US2127727A - Rotary hearth furnace - Google Patents

Description

Aug. 23, 1938.

K. R. GOHRE ET AL ROTARY HEARTH FURNACE Filed April l3, 1937 5 Sheets-Shet 1 Inventors.-

KlLl/ZR. G'b'bre Jakob Sc]: M216 Paul Jpez'c/zerf Atlbrneg .23.'1938. MGQHREETAL 2,1 7 127 ROTARY HEAR'I'H FURNACE Filed April 13. 1937 5 Sheets-Sheet F1912 A flax-neg Aug. 23, R Q AL 2,127,727

ROTARY HEARTH FURNACE:

Filed April 13, 1937 5 Sheets-Sheet 4 PM! 5 eicherl B, OVWM Attorney In renters.-

Patented Aug. 23, 1938 UNITED STAT ES PATENT OFFICE ROTARY nnaa'rn FURNACE Application April 13, 1937, Serial No. 186,576

In Germany September '1, 1935 18 Claims.

1 The present invention relates to rotary hearth furnaces, and, more particularly, to rotary hearth furnaces for producing lead from sulphide ores by means of the roasting reaction process.

As those skilled in the art know, furnaces having a rotatable hearth of annular form provided the best results in treating sulphide ores of lead. These rotary hearth furnaces were generally provided with a stationary hood which extended into the close proximity of the exterior rim of the hearth. The hood was provided with one or with a plurality of working openings or doors for the treating of the charge and charging and stirring apparatus were provided at both This arrangement provided such conditions that even in the case of relatively small air leakage, the gas velocity in the openings of ,the hood and in the gap between the hood and the rim of the hearth was sufficient to completely prevent the gases or fumes formed during the reaction to, escape to The rotary hearth furnace provided a very substantial progress in the production of lead from sulphide ores in that it eliminated the presence of poisonous fumes in the proximity of thefurnace, increased the output and reduced the cost of production. It has been found, however, that even in rotary hearth furnaces substantial losses occurred through the formation of flying dust and of lead fumes. Particularly in proximity to the rabbles employed for the purpose of breaking up the charge, the air blast continuously emitted through the nozzles readily picked up small particles of the charge and cause the production of clouds of ying dust and lead fumes. Likewise, the rabbles had to be operated at a relatively high speed in order to break up the charge and to remove incrustations on the hearth in the proximity of the nozzles and the necessary quick movements of the rabbles had the detrimental effect of spraying substantial amounts of lead and to produce additional amounts of flying dust and lead Further amounts of flying dust and lead fumes have been caused by the irregular circula- 1 tion and currents of the air entering through the working doors and through the gap between the of the furnace. Of course, the losses and' other inconveniences caused through flying dust and lead fumes reduced the eiiiciency of rotary hearth furnaces and prevented to fully realize all of the advantages connected with the employment of the sides of these working doors.

the exterior.

fumes.

hood and the exterior rim rotary hearth principle.

We have discovered that all of the above difficulties may be eliminated in a remarkably simple manner.

It is an object of the present invention to provide an improved rotary hearth furnace .which is substantially free from the disadvantages and inconveniences connected with conventional hearth furnaces.

Another object of the present invention is to provide an improved rotary hearth furnace particularly for the production of lead from sulphide ores by means of the roasting reaction process which substantially eliminates and pre vents the formation of flying dust and of lead fumes and the losses connected therewith.

A further object of the invention is to provide a rotary hearth furnace for smelting lead 'ores involving an improved control of the air blast through the charge. 1

It is also within the contemplation of the in- .vention to provide a novel and improved rabbling and stirring apparatus for breaking up the charge and for the removal of incrustations substantially without causing the formation of flying dust and of lead fumes. 1

Moreover, the invention also contemplates an improved arrangement of the hood and its cooperating parts with respect to a rotatable hearth of annular shape in order to prevent the formation of irregular and eddy currents of air within the hood and to eliminate the production of localized overheating of the charge, formation of flying dust andof lead fumes.

