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bromoaniline was 2.83 per cent ferric oxide. This corresponds to a rate of corrosion amounting to about 3 pounds of iron per hour. This actually resulted in eating through the short iron nipple a t the point a t which the chlorine water was introduced into the pipe line, because the rubber lining originally provided was faulty and broke away completely from the iron. It is believed, however, that the corrosion problems can be satisfactorily met by rubber-lining pipes and mixing chambers, and that this difficulty could therefore be permanently eliminated. It is believed that the plant would operate satisfactorily in practically any weather up to half a gale. It may be stated that the Wallace and Tiernan chlorinators performed in a satisfactory manner even when the ship was rolling a t a n angle from 5 to 10 degrees from the vertical. Figure 9, a photograph of the chlorine room, shows this installation as actually set up. The armored rubber hoke leading from each individual control conveyed the chlorinated water to the mixing chambers. Figures 10 and 11 show the chemical equipment actually in place on board the S. S. Ethyl. Figure 12 shows the route the ship followed on her first and only actual operating cruise. The figures plotted on
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the chart are the bromine content of the samples of sea water taken a t various stations. Altogether it would appear from the results obtained on the S.S. Ethyl that the recovery of bromine from sea water is quite feasible by a continuous pipe-line process, pumping huge volumes of water through the system a t high velocities. There should, therefore, be no reason to fear exhaustion of the world’s bromine resources, no matter how great the industrial demand for this element may become, so long as the “eternal sea” awaits the chemist’s demands. Acknowledgment
It should be pointed out that the suggestion to recover bromine from sea water was made by the Ethyl Gasoline Corporation, and the early laboratory work was carried out in their laboratory and that of the General Motors Research Corporation. Credit for reduction to manufacturing practice of the process is due to E. R. Armstrong and R. L. Kramer and their assistants, members of the technical staff of the du Pont Company. Acknowledgment is made of the assistance of Graham Edgar, of the Ethyl Gasoline Corporation, in reading and criticizing this paper. ~
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Stainless-Iron Nitric Acid Equipment‘ Walter M. Mitchell CENTFALALLOYSTEELCORPORATION, MASSILLON. 0x10
T
HAT ammonia may be converted into nitric acid by an oxidation process has been known as a theoretical possibility for many years,Z but it is only within the last few years that the process has been developed to the point of commercial production. The success of this development should not be considered as due solely t o the skill and ingenuity of the chemist. It is due as well t o the availability of suitable materials which, satisfactorily resisting the corrosive action of the acid and capable of fabrication into the required forms, have made possible the construction of the necessary plants and equipment. These materials3 are the iron-chromium alloys containing 16 to 20 per cent chromium and known, because of their low carbon content, as stainless irons. Nature and Properties of Stainless Iron
Stainless steels, alloys of iron and chromium containing 11 t o 13 per cent chromium with sufficient carbon (0.30 to 0.40 per cent) t o confer hardening properties, have been known in the form of cutlery for some a t e e n years, but owing to their intrinsic hardness cannot be produced in forms suitable for the construction of chemical plant equipment. The desire for a more workable material-one that could be produced in such forms as sheets, tubing, wire, etc.-led to the development of the stainless irons. Since the corrosion resistance of the stainless alloys depends upon the ratio of chromium to carbon content, and since carbon was not desirable on account of the hardening effect produced by it, stainless irons have been produced by lowering carbon and increasing chromium content. Experiments and tests with stainless iron showed remarkable resistance to oxidation and in particular t o nitric acid. The first stainless iron produced may be considered an off1 Received
March 2, 1929. * Taylor, IND.ENO.Cas=., 19, 1250 (1927). * Mitchell, Trans. A m . SOC.Steel Treating, February, 1929.
