REDUCTIOIN OF TUNGSTEN OXIDES BY CARBON OR CARBON-CONTAIMNG COMPOIJNDS - 7

Carbothermal reduction of tungsten oxides with carbon monoxide, or gas mixtures of CO/CO₂, CO/H₂, CH₄/H₂, C₂H₄/H₂, and C₂H₄/H₂, as well as by reaction between metal oxide vapor and solid carbon have recently attracted attention for produci.ng high surface area tungsten carbides (up to 100m²/g), for use as catalyst, and for nanophase WC/Co composite powders.

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REDUCTIOIN OF TUNGSTEN OXIDES BY CARBON OR CARBON-CONTAIMNG COMPOIJNDS - 6

Direct carburization is advantageous mainly in the production of submicron tungsten carbide powders. The idea behind the process is to circumvent the expensive hydrogen reduction of fine tungsten powder by excluding water vapor during the reduction step, which is responsible for grain growth reactions and low capacity. Roughly speaking, the grain size of the final carbides is related to the size of the internmediate oxides.

Only recently was a new carbothermal reduction process developed in which the WC is synthesized by a rapid carbothermal reduction of tungsten oxides in a vertical graphite transport reactor (RCR entrainment process). Rapid heating of the WO3/C mixture driven by thermal radiation allows conversion of the mixture into a carbide precursor WC_1-x) within very short reaction times (a matter of seconds). In a second step, additional carbon is added to the carbide precursor to form a mixture, which then undergoes a second heat treatment to convert the precursor into substantially pure WC.

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REDUCTIOIN OF TUNGSTEN OXIDES BY CARBON OR CARBON-CONTAIMNG COMPOIJNDS - 5

Under industrial conditions, the process can be performed in two steps, one following the other. The first step-reduction-affords the complete exclusion of hydrogen and is performed in nitrogen atmosphere. In the presence 0f hydrogen, water vapor would form which generates tungsten crystal growth and consumption of carbon (C+H₂O→CO+H₂). This carbon consumption is not only restricted to the carbon black or graphite used as reagent (disturbance of the carbon balance), but also to the furnace carbon tube (shortening its lifetime). The second step-carburization-needs a hydrogen atmosphere, which supports the carbon transport via methane to the tungsten particle surface.

Accordingly, the direct carburization process starts with a pelletized mixture of WO₃ and graphite passing step-by-step two rotary furnaces, the first operating at 950-1250℃ under nitrogen (reduction) and the second at 1400℃ under hydrogen (carburization).

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REDUCTIOIN OF TUNGSTEN OXIDES BY CARBON OR CARBON-CONTAIMNG COMPOIJNDS - 4

Carbothermic reduction of WO₃ 0r ore concentrates is of technical importance in melting metallurgy-preparation of ferrotungsten, melting base, and cast carbide.

The reduction of WO3 by carbon in the solid state, however, has gained technical interest and importance m the case when the desired final product is tungsten carbide. This process is named “Direct Carburization” and the basic reaction is described by the following equation:

   WO₃+4C→WC+3CO

In contrast to the conventional procedure for WC production-W03 reduction by hydrogen followed by W+ C mixing and carburization-this method allows the formation of WC in only one step, while, carbon acts as carburizing and as reducing agent.

The above equation is only of theoretical value because, besides the main reaction, the following reactions also occur:

WO₃+5/2C→WC+3/2CO₂

C+CO₂→2CO

Consequently, in this procedure, an uncertainty in the carbon demand is given. This means that, depending on the operating conditions, the carbon content of the final product may vary significantly.

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REDUCTIOIN OF TUNGSTEN OXIDES BY CARBON OR CARBON-CONTAIMNG COMPOIJNDS -3

Although the reducing agent-carbon-is very cheap, and carbon reduction was the early basis of tungsten powder production, so far none of the numerous carbothermic procedures has been established in the production of pure tungsten.

