Effect of High Voltage Electrical Discharges on Sulfur Hexafluoride WALTER C. SCHURIB, JOHN G. TRUMP, AND GR-4CE L. PRIEST Massachusetts Institccte of Teclanology, Cambridge, Mass.
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VIEW of the interest because of their delrterious T h e application of certain gaseous fluorine compounds, which has been aroused effects, to eliminate the lower such as dichlorodifluoromethane and sulfur hexafluoride, in the application of certain fluorides of sulfur from the as voltage insulating gases in high voltage generators and gaseous fluorine compounds, gas mixture either by somc similar apparatus, has led to an examination of the stasuch as dichlorodlfluoiocontinuous absorption procbility of these gases when exposed to corona or spark-over methane and sulfur hexafluoess, or intermittently. A discharges which attend the operation of such equipment. ride, as the voltage insulating satisfactory method oi reIn common with other compound substances sulfur hexamcdia for electrostatic generconditioning the gas for insufluoride is decomposed under corona discharge, and more ators and other high voltage lation purposes by absorption rapidly under spark-over conditions, forming lower valence apparatus, i t seemed desirable of the undesirable decomposifluorides of sulfur and fluorides of the metals forming the to study the stability of these tion products will be dependelectrodes of the electrical apparatus. The corrosion compounds when exposed to ent largely on utilization of damage thereby brought about may be of serious magnithe corona or spark-over disthe difference in properties tude when operation is extended over many hours. Possicharges which may be anticiof individual lower valence> bility of reducing the extent of such damage by- use of pated in the operation of such fluorides of sulfur. Accordsuitable absorbents, such as activated alumina and soda apparatus over extended ingly, a summary of the publime, is indicated. Presence of organic matter, such as periods of time. The first gas lished information concerning fabric belts, rubber, plastics, etc., used in construction of examined in this way was sulthese compounds is included such generators, complicates the corrosion problem and fur hexafluoride, which has below. the problem of removal of decomposition products. Bebeen shown to have superioi The high degree of thermal cause the decomposition of SFs under discharge forms characteristics when used in and chemical stability of fluorine and lower valence fluorides, a r6sumC of the present high voltage x-ray generators sulfur hexafluoride has long state of knowledge concerning these little-known combeen regarded with interest. (2). pounds is included, and a method is suggested whereby In the passage of electricity That this gas, however, bv the hexafluoride may be freed o€ contamination bj- the by the mechanism known as no means is to be regarded lower valence compounds. corona through a gas a t high as possessing the same dopressure, excitation and iongree of inertness and staization of the gas molecules occur primarily by impact of eleotroiis ldity as one of the rare gases of the atmosphere may be deniaccelerated in the high electric field. The superior insulating peronstrated by considering some of its ltnown chemical reacformance of a gas such as sulfur hexafluoride, as compared with air tions. I n a compilation of published data ( I C ) , the critical or nitrogen, results both from the relatively large collision crosstemperature, rcported by Prideaux in 1906 as 54" C., i s probsection of such molecules which extract energy from free elecably too high by eight or nine degrees. I n 1900 Rcrthclot) trons in the field and from the tendency of such molecules or their ( 1 ) reported that the gas mas not affected by the silent dissociation products to att,ach electrons and form impotent electric discharge, but in the same year Moissan and Lebrau negative ions. Because of the Maxwellian distribution of (11) showed t,hat a t t,he temperature of the induction spark collision paths, many elect,rons liberated in a corona discharge 89y0 of the gas was decomposed in 2 hours, although no acquire energy well in excess of the binding energy and cause decomposition was noted a t the softening temperature of hard glass. Sparking in the presence of hydrogen caused complete dissociabion of the molecules on which they impinge. Under decomposition, forming hydrogen fluoride and hydrogen sulfide. spark-over conditions, the high temperaturc in the spark causes When mixed with oxygen and sparked, decomposition occurrrd thermal dissociation of t,he complex gas molecules. Thus, the Ivith formation of some oxyfluoride. maintenance of corona or spark-over mith any complex gasSulfur hexafluoride is chemically resistant to attack by such eous compound, however stable, will result in its gradual decomposit'ion into simpler substances. iiiet>aIsas copper and silver