Thermodynamic Aspects of Chemical Reactions, As Applied to the Smelting and Refining of Lead

THERMODYNAMIC ASPECTS OF CHEMICAL REACTIONS, AS APPLIED TO THE SMELTING AND REFINING OF LEAD

A. B. CHATTERJEA & B. R. NIJHAWAN National Metallurgical Laboratory, Jamshedpur

Abstract entropy of the system. All systems tend to reach maximum stability. The free energy With a reference to the formulae employed in chemical thermodynamics , available data on thermo- of - any system, therefore, tends to attain a dynamics of chemical reactions involved in the minimum value under conditions of constant extraction and refining of lead are brought together. pressure and temperature. Hence, the Equilibrium constants of the reactions have been change in free energy AG is indicative of the calculated from the standard free energy data to direction and extent to which a chemical explain the desideratum of optimum working conditions required during roasting , smelting and reaction, phase transformation and change of refining of lead . Graphs showing the changes with state can proceed. It is the driving force of temperature of standard free energies of formation any chemical reaction. The standard free of oxides, sulphides, chlorides , fluorides, sulphates, energy change of a reaction (A.G°) is related carbonates and silicates have been incorporated for to standard enthalpy change of a reaction ready reference. ( OH°) and standard entropy change of a reaction (AS°) at constant temperature and Introduction pressure by the equation A PPLICATION of the laws of thermo- AGO = AH°-TAS° ...... (2) dynamics permits the study of che- Whether a reaction can proceed in the mical reactions involved in different desired direction or not depends on two metallurgical processes . Thermodynamics factors, viz. the sign of the free energy change thus provides a useful method of predeter- and the availability of a suitable reaction mining the possibility of a chemical reaction path. If the free energy is lowered by the to occur under given conditions of tempera- reaction proceeding in the intended direction ture, pressure and composition . Review of and a path is available, the necessary pre- thermodynamical factors of chemical reac- requisites exist for the reaction to proceed. tions involved in the smelting and refining If the free energy change is not favourable, of lead has been dealt with based on the the mere existence of a reaction path is not conventional processes in use thereof. In enough. Similar reasoning applies to the doing so it has not been possible to include other case where the free energy change is physico-chemical studies of all aspects of lead favourable, but the reaction path is either not metallurgy. available or not straightforward or when the chances of the reactants meeting each other Free Energy of Reaction are small. Available portion of the total energy of a system is known as free energy (G) (F by Equilibrium Constant and Free Energy some American authors) and is defined by For any chemical reaction at given tem- the relation perature and pressure G=H-TS...... ( 1) bB+cC=dD+eE where H is the heat content and S is the in which b, c, d and e represent the number 108 CHAtrERJEA & NIJIIAWAN-THERMODYNAMIC ASPECTS OF CHEMICAL. REACTIONS 109 of molecules of the reactants B and C, and the known value of heat of reaction AH° and products D and E taking part in the re- the value of change in entropy OS°, when action, the change in free energy at a given the substances are in their standard states. temperature and pressure is given by the The variation of the equilibrium constant equation with temperature is expressed by van't Hoff d e equation AGT -_ AGO. -}- RTlnab x aE , ... (3) aBxae d1nK _ LH° (7) dT RT2 '"" where ae = activity of a constituent E; If AH ° is positive , i.e. endothermic reac- T = absolute temperature ('K); and R = tion, lnK, or - AG° becomes more negative gas constant (1-987 cal./degree/mole/mole). AG' and the reaction proceeds further with the is conventionally expressed in cal . per gram rise of temperature . If AH is negative, atom. When the reaction attains a state of ° then - AG°/T becomes more positive with equilibrium, the free energy change AGT is the increase of temperature . The possibility zero, and, therefore, of AH ° changing sign with temperature d e should be considered . From equation (6) AG°_-RTinDXaE _ - RT1nK ... (4) aB xaG LH° log K - 4-575T plus-constant ° or log K = - 4 -575 T ...... (4a) as the heat of the reaction does not change appreciably with temperature and pressure. where K is the equilibrium constant of the When logarithm of the equilibrium constant reaction. This equation shows the relation is plotted against the reciprocal of absolute of standard free energy change AG° to the temperature, a straight line is obtained, the equilibrium constant (K) of a chemical slope of which multiplied by 4-575 will give reaction. K is a ratio which indicates the the heat of reaction in calories. extent to which a chemical reaction can Thermodynamic data, therefore, provide proceed. The high magnitude of K, there- means of ascertaining the possibility of any fore, indicates that the reaction proceeds chemical reaction taking place under given substantially to the right-hand side of the conditions. It must be mentioned that the equation. The unit in which K is expressed reaction rate will depend on factors such as is generally related to the active mass (or the form of the reactants and is not predict- activity). Change of the equilibrium con- able from thermodynamic considerations stant (K) with temperature is given by the alone, e.g. the diffusion of the reactants and equation products from the zone of reaction may determine the rate of reaction. Metallurgical log "K 1 775 x TG° ..... (5) reactions usually take place at high tempera- If for any chemical reaction at a particular tures at which diffusion rates are high. So, temperature AG° can be found, K can be de- if the free energy change is favourable, a termined, and vice versa. It is thus possible metallurgical reaction is expected to pro- from equation (4) to calculate equilibrium gress at a reasonable rate in the desired activities for a reaction. direction.. From equations (2) and (4) it follows that as" AH° Activity 1nK = - RT ..... (6) By activity is meant the effective concen- which shows that the equilibrium constant tration of a constituent, which may not of any temperature T can be calculated from always be equal to its concentration. Activ- 11o SYMPOSIUM ON NON -FERROUS METAL INDUSTRY IN INDIA

