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 I n the last fifty years there have been numerous articles concerning the influence of impurities in lead. Such investigations have greatly advanced the art of using lead. Since lead is difficult to analyze and many of the investigators based their conclusions upon unreliable analyses, much of their work is of questionable value. I n contending with this situation the American Society for Testing Materials8 has published methods of analysis of pig lead, representing the best practice to date in this country. A table of analyses of varieties of pig lead follows: ANALYSESO F P I G LEAD Southcast Missouri undesilverized Southeast Missouri desilverized Southwest Missouri undesilverizcd ( d ) Ordinary common (e) Ordinary corroding, or refined (0:
(GI
(b
(a)
...... . ........ .. . ... . . .. .. .. .. ... ... .... .... ....
Silver., , Arsenic Antimony.. , Tin.. , , Bismuth.. Copper.. , Cadmium.. Iron.. , Zinc.. . . . . . . . Cobalt and nickel Manganese.. ,
.
(b)
Per cent Per .~~ . .cent .~ 0.0004 0.0070 Trace Trace 0.0030 0.0030 None None 0.0030 0.0030 0.0003 0.0600 None None 0.0015 0.0015 Trace Trace None 0.0080 None None ~
~
(4
(d
Per cent .~ Per cent 0.0005 0.0005 Trace Trace 0.0020 0.0100 None None 0.0800 0.0030 0.0006 0.0190 None None 0.0015 0.0015 Trace Trace None 0.0018 None None ~
~
(4
Per cent 0.0005 Trace 0.0050 None 0.0500 0.0006 None 0.0015 Trace None None
. .. - - - - -
Total impurities 0.0825 Lead 99.9175
0.0082
0.0278
0.0926
0.0576
100.0000
100.0000
100.0000
100.0000
100.0000
........... - 99.9918 - 99.9722 - 99.9074 - 99.9424 -
The (a) read is chemical lead. It is produced from the sulfide ore, galena, in a blast furnace after roasting or in a Scotch hearth furnace, and refined without desilverization . The composition of this lead produced year after year is remarkably constant. The presence of copper is considered advantageous, whereas the other impurities are present in practically negligible amounts. With the modern intensive studies of corrosion and its nature, even greater usefulness of lead in chemical equipment may be predicted.
ACKNOWLEDGMENT Acknowledgment is gratefully made to W. A. Cowan for his kind assistance in the preparation of this paper. 8
Book of Standards for 1921.
469
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
E
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 compounded of a mixture of borates and silicates. This mixture, originally colorless, combines with greatest ease with all, or almost all, metallic oxides under the influencesof a pyrotechnic operation, thereby acquiring various bright or sober colors, according to the nature of the oxide, which the enameler can vary a t will.
VARIETIES This is a broad definition, but such a wide scope is necessary in order to cover the field of modern enamels. Many different kinds of oxides and different proportions of flux materials are used in making enamels. The manufacturer of enameled signs uses cobalt oxides for blue colors, chromium or copper compounds for green colors, and lead or antimony oxides for whites. A low-fusing, glossy enamel with brilliant colors is all that is required. Lead oxide is used in enamels for bathtubs and sanitary ware. When fused it presents a smooth surface with a wonderful luster. Such enamel has a very low acid resistance, being etched readily by the juice of a lemon. Smoothness to permit easy cleaning and resistance to water are its main requirements. The opacity in cooking ware is frequently obtained by using tin oxide or cryolite. Cooking utensils are subjected to the action of organic acids present in fruits and vegetables. T h e enamel must have greater acid resistance than bathtub ware. The oxides used may help to give greater acid resistance b u t they are chosen mainly because they are not poisonous. The proportions of borax, soda, and silica can also be varied so as to improve the acid resistance. It is quite evident from these few familiar examples that the enameler can and has varied the nature of the oxides in enamel almost without limit, in order to obtain the results he desired. And the properties of the enamels have varied just as widely. These possibilities of variation present several points of interest to the equipment engineer who plans to use enameled ware in his processes. One cannot judge the serviceability of enamel for certain chemical conditions from the success or failure of another enamel under those conditions. This is especially the case when comparing a given enameled piece with another made primarily for an entirely different purpose. More important than this is the fact that enamels are varied or developed to meet certain conditions, and if the require-
470
INDUSTRIAL A N D ENGINEERING CHEMISTRY
