Note: Descriptions are shown in the official language in which they were submitted.
~ ~7~3~
ML-l 84 8A
PROCES~ FOR M~NUFACl?tlRING TITANItJM COMPOUNDS
The present invention relates to the manufacture
of titanium compounds and particularly titanium dioxide
pigmentary material. More particularly, the present inven-
tion is directed to a novel process for reacting titani-
ferous bearing material with dilute sulfuric acid for
preparing salt solutions of titanium that may be hydrolyzed
to provide titanium dioxide pigment.
Titanium dioxide is a well known material having
desirable pigment properties which is useful in paint and
coating compositions and in plastic materials. Several
different processes are known for manufacturing titanium
dioxide material including, for example, the sulphate
process and the chloride process. The present invention
concerns the manufacture of titanium compounds and particu-
larly titanium dioxide by the sulphate process.
In the usual sulphate process for the manufacture
of titanium compounds, titaniferous bearing material
such as ilmenite ore which includes massive ilmenite~
ilmenite sands and titaniferous or furnace slag are
.~
~ :~57~3~
~2--
reacted with concentrated sulfuric acid (e.g., 90~-96%
sulfuric acid). The reaction is sometimes referred to as
"digestion" or "ore digestion." The digestion reaction of
the titaniferous material and concentrated sulfuric acid is
exothermic in nature and proceeds very violently. Typically,
10 the titaniferous material and the concentrated sulfuric acid
are placed in a reaction vessel called a digestion tank.
Water is usually added to the digestion tank to initiate and
accelerate the acid-ore reaction because of the heat of
dilution of the acid which results in a vigorous boiling
action of the water-acid solution at about 100C to about
190C and the release of vast quantities of steam and
vapor having entrained particulate material. As the
violent reaction proceeds, water is expelled and the reaction
mass becomes solid; the reaction is completed in the solid
20 phase at a temperature of approximately 180C. The solid
reaction mass, referred to as a "cake," is allowed to cool.
Thereafter, the solid cake is dissolved with water or dilute
acid to provide a solution of sulphate salts of iron r
titanium and other trace metals present in the titaniferous
25 material. The digestion operation is a batch procedure
carried out in a single digestion tank. As many digestion
tanks are used as necessary according to the desired capacity
of the manufacturing plant to prepare a titanium sulfate
solution.
After digestion, the resulting sulphate salt
solution (containing iron and titanium salts) is further
processed by known measures to remove the ferrous sulphate,
usually referred to as "copperas," to provide a solution of
titanyl sulphate which, upon hydrolysis r yields hydrated
35 titanium dioxide. The titanium dioxide hydrate is usually
subjected to a calcination treatment in a suitable kiln
device to remove the water of hydration and to provide the
anhydrous titanium dioxide pigment. The foregoing process
is described in greater detail in, for e~ample, U.S. Patent
Nos. 1~504r672; 3r615r204 and 3~071r439
i ~7~
--3--
The sulphate process for the manufacture of
titanium compounds described hereinabove has several environ-
mental drawbacks. For example, the violent reaction thatoccurs in the digestion ~ank results in undesirable emission
problems. Also, solutions of dilu~e sulfuric acid, usually
termed n spent acid," that result from the removal of coppera5
and the hydrolysis of the titanyl sulphate present severe
10 disposal problems because large quantities of such spent
acid cannot be recycled to the diges~ion tank which utilizes
concentrated sulfuric acid or reclaimed on an economic
basis.
Accordingly, the present invention provides a
lS novel sulphate process for manufacturing titanium compounds
that substantially avoids or eliminates the drawbacks
mentioned hereinabove resulting from the conventional
sulphate process. As used herein, the term titanium sulphate
is used collectively to mean sulphate salts of titanium,
such as titanyl sulphate and titanous sulphate.
According to the present invention, there is
provided a process for preparing titanium compounds which
comprises: reacting
a) a titaniferous bearing material in an amount
between about 10~ and 4no~ above the stoichiometric
amount of said material necessary to react with
sulfuric acid ~o provide titanium sulphate,
and
b) a dilute sulfuric acid solution having a
concentration between about 25% and about 60% by
weight, based upon the total weight of said
solution,
at a temperature below about 140C, and thereafter cooling
the resulting reaction mixture to a temperature below
about 110C withoùt precipitating the reaction products to
produce a reaction mixture containing titanium sulfate, and
separating undissolved solids to produce a titanium sulfate
solution.
