March, 1928
INDUSTRIAL AND ENGINEERING CHEMISTRY
283
A System of Boiler Water Treatment Based on Chemical Equilibrium’v’ By Ralph E. Hall3 PITTSBURGH EXPERIMEXT STATION, BUREAU OF hfINES, PITTSBURGH, P4.
From the results obtained in extended tests a t two power plants-one using sulfate and the other bicarbonate water-the conclusion has been drawn t h a t the formation of adherent scale is due mainly t o crystallization in situ. Hard, adherent anhydrite scale and presumably, also, t h a t containing magnesium silicate in relatively large proportions form on the evaporating surfaces because the solubility of these components decreases with temperature increase. Calcium carbonate scale forms on the walls of t h e feed lines because they are cooler than the water in their interior, and because of the condition of supersaturation as regards calcium carbonate resulting from gradual bicarbonate decomposition and the slow attainment of equilibrium. A general formula has been deduced for preventing the growth of hard adherent scale on evaporating surfaces. A specific example has been given of the application of
this general formula to a boiler operating a t 150 pounds gage pressure, when soda ash is used to condition the boiler water. From the analyses of several boiler waters, the degree of constancy of the carbonate-sulfate solubility-product ratio has been deduced. The analyses of the solid phases resulting from the evaporation of various treated waters a t different pressures have been presented. Since the solubility curves of the desirable and undesirable solid phases are divergent, the criteria upon which the concentration of desirable radical in the boiler water is based are the concentration of undesirable radical therein and the pressure a t which the boiler operates. A t high operating pressure it becomes impossible to use soda ash for treatment, unless the sulfate concentration is maintained extremely low. For high pressure conditions, therefore, the use of phosphate in place of carbonate is advisable.
N COKDITIOXIXG water for its utilization in the generation of steam, it is essential to recognize three phases in the process: (1) the maintenance in the water in the boiler of ionic ratios which insure the precipitation in desirable forms of any solid phase separating; (2) the disposal of the sludge developed by treatment or evaporation; (3) the elimination of boiler water from the steam before it leaves the nozzle of the boiler. To meet these conditions the quality of the water in the boiler must be systematically controlled. It is the purpose of this article to present the chemical considerations involved in such control.
I
first the principles which govern the formation of adherent scale and then turn to the soft carbonate scale.
Mechanism of Scale Formation
The conditions existing in a boiler during formation of anhydrite scale are shonm by the following statistics taken from an unpublished report6 of this investigation regarding tests made on a boiler using city water (taken from the Monongahela River a t Beck’s Run and filtered). Figure 1 shows the composition of the feed water and Figure 2 that of the boiler water over the period during which a balance was run. (Silica and iron were low and are not shown.) I n Figure 2 the curves representing the concentration of sulfate, chloride, sodium, and magnesium reach welldefined maxima and minima in cycles. The minima always occurred on days when the amount of steam generated hadbeen greatly decreased (Table 111) while blow-down practice had been only slightly altered. Since the scale on which the sulfate, choride, and sodium concentrations are plotted is only one-fifth of that for magnesium, it is all the more noticeable how plainly these cycles occur, and how closely the curves agree in their general form. The concentration curves for calcium and alkalinity as bicarbonate (presumably hydroxyl in the boiler water) reached a steady state on June 17, and thereafter did not show the regular fluctuations of the other curves. Careful examination of the calcium curve indicates that the maxima and minima, although slight, are coincident with the minima and maxima, respectively, of the sulfate,
Types of Boiler Scale
When precipitation occurs in a boiler water,4 the solid phase appears either as nonadherent sludge (favorable condition) or adherent scale (unfavorable condition). Table I contains analyses of a number of adherent scales which have been classified roughly on the basis of their sulfate, carbonate, and silicate content. The relatively soft carbonate scales are found in the pipes between the feed-water heater and boiler, or in the coolest boiler tubes through which the feed water passes on first entering the boiler. All the other scales are the ordinary hard adherent type so characteristically found on the evaporating surfaces Df the boiler. The mechanism of formation of the soft carbonate scales is different from that of the other types. We shall establish 1 Presented before the Division of Industrial and Engineering Chemistry a t the 67th Meeting of the American Chemical Society, Washington, D. C., April 21 to 26, 1924. *Published by permission of the Director, U. S. Bureau of Mines. This article is based upon the results obtained by the Bureau of Mines, cooperating with the Hagen Corporation of Pittsburgh, in a plant and laboratory study of the mechanism of scale formation and its prevention by suitable boiler-water conditioning. This investigation was begun in January, 1922. 8 The following have been associated with the writer in this work: G. W. Smith, H. A. Jackson, H. A. Otto, Grace Thomas, C. E. Coleman, of Hagan Corporation, Pittsburgh. 4 In this article the expression “boiler water” refers to the water in the boiler and undergoing evaporation.
Formation of Hard Adherent Scale
The most common of the hard adherent scales are those composed largely of anhydrite, illustrated by Nos. 1, 2, and 3 of Table I. They are deposited in crystalline layers, which record the history of operating conditions, have a specific gravity of about 2.9 or nearly that of pure anhydrite, and adhere tenaciously to the surface of the metal. Statistics from Boiler Operation
6 Hall, Smith, Coleman, and Fitch, “A 42-Day Test on a Permanently Hard Water in a 545-H. P. Boiler and a 76-Day Test on a Temporary Hard Water in a 1000-H.P. Boiler,” Bureau of Mines Report, Chemical Division, November 19, 1923, pp. 1-58 (unpublished).
INDUSTRIAL A N D ENGINEERING CHEMISTRY
284
chloride, sodium, and magnesium concentration curves. The range of calcium in the feed water was from 30 to 40 parts per million except for three days (July 6 to 8), when it was only 24.5. I n the boiler water the range was from 28 to 32 parts per million. Since the concentration of calcium in the boiler water, notwithstanding constant evaporation, was less than in the feed water, and since anhydrite was the solid phase deposited, it is quite evident t h a t saturation equilibrium with respect to calicum sulfate existed in the boiler water a t all times. T h e concentration of hydroxyl was fixed by its equilibrium relations with magnesium and ferric ions. T a b l e I-Analyses AlsOa No.
