Sodium Silicates in Water to Prevent Corrosion WILLIAM STERICKER Philadelphia Quartz Company, Philadelphia, Pa.
so located that the silicate has to be added on the pressure side. A need, therefore, still exists for a relatively inexpensive device for operating under these conditions.
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HE addition of sodium silicates to waters for the prevention of corrosion has been practiced in a small way for many years. The fist proposal to use the method for the protection of entire water systems was made by Thresh (14). The initial use was to prevent the solution of lead from lead pipe in localities where lead poisoning had occurred. In connection with this work it was noted that the corrosion of ferrous materials was reduced. For example, the moorland water used in one town yielded a very turbid water supply, and the mains had to be flushed and scraped a t considerable expense to maintain sufficient flow. Then silicate was added to prevent the action on lead. The water cleared and became colorless and “brilliant”; the mains remained clear so that the flow of water was good. About the same time Speller found that natural silicabearing waters were less corrosive than those in which the silica content was low. The addition of sodium silicate to hot water in order to form self-healing protective coatings was developed by Speller, Texter, and Russell (12). A convenient method of feeding the silicate was developed and patented by Speller (IS). This method has been satisfactory in many cases, but its use is limited to relatively small installations. The adaptation of the silicate method in entire municipal supplies was slow for several reasons:
Physiological Action Many natural waters contain as much silica as or more than the 8 parts per million which it is proposed to add in this treatment. For example, Texarkana, Ark., and Kansas City, Kans., have between four and five times as much silica as i t is proposed to add to the water, and more than one hundred cities have over 8 p. p. m., according to Collins (4). Although not all of the silica in these waters is present as sodium silicate, there are waters in which the presence of sodium silicate has been confirmed. Hefferman (8) reported the presence of silica in “medicinal” waters from various European spas; in some cases, the silicate is almost certainly present as sodium silicate-for example, in Vichy water, which is reported to have 65 p. p. m. of silica. I n addition, the British Ministry of Health followed the health records in towns where silicate was added to the water and reported (7) that they saw no reason why silicate should not be added to a water from the health standpoint. King studied the physiological effects of silicates, and the results of his work and that of his co-workers indicate that silica is probably a n essential ingredient of the human body (10). The amounts of silica used by King were much greater than it is proposed to use in the water treatment.
1. Health departments hesitated to permit the use of sodium silicate until more was known about its physiological action. 2. The coat of the silicate treatment under some conditions was greater than that of other methods. 3. No operating data were avaiIable t o serve as guides in the use of the process. 4. Satisfactory and inexpensive devices for accurately proportioning the amount of silicate to the water were not available until recently.
Method of Feeding Silicate Before undertaking a discussion of cost, it will be necessary to describe the method of feeding silicate for the prevention of corrosion. Thresh suggested the use of 8 p. p. m. of silica as the proper dosage, and experience here has indicated that this is probably the minimum initial concentration that will afford protection within a reasonable length of time. One essential difference between the silicate process and the other processes which have been proposed is that the amount of silicate required is independent of the hardness Qr carbon dioxide content of the water. Baylis (2) found that a concentration of a t least 30 p. p. m. of calcium carbonate was necessary to form a film which would prevent or retard corrosion. In addition, the p H of the water must be controlled between 8.0 and 8.5. Neither soda ash nor sodium hydroxide has been efficient in water containing less than 30 p. p. m. of calcium carbonate. Many waters contain this amount of hardness, but some do not. I n the latter the injection of this amount of carbonate would greatly increase the hardness. The silicate treatment is particularly desirable on such waters since it does not increase the hardness. The amount of any of the other alkaline materials added will depend upon the acidity of the water. With waters that fluctuate considerably, this means that the dosage must be analytically controlled in order to produce good results without using an excess of reagent. Many small systems do not have means of analytical control, and these systems will find sodium silicate particularly advantageous.
Experience has shown under what conditions the silicate treatment is advantageous, and the present status of the method is therefore reported here.
