Steel Scrap Melting Process

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  • 8/2/2019 Steel Scrap Melting Process



    D e v e l o p m e n t o f S t e e l S c r a p M e l t i n g P r o c e s sK a z u o O h n u k i' "T e r uy o s h i H i ra o k a '"T a ka s hi I no u e*3

    K a z u s hi ge U r n e z aw a ' "N o z om i M a t su m o to "

    Abstract:Scrap Steel is a valuable iron source. R ecently in the Japanese steel industry,

    a problem of surplus scrap steel is com ing to surface w ith a steady increase in scrapg en era tio n a ga in st sta gn an t stee l p ro du ctio n tha t u se s sc rap ste el. A ga in st th is b ack-ground, N ippon Steel has been w orking on the developm ent of technology for m elt-ing large am ounts o f scrap . In 1993, H irohata W orks shut down its b last furnaceand commercialized the process of m elting scrap in the basic oxygen furnace (B O F).In th is paper, the im portance of scrap m elting is described , N ippon Steel's BO Fsc ra p m eltin g p ro ce ss is e xp la in ed , a nd its a ctiv itie s c on ce rn in g th e re cy clin g o f s cra pas typified by steel cans are in troduced .

    1. IntroductionJapan's steel accumulation has been yearly increasing, exceed-ed 1 billion tons in 1992, and is expected to grow further in thefuture. Along with the increasing steel accumulation, scrap sup-ply has been increasing and amounted to some 50 million tonsin 1990.Scrap is consumed mainly in electric arc furnaces (EAFs) andpartly in the blast furnace (BF)-basic oxygen furnace (BOF)process. Scrap is an iron source that needs no reduction energy,as compared with iron ore. Increasing the scrap consumption bydeveloping a process that can increase the scrap recycle ratio willnot only contribute to energy conservation, but also solve.suchproblems as exhaustion of natural resources and global warm-ing due to increasing emissions of carbon dioxide gas (C02)' Thisis one of the challenges confronting the steel industry.Nippon Steel commissioned a BOF scrap melting process atHirohata Works in June 1993 to pioneer a new form of scrapmelting. Steel cans and other scrap generated in the market, letalone return scrap or home scrap, are recycled and melted intosteel products.*1 Technical Development Bureau*2 Hirohata Works*3 Muroran Works

    This article explains the importance of scrap melting, describesNippon Steel's scrap recycling activities, and introduces the BOFscrap melting process developed to support the scrap recyclingactivities.2. Importance of Scrap Utilization TechnologyScrap may be broadly classified into home scrap generatedat steelworks, prompt industrial scrap generated in the courseof steel processing at machine shops anti the like, and dormantscrap comprising obsolete, worn or broken products of steel con-suming industries. Generation of home scrap has little changedin recent years thanks to various measures taken to improve yield,such as continuous casting. This is also true of prompt industri-al scrap, which reflects the cost-saving effort of users. Dormantscrap now accounts for a little over one half of the total scrapsupply, and is predicted to increase further with increasing ac-cumulation of steel on the market.The above situation is graphically represented in Fig. 1 by theannual changes in steel accumulation and market scrap (or promptindustrial scrap and dormant scrap). Japan's steel accumulationhas yearly increased, marked a little less than 1 billion tons in1991, and is projected to grow further in the future. Generally,generation of dormant scrap can be estimated from the servicelife and accumulation of various steel products. Roughly speak-

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    40 10Steel accumulation ~

    4 : : = ~ : : ~ ~ : : : : : : : : : ~ ; ; - - ~ ~ ~ ; R *:::, 25 ... F . c . : : r : : 1 Dormantscrap 7c:o 20.~c:~ 15Coe~

    ~8c:0S ~S~ 0.~c: "'0.~ o'3 "s p:::Ioo'"ilsen

    -----------------------------. 6

    10 4- - - - - - - - - . - . - - I - ~ - 3Wromptndustrialscrap0~~~~~ww~~~~~f~~.~~.~~1~~.~wu1977 80 85 90 91Year

