June, 1931
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
B was immersed in a mixture of nitric and hydrochloric acids. The iron was rapidly attacked by this mixture, while the tantalum plate was entirely unaffected. Although it is not nossible a t this time to give comdete
637
and is not attacked by any of the well-known corrosive agents except acid. - hydrofluoric Literature Cited
Prevention of Silica Scale with Sodium Aluminate' C. H. Christman, J. A. Holmes, and H. Thompson NATIONAL ALUMINATE CORPORATION, CHICAGO, ILL.
Silicate scale in boilers has caused loss from blistered For the aid of that large H E treatment of water tubes as well as heat losses from reduced heat transfer. group of operators using cold is on a m u c h m o r e Silica reduction in external softening may be imsoftening of boiler feed makescientific basis t o d a y proved by using sodium aluminate as the coagulant. up, sodium aluminate has inthan e v e r before. M a n y Sufficient soluble alumina in boiler feed water will efdeed proved a boon. This problems arising from diverse fect silica precipitation in boilers as calcium or magalkaline coagulant, of itself, types of water and rapidly exnesium aluminosilicate. This material is flocculent has softening power. Reacpanding conditions of operaand permits reduction of soluble silica in boiler water tions are accelerated t o such tion, which were scarcely exto a minimum. With sodium aluminate treatment extent that water c o n t a i n istent in the minds of operatsilicate scale is prevented through removal of silica ing less than 17 p. p. m. caling engineers a decade ago, together with the hardness, rather than by removal of cium a n d m a g n e s i u m exhave called f o r s o l u t i o n . hardness alone. p r e s s e d as calcium carbonHigher pressures and higher An experimental boiler duplicating plant operating ate may be secured a t temr a t i n g s h a v e become the peratures much below norconditions at high pressures is described. This is used vogue and the end seems to be for determination of reactions of silica in boiler waters. mal. ST'ith sodium a l u m i in the distant future. Water Data from actual plant operation are included. nate coagulation better sofsupplies are being drawn upon tening may be accomplished which are increasingly complex owing to greater-tillage of the soil as well as increased do- with much lower excesses of sodium alkalinity than is possible mestic and industrial pollution. Periods of drouth and flood without coagulants or with acid coagulants, where higher which have succeeded each- other have made conditions of one soda excess must be carried to effect hardness reduction. day vastly dissimilar from that of other days. All these have The ability of sodium aluminate effectively to reduce the centralized the attention of efficient operators on the need for silica content of water has proved of greater importance. It is the purpose of this article to discuss this property and to control in treating water for boiler feed supplies. Present methods for treating water for boiler feed purposes describe some of the tests which clearly demonstrated the may be divided into two main classes, external and internal effectiveness of sodium aluminate in silica-scale prevention. Examination of boiler scales has indicated the presence of treatment. Each class of treatment may, in turn, be subdivided into groups designed to give a final water which is much silica. This hard silicate scale is particularly detrisuitable for operation under set conditions established in the mental in boiler operation, causing heat losses due to reduced individual plant. The problems associated with this proper heat transfer as well as blistered and burnt tubes. Sodium preparation of boiler feed water may be relatively simple or aluminate, used as a coagulant in external and internal treatcomplex, according to the nature of plant operation, water ment of boiler water, reacts with silica and calcium or magtreatment, and character of the raw water supply. nesium to form complex insoluble aluminosilicates and effects The first aim in external treatment of water is removal of a silica reduction to a concentration not otherwise obtainable. maximum amount of scale-forming chemicals. These chemi- Silicate scale is prevented by removal of silica and hardness, cals are commonly accepted to be salts of calcium and mag- rather than by the removal of hardness alone. nesium. A careful study of scales taken from boilers in all Reactions of Silica in Boiler Feed Waters sections of the country has pointed out the serious effect of silicate scale as well as its prevalence. In fact, none of the REACTIONS OF CALCIUM AND MAGNESIA WITH SILICAolder methods of chemical treatment in external equipment Concentrations of silica which exist normally in raw water materially affected silica. In places where it proved an acute will not precipitate silica upon contact with permanent hardproblem, evaporator equipment for make-up water was the ness in concentrations up to 300 p. p. m. as calcium sulfate. only recourse. Even with evaporators instances have been Sodium hydroxide gives a partial precipitation of silica only in recorded where carry-over was appreciable and the very the presence of magnesium. purpose of the operation was nullified. REACTIOSSOF SODIUMALUMINATE WITH CALCIUM AND I n lime-soda softeners calcium and magnesium are reduced SILICA-Sodium aluminate does not form a precipitate with to a lower concentration by the application of lime and soda ash. The rate of reaction in cold water is notoriously slow, calcium sulfate in the absence of silica. However, silica is and this led early to the use of hot softeners in which chemical precipitated by sodium aluminate in the presence of calcium reactions were materially accelerated a t elevated tempera- as a calcium aluminosilicate. With proper ratios of the three constituents, silica may be reduced below 2 p. p. m. Silica tures. reduction in this manner is retarded by higher temperatures 1 Received February 28, 1931. Presented before the Division of and increased alkalinity. Water, Sewage, and Sanitation Chemistry at the S l s t Meeting of the AmeriREACTIONS O F SODIUM A4LUhlINATE WITH >\lAGNESIA AND pan Chemical Society. Indianapolis, Ind., March 30 to April 3, 1931.
