Solid carbide is more expensive than steel tools or carbide-tipped drills. In addition, solid carbide is brittle and must be used in a rigid, stable setup that minimizes vibration and movement. Solid carbide also requires a steady temperature or the tools may fracture. Flood coolant can keep carbide cool.

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By drying, calcination tungstate and ammonium paratungstate can be prepared from tungsten trioxide. Drying temperatures around 100℃, the main material is physically adsorbed water vapor but only at higher temperatures the material is calcined to remove ammonium paratungstate and tungstic acid molecule of water and ammonia compounds.

Acid usually calcination in air, 180 ℃ starts dehydration at 500 ℃ for complete dehydration, tungsten oxide formation: H2WO4 = WO3 + H2O ↑

Ammonium paratungstate calcined by heating in air at different temperatures will produce different phase, when the temperature is raised to about 500 ℃, can be completely converted to tungsten trioxide.

Shao-Yi, who used differential thermal analysis and X-ray diffraction observation during calcination of APT and analysis, that the heating APT in the air phase change its course as follows: at a temperature of not less than 240 ℃, APT converted to ammonium metatungstate (AMT), continue heating to 300 ~ 350C, the conversion of the amorphous material (ammonium tungsten bronze ATB), temperature was raised to 400C, into a hexagonal type W03, temperature reached 500 ℃, to obtain a three triclinic crystal tungsten oxide. The purity determined APT purity tungsten trioxide, APT crystal form of the crystal tungsten trioxide hereditary, calcination temperature and the rate of thermal decomposition of the product physical properties of tungsten trioxide has a significant impact.

Calcined at different temperatures with a number of APT, WO3 products vary greatly surface area and particle size, the higher the calcination temperature, the more coarse particle size of the obtained WO3.


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Blue tungsten oxide (TBO) is an important raw material manufacturing of tungsten powder , tungsten oxide reduction due from tungsten powder blue easier to control the particle size and particle size composition , is conducive to the reduction process in the tungsten powder and mixed with other elements, so the TBO is gradually replace the tungsten trioxide production of special tungsten material as tungsten powder materials.

Blue tungsten oxide is an industrial product name , its composition mainly by WO2.9, WO3 and WO2.72 composed of several tungsten oxide , the proportion of each component depending on APT or restore the temperature calcination atmosphere and speed.

Preparation of blue tungsten oxide there are three main methods : APT sealed calcination , APT hydrogen reduction and inherent mild reduction . At present in China more industrial application is the first two methods .

Xue Kam et al APT decomposed under non-oxidizing atmosphere, blue tungsten oxide process of the study found that the decomposition experienced APT six stages :

First stage: the temperature between 90-180C , APT lose some water :

Second stage: temperature range of 180-260 ℃, ammonia begins to escape from ammonium paratungstate , the higher the temperature , the faster the loss of ammonia . Loss of ammonia product was obtained as ammonium metatungstate (AMT), soluble in water, reconstituted after preparation of high purity ammonium metatungstate products ;

Third stage: temperature range of 260-350 ℃, AMT decomposition, further loss of water and ammonia , ammonium decomposition products appeared tungsten bronze (ATB);

Stage IV : The temperature range 350-450C, at this time begins to decompose ammonia in the gas phase , to produce ammonia tungsten bronze (ATB), and to produce hydrogen and nitrogen from the gas phase can be detected in the product hydrogen. Decomposition of ammonia nitrogen and hydrogen atoms is first generated , the new ecological chemical activity of hydrogen atoms , ammonium tungsten bronze to the oxygen index ( tungsten compound in the x (O) / x (W) molar ratio ) decreases. Formation of ammonium tungsten bronze fixed composition , the ammonia content and oxygen index can vary , generally consisting of (NH4) xWO2.8 ~ 3.0;

Fifth stage: temperature range of 450-550 ℃, decomposition ATB ​​this stage , the hydrogen produced by decomposition of ammonia in the reaction , ATB decomposition products occurs WO3 and WO2.9, ATB under reducing atmosphere in the first section and then converted into the WO2 WO3.9in strong reducing atmosphere conditions , ATB will translate directly into W20O58 (WO2.9).

