Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A PROCESS FOR THE RECOVERY OF SULFURIC ACID
This invention relates to a process for the recovery
of sulfuric acid from wasteacids containing metal sulfates
by concentration through evaporation and removal of the
solid metal sulfates.
Sulfuric acid may be recovered from metal-sulfate-
containing waste sulfuric acids, for example from spent
pickling solutions or from thewasbe acid accumulating in
the production of Tio2 by the sulfate method, by ~arious
methods which can be attended by more or less serious tech-
nical problems, according to the composition of the contam-
inated waste acids. Thus, concentration by evaporation to
approximately 65% H2S04 in the liquid phase in submerged-
burner evaporators (DE-A-l 173 074) or similar, directly
heated systems is characterized by high energy consumption,
high maintenance costs and emission problems. Multistage
concentration by evaporation in forced-circulation vacuum
evaporators has proved to be much more favorable in terms
of energy and less expensive in terms of maintenance costs,
temperatures of 80 to 120-C being preferred (EP-A-133 505).
In this case, however, problems can arise through the rela-
tively low temperatures and sulfuric acid concentrations at
which the metal sulfates crystallize on account of incrus-
tation of the apparatus, particularly the heat-exchange
surfaces, and on account of corrosion as a function of the
particular acid concentration in the evaporation stages;
these problems are not observed where concentration by
evaporation is carried out in submerged-burner evaporators
(DE-A-1 173 074), in which the waste acid is directly con-
centrated by evaporation in a single stage at around 150C
to form a suspension of metal sulfates, particularly iron
sulfate monohydrate, in approximately 65% sulfuric acid.
According to DE-A-2 618 121, concentration of the wasteacid
by evaporation is carried out, optionally in several
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stages, by crystallization of the iron sulfate monohydrate
in an evaporation stage in which the sulfuric acid concen-
tration in the liquid phase is between 60 and 70%, whereas
in optionally preceding evaporation stages concentration by
evaporation is only continued to such an extent that no
iron sulfate monohydrate crystallizes. This is said to im-
prove the deposition of chromium sulfate with the iron
sulfate.
On the other hand, it is pointed out in DE-A-2 807 380
that, in the concentration of the waste acid by evaporation,
the deposition of CaSO4 and Tio2 on pipe walls and apparatus
of the evaporation plants leads to considerable problems
because the deposits in question are difficult to remove.
To avoid such deposits, it is proposed that concentration
by evaporation be carried out in the presence of solid iron
sulfate monohydrate in all evaporation stages, including
those with low concentrations of sulfuric acid, any iron
sulfate monohydrate deposited at relatively high concen-
trations of sulfuric acid being recycled.
Now, the expert is confronted by the problem of recon-
ciling these conflicting requirements which, in addition,
are an obstacle to optimal process control in terms of
energy. In addition, the last two processes mentioned
above are designed for waste acids rich in iron sulfates of
the type which accumulate in the processing of ilmenite as
a titanium raw material. The relevance of the particular
teaching to the working up of waste acid from processes in
which titanium slag is used is questionable because the
slag has only a very low FeSO4 content.
Accordingly, the problem addressed by the invention
was to provide a process which would enable waste acids both
from the processing of titanium slag and from the proces-
sing of ilmenite to be worked up to produce a sulfuric acid
which could be reused in the digestion of titanium raw
material. At the same time, the energy and maintenance
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costs involved in the evaporation process were to be as low
as possible.
Surprisingly, this problem was solved by the process
according to the invention.
The present invention relates to a process for the
recovery of sulfuric acid from waste sulfuric acid contain-
ing metal sulfates by concentration through evaporation and
removal of the solid metal sulfates, the waste sulfuric
acid being concentrated by evaporation to a concentration
of 26 to 32% by weight HzSO4 in an evaporation stage 1, the
acid obtained is then concentrated by evaporation in a
steam-heated evaporation stage 2 to such an extent that
water-soluble metal sulfates are suspended in solid form in
the acid concentrated by evaporation, preferably in a
quantity of at least 1% by weight, the vapors from evapora-
tion stage 2 being used as heating medium in an evaporation
stage 3 in which the suspension from evaporation stage 2 is
concentrated by evaporation to a concentration of 60 to 75%
by weight H2SO4. ~
One particular embodiment of the process according to
the invention is characterized in that, without preliminary
removal of the solid metal sulfates, the suspension dis-
charged from evaporation stage 3 is concentrated by evapor-
ation in a steam-heated evaporation stage 4 to a concentra-
tion of 72 to 85% by weight sulfuric acid.
In evaporation stage 1, the waste sulfuric acid is
preferably concentrated by evaporation at temperatures
below 100C and preferably at temperatures below 90C.
