UDC 669.046.58 2 : 532 f!irst Yukawa Memorial Lecture*

Drops and Bubbles in Steelmaking

By F. D. RICHA RDSON

Professor F. D. Richardsun, D. Sc, Ph. D., F. R . S, Professor of Extractive M etallurgy, Department of M etallurgy, Imperial Col lege ofScienee ancl T echnology, London, U.K.

I am greatfy honoured that you should have invited me to this meeting ojyo ur J apanese fron and Steel institute, in order to give the First Yukal a lv/emorial L ecture. But i am here today not only as your guest lecturer, but also as a M ember of Council oj our British fron and Steel Institute in L ondon . So our President has asked me to convey his warmest greetings to y our Institute and its members, and to tell you that our Council JeeL particularly pleased that y ou have invited one of them to give this Yukawa L ecture. It was in the period when Mr. Nlasao Yukawa was President of the fron and Steel institute of J apan that our two institutes in Tokyo and London began to co-operate on the cordial basis that now exists between us. In i 963, a delegation oj senior technical p eopleJrom the British fron and Steel industry, sponsored by our Institute, came to Japan under the leadership of Mr. W. F. Cart­ w right, and were made most welcome through the good offices of Mr. Yukawa. In the JoLLowing year, a J apanese delegation came to the United Kingdom under the leadership of Mr. Yukawa, himself, and spent nearly three weeks visiting steelworks and research laboratories in Britain. W e were aLL delighted that during this visit, Mr. Yukawa accepted the award of H onoraryl M embership oJ our institute. H is death in i969 was deeply regretted by all who knew him and admired his marry achievements. W e aLL know the wide ranging metaLLurgical work which Mr. Yukawa carried out during his long career with the Yawata fron and Steel Company. H is main interest lay in iron and steel production, and he made an enormous impact Oil the Japanese iron and steel industry. He was responsible Jor the worLd'sjirst large and efficient seashore steelworks, and his ideas have influenced iron and steel production throughout the world. Very oJten men oj great practical perception have little regard Jor basic knowledge or for those academic institutions in which basic knowledge is created. But Mr. Yukawa saw the importance of university work and as President of the Japanese Society Jor Engineering Education, he tried hard to improve education over the wllOlejield of engineering. H e certainly approved of the growth of metallurgical research in both industry and universities and recognised the importance of basic knowledge. This knowledge which is so painfully won in our universities is often taken for granted by industry, and often neglected. Thus, despite the Jact that it provides the bases for all thought, argument, and discussion of new processes and is the springboard from which the inventor leaps into the unknown. So I have chosen for this Yukawa M emorial L ecture, a subject which is of growing academic interest and at the same time, of great industrial application, the use and nature of drops and bubbles in steelmaking.

1. Introduction us drops of metal dispersed in a foam of slag and gas In steelmaking, we make tremendous use of drops so that the true metal surface available for reaction and bubbles. The old open-hearth process depends with slag and gas is I 000 or so times greater than the on bubble stirring of both metal and slag ; the Bes­ nominal metal-slag interfacial area given by the cross­ semer converter gives us many drops of metal dis­ section of the converter. persed in slag; the top-blown oxygen converter gives Drops and bubbles are also widely used in the

* Delivered by Prof. Richarclson in the evening of October 18, 1972, at Nagoya University, agoya.