In a preferred embodiment, the process of the
present invention comprises: (1) reacting
l 157~3~
a) a titaniferous bearing material in an amount
between about 10% and about 400% above the stoi-
chiometric amount of said material necessary to
react with sulfuric acid to provide titanium
sulphate, and
b) a dilute sulfuric acid solution having a
concentration between about 2~% and about 60~ by
weight, based upon the total weight of said
solut iOII,
at a temperature below about 140C; (2) cooling the
15 resultina reaction mixture to a temperature below about
llCC, without precipitating the reaction products; (3)
removina undissolved solids and iron sulfate from said
reaction mixture to provide a titanium sulphate solution;
(4) hydrolyzing said titanium sulphate solution to provide a
~ hydrate of titanium dioxide; (5) calcining said hydrate of
titanium dioxide to provide titanium dioxide and (6) recover-
ing the titanium dioxide.
In still another embodiment, the present invention
provides a continuous process for production of titanium
25 dioxide which comprises:
(1) continuously reacting (a) a titaniferous
bearing material in an amount between about 10%
and about 400% above the stoichiometric amount of
said material necessary to react with sulfuric
3~ acid to provide titanium sulphate, and (b) a
dilute sulfuric acid solution having a concentra-
tion between about 25% and about 60% by weight,
based upon the total weight of said solution, in a
first reaction vessel at a temperature below about
140C
t2) cooling the resulting reaction mixture to a
temperature below about 110C in a second
reaction vessel without precipitating the reaction
products
.
.
J ~ 3 ~
~3) separating unreacted titaniferous bearing
material from the reaction mixture to provide a
solution of iron sulphate and titanium sulphate;
(4) removing iron sulfate from said solution of
iron sulfate and titanium sulfate to provide a
solution of titanium sulphate;
(5) hydrolyzing said solution of titanium sulphate
to provide a titanium ~ioxide hydrate, and spent
sulfuric acid solution;
~fi) calcining said titanium dioxide hydrate to
provide titanium dioxide and
~7~ recovering titanium dioxide.
The attached figure depicts one aspect of the
present invention using a continuous process scheme for
preparing titanium dioxide.
The salient features of the inventive process
reside in the discovery that a titaniferous bearing material
may be reacted in a completely liquid phase without the need
for a separate reduction step with dilute sulfuric acid to
provide a stable hydrolyzable titanium sulphate solution
25 which may be used for making titanium compounds and titanium
dioxide pigments.
The digestion reaction is conducted with a
titaniferous bearing material. As used herein, the term
titaniferous bearing material means a material containing
30 recoverable titanium values when treated according to the
process of the invention. Exemplary materials include
titaniferous slag, furnace slag, ilmenite ores such as
magnetic ilmenite and massive ilmenite and ilmenite sands.
The digestion reaction is conducted with a suf-
35 ficient amount of the titaniferous bearing material toprovide an excess of said material in an amount between
about 10% and about 400% above the stoichiometric amount.
This amount may also be represented as being l.l to 5 ~imes
the stoichiometric amount. The following formula depicts
40 the stoichiometry of the digestion reaction:
3 i~
--6--
FeTiO3 + 2H2S04 ~ TiOSO4 + FeS04 t 2H20
The use of excess titaniferous bearing material in
the digestion reaction is effective and desirable for
achieving a successful and workable process according to the
present invention without the need for excessive grinding of
the ore. The titaniferous bearing material preerably has a
surface area ranging between about 0.05 m2/cc to about 0.6
m2/cc. Ore having a higher surface area could be used but
provides no advantage because of increased grinding costs.
As indicated hereinabove, an excess of titaniferous bearing
material between about 10% and about 400% above the stoichio-
metric amount necessary for reacting with sulfuric acid
should be employed in the digestion reaction of the process.
The use of lesser amounts of said mixture results in unac-
ceptably low reaction rates and long processing times sothat the process becomes economically unattractive. Using
amounts of excess material higher than recommended is
undesirable due to greatly reduced fluidity of the reaction
mixture and the need to recycle large quantities of unreacted
titaniferous bearing material to the digestion reactors.
It has been unexpectedly observed, for example, that doubling
the amount of titaniferous bearing material such as MacIntyre
ore above the stoichiometric amount for reacting with dilute
sulfuric acid increases the rate of reaction in the order of
at least 10 times in the last digestor. It should be
recognized ~hat reaction rates will vary with the source of
titaniferou~ material employed during digestion,
The sulfuric acid utilized in the process of the
invention should have a concentration of between about 25%
and about 60% by weight, based upon the tokal weight of the
acid solution. An acid concentration below about 25% by
weight is not desirable because hydrolysi~ of the titanium
dio~ide occurs during and in conjunction with the digestion
reaction when using such acids~ Premature hydrolysis of
~ ~ 57~3~3
--7--
titanium salt solutions precludes the formation of pigment
grade titanium dio~ide at a later stage of the process.
