610 % 5.35 5.18 4.5715 % 5.65 5.46 6.0050 % 7.06 7.02 7.15

0

1

2

3

4

5

6

7

8

Line

ar D

ryin

g Sh

rink

age

Valu

es (

% )

Figure 8. The linear drying shrinkage test results

850 °C 950 °C 1050 °C

0 % 4.27 4.74 5.022.5 % 4.51 4.83 5.525 % 4.72 4.90 5.617.5 % 4.93 4.97 5.9310 % 4.98 5.21 6.7315 % 5.36 5.66 7.0850 % 7.61 8.89 11.55

0

2

4

6

8

10

12

Firi

ng S

hrin

kage

Val

ues

( % )

Figure 9. Firing shrinkage test values

Furthermore, the dimensional measurements made on the 50% waste clay additive sample as length and width did not exceed ±0.40. √A (remembrance dimension), or not to exceed 3 mm for the length and not to exceed 1 mm for the width tolerances according to TS EN 771. Based on the TS EN771,

the shape failure values were found to be out of specification for the 50% additive clay samples (Fig. 10). Because of this reason, the standard tests were not performed for brick species with 50% additive group.


Figure 10. Photos of shape failures from brick samples made with 50% waste clay

3.4 The loss on ignition (LOI) test

The loss on ignition values were shown to increase with increasing waste clay addition. (Fig. 11). This increase is again related to the plasticity water. The

water in the structure increases the value of LOI during evaporation. The dried shrinkage values also increase depending upon the amount rate of clay additive resulting in water addition in order to adjust plasticity.

850 ºC 950 ºC 1050 ºC0 % 10.63 10.70 10.792.5 % 10.75 11.04 11.125 % 10.98 11.40 11.967.5 % 12.07 12.15 12.4510 % 12.24 12.37 12.7115 % 12.91 12.95 13.72

0

2

4

6

8

10

12

14

16

Los

s on

igni

tion

valu

es (

% )

Figure 11. Loss on ignition values


3.5 Water absorption tests

The water absorption property for bricks is a crucial factor from the point of resistance to frost. Therefore, it depends very much on the ratio of apparent porosity of bricks. The pores which are formed during drying process increases water

absorption by leaving the structure. The quantity of water absorption determines the quality of fired product. This value is reported to be limited to max 18% for structural brick materials (5). The changes of water absorption values according to temperature and waste clay additive ratio are presented in Fig. 12.

850 ºC 950 ºC 1050 ºC

0 % 18.18 17.89 13.732.5 % 18.07 17.37 12.895 % 17.88 16.89 12.017.5 % 17.34 16.70 11.0210 % 17.12 16.52 10.2915 % 16.68 15.38 9.92

0

2

4

6

8

10

12

14

16

18

20

Wat

er A

bsor

ptio

n V

alue

s ( %

)

Figure 12. The water absorption values of bricks with different clay additive

In general, the water absorption values were found to decrease (for better) with increasing sintering temperature and decreasing waste clay additive ratio. The decrease in water absorption with increasing fire temperature on the other hand is believed to be caused by the existence of mica and similar minerals, due to filling of these minerals into pores during smelting. The minerals in the raw material like mica and feldspar melts during firing forming a viscous liquid. This liquid can penetrate into the pores as glassy material due to lack of crystallization during cool down (6). The water absorption value was found to be at maximum with samples fired at 850°C. The reduction in water absorption at 950°C, on the other hand, continued to increase at 1050°C. The maximum water absorption limit (18% by TS 705) therefore exceeded for the 2.5% additive and no additive samples fired at 850°C.

3.6 Compressive strength test

The compressive strength test results are presented in Fig. 13 as a function of additive ratio and temperature. It should be noted that compressive strength values were shown to increase similar to reference bricks with increasing temperature. On the contrary, the compressive strength was found to

gradually decrease with increasing waste clay additive ratios resulting in shape deformations.

The increase in fire shrinkage with increasing the waste additive is thought to reduce the percentage of total pores. This may be due to the increase of liquid phase at rates over 2.5% waste additive. The shape failures may be formed when the liquid phase content is over normal values as well as at higher additive ratios. Under these circumstances, the probability of shape failures is expected to be high. The compressive strength values decreased with increasing additive ratios due to formation of non-uniform cross section leading to shape failures.

The other reason for the shape failures seen in Fig. 10, is swelling due to trapped gas pressure. The forming of liquid gas can block the pores causing the gas to be trapped inside the pores. Therefore, local swelling around the trapped gas should be expected. The shape failures seen with the 50% additive samples at alarming rates may be a good indication for this mechanism. It seems that the shape failures were affected negatively by the compressive strength results and the strength values increased rapidly at over 2.5% additive levels. The compressive strength results were over the limit (50 kgf/cm2) according to TS 705 for the bricks with and without additive material.


