Copper and its alloys have a series of excellent properties, and its processed products are widely used in electrical, instrumentation, thermal, machinery manufacturing and military industries. However, the oxidation, gettering and gettering of copper are easy to occur in the smelting process, which pollute the copper melt and reduce the technological properties of the material and the properties of the product. Oxidation of molten copper is the reaction of copper melt with oxygen in the environment during smelting to form Cuprous oxide and copper oxide during the subsequent cooling process. These oxide inclusions will deteriorate the technological properties of copper and reduce the conductivity of copper. The dissolved hydrogen in the melt is the main reason for the porosity, porosity, bubble and delamination of the INGOT, which leads to the hydrogen embrittlement during the subsequent processing. Hydrogen mainly exists in copper melt in the form of solid solution, which comes from hydrocarbons used in melt reduction, furnace gas and water vapor and crystal water in the environment. The molten copper also often absorbs impurities in the smelting process. These impurities include various metallic impurities and non-metallic inclusions, which often adversely affect the final performance of copper products. In order to obtain the final quality copper products, the copper melt generally needs to carry out the necessary refining treatment before pouring.

Technical status 2.1 inclusions are removed by density difference, adsorption, combination and mechanical filtration in the smelting process of removing impurities. The density difference is that the density of non-metallic inclusion is less than that of copper liquid when the copper liquid is standing at high temperature. The role of adsorption is to introduce inert gas into the melt or take flux to produce neutral gas, in the bubble floating process, and suspended slag meet, slag may be adsorbed on the surface of the bubble and be taken out of the melt. The combination action is that there is a certain affinity between the slag and the flux, and the compound or complex can be formed, and finally separated and removed by standing. The so-called mechanical filtration, is refers to when the copper liquid passes through the filter medium, the filter medium will have the mechanical barrier effect to the non-metallic inclusion. 2.1.1 static clarification this method generally involves holding the copper solution for a period of time at the refining temperature and covered with a covering agent, allowing the inclusion to float up and be removed. 2.1.2 floatation is the use of inert gas or flux into the melt produced by the bubble, in the process of flotation and the suspension of slag encounter, the slag is adsorbed to the surface of the bubble and carried to the flux at the liquid level of the melt, as shown below. The more the number of bubbles and the larger the size, the better the flotation effect. The inert gases used are usually nitrogen or argon.

2.1.3 flux method is to remove slag by the action of adsorption, dissolution and combination between flux and slag. As shown in the image below. The flux is added to the surface of the liquid copper, and the slag in the upper layer of the molten pool contacts with the flux, which is absorbed, dissolved or combined into the flux. At this point, a thin layer of liquid copper in contact with the flux is purer and moves downward with a greater density than the molten metal containing slag, while the lower layer of liquid metal containing more slag rises in contact with the flux, in addition, the slag is constantly adsorbed, dissolved or combined with the flux and remains in the flux. This process continues until the slag in the entire bath is almost absorbed by the flux.

2.2 The gas contained in the degassed copper solution is mainly hydrogen, and degassing refining is to remove hydrogen from the melt. There are three ways to remove gas from MOLTEN METAL: first, gas atom diffuses to the surface of molten metal, and then escapes from the adsorption state; Thirdly, it forms a compound with the elements added into the liquid metal and is eliminated in the form of non-metallic inclusion. The first two methods are usually used for dehydrogenation of copper liquid.

2.2.1 when oxygen is fed into the melt by oxidative degassing, a large amount of copper is oxidized. The reaction formula is: 4CU + O2 = 2Cu2O Cuprous oxide is first dissolved in the copper solution. The Cuprous oxide then reacts with hydrogen in the Liquid Copper: CU2O + H2 = 2Cu + H2o Copper is reduced and Water Vapor escapes from the melt. When the reaction is continuous, the hydrogen content in the liquid copper will decrease continuously. During the oxidation process, samples should be taken continuously to check the degree of oxidation of the melt, and the oxidation should be stopped immediately when the oxygen content in the liquid copper is found to have reached the required level. The oxidized copper liquid should be deoxidized to remove the excess oxygen before it comes out of the furnace. 2.2.2 when nitrogen, Argon and chlorine are introduced into the molten metal in a steel tube for degassing inert gases, the partial pressure of hydrogen in the bubble is zero, while the partial pressure of hydrogen dissolved in the melt near the bubble is much higher than zero, based on the difference of partial pressure of hydrogen between inside and outside the bubble, the hydrogen dissolved in the melt is diffused to the bubble continuously, and is expelled to the atmosphere with the rise and escape of the bubble to achieve the goal of degassing. The smaller and more bubbles are, the better for degassing. It is not easy to degassing completely by this method because of the fast speed of bubble floating up and the short time of passing through the melt, and the bubble can not be evenly distributed in the whole melt. In order to improve the degassing refining effect, the gas purity should be controlled. The results show that the oxygen content of refining gas should not exceed 0.03% (volume fraction) and the moisture content should not exceed 3.0 g/l. If the oxygen content in nitrogen is 0.5% and 1% , the degassing effect decreases by 40% and 90% respectively.