Also, utilizing an acid having a concentration greater than
about 60% by weight is not desirable because (1) the result-
ing reaction solution is more viscous and difficult to
10 handle, ~2~ the economics of recycling spent acid are not
reali~ed unless the spent acid is concentrated, which
unnecessarily increases the cost of operation, and (3) the
higher concentration of reaction products in solution
promotes the precipitation of ferrous sulphate monohydrate
and recoverable titanyl sulfate dihydrate. The presence of
the ferrous sulphate monohydrate makes gravity separation
ineffective and is difficult to remove by filtration.
The process operating conditions for conducting
the digestion reaction may readily be adjusted, depending
20 upon the concentration of the dilute sulfuric acid and the
specific amount of excess titaniferous bearing material that
is employed, to provide optimum process operation. To
illustrate, utilizing dilute sulfuric acid of low concentra-
tion, e.g., below 40% by weight, initially requires operating
25 the process at a lower temperature of the preferred tempera-
ture range because of the lower boiling point of the dilute
sulfuric acid. It is desirable to increase the amount of
titaniferous bearing material employed so as to digest as
much material as possible in the first digestor reactor at
3~ which point the operating temperature and reaction rate are
usually higher. As noted hereinbelow, the temperature in
subsequent digestor reactors is maintained at a level lower
than the first digestor reactor and, ultimately, must be
reduced to preclude or avoid premature hydrolysis of the
35 titanium salt solution.
The temperature at which the digestion reaction
occurs is below about 140C and preferably between about
55C and the boiling point of the reaction solution, i.e.,
between about 55C and about 140C. Selecting a tempera-
40 ture that is too low in a digestion reaction should be
I ~ 5~3'J
avoided because the digestion reaction will proceed tooslowly and thus require increased residence time o~ the
reactants in the digestion reactor. Also, increased
residence times should be avoided to preclude the risk of
undesirable nuclei formation in the reaction solution due to
10 premature hydrolyzation of the titanium salt~ Selecting a
temperature above 140C is not recommended because the
titanium salt hydrolyzes at much faster rates at higher
temperatures. Operating the digestion reaction below about
55C should be avoided because the reaction products begin
15 to precipitate from solution and the viscosity of the
reaction mixture increases, making removal of unreacted
solids very difficult. A preferred operating temperature
for conducting the digestion reaction is between about
70 C and 110C. It should be noted that the digestion
20 reaction of the process of the present invention may be
accomplished as a batch reaction, e.g., in a reaction vessel
~rom which the reaction mixture, after the digestion reaction
has proceeded to a desired extent, is withdrawn and processed
further in other vessels. A preferred embodiment of the
25 process of the invention is where the digestion reaction is
performed continuously in at least two reaction vessels and
wherein the titaniferous bearing material and the dilute
sulfuric acid are made to flow concurrently.
When conducted in a continuous manner, the process
30 is preferably performed using two or more aigestor reactors.
The total number of digestors being dependent upon the ease
of reaction control, plant output and process handling.
The preferred operating temperatures or conducting
the digestion reaction in two digestor reactors or stages
3~ are wherein the first digestor is maintained below about
140C preferably below about 110C and the second
digestor is maintained below about 100C, preferably below
about 75C.
~ ~5~3~
The preferred operating temperatures for conducting
the digestion reaction in three digestor reactors or sta~es,
are wherein the first digestor is maintained below about
140C preferably below about 11~C, the second digestor
is below about llOQC preferably below about 100C and
10 the third digestor is maintained below about 80C preferably
below about 75C.
The preferred Qperating temperatures for conducting
the digestion reaction in four digestor reactors or stages
are wherein the first digestor is maintained below about
15 1~0C preferably below about 110C, the second digestor
is maintained below about 11~C preferably below about
90C, the third digestor is maintained below about
10~C, preferably below about 86C and the fourth
digestor is maintained below about g0C preferably below
20 about 75C.
The preferred operating temperatures for conducting
the digestion reactor in five digestor reactors or stages
are wherein the first digestor is maintained belo~ about
140C preferably below about 110C, the second digestor
25 is maintained below about 110C, preferably below about
90C, the third digestor is maintained below about
100C preferably below about 85C, the fourth digestor
is maintained below about 90C, preferably below about
80C and the fifth digestor is maintained below about
85C, preferably below about 75C.
