Removal of Silica from Water Sodium Aluminate F. K. LINDSAY AND J. W. RYZNAR
The data presented indicate that hydrous aluminum oxide is an effective material for the removal of silica from water, and that sodium aluminate is a satisfactory source for the hydrous oxide. The control of pH is essential, as is also the build-up and recirculation of the precipitate formed. The completeness of the removal obtained is dependent upon t h e amount of sodium aluminate used in the auxiliary treatment.
National Aluminate Corporation, Chicago, Ill.
M
OST natural waters contain silica. I n deep well water
supplies, the silica will usually be present in true solution, while water pumped from shallow surface wells or streams may contain silica in colloidal suspension, as well as in solution. The presence of silica in water to be used for boiler feed purposes is very undesirable since it may react with any calcium present to form a hard, dense deposit of calcium silicate. If the silica content in boiler feed water is kept below 5.0 parts per million, the possibility of its concentration and subsequent precipitation as silicate scale is greatly reduced. Neither standard clarification practice nor lime soda softening will remove more than a small percentage of the silica present in most water supplies. Where recirculation of sludge has been employed, further silica reduction has been observed on some types of waters. The zeolite method of softening water by means of either the natural or synthetic silicate base type of product may add silica to the water rather than diminish the amount of silica present, while the newer carbonaceous exchangers apparently do not alter the silica content of a water passed through them. A review of the ohemical literature would indicate that most methods pertaining to silica removal from natural waters make use of the adsorptive properties of the materials being studied ($7 67 6).
jected to test. Ten-gram samples of the hydrous oxide, as iU.208, were added to 200-ml. samples containing varying concentrations of soluble sodium silicate. The results indicated that aluminum oxide had silica-removal properties. A plot of the data obtained follows the Freundlich adsorption isotherm which is expressed by the equation:
LOG MOLES OF SILICA IN SOLUTION
-o.50
,
-0.2
-0.4
-0.6
-0.8
-1.0
-I.2
-1.4
-1.6 I
%8
,
-,?a a =
kC-n1
1
where k, = constants 12 a = amount of solute adsorbed by adsorbent C = concentration of solute in the solution Figure 1 shows that the logarithmic plot, log a = log k
+- ;log c
is a straight line with the slope l / n = 0.815 intercepting the log a axis a t log k very close to 1.00. TABLEI. ADSORPTION OF SILICABY 10 GRAMSOF HYDROUS ALUMINUMOXIDEIN 200 Cc. OF SOLUTION
Since it was known that hydrous aluminum oxide,. precipitated either from sodium aluminate or other alurmnum salts, has definite coagulating and adsorptive properties (1,3, 6),this material was made the subject of the present investigation. A quantity of the oxide was prepared by the addition of ammonium hydroxide to a solution of aluminum chloride. After washing the precipitate so formed a number of times by stirring and decantation, it was filtered and sub-
Moles of SiOz, C 0.02 0.03 0.04 0.10 0.25 0.50 0.75
0.80
859
Grams Si02 ger Cc. in oln. after Adsorption 0.0546 0.0568 0.0425 0.0366 0.0238 0.0135 0.0235 0.0238
Millimoles Si02 Adsorbed per Gram A1208 , Exptl., A Calod.
.
0.40 0.60 0.79 1.78 3.73 5.65 7.15 8.05
0.41 0.57 0.73 1.53 3.23 5.68 7.91 8.34
Log A
Log C
-0.40012 -0.22344 -0.10182 +O ,25042 f0.57171 +O. 75205 +0.85431 +0.90580
-1.6989 -1.5229 -1.3979 -1.0000 -0.6021 -0.3010 -0.1249 -0.0969
860
INDUSTRIAL AND ENGINEERING CHEMISTRY
Experimental Procedure In all of the work outlined below, the precipitation of the hydrous aluminum oxide was made directly in the water being treated. The tests were carried out on 0.2-gallon samples in 1-liter Pyrex beakers. The mixing apparatus had stainless steel blades and rods. All tests were run a t room temperature, 22’ C. Twenty minutes of agitation was adopted as standard with 5-minute settling time; the samples were then filtered prior to analysis. Silica mas determined photelometrically by reading the molybdenum blue formed by the reduction of the silicomolybdate by means of l-amino-2-naphthol-4-sulfonic acid reagent. Gravimetric analysis was also used to verify the accuracy of the colorimetric method. Hellige colorimetric equipment was used for pH determinations. I n the majority of cases a completely water-soluble stabilized sodium aluminate containing 75 per cent Na2Al204 was used and designated “680” for convenience. A 90 per cent Na2A1,04,not stabilized and designated “610”, was also used. The term “stabilized” means that aluminum hydroxide will not precipitate from a water solution of the product owing to hydrolysis. The numbers referred to are the trade numbers for these two particular grades of sodium aluminate marketed by the National Aluminate Corporation. The aluminates were used because of higher A1203contents than other available aluminum salts.