T
IMDUSTRIAL A N D ENGI ‘NEERING CHEMISTRY
638
SILICA-Magnesium sulfate reacts with sodium aluminate to form magnesium aluminate. With silica present there is a marked reduction of silica with the formation of magnesium aluminosilicate, a flocculent precipitate. Silica removal is much more effective a t advanced temperatures and in the presence of high alkalinities than in the calcium series. ZEOLITENATUREOF REACTIONPRoDucTs-Dehydrated precipitate formed by magnesium sulfate, sodium aluminate, and silica resembles a synthetic zeolite. Following regeneration with salt, it exhibited softening qualities when in contact with hard water. REACTIONS OF SILICA AND ALUMINA WITH MAGNESIUM AT 400 POUNDS PRESSURE-The precipitation of silica by magnesia alone is very incomplete a t this pressure. With sodium aluminate present, silica precipitation is much greater. As the proportion of sodium aluminate is increased, more silica is removed from solution with maximum reduction of silica occurring in solutions which contain soluble alumina. With sufficient magnesia present, soluble alumina cannot be found until silica has been reduced to a minimum. I n the absence of soluble alumina, silica may exist in colloidal condition. Excellent coagulation is produced by magnesium aluminosilicabe a t high pressures. Table I-Reduction of Silica in Presence of Magnesium Sulfate and Sodium Hydroxide, or Sodium Aluminate a t 20° C. PHENOL- METHYL PHTHALEIN ORANGE ALALIvIgSOd MgSOd PPTD. Si02 Pprn. KALINITY KALINITY’ P.p.m.P.p,m. % P.p.m. % P.0.m. CaCO, ( a ) 50 p. p. m. SiOz, 85 p. p. m. NaOH 88 11.5 23 82 106 50 44 71.1 19.4 38.8 55 72 100 71.1 25.0 50.0 38 55 96.5 64.3 150 30.5 61.0 27 44 115.5 57.8 200 20 p. p. m. SiOz, 85 p. p. m. NaOH (b) 8.0 40.0 77 94 29.6 59.2 50 12.0 60.0 51 67 55.0 55.0 100 14.0 70.0 41 53 64.0 42.7 150 70.0 29 41 90.8 45.4 14.0 200 (6) 5 p. p. m. SiOz. 85 p. p. m. NaOH 0.5 10.0 63 80 31.5 63.0 50 44 56 1.0 20.0 56.8 56.8 100 30.0 31 41 1.5 76.0 50 6 150 40.0 20 29 46.4 2.0 92.8 200 ( d ) 50 p. p . m. SiOz, 44 p. p. m. NanAlzOd ~~
REACTIONS OF SILICAAND ALUMINA WITH CALCIUM AT 400 POUNDS PREssuRE-Silica precipitation by sodium aluminate and calcium is incomplete a t high pressures. Colloidal solutions are rapidly formed and no coagulation is secured. At high pressures calcium is much less effective than magnesium, in securing complete removal of silica, and in producing desirable coagulation. SILICAREMOVAL IN SOFTENING TESTS-In softening waters without sodium aluminate as the coagulant, silica reduction is very incomplete. Sodium aluminate may be used with decreased excess treatment to secure a lower total hardness, better coagulation, and improved silica reduction. Hot process softening with sodium aluminate gives better silica reduction than cold softening. Sufficient sodium aluminate should be used in softening to give proper coagulation and leave soluble alumina in the treated water. Any unprecipitated silica remaining will then be thrown down in the boiler as the flocculent aluminosilicate. SOLUBLE ALUMINA I N PROPERLY SOFTENED \$’ATER-SOme waters show no soluble alumina when softened and coagulated with sodium aluminate. Additional aluminate may be added to the boiler feed water to establish proper alumina-silica ratios for final precipitation of silica as a flocculent sludge rather than as a hard silicate scale. SILICA DECONCENTRdTION I N BOILEROPERATION-When soluble silica is present in boiler water, it may be removed by
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adding sodium aluminate and magnesium salts. Internal treatment with sufficient sodium aluminate gave excellent silica removal a t 500 pounds pressure. SILICACONTROL I N PLANT OmRmIoN-In plant operation sodium aluminate has been used successfully for the removal of silicate scale. Scale formation has been prevented and precipitated silica removed from the boiler with blowdown. I n successful operation with sodium aluminate, enough must be used to show soluble alumina in boiler feed water as well as in boiler water samples. Reactions of Calcium and Magnesia with Silica