~ 11 of the foregoing digestor temperatures may be
varied depending upon the desired yield and reaction times
present in each stage. One of the essential and salient
features of the invention in providing an operable process
is that the temperature of the digestion reaction is decreased
as the reaction progresses to preclude or avoid premature
hydrolysis of the resulting titanium salt solutions~
- Premature hydrolysis of the titanium salt solution precludes
the formation of pigment grade or quality titanium dio~ide.
~ ~ 5V~3~3
--10--
The duration of the digestion reaction in a
digestor is controlled by the optimum degree of conver-
sion or digestion of the titaniferous bearing material at
that stage. Generally speaking~ it is preferred to digest
or react as much of the titaniferous bearing material a~ is
possible in the first diqestor reactor or stage where the
10 temperature is maintained at the highest level to preclude
hydrolysis of the titanium sulfate in solution~ For example,
in a continuous multiple stage system employing MacIntyre
ore as the source of titaniferous bearing material it is
so~etimes possible to digest in the first stage up to about
15 90% by weight of the stoichiometric amount of the ore
charged to the process, excluding the excess ore. Preferably,
between about 30% and 80~, and most preerably between about
6n% and 80% by weight of the stoichiometric amount of the
ore is digested in the first stage, excluding the excess
20 ore. Conversion is measured by the amount of reaction
completed based on the stoichiometric quantity of titani-
ferous bearing material employed.
Temperature is used to control the digestion reaction
preferably by monitoring the ratio of active acid to titanium
25 in the reaction solution. This ratio is an indication of
the degree of conversion or digestion. The term `'active
acid" means the total quantity of free acid in the reaction
solution plus the acid combined with the titanium in the
reaction solution. The ratio of active acid to titanium
dioxide (active acid:titanium dio~ide) is calculated as the
sum of both the free acid in solution plus the acid combined
with the titanium in solution divided by the titanium in
solution (calculated as TiO2). For example, the active
acid content of a solution may be determined by titration of
a selected sample (by weighing or pipeting techniques) with
a 0.5N caustic solution (NaOH) to a pH of 4~0 in a barium
chloride/ammonium chloride buffered solution. ~he titration
yields the content of free acid plus the acid combined with
the TiO~ which is referred to as active acid. To illu_
strate, ~0 mls of buffer solution containing 75 g/l of
'J
barium chloride and 250 g/l of ammonium chloride is added to
the beaker containing the related sample and diluted with
water to 250 mls and titrated with 0.5N caustic to the
methyl orange end-point.
In a batch process, the active acid content can
vary widely and is not critical except to the extent that
digestion and reduction occur in a liquid phase. In a
continuous process, the active acid ratio is permitted to
drop from infinity at thecommencement of the reaction to
between 1.50 and 7.0 a~ the completion of the reaction
dependent upon digestion conditions. Typically, the active
acid to TiO2 level varies between 2.0 and 3.5. As the
active acid level decreases, the stability of the titanyl
sulfate solution to the hydrolysis decreases. Generally,
the temperature of the reaction solution should be maintained
below about 140C and preferably below about 110C as
the ratio of active acid to titanium (calculated as titanium
20 dioxide) falls to about 2Ø To illustrate in a two-stage
digestion process, the temperature of the reaction solution
in the first stage or digestor of the digestion reaction
should be maintained at a temperature below about 140C,
e.g. r 110C, until the ratio of active acid to titanium
25 dioxide of the reaction solution falls to about 3.0, at
which time the temperature of the reaction solution is
reduced to below about 100C, e.g., 70C. In contrast,
in a three stage digestion process, wherein the temperature
of the first stage is maintained at about 110C to provide
3~ a reaction mixture having a ratio of active acid to titanium
dioxide in the reaction solution in the range of between
about 2.5 and about 3.0, and thereafter the reaction is
conducted in a second stage at a temperature of about
100C to provide a reaction mixture having a ratio of
35 active acid to titanium dioxide in the reaction solution in
the range between about 2.2 and about 2.5. The reaction can
then be completed in a third stage at a temperature below
about 80C to provide a reaction mixture having a ratio of
active acid to titanium dioxide in the reaction solution of
40 about 2.0-
3 ~ 3 ~
-12-
Upon completion of the digestion reaction, the
resulting reaction mixture containing titanium s~lfate, iron
sulfate and trace element from the titaniferous bearing
material may be treated to recover a titanium sulfate
solution to prepare titanium compounds or processed according
to conventional sulfate processing techniques to prepare
10 titanium dioxide pigment.
