INDUSTRIAL AND ENGINEERING CHEMISTRY
910
(22A) Ibid., p. 134, Fig. 215. (23) Hart, L. P., Ball, G. L., Jr., and Johnson, E. E., Natl. Paint, Varnish Lacquer Assoc., Sei. Sect., Circ. 567, 175 (1938). (24) Haslam, J. H., and Werthan, S., IND.ENG. CHEM.,23, 226 (1931). (25) Jacobsen, A. E., Oficial Digest Federation Paint & Varnish Production Clubs, 146, 215 (1935). (26) Mills, B. S., Painter’s Hand-Book, p. 42, Eckstein White Lead Co., 1887. (27) Morrell, R. S., “Varnishes and Their Components”, p. 21 (1923). (28) Pittsburgh Plate Glass Co., “Wdlhide Exterior Primer”, April, 1933. (29) Robertson, D. W., “House Paint Primers and Undercoaters”, Titanium Pigments Go., May, 1936.
Vol. 33, No. 1
(30) Robertson, D. W., Oficial Digest Federation Paint & V a r n i s h Production Clubs, 146, 228 (1935). (31) Robertson, D. W., and Jacobsen, A . E., IND.ENQ.CHEM.,28, 403 (1936). (32) Schmutz, F. C., Palmer, F. C., and Kittelberger, W. W.,
“Improving Paint Service on Wood with Special Priming”, New Jersey Zinc Co., March, 1936. (33) Schmutz, F. C., Palmer, F. C., and Kittelberger, TV. W., IND. ENG.CHEM., 22, 856 (1930). (34) Schmutz, F. C., Palmer, F. C., and Kittelberger, W. W., Oflcial Digest Federation Paint & Varnish Production Clubs, 141, 355 (1934).
PRESENTED at a joint meeting of the Twin-Cities Paint, Varnish, and Lacquer Association and the Northwestern Paint and Varnish Production Club, a t St. Paul, Minn.
Drying Air with Phosphoric Acid in a Packed Tower M. M. STRIPLIN, JR. Tennessee Valley Authority, Wilson Dam, Ala.
a drying agent for gases. From their data it is apparent that dustrial use is to pass it phosphoric acid can be employed to dry air sufficiently for through apacked tower counteruse in processes where atmoscurrent to a stream of a liquid pheric air would not be suitdesiccant. There are installaable. tions of this type in operation using sulfuric acid. Greenewalt I n addition to vapor pressure ( 2 ) published some data on data, i t is essential in the design drying air with sulfuric acid in of large-scale drying towers to a 2-inch-diameter wetted-wall have data from which to calculate the required volume of a column. given tower packing. I n the The present work was underabsence of such data the design taken to supply data for the may result in a tower either too design of drying towers using small to remove the desired phosphoric acid as the desicK c a = 0.01 G lb. moles/(hr.) (cu. ft.) (atm.) quantity of moisture from the cant. The development of this air or one larger than necessary. p a r t i c u l a r m e t h o d w a s of New vapor pressure measurements for Absorption tower performance interest in connection with proc95.0 and 99.0 per cent H3P04 are correlated data suitable for large-scale esses requiring dry air which in convenient form with previous data of plant design must be obtained yield phosphoric acid in such from similar large-scale equipquantities and concentrations other concentrations to permit the calcument or from moderately large that it may be employed in the lation of ICGU. pilot plants. Sherwood (9) drying tower. The use of a stated that data obtained in drying - _ agent _ .produced continutowers less than 6 inches in diameter are of relatively little ously in the process is an advantage because such a procedure value for design purposes, but that data obtained in larger eliminates the cost of regenerating a drying agent. towers serve as a reasonably adequate basis if used properly. The importance of phosphoric acid as a drying agent for The purposes of this paper are to present experimental gases has been overlooked to a great extent until the last few data on the drying of air with phosphoric acid obtained by years, possibly because of its cost and the lack of equilibrium operation of a 10-inch packed tower, and to illustrate the use partial pressure data for this acid. Such data are necessary of the data in the design of a large-scale drying tower. before the degree of dryness that will be obtained can be predicted and before a rational design of large-scale drying equipDrying Equipment and Procedure ment can be undertaken. Perry and Duus (6, 7) gave some data for the partial presFigure 1 is a diagram of the apparatus used for drying air and sure of water over aqueous solutions of phosphoric acid, and Figure 2 shows the design of the acked tower. The latter was constructed from 10-inch standarrf steel pipe, coated inside with called attention to the technical importance of this acid as
A
N ESTABLISHEDmethod of drying air for in-
~