TYPEOF SCALE Sulfate Sulfate Sulfate Sulfate-carbonate Carbonate Carbonate Carbonate-silicate Carbonate-silicate Carbonate-silicate Carbonate-silicate-sulfate Carbonate-silicate-sulfate Sulfate-silicate Sulfate-silicate Silicate Silicate
Sios 1.5
3.1 0.7 4.2 0.7 1.5
17.1 9.7 10.4 12.0 6.9 6.4 6.0 45.4 48.3
SiOp
Fe
C1
solvedsolids Total dis-
4.10
The agreement of the sodium and chloride ratios is very close, and represents the real number of concentrations which the boiler water has undergone, since neither of these radicals forms insoluble salts. Because of the slight solubility of ferric and magnesium hydroxide and of calcium sulfate, respectively, the hydroxyl and the calcium ratios are very small; but whereas the iron and magnesium were forming in part as loose sludges, removable in blow-down and wash-back, the
CaO
Me0
2.2 1.2 0.4 1.2 1.2
38.6 36.9 39.0 37.5 51.1 52.9 26.1 29.1 22.1 34.1 4.5.7 34.3 31.8 32.9 29.9
1.6 5,9 2.5 11.2 3.7 1.8 22.8 24.2 35.0 14.1 5.8 8.0 9.2 1.8 3.2
1.0 1.8
1.7 1.2 1.4 0.6 0.4 4.2 6.9 2.5
so. ~-
NalO
... ... ... .. .. .. ... ... ... ...
6.7 5.7
Loss a t
COI 0.0
54.5 45.6 55.4 24.1 0.4 0.8 0.9 0.8 0.2 10.4 4.5 44.0 43.4 0.7 2.8
... ... ... ...
I n Table 11, Nos. 1 to 4 are analyses of the loose material removed in blow-down and wash-back, and KO.5 is the analysis of the adherent scale formed. Note-A filter through which a portion of the boiler water was continuously passed, was attached to the boiler. I t was cleaned every 24 hours by back-washing into tanks. Analyses were made of the sludge which settled out. A measured portion of each blow-down was also collected and treated similarly.
No. 6 sludge will be discussed later. With the exception of No. 3, in which much broken scale was present, the sludges of blow-down and wash-back were high in silica, iron oxide, and magnesia, and relatively low in anhydrite, whereas the adherent scale contained more than 93 per cent of anhydrite. Table I11 contains data regarding the amount of water evaporated and the blow-downs, and shows the remarkably small amount of suspended material in the boiler water a t all times. It shows, also, the very small amount of insoluble solids removed by filter or blow-down, in comparison with the amount deposited as adherent scale. We must conclude that a very high percentage of the material deposited by the boiler water crystallized as adherent scale in situ on the evaporating surface and never existed as independent crystals subject to movement with the flow of the boiler water. From the balance made over the entire period the distribution of calcium sulfate entering the boiler was found to be as follows: D i s t r i b u t i o n of C a l c i u m S u l f a t e Per cent 3.2
7.7 89.1
The average ratio of the boiler-water concentration of the various radicals to their weighted average feed-water concentration for the period July 10 to 22 was as follows:
MgO 10.6
0.5 0.2 0.2 0.4 0.1 0.1 1.6 1.5 0.5 0.5 0.4
3.3 0.0
15.7 42.1 40.1 20.2 22.7 17.1 22.2 31.6 1.7 0.2 2.1 2.2
15.3 3.5 20.0 1.6 1.8
NarO
0.6 2.1 Not Not Not Not
detd. detd. detd. detd.
N e t ignition loss 1.5
.~ 105O C.
4.0 1.7 5.5 1.0 2.0 9.4 10.1
13.9 8.8 5.6 4.3
... ...
6.6
4.6 5.2
1.4 1.6
of S l u d g e a n d S c a l e F o r m e d in Boiler (Per cent)
Date taken SiOz FerOs CaO SAMPLEFROM: 12.7 25.3 13.8 Sludge in blow-down 6-16-22 6-16-22 11.4 22.2 12.9 Sludge in wash-back 7-27-22 7.1 13.3 29.4 Sludge in blow-down 7-27-22 37.1 15.9 1.4 Sludge in wash-back 1.5 2.2 38.6 10-10-22 Scale boiler N o . 1 39.7 10-23-22 0.8 0.7 Loo& sludge in mud-drum of boiler Much broken scale in this sample. b Analysis made on sample dried a t 105' C. c A treatment of 15 pounds a day of sodium silicate was used when these' samples were taken,
......................
so4
OH
Na
a n d Classification of Boiler Scales (Per cent)
+
Removed in blow-down and wash-back sludge. ........... Removed in blow-down water.. ......................... Deposited in situ as adherent scale
Mg
4.48 4.50 0.86 8.35 11.19 0.68 3.72 11.23
FerOa
Table 11-Analyses
Ca
Vol. 17, No. 3
SOa
19.3 19.0 40.4 1.5 54.5 56.3
Moisture N e t ignia t 105" C. tion loss
1.0 0.7 0.1 5.8 0.5
. . .b
15.1 19.3 2.4 18.0 1.5 1.2
calcium sulfate remained as adherent scale. The ratio of 8.35 for magnesium shows t h a t only a small part of it was forming insoluble solids. I n June and July the calcium content of incoming feed water was just about equal to or slightly greater than t h a t of the boiler water. I n the dry weather of September and October, 1922, however, the calcium content of the feed water increased largely (from 30-40 to 70-80 p. p. m.), with the result t h a t as it entered the mud-drum and reached boiler temperature, it deposited the calcium sulfate in excess of saturation a t boiler temperature as loose crystals. When the boiler was opened on October 23, large quantities of these loose crystals were found in the mud-drum, which through inadvertence had not been blown down for some time. The analysis of these crystals (No. 6, Table 11) is evidence of almost pure calcium sulfate. A like condition was found in a boiler using Youghiogheny River water, which is exceedingly high in calcium sulfate. The deposition of these loose crystals means in no sense that the formation of adherent hard scale had stopped, but merely that precipitation occurred to the extent of the saturation values for the boiler pressure, and t h a t on the evaporating surfaces scale was being laid down just as noted in the figures of Table 111. Table IV contains analyses of the feed waters used in a test on waters containing temporary hardness, and Nos. 7 and 8, Table I, show the composition of the hard adherent scale produced on the evaporating surfaces.6 As far as possible, the water was drawn from the wells, so that the influence of calcium sulfate in the city water is not apparent. In its stead magnesia and silica are present in large proportion, in part as brucite and in part as hydrous 8
Hall, Smith, Coleman, and Fitch, loc. c i t . , p p , 58-68
INDUSTRIAL AND ENGINEERING CHEMISTRY
,March, 1925
magnesium silicate.' It is quite apparent that the equilibria in the boiler water which produced this type of scale must be radically different from those responsible for anhydrite scale. Laboratory Investigation
The solubility relations of calcium sulfate a t higher temperatures have been investigated by Tilden and Shenstone,8 Boyer-G~illon,~ and ?vlelcher.lo Melcher obtained data on insoluble anhydrite which confirmed the work of van't Hoff and his co-workers. At temperatures above 40" C. the solubility of calcium sulfate decreases with increase of temperature for the ordinary solid phases-that is, gypsum, soluble anhydrite, or insoluble anhydrite-below 40" C., the solubility increases.
