Type: Process Essays
Sample donated: Dominick Farmer
Last updated: May 21, 2019
The influenceof silica nanoparticles addition on the physical, mechanical, thermo-mechanicalas well as microstructure of Mag-Dol refractory compositesHassanGheisari Dehsheikh1, Salman Ghasemi-Kahrizsangi*21- Department of Mechanical Engineering, KhomeinishahrBranch, Islamic Azad University, Khomeinishahr, Isfahan, Iran 2- Department ofMaterials Science and Engineering, Sharif University of Technology, Tehran, Iran.2*CorrespondingAuthor, Tel: +98 9137541686 , E-mailaddress: salman.
[email protected] Abstract:Thehigh hydration potential of CaO and MgO phases restricted the application of Mag-Dolrefractory composites. In this study, the impact of nano-silica (SiO2) addition on the physical, mechanical,thermo-mechanical as well as microstructure of Mag-Dol refractory composites isinvestigated. Mag-Dolcompositions were prepared by using calcined dolomite and magnesite particles(micron, 0-1, 1-3, 3-5, and 5-8 mm), liquid resin, and 0, 0.5, 1, 1.
5, 2, and2.5 wt% nano SiO2 as additives.Specimens were heated up to 1650?C for the 3h soakingperiod. Fired specimens were characterized by physical (apparent porosity, bulkdensity, and hydration resistance), mechanical (cold crushing strength), andthermo-mechanical (flexural strength at 1200?C) measurements. XRD andSEM/EDS analysis were done to study phases and microstructure analysis of thefired samples, respectively.
Results showed that by adding up to 2.5 wt% nano-SiO2, due to the formation of CaO.MgO.2SiO2 (Diopside), 2CaO.MgO.2SiO2 (Akermanite),and CaO.MgO.SiO2 (Monticellite) phases, physical andmechanical properties were enhanced.
But the highest flexural strength value isachieved for 1 wt% nano-SiO2 containing sample. Keywords:Hydration;Mag-Dol Refractory; Nano-SiO2;Microstructural; XRD. 1. Introduction:Mag-Dol (or Magnesite-Dolomite) refractory composites are widelyused in the industries such as secondary metallurgyArgon Oxygen Decarburization(AOD), Vacuum Oxygen Decarburization(VOD, non-ferrous kilns (copperconverter), and also cement, lime, and glass making furnaces 1-4.
Highattention to the application of Mag-Dol refractory composites is fortheir desirable properties such as high corrosion and erosion resistance (inalkaline environment), high melting point (Tm>2200), affordable (the low cost of the raw materials), lowthermal expansion, acceptable thermal shock resistance, and ability to generate clean steel melt1, 5-7. Conventionally,Mag-Dol refractory composites are made of about 50-80 wt% magnesia, 2.5-5 wt. %binder (resin, peck, tar, paraffin or other organic binders), which is utilizedin order to create a strong linkage among the matrix and aggregates, and0.25-0.
5 wt% hardener (hexamine) 2-4, 8-10. Various methods have beenmentioned to create Mag-Dol refractory composites. For example, some sources have been suggested using of fired or fused Co-clinker of Mg (CO3)2and Mg.Ca (CO3)2 as a starting material for producing Mag-Dol refractory composites, which itwould result in more homogenous composite with high favorable properties.
Othersuggested method is mixing Mg (CO3)2 and Mg.Ca (CO3)2ores and calcined them at high temperature (more than 1550?C) that lead to generate in–situ Mag-Dol refractory composites 1-3, 11, and12. Also, Mag-Dol refractory composites havesome benefits compared to magnesite and calcia-based refractory composites (Table 1). As wellas recently, Mag-Dol refractory composites are used as substitutes formagnesia-chromite and magnesia-spinel refractory composites in the various industries1-3. In spite of the high mentioned benefitsproperties, these refractory composites have low hydration resistance in theatmosphere. CaO and MgO phases quickly react with humidity in the atmosphereand generate CaO (OH) 2 and Mg (OH) 2 phases (Equation. 1and 2). The volume expansion (?V=15-20%)of the created phase CaO (OH) 2 and Mg(OH) 2, lead to the destruction of these refractory composites (Fig.
