DESULPHURIZATION OF LEAD CAKE BY SODIUM CARBONATE AND SODIUM HYDROXIDE

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1 Journal of the D. Atanasova, University of N. Chemical Lyakov, V. Technology Vassilev, G. and Angelov, Metallurgy, G. Haralampiev 43, 2, 2008, DESULPHURIZATION OF LEAD CAKE BY SODIUM CARBONATE AND SODIUM HYDROXIDE D. Atanasova 1, N. Lyakov, V. Vassilev 1, G. Angelov 2, G. Haralampiev 1 University of Chemical Technology and Metallurgy 8 Kl. Ohridski, 1756 Sofia, Bulgaria venciv@uctm.edu 2 KCM 2000 S. A. Plovdiv, Assenovgradsko shosse, Bulgaria Received 11 March 2008 Accepted 12 May 2008 ABSTRACT Investigations are carried out related to desulphurization of Pb-cake by Na 2 and NaOH and the optimum conditions for process running are defined in two installations: laboratory agitator and drum type rotary reactor. The degree of desulphurization by Na 2 reaches %, while by NaOH %, values which satisfy the technological and ecological requirements for processing of Pb-cake. The content of impurities in the solutions is also examined with a view to produce sufficiently pure crystalline Na 2. The obtained solutions desulphurized by Na 2 are purer concerning the impurities Pb, Zn, Cd and Fe compared to those desulphurized by NaOH. The produced crystalline Na 2 is suitable to be used in other productions. Keywords: lead sulphate, desulphurization of Pb-cake, processing of Pb waste. INTRODUCTION Some secondary lead containing products contain considerable quantity of sulphur in the form of and the following ones refer to them: damped battery paste containing: Pb ( %) in the form of Pb (50-60 %), PbO 2 (30-35 %), PbO (10-15 %) and Sb (0,2-0,7 %) [1, 2]; Pb-cake from the hydrometallurgical cycle of Zn-production which contains: Pb ( %), Zn (8-10 %) and S (9-11 %), in the form of Pb, Zn and, respectively. As a rule Pb is desulfurized before melting [3-6]. The most often used reagents for that purpose are Na 2 and NaOH [1-8]. The interaction between Pb with Na 2 and NaOH goes according the reactions: Pb (solid) + Na 2 (liquid) Pb (solid) + Na 2 (liquid) (1) Pb (solid) + NaOH (liquid) Pb(OH) 2 (solid) + Na 2 (liquid) (2) The produced saturated solution is subjected to evaporation, centrifuging and precipitation of the crystalline Na 2 [7-9]. The following values are obtained for Gibbs energy and the equilibrium constant of reaction (1) at T=298 and T=343 Ê: o G = - 41,91 kj mol 298-1, K c = 2, (T=298 K) and K c = 2, (T=343 K The equilibrium constant values are high and it can be considered that in this temperature range the reaction continues to Pb exhaustion (if there are 267

