Note: Descriptions are shown in the official language in which they were submitted.
A process for recycling sulphuric acid containing me~al
S sulphate
The present invention relates to a process for re-
cycling used sulphuric acid conLaining metal sulphates by
evapora~ing the dilute sulphuric acid containing ~he dis-
solved metal sulphates to a content of from 40 to 85% ofH250~, based on sal~-free acid, separating the metal sul-
phates by filtration and reusing the separated sulphuric
acid.
Metal sulphatP-cDntaining dilute acids, as occur in
the production of TiO2 or as pickling acids, can be re-
moved and supplied for reuse by evaporating these metal
sulphate-containing sulphuric acids up to a concentration
of the acid which is suitable for reuse or up to the con-
centration at which the metal sulphates have a solubility
minimum. Optionally after cooling the suspension, the
metal sulphates are conven~ionally separa~ed by filtration
frDm the sulphuric acid, which is ~hen recycled to the
production prDcess DE-A 33 27 769, US 2 28D 5D8,
US 2 098 056).
According to ~he prior art~ vacuum drum fil~ers are
used for the filtration of these suspensions which gene-
rally contain from 10 to 40X. ~y weight of solids material
with grain sizes of predominantly from 5 to 100 ~m
tUllmanns Encyklopadie d. techn. Chemie, Verl. Chemie,
Weinheim, 4th Edition, 1972, Volume 2, p. 188 -190). The
advantage of these apparatus lies in the simple and eco-
nomical ccntinuous operation with low operatiDnal CDst5.
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The disadvan~ages o~ vacuum filtra~ion are ~ha~,
depending on the grain size and proportional composi-ion
of ~he me~al sulphates of ~he group of sulphates and
hydrogen sulphates of iron, aluminum, magnesium, titani-
um, manganese, chromium and vanadium, the filter cake
tends to form cracks and i5 thixo~ropic
The residual moisture of this filter cake is usually
from 20 to 40% by weight, corresponding to from 12 to 25%
by weight of H2S04 This sulphuric acid is not only re-
moved from the production process, but also makes the
handling and safe removal of the metal sulphates substan-
~ially more difficult In the case Df the ecologically
op~imum removal of the metal sulphates by thermal decom-
position a substantial proportion of the decomposition
capaci~y is taken up by ~he residual moisture of the metal
sulphates The use of this process by residual moisture
i6, moreover, uneconomical, since it requires a specific
energy of decomposition which is about 80% higher than in
the case of metal =ulphate~
Many a~tempts have thus been made to reduce the
residual moisture by additional demoisturizing of the
fil~er cake obtained with vacuum drum filters A certain
degree of success i5 achie~ed with compression s~rip
filters, bu~ ~he technical cost for the double filtration
is too high
The object of the presen~ invention is thus ~o
provide a process which does not suffer from the above-
mentioned disadvanLages
It has surprisingly now been found that an unex-
pectedly good filtration with a minimum residual moisture
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382~
conten~ of the fil~er cake is possible by fil~ration in
pressure filters which per~iLs uniform demoisturizina of
the cake by blowing compressed air ~hrough ~he fil~er
cake.
The presen~ inven~ion ~hus provides a proce~s for
recvcling used sulphuric acid con~aining metal sulpha~es
10 b~ evapora~ing ~he dilute sulphuric acid con~aining ~he :~
dissolved me~al sulpha~es ~o a con~en~ of from 40 to 85%
of H2S04, based on salt-free acid, separa~ing the me~al
sulpha~es by filtration and reusing the separa~d sul-
phuric acid~ the filtration taking placa under ~xcess
pressure and compressed air being blown through the fil~er
cake thereby formed for demoisturiza~iorl.
The filters used accordin~ to the invention sre con-
ventionally used for separating very fine-grained solids
materials and, owina ~o the discontinuous operation, for
2~ suspensions with a low solids contentt The suppliers are
~hus prejudiced against recommending such ~ypes of filter
for the filtration of the metal sulphate-~ulphuric acid
suspension which is high in solids material and relatively
easy to filter. It i~ thus all the more ~urpri~ing that
the residual moisture of the fil~er caka obtained with
these types of filter, in contrast to ~he cake ob~ain2d
on vacuum drum filters, can even be reduced by from 30 to
50%, so as ~o produce a correspondin~ increase in the
yield of reusable sulphuric acid. In view of the above-
described problems in ~he handling and ~sfe remo~al of the
filter cake, the advanta~es of the me~hod of fil~ra~ion
accordina to the invention are ex~remely great,
particularly since the fil~er cake ob~ained accordina to
: the invention is not thixotropic~ but crumbly and ::
35 apparently dry- ~
.
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8~11
In con~ras~ ~o ~he fil~ers oLherwise used, ~he
~iltra~ion ~akes place ba~chwise wi~h the ypes of fil~er
according Lo ~he inven~ion.
I~ has been shown to be advan~ageous Lo carry out the
fil~ra~ion according ~o Lhe invention until the thickness
vf ~he ca~e is at least 1~ mm, preferably from 20 to
60 mm.
