EVALUATION OF REFRACTORY BRICKS FOR …...tests done on magnesia, magnesia-chrome and...

$: EVALUATION OF REFRACTORY BRICKS FOR …...tests done on magnesia, magnesia-chrome and magnesia-carbon refractory bricks in contact with titania slag to evaluate these bricks for the$
The Southern African Institute of Mining and Metallurgy

Refractories 2010 Conference

J van der Westhuizen and D Bessinger

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EVALUATION OF REFRACTORY BRICKS FOR TITANIA SLAGS


Research and Development, Exxaro Resources

Abstract

High titania slag (containing approximately 86 per cent TiO2) is produced from the

reduction of ilmenite in electric furnaces. Three major areas in the magnesia refractory

lining of an ilmenite furnace can be defined:

• Refractory in contact with molten iron,

• Refractory in contact with titania slag,

• Freeboard refractories in contact with the process gas, titania slag from splashing, and

unreacted or partially reacted feed materials from the process.

Prolonged stoppages can cause a large decrease in freeboard temperature putting the

refractory lining through unfavourable thermal cycles. Due to unforeseen stoppages and

planned maintenance the free board area is more exposed to thermal cycling than lower

down in the slag and metal bath. This paper describes the results obtained from rotary slag

tests done on magnesia, magnesia-chrome and magnesia-carbon refractory bricks in contact

with titania slag to evaluate these bricks for the freeboard refractory lining. The results of

these tests show that magnesia-carbon refractory bricks performed the best.

1 Introduction

During ilmenite reduction with carbon, three main product streams evolve, namely titania

slag, low manganese pig iron and a carbon monoxide rich off-gas. In most ilmenite

smelting furnaces magnesia bricks are the primary refractory material used to contain the

molten slag and liquid metal. As in most pyrometallurgical processes this situation is safe

as long as the refractory material has a higher melting point than the slag and the iron. The

high titania slag which are produced during ilmenite smelting is corrosive to all known

refractories, and hence ilmenite smelters operate with a “freeze lining” of solidified slag1.

Because of these three process products, three major areas in the refractory lining of an

ilmenite furnace can be defined:

• Magnesia refractory in contact with the molten iron

• Refractory in contact with the titania slag

• Refractory in contact with the gas phase of the process

In the molten iron area the main wear on the refractory material is erosion due to bath

movement and iron penetration between the bricks2. Chemically the iron does not have a

significant influence on the wear mechanism of the bricks in contact with the iron.




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The situation in the slag layer is completely different, because titania slag at elevated

temperatures has a high affinity for MgO. It is therefore important that the titania slag does

not come into contact with the refractories. Any magnesia refractory material in contact

with molten slag will be dissolved, which will lead to a reduction in the furnace refractory

life. In the slag zone it is essential to operate the furnace under such conditions that some of

the slag freezes onto the refractory material. This “freeze lining” results by utilising the

relatively high thermal conductivity of the magnesia bricks to extract heat from the inside

of the furnace to the shell, where the shell is cooled on the outside by either water sprayers

or a water jacket against the shell. Operating a furnace with a “freeze lining” is still no

guarantee that the lining will last and therefore from time to time the lining need to be

replaced.

In the area above the slag zone the refractory materials are exposed to high temperatures,

thermal cycling, gaseous atmosphere and slag. During stable and continuous furnace

operations the risk of refractory failure in this zone is relatively low. Thermal cycling

normally occurs in the free board area when the furnace is switched off for maintenance,

routine inspections or due to operational problems. This causes the “freeze lining” to fall

off and the exposed refractory material to spall, exposing “fresh” refractory material to

subsequent operational conditions.

The purpose of this study was to evaluate a number of different refractory bricks for

placement in the freeboard refractory lining. The bricks that were investigated included

magnesia, magnesia-chrome and magnesia carbon bricks.

2 Experimental Procedures

2.1 Test materials

Fused magnesia-carbon, magnesia-chrome, magnesia-spinel and magnesia bricks were

received from three different companies for evaluation. The chemical compositions of the

bricks discussed in this study are shown in Table 1. The reason that the analysis total for

the magnesia-carbon brick is more than 100 % is that the residual carbon content of 14 % is

not included in the analysis total. The chemical composition of the titania slag supplied by

Namakwa Sands is shown in Table 2.

