Technical field

[0001 ] The present invention relates to a sinter mixer for the production of sinter mix for use in a metallurgical furnace.

Background of the Invention

[0002] In steelmaking, the quality of the raw material introduced into the metallurgical furnace is a critical feature in order to improve the efficiency of the production process. Studies of the influence of the quality of the raw material have permitted the development of sinter as a result of a pre-treatment of ore and other materials. Sintering is now a key process in steelmaking.

[0003] The sintering process starts with the preparation of a raw mix of solid particles, generally comprising return fines, coke fines, limestone fines, ore and/or additives. This raw mix is then fed into a sinter mixer, wherein the raw mix is thoroughly mixed to produce a sinter mix. The sinter mix exiting the sinter mixer is then fed onto a sinter strand, which feeds the sinter mix through an ignition hood where sinter mix is ignited. The sinter strand is a moving conveyor of hot sinter, which continues to 'cook' after leaving the hood. At the end of the sinter strand, the cooked sinter is discharged into a screening area, where it is passed through crushers and screens to produce particles in a specific size range. Sinter below the required size is fed back to a return fines bin to be fed into the raw mix. Sinter above the required size may be fed into the metallurgical furnace. This general sintering process is well known and is thus not further detailed herein.

[0004] For the production of adequate sinter, a homogenously mixed sinter mix needs to be uniformly spread on the sinter strand. Accordingly, homogeneous mixture of the raw materials to form of the sinter mix is an important characteristic in the production of sinter.

[0005] Traditionally, the sinter mix is obtained in a mixing drum, but conventional mixing drums often only offer a limited homogeneity of the sinter mix. More recently, intensive mixers are preferred as they offer a better homogeneity, but also a lesser energy consumption with higher filling capacities. There are generally two types of intensive mixers, vertical mixers and horizontal mixers. While vertical mixers generally offer a better control on the repartition of the material in the mixer and consequently on the homogeneity of the sinter mix, they are usually operated in a batch-wise process, which involves charging and discharging delays. Horizontal mixers, on the other hand, are usually used in a continuous process.

[0006] An example of a vertical intensive mixer is disclosed in EP 0 204 127, wherein the mixer has a rotatable mixing chamber inside a pressure container. Mixing tools are disposed in the mixing chamber at offset locations in relation with the rotation center of the mixing chamber. The mixing chamber and the mixing tools are rotated in different directions in order to create a greater speed differential for the material inside the mixer. By reaching high mixing speeds, the mixer of this solution produces a sinter mix in a shorter time than a classical drum mixer.

[0007] Nevertheless, this solution has several drawbacks: the mixer requires an important energy input; the high speed is responsible for rapid wear of the mixing blades of the mixing tool; the batch-wise process of the mixer mitigates its time efficiency; and finally, the high pressures created in operation inside the mixer involve construction constraints with a direct impact on development and construction costs.

[0008] An example of a horizontal intensive mixer is disclosed in US 4,214,376 or US 5,275,485, wherein the mixer comprises horizontal drum with plowshare-type mixing tools arranged therein. These mixing tools are operated at high rotation speed in order to mix the material therein and to feed the material from a charge region to a discharge region.

[0009] The latter solution requires an important energy input when operating at high rotation speed. The high rotation speed also produces high frictional forces responsible for rapid wear of the mixing tools.

[0010] US 3,938,787 discloses a mixer in the form of a rotatable bowl with a central outlet aperture. Upon rotation of the bowl, the raw mix is transported along a circular path. Static mixing tools in the form of vanes are provided along the path of the raw mix to mix the material in the bowl. These static mixing tools are fixedly attached to the roof covering the rotatable bowl. A discharge vane is further provided to direct a portion of the material towards the central outlet aperture. The discharge vane can be raised and lowered in order to adjust the amount of material discharged. The material that is not caught by the discharge vane is allowed to further rotate in the bowl and encounters the static mixing tools a further time to further mix the material. The mixing in such a device requires the raw material to pass through the mixing tools a number of times, i.e. requiring a long residence time of the material in the bowl to ensure adequate mixing.

