FR2800057A1 - New stabilized mixed ferrates and their synthesis, useful for the treatment of water - Google Patents

Abstract

The invention describes a method for the synthesis of stabilized metal ferrates. Stabilization is effected by partial substitution of Fe by a stable cation and also stabilization by OH ions. The ferrates have the general formula :- M1<+>(3-a)xM2<+>(3-a)1-x((Fe<6+>yX<6+>1-y)a,(Fe<5+>z'Y<5+>1-z)1-a)O ; or M1<+>(3-a)bM3<++>(3-a)(1-b)/2((Fe<6+>y,X<6+>1-y)a,(Fe<5+>Z,Y<5+>1-z)1-a) O4-w(OH)2w ; in which, M1 and M2 are monovalent cations, M3 is a divalent cation ; X and Y are stable cations with a rare gas electronic structure partially substituting Fe<6+> and Fe<5+> ; a is greater than 0 and less than or equal to 1; x and b are each 0 - 1 inclusive ; y and z are each 0 - 1 inclusive ; and w is 0 - 2 inclusive. An Independent claim is also included for the synthesis of the compounds.

Description

The present invention relates, as new products, to stabilized mixed ferrates.

A subject of the invention is also a process for the synthesis of said ferrates.

Whatever the synthesis method used, wet or dry, the literature does not cite any crystallized sodium ferrate.

Prior patents report the synthesis of K2Fe04 after oxidation in a strongly alkaline aqueous solution of a ferric iron salt by chlorine or a solution of sodium hypochlorite then K2Fe04 is precipitated using the addition of sodium hydroxide. potassium.

This method of synthesis has major drawbacks, namely - ferrate (VI) oxidizes water regardless of the pH and synthesis in aqueous solution affects the reaction yield; - the precipitation of K2Fe04 requires very large quantities of potassium hydroxide, which entails a significant additional cost; - potassium ferrate is not stable over time and requires special storage conditions.

Therefore, such a synthesis cannot be industrially applicable. In an attempt to remedy this, European patent EP-A-0 504 156 proposes a method of synthesizing ferrates (VI) stabilized by the dry route at room temperature.

Said synthetic route makes it possible to produce a stabilized ferrate corresponding to the general formula M (Fe, X) 04, an example of which is K2 (Fe, S) 04.

Such a synthesis process is based on the mixture of three solids - a ferrous or ferric iron salt such as FeS04.7H20 - an oxidant such as Ca (CIO) 2 - a strong base such as KOH The mixture of these three reagents takes place in the stoichiometric conditions given by the equation (1) FeSO ,, 7H20 + Ca (CIO) 2, Ca (OH) 2 + 6KOH @ 2 K2 (Feo, sSo.s) O4 +2 KCI + 2Ca (OH) 2 + 9H20 Stabilization of Fe04 - by S04 - is materialized by the formation of a solid solution between K2Fe04 and K2S04 which are isomorphic and which have similar lattice parameters.

Nevertheless, by simple substitution of potash by soda under the conditions given by equation (1), it is impossible to produce a stable sodium ferrate (VI). The object of the invention is to remedy this drawback, the need being felt for stabilized ferrates for specific industrial applications.

The present invention relates to ferrates stabilized - mixed by the valence of iron: these ferrates include the valences (+ V) and (+ VI); - mixed by the nature of the cation associated with the ferrate group: substitution of monovalent metals by other monovalent metals or by divalent metals.

The stabilization of said ferrates is effected by partial substitution of iron with a high degree of oxidation by a stable cation (having the electronic structure of a rare gas) as well as by the effect of other stabilizers such as the OH radical.

More precisely, the invention relates to ferrates of general formula <B> MI </B> (3-a) xM2 (3-a) 1-xL (Fe y7x 1-y) a, (Fe z7Y 1J1- J04 <B> or </B> M1 + (3-a) bM3 ++ (3-a) (1-b) / 2L (Fe6 + y, x6 + 1-y) a,) (Fe5 + z, Y5 + 1 -z) la] 04_w (OH) 2w according to the structure of the substituted metal, in which - Mi` is a monovalent metal, - Mi ++ is a divalent metal, - 0 <a <1 - 0 <x <_1 and 0 <_b <1 - 0 <y <1 and 0 <z <I - 0 <w <2 The main objective of the present invention is the use of ferrates in the treatment of drinking water where the tolerated sodium doses are ten times greater than those relating to potassium, hence the need to synthesize a solid sodium ferrate (VI) or (V).

If we look at the different degrees of oxidation of iron, it appears that the solid soda compounds are preferentially at low degrees of oxidation while potassium prefers the high degrees of oxidation.

