Precipitation of hydroxides by ammonia solution

Preparation of tantalum and niobium oxides based on the precipitation by ammonium solution of tantalum or niobium hydroxides from strip solutions is the most frequently used method in the industry and consists of several steps. Fig. 135 presents a flow chart of the process. 

The interaction between fluorotantalic or oxyfluoroniobic acids and ammonia solution initiates the hydrolysis of complex fluoride compounds. The complete process can be represented as follows  

2 H2NbOF5 + 10 NH4OH = Nb205-nH20(solld) + 10 NH4F + (7-n) H20 (142) 

Equations (141) and (142) describe the equilibrium between the hydrolysis of complex fluoride acids (shift to the right) and the fluorination of hydroxides (shift to the left). Near complete precipitation of hydroxides can be achieved by applying an excessive amount of ammonia. Typically, precipitation is performed by adding ammonia solution up to pH = 8-9. However, the precipitate that separates from the mother solution can be contaminated with as much as 20% wt. fluorine [490]. Analysis of niobium hydroxides obtained under different precipitation conditions showed that the most important parameter affecting the fluorine content of the resultant hydroxide is the amount of ammonia added [490]. Sheka et al. [491] found that increasing the pH to 9.6 toward the end of the precipitation process leads to a significant reduction in fluorine content of the niobium hydroxide. 

Agulyansky et al. [492, 493] investigated the complex structure and composition of solid phases precipitated by ammonia solution from experimental and industrial niobium and tantalum strip solutions. Fig. 136 shows isotherms (20°C) of Nb2Os content versus pH for solutions prepared by the dissolution of (NH4)3NbOF6 and (NH NbOFs in water and of Nb metal in 

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Fig. 135. Flow chart of tantalum/niohium oxide preparation - precipitation of hydroxides by ammonia solution.

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