EP0839920B1 - Process for preparing a starting powder material for fine grained hard metal - Google Patents

Abstract

In the production of a starting powder for fine grained cemented carbide manufacture from a metal oxide or reducible metal compound powder by reducing to metal, carburizing and subjecting to a mechanical-chemical powder treatment and/or mixing operation, the reduction and/or carburization step employs a cyclone unit which is a heated rotationally symmetrical reaction chamber having inlet and outlet openings for the powder, reagent and carrier gas. The continuously supplied powder retains its solid state during high velocity passage through the reaction chamber along predetermined flight paths which are tangential to the chamber wall and which are controlled by means of the powder flight direction and velocity at the inlet and by means of the flow parameters of the carrier and/or reaction gas. The unit produces ≥ 90 vol.% chemical reaction of the powder in a dwell time of 0.1-60 (preferably 0.2-10) s.

Description

• Reduktion des Oxids oder der Metallverbindung zu Metall
• Karburierung des Metalls
• mechanisch-chemisches Pulverkomaufbereitungs- und/oder Mischverfahren mit einschließt.

The invention relates to a method for producing powder press batches for fine-grained hard metal based on metal carbide, which starts from metal oxide powder or reducible solid metal compounds, and the process stages

• Reduction of the oxide or metal compound to metal
• Carburizing the metal
• includes mechanical-chemical powder preparation and / or mixing processes.

The quality that can be achieved for a type of hard metal depends to a large extent on the nature of the starting powder, which is pressed and processed into a shaped hard metal body by sintering. The chemical-metallurgical composition is just as important as the powder morphology, powder structure and consequently the powder preparation before pressing and sintem.
In recent years, efforts to produce improved carbide grades have focused primarily on achieving fine grain and homogeneity of the carbide in the carbide.

The metallic components in the carbide hard material phase of the hard metal are mainly tungsten or titanium, in addition in the form of stabilizing Mixed carbides small amounts of the high-melting metals tantalum, niobium, Molybdenum, vanadium and chrome.

In the long series of processes for ore processing these metals are Processing steps, starting with the reduction of powdered metal oxides or comparable compounds such as ammonium metallate and metallic acid, to towards metal carbide extraction essential for the later quality of Carbide grades, especially with regard to their structure.

Metal oxides or comparable compounds familiar to the person skilled in the art are reduced to pure metal in one or more process stages and then the metal, usually in a separate process step, converted into metal carbide. Reduction and carburization are isolated also carried out in a common, ongoing process.

Metal oxides are used for metal oxide reduction in a solid-gas reaction on carrier ships in comparatively thin layers led a reduction furnace. The reduction in the Rotary kiln and in the fluidized bed.

Usual processes for carbide formation are the intimate mixing of metal powder, for example tungsten metal powder, with carbon (soot particles) and subsequent reaction in a carburizing furnace.
In addition to powder quality, commercial production also plays an important role in commercial production. It is determined by the price of the device in accordance with the complexity of the process, the amount of energy and reaction gas consumed per reaction unit and, above all, the factor of production time.
The reaction or throughput times of the powders in the devices listed are without exception in the range of hours, at best 1 to 2 hours, at worst 15 to 20 hours of reaction time.

In addition to the chemical process steps, the overall production also includes Reduction and carburization process steps such as grinding and mixing processes, which usually take many hours. Are next to it Powder preparation processes, e.g. granulating the powder by means of Spray drying, largely indispensable in the carbide industry.

In connection with the demand for submicron and nanophase powder for Tungsten carbide powder batches, new processes were developed in which the usual reduction and carburization, including special ones Powder preparation and preparation processes, simultaneously or in one mandatory sequence of steps.

Representing such processes and devices used for them may be mentioned WO 91/07244 with the title "spray conversion process for the production of nanophase composite powder "and WO 95/04703 with entitled "Process for the Production of Submicrometer Carbides, Submicrometer mixed crystal carbides and resulting materials ", in this patent reference is made to US 5 110 565, in which a Reactor is described as it is for carrying out the process WHERE application was used.

The disadvantage of these processes is the high manufacturing and Device costs. In total, the response times are indispensable process steps to be carried out despite individual reductions the previously known state of the art is still in the order of magnitude from one to several hours and thus an essential cost factor. This All in all, procedures are uneconomical, especially when it comes to profit powder fine grain compared to that described above Standard procedure applies.

