High Temperature Properties of BOF Slag and its Behaviour ... · Introduction At blast furnace (BF)...

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Process Metallurgy - lronmaking --̂_

High Temperature Properties of BOF Slag and its Behaviour in the Blast Furnace Lena Sundqvist okvist

SSAB Tunnplat AB, Luleii and Lulea University of Technology, Lulea / Sweden

Slag formation in the bosh and raceway is an important issue in the blast furnace process. SSAB works in Lulea operate with 100 % olivine pellets. A small amount of basic fluxes is added from the top, slag and limestone are used. To improve the control of slag formation, a con- cept with injection of BOF slag was evaluated and tested in the LKAB experimental blast furnace (EBF). In this paper, the behaviour of BOF slag as a slag former, when it is top-charged or tuyere-injected, is evaluated based on the results from a laboratory study including reduction tests, softening and melting tests, XRD analyses and SEM analyses. Samples taken from the EBF during excavation, and with a burden probe during operation, are examined. The evaluations show that the melting point of BOF slag is quite low and will not be increased, because of the partial reduction that occurs in the BF shaft. When carbon is present at high temperatures, reduction proceeds and a high basicity slag with a high melting point, consisting of di- and tricalcium silicates, is formed. When pellets with a basicity of 82 - 1 are used, a slag with similar properties can be formed as a result of interaction with the BOF slag. BOF slag in combination with olivine pellets with a low basicity generates a slag with intermediate basicity and the reduction of iron oxides in the slag has a small effect on the melting temperature. The BOF slag decreases the melting point of coal and coke ashes.

Keywords: BOF slag, basic flux, bosh slag, blast furnace, slag formation, reduction, melting point, dissection.

Introduction

At blast furnace (BF) No. 3 in LuleS, approximately 40- SO kg/tHM of BOF slag is top charged together with two quite similar types of olivine pellets, MPBO and KPBO, produced by the Swedish pellet producer LKAB. The amount of BOF slag used varies from time to time, depending on the ratio between the two pellet types because KPBO contains more P and less V compared to MPBO. When KPBO is used, the addition of BOF slag has to be decreased to meet the demand of P content of hot metal. Average data of 2003 for BF No.3 in Lule?~ are found in table 1.

Table 1. Averages of production data in 2003 of BF No. 3 in LuleB.

Burden

Additives

Reducing agents

Blast

Productivity

Hot metal

Slag

MPRO pellets

KPRO oellets

Lime stone

BOF slag

Manganese slag

Dusr briquettes

Coke

PCI

Specific boltme

Temperature

Or content

c Si Si std. dev

S

Trmnerature

837

5 14

42

42

4

51

339

134

95 1

I099

24. I

2.51

3.64

0.34

0.117

0.04 I

1482

166

1.01

17.5

wt 00

In a Finnish study of BOF slag the main mineral phases occurring were dicalcium silicate, tricalcium silicate, wustite, dicalcium and calcium ferrite and lime. Dicalcium ferrite and wustite in solid solution with MgO and MnO formed the matrix. VzO5 and P 2 0 ~ were found in the edges of the tricalcium and dicalcium silicate grains [ I ] . In previous studies [2,3] made within this project, it was found that the iron in the BOF slag is present as Fe,,,,, Fe2+ and Fe3* in metal, wustite and calcium ferrite. Calcium silicates and free lime were also found. Interaction between BOF slag and olivine pellets was observed locally at the contact sur- faces between BOF slag and olivine pellets at the level of the cohesive zone ofthe BF. The addition of BOF slag was shown to improve the melting properties of a bed of olivine pellets up to a certain amount of addition, but deteriorates the melting properties of a bed of self-t’luxed pellet with CaO/Si02=11 Interaction between the latter pellet and BOF slag caused formation of a high basicity slag. The melting point of this slag was increased. when it was further reduced and the Fe0 content of i t decreased [2,3] . This verified the results from theoretical analyses (4 1 of the operational problems with the same pellets in a full-scale test made at SSAB. The melting temperature of the BOF slag was found to increase in the presence of C during melting and no dripping material was collected when BOF slag was sandwiched between coke layers in a graphite crucible 1231. Previous laboratory tests have showed that addition of BOF slag improves the melting properties of tuyere slag when added to coke and coal ash [ 5 ] .

