DEVELOPMENT OF PROCESSES FOR METAPHOSPHATE PRODUCTIONS RAYMOND L. COPSON,GORDON R. POLE, A m
WILLIAM A. BASKWVILL Tann~ssaeValley Aubrity, Wilson Dam, Ah.
estimate &o* that the available P a s is xmewhat lower than that of concentrated (50 per cent available PO!) operstions,and in the textile industry. During the paat few superphosphate, when both are produoed from electno furnace phosphorus and when the quantities obtained are years, calcium and potassium metaphmphates have attracted equivalent. attention for use aa fertilirars. Some of the work of the TenIn the past four years the TVA has produced over 24,ooO V e Authority in amnection with calcium metaphostons of calcium metaphosphate f e r t i i . The product has phate has been described in previous publications (H,7, been teated and used on demopstration farms in every sadion 11, la). Potassium metaphosphate hss been studied by of the United States, by n b t e land-grant collegea and unithe Bureau of Cbemistry and Soils (8). In the plreent paper versities, by the Department of Agriculture, and by organhiprevious work is reviewed, and more recent research of the tiom of fmera. The renults of many of thw b t a are now TVA in developing profor the production of metaphok available. Inmost casesand pbates is describedfor most mila calcium me& The advantage of calcium phosphate has given reaults and pobmium metaphasD. .a fully equal to thm obtained p h a b aa f e r t i l k msterhb with superphosphate. Jacob lieu in their hi& Concenb SLna the expdmental production of cnlcium and Ross (6) reoently pubtion of plant nutrients. Pure m&aphosphate featilbr hy the TVA X M 6mt delished a Eomprahensive recalcium metaphosphate conscribed in 1931, a conaidemable -e of this maport on the nutrient value of tains 71.7 per cent P,O, and tsaiel has been produced and mppbd for tent and the phasphorus in m r a l pure potassium metaphosdemomtntlon in mew d n of the united new phosphatic materials,inphate contains 39.0 per cent StatW. cluding calcinm metaphosE@ and 80.1 par cent Pas. One metbod ofPpDduotb~,d d pmvbdy, pbate. Them authora conIn crude metaphosphatespreavwlstsofburnin(lphoaphrrtuwithatandbrtyclude that the msterials pared for fertiliser we, the ing the hot products of c o m b u s ~ u into contaet tested "were approximately pementagesof plant nutrients w i t h lump rock p h p b t m peaked in a vertical to superphosphate aa equal ordinarily are somewhat ahaft. A W n d method in being developed to BOUTCBB of phasphonu, for the lower. 6uch concentrates utUh phosphate s d . which w blown h t m the growth of planta on acid and convey more plant nutrient ahamba in which the phospaorOs is burned. In neutral soils". in the eame weight of maboth most of the flu& o f t h e mck ~40.Potassium metaphosphate e ta aa campared with less phate is volatilized, and fluorha compound. may has been produced in much concenhted fwtilhm, and be recavexed 8.a by-pmdncm. d e r quantity and han consequently such cwta as Similar method. alw IM being developed for the been available for test for a storape, hsndlinp, bagging, production of metaphosphatesof the alkali metals shorter pericd. Tests in the and tramportation are refmm their ehlorided with recovew of by-pmduot greenhouse and in the field duced. TheBavinginahiphyd8uchloria acid. In muu pilot plants in whieh have indicated that both the Ping apace W Y p b o i p a M u WM burned w i t h moist air, t k potassium and the phasimportant in times such aa diUtmnt method.of introdudng the d o r i d e w u w phorus in potseSium me& the prasent, when transhiedt 0)am a solid agglomemte packed in a phosphate are at le& 88 portation facilities are at a V d c d S h d h (a) . I Ithe fuwd M k h W h g OVIV a readily available to gmwing plWIliURl. &ctory paking material in the shaft, and (3) plants as equivalent quantiThe experimental prodnoan a powdslsd wlid hlom into the phosphorlu ties of the me materials in tion of calcium metaphosODmbuStion chamber. Satidrctor). operation WM the usual commercid fertiphate by the TVA wan first d e v d only in the third C ~ MPmdwtn . eonlima. The paper by Jacob described in 1837 (a). SubtJninlr metaphosphatm of both potacaium and Roaa (6) contains a reand sequently, the operation of a doium. and of vuisbLs composition. also wera ' view of the literature dcalfnll-male unit WEIS reported p r o d u d from mixturn of potaaum ohloride ing with the nutrient value (8). The second paper conwith either paaph.te # a d s or h a o n e . of the phosphorus in alkalitained a layout ot a 000nplet.e metsl metaphosphate. plant for the pmduction of
ETAPHOSPJL4TES am of increasing hprtsnos in M industry and in agriculture. Sodium metaphosphate is widely nsed in treating boiler feed water, in laundry
-
-
-
28
INDUSTRIAL A N D BNQINBERING CHEMISTRY
January, 1942
Preparation and Properties of Metaphosphates Metaphosphates may be prepared by any one of three general types of reactions,as follom: 1. By dehydrationof monobasic orthophc%phatesor p w phosphates-fm example, NaH,PO,
-
Napol+ Hlo
2. By addition of phcaphorun pentoxide to oxides, tribasic orthophosphates, dibaeio pyrophcaphates, etc-for example, C&@%)r 2px)1- 3b(P'&)r
+
3. By metathesis of salts with metaphosphoric acid-for
exampla, KC1
+ HPO.
