Indian Journal of Chemical Technology Vol. 19, November 2012, pp. 420-426

A novel binder-free and energy-efficient process for making ceramic tiles using red mud and sericitic pyrophyllite

Jeeshan Khan1, *, S S Amritphale1, Navin Chandra1 & M Patel2 1Environment, Industrial Waste Utilization and Nanomaterials Division,

CSIR-Advanced Materials & Processes Research Institute, Hoshangabad Road, Bhopal 462 064, India 2Indian Agro and Recycled Paper Mills Association, 4 Rajendra Place, New Delhi 110 008, India

Received 27 December 2011; accepted 18 June 2012

A new process for utilizing the waste from alumina refinery plant such as red mud in production of ceramic tiles has been developed. Pyrophyllite mineral has been added to the red mud to improve the strength properties. The tiles are produced at comparatively lower temperature (950-1000°C) then the conventional process of making ceramic tiles and without addition of phosphatic binders. Impact strength of the optimized composition of 40-60% (w/w) sericitic pyrophyllite in red mud system meets the acceptable limit of impact strength (19.6 J/m) and other properties for ceramic. The achieved impact strength has been attributed to the densification in the matrix of ceramic tiles due to the formation of new phases like calcium aluminum silicate, iron silicate, potassium titanium oxide and magnetite by thermal reaction and transformation of various mineral phases present in the sericitic pyrophyllite and red mud. The structural features of the red mud have been studied using scanning electron micrographs, exhibiting rhombohedral shaped crystals of calcium aluminum silicate and elongated crystal formation of metals silicates, which provides reinforcement to the ceramic tiles matrix.

Keywords: Ceramic tiles, Energy efficient process, Red mud, Sericitic pyrophyllite

The Bayer process, used for the extraction of aluminum from bauxite using caustic soda, generates voluminous quantity of red mud as waste. There are 85 alumina plants in the world and in each plant, for every tonne of alumina produced, 1–1.5 tonnes of red mud is generated. It is estimated that nearly 90 million tonnes of red mud is produced annually worldwide, and presently in India alone nearly 3 million tones red mud is generated. The major constituent of red mud are Fe2O3, TiO2, Al2O3, SiO2, Na2O, CaO, MgO and K2O with trace amounts of V2O5, Zr, Y, Th, U and rare earth elements as trace constituents1-3. The disposal of such a large quantity of this waste is expensive4 and possesses a very serious and alarming environmental problem5. Huge red mud ponds are made to store the red mud for which the civil work becomes very expensive. Moreover, from safety point of view, the alumina refinery plant has to monitor and supervise the red mud pond permanently. Dry process to dispose the red mud has been tried but they have not been found to be techno-economically viable. To solve the disposal problem, considerable research and development work has been carried out2, for

making value-added products using red mud, such as (i) inexpensive and efficient adsorbent for removal of nickel6, dyes7, arsenic8, lead and chromium9 from aqueous solutions; (ii) sequestration of carbon dioxide (CO2)

10; (iii) radiopaque shielding materials11,12; (iv) Nano-crystal glass-ceramics13; and (v) building materials, namely iron rich cement4,14, red mud–polymer composites panels as wood substitute15, glazes16, bricks17, ceramics, tiles18, etc. Red mud has also been used for the treatment of gold ores19, in making silicate bonded un- sintered ceramics20, heavy-clay ceramics21 and building bricks22.

The conventional process of making ceramic tiles is highly energy intensive and involves sintering of various raw materials, namely feldspar, quartz and clay at 1350 – 1450°C. The sintering is necessary to carryout thermo-chemical reactions among the various mineralogical phases of raw materials for obtaining newer crystalline and glassy phases capable of providing reinforcement in matrix, and thus leading to ceramic materials with desired characteristics. Further, it has been reported in the literature that the use of alkali and alkaline earth metal compounds as flux materials not only reduces the sintering temperature to some extent23-25 but also improves the mechanical strength of ceramic bodies26,27.

————— *Corresponding author. E-mail: jeeshank@gmail.com

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Sericitic pyrophyllite mineral has been found to be a potential material for multifarious applications17,28, as it has a low coefficient of thermal expansion and a reduced moisture expansion and also contains silica and alumina in an optimum ratio along with potassium as flux required for making ceramic wares26 and ceramic tiles29,30.