850 ºC 950 ºC 1050 ºC

0 % 78.25 91.96 101.202.5 % 83.31 110.04 111.345 % 75.82 104.31 108.797.5 % 71.86 102.56 107.8410 % 67.61 96.36 99.5615 % 66.53 82.56 86.55

0

20

40

60

80

100

120

Com

pres

sive

Str

engt

h V

alue

s( k

gf /

cm² )

Figure 13. The compressive strength test results of bricks at different waste clay additive

3.7 Ultrasound tests

The blow-echo method cannot be performed using one transducer type test devices for porous samples. Thus, in this study a direct conduction technique was used. For this measurement, comparison between porous and non porous samples was not made. The principles of all ultrasound techniques are based on the measurements of the amplitude from one end to the other end. There are many cited references

available for the ultrasound measurement of ceramic materials regardless of shape and porosity (3, 4, 8, 15). It should be noted that ultrasound passing velocities were shown to decrease with increasing fire temperatures (Fig 14.). The pores are expected to be closed with increase in temperature allowing less ultrasound wave passing through the brick samples. The optimum conduction velocity was achieved with the 2.5% additive samples sintered at 1050°C perhaps due to its relatively less porous structure.

850 ºC 950 ºC 1050 ºC0 % 1885.50 1732.33 1509.502.5 % 1771.00 1448.70 1362.035 % 1945.93 1527.23 1404.197.5 % 2054.16 1554.29 1426.5610 % 2185.23 1653.23 1536.3715 % 2210.65 1912.57 1767.01

0

500

1000

1500

2000

2500

Ultr

asou

nd T

est V

alue

s ( m

/sn

)

Figure 14. Ultrasound test results

3.8 Frost resistance tests

The compressive strength test values performed after frost resistance were found to be within the 85% of

the standard compressive strength values. (Fig. 15) No damage or cracks were reported on the samples following the frost resistance tests.


Figure 15. Frost resistance test results

3.8. Harmful magnesia and lime experiments

All the brick samples were inspected with naked eye and no break off and decorations were observed on the samples with or without additives. Therefore, compressive strength tests were not performed on these samples.

4. CONCLUSIONS

In this research study, the possibility of using clay rich coal preparation tailings was explored for hollow brick production. The clay was natural waste product of coal preparation plant taken from overflow of a hydro cyclone from Tunçbilek Coal Preparation Plant. As for the reference raw material, Çobanlar clay from the Afyon region was used.

Çobanlar clay from Afyon was found to contain quartz, montmorillonite, illite, kaolinite, dolomite and feldspar while hydrocyclone overflow waste fraction was found to contain quartz, siderite, kaolinite, mica, illite, simectite, and feldspar. In general although both clay groups show similar composition of minerals, the concentration on both groups was found to be different.

Based on the DTA-TG tests, the loss on ignition values for hydrocyclone overflow fraction was found to be lower (25.81%) than that of material fed to hydrocyclone (37.60%) due perhaps to the preference departure of organic matters to underflow. In addition, based on the chemical analysis, quartz was shown to be concentrated in the overflow fraction.

Çobanlar clay and hydro cyclone overflow clay samples density was measured to be 2.72 g/cm3 and

2.01 g/cm3 with carbonate content of 8.30% and 6.72%, respectively.

According to Winkler diagram the waste clay product was shown to be a proper brick raw material that can be used for structural brick production.

In order to determine the quality of bricks made from waste clay, a series of qualitative and quantitative test were performed and results were compared with that of reference bricks.

The plasticity is a crucial factor for the hollow bricks shaped with extrusion technique. It should be noted that waste clay’s plasticity behavior was found to be not different from that of reference clay though quartz and mica content of waste clay was reported to be much higher.

The water addition needed for the proper plasticity were found to increase from 20% to 25% with the addition of waste clay, as well as due to existence of the fine coal particulates. As a result, shape deformations were observed with the bricks made with high waste clay additions (50%).

The loss on ignition value was shown to decrease with increasing waste clay addition. This behavior was attributed to the loss of water during the drying.

The water absorption of bricks was reduced with increasing concentration of waste clay added along with increasing sintering temperature. More specifically, the maximum water absorption suggested by TS 705 (18%) already exceeded with bricks made as small as 2.5% clay addition and reference bricks sintered at 850°C.


The compressive strength of bricks decreased as a function of waste clay added due possibility of the volatile nature of coal constituent in these samples. The compressive strength of bricks samples sintered at 950°C on the other hand was found to improve by 16.43%. Therefore, the compressive strength values exceeded the required value of 50 kgf/cm2 given by the TS705.

Finally, the compressive strength, durability, frost resistance, and water absorption were all improved at lower concentration of waste clay addition, in particular at higher baking temperatures. The ultrasound conductance again was found to be improved at higher temperatures due perhaps to the diminishing pore density.

Acknowledgements

This research presented in this paper is funded by TÜBİTAK (The Scientific & Technological Research Council of Turkey) with the project number of 104I080. The authors highly appreciated for the financial support provided by TÜBİTAK.

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Utilization of Waste Clay from Coal Preparation Tailings for Brick Production

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