Theimpact of Nano-technology on recent advances in monolithic refractories: Areview HeidarAli*1Department of Materials Science, UNSW University,Australia*CorrespondingAuthor, Tel: +98 9133330848, E-mail address: [email protected] Abstract:In recentyears, the use of Nano-technology (Nano-particles, Nano-material and Nano-additives)has attracted attention of scholars, engineers, and scientists in allscientific fields such as chemistry, medicine, material, agriculture, electric,and etc. The use of Nano-technology has also become widespread in therefractories products (which mainly used by various industries such as steel,casting, cement, glass, and etc.
). Therefore, many researchers have evaluatedthe effect of using different types and contents of Nano-materials (oxides andnon-oxides) on the properties of shaped (bricks) and un-shaped (monolithic)refractories products and they have achieved very interesting results. One of the most consumable refractory productsin various industries is monolithic refractories, which has been widely usedbecause of their great benefits to the other refractories products (bricks).Hence, in this paper, recent advances in monolithic refractories by Nano-technology are presented.
This article can be considered as a completereference and guidance for researchers, students and artisans in order to easyaccess to experimental research results of the impact of Nano-technology on monolithicrefractories.Keywords: Nano-technology, Nano-particles, Refractory, Monolithic 1. Introduction:1.2. Nanotechnology(Introduction) The Nano-technologyphrase originating from two words consist of the Greek numerical prefix nanoreferring to a billionth and the technology word 1-3. As an outcome,Nano-technology or Nano-scaled technology is commonly considered to be at asize under 100 nm (a Nano-meter is 10-9 m) 1-2.
Introduction: According to the ASTM C 71 , the refractories are a “non-metallicmaterials having those physical and chemical properties that lead to themapplicable for structures or as components of systems that are exposed toenvironments above 1000 °F (538°C) 11, 16. Also,some references mentioned that refractories are in-organic non-metallicmaterial which can withstand high temperature without changing in theirchemical or physical properties while remaining in contact with molten slag,metal and gases 11-13, 16-20. As well as, according to the operating situation,they should to have high thermal shock resistant, be chemically inert, and have defined ranges of thermal conductivityand thermal expansion coefficient 11- 21, 22. It is clear that refractories have an important role inglassmaking, metallurgical, and ceramic industries, where they are generatedinto a variety of shapes to line the interiors of furnaces or kilns or otherdevices for processing the materials at high temperatures 23-25. Some of thetechnological and scientific inventions and progresses would not have beenpossible without refractory materials. Producing 1Kg of any metal withoututilize of refractory is almost quite impracticable 26-29.
The history of using refractory materials dates back to since mankind start to develop metallurgicalprocess. The first refractor raw material was clay. Up to the nineteenth century, refractoryproducts were made of natural ores, such as magnesite, dolomite stones and clay.itwas at the end of the eighteenth century and beginning of nineteenth century thatthe basis of modern metal beneficiation, the development of Portland cement andof modern glass processes started to inflict higher requirements to therefractory industry 30-33. The mainmaterials used in the producing of refractories are based to Fig.1 34-36. In recent years, withthe changing trends in steelmaking, the high performing shaped refractories areon an increasing demand.
The higher campaign lives and the mutability of thenewer steelmaking operations are decided by the accessibility and performanceof such shaped refractories with superior high-temperature mechanical strength,erosion and corrosion resistance the selection of refractories to be utilizedis often according to the conditions dominating in the application zone36-40. 2.2. Classification:Generally, refractoriesare divided based on chemicalcomposition, manufacturing method, and physical shape or based on their applications(Fig.
2) 11-20, 40-188.8.131.52. Based on chemicalcomposition:a) Acidic refractories:These types of refractories are used in region thatslag and atmosphere are acidic. They have high resistance to acids but corrodedby alkalis. The main raw materials belong to the RO2 category, such asSiO2, ZrO2 and etc.b) Neutral refractories:These categories of refractories are used in areathat atmosphere and slags are chemically resistant to both acids and bases.
