OCTOBER, 1940
INDUSTRIAL AKD ENGINEERING CHEMISTRY
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Conclusion
Literature Cited
The analogy between conventional full-scale lime-soda softening and the laboratory tests on which these results are based may not be exact. Consequently, conclusions are drawn between tests under similar laboratory conditions. Further work is desirable before application of this process is attempted for full-scale plant operation in the removal of silica from boiler feed water. It is hoped that the basic principles outlined in the use of aluminum hydroxide, such as control of temperature, pH, retention time, and age of precipitate, will serve to stimulate interest of other investigators in this process.
(1) Am. Pub. Health Assoc., Standard Methods of Water Analysin, 8th ed.. p. 77 (1936). (2) Betz, L. D., Noll, C. rl., and Maguire. J. J.. IND.EXG.CHEM., 32, 1323 (1940). (3) Germer, L. H., and Storks, K. H., IXD. EXG.CHEX.,Anal. Ed., 11, 583 (1939). ( 4 ) Lindsay, F. K., and Ryznar, J. W . . IND.ENQ.C m x , 31, 859 (1935). (5) Powell, S. T.,Combustion, 4,15 (1933). (6) Schwartz, M. C., J . Am. Water Works Assoc., 30, 551-70 (1938). (7) Straub. 17. G.,IND. ENG.CHEM.,28, 113 (1936).
PRESENTED before t h e Division of Water, Sewage, and Sanitation Chemistry at t h e 99th Meeting of t h e American Chemical Society, Cincinnati. Ohio.
Removal of Silica from Water by Hot Process
L. D. BETZ, C. A. NOLL,
AND
J. J. MAGUIRE
W. H. and L. D. Betz, Philadelphia, Penna.
The presence of silica in water used for boiler feed purposes is undesirable for many reasons. The adient of higher pressures in boiler operation has made this problem particularly troublesome. When added to water, magnesium oxide reduces crystalloid or soluble silica to practically zero. Various types of the oxide behave differently in the amount of silica removed per part magnesium oxide. One type, specially prepared for this purpose, is particularly effective. Greater silica removal and a lowering of hardness occurred a t 95’ C. Retention time affects the amount of silica removed; a period of 15 minutes is sufficient. Alkalinity and pH value must be controlled to provide for the greatest effectiveness but closely approximate those obtained in a n ideal effluent from a lime-soda softener. The method outlined lends itself admirably to use in hot-process lime-soda softening equipment, either with or without softening agents, with no increase in either lime or soda ash requirements. When used with softening agents, magnesium oxide produces better flocculation with a hardness lower than normal. Silica removal by this process compares favorably with other previously suggested methods, such as treatment with magnesium sulfate and ferric sulfate, and produces equal or lower residual silica. Dissolved solids in the treated water are greatly in the favor of magnesium oxide treatment. Magnesium carbonate also can be employed for silica removal with a lesser degree of efficiency.
I
N RECENT years the conditioning of water for industrial purposes and particularly for use as boiler feed water has reached the stage whereby it can be termed a “science”. Application of chemical engineering principles has in most cases made possible the operation of high-pressure boilers with the reasonable assurance of freedom from scale, corrosion, pitting, embrittlement, and carry-over. The most
J
serious problem remaining in boiler feed water conditioning today involves the prevention of siliceous deposits in boilers and turbines. Control over ordinary scale-forming salts of calcium and magnesium is well established, but treatment for silica in its various forms leaves much still to be desired. Powell stressed the importance of this problem ( 3 ) . An excellent summary of various means for silica removal was presented by Behrman and Gustafson (8). Silica is conventionally expressed in water analysis as SiOl. Actually silica exists in both the crystalloidal and the colloidal forms (4). The latter form can normally be removed by proper coagulation and fltration. It is silica in the soluble form that presents the major problem. The results in this study deal with the removal of soluble silica only. Up to the present time the processes suggested have been relatively inefficient in the parts of silica removed per part of reagent employed and, regardless of the degree of efficiency, have usually resulted in high-solids content of the treated water. This increase in solids content is normally sufficient to discourage their practical use. Magnesium oxide has been found, as shown by the following data, to have properties for highly efficient silica removal. I n addition, the added advantage of reduced solids content is secured rather than an increase in the solids of the treated water. Data presented by Table I illustrate removal of silica from water in conjunction with hot-process lime-soda softening. Successive increases in the magnesium oxide employed resulted in lower residual silica content of the treated water.
Conditions of Tests In each case 3.0 liters of water were taken in 3-liter Pyrex beakers and heated to the temperature desired except where tests were conducted at room temperature. On reaching that temperature, t h e various reagents were added immediately, each having been individually mixed into a slurry with a few milliliters of distilled water. Contents of beaker were stirred during retcntion just enough t o keep precipitates in suspension. Distilled water was added from time to time, as required, t o make up for evaporation loss. Retention time was measured from addition of r e agents. Resultsobtained in this fashion in the laboratory approximated closely those results obtained in full-scale plant operation. Except in those cases where it is stated otherwise, the magnesium
INDUSTRIAL AND ENGINEERING CHEMISTRY
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TABLE I. SILICAREMOVAL BY MAGNESIUM OXIDE@ Ma nesium Silica &de Removed Added, as SiOz, P. P. M. P. P. M. 0 .... 0 .... 5 4.8
Silioa Removed per P a r t MgO
.....
