1. Introduction

In hot metal dephosphorization process, usually the CaO-based fluxes are used to form basic slag with high phos-phate capacity. Because of the high melting point, solidCaO does not dissolve completely and remains in the slag,which causes problems such as increase of slag volume anddifficulty of slag recycling. Fluorspar (CaF2) was com-monly used as an additive to promote the melting of CaO-based flux. However owing to the release of fluoride ionfrom the disposed slag, the use of fluorspar is strictly lim-ited based on the concept of the eco-friendly steel produc-tion. Recently it is required to reduce the steelmaking slagemissions further. Therefore, solid CaO should be utilizedmore efficiently without adverse technical and environmen-tal effects.

A lot of studies have been done to clarify the dissolutionbehavior of solid CaO into the liquid slag. Schlitt et al.1)

discovered that the dissolution rate of CaO could bemarkedly enhanced by increasing the FeO content inCaOSiO2FeOx slag. The dissolution rate was also foundto vary with different additives such as CaF2, CaCl2, Al2O3and B2O3

2) and the size of CaO particle.3) Formation of2CaOSiO2 layer on the surface of CaO particles was as-certained by many observations,4,5) and the dissolution ofCaO proceeded by migration of molten slag through crackson the layers of 2CaOSiO2. The effect of 2CaOSiO2 onCaO dissolution and dephosphorization should be con-firmed to promote the utilization efficiency of CaO in hotmetal dephosphorization process.

It is known that 3CaOP2O5 can dissolve into solid2CaOSiO2 phase

6) and many reports confirmed that P2O5existed in 2CaOSiO2 phase in steelmaking slag.

4,712)

Inoue and Suito9) found that the mass transfer of phospho-rus from 2CaOSiO2 saturated slag to the 2CaOSiO2particles was fast and a uniform CaOSiO2P2O5 solidphase was formed within 5 s. Ito et al.10) and Hirosawa etal.11) measured phosphorus partition ratio between solid2CaOSiO2 and molten CaOSiO2FeOxP2O5 slag at hotmetal temperatures. Kitamura et al.12) investigated the P2O5content in different phases for hot metal dephosphorizationslag and found that partition ratio of phosphorus betweenliquid slag and 2CaOSiO2 phase increased as the increasein the slag basicity. However, the formation mechanism ofphosphate compound in the solid 2CaOSiO2 phase wasexplained in few researches.

The authors have clarified the reaction mechanism be-tween solid CaO and slag by dipping solid CaO intoCaOSiO2FeOxP2O5 slag at hot metal temperatures.

4,13)

The P2O5 condensed phases were observed partly in theformed 2CaOSiO2 phase on the surface of solid CaO. TheP2O5 content in the condensed phases increased fromCaOslag boundary toward bulk slag. The reaction betweensolid 2CaOSiO2 piece and slag was also studied by dip-ping solid 2CaOSiO2 piece into CaOSiO2FeOxP2O5slag.14) However, since the experiments were only con-ducted at 1 673 K with particular slag composition, an inte-grated understanding of the formation mechanism of thephosphate compound in the 2CaOSiO2 phase is still un-available.

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Condensation of P2O5 at the Interface between 2CaOSiO2 andCaOSiO2FeOxP2O5 Slag

Xiao YANG, Hiroyuki MATSUURA and Fumitaka TSUKIHASHI

Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5Kashiwanoha, Kashiwa, Chiba 277-8561 Japan.

(Received on February 24, 2009; accepted on May 18, 2009)

In order to clarify the reaction behavior of phosphorus in the multi phase flux, the solid 2CaOSiO2 piecewas reacted with the CaOSiO2FeOxP2O5 slag for 1 to 180 s at 1 573 or 1 673 K. The interfaces betweenthe original solid and liquid phases were observed and compositions of both phases were analyzed bySEM/EDS.

The result shows that P2O5 is condensed at the rim layer of 2CaOSiO2 piece very fast in less than 1 s.The P2O5 condensed phases are identified as the mixture of 2CaOSiO23CaOP2O5 solid solution and thesurrounding liquid slag. After reaction for longer time, the reaction behavior of P2O5 depends on the reactiontemperature and initial slag composition. Reaction temperature and mole ratio of CaO/SiO2 in the initial slaginfluence the stability of P2O5 condensed phases. Higher temperature induces the dissolution of P2O5 con-densed phases while larger mole ratio of CaO/SiO2 has the opposite effect.

