Chapter 6
New Processes for Alkali Ferrate Synthesis 1,*
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L. Ninane , N. Kanari , C. Criado , C. Jeannot , O. Evrard , and N. Neveux Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 14, 2013 | http://pubs.acs.org Publication Date: July 25, 2008 | doi: 10.1021/bk-2008-0985.ch006
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Solvay SA, 1 rue Gabriel Péri 54110 Dombasle sur Meurthe, France institute National Polytechnique Lorraine, Nancy, France University Henry Poincare, Nancy, France NanciE, Nancy, France Corresponding author:
[email protected],
[email protected] 3
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A new process for the synthesis of the potassium ferrate(VI) salt was developed at the University of Nancy, France under an EEC program; started in 2001. This program had an objective of synthesizing a large quantity of ferrate in order to feed large scale applications in the field of water treatment such as drinking water, municipal waste water, and industrial waste water treatment. The raw materials used were ferrous sulphate, potassium hydroxide, and calcium hypochlorite (or chlorine). In this process, mixing of three solids took place in a mixer in which the potassium ferrate(VI) salt was stabilized. Another objective of the program was to develop synthesis of solid sodium ferrate(VI), which is cheaper to produce because its preparation requires less expensive materials: caustic soda instead of potassium hydroxide and sodium hypochlorite (or chlorine gas) instead of calcium hypochlorite. The final objective was to develop a better technology, which could be cheaper and easier to scale-up. This chapter also describes successful results of lab and small pilot tests.
Introduction Iron(VI) has been known for a long time; although it has been observed in 1841 by Fremy, it is still not well known (1,2). Iron salts are known only in the ferrous (Fe(II)) and the ferric forms (Felll); the higher oxidized forms Fe(IV, Fe(V), Fe(VI) give unstable compounds designated under the general name of ferrate. Sometimes all iron salts from Fe(II) to Fe(VI) are called ferrate. Ferrate(VI) is not registered at the European inventory of chemicals (Einecs). The FeO ion or ferrate (Fe(VI)) 2-
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© 2008 American Chemical Society
In Ferrates; Sharma, V.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
103 has raised considerable interest because of potentially interesting applications in the treatment of water and wastewater.
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Physicochemical Properties of Ferrate(VI) The dry crystalline potassium ferrate(VI) is stable if protected against humidity. When this salt is heated, it decomposes at temperature higher than 250 °C with a release of oxygen. This release is not accompanied by immediate reduction of Fe(VI) to Fe(IV). Ferrate(V) may appear as intermediate state of reduction of Fe(VI) depending on temperature. The red-violet aqueous solution of ferrate(VI) is unstable and gradually decomposes according to the equation: 2
2Fe0 "+5H 0 4
2
=
2 Fe(OH) + 4 OH-+ 3/2 0 3
2
(1)
The rate of decomposition depends on the temperature and the alkalinity. The ferrate is more stable at high alkalinity and low temperatures. The decomposition of ferrate(VI) ion in solution is also accelerated by the presence of metal ions or impurities in solution (e.g. Ni, Co). Ferrate(VI) is a powerful oxidant, often superior in oxidizing power than permanganate.
Oxidation potentials of ferrate at several pH values: • • •
2.2 volt at pH = 0 1.4 volt at pH =7 0.7 volt at pH =14
Species: H Fe0 Species: HFe0 " Species: Fe0 " 3
+ 4
4
2
4
This potential is always higher than water over the entire pH range. Therefore water is a potential reductor of ferrate in the whole range of pH.
Comparison with other oxidants Mn0 /Mn0 4
ci /cr 2
Hcio/cr cio/cr o /o 3
2
2
1.68 V in acidic conditions 0.59 V in alkaline conditions 1.51 V in acidic conditions 1.3 V in neutral conditions 75 % in best tests Conclusions made were • Fe(VI)/ Fe yield ratio was significantly improved • Crystal bed temperature was easier to control than in rotating drum • Ferrate obtained in the Fluid Bed was already dried (no after- extra drying) and no sieving needed tot
Process 2: Fluid bed with chlorine A new process using fluid bed technology and chlorine was also disclosed at the ACS meeting. Synthesis with Chlorine: The overall reaction with chlorine, caustic soda and ferrous sulfate is given below: FeS0 .H 0 + 2 Cl + 8 NaOH 4
2
2
=Na Fe0 (?) + Na S0 + 4 NaCl + 5 H 0 2
4
2
4
2
The process can be divided into individual steps: Premixing of solid sodium hydroxide and solid ferrous sulphate in a mixer. During this operation, ferrous sulphate reacted with potassium hydroxide and the reaction progressed inside the pellets. 2 NaOH + FeS0 . H20 * Na S0 + Fe(OH) + H 0 4
2
4
2
2
This premixed solid, which had almost the same particle size as the original solid caustic soda introduced in the mixer. More consistent information was revealed by SEM investigation.
