Note: Descriptions are shown in the official language in which they were submitted.
5~757
This invention relates to a process ~or the production of highly
pure, low-sulfur chromium(III)oxide suitable for use as a pigment by reducing
alkali metal chromates with hydrogen at elevated temperatures.
United States Patent Specification No. 3,723,611 relates to a pro-
cess in which finely dispersed alkali metal chromates are reduced in a heated,
hydrogen-containing reaction zone at temperatures in the range of from 900 to
1600 C, reduction optionally being carried out in the presence of a gas which
binds the alkali metal ions to form salts during~reduction of the alkali
chromates, and the chromium(III)oxide formed being separated off in the form
of an alkaline dispersion. The alkali meta] chromates are used either in the
form of an aqueous solution or in the form of solid substances. Chlorine or
hydrogen chloride is preferably used as the salt-forming gas. The products
obtained by this process have a s~lfur content of less than 0.005% and an ig-
nition loss of less than 1%. They show pigment properties. According to
United States Patent Specification No. 3,723~611, heating is carried out by
introducirg and burnirg gases containing hydrogen and oxygen at the upper end
of the reactor. The temperature is adjusted by varying the gas inputs. In
another embodiment, the reactor is heated by introduci~g hot waste gases, for
example from the combustion of natural gas with air outside the reactor~ at
the upper end of the reactor. In either case, the quantity of hydrogen
stoichiometrically necessary for reducing the alkali chromates~ plus an excess
of approximately lO to 20%, has to be present. The dimen~ions of the reactor
are such that, below the reaction zone, there is a residence zone where any
CrOOH formed can react to form Cr203 and the particle size can increase to
from 0.1 to 10 microns.
~hereas the purity of chromium(III)oxide is of particular signifi~
cance in the manufacture of chromium metal, a pure, full color is the prime
requirement where chromium(III)oxide is used as a pigment. At the same time,
ar~ worthwhile pigment must have as high a coloring strength as possible.
:............................................ . . :
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S~57
Although the products according to United States Patent Specification No.
3,723,611 are eminently suitable for thc manufacture of chromium metal, their
pigment properties are not altogether satisfactory insofar as, in terms of
color, they are much more dirty than chromium oxide pigments produced by
conventional processes, for example by reducing alkali chromates with sulfur.
Accordingly, the object of the present invention is to produce
chromium(III)oxide pigments with pigment properties comparable with those of
standard commercial chromium oxide pigmentsg without losing any of the advan- .
tagles of the process disclosed in the aforementioned United States Patent ~`
Specification, namely allow sulfur content of less than 0.005% in the chrom-
ium(III)oxide and continuous operation with high volume-time yields.
The present invention relates to a process for continuously pro-
ducing low-sulfur chromium(III)oxide with improved pigment properties by re-
ducing finely divided alkali metal chromates at temperatures in the range of
about 900 to t600 C, optionally in the presence of a gas which binds the
alkali metal ions to form salts during reduction of the alkali metal chromates,
the reaction zone being followed by a dwell zone, and separating the resulting
chromium(III)oxide in the form of an aqueous dispersion, distinguished by the
fact that temperautres in the range of about 900 to 1600 C are set up in the
dwell zone through additional, direct heating
In the context of the invention, the reaction zone is the zone in
which the alkali metal chromate is reduced to Cr203 or CrO(OH) with the
hydrogen introduced into the reduction zone. The dwell zone is the zone
following the reaction zone in which any CrO(OH) formed is reacted to form
Cr203 and the particle size increases to around 0.1 to 10 ~m.
The total energy input is advantageously divided in such a way that
between about 30% and 80~ is delivered to the reaction zone and between about
70% and 20% to the dwell zone. The temperatures prevailing in the two zones
do not have to be the same~ The temperatu~e~prevailing in the dwell zone is
~ 5~
preferably adjusted by additional, direct heating to the temperature pre~ail-
ing in the reaction zone to such an extent that the temperatures in the two
zones differ from one another by at most about 200 C.
In the absence of additional heating in the dwell zone, the tem-
perature prevailing in that zone quickly falls below 900C as a result of -
losses attributable to radiation and thermal conduction which are particularly
severe in the preferred wet precipitation of the pig~merlt.
