1038 INDUSTRIAL A N D ENGINEERING CHEMISTRY Vol. 16, No. 10

BIBLIOGRAPHY

I-Muller, Schoeller, and Schrauth, Biochem. Z., 33, 381 (1911). 2-Schoeller and Schrauth, Med. Klinik, 1912, No. 29. a-Davis, White, and Rosen, J. U r d . , 2, 277 (1918). 4-White, Hill, Moore, and Young. J . Am. Med. Assoc., 79, 877 (1922). 5-Schoeller and Schrauth, 2. Hyg. Infektionskrankh., 66, 497 (1910).

’ 6-Ibid., 82, 279 (1916). 7-Ibid., 70, 24 (1911). 8-Young, White, and Swartz, J . Am. Med. Assoc., 73, 1483 (1919). 9-Dimroth, B e y . , 31, 2154 (1898).

10-Brown and Pearce, J . Expll . Med., 31, 475 (1920); 36, 39 (1922);

11-Blumenthal, 2. Immunitdtsfouschung, rei1 I , Orig., 378 (1914). 12-Mulzer and Bleyer, Milnch. med. Wochsch., 67, 1163 (1920). 13-Launoy and Levaditi, Compt. rend. SOC. biol., 74, 18 (1916). 14-White, J . Am. Chem. Soc., 42, 2355 (1920). 15-Hill and Young, J . Am. Med. Assoc., 80, 1365 (1923). 16-Moore and Wasserman, Ibid., 81, 1840 (1923). 17-t3nodgrass, Lancet, 206, 117 (1924). 18-Hadjopoulos, Burbank, and Kyrides, New York Med. J . , 1921,

19--0’Conor, J . Am. Med. Asso;., 78, 1088 (1921).

Am. J . Syphilis, 6, 1 (1921).

532; Cole, Driver, and Hutton, J . Am. Med. Assoc., 79, 1821 (1922).

2O--Schamberg, Kolmer, and Raiziss, Ibid., 68, 1458 (1917); J. Infectious

21-Raiziss and Severac, J . Lab, Clin. Med., 9, 71 (1923). 22-Kolmer and Idzumi, J . Infectious Diseases, 26, 355 (1920). 23-De Witt, Ibid., 28, 160 (1921); Ibid., 30, 363 (1922); J . Am. Med.

24-Hill and Colston, Johns Hopkins Hospital, Bulls. 34, 220, and 373. 25--Engelmann, U. S. Patent 1,074,781 (October 7, 1913). 26-Zieler, Munch. med. Wochschr., 39, 1257 (1917). 27-Sax1 and Heilig, Wien. klin. Wochschr., 33, 943 (1920); Muhiing,

Munch. med. Wochschr., 68, 1447 (1921); Nonnenbruch, Ibid., 68, 1282 (1921); Marlinger, Med. Klinik , 18, 113 (1922); Hassencamp, Zentr. inn. Med., 43, 105 (1922); Issel, Deut. med. Wochschr., 48 (1921); Doll, Z. Heus-Gefdsskrankh., 14, 315 (1922) ; Schilsky, Therap. Gegenwaut, 63, 359 (1922); Schur, Wiener Arch. inn. Med., 6, 175 (1923); Bohn, Z. ges. esptl. Med., 31, 303 (1923).

28-Crawford and McIntosh, Proc. SOC. Exptl. B i d . Med ,21, 253 (1924). 29-Issel, loc. cit.; Marlinger, Eoc. cit. 30-Burwinke1, Munch. med. Wochschr., 69, 202 (1922). al-Piper, Am. J . Obstet. Gyn. , 4, 532 (1922). 32-Young and Hill, J . Am. Med. As~oc . , 82, 669 (1924). 33--Young, White, Hili, and Davis, Proc. A m SOL. Pharmacol. Exptl.

Therap., St. Louis Meeting, December 27, 1923, J . Pharmacol. Expt l . Therap., 23, 139 (1924).

Diseases., 24, 547 (1919).

