metal oxides would have occurred throughout the reactor, particularly
in those areas subjected to low density supercritical fluid
and in the heat exchanger where the fluid was cooled [4]. The
11.1 g of metal oxide filtered from the effluent is clear evidence
of this occurrence. Sue et al. have reported a slightly different order
of solubility based on both theoretical calculations and experimental
measurement: Fe > Ni > Cr [17]. The increased solubility of Ni
(with respect to Fe) in the case of our experiments is almost certainly
due to the stability of ammine complexes in the effluent. It
is well known that ammonia is relatively stable under supercritical
water conditions particularly at temperatures less than 500 C
[18,19]. However, the effluent total ammonia and total sulphur
concentrations were also measured and found to be approximately
one quarter of the influent concentrations. Sulfur was obviously
adsorbed or co-deposited with the metal oxides as indicated by
the filter cake composition which would account for its decrease
in concentration. The loss of ammonia throughout the reactor
likely occurred by degassing in the gas liquid separator and by
partial conversion to molecular nitrogen. Nevertheless, the remaining
ammonia concentration (4200 mg L1
) was more than enough
to stabilize nickel ammine complexes [3,20,21]. This order of
solubility and propensity for corrosion has also been measured
electrochemically in a previous investigation on the corrosion of
Ni–Cr alloys in ammoniacal solutions [