The invention relates to ring furnaces for baking carbonaceous articles, especially open type ring furnaces. The invention relates more specifically to the internal structure of ring furnaces and methods of baking carbonaceous articles.
Open type ring furnaces are well known. British application GB 2,129,918 and U.S. Pat. Nos. 5,683,631 and 6,339,729 describe such ring furnaces.
Open type ring furnaces comprise series of baking pits that are delimited by hollow partitions, often called flue walls, and transverse walls. The partitions and walls are made of refractory bricks, such as those described in International Applications Nos. WO 95/22666 and WO 97/35150.
The baking pits are designed to receive green carbonaceous articles and packing material therein during the baking operations. The hollow partitions are intended for the circulation of heating flue and cooling gas during the baking operations. The transverse walls partition the furnace into a plurality of separate firing sections.
Groups of successive sections are simultaneously utilized to make up a baking sequence for a batch of carbonaceous articles. For that purpose, heating equipment is installed at a specific location of groups of sections while exhaust equipment is installed downstream of the heating means and blowing equipment is installed upstream of the same. After completion of a firing operation, all pieces of equipment are shifted downstream by a specified number of sections.
The applicants addressed the issue of finding ring furnaces that are more economical to make and operate than the known ring furnaces.
A first object of the invention is a ring furnace having a longitudinal axis X-X′ and comprising a first elongated bay and a second elongated bay, each bay being parallel to said axis and including a casing, a plurality of hollow partitions arranged within said casing of each bay so as to be parallel to said axis, a first end transverse wall located at a first end of each bay, a second end transverse wall located at a second end of each bay and possibly one or more intermediate transverse walls located between said first and second ends, said transverse walls being arranged so as to be perpendicular to said axis, said hollow partitions and transverse walls defining baking pits within said bays for stacking carbonaceous articles therein, said baking pits having a length Lp and a width Wp, said length Lp being at least 15 times greater than said width Wp.
Said ring furnace thereby comprises baking pits with a large horizontal aspect ratio Lp/Wp.
The inventors noted that for industrial furnaces the brickwork amounted to a significant item for the cost of production of carbonaceous articles and that the operating costs could be reduced by reducing the number of transverse walls. A furnace according to the invention makes it possible to reduce the number of transverse walls and the required brickwork compared to known ring furnaces.
The inventors further noted that it was possible to operate a ring furnace according to the invention without unacceptable collapse or deformation of the hollow partitions.
The large horizontal aspect ratio Lp/Wp of the baking pits makes it possible to more finely adjust the baking process since the creeping step or “pitch” of the baking process, i.e., the distance between two successive positions of an exhaust ramp, is not necessarily the same as the length of a loading and unloading step (Pa).
Another object of the invention is a method of baking carbonaceous articles including providing a ring furnace according to the invention, stacking said carbonaceous articles in at least one baking pit of said furnace so as to form at least one stacking arrangement having a length La, a first end and a second end, adding packing material around said stacking arrangement so as to protect said articles during said baking, providing at least one heating ramp, an exhaust ramp, a blowing ramp and, optionally, a cooling ramp, and firing a specific part of said arrangement using said heating ramp.
According to an advantageous embodiment of the invention, said method further includes providing at least one removable strut comprising a support member, such as a bar, and at least one spacer and positioning said removable strut on said furnace so that said spacer is inserted in one of said baking pits. Preferably, said removable strut is positioned in a zone of said baking pit that contains no carbonaceous article.
Said removable strut typically comprises a plurality of spacers evenly distributed on said support member so that said spacers can be inserted in adjacent baking pits.
Said removable strut further strengthens said hollow partitions.
Said ring furnace and methods of operating the same are particularly intended for the baking of carbonaceous anodes designed for use in electrolysis cells intended for the production of aluminium through the Hall-Héroult process.
The invention is described in detail below with reference to the appended figures wherein:
As illustrated in
Said bays (10, 10′) include external lateral partitions (15A, 15A′, 15B, 15B′) and a plurality of inner partitions (3, 3′, 4, 4′) that form series of baking pits (2, 2′) having an elongated shape parallel to said longitudinal axis. Said baking pits (2, 2′) have a length Lp in the longitudinal direction and a width Wp in the transverse direction.
Refractory lining is usually provided within said casing (15, 15′) at the bottom and on the sides thereof.
Each of said bays (10, 10′) includes a first end transverse wall (41, 41′) at one end and a second end transverse wall (42, 42′) at an opposite end and, usually, a plurality of intermediate transverse walls (43, 43′) evenly distributed between said end transverse walls (41, 41′, 42, 42′). Said transverse walls (41, 41′, 42, 42′, 43, 43′) partition said bays (10, 10′) into series of distinct sections (So, S1, S2, S3, . . . ).
