3. ANÁLISIS EXTERNO
3.2. Análisis del entorno competitivo (PORTER)
milling on the structure of crystal colemenite, it was milled and the milled products were analysed by using XRD.
Milled colemanite samples were then roasted to determine the solid phases remained. (Figure 1) shows XRD patterns of unmilled and milled, and then roasted colemanite. In the original colemanite (K00) sample, there are some calcite (CaCO3) and gypsum (CaSO4.2H2O). All other peaks belog to colemanite. As also seen from the Figure 1, intensive milling for 45 min (Sample K4-45), not completely but partially, altered the crystal structure of colemenite. At the examples subjected to mechanical activation, colemanite crystal peak intensities decreased with milling.
Accordingly, mechanical activation caused disruption of the crystal structure of this borate mineral. However, this disorder is not very clear and maybe gradual. When these samples were roasted up to 500°C it was found that colemanite‘s peaks disappeared and
almost amorphous structure occurred. It can be proposed that colemanite begin to transform into dehydrated form, maybe just calcium borate. When the temperature was increased to 800°C, new XRD peaks occurs, depending on the recrystallisation of anhydrous colemanite.
Figure 1: Comparison of XRD patterns for unmilled (K00) and 45 min milled (K4-45) colemanite samples, and of roasted at 425°, 500° and 800 °C (Symbols: ♦, calcite; ■ gypsum).
Seen in (Figure 2) are TG curves for unmilled and 45 min milled colemanite samples. Thermal decomposition depending on initially loss of crystal water begins nearly but not very significantly at 337 °C and continues up to 700 °C for unmilled colemanite. At 363 °C, other strong hydrogen bonds of water molecules are broken and then borate structure is began to decompose. When the temperature is at between of 393°-400 °C, decomposition rate reaches to maximum depending on the final release of water molecules in the pores. This phenomenon causes sudden crash of the samples, known as decrepitation [Uzunoğlu, 1992; Çelik et al., 1994; Şener and Özbayoğlu, 1995].
After 700 °C, colemanite converts to sintered colemanite. It was also
demonstrated by Yıldız (2004) that colemanite loses its crystal water through endothermic reactions at 300°-460 °C and that decrepitation and decomposition of colemanite to amorphous B2O3 and CaO takes place at temperatures lower than 600
°C, and finally CaB2O4 and Ca2B6O11
appear as new crystalline boron compounds at 800 °C. When compared to TG curve of unmilled colemanite, 45 min milled colemanite losses its water at very low temperatures. Since the interval between onset and offset temperatures appears within very big interval, decrepitation of milled colemanite does not occur. In addition, decrepitation of the milled colemanite was not observed during atmospheric roasting experiments performed at isothermal conditions.
Figure 2: Comparison of TG curves obtained for unmilled and 45 min milled colemanite samples.
(Figure 3) collectively shows XRD patterns for pyrophyllite samples which unmilled (P0, just gently milled), 45 min milled (P45), and then roasted at various temperatures. Major minerals determined in pyrophyllite samples are pyrophyllite (Al2Si4O10(OH)2), quartz (SiO2), kaolinite (Al2Si2O5(OH)4) and dickite (Al2Si2O5(OH)4). It was found that milling for 45 min significantly results in decrease mainly at the peak intensities of pyrophyllite, kaolinite and dickite. Peaks which remain after 45 min of milling fully belong to quartz.
It is reported that when the milling time increases, dry milled pyrophyllite losses its original crystal structure depending on the
creep of tetrahedral-octahedral layers [Pérez- Rodriguez et al., 1988]. Erdemoğlu and Sarıkaya (2002) was also reported that collectorless flotation recovery of pyrophyllite decreases with prolonged milling due to structural deformation occurred during the milling.
In order to determine the effects of heat treatment on the thermal behaviour unmilled and milled pyrophyllite samples were roasted at different temperatures and the raosted samples were also analysed for their crystal structure.
As seen in (Figure 3), peaks of pyrophyllite and kaolinite are disappeared in the unmilled sample roasted at 800°C, whereas they are not present in the milled sample even at roasting temperatures as low as 400°C. Peaks of kaolinite found in the unmilled sample disappeared at 800°C, whereas 700°C was enough for decomposition of kaolinite present in the milled pyrophyllite sample. Morover, new peaks occurred at high temperatures belonging to mullite with a nominal composition of 3Al2O3.2SiO2 are very common in the milled pyrophyllite samples roasted at temperatures as low as 400°C, when compared to those of unmilled samples.
Figure 3: Comparison of XRD patterns for unmilled (P0), 45 min milled (P45) and then roasted phyrophyllite samples.
Thermograms obtained by roasting of unmilled and milled pyrophyllite at isothermal heating conditions are shown in Figure 4. It seems that pyrophyllite losses its bound water without any structural changes at temperatures between 400° and 700°C. At temperatures near to 800°C, pyrophyllite converts into a mullite-like aluminium silicate form and stays steady up to 1000°C. After this temperature, mullite-phase conversions begin and free quartz present converts into the crystobalite which is a high-temperature polymorph of quartz. It was found that intensive milling significantly changes the thermal behaviour of pyrophylite. Mass loss in 20 min of milled pyrophyllite sample begins at 400°C, whereas it is almost 500°C for unmilled sample. Besides, mass loss of unmilled pyrophyllite at 700°C was calculated as 2.5%, whereas it is 3.8% pyrophyllite sample which was milled for 60 min.
