Specialized metallurgical processes – compositions for use therei – Processes – Producing or purifying free metal powder or producing or...
Reexamination Certificate
2000-09-22
2003-02-25
Wyszomierski, George (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or purifying free metal powder or producing or...
C075S353000, C075S371000, C148S217000, C148S237000, C423S440000
Reexamination Certificate
active
06524366
ABSTRACT:
BACKGROUND OF THE INVENTION
Tungsten carbide is used in a wide variety of products such as cutting tools, wear parts and mining drill bits. Its toughness and hardness makes it excellent for these applications. In order to improve the hardness of tungsten carbide, the grain size of the carbide should be as small as possible. It is known that nanograin size tungsten carbide powders can be produced by a combination reduction carburization process. This permits the tungsten carbide to be used in very demanding applications. One such process of forming tungsten carbide is disclosed in Gao, U.S. Pat. No. 5,919,428. This requires very careful control of operating parameters. Other processes used to manufacture nanophase tungsten carbide as well as tungsten carbide cobalt composites also typically involve several processing steps including reductive decomposition and gas phase carburization. Unintentional formation of stable intermediates in these processes significantly increases the carburization cycle time. Further, most of these products are not particularly well suited for recycling scrap tungsten carbide. Generally it is desirable to recycle spent or unused scrap tungsten carbide for both environmental and cost reasons.
There are several processes currently used to recycle tungsten carbide and tungsten products. These processes include the zinc process, the cold stream process, an alkaline leach process, chlorination systems, electrolysis and high temperature smelting. With the exception of the zinc and the cold stream process, the other chemical methods involve numerous conversion extraction and precipitation steps that increase the cycle time.
The zinc and cold stream process do permit a direct conversion into useful powder but material purity is a problem and thus the application of this type of reclaimed material is extremely limited. Seegopaul U.S. Pat. No. 5,613,998 and Seegopaul U.S. Pat. No. 5,728,197, both disclose methods of reclaiming scrap tungsten carbide and using this to form cobalt tungsten carbide composites. The methods utilized to form cobalt tungsten carbide composites are very different than methods used to form tungsten carbide per se. Basically the carbothermic process typically used to make tungsten carbide cobalt composites will not form nanoscale tungsten carbide. In the carbothermic process, the presence of the cobalt helps catalyze the decomposition of carbon monoxide to promote carburization. Thus, there is currently no method suitable to both reclaim tungsten carbide and at the same time use this to form small grain tungsten carbide particles.
SUMMARY OF THE INVENTION
The present invention is premised on the realization that a tungsten salt such as ammonium tungstate can be formed into particles which can then be calcined in nitrogen or air and then subsequently subjected to a gas phase carburization to form tungsten carbide powder. This will form a nanograin size powder with grain sizes less than 50 nanometers. Grains can then be formed by utilizing a high temperature annealing process.
In particular the present invention can utilize scrap tungsten carbide by oxidizing the tungsten carbide to form a mixture of tungsten yellow oxide (WO
3
), and cobalt tungstate, if cobalt is in the starting material. This is then subjected to an acid leach which will remove binder metals such as cobalt and nickel leaving a mixture of tungsten trioxide and tungstic acid. This is digested with a base such as ammonium hydroxide to dissolve the tungsten which is spray dried. The spray dried particles form an excellent source material for the calcination carburization formation of tungsten carbide particles.
The objects and advantages of the present invention will be further appreciated in light of the following detailed description.
DETAILED DESCRIPTION
The present invention is a method of forming tungsten carbide and will be disclosed with respect to its preferred embodiment which utilizes scrap tungsten carbide as a starting material. However, it should be stated that the present invention can use any tungsten salt as a starting material and does not require utilization of tungsten carbide scrap as a starting material.
The first step of the present invention is to convert the scrap tungsten into oxide powder. Tungsten material that can be used in the present invention includes elemental tungsten, tungsten oxides, tungsten-copper, tungsten-silver, tungsten nitride, tungsten boride, tungsten salenide, tungsten sulfide, tungsten silicide and tungsten carbide which can be found either with a binder such as cobalt, nickel or iron or without the binder.
This material would generally be waste tungsten carbide and is referred in the industry as hard scrap or soft scrap. Hard scrap is previously sintered material and soft scrap is unsintered powder. Hard scrap should be crushed to a small size suitable for loading into a rotary furnace. While it is preferred that the crushed size be down to a few millimeters to increase the reaction rate, fragments in the range of 1 to 6 inches can also be utilized. It may be preferred to subject the scrap to a pre-oxidation treatment such as a washing to remove surface contaminations. But this should not be necessary in the present invention.
The scrap particles are then oxidized preferably by heating to a temperature of 600° to 1050° C. in the presence of air for a period of 10 to 24 hours. As a general rule, high oxidation temperatures shorten the oxidation time. Coated scrap will also require a higher oxidation temperature than powder or uncoated scrap. The scrap can be converted in any furnace which utilizes flowing air. However, the rotary furnace with flowing air is preferred due to the grinding action provided by the rotation of the furnace.
The oxidation forms tungsten trioxide, also referred to as tungsten yellow. If a binder is present such as cobalt, the tungstate such as cobalt tungstate will be the reaction product, together with the oxide.
After the tungsten oxide and tungstate are formed, they are subjected to an acid leaching at a pH of less than 1. The preferred leaching agent is concentrated hydrochloric acid (230 g/l). This will act to dissolve binder elements such as cobalt or nickel as well as impurities to enable them to be separated from the oxide powder. Leaching is accomplished by elevating the temperature to boiling temperature in an acid resistant tank and leaving the oxide powder in the tank for a period of about 4 to about 16 hours with about twelve hours being preferred. Preferably the evaporated acid is recovered.
This will act to dissolve the binder as well as impurities leaving the tungsten in the form of tungsten trioxide or tungstic acid behind. The tungsten compound is then separated from the liquid acid by filtration. Binder metals can be recovered from the acid in a separate recovery process.
The recovered filter cake which is a mixture of tungsten trioxide and tungstic acid is then dried and preferably ground into a powder. Next the tungsten trioxide and tungstic acid are dissolved in a solution of ammonium hydroxide. Preferably the digestion utilizes a twenty-five (25%) percent ammonium hydroxide solution at about 90° C. for about ten hours. The digestion pressure is preferably superatmospheric generally 50 to 70 psig. The pH of the digested solution at room temperature will generally be above ten. If particulate material remains undissolved in the ammonium hydroxide solution, it should be removed by filtration. The filtered solution can be spray dried and formed into tungsten carbide directly or preferably the solution is treated with additional agents which will further facilitate formation of tungsten carbide.
In particular carbon containing compounds can be added to the solution so that when the solution is spray dried carbon atoms will be intimately associated with the tungsten atoms. In particular lower molecular weight i.e., water soluble carboxylic acids such as formic acid, ascorbic acid and citric acid can be added as well as amino acids. Generally any carbon containing composition which doe
Gao Lin
Seegopaul Purnesh
N.V. Union Miniere S.A.
Wood Herron & Evans LLP
Wyszomierski George
LandOfFree
Method of forming nanograin tungsten carbide and recycling... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of forming nanograin tungsten carbide and recycling..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of forming nanograin tungsten carbide and recycling... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3121944