tungsten processing

Ammonium paratungstate

Tungsten ores frequently occur in association with sulfides and arsenides, which can be removed by roasting in air for two to four hours at 800° C (1,450° F). In order to produce ammonium paratungstate (APT), an intermediate compound in production of the pure metal, ores may be decomposed by acid leaching or by the autoclave-soda process. In the latter process, the ground ore is maintained for 11/2 to 4 hours in a solution of 10–18 percent sodium carbonate at temperatures of 190° to 230° C (375° to 445° F) and under a pressure of 14.1–24.6 kilograms per square centimetre (200–350 pounds per square inch). Prior to the removal of unreacted gangue by filtration, the acidity is adjusted to pH 9–9.5, and aluminum and manganese sulfates are added at 70°–80° C (160°–175° F) and stirred for one hour. This can eliminate phosphorus and arsenic and reduce silica to a level of 0.03–0.06 percent. Molybdenum is removed by adding sodium sulfide at 80°–85° C (175°–185° F) at a pH of 10, holding for one hour, and then acidifying the solution to pH 2.5–3 and stirring for seven to nine hours to precipitate molybdenum sulfide. The remaining sodium tungstate solution can be further purified by a liquid ion-exchange process, using an organic extractant consisting of 7 percent alamine-336, 7 percent decanol, and 86 percent kerosene. During the countercurrent flow of the extractant through the solution, tungstate ions transfer from the aqueous phase to the organic phase. The tungsten is then stripped from the extractant into an ammonia solution containing ammonium tungstate. The resultant APT solution is sent to an evaporator for crystallization.

In the acid-leaching process, scheelite concentrate is decomposed by hydrochloric acid in the presence of sodium nitrate as an oxidizing agent. This charge is agitated by steam spraying and is maintained at 70° C (160° F) for 12 hours. The resultant slurry, containing tungsten in the form of a solid tungstic acid, is diluted and allowed to settle. The tungstic acid is then dissolved in aqueous ammonia at 60° C (140° F) for two hours under stirring. Calcium from the resulting solution is precipitated as calcium oxalate, while phosphorus and arsenic may be removed by the addition of magnesium oxide, which forms insoluble phosphates and arsenates of ammonium and magnesium. Iron, silica, and similar impurities that form colloidal hydroxides are removed by adding a small amount of activated carbon and digesting for one to two hours. The solution is clarified through pressure filters and evaporated to obtain APT crystals.

Tungsten powder

When APT is decomposed to tungsten oxides, it displays different colours according to its composition: the trioxide is yellow, the dioxide is brown, and the intermediate oxide is purple-blue. APT can be decomposed to yellow oxide when heated to above 250° C (480° F) in a furnace under a flow of air. In the industrial production of tungsten, however, APT is usually decomposed to the intermediate oxide in a rotary furnace under a stream of hydrogen, which partially decomposes the ammonia in the crystals into nitrogen and hydrogen while maintaining a reducing atmosphere. The rotary furnace is divided by partitions into three zones maintained, respectively, at 850°, 875°, and 900° C (1,550°, 1,600°, and 1,650° F). The furnace is tilted at a small angle and rotated to provide a continuous flow of powder through the central holes of the partitions.

The blue oxide is then reduced by hydrogen to metallic tungsten powder in stationary furnaces at temperatures ranging from 550° to 850° C (1,025° to 1,550° F). In this process the oxide is loaded into “boats” made of Inconel, a nickel-based alloy noted for its strength at high temperatures. These are stoked into tubes, usually arranged in two rows, and the tubes are heated in three separate zones along their lengths.

APT may also be reduced by carbon, although the powder is usually contaminated with tungsten carbide and some mineral elements contained in the carbon. When APT and carbon are mixed and reacted at 650°–850° C (1,200°–1,550° F), the product is a blue oxide. When heated in the range of 900°–1,050° C (1,650°–1,925° F), the brown oxide is formed. For complete reduction to metal, a temperature higher than 1,050° C is required. The purity of the metal is about 95 percent.

Consolidation

Tungsten powder is compacted into bars or billets with a mechanical or isostatic press prior to sintering. The “green,” or unfired, density of these compacts, obtained from powder particle sizes ranging from 1 to 10 micrometres, is usually 65 to 75 percent of the theoretical. After being presintered at 1,000°–1,200° C (1,800°–2,200° F), tungsten bars of small diameter are sintered in a hydrogen atmosphere, with heat being provided by the direct-resistance method—that is, by an electric current passed through the bar. A spring attachment to the water-cooled clips holding each bar is necessary so that one end is free to move as the bar shrinks during sintering. The current is gradually increased to raise the temperature from room temperature to 2,700°–3,100° C (4,900°–5,600° F). After holding at the final temperature for 30 to 60 minutes, the density reaches 88.5 to 96 percent of the theoretical.

An indirect sintering process is used for large tungsten billets. The heating elements of the furnace are constructed of molybdenum strips and supported by molybdenum or tungsten frames, and they are surrounded by molybdenum heat shields. A slow heating in the early stage of sintering is essential for deoxidizing the material and releasing gases at a controlled rate. At higher temperatures—i.e., from 800° C up to the final sintering temperature of 2,400° C (4,350° F)—the heating rate also should be controlled, since too fast a temperature buildup within the billet would cause thermal stresses and would result in the cracking of the material. A final sintering for 10 hours is required for densification.

Tungsten carbide (WC)

Tungsten carbides are divided into two categories. The first is the cemented tungsten carbides, also called hard metals, which are essentially WC produced from sintering a mixture of carbon black and hydrogen-reduced tungsten powder at 1,500° C (2,700° F). These are cemented using a cobalt or nickel binder, with or without other refractory carbides. The major uses of cemented carbides are for cutting and drilling tools, forming and drawing dies, and tire studs.

The second group is called fused or cast carbide, consisting of W₂C and a eutectic mixture of WC and W₂C. Harder but more brittle than the cemented carbide, it is used in wear-resisting applications such as anvils, guide sleeves in machines, and teeth and jaws for excavators.

Other compounds

Tungsten bronze, composed of tungstates of the alkali and alkaline-earth metals, is employed as a substitute for bronze in ornamental paints. Sodium tungstate is also used to produce phosphotungstic acid-type organic dyes and pigments, which are brilliant, light-resistant, and insoluble in water and linseed oil. Calcium and magnesium tungstates are used as phosphors in fluorescent light and television tubes. Ammonium tungstate and other compounds are used as catalysts in the petroleum industry for hydrotreating, hydrocracking, and polymerization.