Note: Descriptions are shown in the official language in which they were submitted.
~Z933~6
A process for ~he production of ti~anium dioxide
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The present invention relates to a process for the
production of titanium dioxide by discontinuous digestion
of titanium slag with sulphuric acid, production of a
titanyl sulphate solution suitable for hydrolysis from the
solid digestion cake obtained during digestion, hydrolysis
of the titanyl sulphate, separation and calcinatinn of the
titanium-containing hydrolyzate, evaporation of the waste
acid separated from the hydrolyzate and re-use of the
sulphuric acid separated from the avaporated ~aste acid
in the digestion of titanium slag.
The requirement for recycling of sulphuric acid from
the Tio2 production process by the sulphate process is
having to be met more and more. In addition to the
expected economic disadvantages in comparison with dumping
of waste acid in the ocean, the process often encounters
~echnical problems.
A process for working up waste acids is known from
DE-A 3 327 769 and is adopted on an industrial scale. De-
pending on the local condition, however, problems can
arise here if the waste acid is to be almost completely
recycled.
According to the working up process currently
adopted, the waste acid is concentrated, pr0ferably by
multi-stage vacuum evaporation, until a 60 to 70%
;~ sulphuric acid with a low content of dissolved metal
sulphates can be separated by filtration from crystal-
lized metal sulphates.
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The concentration of this recovered sulphuric acid
is generally too low, however, to produce a sulphuric acid
initial concentration suitable for the discon~inuous
digestion of raw material when mixed with fresh sulphuric
acid or oleum, which is required for replacing the sul-
phuric acid losses ~by metal sulphate binding, residual
1~ moisture in the separated metal sulphates, waste water).
It is therefore necessary further to evaporate the sul-
phuric acid separated from the metal sulphates, and this
can be effected, for example, by using the waste heat from
Tio2 calcination (EP-A 97 259).
The greater the content of recycled sulphuric acid
in the total mixture, the higher the concentration of the
recycled acid must be. As the concentration incr~ases,
however, the requirements in energy and equipment also
increase. In particular, when using the titanium slags
2~ which are advantageous from an ecological point of view
as raw material, a particularly high concentratior. of the
recycled acid is required if maximum possible industrial
recycling is desired.
A more desirable situation arises if the relatively
low sulphuric acid losses during digestion of the titanium
slags can be replaced by 95 to 98% sulphuric acid ("fresh
acid") instead of oleum. The prod~ction of oIeum can thus
be omitted.
In this case, according to the prior art it is only
~ possible to concentrate recycling acid in a high concen~
tration apparatus (for example a Pauling distillation
vessel) ~o about 96% (~llmanns Encyklopadie d. techn.
Chemie, 4th edition, volume 18, page 579, Verlag Chemie,
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Weinheim 1979)~ However, this stage of the process is
extremely expensive with respect hoth to the investment
costs and to the energy and maintenance cost5 .
It is accordingly the object of the prssent invention
to propose a method which overcomes the above-mentioned
disadvantages of the prior art in an economical manner.
This object can surprisingly be achieved by the
following combination of measures:
- evaporation of the 60 to 75% sulphuric acid separated
from the metal sulphates and containing dissolved
metal sulphates under vacuum in horizontal or circu-
lation evaporators at from 120 to 190C to 76 to 87X.
H2504 ~based on salt-free sulphuric acid),
- mixing of the ground ~itanium slag with ~5 to 98%
sulphuric acid at from 30 to 80C and addition of the
- recycled acid at a temperature of from 80 to 190C,
preferably from 100 to 160C.
The present invention accordingly relates to a
process for the production of titanium dioxide by discon-
tinuous digestion of titanium slag with sulphuric acid,
productio~ of a titanyl sulphate solution suitable for
hydrolysis from the solid digestion cake obtained during
: : digestion, hydrolysis of the titanyl sulphate, separatioh
and calcination of the titanium-containing hydrolyza~e,
:~ evaporation of the waste acid separated from the hydro-
~ lyzate and re-use of the sulphuric acid separated from the
: evaporated waste acid in titanium slag digestion, a 60 to
75% pre-concentrated sulphuric acid being separated from
the sol:id metal sulphates and hydrogen sulphaLes after
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~Z933~6
evaporatiDn of the waste acid, this sulphuric acid being
further evapDrated under vacuum to a concentration of
between 76 to 87% H2S04 ~calculated a5 salt-free sulphuric
acid) and being used together with 95 to 98% sulphuric
acid in the digestion of the ground titanium slag.
The inventiDn also teaches the conditions under which
the titanium slag is to be digested in order to keep the
nece~sary concentration of the recycled sulphuric acid and
therefore the costs of sulphuric acid concentration as low
as possible.
