TITANIUM DIOXIDE Precipitation Factors Affecting Pigment Properties
Basic sulfate solutions in which the ratio of titanium dioxide to sulfuric anhydride is varied have been prepared from commercial titanium sulfate by three methods-namely, by direct calcination, by extraction with acetone, and by reaction in aqueous solutions with orthotitanic acid. The ratios obtained were, respectively, 1/1, 1/1.59,1/0.65. Titanium dioxide pigment of good hiding power has been produced from solutions of basic titanium sulfate by diluting the solution to create seed nuclei, a process called “autoseeding.” The turbidities or hiding powers of pigments prepared from different compositions of the system titanium dioxide-sulfuric anhydride-water have been plotted in a ternary diagram. These results show that the pigments of the best hiding powers were prepared from solutions with a titanium dioxide concentration falling between 13 and 19 per cent and ratios ranging from 1/1 to 1/0.65. Seeding opalescence occurs at different hydrogen-ion concentrations with different concentrations of solutions.
C. LIGORIO AND L. T.WORK Columbia University, New York, N. Y.
hydrolysis of titanium sulfate solutions of various titanium dioxide concentrations and acid strengths over varying periods of time, and presented curves and data showing the nature of the precipitation. The factors affecting the properties of titanium dioxide pigment have been broadly studied by Parravano and Caglioti (9) who showed that the effect of adding precipitation nuclei is to speed up the hydrolysis and to alter the quality of the finished pigment. Work and Tuwiner (16) first pointed out certain relations between the character of the solution and the hiding power of the pigment, and found that pigments with improved turbidity could be prepared from solutions of low acidity and high titanium dioxide concentration. They studied solutions on the acid side of the normal titanium sulfate, Ti(SO&, and brought out the trends by plotting turbidities on the ternary diagram for the system titanium dioxide-sulfuric anhydride-water.
Titanium Sulfate The commercial titanium sulfate used in this investigation was made from Indian ilmenite ore and was supplied by the Grasselli Chemical Company. It was found to contain 18.2 per cent titanium dioxide and 40.78 per cent sulfuric anhydride. It therefore contained 5.36 per cent sulfuric acid above the stoichiometric requirement for the formula Ti(SO&. It was used as such and also after processing to a more basic form as will be described in a subsequent section. The mole ratio of titanium dioxide to sulfuric anhydride of this compound was 1 to 2.26. This is also the weight ratio, in view of the fact that the molecular weights of titanium dioxide and sulfuric anhydride are 79.9 and 80, respectively. Although this nomenclature differs from that used in the industry, it was found convenient in expressing the basicity of titanium sulfate solutions and for this reason is used throughout this investigation. The ratio of titanium dioxide to sulfuric anhydride will henceforth be referred to as “the ratio,” and will be expressed as one part of titanium dioxide to X parts of sulfuric anhydride-for example, 1 to 0.65, 1 to 1.5, 1 to 2.26, and the like.
I
N THE manufacture of titanium dioxide pigment,
titanium sulfate solution obtained by leaching ilmenite ore with sulfuric acid is hydrolyzed by boiling. The resulting precipitate is then filtered and calcined in order to produce a pigment with the desirable properties of high hiding power and tinting strength, These properties are largely determined by the nature of the precipitating solution and the conditions of hydrolysis, which include concentration of titanium dioxide, acidity, and the creation of proper size and number of precipitation nuclei. The present investigation comprises the study of a number of phases of the titanium pigment problem, chiefly centered around the ultimate turbidity values of the pigment as affected by the character of the solution. The study covers the preparation of basic titanium sulfate solutions from the commercial sulfate and the resulting turbidity of pigments made by controlled hydrolysis of solutions over a wide range in the system titanium dioxide-sulfuric anhydride-water. Although the conclusions arrived a t in this work are based solely on the turbidity tests, sufficient published information ( I ) exists on the relation between turbidity, hiding power, and tinting strength to make this turbidity a useful guide in further work. Among the recent investigations in the titanium field is the work of Hixson and Plechner (2) who studied the thermal
Basic Solutions of Titanium Sulfate In the preparation of basic solutions of titanium sulfate, three methods were used-namely, partial calcination of the commercial sulfate, extraction of part of the sulfuric anhydride with acetone, and reaction of titanium sulfate solutions with freshly precipitated titanic acid: METHOD1. The commercial product was calcined in a Hoskins electric pot furnace and the temperature slowly raised to 280’ C., where sulfuric anhydride fumes were liberated. The heating was continued for 4 hours, and test samples were taken at hourly intervals. The products were soluble in water, and finally reached a ratio of 1 to 1.59. Heating for more than 4 hours produced a substance only partly soluble; heating for 10 hours pro213
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duced an insoluble product which, upon analysis, proved to be titanium dioxide. It was gritty and had oor color and low turbidity. The effect of heating time on sogbility is as follows: Ratio TiOt/ SO8
1/2.26 1/2.18 1/1.82 W.67
Heat-
ing SpluTime bilitv Hr. 0 Sol. 1 Sol. 2 Sol. 3 Sol.
