CN214470031U - Smelting furnace - Google Patents

Abstract

The utility model discloses a smelting furnace. Including the furnace body, set up respectively the fire door of the corresponding one side of furnace body to and smelt the liquid discharge port, locate the furnace gate of fire door, locate the multi-functional platform of furnace body, the fire door setting is width direction's full open fire door, multi-functional platform includes wall body under the fire door to and the multipurpose coupling platform of protrusion in the outside wall at the fire door position of furnace body, the bottom of the inboard slope side wall of furnace chamber is the arc angle with the setting of furnace chamber diapire joint portion. The smelting furnace has simple and reasonable structure, energy conservation, simple, convenient, safe and reliable operation.

Description

Smelting furnace

Technical Field

The utility model relates to a smelting furnace.

Background

The existing metal regeneration smelting system and smelting mode, the structure of a smelting furnace device and the like are unreasonable to some extent. For example, the structure of the mobile connecting device of the furnace door and the furnace body is very complex, the reliability is low, the operation is not flexible, the sealing performance of the furnace door is poor, and fire leakage is easy to occur, so that the energy consumption is high and the production cost is high. The inner side wall of the furnace mouth for smelting raw materials is a steep straight wall, the whole furnace bottom and the side of the furnace mouth are planes, which bring adverse effects to slag removal, charging, stirring and/or in-furnace smelting, and slag removal is carried out when the slag is cleaned in the furnace. When feeding, the feeding device almost extends into the furnace chamber, so that the device is burnt out, a large amount of heat of the furnace chamber is absorbed, the feeding amount is very small, and the time is consumed. In addition, most of the raw materials added into the existing smelting furnace are accumulated on the side of the furnace chamber close to the furnace mouth, and need to be repeatedly finished. The whole operation time is long, the steps are multiple, the difficulty is high, a large amount of heat flows out of the furnace mouth, the energy consumption is high, and the metal regeneration cost is high. And the modes of furnace top feeding, furnace side port stirring and the like all cause complex operation process, complex structure, energy waste and the like.

In addition, the existing metal smelting furnace has the problems of difficult coordination between the displacement motion of the furnace door and the effective closing and sealing of the furnace door, complex structure, easy collision between the furnace door and the furnace body and poor sealing, large running resistance, heavy load of a driving device, high energy consumption, low running reliability and high running cost. The existing smelting mode is not reasonable, the recovered metal is directly heated and smelted, and the loss is very large. The recovery rate is low.

SUMMERY OF THE UTILITY MODEL

The utility model aims at providing a smelting furnace to the problem that above-mentioned prior art exists. The smelting furnace has simple and reasonable structure, energy conservation, simple, convenient, safe and reliable operation.

The utility model discloses the technical scheme of smelting furnace includes the furnace body, sets up respectively the fire door of the corresponding one side of furnace body to and smelt the liquid discharge port, locate the furnace gate of fire door, locate the multi-functional platform of furnace body, the fire door sets up the full open fire door that is width direction, wall body under the multi-functional platform includes the fire door to and protrusion in the multi-purpose coupling platform of the outside wall at the fire door position of furnace body, the bottom and the furnace chamber diapire joint portion setting of the inboard slope side wall of furnace chamber are the arc angle.

The utility model discloses its furnace chamber, fire door, furnace gate device's peculiar rational structure feeds in raw material to one fire door, arranges the sediment and provides very big convenience, and the furnace gate operation is safe, reliable, and required power is little, and is sealed good, has improved smelting efficiency, has improved the effect of smelting, has reduced regeneration smelting loss, has simplified operating procedure, has improved operating efficiency, simultaneously, has prolonged the life of smelting furnace, has reduced substantially calorific loss, has practiced thrift the energy, has improved resource regeneration utilization ratio. The melting smelting is adopted, the loss is small, and the recovery rate is high.

Drawings

FIG. 1 is a schematic perspective view of an embodiment of a raw material melting furnace according to the present invention; FIG. 2 is a schematic view of a vertical cross-sectional view of the furnace body in FIG. 1; FIG. 3 is a view of the oven door of FIG. 2 in an open state; FIG. 4 is a schematic perspective view of an embodiment of the oven door operation control device of the present invention; FIG. 5 is a longitudinal sectional view of the connecting structure of the trigger plate and the stroke elastic piece in FIG. 4; FIG. 6 is a schematic view of the three-dimensional structure of the melting furnace of the present invention; FIG. 7 is a schematic perspective view of another embodiment of the raw material melting furnace according to the present invention; fig. 8 is a schematic perspective view of another embodiment of the oven door operation control device.