Assoc , 68, 1458 (1917).

Electrical Precipitation in Retrospect’ By Walter A. Schmidt

WESTERN PRECIPITATION Co., Los ANGELES, CALIF.

HEX the suggestion was made to the writer to pre- pare a short discussion on electrical precipitation W with especial reference to the service which has been

performed by some of the older installations, the first thought was that these older plants have been written about so many times that it would be a waste of good space to print anything further about them. This very thought, however, brought with it a peculiar mental reaction. While thinking about these older installations and considering them from the general reader’s viewpoint, many of the facts which appeared to stand out prominently were not a t all those which formed the basis of former articles and discussions, but rather were those that were left untouched because of the uncertainties and ap- parent mysteries which surrounded this entire art. Per- haps, therefore, it may not be amiss to wander backward over a period of fourteen years and view the ar t of electrical pre- cipitation in retrospect, as it were. We are as a rule too prone to indulge in predictions and forecasts as to future accomplish- ments and achievements in connection with every new line of endeavor, and this reverse mental process may be a healthy diversion even if the resultant discussion fails completely in adding any useful information to our technical literature. So, with the editor’* permission, we will travel backwards instead of forwards to ascertain what we can find of general interest.

Our starting point in this backward journey is, of course, the present-day situation and knowledge of the art of electri- cal precipitation, with regard to which it can be safely stated that every kind and nature of dust and fume can now be pre- cipitated from the gas stream with any desired degree of efficiency-provided, always, that the economic balance be left out of consideration, as there are only too many smokes, fumes, and dusts which it does not pay to precipitate. At the other and far end of the line we emerge into a field only sparsely dotted with investigators, each of whom conducted investigations in his own way and laid down his fragment of the approach to the road over which we have to travel. Furth-

1 Received August 14, 1924.

est out in this field we find Hohlfeld, who a full century ago, in 1824, accidentally found that if he introduced a charged wire into a jar filled with smoke, the smoke disappeared. This was an interesting stunt which was dutifully recorded but soon forgotten. Guitard again made the same accidental discovery in 1850 and once more dutifully recorded it and the record likewise was forgotten. Sir Oliver Lodge in England and Karl Moller in Germany, in 1880, more or less simulta- neously and entirely independently, rediscovered the same phe- ncgnenon, but this time the matter was taken seriously a s static machines had by that time been developed and satis- factory experiments could be conducted. Lodge carried his investigations forward energetically over several years of dili- gent work and eventually built a commercial plant a t the Dee Bank Lead Works in cooperation with Walker. This plant was a complete failure, as we now can well understand. But a t that time this must have been a profound mystery. Why was it that smoke in a bottle would be precipitated a t a rela- tively low voltage when the same smoke in a flue subjected to a relatively high voltage went merrily on its way down the flue with total-disregard of the electrical screen which it was presumed was placed across the flue to trap it?

It was for the same reason that some of the more recent field experiments failed in giving any useful results, as we shall see. For the next twenty-five years speculation ran rampant and every now and then an investigator thought he had found the answer in the use of some special fanciful apparatus, but none of these creations grew to bear any commercial fruit even though a number found their way into the patent archives and were thus preserved for us. The work of these early pioneers must a t best have been disconcerting as well as mys- terious, for these investigators had only the static influence machines as their source of power and were totally ignorant of the phenomenon, to say nothing of the mechanism, of the ionization of gases, which after all lay a t the very foundation of their problem. Looking backward, it appears strange that Lodge, who had failed to precipitate fume in a small flue, should tackle the job of precipitating or rather dispelling

October, 1924 INDUSTRIAL AND ENGINEERING CHEMISTRY 1039

fog in the open as he tried to do for the cities of Liverpool and London--a job which precipitation engineers today would not tackle with much confidence of success.

