Adsorption of Soluble Silica fromwater L. D. BETZ, C. A. NOLL, AND J. J. MAGUIRE W. H. and L. D. Betz, Philadelphia, Penna.
Analysis of the treated water was conducted in each case on a sample filtered at 95" C. and subsequently cooled t o room temperature. At no time were samples in contact with glass; hard rubber funnels together with stainless steel beakers and stirring rods were employed. Silica determinations were made by the Betz photoelectric method involving the use of a Klett-Summerson photoelectric photometer (3). Gravimetric determinations were made on occasional samples for check purposes.
HE necessity of maintaining scale-free heating surfaces in modern high-pressure high-rated boilers has intensified the efforts of the chemical engineer in the development of new methods for the prevention of scale formation and their control. Silicate scale is one of the most obnoxious formations; until recent date no process was available that would remove soluble silica efficiently from water without the introduction of other factors, such as greatly increased solids content of the treated water. In the conditioning of water for boiler feed purposes, i t is not possible to tolerate such large increases in solids content. Efficient removal of silica from solution, together with an actual decrease in the solids content of the treated water, has been made possible through the development of a specially prepared adsorptive form of magnesium oxide, manufactured from sea water bitterns, which has been termed "Remosil". The characteristics of this material and the practical applications of the process for silica removal were previously described (2). Previous data indicated that the removal of silica by magnesium oxide corresponds better with the principles of adsorption from solution rather than with a stoichiometric chemical reaction. Further investigations bearing on this point are herewith presented, and confirmation of previous conclusions is secured that the mechanism of the reaction is one of adsorption.
T
Adsorption Data The results obtained in the treatment of four different waters of varying soluble silica contents of 26.2, 15.6, 11.6, and 7.1 p. p. m. are shown in Table I. These waters cover the range of silica normally encountered in natural supplies. All tests were conducted with 15-minute stirring and retention time, with the exception of the water of 7.1 p, p. m. original silica content on which one-hour stirring and retention time was used. TABLE I. ADSORPTIONSTUDIES O N SILICA REMOVAL EMPLOYING
MAQNESIUM OXIDE
Magnesium Residual SiOz, Oxide, P. P. M. P. P. M. 26.2 0 14.3 50 9.2 75 100 6.2 150 2.9 1.0 200 15.6 0 50 8.3 70 5.4 90 3.4 110 1.6 130 1.0 11.6 0 30 6.75 50 3.5 70 2.6 1.8 90 100 1.5 0 7.1 10 4.1 20 3.0 2.5 50 2.2 60 30 2.0
Conditions of Tests In each case 1.0 liter of the original water was heated in stainless steel beakers t o 95' C. The reagents employed were then added immediately after each had been individually mixed into a slurry with a few milliliters of distilled water. Contents of the beaker were stirred during retention just enough to keep the precipitates in suspension. Distilled water was added from time t o time to make up for evaporation loss. Retention time was measured from the addition of reagents. In all tests a constant temperature of 95" C. was maintained, and the magnesium oxide employed was a specially prepared technical grade, obtained from the California Chemical Company.
z 6 h h l
$
24
p: 5.
22 -"
+ /,VIT/AL S O , = 26.2 f?EP% 0 '' " =,5.' " A ,r
MG
u
=/I.&
"
=I/
"
SiOz Removed,
P. P. M.
Silica Removed per Part Me0
0 11.9
o:iis
17.0
20.0 23.3 25.2 0 7.3 10.2 12.2 14.0 14.6 0 5.85 8.1 9.0 9.8 10.1 0 3.0 4.1 4.6 4.9 5.1
0.227 0.200 0.155 0.127
0 :iie 0.146 0.135 0.127 0.112 o:iQ5 0.162 0.129
0.109 0.101
0:300 0,205 0.153 0.098 0.085
%
Original Si02 Removed 0.0 45.5 65.0 75.3 88.9 96.2 0.0 46.8 65.4 78.2 89.8 93.6 0.0 50.5 69.8 77.8 84.5 87.1 0.0 42.3 57.7 64.7 69.0 71.8
%
Original Si02 Remainine 100.0 54.5 35.0 24.7 11.1 3.8 100.0 53.2 34.6 21.8 10.2 6.4
100.0 49.6 30.2 22.2 15.5 12.9 100.0
57.7 42.3 35.3 31 .O 28.2
I Figure 1 illustrates the marked increase in the quantity of magnesium oxide necessary for the removal of the last traces of silica. For each water the first portion of the curve is practically a straight line, but as the concentration of silica remaining in solution is decreased, equal increments of magnesium oxide remove successively less silica from solution. This phenomenon is characteristic of adsorption reactions. Figure 2 shows silica removed per part of magnesium oxide us. residual or equilibrium silica concentrations on a logarithmic scale. Such plots, known as Freundlich adsorption isotherms, are considered proof that the reaction proceeds by adsorption when a straight line results.