2 Journal of the University of Chemical Technology and Metallurgy, 43, 2, 2008 present the necessary quantity of Na 2 and possibility for reagents contacts Another ground is the difference between the solubility products of Pb and Pb. At T=298 K they are: L(Pb ) = 1, and L(Pb ) = 7, These values provide the basis for the conclusion that if the reaction goes in solution then Pb shall pass in the solid phase, i.å. the 2 ions will replace 2 SO4 ions because of the lower Pb solubility [10]. The above said refers also to equation (2), where for desulphurization of Pb-containing products NaOH is used (at T=298 Ê, L(Pb(OH) 2 )=1, Pb- cake contains, besides Pb (30-45 %) and S (9-11 %), also Zn (8-10 %), Cu ( 0,5 %), Cd, Fe, As, Sb, Bi, etc. The solubility products of some of these metals are as follows [10]: When Na 2 is used: L(Pb )=7, ; L(Zn )=1, ; L(Cd )=5, ; L(Fe )= 3, ; L(Cu )=2, ; When NaOH is used: L(Pb(OH) 2 )=1, ; L(Zn(OH) 2 )=7, ; L(Cd(OH) 2 )=2, ; L(Cu(OH) 2 )=2, ; L(Fe(OH) 2 )=1, ; L(Fe(OH) 3 )=3, ; L(Bi(OH) 3 )=3, The desulphurization of lead containing products gives pure Na 2 solutions, which contain insignificant quantities of heavy metals and iron [11]. In connection to the above said and the fact that no data are found in the publications in relation to Pb- cake desulphurization from different authors, the aim of the present study is to: (i) determine the effects of the factors initial concentration of (Na 2 or NaOH) in the solution, liquid to solid phase ratio, process duration and temperature on the process characteristics - degree of Pbcake desulphurization and degree of reagent utilization; (ii) specify the optimum conditions for carrying out Pb-cake desulphurization by Na 2 or NaOH in dependence of the type of the reactors used; (iii) examine the content of impurities in the solutions with the purpose to produce sufficiently pure crystalline Na 2. EXPERIMENTAL Before desulphurization the Pb-cake is dried to constant weight. The experiments are performed in standard reactor with laboratory agitator (ER 10) or in drum type rotary reactor VEB Elmo Hartha (DDR) and for every experiment 200 g of Pb-cake is used. There is studied Pb-cake from the hydrometallurgical cycle of Zn-production of the following composition [%]: Pb - 38,35; sulphate sulphur ( SSO ) - 9,9; sulphide sulphur 4 (S S ) - 0,5; Zn - 8,34. The paste composition is determined by chemical analysis and the content of sulphate sulphur S 2 SO 4 by gravimetric analysis. The lead cake is treated by Na 2 or NaOH solution of preset initial concentration and liquid to solid phase ratio at defined temperature and duration of the desulphurization process. After completion of the chemical treatment the pulp is filtered and the solid phase washed by warm water on the filter, dried to constant weight and analyzed 2-2- for content. The content in the solid phase is determined by gravimetric method. The liquid phases 2 are analyzed for SO - 4, Na 2 or NaOH contents. The content of free Na 2 and NaOH in the liquid phases (residual concentration) is determined by titration with 0,1 Ì HCl and indicator methyl orange. The desulphurization degree á [%] is calculated by the formula: where: Ds and Ds are initial and residual SO 2- (initial) (final) 4 concentration. The reagent utilization degree â [%] is calculated by the formula: where: C (initial) and C (final) - initial and residual reagent concentration, g L -1. RESULTS AND DISCUSSION Investigation of the desulphurization process in the lead cake treatment by Na 2 Effect of Na 2 concentration The effect of the Na 2 initial concentration (C initial ) in the solution during the Pb-cake treatment by 268

3 D. Atanasova, N. Lyakov, V. Vassilev, G. Angelov, G. Haralampiev Fig. 1. Effect of Na 2 initial concentration on the degree of Pb-cake desulphurization (α) and degree of Na 2 utilization (β) for m=2,5; t=40; τ=90 min and C initial =140 g L -1 Na 2 (6,7 % excess), 155 g L -1 (18,2 % excess), 170 g L -1 (29,6 % excess) and 185 g L -1 (41,0 % excess): - laboratory agitator (α 1 ); - rotary reactor (α 2 Fig. 2. Effect of the process duration (τ) of Pb- cake desulphurization by Na 2 on the degree of Pb-cake desulphurization (α) and degree of Na 2 utilization (β) for m=2,5, C initial =155 g L -1 Na 2 and τ= 5, 15, 30 and 60 min : - laboratory agitator (α 1 ); -rotary reactor (α 2 laboratory agitator and drum type rotary reactor (Fig.1) is studied for liquid to solid phase ratio = 2,5:1 (m=2,5), temperature t=40 o C, process duration ô= 90 min and C initial =140 g L -1 (6,7 % excess), 155 g L -1 (18,2 % excess), 170 g L -1 (29,6 % excess) and 185 g L -1 (41,0 % excess The results obtained clearly display the above described kinetic dependences, i.e. with increase of the concentration the desulphurization degree grows but the degree of reagent utilization decreases. For optimum concentration C initial =155 g L -1 Na 2 could be accepted. It should be noted that the results of the cake treatment in a rotary reactor are higher (á 2 = 94,8 %) compared to those of the treatment by standard reactor with laboratory agitator (á 1 = 92,4 % The inconvenience when using rotary reactor is the maintenance of a constant temperature as the heating of the pulp is difficult. Our previous investigations [11] reveal that when the ratio m is increased from 2,0 to 3,5 the degree of Na 2 utilization (â) decreases from 99,04 to 13,92 %, while the degree of Pb-cake desulphurization (á) increases from 80,41 to 95,93 %. This rule is more strongly expressed in the range m=2,0-2,5. At greater values of m the desulphurization degree grows slightly but on the account of this the degree of Na 2 utilization decreases and the total volume of the system increases. As a result of this the reagent consumption is increased which in its turn results in the formation of insoluble double salts Na 2 Pb 2 ( ) 2 OH and NaH [5]. The value of the relation m=2,5 is recommended as optimum. Effect of the duration of the desulphurization process by Na 2 The effect of the duration of the cake desulphurization by Na 2 (ô= 5, 15, 30 and 60 min) is studied in standard reactor with laboratory agitator and in drum type rotary reactor (Fig.2) under the following conditions: C initial =155 g L -1 Na 2 (18,2 % excess), m= 2,5, t= 40 o C. In case of stirring by laboratory agitator optimum desulphurization degree is obtained for ô= 30 (min - 92,4 % Optimum values for the desulphurization degree in rotary reactor are obtained in the range min (92,8 % - 93,6 % This fact suggests that the process may be carried out in continuous mode of operation. The concentrations of impurities in the solutions are as follows [mg L -1 ]: 30 min: Pb < 0,1; Zn < 0,5; Cd - 0,04-0,28; Fe < 1,00-1,38; 45 min: Pb < 0,1; Zn < 4,0-16,0; Cd - 1,3-10,0; Fe - 1,2-1,7. By experiments performed in advance it is found out that the temperature affects slightly the process char- 269

