Process for preparing nodular pigmentary titanium dioxide

A process for preparing nodular pigmentary titanium dioxide by grinding and mixing a titanium-bearing material, such as sorelslag, with an alkali metal compound such as sodium hydroxide, and roasting the mixture. The roasted material is ground followed by washing and filtering. Thereafter, the solid residue is digested with hydrochloric acid for a time and temperature sufficient to form nodular-shaped solids. After removing the acid by filtration and washing the solid residue, the residue is calcined to provide a nodular titanium dioxide pigment.

BACKGROUND OF THE INVENTION 
This invention relates to the preparation of titanium dioxide, and more 
particularly, to a process for preparing nodular pigmentary titanium 
dioxide from titanium dioxide-bearing materials. 
Titanium dioxide (TiO.sub.2) is a well known opacifying pigment useful in 
paint and coating compositions, in plastic materials and as a filler in 
paper and other materials. Various known processes for producing TiO.sub.2 
include, for example, conventional processes commonly referred to as the 
"sulphate" process and the "chloride" process. 
The "sulphate" process involves solubilizing the titanium values in low 
grade titanium ores, such as ilmenite or sorelslag, with concentrated 
sulfuric acid and meticulously removing ferrous sulfate formed in the 
process. This is followed by precipitation, washing and calcining to form 
pigmentary TiO.sub.2. 
The "chloride" process involves volatilizing, as tetrahalide, the titanium 
values in high grade titanium ores, such as Australian rutile (containing 
about 95 percent TiO.sub.2) or highly beneficiated ilmenite. This is 
followed by purification and oxidation. 
The sulphate and chloride processes are very complex and capital intensive 
which accounts for the relatively costly product of TiO.sub.2 pigment made 
by such processes. It is, therefore, desired to provide a simplified 
process, and relatively less expensive process for preparing a TiO.sub.2 
pigment whereby the titanium values in titanium dioxide-bearing materials 
or ores are not solubilized or converted to a vaporizable liquid compound, 
but are separated, through solid-liquid reactions, from the ore's 
impurities and mechanically comminuted to pigmentary size. 
It is further desired to provide a TiO.sub.2 pigment which is nodular in 
shape. 
SUMMARY OF THE INVENTION 
The present invention is directed to a process for preparing nodular-shaped 
titanium dioxide pigment by mixing a titanium dioxide-bearing material 
with an alkali metal compound; roasting the mixture; digesting the roasted 
material in hydrochloric acid, said digestion including heating the 
roasted material/acid solution at a rate sufficient to form nodular-shaped 
solids and refluxing the roasting material/acid solution; and calcining 
the digested material.

DETAILED DESCRIPTION OF THE INVENTION 
The starting material for the process of the present invention is a 
titanium-bearing material, for example, sorelslag. Various grades of 
sorelslag may be used in the present process. For example, the composition 
of one typical grade of sorelslag, expressed as oxides, may consist of 
approximately 70 weight percent (wt. %) TiO.sub.2 and approximately 11 wt. 
% FeO as an impurity with the remainder being impurities including, for 
example, CaO, MgO, SIO.sub.2, Al.sub.2 O, MnO, V.sub.2 O.sub.5, Cr.sub.2 
O.sub.3 and other oxides as trace impurities. Another grade of sorelslag 
useful in the present process may consist of approximately 78 wt. % 
TiO.sub.2, approximately 8 wt. % FeO as an impurity, and the remainder 
impurities such as those listed above. 
It is to be understood that the present invention is not limited to 
sorelslag. Other titanium-bearing materials or ores as starting materials 
for the present invention are within the scope thereof. For example, 
another titanium-bearing slag referred to as "chloride slag" may be used 
in the process of the present invention. A typical chloride slag may 
consist of approximately 85 wt. % TiO.sub.2, approximately 10 wt. % FeO as 
an impurity and the remainder impurities such as those listed above. 
Another suitable raw material for the process of the present invention can 
be an intermediate product formed during a beneficiation process of 
ilmenite such as that described in U.S. Pat. No. 3,825,419. A typical raw 
material formed during the beneficiation process above may consist of 
approximately 95 wt. % TiO.sub.2, approximately 1 wt. % FeO as an impurity 
and the remainder impurities such as those listed above. 
Examples of other titanium-bearing materials which can be used in the 
process according to the present invention are any titanium-bearing 
material which is so treated that the titanium dioxide portion thereof 
becomes reactive with an alkali metal compound when heated at about 
700.degree. C. to about 950.degree. C. An alkali metal compound, as used 
herein includes, for example, an alkali metal hydroxide, an alkali metal 
carbonate, or an alkali metal oxide or mixtures thereof. 
