Establishment of water quality index (Na+, Ca2+) for purified water reused to zinc electrolysis proc

Available online at www.sciencedirect.com●GIENCEdoinet.Transactions ofNonferrous MetalsScienceSociety of ChinaELSEVIER PressTrans. Nonferrous Met. Soc. China 19(2009) 484-488w ww .nmsc.cnEstablishment of water quality index (Na, Ca2*) forpurified water reused to zinc electrolysis processCHAI Li-yuan(柴立元), XIAO Hai-juan(肖海娟), WANG Yun-yan(王云燕),PEI Fei(裴斐), SHU Yu-de(舒余德), ZHANG Jin-long(张金龙)School of Mtallurgical Science and Engineering. Central South University, Changsha 410083, ChinaReceived 12 December 2007; accepted 13 June 2008Abstract: The efects of Na* and Ca+ in the purified water on the conductivity of zinc electrolyte and the current eficiency of zincelectrolysis were studied by the alternating current bridge method and the simulated electrolysis experiments, and the water qualityindex of reused water was established. The results show that the conductivity of the solution and the current efficiency decrease asthese two kinds of positive ions are added in the electrolyte. The efect of Ca2* is much more remarkable than that of Na*. ρ(Na')≤8 g/L and p(Ca*)≤20 mg/L are the quality indexes in the zinc electrolysis process and the concentrations ofNa' and Ca2 in thepurified water reused to the process should be less than the limited values, i.e. the water quality index of the purified water should becontolled by its reused amount.Key words: water quality index; purified water; zinc electrolysis process; alternating current bridge methodwhich will inevitably enter into the electrolyte during the1 Introductionproduction process. The existence of these impurity ionsnot only affects the physical and chemical properties ofThe nonferrous production is one of the high waterinc electrolyte[8-10]， such as density, viscosity,consumption industries in China. Recently, although theconductivity, surface tension, specific heat and thewater consumption per ton products has declined, thediffusion ceofficient of metal ions, but also influenceswater recycling ratio is still lower than 85%, which lagsthe purity and surface conditions of the electrolyticbehind 15- -20 years compared with developedsediment and the power consumption [11-13]. .countries[1-2]. Addionally, the discharge of heavyConsequently, it is of great significance to study themetal-containing wastewater leads to lavishness ofeffect of impurity ions on zinc electrolysis and establishresource and the toxic heavy metals also seriously afectthe water quality index of the purified water.the safety of drinking water(3- -4]. Therefore, it is veryimportant to treat the wastewater and improve the2 Establishment of quality index for purifiedreeycling ratio of the purified water. At present, there arewater reused to zinc electrolysis processmany treatment methods for the heavy metal- containingwastewater[5-7], however, whether the treated water canTable 1 lists the permissible concentrations of mainbe reused to the smelting process is still a highlymetallic impurity ions[11] in zinc electrolyte as well asconcerned problem today. For example, in the lead-zincthat in the purified water treated by biologics.smelting system, zinc electrolyte is constituted 0From Table 1 it can be seen that the concentrationsZnSO,-H2SO4 solution with certain concentrations, buofCu", Pb3*, Zn', Cd- and As+ in the purifed waterthere are many other metallic and non-metlic impurityare lower than the permissible values, respectively, butions, such as Cu, Pb, Fe, Ni, Na, Mg, K, Ca, CI, and F,the permissible values ofNa~ and Ca2- in zinc electrolyteFoundation item: Proce(2007BAC25B01) supported