285
should have no influence so long as we work at a temperature in which the solubility curve gradient is in the desired direction. Table IV-Analyses of Waters Used in Test on Temporary Hardnessa (Parts per million) City CONSTITUENTS No. 1 No. 2 No. 3 water 6 SiOt 29 5 16 4 7 4 22 FezOa AlzOa Ca 98 152 82 4; 16 18 53 Mg 16 Na K 1:; 1:; 82 64 26 C1 60 128 196 84 so4 HCOI 81 654 551 517 Totai solids 201 850 832 657 Temporary hardness as CaCOa 67 310 435 423 Permanent hardness as Cas04 0 0 0 86 NazCOs excess present 240 0 0 0 Analyses by E.L. Folk, West Penn Power Co , Pittsburgh, Pa.
+
+
(I
When a solution of calcium sulfate was evaporated in this boiler at atmospheric pressure, the nichrome element became coated with a heavy scale, and a few crystals appeared at other points in the boiler. When evaporation occurred at reduced pressure (below 40" C.), crystals were deposited on the glass surfaces, and but slightly on the heating element. When tests were made with calcium chromate and calcium hydroxide whose solubility curves are similar to that of calcium sulfate at the higher temperature, thesolid phase appeared in the main as an adherent scale on the heating element; but wheii calcium iodate and potassium sulfate, with solubility curves of slope, were evaporated, the solid phase appeared as ,";:: opposite crystals in the boiler water and in the filter, only minor amounts of it collecting on the heating element.
Table 111-Chemical Balance on 545-H. P. Heine Boiler Operating pressure: 150 pounds gage Water heating surface: 5450 square feet Water in boiler: 34,800 pounds Feed water: South Pittsburgh filtered (Monongahela River) water Total insolTotal insol- uble solids Total blowuble solids accumulatdown and Suspended removed by ing in boiler Total moisture in matter in blow-down a s adherfeed water steam boiler water and wash-back ent scale Date Pounds 1000 Ibs. P. p. m. 1000 Ibs. Pounds (6) (3) (4) (1) (2) (5) 22 22.1 6-17-22 144.1 3.3 5.2 .. 18 172.5 9,B 1.7 20.2 19.' 295.7 20 19 4.8 33.3 22.1 10 280.1 20 35 . 92 9 21 22.0 354.0 22 20.6 38.4 7 313.6 3.8 23 21.3 10 4.0 37.4 292,3 24 18.6 7 (4.0)" 7.8 120.6 -A R 8.6 .. 25 8.6 26 13 19.0 279.0 27 21.3 13 361,3 28 10 20.3 160.3 29 16 22.3 255,3 20.9 30 14 249.9 7-1-22 20.9 13 135.9 10.0 .. 2 10.0 18.9 3 9 181.9 .. 10.2 4 10.2 5 20.2 300,2 19 21.1 245.1 16 6 21.9 16 281.9 11 21.1 177.1 9 10.2 10.2 .. 10 21.4 17 2i4,4 23.3 11 14 261.3 12 23.5 9 299.5 24.1 14 295.1 13 25.2 14 356.2 .. 15 14.6 1.8 9.9 128.6 16 11.7 .. -6.1 1.5 11.7 17 19.4 .. 2.2 44.5 282,4 18 .. 26.8 378.8 1.2 56.9 19 27.6 327.6 0.8 43.8 20 3.1 46.5 .. 26.4 366.4 21 .. 26.0 358.0 3.2 48.3 TOTAL9 4 1 . 7 The numbers in parentheses are estimated. Possible errors are of trifling significance in view of the values in Column 6.
.. I
.
I n a boiler tube under operating conditions the hottest portion of water is the layer next to the surface of the tube. If there is calcium sulfate in the water, its point of least solubility is at the tube surface, and any precipitation might be expected to take place thereon primarily. On the other hand, materials whose solubility increases with increase of temperature are more soluble a t the tube surface than in the interior of the tube, and therefore will not precipitate so readily at the surface. Proof that this relation is general and independent of the nature of the solid phase separating was obtained with the simple boiler of Figure 3. The nichrome element is the hottest section of the boiler, and therefore corresponds to the heating surface of the tubes. The operating temperature is different from that of a boiler in service, but this difference The writer is indebted t o H. E.Merwin of the Geophysical Laboratory, Washington, D. C., for petrographic examination of the scales. 8 Phil. Tuans., 175A,31 (1884). .4nn. Cons. A v f s Metiere, 3, 2 (1900). J . A m . Chem. S O L . , 32, 50 (1910). 1' Van't Hoff, Armstrong, Hendrickson, Weigert, and Just, 2. physik. Chem., 45, 257 (1903). @
JUNE
JULY
Figure 1-Concentration of Inorganic Constituents in the Feed Water during the Test on a Sulfate Water
I n addition, tests were made on the following solutions (parts per million) : No 1 2 3 4 5 6
Cas04 Saturated Saturated Saturated Saturated Saturated Saturated
Na2SOI 4000 2000 2000 4000 2000
NaCl 4000 2000 2000 2000
NaOH
Tannic acid
600 600 200
200
...
. .
AI(OH)J
...
.. ..
.. 210
I n every case adherent scale formed on the heating element. I n Kos. 1to 4,some loose crystals were formed simultaneously; in Nos. 5 and 6 their appearance was masked by the flocculent precipitate throughout the solution in the boiler.