1) 1-7, 13-16. CaO + H2O = Ca (OH) 2 Eq 🙁 1) 1, 2 and 5 MgO+H2O = Mg (OH) 2 Eq 🙁 2) 1, 2 and 5For these reason, several research has been done to enhance thehydration resistance of Mag-Dol refractory composites. For example, itsuggested that application of organic binders such as peck, tar, and etc. can improvethe hydration resistance of Mag-Dol refractory composites.
This method is notavowed as it lead to released mono-oxide carbon (CO)and di-oxide carbon (CO2) gases into the atmosphere and polluting it1-3, 5-9. Another suggested way is totreading mag-dol refractory composites in a CO2 space or coatingtheir surface by phosphate, which leads to the formation of a denselayer on the surface of CaO and protects CaO grain from hydration. This methodis also not economically desirable 1, 17. Another method that has recently been highly used by themanufacturers of this refractory composites is the use of oxide compounds suchas TiO21, 2, Fe2O31, 3-7, Al2O33,8, 9, Cr2O33,10, ZrO21,7, 11, 12, CuO13,V2O514, FeTiO315, MgAl2O416,ZrSiO417, and etc. Although use of the aforementioned oxide compounds have somepositive results, but application of them generated some restriction such ashigh cost, decreasing refractoriness, and not available and etc 1, 3-8.
Recently,the use of nano-scale additives has attracted the attention of manufacturers inmany industries due to their excellent, singular, and unique properties 18-21. On the other hand, silica (SiO2) is used extensively in refractoryindustry due to its high refractoriness (Tm>1700?C), availability and reasonable prices 1-3. Accordingto above mentioned, in this research study, the addition effect of nano-SiO2as an inexpensive and affordable additive, on the physical, mechanical, thermo-mechanical as well as microstructureof Mag-Dol refractory composites was evaluated. 2. Experimental procedure:2.
1.1. Materials (raw materials, binder, additive, and hardener)Calcineddolomite and magnesite (extracted from Zefreh andBirjand mines in Iran, respectively, Table 2)with size range micron, 0-1, 1-3, 3-5, and 5-8 mm were used as the startingmaterials. Also, Nano-SiO2 (supplier: Sigma-Aldrich, CAS Number 112945-52-5, Table 3, Fig.
2), liquid resin (Table 4), and hexamine (supplier: Kanoria Chemicals & Industries Ltd.) were utilized in this research as anadditive, binder, and hardened, respectively. 2.
1.2. Compositions preparations:Allbatch compositions were formulated according to the following equation: 47 wt%Magnesite + (53-X) wt% Dolomite + X wt. % Nano-SiO2 Eq 🙁 3)X = 0,0.5, 1, 1.5, 2, 2.5 wt.
%As wellas batch codes, mixing times, mixing order, and other characterizes ofcompositions preparation are presented in Table 5and 6. Then the refractory compositions were uniaxial pressed (at150 MPa, SACMI Model, Italy) in the shape of cylindrical whose dimensions were:50mm* 50mm. 2.1.3.
Aging, tempering and firing processes:Preparedcomposition were aged for 12 h at the air atmosphere then were thermaltreatment at 240?C for 12 h, and finally fired up to 1650 ?Caccording to following diagram program (Fig. 3). 2.2. Characterizes measurement:Physical(bulk density, apparent porosity, and hydration resistance), mechanical (coldcrushing strength), and thermo-mechanical (flexural strength at 1200?C)properties of the fired specimens were measured according to the followingstandard methods. Also, presented values for each test are the average of 5determinations for each refractory composite. – Bulkdensity and apparent porosity: ASTM C-20Bulk Density (g/cm3) = (M1-M2)/M3 Eq:(4) Apparent Porosity (%) = (M2-M3/M2-M1)*100 Eq 🙁 5)M1=initial weigh M2= Saturation weightM3= Immersionweight- Hydration resistance: ASTM C456 – 13 Hydration Resistance (%) = Eq: (6)W2= mass gain after hydration resistance test.W1= initial mass gin before hydration resistance test.