The invention likewise contemplates an improved rotary hearth furnace which is simple in construction and which may be built and operated on a practical and industrial scale at a low cost.

Other and further objects and advantages of the invention will become apparent from, the following description taken in conjunction with rotary hearth furnace embodying the principles of the present invention;

Fig. 2 depicts a vertical sectional view of a modified embodiment of the invention, taken on the line 2-2 of Fig. 3;

Fig. 3 shows a section taken on line 3-3 of Fig. 4 is a vertical sectional view of a further modified embodiment of the invention taken on line 4-4 01 Fig. 5;

Fig. 5 illustrates a top elevational view of the rotary hearth furnace shown in Fig. 4;

Fig. '6 shows a fragmentary top elevational the accompanying drawings, in which: i

Fig. 1 illustrates a vertical sectional view of a view having parts in sectionof a portion of the furnace shown in Fig. 1 and illustrating the arrangement for controlling the air blast to the charge and for cleaning thenozzles;

Fig. 7 is a similar view of the rabbling mechanism.-

Fig. 8 illustrates a vertical sectional view of a further modified embodiment of the invention. taken on .line 8--8 of Fig. 9; and

Fig. 9 depicts a top elevational view of the hearth shown in Fig. 8.

Broadly stated, according to the principles of the present invention, the formation of flying dust and of lead fumes is reduced to a minimum or is completely eliminated by providing intermit- 'tent operation of the nozzles employed for the the time they are in immediate proximity of the rabbling apparatus. This removes one of the principal causes of flying dust and lead fumes.

In addition to this, the rabbling apparatus is so constructed and arranged that the breaking up and the rabbling of the charge is carried out cautiously and at a relatively low speed without causing spraying of the lead. Moreover, the charging apparatus and the stationary hood located above the hearth with its gas conduits are so constructed that the production of dust in the newly added charge and the volatilization of lead are greatly reduced and that the small amounts of flying dust accidentally produced are deposited already within the hood. In this man.. ner, the principles of the present invention make it possible to eliminate all of the causes for the formation of flying dust and for the occurrence of volatilization of lead.

In the elimination of flying dust and of lead fumes, the proper control of the nozzles is of the greatest importance. In prior rotary hearth furnaces, the nozzles have been generally. freed from the covering charge materials in the portions of the hearth where the rabbles are breaking up the charge so that the air blast from the corresponding nozzle was blowing over the charge. Of course, this circumstance greatly increased the production of flying dust and of lead fumes. According to the principles of the present invention, this most important cause of the formation of flying dust is eliminated by shutting off the nozzles which are in immediate proximity of the rabbling apparatus. Preferably, the nozzles are shut off by means of an apparatus consisting of a plurality of rods which are rotatably mounted on a common vertical shaft, for example, in the form of a star arranged in the plane of the nozzles. This nozzle controlling apparatus is so arranged within the windbox that its axis of rotation is substantially in a vertical plane passing through the axis of rotation of the hearth and through the rabble of the rabbling apparatus. The rods have such dimensions that the one which happens to be in proximity to the rabble will just about protrude through the nozzle which is in the corresponding position. The. distance of the free end of the rods from each other corresponds to the distance between the nozzles so that rotation of the hearth will cause successive engagement of the rods-with the nozzles of the hearth and will set up rotation of the rods around their common axis. As soon as one of the rods engages a nozzle, the passage of air through said nozzle is greatly reduced or completely prevented, so that in the region of the rabbling apparatus no air blast is blown into the hearth. A further important advantage of this arrangement is that the engagement and disengagement of the rods with the nozzles automatically clean the nozzles, in that charge particles which entered the nozzles and incrustations formed within the nozzles are removed by the rods and are returned to the hearth. Thus, manual cleaning of the nozzles, which has been absolutely necessary in prior rotary hearth furnaces, is completely eliminated and the servicing and supervision of the furnace are greatly facilitated.

