Note: Descriptions are shown in the official language in which they were submitted.
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2251 1-151
Mcthod for re-u~in~ waste sulphuric acid
The invention relates to a method for re-using the
low-concentration waste sulphuric acid resulting from
the production of titanium dioxide according to the
sulphate method, in which the resulting waste sulphuric
acid is concentrated from an initial concentration of
20 - 24% by weight to 65 - 82~ by weight, preferably to
- 75% by weight, some of the dissolved metal saltes
are precipitated and form a suspension with the
concentrated sulphuric acid, and highly concentrated
sulph~ric acid is returned to the raw titanium material
digestion process.
A method of this kind i~ known from the DE-A-2 729 755,
for example.
The production of titanium dioxide entails the
compelling problem of disposiny of the waste products,
in particular the resulting low-concentration sulphuric
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acid and the metal salts which it contains. The only
solution which is feasible on a long-term basis lies in
re-using the waste sulphuric acid, the so-called weak
acid, for the digestion of raw titanium materials.
However as sulphuric acid o~ a higher concentration is
required for this, i.e. a concentration of between
approximately 85 and 92~ by weight, accorcling to the
starting material, the weak sulphuric acid has to be
brought to a higher concentration. Yet, for reasons
relating to practicalities and energy, it is advisable
and usual only to concentrate the sulphuric acid to a
final concentration of approximately 65 to 82% by
weight, if possible of only 65 to 75~ by weight, and to
mix the concentrated sulphuric acid with sulphuric acid
of a very high concentration of at least 95% by weight,
so as to obtain the concentration required for starting
the raw titanium material digestion reaction.
The removal of the metal salts contained in the waste
sulphuric acid, in particular heavy metal sulphates,
presents a further problem. In fact most of the metal
salts are precipitated and form a suspension when the
weak sulphuric acid is concentrated. The majority of
the precipitated metal salts can be separated and
removed by filtering the concentrated acid, so that,
apart from metal salts which are still dissolved, the
concentrated sulphuric acid only contains traces of
precipitated metal salts.
However the filter cake, which preferably consists of
metal sulphates and adhering sulphuric acid, is
processed further in a roasting plant, with the
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resulting sulphur oxide gases being used to produce
very highly concentrated sulphuric acid, which is
returned to the raw titanium material digestion
process.
.he known method entails the disadvantrage that the
metal salts remaining in the concentrated sulphuric
acid have a negative effect on the quality of the
titanium dioxide which is produced, in particular the
chromium or vanadium salts which, due to their
colouring action, have a particularly powerful effect
on the pigment quality. Although an improvement could
be achieved by further concentration of the sulphuric
acid in a high concentration plant, in which more metal
salts would be precipitated, this requires expensive
and energy-intensive auxiliary equipment.
The object of the present invention is to eliminate the
above-mentioned disadvantages of the prior art and
provide a method for re-using the waste sulphuric acid
resulting from the production of titanium dioxide
according to a simple concept which does not require
any expensive auxiliary components, in which the
sulphuric acid does not need to be concentrated to more
than 65 - 75% by weight and in which the recycled acid
nevertheless clearly contains few metallic impurities.
It should also be possible to use the method for
different starting products, in particular titanium
slag and ilmenite, as the raw material for the
production of titanium dioxide.
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- 4 - 22511-151
According to the invention, none, or at most only a por-
tion of the concentrated sulphuric acid which has been obtained
from the waste sulphuric acid is directly returned to the di-
gestion process after separating the precipitated metal salts,
while the remainder of the suspension is roasted, without the
metal salts being separated, and possibly also the metal salts
which have been separated from the recycled portion of the concen-
trated sulphuric acid, the sulphur oxides resulting from the
roasting are converted to highly concentrated sulphuric acid, and
that this highly concentrated sulphuric acid is returned to the
digestion process.
