Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Use o ductile-working ~nd casting mat~ri~l~ togeth~r with
w~ldi~g ~ill~r mat~risls ~or compon~nt~ axposed to hot,
conc~ntrnted ~ulphuric ~cid or olaum
The present invention relates to the use of materials for
components exposed to hot, concentrated sulphuric acid and
0 to 10 wt.~ of oleum and to the production of sulphuric
acid using these materials.
The literature contains much information relating to the
corrosion resistance of materials against hot, concentrated
sulphuric acid.
Since the solubility of lead sulphate in sulphuric acid
rises with the concentration of the acid, lead and its
alloys may be used only at concentrations of up to 78%
H2SO4 and only up to 110C (Ullmanns Encyclopadie der
technischen Chemie [Ullmanns encyclopaedia of industrial
chemistry], 4th edition, vol. 21 (1982), p. 157).
.
Unalloyed steel may be used in 68-99% sulphuric acid at up
to 70C, wherein however, surface removal rates of up to
1.3 mm/year must ~e expected (G. Nelson, Corrosion Data
; Survey, Shell Development Co., San Francisco, 1950,
p. ZT-102A). The resistance of unalloyed steel decreases
sharply in the concentration range 99 to 100% H2SO4.
Elevated flow rates should be avoided with unalloyed steel
(Ullmann, loc. cit.; Z.f. Werkst.-Techn. 4 (1973),
p. 169/186; R. J. Borges, Corrosion/87, Paper N 23, NACE,
Houston, Texas, 1987).
Chromium or copper alloyed grades of cast iron are
resistant to sulphuric acid concentrations of 90-99% at up
to 120C (Ullmann, loc. cit.), however, flow dependency
ofcorrosion must be taken into account here as well (Z.f.
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Werkst.-Techn., loc. cit.). An iron-silicon casting
material with 14-18% Si has very good corrosion resistance
over a wide range of concentrations and temperatures
(Ullmann, loc. cit.); however, the hardness and brittleness
of this special cast iron are very disadvantageous (R. J.
Borges, Corrosion/87, loc. cit.; Ullmann, 4th edition,
vol. 3 (1973), p. 21). Rust-resistant standard austenitic
steels, as per material number 1,4571, are used with
concentrated sulphuric acids at temperatures of up to 85C.
As temperature increases, surface removal rates rise
steeply. At temperatures of only 150C, surface removal
rates of some 1 mm/year may be expected (Z.f. Werkst.-
Techn. 8 (1977), p. 362/270 and 410/417), wherein corrosion
is markedly flow dependent.
The use of nickel based alloys is not advantageous. In
; plate heat exchangers made from NiMol6Crl5W, material
number 2,4819 (type Hastelloy alloy C-276), which
exchangers are used to cool concentrated sulphuric acid,
product temperature is restricted to 95C (N. Sridhar,
Materials Performance, March 1988, p. 40/46).
There has, therefore, been no lack of proposals to improve
sulphuric acid resistance by alloying methods. Thus the
rust-resistant, austenitic grade of steel X 1 CrNiSi 18 15,
material numbers 1,4361, which contains 3.7-4.3~ Si, is
substantially more resistant in for example 98.5% sulphuric
acid at 150 and 200C than material number 1,4571 (Ullmann,
vol. 3, p. 21); corrosion is very slightly flow dependent
(Z.f. Werkst. Techn. 8 (1977), p. 362/370 and 410/417;
M. Renner & R. Kirchheiner, "~orrosionsbestandigkeit von
hochlegierten nichtrostenden Sonderstahlen in stark
oxidierenden Medien" [Corrosion resistance of high-alloy
rust-resistant special steels in strongly oxidizing media],
paper presented to conference entitled "Nickelwerkstoffeund
hochlegierte Sonderstahle" ~Nickel materials and high-alloy
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special steels], Esslingen, 7-8 April 1986). By further
increasin~ the Si content of 4.5 to 5.8~, preferably 5.0 to
5.6~, corrosion resistance of austenitic, rust-resistant
steels in hot 85%, preferably 90%, sulphuric acids may be
increased within certain limits (US 4,543,244;
DE-OS 33 20 527). Such a special steel can scarcely be
considered for practical use at higher temperatures due to
the marked temperature dependence of corrosion. The
following surface removal rates in 98.2% sulphuric acid
were determined on a rust-resistant, completely austenitic
steel of composition 17.5% Cr, 17.5% Ni, 5.3% Si, remainder
substantially iron (above-cited references):
125C 0.1 mm/year,
135C 0.8 mm/year,
145C 1.6 mm/year,
in 93.5% H2SO4 a corrosion rate of 0.25 mm/year was found at
85C. In order to reduce corrosion, anodic protection of
plant and equipment may be provided; under these
~ conditions, the surface removal rate is still however
-~ 1.1 mm/year in 93.5% H2SO4 at 200C. The noticeable flow
~; dependence of corrosion of the rust-resistant chrome-
nickel-silicon steel in sulphuric acids may also be
considered a disadvantage; for example the surface removal
rate of a rotating disk (diameter, 30 mm; rotational speed:
2000 min-1) in 96% sulphuric acid at 150C is as high as
3.7 mm/year.
