Betts process

The bismuth that is found in the lead ore accompanies the lead through the smelting operation right up to the last refining steps. The removal of bismuth then requires special techniques, the most common being the Betterton-KioU and the Betts processes (5). 

Betts Electrolytic Process. The Betts process starts with lead bullion, which may carry tin, silver, gold, bismuth, copper, antimony, arsenic, selenium, teUurium, and other impurities, but should contain at least 90% lead (6,7). If more than 0.01% tin is present, it is usually removed from the bullion first by means of a tin-drossing operation (see Tin AND TIN ALLOYS, detinning). The lead bullion is cast as plates or anodes, and numerous anodes are set in parallel in each electrolytic ceU. Between the anodes, thin sheets of pure lead are hung from conductor bars to form the cathodes. Several ceUs are connected in series. 

In a modified process, potassium substitutes for calcium to form BiTMgeCag which liquates to the top of the bath and is removed from the molten lead. The Betts process is based on electrolytic refining using a solution of lead flu-orosilicate and fluorosilicic acid. While lead is deposited on the cathode, bismuth goes to the anode where it is collected with other impurity metals. It is then filtered, dried, smelted, and further refined, depending on the purity desired. Impurities are removed by adding molten caustic and zinc, and finally by chlorination. 

Electrolytic refining of lead bullion is commonly employed in many modern plants to obtain high purity grade metal. Various separation processes for removal of individual metals are not required. In such refining (Betts process), a solution of lead fluosilicate is used as an electrolyte, while the anode consists of impure lead bullion and the cathode constitutes a thin sheet of pure lead. Lead deposits on to the cathode during electrolysis. Impurity metals remain undissolved and attached to the anode, forming a slime which may be removed after electrolysis and treated for recovery of these metals. 

In the Betts process, the lead content of the electrolyte rises slowly because anodic current efficiency is higher than the cathodic one. The lead content is usually maintained at the desired level by electrolysis in separate cells with insoluble graphite anodes. The cathodic reaction is (35)... 

Betts process. An electrolytic process for removing impurities from lead in which pure lead is deposited on a thin cathode of pure lead from an anode containing as much as 10% of silver, gold, bismuth, copper, antimony, arsenic, selenium, and other impurities. The electrolyte is lead fluosilicate and fluosilicic acid. The scrap anodes and the residues of impurities associated with them are either recast into anodes or treated to recover antimony lead, silver, gold, bismuth, etc. 

Method of purification Desilvering (Parkes process), electrolytic refining (Betts process), pyrome-tallurgical refining (Harris process). Bismuth is removed by Betterton-Kroll process. 

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. 

Purification is carried out by adding zinc, which removes silver (Parkes process), or by electrolysis in a solution of lead hexafluorosilicate and hex-afluorosilicic acid (Betts process). 

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. 

Electrolysis of lead fluosilicate solutions is well established for lead refining by the Betts process, as detailed in Chapter 13. The principal advantage is the ability to produce a dense cathode deposit rather than a powder deposit. The process has largely been examined for the treatment of secondary lead materials, which are converted either into lead carbonate or PbO from mixed sulfate-oxide residues, and are then leached in fluosiUcic acid. The US Bureau of Mines developed an approach along these lines as an extension of the Betts electrorefining process to electrowinning (Cole, Lee and Paulson, 1981). This approach was also developed and applied on a commercial scale by RSR Corporation in the USA (Prengaman and McDonald, 1990). 

A generalised flow sheet of lead refining nsing the Betts process is shown in Fignre 13.1 and details of various operations are given in Table 13.3. 

Fig 13.1 - Flow sheet for electrolytic lead refining (Betts process). 

Electrolytic lead refining operations by the Betts process (Gonzalez-Dominguez, Peters and Dreisenger, 1991 Siegmund, 2000). 

The cell uses a graphite anode within a fabric diaphragm compartment from which the spent electrolyte is drawn. Filtered leach solution as feed electrolyte is fed to the open topped cathode compartment containing a stainless steel cathode from which the deposited lead is stripped. Current densities are of the order of 300 amps/m, which are 60 per cent higher than commonly used in the Betts process. 

Virtually all impurities in the lead bullion are retained in a leach residue, comparable to the anode shmes in the Betts process, but the leaching process can be continuous with the addition of fresh granulated lead and the separation of residue by filtration of the leach solution. There is no recycle of unconsumed lead. 

The disadvantage with this approach is the higher cell voltages resulting from the use of a diaphragm and the oxidation of iron at the anode with an E of -1-0.77 volts and an equilibrium cell voltage of 0.91. The resultant energy consumption is around 500 kWh/t of refined lead compared with around 150 kWh/t for the Betts process. 

Gonzalez-Dominiguez, J A, Peters, E and Dreisinger, D B, 1991. The refining of lead by the Betts process, J of Applied Electrochemistry, 21 189-202. 

Betts process An electrolytic process used for the refining and purification of lead with the associated recovery of silver and gold. Theprocess involves a solution of lead fluorosili-cate and hydro fluorosilicic acid as the electrolyte and lead electrodes. Invented by Anson G. Betts in 1901, the process is dependent on cheap forms of electricity. SeePAKKES process. 

Usually, electrolytic refining is based on the Betts process which was developed in the USA in the early 1920s. This process uses a property of lead which allows it to be deposited as metal in a bath of fluosilicic acid. A more recent process, developed in Italy in the 1950s, uses a sulphamate electrolyte, which is apparently easier to prepare. 

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