The reasons for that are:

·The carbon balance is difficult to control; furthermore, the metal forms carbides. Although the theoretical carbon consumption to reduce W03 to W is 15.5 wt%, only 12-14 wt% are normally sufficient at 1200℃ t0 1400℃because the reaction does not only proceed via WO₃ + 3C→ W +3CO but also to a certain extent via 2W03+3C W+3.C02. Therefore, in most cases the reduction is either incomplete or the metal powder is contaminated by carbides.

·Carbon is always a source of increased tungsten contamination because it contains impurities like Ca, Si, Fe, S, and P.

·Another disadvantage of the carbothermic reduction is the limited possibility of steering the grain size by varying the reduction conditions as compared to the hydrogen reduction process.

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REDUCTIOIN OF TUNGSTEN OXIDES BY CARBON OR CARBON-CONTAIMNG COMPOIJNDS-1

The reduction kinetics are based on an adsorption-autocatalytic reaction. Reduction enthalpy and temperature are linked by a linear relationship. The calculated activation energy is about 75 kJ•mol^-1. Carbide formation occurs only when all oxygen is removed.

The apparent activation energy at the beg, inning of the reaction is 121kj/m0l WO₃ and increases during further progress t0 205 kJ•mol^-1 WO3, due to the fact that the reaction becomes diffusion-controlled (diffusion of oxygen to the particle surface).

The assumed reaction mechanism is as follows: direct reaction of WO3 and C at contact zones and. For the main portion, sublimation of W03 to the carbon particles, reduction at the surface of the carbon particles, and desorption of CO and CO₂.

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REDUCTIOIN OF TUNGSTEN OXIDES BY CARBON OR CARBON-CONTAIMNG COMPOIJNDS-1

The reduction of tungsten oxides by carbon or carbon-containing compounds can be easily performed. Statements about the starting temperature for the reaction between WO₃ and solid carbon (carbon blacks, graphite) vary in the current literature between 655℃ and 783℃. Differences in WO₃ and C properties (particle size of the powders, preparation history, crystallinity, etc.) as well as in atmospheres may be responsible for that. The temperature range coincides with the beginning of self-conductivity and sublimation of W03: Carbon monoxide starts to react with W03 at 535℃.

The reduction/carburization sequence can be assessed from so-called predominance area diagrams (Kellogg diagram s), which can be derived from free energy of formation data of the compounds occurring in the W-C-O system (WO_3, WO_2.9, WO_2.72, WO₂, W, W₂C, WC, CO, CO₂). It predicts that at 1100℃ the reduction/carburization proceeds via the oxide phases（WO2.9→ WO_2.72→WO₂→W→W₍₂₎C）, with the possibility of a direct carburization of WO_2.72 and WO₂.

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MINING AND ORE BENEFICIATION - 2. Ore Beneficiation - 2.6 High Tension Separation

Electrodynamic or electrostatic separators are used only for scheelite-cassiterite mixtures. In contrast to scheelite, cassiterite is conducting.

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MINING AND ORE BENEFICIATION - 2. Ore Beneficiation - 2.5 Magnetic Separation

Magnetic separation can be applied for two possible reasons: to clean scheelite by removing magnetic minerals like garnet, magnetite, and roasted pyrite; to concentrate wolframite by separation from nonmagnetic minerals like cassiterite or unroasted pyrite. Dry and wet magnetic separators are employed.

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MINING AND ORE BENEFICIATION - 2. Ore Beneficiation - 2.4 Wolframite Flotation.

Wolframite flotation is performed similarly to the above-described scheelite flotation, but is not pH sensitive and can therefore be undertake in both acidic and alkaline solutions. Generally, flotation is rarely applied to wolframite The reason is that wolframite occurs mainly in vein-type deposits whose mineralization is much coarser than to most scheelite ores. Therefore, gravity and magnetic methods are more desirable than flotation.

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MINING AND ORE BENEFICIATION - 2. Ore Beneficiation - 2.3 Scheelite Flotation

The existing literature on scheelite notation is quite extensive, because the gangue minerals vary in quality and quantity not only from mine to mine; but also within the same deposit with progress in mining, in principle, each ore dressing plant has its own recipe, and this will always be adjusted to the variations in ore constitution. Although patents have been granted, the detailed procedures are kept secret by most companies.

