MCNICIPALZEOLITE WATER SOFTESING P L A N T , LANCASTER, OHIO
QT
HE p H range of waters generally considered to be most favorable for zeolite softening is about 7.0 t o 8.0. Several factors other than p H are important in determining the optimum conditions for zeolite softening, but these will not be discussed in the present paper. Too high a p H results in a gradual peptizing and eventual distintegration of zeolites, an effect which is not surprisipg in view of the well-known dispersing effect of alkalies on practically all siliceous colloids. The principal manufacturers of zeolites have taken cognizance of this deleterious effect of highly alkaline waters by stipulating in their guarantees that the water deliyered to the zeolite softener must contain no phenolphthalein alkalinity, which is to say that the p H shall not be greater than about 8.3.
Effect of Low-pH Waters Zeolites A. S.BEHRMAN AND H. GUSTAFSON International Filter Co., Chicago, Ill.
The reduced base-exchange capacity of zeolites obtained when softening waters of relatively low pH has led to the general belief that such waters exert a deteriorating and destructive effect on the zeolites. The experimental work reported in this paper indicates that the reduction in exchange capacity is not due to any disintegrating effect on the zeolite (in the absence of true mineral acidity) but simply to new equilibrium conditions a t and with the increased hydrogen-ion concentration in the water. This new equilibrium at a lower level of baseexchange capacity is established quickly ; and the active life of the zeolite at this lower level is no shorter than that of the zeolite when used for softening waters of a more favorable pH range.
Too low a pH in the water to be softened also results in a reduced exchange capacity of the zeolite; and itr has been generally assumed that this effect, as in the case of the excessively alkaline waters, is likewise due to a destructive and disintegrating effect of the low p H involved. A few manufacturers of zeolites either stipulate in their guarantees or tacitly work by the rule that the p H of the raw water should not be materially less than about 7.0; and in a considerable number of cases zeolite softening of waters of p H less than about 7.0 has either been approved reluctantly or abandoned altogether. I n view of the obvious importance of the effect of low-pH waters on zeolites, both to the manufacturer and user, it was decided t o undertake some experimental work in the hope of throwing at least partial light on the question. As a result of the experimental work to be described, the lowered exchange capacity obtained when using low-pH waters appears to be due not to actual disintegration or destruction of the zeolite, but apparently to the new equilibrium conditions established at the lower pH; once this new equilibrium is established, which occurs in a relatively short time, the exchange capacity is stabilized at the lower value. It was also found that the active life of the zeolite under the new equi279
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INDUSTRIAL AND ENGINEERlNG CHEMISTRY
librium conditions is no shorter than a t a more favorable pH, such as the range 7.0 to 8.0 previously mentioned. The designation ‘‘low-pH waters” as used in this paper includes those in the p H range of about 6.9 down t o about 4.3, the turning point of methyl orange indicator. Below this lower limit is the region called “mineral acidity” by the water chemist, which can be expected to have a definitely destructive effect on the zeolite structure.
Capacitq 4t pH 6.0
9Si.3 8 R
50
100
150
200
Number o f Cqcirr
250
300
FIGURE1. EXCHANGE C.4PACITY OF ZEOLITE A (REGUL.4R GREENSAND)
The low-pH waters with which this paper is concerned comprise the major portion of the surface watem of the Atlantic seaboard and a sprinkling of surface supplies throughout the country, together with a considerably smaller number of ground water supplies. The hardness of such waters will usually not exceed 5 or 6 grains per gallon (about 85 or 100 parts per million), accompanied by total alkalinities of not over 50 or 60 parts per million, and p H from about 5.0 to just under 7 . Most, ground water supplies, particularly in the Middlewest and West, are much higher in hardness, alkalinity, and p H than the ranges just mentioned, and therefore are not greatly involved in the present discussion.
