1 N D U S . T R I A L A N D E N 0 I N E E R I N 0 CHEMISTRY
32 PtOi M.S m.4
KsO 88.1 a6.a
OIO 0.8
4.3
Blol 1.8 4.6
Frol ALOI 0.6
0.6 1.6
1.8
Cb 8.7
...
Other 0.7 -0.8
The cokepacked tower and the oooler of the pilot plant were not completely effective in preventing losses of p h w phorus pentoxide and of potSaeium, aa the data of Table I1 show. Undoubtedly improvements could be made in the recovery sptem which would increase the yield of potassium metaphosphate. The reaovery of hydrochloric acid was not studied quantitatively, although it waa found that the hydrochloric acid contained d percenbgea of phosphoric acid and potassium chloride.
Conclusions A general method for the production of metaphosphates
has been developed, in which phosphorus is burned with air in the presence of moisture and the hot products of combustion are brought directly into contact with the second reactant. This method has the admntage of utilimng the combustion of the phosphorus to supply the needed heat; also, the step of converting phosphorus into orthophosphoric acid is eliminated. The development of the proceas baa been carried to full plant scale in the production of calcium metaphosphate from lump phosphate rock. In pilot plants, calcium metaphosphate also has been produced from phosphate sands, and potassium metaphosphate from potassium chloride. A
Vd. 34, No. I
limited amount of work has been done on the production of sodium metaphosphate from d u m chloride. Calcium and d u m metaphosphate are produced by this method in g k q or vitreous form, whereaa the potgssium metaphosphate is a white crystalline solid. Upward of 90 per cent of the fluorine in the phosphate rock and of the chlorine in the potaaeium chloride may be volatilised in the proceas. Fluorine compounds and hydrochloric acid, respectively, may be m o v e r d 8% by-products.
Literature Cited bmadori, AM; omad. fincsl, 21. 11. 182 (ieia): ~ n t s - t i ~ ~ s l Criticd Tables. VoI. IV. p. 70 (1928).
Curtis, H.A., Copaon. R. L., and Abrams. A. J., C h . & Md. Em.. 44. l a 2 (1937l. Curtii.'H. A.,Cap&, li. L.. Abrams, A. J., snd J u n h , J. N., m.. 45, 3 i a 2 a (1838); TW.. ~ mI&. . CM. E W ~ . . 54, No. 3 (1938). Frear. G. L.,D-. E.F., and Leflarge, J. W., to be published. Frear. 0.L., snd Hull. L. H.. IND. ENa., CHEU.,35,1680 (1941). 61. 638-80 Jacob, K. D., and Roas. W. H.. J . A v . Re&, f,oAn> \ _ "__, . MsoIntire. W.H., Hardin. L. J., and Oldham. F. D., h. Eaa. C w . , 29,22864 (1937). Madoraky. 8. L.. and Clark,K. G.,Ib*I.,32,244 (1940). Metallgeaellaehaft A&.. ~ e m mPatent 618,203 ( D ~ o . ao.
in%\.
Partridge. E. P.. H d Laborstaries. Duol S &
Nme. June,
,"27
I-",.
Pole, G. R., and Beinliah. A. W.. B d l . Am. Cwam. Soc.. 20, 229 (1941). Tarbutton. Grady. E m ,E. P., Jr., and F r w . 8. G., J . Am. C h m . SOC.63.1782 (1941).
PHOSPHATES IN WATER CONDITIONING CHARLES SCHWARTZ AND C. J. MUNTFX Hall Laboratories, Inc., Pittaburgh, Penna.
BY.
1930 Phosphate compounds had an established position in the field of water treatment, largely due to the wide use of hydrated trisodium phosphate (NaIPO4.12H20) in boiler water conditioning (11, 26, 46, 96), water softening (11, 46, 91, 96), corrosion control (18, 19), and all typea of detergent proceases (66, 91,96). As a detergent, alone or in combination with other agents, trisodium phosphate played a part in textile procesaing, laundering, dishwashing, and metal cleaning, and was incorporated in many household soap compounds and cleaners. Modi6cations of this salt, other orthophosphates, orthophosphoric acid it&, and tetrasodium pyrophosphate, with its hydrate, were also commercially available and were used in water treatment to some extent. Of the modifications, combinations with d i u m chloride (98), sodium fluoride ( 6 4 , sodium hypochlorite (66), sodium carbonate (80),and sodium hydroxide (90)were available and used in the mme applicationsaa the ordinary salt. The hypochloriteformo5eredsomeadditionalpossibilities as a sterilizing and mild bleaching agent to supplement the usual propertiea of trisodium phosphate. The remaining orthophosphates and orthophosphoric acid were used only in certain Limited situations where their alkali-reducing power or buffering a+ tion, or both, were needed. In so far as water treatment went, the hydrated disodium phosphate (NuaHP0.12II10) was largely consumed in boiler-water applications where the greater alkalinity of the trisodium phosphate was trouble
some (46). This salt was also used widely in certain t y p e of silk dyeing where its bufIering power in controlling dye-bath pH values was ostensibly sought; however, its effect as an added salt in controlling dyest& ahsorption and in protecting phosphate weighting on silks should not he overlooked. Monosodium phosphate (NaH,PO&O) served in boiler water conditioning for its akali-reducing power (&), hut difficulties in handling and feeding its acid solutions p r e vented any wide usage. The same factor worked against the use of orthophosphoric acid in water treatment, though this material was used occasionally in sewage treatment (73,74), and in the prevention of afterprecipitation from lime-soda softened water in filters and mains (60). The pyrophosphate was applied in detergent and dyeing practices (2,71).