.....
0.96
Analysis of Treated Water, P. P. M. Hardness P alka- M alka- p H of Silioa 88 as linityb linityc aa Treated SiOe CaCOs asCaCOs CaCOa Water 22.1 54 0 36 7.1 21.3 32 36 58 9.7 16.5 32 36 68 9.7
34 7.8 13.5 34 0.78 38 11.9 9.4 30 0.626 36 32 10.0 11.3 0.565 42 12.3 32 0.492 9.0 40 32 5.2 16.1 0.322 50 36 32 1 6 . 6 0.221 4 . 7 75 48 24 2.4 18.9 0.189 100 32 38 19.7 0.131 1 . 6 150 a 30-minute retention and stirring time; temperature, 95' C.: b Phenolphthalein alkalinity. c Methyl orange alkalinity. 10
15 20 26
Remarks Original water Lime-soda softened Lime-spda .softened. MgO added 60 Same 9.7 Same 62 9.7 Same 9.7 58 Same 62 9.7 Same 60 9.7 Same 58 9.7 Same 70 10.3 Same 9.7 60 silica determined gravimetrically.
TABLE11. EFFECT OF TEMPERATURE
Conditions: 15-minute retent,ion and stirring time, 100 p. p. m. magnesium oxide added, 40 p. p. m. sodium hydroxide added
Temp.,
30 50 70
95 E
C.
--Analysis Silica 88 Si09 16 8 3 1
of Treated Water, P. P. M.Hardness aa P alkalinity hl alkalinity CaCOa 88 CaCOs as CaCOa 88 76 108 72 60 100 60 48 92 28 32 68
Silica determined oolorimetrioally exoept on original sample.
oxide employed in these tests RWI of a s ecially prepared technical grade (Remosil) obtained from the 8alifornia Chemical Company. Analysis of treated water in each case was conducted on a sample atered a t the temperature of test. All determinations, including colorimetric or gravimetric silica, were immediately effected and in no case was the filtered treated sample permitted to remain in contact with glass container for more than one or two hours before acidifying in the usual manner for silica determination. Colorimetric determinations were made with a Taylor analyzer on samples where gravimetric accuracy was not required. Gravimetric silica was determined by the conventional method (1).
VOL. 32, NO. 10 measures would be necessary for the removal of this increased hardness. Chemical treatment costs would be increased by the reagents required for the additional softening. At 95' C., however, the hardness is actually reduced below that of the original sample, as Tables I1 and I V show. Silica removal can proceed simultaneously with softening, and both effects are favored by higher temperatures.
Use with Lime and Soda Softening Silica removal by magnesium oxide can be carried out in the same container or softener as the softening by lime and soda ash. Removal of silica by magnesium oxide can proceed simultaneously with the removal of hardness from water by lime and soda ash. Experimental data illustrating this point are shown in Table V. The silica content of the original sample of water was 20 p. p. m.; softening by lime and soda ash decreased silica to 19 p. p. m., a reduction of 1 p. p. m. silica. This amount of silica removal is normal for the lime and soda process on a water of entirely calcium hardness. Addition of 100 p. p. m. magnesium oxide reduced the silica concentration to 1.0 p. p. m. A decrease in hardness and alkalinity was also effected which rendered the water more desirable for industrial purposes.
Effect of Temperature and Retention Time Increase in temperature increases the efficiency of silica removal by magnesium oxide. In addition, a t higher temperatures the hardness of the treated water is reduced, which is of considerable importance in the conditioning of water for industrial purposes. Data are given in Table 11. These results show that, with all other factors held constant and with only temperature varied, an increase in the efficiency of silica removal resulted with increase in temperature. Over the temperature range investigated the best results were obtained at 95' C., the approximate temperature maintained in hot-proces water softeners. The less efficient removal of silica from solution a t relatively low temperatures (23" C.) is also shown by the data in Table 111, which in addition illustrates the effect of varying retention and stirring time. Increase in retention and stirring time resulted in a decrease in the quantity of silica remaining in solution, but even with a twelyefold increase in retention time (180 minutes) silica removal remained relatively inefficient in comparison with the results obtained in 15 minutes at 95' C. At the lower temperatures some of the magnesium oxide is placed in solut.ion, as indicated by the increased hardness of the treated water. This constitutes a marked disadvantage in that before the use of this water as a boiler feed water other
Courtesy, Cochrane Corporation
EQUIPMENT FOR PROPORTIONING ADDITIONS OF MAGXESIUV OXIDEIN HOT-PROCESS SOFTENING
1325
INDUSTRIAL AND ENGINEERING CHEMISTRY
OCTOBER, 1940
The effect of higher alkalinities in the treated water was obtained by adding sodium hydroxide along with magnesium oxide. This produced higher alkalinity in the treated water. The same effect could be achieved with the use of additional lime or soda ash. However, sodium hydroxide was used for convenience. Additions of higher quantities of sodium hydroxide showed that increased alkalinities in the treated water resulted in the resolution of silica and rendered the percentage removal of silica less efficient. This is further confirmed by data in Table IV. OF RETENTION AND STIRRIKG TIME TABLE111. EFFECT
Analysis of original sample, p. p. m . Hardness as CaC03 40 P alkalinity as CaCOs 0 M alkalinity as, CaCOs, 28 Gravimetric silica as Si02 2 2 . 0 Conditions: 100 p . p. m. magnesium oxide added, 40 p. p . m. sodium hydroxide added, temperature 23' C. Stirring and Retention Time, Min. 15 30 45 80 ..