Experimental Procedure I n order to secure the maximum amount of representative data with a minimum of work, it was decided to employ a water of pH 6.0 prepared from Chicago city water by the addition of the proper amount of sulfuric acid. The essential characteristics of the t a p water (Lake Michigan water) a t the time of the experimental work reported were a total hardness of 125 p. p. m., total alkalinity of 110 p. p. m., and p H 7.8. Addition of t h e amount of sulfuric acid necessary t o reduce the p H to 6.0 lowered the alkalinity to 42 p. p. m. Since the results of accelerated tests are almost always open to question, it was decided t o employ the procedure of actual softening and regeneration, continuing the work until a sufficient number of cycles had been carried out t o furnish the data required. Comparable information was already available for Chicago water of normal pH, which was another reason for the decision t o work with Chicago t a p water as a basis for the test at p H 6.0. The zeolites tested were placed in special straight-sided percolators about 2.5 inches (6.35 cm.) inside diameter and about 17 inches (43 cm.) over-all height. A zeolite bed 5 inches (12.7 cm.) deep was employed, supported on 2 inches (5.1 cm.) of graded silica gravel. Provision was made for the automatic . Backcarrying out of the softening and regeneration washing was done manually but was necessary on$&osnee or twice a week, so that the test units operated with little attention except occasional inspection. Softening and regeneration were both carried out downflow. The results of tests of three representative zeolites are shown in Figures 1 to 5. Zeolite A is an excellent grade of regular greensand, with a commercially rated exchange capacity of 3000 grains of calcium carbonate equivalent per cubic foot of greensand. Zeolite B is a high-capacity greensand, prepared from regular greensand by rather drastic chemical treatment; the commercial rating of this material is about 5000 grains per cubic foot. Zeolite C is one of the principal “gel zeolites” on the market; its exchange capacity is rated for industrial work at 9OOO to 10,000 grains per cubic foot. The exchange capacities
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shown in the figures for all three zeolites with Chicago water of normal pH are well above the commercial ratings in all cases. This excess is due partly to the favorable laboratory test conditions, as compared with industrial service, and partly to the fact that a liberal margin of safety is usually provided by softener manufacturers. Periodically the percolators were removed from the automatic set-up so that careful determination of the exchange capacity and other characteristics could be made at the time in accordance with our usual control laboratory procedure. These eriodic removals and the data secured at such times were the Eases of the points shown on the curves. The softening rates of flow during automatic operation for zeolites A, B, and C were 1.6, 1.8, and 2.6 gallons per square foot per minute in the order named. When the zeolites were removed from the automatic percolator apparatus for periodic and manually controlled tests, a softening rate of 3 gallons per square foot per minute was employed in all cases. For regenerating the three beds during the manually controlled tests, a 4 per cent sodium chloride solution was employed in volumes equivalent, respectively, to 2.1, 3.2, and 8.0 pounds sodium chloride per cubic foot of zeolite. During automatic operation the weights of salt employed were maintained as closely as practicable to the figures just given for manual operation, but the concentration of the regenerating solution in the case of zeolite C was made 10 per cent (instead of the 4 per cent used for A and B) for operating considerations arising from the large volume of more dilute solution which would have been required.
Discussion of Data Figure 1 shows t h a t t h e exchange capacity of zeolite A drops from its original value of 4000 grains per cubic foot down to 3500 grains in only 3 cycles, t o 3300 grains in 10 cycles, and to 3000 grains in about 175 cycles. The figures in parentheses a t each point on the curve represent the cumulative equivalent number of gallons of water softened by a cubic foot of the zeolite. g r - - C a p r c i t y with Chicapo tap water pH 7.8
4-
.&
(2300 qrls. per cu. ft.)
(272,000 aalr.)
8
200
400
600
800 1000 Number of Cqcc1.r
FIGURE 2 . EXCHANGE C.4PACITY
1200
O F ZEOLITE CAPACITY GREENSAND)
1400
B (HIGH-
In the case of zeolite B (Figure 2) the initial capacity of 7000 grains per cubic foot drops in 3 cycles to 5300 grains, in 528 cycles t o 3900 grains, and in 1598 cycles t o 3200 grains, the last value being substantially the same as the stabilized capacity of regular greensand shown in Figure 1. In t h e case of zeolite C (Figure 3) the initial capacity of 16,000grains per cubic foot drops t o 14,700 grains in 5 cycles t o 13,900 grains in 10 cycles, and t o 9800 grains in 56 cycles, The exchange capacity curves for t h e three zeolites when using Chicago water of p H 7.8 may be considered for the portion of the curves shown as almost, but not quite, horizontal lines extending from the point on the curve indicated. The actual slopes of the curves-that is, the deviations from true horizontal representing the inevitable loss in capacity of all zeolites on long-continued use-depend on the zeolites themselves and on the conditions of their use. There are different grades and qualities of regular greensands, high capacity greensands, and gel zeolites. With these differences in quality this paper is not concerned. The primary object here is t o point out the characteristic behavior of all three
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INDUSTRIAL AND ENGISEERIKG CHEMISTRY
Although the curves show the results of the tests to the point where the new equilibrium conditions had evidently been established, the number of cycles actually c:trried out was usually much greater than those shown, in the majority of cases being equivalent to a t least 500,000 to 1,000,000 gallons per cubic foot. Space mill permit neither the tabulation of the subsequent data nor their graphic representation. These results may be summarized, however, by the statement that once the new equilibrium conditions had been established, the slope of the capacity curve with the water of p H 6.0 paralleled closely that of the water of pH 7.8; in other words, the active life a t the lower p H was no shorter than a t what has been considered the more favorable pH.
