D0I:10.13374/j.issn1001-053x.1989.06.022 北京科技大学学报 第11卷第6期 Vol.11 No.6 1983年11月 Journal of University of Science and Technology Beijing Nov.1989 The“Point”and“Volume”Fusion Behaviour of Prefused Type Powder Fluxes' Jin Shantong(金山，问)，Zou Mingjin（邹明金)* ABSTRACT:The relationship among particle size,properties of heat transfer and melting rate of fluxes as well as the main factors which have effect on the prefused fluxes have been studied.Under the conditions of same components and particle size,the melting temperature of prefused fluxes is lower than that of mixed fluxes.With the decreasing of particle size,the difference of melting te- mperature between the two type of fluxes decreases also.The larger the parti- cle size,the greater the melting rate of prefused type fluxes. KEY WORDS:fusion behaviour,flux,prefused type The proeprties of cast fluxes in the mold is usually determined by their che- mical compositions,the components of raw materials and the physical properties of the fluxes produced by different ways which include the processes of mixing, prefritting and prefusing. A series of researches has been done for simple mixed type powder fluxe- st2-01,and sufficient know ledges and experience are available.The relationship betwcen the properties of mold fluxes,such as viscosity,interfacial tension,abi- lity of absorbing inclusions,activity of the compositions and the temperature are sufficiently clear,but the influence of the properties of the raw materials on the melting characteristics difference between the prefused and the simply mixed pow der fluxes must be understood clearly.Therefore,this paper mainly focused on studying the difference of the "point"and "volume"fusion characterislics be- tween them,with the aims of finding the effect of some primary faclors,such as the major composition of fluxes,kinds of additives and particle size,on the be- haviour of prefused fluxes,and studying the relations of the particle size with the heat transfer as well as the fusion rate. Manuscript Reccived March 1,1989 ·Dcpt.of Metallurgy 516
第 卷第 期 北 京 科 技 大 学 学 报 ， 年 月 “ ， “ ” ’ 为 夕 金 山 同 ， ’ 夕 ， 邹明金 ， ， · ， · ， 从 一 认 ， · ， 卜 一 ， ， 声沪 ， 、 ， 一 ， 、 一 、 从 ， 、 ， 、 ， ， 手 · ， 、 ， 少 ， 、、 、、 一 、 、 一 手一 ， · “ ” “ 一 ” 〕 ， 、 、 一 ， ， ， ， 从 · 。 ， ， DOI ：10．13374／j ．issn1001－053x．1989．06．022
1 Experimental In order to compare the fusing behaviour of two kinds of fluxes,chemically pure reagents were used to compound fluxes which were carbon-free.The ingre- dients of fiuxes and the particle size are shown in Table I. Table 1 The ingredient of fluxes and the particle size Composition Composition,% Particle size type SiO2 CaO Al2O3 CaF Na2 CO3 Fe2Os mesh mixed <10055-75,75- 50 40 10 100,100-150, prefused 150-200,<200 mixed <10055-75,75- Il 44 35.5610 10 100,100-150, prefused 150-200,<200 III mixed 4444 35.5610 10 <100 IV mixed 52.78 42.2210 5 <100 At the same lime,an investigation of the fusion behaviour of prefused powder fluxes used in industrial production has been shown as follow:composition(%)- Si02,3982;Ca0,37.47;Mg0,318;Fe20g,331;A1,0g,9.31;Na20,2.23: K20,0.57;size range of fluxes(mm):0.71~1.4;1.4~3.0;3.0~5.0;5~7; 7~11.8. The drip-melt method was adopted to measure the "point"melting behaviour of flux in the experiment.Because the heat capacity of samples were much less than that of furnace,it was supposed that the samples were so small (63mmx 3mm)that they had the same temperature of the furnace and the temperature homogenization of the samples could be reached in a twinkling,ie.,the effect of interior heat transfer of sample on its melting behavior could be neglected. The precedure of experiments was as follows: (1)Heating the furnace up to 1000C; (2)Charging the sampling into the furnace; (3)Keeping the heating rate in the range of 5.5-6.8C/min; (4)Observing the change of sample height with the increase of furnace tempe- rature; (5)Recording the starting melting temperature toto finish melting tempe- rature (1.)and the rate (dHi/dti)during melting (shown in Figl.) 517
