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ANew Scale of Basicity in Slag and the Basicity of the Slag Containing Amphoteric *

By K azumi Mori * *

systems. According to this definition, an "" I. Introduction is either a donor of (0 1' cations) or an The acidic or basic beh avior of s lag-forming acceptor of (or anions), while a "" oxides a nd the basicity of slag have a n important is either a donor of electrons (or a nions) or an bearing on the iroll- a nd -making processes. acceptor of protons (or cations) . Based on this In most cases some ratio of the amounts of basic theory, a definition of acid-base for oxid e a nd slag oxides to those of acidic oxides has been used as systems may be m ade as follows: A a measure of basicity, but it is only an empirical is one that can donate ions, whi le an acidic measure a nd cannot be cons ider ed a rea l measure oxide is one t hat can accept oxygen ions from of chemical properties. Moreover, in the expr e'­ bas ic ox id es. Such a definition has a lready been s ion of s lag basicity little attent ion has been pa id made wit h glass systems by Sun and SilvermanC,)G) to the behavior of a mphoteric oxides such as AI "O" and wit h slag syst ems by Chi pma n a nd Chang/)_ a nd TiO". Thus, defining and measuring the slag On t he basis of t his definiti on, it is po sible to basicity in a theoretically satisfactory manner has expr eo: s t he scale of basicity or acidity for p ure been an unsolved problem. It is from t his aspect oxides by the strength of the -oxygen (M-O ) that the a uthor has an interest in the s ubject. bond. Sun,n) X) us ing thej'mal data, estimated the In the present repor t a new scale of bas ic ity is r elative M-O bond strength, which he identified given, based on the fact that t he ratio of ferric with t he scale of basicity or acidity of oxides. An to t otal iron (F e:l+/ L:Fe) in ox ide s lag is greatly analogous sca le may be expressed by t he binding affected by the change in bas icity, together with energy between a cation a nd an oxygen ion r epre­ the method of experimenta l determination of the sented by: basicity valu es and the r esults of the study on I = 2 z/a2 slag containing amphoteric oxides. where z is the cha rge of the cation and Cl is the II. General Rev iew of the Slag Bas icity dis tance between the cation and the oxygen ion. The most conventional expression of s lag basi­ The larger I val ue an ox ide has, the stronger is city, which is implied as the strength of base in its acidic prope rty ( 01' t he weaker its basic pro­

the s lag, has been 11= CO ~ CaO) Co;, Si0 2 ) or \" pe rty). The experimental value of acid-base = L: (0;, base) / L: (";, acid ). In some cases in the str engt h of oxides could be obtained f rom t he V' ratio some factors relating to the r elative acid­ coloring transition point in glass or f rom the mea­ base strength of the oxides were given in the s urement of e. m.f. in t he cell where molten g lass weight percentage.! )~) "Excess base" expr essed by was used as the .!) Grant and Chipman';) or the "concentration of free Thu" based on the fundamental definition by oxygen ions" devised by H erasymenko a nd Speight4 ) Usanovich, the definite concept for t he bas icity of could also be used as the expression of s lag basi­ pure cxides has bee n well establis hed. However, city. The common factors in these expl'essions of t his idea of basicity has not been taken into con­ s lag basicity a re that the constituent oxides were sideration in the conventional expr ession of bas icity c lassified distinctly either as acidic 0 1' as basic and in mu lticomponent systems. It is t he chief object that based on this classification the bas icity value of this paper to give a new scale of basicity w h ich was calculated in an a rithmetical ma nner from is able to deal with pure oxides as well as multi­ s lag analyses. However, s ince s uch a procedure component oxide systems on t he same ground. to express s lag basicity is not based on theoretical a rguments, it is difficult to understand what is im­ I I I. A New Scale of Bas icity and Its Measure­ plied theoretically by the basicity thus expressed. ments On the other hand, in t he purely fundamental I. A New Scale of Basicity fie ld of physica l , various definitions have been given to the concept of acid and base Because oxide s lag contains large amounts of by Arrhenius, Germann, Bronsted, Lewis, Usano­ oxygen ions, it seems most r easonable to express vich a nd BjelTum. Of these definitions, Usanovich's the slag bas ic ity by the oxygen-ion activity. definition is most easily applied to oxide a nd slag The oxygen-ion activity may be obtained f r om t h e measurement of e.m.f. by the methods developed by Lux!") a nd Didtschenko and Rochow. !) How­ * Lect ure delivered before the 53 rd & 55 th Grand Lecture Meeting April 1957, 19 58 in Tokyo. Tetsu-to-Hagane ever, the measurement on general s lag systems will (Journal, Iron & Steel Institute, Japan ) Vo l. 46, No. 4 be extremely difficult. pp. 466--473 It is well-known that the ratio of ferric to total * * Assistant Professor, Department of Metallurgy, Faculty ir on (F e+ 3/ L:Fe) in oxide s lag is increased w ith of Eng ineering, Ibaraki University. increase of the basicity of a dded slag under con-

