MXPA98001927A - Method to produce a carbide of transition metal from a partially reduction transition metal compound - Google Patents

Abstract

A transition metal carbide is formed from a precursor mixture comprising at least one of the group consisting of a transition metal, a transition metal carbide and transition metal oxide. The precursor mixture may contain the desired transition metal carbide (eg, WC), but if the desired transition metal carbide is present in the precursor mixture, there is necessarily a significant amount of another compound, such as metal oxide. of transition a desirable carbide (for example W2O) or a transition metal. The method involves forming a mixture by combining the precursor mixture with a sufficient amount of carbon to carburet the precursor mixture to the transition metal carbide and reacting the mixture in an inert atmosphere or reducing atmosphere at a temperature a and for a sufficient time. to convert the mixture to the transition metal carbide, wherein the mixture is reacted in the presence of means to improve heat transfer to the mixture, an inert or reduction gas flowing through at least a portion of the mixture or a combination of the same

Description

METHOD TO PRODUCE A TRANSITION METAL CARBIDE FROM A COMPOUND OF METAL PARTIALLY REDUCED TRANSITION DESCRIPTION OF THE INVENTION This application claims the benefit of the Provisional Application of E.U.A. No. 60 / 003,631, filed on September 12, 1995. The invention is directed to the production of carbides of the transition metals Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W and to carbides in solution of said transition metals. Generally, there are two forms of tungsten carbide, monotungsten carbide (WC) and ditungstene carbide (W2C). It is well known that WC is useful in the manufacture of commercially useful articles, such as cutting tools, dies and drilling tools, whereas W2C is generally not. In fact, W2C degrades properties, such as resistance of WC objects, even though they are only present in small quantities. In producing such WC articles, it is common for the tungsten carbide powder to be combined with a metal such as cobalt and subsequently densified to a WC / Co cemented carbide through heating. Heating can take place at a pressure that varies from vacuum to pressures greater than atmospheric pressure. In a part of cemented carbide, tungsten carbide, grain size and grain size distribution and grain chemistry greatly influence in the properties of the final parts. As previously stated, W2C should be avoided when making cemented tungsten carbide parts. Generally, the smaller grain size is a cemented part resulting in improved strength. In addition, smaller grain sizes usually result in a hardness greater than a given cobalt addition. Non-uniformity of grain size in a Part of cemented tungsten carbide adversely affects the strength of and the condition of the surface of the part after grinding The non-uniformity of the grain size in the cemented WC part is mainly due to an exaggerated grain size during the densification of the Part The grain growth can be controlled through the addition of grain growth inhibitors such as VC, Cr3C2 or TaC or starting with a WC powder having a particle size distribution as narrow as possible (ie, uniform) WC powder, which has an average particle size less than 02 to 03 microns, can cause an exaggerated grain growth. due to the increased reactivity associated with the fine particle size. It has also been reported that normal grain growth inhibitors, as described above, are not effective when concreting a portion of cemented WC using said fine WC powder. The critical parameter for concreting said fine WC powders was reported as the WC powder grain size distribution (Suzuki et al, J Jap Soc Powder and Powder Me, Vol 19, pp. 106-112, 1972) In this way, it is desirable to be able to increase the particle size or control the particle size distribution of the very fine WC powder (less than 02 to 0.3 microns) to reduce the possibility of grain growth during densification of a cemented WC part. Typically, monotungsten carbide is formed through the carburation of tungsten metal The basic steps of the process are (a) calcining ammonium paratungsten or tungstic acid to one of the stable forms of tungsten oxide, such as WO3, WO2 83, WO2 65 and WO2, (b) reducing the tungsten oxide to tungsten metal powder, (c) mixing the tungsten metal powder with a carbon powder form, (d) carburing the tungsten and the carbon mixture to a temperature in excess of 1100 ° C in a reducing atmosphere (containing hydrogen) The resulting WC particle size is controlled through the size of the metal powder W formed in step (b) The particle size of the tungsten metal, as described in the U.A. 3,850,614, is controlled mainly through. (1) the depth of the powder bed during the reduction, (2) the flow velocity of the hydrogen, (3) the dew point of the hydrogen gas and, (4) the reduction temperature The particle size tungsten powder Smaller is produced by increasing the gas flow, reducing the depth of the bed, reducing the dew point of hydrogen gas and decreasing the reduction temperature Reducing the depth of the bed and reducing the temperature, the amount of tungsten powder that can be carbureted a WC is a given period, is reduced The mechanism of growth has been attributed to a volatile species of WOH directly associated with the concentration of water in the gaseous environment (US Patent 3,850,614) The procedures that require the carburation of tungsten metal The formation of monotungsten carbide is typically limited to the production of WC dust with a particle size of 08 microns or larger, due to the difficulty The metal w is much smaller than this size due to, for example, the pyrophoric nature of said tungsten metal powder. Due to the high hardness of the WC, it is also difficult to grind the WC to a small particle size. WC is easily ground to the fine particle size, the milling process inherently produces a broad particle size distribution compared to a controlled synthesis method. Other methods to produce tungsten carbide include the following Steiger (US patent 3,848,062) describes reacting a volatile tungsten species such as WC15, C14, WC2C? 2, WOC,, WCF4 and W (C) 6, with a vaporous carbon source such as a volatile hydrocarbon or halogenated hydrocarbon. The vaporous carbon source is present in an amount at least equal to the stoichiometry of the WC during the previous vapor phase reaction The product of this reaction, a mixture of WC, W2C and c arbono, then it is calcined at a temperature of 1000 ° C for 1 to 2 hours resulting in carbide substantially free of dithungstene carbide. Miyake (US patent 4,008,090) describes a process having a first step of reacting a tungsten oxide with a carbon powder at a temperature higher than 1000 ° C, thus removing the oxygen, and a second step of reacting the first pass product at a higher temperature than the first step in hydrogen to produce monotungsten carbide Miyake specifies that the temperature must be greater than 1000 ° C in the first step to remove oxygen The removal of oxygen is necessary to avoid the reaction of hydrogen with oxygen, forming water vapor, which consequently reacts with carbon forming a carbon-oxygen species volatile, thus affecting the carbon content of the second step product (i.e., desired monotungsten carbide) Kimmel (US patent) A 4,664,899) discloses a method for forming monotungsten comprising mixing tungsten oxide or ammonium paratungstate with carbon powder to form a resulting mixture, reducing said mixture in a non-reducing atmosphere for a sufficient time at a suitable temperature to produce a reduced mixture resulting which comprises tungsten carbide, dithungstene and monotungsten carbide, said reduction being carried out in the presence of sufficient carbon to produce a carbon content of less than 6 13% by weight in said resulting reduced mixture, determining the carbon content of the mixture resulting reduced, add enough carbon to the resultant reduced mixture to increase the carbon content to at least the stoichiometric amount necessary to form monotungsten carbide and carburet said reduced mixture adjusted to form Kimmel monotungstene carbide furthermore discloses that the product of the reduction of tungsten oxide or is a mixture of W, W2C, WC and free carbon and all the oxide is reduced All the above-described processes for producing monotungsten carbide require the reduction of a tungsten oxide or a tungsten compound (eg, WC14) either to tungsten or to a mixture of tungsten metal, tungsten carbides and free carbon. The tungsten or mixture is substantially free of oxygen (ie, tungsten oxide) before carburetion to form monotungsten carbide. Oxygen is essential and completely removed to avoid volatile carbon loss through oxidation or hydrolysis during the carburization of the tungsten metal or of said mixture. Removal of carbon during carburation results in uneven carbon contents of the resulting carbide product (ie, W2C in the product). Non-uniform carbon contents are particularly a problem in industrial processes due to the large volume of carbide that must be processed In an industrial process, it may be desirable to provide a method for producing a transition metal carbide, which