First Yukawa Memorial Lecture [ 369 ) ( 370 ) Transactions ISI1. Vol. 13. 1973 la ter stages of refining . In stream degassing, the Such bubbles rise with velociti es which a re pro­ metal is dispersed into a spray by the emergent gas portional to the square root of the diameter and be­ and the droplets react with their surroundings at the hind them they carry wakes as shown in Fig. I . In low pressure obtaining in the tank. In argon-purging, a bath of metal, such bubbles cause vertical mixing gas bubbles react with liquid metal a nd in electroslag by carrying up liquid in their wakes, which are ex­ refining, drops of metal react with liquid slag as they changed only slowly with their surroundings as vor­ form on the electrode a nd fa ll to the pool of metal tices are shed. They a lso cause lateral mixing by beneath. elearing the liquid through which they pass. In The whole purpose of using drops a nd bubbles is to swarms such bubbles tend on the one hand to break increase the surface areas over which reactions take up due to turbulence, but on the other to join up, be­ place, per unit volume of reactor, so as to maximise cause large bubbles sweep up small er bubbles which output a nd minimise capita l cost. Indeed , it is true they overtake, by drawing them into th eir wakes as to say that even today, steel would be much more ex­ illustrated in Fig. I. pensive if our processes depended entirely on the rates Bubbles rise through liquid metals in various smelt­ at which massive phases, as distinct from dispersed ing and refining operations a nd in gas purging. We phases, can react. With such important and growing are, therefore, interested in how these bubbles react use of drops and bubbles it is important that we should with the liquids through which they pass, a nd how understand how they behave and what factors, they excha nge solutes with them. From the stand­ chemical a nd physical, determine the rates of the re­ point of absorption, the bubble consists of two pa rts, actions in which they take part. the spherical cap over which flow is lamina r, and the circul ating wake which excha nges liquid with the II. Bubbles in Metals bulk phase relatively slowly. As the bubble rises, W e metallurgists find bubbles of a ll shapes and fres h elements of liquid arrive at the top a nd slide sizes in our processes. Only very sma ll bubbles a re over the surface from top to edge. Since now is spherical. As we can see from Photo. I , which shows leminar over the cap, the solute gas is transferred by bubbles in water, they begin to deform when they ex­ diffusion at right angles to the direction of flow. ceed 0.2 em in diameter and to adopt the spherical The rate of transfer can be expressed in terms of a n cap or mushroom shape, onee they exceed a em in average mass transfer coefficient, the bubble area, a nd diameter. The geometry of full y developed spherical concentrations as indicated by the equation, cap bubbles is independent of their sizes, a nd their Ii = k.llA(Ch-Ci) ...... (1) shap es a nd velocities are virtually independent of the liquids through which they rise, whether these are where Ii is the total flux over the cap, A is the a rea, kg mercury, steel, water, or slag. is the mass transfer coefficient in the metal, a nd Cb a nd

o o o

d. = 0.23 em d. = 0.43 em d. = 0.54 em I e ~ ,'f'----) Q >~\ ~ I ) \ d.=O.71 em d. = 1.06 em d. = 1.42em I \ I \ I (\ \ II" 1~ \\ I \ 1/\ \ I ~ \ !.....I '- ~\ d. = 1.61em d. = 2.14 em d. = 2.27 em I \ I \ \

I1 \\\ I \ \

Fig. I. i\ spherical cap bubble rl smg

d. = 2.77 em d. = 3.67 em d. = 4.49 em thro ugh a liquid, with a rrows showing directions of flow rela­ Photo. I. Bubble shapes in water for diff

First Yukawa Memorial Lecture Transactions ISI1, Vol. 13, 1973 ( 371 )

Ci a re bulk a nd interfacial concentrati ons in the metal. When transport in the meta l is rate controlling, Ci is Ill. Bubbles Crossing Interfaces a lmost equal to the concentration in equilibrium with Large bubbles a re commonly used as a means of the pressure of solute gas in the bubble. helping mass transfer across a n interface between two Ba ird a nd Davidson ,2) fr om a considera tion of liquids. Open-hearth steelma king provides a class ic la mina r fl ow a nd diffusion in the liquid passing over example. The events which occur when a spherical the cap , have developed the foll owing equa tion for cap bubble passes through such a n interface are shown the mass transfer coefficient, in Photo. 2 for the case of mercury and water. The