a t red heat, as well as by hot copper I n the synthesis of sulfur hexafluoride from the elements ( 1 4 ) oxide, lead chromate, or fused potassium hydroxide, or most nonthere is formed inevitably a certain proport,ion of lower valence metallic elements (except sulfur and selenium). The gas is rapidly decomposed by molten sodium, forming sulfide and fluorides of sulfur; their presence in the high voltage apparatus fluoride. It is practically insoluble in water or aqueous solutions would be undesirable, either because of their reactive nature in of alkalies. the presence of even small amounts of moisture, resulting in the The use of sulfur hexafluoride for high voltage insulat'ion vas formation of acidic substances which viould attack the materials examined by Watson and Ramaswamy as early as 1934 ( 1 9 ) , of construction of the apparatus, or because of the toxic character n-ho measured the dielectric constant of the gas and found that it of some of these compounds or their hydrolysis products. has no moment, and by Charlton and Cooper ( 5 ) of t'he General It is therefore of importance to point out that when sulfur 1;lectric Company. hexafluoride is subjected to electrical discharges, such as corona I n 1939, Pollock and Cooper (1%') published data on the breakor spark-over, a cert,ain amount of decomposition v d l take place down characteristics of the hexafluoride under spark-over and with the formation of lower valence fluorides of sulfur. l n the corona conditions. Sudzuki (17) also compared the sparking use of such gases it is important to maintain the absorbed elccvoltage in sulfur hexafluoride and in air a t 1 atm., using 50-cycle trical energy a t a minimum. It is generally further necessary,
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INDUSTRIAL AND ENGINEERING CHEMISTRY
alternating current and found the dielectric strength of the hexafluoride t o be three times that of air. Hokberg (9) and coworkers have published several papers on the electrical breakdown strength of various gases, including sulfur hexafluoride, and recommended this gas over nitrogen as a dielectric in condensers, for greater capacity and voltage, for less pressure and higher temperatures without decomposition. I n his most recent paper, Hokberg tabulated the boiling points and dielectric strengths, s, relative to Nz for the following gases which are arranged in the order of decreasing s: CCl,, SeF6, CClsF, CCl,H, CzHsI, CzClzFz, SFe, SOFz, CChFz, SO2, CsHla, C5H12,CzHsBr, C2HsC1,CF,, N2, and COz. Hokberg concluded that SFs (which he called "elegas"), with s = 2.3 t o 2.5 and boiling point = -62", and with outstanding chemical stability in the electric discharge, is the most suitable.
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weighed. The metal samples, such as copper, aluminum 24ST, aluminum (soft), cold rolled steel, stainless steel, and brass, were polished on one surface, degreased with toluene, dried a t 110" C., and weighed. A complete run was made as follows: The flask was prepared for the run with either samples or absorbent in the flask; after which it was evacuated for a t least 24 hours t o a vacuum of approximately 5 microns. Sulfur hexafluoride of known purity then was admitted to the system through a 4-fOOt tower of potassium hydroxide pellets, followed by a 1-foot tower of porous barium oxide. This procedure was used to remove acidic constituents and moisture. The flask was filled t o a pressure slightly above 1 atmosphere and corona discharge, or spark-over, was maintained for the desired time. Corona studies covered periods from 50 to 200 hours with currents in the range of 50 to 200 microamperes. At the end of a run, analysis of the gas was made t o determine acidic constituents and oxygen, using Hempel pipets filled with 30% potassium hydroxide and alkaline pyrogallol, respectively. The presence or absence of oxidizing substances, after removal of acidic material by 30% potassium hydroxide solution, was dekrmined by means of potassium iodide in dry acetone. The results are shown in Table I. From our experiments, the following conclusions may be drawn: 1. When subjected to direct current corona discharge a t 10 to 20 kv. or alternating current spark-over, sulfur hexafluoride decomposes to give products which are absorbed by 30% potassium hydroxide and a portion which is not so absorbed and possesses oxidizing properties. These decomposition products are probably lower sulfur fluorides. 2. Activated alumina effectively removes both the acidic and the oxidizing portions from the sulfur hexafluoride mixture. 3. Depending on the material tested, both the metal and the nonmetal samples showed varying degrees of attack by the decomposition products of the sulfur hexafluoride. I n order of decreasing sensitivity to attack are brass, copper, steel, stainless steel, and aluminum, the latter two showing negligible effects. LOWER VALENCE FLUORIDES OF SULFUR
Figure 1.