Heat-treatable nickel-based alloys with 2-4% Si have also
been proposed for handling hot, at least 65~ sulphuric acid
(German patent DE 21 54 126). Surface removal rates in
sulphuric acid heated to 120C are, however, really very
high at some 0.6 mm/year. Surface removal rates of 0.25
mm/year in 98~ H2SO4 heated to 140C have been reported for
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another, hardenable nickel-based alloy which is not flow
sensitive (R. J. Borges, Corrosion/87, loc. cit.).
A rust-resistant austenitic steel with 17% Cr, 16% Ni, 3.7
Si and 2.3~ Mo on ~he other hand may only be used in cold
sulphuric acids at concentrations of below 10% and above
80% (publication number 235 from CAFL: Uranus, rost- und
saurebestandige Stahle fur schwierige ~orrosionsprobleme
~Uranus, rust and acid resistant steels for difficult
corrosion problems], p. 37).
GB 1 534 926 furthermore describes rust-resistant
austenitic chromium-nickel-silicon-copper-molybdenum steels
with elevated corrosion resistance in concentrated
sulphuric acid; these steels are characterized by the
following composition (parts by weight in ~):
C max. 0.06%
Si 4.1 to 12% (2 4.7%; 6.5 to 12%; 7 to 11%;
7.5 to 10%)
- Mn max. 4~ (3~; 1%, 0.5~)
Cr 6 to 22% (6 to 17%; 8 to 15%; 9 to 14%)
Ni 10 to 40% (10 to 25%; 12 to 23%; 14 to 20%)
; (Mo+1/2W) max. 1.5% (0.5 to 1~)
; 25 Cu 0.6 to 4% (2 1%; 1.5 to 3.5%; 2 to 3%)
N max. 0.2% and
Nb+Ta+Zr+V max. 2%.
Such steels have a shortcoming in that the alloy element
moly~denum distinctly increases the tendency of the
rust-resistant austenitic chromium-nickel-silican steels to
embrittlement, which can cause serious di~ficulties .
during hot shaping, for example when pressing bases.
Furthermore, the alloy element copper also increases
hot-workability problems (Ullmanns Encyclopadie der
technischen Chemie [Ullmanns encyclopaedia of industrial
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chemistry], 4th edition (1982~, vol. 22, p. 56) and
susceptibility to hot cracking. It should also be borne in
mind that copper-alloyed rust-resistant austenitic steels
are susceptible to stress corrosion cracking in hot
sulphuric acids (I. Class & H. Grafen, Werkst. u. Korros.
1964, p. 79/84; H. Grafen, Werkst. u. Korros. 1965, p.
876/879).
Finally, iron-chromium-nickel alloys containing 4-6%
silicon have also been described, the delta ferrite content
of which is restricted to 5 to 10%, so that no coherent
delta ferrite network can be formed (D. J. Chronister &
T. C. Spence, Corrosion 85, Paper 305, NACE, Boston/Mas.,
March I985). Such a network may be expected at delta
ferrite contents from 10%. In an alloy described by D. J.
Chronister et al. containing 4.8% Si, surface removal rates
in 95% H2SO4 heated to 110C are initially relatively low
(0.4 mm/year), but quickly rise on extended exposure to
2.4 mm/year. With alloys containing 5 to 5.2% Si, corrosion
rates of 0.11 to 0.56 mm/year were found under these
conditions. It is only at levels of 5.6% Si that surface
removal rates of approximately 0.1 mm/year are observed. If
the temperature of the 95% H2SO4 is raised to 130C, at a
Si content of 5.6~ rising surface removal rates are again
observed, which in the first stage of the test (48 h) are
0.66 mmtyear and in the second stage as high as 1.24 mm/a;
at a Si content of 5.9%, the surface removal rates are
0.45-0.54 mm/year.
