WO2024177558A1

WO2024177558A1 - A process for the production of sponge iron

Info

Publication number: WO2024177558A1
Application number: PCT/SE2024/050173
Authority: WO
Inventors: Gunilla HYLLANDER; Reine GRANSTRÖM
Original assignee: Hybrit Development AB
Current assignee: Hybrit Development AB
Priority date: 2023-02-22
Filing date: 2024-02-22
Publication date: 2024-08-29
Anticipated expiration: 2025-08-22
Also published as: CN120530209A; SE2350202A1; SE546681C2; KR20250150008A; MX2025008361A; AU2024224346A1

Abstract

The present disclosure relates to a process for the production of sponge iron from iron ore using a direct reduction shaft, wherein the process utilizes a reducing gas comprising greater than 85 vol% hydrogen gas, and wherein the process comprises steps of - determining a first temperature T1 measured in the direct reduction shaft at a first temperature determination point P1; the first temperature determination point P1 being situated in the reducing zone of the direct reduction shaft (s211); and - controlling the process based at least upon the determined first temperature T1 measured (s213). The disclosure further relates to a system adapted for performing such a process and a sponge iron product obtained from such a process.

Description

A process for the production of sponge iron

TECHNICAL FIELD

The present disclosure relates to a process for the production of sponge iron from iron ore using a direct reduction shaft. The disclosure further relates to a system adapted for performing such a process and a sponge iron product obtained from such a process.

BACKGROUND ART

Steel is the world's most important engineering and construction material. It is difficult to find any object in the modern world that does not contain steel, or depend on steel for its manufacture and/or transport. In this manner, steel is intricately involved in almost every aspect of our modern lives. In 2018, the total global production of crude steel was 1 810 million tonnes, by far exceeding any other metal, and is expected to reach 2 800 million tonnes in 2050 of which 50% is expected to originate from virgin iron sources.

Although steelmaking processes have been refined over decades and are approaching the theoretical minimum energy consumption, there is one fundamental issue not yet resolved. Reduction of iron ore using carbonaceous reductants results in the production of CO2 as a byproduct. For every ton steel produced in 2018, an average of 1.83 tonnes of CO2 were produced. The steel industry is one of the highest CO2-emitting industries, accounting for approximately 7% of CO2 emissions globally. Excessive CO2-generation cannot be avoided within the steel production process as long as carbonaceous reductants are used.

The HYBRIT initiative has been founded to address this issue. Central to the HYBRIT concept is a shaft-based direct reduction to produce sponge iron from virgin ore. In direct reduction, the ore is reduced in a solid-state reduction process at temperatures below the melting point of iron. Shaft-based direct reduction processes utilize pelletized iron ore as the feedstock and produce a porous crude iron product known as sponge iron or direct reduced iron (DRI). Instead of using carbonaceous reductant gases, such as natural gas, as in present commercial direct reduction processes, HYBRIT proposes using hydrogen gas as the reductant, termed hydrogen direct reduction (H-DR). The hydrogen gas may be produced by electrolysis of water using mainly fossil-free and/or renewable primary energy sources. Thus, the critical step of reducing the iron ore may be achieved without requiring fossil fuel as an input, and with water as a by-product instead of CO2.

The shaft-based direct reduction process using hydrogen as reductant differs fundamentally from carbon-based DR processes and has not previously been implemented at commercially relevant scales. There remains a need for improved means of controlling such shaft-based direct reduction processes utilizing hydrogen as reductant.

SUMMARY OF THE INVENTION

The HYBRIT initiative has been operating a pilot direct reduction shaft since 2020 where it is possible to produce DRI using either conventional fossil-based methods or hydrogen-based methods in a semi-industrial and commercially relevant scale. Based upon process experience from pilot operations, the inventors of the present invention have observed that shaft-based direct reduction using hydrogen as the reducing gas behaves fundamentally different to direct reduction using conventional fossil reductants. The ultimate consequence of this difference in behaviour is that there is a significant risk of a product being obtained that has a large variation in the properties of individual DRI pellets, unless the process is appropriately controlled. Controlling the process on the basis of measured product properties at the shaft outlet is potentially an option. However, since direct reduction shafts may typically have a throughput of multiple tonnes per hour and since residence time in the shaft may typically be measured in tens of hours, such an approach would risk producing large quantities of off- specification product before the optimal process window is found.

It would be advantageous to achieve a means of overcoming, or at least alleviating, at least some of the above mentioned drawbacks. In particular, it would be desirable to enable a means of controlling a hydrogen-based direct reduction shaft process in order to rapidly enter a suitable process window without excessive production of off-specification product. Such a means would be particularly valuable in situations where the process has been perturbed and is not yet operating under steady-state conditions within a suitable process window, for example during process start-up, production ramp-up or when the properties of the iron ore input to the process have been altered.

The objects of the invention are achieved by a process according to the appended independent claim. The process for the production of sponge iron from iron ore using a direct reduction shaft utilizes a reducing gas comprising greater than 85 vol% hydrogen gas, and comprises the following steps:

- charging iron ore into the direct reduction shaft at a charge rate m^ore;

- introducing reducing gas at a reducing gas temperature T^RG and a flow rate Q^RG into the direct reduction shaft at a reducing gas inlet arranged at a lower end of a reducing zone of the direct reduction shaft;

- removing a top gas from the direct reduction shaft at a top gas outlet;

- removing sponge iron from the direct reduction shaft at a discharge rate m^DRI _;

- determining a first temperature Treasured in the direct reduction shaft at a first temperature determination point P¹; the first temperature determination point P¹ being situated in the reducing zone of the direct reduction shaft; and

- controlling the process based at least upon the determined first temperature Treasured-

