WO2025093899A1

WO2025093899A1 - Method for controlling a sintering machine, associated controller and sintering machine

Info

Publication number: WO2025093899A1
Application number: PCT/IB2023/060897
Authority: WO
Inventors: Flavio Tulio BUSATTO; Gabriel Nazareth Guedes ALCOFORADO
Original assignee: ArcelorMittal SA
Current assignee: ArcelorMittal SA
Priority date: 2023-10-30
Filing date: 2023-10-30
Publication date: 2025-05-08
Anticipated expiration: 2026-04-30

Abstract

The present invention relates in particular to a method for controlling a sintering machine (1) that comprise a belt conveyor, the method comprising: - acquiring wind boxes temperatures (WBT_i) in several successive wind boxes (WB_i) arranged along a belt (2) of the conveyor, below the belt, - determining a temperature rise slope from at least some of the wind boxes temperatures, the temperature rise slope representing the slope of the temperature as a function of a position along the belt of the conveyor, upstream of a maximum temperature position, - controlling a speed of the belt of the conveyor depending on the temperature rise slope.

Description

Method for controlling a sintering machine, associated controller and sintering machine

[001] The technical field is that of ore sintering, in particular iron ore sintering. It concerns more particularly control and regulation techniques for sintering machines comprising a belt conveyor and a forced draft system.

Technical background

[002] A sintering machine for sintering a material such as iron ore usually comprises a belt conveyor with an endless belt for conveying the material. The belt has a grate-like structure to allow air to flow through the material disposed upon the belt. Adjoining wind boxes are arranged along the belt of the conveyor, below the belt, for sucking air through the material. When the sintering machine is in operation, the layer of material is ignited at one end of the conveyor, and the height over which the material is sintered gradually increases during the displacement of the material by the belt, until the material is sintered over its whole height. The position where the material becomes sintered over its whole height is called the bum through point (also denoted BTP, hereinafter).

[003] Some of the wind boxes may be equipped with temperature sensors, to measure the temperature of the gas (air mainly) sucked through the material. Along the belt, the position where this gas temperature reaches its maximum is the BTP. The position of the BTP can thus be derived from measurements of the wind boxes temperatures. The speed of the belt may then be controlled so that the position of the BTP equals a given set position. In practice, this set position is usually chosen close to a discharge end of the conveyor. Indeed, the conveyor length is typically chosen so that it is just higher than the length required for a complete sintering (to avoid building unnecessarily long conveyors).

[004] Another way for controlling and regulating the operation of a sintering machine is to adjust the speed of the belt so that a temperature of the exhaust, waste gas equals a given set temperature.

[005] Anyhow, the control and regulation of the operation of a sintering machine is intrinsically challenging. Indeed, when the material is moved by the belt, firstly, over a rather long distance (typically about half the conveyor total length, of more) the temperature of the gas sucked though varies little, and there is no direct way to monitor how the process is evolving. And then, this temperature rises quite quickly, at the end of the conveyor. So, when regulating such a process, there are both important time lags, and a kind of suddenness of the process. Besides, the position of the BTP has preferably to be regulated so that it is close to its maximum acceptable limit (that is, close to the discharge end of the conveyor), which is not convenient in terms of automation. [006] In this context, document EP2737094 describes a method enabling to control and regulate the position of the BTP, event if this position is out of the zone of the conveyor equipped with temperature sensors. Still, the efficiency of this control method remains limited by the time lag effects above mentioned, and because other parameters, useful for monitoring the process (such as the temperature of the burn through point), are not taken into account when adjusting the speed of the conveyor.

Summary

[007] In this context, a method for controlling a sintering machine according to claim 1 is provided.

[008] This control method is beneficial, compared inter alia to prior art methods in which the speed of the belt is controlled based mainly on the position of the burn through point BTP_pos.

[009] Indeed, the temperature-rise slope TRS, upstream of the BTP: provides more information than just the position of the BTP (as this slope reflects the position and temperature of the BTP, and also the position and temperature at which temperature starts to rise, upstream of the BTP), and provides it in a concise integrated form (which is well suited to as a controller input),

- enables a better anticipation than using the position or temperature of the BTP, as it depends on temperatures upstream of the BTP (in temporal terms, it depends on what is happening before the BTP) and is representative of the dynamic of the process of material sintering in the key zone which is just upstream the BTP.

[0010] The method according to the instant technology may comprise one or several additional features, defined in claims 2 to 16, considered alone or in combination.

[0011] The instant technology also concerns an electronic control unit, as defined by claim 17 and a sintering machine as defined by claim 18.

