WO2014037385A1

WO2014037385A1 - Iron and tungsten containing pellets and iron, tungsten and molybdenum containing pellets

Info

Publication number: WO2014037385A1
Application number: PCT/EP2013/068263
Authority: WO
Inventors: Dag SJÖBERG
Original assignee: Ab Ferrolegeringar
Priority date: 2012-09-05
Filing date: 2013-09-04
Publication date: 2014-03-13
Also published as: TW201430143A

Abstract

The invention relates to iron and tungsten containing pellets and a process for producing the pellets. A green pellet is produced from mixing an iron powder, a tungsten containing powder, and pelletizing the mixture.

Description

IRON AND TUNGSTEN CONTAINING PELLETS AND IRON, TUNGSTEN AND MOLYBDENUM CONTAINING PELLETS

TECHNICAL FIELD

The present invention relates to a process for producing iron and tungsten containing pellets. The invention also relates to pellets produced by the process.

BACKGROUND

WO 11053231 discloses a method for producing an iron- and tungsten containing powder or powder agglomerate. A tungsten carbide containing powder is mixed with iron oxide powder and/or a tungsten oxide containing powder and optionally an iron powder. The mix is heated in a neutral or weakly reduced atmosphere.

WO2008091210 discloses an iron a tungsten containing powder comprising 30-60 % by weight of tungsten and balance iron. The powder is made by mixing an iron powder with a W0₃-powder. A bullet can be produced from the powder.

OBJECTS OF THE INVENTION

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

DESCRIPTION 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 tungsten containing pellets including the steps of: a) providing a mixture comprising (in weight-%):

2-97 tungsten containing powder containing at least one of

tungsten oxides and tungsten carbides,

optionally

0.1 -25 carbon powder,

2-90 molybdenum containing powder, and,

balance

1-40 iron powder;

b) adding to the mixture:

a liquid, preferably water,

optionally one or more of: binder,

slag former,

desulfurizer;

c) pelletizing to provide a plurality of green pellets

By this process it is possible to produce iron and tungsten containing green pellets and reduced green pellets described below.

When preparing the mixture and during pelletizing the total amount of added water is around 5-25 % by weight the mixture, more preferably 10-20 % by weight.

The green pellets are preferably dried to reduce the moisture content to less than 10 % by weight, preferably less than 5 % by weight, more preferably less than 3 % by weight. By reducing the water content the pellets may be heated at higher temperatures, e.g. above 200 °C, without cracking from quick vaporisation of the water present in the pellets.

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

The pellets may be dried in ambient air without heating, preferably for at least 12 hours.

When drying the pellets there is a temperature increase even when no external heat is used. This is believed to be from reactions when the iron oxidises. The strength of the pellets also increases. This makes it possible to provide sufficiently strong pellets that can be handled in rotary ovens without disintegrating, and without the need of adding binders, i.e. the iron powder replaces the need of a binder. Dust problems are also minimised.

The pellets may also be actively dried in as heater at a temperature up to 200 °C, preferably around 80-150°C. This shortens the required time of drying. The heater may be a rotary dryer; in particular a first section of rotary kiln that downstream includes a reduction zone. However, in some applications it may be advantageous to have a moisture content of the green pellets around 3-15 % by weight, preferably around 5-10 %. For example if the green pellets are added directly to a steel melt a "wet" green pellet may dissolve quicker.

Preferably reducing the dried pellets at a temperature in the range of 1050-1400 °C, preferably 1100-1300 °C, more preferably 1150-1250 °C, during at least 0.5 hours. Depending on the reduction time, the amount of carbon and the temperature the reducible oxygen in the pellets can be partially or essentially fully reduced. Preferably the reduction in step e) is performed during 0.5 - 10 hours, preferably 0.5-4 hours, more preferably 0.5 - 3 hours, most preferably 0.5-2 hours. Preferably, the heat treating step d) and the reduction step e) are performed at 0.8-1.2 bar, more preferably at atmospheric pressure. Optionally pre-reducing the green pellets derived from step c) at a temperature in the range of 400-800 °C during 0.5-2 hours, preferably less than 1 hour. The optional step is preferably employed when the green pellet includes molybdenum trioxide. By having pre-reducing at lower temperatures, vapour losses of Mo can be minimised by pre- reducing most of the M0O3 to Mo0₂ at a lower temperature. The optional pre-reduction step can be performed in the same furnace as the reduction step, or alternatively it would be possible to transfer the pre-reduced green pellets to another furnace for the reduction step.