Said baking pits (2, 2′) are delimited by hollow partitions (3, 3′), said transverse walls (41, 41′, 42, 42′, 43, 43′)—more particularly the pillars (5) thereof—and a floor (16). Said hollow partitions (3, 3′) and transverse walls (41, 41′, 42, 42′, 43, 43′) form the substantially vertical sides of said baking pits while said floor (16) forms a bottom that is substantially horizontal.
Each bay (10, 10′) comprises alternately, in a transversal direction (Y axis), baking pits (2, 2′) and hollow partitions (3, 3′), usually called flue walls, that are parallel to said longitudinal direction (X axis).
The hollow partitions of one bay are connected to the hollow partitions of the neighbouring bay by a first by-bass conduit (13) at one end of said bays and by a second by-pass conduit (14) at the opposite end of said bays, so as to allow the circulation of flue gas from one bay to the other one, especially when a baking sequence overlaps said two bays.
Said hollow partitions (3, 3′) have a width We and, as illustrated in
Said inner partitions (3, 3′, 4, 4′) usually include bricks that comprise interlocking means, which typically include recesses on one surface of said bricks and projections on an opposite surface of said bricks. Said projections of a brick are generally so dimensioned that they can fit in corresponding recesses of an adjacent brick so as to provide interlocking action during use in said furnace.
Said inner partitions may be so assembled that the bricks are directly in contact with each other (“dry” assembly) and/or so that an embedding material, such as grout or mortar, is placed between the bricks.
As illustrated in
A ring furnace according to the invention typically includes few or no intermediate transverse walls. In a preferred embodiment, the number of said intermediate transverse walls (43, 43′) within each bay (10, 10′), if any, is smaller than or equal to 5, and more preferably smaller than or equal to 3.
Said hollow partitions (3, 3′) include a plurality of refractory bricks and, preferably, further include means to allow thermal expansion of said bricks while limiting the deformation of said hollow partitions (3, 3′), such as their bowing.
For example, said hollow partitions (3, 3′) may include bricks (110) comprising at least one projection (116) and at least one recess (117) that are designed to match when said bricks are assembled within said hollow partitions (3, 3′), are capable of providing interlocking action between superposed bricks and have sufficient clearance between said projection (116) and said recess (117) to allow sliding of said superposed bricks on one another over a specified distance along said longitudinal axis and, thereby, absorb their thermal expansion during use. In such an embodiment said hollow partitions (3, 3′) may, at least partly, be assembled without mortar or grout, i.e. may be dry assembled. Sufficient clearance is typically from 2 to 5 mm.
Said recesses (117, 119) are typically selected from the group consisting of grooves. Said projections (116, 118) are typically selected from the group consisting of tongues.
Alternatively, or in combination, said hollow partitions (3, 3′) having lateral walls (11, 11′) may include one or more expansion joints. Advantageously, said expansion joint is a built-in expansion joint comprising at least one gap between specified bricks in each said lateral wall (11, 11′) and sealing bricks arranged within said hollow partition so as to overlap said gap and thereby seal the same, i.e., so as to make it substantially impervious to gas and packing material. Said gap enables thermal expansion of said lateral walls (11) while said sealing bricks limit the flow of gas and packing material (34) between the inside of said hollow partitions (3, 3′) and said pits (2, 2′). Said gap typically extends over several layers of bricks, vertically.
The width Wg of said at least one gap is preferably comprised between 10 and 75 mm, and typically comprised between 40 and 60 mm.
Said pillars (113, 114) are typically made of at least one vertical stack of bricks that preferably includes transverse bricks (1111, 1121) that fit into the lateral walls (11, 11′) of said hollow partitions (3, 3′) so as to stabilize the same. Said pillars (113, 114) typically also include inner bricks (1112, 1122) that are adjacent said lateral walls (11, 11′), and more specifically adjoin wall bricks (1101). At least part of the bricks (1111, 1112, 1121, 1122) that form said pillars (113, 114) overlap said gaps (111, 112) and, preferably, snugly fit between said lateral walls (11, 111, so as to seal said gaps (111, 112) while enabling relative longitudinal displacements between said bricks and part of said lateral walls.
In this example a hollow partition has a top (130), a bottom (131) and a height H and includes at least one first vertical gap (111) running from said bottom (131) to a specified height H1, at least one second vertical gap (112) longitudinally shifted, having a length H2 and running from height H1 to said top (130), a sliding plane (115) located at height H1, between said first and second gaps, at the interface between a first horizontal row of sliding bricks (1151) and a second horizontal row of sliding bricks (1152), at least one first vertical pillar (113) running from said bottom (131) to a specified height H1, and at least one second vertical pillar (114) longitudinally shifted, having a length H2 and running from height H1 to said top (130).