4 Journal of the University of Chemical Technology and Metallurgy, 43, 2, 2008 a) Fig. 3. Effect of the NaOH initial concentration on the degree of Pb-cake desulphurization (α) and the degree of NaOH utilization (β) for m=2,5; t= 40; τ=30 min and C initial =104 g L -1 NaOH (5 % excess), 118 g L -1 (19 % excess), 135 g L -1 (36 % excess) and 150 g L -1 (51 % excess): - laboratory agitator (α 1 ); - rotary reactor (α 2 b) acteristics. If there is such influence it is related to the properties of Na 2 and Na 2 [10,12,13]. The best results are obtained in the temperature range of o C, where the values of the desulphurization degree are approximately equal. Investigation of the desulphurization process in the lead cake treatment by NaOÍ Effect of NaOH concentration The studies are performed under the following conditions: C initial =104 g L -1 NaOH (5% excess), 118 g L -1 (19 % excess), 135 g L -1 (36 % excess) è 150 g L -1 (51 % excess); m=2,5, ô=30 min, t= 40 o C (Fig.3 With concentration of 104 g L -1 (5 % excess) very good results are obtained concerning (á), but NaOH utilization (â) is high which imposes increase of C initial NaOH for practical assurance of the process run. The optimum NaOH concentration in relation to the desulphurization degree in reactor with laboratory agitator and in rotary reactor is 118 g L -1 (19 % excess The Pb concentration in the solution for 118 g L -1 NaOH is 1724,6 mg L -1 and for 104 g L -1 it is 2,18 mg L -1. Fig. 4. Effect of the liquid to solid phase ratio (m) on the Pbcake desulphurization degree (α) and the degree of NaOH utilization (β) for C initial =104 g L-1 NaOH, respectively - m=2 (16 % insufficiency), m=2,5 (5 % excess), m=3 (26 % excess) and m=3,5 (47 % excess); t=40 o C and τ=30 min: a) laboratory agitator (α 1 ); b) rotary reactor (α 2 Effect of the liquid to solid phase ratio The investigations are performed in both types of the reactors under the following conditions: C initial =104 g L -1 NaOH, at liquid to solid phase ratio - m=2 (16 % insufficiency), m=2,5 (5 % excess), m=3 (26 % excess), m=3,5 (47 % excess); at t=40 o C and ô=30 min, respectively (Fig.4 The optimum value of the liquid to solid phase ratio is m=2,5 (á=92,8 % - in reactor with laboratory agitator and á=93,2 % - in rotary reactor 270