Sericitic pyrophyllite is relatively much inexpensive mineral and abundantly available in Madhya Pradesh. Therefore, it is worth exploring application of this mineral as supporting fluxing agent because it contains alkali and alkaline earth metal compounds. Characterization studies of sericitic pyrophyllite has been carried out by X-ray diffraction and it is found to contain mainly pyrophyllite, quartz, kaolinite and muscovite. Due to the presence of muscovite phase, the pyrophyllite sample used in the present studies is termed as ‘sericitic pyrophyllite’.

In the present study, a novel binder-free and energy-efficient process for making ceramic tiles has been developed by designing and optimizing the mineralogical contents of red mud and sericitic pyrophyllite. The developed novel process obviates the use of costly phosphates binder system and enables the preparation of tiles in the significantly reduced sintering temperature (950-1000°C). Experimental Procedure

Raw materials

The red mud procured from Hindustan Aluminium Company (HINDALCO), Renukoot, India was used in the form of a mud type residue composed of fine fraction (mud) and a relatively coarse fraction (sand) with small granules. Sericitic pyrophyllite mineral collected from Khari Mines of Tikamgarh district of Madhya Pradesh was used in the present study.

Representative samples of red mud and sericitic pyrophyllite were characterized using X-ray diffraction and scanning electron microscope. The chemical analysis of both the raw materials was carried out using standard wet chemical analysis method.

Preparation of green tile samples and their sintering

The green tile samples were prepared, by using red mud and 0–100% (w/w) sericitic pyrophyllite in selected weight ratios following thorough homogenization and then compressing in a steel mold of 10.2 × 10.2 cm2 size and thickness of 0.5-0.7 cm at a pressure of 50 kg/cm2 for 2 min based on our earlier research12,31. The tile samples have been named as RMSP signifying red mud sericitic pyrophyllite.

The green samples were dried in an air oven at 110°C for 6 h and then sintered in an electrical muffle furnace. The firing cycle was programmed as follows: heating from ambient temperature to 400°C at a heating rate of 10°C per min, holding for 30 min at 400°C, heating again to 1000°C final temperature, soaking for 1 h at the rate 10°C and finally cooling of samples in the furnace itself down to ambient temperature.

To optimize the temperature and soaking time of the ceramic tiles of optimize composition RMSP 40, the tiles were sintered at the temperature range of 900 – 1100°C for a duration ranging from half an hour to two hours and the results are given in the Table 1. Evaluation of Physico- mechanical properties of the

sintered tiles

The apparent density determination has been performed as per the standard procedure prescribed for ceramics32. The sintered tiles samples were evaluated for their impact strength following the procedure laid down in the specifications drawn for ceramic tiles. The linear shrinkage and water absorption have been determined following the procedures described earlier26, 31, 33, 34.

Flexural strength (MOR) was measured by Instron testing machine (Model 8801) as per the standard procedure35. The procedure for measuring impact strength involved, the use of failing weight type instrument. The impact strength measurement was carried out by placing the bottom surface of the tile on a 60 mm equilateral triangular support. A steel ball of 30 g weight was allowed to drop on the top surface of the tile sample from an initial height of 25 cm. The height of the free fall of the steel ball was increased in small increments till failure. Impact strength was calculated as per the following formula36.

Impact strength = t

hw×

where w is the weight of the steel ball in (kg); h, the height of free fall of steel ball in (m); and t, the thickness of the tile in (cm).

Table 1Sintered properties of optimized RMSP 40 composition based ceramic tiles

Temperature Impact strength, J/m

°C For 30 min soaking

For 1 h soaking

For 2 h soaking

900 15.87 18.37 19.08 1000 17.98 20.29 19.68 1100 18.78 18.58 19.76

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Characterization using XRD and SEM techniques Different instrumental techniques such as X-ray

powder diffraction and SEM were used for characterization of the raw materials as well as of all the sintered tile samples. The identification of the various mineral phases of the finely ground sample of raw materials and ceramic tiles was carried out with the help of a Philips X-ray diffractometer (Model PW1710) using Cu-Kα radiation and operated at 40 kV and 20 mA. The XRD patterns were recorded from 10 to 70 2θ° with a scanning speed of 0.02 2θ° per second. In order to examine the morphology of the various mineral phases in the samples a scanning electron microscope (JEOL, Model JSM 5600) was used.