is relatively insensitive to the oxygen concentration of the precursor mixture used to make said carbide. In addition, it may be desirable to have a process, in which the carbide particle size can be significantly controlled by the method and not only depends on the particle size of the precursor mixture (i.e., starting with a small particle size and growing to a desired size) it may be desirable to provide such a method to produce monotungsten carbide. object of this invention is a method for producing a transition metal carbide from a precursor mixture, in which the precursor mixture is composed of a product of at least partial reduction or partial carburization of a compound of transition metal, the method comprises a) forming a mixture by combining the precursor mixture with a sufficient amount of carbon to carburet the precursor mixture to the transition metal carbide, and b) reacting the mixture in an inert or reducing atmosphere. a temperature and for a time sufficient to convert the mixture to the transition metal carbide, wherein the mixture is reacted in the presence of (i) means to improve heat transfer to the mixture, (ii) an inert gas or of reduction flowing through at least a portion of the mixture, or (iii) combinations thereof. The method of this invention, in particular, allows the monotungsten carbide to be uniformly formed even when the precursor mixture has a sufficient amount of tungsten oxide present. It is believed that the present invention overcomes the chemical non-uniformity of the product caused by the hydrolysis or oxidation of the carbon, which occurs during the carburation of the precursor mixture, reducing the inhomegency of the reaction occurring in the mixture during said carburation. For the monotungsten carbide, it has now been discovered that the particle size of the monotungsten formed is inversely proportional to the flow velocity of the gas through the mixture described above. The effect of the gas luxury velocity is unexpected and surprising since the US patent 3,850,614 describes the particle size of tungsten and, therefore, the particle size of monotungsten carbide being directly proportional to the flux or gas passing over, not through, a precursor mixture to form a tungsten powder In addition, the particle size of said monotungsten carbide can also be controlled by temperature Transition Metal Carbide Product: The method of this invention is directed to the formation of transition metal carbides, to transition metal carbides of solid solution or a combination thereof Transition metal carbides that are formed can be any carbide containing a transition metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W and a carbide in solution of said transition metals The transition metal carbide is preferably monotungsten carbide (WC), monotitanium carbide (TiC), monotantaho carbide (TaC), monovanadium carbide (VC), monohafnium carbide (HfC), monochirconium carbide (ZrC), mononiobio carbide (NbC), carbide of dimolybdenum (NbC), dimolybdenum carbide (Mo2C) or tpchrome dicarbide (Cr3C2) Preferably, the method according to this invention produces monotungsten carbide or monotungsten carbide in combination with at least one of the ca above transition metal carbides or metal carbides in solid solution Most preferably, the invention forms a monotungsten carbide. When monotungsten carbide is produced through the method of this invention, the monotungsten carbide formed, as determined through diffraction X-ray, preferably has no detectable tungsten target, no detectable tungsten oxide and at least 5% by weight of di tungsten carbide, most preferably less than 1% by weight of di tungsten carbide, and preferably no carbide of detectable tungsten The quantitative analysis is performed through the X-ray diffraction described below. The concentration of free carbon in the mono tungsten carbide produced is desirably less than 0 5% by weight. Preferably, the free carbon in the WC produced is less than 02, preferably less than 0 1 and most preferably less than 005% by weight Free carbon is determined through an acid digestion procedure described later Method for Forming Said Transition Metal Carbide First Step (a): The first step of the method according to this invention is to mix the precursor mixture with a sufficient amount of carbon to form a mixture having enough carbon to carburet the mixture of precursor to transition metal carbide Carbon: Carbon can be amorphous crystalline organic material or a combination thereof Suitable crystalline or amorphous carbon includes graphite or carbon black such as acetylene carbon black, which is commercially available under the trade name of "SHAWANIGAN" Suitable examples of organic material include organic polymers such as phenol-formaldehyde resins, epoxies, polymers of interwoven and cellulosic polystyrenes, carbohydrates such as sugars and starches and hydrocarbons Precursor Mixture: The precursor mixture is composed of the product of at least a partial reduction or carburization of a transition metal compound such as a transition metal oxide described herein. The precursor mixture is composed of at least a transition metal compound selected from the group consisting of a transition metal, a transition metal carbide and a transition metal oxide. In addition, free carbon may be present in the precursor mixture. The transition metal may be a metal. of transition selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W and their solid solutions The transition metal carbide can be one or more carbides of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo or W, or a carbide compound containing at least two of said transition metals. The transition metal oxide may be one or more oxides containing Ti, Zr, Hf, V, Nb, Ta, Cr, Mo or W, or a compue oxide that contains at least two of said metals. The carbon, which may be in the precursor mixture, is either a residual reagent or is formed during the formation of the precursor mixture. The free carbon can be crystalline or amorphous. Free carbon can originate from such carbon sources such as carbon black, graphite or organic material, each one described here below. The product of at least a partial reduction or carburization of a transition metal compound can result in a precursor mixture having the desired transition metal carbide (e.g., WC). However, if the transition metal carbide is present in the precursor mixture, there must be, according to this invention, a significant amount of another transition metal compound such as a transition metal oxide, a metal carbide. unwanted transition (for example W2C), a transition metal or combinations thereof. Here, more than 5% by weight of the precursor mixture is a significant amount of another transition metal compound. When the transition metal carbide to be produced is monotungsten carbide, the precursor mixture may be composed of: (1) tungsten; (2) tungsten, ditungstene carbide and monotungsten carbide or (3) tungsten, ditungstene carbide, monotungsten carbide, carbon and at least one form of tungsten oxide The amount of oxygen in the precursor to form monotungsten carbide ( WC) can be as large as 5% by weight Preferably, the oxygen is less than 3, preferably less than 2, and most preferably less than 1% by weight, as determined through the combustion analysis described below. Practically, the concentration of oxygen is never 0% by weight, since the formation of a tungsten oxide is thermodynamically favored over the carbide at room temperature in the air. The oxygen level can be as low as practically convenient, but is not necessary for the method of this invention For example, in carrying out the method of this invention, the precursor oxygen may be greater than 0 5% by weight Oxygen is taken is taken to be in the form of a tungsten oxide and suitably is assumed to be WO3 for the calculation of the stoichiomepa described above. Generally, the precursor mixture is formed through at least a partial reduction of a transition metal oxide powder, either by carburetion or reduction by hydrogen The methods that are suitable for forming the precursor mixture described below include methods described in US Patents 4,008,090, 4,664,899, 3,850,614 and 3,848,060, each being incorporated herein by reference. Preferably, the precursor mixture is prepared by the rapid carbothermal reduction method, described below and by U.S. Patent 5,380,688, incorporated herein by reference. The precursor mixture is preferably prepared by mixing a carbon source into solid particles (e.g., carbon black), as described below, with a transition metal oxide The amount of carb or added to the oxide is an amount sufficient to carbure a greater part of the oxide forming a transition metal carbide. Preferably, the amount of carbon is added in an amount less than or equal to the stoichiomede of the desired transition metal carbide. , in the production of a precursor mixture to make WC, the amount of carbon present should be a stoichiometric amount or less than stoichiometc The stoichiometric amount corresponds to 4 moles of carbon per mole of W03 (ie WO3 + 4C = WC + 3CO) The transition metal oxide may be an individual transition metal oxide (oxide) an oxide containing more than one transition metal, hereinafter referred to as an alloy of transition