by the National Key Project of S中国煤化工s Funded by Minityor .Science and Tchnology of China during the lIh Five-Year Plan; Priic and Technical Project.Minisry of Education of China: Prie(08113020)1) supported by:YHC N M H Gaion of Chins: Pojet(50830301) supported by the Key Projeet of the National Natural Science Foundation of ChinaCorresponding author; WANG Yun-yan: Tel: +86-731-8830875; Fax: +86-731-8710171; E-mail: wyy@.mail.csu edu.cnDOl: 10.1016/S1003-6326(08)60300-3CHAI Li-yuan, et al/Trans. Nonferous Met. Soc. China 19(2009) 484-488485Table 1 Concentrations of main cations in zinc electrolyte and that in purified water treated by biologicsCu2+Cd2As3-Permissible concentraion in zinc electrolyte(mg:L) <0.2≤1.5≤0.24Average concentration in purified water treated by0.0720.150.0330.140.62~1.0058.70biologis/(mgLhave not been identified for lacking of detailedinvestigations. Additionally, according to Ref.[12], thepotential of such cations as Na*, Mg*, K' and Ca2* aremore negative than that of Zn2- and they will not deposit42on the cathode, but can absorb on the cathodic activepoint selectively as surfactants， resulting in thecrystallization overpotential to some extent and afectingthe surface properties. The hydrogen evolution andimpurity precipitation will also be accelerated, in the end,the current efficiency will decrease. Moreover, when thequantity of these impurities is too high, the viscosity andFig.1 Schematic diagram of experimental equipments foresistance of the electrolyte will increase, the diffusion ofZn2 will be obstructed, and then the power consumptionaltemating current bridge method: 1- -ZX3N4 AC resistancewill increase. Too high concentrations of Ca? and Mg2box; 2- ZX32 AC esistance box; 3- -SBQ6-ocilloscope;will easily from crystal and precipitate, which makes the4- -FMBa teaching denary capacitance boxes; 5- -Audio signalpipelines block and operation trouble[8 -9]. Therefore,generator, 6- Selfmade conductance poolNa' and Ca were chosen in this study as the mainresearch objectives and the establishment of the indexesofNa" and Ca'+ in purifed water were carried out.Lo古3 ExperimentalThe effects ofNa* and Ca2+ in the purified water onthe conductivity of zinc electrolyte were studied by thealtermating current bridge method[14] and the curentefficiency was determined by the simulated electrolysisexperiments to establish the water quality indexes of Na*and Ca2* in purified water finally.Fig.2 Schematic diagram of experimental equipments forsimulated electrolysis experiment: 1- Power supply; 2- -DC3.1 Experiments of conductivityThe zinc electrolyte was sampled from Zhuzhouammeter; 3- -Electrolytic cell; 4- -Selfmade high groove; 5-Smelter Group, which consisted of 4.071 g/L Na* andSelfmade low groove; 6- Lead-silver anode plate (4 cmX4305 mg/L Ca2*. The chemicals were Na2SO(AR),cm); 7- -Aluminum cathode plate (5 cmX4 cm)CaSO4H2O(AR)，ZnSO:7H2O(AR)andtheconcentrated H2SO<(98%, AR). The temperature of water500 A/m', lasted for 8 h; the volume of bone glue wasbath was controlled at 40 C. The schematic diagram of300 mg/L; the electrolyte flow was controlled at 60experimental equipments for the altemating currentdrop/min, and the distance between the anode andbridge method is shown in Fig.1.cathode was 58- -62 mm.3.2 Simulated electrolysis experiments4 Results and discussionThe concentrations of Na^ and Ca2* in zincelectrolyte used to the electrolysis experiments were4.1 Effect of Na* in purified water on conductivity of4.022 g/L and 323 mg/L, respectively. The chemicalszinc electrolytewere Na2SO4(AR) and CaSO4H2O(AR). The schematic中国煤化工1 soluion can bediagram of experimental equipments in the