Chemical Engineering January 2017 - 29
rite) using " wet " process techniques. The
wet method is favored above more expensive
thermal processes, and entails dissolving
the rock in sulfuric acid (H2SO4) to yield
both phosphoric acid and calcium sulfate
together. The concentration of phosphoric
acid is most commonly achieved through
forced-circulation evaporation, usually
performed in a phosphoric-acid heater as
shown in Figure 1. The sorts of corrosion
phenomena encountered in wet phosphoric-acid
processes can be quite complex
and are dependent on several factors,
such as the presence of certain impurities
or phosphates, and how these substances
react with the acid itself.
Composition of the phosphates may vary
considerably depending on the source. For
instance, phosphates from certain geographical
locations are more corrosive than
others. Also, varying temperature levels
across different areas of the plant may affect
composition. These changes result in
large variances in corrosion parameters,
as seen in Figure 2. Such variations in the
content of the elements, and in the temperatures
during the concentration of the
acid, can make it quite difficult to estimate
the corrosivity.
Graphite versus metallic tubes
Impervious graphite, either in the form of
tubes or blocks, is the traditional choice of
material for phosphoric-acid heaters. This is
due to graphite's excellent corrosion properties.
Nevertheless, it has subsequently been
found that metallic materials exhibit some
advantages over graphite. This is partly
due to superior mechanical strength, which
eliminates the risk for brittle fractures that
can readily occur in graphite when cleaning
scale, a common headache for phosphoric
acid producers.
Metallic tubing is also not susceptible
to the types of erosion that are otherwise
common at the inlet of graphite-block heat
exchangers. If the corrosion-resistant properties
are sufficient, metallic heat exchangers
can offer extended trouble-free service,
yielding substantial economic advantages
for operators.
This was demonstrated in one case where
a doubling of the interval between cleaning
operations was achieved with metallic tube.
The increase from five to ten days (assuming
a stoppage of 8 h for cleaning) resulted in 15
more days per year of access to the production
line for the operator.
However, from the heat transfer point of
%F (total, soluble form)
Acid A
Acid B
Pressure: 60 torr
Dotted line is the
boiling popint curve
% CaO
% Na2O
% CI
oC
80
0.5
70
1
60
0.25
50
122
140
158
oF
176
28 30
40
view, it should be noted that proper design
of metallic phosphoric-acid heaters is vital
for achieving these advantages, and also for
ensuring that the tubes compare favorably
with graphite exchangers.
The effects of high alloys
In light of the advantages that have been
achieved with metallic heat exchangers over
traditional graphite, stainless steels - particularly
the standard grades AISI 316L and
317L - are now used extensively in phosphate
plants. Furthermore, extensive tests
and installations have also found that specially
developed high alloys are required for
the most severe conditions.
The corrosion resistance of a material is
determined by its chemical makeup. In this
context, chromium (Cr) is the most important
alloying element in stainless steels, as established
both in laboratory tests and through
practical experiences.
This element is so essential because it
imbues stainless steels with good corrosion
resistance in wet-process phosphoric
acid processes. The advantages of chroCHEMICAL
ENGINEERING WWW.CHEMENGONLINE.COM
JANUARY 2017
29
50
52
%P2O5
FIGURE 2. A number of factors
impact acid composition during
the concentration process
SIF4(g) + HF(g)
CI(g)
SIF4(g) + HF(g)
+ Na2SIF6(s)
CaO(s) CaSO4 (s)
Na2O(s) Na2SIF6(s)
HCI(g)
http://WWW.CHEMENGONLINE.COM
Chemical Engineering January 2017
Table of Contents for the Digital Edition of Chemical Engineering January 2017
Contents
Chemical Engineering January 2017 - Cover1
Chemical Engineering January 2017 - Cover2
Chemical Engineering January 2017 - Contents
Chemical Engineering January 2017 - 2
Chemical Engineering January 2017 - 3
Chemical Engineering January 2017 - 4
Chemical Engineering January 2017 - 5
Chemical Engineering January 2017 - 6
Chemical Engineering January 2017 - 7
Chemical Engineering January 2017 - 8
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Chemical Engineering January 2017 - Cover3
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