metal oxide The oxides are metal oxides of transition previously described Preferably, the transition metal oxide is an individual oxide or a combination of individual oxides When In a combination of individual oxides in the rapid carbothermal reduction method, a carbide in solid solution can be formed as described in US Pat. No. 5,380,688. Preferably the transition metal oxides have a purity greater than 99% and a uniform particle, where all the particles and agglomerates pass through a 325 mesh screen (ie, the largest oxide particle or the agglomerate is less than 45 microns in diameter). Suitable examples include individual oxides such as tungsten trioxide (WO3) available under the tradename "TO-3" from GTE Products Corp., titanium dioxide (TiO2) available under the tradename "TITANOX" from Veliscol Chemical Corp ., and ditantalium pentoxide (Ta2O5) available from Aldrich Chemical Company. The transition metal oxide can be mixed in any suitable apparatus for mixing powders. Examples of mixing apparatus include, but are not limited to, a sigma mixer, sand mixer, V mixer, and cone mixer., if the reduction of the particle size of the transition metal oxide or of carbon is desired, said oxide or carbon can be ground before mixing or mixing to grind simultaneously in any apparatus capable of moles (reducing the particle size of the powder), such as in a ball mill, jet mill, vibration mill or a stirred mill such as a grinder. If grinding is carried out using grinding media, for example, in a ball mill, the grinding media are preferably grinding media of cemented-tungsten carbide-copper. The carbon and the transition metal oxide (s), transition metal oxide alloy (s) or combinations thereof, after being mixed, will be referred to hereafter as the reactive oxide- carbon The reactive oxide-carbon mixture is advantageously reacted through rapid carbothermal reduction in a dropping or pulling method The dropping method involves heating a graphite crucible in the hot zone of an induction furnace at the reaction temperature The crucible is heated in a non-oxidizing atmosphere such as flowing argon. The crucible is kept at the reaction temperature for a sufficient time (30 minutes) to balance the crucible and the furnace at that temperature. The aliquots of the reaction mixture fell in said graphite crucible in the induction furnace producing heating rates in the range of 100 to 10,000 ° C (K) per second The degree of operation is verified midiend or the reagent through the CO level in the product in the crucible as a function of time When the CO level is reduced to a CO level equivalent to approximately the CO concentration before the start of the reaction, the reaction stops cooling the crucible as quickly as possible at room temperature, to minimize the agglomeration of particles and the grain growth of reactive oxide-carbon mixture reacted The entrainment method involves the use of a vertical graphite tube reaction furnace of the type described in US Pat. No. 5,110,565, incorporated herein by reference. The reactive oxide-carbon mixture is placed in a feed hopper, which allows the non-oxidizing gas, such as argon, to flow to entrain the powder mixture and supply it. to the furnace reaction chamber as a dust cloud The dust or particulate mixture is heated in the reaction chamber at speeds of 10,000 to 100,000,000 ° C r second, while the average residence time of the powder in the furnace is of the order of seconds. As for the exit of the hot zone of the reaction chamber, the flowing gas carries the powder to a stainless steel jacket cooled with water , which rapidly cools the already reacted powder below 10 ° C. The preferred method for making the precursor mixture of this invention from the reactive carbon-oxide mixture is through the entrainment method, since the entrainment method is capable of more uniform reaction conditions, and, thus, a greater capacity in the formation of the precursor powder of uniform small particle size The oxide-cabergone mixture is reacted (ie carburetically oxide) by heating ambient temperature up to the reaction temperature of at a rate of at least 100 to 10,000 ° C per second to optimally in the order of 10,000 to 100,000,000 ° C per second The reaction temperature should high enough so that the reaction is thermodynamically favorable One way of expressing this is that the change in Gibbs free energy for the reaction must be negative In other words, the free energy formation of reaction products must be less than the Free energy from the formation of components of the reaction mixture It must also be lower than the melting point of any reaction product (s), For tungsten carbide, a reaction temperature of at least 1400 ° C is considered beneficial, while temperatures from 1550 ° C to 2400 ° C are preferred The approximate temperatures, at which the free energy of formation of reaction products is lower than the free energy of formation of components of the reagent mixture necessary to form the products of reaction, are as follows tungsten carbide (WC) 677 ° C, titanium carbide (TiC) 1282 ° C, tantalum carbide (TaC) 1108 ° C vanadium carbide (VC) 659 ° C hafnium carbide (HfC) 1661 ° C, niobium carbide (NbC) 955 ° C, zirconium carbide (ZrC) 1657 ° C, dimolybdenum carbide (Mo2C) 469 ° C, and chrome carbide (Cr3C2) 1110 ° C The residence time of the The mixture of carbon-oxide reactant at the reaction temperature depends in part on the heating rate, but it must be high enough to reduce at least a large portion of the metal oxide from the reaction temperature. The residence time is preferably on the scale from 0 1 seconds to 1/2 hour a, depending on the heating method, heating rate, reaction temperature and the desired final particle size In the drop method, typical preferred residence times are from 5 minutes to 2 hours for a reaction temperature of 1500 ° C with a heating speed of 100 to 10,000 ° C per second In the drag method, a residence time of 02 to 10 seconds is preferred for a reaction temperature of 1550 ° C or above, with a heating rate of 10,000 to 100,000,000 ° C per second. At the higher heating rate, residence times substantially greater than 10 seconds may undesirably produce concreted aggregates in place of the particulate product. Any combination of reaction temperature, residence time and heating that is selected, however, must be suitable for converting the reactive mixture of carbon and metal oxide to a product composed mainly of metal carbide. In other words, the product, for example, to make WC could be the precursor mixture previously described Mixture Formation: The mixture is mixed by combining, together, the previously described carbon and precursor mixture. Preferably, the carbon is a solid carbon such as carbon black. The carbon and the precursor are mixed or co-milled through them. techniques and methods previously described for mixing the oxide and the carbon to form the reactive mixture of carbon-oxide Preferably, the mixture of carbon and precursor are combined in a ball mill having cobalt and cemented tungsten carbide media The amount of carbon, which is added to the precursor mixture is typically determined with respect to the desired transition metal carbide. For example, when WC is produced from a mixture of the precursor mixture and carbon in a mixture of 5% hydrogen and 95% argon. Applicants have found that an amount of carbon comprising the sum of about 0.67 stoichiometric with respect to oxygen (ie, WO3 + (0.67) 4C = WC + 3CO) in the precursor mixture, and stoichiometric with respect to W2C and metal Free (W) in the precursor mixture typically converts the precursor mixture to a monotungsten carbide having a low carbon content and a total oxygen low Typically, the amount of carbon that will be added to the carbide precursor made by the method trawl usually represents from 1 to 5% by weight based on the weight of the precursor mixture, more typically, the amount is in the range of 2 to 3% by weight. The amount of carbon that is necessary to convert the mixture to a desired transition metal carbide can change depending on, for example, the reactor, the atmosphere and the precursor mixture. For many conditions according to this invention, the amount of carbon can be determined through experimentation by those skilled in the art Second step (b): The second step of the method according to this invention is to react the mixture in an inert or reduction atmosphere at a temperature for a sufficient time to convert the mixture to the transition metal carbide, where the mixture reactions are made in the presence of (i) means for improving heat transfer to the mixture, (i) an inert or reduction gas flowing through at least a portion of the mixture, or (ii) combinations of the same. The mixture is reacted at a sufficient temperature and time to substantially convert all of the precursor mixture to the desired transition metal carbide. For example, when tungsten carbide (WC) is formed, the mixture can be heated to a temperature of 900 ° C to 1800 ° C for a sufficient time to convert the mixture to the WC described above. Preferably, the reaction temperature of the mixture is between 1000 to 1600 ° C. It has been shown that the temperature of the reaction is directly proportional to the particle size of the WC product. The reaction