4 bubble first pushes a large dom e of m etal into the up­ k", = 0.975de- / D'/2g '/4 ...... (2) ' per liquid ; it then carries throug h into the upper where de is the dia meter of a spheri cal bubble with a phase a skin of liquid metal whic h dra ins away at the volume equa l to tha t of the spherical cap bubble (the base, then breaks, and finall y produces a fin e spray of so-called equivalent spheri cal bubble), D is the dif­ metal in the upper phase. In addition, the bubble fusivity in the metal, and g the acceleration due to produces waves and ripples over a n a rea much la rgel' gravity. This equation has been tes ted fo r so lutes in tha n its own cross-section. Th is series of even ts looks water and alcohol and for oxygen in liquid silvel-3 ) a nd so complicated that one may justifia bly wonder wheth­ it is correct to within about 20% . In a n experiment er mass transfer coeffi cients can be related in a re­ with rising bubbles one can, of course, only measure li a ble manner to the conditions of bubbling. the total transfer through both cap a nd wake. But, A number of reactions involving liquid metals have from work in which bubbles have been held stati onary been studied in the presence of bubble stirring, no­ in downward flowing water, with plastic caps pre­ tably the transfer of In, Zn, a nd Cd from mercury venting transfer over the fronta l surface, it looks·) as into aqueous oxidising solutions7- 9 ) a nd 0 (' tha llium tho ug h tra nsfe r from the wake accounts for about from molten lead into a molten salt. IO) 30 % of the tota l. U nfortuna tely, in a n experiment of The chemical equations are, th is nature, the fl ow over the cap is inevi tably retard­ ed by the ri gid plasti c cover, so this can only he ac­ [In J+3 / 2(Hg~ + ) = (In3+)+3Hg ...... (3) cepted as a rough value. [In) + 3(Fe3+) = (In3+)+3(Fe2+) ...... (4) One would , of course, like to apply this knowledge [ZnJ+2(Fe3+) = (Zn2+)+2(Fe2+) ...... (5) to process situations where we have streams or swarms [C d) = (Pd2+) = (Cd 2+)+ [Pb) ...... (6) of bubbles. Although we know ra ther little a bout [TIJ+ 1/2( Pb2+) = (TI+)+ 1/2[Pb) ...... (7) swarms, it is likely tha t provid ed the gas content of the liquid does not exceed a bout 10 % (i.e. is not so From work on the las t two reacti ons, we find values great that flow is restricted between the bubbles) the for the mass transfer coeffi cients k", in th e lower metal mass tra nsfer coefficients wi ll be much the same as for phase and for kw in the upper aqueous or salt m elt single bubbles. W e can, therefore, predict purging phase, wh ich conform to the fo ll owing equations, efficiencies, but unfortuna tel y we have nothing to compare our predictions with for th ere a re no com­ k w ex: 1°·5VO.4 2(D /v)0 .27±0.5 ...... (8) ple te d ata with bubble sizes for practi cal opera ti ons. kM ex: fO. 5 V O. 4 2D o.~) ...... (9) W e can infer, however, tha t the bubbles we used to observe bursting in open-hearth steelma king a re about where f is the bubble frequency per unit area of in­ the size we would expect if they nucleated on the terface, V is the individual bubble volume D the dif­ furnace bottom a nd rose through the metal coll ecting fu sivity in the appropriate phase, a nd !.I th ~ kinematic carbon a nd oxygen at rates determined by mass viscosity of the upper phase. These coefficients a re tra nsfer from the bulk m etal, at the bulk carbon and rela ted to the interfacial area which exists under oxygen concentrations actually measured .a> We can static conditions: the true area increases with bubble a lso expect (Eq. (2» purging efficients to be propor­ size a nd freq uency and this is one of the factors causing tiona l to D I/2 for the gaseous solute, when tra nsport in the mass transfer coefficients to increase. the m eta l is rate controlling and this will commonly" Because both 1 a nd V enter Eqs. (8) and (9) as the be the case at I 600°C. It is interesting to note that 0.5, or a lmost the 0. 5 power, it follows that both kM big savings in purge gas can in principle be achieved a nd kw a re proportional to the square root of the by using low pressures over the m etal. With low volume rate of gas flow . For open-hearth steelmaking total pressures, for the same partia l pressure of solute (without inj ection of oxygen) we may write for the gas in the bubbles, each molecule of purge gas carries rate Ii at which iron oxide is transferred from slag to away more solu te gas and a t the same time produces metal more gas- metal interfacia l area for mass transfer or 1m )-1 reaction. However, the total press ures in the bubbles 11= ( k + k A( cM-mC.~l ) ...... ( 10) near the interface are higher than the equilibrium '" Sl pressures, because of the in ertia of the meta l being where kM and k.~l are mass tra nsfer coefficients in pushed back by a rapidly expanding bubble. metal and slag, A is the nomina l m etal- slag interfacia l

