US2689176A

US2689176A - Method for roasting ores

Info

Publication number: US2689176A
Application number: US193096A
Authority: US
Inventors: Klepetko Ernest; Philip De B Kaye
Original assignee: Combined Metals Reduction Co
Current assignee: Combined Metals Reduction Co
Priority date: 1948-08-18
Filing date: 1950-10-31
Publication date: 1954-09-14
Anticipated expiration: 1971-09-14

Description

Sept. 14, 1954 E. KLEPETKO ET AL METHOD FOR ROASTING oREs 4 Sheets-Shea?l l Original Filed Aug. 18, 1948 ATTORNEYS Sept. 14, 1954 original Filed Aug. 18, 1948 E. KLEPETKO ET AL METHOD FOR ROASTING CRES 4 Sheets-Sheet FIG. 2

Sept- 14, l954 E. KLEPETKO ET AL METHOD FOR ROASTING ORES 4 Sheets-Sheet 3 Original Filed Aug. `18, 1948 FIG. 7

Sept. 14, 1954 E, KLEPETKQ ET AL 2,689,176

METHOD FOR ROASTING ORES Original Filed Aug. 18l 1948 4 Sheets-Sheet 4 FIG. 4 "a INVENTO Rs L'rnesf /f/epf im ATTORNEYS Patented Sept. 14, 1954 METHOD FOR ROASTING ORES Ernest Klepetko, Bauer, and Philip de B. Kaye, Salt Lake City, Utah, assignors to Combined Metals Reduction Company, Stockton, Utah, a

corporation of Utah Original application August 18, 1948, Serial No. 44,866. Divided and this application October 31, 1950, Serial No. 193,096

6 Claims. (Cl. 75--8) This invention relates to roasting ores, concentrates and like metallurgical products, and has for its principal object the provision of a new and improved roasting method. This application is a division of our copending application Serial No. 44,866, filed August 18, 1948, now Patent No. 2,558,963, granted July 3, 1951.

The method of the present invention, which combines the advantages of flash roasting and of hearth roasting, and which comprises a method of interrupted iiash roasting, involves causing the particles of the charge being roasted to fall freely in a stream of low particle density in a roasting chamber, but only for a small fraction of the height of the chamber in any one uninterrupted drop. Each period of free fall of the charged particles is interrupted for a length of time that may be varied to suit individual conditions, but which generally does not exceed about one-quarter hour, and which in any event is substantially less than the time during which charge particles are held on any given hearth of a conventional multiple hearth roaster.

It is the purpose of the invention to combine the advantageous features of flash roasting, or suspension roasting as it is sometimes called, with the benefits of the bed roasting that proceeds on the hearths of conventional multiple hearth furnaces, and at the same time to' avoid to the greatest extent possible the disadvantages inherent in the use of either of these methods alone. Flash roasting involves causing a nely divided charge of oxidizable ore or concentrate to fall freely through a heated oxidizing atmosphere in a suitable chamber. Roasting time is very short, being substantially no, longer than the time required for the charge to fall the height of the chamber. The particles of charge must he very finely divided or they will be incompletely roasted in this short period. Multiple hearth furnace roasting, on the other hand, involves heating a bed of the charge on each hearth of the furnace in contact with the usually oxidizing furnace atmosphere. The charge is continually turned over by rakes or rabbles to expose fresh unoxdized portions to the atmosphere, and is slowly advanced across each hearth and from hearth to hearth by the action of the rabbles.

Flash roasting has the advantage of greater roasting capacity per unit installation cost, elimination (in most cases) of external fuel requirements, and production of roaster gases rich in sulphur dioxide and Well suited to sulphuric acid manufacture. These advantages stem from the very short period of intensive oxidation to which each particle of the charge is subjected. Great though these advantages are, they have not enabled flash roasting to supplant bed roasting in multiple hearth furnaces, because of the greater exibility and ease of control of the latter type of roasting. Flash roasting requires a much more finely divided and more carefully dried charge than hearth roasting, and generally results in production of undesirably large quantities of dust in the vflue gases (especially when the ash roaster is strongly up-drafted to hinder the free fall of the particles and thus increase the time allowed for roasting to proceed to completion). Control of the roasting temperature is exceedingly diflicult in flash roasting operations, so that control of the chemistry of the roasting process is correspondingly diicult. The nue gases themselves commonly are at such a high temperature as to make it difcult to handle them. Because multiple hearth roasting is not subject to these disadvantages it has heretofore been favored for most metallurgical roasting operations.

Various proposals have been made heretofore to achieve some of the advantages of flash roasting in ordinary multiple hearth roasting operations. For example, it has been recognized that a very substantial part of the sulphur is burned from a charge of sulphide ore or concentrate in a multiple hearth roaster during the time that the material drops from one hearth to the hearth next below. To capitalize on this factor, the number of drop holes on the out hearths of multiple hearth furnaces have been increased from 6 per hearth on early Evans-Klepetko furnaces to 16 per hearth on furnaces built in the last decade. Another proposal, used with some success, has been to remove the central hearths from a conventional multiple hearth furnace, so as to form a flash roasting chamber between the top few and bottom few hearths. To the extent that these expedients have attained the advantages of dash roasting, however, they have resulted in introducing to a corresponding extent the disadvantages of that method. Very nely divided and well dried charges have been necesary for optimum results, and control of the flash roasting temperature is not achieved to any very great degree.