time is desirably as short as a sufficient period. to convert the mixture to WC Preferably, the reaction time is at least 5, preferably at least 15 and most preferably from at least 30 minutes to almost 10, preferably at case 5 and most preferably at almost 2 hours The atmosphere can be any reducing or inert atmosphere. Gases that are suitable for creating the atmosphere include hydrogen, a mixture of hydrogen-argon or argon. Preferably, the gas of 2-6% hydrogen in a mixture of argon gas, since these mixtures provide a reduction environment while the amount of hydrogen in the mixture is below the explosive limit It is also preferred that initially the atmosphere is inert, for example, argon, until substantially all the oxygen present in the mixture is removed (ie, little or no oxygen is generated). CO through the reaction), and then subsequently the gaseous atmosphere is changed to a reducing atmosphere such as 5% hydrogen in argon. The use of an inert gas followed by a reducing gas can prevent the formation of water vapor., which can subsequently react with the carbon, forming CO (ie, H2 + O = H20 and subsequently H2O + C = CO + H2). During the reaction of the mixture, the atmosphere is preferably created through flowing gas. The gas is desirably flowing to allow the removal of unwanted gaseous species such as water vapor. Excess water vapor can react with carbon to form CO gas, thereby changing the carbon concentration of the transition metal carbide formed. The amount of gas flowing by weight of the mixture can be any flow sufficient to react the mixture to form the desired transition metal carbide. Preferably, the gas flow is from 5 to 500 normal liters / min-kg, and very preferably from 25 to 250 normal liters / min-kg. The mixture can be reacted in an intermittent or continuous apparatus. Suitable apparatuses for reacting the mixture include, for example, a tube furnace, a pusher furnace, a band furnace, a rotary kiln, a lifting furnace, a fluid bed reactor and a rotary crucible furnace. It is desirable that the apparatus be constructed of materials, which do not contaminate the mixture during the reaction. Preferably, the furnace or reactor is constructed of carbon materials in at least the hot sections of the furnace or reactor. The carbon material must have a purity as described for a carbon object in the following paragraph. When the mixture reacts, the mixture is reacted in the presence of means to improve heat transfer to the mixture, an inert or reduction gas flowing at least partially through the mixture, or combinations thereof. The means for improving heat transfer to the mixture are any object that conducts heat at a rate significantly greater than the volume mixture, such as a dense charcoal or a ceramic object. The object is significantly larger than the particle size of the mixture, where the object can be separated from the mixture by simple mechanical means such as picking it up by hand. A dense carbon object or objects include graphite or carbon-carbon composite object (s). Preferably, the object or objects are carbon objects. The carbon object (s) must be of a purity, which does not significantly contaminate the mixture. Commercial graphite commonly has a significant silicon contamination Therefore, a graphite or carbon object, which is used to carry out the method of this invention, preferably has a silicon and total metal contamination of less than 25 parts by weight. million (ppm), and most preferably less than 10 ppm The objects can be of any geometry such as, a plate, tube, bar or an arrangement of plates, tubes or bars The object of preference is placed in the mixture, so that at least a portion of the object is not inside the mixture. For example, the mixture is placed in a circular, square or rectangular graphite pot having a bar or plate in the center of the pot, where the bar or plate is in contact with the bottom of the boat (for example, the longitudinal axis of the bar extends from the bottom of the boat to the top of the boat) The upper part of the boat with the end open e the object or objects extend beyond the top of the mixture in the can The object can be in contact with the can so that the object is an integral part of the can For example, the object can be screwed, cemented or slotted to the bottom of the pot An example of a suitable cement is a phenol-formaldehyde resin, which decomposes to a carbonaceous material after heating to a high enough temperature in a non-oxidizing atmosphere. In addition, the pot can be machined in a that the object is a contiguous part of the can The mixture in the can can be covered or uncovered and subsequently reacted, as described herein, in an intermittent furnace or continuous furnace, such as a push oven. Preferably, the furnace is a continuous furnace, and most preferably a push furnace. Another example for forming a transition metal carbide with said furnace object or objects, is the reaction of the mixture, as described above, in a rotary kiln In this example, the rods are brought into contact with the inner diameter of the rotary kiln, where the longitudinal axis of the rods extend radially towards the center of the rotary kiln Desirably, the rotary kiln has a rotating section (pipe), which is made of graphite or a carbon / carbon composite and said rods are periodically bolted to the inner diameter and along the longitudinal axis of said pipe. bars can also beneficially mix and agglomerate the mixture in the rotary kiln When the mixture is reacted in the presence of means to improve the transfer The heat transfer to the mixture (eg, carbon objects), a chemically more uniform transition metal carbide, particularly WC, is formed, compared to when no such means are present to improve the transfer of heat to the mixture. A more chemical uniformity leads to a less desirable transition metal carbide (eg, W2C) less oxygen less carbon or combinations thereof present in the transition metal carbide product When the mixture is reacted in the presence of an inert or reduction gas , at least partially flowing through the mixture, similar apparatuses described above can be used. For example, the circular, rectangular or square graphite canister has an outlet at the bottom of the can, where the gas can pass through, but the mixture can not. Suitable outlets may involve at least one hole through the bottom of the canister, in which the hole (s) has a porous membrane capable of passing an inert or reduction gas, but not mixing. Suitable membranes include granite felt, carbon fiber mesh or porous graphite. An example could be a canister having a first bottom having a hole in the center of said bottom passing from the outside of the can towards the inside of the can. First fund is an integral part of the pot. Said boat has a second bottom that completely covers and placed on top of the first bottom inside the boat, where there is a space between the first and second bottom The second bottom has multiple holes through it, which are parallel to the hole in the first background The second background also has a graphite felt on it. When the mixture is reacted, as described, in the above-described canister, the mixture is placed in the can on the graphite felt and placed, for example, in an intermittent furnace which has means for introducing or removing gas to the Through the hole in the first bottom of the can The means for removing or introducing the gas are, for example, a graphite tube that runs through the bottom of the furnace, which then engages (for example, through screw threads) the bottom of the boat on the internal diameter of the hole in the bottom of the boat. The boat already described, of course, has a previously described object to improve heat transfer to the mixture. Another example for reacting the mixture, as described above, in the presence of an inert or reduction gas flowing through at least a portion of the mixture, can be to insert a graphite tube, described above, through the top of the furnace, where the end of the tube is placed at some depth in the mixture contained in a can that has a solid bottom. The gas is then flowed into the mixture, the gas passing through some portion of the mixture and subsequently passing out of the oven to any part. The end of the tube placed in the mixture can be configured through any number of configurations to better disperse the gas into the mixture. For example, the end of the tube in the mixture may have a sealed end, where the gas leaves the tube through passages that run from the external diameter to the internal diameter at the end of the tube. In this example, the tube may also function as means of thermal improvement. The inert or reduction gas flowing through at least a portion of the mixture unexpectedly results in a transition metal carbide particle size (particularly WC), which is inversely proportional to the gas flow rate In addition, an increase in the flow rate results in less or no W2C formation at otherwise identical reaction conditions. The transition metal carbide powder (s) and, in particular, the WC formed from According to the method of this invention, articles such as a constituent in the formation of wear-resistant parts of cemented carbide, such as cutting tools and blasting nozzles of all types and dies are useful.