Ratio TiOl/ SO8
Heating Time
So!u-
bility
Xr, 1/1.59 1/1.43 1/0
4 5
10
Sol. Partly Insol.
801.
By this method, solutions of ratios ranging between 1/2.26 and 1/1.59 were prepared. METHOD 2. Commercial titanium sulfate was refluxed with acetone in a large Soxhlet a paratus until the ratio was constant as shown by analysis. Tfis point was reached in about 10 hours. The resulting product was air-dried at room temperature until the odor of acetone was completely eliminated. It was white and fluffy, and slowly soluble in water. Upon analysis it was found to have a ratio of 1 to 1.04. Solutions containing about 10 per cent titanium dioxide were prepared with this salt. They were filtered to remove minor impurities present in the original material and concentrated by evaporation at 60' C. under reduced pressure to a thick sirupy consistency. This solution was clear, amber in color, and contained 24.9 Der cent ti-
VOI,. 29, NO. 2
tanium dioxide. The method of analysis is described in the next section. This solution is called "stock solution A." It was used for preparing more dilute solutions for hydrolysis and also for making solutions of more acid ratios by adding calculated amounts of sulfuric acid. In all cases the ratios were checked by analysis and were found to correspond within 1 per cent to the calculated values. METHOD 3. Freshly precipitated orthotitanic acid was added to stock solution A. The precipitate was prepared by adding an excess of dilute ammonia water to a hot solution of 10 per cent commercial titanium sulfate, filtering through a Buchner funnel, and washing with hot water.unti1 neutral to litmus. After partially drying by suction, it was added to the stock solution with constant stirring. If too much orthotitanic acid was added, the solution became cloudy and required additional portions of the stock solution to clear it. The solution was filtered and concentrated to a thick sirup; when analyzed it showed a ratio of 1 to 0.65 and a titanium dioxide content of 22.5 er cent. This solution is called "stock solution B." It was used For making dilutions for hydrolysis and as the starting point for basic solutions of ratios ranging from 1/0.65 to 1/1. All solutions preared from stock solution B had a great tendency to hydrolyze. Etock solution A, as well as all solations made from it, remained clear for several months. When stock solution B was evaporated to dryness under reduced pressure at about 60" C., it yielded a straw-colored. vitreous, hygroscopic solid which slowlv dissolved _in water. It was -possible t o prepare -small portions of solutions of different ratios by dissolving this material in water and adding calculated amounts of sulfuric acid.
Analyses and Tests
2
(E)
The analysis of titanium sulfate for titanium content was made oxidimetrically according to the method of Jones (4, 6, I d ) . The sulfuric anhydride was determined by titration with 0.1 N sodium hydroxide, using phenolphthalein as the indicator. The turbidity tests were made by dispersing a definite quantity of the pigment with acacia and saponin according to the method of Stutz and Pfund ( l o ) , using a modified Jackson turbidimeter, as described by Work and Tuwiner (16). A good commercial sample of titanium pigment was given the arbitrary value of 100 and used as the standard for comparison.