ity is always referred to a standard state. AG° = IT + AH° - AaT1nT - It represents the effective concentration in AbT2 AcT3 (9) any desired state when compared to the 2 6 ... effective concentration in the state chosen as where I and H° are integration constants. the standard or reference state. In a large The value of the other three constants a, b number of instances, the pure substance is and c can be calculated from specific heat taken to be the standard state. In such data of the constituents while H° and I can cases, if the solution obeys Raoult's law, the be evaluated from one value of heat effect on activity is equal to the mole or atom fraction. the reaction and a known value of G° at a When considering dilute solutions, it is some- specified temperature respectively. It is thus times inconvenient to refer to the pure sub- possible to calculate the standard free energy stance as the standard state. Hence, we have change of a chemical reaction at a given tem- the infinitely dilute solution standard, and perature, but the equation is cumbersome. the 1 per cent solution (vide Chipman ) standard. In dilute solutions, the activity Graphical Representation of Standard is proportional to the considerations till the Free Energy Changes with Temperature latter become excessive. This is expressed by Henry's law. Activity of a pure solid or Standard free energies of formation of liquid is conventionally considered as unity, various compounds at different temperatures and that of a gas as equal to its partial have been expressed in the form of equation pressure. (9) by Kelly". The ratio of activity (az) to its concentra- Ellingham2 plotted the standard free energy tion is called the activity coefficient (Y1) changes with temperature in a graphical form. where the concentration is defined by the From such a chart, one can directly see the mole fraction (N; ). conditions for a chemical reaction to occur. As 11° and S° change only slightly with tem- Yi - a...... (8) perature, such a plot will result approximate- Ni ly in straight lines having a slope of S°, These remarks can be generally applied to because of the relation heterogeneous systems. dAG° _ - A$° In equation (3), when the reactants and dT products are not in their standard states and, If AG° is plotted downwards, as is common therefore, the activities are not unity, the practice, the slope will be - bS°. With the Cree energy change of the reaction AG can abrupt change of entropy at transition, be obtained by adding an activity correction fusion, vaporization and sublimation tern- to AG°. It follows from equation (3) that peratures, the slope is usually changed. AGr AGT + RT1nJa, where Ja is the The standard free energy for chemical reac- activity function at temperature T. An tion is related to standard electrode potential activity correction chart has been shown ( or decomposition voltage) of an electrolytic with a AG°/T diagram for chlorides (Fig. 4 ). cell in which the reaction is carried out by The correction has to be multiplied by the the following equation: number of molecules. This is to be added for products of reaction and subtracted for AG° _ -jnE°F ...... (10) reactants. If a is less than unity, reciprocal where E° = standard electrode potential in of a to be added to AG- for reactants and volts; F = faraday constant, 96,500 coulombs subtracted for products. per mole; n = number of faradays per mole The change in free energy with temperature of reaction; and j = conversion factor from at constant pressure can be expressed as Joules to calories (= 0-239). CI-IATTP-RJEA & NIJHAWANT--`I1TIERMODiINAIIIC ASPECTS of CHEMICAL REACTIONS Ili

TEMPERATURE °C. 00 X00 1200 100 (6oo 1800 2,000 2200 2400

-20 Yr 4 i'0 ^ZGflZ ,PZ Z 02 0^ -40 -40 a be¢^ ^.,rOZ ^4 ^2 XZ Z q ^ ^^ r 2 -C -60 Ar" -60 a M M Fe d -80 -80

l00 3 M O S ,.^ ^ 1® Irj^' /.^ rk1 { 120

C 140

x0 x^ 160 0 180 oil -18 A2p3 ^^qp i -20 -zoo I A M ^3 r -2z -720 8 ^ cpa ,^2 -240 24 } Go,A Prob able accurncy of dol-a M A ± I Kilocalorie M ± 3 Kilocalories c±t0 ., .. 2b Chun esa slate Flemerit Oxide Melting po'ihr m l] -28 Boiling poirF' a FB7 -280 5ublimation,f S Tronslhon -300 -3000 Zoo 400 600 $00 A 85oL U rI ZERO TEMPERATURE

FIG. I - STANDARD FREE ENERGIES OF FORMATION AS A FUNCTION OF TEMPERATURE OF OXIDES INVOLVING ONE GRAM-MOLE OXYGEN ( RICHARDSON AND JEFFES4 III SYMPOSIUM ON NON- `ERROUS M1TAL INDUSTRY tN INDIA