Referring to the diagram depicted in the ac-
companying Figure for prepariny titanium dioxide in a
multistage reactor system, reference numeral 10 represents a
digestor reactor. Titaniferous bearing material such as
ilmeni~e ore is adapted to be fed into digestor reactor
10 from ore storage bin 11. Dilute sulfuric acid having a
concentration between about 25% and about 60% by weight,
based upon the total weight of the acid solution, is adapted
to be fed ei~her from a mixture of strong acid (96% by
20 weight) from a source 12 of fresh acid, combined with
recycled acid (15% to 45% by weight~ or water directly to
digestor reactor 10. The ilmenite ore and dilute sulfuric
acid in digestor reactor 10 are agitated continuously at a
temperature up to the boiling point of the reaction solution
25 in the reactor.
The reactants in digestor reactor 10 are maintained
at a temperature below about 140C and preferably between
about 55C and about 140C. More specifically, the
reactants in digestor reactor 10 are preferably maintained
30 at 110C. Digestor reactor 10 may be maintained at any
convenient pressure; atmospheric pressure is preferred for
reasons of economy.
When operated continuously in the depicted three
stage digestion system, the reaction mixture is transported
35 from digestor reactor 10 to a conventional separator device
13, e.g., a filter or cyclone ~eparator, in which a portion
or all of the unreacted ilmenite ore is separated and
recycled by way of recycle conduit 14 to digestor reactor
10. Alternatively, the reaction mixture may be continuously
40 transported from digestor reactor 10 to digestor reactor 15
~ :~5723~
-13-
unaccompanied by recycling the unreacted ilmenite ore to
digestor reactor 10.
The reaction solution in digestor reactor 15
is preferably maintained at a temperature somewhat lower
than the temperature in digestor reactor 10. For example,
the reaction mixture in digestor reactor 15 is maintained at
about 100C. Control of the temperature in digestor
reactor 15 may be achieved by the addition of recycled acid
or water. The pres~ure in digestor reactor 15 is preferably
atmospheric, but higher pressures may be utilized if desired.
The reaction mixture may be continuously transported
from digestor reactor 15 to a conventional separator device
16, e.g., filter or a cyclone separator, in which a portion
or all of the unreacted ilmenite ore is separated and
recycled by way of recycle conduit 17 to digestor reactor
15. Alternatively, the reaction mixture may be continuously
transported from digestor reactor 15 to digestor reactor 18
unaccompanied by the recycling of unreacte ilmenite ore to
digestor reactor 15.
The reaction mixture in digestor reactor 18 is
preferably maintained at about 70C and atmospheric
preSsure-
The reaction mixture from digestor reactor 18is continuously fed to a suitable separator device 19, e.g.,
a filter or gravity separator (or multiples thereof in
series and/or parallel flow arrangement), in which the
unreacted ilmenite ore is separate~ from the liquid reaction
product. The excess or unreacted ilmenite is recycled by
way of conduits _ and 21' to either or both digestor
reactor 18 and/or digestor reactor 10. The liquid reaction
product from separator device 19 is conveyed to settler
device 20, e.g. r a conventional settler device or a LAMELLA~
device, in which gangue or other undesirable solid material
is removed from the reaction product. The use of a LAMELLA~
settling device is not considered a part of this invention.
1 ~$'^~3~)
- -14-
As indicated above, the digestion reaction
is conducted in digestor reactors 10, 15 and 18. It is not
essential that the digestion reaction be conducted in three
digestor reactors. In fact, the process may be conducted
batch-wise using only one digestor reactor. ~owever, it is
preferred to use ~wo or more digestor tanks in order to
10 practice the process in a continuous manner. When utilizing
only two digestor reactors, the temperature of the second
digestor reactor, such as digestor reactor lS, may be
adjusted to a lower temperature as, for example, 70C.
Also, each digestor reactor should be equipped
15 with suitable agitation means, indicated by reference
numeral 9 in the accompanying flow diagram, in order to
maintain the reactants and the reaction solution well
agitatedc
A suitable reductant, for example, iron or titanous
20 sulphate, from container 22 may be added to digestor reactor
10 or digestor reactor 15, or to both reactors, for the
purpose of reducing trivalent ferric iron in the digestion
solution to divalent ferrous iron . The presence of a
reduced condition precludes contamination of later obtained
25 titanium hydra~e with ferric salts. The amount of reductant
added to the reaction solution in the digestors is dependent
upon the amount of ferric iron in the ilmenite feed ore.