rnents are known it is quite probable that an enamel can be or has been developed for such conditions. Enamels for industrial and chemical-engineering operations have been developed in this way. The demands of industry have dictated the general requirements. The enameler has varied the constituents of his formulas to meet these demands. The next step has been the preparation of the least number of different enamels whose chemical and physical properties overlap so as to cover the more important range of requirements. Thus, for fine pharmaceutical work and food work an enamel without heavy metals is required, and heavy metal oxides have been eliminated by most manufacturers of industrial enameled ware. There are many problems where a nonpoisonous enamel coating is desired to keep the product away from metal, and such enamels are available. If the product is a perishable food such as milk, it is, in addition, advantageous to have a smooth, glossy surface which can be easily cleaned and made sterile, and the glass-like surface of these enamels has been developed to a high degree. If the materials to be processed contain organic acids which gradually corrode metal, an enamel lining of a somewhat different character is needed. The enamel itself must have sufficient acid resistance so as not to be etched. It must cover the surface completely so that it does not expose the iron, even in an area as large as a pinhole, as that may be a starting point for action which will gradually increase the exposed area. There is a gradation of such acid materials starting with weak acids and continuing through to the most severe organic acids and to the inorganic acids, the latter being a great deal more corrosive. The stronger the acid conditions the more acid resistant must be the enamel. It must cover the metal completely and must present no irregularities in the composition of the enamel itself. For the strongest inorganic-acid conditions it is found preferable to apply the enamel as a powdered glass which is fused on the ware, because this permits a more acid-resistant base material. It is homogeneous when applied, preventing irregularities in the enamel coat itself, and also permitting a more perfect coating. It must be remembered that throughout this entire range of conditions the enameler is limited to materials which will fuse below the softening point of iron, and which have the proper coefficient of expansion compared with iron so that the enamel will adhere properly. Such limitations make the problem of meeting the chemical requirements with a suitable enamel much more difficult than if these restrictions were not imposed. Enameled equipment coated with a lining which has been developed primarily to avoid contact with metal is used for industrial work of the following nature: I n food work, for example, milk and ice-cream products are mixed, pasteurized, and stored in such vessels. Sugar sirups and beverages are mixed and stored therein. Along chemical lines, vegetable oils are refined, deodorized, and stored in appropriately designed enameled units. Ammonium nitrate is crystallized in enameled crystallizing pans. Phenol is stored in tanks with such an enamel coating. These are but a few typical examples for which it is perfectly obvious that protection from metallic contact is desired, but beyond that there is no great acid action. As typical of materials requiring more acid resistance than those above, fruit juices, such as grapefruit juice, are stored in enameled vessels having good resistance to organic acids. Citric acid-beverage sirups and especially beverageflavoring concentrates containing large amounts of citric acid are mixed and stored in such tanks. Tincture of iodine, chlorinated water, and other chlorinated products are made in such equipment. Aspirin is made in enameled units of this class. Various photographic developing chemicals are made and crystallized in similar equipment. This enamel
Vol. 15, No. 5
can also be used for storing tin chloride or zinc chloride containing a slight excess of inorganic acid. Inorganic acids have a much more severe corrosive action on enamel itself than the materials so far listed. Also, even the slightest imperfection makes the equipment unfit for use. Such conditions have been satisfactorily met by enamels of the highest acid resistance. As we have previously mentioned, the enamel is usually applied by dusting onto the hot metal piece, or occasionally the moist sprayed piece is dusted over with powdered enamel. Typical problems for such enamel are the evaporation of aluminium chloride, copper chloride, or silver nitrate containing excess acids, the refluxing of organic compounds with nitric or hydrochloric acids, or the dissolving of platinum in aqua regia.
BEHAVIOR Uh?)ER
V A R Y I N G CONDITIONS
It is quite evident that enamels may vary greatly in their ability to resist corrosive conditions. I n the same way a given enamel may resist acid action quite differently under varying conditions. It has been found possible to get some idea of the resistance of enamels under changing conditions by subjecting a weighed amount of powdered enamel frit to the action of the corrosive solution and then filtering off and weighing the residue. An enamel which is somewhat susceptible to the action of the material must be taken if the effect of various strengths of the same solution is to be noted, as otherwise the figures will all be uniform. Also, the results do not represent the resistance of the enamel on the ware as the powdered frit is in a different condition from the fused enamel on the finished piece. The accompanying graphs are results obtained from such a study. Fig. 1 shows the relative corrosive action of acetic, hydrochloric, sulfuric, and nitric acids of various strengths on the same enamel frit. It will be noted that the action is much more severe where the ion concentration is greatest. Moreover, an organic acid such as acetic acid is much less corrosive than inorganic acids.