KEY WORDS: multi phase flux; hot metal dephosphorization; dicalcium silicate; P2O5 condensed phase; re-action mechanism.

The present study is aimed to clarify the microscopic re-action behavior of phosphorus at the interface betweensolid 2CaOSiO2 and liquid CaOSiO2FeOxP2O5 slag onthe basis of the previous studies. A solid 2CaOSiO2 piecewas reacted with the CaOSiO2FeOxP2O5 slag with dif-ferent composition at 1 573 or 1 673 K. The interface be-tween the original solid and liquid phases was observed andanalyzed by SEM/EDS. The formation mechanism of P2O5condensed phase at interface between 2CaOSiO2 and theP2O5 containing slag was clarified. The influence of temper-ature and initial slag composition on the condensation be-havior of phosphorus was discussed.

2. Experimental

The CaOSiO2FeOxP2O5 slag was prepared by mixingthe synthesized wustite, CaO obtained by the calcination ofreagent grade CaCO3, reagent grade SiO2 and 3CaOP2O5.With different FeOx content and CaO/SiO2 mole ratio, threetypes of slags were used in the present study as shown inTable 1. The 2CaOSiO2 piece was prepared by pressing amixture of CaO and reagent grade SiO2 on mole ratio of2 : 1 at 50 MPa, followed by heating at 1 773 K for 24 h.About 1 mass% of 3CaOP2O5 was also added into themixture to prevent the dusting of 2CaOSiO2.

Ten grams of slag were charged in an alumina crucible(I.D.: 34 mm, O.D.: 38 mm, height: 45 mm) with the coexis-tence of solid iron (3 g) and melted in a furnace at argon at-mosphere. In the previous study on reaction between CaOand slag at 1 673 K, both the Al2O3 crucible and Fe cruciblewere used. The results proved that about 15 mass% of dis-solved Al2O3 at maximum does not affect the reaction sig-

nificantly.13) Accordingly, it is supposed that the influenceof Al2O3 impurity is also limited in the present study. Solidiron was used to control the oxygen partial pressure deter-mined by the Fe/FeO equilibrium. The solid 2CaOSiO2piece (0.5 to 1 g, f101 mm cylinder shape) attached to thetip of the mullite tube was dipped into the liquid slag andreacted for 1 to 180 s. Reaction temperature was controlledat 1 573 or 1 673 K. After the reaction, the solid sample wasquickly taken out from the furnace and quenched by im-mersing in liquid nitrogen, followed by embedding in thepolyester resin. The surface was polished and the interfacebetween 2CaOSiO2 and slag was observed and analyzedby SEM/EDS. Since the electron beam diameter is approxi-mately 0.2 mm, accurate composition of solid particlesmaller than the spot size can not be obtained, and in thatcase the measured composition will be the average of com-positions of solid particle and that of surrounding liquidphases.

3. Results

3.1. Slag A (FeOx 20.0 mass%, CaO/SiO21.0)

The SEM images of interfaces between solid 2CaOSiO2and slag A after reaction for 1, 10, 60, and 180 s at 1 573and 1 673 K are shown in Figs. 1(a) to 1(d) and Figs. 2(a)to 2(d), respectively. The symbols shown in figures corre-spond to the positions analyzed by EDS.

The results of EDS analysis for Fig. 1 are listed in Table2, where the iron oxide is calculated as FeO. At 1 573 K,only after reaction for 60 and 180 s, the phases of which theCaO/SiO2 molar ratio is larger than 1.5 while P2O5 contentis over 1.0 mass% can be observed, such as position num-bers 3, 6 and 8 in Fig. 1(c), and position numbers 3, 4 and 8in Fig. 1(d), as labeled by open circles. These phases wereidentified as P2O5 condensed phases. On the contrary, fewP2O5 condensed phases were observed after reaction for 1and 10 s.

It is considered that the condensation of P2O5 was re-strained by the formation of the 3CaO2SiO2 layer which

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Fig. 1. SEM images of interfaces between 2CaOSiO2 and slag A at 1 573 K.

