US3770257A

US3770257A - Level control for rotating furnaces

Info

Publication number: US3770257A
Application number: US00159259A
Authority: US
Inventors: J Bell
Original assignee: International Nickel Co Inc
Current assignee: Huntington Alloys Corp
Priority date: 1970-07-08
Filing date: 1971-07-02
Publication date: 1973-11-06
Anticipated expiration: 1990-11-06
Also published as: JPS5327203B1; GB1322427A; NO129587B; ZA714231B; DE2133918A1; DOP1971001824A; FR2100442A5; CA946148A

Abstract

A horizontally mounted rotary cylindrical furnace is provided with an internal helical flight for at least the preponderant part of one revolution but not more than two revolutions of the furnace so that two zones are formed within the rotary furnace. The pitch of the flight can be controlled so that material fed to the first zone is conveyed to the second zone at a rate greater than the rate of feed to the first zone. The height of the helical flight can be set so a deeper bed and greater retention time is obtained in the solids down stream of the flight. Also disclosed is a discharge mechanism which is a tapered helical rotor with its larger vanes being coextensive with and fixed to the furnace diameter and the smaller vanes being mounted in an co-extensive with and fixed to a take-off conveyer tube.

Description

United States Patent 1 Bell 11 3,770,257 51 Nov. 6, 1973 I LEVEL CONTROL FOR ROTATING FURNACES [75] Inventor: James Alexander Evert Bell,

Port Colborne, Ontario, Canada [73] Assignee: The International Nickel Company, Inc., New York, N.Y.

[22] Filed: July 2, 1971 21 Appl. No.: 159,259

[30] Foreign Application Priority Data July 8, 1970 Canada 087,693

[52] US. Cl. 266/18, 75/33 [51] Int. Cl. F27b 7/16 [58] Field of Search 266/36 H, 39, 18,

[56] References Cited UNITED STATES PATENTS Primary Examiner-Gerald A. Dost Att0rneyMaurice L. Pine] 57 ABSTRACT A horizontally mounted rotary cylindrical furnace is provided with an internal helical flight for at least the preponderant part of one revolution but not more than two revolutions of the furnace so that two zones are formed within the rotary furnace. The pitch of the flight can be controlled so that material fed to the first zone is conveyed to the second zone at a rate greater than the rateof feedto the first zone. The height of the helical flight can be set so a deeper bed and greater retention time is obtained in the solids down stream of the flight. Also disclosed is a discharge mechanism which is a tapered helical rotor with its larger vanes being coextensive with and fixed to the furnace diameter and the smaller vanes being mounted in an coextensive with and fixed to a take-off conveyer tube.

7 Claims, 3 Drawing Figures LEVEL CONTROL FOR ROTATING FURNACES The present invention pertains to furnaces, and more particularly to rotary furnaces and processes conducted therein.

The present invention is particularly applicable to rotary furnaces which are employed in the reduction, oxidation or other heat treatment of ores, ore concentrates, or other metallurgical intermediates and will be described with particular reference thereto although it will be appreciated .that the invention has broader applications such as in the use in cement kilns or in any other process apparatus in which a continuous process is dependent on distinct chemical or physical mechanisms that involve different treating times.

Rotary metallurgical furnaces have heretofore comprised a horizontally mounted and refractory-lined cylindrical steel shell, heat generating means to heat and to maintain the material being treated at treating temperatures, atmosphere control means and means for rotating the furnace. In order to transport material from one end of the furnace to the other, rotary furnaces have most often been mounted at a slight inclination from the horizontal and in some instances, flights, both lateral and helical, and dams have been employed to control the rate of flow of material through the entire furnace.

Rotary furnaces have also been provided with means for feeding material to one end and with means for withdrawing the material from the other end. Charging raw material to rotary furnaces has not presented many problems but difficulties are often encountered in discharging processed material therefrom. Occasionally, the desired metallurgical treatment is conducted at other than atmospheric pressures and such pressures, whether subatmospheric or superatmospheric, must be maintained throughout the furnace during discharge which, if continuous, presents numerous problems.