- <SEP> Ferrites <SEP> (III) <SEP>: <SEP> Na <SEP> Fe02 <SEP> synthesized <SEP> at <SEP> ambient <SEP> temperature <SEP>;
<tb> KFe02 <SEP> requires <SEP> a <SEP> temperature <SEP> of <SEP> synthesis <SEP> of <SEP> 800-900 C.
<tb> - <SEP> Ferrates <SEP> (IV) <SEP>: <SEP> Na4Fe04 <SEP> synthesized <SEP> to <SEP> 380 C <SEP>;
<tb> K4Fe04 <SEP> never <SEP> observed <SEP>;
<tb> - <SEP> Ferrates <SEP> (V) <SEP>: <SEP> none <SEP> ferrate <SEP> V <SEP> of <SEP> sodium <SEP> nor <SEP> of <SEP> potassium <SEP>;
<tb> - <SEP> Ferrates <SEP> (VI) <SEP>: <SEP> K2Fe04 <SEP> known <SEP>;
<tb> Na2Fe04 <SEP> never <SEP> observed. The present invention further provides a method of synthesizing mixed ferrates by the dry route at room temperature.

The synthesis process of said invention consists of a mixture of a ferrous or ferric salt with a solid or gaseous oxidant and a strong base, alkali hydroxide MiOH or alkaline earth Mi (OH) 2 or a mixture of the two in proportions different from those given by the reaction stoichiometry (1). The oxidant / iron ratio making it possible to end up with a ferrate (VI) or a mixed Fe (VI) -Fe (V) salt.

The recommended ferrous iron salt is partially dehydrated ferrous sulphate by bringing to 125 C ferrous sulphate heptahydrate, a by-product of the titanium industry (production of TiO 2) or of pickling of steel with sulfuric acid.

After partial dehydration, the ferrous sulfate used in the synthesis has the formula FeS04, nH20 with 1 <n <7 FeS04, nH20 is then mixed with a solid oxidant, calcium hypochlorite and soda in the minimum proportions given by equation (2).

Example Mixing at room temperature in a rotary reactor of - 100 molar parts of FeSO4, nH20 - 200 molar parts of Ca (CIO) 2, Ca (OH) 2 - 1000 molar parts of NaOH according to the stoichiometry of reaction (2) FeS04, nH20 + 2 Ca (CIO) 2, Ca (OH) 2 + 10 NaOH -> 2 Na3 (Feo, 5So, 5) 04 (OH) + 4 NaCl + 2Ca (OH) 2 + (n + 2) H20 Fe (VI) is stabilized by S (VI) which is part of the composition of ferrate but unlike a potassium ferrate where K2 (Fe, S) 04 is stable, Na2 (Fe, S) 04 is not stable and it requires a more basic medium to be able to be preserved. This stabilization is ensured by the presence of hydroxyl ions in the sodium ferrate.

These new basic sodium ferrates correspond to the general formula Nax + 2 (FeyS, _y) 04 (OH) X With x <_1 and a NaIFe molar ratio> 4.

The products according to the invention find a particularly advantageous application in the treatment of water, in particular but not limited to the following fields - disinfection of urban waste water, - decontamination of water containing fungicides, - reduction of heavy metals, phenols , - oxidation of cyanides, - action on COD, suspended solids and turbidity,

Claims (6)

1. Ferrates having the formula MI (3-a) xM2 (3-a) I-xL (Fe Y, X 1-Y) a, (Fe <B> Il </B> Y Iz) IJ04 or MI + (3 -a) bM3 ++ (3-a) (1-b) n [(Fe6 + Y'X6 + 1-Y), (Fe5 + z, Y5 + l-1) 1-aJ04-w (0H) 2, in which - M, and M2 are a monovalent metal cation - M3 is a divalent metal cation - X and Y are stable cations possessing the electronic structure of a rare gas partially substituting Fes + and Fes + - 0 <a <_I - 0 <_x51 and 0 <b_ <1 - 0 <y <1 and 0 <z <1 - Onnr <2 2. Ferrates according to claim 1, characterized in that the molar ratio M ilFe> 4. 3. Process for the synthesis of ferrates according to any one of claims 1 and 2, characterized in that it is carried out by a dry route at room temperature, and that it consists in mixing a ferrous or ferric salt with a solid oxidant. or gaseous and a strong base, alkali hydroxide MiOH or alkaline earth metal Mi (OH) 2 or a mixture of the two hydroxides in proportions different from those given by the reaction stoichiometry. 4. Method according to claim 3, characterized in that the ferrous iron salt is partially dehydrated ferrous sulfate by bringing ferrous sulfate heptahydrate to 125 C. 5. Method according to any one of claims 3 and 4, characterized in that the solid oxidant is calcium hypochlorite. 6. Method according to any one of claims 3 and 4, characterized in that the gaseous oxidant is chlorine.