Application of Physico-chemical Principles in Extraction Metallurgy

T. BANERJEE and P. P. BHATNAGAR

Physical chemistry has two important tools namely, thermodynamics which indicates the possibility of a reaction, and kinetics giving the rate at which a reaction proceeds. This paper indicates how these tools can profitably be utilized jor the extraction of metals. The authors illustrate as to how physical chemistry has been helpful in the clear understanding of the high-temperature reactions and in search-ing out new methods for metal-winning. A few instances where thereto-dynamic data have been applied for developing scientific extraction methods are pointed out and their limitations indicated. The instances cited include the separation of nickel and zinc from a mixture of their oxides, the prepara-tion of titanium tetraiodide and the development of Pidgeons process for the production of magnesium.

p HYSICAL chemistry is ;interested- pin estab-lishing energy relations obtained in physical

and chemical changes in order to assess the feasi-bility, extent and the rate at which they occur, and finally to determine the controlling factors. The object of this paper is to show how these relations can act as a guide in deciding the feasibility of a process and help in materialising the same. Men-tion has also been made about a few of the pro-cesses so developed at the National Metallurgical Laboratory. For detailed information about the various principles" and their applications' various literature on the subject may be referred to.

In a reaction, the change in heat content o H is not fully convertible into work of chemical infor-mation. The energy available for such change is called Free Energy and is expressed by the equation

AF= AH— T AS (1) where S is the entropy, T A S represents a change in the bound energy which is not available to do any useful work.

Again in a reaction tending to equilibrium A F is given by the relation A F = —RT in K+RT In (na product/7ra reactant)

non equilibrium (2) where K stands for the product of the active con-centrations of the products (i.e. resultants) divided by that of the reactants when equilibrium has been established and is called the equilibrium constant, while the term in brackets stands for the activity product ratio of the same nature as K but pertain-ing to the actual condition.

Dr. T. BANERJEE, Asst. Director, and Mr. P. P. BHATNAGAR, Sr. Scientific Officer, National Metallurgical Laboratory.

From Eq° (2), we have A F = A F° + RT In (Tra product/aa reactant)

non-equilibrium ... (3) where F° stands for — RT log K and is called the standard free energy. It is related to temperature by the equation'.

A F° = A Ho — A LI log eT — 4 A — 1/6 A yT3 + IT ... (4)

where A Ho, A £. A Ii, p y and I constants, I, being equal to KR.

In equation (2), if the activity ratio in bracket is smaller than K or the activities of the reactants are more predominant than the ratio indicated by K, then F is negative and the reaction tends to occur in the direction implied by the chemical equation. If the converse is true, F is positive to the reaction as written but negative for the reverse reaction then the reaction occurs in the opposite direction.

GRAPHICAL REPRESENTATION AND COMPILATION

OF STANDARD FREE ENERGY CHANGES

The standard free energy change for any reactions assesses the possibility of such a reaction and these data for various temperatures and pressures are of great advantage in determining the optimum con-ditions of working. The greater the free energy change, the greater are the chances for the reaction to occur. The standard free energy change with respect to temperature have been plotted by Effin-gham' and Richardson & Jeffes6 for oxides, and by Kellogg' for chlorides, while Kelley and his co-workers8 calculated data for carbonates, sulphides, sulphates, carbides and nitrides etc. The free energy diagrams for oxide, sulphide and chloride

310 INDIAN MINING JOURNAL, Special Issue, 1957

TEMPERATURE •C. 200 400 600 1000 1200 -0 1600 1800 2000 2200 gaga

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ABSOUrr ZERO TEMPERATURE 6C

are given in Figs. 1-3. The standard free energy change in these diagrams is associatd with one gram molecule of the combining gas with the cor-responding stoichiometric amounts of the metal.

The .6 F°—T plots are linear and show a break or a change in gradient at points where a change in entropy takes place followed by fusion or melting etc. A pressure correction chart is also incorporated and the free energy data may be extrapolated for the gaseous participants at an atmospheric pressure.

APPLICATIONS

The free energy diagrams are very useful in studying the possibilities of new reaction and show immediately as to which metal forms more stable compounds than others. The lower the metals in the diagram the stabler the compound it forms and the vertical distance at any temperature between two compounds indicates the tendency of the metal to reduce the compound higher in the diagram forming stabler compounds. From the oxides chart, it is observed that aluminium forms a stabler

INDIAN MINING JOURNAL, Special Issue, 1957

oxide than silicon and thus on reacting aluminium with silica, the oxygen of the latter should be trans-ferred to the former producing silicon metal. This reaction has been studied by Nijhawan and co-workers', and aluminium silicon alloy produced. Applying similar consideration of free energy changes it is observed that aluminium could also be used to reduce titanium dioxide and in fact an alloy containing 60 percent titanium has been pro-duced at the National Metallurgical Laboratory".

