The present invention relates to the operation of shaft furnaces.

The term shaft furnace refers to vertical shaft- or column-shaped furnaces.

Shaft furnaces are used in a wide range of processes. Examples of such processes include, but are not limited to the burning of solid waste, the reduction of iron ore to produce pig iron, the melting of metals and mineral wool, etc., whereby a solid charge is fed to the furnace via the top.

It is known to inject combustion oxidant or oxidizing agent, such as air, which may or may not be preheated, into a shaft furnace via a number of injectors distributed around the circumference of the furnace.

In spite of such a distribution of the oxidant injection around the circumference of the furnace, the combustion process is not always homogeneous across the cross-section of the furnace and hot spots can often be observed in shaft furnaces. These may arise due to asymmetry of the structure of the shaft furnace itself and/or due to a non-homogeneous distribution of the charge or of the composition of the charge across the cross-section of the furnace.

The location of a hot spot may be constant or invariable, for example when the charge- loading system systematically creates the same non-homogeneous distribution of the charge across the furnace, or may vary over time, for example, when the nature of the charge varies over time.

The presence of one or more such hot spots and can lead to reduced furnace efficiency, a lower or less constant quality of the product of melting processes and damage to the refractory furnace wall at or near the hot-spot area.

Similarly, cold spots have also been observed in shaft furnaces. In cold spots, the temperature drops below the optimum temperature for the process taking place inside the furnace, thereby affecting the efficiency of the furnace and the quality of the product in case of melting processes. For example, the presence of a cold spot in a shaft furnace may lead to the formation of so-called "bridges" and prevent a uniform descent of a solid charge within the furnace.

AU-B-660238 discloses a blast furnace for producing molten iron from iron ore with tuyeres, which continuously blow hot blast into the blast furnace, located at a lower part of the furnace. As explained in AU-B-660238, where scabs form on the wall of the furnace above the tuyeres, the temperature of the wall is detectably lower. In order to remove said scabs, it is proposed in AU-B- 660238 to reduce the volume of hot blast injected by one or more tuyeres underneath the colder area of the furnace wall for a number of days. This reduces the penetration depth of the hot blast injected by those tuyeres and causes said volume of hot blast to flow close to the part of the furnace wall where the scab is formed so that the scab is removed. In addition, abnormally high hot metal flow in portions of the hearth in the lower part of the furnace, which may cause erosion of the adjacent furnace wall. Such abnormally high hot metal flows can be detected via the higher temperature of the corresponding part of the furnace wall. In that case, it is proposed in AU-B- 660238 to reduce the volume of hot blast injected by one or more tuyeres above the area of abnormally high hot metal flow, again for a number of days, so as to decrease the temperature and the hot metal flow in said area. As, in the method according to AU-B-660238, the reduction of the injected volume of hot blast also reduces the penetration depth of said hot blast, the energy efficiency of the furnace is likewise reduced.

It is known in the art to improve the efficiency of shaft furnaces by injecting a combustion oxidant with a higher oxygen content than that of air into the furnace, in particular oxygen-enriched air and substantially pure oxygen. It is also known, in that case, to inject the combustion oxidant with supersonic velocity in order to achieve increased penetration of the combustion oxidant into the shaft furnace in spite of the lower volumetric flow. It is, in particular, known in the art to inject oxygen-rich combustion oxidant in a pulsed or sequenced manner into the shaft furnace, whereby each of the supersonic injectors between an active phase, during which it injects combustion oxidant at supersonic velocity, and a passive phase during which it does not. Such shaft furnace operation methods are for example described in EP-A- 1242781 , EP-A- 1739194 and DE-A- 10249235.

Unfortunately, it has been observed that the use of oxygen-rich combustion oxidant does not overcome the problem of hot and cold spots and can even increase the occurrence and intensity of hot spots and their consequences.

There is therefore a need to be able to inject combustion oxidant into the shaft furnace in such a way that the occurrence and negative effects of hot and/or cold spots in the shaft furnace are reduced or even remedied.

There is in particular a need to be able to inject oxygen-rich combustion oxidant into the shaft furnace so as to increase furnace efficiency while reducing the occurrence and negative effects of hot and/or cold spots in the shaft furnace. Thereto, the present invention proposes an improved method of injecting oxidizing agent into a vertical shaft furnace in which a combustion process takes place.