    Fig. 1 Annual changes in steel accumulation and market scrap generation inJapan

    ing, the generation of dormant scrap is proportional to the ac-cumulation of steel on the market. In recent years, generationof dormant scrap has increased with increasing steel accumula-tion. Market scrap now totals more than 30million tons per year.In contrast with this scrap generation, the steel demand isnotexpected to grow under the current environment surroundingJapan's steel industry, and the increasing trend of scrap supplyas an iron source cannot be overlooked. In other words, scrapis an iron source that does not need reduction energy as iron oredoes. Efficient melting of scrap is considered to reduce the totalcost of steel production, depending on the scrap price, to pre-vent natural resources from being exhausted, and to reduce car-bon dioxide emissions. Increase of scrap supply against scrapdemand means that if scrap consumption does not increase, scrap,particularly dormant scrap, will be left discarded as pollutingwaste in increasing quantities.Against this background, Nippon Steelhas been positively car-rying out scrap recycling activities. As an example, steel can recy-cle is introduced below. Nippon Steel's technology for increasingthe amount of scrap melting in the BOF and the new BOF scrapmelting process it developed as an extension of the conventionaltechnology are also described.3. Actual State and Future Targets of Steel CanRecycleThe Japan Used Can Treatment Association has been carry-ing out a steel can recycling campaign. Nippon Steel is participat-ing in the campaign.Fig. 2t) shows the recent statistics of steel can recycle ratio

    and recycled steel can consumption in Japan. Production of steelcans in 1992was 1.4 million tons, of which 800,000 tons or 570/0were recycled. Production and recycle figures are as shown inFig. 3.Beverage cans are either steel cans or aluminum cans. Steel

    sheet for steel cans can be produced with only one-eighth of theenergy required for the production of aluminum sheet for alu-minum cans. Steel cans can be easily separated from other wastesby magnets. Ifsteel cans are collected as separated at sources inincreasing amounts, their recycle ratio will improve further.The Japan Used Can Treatment Association in which Nip-pon Steel taken part is working toward a target steel can recycle

    NIPPON STEEL TECHNICAL REPORT No. 61 APRIL 199470r--------------------------

    Recycle ratio '?~ 56.8 1,000 8050.y 0~

    43.6 44.~ 900 e4~--o 795 .9800 i5 .e721 ::I~n 700 0654 c:'"81 600 "il"Consumption 500 '0.~o1'95 "'88 '89 '90 '91 '92 p:

    Year TargetFig. 2 Changes in recycled steel can tonnage and recycle ratio

    P ro du ct i o n G en er a ti o n D i s p o s a l R e u s eo l l e c t i o n

    T ip pe d a tl a n d f i l l43 %

    R e m e l t i n gE A F s9 0%S F s1 0 %

    R e c y c l e d ~57~o8 0 0 , 0 0 0

    t o n s

    Fig. 3 Steel can ret rieval in 1992

    ratio of over 600/0in 1995. Particular efforts are exerted for in-creasing the consumption of recycled steel cans to more than200,000 tons annually in the blast furnace operation.The history of technology development for melting largeamounts of scrap is described in the next chapter.4. Progress in Technology of Melting Scrap in BF-BOF ProcessIn the BOF steelmaking process that uses the heat of carbonoxidation, the heat of reaction from C to CO ismainly used. Theheat of reaction from CO to C02, however, ismuch greater thanthat from C to CO as given by Eqs. (1) and (2). Ifthe secondarycombustion (post combustion) step from CO to C02 is utilized,the amount of scrap melting can be increased without prolong-ing the tap-to-tap time.C + 1/202 = CO + 9,209 kJ/kg-C (1)

    CO + 1/202 = C02 + 23,567 kJ/kg-C (2)In normal BOF operation, the ratio of CO burned to C02 (thepost combustion ratio) is 5 to 10%. It has been confirmed thatthe amount of scrap melting can be increased by increasing thepost combustion ratio. For example, the scrap ratio can be in-creased by 3.4% by increasing the post combustion ratio by 10%.In view of the importance of grasping the post combustionbehavior during BOF refining, Nippon Steel worked to clarifythe mechanism of post combustion. The post combustionmechanism isexplained hereunder from the behavior of an oxy-gen jet in the BOF as illustrated in Fig. 4.As shown in the figure, oxygen jet discharged from the topblowing lance nozzle propagates through the supersonic and tran-

