EP2961856A1

EP2961856A1 - Iron and niobium containing agglomerates

Info

Publication number: EP2961856A1
Application number: EP14756421.5A
Authority: EP
Inventors: Bo Zander; Dag SJÖBERG
Original assignee: Ferrolegeringar AB
Current assignee: Ferrolegeringar AB
Priority date: 2013-03-01
Filing date: 2014-02-28
Publication date: 2016-01-06
Also published as: EP2961856A4; WO2014133447A1; TW201446971A

Abstract

Iron and niobium containing agglomerates and a process for producing the agglomerates are disclosed. Green agglomerates are produced from a mixture of an iron containing powder, a niobium oxide powder, and a carbonaceous powder. In a preferred embodiment sodium carbonate is added to the agglomerates. The green agglomerates can be reduced at a temperature in the range of 1000-1500 °C.

Description

IRON AND NIOBIUM CONTAINING AGGLOMERATES

TECHNICAL FIELD

The present invention relates to a process for producing iron and niobium containing agglomerates and agglomerates produced by the process.

BACKGROUND

Ferroniobium is an iron niobium alloy having a niobium content of around 60-70%. Niobium is mined from pyrochlore deposits and is subsequently transformed into niobium pentoxide Nb205. This oxide is mixed with aluminium iron scrap and/or iron oxide and is reduced in an aluminothermic reaction to niobium and iron in an electric arc furnace.

The liquid ferroniobium is cast into moulds and the solidified ingots are subsequently crushed into smaller fractions that are sieved into various lump size distributions. Lumps of ferroniobium have densities around 8 g/cm³.

These lumps consist of several phases and the melting points of these phases are different, resulting in a melting range from around 1550°C to more than 1800°C. Thus ferroniobium does not melt when added to a steel melt but rather dissolves.

The dissolution time is affected in that a steel shell solidifies around the lumps after entering the melt. Inside this steel shell the lumps heats up before the steel shell re-melts and finally dissolution of ferroniobium take place. Smaller lumps have shorter dissolution time than larger ones. The dissolution time varies with size of the lumps and the temperature of the melt. For 5 mm lumps in a melt of 1400 °C the dissolution time can typically be around 5 minutes, for 10 mm lumps 10-15 minutes, and for 30 mm lumps 30-40 minutes. Due to the high specific weight of ferroniobium larger lumps may sink to the bottom of the ladle. With finer lumps dust losses can occur and the risk of entrapment in the slag is increased. One way of solving the latter problem has been to feed fine ferroniobium by cored wire.

Niobium can also be produced by the two stage Balke process in which the first stage includes carbothermal reduction of niobium pentoxide in a vacuum furnace to produce niobium carbide. In the second stage niobium carbide is reacted with niobium pentoxide.

OBJECTS OF THE INVENTION

It is an object of the invention to provide a novel iron and niobium containing matenal suitable for niobium addition in melting industries e.g. steel, foundry and superalloy industry, and a process for producing such material in a comparably cost efficient manner.

A further object is to provide a novel iron and niobium containing material that has a comparably quick dissolving time in a steel melt.

A further object is to provide a novel iron and niobium containing material low in carbon, preferably less than 5 % by weight, and high in Nb, preferably at least 75 % by weight, and a process for producing such material in a comparably cost efficient manner

SUMMARY OF THE INVENTION

At least one of the above mentioned objects is at least to some extent achieved by a process for producing an iron and niobium containing agglomerates. The agglomerates are preferably pellets or briquettes.

Said process for producing an iron and niobium containing agglomerates including the steps of: a) mixing an iron containing powder, a niobium oxide containing powder, a

carbonaceous powder, a liquid and optionally a binder and/or a slag former and/or an sodium carbonate containing powder

b) forming a plurality of green agglomerates from the mixture.

The term green agglomerates is meant agglomerates of powders that has not been reduced in a reduction process.

When the agglomerates are pellets, the process for producing an iron and niobium containing pellets including the steps of:

c) mixing an iron containing powder, a niobium oxide containing powder, a

d) adding a liquid to the mixture and pelletizing to provide a plurality of green pellets.

When the agglomerates are briquettes, the process for producing an iron and niobium containing briquettes including the steps of:

a) mixing an iron containing powder, a niobium oxide containing powder, a

carbonaceous powder, a liquid and optionally a binder or a lubricant and/or a slag former and/or an sodium carbonate containing powder, and

b) briquetting to provide a plurality of green briquettes. When briquetting liquid may optionally be added to the mixture.

The process may additionally include at least one of the steps:

c) drying the green agglomerates to reduce the moisture content to less than 10 % by weight, more preferably less than 5 % by weight, preferably less than 3 % by weight;

d) reducing the green agglomerates at a temperature in the range of 1000-1500 °C, preferably 1100-1350 °C;

e) cooling the reduced agglomerates in a non-oxidising atmosphere (e.g. reducing or inert) to a temperature below 200 °C to avoid re-oxidation of the agglomerates, more preferably below 150 °C in an inert atmosphere;

f) crushing and/or grinding the agglomerates;

g) sieving the crushed and/or ground agglomerates.

Dry matter refers to the composition for a dried specimen, i.e. excluding any moisture present in the green agglomerates. The moisture content is defined as water present in the green agglomerates apart from water of crystallization. The moisture content can be determined by a LOD (loss on drying) analysis in accordance to ASTM D2216 - 10.

In one embodiment the dry matter of the green agglomerates (e.g. pellets or briquettes) comprises in weight %:

1-40 iron containing powder;

5-40 carbonaceous powder;

optionally

1-20 sodium carbonate containing powder;

1-10 binder and/or slag former;

balance at least 50 niobium oxide containing powder.