Said shaft furnace presents n injectors (at least 3), distributed around the circumference of the shaft furnace at a level or height [h, h + Ah] of the shaft furnace. Each one of said n injectors is adapted for sonic or supersonic injection of a gaseous fluid into one of n sections of a cross-section of the shaft furnace at level [h, h + Ah] .

The n injectors and the n sections are thus in a one-to-one relationship, with each injector being associated with one of the sections into which the injector is capable of injecting the fluid, and each section being associated with one of the injectors by which the section can be supplied with the fluid. The n sections together form the internal cross-section of the shaft furnace at level [h, h + Ah].

According to the invention, the shaft furnace is part of an installation including a source of an oxidizing agent and a control unit.

The source of oxidizing agent is adapted to supply an oxidizing agent with an oxygen content higher than 21%vol and at most 100%vol, said source being fluidly connected to each of the n injectors. The oxygen content of the oxidizing agent supplied by said source is preferably at least 50%vol, more preferably at least 90% and even more preferably at least 95%vol.

The control unit is programmed to control both:

• a total amount of the oxidizing agent injected into the shaft furnace by means of the n injectors considered together and

• an amount of oxidizing agent injected into the shaft furnace by each one of the n injectors individually.

The sum of the amounts of oxidizing agent injected into the shaft furnace by each one of the n injectors at a given moment in time corresponds to the total amount of the oxidizing agent injected into the shaft furnace by means of the n injectors at said moment in time.

The control unit controls the total amount of the oxidizing agent injected by the n injectors so as to meet a demand for oxidizing agent by the combustion process which takes place in the shaft furnace.

As regards the amount of oxidizing agent injected by each individual injector of the n injectors, this is controlled by the control unit so that the injection of oxidizing agent by each one of the n injectors takes place cyclically according to a cycle with a duration t _c . During each cycle, the control unit controls the injection of oxidizing agent by each one of the n injectors so that each one of the n injectors injects oxidizing agent in a sequenced fashion, i.e. in a fashion alternating between an active phase and a passive phase. More specifically, the control unit controls the injection of oxidizing agent by each one of the n injectors so that the injection of oxidizing agent by each one of the n injectors alternates between:

(a) an active phase having an active duration t _a , whereby during the active phase the oxidizing agent is injected by the injector at an active flow rate and with sonic or supersonic velocity, and

(b) a passive phase having a passive duration t _p , whereby during the passive phase either the injector injects no oxidizing agent or the injector injects oxidizing agent at subsonic velocity and at a passive flow rate which is lower than the active flow rate. Even though, as will be explained below, the duration of the active phase ta and the duration of the passive phase t _p may be different for different injectors, for each one of the n injectors, the sum of the duration (t _a ) of the active phase and the duration (t _p ) of the passive phase is equal to the duration (t _c ) of the cycle of the cyclical operation of the control unit (i.e. t _a + t _p = t _c ).

In this manner, the control unit can ensure that oxidizing agent is injected at sonic or supersonic velocity in each section of the cross-section of the shaft furnace at level [h, h + Ah] at some point during cycle duration t _c .

According to a first aspect, the present invention relates to a method of injecting oxidizing agent into a vertical shaft furnace in which a combustion process takes place and whereby at least one of the n sections has been identified as a hot-spot section or as a cold-spot section.

When a section has been identified as a hot-spot section, the control unit controls the injection of oxidizing agent by the injector corresponding to the hot-spot section so that the active duration t _a of said injector is shorter than the active duration t _a of the injectors not corresponding to a hot- spot section (so that the passive duration t _p of a hot-spot section injector is in turn longer than the passive duration t _p of an injector corresponding to a section which is not a hot-spot section). As a consequence, the amount of oxidizing agent injected per cycle into a hot-spot section is lower than the amount of oxidizing agent injected per cycle into a section which is not a hot-spot section. This in turn makes it possible to reduce the intensity of combustion in the hot-spot section and to lower the temperature within a hot-spot section or to prevent a further increase of said temperature. In an analogous manner, when a section has been identified as a cold-spot section, the control unit controls the injection of oxidizing agent by the injector corresponding to the cold-spot section so that the active duration t _a of said injector is longer than the active duration t _a of the injectors not corresponding to a cold-spot section (and the passive duration t _p of a cold-spot section injector is consequently shorter than the passive duration t _p of an injector corresponding to a section which is not a cold-spot section). As a consequence, the amount of oxidizing agent injected per cycle into a cold-spot section is higher than the amount of oxidizing agent injected per cycle into a section which is not a cold-spot section. This makes it possible to intensify the combustion in the cold-spot section and to increase the temperature within a cold-spot section or to prevent a further decrease of said temperature.

