Manufacture, Handling, and Use of Hydrocyanic Acid P. J. CARLISLE R. & H. Chemicals Department, E. I. du Pont de Yemours & Company, Inc., Niagara Falls, N. Y as assurance that the liquid is in satisfactory condition a t the time of observation. A sample of liquid which eventually decomposes Usually remains colorless over an i n d u c t i o n period the length of which varies with temperature, pressure, and the n a t u r e of a n y f o r e i g n substances which may be in the liquid. The first and unerring i n d i c a t i o n of decomposition is a faint yellow color which, under O r d i n a r y conditions Of FREEZING s t o r a g e , gradually deepens in HC?; TEMP intensity, finally passing through k " C 100 -14 86 yellowish brorvn, dark brown, -21 90 and b r o w n i s h black s h a d e s . 80 -23 60 - 17 During the e a r l y s t a g e s the 40 -16 20 13 odor of ammonia f r e q u e n t l y 10 - 8 can be d e t e c t e d , and in the The latent heat of vaporization a t the boiling point is ap- later stages a black substance is precipitated, the so-called proximately 247 gram-calories per gram or about 445 B. t . ~ . azulmic type of compound, which has substantially the same ultimate analysis as hydrogen cyanide itself and which is reper pound ( 2 ) . The vapor pressure of hydrocyanic acid has been reported ferred to here as hydrocyanic acid polymer. Gases are liberby Sinozako, Hara, and Mitsukuri (S), Perry and Porter ( 2 ) , ated consisting largely of ammonia and carbon monoxide with and Bredig and Teichmann.(I). The vapor pressure curve smaller amounts of nitrogen, oxygen, hydrocarbons, and hyshown in Figure 1 was plotted from the data of Sinozako, drocyanic acid. Mixed in the hydrocyanic acid polymer can Hara, and Mitsukuri (3) because of the temperature range be found also colorless crystals of ammonium cyanide. The phenomena of decomposition are covered. The relatively slight discrepanstrongly e x o t h e r m i c and the process is cies in the results from the above three inI ! ! ! ! ! i ! ! ! ! I ! ! / autocatalytic. If the liquid is s t o r e d a t vestigations are not regarded as of importance to the chemical engineer. atmospheric temperature under such conditions that the heat of r e a c t i o n is completely dissipated by radiation, it is posDECOMPOSITION OF LIQUIDHYDROCYANIC sible for t h e polymerization reaction to ACID occur almost completely, with little gas formation and little increase of pressure in Walker and Eldred (4) have described the container. However, if the heat is inthoroughly the decomposition of liquid hycompletely dissipated, the increased temdrocyanic acid. Pure liquid hydrocyanic perature may accelerate the decomposition acid is a very stable substance, but aqueous solutions are less stable. A trace of mineral r e a c t i o n s , the process quickly gathers acids, notably sulfuric, renders aqueous momentum, and the liberated gases may s o l u t i o n s more stable. Any alkaline or cause a sudden rise of p r e s s u r e to more alkali-forming substance, added to liquid than 1000 pounds per square inch with hydrocyanic acid or its aqueous solutions, possible rupture of the container. Several greatly accelerates the rate of decomposicases of such phenomena are reported in tion. Such substances as ammonia, sodium the paper by Walker and Eldred (4). hydroxide, sodium cyanide, and soap beVarious stabilizers for liquid hydrocyanic long to this class while, as noted above, acid have been d i s c o v e r e d , and many even water renders hydrocyanic acid less samples of stabilized product have been stable. Decomposition and polymerization preserved satisfactorily over periods of a appear to proceed simultaneously, but the feT7 years. Sulfuric acid to the extent of combined process is referred to simply as about 0.005 p e r c e n t b y w e i g h t is the decomposition. stabilizer commonly used in the factory d e s c r i h e d here and is quite effective for Liquid hydrocyanic acid is n o r m a l l y FIGURE1. VAPORPRESSURE OF the conditions encountered in the citrus colorless, and absence of color can be taken HYDROGEN CYANIDE (3)