Detailed Description

In order to facilitate a better understanding of the present invention, the present invention will be further described with reference to the following embodiments and drawings. As shown in fig. 1 to 6, the melting furnace includes a raw material melting furnace 1, a melt heating furnace 10, a furnace opening 2 opened on a corresponding side of the raw material melting furnace 1, a furnace door device provided on the furnace opening, and a corresponding multifunctional table 7. The multifunctional platform 7 comprises a lower wall surface 7b of the furnace mouth and a projecting wall body (or wall body) 7a projecting from the inner side wall surface of the furnace mouth part of the furnace chamber. The inner side of the protruding wall body is formed by thickening the corresponding side wall of the furnace chamber. That is, the lower wall surface of the furnace mouth and the upper end surface of the projected wall body form the table surface of the multifunctional table 7, and the multifunctional table is built integrally by refractory materials forming the furnace body. The multifunctional table 7 is provided with an inner chamfer inclined flat wall surface, the inner side wall surface of the multifunctional table is provided with an inclined side wall which inclines towards the inner side direction of the furnace chamber from top to bottom, and the combination part of the bottom end of the inclined side wall inside the multifunctional table or the furnace chamber and the furnace bottom surface (the furnace chamber bottom wall surface) is provided with an arc-shaped angle. The multifunctional platform and the full-open furnace mouth thereof can provide great convenience for slag removal, non-meltable slag removal and/or relatively high-melting-point substances removal, feeding, stirring and the like, and shorten corresponding operation steps and time, such as temporary storage filtration, transition transfer, heat loss reduction and the like.

The furnace mouth is arranged at the position close to the smelting liquid level in the furnace cavity, and the opening width of the furnace mouth is equal to the corresponding width of the furnace cavity, so that a fully-open furnace mouth is formed. The fully-open furnace mouth is a multifunctional operation furnace mouth integrating feeding, impurity removal, stirring and/or slag removal and the like. The outer wall surface (outer wall surface) of the furnace body close to the furnace opening and the outer wall surface of the furnace opening are provided with inclined planes which incline towards the downward outer side direction so as to adapt to the furnace door which is arranged in an inclined way.

The furnace chamber is provided with a self-adaptive immersion melting device. The self-adaptive immersion melting device comprises an inclined furnace bottom wall 8 which is arranged to incline upwards from one side of a furnace opening (the front side of the furnace body) of the furnace body to the opposite side of the furnace opening (the rear side of the furnace body). The self-adaptive immersed inclined furnace bottom has a good fluxing action effect, automatically conforms to the natural stacking state of charging and charging, namely is suitable for the charging characteristic that the material on one side of the furnace opening is stacked thickly, can relatively reduce or reduce the additional material pushing and arranging on the inner cavity of the furnace, saves the process operation steps, enables the high-temperature smelting liquid or the soaking smelting liquid in the furnace to carry out self-adaptive balanced immersion on the thickness of the stacked material, greatly accelerates the melting speed of the material and improves the smelting efficiency.

The furnace door device is a self-service furnace door device which comprises a furnace door 3, a guide rail 4 of the furnace door and the like. The furnace door guide rails are respectively and fixedly arranged at two opposite sides of the furnace opening, and the guide rails of the furnace door are arranged in an inclined shape which inclines along the lower and outer side directions.

The left side and the right side of the furnace door 3 are respectively provided with two walking wheels 6, and the guide rail of the furnace door is provided with a self-service anastomat. The self-service anastomat comprises cambered and/or curved surface concave parts 5 which are arranged on the guide rail and respectively correspond to the corresponding walking wheels 6. The arrangement distance of the two walking wheels on two opposite sides of the furnace opening and the corresponding cambered surfaces and/or the cambered surface concave parts 5 on the guide rails of the two walking wheels is equal to the opening degree or the opening stroke of the furnace door. The radian of the upper half part of the cambered surface and/or the curved surface sunken part at the upper part and the lower part of the guide rail is smaller than that of the lower half part of the guide rail, so that an asymmetric cambered surface and/or a curved surface sunken part is formed. Namely the cambered surface and/or the cambered surface depressed part of the guide rail, the cambered surface of the upper part is shallow, and the cambered surface of the lower part is deep. The resistance of the furnace door during lifting and starting can be reduced, and the power loss is reduced.