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INDUSTRIAL AND ENGINEERING CHEMXSTRY
The solubility of calcium carbonate, as far as possible in the absence of any partial pressure of carbon dioxide in the gaseous phase, has been shown by Kendall12 to increase with increasing temperature. While the rapid decomposition of sodium carbonate,la and doubtless also of calcium carbonate, in pure
sm
E m
62 2-
8 d
Vol. 17, No. 3
sulfate scales contaminated with carbonate, also sulfate scales mixed with carbonate and silicates. There are also silicate-carbonate scales, sulfate-silicate scales, and pure silicate scales-all of the adherent type. If the carbonate in these various scales is linked with calcium as calcium carbonate, then, except in Nos. 14 and 15, there are present in addition, other than calcium sulfate, magnesium hydroxide and magnesium silicate of the approximate composition 6Mg0.3SiOdH~O. The solubility relations are such that calcium carbonate must be the solid carbonate phase; petrographic examination also establishes this point. On the basis of petrographic examination and work with the experimental boiler, it is believed that the presence of calcium carbonate in adherent scales is incidental-being largely a function of its retention, during the process of formation of anhydrite and hydrous magnesium silicate. It is quite noticeable that in all the adherent hard scales of Table I in which carbonate appears, the proportions of magnesium oxide and silica are relatively large. It is inferred, therefore, that the natural silicates, of which magnesium silicate in most of the scales and calcium silicate in Nos. 14 and 15 are examples, are substances whose solubility decreases with temperature increase, and whose deposition as adherent scale is therefore analogous to that of calcium sulfate. Nothing is known in regard to ferric oxide, and in general its quantity is so small that it may be disregarded. Formation of Soft Carbonate Scale
4
V 0
1 1 I I I
I I I I I I I I I I I I I I 1 I
I I I I I I I I H I I I
y8I 1111
13 18 23 28 301 3 8 JUNE JULY Figure 2-Concentration of Inorganic Constituents in the Boiler Water during the Test on a Sulfate Water
solution a t boiler water temperatures renders fictitious any definite solubility values, the maintenance of excess carbonate-ion concentration by continuous introduction of sodium carbonate into the boiler water retards greatly any decomposition of the calcium carbonate. I n keeping with the slope of the solubility curve, where conditions are maintained in the boiler water such that calcium carbonate is the solid phase in equilibrium with the dissolved calcium salts, a calciumcarbonate scale does not "grow" on the heating surfaces, although a slight film of it is always present when the boiler is opened (Table VII) Magnesium hydroxide probably increases considerably in solubility with temperature increase. The value of Kohlrausch and Rose" a t 18" C. fixes one point of the curve; that obtained from water No. 1, Table VI, the other a t 185' C. However, crystals of brucite are found frequently in boiler scales-for example, Table I, Nos. 7 and 9. Their deposition in scales must be analogous to that of calcite. I n general, if S represents solubility of the solid phase in equilibrium with a solution a t temperature T , when dS/dT is positive, precipitation occurs in small degree on heating surfaces; when negative, the reverse is true. Classification and Structure of Hard Adherent Scales
.
The formation of the purely anhydrite scales has been explained. It is found, however, (Table I) that there are Phil. Mag., [6]2 3 , 958 (1912). See, however, Johnston and Williamson, J . A m . Chcm. SOC.,88,975 (1916). 1: Hall, Fischer, and Smith, Iron Steel Eng., 1, 312 (1924). 14 Z . ghysik. Chcm., 14, 241 (1893). 13
The conditions surrounding the precipitation of soft adherent carbonate scales such as Nos. 5 and 6 of Table I are quite different. When a water containing bicarbonate is heated, the carbonate of calcium appears in the feed water heater in part as a sludge and in part as soft scale and in the lines leading from the feed water heater to the boiler as a soft adherent scale. In some instances it appears also in the boiler tubes through which i t flows before reaching regions of rapid evaporation. While the solubility curve of calcium carbonate a t a minimum partial pressure of carbon dioxide is upward, that under atmospheric partial pressure of carbon dioxide is downward with temperature increase.12Js I n the feed water heater the increase in temperature. and decrease of partial pressure of carbon dioxide lead to the precipitation of a considerable portion of the calcium carbonate. However, the rate a t which decomposition of the bicarbonate occurs is not sufficient, in general, for its complete transformation into carbonate, and hence the water passing into the feed lines contains some calcium bicarbonate. Several factors undoubtedly influence the building of scale in these lines. I n the first place, the temperature of the pipes is less on the outside than on the inside, and hence there is a tendency for calcium carbonate to precipitate thereon. I n the second place, the equilibrium of calcium salts is slow in its attainment, and the rough surfaces of the pipes furnish excellent accelerators to the precipitation of calcium carbonate maintained in excess of saturation,l6 by the gradual decomposition of bicarbonate radical. Once crystal growth has started the crystals themselves hasten the attainment of equilibrium in the solution. Moreover the larger crystals already formed on the pipes will tend to grow at the expense of any small crystals in the solution. When a solution of calcium bicarbonate in water, or one containing in addition sodium sulfate and sodium bicarbonate, is fed into the boiler (Figure 3), evaporation results in the formation (a) of a sludge, which collects in the filter, 15 Wells, J . Wash. Acad. Sci., 6 , 617 (1915). 10 Aragonite is prevalent in these scales, whereas only calcite has been found in the scales from the evaporating surfaces. For a discussion of the stable forms of calcium carbonate under these conditions, see Johnston, Merwin, and Williamson, A m . J . Sci., 41, 473 (1916).
March, 1925
INDUSTRIAL AND E,VGINEERING CHEMISTRY
(b) of a scale which covers the entire glass tube in the vicinity of the heating element and the regions above it, (c) of a scale on the heating element. The last mentioned corresponds to the scale on the feed water heater, and is deposited on the heating element because of the rapid decomposition of bicarbonate radical a t its surface. The scale on the glass surfaces corresponds to that formed on the surfaces of the pipe lines, where decrease in temperature and supersaturation are potent factors. The solution at d and e (Figure 3) is clear, and practically no scale deposition occurs a t these points, illustrating the influence of the sludge collected in the filter in removing supersaturation and the lessened rate of bicarbonate decomposition in these cooler sections.
