US1329817A - Art of precipitating suspended material from gases - Google Patents

Description

E. R. WOLCOTT.

ART 0F PHEclPxTATxNG SUSPENDED MATERlAL FROM GASES.

APPLICATION FILED JULY 5, 1917.

Patented Feb. 3, 1920.

EDSON RAY WOLCOTT, 0F LOS PRECIPITATION COMPANY, CALIFORNIA.

ANGELES, CALIFORNIA, :ASSIGNOR TO INTERNATIONAL OF LOS ANGELES. CALIFORNIA, A CORPORATION ART 0F PRECIPITATING- SUSPENDED MATERIAL FROM GASES.

Specication of Letters Patent.

Patented Feb.' 3, 1920.

Application led July 5, 1917. Serial No. 178,838.

To all whom, t may concern.' i

Be it known that I, EDsoN RAY WoLCoTT,

a citizen of the United States, residing at Los Angeles, in the county of Los Angeles and State of California, have invented a new and useful Improvement in the Art of Precipitating Suspended Material from Gases, of which the following is a specification.

This invention relates to theprecipitation of dust or fume from gases by the action of an electrical field, and the main object of my invention is to overcome certain diflculties which arise in the precipitation of certain materials, particularly those which are substantially non-conducting under cond 1- tions usually existing in processes of this kind.

In the electrical precipitation of suspend ed material from gases, the process now 1n general use involves production of an electrical field between one or more discharge electrodes andv one or more collecting or receiving electrodes, the electrical field being maintained between said electrodes by applying to said electrodes unidirectional current of sufficiently high tension to produce discharge of electricity from the discharge electrodes in such manneras to charge `the particles of suspended material in the gas and cause precipitation of such partlcles on the collecting or receiving electrodes. In order to insure effective precipitating action, it is usual to make discharge electrodes of such form as to produce a concentrated field adjacent thereto, and to thereby facilitate discharge of electricity therefrom, while, on the other hand, the collecting or receiving electrodes are usually formed with comparatively extended surfaces so as to minimize the field intensity adjacent thereto, and prevent as far as possible, any discharge therefrom. In order to provide for the greatest possible efficiency in precipitation, it is desirable to maintain as high a potential dif ference between the electrode as possible without causing disruptive discharge or arcing between the electrodes. For this purpose, it is desirable in general to apply the current to the electrodes in such manner that the discharge electrodes are negative, as ordinarily it is possible with this arrangement to maintain a higher potential difference between the electrodes than is practicable with the reverse polarity. In the operation of precipitating apparatus of this kind, there is, in general, in the case of deposits of solid material, continuous precipitation of the suspended particles on the collecting or receiving electrode under the electrical action and in most cases, the deposit so precipitated tends to remain on the collecting or recelving electrodes until it is forcibly removed by jarring or otherwise.

In the operation of electrical precipitation apparatus, it has been found that as the deposit accumulates on the receiving electrodes, there is, in some cases, a marked decrease in the efliciency of precipitation or the clearance of the gas, and this has also been found, in general, to be associated with a decrease in the arcing voltage-that is, the maximum voltage that can be maintained between the electrodes without arcing. This eect varies with the nature, material and condition of the deposit, being especially marked in the case of deposits, such as those of Zinc oXid produced in zinc smelting furnaces, wherein the deposit is of poorly conducting material and is of a light porous nature, and the effect also depends on the configuration of the deposit, being increased by any roughness or projection on the deposit tending to produce localization of the electric field, and in this connection, it may be stated that a much slighter degree of roughness or surface projection of any part l of the receiving electrode is sufcient to cause break down of the electrical field under given voltage conditions, in case of a poorly conducting deposit than in case of an effectively conducting surface.

lVhatever may be the cause of the phenomenon above referred to, it may be stated that it only occurs to a degree suiiicient to cause serious lowering of the arcing voltage when the deposit is of non-conducting or poorly conducting material, and I have found that in such cases, normal precipitation conditions may be restored and full electric voltage may be maintained by applying to the deposit a suitable agent which increases its conductivity to a certain extent.

An investigation of the phenomenon herein described has led lne to the conclusion that it requires, for its production, a configuration of surface, by reason of roughness or porosity, which leads to local ionization, and that there is also required, a sufficiently low conductivity of the deposited deposit in such material to enable the ionization to build up or accumulate an electric charge on the material, which eventually becomes of sufficient intensity to break down the dielectric. As far as I have found, the lowering of the arcing voltage is only marked when the discharge electrode is negative and the effect of the deposit is, in general, to lower the arcing voltage to or toward the limit that could be maintained in case the discharge electrode .were positive, and to thus lose the advantages of Working with negative dis charge electrode as set forth in patent to F. G. Cottrell, No. 1,067,974, dated July 22, 1913.