spring of the original stainless steel and was made with virtually the same chromium content (12 to 14 per cent) but with materially lower carbon (0.12 per cent maximum). This alloy is resistant t o a great degree to all concentrations of nitric acid, but not sufficiently so to the more dilute acids. T o obtain the necessary resistance t o the more dilute acids, it has been found necessary to increase the chromium content (16 to 20 per cent), and specifications for stainless iron for nitric acid equipment virtually all demand a minimum of 16 per cent chromium. Higher chromium (above 20 per cent), while increasing corrosion resistance to a slight degree, does not improvemechanical properties of the alloy, so that fabricating difficulties may be increased. To lessen the hardening tendency carbon should be maintained low, under 0.10 per cent. Other elements, with the exception of nickel or silicon, are present in stainless iron largely as impurities and should be maintained as low as is consistent with good melting practice. A word of caution at this point is advisable. While the 12 to 14 per cent chromium stainless irons cost less than those of higher chromium content, a reasonable margin of safety as regards corrosion resistance is a most desirable precaution. Resistance to nitric acid increases with chromium content, and the insurance of longer plant life by the use of a higher chromium alloy will more than compensate for the questionable advantage of a slight saving in the initial cost of the installation. The stainless irons are suitable only for the construction of plant equipment for acid that is produced from ammonia. The acid prepared from Chile saltpeter has too high a content of chlorides, iodides, etc., and unless these impurities are removed stainless iron will be attacked. Development of Use in Nitric Acid Industry
The first experiments with stainless irons in connection with their application t o the nitric acid industry began in the sum-
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selves was learned. Fortunately, such difficulties have now been overcome and stainless-iron plates may be riveted together with joints, which careful tests show to be fully as strong and enduring as if made from ordinary boiler steel. However, care is required in the heating of rivets, and riveted fitruotures have been confined to such size as conld be fabricated in the shop and transported to the plant site. For this reason field fabrication has not yet been attempted. Figures 1 and 2 are riveted absorption towers and show what may be done in this direction. The tower in Figure 1 is 10 feet in diameter and 52 feet high, and represents about tile practical limit in size, beciiuse of the difficulties of railway transportation of larger pieces of equipment. Production of seamless drawn tubing presented many difficulties, both in tlie piercing of the billets and the subsequent draving operations. Careful control of temperature is absolutely essential, as is tborougb annealing between successive draws. At present the external diameter of drawn tubing is limited to inches maximum; but there is iio inherent reason why larger tubing cannot be drawn provided there is a sufiicient demand to warrant the outlay for the larger and heavier machinery necessary. Seamless tubing larger t,Iian 5’/%inches outside diameter is a t present obtainable, but this is produced either by forging a Figure ]-Riveted Absorption Tower. Enduro “A” StainleRI Iron Plates and Rivets Diereed hille$ oll a mandrel, in the Same as Fabricated by sriulbcrr-Wells co., Warren, Pa. large gun forgings are made, or by turning u p the the Atmospheric Nitrogen Corporation, a subsidiary of the outside of a large cyliiidrical billet and removing the core by Allied Chemical and Dye organization. The satisfactory a trephining operatinn--in either case a laborious and costly resuits of their experiments resulted in the construction of ii procedure. Large-size pipe for conduits is successfully made by riveted large unit a t Hopewell, Va., in which upwards of 500 tons of stainless equipment were used. Since t.hat time additional construction, as shown in Figures 3 and 4. While this replants have been erected by the du Pont Company, the Gen- qnires skill and carefiil work on tlie part of the fabricator, eral Chemical Company, the Nevport Compaiiy, the Grasselli it is an entirely feasible proposition. Such pipe can he Chemical Company, the Eercules Powder Company, the caulked so as to be both gas- and water-tight. Generally speaking, stainless iron of the proper analysis Atlas Powder Company, and others. The total stainless iron now in service in nitric acid equipment is between 4000 for nitric arid plant construction can be fabricated into any and 5000 tons, and represents a value of upwards of $5,000,000.
mer of 1923 when the du Pont Company commenced plant corrosion tests on these alloys. About a year later a small ammonia oxidation unit was constructed, and the successful completion of this experiment led to the designing and construetion of a large plant at Gibbstown, N. J., containing u p wards of 200 tons of stainless equipment. This plant has now been in operation for over two years and has apparently been entirely successful. Shortly after experiment>s were begun by the du Pant Company, siniilar experiments were made independently by
Methods of Fabrication Equipment for an ammonia oxidation plant includes absorption towers, heat exchangers, converters, pipe lines, valves, etc., ete. The first question encountered in the consideration of stainless irons for plant construction was the fabrication of t h e equipment. Could large-size plates be rolled, single-piece flanged arid dished heads of iarge diameter be formed, was seamless drawn tubing available, etc.? And, if these articles were procurable, conld they be welded or riveted together in permanent fashion? Fortunately, the production of large plates and single-piece dished heads presented no unexpected difficulties. The procurement of these articles is now limited only by the range of the equipment available for their production and by the willingness of the steel mills to cosperate with the expectant customer. The usual methods of fabrication are welding and riveting. When fabrication of shinless equipment was first considered, welding was a t once ruled out because of the very unsatisfactory state of the art at that time. The production of rivets that would stay put also presented difficulties, hecause of the propensity of the straight iron-chromium alloys to become brittle if overheated. Workmen accustomed to driving ordinary steel rivets heat.ed the stainless rivets to the usual temperatures (1900“ to 2100” F.) only to find that when cold the heads snapped off spontaneously or at the slightest blow of the hammer. Many experiments were necessary before the proper processing of the rivet stock and the rivets them-
Figure 2-Stainless Iron Bubbler Tower Fsbrieilted by Downingtown Iron W w m , Downingt~wn,Pa.