Themajor raw materials related to, but not confined to, R2O3category. The general examples of these materials are Al2O3,Cr2O3 and carbon(C).c) Basic refractories:These categories of refractories are used in areathat atmosphere and slags are basic; these categories high resistance toalkaline materials but corroded by acids.The major raw materials related to the RO categoryto which MgO is a very general example. Also, (Mg.Ca (CO3)2and (MgO-Cr2O3) are in these categories.
2.2.2. according toproducing method:a) Dry press. b) Fused cast. c) Hand molded. d) Formed (normal, fired or chemically bonded).e) Un-formed (monolithic- plastic, ramming andgunning mass, castables).
2.2.3. according to physicalshape:a) Formed:These types have determined shapes and size.These types divided into standard shapes and special shapes. The first type hassize that is confirmed by most refractory producer and is generally suitable tofurnaces or kilns of the same types. The second type specifically made for specialfurnaces or kilns.b) Un-formed:These categories are without clear format andare only given shape upon application.
Un-formed are known asmonolithic refractories. The common examples castables are, plastic masses, gunningmasses, ramming masses, fettling mix, mortars etc. Monolithic Refractories:Monolithic refractory phrase is thename usually given to all un-shaped refractory products, the word “monolithic”extracted from the word monolith which means ‘big stone’56-58. Monolithicrefractories are specific batches or blends of dry granular or cohesive plasticmaterials utilized to form nearly joint free linings.
Monolithic refractoryare un-shaped products which are installed as some form of suspension thatfinally harden to create a solid shape. Most monolithic formulations aremade of three constituent such as: large refractory particulates (anaggregate), fine filler materials (which fill the inter particle voids) and abinder phase (that gels the particulates together in the green state) Fig359-65. Monolithic refractories show a great range of mineralcompositions and vary greatly in their physical and chemical properties. Someof them have low melting point (low refractoriness) whiles others approach highpurity brick compositions in their ability to tolerate severeenvironments. Monolithic refractories are replacing the conventional typefired refractories at a much faster rate in many applications including thoseof industrial furnaces 53-55, 66-68.These refractories are used to advantage compare to brick construction indifferent type of furnaces.
Their use enhanced fast installation. Utilizeof monolithic refractories often delete difficult brick laying tasks, which maybe accompanied with looseness in construction. Protect of furnaces is veryimportance because substantial repairs can be made with a minimum loss of time69-74. Sometimes, monolithic refractories linings of the samecomposition as firebrick provide better insulation, lower diffusion andenhanced spalling resistance to the effects of repetitive thermalshock. Other major benefits of monolithic refractory linings are asfollows 75-80:ü Removing joints which is an inherent weakness.ü Easier and faster application.
ü Better properties than pressed (sintered or tempered) bricks.ü Simpler transportation and handling.ü Better volume stability.
ü Possibility to install in hot standby state.ü Higher mechanical resistance to vibration and impact.ü confirming shrinkage and expansion to the application.Different methods are used in the placement of monolithic refractories such asramming casting, spraying, gunning, sand slinging and etc.
Heat setting monolithicrefractories have a very low cold strength values and rely on relatively hightemperatures to progress a ceramic bond 81-83. Furnaces wall having the usualtemperature drop across its thickness, the temperature in the cooler part isgenerally not enough to progress a ceramic bond. However with the use of aproper insulating material as backup, the temperature of the lining can be highenough to progress a ceramic bond throughout its entire thickness. Inorder to the installation and curing, monolithic refractories need an intentlycontrolled dry-out program. This led to the filler, binder and aggregateto fire generating a high strength material 84-86. 3.1. Types of monolithic refractoriesUsually the monolithic refractories are dividedaccording to Fig.
4 56-60, 65-88a) Castable refractories Materials with hydraulic setting in nature arename of Castables. These refractories are containing cement binder (commonlyaluminate cement), which creates hydraulic setting properties when blended withwater. By heat-up temperature, the material and binder either transforms orvolatilizes simplifying the generation of a ceramic bond. The most commonbinder used in castables is high alumina cement. Other binders are consistingof hydratable alumina and colloidal silica. These materials are installed bycasting and are also known as refractory concretes. Insulating castables arespecialized monolithic refractories that are used on the cold surfaces of applications.These monolithic castables are composed of lightweight aggregate aggregatessuch as vermiculite, bubble alumina, perlite and expanded clay.
