1-Thermodynamics of Zn_NH_3_NH_4Cl_H_2O system

[Article ID]1003-6326(2000)06-0830-04Thermodynamics of Zn(Ò)-NH3-NH4C-l H2O system¹YANG Sheng-hai(杨声海),TANG M o-tang(唐谟堂)(Department of Metallurgy Science and Engineering,Central South University,Changsha410083,P.R.China)[Abstract]According to the principles of simultaneous equilibrium and aqueous electronic char ge neutrality,t he complex equilibrium of zinc-ammonia and zinc-chloride in the system of Zn(Ò)-NH3-NH4C-l H2O ex isting zinc ox ide has been studied.T he solubility rule and high solubility area of zinc ox ide w er e discovered in the system w hose zinc so lubility is hig her than other two ammo nium systems,ammonium sulfate and ammo nium carbo nate.T he absolute av er ag e erro r be-tw een the ex perimental v alues and the theoretically calculated values of zinc equilibrium concentr at ion is10.77%in aque-ous solution of ammonia and ammonium chloride except in pur e ammonium chlor ide so lution.It shows that the cr itical sta-bility constants and the t hermody namic data ar e believ able.[Key words]zinc;ammonia;complex;thermodynamic[C LC number]T Q013.1[Document code]A1INTRODUC TIONAmmonia zinc metallurgy has the merits of easy purification and simplified process,so it has been re-searched extensively.We have studied up the thermo-dynamics in the systems of Zn(Ò)-NH3-(NH)2SO4-H2O[1]and Zn(Ò)-NH3-(NH)2CO3-H2O[2]and dis-covered the rules of zinc solubility,and the tw o new systems were used to produce the g raded zinc oxide[3] from zinc oxide materials.T he solubility of diamm ine zinc chloride in the concentrated solution of ammon-i um chloride in the present of ammonia at temperature range of30~60e was studied[4~5]and w as used to leach complex zinc sulphide mineral throug h oxidiz-ing[6].SH I[7]studied the thermodynamics in the low concentration system of Zn(Ò)-Cl--NH3-CO2-3-H2O,but the electronic charge neutrality of solution has not been taken into account.The authors studied the electrow inning zinc from the Zn(Ò)-NH3-NH4C-l H2O system[8]w hich could economize15~20 percent energy consumption than the traditional zinc hydrometallurgy.In this article the thermodynamic equilibrium of zinc solubility in ammonia and ammo-nium chloride aqueous solution at the present of zinc ox ide are analy zed,the zinc solubility rule is discov-ered.2THERMODY NAMIC DATA AND EQULIBRIL-IUM EQUATIONST here are tw enty species as Zn(NH3)2+,Zn (NH3)2+2,Zn(NH3)2+3,Zn(NH3)2+4,ZnCl+,Zn-Cl2,ZnCl-3,ZnCl2-4,Zn(OH)+,Zn(OH)2(aq), Zn(OH)-3,Zn(OH)2-4,NH3(aq),NH+4,H+,OH-,Zn2+,Cl-,HZnO-2and ZnO2-2,in the Zn(Ò)-NH3-NH4C-l H2O system.On base of the s-imultaneous equilibrium principle,every zinc complex-es are equilibrium w ith zinc oxide at the present of zinc oxide in the system.ZnO+i NH3+H2O Zn(NH3)2+i+2OH-(1)ZnO+(i-1)H2OZn(OH)2-ii+(i-2)H+(2)ZnO+H2O+i Cl-ZnCl2-ii+2OH-(3) According to the ex ponential com putation method[9],the concentration of these species can be ex pressed as[R]=exp(A+B#pH+C#ln[NH3(aq)]+D#ln[Cl-])(4) w here[R]is every species mole concentration;A is the constants calculated from equilibrium constants or thermodynamic data;B is the multiplication of ln10 and gained or lost proton number;C is the complex numbers of ammonia;D is the com plex numbers of chloride.The critical stability constants of zinc complexes, presented in T able1,w ere chosen from Ref.