M ETHODS OF TEST The following are typical methods for analyzing a transition metal carbide, described herein Carbon: The carbon concentration in a transition metal carbide of this invention is determined using a "LECO" IR-12TM carbon analyzer. A "LECO" which provides normal tungsten carbide having 6 16 wt.% Carbon, is used to calibrate the analyzer The analyzer is calibrated using at least 4 normal analyzes, as described by the manufacturer (LECO) Each sample and the normal one is analyzed with a LECOCEL IITM bucket and iron wafers The bucket is provided by the manufacturer ( LECO) At least four carbide samples are analyzed Oxygen: The oxygen concentration in a transition metal carbide of this invention is determined using a "LECO" oxygen determinant TC-136 ™. 0.0246% by weight of oxygen is used. The oxygen determinant is calibrated using at least 4 normal analyzes as described by the determiner manufacturer. A carbide sample is analyzed by placing 0.2 g of the sample in a tin capsule supplied by the manufacturer and a nickel basket. At least 4 carbide samples are analyzed Surface Area: The surface area of the transition metal carbide is determined by nitrogen gas adsorption, as described by the BET method (Brunauer, Emmett and Teller). The analysis is performed on a Quantachrome Autosorb 1 analyzer. .

Free Carbon: The free carbon in a transition metal carbide of this invention is determined through the acid digestion of a quantity of transition metal carbide (eg, WC) in hydrofluoric and nitric acid, then the residue is filtered of carbide on a silver filter and the carbon in the silver filter is determined by the method described above to determine the carbon concentration. Phase Determination: The phases and number of different transition metal carbide phases are determined at Through X-ray diffraction The number of phases is determined through a method that involves the ratio of peak heights or peak areas integrated between peaks caused by different phases. For example, the amount of W2C is calculated from the ratio between 2 times the peak height of the W2C peak at a "d" separation of 2276 Angstroms, divided by the height of the WC peaks at a "d" separation of 2 5 18 and 1884 Angstroms The following are specific examples within the scope of the invention and comparative examples. The specific examples are for illustrative purposes only and in no way limit the invention described herein.