2.2 Rotary Slag test procedure

The bricks and the slag were taken to a third party test laboratory for rotary slag tests. The

rotary slag test gives an indication of the resistance of the refractory material against slag

attack. A total of three rotary slag tests were preformed, one that was run for 10 cycles and

two that were run for 15 cycles.




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Table 1: Approximate chemical analysis of refractory material as supplied by the

various manufacturers.

Brick type Brick no.

MgO

(%)

Al2O3

(%)

Fe2O3

(%)

CaO

(%)

SiO2

(%)

Cr2O3

(%)

Residual

C (%)

Magnesia

A

(Reference) 97 0.1 0.2 1.9 0.6 - -

C 96 0.3 0.5 2.0 0.8 - -

E 98 0.1 0.1 0.8 0.3 - -

Fused Magnesia-

carbon B 98 0.1 0.4 1.1 0.4 - 14

Magnesia-

Chrome D 54 9.6 10.8 1.0 2.0 22 -

Magnesia-Spinel F 87 10 0.5 1.2 0.5 - -

Table 2: Chemical compositions of the titania slag used in the rotary slag test.

SiO2

(%)

Al2O3

(%)

Fe2O3

(%)

TiO2

(%)

CaO

(%)

MgO

(%)

Na2O

(%)

K2O

(%)

1.16 1.04 9.49 87.0 0.09 0.93 0.008 0.026

Samples were prepared from refractory bricks received from the different suppliers. Each

of the samples was measured before it was built into the furnace. (See Figure 1 for

experimental set-up). After the rotary furnace was relined with bricks it was gas fired until

the test temperature of approximately 1680°C was reached. Time to heat the rotary furnace

to this temperature was 3 to 4 hours. Once the test temperature was reached, 300 g of slag

was placed inside the furnace. The slag samples were put in paper bags and then placed into

the furnace. Putting the slag into paper bags helped to prevent the slag being blown out of

the furnace by the gas flame. Rotation of the furnace was only started once the slag started

to melt. The slag sample was replaced with fresh slag after every 20 minutes.

Two types of tests were carried out; the ten cycle test and the fifteen cycle test. For the ten

cycle test removal and replacement of the slag was done for ten continuous cycles of 20

minutes, whereafter the furnace was stopped and cooled down and the bricks removed for

evaluation. For the fifteen cycle test the same procedure was followed as for the ten cycle

test. After the tenth cycle the furnace was stopped and the slag removed, allowing the

furnace to cool overnight. After cooling down overnight the furnace was reheated and the

procedure was repeated for 5 more slag changes. After the 5 cycles the furnace was

switched off and the furnace cooled to room temperature.




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After completion of the tests the bricks were carefully removed and each brick cut in half

and photographed. The actual size of each brick was recorded and the wear rate determined.

Visual inspection showed slag penetration that varied from severe to almost zero. The

degree of slag penetration was measured in millimetres.

Figure 1: Schematic illustration of experimental set-up for rotary slag test

Sample size (in mm):

Sketch of samples inside rotor furnace

2.3 Scanning electron microscopy

A number of samples were chosen that represented the range of refractory material that was

built into the rotary furnace and submitted for scanning electron microscopy (SEM)

investigations.

Titania slag

Brick Samples

Insulating Material& Koawool

±60-75

±70

±130

Rotation




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2.4 X-Ray Diffraction Analysis

Ten samples were prepared from five different bricks that represent the range of refractory

materials tested. Two samples of each brick were prepared, one sample from the reaction

zone and the second sample from the brick, away from the reaction zone.

The samples were analysed with a PANalytical X’Pert Pro powder diffractometer with

X’Celerator detector and variable divergence- and fixed receiving slits with Fe filtered Co-

Kα radiation. The phases were identified using X’Pert Highscore plus software. The

relative phase amounts (weight %) were estimated using the Rietveld method (Autoquan

Program). Amorphous phases, if present, were not taken into account in the quantification.

X-ray diffraction and SEM analyses were only carried out on samples that were used in the

15 cycle tests.

3 Results and Discussion

3.1 Wear results from rotary slag tests

A summary of the maximum wear, wear per cycle and slag penetration for 5 selected bricks

are shown in Table 3. Also shown is the total porosity for each brick as reported by each

manufacturer.

Table 3: Results of rotary slag test visual inspection

Brick type Brick

no.