25 11 stant oxygen pressure. )- I:l) This phen omenon I. wire ..,. ------1 might be derived from the fact that the iron ratio 2. Rubber tublnJ: is aff ected by the compound strengt h of M-O bonds 3. Pinch cock r elating to the constituent cations other than ferric 4, Gil S in let and ferrous ions. Since, as described in the pre­ S. !'t·Pt. Rh thermocouple vious section, the M-O bond strength itself is the 6. As bestos head scale of basicity for pure oxides, this phenomenon 7. Nicke l plate for radiation has been chosen in t he present pa per to meas ure shie ld t he bas icity of slag, as was s uggested by La rson 8. Gland for rubber packing a nd Chipman.!'!) 9. -c ooling tube 10. Semi·sinter corundum tu b,: Since oxygen ions a r e common to a ll the com­ 10 -'------II . Ere ma hea l ing e lement s ponents in an ox id e s lag syst em, it is adequate to ~ ------II 12. Iso Ji le board Li se ionic f l'action of cation s in the expressions of 11------1------1 2 13. A lu ndum board --- 13 the concentration of the added s lag, of which the 14 . Alumina tu be basicity is to be determined. Then the concentra­ 15. Thermocouple protection 15 ------14 tion of the added s lag ( C ) is represented by the tube 17 ------expression: 16. Platinum crucible --- 16 18 -- - - - ..... • 17 . Alum ina

C = LM / (LFe + LM) (1 ) 18. Chamotle -- - 19 LFe = Fe2+ + Fe:l+ 19. Alundum tune

LM = Ca2+ + SiH + Ti4 + Al:l+ + 20. S tee l plate

21. Water·cooled Since in general the ratio: \('sse l ( 2 ) 22. Gas outlet.. cha nges with C, it is not adequate to choose r at some concentration as the scale of basicity. There­ fore, the gradient of an roC c urve at the infinitely dilute concentration, or (dr / dC)c 0' was ch osen as the scale of basicity. In t h e concentration ra nge where r cha nges linearly with C, (dr/dC)c 0 m ay be put in t he following form: Fig. 1 Experimental furnace (dr / d C)c -0 = (r - ro ) / C ( 3 ) which is independent of e. H er e i'" r epresents t he in Fig. 1. The h eating element cons isted of s ix r of t he pure iron oxide. Erema rods (SiC) . A long the center of the fur­ According to the study by La rson a nd Chi p­ nace was a semi-sinter co rundum tube. A sm all man,I :!) a linear relations hip between rand C seems p latinum crucible with a capacity of 1 cc was to be approximately establi s hed below 20-25 m ol";, s Li spended on a platinum wi re inside the tube. In of the added s lag. In t he present study, based on t his crucibl e 1. 5 g sample was melted. t he ass umption of linear relationship between r Samples were prepared by mix ing fe rrous oxide a nd C, the scale of basicity is given by the equa­ with the slag whose bas icity is to be determined. tion: F errous oxide material was made by melting ferric B = ((r - r o) / C) x 10 ( 4 ) oxide reagent in an iron crucible. The CO2 -CO gas m ixture, which passed through in which the numera l 10 ser ves to make the B chloride, at 500°C and phos phor ous valu e di sposable. pentoxide, was a nalyzed by the Orsat gas a nalysis appara tus. After the temperature of the furn ace Experimental Method was r aised to 1150°C, the platinum crucible was The experimental procedure used in this study s us pended and the furnace tube was sealed. Then is as follows: the iron oxide with sma ll additions t he tube was evacuated and immediately the pre­ of the s lag whose basicity is to be determined pared gas mixture was introduced into the furnace was equilibrated with a gaseous atmosphere of CO2 tu be. At the same time t he temperatu re was again /CO= 13.3. (Po"= 2.87 x 10-'; atm ) at a temperature raised ra pidly to 1480°C. All runs were conducted of 1480 °C, and the quenched samples wer e s ubject for about 3 hI' S, du ring which time the gas mixture to chemical analysis. passed at the (-Jow r ate of 100 cc per min. When The design of the furnace is s hown schematically the s lag had reached an equilibrium, t he crucible