Generally speaking, between about 3% and about 8% by weight,
based upon the total weight of ilmenite ore reacted, of
reductant is adequate to provide satisfactory results in a
process operation using an ilmenite ore that contains
5% to 13% ferric iron. The addition of a reductant such as
iron has another beneficial effect in that it accelerates
the rate of the digestion reaction. Thus, it is possible by
this measure to avoid a separate reduction stage for the
digestion solution as would otherwise be necessary. The
reductant may be added at any point in the digestion opera-
tion. The quantity of reductant used is chosen so that no~
only all of the trivalent iron in the ilmenite ore is
converted to the divalent state, but also part ~f the
I 1 5~3~
-15-
titanium in the reac~ion solution is reduced to the trivalent
state in order to obtain a titanium sulphate solution for
hydrolysis that contains trivalent titanium. The presence
of trivalent titanium inhibits the formation of ferric iron
which would adsorb on the titanium dioxide particles in the
subsequent hydrolysis step of the process. Incidentally,
the accelerating e~fect of the iron on the rate of the
digestion reaction increases as the particle size of the
iron decreases.
A certain quantity of metal sulphates, i.e.
ferrous sulfate monohydrate is usually precipitated during
lS the digestion reaction without any noticeable deterioration
in the fluidity of the reaction mixture. The ferrous
sulphate monohydrate may easily be dissolved at the end of
the digestion reaction by the addition of water. At least
part of the water may be substituted by titanium sulphate
solution which has been freed from a large part of the iron
sulphate (by crystallization and separation of ferrous
sulphate heptahydrate at a later step of the process discussed
below). By this measure, the addition of extra water to the
system can be minimized or avoided. Ordinarilyl additional
water must be removed at a later stage in the process, e.g.,
by vaporizing.
The water or solution of water and titanium
sulphate may be added to the reaction solution in the last
digestor tank or at some convenient point bet~een the last
3~ digestor tank and separator device 19 to provide cooling.
The addition of water or solution of water and titanium
sulphate is not considered a part of this invention.
The resulting solution containing titanium sulfate,
iron sulfate and trace elements from the ilmenite ore may be
recovered and processed to prepare titanium compounds.
Alternatively, the solution may be processed to prepare
titanium dioxide pigment wherein the reaction solution is
passed to settler device 20 to remove solids from solution.
~ ~ 57~
-16-
When preparing titanium dioxide, the reaction
solution is then conducted from settler device 20 to a
crystallizer device 23 wherein the copperas (i.e., ferrous
sulphate heptahydrate) is crystallized and removed by known
process measures. For example, the solution is cooled in a
continuous or batch vacuum crystallizer to about 10C to
10 20C by pulling a vacuum of 29 inches of mercury to form
large crystals of copperas (FeSo4t7H2O) which can easily
be filtered on a drum or table filter. The cake of copperas
may be washed to recover the soluble titanium values. The
reaction solution from the filter may be concentrated by
known measures, e.g., evaporation, prior to being subjected
to hydrolysis. Also, the reaction solution may be clarified
either before or after crystallization and removal of the
ferrous sulphate heptahydrate. A clarification s~ep prior
to crystallization is favorable if it is desired to obtain a
ferrous sulphate heptahydrate of high purity which may be
processed further, e.g., for producing reagents which will
be employed for the purification of water and sewage.
Preferably, the reaction solution is subjected to
a fine filtration step prior to hydrolysis. After removing
the ferrous sulphate heptahydrate and, if necessary, clari-
fying and fine filtrating, a titanium sulphate solution is
obtained having a favorable ratio of Fe:TiO2 that can be
directly hydrolyzed or, optionally, evaporated by known
means in a vacuum evaporator to the desired TiO2 concentra-
30 tion, followed by hydrolysis.
The reaction solution from crystallizer 23 consistsof a titanyl sulphate ~TiOS04~ solution which is fed into
hydrolyzer device 24 wherein the titanyl sulphate is hydro-
lyzed by known process measures to provide titanium dioxide
hydrate Specifically, the titanyl sulphate solution is
hydrolyzed to provide insoluble ~itanium dioxide hydrate by
diluting the titanyl sulphate solution with water at elevated
temperatures. For example, a predetermined amount of
4 titanyl sulphate solution having a titanium dioxide content
of preferably greater than 200 yrams per liter is preheated
~ 157~3~?