The size of the walking wheels and the matching depth of the concave parts are based on the fact that the furnace door can be suspended in the air or a gland is sealed at an outer port of the furnace opening when the furnace door is in an operating state.

The upper part of the furnace door 3 is provided with a lifting lug, and the furnace door is connected with a driving device arranged on the furnace top through the lifting lug and a corresponding steel wire sling 13, so that the vertical opening and closing of the furnace door are realized. The furnace door realizes the cover pressure sealing of the furnace opening by utilizing the dead weight in an inclined state.

During operation, the furnace door is closed: the furnace door is fully covered at the furnace opening, the walking wheels fall into the cambered surface and/or the cambered surface concave part of the guide rail, and the furnace door cover is pressed at the furnace opening. When the furnace door is opened and the furnace door begins to rise, the travelling wheels are separated from the cambered surfaces and/or the curved surface concave parts of the guide rails, so that the furnace door is suspended from the outer end surface of the furnace opening and moves upwards, and the travelling wheels stay at the positions of the guide rails without the cambered surfaces and/or the curved surface concave parts when the furnace door is in a fully-opened state.

The furnace door operation control device comprises a limiting body 12 and a protective automatic limiting control switch, wherein the limiting body 12 is fixed on the inner side of the guide rail 4 close to the upper part and the lower part of the furnace opening and corresponds to the upper wall surface and the lower wall surface of the furnace door. The protection type automatic limit control switch device comprises a containing cavity 12a, a guide coupling concave table 12b, a telescopic touch device, an insulating plate 16, an electric contact fixed contact piece 17, an electric contact movable contact reed 15 and the like, wherein the containing cavity 12a is oppositely arranged on an upper limit body and a lower limit body respectively, the guide coupling concave table 12b is wound on the opening of the containing cavity, the insulating plate 16 is fixedly arranged on the bottom wall of the containing cavity, and the electric contact fixed contact piece is fixed on the insulating plate.

The telescopic touch device comprises a trigger plate 14, a plurality of stroke elastic pieces 18, limiting grooves 14-1 arranged on two opposite sides or the periphery of the inner wall surface of the trigger plate and the like. A stroke elastic sheet 18 is arranged between the inner wall surface of the trigger plate 14 and the guide coupling concave table 12b through a limit groove 14-1, the stroke elastic sheet 18 is always in contact connection with the guide coupling concave table and the limit groove of the trigger plate, one end of an electric contact movable contact reed 15 is fixedly connected with the inner wall surface of the trigger plate, and the other end of the electric contact movable contact reed corresponds to an electric contact fixed contact reed 17.

The outer side of the periphery of the stroke elastic piece 18 is wound with a sealing rubber sheet or an adhesive tape 14b, and the telescopic touch device forms contact type sealing with the side wall of the periphery of the guide coupling concave station 12b through the sealing rubber sheet or the adhesive tape. The lower end of the sealing rubber sheet or the adhesive tape is connected with the guide coupling concave table surface in a bending way, the sealing rubber sheet or the adhesive tape can be manufactured into a corrugated sleeve shape, and the cross section of the sealing rubber sheet or the adhesive tape is L-shaped. The length of the stroke elastic sheet is smaller than the corresponding lengths of the limit groove 14-1 and the guide coupling concave table 12b so as to ensure that the stroke elastic sheet can freely stretch and contract.

The height of the travel elastic sheet and the trigger plate thereof protruding on the wall surface of the limiting body contacting with the walking wheel in a free state is larger than or equal to the working travel of the electric contact movable contact spring piece 15 moving up and down. The lower part of the stroke elastic sheet 18 and the trigger plate thereof form contact connection with the guide coupling concave table 12b under the free state of not being pressed, so that the containing cavity on the limiting body is always in a basically sealed or closed state. The elasticity of the stroke elastic piece 18 corresponds to the working stroke of the movable contact reed. The stroke elastic piece 18 is a wave-shaped stroke elastic piece which is wave-shaped in side view. The utility model discloses a furnace gate operation control device has good harmony of seal protection and operational reliability.