Proportioning Devices A number of proportioning devices now on the market have been described by Cox (6). Some of these devices are actuated by the flow of the water itself, and some depend on the use of synchronized motor operation. When the head operating against the silicate feed is constant, the synchronized-motor devices are very satisfactory. However, this condition often does not exist, and the other type of machine should be used. For example, a t Eagles Mere, Pa., the pumping was done against a variable pressure, and a synchronous pump would not have delivered the correct amount. Therefore, a proportioning feeder is used. The feeder should be located on the suction side of the pumps if possible. The difficulties of obtaining smooth action without leakage a t high pressure (65 to 150 pounds per square inch) are great. Unfortunately in many cases the pumps are 348
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One other case requires special mention. In the past few years many municipalities have installed zeolite water softeners. These softeners can be run with the minimum attention; by mixing the softened with raw water in a definite proportion, it is possible to reduce the hardness of the supply. Some of these waters become much more corrosive under this treatment than would be expected from the analysis of the water. After going to the expense of removing calcium, it hardly seems correct to add lime. Here again the use of silicate seems desirable.
Cost of Treatment I n order to obtain a concentration of 8 p. p. m. of silica in the water, it is necessary to use 28 p. p. m. of the sodium silicate solution recommended for this purpose. This is equivalent to 232 pounds of the silicate solution per million gallons of water. Let us consider the delivered cost of the silicate to be $1.OO per 100 pounds (purchased in carloads). The generally accepted figure for water consumption is 100 gallons per capita per day. Under these conditions the daily cost per municipality of 10,000 people would be $2.32, or 8.5 cents per person per year. Actually Capen (3) found that towns below 75,000 population used less than the average. From his curve it may be judged that a unit of 10,000 people would use 800,000 gallons of water per day. The cost of the silicate treatment under these conditions would be $1.86 per day, or 6.8 cents per person per year. How does this compare with the cost of other methods of treatment, all of which depend upon the hardness of the water and the acidity? The situation was recently summarized by Hutton (9) as follows: Baylis gives a table estimating the cost of converting 1 p. p. m. of carbon dioxide t o carbonate by lime to be about 3 cents per million gallons, while the cost for sodium hydroxide averages 19 cents and soda ash 24 cents. However, it must be remembered that for each p. p. m. of carbon dioxide converted to calcium bicarbonate by lime, the hardness of the water is increased slightly more than 1 p. p. m. If we take Hudson and Buswell’s estimate that the increase of 1 p. p. m. of hardness causes an expenditure for soap, etc., approximating 1 cent per capita per
year, we find the total cost of lime treatment averages 24 cents, exactly the same as soda ash and more than the 19 cents for sodium hydroxide. The latter two do not increase the hardness of the water. Neither does the silicate increase the hardness of the water. Both the free and half-bound carbon dioxide have to be neutralized before calcium carbonate will be precipitated. If these two total 10 p. p. m., the silicate treatment becomes slightly cheaper than soda ash as far as the cost of chemicals is concerned. Likewise, when the total is 1 3 p . p . m . of free and half-bound carbon dioxide, the silicate will cost less than sodium hydroxide. I n many plants, particularly those which operate semiautomatically, the cost of supervision would be so much less that it would offset any difference in the cost of materials even if lime were used.
Mechanism of the Action The action of the silicate is to form a f ilm on the surface of the pipe, apparently by interaction with the ferrous hydroxide formed as the first step in corrosion. The film formation is usually slow, taking about a month, although a t Paignton, England, the water cleared in 3 days. Likewise, if the silicate feed is discontinued, the film continues to protect the pipe for some time. A mechanism of the film formation was suggested by James G. Vail and reported by Speller in 1926 (11, l a ) . The first step was postulated to be the attraction of the negatively charged silica micelles by the positively charged metal ions. T h e filmwas supposed to be formed by the coalescence of the
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silica particles into larger aggregates which eventually formed a gel. The iron in the film was supposed to be present as hydroxide. Further experience indicates that the ferrous hydroxide actually reacts with the silicate to give a coating of light green ferrous silicate, although no positive proof has been secured. On galvanized pipe the formation of zinc silicate is evident. Experience has also shown that the addition of sodium silicate solution gives protection over much greater lengths of pipe than does the use of the silicate glass. At Paignton the first take-off from the main is 17 miles from the pumping station, and protection is secured not only in this length but also in the town itself.
Kind of Silicate Except on acid waters, Naz0:3.25Si02 is recommended. When the pH drops below 6.0, the use of a more alkaline silicate, such as Naz0:2Si02, is preferable. Some concerns have recommended this or even more alkaline silicates for all waters. This recommendation does not seem desirable because of the added expense and also because the higher alkalinity removes the products of the former corrosion, whether in the form of a soft coating or of hard tubercles, so rapidly that very red water is produced; thus the cure, a t least for a time, is worse than the disease. When such silicates are to be used, it is desirable to clean the pipes before adding the silicate.
