Sodium tungstate solution can be prepared by decomposing wolframite with sodium hydroxide. A new technique is developed for precipitating scheelite from the sodium tungstate solution by the addition of calcium hydroxide. In this study, the effects of several variables on the conversion of sodium tungstate are investigated. Results show that the effects of temperature and sodium tungstate concentration are significant. Specifically, 96.4% WO₄^2 − was converted to scheelite in 2 h at 100 °C with a calcium hydroxide stoichiometric ratio of 1.4 and a stirring speed of 350 rpm. The parent solution containing sodium hydroxide can be recycled as a leaching agent return to the wolframite decomposition process.

Highlights

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Synthesis of scheelite from sodium tungstate solution with addition of calcium hydroxide was successfully achieved.

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Effects of processing parameters on the conversion of sodium tungstate were presented.

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Optimal processing parameters in the process of synthesizing scheelite were selected.
 

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As mentioned in some researches, the cutting edge geometry could affect the deposition stresses around the tool tip in coating process, in addition, during the machining, the thermal and mechanical loads will be complicated and imposed as well. It is also indicated that chip formation and cutting processes could be severely affected by the radius edge, especially there is a size effect, which is referred to the thickness of the uncut chip is close to or smaller than the edge radius.

In general, for the coated tungsten carbide inserts,on which the cutting edge effects include, with the edge radius increasing, there is a noticeable decrease in deposition residual stresses. On the contrast, the increasing of edge radius will bring up the cutting forces and the with different edge radii and  combined effects, the coated tungsten carbide inserts’wear behavior are complicated.


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Below are the main producing processes of ammonium metatungstate:

1.Use ammonium tungstate solution as raw material, solvent extraction and ion exchange method is first prepared by metatungstate solution, then by evaporation and crystallization Get ammonium metatungstate.

2.Use sodium tungstate as raw material. By solvent extraction solution as raw material in AMT solution, followed by evaporation and crystallization to obtain ammonium metatungstate.

3.The heating thermal dissociation of APT, which lost its part ammonium And the water of crystallization, converted to ammonium metatungstate easily soluble in water, then evaporation and crystallization Get ammonium metatungstate.

4.Mixing APT in the water with acid and neutralize to PH3-4 condition. APT transforms into AMT solution, followed by evaporation and crystallization to obtain ammonium metatungstate.

U.S. mostly uses ammonium tungstate solution as raw material, first obtained ammonium metatungstate solution using solvent extraction or ion exchange method. Some European countries use sodium tungstate solution as raw material using solvent extraction. China mainly uses thermal decomposition method that use ammonium paratungstate (APT) as raw material.



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In practical machining application, it has been pointed out that the cutting edge geometry has compound effects on tungsten carbide inserts coating process, and the cutting edge radius affects the deposition stresses and machining loads in an opposite way. Specially, under different cutting condition, the optimal radius maybe varied from one to another since the edge radius effects on machining loads are depended on machining parameters.

In the coating process like CVD processes, the thermal mismatch between the coating and substrate materials are likely to generate high residual stresses in the tool, and for the tungsten carbide insert’substrate, the cobalt cemented tungsten carbide receive a tensile stress, which tends to have  impacts on the coating functions. Moreover, the geometric changes like cutting edges, will severely alter the local stress fields.


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Use sodium tungstate solution as raw material, preparing ammonium metatungstate solution by solvent extraction or ion exchange. Solvent extraction and ion exchange which is basically the same process and the preparation of ammonium paratungstate. The difference is in the back-extraction (or desorption) operation, stripping agent (or desorbent) and amount of different concentrations in the solution was maintained at pH 3 to 4 cases, back-extracted (or desorption) obtained metatungstate ammonium solution. This method in general APT production process, unnecessary use of stripping agent (or desorbent) can be produced under conditions of AMT solution, so it is a simple method.

Ammonium tungstate solution as a raw material by solvent extraction or ion exchange, then the cationic extractant (or a cation exchange resin) and extracted (or adsorbed) ammonium tungstate solution [NH4] + ion, and then with a suitable stabilizer and can output under the condition of pH AMT solution, which is both simple and economical method. Ammonium metatungstate solution solvent extraction or ion exchange method was the same concentrated crystals or spray drying to take ammonium metatungstate products.