In one embodiment the reduced iron and niobium containing agglomerates have a composition in weight % of:

2-50 Fe;

< 10 O, preferably < 5, more preferably < 3;

<15 C, preferably < 5, more preferably < 3;

optionally 1-15 Na < 15 of other elements, preferably < 5 of other elements, and

balance at least 50 Nb.

The agglomerates are porous and have a geometric density in the range of 1.0-8.0 g/cm³. It has been found out that their porous structure enables rapid dissolution in the steel melt. The dissolution rate is comparable to cored wire injection of powder. The production costs are however much lower than for ferroniobium cored wire products.

BRIEF DESCRIPTIONS OF THE DRAWINGS

Fig. 1 is a schematic overview of the process of producing iron and niobium containing pellets according to the invention, and

Fig. 2 is a schematic overview of the process of producing iron and niobium containing briquettes according to the invention.

DESCRIPTION OF THE INVENTION

Fig. 1 shows a schematic overview of the process of producing iron and niobium containing pellets according to the invention. Further, Fig. 2 shows a schematic overview of a corresponding process of producing iron and niobium containing briquettes according to the invention.

Referring firstly to Fig. 1, in the mixing station 3, a powder mixture is prepared by mixing an iron containing powder, a niobium oxide containing powder, and a carbonaceous powder.

Iron containing powder can be added in amounts of 1-40 % by dry weight of the mixture, preferably 1-30 % by dry weight of the mixture, more preferably 2-15 % by dry weight of the mixture, most preferably at most 10 % by dry weight of the mixture. In one embodiment 5- 30% by weight of iron containing powder is added. In another embodiment 10-20 % by weight of iron containing powder is added. The iron containing powder has the surprising effect of strengthening the pellets (e.g. acts as a binder) and can also be used to balance the desired amount of Fe and Nb in the final product. Niobium oxide powder is added in amounts of 50-94 % by dry weight of the mixture, preferably 60-90 % by dry weight of the mixture, more preferably 65-90 % by dry weight of the mixture, most preferably 70-85 % by dry weight of the mixture,. The carbonaceous powder is added in amounts of 5-40 % by dry weight of the mixture, preferably 10-35 % by dry weight of the mixture, more preferably 15-30% by dry weight of the mixture. In one embodiment sodium carbonate is added when mixing in amounts of 1-20 % by dry weight of the mixture, preferably 1-10 % by weight, more preferably 2-7 % by weight. When adding sodium carbonate the carbonaceous powder is preferably in the range of 5-25 % by dry weight of the mixture,, preferably 10-25 by dry weight of the mixture, more preferably 13-22 % by dry weight of the mixture,

Optionally, binders and/or slag formers can be added when mixing. The optional binders may be organic or inorganic binders. The binders may e.g. be a carbon containing binders partially replacing the carbonaceous powder. Other binders may e.g. be bentonite and/or dextrin and/or sodium silicate and/or lime. The optional slag former may be limestone, dolomite, and/or olivine. The total amount of optional binders and/or optional slag forms can be 1-10 % by by dry weight of the mixture, more preferably less than 5 wt%, by dry weight of the mixture. The binders are optional since the iron containing powder can provide pellets that are sufficiently strong. Obviously, in the context of the application the term binder do not include the iron containing powder or sodium carbonate containing powder. Preferably neither binder nor slag former is used.

The powders may be mixed in a dry condition, i.e. without adding liquid during mixing, but are preferably mixed in a wet condition by adding liquid, preferably water, in the mixing station 3. Preferably 5-15 % by weight of water is added during mixing. By adding water during mixing dusting problems are minimized.

Before being added to the mixing station 3 each of the powders may be may be milled in the rod mill 1. Of course other mills, grinders, or crushers may be used to disintegrate respective powder into smaller particles. The different powders may also be ground and/or milled and/or crushed jointly. In particular the niobium oxide containing powder may be mixed and ground jointly with the carbonaceous powder and/or the sodium carbonate before being mixed with the iron powder. The ground and/or milled and/or crushed particles may be sieved in a sieve 2 to provide a desired particle distribution.

The mixing in the mixing station 3 can be executed in a batch process or continuously. From the mixing station 3 the prepared powder mixture is transferred to a pelletizer 4. In the pelletizer 4 the powder mixture is pelletized, providing a plurality of green pellets. If the powders were dry mixed in the mixing station 3, liquid is supplied when pelletizing. If the powders were wet mixed in the mixing station 3 additional liquid is optionally supplied when pelletizing. The pelletizer 4 is preferably a disc pelletizer or a rotary drum pelletizer. However other kinds of pelletizers or granulators may be used. In total, during mixing and pelletizing, the amount of added liquid is around 5-25 % by weight of the mixture, more preferably 10-20 wt%, e.g. adding 10 wt% during mixing and 5 wt% dunng pelletizing.

The pellets produced from the pelletizer 4 are here referred to as green pellets. The shape of the green pellet is typically spherical, spheroidal, or ellipsoidal.

To lower the moisture content the green pellets are transferred to a dryer 5, e.g. a rotary dryer. Many other kinds of industrial dryers can of course be used. Vapour is preferably removed by a gas steam or by vacuum. The pellets are dried until desired moisture content has been reached. Preferably the green pellets are dried to a moisture content less than 10 % by weight, more preferably less than 5 % by weight, most preferably less than 3 % by weight. Preferably the green pellets are dried at a temperature in the range of 50-250 °C, more preferably 80-200 °C, most preferably 100-150 °C. For improved process economy, drying time is preferably in the range of 10- 120 minutes, more preferably 20-60 minutes. But longer drying times are of course viable. Furthermore the green pellets may also be dried without active heating, e.g. in ambient air temperature. After drying the green pellets have a maximum moisture content of 10 % by weight. Hereafter referred to as dried green pellets. The green pellets may be dried in a nitrogen free atmosphere to avoid nitrogen uptake drying.