Results at Eagles Mere, Pa. This summer resort is located on top of a small mountain, in the center of which is a lake. The water supply for t h e village is pumped from the lake, approximately 0.75 mile long and 0.25 mile wide. The drainage area of the lake is less than one square mile, and no appreciable area in the vicinity is any higher. The surface of the lake is 1998 fee% above sea level. During the summer up to 140,000 gallons of water are pumped from the lake daily without change in level. Most of the water supply comes from springs; t h e original source of the spring water is unknown, but it must be underground streams traveling from some distance. The water is clear and colorless, and contains little organic material. The total solids vary betweep 15 and 30 p. p. m., and the pH between 5.0 and 7.0. Because of the lack of solids and the low pH, the Rater is very corrosive. Some years ago an aerator was installed to reduce the acidity of the water. This was evidently done without previously making an analysis, because the acidity was not reduced and it is not due to dissolved carbon dioxide. Until sodium silicate was used, the Eagles Mere Water Company had found that the best method of preventing corrosion was to add sufficient soda ash to raise the p H to 8.6 or higher. This checked the action of the water, but the water was quite yellow a t the extremes of tbe pipe line and the iron content was 2.3 p. p. m. as ferric oxide a t the end of the line. The day after the silicate feed was started, this iron content dropped to 1.0 p. p. m. and remained a t not over 1.3 p. p. m. for the rest of that season. Later it dropped to 0.4 p. p. m. or lower. During the winter months the waterworks do not operate, and the mains are drained. This leads to further corrosion. A silicate film would be expected to reduce the winter corEagles Mere, rosion. Carl Bigger, the superintendent reported: “This spring when I turned e water in and flushed the lines, the water was red but not black as it has been before. So far it has been quite clear-in fact, much clearer, I think, than before a t this time of the year.” Since then there has been a steady improvement in quality and a decrease in the amount of flushing necessary. Because of the low pH which occurred from time to time
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in this water, it was decided to feed the more alkaline type of silicate (Naz0:2Si01). The periods of acid water came suddenly and without warning so that it was not practical t.o add alkali only when the water was acid. Besides, the plant ran automatically and there were considerable periods when nobody was in attendance. Figures obtained when the change was made to the more alkaline silicate illustrate the danger of causing red water by such additions. The iron fignre had dropped to 0.97 p. p. m. as ferric oxide uith the low alkali silicato. The first effect of the change was to bring the iron to 5.43 p. p. m. Fortnnately almost all of the cottages and hotels were closed, and there had been times when very red water had appeared in spite of the soda ash treatment, so that there were no complaints. Two days later the iron had dropped to 0.50 p. p. m. at the same point.
Results at East Rochester, N. Y. The addition of silicate to water of a decidedly different type is being made at East Rochester, N. Y. The initial hardness of this well water is approximately 235 p. p. m. About half the water is softened in a zeolite softener and then mixed with raw water to give a hardness of 115 p. p. m. The mixed water has a pH of 7 . 5 4 0 and contains about 10 p. p. m. of free carbon dioxide as it comes from the wells. In spite of the considerable residual calcium and magnesiuni hardness and the very small oxygen content, t.he water proved to be corrosive as shown by rust on samples of pipe (Figure 1) and the iron content in the water at dead ends. The water contains organic matter, and during the summer months there has been a growth of organisms in the dead ends. Speller has snggested that this is “a case of anaerobic corrosion, which takes place when water contains sulfate-reducing hacteria with organic matter, with very little oxygen.” After considering various methods of treatmcnt, the addition of sodium silicate was selected. The principal reason for this decision was that the addition could he made automatically without regard to changes in the water. This plant operates entirely without supervision for as long as 48 hours at a time. The raw water enters and is mixed with the
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softened water (Figure 2). The operation of the pumps at the wells is dependent on the pressure on the system which operates an electrical control. The silicate feeder, a modified Chlorofeeder, is synchronized with the main pumps. In this proportioning pump the stroke can be regulated to feed varying amounts of silicate. A small soft-water line furnishes lnbrication for the piston. Before the treatment was started, the Bureau of Water of the New York State Department of Health made a series of analyses for comparison with similar analyses to be made a t intervals after the treatment had started. The figures before treatment were as follows: Well 1 Fell 2
Total hardness, p. p. m. Alkalinity. P. n. m. Dissolved 0 3 , p. p. an. O1oonsiimod, P. p. m. COI, D. 0. m. pH Iron, I). P. m.
230
240
142
