The effect of particle size on the oxidation rate of ferrotungsten was determined using a procedure similar to the method used to evaluate the effect of temperature on the oxidation rate. In this set of experiments, the furnace temperature was maintained at a constant 950° C. and the crucibles were removed at approximately 1 hour intervals. The ferrotungsten was separated into five fractions by sieving through a series of screens. The screens sizes were "big," 1/4 mesh, 10 mesh, and 30 mesh. The "big" screen had diamond shaped opening measuring 5/16 by 7/8 inches. The oxidation curves for the different fractions are given in FIG. 2. The finer fractions (-30 mesh, +30 to -10 mesh, and +10 to -1/4) appear to completely oxidize in about 9.5 hours. The two larger fractions require a much longer time, approximately 23 hours. Accordingly, it is desirable to reduce the particle size of the ferrotungsten to -1/4 mesh in order to minimize the time required to completely oxidize the ferrotungsten.

Two additional sets of experiments were conducted in a laboratory tube furnace to determine the effect of the gas flow rate and oxygen content of the gas on the rate of oxidation. Collectively, both parameters dictate the amount of oxygen present in the furnace atmosphere during oxidation.


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FIG. 1 demonstrates the effect of temperature on the oxidation rate of ferrotungsten. In FIG. 1, the percent weight gain of the ferrotungsten (FeW) at different temperatures is plotted against time. The data in FIG. 1 reveal that there is a significant increase in the oxidation rate of ferrotungsten above about 900° C. Unfortunately, the increase rate of oxidation at higher temperatures becomes limited by the ability of the calciner to effectively operate at the higher temperatures while handling production quantities of ferrotungsten. For a typical commercial calciner, the calciner is normally limited to operating at temperatures less than about 1000° C. because the calciner tube must be made out of alloys which have sufficient strength to handle the heavy and abrasive ferrotungsten. This excludes higher temperature tube materials such as alumina and silicon carbide which do not possess the needed strength. Therefore, to increase the rate of oxidation further, it is necessary to either decrease the particle size of the ferrotungsten being calcined or increase the oxygen content of the calciner atmosphere. The effects of these parameters on the oxidation rate of ferrotungsten are described below.

FIG. 1 is a graphical representation of the effect of temperature on the oxidation of ferrotungsten.


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The rate of oxidation can be further enhanced by reducing the ferrotungsten particle size to -1/4 mesh size. The size reduction may be achieved by mechanical milling or sieving to remove the larger fraction. Additionally, the rate of oxidation may also be increased by enhancing the concentration of oxygen in the calciner atmosphere. This may be accomplished by increasing the flow rate of the oxidizing gas into the calciner or by augmenting the amount of oxygen in the gas.

The following examples are provided to enable those skilled in the art to more clearly understand and practice the invention. These examples should not be considered as a limitation upon the scope of the invention, but merely as being illustrative and representative thereof.

Oxidation tests on representative ferrotungsten samples were performed in air at different temperatures in a muffle furnace. Samples of ferrotungsten were weighed and placed in previously weighed crucibles. The crucibles were then placed in a muffle furnace which had been heated to the desired temperature. The crucibles were removed at approximately 2 hours intervals, cooled, and weighed. Heating in the muffle furnace was continued until no further weight gain was observed. The data for these experiments are represented in FIG. 1.

FIG. 1 is a graphical representation of the effect of temperature on the oxidation of ferrotungsten.


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In order to achieve high recovery efficiencies for tungsten, it is necessary that the ferrotungsten be completely oxidized. The amount of oxidation can be monitored by measuring the weight gain of the ferrotungsten during calcining. As defined herein, the ferrotungsten is completely oxidized when there is no further weight gain in the calcined material. It is believed that the ferrotungsten is essentially oxidized to WO₃, FeWO₄ and Fe₂ O₃. The rate of oxidation of the ferrotungsten is affected by the calcining temperature, the size of the ferrotungsten scrap, and the oxygen content of the atmosphere in the calciner. In particular, it has been found that above about 900° C. the rate of oxidation of ferrotungsten is greatly enhanced. For example, at 1000° C., complete oxidation of ferrotungsten can be achieved in about 14 hours. Under similar conditions at 925° C., the time to complete oxidation is about 20 hours. At 880° C., about 60 hours are needed. And, at 850° C., the ferrotungsten is only about 80% oxidized after 68 hours. Thus, there is a dramatic increase in the oxidation rate above about 900° C. The rate of oxidation is an important consideration in being able to reclaim tungsten from ferrotungsten on a commercially feasible scale.


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Description of The Preferred Embodiments

A method is provided whereby tungsten values are recovered from ferrotungsten by completely oxidizing the ferrotungsten at high temperatures in an oxygen atmosphere followed by digestion with sodium hydroxide to form soluble sodium tungstate. The sodium tungstate is then purified by conventional means. The method may be used to recover at least about 90% of the tungsten values present in the ferrotungsten.

For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims taken in conjunction with the above-described drawings.