0 USPD, PPN.
FIGURE 1. MAGNESIUM OXIDE REQUIREMENTS 814
June, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
Studies are presented on the mekhanism of silica removal by adsorptive forms of magnesium oxide. Results are illustrated for the removal of silica from different waters of varying silica content; data on the removal of low concentrations of silica correspond to the known characteristics of adsorption reactions. Further work on the effect of retention time indicates that, if a relatively high degree of silica removal is to be accomplished, little is to be gained by increase i n retention time above 15 minutes. Where a relatively low removal of silica is to be obtained, increase in retention time is beneficial. Data on effect of pH indicate that with lower concentrations of silica in solution control of pH becomes less critical. The use of magnesium oxide has been investigated with respect to application of silica removal in conjunction with hot-process external phosphate softening. Two-stage operation is necessary to
Figure 3 shows typical adsorption curves illustrating the quantity of silica removed per part of magnesium oxide with decreasing silica concentrations. This effect is further shown graphically by Figure 4, a plot of the decrease in the quantity of silica removed per part of magnesium oxide with increased percentage removal of the original silica content. This process for silica removal corresponds with the known characteristics of adsorption reactions in all particulars investigated.
815
avoid adsorption of phosphate; with this provision, efficient silica removal and reduction i n the hardness of the treated water to practically zero are accomplished. High-magnesium lime can, under controlled conditions, effect some removal of silica. Owing to a fixed ratio of calcium to magnesium, however, control is difficult, increase in solids may result, and additional reagents may be required to remove excess calcium. Use of sludge recirculation considerably increases the quantity of silica removed per part of magnesium oxide. By this means the amount of magnesium oxide necessary for the removal of a given amount of silica may be reduced to 30-40 per cent of the quantity required without recirculation. Data are presented on the results obtained under actual plant conditions. In general, efficiency of silica removal has been found greater under plant conditions than that secured in the laboratory.
These curves are plotted from the data used for Figure 5 .
As commonly found with adsorption reactions, the amount of silica removed per part of magnesium oxide is lower where higher quantities of magnesium oxide are employed and where the silica content of the treated water is lower. With the use of smaller quantities of magnesium oxide, permitting higher residual silica in the treated water, the quantity of silica removed per part of magnesium oxide increases. Figure 6
Effect of Retention Time at 95' C. Equilibrium conditions in the removal of silica are attained rapidly in the hot by treatment with quantities of magnesium oxide sufficient to reduce silica to low concentrations. Under these conditions a relatively high percentage of the original silica is removed. When the quantity of magnesium oxide employed is not sufficient to effect a large percentage removal of silica, equilibrium conditions are attained much more slowly. The curves in Figure 5 are based on the silica removal obtained with the respective amounts of magnesium oxide at 240-minute retention as 100 per cent removal of silica. Obviously, greater silica removal and consequently lower residual silica in the treated water are obtained from the use of 100 p. p. m. magnesium oxide than with the use of 20 p. p. m. Figure 5 illustrates that with the use of a relatively high quantity of magnesium oxide, such as 100 p. p. m., a high percentage removal of silica is accomplished in the first 15 minutes of retention time. Increasing the retention time up to 240 minutes results in very little increase in percentage of silica removed. With lower amounts of magnesium oxide, such as 20 p. p. m., equilibrium is reached much more slowly, and added retention time greatly improves the percentage removal of silica. It is obvious from these curves that in cases where the amount of silica removal to be effected by magnesium oxide is only a relatively low percentage of the total, increased retention time aids materially in the percentage removal of silica and also that with the use of a relatively low quantity of magnesium oxide, increased retention will effect a larger percentage silica removal. Figure 6 illustrates the relation between the silica removed per part of magnesium oxide and the period of "retention.