The titanium slag is advantageously mixed with warm
95 to 98% sulphuric acid ~"fresh acid"j at from 30 to
80C, preferably from 50 to 80C.
Digestion is initiated by addition of the uncooled
or slightly cooled recycled acid, supplied from sulphuric
acid evaporation to the warm fresh acid/slag mixture. The
process corresponding to the prior art involving ini-
tiating digestion by addition of water or introduc~ion of
steam would require a substantially higher concentration
of the recycled acid than the process according to the
invention.
A particularly preferred embodimant of the process
according to the invention ther~efore involves initiating
he digestion reaction by addition of hot 76 to 87%
sulphuric acid ("recycled acid") at from 80 ~o 190C,
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preferably from 100 to 160C.
~A further reduction in the minimum sulphuric acid
concent~ra~:ion required for discontinuous slag digest:ion
by 2 to 3%, without losses of TiO2 yield occurring, is
successful because steam is blown through the digestion
:
cake instead of air during the maturing time once the
5~ maximum reaction temperature has been reached.
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~L29335~
In a particularly desirable embodimen~ of the process
according to the invention, the concentration of the
sulphuric acid is advantageously between 87 and 91%,
preferably 88 and 90% H2S04 (calculated as solids-free and
salt-free acid) at the start of the digestion reaction and
from 35 to 70% of the required sulphuric acid is recovered
as recycled acid from the process.
The necessary temperatures of the fresh acid and the
recycled acid can be lowered according to the invention
by grinding the titanium slag more finely.
lS The production of the 76 to 87% acid from 60 to 75%
pre-concentrated sulphuric acid is preferably carried out
by evaporation at from 120 to 190C and from 40 to
150 mbar~
Horizontal evaporators with tantalum heat exchangers
or circulation evaporators are particularly suitable for
these evaporation processes not only for reasons of energy
consumption.
Valuation of the process according to the invention
has to consider the fact that secondary energy can be used
ins~ead of primary energy for producing the recycled acid
and that the investment and maintenance costs are sub-
stantially~lower compared with the prior art.
The drawing serves to illustrate the process
according to the invention.
~ The frQsh~acid tl) required for the digestion of raw
material~is mixed in the digestion reactor (3) or in a
preceding mixer with the ground titanium slag ~2)
optionally with addition~of ground ilmenite). The fresh
; acid should have a temperature of from 30 to 80C,
pr~ferably from 50 to 80C~ The necessary quantity of
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recycled acid tl7) is introduced at a temperature of from
80 to 190C, preferably from 100 to 160C, into the
digestion reactor (~) while blowing air ~hrough the
mixture present and the exo~hermal rection is thus
initiated. If steam (4) which is at least 150C hot is
blown in instead of air once the maximum rsaction temper-
ature has beer. attained, thP temperature drop of the
digestion mixture can be reduced and the TiO2 yield
improved. After a maturing time of from 5 to 8 hours,
after which, from experience, no further improvement in
the TiO2 yield can be expected, the digestion cake is
dissolved with water or preferably with sulphuric acid-
containing waste water (5) from the TiOtOH)2 hydrolyzatefiltration process.
The titanyl sulphate solution i5 prepared in known
manner for hydrolysis. During th~ hydrolysis process (6),
sulphuric acid-containing waste water ~5) i5 preferably
used as diluent water. The hydrolyzate (8) is calcined in
known manner to titanium dioxide (10).
The metal sulphate-containing sulphuric acid (waste
acid) (11) produced during fil~ration (7) normally has a
concentration of from 20 to 27% H2504. It can be pre-
evaporated using process heat before being evaporated in
known manner in an evaporator which is preferably a
multistage vacuum evaporator (12) until most of the metal
sulphates are crystalli~ed and a 60 to 75% sulphuric acid
remains (- evaporation 1).
After cooling the suspension supplied from the
evaporation I to from 40 to 70, the metal sulphates (14)
are separated from the sulphuric acid, preferably by
filtration (1~).
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The metal sulphates are partly in the for~ of
hydrogen sulphates and contain 60 to 75% sulphuric acid
as moisture. It is therefore advantageous ~o decompose
this '`filter salt" thermally with formation of S02 and to
produce therefrom the 95 to 98% sulphuric a~id required
as fresh acid. However, reaction with Ca compounds (DE-~
10 3 327 770) or a different harmle6s elimination process is
also possible.
Sulphuric acid losses occur mainly through the filter
salt, but also through the moisture of the solid residues
resulting from raw material digestion, the sulphuric acid
bound in the TiO(OH)2 and the unavoidable waste watar. As
pre-concentra~ed acid, therefore, only about 40 to 60X of
the sulphuric acid u~ed during digestion can be recovered.
