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INDUSTRIAL A N D ENGINEERING CHEMISTRY
Acid- and Chemical-Proof Stoneware By M. A. Knight EASTAKRON,OHIO
HEMICAL stoneware is a ceramic material made from specially selected clays and fired to the vitrification point. Properly manufactured chemical stoneware does not rely on a glaze, enamel, or veneer for its resistance to corrosive acids and chemicals, but the entire body throughout should be resistant. On such ware a plain salt glaze is put on for appearance but plays no part in the wearing qualities. The salt glaze is applied by throwing salt on the kiln fires a t their highest heat, which breaks up the salt, and the sodium combines with the clay on the surface of the ware to form sodium aluminium silicate, giving a smooth appearanre and a neat look to the ware. The clays as used for chemical stoneware (more than one clay is used and the clays are blended before using) are nearly pure aluminium silicate with but small traces of lime, magnesia, or iron. They should, for most of the product, be also of the plaslic type so they can be readily molded. For the various types of bodies produced to meet the various demands in the chemical industry, various types of clays from different parts of the United States and foreign countries are used, and blended as required.
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MANUFACTURE OF CHEMICAL STONEWARE
VOl. 15, No. 5
When the piece has been thoroughly dried both inside and out, it is ready for the kilns or firing. Firing in chemical stoneware requires two weeks, including the cooling of the kiln. The kilns used are circular with hemispherical roof about 30 ft. in diameter, and are known as periodic downdraft kilns-the fire passing downwards through the ware, through a false floor into the stack flues. The ware is piled in the kiln with the object of getting as much into the kiln as possible, consistent with getting it out again in perfect condition, and this setting of a kiln requires considerable experience on the part of the boss kiln-setter. After the ware has been set, the openings are bricked up, and a moderate fire started in the fireplaces surrounding the kiln. The moderate fire is to drive off any remaining moisture mechanically held in the ware and to warm up the kiln slowly. T h e fires are gradually increased, and during this period--“the water-smoking period,” as it is termed-the mechanically held moisture is driven off. After tests made on the smoke passing through the flues to determine if further moisture is present, and none being indicated, the temperature is rapidly run up to around 1000” F., a t which temperature the water of crystallization of the clay is driven off, the temperature being held a t this and a slightly higher point for a predetermined time. (The recording pyrometers used indicate the general time.) This converts the clay into stoneware, but as far as chemical stoneware is concerned the firing is but half completed. After the temperature for driving off the water of crystallization has been held for the proper period, as mentioned above, the fires are crowded to their limit and the temperature gradually pushed up until it reaches the neighborhood of 2500’ F., when complete vitrification or fusion of the body of the ware takes place. After this point is reached and held for a certain period, the fires are permitted to die down slowly, and when sufficiently low the fireplaces are bricked up and the entire kiln allowed to cool slowly by natural radiation. Thus, the ware is cooled slowly or, in other words, annealed, producing tough, strong structures which will withstand considerable rough treatment. When the kiln is cooled off, the ware is removed from the kiln, taken to the warehouse, inspected, tested, checked, and packed for shipment.