Apparatus for Study of Electrical Discharges in SFe Gas
I n these experiments on the chemical stability of sulfur hexafluoride, either corona discharge or spark-over was maintained under varying conditions. The gas was contained in a 1-liter round bottom Pyrex flask (Figure 1) fitted with a 40/50 standard taper female joint, and tubulated with a 4-mm. oblique stopcock for evacuation (not shown in Figure 1). Two tungsten electrodes were sealed in through re-entrant tubes serving to keep the surface leakage path long. T o one tungsten lead was fastened a polished aluminum plane and to the other an aluminum rod carrying light, sharpened, stainless steel points adjusted so that the point-to-plane distance of approximately 6 mm. was constant for all the points. The power was supplied by a 20-kv. direct current power pack. The currents were measured with microammeters. I n the early runs the standard taper joints and the stopcocks were sealed with Apiezon N grease, but in the later runs the standard taper joints were assembled dry and waxed tight with Apieaon W wax and the stopcocks were lubricated with a nonorganic lubricant. Samples to be tested were hung in the bottom of the flask on a glass rod fitted with hooks. Absorbents t o be tested were contained in nickel mesh baskets which were placed in the bottom of the flask. The nonmetal samples, such as Lucite, Butyl rubber, Bakelite, natural rubber, Teflon, Textolite (paper), Textolite (cloth), and Fabreeka, were wiped free of any surface dirt and
These substances include the monofluoride, S2F2; the difluoride, SF2; and the tetrafluoride, SF4, which are analogs of the corresponding chlorides of sulfur, as well as disulfur decafluoride, S2Flo, which has no chlorine counterpart and may be considered as a dimeric form of the pentafluoride. Some question has been raised on theoretical grounds concerning the individuality of the tetrafluoride (16), but the weight of evidence, as detailed below, is in favor of the existence of this compound. Sulfur Monofluoride, S2F2, was first identified by Centnerszwer and Strenk (4) who heated a mixture of silver fluoride or mercurous fluoride with sulfur. The molecular weight and the analysis of the gas confirmed its formula. The colorless gas was described as having a disagreeable odor somewhat resembling that of sulfur monochloride, a meltingpoint of - 105.5' C. and a boiling point of -99" C. The specific gravity of the liquid was 1.5 (at -100" (3.). I n addition to its thermal instability the gas deposited sulfur when exposed to moisture, even the moisture of the air, forming also sulfur dioxide and hydrogen fluoride; the hydrolysis was complete in 30 to 45 seconds. It was absorbed rapidly by solutions of alkalies, and if pure, did not blacken mercury. It reacted with sodium a t room temperature, likewise Kith sodium peroxide, whereas aluminum powder, zinc, or silicon failed t o react appreciably a t 100" C. Potassium permanganate reacted a t 100" C. but the chlorate did not. These results were largely confirmed by Strenks (16) who noted that the gas attacked glass even a t low temperatures, forming sulfur, sulfur dioxide, thionyl fluoride, and silicon tetrafluoride. It likewise attacked quartz, but not steel or iron, tin, or platinum. It was stated to be quite toxic, as is t o be expected of any substance capable of hydrolyzing to form hydrofluoric acid.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Run
No.O,b
1 2
3
f
6 7
Samples i n Flask Sonmetals Noninetals 1\f etala Metals Sone Xone Sone
.iti*ortn,nt
Sonr Sone Sone Potassiuni hydroxide pel1rt.i Sodium fluoride pellets P0rou.s barium oxide ( H a O ) 4 8 l I e ~ hxetinttml aliirninx
Power, watts Total Titile, (Weighted AI..) IToiirs C o ~ o s . 4DI~CH.LRC;F. 1.7 1.7 2.7 2.7 1.8 2.2 3.0 6P.LF.H
1 7 1 . Li5 290 240 171. lti 159.6
l59.R 15R.3
Vol. 41, No. 7
Materials Acidic r l p , Mtn.
Ph-k
A p , L%,
+ 30.5
+- 34 .. 85
+ 354.7 5.7 + -100.5
+c -
f 6 . 8 -12.6
t
4.6
+-
12.1 13.J
0.6 1.2
I,,
in Products, % ' b y Tol.
Oxidizing Substances
3.6 28.6 36.1 1.1 '1.5 17.8 0 I1
Present Present Present Absent Present Present Abwnt
DI,5LH.&l%l4I..
