Removal of Silica from Water bv Cold Process J
L. D. BETZ, C. A. NOLL, AND J. J. MAGUIRE W. H. and I,. D. Betz, Philadelphia, Penna.
result of this method, than HE difficulties resulting Freshly precipitated aluminum hydroxide would be the case were ferric sulfrom the presence of silica can remove soluble silica from water withfate or magnesium sulfate used in water which is to be out introducing large quantities of sodium for this purpose, since sodium used for boiler feed have previsalts into the treated water. Removal of sulfate is formed in the precipiously been stressed. Although silica is most effective in the pH range 8.3 to tation of these salts and thus some effort has been devoted adds to the solids content of the to removal of silica from boiler 9.1. In order to avoid introduction of treated water. water, it is generally agreed aluminum ion into solution, however, a pH The data in Table I ahow that in view of present knowlof 7.6 to 8.0 will probably provide the most reduction of silica content of edge of this subject the proper effective range for practical use. water through the applicstion point for silica removal is prior Efficiency of silica removal is greatest at of a l u m i n u m h y d r o x i d e in to the water entering the boiler. conjunction with sodium hyRemoval of silica can therefore low temperatures comparable to those droxide. be accomplished in either a hotmaintained in cold-process lime-soda sofor a cold-process type external teners. One-hour retention has been found treatment plant. Conditions of Tests sufficient. Aluminum hydroxide can be Magnesium oxide posseeats used in conjunction with cold-process limehigh efficiency in silica removal In each c888 8.0 liters of water were t s k e n . i n 3 - l i t e r P y r e x at the temperatures employed soda softening and causes no increase in beakers. I n most cases tests in hot-process water softeners lime and soda ash requirements. were conducted at room tempera(9). Lindsay and Ryznar have ture, but where elevated temperashown t h e e f f e c t of s o d i u m tun$ were em loyed, the various reagents were added immediately 11 n reaciing that tem eraaluminate (4). Aluminum hydroxide, freshly precipitated, ture. Each reagent had been ilrcJv%ally mixed into a s f k y is shown by the following data to possess properties for silica with a few milliliters of distilled water. During retention, stirring removal at the temperatures used in cold-process water sofof water in the beaker wv8d continued in order to keep precipitates in suspension. Any evaporation loss was made up with distilled tening. water. Retention time wae measured from addition of reagents. The results presented here refer only to soluble silica since Except where otherwise Btsted, the aluminum hydroxide was various coagulants and filtration result in the removal of susfreshly preci itated at room temperature from commercial pended or colloidal silica. Distinction between crystalloidal aluminum suyfate, AL(S0,)). 18H20,by sodium carbonate or and colloidal silica has been made by Schwarte (6). The sodium hydroxide. The aluminum hydroxide was washed free of sulfate but not dried. Solids content ranged from 6 to 8 per freshly precipitated aluminum hydroxide, having been washed cent, and additiona were based on the dry weight of the slurry. free of soluble salts, does not add to the solids content of the The water taken for test was in each case prepared synthetitreated water aa would be the case if precipitation took place cally and hardnms was entirely in the calcium form. Analysis of the treated water was made on a sample filtered at the temperain the water to be treated. Solids content is also lower, as a ture of the test in each case. Analyses were made immediately, and in no caw was the filtered treated sample permitted to remain in contact with a glass container for more than 1 or 2 hours before beinK acidified in the usual manner for silica determination. Colorimetricdeterminations were made with a Taylor analyzer. OF SILICA BY ALUMINWMHYDROXIDE TABLE I. REMOVAL Gravimetric silica was determined by the conventional method
T
Analysis of original sample, p. p. m. Hardness 88 CaCO: P alkalinity as CaCOa M alkalinit as CaCOa silica 88 s i &
(1).
46 0 16 23.9
Effect of Temperature and Retention Time
Conditions: 2-hour stirring and retention time, temperature 20' C.
