2018年1月7日星期日

APT Crystallization Liquor Treatment -- Residual Alkali Decomposition Method


The crystalization method of evaporation and extraction method in the preparation of APT crystallization liquor treatment. 

Leaving about 5% -10% of the WO3 and part of the S, P, As, Mo, K, Na and other impurities, 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.

APT crystallization liquor treatment image

The residual alkali decomposition method is characterized by the use of tungsten concentrate decomposition of sodium tungstate in the process of recycle of residual NaOH solution with the liquor reaction, turn the WO3 directly into Na2WO4 solution, after remove the molybdenum solution into the main purification process and deal with Na2WO4, and also remove the phosphorus, arsenic, tin and other impurities, the transformation equation is:

2NaOH +(NH4)2WO4===Na2WO4+ 2NH4OH

2NaOH +(NH4)2MoO4===Na2MoO4+2NH4OH

The solution recycled by the residual alkali decomposition method is only applicable to the classical precipitation process, which can directly recycle the WO3 in the crystallization liquor, metal yield can be increased by 0.52%, and the use of residual alkali molybdenum acid solution can replace by some of the hydrochloric acid, APT alkali consumption, acid consumption were reduced by 8% and 7%, but the residual alkali decomposition method is only used in the precipitation process.

Some scholars use residual alkali method and ion exchange method to test, and the result shows that the recycle of liquor has an effect on the adsorption capacity of S2- ion exchange resin, and lead to the entire ion exchange waste water ammonia nitrogen seriously exceeded, and the arsenic in the liquor can also lead to ion exchange resin poisoning, so the alkali can only be docked with the classic precipitation method. Unfortunately, the classic precipitation method of the old process is gradually being eliminated, so there is no place for residual alkali decomposition method.

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APT Crystallization Liquor Treatment -- Ion Exchange 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.

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

APT crystallization liquor treatment image

To contain large amounts of NH4Cl crystallization liquor of ammonium paratungstate, ion exchange method can be used to recycle tungsten concentrate and transformed into Na2WO4 and return to the main process.

Ammonium paratungstate crystallization liquor treatment of ion exchange need to adjust the pH value of hydrochloric acid to 3-4, through the weak alkaline anion exchange resin layer, where the tungsten, molybdenum and impurity phosphorus, arsenic, silicon with the same polyacid or heteropoly acid acid anion adsorption, the presence of chloride ions does not affect the adsorption effect, and the supported resin is desorbed by NaOH solution to obtain a concentrated sodium tungstate solution.

Ion exchange method began to be applied to the production of ammonium paratungstate (APT) in the 1980s. The method has the advantages of short process flow, simple equipment, low investment, good environment and high yield, in China has been widely used, many manufacturers have turned to the liquor treated ion exchange techniques, ion exchange method and treatment of crystallization liquor mainly used macroporous weakly basic anion exchange resin, in the acidic conditions, the operation has the characteristics of large exchange capacity, easy to adsorption and desorption, and obtained the sodium tungstate.

The solution can be returned to the ion exchange main process. The method has made great progress compared with the artificial scheelite method, but there are still some shortcomings of acid and alkali consumption and excessive ammonia nitrogen.

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APT Crystallization Liquor Treatment -- Hydrochloric Acid Adjustment

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.

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

APT crystallization liquor treatment image

Hydrochloric acid adjustment method is a new occurrence of ammonium paratungstate crystallization liquor treatment process, it low cost, short process, and the equipment is simple.

Hydrochloric acid method is mainly by using the reaction of strong acid against weak acid, adding suitable amount of hydrochloric acid into APT crystallization liquor, S2- is generated to H2S and volatilize from the liquor, tungsten is precipitated in the form of tungstic acid. Other few phosphorus, arsenic, tin and some impurities with most of cations like Ca, Mg, Cu form into a precipitate. A small amount of impurities that did not form a precipitate were mixed into the peak ammonium tungstate solution as the crystallization liquor was formulated as a desorbent, in the subsequent molybdenum removal process, the removal of the molybdenum reagent is carried out.

The starting point of this method is the recycle of tungsten and ammonium chloride in the APT crystallization liquor, but the liquor in the high concentration of S2- not only can damage the ion exchange resin, can also lead to resolve the peak concentration of S2- on the high side, eventually the final crystallization of the APT sulfur exceeded. Therefore, a more appropriate treatment process is taking out a sulfur removal treatment. Hydrochloric acid adjustment method, while the cost is low, the process is short, but after all, the main material acids chemicals will inevitably increase the cost of environmental protection, so it needs to think twice before using it.

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APT Crystallization Liquor Treatment -- Nano Membrane Filtration

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.

Since the APT crystallized  liquor contains a large number of impurity elements, it can not be directly returned to the process, in order to improve the metal recycle, it is necessary to set the auxiliary process to recycle the  of WO3.


APT crystallization liquor treatment image

Due to the low energy consumption of membrane separation technology, it’s easy to achieve continuous operation and management, the process is easy to enlarge, the equipment is compact, it occupies a limited area and easy to use with other processes. In recent years, it widely attracted by the industry. Nano-Membrane filtration method is based on nano filtration equipment and put forward on the basis of great achievements.