The clays as received in the raw state are ground, washed, sieved, filter-pressed, and then blended and mixed and ground in a special machine for that purpose. The blends are then put into aging pits of concrete where they age for various periods. After aging the clays are ready for the workmen. Chemical-stoneware manufacture differs from ordinary stoneware, porcelain, or sewer-pipe manufacture in that in chemical stoneware but very little machinery is used. The principal use for machinery in chemical stoneware is in preparing the clays, after which the work is almost entirely hand work by skilled men who must know how to read and interpret a blueprint. I n the other ceramic industries as much of the manufacturing is done by machinery as possible; ADVANTAGES AND LIMITATIONS hence, they are “quantity” producers, whereas chemical In all industries where corrosive chemicals are manustoneware is “quality” production. Machines for making chemical stoneware have been tried but so far have not been factured or used it is a necessity that the apparatus constituting the plant equipment be of materials that wiIl withfound as satisfactory as hand production. Orders for chemical stoneware are often accompanied by stand the action of the chemicals, for two reasons-first, blueprints or special specifications, which are turned over to on account of the excessive cost of constant replacement of the proper department in the plant devoted to the group apparatus, and second, on account of the contamination of in which the design belongs. The job is then assigned to a the manufactured product due to the corrosion of the appaspecific workman, who proceeds, from the clay already pre- ratus, On this account chemical stoneware was developed, pared and of the proper body, to mold the piece of ware in as of all the materials offered for this kind of work it is probaccordance with the blueprint or specifications. The time ably the most versatile one in that it is practically immune required for molding may be hours, days, or weeks, depending from the attacks of most every chemical and acid except on the size and the design. When the piece has been molded hydrofluoric acid, which acts on the silica in the clay. Of in accordance with the specified design, it is ready for the course, chemical stoneware also has its limitations and is not the Utopia in acid-resisting material, but for generaL drying: Drying is accomplished in three ways-by natural air all-round use there has still to be found a better material. drying, in heated large rooms, and in automatic tunnel driers. The limitations of chemical stoneware are briefly-limits Each has its specific use in accordance with the design, as to size in which it can be made, tensile strength as compared size, and thickness of the piece. Drying is one of the critical to the ordinary metals, and heating by direct flame cannot periods in the passage of the piece through the plant. The safely be done. To overcome the first limitation has been ware must be dried from the inside out, not from the outside the aim of the manufacturers of chemical stoneware ever in, or stresses will be set up in the structure which will later since they have been in business, with the result that every develop trouble in the firing. Also, the drying must not be year larger and more complex pieces are produced, some too fast, for the same reasons. The clay shrinks about reaching a capacity of 1000 gal. and over. To overcome 5 per cent in volume during drying so this shrinkage has to the second, constant experiments are being conducted to improve the strength of the body, with the result that we be taken into account when molding the piece.
May, 1923
INDUXTRIAL A N D ENGINEERING CHEMISTRY
have to-day ware that will withstand in some cases more punishment from mechanical injury than some of the compounded ferrous alloys. For the last, heating is substituted in the form of water, oil, and sand baths, or by steam injected into the solution by means of chemical-stoneware perforated pipes. DESIGNS
As t o the designs in which chemical stoneware is made, it can be said that a t this stage of its development most anything mechanically possible can be and is made. In addition t o the commoner forms or designs, the greater part of the manufacture of chemical stoneware is from blueprints or speciflcations furnished by the customer in order that the apparatus may be especially adapted for the specific process for which it is intended. As an illustration of the variety of products of chemical stoneware a few of the variety in most general use are mentioned: towers and accessories therefor; tower packing; tile, brick, and blocks for construction of tanks, towers, vats, etc.; elevating apparatus for acids and corrosive liquids; acid receivers and tourills in the manufacture of acids; kettles, with or without stirrers; gas generators; arsenic acid generators; jars; decanting jars; subliming pans; tanks; laboratory sinks; filters; coils; manifolds; pipe and fittings in many styles and bores, return bends; S-pipe, vapor pipe; sleeve pipe, troughs, pipe dampers; faucets, cocks and valves; carboy stoppers; pitchers, jugs and buckets; dipping baskets, funnels, burner guards; Mariotte jars, etc. I n other words, the field of requirements of the chemical and allied industries is practically covered. hpecific mention may be made that chemical stoneware is used in plants manufacturing acids, alkalies, explosives, dyestuffs, chemicals, pharmaceuticals, fertilizer, vinegar, foodstuffs, and mineral products, and for plating, lithographing, developing, and pickling processes.
