Technical Field

The present invention relates to the reduction of metal ores using a reduction gas, specifically a hydrogen containing reduction gas such as methane, syngas or particularly hydrogen. The invention relates too to the reduction of iron ores containing high percentages of ‘volatile’ metals, e.g. zinc, or ‘volatile’ non-metals , e.g. phosphorous minerals, into bare metal, e.g. DRI or HBI.

Terminology

DCR Furnace:

Acronym for Down-draft Co-current Reduction Furnace

Down-draft:

A down-draft reactor is a configuration for a gas - solids reactor where the gas flow within the reactor is directed downwards. This means that the gas supply towards the reaction process is placed higher and above the gas exit for the off-gas from the reaction process.

Co-current:

A co-current reactor is a configuration for a gas - solids reactor where the gas flow and the reacting solids have the same direction of movement within the reactor, more specifically, in the area where the main reactions take place.

Volatile metals:

Volatile metals are those metals which have a boiling point below or not far off from the operational temperature within the specific reactor. In the field of iron ore reduction, relevant volatile metals are arsenic, cadmium and specifically zinc.

DRI:

Acronym for Direct Reduced Iron: Iron made from iron ore by a reducing gas where the ore does not reach the melting point of either ore or metal.

HBI:

Acronym for Hot Briquetted Iron: Briquettes pressed from hot iron lumps or pellets, e.g. those leaving a DRI reduction furnace. Prior Art

Many metal ore or iron ore reduction processes and technologies already exist or are described in the public domain. To describe what this process discriminates, these prior art processes are described along common properties which group those prior art processes or methods in different aspects in which they differ from the down-draft co-current reduction process. In this respect, only reducing gas - based reduction processes are considered (thus not coal based or cokes-based processes).

• DE1003776B discloses a vertical shaft furnace for facilitating endothermic reduction processes for the production of metals. A feed in the shaft is heated via electrical resistance heating. Multiple electrodes are arranged within the shaft and induce alternating or direct currents in the feed, causing heating of the feed. For the reduction reaction hydrogen gas flows counter-current through the shaft.

A disadvantage thereof is that the electrical resistance electrodes are required directly in the reactor shaft, which causes them to be in contact with the feed. In such a position, the electrodes are very fragile and difficult to replace.

Another disadvantage thereof is that the use of electric resistance heating will be very expensive due to the high energy demand.

• GB838067A discloses a process for the endothermic reduction of iron oxide by a reaction with a hydrogen-containing gas while a bed of iron oxide particles is maintained in a mobilized condition. The mobilized iron oxide is heated via pre-heated hydrogen gas that flows through tubes in the bed. Hydrogen is then released from the tubes into the bed in a counter-current manner for reducing the iron oxide.

A disadvantage thereof is that the hydrogen gas must first be preheated, before indirectly heating the iron oxide particles. This way of external heating a gas to heat the reactor will consume a lot of energy.

• Patent US1871848

• There are reactors where the last reduction step (from FeO to Fe) takes place while the Wustite and iron are in a molten, fluid or in a melting state. These so-called melting reactors are the blast furnace, the COREX reactor and Hi-melt and Hisarna reactors.

For the DCR Furnace, the full reduction process takes place in solid state of all metal and metal oxide components in the process. Reactors where the reduction takes place while iron and its ores are in the solid state are so called DRI reactors. • Within most reactors, the iron ore and iron phases move in vertical, downward direction. Some DRI processes, however, have their dominant movement direction in the horizontal plane, like the ITMK3 process, Sidcomet, SL/RN process and Redsmelt NST. A special case is the so-called Primus process.

For the DCR Furnace, there is a single, vertical, gravitational driven downward path of movement for the iron ore in a reactor shaft.

• There are several Iron ore reduction processes where the primary reduction steps take place in a single or in multiple connected fluidized bed reactors. The gas phase moves upward through the ore phase which forms or maintains a fluidized mass where loose particles constantly mix with the gas phase. Examples are the Finmet, the Fior, the Finex, the KDRI and with hydrogen the HYFOR processes.

Within the DCR Furnace reaction shaft, the iron ore moves downward as a solid, permeable mass while gas phase is moving downward as well (at higher velocity). The solids are not fluidized.

• Current art shaft-reactors are up-draft processes: The location where a gas phase is injected into the reaction process is located at a lower level relative to the location where the gas exits the active process phase, the gas(ses) draft upwards through the reactor shaft. Examples are, aside the Blast Furnace, the Midrex, the Hyl and specifically for hydrogen the Hybrit processes.

The DCR process again is a down-draft process where the reaction gas moves downwards through the permeable ore phases while reducing them.

