Behavior of Tin in Producing Ammonium Paratungstate 2/3

Tin compound is of amphiprotic, it exists in water, acid and alkali in the form of +4 valence and +2 valence compounds, ionic radii were 0.71*10^-10m and 1.2*10^-10m. Adding moderate amount of NaOH into the solution containing Sn2+ precipitated white Sn(OH)_2. Sn(OH)₂ can not only dissolve in acid to generate Sn2+, but also dissolve in excess of NaOH solution to generate [Sn(OH)₃]^-、[Sn(OH)₄]^2-. At the same time, add alkali in the solution containing Sn⁴⁺ ions can generate the alpha tin acid gel (H₂SnO3) which is difficult to dissolve in water. H₂SnO₃ react with acid to generate Sn⁴⁺, but also can react with alkali to get SnO₃^2-. In addition, tin has another important compound—tin sulfide with two forms SnS and SnS₂, SnS₂ will decompose and get SnS when heated to 520℃. Both SnS and SnS₂ can not soluble in water and dilute acid, but can react with high concentration of HCl. SnS is not soluble in alkali in the low oxygen state, however SnS₂ can produce sulfostannate and stannate in alkali. In APT production process, the solution is sometimes acidic, and sometimes alkaline, and the strength of the acid and alkali is different, which makes the varieties forms of tin.

In the general alkali decomposition conditions, choose concentration of 500g/L NaOH solution, 1.6 times as much as that of the theoretical amount to ensure reaction in the alkali excessive state, reaction time of 4 h, setting temperature in three value of 150℃, 160℃, 170℃, and selecting two different types of minerals to observe under different temperature. The effect of reaction temperature on the leaching rate of tin is shown below:
 
From the above, we can draw that leaching rate increased with reaction temperature increasing, and the increasing speed is far greater than leaching rate of tungsten. That means compared to tungsten leaching reaction, tin leaching reaction shows more tendency to thermodynamic control.

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