Abstract
Oxydation rates of high-carbon materials such as coals, cokes, graphites, and active charcoal have been investigated by using low-temperature plasma asher with the maximum high-frequency power of about 30 W. A sample of 200400 mg weighed in a sample tube with a vibro-spatula was oxydized in the plasma gas at an ordinary ashing condition. Because of a relatively low H. F. power of this apparatus, the sample was located at the center of the high-frequency coil, and an air blower was used for cooling the reaction tube. Two types of sampling boats were provided as shown in Fig. 2, but the cylinder type was suggested for better protection of flying particles from the charged surface of the sample.
When the surface of the sample became covered with ash, the oxidation was markedly retarded due to the decrease of effective surface area for the oxidation. Mixing of the sample with a Pt wire should be therefore recommended at certain intervals stopping the operation of the plasma asher.
The end of ashing was determined at the time when the sample weight no more decreased, and this end was identical when the light blue glow at the discharge region turned to pale pink due to the initial and pure oxigen plasma.
Spectrophotometric study of some metal elements in the ashes prepared by the different ashing methods supported the higher recovery of metal elements in the ash left in the plasma asher. Wet combution with perchloric acid occasionally yielded serious loss of metal elements probably because of a presence of relatively volatile organometalic compounds in the sample.
The oxidation rate was essentially dependent upon the condition of the exposed surface and the ash contents of the sample. The pure graphite powder which has practically no ash and was spread in about 2 cm2 area, was oxidized at the rate of 45 mg (in 50 mg of sample) or 85 mg (in 200 mg of sample) per hour. On the other hand, oxidation of active charcoal having moderate ash contents but having extremely large surface area also proceeded in the similarly high rate. Both the pure graphite and the active charcoal were oxidized following the zeroth order reaction. The other high-carbon materials having relatively high ash contents were oxidized in the first order reaction because of the decreasing surface area for oxidation by increasing ash concentration during the ashing.