These diagrams could also be used for studying the reducibility or the oxidisability of the com-pounds, this being one of the most important factors in pyrometallurgical operation. It is indi-cated from the diagrams that in the reduction of manganese ore high in phosphorus and iron, iron and phosphorus would be preferentially reduced leaving a manganese-rich slag.

As a part of a scheme worked out at National Metallurgical Laboratory for recovering the nickel and zinc separately from the waste liquor from the silver hefinery, under construction in Calcutta,

311

Bhatnagar & Banerjee" separated the two metals by first precipitating them as hydroxides and then chlorinating the oxides mixture with hydrochloride acid gas.

The reaction of a metallic oxide with hydro-chloride acid may be presented as :—

MO + 2HCI = MCI, + H2O (g) (5) The possibility of separation of nickel and zinc

oxides from this mixture is conceived from ther-modynamic considerations of the reactions :-

NiO + 2HC1 = NiC12 + H2O (g) (6) ZnO + 2HC1 = ZnCl2 + H2O (g) (7)

The standard free energy change for the reactions (6) and (7) have been calculated at 1000°K (727°C), 1500°K (1227°C) and 2000°K (1727°C) to supplement the data of Kellogg' at 737°K (500°C) and 1273°K (1000°C) and reported in Table I.

1425/112. RATIO

00 los

TABLE I—STANDARD FREE ENERGY CHANGES AT DIFFERENT TEMPERATURES FOR THE REACTIONS

OF HC1 WITH NiO AND ZnO

Reaction 773 K* 1000°K 1273°K* 1500°K 2000°K

Standard Free Energy Change at °K in K. Cals.

■ Ni0 -2HCI

NiCI, -11„0 _10.8 _5.25 + 020 _ 2.47 -9.78 ZnO - 2HCI

ZnCl, -H,0 _10.5 _7.65 _11.0 _15.7 * Values taken from Kellogg's' data

o F° for nickel chloride, zinc chloride and HC1 have been taken from Kellogg's data'; the o F° for NiO and H10 from paper by Richardson6 and o F° for ZnO from data by Kelly'.

The trend of the free energy change for reaction (6) and (7) has been shown in Fig. 4. It is noticed

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INDIAN MINING JOURNAL, Special Issue, 1957 412

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from the free energy diagram for both the reactions that separation should be most effective at about 1003°C, and nickel oxide should not react with the HCl at that temperature due to a positive free energy change. These ideas, of course, presume unit activity for the reactants and resultants.

The experiments carried out by the authors in-dicated that though zinc oxide was preferentially removed, a part of nickel oxide was also removed at 1000°C and a residue containing 95-97 percent nickel 'oxide was obtained, from a 50:50 mixture, The removal of part of nickel oxide has been ex-plained due to various factors e.g. removal of one of the reaction products i.e. NiCL from that zone of -reactions, the formation of a cake-like residue where

'INDIAN MINING JOURNAL, Special Issue, 1957

the surface has become very much depleted of zinc oxide with consequent increase of nickel oxide activity.

The investigation was then directed to a 95 per cent nickel oxide and 5 per cent zinc oxide mixture obtained by Dutta & Baneriee" by the addition of caustic soda and sodium hypochlorite in the neutra-lised spent liquor. After 1 minute of chlorination more than 99.9 per cent of zinc oxide was removed while the amount of nickel oxide removed was only 1.25 per cent.

It is, however, not always necessary that the free energy curve for the reducing agent should be be-low the curve of the metal to be reduced and the

313

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110

120

130

140

150

30

20 FIG. 3(b)—Standard free' energies of formation / temperature of chlorides involving one gram-mole of chlorine gas between —90 and —150 K. Cal. [Kellogr]

10

free energy change justify the reaction : the use of such conditions whereby the activity of the reducing agent may be increased does initiate the reaction. This is particularly true where the gaseous phases are concerned as the activity or fugacity may be effectively controlled. The reduction of CrCI, with hydrogen as developed by Maier" clearly indicates the feasibility of such reaction. The process is operated at 800°C and the free energy changes for the reactions involved are :--

o F° at 800°C H. + Cl, = 2HC1 48,350 cats (8)

Cr (S) + Cl. = CrC1,(S) —63,150 Cals (9) H,+CrC1,(S)=Cr(S)+2HCI— 14,800 cals (10)

and thus apparently the reduction of CrCI, with

hydrogen should not be possible. The limiting case where the reaction may be possible from left to right is when :

Rt In = — 14,800 at 800°C this assumes PH,

unit activity of Cr and CrCI, and thus

PHC1

log — 3.017 (11) YHA

For one atmospheric pressure when the partial pressure of HCI and hydrogen will be 0.031 and 0.969 atmosphere respectively, the reaction is pos-sible. Thus if the amount of HCI in the reducing gas is kept below 3.1 percent by supplying fresh .