It is, therefore, clear that the ordinate of oxidation of a sulphide, or of the chlorination the iXG°/T chart can represent LG° and E°, of an oxide or a sulphide at any temperature if n is constant for all the reactions given in a T is shown by the free energy of formation of chart. In practice, due to internal resis- the oxide at T minus that of the sulphide at T, tance, over-voltage and polarization, voltages or the free energy of formation of the chloride higher than the requisite decomposition at T minus the free energy of formation of voltage are necessary for electrolysis. the oxide or sulphide at T. In other words, feasibility of reactions like Standard Free Energy/Temperature MS -}- 3/202 = MO + SO2 ...... (12) Diagrams MS +2MO = 3M--{-S02 ...... (13) Figs. 1-9 show the dependence of standard MS ± C12 = MC12 + 1 /2S2 .... (14) free energies of formation of oxides3, sul- MO ± C12 = MC12 + 1 /202.... (15) phides, chlorides5, fluoridess, sulphates7, MO -{- 2HCl = MCl2 + H2O ..... (16) carbonates7 and silicates8 on temperature. can be predetermined by the use of the oxide, The application of these charts has been sulphide and chloride charts. admirably described , by Ellingham2 and others3,5-7,s. In each' case, free energy change of the reaction in which 1 gram-mole Applications to Metallurgy of Lead of the gas 02, S2, Cl:,, F2, N2, SO3 and CO2 or The basic concepts of thermodynamics as the compound 5102 reacts with requisite applied to equilibrium in chemical reactions quantity of the metal or oxide has been will now be applied to review the important represented, as it shows directly the relative chemical reactions in the roasting, smelting stability of the compound or affinity for the and purification of lead. gas. The further down a compound occurs It can be seen from the L G°/T diagram in the chart, the more stable it is. When for oxides that the free energy of formation of two reactions are compared, the vertical dis- CO becomes more negative with the rise in tance at a particular temperature separating temperature and consequently it becomes the two characteristic lines represents the free theoretically possible to reduce all metallic energy change and, therefore, indicates the oxides with carbon at very high temperatures. tendency of the element represented in the Due to the lower free energies of formation of lower line to decompose the compound re- sulphides like CS, CS2 and H2S compared to presented by the upper line. For example, the oxides CO, CO2 and H2O, the reduction of if at a particular temperature T, the curve for metallic sulphides by carbon and hydrogen oxidation of any element X is below that for is, however, not possible. Sulphides of low element M, then the oxide of the element X stability can be reduced to metal either by is more stable than that of M and the reaction the application of heat when at moderate M0+X=XO+ M.....(11) temperature the free energy of formation will take place when the constituents are in becomes zero (e.g. of mercury sulphide) or their standard states. If, however, X forms by oxygen, which is adopted in the roast an oxide of lower valency, the reverse reac- reaction method of obtaining lead to be de- tion may be possible. scribed later. In such cases no reducing agent Thermodynamic data can be added or is required. The smelting of galena, like subtracted algebraically. The possibility of most of the sulphide ores of other metals,

FIG. 2 - STANDARD FREE ENERGY OF FORMATION AS A FUNCTION OF TEMPERATURR OF SULPHIDES FOR REACTIONS INVOLVING ONE GRAM -MOLE OF S2 GAS ( RICHARDSON AND JEFFES4) CHATT'ERJEA & 1tiIJL AWAN-THERMODYNAMIC A E CTS 01' CHtMICA1. 12EACTIONS 1 3

ps2 a atwm H2Sf HZ RATIO `yf IO^• 3 s 10 t02Q t0'S 1010 108 106 103 i0 10 10 101 20

"Ir r 1Yr53 ® Q`Lr7 0 t +CS2 S !-P^'r^y yo Ia' 5o' _1< G 4 ,,.^,-z = o

=Zpgz 9 Q ,a Ag+59 ^= t b o_►wo5 s_5►53 Sa Gee,

FaS ^

1 Pb c Ft J V 1d4 -80 a f7 R, M

M y a -I0o 1O J 7, } S•Ls 4 QQ -10 0-120 ID J Y 11 y Q 5® X 440 ey g5 /y 101

..I -16O M f.. ^ U 110 IO

tsa ti, r yGo. 10^ - 0a ^Z.`^ SUcIGE5TED ACCUR AC G6" ; IKg.C01. ^^ rJ1. M II '0 O .3K .C0%. 10 ` .5 i O1 Io8 ^^ 3> IO 1' 9CoI . g " CI1ANG .5 OF EL.^- SUL- f STATE . MENT. PNIgE li C^ MELTJNO PT M I t'10 I^ 20 BO,UN6 PT S 5t]BLtiMATlON 5 1 TlIANS,TIOMPT. T 1 ^2 Zoo 400 600 BOO 1000 1200 1400 1600 WQQ ?A 40 y TE PEF1A'i'LIRE °C . !O o v c P c S (/10 1hb2 t/1o^p rh0% j1C " 1JIG 1h10 1/t0 ABSOLUTE HzSJ HZ RAT10 Id 1O 42 -34 -30 2$ 1p!$O ;p14° 10 lbs0 10 0 10 3$ t0 10 Id 10 1 psz atm. I-- i FIG. 2 114 87VNMPOSI171i ON NON-FERROUS METAL INDUSTRY IN INDIA commonly involves roasting of the ore as the will depend on the partial pressure of SO3i first stage. temperature and on the characteristics of the oxide MO. If the partial pressure of SO3 at the selected temperature is less than its General Aspects of Roasting equilibrium value, then MSO4 will decompose Some general aspects of roasting of sulphide into MO + SO3 and more of the oxide will. concentrates have been described recently by form. Conversely when the partial pressure Djingheuzian10. In oxidizing roasting, the of SO3 is higher than the dissociation pressure ore concentrate is heated below its fusion of sulphate, SO3 will combine with the oxide point, to convert the sulphide into the oxide. to form more sulphate till the partial pressure The reaction may be expressed as of SO3 is reduced to equilibrium value. Thus the roasting reactions can be controlled by MS + 1O2 = MO + SO2 ..... (17) controlling the temperature and the composi- For the reaction to proceed substantially tion of the gas in the roaster. to the right, the free energy of the formation of the oxide MO should be higher than that of the sulphide MS, and the concentration of Roasting of Galena SO2 in the roaster gases should be low to The thermodynamical aspects of roasting of prevent the reverse reaction. Perettill has galena have been described by Wenner12. mentioned that at temperatures commonly But whereas he calculated the free energy employed for roasting of sulphides of metals data from equations, use has been made of like copper, zinc and lead, the reaction appropriate OG°/T charts in this review. MS + 9 02 = MO + SO2 and the oxidation In roasting, the following reactions are likely of sulphur to sulphur dioxide involve such to take place: large free energy changes that the S02/02 equilibrium ratios need not be considered. (a) PbS + 3/202 - PbO + SO2 .... (20) This will be later considered in relation to The equilibrium constant of the reaction is roasting of galena. Though the oxidation of the sulphide is the main chemical reaction aPbo.pS02 aimed at, other side reactions usually take K 20 - aPbs.p01-5 place and some of these reactions affect the production of a desirable calcine. In the Assuming that the activities of PbO and presence of catalysts, SO2 is converted into PbS are unity, the equilibrium constant SO3 becomes

