Evaluation of the thermal and rheological characteristics of minerals in coal using SiO2-Al2O3-CaO-FeOx quaternary system
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摘要: 利用SiO2-Al2O3-CaO-FeOx四元体系研究了煤中矿物质热和黏温特性规律。利用热机械分析、差热、X射线衍射和热力学模拟以及高温旋转黏度计分析了矿物质的高温演化、反应和黏温特性（1 700℃到固化温度）。结果表明，热机械分析和差热可以较好地在线考察矿物质高温特性。二氧化硅和氧化铝从低温矿物转变成高温形态，表现出稳定的结构和高黏度；另一方面，加入氧化钙或氧化铁可以起到助熔剂的作用，显著加快二氧化硅和氧化铝的反应和转化并降低黏度。氧化亚铁相比氧化铁和四氧化三铁表现出更明显的降低黏度的作用，而氧化铁还原成金属将显著提高体系黏度。三价铁离子与铝类似，可构成网架结构。另外，氧化铁在弱还原或高温下将产生部分二价铁离子对网架机构进行调整。氧化钙的存在可以加强四元体系中氧化铁的溶解能力，进而降低黏度。Abstract: In the present work, the synergistic effect of components on the mineral behavior in the SiO2-Al2O3-CaO-FeOx quaternary system was tentatively evaluated. The mineral transformation and reaction were analyzed by thermo-mechanical analyzer (TMA), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and thermodynamic calculation (FactSage). In addition, the apparent viscosities of synthetic slags, expressed as a function of temperature and composition, were determined using a high temperature rotary viscometer with temperature ranging from 1 700℃ to re-solidifying temperature. The results demonstrated that the TMA and DSC approaches were applicable to characterize the mineral behavior under in-situ conditions. Silica and alumina preferred to be transformed to high temperature cristobalite and α-alumina with relatively stable structure and high viscosity, respectively. On the other hand, the reaction and transformation of silica and alumina could be accelerated by some valuable fluxing agents, particularly calcium oxide and iron oxides. The addition of ferrous oxide into synthetic slags could lower its viscosity compared with that of ferric oxide and ferroferric oxide. Furthermore, the reduction of iron oxides to metallic iron remarkably increased the viscosity. Ferric oxide may take part in the random glass network in a similar fashion with alumina. Besides, iron oxides with the oxidation state of Fe2+ may also act as a modifier under slightly reducing conditions and higher temperatures. The sensitivity of viscosity of mineral matters to temperature excursion decreased with increasing calcium oxide content as calcium oxide was able to enhance the solution ability of iron oxides in the SiO2-Al2O3-CaO-FeOx quaternary system.
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Key words:
- mineral behavior /
- viscosity /
- TMA /
- quaternary system /
- iron oxides
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Figure 1 Schematic diagram of high temperature rotary viscometer for viscosity measurement
Figure 2 TMA analyses of mineral transformations in SiO2-Al2O3-CaO-FeOx quaternary system
Figure 4 Phase diagram of the Al2O3+SiO2+CaO+“Fe2O3” system with SiO2/Al2O3(mol ratio)=0.8/0.2 showing the phase compositions and equilibrium of the minerals used in this investigation
Figure 6 XRD patterns of S01-S03 after viscosity measurement
C: 74-1081# corundum-Al2O3; A: 82-1584# aluminum iron oxide-Al3Fe5O12; H: 24-0072# hematite-Fe2O3; Cr: 85-0621# cristobalite, syn-SiO2; M: 15-0776# mullite-Al6Si2O13; Ca: 72-0767# calcium aluminum oxide-CaAl4O7; P: 74-0874# pseudowollastonite-Ca3(Si3O9)
Figure 7 XRD patterns of S04-S07 after viscosity measurement
A: 41-1486# anorthite-CaAl2Si2O; M: 39-1346# maghemite-Fe2O3; H: 70-1876# hedenbergite-Fe1.3CaO0.7(Si2O6); I: 77-2355# FeO-iron oxide
Figure 8 XRD patterns of S08-S11 after viscosity measurement
M: 15-0776# mullite-Al6Si2O13; Ma: 75-0449# magnetite-Fe3O4; I: 80-1625# iron silicate-Fe2(SiO4)
Table 1 Composition in SiO2-Al2O3-CaO-FeOx quaternary system (as equivalent oxide)
Sample Composition w/% Mol ratio SiO2 Al2O3 CaO Al2O3-crucible C-cruciblea SiO2/Al2O3 B/Ab Fe2O3 Fe3O4 FeO Fe2O3 S01 65.36 34.64 - - - - - 1.89 0.00 S02 49.29 26.09 - 24.62 - - - 1.89 0.33 S03 49.18 25.98 24.83 - - - - 1.89 0.33 S04 39.52 20.89 19.94 19.65 - - - 1.89 0.66 S05 39.78 21.02 20.07 - 19.13 - - 1.89 0.66 S06 40.31 21.31 20.34 - - 18.04 - 1.89 0.66 S07 39.52 20.89 19.94 - - - 19.65 1.89 0.66 S08 39.78 21.02 8.25 30.94 - - - 1.89 0.64 S09 40.19 21.24 8.34 - 30.22 - - 1.89 0.64 S10 41.05 21.70 8.52 - - 28.74 - 1.89 0.64 S11 39.78 21.02 8.25 - - - 30.94 1.89 0.64 a C-crucible was used to simulate the reduction atmosphere, Fe2O3 was reduced to iron metallic particles during measurement; b base/acid ratio (base=w(Fe2O3)+w(CaO), acid=w(SiO2)+w(Al2O3)), where only four analytical compounds were calculated, and the FeO, Fe3O4 were normalized to the Fe2O3 
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