Abstract:
Alkali metal iron (IV) and iron (VI) ferrates are produced by forming a particulate mixture of reactants including an alkali metal nitrogen oxygen compound and an iron material selected from the group of iron oxide, Fe 2  O 3 , Fe 3  O 4  and an iron compound which self-reacts at a temperature less than about 1100° C. to form Fe 2  O 3 . The mixture of reactants is subjected to a predetermined elevated temperature for a predetermined time duration sufficient to bring about a reaction between the reactants which produces at least one of iron (IV) and iron (VI) ferrates. The molar ratio of alkali metal nitrogen oxygen compound to the iron material is preferably in the range extending between about 4:1 and about 8:1.

Description:
BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to alkali metal ferrates and, in particular, to methods for the preparation of alkali metal ferrates where the iron therein has a valance of +4 or +6. 
     Although the most common and familiar forms of iron in combination with other elements are those wherein iron has a valence or oxidation state of +2 or +3, other compounds of iron such as iron (IV) and iron (VI) are known in the art. Two of the more useful iron (IV) and iron (VI) compounds are the ferrates, i.e., salts of iron (IV) and iron (VI). Iron (IV) ferrates (sometimes referred to as &#34;perferrites&#34;) (FeO 3   2- ) and iron (VI) ferrate (FeO 4   2- ) are generally useful in a variety of chemical reactions as oxidizing agents. Iron (VI) ferrates, in particular are very strong oxidizing agents in aqueous solution, and stable, water soluble ferrates such as potassium or sodium ferrate are, therefore, particularly useful in removing electrons (i.e., oxidizing) from other chemical species. A limiting factor in the broader utilization of iron (IV) and iron (VI) ferrates is the unavailability of an inexpensive and simple means for the synthesis of the pure forms of these compounds in relatively high yield. 
     (b) Description of the Prior Art 
     A typical method utilized for the production of alkali metal ferrates comprises electrochemical techniques wherein a 35-40% NaOH solution is used to convert scrap iron to a concentrated solution of Na 2  FeO 4  (iron (VI)) using 10-15 cm 2  electrodes with a 2 cm separation and an initial resistance of 2-5 ohms. Another method comprises the wet chemical oxidation of a soluble iron (III) compound by hypochlorite, followed by chemical precipitation of FeO 4   2-  with potassium hydroxide to form K 2  FeO 4 , and followed by recrystallization to obtain a high purity solid. Still another method comprises the fusion of iron filings with potassium nitrate and extracting with water. 
     U.S. Pat. No. 2,835,553 of Harrison, et al. discloses a multi-step process for preparing alkali metal ferrates wherein an alkali metal iron (III) ferrate (typically known as a &#34;ferrite&#34;) is reacted at elevated temperature in the presence of free gaseous oxygen, with an alkali metal compound (which may be the same or different than the alkali metal present in the alkali metal (III) ferrate) to produce the ferrate (IV) of the alkali metal or metals. The alkali metal ferrate (III) itself requires synthesis from more readily available materials, e.g., iron oxide. The ferrate (IV) produced in this manner may then be dissolved in water to produce ferrate (VI) according to the following equation (where, e.g., the alkali metal is sodium): 
     
         3Na.sub.2 FeO.sub.3 +5H.sub.2 O→2Fe(OH).sub.3 +Na.sub.2 FeO.sub.4 +4NaOH 
    
     In the foregoing processes, electrical energy is consumed and therefore expensive procedures are employed (e.g., electrolysis) or complicated multi-step procedures are required to produce iron (IV) or iron (VI) ferrates from more readily available materials. Moreover, in most cases, the iron (VI) ferrates produced in these methods are in solution and require a crystallization therefrom in order to avoid the obvious handling, shipping and storage disadvantages associated with aqueous solutions. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method for the direct preparation of iron (IV) and iron (VI) alkali metal ferrates from readily available starting materials. 
     A further object of this invention is to provide a simple method for the preparation of alkali metal iron (IV) and iron (VI) ferrates. 
