Bismuth removal from lead

Bismuth removal, from lead, 14 755. See also Kroll-Betterton debismuthizing process... 

Crude lead contains traces of a number of metals. The desilvering of lead is considered later under silver (Chapter 14). Other metallic impurities are removed by remelting under controlled conditions when arsenic and antimony form a scum of lead(II) arsenate and antimonate on the surface while copper forms an infusible alloy which also takes up any sulphur, and also appears on the surface. The removal of bismuth, a valuable by-product, from lead is accomplished by making the crude lead the anode in an electrolytic bath consisting of a solution of lead in fluorosilicic acid. Gelatin is added so that a smooth coherent deposit of lead is obtained on the pure lead cathode when the current is passed. The impurities here (i.e. all other metals) form a sludge in the electrolytic bath and are not deposited on the cathode. 

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. 

Refining. The alloy of bismuth and lead from the separation procedures is treated with molten caustic soda to remove traces of such acidic elements as arsenic and teUutium (4). It is then subjected to the Parkes desilverization process to remove the silver and gold present. This process is also used to remove these elements from lead. 

Betterton (1) A process for removing bismuth from lead, A calcium-magnesium alloy is added to the molten lead. The bismuth concentrates in an upper layer and is skimmed off. 

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. 

BETTERTON-KROLL PROCESS. A process for obtaining bismuth and purifying desilverized lead that contains bismuth. Metallic calcium or magnesium is added to the molten lead to cause formation of high-melting intennetaHic compounds with bismuth. These separate as a surface scum and are skimmed off. The excess calcium and magnesium are removed from the lead by use of chlorine gas as mixed molten chlorides of lead or zinc. Bismuth of 99.995% purity is produced in this way. 

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. 

Refining secondary lead is not as complex as refining primary lead. The low levels of bismuth and silver contained in the product, as shown in Table 15.2, are generally below the specification limits required for most lead alloys and do not require removal. As mentioned above, the processes to remove these elements from lead are quite complex and there are no eeonomie incentives to recover them, as the levels are far too low. 

As can be seen in Figure 1, radon itself and its polonium daughter products are alpha emitting nuclides, while the isotopes of lead and bismuth produced are beta/ gamma emitters. The short half-lives of the daughter products prior to Pb (Table 2) result in the rapid production of a mixture of airborne radioactive materials which may attain equilibrium concentrations within a relatively short time. The half-life of °Pb is 22 years and at this point in the decay chain any activity inhaled is largely removed from airways in which it is deposited before any appreciable decay occurs. 

Lead-bismuth purification from polonium. Some studies in this field have been carried out at the IPPE (Obninsk, Russian Federation). In particular, the possibility of polonium alkaline extraction from lead-bismuth was studied, coolant flow rate of-0.1% being sufficient to decrease polonium activity in the circuit by over 4 orders and transform extracted polonium into non-volatile compounds. Periodic polonium removal from the circuit is possible for instance before scheduled repair. 

The lead was subsequently removed from the filtrate by precipitation of the blsmuthlol I (2,5-dlmercapto-l,3 4 thlodlazol) complex. This procedure was reported to give > 90 6 yield of pure lead and bismuth In less than 15 minutes (L3) ... 

A convenient iriethod for removing polonium from bismuth or lead has been patented by Karraker (Kl). The polonium Is removed from dilute hydrochloric acid solution by extraction with tr lbutyl phosphate ether or by a 20% sdlutlon of tributyl phosphate In dibutyl ether (H9).. ... 

The method of extracting both the bismuth and lead together from basic cyanide solution and then removing the lead by shaking with dilute acid has been used In the separation of lead from monazlte (P2) and from bismuth-lead mixtures by Moore (M5). The lead is normally removed Into 0.01 N nitric acid solution. Boulssleres and Perradlnl have Investigated the purification of RaD(Pb °), Ra(Bl ) and Po mixtures by dlthlzone extraction (b4). Their results show that either of the above methods can be used effectively. They also. note that Polonium behaves similarly to bismuth and can be extracted from add solutions. 

Anion exchange separations have been used for example to separate lead Isotopes of a few seconds half life from bismuth parent activities (S9). The column was maintained at 82.3 C by refluxing Isopropyl alcohol through an external Jacket In order to Increase the rate of equilibration. Bismuth was adsorbed on the resin and lead was eluted. Lead daughters of half life down to one or two seconds could then be removed from the resin rapidly enough with very dilute hydrochloric acid to allow their lifetimes and radiation characteristics to be measured. A similar method has been used by Campbell and Nelson to make measurements of the characteristics of 0.8 sec lead-207M produced from decay of blsmuth-207 (C3). In this work 200 meah Dowex-1 (1056 cross linked) resin was used at room temperature. These workers have utilized a flow... 

This method will remove the lead from radium, mercury and other common contaminants of RaD deposits but will not provide separation from bismuth (RaE). 

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 bismuth dross formed is removed from the surface of the lead on completion of the batch. Drosses normally contain between three and ten per cent Bi, and commonly around six per cent Calcium plus magnesium total around two per cent and the remainder is lead. Drosses can be upgraded by pressing or centrifuging to remove entrained lead. 

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