In the course of an exhaustive study of roasting problems, we have sought to balance idealized roasting conditions against the practical requirements of economical commercial practice, and have arrived at a set of criteria which has served as the basis on which we have developed our new roasting method. The dominant elements of the criteria by which we have been guided may be summarized as follows:

1. Maximum use should be made of suspension roasting, within the limits imposed by the desirability of avoiding excessive ne grinding and thorough drying preparatory to roastng.

2. Provision should be made for holding the coarser charge particles in the roasting operation long enough to effect completion of the roasting reactions, while preferably allowing the finer and so more rapidly roasted particles to pass through the roasting operation with the least possible delay.

3. An adequate supply of air or other oxidizing gas should be available wherever roasting is proceeding most rapidly, and especially where flash roasting is taking place.

4. Provision should be made for controlling the temperature at which roasting proceeds.

5. The roasting operation should be conducted so that suitable auxiliary reagents (e. g. sodium chloride) may be introduced into the charge at optimum points, when such is desired.

6. The roasting apparatus should be simple, of rugged construction, utilizing known and tried components to the maximum extent possible.

7. Replacement o1' repair of roaster parts subject to most rapid wear or damage should be accomplished quickly and easily,and to the greatest extent possible without shutting down and cooling the furnace.

The improved metallurgical roasting furnace we have designed on the basis of these criteria (and which is claimed in our aforesaid application Serial No. 44,866) is formed with a cylindrical wall and has therein a plurality of vertically spaced hearths. Each roasting hearth, however, instead of being of the circular arched refractory design common to multiple hearth roasters as heretofore known, comprises several substantially sector-shaped hearth segments (or pallets) arranged in a common plane, with the side edges of each segment spaced laterally from the adjacent side edges of the neighboring hearth segments in the same plane. Thus a plurality of substantially radial drop holes are provided in each hearth, through which material on any hearth segment may be showered to the hearth below. The hearth segments in any one plane, or hearth level, are offset laterally with respect to the segments in the plane next above to the extent necessary that material falling vertically through the drop holes will be caught on a hearth segment of the next lower hearth.

Each hearth is provided with rabble arms carrying rabbles that may be set radially of the furnace to push the charge on each hearth segment primarily in a circumferential direction toward the drop hole; or that may be set at an angle (which alternates in direction of pitch with each successive rabble arm) to keep turning the charge over as it is advanced thereby circumferentially toward the drop hole.

Provision advantageously is made for delivering a current of air laterally into contact with the material being roasted as it falls over the edge of each hearth segment through the drop hole.

The hearth segments themselves are preferably made of heat-resisting alloy steel, generally Without any refractory protective insulation, and are hollow to permit circulation therethrough of a cooling fluid. Each sector-shaped hearth segment is formed with a mounting ange at its outer arcuate or circumferential edge for fastening it to the cylindrical furnace wall. As this is the only support that need be provided for the hearth segment, it may be withdrawn through the furnace wall and a replacement may be substituted without it being necessary for workmen to enter the interior of the furnace, or even to discontinue furnace operation.

The roasting method of the invention, which is carried out in the above-described roasting apparatus, involves causing a charge of finely divided ore or concentratey heated to the roasting temperature, to fall freely in the form of a thin elongated stream through the oxidizing furnace atmosphere. The density of the falling particles in the stream is kept low enough so that every particle is in contact with the oxidizing atmosphere. This low particle density is more or less characteristic of the best practice in flash roasting, but, contrary to flash roasting, the method of the invention further involves interrupting the free fall of the particles after they have descended only a small fraction of the height of the roasting chamber. Such interruption occurs, of course, by catching the falling particles on the hearth segment below the drop hole through which they have fallen, and here they are held long enough to further the roasting of the coarser particles. Thereafter the material is again caused to fall another small fraction of the height of the roasting chamber, and again the fall is interrupted. This sequence of interrupted falls continues until the charge reaches the bottom of the furnace in roasted condition, resulting in what we term interrupted flash roasting.

The duration of each interruption between successive falls of the charged material is long enough so that the coarser particles may be completely roasted, but it is much less than the time during which the charge is held on each hearth in ordinary multiple hearth roasting. Usually the duration of the interruption is not more than about one-quarter hour. In normal operations according to the invention, some part of the charge is at almost all instants undergoing flash roasting while the remainder is undergoing bed roasting on the hearth segments, and all parts of the charge undergo flash roasting and bed roasting alternately.

In a preferred embodiment of the invention,

a stream of air (or oxygen, or oxygen-enriched air) is directed laterally against the charge as it begins to fall from one hearth level to the next. In this manner an ample supply of air is provided during the period of flash roasting, when the roasting process is proceeding most intensely. The air stream also can be made to result in some size classification of the charge particles, by delivering it with sufficient force to blow the finest particles out of the path of substantially vertical fall traversed by the coarsest particles. The force of the air stream may be so controlled that the total duration of the interruptions in the fall of the finest particles as they traverse the roaster is substantially less than the total duration of the interruptions in the fall of the coarsest particles as they traverse the chamber. In this manner the fine particles which roast to completion most rapidly, pass through the furnace in a shorter period of time than the coarse and hence more slowly roasted particles.

Control of the roasting temperature is effected by controlling the duration of the interruptions between falls of the charge particles (i. e. by controlling the number and speed of rotation of the rabble arms), and by controlling the extent of cooling of the hearth segments. Additional control over the roasting temperature is also effected by the conventional expedient of controlling the amount of air admitted to support combustion of the roasting charge.