EXAMPLES EXAMPLE 1 A mixture of precursor, prepared through the entrainment method described herein and in US Patent 5 380688 and acetylene black of SHAWNIGAN ™ (carbon black), were reacted to form essentially monotungsten carbide. The precursor mixture comprised WC , W2C, carbon, tungsten oxide and tungsten metal, wherein the mixture has an oxygen concentration of 1.75% by weight, a carbon concentration of 4.88% by weight and a surface area of 5.8 m / g. of precursor-carbon mixture was made by mixing 2. 12 parts by weight of the carbon black with 97.88 parts by weight of the above precursor mixture in a urethane-lined ball mill partially filled with grinding media of cemented WC-Co. The mixture has an overall density of 1.2 g / cm3. The above mixture was placed in a graphite can with a length of 23 cm by a width of 23 cm and a depth of 10 cm, which had been divided into 6 cavities of 23 cm. cm in length by 4 cm in width by 10 cm in depth, using 5 graphite plates (means to improve heat transfer to the mixture). The plates had a length of 23 cm by 10 cm of depth by 064 of thickness The mixture after it was placed in each of the 6 cavities at a depth slightly lower than the depth of the can and covered with a graphite plate. The mixture of Example 1 in the previous can was reacted in a graphite furnace. The furnace was heated to 1525. ° C and was maintained at that temperature for 3 hours and subsequently cooled to room temperature. The reaction was carried out in a flowing mixture of 5% hydrogen-95% argon gas. The samples of Example 1 which react with the mixture, were taken from the center of the can (a central cavity) both the upper part and 5 cm below the upper part of the reacted mixture. The sample taken in the upper part of the reacted mixture was named, in the present, as the shows "superior", in the present. A sample taken at 5 cm below the top of the top mixture was named as a mean sample, in the present After mixing the remaining reacted mixture in a ball mill previously written, another sample was taken. The sample taken from a combined mixture was designated as a combined sample, in the present The properties of monotungsten formed through the method of this example are shown in Table 1 MPLO 2 AXIS The monotungsten carbide of Example 2 was prepared by the method described in Example 1, except that the reaction was carried out at 1375 ° C for 3 hours. The mixture of Example 2 was reacted simultaneously in the same oven, as The mixture of Comparative Example 2, below. The properties of the monotungsten carbide formed by the method of this example are shown in Table 1.