• With gasses moving upwards in most reactor types, and solid components moving downwards, most non-fluidizing shaft type reactors house a so-called counter-current reaction process.

Within the DCRF reactor, the gas moves downwards through the reaction process, in the same direction as the ores move. It is a so called co-current process.

• Further with respect to geometry, most vertical reaction vessels have a height - diameter ratio between 2 and 5, specifically so when the hot reaction zone is considered. Many of those shafts have some form of a pear-shape with a larger shaft diameter in the area where reaction intensity is highest.

The DCR process runs in one or multiple parallel shafts with a height - diameter ratio of between 6 and 12. Furthermore, the reaction shafts of the DCR process are linear in shape with little or no diameter variation over the length of the shaft.

A distinctive aspect of many reduction processes is that, to provide the necessary thermal input to the processes, some of the reductant is combusted by injecting air or oxygen into the process, more particularly into the reducing mass or into the space where the actual reduction reactions of the ore take place.

The DCR process distinguishes itself by an enclosed reaction space without oxygen addition and a thermal input to the process by radiation and conduction through the refractory wall of the reduction shaft.

• Many laboratory scale reduction furnaces are descried in many scientific articles. Some mimic one of the above described large-scale processes, some are used as laboratory set-ups. All those furnaces facilitate badge processes without a continuous flow of the ore - metal mixture through the reactor.

Summary (of the invention)

The invention is claimed in the attached claims.

This invention comprises a process and a possible embodiment for the reduction of metal ores (metal oxides, metal sulfides, metal hydroxides, metal oxides containing slags or any other form) to metal or a high metal-grade product. This invention is particularly aimed at reducing those ores with hydrogen or with a hydrogen containing reduction gas (such as methane or syngas).

Current art processes, when hydrogen is used as an important element in the reduction process, result in a very limited utilization of the hydrogen in the process. With coal-based processes, around 50% of the carbon is fully oxidized, forming CO2. The other 50% of the carbon is half-oxidized, forming CO. With hydrogen, in a counter current process, the utilization can drop to well below 20%. This can be explained with the graphics of figure 9. We see the balance (equilibrium) condition of both CO (carbon monoxide) and CO2 and of H2 (hydrogen) and H2O in the presence of Fe, FeO and Fe ₃ C>4. The graph shows that in case of CO/(CO+CO2), the balance gets better (higher utilization) when temperature of the elements drops (as is the case in a counter current process). In the case of H2/(H2+H2O), the balance shifts to the H2 side which means the utilization of the hydrogen drops when the gas temperature drops before leaving the process.

A second problem which many current art processes share is the enrichment of “semivolatile” elements from the ore, specifically zinc and phosphor minerals in iron ore. Those elements evaporate when coming closer to the reaction zone but condense again in the colder ore at the entry side of a reactor as when gas flow is counter current to the ore flow. This can lead to either clogging of the load inside the reactor or to unwanted enrichment of those elements within the metal output.

This invention and the proposed embodiment aim at solving both problems. The down-draft co-current nature of the process results in a situation where the reduction gas (including any volatile elements from the ore, if present) leave the process where both gas and ore I metal reach their highest temperature within the reduction process. This leads to the best possible utilization of the hydrogen in the reducing gas, and it prevents volatile elements to condense within the to be reduced ore again or to enrich within the metal output.

The DCR-process in the proposed embodiment is based on one or more basically cylindrical reduction shafts within a refractory structure. It is characterized by that both the ore and the reducing agent are loaded from the top or upper part of the shaft. It is a down-draft, co-current process. The metal ore can consist of any substance containing metal oxides, metal sulphides or metal hydroxides, or combinations thereof, that can be reduced thermally, using carbon monoxide, methane, or hydrogen as a reducing agent, to bare metal.

The process is, additionally to the down-draft co-current material flows, characterized by that, within the reduction shafts, nor upstream in the reducing gas, no oxygen is added to the reducing gas and that the necessary thermal input is added, not from the thermal energy in the reducing gas, nor from a chemical reaction inside the shaft but from outside the shaft through the wall of the reduction shaft. This can be either by heat conduction through the wall of the shaft or electromagnetically by electrical currents induced by coils or magnets outside core of the shaft.

An additional characteristic of the process can be that the reduction shaft is surrounded by a refractory structure in which the off gas is led to combustion channels that run, parallel to the reduction shaft(s), within the same refractory structure. This way, the off gas can be used, without in-between loss of thermal energy, to heat up the reducing ore mass by adding oxygen or an oxygen containing gas to the off gas within the combustion channels.

A specific use of this invention can be the reduction of iron ores to DRI metal using hydrogen as a reducing agent.

