JANUARY, 1939
INDUSTRIAL AND ENGINEERING C H E M I S T R Y
water by clay, bentonite, colloidal oxides, and similar emulsifying agents is due to the attraction between the colloidal particles of these emulsifying agents and the hydroxyl group of water. In the presence of citric acid and in the absence of flocculating concentrations of flocculating ions, the emulsifying agents selectively adsorb the citric acid in preference to the water by virtue of the hydroxyl group possessed by the citric acid, and consequently the water adsorption of the emulsifying agent is reduced. This enables water that would otherwise be adsorbed by the emulsifying agent to remain in the intermicellar space of the emulsion system. In lieu of the above or perhaps in combination with it, the strictly chemical action of acids such as citric or oxalic as opposed to that of acids such as sulfuric may be a factor in the viscosity-pH relations. Small amounts of iron oxide present in the bentonite are precipitated by sulfuric acid as insoluble sulfates. The action of citric and oxalic acids is to dissolve the iron oxide. As a result, the reduction in viscosity of the bentonite slip is more than sufficient, with small concentrations of the organic acids, to offset the increase in viscosity due to the flocculating action of the hydrogen ions. With increase in acid content, the latter effect becomes increasingly important and the viscosity becomes greater.
51
Literature Cited (1) Bechhold, H., Dede, L., and Reiner, L., Kolloid-Z., 28, 6 (1921). (2) Bhatnagar, 5. S., J . Chem. SOC.,119, 1760 (1921). (3) Briggs, T. R., J. IND. ENG.CHEM.,13, 1008 (1921).
(4) Davis, C. W., and Vacher, H. C., U.9. Bur. Mines, Tech. Paper 438 (1928). (5) Edser, E., Brit. Assoc. Advancement Sci. 4th Rept., 1922,289. (6) Finkle, P., Draper, H. D., and Hildebrand, J. H., J . Am. Chem. Soc , 45,2780 (1923). (7) Kirschbraun, L., U. 9. Patent 1,691,768 (Nov. 13, 1928). (8) Ibid., 1,918,759 (July 18, 1933). (9) Kirsohbraun, L., and Levin, H. L., Ibid., 1,691,767 (Nov. 13, 1928). (10) Larsen, E. S.,and Wherry, E. T., J . Wash. Acad. Sci., 15,465 (1925). (11) Minne, J. L. van der, Chem. Weekblad,35, 125 (1938). (12) Pickering, S.U., J. Chem. SOC.,91,2001 (1907). (13) Pickering, S.U., J . SOC.Chem. Ind., 29, 129 (1910). (14) Reinders, W., Kolloid-Z., 13, 235 (1913). 9,77 (1926). (15) Ross, C. S.,and Shannon, E. V., J . Am. Ceram. SOC., (16) Ross, C. S.,and Shannon, E. V., J . Wash. Acad. Sci., 15,467 (1925). (17) Schlaepfer, A. U. M., J. Chem. SOC.,113,522 (1918). (18) Thomas, A. W., J . Am. Leather Chem. Assoc., 22,171 (1927). (19) Wherry, E. T., Am. Mineral., 10, 120 (1925). RECEIVED September 12, 1938.
SURFACE-ACTIVE PROPERTIES OF HEXAMETAPHOSPHATE G. B. HATCH AND OWEN RICE Hall Laboratories, Inc., Pittsburgh, Pa.
Besides possessing definite surface-active properties of its own, hexametaphosphate has the ability of forming soluble complexes with many multivalent cations; their concentration is thereby reduced to such an extent as practically to eliminate their agglomerating action on numerous colloid systems. Examples of these properties are found in its action as a peptizing agent, as a depressor in selective flotation, and in its effect upon monolayers. Recently amounts of hexametaphosphate very much below those required for complete calcium sequestration have been found effective in preventing calcium carbonate deposition upon moderate treatment of bicarbonate waters with heat or alkali.
G
LA8SY sodium metaphosphate, (iTaPOB),, commonly termed Graham’s salt or sodium hexametaphosphate, was discovered in 1833 by Graham ( 3 ) . For almost a century it remained a scientific curiosity, and not until Hall’s use of this glassy form as a water-conditioning agent (4, 6) did it become commercially available. Though it is still commonly known chiefly as a water-treating chemical, in the past’few years numerous other uses for sodium hexametaphosphate have been developed, ranging from the treatment of occupational dermatoses (8) to the tanning of leather (14). This paper will be limited to the discussion of the behavior of hexametaphosphate as a surface-active agent, with
Thus the addition of 2 p. p. m. of hexametaphosphate to a water containing 200 p. p. m. of calcium bicarbonate will obviate precipitation when the water is treated with 500 p. p. m. of sodium carbonate or is heated to 80” C. for one hour. This “threshold treatment” with amounts of hexametaphosphate of the order of 1 to 5 p. p. m. has proved very useful in the prevention and removal of carbonate scale in many industrial processes. Data are presented showing the effect of temperature and metaphosphate concentration on the efficacy of this treatment, and demonstrating hysteresis effects on calcium carbonate and metal surfaces. Indications of the colloidal nature of threshold treatment are discussed.
particular emphasis upon the peculiar ability of as little as 1 or 2 parts per million to inhibit the precipitation of calcium carbonate.