4 3 A61 10
?
2.0
t?€S/OU&L
so 70 iao ao S/LICA /N SOL U7/0N, r5%M.
I .O
FIGURE2. FREUNDLICH ADSORPTION ISOTHERMS illustrates the minor increase in silica removed per part of magnesium oxide with increased retention time when a relatively high amount of magnesium oxide such as 100 p. p. m. is employed. It is also evident that appreciable increase in silica removed per part of magnesium oxide results with increased retention time when relatively low quantities of magnesium oxide such as 20 or 60 p. p. m. are employed. The conclusions evident from Figure 5 are confirmed by Figure 6; namely, if a relatively high degree of silica removal is to be accomplished, very little is gained by increase in retention time above 15 minutes, and for practical purposes almost nothing is to be gained by increase in retention time
INDUSTRIAL AND ENGINEERING CHEMISTRY
816
Vol. 33, No. 6
p. p. m. is normally not feasible economically. Such waters may not be particularly suited for seolite softening, and in such cases it is frequently desirable to employ hot-process phosphate softening in which practically complete precipitation of the hardness of the raw water can be accomplished in the softener. The softened water will contain a hardness of only 1 or 2 p. p. m. as calcium carbonate with an excess phosphate content of 5 to 10 p. p. m. This process was investigated for use in conjunction with silica removal by means of magnesium oxide. Characteristic data are shown in Table 11. Table I1 indicates that reduction of the silica content of the raw mater was first accomplished through treatment with magnesium oxide and sodium hydroxide. I n test 3 the treated water was then filtered, and disodium phosphate was added to the filtered water for completion of the softening reaction. I n test 4 no filtration of the suspended precipitates from the silica removal treatment was effected, and the same amount of disodium phosphate was added for softening. A portion of the phosphate was adsorbed by the magnesium oxide in test 4, and full efficiency of the phosphate for softening was not achieved.
O
5
/O
13
RLSIOUAL S / f /CA IN SOL UT/ON, /?E,/+?,
FIGURE 3. ADSORPTIONCURVES above 60 minutes. Ho\Terer, where a relatively low percentage removal of silica is t o be accomplished, much is to be gained by increasing the time of retention.
Effect of pH Figure 7 illustrates the relation between residual silica in solution following treatment with magnesium oxide and the pH of the treated water. The curves were secured by the treatment of three waters of different initial silica content with varying quantities of magnesium oxide. For each individual curve, the quantity of magnesium oxide was held constant and the only factor varied was the pH of the treated water. The minimum silica content of the treated water in each case is obtained with the resultant pH of approximately 10.1. Where the silica content of the treated water has been reduced to low concentrations, the control of pH is considerably less critical than in the cases where silica reduction has resulted in relatively incomplete removal of silica from solution. The lower of these three curves illustrates little change in the residual silica content of the treated water over the rather broad pH range of 9.5 to 10.6. The optimum pH for silica removal in each case is readily obtainable with hot- or cold-process lime and soda softening, and as the residual silica in the treated water decreases, control of pH becomes less important.
Silica Removal by Magnesium Oxide with Hot Process Phosphate Softening On waters of relatively low hardness (50 p. p. m. or less) i t is normally not possible to secure so great a percentage removal of hardness by the lime-soda process as might be desirable. Reduction of hardness of the raw water to approximately 20-30 p. p. m. is usually all that can be accomplished in the lime-soda softener without unduly increasing alkalinity. With a raw water averaging 30 p. p. m. in hardness, installation of a complete softener simply to reduce hardness t o 20
60
SO
70
90
80
'A- S/L/CA R€/+?OV€O
FIGERE 4. DECRE.4SE IN ADSORPTION WITH INCREASE IN PERCENTAGE SILICA REMOVAL Other laboratory tests have corroborated these data and indicated that there is a definite adsorption of the phosphate by the magnesium precipitates, together with consequent increase in phosphate requirements and lowering of efficiency in hardness removal. While it is possible that satisfactory results could be obtained in the simultaneous removal of hardness and silica in the same tank under actual plant conditions, laboratory experiments have provided no basis for such an assumption. The operation of a phosphate softener is more difficult t o duplicate under laboratory conditions than is the
IN CONJCNCTION WITH HOTTABLE 11. SILICAREMOVAL PROCESS PHOSPHATE SOFTENING ~ ~ neSS
Test No. 1" 2b 3c
4d
~
Analysis of Treated Water as
, Alkalinity j -
tacos, P. P. AI.