Object of the invention

[001 1 ] It is therefore desirable to provide improvements to the solutions used to produce sinter mix. Particularly, it is an object of the invention to provide a sinter mixer with improved energy consumption and efficiency.

General Description of the Invention

[0012] The invention overcomes the above discussed deficiencies and disadvantages by providing a sinter mixer for the production of sinter mix for use in a metallurgical furnace. The sinter mixer comprises a mixing chamber with a charge aperture for introduction of a raw mix and a discharge aperture for extraction of a sinter mix. At least one processing tool is mounted inside the mixing chamber for mixing the raw mix into the sinter mix.

[0013] In the context of the invention, the raw mix is a non-homogeneous mix of raw material that still has to be mixed in order to obtain a sinter mix that can subsequently be fed to a sinter strand to form sinter material for use in a metallurgical furnace.

[0014] According to the invention, the mixing chamber is of essentially annular shape and the sinter mixer comprises a ring-shaped, horizontal conveyor for continuously conveying the raw mix within the mixing chamber from the charge aperture to the discharge aperture. The at least one processing tool is arranged in the path of the raw mix between the charge aperture and the discharge aperture.

[0015] The conveyor allows for the raw mix to be continuously transported through the sinter mixer. This continuous process allows for a continuous supply of sinter mix to the sinter strand. An advantage of the invention is that by using a continuous process instead of a batch-wise process, it suppresses production delays due to charge and discharge operations. [0016] The processing direction of the present sinter mixer is horizontal, so the overall elevation of the system may be reduced for easier implementation at the operating site. Due to the annular shape of the sinter mixer, the empty space in the middle of the sinter mixer offers a better accessibility for maintenance operations. This empty space is even accessible while the sinter mixer is in operation. This may be useful to carry out some checks without requiring stoppage of the sinter mixer. The rotatable conveyor is a low energy solution for the transport of the raw mix inside the sinter mixer and offers a precise control over the speed and quality of the sinter mix production.

[0017] The at least one processing tool comprises at least one static mixing tool and at least one dynamic mixing tool. The combination of static and dynamic mixing tools allows a thorough mixing of the material. The static and dynamic mixing tools are preferably arranged one downstream of the other along the path of the raw mix. Advantageously, the sinter mixer comprised a plurality of static mixing tools and/or a plurality of dynamic mixing tools. Static and dynamic mixing tools may be alternately arranged along the conveying path of the raw mix within the mixing chamber.

[0018] The at least one static mixing tool may be formed by one or more obstacles arranged in the path of the raw mix. The movement of the conveyor relative to the static mixing tool generates a movement of the raw mix against the static mixing tool resulting in a mixing effect. This type of mixing uses energy from the movement of the conveyor and requires no additional energy supply. It is also beneficial for the mixing tool that suffers very little wear through abrasion of the raw mix particles.

[0019] The at least one dynamic mixing tool preferably comprises any of the following: rotating plowshares, shovels, self-cleaning twin-shaft tools, or any moving tools. Advantageously, the dynamic mixing tool rotates through the raw mix in a direction opposite to the movement of the conveyor. The opposite movements of the conveyor and the dynamic mixing tools are hence added in order to increase the differential speed between the raw mix particles and the dynamic mixing tool and the energy used to rotate the dynamic mixing tool is reduced. [0020] Advantageously, the dynamic mixing tools have a rotational speed of between 40 and 120 rpm, preferably about 60 rpm. Advantageously, the conveyor has a rotational speed of between 0.2 and 2 rpm, preferably between 0.5 and 1 rpm. Due to such slow rotational speeds, wear of the mixing tools, especially the dynamic mixing tools, is reduced. Consequently, the time between maintenance operations of the mixing tools can be increased, which leads to fewer stoppages of the sinter mixer. The slow operating speed of the mixing tools and the slow travelling speed of the conveyor may be compensated by multiplying the mixing tools and the mixing operations along the course of the mixing chamber between the charge aperture and the discharge aperture.