Said sliding plane (115) that joins said gaps (111, 112) has width W and allows relative longitudinal displacement of a first part (11A, 11A′) of said lateral walls (11, 11′) that lie on one side of said gaps (111, 112) and a second part (11B, 11B′) of said lateral walls (11, 11′) that lie on the other side thereof longitudinally. Said sliding bricks (1151, 1152) advantageously comprise at least one longitudinal projection and/or at least one longitudinal recess on an upper or lower surface of said sliding bricks, such as the tongues (118) illustrated in
In this embodiment, all bricks may be assembled using mortar or grout, except within said gaps (111, 112) and sliding plane (115).
The shift between said vertical pillars (113, 114) creates a passage for flue gases and air within the partition while allowing thermal expansion.
The number of said built-in expansion joints depends on the length Lp of the baking pits. Said built-in expansion joints are typically separated by a distance comprised between 5 and 10 meters.
Said hollow partitions (3, 3′) may further include tie bricks (8) and/or baffles (9) to direct the flow of flue gas.
Said hollow partitions (3, 3′) typically further include upper blocks or caps (19A) that bridge said lateral walls (11, 11′).
The invention also relates to a method of baking carbonaceous articles (30) including:
providing a ring furnace (1) according to the invention,
stacking carbonaceous articles (30) in at least one baking pit (2, 2′) of said furnace so as to form at least one stacking arrangement (31) having a length La, a first end (311, 321) and a second end (312),
providing at least a first heating ramp (24, 25, 26), an exhaust ramp (21), a blowing ramp (27) and, optionally, a first cooling ramp (28), and
firing a specific part of said arrangement using said heating ramp (24, 25, 26).
Said stacking arrangement (31) may be constituted by a train of articles stacked side-by-side or by a train of groups (315) of articles stacked side-by-side, as illustrated in
Said groups (315) of articles may constitute a step Pa for the loading of green articles (313, 323) or the unloading of baked articles (314). Said groups (315) of articles may contact each other or be separated by a determined distance that is preferably small compared to their length so as to optimize the yield of a furnace.
Said packing material (34) is typically added by pouring the same into the baking pits (2, 2′) containing said stacking arrangement (31) so as to cover most of the top and sides thereof. Said packing material (34) typically forms embankments (35) at said ends (311, 312) of said stacking arrangement (31). Said embankments (35) preferably do not extend beyond said first end (311, 321) of said stacking arrangement (31), as illustrated in
Said packing material (34) is poured over substantially the entire length La of said stacking arrangement (31) when baking operations are first started. Afterwards, said packing material (34) is poured only at said first end (311) of said stacking arrangement (31) after fresh green articles have been added to said first end.
Typically, the packing material contained at said second end (312) of said stacking arrangement (31) is removed by suction before specified baked articles are removed from said furnace.
The part of said furnace that is located between said exhaust ramp (21) and said first heating ramp (24) is a pre-heating zone Za. The carbonaceous articles that are included in the part of said stacking arrangement (31) that is located within said pre-heating zone Za are normally green (i.e., are not yet baked) and are warmed up before firing during a baking process. A blanking or closing means (17) is preferably inserted within the peepholes (19) of the hollow partitions (3, 3′) that are close to said exhaust ramp (21) so as to prevent the suction of air coming from a neighbouring loading/unloading zone Ze. Said blanking or closing means (17) may be, for example, a closure arrangement such as described in European Patent Application No. EP 295 192 or fine, flexible steel plates, or any equivalent arrangement and means.
The part of said furnace that is located in the vicinity of said first heating ramp (24) or, if applicable, in the vicinity of a group of heating ramps (24, 25, 26) is a firing zone Zb. The carbonaceous articles that are included in the part of said stacking arrangement (31) that is located within said firing zone Zb undergo a firing operation at elevated temperature (typically from 1100 to 1200° C.) during a baking process.
The part of said furnace that is located between said first heating ramp (24) or, if applicable, between said group of heating ramps (24, 25, 26) and said blowing ramp (27) is a first cooling zone Zc. The carbonaceous articles that are included in the part of said stacking arrangement (31) that is located within said first cooling zone Zc are normally baked and are cooled down after firing during a baking process. The peepholes (19) that are located in said first cooling zone Zc are preferably closed so as to direct the flow of cooling air towards said firing zone Zb and favour its being warmed up before reaching the same.
One or more cooling ramps (28, 29) may be used during a baking process in order to further cool the carbonaceous articles after a first cooling-down phase in said first cooling zone Zc. In such a case, the part of said furnace that is located between said first blowing ramp (27) and up to the last cooling ramp (28, 29) is a second cooling zone Zd. The peepholes (19) that are located in said second cooling zone Zd are preferably open so as to favour rapid cooling of the same.
