Formaldehyde Deodorization Effect and Far-Infrared Emission Characteristics of Ceramics Prepared with Sawdust, Risk Husk, and Charcoal: Effect of Material Mixing Ratio

Sawdust, which had been produced in the sawing process of Larix kaempferi C., was purchased from a commercial sawmill, and rice husk was purchased from a rice mill (Jeonju, Korea), shade-dried, and ground with a grinder (Hanshin, J-NCM, Ansan, Korea) to increase adhesion during the preparation of boards. The particle size of the sawdust and rice husk was sorted in 18 mesh, and the moisture content was adjusted to 6% or less. Oak charcoal was purchased in the size of 100 mesh from Geumhan Farm (Imsil, Korea).

In addition, powdered phenol resin (KNB-100PL, Kolon Chemicals, Seoul, Korea) was used to prepare boards, and liquid phenol resin (KPD-L777, Kolon Chemicals) was used for impregnation to prepare ceramics. The characteristics of the resins are summarized in Table 1.

To prepare boards, the phenol resin powder was sufficiently mixed with uniformly selected sawdust, rice husk, and charcoal, and the mixture was placed in a square stainless mold on a heat plate of a hot press (Daeyang Machinery, DYMSP-001-026, Bucheon, Korea) and was pressed into 260 mm × 260 mm × 11 mm boards while maintaining the height at the top of the samples constant. As the conditions for preparing boards, the resin content and density were fixed at 10% and 0.6 g/cm³, respectively, and four types of boards were prepared by varying the mixing ratio between sawdust, rice husk, and ash to 50:35:5, 50:30:10, 50:25:15, and 50:20:20. During this stage, the hot press temperature was maintained at 170°C, the applied pressure was 40 kgf/cm² → 30 kgf/cm² → 20 kgf/cm² (three-stage press), and the press time was 9 minutes → 2 minutes → 1 minute (three-stage press time). A thickness bar was used to keep the thickness of the boards constant during the hot press process, and Teflon plates were used to prevent the top and bottom of the boards from adhering to the heat plates due to heat.

The prepared boards were cut to a thickness of 11 mm and a cross-sectional size of 120 mm × 120 mm, and their density was calculated. In order to keep the test conditions constant, samples with a density of ± 0.03 g/cm³ were selected. The prepared samples were placed in an impregnation tank (inner diameter Φ31 cm) containing the liquid phenol resin and were subjected to decompression (76 cmHg) and ultrasonic treatment (frequency: 28 kHz, output power: 564 W) simultaneously for 20 minutes, followed by a 20-minute rest at the atmospheric pressure. The impregnation rate was adjusted by changing the interval time. After impregnation, the samples were shade-dried for 8 hours and were placed in a drier again to be dried and cured at 60? for 8 hours and 130? for 8 hours consecutively.

Ceramics were prepared using the impregnated boards with a resin impregnation rate of 40 ± 2% and mixing ratios of sawdust, rice husk, and charcoal of 50:35:5, 50:30:10, 50:25:15, and 50:20:20 in a vacuum carbonization kiln (KOVAC KSF-200V, Korea Vacuum, Daegu, Korea) at a carbonization temperature of 800?. During this process, the temperature was raised by 4?/min from room temperature to the set temperature, which was maintained for 2 hours before being cooled down by circulating cooling water around the carbonization kiln.

The samples prepared with sawdust, rice husk, and charcoal at various mixing ratios were cut to 40 mm × 40 mm and were sealed in a 5 L reactor. The test gas was injected at an initial concentration of 20 μmol/mol, and the concentration of the test gas was measured at the start and after 30 minutes, 60 minutes, 90 minutes, and 120 minutes using gas detection tubes. The temperature and humidity during the test were maintained at 23°C and 50%, respectively. A separate test was conducted without any sample to measure the blank concentration. Deodorization rate of the test gas for each time period was calculated by the following equation (1).

where DR is the deodorization rate, CB is the blank concentration, and CS is the sample concentration.