Nano membrane filtration was developed since the 1980 s, a new type of pressure driven membrane, its property between ultra filtration and reverse osmosis membranes, using nano filtration membrane (membrane aperture at the nanoscale range), the price of 2 and multivalent ions and the molecular weight between 200-1000, the characteristics of the substance has a higher removal efficiency of chloride ions from the  liquor and containing tungsten ion separation, so as to realize the recycle of tungsten and ammonium chloride in the  liquor.

Since the tungsten in the  liquor is present as WO42-, in order to allow chloride ions and tungsten ions to be better separated, it is necessary to use tungsten in the acidic conditions in the form of partial tungstate, the liquor is acidified and then ionized. In nano filtration method, concentration, pressure is the key process parameters, the lower the pressure and the concentration, the higher the dechlorination rate, the lower the tungsten loss . But low concentration and pressure will make the nano filtration membrane permeability and dechlorination rate decreased, energy consumption increases. Therefore, from the comprehensive benefits, it should increase the membrane area, and adopt the appropriate material liquid concentration, pressure in order to improve the dechlorination rate.

Nanofiltration has strong adaptability. Compared with other methods, nano membrane filtration method also has the advantages of short process and and small consumption, more importantly, in the process of ammonium paratungstate crystallization liquor treatment, it almost has no additional pollution of waste water.

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APT Crystallization Liquor Treatment -- Selective Precipitation 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.

Resulting in it can not directly return to the process of use. In order to improve the metal recycle, it is necessary to set up auxiliary processes to recycle WO3 in the APT crystallization liquor.

ammonium paratungstate crystallization mother liquor treatment image

Selective precipitation method is developed from the tungsten and molybdenum separation technology. Scholars have found that the use of tungsten, molybdenum compounds of some of the structure and nature of the differences, learn from the successful experience of mineral processing flotation synthesis, synthesis of a precipitant, it can interact with molybdenum and other impurities to form a very small solubility of the compound, and the tungsten compounds remain in the solution to achieve selective separation of tungsten and impurities.

In the treatment of ammonium paratungstate crystallization liquor, some scholars use M115-a treatment of crystallization liquor, the mass concentration of Mo in the liquor is 0.1 ~ 0.5 g / L, WO3 is 15 ~ 18 g / L, after adding the precipitant, the Mo content of the liquor can be reduced to below 0.005 g / L, and the valent metal tungsten is retained in the solution. Compared with artificial scheelite method, WO3 recycle rate can be increased by 10%, the method has a wide adaptability, it can be used to connect with the classic process, and can also use ion exchange process,the advantage is that the process and equipment is simple, and it's low cost. It should be noted that the use of selective precipitation method, direct evaporation of the APT obtained by the particle size and crystal distribution and other physical and chemical properties are too uniform, will affect the quality of products after calcination.

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APT Crystallization Liquor Treatment -- Ammonia Liquid Recycle

At present, in Chinese ammonium paratungstate (APT) enterprises, the most commonly used crystallization method often requires a lot of ammonia, in the production of ammonium tungstate process there are two cases will escape ammonia.

ammonia recycle image

There are two cases will escape ammonia: One is the free ammonia in the ammonium tungstate solution, which quickly escapes in the early stages of the heating ammonium tungstate solution; the other is the reaction of ammonia, it is with the loss of the ammonia concentration in the solution and the precipitation of ammonium paratungstate, and constantly to escape.

Ammonia liquid is a high energy consumption products, the current market price is about 2400 RMB / ton. If the recycle rate of ammonia is 80%, the recycle value is considerable, and the recycle cost is low, it don’t need to increase in capacity, using the cooling cover to recycle,  only add a small amount of vacuum pump and water pump for electricity, thus removing the recycling cost, annual net profit is bigger; If you do not recycle ammonia, not only cause undue economic losses, more importantly, the environment is seriously polluted.

There are many liquid ammonia recycle methods, in the preparation of APT is mainly used the condensation method and hydrochloric acid leaching method.

Condensation absorption method is based on NH3 have a certain degree of solubility in the water, and in a certain NH3 partial pressure conditions, the lower the temperature, the greater the solubility, so the evaporation of the mold out of the H2O + NHmixed gas turn into the jacketed condenser, congeal the water, meanwhile NH3 dissolved in the condensed water into ammonia, can be returned to use. This method and the disadvantage lies in the periodic evaporation of the crystallization process, the early and late evaporation crystallization NH3 concentrations of the gas phase is different, the early stage of the gas phase NH3 high solubility, and the ammonia concentration is high, easy to return to use, and the late ammonia concentration is low, there is no value.

Hydrochloric acid leaching method is the evaporation of the mold output of H2O + NH3 mixed gas leaching with HCl, resulting in NH4Cl. NH4Cl solution can be returned to the cycle of leaching, so that the concentration increased to a certain extent, for crystallization recycle or with desorbent.

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