It may, therefore, be stated that in the case of electric-al precipitation of fumes or dust of non-conducting material,there is a tendency to an accumulation of an electrical charge by the deposit produced on the collecting or receiving electrodes and this charge tends to ionize gases in the vicinity ofthe collecting electrode, with the result that there is a considerable lowering of the voltage that can be maintained between the discharge elec-trode and the collecting or receiving electrode, and this lowering of the arcing voltage considerably. decreases the efficiency of precipitation.

My present invention consists essentially in conducting away the electrical charge from the deposit so as to prevent accumulation of charge thereon. I provide for this by applying to the deposit a suitable agent which increases the electrical conductivity thereof suiiciently to prevent accumulation of a charge thereon. My invention may be carried out in various ways as hereinafter described. v

A simple and effective way of carrying out my invention is to apply to the deposit, a conducting material which becomes intimately mixed with, oi distributed on the manner as to i'eiider the deposited body of material conducting, as a Whole. For this purpose, I prefer to use a conducting liquid, on account of the facility and thoroughness with which a liquid may be distributed in or upon the deposited niaterial. Any conducting liquid may be used, such as a solution, either acid or otherwise, according to the special results desired, or the special conditions of any particular case, but in general, ordinary water is the most available and suitable liquid. Thus, in the case of precipitation of a fume of Zinc oXid or other substantially non-conducting material, the resulting deposit may be made conducting in any of the following ways:

Water may be sprayed on to the deposit from suitable spray means located adjacent thereto, and within the precipitating chamber, said spray means being, for example, formed as the discharge electrodes themselves, which may be formed as perforated pipes through which the water.' may flow; or the water may be supplied to the deposit through perforations or pores in the collecting or receiving electrodes, which may, lfor example, be formed as porous walled fines, the water being supplied to the outside of said flues and seeping through the pores thereofso as to penetrate the deposit; or steam may be supplied to the gases containing the suspended materials either before or during their passage through the electrical ield, and the collecting or receiving electrodes may be maintained suiiiciently cool to cause condensation of water on the deposit. VVhei'e practicable, it is desirable to maintain the temperature of the collecting electrode below 100O C. when the process is being carried out under ordinary atmospheric pressure so as to insure condensation of moisture in or on the deposit. In some cases, however, even when working at a high temperature, it is possible to precipitate and maintain sufficient moisture in or on the deposit by supplying water thereto faster than it can be evaporatedfor exf ample, in some cases, it is suiiicient to spray or inject water into the intake end of the precipitatingchamber, or flue, at such a rate and in such quantity that part of the Water is precipitated in the form of spray on the deposit before it has had time to evaporate. Other methods of applying my invention depend on the precipitation, together with the deposit, of a conducting solid material, such as carbon or metallic particles which are supplied to the gases in sufficient quantity to render the deposit precipitated on the collecting` electrodes slightly conducting.

Another way of rendering a deposit conducting is to supply to the gases containing suspended particles, suitable reagents which will react either with the gases, or with the suspended particles to form substances which are sutliciently conducting to render the deposit as a whole, more or less conducting.

rll`he accompanying drawings illustrate several forms of apparatus, suitable for carrying out my invention, and referring thereto,-

lfigure 1 is a vertical section of a pre-V cipitating apparatus in which water is sprayed into the gas to be treated.

Fig. 2 is a section on line '2 2 in Fig. 1.

Fig. 3 is a partial vertical section of a 120 forin'mc the invention in which water is supplied through a perforated discharge electrode.

Figs. 4 and 5 are partial vertical sections of forms of discharge electrode, especially 12.t adapted for supply of water therefrom.

Fig. 6 is a vertical section of a precipitat ing apparatus provided with means for supplying moisture to the deposit through perforatioiis in the collecting electrode.

pipe form, or of the form in which the' collecting electrodes consist of plates or screens. v

Referrin to Fig. 1, the collectlng electrode 1 is s own as a vertical Hue, of cylindrical' or other suitable shape and provided, for example, at its upper end, with a chamber or header 2, into which leads a Hue 3 for supplying the gas to be treated The electrode Hue l may provided at its lower end with a suitable chamber or header 12 communicating with an outlet Hue 13. If desired, the apparatus may be arranged for upward How of the gases. Suitable means, such as hammer device 15 may be provided for dislodging the deposit from the receiv ing electrode and causing it to fall into a collecting bin 11, from which 1t may be removed by screw conveyer 9 or other means.