CaCOs,
Phenol-
Methyl
2 9
16 20
76 82
P.P. M . phthalein orange 24 0 56 20 70 20
SiOr, P.P. M. 32.6 0.9 0.9
1.0
pH 7.3 9.5
9.5 9.5
Phosphate as Pod, P. P. M. 0
0 15 7
Original sample no treatment. b Original sample'treated with MgO and NaOH for silica removal. c Sample 2 treated with Na2HPO4 for softening after first filtering o u t precipitates from silica removal treatment. d Sample 2 treated with same quantity of NasHPOa for softening b u t without filtering out precipitates from silica removal treatment. 0
INDUSTRIAL AND ENGINEERING CHEMISTRY
June, 1941
817
LIME TABLE111. USE OF DOLOMITIC MgO Added Dolomitic in Form of Lime as Lime Dolomitic Ca(0H)n. P . P. M. Lime, P . P. M. P . P . M .
...
...
270 270 291 291 0
96 103 103
0
0 96
0
..
Soda Ash as NaaCOa, P . P . M.
MgO (Remosil),
P. P. M.
Silica as Hardness Si02 as CaCOa, P . P . h. P. P. M.
55 0 0 0 0 55
operation of the lime-soda softener, and it is possible that under actual field conditions the simultaneous process of silica removal and phosphate softening could proceed efficiently. These laboratory results have, however, definitely indicated that hardness and silica removal can be carried out in two-stage operation efficiently where the phosphate does not come in contact with the voluminous magnesium precipitates. First stage of operation is removal of silica, and second stage is removal of hardness by means of phosphate. Since extremely good flocculation is obtained with the magnesium oxide, filtration after the first stage is not necessary if facilities are provided for sedimentation of these precipitates so that there is no carry-over of the magnesium precipitates into the second-stage softening. I n the design of a unit to accomplish both silica removal and phosphate softening, it is not necessary that two sedimentation tanks be employed. Through proper internal design and baffling of the unit, one tank only is required for this dual purpose.
Dolomitic Lime The possibility of employing dolomitic lime for silica removal was investigated by Behrman and Gustafson (1) who concluded that this material could not be successfully utilized for silica removal. Their tests indicated that magnesium hydroxide, to be really effective in silica removal, must be formed in situ in the water to be treated. This latter contention has been verified ( 2 ) . Our investigations, however, have indicated that dolomitic lime has some value for silica removal under controlled conditions, although this operation usually introduces other factors which make the process inoperative from a practical standpoint. Dolomitic lime is a mixture of calcium oxide,or unslaked lime and magnesium oxide obtained from roasting of dolomite. While its magnesium content, if in the proper form, can effect some silica removal, a large amount of calcium oxide is introduced a t the same time into the treated water. Quantities of magnesium oxide required for silica removal greatly exceed the quantity of calcium hydroxide required for hardness removal. On a soft silica-bearing water, therefore, in order to add sufficient magnesium oxide combined in dolomitic lime to remove the silica, excessive quantities of calcium oxide are also introduced which increase the hardness of the treated water. This added calcium hardness must be removed through the use of additional excess quantities of soda ash but only a t the expense of releasing an equivalent amount of objedtionable sodium hydroxide alkalinity and thus making the treated water unsuitable for use as boiler feed water because of its high content of total solids and alkalinity. The increased pH resulting will cause resolution of the silica and increase the residual silica in solution. This particular property of dolomitic lime, as well as the fact that silica removal can be effected with its use, is shown in Table 111. A raw water originally containing 46 p. p. m. of hardness and 20.5 p. p. m. of silica was first treated with lime and soda ash with resultant reduction in hardness to 24
Analysis of Treated Water Calcium as CaCOa as CaCO8. p. M. P . P. M.' Phenolphthalein Methyl orange 0 44 24 68 126 216 88 104 172 236 128 140 28 52
pH 7.1 9.5 10.5 10.9 10.9 11.1 9.9
Disaolved solids P. P.M.
