Ammonium Paratungstate (APT) Nucleation Rate

Evaporation and crystallization is a process that feed liquid and solid of ammonium tungstate to get to oversaturation, which makes the material’s phase change and precipitate purified crystals from the solution. 

APT crystals precipitate from ammonium tungstate solution can be achieved, by that to adjust the PH of solution through loss of ammonia and get into the secondary salt formation region. The loss ammonia -adjust PH process can be achieved by evaporation crystallization process and neutralization crystallization process.

The physical properties of ammonium paratungstate crystals are decided by nucleation rate and grain rate. APT nucleation rate depends on: 

(1) the velocity of APT periphery crystallized from a liquid phase, is proportional to the amount of oversaturation (difference between the solution concentration and the equilibrium concentration) of nuclei forming;

(2) the dissolution velocity of APT, that is a velocity that precipitated APT crystals goes into the solution again, it depends on the equilibrium solubility of solute. A major factor of nucleation rate is oversaturation. A new root of nucleation can only form when oversaturation up to a certain level. Growth mechanism of APT grain is still within reach. It is generally believed: After nucleation, the solute is deposited onto the lattice and gradually grow.

Ammonium Paratungstate (APT) Production Conditions

Ammonium paratungstate (APT) is an important intermediate product of the production of tungsten metal powder. To ensure the quality of tungsten powder, the average particle size, particle size distribution and crystal morphology should also meet certain requirements while the APT chemical purity does. With the rapid development of modern science and technology, there’re more and more demands for different grain size and shape of tungsten powder production, and thus the requirements of raw material APT are also getting higher and higher.

For APT crystallization conditions, the scholars have carried out extensive exploration and discussion, but the overall reports are rare, and the results are not entirely consistent. In order to investigate the basic conditions of APT production, for the preparation of APT crystallization problems, this article attempts to make some analysis and discussion on theory and the process practice based on a number of experimental studies made recently and some views are put forward by reference.

WO3 Concentration’s Effect on Ammonium Paratungstate Production

Here is the disscusion about the effect of WO3 concentration in ammonium tungstate solution.

As it can be seen from Figure 1, the bulk density of the APT increases with WO3 concentration in solution decreases. When WO3 concentration is reduced to a certain extent, the bulk density of APT has no obvious change. It shows that when the WO3 concentration in solution is high, the supersaturation is high. Nucleation is fast, so it’s not easy for crystalline particles to grow. When the WO3 concentration in solution is low, oversaturation of the evaporation process does not change significantly in the solution, the oversaturation is low so it helps nuclei to grow. However, when the WO3 concentration in solution is too low, the oversaturation is too low, less chance of contact between the solute and molecule nuclei, affecting the growth of grains.

Temperature’s Effects on Ammonium Paratungstate Production

Here is temperature’s effects on Ammonium Paratungstate production.

Test results are shown on Figure. With the increase of evaporation temperature, the crystal growth rate of APT is faster than the nucleation rate, which makes a particle size enlargement and increases the bulk specific gravity of APT (see figure). The reasons are increasing temperature , faster molecular motion, more opportunities of molecules colliding with each other, which are conducive to grain growth. 

Plane grain formation rate on the grain surface is affected a lot by temperature. When temperature increases to 10 ℃, speed needs to increase 2 to 4 times, so the increasing temperature is most significant for grain growth. It was also observed in the experiment that APT crystalline grains were much coarser than other parts on the crystallizer wall. This may be due to an indirect steam heating, the temperature on wall is higher than other parts , which is conducive to grain growth. It was also found in the experiment that the crystalline particles is more regular in a higher temperature than a lower one, and size distribution is more well-distributed.

Effect of Time on Ammonium Paratungstate Production

Here is the discussion of effect of time on ammonium paratungstate production.

Evaporation and crystallization including the heating time (heating rate) and the evaporation time (evaporation rate). When the heating time is quick, most of the free ammonia in solution were expelled in a short time, so that PH of the solution decreased rapidly, and the over-saturation increases, and a lot of nuclei was formed rapidly, the particle size became finer. Long evaporation time, it will help small crystals dissolve constantly, big crystals continue to grow. the situation of crystal growth becomes more apparent when APT crystalline precipitate for a while before filtered. It was also found in the conditions of a certain degree of vacuum, evaporation speed increases, the bulk density of the resulting APT reduces significantly.