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314 INDIAN MINING JOURNAL, Special Issue, 1957

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dry hydrogen and continuously removing the HCl formed, the p F° will be negative and the reduction reaction will proceed.

In many cases limitations to the completion of a reaction are set up by the conditions of equili-brium when the value of constant K is so small that only an insignificant amount of products are formed. Such reaction, however, are very important for metal winning and in fact offer the only con-venient method. The adjustment of factors which control the equilibrium can, however, be used to complete the reaction.

The measures adopted for the completion of the reaction without change of temperature are the pressure over the system, removal of one of the reaction products and/or a combination of the re-action product to form a stable compound. These principles are liberally adopted in vacuum metal-lurgy. The principles of vacuum metallurgy have already been discussed in detail by the authors". Two typical examples will however be discussed to show how physico-chemical principles can help in determining the line of work to be adopted for such processes.

REDUCTION OF ZnS WITS Fe

Kelly" studied the direct reduction of zinc sul-phide with iron, the vapour phase at 1 atmosphere being mostly zinc but a very high temperature was required for displacing the equilibrium (in the liquid phase).

Iron sulphide is formed during the reaction and at 1188,°C, a eutectic of ZnS—FeS is formed.

If the temperature is kept below 1188°C, the ,-system

ZnS + Fe (y) = FeS (0) Zn (vapour) (12)

INDIAN MINING JOURNAL, Special Issue, 1957

TABLE II

T°C PZu S mm Hg mm Hg

880

1

1.4 x

1030

10

2.0 x 10-'

1170

100

4.5 x 10-1

1170

100

4.5 x 10-'

become monovariant and the vapour pressure of the system was found to change with temperature, as will be evident from the Table II. Experiments were performed with an ore concen-trate containing 79.5% ZnS and a commercial (syn-thetic) product containing 97.7% of zinc sulphide using a vacuum induction furnace with 0.1 mm residual pressure. Practically same rate of conver-sion was observed in both the cases.

In one series of run, the following results were obtained :

TABLE III T° Time Zinc Distilled

940' 0 hrs. (after heating 33.0% 2 hrs 83.5% 3 hrs 97.3%

1000°C 45-60 min complete

A typical analysis of zinc distillate obtained from the ore concentrate was as follows :

Pb-0.11%, Cd-0.11% and rest zinc. Contamination of the distillate with zinc sul-

phide was prevented by controlling the condensation temperature so that rate of evaporation could never exceed the rate of reaction and by filtering the vapour through iron.

315

The reaction offers interesting possibilities as stages of roasting and carbon reduction are avoided and the metal is produced in one stage. Any noble metals present would be left in the residue. Pos-sibilities of experiments on simlar line with zinc concentrate from Zawar Mines under reduced pres-sures have already drawn the attention of the authors.

DEVELOPMENT OF PIDGEON PROCESS

The principles of increasing the partial vapour pressure of the desired component by converting the other component into a material of lower vapour pressure is exemplified in the Pidgeon process for the production of metallic magnesium.

The basic methods exploited for the production of magnesium metal by thermal reduction are (1) reduction of MgO by carbon and (2) reduction by a metal or metallciid, the oxide of which is non-volatile at the temperature of reaction.

For the reduction of MgO by carbon according to equation MgO+C< >C0+Mg (g), a high tem- perature of 1900°C has to be applied to make the reaction proceed to the right, as at lower tempera-tures CO and Mg combine to reform MgO". This difficulty of reverse reaction was, however, over-come by Hansgirg" through shock cooling the equi-librium mixture at high temperature, by blasting with large excess of cold hydrogen. The product so obtained was, due to interference of carbon monoxide, in the form of a fine powder of sub-microscopic particle size and of pyrophoric nature, the melting of which was risky. It required to be distilled either in vacuum or in presence of an inert gas, thus sacrificing the simplicity of the process.

