System for the thermal treatment of solid bulk material

Abstract

A heat treatment system (100) for solid bulk material in pieces having a tower configuration and mainly comprising: - a plurality of belt conveyors (11-15) for the dry transportation of the bulk material, which belt conveyors have a stacked arrangement one above the other and thermally in series, each of said belt conveyors being configured to transport the bulk material transversely to the stacking direction; - one or more heater devices (31-35) arranged at each belt conveyor of said series to heat, e.g. by radiation, the bulk material transported thereupon; - a plurality of material discharging conduits (21-25), each interposed between an upstream belt conveyor and a downstream belt conveyor adjacent thereto in said series arrangement.

Description

[0001] SYSTEM FOR THE THERMAL TREATMENT OF SOLID BULK MATERIAL

[0002] DESCRIPTION

[0003] Field of the invention

[0004] The present invention relates mainly to a system for the thermal treatment, or processing, of solid bulk material.

[0005] The invention may provide, as well, a plant including said system and an associated treatment method.

[0006] Typical applications under the present invention include, but are not limited to, mineral ore embrittlement or beneficiation, limestone calcination and metal heating.

[0007] Background of the invention

[0008] Many industrial processes - such like mineral ore embrittlement or beneficiation, limestone calcination and metal heating - require bulk materials to be heated up typically to a temperature range as high as 500-900°C or more.

[0009] In order for the thermal treatment process to happen correctly, the heating system has to ensure that a specific, target process temperature is reached uniformly in the entire bulk material mass to be treated and that such temperature is maintained - within quite strict tolerances - for a given residence time.

[0010] In addition, the process should, of course, be accomplished in an efficient way, with minimum heat losses to the environment, which requires the related systems be duly thermally insulated. As a consequence, all equipments and components inside the thermally-insulated environment must be resistant to high temperature and highly reliable to operate on a continuous basis, all year long, without (major) failures.

[0011] Moreover, other critical operative conditions also apply. In particular, in most cases said processes deal with bulk materials having low thermal conductivity (e.g. 1-5 W / mK or lower), medium-large particle size distribution (e.g. up to 50-100 mm or larger) and very high mass flow rates (e.g. 100-1000 t / h or higher). The heating to a homogenous temperature of a large number of particles made of a low thermal conductivity material is not an easy task, since the particles whose surfaces are directly exposed to the heat sources will be at a higher temperature than those shielded from the heat source by other particles. Therefore, there is a risk of over-heating those particles exposed to the highest heating and under-heating those whose surfaces are not directly exposed to the heat sources.

[0012] Some known-art systems heat the material on a single conveyor from its top side. While this method may be effective for the upper layer of the conveyed thickness, which is directly exposed to the heating source, heating the core and the lower layer of the material being conveyed may be ineffective and / or take too much time, due to the low material thermal conductivity.

[0013] In general, homogeneity of temperature in the entire mass of material is not satisfactorily achieved by the known systems and configurations.

[0014] Known-art heating systems and methods should, thus, be optimized to better achieve the stated needs.

[0015] Summary of the invention

[0016] The technical problem underlying the present invention is therefore that of providing an improved heating system with respect to the state of the art.

[0017] The above problem is solved by a system according to claim 1 and by a method according to claim 26.

[0018] Preferred features of the invention are object of the dependent claims.

[0019] The system according to the invention comprises a plurality of high-temperature resistant conveyors, in particular belt conveyors, arranged according to a tower configuration, preferably inside a common thermally insulated casing. By way of example, each of said conveyors may have a general mechanical design as disclosed, e.g., in W02007 / 034289A1, WO2014 / 013472 A2 or WO2017 / 013517A1.

[0020] The aforementioned “tower configuration” means that the conveyors are positioned, or stacked, one above the other along a longitudinal direction, typically a vertical direction. The bulk material to be heat-treated is fed from an upper conveyor to a lower, adjacent one according to an arrangement providing a serial heating of the material. In other words, the arrangement is such that a first, upstream conveyor receives cold bulk material, accomplishes a first heating step by one or more heater devices, or heating flow(s), and discharges the material onto a second, downstream conveyor, that, in turn, accomplishes a second heating step by dedicated heater device(s), or flow(s), and discharges the material onto a third conveyor and so on, until the material lands the last downstream conveyor, wherein a target process temperature is reached homogeneously in the full material mass being conveyed. Discharge of the material from an upstream conveyor to the adjacent downstream one may occur (mainly) by gravity.