Colloidal Properties Hexametaphosphate has two properties which are of considerable interest with respect to colloidal phenomena. It possesses definite surface-active properties of its own at solidaqueous solution interfaces, and it has the ability of forming soluble complexes with numerous multivalent cations, thereby reducing their concentration to such low values as practically to eliminate their agglomerating action towards various col-
INDUSTRIAL AND ENGINEERING CHEMISTRY
52
loidal systems. In the applications of hexametaphosphate in colloidal processes, use is made of both of these properties; the two often act simultaneously in the same system. Numerous investigators have used hexametaphosphate to modify the properties of disperse systems of finely divided solids in aqueous solutions. It exerts a peptizing and dispersing action upon many solid materials, chiefly the heavy metal salts and oxides. Thus Feldenheimer (2) employed hexametaphosphate for the deflocculation of clays; Robinson (11) used it to impart to drilling muds desirable properties, such as high specific gravity with an attendant low viscosity, and to prevent the deterioration of such properties through the action of multivalent cations. In these two cases the hexametaphosphate probably functions both by an actual dispersing action upon the clay and by the sequestration of multivalent agglomerating cations. Chwala (I) used hexametaphosphate as a peptizing agent for various solids; Hall (6) employed it to deflocculate magnesium hydroxide and thus decrease the viscosity and settling rate of milk of magnesia. Use has been made of the surface-active properties of hexametaphosphate in flotation. Some apparent anomalies in the results of various investigations exist, since it has been used both as an activator and a depressor; with the present available data, the reason for this is somewhat obscure. Keck, Eggleston, and Lowry (9) found hexametaphosphate to activate the flotation of massive hematite when used with unsaturated soaps or fatty acids, but to have little effect or to act as a depressor with the saturated collectors. Rose and MacDonald (12) used hexametaphosphate as a depressor in selective flotation. They found that by using the proper I
I
I
10 UNTREATED
SODIUM OLEATE
- PERCENT
FIGURE1. EFFECTOF SODIUM HEXAMETAPHOSPHATE ON THE SETTLINGVOLUMEOF CALCIUM PHOSPHATEIN SODIUM OLEATE SOLUTIONS
concentration of hexametaphosphate it is possible t o prevent the flotation of a given mineral by soaps or fatty acids; since the concentration necessary to accomplish this is determined by the metallic constituent of the mineral, they are thus able to obtain selective flotation with these collectors. They ascribe this depressing action of hexametaphosphate to the prevention of heavy metal soap formation upon the surfaces of the mineral particles. This prevention of the formation of heavy metal soaps on the surface of mineral particles by hexametaphosphate is illustrated by the effect of sodium hexametaphosphate' upon the equilibrium settling volume of tricalcium phosphate in sodium oleate solutions (Figure 1). In these tests 2 grams of calcium phosphate were thoroughly mixed with 15 ml. of solution and allowed to stand, protected from vibration, until there was no further change in the apparent volume occupied by the solid. In the absence of the metaphosphate, the 1 In all of the experimental work reported in this paper, the sodium hexametaphosphate employed was a technical produot supplied by Calgon, Ina., Pitteburgh, Pa.
VOL. 31, NO. 1
sodium oleate caused a considerable increase in the settling volume; the calcium phosphate particles agglomerated and were poorly wetted by the aqueous solution. This decrease in wettability was evidenced by the tendency of these particles to adhere to the liquid-air interface and to float if the mixture was shaken in such a way as to introduce air bubbles into the solution. The presence of hexametaphosphate in a concentration of 0.5 per cent resulted in the deflocculation of the calcium phosphate, with a resulting decrease in the settling volume, and entirely prevented the agglomerating action of the sodium oleate. That the sodium oleate had not reacted with the calcium phosphate in the presence of hexametaphosphate was shown by the foaming of these mixtures, in contrast to those containing no hexametaphosphate; the latter showed no true foam but rather a calcium soap scum on the liquid surface. S o t only will hexametaphosphate prevent the formation of insoluble calcium soap on the calcium phosphate particles, but it will also remove that already formed, as is shown by the reduction in settling height and by the foaming of the mixtures of calcium phosphate and soap solution upon the addition of sodium hexametaphosphate to the extent of 0.5 per cent. Calcium sequestration alone cannot explain this action of hexametaphosphate in the prevention of calcium soap formation on the surface of the calcium phosphate