TYPESOF BODIES It has been stated above that various types of bodies are produced to meet the needs of the different classes of equipment. It is not to be understood that there are an indefinite number of bodies in general use, but rather that a reasonable number are employed. To be specific, the bodies employed by one manufacturer may be referred to as Nos. 1,2,3,4,5,6,and7. No. 1 is a porous, open body, through which liquors will readily run, but which will hold hot acid gases. Finished unglazed, it serves for such uses as carboy stoppers, filtering plates and cups, electrolytic diaphragms, or any pieces subject t o high temperatures and sudden changes. No. 2 is a semiporous, open, rough body, through which liquors will seep, but which is excellent for hot gases and for sudden changes in temperature. It finds use in baffle plates, heavy tile, covers and blocks for furnaces, and large pipe for hot gases. It is finished either glazed or unglazed. No. 3 is a fine, close-grain structure, through which liquors are apt to sweat, but which is good for hot liquors and gases and uneven heat action. It is used for standpipe or fittings next t o furnaces, muriatic acid sleeve pipe, boiling kettles, hot towers, burner guards, ejectors, and steam jets, and is finished smooth, bright, and glazed. No. 4 has a tough, rugged, vitrified structure, not as close and %ne a body as No. 3 or No. 5, but more vitrified and denser than No. 2. It will hold acid without seepage, and is used for large pieces of 250-gal. capacity and over, for large tower pipe, kettles, and blow-cases, grillage tile, brick, heavy tanks. It is good for ordinary heat changes in temperature and is finished glazed and somewhat rough. No. 5 has a close, dense, fine structure. This can be considered the general average body. It will withstand ordinary temperature changes but not sudden changes. It is finished smooth with a bright glaze and is used for receivers, tanks, jars,
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drainage pipe, tourills, evaporating pans, condensing pipe, sinks, and arsenic equipment. No. 6 is very fine, close, vitrified structure-closer and denser than No. 5 . It will not stand heat action as well as the bodies previously described. It is used for acid pitchers, monkey pumps, laboratory sinks, vacuum filters, funnels, flanged pipe, coils, tanks, jars, and other pieces where heat changes are small and slow. It is good for storage vesseIs, etching machines, chlorine pipe and fittings, etc, It is finished bright, clean, and smooth, with a heavy glaze. No. 7 has a very fine, tight, close body, vitrified to almost a glass texture. It will not withstand changes in temperature or high temperature, but with this body there is no absorption whatever. It is used for valves, faucets, cocks, pebble-mill jars, distilled-water jars, small bore pipe and fittings, or any small pieces up to 50-gal. capacity where there is very little heat action. It is finished very bright, smooth, with a glazed surface, and made up into only thin, light-weight, or small pieces.
GENERAL CHARACTERISTICS
A few words about the general characteristics of chemical stoneware are in order. At first i t was deemed necessary to make walls of apparatus heavy and thick, for mechanical strength as well as for resistant purposes. The present tendency is to make the walls as thin as possible, and it is surprising to note how thin they are made and yet combine strength and immunity to corrosion. In addition to the fact that the thinner malls make the apparatus lighter, they carryialso the property of being better able to withstand the action of heat and cold (expansion and contraction) and permit better radiation of heat in the case where heating is to be done or solutions are to be cooled off. It is the practice with chemical stoneware, where there is alternate heating and cooling, to avoid designs having sharp edges or corners, and to substitute rounded designs, since, inasmuch as chemical-stoneware bodies are of a crystalline nature and therefore require time for the molecules to adjust themselves to changes in temperature, the square design or sharp corners do not permit of even expansion or contraction and hence increase the liability to cracking. In this respect it should be mentioned that it is not the degree of heat or of cold to which the ware is subjected that has to be guarded against, but the suddenness with which the change from hot to cold or cold to hot is made, as well as the evenness with which heat or cold is applied. Where chemical stoneware apparatus to an exact measurement is required, this is obtained by having the piece run slightly over in measurement and then grinding back to measurement. However, this is a long, tedious, and expensive operation, requiring the use of carborundum and machinery, and if exact measurements are not an absolute essential it is better to avoid this expense. Chemical-stoneware bodies are so hard and tough that nothing softer than carborundum will cut them with economy, and then only a t a comparatively slow rate of speed. TIMEREQUIRED FOR MANUFACTURE The time required to manufacture chemical-stoneware apparatus varies with the size, design, and thickness of the piece, and runs from four to ten weeks. The greater part of the time is consumed in the drying and the firing of the piece, and in those two stages the time cannot be shortened with safety, no matter how necessary it may be to get the piece out faster. Hurrying the manufacture during drying or firing usually means the entire loss of the piece, with the result that the work must be started all over again from the beginning with consequent still longer delay. It is therefore best to allow as much time as possible for manufacture when preparing plans for new installations or the repair or replacement of old.
