The loss of metal due to oxidation during smelting is called oxidation burning loss, the extent of which depends on the thermodynamic and kinetic conditions of metal oxidation, it is related to the properties of metal and oxide, melting temperature, gas property, charge state, furnace structure and operation method, etc. . 

1. Properties of metal and oxide: The burning loss of pure metal depends on the affinity between metal and oxygen and the properties of oxide film on metal surface. The metal and oxygen affinity is big, and its oxide film is loose and porous, then its oxidation burning loss is big. Cuo is of medium affinity, but its strength is small and its linear expansion coefficient is different from that of metal. Cuo is stratified, fractured and detached when heated or cooled. CU2O is liquid or soluble at melting temperature. The degree of Oxidation burn-out of the alloy varies with the addition of alloying elements. The surface active elements with high affinity to oxygen are oxidized preferentially or simultaneously with the base metal. The addition of aluminum and rare earth in brass and aluminum and cerium in nickel alloys have certain antioxidant effects. Iron, nickel and lead in copper alloys generally do not promote oxidation, nor do they oxidize obviously

2. Melting temperature: when the temperature is not too high, the oxidation of metal more follow a straight line, high temperature more follow parabola law. Because the diffusion mass transfer Coefficient increases and the strength of oxide film decreases at high temperature, and the linear expansion coefficient of oxide film is different from that of metal, the oxide film is easy to break. Sometimes due to the oxidation of the film itself dissolved, melted or volatilized so that the loss of protection.

3. The nature of the furnace gas: The nature of the furnace gas depends on the type and structure of the furnace used, the heat source or the degree of combustion of the fuel. The furnace gas often contains various proportions of oxygen, water gas, carbon dioxide, hydrogen and other gases, the property of furnace gas depends on the relative value of partial pressure of oxygen and decomposition pressure of metal oxide in the furnace gas equilibrium system. For example, when oxygen-free copper is smelted in the low-frequency induction furnace, the oxidation of copper can be minimized by adding active charcoal to cover and closing the furnace cover. Because in the weak oxidation atmosphere, the oxygen concentration is small, the copper oxidation rate is very slow.

4. The production practice shows that the degree of metal oxidation loss is very different when using different types of furnaces, because the shapes, areas and heating methods of the Molten Pool are different for different furnace types. It is easy to break the protective oxide film on the surface of melt and increase the oxidation loss of metal when the operation methods such as stirring and slag scraping are not reasonable.

The method of reducing oxidation burning loss is considered from two aspects: melting equipment and melting technology. A. Select a reasonable furnace type, try to choose a smaller bath area, heating fast furnace, currently widely used power frequency and intermediate frequency furnace. B. With reasonable charging sequence and charge treatment process, the charge which is easy to be oxidized and burned should be added to the melt in the lower layer of the furnace or after other charge is melted, or in the form of intermediate alloy, detritus shall be remelted or compacted into high-density packages for use. C. The oxidation loss can also be reduced by using a covering agent or refining agent to cover the charge surface with a thin layer of flux during charging. Readily oxidizable metals and various metallic detritus shall be melted and refined under a flux cover. D. The furnace temperature should be properly controlled under the conditions of ensuring the fluidity of the molten metal and the requirements of the refining process. In general, the furnace charge should be rapidly heated and melted at high temperature before melting, and the furnace temperature should be adjusted after melting, do Not superheat the melt strongly. E. In order to control the properties of the furnace gas correctly, the redox furnace gas should be used in the reduction stage of red copper smelting and in the oxygen-free copper smelting for the alloy which is easy to absorb hydrogen, the matrix metal oxide is reduced by reducing agent, so the active refractory metal can only be melted in protective atmosphere or vacuum. F. Reasonable operation method, should pay attention to operation method when smelting bronze containing aluminum and silicon, avoid frequent stirring, so as to keep oxide film intact, even if not using covering agent to protect, can effectively reduce oxide burning loss.

 Source: Copper Alloy Casting

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