Results and Discussion

Characterisation of raw materials

The chemical composition of red mud (RM) and sericitic pyrophyllite (SP) was determined by standard wet chemical analysis method of chemical analysis37. The chemical analysis confirms the presence of various oxides in red mud and sericitic pyrophyllite (Table 2). The results of chemical analysis of red mud and sericitic pyrophyllite samples are found to tally fairly well with the results reported elsewhere38,39.

Phase identification in red mud, sintered red mud and RMSP

10-50 system based tiles

The X-ray diffractograms of the tiles made from the red mud (RM), sintered red mud (SRM) and red mud with 10-50% sericitic pyrophyllite (RMSP10-50) at 1000ºC are shown in Fig. 1. The RM sample is found to contain anatase (d values: 1.692, 2.420, 1.830), rutile (d values: 1.692, 2.206, 2.115, 1.482), quartz (d values: 3.355, 4.18), hematite (d values: 2.70, 2.514, 1.692), boehmite (d values: 2.206, 1.482), gibbsite (d values: 4.843, 2.206 ), bayerite (d values: 1.692, 1.452, 2.206,), Na5Al3CSi3O15 (d values: 3.67, 2.115, 6.36), cancrinite (d values: 1.452, 3.67, 2.115, 1.482), chantalite (d values: 3.355, 1.452, 4.18, 1.482,) and calcite (d values: 3.036). The prominent peaks of hematite, anatase and rutile, gibbsite and quartz can be seen in the diffractograms. The red mud is reach in hematite. The gibbsite, titania and quartz are expected to help in consolidation of the particles in formation of the strong ceramic tiles, while the hematite particles may be serving for the minimum porosity needed.

From the x-ray diffraction pattern of SRM and RMSP10-50 at 1000ºC (Fig. 1), it is observed that the intensity of the peaks corresponding to gibbsite

Table 2 Typical chemical analysis of red mud and sericitic pyrophyllite samples

Sample Fe2O3 TiO2 Al2O3 SiO2 CaO Na2O MgO K2O LOI*

Red mud, % 31.21 21.20 20.10 8.50 2.99 6.00 - - 10

Sericitic pyrophyllite, % 0.61 0.10 28.14 53.35 1.63 0.32 1.09 9.21 5.55

*LOI loss on ignition.

Fig. 1 X-ray diffraction patterns of red mud (RM), sintered red mud (SRM) and RMSP 10-50 system at 1000ºC

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vanishes completely, and NA5Al3CSi3O15, chantalite are observed to decrease significantly. This leads to the formation of an array of new additional phases, namely calcium aluminium silicate (d values: 2.677, 1.828, 2.191), iron silicate (d values: 2.499, 1.683, 1.476), potasium titanium oxide (d values: 2.677, 2.499, 3.332), sodium calcium silicate (d values: 2.677, 3.332, 1.828, 2.191), sodium calcium iron silicate (d values: 2.677, 3.332, 2.191), aluminium iron oxide (d values: 2.677, 1.444, 1.476), calcium iron silicate (d values: 2.677, 3.649, 1.828) and magnetite (d values: 2.499, 1.476). All phases are identified in XRD.

The major phases in optimized RMSP 10-50, given in the Fig. 1, are the calcium aluminum silicate, iron silicate, potassium titanium oxide and magnetite, which are responsible for observed reinforcement and strength leading to the densification in the matrix of ceramic tiles. The decomposition and chemical reactions among the various constituents of gibbsite, cancrinite and chatalite phases present in the red mud lead to the formation of calcium aluminium silicates. The X-ray diffraction pattern shows the amorphous characteristics in 2θ range from 10° to 30° and is responsible for the observed binding and mechanical strength in the system. The formation of potassium titanium oxide results from the reaction between potassium of sericitic pyrophyllite and titanium oxide content of red mud39-41.