It has been found that the temperatures ensuing in the reducing zone of the direct reduction shaft are reliable determinants of whether the process is operating within a suitable window in order to obtain a product with high and even quality, as determined by product metallisation. The temperatures prevailing in this part of the direct reduction shaft have been found to be highly sensitive to whether the process is operating with a suitable process window or not when using hydrogen gas as the reductant, in contrast to conventional fossilbased reductants where a more even temperature is obtained throughout the entirety of the reducing zone. Without wishing to be bound by theory, it is thought that this may be due at least in part to the endothermic nature of the reduction of iron oxides with hydrogen, as well as differences in reaction kinetics between hydrogen and carbon monoxide as reductant. Moreover, it has been found that the temperatures prevailing in the reducing zone of the direct reduction shaft are more indicative of whether the process is operating in a suitable process window than temperatures measured elsewhere and otherwise typically used to control direct reduction processes, such as the temperature of the ingoing reducing gas or the top gas temperature.

The step of controlling the process based at least upon the determined first temperature T'measured may comprise the steps of

- comparing the determined first temperature T^measured with a predetermined first lower temperature limit T^lim lower and

- if the determined first temperature Treasured is less than the first lower temperature limit T¹iimjower then controlling the process to increase the temperature determined at P¹ until the determined first temperature Treasured is greater than the first lower temperature limit

T I ¹! h-mj Iower-

It has been found that ensuring that the temperature measured at a certain point in the reducing zone of the direct reduction shaft exceeds a critical temperature is a simple and reliable means of ascertaining whether the process is operating within a suitable process window.

The point P¹ for determining the temperature may be situated at a point in proximity to the wall of the direct reduction shaft. Determining the temperature at the wall of the shaft furnace, or in close proximity to the wall, provides a robust, simple implementation as compared to measuring the temperature at other positions in the shaft, such as more centrally. Moreover, the point P¹ for determining the temperature may be arranged a distance of from about 50% to about 90% of the length of the reducing zone from the reducing gas inlet. The length of the reducing zone is defined as extending from the reducing gas inlet to a top end of the reducing zone of the direct reduction shaft, i.e. to the normal burden level of the shaft. It has been found that determination of the temperature at such an intermediate position in the reducing zone provides maximum differentiation between satisfactory and sub- optimal process conditions.

The point P¹ for determining the temperature may be situated at a point in proximity to the wall of the direct reduction shaft, at a point below the normal burden level of the shaft, i.e. at a point between the normal burden level and the reduction gas inlet. For example, the point P¹ may be situated at a point immediately below the normal burden level of the shaft, such as at a point 95% of the length of the reducing zone from the reducing gas inlet. In such a case, T^lirnjower may be at least 550 °C.

For example, the point P¹ may be situated at a distance of about 78% from the reducing gas inlet. In such a case, T¹ii_mjower may be at least 630 °C, preferably at least 645 °C. By "at least" it is meant that the temperature limit may be set higher.

The process may further comprise the following further steps:

- determining a second temperature Treasured in the direct reduction shaft at a second temperature determination point P²; the second temperature determination point P² being situated in the reducing zone at a point higher or lower than the first temperature determination point P¹; and

- controlling the process based at least upon the determined first temperature Treasured and the determined second temperature T²measured-

Determining the temperature at multiple points in the reducing zone and controlling the process on the basis thereof may provide a further degree of certainty as to whether the process is operating within a suitable window or whether the process parameters should be adjusted to enter such a suitable window.

The point P² may be situated at a point in proximity to the wall of the direct reduction shaft.

The point P² may be situated above P¹, at a distance of from about 80% to about 99% of the length of the reducing zone from the reducing gas inlet. In such a case, the determined second temperature T²measured may be compared with a predetermined second lower temperature limit T^hmjower and if T^measured is less than the T²|im_lower then the process may be controlled to increase the temperature determined at P² until the determined second temperature T²measured is greater than the second lower temperature limit T²|im_lower-

For example, the point P² may be situated at a distance of about 95% from the reducing gas inlet. In such a case, T²ii_m_iower may be at least 550 °C.

Alternatively, the point P² may be situated below P¹, at a distance of from about 25%to about may be controlled based at least upon a temperature drop ATdetermined determined between P² and P¹, calculated as T²measured " T^measured- Also in such a case, the point P¹ may be situated at a distance of from about 50% to about 90% of the length of the reducing zone from the reducing gas inlet. It has been found that the temperature drop over this section of the reducing zone may be particularly indicative of whether the process is operating within a satisfactory window or not, with an excessive temperature drop in this region being indicative of the process being operated outside of the optimal window.

A parameter AT_rei may be calculated as ATdetermined relative to the total temperature drop in the reducing zone. That is to say AT_rei = (Treasured ■ Treasured) / (Tlower end of reducing zone " Ttop end of reducing zone). If the parameter AT_rei is greater than an upper limit value AT_rei_iim_upper, then the process may be controlled to decrease AT_rei until AT_rei is less than AT_rei_iim_upper.

For example, P¹ may be situated at a distance of about 78% from the reducing gas inlet, P² may be situated at a distance of about 47% from the reducing gas inlet, and in such a case ATrel lim .upper may be about 0.4 or less.

The process may be controlled to increase T^measured, to increase Treasured, and/or to decrease ATrei by increasing the reducing gas temperature T^RG.

The process may be controlled to increase T^measured, to increase Treasured, and/or to decrease ATrei by increasing the reducing gas flow rate Q^RG.

The process may be controlled to increase T^measured, to increase Treasured, and/or to decrease ATrei by decreasing the discharge rate from the direct reduction shaft m^DRI. In such a case, the charge rate m^ore to the direct reduction shaft may also be decreased in order to maintain the burden level and avoid overfilling.