Based on the proposal to inject BOF slag to get a smooth slag formation 141. a test was made in the LKAB experimental blast furnace (EBF). The process was stable, the Si content of hot metal and the rate of reducing agents were significantly decreased and Ihe slag rate was around 100 kg/tHM when the BOF slag was injected 161. In the litera- ture, injection of fines of fluxes and iron oxide into the blast furnace was reported to decrease the Si content of hot metal [7,8]. The effect of adding MgO containing material by tuyere injection was shown to decrease the silicon content

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Process M~tallurgy - lronmaking ~-

Limestone

more efficiently compiired to when top charging it. The cf- fect of top charging wils more significant when the flux was charged in to the coke layers, compared to when it was mixed with ore 171.

The research W;IS made t o achieve a more fundamental iinderst~in~iing o f how BOF slag behaves as basic flux at high temperatures in a BF when top charged or tuyure injected. This study includes reduction tests. softening and melting tests, melting point measurements. XRD. high temperature XRD, chemical analysis and SEM analysis. BOF slag samples taken from the EBF are examined. The results from these studies are presented in this paper.

54.2 1.06 1.23 0.64 0.34 I

Experimental technique

Sample material. The original chemical compositions of raw rnaterials used in the laboratory tests and charged into the EBF [9] are found in table 2. Samples taken both with the inclined probe at operation of the EBF and during excavation of the EBF after two different test campaigns are examined. In the 1" and 2"" campaigns the burden consisted of 100 % of MPBO and KPBO, respectively. The 1" campaign was operated with all coke and during the 2"d campaign powdered coal was injected at a rate of 86 kg/tHM. The basic tluxes added were BOF slag and limestone. The main difference compared to the burden used at SSAB is the addition of quartzite that is required to compensate for the high Si content of the hot metal and the considerably higher consumption of reducing agents. At the end of the campaigns. the EBF was quenched in nitrogen. The excavation of the EBF started from the top and the pellet layers are numbered beginning from the top. The positions of the samples taken were in the centre. at the intermediate of centre and wall and at the wall. The position of the cohesive zone was approximately at layer 23 in the I " campaign [ 101 and at layer 2 1 in the 2"" campaign [ 1 1 1 and the cohesive zone consisted of one and three pellet layers, respectively. The 23"' pellet layer ( I " camp.) was located just above the tuyere level and the 21" (2"" camp.) pellet layer wits located approxiniately 1.2 m above the tuyere level. BOF slag "as not present below the cohesive zone, but lime could be

registered. Softening starts when the contour of the sample pets smooth and n o sharp border can be distinguished. Ac- tive melting occurs when the melting is rapid and the shape of the specimen changes quickly. Flow point is detcrininrd as the temperature where the mnterial ilows out and the height o f the specimen decreases to 1/3 of the original height.

Reduction. Isothermal reduction tests were perfonned with approximately I IS g of the BOF slag arranged in a basket with several floors. Eiich layer of BOF slag had :I thickness ctf about 1 cm. The BOF slag samples were reduced during 160 or 140 min at 950 "C or 1 IS0 "C, respectively, in a reducing gas consisting of 5 % Hz, 40 %, CC1 and 55 %, Nz.

BF simulating reduction under-load tests were made at the LKAB metallurgical laboratory, in an equipment according to ISO7992. Samples of I2OU g BOF slag were reduced in two different blast furnace simulating programmes (TI and T2). Figure 2 shows the rate of temperature increase and the reducing gas composition during the tests. The total gas flow was approximately 59 limin and 77 I/min in T1 and T2. respectively. The maximum load used, 1000 g/cm', was reached after approximately 1 (XI min and I60 min in T1 and T2, respectively.

Softening and melting tests were performed in N2 atmosphere. Approximately 80 g of BOF slag was put in a bed in a crucible in the softening and melting equipment 131. The different combinations of crucible. coke addition, alumina addition and prereduction degrees tested are shown in figure 3. Time, temperature, bed height and the weight of collected material were continuously registered during the test.

Experimental results

Characterisation of BOF slag. The porosity of BOF slag was measured to 3.8 %. This can be compared with the

found below the tuyere level.

Melting point measurements. The te5t method u\ed was a standard method to determine the melting properties of a Flag in coal and coke [ 12 j and i t wa\ deicribed in detail pre- viously ls1. Melting Point m e a ~ ~ e m e n t s were performed with untreated BOF \lag, BOF clag reduced to - 60% reduction degree, BOF rlag quenched in nitrogen after a \oftenrng and melting test and

Figure 1. Contour of sample at start of test (a), softening (b), active melting@), and flow point (d)

was tested on both alumina and platinum sample holder plates with similar results. Sketch- es of the samples were constantly drawn to show the changes in the shapes of the samples during heating. Figure 1 shows an example of the course from softening to flow out of a specimen. The temperatures when softening, active melting and flow point started were

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Process Metallurgy - lronmaklnq

12W 60%

. . . . . . . . .