A discharge electrode 4 is mounted axially in electrode Hue 1 and may consist of a wire suspended from an insulated support 5 and tensioned by means of a weight 6 at its lower end. Any other suitable form of discharge electrode may beused.

A Water supply means is provided at the uimer 'end of the electrode Hue 1, and also, preferably, at one or more levels below the upper end, such water supply means'consisting. for example, of nozzles 7 opening tangentially or obliquely into the electrode Hue and adapted to receive water from a suitable supply pipe S, and to deliver such water m the form of spray into electrode Hue l. Similar nozzles 1G and 17 may be provided in the inlet header 2 and in the inlet Hue 3. Electrode Hue 1 is preferably grounded as indicated at 10 and discharge electrode 4 may be connected b v wire 10 to suitable .means for. supplying high tension unidirectional current, for example. in the manner set forth in patent to F. G. Cottrell, No. 895,729, (lated August 11, 1908.

AIn the Operation of the invention as carried out in this apparatus, the gas to be treated is passed downwardly through the collecting electrode or Hue 1. and suitable high 4tension unidirectional current is supplied to the electrodes in such manner as to cause fume or dust suspended in the gas to be precipitated, b v the action of the electrical Held, onA the collecting electrode 1. The gas to be treated is generally a hot furnace Hue gas, and in such case, Water'may be supplied thereto in the form of steam or in the form of inespray in such quantity as to be converted wholly or partly into va or, such supply of water or steam being e ected in the supply Hue 3 by nozzle 17, in the inlet header 2 by nozzle 16, or in the collecting electrode, by nozzle 7. By radiation from the collecting electrode, or by the cooling effect of injected water, a portion of the Water vapor so formed or supplied, is condensed on the solid fume or dust particles, or otherwise, and is precipitated together with such particles so that the resulting deposit is conducting. A cloud or mistofliquid particles may be also produced Within the precipitating chamber, by direct atomization ot' water, and may be precipitated with the fume or dust to render the latter conducting.

In case the gas is not hot, the water may be supplied thereto in the form of steam, and may be condensed to form of a mist by the cooling action of the as. In any case, the supply of water in liquid or vapor form may be just sufficient to slightly moisten the 90 deposit. Thus, I have found that zinc oxid fume, which, as it is deposited from hot zinc smelter or furnace gases, is an extremely poor conductor, may be made suliciently conducting to maintain eHective precipitation by applying thereto about one per cent. of water.

The nozzles 7, etc., may be used to inject any other suitable agent for making the deposit conductive, such as a finely divided conducting materialfor example, oxid of iron, or a chemical reagent capable of acting on a constituent of the gas or fume to form a conducting material. For example, in case lead sulfate is being precipitated, hydro- 105 gen sulfid gas may be supplied to the Hue gases, so as to convert the lead compound to lead sulfid, which is more conducting than the sulfate. v

As shown in Fig. 3, the discharge elec- 110 trode may be formed as a pipe having perforations 21 whereby water may be supplied to the gas within the treater from a suitable source, such as a tank 23 mounted on the insulated support 24 for said elec- 115 trode. The collecting electrode 19 may, in this case, be of any usual or suitable construction. The perforated pipe constituting the discharge electrode maybe covered with a porous sleeve 25, as shown in Fig. 4. Or, 120 as shown in Fig. 5, the discharge electrode vmay be formed as a wire or rod 26, covered, if necessary, with a porous jacket 27, and water may be supplied to the upper end of said wire from a Water dropping device 28. 125

The water for moistening the deposit may also be supplied through the receiving electrode, as shown in Fig. 46, wherein the receiving electrode is provided with perforations 31, through which water is supplied 130 from troughs or conduits 32, communicating with a supply pipe 33, said perforations being arranged to deliver Water at one or more levels in the receiving -e-lectrode, 1n such manner as to distribute the Water throughout the deposit on the electrode. The material may be washed olf of the` electrode When required, by momentarily.lncreasmg the amount of Water supplied, 1n the material so Washed ofi' being Withdrawn 'through funnel 35 and outlet 36 at the bottom of the apparatus; or, if desired, the material may be removed by mechanical knocking means of usual type as above explained in connection With Fig. l.