Percenf A c d
FIG. A ACTION
OF DIFFERENT ACIDSON THE
SAMEE N A M E L FRIT
I n Fig. 2 three different enamel frits were subjected 'to the action of sulfuric acids of various strengths. The three frits represent more or less the three different classes of industrial enamels heretofore discussed. According to the curves an enamel represented by Curve I1 would be satisfabtory for cold sulfuric acid a t 50 per cent strength, but entirely impossible for 15 per cent acid. The enamel represented by Curve I11 would be entirely satisfactory for either case. Under the conditions of the test, the Frits I and I1 were entirely out of range near the point of maximum activity, as all the acid-soluble material had been removed. The dotted lines show the probable nature of the curve which would result if this had not been the case. The tests were carried out at room temperature. If higher temperatures were used the general nature of the curves would be the same, but the degree of action would be greater. This change would be
May, 1923
I N D UXTRIAL A N D ENGINEERING CHEMISTRY
471
put into the furnace and when red hot is taken out and powdered enamel dusted uniformly over the piece by means of a mechanically agitated sieve. The piece is then returned to the furnace while still hot, and is heated until the enamel has fused down uniformly, The enameling of bathtubs is done in very much the same way, but the enamel used is much softer and fuses a t lower temperatures than 'enamel for chemical equipment. The enamel itself is a thin layer of glass and must be considered as such. It cannot be applied over sharp corners without having strains which will later cause the enamel to jump. h piece such as a plain pipe end cannot be enameled over the edge and be sure the enamel will cover it thoroughly and be acid-resistant. Surfaces to be enameled should be of simple, uniform contour. There should be no projecting ledges on the inside of a tank which is to be covered with enamel. The piece should have a fairly uniform thickness. Parts should not overlap one another so that one shades the other from the furnace heat, because the piece should heat OWO IO 20 30 40 50 60 70 BO 90 /OD up uniformly when the enamel is being fused. Shapes Fer cent Sulfuric Acld must be such that the enameler can spray or slush the moist FIG 2-AC'rIoN OF VARYING STRENGTHS O F SULFURIC ACID ON ENAMEL enamel uniformly over the surface to be covered. Where FRITS O F DIFFERENT ACIDRFSISTANCE the dusting process is used the unit to be enameled must have a plain, open shape so that the powdered enamel can As we have said before, such curves do not represent the be quickly and uniformly applied. resistance of an enamel to acid corrosion, because for the A softer enamel of the less acid-resisting type can be apenamel which is to meet the entire range of conditions the plied to more varied shapes than the harder varieties. It has graph would be practically a straight line. On the other hand, a more uniform covering power and produces a finished job from such curves it is possible to judge which conditions with less effort. It may fuse down more uniformly. It has are most corrosive toward enamels, and one can be guided greater elasticity and may resist heat shocks more easily. in the selection of a proper enamel. Where it is a matter of merely covering the surfaces to prevent metallic contact, it is preferable to use an enamel of that ALKALIRESISTANCE type. Perhaps a statement shouId be made regarding the alkali I n enameling a vessel for organic acids other probIems preresistance of enamels. I n general, enamels are acid mate- sent themselves. The enamel itself must be more acid-rerials with silica as the main acid. Their acid resistance sistant. The unit must be designed so as to permit perfect varies according to the ion concentration of the acid solu- enameling of the entire inside surface. The determination tions involved. Enamels which have beezl developed to of such limitations is dependent on various conditions learned meet certain conditions may or may not resist other condi- only by experience. I n general, the simpler the design of such tions, and the chemical nature of the solutions involved must a piece, the more easy it becomes to get a perfect coat. The be considered. When we come to alkali resistance of enam- sizes are usually limited from 500 to 1000 gal., depending on els we must look a t them from the same point of view. Most conditions, as it has been found that beyond those sizes the industrial enamels will resist ammonia or alkali carbonate difficulties of manufacture become increasingly greater. solutions as they have no strongly basic reaction. Free Enamels that are dusted on the hot piece can be applied caustic soda in solution, especially if hot, will etch or decom- only to simple, open shapes. The process of applying the pme enamels. Some salts, such as sodium sulfide or potas- enamel is essentially a hand operation and this limits the size sium cyanide, which have a strong alkaline action in solution of piece which can be properly handled. The limiting will also etch enamels. Contrary to the usual opinion, the figures vary, some manufacturers stopping a t 300 gal., while enamels higher in silica which are most acid-resistant also others produce pieces of 500-gal. capacity. have the greater resistance to alkali. CONCLUSION PROBLEMS IN APPLYIXG ENAMEL
more pronounced with Frits I and I1 than with the more acid-resistant Frit 111.
The fact that an acid-resistant enamel is more resistant to most other conditions often prompts the question-why notuse this enamel on all equipment and have it one standard grade? This brings us to the problem of considering the nature of the limitations of applying enamel to industrial equipment of various shapes and sizes. The enamel is usually applied in one or two different ways. It may be ground up with water and a small amount of colloidal clay, the latter keeping it in suspension. The enamel in this form is sprayed over the surface t o be enameled. It is dried. The tank is then put into a heated furnace and brought to a red heat to fuse the enamel. Several coats are required to build up a uniform surface. This system is much the same as that used for cooking ware, except that the small steel shape can be dipped, then drained, dried, and burned. The other system of applying enamel is to cover the iron with a wet, ground-coat enamel as above, to protect it from oxidation The piece is
It has been our aim, in the foregoing paragraphs, to give the manufacturer who uses equipment an idea of the differences between various classes of commercial enamels and how they have been developed to meet different requirements. It is impossible to study the acid resistance of enamels as a class, because enamels have been developed to meet the chemical requirements given and the range includes enamels which are resistant to most acid conditions encountered. As the severity of the acid corrosion becomes greater it is necessary to limit the size and shape to a greater extent to insure a unit with a perfect enamel coating which will give good service for the operation in which it is to be used. The enameler is developing his ware more and more to meet the requirements of industry and the limitations are growing less in proportion. Industrial enameled equipment is consequently finding an increasing field of service in chemical and industrial engineering work.
