Detinning Process of APT Production- Ion Exchange Method

Ion exchange is an exchange of ions between two electrolytes or between an electrolyte solution and a complex. In most cases the term is used to denote the processes of purification, separation, and decontamination of aqueous and other ion-containing solutions with solid polymeric or mineralic 'ion exchangers'.

Typical ion exchangers are ion exchange resins (functionalized porous or gel polymer), zeolites, montmorillonite, clay, and soil humus. Ion exchangers are either cation exchangers that exchange positively charged ions (cations) or anion exchangers that exchange negatively charged ions (anions). There are also amphoteric exchangers that are able to exchange both cations and anions simultaneously. However, the simultaneous exchange of cations and anions can be more efficiently performed in mixed beds that contain a mixture of anion and cation exchange resins, or passing the treated solution through several different ion exchange materials.

In the ion exchange method, the affinity of tin acid radical to resin is far less than the affinity of tungstate to resin in the solution, remove rate of tin in adsorption stage is about 90%, but also to remove 80% to 90% tin when rinsing, so the total remove rate of tin is 98% to 99%.

Although the ion exchange capacity in addition to tin acid radical is strong, there are still a small amount of tin acid radical adsorbed by resin, and during the desorption, some goes into the ammonium tungstate solution with tungsten. If the ammonium tungstate solution crystallize directly without treatment, tin will hydrolyze and separate out and enters the APT product, it is easy to make tin content exceeded in the products. Therefore, it has been studied all this time that how to remove the depth of tin acid radicals by ion exchange methods in an attempt to tap the potential in addition to tin of ion-exchange method.

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.