600

400 . . . . . . . . . . . r

. . . . .

0 0 50 1W 150 2W 250 0 50 100 150 200 250

Time. mm Time. min

Figure 2. Test conditions in BF simulated reduction tests (T1 to the ieH and T2 to the right).

porosities of MPBO and KPBO which were measured to 25.8 % and 25.4 %, respectively. The tumbler strength tested according to IS0 3271, hut with a 6 mm sieve was measured to 95.6 % > 6 mm. The chemical composition of the BOF slag is given in table 2. As can he seen, the basicity B2 was approximately 4. The total Fe content normally varies in the range of 16-26 %. Fe in the BOF slag was present as Fe,,,, Fe’+ and Fe”. The Fe contained in the sample of BOF slag in table 2 consisted of 1.3 ?+ Femet, 12 % Fe2+ and 5.0 % Fe’+.

Mineral structure of ROF slag from EBF. The BOF slag sample taken from pellet layer 21, which was located approximately 0.7 m above the cohesive zone at excavation of the I ” campaign, consisted of a matrix of a calcium silicate phase (di- and tricalcium silicates) as shown in figure 4. On the rim of the calcium silicate phase, Si was partly replaced by V. AI was also detected in the calcium silicate phase. (Fe,Mg,Mn)O was spread over the matrix as well as some areas with Ca and Fe indicating calcium ferrites. Small grains of Fe,,, can be seen and CaO is found. The BOF slag sample shown in the SEM image in figure 5 was taken with an inclined probe at the level of the cohesive zone. The temperature was higher at this sample position and the BOF

BOF slag prereduced at 950T 10 25% reducuon degree

Gl U U i c l

slag panicle was in contact with an olivine pellet. This sample contained the same main phases as the previous sample, dicalcium silicate and (Fe, Mg, Mn)O, but the structure of dicalcium silicate changed into elongated crystals. The basicity of the BOF slag decreased and tricalcium silicate and free lime were not detected in this sample.

Chemical composition of BOF slag samples from EBF. Samples of BOF slag were taken from the hurden inside the EBF during excavation after two different campaigns. As can he seen from table 3, the chemical composition of BOF slag was almost unchanged in the shaft of the BF. Particles of BOF slag were not found below the melting zone, hut calcinated lime was found helow the tuyere level. Visually the pieces of BOF slag looked similar with BOF slag before charging, until the cohesive layers were reached. The chemical content of alkalis was very low in the original BOF slag and reached the highest levels in ROF slag taken from the pellet layers just above the cohesive zone (figure 6). The S content increased as the slag descended in the EBF, as il- lustrated in figure 7. Drops of metal and sldg were found at the levels of pellet layers 23 and 21 during dissection after the I ” and Znd campaign. respectively. The loss of ignition (LOI) of BOF slag samples decreased as the material de-

BOF slagprereducedat llsO*C U 10 SS% mductim degree

Ba 10 63% reduction degree BOF slagprereducedat 1150aC

G , >\I

BOF slag = Cake AI iUuminaerucible BOF slag mixedwith Gr Graphire crucible alurmna

BOF slag mixed with coke

Figure 3. Combinations of reduction degrees, crucibles and addi- tions tested in soHening and melting tests.

794

Figure 4. SEM image of an area in a BOF slag sample (C1 21 centre in table 3).

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Process Metallurgy - lronmaking

Table 3. Chemical compositions of BOF slag samples. All values are given in weight%. Cl. C2=E8F campaign No. 1 and 2, sample position is given by e.g. 9 centre=pellel layer 9 in the centre, SM=soflening and melting test, LOI=loss of ignition.

..

EBF samples

C I 9-centre

C I 14-centre CI 21-centre

CI 23-centre

c2 9-cenme

c2 I k r n t r e C2 I h-centre

C2 19-intermediate

C2 19-wall

CZ 20-intermediate r2 mmre Laboratoly samples

Dnginal sample

14% reduction degree

j S " o reduction degree

Sample from SM test

I IiHF silmdes

Active Softening melting Flow point

1306 I354 1377

1293 1323 I344

1255 1315 I334

u n x 1323 1344

I265 I300 1315

I244 1276 I300

1240 1275 I 10s

I240 I265 1286

1239 I260 1307

1255 1313

I220 1327 I502

1278 1292 1325

1271 I299 1339

1270 I?X5 I330

I250 I264 1335

E C2 C2

c2 ('2 C?