The demoisturizing of the filter cake takes place by
blowing compressed air through the cake. The required
pressure and dura~ion of blowing are ~hereby dependent on
the thickness of ~he cake, the composition of ~he me~al
lS sulphates mixture, the concen~ration of the sulphuric acid
contained in ~he filter cake and the required residual
mois~ure. The econDmical op~imum is easy ~o determine by
tests. The ~se of hot air with ~empera~ures of at most
150C, preferably from 40 ~o 120C, is advantageous.
Compressed air is blswn ~hro~gh according to the invention
for at least 2 minutes, preferably from 4 to 10 minutes.
The usefulness of the sulphuric acid is considerably
influenced by its con~ent of dissolved metal sulphates.
To keep this salt con~ent low special conditions have to
be observed during crys~alliza~ion (cf. EP 0 133 505). In
addition, the salt content is influenced by the
tempera~ure during the sulphate separation~ which is be-
tween 40 and 70C. Although the dissolved sal~ content in
the acid is lower at a low Lempera~ure ~han at a higher
temperature, ~he filterability of the colder suspension
is considerably poorer and after the demoisturization with
compressed air subs~antially larger quantiLies of adheren~
mois~ure remain in the filter cake. A preferred embodimen~
of the
~S
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process accordina ta ~he invenLion avoids Lhe disadvan-
~ages of fil~ra~ion a~ a low ~empera~ure and of ~ha~ a~
a high ~empera~ure in such a wav ~hat ~he ruspension i5
fil~ered a~ a low ~empera~ure and ~he filLer cake i5
demois~urized wi~h ho~ compressed air.
A parLicularly preferred embodimen~ of the process
accordine ~o ~he invenLion consists in carrying ou~ ~he
fil~ra~ion aL a ~emperature of ~5 ~o 50C and blowing
~hrough compressed air having a tempera~ure of 70 ~o
12~C.
A no~iceable improvemen~ in relation ~o ~he conven-
tional vacuum fil~ration can be achieved bv fil~ra~ion
wi~h membrane fil~er presses. Still more favorable resulLs
c~n be achieved wi~h pressure fil~ers of ~he car~ridge
fil~er or pla~e fil~er types, which permit unexpec~edlv
low residual mois~ure con~en~s in the fil~er cake. In this
case, and in view of ~he simple cons~ruction of such
fil~ers, subs~an~ial progress is ~hus made in overcoming
~he disposal problems.
The fil~ers par~icularly preferred according to ~he
inven~ion are thus car~ridge fil~ers. Particularly good
results are achieved when several fil~er tubes are used,
~he minimum spacing be~ween ~he fil~er car~ridges being
from 100 to 150 mm, preferably from 120 ~o 1~0 mm and the
minimum spacing between ~he car~ridges and the fil~er
housing being from 70 ~o 100 mm.
~ The inven~ion is explained below by means of non-
limi~ ng Examples. In ~hese Examples ~ons means me~ric
tons and m3 of air means cubic me~ers under s~andard
condi~ions.
3~
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Exam~le 1 (Compara~ive Example)
In this and ~he foll~wing Examples, a me~al sulphate-
con~aining 65% sulphuric acid is used, which i~ obLained
by evapora~ing a waste acid from the production of Tio2.
This me~al sulpha~e suspension has the following composi-
tion ~data in % by weight):
FeS04 H2010.6 Fe2(54~3 H2504 . 8 H20 1.1
Fe2H2~S04)3 . H20 11.4 Al2(504)3 H2SOq 8 H20
~954 ~ H203.6 TiOS04 2.9
MnS04 H200.7 VOS04 0-3
H2504 43-4 H20 23.3
The particle size of these sulphates, as far as can
be seen with a microscope, is from 5 to 120 ~m, predomi-
nantly from 15 to 80 ~m. The temperature is 55C.
When filtering with a vacuum drum cell filter (drum
diameter = 2.2 m, rotational speed = 0.3 min 1) an 8 to
10 mm thick filter cake i5 obtained. The thixotropic
filter cake contains 28.2X by weight of mois~ure,
corresponding to 18.3% by weight of H2504.
540 kg of reusable 65% sulphuric acid and 460 kg of
filter eake are Dbtained per ton of suspension.
Example 2
The suspension described in Example 1 is filter~d
with a membrane filter press (Pressfilter-Automat P of
Hoesrh g Sohne, Duren~ up to cake thickness of 25 mm.
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-- 7
The filter cake is then pressed by pre~sirg ~he mem-
branes wi~h 16 bar and compressed air (5 bar) i5 blown
~hrough for lO minuLes for demoisturizaLion. Ihe residual
moisture of the resulting cake is 20.3% by weight, corres-
ponding to 13.2% by weigh~ of H2504.
With a pressure of 8 bar during pressing the mem-
~ branes, a residual mois~ure of 18.9X. by weight, corres-
ponding tD 12.3~/. by weighL of H2504, i5 produced. The ne-
cessary blowing air (50C) corresponds to a requirement
of 194 m3 per ton of filter cake.