iii
333 354 205 228 143
p, p. m. and of silica to 16.0 p. p. m. DoIomitic lime containing 35.5 per cent magnesium oxide was then added to additional samples. Soda ash, added in the theoretical quantity t o precipitate the calcium oxide introduced by the dolomitic lime, reduced silica to 6.3 p. p, m. As a result the total alkalinity of the treated water increased to 216 p. p. m. Total solids rose to 333 p. p. m. I n the next test no soda ash was added for the precipitation of the calcium hydroxide introduced by the dolomitic lime. Silica was reduced to 1.8 p. p. m. but only at the expense of introduction of considerable calcium into solution, increasing the hardness of the treated water to a total of 128p. p. m. Total solids rose to 354 p. p. m.
p$ 8r
0
44 80 I20 160 200 7/M€ OF RE 7€N7/W,M/N.
I
240
FIGURE5 (Above). SILICAREMOVAL AS RELATED TO RETENTION FIGURE6 (Below). ADSORPTION EFFICIENCY AS RELATED TO RETENTION
with 104 p. p. m. methyl orange alkalinity. It is evident from these tests that, even if no soda ash is added to remove the objectionable introduction of hardness, the minimum alkalinity which can be produced by this process on this water is 104 p. p. m. Additional quantities of soda ash added for removal of calcium hydroxide hardness serve only to liberate
818
INDUSTRIAL AND ENGINEERING CHEMISTRY
a greater amount of sodium hydroxide alkalinity and give for this particular series of tests a minimum alkalinity in the softened water of 104 p. p. m. Additional tests with higher quantities of dolomitic lime are shown together with the results produced by the use of 103 p. p. m. magnesium oxide. With the same equivalent quantity of magnesium oxide as in the case of dolomitic lime, silica reduction was effected down to 0.4 p. p. m. I n addition, a desirable low hardness was effected equal to that obtained through the limesoda softening. Total alkalinity was only 52 p. p. m., or 16 p. p. m. less t h a n t h a t achieved with limesoda softening. Total solids con90 9.4 %e /Q2 106 L I LY tent of the sample PH was 143 u. u. m. This latte; test inFIQURE 7 . EFFECT OF PH dicates the opportunity afforded by magnesium oxide for economicallyreducing the silica content of a given water without increasing the requirements of lime and soda ash. I n addition, the solids content of the treated water is markedly lower, together with an improvement in the quality of the softened water from the standpoint of lower alkalinity. These tests show that any desired degree of silica removal can be achieved with the use of magnesium oxide without any increase in requirements of lime and soda ash and with an actual reduction of the solids content of the treated water. With dolomitic lime, on the other hand, silica removal can be obtained only a t the expense of greatly increased hardness or alkalinity or both of these detrimental features.
Vol. 33, No. 6
3
3
Effect of Sludge Recirculation
lation the silica removed per part of magnesium oxide increased sharply with the second cycle and continued to increase up to the tenth cycle.
3 ,lo
5
0
W/TH REC/ffCULRT/ON
8'" & ./4 ,2
$
.IO
ent Original 'Vater Of FIGURE 8. EFFECTOF RECIRCUhigher silica content LATION in Table IVB that the recirculation of sludge provides a means of increasing the quantity of silica removed per part of magnesium oxide.