At the same time, the penetration of the oxidant in the furnace is maintained in that, during each cycle, the n injectors continue to inject oxidant with sonic or supersonic velocity during their active phase.

When it is known that no problems arise with cold-spot sections during the operation of the shaft furnace, the control by the control unit of the injection of oxidizing agent by injectors corresponding to cold-spot sections may be omitted from the method.

Likewise, when it is known that no problems arise with hot-spot sections during the operation of the shaft furnace, the control by the control unit of the injection of oxidizing agent by injectors corresponding to hot-spot sections may be omitted from the method.

The passive flow rate is preferably less than half the active flow rate, more preferably less than 30% and even more preferably at most 15% of the active flow rate. The reason for injecting some (subsonic) oxidizing agent during the passive phase of an injector is generally to protect the passive injector against overheating and/or to prevent the formation of solid deposits onto and into the passive injector.

In order to continuously meet the demand for oxidizing agent by the furnace, the control unit controls the number of the n injectors which is in the active phase at any one time, the other injectors of the set of n injectors being in the passive phase. Thus, when the demand for oxidizing agent is high, more injectors will be simultaneously in the active phase than when the demand for oxidizing agent is low.

According to a first embodiment of the present method, the active duration t _a of each of the injectors not corresponding to a hot-spot section or to a cold-spot section is identical. Alternatively or in combination therewith the active duration t _a of each of the injectors corresponding to a hot- spot section can be identical and/or the active duration t _a of each of the injectors corresponding to a cold-spot section can be identical.

The control unit may activate the start of the active phase of the n injectors so that the next injector for which the active phase starts is located in clockwise succession around the circumference of the furnace of the last injector to have started its active phase (as seen from above).

The control unit may alternatively activate the start of the active phase of the n injectors so that the next injector for which the active phase starts is located in counterclockwise succession around the circumference of the furnace of the last injector to have started its active phase (as seen from above).

According to a further embodiment, when the number n of injectors is at least 5 and preferably at least 6, the control unit may activate the start of the active phase of the n injectors so that the next injector for which the active phase starts is located in a semi-circumference of the furnace opposite the last injector to have started its active phase. This may result in a more even instantaneous distribution of the injectors in active phase across the cross section of the shaft furnace.

It may be known from experience where a hot spot or a cold spot will invariably arise in the shaft furnace. In that case, the identification of the one or more hot-spot sections may be predetermined and/or the identification of the one or more cold-spot sections in the method according to the invention may be predetermined and the corresponding data may be stored as predetermined data in the memory of the control unit, i.e. independently from any real-time feedback from the shaft furnace. For example, only one of the n sections may thus have been identified as a hot-spot section and/or only one of the n sections may have been identified as a cold- spot section in a predetermined manner.

Likewise, the active duration t _a of an injector corresponding to a hot-spot section and/or the active duration t _a of an injector corresponding to a cold-spot section may be predetermined, in which case said active duration(s) t _a and the corresponding passive duration(s) t _p are stored as predetermined data in the memory of the control unit and are not varied on the basis of real-time feedback from the shaft furnace. Alternatively, the active duration t _a of an injector corresponding to a hot-spot section and/or the active duration t _a of an injector corresponding to a cold-spot section may be varied in function of real-time feedback from the shaft furnace.

According to such an embodiment, when a section has been identified as a hot-spot section, the method may advantageously further comprise a step of continuously or intermittently determining a hot-spot temperature inside the hot-spot section or of a wall element adjacent the hot-spot section. The control unit compares the thus determined hot-spot temperature with a predetermined upper hot-spot limit value. When the hot-spot temperature exceeds a predetermined upper hot-spot limit value, the control unit reduces the active duration t _a of the injector corresponding to said hot-spot section. The control unit also compares the hot-spot temperature with a predetermined lower hot-spot limit value. When the hot-spot temperature is below said predetermined lower hot-spot limit value, the control unit increases the active duration t _a of the injector corresponding to said hot-spot section.