All the heat required for the reaction is preferably supplied to
the reaction 7one and dwell zone by the combustion of hydrogen. It is par-
ticularly advantageous in this respect to introduce all the hydrogen at the
upper end of the reactor and the oxygen-containing gases required for its
combustion into the reaction zone and dwell zone in the specified ratios of
about 30 to 80% and 70 to 20%, respectively. In this way, a large excess of
hydrogen is obtained in the reaction zone, favorably affecting chromate reduc-
tion. It is particularly preferred to increase the qu~ntity of oxygen intro-
duced into the dwell zone to such an extent that it is at least sufficient for
complete combustion of the hydrogen. In this way, an inert or oxidizing
atmosphere is created both in the dwell 7one and in the downstream parts of
the apparatus, which also increases safety.
It is possible by the process according to the invention to obtain
a pigment with the required, pure full color and high coloring stren~th, while
at the same time the high yields oE the process accord:ing to United S:tates
Patent Specification No. 3,723,611 remain intact. This factor is particularly
surprising in cases where an oxidizing atmosphere prevails in the dwell zone
on account of an excess of oxygen. It is only when, in an instance such as
this, a salt-forming gas is not added that the conversion falls fairly
drastically. For this reason, it is preferrrd to add a salt-forming gas when
the oxygen is introduced into the dwell zone in a quantity at least sufficient
for complete combustion of the hydrogen. In the process according to the in-
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~ . . -
.
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i7~i7
vention, the sulfur content of thecstarting material, i.e., approximately
0~1% by weight, is reduced in the chromium(III)oxide obtained to less than
0.005% by weight, generally to even less than 0.002% by weight, irrespective
of whether the product is precipitated in the form of an alkaline or acid
suspension. Another advantage is that the chromium(III)oxides obtained by
the process according to the invention have a particularly low alkali content
and a very low ignition loss. Typical values are less than 0.2% by weight of
sodium and an ignition loss of less than 0.4% by weight, preferably less than
0.2% by weight.
Both monochromates and also polychromates may be used as the alkali
metal chromates. It is of advantage to use, for example, Na2Cr207O 2 H20,
Na2Cr207, Na2Cr04, ~2Cr207 or ~2Cr0~. The starting materials should have a
..
partical size in the range of about 10 to 500 pm.
It has proved to be advantageous to carry out reduction in the
presence of a gas which reacts to form salts with the alkali metal hydroxides
formed during reduction of the alkali metal chromates. In this way, the yield
of the process, based on the alkali metal chromate used, can be considerably
increased without affecting the low sulfur content in the chromium(III)oxide
obtained. Whereas the conversions obtained in the process according to the
aforementioned United States Patent Specification amount to just about 90% in
the absence of a salt-forming gas and to more than 95% in the presence of such
a gas, the conversions obtained in the process according to the invention in
the absence of a salt-forming gas are to a greater or lesser extent below the
comparable values of the known process, depending on whether an oxidi7ing or
reducing atmosphere prevails in the dwell zone, but can be brought up to their
level by the addition of a salt-forming gas.
It has also proved to be advantageous to add to the solid alkali
metal chromate a smalllq~antity of finely divided silicon dioxide of the kind
formed~ for example, in the vapor-phase hydrolysis of silicon tetrachloride,
-- 4 --
5~5i7
beEore it is introduced into the reaction zone. The finely di~ided silicon
dioxide is preferably added in quantities of from about 0.1 to 2% by weight,
based on the alkali metal chromate. This ends up in the product in about 0.2
to 3.5% by weight.
In one preferred embodiment, the process is carried out as follows:
the alkali metal chromate is introduced into the reactor by means of a gas,
pneumatically or in the form of a solution. The reaction zone of the reactor
is directly heated, the temperature being regulated through the input of
heating gases.
It is best to use an auxiliary gas, for example nitrogen, for
pneumatic delivery of the starting material into the reactor. However, it is
also possible to use the hydrogen required for reduction and/or heating for
this purpose. Instead of pneumatically introducing an alkali metal chromate
with a certain fineness, it is also possible to obtain uniform distribution of
the alkali chromate over the cross-section of the reactor by starting with an
aqueous solution of the corresponding alkali metal chromate and dispersing
it through nozzles by means of a gas in such a way that the droplets formed
are less than about 100 microns and, with advantage, between about 10 and 30
microns in si~e. In this case, the solution has a concentration of about ~0
to 85% by weight.