C2

9 centre I4.CC"tK

2 I -centre 23-cenlre 9-centre I ?-centre 16-centre

I 9-internirdiate IY-waIl

20-intermediate 20-Ce"tre

Original 14?6 red.degrer 59% red. degree

- Fe

16.9 19.7 IX.3

__

I 7.0 _. 17.3 17.6 17.8 17.2 23.5 22.9 37.2 Fe

I x.9 17.5 19.1 19.2

I

-

CaO S i 0 2

40.4 10.6 38.9 9.60 44.3 11.3 41.2 12.1

41.8 9.71 41.4 I0.X

44.3 1 I . X 39.6 9.95 39.6 12.1 25.1 15.2

CaO SiO?

41.4 9.76 41.7 10.8 43.0 10.1 I 43.0 10.4

42.0 I 1.0

xended, meaning that the samples taken at the layers close to the cohesive zone increased more in weight when heated up in oxidising atmosphere, compared to original BOF slag or samples taken in the upper part of the shaft. The basicity B2=CaO/Si02 decreased at this level also, especially in the samples taken from the dissection.

Melting point measurements. The results from melting point measurements of the samples listed in table 3 are con- cluded in table 4. The melting points of BOF slag are in general quite low. When measuring the melting point some samples of BOF slag swelled before softening and melting. The relative change of the size of the specimens can be seen in figure 8. The samples from layer 19 at the wall and from the intermediate area of layer 20 swelled most. The chemical compositions of these samples were quite similar as compared to original BOF slag, but the reduction degree

Figure 5. SEM image of an area In a BOF slag sample taken from the level of cohesive zone in EBF (W=FeO, RO=(Fe,Mg.Mn)O. C2S=2Ca0 Si02)

0.010 0.010

- LDI

-2.6 -2.X -?.U

-6.0

-2.2 -2.3 -3.3 -7.5 -7.9

- i 0.3

-14.9

LOI

-2.9 -2.6 -6.2 -7.0

-

-

_.

__

-

- c

0.07 1.62 0.21 0.10

11.19 0.17 0.17

0.16

__

-

0.09 0.13 0.23

C

0.06

- -

0.0X 0.30 -

- S

0.11 0.27 0.21 0.47 0.14 0.16 0.30 0.92 0.61 0.76 0.75

S

0.07

-

-

- -

0.07 0.ox -

and the content of alkalis and sulphur were higher. The specimen from the centre of layer 20 swelled less than the other, although, it also showed the same difference in potassium content compared to the original ROF slag. However, the basicity of this sample was lower compared to that of the other samples.

Reduction. Isothermal reduction tests were performed at 950 "C and I I50 "C in a reducing gas composed of 5 % Hz, 40 % CO and 55 % N?. The reduction rate of BOF slag was quite low at 950 "C and the reduction degree was -20-25 % after -160 min. The reduction rate rose when the reduction



$4

............... 2 5 ., i r - . 2 0 ~

.5 I~ i~ Na?O+K?O irlternediate. 51.L ' ..*. Na2'3-UO riternediale

* . Na2O+K2O centre .. __ ..... f Na?O+KZOwall 1 5 - .*. Na20-K20 wll , ..

. . . 0 c.o ..... . . . . . . i.T ... i g1a ;. 2'

o 5 ,~~~~ ~~ ~~ ~~~ ~~ ~.~ ~ + , , b' f~ ~~ ~ . 3.5 - . . .c . I

.~~ -- + .i. i +* , o ; . . . . . .- ::~ t~ ~: 3

I .i

0 0 I 5 10 15 20 25 0 5 10 15 20 25

Pellet layer. No tom top Pellet aver, No from top

I 0 % "C

Figure 6. Contents of alkalis in EOF slag samples taken at excavation of EEP aHer Is' and Znd campaign.

23

...... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ....

i -Scentre S w ~~ ~~~~~~~~ .- 2.0 - ~~~~

E 1.5 I..-. ~~ ~

'$0:5 i-~~.~ ...... ...

~

s : 0) 1 0 i---~ ._

0.0 4 ...... .-..

0 5 10 15 20 25 0 5 10 15 20 25 Pellet layer, No from lop Pellet layer, No from top

Figure 7. Content of S in EOF slag samples taken a1 excavation of EEP aHer Is' and Znd campaign.

...... A RX 1401 H I

Table 5. Experimental conditions in softening and melting tests (Gr=graphite, A=alumina. C=coke. L=layen. M=mixed).

Dripping Red. Shrink- 1 Degree I Crucible 1 Addition 1 W Dripping ",

. - C2-9 centre -C2-14 centre + C2-16 centre - s- C2-19 wall

I C2-20centre -C2-20 intenedlate

Figure 8. Relative change of shape of the specimens of EOF slag tested for melting point in the high temperature microscope.

temperature was increased to I150 "C and the reduction degree reached after -140 min was -55-65 %,. The olivine pellets reduced under the same conditions showed a reduction degree o f -90 %.