For convenience in making a comprehensive study of the reactions of calcium and magnesia with silica, consideration was given first to the probable concentrations of silica which might be met in actual practice. With this in mind a study was made of raw-water analyses secured in this laboratory. It was decided to consider a content of 5 p. p. m. (0.29 g. p. g.) silica as a low average, 20 p. p. m. (1.17 g. p. 8.) as a medium average, and 50 p. p. m. (2.9 g. p. 9.) as a high average. In later tests higher concentrations of silica were also used to show the trend of reactions and to duplicate concentrations which might be built up in the boiler by evaporation. The first step involved a study of possible reactions of these three concentrations of silica with calcium and magnesium salts to determine whether varying concentrations of calcium and magnesium would produce a reaction resulting in the precipitation of silica. In these and later tests the sulfates of calcium and magnesium have been used unless otherwise noted. Silica, when added to a water, was obtained from a sodium silicate solution which contained 28 per cent Si02. Sodium aluminate, unless otherwise noted, was a pure commerical grade containing 90 per cent NaA1204and 10 per cent NaOH. Other grades used were “regular dry sodium aluminate” containing 82 per cent Na&04, and “No. 2 K. W. S. sodium aluminate solution” containing 32 per cent NazA1204. B procedure for making these reaction tests was developed, keeping in mind the practical relationship of the test to actual operation and avoiding conditions which would have little bearing on field work as normally experienced. Solutions of chemicals were prepared of a strength which permitted dilution to secure the desired concentration in each test. After the chemicals had been mixed, the solutions were stirred a t a constant rate for 3 hours a t 20’ C. Where visible precipitation had occurred, analyses were made of filtered samples to determine hardness, pH, alkalinity, and silica removal. Silica was determined colorimetrically by the method of Thayer (1). It was necessary, however, t o use a somewhat higher concentration of picric acid for standards. This was established by comparison of the recommended standards with solutions of known silica strength. The practice has been to limit the concentrations read to a maximum of 10 p. p. m. Si02,making dilutions when necessary to hold within this range. Fading of the standards on long standing has been observed so new standards were prepared biweekly. The method has been carefully compared with gravimetric data on raw, treated, and boiler waters and found to be suitable and much more rapid. With a concentration of 5 p. p. m. SiOz there was no precipitation with calcium sulfate a t concentrations under 300 p. p. m. (17.5 g. p. g.). Tests made with 20 and 50 p. p. m. silica and these concentrations of calcium sulfate gave no evidence of precipitation of silica. The addition of sodium carbonate or sodium hydroxide in amounts up to 85 p. p. m. ( 5 g. p. g.) did not produce a reduction of soluble silica. Tests made with magnesium sulfate and silica in the various concentrations were negative with respect to silica precipitation. The introduction of sodium hydroxide produced ap-
INDUSTRIAL A N D ENGINEERING CHEMISTRY
June, 1931
preciable silica reduction. The data in Table I illustrate the effectiveness of the silica reduction in the presence of magnesium sulfate and sodium hydroxide. Included in this table are tests which were made with sodium aluminate in place of sodium hydroxide. The amount of aluminate used was calculated to give the same sodium equivalent as that in the sodium hydroxide series, the additional precipitation of silica being a function of the added alumina. All tests with sodium hydroxide show by titration the presence of hydroxide alkalinity (2 P-M positive) after establishment of an equilibrium. Theoretically 85 parts of sodium hydroxide should precipitate the magnesia from 128 parts of magnesium sulfate. I
I
I
I
1
1
PPm Mg S O ! Figure 1-Comparative Effectiveness of Sodium Hydroxide and Sodium Aluminate i n Silica Precipitation
The effectiveness of sodium hydroxide and sodium aluminate in silica precipitation is compared in Figure 1. With equivalent sodium contents, the added silica reduction is shown to be due to the presence of alumina. Reactions of Sodium Aluminate with Calcium Oxide and Silica
The ratios of the three materials used were based upon the oxide content and expressed as CaO, SiOz, and A1203. All solutions were made with distilled water and the strength was determined by 'gravimetric analysis. LIMEADDEDIN ExcEss-The first series of these tests was made with an excess of lime and variable quantities of silica and alumina in order to provide ample calcium to insure maximum precipitation of silica and alumina. All solutions were stirred at a uniform rate and temperature (20" C.) for the same period of time. After filtration, calcium was precipitated from an aliquot as the oxalate and determined by potassium permanganate titration; silica and alumina were determined gravimetrically. The results of the tests are compiled in Table 11. of Silica, Alumina, and Excess Lime on Silica Reduction a t 20° C. A1203 4 D D E D Ah03 PPTD. SiOn PPTD. P . P . m. % % ( a ) 4 2 7 . 5 p . p. m. CaO, No Silica 17 1 0 ..