All of the equipment used in the process of the present invention for 
grinding, mixing, roasting, filtering and calcining and all other 
operations are carried out by conventional equipment suitable for the 
purpose of continuous or batch type operation. For comminuting the 
titanium-bearing starting material to micron size it is preferred to use 
"sandmills" of the type described and illustrated, for example, in U.S. 
Pat. No. 2,581,414. A "sandmilling" process will refer herein to a process 
of grinding a material to micron particle size using the type of equipment 
described and illustrated, for example, in U.S. Pat. No. 2,581,414. 
However, the grinding media used in such equipment is not limited to sand, 
but can be glass, steel, ceramic, or any other suitable grinding media 
having or spherical or bead shape, generally, in the range of about 0.5 to 
about 3 millimeters in diameter. 
The digesting step has to be carried out in vessels with inner surface 
portions or linings resistant, under normal operating conditions, to the 
acid utilized in the process. Suitable materials for such resistant 
surface portions are, for example, glass, FRP (glassfiber reinforced 
plastic), polyester, vinylester, epoxy, other suitable plastics, Hasteloy 
(Ni/Mo alloy), rubber, refractory metals (Ta, Zr, Cb) or acid resistant 
brick. 
According to a preferred embodiment of the present invention, the size of 
the titanium-bearing material should be small enough for all or 
substantially all of the material to react with an alkali metal compound. 
With reference to FIG. 1, the titanium-bearing starting material is first 
ground, for example, by hammermilling to a size suitable for sandmilling. 
Hereinafter, the process of the present invention will be described with 
reference to sorelslag as the titanium-bearing material but, as 
aforementioned, the material is not limited to sorelslag. 
After the hammermilling step, the sorelslag is preferably sandmilled to an 
average particle size of 15 micron or less. More preferably, the sorelslag 
is sandmilled to an average particle size of about 10 micron or smaller. 
Even more preferably, the starting material may be sandmilled to a top 
size of 10 micron, i.e., to a maximum particle size of 10 micron or less. 
Particles larger than about 10 micron may occlude impurities which may not 
be readily removed from the starting material in the subsequent process 
steps of the present invention. 
After the sorelslag is ground to the preferred particle size, the sorelslag 
is then intimately mixed with an alkali metal compound for example, 
selected from the group consisting of an alkali metal hydroxide, an alkali 
metal carbonate, an alkali metal oxide or mixtures thereof. Alkali metals 
such as sodium, potassium, lithium, rubidium, or cesium or mixtures 
thereof may be used. The preferred compound is an alkali metal hydroxide, 
and more preferably sodium hydroxide, because it is readily reactive with 
the finely ground sorelslag material. Hereinafter, the process of the 
present invention will be described with reference to sodium hydroxide as 
the alkali metal compound but it is understood that the present invention 
is not limited thereto. 
Sodium hydroxide may be mixed with the sorelslag material prior to roasting 
and preferably during the sandmilling step above or, alternatively, prior 
to the sandmilling step. The mixture of sorelslag and sodium hydroxide can 
contain about 30 parts by weight or above of sodium hydroxide to about 100 
parts by weight of sorelslag. Preferably, about 30 to about 60 parts by 
weight of sodium hydroxide to about 100 parts by weight of sorelslag is 
used. More preferably, the ratio by weight of sorelslag to sodium 
hydroxide is about 100:35 to about 100:45. Using a ratio of sorelslag to 
sodium hydroxide above or below the range of about 100:30 to about 100:60 
is operable, however, it may result in an unsatisfactory TiO.sub.2 
pigmentary product. When a material bearing a higher TiO.sub.2 content is 
used, the hydroxide portion of the mixture is increased accordingly. 
After the sandmilling step, the mixture of sorelslag and sodium hydroxide 
is heated or roasted at temperatures ranging from about 700.degree. C. to 
about 950.degree. C. for a length of time ranging from about 1 to about 3 
hours. At roasting temperatures above about 950.degree. C. a hard material 
or clinker may result, and below about 700.degree. C. the reaction between 
the sodium hydroxide and sorelslag may not be complete. It is, therefore, 
preferred to roast the mixture at about 800.degree. C. to about 
870.degree. C. for about 11/2 to about 2 hours. Preferably, the roasted 
mixture is subsequently ground to about 0.5 to about 2 microns, because, 
as mentioned above, fine particles will enhance the removal of impurities 
from the TiO.sub.2 product during subsequent treatment of the roasted 
material. 
During the roasting step, it is believed that the TiO.sub.2 contained in 
the mixture reacts with the sodium hydroxide forming a sodium titanate. 