particles; there is not nearly enough hexametaphosphate present to sequester all of the calcium in the form of the soluble complex. In fact, if such were the case, the solid would be entirely dissolved. Nor can it be due solely to the formation of a simple calcium hexametaphosphate surface, since all of the calcium in this compound is not sufficiently tied up to prevent its reaction with soap. Evidently the hexametaphosphate, or its calcium complex, is so strongly adsorbed on the calcium phosphate surface that the reaction of the latter with soap is prevented. One of the great objections to the use of soap as a wetting agent in many cases is its reaction with solid surfaces to form organophilic heavy-metal soap films. Since hexametaphosphate prevents the formation of such films, it greatly increases the applicability of soap as a wetting agent. I n contrast to the more common anions, hexametaphosphate was found by Langmuir and Shaefer (10) to exert pronounced effects upon certain monolayers, imparting extreme rigidity to these films. The effect is particularly pronounced on monolayers of divalent metal stearates a t high pH or of long-chain amines a t low pH. Hexametaphosphate has no action upon pure stearic acid films, nor does it increase the rigidity of stearate films if present in sufficient quantities to sequester all of the polyvalent metal ions. These investigators attribute the stiffening action to the formation of cross linkages between the divalent metal atoms on the lower surface of the film. A similar explanation is valid in the case of the amine monolayers, since hexametaphosphate is known to combine with this group a t low pH values. I n their studies of monolayers, Langmuir and Shaefer also found hexametaphosphate extremely useful in preventing the disturbing effects of polyvalent cations present as impurities; the action appeared to be due to the sequestration of such cations. The ease with which stearate monolayers are successively laid down on solid surfaces in the presence of calcium offers a possible explanation for the formation of lime soap deposits in detergent processes in which hard water with no hexametaphosphate is employed. Wilson (16) found hexametaphosphate to increase the rate of penetration of tannin into hides and thus greatly increase the rate of vegetable tanning. He also discovered that hexametaphosphate alone is effective in tanning leather (14) and ascribed this action to its power of combining with amine groups a t low pH values.
JANUARY, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
Inhibition of Calcium Carbonate Precipitation Use of hexametaphosphate in the water-conditioning field was first made by Hall and Jackson (6), who employed it as a means of introducing phosphate into a boiler without encountering precipitation in the feed lines. With a ratio of calcium to sodium hexametaphosphate of approximately 1 to 14,the concentration of the calcium ion is reduced by complex formation to such an extent that it cannot he detected by the usual precipitants, such as soaps, carbonate, phosphate, silicate, and oxalate. Since the difficulty in wa8hing with
53
moderate increase in the carbonateion concentration of a calcium bicarbonate solution by the addition of other alkalies or of sodium carbonate, or by heating. Figure 2 illustrates the effect strikingly. The term "threshold treatment" has been applied to this process of the prevention of calcium carbonate deposition by very small amounts of sodium hexametaphosphate of the order of 1 t o 5 p. p. m. Although threshold treatment can prevent calcium carbonate deposition, it does not soften the water measurably. The precipitation of calcium soaps is prnctically unaffected by the presence of threshold amounts of sodium hexametsphosphate. Moreover, threshold treatment will not prevent calcium carbonate precipitation upon the addition of excessive amounts of alkali or of sodium carbonate or upon prolonged boiling of a bicarbonate solution, Thus threshold treatment is not applicable to detergent solutions. Neither can it he used with orthophosphate to introduce the latter into a boiler, since 1 to 5 p. p. m. of sodium hexametaphosphate is not very effectivein preventing calcium orthophosphate precipitation. There are, however, numerous industrial fields where threshold treatment is applicable, such as the prevention of calcium carbonate scale in cooliiig systems, heat exchangers, feed-water heaters, and boiler feed lines, if temperatures are not excessive. hfterprecipitatioo from lime-soda softened waters also is inhibited. Moreover, this treatment has considerable value io combrtting corrosion in water lines, since by its use the pH of a bicarbonate water may he raised wit.hout encountering the difficulty of calcium carbonate incrustation of the lines. In field applications, threshold treatment has shown the property not only of preventing calcium carbonate scale, but of slowly removing that already present.
