2018年2月27日星期二

Ammonium Metatungstate Preparation -- Neutralization Method

Ammonium metatungstate (AMT) is an important compound product in the process of tungsten smelting. It is widely used.

The preparation methods of ammonium acid are heat degradation and ion exchange, extraction, electrodialysis and neutralization.

ammonium metatungstate preparing image

The neutralization method called ammonium tungstate acid, ammonium paratungstate production is one of the commonly used method, but this method can also be used for the preparation of ammonium metatungstate, it is used in acid solution and the crystallization of APT, by controlling the pH value of 3-4, heating, stirring and other means to strengthen the ammonium tungstate solution is gradually converted into ammonium metatungstate solution then, the precipitation of AMT crystals.

Some scholars use ammonium paratungstate as raw material, adopt continuous solution conversion method,compare the effect of two kinds of acidizing agents, nitric acid and cationic resin on the basis of experiment. After that, nitric acid is selected as an acid agent, at a temperature of 95 degree and a stirring rate of 80 r/min,then obtained ammonium tungstate with similar impurity content as ammonium tungstate, the product quality is stable after the several tests. The direct yield of tungsten metal is 88%, and the total yield is 98.4%.

The most important characteristic of neutralization process is simple process and high rate of solution conversion, It is a common preparation process of ammonium tungstate at early stage. But it is a pity that acid consumption of neutralization method is large, It hashigh cost and environmental risks. Compared with Thermal degradation method and Ion exchange method, Now it has no more advantages.


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Ammonium Metatungstate Preparation -- Electrodialysis Method

Ammonium Metatungstate (AMT) is an important compound product in the process of tungsten smelting and it is widely used. 

The common methods of preparing ammonium metatungstate include thermal degradation and ion exchange, extraction, electrodialysis and neutralization.

ammonium metatungstate preparing image

Electrodialysis is the process of separating the electrolyte from the water by the selective conductivity of the ion exchange membrane under the action of the DC electric field. Under the action of the voltage, ammonium ion is used to form ammonium sulfate solution in the cathode solution of the circulation through anion exchange membrane. Electrodialysis method circulating pH = 9 ammonium metatungstate anode solution through the electrodialysis. Under the action of the voltage, the ammonium ion exchange membrane generates an ammonium sulfate solution through the anion in the circulating catholyte. The excess of ammonium ions in the anolyte begins to mass transfer, causing the pH to drop sharply due to the removal of the "associated" ions. At pH 8.7-8.3, the anode becomes turbid due to the insoluble compounds, but the compounds dissolve between pH4-5. At pH3.0, the process stops.

ammonium metatungstate preparing image

The anodic liquid is clarified and evaporated to a thick slurry, from which the coarse-grained white AMT crystals can be recycled. The final product recycle rate is higher than 99%.

Electrodialysis method is an effective way to obtain ammonium metatungstate. Its advantage is that AMT is prepared with high purity and high recycle rate. It has the potential of research and development.


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APT Crystallization Liquor Treatment -- Artificial Scheelite Method

The crystalization method of evaporation and extraction method in the preparation of APT will produce crystallization liquor, leaving about 5% -10% of the WO3 and part of the S, P, As, Mo, K, Na and other impurities. 

It resulting in it can not directly return to the process, in order to improve the recycle of metal, it is necessary to set up auxiliary processes to recycle WO3 in the liquor.

Artificial scheelite was the method of ammonium paratungstate crystallization liquor treatment, the main precipitant is CaCl2, the main purpose of the artificial scheelite method is to convert the WO3 in the crystallization liquor into a very insoluble calcium tungstate (artificial scheelite), then return to the acid process or hot ball milling process.

artificial scheelite image

Artificial scheelite made in three directions: 1. CaCl2 precipitated artificial scheelite → acid decomposition and partial molybdenum → crude tungstic acid → return to the main process; 2. Remove the molybdenum → precipitated artificial scheelite → return to hot ball grinding alkali decomposition; Recrystallized and remediation of artificial scheelite.

Artificial scheelite method is the traditional method of recycling APT crystallization liquor, its advantages are simple, and more selective. But also has a lot of shortcomings, mainly reflect in: 1. Scheelite precipitation rate is low, the gradual addition of CaCl2 resulted in an increase in the acidity of the crystallization liquor. For the synthesis of pH degree, it generally add calcium chloride before adding some of the alkali to the liquor, to ensure a certain degree of alkalinity, which in turn increase the consumption of alkali. 2. The waste liquid has high impurity elements, especially ammonia nitrogen, which is far beyond the national emission standard, it’s not conducive to clean production, it has to re-manage before the emission. 3. Consumption is on the high side, artificial scheelite from the synthesis to return to the alkali decomposition process to do raw materials, need to re-consumption of raw and auxiliary materials, the more synthesis, the higher the consumption.

To sum up, artificial scheelite method played a certain historical role, but has failed to keep pace with the production requirement of modern industry, so it belong to the brink of elimination of backward technology.