EXAMPLE 3 The monotungsten carbide of Example 3 was prepared by the same method as in Example 1, except that the reaction was carried out at 1445 ° C for 4 hours. The mixture of Example 3 was reacted simultaneously in the same oven as Example Comparative 3, described below The properties of monotungsten carbide formed by the method of this example are shown in Table 1 COMPARATIVE EXAMPLE 1 The monotungsten carbide of Comparative Example 1 was prepared by the method described in Example 1, except that the mixture of Comparative Example 1 was placed in a graphite canister, which had a length of 23 cm by a width of 23 cm by a depth of 10 inside the canister The canister in Comparative Example 1 was identical to the canister of Example 1, except that the canister in Comparative Example 1 was not divided into plates (ie no means to improve heat transfer to the canister). mixture) The depth of the mixture in Comparative Example 1 was slightly less than the depth of the can and was not covered through a graphite plate. The mixture of Comparative Example 1 was reacted simultaneously in the same oven as in Example 1 COMPARATIVE EXAMPLE 2 The monotungsten carbide of Comparative Example 2 was prepared by the method described in Comparative Example 1 except that the mixture of Comparative Example 2 was reacted simultaneously in the same furnace as the mixture of Example 2 The properties of monotungsten formed by the method of this comparative example are shown in Table 1 COMPARATIVE EXAMPLE 3 The monotungsten carbide of Comparative Example 3 was prepared by the method described in Comparative Example 1, except that the mixture of Comparative Example 3 was reacted simultaneously in the same furnace as the mixture of Example 3 The properties of the monotungsten formed by the method of this comparative example are shown in Table 1 t n O n in TABLE 1 EFFECT OF TRANSFER MEANS ON THE PROPERTIES OF MONOTUNGSTEN CARBIDE POWDER co in The contents of the samples of the reaction mixture taken at different places within the cans of Examples 1-3 and Comparative Examples 1-3 are shown in Table 1. The results in Table 1 show that the carbon concentration of the Reacted mixtures of Examples 1-3 were more uniform throughout the mixture than the carbon concentration of the reacted mixtures of Comparative Examples 1-3 For example, the reacted mixture (tungsten carbide) of Examples 1-3 has a carbon gradient that was about one third of the carbon gradient compared to the corresponding comparative example (ie, Example 1 compared to Comparative Example 1) In addition, the amount of free carbon in the tungsten carbide of Examples 1 to 3 was one third of the amount of free carbon present in the monotungsten carbide of Comparative Examples 1 to 3, respectively The data in Table 1 show that the addition of said plates to a can when a mixture reacts (Examples 1-3), forms a more uniform monotungsten carbide. In addition, the data in Table 1 show that the surface area of the monotungsten carbide formed was inversely proportional to the reaction temperature. example, the monotungsten carbide of Example 2 (reaction temperature = 1375 ° C) has a larger surface area than Example 1 (reaction temperature = 1525 ° C) In other words, the particle size of the reacted mixture was proportional to the reaction temperature, as described below.

EXAMPLE 4 A mixture of precursor, prepared by the entrainment method described herein and in US Patent 5,380,688 and acetylene black of SHAWNIGAN ™ (carbon black), were reacted to form monotungsten carbide essentially. The precursor mixture comprised WC, W2C, carbon, tungsten oxide and tungsten metal, wherein the mixture has an oxygen concentration of 1 46% by weight, a carbon concentration of 380% by weight and a surface area of 4 1 m2 / g The precursor It was also contaminated by 1000 ppm of silicon. The mixture of precursor-carbon mixture was made by mixing 2 8 parts by weight of the carbon black with 972 parts by weight of the above precursor mixture in a partially filled urethane-coated ball mill with grinding media of cemented WC-Co The mixture has an overall density of 1 2 g / cm 3 A portion of the above mixture was placed in a short graphite container. The container comprised a short tube, which was capped at each end with a graphite lid. Each lid could be removed and has a concentric past hole with the inner diameter of the container. The diameter of the past hole of the lids was smaller than the internal diameter of the container. The powder was placed in the container by uncovering one end of the container and inserting a graphite disk having a plurality of holes through the longitudinal axis of the disk. The discs lie flat on the capped end. The diameter of the graphite disc was approximately equal to the internal diameter of the container. A thin piece of graphite felt was inserted and placed on the disk. The graphite felt completely covered the disc. The mixture was uniformly placed in the container on the graphite felt. Another piece of graphite felt and then a disk with longitudinal holes was placed on the mixture and the container subsequently covered with the above-described lid. The capped container was joined in a hole passed from one of the caps to a graphite tube. This assembly was inserted into a SiC furnace tube of a tube furnace. The internal diameter of the furnace tube was greater than the external diameter of the container. The container was inserted first. The graphite tube was passed through a water-cooled flange having an attachment, which seals the graphite tube and also allows the graphite tube to be removed through the flange. The flange was attached to the furnace tube, thus sealing the furnace and allowing the container to be moved along the longitudinal axis of the furnace tube. The other end of the furnace tube was sealed through another flange, which allows the gas to leave the furnace. The graphite tube was then connected to a gas source, to allow gas flow in a controlled manner through the container (i.e., mixture). The mixture was reacted by placing the container in the hot zone of the oven tube. The oven was heated to 1325 ° C at a speed of 20 ° C / min. The mixture was reacted for 10 minutes in a mixture of 5% hydrogen-95% argon gas at this temperature. The gas flow per kg of the mixture was 40 normal liters / min-kg. At the end of 10 minutes, the furnace was cooled to room temperature. The properties of monotungsten formed by the method of this example are shown in Table 2 EXAMPLES 5-11 The monotungsten carbide formed by Examples 5-11 was prepared by the same method described in Example 4 except for Examples 5-12, the gas flow, the reaction time and / or the reaction temperature was They varied as shown in Table 2 EXAMPLE 12 The monotungsten carbide formed by the method of Example 12 was prepared by the same method described by the method described in Example 4, except that the heating rate was varied as described below. The mixture was reacted by first removing the graphite tube from the furnace tube to a point where the container was adjacent to the inner surface of the flange (ie, the container is not in the hot section of the oven, but was essentially at room temperature when the oven was heated). The oven was heated to 1325 ° C. The container was inserted into the hot section of the tube furnace in 2 minutes (i.e., heating rate at 650 ° C / min.). the mixture was reacted for 10 minutes in a mixture of 5% hydrogen-95% argon gas. The gas flow per kg of the mixture was 40 normal liters / min-kg. At the end of 10 minutes, the container was removed from the hot section in 2 minutes and the whole oven was allowed to cool. The properties of monotungsten formed through the method of this example are as shown in Table 2 EXAMPLES 13 AND 14 The monotungsten carbide formed through the methods of Examples 13 and 14 were prepared by the same method described in Example 12, except that the reaction time varied as shown in Table 2.