[0021] Dedicated discharge elements - such as chutes, slides, channels or download regions - are provided for feeding the material, or let it flow, from one conveyor to the adjacent one of the series.

[0022] As mentioned, at each, or at some, conveyor(s) one or more heater devices may be arranged. Such devices may be based upon different heat sources, e.g. being fuel burners and / or electrical heaters.

[0023] In case of burners, to avoid potential material sintering, underheating or overheating, and, also, to prevent undesired chemical reactions, preferably direct flame impingements on the bed of material being transported is avoided.

[0024] More preferably, heat sources are green hydrogen burners and / or electrical heaters powered by renewable electricity, so as to realize the heating process with net-zero CO2 emissions. In other words, the heating process is realized with zero emissions of CO2.

[0025] It should be noted that the term “green hydrogen” refers to hydrogen produced through a process of water electrolysis powered exclusively by renewable energy sources, such as solar, wind, or hydroelectric energy.

[0026] In some embodiments, each, or some, conveyor(s) in the series may include heater devices distributed, or extending, along its(their) length, i.e. in a direction generally transverse to the stacking one, to progressively heat up the material during transportation.

[0027] According to preferred embodiments, in each conveyor of said plurality a bulk material heating step may be obtained (also) by means of: radiation from said burners, electrical heaters or other and / or re-radiation from walls of the heating environment; and / or convection via flue gases and / or hot air.

[0028] By way of example, the thickness of the material conveyed upon each conveyor may be about 50-300 mm.

[0029] Preferably, the (target) temperature of the bulk material at the end of the treatment is comprised in a range of about 500-900 °C.

[0030] In some embodiments as disclosed further below, the system can handle flow rates included in a range of about 100-1000 t / h.

[0031] Preferably, the bulk material has low thermal conductivity of about 1-5 W / mK or lower. The system achieves a satisfactory temperature homogeneity also with medium-large particle size distribution, e.g. up to 50-100 mm or larger.

[0032] In some embodiments, each conveyor of the heating system may include a material levelling plate, to allow a homogenous material distribution over the entire conveyor length and width. Preferably, each conveyor in the heating system operates with a controlled and constant material thickness on its transport surface. Such thickness might be set large enough to ensure a continuous and safe material extraction, e.g. with a typically recommended value greater than three times the larger particle diameter.

[0033] The configuration herein disclosed, based upon a heating tower arrangement with several conveyors in sequence, helps overcoming the drawbacks of the known art systems, by obtaining a homogeneous heating in the material, as explained below.

[0034] ■ When the bulk material falls from one conveyor to the next, material layers are mixed, and new “cold” material is exposed to the heating source or flow, so as to be more effectively heated.

[0035] ■ In each conveyor discharge element falling material can be exposed to countercurrent heat exchange with ascending hot flue gases from burners or other heater devices of the downstream conveyors. Alternatively, or in addition, dedicated heaters can be installed in said discharge elements, e.g. of a radiant type. Advantageously, during the material discharge step each material particle is exposed to such heating, i.e. not only the top layer. Falling time and heating efficiency can be increased by means of properly designed discharge elements.

[0036] As mentioned, the disclosed arrangement allows heating the full mass of bulk material in a uniform way.

[0037] Other advantages, features and use modes of the present invention will result evident from the following detailed description of some embodiments, provided by way of example and not with limitative purpose.

[0038] Brief description of the drawings

[0039] Reference will be made to the figures of the annexed drawings, wherein: - Figure 1 shows a concept scheme of a tower heat exchanger according to a preferred embodiment of the heat treatment system for bulk material of the present invention;

[0040] - Figure 2 shows a more detailed scheme, in a longitudinal sectional view, of a tower heat exchanger according to an embodiment of the present invention, providing two lateral ascending air or gas streams for heating the descending bulk material;

[0041] - Figure 3 shows an enlarged and more detailed longitudinal sectional view of a preferred configuration of part of the exchanger of Figure 2;

[0042] - Figure 4 shows a frontal transverse sectional view of a belt conveyor and other elements of the exchanger of Figure 3 according to a variant embodiment;