According to another aspect, the objects of the invention are obtained by a system according to the appended independent claim. The system for the production of sponge iron from iron ore comprises:

- a direct reduction shaft comprising a reducing zone, the reducing zone comprising a reducing gas inlet arranged at its lower end;

- a source of hydrogen gas arranged in fluid communication with the reducing gas inlet; - a temperature determination device for determining temperature at a point P¹ situated in the reducing zone of the direct reduction shaft; and

- a control device arranged to control a process for production of sponge iron based at least upon an output of the temperature determination device.

The temperature determination device may comprise a thermocouple, such as a type K thermocouple or a type S thermocouple. Such thermocouples are cost-effective and relatively robust under the prevailing process conditions.

The temperature determination device may be arranged in a wall of the direct reduction shaft at a distance of from about 50% to about 90% of the length of the reducing zone from the reducing gas inlet.

According to a further aspect, the objects of the invention are obtained by a bulk sponge iron product according to the appended independent claims. The bulk sponge iron product comprises sponge iron pellets, wherein the sponge iron pellets are essentially free of carbon, have an average metallization of greater than or equal to 97%, and wherein the standard deviation in metallization is less than 1.5 %.

Further objects, advantages and novel features of the present invention will become apparent to one skilled in the art from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the present invention and further objects and advantages of it, the detailed description set out below should be read together with the accompanying drawings, in which the same reference notations denote similar items in the various diagrams, and in which:

Fig 1 schematically illustrates a system for the production of sponge iron according to an exemplifying embodiment of the present invention;

Fig 2 is a flowchart illustrating a process according to an exemplifying embodiment of the present invention; Fig 3 is a graph illustrating in-shaft temperature profiles of various operational states pre-quench;

Fig 4a is a contour plot illustrating the degree of metallisation in observed by excavation of the direct reduction shaft after quench of operational state K4;

Fig 4b is a contour plot illustrating the degree of metallisation in observed by excavation of the direct reduction shaft after quench of operational state K3;

Fig 4c is a contour plot illustrating the degree of metallisation in observed by excavation of the direct reduction shaft after quench of operational state K2; and

Fig 5 is a graph illustrating the in-shaft temperature profile of various process points.

DETAILED DESCRIPTION

The present invention is based upon unique insights into shaft-based hydrogen direct reduction processes obtained during operation of the HYBRIT pilot direct reduction shaft. It has been found that when using hydrogen as the reducing gas, the measurement of the temperature prevailing in the reducing zone of the direct reduction shaft is a particularly sensitive means of determining whether the shaft is operating under satisfactory or sub- optimal conditions. During satisfactory operation, a relative smooth temperature curve is observed, whereas during non-satisfactory operation an inflection point may be observed in the temperature curve, resulting in the temperature at a point in the reducing zone decreasing below a critical value. If the shaft is operated under such non-satisfactory conditions, a DRI product having widely varying degree of metallisation may be obtained. This is thought to be because the pellets in proximity to the wall of the shaft will still be near-fully reduced, but pellets at the core of the shaft will be reduced to a much lesser degree. Temperatures measured at more conventional points in the process, such as top gas temperature and reducing gas inlet temperature have been found to be less reliable indicators of whether the process is operating in a satisfactory window or not.

The observed effects for hydrogen direct reduction also stand in contrast to the temperatures observed in the reducing zone during conventional direct reduction using a fossil-based reducing gas comprising methane, carbon monoxide and hydrogen. In the conventional process, the temperature curve along the reducing zone is somewhat flatter, and this process has correspondingly been found to be less at risk of producing DRI product having widely varying metallisation.

Without wishing to be bound by theory, it is thought that such differences in process behaviour are due in part to differences in the thermodynamics and kinetics between the different reductants. The reduction reactions of iron oxides using hydrogen to ultimately provide metallic iron are mostly endothermic, as illustrated below.

6Fe2O3 + 2H2 -> 4Fe3O4 + 2H2O +32.7 kJ/mol

2Fe3O4 + 2H2 -> 6FeO + 2H2O +127.6 kJ/mol

6FeO + 6H2 -> 6Fe + 6H2O +171.4 kJ/mol

2Fe3O4 + 8H2 -> 6Fe + 8H2O +299.0 kJ/mol

Thus, the process gas in the direct reduction shaft will cool as the reduction proceeds, until eventually, the lowered temperature will lead to unfavourable kinetics and the reduction reaction will falter.

In contrast, the reduction and carburization reactions that carbonaceous reductants undergo are mostly exothermic, as illustrated below.

Reduction

3Fe2O3 (hematite) + CO 2Fe3O4 + CO2 -24.9 kJ/mol

2Fe3O4 (magnetite)

+45.3 kJ/mol

6FeO (wustite)

-75.8 kJ/mol

Carburization (graphite production)

2CO CO2 + C -173.7 kJ/mol

CO + H2 ^ C + H2O -131.8 kJ/mol

CH4 ^ C + 2H2 +74.5 kJ/mol Carburization (cementite production)

3Fe + CH4 Fe3C +2H2 +98.3 kJ/mol

3Fe +2CO Fe3C + CO2 -148.8 kJ/mol

3Fe + CO + H2 ^ Fe3C + H2O -107.6 kJ/mol

Thus, the reduction using such carbonaceous reductants will be less prone to temperature decreases and will have a lesser tendency to falter.

The invention will now be described in more detail with reference to certain exemplifying embodiments and the drawings. However, the invention is not limited to the exemplifying embodiments discussed herein and/or shown in the drawings, but may be varied within the scope of the appended claims. Furthermore, the drawings shall not be considered drawn to scale as some features may be exaggerated in order to more clearly illustrate certain features.

