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Fire refining (S and O removal) and anode casting
Abstract
Virtually all the molten copper produced by smelting/converting is subsequently electrorefined. It must, therefore, be suitable for casting into thin, strong, smooth anodes for interleaving with cathodes in electrorefining cells. This requires that the copper be fire refined to remove most of its sulfur and oxygen. Fire refining removes sulfur and oxygen from liquid blister copper by (a) air oxidation removal of sulfur as SO2 down to ∼0.003% S, and (b) hydrocarbon reduction removal of oxygen as CO and H2O down to ∼0.16% O.
... The flash converting and bath converting technologies have been developed to overcome the disadvantages [4,5]. Copper fire refining is mostly carried out in a rotary refining furnace, blowing gas, such as O2, CO2, and N2, for the removal of sulfur and adding reductants for the removal of oxygen [6,7]. ...
Arsenic content in copper concentrates is continuously increasing worldwide. It is desirable to remove arsenic from copper in the earlier stages of copper making due to the deposition of arsenic to cathode copper during the electrorefining process. Effects of temperature, flux, and oxygen on the distribution of arsenic during copper converting and fire refining processes were studied using FactSage 8.2. The results showed that arsenic can be effectively removed by proper selection of converting and refining slags. The decrease in Fe/SiO2 or Fe/CaO ratio in the converting slag is favorable for arsenic distributed to slag. CaO is more effective than SiO2 in decreasing the liquidus temperature of the slag and arsenic content in the blister copper during the converting process. Na2O or CaO as a flux is effective to remove arsenic in the fire refining process.
The evolution of methods used for deoxidation of anode topper at the Noranda smelter is summarized. The processes range from birch poles and reverberatory furnace to present day natural gas and rotary furnace. Current deoxidation practice is discussed in detail and comparisons made between Noranda and some alternative procedures.
Anode casting is the link between pyro and hydrometallurgy of copper, and it must possess a certain chemical and physical quality to achieve good electrorefining performance. The aims of anode casting such as high output and long mould lifetimes are not consistent with the objectives of electrorefining, which include uniform corrosion, minimum anode scrap, optimum current efficiency, and high cathode quality. The anodes should have a homogeneous chemical composition, so that the electrolysis can be adjusted properly, for optimum electrorefining operations. The accompanying elements in the anode should form soluble compounds with the copper or solid solutions, as insoluble compounds lead to high amounts of Cu in the anode slimes or the formation of passivating layers on the anode surface. The grain size and the elemental distribution can be adjusted by altering solidification conditions such as change of cooling rate and thermal conductivity.
Copper refining comprises two steps: firstly, the production of anode copper by fire refining in an anode furnace and anode casting, and secondly electrorefining. The anode should have a certain physical and chemical quality in order to meet the requirements of electrorefining, i.e. to achieve a high current efficiency, low energy consumption, low amount of anode scrap, and low effort together with high cathode quality. Hence, the anodes must provide uniform corrosion, weight, and geometric dimensions (especially thickness), smooth surfaces, minimal edge effects as well as minimal distortion of the body and the lugs, closeness of anode spacing, and reduced tankhouse cell loading time. Further quality criteria are minimum content of harmful impurities, surface conditions of anode body (especially absence of passivating films), density, and gas-saturation capacity. The anode quality is influenced by the casting process. Part I of this work focuses on chemical anode quality, Part II (see p. 83) on physical anode quality.
The use of gas purging plugs in the copper industry is a widely used practice. They provide substantial improvements especially in anode refining furnaces, where savings in process time and energy are obtained as well as refractory wear and buildups inside the furnace are minimized. The newly developed system, enables purging plugs to be implemented in both stationary and tilting furnaces, is in operation in vessels on three continents. An additional advantage in these furnaces above all other benefits is the significantly improved melting rate. This innovative and patented gas purging system consists of a purging plug cooling device and a slidable, cooled piping system. It provides compensation for any thermal expansion in the furnace lining and minimizes the risk of undesirable copper infiltration. User experiences and results of this widely used system are reported in this paper.
Based on estimated distribution ratios of several impurity elements between Na2CO3 slag and molten copper, a mathematical model has been developed to discuss the application of Na2CO3 flux to the removal of impurities from molten copper, in terms of the Na2CO3 flux consumption. The effectiveness of the Na2CO3 flux treatment to reduce the impurity contents in molten copper to 0.1 mass ppm, the level required for the high purity copper, is discussed by the model and confirmed by equilibrium distribution experiments.
The decreasing quality of the input materials in copper recycling leads to a higher content of impurities in the anode copper. Therefore an improvement of the pyrometallurgical refining process is necessary to produce high quality anodes for the copper refining electrolysis. In order to improve the metal/slag reactions as well as the volatilisation by selective oxidation in the anode furnace, the behaviour of the most important accompanying elements (e.g. nickel, tin, lead, zinc etc.) at different reaction conditions has to be investigated. This requires knowledge about thermodynamic conditions like the reaction order and the activity coefficient at the copper refining process. Additionally the interactions between different elements, but especially those for nickel, have been investigated as a function of the temperature, the content of the elements and the slag composition. These investigations were done at the Christian Doppler Laboratory for Secondary Metallurgy of Nonferrous Metals.
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