Rotary furnaces, as described hereinbefore, have been used, and are currently being used, to reduce ores, ore concentrates and other metallurgical intermediates. For example, iron oxides containing nickel are selectively reduced to reduce substantially all the nickel values to metallic nickel while reducing only controlled amounts of iron. The selectively reduced ore is then treated, either hydrometallurgically or vapometallurgically, to recover the reduced nickel values.

v the furnace if the discharging end is provided with a As with many chemical reactions, the chemical reduction of metallurgical ores involves two distinct processes. The ore must first be preheated to the reduction temperature, and thereafter the heated ore is reduced with an appropriate reductant. Preheating involves well understood heat exchange principles and is a relatively fast process since the ore, which is not generally massive, presents large surfacevar eas at and through which heat exchange rapidly and readily occurs. On the other hand, chemical reduction, when effected in the solid state, is a relatively slow process since diffusional processes are involved. Advantageously, the ore is preheated as fast as possible while ja longer period is allowed for chemical reduction, but this ideal has rarely been realized in actual practice.

7 In order to realize more efficient chemical reduction of metallurgical ores in rotary furnaces, the bed of ore in a preheating zone should be relatively shallow in order to increase the rate of heat exchange while the bed of ore in a reducing zone is maintained relatively dam. However, the increase in depth of the bed is not so great as to significantly improve the efficiency of the preheating operation or to materially increase the time of residency'in the reducing zone. Also, it is possible, as has been suggested, to decrease the depth of the bed in a preheating zone-by employing flights, both' lateral and'transve'rse throughout the entire preheating zone. Although flights in a preheating zone can increase the preheating'rate, problems associated with dusting are encountered and the flights, extending throughout the preheating zone, provide extensive surfaces and angles over which and inwhich massive accretions can rapidly form. At times, such massive accretions build up so rapidly that any advantage gained by improved preheating is lost by the time required to remove such accretions.

As noted hereinbefore, metallurgical rotary furnaces can be operated at subatmospheric or superatmospheric pressures for thermodynamic and kinetic reasons. When furnaces are operated under subatmospheric or superatmospheric pressures and on a continuous basis, as rotary furnaces are, discharging treated materials can present numerous problems. Even when operated at ambient pressures discharging treated material from rotary furnaces can be troublesome if it is required to maintain a neutral or protective atmosphere over the treated material while leaving the atmosphere in the furnaceundisturbed. Frequently, the problems associated with discharging treated material were overcome by enclosing the entire discharge end of the furnace with a separate and distinct housing which acted as a pressure seal. The discharge ends of rotary kilns have also been provided with pressure seals, but the manufacture and maintenance of gas tight sealsof the size required to seal commercial rotary furnaces whereboth difficult and expensive. Although many attempts were made to overcome the foregoing difficulties and other disadvantages, none, as far as I am aware, was entirely successful when carried into practice commercially on an industrial scale.

It has now been discovered that horizontally mounted rotating cylindrical furnaces can be improved by providing the furnace with a helical flight to establish two zones along the longitudinal axis of the furnace to control the rate of flow of material from one zone to the other. The diameter of the helical flight can be arranged so that the bed depth down stream from the flight is increased-Such rotary furnaces can be further improved by providing the discharge end of the furnace with a discharge tube and a helical'rotor with the larger vanesof the rotor being coextensive with and fixed to the inside diameter of the furnace and the smaller vanes being coextensive with and fixed to the internal diameter of the discharge'tube.

In accordance with the present invention, a rotary furnace of the type described is provided wherein the furnace has an internal helical flight for at least a pre ponderant part of one revolution but not more than about two revolutions of the furnace so that the furnace is separated into two zones along its longitudinal axis and so that the rate of passage of material through the two zones can be independently varied. In this fashion the depth of material in each section and the retention time in each section can be varied. I

It is an object of the present invention to provide a rotary furnace which has at least two zones and in which material passes through the zones at different rates. 1

Another object of the present invention is to provide a cylindrical furnace in which the rate of flow of material through various zones therein can be controlled and which has an improved discharge mechanism.

