tungsten monocarbide is prepared by sparging a molten composition comprising an alkali metal halide and an oxygen compound of tungsten with a gas comprising a gaseous hydrocarbon, particularly methane.

Patent
   RE32612
Priority
Oct 21 1983
Filed
Apr 24 1986
Issued
Feb 23 1988
Expiry
Feb 23 2005
Assg.orig
Entity
Large
3
7
all paid
1. A process for preparation of tungsten monocarbide comprising:
(a) providing a molten composition comprising an alkali metal halide and an oxygen compound of tungsten;
(b) sparging said composition with a gas comprising a gaseous hydrocarbon which is selected from the group consisting of natural gas, methane, ethane, acetylene, propane, butane, mixtures thereof, and admixtures of these gases with H2 or CO, at a temperature of about 900° to 1100°C for a sufficient time for the tungsten compound to be substantially converted to tungsten carbide; and
(c) decanting the molten halide from the tungsten carbide product.
2. The process of claim 1 wherein the alkali metal halide is sodium chloride.
3. The process of claim 1 in which the oxygen compound of tungsten is tungstic oxide, an alkali metal tungstate, or ammonium tungstate.
4. The process of claim 3 in which the tungsten compound is sodium tungstate.
5. The process of claim 1 in which the gaseous hydrocarbon is methane or natural gas.
6. The process of claim 1 in which the tungsten carbide product is cooled and purified by water leaching.
7. The process of claim 1 in which the molten composition consists essentially of a mixture of sodium chloride and sodium tungstate.
8. The process of claim 7 in which the NaCl-Na2 WO4 mixture is prepared by forming a melt comprising a tungsten ore concentrate, sodium chloride and sodium silicate, whereby the tungsten concentrates in the sodium chloride phase.
9. The process of claim 8 in which the tungsten ore is scheelite or wolframite.
10. The process of claim 1, wherein a reductant is included in the molten composition. 11. The process of claim 10, wherein the reductant is selected from the group consisting of powdered charcoal, coke breeze and calcium carbide.

This application is a continuation-in-part of pending application Ser. No. 544,299, filed Oct. 21, 1983 now abandoned.

This invention relates to a process for preparation of tungsten monocarbide (WC) from oxygen compounds of tungsten.

Recent technological advances have resulted in an ever increasing demand for materials that are capable of delivering constant chemical and physical performances under a variety of extreme operating conditions. For example, the development of supersonic aircraft and rockets is based upon the availability of such materials.

One material of relatively recent importance is tungsten. Tungsten possesses a high melting point and is useful in its elemental form, particularly as an alloy for high-temperature applications. Tungsten is also useful in the form of tungsten monocarbide for use in abrasives and cutting instruments. As the demand for increased availability of such materials has advanced, so has the necessity for improved techniques for their production and recovery.

In the past, recovery of tungsten from its ores was an expensive and relatively complex process involving both physical and chemical separation procedures. Tungsten was usually recovered either from wolframite, (Fe,Mn)WO4, or scheelite, CaWO4, by various pyrometallurgical and extraction procedures. The carbide final product would then be obtained by reacting the purified metal with carbon in another sequence of reactions.

The predominant commercial process for the preparation of tungsten carbide from wolframite or scheelite concentrates requires many steps because the preparation of high purity tungsten powder is necessary before synthesis of tungsten carbide.

In the commercial production of tungsten monocarbide by direct synthesis, tungsten powder is mixed with 6.2-6.9% carbon. This amount of carbon is 2-10% in excess of the stoichiometric amount in WC (6.13%) and is required to decrease the oxygen content. The mixture is usually blended in a rotary blender containing steel or tungsten carbide balls. Typically one-half of the tungsten powder will be added to all of the carbon, blended for several hours, and this is followed by the addition of the remaining tungsten powder and additional several hours of blending. Careful blending procedures are required, particularly in the case of coarse tungsten powder, because of great disparity in density of the constituents and the size effects of the particles. The tungsten-carbon mixture is next put into graphite boats and passed continuously through a tube furnace. The furnace may be either gas fired or electrically heated, but must be maintained in the temperature range of 1,400° to 1,600°C Hydrogen is usually introduced into the discharge end of the tube and is burned off at the charging end. The time per furnace cycle is 4 to 5 hours.

