The Impact of Nanotechnology on Recent Advances in Monolithic Refractories

In recent years, the use of Nano-technology (Nano-particles, Nano-material and Nano-additives) has attracted attention of scholars, engineers, and scientists in all scientific fields such as chemistry, medicine, material, agriculture, electric, and etc. The use of Nano-technology has also become widespread in the refractories products (which mainly used by various industries such as steel, casting, cement, glass, and etc.). Therefore, many researchers have evaluated the effect of using different types and contents of Nano-materials (oxides and non-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 products in various industries is monolithic refractories, which has been widely used because 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 complete reference and guidance for researchers, students and artisans in order to easy access to experimental research results of the impact of Nano-technology on monolithic refractories.

Nanotechnology(Introduction)

The Nanotechnology phrase originating from two words consist of the Greek numerical prefix nano referring to a billionth and the technology word [1-3].

As an outcome, Nano-technology or Nano-scaled technology is commonly considered to be at a size under 100 nm (a Nano-meter is 10-9 m) [1-2].

According to the ASTM C 71 , the refractories are a "non-metallic materials having those physical and chemical properties that lead to them applicable for structures or as components of systems that are exposed to environments above 1000 °F (538°C) [11, 16]. Also, some references mentioned that refractories are in-organic non-metallic material which can withstand high temperature without changing in their chemical 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 conductivity and thermal expansion coefficient[ 11- 21, 22]. It is clear that refractories have an important role in glassmaking, metallurgical, and ceramic industries, where they are generated into a variety of shapes to line the interiors of furnaces or kilns or other devices for processing the materials at high temperatures [23-25]. Some of the technological and scientific inventions and progresses would not have been possible without refractory materials. Producing 1Kg of any metal without utilize of refractory is almost quite impracticable [26-29].

The history of using refractory materials dates back to since mankind start to develop metallurgical process. The first refractor raw material was clay. Up to the nineteenth century, refractory products were made of natural ores, such as magnesite, dolomite stones and clay.it was at the end of the eighteenth century and beginning of nineteenth century that the basis of modern metal beneficiation, the development of Portland cement and of modern glass processes started to inflict higher requirements to the refractory industry [30-33]. The main materials used in the producing of refractories are based to Fig.1 [34-36]. In recent years, with the changing trends in steelmaking, the high performing shaped refractories are on an increasing demand. The higher campaign lives and the mutability of the newer steelmaking operations are decided by the accessibility and performance of such shaped refractories with superior high-temperature mechanical strength, erosion and corrosion resistance the selection of refractories to be utilized is often according to the conditions dominating in the application zone [36-40].

Generally, refractories are divided based on chemical composition, manufacturing method, and physical shape or based on their applications (Fig.2) [11-20, 40-55].

Based on chemical composition:

Acidic refractories: These types of refractories are used in region that slag and atmosphere are acidic. They have high resistance to acids but corroded by alkalis. The main raw materials belong to the RO2 category, such as SiO2, ZrO2 and etc.

Neutral refractories: These categories of refractories are used in area that atmosphere and slags are chemically resistant to both acids and bases. The major raw materials related to, but not confined to, R2O3 category. The general examples of these materials are Al2O3, Cr2O3 and carbon(C).

Basic refractories: These categories of refractories are used in area that atmosphere and slags are basic; these categories high resistance to alkaline materials but corroded by acids. The major raw materials related to the RO category to which MgO is a very general example.

Also, (Mg.Ca (CO3)2 and (MgO-Cr2O3) are in these categories

According to producing method:

Dry press.

Fused cast.

Hand molded.

Formed (normal, fired or chemically bonded).

Un-formed (monolithic- plastic, ramming and gunning mass, castables).

According to physical shape:

Formed: These types have determined shapes and size. These types divided into standard shapes and special shapes. The first type has size that is confirmed by most refractory producer and is generally suitable to furnaces or kilns of the same types. The second type specifically made for special furnaces or kilns.

Un-formed: These categories are without clear format and are only given shape upon application. Un-formed are known as monolithic refractories.

The common examples castables are, plastic masses, gunning masses, ramming masses, fettling mix, mortars etc.

Monolithic refractory phrase is the name usually given to all un-shaped refractory products, the word “monolithic” extracted from the word monolith which means ‘big stone’[56-58]. Monolithic refractories are specific batches or blends of dry granular or cohesive plastic materials utilized to form nearly joint free linings. Monolithic refractory are un-shaped products which are installed as some form of suspension that finally harden to create a solid shape. Most monolithic formulations are made of three constituent such as: large refractory particulates (an aggregate), fine filler materials (which fill the inter particle voids) and a binder phase (that gels the particulates together in the green state) Fig 3[59-65]. Monolithic refractories show a great range of mineral compositions and vary greatly in their physical and chemical properties. Some of them have low melting point (low refractoriness) whiles others approach high purity brick compositions in their ability to tolerate severe environments. Monolithic refractories are replacing the conventional type fired refractories at a much faster rate in many applications including those of industrial furnaces [53-55, 66-68].