2.2.3 vacuum degassing of copper can be divided into static vacuum degassing and dynamic vacuum degassing. The static vacuum degassing method is to put the copper melt under a certain vacuum degree (usually 1333 ~ 4000PA low vacuum) and keep it for a period of time. In the process of dynamic vacuum degassing, the copper liquid is led into the vacuum chamber through the flow tank, and a dispersed droplet of the melt is sprayed into the molten pool, and the refined gas is blown into the molten pool through the mechanical stirring fire cell for the melt, or through the low porous brick of the furnace, accelerate the degassing process. 2.2.4 in other degassing methods using solid flux degassing, the degassed flux is forced into the molten pool with a dry perforated cover, which is subjected to flux thermal decomposition or displacement reaction with the metal, hydrogen is removed by the formation of volatile bubbles that are insoluble in the melt. For example, aluminum bronze commonly used cryolite degassing, white copper commonly used fluorite, Borax, calcium carbonate and other flux degassing. 2.3 deoxidation is the reduction of copper from its oxides by adding to the melt a substance which has a greater affinity for oxygen than copper has for oxygen, a process in which solid, liquid, and gaseous products insoluble in copper liquid are formed and eliminated. A substance that reduces the oxide of copper is called a deoxidizer. The deoxidization reaction is: x [ m ] + Y [ O ] = (Mxoy)(4)[ m ] is the deoxidizer. MxOy is a deoxidation product, which is insoluble in copper solution. 2.3.1 the deoxidation reaction of diffusion deoxidizer mainly takes place on the surface of the molten pool, and the Deoxidation of the internal melt is mainly realized by the continuous diffusion of Cuprous oxide to the Molten Pool. The Cuprous oxide is less dense than copper and tends to float towards the surface of the bath. The Cuprous oxide on the surface of the molten pool is continuously reduced and the concentration is constantly reduced. As a result of the concentration difference, the internal Cuprous oxide of the molten pool is constantly raised. Under the cover of charcoal, the temperature is 1200 °C and the holding time is 20 min, the content of Cuprous oxide in copper liquid can be reduced from 0.7% to 0.5% . The deoxidation reaction of charcoal is:In addition to charcoal, some reducible cupric oxide flux with much less density than copper can also be used, such as magnesium boride (Mg3B2), calcium carbide (CaC2), boron slag (Na2B4O6·MgO) as surface deoxidizer. Diffusion deoxidation is slow and takes a longer time to reach complete deoxidation without contaminating the melt. 2.3.2 Precipitation deoxidizing copper and copper alloys commonly used precipitation deoxidizing agents are phosphorus, silicon, manganese, aluminum, magnesium, calcium, titanium, lithium, etc., deoxidizing results in the formation of gaseous, liquid or solid products. The main reaction is as follows :5Cu2O 2P = P2O5← 10CuCu2O P2O5= 2CuPO3(L)Cu2O Mg = MgO(S) 2CuCu2O Li= Li2O(S) The fine solid oxides produced by the 2Cu deoxidation reaction increase the viscosity of the metal or become non-uniform inclusions in the metal. When using this kind of deoxidizer, the amount should be controlled. Precipitation deoxidation can be carried out in the whole molten pool, the deoxidation effect is remarkable. The disadvantage is that deoxidizer residue may form inclusions. 2.3.3 Composite deoxidation in industrial production, sometimes "charcoal-argon (nitrogen)" composite deoxidation, which is a composite deoxidation method by diffusion deoxidation. The method is to blow argon (or nitrogen) deep into the molten pool by rotating a hollow graphite tube or a graphite tube with a porous nozzle at the head under the condition that the melt surface is covered with charcoal. Blowing argon into the liquid copper helps to expand the contact area between the carbon dioxide and charcoal of the molten copper bell, so that the diffusion rate of the carbon dioxide of the molten copper bell is accelerated and the distribution is more uniform. This method not only facilitates deoxidation but also dehydrogenation

3. The metal oxides produced in molten copper and copper alloys are almost all alkaline and can be eliminated by acidic fluxes such as quartz sand or boric acid. The flux should have the following basic conditions: (a) after dehydration or drying, it has strong ability of adsorption, dissolution and combination slagging; (b) the melting point is lower than the temperature of the metal melt; (c) the density is lower than the metal melt, and has a larger density difference with the copper liquid; (d) the interfacial tension between the copper liquid and the copper liquid is big, easy to separate from the melt; (e) the suitable viscosity; (F) has higher chemical stability and thermal stability, does not react with the lining Refractory, does not contain harmful gas impurities; (g) The hygroscopicity is small, the vapor pressure is low, the manufacture is convenient, the price is cheap. 

Source: Copper Alloy Casting