In order to further reduce the formation of lead fumes in the proximity of the rabbling apparatus, the invention provides an improved rabbling apparatus which permits relatively slow operation of the rabbles. As those skilled in the art know, the object of the rabbles in the hearth furnaces of the type contemplated is not merely to break up the charge but also to remove the incrustations formed on the back wall of the hearth in the proximity of the nozzles. This object can be accomplished only if the rabble sweeps over all of the surface of the back wall of the hearth during the time the portion of the rotating hearth is within the reach of the rabble. Generally speaking, in the operation of rotary hearth'furnaces it is highly' desirable to maintain relatively fast rotation of the hearth in order to obtain a high output. Of course, under these operating conditions it is necessary to make the rabble relatively wide and to operate. the same at a high speed, because otherwise it would be impossible to obtain overlapping of the individual rabble strokes on the surface of'the hearth. The great width and the fast operation of the rabble, however, has the disadvantage that substantial amounts of lead are sprayed about and that simultaneously with the breaking up of the charge very great quantities of dust are formed.

In order to eliminate also this cause of the formation of dust andndi fumes, the presentinvention provides a plurality of rabbles, generally two, arranged in close proximity of each other. Preferably, the rabbles arerelatively narrow and are operated alternately. This makes it possible .to operate the rabbles at a relatively lowspeed so that splashing -orspraying of the lead is avoided and a cautious but completely satisfactory breaking up of the charge is obtained. It is preferred to provide the rabbles at a slight distance from each other and to operate them by means of individual eccentrics removably connected to a common drive. This makes it possible to individually disconnect each rabble and to remove the same from the charge for the purposes of inspection, repair, or replacement.

Thus, the furnace embodying the principles of the invention successfully eliminates the principal causes of the formation of dust and of fumes and the only remaining possibility of the formation of dust .and of fumes is in the irregular movement or streaming of the air entering through the working doors and through the gap between the hood and the exterior rim of the furnace, the so-called'air leakage, or false draft. Damming up of this air and the formation of eddy currents therein may cause picking up of increasedpuantities of dust from the charge dropped'onto the hearth. Moreover, it can easily happen that certain portions of the hearth are not suiliciently exposed to theeilect of this draft which may cause localized overheating of the charge and consequently volatiiization of subto strong currents of gas.

Thus, for example, the fresh charge material consisting of well moistened ore mixed in a suitable proportion with coalmay be fed to an annular charging plate provided within or without the hood and rotating with the hearth. This charging plate is only at a slightly higher level than the charge material on the hearth so that the height of fall from the charging plate to the hearth is very low and is incapable of caus-- ing the production of flying dust. In addition to this, premature drying 'out of the charge ma-- terial in the charging apparatus is prevented by providing the discharge end of the charging bin size where the charge is fed simultaneously to the hearth at a plurality of points. It is of advantage to provide the charging bin with a plurality of adjustable discharge openings which, if desired, may be completely disconnected or shut oil. In smaller hearth furnaces where the charge is fed at a single point, insteadof the above arrangement, a stationary charging device mounted on the inner or outer side of the hood may be employed, which is provided with discharge discs and which is completely encased. In all cases the charge material from the charging plate or disc may be conducted through pipes or slide surfaces to the charge material on the hearth whereby the possibility of dust formation is still further reduced.

In the case when ores causing very high temperatures during the operation of the furnace are employed, it is of advantage to avoid throwing the fresh charge material on the charge present on the hearth and to embed the fresh charge into the old and partly treated material. To this purpose the charging of the fresh material is provided in the region of the working door so that the freshly added charge falls in proximity of the back wall of the hearth on top of the broken up charge where the removal of the slag is approximately finished. By returning the portions of the charge which during the breaking up operation have been displaced towards the exterior rim of the hearth and into their former position, this returned charge will cover the freshly added and cold charge matei-ial. Preferably, this procedure is carried out manually by the smelter in charge of the hearth with a scraper, in the same manner as in the conventional manually operated simple lead ,hearths. Thereby the intermediate and the lower portions. of the charge are cooled and excessive temperatures within the charge are 7 avoided. As those skilled in the art know, m n