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    sition speed regions until it becomes a free jet. The reaction be-tween the oxygen jet and the CO gas (gas produced bydecarburization) entrained by the free jet forms a jet predominant-ly composed of CO2. In the surface layer of the C02 jet or atvelocities lower than the critical velocity, some CO2 escapes fromthe C02 jet while accompanying the CO gas flow generated atthe lower hot spot. The post combustion ratio depends on theratio of the amount of fugitive C02 to the amount of CO gener-ated by the decarburization reaction", The possibility of con-trolling the post combustion ratio from the jet behavior has beenelucidated.A coal top blowing process (ALCI process) was developed asa positive method of adding a heat source in the BOF3). TheALCI process injects coal at high speed into the bath in the fur-nace and promotes the post combustion of CO by a lance withmain and auxiliary oxygenholes. The benefit of the ALCI processis shown in Fig. 5. As compared with the conventional top cokeaddition process, the ALCI process was found to provide a higherscrap ratio at the same amount of carbon combustion. Thisbenefit may be explained from improvements achieved in the car-bon yield and post combustion ratio.The new BOF scrap melting process developed against theabove-mentioned technological background is described in thechapter that follows.

    Lance nozzle

    CO entrained by 02 jet

    ~Supersonic jet

    CO2 leaving jet!CO by decarburizationreaction

    Free jet region

    Fig. 4 Schematic illustration of post combustion mechanism

    5. BOF Hot Heel Scrap Melting Process5.1 Outline"Nippon Steel commercialized a new BOF scrap melting processat Hirohata Works in July 1993. When the switchover wasmadefrom the conventional BF-BOF process, the new process conceptwas formed from the following considerations:(1) Utilize the existing BOF to reduce the equipment cost and re-tain the freedom of raw materials and fuel use.(2) Separate the scrap melting furnace and the decarburizing fur-nace to secure high productivity and high product quality.(3) Minimize the total production cost through the effective utili-zation of waste energy, such as recovering the off-gas via the00 system of the BOF plant.Accordingly, Nippon Steel developed the new BOF hot heelscrap melting process as shown in Fig. 6.

    Fig. 7 shows the flow chart of melting operation by the newprocess. Scrap is charged into the melting furnace from abovethrough a chute, and coal and oxygen are blown into the chargeas fuels through the bottom of the furnace. The scrap is rapidlymelted by the heat of post combustion by the top lance as well.Upon completion of the scrap melting period, a half of the hotmetal is tapped from the melting furnace, desulfurized and other-wise treated, and charged into the decarburizing furnace. Fea-tures of this process may be summarized as follows:

    25r-------------------------, ALC! process

    20 0 Coke addition process~o.~ 150.~~c"ij>0 g .~

    o 10 15 20 25 30Carbon combustion (kg-CfTS)

    Fig. 5 Benefit of ALC! processDecarbur iz ingf u r n a c e

    D l~h ot CO , \ B Olten stee lme t a lFig. 6 Hot-heel BOF scrap melting process

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    (1) Fast and stable melting of scrap by leaving a half of each heatin the melting furnace(2) Stable production of clean steel through the treatment ofquasi-steel by the process similar to the conventional process(3) Extended refractory life under low-temperature melting(4) Simultaneous securement of melting heat source by control-ling the post combustion ratio and utilizing the recoveredoff-gas(5) Bottom injection of pulverized coal in the utilization of low-cost coal and from the viewpoint of hot metal carburizationin the vicinity of scrap meltingThe constituent techniques that support the new process aredescribed in the sections that follow.5.2 Importance of post combustion in melting furnace; ControlmethodAn example of heat balance in the melting furnace is shownin Fig. 8. The heat of post combustion from CO to CO2 is ex-tremely large, and an increase in the amount of post combustionleads to an improvement in the melting productivity. From thestandpoint of recovered gas quality, however, an excessive in-crease in the amount of post combustion is not preferred, andtherefore the control of post combustion is important.As to the mechanism of BOF post conbustion, only the be-havior of the top-blown oxygen jet described in the precedingchapter has been given due consideration". Since this model isintended only for top-blowing conditions, it was reviewed for theaddition of coal and oxygen bottom injection. Assuming that theCO evolving from the bottom-blown oxygen and hydrogen in-

    0,Scrap chute Top Ic : : > bl~wing I I Pig ironHigh-carbon C J an8e( tf"quasi-hot metal c : : : : > _ . c : : : : > ~- - - 0 2 , coal U: : : : > O e s u i r u r i z a t i o n c : : : : > D e c a r b u r i z a t i o nQuasi-hot metal