Table 11-Effect
85.5 0 .. 171.0 0 .. ( b ) 4 2 7 . 5 p . p. m. CaO, 1 7 . 1 p p. m. Si02 0 ... 0 17.1 65 53 85.5 58 91 171,O 51 92 ( c ) 427 5 p. p. m CaO, 85 6 P . P. m Si02 0 0 17.1 io6 60 85.5 60 96 171.0 39 99 ( d ) 4 2 7 . 5 p , P. m. CaO, 171 P. P . m. Si02 0 ... 0 17.1 100 54 85 5 100 ' 91 171 0 72 98
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The calcium aluminosilicate precipitate was flocculent, light in weight, and slow settling. The precipitated matter was completely agglomerated leaving perfectly clear water after a time. The advantage of adding increasing amounts of sodium aluminate to secure more complete silica precipitation was definitely illustrated in this series. CHEMICALS ADDEDIN NEARLYEQUALPROPORTIONS-In the preceding tests the amount of calcium was purposely carried in excess to permit completeness of reaction. The following series covers ranges in which the proportions between the chemicals are more nearly equal. The concentrations were held above those which would be encountered in actual operation in order to determine the limits of solubility of the several constituents after precipitation was complete, the soluble products being considered of greater importance than the mass of precipitate which was produced. A material increase in temperature reduced the precipitation of silica slightly in the absence of added alkali, while an increase in the alkalinity effected a marked decrease in the quantity precipitated. As the amount of sodium aluminate added was increased, the quantity of silica removed was also increased, the low range of solubility of silica in the presence of excess sodium aluminate being approximately 5 p. p. m. in cold reactions and lower a t advanced temperatures. The presence of excess alkali definitely retarded precipitation of alumina from sodium aluminate, the effect being more pronounced at 100" C. than at lower temperatures. The regular procedure was followed in the cold stirring tests. A control test in the cold with 855 p. p. m. added alkali showed that no silica was dissolved from the Pyrex glass beaker during the period of use. I n the boiling tests the rate of solution of silica from Pyrex was too great to permit its use for these tests. Accordingly, the samples were boiled in iron pots of 3 liters capacity. Each pot was connected to a reflux condenser to avoid concentration of the solution. The alkali used in these tests was made of a mixture of 3 parts of sodium hydroxide to 1 part of sodium carbonate to correspond to alkalinities which might be encountered in boiler operation. The series covers four tests on each composition: (1) cold stirring for 3 hours a t 20" C.; (2) boiling for 1 hour; (3) cold stirring for 3 hours with 855 p. p. rn. (50 g. p. g.) added alkali a t 20" C.; (4) boiling for 1 hour with 855 p. p. m. added alkali. The results of these tests are given in Table 111. Table 111-Effect TREATMENT
(a)
1 2 3 4
(b) 1 2 3 4 (c)
1 2 3 4 (d)
1 2 3 4 (e) 1 2
3 4
(f) 1 2 3 4
of Various Treatments on Silica Reduction with Lime. Silica. and Alumina