-17-
to a temperature above 90C and added with agitation to
clear water at substantially the same temperature and in the
ratio of 3 to 4 1/2 parts of solution to one part of water.
The solution is subjected to boiling and titanium dioxide in
the form o colloidal particles is precipitated; the colloi-
dal particles flocculate to produce a filterable titanium
dioxide hydrate. The manner and means of conducting the
hydrolysis step is well known in the art and described, for
example, in U.S. Patent 1,851,487 and 3,071,439.
- Following hydrolysis, the titanium dioxide
hydrate is filtered by filtering device 25, such as a Moore
filter, and the resulting filter cake is fed into calciner
26 wherein it is heated in a known manner to remove water of
hydration and adsorbed sulfuric acid to provide titanium
dioxide which is suitable for pigment.
A significant advantage of the process of the
present invention is that it can reduce or even eliminate
the severe "spent acid" disposal problem that is characteris-
tic of the conventional sulphate process for the manufacture
of titanium dioxide pigment. Specifically, the spent acid
resulting from the digestion, crystallization and hydrolysis
steps of the process are reprocessed or recycled for use in
conducting the digestion reaction with ilmenite ore. Thus,
the process of the present invention can be free or substan-
tially free of waste spent acid.
To illustrate, the spent acid from filter 25
is conducted by conduit 27 and returned to digestor reactor
10. If desired, the spent acid from filter 25 may be
concentrated as by evaporation in a known manner in concen-
trator device 28 prior to being returned to digestor reactor
35 - -
An additional and significant advantage of the
process of the present invention is that the recycled spent
acid may be introduced directly into any one or all of the
digestor reactors to control the temperature in each digestor
reactor. The foregoing provides a convenient and effective
manner for balancing and controlling the reaction temperature
~ ~ .S~3 j
-18-
between the reactors.
The principle and practice of the present invention
is illustrated in the following Examples which are exemplary
only and it is not intended that the invention be limited
thereto since modifications in technique and operation will
10 be apparent to anyone skilled in the art. All parts and
percentages specîfied herein are by weight unless otherwise
indicated. Conversions are measured by the amount of
reaction undergone by a stoichiometric quantity of ore
treated.
Procedures and tests specified herein and in the
Examples herebelow were conducted as follows:
SURFACE AREA was measured by the sedimentation
method described in: Jacobsen, A.E. and Sullivan, W.F.,
"Method For Particle Size Distribution for the Entire
20 Subsieve Range," Vol. 19, Page 855 Analytical Chemistry
(November, 1947).
Example 1
800 grams of ilmenite ore ~MacIntyre Ore) having
25 a surface area of 0.39 m2/cc was charged to a digestor
reaction vessel. 1.16 liters of 43% by weight sulfuric acid
~as added to the reaction vessel. The temperature of the
reactants was raised to 108C by heating under constant
agitation with an agitator made of TEFLON~ material. After
30 fifty minutes, a 15cc sample of the reaction mixture was
filtered by gravity through a glass filter paper into a 100
ml polypropylene beaker. The filtrate was analyzed for
active acid content and titanium content (expressed as
TiO2). The active acid content was 430 g/l to provide a
35 ratio of active acid: titanium of 7.1.
The conversion of the reaction was determined
after about 1 1/4 hours by analyzing a sample of the reaction
solution. The filtrate analyzed had an active acid content
of 396.9 g/l H2SO4 and a titanium content of 78.5 g/l
40 (TiO2
) -
~ 1 5723~
-19-
After about 1 1/4 hours, 17 grams of powdered
iron was added to the reaction vessel to provide a reductant
for th~ ferric iron content of the reaction mixture.
After about 1 3/4 hours, the temperature of the
reaction mixture was lowered to 70C by placing the
reaction vessel in a tray of cooling water. Analysis of a
portion of the reaction solution, after cooling and removal
of undissolved solids, showed an active acid content of
353.3 g/l H2S04 and a titanium content of 89O25 9/1
(Tio2) providing a ratio of active acid:titanium of 3.96.
The reaction mixture was maintained at a tempera~
ture of 70-74C for about 15 hours. The reaction mixture
was cooled to about 50C to ~uench the reaction, filtered
to remove undissolved solids,and analyzed for active acid
and titanium content. The active acid content was 275.8
g/l and the titanium content was 136.2 g/l (TiO2) to
provide a ratio of active acid: titanium of 2.025.
The reaction solution was stable and suitable
for hydrolysis to prepare titanium dioxide pigment. A
titanium dioxide pigment may be prepared from the reaction
solution according to conventional processing techniques.
