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Virginia Kyanite and Mullite

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Virginia Kyanite and Mullite VirginiaVirginia KyaniteKyanite andand MulliteMullite TheThe MineralMineral ofof ChoiceChoice forfor ControllingControlling thethe ShrinkageShrinkage ofof HighHigh AluminaAlumina MonolithicMonolithic RefractoriesRefractories andand provideprovide highhigh hothot strength,strength, volumevolume stabilitystability andand highhigh creepcreep resistanceresistance Hank Jamerson, Dilip Jain KyaniteKyanite MiningMining CorporationCorporation Dillwyn,Dillwyn, VAVA USAUSA MaterialsMaterials UsedUsed inin HighHigh AluminaAlumina MonolithicMonolithic RefractoriesRefractories ActiveActive Inert IngredientsIngredients* Ingredients 1) Bonding Agents Calcined Clay 2) Plasticizers Andalusite & Sillimanite 3) Shrinkage Control Aids Calcined Kyanite 4) Nucleators & Catalysts Calcined Bauxite 5) Flow enhancers Corundum (BFA, WFA) * Active Ingredients are “additives” such as Kyanite, Clays, Calcium Aluminates, etc. Kyanite is an additive used in monolithic mixes to regulate volume change, while enhancing refractoriness. But it has many other desirable & beneficial attributes TheThe PropertiesProperties ofof KyaniteKyanite • Large “Irreversible” Volume Expansion (upon conversion)* • Very Low “Reversible” Thermal Expansion* • Unique “Lath-Like” Particle Shape* • Forms Mullite at a Low Decomposition Temperature* • Alumina Content & Purity Promote High Refractoriness (56-60% Al2O3 , very low iron and virtually no alkalis) • High Particle Packing Density Kyanite is a raw material additive that has wide variety of beneficial properties which can be utilized by refractory engineers. Some of these properties* are unique ThermalThermal ExpansionExpansion && ConversionConversion ofof KyaniteKyanite 8 n 7 o i Mullite 6 Phase ns 5 pa x 4 E Conversion r 3 a ne 2 Kyanite Li 1 Phase % 0 0 500 1000 1500 2000 Temperature (Celsius) Kyanite demonstrates unique expansion characteristics when heated to (and through) its decomposition temperature TheThe ThermalThermal ExpansionExpansion ofof KyaniteKyanite 1.6 Quartz 1.4 n o i 1.2 ns Corundum & 1 Calcined Bauxite pa x E 0.8 r a 0.6 ne 0.4 Li Kyanite % 0.2 0 0 200 400 600 800 1000 Temperature (Celsius) Kyanite has a low “reversible” coefficient of thermal expansion – at temps below 1,000oC. This is a very useful characteristic for lower temp refractory applications KyaniteKyanite’’ss ConversionConversion toto MulliteMullite The Mullitization of kyanite begins at about 1,250oC, and is completed virtually instantaneously at 1,450oC. (much faster & at a much lower temp than either Andalucite or Sillimanite TheThe IrreversibleIrreversible ExpansionExpansion ofof aa SingleSingle KyaniteKyanite CrystalCrystal A single Kyanite crystal almost doubles in volume as it is heated to 1450 c and converts to mullite EffectEffect ofof particleparticle sizesize onon ExpansionExpansion n 35 o i ns 30 35 Mesh pa x 25 E r 48 Mesh a 20 ne 15 Li 100 Mesh nt e r 10 200 Mesh ppa 5 325 Mesh A % 0 050100 Weight % Kyanite Apparent volume expansion of 35 mesh Kyanite is much more than the 325m Kyanite. This is simply because of the particle packing effect. Specific gravity of Kyanite is 3.55 gms/cc where as the specific gravity of Mullite is 3.0 gms/cc TheThe EffectsEffects ofof particleparticle sizesize onon thethe IrreversibleIrreversible ExpansionExpansion ofof KyaniteKyanite The apparent volume expansion of finer Kyanite is lower because the Volume expansion of fines take up the void space. For this reason Kyanite fines are an Excellent additive for high alumina bricks to help reduce porosity and improve creep Resistance of bricks. ThermalThermal ExpansionExpansion ofof VirginiaVirginia