Production of zinc and alkaline earth metal titanates

Titanates of zinc and/or alkaline earth metals are produced by simultaneously adding to a suspension of finely divided TiO.sub.2, maintained at a substantially constant pH above about 8, a solution of a salt of zinc and/or alkaline earth metal and alkali to form a hydroxide precipitate. Filtration produces a filter cake mixture of TiO.sub.2 and hydroxide and calcination at above 500.degree. C. produces the desired titanate of narrow particle size distribution and desirable optical properties. Non-simultaneous addition results in a non-filterable material. Zinc, magnesium and/or calcium are preferred and doping elements such as aluminum, phosphorus, boron and/or silicon may also be present.

This invention relates to a process for the production of zinc titanates 
and alkaline-earth metal titanates, more especially magnesium and calcium 
titanates, which are particularly suitable for use as white pigments for 
special applications, e.g. lacquers, plastics and paper, and as 
ferro-electrics. In the production of pigments, it is necessary to ensure 
that the starting materials are highly pure and to develop a production 
process which enables substantially uniform particle sizes with a narrow 
particle size distribution to be produced. The desired average particle 
size is dependent upon the refractive index of the pigments and increases 
with decreasing refractive index. However, the average particle sizes of 
materials having particular optical activity is distinctly below 1 .mu.m 
and, in the case of titanium dioxide, the most frequently used white 
pigment, normally amounts to 0.23 .mu.m. In order to produce finely 
divided and substantially uniform particles such as these, it is necessary 
to use reactive starting compounds. 
It is known that magnesium and calcium titanates can be produced in 
accordance with British Pat. No. 417,699 and German Auslegeschrift No. 
1,014,972. Using titanium dioxide and the corresponding oxides or 
decomposible salts of magnesium or calcium as the starting materials, the 
alkaline earth metal titanate is obtained by calcination at high 
temperatures. According to U.S. Pat. No. 2,434,079, equivalent quantities 
of TiO.sub.2 -hydrate and magnesium carbonate are mixed and calcined at 
1000.degree. C. In German Auslegeschrift No. 1,014,972, it is pointed out 
that it is not possible by solid-phase reactions of this type to produce 
individual crystals with the necessary particle size. Accordingly, German 
Auslegeschrift No. 1,014,972 relates to the precipitation of calcium 
titanium oxalate from TiCl.sub.4, CaCl.sub.2 and oxalic acid, of which the 
calcination enables suitable particle sizes of from 0.1 to 1 .mu.m to be 
adjusted. Unfortunately, this process has the disadvantage that is 
requires extremely expensive starting compounds. 
The object of the present invention is to provide a process for the 
production of alkaline-earth metal titanates and zinc titanates which can 
be carried out inexpensively on a large scale and which enables the 
average particle size to be adjusted to a favorable, desired value. 
Accordingly, the present invention provides a process for the production of 
zinc or alkaline-earth metal titanates by precipitating zinc or 
alkaline-earth metal hydroxides in the presence of finely divided titanium 
dioxide, followed by filtration, optionally washing and calcination, 
wherein the hydroxides are precipitated by the simultaneous addition of a 
zinc or an alkaline earth metal salt solution and an alkali solution to 
finely divided titanium dioxide, e.g. titanium dioxide hydrolyzate sludge, 
at a substantially constant pH-value above about 8, preferably in the 
range from about pH 10 to pH 12, and the resulting suspension is calcined 
at a temperature above about 500.degree. C. and preferably at a 
temperature of from about 700.degree. C. to 900.degree. C. 
