August, 1934
.
INDUSTRIAL AND ENGINEERING CHEMISTRY
ton of phosphate recovered. With the washer supplemented by flotation, the total tonnage recoverable was increased to 4000 and 8000 tons per acre, thus making a practical mining proposition of an otherwise uneconomic undertaking. The yardage handled is correspondingly reduced to 6 to 7 cubic yards of overburden and 2 to 3 cubic yards of matrix per ton. One property as prospected yielded 2000 tons per acre of 68 per cent B. P. L. plus 14-mesh washer fraction. Flotation recovered an additional 11,000 tons per acre of 74 per cent B. P. L. The whole matrix if treated by flotation would grade up to 74 per cent B. P. L. When using a washer alone, there is a tendency to save the finer fractions where the silica grains cannot be effectively separated by screening or classification. This practice either degrades the entire washer output or results in an additional smaller tonnage of lower grade. The resulting tailing, in either case, represents an important loss that is recoverable by flotation a t the higher grade. For example, with a masher screening a t 1 mm., the product may often run 7 2 per cent B. P. L., where with a coarser screen the grade may be raised to 74 or even 76 per cent. The same is true with other grades. Flotation allows the use of the larger screen openings while maintaining the maximum grade with a larger over-all recovery. There are several possible combinations of flotation with the present washing systems in the Florida field. One involves the use of a semi-portable, low-head washer of the conventional flow sheet, followed by a thickener handling all the masher tailing. This rejects slimes and diminishes the volume which may be economically pumped to a central flotation plant. In this scheme the flotation unit may be placed in the middle of a 1000- to 1200-acre tract of phosphate territory which can be mined out with four moves of the washer. The total tonnage thus recovered by one float plant may range from 5 to 6 million tons or more. The total amortization of the flotation unit plus the cost of the
Oiling Earth Roads Application of Surface Chemistry HANSF. WINTERKORN Missouri State Highway Department, Columbia, Mo.
M
ODERN surface chemistry has shown the great influence of the amount and character of surfaces and interfaces upon the mechanical and chemical stability of dispersed systems. Since most of our road courses represent systems with relatively large surfaces and interfaces, it seems promising to analyze road courses from the standpoint of surface chemistry. This has been done to a certain degree in types of courses ranging from sheet asphalt and asphaltic concrete to gravel and dirt surfaces. An attempt is made in this paper to apply surface chemical considerations to the successful oiling of low-cost dirt and gravel roads. Each surface or interface represents a certain amount of free energy which is proportional to the amount of surface and a function of its physico-chemical character, expressed by the surface tension. In the case of liquid-liquid or liquidgas interfaces the surface (or interfacial) tension, or the surface energy per unit surface, which is numerically the same, can easily be determined by experiment. The knowledge of surface and interfacial energies involved in any such system
815
washer moves is thus a fraction of a cent per ton of total output. It will be seen from the above that Florida phosphate operations now involve the same closeness of mechanical and metallurgical control that has contributed so largely to the success and low costs of the sulfides and metallics in general. In Tennessee the problems are somewhat different. There the washing practice is to save all the granular phosphate economically possible, and there is ordinarily no current washer tailing that is recoverable by flotation as in Florida. The province of flotation in Tennessee is mainly to raise the grade of the washer output or fractions. For instance, the grade may be 68 to 70 per cent B. P. L., whereas the demand is for higher grades. Flotation will raise the grade on an average to 77 per cent B. P. L. or better, with a tailing that may be wasted or regulated to about 65 per cent B. P. L. which is used locally for the manufacture of phosphoric acid, or dried and ground for direct application to the soil. The effect of flotation is to enable the Tennessee miner to exploit the lower grades as this now becomes necessary with the depletion of the higher grade reserves. For example, one washer operation had a remaining life of 5 years if confined to areas producing 72 per cent B. P. L. or better. By adding flotation and mining the lower grades, the life of the operation is being extended to 15 years. LITERATURECITED Broadbridge and Edser, U. S. Patent 1,547,732 (1925). Gaudin, "Flotation," p. 7, McGraw-Hill Book Co., N. Y., 1932. Ibid., p. 358. Heinrichs, C. E., paper presented before meeting of Am. Inst. Mining Met. Engrs., New York, February, 1933. (5) Martin, H. S., Am. Inst. Mining Met. Engrs., Contrib. 51 (1933). (1) (2) (3) (4)
RECEIVEDMarch 14, 1934. Presented before the Diviaion of Chemical Education a t the 87th Meeting of the American Chemical Society, S t . Petereburg, Fla., March 25 to 30, 1934.
