In this study, it hopes to use recycled ceramic shell mould as refectory coating materials and geopolymer technology to shorten the process of coating

Transcription

1 A Study on Recycling and Application Waste Ceramic Shell Mould by Using Geopolymer Technology Yi-Fong Wu a, Ta-Wui Cheng a, Steve Hsu b, Gary Hsu b a: National Taipei University of Technology b: Chips Investment Casting Inc. ABSTRACT Lost wax casting is one of the common methods for investment casting. An investment casting mould consists of individual layers of fine zircon particles and granular mullite particles held together by a binder that has been set to a rigid gel. However, the whole process needed several days and the waste ceramic shell mould after sintering required further disposal. In this study, outer layers casting mould was replaced by geopolymer, which was fabricated by mixing waste ceramic shell mould, metakaolin, and the ground calcium carbonate with alkali solution. Physical/mechanical properties, viscosity, setting time of geopolymer with different concentration of KOH and SiO 2 /K 2 O mole ratio were tested. The experimental results showed that the waste ceramic shell mould based geopolymer can fast setting in few hours, thus can save process time. On the other hand, the additional calcium carbonate particles in geopolymer can be decomposed at above 85 at pre-heat process, and results CaO and CO 2 production. CO 2 will increase the porosity and decreased the fired strength of casting mould. After casting, the mould can be easier de-molded by spray water due to generate expansion calcium hydroxide as the reaction of CaO and H 2 O. Geopolymer technology could be a potential method to use in investment casting process. Keywords: Investment casting ; Geopolymer ; Metakaolin ; Ground Calcium Carbonate 1 INTRODUCTION Waste ceramic shell moulds were produced from investment casting [1]. According to the Environmental Protection Administration of Taiwan statistics in 214, foundry industries in Taiwan produced about 16, tons of waste ceramic shell mould per year [2]. Investment casting can be obtained precise size, complex shape and smooth surface products [3]. Currently, lost wax casting is a major method for investment casting. The first step is to create a wax, and then several layers refractory material is coated on the wax surface. After de-waxed at high temperature, a mould cavity is formed, and the molten metal can be poured into the mould cavity to obtain the accurate products. Two problems need to solve: (1) the procedures of lost wax casting is cumbersome and need around 58 hours long time for drying after dipping ceramic slurry [4]; (2) the disposal of waste ceramic shell mould. If the procedures of lost wax casting can be shorter and the waste can be recycled, it is valuable for foundry industry. Geopolymer is a type of inorganic polymers synthesized by polycondensation of AlO 4 and SiO 4 tetrahedral units in alkali solutions. These tetrahedral units are interlinked alternatively where the charge balancing cations are provided by alkali metal cations. It was firstly introduced by Joseph Davidovits in the late 197s [5]. Moreover, geopolymers are three-dimensional materials of silicon oxygen aluminum containing amorphous to semi-crystalline phases [6]. Based on such a unique structure, geopolymers exhibit higher mechanical strength, lower curing time, excellent chemical and fire resistances, lower thermal conductivity and shrinkage [7]. O14-1

2 In this study, it hopes to use recycled ceramic shell mould as refectory coating materials and geopolymer technology to shorten the process of coating and drying time. 2 EXPERIMENTAL 2.1 Materials Waste ceramic shell mould was obtained from the Chips Investment Casting Corporation. The waste ceramics were crushed and dried. Metakaolin and the ground calcium carbonate were purchased from PEI LONG Industrial Corporation and First Chemical Manufacture Corporation individually. The major elements in all materials were analyzed by XRF and listed in Table 1. The particle sizes of d 5 of waste ceramic shell mould, metakaolin, and the ground calcium carbonate were 296.9μm, 6.1μm and 15.1 μm, respectively. Potassium silicate solution (8.3 wt% K 2 O, 2.8 wt% SiO 2 ) was provided by Rongxiang Industrial Corporation. Potassium hydroxide was obtained from the First Chemical Manufacture Corporation. Table 1 Chemical composition of waste ceramic shell mould, metakaolin and ground calcium carbonate Elements (wt%) SiO 2 Al 2 O 3 ZrO 2 CaO Others Waste ceramic shell mould Metakaolin Ground Calcium Carbonate Methods The alkali activator solution contains potassium hydroxide and potassium silicate solution. The molar ratio of SiO 2 /K 2 O of the alkali activator is showed in Table 2. Table 2 SiO 2 /K 2 O molar ratio of the alkali activator Number KOH (M) SiO 2 /K 2 O molar ratio M125Si57.57 M125Si M125Si82 Geopolymer preparation Geopolymer was synthesized by adding 8 wt% of waste ceramic shell mould and 2 wt% of ground calcium carbonate/metakaolin (1:1 ; 14:6 ; 18:2 by wt%). They named G5MK5, G7MK3 and G9MK1, respectively. Initially, the ratio of solid to liquid of the alkali solution was After couple experiments, the optimized one was test with the other two different ratio (3.16 and 2.86). In the process, all samples were casted into prismatic beam moulds ( mm 3 ) and cylindrical moulds (Ф 3 6 mm) after geopolymer slurry was mixed for 1 minutes. The cast specimens were stored in ambient temperature for 1 day. The geopolymer specimens were then removed from the moulds and further 1 day dried in the same environment. Before the physical/mechanical properties tests the completely dried geopolymer specimens were sintered at 12 for 3 minutes.. Geopolymer used as outer layers casting mould preparation As shown in Figure 1, geopolymer casting mould used as outer layers was applied in the investment casting process. The process started from a wax pattern which closely matched the shape and size of the required casting. The ceramic shell mould was formed around this pattern by.82 O14-2