Lowering the moisture content has several advantages. One advantage is that the risk of cracking in the reduction furnace 6 is minimised. Green pellets may crack due to quick vaporisation of the remaining liquid in the pellets when heated at high temperatures.

Additionally, after drying, the dried green pellets become surprisingly strong and they are therefore not required to be compacted at all before, during or after reduction. The iron containing powder act as a binding agent when mixed in wet condition. The optional addition of sodium carbonate strengthens the pellets further as well as lowers the amount of required carbon. Therefore it is an optional step to add a binder during mixing (with the term binder we exclude the iron containing powder and the carbonaceous powder). The dried green pellets can reach compression strength in the range of 100-1000 N/pellet, preferably the compressions strength is 200-800 N/pellet. This compression strength is sufficient for effective handling of the pellets including reduction in a rotary kiln. Stronger pellets may be produced by adding binders.

Optionally, the non-reduced dried green pellets may be used as alloying additive in iron and steel making. The strength and the shape of the green pellets make them easy to transport and handle with low shredding losses. Preferably, the dried green pellets may be reduced in a reduction furnace, such as a rotary kiln furnace 6. In the rotary kiln furnace 6 the green pellets are reduced at a furnace temperature in the range of 1000-1500 °C, preferably 1100-1350 °C. Preferably the reduction is least 10 minutes and at most 10 hours, more preferably at least 20 minutes and at most 2 hours, most preferably at least 30 minutes to 1.5 hour. By monitoring the formation of CO/C0₂ it can be determined when the reduction process is finished. Mixing iron

In green pellets consisting of iron powder, a niobium oxide powder, and carbonaceous powder, niobium oxide can be reduced to niobium carbide. Such pellets contains from 5 to less than 15 %. Oxygen can be controlled to less than 5 % by weight, preferably less than rotary 1 % by weight, more preferably less than 0.5 % by weight.

By adding sodium carbonate and lowering the amount of carbonaceous powder in the green pellets, the carbon content in the reduced pellets can be less than 5 % by weight, preferably less than 3 % by weight, most preferably less than 1% by weight. Oxygen content can be less than 5 % by weight, preferably less than 3 % by weight, most preferably less than 1% by weight. Suitable furnace types for the reduction step are for example rotary kilns, rotary heart furnaces, shaft furnaces, grate kilns, travelling grate kilns, tunnel furnaces or batch furnaces. Other kinds of furnaces used in solid state direct reduction of metal oxides may also be employed.

In a one embodiment a rotary kiln is used to reduce pellets. In a rotary kiln furnace the green pellets from step c) are fed to a rotary kiln rotating on a slightly inclined horizontal axis, and propagated from an inlet of the kiln towards an outlet of the kiln, as the kiln is rotated about its axis.

The atmosphere within the furnace 6 is preferably controlled by supplying an inert or a reducing gas, preferably a weakly reducing gas, e.g. H₂/N₂ (5:95 by vol.), at one end of the furnace and evacuating gases (e.g. reaction gases (e.g. CO, CO_¾ and H₂0) and the supplied gas) at the opposite end, more preferably, supplying the inert or reducing gas counter current at an outlet side 8 of the furnace 6, and evacuating gases at an inlet side 7 of the furnace 6. I.e. the inert or reducing gas is preferably supplied counter flow.

The gas supplied may include argon, N_2j H_2> CO or any mixture of them. For instance H2/N2 having relations such as 5:95, 20:80, 40:60, 80:20, and 95:5 by vol. . In one embodiment the atmosphere comprises 20-60 vol % of H₂ and balance N₂. Such atmosphere may reduce N₂ uptake, compared to e.g. H₂/N₂ (5:95), and it may increase the density of the reduced pellets. The atmosphere may also be supplied with CO, e.g. from burning natural gas. Of course, other gas mixes being inert or reducing may be supplied to the furnace. Preferably the furnace operates at pressure in the range of 0.1-5 atm, preferably 0.8-2 atm, more preferably at a pressure in the range of 1.0-1.5 atm, most preferably 1.05-1.2 atm. In an alternative embodiment, in order to reduce the amount of required external heat, oxygen gas or air can be provided in the pre-heating zone to react with the formed carbon monoxide to form carbon dioxide gas. If air is used the nitrogen uptake of the pellets may increase. Using oxygen the nitrogen uptake during the heating and the reduction step can be minimised.

At the outlet 8 of the reduction furnace the pellets are transferred to a cooling section 9, providing a step e): cooling the reduced pellets in a non-oxidising atmosphere (e.g. reducing or inert) to a temperature below 200 °C to avoid re-oxidation of the pellets, more preferably below 150 °C in an inert atmosphere. The atmosphere may e.g. be a 95 vol-% N₂ and 5 vol.% H₂ atmosphere. To minimize nitrogen uptake the pellets are preferably cooled in a nitrogen free atmosphere such as for example an argon gas atmosphere. The produced pellets may further be subjected to additional process steps including:

f) crushing and/or grinding the pellets;

g) sieving the crushed and/or ground pellets;

h) hot briquetting at a temperature in the range of 250-1000 °C, preferably 400-800 °C, and more preferably between two counter-rotating rollers

h) agglomerating the pellets to pellet agglomerates comprising 2-300 pellets.