Direct reduction

A suitable system for the production of sponge iron 101 from iron ore 103 is illustrated in Figure 1 and comprises:

- a direct reduction shaft 105 comprising a reducing zone 107, the reducing zone comprising a reducing gas inlet 109 arranged at its lower end;

- a source of hydrogen gas 111 arranged in fluid communication with the reducing gas inlet 109;

- a temperature determination device 113 for determining temperature at a point P¹ situated in the reducing zone 107 of the direct reduction shaft; and

- a control device 115 arranged to control a process for production of sponge iron based at least upon an output of the temperature determination device 113.

The direct reduction shaft 105 may be of any kind commonly known in the art. By shaft, it is meant a solid-gas countercurrent moving bed reactor, whereby a burden of iron ore 103 is charged at an inlet 117 at the top of the reactor and descends by gravity towards an outlet arranged 119 at the bottom of the reactor. The level of the bottom of inlet 117 determines the normal burden level. Since the flow of material proceeds by gravity and the vertical orientation of the shaft is a given, use of terms of orientation with reference to the shaft, such as "top", "bottom", "above", "below", and so forth, are well established in the art and well understood by the skilled person.

A heated reducing gas 121, also known as "bustle gas", is introduced in order to reduce the iron ore burden. The reducing gas 121 is introduced into a reducing gas inlet 109 arranged at the lower end of the reducing zone, flows mainly upwards counter-current to the burden and exits the shaft as top gas 123 at a top gas outlet 124 at an upper end of the shaft. Reduction is typically performed at inlet temperatures of from about 750 °C to about 1000 °C. The temperatures required are typically maintained by heating the reducing gas introduced into the reactor, for example using a heater 125, such as an electric heater. Further heating of the gases may be obtained after leaving the heater and prior to introduction into the reactor by exothermic partial oxidation of the gases with oxygen or air (not shown). Reduction may be performed at a pressure of from about 1 Bar to about 10 Bar in the DR shaft, preferably from about 3 Bar to about 8 Bar.

The iron ore burden typically consists predominantly of iron ore pellets, although some lump iron ore may also be introduced. The iron ore pellets typically comprise mostly hematite, together with further additives or impurities such as gangue, fluxes and binders. However, the pellets may comprise some other metals and other ores such as magnetite. Iron ore pellets specified for direct reduction processes are commercially available, and such pellets may be used in the present process. Alternatively, the pellets may be specially adapted for hydrogen direct reduction.

The top (spent) gas 123 from the DR shaft is at least partially recycled, whereby it may be cleaned and treated to remove by-products such as water and/or fines prior to re-introduction to the DR shaft (illustrated as treatment arrangement 127). This recycled top gas 129 may be mixed with fresh reducing gas 131, known as "make-up gas" prior to reintroduction, or may be introduced separately from any fresh make-up gas supply. The reducing gas may consist essentially of reducing make-up gas and recycled top gas.

In contrast to present-day commercial direct reduction processes, the make-up gas 131 used to replenish the reducing gas comprises little or essentially no carbonaceous substances. The make-up gas may for example comprise, consist essentially of, or consist of, hydrogen. For example, the make-up gas may comprise, consist essentially of, or consist of at least 85 vol%, preferably greater than 90 vol%, even more preferably greater than 95 vol% hydrogen gas (vol% determined at normal conditions of 1 atm and 0 °C).

A cooling gas 133 from a cooling gas source 134 may be provided to a cooling zone 135 of the shaft in order to cool the DRI after reduction and prior to discharge. The cooling zone 135 is typically arranged at a lower end of the direct reduction shaft. Suitable cooling gases may include, for example, nitrogen, hydrogen or a combination thereof if a carbon-free DRI is to be produced, or natural gas (diluted as appropriate) if a carbon-containing DRI is to be produced. Typically, cooling gas may be provided at an inlet 137 arranged at the lower end of the cooling zone and may be removed from the shaft via an outlet 139 arranged at the upper end of the cooling zone.

If the cooling gas 133 consists mainly of hydrogen, a proportion or all of the cooling gas may be allowed to proceed upwards in the shaft into the reducing zone, where it will form a proportion of the reducing gas. In some cases, where essentially all cooling gas is allowed to proceed upwards in this manner, there may be no need for a cooling gas outlet and/or a cooling gas treatment circuit.

In some cases, no cooling gas is circulated in the cooling zone 135 and the hot DRI is instead discharged to a separate shaft where it is cooled and optionally carburized using a circulating gas. Such a separate shaft arrangement is disclosed in W02021/225500 Al, which is hereby incorporated by reference.

In order to permit control of the direct reduction process according to the present disclosure, the direct reduction shaft is equipped with a temperature determination device 113 for determining temperature at a point P¹ situated in the reducing zone of the direct reduction shaft. Point P¹ may for example be situated at a point in proximity to the wall of the direct reduction shaft at a distance of from about 50% to about 90% of the length of the reducing zone from the reducing gas inlet. The length L of the reducing zone is defined as extending from the reducing gas inlet 109 to a top end of the reducing zone 107 of the direct reduction shaft. P¹ may for example be situated at a distance of about 78% from the reducing gas inlet. The shaft may optionally be equipped with a second temperature determination device (not illustrated) for determining temperature at a point P² situated in the reducing zone of the direct reduction shaft either above or below the point P¹. P² may for example be situated above P¹, at a distance of from about 80% to about 99% of the length of the reducing zone from the reducing gas inlet, such as a distance of about 95%. Alternatively, P² may be situated below P¹, at a distance of from about 25% to about 50% of the length of the reducing zone from the reducing gas inlet, such as at a distance of about 45% from the reducing gas inlet. Naturally, the shaft may be equipped with further such devices for measuring temperature, and for example, the temperature may be determined at points in the reducing zone both above and below the point P¹.