[0012] The instant technology also concerns a computer program comprising instructions whose execution on a computer (possibly connected to adequate sensors and actuator) makes the computer to execute the steps of the method presented above. The instant technology also concerns a non-transitory computer-readable medium (such as a hard drive, an USB stick or other flash memory) including such a computer program.

Detailed description

[0013] The instant technology will now be described in more detail and illustrated by examples without introducing limitations, with reference to the appended figures.

[0014] Figure 1 is a schematic side-view representation of a sintering machine.

[0015] Figure 2 is a schematic block-like diagram representation of different control loops implemented by an electronic control unit of the sintering machine of figure 1 , in order to control that machine. [0016] Figure 3 schematically represents values of the standard deviation of a temperature measured in a specific wind box of the sintering machine of figure 1 , together with values of an associated correction coefficient employed to correct this temperature before using it to regulate the sintering machine.

[0017] Figure 4 schematically represents values of wind boxes temperatures, as a function of a position x along a belt of a conveyor of the sintering machine, and a curve fit of these temperatures which enables determining a position of the burn through point.

[0018] Figure 5 schematically represents values of wind boxes temperatures, as a function of the position x, together with a corresponding Temperature Rise Slope, and Temperature Rise Slope setpoint.

[0019] Figure 6 schematically represents a histogram of sintering temperatures, when the sintering machine is controlled by its electronic control unit and also when it is controlled manually, directly by an operator.

Sintering Machine

[0020] Figure 1 schematically represents a sintering machine 1 for sintering a material M such as iron ore. The sintering machine 1 comprises a belt conveyor with an endless belt 2 for conveying the material M from a loading end 21 of the conveyor, where the material is loaded on the belt by the hopper 5 (possibly using a charging rotating cylinder), to a discharge end 22. The belt 2 has a grate-like structure to allow air to flow through the material M disposed upon the belt. The belt 2 may form a single continuous strip, or it may be formed by several successive pallets joined to each other (in an articulated manner). The belt conveyor extends from its loading end 21 to its discharge end 22, along a longitudinal axis x (directed from the loading end to the discharge end). The belt 2 movement is guided, at each end 21 , 22, by one or more rotating shafts. The belt 2 is moved continuously in the direction of axis x by an actuator 3, such as an electric motor mechanically coupled to one of the rotating shafts.

[0021] Wind boxes WBi, WB2, ... , WB23 are arranged along the belt 2 of the conveyor, below the belt, for sucking a gas (in practice a mix of air and combustion gases), through the material M disposed upon. The wind boxes are arranged one after the other, along axis x. Their respective openings are adjacent to each other (and possibly even contiguous). The first wind box WB1 is located near the loading end 21 of the conveyor while the last wind box WB23 is located near its discharge end 22. One of the first wind boxes, here the first one WB1 , is located right below an ignition hood 4 for igniting the material M. In the example of figure 1 , the sintering machine 1 has 23 wind boxes WB1 - WB23 but the number of wind boxes can be different. It is typically from 10 to 40. Here, the wind boxes are each in fluidic connection with a common suction duct 6 which is equipped, at an output end, with an extraction blower 8. When the blower 8 is operated, air is sucked through the material M by the wind boxes, creating a forced draft favorizing the gradual ignition and sintering of the material M. [0022] The sintering machine 1 is equipped with several sensors.

[0023] First, at least some of the wind boxes are each equipped with one or more temperature sensors. In this example, all the wind boxes are each equipped with one temperature sensor (not represented in the figures), except for the last three ones which are each equipped with two temperature sensors, one on a left side of the belt and the other on its right side (right and left with respect to a central, longitudinal axis of the belt, extending from the loading end to the discharge end). Each of these sensors is arranged so as to measure the temperature of the gas sucked by the wind box, through the layer of material M. The temperatures measured by these sensors are called wind boxes temperatures in the following. For the wind box WBi (the index i, from 1 to N, being the number of the wind box considered, N being the total number of wind boxes, here equal to 23), the corresponding wind box temperature is denoted WBT. For the wind boxes equipped with two temperatures sensors (right and left ones), WBT is an average of the left and right temperature measurements (while the left and right measurements are WBTi and WBT_ir respectively). In alternative embodiments, all the wind boxes could each comprise just one temperature sensor (or, on the contrary, they may all be equipped with two, left and right, temperature sensors).

[0024] In this embodiment, another temperature sensor, 7, is located in the suction duct 6 or at an output of the blower 8. It measures a waste gas temperature WGT representative of the temperature of the global mix of gas exhausted by the different wind boxes.

[0025] The sintering machine 1 may also comprise a pressure sensor (not shown on the figures) arranged in the wind box that is located underneath the ignition hood 4, and possibly other sensors, like a height measurement device for measuring the height of the layer of material disposed on the belt, or a flow meter or other type of sensor for estimating a total flow rate through the ignition hood 4.

