2017年12月26日星期二

Ammonium Paratungstate Waste Recycle

In the production, sometimes for some reason lead to part of the APT products in the classics or physical performance on the failure, may also occur in the production process a small amount of ammonium paratungstate waste recycle, which are recyclable.

ammonium tungstate image

Recycle of unqualified ammonium paratungstate method is mainly after the dissolution of the filter, and recrystallization, for this reason, the APT is dissolved at a higher temperature directly with ammonia as a solvent, however, APT solubility in aqueous ammonia is limited, resulting in dilute solution concentration, it takes for a long time to dissolution. Therefore, the modern industry began to use of APT thermal decomposition into ammonium metatungstate for recycle.

Thermal decomposition of the working mechanism is to control the appropriate temperature, calcined APT can be converted to ammonium metatungstate (AMT).

According to the high temperature decomposition performance of APT·4H2O, when heated APT·4H2O in the air, First dehydrated at about 100 °C, and then decomposed at 190 °C to 250 °C to dissolve the amorphous ammonium formate. Some scholars have experimented with unqualified calcined tungstate calcination temperature at 280 °C is better, the product has the highest leaching rate in aqueous ammonia at 25% concentration, it can up to 99.6%. Therefore  the calcination treatment is carried out at an appropriate temperature in advance so that the unqualified APT can better soluble in the NH3 solution and can create better recycle conditions for the unqualified ammonium paratungstate.

After the calcination treatment of APT by pure water leaching, leaching pH control in 3-4, after leaching filtration, evaporation crystallization can be obtained ammonium metatungstate.

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Ammonium Paratungstate Preparation Method

Ammonium paratungstate (APT) preparation is an important part of smelting tungsten, whether it is using chemical precipitation method, ion exchange method or extraction method, it all have to go through the process of ammonium trioxide to produce qualified APT.

preparation of ammonium paratungstate image

APT is an important tungsten intermediate product, the higher the purity the greater the value, so APT preparation process is also the further purification of tungsten.

Commonly used method for preparing ammonium paratungstate is evaporation crystallization method, neutralization crystallization method and frozen crystallization method.

Evaporation crystallization method is heating evaporation solvent, so that the solution from unsaturated to saturated, continue to evaporate, the excess solute will be crystal precipitation, called evaporation crystallization method. This method of technology and equipment is simple, easy to large-scale, this method technology and equipment is simple, easy to large-scale, at the same time it has a purification effect, so it’s universal.

And the crystallization is at room temperature neutralizing (NH4)2WO4 solution with hydrochloric acid, dissolving out the ammonium paratungstate crystals.This method has small energy consumption, fast crystallization, and easy to recycle fine particles APT.

Frozen Crystallization In water-cooled vacuum drying, water and APT crystallize at the same time, but then remove the water and leave the dry APT. The advantage of this method is that it can produce ultrafine powder and obtain a homogeneous composite powder.

In addition to the above three common methods, the industry also uses other methods to produce high purity ammonium paratungstate, such as neutralization - evaporation crystallization, extraction, recrystallization, electrolysis. There are many methods and they all have their own merit and demerit. In conclusion, In addition, the main problems of the preparation of ammonium paratungstate crystals are the removal and control of the crystalline morphology to meet the needs of the final tungsten product as the standard.

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The Crystallization of Ammonium Paratungstate

Ammonium paratungstate (APT) is an important tungsten intermediate product, mainly with white crystals.

It have flakes, needles and strips three kind, used in the manufacture of tungsten trioxide or blue tungsten oxide, TP,  also used for manufacturing ammonium metatungstate and other tungsten compounds.

ammonium paratungstate image

Because the production of tungsten oxide, morphology and particle size of tungsten powder has genetic relationship with APT, so the production process is not only to the tungsten APT purification, but also to control the crystal morphology and particle size of paratungstate tungstate.

In the 1950s, in the book "Tungsten and Molybdenum Metallurgy" by A.H rickman, used the reaction 5(NH4)2O·12WO3·nH2O to describe the APT formula, used the evaporation crystallization process to get n = 5 flaky crystal, and used the acid neutralization process for n = 11 needle crystal. After that in China are all using this view, that of ammonium tungstate solution producing is a ammonia chemical process of paratungstate tungstate lost, the crystallization process of chemical reaction is:

12(NH4)2WO4===5(NH4)2O·12WO3·5H2O+2H2O +14NH3

(N = 5 temperature> 50 °C to obtain flaky crystals)

12(NH4)2WO4+4H2O===5(NH4)2O·12WO3·11H2O+14NH3

(N = 11, temperature <50 °C to obtain needle-like crystals)

APT crystal water content and crystal shape depends on the crystallization conditions, the current study that APT has three crystalline morphology.

 APT·10H2O is needle-like or strip-like orthorhombic structure, it is unstable in the industrial crystallization conditions, at least 50 °C below the crystal can be obtained in this crystal.

2. APT·12H2O is a triad flake crystal, which is a crystal in the state of metastable state in water. It is a metastable phase at the boiling point of (NH4) 2WO4 solution, and only when the concentration of WO3 in the solution is more than 30%, the concentration of NH3 more than 5% of the conditions can be generated, with the extension of storage time, it will happen recrystallization and turn into APT·4 H2O.

APT·4H2O is a monoclinic cubic structure, which is always stable under industrial crystallization,  means from WO3 concentration higher than 30%, NH3 concentration is greater than 5% to WO3 concentration less than 10%, in a solution of NH3 was 1%, slightly better in 90-100 °C temperature range as long as there is APT·4 H2O exists, the seed can produce APT·4 H2O crystallization.