     A further object of this invention is to provide a method for the preparation of alkali metal iron (IV) and iron (VI) ferrates wherein the ferrate is produced in a non-aqueous state. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with the invention, alkali metal ferrates are formed by reacting: 
     (a) either alkali metal nitrate or alkali metal nitrite with 
     (b) either an ore known as hematite (primarily comprised of Fe 2  O 3 ), an ore known as magnetite (primarily comprised of Fe 3  O 4 ), or an iron compound which self-reacts via thermal decomposition to form Fe 2  O 3 . 
     The significance of these reactions of the invention primarily resides in their ability to produce iron (IV) or iron (VI) in high yield amounts, and from inexpensive, readily-available starting materials (e.g., iron oxides, hematite, magnetite, iron compounds which form hematite, and potassium nitrate and nitrite) without the need for complicated electrochemical procedures, initial preparation of reactants and the like. 
     A further significance of these reaction schemes of the invention resides in their ability to form iron (VI) alkali metal ferrate directly and not in solution. 
     Prior methods used in the thermal fusion synthesis of iron(IV) ferrate involve the initial formation of alkali metal ferrite (MFeO 2  or M 2  Fe 2  O 4 ) which is the fusion product of Fe 2  O 3  and a suitable alkali metal base, e.g. MOH or M 2  CO 3 . The alkali metal ferrite is then fused with alkali metal oxide or peroxide to form iron(IV) ferrate. Other thermal fusion methods used in the synthesis of iron(IV) ferrate involve the thermal fusion of alkali peroxide and alkali oxides with iron oxides to form a product which is entirely iron(IV) metal ferrate. 
     The formation of iron (VI) alkali metal ferrate has been achieved by such reactions only as a result of the ability of iron (IV) itself to form iron (VI) when dissolved in water by the following reaction; 
     
         3Fe.sup.+4 →Fe.sup.+6 +2Fe.sup.+3 
    
     The significance of the present reactions of the invention therefore resides in the direct formation of iron (VI) ferrate without the need for procedures which involve indirect production of the ferrate in solution. Preferred alkali metal compounds for use in the reactions of the invention are those of potassium, although sodium compounds may also be used to advantage. 
     In accordance with the method of this invention, reactions are carried out between alkali nitrates (or nitrites) and hematite or an iron compound which thermally decomposes to form hematite (Fe 2  O 3 ), with the reactants being heated to produce the desired reaction. 
     Initially the solid reagents are preferably comminuted and formed into a mixture in proportions suitable for the desired reaction. The molar ratio of hematite to the alkali nitrate of the mixture is in the range extending between about 1:2.3 and about 1:8.5, and preferably is from about 1:4 to about 1:5. 
     Where magnetite (Fe 3  O 4 ) is chosen as the iron oxide, the molar ratio is preferably in the range extending between about 1:7 to about 1:8. 
     Ores comprising hematite and magnetite can contain small amounts of silicon dioxide (SiO 2 ) as well as trace amounts of first transition metal oxides which do not adversely affect the reaction. 
     Where an iron compound which thermally decomposes upon heating to form hematite is used in the reaction, the respective molar ratio of iron compound to alkali nitrate, based upon the ratio of atomic iron content to nitrate, is preferably in the range extending from about 1:4 to about 1:5. 
     Preferred iron compounds which thermally decompose to form Fe 2  O 3  are alpha FeO(OH) (geothite) and gamma FeO(OH) (lepidocrocite). Geothite decomposes at about 500° C. to form alpha hematite (&#34;native&#34; hematite). Lepidocrocite which is formed during the rusting of iron decomposes when heated above about 250° C. to form gamma hematite (not naturally found). Alpha and gamma hematite differ only in their crystalline structure and magnetic susceptibility, and are equally efficacious in producing alkali metal iron(IV) and (VI) ferrates according to the present invention. Gamma Fe 2  O 3  is converted to alpha Fe 2  O 3  when heated in air above 400° C. Inasmuch as the subject reactions of the invention are carried out at or above about 800° C., the transformation of geothite and lepidocrocite to hematite begins well before the subject reactions begin. 