The temperature determination device may be any device capable of determining the temperature prevailing at the point P¹ and/or P² to a reasonable degree. The temperature measurement device may be arranged to directly measure the temperature at point P¹ and/or P². In such a case, the temperature measurement device may comprise a thermocouple, such as a type K thermocouple or a type S thermocouple. However, indirect measurements of the temperature at point P¹ and/or P² are also envisaged, using for example a probe arranged within the refractory of the shaft or outside of the shaft refractory.

Control of the direct reduction process

In order to ascertain whether the hydrogen direct reduction is operating within a suitable window and to correct the process parameters to reach a suitable process window in the event that the process is determined to be presently outside of a suitable process window, the following process may be used, as illustrated in Figure 2.

Step s201 denotes the start of the process. In step s203 iron ore is charged into the direct reduction shaft at a charge rate m^ore. In step s205 reducing gas is at the reducing gas inlet at a reducing gas temperature T^RG and a flow rate Q^RG. In step s207 a top gas is removed from the direct reduction shaft at a top gas outlet. In step s209 sponge iron is removed from the direct reduction shaft at a discharge rate m^DRI. All of the steps listed hereto are conventional within the art for operation of a direct reduction shaft. In step s211 a first temperature Treasured is determined in the direct reduction shaft at the first temperature determination point P¹. As described above, P¹ is situated in the reducing zone of the direct reduction shaft, preferably at a point in proximity to the wall of the direct reduction shaft at a distance of from about 50% to about 90% of the length of the reducing zone from the reducing gas inlet. In step s213, the process is controlled based at least upon the determined first temperature "remeasured- Step s215 denotes the end of the process.

Step s213 may be conceptualised as involving at least two separate sub-steps. Sub-step s213a involves comparing a value derived from at least T^measured with at least one predetermined limit, in to determine whether the process is operating within a satisfactory process window or not. Sub-step s213b involves taking appropriate action to bring the process to operation within a suitable window if the process is found to be operating outside of a suitable window.

A number of variants of sub-step s213a are envisaged.

Sup-step s213a may involve comparing f rneasured with a predetermined first lower temperature limit T^lim lower- If T^measured is leSS than T^lirnjower then the process may be controlled in order to increase the temperature determined at P¹ until Treasured is greater than T¹iimjower.

A second temperature "^measured may be determined in the direct reduction shaft at a second temperature determination point P² as described above. In such a case, step s213 may involve controlling the process based at least upon the determined first temperature "f rneasured and the determined second temperature T²measured-

In the case that the second temperature Treasured is determined at a point P² above P¹, substep s213a may involve comparing T²measured with a predetermined second lower temperature limit T²|im_lower. lf T²measured is leSS than T²|im_lower then the process may be controlled to increase the temperature determined at P² until Treasured is greater than T²ii_m_iower.

In the case that the second temperature Treasured is determined at a point P² below P¹, substep s213a may involve controlling the process based at least upon a temperature drop ATdetermined determined between P² and P¹, calculated as T²measured " T^measured- This may involve determining a parameter AT_rei, which is calculated as ATdetermined relative to the total temperature drop in the reducing zone. If AT_rei is greater than an upper limit value AT_rei_iim_upper, then the process may be controlled to decrease AT_rei until AT_rei is less than ATrel lim upper- The predetermined limits T^lim lower, T^lim lower and ATrei iim upper may each be utilized, in isolation or in any combination, to avoid the process being operated with insufficient energy in the process gas to ensure complete reduction across the entire cross-section of the shaft, i.e. to avoid the shaft being run too "cool". However, complementary parameters T^lim upper, T^lim upper and ATrei iim upper may also be predefined and used, in isolation or in any combination, to avoid operating the shaft with excessive energy in the process gas, i.e. avoid running the process too "hot". Such limits may be used to improve energy effectivity and/or avoid problems typically associated with excessively high shaft temperature, such as clustering and/or excessive wear of equipment.

Sub-step s213b involves taking appropriate action to bring the process to operation within a suitable window if the process is found to be operating outside of a suitable window. This may generally involve controlling one or more of the ingoing process parameters reducing gas temperature T^RG, reducing gas flow rate Q^RG, discharge rate from the direct reduction shaft m^DRI, and/or charge rate to the direct reduction shaft m^ore in order to bring the process into a suitable window. If the process is operating with insufficient energy in the process gas, it is appropriate to increase T^measured, to increase Treasured, and/or to decrease AT_rei. This may be done by increasing the reducing gas temperature T^RG, increasing the reducing gas flow rate Q^RG, and/or decreasing the discharge rate from the direct reduction shaft m^DRI. If the discharge rate m^DRI is decreased, it may also be appropriate to decrease the charge rate m^ore to the direct reduction shaft, in order to maintain a steady burden level.

On the contrary, if the process is operating with excessive energy in the process gas, it is appropriate to decrease Treasured, to decrease Treasured, and/or to increase AT_rei. This may be done by decreasing the reducing gas temperature T^RG, decreasing the reducing gas flow rate Q^RG, and/or increasing the discharge rate from the direct reduction shaft m^DRI. If the discharge rate m^DRI is increased, it may also be appropriate to increase the charge rate m^ore to the direct reduction shaft, in order to maintain a steady burden level.

Other ingoing parameters than the ones mentioned above may be used to control the process. For example, the proportion of recycled gas or amount of cooling gas permitted to enter the reducing zone may also be suitably controlled in order to bring the process within a suitable window or maintain the process within a suitable window.