Other objects and advantages will become apparent from the'following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a longitudinal cross-sectional view of a rotary furnace in accordance with the present invention;

FIG. 2 is a cross-sectional view of the furnace taken along the lines 22 in FIG. 1; and

FIG. 3 is a cross-sectional view showing an improved discharge mechanism of a rotary furnace in accordance with the present invention.

Referring now to the drawings wherein the showings are for the purpose of illustrating the preferred embodiments of the invention only and not for the purpose of limiting same, FIG. 1 shows an improved rotary furnace in accordance with the present invention. The rotary furnace A includes a cylindrical shell 12, generally steel, which is suitably lined with refractory 13. The charging end of the furnace A is generally represented at B and includes a flue 14 and a feeding device 15. The discharging end of the furnace A, generally depicted at C, is provided with burner 16 and a housing 17, which housing acts as a gas seal and a discharge hopper. The cylindrical shell is provided with steel tires 18 which in conjunction with driving means 19 rotate the cylindrical furnace at preselected speeds. Optionally, the rotary furnace can be provided with air vents or side burners 20. In accordance with the present invention, the cylindrical furnace is provided with a helical flight 21 which can be constructed of the same material as that of the refractory lining 13, or a suitable metallic material. Since, as described hereinafter, helical flight 21 has a pitch such that particulate material is conveyed from one zone to another at a rate greater than that provided by gravity and the inclination and rotation of the furnace, dam 22 can optionally be located immediately before flight 21 to provide more readily controllable depths in the first zone.

The helical flight is an important feature of the present invention. The flight can be of any height as long as it is at least as high as the depth of the deepest bed. For example, as shown in FIG. 1, if the furnace is a metallurgical furnace in which an oxide is preheated in zone D and reduced in zone B, the ore in zone E is maintained at a greater depth than in the preheating zone D to provide a longer residence time in zone E and the height of the flight is at least equal to the depth of the ore in zone E. Placement of the helical flight along the longitudinal axis of the cylindrical furnace is not mechanically critical. However, such' placement can be of great importance for particular chemical or metallurgical operations. If the furnace is employed to directly reduce metal oxides, as iron oxide or nickel oxide, the helical flight is placed at the position in the furnace where a shallow bed and the metal oxide will have been preheated to effect the reducing temperatures.

When reducing nickeliferous iron oxide ores with liquid hydrocarbons, as described in Canadian Pat. No. 744.329 the helical flight is placed at that position where a shallow bed of the ore has been preheated to a temperature of at least about 900C.

The pitch of the helical flight can be varied to suit individual requirements. In metallurgical application, the inclination of the furnace will cause material to flow through the furnace, and the helical flight will have a pitch such that the flow of material will be greater than that caused by the inclination of the furnace, i.e., the leading edge of the helical flight cuts into the ore flowing along the furnace longitudinally under the impetus of gravity and the usual slight inclination of the furnace toward the discharge end. For example, in reducing iron oxide, the pitch of the helical flight can be such that preheated ore in zone D is transferred to the reducing zone E at a rate greater than the rate of feeding iron oxide to the preheating zone D whereby the preheating operation is made more efficient and the residence time of iron oxide in the reducing zone is increased. It will be noted that in metallurgical furnaces drying or preheating will be conducted in the first zone or zones and chemical processes are effected thereafter. Since these chemical reactions will most frequently be diffusion controlled processes, it is desirable to have the bed of material shallow in the initial zones and deeper thereafter.

In addition to the foregoing embodiments, it might be noted that more than one helical flight can be employed in a single cylindrical furnace so that three or more distinct zones can be established. By regulating the pitch of each of the flights, zones of different depths can be formed so that surface area dependent operations can be conducted more efficiently in shallow beds while diffusion controlled (time dependent) operations are affected with deeper beds.