纽 从 ， ， 注 改 一 · 工 里以 又 王 认 · 里 卜 、 。 ， 。 。 ，、 必 又 ， ‘ ， ‘ 马 匕一 ‘ 口 呀匀 口 口 以 』‘ 一 ， ， 一 ， 一 ， 一 ， ， 一 ， 一 ， …吮丫刀﹄ 往口 ，土 八 任号只，哇曰山月 气 吸‘ 土 。 ， 主 士 。 几 。 。 、丫 。 二 一 ， · ， ， 。 ， ， 。 ， 一 ， · 一 一 一 。 一 一 。 。 一 “ ’， 从 。 ， 价 玉 一 ， ， 一 · ， · 五 从 一 从 一 “ 。 。 一 皿 飞 。 、 、 主 一
He Temperature,℃ Eig,1 Change of sample height of mold flux with furnace 妞 temperature Fig,2 Schematic of the apparatus used for measuring the "volu- In fact,the melting of fluxes is me"melting behavlour of flux completed in three-dimensional space 1.flux; 2.corubin tube; 3.coil 4.Al2O3 crucibler in practical utilization,that is,a tempe- 5.Al203 packing;6.graphite base; rature gradient exists in the vertical 7.corubin tuber 8.supporty 9.Pt/Rh themocouples direction of flux layer. 10,corubin crucible, The graphite base heated by induction was used for finding the "volume" melting characterislics of flux.The mold flux whose weight was 140g was set in the furnace and whose temperature had been rised to 1550C and kept stable. Absorbed heat in horizontal direction,the molten flux flowed out from the cen- tral hole of the graphite base.The weight of flowout liquid flux were measured at regular intervals,the observation of the tendency to forming glass phase and the homogenization of samples after it was melted as well. 2 Experimental Results The difference of the melting characteristics between prefused fluxes and simply mixed fluxes with variation of their principal flux systems,additives and particle size has been studied. 2.1 Melting Curves of Principal Composition of Flux Systems Bascd on the SiO,-CaO-Al2Oa flux system,the prefused type powder flux and simply mixed type powder flux were compounded separately according to the ingredients of fluxcs of type I in Table l.The experimental results of the me- Iting characteristics measured with drip-mcthod in the case of same particle size are shown in Fig.3. 518
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1.0 1.0 0.8 0.8 0. 0.6 0.4 0.4 0.2 0.2 0 .1140 12201300 1220 1260 1300 1340 t,℃ t,℃ Fig.3 The melting characteristics of Fig,4 The effect of CaF2 additive prefused type (A)and simply on the melting characteristics mixed type (B)fluxes based of prefused (A)and simply on SiO-CaO-Al2O3 flux system mixed (B)fluxes (CaF2 =10%) Experimental results for the prefused powder flux (A),the starting melting temperature was about 1245C,and finishing melting temperature was 1285C,so the range of melting temperature was 40C.For the simply mixed powder flux (B),these three values were about 1300C,1340C and 40C respectively.That is,both of them had same fusion rate.This conclusion indicated that the melting behaviour of them could be kept as the same unless the temperature of hot metal with using simply mixed flux was 60C higher than that with using prefused flux.And it was predicted that the difference of melting behaviour between them should vary with the change of chemical compositions and particle size of fluxes. The prefused fluxes had a strong tendency to form glass phase themselves after re-melted.This glass phase was homogeneous (photograph 1.A).But the tendency to form homogeneous glass phase for the melted simply mixed fluxes were related to the compositions and componenls of raw materials.Excepted few cases,the tendency to form glass for most of the simply mixed fluxes were not completed because the small solid particles were often surrounded by a layer of two-phase mixture (photograph I.B). 2.2 Effect of Additives The melting curves of fluxes in which CaF2 was added as additive to the above basic composition of flux system were shown in Fig.4.for prefused and simply mixed fluxes. The results showed that,for both kinds of fluxes,the melting temperature of prefused flux was lower than that of simply mixed flux,but the difference between them slightly decreased. 519
从 一 、 、 甲 、 架卜 气 、 一 ，、 、 、一 口川》 目 之 ‘ ︸ 卜九 一二衬加。 一 一二词切。 欢 、 、 人、 、、 、 丫 勺 ” 入 ‘ 义 一 、日 】 之心 。 、 亡 ， “ ， ℃ 汽 气 、 ‘ 卜 万 。 卜 卜 一 一 。 ， “ ， “ ， 五 扭 ， “ ， ” ，， 七 升 。 主 至 “ 研 一 皿 。 一 · · · 一 。 ， 一 义 。 从 从· 甲 · 一 从 ， ， ， 五
