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Recent Development and Trends in Electrorefining

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The production of copper by electrorefining has improved substantially since it was established in the late nineteenth century. Improvements in current efficiency, materials handling, electrolyte purification, and automation of process control have led to substantial production increases, particularly in the last several decades. According to Schloen and Davenport, the electrorefining capacity of 53 commercial tankhouses located throughout the world was 10.8 million metric tons in 1995. Some of the recent developments that have led to increases in production and cathode quality have been reusable cathodes, improved electrolytic bleeds, monitoring of additives, periodic current reversal, and anode preparation machines.

Reusable Cathodes

From the early days of electrorefining, copper starter sheets have been used as the cathode. The sheets are electrowon from stripper cells usually within the refinery. These thin sheets can create difficulties due to warping or bending. In recent years, the use of reusable starter sheets of titanium and stainless steel have become prevalent. Copper is electrodeposited on the mother plate for a suitable time, typically 7-14 days. The copper is then removed, usually by a mechanical stripping machine. Following a visual inspection, the starter sheets are returned to electrorefining cells.

Stainless steel technology has been growing in popularity compared to titanium because of the significantly lower initial capital expenditure. There are two different stainless steel technologies, the ISA process and the Kidd Process . The major difference between the methods is on the bottom of the cathode. The ISA process uses wax to prevent copper deposition and thus produces two cathode sheets (one on each side of the stainless steel blank). The Kidd Process leaves the bottom exposed, which creates two sheets connected at the bottom. The major advantages of the stainless steel technology over traditional copper starter sheets are the avoidance of starter sheet manufacturing and improved verticality. Improvements to the verticality of the cathode have led to fewer shorts, greater current densities, and purer cathodes.

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Electrolyte Bleeds

While the removal of impurities from the electrolyte has been practiced for many decades, there have been some recent changes in the removal method. Traditionally, the bleeding of impurities involves the use of liberator cells. In a liberator cell, there are several stages of electrowinning followed by crystallization. The electrowinning stages remove copper and the Group VB elements (As, Sb, and Bi). The first electrowinning step usually deposits a fairly pure copper product. The second and sometimes third electrowinning stage removes Cu, As, Sb, and Bi. Following electrowinning, the solution is heated to evaporate water and thus crystallize nickel, cobalt and iron as sulfates.

The removal of As, Sb, and Bi from the bleed streams using solvent extraction and chelating resins has received considerable attention within the last twenty five years. Hoey et al. indicated that solvent extraction can be utilized for As removal with a final salable product of bicupric arsenate, which is a precursor for chrome copper arsenate, a lumber preservative. Rondas et al. reviewed the solvent extraction of arsenic with TBP, which has been employed at Union Miniere since 1974. Ferric ions hinder the arsenic removal by TBP because of an undefined relationship between As5+ and Fe3+. Shibata et al. related the ability of UR-3300, a chelating resin manufactured by Unitika Co., to remove Sb and Bi selectively from commercial electrolyte. A final product of antimony chloride and bismuth chloride is produced by stripping the resins with 6 N HCl. Dreisinger and Scholey performed laboratory studies on two phosphonic acid based resins, UR-3300 and C-467, indicating a process to minimize ferric loading on to the resins. Sasaki reported using another chelating resin for Sb removal. The resin was EPORUS MX-2 produced by Miyoshi Oil and Fat Co. Ltd. and could be used with a 2-4 M HCl stripping solution. Toyabe et al. demonstrated that activated carbon can also be used to absorb antimony from the electrolyte. The loaded activated carbon is used as a reducing agent in a furnace within the silver refinery attached to the copper refinery.


Additive Monitoring
Additives, such as chloride, thiourea, glue and Avitone, are used extensively in electrorefining as grain refiners and leveling agents for deposition. They allow the production of smooth dense cathodes which will not encapsulate impurities either in the electrolyte or secondary phases. While additives have been used for many decades, the monitoring of these has only recently been employed. The development of the Reatrol Process by Asarco to monitor thiourea and the CollaMat system for glue by Norddeutsche Affiniere have led to substantial improvements in deposition consistency and improved current efficiency. The Reatrol Process is a patented technique, which utilizes differential pulse polarography to ascertain the active concentration of thiourea by complexation with a dropping mercury electrode. It is said to be accurate to 100 ppb of thiourea. A disadvantage with the Reatrol Process is that it is an off-line measurement. The CollaMet system is a proprietary real-time glue monitoring apparatus. There has been no literature on the details of the system except that it is based on evaluation of potential.


Periodic Current Reversal
Periodic current reversal (PCR) is being employed in at least 11 refineries to increase the rate of cathode production by increasing the applied current density. PCR is a method by which a forward current is applied for a length of time followed by a quick current reversal. The forward to reverse period ratio is typically between 20/1 to 30/1. The reversal of current allows for higher current densities to be employed without causing anode passivation. The advantage arises from the reversal current depleting the built up copper concentration within the anodic boundary layer. This helps avoid the precipitation of copper sulfate, which is one of the causes of passivation. Depleting the copper concentration also affects the localized pH, which can significantly affect the stability of copper oxide films, another cause of passivation. The major disadvantage of PCR is higher energy costs. This has limited the use of this technology.


Anode Preparation Machines
Anode preparation machines are growing in usage by allowing greater current densities, decreasing shorts, and decreasing damage to the electrolytic cells. This is caused by the improved verticality caused by pressing and lug machining and greater control of anode physical dimensions and weight. Anode preparation machines are now found in 23 of the 53 electrorefineries reported in Schloen and Davenport's most recent survey.

Anode preparation machines usually consist of at least five critical components: 1) receiving station, 2) anode weighing unit, 3) anode press, 4) lug press and milling and 5) spacing conveyor. The receiving station is where the anodes are loaded into the process stream of the electrorefinery. This is typically done with a forklift or straddle car. The anodes are then weighed. Anodes with out-of-spec weight are returned for re-casting. By controlling the anode weight, shorts are minimized and post-refinery scrap is minimized. While the least amount of anode scrap is ideal, anodes that break apart in the electrolytic cell can cause damage to the liner and hinder production. After weighing, the anodes are pressed to ensure proper physical dimensions. Pressing can be associated with measuring of anode thickness. Anodes that are too thick or thin are rejected. The lug press and milling is performed to ensure that the anodes will hang straight in the tankhouse and have a proper electrical contact. The final stage is the spacing conveyor. From the conveyor, the anodes are picked up, typically with an overhead crane and placed in the electrorefining cells. Proper spacing is critical to ensure uniform dissolution and minimize shorts. Spacing between anode and cathode (center to center) ranges from 60 to 177.8 mm with most tankhouses operating around 100 mm.


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