[0043] - Figure 5 shows a frontal transverse sectional view of a belt conveyor and other elements of the exchanger of Figure 3 according to another variant embodiment;

[0044] - Figure 5bis shows a schematic top view of an embodiment of a belt conveyor and other elements of the exchanger of Figure 3;

[0045] - Figure 6 shows a preferred plant configuration including a thermal treatment system according to a preferred embodiment of the invention;

[0046] - Figure 7 shows an exemplary longitudinal sectional view of a contact heater that can be used in the plant configuration of Figure 6;

[0047] - Figure 7A shows an enlarged detail of Figure 7;

[0048] - Figure 8 shows a schematic longitudinal sectional view of an embodiment of a tower cooling arrangement that can be used in combination with, and downstream of, the tower heat exchanger according to any of the preceding figures;

[0049] - Figure 9 shows a schematic longitudinal sectional view of another embodiment of a tower cooling arrangement that can be used in combination with, and downstream of, the tower heat exchanger according to any of Figures 1 to 7A;

[0050] - Figure 10 shows a block diagram of an embodiment of a plant configuration including a tower cooling arrangement as that of Figure 8 or 9.

[0051] Detailed description of preferred embodiments of the invention

[0052] Several embodiments and variants of the invention will be described below, with reference to the figures already introduced. Generally speaking, analogous components are indicated in the various figures using corresponding reference numbers.

[0053] Further embodiments and variants other than those already described will be explained solely in conjunction with the relevant differences, if any, with respect to the preceding ones.

[0054] Moreover, the features of the various embodiments and variants described below are to be understood as combinable.

[0055] With reference initially to Figure 1 , a concept scheme of a thermal treatment system for solid bulk material in pieces according to a preferred embodiment of the invention is shown and globally denoted by 100. The system 100 comprises a main heat exchange body 101, which in the present example includes a common casing, it also denoted by 101 , preferably having thermally-insulated walls or wall parts. The system 100 has a tower configuration, extending according to a main longitudinal direction L, in particular a vertical direction. For this reason, the system 100 may also be denoted as tower exchanger or a tower heater. The longitudinal direction L may also be defined as the direction along which a plurality of belt conveyors are stacked, as explained below.

[0056] The casing 101 may include an external skirt made of steel and, preferably, is configured to prevent dust dispersion into the environment. Casing 101 may include walls, or wall parts, lined with thermal insulating and / or refractory panels or material, and, generally speaking, may be configured to withstand severe operating conditions in terms of high temperature, heat radiation, abrasion and possible material impact.

[0057] As mentioned above, a plurality of heating conveyors, in particular belt conveyors, are housed within the exchange body 101 according to a stacked arrangement, in particular one above the other and thermally in series with respect to a flow of the bulk material to be treated. Each belt conveyor of the series is configured to transport the bulk material transversely to the longitudinal direction L. In the concept scheme of Figure 1, there have been indicated five belt conveyors, denoted by respective reference numbers from 11 to 15. Each conveyor is in material flow connection with a downstream conveyor by a discharge, or connecting, element, such as a chute, slide, channel, conduit or region. In the example shown, discharging elements are denoted by respective reference numbers from 21 to 25. Still according to this example, the material discharge path through the discharge elements develops, at least prevalently, according to said longitudinal direction L. Of course, the discharge elements 21-25 are arranged alternatively on one side or the other of the stacked conveyors 11-15, following the opposite verse / direction of transportation of the material from one conveyor to the downstream, adjacent one. Figure 1 also shows an inlet 110, or material feeding input, for the cold material into the treatment system 100 and an outlet 120, or material release output.

[0058] The resulting material conveying path on the conveyors and through the discharge elements is indicated by arrows F, in particular a transverse and a longitudinal direction are alternated to define the overall transport path of the bulk material.

[0059] Each conveyor of the series, or a subgroup thereof, is associated with one or more heater devices arranged so as to heat by radiation the bulk material transported thereupon. In the example shown, one heater device is provided for each conveyor, such heater devices being denoted by respective reference numbers 31 to 35.

[0060] Additional heating devices can be provided at the discharge elements and configured to heat the bulk material during discharge from one belt conveyor to the adjacent, downstream one. In the present example, such additional heating devices are represented as a single body or device with the main heating device associated with the conveyors.