A second specific use of this invention can be the reduction of zinc containing iron (BOF-) slags to win back as much as possible both the iron and the zinc from those slags.

Detailed description of the invention

The invention is illustrated in more detail in the attached drawings.

Figure 1 represent the basic principles of the process. In this figure, the following items are depicted:

1. Input of the to be reduced material from the upper side of the reactor.

2. An entry gate where the to be reduced material can enter the reaction space without gas exiting the reaction space.

3. Entry of the reduction gas from the upper side of the reactor.

4. The enclosed reduction shaft.

5. A source of heat that can add thermal energy from outside of the reaction space into the reaction space, either by radiation, convection, conduction or a combination thereof.

6. The movement direction of the to be reduced mass, moving slowly downward.

7. The movement direction of the reducing gas, moving (much) faster downward than the to be reduced solids. The number 6 and 7 depict the co-current, down-draft character of the process.

8. The exit of the (partially) utilized deduction gas from the reduction shaft. By applying some form of grating or sieve, or by letting the gas flow upward, or by both measures combined, it is prevented that the solid mass leaves the reduction shaft together with the reducing gas.

9. The exit of the reduced or partially reduced ore. Some form of gated exit either by mechanical slide gates or cut-off valves, or a fluid siphon shall prevent that gas exits the reduction shaft together with the reduced ore.

Figure 2 presents a possible cross section through a reduction shaft, perpendicular to its length axis in a more realistic configuration. In this figure:

10. Is the mass of ore in the form of pellets or porous lumps. Sufficient macro - permeability is important for the gas flow through the shaft. Sufficient micro - permeability (within the pellets and or lumps) is important for the gas to reach all metal oxide, metal sulphide or metal hydroxide molecules.

11. Along the edge of the reduction shaft, the dark rim represents the already reducing c.q. reduced material.

12. Is the refractory structure forming the reduction shaft.

13. Around the primary refractory structure, a layer of thermal insulating refractory material is foreseen.

14. Is a combustion channel around the reduction shaft. In this embodiment, twelve combustion channels are grouped around a single reduction shaft. Within each of the combustion channels, all or a fraction of the non-utilized reduction gas can be combusted by oxygen (or air) injection into the left-over hot reduction gas.

Figure 3 presents a possible cross section through multiple parallel shafts. Multiple shafts are a better way to increase the output in comparison to one large diameter shaft. This is because of the heat conduction towards the centreline of the shafts will take too much time in very large diameter shafts. This slows down the reaction processes and leads to low the plug stream velocities and to little gain in the amount of matter that can be processed per unit of time.

Figure 4 presents a cross section along the length axis of the reduction shaft of a possible embodiment for a single vertical shaft DCR reactor.

In this figure:

The numbers 10 to 14 are identical as the same numbers in figure 2. The other numbers indicate as follows: 15. An input funnel or hopper where the to be reduced material can be collected.

16. A gated entrance to the DCR shaft. In this figure, a “zellen rad” gate is depicted. In practice, any gated entrance or feed system can be used.

17. Entry pipe(s) for admission of the reduction gas into the DCR shaft.

18. Exit pipes for the off gas of the process.

19. Oxygen (or air) entry points where oxygen (or air) is added to or injected into the rising off gas from the reduction shaft, oxidizing or combusting (part of) the non-utilized fraction of the reduction gas. This way, heat is generated to supplement the necessary heat for the reduction process. By using entry points at different heights, the thermal profile of the reactor along its vertical axis can be controlled.

20. Channels connecting the combustion shafts with the central DCR shaft. The reduction gas enters the reactor at the entry points at the top of the reactor (17), flows trough the to be reduced ore (10) and near the lower end of the reduction shaft, the (partially utilized) reduction gas is led to the combustion channels (14) circumventing the reduction shaft. Within the combustion channels, oxygen (or air) can be added to the partially utilized reduction gas, leading to further oxidation of the gas, generating the necessary heat for the process.

21. Are rotary mills or breaker rolls, closing of the bottom end of the DCR shaft for the solid material. The rotation speed of these mills or rolls determines the speed at which the material can be let out of the reactor. The speed is adjusted such that the required degree of reduction is reached for the full cross section of the reduction shaft.

A second function of the mills can be to break the reduced, coagulated metal into smaller pieces, easier to handle in further downstream processes.

Alternative to breaker rolls, or directly below, briquetting rolls can be placed to compact the hot metal to metal briquettes, in the case of iron as metal, into HBI.

22. A gated exit lock or system to prevent gas exiting at the bottom end of the reduction shaft while letting out the reduced metal material.