The object of the present invention is therefore to compare one with the state of the art Technology more economical process for the production of pressable To provide powder mixtures for hard metal production, which the Manufacture of high quality, especially fine-grained carbide grades allowed.

The object of the present invention is also to select one for Implementation of the method suitable device, such Devices for the chemical conversion of various organic and inorganic materials are known for the production of hard metal powder batches to achieve homogeneous, fine-grained carbide grades have not been used so far.

The above object is achieved according to the invention by a method according to claim 1.
The term "hard metal" in claim 1 also includes materials known under the name of cermet, which in addition to carbides also contain substantial amounts of nitrides or carbonitrides in the hard material phase.

The inventive method requires the use of a Term "cyclone" common reaction chamber.

Cyclones are characterized in that they generally have axially or rotationally symmetrical chamber walls. The material to be reacted in the form of solid particles is intimately mixed and swirled with carrier and / or reaction gas immediately upon entry into the reaction chamber and continuously blown in as a mixture in a direction deviating from the longitudinal axis of the chamber. The substances introduced in this way move under the action of gravitation and centrifugal forces in accordance with the gas flow conditions prevailing in the chamber on essentially predetermined trajectories, that is to say not in a statistical movement, for example in a fluidized bed furnace. The gas and particle flow is determined by the chamber walls, including any guide elements that may be attached there.
There are high flow velocities tangential to these chamber walls. High relative speeds between solid and gaseous substances occur in the reaction chamber. High speed gradients between individual substances mean high turbulence intensities and result in high heat and mass transfer numbers for the individual reactants in the desired chemical reaction.

The duration of the reaction material in the chamber is device and process-related small. The stay or reaction times depend on Plant design between tenths of a second and about a minute.

Such cyclone reactors are already used in the pyrolysis of Sawdust: J. Lede et al, "Flash Pyrolysis of Wood in a Cyclone Reactor", Chem. Eng. Proc. 20 (1986), pp. 309-317; J. Cousins et al, "Gasification of Sawdust in an air blown cyclone gasifier ", Ind. Eng. Chem. Process Des. Dev. 24 (1985), pages 1281-1287; in the slag combustion and the Incineration of sludge residues, T. Murakami et al, "Characteristics of Melting Process for Sewage Sludge ", Wat. Sci, Tech. 23 (1991), pages From 2019 to 2028. The processes ultimately also find for exothermic metallurgical Processes, e.g. for melting copper, lead and zinc Copper concentrates application, as described in DE 33 41 154 and in the technical essay "The levitation and other performance-intensive Processes ", A. Lange, Erzmetall 13 (1960), pages 151-159.

However, all processes previously carried out in cyclones, in particular also metallurgical processes, have in common that the material to be reacted is introduced into the cyclone as solid particles, but that the solid particles are converted into the liquid or mostly gaseous phase in order to carry out the desired reaction and that in the end, the desired reactants, especially also reduced metals, leave the cyclone in gaseous or molten form.
As a rule, chemical reactions take place faster in the molten or gaseous phase than in the solid phase and thus meet the short dwell times and reaction times in the cyclone or appeared indispensable to the person skilled in the art.
Experience from cyclone reactions while maintaining the solid phase for at least one reactant was therefore not available. The technically and commercially satisfactory implementation of such a cyclone process was therefore neither specified nor suggested

Unlike with chemical reactions in the cyclone according to the prior art must reduce and / or carburize corresponding solid particles done while maintaining the solid phase. Liquefaction and / or Volatilization of the solid particles and subsequent sublimation would occur lead to end products (powders) in the cyclone reaction which, owing to their structural powder texture not for further processing into hard metal usual quality are suitable today.

A major advantage of the present method over the known methods for producing submicron or nanophase powder for powder press approaches for the production of hard metal is that Raw material powder (metal compounds to be reduced) can be used can, as they do without special additional treatment from the ore processing be provided and after applying this procedure to carbide very uniform and fine-grained structure can be processed.

Those made very economically from the process in question Powder batches allow hard metal qualities to reach those of those correspond to or are even superior to those after the ones above described method, production of nanophase composite powder, Manufacture of submicron carbides.