The results from the blast furnace simulating reduction tests are found in figure 9. The reduction of BOF slag was very slow at rempera- lures below 950 'C. In the temperature interval of 900 'C to 990 "C the ratio CO/(CO+CO2) was higher in T2 compared with in TI. In this temperature interval the reduction degree was almost unchanged in TI. hut increased from 2 % to 20 % in T2. When a temperature of -I000 "C was reached and CO/(CO+CO2) was increased, the sample was further reduced in the TI test. The final

reduction degrees reached were 16.7 % and 23.4 %, for TI and T2, respectively.

Softening and melting tests. The results o f softening and melting tests o f BOF slag up to a temperature of 1550 "C are summarised in table 5. When BOF slag prereduced to 23 %, 25 %. 55 % and 63 % were sandwiched between coke layers in a graphite crucible, no dripping material was collected and the sample bed shrank little. The shrinkage of the sample bed decreased with increased prereduction degree. The maximum pressure drop measured during these tests occurred at 1525 "C to 1532 'C. The addition of crushed alumina to the BOF slag of 23 % reduction degree did not improve the melting properties. when the sample was heated up in a graphite crucible without coke addition.

BOF slag prereduced to 23 lo and 63 % was tested in alumina crucibles without the addition of coke. Dripping material was collected at 1401 "C and 1473 "C for 23 % and 63 % pre-reduction degrees. respectively. The shrinkage of

500 600 700 800 900 1000 1100 Temperature, "C

Figure 9. Reduction degree of EOF slag and COI(CO+C02) of the gas during EF simulated reduction tests.



the sample bed decreased when the prereduction degree was increased. If the BOF slag, prereduced to 23 %, was charged between coke layers in an alumina crucible, the temperature for dripping start temperature increased by almost 30 "C and the shrinkage of the sample bed and the ratio of dripping material decreased. If some coke wits mixed with the BOF slag, the dripping temperature was further increased.

XRD analysis. Qualitative XRD analysis was conducted for three samples of BOF slag produced in the laboratory tests. The first sample consisted of untreated BOF slag, the second of BOF slag reduced to 63 r/c reduction degree and the third one of BOF slag pre-reduced to 63 % reduction degree and heated in a graphite crucible in a softening and melting test. As is shown in figure 10, calcio-wustite, magnesio-wustite and calcium silicate were present in all samples. Dicalcium ferrite was detected in the original sample as well as in the prereduced sample. Peaks corresponding to Femet were detected in the prereduced sample and in the sample from softening and melting test.

Figure 11 shows the results from qualitative high-temperature XRD performed for one sample of prereduced BOF slag and for one sample of prereduced BOF slag mixed with coke. The prereduced sample contained calico-wustite, magnesio- wustite. calcium ferrites, dicalcium silicate and Feme,. When the prereduced BOF slag sample was heated up in Nz, Fe present as Fe3+ in ferrites and Fe,,,, were transferred to Fe2+ and Fernet. At 750 "C an intermediate phase of magnetite was detected before wustite was found at 1000 "C, at the same time as the intensity of magnetite decreased. At 1250 "C the sample melted and a residual phase containing dicalcium silicate was registered. When the sample of prereduced BOF slag mixed with coke, was heated up in a nitrogen atmosphere. the peak corresponding to Femet measured at 750 "C decreased significantly in the diffractogram,. At 1000 "C peaks representing Fe3C were observed and the intensity of magnesio-wustite and calico-wustite decreased. After a further temperature increase to 1300 "C, peaks corresponding to dicalcium silicate dominated, but magnesium silicates and Fe3C were detected as well. Signs of melting appeared in the diffractogram registered at 1250 "C. The final residual solid phase contained dicalcium silicate, tricalcium silicate, magnesium silicate and some FqC. The ash released from the used coke dissolved into the slag.

Discussion

BOF slag is a dense material with low porosity. The low shrinkage of bed during reduction and what can be visually seen when studying the samples of BOF slag taken at excavation of the EBF indicate that i t is only slightly defomied

30 35 40 45 50 55 60 65 70 75 80 85 2-Theta scale

Figure 10. Results from XRD analysis of BOF slag.