Temperature and Concentrations of Hexametaphosphate and Calcium Bicarbonate FloURE 2. EFFECT OF TEZRE~EZOLD 'hEATMEEiT ON CALCIUM
BICARBONATE WATER
hard water is due to the precipitation of calcium and magnesium salts, Hall (4)also found hexametaphosphate t o be an effective water softener for detergent operations. In these applications of hexametaphosphate, aniounk sufficient to sequester all of the calcium in the form of the soluble complex were used. Rosensteio ( I S ) recently patented the use of the molecularly dehydrated phosphates, such as metaphosphate and pyrophosphate, for the prevention of the deposition of calcium carbonate when the carbonateion concentration of a calciumcontaining water is increased, as by the addition of alkali. Sodium hexametaphosphate, which is the most effective of the commercially available rnolecularly dehydrated phosphates, was first applied for this purpose by Roseiistein in preventing precipitation of calcium carbonate when ammonia was added as a fertiIieing agent to irrigation water. In this process, in contrast to the use of hexametaphosphate for water softening, only a small fraction of the amount required for complete calcium sequestration is employed. For example, precipitation of calcium carbonate upon the addition of 500 p. p. m. of animonia to a bicarbonate water with a hardness of 200 p. p. m. of calcium carhonate can be prevented by 2 p. p. in. of sodium hexametaphosphate; on the other hand, rtctually to soften this water would require ahout 1,100 p. p. ni. of sodium hexametaplimphate. The addition of these small amounts of hexametaphosphat,eis also effective in preventing calcium carhooate deposition resulting from a
Tests were conducted in order to determine efficacy of hexametaphosphate in preverlting the precipibtion of calcium carbonate resulting from heating hicarbonate waters, and the effect thereon of temperature, calcium bicarbonate concentration, and hexametaphosphate concentration. In these tests calcium bicarbonate solutions, prepared from equivalent amounts of calcium chloride and sodium bicarbonate contained in loosely stoppered ghss bottles, were heated for one hour in a water bath controlled to *0.5' C., cooled, filtered, and titrated with standard acid to the methyl orange end point; the drop in alkalinity upon heating served as a measure of the calcium carbonate deposition. The results of individual determinations by this method were reproducible to within ahout 10 p. p. m. of calcium carbonate, except for untreated SW-p. p, m. water where the variation was about double that encountered in the other tests; this behavior was attributable to the instnbiiity of 8his water a t room temperatiires, since any precipitation occurring prior to heat treatment considerably affects the results. In general, the results obtained from the waters treated with hexametsphosphate were somewhat more reproducible than those for the untreated waters, probably because of the greater stability of these waters a t room temperature. In all cases in which zero precipitation was indicated by titration, this was confimied by visual examination of the water and bottle after the heat treatment; it had been foimd that deposits so small as to be within the limits of error of the titrations could rather easily be detected visually. The effect of variations in the concentration of sodium hexametaphosphate upon the deposition of calcium carbonate resulting from the heat treatment. for one hour at 80" C. of a calcium bicarbonate solution with a hardness of 600 p. p. m. of calcium carbonate is given in Table I and Figure 3. From these results it is evident that 2 p. p. ni. of sodium hexameta-
INDUSTRIAL AND ENGINEERING CHEMISTRY
54
phosphate has just as much effectin inhibiting the deposition of calcium carbonate as do higher concentrations; the results even indicate that this amount is slightly more efficacious. Although the differences are within the limits of experimental error, in all cases in which parallel tests of 2 p. p. m. and higher concentrations were made, the former was slightly more effective. These tests were run under severe conditions where even 2 p. p. m. would not give complete protection against precipitation, and it should not be inferred from these results that less than 2 p. p. m. are unsatisfactory in all cases. Under less severe conditions, smaller concentrations are often sufficient to prevent deposition entirely. If concentrations as high as 10 or 20 p. p. m. are used, a nonadherent, flocculent precipitate (apparently calcium hexametaphosphate) will be formed. In practical applications no trouble has been encountered with this calcium hexametaphosphate precipitate. TABLEI. EFFECTOF HEXAMETAPHOSPHATE CONCENTRATION PRECIPITATION OF CALCIUMCARBONATE BY HEATTREATMENT OF CALCIUM BICARBONATE SOLUTIONS
ON
(NaPOs)z P. p . m. 0 *
CaCOa P p t d . P. p . m. 289 .. 180 122 49 ~
6.5
1.0 2.0
(NaPOdz P. p . m.
CaCOs Pptd.
3.0 4.0 10.0 20.0
58 61 59
P. p .
60
TABLE 11. EFFECT OF HEXAMETAPHOSPHATE UPON DEPOSITION OF CALCIUM CARBONATE ON HE.4TING CALCIUM BICARBONATE SOLUTIONS
--
0
.*.
400
0
2
500
2
CaCOePptd., p. p. m.----5 0 ° C . 60" C. 70°C. 80'
...
...
...
...
...
0
0
28
75
0
...
...
600
o
i39
800 1,000
2
0 5
159 0 5 11
2
2
2
...
...
0
...
91 0
115
0 '190 5
5 263 16 77
... 8
66
0
,
.,
...
:briy_i FIQURE 3. EFFECT OF CONCENTRATION OF SODIUM HEXAMETAPHOSPRATE ON THE THERMAL PRECIPITATION OF CALCICM CARBONATE
>
100
The more common constituents of natural waters, other than calcium, magnesium, carbonate, and bicarbonate, do not appear to exert much effect upon threshold treatment. Sodium chloride or sodium sulfate in amounts upon to 2,000 p. p. m. or moderate amounts of silicate have no adverse
SODIUM HEXAMETAWOSPMTE- P.P.M.
m.
The effect of the calcium bicarbonate concentration upon the calcium carbonate deposition resulting from heat treatment of the solution for one hour at various temperatures from 40" t o 80" C., and the effect of 2 p. p. m. of sodium hexametaphosphate upon this precipitation are given in Table I1 and Figure 4. From these results it is evident &at for each temperature there &-kmx&gm calcium bicarbonae co5centration above which- pKe&itation cannot be entirelv preyented by 2 p p. m-.-of the_metaphosph_ate,and_t&t this
Ca(HC0s)z Concn.. (NaPOa)z, p. p. m. CaCOa p. p. m. 4 0 ° C .