In Ferrates; Sharma, V.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
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The external part of the caustic pellets was essentially composed of Na,0,S and Fe; indicating a reaction of NaOH with iron sulphate. NaOH pellets became porous; facilitating the diffusion of reactive gas during the Sodium ferrate(iv) synthesis The core of the pellet showed the presence of unreacted NaOH
The premixed solid was fluidised by air with a small content of chlorine gas. Visual observation indicated a good fluidisation without dust formation. The solid after chlorination was examined by optical microscope, which confirmed that the shape after treatment was similar to the shape of the original caustic soda pearls. The solid ferrate(VI) colour was purple; indicating the presence of sodium ferrate(VI). Comparison of the Two Fluid Bed Processes Process 1 Process 2
Miscellaneous iron salts NaOH, Fe , Cl 2+
2
Yield Fe(VI)/Fe =75 % Yield Fe(VI)/Fe =50%
K or Na Ferrate
Tot
Na Ferrate only
Tot
Main Results in Applications Results in drinking water treatment •
From surface water resource: little technical and economical advantage of ferrate(VI) or ferrate(VI) in combination with ferric chloride compared to traditional solution • From sub surface resource: these water are often contaminated with As and/or Mn • Ferrate(VI) used in combination with ferric chloride is a good technical and economical alternative to existing treatment of As with a target level of 10 ug/L • Ferrate(VI) used in combination with ferric chloride is a good technical and economical alternative to existing treatment of Mn with a target of 50 ug/L Urban waste water • •
Ferrate(VI) used at small dosage in combination with ferric chloride Few specific applications are possible in tertiary treatment
Industrial waste water • •
Some specific applications have been found in treatment of waste water from textile industry allowing direct discharge of treated water in municipal plant Some specific applications have been found in treatment of waste water from tannery industry allowing direct discharge of treated water in municipal plant
In Ferrates; Sharma, V.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
Ill •
Some specific applications have been found in removing aromatic residue in water from coke.
Acknowledgements
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This work was performed in the frame of contract No G5 RD-CT-2001-03011 and we want to thank the support of the EEC.
References 1.
Evrard, O.; Geradin, R.; Schmitt, Evrard, L.L. Ferrate of Alkali or Alkaline Earth Metals, Their Preparation and Their Industrial Applications. International patent WO 91/07352, 1991. 2. Ninane, L.; Veronneau, C ; Neveux N.; Jeannot C ; Dupre B. Manufacture of super oxidants for the treatment of industrial wastes. In proceedings of environmental clean technologies for sustainable production and consumption, Vancouver, 2003. 3. Kanari, O.; Ninane, L.; Neveux, E. Synthesis of Alkali Ferrate using a waste as Raw Material, JOM, 2005. 4. Gmelin Handbuch der anorg. Chemie, Eisen Teil B, 1932. 5. Kokarovstseva, B. Russ. Chem. Rev. 1972, 41(11), 929-936 6. Asahi Glass. Japanese patent 57/19827 of 27/5/1981 7. Olin Corp, US 4435256 of 23/3/1981 8. Olin Corp, US 4435257 of 23/3/1981 9. Four Ph.D. Thesis. N. Neveux, N. Aubertin, A. Lechaudel, C. Jeannot at the University of Nancy, France. 10. Sharma, V.K. Potassium Ferrate (VI): an environmentally friendly oxidant Adv. Environ. Res. 2002, 6, 143-156. 11. Neveux, N. Le sulfato-ferrate de potassium, un nouvel agent oxydant coagulant.
Informations Cimie no 386, 1997.
In Ferrates; Sharma, V.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.