In Fig. 2 shows a schematic overview of a corresponding process of producing iron and niobium containing briquettes from niobium oxide containing powder mixture, according to the invention. In this process briquettes are formed instead of pellets. In a corresponding manner as previously described for pellets production, powders milled in a rod mill 11 are added to a mixing station 31, but in Fig. 2 the prepared powder mixture is transferred from said mixing station 31 to a briquetting machine 41 instead of a pelletizer 4. In the briquetting machine 41 the powder mixture is briquetted to provide a plurality of green briquettes.

In one embodiment the powder mixture is briquetted at a comparably low briquetting pressure, preferably employing a briquetting pressure in the range of 80-1000 kg/cm², more preferably 100-500 kg/cm². The low briquetting pressure has been found out to improve the quality of the produced green briquettes.

In another embodiment the briquetting machines operates at higher pressures, e.g. 1 000-10000 kg/cm². Higher pressure can be used to increase the geometric density of the green briquettes. Preferably the briquetting machine 41 is a roller press. However, other kinds of briquetting machines may be used. In one embodiment the powder mixture is hot briquetted at a temperature in the range of 250- 1000 °C, preferably 400-800 °C, and more preferably between two counterrotating rollers, most preferably at a pressing force in the range of 60-200 kN per cm active roller width. Suitable hot briquetting machines are for instance sold by Maschinenfabrik Koppern GmbH & Co. A binder may optionally be added in the hot briquetting step.

The green briquettes produced from the powder mixture are preferably reduced in a reduction furnace 61. Alternatively the non-reduced green briquettes can be used as alloying additive in iron and steel making. Optionally the green briquettes are dried before being transferred to the reduction furnace 61. Many different kinds of industrial dryers 51 can be used. The briquettes may also be dried without active heating, e.g. in ambient air temperature. In a dryer vapor may be removed by a gas steam or by vacuum. The green briquettes can be dried until desired moisture content has been reached. The green briquettes may be dried to moisture content less than 10 % by weight, more preferably less than 5 % by weight, most preferably less than 3 % by weight. The green briquettes may be dried at a temperature in the range of 50-250 °C, more preferably 80-200 °C, most preferably 100-150 °C. For improved process economy, drying time is preferably in the range of 10- 120 minutes, more preferably 20-60 minutes. But longer drying times are of course viable.

The green briquettes are preferably reduced in a reduction furnace 61. The reduction furnace is preferably a continuous furnace hut may also be a batch furnace. The continuous furnace 61 having an inlet 71 and outlet 81, and the briquettes are conveyed during reduction from the inlet 71 to the outlet 81. In a preferred embodiment a belt furnace is used.

The green briquettes are reduced at a temperature in the range of 1000-1500 °C, preferably 1100-1350 °C. Preferably the reduction is least 10 minutes and at most 10 hours, more preferably at least 20 minutes and at most 2 hours, most preferably at least 30 minutes to 1.5 hour. By monitoring the formation of CO/C0₂ it can be determined when the reduction process is finished. Depending on the reduction time, the reduction temperature, and the relation between carbon and reducible oxides in the briquettes; the reducible oxides of the briquettes can be partially or fully reduced. Optionally the green briquettes are heat treated at a lower temperature before reduction.

Preferably heat treating the green briquettes at a temperature in the range of 200-800 °C, more preferably 400-700 °C. Preferably, the optional heat treating at lower temperature is performed from 10 minutes to less than 2 hours, preferably less than 1 hour. By heat-treating at lower temperatures the optional lubricant can be burned off in a controlled manner. The optional heat treating at 200-800 °C, can be performed in the same furnace as the reduction. The optional heat treating and optional drying may also be combined. During the reduction CO and C02 can form from reactions with the carbon source and the reducible oxides in the briquettes. Additionally remaining moisture may vaporise. The reduction time can be optimised by measuring the formation of CO and C02; in particular CO since C02 is mainly formed during the first minutes of reduction where after CO formation is dominating until the carbon source is consumed or all reducible oxides have been reduced.

The reduction reactions are endothermic and require heat. Preferably heat is generated by heating means not affecting the atmosphere within the furnace, more preferably the heat is generated by electrical heating. The atmospheric and pressure conditions within the furnace 61 are the same as described for the furnace 6 of Fig. 1.

At the outlet 81 of the reduction furnace 61 the briquettes are transferred to a cooling section 91 where they are cooled in the same way and under the same conditions as described for the cooling section 9 of Fig. 1.

Niobium oxide containing powder

The niobium oxide powder preferably includes at least 80% by weight of Nb₂0₅, more preferably at least 90 % by weight of Nb₂0_5> most preferably at least 95 % by weight ofNb₂0₅.

Preferably at least 90% by weight of the particles of the niobium oxide powder pass through a test sieve having nominal aperture sizes of 300 μιη and at least 50 % by weight of the particles of the niobium oxide powder pass through a test sieve having nominal aperture sizes of 125 μιη. More preferably at least 90% by weight of the particles of the niobium oxide powder pass through a test sieve having nominal aperture sizes of 125 μηι and at least 50 % by weight of the particles of the niobium oxide powder pass through a test sieve having nominal aperture sizes of 45 μπι. Nominal aperture sizes in the present application are in accordance with ISO 565: 1990 and which hereby is incorporated by reference.

Iron containing powder

The iron containing powder is preferably an iron powder containing at least 80 wt% Fe, preferably at least 90 wt % Fe, more preferably at least 95 wt% Fe, most preferably at least 99 wt% Fe. The iron powder can be an iron sponge powder and/or a water atomised iron powder and/or a gas atomised iron powder and/or an iron filter dust and/or an iron sludge powder. For instance filter dust X-RFS40 from Hoganas AB, Sweden is a suitable powder.