600 6 0D 5 00 5 00 400 400 3 00 3 00 2 00 2 00 t 00 1 0D 0 0

6 00 000 5 00 500 400 400 3 00 300 2 00 200 1 00 100

0 0 25 30 35 40 46 50 55 25 3l 35 43 46 50 55

600 000 5 00 500 400 400 3 00 300 2 00 200 100 100

0 0

6 DO 6 00 5 00 5 00 400 400 3 00 3 DO 2 00 2 00 100 1 00

0 0 25 30 35 40 46 50 55 25 30 35 40 46 50 55

600 500 400 3 00 2 00 I 00

0 25 30 35 4O 45 2!&eta55

M Fe301 WFeO &CaO Fe203 $ 2CaO Fe203 + CaO SI@ # 3CaO SiO2 $ MgOStO. h FeiC O F e o Magnesio wustite * Calcio wustite

600 500 400 300 200 100

0

6 00 5 00 400 3 00 2 00 1 00

0 25 30 35 40 45 25beta55

Figure 11. High-temperature XRD of BOF slag on platinum strip (left) and BOF slag mixed with coke on alumina plate (right). The y-axis corresponds to the intensity in counts per minute.


Process M~tallurgy - lronmaking

in the shaft. I t is likely that BOF slag can have some effect on keeping the permeability of the pellet layers if the pellets are deformed in the BF shaft. BOF slag picks up S and alkalis in the shaft. If the basicity is high. an increased KzO content makes the BOF slag swell more when the melting point is measured. If the basicity is decreased as a result of interaction with the pellets. swelling is similar as for BOF slag with low potassium content.

The reduction of BOF slag is quite low at low temperatures and complete reduction to Ferr,,, probably occurs when softening and melting starts. The reduction rate and the maximum reduction degree reached increased significantly when the temperature rose from 950 "C tu I150 "C in the isothermal reduction tests. The faster reduction in T2 compared to in TI in the temperature interval of 9 0 ~ - I O ~ ) "C can be explained by the stronger reducing power of the gas in terms of a higher CO/(CO+COz) and higher Wz content in T2. A high content of Hz is probably beneficial for reduction because of the low porosity of BOF slag. The analyses of the samples taken during excavation of the EBF indicate that the mineraiogical structure of BOF slag is almost unchanged and that the reduction of BOF slag is quite slow until it reaches the level of the cohesive zone, where the temperature and reducing power of the gas are quite high. A low melting point results in effective interaction with the olivine pellets and formation of a slag with properties suitable for S refining. Pieces of calcinated lime were found at the level of raceway in the EBF and in samples taken by core drilling into BF No. 3 recently. The fact that BOF slag takes part in bosh slag formation earlier than lime can be seen by an increased S content and a decreased C content of hot metal produced at BF No. 3. when the main part of BOF slag is replaced by limestone. In softening and melting tests. in a previous study [3j made with a fluxed pellets of basicity B2=I, the lime had less effect on the melting properties of the slag formed. This indicates that lime is only partly dissolved in the bosh slag and as a result the bosh slag will get a lower basicity compared to when BOF slag is used.

The XRD analysis of quenched samples shows that the original BOF slag, the 63 % prereduced BOF slag, and the BOF slag after softening and melting all contain wustite (with content of Mg and Mn), calico-wustite, calcium ferrite, dicalcium silicate and tricalcium silicate. The prereduced sample and the BOF slag after softening and melting tests also contain metallic iron (Femet ). From the chemical analysis it is known that the original BOF slag also contains Fe,,,,, , but the content is below the detection limit of the XRD measurement. When the BOF slag sample prereduced to -63% reduction degree is heated up in a N2 atmosphere. Fe,,,, and calcium ferrite react and magnetite and wustite are formed. When coke is present, the peak corresponding to Fe,,,, disappears itt 1000 "C as well but magnetite and wustite are not detected. Peaks corresponding to F e K are appearing at 1200 "C. The calcium ferrites seem to tx reduced prior to magnesio-wustite. Themiodynamic calculations 1131 carried o u t in 100 "C intervals from 1000 "C to 1300 "C indicate similar behaviour in principle. Fe,,, and dicalciuni ferrites in the sample were transferred to wustite

at a temperature of 500 "C, but some Fe,,,,, was still present. At 600 "C the reduction o f wustite started, and in the sample containing coke all wustite was reduced to Fe,,, at 700 "C. In this sample a melt containing all Fe and approx- itnately 5.4 56 of C and 0.8 % of Mn was formed at 1000 "C, The C and Mn contents rose as the temperature was further increased. A small amount of slag of approximately the same basicity B2 as the BOF slag, but with B4=(CaO-t.SiO.)/(SiO~~Al~O~) -1.3 was estimated at 1401) "C. In the sample with BOF slag only, a slag melt was estimated at 1000 "C. The main components in the slag were FeO, MnO, CaO, AI:@, SiOz and MpO with - 70 $6 Fe0 and - 10 c/c MnO in the first slag melt formed. Fe,,, was estimated in the solid phase up to 1300 "C. Carburisa- tion of metal and reduction of MnO followed by dissolution in the metal did not occur because C was not present. In re- ality. kinetics limit the possibility to reach 100 % reduction of the BOF slag. Therefore, the BOF slag will contain iron oxides and its melting temperature is still quite low. when the interaction with pellets starts at the level of the cohesive zone.