Effect of Stirring Speed on Ammonium Paratungstate Production

Here is the discussion of effect of stirring speed on ammonium paratungstate production.

The test results are shown in Figure 2. It’s complicated that stirring speed affects nucleation. On one hand, the stirring speed is accelerated, so that the newly formed crystals were minced and the form much crystal nucleus, particle size becomes fine, on the other hand, in a certain stirring speed range, increasing the stirring speed can increase the relative velocity between solid and liquid, thereby increasing the speed of nuclei grow. 

To prepare coarse APT, the best speed is when the resulting APT does not precipitate,  APT particle formed can make full contact with the solutes. It was also found  the effect is not the same with different paddles stirring or different stirring intensity. Stirring has a great impact on the preparation of the different particle size of APT, APT prepared by gas stirring is more regular . For different production conditions, it’s generally required to determine the proper form and speed of stirring through practice.

Structural Evolution of Ammonium Paratungstates during Thermal Decomposition

Mixed metal oxide systems (e.g. Mox(V,W)yO3-z) are employed for the partial oxidation of light alkenes. Ammonium paratungstate (APT) and ammonium heptamolybdate (AHM) are used as precursors for the production of WO3 and MoO3, respectively. The catalytic activity of these materials may depend on the treatment of the precursors. Therefore, studies of the decomposition process in order to identify and quantify tungsten oxide phases and their formation under various atmospheres, reveal correlation between catalytic activity and structural evolution of APT is very important. Here, we present results obtained from bulk structural studies on the thermal decomposition of APT in various reducing and oxidizing atmospheres. In this work the decomposition of APT is studied in situ by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). Using these two complementary methods allows us to follow the evolution of the short-range and long-range structure of the phases formed during the decomposition of APT, to elucidate the evolution of the primary and secondary structure under different conditions. 

Experimental 

Ammonium paratungstate (APT), (NH4)10H2W12O42*4H2O (OSRAM) was used as purchased. Transmission X-ray absorption spectra were measured in situ with the sample pellet in a flow reactor (4 ml total volume) under a controlled reactant atmosphere. In situ XAS experiments were performed at the W LIII edge (10.204 keV) (Hamburger Synchrotron Radiation Laboratory, HASYLAB, beamline X1), using a Si (311) double crystal monochromator. Temperature programmed decomposition was carried out at temperatures between 300 and 773 K in atmospheres of pure helium, 5 % hydrogen in helium, 20 % oxygen in helium, 10 % propene in helium, and 10 % propene/10% oxygen in helium. For the in situ XAS measurements APT was mixed with boron nitride and pressed into 5 mm in diameter pellets. Analysis of the gas phase was carried out with a quadrupol mass spectrometer, QMS 200 (Pfeifer), with a time resolution of ~ 2 s. Further details about the experimental XAS set-up used can be found in .

Metallo-Organic Deposition of Tungsten Oxide Films from Alkylammonium Tungstate Solutions

Metallo-organic deposition of tungsten oxide films from alkylammonium tungstate solutions

This paper describes a simple and inexpensive metallo-organic deposition (MOD) process for forming electrochromic tungsten oxide (WO₃) films on glass. The thin films of WO₃ were made by air firing (500–700°C) films from xylene/2-propanol solutions of bis-(di-n-octylammonium) tetratungstate, [(n-C₈H₁₇)₂NH₂]₂[W₄O₁₃]. The process coats glass with undoped films ranging in colour from faint yellow to dark brown, and can be used to make gradients of these colours. The colour is determined by the firing parameters and results from residual carbon and tungsten suboxides in the film due to incomplete firing. Increased firing temperatures or longer firing times removes the carbon and produces films with higher crystallinity. Electrochemical doping with acid (H⁺) switches the colour gradient films to a uniformly blue colour.

