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Russian Journal of Applied Chemistry, Vol. 76, No. 2, 2003, pp. 191�194. Translated from Zhurnal Prikladnoi Khimii, Vol. 76, No. 2, 2003,pp. 201�204.Original Russian Text Copyright � 2003 by Zarubitskii.

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AND INDUSTRIAL INORGANIC CHEMISTRY

Refining of Lead To Give Bismuth-Enriched Drosses

O. G. Zarubitskii

Vernadskii Institute of General and Inorganic Chemistry, National Academy of Sciences of Ukraine, Kiev, Ukraine

Received June 3, 2002

Abstract�The results obtained in testing an improved technique for reagent refining of crude lead, whichyields bismuth-enriched drosses in the form of a powder, are presented. The advantages of the method putinto industrial practice are described.

The pyrometallurgical method proposed by Krolland Betterton [1, 2] is mainly used at lead works toremove bismuth admixtures from refined crude lead(lead bullion). This, the most complicated procedureis carried out after silver removal. It involves dissolu-tion of blocks of circulating bismuth drosses at 400�Cand subsequent introduction of magnesium and cal-cium into melted crude lead at 360�370�C. Thesemetals react with bismuth to form intermetallic com-pounds: Ca3Bi2, Mg3Bi2, Mg2CaBi2, which are poorlysoluble in lead and are deposited on the bath surface.They are extracted as the so-called �enriched� bismuthdrosses with bismuth content of 3�5%. Then the bathis cooled to 340�350�C and �lean� circulating drossescontaining 0.5�2% Bi are extracted. Repeated circu-lation of drosses can increase the bismuth content upto 5�11%, to give a lead�bismuth alloy containing8�15% Bi [1]. The increase in the bismuth contentfrom 5�11 to 8�15% results from removal of mag-nesium and calcium from the drosses with moltenNaOH. A 5�6% fraction of Bi passes into the result-ing alkaline melts.

The existing technology for Pb�Bi alloy productiondoes not ensure isolation of bismuth-enriched (to morethan 8�15%) products and is characterized by repeat-ed dross circulation, which leads to loss of the targetmetal and involves additional consumption of reagents(magnesium, calcium, and sodium hydroxide).

Various methods for dross enrichment with bis-muth have been proposed. For example, centrifugalrefining can enrich drosses to Bi content of 18�21%at its initial concentration of 5.7% [3]. Air bubbl-ing through liquid metal (cupellation) at 900�950�Ccan raise the bismuth content of a Pb�Bi alloy beingtreated from 14 to 40�60%, with simultaneous re-

moval of antimony, tin, arsenic, and tellurium [4].This method has been implemented in industry.

Of special interest are the publications [5�7] de-scribing the possibility of using organic compoundsin pyrometallurgical processing of lead�bismuth al-loys. For example, in purifying bismuth to removelead and copper by adding sulfur to a metallic melt,friable yields can be obtained if carbon or masut isintroduced into the metal being refined [5]. It is re-commended [6] that coal-tar pitch should be addedto a melt to enrich dross with bismuth, which yieldsan alloy containing 40% Bi and 60% Pb. Dry drossesare obtained by adding sawdust to crude lead (modi-fied Kroll and Betterton method [6, 7]). However,the authors of [5, 7] did not consider the mechanismby which the above-mentioned organic compoundsaffect the metallurgical processes under consideration.

EXPERIMENTAL

Our experiments and pilot tests made it possible todevelop and put into industrial practice an improvedmethod for lead refining, which provides lead alloyswith high bismuth content. The technological processconsists in the following. First, magnesium and cal-cium are introduced into crude lead at 375�400�C(rather than the commonly used circulating dross),and then the temperature of the refining bath is raisedto 475�490�C and circulating drosses are loaded.After settling for 25�35 min, dead oil (0.13�0.18 kgper 1 ton of lead to be refined) is introduced in por-tions with continuos agitation at 400�470�C. Afterliquation, solid powdered drosses (30�40% Bi, 50�60 Pb, 5�10 Zn, 0.015�0.02 Ag) are extracted, cooledto room temperature, and classified by sieving with0.10�0.15 mm mesh. Drosses remaining on the sieve

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are loaded in the lead bath for recirculation, whereassifted drosses (�0.10�0.15-mm class) are remelted toobtain a lead�bismuth alloy. After the removal of drydrosses, the temperature is lowered to 330�C, and cir-culating bismuth drosses are taken off. The refiningprocess is finished according to the standard scheme.

Powder drosses of the +0.10�0.15-mm class, con-taining (%) 12�17 Bi, 50�55 Pb, 25�40 Zn, and0.06�0.07 Ag, are purified to remove silver, whichdiminishes the expenditure of metallic zinc by ap-proximately 50%, and loss of silver and bismuth, byapproximately 30%. Thus, in contrast to the commontechnologies, this method does not involve a labor-consuming procedure of lead bullion purification toremove zinc introduced into lead to recover silver.