The atmosphere in the furnace during heat treating and reduction may be an inert or a reducing gas, e.g. argon, N₂, H₂, or any mixture of H₂/N₂ (e.g. 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. Depending on purities of the powders, the mixture the pellets may contain further elements including oxides that are difficult to reduce. The amount of such elements are mainly determined by the purity of the tungsten containing powder and the optional molybdenum containing powder, but may also come from impurities in the iron powder, the carbon powder, and from reactions with elements in the surrounding atmosphere during heating, reduction, or cooling. The total process is endothermic and requires heat. To reduce the amount required external heat, oxygen gas or air can be provided in a 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.

Preferably cooling the 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 argon, N₂, H₂, or any mixture of H₂/N₂ (e.g. 5:95, 20:80, 40:60, 80:20, and 95:5 by vol.). If it is desirable to have very low levels of nitrogen, the pellets may be cooled in a nitrogen free

atmosphere such as for example an argon gas atmosphere.

The process may further crushing and/or grinding the pellets, and optionally sieving the crushed and/or ground pellets.

Suitable furnace types for the pre-reduction step and 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 preferred embodiment a rotary kiln is used to reduce the pellets. In a rotary kiln furnace the dried green pellets 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.

Instead of drying the green pellets before entering the kiln, the kiln may have a drying zone operating at a temperature in the range of 80-200 °C, preferably 100-150 °C. The green pellets are dried in this zone to a moisture content of less than 10 % by weight, preferably less than 5 % by weight.

The kiln may also include a pre-reduction zone, downstream the drying zone if such is used, and operating in the range of 400-800 °C, preferably 500-700 °C. A pre-heating zone may in particular be useful if the green pellet includes M0O3. I.e. to reduce at least a significant part of M0O3 in the green pellets to Mo0₂. Thereby vaporisation losses of Mo can be minimised. The kiln further includes a reduction section, downstream the drying and pre-heating sections if they are present. The reduction section provides a temperature zone in the range of 1050-1300 °C in which a significant part of remaining molybdenum oxides are reduced by the remaining carbon powder to MoO and/or Mo.

Mixture

The mixture provided in step a) comprises (in weight-%):

2-97 tungsten containing powder,

optionally

0.1-25 carbon powder,

2-90 molybdenum containing powder, and

balance

1-40 iron powder.

Preferably the iron powder is 2-25 % by weight, more preferably 3-15 % by weight. Preferably the tungsten containing powder is at least 20 % by weight.

Preferably, the tungsten containing powder + molybdenum containing powder is more than 50 % by weight of the mixture, more preferably more than 70 % by weight of the mixture.

In one embodiment the mixture consists of (in weight-%):

1-40, preferably 3-15 of a iron powder, and

75-99, preferably 85-97 of a tungsten containing powder.

Preferably, the tungsten containing powder includes tungsten oxides and tungsten carbides. Preferably, the reducible oxides in the tungsten containing powder and the iron powder are stoichiometric matched with carbon of the tungsten carbides, so that after a reduction the carbon content is less than 10 % by weight preferably less than 5 % by weight, more preferably less than 1 % by weight, most preferably less than 0.5 % by weight, and oxygen is less than 10 % by weight, preferably less than 5 % by weight, most preferably less than 3 % by weight.

Thereby iron and tungsten containing pellets can be produced that essentially consists of iron and tungsten and unavoidable impurities. Iron and tungsten containing pellets that consist of iron and tungsten and unavoidable impurities can also be produced from a mixture where tungsten carbides is partially or fully replaced by a carbon powder, i.e. so that carbon of tungsten carbides and/or carbon powder stoichiometric matches the reducible oxides in tungsten containing powder and the iron powder.