form or shape which can be made from ordinary boiler steel, provided due regard be given to the individual peculiarities of the metal. I n handling this metal it must be remembered that i t is both harder and tougher than steel. Forming and bending operations should therefore be conducted more slowly than with plain steel; furthermore, excessive temperatures must be strictly avoided. Figures 5 and 6, showing a heat
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e x c h a n g e r and a tray plate, caused almost endless controi l l u s t r a t e what can be done versy between the foundries, who desire to salvage as many with proper handling. There has been much discuscastings as possible, and the chemical manufacturers, who sion on the relative merits of w e l d i n g a n d r i v e t i n g as a do not desire material which m e t h o d of fabrication. The inay eventually prove defecmajority of tbe chemical manutive. hluch can be said on facturers, the larger companies both sides and the decision will depend largely upon the nature in particular, absolutely b a r welded construction for any of of tbe defect. the larger equipment unless it Forgings are used for Ranges, is physically impossible to propipe fittings, p u m p s h a f t s , duce the desired piece of equipnozzles, and similar pieces of ment in any other way. The equipmentbherea better metal high temperatures of welding than that obt,ninablein castings produce a crystalline structure is required. Forgjngs present Fisure 3-L.arse-Diamefer Stainless Iron Conduit essentiallv different from that n o p a r t i c u l a r bifficulty in in the a;?jacent m e t a l . I n Pnbrieated by Downingtown Iron Works, Downingtown. Pa. manufacture other than that addition, there is always opintroduced bv the ereater stiffportunity for inclusions of slag, particles of oxide, etc. In ness of the metal as compared with ordinary carbon steel. the straight chromium alloys it is practically impossible t o Forgings may be made from billets or bar stock of the same produce a homogeneous structure in the weld and the ad- analysis as plabes and tubin: and, if properly made, are free jacent metal by any process of heat treatment. Therefore, from such defects as blowlioles or porosities. If high presthere may be, and generally is, localized corrosive attack, sures are to be withstood, forgings have a great advantage either in the weld itself or in the over castinps in their freedom metal closely adjacent to it. from such defects. However, In addition to the probability owing t o tlie considerable of local attack, there is to be machine work required, pipe considered the brittleness, or fittings, such as ells, tees, ctc., lack of ductility, due to the made from forgings will he coarse crystalline s t r u c t u r e high in price compared to castformed during welding, which ings. is characteristic of s t r a i g h t Stainless-Iron Tank Cars i r on-c h r o m i u m alloys, and which, as already stated, can For bulk shipment of acid, be removed to a limited exstainless-iron tniik cars haye tent only by heat treatment. beeu under consideration for Naturally, tliere has been a some time, but their high cost great improvement in welding tins preveiitcd actual constructechnic since the stainless irons tiou until very recently. A were first i n t r o d u c e d , b u t uuinber of theso are now on limitations due to the char4-Ten.mch Stainless Conduit Pipe order and the first sttninlessand Fittim,s acteristics of the metal necesiron tank car was pot in sersarily romaiii. Tire stainless Plnbricated b y Striithers-Weiis Co.. Weiren, Pr. vice by the du Pont Company irons with added nickel are far a b o u t e i g h t m o n t h s ago. superior in this respect, and will be discussed in some detail Fabrication of tank cars presents no difficulties above those later. iiicurred i n tlie construction of stationary tanks. The conFlanges, nozzles, valves, and pipe fittings are made from tairier t,ariIilk only is made from stainless iron. The frrtmeforgings or castings. Castings are made with higher carbon work and trucks are of the customary steel construction. than plates. tubinp, etc. The low-carbon analvsis. when cast. Naturallv, because of the shocks and vibrations to which the will u&ally have b l o w h o l e s , tank oar is subjected, great porosities, etc., and to overcare must be observed in the come this, carbon must be inspection of tlie materials, raised to 0.30 to 0.40 per cent. and tbo general desigu must be With this high carbon, resistsufficientlyheavy to insure sufance to nitric acid, provided ficient strength and stability. the chromium is maintained Alloys Containing Nickel h i g h , is a p p a r e n t l y not. affected. Casting technic has All of the equipment, so far improved greatly in the last described, has generally been four years and reasonably good made from the straight ironcastings may now be procured chromium alloys. But for cerfrom a number of foundries tain piirposes, where welding which specialize in s t a i n l e s s is unavoidable, the stainless metals. irons with added nickel have a Welding, plugging, or caulknotable advantage. Alloys of ing of defective castings is usuthis kind will u s u ~ l l yrun i8 to FiPure 5-Enduro "A" stainless Iron Air eaf fer a l l y p r o h i b i t e d . This has 20 per cent chromium, 7 to 10 Pmbricaied by Strutherr Weiir Co , Warren, Pa
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INDUSTRIAL A N D ENGINEER1hrG CHEMISTRY