Necessary Conditions for Prevention of Scale Formation Regardless of variations in feed water, the conditions existing in the boiler water determine the type of solid phase which may separate. Since the ionic concentrations in the boiler water are the integrated result of feed water inflow, blowdown, and moisture in the steam for a period of several daysJ6 the maintenance of desirable conditions should be based on the boiler water itself. To inhibit the formation of hard adherent scale on the evaporating surfaces, ionic concentrations in the boiler water must be so regulated that any solid phases in equilibrium are substances whose solubility increases with temperature increase. The growth of soft carbonate scale is controlled by imposing upon the feed water, either before or during its passage to the boiler, the conditions that its calcium salts shall rapidly reach equilibrium with the solid phase precipitating.
287
composition of the carbonate ion a t boiler temperatures minimizes magnesium silicate formation. For this case the general inequality assumes the following form:
From the electrical conductivity of salts of the different ionic types Xoyes and his co-workers17 have determined their degree of dissociation over a wide temperature range. Since the ionization values for salts of any one ionic type are nearly the same,'* the ionic concentration in saturated solutions of the sulfate and carbonate of calcium may be readily approximated from Noyes' data, and their solubility products fixed for the range of temperature characteristic of boiler operation. Thus, at 185' C., K sol. prod. CaS04 = 0.545; and K sol. prod. CaC03 = 0.077l9 (concentrations expressed in milliequivalents per 1000 grams of solution). Also, the degree of ionization of calcium sulfate and carbonate-salts of the same ionic typeby reason of application of the isohydric principle, is based upon the same total ionic concentration in the boiler water. This simplifies practical application by permitting the substitution of COS and SOc determined analytically in place of COS-- and SOa--.
Prevention of Hard Adherent Scale
GENERALIZED CONDITIOriS-Let A oBb represent a salt Of relatively low solubility, the slope of whose solubility curve dS/dT is positive, and let A,Dd be one of the same order of solubility, but with dS/dT negative. At any temperature To, at saturation, [ A la (ion) X [Bib (ton) = K [AIc (ion) X [Did (ion) = K
sol. prod. SOL prod.
A J b at AcDd a t
TO TO
Khen conditions are so adjusted in a solution that it is in equilibrium with both solid phases, the value of A (ion) in both solubility product equations must be the same. Then
n Figure 3-Experimental
I n boiler operation, however, as evaporation occurs it is essential that the solid phase precipitating be one in which dS/dT is positive. This condition is fulfilled if at all times the concentration of B (ion) in the boiler water is maintained as follows:
Finally the formation of adherent anhydrite scale on the evaporating surfaces of a boiler operating a t 185' C. (150 pounds gage) is prevented by using soda ash to maintain the following condition in the boiler water: COI p. p. m.
Noie-If the B ion salt is one of extremely low solubility, the condition regarding the sign of dS/QT becomes of less significance. However, the presence of magnesium and calcium silicates in hard adherent scale (Table I) in relatively large proportions is indicative of how small the solubility must be.
Boiler
a-Feed water inlet b-Filter g-Nichrome heating element
> 0 0883 SO,
p. p. m.
The condition set up prevents formation of solid calcium sulfate in the boiler water, but deposition of hydrous magnesium silicate must also be controlled, else a hard adherent carbonate-silicate scale may form if silicate is present in any '1
CarnegieInstitule P u b , 63 (1907).
18 Noyes and Falk, J . Am. C h c n . SOC.,34, 484 (1912). This inequality is the general criterion for preventing the for calcium carbonate, those ob19 The solubility values used were: growth of hard adherent scale on the evaporating surfaces. tained by extrapolating Kendall's curve; for calcium sulfate, Melcher's OF ADHERENT SCALEPREVENTIOX- data. The data of Boyer-Guillon would give a considerably higher value SPECIFICEXAMPLE As an example of its application in practical operation, we will for the solubility product of calcium sulfate: but an investigation under in this laboratory supports Melcher in his contention that Boyershow how calcium sulfate scale formation may be prevented way Guillon did not have insoluble anhydrites as solid phase, although our values by the use of soda ash as treating chemical, and how the de- are slightly higher than those of Melcher.
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I N D U S T R I A L A N D ENGINEERING CHEMISTRY
quantity. It was under such conditions of no control that scales 7 and 8, Table I, were formed. If the hydroxyl concentration in the boiler water is sufficient to depress the magnesium-ion concentration, however, the relatively small amounts of silicate in solution are insufficient to form magnesium silicate in troublesome quantity.
TableVIshows the division of the salts in the samples shown in Table V into molecules and ions, when calculated by the method proposed by SherrillJalbased on the isohydric principle and the Storch formula. I n the last two lines of the table are given the product of the calcium and carbonate ion and the calcium and sulfate ion, respectively. These data,
Table V-Boiler Water Analyses (Parts per million) No.