Table 1. Chemical compositions of slag.

separated the solid 2CaOSiO2 and the slag as shown inFig. 1. The compositions of the new layer, position numbers2, 5 and 7 in Fig. 1(a), position numbers 7 and 9 in Fig.1(b), position numbers 2, 4 and 10 in Fig. 1(c), positionnumbers 2, 7 and 9 in Fig. 1(d), were analyzed that FeOcontent is less than 6.0 mass% and mole ratio of CaO/SiO2approximately equals 1.5. With the increase of reactiontime, the initially uniform layer grew up and split into anindented appearance. Then, the slag penetrated through thecracks formed in the layer of 3CaO2SiO2 and reacted withsolid 2CaOSiO2. Therefore, the P2O5 condensed phasescan be observed after reaction for longer time.

As shown in Fig. 2, the formation of 3CaO2SiO2 layerwas not observed at 1 673 K, that is different from the caseat 1 573 K. It is considered that the enlarging of liquid areaon the CaOSiO2FeOP2O5 phase diagram at higher tem-perature avoids the formation of the additional layer. How-ever, the detailed formation mechanism of 3CaO2SiO2 atthe interface is still not fully understood since the phase di-agram for the CaOSiO2FeOP2O5 system is unavailableyet. In addition the P2O5 condensed phases can be easilyobserved even after short reaction time. The compositionanalysis by EDS at different positions across the interfacewas conducted as the pattern shown in Fig. 3 to observe theconcentration profiles of the main components in the sys-tem. An arbitrary reference location was chosen inside the2CaOSiO2 phase. The composition at each analyzed posi-tion is recorded as the function of its distance from the ref-erence location as shown in Figs. 4(a) to 4(d). Similar infour cases, CaO content is decreasing from 2CaOSiO2 toslag and FeO content shows the incremental trend whileSiO2 content almost keeps constant. The P2O5 condensedphases with CaO/SiO2 molar ratio larger than 1.5 and P2O5content over 1.0 mass% are labeled by open symbols, whileall other phases are labeled with solid symbols. As can beseen, the condensed phases contain more CaO and P2O5 butless SiO2 and FeO than surrounding phases.

The solid line is drawn from the tendency in Fig. 4 to de-scribe the changing of FeO content. In order to describe thelocation of the P2O5 condensed phases, the area near the in-terface can be divided into three parts according to the con-centration profile of FeO: solid 2CaOSiO2 area (constantlow FeO), solid 2CaOSiO2 and liquid slag coexisting area(increasing FeO), and slag area (constant high FeO), asshown in Fig. 5. In the present study, the term liquid slagmeans the phase other than solid 2CaOSiO2 phase andP2O5 contained solid phase, while the slag area denotes

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Fig. 2. SEM images of interfaces between 2CaOSiO2 and slagA at 1 673 K.

Fig. 3. The pattern of EDS analyzing across the interface.

Table 2. Results of EDS analysis for Fig. 1.

the area distinguished from the solid 2CaOSiO2 and theintermediate solid and liquid coexisting (multi phase) area.Since the P2O5 condensed phase is formed very little andscattered, the existence of the condensed phases in slag areais not considered while discussing the partition of the areanear the interface. The solid 2CaOSiO2 and liquid slag co-existing (multi phase) area is defined since the composi-tions of the phases in this area locate in the two phase areain the CaOSiO2FeO phase diagram. As shown by thegrey field in Fig. 4, the multi phase area is formed by thepenetration of liquid slag into the solid 2CaOSiO2. Be-cause of the dissolution of 2CaOSiO2, the solid will be-

come thinner with the increase of reaction time. The solidarea will be shrinking and slag area be expanding. There-fore, it is reasonable to realize that the multi phase area asthe intermediate region between pure solid 2CaOSiO2 andslag will shift towards the inner 2CaOSiO2 area even with-out observation.

The P2O5 condensed phases only appear in certain re-gions, as enclosed by dotted line in Fig. 4. After reactionfor 1 and 10 s, the condensed phases lie in both the multiphase area and slag area. After reaction for 60 and 180 s,the condensed phases can only be observed in the multiphase area. The width of the region with condensed phasecan be obtained by reading the width of the region withopen circles in Fig. 4. With increase of reaction time, thewidth firstly expanded before reaction for 10 s, then becamenarrower from 10 to 180 s.