Preferably at least 90% by weight of the particles of the iron containing powder pass through a test sieve having nominal aperture sizes of 300 μιη and at least 50 % by weight of the particles of the iron containing powder pass through a test sieve having nominal aperture sizes of 125 μιη. More preferably at least 90% by weight of the particles of the iron containing powder pass through a test sieve having nominal aperture sizes of 125 μηι and at least 50 % by weight of the particles of the iron containing powder pass through a test sieve having nominal aperture sizes of 45 μηι.

In one embodiment at least 90 % by weight, more preferably at least 99 % by weight, of the particles of the iron containing powder pass through a test sieve having nominal aperture sizes of 125 μηι, more preferably 45 μηι. In one example at least 90 % by weight, more preferably at least 99 % by weight, of the particles of the iron containing powder pass through a test sieve having nominal aperture sizes of 20 μηι. The iron powder may partly or fully be replaced by an iron oxide powder, for instance but not limited to: powder consisting of one or more from the group of FeO, Fe₂0₃, Fe₃0₄, FeO(OH, (Fe2O₃*H₂0). The iron oxide powder may e.g. be mill scale. In one embodiment the iron containing powder contains at least 50 % be weight of metallic iron, more preferably at least 80 wt% metallic Fe, most preferably at least 90 wt% metallic Fe.

Carbonaceous powder

The carbonaceous powder is preferably chosen from the group of: sub-bituminous coals, bituminous coals, lignite, anthracite, coke, petroleum coke, and bio-carbons such as charcoal, or carbon containing powders processed from these resources. The carbonaceous powder may e.g. be soot, carbon black, activated carbon, fly ash. The carbonaceous powder can also be a mixture of different carbonaceous powders. Regarding the choice of carbonaceous powder a high reactivity is desired. For instance German brown coal (lignite) is normally reactive at lower temperatures than petroleum coke, and is hence suitable since it has comparably high reactivity at low temperatures. Also charcoal, bituminous and sub-bituminous coals can exhibit comparably high reactivity. Particularly suitable examples are soot, carbon black, and activated carbon.

Preferably the carbonaceous powder contains at least 80 % by weight of C, preferably at least 90 wt%, more preferably at least 95 wt%, most preferably at least 99% by weight. Preferably, at least 90 % by weight, more preferably at least 99 % by weight, of the particles of the carbonaceous powder pass through a test sieve having nominal aperture sizes of 125 μιη, and at least 50 % by weight of the particles of the carbonaceous powder pass through a test sieve having nominal aperture sizes of 45 μηι.

In one embodiment at least 90 % by weight, more preferably at least 99 % by weight, of the particles of the carbonaceous powder pass through a test sieve having nominal aperture sizes of 45 μπι, and at least 50 % by weight of the particles of the carbonaceous powder pass through a test sieve having nominal aperture sizes of 20 μηι. In one example at least 90 % by weight, more preferably at least 99 % by weight of the particles of the carbonaceous powder pass through a test sieve having nominal aperture sizes of 20 μηι.

The amount of carbon sources (e.g. carbonaceous powder and optional sodium carbonate powder) can be optimised by analysing the oxygen content in the niobium oxide containing powder and optionally in the iron containing powder. If sodium carbonate is added its influence should also be taken into account. The oxygen content can be analysed by an oxygen measuring equipment such as e.g. LECO® TC400. Preferably the amounts of the carbon sources are chosen to stoichiometric match or slightly exceed the amount of reducible metal oxides in the niobium oxide containing powder and the iron containing powder. However, the amounts of the carbon sources may also be sub-stoichiometric. The amount of carbon sources can further be optimised by measuring the carbon and the oxygen levels in the produced agglomerates (e.g. by producing agglomerates in a lab furnace and measuring carbon and oxygen levels). Based on the measurements the amount of carbon sources can be optimised to achieve desired levels of carbon and oxygen in the produced agglomerates.

Sodium carbonate powder

Preferably the sodium carbonate containing powder contains at least 80 % by weight of Na₂C0₃, preferably at least 90 wt%, more preferably at least 95 wt%, most preferably at least 99% by weight.

Preferably at least 90% by weight of the particles of the iron containing powder pass through a test sieve having nominal aperture sizes of 300 pm and at least 50 % by weight of the particles of the iron containing powder pass through a test sieve having nominal aperture sizes of 125 μηι.

Iron and niobium containing green agglomerates

In an embodiment the iron and niobium containing green agglomerates have a dry matter composition in weight % of:

1-40 Fe, preferably 1-15, more preferably 1.5-10;

10-35 O, preferably 15-30;

10-40 C, preferably 15-25;

Optionally 0.5-10 Na, preferably 1-5 Na;

< 10 of other elements, preferably < 5 of other elements, and

balance at least 20 Nb, preferably at least 30, more preferably at least 40.

Other elements (i.e. apart from Nb, Fe, C, O, Na) are preferably less than 5 % by weight, more preferably less than 2 % by weight. Such elements may come from impurities in the niobium oxide containing powder, the iron containing powder, the carbonaceous powder, the optional sodium carbonate containing powder, and the optional binder and/or slag former.

Na in green agglomerates comprising sodium carbonate is preferably in the range of 0.5-10 % by weight, preferably 1-5 % by weight. For agglomerates not having the optionally added sodium carbonate Na is preferably only present as impurities.

In one embodiment the iron and niobium containing green agglomerates have a dry matter composition in weight % of:

20-40 Fe;

10-35 O;

10-40 C;

Optionally 0.5-10 Na;

< 10 of other elements, preferably < 5 of other elements, and

balance 20-60 Nb. The green pellets may in some embodiments be defined by the powders that they comprise.