Effect of pH on Silica Removal Early in the work on the application of hydrous aluminum oxide, it was seen that reproducible results were not being obtained. An investigation showed that pH control was necessary for optimum results. Inasmuch as considerable work has been done on the application of iron salts to silica removal (2, 4, 6 ) , the effect of pH on the use of anhydrous ferric sulfate was also determined.
VOL. 31, NO. 7
Sodium Aluminate and Ferric Sulfate without Recirculation A series of experiments was carried out comparing the effect of varying dosages of No. 680 sodium aluminate with anhydrous ferric sulfate. These tests mere made on two natural waters with different silica contents. Each value is the average of several experiments. TABLE11. COMPARISON OF No. 680 SODIUM ALUMINATE AND FERRIC SULFATE I
Treatment, P. P. M. 17.0 34.0 51.0 68.0 103.0
Si02 Left in Soln., P. P. M. Hinsdale water Chicago t a p water (SiOp = 16.6 p. p. m.) (Si02 = 3.3 p. p. m.)
,
r
Naz.4ldh 11.60 8.90 5.52 4.14 2.72
Fez(S04)a 8.75 8.39 8.50 7.34 6.12
NazAlnOd 2.43 1.64 0.96 0.70 0.34
Fea(S04)a 2.84 2.07 1.64 1.06 0.51
Sodium Aluminate and Ferric Sulfate with Recirculation Examination of Table I1 shows that the efficiency of silica removal increases much faster with increase in concentrations of the N o . 680 sodium aluminate than it does with the ferric sulfate. Tests were therefore made in which higher initial concentrations of the oxides were used. As a typical example, a sample of water was treated with a quantity of sodium aluminate equivalent to 255 p. p. m., and the water was adjusted to the proper pH with lime and hydrochloric acid. After this first treatment the water was decanted from the settled sludge, and this sludge was used in conjunction with an auxiliary treatment of 34 p. p. m. of sodium aluminate, plus the required amount of lime for pH control, for treating a subsequent sample. I n every case the precipitate from the preceding cycle and the precipitate added by means of the auxiliary treatment were agitated in the silica-bearing water. Because the quantity of precipitate increases in each cycle by the amount of auxiliary treatment added, after a number of cycles part of the silica bearing sludge was “bled off” or discarded, and the remaining sludge used in conjunction with the auxiliary treatment for treating additional water. Inasmuch as this process could be continued indefinitely, it became evident that if used on an industrial scale, the initial treatment was of little consequence from a cost standpoint. TABLE111. COMPARISON OF No. 680 SODIUM ALUMINATE AND FERRIC SULFATE ON SILICA REMOVAL FROM CHICAGO TAP^ AND HINSDALE~ WATERS WITH RECIRCUL-TION Cycle
No.
1 2 3 4 5 6 7 8 4v.
P.P.M.S/LICA IN SOLUTION
The data given in Figure 2 indicate that best results are obtained from the use of the hydrous aluminum oxide in the pH range 8-3 to 8.7. The results of Schwartz (4) were confirmed, which indicated that the ferric sulfate is most effective at a pH of approximately 9.0. I n all the following work, unless otherwise stated, the pH was adjusted with lime to approximately 8.5 in tests involving sodium aluminate, and to approximately 9.0 in tests involving ferric sulfate. The ferric sulfate used was the commercial product “Ferrisul”, which contains a minimum of 90 per cent Fe2(S04)8.
-Chicago
IC
0.36 0.56 0.68 0.99 0.97 0.73 0.80 1.02 0.76
Tap W a t e r IId 1110 0.43 0.63 0.68 1.06 1.48 1.49 1.65 1.76 1.24 2.04 1.70 1.93 1.30 1.76 1.65 1.98 1.56 1.29
-Hinsdale IC
0.94 3.48 3.74 3.82 3.83 4.17 3.83 3.32 3.39
IId
Water-
2.32 7.65 7.99 9.01
9.69 10.37 10.37 10.37 8.47
1116 1.84 5.36 6.53 4.42 6.12 7.14 5.78 6.12 5.29
*
Chicago t a p watef, Si09 = 3.3 p. p. m. b Hinsdale water Si02 = 16.6 p. p. m. c Initial t r e a t m e h , 255 p. p. m. No. 680 sodium aluminate and lime; auxiliary treatment 34 p. p. m. d Initial treatmedt, 255 p. p. m. ferric sulfate and lime; auxiliary treatment 34 p. p. m. e I h t i a l treatment, 510 p . p. m. ferric sulfate and lime; auxiliary trestment, 68 p. p. m.