Likewise, when a section has been identified as a cold-spot section, the method may advantageously further comprise a step of continuously or intermittently determining a cold-spot temperature inside the cold-spot section or of a wall element adjacent the cold-spot section. The control unit then compares the thus determined cold-spot temperature with a predetermined lower cold-spot limit value. When the thus determined cold-spot temperature is below said predetermined lower cold-spot limit value, the control unit increases the active duration t _a of the injector corresponding to said cold-spot section. The control unit also compares the cold-spot temperature with a predetermined upper cold-spot limit value and when the determined cold-spot temperature exceeds the predetermined upper cold-spot limit value, the control unit decreases the active duration t _a of the injector corresponding to said cold-spot section.

Obviously, the upper hot-spot limit value is higher than the lower hot-spot limit value and the upper cold-spot limit value is higher than the lower cold-spot limit value. Both hot-spot limit values are normally higher than both cold-spot limit values.

It will be appreciated that these embodiments enable a more refined response to the occurrence of hot spots and/or cold spots in the shaft furnace.

Although, as indicated above, in some cases it may be possible to predetermine that hot spots and/or cold spots will occur at certain locations, and thus to predetermine hot-spot sections and/or cold-spot sections, in many cases it will not be possible to predict when and/or where a hot spot or a cold spot may occur in the shaft furnace.

For this reason, the present invention also includes a method of injecting oxidizing agent into a vertical shaft furnace in which a combustion process takes place and whereby the occurrence of one or more hot-spot sections and/or cold-spot sections is detected in real time.

According to such an embodiment, the method further comprises the step of continuously or intermittently determining a control temperature inside each of the n sections or of a wall element adjacent each of the n sections.

Thereafter, when the method includes the identification of hot-spot sections, each control temperature is compared with a hot-spot reference temperature. When the control temperature of a section exceeds the hot-spot reference temperature, said section is identified by the control unit as a hot-spot section.

Likewise, when the method (also) includes the identification of cold-spot sections, each control temperature is compared with a cold-spot reference temperature and when the control temperature of a section lies below the cold-spot reference temperature, said section is identified as a cold-spot section by the control unit.

The hot-spot reference temperature and/or the cold-spot reference temperature are generally predetermined. However, said reference temperature(s) may also usefully be determined in realtime, for example in function of the average of the control temperatures of the n sections. The hot- spot reference temperature could then be a first predetermined number of degrees above said average or a first predetermined percentage above said average. Likewise, the cold-spot reference temperature could be a second predetermined number of degrees below the average or a second percentage below the average, whereby, when both hot-spot sections and cold-spot section are determined, the first and second predetermined number of degrees or the first and second percentage may be identical or different. The exact value of the first and/or second predetermined number of degrees or the first and/or second percentage will depend on the sensitivity to differences in temperature of the process taking place in the furnace and/or of the furnace refractories. Indeed, differences in temperature which do not produce a noticeable detrimental effect on the process nor on the refractories do not as a rule justify the adjustment of the injection of the oxidizing agent into the furnace. The determination of the control temperatures may be done manually or automatically. When the control temperatures are determined intermittently, this may be done manually or automatically. When the control temperatures are determined continuously, this is normally done automatically. Likewise, the identification of hot-spot sections and/or cold-spot sections and in particular the comparison between the control temperatures and the reference temperature(s) may be performed by the furnace operator and the results (temperature difference or the identification of the hot- and/or cold-spot sections) may be inputted into the control unit by the operator. Preferably, the control unit automatically receives the determined control temperatures, compares same with the reference temperature or temperatures and automatically identifies any hot-spot sections and/or cold-spot sections in function of said comparison.