The limiting body at the lower part of the guide rail simultaneously forms a support body after the furnace door is closed, the support body can support the furnace door in a closed state of the furnace door, the energy consumption of the driving device can be reduced or reduced, and the service life of corresponding equipment can be prolonged; meanwhile, a triggering protection device of the furnace door protection type automatic limit control switch device is formed.

The electric contact fixed contact piece 17 and the electric contact movable contact reed 15 form a press-contact type electric contact switch for controlling the up-down limit of the oven door. The electric contact fixed contact piece and the electric contact movable contact reed are respectively led to be connected with a corresponding furnace door controller through a lead wire outside a lead wire hole on the inner side of the containing seat. One end of the limiting body close to the inner side is embedded into a mounting groove formed in the bottom wall of the guide limiting groove of the guide rail and is locked to the guide rail by a screw 19 to form limiting, so that the furnace door is supported in strength. The operation response speed is high, and the switch action is sensitive. The operation mode of the furnace door operation control device is as follows:

when the furnace door descends until the walking wheels of the furnace door fall into the lower cambered and/or curved surface concave part 5, the lower part of the furnace door touches the limiting body positioned at the lower part of the furnace opening, the trigger plate is pressed to overcome the elasticity of the stroke elastic sheet, so that the trigger plate moves downwards along the guide coupling concave table 12b, the electric contact movable contact sheet is driven to move towards the inner cavity to be in contact connection with the electric contact fixed contact sheet, and the controller controls to stop the descending of the furnace door and close the furnace door. When the furnace door moves upwards to the upper part of the furnace door and touches the upper limit body of the guide rail, the trigger plate is pressed and moves upwards, the corresponding electric contact movable contact piece moves upwards to be in contact with the electric contact fixed contact piece, the switch is closed, and the controller controls the furnace door to stop. The furnace door is opened in place. The corresponding control circuitry may be similar to the prior art.

As shown in fig. 6. The raw material melting furnace 1 and the melt heating furnace 10 are in liquid phase communication with each other through corresponding melting launders 20. Two ports of the melting launder 20 are connected to the melt ports of the raw material melting furnace 1 and the melt heating furnace 10, respectively. The smelting launder is provided with a melt pump 21, two ends of the melt pump of the smelting launder are connected with a return channel in parallel, a melt port of the raw material melting furnace is provided with a control valve 20b, and an inlet and an outlet of the melt pump 21 are respectively connected with the melt heating furnace and the raw material melting furnace. The bottom wall of the corresponding end of the smelting launder is provided with a settling pit 20 a. The sedimentation pit is provided with a slag discharge port. The lower part or the bottom of the melt heating furnace is provided with a smelting casting discharge port and a discharge valve thereof.

The upper opening of the smelting launder is provided with an assembled heat preservation cover plate. The melt heating furnace 10 is provided with a heater. The raw material melting furnace is arranged at a position higher than the corresponding position of the melt heating furnace, namely, the melt of the raw material melting furnace can be directly replayed to the melt heating furnace through a melt port of the raw material melting furnace through a melting launder by utilizing the relative position height difference. The structure is simplified, the energy and equipment investment are saved, and the service life of the equipment is prolonged. A gas phase balance communicating hole is arranged between the raw material melting furnace and the gas phase part of the melt heating furnace. The method can avoid the excessive gas pressure formed by the melt heating furnace and/or the raw material melting furnace in the process of heating or soaking the molten metal, ensure the safe production and ensure the relative balance of the pressure of the two furnaces. The top wall of the melting heating furnace is provided with a feeding port 10b and a sealing cover thereof. Is only used for the first time of blow-in operation. And (3) performing primary blow-in production, namely putting the recycled metal raw material into the melt heating furnace from the feeding port, and heating, burning and smelting the raw material in the melt heating furnace by the smelting heater to obtain the running primary smelting immersed melt. The melt pump can be an existing pump with a similar structure.

The smelting is to heat the melt stored in the melt heating furnace through a heater of the melt heating furnace to increase the temperature of the melt, the metal raw material is put into the raw material melting furnace through a furnace mouth of the raw material melting furnace, the high-temperature melt of the melt heating furnace is conveyed to the raw material melting furnace through a smelting launder by a melt pump, and the raw material materials in the melting furnace are subjected to immersion type smelting. The melt melted by the raw material melting furnace returns back to the melt heating furnace through a control valve through a backflow channel of the melting launder by a potential difference for ingot casting.

