Preparation and Properties of Titanium Pigments Properties of Titanium Sulfate Solution LINCOLNT. WORK,SIDL\;EY B. TUWINER. AND ARTHURJ. GLOSTER, Columbia University, New York, N . Y . HE attainment of pigment value in c a l c i n e d tita-
T
The attainment of pigment ualue in calcined titanium oxide is affected by many factors, beginning with the character of the solution. I n the system titanium oxide-sulfur trioxide-water certain zones fend to produce more suitable precipitates, and analysis of the solution from the partial volume viewpoint has shown that dilute solutions are hydrolyzed appreciably and that such hydrolysis is materially affected by the acid conient. After the precipitation is complet?, the residual concentration may play a n important part in the state of the precipitate. A brief mathematical treatment reveals thal this end condifion m a y be used as one criterion of the character of the titanium pigment which will result.
a small stream of air through the solution during the period of hydrolysis by means of a glass tube nium oxide i s affected by passed through the condenser and many factors, beginning with the connected with an air line through solution from which precipitaa wash bottle. A Hoskins pot furnace was used tion takes p l a c e . Concentrafor calcination. This consisted tion of titanium oxide and the of an insulated nichrome-wound amount of acid in excess of stoic e r a m i c core i n which was chiometric requirements have placed a 4-inch (10.2-cm.) quartz c r u c i b l e w i t h a quartz cover. been stressed from the viewpoint Top insulation was provided by of economy and of yield. In a T r a n s i t e board cover with a industrial practice, factors such 4.5-inch (11.4-cm.) r e m o v a b l e as the effect of iron salts and of center. Above this was placed a second layer of board, covering lesser impurities and the use of the entire f u r n a c e t o p , which closely controlled precipitation could be removed w h e n e v e r i t nuclei also play a dominant role was necessary to add or remove in the ultimate quality of piga c h a r g e . A small hole drilled a t the center t h r o u g h t h e two ment. I n this section the results lavers of board and through the of some studies on the system cqucible cover permitted tThe intitanium oxide-sulfur trioxidesertion of a thermocouple. water are presented. The temperature of the furnace was varied by adjusting a 7-ohm rheostat connected in series PREPARATION O F P U R E TITANIVM SULFATE with the winding across the 110-volt d. c. line. Having once established a steady state, the variation over a period of hours Pure solutions of titanium sulfate mere prepared from the could be kept to within 5" C. anhydrous tetrachloride essentially a s described by Hixson A platinum-rhodium thermocouple with an ice junction was and Plechner ( 1 ) . Modifications consisted chiefly in the use employed. The portion of the wire inserted into the furnace enclosed in alundum tubing, the end being sealed with of a small amount of sulfuric acid in the hydrolyzing solution was alundum cement to protect the entire heated portion from corroand in the method of clarifying the final solution of the sulfate. sive action. The temperature was read upon a calibrated highThe sulfuric acid aided in the formation of readily filterable resistance millivoltmeter. After the desired temperature had been reached, a porcelain titanic acid, thereby permitting the hydrolysis of chloride in crucible containing the sample was placed a t the bottom of the greater concentration than previously had been possible. quartz crucible of the furnace. The top covering was then imCommercial titanium tetrachloride was redistilled once a t mediately replaced. In this way a.charge could be added, or 134" to 138" C. About 400 cc. were dissolved in 1400 cc. of similarly removed, with no appreciable temperature drop in the water. The resulting solution was reduced by stick zinc to a interior. There was but little question of the effective period of light brown color, and 100 cc. were further reduced to a deep calcination since the charge assumed the temperature of the violet. The highly reduced solution was added drop by drop furnace almost immediately after its entrance, and dropped to to a boiling solution of 5 grams of oxalic acid and 10 cc. of concen- room temperature after its removal. With this arrangement trated sulfuric acid in about 5 liters of water. Then 800 cc. of as many as four samples could be calcined simultaneously. This procedure was used generally in the precipitations and the lightly reduced solution were added slowly, and boiling was continued for 6 hours, water being added from time to time to calcinations of test compositions. In this portion of the study the calcined precipitates from the pure solutions were milled in an maintain the volume. The precipitate of titanic acid formed was filtered on a large agate mortar and tested for obscuring power with the turbidimBuchner funnel, washed free of chloride and iron with 15 liters eter. Turbidity was measured with an improved form of the