Data are given for the absorption of water vapor from air by phosphoric acid in a 10inch-diameter tower packed with 1-inch Raschig rings. The influence of gas and liquor rates on the over-all capacity coefficient, is given. Variations in the air rate are shown to have a much greater effect on KGu than variations in the acid rate. With an acid rate, L , of about 460 pounds/ (hour)(square foot), the effect of variations in the air rate, G, between 128 and 556 pounds/(hour)(square foot) is represented by the equation,
INDUSTRIAL AND ENGINEERING CHEMISTRY
July, 1941
911
curve for air and phosphoric acid is approximately linear over the concentration range involved. Since there was only a small difference in terminal acid concentrations, i t was assumed that a linear relation existed. However, the method was checked by plotting the equilibrium curve for several values and determining Koa by graphical integration. Excellent agreement in the values was obtained by the two methods. Values for the vapor pressure of phosphoric acid used in calculating Koa were obtained as follows.
Vapor Pressure of Orthophosphoric Acid The vapor pressure of aqueous solutions of orthophosphoric acid as given in the literature (4,6) and unpublished TVA data were found to be of limited range as to concentration and temperature. The data for specific concentrations were found to be consistent with respect to temperature, in that each
mTL%$ LEGEM) @ TmRMOMETER
0
U
ORIFICE METER
@ I"ROh4ETER
ti
OF APPARATUS FOR DRYING AIR FIGURE 1. DIAGRAM
acid-proof paint and then acked to a height of 7 feet with 1-inch Raschig rings. Acid was Zistributed over the packing by a fourpoint distributor. The acid tank was lined with sheet brass and contained a coil of 1-inch brass pipe through which either steam or coolin water could be circulated to control the temperature of the acif. The tank had the relatively large capacity of about 160 gallons. Samples of the inlet acid were taken from the pi e supplying acid to the tower. The s ecific gravities of the sampgs were measured with a Westphal batance and the concentrations obtained from data given in International Critical Tables (3). The concentration of the exit acid was calculated from that of the inlet acid and the quantity of water absorbed from the air passing through the tower. The acid rate was measured with a calibrated orifice meter. A mechanical mixer was used in the tank to ensure uniform temperature and concentration. The humidity of the air entering the packed tower was adjusted by first passing it through a humidifier (not shown in Figure 1) consisting of a shallow metal box partially filled with water which was heated by a steam coil. By maintaining the water at a constant elevated tem erature, air of a humidity corresponding to that which might &e encountered during summer was obtained. The air rate was measured by a calibrated Venturi meter. Wet-bulb psychrometers containing distilled water and matched thermometers with 0.2' C. graduations were used in determining the water vapor content of the inlet and exit air. The velocity of the air past the thermometers was held constant, as indicated by orifice meters, at 10 feet per second t o provide adequate ventilation of the wet bulbs. As a precaution against getting acid in the outlet psychrometer, an entrainment separator was installed in the exit air line. The air line and the entrainment separator were dry at all times, however, which indicated that no acid was carried from the tower as entrainment. The acid rate and the air rate were varied to determine their effect upon the capacity of the tower. The results are given in Table I. The data were correlated in terms of an over-all capacity coefficient, Koa. I n calculating the values of KQa given in Table I, the logarithmic mean of the terminal partial-pressure differences was used for the driving force. This method is applicable provided the equilibrium
'1 C-
L&-l*-
PIPE
FIGURE 2 . PACKED TOWER concentration could be well represented by a linear relation between the logarithm of the vapor pressure and the reciprocal of the corresponding absolute temperature. However, the data were discordant as to the effect of concentration changes on the vapor pressure a t constant temperature. This discrepancy is believed to be due mainly to uncertainties as to the compositions of the acids studied. The vapor pressures of two acids, prepared by mixing reagent-grade phosphorus pentoxide and orthophosphoric acid and containing 95.0 and 99.0 per cent HsP04, respectively, were determined over a temperature range from 30"
912
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE I. DATAFOR a,
L b / Hr.) (Si.b t . ) 256 254 254 254 255 255 255 255 255
Temp. of Air, Inlet 31 33 32 32 31 32 31 31 31
' c.