H
YDROCYAKIC acid is a colorless, mobile liquid with a b oil i n g point of approximately 25.7" C . a t 760 m m , a n d a f r e e z i n g point of about -14.86' c. ( 2 ) . The specific gravity of anhydrous liquid hydrocyanic acid is 0.682 at 25" '*, and Of the 97 per cent aqueous solution, used in citrus fumigation, is about 0.692 a t 25' C. ( 5 ) . The freezing points of aqueous solutions are as follows:
Hydrocyanic acid has been used in the fumigation of citrus trees f o r control scale insects since 1886'. u n t i l about 1917 hydrocyanic acid was by generating the gas in the Orchard f r o m sodiumandpotassium cyanides andsulfuric acid. I n 1917 liquid hydrocyanic acid was intraduced commercially in California, and a f e w years in that state the practice of generating hydrocyarlic acid f r o m cyanide and acid in the orchard had been displaced completely by the use of liquid hydrocyanic acid. This paper mentions some of the pertinent properties of liquid hydrocyanic acid, and describes one of the factories in Chlifornia f o r making it and the methods of handling and using this material in the orchards.
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fumigation industry of California. Nevertheless, frequent checks are made on the color a i d acidity of liquid Iiydrocyaiiic acid in storage, and any liquid displaying the slightest yellow color is given a protective a d d i t i o n of suliurir acid arid is “reworked” by passing it back t h r o u g h the factory purifieat.iori process. Traces of a l k a l i n e s n b stances are so potent t h a t practically e v e r y t h i n g used iir handline or testinv the liijui;i in the Iahc>I ratory and plant, after c l e a n i n g , is rinsed m4t.h sulfuric aeid solutioii. 1,almratory glassware is s(mi(itimes soaked for hours in a.(.id solutiim before using, especially are involred. In the factory, ii a unit i s started in operation after a period of iillnness, it is first rinsed thoroughly with dilute acid soiubiim. Shipping drums arc always w a s i d , inspected interiinlly, t d a t 10 to 1 5 pounds per square inch pressure, and rinsed with dilutc acid a h t i o n before thev are filled. A peculiarity < t i p o l g r i i ( or “decomposed” Iiydrocyaiiic acid is introduced by th that tlie volume of the solid, black product formed is greatcr than that occupied by tlie original liquid and by the tendency of the black m a s to expand in directions of greatest resistance rather than in directions of least resistance. In tiic laboratory t,he lower p r tiou of an open glass bottle of deeomposed iiydri~cyanicacid was literally yiisiied off even though there was ample room for expansion in tlie upper part of tlie bottlc. In the mrly days of tile factory, in a vertical sliell-and-tube ciiii~lt~n~er in which cooling medium floved through the tubes and liyiirocyaiiic acid vapor vas coiidensed at atmospheric pressure iii tlie space surrounding the tubes (the revme oi present practice), the black decomposition or polymerizatioii product accumulated on top of tlie bottom tube slrcet t o depth of several inches, and expansion of the inat,erial proiluced liuge bulges in the alumirrum shell wall and, in some cases, upt ti ire. It also contaminated the liquid hydrocyanic acid, producing off-color and an unstahle product. Other diffieultics in tlie shape oi plugged pipe lines, condenser tubes, and pintially obstructed still plates were caused by tlie polyrneri~,ation product. These difficulties, of course, lime long siiice brim eliminated. Toxiwry OF i l u ~ ~ ~ o c ~An~x: Ii Dc The toxicity of hydrocyanic acid to most forms i,S life is too well k n o m to require discussion here. ?;atorally in the early days of the factory many of the problems vere assmiated with the safety and health of persmncl. TVliile in former years operators were occasionally overcome, in this factory not a single man has lost his life or had his iiralth permanently impaired. TlIE hfANUFACTCIllhG I’IlOC
