Canada - Memorial University of Newfoundland

2 FcIlFc[[J204(S) + 26 Na+(aq) + 12 HPOi'(aq) + 2 OH'(aq)'"
6 Nu)Felll(P04no(N3.trJH2I30)(s) + H2(g) +4 HP(I) (1.38)
The schematic diagram ofthe phosphate hideout processes involved can be seen in
Figure 1.3. Extensive damage to the magnetite film covering the boiler tube is observed
at low NalP04 mole ratios producing maricite and hematite, and an acidic environment.
In the range 2.0:S NalP04 :s 3.0, maricite and SIHP are both produced, whereas, at
NalP04 > 3.0, SIHP is the only reaction product and the presence ofHz(g) produced very
strong reducing conditions and a more basic environment (Tremaine el aI., 1998).
Figure 1.4 is a stability diagram that represents the paths ofthe hideout reactions.
In other words, the diagrams can be used to identify thc hideout reactions Ihal occur when
magnetite is exposed to different temperatures and H 2 pressures while in equilibrium with
a given sodium phosphate solution, and to understand the effects ofredox conditions
arising from hideout processes. Figure 1Aa shows iron(ll) being removed from
magnetite to form maridte, thus oxidizing magnetite to form hematite. Figure lAc
shows the formation ofSlHP and H2. AI NalP04 = 2.2, a true invarianl point is observed
in which the four solid phases co-exist, as seen by Figure lAb. At this point, the redox
potential is buffered al much lower reducing conditions. Tremaine et al. (1996)
concluded that the redox chemistry of magnetite in boiler water during hideout is
controlled by the sodium phosphate species concentrations, not those ofdissolved O2 or
redox buffers which were added.
Several chemical equilibrium models have been developed to describe the
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