The foregoing is merely an outline of some of the major features of the invention. These and other features are described in greater detail below with reference to the accompanying drawings, which show a roasting furnace in which the method of the invention can be carried out. In the drawings,

Fig. 1 is a perspective, with parts broken away, showing the hearth levels of a furnace in which interrupted flash roasting is proceeding in aocordance with this invention;

Fig. 2 is an elevation of the multiple hearth furnace shown in fragmentary section in Fig. 1;

Fig. 3 is a plan of the furnace shown in Fig. 2;

Fig. 4 is a vertical section through the furnace shown in Fig. 2;

Fig. 5 is a horizontal section through the furnace of Fig. 4, taken substantially along the line 5 5 of Fig. 4;

Fig. 6 is a horizontal section through a hearth segment for the roasting section of the furnace, taken substantially along the line G-S of Fig. 7;

Fig. 7 is a cross section taken substantially along the line 1 1 of Fig. 6; and

Fig. 8 is a vertical section taken substantially along the line 8--8 of Fig. 6.

rIhe roasting apparatus shown in the drawings is first described below, and then follows a description of the method of the invention.

Apparatus The general arrangement of the new roasting furnace in the region wherein interrupted flash roasting takes place is best shown in Fig. l. The

furnace comprises a cylindrical steel shell lll havl ing a refractory lining Il and an axial rotatable hollow metallic column I2 provided with a refractory facing I3. A series of hearths at vertically spaced hearth levels indicated by the arrows A., B and C are provided within the furnace wall. The hearth at each level A, B, etc., comprises a series of sector-shaped hearth segments lll. Hearth segments indicated by reference numerals MA, plane of the hearth level A, hearth segments indicated by reference numerals lzlB, MB', etc., are arranged in the common plane of the next lower hearth level B, etc. The radial side edges i5 (herein called the forward side edges) of each hearth segment are spaced laterally from the adjacent radial side edges l (the rearward side edges) of the neighboring hearth segments in the same common plane or hearth level. Thus between each adjacent pair of hearth segments in a given hearth level is a fairly wide radial opening (such, for example, as the opening between the forward side edge l5 of hearth segment MA and the rearward side edge it? of the hearth segment MA) which serves as a drop hole through which ore or concentrate being roasted may fall from one hearth level to that next below.

It will be noted that the hearth segments in any given hearth level are staggered laterally with respect to the hearth segments in the levels next above and next below. For example, the hearth segments 14B, MB', etc., in the B hearth level are not arranged directly below the hearth segments MA, I4A, etc., in the A hearth level MA', etc., are arranged in the common 2l into two

compartments

22 and 23.

next above, but are offset laterally with respect thereto so as to underlie the radial drop holes between these latter hearth segments. Thus each hearth segment is in position to receive material falling vertically over the forward edge i5 of a hearth segment in the level next above it.

Rabble arms Il project laterally from the axial column l2 at each hearth level. Each rabble arm carries a series of rabbles I8. The rabbles, contrary to usual multiple hearth furnace practice, are shown in Fig. 1 arranged parallel to the radially extending rabble arms, and they serve to push ore or concentrate being roasted on any of the hearth segments circumferentially toward the forward side edge l5, over which it may fall through the wide radial drop hole to the hearth below. In view of the arrangement and action of the rabbles i3, they might more accurately be termed pushers, but we prefer to use the more conventional term rabbles to identify them. The rabbles i8 may, however, be set at an angle to the axis of the rabble arm. If this is done the pitch of the angle should alternate from one rabble arm to the next, so that while the rabbles then will keep turning over the charge on each hearth segment, they will not, in net effect, tend to move it radially in or out along the hearth, but will move it circumferentially to the dropl hole.

As shown in Figs. 1, 6, 7 and 3, each hearth segment Ui is a hollow metallic sector-shaped elementy having top and bottom walls i9 and 2e joined together by the side edges and ifi. such segment is divided interiorly by a partition The partition 2l docs not extend all the way to the apex end 2d of the hearth segment, but instead terminates at a point 25 short of the apex, so to provide an opening 26 for communication be'- tween the lower and upper compartments and 23 within the segment.

The arcuate or circumferential end portion El of each hearth segment is enlarged to provide cooling fluid inlet and exhaust passageways and 29. The inlet passageway 28 communicates with the compartment 22 below the partition 2 i, and the exhaust passageway 29 communicates with the compartment 23 above the partition. The inlet and exhaust paesageways advantageously are formed integrally with the hearth segment, and integrally also with mounting flanges 3i] by which the segment is fastened in place in the furnace. Each hearth segment extends thro-ugh a circumferentially-extending slot in the steel shell lil of the furnace, and is secured in place by bolting the mounting @il to the furnace shell. This is the only support necessary to retain the hearth segment in place, and to enable it to carry its own weight and that of the charge which falls upon it. Consequently it is a simple matter to remove cr insert any particular hearth segment, without entering the interior of the furnace or even discontinuing its operation. To facilitate inserting a hearth f segment into the furnace, or removing it therefrom, the top and bottom walls of the cooling iiuid inlet and exhaust passageways are tapered as indicated at 5I (Fig. 8).