26 was quenched by being lowered rapidly until it SiO , reached the water-cooled vessel at the bottom of the furnace tube. The sample was crushed care­ fully in an agate mortar and analyzed for ferrous a nd ferric iron. From the analytical res ults the basicity value was calculated by the eq. (4) . IV. Basicity of S lag Containing Titanium Ox ide The studied systems a re as follows: Ca O, SiO"

TiO, Ca O-SiOt, CaO-TiOt, SiO,-TiOt , CaO-SiOz-TiO,. Samples used in this study cons isted of 80 mol% FeO and 20 mol";, slag in question. Fig. 2 shows the basicity of three binary systems. The values of the basicity for CaO, Si02 and Ti02 are s uch as would be expected from t he scale of basicity given by Sun';)K) and the metal-oxygen bond strength ( I). The acid-base property of Ti0 as a pure 2 -TiO,(%) ox ide is rathel' s imilar to that of SiO" t hough the of TiO, is somewhat weaker t han - - Obser ved that of Sial' ----- Caiculuted by the e mp ir ica l equation ( 5) In the systems CaO-SiO" a nd CaO-TiO", t he basicity changes a pproximately linear ly with mol";,. Fig. 3 Isobasicity lines in the CaO-SiOz-TiOz system Comparison between t hese two systems reveals that

Ti02 has the same behavior as SiO". On the other not change with additions of Ti02 • At lower ratios hand, in the system SiO ,-TiO" a considerably dif­ of CaO/SiOl, basicity ( B ) is increased by additions ferent curve appears, showing a maximum at nearly of TiO" along the lines of the constant CaO/SiO, 50 mol'Yo TiO,. and at higher ratios decreased. This is a very In Fig. 3 the basicity of t he CaO-SiO,-TiO, ter­ interesting fact, because it shows the so-call ed nary system is shown in terms of isobasicity lines amphoteric behavior that an a mphoteric oxide plotted on the ternary diagram (in wt";,) . Isobasi­ added to an acidic s lag behaves as a base and it city line for B = -2.2 below 40'';, TiO, coincides behaves as a n acid when added to a basic slag. with the line of constant CaO SiO" which s hows However, at higher ranges of TiO, the feature of that at t he constant CaO SiOz, t he basicity does the isobasicity curve changes in s uch a way that the bas icity rapidly draws near that of TiOz itself. For t he CaO-Si 0 ,-Ti 0 , system the empirical eq uations have been obtained as foll ows: / ~ B = - 17.95 / [1.45 + (CaO) V "-, ((Si02) + !T(Ti0 2)} J + 7.27 ( 5 ) ~ !T = gd(CaO / Si02)} • hd(TiOz)} / " 0 '" 0 = [ - 1 / {O.005 + (CaO / Si02)} + 2.350J . · 2 · 2 ~ (-0.0066(TiOz) + 0.598} ( 6 ) ., /V ., ~ "-, wher e concentrations are expressed in wt";,. These ·6 V · 6 ., ·8 equations hold good in t he concentration range 25 50 75 (an (aO 25 50 75 CaO/SiOl = 1/3-4, TiO,= 0 - 35";,. The dotted lines TiO . : mol . in Fig. 3 are isobasicity lines corresponding to the 2 empirical equations. In eq. (5) , considering the

s imilar acid-base behavior of pure TiO t to that -, of SiO" the weight percentage of Ti0 2 is put in ., ----c t he denominator of t he fraction r elating to con­ "0.. centration, being multiplied by the factor!T. This -6 ~ factor could be expressed by the product of a ·8 Sin 2;) 50 75 function of CaO /SiO, (gT ) a nd that of TiO, wt";, TiO , mol o~ (c S,Oo-TiO. s~·sl e m (hT) as shown by eq. (6) . Fig. 4 illustrates the

r elation between the factor !T and Ca O/Si02 at Fig. 2 Basicity of binary systems TiO, 20% f or the CaO-SiOz-TiO" system.