In an embodiment the dry matter of the green agglomerates comprises in weight %:

iron containing powder;

carbonaceous powder;

1-20 sodium carbonate containing powder;

1-10 binder and/or slag former;

balance at least 50 niobium oxide containing powder. The balance of niobium is Niobium oxide powder is preferably 60-90 weight %, more preferably 65-95, most preferably 70-85.

The iron containing powder can be 1-30 % by weight, preferably 1-15. To improve strength of the agglomerates the iron powder may be at least 2 % by weight, preferably at least 3 % by weight. In some applications high iron contents are desirable, e.g. at least 10 % by weight of iron containing powder, or at least 20 % by weight.

The carbonaceous powder is preferably 10-35 % by weight, more preferably 15-30% by weight. If used sodium carbonate may be 1-10 % by weight, more preferably 2-7 % by weight.

1-15 iron containing powder;

10-40 carbonaceous powder;

optionally

1-20 sodium carbonate containing powder;

1-10 binder and/or slag former;

balance at least 50 niobium oxide containing powder.

1-10 iron containing powder,

10-25 carbonaceous powder;

1-10 sodium carbonate containing powder;

balance at least 50 niobium oxide containing powder. The agglomerates are preferably pellets or briquettes. The agglomerates may also be a compressed filter cake.

Green agglomerates in the form of pellets

For agglomerates in the form of pellets; the average diameter of the green pellets is preferably in the range of 3-35 mm, preferably 5-25 mm. Too large pellets may prolong the needed reduction time, while too small pellets can be difficult to handle.

The green pellets may have a geometric density in the range of 1.0-4.0 g/cm³, preferably 1.2-3.5 g/cm³. A lower geometric density results in higher porosity, which is believed to yield a shorter dissolution time of the pellets. The geometric (envelope) density can be measured in accordance to ASTM 962-08.

The shape of the green pellet is typically spherical, spheroidal, or ellipsoidal. When handled, this form compared to the form a compressed briquettes reduces the risk of shredding.

Furthermore the flow properties are better than that of briquettes.

The green pellets are preferably dried to have a moisture rate less than 10 % by weight, preferably less than 5 % by weight.

Green agglomerates in the form of green briquettes

The density of the green briquettes can be modified by the compression pressure during briquetting. The green briquettes may have a geometric density in the range of 1.0-7.0 g/cm³, preferably 1.5-6 g/cm³. The density of the green briquettes may be less than 4.0 g/cm³. The density may be controlled to be less than 3.5 g/cm3. A lower geometric density results in higher porosity, which is believed to yield a shorter dissolution time of the briquettes. The geometric (envelope) density can be measured in accordance to ASTM 962-08.

Reduced iron and niobium containing agglomerates

By the suggested process iron and niobium containing agglomerates can be produced having a composition in weight % of:

2-50 Fe, preferably 2-30;

< 10 O, preferably < 5, more preferably < 3;

<15 C, preferably < 5, more preferably < 3;

optionally 1-15 Na

< 15 of other elements, preferably < 5 of other elements, and

balance at least 50 Nb, preferably at least 60. Less than 80 % by weight of the niobium is present as niobium carbide, the niobium could even be present as niobium carbide lower than 50 % by weight. To match the content of commercial grades of solid ferroniobium, the continent of Fe could be 20-50 % by weight, preferably 20- 40 % by weight. For such application the niobium content is suitably within the range of 50-80 % by weight, preferably 60-80% by weight.

For higher yields of niobium, the niobium content is preferably at least 75, more preferably at least 80, most preferably in the range of 85-97 % by weight. Such application

Agglomerates in which niobium oxide is reduced to niobium carbide may contain from 5 to less than 15 % by weight of C, whereas oxygen can be controlled to less than 5 % by weight, preferably less than 1 % by weight, more preferably less than 0.5 % by weight. Agglomerates in which niobium oxide is reduced to Nb, the carbon content may be less than 5 % by weight, preferably less than 3 % by weight, most preferably less than 1% by weight. Oxygen content can be less than 5 % by weight, preferably less than 3 % by weight, most preferably less than 1% by weight.

Sodium carbonate facilitates reduction. Agglomerates produced from green agglomerates comprising sodium carbonate may contain in weight % 1-15 Na, preferably 1.5-10 Na, more preferably 2-7. Sodium is not a problem if present in the agglomerates. When using the agglomerates as an alloying additive, any present sodium will evaporate from the melt. Sodium may also evaporate during reduction depending on reduction temperature. Other elements (i.e. elements apart from Fe, Nb, C, O och Na) are preferably less 10 % by weight, more preferably less than 5 % by weight, most preferably less than 3 % by weight. Other elements are preferably only present as impurities. Such elements may come from impurities in the niobium oxide containing powder, the iron containing powder, the carbonaceous powder, the optional sodium carbonate containing powder, and the optional binder and/or slag former.

The nitrogen content may be affected by the atmosphere during drying, reduction and cooling of the agglomerates. By controlling the atmosphere in these steps the nitrogen content can be made lower than 3 wt%, preferably lower than 1 wt%, more preferably lower than 0.5 wt%. Other remaining elements are each preferably max 1 % by weight, more preferably max 0.5, most preferably max 0.1. In an embodiment the reduced iron and niobium containing agglomerates have a composition in weight % of:

2-30 Fe;

< 10 O, preferably < 5, more preferably < 3;

<15 C, preferably < 5, more preferably < 3;

optionally 1-15 Na

< 15 of other elements, preferably < 5 of other elements, and

balance at least 60 Nb, preferably at least 80.

