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www.ireng.org grains is composed of 32.5 wt.% MgO, 43 wt.% SiO2


Technical Paper , and 24.5 wt.% CaO


(EDX spot 7). This composition lies next to the field of stability of MgO. By interaction with MgO, monticellite (CMS) will be formed until the stability field of MgO will be reached very fast. This will disrupt further corrosion.


Material


MgO-C- REF


MgO-C- MA


MgO-C- CMA


Spot 1


2 3 4 5 6 7


8


Al2O3 46.3


37.6 39.9 40.2 4.9 - -


47.5


MgO SiO2 4.7


7.2 3.8 4.5 1.8 -


32.5 3.6


18.9 15.5 5.2


Compound (wt.%) CaO 29.9 35.4 34.0 30.1 52.7 36.7 24.4


22.6 33.8 -


43.1 10.4


Table 3: Results of EDX spot analysis in SEM


2-3: Conclusion of laboratory corrosion tests After corrosion test all three sample bars show oxidation of carbon. The slag-refractory interface of reference sample MgO-C-REF is fissured and an infiltration depth of more than 1.2 mm can be calculated. Sample MgO- C-MA shows a wavy slag-refractory interface with an average infiltration depth of 800 µm. MgO-C-CMA shows a very sharp slag-refractory interface with slag infiltration depth in the range of 100 – 200 µm.


SEM investigation of the slag-refractory interface of MgO-C-REF shows an interaction with CaO of the slag, which results in a eutectic slag with lower melting point and lower viscosity, which can easily penetrate the MgO-C reference material. Al2


O3 from slag seems to play only a minor role on


the MgO-C-REF corrosion. In contrast to this, the MA-spinel containing sample MgO-C-MA reacts with CaO and Al2


O3 . This will intensify the basic


character of the residual slag, which results in higher melting point, higher viscosity, and reduced slag penetration. Despite the higher viscosity, the penetrating slag has a chemistry which has a potential to further react with the sample material. A similar mechanism applies for sample MgO-C-CMA that contains an addition of calcium magnesium aluminate, as it will react with CaO and Al2


O3 . But in contrast to sample MgO-C-MA, the composition


of the slag in the infiltration zone lies in the stability field of spinel, which will result in less interaction. Additionally, the residual penetrating slag inside the coarse MgO grains lies next to the field of stability of MgO. Hence, interaction with MgO results in formation of monticellite (CMS) until the stability field of MgO will be reached quickly. At this point, further corrosion will be disrupted which results in the highest corrosion resistance of sample MgO-C-CMA against molten steel and the synthetic basic slag.


3: Steel ladle trials 3-1: Results from a 205t ladle trial CMA-doped MgO-Al2


O3 37.2


Fused Alumina CMA aggregate MgO Sinter


TiO2 0.3


1.0 8.0 -


1.0 - -


1.3


SO3 -


3.3 9.1 2.6 5.9


63.3 -


-


Graphite (wt.%) Total C


Antioxidants


Density Porosity CCS


HMOR Elastic Modulus


Density Porosity CCS


HMOR PVE


Elastic Modulus


Density Porosity


After curing at 200°C (g/cm3) (%)


(MPa)


(MPa) 1400ºC (GPa)


After coking at 1400°C (g/cm3) (%)


(MPa)


(MPa) 1400ºC (%)


(GPa)


After coking at 1600°C (g/cm3) (%)


CCS (MPa) HMOR PVE (%)


Elastic Modulus (MPa) 1400ºC (GPa) O3 A1


+++ -


+++ 5


+++ ++


3.09 9.1


71.6 10.6 65


2.95 14.5 54.1 5.9 4.1 39


2.70 20.7 23.8 2.1


14.0 31


aggregate (A2) [10] A2


+++ +


+++ 5


+++ ++


3.06 9.6


59.2 10.0 60


2.92 14.9 51.7 6.2 4.4 36


2.73 19.6 23.9 2.0


14.0 30


Table 4: Compositions of conventional MAC (magnesia-alumina-carbon) brick (A1) and MAC with CaO-MgO-Al2


3-2: Results from a 130t ladle trial


An MgO-C brick with an addition of 5% CMA were produced. In the metal zone, three layers of the CMA-free MgO-C reference bricks were replaced by the CMA-containing material to allow a direct comparison between these 2 materials under equal conditions. After 89 heats the ladle went offline. Although the residual maximal thickness of the material was similar, the CMA-containing material had a very smooth surface while the reference material had a very rough surface and showed also stronger


-C bricks (A2) were tested in a 205 t steel ladle in


which both Al- and Si-killed steel is produced [10]. Although no significant differences in mechanical properties (Table 4) were observed compared to the CMA-free reference material A1, the material A2 performed better inside the ladle wall. Unlike for A1, during ladle operation it was observed that on the surface of A2 a protective slag coating was formed that seems to be responsible for the improved service life. This confirms laboratory corrosion trials in an induction furnace where better slag resistance has been seen with A2.


November 2018 Issue


Figure 8: MgO-C bricks with and without CMA addition in ladle metal zone after 89 heats


ENGINEER THE REFRACTORIES 19


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