SOLUBLE SOLUBLESOLUBLE CaO AhOs Si02 CaO AI203 Si02 PPTD PPTD. PPTD. P.p.m. P.0.m. P.0.m. 70 % % 342 p. p. m. CaO, 171 .p. p. P. . P . m. SiOr . m. AlzOa. 171 . 267.0 15.1 11.0 22 91 94 275.0 18.9 10.0 20 89 94 92.5 56.6 6.0 73 67 96 39.3 94.5 50.0 89 45 71 171 p . p. m. CaO, 171 p , p. m. AlzOa, 342 p. P. m. Si02 79.2 0 18.0 54 100 95 65.0 18.9 60.0 62 89 83 35.4 56,6 20.0 79 67 94 5.8 151.0 200.0 96 13 4'2 171 p . p. m. CaO, 342 p . p. m. AlzOa, 342 p. p . m. Si02 49.7 37.8 2.; 71 89 99 24.8 28.4 7.0 86 92 98 18.1 142.0 15.0 90 5s 96 4.8 276.0 200.0 97 19 42 171 p . p. m. CaO, 171 p. P . m. AhO3. 171 P P . m. Si02 .. . 18.9 93.7 15.0 45 89 91 37.8 92.5 11.0 46 78 94 37.8 78.6 25.0 53 78 85 7.7 171.0 100.0 95 0 42 342 p. p , m. CaO. 342 p. p. m. AlzOa, 171 p . p m. Sin? 256 0 161.0 0 25 53 100 75 5 21s 0 3 0 36 78 98 64 0 179 5 5 0 81 47 97 37 6 340.0 50 0 89 1 71 171 p. p. m. CaO, 342 p. p. m .AlzOs. 171 P. P . m. Si07 57.5 123.0 4 0 66 64 98 65 0 161.0 5 0 62 53 97 264 0 7 5 16.2 91 23 96 7.7 340 0 100 0 97 1 42 ~~
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640
To illustrate graphically the relationships between soluble calcium oxide, alumina, and silica, data for 100" C. (treatment 2) from Table I11 are plotted in Figure 2. Here it is observed that the soluble silica remaining after completion of the reaction a t 100" C. is steadily reduced with increased soluble alumina. Since the data in this series were obtained a t 100" C., the conditions are comparable to those existing in hot-process softening with respect to the action in silica reduction by calcium and alumina.
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silica, with increased suppression of precipitate formation as the alkalinity was increased. For example, 102.5 p. p. m. CaO, 22.8 p. p. m. AlzOa, 16.3 p. p. m. SiOz, and 142 p. p.m. NazCOa gave a crystalline precipitate having no flocculent appearance. The filtered solution tested: hardness, 13.7 p. p. in.; phenolphthalein alkalinity 53 p. p. m.; methyl orange alkalinity, 132 p. p. m.; soluble silica, 16.3 p. p. m.; soluble alumina, 22.8 p. p. m., showing the formation of calcium carbonate as the precipitate with no precipitation of silica or alumina. In a second test in which sodium carbonate was omitted in the initial precipitation, the silica precipitated was 3.3 p. p. m. and the alumina, 9.2 p. p. m. Upon addition of the same quantity of soda ash, some precipitated alumina and silica was dissolved, yielding a final solution from which some 0.3 p. p. m. of silica and 3.9 p. p. m. of alumina had been precipitated. There was definite evidence here that the conipleteness of precipitation of alumina and silica was prevented by an excess of soda ash. From these tests it may be concluded that in reactions which occurred in cold softening calcium carbonate was a more insoluble compound and that removal of silica as the calcium aluminosilicate was less certain. Reactions of Sodium Aluminate with Magnesia and Silica
SkOz
/I/
171
342
171
J7/
342
542
17/
346 /?I
34d 171
PP m Figure 2-Relationships between Soluble Calcium Oxide, Alumina, and Silica in Precipitation of Silica at 100' C.