To keep the hearth segments from overheating during operation of the furnace, cooling air (or other cooling fluid) is admitted to the interior of each segment through a supply pipe 32 communicating with the inlet passageway 28. The air flows thence through the lower compartment 22 to the opening 26 and back through the upper compartment 23 to the exhaust passageway 29, from which it escapes through an exhaust pipe 33. The longitudinal partition 2l thus serves primarily as a baflle for directing the cooling air to the apex 24 of the segment before it escapes. In place of using a horizontal longitudinal partition 2| (as shown in the drawings) for this purpose, a vertical longitudinal partition can be used instead.

A vertical longitudinal partition 34 (Figs. 3 and 6) is arranged interiorly of each roasting hearth segment near its forward side edge I5, to form a combustion air inlet manifold 35. The forward side edge l is itself formed with a series of openings 35, which serve the purpose of directing a stream of air laterally against the charge being roasted as it drops over the forward side of the hearth segment to the hearth next below. Combustion air is admitted to the manifold 35 through inlet pipe 31 controlled by a valve 38 (Fig. 6) and may be taken with advantage, as preheated air, from the exhaust pipe 33 through which spent cooling air escapes from the outlet passageway 29. If some other medium than air is used for cooling the hearth segments, the valved inlet pipe 31 is connected to some other convenient source of air or other oxidizing gas.

A substantially complete roasting furnace, in which interrupted flash roasting hearths according to Fig. l are incorporated is shown in Figs. 2 to 5. The furnace is shown as having six vertically spaced hearths 49 to 45, but additional hearths may be provided if desired. The uppermost hearth 40 receives the charge and serves as a preliminary drying hearth. The second hearth 4i is a drying and distributing hearth, and is arranged to insure proper distribution of the charge on the hearth segments of the rst interrupted nash roasting hearth 42 (shown as the third hearth of the furnace). Additional interrupted flash roasting hearths, in any desired number, are arranged below in the manner described above with reference to Fig. 1 (only two such

additional hearths

43 and 44 are shown in Fig. 4) The lowermost hearth 45, which forms the floor of the furnace, is a finishing hearth from which the roasted product is withdrawn.

The two upper drying hearths 40 and 4i are full circular hearths. They may be of the usual arched refractory brick construction, but advantageously they are made of metallic segments of substantially the same construction shown in Figs. 6 to 8, but without the longitudinal partition 34 as there is here no need for a combustion air manifold 35 or openings 36. The upper drying hearths may be heated rather than cooled to facilitate drying of the charge.

Ground ore or concentrate is delivered to the upper drying hearth 40 and is spread evenly on it by a charge distributor 45 (Fig. 4). The charge distributor receives the material to be roasted from a hopper 41 to which it is delivered through a feed pipe 48. A screw conveyor 49 inside the charge distributor moves charge radially out from the hopper 41, discharging it through distributing outlets 50 along the radius of the hearth. At the same time the distributor is moved slowly in a circular path about the axis of the central column, thus spreading the charge evenly over the upper hearth 40. A furnace cover 5|, on which the charge distributor is mounted, is itself mounted on wheels 52 riding on a circular rail 53 at the top of the furnace shell IE), so as to permit the circular travel of the distributor. A motor 54 drives the distributor and cover in their circular path, and also operates the worm conveyor 49. A seal plate 55 extending up from the furnace shell provides a fairly gas-tight joint between the cover 5l and the main body of the furnace.

The upper hearth 40 is provided with a series of radial drop holes 5S (Fig. 5) and is served by

rabbles

51, 51 carried on

rabble arms

58, 55 which in turn are mounted on the central column I2 of the furnace. The rabbles are pitched at an angle to the axis of the rabble arms, but the direction in which the rabbles 51 on one arm 58 are pitched is opposite to that of the rabbles 51 on the other arm 58. The net result of this arrangement is that the rabbles do not move 'the charge substantially either in or out along the radius of the furnace, but keep turning it over and working it circumferentially toward the drop holes.

The second drying hearth 4I is of essentially the same construction as the upper drying hearth 4t. It is provided with radial drop holes 59, and is served by rabbles '59, 60 arranged in the same way on rabble arms 6|, 5I as the rabbles 51, 51' serving the upper hearth. However, the radial drop holes 59 of the second hearth are located so as not to lie directly under the drop holes 55 of the upper hearth (otherwise charge falling from the upper hearth 40 would not be caught on the second hearth il The next three

hearths

42, 43 and 44 shown in Fig. 4 are interrupted flash roasting hearths constructed and arranged as described above in cennection with Fig. 1. The hearth segments of the upper of these hearths 42 are so located that their rearward edge portions underlie the radial drop holes 59 of the second drying hearth lli, so as to catch the charge as it falls from the lowermost drying hearth. The interrupted flash roasting hearths 42 to 44 are served by rabbles l5 carried on rabble arms l1, as previously described, and these rabbles may if desired be set parallel to the rabble arm so as to act more like pushers for moving charge directly across the hearth segments to the radial drop holes between segments. However, the roasting hearth rabbles i8 mai`T alternatively be arranged in the same manner as the rabbles 51, 5l and 5G, Gil serving 'the drying hearths, so as to move the charge more slowly across the roasting hearth segments and turn it over more effectively.

The lowermost hearth 45 is in effect the bottom of the furnace, and is advantageously of refractory brick construction. It is served by rabble arms 52 carrying rabbles 53 which are set at an angle to move the charge outwardly from the center of the furnace to the periphery, in 'the conventional manner of out hearths of ordinary multiple hearth furnaces. The roasted product is discharged from the bottom hearth 5:5 through one or more outlets 54 at furnace periphery.