SO2 + 1/202 + catalyst = SO3 .... (18) K20 = pros SO3 then reacts with the oxide MO to form 2 sulphate The change in free energy of reaction (20) MO + SO3 = MSO4 ..... (19) can be found from the known values of standard free energies of formation of PbS, which is undesirable. PhO and SO2 and from the expression The roaster gas consists of a mixture of 02, OG20 = -RT1nK20, K20 can be calculated. N21 SO2 and SO3. In a mixture of gases, the Other reactions are d partial pressure of each is, of course, proportional to its volume. The formation of MS04 (b) PbS 1- 202 = PbSO4 ...... (22)

FIG. 3 - STANDARD FREE ENERGIES OF FORMATIONJTEMPE RATURE OF CHLORIDES INVOLVING ONE GRAM- MOLE OF CHLORINE GAS BETWEEN 0 AND -90 K. CAL. (KELLOG5 0- CHANGES OF STATE. ELEMENT CHLORIDE. MELTING POINT. M El BOILING POINT. B. SUBLIMATION PT. S m TRANSITION PT. T 0 0.2-

0.4-

0.6

O.8 SnCIY+Ct2= Snc t

149, Cit e IASC%2. H =+C12=2HC1^ 1.0- ^y`}3' AS+Clts 3AsC1 2Hg-I-clj- H91C1z 2/3gi, C12=21ab;Of t Sb +CI1=213SbCI ZAg+C12= 2AgcI 1.2 - 2CrGI2+C122 2Crcl 2Cu+C1t=Cu1CIZ

2135+CI2=2f3 BCI

1.4 - NI+Ct2 t NiCI2 =--r Co- C12 CoCli 1!2Si+Clt=ij2SiCl4-, Fe+Ct2= FeCIx 1.6 -I Sn}Cti:SnCI1 Pb+C12=PbC12 Cd+ C12: CdC l2

1-8 - Ij2Ti +CI2. 'jZTiC14

Cr+CI1= CrC12

Zn+ CI2. =ZnC%

FXG, 3 116 SYMPOSIUM ON NON-FERROUS METAL INDUSTRY IN INDIA

For the reasons mentioned above The high values of the equilibrium constants K2Q., K22, K24 and K26 show that the K22 = 02 . (23) reactions (20), (22), ( (26) are thermo- P 2 24) and dynamically feasible in the temperature (c) 2PbS±71f202 =PbSO4PbO±SO2 ... (24) range of 600°-500°C., and the reactions will proceed substantially to the right of the K24 - PS02 ...... , . . (25) or equation. The reaction 11252 -t- 02 = SO2 has an equilibrium constant of 1.12 x 101$ (d) PbS ± 02 - Pb + SO2 ...... (26) at 825°C. PbO and SO2 will, therefore, be Kos = PS©2/PO2 ...... (27) stable phases where there is a minute partial pressure of oxygen. On the other hand, it (e) S02 ± 1 /202= SO3 ...... (28) can be shown from the free energy data that pSO3 the basic lead sulphate formed in equation K28 = PS02 x p00 5 . : . (29) (24) should react with SO2 and oxygen to form PbSO4 thus : (f) PbO -}- SO3=PbSO4 ...... (30) PbSO4PbO+S02+11202 = 2PbSO4 .. (32) I K3o pS03 ...... (31) which has an equilibrium constant of 5.01 X 10 5 at 800°C. It means that The approximate AG values, as obtained from the appropriate AG°/T diagram, and PSO2 X Pa(-5 = 1 (33) the calculated values of the equilibrium 5.01x105''' constants of these reactions at 600°, 700° and The partial pressure of oxygen is 0-21 atm. 800°C. are given in Table 1. Substituting value of partial pressure of oxygen in the above equation, it can be seen that the partial pressure of SO2 for the con- TABLE 1-FREE ENERGY OF FORMATION version of basic lead sulphate to lead sulphate OF COMPOUNDS AND EQUILIBRIUM is extremely low, which means that very low CONSTANTS OF REACTIONS (a) TO (d) DURING ROASTING OF GALENA concentrations of SO2 in the roaster gases in presence of oxygen can react with basic lead TEMPERATURE, C. sulphate to form lead sulphate. Conversely 600 7 00 800 high SO2 concentrations in the presence of AGO PbS cal. -21500 -19500 -17500 traces of oxygen will convert basic lead AGO PbO -32000 -29300 -27000 sulphate to lead sulphate. The roasted pro- AG- PbSO4 „ - 144280 - 135700 - 127130 duct will, therefore, consist of little unreact- AG- PbSO4 „ - 180900 -169000 -158300 ed lead sulphide and mainly of lead oxide Pb0* and lead sulphate. AG° SO2 : - 72000 - 70000 -68000 (a) K2O 4. 57x1020 . 8.51x1(117 6.31x10115 (b) KS2 5.9 x 100 1 - 3 x 1026 2.14 x 1022 Smelting of Lead ( c) K24 2.5 x 1052 8.51 x 1044 9 .55 x 1028 (d) Kee 4.4 x 1012 2.2 x1011 2- 0 x 1010 The smelting of lead follows three methods, *Calculated. viz. (i) the reduction of lead oxide by carbon and carbon monoxide; (ii) reaction between