VOL. 31, NO. 1
C. 0
0 144 6 220 22 289 53 148
maximum concentration for complete protection against deposition decreases with increasing temperature. It is also apparent that even where 2 p. p. m. does not afford complete protection against precipitation, it still causes a marked reduction in the amount of calcium carbonate deposited. In industrial applications_-ofthruhold treatment, complete prot@>?m!gainst calcium carbonaET'ep?+kri may be ensqred by maixaining the calcium bicarbonate concentration below that value at which precipitation starts a t the particular temperature involved; this is accomplished in recirculating systems by adequate blowdown. In applications where complete freedom from deposits is not essential, the system automatically adjusts itself. Scale is deposited upon the heated surface until the heat transfer is lowered to such an extent that the hexametaphosphate can afford complete protection; further deposition is then prevented.
Water containing calcium bicarbonate equivalent t o 600 p. p. m. calcium carbonate heated 1 hour at SOo C.
influence. The treatment will inhibit the precipitation-of magnesium carbonate but is not very effective in preventing the precipitation of magnesium hydroxide.
Adsorption of Hexametaphosphate on Metalland Calcium Carbonate Surfaces After exposure to threshold-treated waters, certain metal surfaces exhibit a definite hysteresis with respect t o scale formation. When threshold treatment of the water is discontinued, the rate of calcium carbonate deposition upon these heated metal surfaces does not immediately return t o the normal value t o be expected with the untreated bicarbonate water. This is illustrated by the data given in Table 111, which show the calcium carbonate deposition resulting upon heat treatment of calcium bicarbonate solutions, prepared from calcium chloride and sodium bicarbonate, for one hour at 60" C. * 0.5" in a copper container; this container was thoroughly rinsed with distilled water between successive runs. From these results it is evident that the dilute hexametaphosphate solution has affected the copper container in such a manner that when it is refilled with a bicarbonate water containing no hexametaphosphate, the crzlcium carbonate precipitation is much lower than is usual for such a water. The normal deposition is not attained until a second portion of the untreated calcium bicarbonate solution is heated in the container. OF THE EFFECT OF THRESHOLD TABLE 111. HYSTERESIS TREATMENT UPON THB DEPOSITION O F CALCIUM CaRBONATE ON A COPPERSURFACE
Successive Run No. 1 2
3 4
(NaP0s)z P. p . ?n.
CaCOp P p t d .
0 2
103
0
0
P. 8. m. 0
54 104
This hysteresis effect appears to be due to the adsorption of hexametaphosphate, eithet as the calcium salt or as a calcium complex, upon the copper surface. The presence of
INDUSTRIAL AND ENGINEERING CHEMISTRY
JANUARY, 1939 200
1
u 0 0
1
l
P
60°C.
55
4 0
GOO
8 0
1000
FIGURE4. INHIBITING EFFECTOF TREATMENT WITH 2 SODIUMHEXAMETAPHOSPHATE ON THE THERMAL PRECIPITATION OF CALCIUMCARBONATE AT THRESHOLD P. P. Y. OF
VARIOUS TEMPERATURES AND CONCENTRATIONS O F CALCIUM BICARBONATE
I
loo+
Ca(HCO&
I
CONCENTRATION- P.P.M.
calcium appears to be necessary for this hysteresis effect, since it does not occur when the metal surface is heated in contact with a distilled water solution of sodium hexametaphosphate; subsequent runs with calcium bicarbonate solution give normal deposition of calcium carbonate under these conditions. The adsorption of hexametaphosphate on both copper and brass surfaces appears to be quite strong; even treatment with 0.1 N hydrochloric acid does not destroy the hysteresis effect. In tests in which glass containers were used, no such hysteresis was observed after exposure to hexametaphosphate. When threshold-treated waters are brought in contact with
calcium carbonate surfaces, the hexametaphosphate appears to be adsorbed on them. As a result of this adsorption, when waters stabilized against precipitation by threshold treatment are passed over large areas of calcium carbonate surface, a certain lag is observed before complete prevention of deposition is attained. The first portions of water in contact with the surface give considerable precipitation, but this gradually decreases to zero as successive portions are passed over the surface. This adsorption of hexametaphosphate on calcium carbonate is illustrated by the results of experiments designed to simulate conditions in the filtration of lime-soda ,softened
_----INITIAL VALUE - PHENOLPHTHALEIN FIGURE5. INHIBITING EFFECT OF SODIUM PHATE UPON
HEXAMETAPHOSDEPO~ITION OF CALCIUMCARBONATE ON INCRCSTED FILTER SAND Calcium bicarbonate water containing ( A ) 50 P. p. m. or ( B ) 100 p. p m of calcium carbonate was passed through 100 grams of -14 +20 mesh incrusted filter s a n d at t h e rate tof 100 ml. per minute.