These refractories are used to advantage compare to brick construction in different type of furnaces. Their use enhanced fast installation. Utilize of monolithic refractories often delete difficult brick laying tasks, which may be accompanied with looseness in construction. Protect of furnaces is very importance because substantial repairs can be made with a minimum loss of time [69-74]. Sometimes, monolithic refractories linings of the same composition as firebrick provide better insulation, lower diffusion and enhanced spalling resistance to the effects of repetitive thermal shock. Other major benefits of monolithic refractory linings are as follows [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 as ramming casting, spraying, gunning, sand slinging and etc. Heat setting monolithic refractories have a very low cold strength values and rely on relatively high temperatures to progress a ceramic bond [81-83]. Furnaces wall having the usual temperature drop across its thickness, the temperature in the cooler part is generally not enough to progress a ceramic bond.

However with the use of a proper insulating material as backup, the temperature of the lining can be high enough to progress a ceramic bond throughout its entire thickness. In order to the installation and curing, monolithic refractories need an intently controlled dry-out program. This led to the filler, binder and aggregate to fire generating a high strength material [84-86].

Usually the monolithic refractories are divided according to Fig.4 [56-60, 65-88]

Materials with hydraulic setting in nature are name of Castables. These refractories are containing cement binder (commonly aluminate cement), which creates hydraulic setting properties when blended with water. By heat-up temperature, the material and binder either transforms or volatilizes simplifying the generation of a ceramic bond. The most common binder used in castables is high alumina cement. Other binders are consisting of hydratable alumina and colloidal silica. These materials are installed by casting and are also known as refractory concretes. Insulating castables are specialized monolithic refractories that are used on the cold surfaces of applications. These monolithic castables are composed of lightweight aggregate aggregates such as vermiculite, bubble alumina, perlite and expanded clay. The main function of castables is to create thermal insulation. Also, they are generally had low density and low thermal conductivity. The castables are classified according to following [48-58]: ü Conventional Castable. ü Low Cement Castables (LCC). ü Ultra Low Cement Castable (ULCC). ü No Cement Castable (NCC). ü Light Weight Castables. ü Self-Flow Castables (SFC). ü Insulating Castable.

Plastic refractories are used to form refractory monolithic linings in different types of furnaces. These refractories are suitable for making quick, economical emergency repairs and they are easily rammed to any shape or contour. Plastic refractories are consisting of refractory aggregates and adhesive clays which are prepared in stiff plastic condition at the proper consistency for use without more preparation. During utilization, the blocks are tasked into pieces and are rammed or casted into place with pneumatic rammer. These refractories can also be casted into place with a mallet. These refractories suitable for many important applications due to the high melting point (high refractoriness), the range of compositions, and the ease with which plastic refractories are rammed into place make them. Also, they have often highly spalling resistant. Plastic refractories can consist of all the, clay-graphite, fireclay, high alumina, high alumina graphite and chrome types adapted for many various operating situations. Specific gunning types are also accessible. These are in granulated shape and are produced at the proper consistency, ready to use. Some examples of plastic refractories are [65-69, 76-80]: ü Heat setting super duty fireclay plastic, ü Super duty heat setting plastics with graphite, ü Plastics in the 50 % alumina class, ü Heat setting 60 % alumina class plastics, ü Air setting high alumina plastics in 80 % alumina class, ü Phosphate bonded high alumina plastics with alumina content ranging from 70 % to 90 %, ü Phosphate bonded alumina chrome plastics, ü And silicon carbide based phosphate bonded plastics.

Ramming mixes composed essentially of ground refractory aggregates, with a semi-plastic bonding matrix. These refractory materials are like to plastic refractories but are much harder. They need some sort of form to maintenance them when formed. The grain sizes are carefully classified and the final product is usually 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 use immediately upon opening. Ramming mixes are placed with pneumatic rammer in layers of 25 mm to 40 mm. Steel making, burner blocks, ports and similar applications used of High purity ramming mixes based on mullite grain. Ramming mixes consist of 80wt. % alumina content have good shrinkage resistance and thermal spalling at high temperatures. Some ramming mixed such as, stabilized high alumina air setting, have good thermal spalling resistance at high temperatures and volume stability up to their temperature limit. Also, phosphate-bonded alumina-chrome ramming mixes typically have very high strength at high temperatures and very good resistance to acid and neutral slags consist of coal ash slags. Alumina-graphite ramming mixes have mixture of high alumina grain and slag inhibitors which give them well slag resistance to acidic and slightly basic slags. In steel making industry, the dry ramming mixes based on high purity MgO and a sintering aid are useful. Magnesite ramming mixes of exceptional purity and stability are used firstly as lining materials for coreless type induction kilns. Magnesia-Chrome fused grain ramming mixes can create special strength and density [52-60, 64-73].