Some of the impurities in the material may also react with the particular 
hydroxide used to form an alkali metal salt, leaving them in an 
extractable form. For example, when sodium hydroxide is used, the 
impurities in their alkali metal salt form include sodium vanadate, sodium 
chromate, sodium aluminate and sodium silicate, which are readily soluble 
in water. These impurities are therefore preferably at least partially 
dissolved in water by washing, after the roasting step, to reduce, or more 
preferably, substantially entirely remove deleterious amounts of the 
impurity from the TiO.sub.2 product. Other compounds present in the 
starting material such as iron oxide, magnesium oxide, and calcium oxide 
are soluble in mineral acids, such as hydrochloric acid (HCl), and are 
removed from the TiO.sub.2 product during the digestion step as discussed 
below. 
After the water soluble impurities are washed off or dissolved from the 
roasted material, the remaining insoluble alkali metal titanate and 
residue is digested in hydrochloric acid. To obtain the nodular form of 
TiO.sub.2 pigment product, the digestion step of the overall process of 
the present invention is critical. The normality of the acid used is about 
6 normal (N) HCl acid. The normality of the acid/roasted material mixture 
must be maintained at about 6N during the digestion step. The digestion 
step must be carried out for a time and temperature sufficient to form 
nodular-shaped particles during the digestion. 
In a preferred embodiment of the present invention, the roasted material is 
mixed with HCl at room temperature (i.e. from about 20.degree. C. to about 
30.degree. C.). The roasted material/acid mixture or solution is then 
heated, slowly, up to the boiling point of the acid at a rate of about 
5.degree. C./minute or less, preferably from about 2.degree. C./min to 
about 3.degree. C./min, and more preferably from about 2.degree. C./min or 
less. It is believed that the nodular sodium titanate material is formed 
during the digestion step. 
After reaching the boiling point of the acid (which variees according to 
pressure) the solution is preferably refluxed; for example, at about 
90.degree. C. to about 111.degree. C., and preferably at about 108.degree. 
C., for a length of time to substantially complete the digestion step. The 
digestion of the solution is substantially complete in about 10-15 
minutes. Digestion times of up to about 120 minutes can be used but 
preferably the digestion time is from about 10 to about 40 minutes. The 
digestion step above may be carried out one or more times, however, it is 
preferred to carry out the digestion step at least two times. Higher 
heating rates and time may be used, however, pressurized equipment may be 
necessary. 
During the digestion step, the alkali metal titanate formed is believed to 
hydrolyze into amorphous hydrous TiO.sub.2.nH.sub.2 O and the iron oxides 
solubilize as ferrous and ferric chloride. The iron chlorides and other 
impurities in the acid suspension are removed by, for example, 
centrifugation or filtration and, optionally, disposed of or further 
treated to recover unreacted acid. The insoluble amorphous titanium 
dioxide residue is washed with a fluid such as water to further remove 
soluble impurities. Thereafter, the TiO.sub.2 is recovered from the water 
by, for example, filtering or centrifuging. A white residual cake results 
after this step is carried out. 
The iron impurities in the TiO.sub.2 pigment are believed to be the cause 
of a non-white pigment. Preferably the TiO.sub.2 pigment contains less 
than 520 ppm Fe and more preferably less than 200 ppm. 
The amorphous TiO.sub.2 is calcined, preferably, for about 30 to about 60 
minutes at temperatures ranging from about 800.degree. C. to about 
1000.degree. C. to convert the TiO.sub.2 to its crystalline rutile form. 
More preferably, a temperature of about 875.degree. C. to about 
925.degree. C. for about 30 minutes to about one hour is employed, because 
undesirable discoloration of the resulting pigment is minimized and lower 
temperatures are not as effective in converting the product into its 
desirable crystalline form. 
The crystalline rutile product obtained by the process of the present 
invention is preferably nodular in shape. Preferably the nodular-shaped 
TiO.sub.2 particle has a particle size of less than about 1 micron and 
preferably about 0.3 micron, because the light scattering ability of the 
pigment and, consequently, its value as an opacifier is dictated by a 
narrow optimum size range. If the crystalline rutile product obtained 
after calcination is severely agglomerated it may be pulverized or 
sandmilled to the preferred particle sizes above. 
The TiO.sub.2 product can be used as a pigment in any of the typical 
applications for which opacifying pigments are used. As an illustration 
only and not to limit the scope of the the present invention, the 
TiO.sub.2 pigment obtained from the process of the present invention can 
be used as an opacifier in paint, paper or plastics. The opacifying power 
and brightness of the product is determined by measuring its light 
scattering coefficient and reflectance. Pigment obtained by the process of 
this invention desirably has a light scattering coefficient of above about 
2000 cm.sup.2 /g and preferably above about 4000 cm.sup.2 /g and a 
brightness of above about 80 percent and preferably above about 85 
percent. 
The example which follows illustrates the present invention but the present 
invention is not to be limited thereby. 