Outlet 47 48 48 48 48 49 49 48 49
THE
0.0251 0.0256 0.0240 0.0248 0.0248 0.0247 0.0250 0.0248 0.0243
AB~ORPTION OF WATERVAPORFROM AIR BY PHOSPHORIC ACID
He0 Removed L from *ir, L b . 1 ' )~ ~ % Hap04 Lb./Hr. (Sq. Inlet Outlet4
Lb. HsO/Lb. Dry Air Inlet Outlet
kt.i
0.0049 0.0046 0.0040 0.0041 0.0038 0.0038 0.0034 0.0033 0.0036
32 56 0.0200 12s 42 0.0164 30 231 0.0200 31 39 341 31 0,0197 39 341 30 0.0208 53 379 32 0.0188 41 448 0.0207 54 30 457 31 0.0208 64 457 44 32 0.0189 556 a Calculated. b From Figure 3.
Vol. 33, No. 7
Vapor Pressure Mean Lb' Moles/ HIP04, Mm. Hg) Partial Pres- (Hr.) sure Differ(Cu. Inlet Outlet ence Atm. Ft.) (Atm.)
Acid Temp., 0
Inlet
c.
Outlet
2.83 2.92 2.78 2.88 2.94 2.91 3.02 3.01 2.91
Varying Acid Rate 96.3 98.7 97.3 95.9 98.4 97.3 97.3 98.4 97.8 98.7 98.4 97.5 98.4 99.1 99.5 98.8 99.1 98.5
55 58 60 60 61 61 63 63 63
59 61 62 62 62 62 63 63 63
0.62 1.32 0.95 0.95 0.87 1.02 0.82 0.68 0.82
1.97 2.70 1.70 1.70 1.40 1.56 1.15 0.97 1.09
0.0174 0.0159 0.0150 0.0159 0.0155 0.0153 0.0151 0.0150 0.0149
2.36 2.67 2.69 2.67 2.75 2.76 2.91 2.89 2.81
1.18 1.76 3.14 3.08 3.73 3.72 4.35 4.39 4.68
Varying Air Rate 467 100.0 99.5 461 98.5 99.3 461 96.6 95.2 461 96.6 95.3 461 100.0 98.5 461 96.4 95.0 461 100.0 98.3 461 100.1 98.3 94.7 461 96.5
51 51 41 41 51 41 50 52 41
64 51 52 52 63 53 62 64 59
0.25 0.37 0.61 0.61 0.25 0.67 0.24 0.24 0.66
0.71 0.53 1.96 1.90 1.09 2.25 1.12 1.26 3.40
0.0140 0.0108 0.0127 0.0125 0.0133 0.0126 0.0140 0,0139 0.0120
1.22 2.36 3.59 3.57 4.06 4.28 4.50 4.57 5.66
to 150" C. A Pyrex bulb containing the acid to be studied was directly connected to the top of one side of a closed Utube mercury manometer, and the entire apparatus was immersed in a constant-temperature oil bath, using a glass battery jar as a container. Both sides of the manometer were evacuated until bubbles ceased t o rise from the acid, evacuation of the acid side of the manometer was discontinued, and the difference in mercury levels was read with a cathetometer. The vapor pressure of the acid 20 30 40 was calculated from this 1000 reading. I n correlating these data 500 for 95 and 99 per cent HaP04 with the data of 300 Kablukov and Zagvozdkin 200 (4), with the unpublished TVA data, and with the 100 data of Keenan and Keyes (6) on pure water, individual equations of the 50 type, 30
were not great. This method of plotting vapor pressure data may therefore be useful in other similar problems. The values of the vapor pressures of arbitrarily chosen percentages o f HsP04 were taken from these curves and used to construct Figure 3. (Reliable quantitative data as to the composition of the vapor over phosphoric acid are not available; however, based on qualitative experiments, it seems safe to assume TEMPERATURE
50
80
31
30
-
70
"C.