operating temperatures have the disadvantage that not only the volatilization of lead but also the formation of slag are greatly increased and the output of metal is considerably lowered. In rotary hearth furnaces generally, water cooling is provided for the back wall of the hearth for the purpose of reducing the working temperatures and for carrying away heat from thecharge. This cooling is further supplemented by the cooling effect of the air entering from the outside underneath the hood. According to the present invention, the intimate contact of this air with the charge and thereby emcient cooling conditions are obtained by the special arrangement and construction of the hood and of the -gas outlets. Since in the prior rotary hearth furnaces the major portion of the air is introduced through the working door into the hood, in the rotary hearth furnace embodying the invention and employing only one working door, this door is located at one side and the gas outlet at the other side of the furnace ring so that the gas outlet is provided in the portion of the hood directly opposite to the working door. Thus the air entering through the working door is uniformly distributed to both sides and a uniform gas stream is developed above each half of the hearth between each working door and'gas outlet without having obstacles in the path of these streams and without causing any eddy currents which facilitate the production of flying dust. The hood may be made large enough to considerably reduce the velocity of the gas stream within the hood.

Likewise, and in consideration of the same principles, in a hearth having a plurality of working doors, the outlets i'or the furnace gases are preferably provided approximately half way between two successive working doors. This makes it possibleto obtain the most favorable conditions for the streaming of the gases and for the dissipation of the heat. It has been ascertained that in this manner the formation of flying dust and particularly the volatilization of lead are still further reduced. Increasing the size of the hood has the further advantage that a great part of the flying dust possibly formed'is separated from the gases within the hood and falls directly back on the charge in the hearth. The separation of dust in hot condition above the hearth can be further promoted by special construction or shape of the hood. These conditions may be obtained, for example, by subdividing the inner space of the hood in the form of a cyclone separator or by changing the direction or speed of the stream of waste gases within the hood. Thus, the space within the hood may be subdivided by means of an inclined annular wall. At suitable portions of the wall openings may be provided for the passage of the gases from the lower space or portion of the hood into the upper space thereof and the gas outlet pipes are connected to suitable portions of the upper space. The dust deposited in the upper space of the hood is returned to the hearth by means of small slots in said inclined annular wall which are preferably arranged in the proximity of the cooled wall of the hearth. The same results, to wit: returning the dust to the hearth, may be *obtainecf by providing a relatively low hood in combination with high and very wide gas outlet pipes, (Fig. 5) 28 above the hood. These gas outlet pipes act in the manner of a dust chamber so that part of the dust contained in the gases is deposited therein and is returned to the charge on the hearth.

For the purpose of giving one skilled in the art a better understanding of the invention, a preferred embodiment will be described in conjunction with the accompanying drawings.

Referring more particularly to Fig. 1, an annular hearth I is rotatably mounted by means of rails 2 on rollers 3. Hearth I is driven by means of a motor (not shown) and a. reducing gear II through a pinion 39 and crown wheel 4. Nozzles 5 are provided around the circumference of the hearth and are supplied with blasting air through a wind box 6 connected to the hearth and rotating therewith. Wind box '6 is in turn supplied with air through a centrally located stationary pipe I located underneath the wind box. Stationary air pipe is connected to a suitable source of air such as blower, or the like having a conduit discharging into pipe I through an opening 8. The air pipe is provided with a man hole 62, through which the inside of the pipe is accessible for inspection and cleaning.

Hearth I is centered and wind box 6 is connected to pipe I in a substantially air tight manner through sliding surfaces 9 and packing seal III. A support IB bearing the rotatably mounted rods I! and I8 is secured to the upper end of the stationary air supply pipe I in such a manner that its axis of rotation is approximately in the same vertical plane as the rabbles I2 and I3 of a rabbling apparatus I4. This will cause rods I'I, I8, etc. to successively engage and disengage corresponding nozzles in the hearth and will close the nozzles during the time when they pass by the rabbles and if necessary also during the time the nozzles are in the region or in a part of the region of the working door. Further similar devices for intermittently shutting off the nozzles may be provided at other portions of the furnace, if necessary.

