Using low grade silicon carbide and leftover construction soil to fabricate green bricks

Using low grade silicon carbide and leftover construction soil to fabricate green bricks *Min-Hsin Liu 1) and Yi-Le Hsu 2) 1), 2) Department of Environmental Engineering and Management, Chaoyang University of Technology, Taichung 41349, Taiwan 1) jliu@cyut.edu.tw ABSTRACT A significant amount of cutting mixture liquid is used in the cutting process of solar wafer slices. When a cutting mixture liquid is cut cyclically, the lubrication efficiency decreases. To maintain lubrication, the liquid needs to be regularly changed, thus producing considerable cutting mixture liquid waste. The cutting mixture liquid is claimed to be a recyclable waste. Low grade silicon carbide is the solid produced inform the solid and liquid separation process of the cutting mixture liquid, whose major components are silicon carbide and silicon; however, low grade silicon carbide has not yet been classified as a recyclable waste in Taiwan. This research discusses the related waste reduction and recycling. In the process of brick specimen making, 5%, 1%, 15%, 2%, 25%, 3%, 35%, and 4% of low grade silicon carbide was added to and mixed with leftover construction soil. Furthermore, the CNS 382 brick testing standard was conducted to determine the strength of the sintered specimen. The results indicated that low grade silicon carbide helps green brick sintering with less than 25% of the total materials. The best green brick result can be achieved with 1-15% addition of low grade silicon carbide. Moreover, the use of low grade silicon carbide helps reduce ignition loss in the sintering process. The use of 5% wasted brick billet in feedstock created a better quality, as did increasing the temperature from 1,2 º C to 1,5 º C. Keywords: Low grade silicon carbide, Solar wafer cutting fluid, Leftover construction soil, Green bricks 1. INTRODUCTION 1) Associate Professor 2) Graduate Student

Increasing environmental awareness has led to the rise of the solar energy industries. In order to maintain the lubrication of the cutting operations in the production of silicon chips in solar panels, cutting fluids, which are made of a mixture of cutting oils and silicon carbide powder, are added during the cutting process. To maintain the lubrication function of the cutting operations, the cutting fluid must be replaced regularly, thus generating large amounts of waste cutting fluid. The waste cutting fluid can become reused waste. Through physical separation methods, the waste cutting fluid can be separated from the solid waste, which mainly consists of silicon carbide, silicon, and a small amount of Diethylene glycol, also known as low grade silicon carbide (LGSC). The silicon carbide particles in the LGSC are relatively complete and do not undergo any changes or deterioration in quality during the cutting process; they account for 46.56% (Gopala and Murthy, 215) of the LGSC. Silicon is the main impurity of the LGSC, together with other impurities such as iron, copper, and zinc (Bidiville et al., 211). To purify and extract the silicon carbide in LGSC, the particles are first filtered through centrifugation and then mixed with the cleaning fluid in hydro cyclone to use the ion-exchange resin method to remove the dissolved solids. However, this method requires several steps and chemicals and produces a large amount of waste water; it also requires the regular cleaning of the ion-exchange resin and other components and thus has a relatively high cost. Green bricks are extensively made through the sintering of industrial waste, thus encouraging the recycling and reuse of industrial waste. In the green bricks certified by the Renewable Resources and Green Product Review (Babatunde and Zhao, 27), the added industrial wastes include ceramic waste, inorganic sludge, construction waste, water purification sludge, and glass waste. However, the current handling methods for low grade silicon carbide (LGSC) consist only of incineration and heat treating or use as a composite deoxidizing agent or heating agent in steelmaking processes. Therefore, the main aims of this research is to study the addition of LGSC to leftover construction soil sintering processes to make green bricks in order to utilize the characteristics of the silicon carbide and silicon to improve the quality of such bricks, in turn leading to widespread application of LGSC green bricks in green buildings. The grade determination of the bricks is complies with the specifications of ordinary bricks pursuant to Chinese National Standard CNS382. In general buildings, Class 3 bricks should have a water absorption rate lower than 15% and compressive strength higher than 15 kgf/cm 2, Class 2 bricks should have a water absorption rate lower than 13% and compressive strength higher than 2 kgf/cm 2, and Class 1 bricks should have a water absorption rate lower than 1% and compressive strength higher than 3 kgf/cm 2. 2. MATERIALS AND METHODS This research has divided the experimental process into three phases. First, two different types of leftover construction soil are selected for soil quality analysis, to find clay loam containing 31.5% sand particles, 6.1% silt particles, and 62.4% clay particles and silt loam containing 2% sand particles, 77% silt, and 3% clay particles. Then the silicon carbide powder and silicon powder are mixed in the ratios of 4:1 and 3:1 to