Stage 6: temperature range 550 N 650 ℃, decomposition products of this phase with the phase composition of systems vary considerably reducing the strength of the hydrogen to further participate in the reaction , so that the low priced tungsten oxide, tungsten oxide, transition , decomposition products , in addition to a small amount of ATB , there WO3, WO2.9 and tungsten oxide WO2.27 etc. , even WO2.


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Metatungstate production methods more, can be divided into solid and liquid phase inversion transformation method two categories. Typical solid phase inversion process is based on the thermal degradation APT method as a raw material, this method is the main industrial production process of the AMT. The phase transformation method of ammonium tungstate solution as raw material, extraction, ion exchange, membrane electrolysis, or a method in the solution and the pH adjusted to the appropriate range, the obtained solution of ammonium metatungstate. Many research phase inversion method, is to develop in the direction of industrial applications.

Production of Thermal Degradation AMT

By controlling the APT and the pyrolysis temperature and the ammonia partial pressure of water vapor, the water-soluble pyrolysis APT metatungstate large amorphous material, and then hydrated into a solution of ammonium metatungstate, the solution was concentrated and cooling crystallization available AMT solid.

APT pyrolysis conversion rate and the temperature, if the pyrolysis temperature is too high, the resulting insoluble tungsten oxide is not, if the temperature is too low, the APT insufficient removal of ammonia and water (by Molecular Formula calculated APT be removed the four NH3 and 4 H2O), insolubles increases. APT suitable temperature to improve the conversion rate of the key.

Preparation of Ion-Exchange AMT

AMT ion exchange process to produce ammonium tungstate solution as raw material, ion exchange method because the process is relatively simple, easy to control, to avoid the formation of APT, not into the new contaminants, the process can be continuous, and thus has potential for development .

Extraction and ion exchange method originally buried in a similar, but in the extraction process is easy to produce solids resulting emulsion, industrial applications, there are certain difficulties. Silicic acid, and the solid is likewise prepared using ammonium tungstate solution, one way to take AMT, but due to the addition of silicic acid may cause contamination of the product AMT, so there is no industrial application. Ion-exchange membrane electrolysis (including ion-exchange membrane electrodialysis) is still in the development stage, to be developed better longer selective membrane, reduce production costs, to be applied to the production.


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Preparation of tungsten metal powder The main methods are: Preparation of tungsten metal powder, the main method

(1) hydrogen reduction of tungsten oxide;
(2) hydrogen reduction of tungsten halides;
(3) tungsten oxide or tungsten acid carbon reduction;
(4) tungsten oxide or metal tungstate thermal reduction method;
(5) molten salt electrolysis.

The most practical industrial method of hydrogen reduction of tungsten oxide. Tungsten halide (WCl6, WF6) can be prepared from hydrogen reduction of tungsten powder with special features, and was gradually extended application of other methods do not apply to the production of plastic tungsten.


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Tungsten oxide behavior at high temperatures

Tungsten oxide, can be stable in four: (1) tungsten oxide yellow; (2) blue tungsten oxide; (3) purple tungsten oxide; (4) of tungsten oxide Brown

Hydrogen Reduction of Tungsten Oxide Thermodynamic Analysis

WO3 +3 H2 = W +3 H2O The overall reaction is endothermic reaction, the temperature increases the equilibrium constant Kp = PH2O/PH2, is advantageous to the reaction. Temperature, the equilibrium concentration of water vapor in the gas phase increases, the reaction proceeds more completely.

Tungsten Oxide Hydrogen Reduction Kinetics Analysis

Thermodynamics of the reaction can be determined, but the actual process far from equilibrium conditions, WO3 hydrogen reduction process does not strictly follow the step by step to restore order. α-W and β-W is W two allotropic crystal. α-W is a cubic crystal, lattice constant of 0. 316nm; pW cubic crystal lattice constant of 0.5036nmoβ-W is the temperature below 823K from the WO2 reduction with hydrogen, which is characterized by a large chemical activity, spontaneous combustion . β-W converted to α-W transition temperature was 798K, does not occur α-W converted to β-W in the reverse transformation. β-W is tungsten atoms of the crystal, but due to the presence of impurities and lattice distortion.

Tungsten Powder Particle Size and Purity Influence Factors

Tungsten particles into a particle and the second particle. The primary particles is a single particle, is first generated and separated from each other and independent of the presence of the particles; secondary particles of two or more of primary particles with particle accumulation easily separated.