[0026] When the sintering machine is in operation, the height over which the material is sintered gradually increases during the displacement of the material M along axis x, until the material is sintered over its whole height. The position where the material becomes sintered over its whole height is called the burn through point BTP. In figure 1 , the material that is sintered (i.e.: the sinter) is represented in dark grey, while the material not yet sintered is represented in lighter grey.

[0027] The temperature of the gas, sucked through the layer of material M, varies along axis x as follow: firstly, over a rather long distance (about half of the conveyor total length, of more) the temperature varies little. Then it begins to increase substantially (at a point called temperature rise point TRP), and reaches a maximum at the burn through point BTP. Then, downstream of the burn through point (that is, after the BTP, in the direction of the discharge end), the temperature decreases. This variation of the temperature of the gas, sucked through the layer of material M, is reflected by the wind boxes temperatures, which somehow sample this temperature, at various positions along axis x.

[0028] The sintering machine 1 also comprises an electronic control unit 10, operatively connected to the temperature sensors of the wind boxes, and possibly also to the other sensors above mentioned. The control unit 10 is also connected to the actuator 3 that drives the belt, and is configured for controlling a speed v of the belt via the actuator 3. The control unit 10 is an electronic device that comprises logic circuitry and possibly also analogic circuitry, arranged to implement a control loop(s) regulation of the speed v of the belt. It may take the form of a programmable corrector, or of an electronic card including a Field Programmable Gate Array (FPGA) chip, for instance. The control unit 10 may have the structure of a computer. The control unit 10 is configured to implement the control method described below.

Control method

[0029] Remarkably, this method comprises:

- acquiring at least some of the wind boxes temperatures WBT (preferably the ones that are in the vicinity of the BTP, in an end part of the conveyor), here all of them,

- determining a temperature rise slope TRS from at least some of the wind boxes temperatures WBT, the temperature rise slope TRS representing the slope of the temperature (of the gas sucked through the material) as a function of the position x along the belt 2 of the conveyor, upstream of a “maximum temperature position” BTP_pos, which is the position of the burn through point BTP,

- controlling the speed v of the belt 2 of the conveyor depending on the temperature rise slope TRS and on a temperature rise slope setpoint TRS^sp , which is automatically calculated by the control unit 10, based on measurements and setpoints for the BTP (figures 2 and 4).

[0030] As explained in the “summary” section, this control method is beneficial, compared inter alia to prior art methods in which the speed of the belt is mainly controlled based on the position of the burn through point BTP_pos.

[0031] More specifically, in this exemplary embodiment, the speed of the belt v is controlled using three different control loops 11 , 12 and 13 (figure 2) and a selection logic 14 for switching from one theses loops to another (depending on specific conditions). The control loop 1 1 is called stabilization loop and it is configured for controlling v based mainly on the temperature rise slope TRS. The control loop 12 is called correction loop and it is configured for controlling v based mainly on the temperature(s) of the gas sucked through the layer of material, this temperature being measured at various positions (eg: in different wind boxes and in the suction duct 6). The control loop 13 is called protection loop and it is configured for controlling v based mainly on the position of the burn through point BTP_pos. Here, each control loop 11 , 12, or 13, is a PID control loop (that is Proportional, Integral, Derivative control loop). Still, it may be noted that in such a PID control loop, the Derivative (or even Integral) corrector may be omitted, and that the P, I and D correctors may either be arranged in series or in parallel. Besides, in alternative embodiments, another type of controller than PID ones could be employed, for these control loops.

[0032] Employing more than one control loop for controlling the speed of the belt is beneficial, for this kind of application. Indeed, there are mostly two type of actuators that could be controlled to regulate the sintering process, namely the actuator 3 coupled to the conveyor belt and the blower 8 that controls the forced air draft, but the last one does not enable an accurate control, usually. So, there is mainly one actuator that can be used to regulate the sintering process, while it is desirable to regulate several quantities (such as the BTP position, the BTP temperature, and other process parameters such as the waste gas temperature), with possibly different constraints and criteria. Employing different control loops and an associated selection logic then enables to regulate efficiently these different quantities, and to cope with different constraints while just one actuator (actuator 3) is controlled.

[0033] Another, complementary technique, also implemented here for regulating more than one quantity with just one actuator, is to combine these quantities in a composite error signal which is then employed in the control loop considered. In this regard, it is noted that combining several quantities in a single error signal is not straightforward, as these quantities have to be selected so as to vary in the same manner, when the actuator is controlled.

[0034] The selection logic 14 and the control loops 1 1 , 12, 13 are now described in more details, one after the other.