The operation of these devices for controlling the nozzles may be more completely understood by referring to Fig. 6 illustrating a top elevational view of the device for closing and cleaning the nozzles and a sectional view of the hearth wall taken in the plane of the nozzles. A

. vertical shaft I5 around which rod supporting member I6 bearing rods I1, I8 and I9 is mounted, is rotatably supported in a bracket 20, which is rigidly connected to pipe I. Rod I8 is shown being within conically shaped nozzle 2I, while.

rod ll just begins to enter nozzle 22 and rod I9 leaves nozzle 23. The rotation of hearth I will cause the nozzles H to engage and displace rod I8 until the next rod I! completely engages nozzle 22. Thus. the rotation of the rods and of their support is caused and is directly controlled by the rotation of the hearth. This arrangement has the great advantage of providing positive control and closing of the nozzles and at the same time deposits and incrustations within the nozzles are automatically removed and returned to the hearth and cannot drop into the wind box. In other words, the device provides positive control and automatic cleaning of the nozzles in a simple and foolproof manner.

Hearth I of the furnace is provided with a water cooled back wall 24' which rotates with the hearth. A gas tight joint between back wall 24 and the inner wall 25 of stationary hood 26 is provided by means of a water or sand seal 21. The gases are removed from the hood through wide gas outlet pipes 28 which are mounted in the cover of the hood between two working doors.

In case more than two working doors are employed, the number of gas outlet pipes is increased accordingly. In order to maintain the lead bath in the hearth at a substantially uniform level throughout the operation of the rotary hearth furnace, a circular channel 48 is provided around the hearth. The lead discharged from the hearth through discharge spout 63 flows into channel 48 and finally arrives into a collecting and casting receptacle 49. in admixture with carbonaceousmaterials and in some cases with other additional reducing agents, is introduced in conventional manner onto an annular charging plate 59 rotated with the hearth and is charged from this plate to the hearth. The outer edge of the hearth is provided with a working plate 64 which facilitates the removal of slag from the broken up charge.

The relatively high and wide hood, such as 26 in Fig. 1, is particularly advantageous in smaller furnaces. In rotary hearth furnaces of larger size the hood may be constructed in the form illustrated in Figs. 2 and 3. From the illustration of this modified embodiment of the invention the drive for rotating the hearth, the air conduits'and the device for controlling the nozzles have been omitted for the sake of sim- The ore plcity. The air entering through the workingdoors 51 and the furnace gases are conducted in a regulated stream over the hearth and rise into the upper portion of the hood through openings 58. The upper portion of the hood is defined by plates 29, 30, 3| and 32. Openings 58 are located approximately above the middle of the hearth portion between the two working doors; whereas the gas outlets 28 are located above the working doors. In this form of the hood the gases not only provide good cooling of the charge on the hood but at the same time due to the change in the direction of their streaming and in their velocity and due to their reflection, most of the dust carried by the gases is precipitated. The precipitated dust slides downwards along inclined surfaces 28 and 30 and arrives at a gap 33 through which it drops back on top of the material on the hearth. Gas outlet pipes 28 are extended into the upper space of the hood so that a streaming of the gases is produced similar to that prevailing in cyclone separators whereby the precipitation of dust is still further improved. The hearth is provided with a rabbling apparatus i4 having a pair of rabbles I2 and i 3.

In the modified embodiment of the invention shown in Figs. 4 and 5 a relatively low hood 26 is employed, which will cause substantially higher velocities of the gases above the hearth. The precipitation of dust is performed in the very high and wide gas outlet pipes 28 which in addition may be constructed in the manner of a cyclone separator. Preferably, the device'conveying the charge to the hearth is provided in close proximity to the hearth. A low cooling sleeve 24 carries a charging ring 34 on which the ore is deposited by the discharge openings 35 of the charging bin. Discharge openings 35 and the conduits associated therewith are of a quadratic cross section and extend through cover 52 of the dust collecting hood approximately to the charging ring. Looking in the direction of hearth rotation, in the back wall of each charging outlet there is an opening cut out, the free width of which may be adjusted by means of a slide 36. Slide-36 may be opened to a smaller or, greater extent, or may be completely closed by means of a threaded spindle 6|. The charge material is thrown on the hearth by means of a scraper60 shortly after it has been deposited on the charging ring. The wind box and the nozzle cleaning device are accessible through the air pipe which to arrange the charging apparatus and the conduits for the cooling water in this space. In some cases it is of advantage to provide a sheet metal ring 31 around the outer wall of the hood whereby the streaming of air along the outer surface of the hood is regulated and the cooling of the outer wall is ftu'ther promoted. A rabbling apparatus l4 having rabbles l2 and His provided at one side of the hearth.