[0061] Therefore, the overall arrangement is such that the bulk material is fed to an uppermost belt conveyor, is transported thereon while heated by the heater device and it is discharged through the respective discharge element onto a downstream conveyor, wherein it is further heated and so on for all the belt conveyors arranged in series. During this discharge path, the bulk material is further heated, during the falling path from one belt conveyor to the adjacent one, by the additional heating devices or by the air or gases ascending from the heater devices associated with the downstream conveyor(s).

[0062] In this way, the material is discharged at a target heating temperature from a lowermost belt conveyor.

[0063] In the above arrangement, material particles, when falling through the discharge element, are mingled and temperature gradients deriving from the upstream conveyor heating are smoothed. The downstream conveyor, in turn, receives the bulk material, which has been mixed during the falling from the upstream chute, and allows the material to be newly exposed to a further heating step during transportation, by means of the respective heater device, for a given additional residence time. The material is then discharged onto the following conveyor, exposed to another heater device I additional heating device, and so on, until the final heating step is accomplished by the lowest conveyor in the tower.

[0064] In the above arrangement, preferably material is discharge from an upstream conveyor to a downstream one by gravity (only).

[0065] As mentioned, in the discharge element from an upstream to the downstream conveyor, material is spread and, preferably, also exposed to an ascending flow of (hot) combustion gas or air generated at the downstream conveyors.

[0066] Therefore, in the present embodiment in each conveyor and relevant discharge element, the bulk material heating step is obtained by means of (direct) exposure to the relevant heater devices along the belt conveyors, optional additional heating devices arranged at the discharge elements, possible re-irradiation from the walls of the casing 101 and / or convection via flue gases and / or hot air.

[0067] According to a variant embodiment, each conveyor of the system 100 is enclosed in its own insulated (sub)casing, or box, and the heater devices are kept partially outside of the (sub)casing, but within a common casing 101.

[0068] With reference to Figure 2, according to a specific embodiment two separate flue gas, or air, channels are provided at the sides of the tower arrangement and designated by 41 and 42. Each channel 41 , 42 guides an ascending hot gas / air flow that thermally interacts with, and heats, the descending bulk material while it is being discharged from an upstream to a downstream conveyor. Each lateral flue gas channel 41 , 42 intercepts and is in material and gas / air flow communication with the discharge elements 21-25 on the respective side, possibly having segments in common therewith.

[0069] Moreover, advantageously, in the example of Figure 2 the belt conveyors may be inclined with respect to a horizontal plane so as to maximise the length of the heating path and minimize the encumbrance of casing 101.

[0070] In the exemplary configuration of Figure 3, each conveyor is equipped with bottom hoppers, one denoted by way of example with 50, to collect possible fine particles or material falling from the conveyor and release them to the downstream conveyor through a fine particle I material conduit 51.

[0071] Alternatively, the system may be provided with openings for the free falling of fines.

[0072] The fine particle I material conduits 51 can be equipped with airlock valves 52 to prevent undesired flows of gas from the downstream conveyor to the upstream one.

[0073] For the same purpose, also the discharge elements (denoted by 21-25 in the embodiment of Figure 1) of each conveyor can be equipped with respective valves, e.g. airlock valves, which, by way of example, can be made by a buffer bin 53 wherein conveyed material accumulates to keep it always full. Advantageously, high / low level probes (not shown in the drawings) can be used within such bins to control the level of the material contained therein. The heating tower 100 may include an airlock system 531 - such as a buffer bin always kept full of material, as the ones just described - also at the inlet of the first conveyor, i.e. the most upstream one, to prevent hot gas flowing in the upstream material feeding system 110, in order to avoid heat dispersion.

[0074] Figure 4 shows, as an example, a section of a conveyor 12 in the heating tower 100, with a possible arrangement of a set of burners, or electrical heaters, 32 placed above the material M being conveyed and facing the transport surface 30 of the conveyor receiving said material. In this case, the heater device 32 is placed onto an upper intermediate wall 103 of casing 101 , facing the transported material.

[0075] Figure 5 shows, as another example, a section of a conveyor 12 in the heating tower 100, with a possible arrangement of a set of burners, or electrical heaters, here denoted by 320, placed at the sides of the material M being conveyed.