    Fig. 7 Flow chart of melt ing in hot -heel BOF scrap mel ting process

    S en si bl e h ea t O th er so f h ot m e t a l

    H e at o f r ea ct io no f o f f- g ase ns ib le h ea t o foff-gas

    Fig. 8 Example of heat balance assumed for full-scale scrap melting furnace

    NIPPON STEEL TECHNICAL REPORT No. 61 APRIL 1994troduced by the bottom-injected coal only serve to dilute the gasevolving from the top blowing, simplified model equations wereprepared for estimating the post combustion ratio".PCR' = 1O(x/d)0.3- lOO(x/d)-0.7 + 1 (3)

    PCR = = RPCR'/(1 + PCR' - RPCR') (4)where PCR' isthe post combustion ratio with top blowing alone;PCR is the post combustion ratio with both top and bottom blow-ing; x is the length of the free jet (mm); d is the lance hole di-ameter (mm); and R is the top blow ratio.The relationship between PCR' back calculated from the meanpost combustion ratio during the melting period and the top blowratio and x/d is shown in Fig. 9. The relationship between theobserved mean post combustion ratio and the post combustionratio calculated by the above model is shown in Fig. 10. The dataobtained verified the validity of the new simplified model forpredicting the post combustion ratio during the melting periodand controlling the amount of post combustion in the meltingfurnace.The target post combustion ratio can be achieved by chang-ing the lance shape, lance height, and other relevant conditionsas required.5.3 Scrap melting behaviorUnderstanding the scrap melting mechanism is important innot only predicting process productivity but also controlling thetemperature and carbon content of hot metal. Described beloware the results of melting behavior experiments using 500-kg and5-ton furnaces and melting model studies.In the 5-ton furnace, copper was added to hot metal beforescrap melting, and the scrap melting ratio was determined fromthe dilution of the copper. The bath temperature variation dur-ing the melting period was also measured. The results are given

    10080 Fo,(Nm

    J Ih/t) eq.(3)0 71

    ~ 141-159~ 40 0UPo .6~0----~--_J-----LO----L---~100 140 18 220 260

    x/d (-)Fig. 9 Relationship between PCR' and x/d


    PCR"k (070)Fig. 10 Comparison of measured and calculated PCR

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    NIPPON STEEL TECHNICAL REPORT No. 61 APRIL 1994in Fig. 11. In the initial melting stage, the bath temperaturedropped, and the melting ratio did not rise; After 10minutes intothe melting process, the melting ratio and bath temperature bothstarted to rise.The carbon and temperature profiles near the scrap-hot met-al interface during the melting period are schematically illustrat-ed in Fig. 12. The melting of scrap ispromoted by the followingconditions:(1) Increase of interface carbon content C, with increasing bathcarbon concentration(2) Increase of interface carbon content C, and interface tem-perature T, with increasing bath stirring force(3) Rise in the bath temperatureTo quantify the factors that promote the scrap melting, a melt-ing behavior model'" that allows for simultaneous heat and masstransfer, and a BOF heat and mass balance model" were com-bined to build a scrap melting model'", Itwas confirmed that themodel brings out results that practically agree with the experimen-tal results shown in Fig. 11.5.4 Effect of coal melting on scrapFor appropriate control of the bath carbon concentration dur-ing the scrap melting period and for the promotion of the above-mentioned scrap melting behavior, the melting behavior of thecoal injected into the hot metal must be clarified. The meltingof coal was analyzed using the 500-kg and 5-ton furnaces.Various carbonaceous materials were placed on the molteniron bath in the 500-kg furnace, and the resultant carburization. . .Melting ratio }Measured80 0Bath temperature values

    1800 t~ 40 .. . _ _Calculated values ~0 1600 ;::I'E 0 ~. . ' 0OIl ....0 1400 0. 5 ...9"'6"0 E-40 1lQj L....--- .c~ w __ ~ _______ . " o s-80 h: 30000 1200 ~u: 0.5-120 10000 10 15 20 25 30 35

    Time (min)Fig. 11 Changes with time in melting ratio and bath temperature (5t pilot

    furnace)e~ Scrap Liquid Fe-C:: Liquidus temperatureg _ . . " TL,b.~ Ts .-.. .1 (_ _ . . _iV,:~~e~~~ure ~.~ CL,b ~~ Cs. .__ g _g ~o .~ug Co- e'"u-----'-- __(a) Temperature and carbon

    concentration profiles


    Carbon con entration (070)(b) Carbon concentration versus

    liquidus temperatureFig. 12 Bath carbon concentration and temperature near scrap-bath interface