Heating is carried out by burning hydrogen with gases containing
oxygen, the temperature being regulated by varying the gas inputs. However,
the quantity of hydrogen introduced must be such that at least the quantity
stoichiometrically necessary for reduction is present in the reaction zone.
The reactor can also be heated by hot waste gases emanating from combustion
outside the reactor. These waste gases, of the type formed for example during
the combustion of natural gas with air, are then introduced into the reactor.
The excess of air required for complete combustion of the combustible gas
outside the reactor is then burnt with hydrogen in the reactor itself.
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~S757
The quantity of oxygen introduced into the dwell zone must be at
least sufficient to maintain the required temperature in the dwell zone. It
is best to use a slight excess of oxygen in relation to the hydrogen still
present in the dwell zone.
In cases where the reactor is heated by the combustion of hydrogen,
it is best to burn;only a portion of the hydrogen in the reaction zone.
Favorable conditions prevail when a proportion of about 30 to 80% of the
hydrogen is burnt in the upper part of the reactor. The rest of the hydrogen
is then available for combustion in the dwell zone~
In tests carried out with a view to improving pigment properties,
it proved to be inadequate to carry out combustion of the hydrogen in two
separate zones as long as an excess of hydrogen was maintained in the after-
combustion zone as well and no salt-forming gas was added. It was only by
adding a salt-forming gas, in conjunction with after-combustion being carried
out under reducing conditions in the dwell zone, that it was possible to ob-
tain the required improvements in the pigment properties. However, if oxygen
is introduced into the dwell zone in such a quantity that the hydrogen is
completely burnt, the improved pigments are obtained without any need to add
a salt-forming gas. However, it is preferred even in this case to add a
salt-forming gas.
The quantity in which this additional gas is used is best such that
the quantity stoichiometrically necessary for binding the alkali hydroxides
liberated during reduction is added. However, an excess, for example of 100~
beyond that quantity, does not have any harmful effects. ~xamples of suitable
salt-forming gases include chlorine, hydrogen chloride, bromine, hydrogen
bromide, nitrogen monoxide, nitrogen dioxide or carbon dioxide. Chlorine is
preferably used. The salt-forming gas when added can be present in up to
about twice the molar amount required for salt formation with the alkali
metal atoms of the alkali metal chromate, or even higher.
-- 6 --
~45i7~i7
This gaseous salt former can be added to the reactor in any way,
for example by previous admixture with the combustion air and/or with the
combustion oxygen and/or with the hydrogen and/or with the gas used for
pneumatic introduction of the alkali metal chromate. However, the gaseous
salt former can also be introduced into the reactor through nozzles provided
specifically for that purpose.
Whereas, in cases where reduction is carried out in the absence of
a salt-forming gas, the precipitation liquid becomes alkaline, the addition
of a salt-forming gas in a ~uantity greater than the stoichiometrically
necessary quantity can produce an acid reaction in the precipitation liquid.
This is the case, for example, in the reduction of Na2Cr207 with chlorine as
salt-forming gas:
2 2 7 2 - ~ Cr203 + 2 NaOH ~ 2 H20 ~l)
Na2Cr207 + 4 H2 + C12 >Cr23 -~ 2 NaCl ~~ 4 H20 ~ (2)
H2 ~~ Cl2 - ~ 2 HCL
In contrast with the process according to United States Patent
Specificakion No. 3,723,611, the pH-value of the suspension does not have to
be kept alkaline in the process according to the invention, because chromium
(III)oxide with a very low sulfur content is also obtained from acid disper-
sion. In addition, the required pH-value can also be adjusted by adding acid
or alkali in appropriate quantities to the pl~lp-recirculated suspension.
The dimensions of the reactor should be such that, below the
reaction zone, there is a reducing or oxidizing dwell zone in which the CrO(OH)
partly formed during reduction is completely dehydrated to for~ Cr203, and
the particle size grows to around 0.1 to 10/um. The rate of gas flow in the
reactor is best selected in such a way that the particles have an average
residence time of about 0.1 to 10 seconds, advantageously about 0.4 to 4
seconds, approximately evenl~ divided between the reaction and dwell zones.
