Note: Descriptions are shown in the official language in which they were submitted.
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The invention concerns a method for precipitating
chromium and/or vanadium from evaporated waste sulphuric acid from
the titanium dioxide production by aging after evaporation.
In titanium dioxide production waste sulphuric acid is
generated having a concentration between 18 and 28 % containing
metal impurities such as iron, aluminum, magnesium, titanium,
chromium, vanadium, manganese, etc., of concentrations up to some
% by weight.
In various publications, for example, DE-A-28 07 308,
DE-A-28 07 360 and DE-A-28 07 304, it is described that such waste
sulphuric acid can be conditioned for recycling by evaporating
water from the dilute acid to a concentration of e.g. 65 to 70 %
~ollowed by an aging process during which the metal impurities are
precipitated and separated. The aging process allows to precipit-
ate the metal impurities within hours or days to such a degree
that the conditioned sulphuric acid is fit for reuse.
It soon became clear that especially chromium and vana-
dium precipitate only very slowly, e.g. at an initial chromium
concentration of between 300 and 400 ppm, the final concentration
after one weekls aging is still more than 200 ppm. Due to the
strong colorific effect, sulphuric acid with such chromium and
vanadium amounts can only be used to a limited extent in e.g. the
pigment or dyestuff industry, especially for the production of
titanium dioxide. A reduction of the chromium concentration to
values below 200 ppm, if possible to below 100 ppm, would be
desirable.
1 329005 22511-150
It is also described in DE-A-26 18 121 and DE-A-27 29 756
that during evaporation the removal of chromium from waste sul-
phuric acid resulting from titanium dioxide production can be
improved by adding metal sulphates especially ferric (II)-sulphate.
When using this method, however, the chromium precipitation is
often insufficient~
In DE-A-24 46 117 and United States Patent3,575,853, it
; is described that chromium complex compounds and chromates result-
ing from industrial waste water, e.g. from the galvanic industry
such as metal chroming can be separated or floculated by contact
with aluminum plates or particles. The application of this method
for the purifica~ion of waste sulphuric acid resulting from
titanium dioxide production, where chromium is found in a different
form, has not shown the expected results.
It is further described, for example, in Swiss Patent
Publication 409,800, DE-A 678 034 and Unexamined Japanese Patent
Publication 79/65 194 that a number of heavy metals such as copper,
;~ iron, mercury or lead can be precipitated from waste waters con-
; taining sulphuric acid by the influence of aluminum. However, also
with this method is~ the chromium s~parati`o~ insufficient to allow
reuse of the sulphuric acid for the titanium dioxide production.
It is attempted in the pr~sent invention to devise a
method which allows a better chromium and/or vanadium separation
from evaporated sulphuric acid resulting from the titanium dioxide
production within a shorter time and achieving a lower concentra-
tion so that the purified sulphuric acid can be reused for the
,
1 32 q 0 0~ 22511 150
titanium dioxide production.
According to the invention, metallic aluminum having an
active sur~ace is added to the waste sulphuric acid after evapora-
tion but before the aging process. The formation of the active
surface may be achieved by comminution, especially cutting or
pulverization of aluminum at a predetermined period of time before
adding to the waste sulphuric acid.
To maintain an active surface, the comminution should
preferably be done not more than 4 hours before adding it to the
waste sulphuric acid when it is stored under normal conditions.
The chromium separation can additionally be improved
by adding seed crystals taken from the solids separated from the
sulphuric acid, preferably in an amount of 1.5 to 2.5% by weight.
It is assumed that the favorable effect of adding
aluminum having active surface is based on the very negative redox
potential of the aluminum resulting in a redo~ reaction, when
chromium is present in the titanium dioxide waste sulphuric acid
in a specific form. The higher valence chromium ions, e.g. 6-3-
valence ions are thus probably reduced to lower valued e.g. 3-
valued chromium ions or to metallic chromium. In this way the
solubility of some salts of the 3-valued chromium especially sul-
phate and calcium chromite is much lower as compared to the corres-
ponding salts in the 6-valence chromium so that in this special
waste acid the reduction by means of active chromium results in a
faster and quantitatively higher precipitation of chromium salts
than with conventional methods. A similar effect is achieved for
-- 3 --
.~ ~
.
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22511-150
the precipitation ofvanadium in a waste sulphuric acid of said
origin.
The evaporated sulphuric acid is usually in a slurry
form and has a sulphuric acid concentration of about 60-70% by
weight.
Usually, the necessary amount of aluminum to be added is
at least 0.2, preferably about 0.3 % by weight based on the eva-
porated sulphuric acid, depending on the initial material. A
higher temperature results in a further increase of the chromium
separation. The aging is performed preferably at 40 to 70C.
Although a remarkably increased chromium separation is achieved
at 40VC, much more favorable results are obtained at temperatures
of up to 60C. Under these conditions and at an initial material
with a chromi~m content of over 300 ppm a reduction of the
chromium concentration in the concentrate to below 200 ppm within
24 hours and to below 100 ppm within two to four days is achieved
whereas the reduction to 200 ppm without the addition of aluminum
re~uired one week, and a reduction to 100 ppm was not achieved at
all~
The basic material, a 22% sulphuric acid from an ilmenit/
slag disintegration was heated up to the relevant boiling point up
to max. 150C and evaporated to a sulphuric acid salt slurry with
a free H2SO4 content of 63.9 ~ and the following metal impurities
in the filtrate:
.
~ _ 4 _
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22511-150
~: Fe 0.40 %
Al 0.45 %
Mg 0.48 %
Cr 320 ppm
V 300 ppm
', Mn 300 ppm
Ti 0.11 %
;: A part of this test acid was aged without additives at
55C during one week (test 1). Another part of the test acid was
mixed with seed crystals (2 % by weight) taken from the precipitat-
: ed, aged substance of an earlier test and then aged at 55C resp.
; 40C (tests 2 and 3). A further part of the test acid was mixed
with 0.3 % by weight aluminum chips which had been cut less than
one hour before adding them to the acid (test 4).
.' The reduction of ahromium content as to time was observea
~ in all ~our samples, and the following results were achieved: -
.'~ Co~parison examples
.,~
', Aging Sample 1 Sample 2 Sam~le 3 Sample 4
time -/-/55C -/2%/40C -/2%/55C 0.3%/2%/55C
'Hours ppm Cr ppm Cr ppm Cr ppm Cr
0 320 320 320 320
19 27~ 290 270 230
39 270 280 245 170
, 63 265 275 220 130
135 260 210 170 90
The test results show that with an addition of 0.3%
. active aluminum the chromium content can be reduced to the in
,' many cases admissible limit value of 200 ppm within 2a houxs.
.,
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After less than 4 days aging the chromium content is almost
reduced to the value required if the acid is reused, i.e. below
100 ppm. The comparison examples show that a reduction of the
chromium content to below 200 ppm can only be achieved by adding
seed crystals in the amount of 2% by weight and aging at a tem-
perature of 55C during four days. In the comparison example
without additives, this value was not achieved even after one week
of aging.
As the comparison examples l to 3 show, chromium separa-
tion at 55C is slightly higher than at 40C. A mere temperature
increase however is not sufficient even at aging periods of one
week to reduce the chromium content to below lO0 ppm.
As has been proved by further tests, an increased
aluminum addition resulted in a faster chromium separation. How-
ever, the addition of aluminum chips made the day before showed a
result which was not substantially better than in example 3.
Similar observations were made as regards to the reduc-
tion of the concentration of other, scarcely precipitatable
impurities, especially vanadium. It is recommended to use well-
aged seed crystals for better reduction of the concentration.