1 2 3 4 5
6 7
SOURCE OF WATER Monongahela River Ohio River Ohio River Monongahela River Monongahela River Ohio River Monongahela River
TYPEOF TREATMENT Si02 South Pittsburgh filtration plant 2.1.2 19.8 Lime-sodaashand filtration 26.4 Lime-sodaashand filtration 7.2 Internal, with soda ash 7.8 Internal, with soda ash 42.4 Lime-sodaasband filtration 24.8 Internal. with soda ash
Fe 4.8 2.5 8.4 5.4 6.2 2.2 7.6
Ca 29.4 13.8 11.5 5.5
8.8 5.0 2.3
Mg 47.0 trace 0.4 0.3 0.4 trace 0.5
Na COS 166,3 0 1045 33.0 1427 69 1301 78 1482 105 1389 288 1742 330
OH 2.4 136.0 167.0 189 119 131 395
SO4 478.6 1478 2092 1935 2435 1712 1860
CI 92.2 223 264 105 134 182 108
Total solids 850 2951 4059 3625 4300 3752 4470
in conjunction with the curve of Figure 4 serve to throw light upon the dependability of solubility product for use in defining the necessary carbonate concentration in the boiler water, and to substantiate the validity of extrapolating Kendall’s data to boiler water temperatures. I n the case of the calcium salts in boiler water, the solubility is slight enough, and the ionization, especially at boiler water temperatures, small enough so that deviations in the solubility product constantz2should not be too great to impair its usefulness in this work. The use of this function is further favored because calcium carbonate and calcium sulfate are salts of the same ionic type; hence deviations in their soluProduct Ratio Of Carbonate-Su1fate bility product relations mustbe similar, and cancel outto Table V contains the analyses of boiler water samples from extent in the ratio of the two. I n Figure 4 the value of C a + + CaS04, as obtained in boilers operating a t 150 pounds gage pressure, arranged in the order of increasing carbonate content. They were with- Table VI, is plotted against sulfate-ion concentration. With drawn from the boiler through a cooling coil, in which the tem- the sole exception of water No. 4, all samples which were perature was reduced below 100’ C., and were filtered either shown by use of the solubility product criterion to be on the in the boiler or immediately after delivery from the cooling sulfate equilibrium lie on thesmooth curve, which in reality representsthe solubility curve of calcium sulfate under the concoil. ditions imposed. Nos. 6 and 7 , which were definitely Table VI-Ionic a n d Molecular Composition of t h e Waters of Table V on the carbonate equilib(Concentrations in milliequivalents per 1000 grams of solution) SUBSTANCE No. 1 No. 2 No. 3 No. 4 s No. 55 No. 6 No. 7’ rium, fall far off the curve. 0.5 NaCl 0.15 0.90 1.19 0.45 0.6 0.56 In Table VI, No. 4, it is 3.96 ... 1.14 1.56 1.69 1.11 1.16 NaOH NarCOs ... 0.48 1.07 1.19 1.61 4.34 noticeable that the products 1 58 .. 73 NazSO4 1.48 13.50 20.24 18.30 23.42 16.10 0.03 0.01 0.01 0.002 0.005 0.002 0.001 of calcium-ion concentration CaClz 0.003 0.0050 0.003 0.01 0.01 . ... .. Ca(0Hh and those of carbonate and :g! 0,044 0.0136 0.024 0.02 0.02 CaCOs Cas04 0... .54 0... .44 0 .. 02 01 00 2 0... .344 0. ..1.6 3 o.068 sulfate are smaller than MgClz 01 .. 01 74 0 ... ... ... 0.0005 ... ... ... those of the other similar ... Mg(OH)a ... ... ... ... ... ... 0.0012 waters; and it is concluded MgCOa 3 . 0 1 0 . 0 1 8 6 MgSO4 39:QZ 35.32 38: o , 0266 4 6 3 o4 ,:05328 3 4i;kl8 that No. 4 was unsaturated 5.60 30:i5 gM+: g +++ 0.30 0.12 0.098 0.045 0.0096 0.79 ... ... 0.004 0.0036 ... both as regards calcium carc15 2 12 3 36 .. 15 42 bonate and sulfate. Thus App. 21 .. 3W5 6 .. 4 80 6 86 .. 2 92 .. 44 19 5 .. 82 90 129 .. 52 OH cos-... 0.62 1.23 1.42 1.89 5.27 5.8 the results obtainedbyuseof 4.34 17.30 23.36 27.28 19.60 20.1 22.0 sod-0.126 0.202 solubilityproduct agreewith C a + + X cos-0,0744 0.1198 0.0639 0.122 0.438 C a t + X sod-i:302 2.074 2.276 0,992 1.760 0.751 those obtained in the more LI Specially filtered samples. exact analysis of Table VI. b Landolt-Bornstein Tabellen, 4th ed., p. 1187. The changes in the ion If the carbonate content in these waters is compared with products (Table VI) in relation to the solubility prodthat necessary for stability of both calcium carbonate and sul- ucts as determined for the pure substances must be fate in the solid phase-i. e., COa = 0.0883 S04-the results considered. In those waters on sulfate equilibrium, the product of Ca++ and Sod-- shows the tendency of soluare as follows: bility product to increase with increasing sulfate concentraNumber 1 2 3 4 5 6 7 tion. The same effect is noted for the product of Ca + and Carbonate found by analysis 0 33 69 78 105 288 330 Carbonate necessary for the COS-- in those waters on carbonate equilibrium. If the double equilibrium 42 130 185 171 215 151 164 average of the value for calcium sulfate in Nos. 2, 3, and 5, Thus in the first five waters carbonate concentration was in- and for calcium carbonate in Nos. 6 and 7, is accepted as sufficient, and was in excess in Nos. 6 and 7 only. characteristic for waters of this general concentration, the relations may be summarized as follows: The intri10 For data on the rate of decomposition see Reference 13. Fortunately, in the sense that it provides sufficient hydroxyl for this purpose, sodium carbonate in solution under boiler conditions readily decomposes into caustic soda and carbon dioxide.20 The elevated temperature is one factor in promoting the reaction; the low partial pressure of carbon dioxide in the rapidly regenerated gas phase is a second. For the extreme condition of a feed water exceedingly low in sulfate and high in magnesium, the decomposition of carbonate might not be sufficiently rapid to provide the essential hydroxyl concentration. I n such a case the addition of hydroxyl as such would be necessarv.
+
+
cate chemistry of sodium carbonate decomposition in boiler water proposed by Paul. “Boiler Chemistry and Feed Water Supplies,” Longmans, Green & Co., 1919, p. 137, and quoted by Pollitt, “The Causes and Prevention of Corrosion,‘’ Ernest Benn, Ltd., p. 101,need not be considered here.
J . A m . Chcm. SOC.,38, 741 (1910). Stieglitz, J . A m . Chem. SOC.,SO, 946 (1908); Noyes and Bray, I b i d . , 33, 1643 (1911). 2’ 2’
0
(a) (b)
289
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
March, 1925
K sol. prod. Cas04 (pure solution). ........................ 0.545 K sol. prod. Cas04 (waters Nos. 2, 3, and 5) . . . . . . . . . . . . . . . . 2.04 0.077 K sol. prod. CaCOs (pure solution). . . . . . . . . . . . . . . . . . . . . . . . . K sol. prod. CaC03 (waters Nos. 6 and 7 ) . . . . . . . . . . . . . . . . . . . 0.164 Ratiob:a . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7 Ratio d:c .............................................. 2.1
The solubility products of both calcium sulfate and calcium carbonate have shown marked increase. The greater effect for calcium sulfate may be due to the solubility data chosen for its pure solution,lg or to the larger increase of sulfate, as compared with carbonate concentration. I n the solubility product ratio a portion, at least, of the deviations disappears. In any event, these relations give a very fair conception of how closely
in Table VII, Nos. 1 to 7 ; the character of sludges and steam line deposits in Nos. 9 to 15. No. 8 is a sulfate scale which developed with incomplete treatment. Thesimilarity throughout the entire table is notable in view of the different boiler pressures and the divergent character of the mineral content in the feed waters (Table VIII). The per cent of magnesium oxide is slightly higher in the sludges, with one exception (No. 6), and very much greater in the steam line deposits. Table VIII-Waters Corresponds to deposit Nos., Table VI1 1, 2, 9, 15 3, 10 4, 12 5 6 7 8 13, 14
No.
