As a result, the Enefit280 WHB ash does not contain a CaS (oldhamite
Geochemists studying the samples found abundant bits of the mineral oldhamite
(calcium sulfide), so touchy that it decomposes after the slightest exposure to water vapor.
PHOTO : CL of periclase (blue) and oldhamite (yellow) in MgO rich sample.
PHOTO : MgO rich buildup showing internally zoned periclase crystals with a minor amount of oldhamite (yellow).(25x)
In addition, black ash contains secondary phases formed during thermal treatment of oil shale: authigenic calcium silicates (14 wt%), periclase (7.5 wt%), free lime (2.1 wt%) and oldhamite (3.6 wt%).
Oldhamite is a CaS phase that is particularly characteristic of black ash and does not occur in thermal power plant (TPP) ashes that are a byproduct of oil shale thermal combustion.
The major difference in mineral composition between the hydrated samples after 7 days and fresh ash is the disappearance of oldhamite (CaS) and the formation of the secondary Ca-Al hydrate phase hydrocalumite and the Ca-Al sulphate-hydrate phase ettringite.
Fresh semicoke contains only trace amounts of free lime, oldhamite, anhydrite and minor secondary Ca-silicate phases that have been formed due to the partial thermal decomposition of semicoke and the reactions taking place in the final stage of combustion .
Another key mineral, ettringite, is a common secondary mineral phase that is formed during the hydration of semicoke and ash in alkaline conditions as a result of the reaction of portlandite with sulphur compounds (oldhamite (CaS) in semicoke and anhydrite (CaS[O.sub.4]) in ash) and dehydroxylated aluminosilicate clays and/or Al-Si glasses.
Also oldhamite [CaS] together with traces of FeS--mackinawite and C3A--tricalcium aluminate were identified in fresh semi-coke.
The formation of ettringite occurs as a result of the reaction of CaO, formed at thermal decomposition of limestone/dolomite, with Al-compounds derived from partial decomposition of clay minerals and/or feldspars, and with dissolved sulfur compounds and/or CaS (oldhamite), emitted in the combustion process of organic matter or decomposition product of pyrite and carbonate phases, respectively.
It is highly probable that ash contains also oldhamite
CaS, hematite [Fe.sub.2][O.sub.3], magnetite [Fe.sub.3][O.sub.4], potassum calcium sulfate [K.sub.2][Ca.sub.2] [(S[O.sub.4]).sub.3], marcasite Fe[S.sub.2], rutile Ti[O.sub.2], zircon ZrSi[O.sub.4] and portlandite Ca[(OH).sub.2].