In addition, according to a preferred embodiment:

• the control unit also selects the active duration t _a of an injector corresponding to a hot-spot section in function of the difference between the control temperature of said hot-spot section and the hot-spot reference temperature so that a greater difference between the control temperature and the hot-spot reference temperature results in a shorter active duration t _a and a smaller difference between the control temperature and the hot-spot reference temperature results in a greater active duration t _a

and/or

• the control unit selects the active duration t _a of an injector corresponding to a cold- spot section in function of the difference between the control temperature of said cold-spot section and the cold-spot reference temperature so that a greater difference between the control temperature and the cold-spot reference temperature results in a greater active duration t _a and a smaller difference between the control temperature and the cold-spot reference temperature results in a smaller active duration t _a .

The control unit preferably has at least one information output element which discloses which of the n sections is a hot-spot section and/or which of the n sections is a cold-spot section or which of the n injectors corresponds to a hot-spot section and/or which of the n injectors corresponds to a cold-spot section. The control unit may, for example, comprise, by way of output element, a screen with a schematic representation of the cross section of the shaft furnace in which any hot- spot sections and/or cold-spot sections are highlighted. The control element may also have an output element which transmits said information to a remote device, in particular a hand-held or mobile device.

Typically, the cross section of the shaft furnace is substantially circular, though different cross sections, such as a rectangular cross section, is also possible.

In general, the n injectors are substantially evenly or uniformly distributed around the circumference of the shaft furnace.

The number n of said injectors is generally greater than 3. A number of up to 14 or 16 injectors may be useful. However, the number n of injectors may also be significantly higher, for example up to 24 or even up to 36.

The source of oxidizing gas may be an installation for enriching air with oxygen, typically when the oxygen content of the oxidizing gas is relatively low, for example more than 21%vol and not more than 90%vol. The source of oxidizing gas may also be an air separation unit, a reservoir of liquefied oxygen or a pipeline transporting liquefied oxygen, for example when the oxygen content of the oxidizing gas is between 90%vol and 100%vol, preferably at least 95%vol.

The control unit usefully controls the total amount of the oxidizing agent injected into the shaft furnace by means of an adjustable control valve unit. Such an adjustable control valve unit may for example control the total amount of oxidizing agent which is supplied to a gas distributor which is in fluid connection with each one of the n injectors, typically a gas supply ring which surrounds the shaft furnace.

The control unit advantageously controls the amount of oxidizing gas to each of the n injectors by means of n individual valve units, each of the n individual valve units controlling the supply of oxidizing agent to a single one of the n injectors. Said n individual valve units may, for example, be positioned on the n fluid connections between the gas distributor (or ring) and the n injectors, one individual valve unit per fluid connection.

The n individual valve units are preferably on-off valve units. When an individual valve unit is in an on-position, a first flow rate of oxidizing agent is supplied to the corresponding injector so that said injector injects oxidizing agent at said first flow rate (active flow rate) and at sonic or supersonic velocity into the shaft furnace. When an individual valve unit is in an off-position, no oxidizing agent is supplied to the corresponding injector or oxidizing agent is supplied to said corresponding injector at a second flow rate which is lower than the first flow rate so that said injector injects no oxidizing agent into the shaft furnace or injects oxidizing agent at said second flow rate (passive flow rate) and at subsonic velocity into the melting zone. The injectors are advantageously equipped with a convergent-divergent nozzle or laval nozzle.

The shaft furnace may be a waste combustion furnace. However, the invention is particularly useful when the furnace is a furnace in which a charge material, other than the fuel which is combusted with the oxidizing agent, is transformed. The invention is thus particularly useful when the shaft furnace is a glass-melting furnace, a mineral-wool-melting furnace or a metal-melting furnace.

The shaft furnace may be a cupola. The shaft furnace may also be an ironmelting blast furnace.

The present invention also relates to an installation for effecting a combustion process in a vertical shaft furnace. This installation comprises the shaft furnace, a source of oxidizing agent and a control unit.

The shaft furnace presents n injectors distributed around the circumference of the shaft furnace at a level [h, h + Ah] of the shaft furnace, whereby n is at least 3, each injector being adapted for sonic or supersonic injection of a gaseous fluid into one of n sections of a cross-section of the shaft furnace at level [h, h + Ah] .

The oxidizing agent which the source of oxidizing agent is capable of supplying has an oxygen content higher than 21%vol and at most 100%vol. Said source is furthermore fluidly connected to each of the n injectors.