A method for the recovery of tungsten from ferrotungsten has been developed whereby at least about 90% of the tungsten may be recovered from ferrotungsten scrap. The method is comprised of calcining the ferrotungsten at a high temperature in an oxygen containing atmosphere. During calcination, an oxygen containing gas is flowed through the calciner to promote oxidation. Calcining is continued until the ferrotungsten becomes completely oxidized. The oxidized ferrotungsten is then digested using sodium hydroxide to form a soluble sodium tungstate, Na₂ WO₄. Once in solution, the Na₂ WO₄ can then be purified and processed by conventional hydrometallurgical techniques. Examples of such hydrometallurgical techniques can be found in U.S. Pat. Nos. 4,360,502 to MacInnis, 4,369,165 to Kim, 4,552,729 to Miller, and 2,339,888 to Smith which are incorporated herein by reference.


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The article will introduce something (Part two) of GTAW (Gas Tungsten Arc Welding) equipment following the Part one, welding torch, on tungsten electrode, and its details can be seen as following.

The size of the welding torch nozzle which depends on the amount of shielded area desired. The size of the gas nozzle depends upon the diameter of the electrode, the joint configuration, and the availability of access to the joint by the welder. The inside diameter of the nozzle is preferably at least three times the diameter of the electrode, but there are no hard rules. The welder judges the effectiveness of the shielding and increases the nozzle size to increase the area protected by the external gas shield as needed. The nozzle must be heat-resistant, and therefore it is normally made of alumina or a ceramic material, but fused quartz, a high purity glass, offers greater visibility. To reduce turbulence and introduction of contaminated atmosphere into the shielded area, devices can be inserted into the nozzle for special applications, such as, gas lenses or valves to improve the control shielding gas flow. Hand switches to control welding current can be added to the manual GTAW torches. 



 

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The article is something of GTAW equipment, power supply, on tungsten electrode, which uses a constant current power source and the detail can be seen as below.

Gas tungsten arc welding uses a constant current power source, meaning that the current (and thus the heat) remains relatively constant, even if the arc distance and voltage change. This is important because most applications of GTAW are manual or semi-automatic, requiring that an operator hold the torch. Maintaining a suitably steady arc distance is difficult if a constant voltage power source is used instead, since it can cause dramatic heat variations and make welding more difficult.

The preferred polarity of the GTAW system depends largely on the type of metal being welded. Direct current with a negatively charged electrode (DCEN) is often employed when welding steels, titanium, nickel and other metals. It can also be used in automatic GTAW of aluminum or magnesium when helium is used as a shielding gas. The negatively charged electrode generates heat by emitting electrons, which travel across the arc, causing thermal ionization of the shielding gas and increasing the temperature of the base material. The ionized shielding gas flows toward the electrode, not the base material, and this can allow oxides to build on the surface of the weld. Direct current with a positively charged electrode (DCEP) is less common, and is used primarily for shallow welds since less heat is generated in the base material. Instead of flowing from the electrode to the base material, as in DCEN, electrons go the other direction, causing the electrode to reach very high temperatures. To help it maintain its shape and prevent softening, a larger electrode is often used. As the electrons flow toward the electrode, ionized shielding gas flows back toward the base material, cleaning the weld by removing oxides and other impurities and thereby improving its quality and appearance.

Alternating current, commonly used when welding aluminum and magnesium manually or semi-automatically, combines the two direct currents by making the electrode and base material alternate between positive and negative charge. This causes the electron flow to switch directions constantly, preventing the tungsten electrode from overheating while maintaining the heat in the base material. Surface oxides are still removed during the electrode-positive portion of the cycle and the base metal is heated more deeply during the electrode-negative portion of the cycle. Some power supplies enable operators to use an unbalanced alternating current wave by modifying the exact percentage of time that the current spends in each state of polarity, giving them more control over the amount of heat and cleaning action supplied by the power source. In addition, operators must be wary of rectification, in which the arc fails to reignite as it passes from straight polarity (negative electrode) to reverse polarity (positive electrode). To remedy the problem, a square wave power supply can be used, as can high-frequency voltage to encourage ignition. 



 

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The article is something Part one) of GTAW equipment, welding torch, on tungsten electrode, and the detail can be seen as below.

Part one
GTAW welding torch is designed for either manual or automatic operation and is equipped with cooling systems using air or water. The automatic and manual torch is similar in construction, but the manual torch has a handle while the automatic torch normally comes with a mounting rack. The angle between the centerline of the handle and the centerline of the tungsten electrode, known as the head angle, can be varied on some manual torches according to the preference of the operator. Air cooling system is the most often used for low-current operations (up to 200 A around), while water cooling is required for high-current welding (up to 600 A around). The torch is connected with cables to the power supply and with hoses to the shielding gas source and where used, the water supply.

The internal metal parts of a torch are made of hard alloys (such as, copper or brass) to transmit current and heat effectively. Tungsten electrode must be held firmly in the center of the torch with an appropriately sized collet, and ports around the electrode provide a constant flow of shielding gas. Collets are sized in accordance with the diameter of the tungsten electrode they hold. The body of the torch is made of heat-resistant, insulating plastics covering the metal components, providing insulation from heat and electricity to protect the welder. 



 

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