20
Evaporated i n Producing the Deposits Listed in Table VI1 (Analyses in parts per million)
-
j16 Fi
Si02 5 2 2 3 3 50 3 18
Fe Ca Mg Na 1 41 6 16 22 5 17 0.4 12 14 4 1.0 2 8 1.0 8 20 7 0.4 7 35 1 . 0 172 17 22 3 68 27 34 16 9 6
HCOs 13 20 29 18 70 462 275 78
so4
C1 10 14 15 11 8 25 15 15
143 76 31 15 26 102 81 70
Value of Definite Carbonate Concentration Limits
5
b EO6
I3
C O L C W R A T I O N OF so1 IMILLIFAENhLEh7s)
Figure 4-Values
of
Ca++
+ Concentration C a s 0 4 a t 185' C. in Their Relation to
sod--
the necessary carbonate concentration may be known. It should be pointed out in this connection that up to the present time there has been no general criterion by which to judge the necessary lower and upper limit of carbonate. Solid Phases Developed by Evaporation
The chemical character of the thin scales which form from the evaporation of properly conditioned water, but which do not increase in thickness as evaporation continues,23is shown 28 See Reference 13. The boiler cited in this test has been in operation since November 21, 1923, and has evaporated 112,000,000 pounds of South Pittsburgh filtered water. Nos. 1, 2, 9, and 15, Table VII, were taken from this boiler. It is in better condition today than when the test began, as the thick adherent sulfate scale in the headers which the turbines had never reached is now completely removed.
The value of knowing the lower and upper limits which are satisfactory for carbonate concentration is inestimable. The lower limit, fixed for preventing precipitation of the solid phase, calcium sulfate, controls magnesium silicate deposition also by the hydroxyl developed. The hydroxyl is sufficient, also, in conjunction with a properly operated feed water heater, to inhibit any corrosion in large measure.*' The definition of the upper limit is of equal importance with that of the lower, since the greater excess is favorable to foaming and therefore wet steam. This is the case because of accelerated development of hydroxyl concentration, which in turn readily saponifies any organic oil having access to the boiler, and peptizes any colloidally disposed material that may be present in the boiler waters.26 A high concentration of hydroxyl leads to the further danger of embrittlement where waters are not high in sulfate.26 The effectiveness of sulfate may be due to the precipitation of sodium sulfate for which dS/dT is negative as the hydroxyl content, and therefore the common sodium ion reaches large proportions. Wilson, THISJOURNAL, 15, 127 (1923). The writer is indebted t o Harry N. Holmes, of Oberlin College, for the last idea. 26 Parr, University of Illinois Eng. Expt. Sta., Bull. 94 (1917). 1'
Y
...... Table VII-Thin
No. 1 2 3 4 5" 6 7 8
9 10 11 12 13 14 15
Scales, Sludges, and S t e a m Line Deposits Obtained when Water Is Treated on the Basis Ratio in the Boiler Water (Analyses i n Per cent) BOILERAlzOa PresCHARACTER OF TYPEA N D POINT Type H. p. sure Rating TREATYENT OF DEPOSIT Si02 Fez08 CaO MgO Heine 545 160 150 Internal with soda Thin scale from 3.7 7.0 44.8 5.1 ash steam drum Internal with soda Heine 545 160 150 Thin scale from 2.2 3.4 49.9 3.0 ash tubes Erie City vertical 580 Internal with soda Thin scale from 150 100 2.2 3.6 50.3 2.7 ash lower drum Internal with soda Thin scale on tubes 150 75 B. & W. 3320 5.5 4.2 44.9 6.9 ash Bigelow Hornsby 750 200 180 External with soda Thin scale 45.4 8.2 7.3 7.3 ash, unfiltered B. & w. 600 . . . 175 Internal with soda Thin scale 7.4 0.9 37.5 15.9 ash B. & W. 300 125 50 Internal with soda Relatively thin scale 6.9 0.6 45.7 5.8 ash, incomplete Internal with soda Thin scale Stirling 600 150 150 1.6 0.6 39.1 4.6 ash, incomplete Heine 545 160 150 Internal with soda Sludge from filter at- 6.1 4.3 39.1 9.0 ash tached to boiler Erie City boiler 580 150 100 Internal with soda Sludge from filter a t - 28.5 18.2 8 . 0 11.4 ash Stirling 250 150 125 Internal with soda 5.9 10.2 33.4 11.0 ash Ladd 500 External with so160 125 10.9 11.7 27.4 10.3 dium phosphate incomplete B. & W. 400 160 125 External lime-soda Deposit from super- 10.6 6.3 1 8 . 9 26.4 ash filtered heater headers 400 B. & W. 160 125 External lime-soda Steam trap deposit, 12.5 7.8 19.2 28.0 ash filtered 400 ft. from boiler house Heine 545 160 150 Internal with soda Steam trap deposit 5.4 7.2 37.6 1 0 . 6 ash This analysis was furnished by L. J. Willien, chemical engineer, Charles H. Tenney & Co., Boston.
+
of t h e Carbonate-Sulfate
SOs
2.6
Loss a t CO1 105" C. 35.5 0.0
Net ignition loss 1.9
2.2
38.3
0.1
1.3
2.5
36.8
0.3
2.4 3.5
0.5
34.2
0.1
Trace
29.1
...
2.5
4.1
27.3
0.6
7.3
4.5
31.6
0.4
5.6
46.0
4.6
0.3
3.0
0.7
30.4
0.3
9.9
2.5
4.8
2.1
24.9
1.7
25.6
0.4
12.9
7.3
PzOs
0.7
11.3
18.5
6.6
14.4
1.1
14.6
2.0
14.7
2.0
15.3
1.2
28.6
0.5
10.2
INDUSTRIAL A N D ENGINEERING CHEMISTRY
290
Determinant Factors in Boiler Water Treatment
Since the solubility curves of the desirable and undesirable solid phases are divergent, it follows that the concentration of conditioning chemical which must be maintained in the boiler water is a function both of the unfavorable radical concentration and of the pressure a t which the boiler operates. Further, the point of introduction of the chemical, whether into the feed water or the boiler water, is a matter of indifference. To illustrate, consider two boilers operating a t 100 and 200 pounds gage pressure, respectively, and with raw water drawn from practically any lake or river in the country. Let the conditioning be done with soda ash. Note-In case of a raw water containing soda ash, as No. 1, Table I V , i t may be necessary, in order t o maintain t h e upper carbonate limit a s desired, t o use a magnesium or calcium salt t o decrease the carbonate concentration.