CYANIDE Soimriox. Cyanide i s dissolved ill vertical steel cylindrical tanks approximately 9 ieet in diameter and 4.5 ieet high. Each tank is equipped ~ i t ha perforated steel basket
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auppwtcd 18 inches above t h e b o t t o m . The b a s k e t is a p proximately 8 feet in d i a m e t e r . Taiik c.overs are of vood and contain opcnings for c h a r g i n g t h e basket with cyanide. The tank is charged with 1250 gallons of rrat.er arid 5 0 0 0 piruiids of sodium cyanide. Rapid solution is effected by circulation caused by diffcrence hetween t h e specific g r a v i t i e s of cyanide solutions and of water. The time r e o u i r e d is 2 to 4 hours, depending on the t e m p e r a t u r e . Thc s~. o l.n t h~eont,siw ..~ ~ ~ approximately 32 per cent sodium cyanide. The solut~ioois transferred by a 2-inch, all-iron ccntrifugal pmnp to a stock tank where it is diluted with water to a conccntration of about 23 per cent sodium cyanide. Mixing is effected liy a small nrecliairical agitator. This solution is pumped to a feed tank from which it Horns by gravit.y through a flowmeter t o the hydrocyanic acid gmerator. SULFURIC ACID. The 66” BA. sulfuric acid flows from a steel shrage tank to a cast-iron egg and is lifted by air pressure to rlex,ated steel feed tanks. The acid then flows by gravity through :t floiTmeter to the hydrocyanic acid generator. TIE fiow oi cyanide sdiitiim is adjusted to give aiiy desired rate of Iiyili.oi.yanic acid pnxiuction. The sulfuric acid flow is adjusted trr maintain about (4.5 per cent by weight of free suliuric acirl in the residue or sodium sulfate solution from the gcneratir. ltesidue samiiles a.re tested at frequent intervals for free sulfuric acid and for hydrocyanic acid content, and, if necessary, tlic rate of flow of sulfuric acid is adjusted. It is important that a slight excess of sulfuric acid be maintnincd a t 811 times in the generator. If even for only a few minutes blrcre is a slight deficienry of sulfuric acid with respect to the cyanide solution, the alkalilie condition results in the lih:mtion of ammonia which induces polymerization of liydrocyanic acid, which, once darted, is certain to result in a temporary shutdown for cleaning and acidifying the stills arid condensers. Polymerization products catalyze further polymerization of hydrocyanic acid, and at certain points in the stills arid condensers the products accumulate and impart a yellon-ish bron-n color to the hydrocyanic acid after it i s liqiieficd. Sucli discoloratioii is accompanied by poor stability and is a serious matter wliich must be corrected a t once. Discolored 1iydroc.yanic acid must be reworked hy acidifying it with sulfuric acid and passing it back through the generators, stills, and condensers. Sulfur dioxide has been found to be a powerful stabilizer for hyrlrocyanic acid and the difficulty just described has been to a considerable extent eliminated by the practice of ailding a small amount of sodium bisulfite to the sodium cyanide solution (ahoot 4 pounds of bisulfite per batch of cyanide diition). The small percentage of sulfur dioxide thus maintained in the vnpors passing tirrougb the system has to a large extent eliminated the above difficulty. TEEHYDXICYANIC Acrn GENERATOR.The generator consists of two parts, called, respectively, the mixer and the ~