FIG. 4 - STANDARD FREE ENERGIES OF FORMATION/ TEMPERATURE OF CHLORIDES INVOLVING ONE GRAM- MOLE OF CHLORINE GAS BETWEEN - 90 AND -150 K. CAL. (KELLOG5 ) 90

lop = A1CI^^) ® F If

I 1 00

/ I

110

120

130 D C

0 In

cl=4CcG1 2 /A r 150 0 22 z fn 30 2P ,C12 = 2NOGI.

ACTIVITY CORRECTION CHART VALUES OF RT InO► . ^ ol I 1-1 20

kip 10 as {o a=^

0 Soo 1000 20000

TEMPERATURE ,'C. Fir.. 4 118 SYMPOSIUM ON NON-FERROUS METAL INDUSTRY IN INDIA

CHANGES OF ELEMENT FLUORIDE. STATE. ajE+F ^° ^ ' ajF 6 0.6 Y MELTING POINT M. ^ ;Fti I 30 BOILING POINT B !^. Al tb 0.8 ^ SUBLIMATION PT. S L ,.' TRANSITION PT. T Q 1.0 CIZ+ FZ = 2CIF so 1.2 - 60 1.4 - M i S 70 !r35e+F2_ p SeF6 1.6 3 t M tiP9 ® ^aF ^3S + Z- p-F,. 80 1-8 - M 2Ag+F2= 2A9F, rr 2.0 - 90

I/37e+Fz=13 TeFs 3 2.2 - 100

2.4 ^ Mj^ 110 m r C 120 0 t z H2+Fz= 2HF--* ,r 1" 130

3.0- AsF3 a f 140 3As+p2. 3AsF3-., 3.2- Pb+F2 - PbF2 C d4 ISO 5 00 1000 1500 TEMPERATURE, C

FIG. 5 - STANDARD FREE ENERGY OF FORMATION OF FLUORIDES AS A FUNCTION OF TEMPERATURE INVOLVING ONE GRAM-MOLE OF FLUORINE GAS BETWEEN -20 AND -150 K. CAL. (KELLOGG

the sulphide and the oxide ; and (iii) displace- reaction is reversible, complete decomposition ment of Pb from PbS by other metals. For is not possible. As regards the other two the precipitation method, any metal whose processes, argentiferous ores are treated in position in the sulphide diagram is lower than the blast furnace and high-grade non-argenti- PbS will be able to combine with sulphur, ferous ores in reverberatory furnaces. In displacing lead from its sulphide. Iron is the the AG°fT diagram for the oxide, the 2C+02 chief element used for displacement. As the =2CO curve cuts that for 2Pb+O,=2PbO CHATTERJEA & NIJHAWAN- THERMOD 'YNAMIC ASPECTS OF CHEMICAL REACTIONS 119

'^^ '^ }AIF, Cd+ FZ = CdF2 ^r•4^ 341 s ^U + Fa x ,.3U% r f ® 3.6 -r J ^^^ y ' ,^^'L 170 2♦36 3.8 -F2 < BF3- r J ♦ Mn+F2 = nF2- F ^- •' 180 La %2Si +F2, i SiF4 z W 4.0 at 0 190 D J 4.2 IL %AI + F2 = / AIF3 20C 4.4

210 4.6 ^ ^. i CoR= .+r B s ^♦ 141 1 22C 4.8

5.0 23(

MHz 2-41

AM 1f 250 M9- FZ - MgF2 2K+ F2 = 2KF - CHANGES OF ELEMENT FLUOI UDE. 3.6 - 2No+F2 = 2.NOF, i STATE, 260 MELTING POINT. M 58- BOILING POINT. B 161 270 SUBLIMATION PT. S Q Ba+ F2 = Ba Fx TRANSITON POINT. T 6-0- Co- F2 = Cafe T^ 28C 2Li+F2 = '2 Lit; 500 1000 1500

TEMPERATURE °C:.