-
KPO.
+ HCl
Metaphosphates are formed also when phosphorun pen& oxide react^ with sal@ of the halogen acids in the a k c e of water vapor. However, in this D B S ~a considerable part ofthe phcuphorun is volatilized as phosphorus oayhalidea and hal-
id-
(1.8).
Regotiom of the 6mt type are readily carried out at mcderate temperstures. Thus,sodium metaphosphate can be prepared by heating monoBodium orthophosphate at about 275' C. (IO). Monodcium and monopotwdum orthophosphatm also may be dehydrated at relatively law tempera-. If the heatim of the nure d t a is &ed above the m e l h p o k of the metaphospbatas and the melt is moled rapidly, a viitreous or Blassy product h readily obtained in the caw of ddum or sodium metaphosphate. G k y sodium me& phoephate chmgea to a crystalline form on heating at moderate temperatures (IO), and the same h true of d c i u m mekphosphab. The crystalline metaphosphates also are obtained when the fd material is cooled slowly. mente in whiab fused potanium metaphcapbate m d e d rapidly prnduced, not a vitreous material,but a whits cryst$line solid. "he melting points of durn,sodium, and p o b sium metaphcaphateu am 97S0 (4), 610' (IO), and 807' C. (8).rrspeotidy. Since oonmtmted s u p r p b ~ h bconsista @ci+ of
monodcium orthophosphate, it may be fused and rapidly moled to pmpare a crude, vitreous dcium metaphosphate. In the caw of resotiom of the m n d and third types, it h advantsgeaw to start with phosphorun. In this procsss the phosphorus is burned with air to phosphorun pentoxide, and the hot combustion gases are bmught into contact with phw phate rock. The hest of combmtion of the p h o s p h m h utilised, and no other fuel is required; also, the plant otherwise d e d to convert phosphoms into pbaephoria acid is eliminated. For resctiom of the third type, su0icient w a k vapor must be supplied to hydrate the phosphorus pentoxide to metaphosphoric acid, and the aeid conwponding to the salt used may be r e o o v e r e d as a by-product. The work d e scribed in the following sections of this paper relatea to the d e velopment of methoda and apparatus for carrying out pmoe88es of the m u d and third type. Calcium metaphosphate can be produced hy the addition of phosphorus pentoxide either to lime @&ne) or to phos phate rock. "he former yields a product of the greater purity; but the latter ordinsrily is the more economical, since, as has been pointed out (a), part of the phosphorun pentoxide in the pmduo%comes from the phosphate rock, which is the cheapat of all BDWWE. Since the prinoipd p h phatio component of phosphate rock is a calcium fiuophos phate, the latter process involves a comhiition of reactions of both the Bamnd and third trpes mentioned above: CawFdPOI). 6P~0, 2apo. 10CaWh WS The chemical and p h f i d properties of d e , vitrema dcium metaphosphste produced from phosphate rock haw been studied in some detail (4, '7). The prcduot is almost completely solublein ammonium citrate solution and in many dilute acids. Thee solubility of the 6nely gmnnd pmduot in water varies widely with the ratio of PI06 to CaO. produata in which this ratio is helow the theoretical Value tsnd to be nmly insoluble in cold water,w h e w the more saidio produota when 6nelygronnd may dissolvealmost mple.telp. Glaany calcium metaphoqhate in whiab the mole ratio of Pa,to CaO is lcss than 1.0 &om low moisture abmptdon capacity.sindlittletendanaytoc&ewhen6nelygmd. When the ratio is high, thematmid abeabs moisture rapidly and
+
+
-.