simulatedexpreselectrolysis tests is shown in Fig.2.:TYHCNMH G,41X10*2 (1)The temperature of water bath was maintainedwithin the range of 38 -40 C; the current density waswhere Kka is the standard conductivity of KCI solution486CHAI Li-yuan, et a/Trans. Nonferrous Met. Soc. China 19(2009) 484- -488in S/cm; t is the temperature in C.electrolysis declines simultaneously. Therefore, the Na^The effects of Na^ on the conductivity of zincions in purified water influence the current efficiency,electrolyte are listed in Table 2. It can be concluded fromtoo, when the Nat ions enter into the zinc electrolysis.Table 2 that when the concentrations of Na* and Zn2* arefixed, the conductivity of the solution increases with the.2 Effect of Na* in purified water on currentaddition of H2SO4. But the conductivity declines whenefficiency of zinc electrolysisthe concentration of Zn2* increases under the sameThe efct of Na" in purified water on the curentconcentrations of Na* and H2SO4. While theconcentration of Na* increases and the conductivityefficiency of zinc electrolysis process is listed in Table 3.decreases. The Na* ions in purified water influence theIt can be seen from Table 3 that the current efficiencydecreases gradually from 88.28% to 87.41% as theconductivity of zinc electrolyte.The effect of Na* in purified water on theconcentration of Na* increases from 0 to 8 g/L. While theconductivity of zinc electrolyte is shown in Fig.3, whichconcentration of Na* is higher than 8 g/L, and the currentillustrates that when the concentration of Nat in purifiedefficiency is lower than 87%. This current eficiency canwater increases, the conductivity of zinc electrolytesatisfy the main technical and economical index whendecreases gradually. The relationship between thethe concentration of Na* is less than 8 g/L (87% is aconcentration of Na* and the conductivity of zinclimited minimum valuc for the technical and economicalelectrolyte can be linearly expressed as follows:index in Zhuzhou Smelter Group).Kywa. -327.896 25-1.788 45p(Na")(24.3 Effect of Ca2+ in purified water on conductivity ofIt is known that the diffusion of metallic ions iszinc electrolyteobstructed due to the increase in the resistance ofThe effect of Ca2* on the conductivity of zincelectrolyte, and the quantity of the metal depositing onelectrolyte is listed in Table 4. The conductivity of zincthe cathode decreases, finally, the current efficiency forelectrolyte increases with the increase of the H2SO4concentration, but decreases with the decline of the. xr-327.896 25-1.788 45P(Na')Ca2' concentration. This also means that the Ca”ions inR=0.99647330purified water affeet the conductivity of zine electrolyte.Fig.4 shows the effect of Ca”in purified water onthe conductivity of zine electrolyte, which shows that therelationship between the conductivity of zinc ectrolyte5 31and the concentration of Ca' can be linearly fitted as theE 300following equation:Kca2- =327.437 14-0.315 94 p(Ca2)(3)90 tEq.(3) obviously ilustrates that the conductivity of280 50520一25zinc ectrlyte decreases with the increase of the Ca2+p(Na*)(gL1)concentration. Comparing Eq.(2) with Eq.(3), it can beFig.3 Effect of Na^ in purfed water on conductivity of zincconcluded that the effect of Ca2+ on the conductivity iselectrolytemuch more remarkable than that of Na'.Table 2 Conductivities of zinc cerolyte at difret concntrations of Na (mS/cm)p(Na)Vp(H2SO,)=I50 g/Lp(H2SO)=180 gLp(H2SO)=210 g/L(g:L)p(Zn^ )=50gL_ p(Zn")=60 g/Lp(Zn")=50g/L p(Zn?