The following chemicals and conditions are generally in use:

Collectors. Oleic acid, mixtures of oleic and linolic acid, sodium oleat, tall oil, saponified tall oil and mixtures of these substances, coriander oil, rape oil, mustard oil, pyridine base, water-soluble carbohydrates of the green oil type, also in saponated form, fatty acids and their salts, fish oil, hydroxyl-ethylene- cellulose, carboxy-methyl-cellulose.

Frothers. Alcohols, cresylic acid, aerosol, oil of turpentine, ketones.

Depressants. Quebracho or tannin for calcite, formic acid for apatite, lactic acid for mica, and sodium salts of hydrolyzed polyacrylnitrile.

The optimal pH value is between 9.0 and 10.5 and must be controlled to a tolerance of ±0.1 units, depending on the ore composition and the reagents in use. Sodium carbonate or sodium hydroxide solutions are used for pH regulation.

Sodium silicate serves as a dispersant for calcite. The addition of polyacrylamide or flocculents increases the flotation yield.

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MINING AND ORE BENEFICIATION - 2. Ore Beneficiation -2.2 Ore Sorting

Ore sorting is a modem preconcentrating stage ahead of the other conventional dressing methods. In former times, sorting was done by hand. Today, mechanical sorting is possible.

Successful application of an ore sorter depends on two important t criteria:

The valuable fraction must be liberated from gangue at a size range which can be handled by the machine.

The valuable fraction must be identifiable within the time available for examination by the machine.

Photometric ore sorting can be applied to wolframite. It can also be used for scheelite, if contained in white quartzite veins. Fluorescence ore sorting is a suitable method for scheelite.

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MINING AND ORE BENEFICIATION - 2. Ore Beneficiation - 2.1 General - 03

For concentration (separation of gangue minerals) several methods can be applied, depending mainly on the composition of the ore. They include ore sorting, gravity methods, flotation, magnetic, and electrostatic separation.

Beneficiation of tungsten ores by gravity was the classical method, followed by a “cleaning” step.  The recovery depends on the ore characteristics (mainly liberation size) and ranges typically between 60 and 85%. The main loss is in the slimes, because the tungsten minerals are the most friable ones present in ores.

As regards“cleaning,” for example, a roast process can be applied to convert pyrite to magnetic separation together with garnet and pyroxene. Another cleaning step in the presence of sulfide minerals would be a sulfide flotation.

Gravity methods can also be applied for both scheelite and wolframite. The usual equipment consists of spirals, cones, tables, and a sink-float.

In order to optimize the yield, modern technology includes the following additional steps or combinations:

Preconcentration. This could be accomplished by sorting, use of jigs, or heavy media separation.

Whole Flotation. If tungsten ore mineralization is too fine, the total amount of mined ore can be subject to flotation.

Scavenging Circuits. These are combinations of gravity separation and flotation to recover the loss via slimes occurring in the classical procedure.

MINING AND ORE BENEFICIATION - 2. Ore Beneficiation - 2.1 General - 02

In principle, two important properties of the ore determine the flow sheet of an ore dressing plant:

1. The particle size of the tungsten ore which determines the degree of disintegrations necessary to liberate the tungsten mineral (liberation size).

2. The type and concentration of the accompanying minerals (gangue) which have to be separated dictate the mode and number of separation steps.

Ore beneficiation consists of two main steps: comminution and concentration. Comminution is first performed by crushing. Equipment in use comprises jaw, cone, or impact crushers working mostly in closed circuits with vibratory screens. The second step in coinminution is grinding, which is undertaken in rod or ball mills working in closed circuits with classifiers.

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MINING AND ORE BENEFICIATION - 2. Ore Beneficiation - 2.1 General - 01

The majority of tungsten deposits only contains some tenths of a percent of WO₃. On the other hand, ore concentrates in international trading require 65-75% WO₃. Therefore, a very high amount of gangue material must be separated. This is why ore dressing plants are always located near the mine (to save transportation costs). Companies which process their own concentrates produce low-grade concentrates (6-40% WO₃) in order to minimize the loss of tungsten minerals which increases with increasing concentration grade.