Maintaining the hydrogen direct reduction process within a suitable process window permits the production of a DRI having a high and uniform degree of metallization. For example, the process may permit the production of a bulk sponge iron (DRI) product comprising sponge iron pellets, wherein the sponge iron pellets are essentially free of carbon, have an average metallization of greater than or equal to 97%, and wherein the standard deviation in metallization is less than 1.5 %. The average metallization may be greater than or equal to 98%, or greater than or equal to 99%. The standard deviation in metallization may be less than 1 %, or less than 0.5 %.

Metallization is defined in a manner conventional within the art as (Fe_metaiiic / Fe_totai) x 100. Metallization was determined using X-ray diffractometry (XRD), but may also be determined using other methods. Where several methods are in conventional use for determining a single property, variations in the determined property are typically within the limits of experimental error.

Such other methods for determination of metallization include:

ISO 2597-1:2006 (Iron ores — Determination of total iron content — Part 1: Titrimetric method after tin(ll) chloride reduction) in combination with ISO 5416:2006 (Direct reduced iron — Determination of metallic iron — Bromine-methanol titrimetric method); and

ISO 10276-1:2000 (Chemical analysis of ferrous materials — Determination of oxygen in steel and iron Part 1: Sampling and preparation of steel samples for oxygen determination) in combination with ISO 10276-2:2003 (Chemical analysis of ferrous materials — Determination of oxygen content in steel and iron — Part 2: Infrared method after fusion under inert gas).

Unless otherwise stated, all DRI samples produced and tested were produced at the Hybrit pilot direct reduction facility in Lulea. In brief, the pilot facility comprises a direct reduction shaft having a total height of approximately 9.3 meters, a widest diameter of approximately 1.22 meters and a total volume of approximately 7.6 cubic meters. Considering only the section of the shaft constituting the reducing zone, this zone has a height of approximately 3.1 meters from reducing gas inlet to normal burden level (approximately 4 meters from reducing gas inlet to upper flange), and a diameter of approximately 0.94 m. The shaft is of a conventional design. That is to say that it is a solid-gas countercurrent moving bed reactor, whereby a burden of iron ore is charged at an inlet at the top of the reactor and descends by gravity towards an outlet arranged at the bottom of the reactor. Commercially available KPRS direct reduction pellets from LKAB were used as the iron ore burden in all studies described herein. However, the same or similar results as described herein may be obtained using any suitable iron ore pellets as the starting material. The DR shaft comprises a reducing zone, an isobaric (transition) zone, and a conical cooling zone tapering towards an outlet of the DR shaft. The shaft has a nominal production capacity of approximately 1 ton DRI/h, although this may be varied as shown in the studies below. The operational pressure in the reactor may be varied up to about 4 barg.

Study 1 - Excavation studies

In order to investigate the properties of pel lets/DRI at various points when passing through the direct reduction shaft, an excavation was performed. This involved operating the DR shaft at a chosen stable state for a determined period of time, followed by quenching the shaft to halt reduction and subsequent excavation to retrieve samples at varying depths within the shaft. The pilot shaft is equipped with thermocouples spaced alone the shaft wall at regular intervals permitting monitoring of the temperature prevailing at various levels in the shaft. Note that the depth of the thermocouple is provided relative to the top flange of the shaft. The normal burden level in the shaft is at a depth of approximately 0.9 m relative to this top flange, meaning that for example a thermocouple located at approximately 1 m depth is immediately below the normal burden level.

Three different operational states were investigated: an operational state using conventional natural gas as reductant (K2), an operational state using hydrogen outside of the optimal process window (termed K3) and an operational state using hydrogen within the optimal process window (termed K4). The in-shaft temperature profile measured for each of these operational states is shown in Figure 3. It can be seen from Figure 3 that the temperatures observed within the shaft during conventional fossil operation (K2) show greater uniformity, with a relative plateau in temperature being observed between approximately 1 m depth and 3 m depth in the shaft. In contrast, the hydrogen direct reduction studies (K3 and K4) show relatively smooth temperature curves, with the temperature decreasing on going from the reducing gas inlet to the top gas outlet. It can be seen that the operational state operating outside of the optimum process window, K3, generally shows lower temperatures in the shaft as compared to K4, but has a higher top gas temperature.

Quench of the various operational states was performed using nitrogen gas. Once the reactor was quenched and cooled, excavation was performed. An excavation consists mainly of sampling along the shaft in both radial and vertical direction descending down into the shaft. The target layer-thickness in the reduction zone was set to 150mm, with thicker layers in the isobaric and cooling zones. 13 layer-samples were taken out for each layer. Each layer-sample weighed approximately 1200g. A contour plot showing the degree of metallization relative to location in shaft is shown in Figures 4a (K4, H2 satisfactory process window), 4b (K3, H2 non- satisfactory process window) and 4c (K2, natural gas direct reduction). Both K3 and K4 were found to have near-complete metallization close to the wall of the reactor. However, in K3, operating outside of a suitable process window, it was found that the central core material in the reactor was reduced to a lesser extent, leading to large variation in product quality at the reactor outlet, despite the product having a relatively high average metallization (95%). K4 had near-complete reduction across the full diameter of the shaft and the resulting product had extremely high metallization (99%) with low deviation. The natural-gas based reference, K2, showed a relatively uniform metallization across the diameter of the shaft, but the metallization was much lower as compared to the hydrogen direct reduction, having an average metallization of approximately 89%.

From the excavation experiments, it could be concluded that even if a hydrogen direct reduction operational state may appear to provide a high-quality DRI product with high average metallization, there is a risk that the product could comprise a proportion of relatively less reduced material. It was further concluded that in-shaft monitoring of the temperature could potentially be used to distinguish between satisfactory and non-satisfactory operational states, but that top gas temperature was an unreliable indicator. Further, it was observed that conventional natural gas direct reduction has a more uniform temperature along the reducing zone and most likely for this reason is not subject to the same problem of variation in product quality across the shaft diameter. However, the product obtained from the conventional process has a much lower average degree of metallization.