The mainfunction of castables is to create thermal insulation. Also, they are generallyhad low density and low thermal conductivity. The castables are classifiedaccording to following 48-58:ü ConventionalCastable.ü Low CementCastables (LCC).
ü Ultra LowCement Castable (ULCC).ü No CementCastable (NCC).ü Light WeightCastables.
ü Self-FlowCastables (SFC).ü InsulatingCastable. b) Plastic refractoriesPlastic refractories are used to formrefractory monolithic linings in different types of furnaces. Theserefractories are suitable for making quick, economical emergency repairs andthey are easily rammed to any shape or contour. Plastic refractories are consistingof refractory aggregates and adhesive clays which are prepared in stiff plasticcondition at the proper consistency for use without more preparation.
During utilization,the blocks are tasked into pieces and are rammed or casted into place withpneumatic rammer. These refractories can also be casted into place with amallet. These refractories suitable for many important applications due to thehigh melting point (high refractoriness), the range of compositions, and theease with which plastic refractories are rammed into place make them. Also,they have often highly spalling resistant. Plastic refractories can consist ofall the, clay-graphite, fireclay, high alumina, high alumina graphite andchrome types adapted for many various operating situations.
Specific gunning typesare also accessible. These are in granulated shape and are produced at theproper consistency, ready to use.Some examples of plastic refractories are65-69, 76-80:ü Heat settingsuper duty fireclay plastic, ü Super duty heatsetting plastics with graphite,ü Plastics in the50 % alumina class, ü Heat setting 60% alumina class plastics, ü Air settinghigh alumina plastics in 80 % alumina class,ü Phosphatebonded high alumina plastics with alumina content ranging from 70 % to 90 %,ü Phosphatebonded alumina chrome plastics, ü And siliconcarbide based phosphate bonded plastics. c) Ramming mixesRamming mixes composed essentially of groundrefractory aggregates, with a semi-plastic bonding matrix. These refractory materials are like to plasticrefractories but are much harder. They need some sort of form to maintenance themwhen formed.
The grain sizes are carefully classified and the final product isusually rendered dry and then mixed with a little content of water just before utilization.Other ramming products are rendered in wet state and are ready for useimmediately upon opening. Ramming mixes are placed with pneumatic rammer inlayers of 25 mm to 40 mm.
Steel making, burner blocks, ports and similarapplications used of High purity ramming mixes based on mullite grain. Rammingmixes consist of 80wt. % alumina content have good shrinkage resistance andthermal spalling at high temperatures. Some ramming mixed such as,stabilized high alumina air setting, have good thermal spalling resistance athigh temperatures and volume stability up to their temperature limit. Also, phosphate-bondedalumina-chrome ramming mixes typically have very high strength at hightemperatures and very good resistance to acid and neutral slags consist of coalash slags.
Alumina-graphite ramming mixes have mixture of high alumina grainand slag inhibitors which give them well slag resistance to acidic and slightlybasic slags. In steel making industry, the dry ramming mixes based on highpurity MgO and a sintering aid are useful. Magnesite ramming mixes of exceptionalpurity and stability are used firstly as lining materials for coreless typeinduction kilns. Magnesia-Chrome fused grain ramming mixes can create specialstrength and density 52-60, 64-73. d) Gunning mixesThe install method of more monolithicrefractories is gunning. The constitution material of gunning mixes are differentparticles sized of refractory aggregate, a bonding compound, and may containplasticizing agent to enhance their stickiness when pneumatically placed onto akiln surface. These refractory materials are sprayed on application surfacesusing a gun device. Usually gunning refractory mixes are supplied dry.
In orderto application, they are pre-damped in a batch mixer, and then continuously pouredinto a gun device. Water is added to the mix at the nozzle to achieve theproper consistency. Typically, Gun mixes are including high alumina, siliceous,fireclay, dead burned magnesite and chrome types. Magnesite and hot gun mixesare not pre-damped and are placed in a batch pressure gun. Gun mixes should providegood coverage in a variety of applications 40-52, 61-68. Some types ofgunning mixes are:ü Fireclaygunning mixes of multipurpose hard fired fireclay and standard calcium-aluminatecement compositions.ü Fire claygunning mixes with high purity calcium-aluminate bonding system. ü Gunning mixesbased on vitreous silica.