In evaporation stage 2, the waste sulfuric acid is
preferably concentrated by evaporation at 120 to 165C to
a sulfuric acid concentration of 40 to 50% by weight.
In another particular embodiment, the vapors produced
in evaporation stage 2 are partly used for concentration of
the waste sulfuric acid by evaporation in evaporation stage
1.
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The acid from evaporation stage 1 is preferably fed
into the circulation pipe of evaporation stage 2 on the
pump pressure side.
The wasteacid is thus brought to a concentration of 60
to 75% by weight by the three-stage evaporation process de-
scribed above. The sulfuric acid may optionally be further
concentrated to 85% by weight in a fourth evaporation
stage. According to the invention, 10 to 26% waste acids
are concentrated in an e~aporation stage 1 to a concentra-
tion of at most 32% by weight H2S04 (based on the liquid asa whole) at-temperatures below 100C and preferably at tem-
peratures below 9ooc. It has been found that, under these
conditions, only small quantities of CaS04 are deposited
from the usual waste acids and that this CaS04 is easier to
remove from walls of pipes and apparatus than CaS04 which
has been deposited at higher temperatures. The concentra-
tion by evaporation may take place in a vacuum evaporator
heated with steam or with part of the vapors from evapora-
tion stage 2 or may be carried out by one of the known ad-
vantageous processes which utilize various types of wasteprocess heat (DE-A-2 529 708, DE-A-2 529 709, EP-A-313
715).
The wast*acid concentrated by evaporation to a concen-
tration of 26 to 32% by weight H2S04 in evaporation stage 1
is concentrated by evaporation to a concentration of 40 to
50% by weight H2S04 (expressed as salt-free sulfuric acid)
in an evaporation stage 2, but in any event to such an ex-
tent that the metal sulfates are partly suspended in solid
form in the sulfuric acid. In evaporation stage 2, evapo-
ration of the water takes place under an absolute pressureof 1 to 4 bar, so that the vapors may advantageously be
used for further concentrating the sulfuric acid containing
dissolved and solid metal sulfates by evaporation to a con-
centration of 60 to 75% by weight H2S04 in an evaporation
stage 3 in which evaporation of the water takes place at 80
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to 140C under a pressure of 40 to 300 mbar.
The suspension of metal sulfates in 60 to 75% sulfuric
acid flowing off from evaporation stage 3 may be suitably
cooled (for example in accordance with EP-A-133 505). A
reuseable sulfuric acid may then be separated off from the
solid metal sulfates.
In the processing of waste acid from the digestion of
titanium slag, it is of particular advantage directly to
concentrate the sulfuric acid/metal sulfate suspension
flowing off from evaporation stage 3 to a concentration of
72 to 85% H2S04 in an evaporation stage 4 without prelimin-
ary separation of the solid metal sulfates. In this evapo-
ration stage 4, which is heated with fresh steam, the con-
centration by evaporation takes place at 120 to 150C under
a pressure of 40 to 200 mbar.
Forced-circulation evaporators are preferably used for
the concentration of waste acid by evaporation in evapora-
tion stages 2 to 4 by the process according to the inven-
tion.
It has been ~ound that the known corrosion problems
which are caused by the chloride in the waste acid (E.
Schmoll, Escher Wyss Mitteilungen 2/1978-1/1979, pages 17
to 20) can be avoided if the 26 to 32% waste acid from evap-
oration stage 1 is fed into evaporation stage 2 after the
circulation pump, pre~erably in the circulated acid stream
be~ore or a~ter the steam-heated heat exchanger.
The process according to the invention affords the
~ollowing particular advantages over the prior art:
a) low steam consumption through multistage evaporation
utilizing the vapors from evaporation stage 2,
b) low consumption of cooling water through multistage
evaporation at a high temperature,
c) brie~ stoppage times ~or the removal of deposits be-
cause the deposits formed in evaporation stage 1 are
minimal and relatively soft while the deposits formed
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in evaporation stages 2 to 4 can be removed by flush-
ing with water,
d) elimination of corrosion problems caused by the chlo-
ride in the wasteacid.
The following ~E~les are intended to illustrate the
process according to the invention without limiting it in
any way.
Example 1 (Comparison Example)
Waste acid of the following composition was wor~ed up
according to the prior art (EP-A-133 505) (all quantities
in % by weight):
24.5 % HzSO4 0.15 ~ VOSO4
6.62 % FeSO4 o.o9 % Cr2(S04) 3
1.84 % Al2(SO4)3 1.42 % MeSO4
2.20 % MgSO4 63.18 % H2O
(Me z Mn, Ti, Ca, Na etc.)