Jackson of distilled water, and sucked dry. No trouble was encountered in the filtration or washing of the material hydrolyzed in this turbidimeter. The tube of this instrument is surrounded by a dark box with top shielding for the elimination of side radiation. way. The cake was transferred to a beaker and mixed thoroughly Constant high-intensity light produced by a lamp passes through with one and a quarter times its weight of concentrated sulfuric a cross-shaped opening below the tube and through the susacid, and the mixture gently boiled until the formation of a solid pension, being viewed by means of an inclined mirror within the cake of crystalline sulfate. After cooling, an equal weight of top shield. The suspension is then poured into the tube through a water was added and the whole allowed to stand for several funnel a t the top until the depth is such as to cause the image of days with occasional stirring. The resulting solution was then the cross to become obscured. With care the readings so obclarified by filtration through a bed of freshly precipitated titanic tained may be relied upon to within 2 per cent. In preparing the suspension, the method of Stutz and Pfund acid and prepared as described above. The clear solution, after addition of the required amount of sulfuric acid, was then aged ( 5 ) was used. A 0.25-gram sample was ground for 8 minutes in a glass mortar with 0.05 gram of saponin and 0.1 gram of gum for at least one week and diluted as required. arabic in 1 cc. of a 0.05 N solution of barium chloride. The resulting thick dispersion was then diluted with 100 cc. of water PRECIPITATION, CALCINATION, AND EVALUATIOX OF PIGMENTS from which suspension an aliquot was taken and diluted as conVarious concentrations of components in the ternary system venience dictated. Three different concentrations were taken from each sample were prepared and hydrolyzed by boiling in 500-cc. flasks attached to water-cooled reflux condensers for periods of 8 hours. and a plot was made of depth of suspension for total extinction Approximately constant agitation n-as afforded by bubbling us. dilution. The best straight line was then passed through 1263
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these points, and the reciprocal of the slope taken as the turbidity. An arbitrary value of 100 was assigned to the best commercial sample; all other samples were thus given values which are relative to this.
Vol. 26, No. 12
PARTIAL SPECIFIC VOLUMES
Colloidal and often gelatinous in nature, the titanium oxide precipitates are of indefinite chemical composition with properties depending upon the mode of preparation. HowPIGMENTS FROM PURE SOLUTIOXS ever, there appears to be established by a state - precipitation - A number of solutions of pure unseeded titanium sulfate of aggregation which controls the character of the calcined covering a wide range of composition were prepared and pigment. The grain size of the product and consequently its hydrolyzed as described. The pigments obtained by filtering, obscuring power are determined primarily b y the composition washing, drying, and calcining the precipitated titanic acids and conditions of the hydrolyzing sulfate solution and only for one-half to one hour were then milled for 15 minutes and secondarily by the calcination. Analogies with other solutions of this type would indicate that dissociation into basic tested. compounds or into molecularly dispersed titanic acid may occur, particularly on dilution. This has been studied briefly with solutions of fixed ratio of titanium oxide to sulfur trioxide. Two stock solutions of titanium sulfate were used. Solution A contained the equivalent of about 9 per cent titanium oxide and had been aged for a year; solution B contained the equivalent of about 14 per cent titanium oxide and was comparatively fresh. Solution A was of approximately stoichiometric proportions (1 SO, - P e r c e n t by W e i g h t mole titanium oxide to 2 moles sulfur trioxide); and solution B was slightly lower in sulfur triFIGURE1. TURBIDITIES OF PIGMENTS PREPARED BY HYDROLYSIS OF S O M E oxide. After the composition of each solution COMPOsITIONs IN THE SYSTEM TITANIUM OXIDE-SULFUR TRIOXIDE-WATERhad been determined, many different solutions of each were made by accurately weighing porThe results reported in Table I illustrate the variation of tions of the stock solution and compounding them with acturbidity with composition of solution. They are obtained curately weighed portions of distilled water. The relative by averaging the turbidity values obtained by calcining por- dilutions were thus accurately maintained. tions of a precipitate a t several temperatures in the range Specific gravities of the several solutions obtained in this from 720" to 900" C. This method eliminated the errors in way were determined precisely from pycnometer measureindividual calcination and turbidity tests and did not intro- ments in a thermostat bath a t 25' C. * 0.05". Specific duce a serious error on account of the temperature range used. volumes were calculated from these values.
