Bismuth removal

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

Solid oxidants Sodium bismuthate is such a strong oxidant that it wiU oxidize Mn(II) to permanganate at room temperature. Ce(III) is oxidized quantitatively to Ce(rV) in sulfuric acid solution, and the excess bismuthate removed by filtration. Silver-catalyzed peroxydisulfate oxidation has largely supplanted the bismuthate method, thus avoiding the inconvenient filtration stq> required with solid oxidants. 

BBOC. Seven of the reporting operations carry out a bismuth removal step with part or the entire amount of lead bullion. Before casting the lead into ingots or blocks, all traces of any remaining impurity elements are removed by a final refining with caustic and niter. 

Kroll Bretterton process for bismuth removal 1946 Vacuum dezincing process... 

In some cases special removal steps are added, such as tellurium removal, where these elements are present in significant amounts in the ores treated. Also, particular smelters may have less than the full range of separation stages depending on the impurities present and the smelter s source of lead concentrates for instance bismuth removal can often be omitted. However, in all cases copper dressing and caustic refining are required. 

Most electrolytic refineries were built before the successful application of the Kroll-Betterton process for bismuth removal as the only reasonable approach for bismuth removal. Since that time no new electrolytic refineries have been installed and the adverse economics are clearly indicated by... 

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. 

Before the fibers can be spun into yams, a certain amount of preparation is necessary for cleaning and removal of undesirable accessory materials such as fat, wax, gum, or pulp. The weighting of sUk is a process to counter the weight loss resulting from degumming the fibers using heavy metal salts of tin or bismuth. This process affects the durabUity and long term preservation. 

Pemoval of Other Impurities. After softening, the impurities that may stiU remain in the lead are silver, gold, copper, tellurium, platinum metals, and bismuth. Whereas concentrations may be tolerable for some lead appHcations, the market values encourage separation and recovery. The Parkes process is used for removing noble metals and any residual copper, and the KroU-Betterton process for debismuthizing. 

Bismuth concentrates in the dorn until the last stages of cupeUation, when it is oxidized and removed with the Htharge. After the last Htharge has been removed, it is often necessary to add bars of refined lead to provide more Htharge to carry off the last traces of bismuth. The dorn metal is then cast into bars for marketing. 

In the Betterton-KroU process the dezinced lead is pumped to the debismuthizing kettie, in which special care is taken to remove drosses that wastefuUy consume the calcium and magnesium. The skimmed blocks from the previous debismuthizing kettie are added to the bath at 420°C and stirred for a short time to enrich the dross with the bismuth being extracted from the new charge. This enriched dross is skimmed to blocks and sent to the bismuth recovery plant. 

Electrolytic Eefming. Electrolytic refining (26,27), used by Cominco Ltd. (Trad, B.C., Canada) and Cerro de Pasco Corp. (La Oroya, Pern), as weU as by several refineries in Europe and Japan, removes impurities in one step as slimes. The impurities must then be separated and purified. Before the development of the Betterton-KroU process, electrolytic refining was the only practical method of reducing bismuth to the required concentrations. 

Metals less noble than copper, such as iron, nickel, and lead, dissolve from the anode. The lead precipitates as lead sulfate in the slimes. Other impurities such as arsenic, antimony, and bismuth remain partiy as insoluble compounds in the slimes and partiy as soluble complexes in the electrolyte. Precious metals, such as gold and silver, remain as metals in the anode slimes. The bulk of the slimes consist of particles of copper falling from the anode, and insoluble sulfides, selenides, or teUurides. These slimes are processed further for the recovery of the various constituents. Metals less noble than copper do not deposit but accumulate in solution. This requires periodic purification of the electrolyte to remove nickel sulfate, arsenic, and other impurities. 

In the calciaation process, a mixture of corresponding oxides and an optional modifier, eg, molybdic acid, are milled together to achieve a homogenous mixture. The mixture is calciaed at 750—950°C and milled to a desired particle size. Wet milling ia an alkaline medium is recommended to remove any unreacted vanadium salts that ate beheved to degrade the pigmentary properties of bismuth vanadate (39). 