[0076] Figure 5bis shows a similar configuration, wherein the heaters have a staggered layout along the conveying direction.

[0077] Figure 6 refers to a possible plant configuration including a thermal treatment system 100 according to an embodiment of the invention.

[0078] The plant includes, in sequence along a material transport path:

[0079] ■ a screen 501 ;

[0080] ■ a multiple-step pre-heating arrangement, preferably made of a plurality of belt conveyors arranged thermally in series, e.g. conveyors 502, 503, 523, the first two of which also implementing a material elevation, or lifting, system; the arrangement transfers the bulk material at the inlet 110 of the tower exchanger 100;

[0081] ■ a cyclone separator 506 arranged at the outlet of conduits 41 and 42, to extract particles from the flow of gas / air;

[0082] ■ a cooling and heat recovery system 504, arranged downstream system 100 and configured so as to transfer sensible heat from the hot solid material leaving the heating tower 100 to a (cold) air flow fed by a fan device 512, by means of a solid-to-air heat exchanger (in the specific case, a bulk material annular cooler is shown); the hot air flow thus generated is released to one, or more, conveyors of the pre-heating arrangement - in particular the conveyors 502 and 503 in the example shown - so as to preheat the bulk material before it enters the heating tower 100; ■ a de-dusting and outlet arrangement 510, e.g. made of bag filters 507, induced fan 508 and chimney 509, configurated to outlet towards the outside the hot air / gases coming from the pre-heating arrangement;

[0083] ■ an additional de-dusting and outlet arrangement 510’ having the same configuration as the above one - preferably with bag filters 507’, cyclone device 508’ and chimney 509’ - and associated with the air / gas leaving the conduit outlets 41, 42;

[0084] ■ an additional heat exchanger 511 arranged upstream the additional de-dusting and outlet system 510 and configured to transfer heat from the air / gas outletting the chutes 41, 42 to an air flow adducted by a fan device 513 and to convey heat to the pre-heating arrangement 502, preferably at a last (hottest) stage thereof, in particular conveyor 523 in the example shown;

[0085] ■ a stock piling arrangement 505, to which dust, fine material and bulk material extracted from the tower exchanger 100, the de-dusting and outlet systems and the heat recovery system are conveyed.

[0086] As an example, Figure 6 shows the directionality and path of the hot gas, in case gas burners are used as heaters: the hot combustion gases leaving the heating tower 100 are first cycloned in device 506, to provide a first cleaning from the coarse fraction of residual air-borne fines and then released to the gas-air heat exchanger 511, to recover heat content from combustion gas and produce an additional flow of hot air, which is delivered to one or more of the preheating conveyors. In this way, hot gases are cycloned from coarse fines, cooled down to produce hot air and, finally, filtered before being released to the atmosphere, e.g. by means of a dedicated fan.

[0087] The additional pre-heating and heat recovery arrangement and system cited above increase efficiency.

[0088] Further features which may be added in variant embodiments, either jointly or independently, are: o mechanical mixing means, such as ploughs, inserted in the material being conveyed, to turn material over, so to expose lower material grains to the heating source located on top; o belt conveyors provided with steel pans with welded steel fins - or other heat transfer improving means - immersed in the bulk material, to increase heat transfer from the heating source across the low conductivity bulk material thickness; o belt conveyor speed and / or material thickness adjusting system, to provide flexibility in the heating operation; o belt conveyors equipped with weighing system, to allow for online mass flow rate detection and control; o bulk material temperature detection system, with contact (e.g. thermocouples) and / or no contact (e.g. IR camera, pyrometers etc) sensors located along all conveyors in the heating tower and / or the pre-heating arrangement; o heating power control system, capable of adjusting the power delivered in each conveyor, to reach the desired profile of bulk material temperature along the transportation.

[0089] Figures 7 and 7A show schematically a contact heater 600 that can be used in the discharge elements 21-25. Arrows G indicate a flow of hot gas and arrows B indicate the flow of bulk material.

[0090] Cold bulk materials enter the heater 600 from an aperture located in its upper part, falling by gravity. Hot gas enters the device from an aperture located in its bottom part and flows upwards, by naturally or forced draft (e.g. by a fan, not shown in the figures).