3 dipping it from the ceramic (zircon or mullite) slurry or geopolymer slurry and coated with ceramic (zircon or mullite) sand (experiment Raw-HL) repeatedly for 34 h. However, it only took 29 h by applying waste shell mould sand on layer V and VI (experiment 5L-GHL and 4L-G1HL). After the ceramic shell mould was dried for 24 h, the wax pattern was removed from the shell by heating. A cavity with the same shape as the required was formed. The shell mould was then dried for another 24 h. For casting process, molten metal was then poured into the cavity and allowed to solidify before the shell was removed from the casting surface. For green strength modulus rupture test, dried shell mould was tested directly after drying 24 h. The fired strength of shell mould was sintering at 1125 before applying to the modulus of rupture test. The permeability properties of fired shell mould were tested with a plate dimension of 1 1 mm 2. The permeability properties were tested by measuring pressure-decayed time. Finally, the casting tolerance was calculated. Pattern mould Wax injection Pattern Raw-5HL Layer Ⅰ(Zircon slurry) Zircon sand 4 hrs Invest (mould) Layer Ⅱ(Zircon slurry) 6 hrs Layer Ⅰ Layer Ⅱ Layer Ⅲ Layer Ⅳ 4 hrs 6 hrs Layer Ⅴ Layer Ⅵ Layer Ⅴ Layer Ⅵ-Geopolymer Layer Ⅴ-Geopolymer Layer Ⅵ-Geopolymer 3 hrs Lost wax Cast Layer Ⅲ(Mullite slurry) Layer Ⅳ(Mullite slurry) Layer Ⅴ(Mullite slurry) Layer Ⅵ (Mullite slurry) Experiment 5L-GHL Layer Ⅴ(Mullite slurry) Layer Ⅵ (GP slurry) Dried Shell Mould Experiment 4L-G1HL Layer Ⅴ(GP slurry) Waste shell mould sand Layer Ⅵ (GP slurry) 3 hrs Remove shell Casting Lost wax (Green strength ) Sintering (Fired strength ) Founding of molten metal Modulus of rupture test Ventilation properties Calculating of casting tolerance Figure 1 Process of the investment casting 2.3 Geopolymer Test Items The viscosity of the geopolymer slurry was determined using a Brookfield Viscometer (type RVF-1, USA). The setting time test was performed according to CNS786/R322. Flexural strength test was performed after drying 1 day and sintering of specimens according to CNS 1233/A346. Physical properties tests were performed after drying 1 day and sintering according to CNS 619/ RESULTS AND DISCUSSION 3.1 Geopolymer samples As shown in Figure 2, the viscosity of all samples decreased as the additional of ground calcium carbonate increased. Besides, the viscosity reduced as the SiO 2 /K 2 O molar ratio increased. Because of the high SiO 2 /K 2 O molar ratio, it leads to lower alkalinity, and higher moisture content in the alkali activator. Thus, its viscosity reduced. As shown in Figure 3, the setting time was reduced with increasing of calcium carbonate content at.57 SiO 2 /K 2 O molar ratio. However, geopolymer made at SiO 2 /K 2 O molar ratio of.7 or.82 had longer setting time. Moreover, the setting time of all samples were reduced with decreasing SiO 2 /K 2 O molar ratio due to the high potassium hydroxide concentration at low SiO 2 /K 2 O molar ratio would provide lower moisture contents to form geopolymer. It would decrease the time of dehydration in condensation reaction and the O14-3