Phosphate Compounds The development of the various modifications of trisodium orthophosphate was paralleled by efforts toward utilising some of the known hut hitherto unused phosphates. This search, which had produced its 6rst evidence of succea8 little more than ten years ago (&),has now brought into wide a p plication compounds with unusual properties. The result has been the chemical counterpart of a gold rush, as attested by the number of patents issued in which theae materials are mentioned. Figure 1 indicates the abrupt upward surge of interest following Hall's announcement (48)of the complete
IanUary, 194a
INDUSTRIAL A N D ENGINEERINQ CHEMISTRY
The u ~ e soutlined here for the phosphates reat upon fundamental properties which are p d by different phosphates in M a n y water softeners varying de-. and detergents for home or industrial use, as well as boiler water conditioning practiees, make use of the long-known alkaline, pH-regulating, precipitating, dispeming, and emulsifying powers. In recent years detergent practiees have been extended in scope by the complex-forming power, which is not exhibited by the ortho-
softening of water simply by dissolving in it a sodium phosphate g k - a profound change from the current concepts of water purification involving distillation, precipitation, ionic exchange, and electrownosis (11,.%, 63). The new developments in general were with phosphates whose molecules, when considered aa acids, contained lese water in proportion to phosphorus pentoxide than orthophoephoric acid (HIPOI); this was the basis for the designation “molecularly dehydrated phosphates”. Reference to the extensive literature ($4, 67,68) showed an amazing array of complex compounds which were the center of a confusing 84 ries of claims,denials, and countkrclaims. One group of these compounds had become hown aa the polypheaphates and w88 characterized by the general formula [nEPO,-(n-l)H a ] . Salts of various members’of this series up to values of n = 10 were reported. The remaining compounds fell into another group known as the polymetaphosphates, cham+ terized by the general formula [nHIP04+aH,0]. Salts of varions members of this series up to n = 14 were reported in the literature. These two series approach one another as n a p proaches infinity, yielding on the way some truly colossal hypothetical compounds. To this situation the ninety-yearold statement of Rose (88)still applied: “No substance offers the chemist greater dficulties than phosphoric acid; the more the behavior of the acid is studied, the more the dficulties increase. Every new investigation presents the chemist with new anomalies, fresh and pussling phenomena make their a p pesrance, whilst the older and already known difficultiesare by no means cleared up.” While even now all questions cannot he answered to full atisfaction, recent work d o r d s a clearer understanding of the relations among the sodium compounds, the ones of most interest commercially (4, 6, 7, 8, 68, 78). Studies in the system between the lowest member of the polyphosphate group (NarpIoI) and the metaphosphate (NaPOS, which may he regarded aa forming the limit to that group, are summarized in Figure 2. Only three molecularly dehydrated phosphates exist in the crystalline state-pyrophosphate, tripolyphosphate, and metaphosphate. Tetrasodium pyrophosphate (Na,F‘&) is known to exhibit four transformations with elevation of temperature to ita melting point, hut only one form is stable a t ordinary temperatures. Pentasodium tripolyphosphate (N%P,OL0)may take either of two stable crystalline forms, depending upon the temperature of preparsr tion; these forma can be converted from one form to the other by proper heat treatment. The metaphosphate may exkt in three cryhllhe forms; two low-temperature forms are insoluble, while the form produced between 475-50O0 C. and the
33
phosphates but only by the molecularly dehydrated phosphates, partieularly the phosphate glasses. Boiler water conditioning makes use of this complex-forming power in the control of feed line deposition and also takes advantage of the alkali-reducing power provided by reversion and the precipitating ability of the reversion products. Proms and municipal water treatment utilizes the stabilizing and corrosion-preventing properties with striking reaults.
melting point is soluble. The two low-temperature forms can be converted to the high-tempratwe form successively by temperature elevation, but the change is not readilyreversihle. Of these crystalline materials only the tetrascdium pyrophosphate w88 commercially available and in u8e over ten years ago (8,71,98); the production of other members of thia group ww carefully avoided in commercial processes (9s). Since then the uses of d i u m pyrophosphate have been extended aa the increaSing application of other members of the molecularly dehydrated phosphate group has shown the way (S’,14,16,W). Potas%iumpyrophosphate bas alw been introduced commercially and found valuable in certgin applications, largely because of ita great solubility (14). The most unusual property of the nominal compositions ranging from the metaphosphate to nearly to the pyrophosphate is their ability to form glasses. This property for t.he
90 80 70 66
$3 70
1“ 11
a4
INDUSTRIAL A N D ENGINEERING CHEMISTRY
metaphosphate extreme waa first reported by Graham (56) in 18-33 when he showed that m o n d u m orthophosphate could he wverted to a glass by meltiig and rapidly quenching the melt. The glass of a 1 to 1 sodium oxide-phosphorus pentoxide ratio, which he called sodium metaphosphate, has home the name “Graham’s mlt” since that time. Fleitmann (B)in 1&49 concluded somewhat dubiously that this material WBB a hexametaphosphate; since then thin name has bean commonly employed for the glass.
/
+ MELT
I
As disclosed by Heddan (SQ,any composition hetween metaphosphate and a mixture corresponding to 60 per cent each of metaphosphate and pyrophosphate-that is, clcee to the peritectic between pyrophosphate and tripolyphosphatewill give a clear glass readily. At compositions of lower phosphorus pentoxide content, glassea over to approximately the tripolyphosphate composition can be obtained by more d r d c quenching. Below that point it is impassible to produce a clesr glass, although mixed producta, part crystalline and part glaay, can he made. F’yrophosphate, however, has not yet been and probably emnot be produced as a glass. The d u m phosphate glaases may he looked upon from a numbat of points of view: (a) They may be considered BB “polyphosphates” (24) of almost any nominal composition or &urea of such phosphates; (b) they may he regarded as undermledsolutions of crystalline phosphates in each other ($7); or (c) they may be looked upon, as are other glasses, as made up of a random nonrepating and nonsymmetrical network of atoms whose relation to each other can be found only by &tietical methods (69,W). It is from this array of glsasy materials that the new phosphates used in water conditioning have come. The &ut material producedand used commercially was made by Hall and Jackson (&) in the manner employed by Graham a century earliethat is, fusion and quenching of m o n d u m orthophosphate. To correct the slight acidity of this material, in subsequent mannfacture it was modified with alkali either by admixture or fusion (48). With the change in ratio of sodium oxide to phosphorus pentoxide resulting from the added alkali, the produds in w e as “metaphosphate” range in phasphom pentoxide content from a b u t 66 to about 69 per mt; it is obvious that they might be c o n s i d d as any appmp4at.e “polyphosphate” from NalSllOu to NsrpaOn or ad mixtures of such polypbosphates. More recently another sodium phosphate glass has been introduced commercially;it isreferredtoassodium~raphosphate(d0,2f,81,8j) which con& 60.4 per cent phosphorus pentoxide. Thus, at present there are in u88 &B water-oonditioning chemicals the
VoL 34, N e 1
orthophosphates, long known, and two typea of molecularly dehydrated phosphates, the crystalline tetrasodium pyrophos phate and the glaea?s ranging in compoeition from about 80 to about 69 per cent of phosphorus pentoxide.