TABLE I. TURBIDITIES OF PIGMENTS PREPARED BY HYDROLYSIS OF
SOMECOMPOSITIONS IN THE SYSTEM TITANIUM DIOXIDE-
SULFURTRIOXIDE-WATER
RUN A
Ti02 SOs % b y weight
B
C
2.22 4.94 8.69
3.58 8.00 13.83
TURBIDITY No. of testa RelaR ~ ~ i o S 0 a : T i O z averaged tive % 1.61
4 6 4
31 32 45
D
4.85
12.21
2.52
4
16
E
8.97
19.09
2.13
2
11
F G H I J
1.92 2.95 3.35 4.99 6.30
3.52 5.38 6.15 9.14 11.53
1.83
4 4
5 3
3
12 13 15 26 29
K
5.76
14.43
2.50
6
29
L M
6.69 6.50
17.80 20.20
3.12
6 6
11 10
N
3.97 2.73 3.04 2.87 1.54
14.89 10.25 11.40 10.76 5.78
3.75
4 2 3 5 2
11 14 12 14 10
0
P
8
The data show that the samples tested were not comparable in turbidity with commercial samples. However, they show certain distinct trends which reveal characteristics of the solution affecting the ultimate quality of pigment. The turbidity values have been spotted on a corner of the ternary system and trend lines of constant turbidity interpolated between them (Figure 1). Turbidities in the range of 15 to 30 are shown to fall near the line of stoichiometric compositions, while on the acid side the values are lower and on the alkaline side they are higher. The curvature of these constantturbidity lines indicates changes in character of the solution probably in the nature of partial hydrolysis.
FIGURE2. PARTIAL SPECIFIC VOLUMES IN TITANIuni SULFATE SOLUTIOIVS The solution concentrations and corresponding specific volumes were then tabulated. Using the method of successive differences (S), the derivative or rate of change of the specific volume function with respect to concentration was determined for each interval. The derivative will be denoted by dV/dT. If T i s the percentage composition of solute, and V the specific volume of the solution, the partial specific
I Iv D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
December, 1934
volume of the solvent is defined by Lewis and Randall (3) as V T(dT‘/dT). It is the volume, in cubic centimeters, b y which a large quantity of the solution mould increase when diluted with 1 gram of solvent. The partial specific volume is, of course, a function of the solute concentration except for a perfect solution, in which it is a constant equal to the specific volume of pure solvent. Deviations of the partial specific volume function from the specific volume of the solvent denote deviations from the laws of ideal solutions. The results from the two solutions are shown in Table 11, and the partial specific volumes of water are plotted against titanium oxide content in Figure 2. Solution A approximates ideal up to a concentration of 4 per cent titanium oxide when there is a zone of metastability and a larger break from the ideal. Below this concentration all solutions appeared turbid, while above it they were clear. Solution B, which was clear a t all concentrations tested, does not show the zone of metastability, and still shows similar marked deviation from the ideal as the titanium oxide content increases. It would appear that there is hydrolysis in both solutions when dilute, and that this diminishes but does not necessarily disappear a t higher concentrations. Further, aging of the stock solution appears to affect the dilute solution through the formation of larger nuclei which cause turbidity.
+
TABLE1 I.
PARTI.4L
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The rate of precipitation of titanic acid from solutions of the sulfate hydrolyzed near the boiling point will, in general, depend both upon the momentary composition of the solution and upon the character and amount of precipitate and colloidal matter with which it may be in contact. If, for simplicity, the solution composition is assumed to be the controlling factor, the following may be argued: Consider a n unhydrolyzed solution of titanium sulfate, and let cl be its
SPECIFIC VOLUMES PARTIAL SP.VOL.,
RUN
%TiOz, T
1
0.5236 1.5511 2.5722 3.5662 4.5519 5.5899 7.6042 8.6344 9.8300
2
1.0740 2.1316 3.0588 4,0458 5.0028 5.6655 6.3658 6.9095 7.9471 8.9597 9.5295
SP. VOL.,
v
if_v dT
dV
VfTpi, 1.00251 1.00214 1.00186 1,00037 1.00226 0.99703 0.99309 0.98980
0.97690 0.95181 0.92996 0.90686 0.88364 0,86837 0.85214 0.83983 0.81686 0.79446 0.78198
0.02373 0.02360 0.02341 0.02427 0.02303 0.02319 0.02267 0.02210 0.02210 0.02191
.....