[0021 ] Another advantage coming from the low speed and combined movements of the dynamic mixing tools is that their driving means only require low power supply. Immediate advantages of a lower power supply are improved energy efficiency, and low power consumption. For example, for a production of 750 tons of sinter mix per hour, the power consumption of the sinter mixer may be around 100 kW, whereas for other known mixers, such as intensive or drum mixers, power consumption ranges from 400 kW to almost 1000 kW.

[0022] Another advantage is a reduction of the electric circuitry scaling, and a better cost efficiency. It impacts positively on the global efficiency of the sinter mixer, particularly in comparison with high-speed rotating mixer as known in the prior art.

[0023] The low speed and combined movements of the dynamic mixing tools also cause relatively low pressure difference between the inside and outside of the sinter mixer, reducing risks of leakage or spillage of material through the openings of the sinter mixer while in operation. This is also advantageous for reducing mechanical constraints when forming sealing connections between rotating and static parts of the sinter mixer.

[0024] Moreover, when compared to high-speed rotating mixer as known in the art, the sinter mixer of the present invention does not require additional protective plates to reduce wear on a specific element or wall of the sinter mixer. The construction, development, and maintenance costs are consequently reduced. [0025] The sinter mixer may further comprise at least one water feeding system for wetting the raw mix in the mixing chamber. The controlled addition of water to the raw mix is advantageous for controlling the quality of the resulting sinter mix. Water may be added before or after a specific mixing tool. The water feeding system may also be integrated within at least one of the mixing tools.

[0026] A plurality of water feeding systems may be arranged at different locations within the mixing chamber. Water may thus be added at different stages of the mixing process at different quantities, thereby enhancing the control of the homogeneity of the resulting sinter mix.

[0027] The mixing chamber may further comprise at least one additive feeder device to feed an additive to the raw mix within the mixing chamber of the sinter mixer. Such additive may e.g. be a binder, a reagent, return fines, coke, or any required material. The additive is added to refine the control of the quality of the resulting sinter mix. Advantageously, additive feeder devices may be positioned at different locations and thus allow the addition of an appropriate additive at different stages of the mixing process.

[0028] Preferably, the charge aperture is positioned adjacent to the discharge aperture. The conveyor transports the raw mix from the charge aperture to the discharge aperture, describing almost a full circle in order to maximize the use of available space, to maximize the amount of mixing tools and to optimize the ground surface required to install the sinter mixer.

[0029] The sinter mixer preferably further comprises a discharge system in a discharge region comprising the discharge aperture. The discharge system is preferably configured to extract essentially the entire flow of sinter mix from the sinter mixer through the discharge aperture. While it is desired to extract essentially all of the sinter mix, it cannot be excluded that some of the sinter mix passes the discharge region without being extracted. This sinter mix then progresses to the charge region and is exposed to further mixing with the newly added raw mix. The proportion of sinter mix being processed again is however preferably reduced as much as possible.

[0030] The conveyor may comprise an annular plate or an annular belt assembly. The annular plate may be a solid plate, for example a metallic plate, capable of supporting the rough conditions inside the mixing chamber. An annular belt assembly made of several belt parts linked together has the added advantage of being repairable in parts. Should it become necessary, individual belt parts may be replaced without having to replace the whole conveyor.

[0031 ] Advantageously, the conveyor is supported by a plurality of static wheels, preferably driven by a single motor. The conveyor may be mounted on one or two circular rails supported by static wheels. This provides low resistance against the rotating movement of the conveyor, such that a single motor driving one of the static wheels may be sufficient to move the conveyor. Additional motors may however be used without drastically increasing the energy consumption of the sinter mixer.