Selection logic

[0035] The selection logic 14 is configured to select the stabilization loop 11 by default. In other words, except if specific conditions are fulfilled (like having the BTP position BTP_pos above a maximum acceptable limit), it is the stabilization loop 11 that is selected. In practice, the stabilization loop 11 is the one selected most of the time.

[0036] Regarding the protection loop 13, it is selected (rather than the two other control loops 11 and 12) when a distance, between the BTP position BTP_pos and the discharge end 22 of the conveyor, becomes smaller than a preset distance limit (in other words, when the BTP_pos becomes higher than the corresponding maximum acceptable limit).

[0037] Regarding the correction loop 12, it is selected (rather than the two other control loops 11 and 13) when a waste gas temperature error signal, WGT_cf ^er (described in more details below) gets out of an admissible range. One purpose of the correction loop 12 is to adjust the speed v more aggressively than the stabilization loop 11 , to maintain or correct the temperatures of the sucked gas when it deviates from corresponding setpoints. The corrector of the correction loop 12 is thus configured to have a higher correction gain and/or shorter response time(s) than the corrector of the stabilization loop 11 .

[0038] The gain and/or response time(s) of the stabilization loop 11 are adjusted to be conservative, to favorize stability. To speed up slightly the regulation of the speed v when the stabilization loop 11 is selected, an optional, complementary signal (not represented in the figures) may be added to the output signal sn of the stabilization loop 11 , said complementary signal being for instance a time-derive of a wind box temperature, or of a sum of wind boxes temperatures.

[0039] Additional components (not represented in figure 2) are added to the control loops 11 , 12, 13, like tracking modules (for mutual tracking between the different loops), to enable bumpless, smooth switching between the different control loops.

Correction loop

[0040] The correction loop 12 outputs a control signal S12, which is a setpoint for the speed v of the belt of the conveyor, and which is transmitted to the actuator 3 (when the correction loop 12 is selected by the selection logic 14). The control signal S12 is produced by the PID corrector of the correction loop 12, whose input is an error signal

is a setpoint, called Waste Gas Temperature setpoint (which may be related not to waste gas temperatures only). WGT^SP may be preset and read in an instruction file during the operation of the sintering machine. It may also be set by an operator during this operation (and possibly be modified during this operation). WGT^P^ is a quantity that depends at least on some of the temperatures WBTi measured in the wind boxes.

[0041] Here, WGT^P^ combines the following quantities:

- a quantity, denoted WBT^P^ , depending on a selected wind box temperature WBT_S; here, WBT^P^ depends more specifically on a deviation kWGT_s of the temperature in the selected wind box WB_S,

- a waste gas temperature WGT^_as representative of the temperature of the global exhaust, waste gas; WGT^_as is measured by the temperature sensor 7 (located in the suction duct 6 or at an output of the blower 8), an estimate of the material permeability Perm^^lc, derived from a pressure measured in one of the wind boxes located underneath the ignition hood 4,

- a temperature BTP_temp (of the sucked gas) at the Burn Through Point (which is determined from some of the wind boxes temperatures WBT, as explained below when describing the stabilization loop 11 ).

[0042] To obtain the WGT^P^ signal, the above quantities are combined together by summing them, according to formula F1 below:

[0043] The quantities thus summed up provide useful complementary information regarding the sintering process. Still, in other embodiments, one or more of these quantities could be omitted, in WGT^P^. For instance, WGT^p%j may take into account WBT^P^ only, or WGT^P^ only, or even BTP_temp only. Besides, the above sum could be a weighted sum, with weighting coefficients different one from another.

[0044] The selected wind box WB_S is a wind box located upstream of BTP, for instance 3 to 10 wind boxes upstream of the BTP. Here, it is a wind box where a temperature rise starts (where a temperature increase with position x starts). The selected wind box WB_S is for instance the wind box where the Temperature Rise Point TRP (presented below) is located.

[0045] Here, WBT^P^ is calculated by multiplying the deviation of the selected wind box temperature &WBT_S by a (positive) corrective coefficient c_m, according to formula F2 below:

[0046] The deviation hWGT_s is for instance a difference between the selected wind box temperature WBT_S and a (temporal) rolling average of WBT_S.

[0047] Remarkably, the corrective coefficient c_m is determined based on a quantity, such as the standard deviation, that is representative of the amplitude of the temporal fluctuations of the selected wind box temperature WBT_S (or, similarly, representative of the temporal fluctuations of AWGT_S). c_m may, like here, increase (not necessarily continuously) with said quantity (e.g.: increase with the standard deviation).