The major disadvantage of this process is the requirement for high purity tungsten powder as the feed material for synthesis of the carbide. The complete process for preparing monocarbide from ore concentrates includes the following steps: (1) pressure digestion to decompose the ore, (2) solution purification, (3) solvent extraction, (4) ammonium hydroxide stripping, (5) ammonium paratungstate crystallization, (6) formation of tungsten metal by hydrogen reduction, and (7) carbide synthesis.

Aluminothermic reduction is also used to produce WC from ore concentrates. This process consists of blending the concentrates, aluminum, calcium carbide and other reagents in a tear-down carbon lined refractory furnace. A thermite reaction is started and the whole mass becomes molten, thereby producing a slag and an iron-manganese alloy containing WC crystals. The furnace is cooled in about 10-14 days and the slag and menstruum separated. The menstruum is then crushed and leached to yield a coarse WC product. The coarse WC crystals next are ground in a WC rod or ball mill and yield a commercial WC product.

This technique, however, is also disadvantageous in that it requires a large amount of energy for size reduction and leaching the menstruum, and also for the size reduction of the recovered WC crystals. Additionally, the carbide thus formed is not suitable for preparing many cemented carbide products.

U.S. Pat. No. 3,482,941 to Palm discloses another method for recovering tungsten in the form of tungsten carbide from scheelite by heating ore with silica and carbon. This process is said to reduce the amount of ditungsten carbide which is commonly formed when tungsten carbide is prepared at elevated temperatures.

U.S. Pat. No. 4,256,496 to Brandstatter discloses an additional process for the recovery of metal ores utilizing a solid state reduction and carburization reaction. The disclosed process utilizes physical separation techniques, such as gravity separation employing water elutriation. The separation is based upon the different physical properties of metal carbides and various gangue oxide constituents of ores at elevated temperatures.

It has now been found, according to the present invention, that preparation of tungsten monocarbide from oxygen compounds of the metal may be efficiently and economically accomplished by subjecting a melt of an alkali metal halide and the tungsten compound to sparging with a hydrocarbon gas. The process of the invention has been found to be particularly applicable to preparation of tungsten monocarbide from tungstic oxide or tungstates such as sodium tungstate and ammonium paratungstate.

The preferred alkali metal halide will generally be sodium chloride since it is usually most readily available and economical. However, the alkali metal halide may also consist of fluorides or bromides of sodium, potassium, or lithium. In addition, mixed metal compounds such as NaAlF or KAlF may also be used.

Proportions of the alkali metal halide and the tungsten compound are not critical, provided the amount of halide is sufficient for ready formation of a homogenous melt for treatment with the hydrocarbon gas. The percent of tungsten (in percent WO3 by weight) will generally range from about 5 to 30. Formation of the melt is accomplished by simply heating an admixture of halide and tungsten compound to a temperature and for a time sufficient to form a melt of sufficient fluidity for sparging with the hydrocarbon gas. A temperature of about 900° to 1100°C and a time of about 3 to 8 hours is usually sufficient.

Formation of the melt, as well as the sparging, may be carried out in a crucible of any conventional refractory material such as graphite, or ceramics such as alumina, magnesia, zircon or silicon carbide. However, it has been found that refractory metal alloys, such as Inconel, are particularly effective in the process of the invention.

The preferred hydrocarbon gas will generally be methane, or natural gas. However, other hydrocarbons, such as ethane, acetylene, propane or butane may also be used. In addition, mixtures of the hydrocarbon gas, or gases, with H2 or CO may be used. Furthermore, other reductants such as powdered charcoal, coke breeze or calcium carbide may be added to the sodium chloride melt to facilitate reduction during the sparging process. It has also been found that addition of small amounts of alkali borates, carbonates, fluorides or hydroxides to the melt may assist in reduction and crystal growth.