+
28
.
Vol. 34. No. 1
INDUSTRIAL AND ENGINEERING CHEMISTRY
tends to cake. It haa been shown that t h w p r o p e r k are dected to some extent by the proportions of silica, iron oxide, and alumina which mpresent. CRlde calcium me& phosphates of high silica content have been shown to be more readily soluble than thoea low in silica. In the absence of silica,iron oxide and alumina tend to decrease the soluhility and the hygracopicity of the pmduct, whereas silica tends to offaet this efiect. The presence of these oxides a h a p p m to inhibit the tendency of the vitrw~uscal@ummetephosphate to crystalk. The propwtiea of the high-temperature form of potassium metaphosphate were studied by Madorsky and Clark (8),who found that the product waa low in moisture ahsorption eapacity at and below a &tive humidity of 81per cent at loom temperature, and contained between 87 and 98 per cent of the potne8ium and phasphom content in water-imlnble although plant-available form. The crude potassium me& phosphate made in the pilot plant, 88 described below, had similar propertiee. Products in which the ratio of Pa‘ to KsO w&8 cloae to the theoretical value s o l i W on cooling to a wbite, 008mdy crystalline material. Photomicrographs of this product are shown in Figure 1. Samples containing an were observed to consist in part of vitreous maexceea of P201 terial. Potaaaium metaphosphate containing a small percenb age of calcium metaphosphate was a more finely crystdine and less hygroscopic product than the purer material.
Furnace Utilizing Lump Phosphate R o d The equipment developed by the TVA for producing calcium metaphosphate from phosphorus and lump phosphate rock waa descrihed in detail in two previous papers (2, 8). The equipment consistsessentially of a comhustion chamber in which phosphorus is burned and a vertical shaft into which lump phosphate rock is charged. The hot gases from the comhustion chamber pasa upward through the vertical shaft, and the phosphorus pentoxide reacts with the phosphate rock. The calcium metaphosphate formed in the shaft melts and then drains into the combustiofi chamber, from which it is m o v e d periodically. One of the main problem in connection with thin type of furnace was the development of a means of supporting the phosphate rock in the wertical shaft, at the same t i e p m riding adequate opefor the comhustion gaeea to p a s into the shaft, and for drainage of the molten hut viscous calcium metaphosphate. The materials of construction must resist the corraive action of the hot phosphorus pentoxide and of the molten calcium metaphosphate. Experience obtained since the second of the previous papers waa published has shown that the type of rock mpport described in that paper aa the ‘Istest design” haa given satisf&ry aervice. In thin design the rock supporta were oonstructed of 8 cast aluminoUS dractory material, cooled by emhedded water pipes and faced with zircon refractory b l o b . In a further improvement of the design, the water-mled tubes were i n 4 in graphite de8ves. Some of the factors decting the rate of reaction under the conditions prevailing in such a furnace have been studied experimentally (6). Although the situation is complex, it is clear: (a) that the optimum temperature in the reaction mne is approximately looOo C.; (a) that the rate of reaction st this temperature is very rapid, and 99 per cent of the phoephom pentoxide originally present in the gas is absorbed hy the phmphate rock in Less thanone second of contact; (c) that the rata of absorption is dected by the gas velocity to a minor extent and by the concentration of phosphom penta i d e in the gas, being proportional to the concentration under some conditions; and (a) that inmessing proportions of silica snd clay in phmphate rock tend to dethe rate d abmrption of phmphom pentoxide.