I n this case sulfate is the unfavorable radical to be considered. The minimum carbonate concentrations which may be maintained in the two boilers are as follows (p. p. m.) : Concentration of
so4
CONCBNTRATION OF COa
Boiler a t 100 pounds gage Boiler a t 200 pounds gage 500 22 72 1000 46 143 2000 88 285 Note-In view of t h e variation of the solubility product ratio with in-
creasing concentrations, t h e carbonate concentration, a s obtained directly from t h e equation, suffices for t h e upper limit at the higher sulfate concentrations.
The relatively high concentration of carbonate radical required a t the higher pressure naturally leads to the question-what is the limiting pressure a t which the use of soda ash is feasible, in view of its ready hydrolysis and decomposition under boiler conditions? Data on its decomposition a t different pressures have been presented elsewhere;13 a t 320 pounds gage it is impossible to keep enough carbonate in the boiler to prevent the precipitation of calcium sulfate13 and the high hydroxyl engendered may lead to the precipitation of calcium hydroxide, for which dS/dT is negative. The amount of carbon dioxide introduced into the steam by carbonate decomposition, although of almost negligible percentage, becomes another factor for consideration at the higher pressures because of its relation to corrosion in condensers and other points of condensation. Two methods of control may be suggested: (a) to maintain a low sulfate concentration by blow-down, or by the use of a barium salt; (6) to substitute a stable radical, such as phosphate, for the carbonate. It has been found satisfactory in practice, for boilers operating a t 150 pounds gage, to establish a suitable sulfate concentration (2000 to 2500 p. p. m. SO,) and maintain it by blow-down, thus making more uniform the inflow of soda ash. When the sulfate concentration of the feed water is high, the operating pressure a t which the substitution of phosphate for carbonate becomes desirable is probably not far removed from 200 pounds gage.
Vol. 17, No. 3
sing on to the boiler contains some carbonate crystals as nuclei. Although they are probably sufficient to lessen the formation of scale, they do not prevent it. TWOmeans of minimizing this formation of scale from a bicarbonate water suggest themselves. One method is the use of a d e a e r a t ~ r . ~ ?By the division of the water into a fine spray and the maintenance of a very low partial pressure of carbon dioxide in contact with it, partial decomposition of bicarbonate is much more quickly accomplished than otherwise would be the case, because of the large surface exposed. A second method is to introduce into the water leaving the feed water heater the hydroxyl and the finely divided calcium carbonate crystals in the effluent of a filter attached to a boiler for the purpose of maintaining a low content of suspended matter in the boiler water. The alkaline effluent water immediately removes any bicarbonate radical and provides nuclei for uniform crystal formation throughout the water in place of on the irregularites of the metal surface. Simple methods suffice for the removal of carbonate scale from feed lines. Cold water, saturated with carbon dioxide under pressure, disintegrates it.28 A somewhat higher hydrogen-ion concentration, under rigid supervision of a competent chemist who would realize the possibility of injury to the feed lines through overtreatment, would be more economical both in time and cost of material. I n either method the water used in the treatment should be kept out of the boiler. Other Factors in Water Treatment
I n addition to maintaining conditions in the boiler water so that hard adherent scale shall not form on the evaporating surfaces, any complete system of boiler water treatment must provide for the inseparably associated factors of sludge formation and wet steam, and must minimize corrosion. The prevention of scale formation is distinctly a chemical problem, and has been considered in this article. For minimizing corrosion both chemical and mechanical means should be employed;29 control of the other two factors is mechanical, and has been discussed e1~ewhere.l~ Acknowledgment
I n addition to those mentioned throughout the paper, the writer wishes to express his especial appreciation to J. M. Hopwood, president of the Hagan Corporation, and to A. C. Fieldner, superintendent and supervising chemist of the Pittsburgh Experiment Station, Bureau of Mines. n McDermet, Mcch. Eng., 42, 273 (1920); Jackson a n d McDermet, THISJOURNAL, 16, 959 (1923). Cross and Irvin, Power, 66, 422 (1922); Jones, Ibid., 60, 578 (1924);
private communication from W. P. Chandler, fuel engineer, Carnegie Steel Co., Duquesne (Pa.) Works. Hall, Meck. Eng., 46, 810 (1924).
Prevention of Soft Carbonate Scale
Production and Imports of Pyrites Continue Downward Trend in 1924
Water that has been given a correct treatment with lime and soda ash and sufficient time for complete precipitation, or with a zeolite exchange process, should cause little trouble a t this point in the boiler, as deposition of calcium carbonate will be a function of change in solubility with temperature only. If errors enter into the treatment, however, the conditions are most favorable for formation of carbonate scale, inasmuch as such waters are very free from suspended materials which may serve as nuclei in crystal formation, and thus more readily deposit crystals of calcium carbonate on the irregularities of the metal surface. I n the case of no outside treatment or treatment with soda ash alone, there is a gradual decomposition of bicarbonate radical and consequent precipitation of calcium carbonate. Some of this occurs in the feed water heater, and hence the water pas-
The production of pyrites in the United States, as indicated by figures received from the Geological Survey, dropped from 181,628 long tons, valued a t $661,000 in 1923 to 160,096 long tons, valued a t $645,262 in 1924 and, with the exception of 1921, when only 157,118 long tons were produced, was the smallest recorded since 1897. The figures for 1924 show a decrease of 12 per cent in quantity but of only 2 per cent in value as compared with 1923. The average value per ton was thus higher in 1924 than in 1923, being $4.03 as compared with $3.64. Although the production of pyrites dropped 12 per cent in 1924, the quantity of pyrites sold and consumed by producing companies dropped only 6 per cent. The imports of pyrites containing more than 25 per cent of sulfur in 1924 amounted to 243,237 long tons, valued at $582,794, according to the Bureau of Foreign and Domestic Commerce. Of this amount 242,786 long tons, or 99.8 per cent, were imported from Spain. The imports records for 1924 and 1923 indicate a decrease of about 50 per cent in the average value per ton of pyrites imported.
I