    rate under gas stirring was measured. The carbonaceous materi-als used in the experiment are shown in Table 1. The apparentmass transfer coefficient was calculated from the obtained car-burization rate and bath cross-sectional area, and was comparedwith the sulfur content of coal, as shown in Fig. 13. The carbu-rization rate decreases with increasing sulfur content. This ef-fect can be explained by the action of sulfur that isa surface activeelement. The carburization rate increases with decreasing vola-tile matter of coal.Assuming that the coal melting rate is controlled by the met-al mass transfer, a model for predicting the coal melting rate wasstudied.The carburization rate measured without top blowing is con-sidered to correspond to the difference between carburizationfrom the bottom-injected coal and carburization from the bottom-blown oxygen, and is given byd[C]/dt = k(A/V)([C]s - [C]) (5)where A is the reaction interface area defined by the amount ofcoal in suspension and the specific surface area of coal. Theamount of coal in suspension is determined from the hot metalrising rate?' and coal injection rate.Using the model, the results of the 5-ton furnace experimentunder the conditions of Table 2 were evaluated (see Fig. 14). Asthe bath carbon concentration increases to approach saturation,the melting rate of the bottom-injected coal declines. When thecoal is high in sulfur and low in carburization nature, the coalmelting rate is lower under a lower bath carbon concentration.Measurements were made also of the carbon concentrationin the dust generated when scrap was melted with the top-blownoxygen. When different types of coals were used, unmelted coalparticles were observed in the dust if the carburization rate fellbelow a certain level.Thus, the promotion of coal melting calls for the selectionof low-sulfur and low-volatile coal and the injection of pulver-ized coal.Control of the post combustion ratio with attention focusedon the lance conditions, and optimization of the scrap melting

    Table 1 Compositions of carbonaceous materia lsMaterial FC

    A (graphite) 98.8B (coke) 84.5C (coal) 78.9D (coal) 79.6E (coal) 59.3

    ~ 0.10 - .,E~" " 0.08C 5C ! 0.060u U' 0.04c.S

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    NIPPON STEEL TECHNICAL REPORT No. 61 APRIL 1994Table 2 Experimental conditions

    Hot metal 5 tons, 1300-1500C[C];n;: 2.0-3.5"7.N2: 1.6-5.8 Nm'/min02: 0.0-3.5 Nrrr'z'minLPG: 0.0-0.2 Nrrr'Zrnin9.0-12.5 kg/min

    Gas flow rate

    Coal feed rate

    ~ 40 0 Large~ 30 coal loss-e Coal E ~s 0c0 20~c 0(ij 10 5i- Coal C, . 0'2 I I I Coal C s< r=:':0.15-' 5 / Estimated by modelG 0.10 / \ -e" I .r"~ I /I ./0.05 I0 I ./.~ I ./ Coal E. ! : : i I ./~ I0 I :~ 0 I 2 3u [C], - [C] (%)

    [C] (%) 4 2Fig. 14 Effect of coal type on carburization rate

    rate that depends on the bath carbon concentration and stirringforce and of the coal melting rate as required to maintain thedesired bath carbon concentration are important in the new hot-heel BOF scrap melting process.6. Future OutlookRecycling of used materials is becoming a worldwide trend,as symbolized by the issues of the global environment- The hot-heel BOF scrap melting process will draw attention as a newprocess that can solve the iron source problem while making ef-fective use of the structure of existing steel plants.As an integrated steelmaker, Nippon Steel has pioneered ef-fective scrap utilization processes, as represented by the new hot-heel BOF scrap melting process, and has moved to address fu-ture changes in the iron source structure, The authors intend topush ahead with studies for an early commercialization of newtechnologies through the activities of the Environmentally Friend-lyAdvanced Steelmaking Process Forum that has been research-ing on the effective utilization of low-grade scrap.

    References1) Japan Used Can Treatment Association: Private communication2) Harada, T. et al. : Tetsu-to-Hagane. 71, S187 (1985)3) Nakamura, K. et al. : Tetsu-to-Hagane, 73, A47 (1987)4) Ozawa, K_et al.: 6th IISC Proc, 1990, p.335) Ohnuki, K. et al.: CAMP-ISIJ. 4, 276 (1991)6) Isobe, K. et al .: CAMP-ISIJ. 3, 130 (1990)7) Ishikawa, H. et al. : Tetsu-to-Hagane, 73, 653 (1987)8) Isobe, K. et al.: CAMP-ISIJ. 3, 1B9 (1990)9) Sawada, 1. et al.: Tetsu-to-Hagane. 70, S160 (1984)

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