Smelting characteristics of oxygen-free copper
Strictly distinguish, oxygen-free copper should be divided into ordinary oxygen-free copper and high-purity oxygen-free copper. Ordinary oxygen-free copper can be smelted in a power frequency iron core induction furnace, while high-purity oxygen-free copper should be smelted in a vacuum induction furnace.
When using semi-continuous casting, the refining process of the melt in the melting furnace and holding furnace can be independent of time constraints. Continuous casting is different. The quality of molten copper not only depends on the refining quality of the smelting furnace and holding furnace, but more importantly, it also depends on the stability of the entire system and process.
In order to prevent the melt from being contaminated, oxygen-free copper smelting generally does not use any additives for smelting and refining. The surface of the molten pool is covered with charcoal and the reducing atmosphere formed is a commonly used smelting atmosphere.
Induction furnaces for smelting oxygen-free copper should have good sealing properties.
Smelting oxygen-free copper should use high-quality cathode copper as raw material. To smelt high-purity oxygen-free copper, high-purity cathode copper should be used as raw material. If the cathode copper is dried and preheated before entering the furnace, moisture or humid air that may be adsorbed on its surface can be removed.
When smelting oxygen-free copper, the thickness of the charcoal layer covering the surface of the molten pool in the furnace should be double that of smelting ordinary pure copper, and the charcoal needs to be updated in time.
Although charcoal covering has many advantages, such as heat preservation, air isolation and reduction, it also has certain disadvantages. For example, charcoal easily absorbs moist air and even directly absorbs moisture, thus becoming a channel through which liquid copper may absorb a large amount of hydrogen.
Charcoal or carbon monoxide have a reducing effect on cuprous oxide, but are completely ineffective against hydrogen. Therefore, charcoal should be carefully selected and calcined before being added to the furnace.
During the smelting, transfer, heat preservation and entire casting process, complete protection of the melt is a necessary condition for the production of oxygen-free copper. In many modern oxygen-free copper smelting and casting production lines, not only the smelting, but also the drying and preheating of the charge, the transfer launder, the pouring chamber, etc. are fully protected.
Some modern large-scale oxygen-free copper production lines use generator gas as protective gas, while most gas generators use natural gas as raw material.
A method of manufacturing protective gas commonly used abroad is: first burn natural gas with relatively low sulfur content and 94% to 96% methane with theoretical air, and use nickel oxide as the medium to remove hydrogen. The resulting gas is mainly composed of Composed of nitrogen and carbonic acid gas. Then, the carbonic acid gas is turned into carbon monoxide by using hot charcoal to obtain an oxygen-free gas containing 20% to 30% carbon monoxide and the remainder being nitrogen.
In addition to generator gas, gases such as nitrogen, carbon monoxide or argon are also used as dielectric materials for oxygen-free copper melt protection or refining.
Vacuum smelting should be the best choice for smelting high-quality oxygen-free copper.
Vacuum melting can not only greatly reduce the oxygen content, but also greatly reduce the content of hydrogen and certain other impurity elements.
When smelting in a vacuum medium frequency coreless induction furnace, graphite crucibles and high-purity cathode copper or remelted copper that have been refined twice are often used as raw materials. Packed into the furnace together with the copper cathode, it also includes flake graphite powder for deoxidation.
In fact, deoxidation is mainly carried out through the carbon in the graphite crucible material. The amount of carbon consumed can be calculated. For example, 1 kg of copper consumes 100 g of carbon. Experience shows that the higher the oxygen content in the copper liquid at the beginning, the faster the deoxidation reaction proceeds in the early stages of smelting.
Oxygen-free copper obtained through vacuum smelting can have an oxygen content lower than 0.0005% and a hydrogen content lower than 0.0001% to 0.0003%. In fact, only when copper is smelted and cast under a certain degree of vacuum, it is possible to obtain castings that are completely free of oxygen and other gases. Therefore, the vacuum degree of the vacuum furnace used to produce copper materials for electronic tubes should be above 10-6.