A B Photo,1 The tendency to form glass phasc and homogenization of melted fluxes(A,prefused flux;B,simply mixed flux) 2.3 Effect of the Particle Size of Raw Materials The strong effect of particle size on the melting behaviour of simply mixed fluxes have been proved by some researcherst10-111.In these experiments,the particle sizes of raw materials were classified into four grades (75-100 mesh,100- 150 mesh,150-200 mesh,and<200 mesh)to compound the prefused flux and sim- ply mixed flux.The melting curves of fluxes under the condition of the heating rate in the range of 5.5-6.8C/min has been recorded and shown in Fig.5. .5-100mesh 100-150mesh 1.00 0.75 0.5 0.25 150-200mcsh <200 mesh 1.0 0.75 0.5 0.25 12601300 13801260 1340 1430 t,℃ Fg.5 The effect of particle size on the melting behaviour of simply mixed (.and prefused (o)flux It was concluded that the melting temperature of prefused powdcr flux was always lower than that of simply mixed flux for all grades of particle size,but 520
鄂 豪 疾汾 魏 。 ， 一 一 ， 一 ， ， 一 ， 。 一 。 “ 。 一 工。 。 ’ 一 一 汀旧 钧 入 飞 ， 、 飞 免 侣 月 蕊 、 中 〔〕 一 、 二盔 主 轰 、， 风 花 飞叨 、 了 ， 、 忽吕兰。 ‘ 讼 。 · ， ℃ 卜 豆
the difference of melting temperature between them was decreased with the redu- cing of particle size.In addition,the sensitive extent of the melting temperature due to the particle size of simply mixed fluxes was larger than that of prefused fluxes. 2.4“Volume”Melting Characteristics With rcducing particle size the "point"fusion rate of the simply mixed fluxes was increased until the size was smaller than 100-150 mesh,especally at the tem- perature of higher than 1400Ct).But,for the "volume"fusion rate of simply mixed fluxes at 1550C,the flow-out weight of liquid flux per minute in the case when the particle size was in the grade of 75-100 mesh was a little more than that of mixed fluxes of particle size in the grade of 100-150 mesh (shown in Fig-6.). When CaF:,Na2COa and Fe2O3 was added in flux from 5%6 to 10%6,the primary experimental results showed that the effect of CaF2 was stronger than that of Na2CO3 and Fe2O3. The effect of particle size on the prefused granule type fiuxes was shown in Fig.7.With the increase of particle size,the flow-out weight of liquid flux per minute varied from 8.75g/min at particle size of 0.7-1.4mm to 12.8g/min at par- ticle size of 7-11,7mm. 120r- 1550℃ 100 00 60 515Dmc型 ino-Mold 40 20 12 Time:min 2 4 81012 Time,min Fig.6The“volume”melting beha- Fig.7 The realation between the viour of simply mixed powder melting characteristic and the fluxes for two grades of particle size of the commercial prefused granule type fluxes particle size at 1550C particle size:]11.8-7mm; 2.7-5mm:3.5-3mm;4.3-0.7 1111 A conclusion has becn madc that the fusion rate of prefused flus is larger than that of simply mixed flux.The melting rate of commercial prefused fluxes particle size of which was in the range of 0.7-11.8mm can be 8.75-12g/min. The melting rate of simply mixed flux with particle size of 75-150 mesh was 521
· ， 笠 ，’ ” 万 “ ，， 一 ， “ 一 ， “ ’， ， 一 从 一 又 一 。 ， 。 、 一百 早百 ， ，。 。 · 手 · · ， 一 、 · · 。 一 。 一 。 。 助一火﹃‘ ﹁目石兰弓亏。琴助 忍左工 雏 军 “ 尹 国 户声尹 一 ， 才华之 尸 沪刁卢 扮产 产含 一 州 忿 又 ， 之二 ℃ 产 户乞 “ 少冷 生 一 庚名 少一 消多厂 臀 夕 卫二节子 一沁工考。 少 卜 “ ，， 、 己 ， 。 卜 飞 。 一 。 一 。 一 。 一 。 」 、 。 又 皿 扭 一 一 · 一
only 5g/min,for prefused flux with same size,it reached about 7g/min.This result showed that the particle size of prefused fluxes was related obviously to the mclting behaviour,which was inconsistent with the conclusion presented by authort7).It was also concluded that there was a little difference some time between“point”and“volume”fusion rate.The further work should be that to study the relationship of the physical properties,heat transfer with the fusion rate. 3 Discussion The melting mechanism of fluxes and the relation between the particle size of fluxes and the heat transfer during melting will be applted to explain the difference of melting behaviour between prefused powder flux and simply mixed powder flux as follow. 