27 SiO ,

TiO , 20"" V-I--- ..:. 0.5 / '0\ o /

-', - 0.5 ~ o" o ~A I 2 0 3 (%) CaO/ SiO ,. --Obsen'ed Fig. 4 Change of the factor fr with CaO,SiO, ----- Calcul ated by the empiri ca l equati on (7) Thoug h eqs. (5) a nd (6 ) have been obtained as Fig. 5 lsobasicity lines in t he CaO.Si0 ·A l,O, empirical equations, t hey will give some s ugges­ 2 tion to the theoretical consideration of basic ity. is increased by additions of A I20 ; a long the lines

According to the curve in Fig. 4, t he fr value is of the constant CaO Si02 a nd at hi gher ratios found to be zero at CaO S iO,= 0.42. It is positive decreased. That is, the same amphoteric behavior

in t he r ange where CaO S i0 2 0.42, whi le it is of AI , 0 3 as that of TiO, h as been s hown to be negative in the range where CaO / S iO ~ 0.42. The evident. In t his case, it s houl d a lso be noted t hat former indicates t he acidic behavior of TiO", while the boundary line classifying the acidic and basic the latter indicates t he basic behavior. behavior coinc ided with t he line of Ca O / SiO ~ = l. The author thus a rrived at t his conclu s ion : Al­ As an expressic n of the correlation between t he t houg h titanium-dioxide itself is c lassifi ed as an basicity a nd the concentration, the foll owing like silica, in the te rnary system CaO­ empirica l equations were obtained:

Si0 2 -Ti0 2 ( TiO ~ , 40" ;,) it is regarded as a n a m­ B = - 17.95 [1.45 + (CaO) / photeric oxide. This amphotel'ic be havior of Ti0 2 co uld be expressed quantita tively in terms of J-r. {(Si02) + f A(A I20 3)} J + 7.27 ( 7 ) 1.4 = gA {(CaO / SiO J . h A {(AI20 3) } V. Bas icit y of Slag Containing A lumina = r -1 {0.OS9 + (CaO Si0 2) } + 0.955J . { - 0.0103(AI 0 ) + 0.S10} ( 8 ) Next the experimental r esults on t he s lag con­ 2 3 taining a lumina, which is a typical a mphoteric These eq uations hold good in t he concentration oxide, w ill be described. The studied systems were range CaO /Si02=1 3-4, AI 20 3= 0 -35%. The dot­

as follows : CaO-AI"O", Si0 2 -AI 2 0 " a nd CaO-SiO,­ ted lines in Fig. 5 a re isobas icity lines plotted from

AI 2 0 :: . The melting point of iron oxide containing the eqs. (7 ) a nd (8 ). The forms of t hese equations a lumina rose sharply with increase of a lumina are the same as those of eqs. (5 ) and (6).

content and it was impossible in this work to In eq . (7 ) the weight percentage of AI 2 0 2 is put obtain the bas icity for a concentration r a nge above in t he denominator of the fraction relating to con­ 25 mol";, AI ~ O " . Samples used in this study con­ cent ration, being multiplied by t he factor fA' Al­ sisted of 85 - 90 mol" ;, F eO and 10 - 15 m ol" ;, slag though in t his case it is likewise permitted to put

containing alumina. Al 2 0 3 concentration in t he numer ator, eq. (7) was Naturally, in t he Ca O-AI ~ 0 1 system the basicity obtained by adopting t he convent ional idea that

was decreased by additions of A I ~ 0 3 ' while in t he t he acid-base property of AI 2 0 3 resembles that of

Si0 2 -AI 2 0 , system it was increased. Si02 rather t han CaO. Fig. 5 s hows t he isobasicity lines in the CaO­ Concerning the ]'e lationship between the factor

SiO,-AI20 3 system. The isobasicity line of B = f A a nd CaO/S iO", a curve analogous to that shown - 0.2 coincides with the line of CaO /SiO, = 1, where in Fig. 4 was obtained. In this case, h ow­ t he basicity does not change with t he addition of ever, the value of fA was found to be zero at CaOI

a lumina. At lower ratios of CaO ISi0 2 the basicity Si02 = 0.9 6, and it was pos itive in the ra nge wher e