Hydrolysis of Titanium Sulfate Solutions COMMERICAL TITANIUM SULFATE.The hy-
[
2 (H20)s Ho\ Ti l O >Ti/ O (H.I,O)a H]++
-
F I Q U R1. ~ PROPOSEDDIAGRAMS OF HYDROLYTIC PRODUCTS FOR AUTOSEEDINQ AND PRECIPITATION IN BASICSOLUTIONS OF TITANIUM SULFATE
drolysis of commercial titanium sulfate solutions for the purpose of obtaining a pigment was carried out in the following manner: A solution of ratio 1/2.26 containing 26.5 per cent titanium dioxide was used as the starting point. Dilutions of 250 cc. each, representing 5, 10, 12, 15, 17, and 20 per cent titanium dioxide, respectively, were refluxed simultaneously with glass beads for 6 hours or until a portion of the solution, after filtration and reboiling, produced no further precipitate. After cooling, each precipitate was filtered through a Buchner funnel, washed with hot water until neutral to litmus, dried in an oven a t 110" C. for 4 hours, and calcined in a Hoskins electric furnace for 1 hour a t 850" C . The product was milled in a mortar for 15 minutes and tested for turbidity. In general, pigments made from these solutions were gritty, had poor color, and gave low turbidity. I n calcination, a platinum rhodium thermocouple with one junction in melting ice was used, and the temperature was read upon a calibrated high-resistance millivoltmeter. BASICTITANIUM SULFAWSOLUTIONS. The starting point for these solutions was stock
FEBRUARY, 1937
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solution A of mole ratio 1/1.04 and stock solution B of mole ratio 1/1.65. As already explained, these ratios were varied by adding calculated amounts of sulfuric acid to weighed portions of stock solution. When t h e required ratios were reached, dilutions were made representing various percentages of titanium dioxide and were refluxed for 6 hours. Under comparable condi$ions Parravano a n d Caglioti (9) found that about 95 per cent of the titanium dioxide was precipitated. The precipitates were filtered, washed with hot water until PER CENT TITANIUM OXIDE. neutral to litmus, calcined, FIGURE2. HYDROLYSIS OF SEEDED SOLUTIONS OF TITANIUM SULFATEBY BOILING milled, and tested for turbidity. The hydrolysis of the solutions made from stock solution A was complete in 2 The process of developing seed nuclei by creating an opalescence by the limited dilution of a basic solution of hours; that of the commercial sulfate required 6 hours, while the one made from stock solution B hydrolyzed in 30 titanium sulfate containing dispersed titanic acid will be referred to as “autoseeding.” The method was used reminutes. This behavior is in agreement with the findings of peatedly and gave concordant results which checked among Mecklenberg (7) and Kayser (6). themselves within 4 per cent. SEEDED BASICSOLUTIONS,When titanium sulfate solutions of ratio more basic than 1/1 made from stock solution B were progressively diluted with water, a point was reached where Autoseeding and Precipitation an opalescence bordering on a slight turbidity developed. The wide differences in hiding powers which are obtained This point can easily be duplicated by visual observation. when different solutions of titanium sulfate are hydrolyzed I n the measurements of hydrogen-ion concentration, however, are chiefly due to the manner in which the several steps in this opalescence was distinguished by looking down through precipitation take place. a glass absorption cell with a 27-mm. depth of liquid; the Much speculation and interest exist in the titanium pigopalescence was considered to be reached when an India ment industry concerning the mechanism by which these ink line on white paper was obscured. With different consteps occur. The similarity in behavior between other centrations and ratios, this point appeared a t different pH hydrous oxides (11) and titanium dioxide (14) is sufficiently values. I n all cases, the more basic the solution the sooner definite to permit comparison. Reasoning from analogy, this opalescence appeared. therefore, and beginning with the prepared solution of titanium