– Cold crushing strength: ASTM C133 – 97 – Flexuralstrength at 1200: ASTM D790 2.3. Microstructure and phase analysis:Scanning electron microscopy (model Philips XL30 TMP) with attachedenergy dispersive analysis (EDS) analysis was performed for microstructureevaluation of the fired samples. Also, for crystalline phases analysis of firedsamples, the X-ray diffraction (XRD analysis) using a Ni-filtered Cu Karadiation with a scanning speed of 28 (2u) per minute was used. 3. Results and Discussion:3.1. Crystalline phases analysis:Figs.
4-5 show the X-ray diffraction patterns (XRD) of the MDS0,MDS1 and MDS2.5 refractory compositions afterfiring at 1650?C for3h. Magnesia (MgO), calcia(CaO), and calcinumhydroxide(Ca(OH)2 phases were detected for MDS0composition. The presence of calcinum hydroxide (Ca(OH)2 phase indicates the high tendency of thissample to be hydrated. Also, rather than magnesia, calcia; CaO.MgO.
2SiO2(Diopside), 2CaO.MgO.2SiO2(Akermanite), and CaO.
MgO.SiO2 (Monticellite) phases were the main identified phases for MDS1 andMDS2.5 compositions. As it can be seen, by increasing thenano-silica content, the peaks intensity for magnesia (MgO) and calcia (CaO)are diminished, and also calcinum hydroxide (Ca(OH)2 peaks notdetected.
As well as the peaks intensityof the CaO.MgO.2SiO2(Diopside), 2CaO.
MgO.2SiO2(Akermanite), and CaO.MgO.SiO2 (Monticellite) phases are promoted. The melting point of the CaO.
MgO.2SiO2 (Diopside),2CaO.MgO.2SiO2 (Akermanite), and CaO.MgO.SiO2(Monticellite) and phase are 1391?C, 1454?C and 1503?C respectively. Formation of the aforesaid phase at the firing temperature(1650?C) leads to the covering grain, grain boundaries, and triple points and enhancing the firing process.
Also,raising additive content helps to the formation more liquid phases between themain particles. Thus the wettability of CaO and MgO particles increases and leadsto grain growth via solution and precipitation. 3.2. Microstructure analysis(SEM/EDX)Fig. 6a-c reveals themicrostructure images relating to the compositions without and with theaddition of nano-silica. In addition of porosities and voids, a light gray phase relating to CaO (calcia) and darkgray phase relating to MgO (magnesia) particles were marked by energydispersive X-ray (EDX) (Fig.6a, Table 7) for MDS0 composition.
With nano-silicaaddition (Fig.6b-c.), the generation of phases with Si, Ca, and Mg elementswas observed. These phase have low melting point (lower than 1520?C). As it cansee a homogeneous and dense microstructure with low apparent porosity weregenerated by increasing nano-silica content.
By using EDX analysis, generationof CaO.MgO.2SiO2 (Diopside), 2CaO.MgO.2SiO2 (Akermanite),and CaO.MgO.SiO2 (Monticellite) were confirmed (Table 7).3.
3. DensificationDensification parameters i.e. bulk density (BD) and apparentporosity (AP) of the Mag-Dol refractory composites fired for 3 h at 1650 ?C with varying nano-SiO2 content are shown in Figs.7 and 8. A gradualenhancement is showed in density value with adding nano-SiO2 contentup to 1.5wt%.
Also, for 2.5 wt% of nano-SiO2 content there isdiminished in density value, which it is related to the weak distribution ofSiO2 nano-particles and the formation of accumulating in the body.Apparent porosity variation (see Fig.8) opposed to bulk density haschanged.
The lowest and highest apparentporosity values (7.54 and 5.34%) related to the specimens with 2.5 wt% MDS2.5and MDS0 compositions, respectively.
Factors such as a good compression of thematrix on filling up of the pores among the calcia and magnesiaparticles, (ii) more complete firing process of the compositions due to theexistence of active nano-SiO2particles, (iii) generation of low melting phase such as CaO.MgO.2SiO2, 2CaO.MgO.2SiO2, and CaO.MgO.
SiO2which leads to filling void and porosities inthe matrix and create a high strength connection between main constitutionparticles i.e. magnesia and calcia. 3.4. Cold crushing strength (CCS):The results of the cold crushing strength testof the fired compositions (at 1650?C for 3h) are shown in Fig. 9. As can be seen, the cold crushing strengthchanges trend of the samples has been progressed by increasing the amount ofSiO2 nano-particles.
