Ⅰ. Mining and Concentrating^[1][2]

Tungsten ores are beneficiated by crushing followed by gravity concentration. Flotation separation is used for scheelite that has been ground to a fine size to liberate the tungsten; this is further supplemented by leaching, roasting, and magnetic or high-tension separation when required.

Ⅱ. Extraction And Refining^[1][2]

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.

Ⅲ. Tungsten powder^[1][2]

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.

Ⅳ. Consolidation^[1][2]

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.

Ⅰ. Tungsten copper alloy

Considering the difference between the melting points of tungsten and copper is large, and the melting point of tungsten is 3410 °C, which is much higher. At the boiling point of copper, tungsten and copper are not miscible. Therefore, tungsten-copper composites can only be prepared using powder metallurgy method.^[3] The main preparation methods are as follows:

• Infiltration method

The infiltration method, also called the melt leaching method, utilizes the action of capillary force to wet a metal matrix with a lower melting point to fill a porous matrix skeleton of a certain density and strength, and the molten metal flows along the interstitial space to fill the pores of the porous tungsten skeleton, thereby obtaining For denser materials, the W-Cu toughness prepared by this method can be greatly improved.^[3]

The infiltration method is divided into two methods: high-temperature sintering of tungsten skeleton after copper infiltration and low-temperature sintering of partially mixed powder and copper infiltration ^[4]. After high-temperature sintering of the tungsten skeleton, the copper is sintered in a high-temperature hydrogen gas to suppress the formation of tungsten powder. Above the melting point of copper, the molten copper penetrates into the tungsten skeleton by capillary action. Using this method, the skeleton has high strength, and the occurrence of burning loss is avoided in the arc operation. However, the high-temperature sintering tungsten skeleton method has its inadequacies, while the production process is complicated, the production cycle is long, and the high production cost also limits its application.^[3]

In the low-temperature sintering process, the tungsten powder is mixed into the copper powder and a small amount of sintering additive (generally nickel powder) when the powder is mixed with the powder, and pre-sintered in hydrogen at a lower temperature, and then the sintered material is subjected to copper infiltration. In the W-Cu alloy produced by this method, the strength of the tungsten skeleton is inferior to that of the high-temperature sintering method. As a contact material in the circuit breaker, ablation is likely to occur. This method requires higher raw material composition, otherwise the product will contain more impurities and gases. However, this method has a simple process and is suitable for the manufacture of W-Cu alloys with a copper content of 20% ^[5]. In general, the infiltration sintering has certain defects: the degree of densification is low, the solidification phase of copper is coarse and the distribution is not uniform, and the high temperature sintering causes the tungsten particles to aggregate and grow, forming coarse and uneven structure. The liquid copper excessively overflows to segregate the components, and the dimensional deformation is severe at high temperature ^[6].^[3]

• Activated liquid phase sintering

Activated sintering refers to a powder metallurgy method in which the sintering temperature is lowered by physical or chemical means, the sintering time is shortened, and the sintering energy is improved. Since the high-temperature liquid phase sintering method cannot obtain a W-Cu alloy close to the theoretical density, compared with the high-temperature liquid phase sintering method, the method also greatly increases the sintering density. Activated sintering has mechanical activation and chemical activation. The high-energy ball milling of mechanical activation on powder includes complex processes such as powder deformation, cold welding, fracture, and composite. Chemical activation is to apply a small amount of alloy salt to the surface of powder particles, and form a thin layer with uniform structure and high activity by chemical reaction to increase the surface activity of the powder ^[8].

In summary, activated liquid phase sintering can obtain higher relative density, hardness, fracture strength, etc., but the addition of activator affects the electrical and thermal conductivity of copper, significantly reducing the electrical and thermal conductivity of the composite, so the materials prepared are only It is suitable for the occasions where the preparation of conductive and thermal conductivity is not high.^[5]

• Chemical coprecipitation

The experiment was carried out by adding concentrated HNO3 to a copper nitrate solution, and then adding the mixed solution to the (NH4)2WO4 solution for chemical coprecipitation in a magnetic stirrer. After the reaction was carried out for 1 hour, the mixed solution was taken out, and the precipitate was placed in a muffle furnace and calcined at 250 ° C for 2 hours to obtain a composite oxide powder. The composite powder was ground in a mortar and reduced to a W-Cu composite powder by a low-temperature (650 to 750 °C) reduction in a strong drainage and ventilating tube furnace. The W-Cu composite powder is formed by a steel mold having a pressure of 100 to 250 MPa, and the relative density of the green body is 40% to 47%. Finally, it was sintered in a hydrogen furnace for 2 h (temperature of about 1150 to 1250 °C), and the heating rate was 10 °C /min to prepare ultrafine grain W-Cu alloy. ^[5]