Powder drosses of the �0.10�0.15-mm class areremelted under a layer of slam (spent PbCl2�ZnCl2�KCl�NaCl electrolyte used at the plant in electro-chemical reactors for separating lead�bismuth alloysin ionic melts [8]). In this case, the following reac-tions occur

Mg3Bi2 + 3PbCl2 = 3MgCl2 + 3Pb + 2Bi,

Ca3Bi2 + 3PbCl2 = 3CaCl2 + 3Pb + 2Bi.

Such a procedure is expedient because lead, whichwas previously dumped as a salt in a chloride mixture,is returned into the production cycle. The resultinglead�bismuth alloy contains 55�65% Bi and the bal-ance lead.

In pilot tests, we used dead oils of BN-IV andBN-V brands, which have softening points of 70and 90�C, respectively. Special bitumens of A andG brands, and also pitches and other products ofstraight-run masut distillation, can be used for thispurpose.

The tests have shown that the initial introductionof calcium and magnesium into crude lead at 375�400�C and subsequent addition of circulating drossesat higher temperature (475�490�C) diminish the lossof active metals (magnesium and calcium) and thusmake lower the process costs. According to this tech-nique, the purification to remove bismuth is carriedout at bath saturation with bismuth higher than 2%.The introduction of calcium and magnesium at 375�400�C ensures high dissolution rate at minimum lossof the active metals, which can be oxidized by at-mospheric oxygen or moisture. At 475�490�C thedissolution of circulating drosses proceeds faster,the solubility of all the components added is higher

at elevated temperature, and, simultaneously, lead isdeoxidated and cooled. Thus, complete use of the re-agents is achieved, and drosses very rich in bismuthare obtained in small amounts. This is also importantfor the subsequent introduction of a solid product ofheavy oil residuals, e.g., dead oil, to be carried outsuccessfully. Under these conditions, fine particles ofbismuth intermetallics do not aggregate, which makesit possible to obtain bismuth-rich drosses.

In carrying out this operation, dry powder drosseswith high bismuth content (33�37%) are formed asa phase on the lead surface. The results of X-ray dif-fraction, mass-spectrometric, microscopic, and chem-ical analyses suggest that the forming particles ofintermetallic compounds, and especially bismuth in-termetallide Mg3Bi2, are wetted with bituminous hy-drocarbons (asphaltenes etc.). This phenomenon elim-inates wetting of intermetallic particles by liquid lead,because the adsorption energy of hydrocarbons onthe surface of solid intermetallic particles is higherthan the energy of lead adsorption. Consequently,bismuth drosses are formed as powders containingno coarse conglomerates with adsorbed or adheringlead. This results in that the bismuth drosses areobtained in powdered (dry) form and contain muchlesser amounts of lead and greater amounts of bis-muth. According to a mass-spectrometric analysis ofthe gas content of samples, the fine powder contains2.5 and 3 times greater amount of, respectively, hy-drogen and carbon dioxide than the coarse fractiondoes. This may indicate that bitumen is mostly ad-sorbed on fine-fraction particles of bismuth drosses.

Our physicochemical study has shown that powderdrosses with particle size less than 0.10�0.15 mm arelargely composed of bismuth intermetallides (mainlyMg3Bi2 in amount of 30�35%, according to X-rayanalysis). Drosses with particles larger than 0.10�0.15 mm additionally contain intermetallic compoundsof zinc and lead. It was established that the fine frac-tion of dry powder drosses with particle size less than0.15 mm contains (%) 43�50 Bi, 35�45 Pb, 0.1�0.5Zn, 0.002�0.003 Ag, and Ca and Mg the rest. Drossesof the +0.10�0.15-mm class are complex intermetal-lic compounds of zinc, bismuth, lead, silver, calcium,and magnesium.

The expenditure for production of bismuth froman alloy containing 55�65% Bi is considerably lessthan that for production of the same amount of Bifrom an alloy with bismuth content of 15% (this is themaximum Bi content ensured by the currently existingprocedures). In this case, the expenditure of electricpower and electrolyte, number of electrolyzers, floor

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REFINING OF LEAD TO GIVE BISMUTH-ENRICHED DROSSES 193

area, and maintenance staff are diminished. This isconfirmed by the following simple calculation of, e.g.,the electric power expenditure.

Let us assume that a plant accomplishes electro-chemical separation of a lead�bismuth alloy contain-ing, on average, 12% Bi and 88% Pb by the formerlyused technology, i.e., it is necessary to transfer elec-trochemically 88 tons of Pb from anode to cathode inorder to produce 12 tons of Bi. If, however, a lead�bismuth alloy enriched with bismuth to 60% by thenew technology is subjected to electrolysis, it is nec-essary to dissolve at the anode and precipitate at thecathode only 8 tons of Pb in order to obtain the sameamount of Bi (12 tons). Consequently, the amountof lead to be electrolyzed in the second variant is 11times less than that in the case of the standard tech-nology. The expenditure of electric power, number ofelectrolyzers, etc. will be equally diminished.

The results obtained in industrial tests of the leadrefining technique under consideration, which yieldsa bismuth-enriched Pb�Bi alloy, are presented below.