EP-A-0 013 507 proposes Ni-Cr-Si steels for handling hot
concentrated nitric acid, which are characte
~content of 5 to 7%, a Cr content of 7 to 16%, a Ni content
of 10 to 19%, a Mn content of up to 10% and contain one or
more of the elements Ti, Zr, Ta, Nb in a quantity of no
less than 4 times the quantity of carbon and no more than
2%, ~herein the C content may be no more than 0.02%
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Austenitic Ni-Cr-Si steels with good corrosion resistance
in oxidizing acids are described in DE-A-4 118 437. These
steels are characterized by the following composition
(parts by weight in ~):
C max. 0.02 wt.%
Ni 10 to 25 wt.~
Cr 8 to 13 wt.%
Si 6.5 to 8 wt.
10 Mn + Co 0 to 10 wt.~
S max. 0.010 wt.%
P max. 0.025 wt.~.
:~ Apart from good nitric acid resistance, these alloys also
exhibit acceptable corrosion resistance in 96% sulphuric
acid at up to 150C; however, at temperatures of up to
200C a corrosion allowance must be made when calculating
wall thicknesses. Si contents of 7.5 to 8% are necessary in
order to reduce the surface removal rate in 96% sulphuric
acid at 200C. However, these high Si contents cause
considerable problems in processing the alloys.
However, in both the production and purification of
sulphuric acid, higher temperature ranges are of particular
technical interest. However, according to the prior art
(see above and U. Heubner, Chemische Industrie 11 (1992),
p~. 54 to 56), the above stated steels with Si contents of
7.5~ cannot economically be used in higher temperature
rànges~ due to high surface removal rates.
Austenitic-ferritic Cr-Ni-Si steels are proposed
(EP-A-378 998) for use in concentrated sulphuric acids at
;~ ~ temperatures of ~ 200C, which however have the shortcoming
of losing ductility on extended exposure to temperatures of
300C.
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The object is therefore to provide materials which are
suitable for use in concentrated sulphuric acids at
temperatures of ~ 200C up to the boiling point of the
sulphuric acid in question and are resistant to corroslon
and may moreover be readily shaped and readily welded, such
that they may be used for components which are used under
the above-stated conditions.
Surprisingly, this object could be achieved with quite
specific materials.
The present invention provides the use of ductile-working
and casting materials together with welding filler
materials made from austenitic iron-nickel-chromium-silicon
alloys with
Cr 8 to 16 wt.
Ni 20 to 30 wt.
Si > 6.5 to 7.4 wt.
20 Mn max. 2.0 wt.%
C max. 0.03 wt.
P max. 0.03 wt.~
S max. 0.01 wt.~
and the remainder as iron, together with minimal quantities
of unavoidable admixtures including the elements magnesium,
aluminum and calcium used for deoxidation, at temperatures
of > 200~C up to the boiling point of the particular
sulphuric acid/oleum used, for components exposed to
concentrated sulphuric acid or 0 to 10 wt.i~ oleum.
The presence of small quantities of nitrosylsulphuric acid
(NOHSO~) in the hot, concentrated sulphuric acid or in the
0 to 10 wt.~ oleum is advantageous for reducing surface
removal rates.
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In order further to improve hot workability, the material
may preferably contain up to 0.01 wt.% boron and up to
0.25 wt.% of rare earth metals.
The materials used possess favorable mechanical and
technical properties. Despite the high silicon content, the
notched impact strength of the materials is satisfactory.
The alloys may be produced in any industrially significant
forms such as sheet, strip, tube, bar and wire. Further
forms include steel castings for example for pumps and
fittings. The nickel-chromium-silicon-iron alloys are
readily weldable, such that it is possible to fabricate
equipment by welding. The welding filler may be of the same
type or may instead have a delta ferrite content of up to
approximately 20%.