create two groups of low grade silicon carbide (LGSC). The silicon carbide mixture is then added to the two types of leftover construction soil in the proportions of 5%, 1%, 15%, 2%, 25%, 3%, 35%, and 4%, respectively. After soil development, formation, and drying, the soil is then sintered in high temperature ovens at 1,2ºC for 1 hour to make brick samples of 5 x 5 x 1 cm. Once complete, the volume reduction rate, water absorption rate, and compressive strength of the brick samples were explored. In the second phase of the experiment, the low grade silicon carbide recycled from the solar energy plants are added to the two types of leftover construction soil. The sources for low grade silicon carbide are obtained from the solid waste extracted from the waste cutting fluid recycled from two solar wafer plants in Taiwan (abbreviated as Plant T and Plant M). Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES, Thermo Scientific) is then conducted to analyze the metal composition of the LGSC, while the elemental analyzer (Elementar vario EL III) is conducted to analyze the element composition of the LGSC. The addition proportion of the LGSC is set between 5% and 4% to explore the changes and brick classes after the addition of LGSC. For the third phase of the experiment, a fixed 5% of waste clinkers from brick factory is added, and the sintering temperature is increased from 1,2ºC to 1,5ºC to reduce the porosity (Huang and Jin, 21) of the brick samples and increase the strength and grade of the bricks. 3. RESULTS AND DISCUSSION 3.1 Sintering results of the addition of silicon carbide and silicon mixture to leftover construction soil in Phase I After mixing silicon carbide powder and silicon powder in the ratios of 4:1 and 3:1 to synthesize the low grade silicon carbide (LGSC), the LGSC is then added in different proportions to the clay loam for sintering. The volume reduction rate for the brick samples are shown in Fig. 1. When the LGSC is added at proportions of % to 15%, the two groups of brick samples show a significant increase in the volume reduction rate but do not exhibit any changes in appearance. When added at proportions of 2% to 3%, the brick sample shows only small shrinkage. When added at proportions of 35%, the brick sample shows an unstable state. When the 3:1 silicon carbide is added at a proportion of 35%, the volume of bricks expands. After sintering, both groups show little differences in volume reduction rate. Therefore, this research deduces that the maximum proportion of silicon carbide that can be added is 3% in order to maintain the quality of the brick sample.

Volume reduction (%) 14. 12. 1. 8. 6. 4. 2.. -2. -4. 5 1 15 2 25 3 35 4 Ratios of LGSC addition (%) SiC:Si (4:1) SiC:Si (3:1) Fig. 1 Volume reduction rate of sintered bricks with different percentages of LGSC added to clay loam Through the changes in water absorption rate of the brick samples, it can be determined that the amount of open pores in the internal structure of the sintered samples. When the brick sample has a higher water absorption rate, it has more pores. The water absorption trend graph of the brick samples in Fig. 2 shows that when the addition of LGSC exceeds a ratio of 3%, the water absorption rate will exceed the minimum specification of 15% water absorption rate of the three classes of bricks used in ordinary construction. As the proportion of the low grade silicon carbide, which is synthesized from silicon carbide and silicon, increases, the porosity of the brick sample will also increase, resulting in an increase in the water absorption rate.

Water absorption ratio (%) 2 18 16 14 12 1 8 6 4 2 5 1 15 2 25 3 35 4 Ratios of LGSC addition (%) SiC:Si (4:1) SiC:Si (3:1) Fig. 2 Water absorption rate of sintered bricks with different percentages of LGSC added to clay loam The compressive strength of the brick samples are shown in Fig. 3. When low grade silicon carbide (LGSC) is added during the brick sintering process, the compressive strength of the brick samples can be increased to a minor extent but the percentage of LGSC added does not have a visible effect on the compressive strength of the brick samples between 23-28 kgf/cm 2. When the product is added with 5%, 15%, or 35% of the 4:1 silicon carbide to silicon LGSC group, the compressive strength of the product can reach a maximum of 26 kgf/cm 2, while the compressive strength of the product added with 25% of the 3:1 silicon carbide to silicon LGSC can reach 28 kgf/cm 2.