Figure 5 Presents a cross section through an alternative bottom construction of a DCR reactor combined with an induction smelter. The numbers 10 to 14 and 19 to 20 are identical as described above.

23. Represent loops of an induction coil to heat up the lower part of the material inside the reduction shaft. Since the refractory structure is non-conducting for electricity, only the hot metal is heated and will melt. Because field strength vanishes with increasing distance from the coil, only the lower part of the reduced metal will melt. 24. Exit for the molten metal. A siphon is drawn to depict any means to prevent any (offer reduction-) gas to escape the reactor together with the hot metal. This function can be placed further downstream as well.

25. Exit for molten slag. Non-metal components within the (solid) input in the reactor will melt on top or sink below of the molten hot metal. The slag exit is placed above or below the level of liquid metal inside the reactor. Here too, a siphon or any other construction shall prevent gas exiting here from the reactor.

Figure 6 Depicts a DCR shaft where heating by the off gas is combined with or even replaced by electrical heating, in this figure in the form of induction coils around the shaft. With a combined heating configuration, energy balancing can be supported by electrical heating when there is a surplus of electricity and combustion heating when there is a shortage of electricity. In the figure, the numbers 10 to 14 and 19 to 20 are identical to the previous figures.

26. Possible alternative solution with heating elements, in this example in the form of induction coil(s). Resistance heating heating elements can be used as well.

Figure 7 Depicts a DCR shaft with an integrated pellet baking top. This shaft can be fed with ‘green’ pellets that are baked (heated through at sufficient temperature) before the reducing gas is added to the ore. When green pellets would be loaded together with the reducing gas, the reduction process would alter the pellets before they get sufficient strength to maintain their shape, leading to the risk of collapsing of the pellets and clogging the shaft. In this figure, the numbers 10 to 22 depict the same components as described above.

27. “Green” pellets that are thermally baked off within a top-extension of the reduction shaft, also called the baking zone. The reducing gas is not entered before all green pellets are heated through and through to the temperature needed to bake them off.

Figure 8 Depicts heat exchangers for the off gas to preheat the solid input and I or the reducing gas before those will enter the reactor. In this figure, the numbers 10 to 22 depict the same components as described above.

28. Depicts a pre-heater for the solid input of the process;

29. Depicts a heat exchanger to pre-heat the reduction gas before entering the reduction shaft.

30. Depicts the reduction zone.

31. Depicts the housing that delimits the reaction chamber. In all cases, (elements of) the figures 5, 6, 7 and/or 8 can be combined.

Figure 9 Equilibrium diagram iron phases with reducing gases at different temperatures

Advantages

Some of the advantages of the DCR - process are:

• No oxygen is added within the reduction part of the reactor, as opposed to most other gas-based (and coal or cokes based) reduction processes. This keeps the oxygen concentration within the reduction shaft at the minimum attainable level. More of the reductant will be used this way to take away oxygen from the metal component in the ore instead of being oxidised to generate heat. This leads indirectly to a better utilization of the reductant.

• With hydrogen as (one of the) reducing agents, the balance between hydrogen (H2) on the one hand and hydrogen plus water vapor (H2O) on the other will shift to the hydrogen plus H2O side with a rising temperature (see figure 9,). The off-gas (the partial utilized reduction gas plus possible volatile elements from the ore) leaves the reduction shaft where process temperatures are highest. As long as reduction takes place, heat is drawn from the process. Therefore will the process temperatures be the highest at the downside (most reduced side) and outer circumference of the reactor channel). The high temperature where the gas leaves the reduction process results again in a higher utilisation of the hydrogen as a reducing agent within the reaction process as compared to updraft processes.

• The utilized reductant still contains a certain percentage of hydrogen. This can be oxidized in the combustion channels within the same refractory structure that forms the reduction shaft(s) to generate the necessary thermal input to the process (the reduction process is overall endothermic in nature). This leads to an even higher utilization of hydrogen within a single pass through the DCR-reactor. Depending on the balance, up to 100% of the hydrogen can be utilized in one pass, avoiding the need to cool down the gas, remove the H2O component and letting the gas be heated again within the reactor.

This vastly improves the overall process energy efficiency.

• More generally speaking, and particularly when only hydrogen is used as the reductant, the gas utilization within a DCR-reactor is at the maximum possible level, up to 100%. This results in low gas volumes being pumped around which strongly contributes to the high thermal efficiency of the process.

When oxygen is used as oxidiser, instead of air, gas volumes remain further at the minimum possible level. • Some ores or metal-oxide containing slags contain low-boiling components, particularly zinc as metallic component and phosphor minerals as non-metal components. Within counter current (up-draft) processes, those low boiling components tent to condensate again within the reactor, leading to unwanted enrichment of those components within the process. These ‘volatile’ components can’t “boil off’ in current art processes since they cool down and condensate within the reactor and can only come out with the solid output of the process.