Phase identification in sericitic pyrophyllite, sintered sericitic

pyrophyllite and RMSP 60 - 90 system based tiles

The results of X-ray diffractogram of the samples made from 60- 90% sericitic pyrophyllite in red mud

(RMSP 60-90) sintered at 1000º C are given in Fig. 2. For the sake of comparison, the X-ray diffractogram of the sericitic pyrophyllite (SP) and sintered sericitic pyrophyllite (SSP) at 1000ºC is also considered. The X-ray diffraction results of sericitic pyrophyllite show the presence of major phases of pyrophyllite (d values: 3.067, 4.603, 4.446) and kaolinite (d values: 3.334, 4.446, 2.416) and muscovite (d values: 3.342, 1.494, 4.446, 2.556). The minor phases of quartz (d values: 3.342, 1.839) and sillimanite (3.334) are also identified42.

The X-ray pattern of SSP shows the presence of dehydroxylated pyrophyllite as major phases and RMSP 60-90 (Fig. 2) at 1000ºC illustrating new phases of potasium titanium oxide (d values: 2.692, 3.656), sodium calcium silicate (d values: 1.632 ), sodium calcium iron silicate (d values: 1.632), calcium aluminium silicate (d values: 1.632), calcium iron silicate (d values: 3.656, 1.632), sillimanite (d values: 3.334, 2.202, 2.692, 3.286), dehydroxylated pyrophyllite (d values: 4.232, 2.540), muscovite (d values: 3.339, 4.473, 2.540) and kaolinite (d values: 3.334, 4.473, 2.540, 1.524), nearly same as a observed in the X-ray pattern of RMSP-10-50 but with the significantly reduced intensity. However, with the increase in sericitic pyrophyllite the phases belonging to sericitic pyrophyllite increases.

Impact and flexural strength (MOR) of RMSP system tiles

The impact strength and flexural strength (MOR) versus percentage RMSP system (containing 0–100% sericitic pyrophyllite) is shown in Fig. 3. The impact strength and flexural strength (MOR) of sintered tiles made from either alone red mud or alone

Fig. 2 X-ray diffraction patterns of RMSP 60- 90, sintered sericitic pyrophyllite (SSP) at 1000ºC and sericitic pyrophyllite (SP)

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sericitic pyrophyllite is found to be very low. However, during addition of SP in red mud, the formation of metal silicate phases namely calcium aluminum silicate, iron silicate, potassium titanium oxide and magnetite takes place respectively, consequently leading to a sharp increase in impact strength as well as in flexural strength because these phases are responsible for providing reinforcement38. These crystal phases act like whiskers and therefore enhances the strength20.

The tiles in the RMSP 30-60% system show the impact strength as per the Indian Standard (19.6 J/m). However, the RMSP 40 sintered tiles optimized composition exhibit highest impact strength and flexural strength.

Beyond 70% of sericitic pyrophyllite content, as the availability of heavy metal from red mud to form metals silicate phases in the RMSP system becomes a limiting factor, the impact strength and flexural strength decrease significantly. Apparent density and % linear shrinkage of RMSP system

based tiles

The effect of addition of sericitic pyrophyllite in the red mud on the % linear shrinkage and apparent density of fired tiles has been studied and the results are given in Fig. 4. Both the % linear shrinkage and apparent density are observed to increase with increase of sericitic pyrophyllite content up to 40% in the tile. The increase in % linear shrinkage indicates enhanced densification of tiles due to increase in sericitic pyrophyllite which contains fluxing agent i.e. potassium.