Hydroxides of alkaline earth metals or zinc are normally obtained in the 
form of finely divided, slimy deposits which are virtually impossible to 
filter, so that hitherto homogeneous admixture of the component containing 
titanium dioxide, followed by filtration, had never been considered for a 
commercial manufacturing process because of the prolonged washing and 
filtration times. In contrast, however, surprisingly it is possible by the 
process according to the present invention to obtain readily filterable 
and washable precipitates when alkaline earth metal hydroxides are 
precipitated onto finely divided TiO.sub.2, e.g. TiO.sub.2 -hydrolyzate 
sludge. The precipitation of the hydroxides has to be carried out at a 
substantially constant pH-value above 8, which preferably should not vary 
by more than .+-.1 pH unit, by the simultaneous addition of an alkaline 
earth metal salt solution and an alkali hydroxide solution. The finely 
divided TiO.sub.2 used for the purposes of the present invention may be, 
for example, a so-called flame hydrolyzate of the type formed by the 
oxidation of TiCl.sub.4 in the vapor phase in accordance with DAS No. 
1,244,125 or DAS No. 1,210,421. Suitable TiO.sub.2 hydrolyzate sludges are 
obtained in the known hydrolysis of solutions containing titanyl sulphate 
by the so-called sulphate process. 
One particular embodiment of the process according to the present invention 
starts from the TiO.sub.2 -hydrolyzate sludge obtained in the production 
of TiO.sub.2 which, after washing, is adjusted to the pH-value required 
for precipitation and introduced into the precipitation vessel. An 
alkaline earth metal chloride solution and an alkali hydroxide solution, 
for example, are then introduced simultaneously with stirring at such a 
rate that the pH-value remains constant during the precipitation process. 
The process according to the invention leads to readily filterable and 
washable deposits of hydroxide which can be filtered and washed in known 
filtration units. It is only by precipitating the hydroxides at a constant 
pH-value that it is possible to obtain commercially processible, reactive 
hydroxides from zinc and alkaline earth metal hydroxides and TiO.sub.2 
-hydrolyzate sludges which can be calcined at considerably lower 
temperatures, preferably below 900.degree. C., to form the corresponding 
alkaline earth metal titanates of suitable particle size and which do not 
react solely at considerably higher temperatures like the mixture of the 
oxides or carbonates. 
Comparison precipitations have shown that the alkaline pH-value prevailing 
during precipitation is not sufficient in itself. If an alkali hydroxide 
solution is initially introduced and an alkaline earth metal salt 
solution, for example, subsequently added, the deposits obtained are as 
difficult to filter as those obtained where the opposite procedure is 
adopted, i.e. when the alkaline earth metal solution is initially added to 
the TiO.sub.2 -hydrolyzate and the alkaline earth metal hydroxides 
subsequently precipitated by the addition of sodium hydroxide. 
In one particularly preferred embodiment of the process according to the 
invention, the alkaline earth metal hydroxides are precipitated in the 
presence of TiO.sub.2 -hydrolyzate sludges, preferably at a pH-value which 
corresponds to the solubility minimum. Thus, Mg(OH).sub.2 is preferably 
precipitated at a pH-value of about 10. The particular pH-value to be 
adjusted is dependent upon the required titanate. Thus, the precipitation 
of Ca(OH).sub.2 is preferably carried out at a pH-value of about 13 while 
the precipitation of Zn(OH).sub.2 is preferably carried out at a pH-value 
of about 8.5. 
Particularly suitable alkaline earth metal salts and zinc salts are highly 
concentrated solutions of the type frequently obtained as secondary 
products in commercial processes, for example the corresponding alkaline 
earth metal chloride solutions, but also magnesium sulphates, calcium 
nitrates, zinc sulphates and other salts. The process is not adversely 
affected by the anions providing no substantially insoluble salts are 
precipitated. Suitable alkalis are the various alkali metal hydroxides 
such as NaOH and KOH. The precipitation rate has no real bearing upon the 
process according to the invention, provided that provision is made, by 
turbulent or intensive stirring, to ensure that no local fluctuations in 
the pH-value of greater than about 2 occur. Thus, equivalent MgTiO.sub.3 
-pigments were obtained from a 20 kg batch for the production of magnesium 
titanate irrespective of whether precipitation was carried out over a 
period of 5 minutes, over a period of 1 hour or over a period of 2 hours. 