Effect of Restoring Higher pH I n several cases test beds of different types of zeolites. which had been run n i t h water of p H 6.0 until equilibrium was reached under the new conditions, were now run d h Chicago lyater with its normal p H of 7.8. I n all cases there was an immediate jump in exchange capacity at the higher pH, the capacity then gradually increasing with successive runs until a t least a substantial part of the exchange capacity lost a t the lower p H had been restored. Sufficient data have
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not yet been obtained to permit the correlation, if any, of the length of exposure to the lower p H and the percentage of lost capacity restored in increasing the pH.
Effect of Low pH Waters on Stability of Zeolite Usually it has been considered that one of the detrimental effects of low-pH waters on zeolites is an increased loss of silica under such conditions. With that point in mind, determinations of silica were made in the effluent from t h e experimental units a t periodic intervals, under carefully controlled conditions, and at comparable points in each run tested, since the silica in the effluent immediately after regeneration is characteristically higher than a t the end of the run, and higher than in the raw water. The silica found in the effluent a t p H 6 was no greater than under comparable conditions a t p H 7.8, and in some cases actually less. These results were really not surprising in view of the fact that silica gel (and silica in general, for that matter) is less soluble under acid than under alkaline conditions. The indications of the silica determinations were confirmed by observation of the hardness and other physical characteristics of the zeolites tested under the two different p H conditione, since there was no perceptible difference in these characteristics a t the end of the test runs.
Changes of pH during Softening
.c
Y
0
5;
5b
A
Id,
Number o f Cqcler
FIGURE 3. EXCHANGE CAPACITYOF ZEOLITE C (GEL ZEOLITE)
I n the preceding section i t was mentioned that the silica in the effluent water immediately after regeneration (in which operation is included washing out excess brine) is higher than during later portions of the softening run. The change in p H is even more striking. The pH immediately after regeneration is considerably higher than that of the raw water and, as the softening run progresses, diminishes steadily until a t
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+'* "
4 ( w u l a r ureenJdnd)-Chicaqo WAter p~ nI zeolite c ( e1 z t o l i t e ) - 3 0 q r i i n water p~ T.B "
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4 -%hieyo w t p r adjytrd t: pH 6;O
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Conatant pH
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pH of influent brina-yL;nd wash water
81-.Conrtant
s;' I
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-
-
-
the end of the run it is lower than that of the raw water. This behavior is typical, regardless of the type of zeolite involved, and even though the p H of the regenerating brine is the same as that of the water being softened. So striking and uniform i n general characteristics is this behavior that it may be employed, with a proper understanding of its limitations and with means for adjustments to fit a particular zeolite and given operating conditions, for automatically regulating (through suitable pH recording and controlling apparatus) the complete softening, regeneration, and backwash operations of the zeolite cycle. The changes in p H of the softened water in the case of zeolites A and C when used with Chicago t a p water both at p H 7.8 and at 6.0 are shown graphically in Figure 4.
Changes i n pH of Regenerating Brine I n view of the nature of these changes in p H of the softened water, we would naturally suspect that p H changes would also take place, in the opposite direction, in the brine during the course of regeneration. That these changes do occur is shown in Figure 5, in which the curves are directly related to those in Figure 4. It is interesting to speculate on the mechanism of the reactions bringing about these changes in pH. Whatever may be the nature of these reactions-whether they be hydrolysis. hydration (possibly with the formation of sodium silicate), or related phenomenon-the fact that there is an appreciable loss of silica with each regeneration indicates that regeneration
m
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i n f l u e n t brine and washwater
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itself acts a t least as a minor destructive force on the zeolite and that, by the same token, the employment of too short a softening run and consequently too frequent regeneration may often be a doubtful economy.
Reduced Exchange Capacity at Lowered pH From the data and curves obtained as a result of the experimental work, the conclusion seems reasonable that the reduced exchange capacity at the lowered pH is due t o the new equilibrium conditions established a t the lowered pH. What is not so apparent, however, is the difference in the nature of the equilibria under different p H conditions. While any attempt to visualize these differences must at this time be based largely on hypothetical considerations, it appears not unreasonable to assume the formation of a certain proportion of hydrogen zeolite, this proportion being greater as the hydrogen-ion concentration of the raw water is increased (that is, as the pH is lowered) ; and the greater this proportion of hydrogen zeolite, the less the proportion of sodium zeolite and consequently the reduced effective tendency (both mass and absolute) to exchange sodium for calcium and magnesium. Whether or not this explanation is eventually upheld by more definitely factual information, it is believed that in the meantime it will serve as a useful guide and tool in further investigations in this field. RECEIVEDAugust 15, 1936. Presented before the Division of Water, Sewage, and Sanitation Chemistry at the 91st Meeting of the American Chemical Society, Kansas City, Mo., April 13 to 17, 1936.