EFFECTOF LOWERCONCEPU'TRATIONS O F CHEMICALS-The effect of sodium aluminate on reduction of silica a t lower concentrations of lime, alumina, and silica was studied with particular reference to the effect of increasing dosages of sodium aluminate. These tests were made in the cold, following the same procedure used in the previous tests. The data obtained are given in Table IV. The precipitated matter in tests involving the use of sodium aluminate was quite flocculent, with clear water in the interstices between floc. Under test conditions each increase in aluminate decreased the amount of soluble silica remaining after establishment of equilibrium. The amount of soluble alumina rose rapidly with increased dosage. This test was of particular importance in that it dealt with concentrations of chemicals which are met in regular softening operation. The extreme value of sodium aluminate in forming insoluble compounds with silica was emphasized, as it illustrated particularly well the advantage of relying upon sodium aluminate for the definite purpose of silica reduction. Table IV-Effect of Sodium Aluminate on Reduction of Silica a t Lower Concentrations of Lime, Alumina, and SiHca a t 20' C. (102.6 p. p. m. CaO, 8 5 . 5 p. p. m. SiOa) AlzOa AhOa PPTD. SiOa PPTD. . . ADDED P. P. m. P. 9. m. P. 9. m. % % 59 50.5 16.3 95 17.1 77 66.5 89 30.4 34.2 85 73.0 70 36.0 51.3 88 76.1 60 41.0 68.4 78.0 91 51 43.6 85.5 79.4 93 45 45.9 102.6 94 80.5 37 44.1 119.7 95 41.0 81.5 30 136.8 96 82.2 25 38.4 153.9 96 82.5 22 38.1 171.0
Silica reduction by sodium aluminate and different calcium oxide contents is plotted in Figure 3. The silica content was constant in all the tests and the calcium oxide contents were those given in Tables I1 and IV. Sodium aluminate was added in increasing amounts. The effect of excess calcium on silica reduction was quite apparent. Previous tests indicated that added alkali interfered with the completeness of reaction between calcium, alumina, and
Coagulation in water softening with sodium aluminate has been considered to be due to the formation of insoluble flocculent magnesium aluminate. Analyses of various waters softened in this manner have shown that some contain soluble alumina, while others do not. In internal treatment the suspended matter coagulated by sodium aluminate consistently included a greater portion of the added alumina. Associated with calcium, magnesium, and alumina in the floc there has been consistently noted a material amount of silica. It was therefore desirable that the effect of magnesia in precipitating silica and alumina be studied under the same conditions as were used for the calcium series. Some of these reactions are reported in Table V. of Various Treatments on Silica Reduction with Magnesia, Silica, and Alumina TREAT-SOLUBLESOLUBLESOLUBLE MgO A1103 SiOt MENT MgO AlzOa Si03 PPTD. PPTD. PPTD. P.p.m. P.p.m. P.9.m. % % % ( a ) 342 p. p. m. MgO, 171 p. p. m. AlzOs. 171 p. p. m. si09 1 294 5 5 14 97 97 2 166 0 22.5 52 100 87 98 78 3 37.5 100 3 0 9 15 100 95 91 4 0 i.b ,) 171 D. D. m. MeO. . P. . m. Sios - . 171 D. - D. m. AlzOi. 342 P. 1 51 5 2;. 5 70 97 94 0 15 89 100 96 19 2 89 52 100 3 0 19 162.5 47 138 100 73 59 4 0 171 . P. . P. m. MgO, 171 p. p. m. AlzOa, 171 p. p. m. Si02 (c) 1 73 5 3 57 97 98 n 13 81 100 92 2 31 3 n 48 87.5 100 72 49 72 68 4 n 47 55 100 ( d ) 342 p. p. m. MgO, 342 p. p. m. AlrOa, 171 p. p. rn. SiOn 1 178 5 1 48 98 99 0 9 2 103 70 100 95 83 85 3 0 57 25 100 4 0 95 14 100 72 92 171 p. p. m. MgO, 342 p. p. m. AlzOa, 171 p. p. m. Si02 (e) 1 12 19 3 93 94 98 96 98 2 3 14.2 2 98 3 56 63 0 151 62.5 100 4 0 170 45 100 50 74 (f) 171 p. p. m. MgO, 342 p. p. m. AlpOa, 342 p. p. m. SiOn 1 1 19 15 99 94 96 2 0 23.6 12.5 100 93 96 3 0 113 125 100 67 63 4 0 132 125 100 61 63
Table V-Effect
-
_ _
The addition of sodium aluminate to magnesia and silica resulted in precipitating virtually all the silica when no other alkali was added. The reactions were of the same order when the solutions were held a t 100' C. The addition of sodium hydroxide-carbonate solution inhibited silica precipi-
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
June, 1931
tation more in cold tests than in those a t high temperature. There was good precipitation of both silica and alumina in those tests where such an excess of caustic was used. The amounts of alumina precipitated indicated the insoluble nature of the magnesium aluminosilicate in sodium hydroxide solut.ion.