180 0
-Analysis silica as SiO+ 19 17 16
13 10
of Treated Water, ,P, P. M.Hardness as P alkalinity JI alkalinity CaCOa as CaCOa as CaCOs 104 72 108 184 76 116 132 40 88 108 76 10 s
...
..
...
Silica determined colorimetrically except on original sample.
HYDROXIDE TABLE IV. EFFECTOF SODIUM Analysis of original sample, P. P . m. Hardness as CaCOs 40 P alkalinity &B CaCOs 0 M alkalinity as CaCOa, 28 Gravimetric silica aa SIOZ 2 2 . 0 Conditions: temperature 95' C., 15-minute stirring and retention time, 100 p. p. m . magnesium oxide used -Analysis of Treated Water,,P. P. M.-Silica aa Hardness aa P alkalinity M alkalinity P. P. M. SiOp CaCOs as CaCOa SR CaCOa 20 1.5 56 32 64 40 1.0 34 40 72 100 3.0 10 84 124 a Silica determined colorimetrioally except on original sample.
Courtesy, Cochrane Corporation
SEDIMENTATION TANKFOR USE IN HOT-PROCESS SOFTENING
Sodium Hydroxide,
The addition of higher quantities of magnesium oxide a t the rate of 150 p. p. m. resulted in a further reduction of silica and a greater percentage of removal; this confirms results previously illustrated and indicates that increase in the quantity of magnesium oxide results in greater removal of the silica and in smaller quantities of silica remaining in the treated water. Also, requirements of lime and soda ash are not in any way affected by the use of magnesium oxide in conjunction with lime and soda softening. No increase in either lime or soda ash is occasioned by the application of magnesium oxide for the removal of soluble silica. Although lime-soda requirements are not affected by the use of magnesium oxide, the results tabulated show that i t is possible to achieve a lower hardness and a lower alkalinity of the treated water by em-
ploying magnesium oxide in conjunction with the lime-soda process-desirable advantages from the standpoint of boiler feed water. Magnesium carbonate can also be employed for the removal of silica in a similar manner to magnesium oxide but with a lesser degree of efficiency. The magnesium carbonate can be used in either the dry form as commercially available or in a slurry form from precipitation of a magnesium salt with sodium bicarbonate or carbonate. Table VI illustrates the results obtained with magnesium carbonate in conjunction with hot-process lime and soda softening. Additional lime was required for the precipitation of the magnesium carbonate used. Increased quantities of magnesium carbonate added in slurry form resulted in increased removal of silica from solution. Using magnesium carbonate in dry form, results showed an increase in residual silica with increase in magnesium carbonate but such results were at least partially caused by the quantity of lime added. I n order to show comparable results, the same quantity of lime was added in each case, but it is evident that somewhat
TABLEV. EFFECTOF MAGNESIUM OXIDEIN CONJVNCTIOX WITH HOT-PROCESS LIMEAND SODASOFTENIXG (AT 95' C.)
Hardness as CaCOs P alkalinity as CaCOs M alkalinity as CaCOa Silica aa SiOna Magnesium oxide Sodium hydroxide Calcium hydroxide Sodium carbonate
Original Sample, P. P. M. 124 0 6 20
38 36 64
19
32 26 54
1.0 0 100 0 0 15 15 140 140 Retention time, min. 15 15 a Silica determined colorimetrically exoept on original sample.
32 22 36 1.0 100 0 15 140 60
Analysis of Treated Water, P. P. M. 20 20 6 52 32 62 84 82 96 2.5 2 0 3.0 100 100 100 20 20 40 15 15 15 140 140 140 15 60 15
34 24 60 to lie rxtwt n o t d . However, in tlir test, .hoe.n, the qiiirntity of reagent rcqiiird hail Iiren p r e h i i + drttmninid, and enoiigli r,f r i i c l i rengent wn.i used, (wept in the case (d ferric juliiste, to redwe silirn to the rangr d U t r t 3 p. 1). i n . , n tolimble limit fur a boiler iw.1 w.rcr. Table IS-ll ~ l i v \ ~similnr j ww,x~rt~tive rrsults w i t h a diiferent origitinl n n t c r of lotver silica coilttxit. l'liu concIi:.iiiiis evidvnt f r m i Tnblr lX-.l xrc n l s ) I h m e out lly rrsults i n t l i i i series td tr:tr. Silica rcniuwl \vas conaidernbly niure clliriwt wit11 ningriL.6ium oxide tlian witl. tlcrotlirr reagent-. \Virl! magnesium oxide, soli