MulliteMullite 1.6 1.4 n o 1.2 1 0.8 a-axis 0.6 Aggregate near Expansi 0.4 c-axis Li 0.2 % 0 -0.2 0 500 1000 1500 Temperature (Celcius) Mullite has low anisotropy – which means that its expansion along the different axial dimensions is almost the same. This contributes to high thermal shock resistance. ThermalThermal ExpansionExpansion ofof VirginiaVirginia MulliteMullite 1.6 1.4 n 1.2 o Quartz i ns 1 pa x E 0.8 r a ne 0.6 Virginia Mullite Li % 0.4 0.2 0 0 200 400 600 800 1000 Temperature (C) The thermal expansion of Virginia Mullite contains a small inflection due to the presence of small amounts of quartz in the commercial concentrate ReasonsReasons whywhy KyaniteKyanite isis usedused inin MonolithicsMonolithics 1) Large & Predictable “Irreversible” Thermal Expansion - Tightens structure of monolithics - Helps in reducing porosity of monolithics - Expansion of Kyanite offsets shrinkages from other ingredients - Helps in maintaining volume stability 2) Alumina Content & Purity - Provides good “refractoriness” - Converts to Mullite upon heating at a fairly low temperature - Promotes the formation of secondary mullite - Contributes to improved Hot MOR and Creep Resistance 3) Unique Lath-like Particle Shape - Enhances Green Strength 4) Low “Reversible” Thermal Expansion - Promotes good thermal shock resistance: for kyanite below 1,200oC, and for Mullite at any temperature UsesUses ofof KyaniteKyanite inin RefractoriesRefractories 1) All low, ultra low cement, no cement castables, self flow, Shotcretes castables 2)2) GunningGunning mixes,mixes, rammingramming mixesmixes 3)3) DryDry Vibratable,Vibratable, 4)4) PlasticsPlastics 5)5) MortarsMortars andand patchingpatching materialsmaterials 6)6) SteelSteel andand ironiron ladleladle lanceslances 7)7) VarietyVariety ofof prepre castcast shapesshapes 8)8) BricksBricks andand otherother shapedshaped RefractoriesRefractories toto reducereduce porosityporosity andand increaseincrease creepcreep resistanceresistance 9)9) HighHigh temperaturetemperature insulatinginsulating firebricksfirebricks EffectsEffects ofof KyaniteKyanite inin aa RefractoryRefractory BodyBody 8 6 n o i 18% Kyanite ns 4 16% Kyanite pa x 14% Kyanite E 2 r a 12% Kyanite ne 0 10% Kyanite Li 0 500 1000 1500 2000 % -2 0% Kyanite -4 Temperature (C) The typical volume change in the size of the final refractory body is controllable by means of the judicious addition of specific amounts and size of kyanite AluminaAlumina ContentContent && PurityPurity A) Commercially available kyanite concentrate is very pure and has an alumina content of 57% (and about 0.6% total iron, and virtually no alkalis) - Kyanite is an excellent refractory in and of itself - Absence of any glass phase during transformation helps limit liquid formation at high temperatures - Contributes to improved Hot MOR - Raises Creep Resistance B) As a bonus, kyanite acts as a nucleating agent for the development of secondary mullite. - A function of particle size, decomposition temp & activation energy - Significantly improves hot strength & high temp mechanical properties - Enhances thermal shock resistance Properties of Virginia Mullite 1) Virginia Mullite has the least amount of impurities than most commercially available Mullite aggregates. 2) Has high Refractoriness (37+ PCE) 3) Is volume stable at very high temperatures 4) Has excellent thermal and physical properties 5) Has low coefficient of thermal expansion (5.3x10-6) 6) Is an excellent thermal and electrical insulator 7) Imparts High hot strength and outstanding hot load-bearing properties 8) Is resistance to acidic corrosion Mullite is a critical ingredient in high temp, high strength, thermal shock resistant & chemically stable refractory products.
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