[0048] Figure 3 represents exemplary values of:

- the standard deviation r>_m (in arbitrary units) of the selected wind box temperature WBT_S, at successive time steps j (j=1 ..24 in figure 3); each value of the standard deviation c_m is computed over the same (rolling) temporal window (which is the same as for computing the rolling average of WBT_S),

- and of the corresponding corrective coefficient c_m.

[0049] The duration of the temporal window in question is related to the machine size and to a typical process speed. It is for instance comprised between 0.1 and 3 times a time (of typically a few minutes) taken in average for transporting the material from the loading end to the discharge end of the conveyor.

[0050] Here, the corrective coefficient c_m is determined from the standard deviation c_m as follow: if c>_m is below a given threshold, c_m is equal to zero, and if c_m is above said threshold, c_m is all the higher than

is high (for instance, c_m is proportional to the difference between c_m and said threshold). [0051] Correcting the selected wind box temperature WBT_S (or, like here, its deviation t\WGT_s) with such a corrective coefficient gives greater importance, in the error signal WGT^^r, to the selected wind box temperature when it fluctuates substantially, thus enabling a more efficient stabilization of the wind boxes temperatures than when using a non-corrected temperature value(or than when using a corrective coefficient independent of the temperature fluctuations). [0052] Regarding the estimate of the material permeability Perm^^Lc, as it is derived from a pressure measured in a wind boxes located underneath the ignition hood 4, it reflects the quality of the not-yet sintered material, at the beginning of the conveyor. It thus enables to anticipate low quality material reaching the last part of the conveyor. Here, the quantity Perm^^lc is determined as explained below.

[0053] First, at regular time intervals, the permeability perm of the portion of the layer of material located underneath the ignition hood is determined, based on the gas pressure measured in the wind boxes located underneath the ignition hood. The permeability perm may be determined for instance according to formula F3 below:

where:

Wind Box Pressure is the pressure (relative to ambient pressure) measured in the above-mentioned wind box, layer height is the height of the layer of material M (measured for instance with the above-mentioned height measurement device),

- sinter Machine area is the area of the opening of the wind box collecting the gas flow underneath the ignition hood.

[0054] The Ignition Hood total Flow is obtained by measuring air (and/or gas) flow in the ignition hood piping (either in the supply piping of the hood, or at the windbox output).

[0055] More specifically, the computation of the permeability perm may be carried out according to the following article (section 2.2 of this article): ‘Optimal mixing and granulation process for fine utilization in sinterplants’ by Gergo Rimaszeki et al., ISSN 2176-3135, Technical contribution to the 46^e Seminario de Redugao de Minerio de Ferro e Materias- primas, 17^e Simposio Brasileiro de Minerio de Ferro e 4^e Simposio Brasileiro de Aglomeragao de Mineriode Ferro, part of the ABM Week, September 26th-30th, 2016, Rio de Janeiro, RJ, Brazil.

[0056] At a given time t, the information regarding the ‘quality’ of the material located above the wind box n°i, WBi, is provided by the permeability perm estimated before, at a time t - At;, where the time shift At; is the time taken by the belt for moving the material from the wind box where the pressure is measured (here WBi), to the wind box WBi. [0057] Here, at each time t, a quantity Perm^c _cf^lc is calculated, as a difference between:

- permit - My), for i' corresponding to a wind box where a temperature rise starts, for instance the selected wind box above-mentioned, and

- permit - &t_i=N), that is the permeability estimation for the material that is in the last wind box WBN, at time t.

[0058] Taking into account such estimates of the material permeability, through Perm^, is beneficial, as it enables to anticipate the quality of the material arriving in the sintering zone. In this respect, is noted that Perm^^lc could be computed, in alternative embodiments, as a difference between:

- permit - M_it), for i corresponding to any wind box noticeably upstream the BTP (for instance in the middle of the conveyor), and

- a permeability value considered as normal.

[0059] Besides, it may be noted that a permeability-related quantity like Perm^^lc, and a temperature-related quantity like WBT^P^ are compatible together in that they can be summed up in an error signal. Indeed, when the permeability is too high (porous material, of low quality), it is preferable to increase the belt speed v. And when the sucked gas temperatures are too high, it is preferable too to increase the belt speed.

[0060] The temperature BTP_temp (of the sucked gas) at the Burn Through Point is also added to WGT^PL (in addition to WBT^PY , WGT.!Z_as and Perm_C ^cf^lc). This is useful, as the WGT^P^ signal is mainly composed of sucked gas temperatures information and as BTP_temp is, among these temperatures, a particularly useful one for monitoring the sintering process.