641
and sodium salts were leached out with distilled water, after which tap water was passed through the regenerated product. Hardness tests on the resultant water showed the absence of soap-consuming salts, thus establishing the completeness of the cycle and demonstrating that the original reaction product was similar in character to a synthetic zeolite. Reactions of Silica and Alumina with Magnesia at High Pressure
-
4 /OZ.Sppm CO 0
IZI
342
5/3
68.4
PPm Figure 3-Silica
853
/026 1/37
l36A
1539 1 7 / 0
4 a,
Precipitation by Aluminate and Silica with
Two Concentrations of Calcium Oxide
A comparison of the magnesia-alumina-silica series with the calcium-alumina-silica series showed a much greater removal of magnesium from solution than of calcium. The alumina precipitation was greater in the magnesium series. In those tests in which 171 p. p. m. (10 g. p. g.) of lime or magnesia were used, silica removal was higher in the calcium series, but in those tests in which an excess of lime or magnesia was used, magnesia and sodium aluminate were more effective in the precipitation of silica. This is attributed to the formation of the more insoluble magnesium aluminosilicate, since in these tests the alumina precipitation was much greater than in the calcium series. The treatment used in this series was identical with that used in the calcium series reported in Table I11 and included (1) cold stirring, (2) boiling under reflux, (3) cold stirring with 855 p. p. m. added alkali, and (4)boiling under reflux with 855 p. p. m. added alkali. The tests secured a t 100" C. are presented graphically in Figure 4. Here the conditions are parallel to those in hotprocess softening. Decreasing soluble silica was observed with complete alumina precipitation. When soluble alumina was found soluble silica was a t a minimum, except in instances where there was insufficient magnesia present to complete the precipitation of the other soluble constituents. Zeolite Nature of Reaction Products
The reactions between silica and alumina in the presence of hardness with formation of insoluble aluminosilicates suggested a relationship to zeolites. I n fact, a commercial zeolite is manufactured from sodium aluminate and sodium silicate in concentrations higher than those used in these tests. To observe more fully the zeolitic character of the products obtained in these tests, a quantity of precipitate was prepared. The ratios used were magnesia 10 parts, alumina 20 parts, and silica 20 parts, since test (f-1) in Table V indicates that in this proportion practically all the reaction products are precipitated. The precipitate formed was washed by decantation, filtered off, and dried. A white, porous, light product was secured. After suitable crushing to uniform size, the material was extracted with sodium chloride to remove the magnesia if possible. Tests of the extract showed a T-ery high concentration of magnesium. The magnesium
Since softened water in a boiler meets with different equilibrium conditions, owing to higher temperatures and higher alkalinities, a series of tests was made a t constant concentration and 400 pounds (26.6 atm.) pressure. For this purpose use was made of an autoclave (Figure 5 ) which was equipped so that samples could be withdrawn through a cooling coi!, thereby avoiding flash with the release of pressure. At stated intervals samples were drawn off, filtered a t once through Khatman OB paper, and tested in the routine manner used in previous tests. The results are presented in Table VI. Coagulation was excellent in all samples except (a) and (h). There was a marked difference in filtration rates of some of the samples, with best results in sample ( e ) . As it was known that slower rates of filtration might be due to a colloidal condition, tests were made on sample (6) since it contained a large quantity of soluble silica. The amount of silica present was also greatly in excess of that in cold (1) and hot (2) tests reported in Table F' (b), in which the same concentrations were used.
A&Oj
0
SLOz /7/
/7/
17/
2/4
342
17/
/7/
2556 /7/
259 /7/
342 17/
/7/
I?/
PPFigure 4-Relationships between Soluble Magnesia, Alumina, and Silica i n Precipitation of Silica a t 100' C.
Night blue, a positive colloid, was used in this test on the assumption that it would combine with any negative colloid (such as colloidal silica) with mutual neutralization of the electric charges followed by precipitation of the colloids. The solution to be tested was filtered free from suspended matter. It analyzed as follows: hardness, 17 p. p. m.; phenolphthalein alkalinity, 0; methyl orange alkalinity, 10 p. p. m.; silica, 85 p. p. m. Increasing quantities of night blue caused rapid coagulation with complete precipitation of all added dye, until all the silica had been precipitated. Excess additions of dye gave no precipitation because the sign of the colloid had changed entirely to positive with a complete stabilization of all material present. As further evidence of this latter phase, filtration rates of night blue precipitated colloids through Munktell OB filter paper were much faster than for those solutions which had been stabilized by an excess of night blue. Tests (a), ( b ) , ( c ) , and ( d ) indicated clearly the need for having higher concentrations of sodium aluminate t o give satisfactory precipitation of silica and provide for sufficient
alunririw to leave a protecting reuidual cont,ent. Tests ( E ) , (f), and (9) showed the advantage of increased aluminat,e dosage. Having established in test (e) the approximate ratio desired between alumina and silica, tcvt (h) was made with a decreased magnesia dose to determine the effect of this factor on the equilibrium. As can be noted. results were unfavorable, indicating the importance of magnesia in securing complete precipitation of silica and alumina. Test (a) sliowed the comparatively small anlount of silica that can be precipitated by magnesia in the absence of sodium aluminate, Of especial note was the shifting equilibrium shown in this test, since it indicated definite evidence of re-solr~tion of silica in colloidal condition.