Purification. Tellurium can be purified by distillation at ambient pressure in a hydrogen atmosphere. However, because of its high boiling point, tellurium is also distilled at low pressures. Heavy metal (iron, tin, lead, antimony, and bismuth) impurities remain in the still residue, although selenium is effectively removed if hydrogen distillation is used (21). 

In hquidation, tin is heated on the sloping hearth of a small reverberatory furnace to just above its melting point. The tin mns into a so-called poling ketde, and metals that melt sufficiently higher than tin remain in the dross. Most of the iron is removed in this manner. Lead and bismuth remain, but arsenic, antimony, and copper are partly removed as dross. 

Iron, copper, arsenic, and antimony can be readily removed by the above pyrometaHurgical processes or variations of these (3). However, for the removal of large quantities of lead or bismuth, either separately or together, conventional electrolysis or a newly developed vacuum-refining process is used. The latter is now in use in Austraha, BoHvia, Mexico, and the CIS (5). 

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). 

Betterton-KroIIProcess. MetaHic calcium and magnesium are added to the lead bullion in a melt and form ternary compounds that melt higher than lead and are lower in density. By cooling the lead bath to a temperature close to the melting point of lead, the intermetalHc compounds high in bismuth content soHdify and float to the top where they are removed by skimming. 

This bismuth—calcium—magnesium dross also contains lead that must be removed. The dross is heated in a ketde to free any entrapped lead that melts and forms a pool under the dross. This lead is cast and returned to the bismuth separation cycle. The dross is then melted and treated with chlorine and/or lead chloride to remove the calcium and magnesium. The resulting molten metal is an alloy of bismuth and lead, high in bismuth which is then treated to produce refined bismuth metal. 

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. 

The washed slime is dried and melted to produce slag and metal. The slag is usually purified by selective reduction and smelted to produce antimonial lead. The metal is treated ia the molten state by selective oxidation for the removal of arsenic, antimony, and some of the lead. It is then transferred to a cupel furnace, where the oxidation is continued until only the silver—gold alloy (dorn) remains. The bismuth-rich cupel slags are cmshed, mixed with a small amount of sulfur, and reduced with carbon to a copper matte and impure bismuth metal the latter is transferred to the bismuth refining plant. 

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. 

The desilverized alloy now contains bismuth as well as lead and ziac. To remove the lead and ziac, advantage is taken of the fact that ziac and lead chlorides are formed before bismuth chloride [7787-60-2J, BiCl, when the alloy is treated at 500°C with chlorine gas. Ziac chloride [7646-85-7] ZnCl, forms first, and after its removal lead chloride [7758-95-4], PbCl2, forms preferentially. This process is continued until the desired level of lead removal has been reached. The bismuth is given a final oxidation with air and caustic soda the refined product has a purity of 99.999%. 

Bismuth tribromide may be prepared by dissolving Bi O in excess concentrated hydrobromic acid. The slurry formed is allowed to dry in air, then gendy heated in a stream of nitrogen to remove water, and finally distilled in a stream of dry nitrogen. Bismuth tribromide is soluble in aqueous solutions of KCl, HCl, KBr, and KI but is decomposed by water to form bismuth oxybromide [7787-57-7] BiOBr. It is soluble in acetone and ether, and practically insoluble in alcohol. It forms complexes with NH and dissolves in hydrobromic acid from which dihydrogen bismuth pentabromide tetrahydrate [66214-38-8] H2BiBr 4H2O, maybe crystallized at —lO C. 

Bismuth trioxide may be prepared by the following methods (/) the oxidation of bismuth metal by oxygen at temperatures between 750 and 800°C (2) the thermal decomposition of compounds such as the basic carbonate, the carbonate, or the nitrate (700—800°C) (J) precipitation of hydrated bismuth trioxide upon addition of an alkah metal hydroxide to a solution of a bismuth salt and removal of the water by ignition. The gelatinous precipitate initially formed becomes crystalline on standing it has been represented by the formula Bi(OH)2 and called bismuth hydroxide [10361 -43-0]. However, no definite compound has been isolated. 

Bismuth [7440-69-9] M 209.0, m 271-273°. Melted in an atmosphere of dry helium and filtered through dry Pyrex wool to remove any bismuth oxide present [Mayer, Yosim and Topol J Phys Chem 64 238 7960]. 

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