The central column l2 on which the rabble arms l1, 58, 5l and 52 are mounted is of more or less conventional construction. It is advantageously in the form of a large hollow metallic shaft, provided with hollow rabble arm socket rings at each hearth level. The column AEs supported on a heavy roller bearing t5 at its base, and is provided with the usual ring gear (il, through which the column may be slowly rotated by a pinion 58 and associated drive mechanism. A bearing 59 mounted in a supporting frame "55 (which may in turn be supported from the furnace shell) is provided near the top of the column !2 to hold it upright in its axial position within the furnace.

The arrangement for cooling the rabble arms is conventional, but is described below in some detail for the sake of clarity. A cooling fluid inlet pipe 'Il communicates with the interior of the column through the central opening of the supporting bearing Se. The interior of the column is divided into two compartments lil and i3 by a longitudinal partition 'lf-3. Cross partitions l5 at the top and bottom of the longitudinal partition 'lll prevent direct communication between the compartments l2 and 'i3 on opposite sides of the longitudinal partition. Cooling air delivered through the inlet conduit l! enters only the compartment E2 on one side of the partition it.

Each of the rabble arms Il etc. below the upper cross partition. 'i5 is hollow and is provided interiorly with a pipe 'i6 or it which extends from the inner end of the rabble arm almost to its outer end. Pipes 'it in rabble arms mounted on one side of the partition lll communicate with one of the compartments 42, and pipes 'li-5 in rabble arms on the opposite side of the partition cornmunicate with the other compartment '13. Each annular rabble arm socket ring G5 is hollow, deiining an annular cooling medium passage ll, and the interiors of the rabble arms il etc., outside the pipes lil, communicate with this passage through ports 'it and lt.

Cooling air or other cooling medium entering the compe. tment 'it through the inlet conduit ll can new only into the pipes it, and then from the ends of these pipes must flow back through the hollow raboe arms and through the ports i8 into the annular passages 'il in the rabble arm socket rings. The air then flows through these passages to and through the ports i8 and into the interiors cf the rabble arms mounted on the opposite side of the partition 1li. Thence the flow is through the rabble arms and .back through the pipes into the compartment i3.

The spent cooling air passes from the compartu ment it through an exhaust conduit 'it at the top of the furnace. A liquid cup seal 8f4 is provided at junction of the exhaust condiut 'iii with the column if. A damper may be provided in this conduit near the top of the furnace, if desired., to control the rate of iiow of cooling air through rabble arms.

In order to prevent overheating of the metallic hearth segments and to provide for control over the temperature prevailing during roasting, a cooling fluid supply header di (Figs. 2 and 3) connected to bustle pipes iii. is arranged to deliver coolinCr fluid to each of the hearth segments of the interrupted flash roasting hearth (i2, d3, riffs, etc., and to the lowemost drying hearth il (since the bottom of this hearth is vexposed. tol the roasting zone of the furnace, it requires effective cooling). The supply pipes 32 of the hearth segments are connected to this bustle pipe. The cooling fluid entering each hearth segment flows through the compartment 22 on one side of the partition 2l and back through the compartment 23 on the other side to the cooling iiuid exhaust passageway 253 (as above described in connection with Figs. 6 to 8) whence it flows through the exhaust pipes S3 to a second set of bustle pipes 83 connected to an exhaust header 3G.

desired, provision may be made to utilize the heated spent cooling air from the interrupted iiash roasting hearth segments for preheating and drying the incoming charge on one or more of the upper drying hearths lili. In the arrangement shown in Fig. 2 for this purpose, a pair of bustle pipes and 86 are connected to` the exhaust header `8d below and above a damper 81, respectively. The supply and

exhaust pipes

32 and 33 of the hearth segments are connected to these bustle pipes (here the pipes 32 that are exhaust pipes on the roaster hearth segments are used as supply pipes, and vice versa, for effective heating and drying of the charge). Any desired proportion of the hot spent cooling air from the roasting section of the furnace may be :by-passed from the exhaust header through the segments of the drying hearth 4U by appropriate adjustment of the damper 81.

To provide for withdrawal of roaster combustion gases, one or more combustion gas outlet flues Se (preferably at least one such flue serves each hearth) connects the interior of the furnace with a combustion gas exhaust stack t9. Dampers et in the fiues 38 are provided for regulating the draft through the furnace. A conventional gate valve at the bottom of the stack is provided for removal of flue dust that settles out in the stack.

At least one access door ti is provided at each hearth level, both to permit observation of and access to the interior of the furnace, and to admit combustion air. Any desired number of fuel burners 92 may also be provided to heat the charge to the roasting temperature when starting the furnace, and also to keep the charge at the roasting temperature if it does not contain enough sulphur to roast autogenously.

Downwardly sloping ,annular protective shields St (Fig. 4) are secured to the rotatable central column l2 at each hearth level to prevent particles of charge from. falling between the refractory facing lof the rotating Central column and the adjacent edges of the stationary hearth segments. vThe hearth segments may be provided at their inner ends with an upstanding lip 9d which extends up behind the cooperating shield 93.