A recovery device is arranged at the mouth of the raw material melting furnace, and comprises an automatic expansion collecting cover 24, an induced draft fan 27, a corresponding automatic control device and the like. The automatic expanding collecting hood 24 comprises a top plate connected with the top of the furnace, a fixed diversion collecting plate 25 which is connected with the top plate and obliquely extends to the front part of the furnace opening, and approximate fan-shaped side coamings 26 which are arranged at the left and right opposite sides of the raw material melting furnace and are used for fixing the diversion collecting plate. The lower end of the fixed diversion collecting plate is connected with a folding and unfolding diversion collecting plate 28 through a rotating shaft.

The automatic control device of the recovery device comprises a gear 23 connected to a rotating shaft of the deflection expansion guide plate, a pneumatic motor 22 connected with the gear in a transmission mode, a pressure-contact type electric contact switch at the lower part of the furnace door protection type automatic limit control switch device and a corresponding controller, wherein the controller is provided with a timer, an induced draft fan 27 is connected to the corresponding output end of the controller, a driver of an action cylinder is connected with the corresponding output end of the controller through the timer, and a limit switch under the furnace door is connected to the input end of the controller. The control circuit is similar to the corresponding prior art.

The operation is as follows: when the furnace door is opened, a furnace door press-touch type electrical contact switch signal is transmitted to the controller, the controller simultaneously controls the draught fan to be opened and the pneumatic motor to rotate forward by an angle according to the input signal, the pneumatic motor drives the draught fan to rotate to turn down the deflection expansion guide plate, the deflection expansion guide plate is enabled to block the front part and the lower part of the furnace opening, all flame gas sprayed from the furnace opening is guided and collected to the collection cover, a time is delayed by the timer for one time, after surge flame gas just opened from the furnace opening is basically dissipated, the controller controls the pneumatic motor to rotate reversely, the deflection expansion guide plate is turned upwards, the main space of the furnace opening is made free, and normal operation places such as feeding are vacated for operators. The flame smoke with weakened strength is collected and recovered by the inclined diversion collecting plate and the like. After the smelting process operation is finished, the furnace door is operated to descend to be closed, the travelling wheels press the pressure-contact type electric contact switch to be closed, and the controller controls the induced draft fan to stop.

The automatic expansion collecting cover 24 has the advantages that the automatic expansion collecting cover can automatically adapt to the operation process change of a smelting furnace system through the deflection expansion guide plate and the automatic control device thereof, the extension change of a collecting channel is expanded to the front lower part of the whole furnace opening, the pressure gas flow mixed with high-temperature flame in the furnace in the stage of just opening the furnace door can be effectively prevented from impacting a smelting operation environment place, and the whole production environment space is filled. The recovery effect is improved and enhanced fundamentally.

The smelting launder is provided with a bidirectional output melt pump, and the raw material melting furnace and the melt heating furnace are arranged on the same horizontal plane. To suit the needs of a particular smelting furnace system process setting. The raw material melting furnace is not provided with a control valve. A bidirectional output injection pump is arranged in a smelting circulation channel. The raw material melting furnace and the melt heating furnace both output melt pumps in two directions and carry out mutual conveying through the melting launder. A control valve and a backflow channel are not arranged, and a bidirectional output melt pump replaces the control valve of the raw material melting furnace by utilizing the small melt level difference between the immersion melting furnace and the smelting and casting furnace. The molten liquid melted by the raw material melting furnace is mutually conveyed by a bidirectional output pump through a melting launder.