In an embodiment the reduced iron and niobium containing agglomerates have a composition in weight % of:

2-20 Fe, preferably 3-15 Fe

< 5 O, preferably < 3;

< 5 C, preferably < 3;

1.5-10 Na, preferably 2-7;

< 15 of other elements, preferably < 5 of other elements, and

balance at least 75 Nb, preferably at least 90. The agglomerates are preferably pellets or briquettes. The agglomerates may also be a compressed filter cake.

Agglomerates in the form of reduced pellets

For agglomerates in the form of pellets; the average diameter of the pellets is preferably in the range of 2-30 mm, preferably 3-20 mm. Too large pellets may prolong the needed reduction time, while too small pellets can be difficult to handle.

The pellets have a geometric density in the range of 1.0-5.0 g/cm³, preferably 1.3 -4.0 g/cm³, more preferably 1.5-3.5 g/cm³. A lower density results in higher porosity, which is believed to yield a shorter dissolution time of the pellets. The density is measured in accordance with ASTM 962-08.

The pellets preferably have compression strength in the range of 100-1000 N/pellet. The compression strength may also be limited to be in the range of 200-800 N/pellet.

The shape of the pellet is typically spherical, spheroidal, or ellipsoidal. When handled, this form compared to the form of a compressed briquette reduces the risk of shreddmg, which typically has sharp edges. Furthermore the flow properties are better than that of briquettes. Furthermore they can be produced at lower costs since a briquetting step is not required.

In some applications it may be desirable to have other shapes than spherical, spheroidal, or ellipsoidal. For instance pellets that are transported on a conveyor belt may roll of the belt depending on how the conveyor belt is configured.

Pellet agglomerates comprising 2-300 pellets are less likely to roll off a conveyor belt.

The pellets may be agglomerated by means of a binding agent such as glue. Preferably such agglomerates contain 2-20 pellets, more preferably 5-15 pellets.

It is also possible to form pellets agglomerates by filling plastic bags with pellets, and preferably hot shrinking the plastic around the pellets and/or vaccum shrinking. Preferably such agglomerates contain 30-300 pellets, more preferably 50-200 pellets, most preferably 75-150 pellets.

Another way to avoid the problem is to fill a container, such as a metal canister, with pellets. Preferably the container has an inner volume in the range of 100- 125000 cm³. Of course, also the green pellets may be agglomerated or put in containers in the manner described above.

The pellets may further be hot briquetted at a temperature in the range of 250-1000 °C, preferably 400-800 °C, and more preferably between two counterrotating rollers, most preferably at a pressing force in the range of 60-200 kN per cm active roller width. Suitable hot briquetting machines are for instance sold by Maschinenfabrik Koppern GmbH & Co. A binder may optionally be added in the hot briquetting step. The volume of a briquette is preferably between 15 and 200 cm³. Of course, also the green pellets may be hot briquetted. These briquettes may have a geometric density in the range of 2.0-8.0 g/cm³, preferably 3.0-7.0 g/cm³.

Agglomerates in the form of reduced briquettes

The density of the briquettes can be modified by the compression pressure during briquetting. During reduction the density may increase. The geometric (envelope) density can be measured in accordance to ASTM 962-08. The briquettes may have a geometric density of at most 8.0 g/cm³, preferably at most 7 g/cm³, more preferably at most 6.0 g/cm³. The geometric density may be less than 4.0 g/cm³, or even less than 3.5 g/cm3. The geometric density is preferably at least 1.0 g/cm³, more preferably at least 2.0 g/cm³. The geometric density in some embodiments is 3.0-4.5 g/cm³. A lower geometric density results in higher porosity, which is believed to yield a shorter dissolution time. However the lowering the density requires more space to transport and store the same amount of niobium.

The volume of a briquette is preferably between 15 and 200 cm³. EXAMPLE 1

Three samples (A, B, C) was prepared by mixing niobium pentoxide powder (High purity > 98,5 % Nb₂0₅, HP311 from CBMM), carbon powder (< 20 urn, Carbon Black), and fine grained iron powder (< 40 μιη, >99 wt% Fe, X-RSF40 from Hoganas AB). The data of the samples are shown in table 1. As can be seen two of the samples (B, C) also included sodium carbonate powder (Na₂Co₃, < 150 μιη). Water was added to the mixture and green pellets were produced in a disc pelletizer. The pellets had a moisture content of about 10 % by weight as measured using by LOD in accordance to ASTM D2216 - 10. The pellets were thereafter dried at room temperature to a moisture of 2 wt%.

The green pellets were reduced in a tube furnace at a temperature of 1300 °C for a time period of 4 hours in argon atmosphere. The pellets were thereafter allowed to cool to a temperature around 100 °C before evacuating the atmosphere and removal from the furnace. The reduced pellets were ground to powder and oxygen and carbon was analyzed. The result shows that with the addition of Na₂C0₃the carbon amount was considerably lowered. These pellets were also stronger.

Table 1

Sample Green pellet Reduced pellet

Nb₂0₅ C-powder Fe-powder Na₂C0₃ Weight C O Weight

% % % % g % % g

A 73 23 4 - 27 11.5 0.43 18

B 77 17 4 2 30 0.85 4.5 14.7

C 77 17 4 2 31 1.6 1.5 18.2 EXAMPLE 2

Green briquettes was produced having the same composition as the green pellets in Example 1. The bnquettes was formed by a briquetting pressure of 400 kg/cm². The green briquettes were reduced in a lab furnace at a temperature of 1300 °C. The reduced briquettes was visually examined to be of sufficient quality.