3.2. Slag B (FeOx 30.0 mass%, CaO/SiO21.0)

Slag B contains more FeOx than slag A. The concentra-tion profile of FeO and P2O5 across the interface between2CaOSiO2 and slag B after reaction at 1 573 and 1 673 Kare plotted in Figs. 6(a) to 6(d) and Figs. 7(a) to 7(d), re-spectively. No additional layer such as layer of 3CaO2SiO2was obviously formed at the interface. The P2O5 condensedphases can be easily observed after various reaction timesat both temperatures.

At 1 573 K, P2O5 condensed phases were located in onlymulti phase area after reaction for 1 s. After longer reac-

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Fig. 4. Change of the concentration of components against the position at the interface between 2CaOSiO2 and slag Aat 1 673 K.

Fig. 5. Partition of the area near the interface.

tion time, the condensed phases can be observed in both the multi phase area and slag area. P2O5 content in the condensed phases shows an increasing tendency from2CaOSiO2 side to the bulk slag side.

At 1 673 K the condensed phases with lower P2O5 con-

tent than that at 1 573 K lay in both areas after reaction for1 and 10 s. On the other hand, after reaction for 60 and180 s, the condensed phases were only observed in themulti phase area. The tendency for the P2O5 content in thecondensed phase is not simplex increasing as that at

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Fig. 6. Change of the concentration of components against the position at the interface between 2CaOSiO2 and slag B at1 573 K.

Fig. 7. Change of the concentration of components against the position at the interface between 2CaOSiO2 and slag B at1 673 K.

1 573 K.

3.3. Slag C (FeOx 20.0 mass%, CaO/SiO21.3)

The mole ratio of CaO/SiO2 in slag C is higher than thatin slag A. The concentration change of FeO and P2O5 withthe position across the interface after reaction at 1 573 and1 673 K are plotted in Figs. 8(a) to 8(d) and Figs. 9(a) to9(c), respectively.

Slag C is considered to be not in strictly pure liquid stateat 1 573 K. However, the condensation of P2O5 from liquidphase in slag C to 2CaOSiO2 can still be observed. Theoriginal solid phases in slag C before reaction can be easilyidentified by microscope due to their larger size and were

not considered in the present study. The term slag area isstill used to set the part of slag with constant high FeO con-tent in Fig. 8 as distinguished from the solid 2CaOSiO2and the intermediate multi phase area formed by slag pene-tration when discussing the partition of the area near the in-terface. At 1 573 K, P2O5 condensed phases lay in the inter-mediate multi phase area after reaction for 1 s, and ex-panded to slag area after reaction for 10, 60 and 180 s. Sim-ilar with slag B at 1 573 K, the P2O5 content in the con-densed phase keeps increasing from 2CaOSiO2 side to thebulk slag side.

At 1 673 K, the condensed phases mostly lay in bothareas. The P2O5 content in the condensed phase shows a

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Fig. 8. Change of the concentration of components against the position at the interface between 2CaOSiO2 and slag C at1 573 K.

Fig. 9. Change of the concentration of components against the position at the interface between 2CaOSiO2 and slag C at1 673 K.14)

tendency of increase-decrease from 2CaOSiO2 side tobulk slag side.

4. Discussion

4.1. Formation of P2O5 Condensed Phases

The results indicate that normally the condensation ofP2O5 in the slag happens very fast at the interface between2CaOSiO2 and slag. The only exception is the case of slagA at 1 573 K. In this case, an additional 3CaO2SiO2 layerwas formed and restrained the reaction of P2O5 by separat-ing solid 2CaOSiO2 and molten slag. P2O5 was not con-densed until the layer split after certain reaction time.