The Cr203 formed can be separated in any way~ for example in
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~45757
cyclones. Below the reactor there is preferably a unit in which most of the
Cr203 formed is wet-precipitated from the waste gas stream and, at the same
time~ the waste gas is brought to a temperature below 100 C. The dispersion
of Cr203 in water initially formed may be used as the precipitation and
cooling liquid. This dispersion can be continuously pump-recirculated through
a condenser and returned to the separation and cooling unit. It has proved
to be particularly advantageous to produce a film of liquid by spraying liquid
or dispersion onto the wall of the cooling unit, as a result of which the
temperature of the gas/solids mixture is reduced below 100 C and most of the
solids present in the aqueous dispèrsion are precipitated.
The residual Cr203 left in the waste gas may be separated in a
following washing tower. The concentrated dispersion is continuously pumped
off to recover chromium oxide and replaced by corresponding quantities of
fresh water.
The pigment properties of the chromium(III)oxide produced in
accordance with the invention can be further improved by adding a sm~ll
quantity of finely divided silicon dioxide, of the kind obtained for example
by the flame hydrolysis of silicon tetrachloride, to the alkali metal chro-
mate. Additions of as little as 0.1% by weight are positively reflected
both in coloring strength and in color, the effect of the silicon dioxide
becoming less positive when it is added in quantities of greater than 2% by
weight. The silicon dioxide is preferably added in a quantity of about 0.25
to 1% by weight. Apart from improving pigment properties, the addition of
silicon dioxide during the dosage of solid alkali metal chromates consider-
ably improves the free-flow properties of the mixture, thus facilitating
dosage.
One particular embodiment of the method according to the invention
is described by way of example in the accompanyirlg drawing wherein the figure
is a schematic sectional view through an apparatus for carrying out the
~ s~
process.
In the drawing the reference 1 denotes a steel cylinder~ the ref-
erence 2 denotes an insulating layer, the reference 3 denotes a heat-resistant
ceramic lining, the reference 4 denotes a reaction zone, the reference 5 de-
notes a dwell zone, the reference 6 denotes a cover with a cooling system, the
reference 7 denotes an inlet pipe, the reference 8 denotes a burner, the ref-
erence 9 denotes a conically tapering pipe, the reference 10 denotes a welded-
on pipe~ the reference 11 denotes a coverplate~ the reference 12 denotes
nozzles, the reference 13 denotes a container, the reference 14 denotes a
vent and the reference 15 denotes inlet pipes.
More particularly, the method according to the invention is car-
ried out as follows:
The actual reduction stage takes place in the reaction zone 4. The
reaction zone 4 is surrounded by a steel cylinder 1 comprising a layer 2 of
insulating material and a ceramic lining 3. At its upper end, the reactor is
closed by a cover, preferably made of fine steel, with a water cooling system
6. The pipe 7 extends centrally through the cover for introducing the alkali
metal chromate. Individual burners 8 inclined relative to the vertical are
arranged coaxially around this inlet pipe 7. ~elow the reactor there is a
separation and cooling unit 9 which is conical in shape, tapering in the down-
ward direction. Liquid or dispersion is pumped in through one or more tan-
gentially arranged pipes lO. An encircling cover plate 11 prevents liquid
from spraying onto the ceramic wall o-f the reactor. In the lower cyclindrical
section, individual noz~les 12 through which the liquid or dispersion can be
sprayed are built in around the peripheryO Waste gas and liquid or dispersion
then enter the container 13 from which the waste gas is passed through the
vent 14 into the washing tower.
The main difference in relation to United States Patent Specifica-
tion No. 3,723,611 lies in the presence of a number of inlet pipes 15, for
.
example for the introduction of oxygen-containing gas, such as air, oxygen-
enriched air or even pure oxygen, allowing for example the post-combustion of
some of the excess hydrogen from the upper part of the reactor. The position
at which the inlet pipes 15 are arranged is best such that the additional,
direct heating takes place after the hottest location in the upper zone, for
example in the vicinity of or slightly below the middle of the reactor. By
virtue of the fact that the excess hydrogen is burnt in the dwell zone 5, it
is possible to regulate temperature to any required level in the dwell ~one
over a much longer section of the reactor.