28 CaO Si0 2 ·0.96, while it was negative in the range a s a base, and if the solu tion is more bas ic, it be­ where CaO SiOz 0.96. Thus it was found that haves as an acid. At the isoelectric point, the the amphoteric behavior of Al"O" could be ex­ degree of electrolytic dissociation of the amphote­ pressed quantitatively by the factor 1.4' rite as an acid equals to that as a base and the con­ centration of undissociated molecule shows a V I . D iscussions maximum value. The concentration of the hydro­ Properties of the Newly Defined Basicity gen ion at the isoelectric point is given by; The newly defined basicity is characteristic in (10) two ways. In the first place, basicity of a pure oxide as well as that of any multicomponent oxide where }'a, k b ; the constant of electrolyti c: dis­ s lag can be expressed on the same sca le. Secondly, sociation as a cid or base in principle, values of the new basicity can be K w ; the constant of e lectrolytic dis­ obtained only by a n actual measurement, which sociation of water is very different from the conventional expression. In s lag, as mentioned before, If or f ll valu e be­

The basicity, in its real nature, has no concern comes zer o at a certain CaO Si0 2 , where there is with the form of constituent ions in s lag. In t his no c ha nge of basicity by addition of the amphoteric way the new basicity very much r esembles a t he r­ oxides, just as in the case of aqueous solutions. modynamic function. Ac:co rdingly it is a lso possible to apply the defini­ Accordingly, t he basicity will r eveal its real tion of the isoelectric point in aqueous solutions natur e in the chemica l reaction between metal a nd to t he s lag systems. The basicity corresponding­ s lag. Recently the author worked on the r educ­ to the isoelect ric point is r epr esented by Bi. Iso­

tion of Ti and Si from the m olten slag containing electric points in t he CaO-Si0 2 system with addi­

titanium oxide to ··saturated iron and suc­ tions of TiOz or Al z0 3 as the third component are ceeded in explain i ng the r es u Its based on the new illustrated in Table 1. basicity. As for this work, the author's other reports 1-1 ) 1'-,) s hould be referred to. Table L Isoelectric points

In aqueous chemistry a solution is classified as Values calculated from Observed values the empirical equations acidic or basic or neutml according to t he pH S lag system value as compared with pH = 7. Simila rly, in (CaO SiO, ). Wl "" Hi CaO S iO,. wl "., Bi oxide slag systems, the classification can be made as follows; CaO SiO, T iO, 0.4 5 2.2 0.42 2.3 CaO-SiO, AI , 0 3 0.2 0.96 0. 15 B "> 0 basic 1 B = 0 neutral ( 9 ) Thus, it has been s hown for the first time that B <" 0 acidic J the so-call ed amphoteric behaviol' can be quanti­ As will be seen from Fig. 2 (a ), t he classifi cation tatively described in terms of the isoelectric point. g iven in (9 ) approximately agrees with t h e con­ Namely, it fo llows that an amphoteric oxide behaves ventional one, which has usually been made accord­ as a base in a m ore acidic s lag than t he isoelectric ing to Ca O/Si0 2 z 1. point, while in a more basic s lag it behaves as an acid. The Behavior of Amphoteric Oxides Here the authol' must make a few r emarks about Up to the present much information has been the difference between the isoelectric point and obtained as to what was implied by acidic or ba ' ie, the neutral point. In aqueous solutions, as a rule, but li ttle has been known of amphoteric. Now it an amphoterite itself does not h ave a neutral be­ is possible to obtain some ins ight into the nature havior and the isoelectric point does not COITes­ of the amphoteric oxides. The behavior of the pond to t he ne utral point. This is quite in con­ a mphoteric oxides which has been described has formity with the amphoteric behavior of TiO" a quite analogous aspect to that of aqueous solu­ whose isoelectric point has its location in the con­ tions. siderable acidic range. On the other hand, in the

When the amphoterite electrolyte is dissolved case of Al z0 3 the isoelectric point nearly coincides in an , it behaves as a n acid or with t he neutra l point. a base, depending on the pH value. At a certain The location of t h e isoelectric point may depend concentration of the , which is termed upon the acid-base strength of the amphoteric the "isoelectric point," a dditions of an amphoterite oxide· In Fig. 6 t h e isoelectric point CBi) is plot­ bring on no change of the pH value. If the solu­ ted against the strength of the metal-oxygen bond tion is more acidic, the added amphoterite behaves which is a measu r e of the acid-ba se strength of

