Direct bonded MgO refractories

Question 1

What causes inferior thermal shock resistance in silicate-bonded MgO?

Answer 1

> the coefficient of thermal expansion difference between MgO grains and silicate bonding phase

Question 2

What is direct bonded MgO?

Answer 2

> the use of higher purity MgO (less CaO and SiO2)
Cr2O3 additions (from chromite ore)
results in second phase bridges of MgCr2O4 SPINEL forming during firing (1700-1800C)

Question 3

What are the qualities of direct bonded MgO refractories? What causes them?

Answer 3

> the liquid phase between MgO and MgCr2O4 forms at 2300C, leads to:
highly refractory, high hot strenght, high spalling resistance (thermal shock resistance)

Question 4

What effect does low porosity have on direct bonded MgO?

Answer 4

> yields good resistance to SLAG PENETRATION

Question 5

What are direct bonded MgO refractories used for?

Answer 5

> used for SLAG LINES (slag-metal interface) of STEEL LADLES
used for formation of non-ferrous metals (Pb, Ca) that form aggressive slages

Question 6

Why are we trying to avoid using chromium in these refractories?

Answer 6

> Cr6+ is a human carcinogen
MgAl2O4 spinel-bonded MgO has been devloped to avoid chromium