Change of the Dissolved Oxygen Content in the Process of Silicon Deoxidation.

Abstract

A new method has been developed to determine directly the dissolved oxygen content in molten iron which has oxide inclusions in it. By using this method, the change of the dissolved oxygen content in molten iron in the process of silicon deoxidation has been clarified. The method is based on such an idea that the primary deoxidation products and the oxides formed during cooling and freezing can be differentiated when radioactive silicon (silicon-31) is added to molten ironjust before the cooling begins.
In such a case, the oxides formed during cooling and freezing are radioactive, while the primary deoxidation products are not since it has been clarified that the exchange of silicon between the metal phase and the oxide phase does not occur. That is, one can estimate the dissolved oxygen content by measuring the radioactivity of SiO₂ extracted from iron samples, since almost all of the dissolved oxygen has combined with silicon due to its large affinity for the latter.
In each heat, 1.2kg electrolytic iron was charged into a magnesia crucible and melted in the indution furnace. The temperature of the molten iron was kept at 1600°C. Radioactive silicon was added to the molten iron at certain time after the addition of 0.5% silicon for deoxidation. SiO₂ was extraccted from iron samples by the nitric acid solution technique and its radioactivity was measured with GM counter. As the half life of silicon-31 is very short (2.62hr), all the measurements were compleed within 8 hours after the radioactive silicon was produced in a nuclear reactor. The change of the dissolved oxygen content with time was determined from six runs in which the radioactive elementt was added at varions times after the primary deoxidation.
The results are as follows:
1) The amount of the dissolved oxygen decreases very rapidly within one minute after the deoxidation and then continues to decrease very slowly. It is confirmed that the time required for the chemical reaction Si+20=SiO₂ (Si=Si⁴⁺+4e^-) is negligible since the observed oyxgen content at each stage of deoxidation was found in agreement with the value predicted by equilibrium relation with silicon. This fact also indicates that the rate of decrease of the dissolved oxygen can be estimated from the rate of increase of the metallic silicon in molten iron.
2) Comparison of changes of the dissolved and the total oxygen shows that the rate of decrease of the total oxygen is determined by the rate of removal of the deoxidation products.
3) The analyses of primary deoxidation products show that these consist only of SiO₂ and do not include any FeO. Thus a small amount of FeO found in final products must have been formed in the processes of cooling and freezing.
This technique is supposed to be applicable to other cases in which elements other than silicon are used for deoxidation.