A second system for reduction was, therefore, searched for where magnesium metal will be the only gaseous product and the product is not ob-tained in fine pyrophoric powder. Free energy considerations, keeping the above mentioned con-ditions in view, advocated the use of silicon and aluminium for commercial production, though ferro-sililon is the mostly used material. The action is represented as

2MgO+Si = 2 Mg (g) + According to Doemer" Pm, was 0.19 mm Hg

and 0.38 at 1200°C and 1300°C respectively. According to Schneider and Hese° it was however, 1.9 mm at 1200°C but they stated that magnesium existed as orthosilicate (forsterite). Forcing the re-action to the right by continuous removal of magne-sium was therefore not found to be commercially feasible.

In an attempt to increase the vapour of mag-nesium by keeping the activity of the other product silicon dioxide extremely low, the well-known Pidgeon process came into existance, wherein dolo-mite was used and stable calcium silicate was formed at moderately high temperature (1100-

1300°C) by reaction with lime according to the following equation :

2Mg0+2Ca0+Si(Fe) =2 CaO, Si02 +2Mg(g)+(Fe) (14)

The rapid distillation rate in the Pidgeon pro-cess was later found by L. M. Pidgeon and J. A. King" to be due to high vapour presure of mag-nesium under the stated conditions being 10-1-30.2. mm Hg within the temperature range of 1100 1190°C.

It is thus seen that thermodynamic data help in understanding the reaction mechanism and in developing new methods for metal extraction. Even in cases where the available thermodynamic data cannot be direotly applied, it gives informaton about the adjustment of factors like pressure, removal of the reaction products etc.. which can be profitably used to take a reaction to completion.

ACKNOWLEDGMENTS

Our Ixst thanks are due to Dr. B. R. Nijhawan, Director, National Metallurgical Laboratory for his valuable comments.

REFERENCES

1. GOODEVE, C. F. A general discussion on the physical chemistry of process metallurgy. Faraday Society. 1948, pp. 9-23.

2. Lecture on physical chemistry of steel making (Insti-tute of Metals, India, 9th Annual Meeting), Lecture No. 1, physico-chemical principles by T. Banerjee.

3. A general discussion on the physical chemistry of pro--cess metallurgy, Faraday Society, 1948.

4. WELCH, A. J. E. Principles of extraction and refining of metals, (Instt. of Metallurgists), March 1951. Refresher course Lecture 1950.

5. ELLINGHAM. H. J. T. J. Soc. Chem. Ind. 63 (1944) 128. 6. RICHARDSON, F. D., and JEFFES, J. H. E. J. Iron &

Steel Inst. 160 (1948) 261. 7. KELLOGG, H. H. J. of Metals, 188 (1950) 862. 8. KFTLEY, K. K. U. S. Bur. of Mines Bull. 384 (1935),.

406, (1937) 407 (1937). 9. KRISHNAN, R. M., CHAT-TONER, A. B., KHANAN and

NISHAWAN, B. R. N. M. L. RR/37/54, March 1954, paper contributed to the Symposium on 'Non-Ferrous Metal Industry in India'.

10. SHARMA, R. A., KAPOOR, A. N., and CHATTERJP:E. A. B. N. M. L. RR/75/56, July 1956.

11. SHARMA, R. A., BHATNAGAR, P. P. and BAN ERJEE, T. J.. Sri. I. Bee. 15B (1956) 378-382.

12. Durra, R. K. and BANERJEE, T. 'bid 11B (1952) 54. 13. Mama, C. G. U. S. Bur. of Mines Bull. 436 (1942). 14. BHATNAGAR, P. P. and BANERJEE, T. Vacuum-a new

tool in extractive metallurgy, N. M. L. LR 25/54,. paper contributed to the Symposium on 'Non-ferrous Metal Industry in India'.

15. KELLY, K. K. U. S. Bur. Minos. Bull., 406 (1937). 16. GROSS, P and WARINGTON, V. The physical chemistry

of process metallurgy. Faraday Society, 1948. 216. 17. SLADE, J. Chem. 93, (1907) 327. 18. HANSGIRG. German Pat. 529, 120. 1029. 19. DOERNER. U. S. Bur. Mines Bull. P. July 1937. 93. SCHNEIDER. and HASSE. Z. Elertrorhem, 48 (1940) 24. 21. PID';EON, L. M. and KING, J. A. Transactions of

Faraday Sor. Symposium on the physical chemistry of process metallurgy, 1948. p. 197-206.

316

INDIAN MINING JOURNAL, Special Issue, 1957'