Study 2 - Systematic variation of process parameters

In order to more fully investigate the process window for hydrogen direct reduction, independent process variables such as reducing gas inlet temperature, reducing gas flow and production rate (charge and discharge rate) were systematically investigated. The resulting temperature profiles in the direct reduction shaft are shown in Figure 5.

Process points PP6_1, PP6_2 and PP6_3 all had the same inlet reducing gas temperature (780 °C) and reducing gas flow (2900 Nm³/h), and differed only in production rate, with the burden in PP6_1 having a relative shaft residence time of 0.94, PP6_2 having a relative shaft residence time of 1.00 and PP6_3 having a relative shaft residence time of 1.09. Only PP6_3 resulted in a DRI product having a high and uniform degree of metallization (98%). PP6_1 and PP6_2 both resulted in metallization of less than 90%, and with a large standard deviation.

Looking at the in-shaft temperature profiles for these three process points, it can be seen that the observed temperatures are very similar at the lower end of the reducing zone close to the reducing gas inlet (RG

N9) and are also very similar at the top gas outlet (TG). The main points of differentiation are observed in the mid- to upper-reducing zone (N 5

N 3 ), and particularly mid-reducing zone (N5, which is positioned at the wall of the shaft at approximately 78% of the distance between the reducing gas inlet and the normal burden level of the shaft). PP6_1 has an N5 temperature of 611 °C, PP6_2 has an N5 temperature of 577 °C and PP6_3 has an N5 temperature of 645 °C. It can be seen that the non-satisfactory process points PP6_1 and PP6-2 demonstrate a relatively large drop in temperature between the N9 point (distance of approx. 47% from the RG inlet) and N5 point (distance of approx. 78% from the RG inlet) in the shaft, leading to a kink or inflection point in the temperature curve. Thus, a further means of differentiating the various process points is by comparing the temperature drop between the N9 and N5 positions (N9-N5), relative to the temperature drop over the entire reducing zone (approximated by N12-N3, N12 having a distance of approx. 13% from the RG inlet and N3 having a distance of approx. 96% from the RG inlet). It is found in this manner that satisfactory process points should typically have a relative temperature drop ((N9-N5)/(N12-N3)) of less than 0.4.

Thus, it can be concluded that transitioning from PP6_1 to PP6_3 by lowering the production rate brings the process into a satisfactory process window, and that this transition into the satisfactory window can be detected in the in-shaft temperature profile, and in particular at the N5 position in the shaft, long before any effect on the product quality will be detectable at the shaft outlet.

Likewise, a comparison can be made between process points PP6_1 and PP7_1. Both process points have very similar production rates, but differ in reducing gas inlet temperature (PP6_1: 780 °C, PP7_1: 820 °C) and reducing gas flow rate (PP7_1 has an RG flow approximately 95% that of PP6_1). In contrast to PP6_1, PP7_1 provides a DRI product having a high and uniform metallization (average 98%). This transition from non-satisfactory to satisfactory process point can be observed in the in-shaft temperature profile, where PP7_1 shows a smooth temperature curve with an N5 temperature of 650 °C. Thus, the temperature profile, and in particular the N5 temperature, indicates that the increase in reducing gas temperature on going from PP6_1 to PP7_1 more than compensates for the decrease in reducing gas flow, and this is borne out in the relative qualities of the resulting products.

In this manner, it can be seen that the temperature profile in the reducing zone, and in particular the N5 position temperature measurement can be used to determine whether the process is operating in a satisfactory process window, with process points showing an N5 temperature lower than 630 °C providing poorer quality DRI, and process points showing an N5 temperature greater than or equal to 630 °C, and in particular greater than or equal to 645 °C, providing high quality (high, uniform metallization) DRI. Note that the exact critical temperature varies with regard to measurement position and may vary with respect to further parameters such as shaft geometry or ingoing pellet properties (e.g. moisture). Therefore, the exact critical temperature may need to be determined for each shaft on a recurring basis.

Study 3 - Modelling studies

In order to ascertain whether the results obtained in pilot scale could be extrapolated to full- scale direct reduction shafts, modelling studies were performed. Models were calibrated using results obtained from operation and excavations at the pilot shaft and then used to investigate the expected performance of full-scale shafts. A series of operational states were modelled, varying a range of operational parameters systematically and ranging from non- satisfactory states with large deviation in metallization to states with uniform, near-complete metallization.

It was found in all modelled states that the material closest to the wall of the shaft is always more-or-less fully reduced, but that in states with lower gas energy (low temperature and/or low flowrate) a non-reduced core column of burden persists throughout the reducing zone and all of the way to the outlet of the shaft, in a similar manner as to Figure 4b. It was also seen that the temperature persisting in any given point in the reducing zone correlates reasonably closely to the degree of reduction at that point, with cool spots correlating to unreduced material.

With regard to features able to be used to differentiate between satisfactory and non- satisfactory states, it was seen that theoretically, temperature measurement at a point on the central axis of the shaft somewhere above the reducing gas inlet would be optimal, since such a point is subjected to a large range of temperatures upon transitioning from a sub- satisfactory to a satisfactory operational state. However, placement of a temperature probe in such a position would be highly impractical. Temperature measurement at the wall of the shaft mid-reducing zone is a reasonable compromise in this regard, since such temperature measurement is relatively easy to implement and such a point is subject to a relatively large temperature range upon transitioning from non-satisfactory to satisfactory states. In this regard, temperature measurement at the wall of the shaft at a lower position closer to the reducing gas inlet, or temperature measurement at a higher point closer to the upper end of the shaft are both inferior since such points are found to be subject to a narrower temperature range upon the transition from sub-satisfactory to satisfactory states.