ExamDle ~
The suspension from Example 1 i5 filtered wi~h 3 tube
filter (Fundabac (R) - Filter of DrM, Mannedorfl Switzer-
land~.
The filter i5 fitLed wi~h 8 ~ubes wi~h a ~o~al area of
filter cloth of 2.2 m2. The minimum spacing between the
tubes is 50 mm. With an excess pressure of 3 bar, filter-
ing is carried out for 30 seconds and compressed air (5C)
i5 blown through the filter cake for 5 minutes to remove
the residual moisture. The air pressure within the filLer
thereby falls from Z.8 to 2.1 bar. The filter cake (86 kg)
delivered from Lhe filter has a ~hickness of about 20 mm
and a moisture content of 18.0% by weight, corresponding
to 11.7% by weight of H2504.
ExamDle 4
Analogously to Example 3, filtering is carried DUt
under 3 bar for 30 seconds and compressed air (55C) is
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s~
-- 8
blown ~ro~gh ~he fil~er cake for 5 minu~es The residual mois~ure is ~hereaf~er 14 6% by weigh~, corresponding Lo
eight of H2504 610 kg of 65% sulphuric acid
(6l 9/ H2S04, 3~ 3% H30, 4 8% dissolved sulpha~es~ are
separa~ed per ~on of suspension (compared wi~h 540 kg/~on
under vacuum fil~ra~ion corresponding to Example 1) The
fil~er cake (390 kgl~on Df suspension) has not thixo~ropic
proper~ies, bu~ is an apparen~ly dry powder For dry-
blowing 84 kg of filter cake, 40 m3 of pressurised air
are required, corresponding to 476 m31ton of filter cake
ExamDle S
In order ~o ob~ain a greaLer cake ~hickness, 6 filter
tubes are removed from the Fundabac(R) - Filter and only
~he central filter tubes is u6ed for the fur~her experi-
ments
The fil~er ~0 31 m2 fil~er surface) is charged for
80 sec under an excess pressure of 5 bar with ~he suspen-
sion correspJnding to Example 1 After emptying the pulp,
dryblowing is carried out for 10 minutes wi~h compressed
air ~emperature 5~C, pressure falling from 4 7 ~o
3 7 bar) 2~ kg of fil~er cake are ob~ained wi~h a moi 5 -
~ure con~ent of 15 9% by weigh~ corresponcling ~o 10 3% bv
weigh~ of H2S04. The average cake ~hickness is 45 mm
Exam~le 6
Fil~ering i5 carried ou~ wi~h a filter equipped wi~h
one tube (analogously ~o Example 5) for 8D seconds and,
~5
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af~er emptyin~ ~he pulp, dry-blowing is carried ouL for
5 minutes wi~h compressed air (tempera~ure 55C, pressure
falling from 4.5 ~o 3.2 bar). The fil~er cake obtained has
a residual moisture content of 14~3% by weight, correspon-
ding to 9.3% ~y weight of H2504. For dry-blowing 27 kg of
fil~er cake, 6 m3 of air are required, corresponding to
~ 222 m31~on of filter cake.
ExamDle 7
Filtering is carried out for 80 seconds analogously
to Example 6 and dry-blowing is carried out for 8 minutes
with compressed air (temperature 75C, pressure 3.3 bar).
The residual mois~ure content of the fil~er cake obtained
(27.5 kg) is 13.5% by weight, corresponding to 8.8% by
weight of H2504. The air consump~ion is 7 m3, correspon-
ding to 255 m3~on of fil~er cake.
Exam~le 8
The suspension according to Example 1 was cooled in
the course of 5 hDurs frDm 55C to 42C and fil~ered with
a tube filter as in Example 6. Eiltra~ion was carried out
for 60 seconds under a pressure of 5 bars and af~er remov-
ing the slurry ~he filter cake was air-blown for 10 mi-
nutes using compressed air (40C 4.2 bars). The filtrate
contained 62.9% of H2504, 33.8% of H20 and 3.3% of dis-
solved sulphates (cf. example 4). 16.4 kg of a filter cake
containing 26.7~/. by weigh~ of residual moisture, corres-
pDnding to 10.9% by weight of H2S04, were discharged. The
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air consumpLion was 8.3 m3, corresponding ~o 506 m3/ton
of fil~er cake.
~xam~e 9
As in ~xample 8 ~he susDension was fil~.ered for
~ 80 seconds under a pressure of 5 bars and a~ a ~emperature
of 42C. Af~er removal of ~he slurry the filter cake was
air-blown for 8 minu~es using compressed air which had a
~empera~ure of 110C and a pressure of 4.2 bars. The
filtrate contained 62.8% of H2504, 33.8% of H20 and 3.4%
~5 of dissolved sulpha~es. 20.9 kg of a filter cake having
a tempera~ure of 55C and containing 13.8% by weight of
residual moisture, corresponding to 9.0% by weigh~ of
H2504, were discharged. The air consump~ion was 5.5 m3,
corresponding to 26.3 m3/ton Df fil~er cake.
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