Compressive strength (kgf/cm 2 ) 4 35 3 25 2 15 1 5 SiC:Si (4:1) SiC:Si (3:1) 5 1 15 2 25 3 35 4 Ratios of LGSC addition (%) Fig. 3 Compressive strength of sintered bricks with different percentages of LGSC added to clay loam After the silicon carbide and silicon are mixed at ratios of 4:1 and 3:1, the grade distribution chart of the bricks made from the addition of the mixtures to the clay loam are shown in Fig. 4 and Fig. 5, respectively. The red box shows the scope of standards for Class 1 bricks, with a water absorption rate below 1% and compressive strength above 3 kgf/cm 2. The brick samples made with either groups of LGSC do not conform to the scope of criteria for Class 1. The green box shows the scope of standards for Class 2 bricks, with a water absorption rate below 13% and compressive strength above 2 kgf/cm 2. For bricks manufactured with zero to 25% addition of either group of LGSC, the bricks can roughly meet the standards for Class 2 bricks. The purple box shows the scope of standards for Class 3 bricks, with a water absorption rate below 15% and compressive strength above 15 kgf/cm 2. In general, with the exception of a few addition percentages, the bricks can meet the standards for Class 3 bricks. The results and data show that the mixture ratio of 4:1 and 3:1 of silicon carbide and silicon does not have an effect on the grade of the brick samples. Meanwhile, for the bricks to achieve Class 2 standards, the maximum percentage of added silicon carbide and silicon mixture is 25%.

Compressive strength (kgf/cm 2 ) 4 35 3 25 2 15 1 5 5% 15% 1% 25% % 35% 2% 4% 3% Class 1 Class 2 Class 3 2 4 6 8 1 12 14 16 18 2 Water absorption ratios (%) Fig. 4 Grade distribution chart of sintered bricks with different percentages of 4:1 silicon carbide and silicon mixture added to clay loam Compressive strength (kgf/cm 2 ) 4 35 3 25 2 15 1 5 5% 1% 2% 25% 4% % 15% 3% 35% Class 1 Class 2 Class 3 2 4 6 8 1 12 14 16 18 2 Water absorption ratios (%) Fig. 5 Grade distribution chart of sintered bricks with different percentages of 3:1 silicon carbide and silicon mixture added to clay loam

To explore if the quality of the leftover construction soil will also affect the quality of brick when the silicon carbide and silicon mixture is added to the leftover construction soil, this research has selected the 4:1 silicon carbide and silicon mixture because it has shown optimal results in the clay loam experiment and added in the silt loam leftover construction soil at percentages ranging from % to 4%. The grade distribution chart of the silt loam bricks is shown in Fig. 6. The results show that bricks made from silt loam are more stable than bricks made from clay loam. When the percentage of the silicon carbide and silicon mixture added is 35%, the resulting bricks can reach the specifications of Class 3 bricks, with only 4% additions going beyond the acceptable standards. The silt loam bricks also show better performance with regard to water absorption and compressive strength than clay loam bricks after sintering. 4 Compressive strength (kgf/cm 2 ) 35 3 25 2 15 1 5 1% % 5% 25% 4% 35% 15% 2% Class 1 Class 2 Class 3 2 4 6 8 1 12 14 16 18 2 Water absorption ratios (%) Fig. 6 Grade distribution chart of sintered bricks with different percentages of 4:1 silicon carbide and silicon mixture added to silt loam 3.2 Sintering results of the addition of low grade silicon carbide from solar power plants to leftover construction soil in Phase II Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) is conducted on the low grade silicon carbide recycled from Plant T and Plant M to analyze the metal composition, as shown in Table 1, while the element composition analyzed using Elementar vario EL III is shown in Table 2. The low grade silicon carbide recycled from the plants mainly contain silicon and iron, while the aluminum content of the solid waste from Plant M (3,56 mg/kg) is much higher than that of Plant T (122 mg/kg), showing that the reactive aluminum content will affect the results of the brick sintering process. Furthermore, the proportion of carbon content from Plant M is