'Test ( a ) sliuwed IIO conciiliitioii and gave definite reaction for a colloidal conditiwi. In test ( 6 ) there was a slight flocculation of thc precipitate formed. On the other hand, there was definite scale formation on the walls of the autoclaw, the iirst instance of this condition noted in any of the pressure tests. In test (c) all calcium was precipitated from solution at the end of tlie first test period and, while there was some flocculation of precipitated material, definite evidence of a colloidal condition mas established. A comparison between these tests and tlie corresponding ones reported in Table VI indicated t.he marked superiority of magnesia in effecting silica reduetion under similar conditions of operation. This efiect of niagneaia and alumina on silica precipitation was studied further in xn experimental boiler. Silica Removal in Softening Tests
,>/ P. P . m. 70 P . 0. m. ,o 171 p . p . m, MgO, 1 7 1 p. p . m. SiOs, No Sodium Aliiniiiiair 3 100 42 ~. ... 6 130 24 .. 7 150 12 .. (b) 1 7 1 p . p . m. M y O , 171 p. p . m. AIaOs, 342 i,.p m Si(>* 3 70 79.5 0 100 5 75 78.0 0 1w 7 86 75.0 0
llours in)
82
1 i
0s
7s
0
52 0 60 2 54 3
Altliougli tests with calcium and niagnesium sulfate have indicated that silica can be precipitated by sodium aluminate, there has been observed a condition which interferes with silica removal, preferential removal of calcium by soda ash. The tests made thus far have been on celcium OF magnesium sulfates rather than rnixt,rire.n such as occur naturally in raw wat,crs.
100 100 100
0
0
m. MgO. 214 p. I). m.
4s
53
(4 3 5
72.0 69.0
63 63.2 171 D. P m, MgO. 856 P. P. m 13 93 10 94
9 . ;. . (1) 171 p. p . m. MzO, W9 p. $3
3 r, 7
m. SiOi 92 89 4 91 m. MzO. 342 p. p. m AbOi, 171 p . p. m. Si02 2 98 s 52 M.8 1 99 4 52 84.8 2.5 9S.b 19 85.7 nt. h 4 ~ 0 258 , p. 5'. -2 A l r O r . 171 p . Y. m SiG 40 77 ee 74 40 77 66 74 40 77 76 io 4 4
(pi
171 p.
17.
3 5
7 (h) 3 5
128 P . I).
7
ni AliOi, 171 p . p .
97.6 97.5 97.6
23
32 27
I n Figure 6 there is a presentation of the fundamental fact established at lower temperatures, the need for an excess of sodium aluminate to give residual soluble alumina with its coincidental condition, a decrease in soluble silica. The effect of a deficiency of hardness on the removal of silica was clearly demonstrated. I n practice this condition couId be met by the addition of magnesia from raw or treated water, or from magnesium sulfate. Reactions of Silica and Alumina with Lime at High Pressure
Figure .+--Autoclave for Sfvdyine Reactions at 4nn Pounds ~reesure
These pressure tests were continued to include the relationships established with calcium, alumina, and silica. The technic used was the same as in preceding tests with magnesia. Data from the tests made arc presented in Table VII.
A series of tcsts wds made to parallel more nearly the actual conditions met in softrning practicc, limited, however, to the removal of sulfate or permaneiit hardness arid its effect on silica removal. The sinount of sodium aluniinate used coresponded closely to those amounts wlricii are used in actual practice. The work was divided into two groups, comparable with cold softelling and hot softening. 1x1 each series the reaction t,inie was limited to 1 hour, cold samples being stirred a t 20' C. and hot samples being boiled in iron pots under reflnx condensers. Regular dry sodium aluminate containing 82 per cent Ka&Ot was used as the coagulant. For the precipitation of magnesia, sodiuin hydroxide was used and for calcium, sodium carlionate was added. The amounts of sodium hydroxide and sodium carbonate added were sufficient to give 9 p. p. 111. hydroxide and 34 p. p. m. finished sxmple. The data are carbonate alkalinity in t . 1 ~ given in Table VIII.
'Table VIl-~ERecf of Slllca end Alumina wich Lime a t 4Ui1 Puvnda Prcsrvre (4490 C.)
(bj 3 ,