A feature of importance of the new furnace, resulting from the use of all-metallic sectorshaped hearth segments fastened only to the furnace shell is the ease with which a hearth segment may be removed for repairs and may be replaced without shutting down the furnace. The hearth segment is simply withdrawn radially through the circumferentially extending slot in the furnace wall in which it is mounted, as indicated in dotted lines at ld', Fig. 3. Each of the hearth segments normally is held in place solely by bolts 95 passing through holes et in its mounting flanges 36 and threaded into tapped holes in the steel shell H3 of the furnace. No fastening means are needed inside the furnace for holding the hearth segment in place. The flange mounting is adequate for supporting both the weight of the hearth segment itself and any charge upon it. Accordingly, by simply removing the bolts 95, the entire hearth segment may be withdrawn from the furnace and another similan hearth segment may be inserted and fastened in place. If there is an adequate draft through the stack 8d, this operation may be performed even while roasting is going on within the furnace.

Method The roasting method of the invention is carried out in the furnace described above substantially as follows: Quite `finely crushed ore, or a notation Iconcentrate of the usual degree of fineness, such as a zinc sulphide or copper sulphide concentrate, is introduced through the feed pipe 48 and is spread even over the top drying hearth 40 by the distributor 4B. The rabbles l and 5l keep turning it over and moving it gradually to the radial drop holes 56, whence it falls to the second drying hearth 4I. Again it is repeatedly turned over, by the rabbles 6U and 6D', and is gradually moved to the radial drop holes 59, through which it falls to the hearth segments (or pallets) of the rst roasting hearth level At2. While the charge is being moved across the drying hearth 40 it is heated by the hot spent cooling air delivered to the interior of the hollow metallic segments of this hearth from the roasting hearths below; and thereby it is dried sufficiently for roasting to begin.

The charge falls in an elongated stream through the radial drop holes 5g and is collected quite uniformly along the radial length of the underlying segments of the first interrupted flash roasting hearth 112. As it falls thereto, it is heated by the hot atmosphere in the roasting section of the furnace to the temperature at which roasting begins. Assuming the charge to be a sulphide mineral capable of autogenous roasting-i. e. able to sustain combustionsome roasting will be initiated as the charge falls from the last drying hearth, and the roasting reaction will spread through the charge as it collects on the segments of the rst interrupted flash roasting hearth.

Quite shortly after a portion of the charge has been deposited on a segment of the first interrupted flash roasting hearth, it will be pushed therefrom over the forward side edge by the aotion of the rabbles i8, and flash roasting will ensue as it showers tc a segment of the next lower hearth level. Bed roasting then proceeds for a further short period of time, until the charge is again pushed by the rabbles over the forward side edge of the hearth segment, to fall in a flash roasting environment to the next hearth. These periods of flash roasting with intervening periods of bed roasting on the hearth segments follow one another until the charge finally arrives at the bottom hearth 65 of the furnace. Here the rabbles move the charge to the outer periphery of the furnace, where it drops out through the discharge conduits Sri.

The combustion gases formed during roasting of the charge pass out through the ues 88 and escape through the stack 8S. With one or more flues 8S serving each hearth level, the nature and extent of the draft through the furnace is readily controlled by suitable regulation of the damper 9S in each flue.

The movement of the charge through the roasting section of the furnace is most clearly visualized by reference to Fig. 1. Once the charge has been distributed uniformly along the radial length of the hearth segments in the first roasting hearth level (say hearth level A), it is moved quite uniformly by the rabbles over the forward side edges of the segments and falls in a thin elongated stream of low particle density to the segments of the hearth next below. It is desirable of course that the particle density in. the stream of charge falling from one hearth to the next be sufciently small so that every particle is in contact with the oxidizing atmosphere of the furnace during its fall. This is necessary to insure optimum ash roasting. Conventional flash roasting furnaces are, of course, designed to shower the charge through the roasting chamber in a stream of low particle density; but in the conventional multiple hearth furnace, the particle density of the stream of charge falling through the drop holes is so great that only very incomplete hash roasting can take place-the high particle density of the falling charge in such furnaces locally depletes the oxidizing constituents of the furnace atmosphere to such an extent that many of the particles fall all or part of the distance to the next hearth below without coming in contact with suiiicient oxidizing gas to be effectively roasted.

Two results of major importance ensue from the above-described method of interrupted flash roasting. The first such result is that the total time of flash roasting, in a furnace of given height, is very substantially increased as compared with a conventional fiash roasting operation. This follows from the fact that as a particle falls it accelerates, and traverses a given distance much more rapidly toward the. end of its fall than toward the beginning. For example, a freely falling body takes 1.12 seconds to fall vertically 20 feet; but if the fall is interrupted at, say, flfoot intervals, the falling time to traverse a total distance of 20 feet is increased to 2.5 seconds. This is precisely what occurs in the new roasting method-neglecting the hindering eiect of an updraft through the furnace, the time of free fall for each particle of the charge traversing the roasting section of the furnace is substantially greater than in a conventional flash roasting operation in a roasting chamber of equal height. At the same time the low particle density in the owing stream of charge that is characteristic of flash roasting but not of conventional multiple hearth roasting is achieved.

The second major result is that during the periods the charge is held on the segments of the several hearths, the larger particles are given time to be effectively roasted. As pointed out above, ordinary flash roasting operations require a very finely divided charge in order that each particle may be roasted completely in the short period of time allowed. In the new interrupted hash roasting method, however, substantially larger particles in the charge are permissible, because the time during which the particles are held in a roasting environment on the hearth segments is very much greater than can be achieved in ordinary iiash roasting. On the other hand, the time of retention on the hearth segments is much shorter than in conventional multiple hearth roasting, so that the charge may be advanced much more rapidly through the furnace by the new method.