The reaction times in the chemical process steps reduction and Carburization according to the present invention until complete reaction for at least 90% by volume of the material to be reacted in the solid However, the phase is significantly lower than that of the known processes. Thereby is a much higher cost-effectiveness of the process in question given compared to the known prior art. The cost advantages of the subject procedure are enlarged due to a comparatively simple construction of cyclone reaction chambers, and because of comparatively cheaper energy and Gas consumption data.

Metal oxides, or standard compounds which are alternatively available for the reduction to metal powder, are usually provided in particle sizes between approximately 2 and 30 μm and, according to the process according to the invention, give metal powders with a particle size approximately comparable to the starting powder size, but with a significant proportion of agglomerated powders.
Agglomerated powders are generally not a good starting point for the production of extremely fine-grained carbide. It was completely surprising, however, that the metal powders produced by chemical reduction according to the present invention consistently have an extremely high, sponge-like microporosity in the range of 0.1 μm. However, this means that the metal powder for further processing to carbide and hard metal has a quality that was previously known only approximately from the nanophase process mentioned as previously known. The entire volume of the metal powder can be completely carburized in a cyclone reaction and leads to a previously unattainable, fine-grained hard metal quality.

According to a preferred embodiment of the chemical process step in Cyclone is the throughput time of the material to be reacted in the solid Phase 0.2 to 10 s, for at least 90% by volume of the solid phase complete chemical conversion to the predetermined reaction state. To increase the volume fraction with complete chemical reaction the individual chemical process step in the cyclone optionally at least in repeated in another run.

According to a further preferred embodiment of the method, the Metal oxide powder or the powdery metal compound as that too reducing material before the process step reduction metallic Filler materials added, especially those in hard metal as binder metal metals Co and / or Ni used. This is done by adding metallic powder, or by making a solid beforehand Solution, that is, by introducing the filler materials into the solid phase of the good to be reduced.

According to individual preferred embodiments, the following result Variants for the invention comprising several partial steps Process for the production of powder press batches for further processing to fine-grain carbide.

A first method variant of the method according to the invention consists in the reduction of metal oxide or comparable metal compounds Cyclone metal powder in the cyclone process; at high Purity requirements for the metal powder to be produced also in one Repeat this reduction step.

Then the metal powder thus obtained is in a in Carbide manufacturing commonly used ball mill with carbon particles intimately mixed. In doing so, agglomerates of spongy Crushed metal powder. This mixing and Milling process powdered additional metals (to form mixed carbides as Grain growth inhibitors in hard metal) added. The powder mixture is further converted to metal carbide in the carburizing furnace by conventional methods converted, according to standard procedures with the binding metal (cobalt and / or Nickel powder) mixed and optionally over attritor grinding and Spray drying transferred into a ready-to-press powder batch. The so obtained Powder press approach can be too much using conventional pressing and sintering processes Process fine-grain carbide with very high phase homogeneity.

According to a second variant, the metal oxides are reduced to metal powder in the cyclone, as above. In contrast to the process described above, the metal powder obtained in this way is also further processed in the cyclone to metal carbide by the cyclone process which is essential to the invention, namely in two sub-variants,
either following prior external mixing with carbon particles (as above) by simultaneously blowing this mixture together with carrier gas and possibly with reaction gas into the reaction chamber,
or according to a second sub-variant, by directly blowing the metal powder into the cyclone reactor together with gaseous carbon compounds, in particular hydrocarbon gases and / or CO. This variant is also supplemented by customary grinding, mixing and granulating processes, with grinding and granulating processes not necessarily having to take place.

According to a third preferred variant of the method, the Metal oxides together with a reducing gas and a carbon containing gas into the cyclone reaction chamber, or blown in and it takes place during a single overall run a first part of a spatially uniform overall reactor Reduction of the oxide to metal powder and immediately afterwards in one second chamber part to carburize the reduced metal powder Metal carbide.

As stated for the first variant, in addition to the base metal, for example tungsten and / or titanium, additional metals for mixed carbide formation, such as niobium, tantalum, vanadium and chromium, can also be added to the carburizing process in the cyclone and simultaneously converted to carbides with the main metal.
The above also applies to the accompanying mixing, grinding and granulating processes, as well as their mandatory or optional use.