FIG, 6 - STANDARD FREE ENERGY OF FORMATION OF FLUORIDES AS A FUNCTION OF TEMPERATURE INVOLVING ONE GRAM-MOLE OF FLUORINE GAS BETWEEN -150 AND -280 K. CAL. ( KELLOG6)

at about 200°C., which means that the (a) PbO - CO = Pb + CO2 ...... (34) reducing effect of CO theoretically commences at that temperature. The reducing action 1.C34 - pCO2pC0 ...... (35) of C commences at 400°C. and the rate (h) PbO+C^Pb+-CO ...... (36) of reduction is comparatively slow. The main reactions of the lead blast furnace and K36 = pCO ...... (37) their equilibrium constants are as follows: (c) CO2 + C = 2CO ...... (38) 10 to 01 , o

to 0 .b^ 20 O a 0 Lij 30 M D o- ! 40

50 w 60 001, 0 10I.-e 70

90 20

.100 CH! AtJ ES OF ELEMEN SULPHATE . STATE

MELTINGr POINT M O0 120 BOILING ^• H I, B t 00, 0,0 SUBLIMATION - S S^ TRANSIT I Ot%I n T I t I 130

20 0 400 600 800 . 1000 I zoo 1400 1600

TEMPERATURE 0C.

FIG. 7 CHATTERJEA & NIJHAWAN-THER&10DYNAMIC ASPECTS OF CHEMICAL REACTIONS 121

K3a = pCO2 (39) PC02 TABLE 2-FREE ENERGY OF FORMATION OF COMPOUNDS, EQUILIBRIUM (d) PbSO4 + 4CO = PbS -}- 4002 . (40) CONSTANTS AT 900°, 1000° AND 1100°C. OF REACTIONS IN LEAD BLAST FURNACE

TEMPERATURE, °C. K4Q = pCO2 . . (41 ) PC04 900 1000 1100 (e} PbS ± 2PbO = 3Pb ± S02 . . (42) AGO PbS cal. -15200 -13000 -11500 K 42 = PS -02 - • - (43) LG° CO -51500 -53000 -55500 (f) PbS + PbSO4 2Pb 4- 2SO2. . (44) AGO CO2 -94000 -94000 -94000 AGO PbSO4'1 -118600 -110000 -101000 K41 = pSO2 . . . . . (45) AGO PbO -24000 -22000 -19800 -20000 -18600 (g) PbS + Fe = FeS ± Pb . . . . . (46) AGO FeS -21500 AGO SO2 -66000 -64500 -63000 (a) Ksa 3.2 x 103 2-2 X103 1.1 X103 K 46 _ aFeS ...... (47) wpbs (b) K36 1.05 x 1015 2.14 x 1012 6-3 x 10'1 All these reactions occur around 900°C. (C) K,a 2.0 X 1013 3-1 x 1011 2.0 x 101° Reaction (36) is really a combination of the (d) Kd2 0- 71 1.30 1-91 1.03 2-31 reactions represented by equation (34) and {e) K44 4-5 x 10-1 the producer gas reaction (38). The approxi- {f) K42 1-17 1-26 1-13 mate values of free energies of formation of *Pb(5)+S(s)+202(g)= PbSO4(S) AGT = various compounds at 900°C. taken from the - 219000+85.6 T. cal. appropriate AG' /T diagrams and the equilibrium constants of these reactions are given TABLE 3-EQUILIBRIUM CONSTANTS OF in Table 2. THE CHEMICAL REACTIONS OF ROAST REACTION METHOD

E" QL'ILIBRI UM TEMPERATURE, °C. CONSTANTS OF r---•------'^^-- ^! Roast Reaction Process CHEMICAL REACTIONS 800 900

K44 0-21 0-71 The desirable reactions in the process are 2.0x10-2 4.5 x10-3 given in equations (42) and (44), but in the K 44 2.14x10-6 presence of excess of PbS or PbO than is Kso 2-0 x 10- 11 required by equations (42) and (44), the K51 excess remains unaltered, while with excess of PbSO4 either a mixture of Pb and PbO, or The equilibrium constants of the reactions PbO alone, results as shown in equations (42), (44), (50) and (51) at 800° and 900°C. (48) to (51). are given in Table 3. PbS+3PbO = 3Pb+PbO ±SO2 . (48) K42 has values of 0-71 at 900°C., 1.3 at 1000°C. and 1-91 at 1100°C. and the corres- 2PbS ± 2PbO = 3 Pb ± PbS -}- S02 . . (49) ponding values of K44 are 4-5 x 10-1, 1-03 and PbS±2PbSO4 = Pb-4-PbO± 3SO2 . (50) 2-31 respectively. These indicate that if the PbS±3PbSO4 = 4PbO± 4SO2 . . . . (51) partial pressure of SO2 attains these values,

FIG. 7 - TEMPERATURE DEPENDENCE OF STANDARD FREE ENERGIES OF FORMATION OF SULPHATES FROM THE OXIDE AND SO3 GAS INVOLVING ONE GRAM-MOLE OF SO3 GAS (OSBORN7 ) 122 SYMPOSIUM ON NON-FERROUS METAL INDUSTRY IN INDIA omv/ 5