Furthermore, ˃40% of sericitic pyrophyllite addition, the apparent density and % linear shrinkage both decrease significantly due to increasing pyrophyllite content, which may be attributed to the increase in quartz content with the further increase in sericitic pyrophyllite content43. Percentage water absorption of RMSP system based tiles

The plot of % water absorption versus percentage of sericitic pyrophyllite in RMSP system is shown in Fig. 5. It is observed that initially, the % water absorption values decrease with increase in sericitic pyrophyllite content up to about 40% and then found to increase on further increase of sericitic pyrophyllite content. The increase in % water absorption above 40% sericitic pyrophyllite may be due to the formation of the micro pores because of escaping

Fig. 3 Effect of addition of sericitic pyrophyllite (0–100%) RMSP system on impact strength and flexural strength (MOR) of tiles sintered at 1000ºC

Fig. 4 Effect of addition of sericitic pyrophyllite (0–100%) to RMSP system on the apparent density and per cent shrinkage of the tiles sintered at 1000ºC

Fig. 5 Effect of addition of sericitic pyrophyllite (0–100%) in RMSP system on per cent water absorption

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water molecules following the dehydroxylation of pyrophyllite. The initial decrease in percentage of water absorption may be attributed to the densification process on addition of sericitic pyrophyllite, which is indicated by the trends of increase in the fired density of tiles also. Since the water absorption value is high (>15%), the tiles can be fixed on the walls by a cementing process.

Morphological studies on red mud, sericitic pyrophyllite and

RMSP 40 system

The SEM micrographs exhibiting microstructure of red mud is shown in Fig. 6 (A), where in the scattered morphology texture of aluminium silicates of red mud is seen. The micrographs also exhibit the distribution of heavy constituent phases, such as tetragonal anatase and rutile, spherical hematite, hexagonal cancrinite and orthorhombic boehmite. The SEM micrograph of the sericitic pyrophyllite is shown in Fig. 6 (B), where the morphology of mainly chunky phases of pyrophyllite can be differentiated.

The morphology of optimized RMSP 40 can be seen in Fig.6 (C & D), revealing the rhombohedral shaped crystals of calcium aluminum silicate and the elongated shaped crystals of metals silicate in the matrix. The morphology features prove that the binding of the individual oxides have been

accomplished by the sintering process and this inference tally with the mineralogical phases, obtained by XRD (Fig. 1). The binding process brings in a state of consolidation of the particles due to sintering and thus attaining high density. However, above 40% addition of the pyrophyllite, the binding process is hindered and cannot be recommended for the formation of tiles with adequate strength. The fine particles in the red mud accompanied by the alkali helps reducing the sintering temperature, as the later acts as fluxing agent. On the other hand, the calcium, potassium and silica rich sericitic pyrophyllite provide the required mineralogical phases in formation of ceramic product. These crystals act like whiskers enhancing the strength, which act as reinforcement to the ceramic matrix and are responsible for providing strength in the sintered tile bodies20. The tiles made using sericitic pyrophyllite above 40% posses low strength of the ceramic tiles due to the presence of chunky phases resulting from the dehydroxylation of pyrophyllite.

Conclusion

A techno economically viable new process for utilizing the alumina refinery waste (red mud) has been developed for production of ceramic tiles. The process is novel as it is binder free and obviates

Fig. 6 SEM micrographs of (A) red mud RM; (B) sericitic pyrophyllite SP; (C) at different magnification & (D) optimized RMSP 40 ceramic tile

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the use of costly phosphate binding system. The developed process is energy efficient as it enables achieving the desired impact strength in the ceramic tiles by sintering of tiles at a temperature as low as 1000°C as against the conventional sintering temperature of 1300-1400°C. Addition of sericitic pyrophyllite for the ceramic tiles is found to be 40% only, sufficient to obtain the impact strength for ceramic tiles as specified in the Indian Standard 777–1970. The major new phases formed in the optimized product are calcium aluminum silicate, iron silicate, potassium titanium oxide and magnetite which are responsible for observed reinforcement and strength, leading to the densification in the matrix of ceramic tiles. The SEM micrographs reveal rhombohedral shaped crystal of calcium aluminum silicate and elongated crystal formation of metals silicate in the ceramic matrix. These crystals act like whiskers, thereby providing reinforcement to the ceramic matrix and adequate strength in the sintered tile bodies. Acknowledgement

One of the authors (J K) is obliged to CSIR, India, for providing Senior Research Fellowship to carry out his research work. The authors are highly thankful to Dr M Singh, Mr A Khare, Mr Shafiq M and Mr Prasanth Nedumbilly for their cordial help in XRD and SEM characterizations. References 1 Genc-Fuhrman H, Tjell J C & McConchie D, Environ Sci

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