Properties Figure 3 preaenta the relations of the variow phosphates in the N&PA-HLI sptem from the standpoint of composition. Thw phosphates posseae some of the most unusual propertiea found in any chemid family. M e m k chmn from this group of mmpounds give solutions which may k strongly alkaline, mildly alkaline, neutral, mildly acid, and strongly acid, BB illustrated by the electrometric titration m e a for the sodium d t s shown in Figure 4. Here the curves are shifted along the axis of abeoissas to avoid confwion fromoverlapping. The solutionstitrated were prepared by diesolving l gram of the indicated material in 100 ml. of water, add= 0.1 N sodium hydr6xide solution to raise the pH to over 11, and then titrating with 0.1 N hydrochloric acid solution wing the glass electrode. The change in pH d u e with added acid is shown in each curve without m o t i o n for d u m - i o n error of the electrode at high pH dues. The vertical linea mark intlections in the curve^ while the pH value of each original solution without any added alkali in indicated by an X. The titration curve for eaoh material alone in water fdows the cnrve to the right of that point. Of the crystalline materids only trisodium phosphate po6 8e8888 extensive neutmlizhg power at high p H dw.The 6rst sharp infldion in ita curve is in about the same p B range BB the upper intlections of the pyrophosphate and tripolyphosphate. Below this inflection all three materials exhibit a slow drop in pH before a second sharp idection at a lower pH. It will be noted that both pyrophosphate and tripolyphosphate exhibit a slight inflection between the two sharp inflections, which indicates‘ht there may be some differenm in the dissociation of the remaining metal ions. The soluble cryutdine metsphqhate is the only soluble m y s t a l k phosphate which dBem from the others in ita hehavior on titration. This material posseasea no neutralizing or butlering power and shows a single sharp inflection (84). Turning to the glasses, it will be noted that the metaphob phate glass doea not show the eame curve as does the soluble mystdine metaphosphate. The curve of the soluble cryetalline metaphosphate can,however, be approached by more complete dehydration of the metaphoephate glass (69, 84). An ordinarily prepared, complete elimination of molecular water is not attained and the titration curve then shows i n t l e c t i ~ ~ in the m e pH rangea as are found in the variou8 “polyphosphate” curvea. Interestingly enough, though the intlections for the more alkslineglasses fall in much the m e range 88 for the mystdine forma having two inflections, the spresd between the idections correlates so well with the p h o s p h m pentoxide (Pto,) content of the glass that ib nominal armposition can be dculated readiiy from the titration C~I’VO
(MI.
Reactions of the phosphates with metal ions am extmnely varied; to summarize them in a reasonable space, considemtion must be limited to those metals such as calcium, magnesium, inm, and aluminum ordinarily met in water treatment. Orthophosphates (Sf,#) precipitate these metsls an phosphates or hydroxides at almost all concentrationsin nentrsl and alkaline solutions; precipitation h m e a legs completa with lowering pH valuea until no precipitation o c m . With pyrophosphate on the other hand, additions to solutions of low concentrations of metal ions will not give a precipitate regsrdless of pH d u e , temperature, or time (6,Jo, 86). As the concentration of metal ions is raised, a range in h d y reached where precipitation occurs as long as pH duea am
January, ler2
INDUSTRIAL AND ENGINEERfNG CHEMISTRY
not too low. However, for any such concentration of metal ion a certain exma of pyrophosphate over the precipitating doaage rediseolves the precipitate already formed. In time, some precipitate may re-form, SI) when calcium e x d s oertSin limiting concentrations, but this does not oaclll with dl metals. this power of forming comTripolyphosphate plmes to a greater extent tban pyrophosphate (6, 69, 86), but it is in turn mrpawd by the phosphate glasses, partitularly thoee approaching the metaphosphate composition (6, IS,J0,6S’, 86,80,06). These materialn give no precipitation at low concentrations, whether pH values are high or low. At bigher concentrations precipitation will occur an long an pH valuw (VB not too low, but an exma of phosphate w i l l prevent precipitation and redholve any precipitate that may form. To s d , four dBerent situations may arise upon the addition of a phosphate to a w a h containing the metal ions ueuslly met with in water treatment: 1. w p i t a t i o n of the metals may occur; tbpmpertypss p”ed valuable in water BOand boiler water con&bm-
?. Prscipitation of ths metala may not occur tecause of low p H duea but this offers nothing of value for practical work.
8. ~rsdipitstionmay not ooouT tb mn-htiom are too low to ‘t eatmation of the solution. A p-at that would p x t h k e&, so that feed l i n ~deposita could b avoided Ill boiler Water C O n d t i O ,Wae hew Bought When the seamh tanned inta the field of= w i d phosphates The apPfiC8tiM Of Dl& hoaphate t0 BcoompLiah this led t0 & discovery that the r o d situation exista~: 4 Precipitation may not occur at highm concentrations beU U W Of the formation of a duble compk of phosphate and metsl ion,
.
FIOWE
have formed, we can rate tbee phosphatea in ordar of &eotiveness. Studies of this type show that the metal ion, the concentration of the metal ion, the conmtration of added agent, the pH value, and tempmature play important roles in this BCtion (6, 13, SO, 69, 86, 80, 06). For holding calcium ions Sgsinst precipitating agents, the effectivenea of the p h m phatea changes greatly with inin pbosphorua pentoxide content, SI) indicated in Figure 5. For magnesium, on the other band, the situation is revered but the relative Merence in e5ciency is less marked. With iron and aluminum generalization is diEcnlt, the complm-forming agenta b e i i specifio in their effects. The concentration of metal iow would be expected to play a part in controlling the amount of agent needed for best results and does so. However, in Borne crwa, once a certain concentration of metal ion baa been exceeded, there is no &ect from the complex-forming agent regadleas of the amount added (SO). Such a situation arisee in the use of pyrophw pbate for preventing the precipitation of calcium carbonate and calcium orthophosphate. A riaa in pH value and a drop in temperature call in general for more phosphate (SO, 86). Another unllsusl propats of the molecularly dehydrated phosphates is their ability to take up water to re-form orthcPhosphates ( ‘ 3 9 14,m,4466 67968). These rea^&^^ in We d are typised by the metaphosphate: NaPOs
+ g0
-
NaFt30.
