4. EXPERIMENTAL
4.2 Apparatus
Experimental set-up mainly consisted of a high temperature vertical tube furnace and a gas supplying system including Argon (Ar), Nitrogen (N2), Carbon monoxide (CO) and Carbon dioxide (CO2) gases. In Figures 4.1 and 4.2 a schematic diagram and a general view of the experimental set-up used in this study are given, respectively.
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4.2.1. Furnace
The vertical tube furnace consisted of a programmable temperature control unit and a recrystallized alumina reaction tube with 50mm inside diameter and 1000mm length enclosed by MoSi2 heating elements which allowed up to 1700 oC maximum furnace temperature. The ends of the reaction tube were closed with silicon stoppers fitted with alumina rods for gas inlet and outlet.
Before the experiments, the hot zone of the tube furnace was determined, and a recrystallized alumina support rod with a plate was placed in this zone. Then, the radiation shields were placed at either end of the reaction tube to provide thermal insulation. After an empty silica crucible was inserted to the hottest region with the help of the support rod, the temperature profile of the furnace was obtained as given in Appendix A. According to the temperature profile of the furnace, the constant temperature zone was maintained in the range of ±3 oC within the length of 80 mm which corresponded to silica crucibles’ height.
Furnace was heated to the desired temperature in exactly 5 hours. After being kept at that temperature for a certain period of time, it was cooled at a rate of ~4 oC/minutes in all experiments.
4.2.2. Gas Supplying System
Gas supplying system included four silica gel columns, two gas cleaning furnaces for CO and CO2, four flow meters (two of them being capillary type for CO and CO2), a gas mixing unit (to mix CO-CO2 gases), a gas control unit (to change the type of gas needed in flushing and in the main experiments), and two bubble flasks filled with H2SO4 (gas washing column) at the entrance as well as at the exit of the furnace to check for any leakage in the furnace.
All gases were initially passed through the columns of silica gel to remove any trace of moisture that may be present. Then, Ar which was used only for flushing and N2 gases were sent to DK-800S-4 model flowmeters to control their flow rates while CO and CO2 were passed through the gas cleaning furnace to remove oxygen present in the gases. Gas cleaning furnaces including pure copper chips were heated to 500 oC and kept at that temperature during the experiments. Since the experiments were performed under different atmospheres (N2 and controlled oxygen atmosphere) after argon flushing of the furnace, a gas control unit was used to obtain the planned atmosphere in the furnace.
To provide the required oxygen partial pressure of the system, carbon monoxide – carbon dioxide (CO–CO2) gas mixture was sent into the furnace. The flow rates of CO–CO2 gases were controlled by two capillary flow meters since they could be easily installed and calibrated for measurement of small flows. As seen in Figure 4.1, this system included two leveling bottles filled with CuSO4 solution to adjust the height of the liquid (dibutly phtalate) in manometers. Hence, the CO–CO2 flow rates were determined by means of these leveling bottles by adjusting the height of liquids in manometers.
Prior to the main experiments, the capillary flow meters were calibrated by soap bubble method which is commonly used to measure the volume flow rate of gases. Calibration measurements and results are given in Appendix B. Consequently, the calibrated CO–CO2
gases were mixed in a glass bead mixer and sent into the furnace in order to assure predefined oxygen partial pressure of the system. The oxygen partial pressure of the system was checked by using a DS oxygen probe (supplied from Australian Oxytrol System Co.) during the required experiments. It was suitable for the measurement of oxygen partial pressures up down to 10-20 atmosphere at a temperature range from 700 oC to 1700 oC, which covered the experimental conditions studied in this thesis (Po2: 10-7–10-11 atm., Temp.:
1200 – 1300 oC). Since the oxygen probe output was DC millivolt, a potentiometer was connected to its output. This millivolt signal was used to calculate the oxygen partial pressure in the furnace by means of Nernst equation. All calculations of Po2 depending on the CO/CO2 ratio and also oxygen probe measurements are given in Appendix B.
4.2.3. Crucibles
Smelting experiments were done in silica crucibles produced in the Metallurgical and Materials Engineering Department of METU by slip casting method. For this purpose, equal amounts of silica (extra pure sea sand-Merck quality) and kaolinite were mixed with half as much as water to a prepare slurry. This slurry was ground in a ceramic ball mill for 8 hours and then it was poured into a previously prepared plaster mold (slip casting method).
Crucibles were left to dry overnight, heated to 1450 oC in 10 hours kept at this temperature for 2 hours in a muffle furnace, and then cooled down to room temperature (firing method).
Cooling rate was very slow (~3oC/min.) to prevent formation of any cracks in the crucibles.
After firing, all crucibles (more than one hundred) were observed to be glazy in appearance without any visible deformation. According to X-Ray Fluorescence (XRF) analysis, the silica crucible consisted of 73%SiO2, 16% Al2O3, 7%K2O and small amounts of other oxides (2%
Fe2O3, 1% CaO, 0.8% P2O5). As a result, each silica crucible had the dimensions within the
limits of 30±1mm inside diameter, 38±1mm outside diameter, 80±2mm height and 7±1mm bottom thickness.