As shown in Fig. 7 the collecting electrode or flue, indicated at 39, may consist of a cylinder of porous materia-l, such as terra cotta, built up in sections if desired, and extending Within an outer cylindrical casing 40,

` forming a Water chamber 41, to Which Water may be provided in the upper header 44 for collecting electrode 39, to enable cleaning the latter by Washing with a hose, or by suitable scraping means, the material falling into a lower header 46, from which it is removed through outlet 47. As shown in Fig. 8, any desired number of porous walled collecting electrodes or flues 48 may be mounted in a single outer casing 49, from which Water is vsupplied to all the said fiues.

That I claim is l. In the art of electrical precipitation of suspended solid material from gases, the method of maintaining the dielectric strength of the precipitating electrical field in cases Where the precipitated material 1s non-conducting and tends to accumulate a charge by the action of the electrical field which consists in applying to the deposit, an agent capable of increasing the conductivity thereof sufficiently to discharge the same Without rendering the deposit fiuid.

2. In the art of electrical precipitation of non-conducting solid materials suspended in gases which consists in passing said gases between discharge and collecting electrodes, applying to said electrodes unidirectional current of suiiiciently high tension to cause precipitation of said particles on collecting electrodes and continuously applying to the deposit formed on collecting electrodes a conducting substance in sufficient quantity l t0 render the deposit conducting and prevent Vthe accumulation of a charge on'the deposit without rendering the deposit fluid.

3. In the art of precipitating from gases, particles of' non-conducting solid material suspended therein, the method of maintaining the dielectric strength of' the electrical field and maintaining the maximum possible field strength which consists in continually discharging` the deposit, so as to prevent the accumulation of a charge thereon, by applying to the deposit an agent capable of increasing its conductivity Without rendering the deposit fiuid.

Jr. In the art of precipitating suspended solid particles Jfrom gases by the action of an electrical eld, the method of maintaining the conductivity of precipitated material to prevent theaccumulation of an electric charge thereon, which consists in supplying water to the gases in such a manner that vit is distributed in the form of vapor in the gases and then cooling the gas so as to condense the Water vapor and precipitating the condensed water together with the solid particles suspended in the gas.

In the art of electrical precipitation from gases of suspended, finely divided, solid material, tending to form a non-conducting, porous deposit and to produce local ionization on such deposit, with resulting lowering of' the dielectric strength of the electrical precipitating field, the method of maintaining such dielectric strength which consists in applying to the deposit a conducting material in sufficient quantity to prevent accumulation of electrical charge thereon without rendering the deposit fluid.

6. In the art of electrical precipitation from gases of suspended, finely divided, solid material, tending to form a non-conducting. porous deposit and to produce local ionization on such deposit, with resulting lowering of the dielectric strength of the electrical precipitating field, the method of maintaining such dielectric strength which consists in applying to the deposit a conducting material in sufficient quantity to prevent accumulation of electrical charge thereon, the conducting material being` supplied to the gases and precipitated along with the non-conducting deposit.

7. In the art of electrical precipitation of suspended particles from gases, the method of maintaining the dielectric strength of the electrical precipitating field in cases when the precipitated material presents portions of such configuration and non-conducting nature as to cause local ionization by accumulated electrical charge, which consists in applying sufficient conducting material to the precipitated material to prevent the accumulation of electrical charges on such portions without rendering the deposit fluid.

8. The process of separating from gases suspended material tending to form a nonto prevent accumulation of electric chargeA thereon,

9. The process of separating from gases suspended solid material tending to form a non-conducting deposit, Which consists in supplving Water vapor to the gases and then passingl the A(rases through an electrical eld maintained between discharge and collecting electrode surfaces to precipitate the suspended material on such collecting electrode surfaces by electrical action and maintaining such collecting electrode surfaces suiciently cool to cause condensation of Water Vapor on the deposit and thereby render the deposit sufficiently conducting to prevent the accumulation of electric charge thereon.

10. The process of separating from gases suspended material tending to form a nonconducting deposit, Which consists in sup` plying Water to the gases in such manner that Water vapor becomes distributed in the gases, passing the gases through an electrical field between discharge and collecting electrodes to precipitate suspended material by electrical action on the collectingT electrodes and subjecting the gases to sufficient cooling action to cause condensation of Water vapor on the precipitated deposit sufficient to prevent accumulation of electrical' charge thereon, the amount of Water so supplied to the precipitated deposit being insufficient to form a fluid deposit, and ythen removing the precipitated deposit from the collecting` electrodes.

In testimony whereof I have hereunto sei' my hand at Los Angeles, Calif., this 27th da)v of June, 1917.

EDSON RAY WOLCOTT. lVitnesses:

MARY W. BROWN, ERNEST E. SHACKLEIT.

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