[0091] Bulk materials, falling inside the heater 600, encounter a series of sloped plates - by way of example, two plates 601, 602 in the figure - allowing the bulk materials to slide on them. At the end of each sloped plane the hot gas crosses the falling bulk materials, and heat is exchanged from hot gas to said material.

[0092] Each sloped plane may preferably be constituted by a series of smaller sloped plates, among which a gap is present, to allow flows of gas. Gas flowing in the gaps crosses the bulk materials falling down, for an enhanced heat exchange, thanks to repeated jumping of materials, from one plate to the next, and repeated cross flow with hot gas.

[0093] The number of plates, the gap dimensions, the arrangement and number of sloped planes and the overall dimensions of the device can be selected to optimize gas velocity and heating performances, depending upon the specific application and needs.

[0094] Heated bulk materials leave the contact heater 600 from the bottom, whereas cooled gas exit from the top.

[0095] Figures 8 and 9 show each a respective embodiment of a cooling system that can be used in the plant of Figure 6 in substitution of, or in addition to, the cooling and heat recovery system 504. Each of said cooling systems of Figures 8 and 9 may be useful when the material treated in the heating tower 100 needs be handled and worked under a controlled, e.g. a reduced, atmosphere and / or when the material needs be brought below an oxidation temperature.

[0096] The system of Figure 8, globally denoted by 800, includes a plurality of conveyors in a stacked, or tower, arrangement. In particular, in the example four conveyors are shown, denoted by reference numbers 811 to 814, respectively. In full analogy with the operating principles and the structure of system 100 and its conveyors 11-15, the hot bulk material falls from one conveyor to the downstream, lower one. However, in discharging elements, here denoted by 821-824, the material is cooled by an ascending, i.e. countercurrent, flow of gas or air, in particular through contact coolers 861-864 similar to the contact heaters 600 of Figures 7 and 7A. The gas may be air, nitrogen, steam, cooled combustion gas from heaters of the same plant or a mixture thereof.

[0097] The embodiment shown in Figure 8 also provides for the hopper arrangement already described in conjunction with Figure 3.

[0098] In Figure 9, a system 900 with a similar arrangement of stacked conveyors, here denoted by 911-914, is shown. Cooling is obtained by an ascending gas flow passing through slots of the transporting surface, or plates, of each conveyor and across the bulk material layer being conveyed. In this way, a cross-flow cooling mechanism is realized between the cooling gas and the bulk material. The hot cooling gas leaving each conveyor 911-914 can be drafted by suitable hoods - one of which denoted, by way of example, by 940 - and at least partially conveyed through two main lateral channels 945, 946.

[0099] Preferably, at the discharge end of each conveyor, a buffer bin (one of which denoted, by way of example, with 950) similar to that described in conjunction with Figure 3 is provided, acting as an airlock valve, in order to prevent undesired gas flow from a conveyor to the upstream one, which might otherwise jeopardize the above described cross-flow cooling mechanism.

[0100] The heater devices associated with the conveyors in the tower exchanger may be operated under either lean-fuel conditions, to obtain an oxidising gas, or under fuel-rich conditions, to achieve a reducing environment within the system, or to achieve an inert gas. In particular, the reducing atmosphere is generated from the products of combustion derived from a flame operated with a slight excess of fuel (e.g., hydrogen or natural gas). Alternatively, the neutral atmosphere can be obtained with a nominally stoichiometric ratio of fuel to air. Typically either mildly reducing or neutral atmospheres are desirable to prevent significant re-oxidation of an ore during cooling. By an appropriate design of the tower, it is possible to achieve neutral roasting environment in some conveyors together with a reducing one in others. Alternatively, all the burners can be operated under lean-fuel conditions but with the conveyors equipped with extra nozzles to allow the use of reducing agents for material process operations requiring reduction (e.g., iron ores).

[0101] Figure 10 shows schematically a configuration of three tower arrangements thermally in series, together with the ducting of gases between them, to achieve heat recovery (thereby increasing efficiency), by the use of convective heating and cooling on either side of a radiatively heated tower. In particular, the left and central heating tower arrangements - herein denoted by 100’ and 100”, respectively - can be, e.g., implemented as those shown in Fig. s 1 to 5bis. The right tower arrangement 900’, i.e. the cooling tower arrangement, can be, e.g., one of those described with reference to Figure 8 or 9.