Varying concentrations of sodium aluminate from 86 to 255 p. p. m. were used for initial treatments. Although the higher concentrations of sodium aluminate resulted in the
INDUSTRIAL AND ENGINEERING CHEMISTRY
JULY, 1939
TABLEIV. COMPARISON OF SODIUM ALUMINATEAND FERRIC SULFATE WITH AND WITHOUT SLUDGE RECIRCULATION
TABLEVIII. COMPARISON O F SODIUM ALUMINATENo. 610 AND FERRIC SULFATE ON A SAVANNAH, GA., WATER" Cycle No. 1 2 3 4 5 6 7 8 9 Av.
7mc
With Without sludge sludge 0.76 1.64 3.39 8.90
Water Chicago t a p Hinsdale
With m'ithout sludge sludge 1.55 2.07 8.39 8.47
With Without sludge sludge 1.06 1.29 5.29 7.34
OF No. 610 SODIUM ALUMINATE, TABLEV. COMPARISON ALUMINUMCHLORIDE, AND ALUMINUMSULFATE ON SILICA REMOVAL"
A I Compound
Formula
No. 610 sodium aluminate
NarAlzOn AlCla.6HzO Alz(S0a)a.18Hr0
Silica in So1n.b
P. p . Aluminum chloride Aluminum sulfate
P. M. Silica in Soh-
-P.
P. P. M. of Silica Left in S o h . 68 P. P. M. 34 P. P. M. 34 P. P. M. NarAlrOa Fez(SOn)3 Fez(S0r)s
-----
861
m.
3.70 4.29 5.15
a Hardness = 83.3 p. p. m: silica = 27.9 p. p. m.; initial treatment, 136 p. p. m. AlzLs and lime; auiiliary treatment, 51 p. p. m. A1203 and lime, The comparisons were made on an equal A1203 basis. b Average of seven c ~ c l e s .
,
Ib
110
0.77 2.51 3.06 3.23 3.23 3.74 3.57 3.74
28.56 24.48 24.48 26.52 26.52 23.80 25.84 24.48
2.99
25.58
..
Q
TABLEIx. COMPARISON OF No. 610 SODIUM ALUMINATEAND FERRIC SULFATE ON A HIGH-SILICA WATER'"
-P.
IQ 0.42 1.53 2.21 1.87 2.76 2.50 3.06 2.63 2.12
8
Av.
P. M. Silica in SohIIb IIIC 2.25 0.64 7.39 2.98 5.78 3.23 5.78 3.31 6.46 3.82 5.78 3.57 6.12 4.16 .. 3.57 4.94 3.16
I 2.55 2.04 2.97 2.89 2.21 2.89 3 31 3.23 2.76
Cycle No. 1 2 3 4 5
COMPARISON OF SODIUM ALUMINATE AND FERRIC SULFATE ON 9.8 P. P. M. SILICA WATER
Cycle No.
14: 96 12.92 13.60 14.28 13.60 11.56 13.52
Hardness = 109.0 p. p. m.; silica = 39.5 p. p. m. b Initial, 255 p. p. m. No. 610 sodium aluminate and 255 p. p. m. lime, then 0.44 cc. per liter qoncentrated HCI; auxiliary, 136 p. p. m. KO.610 sodium aluminate and lime. C Initial, 255 p. p. m. ferric sulfate and 255 p. p. m. lime; auxiliary, 136 P. P. m. ferric sulfate and lime. d Initial, 510 p. p. m. ferric sulfate and 510 p. p. m. lime: auxiliary, 272 p. p. m. ferric sulfate and lime.
7 -
TABLEVI.
...
IIId 12.92 14.28
6
7 8 Av.
P. P. hl. Silica in Soln.6I1 I11 24 48 50.32 17.00 44.20 53 57 17.68 41 48 17.68 17.68 55.88 44 20 20.68 23.12 44.20 44 20 25.84 47 26 20.52
Silica = 95.2 D. D. m.: hardness = 17.0 u. n. m. b Treatment with recirculation of sludge: 1, i 5 5 p. p. m. No. 610 sodium aluminate and 265 p. p. m. lime, then 0.42 cc. concentrated HCI per liter; 11, 255 p. p. m. ferric sulfate and 323 p. p. m. lime; 111, 510 p. p. m. ferric sulfate and 510 p. p. m. lime.