Iron and tungsten containing pellets that consists of iron, tungsten and molybdenum and unavoidable impurities can be produced from the mixture by adding the optional molybdenum containing powder. Here, carbon from the tungsten carbides and/or carbon powder is stoichiometric matched with the reducible oxides in molybdenum containing powder, the tungsten containing powder and the iron powder. Here, the tungsten containing powder preferably is a tungsten carbide powder comprising at least 70 % by weight of WC, preferably at least 95 % by weight of WC, or a tungsten oxide powder comprising at least 70 % by weight of W0₃, preferably at least 95 % by weight of WO₃, or a mixture of these powders.

Preferably the carbon and oxygen is balanced so that after reduction, the carbon content is less than 10 % by weight preferably less than 5 % by weight, more preferably less than 1 % by weight, most preferably less than 0.5 % by weight, and oxygen is less than 10 % by weight, preferably less than 5 % by weight, most preferably less than 3 % by weight.

The relative amounts of molybdenum and tungsten can be varied by changing the relative amounts of the tungsten containing powder and molybdenum containing powder, while considering the carbon and oxygen balance.

In a preferred embodiment the weight ratio between tungsten carbide and tungsten oxide (WC/W03) is within the range of 0.5-5, preferably 1-4, more preferably 1.5-3. An optimal balance is about 2. Thereby the tungsten carbide can match the tungsten oxide without the need of carbon powder addition.

In one embodiment the weight ratio of molybdenum and tungsten (Mo/W) are determined to be within the range 0.25 - 4, preferably 0.5 - 2, more preferably 0.8-1.25.

Tungsten containing powder

The tungsten containing powder is preferably one of: a tungsten carbide containing powder,

a tungsten oxide containing powder,

a mix of tungsten carbide containing powder and tungsten oxide containing powder.

Tungsten carbide containing powder

The_tungsten carbide containing powder is a powder that comprises tungsten carbides contained in a metal matrix. Preferably the tungsten carbide containing powder is obtained from tungsten cemented carbide scrap. The tungsten carbide containing powder preferably comprises 1-10 % by weight of carbon, balance tungsten and incidental impurities. The tungsten carbide containing powder may also include alloy elements which have formed a matrix (binding material) for the cemented tungsten carbide material. The proportion of carbide phase is generally between 70-97% of the total weight of the composite. The carbon is present in the powder particles in the form of tungsten carbide grains, and typically the grain size averages between 0.10 μιη and 15 μιη. Any powder particle may include several tungsten carbide grains, in particular if the particle sizes are large. Further, the tungsten carbide containing powder may include powder particles that are void of any tungsten carbide grains; however most of the powder particles will include one or more grains of tungsten carbide.

Some tungsten carbide powder can contain cobalt up to 15 %.by weight; typically around 1-10 % by weight of Co. For instance, the tool material in circuit board drills typically comprises fine grained, cemented tungsten carbides existing in a cobalt matrix, the amount of which represents 6 percent of the total weight of the tool material, while coarse grain tungsten carbide materials typically are used for the tool material of mine drills, where the cobalt content of the cemented carbide material is about 10 weight-%. These powders can be used if cobalt can be allowed or is desirable in the pellet to be produced. If not, these powders can be used after being leached from cobalt. For instance a commercially available tungsten carbide containing powders from scrap that comprises 1-10 % by weight Co, usually in amounts of 3-8 % by weight Co, can be hydrometallurgical leached to reduce the cobalt content to be less than 1 % by weight Co, preferably less than 0.5 % by weight Co, more preferably less than 0.2 % by weight Co. The cobalt from the leaching process can be recycled and employed as a

commercial product per se.

Of course a tungsten carbide powder that already is low or void of cobalt can be used. I.e. a powder that contains less than 1 % by weight Co, more preferably less than 0.5 % by weight Co, even more preferably less than 0.2 % by weight Co.

Preferably the tungsten carbide powder contains at least 90 % by weight of WC, more preferably at least 95 % by weight.

Preferably at least 90 % by weight, more preferably at least 99 % by weight, of the particles of the the tungsten carbide containing powderpass through a test sieve in accordance to ISO 3310-1 :2000 having nominal aperture sizes of 250 μιη, more preferably 125 μιη, most preferably 90 μιη. Very fine powder where at least 99 % by weight, passes through a test sieve of 45 μιη can suitably be used.