80
90
100
110 120 130
20
were deduced for each acid on which experimental data were available. The logarithms of the vapor pressures of each of these acids were then calculated a t 10" C. intervals by the equations. These calculated values were plotted against the mole fraction of HsP04, and curves were drawn connecting points of constant temperature. This resulted in a number of smoothly curving isotherms connecting the logarithms of the vapor pressure of pure water with those of pure orthophosphoric acid. The curvatures of the isotherms
10 5
3 2 I
0.5
0.3 0.2
-..
01
34
33
32
29
28
27
RECIPROCAL OF AESOLUTE TEMPERATURE ~ K Jx
26
25
io4
FIGURE 3. EQUILIBRIUM VAPOR PRESSURE OVER ORTHOPHOSPHORIC ACID
24
INDUSTRIAL AND ENGINEERING CHEMISTRY
July, 1941
for the present problem that the vapor does not contain any of the acid anhydride.) The data are now consistent with respect to composition and temperature and are believed to be accurate within about 10 per cent, corresponding to an uncertainty of about 0.5 per cent of HsP04 contents.
913
While this method is not considered a rigorous test of the data, it is helpful in determining whether to consider both films when using the data for design purposes. If the factors affecting one of the films may be considered to have negligible effect on KQU,the design calculations are simplified considerably.
Effect of Acid and Air Rates
Pressure Drop
Table I gives the experimental data arranged in two groups according to whether acid rate or air rate was the main variable. An increase in acid rate L from 210 to 822 pounds/ (hour) (square foot) resulted in an increase in Koa of only about 22 per cent. The effect of air rate G on Kea was determined by holding L constant and varying G from 128 to 556. These data are plotted in Figure 4. Variations in the air rate have a much greater effect on Koa than variations in the acid rate. This indicates, as would be expected in the dehumidification of air, that the gas film is controlling and that the resistance offered by the liquid film may be negligible. I n order to substantiate this assumption, the data were plotted as shown in Figure 4. If the vapor pressure of phosphoric acid over the narrow range of concentration and temperature of the inlet and exit acids may be expressed as a linear function of concentration ( p = HC b), Equation 2 may be written:
Since few data are available for the pressure drop through packing wet with liquids other hhan water, some measurements were made with the present tower a t various gas velocities when circulating 460 pounds/(hour) (square foot) of 98 per cent Hap04 a t 60" C. The results are given in Table 11. The experimental values for the pressure drop given in this table were found to average about 25 per cent greater than those calculated for water by Chilton and Colburn's method (1) 0 as given in the literature (0).