EXAMPLE 15 The monotungsten carbide formed through the method of Example 15 was prepared by the same method described in Example 4, except that a different precursor was reacted with a different amount of carbon black at a gas flow of 50 normal liters / min-kg. The precursor mixture was prepared through the trawl method described herein and US Pat. No. 5,380,688. The precursor mixture comprised WC, W2c, carbon, tungsten oxide and tungsten metal, wherein the precursor mixture has an oxygen concentration of 035. % by weight, a carbon concentration of 5 16% by weight and a surface area of 52 m2 / g The precursor-carbon mixture was made by mixing 1 4 parts by weight of the carbon black with 986 parts of the precursor mixture. above in a urethane-lined ball mill partially filled with cemented WC-Co grinding media The properties of the tungsten carbide formed by the method of Example 15 are shown in Table 2 EXAMPLES 16 AND 17 The monotungsten carbide formed by the methods of Examples 16 and 17 were prepared by the same method described in Example 15, except that the reaction time was varied as shown in Table 2 in or in Table 2.

-Other Not Determined Table 2 shows the flow results of a 5% hydrogen-95% mixture of argon gas through a mixture when monotungsten carbide is formed. The first effect was a reduction in surface area with an increase in the gas flow. A reduction in the surface area was equivalent to an increase in the particle size given by the equivalent spherical diameters (ESD) The ESD, in micrometers, was equal to 6 divided by the density of the material (1563 g / cm3 for the WC ) and divided by the surface area of the powder in m2 / g The effect of increasing the gas flow through the mixture during the reaction was evident when Examples 4-6 were compared with Examples 7-9 The surface area it is reduced by a factor of 2 (for example, Example 4 vs. Example 7) and subsequently the ESD is increased by a factor of 2. The gas flow through the mixture also allows a greater control of the chemistry of monotungsten carbide. For example, when flow is increased through the mixture ("high flow", Examples 7-9 vs. "low flow", Examples 4-6), the oxygen in the resulting monotungsten carbide was reduced by a factor of 3 Carbon increased to close to a stoichiometric value for WC (6 13% by weight) in the "high flow" examples. The monotungsten carbide that formed also had no detectable W2C, when the gas flow was increased. Even at a small flow, the increments may have a effect on the formed monotungsten carbide (Examples 15-17 vs. - Examples 4-7). Examples 15-17 were reacted under the same conditions as in Examples 4-7, respectively, except that the gas flow was increased from 40 to 50 normal liters / minute-kg. Examples 4-7 exhibit a higher concentration of oxygen, a low carbon stoichiometry for WC and the presence of W2C in monotungsten carbide, while Examples 15-17 have a significantly lower oxygen concentration than Examples 4. -7, a concentration of carbon near the stoichiometry of the WC and no detectable W2C The effect of temperature on the reaction of a mixture with gas flowing through the mixture is shown in Examples 4 and 10-11 The area of The surface of the resulting monotungsten carbide is reduced proportionally with an increase in the reaction temperature. For example, the reaction at 1150 ° C (Example 10) produces a monotungsten carbide having a surface area of 32 m2 / g and the reaction at 1500 ° C. C produces a monotungsten carbide having a surface area of 06 m / g. Subsequently, the particle size given by ESD is increased proportionally with an increase in the reaction temperature. The effect of a rapid heating rate on the formation of monotungsten carbide is shown by Examples 12-14, compared to Examples 4-6. The rapid heating of Examples 12-14 appears to result in a monotungsten carbide with a lower oxygen concentration and a surface area compared to slowly heated Examples 4-6, respectively. The rapid heating has very little effect on ca concentration (Example 12, compared to Example 4). In this way, there does not appear to be any significant advantage to rapidly heat the mixture when monotungsten carbide is formed in accordance with the present invention.

Claims (3)

1. - A method for producing a transition metal carbide from a precursor mixture, wherein the precursor mixture is composed of a product of at least partial reduction or partial carburization of a transition metal compound, The method comprises the steps of a) forming a mixture by combining the precursor mixture with a sufficient amount of carbon to carburet the precursor mixture to the transition metal carbide, and b) reacting the mixture in an inert or reduction atmosphere at a temperature and for a sufficient time to convert the mixture to the transition metal carbide, wherein the mixture is reacted in the presence of (i) means to improve heat transfer to the mixture, the media being at least one dense carbon or a ceramic object extending to and contacting at least a portion of the mixture, (n) an inert or reduction gas flowing through and at least a portion of the mixture, or (ni) combinations thereof.

The method according to claim 1, wherein the temperature is from 1000 ° C to 1600 ° C.

3. - The method according to claim 1, wherein the inert or reduction gas is hydrogen, a hydrogen of 2-6% in admixture with argon gas, or argon 4 - The method according to claim 1, wherein the Time is 5 minutes to 2 hours. 5 - The method according to claim 1, wherein the heat transfer means are a plate, tube, bar or an arrangement of plates, tubes or bars 6. The method according to claim 1, wherein the Mixing is carried out in a ball mill 7 - The method according to claim 1, wherein the transition metal carbide is selected from a carbide or carbide solution of the transition metals T, Zr, Hf, V, Nb , Ta, Cr, Mo, or W 8 - The method according to claim 7, wherein the transition metal carbide is monotungsten carbide 9. The method according to claim 8, wherein the precursor mixture is composed of tungsten 10 - The method according to claim 1, wherein the inert or reduction gas flowing through at least one mixture potion has a flow rate by weight of the mixture which varies from 5 to 500 normal liters / minute-kg 11 - The method according to claim 1, wherein the transition metal carbide is monotungsten carbide and the precursor mixture has an oxygen concentration of less than 2% by weight as determined by the combustion analysis. 12. The method according to any of the preceding claims, wherein the atmosphere is changed from an inert atmosphere to a reducing atmosphere containing hydrogen, during the reaction of the mixture.