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Ammonium Paratungstate Preparation -- Neutralization Method

Ammonium paratungstate (APT) preparation is an important part of smelting tungsten, whether it is chemical precipitation method, ion exchange method or extraction method,they all have to go through the process of ammonium trioxide to produce qualified APT. 

preparation of ammonium paratungstate image

 APT is an important tungsten intermediate products , the higher the purity the greater the value, so APT preparation process is also the further purification of tungsten.

Neutralization is one of the most commonly used manufacturing method of para-ammonium tungstate, and it's settlement with hydrochloric acid and (NH4) 2  wo4 solution, precipitation crystallization of ammonium paratungstate, and the energy consumption is small, crystallization speed is fast, easily to recovery the fine particle APT.

The neutralization method is carried out in a reaction tank with a liner rubber. Heating with hydrochloric acid and (NH42WO4 solution, ammonium paratungstate crystallization, the reaction is:

12(NH4)2WO4+14HCl+4H2O==5(NH4)2O·12WO3·11H2O+14NH3Cl

Putting (NH4) 2WO4 liquid into the reaction tank, and heat the temperature according to the requirements of the granularity of APT, general particle in about 75 °C, coarse particle above 90 °C, the fine particles under 50 °C.Under the intense agitation, slowly add concentration about 10% - 20% of hydrochloric acid, neutralization to pH 7-7.5, stop adding hydrochloric acid, thermal mixing 0.5 hours,make the pH stable, stand for 8-10 hours, 90% to 95% of tungsten in the solution generated APT precipitation. To get the needle after washing with deionized water containing 11 crystallization water of crystallization of ammonium paratungstate.

Compared with the evaporation method, the neutralization method has the advantages of low energy consumption, fast crystallization and easy access to fine particles APT, but sometimes the grain size of the crystal is very fine, which is not conducive to filtration and washing, but also easy to bring impurities.the acid reagent corrosion equipment, the reagent consumption is large, virtually increases the environmental cost.

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Ammonium Paratungstate Preparation -- Evaporation Crystallization

Evaporation crystallization method is the most commonly used method of preparation of ammonium paratungstate, this method of technology and equipment is simple, easy to large-scale, have a certain role in the purification at the same time,it has been favored by industrial production.

preparation of ammonium tropsanate image

When the ammonium tungstate solution heated, the ammonia concentration decreased, (NH4) 2WO4 solution concentration gradually increased, ammonium paratungstate crystallization, the reaction is:
12(NH4)2WO4==5(NH4)2O·12WO3·5H2O+14NH3↑+2H2O

In industrial production, the degree of evaporation crystallization is generally based on the amount of molybdenum allowed in the solution and the product. There are usually two ways of batch intermittent operation and continuous crystal operation.

The batch intermittent operation was carried out in a jacketed heated stirred tank, and a solution of (NH4) 2WO4 having a density of 1.16-1.28 g / cm3 was added to the reactor and stirred to boil the boiling ammonia.The reactor roof is generally installed with the vacuum system connected to the pipeline, discharged the ammonia to outside of the reactor in time to cooling and recovery. Solution pH value decreased to 7.0-7.7, separated out APT.

Field sampling measure mother liquid density is 1.06 1.02 g/cm3, namely stop heating and cooling for 0.5 hours,The material is discharged to a vacuum filter and filtered and washed. The resulting wet APT is dried at a temperature of 90 ° C to 120 ° C and then sieved to obtain an APT product.

The continuous crystallization operation is carried out in a continuous mold. Material liquid outer heater after heated to a certain temperature and then sent steam into the room, ammonia evaporation and gives some supersaturated solution in the steam, the supersaturated solution by the middle tube into the crystallization of chamber, with which contact with the crystal crystal, crystal send continuous filter to filter washing, part of the return to the original solution crystallization mother liquor mixed circulation, mixing part into the batch operations last reactor for secondary crystallization.

The discontinuous operation is simple and easy to recycle, but the distribution range of APT particle size is too large. On the contrary, the continuous crystallization method has the same composition and particle size. The process is continuous and the quality is stable, but the crystallization rate is not high, so it needs more time.

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Ammonium Paratungstate Preparation -- Freezing Crystallization Method

Ammonium paratungstate (APT) is an important part of tungsten smelting, whether it is using chemical precipitation method, ion exchange method or extraction method, they all have to through the process of ammonium trioxide to produce qualified APT. 

freezing crystallization method image

APT is an important tungsten intermediate products , the higher the puritythe greater the value, so APT preparation process is also the further purification of tungsten.

Freezing crystallization method is one of the most commonly used methods for the preparation of ammonium paratungstate, which is the use of liquid ammonia to freeze the APT, so called frozen crystals. 

The freeze crystallization method was carried out in a liquid nitrogen cooled vacuum drier, and the ammonium tungstate solution was poured into an enamel plate pre-filled with liquid nitrogen (the volume ratio of ammonium tungstate solution to liquid nitrogen was 1: 2), stirring it and then   crystallized water and APT at the same time, Along with enamel disc quickly moved to the vacuum dryer, removed the water and free ammonia distillation under vacuum conditions, leaving a dry white amorphous and loose with 11 crystal water of ammonium paratungstate.

Produced APT by freezing crystallization method, chemical purity and raw material liquid did not change, but the average particle size can be obtained in 0.6 to 1.3 microns of superfine powder of ammonium paratungstate, and composition of paricle size was balanced, and the distribution of various impurities or additives was uniform. So often use this method to making W - the composition such as Re, W - Cu composite powder evenly.

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