[0102] The arrows M1 show the flow of solid material, while the arrows F1 and F2 show the flow of heated and cooled combustion products, respectively, e.g. combustion gases. According to the arrangement shown, the bulk material undergoes progressive heating, in particular by convection and radiation, in the first two tower systems 100’ and 100” and it is subsequently cooled by convection in the third tower system 900’. The combustion products basically follow an opposite path.

[0103] The use of cooled combustion products in the cooling section 900’ allows the hot material to be cooled to below the oxidation temperature without any significant exposure to oxygen. This prevents the re-oxidation of reduced material, as would otherwise occur were air to be used as the cooling medium.

[0104] The system also provides means for preventing ingress of air through the walls of the towers, ducts and fans, in order to avoid oxidation.

[0105] Inducted draft fan(s) may be installed into one or more the ducts transporting the cooled combustion products.

[0106] As shown, liquid water spraying means 930 could optionally be provided to further lower the temperature of the cooled gases used after reductive roasting.

[0107] Where a reducing environment is not desired in the radiant section, ambient air can be used in the convective cooling section 900’, instead of cooled combustion products. This may be desirable where more rapid cooling is desired and would also reduce costs.

[0108] Regarding the exhaust gas, the following is noted. (i) Where hydrogen and oxygen is used as the fuel, the product gas is steam, making it desirable to co-locate an application needing steam;

[0109] (ii) Where hydrogen and air are used as the fuel, the product steam could likely be vented with little treatment; (iii) Where hydrocarbons and air were to be used for the radiant heating, an afterburner may be provided to meet emission standards for CO.

[0110] The cooling systems of Figures 8 and 9, also as including features disclosed in conjunction with system 100, may define a separate aspect of the present invention for which protection may be sought independently from their combination with a heating or treatment system.

[0111] The present invention has been described so far with reference to preferred embodiments. It is intended that there may be other embodiments which refer to the same inventive concept as defined by the scope of the following claims.

Claims

CLAIMS1. A thermal treatment system (100) for solid bulk material in pieces, comprising:- a plurality of belt conveyors (11-15) for the dry transportation of the bulk material, which belt conveyors are stacked one above the other according to a tower configuration along a longitudinal direction (L), in particular a vertical direction, and are arranged thermally in series, each of said belt conveyors being configured to transport the bulk material transversely to said longitudinal direction;- one or more heater devices (31-35), arranged at one or more of said belt conveyors and configured to heat the bulk material transported thereupon;- a plurality of longitudinal bulk material discharging conduits or regions (21-25), each interposed between an upstream belt conveyor and a downstream belt conveyor adjacent thereto in said arrangement in series;- additional heating means (41, 42) configured to heat the bulk material during discharge from one belt conveyor to an adjacent, downstream one, through said discharge conduits or regions, the overall arrangement being such that the bulk material is fed to an uppermost belt conveyor, is transported thereon while heated by said one or more heater devices and it is discharged through the respective discharge conduit or region onto a downstream conveyor wherein it is further heated and so on for all the belt conveyors arranged in series, the bulk material being further heated, during the discharge from one belt conveyor to the adjacent one, by said additional heating means, the bulk material being discharged at a target heating temperature from a lowermost belt conveyor.

2. The thermal treatment system (100) according to claim 1, comprising a main body or casing (101) extending according to said main longitudinal direction (L) and housing said plurality of belt conveyors (11-15).

3. The thermal treatment system (100) according to any of the preceding claims, wherein said additional heating means comprises one or more lateral channels (41, 42), preferably defined within said main body or casing (101), extending longitudinally on a respective side of said belt conveyors (11-15) and configured to feed an ascending heating gas or air flow.

4. The thermal treatment system (100) according to any of the preceding claims, wherein said additional heating means comprises at least one contact heater (600) between the bulk material and an ascending heating gas or air flow, which contact heater (600) includes one or more sloped plates (601, 602) for the material to slide onto.

5. The thermal treatment system (100) according to any of the preceding claims, wherein said additional heating means comprises one or more additional electric heaters arranged at said discharge conduits or regions (21-25).

6. The thermal treatment system (100) according to any of the preceding claims, wherein said heater devices (31-35) are arranged above a transport surface (30) of said belt conveyors (11-15) and facing such surface.