Factors affecting the qualities of oiled dirt roads are examined theoretically and experimentally, and new surface treatment methods for oiling jobs tried out on the experimental roads are described. I n all cases it has been found that the addition of moislure and of surface-active substances, such as soaps, are able to improve the .final road. However, it is necessary to consider the general properties, such as the clay content, organic matter, etc., of the soil to be treated. A new melhod of making and applying road oil emulsions has been worked out, which promises to be of use infuture road construction. would make possible the calculation of the mechanical stability of the system as a function of its cohesional and adhesional forces. The surface energy data also permit the formation of conclusions as to the chemical or thermodynamical stability of the system-in the present case the resistance of the road towards weather influences. Unfortunately, it is as yet impossible to obtain reliable data on the surface tension of solids against either air or liquids. Even if the necessary data could be obtained, the aforementioned calculations would have a practical value only in the case of higher type pavements, where purity and uni-
On a dirt road, in the optiiiiiui~condition for oiling @), i l i e soil p a r t ~ i d eare ~ surroiindcd by wabcr tilrns. When the oil has lieen added, tlir wnplike molecules and inicellae will he positively adsorbed at tlre water-oil interface and will orient themselves in such a way that the Iiydrophilic head is strctclied toward tire aater aud the inert hydrocarbon tail djs.wlved in tlie oil. By evaporation the water film gradually decreases in thickness, and tinally the oil is linked directly with the soil particle by means of the oriented surfaco-active rnolcci~lea aiid nricellae i n the oil-phase boundary. This oriented striicture hrings aimit the greatest resistance against water tliiit. can be iittaincd with the materials einployed. 'rntiLE
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surface chemistry will finally lead to a quantitative evaluation of the stability factors in higher type asphaltic pavements, iii the case of low-cost oiled roads it can provide only general principles to guide in experimental attempts to improve construction methods and final quality of this type of road. According to tlie second principle of tliermodynainics, the free energy in any system tends to decrease. Free surface energy can decrease by reduction of the amount of surface ani1 by accumulation of substances in tlie surface, eitlicr comlined o r not vith oricntation. Consequently, in any system tlime interfaces will he most stable wliicli possess the least amount. of free energy, or, since alfnity is rncasureil by tlie iltx:rease in free energy, those p!t;a.ses will tcnrl to co~nein contact with each otlicr tliat show tlie highest affinity toward each other, Tire affiiiit.y of different rodbuilding materials for water and for oil and asohalt is shown in Ta,lile I, frorn a paper of Sieliolsoii (1). Appareiit.ly there is nut iniich jrroiiiise of obtaining eitlier ail a@ilt or an oiled dirt road that is atisolutely stable against the influence of weather. Disc o u r a g i n g as t h i s outlook is for a11 asphalt road, it is even worse for uildirt surfaces, sirice we are better able to prevent the entrance of water in an amount suficicnt to endanger a wellcoiistructotl asp11;alt paveinent. Is i t then a t all jiossilik to have stable, oilcd dirt roads? In ilraA-ingconclusions from Table I, however, a consideration of tile composite nature of the different plinses in the system was neglected. Both tlic soil and tlie oil contain an appreciable amount of various substances, dissolved and dispersed, some of wliich have tlie same general character and show the same behavior as map Inolecuies. This dual c h a r a c t e r is expressed by the following formula:
I,anoa.vroar ESPEHIMENTY On tlie basis of lliesc facts and considerations, the problem was investigated as to whether tlie amount of surface-active substances in road oil a i d soil is sufficient for the attainment of the above-described microstructure-the soil and oil only requiring the activating effect of water-r whether tlie introduction of inore surface-active substances would improve the qualities of an oildirt road. For this purpose a large nriinbcr of litboratory experiments was undertaken axid in addition two experimental roads were built, The laboratory cxperirnents were mostly concerned with t,tie effect of soap on the resistance of a soil-oil system against the slaking action of water, and with tlie effect of soap on the cohesional properties of such a system. 1?~,rt.lie investigation of tlw effect of oil and of oil plus soap tance of a soil system to the slakini: action of water, and also for tlie tensile strength measurements, bricks of an m s h a p e were made with the mold used for preparing mortar test specimens for tensile strength d e t ern! inn t io n s. The test saniplcs were inado by molding to the desired shape and drying: (a) Moist field soil (b) Dried nulverized soil with added
soap solutiori (8) The moist ficld Boil with oil nnd sunp Sdution
All tlie samples were dried at rooiii ternlxrature to the same moisture content, then either suspended in water for t h e s l a k i n g experiment or tested for tensile strength in the mortar testing maefiine.