The control unit of the installation according to the invention is programmed to control both

(a) a total amount of the oxidizing agent injected into the shaft furnace by means of the n injectors so as to meet a demand for oxidizing agent by the combustion process and (b) an amount of oxidizing agent injected into the shaft furnace by each one of the n injectors, said control unit being more specifically programmed to control the amount of oxidizing agent injected by each one of the n injectors in accordance with any one of the embodiments of the method of the invention as described above.

The different installation features described hereabove in the context of the method, such as the number and type of injectors, the types of shaft furnace, etc., also apply to different embodiments of the installation of the invention.

The present invention and its advantages are illustrated in the following examples, reference being made to figures 1 to 4, whereby: • figure 1 is a schematic representation of a first embodiment of an installation for melting cast iron and suitable for use in the method of the invention, including a cross- section representation of the cupola-type shaft furnace of the installation,

• Figure 2 is a schematic representation of a second embodiment of such an installation for melting cast iron,

• Figure 3 is a schematic representation of how, on the basis of a detected section temperature, sections are identified as hot-spot or cold-spot sections, and

• Figure 4 is a partial schematic representation of a screenshot of a user interface suitable for use in the context of the present invention.

The shaft furnace 10 of figures 1 and 2 has a substantially circular cross section. A charge 20 of metal (cast iron) to be melted and coke is introduced into the top end 1 1 of the shaft furnace 10. In the embodiment shown in figure 1, the charge 20 is introduced into shaft furnace 10 via a feed opening 21 in the mantle 17 of the shaft furnace at its top end 11. In the embodiment shown in figure 2, the charge 20 is introduced via the roof 18. Flux materials are generally also introduced in this manner. The charge 20 is typically introduced so as to form successive substantially horizontal layers inside shaft furnace 10, for example a layer of metal, followed by a layer of coke, followed by a layer of flux material, followed by a layer or metal, etc.

The coke is combusted with combustion oxidizing agent in a combustion zone 12 located further down in the shaft furnace 10. Thereto, the combustion oxidizing agent is injected into the shaft furnace 10 by means of injectors or tuyeres 30 which are positioned around the combustion zone 12 at a level [h, h + Ah] from the bottom 19 of the furnace 10. Each one of the six injectors 30 is adapted to inject oxidizing agent at sonic or supersonic velocity into one of six sections, identified as sections 1 to 6 in figure 4, of the cross-section of the combustion zone 12 of the shaft furnace 10 at level [h, h + Ah] . Each section 1 to 6 has one corresponding tuyere 30 and each tuyere 30 has one corresponding section 1 to 6. Together, the six sections 1 to 6 cover the entire cross section of the shaft furnace 10.

In the illustrated embodiments, all tuyeres 30 are located at a same level h' from the bottom of the furnace. However, in other embodiments, injectors may be present at different levels around the combustion zone 12 within an area of height Ah upwards from level h.

The heat of combustion causes the metal in the charge immediately above the combustion zone 12 to melt and the molten metal trickles through the combustion zone 12 to the bottom area through the layered charge, thereby preheating the charge, until they are removed from the shaft furnace 10. In the embodiment illustrated in figure 1 , the combustion gases leave the furnace 10 via a gas outlet in the roof 18, in the embodiment illustrated in figure 2 via a flue gas outlet 16 in the mantle 17 of the furnace 10. The molten metal is removed from the bottom area 13 of the shaft furnace 10 via tapping spout 14. The slag which is formed during the melting process is removed from the shaft furnace 10 via slag spout 15 located at a level above the level of tapping spout 14.

A control unit 40 controls the operation ofthe shaft furnace 10.

In the illustrated embodiment, six (6) tuyeres 30 for oxidizing agent are evenly distributed around the combustion zone 12 of the shaft furnace 10. Each tuyere 30 is individually connected to an distributor for oxidizing agent in the form of an oxidant ring 31 which surrounds the shaft furnace 10.

In the illustrated embodiments, tuyeres 30 are tuyeres for the injection of oxygen with a degree of purity of between 90%vol and 100%vol, preferably of at least 95%vol.

In order to enable the sonic or supersonic injection of oxidizing agent into the shaft furnace

10, each tuyere 30 is equipped with a laval nozzle 34.