+
where 1/&a
= over-a11 resistance l/k@ = as-& resistance l / H k ~= $quid-& resistance
Furthermore, assuming kQuto be proportional to some power of G, (k,a = rGm),Equation 3 is obtained: 1
1 % a
=
1 f
ZH
(3)
Assuming k ~ to a be independent of G , a plot of 1/&a us. 1/G" using the proper value of n, which is usually about 0.8, should give a straight line with a positive intercept representing the resistance of the liquid film, l/HkLa. If the liquid film resistance is negligible, as assumed, the data should be represented by a straight line passing through the origin. Figure 4 shows that the present data conform satisfactorily to this method of interpretation when a value of 1 is assigned to exponent n. The intercept is zero, substantiating the assumption that the liquid film resistance is negligible;
TABLE11. EFFECTOF AIR VELOCITY IN IO-INCHTOWER ON PRESSURE DROPTHROUGH 7 FEETOF ~ - I N CRINGS K [L
-
460 lb./(hr.)(ss. ft.). 98% HsPO4 at 60' C . ] Ab!. Stet10 Air Velocityb Air Rate (S.T.P.) Air Temp." Pressure Pressure Drop In. H g Cu. ft./hr. C. Ft./sea. In. Ha0 40 28.6 1.70 2795 1.47 40 1.73 2850 1.52 28.6 1.84 3010 28.5 41 1.66 1.94 3140 28.2 41 1.78 28.3 2.04 41 3320 1.94 3460 28.1 2.15 41 2.08 2.25 41 3630 2.26 28.1 3670 27.4 2.34 41 2.40 3770 27.5 2.40 42 2.44 3940 27.8 2.48 42 2.59 4125 28.1 2.56 42 2.75 4260 28.1 2.66 42 2.87 4450 28.0 2.78 42 3.10 4525 2.83 42 3.23 28.0 4600 28.0 2.88 42 3.31 a Mean of inlet and exit air temperature. b Based on empty tower.
Discussion
-E
0
0
During this investigation, which covered only a nar0 row range of acid concentration, the 0 0.002 0.004 0.006 0.008 driest air obtained I contained 0.0025 G pound of water per FIGURE 4. EFFECT OF AIR RATE pound of dry air. Since the initial moisture content of the air in this case was 0.0164 pound of water per pound of dry air, 85 per cent of the initial water vapor was absorbed. I n this case 99.3 per cent HaPOd, equivalent t o 72.0 per cent PzOS, was used. Little or no information concerning the vapor pressure of more concentrated acids containing greater than 72.4 per cent Pz06is available a t present. However, since the equilibrium vapor pressure of water over phosphoric acid decreases markedly with increasing acid concentration, drier air undoubtedly could be obtained with acids of higher Pz06content than were used in the present study. Table I shows that the most concentrated acid is reported as containing 100.1 per cent HaPOI, which is equivalent t o an aqueous solution containing 72.5 per cent PzOs.While this method of reporting acid concentration may seem unusual, it should not be unduly confusing if the various phosphoric acids are considered as aqueous solutions of P20s rather than HaPOd. I n designing an absorption tower, it is important to have sufficient information to determine the optimum liquor and gas rates. Selection of the optimum rates is complicated because several factors other than the effect of gas and liquor rates on tower capacity are involved. Both the energy required to pass gas and liquor through the packing, and the capacity of the drying tower increase with an increase in gas and liquor rates. This involves the cost of power and the size of the tower. Therefore, an economic balance of operating and fixed costs, which is beyond the scope of the present study, must be made before the optimum gas and liquor rates can be determined.