Thus, the invention provides a method for re-using a low
concentration waste sulphuric acid resulting from a digestion pro-
cess of a raw titanium material in the production of titanium
dioxide according to a sulphate method, the said low-concentration
waste sulphuric acid having a sulphuric acid concentration of 20
to 24% by weight and containing metal salts dissolved therein,
which method comprises:
[A] concentrating the low-concentration waste sulphuric
acid to a sulphuric acid concentration of 65 to 82% by weight and
at the same time causing a precipitation of a part of the dis-
solved metal salts, thereby forming a suspension;
[B] roasting the entire or a portion of the suspension
without separating the precipitated metal salts, thereby producing
sulfur oxide and roasted metal salts;
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- 4a - 22511-151
[C] mixing the thus-produced sulfur oxide with water,
thereby producing highly concentrated sulphuric acid;
[D] returning the thus-produced highly concentrated
sulphuric acid to the digestion process of a raw titanium
material; and
[E] where only a portion of the suspension is roasted
in step [B], (i) separating the precipitated metal salts from the
remainder of the suspension, to produce separated metal salts and
a concentrated sulphuric acid having a concentration of about 65
to 82% by weight and substantially free of precipitated metal
salts, and (ii) returning the said concentrated sulphuric acid to
the digestion process.
In a preferred embodiment, the proportion of concentra-
ted sulphuric acid which is directly returned to the digestion
process may even be zero, i.e. all of the concentrated suspension
is conveyed to the roasting plant. The method thus becomes parti-
cularly simple, as no filtration system is required for the sus-
pension.
Even if some of the concentrated sulphuric acid,
advantageously at most 25~, although usually less, i9 directly
returned to the digestion process (namely, at least 75% of the
suspension is roasted), the amount of metal salts contained in the
concentrated acid is reduced to a fraction of the impurities in
known methods, which means that the pigment quality is distinctly
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- 4b - 22511-151
improved.
The accompanying figures are diagrams showing the
circulation of the sulphuric acid in methods according to the
invention with different starting materials and
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different proportions of recycled concentrated
sulphuric acid. In the figures:
Fig. 1 is a circulation diagram using QIT slag and with
the entire suspension being transferred for
roasting,
Fig. 2 is a circulation diagram for QIT slag with
direct recycling of approximately one fifth of
the concentrated sulphuric acid, and
Fig. 3 is a circulation diagram using an ilmenite
mixture as the raw material.
The numerical values given in the circulation diagrams
denote the respective quantities of 100% sulphuric acid
in the acid or suspension of the indicated sulphuric
acid concentration in tonnes, related to one tonne of
produced titanium dioxide.
Canadian QIT titanium slag from the Quebec Iron and
Titanium Corp. with a TiO2 content of 78.5% is used in
the diagrams reproduced in Figures 1 and 2, a
considerable portion of the iron sulphate contained in
the ore, in particular in the ilmenite, already haviny
been separated, so that the titanium slag only
comprises a small portion of iron, although greater
proportions of other metals, for example aluminium and
magnesium, and in particular undesirable proportions of
chromium and vanadium.
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22511-151
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2.323 t of sulphuric acid, related to 100%, with a
concentration greater than 90,5% by weight, e.g. 95% by
weight, as well as steam are required for each tonne
(t) of titanium dioxide which is produced in the
example illustrated in Pigure l. In the titanium
dioxide production process which follows the digestion
of the raw material 1,3961 t of waste sulphuric acid,
related to 100%, with a metal sulphate proportion
corresponding to 0.5779 t of sulphuric acid is obtained
as a weak acid DS, as well as 0.3490 t of 5% sulphuric
acid as a weak filtrate DF. ~he latter has to be
disposed of, as it would be highly uneconomical to
reprocess it. ~he weak sulphuric acid is concentrated
in a multistage evaporation plant C to a concentration
of approximately 70% by weight, with the majority of
the salts contained in the waste acid, in particular
the metal sulphates, being precipitated and passing
into suspension. Steam of the usual pressures and
temperaturs, some of which results from the subsequent
roasting, yet some of which may also be taken as waste
heat 'rom the titanium dioxide production process, can
be used for the concentration process. This also has a
favourable effect on the energy costs, in addition to
the advantages of the multistage system.