.. Transport on the gas side might become important when the partia l pressure of solute gas in equilibrium with th e metal, becomes f'xct'ed ingly small because of Seiverts L:lw.

First Yukawa Memorial Lecture ( 372 J Transactions ISIJ, Vol. 13, 1973

(a ) 0.000 sec (b) 0.166 sec

( c) 0.1 70 sec (d) 0.176 sec

(e) 0.184 sec (0 0.196 sec Photo. 2. Stages in the transit of a spheri cal cap bubble 5 cm in basal diameter, from mercury into acidifi ed water" '

a rea, CM a nd Cst are concentra tions of FeO in m etal driving force in terms of concentra tion, and this is why a nd slag, a nd m is th e ra tio (CM /Cst) at equilibrium. steelma king by th e old open-hea rth process was com­ The rate of tra nsfer of iron ox ide from slag to m etal is monly referred to as being autocatalyti c-doubling the virtually equal to the rate of removal of carbon [rom concentration of FeO in the slag could quadruple the the metal, because th e oxygen concentra tion in the rate. There is little doubt that the surges experi enced metal rises very little during decarburisati on. Thus with top blown converters, a rise from somewhat simi­ (fi /A) is proportional to the ra te of passage of gas la r reasons. through the slag-metal interface. We m ay, there­ R eturning now to the list of reacti ons (3) to (7), the [ore, wri te, first three have something special about them when they occur at high driving forces, tha t is when the ( ~ ) oc ( ~ r \CM - m CSt ) ...... (11 ) reacting phases are fa r removed from equilibrium. Under these conditions, the interface becomes turbu­

( ~ ) oc (C M - mC.,., )2...... ( 12 ) lent even in the absence of stirring. This turbulence a rises from interfacia l phenomena, which are de­ Thus, (fi /A) b ecomes proportional to the squa re of the scribed in my second lecture a t this m eeting. It oc-

First Yukawa Memorial Lecture Transactions ISJJ, Vol. 13, 1973 ( 373 )

curs in such a vital region that even when stirring is vigorous, the mass transfer coeffi cien ts are much enhanced. With bubble tilTed systems kw can be in creased by factors up to 3 times and kft/ by some 20 to 40%, depending on the speed of bubbling. Even under these conditions however the influence of bub­ ble fr equency a nd volume a nd of diffusion coeffi cients, is much th e same as a lready described for sys tems without interfacial turbulence. The mass tra nsfer Eqs. (8) a nd (9) can be applied to the case of a boiling open-hearth where, as already stated, the ra te of decarburisation to virtually equal to the ra te of transfer of FeO from slag to metal. When this is done, we find the ma in resistance to tra nsfer of the FeO li es in the metal phase. The ra tes we predic(5) for various FeO concentrations in the slag can then be compared with tho e observed in practice. The predictions are equal to about 67 % of the observed ra tes, * a nd this is qui te good agree­ ment, when account is taken of the fa ct that the rates of gas evolution per unit a rea of slag a nd metal in­ Photo. 3. Circulation patterns within a drop of wate r supported terface are one hundred or more times greater in in upward fl owing casto r o il , ta ke n from Kintne r'3) open-hearth steelmaking than in the laboratory ex­ periments on which Eqs. (8) and (9) are based . W e are now able to think at least semi-quantita­ tively about mass transfer between slag a nd metal I phases stirred by bubbles, a nd we should be able to ,, I , I apply our knowledge not only to the old open-hearth 0 I I which is now ra ther uninteresting, but a lso to arc I furnaces a nd to counter current refining operations such as those envisaged by Wornerll) and Thring. 12)