F I G . 1 -DIAGRAMMATIC S K E T C H OF EARLY LODGE A N D W A L K E R PLANT, A S ILLUSTRATSD BY PATENT DRAWING

COTTRELL’S DISCOVERIES

Cottrell made the first big advance when in 1907 he took hold of the weapon which was given to him through the de- velopment of alternating current and the transformer. He rectified this high-tension alternating current and a t last had a positive source of power with which to work. His first little outfit gave him only a few thousand volts, but with this he made an accidental discovery one evening while working in his laboratory with the lights turned out. He observed that cotton-covered wires which were strung across the room and which served as conductors were luminous in the dark. Here was the answer to the question of how to produce proper discharge-namely, cover the electrodes with innumerable fine hairy points from which the charge could leak away. A small laboratory apparatus was built and tried out on sulfuric acid fume. It worked. A larger plant was built a t the plant of the E. I. du Pont de Nemours Company’s acid works on San Francisco Bay. This also worked. A still larger plant was then built a t the Selby smelter, but here some difficulties were encountered and the pubescent electrodes were replaced by serrated mica strips clamped between lead rods. This Selby plant has since been remodeled, but in all essential de- tails it is still the same plant which Cottrell built in 1907, and it is still performing its daily duties. Tests were then con- ducted on the Selby roaster flue by Cottrell, and the work was then carried to the Balaklala smelter where a zinky fume in- stead of acid mist had to be treated. Here trouble was encountered with a vengeance. At times this plant would function satisfactorily, while a t other times it refused to oper- ate for no apparent reason and the mystery of the ultimate cause was perplexing to say the least. By reason of the fume and smoke agitation then in progress in the Shasta Valley, the Ralaklala plant was shut down before the answer to the precipitation problem could be found. We now know that the mystery of this specific problem lay in the nature of the fume

APPLICATION OF GASEOUS IONIZATION TO PRECIPITATION

This is the distant approach to the road over which we are looking backwards. The writer’s association with electrical precipitation started in 1910 a t the plant of the Riverside Portland Cement Company, where the problem consisted of preventing the dust carried by the cement kiln gases from dis- charging into the atmosphere and settling over the surround- ing orange groves. The gases to be treated had a temperature of approximately 500’ C., and pubescent and mica electrodes, as previously used by Cottrell and Burns, ceased to function.

About this time Whitehead, Ryan, and Peek were carrying on their researches on corona losses, and the phenomenon of gaseous ionization was commencing to be well understood through the researches of J. J. Thompson and others. It was a relatively simple matter to apply this new knowledge to the precipitation art, and by running up the impressed voltage to approximately 50,000 it was found that a bare, smooth conductor served admirably well as an ionizing elec- trode, although it took a long time to definitely prove this. I n the commercial plant subsequently built a t the Riverside factory and put into operation in 1912, the ionizing electrodes consisted of No. 16 bare iron wires suspended between grounded plate-collecting electrodes 12 inches apart. The impressed voltage was approximately 40,000. The plant treated approximately 1 million cubic feet of gas per minute and collected an average of 100 tons of dust per day.

INVESTIGATIONS AT RIVERSIDE

This Riverside plant is of interest in several ways and a few words can be said about it to advantage, as it was here that much of the present-day knowledge of the art of elec- trical precipitation was gained. It was also here that many of the unprofitable investigations were started. It is of particu- lar interest to note that this plant, as designed over twelve years ago, is in all essentials the modern plate-type installation now being quite generally adopted for smelter practice and often erroneously referred to as the newer type of treater. The Riverside treaters have been in continuous operation ever since 1912 without alteration and with only minor and inconsequential repairs. When these treaters went into

F I G . %-EARLY EXPERIMENTAL APPARATUS USSD BY COTTRELL

operation they collected an average of 100 tons of dust daily. Since then modifications in kiln operations have cut the dust loss from the kilns so that now the average treater collection is approximately 65 tons per day. Throughout this period of twelve years these treaters have collected between 95 and 98