Processing of Ammonium Paratungstate from Tungsten Ores

To obtain highly purified, lamp grade ammonium paratungstate crystals from any of several different tungsten ores, the ore is reduced to finely divided status and slurried in heated HCl solution to convert tungsten values to WO.HO. Recovered tungstic oxide is washed and dissolved in heated aqueous solution of sodium carbonate or sodium hydroxide with the pH maintained at about 8 to 8.5 to form soluble sodium tungstate. Sodium hydroxide is added to raise the pH to about 10.5 to 11.5, and magnesium chloride is added in amount sufficient to somewhat neutralize the solution. Sodium hydroxide is added to raise the pH to about 10.5 to 11.5 to precipitate as hydroxide the magnesium and additional metallic impurities. At least one of ammonium sulfide and thioacetamide is and the heated solution is acidified to a pH in the range from about 2 to 3 to precipitate any molybdenum as MoS. The tungstate solution is then contacted with an organic, water-immiscible ion exchange liquid in which the active ingredient is an amine salt to extract the tungsten values. Tungsten values are then stripped from the ion exchange liquid with ammonium hydroxide to form ammonium tungstate solution, which in turn is separated. From the ammonium tungstate solution is crystallized highly purified ammonium paratungstate. The process is adaptable to continuous type operation.

Process for Producing Ammonium Paratungstate

Ammonium paratungstate (or APT) is a white crystalline salt of ammonium and tungsten, with the chemical formula (NH₄)₁₀(H₂W₁₂O₄₂)·4H₂O.

Ammonium paratungstate is produced by separating tungsten from its ore. Once the ammonium paratungstate is prepared, it is heated to its decomposition temperature, 600 °C. Left over is WO₃, tungsten(VI) oxide. From there, the oxide is heated in an atmosphere of hydrogen, reducing the tungsten to elemental powder, leaving behind water vapor. From there, the tungsten powder can be fused into any number of things, from wire to bars to other shapes.

A process is disclosed for producing ammonium paratungstate which involves adding hexamethylenetetramine to a first solution of ammonium tungstate, adjusting the pH to about 2 with an acid to form a precipitate which contains the major portion of the tungsten and the hexamethylenetetramine and separating the precipitate from the resulting mother liquor. The tungsten hexamethylenetetramine precipitate is then dissolved in aqueous ammonia to form a second ammonium tungstate solution which is then heated at from about 90℃ to about 100℃ to form a precipitate essentially all of which is ammonium paratungstate and a mother liquor which contains essentially all of the hexamethylenetetramine. The ammonium paratungstate precipitate is then separated from the mother liquor.

Calcium Tungstate Crystals Aim to Light up Dark Matter

German scientists hunting dark matter are set to produce half a tonne of high-purity calcium tungstate for their detectors, one 1kg crystal at a time. The CRESST-II experiment based in Gran Sasso, Italy is currently seeking this enigmatic substance, thought to explain the universe’s structure, with 10kg of calcium tungstate (CaWO4). Now Andreas Erb and Jean-Côme Lanfranchi are preparing crystals for its larger successor EURECA, which will begin operation in the French Alps in 5–10 years.

Gravitational effects suggest as-yet-unobserved dark matter in the universe outnumbers more familiar atomic matter four to one. Erb, Lanfranchi and their colleagues are hunting leading theoretical candidates, Weakly Interacting Massive Particles (WIMPs). That name reflects their size – up to a lead atom’s mass – and the limited interaction with atomic matter that makes them hard to find, or ‘dark’. ‘They have to interact weakly to agree with the matter needed,’ says Richard Gaitskell from Brown University in the US, who isn’t involved in the calcium tungstate experiments.

Erb explains that the detectors should be able to pick up dark matter particles when they hit atomic nuclei in the crystals. ‘A higher sample mass gives a higher probability of such events.’ But distinguishing the miniscule amount of heat WIMP–nucleus collisions would produce requires detectors cooled to 10mK and shielded from ambient radioactivity.

CRESST/EURECA is the only team hunting dark matter with calcium tungstate, which has two advantages. Firstly, its different atoms cover a range of possible WIMP masses. ‘No matter the mass, you always have a nucleus with a high probability of interacting,’ Erb says. Second, it would also emit light when a WIMP hits it and monitoring the different signals will help the scientists eliminate background noise. 