The process according to the invention may be carried out in the usual 
apparatus of the type commonly used for the production of TiO.sub.2. Thus, 
the usual filter units, for example rotary filters or filter plates, may 
be used for separating the solids from the TiO.sub.2 -hydrolyzates and 
alkaline earth metal hydroxides by filtration. Washing of the mixture 
which is obtained during the precipitation of alkaline earth metal 
hydroxide or zinc hydroxide in the presence of finely divided TiO.sub.2 or 
TiO.sub.2 -hydrolyzate may be carried out on the filter itself or by 
resuspension and subsequent filtration. However, there is no need for the 
mixture to be washed before calcination. In this case, however, the 
water-soluble salts must be removed by washing after calcination. However, 
washing is preferably carried out at pH-value of about 9 to 11. 
In the case of the deposits obtained by the process according to the 
invention, the hydroxides become washable much more quickly and more 
completely than in cases where the lyes or salt solutions are initially 
produced during precipitation. 
The present invention is also applicable to the precipitation of mixed 
hydroxides. Thus, it may be advisable to control certain properties of the 
alkaline earth metal titanates or zinc titanates by the incorporation of 
foreign metal ions, e.g. about 0.1 to 10% by weight of TiO.sub.2 of 
aluminum hydroxide or the oxide of phosphorus, boron and/or silicon. 
In one particular embodiment, an aluminum-doped magnesium titanate may be 
produced by adding an aluminum chloride solution to a magnesium chloride 
solution and precipitating the hydroxides together after calcination. In 
this case, too, the filterability and washability of the precipitated 
metal hydroxides is considerably accelerated. In this way, it is possible 
to incorporate as doping ions any metals which form substantially 
insoluble deposits with the alkaline earth metal hydroxides in the 
alkaline pH-range or which are absorbed on alkaline earth metal hydroxides 
or titanium hydrolyzates. 
The quantitative ratio of titanium to zinc or alkaline earth metal is 
dependent upon the required solids phase. Thus, in the production of Ca or 
Ba titanates, the ratio of titanium to calcium or to barium will be close 
to 1 if the phases crystallizing in the Perowskit lattice are to be 
produced in this case. By contrast, a variety of different molar ratios is 
possible in the precipitation of Mg(OH).sub.2 and calcination to form 
magnesium titanates because both MgTiO.sub.3 and also Mg.sub.2 TiO.sub.4 
and MgTi.sub.2 O.sub.5 exist as defined compounds and are of interest for 
various fields of application. Thus, with a molar ratio of Mg(OH).sub.2 to 
TiO.sub.2 of 2:1, Mg.sub.2 TiO.sub.4 is preferentially obtained during the 
subsequent calcination step, whereas with a ratio of 1:1 MgTiO.sub.3 
represents the most preferential phase and, where TiO.sub.2 is present in 
excess, MgTi.sub.2 O.sub.5 is formed. The average particle size of the 
alkaline earth metal titanates of about 0.25 to 0.5 .mu.m is adjusted in 
particular through the calcination temperature or time. To obtain a narrow 
particle size distribution, it is essential to carry out calcination at a 
low temperature which should be below about 1000.degree. C. and preferably 
in the range from about 700.degree. C. to 900.degree. C.

The invention is illustrated by the following examples: 
EXAMPLE 1 
In a stirrer-equipped vessel, 15.8 kg of TiO.sub.2 --hydrolyzate sludge 
containing 31% by weight of TiO.sub.2 were initially introduced and 
adjusted, with 1.46 kg of 40% NaOH, to a pH-value of 10. Over a period of 
1 hour, 19.8 kg of an MgCl.sub.2 -solution containing 12.2% of MgO and 
11.3 kg of a 39.5% sodium hydroxide solution were simultaneously 
introduced by two metering pumps at a constant pH-value of 10. On 
completion of precipitation, the suspension was stirred for 5 minutes and 
subsequently filtered. The filtration rate was determined by way of three 
filter candles which were covered by Dralon acrylic cloths and operated 
under a reduced pressure of 600 mm of mercury at room temperature 
(25.degree..+-.2.degree. C.). The precipitated hydroxide sludges 
containing 14.7% of solids and 14.8% of water-soluble salts were initially 
introduced and the filter candles with a filter surface of 250 cm.sup.2 
were immersed therein and charged over a period of 3 minutes. The charging 
time was selected so that a filter cake layer approximately 20 mm thick 
was formed. After charging, the initial filter liquid was replaced by 
washing water, the washed water was sucked through the filter cake for a 
certain time and the proportion of water-soluble salts thus reduced. In 
order to establish comparable conditions, the washing time in the examples 
selected was 9 minutes. This was followed by suction drying for 2 minutes. 