Using this method, the prepared W-Cu composite powder exhibits high sintering activity; its electrical conductivity, thermal conductivity, flexural strength and hardness are greatly improved compared with conventional products.^[5]

• Explosive powder compaction

Explosive powder compaction is a process in which a metal or non-metal powder is compacted and sintered by a shock wave generated by an explosive explosion. When the shock wave passes through the metal powder, the powder is squeezed and collided, and friction, thermoplastic shearing and microjet flow are generated between the particles ^[8-10].

The experiment used a purity greater than 99. 9% of electrolytic copper powder with a purity of 99. 8% of tungsten powder, copper powder and tungsten powder have a particle size of 200 mesh. Ball milling was carried out in a ratio of 92% W - 8% Cu (mass fraction). In the explosion consolidation device, the inner and outer diameter of the steel pipe is 18 / 22 mm, the alloy powder is filled with steel pipe, the two ends of the pipe are sealed with a metal plug, and the end plugs at both ends are each provided with a 2 mm small hole for hydrogen reduction, mixed powder The initial loading density is 50%. The W-Cu alloy powder was reduced by hydrogen at 850 °C for 3 h, and a composite material with a density of 98% was obtained. The distribution of the components and elements in the composite structure remains uniform. The fracture form of the composite is intergranular fracture ^[9]. The W-Cu sample prepared by the explosive compaction method has an hardness of 330 HV after annealing. The conductivity is 20 MS / m, good physical properties.

 Ⅱ. Tungsten nickel alloy^[12]

The preparation process of tungsten-copper alloy is divided into the following four steps:

•  Powder mixing

Powder mixing is an important part of the preparation of tungsten alloy. The mechanical mixing (dry mixing) method is used to mix the mixed powder into the V-type mixer for thorough mixing. The mixing time of the tungsten alloy powder is 24 hours to ensure the uniformity of the mixed powder and avoid uneven mixing which results in segregation, uneven composition and other issues.

•  Forming

In the past, the traditional forming methods of high-specific gravity tungsten alloys were mostly molded by hydraulic presses. Among them, the cold isostatic pressing method has the characteristics of uniform pressure, high density of the body, simple manufacturing process, and can obtain a preform having a complicated shape and a large size without using a binder. Before pressing, the uniformly mixed alloy powder was placed in a custom rubber mold sleeve, which was hollow cylindrical, 2 mm thick, 15 mm in diameter and 150 mm long. In order to obtain a higher bulk density, the rubber sleeve should be vibrated during powder loading. After filling, tighten with a rubber stopper and fasten the sealing with copper wire to avoid the infiltration of the emulsion during pressing.

•  Sintering

Sintering is one of the core parts of the tungsten alloy preparation process. The alloy powder is oxidized during mixing, storage and pressing. In order to avoid the formation of oxides during sintering and to reduce the properties of the alloy, the alloy powder is pre-reduced and kept at 800 ° C for 2 hours under a hydrogen atmosphere. Since the tungsten alloy has a liquid binder phase during the sintering process, in order to prevent it from adhering to the molybdenum boat, a certain amount of quartz sand is uniformly spread on the bottom of the molybdenum boat before the compact is placed in the molybdenum boat. The sample in the molybdenum boat is uniformly heated during the sintering process, and the molybdenum boat is placed in the middle position in the furnace. After that, process parameters such as sintering temperature and holding time are set, and sintering is started by introducing a corresponding sintering atmosphere. After the sintering is completed, the cooling method of cooling with the furnace is adopted, and after cooling to room temperature, the molybdenum boat is taken out, and the macroscopic deformation of the sample is observed.

•  Vacuum annealing

When a high-density tungsten alloy is sintered in a hydrogen atmosphere, a part of hydrogen remains in the sintered body, causing hydrogen embrittlement. In order to eliminate the influence of hydrogen embrittlement on the performance, the vacuum annealing heat treatment of the tungsten alloy in the vacuum tube furnace after sintering is carried out. The specific process is to keep the temperature at 1100 ° C for 5 hours under vacuum conditions.

Tungsten carbide ^[11]

• Synthesize tungsten from source

Approximately 85% of the world’s tungsten comes from China and is extracted from various ores. Tungsten ore is refined to form tungsten oxide or pure tungsten powder.