In the operation of the furnace the composition of the product is controlled largely by temperature. As phosphom
pentoxide is absorhed by the phosphate rock, products having continudly lower melting points m formed. The melting point curve of CaO-PrOr compounds (4) is reproduced in Figure 2. By maintaining the temperature in the reaction mne at shout the melting point of calcium metaphosphate, absorption of phosphorus pentoxide proceeds until the surface layers of the lumps ib the active m e of the shaft have the
FIQURE2. PLETE
TEMPERATWE OF COMFUSION OF PWE CaO-P208 COMPOSITIONS
composition of calcium metaphosphate, which then melts and drains away. However, because of the shape of the melting point-composition curve and because of variations in the percentage of silica and other constituents, the composition of the product is not fixed accnrately by the operating temperature. Consequently, it is neceasary for the operators to acquire a certain amount of skill and experience in order to control the composition of the product within the desired range. The full-scale experimental plant, known aa TVA Metaphos Unit No. l, has now been in continuous operation for periods totaling over tluw years. Operating data taken d w ing a 15day test period follow:
The lump phosphate rock charged during thii test waa mostly to 3 inches in size. The product over a period of nine month had an average analyein of 65.5 per cent Proaand 25.4 per cent CaO, which corresponds to a P&CaO mole ratio of 1.02. The fluorine content of the product during the m e period averaged 0.6 per cent. However, at other times the fluorine in the calcium metaphosphate waa aa low 88 0.2 per cent. The latter 6gum corrwponda to volatilization of approximately 90 per cent of the fluorine present in the raw phmphate. As might be expeded, the proportion of fluorine volatilimd increassd with an incregse of the amount of moisture supplied to the phwphom comhustion chamber. An analysis of a typical sample of calcium metaphmphata is given below (in per cant): Pan
CsO
M.O
aa.0
Si@ 7.1
F& 1.u
&Os 1.7
&O 1.0
F
Otba
0.9
0.0
JMW,
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
Proms Utilizing Phosphate Fines The process embodied in Metaphos Unit No. 1 requirea natural lump phosphate rock, or agglomerated phosphatic mateaisl. since much of the phosphate rock now mined ie m w w d aa h e s or mds, and since the agglomeration of such materials adds to the cost of operation, it in evident that B method permitting the direct utilization of phosphate sands or 6nea would posseae a d ~ ~ ~ ~ t s g e s . P I Wsxperiments, ~ made early in 1937,demonstmted that fused calcium metaphosphate could be prepared by blowing phosphate hea into a chamber where phosphorua waa burned. A pilot plant was designed and constructed for the pmluction of cslcium metapbosphate from phosphate clues. A diagram of the plant in shown in FEgnre 3, and a photograph in Figure4. The pilot plant consisted eesantially of a furnace having a horiwntal combustion chamber connecting with a vertical reaction chamher and, mounted above the latter, a cokepacked tower. The combustion and reaction chamber and the packed tower were for the most part lined with dense, stifl-mud, fire-clayre.fractories. The thermocouple proteotion tubes, the lower two c o r n of brick on the combustion chamber walls, the dam acm the bottom of the combustion chamber, and the tapping block were made of Simon dractory which had been shown in testa to be resistant to molten calcium metaphosphate (11). It waa observed after operation that the zircon was much leas aeverely attacked than the s M mud r&actoriea. The water-cooled grates used to support the coke packing wem made of stainleas steel (American Iron and Steel Institute type 304) tubes covered with graphite dwe. A short section of the tower just above the grateswaa lined with carbon brick. Both the graphite and carbon were d e c t e d during operstion of the pilot plant. Phwphom waa supplied from steel drums, i m m d in hot water tank%,by displacement with hot water and was burned in the combustion chamber. Phosphate h e s were supplied from a storage bin by a feeder, paesed through suitable feed pipe, and blown by o o m p d air through three openings
29
arrsnged oimumfarentislly m u n d the top of the reaction chamber. The hot gases containing phosphorua pentoxide from combustion of the phosphom t h w paesed upward through the reaction chamber oonntercurmnt to the falling phosphate partidea. Water was intduced at the top of the de-packed tower, and part of the phmphom pentoxide wan collected as acid in the tower. The partidy reacted phosphate particlea, and the acid from the coke-pscked tower fell into the fumaoe where the chemical reaction wea completed to form molten calcium metaphosphate. The dcium metaphosphate waa tapped from the fumaoe periodically. The gaa leaving the top of the &packed tower contained water vapor and =me entrplped phosphoric acid. The gas wan p a s 4 through a cooler, where an additional amount of dilute phosphoric acid was collected. This acid was recycled to the packed tower, being intmluced either at the top or at a point just a b e the water-cooled grates. Several diiTerent coolers were tried. In the one which p r o d to be most s u d u l , w a t e r - d d Btainleas steel tubes covered with graphite dewen wem used. The pilot plant wan operated successfully at production rates of about 5 tons per 24 houra In o p t i o n the tempem ture in the combustion chamber was maintained at about llOOo C., at the top of the reaction chamber the g a t~empem ture was about 950° C., and the exhaust &as temperature leavingthepackedtowwaaabout195'C. A t a l l t i i t h e r e wan a pile of reacting and fusing material on the floor of the reaction ahamber. By suitable control of the temperature, this pile wea maintainedat approxhately eonstant size. The desired composition of product thus was obtained by controlling the relative rata of supply of phosphom and of p h w phate hes, and through control of the combustion chamber temperature. Experience indicated that it was easier to control the compdion of the product in this type of operation than in that of the No. 1Metaphos Unit. The metaphosphate produced waa similar in composition to that from the No. 1 Unit, although the average fluorine content was somewhat lower. Experimental data taken during a d a y test of this pilot plant are aa follom:
The phosphate hea charged during this test were lees than I/. inch in si=. Following the suc&ul operation of the pilot plant, an experimental f u l l d unit dwigned along similar lines was constructed. This plant, known aa TVA Metaphos Unit No. 2 , h been in operation for a short time only. ___WATER-COOLED
PHOSPHORUS PHOSPHORUS
I
DAU
FUSED YETAPHOSPHATE
Cooling, Storage, Grinding, and
w&%
Whether calcium metaphosphate be produced from lump phosphate rock or from phosphate tinea, it leaves the furnace as a fused &dike material. various methods of cooling and handling the product have been tried. The Simplest method consisted of tapping the fusedmaterial onto a hearth in layem 1 to 2 inches thick. On solidifying and owling, the p d u d frsctured into pie-cw averaging 1 to 2 inahea in maximum d i m d o n . Theae pieoes were then ahoded from the hearth and handled in trU& or O t h e r w k . This procedure waa followed in m& of the pilobplant work.
.:..
1.
.
.
. . . . ,:
,,..;
. ..
~ .:." : 3 N D U S T R I A L A N D E N G - I X E E R I N G C H E M I S T R Y
In order to obtain the product in the form of smaller piece8 and thus reduce grinding costs, quenching of the molten metaphosphate with watar was tried. The Btream of molten metaphosphate was broken up into small pellets by means of a water-cooled r o t a t i i paddle wheel. Water sprays were provided for cooling. This method gave promise when d i n the pilot plant and was first installed aa part of Metaphos Unit No. l. In the f u U d e plant the pellets from the paddle wheel were projected into a revolving steel drum. However, it waa not found poeaible to quench the product sstisractorily without at the m e time producing a wet matarial which tended to hydrate and set up in storage. Cones quently this method of cooling waa abandoned. The apparatus waa then mcdi6ed so that the molten me& phosphate was allowed to flow into the ineide of the revolving steel drum, which waa oooled by water sprays on the outside. The drum waa rotated very slowly, thus causing the metaphosphate to solidify in a layer adhdng to the inside wall. When the layer cooled, it cra&d and fell away from the wall, yielding a product in lump form similar to that obtained on the hearth. The product from the cooling drum was d e l i d by B buoket elevator into a storage hopper. This method has been used Batiifactorily during most of the time the No. 1 Unit has been in operation. A somewhat diflerent arrangement has been installed for the No. 2 Unit. In this CB%B the molten metaphosphate is tapped directly onto a moving pan conveyor, the pa- being cooled by water sprays directed on the bottom. The metaphosphate remains on the conveyor until it is cooled and is then delivered directly into storage bins. Unlike superphosphate, "curinp" of calcium metaphas. phate is not nece8882y, and storage is solely a matter of condeuce.
The possibility of storing vitreous, lump calcium metaphosphate outdwm waa suggested in a previous paper (8). However, aa d i d above, the stability of vitreous calcium metaphosphate toward moisture varies markedly with the compositjon. For example, a teat waa made on two m p l e a of vitreous, lump calcium metaphosphate; one had a PzOr G.0 mole ratio of 0.94 and the other of 1.26. They were stored outdoom in layers about 3 inches thick for 368 days, during which the total rainfall waa 39 incheo; the former eample retained its lump form although the lump became coated with a thin white layer, whereas the latter eample d i s integrated into a porous mass and most of it leached away. Quantitative data on the Iowa by leaching in this test am given in Table I. It a p p m from t h e e data that vitreous, lump calcium metaphosphate having a PIOrCaO mole ratio of less than 1.0 might be exposed to the weather for periods of a few weeks without appreciable loss, but that long-time outdoor storage is not practical. At the TVA plant the lump calcium metaphosphate is stored under cover, and is ground, screened, and bagged prior to shipping. The grinding is done in a hammer mill. The equipment waa desarihed and illustrated in a previous paper (3). A typical screen analysis of the ground product follows: Cumulative Per Cant Throw& lCO.mesh aoc-m-h
ZO-md 95
Vol. 34. No. 1
I
24
12
The tendency of the product to cake when 6nely ground
has been noted. The d i n g apparently reeults from the a b sorption of atmospheric moisture. Since the hygroscopioity of the material varies with the composition, aa discussed above, products havihg a P,O,CaO mole ratio somewhat Mow 1.0 show much lass tendenw to cake than do those having mole ratios above 1.0. Even with the more bygrp scopic products caking may be prevented by the w of conditioning agents in suitable proprtione. It has been
found that the addition of 100 pounds of ground limestone per ton of metaphosphate ordinarily prevents caking. k t ment of the finely ground metaphosphate with a few pounds of ammonia per ton is slso etrective, the treatanant being carried out in a manner similar to the ammoniation of super phosphate.