8 Metal* Nor1r 3 0 (Estd.) 27 -- 2 4 . 3 - 3 AbeenL 9 illet a16 None 50 (Estd.) 5 . 6-7 15.1 - 1.9 24: Lis Presont Apiezon X grease was used in I'IIIIF 1, 4, a n d 8 ; tllr grease blackened a n d the flask was more or less etched. No grease was used in the other runs; t h e gaseous products fumed in the presence of moisture, noted especially in runs 2, 3, 8, a n d 9. I n runs 8 a n d 9 t h e capillaries of the Oas analysis apparatuh became p l u p e d with hydrolg7sis prodiirrs o f the gases. The Lionel rlectroden were badly pitted and a solid deposit, cnntaininp cogprr%nd nickel. forincd o n t h e flask wa11sP Air v a s found t o be absent in all samples of products
+
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Deribigh and Khj-tlaw-Gray ( 6 ) also obtained a, gas which they believed t o be sulfur nionofluoride as a by-product of t,hc preparation of the hexafluoride from the elements. Their gas, hoivever, boiled a t approximately - 50 ', and they considered that Crntnerszrver's ,product contained some volatile impurity. Trautz and Ehrmann ( l a ) , discussing thc fluorides and oxyfluorides of sulfur, concluded that the monofluoride could not be ob t'ained entirely free from the difluoride; that thermal decomposition began a t 90 O C. and proceeded rapidly a t 200 O t o 250 O C. Decomposition by heating or an electric spark caused precipitarion of sulfur and a contraction in volume; t'he gaseous products were partly difluoride with a small amount of a substance boiling about 30" C. which they believed to be probably a polythiofluoride (presumably the S2F,o of Denbigh and Whytlaw-Gray) . When mixed with hydrogen and sparked, hydrogen fluoride and hydrogen sulfide were formed. Thr structure of the monofluoride was believed t o be S-SF,. In considering the status of the sulfur fluorides in 1936, ('art.er and Wardlaw (3) expressed the brlief that up t o that time neither the mono- nor difluoride had h i ~ p~r ei p a r d in a pure statc. More recently, Dubnikov and Zoriri ('7) have re-examined the reaction of silver fluoride with sulfur a t various temperatures and iourid that under no conditions was pure sulfur monofluoride obtained; some sulfur difluoride was formed by decomposition, and if glass vessels were employed, attack of t,he glass by sulfur monofluoride prod-uced some silicon tetrafluoride. Reaction became apparent first at about 110"; optimum yields (about 667,) of sulfur monofluoride were obtained, with least decomposition, a t about 140" C. Sulfur Difluoride. SF2is the least wcll known of the fluorides of sulfur and has perhaps not been prepared in a purc stat,(,. Its formation by the thermal deconiposition of the nionofluoride has heen indicated by several investigators, and it,s existence: is reasonably to be expected by analogy with the corresponding chlorides of sulfur. It is also reasonable to expect that it will undergo hydrolysis, probably with formation of sulfur, sulfur dioxide, and hydrogen fluoride, so that an alkaline absorbent should be effective in removing this gas from a mixture wiOh sulfur hexafluoride. Sulfur Tetrafluoride. SF4 was prepared by Fischer and Jaencker (8) by the interaction at room temperature of cobaltic fluoride, CoF8, with sulfur in quartz apparatus: 4CoFa f S - t 4CoFa f SF,. As the reaction may become violent if the mixture is heated, dilution of the latter with calcium fluoride was resort'ed to. This preliminary communication (8) was not followed by the promised more definitive article, which led Sidgwicli: (16) to conclude that there was some mistake coiicerning its identity. It is described as a colorless gas, condensing t o a colorless mobile liquid (boiling point -40" C.; melting point -124" C.), with an observed mole weight of 107 (theory 108), which when