• Carburization:

The process of combining a tungsten metal with carbon to form tungsten carbide can be done in several different ways. A method of manufacturing Kennametal special powder is the use of high temperature, more than 2200 ℃, chemical reaction, the temperature is generated by burning aluminum, the purpose is to make tungsten react with carbon. WC powder is cooled to form crystals. Then further cleaning treatment, extract WC powder. The unique high temperature carburizing process enables tungsten and carbon atoms to produce perfect stoichiometric characteristics at crystal sizes of 100 microns or larger.

• Mill and blend

The coarse WC powder is wet milled into a finer particle size. The size of the WC particles is tailored for specific applications since it significantly affects the physical properties of the final product. The WC powder is also blended with a metal binder, such as cobalt and possibly other hard materials, as well as a soft wax lubricant used to temporarily hold the particles together after compaction.

• Dry pelletize

The wet slurry of powders is dried using either a vacuum dryer or a spray dryer to remove most of the moisture. The resulting agglomerated powder particles may need to be reshaped into a better flowing particle through a pelletizing operation.

• Shape

A number of different processes can be used to compact the powder into various shapes. A couple of the most common processes used for shaping WC powders include die pressing and injection molding. An emerging process to shape WC powders into components is additive manufacturing (aka 3D Printing). After this shaping step, the parts are not fully dense and considered to be in a “green” state held together by the wax binder

• Remove Wax & sinter

"Green" tungsten carbide shapes are heat treated to remove temporary wax adhesives and allow permanent metal adhesives to melt and flow around hard particles. The parts are then cooled to freeze the adhesive and hold the hard particles in place.

For example, think rice kipsies. Hard WC particles are similar to rice chips, while metal adhesives (such as cobalt) are cotton candy. By increasing the amount of adhesive - in our case, marshmallow - the resulting product has more "forgiveness" or toughness than a small amount of adhesive, which produces a product that is harder but more brittle. We vary the amount of adhesive in the final product to meet the requirements of a specific application.

• Post-processing

After the sintering process the very hard, fully dense parts receive final post treatments which may include a final grinding step to ensure the part meets final dimensional specifications. Also, often times, tungsten carbide components will receive a coating that will extend the useful life of the part in the customer’s process

[1]Tungsten processing. Chun Tsin Wang Alexander Sutulov See Article History. https://www.britannica.com/technology/tungsten-processing#ref82035. Retrieved April 21, 2019.

[2]Scheelite Locality. Rob Lavinsky. Created before March 2010. CC-BY-SA-3.0. https://en.wikipedia.org/wiki/Scheelite#/media/File:Scheelite-224167.jpg. Retrieved April 21, 2019.

[3]Lingling,. W. Linxia,. F, Xue,. L. Zhiwei,. W. Qingrong,. Q(2012).Analysis of Preparation Process of Tungsten-Copper Alloy.GuangZhou Chemical Industry,40(04):10-11+33.

[4]Wenge,. C. Factors affecting tungsten-copper electrical contact materials(1998). Electrical Alloys, (3): 35 - 36.

[5]Franeine Alves da Costaa，Angelus Giuseppe Pereira da Silvab, Uilame Umbelino Gomese. The influence of the dispersion technique on the characteristics of the W － Cu Powders and on the sintering behavior. Powder Technology，2003，134: 123 － 132．

[6]Benjamin JS．(1970）. Dispersion Strengthened Super － alloys by mechanical Alloy － ing Metal Trans，( 1) : 2943 － 2951．

[7]Daming,. L. Keqiang., Y. (1990). Hot isostatic pressing of tungsten-copper contact materials. Powder Metallurgy Technology, 8(1) : 19 - 23.

[8]Mamalis A G，Vottea I N，Manolakos D E．(2001). On the Modelling of the Compaction Mechanism of Shock Compacted Powders． J Mater Process Technol, 108( 2) : 165 － 178．

[9]Morris D G．(1983).  Bonding Processes during the Dynamic Compaction of Metallic Powders． Mater Sci Eng A, 57( 2) : 187 － 195．

[10]Shao B H，Gao J X，Li G H. (1989). The Mechanism of Energy Deposition at the Interface of Metal Powder in Explosive Consolidation． Explosion and Shock Waves, 9( 1) : 17 － 27．

[11]Xiaojie,. L,. Zhanlei,. W. Honghao,. L. et al. (2010). Experimental study on the preparation of W-Cu nanocomposites by explosive compaction [J]. Journal of High Pressure Physics, 24( 5) : 368 - 372.

[12]Hong,. Z.(2015) Study on preparation technology of low tungsten content tungsten alloy. Beijing Institute of Technology.