The suspension ~slurry) CS which i9 formed and which
has a sulphuric aoid content in liquid form of 1.3961 t
and in the form of metal sulphates of 0.5779 t is
conveyed to a roasting plant R, where it is
disintegrated. l.7766 t of highly concentrated
sulphuric acid HCS i9 'ormed from the resulting gaseous
sulphur oxides andthe addition of water.
.. . .
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This highly concentrated sulphuric acid HCS, which is
obtained from the roasting, is returned to the
digestion process, the sulphuric acid loss resulting
from the operation being compensated by the addition of
0.5464 t of fresh acid FS. The metal oxides ~leO
resulting from the oxidation, which are largely free of
sulphuric acid, are removed and conveyed to a waste
dump.
Figure 2 shows an embodiment in which QIT slag is also
used as the raw material. However, in contrast to the
example illustrated in the preceding figure, after
being concentrated to 70% by weight, some of the
suspension CS, with a 0.270 t proportion of sulphuric
acid~ related to 100%, is separated off. This portion
is conveyed to a filter press F, in which the
precipitated salts ~IS are separated. ~he filtered
concentrated sulphuric acid FCS is returned to the raw
titanium material digestion process, while the filter
salts ~IS are conveyed to the roasting plant R with the
unfiltered portion of the suspension CS having a liquid
sulphuric acid content of 1.1261 t and a sulphuric acid
content in the form of sulphates of 0.5779 t. 1.5336 t
of sulphuric acid HCS with a concentration of 98% by
weight is obtained from the roasting gases and the
addition of water and returned to the digestion
process. In this case 0.5194 t of 98% fresh acid FS has
to be added to compensate for` the losses.
. .
Figure 3 is a cicu~ation diagram in which a mixture of
different ilmenites, i.e. a mixture-of ,50% Malaysian,
20% Thai and 30% Australian ilmenite, is used as the
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xaw material for digestion. 3.3782 t of sulphuric acid
are required in the digestion process, and the waste
acid amounts to 2.023 t, of which 0.5229 t is lost in
suspension as 5% weak filtrate DF with 0.0757 t of
metal sulphates, as well as 0.7249 t as adherent acid
of the precipitated copperas. 1.5001 t of 23% weak
sulphuric acid DS with 0.5546 t of acid in the
sulphates are then still available for recirculation.
In this case it is advantageous, after concentrating
the weak acid to approximatelxy 70% by weight, to
separate off this proportion and free it from the
precipitated metal salts ~IS by filtration, so that
1,24 t of 70% sulphuric acid FCS can be returned to the
digestion process. ~he remainder of the suspension CS
with 0.2601 t of sulphuric acid and 0.5546 t in the
form of sulphates is roasted and processed to form
0.7332 t of 98% sulphuric acid, which is returned to
the digestion process. A further 1.4050 t of highly
concentrated fresh acid FS has to be added to the
digestion process to compensate for the losses.
When compared with the example to be found in the
DE-A-2 729 755, the content of metallic impurities in
the reprocessed and recycled sulphuric acid is
substantially reduced, i.e. to at most 20~ of the
comparative example, althougll usually quite a lot less,
as in the method according to the invention the
concentrated suspension is completely, or at least for
the most part, conveyed to the roasting plant, in wich
a substantially greater portion of the impurities is
separated than in a conventional filtration system. ~he
quality of the titanium dioxide pigment which is
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produced is therefore clearly improved and its impurity
content considerably reduced, without having to bring
the waste sulphuric acid to higher concentrations by
means of expensive equipment and an increased use of
energy.
The individual numerical values must of course differ
somewhat according to the origin of the raw materials.
When using South African, Canadian, ~orwegian or
Australian titanium slag, or ilmenites of a different
origin, the required quantities have to be adapted to
the respective raw material composition, taking account
of the teaching of the invention.
