Letters to the Editor
Table 1 Minerals studied and their reactions upon heating to 900°C Mineral Chemical formula Reactions Kaoiinite lllite Calcite Dolomite Quartz Gypsum Rutile Pyrite Siderite Al&~O~ofOf-fI~ Release of adsorbed water and dehydroxylation K~fAI,Fe)y(Mg,Fe)r.t$ixAlyOlo(OH)~ Release of interlayer water and dehydroxylation CaCOs Decarbonation CaCOa . MgCO3 Decarbonation SiOz Nil CaS04.2HzO Dehydration TiO:, Nil FeSt Conversion to triolite FeC03 Decarbonation
Siderite
Upon heating in N2 to 9OO"C, siderite loses CO2 and is converted to FeO. In the CH4-N2 mixture, Fe0 is first reduced to iron within about 30 min, as seen in Figure 1. This is followed by a continuous increase in weight as carbon is deposited, at least up to 23 h. The maximum rate of weight increase was about 0.11 mg h-l mg-l starting weight of siderite. The deposited carbon can be completely gasified in a 10% Hz-90% N2 mixture at 9OO"C, again yielding iron.
The carbon deposited over a 24 h period was characterized by X-ray diffraction, using silicon as an internal standard. Considering that it was only formed at 900°C, it was highly crystalline 3,4 having an interlayer spacing of 0,3371 nm and an &erage crystallite height, L,, of 50 nm.
Calcite
Upon heating in N2, calcite loses CO2 in the temperature range 500-750 C producing CaO. In the CH4-N2 mixture at 9OO"C, a slow, continuous weight increase occurs. At the end of 3 and 15 h, it corresponds to about 3 and 11% of the * 'Sulphate' sulphur as determined by ASTM D 2492-68 was reported as negligible or at trace level for all the coals of Table I 454