**5. Conclusions**

300 Ion Exchange Technologies

OH-

T(°C)

are equal.

3Al6Si2O13+8Na+(aq) + 8OH-

**Reaction A)**

**Reaction B)** 

**Reaction C)**  5Al6Si2O13 + 6Na(+a) + 6OH(-a) + 45H2O = Na6Al6Si10O32\*12H2O + 24Al(OH)3

**Reaction D)**  3 NaAlSi2O6\*H2O + 5Na(+a) + 5OH(-a) + 3Al(OH)3 = Na8Al6Si6O24(OH)2\*2.7H2O + 6.3H2O

**Reaction E)**  3 Na6Al6Si10O32\*12H2O + 12Al(OH)3 + 22Na(+a) + 22OH(-a) = 5Na8Al6Si6O24(OH)2\*2.7H2O + 46.5H2O

**Reaction F)**  Na6Al6Si10O32\*12H2O = 5 NaAlSi2O6\*H2O + Al(OH)3 + Na(+a) + OH(-a) + 5H2O It was considered the activities of solid phases equal to 1 and that the activities of Na+ and

**Figure 10.** Phase diagram of stability of zeolite formed from the coal ash. (Abreviations: ANA –

It was possible to make a survey of pH versus temperature diagram as shown in Figure 10. It is possible to check, at 25 ° C, a large region of stability of the zeolite P type, ie the reaction of the mullite forming zeolite P, at pH values between 8 and 15. This region decreases as the

Analcime; MUL – Mulite ; ZEOP – Zeolite P ; HCAN- Hidroxycancrinite)

+ 5Al (OH)3

(aq)+17.7H2O = Na8Al6Si6O24(OH)2\*2.7H2O +12Al(OH)3

Al6Si2O13 + Na + (aq) + OH- (aq) + 8H2O = NaAlSi2O6 \*

The study demonstrates the application of Brazilian fly ash for zeolite synthesis by hydrothermal treatment. Statistical analysis shown that an increase of time and temperature in zeolite synthesis tends to increase the capacity of sorption of ions tested onto products.

The products obtained are effective sorbent for removal of Pb2+, Zn 2+, Mn 2+ and Cu 2+ ions from aqueous solutions. The performance of sorption increased up to 25 times when compared with the original ashes.

From the results obtained and discussed, it was concluded that the formation of zeolitic phases from coal ash is thermodynamically possible. The thermodynamic values obtained for the models in the literature and the proposed in this study had similar, and allowed the establishment of equilibrium diagrams that define the regions of thermodynamic stability of the species present in the reaction medium.

The analysis of these diagrams shows that the temperature and concentration of alkali present in the medium are the most important factors affecting the stability of species to be formed during the hydrothermal treatment of coal ash with alkaline solution of NaOH. Among the equilibrium reaction studied, we found that the phase hydroxycancrinite phase tends to be more stable zeolite with increasing reaction temperature and the NaOH concentration, rather than formation of zeolite P and analcime phases. Therefore, to obtain phase zeolite P and zeolite such as analcime, are required low reaction temperatures and low concentrations of NaOH.

These results show the existence of a potential reduction of manufacturing costs of these products zeolites which may eventually enable the production of zeolites from an industrial waste such as coal ash.