This reaction is just the revem of that for the formation of metaphosphate. For any one form thisrebyohtionis speeded up by +in temperature, by -tion in pH duein either direction from neutrality, and by increae~in metal-ion con-
centrations. For various forma the stability increaw with decrense in apparent molecular complexity, but all forms are sufficiently stable to be applicable in many w a h treating problema wbere the complex-forming &ect is desired. The complex-forming power of these mat&& bad brought them into wide we when Rosenstain (831disoovered that low conce.ntrations of these agents, in the range of leas than one up to a few parts per d o n , were able to p m t the p r e cipitation of compounds wbich o r d h d y cause great tmuble in water. The amount of material needed for this effeat waa so far below that required for complete complex formation zI N N*W. ,. PO. izn,o with the metal ions that this phenomenon could only be accounted for SI) a stabilim 3 Ne* H PO+ 4 Ne*HPOI’IZHzO tion by adsorption. Because of the low 5 N e Y Par concentrations required, the term “threah6N e HI PO. *H. 0 old e5ect” wrm coined (IO, .cS, 76-70). An example of this effect in a water from whkb calcium carbonate prccipitatea is shown in Figure 8 (@), where the change id pH value reilecta the extent to wbich precipitation OCCIUS. The relative concentrations of metal ions and preaipitatii ions are factors in the utilization of this property, but fortunately in water treatment work a condition rareIy arises where the deet cannot be obtained. The amount of phosphate reagent used must be carefully controlled since some materials, once they have exceeded certain limits, will fail to give results. The glasey forms do not have this limitation, but the we of dosages beyond those effective may represent an unnecwary waste of material. Wrrcxr Fm Cwrr N+O Incream in alkalinity and tempeaturea cut 3. C-ITION 09 vmom BODrno-ms down the &ectiveneas of the phosphate
The practical importance of such a proma dependn upon the stability of the complex, an the phosphates must compete for metsl ions with other agenta wbich tend to precipitate the m e a . For example, in addition to the orthophw phatea, a long list of calcium-precipitating agenta such an carbonates, silicatea, soaps, eta, are used in water treatment. By measuring the ability of the new phosphates to prevent such precipitation and to rediseolve the precipitate once they
4 OS
33
36
INDUSTRIAL A N D ENQINEERING CHEMISTRY
reagents and make the use of greater concentrations n e w ssay.
VoL 34, No. 1
. Boiler Water Conditioning
One Of the earliest uses of phosphates in water conditioning was that of scale prevention in high-pressure boilers. The reSuirements for S d e f W evaporative surfaces arise from the su,,fws and on itlsoluble ~onipoundsinvolving metal ions limitations in temperature to which boiler tubes may safely (48). rn the of met&, the flow of water containing low he 3and from the fact that even 8 thin Of scale concentrations of molecularly dehydrated phosphates develmult in an excessive and d e rise in tube temperaowa film on the m~ surface, which shows its presence by may ture. Also, operating dciency demands that rapid heat m a r w y reducing the rate of corrosion. In Figure 7 the pmh m f e r from furnace to water should not be interfered with tection of iron is indicated by the lower oxygen absorption, by a blanket of heat-insulatingsoale. lower degree of oxidation of iron, and lower &tance to flow While scattered references to the haphamd use of p h w in the presence of the phosphate. In the lower graph the 4 phatea in the t r a h e n t of boiler water may be found in the p. p. m. curve f& on the axis of abscissas. The effect a p not until about l i h t n r e of the psst ninety years (4a), it to be largely controlled by concentration of phosphate 1925 that their utility k a n to become more widely recoguaed,but other factors such as rate of &ow,pK value, and temnized. Hall (41) pointed out a t that time that with proper conperature play a part. Other conditions being favorahle, adtrol of the relative concatrations of the substanaea in boiler sorption on iron will take place as long as sufficient material water, evaporation would result in the separation of a nonadis fed to build up the film and maintain it; an excess beyond herent sludge as the solid phase rather than in the separation that requirement is unnecessary. of an adherent scale. In boilers operating a t low pressures, In the c88e of the insoluble compounds involving metal say up to 150 pounds per s q w e inch, the use of sodium carions, the &ect of these agents is best deacrihed 88 a “dispersbonate as the trmting chemical resulted generaUy in relative ing” action (1J,14, &, 76). The effectsof the dserent p h w freedom from scale. At higher pressures and rates of evapora&tea appear to be highly specific, so that generalizations tion, the instability of the carbonates made their use in-0t be made and each c&8e must be considered as a dit+ feasible,and Hall with his associates developed the controlled tinct problem. However, for a material which responds to a use of the orthophosphates; they do not decompose even at phosphate reagent it is generally noted that the effective the high temperatures characteristic of modern boiler operaconcentratiom of agent fall certain definite limits, tion and, when properly n d , do result in the formation of a outside of which no appreciable edfect is obtained. n o d h e r e n t sludge in the boiler (46). As the poaeibility of The molecularly dehydrated phosphates have found wide cleaner surfaces by phosphate trwtment has been recognid, application in water treatment by adding to the properties of their use has hecome almost u&md. the long hown phosphates some unusual features of great Since the scale found in boilers consists largely of d c i u m practical value. A summary of these properties, both old and sulfate, proper treatment with phosphate resolves itself into new, shows that phosphates have the ability: the maintenance of enough phosphate ion and sufficient, but not excessive, alkalinity 80 as to attain the dubility pmduct To rovide wid, neutral, or alkaline solutions for control of 1. of the insoluble calcium phosphate before the solubility prodpH value f y both neupalization and b d e r action. 2. To act as emulslf$ng agents. uct of calcium sulfate is reached. The exact ratio of phot+ 3. To reduce metal-ion concentration by precipitation. phate to sulfate which must be maintamed depends upon EO 4. To reducs &-ion coacmtration by wmple~formation. many factors that recommendation of d e h i t e rstios h a been 6. To reduce alkalinity by hydrolysis to orthophosphate. 