ISSN 0012-5008, Doklady Chemistry, 2008, Vol. 420, Part 2, pp. 144–146. © Pleiades Publishing, Ltd., 2008.Original Russian Text © D.L. Motov, M.V. Vasyokha, 2008, published in Doklady Akademii Nauk, 2008, Vol. 420, No. 5, pp. 632–634.

144

Ferrous ions are removed from acidic solutions ofnonferrous metal compounds by precipitation ofFe(OH)

3

after oxidation of Fe

2+

in solutions whileincreasing their pH. The mother solution of this productcontinues to contain valuable components, in particu-lar, nonferrous metal ions irremovable by repeatedrepulping of the solution by acidified water. The utiliza-tion of ferric hydroxide is an important problem for var-ious industries, first of all, nonferrous metallurgy [1].

We solved the problem of utilizing ferric hydroxide,which is now a production waste, by developing a tech-nology based on sulfite conversion, the theoretical basisfor which was a study of the pseudoquaternary systemFe(OH)

3

(H

2

SO

4

)–Na

2

SO

3

–H

2

O [2]. For the first time,information was visualized by plotting on a plane of themulticomponent system a combination of two surfacesusing two types of isolines: isolines of equal Fe(III)reduction state FeO

1.5

(final)/FeO

1.5

(initial) (%) and iso-lines of equal molar ratio Na

2

SO

3

/FeO

1.5

. This relatedthe redox process with the reactant ratio and, thereby,clarified the mechanism of interaction in the systemthat is necessary for optimization and practical applica-tion of the process [3].

In the system Fe(OH)

3

(H

2

SO

4

)–Na

2

SO

3

–H

2

O, threemoles of Na

2

SO

3

react with one mole of Fe(OH)

3

tolead to virtually complete precipitation of ferroussulfite. In this reaction, the reduction of Fe(III) to Fe(II)consumes two moles of Na

2

SO

3

, which is oxidized todithionate Na

2

S

2

O

6

, and only one mole of Na

2

SO

3

isused to precipitate Fe(II) as sulfite FeSO

3

· 2.5H

2

O.

In the technological process of sulfite conversion offerrous cake, cake is initially separated by oxidizing thesolution while increasing pH and then sulfitized to con-vert Fe(III) to Fe(II); ferrous sulfite precipitates; andsodium sulfite is recovered by thermal hydrolysis of thesuspension, with SO

2

being distilled off and trapped bya Na

2

CO

3

solution.It seemed reasonable to develop a process of direct

removal of ferrous sulfite from solution without a com-plex step of ferric hydroxide separation. For this pur-pose, it is necessary to investigate the FeCl

2

–Na

2

SO

3

–H

2

O system at room temperature. This system has notyet been studied.

The FeCl

2

–Na

2

SO

3

–H

2

O system was producedfrom a 0.18 M FeCl

2

solution (pH 2.6) and a 0.18 MNa

2

SO

3

solution (pH 8.4). The FeCl

2

solution wasadded to the H

2

SO

3

solution, and the mixture was sup-plemented with a 1 M H

2

SO

4

solution or a 1 M NaOHsolution until a certain pH value from 4.0 to 1.5 at aninterval of 0.5 was reached.

H

2

SO

4

or NaOH was not a component of thepseudoternary system FeCl

2

–Na

2

SO

3

–(H

+

or OH

–

)–H

2

O since either of the two substances was introducedonly for reaching a certain pH value and was notinvolved in phase formation.

Seven sections of the system at Na

2

SO

3

: FeCl

2

molar ratios of 5 : 1, 5 : 2, 5 : 3, 5 : 4, 5 : 5, 5 : 10, and5 : 15 were studied. Each section contained six pointscharacterized by different pH. A total of 42 points wereinvestigated.