Tungsten oxide containing powder

The tungsten oxide containing powder may be an iron and tungsten oxide containing powder, more preferably iron tungstate in the form of the mineral Ferberite. Preferably a feberite that contains over 60 % of W03, more preferably at least 70 % W03. The Ferberite is crushed and/or milled and/or ground to a powder so that at least 80 % by weight of the particles, preferably at least 90 %, passes through a test sieve in accordance to ISO 3310-1 :2000 having nominal aperture sizes of 250 μιη, more preferably 125 μιη.

The tungsten oxide containing powder may also be a pure tungsten oxide powder containing less than 5 % by weight of other elements besides W and O, preferably less than 1 % by weight of other elements. E.g. a powder that includes at least 95 % by weight of W0₃, preferably at least 99 % by weight.

Preferably at least 80 % by weight of the particles, more preferably at least 90 % by weight of the particles of the tungsten oxide powder pass through a test sieve in accordance to ISO 3310-1 :2000 having nominal aperture sizes of 250 μιη, more preferably 125 μιη. most preferably 90 μιη. Very fine powder where at least 99 % by weight, passes through a test sieve of 45 μιη can suitably be used.

The tungsten oxide containing powder may also be a mix of iron tungstate and pure tungsten oxide powder.

Other available grades of tungsten oxide powders may also be used. Molybdenum containing powder

The molybdenum containing powder is preferably a molybdenum oxide powder. The powder preferably consists of molybdenum dioxide and/or molybdenum trioxide powder.

The molybdenum oxide powder should contain 50-80 % by weight of Mo, the remaining elements being oxygen and impurities. Preferably the impurities are less than 10 % by weight, more preferably less than 5 % by weight, most preferably less than 1% by weight.

Preferably at least 90 % by weight, more preferably at least 99 % by weight, of the particles of the molybdenum oxide powder pass through a test sieve in accordance to ISO 3310-1 :2000 having nominal aperture sizes of 250 μιη, more preferably 125 μιη, most preferably 45 μιη.

Iron powder

The iron powder is preferably an iron powder containing at least 80 wt% metallic iron, preferably at least 90 wt% metallic iron, more preferably at least 95 wt% metallic iron, most preferably at least 99 wt% metallic iron. 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.

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.

Preferably 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 in accordance to ISO 3310-1 :2000 having nominal aperture sizes of 125 μιη, more preferably 90 μιη. Very fine powder where at least 99 % by weight, passes through a test sieve of 45 μιη can suitably be used. Optional carbon powder

The green pellets preferably includes a carbon source. In the preferred embodiment the carbon source is a tungsten carbide containing powder, where the carbon content stoichiometric matches the oxide contents in the green pellets. However, a carbon powder may also be used as the carbon source, either in combination with a tungsten carbide containing powder or as the sole carbon source. The carbon powder may be added from 0.1% by weight. It may be between 1-25 % by weight.

The carbon 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 carbon powder may e.g. be soot, carbon black, activated carbon. The carbon powder can also be a mixture of different carbon powders.

Regarding the choice of carbon powder, the reactivity of the carbon is preferably taken into consideration. Preferably carbon black is used. German brown coal (lignite), charcoal, bituminous and sub-bituminous coals also have comparably high reactivity.

Preferably at least 90 % by weight, more preferably at least 99 % by weight, of the particles of the carbon powder pass through a test sieve in accordance to ISO 3310- 1 :2000 having nominal aperture sizes of 125 μιη, more preferably 45 μιη, most preferably 20 μιη.

The amount of the carbon source (WC -powder and/or carbon powder) is preferably determined by analysing the amount of reducible oxides in the tungsten containing powder, the iron powder, and the optional molybdenum containing powder. Preferably the amounts of the carbon source is chosen to stoichiometric match or slightly exceed the amount of reducible oxides in the tungsten containing powder, the iron powder, and the optional molybdenum containing powder. However, the amount of the carbon source may also be sub-stoichiometric.

The amount of the carbon source can be optimised by measuring the carbon levels and the oxygen levels in the reduced pellets - increasing or decreasing the amount of carbon source to achieve desired levels of carbon and oxygen. Oxides which are difficult to reduce with carbon such as Si, Ca, Al, and Mg may be allowed up to certain levels depending on in which applications the pellets are to be used in. For instance in many applications of steel metallurgy these oxides can be handled e.g. by removing them in the slag of steel melt. If lower amounts of these oxides and elements are desired, purer grades of the tungsten containing powder, the iron powder, and the optional

molybdenum containing powder can be used, e.g. grades that contains less or no amounts of these oxides.