Another important aspect of the beneficiation process today is the disposal of the separated gangue materials, which in case of flotation also contain chemicals. Especially in areas with rigid environmental restrictions, a deposition near the plant is sometimes not possible and transportation over longer distances is therefore necessary. Depending on the mine conditions, a total or partial refill of the tailings into the mine is possible.

In regard to the beneficiation of ore, the positive properties of the tungsten minerals are the high specific gravity (scheelite and wolframite) and the ferromagnetism (wolfram-mite). A negative property is their brittleness, leading to a partial joss by too fine particles formed during the disintegration steps.

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MINING AND ORE BENEFICIATION - 1. Mining

Tungsten mines are relatively small and rarely produce more than 2000 t of ore per day. Mining is mainly limited by the size of the ore bodies, which are not very large. Open pit-mining is the exception. These mines are short-lived and soon convert to underground operations.

Mining methods for tungsten ore are not at all exceptional and usually are adapted to the geology of the ore deposit.

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Tungsten Products - Tungsten Carbide Powder

Tungsten carbide powder is the intermediate in the line from W powder to cemented carbides. It can be produced from different raw materials and by different processes. By far the biggest percent-age is manufactured by the conventional method –carburization of tungsten powder- and covers the widest range of powder qualities in regard to average grain size (0.15 – 12 pm). All other methods in use yield in very fine or very coarse powder grades.

The conventional process of carburization comprises mixing of the respective tungsten powder of desired grain size with high purity carbon (lamp black or graphite) and reacting at temperatures between 1300 t0 1600 0C in hydrogen atmosphere, The average grain distribution of the WC powder.

The final carbon content of the WC Powder depends on the production mode of the hardmetal producer and is one item of the rigid specification, and varies from slightly sub-stoichiometric to stoichiometric (6.13 % C) to slightly over-stoichiometric, But not only is the carbon content speciffed but also a series of physical properties as there are: Average particle size, particle size distribution, apparent (bulk) density and homogeneity. High temperature carburized WC powders (1700 -2200 0C) are usually coarse 10 – 50 µm, but sometimes also 5 to 10 µm grades are treated that way, The percentage of high temperature WC is small.

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Tungsten Products - Tungsten Metal Powder

Technical tungsten powder qualities are prepare d by hydrogen reduction and are available in average particle sizes from 0.1 to 100µm. The reduction process is in some respect, unique. It offers the possibility to produce tungsten powder of any desired average grain size within the above limits only by changes in reduction conditions.

The whole palette of grain sizes finds their applications in cemented carbide production. The main portion of tungsten powder is directed to that manufacture. Starting tungsten powder for ductile tungsten and powdermetallurgically produced tungsten alloys covers grain sizes between 2 and 6µm. Extremely coarse powder gained by screening to separate any finer particles has excellent flow character is used in plasma spraying.

The purity of the tungsten powder is of particular importance in all application and is mainly influenced by the purity of the origin al APT. Typical upper limits of foreign element concentrations in pg/g are: Co, Cu, Mg, Mn, Pb≤2 Al, Fe, Ni, Si, Ca, Cr, Sn≤<10 Mo≤20.

The crucial physical properties are average particle size, particle size distribution, apparent, tap and compact or green density, specific surface area, degree of agglomeration and morphology,  They are to a certain extent related to each other and can be influenced between limits by the oxide properties an d the reduction conditions.

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Tungsten Products - Tungstic Acid H2W04

Tungstic acid, the most important intermediate in former tungsten chemistry, is today exclusively manufactured out of APT, in order to make use of the high purity APT level. For that purpose an aqueous APT slurry will b e treated with hydrochloric acid. By that tungstic acid is precipitated, and afterwards filtered, washed and dried. Tungstic acid has a very high active surface and is only used in small quantities for special purposes as there are: Production of ultrafine W and WC powders and production of tungsten chemicals.

 

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