Al(OH)s,
---Analysis of Treated WaterDry P M Basis, Added in Silica RealkaalkaHard- linity linity Slurry moved as Form, SiOg. Silica as ness as as P. P. M. P. P. M. SiO@ aa CaCOa CaCOa CaCOs 14.4 42 0 20 25 9.5 12.1 12 0 20 bo 11.8 3.2 34 2 44 100 20.7 2.5 40 0 32 160 21.4 2.3 40 0 23 200 21.6 a
Silica determined gravimetrically.
pH of Treated Water 6.9 7.1 8.3 7.1 7.1
The results obtained in the removal of silica from solution by aluminum hydroxide have been more efficient a t the lower temperatures. Increased temperatures tend to decrease efficiency of silica removal and result in higher residual silica content of the treated water. This effect is illustrated by data in Table 11. I n this series of tests all factors were held constant with the exception of temperature. Over the range investigated, the most efficient removal of silica was obtained a t 23' C. At 95' C. the removal of silica was relatively inefficient. It is evident, therefore that use of this process on a commercial scale should be confined to low-temperature applications. 1320
OCTOBER, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE 11. EFFECTOF TEMPERATURE Analysis of original sample, p. p. m. Hardness as CaCOr 44 P alkalinity aa CaCOa 0 M alkalinity 88 CaC03, 24 Gravirnetrio silioa aa S i 0 ~20.0 Conditions: 200 p. p. rn. aluminum hydroxide (dry basis) added i n elufry form, 40 p. p. m. sodium hydroxide added, 1-hour retention a n d stirring time
0
-Analysis of Treated Water, P. P. M.Silica P alka- M alkaTempera- Removed, Silica as Hardness linity a s linity as P. P. M. Si025 as CaCOa CaCOa CaCOr ture. O C. 1.0 4s 4 40 23 19.0 1.5 52 4 46 30 18.5 2.0 52 4 40 50 1s 4.0 56 4 44 70 16 9.0 60 4 44 95 11 Silica determined colorimetrically except on original sample.
OF TABLE 111. RETENTION TIMEWITH VARYINGQUANTITIES SODIUMHYDROXIDE
Analysis of original sample, p. p. m. HardneRs as CaCOa 40 P alkalinity a8 CaCOa 0 M alkalinity as CaCOa 30 Gravimetric silica as SiOt 20.0 basis) added in slurry Conditions: 200 p. p. m. aluminum hydroxide form, temperature 23" C. Betention -Analysis of Treated Water, P. P. M.and Stkring NaOH Silica Hardness P alka,alkaTime. Added. Removed, Silica as aa linity as linity as Man. P. P. M. P. P.M. SiOP CaCOs CaCOa CaCOa so 0 5 15 56 0 24 80 0 4 16 60 0 12 30 10 9 11 44 0 20 60 10 9 11 48 0 24 14 6 60 0 12 30 80 a0 2o 14 6 52 0 24 17.5 2.5 52 0 32 30 SO 0 28 60 30 17.0 3.0 56 30 40 19 1.0 56 4 36 60 40 19 1.0 48 4 34 I
0
-..
coagulant, considerably increased the efficiency of hardness removal, and also lowered the alkalinity of the treated water somewhat. Not only was silica removed from solution by the process, but a lower hardness and a lower alkalinity of the treated water were secured than was possible simply through cold-process lime-soda softening. This reduction in hardness and alkalinity is of material advantage in the treatment of water for industrial purposes and particularly for use as boiler feed water.
Effect of pH and Alkalinity Figure 1 shows the results obtained in a study of the influence of the pH of the treated water. The lowest residual silica content was obtained between a pH of 8.3 and 9.1. It cannot be stated, however, that the optimum pH value for practical silica removal by aluminum hydroxide is between 8.3 and 9.1. The residual aluminum content of the treated water increases sharply above a pH of 8.3. This introduction
TABLEIv. ALUMINUMHYDBOXIDE IN CONJUNCTION WITH COLD-PROCESSL I M ~ ~ OSOFTENING DA
Analysis of original sample, p. p. m. 124 Hardness aa CaCOa P alkalinity as CaCOa 0 M alkalinity as CaCOa 6 Gravimetric ailioa aa SiOn 2 0 . 0 Conditions: cold-prooeea lime and a d s softening, temperature 23O C. Al(OH)r, Retention Dry Basis. Analysis of Treated Water, P. P. M. and Addedin Stirring Slurry NsOH Hardness P alka- M alkaTime, Form, Added Silica 88 88 l i n i t y a s h i t aa ~ o u m P. P.M . P. P. Si020 CaCOa CaCOa Ca&a
d.
1
0
4
60 100 100 100 100 200 200 200 200 200 200 200 200
4
4 4 4
Silioa determined oolorimetrioally except on original sample.
4
The results in Tables 111 and IV indicate that little advantage in further silica reduction of the treated water is secured by a 4-hour retention time as compared with the results secured with 30-minute or 1-hour retention. I n no case did additional retention effect a further decrease in silica greater than 1.0 p. p. m.
1321
a
0
0 0
19
19
0
6.0
0 5 10 15
3.0 1.0
0 10 20 0 10 20 30 40
2.0
2.0 11.0