The experiments were carried out in cauldronswith capacity of 125�130 tons of crude lead (leadbullion). The sequence of operations was describedabove. The size classification of powder drossesyielded 5.3�6.0 tons of a fine fraction containing43.7�50.6% Bi and 2.8�3.8 tons of grit with Bi con-tent of 12.6�17.2%. After remelting of the fine frac-tion under a fluxing agent (PbCl2�ZnCl2�KCl�NaCl),4.2�4.5 tons of alloy with Bi content of 55.1�59.9%were obtained. The process was implemented indus-trially and ensured a significant technological andeconomic effect. The environmental safety was im-proved, because we used a lesser number of electro-chemical reactors producing lead by high-temperatureelectrolysis of a lead chloride melt.

A somewhat different technique for lead�bismuthalloy enrichment was also developed and applied in in-dustry. At one of lead-manufacturing plants, a Bi�Pballoy is separated by electrolysis in hexafluorosili-con electrolyte, following the Betts procedure [1, 9].A semi-finished product with Bi content of 8�15%is used as the starting alloy. It is desirable to raisethe content of this metal, but to no more than 30%. Ifthe bismuth content in Pb�Bi anode plates is greater,the anodic process is significantly complicated. Aslead is dissolved, the solid alloy lattice is disruptedand narrow channels are formed, in which lead hexa-fluorosilicate is crystallized. At the same time, a largeamount of bismuth is dissolved and deposited at thecathode. Small pieces of the anode are crumbled and

entrained by slam. The surface layer of the piecesconsists of virtually pure bismuth, and the inner layer,of electrochemically unreacted lead. Industrial testshave shown that the Betts method for processing ofPb�Bi alloys containing more than 30% Bi is inex-pedient.

Taking into account the aforesaid, we proposed,developed, and implemented industrially a somewhatdifferent method for enrichment of bismuth drosses.The method, which is simpler and less expensive,consists in the following. To refine lead containing0.5 � 0.1% Bi, a mixture of bitumen (45�75 wt %)and sawdust (25�55 wt %) is added, and the processis performed at lower temperature (370�390�C). Asa result, powder drosses containing 23�27% Bi areobtained and delivered to hydroelectrolytic processingby the Betts method. This procedure involves separa-tion of drosses and thus eliminates the additional stageof bath heating to a temperature of 400�470�C, atwhich drosses are separated in the method describedabove. The addition of sawdust (which is less expen-sive than bitumen) prevents the possible inflamma-tion of bitumen particles floating-up to the surfaceof a lead bath. Sawdust contains moisture, whichevaporates to cool the upper layers of molten lead. Itshould be noted that sawdust is introduced into theliquid metal being refined not simultaneously withdead oil, but later, when obtaining �dry� powderdrosses. In addition, sawdust plays the same part inthe separation of intermetallic compounds of bismuthas dead oil.

This method makes lower the cost of bismuth re-moval from lead because of the use, in addition tobitumen, of such a less expensive material as sawdust.Labor conditions are improved owing to the elimina-tion of the possibility of bitumen inflammation. Theenergy expenditure, labor intensity, and reagent con-sumption are much lowered owing to a significantdecrease in the amount of lead�bismuth alloy to beprocessed by an electrolyzer in a hexafluorosiliconacid solution.

CONCLUSIONS

(1) A modified Kroll and Betterton method forlead purification to remove bismuth has been pro-posed and subjected to pilot tests. The improvementconsists in that dead oil or its mixture with sawdustis introduced into metal melt in a certain stage of re-fining to yield rich bismuth drosses in the form ofa powder. The separation of the product is also a spe-cific feature of the process.

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(2) The method for lead and bismuth separation,which was developed and implemented industrially,makes it possible to produce a bismuth�lead alloycontaining up to 55�65% Bi.

ACKNOWLEDGMENTS

The author is grateful to I.P. Brovin and V.A. Shle-enkov (DVGMK, Far-Eastern Mining and SweltingCombine) for help in the study.

REFERENCES

1. Smirnov, M.P., Rafinirovanie svintsa i pererabotkapoluproduktov (Lead Refining and Semiproduct Proces-sing), Moscow: Metallurgiya, 1977.

2. Betterton, J.O. and Lebedeff, Y., Trans. Metallurg.Soc. Am. Inst. Met. Eng., 1936, vol. 121, pp. 205�209.

3. Khodov, N.V., Suturin, S.N., Meshkov, E.I., et al.,Byull. Tsvet. Metallurgiya, 1982, no. 6. pp. 22�24.

4. USSR Inventor Certificate, no. 653 916.

5. Polyvyannyi, I.R., Ablanov, A.D., Batyrbekova, S.A.,and Sysoev, L.N., Metallurgiya vismuta (Metallurgyof Bismuth), Alma-Ata: Nauka, 1973.

6. Zelenova, E.I., Byull. Tsvet. Metallurgiya, 1971,no. 15, pp. 33�37.

7. German Patent 1 132 731.

8. Zarubitskii, O.G., Tsvetn. Met., 1990, no. 6, pp. 41�42.

9. Shivrin, G.N., Metallurgiya svintsa i tsinka (Metallurgyof Lead and Zinc), Moscow: Metallurgiya, 1982.