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Ammonium Metatungstate Preparation -- Ion Exchange Method

Ion Exchange method is the process of preparing ammonium metatungstate.It is made from ammonium wolframate solution or ammonium paratungstate. 

This method is also one of the main methods for preparing ammonium metatungstate (AMT) at present.

ammonium metatungstate preparing image

The main mechanism of ion exchange method is that use NH4+ in H+ solution of strong acid or weak acid type H+ cation resin to process ion exchange. Under the condition of pH 3.5, H+ enters solution , cook for 5 hours at 98 °C. Converting ammonium tungstate into stable ammonium metatungstate solution. Then after evaporation and concentration, ammonium tungstate was converted to stable ammonium metatungstate solution. Again after evaporation and concentration, AMT crystals were prepared by freeze crystallization or spray drying. The resin saturated by NH4+ is recycled after treatment with hydrochloric acid solution. 

ammonium metatungstate preparing image

Because of the simple process and convenient control, the ion exchange method can avoid the formation of APT. It does not bring new pollutants into the process, it has gained popularity because of process continuity. The main drawback is that a small amount of colloidal tungstic acid may be produced during the ammonium removal process and plug the resin layer and resin pores. In addition, the tungsten liquid entrained by the resin must be removed.

The resin should be regenerated by acid, which will increase the operation step and bring troubles to the production. Of course, there is no denying that the production of AMT by ion exchange method has the potential for development.

Recently, some scholars have proposed a new method of making high-purity ammonium  metatungstate (AMT). The method adopts cation interchange under the low pH condition. A mixture of ammonium paratungstate and water is exchanged with a cationic material to obtain a high-purity ammonium metatungstate product.


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Ammonium Metatungstate Preparation -- Nanofiltration Method

Ammonium Metatungstate (AMT) is an important compound product in tungsten smelting process.It is widely used in areas. 

The common methods of preparing ammonium metatungstate include thermal degradation and ion exchange, extraction, nanofiltration, electrodialysis and neutralization.

nanofiltration method image

Nanofiltration membrane is a new type of pressure driven separation membrane developed since 1980s. Nanofiltration method is a membrane separation technique, which is independent from reverse osmosis membrane technology.

Filter membrane is the main focus of nanofiltration. The separation performance of nanofiltration membrane is between reverse osmosis and ultrafiltration membrane. It has a good removal effect on low molecular weight substances, high valence ions, low valence cations and anions. Therefore, some scholars used the nanofiltration method to prepare ammonium metatungstate (AMT) from the original solution of ammonium paratungstate (APT) crystallization.

The separation of chloride ions from the crystallization mother liquor by nanofiltration can overcome the shortcoming of the difficulty in removing the chloride ions in the crystallization liquor of the APT process by ion exchange. At the same time the use of nanofiltration membrane acid anion NO3-, but also the main material of the tungsten does not wear, to achieve the purpose of producing pure AMT crystal.

It is important to note that when APT crystallization liquor is dealt by nanofiltration method, the appropriate hydrochloric acid is added to neutralize and adjust the acid, and then the impurity is removed by nanofiltration. The quality of the obtained AMT is better.After the industrial proof, our country's nanofiltration membrane production technology has made great progress. Nanofiltration method has no pollution, high impurity removal efficiency, and has been able to control costs, and has the potential for popularization. 


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Ammonium Metatungstate Preparation -- Extraction Method

The extraction method is a preparation process that use ammonium tungstate with ammonium tungstate solution as raw material. 

This method is also one of the processes for preparing Ammonium Metatungstate (AMT) at presnt. The extraction principle of ammonium metatungstate (AMT) extraction method is similar to that of ammonium tungstate extraction method, which needs to experience four stages including extraction, water washing and stripping. The whole extraction process is carried out in a box-type mixer-settlers, the difference is that in the reverse extraction operation, the concentration and dosage of the extractant are different.

ammonium metatungstate extraction preparing image

In the process of production, using ammonium tungstate solution as raw material, ammonium metatungstate was extracted by solvent extraction, extraction of NH4+ ion from ammonium metatungstate solution by a cationic extractant, then, the solution of pH in the condition of 3~4, ammonium metatungstate solution was obtained by reverse extraction. This is a simple and economical method. Ammonium metatungstate solution obtained by solvent extraction, ammonium nitrate was prepared by concentrated crystallization or spray drying.

ammonium metatungstate preparing image

Some scholars used EPA-88-1 extractant to prepare ammonium tungstate from ammonium metatungstate solution.The most important feature of this method is that two product of ammonium tungstate and ammonium metatungstate can be obtained simultaneously by changing the pH value of different raffinate. The production cost is correspondingly reduced, the cycle is shortened, and the energy consumption is low, and the utility model is especially suitable for the technical transformation project of the existing ammonium tungstate factory.

Ammonium metatungstate has the same advantages as ion exchange method. It is simple, continuous and low cost. But the biggest weakness of the extraction method is that the solid substance is easy to cause emulsification during the extraction process. The whole process is easy to fail in some simple situations, and that's why it hasn't been as popular as ion exchange.


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