7. The thermal treatment system (100) according to any of the preceding claims, wherein said heater devices (31-35) are arranged laterally with respect to a transport surface (30) of said belt conveyors (11-15).

8. The thermal treatment system (100) according to any of the preceding claims, wherein said heater devices (31-35) comprise fuel burners and / or electrical heaters.

9. The thermal treatment system (100) according to any of the preceding claims, wherein said heater devices (31-35) comprise green hydrogen burners and / or electrical heaters powered by renewable electricity, so as to realize the heating process with zero emissions of CO2.

10. The thermal treatment system (100) according to any of the preceding claims, wherein said heater devices (31-35) provide heat to the bulk material by radiation.

11. The thermal treatment system (100) according to any of the preceding claims, wherein one or more of said belt conveyors (11-15) extend according to a main direction which is inclined with respect to a horizontal plane.

12. The thermal treatment system (100) according to any of the preceding claims, wherein the configuration is such that the bulk material is discharged by gravity only from one conveyor to the downstream, adjacent one.

13. The thermal treatment system (100) according to any of the preceding claims, comprising, at one or more of said belt conveyors (11-15) or one or more of said conduits or regions (21-25), an end material buffer bin (53) configured to prevent hot gas flowing to the upstream conveyor.

14. The thermal treatment system (100) according to any of the preceding claims, comprising, at one or more of said belt conveyors (11-15), a material levelling plate configured to determine a homogenous bulk material distribution over the conveyor length and / or width.

15. The thermal treatment system (100) according to any of the preceding claims, comprising, at one or more of said belt conveyors (11-15), bottom hoppers (50), configured to collect fine particles or material falling from the conveyor and release them to the downstream conveyor through fine particle I material conduits (51).

16. The thermal treatment system (100) according to any of the preceding claims,wherein the bulk material has low thermal conductivity of 1-5 W / mK or lower, and a medium-large particle size distribution, up to 50-100 mm or larger, and wherein the target heating temperature of the bulk material at the end of the treatment is comprised in a preferable range of 500-900 °C.

17. A plant for the thermal treatment of bulk material in pieces, comprising a thermal treatment system (100) according to any of the preceding claims and one or more upstream belt conveyors (502, 503, 523) configured for the pre-heating of the material and arranged upstream said thermal treatment system.

18. The plant according to the preceding claim, comprising a recovery heat exchanger (511) between a hot gas or air flow outing from said thermal treatment system (100) and an air flow.

19. The plant according to claim 17 or 18, comprising a cooling system (504; 800; 900) of the bulk material, arranged downstream said heat treatment system (100) and configured to bring the material below a threshold temperature, in particular an oxidation temperature.

20. The plant according to the preceding claim, wherein said cooling system (800; 900) comprises a plurality of belt conveyors (811-814) for the dry transportation of the bulk material, which belt conveyors are stacked one above the other according to a tower configuration along a longitudinal direction (L), in particular a vertical direction, and are arranged thermally in series, each of said belt conveyors being configured to transport the bulk material transversely to said longitudinal direction.

21. The plant according to the preceding claim, wherein said cooling system (800; 900) comprises cooling means configured to adduct a flow of cooling gas in thermal contact with the bulk material.

22. The plant according to any of claims 19 to 21, wherein said cooling system (800) comprises one or more contact coolers (821-824) between the bulk material and an ascending heating gas or air flow, which contact coolers include one or more sloped plates (601, 602) for the material to slide onto.

23. The plant according to any of claims 19 to 22, wherein said cooling system (900) comprises slotted plates defining a transport surface (30) of said belt conveyors (911-914), through which a cooling gas can pass through.

24. The plant according to any of claims 19 to 23, comprising two thermal treatment systems (100’, 100”), each according to any of the claims 1 to 16, and said cooling system (800; 900; 900’) arranged thermally in series.

25. The plant according to the preceding claim, wherein said two heating systems are a convective heating system (100’) and a radiant heating system (100”) and saidcooling system (900’) is a convective cooling system.

26. A method for the thermal treatment of a bulk material, which uses a thermal treatment system (100) or a plant according to any of the preceding claims, which method is a mineral ore embrittlement or beneficiation method or a limestone calcination method.

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