Fig. 7 is a detail view of the rabbling apparatus l4 for actuating rabbles l2 and I3. .A pair of discs or ecce'ntrics 40 and 4| is provided, supporting rabbles l2 and I3 displaced by about 180 with respect to each other. Discs 40 and 4| are rotated by means of a motor 42, through a reduc-' ing gear 43, a shaft 44 and drive wheels 45 and 46 of which one, wheel 46, may be disengaged by means of a clutch 41. The speed of rotation of discs 46 and 4| and therotation' of the hearth are so adjusted that the strokes of the rabbles overlap and that all of the surface of the hearth and the charge therein are uniformly subjectedto the rabbling treatment.

The rotary hearth furnace illustrated in Figs. 8 and 9 is similar in construction to the ones shown in Figs. 1 to '7 and similar reference characters are used to denote corresponding parts.

This hearth is provided with a single charging apparatus 66. The ore is charged from a bunker 66 into a tube 68 through the intermediary of a discharge plate 61 arranged within a closed housing 66. Tube 68 is carried through hood 26 of the hearth which extends downwardly almost to the hearth. This hearth furnace is provided with a single working door 69. Opposite to the working door 691s located draw-off ill for the hearth gases. Thus, the air introduced underneath the hood through working door 69 is divided into two branches streaming right and left towards drawoff 10 above the twmhalves of the hearth. These .two branches of streaming air take up the hearth gases and are again united after leaving the hearth.

We claim: 1. A rotary hearth furnace for the production of lead from sulphide ores comprising an annular rotary hearth, a stationary hood above said" hearth, said hood extending close to the outer rim of the hearth, a windbox in the center of the hearth, a plurality of nozzles operatively assosaid hood to said hearth, a plurality of nozzles associated with the inner rim of said hearth and in Communication with said windbox, and means windbox in the center of the hearth, a plurality oi."

nozzles associated with the inner rim of said hearth at spaced intervals of the circumference thereof in communication with said windbox, means for maintaining said hearth in continuous rotation, and means for intermittently shutting off the passage of air through said nozzles during the time they are in proximity of said rabble arms.

4. A rotary hearth furnace for the production of lead from sulphide ores comprising an annular rotary hearth, a stationary hood above said hearth, said hood extending close to the outer rim of the hearth, at least one rabble arm extending through said hood to said hearth, a windbox in the center of the hearth, a plurality of nozzles associated with the inner rim of said hearth at spaced intervals of the circumference thereof in communication with said windbox, a rotatable member in said windbox, and a plurality of rods radially and equidistantlally mounted on said member, said rods being adapted to engage said nozzles during rotation of said hearth and to impart rotary displacement to said member for intermittently shutting off and to clean said nozzles.

5. A rotary hearth furnace for the production of lead from sulphide ores comprising an annular rotary hearth, a. stationary hood above said hearth, said hood extending close to the outer rim of the hearth, at least one rabble arm extending through said hood to said hearth, a windbox in the center of the hearth, a plurality of nozzles associated with the inner rim of said hearth at spaced intervals of the circumference thereof in communication with said windbox, means for maintaining said hearth in continuous rotation, a rotatable member in said windbox, a'

shaft for said member located substantially in the plane of said rabble arm, and a plurality of rods radially and equidistantially mounted on said member, said rods being adapted to be engaged by said nozzles during rotation of said hearth and to impart rotary displacement to said members for successively and intermittently shutting off and for cleaning the nozzles within the. region of said rabble arm.