Total

Porosity

(%)

Max

Wear

after 15

cycles

(mm)

Wear/cycle

after 15

cycles (mm)

Slag

Penetration

(10 cycles)

(mm)

Slag

Penetration

(15 cycles)

(mm)

Magnesia

A 15 12.86 0.86 3.2 3.0 (Average

of 3 tests)

C 17 14.16 0.94 9.2 8.0

E 12 12.51 0.83 3.2 1.0

Fused

Magnesia-

carbon

B 4.5 7.82 0.52 No 10 cycle

test 1.0

Magnesia-

Chrome D 18.7 15.06 1.00

No 10 cycle

test

20 mm colour

change




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A comparison of the magnesia bricks after the rotary slag test is shown in the photographs

in Figure 2. Brick A is the brick currently used at the freeboard hot face. From these

photographs it is clear that slag penetration occurred with all the bricks, with brick E

showing the least slag penetration. Cracks can be observed in all the bricks. From Table 3 it

is clear that the slag penetration and wear of the magnesia bricks are a function of the brick

porosity, with brick C (highest porosity of 17 %) having the highest wear and slag

penetration when compared to brick E (lowest porosity of 12 %).

Figure 2: Photographs of magnesia bricks after the rotary slag test (On the left after

10 cycles, on the right after 15 cycles)

Brick A-Sintered magnesia brick. Ceramic bonded. MgO= 97%

Brick C-High quality Magnesia. MgO= 96%

Brick E-Sintered Magnesia brick. MgO = 98%,




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From Figure 2 it is also clear that all the magnesia bricks showed significant more wear

after 15 cycles when compared to the wear after 10 cycles. Slag penetration was limited in

the 98% magnesia brick with a 12% apparent porosity (brick E). Severe cracking of the

brick was also observed after thermal cycling of the brick. Due to cracks that formed inside

the brick the maximum wear on the brick was almost the same as the 96% magnesia brick

with 17% porosity (brick C).

Figure 3 compares the performance of the three magnesia bricks with different porosities as

tested in the rotary furnace after ten and 15 cycles respectively. The brick with the highest

porosity and the most slag penetration outperformed the dense bricks with less slag

penetration during the 10 cycle test. This phenomenon could be that slag penetrating the

brick causes densification and also freezes inside the matrix acting like a “freeze lining”

preventing future brick wear. This situation changes as soon as the bricks were put through

a thermal cycle in the fifteen cycle test. Thermal cycling causes the densified layer to spall,

causing excessive brick wear.

Figure 3: Average wear rate of different porosity magnesia bricks after 10 and 15

cycles

Comparison of magnesia bricks in rotary slag test with titania slag

0.75

0.80

0.85

0.90

0.95

1.00

Brick A: 97% MgO brick, App

porosity 15%

Brick C: 96% MgO brick, App

porosity 17%

Brick E: 98%MgO brick, App

porosity 12%

Brick quality

Avera

ge w

ear

rate

/cycle

10 Cycles 15 cycles

Photographs of the magnesia-carbon brick and the magnesia-chrome brick are shown in

Figure 4. Slag penetration into the brick containing 14% residual carbon was less than for

any of the other bricks used in the test. Some oxidation of the carbon is visible in the Figure

as indicated by the whitish discolouration. No visual cracks formed in the brick after the

tests. For the magnesia-chrome brick a darkish discolouration was observed, and was

measured at more than 20 mm (see Table 3). This was thought to be due to slag penetration

into the brick. This could however not be confirmed by SEM work, with no titanium

containing phases found in the sample analysed. The wear rate on the magnesia-chrome

brick was the highest of all bricks tested.




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Figure 5 compares the average wear rates of the various bricks after 15 cycles, with the

magnesia-carbon brick performing best and the magnesia-chrome brick performed the

worst under conditions of thermal cycling.

Figure 4: Photographs of magnesia-carbon and magnesia-chrome bricks after the slag

rotor test (On the left after 10 cycles, on the right after 15 cycles)

Brick B-Magnesia-Carbon brick with 14% residual carbon.