Combustion oxidant (oxidizing agent) is supplied to the oxidant ring 31 from a source of oxidizing agent, such as an air separation unit or an oxygen reservoir (not shown). Valve 32 is used to control the flow of oxidizing agent from the source of oxidizing agent to the distributor 31 , and, in this manner, the instantaneous total amount of the oxidizing agent injected into the shaft furnace 10 by means of the six injectors 30. Valves 33 are used to control the flow of oxidizing agent from distributor 31 to the individual tuyeres 30, one valve 33 per tuyere 30. The functioning of the individual valves 32, 33 is controlled by or through control unit 40. Control unit 40 more specifically controls the total amount of the oxidizing agent injected into the shaft furnace 10 by means of the six injectors/tuyeres 30 so as to meet a demand for oxidizing agent by the combustion process taking place in the shaft furnace 10. By means of valves 33, control unit 40 moreover controls the injection of oxidizing agent by each one of the six injectors 30 so that each injector 30 injects oxidizing agent into the furnace 10 in a pulsed fashion, i.e. in a manner whereby said injector 30 alternates between

(a) an active phase having an active duration t _a and during which the injector 30 injects the oxidizing agent with sonic or supersonic velocity and this at an active flow rate, and (b) a passive phase having a passive duration t _p and during which the injector 30 either injects no oxidizing agent or injects oxidizing agent at subsonic velocity and this at a passive flow rate which is lower than the active flow rate.

For each injector 30, the sum of the active duration t _a and the passive duration tb is equal to the cycle duration t _c .

It will be appreciated that the embodiments illustrated in figures 1 and 2 are only two of many possible embodiments.

The number of tuyeres 30 may be greater or smaller than in the illustrated embodiment. Additional fuel, such as coal, fuel oil or gaseous fuel, may also be introduced into the combustion zone 12. The additional fuel may be introduced into the furnace 10 via burners, via fuel tuyeres, which may be separate from the tuyeres or which may form a tuyere ensemble with (some of) the tuyeres 30, or, in particular in the case of solid particulate additional fuel, directly through (some of) the tuyeres 30. The furnace 10 may also comprise multiple sets of tuyeres for combustion oxidant. For exemple, a set of air tuyeres for the injection of air, which may or may not be enriched with oxygen, may be connected to a wind ring around the shaft furnace and a set of tuyeres for an oxidizing agent such as oxygen with a purity of between 90%vol and 100%vol may be connected to a separate oxygen ring around the shaft furnace.

With the type of side-charged furnaces as illustrated in figure 1 , it was observed at the end of each operation campaign that the refractory material of the mantle 17 of the furnace had suffered significant thermal damage in the area adjacent one particular section of the combustion zone 12, said damage being indicative of the occurrence of a hot spot in said section. During a subsequent campaign, the temperature of the furnace mantle 17 adjacent the different sections was regularly measured and the difference in temperature between the mantle adjacent the identified section and adjacent the other sections was calculated. These measurements confirmed that the identified section was indeed a systemic hot-spot section, hereafter referred to as "the predetermined hot-spot section". A test was then conducted whereby control unit 40 reduced the active duration t _a of the tuyere 30 corresponding to the predetermined hot-spot section, while increasing the identical active duration t _a of all other tuyeres 30 so as to compensate for the reduced amount of oxidizing agent injected by the first tuyere 30, until an optimum active duration t _a was found for the tuyere 30 corresponding to the predetermined hot-spot section, i.e. until no significant difference could be detected between the mantle temperature adjacent the pre-determined hot-spot section and the mantle temperature adjacent the other sections. Thereafter, control unit 40 continued to control the individual valves 33 so as to maintain the thus determined shorter optimum active duration t _a for the tuyere 30 corresponding to the predetermined hot-spot section and the identical, somewhat longer active duration t _a for the other tuyeres 30. As a consequence, the previously observed thermal damage at the level of one particular section no longer occurred and the absence of a hot spot further resulted in a molten product of improved quality.

The furnace 10 shown in figure 2 is operated at varying loads and with a charge of somewhat varying composition.

The total oxidizing agent requirement of the process or of the different stages of the process are stored in the memory of the control unit 40.