Sparging is by conventional means using sparging tubes of any suitably inert material such as graphite, alumina or Inconel. A temperature of about 900° to 1100°C and a methane flow rate of about 4 to 12 liters per minute per liter of melt for a period of about 3 to 8 hours is usually sufficient to give substantially complete conversion of tungsten to WC. A vacuum or inert atmosphere may also be beneficial in the sparging process.

The WC product has a density greater than the melt, and forms a separate phase. The phases are readily separated by decanting the tungsten-depleted phase from the melt, after which the WC-rich composition is cooled and water leached to remove adhering salt from the WC product. The WC product may then be further purified by leaching with dilute HCl solution, followed by dilute caustic solution, and water washing, and is then dried. The dried WC product may be further purified by treatment for removal of any free carbon by a conventional heavy liquid treatment, resulting in gravity separation of the carbon from the heavier WC. Diiodomethane has been found very effective for this purpose, although other heavy liquids such as acetylene tetrabromide may be used. The free carbon can also be removed by floation or gravity methods.

The process of the invention has been found to be particularly effective in preparation of WC from concentrates of tungsten ores such as scheelite or wolframite. A sodium tungsten-sodium chloride compostion is initially prepared by means of the process of U.S. Pat. No. 3,373,097, the disclosure of which is incorporated herein by reference. In this process a melt comprising the ore concentrate, sodium chloride and sodium silicate is held at elevated temperature for a time sufficient to form separate halide and silicate phases. Sodium tungstate is formed by the reaction (in the case of wolframite)

2(Fe,Mn)WO4 +3Na2 SiO3 →2Na2 WO4 +Na2 (Fe,Mn)2 Si3 O9

and concentrates in the halide phase, with gangue materials concentrating in the silicate phase. The phases are separated, and the resulting NaCl-Na2 WO4 composition is then treated according to the process of the present invention for formation of WC.

The invention will be more specifically illustrated by the following examples.

Feed material-Colorado wolframite concentrate

______________________________________
Analysis, pct
WO3 Fe2 O3 MnO
69.1 9.1 13.5
High temperature phase separation
Charge grams
______________________________________
Wolframite
484
NaCl 704
Na2 SiC3
291
1,479
______________________________________

The charge was melted for 2 hours at 1,075°C in a silicon carbide crucible. Molten charge was removed from the furnace and the two molten phases were decanted into separate slag pots. The upper halide phase was fluid and easily separated from the lower silicate phase which was very viscous.

______________________________________
Analysis of the phases, pct
Wt. g WO3 FeO MnO
______________________________________
Halide 980 31.4 0.05 0.17
Silicate 440 4.5 9.3 13.4
1,420
______________________________________

The halide phase containing 92 pct of the WO3 in the wolframite was treated by methane gas sparging.

A 600-gram charge of the halide phase was melted in a 3" I.D.×7" high graphite crucible at 1,100°C A graphite tube with four 1/16" holes at the bottom was used as a gas sparger. Methane was passed through the tube for 6 hours at a rate of 4 liters/min STP. The gas was turned off, the sparging tube removed and the remaining molten salt in the crucible was decanted into a slag pot. The cooled crucible was then water leached and scraped to recover the WC product. The WC product was leached in dilute HCl and NaOH solutions, and free carbon removed by heavy liquid treatment.