per cent nickel, and carbon under about 0.16 per cent. They were introduced by Straws, of the Friedrich Krupp Company, in Germany about fifteen years ago, but until recently have attracted little or no attention in this country. Their advantages are now beginning to be appreciated and they are rapidly supplanting the straiglit iron-chromium stainless irons. The addition of nickel results in a number of advantages: first, increased corrosion resistance; second, prevention of coarse crystalline growth a t elevated temperatures; third, it produces what is metallurgically known as an "austenitic" alloy. Such alloys, of which manganese steel is
Plkure 6-Stainkart Iron Tray Plate Pabrlrzted by Struthew-Wells Co., Warren. Pa
well-known example, are characterized by great toughness and ductility, entire freedom from any tendency to hardening on sudden cooling, and the property of being non-magnetic. These alloys possess higher resistance t o both hot and cold nitric acid than the straight iron-chromium alloys. As they are practically free from the grain growth associated with high temperatures, welds remain tough and ductile, and fahrication by welding is a reasonably safe procedure as far as maintenance of physical properties is concerned. The qiiestion of preferential corrosion resistance, however, remains; hut it is possible to heat-treat these alloys so that structural differences between the weld and the adjacent metal may he greatly broken down, thus materially lessening the tendency to localired attack. Patents covering the heat treatment of this alloy were granted to Straws, ns well as patents covering the analysis. Shipping drums for nitric acid have been somewhat of a problem since their suggestion some five years ago. The glass carboy, long a standard shipping container for acids, is far from ideal. Hence, stainless irons were welcomed by chemical industries as the potential means of solving the transportation phase of the nitric acid problem. The straight iron-chromium alloys were originally used for construction of experimental drums, but owing to the brittleness of the welds, by which the drums were of necessity fahricated, it was found practically impossible to meet tests imposed hy the Bureau of Explosives and the Interstate Commerce Commission, notwithtanding that various types of construction were used. Because of the greater ductility and toughness of welds made from the chromium-nickel stainless alloys, later experiments have been made with drums constructed from them and these drums have passed successfully all varieties of mechanical tests that could be devised. I n use, however, the first lot experimentally made showed the effects of unrelieved strains in the metal after some six or seven months in service. This is thought to be the result of insufficient annealing after welding. Other drums, which have had a more complete annealing treatment, are now in service. It is believed that these will be satisfactorv. althoueh the time in service is too short for confirmation oj'tliis. 'fhey are constructed of vari-
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ous sizes, usually 15-,30-, and 55-gallon capacity, and provided with cast or forged flanges and bungs. The Hackney, or two-piece eonstruetioil (Figure 7), with equatorial weld, has proved the best type in all mechanical t.csts. Outlook
Looking toward the future, any prediction of what may be expected is extremely hazardous. When we consider the advances made in metallurgy of corrosion-resisting alloys during the last five years, any assertion of what can or what cannot be done is manifestly unwarranted. The two years' successful operation of the first du Pont plant indicates that stainlessirons have attained an established position in nitric acid manufacture. However, the greater advantages of the chromiumnickel stainless irons over those containing chromium as the only alloying agent indicate that they will be carefully considered in the future, and may replace the latter altogether. It is rather doubtfulif newalloys will he produced with qualities widely different from those at present known. The comhinations of chromium, nickel, iron, silicon, etc., which produce workable alloys are fairly well known. Numerous experiments have determined the limits of the useful composition ranges with considerable certainty. There is a possibility that small additions of some other metal, such as molyhdenum which is knowii to incrense resistance to mineral acids, will prove desirable. But until the present equipment has been subjected t o longer tests in service, we do not know that such additions are even necessary.
Piwre 7-30-Gallon Enduro KAZ (ChromiumNickel Stainless Iron) Shlpplng Drum Fahrlested by Prensrd Steel Tank Co..Milwaukee, Wis.
If any improvements are to be expected, these lie in the direction of improved quality of metal, which may be looked for with increased demand and resulting increased tonnage production. Increased tonnage will result in mills and equipment devoted exclusively to the production of these alloys, and under such advantageous conditions improvement in quality should inevitably follow. Thermal Emaosion of Tantalum--The Bureau of Standards will shortly publish t h e results of an important investigation on the expansion of pure tantalum for various temperature ranges between -310" and +930" F. Tantalum expands more than tunxsten or molybdenum. but less than iron.
in electrochemical aualys& and electrolytic val;es for t h e manukacture of rectifiers