6. To stabilize solutions supersaturated mth respot to abandoned in favor of maintaining constantly a alight meem ordinarily insoluble salts. of phosphate over calcium. This is generally sufficient to 7. To form adsorbed films on metal SUI~WS, good rerrults due to the frequent Or COUthuOUS blowdown a T~, i,,solub~ wmpom,& tbmugh adsorption employed in modern boiler operation. Under conditions of on the sohd particles. careful control a 30 p. p. m. excw of phosphate ion (PO,)is sufficient,although ordinarily 60 p. p. m. of excess phosphate is maintained. While a certain amount of alkalinity is necessary in order that the calcium may be precipitated as a sludge, excessive alkalinity is undeairahle and has led to the use at timea of mono- and disodium phosphate. When the former is utilized, s p cia mater& must be used for p i m containers, pumps, etc., to limit corrcaion. The use of the orthopbosphatea under proper control has, by maintaining a d * free snrfacea, helped lead to boiler operations a t higher pressures and higher ratea of evaporation. However, certain di5cultiea have been encountered. Among the greatest of these is the precipitation of the residual calcium of the water in the feed lines and economizers. These deposita accumulate and may seriody interfere with the operation of the equipment. In ddiML. Or a/ N UYPROCHLORIC Acio tion, trisodium phoephate, which is most widely used, may lead to undesirable F I Q 4.~ Emm-c TITEATION Cwms OF V*BIOVBSODIUM PEOSPEIAT%S excessivealkslinityitheboiler. Forthese Closely related to the “threshold effect” is the ability of low concentrations of these ,,,&kk to be adsorbed on me^
18nlmtY, 1942
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
37
from the properties of the materisls, the metaphosphates are more effective although somewhat higher in price. In the processing of a e s these phoaphates are used primarily t o overcome difficulties due to the presence of heavy metals. While water, even zeolibsoftened, is in general the s o w of these materials, it is not the only one. Lime-prilled wool and raw silk, which at time8 contain enough metal ions to raise the hardness of the water to several grains per gallon, may be cited as materials supplying contsminsnta in addition to those found in the water. The firstgeneral type of operation to which the phosphates are applicable is that of scouring the textiles, either in early or late oc /i?PPeM. stages of the proeesSing. Wool, which may contain from 1 to 2 or more per cent of lime, h o r n 5. R m m OD ~ W A T ~&rcmmo B ET CowLmx FOBMATION AT ROOM ~ K D W A ~ L8s) , TWrrrmm (woy Ronr, is generally B c o d at low t4!mperature8 (11C1-140~ F3. low alkalinities. and slow operation 8 4 . Since alkaliesBIB injurious to wool, it isdesiredtomaintainaslowanalkalinityasisconreaaons, Hall and Jackson (4.4) introduoed the we of the mo&tent with good sconring. In the event that map is used, lecularly dehydratedphosphates such astetrssodium pyrophosit is obvious that much of the lime in the wool would not be phate, sodium metaphosphate, and substsnces of intermediate removed, and m e of the soap would merely add to the forcomposition in boiler w a k treatment. When put into the eign matter on the wool. The addition of enough phosphate feed line, they prevent the formation of scale by virtue of to soften the water plus an amount in e x w , depending forming slightly i o n i d comphes with calcium; when in upon the lime content of the wool, enahlea the proeeaeor to the boiler, they revert to orthophospbtes, reducing excea obtain a bright, soft, welliscouied wool. In the scouring of alkalinity and causing a node-forming solid phsse in the woolen piece goods, the use of the molecularly dehydrated b o i i water. It is obvious that t k a aubetances would dl phosphates in the riose decressea rinaiing time (38,70). behave in a similar manner hut would be somewhat less In the kier boil&aient, both in preventing feed-line scale and in reducing ing of cotton, the b o i i water alkalinity, an the phosphate content waa diminuse of phosphates ished. le& to production Another method of using phosphate in boiler water congoods which are free n ditioning involves the pmftening of all &up water with from s t a i n s , a r e I one of the orthophosphates, with or without added sodium UIWW moreaheorbent, and hydroxide (64). The treathg chemicals are added to the are of uniform color. water at elevated temperatwee in a sedimentation tank. The easier removal The sludge is removed by filtration and a water of “zero” of pectates, p m t hsrdness is delivered to the boiler. The method is applicable M-mm aa calcium d t a in to waters of hsrdness lower than that normally softened by the cotton, and the limesodsbut harder than that which could be easily handled h O n S n 6. haSILWA’IlON OD A supmFau.lwa.4m SOLUTION OF car, prevention of limeby internal treatment. CIM CABEONATE BY 2 P. P. M. soap deposita is efThe last twenty yeara have Been a remarkable rise in the !or GLASSYM E T A P E O W ~ A ~ fected. W h i t e r operating premreu and evaprstive capacities of boilers. goods coming from It is di%dt t o d b e an exact value t o t h e part played by the kier quire less the phasphates in thin chsnpa; it is fair to state, however, bleach and reanlt in a material improvement in tensile that the protection of high-pressure b o k is almost universtrength (70). sdly attained by the UBI) of phosphates in treating the boiler Rayon fibers q u i r e carefnl muring in order to remove any water, and that without them and withont well defined mbstanw which might interfere with proper dyeing. The methods for their we, power plant &ciency would lag far use of the calcium-sequesteriug phosphates, together with behind ita present atbhmenta. soap or other detergent in the d n g operation, bss been markedly BUCoessfUl. TextUeProceeeing Silk contains sn5cient calcinm to harden the making bath, in some iastsnces as much as 17 grains per gallon (6f). The wen of the phosphates in the conditioning of water It is ohviow that preliminary softening cannot cope with for the text% indnatryare too numeronstodiscnss here in detsii. Of the orthophmphoks, ody trisodium and dieodium hardness BO intmd~ced;the UBI) of the molecularlydehydmted phosphates will lead to more d o r m &ta and prevent the phosphate enjoy any marked utility in tatiki plwaaeiqg; the former is Woyed in wanring operatiom (S8) where ~ta di$oulties normally e n m t e m l hy virtue of the presence of t h e e SlLSline earth metala. Silk degumming with the p b rlkslinity is deairahle, the latter in dyeing operatiom 110 phatea is genemlly carried out in a map bath of mild dLspreviouaIy mentioned. Of the mohdacly dehydrated p h a s Iinity at b p e r a t u r m lvonndZOE-212° F. p h a t a both the glassy phosphaten and p y m p h q h t e are Not only in acoUring we the phosphates found to be of uaed *in thetypea of u p e r a h , the choice beha d n e . In the dyein@of wd, cotton, and rayon, the UBI) of made generdy on the beais of dseLed d t and sconomy mt&& d l &&&e &,reaky dyeing and the deeffeoted. &nerdy nwakiug d an &t be
1- J
5, ~
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
.a8
VoL 34, No. 1
&o greatly minimised red water from hot water heatera (47,48,61, 77-79).
lb
10
M
+o
I so
The applications of d amounts of molecularly dehydrated phosphates (threshold treatment) are many and varied. They me being used to prevent deposition from &oling water in power plants, steel mills, and distilleries, both in oncethrough and recirculatiing systems, in railroad and industrial softening plants, in air conditioning and refrigeratingunitein fact, in all typea of industrial applications where it is neceeeary to reduce corrosion and to prevent the preoipitation by heat or alkali of calcium carbonate from water (79).