Stabilization loop

[0061] The stabilization loop 1 1 outputs the control signal Sn, which is a setpoint for the speed v of the belt of the conveyor, and which is transmitted to the actuator 3 (when the stabilization loop 11 is selected by the selection logic 14). The control signal Sn is produced by the PID corrector of the stabilization loop 11 , whose input is an adjusted error signal TRS^, which depends at least on the difference between a temperature rise slope setpoint TRS^sp and the temperature rise slope TRS. As above mentioned, the temperature rise slope TRS represents the slope of the temperature as a function of a position x along the belt 2 of the conveyor, upstream of the burn through point BTP. Regarding the setpoint TRS^sp, it is not directly input to the controller 10 (by an operator, or by reading an instruction file): it takes also into account process parameters to together with input setpoints, as described below. The setpoint TRS^sp has a non-zero value. [0062] In the instant embodiment, the adjusted error signal TRS^err is equal to a difference between an adjusted temperature rise slope setpoint TRS^aj and the temperature rise slope TRS) wherein the adjusted temperature rise slope setpoint TRS^ combines:

- the temperature rise slope setpoint TRS^sp and

- the waste gas temperature error signal WGT^^r presented above when describing the correction loop 11 .

[0063] Here, the waste gas temperature error signal WGT^^r is filtered by a rate controller before being combined to the temperature rise slope setpoint.

[0064] The adjusted temperature rise slope setpoint TRS^ is obtained by summing these quantities:

[0065] The adjusted error signal TRS^err thus reads:

[0066] Regarding the temperature rise slope TRS, not all the wind boxes temperatures WBT are necessary to determine it. Indeed, over typically the first half of the conveyor, the wind boxes temperatures WBT vary little, from one wind box to other, and most the temperature variation occurs on the second (last) half of the conveyor. So, the temperature rise slope TRS may for instance be determined from the temperatures measured in the wind boxes distributed over the second half (last half) of the conveyor (here, wind boxes n°13 to 23, 14 to 23 or 15 to 23, for instance), or even from the k last wind boxes temperatures WBT (for instance the last five ones, or the last ten ones).

[0067] The temperature rise slope TRS may, like here, be representative of an average slope of the temperature T (temperature of the gas sucked through the material), as a function of the position x, for a range of positions extending from a temperature rise point TRP, to the burn through point BTP.

[0068] The temperature rise point TRP is a point where the substantial temperature increase, which occurs just upstream the BTP, starts. The position of the TRP may be determined either as a position where T(x) becomes higher than a given threshold, or as a position for which dT/dx is null.

[0069] In the embodiment considered here, the temperature rise slope TRS is determined as follow. The ensemble of measurements {WBT}_i=i3.. 23, which gathers the last 1 1 wind boxes temperatures, is numerically fitted by a function T(x), which, here, is a polynomial function of degree d (see figure 4). In this example, d is equal to 3. It may more generally be comprised between 3 and 7, for instance. More generally, this fit can be based on the ensemble of measurements {WBT}_i=N-k+i.. i=N (for instance, for i=15 to 23). [0070] The temperature at the burn through point, BTP_temp, and its position BTP_pos, are determined as the coordinates of the maximum of T(x) (where dT/dx cancels). Similarly, the temperature at temperature rising point, TRP_temp, and its position TRP_pos, are determined as the coordinates of the minimum of T(x) (where dT /dx also cancels).

[0071] The temperature rise slope TRS is then computed according to formula F5 below:

[0072] Alternatively, TRS could be computed as (BTP_temp - TRP_tempj/(BTP_rj0S - TRP_pos) .

[0073] Besides, TRS could be determined using another fitting method, for instance by fitting f(x) with a straight line, on a given range of positions.

[0074] Regarding now the TRS setpoint TRS^sp, it is determined using both the values of

TRPtemp and TRP_pos and setpoint values for the position and temperature of the burn through point, noted BTP_p ^p _s and BTPt ^P _mp. BTP_p ^p _s and BTPt ^P _mp are input by an operator (by means of a human-machine interface connected to the controller 10), read by the controller in an instruction file, or otherwise received by the controller 10. TRS^sp is determined according to formula F6 below (as illustrated in figure 5):

[0075] The rate controller (for instance a rate limiter, or a setpoint filter or the like) employed to filter the waste gas temperature error signal WGT^^r (before adding it to TRS^sp) is not represented in figure 2. It is configured to avoid overshooting in the evolution of IVGri or (when the stabilization loop is selected). The stabilization loop 12 is thus provided with a signal reflecting desirable corrections for the gas temperature(s), but conditioned to favorize stability. The rate controller thus enables to use a rather aggressive correction (high gain and/or short response times) for the correction loop 11 (which is desirable, as inertia is very important in a sintering machine), while providing a conditioned, stability-oriented version of the waste gas temperature error signal to the stabilization loop 11 .