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s t e a m e r (Figure 2). All internal surfaces are covered with antirnntiiallead. Tlic mixer coiisist.s of a s t e e l f u n n e l whose diameber at tlie top The is 5 feet. hciglit of t l w funncl is a p p r o xi 1x1a t el y 4 feet 8 inches, and tlie diameter of the hottorn stem or disc h a r g e is 6 inches. The cover c o 11 t a i 11 s coniicctioiis for cyanide am1 acid s o l n tions, vapor o u t l e t pipes, sight g l a s s e s , thermomet,er well, ete., and also carries a iriec'lianical agitator. Supported n e a r t h e top of tlic funnel is a sliallow lead mixing basin, 36 inches in diameter a i d 4.5 inahcs deep. To tlie periplieiy of t,liis basin is secured a lead apron. The vertical shaft. of the agitator carries a three-bladed propeller, 20 inches in diameter, which rotates a t a speed of 90 1. p. ni. in the mixing basin. 'rhe cyanide and acid solutirms are led thsough tlie cover tlirougli lcad pipes externling to a point j u t above the propeller arid at tlic tnid~llcof ilie mixing basin. The solut,ions are mixed very quickly, omrflow from the basin ani1 down over tlir n l ~ r i i n nnd , spill on to the sloping wall of tlic fuiinel. Rapid iiiixing of the cyanide arid acid sohitiuns is inrportarit since it prevents loeal overheating and hydrolysis of hydrocyanic acid slid makes possible a high hydrocyanic acid recovery efliciency. As the solution flows down the ~ m lriEl the funncl in a film, about SO per cent of the hgrlrocyaiiic a,cid is liberated and escapes tlirou~lia lead vapor iiutlct pipe. The solutiun leaves the funncl by way of it 6incli elbow and flows into tlie stearner, tlie function of diicli is to remove the remainder of tlie Iiydrocyariic acid. The steamer coiisists of a horizontal, lead-lined, steel trough about 12 ieet lotig whose cross section is nppnximately that of an cqriilatoral triangle with 2-foot sides. The outlet end is cquippcd with a discharge coiiiiection niailc adjustable for regulating the dcptli (if liquid to be maintained in the steamer, whicb usually is 10 inclres. The flat cover is equipped with twvo sight glasses, six thermomcter wells, six connections for aciinitting steam through perforated lead pipes whicli lie in the bottom of the trough, a 4-inch vapor outlet connection, and near the liquid inlet end :in ope16ng for admitting rctiirn liquors, which will he referred to later. Cyanido and acid solutions, as they oiiter tho mixing basin, are joined by a third stream of dilute aqueous hydrocyanic acid solution which consists of liquid from the bottorn of the hydrocyanic acid still and solution frorri a r a t e r scrubbing towcr to be referred to later. Tire hydrocyanic acid in tliese solutions is thus recovered, wliile the water introduced dilutes tlic reacting d u t i o n s to an extent which avoids difficulty from crystnllizatiun of sodium sulfate in the residue solution. The temperature rise in tlre mixing basin is very rapid and scrves to expel about SO per cent of tlie liydroeyaiiic acid as the reaching iiolut,ions Row dowii tlie wall of tlle fon~tel. As t l l c solution their flows through the steamer, the stcam is regulated to give a progressively incrcasing temgmzture which, at the liquid discharge of the sleniner, is about 103" to 104" C. Sornially tlir residue solut,ion contains 0.02 to 0.05 per
c e n t Iiy w e i g h t of h y d r o c y a n i c acid, corresponding t o a loss or aboot 0.2 to 0.5 per cent referred to the cyanide input. Steam consumption is about 1230 p o u n d s per hour. The total exposure time of cyarriilc and acid solutions in the mixcr and s t e a m e r is about 2.5 niinrttes. T h e p r i n c i p l e of q u i c k m i x i n g and short exposure time makes it possible to employ Mi0 1%. sulfuric acid in this continuous unit with low