General Detergent Applications
7i"C -,4ouXs
WOBPEWFE
W
wm
ON COBBWION PllTSBUBQE TAP T " STEEL WOOL
A Is ~PA-
position of precipitated color on the d a c e of the fiber.
Dyea which may be precipitated by metal ions are maintained in solution, and gocd penetration and levelness are
Bs8ured. From this rather hurried mview of textila opmtions, it is obviow that the properties of the molecularly dehydrated phosphstes fit in remssksbly well with the requirements of the industry.
Municipal and Proenss Water Treatment The d B d t i e a encounted in the field of municipal and prucea water treatment are generally of two kin&: (a) the necessity of preventing precipitation and scaling up of the linea, and (b) the necessity of preventing corwion of the metal throughout the syatem. Both of thew Wdtiea have been materidy alleviated with the we of phosphates (66), espeoiauy of the molecularly dehydrated phosphates. Of prime importance is the fa.&that the addition of a few parts per million of glassy phosphate to water going through iron or steel pipes reduce0 the corrosion markedly over a wide range of pH d u e s and produma am gwd results at pH 5 as at 10. The lesult of this treatment is to reduce the corwion to the extent of about 90 per cent, and to minimize "red wster"difhcUlties(lb,b8,~,81,94). The we of damounte of molecularly dehydrated phosphates in water has another etfect of marked utility. Many waters are unstable, tending to precipitate calcium carbonate when heated or c h e m i d y trested. To avoid scale formstion by c h e m i d y treated watera, recarhanation has been practiced but hss not been completely s u d d . For water mdered unstable by heat, no treatment so simple and eatisfsctmy hed been availsble. One to two parts per million of metaphosphate have bean found sufficient for the d b h tion of treated or heated watera o m a wide range of concenh t i o n s , pH d u e s , and temperatuns, and thus to eliminate the need for reearhanation. The higher pH values and dmum oarhanab concentrations maintained, together with the conusion-inhibiting property of the phosphates, have
The most widespread we of phosphates in water conditioning is found, perhapl, in the general 6eld of detergents. Here phosphates are used for every conceivable purpose and reason. It is obvious, since most detergent operatiom are carried out in alkaline solution, that difficulties are encountered from the formation of insoluble deposits ranging in scope from the bathtub "ring" through lime soap on laundered materials to heavy calcareous deposita on utensils in the food prowaing industries, such 88 those found on milk pasteurizers and the l i e . The source of the W d t i e s in detergent operations haa long been recognimd, and one of the first u8e8 of the orthophosphates wea in &urea with d u m carbonate EO as to change the ohsrscter of the precipitate, which formed when d u m carbonate by iW waa added to a hard water, from a granular adherent type to one less granular and nonadherent. Since that time the orthophosphate hss gradually replaced d u m carbonate and is now widely regarded aa the superior mated from practically all points of view. Howew, the diflicultiea of m i n d precipitation are not overcome merely by suhstituting salts of orthophosphoric acid for sodium carbonate, since the former also react with water hardness to form insoluble ~~bstance0 (SO) although of a nature somewhat less troublesome. The elimination of these difTidtiea haa been made pcwible only by the uw of the molecularly dehydrated phosphates such as the soluble glassy d u m phosphates and to a -1 extent with tetraaodium pyrophosphate. These mater& wiU, when properly used, render the hardness of water incapable of reacting with the commonly used detergent mater&,such 88 soap or alkalies, and enable them to do a thorough job of cleaning without the formation of film or deposit (Sf). This property of complex formation with calcium and magne8ium has been found of advantage in laundry and dishwashing operations, dairy and metal cleaning,and domedically for bathing, hair shamping, etc. In the laundry the first u88 of this type of materid wea that of redissolving lime soap deposited during previoua operations, and to mist in rinahg4ecreasing rinsing time and preventing deposition of insoluble soaps during the rim ing p r o w (89). More recently, however, it has been found that the glaasy phosphates, when added directly to the soap and alkali mixtures used in laundering, effect a marked savings in supplies without any decresae in the quality of the results obtained. This d e c t haa bean attributed to detergent propertiea p o d by the glassy phosphates, in addition to their water sofbning properties (33). A similar etfect is ohserved when pyrophosphates are used aa soap buildera, although in this O B B ~the phosphate in plpsumed to act as a diapming agent rather than a complex-forming m a W (9,lS, 14, SO, 76). In the diahwaahing field, the formation of lime film on dishea and machines has long been recognized 88 a W b l e WUMB of the transfer of communicable -,and much w r k has been donein an dort to eliminate,insofar aspoasi-
Janlmry, 1s42
INDUSTRIAL A N D ENGINEERING CHEMISTRY
ble, the formation of this tspe of deposit. The use of the molecularly dehydrated pbsphates together with alkaline washing materisls has led to axcellant d t a in this field and has made possible the thomugb wading of dishes meabanioally (43). Many inveatigatorn have &own that the w of these mstpaialq reeulting in cleaner dkhen, lesds to mar!fed reductions in the number of bactrnis found on the washed dishes (16,30, 40, M, ea,8s). In the washing of milk ems, milking machines, pasteur iringequipent, etc., the elimiuation of deposita has long been the objective. Mi& handling equipment has alwaya been di6icult to clean, not only hecause of water wnditions but becsuae of the relatively h g e quantith of d e f o r m i n g materiala, mob as calcium and protein, in the milk itealf. The use of ordinary alkalies muat be wpplemented with wnsiderable amounta of band scrubbing, and at timen even strong acids are required to move the incruststion known as milkstone. The glassy phosphates, by virtue of their proprty of preventing the formation and redissolving calclv~ousdeposita, and the p y r o p h ~ because , of their dispersive pmpertiea, am now a e l y mwd in the olesning of dairy equipment, and have RsuEted in 8 marked reduction in the time and &ort requid to @arm the daily c h u p in the dairy (f3$,8@,87). The orthophosphates are used widely in metsl cleaning for their detergent d e e t , as are the msleoulsrly debydmtad phosphates for their disper%iog and wmplex-forming propar-
th. In the home the pbosphatea have been used for detergent operations ever since they w m first manufactured. The Orthoph~phates,especisUs trisodium & ~ p l ~ ~ ~ p h a tene, joy s wide sale in small packages and are used for dishwashing, wallwashing, laundering, and many other household taslrs. The moleculsrly dehydxated phosphatea are gradually entering this field ala0 and, while not so widely used because be eXpeoted to 6Ud d y SC0eptanat of their ne-, especially when mixed with more alksline materisls (38).