[0032] A further advantage of the sinter mixer according to the present invention is that it does not require hydraulic or pneumatic systems, and their associated pressure generator to operate the conveyor or the dynamic mixing tools. The invention relies on simple mechanical driving systems.

[0033] The annular mixing chamber may comprise a plurality of removable annular sections, which may be fixed together by any suitable means. Such removable annular sections are advantageous for making maintenance easier on a particular part of the sinter mixer. Sections may be individually removed to open the sinter mixer without having to perform a complete dismantlement of the installation.

[0034] Another advantage of the invention is that, due to annular shape of the sinter mixer, the mixing tools are easy to access, replace or remove. In general, the invention provides a sinter mixer which is easy to assemble, disassemble and maintain.

Brief Description of the Drawings

[0035] Further details and advantages of the present invention will be apparent from the following detailed description of not limiting embodiments with reference to the attached drawing, wherein:

Fig.1 is a perspective schematic top view of a sinter mixer according to one preferred embodiment of the invention;

Fig.2 is a cross-section top view of the sinter mixer through plane A of Fig.1 ; Fig.3 is a schematic side view of the sinter mixer of Fig .1 ; and

Fig.4 is a cross-section side view of the sinter mixer through plane B of Fig .1 .

Description of Preferred Embodiments

[0036] A sinter mixer according to one embodiment of the invention is shown in Fig.1 . The sinter mixer 10 is used for the production of a homogenous sinter mix to be fed to a sinter strand (not shown), where the sinter mix may be transformed into sinter for use in a metallurgical furnace. The sinter mixer 10 comprises a mixing chamber 12 supported by a support base 14.

[0037] The mixing chamber 12 is a hollow ring of rectangle section. It is understood that the shape of the mixing chamber may be different but geometrically remains a toroid. The mixing chamber 12 comprises a charge aperture 16 for introduction of material, herein referred to as raw mix, to be mixed. The mixing chamber 12 also comprises a discharge aperture 18 for extracting the mixed raw mix as sinter mix from the sinter mixer 10.

[0038] The mixing chamber 12 comprises three walls mounted on top of the support base 14. An inner wall 20 facing the center of the toroid of the mixing chamber 12, an outer wall 22 parallel to the inner wall 20, and an upper wall 24 sealing the top of the mixing chamber 12 by connecting to the inner and the outer walls 20, 22.

[0039] The mixing chamber 12 comprises a plurality of ring sections connected together through sealed connections 25. The sealed connections may be any suitable connection satisfying the dust-tight needs of the sealing required in sinter mixers. The connections are preferably individually removable in order to perform partial maintenance operations.

[0040] Fig.2 shows a cross-section top view of the mixer through a plane A shown in Fig .1 , just below the upper wall 24. As shown in Fig.2, processing tools are installed inside the mixing chamber 12 in order to transform the raw mix into a homogeneous sinter mix. The processing tools in the represented embodiment comprise mixing tools 26, water feeding systems 30 and an additive feeder device 32. [0041 ] The support base 14 comprises a ring-shaped, horizontal conveyor 28. The conveyor 28 is a plane metallic plate for transporting the raw mix inside the mixing chamber 12. The conveyor 28 is rotatable relative to the mixing chamber 12 in order to transport the raw mix from the charge aperture 16 to the discharge aperture 18 via the processing tools 26, 30, 32.

[0042] The charge aperture is advantageously provided on the upper wall 24 of the mixing chamber 12. The raw mix is brought to the charge aperture 16 by another machine or system, like for example a conveyor belt or any suitable means. The raw mix is poured onto the conveyor in order to form a shallow bed of raw mix. The height of the bed of raw mix may be controlled by any suitable means and may e.g. be around 250 mm. The raw mix is continuously poured over the conveyor 28, as the conveyor 28 rotates continuously.