A . :
10
VOLUME OF WATER THWGW SAND-LITERS
VOLUME OF WATEQ THROUGH SAND-LITERS
INDUSTRIAL AND ENGINEERING CHEMISTRY
56
water through filter sand incrusted with calcium carbonate. I n these tests waters supersaturated with respect to calcium carbonate were passed at a constant rate through a column of washed and sized, incrusted filter sand ohtsined from a municipal water purification plant, and the alkalinity of the effluent was determined at definite intervals; the drop in alkalinity sewed as an index of the amount of calcium carbonate deposition taking place. The results of such experiments for a water containing 50 p. p. m. of calcium carbonate prepared from calciuni chloride, sodium carbonate, and sodium hydroxide, and for t.lie same water treated with 2 p. p. m. of sodium hexametaphosphate are shown in Figure SA. These results show that, after the first few portions of the untreated water are pased through the incrusted sand, the alkalinity of the effluent becomes constant at a value appreciably lower than that of the initial water; considerable calcium carbonate deposition upon the sand is thus indicated. I n the case of the water containing 2 p. p. m. of sodium hexametaphosphate, the drop in alkalinity, though considerable for the initial portions of water passing through the incrusted sand, decreases rapidly at first and then more gradually, until no drop is observed; the deposition is then indicated to have fallen off to zero. Upon saturation of the calcium carbonate surface with hexametaphosphate, further deposition on i t from the water is prevented; the surface is apparently rendered inactive in relieving the supersaturation of the water wit,h re. spect to calcium carbonate. The first 100 tni. of untreated water passed throngh the sand shows an abnormally high drop in phenolphthalein alkalinity as compared with the value obtained when more water has been passed through the sand. At the same time the methyl orange alkalinity shows no such abnormality hiit only a slight and gradual decrease. This suggests that the high initial drop in phenolphthalein alkalinity may he due to the release of some acidic constituent, perhaps carbon dioxide, previously adsorbed on the filter sand. When a water containing 100 p. p. m. of calcium carbonate is treated with 2 p. p. m. of sodium hexametaphosphate and passed through the incrusted sand, the initial alkalinity drop and subsequent decrease of this drop are even more marked than for the softer water, as shown in Figure 5B. In the case of this harder water no tests were made in the absence 01sodium hexametaphosphate; for under such conditions the water was too unstable, and precipitation occurred even before it was passed through the incrusted filter sand. When 4 p. p. m. of sodium liexanietaphospliate and a 50-p. p. m. calcium carbonate water are used, the results are similar in nature to those obtained with this water when treated with 2 p. p. m.; the chief difference is that the alkalinity drop decreases to zero more rapidly. Preliminary experiments indicate that for this 50-p. p. m. calcium carbonate water, 1 p. p. m. of sodium hexametaphosphate is sufficient to prevent deposition entirely, although the decrease in the alkalinity drop to zero is s!oomcr than when 2 p. p. m. is iisecl.
c m a s aeotion 01 orieioa1 . XOSk
VOL. 31, NO. 1
When the usual stability test to determine the incrustant or corrosive properties of a water (7) is applied to thresholdtreated waters, results open to somewhat misleading conclusions are obtained. I n this test the change in alkalinity of a water produced when shaken with finely divided calcium carbonate is determined; a decrease in alkalinity indicates the deposition of calcium carbonate from the water. I n this test, as generally employed, the ratio of the calciu~ncarbonate snrface to the water volinne is quite high and, as might be expected, when fresh calcium carbonate is used, 9. threshold-
F I G U R E 6. &P&RILlZATION OB CALCIUM BICARBONATE WATERCONTAIUINC 200 P. P. m. OF CALCIOM CARBONATE BY 3 P. P. M. OF
Somm I I E X A F - I ~ ~ A P I ~ O S P H A ~ E
treated water will show a considerable drop in alkalinity; the amount of hexametsphosphate is insufficient to saturate the calcium carbonate surface when adsorbed on it. If, however, the same calcium carbonate is used repeatedly with successive portions of threshold-treated water, the alkalinity drop will decresse as the calcium carbonate surface appronches saturation with respect to adsorbed hexametaphosphate. I n view of the usual high ratio of calcium carbonate surface to water volume in this test (much higher than is generally encountered in practice), a rather large number of portions of threshold-treated water will be required hefore the alkalinity drop will decrease to zero, although it will decrease to a low value quite rapidly. After exposure to threshold-treated waters, calcium carbonate surfaces exhibit a hysteresis which is similar in nature to that previously discussed for copper surfaces. When incrusted filter sand, through which threshold-treated water has been passed until no drop in alkalinity occurs, is exposed to the action of a 5033 p. m. calcium carbonate water containing no hexametaphosphate, appreciable amounts of this untreated water may be passed through the sand before any deposition occurs. These hysteresis effects are sufficient to compensate for slight variations in the rate of addition of the hexametaphosphate; the necessity for very precise and costly feeding devices is thus obviated, although they do not appear strong enough to justify intermittent feed. The rate of solution of calcium carbonate in distilled water is considerably retarded by the presence of threshold amounts
Surfaae after 9 month
FIGURE7. EFFECTOF WATERCONTAINING 2 P. P. m. OF S o ~ r HE~AXBTAPE~SPHATE n~ ON CALCIUM CAREONATE SCALE IN PIPE L m
_ ...