The process according to the invention is illustrated by the examples below described in more detail.

EXAMPLE 1:

A cyclone with the features of the present invention and corresponding to the representation in FIG. 1 is used as the device for carrying out the reduction process. The overall system shown in FIG. 1 is composed of a steel reaction chamber designed as a cyclone, which is followed by a second reaction chamber designed as a downpipe for chemical aftertreatment of the reacted material, this aftertreatment and the associated reaction chamber not being part of the invention.
It is a pilot plant that is smaller in comparison to plants for industrial production in terms of the throughput quantity of goods to be reduced per unit of time.

In the first stage, powdered W ₄ O _{11 is} blown in via a feed device (1) together with reaction and / or carrier gas into the head part of a reaction chamber (2) which is approximately rotationally symmetrical with respect to the direction of fall. The amount of gas is metered by means of a flow meter (7). The reaction chamber is brought to a reaction temperature of 1100 ° C. by means of an electrical heating device (6). The pulverulent reaction product leaves the chamber at the lower end, falls into a storage room with screw conveyor (3) and is introduced into the second reaction chamber (4) with heating device (6). The exhaust gas (8), reaction and / or carrier gas and H ₂ O vapor as the end product of the reaction leave the first chamber at the top.
In the second process stage, both the reacted material, that is high-purity tungsten powder, and the exhaust gases emerge at the lower end of the vertically aligned tubular chamber. The tungsten powder is collected in a container (5).
The temperature of the entire two-stage process is controlled by means of a thermocouple (9) at the exhaust outlet of the first reaction chamber.

With a continuously added powder throughput of 1000 g W ₄ O ₁₁ (tungsten oxide blue) per hour, a gas amount of 4000 l H ₂ gas is used, ie a large excess of gas based on the stoichiometric reaction amounts. Tungsten oxide as the material to be reduced and H ₂ as the carrier or reducing gas are fed to the cyclone separately. The carrier or reducing gas is preferably introduced horizontally into the chamber at the upper end at a high flow rate. The pulverulent material to be reduced is brought up to the gas inlet nozzle in such a way that it is entrained by the gas jet as it enters the chamber, is swirled and mixed intensively with it, and passes through the chamber in accordance with the guidance of the gas flow on predetermined trajectories. The material reduced to tungsten powder leaves the reduction chamber after a passage time of 1-2 s and has a residual oxygen content of 10,500 µg / g at the outlet. The emerging tungsten powder has a grain size comparable to that of the powder taken in, of the order of 20 µm in diameter, but the individual powder particles or grains have a large porosity throughout their entire volume. The spatial expansion of the substructure in the tungsten particle is 0.1 µm.

For the production of high-purity tungsten powder with a very low residual oxygen content the reduction process in the cyclone is repeated once.

The tungsten powder obtained in this way is converted into carbide by customary methods. For this purpose, the tungsten powder is first intensively mixed in the ball mill with a stoichiometric proportion of fine soot particles for tungsten carbide, WC. Individual agglomerates of the tungsten powder are crushed. The batch obtained in this way is carburized in a graphite furnace with induction heating under an H ₂ atmosphere at 1300 ° C. for 3 hours. Pure tungsten carbide with a carbon content of 6.12% and a residual oxygen content of 1200 µg / g is formed.

The carbide is made with binder metal and usual amounts of mixed carbides (Niobium carbide, tantalum carbide) mixed and optionally via attritor grinding and Spray drying processed into free-flowing granules.

From such powder batches by means of pressing and sintering according to the usual Processed carbide samples have exceptionally large Fine grain with a very homogeneous carbide structure.