Tungsten powder particle size and purity influence of many factors, the following factors will be analyzed as follows: raw materials, water, impurity elements, hydrogen


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Acid decomposition method in principle can be used to decompose scheelite, wolframite can be used to decompose . Decomposed with acid concentrates obtained crude acid can be obtained and then the ammonia dissolved ammonium tungstate solution , this solution can be prepared by crystallization of the pure compound of tungsten APT . Therefore, the acid decomposition process concentrates with short process, low cost. Mineral raw materials such as tungsten high impurity content , and ammonia dissolved in acid decomposition alone purification is difficult to obtain high -purity products . Therefore acid decomposition method used for processing low impurity content of scheelite . If the process to make appropriate adjustments for the poor quality of scheelite ( tungsten containing 40% to 60 %, including CaCO3 and other minerals less susceptible to acid dissolution ) can also be acid decomposition method.

Hydrochloric acid, nitric acid can be used to decompose scheelite . However, economic benefits, production results and other factors to consider, at present is generally used industrially acid decomposition method .

Thermodynamic Analysis

Tungsten minerals and chemical reaction of hydrochloric acid , and hydrochloric acid scheelite occurs when the following chemical reactions : CaWO4 (s) +2 HCl (aq) = H2WO4 (s) + CaCl2 (aq)

Wolframite and hydrochloric acid reaction : FeWO4 (s) +2 HCl (aq) = H2WO4 (s) + FeCl2 (aq); MnWO4 (s) +2 HCl (aq) = H2WO4 (s) + MnCl2 (aq)

Production practice, each larger than the concentration of the reaction solution , the concentration equilibrium constant at this time has more practical significance. Concentration equilibrium constant can be measured directly by experiment .

Acid decomposition process, the mineral raw materials of impurity elements in different reaction conditions with different chemical behavior .

Iron: elemental iron in tungsten minerals usually FeWO4, FeS2, Fe2O3, Fe3O4 and other forms exist. Concentrate milling , may also be part of the metal into iron .

Mo : Mo ore mainly CaMoO4 and molybdenite (MoS2) exists in the form of calcium molybdate molybdenum reacts with hydrochloric acid

CaMoO4 +2 HCl = H2MoO4 + CaCl2

MoS2 not with hydrochloric acid , but in the presence of an oxidizing agent can react molybdate :

MoS2 +2 HCl +6 NaNO3 = H2MoO4 +2 Na2SO4 +2 NaCl +6 NO ↑

Molybdate in hydrochloric acid greater than the solubility , and with the temperature increase and the concentration of hydrochloric acid increases. Therefore, improving concentration of hydrochloric acid , can make most of the molybdenum and tungsten into the solution to achieve separation .

In the acid decomposition process , you can also add to the addition of molybdenum ferrosilicon . Ferrosilicon (SiFe) as a reducing agent , where the silicon does not play a role with hydrochloric acid , and the iron will be oxidized to iron ions.

2H2MoO4 + SiFe +8 HCl = 2MoOCl3 + FeCl2 + Si +6 H2O

Treatment of high molybdenum content scheelite , the mother liquor by concentration of hydrochloric acid by acid and adding a reducing agent (SiFe) approach to the removal of molybdenum . Controls the right conditions , in addition to molybdenum ratio can reach 80%.


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Many have studied the decomposition of tungsten mineral raw materials, in addition to several domestic and industrial production in the application of methods, chlorination and fluoride salt solution is also moving towards industrialization, while the high temperature carbonization melt extraction from the direct production of tungsten tungsten carbide technology have reached the industrial applications. In addition, the complexation of scheelite acid leaching method, EDTA sodium salt leaching method has also been reported.

Scheelite Chloride Leaching

Fundamental

Scheelite sodium or ammonium fluoride can be used to break down. Sodium fluoride and scheelite reaction following formula: CaWO4 (s) +2 NaF (aq) = Na2WO (aq) + CaF2 (s)

The use of NH4F + NH4OH as a leaching agent, the reaction with scheelite obtained directly (NH4) 2WO4 solution. At 225 ℃, the measured decomposition scheelite NaF concentration equilibrium constant Kc 24.5, and sodium carbonate decompose under the same conditions scheelite, measured Kc value of only 1.52, so the sodium fluoride and ammonium is oxidized one kind is more effective than sodium carbonate decomposition scheelite reagents. Scheelite NaF decomposition below 100 ℃ under the conditions of mineral particles CaF2 film formed on the surface, hindering further reaction; temperature higher than 100 ℃, the CaF2 film off, the reaction speed is mainly affected by the chemical reaction speed control.