With the DCR-process, the low-boiling components remain gaseous and leave the process within the gas phase together with the other hot gasses instead of remaining with the reduced metal. The low-boiling components can be caught (condensed and/or solidified) from the off-gas after the off-gas leaves the refractory structure in dedicated condensation traps.

• With pure oxygen to oxidise the remaining reductant within the reduction shaft, nitrogen levels will be extremely low. This reduces or prevents the forming of nitric oxides (NOx) and e.g. of dioxins within the off-gas.

• Many ores contain salts that disintegrate to halogens when heated. By loading ore that hasn’t been heated up upfront of the process, e.g. by loading green ore pellets (and bake them within the reduction shaft), the only one source of polluting off gas remains for the reduction process. When the process is configured such way that all hydrogen is fully utilised, off gas volume will be extremely low. This way, pollution from metal reduction processes can be reduces to virtual zero.

Clauses

1 . A process is claimed for the reduction of a metal ore (metal oxides, metal sulfides, metal hydroxides, metal oxides containing slags or any other form) to metal or a high metal-grade product with the following characteristics:

• Metal ore [1] is loaded from the top into a shaft reactor [4] and moves slowly downwards [6] as a ‘prop-stream’ or ‘plug-stream’ through the shaft in which a reduction reaction takes place (the reduction shaft). The resulting metal or high metal-grade product is off-loaded from the downward side of the reduction shaft;

• The topside of the reduction shaft is closed off, the ore is entered into the reduction shaft through some form of an entry-lock, e.g. like [2];

• The bottom side of the reduction shaft is closed off as well while the resulting metal and possible slags are off-loaded by a gated exit system, e.g. like [9];

• Inside the reduction shaft, the ore is heated from outside the reduction shaft through the shaft’s wall [5];

• A reducing gas is entered into the reduction shaft [3], higher up than where the off-gas leaves the reduction shaft [8], creating a downward draft inside the reduction shaft [7],

2. A possible embodiment is claimed for a process according to clause 1 where the reduction shaft is made within a refractory structure [12],

3. A possible embodiment is claimed for a process according to clause 1 where multiple reduction shafts are combined within a combined refractory structure.

4. A possible embodiment is claimed for a process according to clause 1 where the length over diameter ratio of the “active zone” has a value of 6 to 12. The “active zone” is the area where the actual reduction takes place. The length is determined by the distance between the input level of the reducing gas into the reaction shaft and the level where the off-gas leaves the reaction zone.

5. A process is claimed according to clause 1 and clause 2 where the off gas from the reduction process, after leaving the reduction shaft, is led to and upwards through combustion channels configured around the reduction shaft within the same refractory structure that forms the reduction shaft. Within the combustion channels, oxygen or an oxygen containing gas, e.g. air, is added to or injected into the off-gas [19], partially or wholly oxidizing the remaining reductant in the off-gas, to create the necessary thermal input to the process.

6. As alternative to clause 5, or in combination with clause 5, a process is claimed according to clause 1 where electrical or electro-magnetic heating elements [26] are used to provide the necessary or additional process heat to the material inside the reduction shaft. A possible embodiment is claimed according to clause 1 , where the reduced metal, instead of being broken into pieces by breaker rolls [21], is rolled between “briquetting rolls” to create hot briquetted metal, particularly HBI. A possible embodiment is claimed according to clause 1 and 2, where the reduced metal in the lowest section of the reduction shaft is heated further [23], above melting temperature of the metal and possible slag components, such that a liquid exit lock can be formed and metal and slag can be off-loaded in liquid form [24] and [25], A process is claimed according to clause 1 where metal ore is fed as “green” (this is non-baked) pellets and where pellets are heated to baking temperature in the upper section of the shaft, above the entry point of the reducing gas. A process is claimed according to clause 1 and possible clause 5 where the high temperature of the off gas is used to pre-heat either the reducing gas, or the metal ore, or both, prior to where the reducing gas and/or the metal ore enters the reduction shaft. A process is claimed according to clause 1 where, additional to the reducing gas, a solid reductant is added to the metal ore. A special application of the process according to clause 1 and one or more of the other clauses is the reduction of iron ore into DRI. A special application of the process according to clause 1 and one or more of the other claims is the reduction of zinc containing iron ore or zinc and iron containing slags. A special application of the process according to clause 1 and one or more of the other clauses is the reduction of phosphorous minerals containing iron ore.