5 D. Atanasova, N. Lyakov, V. Vassilev, G. Angelov, G. Haralampiev a) b) desulphurization degree has high values 95,7-96,3 % and 96,5-96,8 %, respectively. In case of desulphurization in reactor with laboratory agitator and 118 g L -1 NaOH, the impurities concentrations in the solution mg L -1 are: Pb-1724,6; Zn- 154,4, Cd<0,01 and Fe-1,07. In case of desulphurization in rotary reactor the impurities concentrations in the solution mg L -1 are: For concentration 104 g L -1 NaOH: Pb - 49,28; Zn - 20,85; Cd < 0,01; Fe - 0,71; For concentration 118 g L -1 NaOH: Pb - 104,35; Zn - 57,12; Cd < 0,01; Fe - 0,54. When using NaOH, it should be operated with excess of not more than %, because it is found out that when the reagent excess is greater the Pb concentration in the solution grows. CONCLUSIONS Fig. 5. Effect of the process duration (τ) of Pb-cake desulphurization by NaOH on the Pb-cake desulphurization degree (α) and degree of NaOH utilization (β) at m=2,5, t=40o C. Samples are examined at τ= 5, 15, 30, 45 and 60 min: a) laboratory agitator C initial =104 g L - 1 (α 1 ) and 118 g L -1 NaOH, (α 2 ); b) rotary reactor C initial =104 g L -1 (α 3, β 3 ) and 118 g L -1 NaOH, (α 4, β 4 Effect of the duration of the desulphurization process by NaOH The investigations are performed in both types of the reactors under the following conditions: C initial =104 g L -1 and 118 g L -1 NaOH, m=2,5, t=40 o C and process duration ô=5, 15, 30, 45 and 60 min (Fig.5 In case of stirring by laboratory agitator and concentration of 104 g L -1 NaOH the optimum process duration is ô=30 min (á=92,8 %), and for concentration 118 g L -1 NaOH - ô=15 min (á=96,4 % The optimum process duration in rotary reactor for both concentrations is 5-15 min, and the On the basis of the experiments performed the following conclusions could be made: it is found out that the process goes more favorably in rotary reactor (for min), which provides precondition for continuous mode of operation; the degree of desulphurization by Na 2 reaches % and by NaOH %, values that meet the technological and environmental requirements for Pb-cake processing; the solutions obtained after desulphurization by Na 2 are more pure regarding the impurities Pb, Zn, Cd and Fe, compared to those by NaOH. In both cases the solutions could be used for production of pure crystalline Na 2. REFERENCES 1. N.K Lyakov, Vtorichna metalurgia na cvetnite metali i splavi, Martilen, Sofia, 1996, pp , (in Bulgarian 2. T.T. Chen, J.E. Dutrizac, The mineralogical characterization battery paste of lead-acid, Hydrometallurgy, 40, 1996, A.G. Morachevskii, Jurnal prikladnoi chimii, 70, 1, 1997, 3-15, (in Russian 4. European Patent Off. No EP A1, 1/ Y. Gong, J.E. Dutrizac, T.T. Chen, The conversion of lead sulphate to lead carbonate in sodium carbonate media, Hydrometallurgy, 28, 1992,

6 Journal of the University of Chemical Technology and Metallurgy, 43, 2, Y. Gong, J.E. Dutrizac, T.T. Chen, The reaction of anglesite (PbSO) crystals with sodium carbonate solutions,. Hydrometallurgy, 31, 1992, V.P. Yanakieva, V Petr, Solution regeneration after desulphurization of lead-acid battery pastes, National Science Fund, Ministry of Science and Education, Bulgaria under Contact No.TH-908/ A.G. Morachevskii, et al., Jurnal prikladnoi chimii, 66, 9, 1993, , (in Russian 9. A.G. Morachevskii, et al., Jurnal prikladnoi chimii, 70, 1, 1997, , (in Russian 10. Narachnik na chimika, Gostchimisdat, Moscow, III, 1962, (in Russian 11. D.A. Atanasova, N.K. Lyakov, V.S. Vassilev, G.D. Angelov, G.A. Haralampiev, Desulphurization of Pbcake by sodium carbonate, X National conference on metallurgy, Varna, Bulgaria, G. Remi, Cours neorganicheskoi chimii, Inostranaja literatura, Moscow, 1963, pp , (in Russian 13. Y. Lurie, Narachnik po analiticheskoi chimii. Gostchimisdat, Moscow, 1962, pp , (in Russian 272

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