With 60 to 75% H2504, however, the concentration of this
- acid is too low to allow auto~hermal slag digestion in
mixture with the necessary amount of from 95 to 98% fresh
acid.
The pre-concen~rated acid (15) which still con~ains
about ~ to 6% by weight of dissolved metal sulphates
~herefore mus~ be evaporated in an evaporation II (16~ to
25 76 to ~7% H2S04 (as salt-free acid), before i~ can be
recycled (17) for the digestion of the raw material. The
evaporation II (16) i6 carried out according to the inven-
tion hy vacuum evaporation at 120 to 190C. Circula~ion
evaporators or horizontal evaporators with tan~alum hea~
exchangers can be used as evaporator ~ystems. Horizontal
evaporators are preferrQd owing to the particularly high
specific evaporation capacity (with respect to the
tantalum heat exchanger surface). Preheating of the pre-
~Z93356
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concentrated sulphuric acid almost to boiling point is
advantageous (at a given evaporation pressure). Steam-
heated graphite heat exchangers are preferred fDr the
preheating operation. Cooling of the recycled acid
supplied from the evapora~or is omitted or is op~ionally
carried out using the pre-concentrated acid introduced as
a cooling liquid, only until the ~emperature of the
recycled acid i5 still sufficient to initiate the di-
gestion reac~ion in ~he manner described above, i.e. until
the tempera~ure lies in the range of from 80 ~o 190C.
A substantial advantage of the process according to
the inventior. lies in the fac~ that problems which arise
during further cooling of this acid are avoided by using
the recycled acid at a temperature of from 80 ~o 190C
because the metal sulphates dissolved in that acid (about
4 to 7% by weight) crystallize in an extremely finely
divided form at lower ~emperatures. As these salts tend
to se~tle on cool surfaces, they lead to frequent inter-
ruptions in the operation. These problems can be avoided
by introducing ~he recycled acid at a temperature at which
the dissolved metal sulphates not yet crystallize.
The process according to the invention, which is
substantially more economical than conventional high
concentration processes according to the prior art
evaporating sulphuric acid to about 96/. H2S04 and
initiating the diges~ion reaction by introduction of
steam into the sulphuric acid-slag mixture will be
described with reference to ~he following non-limiting
Examples.
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Example 1 (Comparison Example)
17.5 t of grDund titanium slag were mixed with 18 t
of metal sulphate-containing recycled acid and with 13.6 t
of fresh 96% sulphuric acid t20C). The digestion reaction
was initiated by addition of 1.4 ~ of water and intro-
duction of 0.6 t of steam. The maximum reaction temper-
ature of 203C was attained after 10 minutes. Af~erinitiation of the reaction, air wa~ blown through the
mixture ~for 30 minutes at 350 m~n/h, then for 7 hours at
20 m3n/h), After a maturing time of 7 hours, the digestion
cake had a temperature of 169C. The Tio2 yield was
95.3%.
The digestion cake was dissolved with a proportion
of the sulphuric acid-containing waste water from the
hydrolyzate filtration (5.4% by weight H2S04), After
working up the titanyl sulphate solution, the mixture was
hydrolysed using a further proportion of the sulphuric
acid-containing waste water as diluent water,
85 t of waste acid containing 23.2% H2S04 and 29 8%
S042-(total) were separated from the hydrolyzate filtra-
tion process. The waste acid was evaporated in a 2-stage
vacuum evaporator until the sulphuric acid (calculated as
~salt-free acid) had a concentration of 66% H2504. 31 t of
5 bar steam were used up for evaporating 47.5 t of H20.
11.3 t of sulphuric acid-containing filter cake were
separated from the metal sulphat2-sulphuric acid suspsn-
sion after cooling to 55C, The pre-concentrated ~cid
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(25.7 t) contained 62.8% of H2S04, 32.4% of H20 and 4.8X.
; of dissolved metal sulphates.
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The pre-concentrated acid was evaporated in a Plinke-
Pauling vessel to 96% H2S04 (calculated as salt-free acid)
at 330C under normal pressure. 1225 m3n of natural gas
(calorific value: 31,380 kJ) were required for this pur-
pose. The 18 ~ of recycled acid were used for digestion
of the slag wi~hout separating the 1.2 t of suspended
metal sulphates.