Having initially purchased crystals, this need for extreme sensitivity drove Erb and Lanfranchi to produce their own. ‘They weren’t pure enough for the background we want,’ Erb recalls. To avoid oxidation at calcium tungstate’s 1600°C melting temperature, the crucibles are made from rhodium, with their 12cm diameter vessel costing €120,000 (£97,300). Erb says that if they can grow two or three 1kg crystals per week then they will have the required amount for EURECA in about five years.

New Report Sheds Light on the Sodium Tungstate Sales Global Market

The report provides a basic overview of the Sodium Tungstate industry including definitions, classifications, applications and industry chain structure.
Development policies and plans are discussed as well as manufacturing processes and cost structures are also analyzed. This report also states import/export consumption, supply and demand figures, cost, price, revenue and gross margins.
The report then analyzes the global Sodium Tungstate market size (volume and value), and the sales segment market is also discussed by product type, application and region.
The major Sodium Tungstate market (including USA, Europe, China, Japan,.) is analyzed, data including: market size, import and export, sale segment market by product type and application. Then we forecast the 2016-2021 market size of Sodium Tungstate.
Finally the marketing, feasibility of new investment projects are assessed and overall research conclusions offered.

With 183 tables and figures the report provides key statistics on the state of the industry and is a valuable source of guidance and direction for companies and individuals interested in the market.

Sodium Tungstate Physical Properties

Molecular formula : Na2WO4
Molecular Weight: 329.86 g/mole
Molar mass：293.82 g·mol−1
Physical state and appearance: Solid.
Melting Point: 692.22℃ (1278°F)
Specific Gravity: 3.25 - 4.15(Water = 1)

Nature : a white crystalline powder, rhomboidal, mineral spinel structure. The temperature of 698 ℃, density 4.179g/cm3. In dry air weathering. Air heated to 100 ℃ or concentrated sulfuric acid can be stripped dry crystalline water. Three tungsten oxide and liquid caustic soda or soda ash by heating system in concentration crystallization. 10 hydrate colorless crystals, the 6℃ easy dehydration two hydrate. Preparation for the metal tungsten and tungsten products, the other intermediate materials, and for the production of paints and pigments Light, heavier fabrics agent, waterproof fabric and fire, as alkaloids precipitant.

Synergistic Inhibition Effect of Sodium Tungstate and Hexamethylene Tetramine on Reinforcing Steel Corrosion

Sodium tungstate (Na₂WO₄) and hexamethylene tetramine (HMTA) are both eco-friendly corrosion inhibitors. In this work, their synergistic corrosion inhibition effects on reinforcing steel in the simulated polluted concrete pore solution containing Cl⁻ were studied by electrochemical techniques including electrochemical impedance spectroscopy and potentiodynamic anodic polarization curve measurements. The morphologies and compositions of the steel surface were characterized by Electron Micro-Probe Analyzer, X-ray photoelectron spectroscopy, and Raman spectroscopy. The results showed that the serious steel corrosion took place in the solution with pH 11.00 and 0.5 M NaCl. However, a stable passive region occurred in the anodic polarization curve of the steel and its corrosion current density decreased dramatically after addition of a mixed inhibitor with 0.01 M Na₂WO₄ and 0.01 M HMTA to the solution. The inhibition efficiency of the mixed inhibitor reached 97.1%. The surface analyses revealed that a protective composite film was formed on the steel in the solution with the mixed inhibitor, which indicated that the mixed inhibitor had a synergistic inhibition effect on the steel corrosion. Our study also indicated that the mixed inhibitor could effectively control corrosion of the reinforcing steel in cement mortar.

Sodium Pentatungstate/Sodium Hexatungstate

Sodium pentatungstate, Na2O.5WO3, is obtained by fusing together sodium tungstate and tungstic anhydride (1:2), or by heating sodium paratungstate to incipient fusion and extracting the fused mass with water, when it remains in brilliant plates or scales which are only slightly soluble in water.

