Forsterite Bursting in Magnesia Chromite Bricks—Two ...

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Macroscopic Appearance. The copper furnace brick sample showed a residual thick- ness of approximately 160 mm from an original brick thick- ness of 300 mm.

RHI Bulletin > 2 > 2011, pp. 49–53

Dean Gregurek, Christian Majcenovic, Alfred Spanring and Marcus Kirschen

Forsterite Bursting in Magnesia Chromite Bricks—Two Case Studies from Lead and Copper Smelting Furnaces During most nonferrous metallurgical processes a common wear mechanism of magnesia chromite bricks results from the chemothermal load caused by iron silicate (fayalithic) slag [1,2]. However, an atypical, exceptionally high SiO2 supply caused by changes in the processing and/or the uncontrolled addition of silica sand results in the considerable formation of forsterite (Mg2SiO4) following contact with periclase, which due to the associated volume expansion causes forsterite bursting and deterioration of the brick structure. Two postmortem studies of magnesia chromite bricks taken from a copper and lead furnace are presented providing detailed chemical and mineralogical examples of this wear phenomenon, which whilst seldom experienced in nonferrous applications is quite typical for regenerator chambers in the glass industry in the case of extreme SiO2 carryover [3]. Introduction Postmortem investigations are an important tool to understand the wear phenomena influencing the performance of refractory products in the nonferrous metal industry. Furthermore, they enable recommendations to be made for appropriate furnace linings and provide a vital basis for innovative product development. The following case studies deal with a quite unusual type of chemothermal attack observed on magnesia chromite bricks from copper and lead smelting furnaces. In the first postmortem study, a magnesia chromite brick (50 wt.% MgO) from a copper furnace was examined. The main components of the basic brick brand include OXICROM sinter, sintered magnesia, and chrome ore. Under the operating conditions, this brick type only showed a refractory life of 4 months when used to line the cylinder or endwall area of a short rotary furnace. The usual service life of the furnace refractory lining is 12–15 months.

Investigation Results—Case Study 1 (Copper Processing) Macroscopic Appearance The copper furnace brick sample showed a residual thickness of approximately 160 mm from an original brick thickness of 300 mm. The immediate brick hot face was rough and covered with a thin slag coating. A deeply infiltrated and completely degenerated, softened brick microstructure could be recognized in the cross-sectional view. Several cracks—partly softening cracks—had formed in the infiltrated brick microstructure (Figure 1). Additionally, melt penetration between the brick joints was also visible.

A

The second study was carried out on a magnesia chromite brick (60 wt.% MgO) from the sidewall of a lead reverberatory furnace after 4 weeks in operation. The nominal service life with continuous processing (no shutdowns) is approximately 12–14 months. This basic brick brand consists of sintered magnesia and chrome ore.

Investigation Methods Samples for the chemical and mineralogical investigation were taken from the brick hot and cold faces (Figures 1b and 2b). The chemical analyses were carried out using X-ray fluorescence (XRF) analysis on the original sample and after dissolution of the sample in Li2B4O7 (according to DIN51001). The mineralogical investigations were performed on polished sections using a reflected light microscope and a scanning electron microscope combined with an energy-dispersive X-ray analyser and X-ray diffraction.

(a)

(b)

Figure 1. Magnesia chromite brick (50 wt.% MgO) from the cylinder/endwall of a copper short rotary furnace. (a) rough brick hot face and crack formation (arrows). (b) cut section showing infiltrated and degenerated brick microstructure with crack formation (arrows). Sample position (A) for the chemical analysis and X-ray diffraction is indicated.

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RHI Bulletin > 2 > 2011 Chemical Analysis An extremely high SiO2 (up to 10 wt.%) and an increased CaO content (up to 3 wt.%) were determined within the area 0–20 mm from the brick hot face (Table I). Additionally, within the infiltrated brick microstructure elevated up to high concentrations of NiO, CuO, PbO, SnO2, and ZnO were detected. Mineralogical Investigation According to the X-ray diffraction the high SiO2 supply resulted in the formation of a huge amount of newly formed Mg-silicate of forsterite type (Mg2SiO4). Ca-Mg-silicate of monticellite type (CaMgSiO4) was also detected (see Table I).

Investigation Results—Case Study 2 (Lead Processing) Macroscopic Appearance The brick examined from the lead reverberatory furnace showed a remarkably high residual brick thickness of approximately 350 mm (Figure 2a) from an original brick thickness of 450 mm. The cross-sectional view through the area 0–40 mm from the hot face revealed the brick microstructure was highly degenerated (see Figures 2b and 2c). Macroscopically visible pores, the formation of softening cracks, and deformation of the brick edges due to the volume expansion process could be clearly recognized.

