Ideal Grain Size Distribution of Refractory Raw Materials Mixture for making Magnesia Carbon Bricks

6-July-2020

Mag Carbon Bricks

Magnesia carbon refractory bricks (MgO-C) or Carbon containing Magnesite refractories have been extensively used by steel makers for the secondary treatment of steel in basic oxygen furnaces, electric arc furnaces, and ladle furnaces. Mag Carbon refractory bricks are widely used in slag lines of BOF (Basic Oxygen Furnace) because of their superior wear resistance. The service life of Magnesia Carbon refractories used in BOFs have been pushed quite significantly (largely due to slag splashing and gunning improvements) even as the service conditions have become more severe due to the increased operating temperature required for continuous casting and the need to produce cleaner steel.

Mag Carbon bricks are made of high purity magnesia, high quality graphite, antioxidants and some additives with a suitable binder (bonding agent). Selection of raw materials, their grading and grain size distribution (Granulometry) and composition together have ultimate role in the development of various physical properties, microstructure and ultimately thermo-mechanical properties of Mag Carbon refractory bricks (MgO-C). Various different types of MgO (Magnesite) grains provide different levels of corrosion resistance.

From the literature and plant applications it has been established that Magnesia-carbon bricks having 3 mm particle size show better wear resistance as well as a few other characteristics as compared to the bricks with 5 mm size grains.

Graphite, Anti-Oxidants (Additives) and Binders used in the composition of Magnesia Carbon Bricks

The graphite flakes used in these bricks impart -

=> High thermal conductivity

=> Good thermal shock resistance

=> Low thermal expansion

=> Non-wettability by liquid slag

=> Low corrosion rates by slags

Graphite contents of typical bricks range from 4 - 35% natural flake graphite. Since oxygen affinity of carbon is very high so different kinds of antioxidant minerals are used (in fines or superfines) in order to protect refractory material against chemical corrosion. The REDOX reactions in magnesia carbon can be reduced by selection of high purity magnesite, large crystal size and use of graphite with low impurities. Slag corrosion resistance of MgO-C refractories can be improved by use of magnesite grains with less reactivity i.e. fused magnesite grains of high Bulk Density (BD) and high purity.

The above are some of the reasons which explain how selection of various raw materials can affect the performance of magnesia-carbon bricks. More on this aspect and the compositions of Magnesia-carbon refractory bricks will be discussed in a separate post:

Refractory composition, production of Magnesia Carbon Refractory Bricks and their properties

Here, our topic is Granulometry i.e. overall grading and the grain size distribution, suitable for the best performance of MgO-C bricks. Grading and the grain size distribution are important as these are directly related with the following properties of Magnesia-carbon bricks:

=> Porosity

=> Mechanical strength

=> Spalling resistance

=> Microstructure and phase development

=> Wear resistance    

From the experience of various trials and performances it has been found that 0 - 4 mm grading is the best for MgO-C refractory bricks for all general applications and also for different shapes like Tap Hole Blocks, Sleeves, etc. (except Slide Gate refractories which will be different).

Bonding agents or binders used in Mag Carbon bricks and other carbon refractory products are immiscible with graphite and other refractory raw materials. At room temperature, they rely on binder to cure. Generally these binders or bonding agents are resin, asphalt or an organic matter and can be divided into three types: phenolic resin, modified asphalt, petroleum cracking by-product category.

The grain size distribution (granulometry) of the press mixture (powder) for MgO-C bricks with different Graphite percentages as they should be are given in the following table:   

Refractory composition, production of Magnesia Carbon Refractory Bricks and their properties

In one of our previous posts on Magnesia Carbon refractory bricks (MgO-C), we have already discussed on the selection of various raw material ingredients required for the production of MgO-C refractory bricks with an emphasis to their grading etc. In this article we bring you one MgO-C (Mag Carbon) refractory brick composition and further guidance (tips) on its production along with the tested results of some of the refractory properties of the brick made from this composition.

Refractory Composition (Formulation) for MgO-C Refractory Bricks

Item No	Raw Materials	Grading (mm)	Quantity
1	China 97FM (Fused Magnesite)	2.0 - 4.0	18 %
2	do	0.6 - 2.0	42 %
3	do	0 - 0.6	20 %
4	China 98FM or Seawater DBM 98 HD	Flour (Ball Mill Fine)	7.5 %
5	Coarse Flaky Graphite (94% LOI)	– 0.10 (> 40 %)	12.5 %
6	Al-metal Powder		3 parts
7	Si-metal Powder		1 part
8	Liq. Resin (Durez-40)		3 parts
9	Solid Resin (Powder)		0.6 parts
10	Hexamine		0.25 parts
11	MEG (Meth. ethyl glycol)		0.6 parts

Mag Carbon Refractory Bricks - Production (Tips)

=> Mixing Sequence:

(A) First co-mix item nos. 4, 5, 6 and 7 together in a Ball Mill for at least 6-8 hrs, unload and keep separate.

(B) Mix MEG with (0-0.6) DBM fraction (item 3). Then add the co-mix powder made in step (A) to it and remix.

(C) Take (2-4) & (0.6-2) DBM grading (item no 1 and 2), add hot Resin (item 8). Mix thoroughly to get perfect coating. Add mixture made in step (B) and remix the batch intensively. Now add powder Resin (item 9) then, Hexamine (item 10) and mix it finally. Keep the mixture (powder) in closed polythene bags 35-40 kg in each bag, for ageing 8-12 hrs.

=> Heat the liquid Resin to 60-65^OC before mixing. Heating reduces the viscosity and thus help for better coating.

=> Don't use any mixture after it has dried up. Try to make mixture in perfectly pre-calculated quantities as reuse of such mixture (powder) even by adding more MEG is not advisable and should be avoided as far as possible.

=> While making any shaped brick by pneumatic Ramming it must be done continuously until the shape is complete.

=> Don't keep mixture (powder) uncovered.

=> Also read: Ideal Grain Size Distribution of Refractory Raw Materials Mixture for making Mag Carbon Bricks

Properties and Test Results

Magnesia Carbon Bricks - Properties

Properties	Results
MgO %	94 (min)
Fixed Carbon %	8 (min)
Green BD (gm/cc)	3.1 (min)
Coked BD (gm/cc)	2.95 (min)
AP % (after coked after coked in reducing atmosphere at 1000OC )	14 (max)
AP % (original condition)	7 (max)
CCS (kg/cm2)	450 (min)
MOR at room temp (kg/cm2)	140 (min)

Related Articles

DBM (Dead Burnt Magnesite) Specifications from Different Sources