The part of said furnace that extends beyond said cooling zones Zc, Zd includes a loading/unloading zone Ze.
Said pre-heating zone Za, said firing zone Zb, said first cooling zone Zc and, when applicable, said second cooling zone Zd are called active zones and make up a full baking sequence.
As illustrated in
Said stacking arrangement (31) may be fully included within a single pit (2, 2′) or spread over one or more pits (2, 2′) by overlapping one or more transverse walls. Said stacking arrangement (31) may also be split into two parts, initially or as a baking sequence creeps along said bays (10, 10′), so that one part is located in one bay while the other part is in another bay and so that one end of each part abuts against an end transverse walls (41, 41′, 42, 42′), as illustrated in
Said first end (311, 321) and second end (312) of said stacking arrangement (31) are often remote from transverse walls, whether they be end transverse walls (41, 41′, 42, 42′) or intermediate transverse walls (43, 43′), although they eventually reach and pass said transverse walls as said baking sequence is shifted along said bays (10, 10′).
A ring furnace may include more than one baking sequence, as illustrated in
For a baking operation, each of said ramps (21, 24, 25, 26, 27, 28, 29) is fitted to said hollow partitions (3, 3′) using said peepholes (19). During a baking process, once a firing operation is deemed to be terminated, said active zones (Za, Zb, Zc, Zd), as well as the loading/unloading zone Ze, are moved in the direction of arrow F in
The removal of baked articles at said second end (312) and the addition of green articles at said first end (311) and the shifting of said ramps may be performed separately and according to different sequences so long as the global rates match and ensure an identical average creeping pace for both.
During baking operations, a gaseous flow containing air, heating gas, vapours given off by the carbonaceous articles or combustion gases (or, most often, a mixture of them) circulates, in the longitudinal direction of the furnace (X axis), in said hollow partitions (3, 3′). Said gaseous flow is blown upstream of said active zones and is sucked downstream thereof. The heat produced by the combustion of the gases is transmitted to said carbonaceous articles (30) contained in the baking pits (2, 2′), which leads to their firing.
Said exhaust ramp (21) provides suction through the peepholes (19) where it is fitted. For that purpose, said exhaust ramp (21) are typically fitted to suction means, such as fans, located downstream thereof.
Said heating ramp or ramps (24, 25, 26) include burners capable of producing one or more flames within said hollow partitions (3, 3′) so as to provide heat at least in the vicinity of said firing zone Zb.
Said blowing and cooling ramps (27, 28, 29) provide the input of air, typically cool air, within said hollow partitions (3, 3′), using blowers that are fitted thereto.
For a baking process, said exhaust ramp (21), said blowing ramp (27) and, if applicable, said cooling ramp or ramps (28, 29) are activated so as to produce a specific gaseous flow within said hollow partitions (3, 3′) and said burners of said heating ramp or ramps (24, 25, 26) are lit so as to produce said heat within said hollow partitions (3, 3′).
In a preferred embodiment of the invention, said method of baking further includes:
providing at least one removable strut (50) comprising a support member (54), such as a bar, and at least one spacer (51), and
positioning said removable strut (50) so that said spacer (51) is inserted in one of said baking pits (2, 2′).
Said removable strut (50) is typically positioned in a zone of said baking pits (2, 2′) that contains no carbonaceous article (30) or optionally positioned in a zone of said baking pits (2, 2′) that contains carbonaceous article (30) without packing material (34). In the former variation the height HT of said spacer (51) is typically comprised between 1 and 2 meters. In the latter variation the height HT of said spacer (51) is typically comprised between 30 cm and 60 cm.
As illustrated in
Said spacer or spacers (51) of said removable strut (50) are typically secured to a support member (54) that is sufficiently long to span the whole width Wb of a bay (10, 10′) and bear on said external lateral partitions (15A, 15A′, 15B, 15B′) of a casing (15, 15′). The width WT of each spacer (51) is substantially equal to the width Wp of the baking pits. The spacers (51) form crenels (52) having a width WE. Advantageously, said removable strut (50) further comprises a pin (53) at each end that can fit in cavities (55) provided in the casing (15, 15′), so as to limit the displacement of the strut during use.
In a simple embodiment of the invention, such as the ones illustrated in
Said spacer or spacers (51) of said removable strut (50) are fit to be inserted in one of said baking pits (2, 2′). The width WT of said spacers (51) may be uniform, as illustrated in
In an alternative embodiment of the invention, such as the one illustrated in
Number | Date | Country | Kind |
---|---|---|---|
07356065.8 | May 2007 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/EP2008/003448 | 4/29/2008 | WO | 00 | 11/10/2009 |