Brick D-Magnesia-Chrome brick. MgO = 54%, Cr2O3 = 22%

Figure 5: Average wear rate of different quality magnesia bricks after 15 cycles

Rotary slag test 15 cycles ( 10 cycles, cooling, 5 cycles)

0.00

0.20

0.40

0.60

0.80

1.00

1.20

Brick A: 97% MgO

brick, App porosity

15%

Brick C: 96% MgO

brick, App porosity

17%

Brick E: 98%MgO

brick, App porosity

12%

Brick B: 98% MgO

carbon brick, Res

carbom 14%

Brick D: Mag-Chrome

brick with MgO = 54%,

Cr2O3 = 22%

Bricl quality

Avera

ge w

ear

rate

/ cycle

Average wear rate




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3.2 X-Ray diffraction and scanning electron microscope analyses of different

bricks

X-ray diffraction results of the magnesia bricks are shown in Table 4, while those of the

magnesia-carbon and magnesia-chrome bricks are shown in Table 5. Samples were taken

some distance away from the slag interface and show the phases present in the bricks after

the rotary slag test. As expected the major phase in each instance was periclase. Samples of

the slag-brick interfaces for these bricks were also analysed, unfortunately these samples

did not contain enough slag, resulting in results similar to those of the bricks only. These

results are not reported here.

Table 4: X-ray diffraction results from the magnesia brick samples after the rotary

slag test

Phase

Brick A: 97%

MgO, Porosity

15%

Brick C: 96%

MgO, Porosity

17%

Brick E: 98%

MgO, Porosity

12%

Ca2SiO4 0.89 0.87 0.23

Lime - 0.09 -

Magnesite (MgCO3) 0.55 0.58 0.55

Magnetite (Fe3O4) - - 0.14

Merwinite (Ca3MgSi2O8) 0.14 - 0.26

Periclase (MgO) 98.42 98.46 98.82

Table 5: X-ray diffraction results from the magnesia-carbon and magnesia-chrome

brick samples after the rotary slag test

Phase

MgO-C brick B:

98% MgO; 14 %

residual C

Brick D: Magnesia-Chrome

brick

Graphite 5.92 -

Monticellite ( CaMgSiO4) 0.46 2.71

Chromite (FeCr2O4) - 41.93

Periclase (MgO) 93.62 55.36

The X-ray diffraction results of the magnesia-carbon brick in Table 5 shows that a

significant amount of carbon remained in the brick after the slag rotor test.




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Figure 6 shows the SEM image and phase analyses of the slag-brick interface area for brick

A. The dark phase is the MgO brick, while the grey phase is mainly a titania containing

spinel phase with a stoichiometry of M3.08O4, M representing the sum of the cations in the

phase. This is similar to the phase found by Garbers-Craig and Pistorius3 in their study. The

light phase at the bottom of the image in Figure 6 is a high titania phase containing a

significant amount of MgO. As mentioned by Garbers-Craig and Pistorius3, refractory wear

in the MgO-based brick proceed primarily through the formation of solid-solution phases.

Slag penetration is clearly visible in the SEM image.

Figure 6: Image and analyses of slag-brick interface for Brick A (Porosity 15 %)

Phase Ti

(%)

Mg

(%)

Fe

(%)

Al

(%)

Cr

(%)

O

(%) Total

Light phase (point 1) 50.08 10.90 4.33 1.63 1.34 31.71 100.00

Grey phase (point 9) 24.46 25.42 6.11 3.38 2.49 38.13 100.00

Dark phase (point 15) 0.00 57.43 0.61 0.00 0.00 41.95 100.00


C. The same phases are present, except for the light phase, which approximates perovskite

(CaTiO3), also identified by Garbers-Craig and Pistorius3 in their study.




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Figure 7: Image and analyses of slag-brick interface for Brick C (Porosity 17 %)

Phase Ti

(%)

Mg

(%)

Fe

(%)

Al

(%)

Ca

(%)

Mn

(%)

O

(%) Total

Light phase (point 9) 39.06 - - - 30.33 - 30.61 100.00


E. The same phases are present as those shown in Figure 5. Comparing the three magnesia

bricks in Figures 6 to 8 shows that least slag penetration is observed for Brick E, which also

has the lowest porosity.