sulfate, the steps in the seeding operation and precipitation may be outlined in terms of Werner’s theory of complex ions (16). A titanium ion with a coordination number of eight in an aqueous solution (8) is said to hold eight water molecules (Figure 1A) by the secondary valence forces postulated in this theory. Upon hydrolysis, one of the water moleculea may ionize, splitting off a hydrogen ion which goes in the solution and leaving behind a titanium PER C E N T TITANIUM OXIDE. complex with a hydroxo group, B, and lowering the charge on the ionic micelle. As hydrolysis pro>gresses, this complex may become sOg attached through this hydroxyl 50 $ bond to another titanium com2 plex, forming a dinuclear titanium 40 01-compound, C. Depending on 5 the hydrolysis conditions, it is also 30 possible for these hydroxo groups 206 to become oxolated through the 10 loss of the hydroxyl hydrogens, D. More water may be dissoPER CENT TITANIUM OXIDE. ciated from the dinuclear comF I G U R3.~ HYDROLYSIS OF UNSEEDED SOLUTIONS OF TITANIUM SULFATE BY BOILINQ pound to form more hydroxo com-
INDUSTRIAL AND ENGINEERING CHEMISTRY
216 ISOTURBIDITY /
/'
FROM
~'UNSEEDED SOLUTIONS OF WORK Et TUWINER ISOTURBIDITY FROM SEEDED
VOL. 29, NO. 2
the compound becomes less stable until the amount penetrated has so reduced the charge that the ratio of mass to charge is so great that the micelle precipitates.
Results of Hydrolysis Tests
SOLUTIONS
From the hydrolysis of a series of unseeded solutions of ratios ranging from the most acid, 1/2.26, to the most basic, 1/0.65, and containing varying amounts of titanium dioxide, precipitates were obtained and tested for turbidity. In a similar manner hydrolyses were made with seeded solutions of ratios ranging 0" " " " " " " \' \' " "L from 1/1 to 1/0.65. The values of turbidity are IO 20 30 100% HzO plotted as indicated in Figure 2 and show that PER CENT SO3 the seeded solutions yielded pigments of high FIGURE 4. HYDROLYSIS OF SEEDED SOLUTIONS OF TITANIUM SULFATE, IN turbidity. The unseeded solutions gave prodTHE SYSTEM TITANIUM SULFATE-SULFUR TRIOXIDE-WATER, B Y BOILING ucts low in hiding power, poor in color, and lacking brightness, pounds, E. By olation the formation of polynuclear comFigure 2 is further used in developing the ternary diagram pounds, F , of titanium follows, growing in size and tending of isoturbidity lines in the system titanium dioxide-sulfuric toward the colloidal state. As long as hydrolysis takes anhydride-water as shown in Figure 4. For the unseeded place, this growth mechanism continues until very large solutions, a, similar set of data is plotted in Figure 3, but no titanium complexes are built up and the colloidal state is ternary diagram is given on account of the irregular behavior reached. The solution then displays all the characteristics of the unseeded solutions. of colloids, such as the Tyndall phenomenon, Brownian movement, nondiffusive tendencies, and the like (9). G Turbidities in the Ternary System represents an oxolated polynuclear titanium compound after The results of the turbidity tests on the calcined hydrolytic dehydration. products obtained by autoseeding are plotted as a ternary Jander and Scheele (3) found that some soluble basic diagram (Figure 4). Smooth curves were drawn through the salts of chromium had an average molecular weight of points of turbidity vs. per cent titanium dioxide for the several 84,000 and contained from 600 to 700 chromium atoms. ratios (Figures la-D), Inaccuracies in individual points Basic titanium salts may also have very large molecular which were caused by seeding technic were thus eliminated. weights, in which case the various compounds illustrated For each ratio the percentages of titanium dioxide correrepresent only the smallest units formed. Through olation, sponding to 40,60,80, and 100 per cent turbidity were plotted complexes containing a great number of titanium atoms may on the corresponding ratio lines on the ternary diagram, and form and still be soluble. Through olation and some oxolathe isoturbidity