     For the purpose of the subject reactions of the invention, the transformation to hematite is essentially complete if the molar ratio of geothite or lepidocrocite (based upon atomic iron content) to alkali nitrate (or nitrite) is kept at about 1:2 to about 1:3. 
     The comminuted reagents are subjected to a temperature of at least about 800° C., preferably from about 800° C. to about 1100° C., and most preferably at a temperature of about 960° C. The reaction is carried out in an appropriate vessel such as a stainless steel, zirconium or other suitable ceramic vessel. The time necessary for reaction will vary depending upon the specific choice of reagents, reagent ratios and temperature, higher temperatures yielding faster reactions. Generally, ferrate formation will begin after about 5 minutes and in most cases will be complete in less than 2 hours, and in essentially all cases complete reaction will occur in less than 3 hours. 
     The product resulting from the reactions involving alkali nitrate is a block solid mass and comprises alkali metal iron (IV) and iron (VI) ferrates together with other products, including unreacted iron oxide, and unreacted alkali metal nitrates and nitrites, peroxides and oxides. 
     The method of the invention can also be carried out with reactions employing alkali metal nitrites (preferably potassium nitrite). The molar ratio of iron oxide to the alkali nitrite generally is between about 1:3 and 1:8, and preferably is from about 1:4 to about 1:5. Generally the reactants are subjected to a temperature greater than about 780° C., preferably in the range extending from about 860° C. to about 1100° C., and most preferably at about 960° C. 
     Where an iron compound which thermally decomposes to form hematite is used in the reaction, the molar ratio of iron compound to the alkali nitrate (or nitrite), based upon the ratio of atomic iron to nitrate (or nitrite), generally is between about 1:2 and about 1:4, and preferably is from about 1:2.5 to about 1:3. 
     The reaction of the reagents in carrying out the method of the invention may be conducted at a substantially constant temperature or, alternatively, may be conducted in a stepwise manner with gradual increases in temperature until the reaction is completed. 
     The product of the reactions involving alkali nitrite is a black solid mass and comprises alkali metal iron (IV) and iron (VI) ferrates, with the exact presence of the iron (VI) ferrate being unknown when the yield of the reaction is less than about 42%. 
     The reaction product remitting from the method of the invention when alkali nitrate or alkali nitrite is employed is useful per se as a source of iron (IV) or iron (VI) ferrate or, preferably, as a progenitor for production of a substantial quantity of iron (VI) ferrate therefrom through dissolution of the reaction product in water. 
     The alkali nitrates and nitrites preferred for use in the present invention are those having potassium, sodium, cesium, or rubidium as the alkali component. Potassium nitrate and potassium nitrite are most preferred. 
     The methods of the present invention are described in further detail with reference to the following illustrative 
    
    
     EXAMPLES 
     EXAMPLE 1 
     Five grams of Fe 2  O 3  (flue dust) and 15.8 grams of KNO 3  (1:5 molar ratio) were mixed and heated for 60 minutes at 850° C. in an atmosphere of N 2  gas. The resulting fusion ferrate yield was less than 1.0%. 
     EXAMPLE 2 
     Five grams of Fe 2  O 3  and 15.8 grams of KNO 3  (1:5 molar ratio) were mixed and heated for 30 minutes at 950° C. in an atmosphere of N 2  gas. The resulting fusion ferrate yield was 34.4%. 
     EXAMPLE 3 
     Five grams of Fe 2  O 3  and 15.8 grams of KNO 3  (1:5 molar ratio) were mixed and heated for 30 minutes at 950° C. in an atmosphere of argon gas. The resulting fusion ferrate yield was 42.0%. 
     EXAMPLE 4 
     Five grams of Fe 2  O 3  and 12.6 grams of KNO 3  (1:4 molar ratio) were mixed and heated for 30 minutes at 950° C. in an atmosphere of argon gas. The resulting fusion ferrate yield was 33.2%. 
     EXAMPLE 5 
     Five grams of Fe 3  O 4  and 15.8 grams of KNO 3  (1:7 molar ratio) were mixed and heated for 50 minutes at 950° C. in an atmosphere of argon gas. The resulting fusion ferrate yield was 33.0%. 