Example 2, as shown in fig. 7 and 8. The multifunctional platform 7 comprises a lower wall body 7b of the furnace mouth and a multipurpose coupling platform 7c protruding out of the outer side wall surface of the furnace mouth part of the furnace body. The end face of the multifunctional table 7 is an inclined wall surface inclined inward downward, and the lower end face of the oven door 3 is provided with an inclined surface corresponding to the inclined wall surface at the upper end of the multifunctional table. The top wall of the furnace chamber is arranged at the upper port of the furnace opening, so that the top of the furnace chamber is approximately parallel to the upper end of the furnace opening. The top wall of the furnace chamber is arranged in a totally enclosed type with the corresponding peripheral side wall of the furnace chamber as a whole. The corresponding gas phase space height of the furnace mouth and the furnace chamber is only adapted to the height of the stirring and feeding device, such as a stirring rake, a slag cleaning rake and the like. The side wall of the furnace chamber close to the furnace door, the inner side wall of the furnace door and the furnace opening are arranged to incline towards the upper part of the inner side. One side of the furnace mouth of the furnace chamber is inclined, the liquid level of the smelting liquid in the furnace chamber is close to the lower end surface of the furnace mouth, and the gas phase part of the furnace chamber is only formed by partial space of the furnace mouth. The remarkable characteristic is that the volume of the smelting cavity is correspondingly enlarged under the condition of realizing the function of basically the same smelting capacity. The upper end of the furnace door guide rail extends to the top space of the furnace body through the bracket. The multifunctional platform has the function of sealing the lower part of the furnace door and the furnace opening and can support the furnace door. The furnace chamber of the furnace body is arranged to form a shape structure, so that the gas phase space of the furnace chamber is reduced, the whole volume of the furnace body is reduced, the heat efficiency of the immersion smelting is relatively improved, and the energy conservation is remarkable.

The furnace door operation control device comprises a containing cavity 12a, a guide trigger block 14a, a table-shaped elastic protective band ring 14c and the like, wherein the containing cavity 12a, the guide trigger block 14a and the table-shaped elastic protective band ring 14c are respectively arranged on the upper limiting body and the lower limiting body, and the ports of the containing cavity and the guide trigger block are flat. Namely, the port of the containing cavity is not provided with a guide coupling concave table, and the guide trigger block 14a is guided to operate by the matching contact of the peripheral wall of the containing cavity 12a and a table-shaped elastic protective belt ring.

The electric contact movable contact reed 15 is fixedly connected to the inner wall surface of the guide trigger block. A plurality of reset springs 18a which are uniformly distributed are respectively connected between the inner wall surface of the guide trigger block 14a and the bottom wall surface of the containing cavity 12a through guide blind holes 12c on the bottom wall of the containing cavity or by gluing. The periphery of the guide trigger block 14a is movably sealed or hermetically connected with the side wall of the containing cavity of the limiting body through a table-shaped elastic protective belt ring. The cross section of the table-shaped elastic protective band ring 14c is in an equilateral trapezoid or a right-angle trapezoid, the small end is positioned at the outer side, and the cross section is mainly in elastic extrusion contact with the peripheral wall of the containing cavity 12a through the peripheral wall of the table-shaped large end surface positioned in the containing cavity. The dynamic sealing is reliable, and the guide trigger block can be prevented from being extruded out of the containing cavity after being pressed to deform.

The guide trigger block 14a is always in closed or sealed contact connection with the peripheral wall of the containing cavity through the inner peripheral wall of the table-shaped elastic protective band ring 14c, and the guide trigger block 14a freely protrudes out of the working wall surface of the limiting body in contact with the upper part and the lower part of the furnace door. When the device runs, the guide trigger block moves up and down in the containing cavity under the matching action of pressure application and the reset spring, and the reset spring expands to jack up and reset the guide trigger block after the pressure of the guide trigger block is relieved. The structure is relatively simpler, and the operation is stable and reliable. The remaining structure and corresponding operation of this example may be similar to the embodiments described above. The rest of the structure, the corresponding operation mode, etc. of this embodiment may be similar to the above embodiments.

The table-shaped elastic protective band ring is characterized in that the big end is positioned at the inner side of the containing cavity, the elastic protective band ring can be a straight wall-shaped elastic protective band ring, and the elastic protective band ring is made of rubber. The rest of the structure, the corresponding operation mode, etc. of this embodiment may be similar to the above embodiments.

Claims (1)

1. The utility model provides a smelting furnace, includes the furnace body, sets up respectively the fire door of the corresponding one side of furnace body to and smelt the liquid discharge port, locate the furnace gate of fire door, locate the multi-functional platform of furnace body, characterized by the fire door sets up the full open fire door that is width direction, multi-functional platform includes wall body under the fire door to and the multi-purpose coupling platform of protrusion in the outside wall at the fire door position of furnace body, the bottom and the furnace chamber diapire joint portion setting of the inboard slope side wall of furnace chamber are the arc angle.

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