FIG. 3 depicts a preferred embodiment of the present invention. Only the discharge end of the cylindrical furnace is shown in FIG. 3 since the preferred embodiment relates to a discharging mechanism. The cylindrical furnace includes a cylindrical steel shell 30 which is lined with a suitable refractory 31 the furnace is rotated about its longitudinal-axis by drive means (not shown in the drawing). Flow of material through the furnace is controlled by helical flights as described hereinbefore. The furnace is provided with a fixed discharge tube 32 which can be a steel shell 33, suitably lined with refractory 34 to handle hot material. The discharge tube is provided with a gas seal 35 so that the escape of furnace gases through the discharge end are minimized. It might be noted that an advantageous feature of the present invention is that the diameter of the discharge tube is vconsiderably smaller than that of the cylindrical furnace so that any gas seals that are provided are more effective since it is well known that smaller fixtures are easier to seal than are larger ones. The discharge mechanism is a conveyor screw 36 with helical flights 38 extending into the furnace being substantially equal to the internal diameter of the furnace while those helical flights 39 extending to the discharge tube are substantially coextensive with the internal diameter of the discharge tube. The helical flights for the discharge mechanism can be made of suitable metals or refractories. The screw 36 is fixedly mounted in the furnace so that it rotates with the furnace. Thus, flights 39 rotate in fixed discharge tube 32 as the furnace rotates.

The pitch of flights 39 is regulated so that the material is fed out faster than it arrives. The rate of discharge can be effectively controlled by regulating the rotation of the conveyor screw.

The shaft upon which the helical screw is mounted can be hollow so that gases such as air and/or fuel can be introduced into the interior of the furnace so that the shaft can act as a burner for supplying heat to the cylindrical rotating furnace. Thus, acting as a burner the rate of air and fuel can be varied to provide oxidizing and reducing conditions at that portion of the rotating furnace into which the shaft extends.

It will be appreciated by those skilled in the art that rotary furnaces equipped with a helical flight in accordance with the apparatus of the present invention pro- I vide optimum conditions for processing particulate material where different operations are dependent on various physical processes. Thus, in a preheating zone where heat exchange is more dependent on surface area a shallow bed is provided to promote heating while in a reducing zone where chemical diffusion, dependent on time and temperature, is the major consideration a longer residence time is provided while heat losses are minimized.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. In a rotary furnace including a cylindrical furnace having a charging end, a discharging end and a substantially uniform cross section and being rotatable about its longitudinal axis and mounted at a preselected inclination so that rotation of the furnace transports particulate material fed to the charging end to the discharging end, the improvement which comprises: at least one helical flight mounted on the furnace walls between said charging and discharging ends forming within said furnace two zones, said helical flight extending for at least the preponderant part of one revolution of said furnace but not more than two revolutions of said furnace, so that the rate of flow of particulate material through the zones can be controlled by controlling the speed of rotation of the rotary furnace and a conveyer tube fixedly mounted at the discharging end of the furnace, a conveyor screw having helical flights that are substantially coextensive with the internal diameter of the conveyor tube and the furnace-and fixed to the furnace wall so that particulate material is conveyed out of the furnace into the conveyor tube by the helical flights by rotation of the furnace.

2. In a rotary furnace including a cylindrical furnace having a charging end, a discharging end and a substantially uniform cross section and being rotatable about its longitudinal axis and mounted at a preselected inclination so that rotation of the furnace transports particulate material fed to the charging end to the discharging end, the improvementwhich conprises: an internal helical flight for a preponderant part of at least one revolution of the furnace mounted on a furnace wall between the ends of the furnace to provide the furnace with two separate zones along its longitudinal axis whereby the depth of particulate material in the respective zones can be controlled by regulating the speed of revolution of the furnace and a conveyor tube fixedly mounted at the discharging end of the furnace, a conveyer screw havinghelical flights that are substantially coextensive with the internal diameter of the conveyor tube and the furnace and fixed to the furnace wall so that particulate material is conveyed out of the furnace into the conveyor tube by the helical flights by rotation of the furnace.