Previous researches712,14) have already proved the forma-tion of 2CaOSiO23CaOS2O5 solid solution at the inter-face between 2CaOSiO2 and slag. Accordingly, the P2O5condensed phases in the present study are identified as themixture of 2CaOSiO23CaOP2O5 solid solution and thesurrounding liquid slag because the size of the formed solidsolution particle is not big enough to be detected separatelyby the current analysis method. The chemistry of P2O5 con-densed phases and the surrounding liquid slag are normal-

ized to CaOSiO2P2O5 system and plotted in the ternarycomposition triangle as shown in Fig. 10. From these com-position triangles, two pieces of information can be ob-tained: the distance of the composition point to the2CaOSiO23CaOP2O5 tie line and the P2O5 content inthe condensed phase. Both are the implications of the ratioof solid solution in the mixture. As more P2O5 rich solid so-lution is contained in the mixture, the composition pointlies closer to the tie line between 2CaOSiO2 and3CaOP2O5 and the P2O5 content in the mixture is larger aswell. The variation of P2O5 content in the mixture can alsoindicate the change of P2O5 content in the solid solution. Inthe present study, both the approaching of the compositionpoint towards the tie line and the increase of P2O5 contentare regarded as the evidences that P2O5 condensation is pro-moted, vice versa.

The first indication of these results is the influence of re-action time on the P2O5 condensation. In most cases, thecomposition points are approaching the tie line between2CaOSiO2 and 3CaOP2O5 which is the implication of the2CaOSiO23CaOP2O5 solid solution. The tendencyshows the progression of P2O5 condensation. With the in-

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Fig. 10. Composition of P2O5 condensed phase at interface between 2CaOSiO2 and slag.

crease of reaction time, the ratio of solid solution in theP2O5 condensed phase, which is the mixture of solid so-lution and liquid slag, is increasing. Therefore, the P2O5condensed phase turns to be the simplex 2CaOSiO23CaOP2O5 solid solution after reaction for longer time.

Reaction temperature influences the reaction rate. Highertemperature decreases the slag viscosity and improves thefluidity, which is favored for the mass transfer in the sys-tem. The reaction of slag B at 1 673 K is so fast that theprogression of P2O5 condensation with reaction time similaras that at 1 573 K can not be obviously observed. Compar-ing the results of slag B at 1 573 and 1 673 K, the P2O5 con-tent in the condensed phase decreases at higher tempera-ture, which implies that P2O5 condensation is promoted atlower temperature by formation of larger P2O5 content solidsolution or increase of solid solution ratio in the mixture.However, the dominant mechanism is not clear yet. For slagA at 1 673 K, the composition point is approaching to thetie line from 1 to 10 s which means the condensation is pro-ceeding. However, from 10 to 180 s, the P2O5 content in thecondensed phase is decreasing which means that the P2O5content in the solid solution or the ratio of solid solution inthe mixture is decreasing, as shown in Figs. 4 and 10. Suchtendency educes the possibility that dissolution of con-densed phase happened. It is suggested that the condensa-tion proceeded from 1 to 10 s, while after reaction for 10 sthe dissolution of condensed phase happened and hence theP2O5 content decreased. More reason to explain the dissolu-tion of P2O5 condensed phases will be given in the follow-ing section. In a word, higher temperature improves the re-action rate and mass transfer, and also induces the dissolu-tion of P2O5 condensed phases.

The initial slag composition is another factor. Largermole ratio of CaO/SiO2 in the initial slag is favored for theP2O5 condensation. Comparing the results of slags A and Cat 1 673 K, the P2O5 content in the condensed phases islarger in the case of slag C, which means the condensationis improved by increasing the CaO/SiO2 ratio. The progres-sion of reaction for slag C at 1 673 K can be apparently no-ticed dissimilar to slag A at 1 673 K, which indicates thatthe dissolution of condensed phases was not as fast. Sinceslag C with a larger CaO/SiO2 ratio is closer to the liquidussaturated by 2CaOSiO2 in the phase diagram than slag B,it is easier to become saturated with 2CaOSiO2. Therefore,the formation of P2O5 condensed phases is easier and corre-spondingly the dissolution of condensed phases into slag isrestrained. On the contrary, the influence of FeO content inthe initial slag can not be clarified yet from the present re-sults.

4.2. Characteristics of the Region with P2O5 Con-densed Phases

As shown by the open circles in the SEM images, theP2O5 condensed phases were not formed as a uniform layer,but scattered unequally in a certain region at the interface.The width and location of this region are summarized inTable 3.