To investigate the far-infrared emission characteristics of the ceramics, samples were prepared as a testing material with a size of 30 × 30 × 2 mm, and far-infrared emissivity was measured with an FT-IR spectrometer over the range of 5–20 μm at 40°C according to the standard method (KICM-FIR-1005) provided by the Korea Conformity Laboratories.

Table 2 shows the results of measuring the HCHO deodorization rate of the boards prepared with sawdust, rice husk, and charcoal at various mixing ratios with the board density fixed at 0.6 g/cm³. Overall, the deodorization rate was found to increase as the running time increased. The deodorization rate also slightly increased from 80% to 81% and 84% as the charcoal ratio increased from 5% to 10% and 20%, respectively. Oh (2014a) reported that the deodorization rate of particle boards prepared at a density of 0.6 g/cm³ using oak wood was 81.9%, similar to the experimental result of this study. However, the deodorization rate of the boards prepared with sawdust, rice husk, and charcoal in this study was at least 70% after 30 minutes, which was higher than that of the boards made of oak wood at 55.7% and demonstrated a superior initial deodorization effect. It is believed that the addition of charcoal powder increases the surface area given the same weight, resulting in effective initial deodorization performance.

Table 3 provides the results of measuring the HCHO deodorization rate of the ceramics prepared with sawdust, rice husk, and charcoal at various mixing ratios with the board density of 0.6 g/cm³, resin impregnation rate of 40 ± 2%, and carbonization temperature of 800°C. The deodorization rate tended to increase from 85% to 88% and 89% as the charcoal ratio increased from 5% to 15% and 20%. It is believed that the surface area increases with the addition of charcoal powder, which improves deodorization performance. Also, all of the samples demonstrated a very high initial removal rate as more than 80% was removed after 30 minutes.

When comparing the deodorization effect of the boards and ceramics at each mixing ratio, the ceramics were overall 5% or higher in deodorization effect than the boards prepared at the same mixing ratios. Generally, adsorption performance is determined by the surface area, and the specific surface area and pore volume of wood or woody materials are increased by the carbonization process (Lee and Kim, 2011). Thus, it is believed that the ceramics that have undergone the carbonization process has an increased surface area and is superior in deodorization effect than the mixed boards. Also, since the deodorization effect is overall measured to be 80%–90% after 120 minutes, it is deemed feasible to use the boards and ceramics prepared in this experiment for deodorization.

Fig. 1 shows the far-infrared emissivity and emission power of the boards prepared at different mixing ratios of rice husk and charcoal with the board density fixed at 0.6 g/cm³. Far-infrared emissivity and emission power tended to increase with the decreasing charcoal ratio and increasing rice husk ratio; 0.928 and 374 W/m² at the rice husk and charcoal ratio of 20:20, 0.925 and 373 W/m² at 25:15, and 0.933 and 0.376 W/m² at 35:5, respectively.

Fig. 1. Emissivity (a) and emission power (b) of ceramics after carbonization with mixing ratio of sawdust, rice husk and charcoal.

Download Original Figure

These far-infrared emissivity values were higher than that of silica sand (0.60 to 0.80), quartz-porphyry (0.90), and SiO₂ (0.83) and were similar to that of charcoal (0.93), graphite (0.93), and ceramic coating plates (0.924; KFIA, 2000). Particularly, the values were higher than the emissivity value of 0.80 of the woodceramics carbonized at 800°C as an MDF material by Okabe (1996) and that of the woodceramics prepared with thinned logs by Oh and Byeon (2006). The values were also higher than the far-infrared emissivity value of 0.923 of the ceramics prepared by Oh (2016) with sawdust and rick husk and measured at the same impregnation rate and carbonization temperature as in this study. Lee and kim (2010) reported that the far-infrared emissivity of commercially available domestic charcoal was 0.89 and 0.87 with traditionally manufactured black coal and white coal, respectively. In this case, it is thought that the addition of charcoal affected far-infrared emissivity. Additionally, the measured values of the ceramics prepared by Hwang (2011) with mandarin peels and sawdust exhibited a similar tendency.