T A BI.~ hams BY h c ~ L ? ~OF c i P a s F R O ~LUMPC m ~ m M E T A P ~ O S P EEXPOSED A~ TU m W m m a Sample A
c 84
R O r C s O mole n t i o P*O,loet 85 dam, 9 P,O&I d 368 dw..
1.8
%
la
8ampla B I a6
a8 94
Production of Alkali-Metal Metaphosphates The production of other metaphosphates by prooesees similar to those used for calcium metaphosphate waa suggested in an earlier paper (8). In particular, the production of metaphosphates of the alkali metals, by reaction of their chlorides with metaphosphoric acid at temperatures ahve the melting point of the product, appeared of intarest. This re action is of the third type mentioned above. The prcnpect that the reaction would go substssWy to completion, with formation of alkali-metal metaphosphate and by-pmduct hydrochloric acid, was confirmed by the finding of Madorsky and Clark (8) that, when mixtUrea of potassiUm chloride and orthophosphoric acid were heated at 700" C. or higher, the products contained relatively little free aoid and unreacted chloride. As compared with phosphate rock as a stating material for production of metaphosphate, the W-metd chloridea, baoaw of their lower melting points and higher volatility, present quite di5erent problems. This is evident from the followingdata which oompare the melting points of phosphate rock, sodium chloride, potassium chloride, and the c o r n sponding metaphosphates: Tenn. B m m PhWDhk W 1 m e c. (.*DrOX.)
Ca M a t . p k h t 4 078*
c. (4)
NaCI 801-
c.
N. M&DhWDht4 610*
c. (10)
KCI 790' C.
K M&SDhmhh 801. c. (a)
In the first prooess developed for making csloium metaphosphate, tbe products of oombustion of phosphorus were passed upward thmugh a bed of lump rock phosphate, molten calcium metaphosphate being formed and draining from the tower. This procedum obviously d d not be used for malring potsssium metaphosphate from potsssium chloride hecaw the melting point of potsssium chloriae is lower than that of potaeaium metaphosphate; furtbtamon, a eutectic mixture of the two materials conbhbg 13.6 per cent potasmum chloride waa reported (I) to ham a still lower melting point (620' C.). However, the packed-tower method aaemed f d b l e for the production of d u m metaphosphate fmm sodium chloride. This procedure was tried on a s o d l pilot-plant scale. Ele mentd phosphorus was burned, the phosphorus pentoxide waa partially hydrated. and the hot gases were passed upward through a tower packed with lump sodium chloride. A BBcond tower waa provided for cooling the gases and a b r b i i the hv& chloride reSUthn7 fromthe d o n . Fused m d &a Gpmsching the oom&tion of pura d u m &LW phate (30.3 per cent N d . 69.7 per mt P&) were prepad, hut all of the ssmplas contained either an exwen of P a ,or up to 10 per cent of uurewted d u m ahloride. However,
J a n w , 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
it appeared thst the composition could be held closa to that of sodium metaphosphate by Sursciently accurate control of the operating temperature. The crude d u m metaphosphate solidilied in the well-horn glassy form when cooled rapidly, but waa cryatdine when cooled alowly. In the next aeries of experbmta, also on a small pilob plant scale, it waa proposed to taka advantage of the relat i d y low melting point of poksium cbloride. The salt was melted in a small electric fumaoe and caused to flow down a vertical shaft packed $vi& mfracbry material,c o u n t e r c m t to Bdcending hot gssaa from the combustion of phosphorus with moist air. Fused material waa produced approximating the composition of potassium metaphosphate, but because of operating di5dtiea the composition varied widely. Considerable diffirmlty waa encounted in preventing the liquid potassiumchloride from mlidifying in the overflow tube from the melter. To overcome this Wculty, the top of the reaction tower was operated at a te~~perature considerably abow the m e l h point of the salt, but a large amount of v o l a t i l i i material, containing about half of the potassium and of the phosphorus introduced, went out with the gaeea from the reaction tower. Very corrosive conditions existed in the reaction tower, and of three tspea of relractaries wed a . 8 pa-, only zircon gave satisfactory service. The loss of material by volatilization in theae experimenta probably could have been overcome. However, no further work waa done w+th this method of operation because of the concurrent development of apparatus for producing calcium metaphosphate from phosphate Iks, and because it appeared that potassium metaphosphate could be made from potassium chloride in very nearly the eane apparatus. Two other methods for the production of metaphosphate of the alkali mefala from their cbloridea were considered and &odd be msntioned. One of these wnaisted of wporiaing the chloride and bringing the vapor into contact with the hot producta of combustion of phosphorus. The other waa d e scribed in a German patent (g), which claimed a batchwise prowae of introducing phosphorus pentoxide vapor and stesm into molten sodium cbloride. Neither of theae methods waa tried because of apparent practical di5dties.