1040 INDUSTRIAL AND ENGINEERING CHEMISTRY Vol. 16, No. 10

per cent of the dust and fume entering them, and the average power consumption has been approximately 40 kilowatt- hours per hour. Over this entire period oftwelve years these treaters have handled approximately 2860 billion cubic feet of gas and have collected approximately 350,000 tons of dust. To visualize this quantity of dust we can picture a fully loaded freight train somewhat over 100 miles in length.

plant was built with this feature of design incorporated. It was also noted that the weaker the electrical field a t the surface of the collecting electrode the less was the tendency for the dust to creep along and this led to the development of the multiple-pipe type of precipitator, subsequently so uni- versally applied to the treatment of smelter fumes. It was erroneously assumed that, as the ease with which gaseous

FIG. 3 - E X P E R I M E N T A L A N D TESL, APPARATUS O N T H E ROOF OIP T H E FACTORY OF T H E RIVERSIDE P O R T L A N D C E M E N T C O M P A N Y

Strange as i t may seem, looking backwards as we are doing, i t was a t this highly successful plant that we got off the track of direct development in several respects. It was found on starting the first Riverside treater unit that the electrical equipment operated very smoothly on air load-that is, before the gases were turned in-and that on admitting the dust-laden gases apparently a complete collection of all solids was effected for several minutes, after which the efficiency dropped and operation settled down to approximately 95 per cent collection. The electrical equipment started to sputter and nothing that could be done would raise the effi- ciency or smooth down the operation of the electrical equip- ment. Innumerable experiments were conducted and ex- pedients without end were tried, but mostly without avail. The factory management, being pressed by litigation on the dust nuisance problem, could not wait and completed the installation consisting of ten double-unit treaters. On start- ing each of the twenty units the same experience was en- countered-namely, efficiency dropped after a few minutes' operation to approximately 95 per cent collection and the operation of the electrical equipment became less steady. This was a real puzzle. Subsequently, supplementary electrodes were installed in each treater unit and the effi- ciency of the entire plant was thereby raised to 98 per cent collection.

During this period of experimentation it was found that the dust had a tendency to creep along the electrode plates, which phenomenon had previously been observed by Burns in his work a t Selby, and the baffled plate was devised to correct this difficulty in a measure and the entire Riverside

ionization could be obtain increased with increase in voltage, a higher voltage with larger electrode spacing should give more effective precipitation and a t the same time give a simplified plant of lower first cost. Nothing could have been more mis- leading. Experimental work a t Riverside along this line was carried to a trial with a 4-foot stack and a voltage exceeding 100,000, and this work was subsequently carried to t h e h a - conda smelter by the Anaconda Smelter Commission, where voltages as high as 250,000 were employed. This expensive and time-consuming work was only useful in a negative way in that it showed the fundamental reasoning to be erroneous, but even then it did not give the explanation. This only de- veloped many years later.

Among these early experiments a t Riverside it was also tried to separate or control the individual electrode sets by split- ting the system into many small electrical units, inserting high resistan'ce in the line of each unit so as to prevent one unit from robbing the energy intended for another. Tests were made with steel-plate electrodes, wire-mesh electrodes, compound so-called box electrodes, tube electrodes, plane electrodes, baffled electrodes, and many other forms of col- lecting electrodes. Fine wire discharge electrodes, heavy wire electrodes, strip electrodes, and many other types of dis- charge electrodes were tested. High-frequency electrical impulses and low-frequency impulses were tried and subse- . quently high tension direct current was tested. Motor- generator-rectifier sets and synchronous motor-rectifier sets were compared. Highly loaded resistance circuits and low- resistance circuits, single large treater units and electrically subdivided treater units, various control and regulating de-

October, 1924 IiYDUSTRIAL A N D ENGINEERING CHEMISTRY 1041

vices in the electrical circuits, and many and diverse other stunts were tried, but always with essentially the same results.

All of this is of much interest as we look backwards] for the history of the next few years is filled with discussions and con- troversies as well as tests in various parts of the country at- tempting to decide what alternative features of design are fundamentally the best. The literature is filled with dis- cussionr; of the questions of motor-generator rectifier sets us.