JANUARY, 1939
. .-
INDUSTRIAL AND ENGINEERING CHEMISTRY
of hexametaphosphate, presumably as a result of the effect of an adsorbed film of the latter on the surface of the calcium carbonate. This retardation in the solution rate may be shown rather simply by adding 5 grams of incrusted filter sand (- 12 f20 mesh) to 100 ml. of distilled water containing phenolphthalein. When the water contains 2 p. p. m. of sodium hexametaphosphate, the development of the pink color is much slower than in its absence; if 10 p. p. m. is present, an even greater retardation in the color development is observed.
Mechanism Colloidal suspensions of calcium carbonate can be prepared with threshold amounts of hexametaphosphate as a protective agent, but they do not exhibit great stability if they are very concentrated. When a cold solution of calcium chloride, equivalent t o 400 p. p. m. of calcium carbonate, containing 2 p. p. m. of sodium hexametaphosphate, is treated with an equivalent amount of sodium carbonate, a faintly opalescent solution showing a strong Tyndall cone is obtained. If such a solution is kept cold, no sign of precipitation will occur in 6 hours, although on standing 18 hours some crystallization of calcium carbonate will be observed. If the solution is slightly warmed, crystallization will occur much more rapidly. Measurements of pH on 200-p. p. m. calcium carbonate waters, prepared by mixing equivalent amounts of calcium chloride and sodium carbonate, show considerably different results when 2 p. p. m. of sodium hexametaphosphate is present than in its absence as is apparent in Figure 6. Five minutes after preparation the 200-p. p. m. calcium carbonate water containing 2 p. p. m. of metaphosphate shows a pH of 10.25, and that containing no hexametaphosphate, a pH of 10.1. After standing for 15 hours the pH of the former is 10.1 and of the latter 9.3. After 5 days the values are 9.9 and 9.0, respectively, the‘threshold-treated sample still showing no visible deposition. If the same amount of calcium carbonate was formed in each casc and the only difference was in its being colloidally dispersed in the presence of the hexametaphosphate rather than being precipitated, one should not expect such a difference in pH, calcium carbonate having been removed from true solution in both cases. If, however, the presence of the hexametaphosphate in threshold amounts allowed the maintenance of a high degree of supersaturation, such pH differw - ences would be expected. In the experiments with incrusted filter sand, previously discussed, upon saturation of a calcium carbonate surface with hexametaphosphate it became inactive in relieving supersaturation with respect t o calcium carbonate of waters brought in contact with this surface. Thus it appears that the function of hexametaphosphate in threshold treatment may be to provide increased stabilization for a condition of supersaturation; deposition upon any calcium carbonate nuclei formed is inhibited by adsorption on it of hexametaphosphate which thus prevents their growth beyond colloidal dimensions. When threshold-treated calcium bicarbonate solutions are heated, as in the tests previously considered, the pH values of the solutions remain near that of the phenolphthalein end point; heating of the untreated bicarbonate waters results in a decrease in pH below this value in all cases in which appreciable precipitation occurs. Since the partial pressure of carbon dioxide from a solution containing a given amount of total carbon dioxide decreases with increasing pH, there is probably considerably less carbon dioxide loss from the solutions containing threshold amounts of hexametaphosphate than from the untreated. Determinations of phenolphthalein
57
alkalinities indicate that little carbon dioxide is lost on heating, the threshold-treated waters, although such determinations might be affected if colloidal calcium carbonate protected with adsorbed hexametaphosphate were present.
Scale Removal In the industrial use of threshold treatment when it is applied to a system which is considerably incrusted with calcium carbonate deposits, not only is further deposition prevented, but the old scale is slowly removed. Figure 7 shows two samples of scale which were removed from a 14-inch line carrying the effluent from a cold-process lime-soda softener a t a rate of 1,400 gallons per minute; one was taken before the start of threshold treatment, and the other 9 months after treatment with 2 p. p. m. of sodium hexametaphosphate was started. The reduction in the thickness of the old scale by threshold treatment is evident. The original scale was hard and compact throughout. After treatment the scale was soft and crumbly on both sides; in fact, before drying, this material on the sides was more of a slime. It appears from this sample that the hexametaphosphate, present in threshold amounts, has worked in behind the scale, loosening it from the metal surface as well as causing its disintegration on the side in contact with the water. Scale removed by threshold treatment tends to come off as a granular sludge or in flakes, the hexametaphosphate appearing to loosen the bonds between the individual calcium carbonate crystals and between the metal and the calcium carbonate, perhaps because of its strong adsorption on such surfaces. Practical experience indicates that the rate of scale removal by threshold amounts of hexametaphosphate increases with increasing temperatures and water velocities. In the preceding discussion the emphasis has been largely upon laboratory studies of threshold treatment. Of the various manifestations of surface-active behavior demonstrated by sodium hexametaphosphate, the use of from 1to 5 p. p. m. of this substance t o prevent the precipitation of cqlcium carbonate from water after softening or upon heating has found wide industrial use during the past 2 years. Some of the results obtained in practice will be presented in the following paper by Rice and Partridge.
