INDUSTRIAL A N D ENGINEERIATG CHEMISTRY
May, 1923
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M A T E R I A L S OF C H E M I C A L - E Q U I P M E N T CONSTRUCTION SYMPOSIUM* Papers presented before the Division of Industrial and Engineering Chemistry a t the 65th Mqeting of the American Chemical Society, New Haven, Conn., April 2 to 7, 1923 *
Lead as a Material for Chemical Equipment By George 0.Hiers NATIONALLEAD Co., BROOKLYN, N. Y.
EAD, principally in the forms of pipe and sheet, is used very extensively in the chemical industries in the United States, chiefly in the manufacture of sulfuric acid and in the diverse industries using sulfuric acid. VARIETIESOF LEAD The lead used in chemical equipment in this country is almost entirely of a brand called “chemical” lead. According to the tentative specifications for pig lead issued by the American Society for Testing Materials in 1922, conimercial lead is classed in three groups-No. 1, corroding lead; No. 2, chemical lead; and No. 3, common lead. The following is quoted from these specifications:
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Chemical lead is a designation that has been used for many years in the trade to describe the undesilverized lead produced from southeastern Missouri ores. This lead contains from 0.04 to 0.08 per cent of copper, from 0.005 to 0015 per cent (1.5 to 4.5 oz. per ton) of silver and carries less than 0.005 per cent bismuth.
These varieties of lead are often spoken of as “soft” lead in contradistinction to “hard” lead containing u p to about 15 of antimony or lead hardened with other elements. PROPERTIES OF LEAD Lead is one of the metals which apparently has been serviceable to man from the earliest days. It is mentioned in the Bible and in other ancient writings. Although a common metal, lead has certain characteristics which make it very valuable in the chemical industries. SOFTNESS-It is a soft, dense metal. Owing to its softness and lack of brittleness, in the forms of pipe and sheet it can readily be formed by bending or coiling without cracking. Sheets and pipes can be placed end to end or edge to edge, and welded or burned together forming a durable tight seam. Thc parts to be joined are scraped clean, and when placed together a bar of the same kind of lead is melted with an oxy-gas or an oxyhydrogen flame so that the three parts melt together. KO flux is used. The ability to readily “burn” pieces of lead together is due to the fusibility of the metal. Lead melts a t 327” C. (621’ F.). Eight per cent antimonial lead, which is commonly used, starts to melt at 245” C. (473” F.) and becomes liquid a t 296’ C. (565” F.). Occasionally a lead-burner will make a futile attempt to join together two pieces of hard lead with a piece of soft lead. This is impractical or impossible; therefore, it is always recommended that on account of the same fusibility soft-lead bars be burned on soft lead, and hard-lead bars on hard lead. No advantage would be derived from burning soft lead to hard lead.
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Other papers forming a part of this symposium will appear in a subsequent issue.
MALLEABILITY AKD COMPRESSIBILITY-The properties of malleability and compressibility account for the use of lead as valve pacliings and as gaskets or washers. Sheet lead is made by rolling. A large ingot is cast, carefully skimmed, and cooled. The ingot is placed upon a roller table and fed back and forth between two large rollers, one or both of which may be power-driven. After many passes the relatively thick ingot is reduced to sheet of the desired thinness. Sheet lead is marketed according to its weight in pounds per square foot. The thickness in sixtyfourths of an inch corresponds closely to the weight in pounds per square foot. Eight-pound sheet lead is about or l / ~ in. thick. The pipe is made by a process called “extrusion.” A cast ingot of lead in an iron cylinder containing a rod through the center, equal in diameter to the inside of the pipe desired, is forced by hydraulic pressure against a piston snugly fitting the cylinder. This piston is hollow and is rigidly fastened in the machine. At the end of the piston is a steel die with a circular hole equal in diameter to the outside of the pipe. The die is concentric with the core. From 10,000 to 60,000 lbs. per sq. in. pressure, varying with the cross-sectional area of the metal extruded, is applied to the lead. STRENGTHAND ELASTIcITY-Lead, as compared with other common metals, such as iron, steel, and copper, lacks strength. TT’hile no exact data are a t hand, save the hardness tests cited below, it is known that the strength of lead decreases with a rise in temperature. It is often said that chemical lead will stretch when heated but will not contract again when cooled. This statement is true only in the event of the metal being stressed beyond its elastic limit a t some time in the interval. With repeated alternate heating and cooling under too great a stress, the metal elongates. This elongation is loosely described as creeping or flowing of the metal. It should be borne in mind that any metal when stressed beyond its elastic limit a t any temperature elongafes permanently. A piece of soft lead of any uniform cross section and a length of 355 ft. could not be supported freely in a vertical position from the top only. .It would elongate and break near the top under its own weight. This figure is derived by calculation using an ultimate tensile-strength value of 1740 lbs. per sq. in. Using a safety factor of five, as is found good practice in calculating the bursting strength of lead pipe, the safe limit of length of lead of uniform cross section supported freely in a vertical position from the top would appear to be 71 ft. a t the ordinary temperature (25” C.). Since the metal is less strong a t elevated temperatures, such as are commonly encountered in tall chambers, towers, or in deep tanks, the vertical sheets used therein should be given attention so as to prevent the metal from breaking under its own weight. There are two precautions that may be taken-the use of tapered sheet thickest at the top, or the use of straps on the outer sides to distribute the load. Very little is found in the literature concerning the elastic limit of lead. Wertheim,l testing wires of small diameter in tension and taking as the elastic limit that force which 1 Burr, “The Elasticity and Resistance of the Materials of Engineering,” John Wiley & Sons, Inc., 1906, p. 361.