FIG. &-CROSS S E C T I O N THROUGH RIVERSIDE COMMERCIAL INSTALLATION

synchronous motor-rectifier sets, pIate-type treaters us. pipe- type treaters, optimum gas velocity through treaters, and the like. It is perhaps no exaggeration to say that all these dis- cussioits were productive of practically nothing, for in the end each plant was built with such features of design as the engineer in charge liked best and probably with as good or as bad results as would have been the case had he built otherwise.

OTHER DEVELOPMENTS

It would take entirely too long to review this interesting development in detail. It is sufficient to say that these con- troversies and discussions continued over several years until two fundamental developments were made as the result of exhaustive and painstaking experimenting.

First, T;lrolcott showed that a fume or dust-covered electrode does not behave like a plain metallic electrode. The accumu- lated discontinuous material is often nonconducting, accumu- lating a charge and causing ionization adjacent to the collect- ing electrode. This establishes what has been termed a “reverse discharge” or “back ionization.” This reverse discharge prevents the free precipitation of particles and is ap- parently particularly detrimental in the collection of finely divided fume, less so when coarser dust is collected. When this condition is establishes both of the opposing electrodes dis- charge, causing heavy wastage of power with little useful re- sult. Electrical disturbances are also bad and the electrical equipment operates far from smoothly. Wolcott showed that all that is needed to obviate these difficulties is to conditionthe gases and fume properly, so that the deposited material will become conducting instead of being nonconducting, thus preventing the accumulation of a charge upon the deposit. This can be accomplished by humidification of the gases, acidification of the gases, or by loading the deposit with con- ducting materials. All these methods have been applied successfully in commercial plants.

Second, Anderson and Horne showed that each combina- tion of fume and gas possesses its own characteristics when viewed from a precipitation standpoint] and that these char- acteristics can be evaluated and expressed numerically as a constant in a precipitation equation. They also showed that

the important factor in determining treater efficiency is the length of time during which the gases are subjected to treat- ment, and not the velocity of the gas, as had been univer- sally assumed heretofore. They further showed that the effectiveness of one type of treater could be expressed in terms of another type. The result of this work of Anderson and Horne is that we can now intelligently express the per- formance of a precipitator by an equation having the general form:

where E = treater efficiency

C = type of treater K expresses the precipitation characteristics of the gas

These fundamental developments of Wolcott and Anderson and Horne have brought order out of chaos, and it is now pos- sible to reason intelligently upon the factors entering into the design of a precipitator. It is now possible to precipitate any type of fume and after simple tests to design a precipita- tor to give any desired efficiency of collection. This does not mean that any fume can be collected economically, for many materials are too difficult to collect to make it worth while in view of the values contained therein. Furthermore] some materials which must be conditioned before satisfactory pre- cipitation is obtainable are of such a nature that they will lose their value when conditioned with either moisture or acid or otherwise. From a precipitation viewpoint the answer may be one thing, while from an economic viewpoint the answer may be something quite different. Then, the proper choice of treater type and of various factors entering into the de- sign of a precipitator are questions which can only be answered by the engineer well experienced in the art of precipitation. It is not the purpose of this discussion to tell how or to lay down rules for the design of precipitators. This is a matter which should be entrusted to the engineers trained and ex- perienced in the art.

1 - E = CKt

t = time of treatment

and fume combination

FIG. RIVERSIDE C O X M E R C I A L INSTALLATION I N C O U R S E OF C O N - STRUCTION

I n closing our retrospective survey we might take another glance a t the high spots of the development of the art-a few simple preliminary experiments by Hohlfelt, Guitard, and Lodge, calling attention to possibilities ahead, a sound and well-planned application to a simple problem by Cottrell, a period of intricacy, complexity, and uncertainty, a gradual un- raveling of the fundamental factors hidden beneath the per- plexing superficial manifestations, and finally an intelligent understanding of the art. This is the history of nearly every development, particularly of all complex processes.