For slags B and C at 1 573 K, the region with P2O5 con-densed phases is expanding and P2O5 content in the con-densed phases always show the increasing tendency acrossthe interface from 2CaOSiO2 side to bulk slag side after

various reaction time. These patterns suggest that moreP2O5 was condensed after longer reaction time for slags Band C at 1573 K. P2O5 condensed phases were only locatedin multi phase area after reaction for 1 s. After reaction forlonger time, the condensed phases turned to be located inboth the multi phase area and the slag area.

The different locations of the P2O5 condensed phaseswith reaction time prove that at early period of reaction thecondensed phases are formed in the multi phase area. Atthe interface between 2CaOSiO2 and slag, the multi phasearea shifts towards 2CaOSiO2 side along with the continu-ous dissolution of 2CaOSiO2 into slag as the reaction timeincreases. After certain time reaction, the location of thepreviously formed condensed phase changes to be slag areaowing to the shift of multi phase area. New condensedphases are formed in the new multi phase area.

Higher temperature improves the reaction rate. HenceP2O5 condensed phases can be observed in both multi phasearea and slag area after reaction as short as 1 s in each slagat 1 673 K. However, higher temperature also enhances thedissolution rate of P2O5 condensed phases. As for slags Aand B at 1 673 K, the condensed phases turn to be locatedonly in multi phase area after reaction for 60 and 180 s.This phenomenon indicates that all of the previouslyformed condensed phases dissolved into the slag after cer-tain reaction time. The condensed phases can hardly be ob-served in slag area as only newly formed P2O5 condensedphases remains. For slag C at 1 673 K, because of the largerCaO/SiO2 ratio of bulk slag, the dissolution rate is not fastenough and thus the condensed phases can still be observedin slag area even after reaction time for 60 s. The width ofthe region with P2O5 condensed phases is shrinking withthe reaction temperature increase from 1 573 to 1 673 K,which also indicates the dissolution of condensed phases athigher temperature.

4.3. Dissolution of P2O5 Condensed Phases

As stated above, the different locations of P2O5 con-densed phases with reaction time and shrinking of the con-densed phase region at higher temperature indicate the oc-currence of the dissolution of P2O5 condensed phase intothe slag. In addition, the different tendency of P2O5 contentin the condensed phases at different temperature is anotherindication of the dissolution of condensed phases. As forslags B and C at 1 573 K, the P2O5 content in condensed

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Table 3. Characteristics of region with P2O5 condensedphases.

phases increases from 2CaOSiO2 side to bulk slag side, asshown in Figs. 6 and 8. However, the increasing tendency isnot apparent in the case of 1 673 K, as shown in Figs. 4, 7and 9. The increasing trend for P2O5 content in the con-densed phase across the interface should be observed in thecase of condensation behavior, while dissolution brings de-creasing trend. At lower temperature, the dissolution is notdominant, hence the tendency is increasing. At higher tem-perature, the dissolution becomes more dominant and thecombining effects of both condensation and dissolution re-sult in the distinct tendency. The distinct tendency at highertemperature is induced by the dissolution of the previouslyformed condensed phases.

The mechanism of the dissolution could be explainedfrom the perspective of P2O5 activity profile. In some cases(slags A, B and C at 1 673 K), The P2O5 content in thephases other than the condensed phases (solid symbol rep-resents) near the interface was obviously decreasing withthe increase of reaction time, as shown in Figs. 4, 7 and 9.For example Fig. 9(a) shows the P2O5 content in the rangeof 6position16 mm is as high as over 6.0 mass% afterreaction for 1 s. However after reaction for 60 s as shown inFig. 9(c), the P2O5 content in the range 16position35 mm is less than 2.0 mass%. Similar tendency can also beobserved in Figs. 4 and 7. It is considered that the P2O5 ac-tivity being the driving force of P2O5 condensation and dis-solution exhibits the similar tendency as the P2O5 content inthe phases other than the condensed phases, as shown inFig. 11. At the early period of reaction, there was a sharpactivity gradient between 2CaOSiO2 and slag as denotedby the dashed line. After the slag turned to be saturatedwith 2CaOSiO2 by the dissolution of solid 2CaOSiO2,P2O5 and CaO in the slag react with solid 2CaOSiO2 toform solid solution or compound. The condensation at oneplace progressed until the P2O5 activity in the condensedphase equaled that in the surrounding slag phases. With theincrease of reaction time, the previously formed condensedphase stayed still with little change while the multi phasearea shifted towards the 2CaOSiO2. However, P2O5 activ-ity in the phases other than the condensed phases near the