______________________________________
Results
______________________________________
WC recovered = 128 g
W recovered as WC, pct = 81
Final melt, g = 310
Final melt, pct WO3 = 10
Final melt, g WO3 = 31
X-ray diffraction
Major WC, trace C
No W2 C detected
Spectrographic analysis of WC product, pct
Al Cu Fe Mg Mn Mo Si Sn Ti V
______________________________________
0.005
0.007 0.10 0.005
0.05 <.001 0.07 0.009
0.01 0.006
Other impurities below detection limits.
Carbon and oxygen analysis, pct
C = 6.19
O = 0.34
Specs for Commercial WC
Max. pct
Free carbon
0.10
Fe 0.15
Mo 0.10
Stoichiometric carbon content of WC = 6.13 pct.
______________________________________
______________________________________
Feed material - California scheelite concentrate
Analysis, pct
WO3 CaO Fe2 O3
69.3 18.1 2.6
High Temperature phase separation
Charge grams
______________________________________
Scheelite
432
NaCl 420
NaF 201
Al2 O3
72
Al2 SiO3
366
1,491
______________________________________

The phase separation was made with the same procedure as that of Example 1.

The molten phases were decanted into separate slag pots.

______________________________________
Analysis of the phases, pct
Wt. g WO3
CaO
______________________________________
Halide 909 31.5 0.22
Silicate 515 2.0 14.5
1,424
______________________________________

The halide phase containing 96 pct of the WO3 from the scheelite was treated by natural gas sparging. The natural gas contained, in pct: methane, 93.63; carbon dioxide, 0.70; nitrogen, 0.47; ethane, 3.58; propane, 1.02; other hydrocarbons, 0.60.

A 600-gram charge was melted at 1,100°C in a 3" I.D.×7" high graphite crucible. A 1/4" O.D.×1/8" I.D. Alundum tube was used for gas sparging the melt. Natural gas was passed through the melt for 6 hours at a rate of 4 liters/min STP. The tube was removed and the remaining molten salt in the crucible was decanted into a slag pot. The WC was recovered from the cooled crucible by water leaching and scraping. The WC product was leached in dilute HCl and NaOH solutions, and treated for free carbon removal in heavy liquids.

______________________________________
Results
______________________________________
WC recovered = 120 g
W recovered as WC, pct = 75
Final melt, g = 360
Final melt, pct WO3 = 9.5
Final melt, g WO3 = 34.0
x-ray Diffraction of WC product
Major WC
Spectrographic analysis of WC product, pct
Al Cu Fe Mg Mn Mo Si Sn Ti V
______________________________________
0.005
0.007 0.1 0.005
0.05 0.001
0.05 0.003
0.005
<.006
Other impurities below detection limits.
Carbon and oxygen analysis, pct
C = 6.31
O = 0.32
______________________________________
PAC Gas Sparging of Halide Phase Prepared from Wolframite in a Metal Alloy Crucible

A 1,600 gram charge of halide phase prepared according to the procedure of Example 1, and containing 25 wt pct WO3, was melted in an Inconel 600 crucible 3" I.D.×12" high. A 1/4" Inconel pipe was the sparging tube. The melt was heated to 1,070°C and methane sparged for 4 hours at a gas flow rate of 5 liters per minute STP. The WC was recovered by decanting off the unreacted melt, cooling the crucible and water leaching and scraping. The WC product was leached in dilute HCl and NaOH solutions, water washed and dried.

______________________________________
Results
______________________________________
WC recovered = 181.5 g
W recovered as WC, pct = 54
Final melt, g = 992
Final melt, pct WO3 = 15.5
Final melt, g WO3 = 153.8
X-Ray Diffraction
Major WC
Spectrographic analysis of WC product, pct
Al Ca Cr Cu Fe Mg Mn Mo Na Ni Si
______________________________________
.005 <.01 0.1 .002 .04 .001 .001 .05 .04 .02 .02
Other impurities below detection limits.
Carbon and oxygen analysis, pct
C = 6.19
O = 0.07
______________________________________

The Inconel crucible was used a total of 5 times with similar results. A major advantage of using the Inconel crucible was that free carbon was not detected in the WC product, thus eliminating a free carbon cleaning step.

______________________________________
WC from Na2 WO4
Charge grams
______________________________________
NaCl 440
Na2 WO4
160
600
______________________________________

The charge was melted in an alumina ceramic crucible 23/4" I.D.×6" high, at 1,100°C Methane gas was passed through the melt for 6 hours at a flow rate of 4 liters/min STP.