Literature Cited (1) Chwala, A., U. S. Patent 1,728,662 (1929); German Patent 504,598 (1930). (2) Feldenheimer, W., U. S. Patent 1,438,588 (1922). (3) Graham, Thomas, Trans. Roy. SOC. (London), 123, 253-84 (1833). (4) Hall, R. E., U. S. Patent 1,956,515(1934),Reissue 19,719 (1935); U. 5. Patent 2,035,652 (1936). (5) Hall, R. E., U. S. Patent 2,087,089 (1937). (6) Hall, R. E., and Jackson, H. A., Ibld., 1,903,041 (1933). (7) Hoover, C. P., “Water Supply and Treatment,” Natl. Lime Assoo., Bull. 211, 141 (1934). (8) Jones, K. K., Murray, D. E., and Ivy, A. C., Ind. M e d . , 6, 459-62 (1937). (9) Keck, W. E., Eggleston, G. C., and Lowry, W. W., Am. Inst. Mining Met. Engrs., Tech. Pub. 763 (Jan., 1937). (IO) Langmuir, I., and Shaefer, V. J., J. Am. Chem. Soc., 59, 2400 (1937). (11) Robinson, W. W., Canadian Patent 366,534 (1937). (12) Rose, E. H., and MacDonald, W. T., U. S. Patent 2,040,187 (1936). (13) Rosenstein, L., U. S. Patent Reissue 20,764 (1938). (14) Wilson, J. A , , J . Am. Leather Chem. Assoc., 32, 113 (1937). (15) Wilson, J. A., U. S. Patent 2,087,849 (1937). RECEIVED September 1 2 , 1938.
(End o j Symposium on Surface-Active Agents)
OWN AND TRAP FLASH TO 0O\LER5
STEAM SUPPLY
FEED PUMP STCR
PUMPS
SYSTEMAT THE PEORIA PLANT OF COMMERCIAL SOLVENTS CORPORATION FIGURE 1. WATERTREATMENT Threshold treatment system is shown a t left. Effluent cooling water from mash coolers is t h e n softened and used for bailer feed, which is further conditioned with sodium sulfite a n d sodium metaphosphate.
THRESHOLD TREATMENT Elimination of Calcium Carbonate Deposits from
Industrial Waters OWEN RICE ARrD EVERETT P. PARTRIDGE Hall Laboratories, Inc., Pittsburgh, Pa.
W
HEN the system calcium oxide-carbon dioxide-water manifests itself in the chalk cliffs of Dover or the architecture of the Carlsbad Caverns, it arouses emotions in the observer which may be more lofty but are no more intense than those produced in an engineer by the unseen workings of this same system when it converts a 12-inch into an 8-inch pipe line or reduces the heat transfer in a heat exchanger to 40 per cent of its design value. The tendency for calcium carbonate to come out of solution in inconvenient places creates an operating problem wherever waters containing appreciable concentrations of calcium and bicarbonate or carbonate ions are used industrially. In the power plant, the distillery, the oil refinery, and throughout the process industries in general, calcium carbonate scales form on the water side of condensers or heat exchangers in just those regions where, for efficient operation, heat transfer should be least impeded. With regard to afterprecipitation, it is probably safe to say that nearly every pipe line following a cold process lime or lime-soda softener and many of the lines following hot-process softeners have accumulated calcium carbonate scale to an extent which has either increased pumping costs, seriously reduced capacity, or pyramided these undesirable effects. Incrustation of filter sand, distribution systems, and domestic water heaters, following a soften-
ing operation, is another variation of this problem which has contributed no little to the troubles of water-works operators. Altogether, calcium carbonate has been a consistent nuisance t o the engineer.
Traditional Methods of Preventing and Removing Scale The means employed to cope with calcium carbonate have varied with the local conditions and the degree to which the engineer in charge regarded scale as a necessary evil. In some industrial plants, tannin or compounds containing tannin as the chief constituent have been added to cooling water in the hope that the calcium carbonate precipitated would be peptized sufficiently to prevent or retard the accumulation of deposits. Some railroads have used tannin similarly in the attempt to prevent afterprecipitation in pipe lines carrying softened waters, and in injectors, boiler feed lines, and feed water heaters on the locomotives. The concentration of tannin necessary t o achieve satisfactory results is, however, high enough t o make the treatment rather expensive and to introduce a definite color into the water, which may be objectionable if the water is to be used in some types of industrial processing or for domestic consumption. 58