Process and Lndicators

Zhen-Gang YAO White tungsten leaching tests carried out NaF, when the temperature is 190 ℃, NaF dosage is 1.2 times the theoretical amount, adding a small amount of additives, pressure cooking 1.5h, decomposition rate remained stable at around 99%. The resulting crude sodium tungstate solution containing P · As · SiO2 impurities is low, basically achieve the technical requirements after purification. For low molybdenum scheelite by NaF pressure cooking crude sodium tungstate solution can be directly used defluorinated plus molybdenum addition of acid curing agent. Obtained by extraction and crystallization APT quality can be reached international zero Need requirements.


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Chloride metallurgy is the recent development of the new technology , with the chlorination process tungsten mineral raw materials have a number of advantages , mainly in:

( 1 ) Chlorine is very active , and thus the decomposition rate is very high, high metal recovery , in addition to deal with the general tungsten , but also especially suitable for processing lean tungsten ore, tungsten fine mud , tungsten ore tailings and polymetallic composite ore.

( 2 ) low boiling point of many of the metal fluoride , there is a gap between each other , and thus the vapor pressure can be used for separation and purification of different , easily obtained pure tungsten chloride.

( 3 ) Hydrogen chloride reduction process for the gas-phase reaction , easily accessible from tungsten chloride to obtain fine tungsten powder or plating .

( 4 ) easy recovery of other valuable elements .

Therefore, the method is the decomposition of tungsten chloride mineral raw materials promising methods. At present chlorination is mainly used in the decomposition of tungsten alloy scrap tungsten and tungsten minerals and thus obtain fine tungsten powder, has small-scale production .

Fundamental

Professor Li Honggui materials under thermodynamic system draws 1100K when W-Cl-O system and Fe-W-Cl-O system thermodynamic equilibrium diagram . In the W-Cl-O system exists WO2Cl2, WOCl4, WCl2 other compounds , oxygen -bit systems is higher, the higher the content of mixed vapor WO2Cl2 , to get WCl4 or WCl2, must reduce the oxygen -bit system , to ensure adequate reducing atmosphere . Therefore, scheelite or wolframite chloride , often adding carbon as a reducing agent.

For scheelite such as the use of sulfur dioxide as a reducing agent , the reaction is : CaWO4 (s) + Cl2 (g) + SO2 (g) = CaSO4 (s) + WO2Cl2 (g)

Scheelite chloride decomposition using carbon as a reducing agent , such as , you must add a certain amount of calcium fluoride decomposition rate was high : CaWO4 (s) +3 C (s) +2 Cl2 (g) + CaF2 (s) = 2CaClF (s ) + WOCl2 (g) +3 CO (g)

Process Lndicators

Tungsten with carbon and mineral raw materials are first additive (CaF2) and binder mixed group ; system group for the purpose of the charge to ensure good air permeability , dehydration briquettes sufficiently dried , and then chlorination, chlorination process is usually vertical chlorination furnace , the temperature controlled at 600 ~ 800 ℃ ( different materials , chlorination temperature is not the same ) , the chlorination of a mixed oxide vapor is condensed , and then purified by means of separation , can be used to obtain pure tungsten chloride obtained by hydrogen reduction of tungsten powder or coating can also be hydrolyzed acid .

Wolframite use chlorine chloride , the temperature to 800 ~ 900 ℃ only when a full , but adding carbon as a reducing agent , the reaction starts 400 ℃ , 500 ℃ under faster , to 600 ℃ has chlorination more completely .

Scheelite plus carbon oxidation , if not additive CaF2, then at 600 ℃, tungsten chlorination rate of only 65% , if by adding the theoretical amount of calcium fluoride , ingredients of carbon and scheelite ratio of 0.075,600 ℃ chlorination of tungsten chloride rate up to 99% .


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