It is also important that the briquettes or pellets formed are of sufficient green strength so that they will not crush or deform when loaded as a static three-dimensional bed in a vacuum furnace, thereby assuring the retention of the porosity of the bed through which the volatile constituents and gaseous reaction products can escape during the vacuum smelting reaction. Adequate green strength to enable a preliminary handling of the pellets, as well as providing the requisite final strength necessary during the initial stage of the vacuum smelting operation, can be imparted to the agglomerates by incorporating any one of a variety of inexpensive binder materials which volatilize without leaving any substantial residue under the temperature and vacuum conditions present in the reactor. For this purpose, binder materials including starches, gelatins, sugars, molasses, sodium silicate, etc., can be employed, of which a dilute molasses solution has been found as being particularly satisfactory. Such binder materials are generally incorporated in amounts ranging from about 2% up to about 10%, with the specific amount used in any particular situation varying in consideration of such factors as the particular size of the tungsten mineral concentrate particles, the manner of agglomerating the particulated mixture and the size of the resultant pellets desired.

 

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Preferably, the carbon reducing agent is employed in excess of that stoichiometrically required and is usually controlled within a stoichiometric ratio of from about 1.05 to about 1.20 times that theoretically required. Amounts of carbon above about 20% in excess of that stoichiometrically required are undesirable due to the retention of excessive carbon in the resultant ferrotungsten alloy pellets, rendering them less desirable as an alloying addition agent in some instances. It is also contemplated that small percentages of the carbonaceous reducing agent, such as carbon, can be incorporated when the iron bearing material comprises metallic iron powders for the purpose of reducing any oxides present on the iron particle surfaces. Generally employing the carbonaceous reducing agent in amount up to about 1% of the iron-bearing material employed is effective for this purpose and provides for ferrotungsten alloys of relatively high purity.

The mineral concentrate, the carbonaceous reducing agent and any supplemental iron-bearing constituent are blended mechanically in appropriate proportions to form a substantially homogeneous or uniform blend. It is important that the particulated mixture is first agglomerated into briquettes or pellets of a size which facilitates their handling and also assures the formation of a porous bed to permit an escape of the volatile constituents and gaseous reaction products of the oxidized reducing agent from the agglomerates during the vacuum smelting operation. The particular configuration and size of the pellets are not critical, and to some extent, will be dictated by the particular type of agglomerating process and equipment employed. Generally, pellets of a spherical configuration, such as derived from a disc-type pelletizing apparatus, having diameters ranging from about 1/8 inch up to about 1/2 inch, are satisfactory.

 

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When a metallic iron powder is employed as the iron-bearing constituent, the average particle size is not critical and may range from about 175 microns to about 74 microns, and preferably is controlled at an average particle size of about 125 microns to about 100 microns. When iron oxide is employed as the iron-bearing material, the iron oxide may suitably be introduced in the form of a fine-sized powder, preferably of an average particle size ranging from about 44 microns to about 10 microns. The iron oxide powder preferably comprises ferric oxide (Fe2 O3), which may be conveniently derived from sources such as millscale, a by-product of hot rolling steel, or the like. When all or a portion of the iron-bearing constituent is introduced in the form of an iron oxide compound, an appropriate amount of carbonaceous reducing material or reducing agent is incorporated in the mixture to effect a substantially complete reduction of the iron oxide to the corresponding metallic state.

In addition to the mineral concentrate and any iron-bearing material, if employed, the particulated mixture contains a fine-sized particulated carbonaceous reducing agent, of which carbon powder itself of an average particle size ranging from about 44 microns to about 10 microns constitutes the preferred material. The quantity of carbon or other carbonaceous reducing agent is employed in an amount at least equal to that stoichiometrically required to effect a substantially complete reduction of the tungsten oxide and any iron oxide present to the metallic state in accordance with the following typical reaction equations: WO3 + 3C ➝ W + 3CO 2Fe2 O3 + 6C ➝ 4Fe + 6CO

 

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Rare earth usually contains one or two kinds of Y, La, Ce, Pr, Gd, Nd, etc. And the main form of adding is rare earth metal, oxide or mixed rare earth. There are some improvements of rare earth: One is due to rare earth lively properties and easily combined with Oxygen nitrogen carbon and sulfur, tungsten carbide impurities will come into being rare earth compounds and distribute in the grain boundary, which prevent liquid Co phase from dissolving and moving grain boundary. Therefore, the region growth of WC grain has been suppressed and refines the particle.

The other one is due to the spherical compound and impurity elements combine to form a rare earth element, it played on the binder phase dispersion strengthening effect, which remove impurities further and purified grain boundaries. In addition, rare earth element can also reduce the sintering temperature of tungsten carbide nozzle, to reconcile the contradiction between grain growth and sintering density and remarkably improve the toughness and bending strength of tungsten carbide.

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Some scientists have developed a new type of inhibitor, which adds the insoluble metal carbides into Co-rich matrix to form a solid solution as inhibitors. It decrease the added inhibitors Co-rich carbide substrate drops below the melting point of 1200 ℃, and formed the stable metal / non-metal atoms in the liquid phase Co group, which is W, V, Cr / C radicals.

This radical prevent W atom and C atom from moving between the grains in liquid phase, which decreases the speed of WC grain growth further and improves the compactness and the lifespan of tungsten carbide buttons. Aside from the grain growth inhibitor above, rare earth element also serves as a common inhibitor used to inhibit tungsten carbide grain growth and improve properties of tungsten carbide nozzle during sintering.