Calculations for Large-Scale Drying Tower The following calculations are given to illustrate the application of the experimental data to the design of a drying
Vol. 33, No. 7
INDUSTRIAL AND ENGINEERING CHEMISTRY
914
tower. The calculations are for a tower capable of reducing the absolute humidity of 3160 pounds of air per hour from 0.018 pound water per pound dry air to 0.0015, equivalent to the removal of 91.5 per cent of the moisture. CONCENTRATIONOF ACID REQUIRED. The moisture content selected for the dried air governs the concentration of the acid required. The use of 100 per cent H3P04will be assumed for the present problem. Acid of this concentration a t 60" C., which is above the temperature of the solid phase, exerts a vapor pressure of 0.45 mm. mercury (Figure 3 ) , which is equivalent to an absolute humidity of 0.00037 pound water per pound dry air. DIAMETEROF TOWER. As pointed out in the text, the optimum air rate should be determined by an economic balance of COMMERCIAL CSIT FOR THE PRODUCTIOS OF PHOSPHORIC ACID operating and fixed costs. However, for the present problem an arbitrarily chosen air rate of 400 -pounds/(hour) (square foot) of CONCENTRATIOS OF EXITACID. The addition of 52 pounds tower cross section was used to calculate the diameter of the of water t o 3950 pounds of 100 per cent &Po4 would result tower: in acid of the following concentration: Tower cross section = 3160/400 = 7.90 sq. ft. 3950 X 100 = 98.770 HsPOa Tower diameter = d ( 4 X 7.9)/3.14 = 3.18 ft. (3950 52)
+
ACID RATETO TOWER. This will be based on an arbitrarily chosen acid rate of 500 pounds/(hour) (square foot) : Acid rate = 7.90 X 500 = 3950 lb./hr. Density of 100% HaPo4 at 60" C. (3) = 15.4 lb./ a1 (approx.) Acid rate = 3950/(15.4 X 60) = 4.27 gal.$mk. PARTIAL PRESSURE OF WATERVAPOR IN ENTERING AIR: 760 X 0.018 (0.62 f 0.018)
21'4 mm' Hg
VAPORPRESSURE OF ENTERING ACID. The vapor pressure of 100 per cent &Po4 at 60" C. is 0.45 mm. mercury (Figure 3). This vapor pressure corresponds to an equilibrium moisture content as follows: o'62
(760
0'45 - 0.45)
=
0.00037 lb./lb. dry air
PARTIAL PRESSURE OF WATER VAPORIN DRIEDAIR : 760 X 0.0015 (0.62 0.0015) =
+
mm' Hg
QUANTITYOF WATERREMOVED FROM AIR: Humidity of entering air = 0.0180 lb. HlO/lb. dry air = 0.0015 Humidity of dried air Water removed = 0.0165 lb. HsO/lb. dry air = 3160 X 0.0165 = 52 lb./hr. = 2.89 lb. moles/hr.
VAPORPRESSURE OF EXIT,4121~. The temperature of the exit acid will be assumed to be 60" C., which is the same as that assumed for the entering acid. The vapor pressure of 98.7 per cent H3P04 at 60" C. is 0.84 mm. mercury (Figure 3). LOGIl'IEAK PARTIaL PRESSURE DIFFERENCE : Partial pressure of water vapor in entering air = 21.40 mm. Hg Vapor pressure of exit acid = 0.84 Partial pressure difference at bottom of tower = 20.56 mm. Hg Partial pressure of water vapor in dried air = 1.83 mm. Hg = 0.45 Vapor pressure of entering acid Partial pressure difference a t top of tower = 1 .38 mm. Hg 20.56 - 1.38 Logmean = 20.58 = 7.10 mm. Hg = 0,00935 atm. 2.3 loglo __
-
1.38
DEPTHOF PACKED SECTION.The experimental value of KGU corresponding to an air rate of 400 pounds/(hour) (square foot) is given as 4.0 pound moles/(hour) (cubic foot) (atmosphere) on Figure 4: 2.89 Packed volume = 4.0 X 0.00935 = 77'2 ft' The packed depth using a tower diameter of 3.18 feet is: 77.2 X 4/3.14 (3.18)'
=
9.7 f t .
July, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
NOTE. I n the above example, the log mean partial-pressure difference was used for the driving force. This method is not applicable to the design of towers employing acid of an appreciably different concentration from that used in the experimental determination of KGU. I n such cases the equilibrium curve for air and phosphoric acid should be plotted and Kaa determined by graphical integration (8).
Acknowledgment Credit is due L. H. D. Fraser for the vapor pressure determinations and correlation of the vapor pressure data, and R. A. Wilson for assistance in the experimental work.
Nomenclature a = area of interphase contact, sq. ft./cu.
.
ft. of packed volume b = constant in equation, p = HC b C = concentration of acid G = air rate, lb./(hr.) (sq. ft. total cross section) H = proportionality constant in equation, p = HC b KQU= over-all coefficient of mass transfer, lb. moles/(hr.) (cu. ft.) (atm.)