lines drawn to show the trends of these points. tion, partly soluble complexes may form. Because of the difficulty in controlling the seeding process, The opalescence developed in autoseeding and the final the steps in this procedure were taken in sequence without precipitation seemed to be the result of all the steps outlined. revision. Under the circumstances a difference of about 10 For example, it was found that the acidity of the solution per cent turbidity may exist in each isoturbidity line. I n increased as hydrolysis progressed, and that the opalescence all cases, however, the trend of peak turbidity is well estabcould be destroyed after prolonged heating a t 60" C. When lished. the opalescence disappeared, it no doubt represented a deThe diagram shows a zone of high turbidity for a ratio of olation process where the polynuclear complexes broke titanium dioxide to sulfuric anhydride ranging from 1/1 to down to form smaller units. When the opalescence failed to 1/0.65 and of comparatively low turbidity on either side of disappear, it showed, no doubt, that oxolation had progressed these values. Concentration of titanium dioxide seems less to the point where reversibility was more difficult to attain. important, although the turbidity is high between 13 and Whenever this happened, the final pigment showed poor 19 per cent. It is evident from the ternary diagram developed hiding power. It might be reasoned that the seed nuclei in the present investigation that in the unseeded region shown had developed in such a manner that they failed to act as in the diagram of Work and Tuwiner (16) there is a slight good starting points for the growth of the titanium dioxide difference which is probably due to the absence of seed nuclei. micelles, It may be that they gave the finished precipitate It was shown earlier in the investigation that high turbidities a poorly oriented structure with respect to crystal habit. could not be secured in the alkaline region by direct hydrolysis Particularly in the case of titanium pigment, it is necessary without autoseeding. Experimental values within the highest to create the particle nuclei in an oriented pattern since the isoturbidity line indicate that the peak turbidity is about 103. calcination only slowly converts the unoriented structure to the oriented form of high hiding power. In accordance with von Veimarn's concept ( I S ) , if the solution was not seeded, Literature Cited the unoriented structure would prevail and would be respon(1) Dunn, E. J., Jr., IND.ENO.CHEW,Anal. Ed., 4, 191 (1932). sible for products which are poor as pigments. It is also pos(2) Hixson, A. W., and Plechner, W. W., IND. ENQ.CHEM.,25, sible that the sulfuric anhydride, which is always present in 262-74 (1933). the final hydrolytic product obtained from solutions of ti(3) Jander, G., and Soheele, V., 2. anorg. allgem. Chem., 206, 241 (1932). tanium sulfate, might be an indication of the state of orienta(4) Jones, C., Trans. Am. Inst. Min. Engrs., 17, 414 (1889). tion of the precipitate. According to theory, it finds its way (5) Kayser, L., 2. anorg. allgem. Chem., 138, 43 (1924). into the complex by penetration, each sulfate radical dis(6) Lundell, G. E. F., and Knowles, H. B., J. Am. Chem. SOC., 45, placing two aquo groups, or an equivalent number of other 2620-3 (1923). (7) Mecklenberg, W., U. 8 . Patent 1,758,528 (May, 1930). groups (Figure 1H). By neutralizing the cationic charges, "
"
"
"
I
_
FEBRUARY, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
(8) Mellor, J. W., “Comprehensive Treatise on Inorganic and Theoretical Chemistry,” Vol. VII, p. 84,New York and London, Longmans, Green and Go., 1925. (9) Parravano, N., and Caglioti, V., Gazz. chim. ital., 64, 429-50 (1934).
(10) Stutz, G. F. A., Jr., and Pfund, A. H., IND.ENG.CHEM.,19,
.
,
51-3 ._ 11927).
(11) Thomas, A. W., and co-workers, J. Am. Chem. Soc., 54, 841 (1932) : 56, 794 (1934) : 57, 4 (1935). (12) Thornton, W., Jr., “Titanium,” A. C. S. Monograph 33, pp. 140-53, New York, Chemical Catalog Go., 1927.