ü High purityalumina mixes which combine high fired alumina aggregate. ü High puritycalcium aluminate binder.ü Basic refractorygunning mixes with magnesia content ranging from 60 % to 95 % with or without aphosphate bond.e) Patching refractories These type refractories materialsare like to plastic refractories though have a very soft plasticity let them tobe casted into place 35-43, 71-76.F) Coating refractories These types of refractories materials areused to maintain refractory linings usually against chemical attack.
Coatingrefractories are usually intended to coat just the working surface of a lining.They tend to be justly thin layers 60-76.g) Mortars Generally, mortars are neither classified asrefractory brick nor monolithic refractories. They are very fine refractorymaterials, which become plastic when mixed with water. These are used to bondthe brickwork into solid unit, to provide cushion among the slightly irregularsurfaces of the brick, to fill up spaces created by a deformed shell, and tomake a wall gas-tight to prevent penetration of slag into the joints. Mortars shouldhave good water keeping properties and must not foul.
In this way, prematurepenetration of water in the refractory bricks after laying, causing the mortarto dry out, can be avoided. Different types of refractory mortars are consistingof 50-58:Ø Mortars withceramic bonding (bonding starting at 800 C)Ø Mortars Withchemical bonding Ø Mortars withhydraulic bonding (bonding starting at 20 C) Also, the important properties of the mortarsare consisting of:ü Composition andcharacteristics of the mortar materials, ü Grain size ü Consistency e) Fettling mixes Fettling mixes are also granular refractorymaterials, with function like to gunning mixes, but are applied by shovelinginto the kilns needing patching 43-51.f) Tap-hole mixesTap-hole mixes are resin bonded. In these mixesthe higher strength which is normally desired for monolithic refractoryproducts, is not that important. Some criteria are necessary for all tap-holemixes. These criteria are consisting of: correct consistency, setting, andcarbonization at the right time, precisely controllable PLC, and above alldrilling capability 72-78.
3. Application ofNano-technology in refractory industrial: Nano-technology is usually introduced by sizeand consist of the visualization, properties, production and manipulation ofstructures which are lower than 100 nm 89- 90. Specific mechanical, optical,electrical, and magnetic properties which can differ substantially from theproperties of the same materials at larger dimensions can show for thestructures that the dimensions of which range from 100 nm down to approx.
0.1nm. Hence, nano-technology is a very active research field and has applicationsin a number of areas. Today, considerable attention has been paid to the use ofnano-technology in the progress of refractories products 91-93.Nano-technology has been entered to refractories. It has been expressed thatthe efficiency ofthe refractories was extremely improved for the well dispersion of nano-sized grainin the microstructure and reaction activity. Some efforts have been done by differentresearchers to enhance the properties of refractories (bricks and monolithic)by using Nano-particles. The application of nano-technology is aimed at achievingthe following specific properties of brick and monolithic refractories 90-95:ü Ultra-high compressive strength, ü Relatively high tensile strength andductility,ü More efficient cement hydration,ü Increased aggregate-paste bond strength, ü High corrosion resistanceü Control of cracks and self-healingü High thermal shock resistance and ü High Abrasion resistance ü High chemical corrosion résistance 4.
Use of Nano-technology in monolithicrefractories:According tothe above, in this section, the results of carried out activities by various researchersusing nano-technology in monolithic refractories have been expressed (Table 1). It is observed that the use ofNano-technology has been heavily used by researchers in recent years.Conclusion:Recently, Nano-technology is used forproduction to refractory products and it is a necessary tool included in many activities.A lot of research has been working on the adding of different types ofadditives in ceramic goods, and some of them have concentrated theirinvestigations on the use of Nano-additives, because of the mentionedadvantageous of adding nano-particles to the ceramic goods. In this review article, all researches which done to enhancethe performance of monolithic refractories is reported and it was concluded that theapplication of nano-additives has the best results.
Results show that recently, researchershave been using nano-technology and have reached interesting results.