Sodium hexatungstate, Na2O.6WO3.9H2O, is obtained according to Marignac by prolonged boiling of tungstic acid with sodium paratungstate. Ullik, by decomposing a solution of sodium metatungstate with hydrochloric or nitric acid and allowing the solution to evaporate, obtained large yellowish crystals of what he considered to be the octa-tungstate, Na2O.8WO3.12H2O, but Friedheim could not confirm his results, and Leontowitsch, using the reagents in different proportions, obtained crystals of the hexatungstate, of compositionNa2O.6WO3.15H2O. The anhydrous octatungstate, Na2O.8WO3, was obtained by von Knorre by oxidation of fused metatungstate at a bright red heat, and extraction of the mass with water, when lustrous scales of the octatungstate remain. The relation of these higher acid salts to one another and to metatungstic acid has not yet been determined.

Sodium Tritungstate /Sodium Tetratungstate

Sodium tritungstate, Na2O.3WO3.4H2O, is prepared, according to Lefort, by gradually adding a concentrated solution of the ditungstate to a boiling 50 per cent, solution of acetic acid. On cooling, a white precipitate results which dissolves in water, and the solution on evaporation yields long prismatic crystals. The existence of a tritungstate is denied by Kantschew.

Sodium tetratungstate, Na2O.4WO3, is obtained by the complete dehydration of sodium metatungstate, and is sometimes called "anhydrous sodium metatungstate." As will be seen, however, water is essential to the constitution of metatungstates. The salt may be obtained by heating the paratungstate and treating the residue with water. It is insoluble in water, but on prolonged heating with water at 120° C. it is converted into the metatungstate.

 

Acid Tungstate

The acid tungstate, 4Na2O.10WO3.23H2O, may be prepared by passing carbon dioxide for several days through an aqueous solution of normal sodium tungstate, or by gradually adding formic acid, until the action is distinctly acid, to a solution containing 100 grams of the normal tungstate in 100 c.c. of water.

The action of glacial acetic acid on a solution of sodium tungstate produces a mixture of the salts 4Na2O.10WO3.23H2O and 5Na2O.12WO3.28H2O. The salt, 4Na2O.10WO3.23H2O, forms monoclinic crystals which effloresce rapidly in dry air and have density 4.3. When heated, the salt loses 17 molecular proportions of its water of crystallisation at 100° C., the remainder only being driven off by strong ignition. It melts at 680.8° C. It is soluble in water - 19 parts of the salt dissolve in 100 parts of water at ordinary temperature - forming an acid solution.

Sodium Tungstate Hydrates Described by Formula

From the solutions so prepared Na2WO4 various hydrates have been obtained and are described under many different formula. There appear, however, to be five distinct salts which show distinctive properties, varying from one another in degrees of solubility, crystalline form, etc.

I.5Na2O.12WO3.28H2O is formed when crystallisation takes place at ordinary or lower temperatures. It yields transparent or milky triclinic pinacoidal crystals with
a:b:c = 0.5341:1:1.1148; α = 93° 56', β = 113° 36', γ = 85° 55',
of density 3.987 at 14° C. and stable in air. On heating, the salt loses, according to Scheibler, 10.42 per cent, of water - 21 of the 28 molecules H2O would correspond to a loss of 10.52 per cent.; according to Rosenheim the loss at 100° C. corresponds to 24H2O, and he therefore suggests the formula Na10H4[H4(WO4)6(W2O7)3].24H2O.
The remaining water is lost at 300° C., and the residue, which has density 5.49, is still completely soluble in water. At a red heat - according to Smith at 705.8° C. - the salt melts to a clear, yellowish, oily liquid and undergoes decomposition, for on cooling it sets to a crystalline mass which is only partly soluble in water, the insoluble residue being the tetratungstate, Na2O.4WO3. According to von Knorre the decomposition may be represented thus:
3(5Na2O.12WO3) → 7(Na2O.4WO3) + 8(Na2O.WO3).
Solubility data for sodium paratungstate have been given as follows:
One part of salt dissolves in 8 or 12 parts of cold water, or 12.6 parts of water at 22° C.
If the salt is boiled for some time with water, a solution is obtained which when cooled to 16° to 20° C. contains 1 part of the salt
after 1 day in 0.68 parts of water
after 12 day in 2.6 parts of water
after 72 day in 6.9 parts of water
after 7 month in 9.7 parts of water
after 14 month in 8.8 parts of water
If the salt is boiled with water, or kept for a considerable time in aqueous solution, it is decomposed into the normal and metatungstates. This accounts for the fact that although the cold fresh solution is neutral in reaction, it gradually becomes acid towards phenolphthalein and alkaline towards tropaeolin, especially after boiling; it also explains the apparent increase in solubility with time indicated above.
The solution has at first a sweetish taste, but it gradually becomes sharp and bitter. Rosenheim has determined the equivalent conductivities of solutions at 25° C. containing 1/10 molecule 5Na2O.12WO3in v litres, as follows: 