IV. Metal Drops The ra tes at which m etal drops react with their

Solid sphere Circ lil atin~ drop surroundings a re determined by three factors : (1) The rates a t wh ich reacta n ts a nd prod ucts Fi g. 2. Graphical illustra tion o l' ve locit y profiles and circulation for a soli d sphere a nd a circ ulating drop fa lli ng thro ugh move between the bulk continuous phase a nd the a li ghter liquid metal in terface (2) The ra te of chemical reaction a t the interface 1.0r----,-----.------,------, (3) The rate at which reactants a nd products 0.9 move between th e surface a nd the interior of the drop. The mass tra nsfer rates both within the drop and 0.8 in the continuous phase surrounding it, depend on

~ 0.7 flow conditions. We shall first consider drops falling U ~ o ~--~------~ through lighter liquids- the kinds of drop that occur x in the slag in converters a nd in the electroslag process. " Such drops deform in greater or less degree as they fall , depending on their size and interfacial tension. Once their diameter exceeds about 0.5 mm, internal circulation is set up inside the drop by the viscous forces operating at the interface. The drops whieh are perhaps of most interest to us, are I to 5 mm in diameter and have Reynolds numbers in slag between 100 and I 000. Thus, they have circul ating wakes 200 300 400 from which vortices are torn as they fall. The kind Lead nit rote millimole litre- I of internal circulation which occurs is illustrated in rig. 3. Extraction curves for merc ury drops of various dia­ Photo. 3 and Fig. 2, which show a drop of water held meters falling thro ugh 35 c m of aqueous phase of IeP. sta tionary in upward flowing castor oi l with alumina The me rc ury contained cadmium at 0.12 mole litre- L and this was oxidised by lead nitrate a t the concentra­ particles revealing the flow. There is, of course, some tions indicated. distortion caused by differences in refractive index.

* The calculatinn in (5) has to be corrected fnr th e new Eq. (9) above, fo r k", and the revised diffusivity o rIn in merc ury (9).