MisceJlaneow Applicationa Among the other u ~ e 8of the soluble phoephstes in Water conditioning may be mentioned photography. where the orthophosphates are utilired largely to imp8Ft slkslinity to dawloping solutions, and the glassy phosphates are employed to prevent the formation of precipitation and scum deposition on the film or paper (Bo). Glassy phosphates are used to prevent the wrrwion of aluminum by certain typea of water (S), and in plant nutrient solutions the property of preventing the deposition of hesvy metals as exhibited by the molemtlasly dehydrate3 phosphates has been found o f value (17). The only use of an ineoluble phosphate in water oonditioning is the removal of fluoride from potable water by be& tricalcium phosphate (I).
Acknowledgment The writam wish to aoknowledge the Sssiatsnca of E. R. Bumett, who carried out the eleotrometric titratim presented in this paper. Literature cited (1) Mlsr. H.. Klein. 0..and L4ndaW, F. K., IUD. &a. clool.. So. 1 8 8 6 (1988). (a) ncrths, C. A.. Brit. Patent 363.664 (1987). (3) h. J. R.. and M s a h E.B.. S m p , 17. No. 4. ab-?. 71, 78-4
,Q
Bo&,
39
"-
An&, Cmz4. r d . 200,BsgSo (1936).
m., 200, 8au (1936). F. c.. smp. 16, NO.4, - 7 2
(8) (9) BO-.
( 1 ~ ) . (10) B u b . T. F.. and Raitemeier. R. F.. J . Phw. Ch.. 4 4 662-74 (1940). ' of Water and h w w e Trrrrtment". BunrsU. A. M.. NsaYork, CbemidC.td~gCa..19%. BuMer. B. W..lm 81as EM., 18,64-7 (1941). chsssla, A,, and Mt&nk, A. M M , T d h n . . 21. -7, 446.67.6264 (1W). (14) Cobb, W. W. S m p . 14, No. 11.2443 (1938). d J. C.. and E& J. R., oil & Smv. (16) cobb.. W. W.. H . 17.. P z i (1940). (16) Cox, W. C., Am. J. Pub. E& 28. 174-80 (1938). U. 8. Patent 2.!228,316 (Nov. 26,1940). (17) W, (181 E-. U. R.. " C o d n of Metsls". London, Edward drnold ,~ & &., 19Zs. (19) Evans, U. R.. J . Sa.Chmm Id.. 46. W 7 4 T (1987). (ZO) Fisks,A. H., andBnan. C. 8.. U. 8. Patent 2,018,886 (1936). 2,019,666 (1936). (21) M., (az) m., a , o a i m (1936). (a3) meitmann. T.. P ~AM., . 78, a33-60,3ages (1818). (24) Fleitmaao. T.. and Henne.ber& W., Ann., 65,304-34 ( 1 8 6 . (25) Foulk, C. W., Gaol. Survey of Ohia, 4th 8erieS. Bull.. 28 (1916). (281 - , Qerber. A. B.. and Mile& FLT.. IUD. ENa. clool.. ha.En.. 10, 619-24 (1988). (87)opamrrin,L.,clrbn*rCA indw6is, 2 2 4 (1936). (as) Gidley. E.T., snd We$ta% R. 8.. J . Am. Wata W w h A m . 32. llscg? (1940). (28) d t . J. M., &a?, 16, No. 1.25-8.89 (19.39). (So) oilmons,B. H., h. ENS. h. 29. . &%4-90 (1937). ..oil &B.clR 12,2342 (1936). (31) Qilmme. B. E (82) Qilmom, B. E., SkwhGimLuundW J., 40 24-9 (1833). (88) G h o m . B. E..MunW. C. J.. and Burnett. E. R.. IUD. h a .
.. .
s*
..
(1937). (40)m.. 30.23-8 (1838). (41) Hall,R. E..lW.. 17, agg-w)(1926). (42) Hall. R. E.. U. 8. Patent 1,868.616 (1934); b k u e 19.719 (1036). (43) Hall,R. E., U. 8. Patent 2,036,652 (Maroh 31,1888). (44) Hall.R. E., and JaOkmn,,E.A . , I M , 1.WW.~1(1933). (46) E d , R. E., d d.. Mbma and MetsllUImd hvedtimtiom, C-gie Innt. T& and Bur. Mines, Bull. 7.4 (1987). (46) Eslvanon, H. 0.. BaJrlia. M.,oldal. E. J., and W b n . J. L .. swp.11, NO.5, HI, i w i a (1936). (47) Hutung. E. 0.. Wata W m h & S6kwWG 86, Mwo (1839). (48) Hatah. Q. B.. and Rim. 0..h. &a. h. 31.61-7 , (1839). (49)IW., 32,1672-9 (1940). (SO) Eomer. C. P.. Enu. N m 8 - M . 86.81 (1921). . lo-a (61) Haover, C. P., and Rice, O., Wata Work & S m w 1 ~ 8BB. (1939). (sa) E u k . E., 2.awm. C h . . 60,3234 (19.37). (6.3) rut,IC. Itid.. 39, losbaa (1926). (64) . I .C. E., Paaa PZatIt Bnu.. 40,638-40(1936). (Sa) Judmn. Wilber. U. 8. htent 1,924,881 (Au. 29.1933). (68) Kiehl, 8. J.. and Clausaen, E., Jr.. J . Am. C h .Soc.. 5 7 , a M (1936). (67) ~ishl. 8. J.. and coats. E. p..m,.58. a m (1837). (a) Kiehl, 8. J.. and H-n. W. C..lbid., 18,2802-14 (1928). (69) Kiehl, 8. J., and Hill. T.M.. M., 5 4 1332-6 (lOaa). (W) Kim.Karl. Phd.lnd.. 34,11657 (1936). Cdorid.58,13-16,60 (1936). (61) Leapar, J. M.F.,T& (62) M d h m n . W. L., A m J, Pub. E& ?7,1Sc?O (1937). (68) M d h m n , W.L.,Bryan.C.8., andmman.L. 8..J.Doby 5.A. 23,821-7 (1W). (Sr) Muum, C. W.. and Anhorsft. E. B.. IUD. h a . caut..SI. 763-74 (1939). (66) mat hi^. L. D.. U. 8. Patent l,E66,474 11926). (66) Meokntroth. J. T.. C h d Md.Ew,. 28, 2?34 (19%). (67) M&. J. W.. "Cnn-ve 'lieatbe on Inorganiio .nd Thmtiwl c-0, Vol. II ( l W . @) M., Val. VIII (19%). (69) M m y . 0. W., "Prwettiw of Oh". A. C. 8. Monop.rh 77, New York. Reinbold Pab. Coop.. 1938. (70) Munter. C. J., and Ben, E. B.. Am. DueW 7.4, 40-7 (1936). (71) Murphy.A. R.. snd O d .J. B., U. 8. Patent 1.788.116 (1831).