6. A rotary hearth for the production of lead from sulphide ores comprising an annular rotary hearth, a stationary hood above said hearth extending close to the outer-rim thereof, a windbox in the center of said hearth, a windpipe communicating with said windbox and coaxial therewith, a plurality of tapering nozzles associated with the inner rim of said hearth at spaced intervals of the circumference thereof in communication with said windbox, rabble arms extending through said hood to said hearth, a rotatable member in said windbox, a vertical shaft for said member mounted on said windpipe substantially in the plane of said rabble arms, and a plurality of rods radially and equidistantially mounted on said member, said rods being adapted to be engaged by said nozzles during rotation of said hearth whereby a rotary displacement will be imparted to said members and the nozzles within the region of said rabble arms will be positively and automatically controlled and cleaned.

7. A rotary hearth furnace for the production of lead from sulphide ores comprising an annular rotary hearth, -a stationary hood above said hearth extending close to the outer rim thereof, a windbox in the center of the hearth, a plurality of nozzles associated with the inner rim of said hearth at spaced intervals of the circumference thereof in communication with said windbox, at least one rabbling device located in proximity of said hood, a pair of rabble arms associated with said rabbling device and extending through an opening in said hood to said hearth in close proximity to each other, a roller underneath said opening for each of said rabble arms for slidably supporting the same, means including a rotatable disc for each of said arms for reciprocatingly and alternately actuating said arms, means for maintaining said hearth in continuous rotation, and means for intermittently shutting off the passage of air through said nozzles during the time they are in proximity to said rabble arms.

8. A rotary hearth furnace for the production of lead from sulphide ores comprising an annular rotary hearth, a stationary hood above said hearth extending close to the outer rim thereof, a windbox in the center of the hearth, a plurality of nozzles associated with the inner rim of said hearth at spaced intervals of the circumference thereof in communication with said windbox, at least one rabbling device located in proximity of saidhood. a shaft associated with said device, a pair of eccentrics driven by said shaft, a rabble arm for each of said eccentrics having one of its ends connected to said eccentric and having its other end extending through an opening in said hood to said hearth, said rabble arms being located in close proximity to each other, a roller underneath said opening for slideably supporting said arms, means for driving said shaft for reciprocatingly and alternately actuating said arms, means for maintaining said hearth in continuous rotation, and means for intermittently shutting -oif the passage of air through said nozzles during the time they are in proximity to said abble arms. F

9.v A rotary hearth furnace for the production of lead from sulphide ores comprising an annular rotary hearth, astationary hood above said hearth extending close to the outer rim thereof, a windbox in the center of ,the hearth, a plurality of nozzles associated with the inner rim of said hearth at spaced intervals of the circumference thereof in communication with said windbox, a

- rabbling device having at least two rabble arms extending through said hood to said hearth in close proximity to each other, means for reciprocatingly and alternately actuating said rabble arms, means for maintaining said hearth in continuous rotation, and means for intermittently shutting off the passage of air through said nozzles during the time they are in proximity of said rabble arms.

10. A rotary hearth furnace for the production of lead from sulphide ores comprising an annular rotary hearth, a stationary h od above said hearth, said hood extending clo e to the outer rim of the hearth, a windbox in the center of the hearth, at least one rabble arm extending through said hood to said hearth, a plurality of nozzles associated with the inner rim of said hearth and in communication with said windbox,

means for intermittently and successively shutting off the passage of air through said nozzles.

means for charging ore to said hearth, means for discharging lead therefrom, and at least one working door for handling the charge on the hearth.