EXAMPLE 2:

The device used corresponds to that of Example 1 without a downpipe being connected downstream of the cyclone.
Tungsten oxide, blue, is reduced to tungsten powder in the cyclone in accordance with the process conditions mentioned in Example 1.
In a departure from example 1, the tungsten powder is then further processed into tungsten carbide in a cyclone reactor lined with graphite using carbon-containing gases plus carrier gas (CH ₄ / H ₂ mixture). Carburization takes place in one step at a cyclone temperature of 1100 ° C.
A gas throughput of 6000 l / h is regulated for a tungsten powder throughput of 1000 g / h. The methane concentration in the CH ₄ / H ₂ mixture is 1.1% by volume. This corresponds to a C activity of 0.8 g / mol at 1100 ° C. The injected tungsten powder leaves the cyclone after 4 s as a mixture of W ₂ C and WC, but without free carbon. The carbon content in the carbide is 4.5% by weight, the residual oxygen content is 2390 µg / g. It is an indispensable, decisive advantage that reaction gas reaches the site of the reaction directly through the microporosity of the starting powder and the rate of reduction is high as a result.
The mixture W ₂ C / WC thus obtained is converted in a second pass through the cyclone reactor under approximately the same conditions as above to pure tungsten carbide WC (C content = 6.12%).

According to Example 1, ready-to-press powder batches are produced by mixing the WC with binder material and small proportions of mixed carbides with optional granulation by means of spray drying.
The hard metal obtained from these powder batches corresponds in its fine-grained structure and homogeneity to that according to Example 1.

EXAMPLE 3

Design of the cyclone device and reduction process of Tungsten oxide to tungsten powder in two successive runs correspond to those of Example 1.

The subsequent carburization is again carried out in the cyclone reactor, but in contrast to Example 2, using CO as the carburizing and carrier gas. The tungsten powder obtained from the cyclone reactor is continuously introduced into the chamber at a throughput of 1000 g / h with a gas quantity of 6000 l / h (CO gas) and at 1000 ° C. chamber temperature in a one-step process to produce W ₂ C and WC (C content 4.2% by weight) and a residual oxygen content of 3240 µg / g reacts. The X-ray diffractometer examination shows that in addition to W ₂ C, small amounts of WC but no free carbon are present in the end product thus obtained. The throughput time for the particles to be carburized in the cyclone reactor is 1-2 s.

The W ₂ C-WC powder mixture obtained in this way is reacted in a second process step in the cyclone under approximately the same test conditions as for the first carburization step to pure tungsten carbide WC with only a very small residual oxygen content and without detectable free carbon residues.
The powder sets are completed by mixing and optionally granulating as in Examples 1 and 2.
A hard metal made from these powder batches by customary processes has a high degree of fine grain and a high degree of material homogeneity.

Claims (5)

1. A process for the production of powder pressing feedstocks for metal carbide based, fine grain hard metal, starting from metal oxide powder or from reducible, pulverulent metal compounds and comprising the process steps

   reduction of the oxide or the metal compound to yield the metal

   carburisation of the metal

   a mechanical-chemical powder grain treatment and/or mixing process

   characterised in that,
   in at least one of the two chemical process steps, reduction and carburisation, a cyclone apparatus and the cyclone process are used, which exhibit the sum of the following features

   the apparatus is a temperature-controllable reaction chamber, at least a portion of which is approximately rotationally symmetrical around the longitudinal axis of the apparatus, with inlet and outlet openings both for the pulverulent material to be reduced and/or to be carburised and for substances reacting with the material and for carrier gas,

   the continuously introduced pulverulent material, while retaining the solid phase state, passes through the reaction chamber at elevated powder velocities on predetermined flight paths which are at least in part tangential to the chamber wall

   the flight paths are controlled by means of the flight direction and velocity of the pulverulent material on introduction and by means of the flow parameters of the carrier and/or reaction gas at the inlet

   the pulverulent material leaves the apparatus, having reacted chemically at least to an extent of 90 vol.%, on average 0.1-60 s after being introduced.
2. A process for the production of powder pressing feedstocks according to claim 1, characterised in that the pulverulent material is converted into the predetermined reaction state during a transit time of 0.2-10 s.
3. A process for the production of powder pressing feedstock according to claims 1 to 2, characterised in that elevated relative velocities between solid and gaseous substances arise in the apparatus.
4. A process for the production of powder pressing feedstocks according to claims 1 to 3, characterised in that at least one of the two process steps, reduction and carburisation, is repeated at least once.
5. A process for the production of powder pressing feedstocks according to claims 1 to 4, characterised in that, prior to reduction, metallic additional materials are added to the metal oxide powder or to the pulverulent metal compound in the form of a separate powder or by prior formation of a solid solution in the material to be reduced.

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