Consequently, conversion of pyrophyllite into mullite shifted to low temperatures, suggesting the mechanical activation. In
the literature, it was reported that transformation in the pyrophyllite begins with the milling; milling longer than 7 min changes the thermal behaviour; according to TG curves, onset temperature at which mass loss begins decreases and endothermic reaction region shifts to occur at low temperatures [Pérez-Rodriguez and Sânchez-Soto, 1991].
Figure 4: TG curves for unmilled and milled (20, 45 and 60 min) pyrophyllite as obtained by isothermal roasting tests.
Silicon carbide is manufactured by charbothermic roasting of high purity silica in the presence of coke:
SiO2 + 3C → SiC + 2CO
Metallurgical-grade silicon used for many purposes including photovoltaics is obtained from the reduction of silicon in the presence of carbon at high temperatures:
SiO2 + 2C → Si + 2CO
In order to determine the effect of intensive milling on the carbothermic roasting and reduction of quartz, high purity silica sand was mixed with metallurgical grade coke;
milled for long periods and finally the milled mixtures roasted at 1200 °C for half a day. Unmilled, milled and roasted materials were characterised using XRD and TGA.
As seen from Figure 5, XRD patterns of unmilled mixture are very simple. Since silica sand is very pure, one and only the crystal mineral phase seems as quartz. All the peaks on the patterns are belongs to quartz. Since coke is in the amorphous phase, it was not determined by XRD analysis. However, intensities of the quartz
XRD peaks decreased and peak areas enlarged gradually with prolonged miling.
Milling 5 h resulted in the amorphisation of quartz in the silica sand-coke mixture.
Since the presence of coke in the mixture behaved as grinding additive, 10 h of milling gave a complete amouphous material.
XRD patterns for unmilled and 10 h milled silica sand-coke mixtures both which were roasted at 1200°C for 12 h were collectively shown in Figure 6. Seen on the XRD pattern of unmilled and then roasted silica sand-coke mixture is quartz with a little bit high peak intensities due to heat treatment. But, the materials including quartz in the 10 h milled mixture were completely amorphous, roasting of milled mixture at 1200 °C gave also rise to appearance of crystal quartz. But in this case, quartz is in crystobalite phase. All of the peaks reappeared belong to crystobalite quartz. It is known that quartz is in trydimite phase after 870°C and in crystobalite phase after 1470 °C. Since the crystobalite phase is obtained just at 1200
°C, this result solely suggests mechanical activation which provides phase transformation of quartz to occur at low temperatures.
Figure 5: XRD patterns of unmilled and milled (1, 5, 10 h) silica sand-coke mixtures.
Figure 6: Comparison of the XRD patterns for the unmilled and 45 min milled silica sand and coke mixtures roasted at 1200°C for 12 hours in the air.
Figure 7: TG curves for unmilled silica sand only, unmilled and milled (1, 5 and 10 h) silica sand-coke mixtures as obtained by non-isothermal analysis in argon.
Shown in (Figure 7) are TG curves for original silica sand only, unmilled and 1, 5 and 10 h milled silica sand-coke mixtures, as obtained by thermal analysis performed up to 1400 °C. On TG curve of the unmilled original silica sand only, mass loss onset temperature is about 1300°C, whereas it is about 1050 °C for the unmilled silica sand-coke mixture. It seems that milling considerably changed the mass loss starting temperature which decreases with milling time be longed from 1 to 5 h.
This may not be resulted from gasification of carbon using O2 originated from the air to form COx gases, since TG analysis was performed in argon atmosphere. According to Sahajwalla et al. (2003), the reaction between SiO2 and C in powdery mixtures has significant rates from about 1400°C onwards in vacuum or in stream of argon.
The reaction can be seen as a combination of two basic reactions:
SiO2(s, l) + C(s) SiO (g) + CO(g) SiO(g) + 2C(s) SiC(s) + CO(g)
The reactions taking place at the carbon surface are also reported to play a role in controlling reaction kinetics. Thus it was suggested that the mass loss occurred in the 1 and 5 h milled mixtures is due to early reactions of silica and carbon to form SiO gas and to release CO. But, TG curve of the mixture milled for 10 is very different. TG pattern is similar to others up to 900°-1000°C, then the material dramatically starts to gain mass up to 1350°C and to loose its mass again with the increasing temperature up to the end of analysis limit. The mechanism causing this thermal behaviour needs further study. But what the observed is the clear effect of
intensive milling on the charbothermic reactions of quartz.
4. CONCLUSIONS
In this study, structural and thermal alterations resulted from intensive milling of selected minerals such as colemanite, pyrophyllite and quartz were investigated, which are processed generally at very high temperatures in their metallurgy.
For each of the minerals examined in this study, it was typically found by XRD analysis that intensive milling appearently alter or deform the crystal structure of the minerals, as leading to become XRD amourphous as a final point. There was a remarkable result so that quartz present in the pyrophyllite sample resists to the action of intensive milling while the quartz in the silica sand-coke mixture easily goes to become amorphous.
Studies performed either at non-isothermal or isothermal heating conditions showed that, as compared to their unmilled counterparts, thermal behaviour of the intensively milled minerals significantly was altered to release their volatile content at low temperatures, mainly due to mechanical activation.
Finally, it was concluded that mechanical activation may be one of the keys to develop existing technologies for manufacturing many of the high temperature processed engineering materials like oxides (Al2O3, ZrO2), nitrides (AlN, BN), borides (CaB6, TiB2), carbides (SiC, TiC, WC, B4C) at low-costs.
Acknowledgement: Financial supports of İnönü University (BAPB Project Numbers:
2012/108 and 2012/14) is gratefully acknowledged.
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