[0043] As seen in Fig .1 , the charge aperture 16 and the discharge aperture 18 are preferably positioned adjacent to one another. The conveyor 28 rotates from the charge aperture 16 in a direction away from the discharge aperture 18. The raw mix entering the sinter mixer 10 is thus substantially transported along the entire length of the mixing chamber 12 before reaching the discharge aperture 18. This installation aims at using a maximum of the available space inside the mixing chamber 12 for mixing the raw mix and form a homogenously mixed sinter mix.

[0044] Preferably, all the elements inside the mixing chamber 12 are connected to the walls of the chamber 12 or through the walls to exterior supports 34.

[0045] There are two kinds of mixing tools 26 mounted inside the mixing chamber 12: dynamic mixing tools 36 and static mixing tools 38.

[0046] As seen in Figs 2 and 4, dynamic mixing tools 36 are here represented as plowshare-type tools. They comprise a rotating shaft 40 parallel to the conveyor 28.

[0047] The represented dynamic mixing tools 36 comprise a series of shovels 42 fixed to the shaft 40. The shovels 42 are designed and attached to the shaft 40 such that the shovels 42 reach into the bed of raw mix arranged on the conveyor 28. The row of shovels covers essentially the whole width of the mixing chamber 12. The shovels 42 are fixed to the shaft 40 in different angular positions relative to the shaft 40 with the resulting effect that the row of shovels 42 describes a helicoid around the shaft 40.

[0048] Advantageously, each shovel 42 comprises a digging plate 46 that is configured to turn around the shaft 40 along with the rotation of the shaft 40. One side of the plate 46 is in contact with the sinter particles and comprises two concave surfaces 47 joined in a central ridge. In that manner, when turning around the shaft 40, the shovel 42 simultaneously lifts the raw mix particles and pushes them away to the side.

[0049] The shaft 40 rotates in the direction such that the shovels 42 move in a direction opposite to the direction of the conveyor 28. Advantageously, the rotating speed of the shaft 40 is relatively slow i.e. it does is between 40 and 120 rpm, preferably about 60 rpm. The multiplication of the mixing tools 26 and the combination with the movement of the conveyor plate 28 allow for the sinter mixer 10 to achieve a very efficient and homogeneous sinter mix without requiring highspeed rotating elements.

[0050] An important feature of the present sinter mixer 10 is that, reducing the rotating speed of the dynamic mixing tools 36 implies a consequent reduction of the wear phenomenon that results from the friction between the highly abrasive particles of the raw mix and the dynamic mixing tools 36. By reducing the wear of the tools, the sinter mixer 10 is able to operate for longer periods of time without being stopped for maintenance. As the sinter mixer 10 operates in a continuous process, this consequently improves the efficiency of the sinter mixer 10.

[0051 ] The mixing chamber 12 also comprises static mixing tools 38, which are preferably connected to the upper wall 24 of the mixing chamber 12. They may however also be connected to the inner and/or outer walls 20, 22. Static mixing tools 38 comprise a series of obstacles 44 configured to separate the flow of raw mix. The obstacles 44 exhibit a sharp ridge directed towards the incoming raw mix. Advantageously, the obstacles 44 are positioned along the whole width of the mixing chamber 12 and shifted in angular and radial position relative to the center of the ring-shape of the mixing chamber 12. In that manner, each obstacle 44 divides the flow of raw mix particles and diverts a part of it in the direction of a next obstacle 44. [0052] Water feeding systems 30, which may be arranged at different locations along the mixing chamber 12 may comprise a horizontal arm (not shown) extending over the entire width of the mixing chamber 12. The arm is advantageously mounted against the upper wall 24 and comprises water feeding pipes with sprayers or nozzles to spray water over the passing raw mix.

[0053] In order to improve the permeability of the raw mix, return fines are added via the additive feeder device 32. The additive feeder device 32 may be used to feed any appropriate material into the mixing chamber 12. Additional additive feeder devices may be provided along the mixing chamber 12 to add different additives at different stages of the mixing process.