i 30

40 JJ

/ i 50

60 t7 el6 65

70 100 200 300 400 500 600 700 800 900

TEMPERATURE

FIG. S - TEMPERATURE DEPENDENCE OF STANDARD FREE ENERGIES OF FORMATION OF CARBONATES FROM THE OXIDE AND CO2 INVOLVING ONE GRAM-MOLE OF CO2 GAS (OSBORN' CHATTERJEA & NIJHAWAN_THERMODYNAMIC ASPECTS OF CHEMICAL REACTIONS 123 the production of lead will stop. But as SO2 takes place- In this method, bismuth com- is allowed to escape from the furnace, the bines with calcium, magnesium and antimony partial pressure of SO2 cannot attain any forming a dross, and leaving lead which con- value excepting 4.5 x 10-1 so that the desired tains only 0.005 per cent Bi. reactions will go to completion. For the same reason, reactions (50) and (51) will not Removal of Zinc from Lead proceed substantially to the right of the equations. From Parkes's process, lead contains 0.5- 0.6 per cent zinc. The older practice was to pass air through molten lead kept at 1200°- Refining of Lead 1300°F. to oxidize zinc, because of higher Besides lead, copper and iron, galena con- free energy of formation of ZnO compared to tains a large number of other metals like gold, lead oxide. But, in practice, oxidation of silver, zinc, tin, arsenic, antimony, bismuth, lead cannot be altogether prevented and nickel, cobalt, which need removal, with the 3-5 per cent lead is lost by oxidation. sulphur, to obtain pure lead. Bettertoni4 has described a method of puri- The elimination of impurities norm illy fication by chlorine gas. In this process the takes place in two main ways, volatilization loss of lead has been practically eliminated of easily volatile oxides, like arsenic and anti- and zinc chloride is recovered as a valuable mony oxides, and slagging, in which the byproduct. The reaction is conducted under elements having higher free energy of forma- vacuum at a temperature of 350°-390°C. The tion of their oxides than that of lead oxide reaction is exothermic, but ZnCla does not are readily oxidized and taken up by the slag. volatilize until a temperature of 730°C. is It can be seen with the aid of Fig. I that attained. At the end of the operation, the tin, zinc, sulphur, iron, nickel and cobalt are lead contains about 0.005 per cent Zn and the oxidized at a low temperature preferentially ` crust ' contains 2.5 per cent lead. Dezinc- to lead and may thus be slagged off. Copper ing by chlorine evidently depends on the fact can only be partially removed by oxidation. that the standard free energy curve for ZnCI2 It, however, forms an alloy of higher melting is below that of PbCl2. The standard free point than lead and, on slow melting, dross energy of formation of ZriCl2 at 390°C. is containing Cu, S, As and Pb separates out. -76,640 cal. and that of PbCI is -61,550 As the free energy of formation of bismuth cal. as shown in Fig. 3. If any PbCl2 is form- oxide is lower than that of lead oxide, re- ed, it will be decomposed by Zn according to moval of bismuth is not possible by either the equation oxidation or vaporization (in a way similar Zn + PbCI2== Pb(l) -}- ZnCl2 (52) to its removal from copper matte). in which the standard free energy change is -15,090 cal. at 390°C. Kellog5 has calculat- Removal of Bismuth ed the residual zinc in lead treated in this way, It is well known that Parkes's process on the assumption that equilibrium between does not remove bismuth, but that it can be the metal and chloride phases is attained. removed by Pattinson's process, but at considerable cost. Betterton and Lebedeff13 AG° = -15090 = -RTin apb x aznc', . . . (53) azn x apbct, have developed a process in which calcium, magnesium and antimony are used for the As the final lead is of over 99.9 per cent removal of bismuth. The principle is similar purity, it can be assumed that apt, = 1. to that of Parkes's process in which partition The final chloride has a mole fraction of of silver between alloys, rich in zinc and lead, 0-983. On the assumption that the solution ox.

Fe0'XFe+ iO, n Pe5i0a Ail fiofi d) Fe XFe+SiOa= e5i03(Atl Liquid) X Q Nodal iGe 1i1C} 3 to x +i0}(EaY n f 5 „ a 5i0i' MnS1 e t 55at Fe 2Fe +2y 1lClineeY15t1tC•} 0 + SiOa ° NI95iO -I -10 10 hnt`l+ S ^tutn 5 iOi(s) 11 10>1^L^ -15 LM 1510a - M 9 i5i0 ( FOrsterite)

-20 A -1 CaO+ 5iO aslo%(Wollastonite o 10 m I^ Cli.,O 4 13W, : Lit i0S D -25 - 2ca0+A1i03,Sioa' C*,AI,Si0, BQO • SiO t = ga Si03 3c ao + Siot - Cassi os @ x -3 -30 2Ca0+ Si Qt = Cn15i04 10

-35 0 u A1t03Si0 = AI15i0 ,,.(Gyante.) Oh40 AI1Oj. SIO1 _ All i03 (AtldaiuSit,.) ®

AIX03 * SiOi a AI=5103 ( Simmanite• z O -50

-55 0s0 • Siol Noa Si4o SUGGESTED ACCURACY. ® ± I K g. col . 0 ± 31cg . 001. -60 © = IoK%.Cai. p ±>to1Sgcal. 10 K10+S iOz a K,SiO3 O CHANGE OF REACTANT PRODUCT -65 STATE. MELTING PT. M BOILING: PT. SUELIMATIOI 0 S $^ JRAr4$tTtCA PT. -70 T 0 200 400 600 800 1000 1200 1400 I600 TEMPERATURE ,°C '107