Recirculation of the sludge formed in the adsorption of silica from solution bv means of magnesium oxide is of definite value in increasing the amount of silica removed per part of magnesium oxide. TABLE IV. EFFECT OF SLUDGE RECIRCULATION Table IV illustrates data obAnalysis of Treated Water tained on this recirculation operation. silica %&-: CaCOa, Alkalinity as MagSodium Silica Silica P.P M. nesium HyRemoved Removed as I n the specific case of Table Cycle Oxide, droxide as SiOz, per P a r t SiO2, CaCOa, Phenol: Methyl IVA, silica removal was deNo. P, P. 1LI. P. P. b. P. P. M. MgO P. P. M. P. P. M. phthalein orange termined a t three concentraA . Series I tions of magnesium oxide, 30, 12.3 44 0 30 0 .. . 0 68 7.2 38 5 :1 0.17 30 40 .. 30 60, and 90 p. p. m. These 64 5.0 14 58 20 0.12 7.3 60 .. 60 36 62 20 3.3 0.10 9.0 .. 90 points were determined as 64 14 58 20 5.0 0.12 7.3 60 1 usual without any recircula16 4.5 0.26 30 58 40 7.8 2 30 4 . 5 20 32 70 0 . 2 6 40 7 . 8 30 3 tion of sludge. A sample of 26 76 4.3 0.27 20 40 8.0 4 30 28 32 80 40 3.9 0.28 8.4 30 the original water was then 5 28 30 76 3.9 0.28 8.4 40 6 30 treated with 60 p. p. m. mag22 34 3.9 0.28 82 40 8.4 30 7 0.29 14 32 86 3 . 7 8 . 6 40 30 8 nesium oxide and 20 p. p. m. 24 32 74 3.5 0.29 40 8.8 30 9 16 3.3 28 82 40 0.30 9.0 30 sodium hydroxide for the first 10 B. Series I1 cycle; a stirring and reten0 26.2 34 20 0 0 .. tion time of 30 minutes was 54 0:205 18 50 18.0 11 s: 2 .. 40 48 6.0 24 58 20.2 0.202 20 100 followed by decantation of .. 40 26 52 24.1 0.172 2.1 30 140 .. the treated water and addi24 58 48 0.205 6.0 20 20.2 1 100 54 18 54 0.416 1 6 . 6 9 . 6 8 40 2 tion of the settled sludge to 62 54 18 10.0 0.400 16.0 8 40 3 50 0.400 56 10.0 14 16.0 40 4 8 a second 1-liter sample of the 46 54 11.2 12 0.375 15.0 8 40 5 raw water. To this sample 62 16 66 0.430 17.2 9.0 8 40 6 62 16 54 0.465 1 8 . 6 7 . 6 40 7 8 of the raw water, represent62 16 54 0.455 18.2 8.0 8 40 S 60 14 48 0.455 18.2 8.0 8 ing cycle 2, 30 p. p. m. mag40 9 nesium oxide and 40 p. p. m.
pH 6.9 10.1 9.9 10.15 9.9 10,05 10.1 10.0 9.8 10.1 10.0 10.0 10.0 10.2 6.9 9.1 9.5 9.7 9.5 9.3 9.1 9.1 8.9 9.1 9.1 9.1 9.1
June, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
FIGURE9. HOT-PROCESS SOFTENER FOR SILICA REMOVAL WITH STAGESOFTENINQ Characteristically, the silica removed per part of magnesium oxide is higher with higher residual silica content illustrated by Table IVB than is the silica removed per part of magnesium oxide illustrated by Table IVA, sincein thelatter case lower residual silica is obtained. Figure 8 indicates that by recirculation of sludge it is possible to reverse this normal characteristic of an adsorption reaction. In this illustration the silica removed per part of magnesium oxide is plotted against the residual silica in solution from the data of Table IVA. Without recirculation and with lower residual silica in solution, the silica removed per part of magnesium oxide is also lower, which is normal. With recirculation, however (Figure 8) and with lower residual silica in solution, the silica removed per part of magnesium oxide has increased. Under certain practical operating conditions, therefore, it is evident that recirculation provides an opportunity of overcoming the normal characteristics of the adsorption reaction and will result in increased silica removed per unit of magnesium oxide, even a t low concentrations of residual silica in the treated water.