29 pure oxides. It is conceivable from Fig. 6 that (4) Isobasicity lines of the systems CaO-SiOz- t he basicity of the isoelectric point is decreased Ti02 and CaO-SiOz-A lz0 3 a re s hown in ternary with increase of t he acid strength. It seems to diagrams. The empirical equations of basicity were obtained for the concentration range CaO/ SiOz= 1/3-4 and TiOc (or A lzO:.) = 0- 35";,. At a certain CaO/Si02, the basicity does not change with additions of T i0 2 or A12 0 3 • 8 (5 ) The basicity corresponding to this CaO/Si02 Ca 2.+ was termed the isoelectric point (Ei) . At lower - ~ r atios of CaO/ Si0 , B is incr eased by additions of ,, 2 , Ti0 or Al 0 and at higher ratios decreased. Thus, , 2 2 3 4 in terms of the isoelectric point the amphoteric 1 \ Ti' · behavior of slag constituents could be descl'ibed - IVl g2 ' , ~ qua ntitatively. It was s uggested that Ei is de­ t:O . f , creased the m or e with increase of the metal-oxygen Fe 3·' ', A J3 t o bond str ength. Q, , , Ti" REFERENCES - U. , ( 1 ) M. Ichinoe: J . Iron Steel Inst. Japan, 34 (1948) No. , , , 4-6, 4 - 4 , , ( 2 ) W. L. Kerlie: J. Iron Steel Inst., 167 (1951 ) 9 ...... , S i' ~ ( 3 ) N. J. Grant, J . Chipman: Trans. AIME, 167 (1946 ) - ...... 134 ( 4 ) P. Herasymenko, G. E. Speight: J. Iron Steel Inst.. - 8 I I I 166 (1950) 289 2 3 ( 5 ) Kuan-Han Sun, A. Silverman: J _ Amer. Ceram, Soc., 28 (1945) 8 2 2Z /a ( 6 ) Kuan-Han Sun: Glass Ind., 29 (1948 ) 73 7 ) J- Chipman, Lo-Ching Chang: Trans. AIME, 185 Fig. 6 Relation between the basicity corresponding to the (1949) 191 isoelectric point and the strength of the metal· ( 8 ) Kuan-Han Su n: J. Amer. Cerm. Soc., 30 (194 7) 277 oxygen bond 1 9 1 R. Didtschenko, E. G. Rochow: J. Amer. Chern. Soc., 76 (1954) 3291 be possible that from this figure s uch an unknown 10 H. Lux : Z_ E lektrochem., 45 (1939 ) 303 H 11 J. White : J. Iron Steel I nst., Carnegie Scholarship value of the isoelectric poi nt as that of Mg~ +, Fe , Memoirs, 27 (1938 ) 1 or T i:H will be estimated. (12) K. L. Fetters, J - Chipman : Trans_ AIME, 145 (1941 ) V I I. Summary 95 (13 ) H. Larson, J. Chipman: J. , 5 (1953 ) 1089 The r esults obtained in this study are sum~ (14 ) K. Mori : J. Iron Steel Inst. Japan, 46 (1960) 548 marized as follows: (15 K. Mori: 1- Japan lnst. Metals, 24 (1960 ) 383 (1) Aftel' a review of the concepts of s lag ba s i~ city was made, the well~known fact t hat the ratio of ferric to total iron in oxide slag is g reatly affected by changes in basicity was selected to give the new scale of basicity. (2) Iron oxide with small additions of the slag whose bas icity is to be determined was equilibrated under an a tmosphere of CO 2 CO = 13.3 at a tem~ perature of 1480°C. The basicity value (E ) wa s determined from the resu lts of the analyses for Fe~ + and Fe::+.

(3) The B values for CaO, Si0 2 and TiOc are s uch as would be expected from the metal-oxygen bond strength. In the systems CaO~Si02 and CaO~Ti0 2 ' B changes approximately linearly with mol";" but in the system Si02-Ti02 a maximum a ppears. The basicity of CaO is decr eased by additions of AI ~ O ;l> while that of Si02 is increased.

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