The materials used are to a great extent corrosion~
resistant against an over 85 wt.% H2SO4, preferably against
90 to 100 wt.~ H2SO4, particularly preferably against 95 to
100 wt.% ~2SO4 and against 0 to 10 wt.% oleum. This
elevated corrosion resistance is present even at high
temperatures of > 200C, preferably of up to 370C. The
materials or components may be exposed to hot, concentrated ~ -
sulphuric acid or 0 to 10 wt.% oleum at pressures of 0.1
25 bar to 10 bar. The materials may be used for components ~`~
which are exposed to such hot, concentrated sulphuric ~ -
acids. Such components are for example reaction vessels,
pumps, fittings, pipework, heat exchangers, and the like. ~-
Such components may be produced by forging and rolling
(ductile working), by casting, by lining, by cladding, by
shaplng welding or by build-up welding. --~
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Under the stated severe conditions, high corrosion
resistance is taken to be a surface removal rate of at most
1 mm/year, normally however at most 0.1 to 0.2 mm/year.
The invention also provides a process for the production of
sulphuric acid by catalytic oxidation of sulphur dioxide to
sulphur trioxide, absorption of the sulphur trioxide in
sulphuric acid with a concentration of between ga and 101%,
wherein heat of absorption is produced, and absorption is
performed in a tower into which the sulphuric acid is
introduced at a temperature of over 200C, the sulphuric
acid leaving the tower has a concentration of over 99~ and
a temperature of over 200C, the heat of absorption is
dissipated in a heat exchanger by transfer into other
liquids or by steam production, which process is
characterized in that the heat exchanger and optionally
other components in contact with the hot sulphuric acid
consist of austenitic iron-nickel-chromium-silicon alloys `~
with
-
Cr 8 to 16 wt.
Ni 20 to 30 wt.
Si ~ 6.5 to 7.4 wt.
Mn max. 2.0 wt.
C max. 0.03 wt.
P max. 0.03 wt.~ ~
S max. 0.01 wt.~ -
and the remainder as iron, together with minimal quantities
of unavoidable admixtures including the elements magnesium,
aluminum and calcium used for deoxidation.
By using the special alloys, it is possible to produce high
pressure steam during sulphuric acid production and thus to~-
make optimum use of the heat of absorption. The sulphuric
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acid may additionally be processed at elevated
temperatures.
The invention also provides a process for concentrating
sulphuric acid to 85 to 97 wt.% H2SO4 and optionally for
purifying this sulphuric acid at temperatures of up to
350C, preferably 200 to 320C, optionally together with
cooling, which process is characterized in that
concentration and optionally purification of the sulphuric
acid and optionally cooling proceed in plant and equipment
having components which are exposed to hot sulphuric acid,
which components are entirely or partially manufactured
from ductile-working and/or casting materials together with
welding ~iller materials, which consist of austenitic iron-
nickel-chromium-silicon alloys with
Cr 8 to 16 wt.%
Ni 20 to 30 wt.%
Si ~ 6.5 to 7.4 wt.%
C max. 0.03 wt.%
Mn max. 2.0 wt.%
P max. 0.03 wt.
S max. 0.01 wt.~
::.
the remainder as iron, together with minimal ~uantities of
unavoidable admixtures including the elements magnesium,
aluminum and calcium used for deoxidation. -~
. - '
The invention is illustrated by way of the following
examples.
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Examples
Example 1
Material samples of a cold-rolled, 2 mm thick sheet of the
following composition are tested
.
0.005 wt.% carbon
6.96 wt.~ silicon
10 1.38 wt.~ manganese
0.011 wt.~ phosphorus
0.002 wt.% sulphur
25.40 wt.~ nickel
9.15 wt.~ chromium ~;
15 remainder iron.
After 700 hours' exposure in 96~ boiling sulphuric acid
(320C), the surface removal rate of this austenitic steel
was determined at 0.15 mm/year.
;~ 20
Example 2
Material samples of a cold-rolled, 2 mm thick sheet of the
above composition were exposed in 96~ boiling sulphuric
25; acid (320C) containing NOHSO~ for over 700 hours. After
this exposure, the surface removal rate of this austenitic
steel was determined at < 0.1 mm/year.
The samples are also capable of being readily shaped and
;j 30 readily welded.
.
It will be understood that the specification and examples
are illustrative but not limitative of the present
invention and that other embodiments within the spirit and
` 35 scope of the invention will suggest themselves to those
skilled in the art.
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