With rabbles i8 set parallel to the rabble arm I1 and acting as pushers, the time of retention of the charge at each hearth level is determined by the number of rabble arms and by the speed of rotation of the central shaft. An increase in the number of rabble arms at any given hearth level for a given speed of rotation of the shaft decreases the time interval between two sucn cessive passes of the rabbles over a given hearth segment; and, of course, such time interval decreases with an increase in the speed of rotation of the central column. Since pusher type rabbles wipe a hearth segment substantially clean each time they pass over it, the retention time can be controlled by the number of rabble arms used and their speed of rotation. The same is for the most part true if the rabbles are inclined at an angle to the rabble arms, except that in this case the rabbles do not wipe the hearth segment clean with each pass over it. Consequently with inclined rabbles the retention time is considerably longer than with 13 pusher type rabbles, but is still much shorter than in a conventional multiple hearth furnace because of the provision of the plurality of radial drop holes.

In general, it is advantageous to retain the roasting charge at each hearth level for a period of time that is long in relation to the length of time required for free fall through the height of the furnace, but which, in general, does not exceed about one-quarter hour. A quarter-hour is a Iconsiderably shorter time interval than that during which the charge is held on each hearth of a conventional multiple hearth roaster (which is generally about an hour or so). lt is a period of time that is ample for effective roasting of charge particles much coarser than can be treated successfully in conventional flash roasting operations; and yet it is so short in relation to the time interval involved in multiple hearth roasting as to greatly increase the rate at which a charge may be roasted.

As indicated above, it is very desirable to direct a stream of air (or other oxidizing gas) laterally against the charge, as it falls from one hearth level to the next. In the apparatus shown in Fig. l, a stream of air is delivered. into contact with the charge through the openings 36 as it falls over the forward side edges l of each hearth segment. Several advantageous results are accomplished in this manner. One result is that an ample supply of oxidizing gas is brought into contact with the charge at just the point where it is required-that is, at the point where flash roasting commences or resumes. A second result is that the stream of air, if delivered with sufficient force, blows the :liner particles of the falling charge farther out from the forward edge of the hearth segment than the coarser particles,

thus forcing a decreased particle density in the v falling stream of charge. In consequence of this second result, the finer particles fall to the hearth segment next below at a point nearer its forward edge than do the coarser particles. Thus these nner particles, which require a minimum of bed roasting, are in position to be among the first to be rabbled to the next drop hole. If inclined rather than "pushen type rabbles are used, the fines falling toward the forward edge of the hearth segment may be rabbled to the drop hole with the very next pass of a rabble arm, while the coarser particles may not be advanced to the drop hole until after several passes of; a rabble arm over the hearth segment. Still greater size classification of the particles results if the stream of air issues from the openings 3B with suicient force to blow the finest particles in the charge far enough forward to miss the hearth segment immediately below, and to continue falling at least to the second hearth below.

To illustrate the foregoing, consider a charge falling over the forward edge I5 of a hearth segment ilA in the A hearth level of Fig. l. Should the charge fall vertically, it would be caught near the rearward edge of a hearth segment HiB in the B hearth level. However, a stream of air issuing from the openings 36 in the upper hearth segment lliA will blow some of the fines forward to be caught near the forward edge of the lower hearth segment llB; and if the force of the air stream is suiiiciently great, only the coarser particles will be caught at all on this hearth segment iflB, while the finest particles will be blown forward sufficiently to fall through the radial drop hole between the hearth segments MB and MB in the B hearth level to the hearth segment |4C two hearth levels below. Thus in this latter case the finest particles miss the B hearth level altogether-their rfall is interrupted at the C hearth level, substantially lower in the furnace than the B hearth level where the fall of the coarse particles is interrupted. In either case, however, finest particles, which roast most quickly, may thus be made to traverse the furnace in a substantially shorter length of time than the coarser particles, which require a longer time for roasting.

Effective control 'of temperature is important in order to secure most satisfactory roasting results. It is virtually impossible to control the temperature of a conventional flash roasting operation, particularly at different levels in the flash roasting zone. Co-nsiderably better success in this respect is attainable in multiple hearth roasting operations, but often at the expense of decreasing the capacity of the roasting furnace. This is because the temperature vcontrol in such furnaces is generally achieved by controlling the rate of speed of the rabble arms and the rate at which air is admitted to the furnace. Temperature control in these manners directly affects the rate at which the charge is or can be passed through the furnace. Furthermore, the high reaction temperature attainable in properly ccntrolled flash roasting cannot be reached in conventional multiple hearth roasting.

In the roasting operation of the invention, the same temperature controls used in ordinary multiple hearth roasting are available, but in addition the temperature may further be 'controlled by regulating the flow of cooling air or other cooling medium through the metallic hearth segments. Temperature vcontrol in this latter manner is often more precise than by the methods available in ordinary multiple hearth roasting operation, and in addition permits different temperatures to be maintained, within limits, at difference hearth levels in the roasting section of the furnace. For example, in roasting an iron-bearing zinc sulphide concentrate, it is generally desirable to avoid the formation of complex zinciron compounds. Such compounds will form if zinc sulphide is in contact with iron sulphide in a bed of charge heated to the roasting temperature of zinc sulphide. However, if the roasting operation is initiated at a relatively low temperature, at which the iron sulphide oxidizes readily but at which the zinc sulphide is not significantly affected, then subsequently the Zinc sulphide may be roasted at the higher temperature required for its oxidation without formation of undesirable complexes. The roasting apparatus and method of the invention permit of achieving such preferential initial roasting of one component of a charge, before roasting of the other component begins to any substantial extent, because sufriciently close control of lthe temperature of the roasting charge at each of the different `hearth levels may be attained.