     EXAMPLE 6 
     Five grams of Fe 2  O 3  and 15.8 grams of KNO 3  (1:5 molar ratio) were mixed and heated for 30 minutes at 950° C. in an atmosphere of air. The resulting fusion ferrate yield was 23.0%. 
     EXAMPLE 7 
     Five grams of Fe 2  O 3  and 15.8 grams of KNO 3  (1:5 molar ratio) were mixed and heated for 15 minutes at 1000° C. in an atmosphere of N 2  gas. The resulting fusion ferrate yield was 33.7%. 
     EXAMPLE 8 
     Five grams of Fe 2  O 3  and 15.8 grams of KNO 3  (1:5 molar ratio) were mixed and heated for 15 minutes at 1050° C. in an atmosphere of N 2  gas. The resulting fusion ferrate yield was 41.5%. 
     EXAMPLE 9 
     Five grams Fe 2  O 3 , 13.3 grams NaNO 3  and 3.3 grams Na 2  CO 3  (1:5:1 molar ratio) were mixed and heated for thirty minutes at 900° C. in an atmosphere of argon gas. The resulting fusion ferrate yield was less than 1%, (Na 2  CO 3  added as fluxing agent). 
     EXAMPLE 10 
     Seven grams Fe 2  O 3  and 10.2 grams KNO 3  (1:2.3 molar ratio) were mixed and heated for 75 minutes at 960° C. in an atmosphere of argon gas. The resultant fusion ferrate yield was 4.3%. 
     EXAMPLE 11 
     Six grams Fe 2  O 3  and 15.1 grams KNO 3  (1:4 molar ratio) were mixed and heated for 75 minutes at 960° C. in an argon gas atmosphere. The resultant fusion ferrate yield was 40.3%. 
     EXAMPLE 12 
     Five grams Fe 2  O 3  and 15.8 grams KNO 3  (1:5 molar ratio) were mixed and heated for 75 minutes at 960° C. in an argon gas atmosphere. The resultant fusion ferrate yield was 54.2%. 
     EXAMPLE 13 
     1.64 grams Fe 2  O 3  and 10 grams CsNO 3  (1:5 molar ratio) were mixed and heated for 60 minutes at 960° C. in an argon gas atmosphere. The resultant fusion ferrate yield was 11.0%. 
     EXAMPLE 14 
     Five grams Fe 2  O 3  (steel mill flue dust) and 15.8 grams KNO 3  (1:5 molar ratio) were mixed and heated at 960° C. for 75 minutes in an argon gas atmosphere. The resultant fusion ferrate yield was 53.4%. 
     EXAMPLE 15 
     Five grams Fe 2  O 3  and 15.8 grams KNO 3  (1:5 molar ratio) were mixed and heated at 950° C. for 75 minutes in N 2  gas atmosphere. The resultant fusion ferrate yield was 52.2%. 
     EXAMPLE 16 
     Five grams Fe 2  O 3  and 15.8 grams KNO 3  (1:5 molar ratio) were mixed and heated at 950° C. for 75 minutes in an atmosphere of air. The resultant fusion ferrate yield was 11.9%. 
     EXAMPLE 17 
     Six grams of Fe 3  O 4  and 19.7 grams of KNO 3  (1:7.5 molar ratio) were mixed and heated for 75 minutes at 960° C. in an argon gas atmosphere. The resultant fusion ferrate yield was 41.5%. 
     The reactions of Examples 1-5, 7-15 and 17 hereinabove were carried out in stainless steel reaction vessels and heated in a tube furnace through which a constant flow of inert gas was maintained at about 100 ml/minute at atmospheric pressure. 
     The percent yield in the above examples was determined by spectrophotometric measurement of the Fe(VI) content of aqueous solutions prepared from the reaction product (pulverized) as compared to solutions prepared from a standard K 2  FeO 4  prepared according to the method of Schreyer, et al., Anal. Chem. 22:691 (wet chemical oxidation of Fe(III) by hypochlorite, followed by chemical precipitation of FeO 4   2-  with KOH, forming K 2  FeO 4  which, upon recrystallization, is substantially pure).