First Yukawa Memorial Lectur'il [ 374 J Transactions ISIJ, Vol. 13, 1973

This flow inside the drop obviously has a profound Fig. 3, the ra te is controlled by tra nsport in the con­ effect on mass transfer in the meta l, a nd it also in­ tinuous phase a nd along the hori zontal pa rt by flu ences m ass transfer in the continuous phase. transport in the meta l phase. Experiments with aqueous and organic systems The mass tra nsfer ra tes observed in the meta l have led to various models a nd hence equa tions, for phase a re 100 times faster tha n for a rigid unstirred mass tra nsfer within and without the drop. But in drop and a bout half as fas t as given by the model the systems investiga ted, the drop surface tensions a nd developed by Handlos a nd Ba ron 15 ) for a circul a ting densities have been very sma ll compa red with those and pulsating drop. For mass transfer in the con­ for meta ls. tinuous phase the ra tes a re a bout 3 times those for a Attempts have, therefore, been made to study the ri gid drop a nd about ha lf those given by the Higbie behaviour of drops of mercury amalgams reacting modeP6) for the fl ow condition shown fo r a fully with aqueous phases a nd of molten lead reacting with circul a ting drop in Fig. 2. It thus a ppears that for molten salts. Reacti ons (4) a nd (5) a bove have been meta ls with high surface tensions circul a ti on inside studied ,14) together with the a ma lgam reacti on, th e drops, at least for short times of fa ll , is not as vigorous as for the aqueous a nd organi c systems for [In]+ 3/2(Pb2+) (In3+ )+ 3/2[Pb) ...... ( 13) = which the two m odels referred to are more successful. and the reacti on, From work so far done, we cannot predict with great accuracy, quite apa rt from the possible com­ [Zn]+(Pb2+) = (Zn2+ )+ [Pb) ...... ( 14) plicati on of interfacia l turbulence. But we can reach some interesting concl usions about th e refining of where the zinc is dissolved in molten lead a nd the lead drops. In such cases, we a re genera ll y concerned ions a re in the LiCI + K C I eutecti c. The drops have with low concentra ti ons of impurities, so we opera te been 1 to 4 mm in dia meter and the dista nces and mostl y under conditions where tra nsport in the meta l times of fall up to 35 em a nd 0.7 sec. drop is ra te controlling. For such a refining pro­ A typical extracti on curve i shown in Fig. 3 for cess, provided there a re no chemical restricti ons, a mercury drops of different sizes conta ining cadmium, fa ll of I metre should be sufficient to achieve a n ex­ oxidised by aqueous lead nitra te. The extracti on is traction of 99% wi th drops ra nging from 0.5 to I mm shown as a fun ction of lead nitra te concentra ti on for in diameter depending on the viscosities and differ­ three different drop sizes. At low concentra ti ons of ences in density of the continuous a nd dispersed phases. oxidising agent in the column the percentage extrac­ These concl usions obviously have releva nce to el ec­ tion from the drop increases linearl y with concentra­ tros lag refining, a nd the ra tes a t which droplets a re ti on until it reaches a maximum a nd then becomes decarburised when dispersed in slag in converters. indep endent of this concentra ti on . This kind of Furthermore, they suggest tha t the refining of metal behaviour is consistent w ith tra nsport being ra te con­ by spraying it through a molten salt mi ght be pos­ trolling and it can indeed be shown fr om d a ta avail­ sible with quite sma ll reactors at least for some non­ able on exchange current densities for the se pa ra te ferrous meta ls. electrode reactions, The vigour of circul a tion set up in a drop fa lling [Cd) = (Cd2+) = 2e ...... ( 15) through a gas is much less tha n in one fa lling throug h a liquid. Initia ll y, the stirring in th e drop d epends [Pb) = (Pb2+) = 2e ...... ( 16) on th e way it is made, but after a fa ll of a bout 20 sec tha t if electrode kineti cs were rate determining the a ny initia l stirring dies away. A stead y state of cir­ extraction rates would be more than 103 times the cul a tion is then a tta ined , this depending on the drop obse rved ra tes. size, the gas press ure, a nd the d rop velocity.l7) But in Al ong the sloping pa rt of the ex tracti on curve in processes such as spray steelmaking or steam degassing, this stead y sta te i. never a tta ined, because the dis­ ta nces of fa ll a re onl y a few feet a nd th e drops a re vigorously stirred by gas bubbles whi ch nucleate within them a nd burst from their surfaces. In m a ny cases, the bubbles cause the drops to disintegra te a nd so greatl y increase the gas- meta l interfac ia l a reas. A good deal of work has been done on d rops re­ acting with fl owing gases, by means of the levita ti on technique, in which the drop is supported a nd heated simulta neously in a vertical tube by RF induction. Photograph 4 shows a drop of iron + carbon a lloy supported in this way in fl owing oxygen, with the levita ti on coil hiding the lower part of the drop . A bubble of CO has just burst from the surface. This burst is too rapid to record a t the speed of the Photo . 4 . A n ejecti on fo ll owi ng the burst of a bubble from a photograph (which is from a film ta ken a t 2000 levitated d rop of iron+ carbon a ll oy held in fl owing fr ames per sec) a nd one can onl y see the j et of m etal oxygen IS ) thrown in to the gas immedia tely foll owing the b urst.