Protection loop

[0076] The protection loop 13 outputs a control signal S13, which is a setpoint for the speed v of the belt of the conveyor, and which is transmitted to the actuator 3 (when the protection loop 13 is selected by the selection logic 14). The control signal S13 is produced by the RID corrector of the protection loop 13, whose input is an error signal BTP^, which depends at least on the difference between the BTP position setpoint BTP$_S , and the BTP position BTP_pos .

[0077] In the instant embodiment, the error signal BTP^err combines more specifically:

- the above-mentioned difference BTP_p ^p _s - BTP_pos,

- an indicator of a left-right dissymmetry for the sintering process, noted BTP_alff, and an indicator of a cooling speed downstream of the BTP, noted BTP_C00l.

[0078] In this embodiment, BTP^err is determined according to formula F7 below:

[0079] The indicator of a left-right dissymmetry for the sintering process may, like here, be a difference (absolute value of the difference) between:

- an estimate BTP_{pos l} of the BTP position on the left side of the conveyor, determined as explained above for the determination of BTP_pos, but using the left-side temperature measurement WBTi in the last three wind boxes, instead of the left-right average temperature WBT, an estimate BTP_{pos r} of the BTP position on the right side of the conveyor, determined based on the right-side temperature measurement WBT_ir in the last three wind boxes.

[0080] The indicator of a left-right dissymmetry for the sintering process reflects a left-right dissymmetry of the material distribution or quality. Adding the above left-right difference to the error signal BTP^err enables to slow down the process (by reducing the speed v) when such a dissymmetry is present (just as when the BTP_pos is too high), and thus to obtain high quality sinter in spite of the dissymmetry.

[0081] The indicator of the cooling speed downstream of the BTP, BTP_cooL may, like here, be a difference between:

- a given cooling amount Co and

- the quantity BTP_tem_P - WBT_N.

[0082] Adding BTP_C00l to the error signal BTP^err enables to slow down the process if the temperature does not go down sufficiently after the BTP, which is useful as a too hot material at the discharge end may deteriorate the equipment installed thereafter.

[0083] A histogram H_A of the sintering maximum temperature, BTP_temp, is represented in figure 6. H_A obtained when the sintering machine 1 is controlled by the electronic control unit 10. In figure 6, n is the number of occurrences, for each temperature sample. Another histogram, H_M, is also represented in figure 6, showing the number of occurrences, for each temperature sample, when the speed of the sintering machine is controlled manually, directly by an operator (based on observations of process parameters by the operator). The width of H_A is clearly smaller than that of H_M which illustrates the efficiency of this automatic control method. Besides, H_Ais well centred on the desired sintering temperature,

Claims

1. A method for controlling a sintering machine (1 ) that comprise a belt conveyor, the method comprising:

- Acquiring wind boxes temperatures (WBT), representative of the temperatures in several successive wind boxes (WBi) arranged along a belt (2) of the conveyor, the wind boxes being arranged below the belt of the conveyor for sucking a gas through a material (M), to be sintered, disposed on the belt of the conveyor,

- Determining a temperature rise slope (TRS) from at least some of the wind boxes temperatures (WBT), the temperature rise slope representing the slope of the temperature as a function of a position (x) along the belt of the conveyor, upstream of a maximum temperature position (BTP_pos),

- Controlling a speed of the belt (2) of the conveyor depending on the temperature rise slope (TRS) and on a temperature rise slope setpoint (TRS^sp).

2. A method according to claim 1 , wherein the temperature rise slope (TRS) is determined from: the maximum temperature position (BTP_pos), a corresponding maximum temperature (BTP_temp), a temperature rising position (TRP_pos) and a temperature at the temperature rising position (TRP_temp), which are determined from the at least some of the wind boxes temperatures.

3. A method according to claim 2, wherein the temperature rise slope setpoint (TRS^sp) is calculated from a maximum temperature setpoint (BTP?^p _mp), a maximum temperature position setpoint (BTP^^p _s), the temperature rising position (TRP_pos) and the temperature at the temperature rising position (TRP_temp).

4. A method according to anyone of claims 1 to 3, further comprising determining a waste gas temperature error signal (WGT^^r), representative of a difference between a global waste gas temperature setpoint (WT^SP) and a global waste gas temperature signal (WGT^Paj), the global waste gas temperature signal depending on some of the wind boxes temperatures at least.

5. A method according to claim 4, wherein the global waste gas temperature signal (WGT^Paj) depends on a selected wind box temperature (WBT_S).