3.1 Melting Mechanism The melting process is a physical-chemical reaction process in which multiple raw materials are involved and interacted on each another.These reactions lead to three layer structures of flux macroscopically in different intervals of tempe- rature.The melting mechanism of prefused flux shows that the flux is only heated and melted,but that of simply mixed flux is more complex. The melting process in which the simply mixed flux is transformed from solid powder into liquid flux must pass throngh several stages as follow:(a)rea- ctions in solid phase at low temperature (generally lower than 500C);(b)si- ntering reactions (500-800C);(c)liquid-solid interactions (800-900C);and (d) reactions in liquid phase (higher than 1000-1100C).With the rising of tempe- rature,the reaction products are activated contineously,and loose bonds between molecules resulting in the diffusion and permcance of them,and then,eutectic mixtures are formed.A new eutectic mixture binds with other eutectic mixture or a kind of raw materials and forms a newer cutectic mixture,thus,a benefi- cial condition for the sintering,melting and chemical reactions of powder flux is establiseed.Most of the substance involved in this process are not liquid until 800C,but a little for cutectic mixture and liquid.While the temperature is hi- gher than 900C or 1000C,the additives or some formed cutectic mixture arc changed into liquid and reached a certain superheat.It is certain that there is much liquid flux with lower viscosity in the system,which is favourable for the dissolution of remained solid.Particularly,when temperature is rised to 1100C, this dissolution reaction will be intensified sharply.R.Satott observed the stru- cture of mold flux samples at different temperature with X-ray and confir- med that the phenomena above existed indeed.So it is concluded that the 522
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solid powder of flux cannot be transfor ned wholly into liquid without the appe- arence of liqiid cuteetic micture or additives,because the melting point of every kind of raw materials is higher than the melting temperature determined by the ingredient of flux although the temperature of flux exceeds its melting point. Furthermore,temperature is a necessary condition for the melting of flux,and the fusion rate of simply mixed powder flux is controlled by the formation of eute- ctic mixture and the dissolution of solid powder.The rate of solid dissolution that depends upon the chemical compositions and physical properties (including particle size,ect.)of raw materials,the type and amount of additives,contacts naturally with the diffusion rate of the dissolution of solid substance,the relation between them can be shown as follows: -dw dt.V=K.A.(C,-C)/V where,V-volume of liquid (cm3) W-mass of solid powder (g) K-mass transfer coefficient (cm/s) 4-area of solid-liquid interface (cm2) C。一saturate concentration(g/cm3) C-concentration at time of (g/cm3) From this equation,it is known that the fusion rate of solid powder in liquid 车 flux is related to the mass transfer coefficient,area of the solid-liquid interface (i.e.,the particle size)and the saturate dissoluability (ie.,the difference of concentrations). In the case of prefused powder flux,this fusion process described above has already completed in pretreatment process of flux.The fusion rate is written as: △m/△t=(T2-T1)q8 herc,m-weight of fused flux (g) t一fusing time(s) A-heat conductivity of flux (J/s cmC) T2-temperature of liquid metal (C) Ti-temperature of flux (C) q-latent heat of flux (J/g) 8-thickness of "saturation"liquid flux layer surrounding a solid particle. This equation shows that the fusion rate of prefused flux is in direct propo- rtion to the heat conductivity of flux and superheat,and is in inverse proportion to the latent heat of flux and the thickness of layer of saturate liquid flux su- rrounding a solid particle.Generally,the values of A,g and 6 of flux are 523
、 、 〕 〕 〕 卫 。 ， 几 以 。 呈、 住 ， ， 〕 五 · ， 扭 ， 爪 ， 。 ， ， ， 一 牙 · 犷 · · 。 一 犷 阅，卜 ， “ 牙一 一 一 一 。 一 “ 一 “ ， 从 一 ， 一 乓 ， 一 · ， 。 ， · △ 。 △ 凡 一 ， · 占 ‘ 、 ， 一 一 几一 “ 一 爪 。 一 “ 叮一 一 “ · 五 ， 五 又 ， 久 ， 占