iiiflucitce was likcly ta decrcasc tlie 1mttei:tiiig Ixoperties of the oil. (3) In all cascs in wliicli the soils had a rclativeiy low co~itent of organic inatter, soai) si~owedbeneficial effects. Soilitiiii soap Iirovcd to he better with low colloid content of the soil, Imtassiuin soap witli high colloid content of the soil. In s m i e ~ a s e swtiere an oil-treated soil slaked (lowti in less t,lioii a day's tirnc, the sample of ti aine soil t.reatei1 vcitli map in addition to thc oil resisted tire water SOT over a year. This high protective effeat, in some eases, was ohlained with tlir iisc OS as lit.tle oil as 3 per cent OS tiic weight of tile dry soil along with 0.3 per cent soap. (1) \$'itti increasing organic matter the soap irifliierice beeaiiie erratic. This \viis first attributed to the effect of sodium on the liydropliilic properties of tlie soil-rxcliange coniplex. Itwever, yinee potassium soap had the sitme effect iLlthi,ogh tlie prit8ssiiim ion iisiially decreases the liydrripliilic jrrriperties of tlie exchange complex appreciably, the reason Cor this behavior undoulitedly must be sought in the dispersing and dissolving effect of alkali ions OIL the organic matter isi the soil. Coagulation of this organic colloidal iiiitterial with calcium clihiride and subseqiient treatment of tlie soil with oil and soap solution resulted in nonslrrinking, water-resistant systems. A sinall addition OS copper sulfate to these systems proved especially beneficial. (5) Soap in all cases riindo the mixing of tlie soil and tile oil easier, and provided a better and more even distribution US tlie oil ttiroiigliout the soil system. To indicate the clraracter of tlie experiineiital results obtained, data S i x four typical soils are given in Tables I1 to VI. ttxd
As sonpi, sixIiun-Ivory snap and potassium soap prepared from the same Fatty acids as contained in the sudium soap iiiidrtals of these test snnqiles were mi& froin a large ninnbcr of diffmxt soils, ranging froin light sandy soils to Waiiii~Iiginnbo, with varin:ions of organic inatt.er as well zts of the other constitiicnts. The results of these t,ests which necesrivily were qiialitative in cliiiracter are surninariaed
T.\nr.c 11. 1ZssrXL'iI'r8ON Fona I t ~ ~ n e s ~ . v r . m v'Tssi x ; SO~LS
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as follo\vs:
(1) There was iiot nincli difference hetwecn the slnkiiig resistance arid tensile strength of soil s steins prepared accordas the oils hiid i~ relatively low ing t,n metliisls ( L i i i i i l h BS l w ~ g
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structuriii c:il,acity. Ifowever, if t,he structural capacity \vas liigli, p:irt,iciilarly owing to liiglr coiit,rnt, of organic matter, then in cas