Plant Results
819
the softened water. Because of the high silica content of the raw water, the silica removed per part of magnesium oxide was greater than in the cases illustrated by Table V, A and B. Silica removal was carried out in conjunction with hot process lime-soda softening of the make-up water for a 450-poundpressure boiler system. It is frequently desirable for power plants employing hot-process lime-soda or hot-process phosphate softeners to recirculate a portion of the continuous blowdown back to the softener. It is possible by this procedure to secure advantages such as increased sulfate-carbonate ratios, saving in quantities of external and internal chemicals, etc. (4). A disadvantage entailed, however, in boiler water recirculation as norTwomally practiced is the increase in concentration of soluble salts developing in the boiler water. Together with increased sulfate and chloride values, silica concentrations will also increase in the softener effluent and in the boiler water. The increase in silica content of the water in the softener, caused by introduction of the concentrated boiler water, affords an opportunity for securing a greater silica removal per part of magnesium oxide. Results in Table VI illustrate this point. With a relatively high percentage of blowdown water present in the softener effluent, a very efficient removal of silica is effected. I n some cases it has been possible to remove as much as 1.5 parts silica per part of magnesium oxide employed.
TABLEV. SILICAREMOVAL IN CONJUNCTION WITH HOTLIMESODA SOFTENING Silica as SiOz Raw Softened
Date
A.
10/17/40 12/ 7/40 9/16/40 9/19/40 9/ 23/ 40 9/26/40
Silica Removed Remosil, per P a r t Remosil P . P. M. Used
Boiler Pressure 250 and 600 Lb./Sq. In.
5.8 7.6 B. 23
...
1.5 2.8
13 13
0.33 0.36
Boiler Pressure 160 Lb./Sq. I n .
5.7
5.7
40 40
0.43 0.43
Data presented so far were from laboratory work. Results .. .. .. 5.3 40 0.44 3.1 40 0.49 obtained in full-scale operation are now given for waters of C . Boiler Pressure 450 Lb./Sq. In. silica content varying from 3.0 to 53.5 p. p. m. 43.0 0 Table VA shows typical data secured in a hot-process lime5/ 3/40 49.3 25.3 0:io 9/14/40 51.5 25 soda softener supplying make-up for boilers operating a t 250 25.4 18.6 1.01 10/24/40 60.6 7.9 53.5 50 0.78 9/17/40 and 600 pounds pressure. Table VB illustrates the use of 51.6 0.68 121 2/40 8.9 50.5 magnesium oxide for silica removal in a hot-process lime soda 52.0 4.5 75 0.55 9/19/40 softener supplying make-up for 160-pound-pressure boilers. With the higher silica content of the raw water as compared with the supply shown in Table VA, a greater silica removal per part of magnesium oxide is obtained. The residual silica on September 26, 1940, was 3.1, with the same amount of magnesium oxide that was used a t the start of this test (9/16/40), a t which time the residual silica was 5.7 p. p. m. The greater silica removal per part of magnesium oxide was due to the accumulation of sludge concentration in the sedimentation tank. All plant tests show this same characteristic; namely, as the test progresses and a greater quantity of sludge accumulates in the sedimentation tank, a higher quantity of silica is removed per part of magnesium oxide employed. Removal of silica from an initially high level H,POq NdaV of 49.3-53.5 p. p. m. is shown by Table VC. B At this plant varying rates of feed of magnesium oxide were employed to determine the silica-reSOFT~NER FOR PHOSPHORIC ACIDTREATMENT AND FIGURE 10. HOT-PROCESS SILICAREMOVAL moval efficiency with varying silica content of
820
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 33, No. 6
for the circulation of sludge to obtain the greatest value from the magnesium oxide employed. I n the second stage disodium phosphate is used for reduction of hardness to practically zero. Expected Figure 10 shows the design of a hot-process Silica Content softener for a plant operating a t 450 pounds presRemoved Silica silica as sios, P. P. M. ~ ~ so&ned ~ o silica ~ er Part sure. The raw water is relatively low in hardness SofBlow- Softened Water, Removed Remosil Eemosil with alkalinity exceeding the hardness. Phosphoric River tened down Water P. P. M." P P. M . ' P. P. M: Used 3 3 52 12.0 8.9 5.9 12 0.49 acid is first introduced, release of carbon dioxide 4 3 52 28.4 17.7 14.7 12 1.20 is obtained in the deaerator, and then sodium 50 4 4 . 0 2 3 . 3 1 8 . 3 12 1 . 