[10]. The thermodynamic data,presented in Table2,w ere chosen from Ref.[11]or calculated by the authors.Table1Critical stability constants ofzinc complex es at T=298KComplex es lg B i Complex es lg B iZn(N H3)2+ 2.38ZnCl+0.10Zn(N H3)2+2 4.88ZnCl20.06Zn(N H3)2+37.43ZnCl-30.10Zn(N H3)2+49.65ZnCl2-40.30Vol.10l.6Trans.Nonferrous Met.Soc.C hina Dec.2000¹[Received date]2000-01-07;[Accepted date]2000-06-23Table 2 Gibbs free energy of related species at T =298K(J/mol)Species $G áSpecies $G áSpecies $G á ZnO(s)-323131ZnOH +-330540*Zn 2+-147773*Zn(OH)2(aq)-537398*Zn(OH)-3-702912*Zn(OH)2-4-868031*HZnO -2-465780*ZnO 2-2-390729*NH 3(aq)-26712*NH +4-79800*Cl --131170*OH --157899*H 2O-238098*Zn(NH 3)2+-188065**Zn(NH 3)2+2-229042**Zn(NH 3)2+3-270303**Zn(NH 3)2+4-309682**ZnCl +-279513**ZnCl 2-410455**ZnCl -3-541853**ZnCl 2-4-674165***)From Ref.[11];**)CalculationU sing the data in T able 2,the values of A ,B ,C ,D in Eqn.(4)can be calculated and listed in T able 3.Table 3 Constants in ex ponential Eqn.(4)for calculating species concentrationSpecies A B C D Zn 2+25.324-4.60600Zn(N H 3)2+30.804-4.60610Zn(N H 3)2+236.562-4.60620Zn(N H 3)2+342.434-4.60630Zn(N H 3)2+447.547-4.60640ZnCl +25.554-4.60601ZnCl 225.461-4.60602ZnCl -325.554-4.60603ZnCl 2-426.014-4.60604Zn(OH)+2.990-2.30300Zn(OH)2(aq)-9.619000Zn(OH)-3-38.915 2.30300Zn(OH)2-4-68.371 4.60600HZnO -2-38.525 2.30300ZnO 2-2-68.819 4.60600NH 4+21.427-2.30310OH --32.370 2.30300Cl -0001H +-2.303According to the principle of substance quantity changeless,the sum concentration of zinc,am monia and chloride can be ex pressed as Eqns.(5),(6)and (7),respectively.[Zn2+]T =[Zn2+]+64i=1[Zn(NH 3)2+i ]+64j=1[ZnCl 2-j j ]+64k =1[ZnOH 2-k k ]+[H ZnO -2]+[ZnO 2-2](5)[NH 4OH ]T =[NH +4]+[NH 3(aq)]+64i=1i [Zn(NH 3)2+i ](6)[Cl -]T =[Cl -]+64j=1j [ZnCl 2-jj ](7)where [NH 4OH ]T is the total concentration of am -monia and ammonium in the system;[NH 3(aq)]isthe concentration of free ammonia in system ;i ,j and k are the com plex numbers of ammonia,chloride and hydrox ide respectively.According to the principle of electronic charge neutrality,the equation of electronic charge equilibr-ium can be expressed as2[Zn 2+]T +[NH 4+]+[H +]=[Cl -]T +[OH -](8)3 C ALCULATED RESULTS AND DISCUSSION The relation betw een the six variables of[Zn 2+]T ,[NH 3(aq)],[NH 4OH]T ,[Cl -]T ,[Cl -]and pH is confined by Eqns.(5)~(8).If tw o of them are g iven,other four variables may be obtained from the above mentioned simultaneous equations by the computation program compiled by ourselves.T he calculated results have been plotted into Figs.1~4.By thoroug h analyses of Fig.1and Fig.2,it show s that:1)the zinc equilibrium solubility in a pure ammonia aqueous solution (line 0in Fig.1)is low er than that in a pure ammonium chloride aqueousFig.1 Influence of ammonia concentration onequilibrium concentration of zinc at variousammonium chloride concentrations[N H 4Cl]/mol #L -1:0)0;1) 1.0;2) 2.0;3) 3.0;4) 4.0;5) 5.0;6) 6.0;7)7.0;8)8.0;9)9.0;10)10.0#831#Vol 110 l 16 Thermodynamics of Zn(Ò)-NH 3-NH 4C -l H 2O systemFig.2 Influence of ammonium chloride concentrationon equilibrium concentration of zinc at various ammonia concentrations[NH 3]/mol #L -1:0)0;1) 1.0;2)2.0;3) 3.0;4) 4.0;5) 5.0;6) 6.0;7)7.0;8)8.0;9)9.0;10)10.Fig.3 Influence of ammonium chloride concentration on