The rate at which iiash roasting proceeds relative to the rate of bed roasting also may be controlled within limits by controlling the rate at which air is admitted through the openings 35 at the forward edges of the roasting hearth segments. This serves also as an added means for controlling the temperature in the furnace, for the relatively high temperature developed in flash roasting is transferred by the furnace atmosphere to all parts of the furnace.

Roasting operations sometimes are carried out in conjunction with an added solid reagent, as

for example when sodium chloride is added to a roasting charge to eiiect a chloridizing roast. It is generally not feasible to incorporate an added reagent in the charge delivered to a suspension roasting operation, because the time interval of the roasting operation is too short for a plurality of diiferent reactions, one dependent upon the other, to proceed to substantial completion. 1n the process of the invention, however, it is thoroughly feasible to incorporate a solid reagent with the charge, either when the charge is rst delivered to the furnace, or at some point in the roasting section of the furnace below that at which roasting first begins, in the same manner as is possible in ordinary multiple hearth roasting. In the method of the invention, the reactions involving the added reagent usually proceed more rapidly than in ordinary multiple hearth roasting, because they may in part proceed at the relatively higher temperatures occui-ring during the intervals of ash roasting.

We claim:

1. In the roasting of ores and concentrates, the improvement which comprises causing a stream of such material in finely divided form and heated to its roasting temperature to fall over the edge of a hearth and through a drop hole, and initially contacting a fresh stream of laterally-directed air with the falling stream of charge substantially immediately as it passes over the edge of the hearth.

2. The method of roasting ores and concentrates which comprises causing a finely crushed charge of such material heated to its roasting temperature to fall in the form of a thin elongated stream through an oxidizing atmosphere, directing a stream of air laterally against the falling stream of charge in such direction and with sufficient force to blow the most finely divided charge particles through a substantial lateral distance in the direction of their movement through the roasting zone but with less than suiiicient force to deect the coarsest of the charge particles substantially from a vertical path of fall, interrupting the fall of at least the coarsest particles after they have fallen a short distance, and thereafter causing the particles whose fall has been interrupted to fall a further short distance through the oxidizing atmosphere.

3. The method of roasting ores and concentrates which comprises causing a nely crushed charge of such material heated to its roasting temperature to fall in the form of a thin elongated stream through an oxidizing atmosphere, directing a stream of air laterally against the falling stream of charge in such direction and with sufficient force to blow the most nely divided charge particles through a substantial lateral distance in the direction of their movement through the roasting zone but with less than sufficient force to deflect the coarsest of the charge particles substantially from a vertical path of fall, interrupting the fall of said coarsest particles after they have fallen a relatively short vertical distance, and interrupting the fall of said most finely divided particles only after they have fallen a relatively substantially greater vertical distance.

4. The method of roasting ores and concentrates which comprises introducing a finely crushed charge of such material into the upper portion of a roasting chamber, heating the charge to its roasting temperature, causing the heated charge to fall in the form of a thin stream across a laterally directed stream of air, said stream of air being directed in such direction and of suincient force to deflect the most finely divided charge particles through a substantial lateral distance in the direction of their movement through the roasting zone but of less than sunlcient force to deflect the coarsest charge particles substantially from a vertical path of fall, interrupting the fall of at least the coarsest particles after they have descended only a small fraction of the height of the roasting chamber, and repeatedly causing the charge particles to fall in the manner just defined across a laterally directed stream of air after each interruption of their fall until they have descended to the bottom of the chamber, the total duration of the interruptions in the fall of the finest particles as they traverse the chamber being substantially less than the total duration of the interruption, in the fall of the coarsest particles as they traverse the chamber, whereby the most finely divided particles pass through the roasting chamber substantially more quickly than the coarsest particles.

5. The method of roasting ores and concentrates which comprises introducing a finely divided charge of such material into the upper portion of a roasting chamber of substantially circular cross-section, heating the charge to its roasting temperature and hash roasting the heated material by causing it to flow freely in the form of an elongated stream through an oxidizing atmosphere, the density of falling particles in the stream being sufliciently small so that every particle is in contact with the oxidizing atmosphere as it falls therethrough, interrupting the free fall of the material before it has descended more than a small fraction of the height of the roasting chamber, bed-roasting the material by retaining it for an appreciable period of time at the point at which its free fall was interrupted, moving it through only a small sector of said roasting chamber and then causing the material to fall freely in the manner just defined through another small fraction of the height of the roasting chamber, again interrupting its fall and retaining it there for an appreciable period of time, again moving it through only a relatively small sector of the said roasting chamber whereby it is once more caused to fall freely in the manner defined through a further small fraction of the height of the chamber, initially contacting a fresh stream of a laterally-directed oxidizing gas with the falling streams of the ore after each bed roasting operation substantially immediately as such streams begin their free fall, and repeating this sequence of operations until the material has been roasted to the desired extent.

6. The method of roasting according to claim 5, characterized in that an auxiliary reagent for reacting with the charge during the roasting operation is added to the charge.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 522,421 Jones July 3, 1894 677,263 Pape et al. June 25, 1901 '799,696 Ward Sept. 19, 1905 879,842 Swart Feb. 18, 1908 1,671,395 Baird May 29, 1928 2,421,542 Connolly June 3, 1947 2,558,963 Klepetko et al. July 3, 1951