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Figure 8: Image and analyses of slag-brick interface for Brick E (Porosity 12 %)

Phase

Ti

(%)

Mg

(%)

Fe

(%)

Al

(%)

Mn

(%)

O

(%) Total

Light phase (point 1) 50.75 11.17 3.16 2.50 - 32.41 100.00

Grey phase (point 6) 16.11 22.64 4.69 15.65 0.79 40.13 100.00

Dark phase (point 7) - 57.73 0.80 - - 41.46 100.00

Figure 9 shows the SEM image and phase analyses of the slag-brick interface area for the

magnesia-carbon brick B. The same phases as in Figure 6 can be identified. The analyses of

the brick however do not include any carbon, as this was not analysed. The presence of

chromium in the grey phase can perhaps be explained by contamination from magnesia-

chrome bricks, as these were included in the same test lining as this brick. Comparing this

brick to the magnesia bricks (Figures 6 to 8), the following comments can be made:

• Slag penetration for the magnesia-carbon brick is much less. The lower porosity of this

brick (4.5 %) certainly seems to play a role as well when compared to the magnesia

bricks with porosities in excess of 12 %.

• Visually it also seems that the grey spinel phase is also less for the magnesia-carbon

brick when compared to the magnesia bricks.




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Figure 9: Image and analyses of slag-brick interface for magnesia-carbon Brick B

Phase Ti

(%)

Mg

(%)

Fe

(%)

Al

(%)

Cr

(%) O (%) Total

Dark phase (point 1) - 56.78 1.60 - - 41.62 100.00

Grey phase (point 10) 22.51 24.20 7.10 4.79 3.31 38.09 100.00

Light phase (point 7) 51.13 11.52 4.07 1.62 - 31.66 100.00

3.3 Comment on magnesia-spinel brick tested

In addition to the evaluation of the magnesia, magnesia-carbon and magnesia-chrome

bricks tested, a magnesia-spinel brick was also evaluated, containing approximately 10 %

Al2O3. This brick however performed poorly and was not evaluated further.

4 Conclusions

The purpose of this study was to determine how different types of brick react in a rotary

slag test in contact with titania slag. The bricks received were from different suppliers with

different characteristics. The bricks differed in chemical as well as physical properties.

The main difference in the physical properties of the magnesia bricks were the porosity.

The porosity of the bricks varied from 12% to 17%. It is evident in the photographs of the

brick cross-sections and the SEM images that the brick with the highest porosity showed

the most penetration. It is also evident that the magnesia brick with the most slag




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penetration showed the least wear during the 10 cycle rotary test, but the highest wear

during thermal cycling. This high wear can be due to the densification effect the slag has on

the brick and as soon as thermal cycling is present the brick loses the slag penetrated layer

due to thermal spalling. It can also be concluded that magnesia bricks with a relatively high

slag penetration can perform well under conditions where no thermal cycling is present.

Dense magnesia bricks out-perform less dense magnesia bricks under thermal cycling

conditions, just because there was less slag penetration in the dense brick.

Magnesia-carbon bricks showed the least slag penetration and out-performed the magnesia

and magnesia-chrome bricks. Under thermal cycling conditions the magnesia-carbon bricks

show no signs of crack formation. Absence of cracks show the superiority of magnesia-

carbon bricks over magnesia bricks and would therefore perform better under thermal

cycling conditions as present in the freeboard area of the furnace. Oxidation of the

magnesia-carbon brick was visible during the rotary slag test due to the oxdising

atmosphere the tests were performed in. Oxidation should however not be a problem under

actual furnace conditions, because the freeboard atmosphere of the furnace during ilmenite

smelting is predominately carbon monoxide. Under these conditions magnesia-carbon

bricks should out-perform normal magnesia bricks.

5 Acknowledgements

The authors are grateful to J. Victor for all test work performed and J Terblanche for his

valued contribution from Arcelor Mittal. The mineralogical work carried out by A Walliser

and J Richards is also appreciated. The authors are also grateful to Exxaro Resources

management for the opportunity and the permission to publish the article.

6 References

1 Pistorius P.C. Fundamentals of freeze lining behaviour in ilmenite smelting. Heavy

Minerals 2003, Johannesburg, South African Institute of Metallurgy, 2003, pp 167-

173

2. Kotze H., Bessinger D. and Beukes J. Ilmenite smelting at Ticor SA. Southern

African Pyrometallurgy 2006, Edited by R. T Jones, South African Institute of

Mining and Metallurgy, Johannesburg, 5-8 March 2006, pp. 203-214.

3 Garbers-Craig A.M. and Pistorius P.C. Slag-refractory interactions during the

smelting of ilmenite, South African Journal of Science 102, Nov/Dec 2006, pp 575-

580

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