The install method of more monolithic refractories is gunning. The constitution material of gunning mixes are different particles sized of refractory aggregate, a bonding compound, and may contain plasticizing agent to enhance their stickiness when pneumatically placed onto a kiln surface. These refractory materials are sprayed on application surfaces using a gun device. Usually gunning refractory mixes are supplied dry. In order to application, they are pre-damped in a batch mixer, and then continuously poured into a gun device. Water is added to the mix at the nozzle to achieve the proper consistency. Typically, Gun mixes are including high alumina, siliceous, fireclay, dead burned magnesite and chrome types. Magnesite and hot gun mixes are not pre-damped and are placed in a batch pressure gun. Gun mixes should provide good coverage in a variety of applications [40-52, 61-68]. Some types of gunning mixes are: ü Fireclay gunning mixes of multipurpose hard fired fireclay and standard calcium-aluminate cement compositions. ü Fire clay gunning mixes with high purity calcium-aluminate bonding system. ü Gunning mixes based on vitreous silica. ü High purity alumina mixes which combine high fired alumina aggregate. ü High purity calcium aluminate binder. ü Basic refractory gunning mixes with magnesia content ranging from 60 % to 95 % with or without a phosphate bond.

These types of refractories materials areused to maintain refractory linings usually against chemical attack. Coating refractories are usually intended to coat just the working surface of a lining. They tend to be justly thin layers [60-76].

Generally, mortars are neither classified as refractory brick nor monolithic refractories. They are very fine refractory materials, which become plastic when mixed with water. These are used to bond the brickwork into solid unit, to provide cushion among the slightly irregular surfaces of the brick, to fill up spaces created by a deformed shell, and to make a wall gas-tight to prevent penetration of slag into the joints. Mortars should have good water keeping properties and must not foul. In this way, premature penetration of water in the refractory bricks after laying, causing the mortar to dry out, can be avoided.

Different types of refractory mortars are consisting of [50-58]:

- Mortars with ceramic bonding (bonding starting at 800 C)

- Mortars With chemical bonding

- Mortars with hydraulic bonding (bonding starting at 20 C)

Also, the important properties of the mortars are consisting of: ü Composition and characteristics of the mortar materials, ü Grain size ü Consistency

- Fettling mixes

Fettling mixes are also granular refractory materials, with function like to gunning mixes, but are applied by shoveling into the kilns needing patching [43-51].

- Tap-hole mixes

Tap-hole mixes are resin bonded. In these mixes the higher strength which is normally desired for monolithic refractory products, is not that important. Some criteria are necessary for all tap-hole mixes. These criteria are consisting of: correct consistency, setting, and carbonization at the right time, precisely controllable PLC, and above all drilling capability [72-78].

Application of Nano-technology in refractory industrial:

Nano-technology is usually introduced by size and consist of the visualization, properties, production and manipulation of structures which are lower than 100 nm [89- 90]. Specific mechanical, optical, electrical, and magnetic properties which can differ substantially from the properties of the same materials at larger dimensions can show for the structures that the dimensions of which range from 100 nm down to approx. 0.1 nm. Hence, nano-technology is a very active research field and has applications in a number of areas. Today, considerable attention has been paid to the use of nano-technology in the progress of refractories products [91-93]. Nano-technology has been entered to refractories. It has been expressed that the efficiency of the refractories was extremely improved for the well dispersion of nano-sized grain in the microstructure and reaction activity. Some efforts have been done by different researchers to enhance the properties of refractories (bricks and monolithic) by using Nano-particles. The application of nano-technology is aimed at achieving the following specific properties of brick and monolithic refractories [90-95]: ü Ultra-high compressive strength, ü Relatively high tensile strength and ductility, ü More efficient cement hydration, ü Increased aggregate-paste bond strength, ü High corrosion resistance ü Control of cracks and self-healing ü High thermal shock resistance and ü High Abrasion resistance ü High chemical corrosion résistance

According to the above, in this section, the results of carried out activities by various researchers using nano-technology in monolithic refractories have been expressed (Table 1). It is observed that the use of Nano-technology has been heavily used by researchers in recent years.

Recently, Nano-technology is used for production 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 of additives in ceramic goods, and some of them have concentrated their investigations on the use of Nano-additives, because of the mentioned advantageous of adding nano-particles to the ceramic goods. In this review article, all researches which done to enhance the performance of monolithic refractories is reported and it was concluded that the application of nano-additives has the best results. Results show that recently, researchers have been using nano-technology and have reached interesting results.