James,
The glass bulk analysis of your slag shows that the furnace (in the
region of formation of these slags) was running under blast furnace conditions.
This is indicated by the low iron content of the slag relative to the Ca+Si,
but what is extraordinary is the very high titanium content.
There will not be any wustite present, this is not compatible with the
slag composition nor the presence of free carbon even if it is in solution in iron
which will reduce it activity.
The properties of this region of this slag would be best described by
the CaO-SiO2-TiO2 phase diagram DeVries R.C.,Roy R., Osborn, E.F. 1955, J.
Amer. Ceramics Soc. 38 161. I expect that the iron will have virtually no
effect, and although the effects of the other elements present in significant
quantities is difficult to predict, but they are likely to act as glass
formers preventing the formation of solid silica, thus lower the liquidus
temperatures a little.
The phase diagram would seem to predict one of two
possible solidification paths from the given bulk composition depending
on how the trace lements shift the phase boundaries. From a quick back of the
spreadsheet calculation of the ternary region for the given analysis it would
seem that the sphene + psuedo-wollastonite+silica (minimum liquidus
temperures 1348 or 1318 if free silica present) was the most likely path.
However, the phases present indicate that the slag took the other possible path
which would lead to sphene, silica and rutile. In both these case there are
probably enough glass forming elements present to prevent the crystallization
of silica.
The ore used to produce such a slag would be unusual, a titanium rich
detrital magnetite sand derived from a basic igneous deposit or its metamorphic
equivalent.
The composition of this slag falls nicely in the lowest melting point range of
this system, and also in the normal working temperature range of a blast
furnace. But, because of the composition of the slag with high Ti and
slightly lower Al is has crystallized Ti phases (probably sphene) and certainly
rutile. It is not surprising that it looks different from a normal blast
furnace.
Chris Salter
---------------------------------------------
Materials Science-Based Archaeology Group,
Department of Materials,
Begbroke Business and Science Park,
Sandy Lane,
Yarnton,
Oxford OX5 1PF,
Tel. Office 01865 283722
Probe 01865 283741
Fax. 01865 848790
Mobile. 0777 6031608
E-mail [log in to unmask]
|