Experiments w i t h Powdered Potassium Chloride The pilot plant (Figum3 and 4) for the production of calcium metaphosphate from phosphate 6nes waa used for experimenta with powdered poksium chloride. Minor m o a cations were made, but the gemeral arrangement and the ma&&t of oonstruction were similsr to th- of the calcium metaphosphate pilot plant. In addition,means were provided for &er and disposing of the by-pi-cduct hydrochloric acid. The normal operation for producing potassium metapbw phate waa 88 follons: Phosphorua waa burned with the pmper amount of air to maintain a temperature of 1OOO1050' C. in the combustion cbamber and 800-900°C. in the reaction chamher. A commercialgrade ( W l per cent Ea) of h e potassium chloride (98 per cent minuus 14 mesh) was Mown in at the top of the reaction cbamber at the proper rate to give the desired P20rKs0ratio in the product. The h e pOtaaeium chloride waa intwdueed at two d i a m e t i d y o p pwitefeedholes by meam of 4air jeta which Bervedtodistribute it evenly thmughout the reaction chamber. Watar wee added at the top of the pscked tower. The gasee left the packed tower at about 100' C. and then p d to the wler where they were cooled to 70-80" C. More going to the hydmchloric acid mcovery system. The m d phos phorio acid from the d e r waa r e to the paokeed tower a h the water-cooled grate. The molten product collected in the combustion chamber and w& tapped from the furnace
31
every two hours into a water-cooled steel pan where it was left to solidify. An example of the data obtained from a run of 78 hours, in which 14 toas of potsssiUm metaphosphate were produced, is given in Table 11.
TAB= 11.
OEEBATTNO DATA (7aHom TNT) FDB TES POTAS. SIM METAPEOSPEA~ PILOTPLANT
The percentage of residual chlorine in the crude potassium metaphosphate made in the pilot plant tended to he somewhat higher than tht in similar producta made on a laboratory scale by Madoraky and Clark (8). During operation of the pilot plant, a total output of about 65 tons of potassium metaphosphate waa made, with the composition varying from 55 to 58 per cent Pror,35 to 38 per cent KO, and 1 to 4 per cent chlorine. The crude product waa ground in a hammer mill to a fineness of 100 per cent through a %mesh screen. pmducta contaking metaphosphates of both potaesium and calcium, and of variable compxition, also were p d u c e d from mixturn of potassium chloride with either phosphate sands or limestone. Anslysea of samples of typical mated~, one produced from potasSjnm chloride and the aecond fmm a mixture of a dproportion of phosphate 5 e s with potwsium cbloride, are given herewith (in per cent):
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
I(I0
OIO
M.S m.4
88.1 a6.a
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). Jacob, K. D., and Roas. W. H.. J . A v . Re&, 61. 638-80 f,oAn> \ _ "__, . MsoIntire. W.H., Hardin. L. J., and Oldham. F. D., h. Eaa. Cmx., 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.
Phosphate compounds had established position BuseYin.ofthehydrated field of water treatment, largely due to the wide trisodium phosphate in 1930
an
(NaIPO4.12H20)
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, orthophosphoricacid 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