Antioxidants and the Autoxidation of Fats
217
(13) Washburn, E. W., “Introduction to Principles of Physical Chemistry,” 2nd ed., New York, McGraw-Hill Book Co., 1921. (14) Weiser, H. B., “Inorganic Colloid Chemistry,” Vol. 11, pp. 257-75, New Y o r k , John Wiley &a Sons, 1935. (15) Werner, A., “New Ideas on Inorganic Chemistry,” by E. P.
Hedley, New Y o r k and London, Longmans, Green and Co., 1911.. ~ ~ (16) Work, L. T., and Tuwiner, S. B., IND. ENQ.CHEM.,26, 1263-8 (1934).
RECEIVED August
10, 1936.
VIII. Autoxidation of Oleic Acid, Methyl Oleate, and Oleyl Alcohol‘ L. A. HAMILTON* AND H. S. OLCOTT University of Iowa, Iowa City, Iowa
ATURAL fats and oils manifest two kinds of changes Previous workers have examined the oxidation of oleic acid by isolation of the end products after prolonged exin the presence of air or oxygen. The more highly posure to oxygen (6, 6,18,20). The numerous compounds unsaturated or drying oils absorb oxygen and polywhich have been detected and isolated indicate not only that merize to form stable films. Fats which are less unsaturated the reactions are complex, but also that some methods which absorb oxygen more slowly and in due time exhibit the have been used to accelerate the rate of oxygen absorption phenomena of rancidity. Purified oleic acid acquires a actually change the course of the reaction so that different rancid odor on exposure to air and light indicating, as Powick end products are formed. (14) pointed out, “that the oleic acid radical is the point of The experiments to be described were an attempt to exattack in the development of rancidity, and that a study of plore the mechanism of the uncatalyzed oxidation of oleic the chemistry of rancidity should begin with a study of the acid and two closely related compounds, methyl oleate and oxidation of oleic acid.” oleyl alcohol, by following and correlating the changes Despite the paucity of experimental data, several theories which are amenable to reasonably accurate measurement. have been advanced to account for the various transitory and These include the absorption of oxygen, the decrease in unend moducts of the reaction between oleic acid and oxygen. saturation, the evolution of water and carbon dioxide, and Bull-(,$) reviewed some of these. It is generally agreed that o n e m o l e c u l e of oxygen is initially absorbed at the double bond. The actual nature of The course of oxidation of oleic acid, methyl oleate, and oleyl the ”compound formed is unalcohol was studied by an apparatus and methods which permit the certain and was recently dissimultaneous measurement of the oxygen absorbed and of its districussed by Morrell and Davis bution among the transitory and final products of oxidation, including (11). From this point to the water, carbon dioxide, and carboxyl, hydroxyl, peroxide, and aldehyde end of the complicated series of r e a c t i o n s , the mechanism compounds. is u n k n o w n . Any proposed In the initial reactions a t the double bond, each molecule of theory must take into account methyl oleate and oleic acid absorbs approximately four, and each the presence of peroxides, hymolecule of oleyl alcohol approximately five, atoms of oxygen. Simuldroxyl groups, water, carbon taneously each of the three compounds loses one molecule of water. d io x i d e , e p i h y d r i n aldehyde (responsible for the Kreis test), The peroxide level in the early stages of oxidation is higher in oleyl and the numerous aldehydes alcohol and methyl oleate than in oleic acid ; conversely, the hyand acids which have been found droxyl content of oxidizing oleic acid is higher than that of methyl in rancid fats.
N
1 Previous papers in this series rtppearedin J . Biol. Chem., 90, 141 (1931): J . A m , Chem. SOC.,66, 2492 (1934); IND. ENQ.CHEM.,27, 724 (1936); Oil & Soap, 13, 98, 127 (1936); J . Am. Chem. Soc., 68, 1627, 2204 (1936). 2 Present address, Fooony-Vacuum Oil Company, Psulsboro, N. J.
oleate or the extra hydroxyl of oleyl alcohol. The destruction of the double bond occurs faster than would be expected i f the reaction proceeded at a unimolecular rate, presumably because of secondary reactions, which become more prominent as the oxidation progresses. A tentative explanation of these observations is proposed.