E mple 2
A two-stage system was constructed consisting of a
heated, agitated 5-liter first stage reactor overflowing
into a heated, agitated 25-liter second stage reactor.
MacIntyre ilmenite ore having a particle size distribution
as follows (U.S. Standard Screens~:
Mesh Wt %
~100 1.2
+200-100 35.8
+325-200 23.0
+400-325 6.0
-400 34.0
4 and containing 46.84 TiO2 was continuously fed into
3 s~
'che first stage at a rate of 3.78 gms/minute. A dilute
sulfuric acid solution having the following analysis:
29.9% Free H2SO4
1.4% Titanous Sulfate ~as Tio2)
3.3% Soluble Titanium (as TiO2)
10 was also fed into the first stage at a rate of 1~.5 milli-
liters/minute. The titanous sulfate was added to the
reaction vessel to provide a reductant for the ferric iron
content of the reaction mixture. Both stages were initially
charged with sufficient ore to provide a 100% excess over
the stoichiometric requirement. Unreacted ore overflowing
from the second stage was recycled to the first stage, in
order to maintain this excess ore in the system. The first
stage reactor was controlled at 106C while the second
stage reactor was controlled at 71C. The first and
second stages had residence ~imes of about 6.8 hours and
34.2 hours, respectively. After sufficient time elapsed for
equilibrium to be established, it was found that 54.2% of
the TiO2 in the ore feed was reacted in the first stage,
and 28.2~ was reacted in the second stage. An overall
25 conversion o 82O4% was achieved with the two stages.
Analysis of the final product was:
9.4~ Soluble Titanium (as Tio2)
9.0% Free H2SO4
0.3% Titanous Sulfate ~As TiO2)
Example 3
The system described in Example 2 was operated
35 with the following feed rates to the first stage reactor:
3.27 g/min ilmenite ore (46.8% Tio2)
12.28 g/min of dilu~e sulfuric acid solution
containing 42.9~ free H2SO4 with no titanous
sulfa'ce.
Powdered iron was also fed into the first stage at a rate of
~ ~5 ~3~J
-21-
0.19 g/min. The powdered iron was added to the reaction
vessel to provide a reductant for the ferric iron content of
the reaction mixture.
A 100~ excess of ore over the stoichiometric
requirement was maintained in the system as in Example 2.
10 The first stage reactor was controlled at 106C and the
second stage at 72C. The first and second stages had
residence times of about 9.4 hours and 47.1 hoursr respec-
tively. After equilibrium had been reached, it was found
that 73.9% of the Tio2 in the ore feed was reacted in the
first stage and 20~9% in the second stage~ An overall
conversion of 94.9% was achieved with the two stages.
Analysis of the final product was:
8.9% Soluble Titanium ~as TiO2)
8.8% Free H2SO4
0.1% Titanous Sulfate ~as TiO2)
Example 4
A reaction acid solution of 41.4~ by weight
sulfuric acid was prepared by combining 96.5% by weight
sulfuric acid, spent sulfuric acid solution containing
16.32% by weight sulfuric acid, and water in a reactor
vessel. The reaction acid was heated to 100C under
constant agitation. 2130 grams of ilmenite ore~ two times
the stoichiometric quantity, was heated to 100C and
charged to the reaction vessel. The temperature of the
reaction mixture was then raised to about 108C and
maintained for 10.5 hours. Samples of the reaction mixture
were taken periodically and analyzed. Analysis of a sample
about 45 minutes after the ilmenite ore addition showed an
active acid content of 35.90% and titanium content of 1.72%
(TiO2) to provide a ratio of active acid:titanium of
20.87.
i ~ 57~
The analy~is of the sample taken after 10.5 hours
at about 108C showed an active acid content of 23.10 and
a titanium content of 7.49% (TiO2) to provide a ratio of
active acid:titanium of 3.08.
Example 5
This example illustrates multi-stage continuous
digestion processes wherein a 41.7% sulfuric acid solution
is reacted with a 100% stoichiometric excess of MacIntyre
15 ilmenite ore assayed at 46.8% TiO2 in the presence of
a powdered iron reductant at an amount equal to 5% by weight
of the ilmenite ore reac~ed. Table I provides digestor
conditions of temperature, residence time, and conversion
for the digestors in various multistage digestion processes.
The invention being thus described, it will be
obvious that the same may be varied in many ways. Such
variations are not to be regarded as a departure from the
spirit and scope of the invention and all such modifications
25 are intended to be included within the scope of the following
claims.
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