involved. The g e n erator is usually operated at a rate of about 650 pounds of hydrocyanic acid per liorir Iiut a rate of SO0 pounds per hour Ims I ~ i ~ rcaclicd ii a t tirnei. This happens t,o he about tlic limit, of capacity of tire still. It is probable that the capacity of the gcnerator is considerably higher, but this has not been determined. In the rernaimier of this paper, operating data are based on a p~otluctionrate sf F50 piiunds of liydroeyaiiic acid per hour. VAPORSYST*:MPiiron TO STILLS.All vapor piping from the generator to the stills consists of ant,imoriial lead. The 4-inch rapor pipcs from the nrixer and steamer enter a 6-inch manifold d i i a h carries tlic vapors to a lead pipe eoolcr. The lattor cmsists iif a vcrl.ical ilnrlk of five 15-loot lengths of 0inch piiie coriiiecteil by return bends. Cooling water trickles dowriward over the pigcs. \'apors enter t.lie cooler a t the bottotn and lcnve by tlie upper pipe. A drain a t the inlet to the bottom pipe conducts condensate back to the liquid inlet end (if tlie steamer. Leaving the top of the cooler thecnriched Irydrocyanic acid vapor enters a separator (where some liquid is separated and returned to the inlet end of the steamer) aiid tlicn passes to a still. Neglecting gaseous impurities, vapors cntering the cooler at 94' C. coiisist of about 40 per cent by weight hydrocyanic acid and 60 per cent steam. They leave the cooler at 80" C. and contain ahout 60 per cent hydrocyanic acid a d 40 per ccirt steam. Cooling water, i n summer at 22" C., is used at a rate of approsimately 6 gallons per minute. STILLS.One of the stills is sliawn in Figure 3. It consists of three parts separated hy adapter or spacer sections, a Iowtx boiling or exhisting section, an upper or rectifying section, aiid above tlie latter a tubular reflux condenser. Vapors enter the still at about 80" C. and (neglecting noncondcnsablc gases) corisist roughly oi 60 per cent hydrocyanic acid and 40 par cent steam by weight. The exhauster section hclow tlie feed inlet is inade of nionel metal and is 18 inches in diameter. The boiling section proper in the bottom is 18 inclies deep and is equipped with a copper steam coil with licnting surface of 11 square feet. Above this are five plates of the bulhlc cap type, each plate containing ten 3-inch bubble caps.
tion abuve tlie feed consists of seven plates >&kr twenty-two 4.75-inoli aluminum caps per plate. The rectifier tion i s inadc up of seven castahrnrinum sections. The hottmi of each section forms a
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plate niid the part above it simply carrsists of a spacer. The height between plates is 8 inches. Eacli spa.cer section is fitted with two sight-glass openings oil opposite sides, and tlrennorneter wells are included a t convenient points. Mounted directly aliol-c the rectifier section is an aluminum reflux condenser 3 feet iii diameter X 8 feet high coritaining two hundred eighty-tlirce Xo. I8 gage aluminum tubrs 16//16inches o. d. nith a cooling surface of approxirnat.ely 740 square feet. Tlic coiidcnser is capped by a cast-aluminum dome.
The average coding-water temperature in sutiiiner is about 22" C., tlie rate of flow through tbe reflux condenser is 150 gallons per minute, and the exit temperature is 25" C. The vapor temperature at the outlet of tlie reflux condcnser is maintained a t 32" C., and, when these vapors are liquefied, the liquid consists of 97.5 to 98 per cent by weiglit hydrocyanic acid. Tlic liquid from the bottom of the still contains only n small perccntage of hydrocyanic acid, hut, as mentioned before, it is used for diluting tho reacting solut,ions in the generator. The reflux ratio is slightly above 1:I. Steam consumption in the heating coil averages 33 pounds per hour. Vapors fruin the reflux condenser pass through G-incli, welded monel metal piping t,o the final condensers diich consist of trvo units, the first cooled by water and the second by alcohol brine. All condensers are constructed of alumitmtn.