In previous laboratory studies [ 2,3] made to explain the operational problems at BF No. 3 experienced with the fluxed pellets of basicity B2=1, it was found that the melting properties of BOF slag differed depending on the presence of C. To clarify the effect of BOF slag on the slag formation, softening and melting tests were made and the effect o f C verified. Dripping material was not collected in the softening and melting tests made in graphite crucible, but when adding coke to alumina crucible. dripping material could still be collected. The dripping temperature was increased when coke was present. The temperatures when the maximum pressure drop was measured were similar for all tests made in graphite crucible, 1525-1532 "C, which is close to the melting temperature of pure iron. Both graphite and coke produce a reducing atmosphere leading to reduction of iron oxides to Fe,,,,. When coke is used, acid ash is released and can be dissolved in the slag. This is indicated at high temperature XRD by the peaks in the diffractogram corresponding to magnesium silicate. The coke is more reactive and can more easily be used for carburisation of the formed metallic phase. Carburisation decreases the melting point from -1 535°C to the eutectic melting temperature. which is 1147°C according to the meta-stable phase dia- gram for Fe and C. A difference is verified both in the high temperature XRD performed and in the thermodynamic calculations. According to the calculations. approximately the same amount of residual solid phase is estimated in both samples. The reason for the different behaviour in the softening and melting test can partly be explained by phenom- ena leading to a melt flow through the packed bed. The viscosity of the slag with a high melting point will be high at these temperatures and slag formed in the crucible will pro- hibit metallic iron from flowing through the bed and drip down. The void fraction between the BOF slag particles is also quite low. Another important thing is the carburisation of Fe,,,, that is necessary if a metallic melt shall be formed early in the test. The fact that the material has to flow down through the sample bed in the crucible before dripping ma-


Process Metallurgy - Ironmaking

terial is registered explains why the drippiiig temperature is always higher than the melting point measured in the high- temperature microscope. Another important reason is that C is never added when this method is used.

When the BOF slag is co-injected with coal. it is reiluced to some extent in the raceway and because of the low melting point i t is easily dissolved into the coal and coke ashes. The resitlues of ROF slag mainly consisting nf cii- and tricalcium silicates. brit probably also some wustite, is combined with the acid slag generated from combustion of coal and coke. The changed chemical composition and increased basicity result in significantly improved melting properties of the tuyere slag, as has been shown in a previous laboratory study 151.

When olivine pellets are used, the bosh slug formed has a much lower basicity than the BOF slag itself, and the melting point of the slag will still be low when it is further reduced. On the other hand, when fluxed pellets ( B 2 = l ) are used. slag formation problems will occur in terms ctf formation of ii high-basicity slag with a melting point that in- creases significantly when the slag is further reduced. The formed slag will have a high viscosity at the operating temperatures o f 3 BF. When addition of basic flux is partly done to neutralize the gangue of the pellet, BOF slag is suitable. Supposed all fluxes added neutralize the coal and ccrke ash, an excessive basicity of the bosh slag can be partly coun- teracted, if ;i less reactive basic flux as limestone is used.

Conclusions

The test results show that the BOF slag has properties suitable for use as top-charged basic f l u x in the BF. I t shows high strength and volume stability during reduction. 'The BOF slag has a single basicity of - 4, does not generate any CO? and contains valuable amounts of Fe and Mn. I t is characterised by quite a low melting temperature (which is almost unaffected by the reduction in the BF shaft) and can therefore interact with the iron burden in the bosh and c m be completely dissolved into the dripping slag.

The melting behaviour of BOF slag differs depending o n the presence of coke. If BOF slag is melted without the presence of C, an early melting slag mainly containing FeO, MnO and CaO is formed. If C is present, Fe0 and MnO in the BOF slag will be reduced and its melting temperature increased. A residual solid phase, with high melting point, consisting of dicalcium silicate, tricalcium silicate and magnesium silicate is formed. Coke ash assimilated in the slag decreases the basicity to some extent. The Fe,,[ formed can melt at quite a low temperature if it is carburised. However. the slag with a high melting point might resolidify, fill the voids in the bed and prevent the hot metal from flowing.