The thickness of the filter cake amounted to 21 mm and the moist filter 
cake was weighed out at 2.8 kg for a solids content of 34%. By 
calcination, this gave a filter capacity of 214 kg/h/m.sup.2. After 
calcination for 1 hour at 900.degree. C. in a muffle furnace, the content 
of water-soluble salts was measured at 1.4% in accordance with DIN 53 197 
B. The MgTiO.sub.3 white pigment formed was characterized by a lightening 
power of 240 in accordance with DIN 53 192. The pigments were highly pure 
and could be used for a variety of applications. 
EXAMPLE 2 
The procedure was as described in Example 1, 1% of Al.sub.2 O.sub.3 in the 
form of aluminum sulphate being added at the end of precipitation. The 
solids content after precipitation amounted to 14.5% for a salt content of 
13.5%. After a suction time of 3 minutes and a washing time of 9 minutes, 
a 2.7 kg filter cake with a thickness of 20 mm was obtained. 
For a solids content of 32.3%, the calculated filter capacity amounted to 
192 kg per h per m.sup.2. After calcination, the content of water-soluble 
salts was determined at 2.4% by weight, the lightening power amounting to 
200. 
EXAMPLE 3 
The magnesium hydroxide was precipitated as described in Example 1 by the 
simultaneous addition of a 33% magnesium chloride solution and 40% sodium 
hydroxide at pH 10 to the TiO.sub.2 -hydrolyzate which was used in a 5% 
excess. After precipitation, the hydroxide suspension contained 15.4% by 
weight of solids and 13.9% of salts in dissolved form. After a suction 
time of 3 minutes and a washing time of 9 minutes, followed by suction 
drying for 2 minutes, a filter cake weighing 2.6 g and having a wall 
thickness of 20 mm was obtained. For a solids content of 37.1%, the 
calculated filter capacity amounted to 212 kg/h/m.sup.2. After calcination 
for 1 hour at 900.degree. C., the pigment contained 1.5% by weight of 
water-soluble salts. Its lightening power amounted to 195. 
EXAMPLE 4 
15.4 kg of TiO.sub.2 -hydrolyzate sludge containing 31% of TiO.sub.2 were 
initially introduced and, as in Example 1, 577 g of NaOH were added to 
adjust the pH-value to 11.0. The Mg(OH).sub.2 was precipitated at this 
pH-value by the simultaneous addition of 19.8 kg of MgCl.sub.2 -solution 
containing 12.2% of MgO and 11.3 kg of a 39.5% sodium hydroxide solution. 
For a solids content in the sludge of 15.3% and a salt content of 15.2%, 
filtration carried out after a suction time of 3 minutes and a washing 
time of 9 minutes, followed by drying for 2 minutes, produced 2.2 kg of a 
filter cake with a thickness of 18 mm, giving a filter capacity of 183 
kg/h/m.sup.2 for a dry residue of 37.6%. The content of water-soluble 
salts after calcination (1 hour at 900.degree. C.) amounted to 1.3% by 
weight and the lightening power to 195. 
EXAMPLE 5 
The procedure was as described in Example 1 except that the Mg hydroxide 
was precipitated over a period of 4 hours at a pH-value of 10. The filter 
capacity was calculated at 229 kg/h/m.sup.2. The pigment corresponded in 
its properties to the product of Example 1. 