[0054] As shown in the embodiment of Fig.2, static and dynamic mixing tools 38, 36 are mounted in successive mixing zones. After every other mixing zone, a water feeding system 30 is installed, and the additive feeder devices is mounted after the last mixing zone. It is to be understood that the disposition of the different systems and tools inside the mixing chamber may be conceived differently in order to optimize the production of sinter mix according to the particular nature and properties of the raw mix.

[0055] As explained above, the conveyor 28 may be a metallic plate shaped as a plane ring. As shown in Figs 3 and 4, the conveyor 28 is mounted on a frame comprising two circular rails 50, an inner rail, and an outer rail. The rails 50 are supported by a plurality of static wheels 52. The static wheels 52 are fixed to the ground via pivotal links 54. In operation, the rotation of the wheels 52 induces a rotating movement of the rails 50 relative to the ground.

[0056] At least one of the static wheels 52 is equipped with a driving motor 56. The driving motor 56 controls the rotation of the wheel 52 and consequently the movement of the rails 50 and the conveyor 28. In other embodiments, several motors may be installed to reduce the load on one single motor. It is understood that the conveyor 28 may be rotated by any driving system that assures a continuous and constant rotation.

[0057] The conveyor 28 transports the bed of raw mix through each of the processing tools 26, 30, 32, inside of the mixing chamber 12. The progression of the bed of raw mix may be precisely controlled by a control system and may be slowed down or accelerated as needed.

[0058] The mixed bed of raw mix is finally brought to the discharge aperture 18. The discharge aperture is advantageously arranged in the outer wall 22 of the mixing chamber 12. The discharge aperture 18 comprises a discharge system 58.

[0059] The discharge system 58 may comprise two rotating worm-screws 60 driven by a motor (not shown). The worm-screws 60 have a length covering essentially the whole width of the mixing chamber 12, and are dimensioned to extract the entire flow of sinter mix through the discharge aperture.

[0060] As the global shape of the sinter mixer is a toroid of essentially rectangular cross-section, it is noteworthy that the center of the sinter mixer 12 is an empty space and that both inner and outer walls 20, 22 are easily accessible for maintenance operations. Accordingly, all of the elements inside the sinter mixer 10, i.e. all of the processing tools 26, 30, 32, are easily accessible during maintenance operations.

[0061 ] Discharge of the produced sinter mix from the sinter mixer 10 is a continuous process and the rotation speed of the worm-screws 60 is controlled in relation with the speed of the conveyor 28 and the quantity the raw mix fed into the mixing chamber 12.

[0062] In other embodiments (not shown) of the invention, the discharging system comprise only one worm-screw, or another system, like for example a diverting plate, in order to divert the flow of sinter mix through the discharge aperture.

[0063] As already explained, the sinter mixer 12 ideally functions in a continuous process. The ground surface necessary for installing the sinter mixer, and consequently the size of the conveyor in combination with the thickness of the bed of raw mix, are elements that influence the quantity of raw mix that is treated by the sinter mixer 12 in a predetermined period of time.

[0064] The sinter mixer 10 may be controlled by a controller (not shown) connected to a display providing a user interface to the technician.

[0065] The person skilled in the art will appreciate that the processing tools of the raw mix may be operated continuously with only one functioning phase. Traditional mixers require implementation of timely scheduled phases for the processing tools, like for example a high speed rotating phase to transport the sinter base forward followed by a slower phase for receiving sinter in a particular area. In embodiments of the invention, each processing tool only performs one action continuously with fixed parameters, reducing the programming and calibration time of the sinter mixer.

List of Reference Symbols

10 sinter mixer 34 support post

12 mixing chamber 36 dynamic mixing tool

14 support base 38 static mixing tool

16 charge aperture 40 shaft

18 discharge aperture 42 shovel

20 inner wall 44 obstacles

22 outer wall 50 rails

24 upper wall 52 static wheels

26 mixing tools 54 pivotal link

28 conveyor 56 driving motor

30 water feeding system 58 discharge system

32 additive feeder device 60 worm-screw element