5 3 4 4 40 42.0 19.8 12.8 12 1.07 hydroxide and magnesium oxide are added for 4 5 48 28.5 16.6 11.6 12 0.97 4 5 48 28.6 16.6 11.6 12 0.97 hardness precipitation and silica removal. Provision is made for recirculation of the partially a Calculated on basis of mixed raw and boiler water. spent magnesium oxide sludge. Because of the advantages possible through recirculation of boiler waterto the softener, provision has also been made for this feature which will further increase the quantity of silica A design of a combination hot-process softener is shown that can be removed per part of magnesium oxide. in Figure 9. This unit was designed for softening and the removal of silica from the make-up water for a 1400-poundLiterature Cited pressure utility installation. Silica is present in the raw water as 33 p. p. m., and the softened water will constitute approxi(1) B e h r m a n , A. S., a n d G u s t a f s o n , H., IXD.ENG.CHEM.,32, 468-72 mately 10 per cent of the boiler feed water. Specifications re(1940). (2) B e t z , L. D., Noll, C. A., a n d M a g u i r e , J. J., Ibid., 32, 1323-9 quire that the silica content of the boiler water shallnot exceed (1940). 10 p. p. m., with total solids not exceeding 1000 p. p. m. Mag(3) K a h l e r , H. L., IND.ENG.CHEM.,Anal. Ed., t o be published. nesium oxide affords the most efficient means of silica removal (4) M a g u i r e , J. J., a n d T o m l i n s o n , W. J., Combustion, 11, 26-32 without increasing the solids content of the make-up water. (1939). Silica and partial hardness removal are accomplished in the I'RESENTBD before the Division of Water, Sewage, and Sanitation Chemistry first stage by magnesium oxide and lime. Provision is made at the 100th Meeting of the American Chemical Society, Detroit, Mich. TABLEVI. SILICA REMOVAL IN CONJUNCTION WITH POT LIME-SODA SOFTENING A N D BOILERWATER RECIRCULATION AT A BOILER PRESSURE OF 225 POUNDS PER SQVARE INCH
;
The System Asphaltic BiturnenRubber Powder J. M. VAN ROOIJEN The Rubber Foundation, Delft, Holland
I
N COMMUKICATIOP\'S 3, 7 , and 15 of the Rubber Foundation the advantage of adding unvulcanised rubber powder to asphaltic bitumen 'was pointed out. I n one of these publications (5) an explanation was given, based on the selective absorption of the light hydrocarbons by the rubber powder, by which a two-phase system is formed, with swollen rubber particles dispersed in a medium of hardened asphalt. An examination was also made into heat resistance, in which it appeared that a mixture containing 5 per cent rubber could be kept heated a t 170" C. for 16 hours without any change in properties. The literature on the subject, however, mentions less stability to heat. Davey ( I ) , for instance, states that by heating a t too high a temperature, the rubber-containing mixtures sometimes become softer than those without rubber, which is attributed to decomposition of the rubber by superheating. At the Research Station a t West-Java (3)it was observed more than once that the penetration increased in the presence of rubber when the mixtures were heated above 100" C. The penetration value of Wonokromo asphalt without rubber powder amounted to 48; that of a mixture of 5 and 7.5 per cent unvulcanised rubber powder (Pulvatex) after being heated a t 125" C. for 2 hours was 62 and 74, respectively,
the surface of the penetration dishes was rough, and the penetration values diverged considerably. These phenomena were not found at first in this laboratory. On the basis of the theory given (6) it was, moreover, not thought probable that softening of the rubber by heat would have an unfavorable influence, since the viscosity of the dispersed phase has little influence on the characteristics of the entire system. When softening accidentally appeared with a special mixture, it appeared desirable to investigate this phenomenon. I n the first place, it was necessary to see in what cases such behavior is to be expected-in other words, what significance this phenomenon has in practice. The second object of the research is the explanation of this phenomenon.
Variables Affecting the Stability of the System Among the factors which may affect the stability of the properties are the percentage of rubber, the type of asphaltic bitumen, the temperature, and the length of the heating period. I n the following investigation these factors were varied as follows: 1. Rubber content: 5 , 7.5, and 10 per cent of the mixture. 2. Asphaltic bitumen: normal asphalt 20/30, blown asphalts R85/40 and R75/55.