1.00239 1.00212 1.00157 1.00504 0.99885 0.99975 0,99645 0.99253 0.99249 0,99077
.....
GOLUTION B b
1
2
a b
0.5067 1.5131 2.5094 3.3658 4.3149 5.2942 6.4785 7.4818 9.0770 11.9671 12.9647 13.8400
0,99083 0.96749 0.94440 0.92462 0.90286 0.88069 0.86432 0.83231 0,79796 0,73732 0.71724 0.70051
0.02320 0.02318 0.02307 0.02293 0.02263 0.02223 0.02195 0.02153
1.00254 1.00256 1,00229 1.00180 1.00051 0.99838 0.99653 0.99339
0.02014 0.01910
0.97834 0.96487
0.9932 1.9835 2.9519 3.9505 4.9568 5.9377 6.9860 8.0516 8.3144 9.3299 10.3823 11.4714 12.5105 13.8400
0.97960 0.95662 0.93428 0.91133 0.88846 0.86639 0.84314 0,82010 0.81446 0.79280 0.77012 0.74776 0.72688 0.70062
0.02320 0.02306 0,02298 0.02275 0,02250 0.02222 0.02163 0.02143 0 . 0 2 137 0.02156 0,02054 0.02013 0.01983
1.00264 1.00236 1.00212 1.00120 0.99999 0.99833 0.99425 0.99265 0.99214 0.99395 0.98338 0 . 97868 0.97497
.....
.....
6 8 /O Percent Ti02 FIGURE3. THEORETIC4L CURVES RELATINGTIT.%NIUM TITANIUM OXIDE OXIDE CORTENT WITH EQUIVALENT CONTENT .4T ACIDITY FACTOR 1.02
2
B O L U T I O i i A“
4
titanium oxide content in weight per cent, a1 its total sulfuric acid content similarly expressed, and f1 its acidity factorthat is, the ratio of the weight per cent of the excess sulfuric acid above that necessary for the formula Ti(SO& to the weight per cent of dissolved titanium oxide. Assume that after a certain period of hydrolytic precipitation these quantities will have changed to c2, az, and f2, respectively, owing to the precipitation of titanium oxide. Such a precipitation will alter both the titanium oxide content of the solution and the acidity factor, but the total weight of sulfuric acid may be assumed to remain essentially constant. Thus, if Ma and Mt are used to denote the molecular weights of the acid and the titanium oxide, respectively:
.....
By numerical substitution:
Solution A: 1.98 moles of SO8 per mole of TiOn: ratio ‘Ti02 t o SOa, 0.505. Solution B : 1.78 moles of SO8 per mole of TIOP: ratio ‘Ti02t o 903, 0.561,
CONDITIOKS DURING HYDROLYSIS This phase of the investigation is based upon a mathematical approach and a study of experimental data. Through the courtesy of Hixson and Plechner the data in their experimental study was made available for comparison with the relations derived mathematically.
It is recognized that there may be sorption effects, basic sulfates, and a change in the volume of the solution due to precipitation of titanic acid. These factors may be significant, but for the present they are neglected and the percentages of acid, aland u2, are equated: Two solutions, 1 and 2, which are related in this way are here called “equivalent” within the limitations imposed. This implies that, upon removal by hydrolytic precipitation of the quantity of titanium oxide, c1 - c2, from solution 1, the composition becomes identical with that of solution 2. When this point has been reached, the further course of precipitation in the two solutions should be the same according to the hypothesis that has been made.