EXAMPLE 6 
For comparison, the procedure adopted differed from the production process 
according to the invention, namely precipitation at a constant pH-value, 
and 15.7 kg of TiO.sub.2 hydrolyzate sludge containing 30.7% of TiO.sub.2 
were initially introduced. The entire magnesium chloride liquor was added 
and the pH-value was subsequently adjusted to pH 10 by the addition of 40% 
sodium hydroxide solution over a period of 1 hour. Further processing was 
carried out in the same was as described in Example 1. This variant of the 
process gave a poorly filterable and washable precipitation product which 
could not filtered under commercial conditions. The solids content 
amounted to 14.9% and the content of water-soluble salts to 16.0%. After a 
suction time of 3 minutes and a washing time of 9 minutes, a moist filter 
cake with a wall thickness of only 6 mm weighing 765 g was obtained. For a 
solids content of the moist filter cake of 39.9%, the filter capacity was 
calculated at only 65 kg/h/m.sup.2. After calcination (1 hour at 
900.degree. C.), the proportion of water-soluble salts still amounted to 
4.5% and the lightening power was determined at 200. 
EXAMPLE 7 
This example is also intended for comparison. The entire sodium hydroxide 
solution was added to the TiO.sub.2 hydrolyzate sludge and the pH-value 
was subsequently adjusted to pH 10.0 by the addition of Mg chloride 
solution. The quantities corresponded exactly to Example 1. After the 
hydroxide had been precipitated, the solids content and salt content 
amounted to 30.9%. Charging of the filter candles for 3 minutes and 
washing for a period three times longer produced a filter cake with a 
thickness of 5 mm and weighing 580 g. For a solids content of 39.6%, the 
filter capacity was calculated at 52 kg per hour per m.sup.2. After 1 
hour, calcination at 900.degree. C. to form MgTiO.sub.3 produced a white 
pigment which contained 15.7% of water-soluble salts and which was 
therefore unsuitable for further use. The lightening power of the Mg 
titanate pigment was determined at 60. 
EXAMPLE 8 
Precipitation in accordance with Example 1 using MgCl.sub.2 and ZnCl.sub.2 
solution at a pH-value of 10 produced a deposit of 0.5 mole of Zn 
hydroxide and 1.5 mole of Mg hydroxide, per mole of TiO.sub.2 in the 
hydrolyzate sludge initially introduced, on TiO.sub.2. After 
precipitation, the solids content amounted to 10.7%, based on the oxides, 
and to 28.2% with the dissolved salts. The product was filtered by suction 
for 3 minutes, washed for 9 minutes and dried under suction for 1 minute. 
In this process, the filter capacity amounted to 192 kg/h/m.sup.2. After 
calcination for 1 hour at 900.degree. C., an (Mg.sub.0.75 
Zn.sub.0.25).sub.2 TiO.sub.3 mixed pigment of spinel structure was 
obtained. It contained 0.8% of water-soluble salts and, after dry 
grinding, could be directly used for pigmenting paper and lacquer. 
EXAMPLE 9 
For comparison, the solutions corresponding to Example 8 were successively 
added to TiO.sub.2 hydrolyzate sludge. After the addition of sodium 
hydroxide to the TiO.sub.2 hydrolyzate, Mg- and Zn-chloride liquor were 
mixed and added dropwise. For the same suction, washing and drying time as 
in Example 8, the filter capacity only amounted to 36 kg/h/m.sup.2. The 
white pigment obtained after calcination for 1 hour at 900.degree. C. 
contained 4.3% of water-soluble salts, showed high solubility in acids and 
was therefore unsuitable for use as a white pigment. 
EXAMPLE 10 
For comparison, the same quantities as in Examples 8 and 9 were reacted 
with the difference that the Mg-Zn-chloride liquor was initially added to 
the TiO.sub.2 hydrolyzate sludge, followed by introduction of the sodium 
hydroxide solution. Precipitation produced a non-filterable sludge which, 
for a solids content of 30% with salt and 10.5% without salt, gave a 
filter capacity of only 61 kg/h/m.sup.2 after suction for 3 minutes, 
washing for 9 minutes and drying for 3 minutes. After calcination, the Zn, 
Mg-spinel contained 25.8% of water-soluble salts and was therefore 
unsuitable for the pigmenting of paper or lacquers. 
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.