v =	32	64	128	256	512
Λ =	68.5	79.8	90.8	100.3	110.0

Tungstate of Soda

Sodium paratungstate is known commercially as "tungstate of soda" and may be prepared on a large scale by fusing wolframite with soda ash and lixiviating the fused mass. On nearly neutralising the boiling solution with hydrochloric acid and allowing to crystallise, large triclinic crystals of the salt separate.

The salt may be formed in solution by any of the following methods:
1.Saturation of a solution of sodium hydroxide, carbonate, or tungstate, with anhydrous tungstic acid.
2.Treatment of a sodium tungstate solution with hydrochloric acid at boiling-point (as described above) until only faintly alkaline to litmus.
3.Addition of a solution of sodium metatungstate (containing 5.8 grams Na2O.4WO3.10H2O) to one of the normal tungstate (containing 2 grams Na2O.WO3.2H2O).
4.Saturation of a solution of normal sodium tungstate with carbon dioxide.
5.Electrolysis of sodium tungstate solution in a cell in which the electrodes are separated by a diaphragm (see above).

Sodium Tungstate as Mordant

The production of colloidal tungsten hydroxide by the electrolysis of a solution of sodium tungstate has already been described. If precautions are taken to prevent the sodium hydroxide formed at the cathode from reaching the anode, for example, by means of a porous partition, it is possible to prepare the paratungstate, or other complex tungstate, from the anode solution.

The use of sodium tungstate has been recommended as a mordant, and it has been used as a fire-proofing material for flannelette, but owing to its solubility it cannot be considered satisfactory and it is not now used.

Sodium ditungstate, Na2O.2WO3, may be obtained by fusing together tungstic anhydride and sodium hydroxide or sodium carbonate, the mixture containing lNa2O:2WO3. On cooling, long needles separate, which on prolonged heating with water dissolve, yielding an alkaline solution which contains metatungstate. The dihydrate, Na2O.2WO3. 2H2O, is described by Rammelsberg as a crystalline precipitate obtained by addition of hydrochloric acid to a solution of the normal tungstate. The hexahydrate, Na2O.2WO3.6H2O, is stated by Lefort to crystallise from a solution containing the normal tungstate (2 molecules) and acetic acid (1 molecule); von Knorre, however, could only obtain the paratungstate from such a solution. The hydrate,Na2O.2WO3.12H2O, has also been described.

Sodium paratungstate is known commercially as "tungstate of soda" and may be prepared on a large scale by fusing wolframite with soda ash and lixiviating the fused mass. On nearly neutralising the boiling solution with hydrochloric acid and allowing to crystallise, large triclinic crystals of the salt separate.

Na2WO4 Refractive Indices

The densities and refractive indices of solutions of various concentrations have been determined as follows:

Grams Na2WO4 in 100 Grams Solution.	Density, d4°20°	Refractive Index, nD20°
2.21	1.0184	1.33586
10.08	1.0949	1.34516
16.56	1.1667	1.35376
20.59	1.2148	1.35933
25.46	1.2789	1.36648
32.68	1.3854	1.37934
38.43	1.4828	1.38890

The equivalent conductivities of solutions containing ½Na2WO4 in v litres at 25° C. are as follows:

v =	32	64	128	256	512	1024
Λ =	95.9	101.8	105.4	110.3	112.9	116.4

The vapour pressures of solutions have been determined.