According to the previous reports, existenceof porosities, cavities and grain boundaries in the specimen’s matrix can leadsto the loss of strength. On the otherhand, the formation of the low melting point phase at the firing processtemperature can result in the filling of the porosities and cavities, the reduction ofgrain boundaries, and also the growth of the main grain in the samples matrix.Based on the phase’s analysis results (see Figs.
4-5),the formation of aforesaid low melting phases lead to the formation of a densebody by removing porosities and cavities, as well as the growth of grains insamples containing nano-silica. Ultimately, this has led to an increase in thecold crushing strength of the samples.3.5. Flexural strength at 1200?C:The flexural strength (at 1200?C)change trends of the fired (at1650 ?C for3h) Mag-Dol refractories composites are depicted in Fig.
10. As it can be observed, by increasing nano-SiO2content (up to 1 wt %) the flexural strength values enhanced and the highestvalue is related to MDS1composition (318kg/cm2). But after it (from 1.
5 to 2.5 wt. %), theflexural strength gradually decreased (reached to284kg/cm2).
Themain important factors that contribute to this initial increase are: (a) thedense (compressed) created body by the removal of porosities and cavities inthe microstructure and (b) the grain growth of the main constituent grain ofthe body and the reduction of the grain boundaries in the microstructure. Also,the secondary reduction of the flexural strength is due to the: formation of more quantities oflow melting point phases at such as (CaO.MgO.
2SiO2,2CaO.MgO.2SiO2, and CaO.
MgO.SiO2) the firingtemperature. Generally, in order toaccess the highest flexural strength value in Mag-Dol refractory composite,selecting 1wt % nano-SiO2 can be considered.as the optimal content. 3.6. Hydration resistance:Composite compositions include the MgO, and inparticular CaO phases, have highly susceptible to be hydrated (high tendency toabsorb moisture) and reaction with moisture in the atmosphere.
In these compounds, CaO and MgO phases will be converted to thelower density phases Ca (OH) 2 and Mg (OH) 2. Formationof Ca (OH) 2 and Mg (OH) 2 phases lead to avolume expansion about 15-20%. The created volume expansion results in the cracking and collapse ofthe compositions containing these phases (Fig.11).For this purpose, in order to enhance the hydration resistance of Mag-Dolcomposite materials, coating of magnesia (MgO) and calcia (CaO) phases or theirconversion into higher hydration resistance phases can be effective. On the other hand, porosities, holes, grainboundaries, and in general all defects in the microstructure (due to highersurface energy) tend to absorb moisture and perform hydration reactions.
Asshown in Fig.12, withincreasing nano-silica content in Mag- Dol compositions the percentage of the gainweight (tendency to moisture absorb) have decreased. It indicates the hydrationresistance improvement of the specimens. By comparing the SEM images of thesamples after firing at 1650?C for3 h (Fig.6A-C), it can be seen that thegrain growth of the particles, porosities and cavities filling (the reductionof the free specific surface (, as well as the loss of the grain boundaries (which all of themare susceptible sites to hydration), occur more for MDS2.5 compositionin comparison with other samples.
These created denser and more uniformmicrostructure is prevents for easier hydration reactions, and ultimatelyimproved the hydration resistance of samples including nano-silica as comparedto the non-additive sample.Conclusion:In the present research, the addition effect of Nano-silica on thephysical, mechanical, thermo-mechanical as well as microstructure of Mag-Dolrefractory composites was evaluated and the following results concluded:1- Nano-silicahelps in densification process by liquid phase generation at the firing sinteringtemperature (1650?C). Amaximum density of 3.36 g/cm3 is achieved with addition 2 wt. %nano-silica.2- Grain-growth of CaO and MgO particles occurred by adding Nano-silica which finallylead to hydration resistance enhancement of Mag-Dol refractory composites.3- Highcold crushing strength values of Nano-silica containing composition is for thedevelopment of a strong and continuous bonding and connection in the matrix.4- 1 wt% nano-silica was selected as the optimum content for access to the highflexural strength value due to increased fired density of Mag-Dol compositionsand limited liquid phase generation.
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