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In addition to the tungsten containing mineral concentrate, the mixture may further contain controlled amounts of an iron-bearing material which may comprise a finely-divided iron powder or an iron oxide powder present in a controlled amount so as to adjust the total iron concentration in the particulated mixture to produce a resultant ferrotungsten alloy having the desired iron content. Ferrotungsten alloys usually contain iron concentrations of from about 0.2% up to about 20%. When employing mineral concentrates derived from wolframite or ferberite, which inherently contain iron concentrations of from about 10% to as high as about 20%, the addition of a supplemental iron bearing material to the particulated mixture is normally not required. However, when employing mineral concentrates derived from heubnerite or scheelite, or when higher iron concentrations are desired in the resultant ferrotungsten alloy, the mixture can be supplemented by the addition of a metallic iron powder or iron oxide powder, as well as a mixture thereof to provide the requisite iron concentration. When the mixture contains an excessive concentration of iron, a controlled amount of iron can be removed by volatilization during the vacuum smelting operation by employing temperatures in the upper portion of the permissible temperature range.

 

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The most common grain growth inhibitors are VC, Cr₃C₂, TaC, TiC, NbC and so on. The inhibitor in the liquid binder phase solubility depends on the chemical stability of the carbide, which has low chemical stability in the binder phase exhibits a high saturation concentration. And the inhibitor effects depend on the solubility of the binder phase itself. Generally, the lower chemical stability of grain inhibitors higher solubility in the binder phase, the inhibitory effect is more obvious. In addition, the eutectic temperature of inhibitor and Co Inhibitor thinning effect is also the main factors of the inhibitor refinement, the lower Co eutectic temperature, the higher solubility in the adhesive phase, the effect of suppressing is better.












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The tungstic oxide (WO3) constituent of the particulated mixture is introduced in the form of a finely-particulated tungsten containing mineral concentrate which preferably comprises predominately tungstic oxide. In accordance with conventional practices, tungstic oxide containing feed materials are commercially available as concentrates derived from various ore beneficiation processes to reduce the gangue and other contaminating constituents to concentrates usually containing at least 60%, and preferably at least 70%, tungstic oxide. Such beneficiation processes usually employ crushing, grinding, magnetic or gravity separation, flotation, or special chemical reactions to produce a particulated tungstic oxide concentrate. Common mineral sources containing tungsten oxide compounds which are suitable for producing concentrates for use in accordance with the practice of the present process include wolframite [(FeMn)WO4 ]; ferberite [FeWO4 ], which is the iron member of the wolframite family and contains some manganese; huebnerite [MnWO4 ], which is the manganese member of the wolframite family and contains some iron; and scheelite [CaWO4 ], which comprises essentially calcium tungstate.
 

The average particle size of the tungsten mineral concentrates for forming the mixture may range from about 10 microns to about 250 microns, while average particle sizes of from about 50 microns to about 125 microns are particularly satisfactory. Usually, beneficiation processes commercially employed effect a grinding or pulverizing of the mineral during the concentration process to an average particle size within the required range and no further supplemental grinding is required. However, in those instances in which the mineral concentrate is of an average particle size generally greater than about 250 microns, the concentrate is first subjected to a grinding operation to reduce its average particle size within the desired range.

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Description of the preferred embodiments

The composition and concentration of the various feed materials, products, by-products and intermediate by-products, are described in the specification and subjoined claims in terms of percentages by weight unless clearly indicated to the contrary.

Sintered, dense pellets or briquettes of a ferrotungsten alloy of the desired composition are produced in accordance with the present process by forming a substantially uniform mixture of a finely-particulated tungsten ore concentrate, a carbonaceous reducing agent and some supplemental iron-bearing material, if necessary, to adjust the iron content within the desired concentration range. The mixture is agglomerated and thereafter heated at an elevated temperature in an environment devoid of oxygen and in a substantial vacuum in a manner to effect a direct reduction of the tungstic oxide constituent to form metallic tungsten and an alloying thereof with the iron constituent present to produce the ferrotungsten alloy.



 

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Summary of the invention

The benefits and advantages of the process comprising the present invention are achieved by forming a substantially uniform mixture composed of controlled amounts of a finely-particulated tungsten ore concentrate derived from minerals, such as wolframite, scheelite, ferberite and/or huebnerite; a supplemental quantity of a finely-particulated iron-bearing material present in an amount to adjust the iron content of the mixture to provide the desired iron concentration in the resultant ferrotungsten alloy produced, and a carbonaceous reducing agent such as carbon present in an amount slightly in excess of the stoichiometric quantity required to reduce the tungstic oxide and any iron oxide compounds present in the mixture to the metallic state. The resultant uniform mixture is agglomerated into a plurality of shape-retaining pellets, which thereafter are heated to an elevated temperature ranging from about 2500° F. to about 3100° F. under a vacuum of less than about 0.5 Torr (500 microns) for a period of time sufficient to effect a reduction of substantially all of the tungsten oxide compounds and any iron oxides present to the metallic state and to effect an alloying of the reduced metallic tungsten and iron to form a ferrotungsten alloy. The vacuum smelting of the pelletized feed material further effects a volatilization of the contaminating constituents in the pelletized feed material, which are continuously withdrawn together with the gaseous reaction products of the oxidized carbonaceous reducing agent. The gaseous effluent can be treated to effect an extraction and recovery of certain valuable volatilized constituents therein, such as manganese, for example, and the balance of the gaseous effluent after appropriate treatment can be harmlessly discharged to the atmosphere. The resultant ferrotungsten alloy in the form of relatively dense sintered pellets are cooled to a temperature of about 300° F. or below, and thereafter are extracted from the vacuum smelting furnace.

Additional advantages and benefits of the present invention will become apparent upon a reading of the description of the preferred embodiments taken in conjunction with the specific examples provided.

 

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