The water-cooled condenser miglit consist of a duplicate of the st.ill reflux condenser. ILiwcver, simply because the equipinent was available, the present unit consists of two
tubular rxeliangers mounted vertically, one above the oilier, &li a 10-inch sp:iner section hct,ween them. Each csciianger consists of a 3-foot diaineter shell,5 feet in length, wntaining t m liuriilred eight.y-three ls/,rincli o. d. KO.16 Stubs gage aluminum t,ulies. Cooling water a t a rate of 90 gallons per niinrite passes tlimiigl! tlir excliangers in series, rnteririg at 22' C. and leaving at 25' C. About 30 per cent f tlie hydrocyanic acid is liqrtefied in this iirrit. Vapors ],ass downu-ard tliroiigli the tuhcs and at tlie lower end pass to the bottom of the !vine condenser. The brine condenser consists of onc excliangrr similar t o lliosc just (lescrii,od. liotli vapors and brine enter this unit a t t.lie bottom, vapors flowing through the tubes and the iirine surrounding thrni. The lirinc a t a ratc of 107 gallons per minute enters at a tcinperature of -4.5" and leaves at -3.0" C. Vapors escaping the brine condenser pass to a water sc.rubbing tomcr 1rliei.e the last traces of hydrocyanic ncid arc removed. Tire solutiom from this tower is added to tile still efiuent and is used for diIut.ing tlir rcaction mixture in the generator.
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I~ISSOLWGD GASELIMIN.AT~R Liquid hyrlroopnic acid leaves the two condenser uiiits through 2-incli i m s s pipes through separate sight, boxes, whcrc tlie color is iibserved. The sodium cyanide contains varying percentages of sodium .ioriiilly amount to as much as carbonate which may o 2.8 per cent,. At 0" C. and one atrnospliere pressure, one volutne of liquid hydrocyanic acid dissolves about eight volumes of carbon dioxide: arid the liquid froni the condensers usually contains about one volume of carbon dioxide per volume of liquid. In the early days this liquid was drurrimed immediately, and later, when drums wliicli had been allowed t.o become very warm uw1: opetiril, a suddcii release of the dissolved gas occurred wliicti caused the drmii to behave more like a geyser. or IPSS Ikinoval of the gas is effected by passing the liquid tiirough a sinall tubular exchanger, lieatcd with hot water to a ternperatwe barely below the boiling point. The liquid is discharged froni t,Siis exchanger into a sight box which also acts as a srparatov. The escaping gas is passed to a small brinecooled condenser from which tbe condensate joins the main stream froin the largc condensers and the gas passes to tlie water scrubbing tower. The Tvnrin liquid is passed through a brine-cooled exchanger and then flows into the mixing taiiks. MIXINGT A N K S . Liquid liydrocyanic acid is allowed to accumulate in one of these aluminum tanks until approximately 1000 pourids have entered. The concentration is checked by R specific gravity determination, t.be correct amount of water is added to briiig it to the standard shipping concentration of 97 per cent, and about, 0.005 per cent I)y wciglit of sulfuric acid is added as stabilizer. The solution is t.liorouglily mixed by means of a 2-incli centrifugal pump. When the concentration is finally adjusted arid the batch is approved by the laboratory, tlie pump transfers the solution to an elevated aluminurn tank which serves as a supply for the (lrumn-filling equipment.. h U w ~ r L L r N G SixATioN. The drun-filling station is sliown in Figure 4. The drums are approximatt:ly 15 inches in diarnetcr and 28.5 inches in height. They are niadc of 18-gage steel and are heavily tinned inside and out. The upper liead of each drum is equipped with a filling opening, a vent connect.ion (used rvlmn the drums are employed on the vnprrrising applicators to tie described Inter), aiid a pressurerelief valve wliieh is set to I h v off a t 7 pounds gage pressure. Each drum is charged with 100 pounds of 97 per cent bydrocyanic acid. ( h c e n t l y an 80-pound drum also has been used and a liquid outlet is placed on the side near one end.) Ten drutirs arc filled a t a time. Ten brass measuring tanks arc mounted a t a convenient heiglit. By means of a manifold and three-wily valves, liquid is allowed to flow by gravity from the supply tank upward tlirougli the measuring tanks, each of which wlieii full delivers 100 pounds or slightly less. Vent pipes from the niensuring tanks are gathered into a header which carries vented vapor from all hydrocyanic acid tanks in thc factory to tlie water scrubhing tower. l