Initial, 255 p. p. m. No. 610 sodium aluminate and 255 p. p. m. lime then 0.1 cc. concentrated HCI per liter; auxiliary, 34 p. p. m. No. 616 sodium aluminate and lime. b Initial, 255 p. p. m. ferric sulfate and 289 p. p. m. lime: auxiliary, 34 p. p. m. ferric sulfate and lime. C Initial, 510 p. p. m. ferric sulfate and 425 p. p. m. lime; auxiliary, 68 p. p. m. ferric sulfate and lime. Q
, TABLEVII. COMPARISON OF SODIUM ALUMINATEAND FERRIC SULFATE ON 21.0 P. P. M. SILICA WATERQ -P.
P. M. Silica in Soln.-
Cycle No.
Ib
Av.
0.68 2.04 3.57 3.57 4.50 3.74 4.68 4.16 3.37
IIC 0.68 1.93 3.12 3.57 3.57 3.31 4.33 3.82 3.04
IIId 2.97 4.68 5.78 5.27 7.14 7.48 9 35 6.46 6.14
Hardness = 88.4 p. p. m.; silica = 21.0 p. p. m. b Initial, 255 p. p. m. No. 680 sodium aluminate and 170 p. p. m. lime; auxiliary, 68 p. p. m. No. 680 sodium aluminate and lime. C Initial, 255 p. p. m. No. 610 sodium aluminate and 170 p. p. m. lime; auxiliary, 68 p. p. m. No. 610 sodium aluminate and lime. d Initial, 510 p. p. m. ferric sulfate and 476 p. p. m. lime; auxiliary, 136 p. p. m. ferric sulfate and lime.
TABLEx. EFFECT O F METHODOF ADDITION O F SODIUM ALUMINATEAND LIMEON SILICAREMOVALS Cycle No. 1
2
3
4 5 6 7 8
9
Av.
P. P. M. Silica in So1n.No. 680,NazA1zOa and No. 680 NasAlzOa Lime added, then lime mixed together added, then lime No. 680 NazAlnOn
A
B
0.40 0.94 1.19 1.53 1.02 1.62 2.38 1.96
0.30 1.02 1.36 1.96 1.62 2.04 1.62 1.96
1.62
1.48
..
*.
0.68 2.04 2.45 1.96 2.04 2.55 2.72 1.87 2.38 2.28
0.23 1.02 1.28 3.23 1.53 6.63b 3.06 2 97
..
2.42
Hinsdale water, low methyl orange alkalinity. silica = 16.6 p. p. m.; initial treatment, 255 p. p. m. No. 680 sodium a1;minate and 255 p. p. m. lime: auxiliary treatment, 68 P. D. m. No. 680 sodium aluminate and lime. b Not used in average.
Q
more complete removal of silica for the first few cycles, the quantity of auxiliary treatment used determined the extent to which the silica was removed. The sole purpose of the, initial treatment was to provide sludge build-up to take care of the first few cycles. Under the conditions employed in the experiments, a maximum of 1.7 p. p. m. of soluble A1203 was found in the samples analyzed. The average amount found was less than 0.85 p. p. m. I n a comparison of the results of Tables I1 and 111, i t was evident that the use of sludge build-up and recirculation was more effective than the same treatment without sludge buildup. In Tables V to X sludge recirculation was used. A
comparison of the three possible methods of applying the lime sodium aluminate treatment revealed that best results were obtained when the two chemicals were mixed together prior to their addition to the water to be treated. Sample I in Table X was run in duplicate (aand b) which indicated that quite reproducible results can be expected from this method of treatment.
Literature Cited (1) Christman, C. H.,Holmes, J. A., and Thompson, H., IND. ENQ.CHEM.,23, 637,849 (1931). (2) Liebknecht, O.,U.S. Patent 1,860,781(May 31, 1932). (3) Ruegg, K., German Patent 642,419(June 17,1937). J. Am. W d e r Works Assoc., 30,551 (1938). (4) Sohwartz, M C., (5) Sen, K.C., J. Phys. Chem., 31,686 (1927). (6) Stumper, R., Wurme, 55,272 (1932). PRESENTED before t h e Division of Water, Sewage, and Sanitation Chemistry a t the 96th Meeting of the American Chemical Society, Milwaukee, Wis.