Iron and tungsten containing green pellets

The iron and tungsten containing green pellets having a dry matter composition in weight-% of:

a) 90-100 of a mixture comprising in weight % of the mixture:

2-97 tungsten containing powder containing at least one of

tungsten oxides and tungsten carbides, optionally

0.1-25 carbon powder,

2-90 molybdenum containing powder, and balance

1-40 iron powder containing powder;

< 10 binder and/or slag former and/or a desulfurizer.

When preparing the mixture and during pelletisation the total amount of added water is around 5-25 % by weight of the mixture, more preferably 10-20 % by weight. The green pellets are preferably dried to reduce the moisture content to less than 10 % by weight, preferably less than 5 % by weight, more preferably less than 3 % by weight.

When producing the green pellets one or more organic or inorganic binders and/or slag formers and/or desulfurizers may optionally be added. The binder may e.g. be a carbon containing binder. Other binders may e.g. be bentonite and/or dextrin. The slag former may e.g. be limestone, dolomite, and/or olivine. The total amount of optional binder/s and/or optional slag former/s and/or desulfurizers should be less than or equal to 10 % by weight, preferably less than or equal to 5 wt%, of the dry matter of the pellet. Most preferably the green pellets are void of binders, slag formers and desulfurizers.

The dried green pellets are surprisingly strong and it may therefore be possible to use the dried green pellets to directly alloy a steel melt with tungsten and optionally tungsten and molybdenum, i.e. without prior reduction of the green pellets. The green pellets can be cost efficient way of alloying with tungsten and optionally tungsten and molybdenum. The green pellets may also be partially or fully reduced in by heating the green pellets in subsequent steps.

The green pellets may have a geometric density in the range of 1.5-5.5 g/cm³, preferably 2-5 g/cm³.

A green pellet comprising 5-15 % by weight iron powder (> 99 Fe) and 85-95% by weight of a tungsten containing powder (WO₃+WC > 95wt%) may have compression strength around 10-50 N/pellet directly after pelletizing. After drying the pellets the compression strength may increase to around 50-150 N/pellet. For pellets where the tungsten containing powder is partially replaced by molybdenum oxide powder (e.g. replacing tungsten oxide) the compression strength is similar directly after the pelletizing, but after drying the compression strength may reach as high as 600 N/pellet, depending how much is substituted. The compression strength determined by increasing the load on a pellet until it is crushed.

In one embodiment the green pellets have a dry matter composition of 80-98 % by weight of a tungsten carbide powder and a tungsten oxide powder balanced being an iron containing powder. The weight ratio between tungsten carbide powder and tungsten oxide (WC/W03) being within the range of 1.5-3, preferably about 2. Thereby the tungsten carbide can match the tungsten oxide without the need of carbon powder addition. Such green pellets may have a geometric density in the range of 3-5.5 g/cm³, preferably 3.5-5 g/cm³.

Iron and tungsten containing pellets

Iron and tungsten containing pellets can be produced by the suggested process that consists of in weight %:

W 3-97, preferably 30-95,

Mo+ W 50-97, preferably 70-95,

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

C <10, preferably < 5, more preferably < 1,

Si < 10, preferably < 5, more preferably < 1,

Co < 10, preferably < 5, more preferably < 1,

Other elements < 5, preferably < 1,

and balance Fe 2-40, preferably 3-25, more preferably 5-20, most preferably 5-15. These pellets can have a geometric density in the range of 2-7 g/cm³, preferably 3-6 g/cm³ and the compression strength can be in the range of 100-1000 N/pellet, preferably 150-600 N/pellet, more preferably 200-500 N/pellet.

O, C may be present from 0.05 % and higher. Si, Co may be present from traces up to the given amounts. They are preferably not deliberately added but may be present as impurities. Other elements apart from W, Mo, Fe, O, C, Si, Co may be present from traces up to the given amounts. They are preferably not deliberately added but may be present as impurities.