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The crystal properties of ammonium paratungstate like crystal morphology, average particle size, particle size distribution, apparent density and flow ability affect the properties of tungsten powder, tungsten wire and tungsten alloy. Single crystal ammonium paratungstate due to its eminent physical properties has become ideal raw materials for preparing tungsten products.

Based on crystallization theory of single crystal ammonium paratungstate, the influences of crystallization device, stirring speed, temperature on crystallization agglomeration of ammonium paratungstate were studies. The results show that the optimal stirring speed is determined as 30, 50, 70r/min, respectively in different developmental stages of crystallization. Under optimal conditions, single crystal rate arrives to optimal value. Suitably reducing pre-crystallization temperature, single ammonium paratungstate crystal rate has more space for enhance. When crystallization temperature is 80 degree Celsius, crystal rate was 96% to achieve the best value. The fluid layer flow control technology and devices for single ammonium paratungstate crystal preparation can effectively reduce collisions between grains, so that the ammonium paratungstate powder single crystal rate can arrive at more than 90%.



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Vanadium dioxide film is a new type of heat-sensitive functional materials. It is with increasing temperature at about 68 ℃ will change from semiconductor monoclinic rutile structure to metallic state tetragonal rutile structure. Besides the resistivity and infrared light transmittance and other physical properties are also mutated, based on this characteristic, it is widely used in optical data storage, optical switches and smart windows and other fields. However, due to the phase transition temperature of vanadium dioxide is too high to be widely used. After the studies found that the incorporation a small amount of tungsten ions in vanadium dioxide film can effectively reduce the phase transition point of the film, which is conducive to tungsten vanadium dioxide film be applied in more areas.
Vanadium dioxide doping tungsten cause phase transition temperature lower there are two interpretations, the first one is because when the metal - semiconductor phase transition the vanadium atoms of lattice center off center octahedral voids, then tungsten which has larger atomic radius replace vanadium atoms, however, vanadium octahedral body no extra space for tungsten atoms from the center so as to stabilize the vanadium dioxide metal phase. The second explanation is tungsten has more than two d electrons into V3d vanadium, making the band gap of the semiconductor phase decreases.

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The physicochemical basis of the heterogeneous reaction between the components of the system (sodium oxalate)-(sodium nitrate)-(tungsten(VI) oxide) has been studied. The reaction of WO3 with sodium oxalate-nitrate mixtures occurs at a lower temperature and at a higher rate than the reaction with pure sodium carbonate. A high-yield process for the synthesis of high-purity sodium tungstate has been developed on the basis of this study.

 

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Single crystal ammonium paratungstate is the ideal raw material for high performance tungsten material due to its high physical properties. Single crystal ammonium paratungstate powder has good flow ability. Tungsten powder calcined by single crystal ammonium paratungstate which is pressed and drew into tungsten wire, the rate of finished products can reacn 90%. On the contrary rate of finished products made by polycrystal ammonium paratungstate is 70%. Thus single crystal ammonium paratungstate has become the required raw material for high quality tungsten wire.

Besides that, single crystal ammonium paratungstate has high apparent density, gaps between the grain is even and small, it has low mechanical defects, high compaction density, and has become the raw materials for tungsten wire and tungsten alloy products. For example, the service life of hammer made by single crystal ammonium paratungstate is 2-3 times longer than polycrystal APT. Single crystal ammonium paratungstate has become the perfect raw materials for ammo, high density alloy and other tungsten products.



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Before beginning the process to convert APT to AMT, a conventional pH, probe was calibrated using standard solutions at pH 4 and 7. 3000 mL of double distilled water was heated to 90-95° C. Approximately 250 g of APT powder was added to the hot water, and the mixture was stirred using a stir bar. Cation exchange resin was added incrementally in portions of approximately 10 g until the pH of the liquid portion of the APT/water mixture was between 3 and 4. The total volume of resin necessary to reach a pH between 3 and 4 was approximately 675 mL resin/Kg APT. As the APT dissolved in the acidified water, it reacted to form AMT, thereby allowing more APT to dissolve in the solution and drive the conversion of the APT toward completion.

The dissolution and reaction of the APT initially occurred relatively quickly. As the reaction proceeded the reaction mixture changed from an opaque white slurry to a clear, slightly yellow or amber solution. When the solution appeared clear, indicating that substantially all of the APT had been converted, additional APT was added in two 250 g increments until the solution contained 750 g APT (250 g/L). The 750 g of APT reacted in the process (250 g initially+500 g added) was dissolved and converted to AMT in approximately 90 minutes. 



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