higher, showing a higher proportion of silicon carbide in the low grade silicon carbide solids (LGSC) solid wastes than Plant T. The hardness of silicon carbide is second only to diamonds and has the characteristic of a low thermal expansion coefficient, so it will contribute to the compressive strength of the bricks. Table 1 Metal analysis of the LGSC solid wastes recycled from the two plants Analytical items Plant T (mg/kg) Plant M (mg/kg) Si 537, 286, Fe 77,3 4,3 Ca 845 1,9 Mg 291 296 Na 1,31 1,13 K 487 1,3 Al 122 3,56 Mn 237 291 Cd 2.36 1.56 Cr 2.1 22.1 Cu 1,51 81 Ni 41 41 Pb 11.3 2.32 Zn 366 527 Table 2 Elemental analysis of the LGSC solid wastes recycled from the two plants Element items Plant T (%) Plant M (%) C 1.9 13.6 N.2.14 H.5 1.69 O.53 5.75 S.25.7 The low grade silicon carbide solids (LGSC) recycled from the two plants are added to the leftover construction soil at percentages of 5%, 1%, 15%, 2%, 25%, 3%, 35%, and 4%. The grade distribution chart of the bricks made from the addition of low grade silicon carbide solids from Plant T to the clay loam and silt loam is shown in Fig. 7. In the experiments, only LGSC added at percentages of %, 5%, and 1% conform to the standards of Class 3 bricks. The bricks without addition of LGSC have a water absorption rate of 1.9% and compressive strength of 25 kgf/cm 2 and are

classified as Class 2 bricks. Bricks added with 5% and 1% of LGSC have a water absorption rate of 12.6% and 13.7%, respectively, and both have a compressive strength of 24 kgf/cm 2 ; thus, they are classified as Class 3 bricks. Bricks added with 15% to 4% of LGSC have water absorption rates that exceed the Class 3 specifications but the compressive strengths remain within the scope of Class 3 standards. The results show that when LGSC is added to clay loam, as the quantity added increases, the water absorption rate will increase, and the compressive strength will decrease. Fig. 7 also shows the experiment results for the addition of LGSC recycled from Plant T to silt loam bricks. After the addition of LGSC, the bricks do not meet the standards of Class 3 bricks. The results show that although the bricks have compressive strengths within the scope of the Class 2 standards of 2 kgf/cm 2, the water absorption rates are greater than the 15% standard required by Class 3; therefore, the groups do not conform to the scope of the standards. When the LGSC recycled from Plant T is added to the different leftover construction soils, the bricks show signs of poor water absorption performance, while the bricks sintered from silt loam have better compressive strength than those from clay loam. Compressive strength (kgf/cm 2 ) 4 35 3 25 2 15 1 5 5 1 15 2 25335 5 1 15 2 25 3 35 4 2 4 6 8 1 12 14 16 18 2 22 24 26 28 3 Water absorption ratios (%) 4 Class 1 Class 2 Class 3 LGSC percentage: Clay loam Silt loam Fig. 7 Grade distribution chart of sintered bricks with different percentages of LGSC (from Plant T) added to leftover construction soil The grade distribution chart of the bricks made from the addition of low grade silicon carbide (LGSC) from Plant M to the leftover construction soil is shown in Fig. 8. When LGSC from Plant M is added to the bricks made from clay loam, the bricks show compressive strengths higher than 2 kgf/cm 2, which is within the scope of Class 2 bricks, but poor water absorption performance. The water absorption rate of the bricks added with %, 5%, and 1% LGSC is 1%, 12.7%, and 11.8% respectively, which is within the scope of Class 2 bricks. The water absorption rates of bricks added with 15%, 2%, and 3% LGSC are within the scope of Class 3 bricks. Bricks with higher percentages of LGSC added do not fall within the scope of Class 3 bricks. When the percentage of LGSC from Plant M added to clay loam is higher, the resulting bricks

