Starter sheets

The tank house is divided into commercial and stripper sections. In the latter, one-day deposits are prepared by electrorefining anode copper onto oiled copper, stainless steel, or titanium blanks. These copper sheets are stripped from the blanks and fabricated into starter sheets for the commercial sections as starting cathodes. After 9—15 days, depending on the tank house, hill-term cathodes are pulled and washed and either sent to the casting department or sold direcdy. 

The cathodes are grown for 5-14 days. The cathodes weigh 40-160 kg [20]. After being removed from the cell, the cathodes are thoroughly washed to remove the electrolyte that would otherwise contaminate the copper during melting. Two types of cathodes are commonly used. Cathode copper can be deposited on consumable thin copper starter sheets or on... 

The hydrogen overvoltage is also influenced by the condition of the surface of the electrodes. The starter sheets must be smooth as smooth surfaces lead to higher hydrogen evolution overvoltages. For this... 

The aluminum cathode sheets are 5-7 mm thick, supported on carrier bars, and provided with copper contacts. The AI2O3 layer prevents sticking of zinc. The cathodes are polished at intervals of a few weeks to prevent excessive adhesion of the deposits. Too high a fluoride (over 50 mg L-1) or chloride (300 mg L-1) level pits the aluminum starter sheets, causing zinc to stick very tightly to the starter sheets. 

Electrolytic refining. Lead of very high purity can be produced from the electrolytic process. Most electrolytic refineries utilize the Betts process [17]. In this process, lead bullion is cast into anodes and placed in an electrolytic cell which contains an electrolyte of fluorosilicic acid and lead fluorosilicate. The cathode is a thin sheet of high-purity lead referred to as the starter sheet . Lead is deposited on the cathode while the impurities form an adherent, but porous, slime layer on the anode. The slimes are collected for recovery and refining as they contain valuable impurities such as silver, gold, copper, and bismuth. 

The Betts electrorefining process was developed by Cominco at its smelter in Trail, British Columbia, Canada, in 1902. Lead from the smelter is melted and cast into anode slabs. Cathode starter sheets are made from refined lead. The electrolyte composi-tionvariesfromGOtolOOg lead as lead... 

For the electro winning of nickel, a tubular anode constructed from titanium with a mixed metal oxide coating is employed. In a standard design EMEW cell, the anode is approximately 50mm in diameter. The cathode is a stainless steel tube approximately 151 mm in diameter. Both the anode and cathode are fixed in the cell. A thin stainless steel starter sheet is rolled and inserted in the cell to deposit nickel. The deposited nickel cathode is removed from the cell as a tube. The stainless steel starter sheet then springs off the deposit to release the nickel cathode. 

Electrorefming involves the transfer of lead from an impure anode sheet, through an electrolyte to a high purity lead cathode. Crude bullion, after copper, arsenic and antimony ranoval, is cast into anodes, which are placed in tank cells. The electrolyte commonly used in the Betts Process is a solution of lead fluorosilicate and free fluorosilicic acid. Lead is deposited on lead starter sheets, which are removed from the cells and melted to high purity refined lead. Impurities are contained in the anode slimes and are collected and processed by pyrometallurgical methods for recovery of precious metals, bismuth and copper. 

The standard approach has been to use thin lead starter sheets for the cathode and to melt the total cathode after removal from the cell. The alternative approach is to use stainless steel cathodes and strip the deposited lead from the starting cathode. The latter has been applied more recently in secondary lead refining using fluoroborate electrolytes (Olper, 1998). There must be a balance between the requirements for producing starter sheets and eqnipment required for cathode stripping. 

Larger electrodes are possible with stronger electrodes rather than using thin starter sheets, which are easily bent. 

As detailed in Chapter 13 the electrical energy required for electrolysis is around 150 kWh/t of product lead. Some additional power for solution handling and fuels is required for the anode casting operation and for melting cathode lead in preparation for casting and starter sheet preparation. Estimates are provided in Table 16.4. 

Conventional electtowinning EW), as well as electioiefining, utilizes rectangular electrodes (planar starter sheets) having surface areas that occupy the space Imxlm approximately. The anodic and cathodic electrodes are suspended vertically and alternatively at a distance 3 cm < x < 10 cm in the cells. The electrodes in one cell are connected in parallel and the cells are connected in series as shown in Figure 7.3. This is a classical electrode-ceU arrangement for reducing the net cell potential drop and ohmic resistance [27,34]. 

Once enough metal deposition is obtained, the cathodes are mechanically removed from the cells and peeled off (stripped off), provided that a weak atomic bonding exists at the sheet-deposit interface. Then, the cathode starter sheets are recycled [1-3,34]. Figures 7.4 shows an electrowinning cells as part of a class project and Figure 7.5 illustrates ASARCO industrial cells for producing copper [33]. 

The technology of these etectrowinning processes is very simple, The cells are open OD iele tanks lined with rubber or plastic and the lines of anode and cathode plates are placed alternatively with a separation of 5 15 cm the cells are connected monopolar and one arrangement is shown in Fig. 4.S, The anodes are lead alloy sheets which in the sulphate medium form a lead oxide coating the metal additions to the lead (c.g. silver) arc to catalyse oxygen evolution and therefore reduce the anode overpotentiaL The starter sheets for the cathodes are aluminium or titanium and the metal is deposited until there is a layer 3-5 cm thick, at which point the cathodes are removed from the cell. The metal is then Stripped from the starter sheets and these are reused. The electrodes are typically Q,3-0.5 m in area but the anodes are always larger than the cathodes to prevent... 

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