ExamDle 2
17.5 t of ground titanium slag were mixed with 13.6 t
of fresh 96~/. sulphuric acid (80C). The digestion reaction
was initiated by addition of 20 t of recycled acid (80.6%
of H25O4, 13.4% of H2O, 6.0% of ~eS04 (dissolved), corre-
sponding to 85.7% of H2504 in the salt-free recycled acid~
having a temperature of 142C. As dig2stion commenced, the
- temperature of the solids-free and salt-free sulphuric
acid was 90% as in Example 1. The maximum temperature
attained after about 15 minutes was 207C. Air was blown
in as in Example 1. After a maturing time of 7 hours, the
temperatur2 was 165C and the TiO2 yield was 95.4%.
The remainder of the process was carried out as in
Example 1. The pre-concentrated acid was no~ evaporated
to form ~6% H2S04 in Plinke-Pauling vessels, however, but
wa~ evaporated to 85.7% H2504 (calculated as salt-free
acid) according to the invention in a horizontal evapora-
tor with tantalum heat exchanger and 15 bar steam as
heating agent. The acid was preheated to from 50 to 100C
in a graphite heat exchanger with 1.3 bar steam. Evapora-
tion in the horizontal evaporator was carried ou~ at
60 m~ar, the ~emperature of the issuing recycled acid
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being 160C. The steam consumption for prDducing 20.0 t
of recycled acid was 10 t of 15 bar steam. The 1.3 bar
steam for preheating the pre-concentrated sulphuric acid
was produced by depressurising the 15 bar staam conden-
sat~. In view of the re-use of the lZ0C hot steam
condensa~e for steam production, the energy consumption
for producing the recycled acid from pre-concentrated acid
was 22,700 kJ. 0.6 t of 5 bar steam were also saved when
initiating the digestion react;on. An advantage of ~3% of
the eneray requirement was therefore achieved in compari-
son with the prior art.
ExamDle 3
17.5 t of ground ti~anium slag were mixed with 13.6 t
of fresh 96% sulphuric acid (75C). The digestion reaction
was initiated by addition of 20.76 t of recycled acid
(77.65% of H2S0~, 16.57% of H20, 5.78% of MeS04, corre-
sponding to 82.4X. of H2504 in the salt-free acid) having
a temperature of 140C. Air ~350 m~nlh) was blown through
the reaction composition until the maximum temperature of
192C was attained after 14 minutes. 5 bar steam (180C,
Z5 0.3 t/h for 20 minutes and 20 kg/h for a further 7 hours)
was then blown through the reaction composition. After
stopping the steam, the temperature of the digection cake
was 172C and the TiO2 yisld was 95.2%.
The remainder of the process was carried out as in
Example 1. Evaporation II during which 25.7 t of pre-
; concentrated acid were evaporated to Z0.76 t of recycled
acid was carri~d out at 60 mbar in the same hori7Ontal
evaporator as in Example 2. The pre-concentratad acid
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introduced had a temperature of 89C and ~he discharged
recyclad acid had a temperature of 145C. Only 4.94 t of
H20 had to be evaporated instead of 5.7 t of ~2- The
total energy consumption for the production of ths
recycled acid from pre-co~centrated acid and the blowing
of steam through the digestion cake was 19,460 kJ in
comparison wi~h 22,700 kJ in Example 2.
Example 4
Labora~ory experiments were carried out to examine
how the temperature of the recycled acid can be lowered
without problems arising due to an excessiYely slow rate
of the digestion reaction. 500 g of ~lag ground to varying
finenesses were mixed with 520 g of 96% sulphuric acid and
preheated to the temperature T1. The mixture was then
mixed in an insulated vessel with 590 g of pure 85% sul-
phuric acid having a temperature of T2. The mixture was
stirred while introducing 250 l of air/h and the temper-
ature gradient was recorded. The experimental results are
compiled in Table 1. T3 is the temperature of the mixture
after addition of the 85% sulphuric acid and T4 the
maximum temperature attained. t i5 the time period until
the maximum temperature i~ attained after addition of the
85% acid.
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Table 1
Exp. Fineness of grinding T1 T2 T3 T4 t
No. % by weight (40 ~m ~C] tC] [C~ tC] rmin]
:
1 ~1 50 110 76
2 81 50 140 ~1 106 36
~ 81 70 140 101 154 67
4 81 80 140 109 193 37
81 70 160 117 192 2
6 100 50 1~0 8~ 16~ 42
7 100 50 140 95 185 28
The experimental results show that the ~emperature
of the recycled acid can be lower if the titanium slag is
- ground more finely (Experiment 7) than with a coarser grinding ~Experiment 2~ achieving approximately ~he same
reaction rate. Also the temperature of the 96% fresh acid
can obviously be lowered instead of the temperaturQ of the
recycled acid.
:~ The slower rise in the temperature of the laboratory
~ 25 digestions in comparison with the rise in temperat;ure on
: an industrial scale is due to the relatively high thermal
~ losses.
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