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will permanently elongate the metal 0.00005 of its original length, determined the elastic limit of lead as being from 284 to 355 Ibs. per sq. in. for the annealed and drawn metal, respectively. Testing cast bars of metal 1 sq. in. cross section and 8 in. test length, the following results were obtained in a n Olsen machine pulling the bars a t the rate of 0.06 in. per min. The initial yield point was arbitrarily taken as 0.1 per cent elongation.
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Initial Yield Point Lbs./Sq. In. Soft lead (S. E. Mo. desilverized).. 328 Hard lead (8 per cent Sb). 2664
... .........
Ultimate Tensile Strength 1400
Total Elongation Per cent 55
7650
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I n testing 2-lb. sheet lead these results were found: Ultimate Tensile Strength Lbs./Sq. In.
Chemical lead.. ....................... 2800 Hard lead (8per cent Sb). . . . . . 5500
Total Elongation Per cent
37 15
Some 11/4-in. diameter extruded bars were tested with the following results: Ultimate Tensile Strength Lbs./Sq. In.
Soft lead (S. E. Mo. desilverized). ....... 1740 Hard lead (8 per cent Sb). . . . . . . . . . . . . . 3340 Chemical lead.. ....................... 2300
Total Elongation Per cent
130 75 50
Brinell hardness tests of different leads were made with these results for cast metal:
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Chemical lead.. 2 per cent antimonial lead, S. E. Mo. desilverized lead.. 8 per cent antimonial lead,
..... ...
25O C. 5.3 7.0 4.2 13.0
100” C. 3.6 5.4
... ...
125’C. 3.2 4.8
150OC.
... ...
2.6 4.5
... ...
STRUCTURE-Lead in the cast condition has a coarse, crystalline structure as compared with iron or brass. By rolling and extrusion the coarse-grain structure is broken up and a finer recrystallization takes place. Growth of the small grains in the worked metal occurs with annealing. I n the course of a year, a t ordinary temperatures, there is a alight grain growth in worked lead. Along or across the direction of rolling the grains due to recrystallization are apparently identical. USE
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SULFURICACID INDUSTRY
Large quantities of chemical lead are used in the sulfuric acid industry. As is well known, sheet-lead walls are used in.the chambers in the chamber process. I n both the chamber and contact processes miles of pipe are used for conveying the acid about the plant and much sheet lead is used for making pans and tanks. Details of construction and use are described in some books on sulfuric acid manufacture.2J Much valuable information is given in recent articles by French‘ and Jones.6 Sulfuric acid ranging in strength up to 60’ BB. (77.67 per cent H2SOd) has but slight action upon lead, even when heated to near the boiling point. When more concentrated (up to 96 per cent), the action is still only slight in the cold, but increases with rising temperatures.2 Hard lead will not resist the solvent action of hot concentrated sulfuric acid so well as chemical lead does, and is not recommended in contact with sulfuric a t temperatures above 200’ C . In lead chambers used in the manufacture of sulfuric acid the sheet ceilings and walls are carefully SUSpended. I n many plants the wall sheets are fastened to the steel or wooden exterior structure or framework, with 2
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Wells and Fogg, Bur. Mines, Bull. 184 (1920). DeWolf and Larison, “American Sulphuric Acid Practice,” McGraw-
Hill Co., 1921. 4 Chem. Met. Eng.,27 (1922), 219. 8 J . SOC. Chem. Ind., 89 (1920),221T.
Vol. 15, No. 5
lead straps burned to the outside of the sheet. These prevent buckling and a t times are used to support and distribute some of the load due to the weight of the sheet. The latter plan is highly commendable.