First Yukawa Memorial Lecture Transactions lSI], Vol. 13, 1973 [ 375 )

From work that has been done of the evolution of a metal with an easily removable gas, such as hydro­ CO from drops of iron and nickel containing car­ gen, in order to obtain sufficien t break up of the bon,18,19) it looks as though gas nucleation d oes not stream to permit elimination of more slowly diffusing require the very high supersaturation pressures of solutes present at low concentrations. about 104 atm which can be predicted by the ap­ plication of Becker and D oring's bubble nucleation 1 alm theory. These levitated drops can be supercooled some 100° to 200°C, so they are very free from in­ Silver clusions sui table for nucleating solids, and probably a lso free from inclusions suitable for nucleating gases. Nevertheless, nucleation undoubtedly occurs at super­ saturation pressures ranging onl y from 10 to 40 atm. This is fortunate for those w ho need to eliminate gases

fr om metal, but the phenom enon is not, at present, 200 mmHg understood. In practical operations, heterogeneous nucleation which can, in theory, occur at very low super-satura­ tion pressures, is likely to be much more important than homogeneous. This appears to be the case in stream degassing20 ) which has recently been under study at Imperial College by Mr. S. Mizoguchi from Nippon Steel, in cooperation with Professor A. V. Bradshaw a nd Dr. D . G. C. Robertson.2l) Some fin e experiments have been done and I re­ gard the project as a very happy example of cooper­ ation between our two coun tries. Mr. Mizoguchi fig. 4. Experimental arrangement for studying tiJe break up has been studying the removal of oxygen from streams of streams of liq uid sil ve r loadecl with oxygen, on entry of liquid si lver at I 100°C flowing fr om a sm a ll cru­ into a low pressure chamber 20 ) cible into a low pressure or vacuum chamber. His experimental arrangement is shown in Fig. 4. The liquid metal is saturated with oxygen at the appro­ priate partial pressure in the crucible and then a ll ow­ ed to pass into the low pressure chamber via a nozzle, designed to give no separation of fl ow. The kinds of streams obtained with a nozzle 0.48 cm in diameter, and 1.27 cm long are shown in Photo. 5. With the metal saturated with oxygen at I atm pressure so that it contains 16 .7 cm3 of gas at STP per cm 3 of sil ver, no nucleation of gas is obtained at 400 mm tank pressure ( Photo. 5 (a)); nucleation begins at about 300 mm and a foamy stream is produced at 100 mm ( Photo. 5 (c)); at 10 mm pressure the stream is completely broken up ( Photo. 5 (d )). When the si lver is saturated with air so that the oxygen content is approximately halved (7.5 cm 3 of gas at STP per cm 3 of m etal) nucleation occurs much less readily: there is virtuall y none at 100 mm tank pressure ( Photo. 5 (e)) and very li ttl e at 10 mm ( Photo. 5 (f)). It is interesting that nucleation starts in the nozzle, and probably occurs heterogeneously on the sides of the nozzle. H omogeneous nucleation possibly ac­ counts for the formation of bubbles in drops produced in a disintegrated stream such as that shown in Photo. 5 (d ). The pressures in the nozzle, just above the exit, never fall below 200 mm, even when the tank pressure falls to 10 mm, and once nucleation occurs readily as in Photos. 5 (b), (c), a nd (d ), the proportion of gas in the metal at this point is a lways about 13% by volume. Photo. 5. Streams of liquid silver issuing from a nozzle 0.48 cm The behaviour of the stream is surprisingly sen­ dia., into a low pressure chamber, the metal saturatecl with oxygen at I atm in (a), (b), (c), and (d); at 0.2 sitive to the concentration of dissolved gas. As it is a tm in (e) and (f). Tank pressures: (a) 400 mm ; (b) important in d egassing operations to obtain good 200 mm; (c) 100 mm; (d) 10 mm; (e) 100 mm; (f) nucleation, it may in some cases, be d es irable to load 10 mm, from Mizoguchi2l )