equilibrium concentration of zinc at constant total concentration of ammonia and am monium chloride[NH 4OH]T /mol #L -1:7)7.0;8)8.0;9)9.0;10)10.0;11)11.0;12)12.0solution(line 0in Fig.2).On the other hand,the former increases slow ly contrary to the latter w ith the increasing of ammonia or ammonium chloride concen -tration;2)when the ratio of am monia concentration to ammonium chloride concentration is low er than 1,the zinc equilibrium solubility increases rapidly with the increasing of ammonia concentration.But w hen the ratio is larger than 1,the solubility increases slow ly and almost forms level lines(in Fig.1);3)w hen ammonia concentration is constant,the zinc e -quilibrium solubility increases rapidly at first w ith the increasing of ammonium chloride concentration,then it increases slow ly.So these curves starting endsFig.4 lg [R])[NH 3]relationship of variousions at [Cl -]T =5mol #L -11)[Zn 2+]T ;2)[Zn(N H 3)2+4];3)[ZnCl 2-ii];4)[Zn(NH 3)2+3];5)[Zn(NH 3)2+2];6)[Zn(NH 3)2+];7)[Zn 2+];8)[Zn(OH)2-ii ]form a line.Fig.3show s that w hen the total concentration of ammonia and ammonium chloride is constant,at first the zinc equilibrium solubility increasing is propor -tionate to the am monium chloride concentration,but almost has nothing to with the ammonia concentra -tion.When the ratio of am monia concentration to ammonium chloride concentration is 1,the zinc equ-ilibrium solubility has a max imum.T hen w ith the in -creasing of ammonium chloride concentration,the zinc equilibrium solubility decreases slowly.But when the concentration of ammonium chloride is large enough,the zinc equilibrium solubility as w ell in -creases because of the form ation of zinc -chloride com -plexes.Fig.4show s that w hen the concentration of ammonium chloride is 5mol #L -1,w ith the increas -ing of am monia concentration,the concentration ofcomplexes,such as [ZnCl 2-ii]T ,[Zn(NH 3)2+],[Zn (NH 3)2+2]and [Zn(NH 3)2+3],decrease rapidly,the concentration of [Zn(OH )2-i i ]T is very low and a-lmost has no change,but the concentration of Zn (NH 3)2+4increases.From the above view s,it is con -sidered that most parts of zinc ions ex ist in the form of Zn(NH 3)2+4with the increasing of ammonia con -centration.But w hen ammonia concentration is too low ,the concentration of zinc -chloride complexes dominates the other species.4 EXPERIMENTAL C ONFORMATION TO CAL -CULATED VALUES Extra zinc oxide of the analysis grade has been added to the aqueous solution of ammonia and am mo -nium chloride at a certain concentration,then agitat -#832#Trans.Nonferrous M et.Soc.China Dec.2000ed for72h at tem perature25e.Finally the zinc con-centration of the filtered solution has been analyzed. T he experimental and theoretically calculated values are show n in Table4and Table5.Table4Ex perimental and calculated values of equilibrium concentration of zinc in pureammonium chloride aqueous solution[NH4Cl] /(mol#L-1)[Zn]/(g#L-1)Cal.valuesExp.values[NH4Cl]/(mol#L-1)[Zn]/(g#L-1)Cal.v aluesEx p.values1.0 1.325 1.993 4.032.2111.042.0 6.4011.23 5.050.8910.293.016.9313.63In Table4,the experimental values do not in-crease as the theoretically calculated values w hen the concentration of ammonium chloride is more than3 mol/L.In fact it decreases.T his phenomenon has been explained in Ref.