11. A rotary hearth furnace for carrying out a continuous roasting reaction process comprising an annular hearth of substantially semi-circular cross section, a stationary hood above the hearth, a rabbling device including a pair of rabbling arms alternately actuated by a pair of eccentrics and extending through the hood to the hearth, a windbox in the center of the hearth, a plurality of tapered nozzles associated with the inner rim circular cross section, a stationary hood above the hearth, a rabbling device includinga pair of rabbling arms alternately actuated by a pair of eccentrics and extending through the hood to the hearth, a windbox in the center of the hearth, a plurality of tapered nozzles associated with the inner rim of said hearth and in communication with said windbox, means for controlling the passage of air through said nozzles in accordance with the rotation of said hearth, an annular charging plate arranged at a slight height above the hearth and rotatingwith said hearth, a charging hopper for depositing charge material on said charging plate, and a scraper for stripping oif said material from said plate and to deposit said material on the hearth.

13. A rotary hearth furnace for the production of lead from sulphide ores comprising an annular rotary hearth, a stationary hood above said hearth extending close to the outer rim of the hearth, awindbox in the center of the hearth, a plurality of nozzles associated with the inner rim of said hearth in communication with said windbox, rabble arms extending through said hood to said hearth, means .for intermittently shutting ofl air through said nozzles during the time they are in proximity of said rabble arms, partition walls separating said hood into an upper and-a lower portion, working doors in said lower portion for handling the charge on the hearth, gas outlet pipes extending into said upper portion, and openings in said partition walls for permitting the passage of gases from said lower into said upper portion of the hood.

14. A rotary hearth furnace for the-production of lead from sulphide ores comprising an annuiarrotary hearth, a stationaryhood above said hearth extending close to the outer rim of the hearth, a windbox in the center of the hearth, a plurality of nozzles associated with the inner rim of said hearthin communication with said windbox, rabble arms extending through said hood to said hearth, means for intermittently shutting of! air through said nozzles during the time they are in proximity of said rabble arms,

inclined partition walls separating said hood into an upper and a lower portion, working doors in said lower portion for handling the charge onthe hearth, gas outlet pipes extending into said upper portion, openings in said partition walls to establish communication between said upper and lower portions, and a gap in the lowermost portion of said inclined partition walls for returning precipitated dust to the hearth.

15. A rotary hearth furnace for the produc-- tion of lead from sulphide ores comprising an annular rotary hearth, a stationary hood above said hearth extending close to the outer rim of the hearth, a windbox in the center of the hearth, a plurality of nozzles associated with the inner rim of said hearth in communication with said windbox, rabble arms extending through said hood to said hearth, means for ining doors to establish communication between said upper and lower portions, and a gap in the lowermost portion of said inclined partition walls for returning precipitated dust to the hearth; v

16. A rotary hearth furnace for the production of lead from sulphide ores comprising an annular rotary hearth, a relatively low stationary hood above said hearth adapted to cause high' gas velocities, a windbox in the center of the hearth, a plurality of nozzles associated with working doors, and adapted to cause the precipitation of dust therein.

17. A rotary hearth furnace for the produc? tion of lead from sulphide ores comprising an annularrotary hearth, a relatively low stationary hood above said hearth adapted to cause high gas velocities, a windbox in the center of the hearth, a plurality of nozzles associated with the inner rim of said hearth in communication with said windbox, rabble arms extending through said 'hood to said hearth, means for intermittently shutting off air through said noz-- zles during the time they are in proximity of said rabble arms, working doors in said hood for handling the charge on the hearth, and high gas outlet pipesprotruding into the top portion of said hood, said pipes being constructed in the form of a cyclone separator and being adapted to precipitate dust therein.

18. A rotary hearth furnace for the production of lead from sulphide ores comprising an annular rotary hearth, a relatively low stationary hood above said hearth adapted to cause high gas velocities, a windbox in the center of the hearth, a plurality of nozzles associated with the inner rim of said hearth in communication with said windbox, rabble arms extending through said hood to said hearth, means for in-- termittently shutting 01f air through said nozzles during the time they are in proximity of said rabble arms, working doors in said hood for handling the charge on the hearth, high gas outlet pipes protruding into the top portion of said hood located equidistantially from said working doors and adapted to cause the deposition of dust therein, a charging hopper for feeding ore to said. hearth, a ledge rotating with said hearth and arranged at a slight height above the hearth upon which-the ore is deposited from said hopper, and an adjustable scraper for scraping ore from said ledge on to said hearth.

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