General information Furnace

Copper short rotary

Magnesia chromite brick (wt.% MgO) Sample

Lead reverberatory

50 A

60 Data sheet

B

Data 330– sheet 350

Sampling position from hot face (mm)

0–20

Chemical analysis

(wt.%) (wt.%) (wt.%) (wt.%) (wt.%)

Determination by XRF

(2)

(1)

42

49.4

Na2O MgO

0–20

C

(2)

(2)

(1)

0.8

0.6

49

57

63

Al2O3

6

8

5

5

6.9

SiO2

10

0.6

11

2

2.0

P2O5

0.6

0.5

2

1.0

0.2

0.4

SO3

4

K2O

0.7

CaO

3

TiO2

0.4

1.0

V2O5

0.3

Cr2O3

14

MnO

0.3

25.0

10

13

0.3

0.3

10

13

6.4

Fe2O3

14

NiO

0.3

CuO

2.9

PbO

0.7

6.3

SnO2

0.5

0.8

ZnO

5.3

16.0

17.1 10.0

(a)

BaO

1

Sb2O3

0.4

(c)

Phase analysis by X-ray diffraction(2) Mineral phase

Formula

(wt.%)

Periclase

MgO

10–50

Magnesiochromite

(Mg,Fe)(Cr,Al)2O4

10–50

Forsterite

Mg2SiO4

5–10

Monticellite

CaMgSiO4

2–5

Donathite

(Fe,Mg)(Cr,Al,Fe)2O4

2–5

Table I. Chemical analysis and X-ray diffraction of the postmortem magnesia chromite bricks. X-ray fluorescence analysis on an ignited original sample (1050 °C) (1) and postmortem sample (2).

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2 > 2011 Chemical Analysis According to the chemical analysis and similar to the aforementioned sample, the brick hot face was highly enriched with SiO2 (up to 11 wt.%), SO3, and PbO (see Table I). Additionally, elevated levels of alkalis (i.e., Na2O, K2O), BaO, as well as Sb2O3 and SnO2 could be detected. A high PbO content was also determined at the brick cold face.

4

1

2

3

Mineralogical Investigation Based on the mineralogical investigations and the chemical analyses, microstructural changes within the area 0–25 mm from the hot face can be summarized (Figures 3–5) as follows: >> Strong degeneration, namely softening of the brick microstructure caused by infiltration and severe corrosion (partly intragranular) of the sintered magnesia. This resulted in the formation of a huge amount of idiomorphic forsterite. The second brick phase chromite had become chemically modified and highly enriched with Fe- and Sn-oxide. Additionally, PbO-bearing silicatic glassy phase was determined. >> Formation of coarse pores, including pore channels. Partly elongated periclase crystals and crystal growth (up to 1 mm in size) towards the thermal gradient. The size of individual periclase crystals within sintered magnesia in the original brick microstructure was approximately 90 μm (Figure 6). >> Crack formation parallel to the hot face surface.

1 mm

(a)

(b) Figure 4. (a) microstructural detail of the postmortem magnesia chromite brick (60 wt.% MgO) boxed region D1 in Figure 3 showing the infiltrated and corroded brick microstructure. Corroded magnesia (1), chemically changed and partly recrystallized chromite (2), massive forsterite formation (idiomorphic crystals) (3), and coarse pores filled with preparation resin (4) are indicated. (b) higher magnification of the white boxed region.

1 4

D2

D1

2 3

500 µm

(a)

R

2 mm Figure 3. Microstructural overview of the postmortem magnesia chromite brick (60 wt.% MgO). Boxed region 1 in Figure 2. Severely degenerated brick microstructure with coarse pores (arrows) (also see higher magnification of boxed regions D1 and D2 in Figures 4 and 5) and crack formation (R).

(b) Figure 5. (a) microstructural detail of the postmortem magnesia ­chromite brick (60 wt.% MgO) boxed region D2 in Figure 3 showing the degenerated brick microstructure and elongation of the periclase crystals (lines). Corroded magnesia (1), chemically changed and partly recrystallized chromite (2), forsterite (3), and coarse pores filled with preparation resin (4) are indicated. (b) higher magnification of the white boxed region.

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RHI Bulletin > 2 > 2011

2

formation of spinel and—in the case of a higher amount of slag in the infiltrated zone—forsterite (see Figure 7). In a magnesia chromite brick with 60 wt.% MgO, the amount of forsteritic secondary phase increases when the brick is infiltrated with fayalithic slag (see Figure 8). The FeO of the slag remains in the liquid oxide fraction. Considering the apparent porosity of the original bricks which is around 17 vol.% and the apparent porosity of the infiltrated layer between 0 and 5 vol.%, the ratio between slag mass and refractory mass that is reacting is estimated to be 12–17 wt.% in Figures 7 and 8. At this ratio the amount of newly formed forsterite is approximately 5–10%. This may be sufficient to destabilize the grain and bonding structure of the brick.