3. In a rotary furnace including a cylindrical furnace having a charging end, a discharging end and a substantially uniform cross section and being rotatable about its longitudinal axis and mounted at a preselected incliulate material fed to the charging end to the discharging end, the improvement which comprises: an internal helical flight for the preponderant part of at least one revolution but not more than two revolutions of the furnace mounted on the furnace wall and between the ends of the furnace to provide two zones along the longitudinal axis of the furnace whereby particulate material treated in the furnace can be controlled to have different depths within the two zones in the furnace and a conveyer tube fixedly mounted at the discharging end of the furnace, a conveyer screw having helical flights that are substantially coextensive with the internal diameter of the conveyor tube and the furnace and fixed to the furnace wall so that particulate material is conveyed out of the furnace into the conveyor tube by the helical flights by rotation of the furnace.

4. The furnace as described in claim 3 wherein the furnace is mounted at a preselected inclination from the horizontal to cause particulate material to be transported through the furnace and the helical flight has a pitch such that the particulate material flow through the flight is greater than that caused by the preselected inclination so that particulate material in the zone nearer the discharging end is deeper than particulate material in the zone nearer the charging end.

5. The furnace as described in claim 4 further including a dam proximately mounted to the helical flight so that the dam controls the depth of particulate material nearer the charging end and the helical flight feeds par-- ticulate material to the zone nearer the discharging end.

6. The furnace as described in claim 5 wherein the helical flight has a height at least as high as the depth of the deepest bed.

a hollow shaft onwhich the conveyor screw is mounted so that the hollow shaft can function as a burner.

Claims

1. In a rotary furnace including a cylindrical furnace having a charging end, a discharging end and a substantially uniform cross section and being rotatable about its longitudinal axis and mounted at a preselected inclination so that rotation of the furnace transports particulate material fed to the charging end to the discharging end, the improvement which comprises: at least one helical flight mounted on the furnace walls between said charging and discharging ends forming within said furnace two zones, said helical flight extending for at least the preponderant part of one revolution of said furnace but not more than two revolutions of said furnace, so that the rate of flow of particulate material through the zones can be controlled by controlling the speed of rotation of the rotary furnace and a conveyer tube fixedly mounted at the discharging end of the furnace, a conveyor screw having helical flights that are substantially coextensive with the internal diameter of the conveyor tube and the furnace and fixed to the furnace wall so that particulate material is conveyed out of the furnace into the conveyor tube by the helical flights by rotation of the furnace.

2. In a rotary furnace including a cylindrical furnace having a charging end, a discharging end and a substantially uniform cross section and being rotatable about its longitudinal axis and mounted at a preselected inclination so that rotation of the furnace transports particulate material fed to the charging end to the discharging end, the improvement which conprises: an internal helical flight for a preponderant part of at least one revolution of the furnace mounted on a furnace wall between the ends of the furnace to provide the furnace with two separate zones along its longitudinal axis whereby the depth of particulate material in the respective zones can be controlled by regulating the speed of revolution of the furnace and a conveyor tube fixedly mounted at the discharging end of the furnace, a conveyer screw having helical flights that are substantially coextensive with the internal diameter of the conveyor tube and the furnace and fixed to the furnace wall so that particulate material is conveyed out of the furnace into the conveyor tube by the helical flights by rotation of the furnace.

3. In a rotary furnace including a cylindrical furnace having a charging end, a discharging end and a substantially uniform cross section and being rotatable about its longitudinal axis and mounted at a preselected inclination so that rotation of the furnace transports particulate material fed to the charging end to the discharging end, the improvement which comprises: an internal helical flight for the preponderant part of at least one revolution but not more than two revolutions of the furnace mounted on the furnace wall and between the ends of the furnace to provide tWo zones along the longitudinal axis of the furnace whereby particulate material treated in the furnace can be controlled to have different depths within the two zones in the furnace and a conveyer tube fixedly mounted at the discharging end of the furnace, a conveyer screw having helical flights that are substantially coextensive with the internal diameter of the conveyor tube and the furnace and fixed to the furnace wall so that particulate material is conveyed out of the furnace into the conveyor tube by the helical flights by rotation of the furnace.

5. The furnace as described in claim 4 further including a dam proximately mounted to the helical flight so that the dam controls the depth of particulate material nearer the charging end and the helical flight feeds particulate material to the zone nearer the discharging end.

7. The furnace described in claim 3 further including a hollow shaft on which the conveyor screw is mounted so that the hollow shaft can function as a burner.