______________________________________
Results
______________________________________
WC recovered = 92 g
W recovered as WC, pct = 86
Final melt, g = 260
Final melt, pct WO3 = 6.2
Final melt, g WO3 = 16
Spectrographic analysis of WC product, pct
Al B Cu Fe Mg Mn Na Pb Si Sn TiZr
______________________________________
0.05 0.004
0.01 0.03 0.02 0.02
0.4 <.01 0.1 <.002 0.01 0.1
______________________________________

WC was also prepared in NaCl-Na2 WO4 melts containing 50 pct WO3. The WC yield was about 50 pct.

______________________________________
WC from WO3
Charge grams
______________________________________
NaCl 500
WO3
100
Na2 CO3
46
646
Procedure:
Temperature, 1,100°C
Graphite crucible, 3" I.D. × 7" high
1/4" alumina sparging tube
Methane gas sparged for 6 hours at
a flow rate of 3 liters/min STP
Results
WC recovered = 68 g
W recovered as WC, pct = 80
Final melt, g = 398
Final melt, pct WO3 = 5.0
Final melt, g WO3 = 19.9
______________________________________

The analyses were similar to those obtained in Example 4.

______________________________________
WC from ammonium paratungstate (89 pct WO3)
Charge
grams
______________________________________
NaCl 500
APT 140
NaOH 43
683
______________________________________

The charge was melted in a 3" I.D.×7" high graphite crucible at 1,100°C Methane gas was passed through a 1/4" alumina tube into the melt for 6 hours at a flow rate of 4 liters/min STP. Ammonia gas was given off during heating of the APT prior to melting.

______________________________________
Results:
______________________________________
WC recovered = 74 g
W recovered as WC, pct = 70
Final melt, g = 307
Final melt, pct WO3 = 7.5
Final melt, g WO3 = 23.0
______________________________________

The analyses were similar to those obtained in Example 4.

Gomes, John M., Raddatz, Andrea E., Baglin, Elizabeth G.

Patent Priority Assignee Title
5096689, Jan 23 1989 KENNAMETAL INC Process for producing tungsten monocarbide
5882620, Jun 07 1995 INTERNATIONAL CARBITECH INDUSTRIES, INC Pyrometallurgical process for forming tungsten carbide
6749663, Sep 06 2000 H C STARCK TUNGSTEN GMBH Ultra-coarse, monocrystalline tungsten carbide and a process for the preparation thereof, and hardmetal produced therefrom
Patent Priority Assignee Title
3373097,
3482941,
3692479,
3902917,
4190439, Dec 28 1973 AVOCET TUNSTEN, INC Process for the preparation of fine grain metal carbide powders and sintered articles therefrom
4256496, Aug 05 1974 Ontario Research Foundation Production of metal carbides in pure form
GB1190713,
/
Executed onAssignorAssigneeConveyanceFrameReelDoc
Apr 24 1986The United States of America as represented by the Secretary of the(assignment on the face of the patent)
Date Maintenance Fee Events
Jun 01 1992M184: Payment of Maintenance Fee, 8th Year, Large Entity.
Sep 14 1992ASPN: Payor Number Assigned.
Jun 06 1996M185: Payment of Maintenance Fee, 12th Year, Large Entity.
Aug 19 1996RMPN: Payer Number De-assigned.


Date Maintenance Schedule
Feb 23 19914 years fee payment window open
Aug 23 19916 months grace period start (w surcharge)
Feb 23 1992patent expiry (for year 4)
Feb 23 19942 years to revive unintentionally abandoned end. (for year 4)
Feb 23 19958 years fee payment window open
Aug 23 19956 months grace period start (w surcharge)
Feb 23 1996patent expiry (for year 8)
Feb 23 19982 years to revive unintentionally abandoned end. (for year 8)
Feb 23 199912 years fee payment window open
Aug 23 19996 months grace period start (w surcharge)
Feb 23 2000patent expiry (for year 12)
Feb 23 20022 years to revive unintentionally abandoned end. (for year 12)