6. A method according to claim 5, wherein said the global waste gas temperature signal (WGT^) takes into account the selected wind box temperature (WBT_S) multiplied by a corrective coefficient (c_m) which is determined based on an amplitude (o_m) of temporal fluctuations of the selected wind box temperature (WBT_S).

7. A method according to claim 5 or 6, wherein the global waste gas temperature signal (WGTadj) combines at least: the selected wind box temperature (WBT_S), possibly corrected, and a waste gas temperature (WGT^_as).

8. A method according to anyone of claims 5 to 7, wherein the global waste gas temperature signal (WGT^p _dJ) combines at least: the selected wind box temperature (WBT_S), possibly corrected, and a material permeability (Perm^^lc), the material permeability being derived from a pressure measured in one of the wind boxes that is located underneath an ignition hood (4) of the sintering machine.

9. A method according to claim 8, wherein the material permeability (Perm^^lc) is determined taking into account a time-shift representative of a time taken by the belt (2) for moving the material (M) from the wind box (WBi) where said pressure is measured, to a given position.

10. A method according to anyone of claims 4 to 9, wherein the speed of the belt is controlled depending on an adjusted error signal (TRS^err) equal to a difference between an adjusted temperature rise slope setpoint (TRS^_dj) and the temperature rise slope (TRS), wherein the adjusted temperature rise slope setpoint (TRS^j) combines at least:

- said temperature rise slope setpoint (TRS^sp) and

- said waste gas temperature error signal

possibly filtered.

11. A method according to claim 10, wherein the waste gas temperature error signal (WGT^^r) is filtered by a rate controller before being combined to the temperature rise slope setpoint (TRS^sp), said rate controller being configured to avoid overshooting in the temporal evolution of the global waste gas temperature signal {WGT^p _d/).

12. A method according to anyone of claims 1 to 11 , wherein the speed of the belt of the conveyor is controlled using; more than one control loops (1 1 , 12, 13), that are used alternatively to control the speed of the belt of the conveyor, and using a selection logic (14) configured for switching the speed control from one of the control loops to another depending on predetermined criteria.

13. A method according to claim 12, in its dependance of any of claims 4 to 11 , wherein said control loops (11 , 12, 13) comprise at least:

- a control loop named stabilization loop (11 ), configured for controlling said speed depending on the temperature rise slope (TRS), and

- another control loop named correction loop (12), configured for controlling said speed depending on the waste gas temperature error signal (WGT^^r) and regardless of the temperature rise slope (TRS), and wherein the selection logic (14) switches the speed control from the stabilization loop (1 1) to the correction loop (12) when the waste gas temperature error signal (WGT^^r) gets out of an admissible range.

14. A method according to claim 12 or 13, wherein said control loops (11 , 12, 13) comprise at least:

- the control loop named stabilization loop (11 ), configured for controlling said speed depending on the temperature rise slope (TRS), and

- another control loop named protection loop (13), configured for controlling said speed depending on a burn through point error signal (BTP^err) and regardless of the temperature rise slope (TRS), the burn through point error signal (BTP^err) depending on the maximum temperature position (BTP_pos), and wherein a selection logic (14) switches the speed control from the stabilization loop to the protection loop when a distance between the maximum temperature position (BTP_pos) and an end of the belt conveyor becomes smaller than a preset distance limit.

15. A method according to claim 14 wherein the burn through point error signal (BTP^err) combines at least:

- a difference between: a setpoint for the maximum temperature position (BTP$?_S ), and the maximum temperature position (BTP_pos), and

- an indicator (BTP_cool) of a cooling speed downstream of the maximum temperature position (BTP_p£_s).

16. A method according to claim 14 or 15, wherein the burn through point error signal (BTP^err) combines at least:

- the difference between: a setpoint for the maximum temperature position (BTP^_S), and the maximum temperature position (BTP_pos), and

- an indicator (BTP_diff) of a left-right dissymmetry for the sintering process.

17. An electronic control unit (10) for a sintering machine (1 ), configured for executing the steps of the method according to any of the previous claims.

18. A sintering machine (1) comprising:

- a belt conveyor,

- several successive wind boxes (WBi, WB₂, WB23) arranged along a belt (2) of the conveyor, below the belt of the conveyor for sucking a gas through a material (M), to be sintered, disposed on the belt of the conveyor,

- temperatures sensors arranged in at least some of the wind boxes (WB1, WB₂, WB₂₃),

- an electro-mechanical actuator (3) coupled to belt (2) to drive the belt,

- the electronic control unit (10) according to claim 17, operatively connected to the temperature sensors and to the electro-mechanical actuator (3).

19. A computer program comprising instructions, whose execution on a computer (10) makes the computer to execute the method for controlling a sintering machine according to anyone of claims 1 to 16.