In a silicon single crystal manufacturing apparatus that is used for manufacture of a silicon single crystal based on the Czochralski (CZ) method, a seed crystal is brought into contact with a highly heated silicon melt in a crucible in an argon atmosphere and pulled up while being rotated, thereby growing a single crystal. It is crucial for recovery, purification, and recycle of the argon gas to discharge the argon gas by a vacuum pump in this silicon single crystal manufacturing apparatus, and then remove an impurity gas through a preprocess of removing silicon oxides contained in this waste gas.
As disclosed in Patent Literature 1, there have been suggested a step of removing solid contents in a waste gas, a step of compressing the waste gas, a step of removing oil contents, a catalytic reaction step of adding hydrogen exceeding a stoichiometric amount required for oxygen in the waste gas to convert the oxygen into water by a catalytic reaction, an adsorption step of adsorbing and removing the water and carbon dioxide produced by the catalytic reaction with the use of an adsorption cylinder at ordinary temperatures, and a method for purifying the waste gas which has been through each step.
Further, in Patent Literature 2, the following method is suggested. That is, a waste gas is taken into a gas holder by a blower and then passes through a heat exchanger for preheating and a heater via a dust collector which removes solid contents, a compressor which compresses the waste gas, an oil removal cylinder, and an oil filter, oxygen in the waste gas and hydrogen previously added to the waste gas are converted into water by a catalytic reaction with the use of a catalytic cylinder, and the water and carbon dioxide are adsorbed and removed by an adsorption cylinder via a cooling device. The waste gas is further introduced into a cold box, cooled by a main heat exchanger, and then subjected to gas-liquid contact in a rectifying cylinder, and an impurity gas having a lower boiling point than argon is separated and removed by liquefaction and rectification, thereby improving purity of a waste argon gas.
On the other hand, Patent Literature 3 suggests a method having a step of heating a waste gas, adding air or oxygen, converting carbon monoxide and hydrogen into carbon dioxide and water with the use of a catalytic cylinder, then cooling them, adsorbing and removing the carbon dioxide and the water in an adsorption cylinder of ordinary temperatures, further cooing the waste gas to −10 to −50°, and adsorbing the remaining carbon monoxide and nitrogen after a step of removing solid contents in the waste gas, a step of compressing the waste gas, and a step of removing oil contents.
FIG. 3 is a flowchart showing a method for recovering and purifying an argon gas according to each of Patent Literature 1 and Patent Literature 2. As shown in FIG. 3, in Patent Literature 1, a waste gas (FIG. 3(1)) from a single crystal manufacturing furnace is first compressed (FIG. 3(2)) by a blower and put into a gas holder (FIG. 3(3)), then solid dust is removed (FIG. 3(4)) by a dust collector and compressed (FIGS. 3(5)) to 730 kPa, and oil contents are removed (FIG. 3(6)). Thereafter, the waste gas is heated to 100 to 300° C. by a heater (FIG. 3(7-a)), hydrogen exceeding a stoichiometric amount required for oxygen in the waste gas is added, oxygen is converted into water and carbon monoxide is converted into carbon dioxide in a catalytic cylinder respectively (FIG. 3(8)), they are passed through a heat recovery heat exchanger and cooled to 10° C. by a cooling device (FIG. 3(9)), and then water and the carbon dioxide are adsorbed by an adsorption tower (FIG. 3(10)). Subsequently, the waste gas is introduced into a cold box, and nitrogen, the carbon monoxide, and the hydrogen are rectified and removed (FIG. 3(11)), and high-purity argon is thereby provided (FIG. 3(12)).
As shown in FIG. 3, in Patent Literature 2, substantially the same steps as Patent literature 1 are carried out until the removal of oil contents in the waste gas, then heating to 100 to 350° C. is performed by the heater (FIG. 3(7-b)), the hydrogen exceeding the stoichiometric amount required for the oxygen in the waste gas is added, and conversion to water is thereby performed in the catalytic cylinder. Subsequently, the waste gas is allowed to pass through the cooling device, the water and the carbon dioxide are adsorbed and removed at ordinary temperatures, then the waste gas is introduced into the cold box like Patent Literature 1, and the nitrogen, the carbon monoxide, and others which are yet to react are removed, thereby providing the high-purity argon.
FIG. 4 is a flowchart showing a method for recovering and purifying an argon gas according to Patent Literature 3. As shown in FIG. 4, in Patent Literature 3, the same steps as Patent Literature 1 and Patent Literature 2 are carried out until the removal of oil contents in the waste gas (FIGS. 4(1) to (5)), then heating to 200 to 350° C. is performed by the heater (FIG. 4(6)), a slightly smaller amount of oxygen than a stoichiometric amount required for the hydrogen and the carbon monoxide in the waste gas is added, and the waste gas is converted into the water or the carbon dioxide by a reaction in the catalytic cylinder (FIG. 4(7)). Thereafter, the waste gas is cooled to ordinary temperatures by the cooling device (FIG. 4(8)) and then introduced into a first adsorption cylinder, and the water and the carbon dioxide are adsorbed and removed with the use of zeolite (FIG. 4(9)). Furthermore, in the second adsorption cylinder, the nitrogen and the carbon monoxide which is yet to react is removed from the waste gas maintained at lower temperatures (−10 to −50° C.) by using the zeolite in an adsorption tower (FIG. 4(10)), thereby providing the high-purity argon (FIG. 4(11)).