Claims

A process for producing iron and niobium containing agglomerates including the steps of:

a) mixing an iron containing powder, a niobium oxide containing powder, a

b) forming the mixture to provide a plurality of green agglomerates.

A process for producing iron and niobium containing agglomerates according to claim 1 wherein the dry matter of the green agglomerates comprises in weight %:

1-40 iron containing powder;

5-40 carbonaceous powder;

optionally

1-20 sodium carbonate containing powder;

1-10 binder and/or slag former;

balance at least 50 niobium oxide containing powder.

1-15 iron containing powder;

10-40 carbonaceous powder;

optionally

1-20 sodium carbonate containing powder;

1-10 binder and/or slag former;

balance at least 50 niobium oxide containing powder.

1-10 iron containing powder,

10-25 carbonaceous powder;

1-10 sodium carbonate containing powder;

balance at least 50 niobium oxide containing powder

A process for producing iron and niobium containing agglomerates according to any one of claims 1-4, wherein the agglomerates are pellets and the forming step b) includes:

adding a liquid to the mixture and pelletizing to provide a plurality of green pellets. A process for producing iron and niobium containing agglomerates according to any one of claims 1-4, wherein the agglomerates are briquettes and the forming step b) includes:

briquetting to provide a plurality of green briquettes.

A process for producing iron and niobium containing agglomerates according to any one of claims 1-6 wherein the niobium oxide containing powder includes at least 80% by weight of Nb₂0₅, preferably at least 90 % by weight of Nb₂0_5i more preferably at least 95 % by weight ofNb₂0_5.

A process according to any one of claims 1-7, wherein the iron containing powder contains at least 80 % by weight of Fe, preferably at least 90 wt%, more preferably at least 95 wt%, most preferably at least 99% by weight.

A process according to any one of claims 1-8, wherein the sodium carbonate containing powder contains at least 80 % by weight of Na₂C0₃, preferably at least 90 wt%, more preferably at least 95 wt%, most preferably at least 99% by weight.

10. A process according to any one of claims 1-9, wherein further including at least one of the steps: c) drying the green agglomerates to reduce the moisture content to less than 10 % by weight, more preferably less than 5 % by weight, preferably less than 3 % by weight;

e) cooling the reduced agglomerates in a non-oxidising atmosphere (e.g. reducing or inert) to a temperature below 200 °C to avoid re-oxidation of the agglomerates, more preferably below 150 °C in an inert atmosphere.

11. Iron and niobium containing green agglomerates comprising:

an iron containing powder;

a carbonaceous powder;

a niobium oxide containing powder; optionally

a sodium carbonate containing powder;

a binder and/or slag former. 12. Iron and niobium containing green agglomerates according to claim 11 having a dry matter composition in weight-% of:

1-40 iron containing powder;

5-40 carbonaceous powder;

optionally

1-20 sodium carbonate containing powder;

1-10 binder and/or slag former;

balance at least 50 niobium oxide containing powder.

13. Iron and niobium containing green agglomerates according to claim 11 having a dry matter composition in weight-% of:

1-40 Fe, preferably 1-15, more preferably 1.5-10;

10-35 O, preferably 15-30;

10-40 C, preferably 15-25;

Optionally 0 5-10 Na, preferably 1-5 Na;

< 10 of other elements, preferably < 5 of other elements, and

balance at least 20 Nb, preferably at least 30, more preferably at least 40.

14. Iron and niobium containing green agglomerates according to any one of claims 11-13, wherein the agglomerates are green pellets fulfilling at least one of the following conditions:

a moisture rate less than 10 % by weight, preferably less than 5 % by weight; a compression strength in the range of 100-1000 N/pellet, preferably in the range of

200-800 N/pellet ;

a geometric density in the range of 1.0-4.0 g/cm³, preferably 1.2-3.5 g/cm³; and a diameter in the range of 3-35 mm, preferably in the range of 5-25mm.

15. Reduced iron and niobium containing agglomerates having a composition in weight % of:

2-50 Fe;

< 10 O, preferably < 5, more preferably < 3;

<15 C, preferably < 5, more preferably < 3;

balance at least 50 Nb.

16. Reduced iron and niobium containing agglomerates according to claim 15 comprising 20-40 weight % Fe.

17. Reduced iron and niobium containing agglomerates according to claim 15 comprising 2-30 Fe;

< 10 O, preferably < 5, more preferably < 3;

<15 C, preferably < 5, more preferably < 3;

optionally 1-15 Na

< 15 of other elements, preferably < 5 of other elements, and

balance at least 60 Nb, preferably at least 80.

18. Reduced agglomerates according to claim 15 comprising in weight %:

2-20 Fe, preferably 3-15 Fe

< 5 O, preferably < 3;

<5 C, preferably < 3;

1.5-10 Na, preferably 2-7;

< 15 of other elements, preferably < 5 of other elements, and

balance at least 75 Nb, preferably at least 90.

19. Reduced agglomerates according to any one of claims 15-18, wherein the agglomerates are reduced pellets fulfilling at least one of the following conditions:

a compression strength in the range of 100-1000 N/pellet, preferably in the range of 200-800 N/pellet ;

a geometric density in the range of 1.0-5.0 g/cm³, preferably 1.3-4.0 g/cm³, more preferably 1.5 -3.5 g/cm³

a diameter in the range of 2-30 mm, preferably in the range of 3-20 mm.

20. Reduced agglomerates according to any one of claims 15-18, wherein the agglomerates are reduced briquettes having a geometric density in the range of 1.0-7.0 g/cm³, preferably 2.0-6.0 g/cm³, more preferably 3.0-4.5 g/cm³.