A Study of Cordierite Ceramics Synthesis From Serpentine Tailing and Kaolin Tailing

Cordierite ceramics was synthesized using a composition prepared by the mixture of three different materials: waste serpentine mine tailing (WST), waste kaolin mine tailing (WKT) and alumina. The formation of cordierite was achieved with the solid-state sintering reactions at 1350 C for 3 h. The synthesized cordierite ceramics was characterized by X-ray diffractometer (XRD), thermal analysis (TG-DTA), and SEM-EDS (Scanning Electron Microscopy-Energy Dispersive Spectrometer). The XRD analysis results showed the cordierite with typical rhombic system as a major phase, and its compositions consisted of O, Al, Si and Mg by means of SEM-EDS.


Introduction
Natural cordierite having a chemical composition of (Mg,Fe) 2 Al 4 Si 5 O 18 is a magnesium aluminosilicate mineral, occurring very rare in nature.Cordierite ceramics have low thermal expansion coefficient, excellent thermal shock resistance, low dielectric constant, high volume resistivity, high chemical durability, high refractoriness and high mechanical strength.Therefore, they are widely used as honeycomb-shaped catalyst carriers in automobile exhaust systems, as substrate material for integrated circuit boards, as thermal shock-resistant tableware, as heat exchanger for gas turbines, as porous ceramics and as refractory materials [1][2][3][4][5][6][7][8].
As a low cost alternative method, reaction sintering is proposed using inexpensive materials such as kaolin and talc, andalusite, industrial wastes and Al-rich anodising sludge [7,[21][22][23][24].This method is quite suitable to directly prepare cordierite ceramics during a one-step sintering process, in accordance with the concept of sustainable development.However, there are few reports on preparation of cordierite ceramics using serpentine (or serpentine tailing) as starting materials.Serpentine, Mg 2 Si 2 O 5 (OH) 4 , is a kind of hydrated magnesium silicate.The major applications for serpentine are dimension stone and flux materials.The serpentine rock is crushed to produce many waste serpentine tailing (WKT), and it is estimated that a total of no less than a thousand million tons of WST was produced from Yiyang county, Jiangxi province, China.WST compositions of SiO 2 (34.3%),MgO (44.1%), and Fe 2 O 3 (6%) is rich SiO 2 and MgO so that cordierite ceramics can be produced if a suitable amount of alumina source is added based on the MgO-Al 2 O 3 -SiO 2 ternary phase diagram [25][26][27][28].
The main purpose of the present study was to produce cordierite ceramics.WST, waste kaolin tailing (WKT) and alumina are selected as the starting materials to achieve this purpose.This could be another way for potential application of WST and WKT in China.
Based on the composition of WST and WKT and the MgO-Al 2 O 3 -SiO 2 phase diagram, WST, WKT, and alumina with the content of 30%, 50% and 20% in the region of cordierite formation) were ground into sizes below 100µm, and they were sufficiently fine to be mixed homogeneously.The mixture was mixed with the organic binder (Glycerol reagent /raw materials: 8%), which was pressed into cylindrical pellets (13 mm diameter and 3 mm height).The samples were placed in a room of 25 o C for two days, and were dried at 105 o C, finally, were roasted in the high-temperature resistance furnace.The heating of sample started at 25 o C, and then the sample was sintered in air up to 1000, 1200, and 1350 o C at the ramping rate is 5 o C min -1 .Finally, the sample was naturally cooled until they reached the room temperature.
The mixture of WST, WKT and alumina weighed from 4 to 10 mg in mass, and they were put into a Pt-Rh crucible, which was analyzed between 25 °C and 1000 °C at a heating rate of 5 °C min −1 in a STA449C integrated thermal analyzer.The sintered disk samples were characterized by XRD (D/max-γβ X-ray diffractometer with 40mA and 40kVv, CuKR radiation, Japan) for the determination phase transformation.Morphology of the sintered bodies was observed using a JSM-6700F scanning electron microscope (Japan) at an accelerating voltage of 20 kV.Fig. 1 The DTA curves of the physical raw materials mixtures.

XRD analysis phase transitions
Fig. 2 shows XRD patterns of WST treated at various temperatures (25, 650,750, 850, and 1000 o C).It is seen from Fig. 2 that Lizarsi is the main crystalline phase below 650 o C, which indicates that there is not the release of structure water in WST corresponding to the result of TG-DTA.From the same figure, it is seen that the forsterite becomes the main crystalline phase, which indicates that WST starts to release the structure water corresponding to the results of TG-DTA.When temperature is increased to 1000 o C, forsterite and ensitatite become the main crystalline phases.

Conclusion
The cordierite ceramics has been successfully synthesized by using raw materials including WST, WKT, and alumina.The XRD results showed that the synthesized material sintered at 1350 o C for 3 h is formed the cordierite phase.SEM observation shows that the cordierite with the compositions of O, Al, Si and Mg shows the typical rhombic system.

Fig. 1
Fig.1shows TG-DTA curves of ceramic sample.It is seen that there are three endothermic valleys at 129.3, 696.2 and 911.6 o C for ceramic sample.The endothermic valley at129.3indicates that the surface absorbing water is vaporized, and endothermic valley at 696.2 and 911.6 o C indicates that the structure water of ceramic sample is released.As can be seen, the exothermic peaks of ceramic sample is detected at 453.9, 727.4 and 862.9 o C. The exothermic peaks at 453.9 o C corresponds to convert WKT into amorphous metakaolin, and

Fig. 3
Fig.3shows XRD patterns of WKT treated at various temperatures (25, 350,600, 1000, and 1200 o C).It is seen from Fig.3that quartz and kaolinite are the main crystalline phase below 350 o C. From the same figure, it is seen that quartz becomes the main crystalline phase when temperature increase from 600 to 1000 o C.This indicates that kaolinite in WKT is converted into amorphous metakaolin corresponding to the results of TG-DTA.When temperature is increased to 1200 o C, mullite and quartz become the main crystalline phases.

Fig. 4
Fig.4 show XRD patterns of ceramic sample sintered at various temperatures (800, 900, 1100, 1200, 1300and 1350 o C).It is seen from Fig.4 that quartz, forsterite and ensitatite are the main crystalline below 1200 o C. When temperature is increased to 1350 o C, the cordierite becomes the main crystalline phase.This result indicates that the cordierite forms at about 1350 o C with the interaction of forsterite, ensitatite, mullite and quartz.

Fig. 5
Fig.5 shows appearance (a), SEM micrograph (b), and cordierite compositions (c) of ceramic sample sintered at 1350 o C. It is seen from Fig.5 (a) that the cordierite ceramics is undeformed.It is seen from Fig.5 (b) that the cordierite shows the typical rhombic system, which has the compositions of O, Al, Si and Mg (In Fig.5c).