EXAMPLE 1 
A sorelslag sample with a particle size of about 200 mesh was obtained from 
QIT-FER ET TITANE, INC., a Canadian company. A 1200 gram (g) sample of the 
200 mesh sorelslag and 480 g of sodium hydroxide (NAOH) were placed in a 
laboratory sandmill containing one (1) liter (1) of 1.2 millimeters (mm) 
in diameter steel shot and about 1,000 milliliters (ml) of water. Then, 
the mixture was sandmilled until the maximum particle size (top size) of 
the sorelslag was 10 microns. The slag/NaOH ratio of the mixture was 
100/40 by weight. After the steel shot was screened out, the mixture was 
dried in an oven at 100.degree. C. overnight. The dried material was 
hammermilled to break up the agglomerates formed after drying. 
The hammermilled material was then roasted at 875.degree. C. for two hours. 
The roasted material was run through a grinder to break up the 
agglomerates formed after roasting and then the roasted material was 
sandmilled for five minutes in about 700 ml of water and one liter or 1.2 
mm in diameter zirconium oxide beads. After the beads were screened out, 
the suspension was vacuum filtered and the filter cake was washed with 
water two times, each wash using about one liter of water. 
A 194 gram sample of the roasted material on a dry basis was digested in 
1,000 ml of 6 N hydrochloric acid (HCl). The digestion step was carried 
out as follows: The temperature of the 6 N solution (i.e. roasted 
material/acid mixture) was at room temperature (20.degree. C.). Then the 
temperature was increased slowly at a rate of bout 5.degree. C./minute up 
to the boiling point of the 6 N acid solution and the solution was allowed 
to reflux for 15 minutes. The amount of water in the wet filter cake was 
taken into account when the normality of the HCl acid was calculated. A 
flocculant, 3 g of a 1% solution of Sepran M6-205 (a trademark of The Dow 
Chemical Company) was added to the digestion suspension. The suspension 
was filtered and washed two times with water, each wash using about one 
liter of water. The filter cake was redispersed in water and digested a 
second time with 800 ml of 6 N HCl in the same manner as the first 
digestion. The same amount of flocculant as in the first digestion was 
added to the suspension. The suspension was then vacuum filtered and 
washed as in the first digestion. A filter cake was dried at 110.degree. 
C. for about three hours. The dried material was then calcined for one 
hour at 900.degree. C. 
The dry brightness of the calcined material was measured by packing a 
portion of the material into a sample vial cap which was about 1/2 inch 
deep by 1 inch in diameter and measuring the reflectance of the sample 
using a Photovolt equipped with a blue filter. The instrument was first 
calibrated by using a white standard chip of known brightness of 86.9 with 
the blue filter. The dry brightness of the calcined material of this 
example was 88.0%. 
The scattering coefficient of the calcined material was measured by the 
following procedure: 
A 15 gram sample of pigment and 1/2 gram of sodium tripolyphosphate in 70 
grams of water was dispersed in a high speed disperser for 5 minutes. Then 
3.1 g of a latex sold under the tradename Rhoplex B-100 by Rohm and Haas 
Co. and 1/2 g of a surfactant sold under the tradename Triton X-100 by 
Rohm and Haas Co. was added to the dispersion and stirred gently by hand 
for about 3 minutes. A drawdown coating of the dispersion was applied on a 
2 mil clear plastic sheet of Mylar (a trademark of E. I. DuPont de Nemours 
& Co.) with a 1.5 mil Bird film applicator and placed in an oven and dried 
at 100.degree. C. for 3 minutes. A 2 inch by 2 inch square sample of the 
coated 178 sheet was weighed and then placed on a 2 inch by 5 inch 
optically flat black glass plate which was coated with propylene glycol. 
The propylene glycol was used to ensure optical contact. Using a Photovolt 
the reflectance over the black glass, R.sub.B, was measured for the 
sample. The coating on the 2 by 2 inch Mylar sheet was washed off and the 
Mylar sheet was dried and then weighed to find the coating weight per unit 
area, W. A practical approximation to an infinitely opaque coating, 
R.infin., was obtained as follows: A drawdown coating was applied on a 
sheet of 2 mil Mylar and dried and this step was repeated until a maximum 
reflectance was reached as measured by the Photovolt. Using the R.sub.B, 
R.infin., and W values, a scattering power value, SW, was found by using 
the Tables in the article Mitton-Jacobsen, "New Graphs for Computing 
Scattering Coefficient and Hiding Power", Official Digest, September 1963, 
pages 871-913. The scattering coefficient, S, was then calculated using 
the formula: 
In this example the scattering coefficient of the calcined material was 
3,336 cm.sup.2 /g. 
The iron content of the calcined material was measured using ionic coupled 
plasma spectrometry. In this example the iron content of the calcined 
material was 200 ppm.