The ammonium isopolytungstate solution from solvent extraction is evaporated, whereby ammonia and water are volatilized. The ammonia concentration in the mother liquor decreases (Ph drop) and at the same time the WO3 concentration increases. Paratungstate B [H2W12O43]10- is formed whose ammonium salt has low solubility. Crystallization of (NH4)10H2W12O42•4H2O is the result. The degree of evaporation depends on the purity of the feed solution and the required purity of the APT. Usually, if proper purification was performed in the forgoing steps it ranges somewhere between 90 and 99%.
The physical prosperities of the crystallized of APT can be influenced by crystallization conditions to some extent. They are of importance for the further processing to metal powder by hydrogen reduction.
The crystallization is not only a conversion of the dissolved to the solid salt, but the last purification step in the hydrometallurgical process. Most of the impurity elements still present in the process solution have much higher solubility than APT and are consequently enriched in the mother liquor.
Impurity concentrations of the feed solution to the crystallizer, of the mother liquor, and of the crystallized APT are related. For a given required APT, purity limits in impurity concentrations of the mother liquor are allowed, and these limits in combination with the impurity level of the feed solution determine the degree of evaporation. Upper impurity levels in the feed solution and corresponding concentrations in the mother liquor and the crystallized APT are given in table as following:
APT crystallization: Impurity Concentrations
Element |
Feed Solution
(g•l-1) |
Concentration of mother liquor
(g•l-1) |
APT
(µg•g-1) |
Al |
<10 |
<100a |
<10 |
|
<3000b |
|
As |
<50 |
<2000b |
<20 |
F |
<250 |
<3000 |
<10 |
Fe |
<10 |
<200 |
<10 |
Mo |
<10 |
<60 |
<20 |
Na |
<10 |
<100 |
<10 |
P |
<50 |
<400 |
<20 |
Si |
<10 |
<200 |
<20 |
V |
<100 |
<1200 |
<20 |
a In the absence of fluoride.
b At fluoride concentrations of about 4000mg/l.
The presence of foreign element impurities not only affects the impurity level of APT, but also in some cases the APT crystal size. The higher the concentration of these impurities, the smaller the average crystal size. Heteropolytungstate-forming ions like silicate, phosphate, and arsenate lead to a considerable increase in the solubility of tungsten in the mother liquor. The consequences are worse crystallization yield and higher cost due to increased recycling of tungsten. A precipitation by aluminum salt solution to separate phosphorus and silicon prior to crystallization is proposed. However, it is preferable to make all advanced purification steps more effective.
Ammonia recovery. The ammonia evaporation together with water during crystallization is condensed and can be reused in the stripped stage of solvent extraction. Sometimes, a concentration by distillation by distillation is applied t maintain the water balance.
Mother liquor recycling. In order to maintain the impurity concentration in the APT low and consistent, the evaporation can never be 100%. This means that a certain amount of mother liquor must be recycled to the former steps of the conversion plant. Usually, an amount of mother liquor corresponding to 5-10% of the volume fed into the crystallizer is pumped back(90-95% evaporation). This can be done continuously or batchwise. There exist several possibilities to treat the recycled mother liquor, which is high in impurities.
In some factories it is added to the feed make-up prior to digestion. The disadvantage of this method is that, during heating, an ammonia pressure is built up that prevents one to reach the desired temperature by steam injection. An intermediate venting of the autoclave is necessary, resulting in loss of time and in increased steam demand. The advantage is the proper separation of all impurities.
A second possibility is to add the mother liquor immediately after the digestion but prior to the purification steps. The disadvantage here is the presence of ammonium ions. As already pointed out, ammonium ions favor the condensation of tungsten ions, and this changes the properties and behavior of the isopolytungstate solution in the solvent extraction process. Therefore, this type of mother liquor recycling affords a separate pretreatment to expel the ammonia. This can be done by addition of NaOH and subsequent heating. The ammonia evolved can be fed into the ammonia recovery system of crystallization. The treatment produces a precipitate which can be separated by filtration.
A third possibility is to further crystallize the mother liquor yielding a more impure APT. Also, treatment of mother liquor with hexamethylene tetramine to precipitate tungsten was proposed.
If you have any interest in ammonium paratungstate, please feel free to contact us by email: sales@chinatungsten.com or by telephone: +86 592 5129696.
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