According to one example the iron and tungsten containing pellets consists of in weight

%:

W 60-97, preferably 80-95,

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

C <10, preferably < 5, more preferably < 1,

Si < 10, preferably < 5, more preferably < 1,

Co < 10, preferably < 5, more preferably < 1,

Other elements < 5, preferably < 1,

and balance Fe 2-40, preferably 3-25, more preferably 5-20, most preferably 5-15.

These pellets can have a geometric density in the range of 3-7 g/cm³, preferably 4-6 g/cm³ and the compression strength can be in the range of 100-1000 N/pellet, preferably 150-400 N/pellet, more preferably 200-300 N/pellet.

These pellets may substitute traditionally manufactured ferrotungsten alloys, when alloying with tungsten in melting practices. The pellets can be produced at lower costs than standard grades of ferrotungsten. Furthermore, due to their porous structures the pellets dissolves quicker than standard grades of ferrotungsten.

According to another example the iron and tungsten containing pellets consists of in weight %: W 20-80, preferably 30-65, more preferably 40-55,

Mo 20-80, preferably 30-65, more preferably 40-55,

Mo + W > 50, preferably >70,

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

C <10, preferably < 5, more preferably < 1,

Si < 10, preferably < 5, more preferably < 1,

Co < 10, preferably < 5, more preferably < 1,

Other elements < 5, preferably < 1,

and balance Fe 2-40, preferably 3-25, more preferably 5-20, most preferably 5-15. Preferably, the weight ratio of molybdenum and tungsten (Mo/W) are determined to be within the range 0.25 - 4, preferably 0.5 - 2, more preferably 0.8-1.25.

These pellets can have a geometric density in the range of 2-6 g/cm³, preferably 3-5 g/cm³ and compression strength in the range of 100-1000 N/pellet, preferably 200-600 N/pellet, more preferably 250-500 N/pellet. These iron, tungsten and molybdenum containing pellets are suitable for alloying with tungsten and molybdenum in melting practices. The iron, tungsten and molybdenum containing pellets can be produced at comparably lower costs. Furthermore, due to their porous structures the pellets dissolves quickly in a steel melt. The amount of other elements is mainly controlled by the purity of the tungsten containing powder and the optional molybdenum containing powder. The purity of the iron containing powder and optional carbon powder may of course influence the amount of other elements. The nitrogen content mainly depends on the nitrogen levels in the atmosphere during heating, reduction and cooling of the pellets. By controlling the atmosphere in these steps the nitrogen content can be made lower than 0.5 wt%, preferably lower than 0.1 wt% and most preferably lower than 0.05 wt%. The average diameter of the pellets are preferably in the range of 3-30 mm, preferably 5-20 mm. Too large pellets may prolong the needed reduction time, while too small pellets can be difficult to handle.

The shape of the 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.

However, in other 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 vacuum 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 have 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 also be crushed to irregular shaped pieces, e.g. a coarse iron and tungsten containing powder, where 90 % by weight of the powder particles are contained by a test sieve in accordance to ISO 3310-1 :2000 having nominal aperture sizes of at least 250 μιη, preferably at least 500 μιη, more preferably at least 1 mm.

The pellets may further be ground and optionally sieved to provide a fine iron and tungsten containing powder. Preferably the fine powder having particle size wherein at least 90 % by weight, more preferably at least 99 % by weight, of the particles pass through a test sieve in accordance to ISO 3310-1 :2000 having nominal aperture sizes of 250 μιη, more preferably 125 μιη, most preferably 45 μιη. The fine powder can e.g. be provided as a core filling of a cored wire for injection alloying or welding application.

The powders may be cold briquetted.

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.

EXAMPLE

1/3 by weight of W03 powder was mixed with 2/3 by weight of a WC powder. To the mixture 5 % by weight of Fe powder was added. The amounts, particle sizes and the purity of the powders are shown in Table 1. When mixing the powders, 5 % by weight of water was added Table 1

The mixture was thereafter fed to a disc pelletizer for pelletizing. During pelletizing additionally about 5-7 % by weight of water was added. The produced green pellets where thereafter dried to a moisture content less than 3 % by weight.

The average geometric density of the dried green pellets was determined to be 4.3 g/cm³ as measured according to ASTM 962-08.