4 setting time. For specimen prepared by G9MK1 and.57 SiO 2 /K 2 O molar ratio, it had the lowest final setting time which was about 45 minutes M125Si57 M125Si7 M125Si82 5 Visicosity (cps) Backup slurries in factory =18(cps) G5MK5 G7MK3 G9MK1 Depth (mm) 1 G5MK5-M125Si57 G5MK5-M125Si7 15 G5MK5-M125Si82 G7MK3-M125Si57 G7MK3-M125Si7 G7MK3-M125Si82 2 G9MK1-M125Si57 G9MK1-M125Si7 G9MK1-M125Si Setting time (min) Figure 2 Geopolymer viscosity Figure 3 Setting time of geopolymer The flexural strength analysis of geopolymer was applied after dried 1 day, including samples sintered and without sintered. Sintered of prismatic beam geopolymer specimens were prepared by various filler and SiO 2 /K 2 O molar ratio are shown in Figure 4, and 5. As indicated, specimens prepared by G5MK5 had the highest green flexural strength. No matter increase or decrease the SiO 2 /K 2 O molar ratio, the flexural strength increased after sintered at 12. Nevertheless, the strength of geopolymer with higher addition of calcium carbonate were reduced after sintered to 12. Flexural strength (MPa) M125Si57 M125Si7 M125Si82 Green strength in factory = 5.5 (MPa) Flexural strength (MPa) M125Si57 M125Si7 M125Si82 Fired strength in factory =6.9(MPa) G5MK5 G7MK3 G9MK1 G5MK5 G7MK3 G9MK1 Figure 4 Green flexural strength of geopolymer Figure 5 Flexural strength of geopolymer after sintered at Geopolymer used as outer layers casting mould Figure 6 shows the result of the modulus rupture of the green and sintered strength in shell mould with various substituted layers of geopolymer. It observed that the green strength was increased with increasing of substituted geopolymer layers. For specimen made by 4L-G1HL at room temperature, the highest of modulus rupture can be reached 1.2 MPa. After sintering, the sintered strength was decreased as the layers substituted by geopolymer increased. For specimen made by 4L-G1HL after fired at the temperature of 1125, a lowest modulus of rupture of 6.5 MPa was made, due to calcium carbonate decomposed at 85 at pre-heat process resulting O14-4

5 production of CaO and CO 2, thus, increased the collapsibility of ceramic sell mould. Therefore, it can be found that geopolymer used as outer layers for casting mould (5L-GHL, 4L-G1HL) had the better mechanical properties than raw shell mould (5HL) Green strength Fired strength Modulus of Rupture (MPa) HL 5L-GHL 4L-G1HL Figure 6. Modulus rupture of shell mould at room temperature and after sintered at Figure 7 shows the sealed shell mould surface on 1 bar CO 2 pressure, the time of gas pressure reduced to zero. Obviously, 4L-G1HL was the fast sample to reach zero. It should be noted that the pressure-decayed time related the thickness of shell mould, and therefore, the normalization of pressure-decayed time can be calculated by pressure-decayed time divided thickness of shell mould, as shown in Figure 8. The results showed that the geopolymer used as substituted outer layers casting mould (5L-GHL, 4L-G1HL) had the better permeability properties than raw shell mould (5HL). Gas pressure (bar) HL 5L-GHL 4L-G1HL Sec/mm Time (sec) Figure 7. Permeability properties of shell mould after sintering at HL 5L-GHL 4L-G1HL Figure 8. Normalization of Pressure-decayed time of shell mould after sintering at 1125 Table 3 shows the surface and dimension change of casting. The surface of the castings would not be affected by the substitution of the geopolymer. Similarly, the gas holes would not be observed on all castings. The castings could be generated by 5HL, 5L-GHL and 4L-G1HL procedures. The dimension of all castings were between mm, mm, and mm which were corresponded to A, B and C. Most important of all, the addition of the substituted geopolymer layers would not change the dimension of the castings. O14-5

6 Table 3 Surface and dimension change of casting Casting Diagram of casting 5HL 5L-GHL 4L-G1HL Surface of casting C A B Dimension of casting A 172.5±.9 mm 172.2±.1 mm 172.1±.2 mm 172.2±.1 mm B 38.6±.4 mm 38.7±.1 mm 38.8±.2 mm 38.8±.1 mm C 35.9±.4 mm 35.6±.1 mm 35.7±.2 mm 35.7±.2 mm 4 CONCLUSION The physical/mechanical properties of the geopolymer used 8 wt% of waste shell mould, 14 wt% of the ground calcium carbonate and 6 wt% of Metakaolin as raw materials can be reached excellent characteristics while prepared by.57 SiO 2 /K 2 O molar ratio alkali solution and 2.86 solid/liquid ratio. The sintered strength of the geopolymer can be achieved 3.2 MPa. When the geopolymer used as substituted outer layers casting mould, the properties of specimen show a good performances. Therefore, recycle and application of the waste ceramic shell mould by using geopolymer technology could be a potential way to use in investment casting process. REFERENCES [1] S. Nycz, R. Johnson, J. Jackson, P. Nipendra, Study Shows Fumed Silica in Combination with Colloidal Silica Provides Potential for Stronger Investment Casting Shells, pp , INCAST, 28. [2] EPA Taiwan, Environmental statistics, [3] W.H. Chen, Casting Applications Practice, Chuan Hwa Book Co. Ltd., Taipei, 199. (in Chinese) [4] S. Jones, M. Jolly, K. Lewis, Development of techniques for predicting ceramic shell properties for investment casting, British Ceramic Transactions, Vol. 11, pp , 22 [5] J. Davidovits, Keynote Conference on Geopolymer Conference, Melbourne, 22. [6] J. Davisovits, Geopolymer chemistry and applications, 3rd ed., 211. [7] Q. Li, H. Xu, F. Li, P. Li, L. Shen, JP. Zhai, Synthesis of geopolymer composites from blends of CFBC fly and bottom ashes, Fuel, Vol. 97, pp , 212. O14-6