A temperature sensor 60, is installed in or near the furnace mantle 17 adjacent each of the six sections of the combustion zone 12 and the temperatures detected by sensors 60 are transmitted to control unit 40 where they are compared, on the one hand, with a hot-spot reference temperature significantly above the normal or target temperature of the furnace mantle 17 and, on the other hand, with a cold-spot reference temperature significantly below said normal or target temperature.

Thus, as illustrated in figure 3, when the detected temperature is higher than a predetermined hot-spot temperature, the section is identified as a hot-spot section and when the detected temperature is lower than a predetermined cold-spot temperature, the section is identified as a cold- spot sections. When the detected temperature lies within the range between the predetermined cold- spot temperature and the predetermined hot-spot temperature, the corresponding section is neither a hot-spot section nor a cold-spot section.

When all detected temperatures lie between the cold-spot reference temperature and the hot- spot reference temperature, control unit 40 ensures that all six tuyeres 30 operate with identical active durations t _a (and thus also with identical passive durations t _p ) so that each tuyere 30 injects one sixth of the total amount of oxidizing agent injected into the furnace 10. Control unit 40 may for example activate tuyeres 30 in clockwise succession (seen from above) with a first tuyere 30 starting its active phase at the start t ₀ of a cycle duration, the next tuyere 30 starting its active phase at t ₀ + l/6*tc, the following tuyere 30 starting its active phase at t ₀ + 2/6*t _c , etc.

When a section 1 to 6 has been identified as a hot-spot section, the active duration of the corresponding injector 30 is reduced to a value ta' which is smaller than the previously mentioned active duration t _a by a predetermined fraction. As a consequence, the amount of oxidizing agent injected per cycle by said tuyere 30 into the corresponding hot-spot section is reduced.

Similarly, when a section 1 to 6 has been identified as a cold-spot section, the active duration of the corresponding injector 30 is reduced to a value ta" which is greater than the previously mentioned active duration t _a by a predetermined fraction. As a consequence, the amount of oxidizing agent injected per cycle by said tuyere 30 into the corresponding cold-spot section is increased.

In order to ensure that the required total amount of oxidizing agent continues to be injected into furnace when one or more sections are identified as hot-spot sections into which a reduced amount of oxidizing agent is injected per cycle and/or when one or more sections are identified as cold-spot sections into which an increased amount of oxidizing agent is injected per cycle, control unit 40 advantageously adjusts, where necessary, the active duration of the remaining tuyeres 30 (via which oxidizing agent is injected into the sections which are neither hot-spot sections nor cold- spot sections) so that the total amount of oxidizing agent injected into the furnace 10 per cycle corresponds to the actual oxidizing agent requirement of the process taking place in the furnace 10.

Figure 4 shows a touchscreen with a schematic representation of a cross section of shaft furnace 10 at the level of tuyeres 30, the corresponding valves 33 and the corresponding sections numbered 1 to 6 in the figure. Oxidant ring 31 and valves 32 are equally shown.

General information regarding the furnace 10 and the process taking place therein are permanently displayed, for example information regarding the charge and regarding the total amount of oxidizing agent injected per cycle or per time unit. Additional information may include the cycle duration t _c and or the standard active duration t _a of the six tuyeres 30.

Specific information on the status or operating conditions of the different elements shown in the representation can be obtained by "clicking" on the element concerned.

Thus, when "clicking" on one of the sections, the touchscreen may display the detected furnace mantle 17 temperature for said section and the actual amount of oxidizing agent injected into said section per cycle or per time unit through the corresponding tuyere 30, or the difference between the actual oxidizing agent injection rate and the standard oxidizing agent injection rate. Three examples are shown in figure 3: section 3 is a hot-spot section, section 4 is a normal section, i.e. neither a hot-spot section nor a cold-spot section and section 5 is a cold spot section. A colour code is preferably used to permanently display the status of each section. For example, a section may be shown in blue when it has been identified as a cold-spot section and as red when it has been identified as a hot-spot section, a different standard colour, such as white, being used for sections which are neither hot-spot nor cold-spot sections.

Preferably, information stored in control unit 40 regarding the evolution over time of the detected mantle temperature of each section and the amount of oxidizing agent injected per cycle into said section (or the cycle-averaged amount of oxidizing agent injected into said section) can also be visualized.