If a tluxed pellet with a basicity B2-1 is used, the use of BOF slag as a basic tlux will result in a higher basicity of the bosh slag compared t o if limestone is charged. The limestone is only partly dissolved in the bosh slag and when it is finally dissolved. coal and coke ash are also taking part in slag formation. The melting point of a bosh slag with a high basicity will increase significantly when the iron oxides are further reduced.

The low melting point of BOF diig niitkes it easily dis- solve into the slag of the olivine pellets forming a bosh stag with a higher basicity compared to if limestone is used. BOF slag in combination with o l i v i n t pellets with a low basicity generates a slag with intermediate basicity and the reduction o f iron oxicles in the slag has ;I small effect o n the melting temperature.

The chemical composition in t e r m of CaO, MgO iinrl

iron oxides, and the high temperature properties are positive if BOF is co-irijected wlith coal. Coal and coke ash can be combined with BOF slag and decrease its basicity. Iron oxides of the ROF slag can consume "char" ;tnd c tke fines when they are reduced.

I am grateful to the Swedish Steel Producers' Associatim for financial support and wish to thank the members t ~ f the committee, Irijection into the Blast Furnace. I thank my company, SSAB Tunnplrt in Lulei, for giving me the op- portunity to make this study and LKAB for supplying EBF

(A2004027; t . c . c ~ c i i . r d O I I I,? hlurclr 2004, i l l r-fl~i.vi~ii ji)r.t?l 0 1 1 4 . l l i t l t~ 31)OLC)

samples.

References

A. Hiltunen, K. Hilttinen: The Influence o l Miner;ilopicnl and C'henii- cal Proprrtiec o n the Utilization of Slags. Seminar o n the Proccs\mg. Lltili~ation nnd Ihpcisal of Wahte i n Steel Industry, UN. Hungary, 1046. 1.. Sundyiist ()kvist ;mrl G. Zuo: Effect of tluxes o n slag formntiwi in a hlast liirn;tce hy lop charging and tuyere injection. 7th Nordic-J:ip;in- ese Symp.. Nagoya, Nov. XKH). L. Siindqvist okvist, G. Zuo and J. blx Influence of Basic Fluxe\ o n Slag Forniation in a Blast Furnace Li\ing LXAB Pellets. 6th Internat. C'rrtiference o n Molten Slags. F l u x \ ;ind Salts. June 3KK). J. M:i: ISlJ Int.. 3Y (1944), 647-704. L. Sundyvist Okvist: ISIJ fnt.. 31 17(X)I ). 1.1.20-1336. P. Sik\triim. I-. Suiiiiq\.ist iikvist. 5 - 0 . Wik\triirn: "Injection of HOF- slag through blast furnace tuyerec -~ trials in an experitnentnl blast furnace". ISS 61st Irtriiniaking C'onf., Naslrville, 'Tennes\ee. USA. 2(102. C. Yamagata, Y. Ka.?jiu;ira, S . Suycima, K. Sato, S . Kornatw: ISIJ Int.. 30 ( l440). 362-37 I , K. Kushima. M. Naito. K. Shibata, H. Snto. H. Yozhida and bl. Ichi- da: "Iron ore injection into blast furnace raceway". Ironmaking Conf. Proc. 1988. pp.357-466. A. Dahlstedt, M. Hallin, M. Tottie: "LKAB'\ Experimental Blast Fur- nace for Evaluation of Iron Ore Products". l st Int. C'onf. o n Proce-,s Development in Iron and Steelmakiiig ,IWO, pp. 73.5-245,

I lO]C. Tranell, T. Hagelian, L. Kolbrinwn. A. U;lhlsiedt. M. Hailin: Ke- ~ u l t s and Visualization from the Fir\t C':rmp:iign in the LKAB's Ex- perimental Blast Furnace in I.ult.5. Sweden, ISS 6 1 \I Iroiimshing Conf.. Na>h\~ille. Tennessee. USA. 7002.

[ I I I M. Hallin. Evaluation of Ferrous Burden Properties in an Experirnen- tal B l a h t Furnace After Quenching and Disection. Int. HF Loiber Zone Symp.. Wollonpong. .4ustralia. ?(NE

[ I21 Ruhrkohlen- Handhuch, 7nd ed., Verlag Gluckauf. 1131G. Eriksson. K.Hack: Met. Trails. B. 2 1 8 t l O Y 0 1 . lOl7-lO23.


High Temperature Properties of BOF Slag and its Behaviour ... · Introduction At blast furnace (BF)...

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