The modelling studies indicate that the means of controlling a hydrogen direct reduction process as disclosed herein are applicable to full-scale direct reduction shafts.

Claims

1. A process for the production of sponge iron from iron ore using a direct reduction shaft, wherein the process utilizes a reducing gas comprising greater than 85 vol% hydrogen gas, and wherein the process comprises the steps of charging iron ore into the direct reduction shaft at a charge rate m^ore (s203); introducing reducing gas at a reducing gas temperature T^RG and a flow rate Q^RG into the direct reduction shaft at a reducing gas inlet arranged at a lower end of a reducing zone of the direct reduction shaft (s205); removing a top gas from the direct reduction shaft at a top gas outlet (s207); removing sponge iron from the direct reduction shaft at a discharge rate m^DRI (s209)_; wherein the process comprises the further steps of determining a first temperature Treasured in the direct reduction shaft at a first temperature determination point P¹; the first temperature determination point P¹ being situated in the reducing zone of the direct reduction shaft (s211); and controlling the process based at least upon the determined first temperature "remeasured (s213).

2. The process according to claim 1, wherein the step of controlling the process based at least upon the determined first temperature Treasured comprises the steps of comparing the determined first temperature T^measured with a predetermined first lower temperature limit T^lim lower and if the determined first temperature Treasured is less than the first lower temperature limit T¹iimjower then controlling the process to increase the temperature determined at P¹ until the determined first temperature Treasured is greater than the first lower temperature limit T^lim lower-

3. The process according to any one of the preceding claims, wherein P¹ is situated at a point in proximity to the wall of the direct reduction shaft at a distance of from about 50% to about 90% of the length of the reducing zone from the reducing gas inlet, wherein the length of the reducing zone is defined as extending from the reducing gas inlet to a top end of the reducing zone of the direct reduction shaft.

4. The process according to claims 2 and 3, wherein P¹ is situated at a distance of about 78% from the reducing gas inlet, and wherein T¹ii_mjower is at least 630 °C, preferably at least 645 °C.

5. The process according to any one of the preceding claims, further comprising the steps of determining a second temperature Treasured in the direct reduction shaft at a second temperature determination point P²; the second temperature determination point P² being situated in the reducing zone at a point higher or lower than the first temperature determination point P¹; and controlling the process based at least upon the determined first temperature Treasured and the determined second temperature T²measured-

6. The process according to claim 5, wherein

P² is situated above P¹, at a distance of from about 80% to about 99% of the length of the reducing zone from the reducing gas inlet; the determined second temperature Treasured is compared with a predetermined second lower temperature limit T²|im_lower; and if the determined second temperature Treasured is less than the second lowertemperature limit T²iim_iower then the process is controlled to increase the temperature determined at P² until the determined second temperature Treasured is greater than the second lower temperature limit T²|im_lower-

7. The process according to claim 6, wherein P² is situated at a distance of about 95% from the reducing gas inlet, and wherein T²ii_m_iower is at least 550 °C.

8. The process according to claim 5, wherein P² is situated below P¹, at a distance of from about 25% to about 50% of the length of the reducing zone from the reducing gas inlet; and the process is controlled based at least upon a temperature drop ATdetermined determined between P² and P¹, calculated as T²measured " T^measured-

9. The process according to claim 8, wherein if a parameter AT_rei is greater than an upper limit value AT_rei_ Jim_upper, then the process is controlled to decrease AT_rei until AT_rei is less than the upper limit value AT_rei_iim_upper, wherein AT_rei is calculated as ATdetermined relative to the total temperature drop in the reducing zone.

10. The process according to claim 9, wherein P¹ is situated at a distance of about 78% from the reducing gas inlet, P² is situated at a distance of about 47% from the reducing gas inlet, and wherein AT_rei_iim_upper is about 0.4 or less.

11. The process according to any one of the preceding claims, wherein the process is controlled to increase Treasured, to increase Treasured, and/or to decrease AT_rei by increasing the reducing gas temperature T^RG.

12. The process according to any one of the preceding claims, wherein the process is controlled to increase Treasured, to increase Treasured, and/or to decrease AT_rei by increasing the reducing gas flow rate Q^RG.

13. The process according to any one of the preceding claims, wherein the process is controlled to increase Treasured, to increase Treasured, and/or to decrease AT_rei by decreasing the discharge rate from the direct reduction shaft m^DRI, and optionally decreasing the charge rate m^ore to the direct reduction shaft.

14. A system for the production of sponge iron from iron ore, the system comprising: a direct reduction shaft (105) comprising a reducing zone (107), the reducing zone comprising a reducing gas inlet (109) arranged at its lower end; a source of hydrogen gas (111) arranged in fluid communication with the reducing gas inlet (109); a temperature determination device (113) for determining temperature at a point P¹ situated in the reducing zone of the direct reduction shaft; and a control device (115) arranged to control a process for production of sponge iron based at least upon an output of the temperature determination device (113).

15. The system according to claim 14, wherein the temperature determination device comprises a thermocouple, such as a type K thermocouple or a type S thermocouple.

16. The system according to any one of claims 14-15, wherein the temperature determination device is arranged in a wall of the direct reduction shaft at a distance of from about 50% to about 90% of the length of the reducing zone from the reducing gas inlet.

17. A bulk sponge iron product comprising sponge iron pellets, wherein the sponge iron pellets are essentially free of carbon, have an average metallization of greater than or equal to 97%, and wherein the standard deviation in metallization is less than 1.5 %.