+
+
915
= as-film coeffcient of material transfer uid-film coefficient of material transfer L = aci rate, lb./(hr.) (sq. ft. total cross section) 11 = exponent in equation,koa = G,, p = equilibrium partial pressure of water over acid Y = Proportionally constant in equation, = Y G”
‘3
Literature Cited (1) Chilton, T. H., and Colburn, A. P., IND. ENQ.CHBM.,23, 913 (1931). (2) Greenewalt, C. H., Ibid., 18, 1291 (1926). (3) International Critical Tables, Vol. 111,p. 61,New York, McGrawHill Book Co., 1928. (4) Kablukov, I. A., and Zagvoadkin, K. I., 2. anorg. allgem. Chem., 224, 315 (1935). (5) Keenan, J. H., and Keyea, F. G., “Thermodynamic Properties of Steam”, New York, John Wiley & Sons, 1936. (6) Perry, J. H., and Duus, H. C., C h m . & Met. Eng., 41,74 (1934). (7) Ibid., 41, 127 (1934). (8) Roas, W. H., and Jones, R. M., J. Am. Chsm. Soc., 47, 2165 (1925). (9) Sherwood, T. K., “Absorption and Extraction”, New York, McGraw-Hill Book Co.,1937. PRHIS~NTPD before the Division of Industrial and Engineering Chemistry a t the 1Olst Meeting of the American Chemioal Society, St. Louis, Mo.
Diatomaceous Earth Base for Chemical Pigments E.
c. BURWELL, The Dicalite Company, New York, N. Y.
D
IATOMACEOUS earth has developed from a position of obscurity to one of value in industry today. Its utility will be apparent from the process literature and patents (2). Processing and refining has accounted for its increased value. The paint industry is one of those which has come to recognize the value of a process-refined diatomaceous earth. I n the research laboratory and later in the development laboratories of paint manufacturers, it was observed that this refined diatomaceous earth seemed to increase the hiding or opacity of pigmented paint products materially even though it has a refractive index of 1.45, which is very near that of the paint vehicles and thus imparts no hiding power or opacity in itself. It was thought that this increase in opacity was due to the diatom particles being coated over by the other pigment so that they did not merely occupy space within the paint film, which is characteristic of the usual paint extenders. Microscopic examination of mixtures containing diatomaceous earth and paint pigments showed that virtually every diatom particle did have attached to it the particles of pigment with which it was mixed. Based on this experience, it was reas6ned that diatomaceous earth would be an excellent base upon which t o precipitate chemical pigments. Accordingly, a research program was instituted to determine any possible advantage and value of such phenomenon. Obviously the first thought would be to use this material as a base for the more expensive pigmentsfor example, the organic dyes to make lake colors. However, in order to take advantage of the characteristics imparted to the paints by the diatomaceous earth in the larger
volume of the business, it was decided to start first with the lead pigments as a type of the white, next the iron blue pigments, then chrome yellow, and later the chrome greens, titanium pigments, zinc sulfides, and chromium oxides.
White Lead I n the case of basic-carbonate white lead the present trend in manufacture is away from the old corroding process. As a result, chemical and electrically precipitated white leads are now available which have better color, better hiding power, better control of oil absorption, and consistency in the finished paints. I n our experiments a lead nitrate solution was used, from which the lead was precipitated in the form of a basic lead carbonate by a mixture of sodium carbonate and sodium hydroxide, according to the followingformula: 3Pb(NOs)a
+ 2NalCOa + 2NaOH +
2PbCOs.Pb(OH)a
+ 6NaN08
Other salts of lead, such as lead acetate or chloride, dissolved in hot salt brine could have been used with comparable results. Pure solutions of these salts were used and molecular proportions taken. Yields were checked in each case to determine the completeness of reaction. Batches of the 100 per cent white lead were compared with a first-grade commercial product for color, brightness, Gardner-Coleman oil absorption ( I ) , and hiding power as determined by a Pfund Cryptometer. After several runs were made on the 100 per cent products and constants were determined, mechanical mixtures were