dry does not at,tack glass, paraEn oil, rubber, or sulfur but blaclrens mercury. It, is absorbed by caustic alkali solutions forming sulfite and fluoride. Its vapor pressure is represented by the q u a t i o n : l ~ a . : ~ p ,=, ~ -1132/Y 7.746. Its odor resembles that of sulfur chloride and is very irritating. Cnsuccessful attempts t o prepare sulfur tetrafluoride rwrc reported also by Ruff and Thiel ( I S ) by such mothods as the int,eraction of hydrofluoric acid with sulfur nitride or sulfur tetrachloride, or with sulfur tetrachloride arid the tetrafluorides of titanium or tin, or with arsenic trifluoride. Carter and Kardlaw (5') pointed out that the ivork of B'ischer and Jaenclcer was not reproducible and that a later study of the same reaction by Luchsinger ( I O ) had shown t,hat pure sulfur tetrafluoride was not produced under t,hese circumstances. Indeed the conclusion was reached that in all reactions of metal fluorides with sulfur, all of the fluorides of sulfur were formed in proportions varying according to the kind and amount of the metallic fluoride used and t,he velocity of the reaction. The mono- and difluoridc of sulfur could bo removed by shaking with mercury aiid the hcxafluoi,ide separated from t>hetetrafluoride h y fractional distillation. Disulfur Decafluoride. The dimeric pentafluoridc, f32F10,mas first pwparcd by Denbigh and TTThytlan--Gray (6') and is described as being a colorlms liquid of specific gravity 2.08 * 0.03 a t 0 " C., boiling a t 29" * 1' C., and melt,ing a t approximately - 0 2 " + 1 C1. (SO); it, is found in small proportions in the fluorination products of sulfur. I t is not hydrolyzed or dissolved appreciably b y water or solutions of alkalies but is decomposed by fused caustic., It also dwomposes when heated to about 300" C., forming the trtra- and hrxsfluorides. Sparking a, mixture of the gas and hydrogen for seiwal hours in the presence of potassium hydroxide causes complete decomposition into hydrogen fluoride aiid hydrogen sulfide which were absorbed by the caustic potash. Ordinary stopcock grease is blackened by the vapors with deposition of sulfur on the gla.ss, a.lt,hough paraffin ~ v a xis not affected. It reacts with some mctals, such as copper or iron, to form sulfides, particularly when heated; platinum at red heat decomposes the gas, a pale yellow substance depositing on t,hc walls. Silica reacts a t bright red heat, arid at a lower temperature in the presence of sulfur, according t o t'he equation 2S2Fi0 S 5SiOaj5SiFa S O n . I t s vapor pressure may be represented by the equation: log,Opmm= - 1530jT 7.95. Since the lower fluorides of sulfur are cit,her hydrolyzable in water or alkaline solutions, or are decomposable t o form hydrolyzable products, the purification of sulfur hcxafluoride from such lower valence fluorides would appear t o consist of thermal decomposition of the unstable ones in a Monel or nickel tube followed by passage through soda lime or similar absorption media. As some of the lower fluorides are toxic substances,
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INDUSTRIAL AND ENGINEERING CHEMISTRY
their removal from the insulating gas is necessary hdore any servicing of the high voltage apparatus is undertaken. LITERATURE CITED
(1) Betthelot, Ann. chim. et phys., [71 21, 205 (1900). (2) Buechner, Van de Graaff, Speiduto, Burrill, McIntosh, and Urquhart, Phys. Rev., 69, 692 (1946). (3) Caiter and Wardlaw, Ann. Rept., Chem. Soc. (London), 1936, p. 148. (4) Centnerszwer and Strenk, Rer., 56B, 2249 (1923); 58, 914 (1925). (5) Charlton and Cooper, Gen. Elec. Rev., 40, 438 (1937). (6) Denbigh and Whytlaw-Gray, J . Chem. SOC.(London), 1934, p. 1346. (7) Dubnikov and Zonn, J . Gen. Chem. (U.S.S.R.), 17, 185 (1947). (8) Fischer and Jaencker, 2.angew. Chem., 42, 810 (1929). (9) Hokberg, Elektrichestvo, 1947, No. 3, p. 15; Hokberg and Oksman, J.Phus. (U.S.S.R.), 5,39 (1941); Hokberg, Reinov, and Gliking, J . Tech. Phys. (U.S.S.R.), 12, 8 (1942); Hokberg and
(10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20)
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Sandberg, Ibid., p. 65; Compt. rend. acad. sci. (U.R.S.S.‘I. 53, 511 (1946). Luchsinger, Disseitation, Breslau (1936). Moissan and Lebeau, Compt. rend., 130,865, 984 (1900). Pollock and Cooper, Phys. Reu., 56, 170 (1939). Ruff and Thiel, Be?., 38, 549 (1905) ; Thiel, Versuche BUT Darstellung eines Schwefeltetrafluoridefi, Berlin, 1905. Schumb, IND.EN^. CHEM.,39,421 (1947). Sidgwick, Ann.Rept., Chem. SOC.(London), 1933, p . 126. Strenks, Acta Univ. Latwiensis K i m . Fakultat, 1, 233 (1930). Sudauki, J . Inst. E k e . Engrs. J a p a n , 61, 636 (1941). Traute arid Ehrmann, J. prakt. Chem., 142, 79 (1935). Watson and Ramaswamy, Proc. Roy. Soc. ( L o n d o n ) . 143A, 558 (1934) ; 156, 144 (1936). Yost and Russell, “Systematic Inorganic Chemistry,” p. 299. New York, Prentice-Hall, Inc., 1944.