The process according to the invention is illustra-ted in the fol-
lowing Examples. The tests to which these Examples relate were carried out
in the apparatus already described with reference to the drawing.
E~AMPLES 1 ~o_Ll
A steel cylinder (external diameter L50 mm, length L100 mm) was
used as the reactor. IInternally it contained an insulating layer of A1203-
tamping compound (internal diameter 80 mm). The burner consisted of three
coaxial quartz tubes. Through the inner tube, sodium dichromate was uniformly
distributed over the cross-section by means of hydrogen and, in addition,
nitrogen. The rest of the hydrogen was introduced through the middle tube,
while oxygen and/or air was introduced through the outer tube. Chlorine was
used as the salt-forming gas. The chlorine was then passed through the outer
tube together with oxygen and/or air. The burner was fixed to the reaction
tube with a water-cooled plug of VA-steel. 600 mm below the burner orifice
there were 4 inlet pipes, staggered at 90 intervals, for post combustion.
The reactor was joined at its lower end to a quartz tube around whose center
were arranged 8 inlets through which aqueous suspension~ which had been cooled
beforehand in a condenser coil, was sprayed in for precipitating the Cr203
formed and for quenching the hot reaction gases. The temperature of the sus-
pension was thus kept at about 50 to 60 C. Suspension and waste gas then
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457~i~
flowed through a fine steel funnel into the glass washing section (diameter
100 mm). The suspension which had become concentrated during the test was
able to flow out through a lower outlet into the storage container. The waste
gases were passed through a glass packing, onto which the suspension was
sprayed from above, and then discharged through a chimney.
Table 1 shows the reaction conditions in the tests carried out,
while Table 2 showsithe~results of those tests. Table 3 shows the tempera-
tures in the reaction and dwelling zones respectively and the SiO2-content
of the resulting products. For comparison, Examples 1 to 3 relate to tests
conducted without post-comb~stion by the process disclosed in United States
Patent Specification No. 3,723,611.
The operand
moles of H2 ~ (3 . moles of Na2Cr2C7) - moles of C12
w ---- _
2 . moles of 2
is a measure of the performance of the reaction in relation to reducing or
oxidizing conditions in the dwell ~one. Where w>17 there is an excess of
hydrogen, while where w <1 there is an excess of oxygen, based on the total
input of oxidizing and reducing agent into the reactor.
Na2Cr207 . 2 H20 was used in Example 11, while Na2Cr207 optionally
admixed homogeneously with the specified quantity of finely divided silicon
dioxide was used in the other examples. For working up~ the suspensiorl was
filtered off, the filter cake resuspended in water, filtered and washed. The
conversion was determined by titrating the unreacted qua~ntity of Cr(VI) in the
filtrate of the chromium(III)oxide suspension. The filter cake was dried for
16 hours at 120 C. The color was assessed by comparing lacquer coatings. A
Standard commercial chromium(III)oxide pigment was used as a comparason stand-
ard. Coloring strength was determined against the s~ame standard in accordance
with DIN 53 234.
,
~l~4S~57
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-- 12 --
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31~4L57S7
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5:~C~ ~ bO ~ s:l ~ 3 ,5:1
a~ ~ c~ P, h ~ a~
~1 ~ P ~ P
~q bD ~ ~ ~bO bO
a) o h h h bO
r~ O ~ ~1 ,h~
r-l rl rl C~ rl ~1 ~1 ~ ~1 0 bn bO
C~ bO ~ h
bO h C~ O ~ ~ ~d1--l
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-
~3~i757
Table 3:
Ex. Temperature Temperature SiO2-content of
No. Reaction zone Dwell zone the product
_ . _ _ _ _
1380C 870C o %
2 1290C 850C 1,0 %
3 1040C 740C o,g %
4 1380 C 1230l,C o %
1220C 1150C 1,1 %
6 1360C 1400C o %
10 7 1390C 1420(C 1,5 %
1250C 1180C o,g %
9 1350C 1430C o,g %
1360C 1360C 0,9 %
11 1420C 1~60C 1,1 %
It will be appreciated that the instant specification and examples
are set forth by way of illustration and not limitation, and that various
modifications and changes may be made without departing from the spirit and
scope of the present invention.
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