TANKLININGS Large quantities of chemical and hard sheet lead are used to line the inside of tanks. The tanks outside are mostly of wood, but may be of concrete, brick, or structural steel. The use of lead-lined wooden containers was discussed by Dorr a t the convention of the American Institute of Chemical Engineers, in his paper concerning the use of wood in the chemical industry.61’ Recently, cast hard-lead tanks about 2 ft. square by 4 ft. long have been produced and used. The sheet linings for tanks, vats, or other containers are made up of sheets seamed together by burning. This process was previously described, but is again mentioned because of its exceeding importance on account of its simplicity, and on account of making the metal especially usable. The linings overlap the top edge of the tanks, and may or may not be fastened, according to the necessity. Where solutions in lead tanks are heated by the injection of live steam, it should be directed away from the lead. To make lead last the longest in contact with hot corrosive liquids or gases there should be no hot spots in the metal-i. e., the metal should be at a uniform temperature. Occasionally, insects from a wooden tank bore through a lead lining. I n such cases the insect or its eggs were present in the wood when installed. To prevent such trouble the wood may be treated with creosote. Before lining a brick or concrete tank with lead it is often recommended to put in a tar-paper lining. OTHERLEADEQUIPMENT Stills are made from sheet-lead parts burned together. Some in use are 0.5 to 1 in. thick, and hold 50 gal. of liquid. In such cases no outside or inside reinforcement is used. Jugs, carboys, pails, and similar containers are constructed in the same way. Steam-heating coils of lead pipe are widely used, in heating corrosive liquids by immersion, in leadlined or wooden tanks. Lead-pipe coils are used for cooling and condensing purposes, Special filter presses are made with lead. Lead pipe is extensively used for conveying corrosive liquids about a chemical plant. On account of superior strength hard lead is employed in cases where it is sufficiently resistant to corrosion. “Crawlproof” lead sheet is chemical lead reinforced with hard lead. Lead-covered and lined steel and iron articles are valuable as chemical equipment or in the construction thereof. Such products are on the market and have been found serviceable in the forms of pipes, fittings with or without flanges, valves, stirrers, agitators, tanks, coils, kettles, stills, pumps, evaporators, drying pans, drums, etc. The linings or coverings are of substantial thickness, and are preferably bonded or united to the iron or steel. Coatings of lead on steel and iron made by hot dipping, the Schoop or spraying process, and electroplating are somewhat thin. Terne plate or lead-tin alloy coating on iron is well known. During the recent war steel gas bombs were satisfactorily coated on the inside by electrodeposition. Lead, acting as insoluble anodes, is being used in the recovery of copper by electrodeposition from solutions. The durability and cheapness of lead account for its wide usage in the chemical industries. I n contact with many corrosive liquids or gases mentioned elsewhere, it has a long life. Many pieces of lead equipment in the chemical industry last for years. Corroding lead or common lead e THISJOURNAL, 16 (19221,94. 7
Chem. Met. Eng., 27 (19221,1207.
IiVDUSTRIAL AhTDENGINEERING CHEMISTRY
May, 1923
containing several hundredths per cent bismuth is not recommended for chemical equipment. Some sulfuric acid solutions form an adherent protective coating of insoluble lead sulfate on the surface of the lead. Without erosive forces or abrasive action of precipitates in such a case, the lead lasts indefinitely. I n the manufacture of sulfuric acid temperatures are sometimes high enough to melt the lead used. A manufacturer of dyestuffs used a cold solution of so-called sulfuric acid in a lead tank, which was quickly eaten away. It was later discovered that the solution contained considerable nitric acid. Such instances as these illustrate the difficulties encountered in attempting to predict how long a lead tank or other lead equipment will last. There is no doubt but that lead in chemical equipment is an economical material to use. It is used mainly in contact with sulfuric acid and solutions thereof. Various chemical industries having lead installations are: phosphoric acid manufacturers, color works, dye works, aniline works, perfume manufacturers, oil refiners, silk manufacturers, coke industry, glue, paste and adhesive manufacturers, manufacturers of hydrofluoric acid, hydrocyanic acid gas, boric acid, ether, soap, gunpowder, inks, phonograph records, graphite for pencils, straw bleachers. Some other uses are-in contact with alum solutions, photographic solutions, brine, pickling solutions, and electrolytes. EFFECT OF IMPURITIES
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Enameled Apparatus from a Chemical Engineering Standpoint By Emerson P. Poste and Max Donauer ELYRIAENAISLSDPRODUCTS Go.,ELYRTA, OHIO
NAMELED metal for commercial operations is a