.,
-..
40
INDUSTRIAL A N D ENGINEERING CHEMISTRY
E. P.. Hioks. V.. . . Partrid-. S O C . . ~&e ~, (isai).
and
172)
Smith. G.
W.. J. Am. C h .
..., .............................. Poaney, J.. and Noad, R. W.. J. Tatila I&..
(73) Philliua, J. W.. U. 8. Patent 1,282,082 (Oet. 22. 1918).
(74 rhci.. 1 . 2 ~ 4 . 4 4 1( N " ~ .la. IDIRI.
(76) (76)
(1939).
30, Tll67-71
Reitemeier. R.F.. and Buehm. T.F.. J. Phw. C h . ,44,635-51
(1940). (77) Rim. 0..Wofa W o r k E w . , 92,690-3 (1939). (78) Rice.0.. andHatah.G. B..J. Am. Wata Work Anao0..31.117186 (1939). 31. 58-63 (79) Rice. O., and Partridge. E. P.. Im. ENQ.C-..
.......
(lQ3Ql.
(80)Ridenour. W.E., U. 8. Patent 850.169 (1910). isij k g e r s , A.H . : J : A ~~~ .o t W a G T ~ A M 32; O ~1498-1500 .. (1940). (82) Rase, H..P w . Ann.. 76, 1-28 (1849). (8.3) Rasenstein. L., U. 8. Patent 2,038,318 (1936); R e h w 20.380 (1937) and 20.764 11938). (84) Ridy, H., and Schloeknr. H.,Bw., 73,484-92 (1940).
(86) . .
Vol. 34, No. 1
Rudy. H.. Elohloesser.. H.. . and Wstsel. R.. Ammo. C h . .. 53.. 626-31 (1940).
(86) Wea, F. M.. Ann. Rapt. N. Y. Stats Aaroc. Dairu Miuc ks. 11. RO ( l a m . (87) Schwart., C., Ibid.. 14,267-7S (1940). (88 Sahwh. C., and Oilmore. B. H.,h. ENa. Cam., 26, 888-
................
1001 (1934). (89) Smith, G. W.. Am. L%.sWRaptr., 23.313-22 (1934). (90) Smith. J. H., J . Soc. C h . Id.,36,420-4(1917). (91) 8neU. F. D.. I m . E m . C u m . , 23,470-4 (1931). (92) 80ciht4 des phwhates tunisiena et des engrsis et produita chimiques. French Patent ffi6.905 (July 2,1926). 193) Teunaut. W. J.. British .-.. Patent 441.474 (1936). (94)Trax. E. C.. J. Am. Wfar W o r k Asam.. 32.14967 (1940). . . (9s) Treier. A&-. smp, 17, NO.6, za-9; 7 0 (1941). (96) Wwaman, W. H..and Essterwmd. H. W.."Phowhoric Aoid. Phomhataa, and Phosphate Fertitinera", New York, Chemic d Cstdos Ca.. 1927. (97) Wmen. B.E.. C k . Rm.. 2 6 . 2 3 7 4 (1940).
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DEFLUORINATION OF PHOSPHATE ROCK IN THE MOLTEN STATE Factors Affecting Rate of Defluorination KELLY L. ELMORE, ERNEST 0. HUFFMAN. AND WILLIAM W. WOLF Tennessee Valley Authority, Wilson Dam, Ala.
DOMEgTIC . phosphate . . rocka are essentially fluorapatite adrmxed mth various proportions of other compounds of calcium, fluorine, iron, aluminum, and silicon (1, 8). Since the greater fraction of the phosphorus is present &8 fluorapatite, the phosphorus of raw phosphate rock is not assimilated by plant life. If the apatite lattice is destroyed and the fluorine is eliminated, however, most of the phosphorus is made availahle. Various inveatigators (6, 7) have shown that the fluorine can be removed by heating the phosphate rock at temperatures above 130O0 C. in the presence of silica and water vapor; their work suggested the possibility of producing a phosphatic fertilizer by calcining the rock in a rotary kiln. Calcination experiments carried out in this laboratory, however, showed that defluorination below the fusion point of phosphate rock containing the oxides of silicon, iron, and aluminum lowered the melting point to that of a eutectic mixture which sintered. The sintering p r o w s caused the rate of removal of the fluorine to be prohibitively slow and favored the formation of rings in the kiln. Accordingly, studies of defluorination in the molten state were undertaken with the hope of eliminating the difficultieg arising from sintering and developing a prows suitable for largescale operation, in which the deflnorinated product could be tapped from the furnace. The kinetics of the removal of fluorine from the melt waa determined by systematically investigating the specific effects of the composition of the charge, the concentration of water vapor in the furnace atmosphere, the velocity of the furnace atmosphere, the temperature of defluorination, and the depth of charge.
Apparatus and Procedure The work was carried out by fusing S to 4apram charges in 60 per cent platinum40 per cent rhodium boats in a high-ternture furnace. eauinwd with a tubular silicon carbide heatins element 24