[4],which ex plained that the follow ing reaction takes place w hen the ratio of am-monia concentration to zinc concentration is no more than2and chloride concentration is concentrated in the solution.Table5Ex perimental and calculated values of equilibrium concentration of zinc in ammonia and ammonium chloride solutions w ithvarious concentrations[NHCl] /(mol#L-1)[NH3]/(mol#L-1)[Zn]/(g#L-1)Exp.valuesT heo.valuesRelative errors/%5.0 1.0443.4443.60-0.375.0 1.1246.17692.10-26.655.0 2.3295.0682.6914.965.0 4.25158.19150.97 4.785.0 5.66163.14157.58 3.534.0 2.0579.7165.3721.94.0 2.85106.8289.8818.854.0 3.80121.67115.05 5.754.0 4.9136.40126.507.803.0 2.2268.7668.96-0.303.0 3.0391.2387.614.133.04.09106.1094.8511.863.04.81112.2796.0116.942.00.8426.3025.72 2.272.0 1.4547.2243.927.512.0 2.5166.8460.7210.082.03.9680.7663.8626.471.00.5215.8414.519.171.0 1.0327.5425.199.331.0 1.5433.4229.3713.79A bs.aver ag e error/%10.77Zn(NH3)2+2+2Cl-=Zn(NH3)2Cl2(9)B s=[Zn(NH3)2+2]#[Cl-]2=2.88@10-2(10)Table5shows that the absolute average error be-tw een the experimental v alues and the theoretically calculated values is10.77%for the solution of v ar-i ously concentrated am monia and ammonium chloride. So it can say that the thermodynamic mathematical model is correct and the critical stability constants and thermodynamic data are basically accurate.[REFERENCES][1]T A NG Mo-tang and L U Jun-le.O n the ammoniation-complex equilibria in the system of Zn(Ò)-NH3-(N H2)SO4-H2O[J].J Cent South Inst M in M etall,(in Ch-inese),1994,25(6):701-7059.[2]OU Y ANG M in.T he Study on A New M etallurg icaland Chemical Process For T reating LA NP ING.S ZincO xide O res[D].Changsha:Centra-l South University ofT echnolog y,1994. 3.[3]T AN G M o-tang and O U YAN G M in.Pr epar at ion ofg raded zinc ox ide by using ammo nium and ammoniacomplex leaching process[J].T he Chinese Journal ofNonferrous M etals,(in Chinese),1998,8(1):118-121.[4]L impo J and L uis A.Solubility of zinc chloride in ammo-nium chloride solution[J].Hy drometallur gy,1993,32(1/3):247-260.[5]L impo J L,Fig ueiredo J M,Amer S,et al.T he CEN-I M-LN ET I process:a new process for the hydrometa-llurgical treatment of complex sulphides in ammo niumchloride solution[J].Hydrometallurg y,1992,28(2):149-161.[6]Limpo J L,L uis A and Gomez C.React ion dur ing theox yg en leaching of sulphides in the CEN IM-L N ET I pro-cess[J].Hy drometallur gy,1992,28(2):163-178.[7]SHI X-i chang,ZHAO Ru-i rong and JIAN G Han-ying.T hermody namic analysis on the system of Zn-C-l NH3-CO2-3-H2O[J].Journal of Central South U niversity ofT echnolog y,1998,29(2):193-196.[8]T A NG M o-tang and YAN G Sheng-hai.Electrowinningzinc in the system of Zn(II)-NH3-NH4C-l H2O andmechanism of ano dic r eaction[J].Jour nal of Centr alSout h U niversity of T echnology,(in Chinese),1999,30(2):153-156.[9]T ANG M o-tang and ZHAO T ian-cong.T he thermody-namic study on the basic and negative potent ial field oft he systems of Sb-S-H2O and Sb-N a-S-H2O[J].J Cent-Sout h I nst M in M etall,1988,19(1):35-43.[10]Rober t M S and Ar thur E M.Critical Stability Con-stants,Inor ganic Complex es[M].N ew York and Lon-don:Plenum Press,1976.14.[11]Pourbaix M.A tlas Dequibres Elex trochimiques et25e[M].Paris:Publication du Centre Belege dEtude de laCorrosion cebelior,1963.407-408.(Edited by LONG Hua-i zhong)#833#Vol110l16Thermodynamics of Zn(Ò)-NH3-NH4C-l H2O system。