constant,so the fusion rate of prefused flux mainly depends on the superheat. Because the temperature of liquid metal in ingot is usually about 1500C and the temperature of melting point of flux is only 1150-1250C,once the heat transfer makes the liquid temperature exceed its melting point,fusion process starts.Even if the superheat is not too high,the fusion rate will also be great. 3.2 Effect of Particle Size and Heat Transfer According to the classical theory,in the casc of using mold fluxes of same chemical compositions,the fusion rate of them becomes higher with the reducing in particle size.This effect is obvious until the particle size of flux is less than 150 mesh.This conclusion is presented based on the observation of the "point" melting behaviour.However,the measured results of "volume"melting behaviour of the prefused powder flux is opposite to it.It is presented that the reason of this phcnomenon is connected with the influence of particle size on heat transfer When temperature reaches the designed melting temperature of flux,(m), the thickness of fusion layer in the vertical direction is described respectively as h or hp in the case that two kinds of flux system which have different particle size.Because is grcater than h,according to the theory of heat transfer, flux mixed up with large particle powder is favourable to increase the fusion rate as a result of causing a large fusible interval.But the fusion rate of simply mixed powder flux is depentent on the dissolution rate of solid powder in liquid,especially for the flux whose particle size is larger,the dissolution will become the limiting step of fusing process.So it is impossible to obtain a conclu- sion that the larger the particle,the faster the fusion of flux.This is connected with the "point"melting behaviour.As long as the particle size of flux reaches a certain extent and has no influence on the dissolution rate,it is possible for flux which is comprised of large particle powder to have a strong effect on heat transfer and lead to a great change in fusion bchaviour. 4 Conclusions (1)The melting temperature of prefused flux is lower than that of simply mixed flux,and the tendency to form glass and the homogenization are also better in the case even the two kinds of fluxes have same chemical compositions and same particle size. (2)The difference of melting temperalure between them was reduced with the decrease in particle size.The sensitive extent of the melting temperature to the particle size of simply mixed flux is greater than that of prefused flux. (3)The expcrimental results of "volume"melting behaviour show that the 524
， · 几 “ 了 一 “ ， ， 一 ， 。 ， ， “ 、 ， · 、 ， “ ” · 〕 · ， 二 ， 、 、 、 几。 ， ， 又 “ 一 ， ， · 一 ， · 、 “ ” · ， 才 月卜 、 ， 、 。 ‘ · 、 。 · ， “ ，， 尹
fusion rate of prefused flux is increased with the increase in particle size.This is due to the relation of the particle size to the heat transfer in flux layer. ACKNOWLEDGEMENT Mr.Wang Xin and Li Xiuwen have kindly assisted in the experimental work,grateful for their help. REFERENCES 1 Sato R.Steel Making Proceedings ISS-AIME,1979;62:48 2 McCauley W L,Apelian D.Iron and Steel Making,1983;(1):1984 3 Gray R,Marston H.Steelmaking Proceedings,ISS-AIME,1979; 62:93 4 Jin Shantong.Iron and Steel of China 1980;15 (5):14 5 Jin Shantong.The 3rd China-Japan Symposium on Science and Te- chnology of Iron and Steel,1985;410 6 McCauley W L.Iron and Steel Engineer,1985;(9):36 7 Jin Shantong.J of Beijing University,of Iron Steel Techno- 1o9y,1981;(1):15 8福井敞。J of the Iron&Steel Institute of Japan,1974;(11):396 GPH Tyde Tubular Jet Heat-Exchanger GPH is a new high performance heat-exchanger.It is constructed by stan- dard special section tubes.Multistage Jetting is used on the air side,so the ef- ficiency of heat-exchanger increases considerably.The overall coefficient of heat transfer is as high as 45~60W/m2C.The pressure loss of air side is about 600 T Pa per stage.It is with lower wall temperature,figure impacted and lighter wei- ght. GPH is suitable for various industrial furnaces and kilns for utilizing waste heat to preheat air used in combustion.The air can be preheated to 300 to 500C according to the temperature of stack gases.At present it is a modern and ef- fective device of energy saving for industrial furnaces and kilns. GPH heat-exchanger and QRF burner can be worked together to form a complete set for industria!furnaces and kilns. 525
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