*)=60 gLp(Zn*)=50g/L p(Zn2 )=60 g/L381.49 .374.01395.28389.82414.19394.07345.16319.11365.09363.20402.68385.31325.24316.12391.80368.922313.20358.51中国煤化工361.2316307.50355.31MHCNMHG353.8620302.01352.16338. I 5371.71346.78297.17337.24326.88353.57339.98CHAI Li-yuan, et al/Trans. Nonferrous Met. Soc. China 19(2009) 484-488487Table 3 Effect of Nat on current cfficiency of zinc eletrolysis processp(Na")(g:L)0812Current fficiency/%88.2888.2587.4186.3586.30Table 4 Conductivity of zinc electrolyle at diferent concentrations of Ca?* (mS/cm)p(Ca2")Vp(H2SO4)=150 gLp(H2SO4)=I80g/Lp(H2SO4)=210 g/L(g:L-)p(Zn2* )=50gL_ p(Zn*)=60g/Lp(Zn')=50g/L p(Zn2")=60gL__ p(Zn2")=50gL_ p(Zn2*)=60 gL381.49374.01395.28389.82414.19394.078(371.71369.15383.30378.99402.68393.21160367.00364.42372.03368.75391.80380.52240362.41359.81366.64363.84374.48320357.94355.31361.40359.05368.634.5 Establishment of water quality index for purified375+ --- r-372.437 14-0.315940(Ca*)water reused to zinc electrolysisR=0.993 69From the effects of Na° and Ca2* on condutivity ofzinc electrolyte and current efficiency of zinc electrolysis,365|it can be concluded that the metal impurities in thepurified water, even in low concentrations, make the白355-conductivity and the cutrent efficiency decrease. Whenthe concentration of Na* in zinc electrolyte is lower than34s-8 g/L or that of Ca2* is below 20 mg/L, the curentefficiency is higher than 87%， which satisfies thetechnical and economical index of Zhuzhou Smelter335- 20406080100120Group. Therefore, the concentrations ofNa* and Ca2* inp(Ca2*)(mg-L-)the purified water reused to the electrolysis processFig.4 Effet of Ca2+ in purified water on conductivity of zincshould be less than the limited values mentioned above.electrolyteThat is to say, the water quality index of the purifiedwater should be controlled by the reused amount.4.4 Effect of Ca2* in purifed water on currentefficiency of zine electrolysis5 ConclusionsThe effect of Ca2* on the current efficiency of zincelectrolysis is listed in Table 5, which indicates that the1) The conductivity of zinc electrolyte and thecurrent eficiency decreases gradually with the increasecurrent efficiency of zinc electrolysis process decreaseof the Ca"' concentration in zinc electrolysis. The currentdue to the presence of various metal impurities inefficiency decreases from 89.88% to 87.91% with thepurified water, even in low concentrations. The effect ofincrease of the Ca^*concentration from 0 to 20 mg/L. TheCa2* on the conductivity of zine electrolyte is much morecurrent efficiency is 86.86%, which is lower than theremarkable than that of Na*.limited minimum of 87% in the Zn electrolysis process,2) p(Ca2)≤8 g/L and p(Ca?*)≤20 mg/L are thewhen the Ca2+ concentration is 40 mgL. Therefore, thequality indexes for the zinc electrolysis. Thecurrent efficiency at the Ca2 concentration less than 20concentrations of Na* and Ca* in the purified watermg/L can qualify the main technical and economicalreused to the process should be less than the limitedindex in Zhuzhou Smelter Group.values, i.e. the water quality index of the purfied watershould be controlled by the reused amount.Table 5 Efect of Ca2*+ on current eficiency of zinc electrolysisprocess .Refe中国煤化工p(Ca*)/204080(mgL)[1]RMHC N M H Gu Sheoqgi Waer reueCurrent89.88 88.46 87.91 86.86 82.72and economical cost-benefit analysis 小Guangdong Chemicalfficiency/%Industy, 2005(5); 36 -38. (in Chinese)488CHAI Li-yuan, et alVTrans. Nonferrous Met. Soc. China 19(2009) 484-488[2] JIANG Pei-bua, LIU Zhen-hai, CHEN Ji-hua. Reusing technique ofzinc concentrate of Zhuzhou Smelter [01. Science and Technology ofindustry wastewater [] Shanghai Environment Science, 1995, 14(2):Zhuzhou Smelte, 1995, 23: 35- -38. (in Chinese)33-37. (in Chinese)9] FU Yun-kang. 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