1 t6 10t4 1010 1012 10

F I G. 't) CHATTERJEA & NIJHAWAN-THE RMO DYNAMIC ASPECTS OF CHEMICAL REACTIONS 125 of ZnC12 PbC12 is ideal, the activity of ZnC12 illustrate the importance of thermodynamics was taken as its mole fraction. Similarly, and free energy data in studying chemical the activity of PbCI2 was assumed as its mole reactions of value in extraction and refining fraction of 0.017 while the azn accepted as of metals. It has been mentioned that the reported value of 29Nzn. Substituting chemical thermodynamics can predict the these values in equation (47) Kellog5 found course of chemical reaction and enables to that under equilibrium conditions, 0.00066 calculate the value of equilibrium constant. per cent zinc should be present in the lead. The application to practical problems de- In view of the assumptions made, the discre- pends on the availability of activity data. pancy between the theoretical value and that In this brief article it has .not been possible obtained in practice is not great. to discuss all applications of physical chem- Recently a vacuum distillationi5 process istry to the extractive metallurgy of lead. for separating silver from silver-zinc alloy The reproduction of standard free energy obtained from Parkes's process has been charts of various compounds in this paper described, The principle of separation by should bring together for ready reference use- vacuum distillation depends on the differ- ful thermodynamic data scattered in the ence of vapour pressures of the metals con- technical literature, which may be applied cerned. As the vapour pressures of lead and on similar lines to other cases than the zinc are much higher than that of silver, metallurgy of lead extraction, provided the distillation under vacuum at 800°C. can simplifying assumptions made in this treat- separate lead and zinc from metallic silver. ment are, and remain, valid, Vacuum dezincing of desilverized lead bullion has also been developed at the St. Acknowledgement Joseph Lead Company and the Broken Hill Associated Smelters ( South Australia). This The authors wish to record their indebted- is carried out under a vacuum of 0-01-0.1 mm. ness to those authorities in this field whose mercury, when the zinc atoms from the works have been referred to in the text. bullion surface diffuse out to a neighbour- They are specially indebted to Richardson ing condensing surface. The design of the and Jeffes and Kellog and Osborn for their apparatus ensures that there is no resistance diagrams which have been reproduced. to the flow of zinc vapour apart from that due to the residual gas pressure within the vacuum dezincer. Davey"' has discussed the References fundamental aspects of vacuum distillation processes with special reference to vacuum 1. KELLY. K. K., " Contributions to the Data on . U.S. Bureau dezincing and has shown that the actual Theoretical Metallurgy ", Bull Mines, 384 (1935 ); 406 (1937); 407 removal of zinc in practice has reasonably (1937). good agreement with theoretical calculations. 2. ELLINGHAM , H. J. T., " Reducibility of Oxides and Sulphides ", J. Sac. Chem. Ind., 63 (1944), Summary 125. 3. RICHARDSON , F. D. & JEFFES, J. H. E., " The The application of thermodynamic princi- Thermodynamics of Substances of Interest ples to some of the chemical reactions involv- in Iron and Steel Making from 0°C. to 2400°C.", ed in the metallurgy of lead has been made to J.I.S.I ., 160 (1948 ), 261.

f-- FiG. 9 --- FREE ENERGIES OF FORMATION OF SILICATES AS A FUNCTLON OF TEMPERATURE, INVOLVING ONE GRAM-MOLE OF SiO^ ( RICHARDSON , JEFFES AND WITHERS" ) 126 SYMPOSIUM ON NON-FERROUS METAL INDUSTRY IN INDIA

4. RICHARDSON , F. D. & JEFFES, J. H. E., " The 10. DJINGHEUZIAN, L. E., " Theory and Practice of Thermodynamics of Substances of Interest in Roasting Sulphide Concentrates ", Canadian Iron and Steel Making ", J.I.S.I., 171 ( 1952 ), lliss.:llet. Bulletin, 45(482) ( 1952), 352. 165. 11. PERETTI, E. A., „ A New Method for Studying 5. KELLOG, H. H., " Thermodynamic Relationship the Mechanism of Roasting Reactions ", in Chlorine Metallurgy Trans. A.I.M.E., Trans. Faraday Soc., 4 ( 1948 ), 174. 188 (1950), 862. 12. WENNER, it. R., Thermochemical Calculations 6. KELLOG, H. H., " Metallurgical Reactions of (McGraw-Hill, New York ), (1941), 298. Fluorides ", Trans. A,I.M .E., 191 (1951), 13. BETTERTON, J. O. & LEBEDEFF, Y. E., " Di- 137. bismuthizing Lead with Alkaline Earth 7. OSBORN, C . J.. " Graphical Representation of Metals Including Magnesium, and with Anti- Metallurgical Equilibria", Trans. A.I.M.E., mony ", Traits. .4.7.1I. E., 121 (1936), 205. 188 (1950), 600. 14. BETTERTON, J. 0., " Chlorine Dezincing in Lead 8. RICHARDSON , F. D., JEFFEs, J. I-f. E. & Refining ", Trans. A.I.M.E., 121 ( 1936 ), 264. WITHERS, G., " The Thermodynamics of 15. SCHLECHTEN, A. W. & SHIN, C. H., " Vacuum Substances of Interest in Iron and Steel Furnace Separation of Silver from Zinc- Making ". J.I.S.I., 166 (1950), 213. Silver ", Engineering & Min. Journal, 150(12) 9. DANNATT , C. W. & ELLINGHAM , H. J. T., (1949 ), 80. " Roasting and Reduction Processes, A 16. DAVEY , T. R. A., " Vacuum Dezincing of General Survey ", Discussion Faraday Soc., 4 Desilverized Lead Bullion ", Trans. A.I.M.E., (1948 ), 126. 197(1953),991.