have poorer water absorption performance, but when added with 25% of LGSC, the bricks show a significant increase in compressive strength. When the LGSC (from Plant M) is added to the silt loam, after sintering, only the water absorption rates of the four groups of bricks with %, 5%, 1%, and 15% LGSC added conform to the specifications of Class 2 and Class 3, while the others fail to meet the standards. However, the compressive strength of the bricks from all groups are higher than the Class 1 standards of 3 kgf/cm 2, so bricks made from silt loam show a significant advantage in compressive strength than bricks made from clay loam. Compreesive strength (kgf/cm 2 ) 4 35 3 25 2 15 1 5 2 3 25 1 15 2 5 15 15 25 3 35 4 35 4 2 4 6 8 1 12 14 16 18 2 22 24 26 28 3 Water absorption ratios (%) Class 1 Class 2 Class 3 LGSC percentage: Clay loam Silt loam Fig. 8 Grade distribution chart of sintered bricks with different percentages of LGSC (from Plant M) added to leftover construction soil 3.3 Sintering results of the addition of low grade silicon carbide from solar power plants and waste clinkers to leftover construction soil in Phase III The results from the addition of LGSC from Plant T and Plant M to clay loam and silt loam show that in all the experiments, the addition of LGSC will increase the water absorption rate of the brick samples. Therefore, in the third phase, a combination addition of 1% to 25% of LGSC from Plant T and 25% to 4% of LGSC from Plant M will be added, together with 5% of waste clinkers, to the leftover construction soil for brick sintering. As the waste clinkers contain defects after sintering, the waste clinkers are first crushed and pulverized into particles, after which the particles with a diameter of.4 mm or less were added to the brick sintering process. During low temperature sintering, the waste clinkers have low compactness, and the increase in pores results in an increased water absorption rate. At high temperature sintering, the waste clinkers have higher compactness, causing the water absorption rate to decrease. Therefore, the temperature of the brick sintering process in Phase III is increased from 1,2ºC to 1,5ºC.

The grade distribution chart of the brick samples made from the addition of LGSC from Plant T and 5% waste clinkers to the leftover construction soil is shown in Fig. 9. The graph shows that the brick samples made from clay loam have a significant drop in water absorption rates and an increase in compressive strength. The addition of waste clinkers increase the grades of the bricks to the Class 2 standards. The brick samples made from silt loam also show a sharp increase in grade levels, with the optimal condition being the addition of 15% of LGSC and 5% of waste clinkers to the silt loam. The resulting brick is elevated to meet Class 1 standards. 4 Compressive strength (kgf/cm 2 ) 35 3 25 2 15 1 5 LGSC percentage: 15 1 Clay loam 2 252 115 Silt loam 25 Class 1 Class 2 Class 3 2 4 6 8 1 12 14 16 18 2 Water absorption ratios (%) Fig. 9 Grade distribution chart of sintered bricks with different percentages of LGSC (from Plant T) and 5% of waste clinkers added to construction leftover soil The grade distribution chart of the brick samples made from the addition of LGSC from Plant M and 5% of waste clinkers to leftover construction soil is shown in Fig. 1. The graph shows that the bricks made from clay loam exhibit a significant drop in water absorption rates while the compressive strengths are not significantly affected. The bricks made with the addition of the waste clinkers belong to Class 2. The bricks made from silt loam show a drastic drop in water absorption while the compressive strengths are not significantly affected. The bricks made with the addition of 25%, 3%, and 35% LGSC belong to Class 2, while the bricks made with the addition of 4% LGSC meet the standards of Class 3 bricks. The results show that the addition of 5% of waste clinkers can reduce the water absorption rates of the bricks made from the silt loam group. Although the increase in compressive strength is minor, the grade of the resulting bricks can be increased.

Compressive strenth (kgf/cm 2 ) 4 35 3 25 2 15 1 5 25 3 35 4 35 25 4 3 Class 1 Class 2 Class 3 2 4 6 8 1 12 14 16 18 2 Water absorption ratios (%) LGSC percentage: Clay loam Silt loam Fig. 1 Grade distribution chart of sintered bricks with different percentages of LGSC (from Plant M) and 5% of waste clinkers added to leftover construction soil 4. CONCLUSION Whether the 4:1 or 3:1 ratio of the silicon carbide to silicon mixture, the mixture does not affect the quality of the bricks and can be added to a maximum proportion of 25%. When the LGSC recycled from solar power plants is added to the leftover construction soil, the water absorption rate of the green bricks is affected. The higher the proportion added, the higher the water absorption will be. The compressive strength of bricks made from silt loam is better than that of bricks made from clay loam. When waste clinkers are added and the sintering temperature is increased to 1,5 º C, a significant drop in the water absorption rate can be observed, while the compressive strength of the brick will experience a slight increase, thus improving the grade of the brick. In this research, the optimal mixture proportion is the addition of 15% of LGSC from Plant T and 5% of waste clinkers to silt loam, which will produce a brick with Class 1 specifications. REFERENCES Babatunde, A.O. and Y.Q. Zhao. (27), Constructive approaches toward water treatment works sludge management: An international review of beneficial reuses, Environ. Sci. Technol., 37(2), 129-164.

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