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Vol. 26, No. 12
contents a t this acidity factor are taken as cz. From these are found corresponding values of c1 at some other acidity factor, fi, and a functional relationship is obtained which may be plotted. The results are shown in Figure 3. These curves, sensibly linear, relating the titanium oxide contents of corresponding solutions of titanium sulfate, reveal an analogy to Duhring’s rule relating the temperatures of two liquids for which the vapor pressures are the same (4). It will be of interest to compare the curves obtained in this way with those derived mathematically from the simple hypotheses that have been made. If in Equation 2 , f 2 = 1.02, then: where K If fi fi
= =
ji = fi
FIGURE4. EXPERIMENTAL CURVESRELATING TITANIUM OXIDE CONTENT WITH EQUIVALENT TITANIUM OXIDECONTENT AT ACIDITY FACTOR 1.02 Solutions which are “equivalent” should therefore show similar curves of titanium oxide yield us. time for solutions hydrolyzed a t the boiling point. Hixson and Plechner (1) have obtained such curves for a number of pure titanium sulfate solutions a t acidity factors of 0.10, 0.19, 0.52, and 1.02. The corresponding types for these acidity factors have been selected from similarities in the shapes of the curves of titanium oxide yield us. time. While admittedly somewhat inexact, these selections are not too difficult, as may be verified from an examination of the data from which they are derived. The highest acidity factor is chosen as a standard of comparison. Thus f2 equals 1.02 and the various titanium oxide
=
+
~2 = (2.44 fi)~i/3.46 Kci (3) a constant depending only upon acidity factor fi 0.10, K = 0.734 0.19, K 0.760 0.52, K = 0.855 1.02, K = 1.000
In Figure 4 the several curves have been plotted from these equations, and they bear a close resemblance to the curves of Figure 3. With due correction for change in concentration of sulfuric acid which has been neglected in this derivation, the curves approach still more closely those experimentally determined. In the industry the situation is further complicated by the presence in the solutions of ferrous sulfate, titanous sulfate, and other salts, and the deliberate addition of seeding material, all of which seriously modify both the rate of precipitation and the character of the product.
LITERATURE CITED (1) Hixson, A. W., and Plechner, W. W., IND.ENQ.CHEM.,25, 262 (1933). (2) Lewis, G . N., and Randall, M., “Thermodynamics and the Free Energy of Chemical Substances,” p. 38, McGraw-Hill Book Co., New York, 1933. (3) Perry, J. H., Chemical Engineers’ Handbook, p. 241, McGrawHill Book Co., Xew York, 1934. (4) Perry, J. H., and Smith, E. R., IND.ENQ.CHEM.,25, 195 (1933). (5) Stutz, G. F. A , , Jr., and Pfund, A. H., Zbid., 19, 51 (1927).
* * * * *
Effect of Precipitation, Calcination, and Subsequent Treatment LINCOLNT. WORKAND SIDKEY B. TUWINER Calcination tests reveal marked diferences in the character of the precipitate. The recognized effect of seeding is of dominant importance. Pigments produced from unseeded solutions tend to be coarse in size when distribution curues are shown. The effect of calcining temperature has been studied and shown lo be of limited importance over a normal range but to vary appreciably
I
N T H I S section a number of variables, besides the composition of the solution, which affect the ultimate quality of the pigment are considered. P I G M E N T S FROM COhlMERCIAL P R E C I P I T A T E S
It has been shown that the pure solution did not yield sufficient nuclei as treated to give a pigment of high hiding power. It is recognized that these nuclei play an important part in the ultimate quality of this type of pigment. I n order to make a comparison on commercial precipitates, two samples, X and Y, were supplied by the Titanium Pigment Com-
at temperatures above 850” C. The character of the pigment appears to be agglomerate rather than crystalline, and milling produces signgcant effects in oil absorption and hiding power. Microscopic examination of milled particles shows the structure of indioidual particles and clusters. The counting of doublets, triplets, etc., is used as a criterion of this effect. pany as representative precipitates from commercial manufacturers. They were calcined in a similar manner to that used for the precipitates from pure solutions. Turbidimetric measurements were made on both the pigment as obtained from calcination and on the same material ground for one hour in an agate mortar. Oil absorption and tinting strength mere measured on milled materials alone. Tinting strength was run in accordance with the procedure of the National Lead Company, and the oil absorption by the method of Gardner ( 2 ) . However, smaller samples were used, and the results are