1 3

Conclusion

500 µm Figure 6. Microstructural overview of the original magnesia chromite brick (60 wt.% MgO). Sintered magnesia (1), chromite (2), and pores filled with preparation resin (3).

FactSage Calculations Phase equilibrium assemblages were calculated using the FactSage software in order to determine the dissolution of brick material or the formation of new phases in the slag contact area. The thermodynamically stable phases of the magnesia chromite brick with 50 wt.% MgO are magnesia, (Fe,Mg)(Al, Fe,Cr)2O4 spinel, minor dicalcium silicate (Ca2SiO4), and merwinite (Ca3MgSi2O8) (Figure 7). The second brick sample (60 wt.% MgO) consists of magnesia, (Fe,Mg)(Al, Fe,Cr)2O4 spinel, and some forsterite (Figure 8).

Generally, it is of particular importance to operate a chemically and temperature controlled process for the production of nonferrous metals, for example in the copper and lead industries. Although, high metal prices on the market nowadays are a driving force for operators to change furnace processes to decrease metal losses in the process slag and to run operation routes that are not usually

100

100

90

90

80

80

70

70

Stable phases [%]

Stable phases [%]

Contact and infiltration with fayalithic FeO-MgO-SiO2 slag with approximately 25% SiO2 results in a dissolution of the minor phases Ca2SiO4 and merwinite of the magnesia chromite brick with 50 wt.% MgO in favour of the

The main wear mechanism of the investigated magnesia chromite bricks was an extremely high chemothermal load caused by massive SiO2 supply. Whilst such a high SiO2 supply is very seldom experienced in nonferrous metal applications, in the postmortem samples it caused degeneration and softening of the brick microstructure by infiltration and severe corrosion of the main brick component: Sintered magnesia. In addition, the chromite was also affected by the chemothermal load. The huge amount of SiO2 supply resulted in the formation of forsterite, an associated volume expansion, and so-called forsterite bursting. Such a wear phenomenon is quite typical for regenerator chambers in the glass industry when there is extreme SiO2 carryover.

60

*

50 40 30

60

*

50 40 30

n Oxide liquid n Forsterite n MA, Cr spinels n MgO periclase

20 10

n Oxide liquid n Forsterite n MA, Cr spinels n MgO periclase

20 10

solid solution

0

0

3

6

solid solution

9

12

15

18

21

24

27

30

Slag to refractory ratio [%] Figure 7. Calculated phase assemblage of infiltrated magnesia chromite brick with 50 wt.% MgO as a function of slag mass to refractory mass ratio (pure brick at coordinate axis, x=0): Mainly periclase and spinels. Infiltrate is forsterite forming. The apparent porosity is approximately 17 vol.% (*).

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2 > 2011 included or part of the common process, overheating the slag and bath, and too high levels of slag additives like SiO2 will strongly support this negative type of brick corrosion and decrease the brick lifetime dramatically. Another important aspect is to run the metallurgical vessel continuously without any shutdowns. However, recent investigations and discussions with customers have brought to light that very discontinuous furnace operations are occurring in certain plants. Especially in the lead industry—where Pb-compounds show partly liquidus temperatures lower than the liquidus temperature of pure lead (327.5 °C)—gaps or joints can become heavily infiltrated before they are fully tight and closed. Therefore, proper brick installation and a strictly followed heating-up procedure is necessary to achieve an acceptable, successful furnace campaign.

References [1] Barthel, H. Wear of Chrome Magnesite Bricks in Copper Smelting Furnaces. Interceram. 1981, 30, 250–255. [2] Gregurek, D. and Majcenovic, C. Wear Mechanism of Basic Brick Linings in the Non Ferrous Metals Industry – Case Studies From Copper Smelting Furnaces. RHI Bulletin. 2003, No.1, 17–21. [3] Barthel, H. Über den Verschleiß von Magnesitsteinen in Gitterungen von Glasschmelzöfen. Presented at the XV. Internationales Feuerfest Kolloquium, Aachen, Germany. Glastechn. Ber. 1973, 46, No. 7, 134–140.

Authors Dean Gregurek, RHI AG, Technology Center, Leoben, Austria. Christian Majcenovic, RHI AG, Technology Center, Leoben, Austria. Alfred Spanring, RHI AG, Industrial Division, Vienna, Austria. Marcus Kirschen, RHI AG, Steel Division, Vienna, Austria. Corresponding author: Dean Gregurek, [email protected]

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