First Yukawa Memorial Lecture ( 376 J Transactions lSI], Vol. 13, 1973

I hope that in th is lectUl-e I have been able to show jISI, 205 ( 1967), 1034, London. yo u the ways in which our knowledge of drops and 7) "V. F. Porter, F. D . Richardson, and K . N. Subramanian : bubbles is increasing and sugges t some directions in " H eat and Mass Transfer in Process Metallurgy ", a which this knowledge might be exploited. I beli eve symposium, Inst. Mining. Met. , London, ( 1967), p.79. 8) K. N. Subramanian and F. D. Richardso n : JIS!, 206 that better control of our dispersed phases could lead ( 1968),576, London. to better steelmaking. 9) J.K. Brimacombc and F. D. Richardson: Trans. hlSt. During my visit to Japa n, I have learned that you Nlining Met. , 80 ( 197 1), C 140, London. are addicted to short poems. I am sorry I cannot 10 ) j . K. Brimacombe a nd F. D. R ichardson: Trans. Inst. give you a Hainku or even a ] anka in pra ise of dis­ Mining Met., ( 1973), L ondon, in press. persions, so I hope yo u will accept the following short II ) H. K . Worner, F. H . Baker, l. H . Lassam, and R . Siddons: verse instead , A IME Natl. Open H earth and Basic Oxygcn Stcelmaking " The bubbles rise, the drops fall , Conference, j. ]\Ile tals, june, 1969. And steelmaking proceeds apace. 12) M. W. Thring: iron and Steel, 42 ( 1969),35. W e master the great 13) R . C. Kintner: Advances in Chemical Eng., cd. T. B. Drew, J. W. H oopes, and T. Vermeulen, ( 1963), p.70, When we control the small. Acad. Press, New Yo rk. You J apanese are in love with both ; 14) S. M. Aeron: "Mass Transfer in Liquid-Liquid System s You a re great steelma kers ." I nvolving Metal Dro ps ", Ph.D. T hesis, 1971 , University o f London. 15) A. E. Handlos and T. Baron : A.I. Chem. E.}., 3 ( 1957), REFERENCES 127. I) P. H . Calderbank: The Chemical Engineer, ( 1967), CE209. 16) R . Higbie : Trans. A. I. Chem. E. , 31 ( 1935),365. 2) M . H . 1. Ba ird and j. F. Davidson: Chem. Eng. Sci., 17 17 ) F. H . Carner and j.j. Lane: Trans. Inst. Chem. t;ng., 37 ( 1962),87. ( 1959), 162. 3) R . I. L. Guthrie and A. V. Bradshaw: Trans. Met. Soc. 18) P. A. Distin, C. D. H a ll et! , and r. D. Richardson: J IS I, AIM E, 245 ( 1969), 2285. 206 ( 1968),821, Londo n. 4) A. G. Szekely, Union Carbide, Linde Division, New York: 19) .1 . W. Mar3hall: unpublished work , Nuffield R esearch private communication. Gro up, Extracti :JI1 ]\I[ctallurgy, I mperial Coll ege, Londo n. 5) A. V. Bradshaw and F. D. Richardson: "Chemical Engi­ 20) N . A. Warner: JISI, 207 ( 1969),44, London. neering in the Iron and S teel Industry ", a symposium , 2 1) S. Mizoguch i: "A Study of the Stream Degassing Pr.:>cess Insl. C he rn. Eng., (1970), p. 130. Using thc Si lver- Oxygen System ", Ph.D. Thes i , Uni­ 6) W. C. Davenport, A. V . Bradshaw, and F. D . Richardson: versit y of London, ( 1972).

First Yukawa Memorial Lecture