K E C E I V ~ DMay 27, 1948. Presented a3 a vart of the Symposium on Inorganic Compounds before the .June 1948, Meeting of Section B of t h e DiviSOCIETY, iiion of Physical a n d Inorganic Chemistry, AMERICAN CHEMICAL Syracuse. Ii. Y.
Methanol Extraction of Lactose and soluble roteins from Skim
Milk Powder ABRAHAM LEVITON Rureau of Dairy I n d u s t r y , U . S . Departmen,t of Agriculture, V a s h i n g t o n , D . C .
w o r k previously reported on the separation of soluble proteins and lactose from whey powder has been extended to skim powder and skim milk. Precipitation of the proteins of skim milk by methanol at subzero temperatures permits the recovery of practically all the soluble proteins of the original milk. Spectroturbidimetric examination of the redispersed protein indicates no significant change i n particle size distribution. Sprayprocessed skim milk powder treated with 62% methanol at -15” C. yields lactose and a “soluble” protein product. The crude lactose is of exceptionally high quality. The protein product, comprising 42.270 of the solids of skim milk, contains 74.1% protein. Of this pro.fein, 81% is casein. Small losses in solubility result from both extraction and drying. Extraction at room temperature leads to equally good results. The solubility of the protein product in cold water, lost in part as a result of the increase in processing temperature, is recovered by heat treatment of the reconstituted suspension.
S
OME years ago a method (6, 7, 9) was developed for the preparation of lactose, a soluble protein product, and a ribo-
flavin concentrate from sweet whey powder by alcohol extraction. This method had as its basis a number of observations. First, in the alcohol treatment of spray-process lactose-containing products, amorphous lactose dissolves to yield supersaturated lactose solutions. Second, there is a lag period between protein precipitation and the start of lactose crystallization; this lag allows sufficient time to separate the lactose from the residual proteins. Finally, the rate of “insolubilization” of the proteins depends upon alcohol concentration and temperature in such a way that even at room temperature a highly soluble product is obtained if a certain minimum concentration of alcohol is exceeded. When the process was projected on a pilot plant scale, it was found that ethanol, which had been used in laboratory experi-
ments, was less satisfactory as a solverit than methanol. Thus whey powder, which is difficult to disperse in dilute ethanol, is readily dispersible in dilute methanol. Furthermore, the stability of the supersaturated solutions of lactose in inethanol is much greater than corresponding solutions in ethanol; consequently, additional time becomes available with the use of methanol for the separation of coagulated proteins. I n subsequent experimentq with skim milk, i t was therefore considered expedient t o carry out the first experiments with methanol. Skim milk is better than whey as a starting material for the application of a n extractiou process because whey is derived from skim milk and has a variable composition, depending upon its mcthod of preparation. For industrial purposes it is necessary t o separate certain constituents of skim milk with rennet or acid, but where no such urgency exists, solvpnts can be used for all separations. The advantages of the extraction process are the same for skim milk as they are for whey. T h e purity of the lactose obtained by a single crystallization is comparable to t h a t of the refined milk sugar of commerce. The milk proteins obtained are readily dispersible in water and as little altered in composition as possible. T h e present study is concerned with (1) the low temperature fractionation of the constituents of milk as a function of mcthm o l concentration, (2) the partition as a function of solventpowder ratio, and (3) the effect of a higher temperature of extraction. Finally, precipitation phenomena in fluid and concentrated skim milk are described and discussed in the light of SZrensen’s views ( 1 4 )on the constitution of proteins. MATERIALS AND METHODS
Spray-process skim milk powder was prepared from freshlyseparated skim milk. The milk was introduced into a commercial dryer a t 16” C. under a pressure of 2100 pounds per square inch through a spray orifice 0.002 inch in diameter. The tunnel temperature was held at 257 O F. (125 (2.). The success of the process depends largely on the method of pre aring the owder. Types of processing which permit the crystalfzation of Yactose or involve