The green pellets were reduced in a batch furnace at a temperature of 1200 °C for a time period of 2 hours, in a 95 vol-%> N₂ and 5 vol-%> H₂ atmosphere. The pellets were thereafter allowed to cool to a temperature around 100 °C before evacuating the atmosphere and removal from the furnace. The average geometric density of the reduced pellets was determined to be 5.0 g/cm³ as measured according to ASTM 962- 08.

Claims

1. A process for producing iron and tungsten containing pellets the process including the steps of:

a) providing a mixture comprising (in weight-%):

2-97 tungsten containing powder containing at least

tungsten oxides and tungsten carbides,

optionally

0.1-25 carbon powder,

2-90 molybdenum containing powder, and

balance

1-40 iron powder; b) adding to the mixture:

a liquid, preferably water,

optionally one or more of:

binder,

slag former,

desulfurizer; c) pelletizing to provide a plurality of green pellets.

2. A process according to claim 1 wherein the mixture fulfilling the condition:

> 50 molybdenum containing powder + tungsten containing powder.

3. A process according to claim 1 and 2 wherein the process further includes drying the green pellets at a temperature less than 200 °C, preferably less than

150 °C, until the moisture content is less than 10 % by weight of the pellets, preferably less than 5 % by weight.

4. A process according to any one of claims 1- 3 wherein the process further includes reducing the green pellets at a temperature in the range of 1050-1400 °C, preferably 1100-1300 °C, more preferably 1150-1250 °C, during at least 0.5 hours.

5. A process according to claim 4, wherein the method includes one or more of the following steps:

d) cooling the reduced pellets in a non-oxidising atmosphere to a temperature below 200 °C , more preferably below 150 °C, preferably in an inert atmosphere;

e) crushing and/or grinding the reduced pellets;

f) sieving the crushed and/or ground reduced pellets;

g) 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 reduced pellets to pellet agglomerates comprising 2-300 pellets.

6. Iron and tungsten containing green pellets having dry matter composition in weight %

a) 90-100 of a mixture comprising in weight % of the mixture:

2-97 tungsten containing powder containing at least one of

tungsten oxides and tungsten carbides, optionally

0.1 -25 carbon powder,

2-90 molybdenum containing powder, and

balance

1-40 iron powder;

b) optionally up to 10 of a binder and/or slag former and/or a desulfurizer.

7. Iron and tungsten containing pellets consisting of in weight %:

W 3-97,

Mo+ W 50-97,

O < 10,

C <10,

Si < 10,

Co < 10,

Other elements < 5,

and balance Fe 2-40.

8. Iron and tungsten containing pellets according to claim 7, wherein the pellets have a geometric density in the range of 2-7 g/cm³, preferably 3-6 g/cm³ and compression strength in the range of 150-600 N/pellet, preferably 200-500 N/pellet.

9. Iron and tungsten pellets according to claim 7 consisting of in weight %:

W 60-97, preferably 80-95,

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

C <10, preferably < 5, more preferably < 1,

Si < 10, preferably < 5, more preferably < 1,

Co < 10, preferably < 5, more preferably < 1,

Other elements < 5, preferably < 1,

and balance Fe 2-40, preferably 3-25, more preferably 5-20, most preferably 5- 15.

10. Iron and tungsten containing pellets according to claim 9, wherein the pellets have a geometric density in the range of 3-7 g/cm³, preferably 4-6 g/cm³ and compression strength in the range of 150-400 N/pellet, preferably 200-300 N/pellet

11. Iron and tungsten containing pellets according to claim 7 consisting of in weight

%:

W 20-80, preferably 30-65, more preferably 40-55,

Mo 20-80, preferably 30-65, more preferably 40-55,

Mo + W > 50, preferably >70,

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

C <10, preferably < 5, more preferably < 1,

Si < 10, preferably < 5, more preferably < 1,

Co < 10, preferably < 5, more preferably < 1,

Other elements < 5, preferably < 1,

12. Iron and tungsten containing pellets according to claim 11, wherein the pellets have a geometric density in the range of 2-6 g/cm³, preferably 3-5 g/cm³ and compression strength in the range of 200-600 N/pellet, preferably 250-500 N/pellet.

13. An iron and tungsten containing pellet according to any one of claims 7 wherein the pellets having an average diameter in the range of 3-30 mm preferably 5-20 mm.