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rather recent addition to the list of equipment-construction materials, when considered along with such products as iron, copper, lead, or earthenware. The information regarding the properties of enameled ware and the possibility of coating with enamel the various shapes and sizes of industrial engineering pieces are consequently not as generally known or as fully understood as with older engineering materials. It should therefore be of interest to consider enameled ware from the standpoint of its resistance to the chemical action of industrial products and the controlling elements which determine the shapes and sizes possible. NATUREOF AN ENAMEL The general conception of the inherent properties of enamel will be made more definite if we consider for a moment what constitutes an enamel. Perhaps the best definition for the equipment engineer is the one given by Popelin. It reads:
Enamel is a glass fusible a t a low temperature, and usually I n the last fifty years there have been numerous articles compounded of a mixture of borates and silicates. This mixconcerning the influence of impurities in lead. Such investi- ture, originally colorless, combines with greatest ease with all, gations have greatly advanced the art of using lead. Since or almost all, metallic oxides under the influences of a pyrotechnic lead is difficult to analyze and many of the investigators operation, thereby acquiring various bright or sober colors, acbased their conclusions upon unreliable analyses, much of cording to the nature of the oxide, which the enameler can vary their work is of questionable value. I n contending with a t will. this situation the American Society for Testing Materials8 VARIETIES has published methods of analysis of pig lead, representThis is a broad definition, but such a wide scope is necesing the best practice to date in this country. A table of sary in order to cover the field of modern enamels. Many analyses of varieties of pig lead follows: different kinds of oxides and different proportions of flux ANALYSESO F P I G LEAD materials are used in making enamels. The manufacturer of ( 0 : Southcast Missouri undesilverized enameled signs uses cobalt oxides for blue colors, chromium ( b Southeast Missouri desilverized Southwest Missouri undesilverizcd or copper compounds for green colors, and lead or antimony ( d ) Ordinary common oxides for whites. A low-fusing, glossy enamel with brilliant (e) Ordinary corroding, or refined colors is all that is required. Lead oxide is used in enamels (a) (b) (d (4 (4 for bathtubs and sanitary ware. When fused it presents a Per cent Per Per cent .~~ .~ Per cent . .cent .~ Per cent 0.0005 0.0005 0.0004 0.0005 Silver., ... . .., . 0.0070 smooth surface with a wonderful luster. Such enamel has Trace Trace Trace Trace Arsenic ........ Trace a very low acid resistance, being etched readily by the juice 0.0100 0.0020 Antimony.. .. , . 0.0030 0.0050 0.0030 None None None None Tin.. .. ., . , . . . . None of a lemon. Smoothness to permit easy cleaning and resis0.0800 0.0030 0.0030 0.0500 Bismuth.. . . . . . 0.0030 0.0006 0.0003 0.0190 0.0006 Copper.. ... , .. . 0.0600 tance to water are its main requirements. The opacity in None None None None None Cadmium.. . . . . cooking ware is frequently obtained by using tin oxide or 0.0015 0.0015 0.0015 0.0015 Iron.. ...., . . . . 0.0015 Trace Trace Trace Trace Trace Zinc.. . . . .. . . . . cryolite. Cooking utensils are subjected to the action 0.0018 None Cobalt and nickel 0.0080 None None None None Manganese.. ., . None None None of organic acids present in fruits and vegetables. T h e - - - - enamel must have greater acid resistance than bathtub ware. 0.0082 0.0278 0.0926 Total impurities 0.0825 0.0576 Lead ........... 99.9175 The oxides used may help to give greater acid resistance b u t - 99.9918 - 99.9722 - 99.9074 - 99.9424 they are chosen mainly because they are not poisonous. 100.0000 100.0000 100.0000 100.0000 100.0000 The proportions of borax, soda, and silica can also be varied The (a) read is chemical lead. It is produced from the so as to improve the acid resistance. sulfide ore, galena, in a blast furnace after roasting or in a It is quite evident from these few familiar examples that the Scotch hearth furnace, and refined without desilverization . enameler can and has varied the nature of the oxides in enamel The composition of this lead produced year after year is almost without limit, in order to obtain the results he desired. remarkably constant. The presence of copper is considered And the properties of the enamels have varied just as widely. advantageous, whereas the other impurities are present in These possibilities of variation present several points of inpractically negligible amounts. With the modern intensive terest to the equipment engineer who plans to use enameled studies of corrosion and its nature, even greater usefulness ware in his processes. One cannot judge the serviceability of lead in chemical equipment may be predicted. of enamel for certain chemical conditions from the success or failure of another enamel under those conditions. This ACKNOWLEDGMENT is especially the case when comparing a given enameled piece Acknowledgment is gratefully made to W. A. Cowan for with another made primarily for an entirely different purpose. his kind assistance in the preparation of this paper. More important than this is the fact that enamels are varied or developed to meet certain conditions, and if the require8 Book of Standards for 1921.
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