interface was decreasing after long reaction time on ac-count of the facts that P2O5 was being consumed with con-tinuous formation of new condensed phases while the diffu-sion of P2O5 from bulk slag to the interface area was notfast enough to fill the loss, as illustrated by the solid line inFig. 11. The difference of the P2O5 activity between the pre-viously formed condensed phase and the surrounding slagled to the dissolution of P2O5 condensed phases into thesurrounding slag. The first formed condensed phase shouldbe the first to dissolve into the slag.

4.4. Reaction Behavior of P2O5 at 2CaOSiO2Slag In-terface

The mechanism of reaction between solid 2CaOSiO2and liquid CaOSiO2FeOxP2O5 slag could be concludedfrom the present results as illustrated in Fig. 12.

(1) The solid 2CaOSiO2 dissolves into the slag andalso the slag penetrates into the solid sample. The slag areanear the solid turn to be the region with gradient of2CaOSiO2 by the dissolution. (Fig. 12(a)).

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Fig. 11. Schematic profile of P2O5 activity across the interfacebetween 2CaOSiO2 and slag.

Fig. 12. Phosphorous behavior at interface between 2CaOSiO2and CaOSiO2FeOxP2O5 slag (C2S is short for2CaOSiO2).

(2) The rim layer of the solid 2CaOSiO2 changes intomulti phase area where solid and liquid phases are coexist-ing. (Fig. 12(b)).

(3) In the multi phase area, CaO and P2O5 in liquidphase react with solid 2CaOSiO2 to form the P2O5 con-densed phases. (Fig. 12(c)).

(4) Since the solid 2CaOSiO2 continues dissolvinginto the bulk slag, meanwhile, the slag continues penetrat-ing into the solid 2CaOSiO2, the multi phase area shiftstowards the side of 2CaOSiO2 and new P2O5 condensedphases are formed. (Fig. 12(d)).

(5) The previously formed P2O5 condensed phaseswould remain (Fig. 12(e1)), partly dissolve (Fig. 12(e2)) orfully dissolve into the slag (Fig. 12(e3)), depending on thetemperature and slag composition.

Reaction temperature and initial slag composition influ-ence the phosphorous behavior at the interface between2CaOSiO2 and slag through changing the stability of P2O5condensed phases. For slags B and C at 1 573 K, the P2O5condensed phases are very stable and the reaction pro-gresses on the sequences of (a)(b)(c)(d)(e1). For slagsA and B at 1 673 K, the P2O5 condensed phases are easy todissolve according to the path (a)(b)(c)(d)(e3). Forslag C at 1 673 K as the stability of P2O5 condensed phasesis intermediate between the two cases above the reactionpath is (a)(b)(c)(d)(e2).

5. Conclusions

The reaction behavior of phosphorus in the multi phaseflux was investigated by analyzing the reaction interface be-tween solid 2CaOSiO2 and liquid CaOSiO2FeOxP2O5slag at 1 573 and 1 673 K. The influence of reaction temper-ature and initial slag composition on the P2O5 condensationwas clarified. The results are summarized as follows:

(1) P2O5 condensed phases are firstly formed in themulti phase area, then turn to be located in the slag area by

the dissolution of 2CaOSiO2.(2) For slag A (FeOx 20 mass%, CaO/SiO21) at

1 573 K, a layer of 3CaO2SiO2 is formed at the interfaceand restrained the condensation. After reaction for longertime, the layer would split into an indented appearance andP2O5 may react.

(3) The P2O5 condensed phases are identified as themixture of 2CaOSiO23CaOS2O5 solid solution and thesurrounding liquid slag.

(4) Higher reaction temperature improves the reactionrate and mass transfer in the system.

(5) Reaction temperature and CaO/SiO2 mole ratio ofinitial slag influence the stability of P2O5 condensed phases.Higher temperature induces the dissolution of P2O5 con-densed phase while larger CaO/SiO2 ratio has the oppositeeffect.

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