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Showing posts with label Grain size distribution. Show all posts
Showing posts with label Grain size distribution. Show all posts

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

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Mag Carbon bricks - representative image
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:

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: - representative image

Effects of Grain Size Distribution and Powder Characteristics on Sintering, Densification and some other Properties of Refractory Bricks



We assume that the reader is already aware with the concept of ‘Sintering’, and also the types and effects of sintering on various properties of refractory bricks. In this article we will discuss on the effects of Powder characteristics (Grains size and their distribution) on sintering and densification behaviour of any refractory brick.

Densification is an important objective of sintering. Characteristics of starting powder e.g., particle size, size distribution, particle shape, particle aggregates, degree of agglomeration have a profound effect on the sintering kinetics as well as densification and Microstructural development of a refractory brick. The current understanding of ceramic powder processing has led to the following description of the desired powder and transport processes which led to the high density [R.L. Coble and R.M. Cannon in “Material Science Research Processing of Crystalline Ceramics, Vol.11” Plenum Press, New York, 1978, p.291]:
  • Small, non-agglomerated, monodisperse, spherical powders,
  • Uniform, dense packing of powder,
  • Mass transport during sintering by volume or grain boundary diffusion, no transport by surface diffusion or vaporization and condensation.
How can grain / particle size help in achieving better sintering at a relatively lower temperature and thus, high sintered density in a refractory composition?

It has been found that a higher percentage of smaller particle size or fines in the starting powder lead to a faster densification rate. The theoretical basis of this argument is due to Herring’s scaling law which states that there are simple laws governing the times required top produce, by sintering at a given temperature, geometrically similar changes in two or more systems of solid particles which are identical except for a difference in particle dimensions. That is why; those who are interested in high sintered density or reduced sintering temperatures and times strive for fines starting powder (mixture). However, the success in sintering of the fine powder relies on the removal of agglomerates and aggregates. De-agglomeration treatments increase the sinterability of refractory mixtures. Homogeneous mixing of ingredients plays an important role in this regard. There are other limitations too as a very high percent of fines in the initial mixture may cause other problems like -
  • Formation of lamination cracks in green bricks during their pressing or ramming which get exposed after the bricks become dry or has been fired.
  • Less compressive strength, less load bearing capacity and low MOR (Modulus of Rupture).
A narrow grain size distribution is imperative for obtaining a high sintered density. H. Kent Bowen stated two postulates for improving the manufacturability of high value added refractory products [H.K. Bowen in “Proceedings of the First China-US Seminar on Inorganic Materials Research”, May 17-21, 1983, Shanghai, Eds. T.S. Yen and J.A. Pask, Science Press, Beijing, 1983, p.55] -
  • Postulate 1: Powders (individual homogeneous or heterogeneous units) with a narrow size distribution are easier to process into uniform microstructures (uniformity of size and distribution of the voids), which results in easier control of the microstructure during densification.
  • Postulate 2: Submicron particles require modification of the interparticle forces by controlling the surface chemistry (usually a liquid phase) such that a small repulsive interaction is achieved by electrostatic, salvation, or steric phenomena.
How can particle or grain size distribution help in controlling the porosity of a refractory composition?

According to Kingery [W.D. Kingery in “Ceramic Fabrication Process, Part IV”, Technology Press, Cambridge and John Willey and Sons, New York], in practice it is found commonly that the porosity is about 40% for a single particle size refractory material, and a combination of cubic and hexagonal packing is observed. In a binary refractory mixture i.e. if two quite different particle sizes are mixed, the apparent volume varies as indicated in the adjacent Figure. with a minimum in apparent volume at about Refractory Technology: Effects of Grain Size Distribution on a Binary Refractory Mixture
Fig.- Particle Packing in a Binary Refractory Mixture
Refractory Technology: Effects of Grain Size Distribution on a Ternary Refractory Mixture
Fig.- Particle Packing in a Ternary Refractory Mixture
70% coarse material. At an infinite size ratio, the lower straight lines are reached, while for identical sizes the top horizontal line is followed. The heavy line is for a diameter ratio of about 10:1. Addition of a third size i.e. in a ternary refractory mixture decreases the pore volume even more as shown in the Figure.

Thus, during processing i.e. before firing of the refractory bricks, the powder characteristics (at first instance can be observed through the green mixture sieve analysis data) can be considered as a set of constant parameters. In conclusion, to achieve a dense fine-grained microstructure it is desirable to have a starting powder with small particle size and a narrow distribution, non-agglomerated particles with equiaxed shapes, and high purity (or controlled dopant / additive content). For all practical purposes and mass productions of refractory bricks and castables of different types like Fire-clay, High Alumina, Basic, Silica, Mag-chrome, Mag-carbon, Slide Gate refractories etc. at industry level there are established standard values for the range of coarse, medium, fine and ultrafine fractions which need to maintained and checked at regular intervals by observing mixture (powder) sieve analysis reports to ensure better control over fired properties of these refractory bricks and castables respectively.