Evaporation stage 1 was operated with waste heat from the
sulfuric acid plant in accordance with DE-A-2 529
708. Concentration by evaporation to 28~ H2S04 took place
at 73C. Further concentration by evaporation to a concen-
tration o~ 68% H2S04 ~expressed as salt-free acid, corre-
sponding to IOO-H2S04:(H2S04 + H20) - % H2S04) took place in
a ~orced-circulation vacuum evaporator.
~he acid was ~ed into the steam-heated first evapora-
tion stage of this evaporator in which concentration by
evaporation took place at 110C under a pressure of 760
mbar to a concentration of 41.5% H2SO4 ~salt-free). The
acid passing into the second evaporation stage from the
first evaporation stage contained all the metal sulfates
except CaSo4 in dissolved form.
In the second evaporation stage, which was heated with
the vapors from the first evaporation stage, concentration
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by evaporation took place at 76C/50 mbar to a concentra-
tion of 59% H2S04.
The suspension of metal sulfates in 59% sulfuric acid
discharged from the second evaporation stage had to be
further concentrated by evaporation at 90C/50 mbar in a
third steam-heated evaporation stage to obtain the desired
concentration of 68% H2S04. The vapors from the second and
third evaporation stages were directly condensed with
cooling water and removed.
After the suspension discharged from evaporation stage
3 had been cooled from 90C to 54~C over a period of 22 h
in a stirred cascade, the solid metal sulfates were sepa-
rated off by pressure filtration. The filtrate had the
following composition:
65.15 % H2S04, corresp. to 68.1% salt-free 0.005 ~ VOS04
0.19 % FeS04 0.08 % Cr2 ( S04 ) 3
2.1 ~ Al2( S04 ) 3 0 . 3 % MeS04
1.7 % MgS04 30.58 % H20
The evaporator was cleaned monthly, deposits in the
thlrd and second evaporation stages being easy to remove
with water. In the first evaporation stage, the deposits
consisted essentially of calcium sulfate and could not be
completely removed with hi~hly pressurized water- The stoppage
time required to clean the evaporator was between 20 and 50
hours.
Exam~le 2
Waste ~cid (composition as in Example 1) was concen-
trated by evaporation to 28.3$ H2S04 (salt-containing) in an
evaporation stage 1 in accordance with DE-A-2 529 708 and
fed into the evaporation stage 2 of a forced-circulation
evaporator in accordance with the present invention. The
acid was fed into the circulation pipe on the pump pressure
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side by way of a heat exchanger which was heated with part
of the vapors from evaporation stage 2 and in which the
acid was heated to 107C. In the steam-heated evaporation
stage 2, the acid was concentrated by evaporation to a con-
centration of 47% H2S04 (expressed as salt-free acid) at
150C/2.2 bar (absolute). Part of the iron sulfate crys-
tallized out. The vapors were passed into the preheating
heat exchanger and the heat exchanger of evaporation stage
3.
In evaporation stage 3, the suspension discharged from
evaporation stage 2 was concentrated by evaporation to a
concentration of 68% ~2S04 at 107C/lOo mbar and directly
discharged into the stirred cascade. The vapors from evap-
oration stage 3 were condensed by indirect cooling with
cooling water. The specific cooling water demand amounted
to approximately 65% of the demand in Comparison Example 1.
The suspension was cooled and filtered as in Example
1. However, the filtration rate was approximately 30%
higher than after concentration by evaporation in accord-
ance with Example 1. The composition of the 68% sulfuricacid obtained as filtrate was identical with that in Ex-
ample 1 within the limits of analytical accuracy. The
monthly cleaning intervals were maintained. However, the
deposits in evaporation stage 2 were relatively easy to
remove with water, so that the overall stoppage time for
cleaning could be shortened to 15 to 20 hours.
Example 3
Concentration of the waste acid (composition as in
Example 1) by evaporation took place as in Example 2,
except that the suspension of metal sulfates in 68% sul-
furic acid discharged from evaporation stage 3 was not
passed into the stirred cascade, but instead into an
evaporation stage 4.
In the steam-heated evaporation stage 4, further con-
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centration by evaporation took place at 140C/40 mbar. The
suspension discharged was cooled from 140C to 90C in a
heat exchanger and introduced into the stirred cascade in
which it was cooled to 50C over a period of 25 hours. The
suspension was readily filtrable. The sulfuric acid ob-
tained as filtrate had the following composition:
79.5 % H2S04, corresp. to 82.3% salt-free 0.008 % VOS04
0.24 % FeS04 0.08 % Cr2(So4)3
1.8 % Al2(S04) 3 0.4 % MeS04
0.9 % MgS04 17.1 % H20
It will be appreciated that the instant specification
ls set forth by way of illustration and not limitation, and
that various modifications and changes may be made without
departin~ from the spirit and scope of the present invention.
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