In the sections at Na

2

SO

3

: FeCl

2

molar ratios

≥

5 : 5,only sulfuric acid was added; in the section at aNa

2

SO

3

: FeCl

2

molar ratio of 5 : 5, the point at pH 4.0was obtained without introducing H

2

SO

4

. In the sec-tions at Na

2

SO

3

: FeCl

2

molar ratios <5 : 5, for the pointsat pH > 3.0, a NaOH solution was added, and for thepoints at pH

≤

2.5, a H

2

SO

4

solution was introduced. Thepoint at pH 3.0 in the section at a Na

2

SO

3

: FeCl

2

molar

Plotting of Isolines of Equal Degree of Precipitation in the FeCl

2

–Na

2

SO

3

–H

2

O System as the Basis for Isolation of Ferrous Sulfite from Solution

D. L. Motov

a

and M. V. Vasyokha

b

Presented by Academician V. T. Kalinnikov December 26, 2007

Received January 9, 2008

DOI:

10.1134/S0012500808060050

a

Tananaev Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials, Kola Scientific Center, Russian Academy of Sciences, ul. Fersmana 26A, Apatity, Murmansk oblast, 184200 Karelia, Russia

b

Murmansk State Technical University, ul. Sportivnaya 13, Murmansk, 183010 Russia

CHEMISTRY

DOKLADY CHEMISTRY

Vol. 420

Part 2

2008

PLOTTING OF ISOLINES OF EQUAL DEGREE OF PRECIPITATION 145

ratio of 5 : 10 was obtained without adding eitherH

2

SO

4

or NaOH.

Of great importance was the sequence of introduc-ing components. Addition of a Na

2

SO

3

solution to aFeCl

2

solution in the formation of the system was inap-propriate because of abundant release of SO

2

. In addi-tion of a FeCl

2

solution to a Na

2

SO

3

solution, the SO

2

release did not exceed 2% at the lowest pH 1.5. AtpH 4.0, the SO

2

release was within 0.1%.

The mixture was stirred and kept for several minutesuntil ferrous sulfite precipitated. The suspension wasfiltered in air. The precipitate, originally light green,gradually became brown, beginning with the surface,because of Fe(II) oxidation by atmospheric oxygen.

In the filtrate, the Fe

2+

ion content was measuredphotometrically. A 10-ml test sample was supple-mented with 1 ml of nitric acid diluted in a 1 : 1 ratio,5 ml of 10% sulfosalicylic acid solution, and 5 ml of10% aqueous ammonia solution. Being readily oxidiz-able, Fe(II) is converted to Fe(III) and, in the alkalinemedium, forms a sulfosalicylate complex, which isphotometrically determined at a wavelength of 416 nm.

The

ion concentration was found by iodometrictitration.

Figure 1 illustrates the properties of the system viaa previously developed [4] approach using isolines of

equal degree of

Fe

2+

and precipitation

Fe

2+

(final)/Fe

2+

(initial) (%) and (final)/ (ini-tial) (%).

The representation method consists of plotting theprojections of surfaces—isolines of equal degree of

Fe

2+

and underprecipitation relative to their ini-

tial amounts—as functions of the Fe

2+

and con-tents and pH of the solution.

In this method for comparing two response func-tions, the previous visualization approach [3] has obvi-ously been developed by using the degrees of precipita-tion of both cation and anion.

Figure 1 shows that, throughout the studied pH

range, at the equimolar :

Fe

2+

ratio, the samedegree of Fe

2

SO

3

precipitation of 98% is reached and

2% of both the cation

Fe

2+

and the anion

remain

in solution. With an increase in the :

Fe

2+

molarratio, there is no salting out in the initial solution andthe degree of

Fe

2+

precipitation decreases to 95% sincethe

Fe

2+

content of the initial solution decreases and,with respect to it, the degree of

Fe2+ underprecipitation

increases. With a decrease in the : Fe2+ molar ratiobelow equimolar, the degree of Fe2+ underprecipitationabruptly increases from 2 to 50% and then drops to

SO32–

SO32–

SO32–

SO32–

SO32–

SO32–

SO32–

SO32–

SO32–

SO32–

25% because of the increase in the Fe2+ content of theinitial solution.

The isolines of equal degree of Fe2+ and pre-

cipitation coincide only at the equimolar : Fe2+

ratio in the initial solution. With an increase in this

molar ratio above equimolar, the degree of under-precipitation increases from 2 to 50%, and with a

decrease in this ratio, the degree of underprecipi-tation increases from 2 to 6%.

Figure 2 presents the compositions of the solid and liq-uid phases forming in the FeCl2–Na2SO3–(H+ or OH–)–

H2O system. At an : Fe2+ molar ratio in the initialsolution in the range of 0.625 : 1 to 2.0 : 1, FeSO3 ·2.5H2O with a refractive index of N = 1.670 ± 0.005precipitates. We showed that this phase can be a precur-sor of ferric oxide pigment [5]. With an increase in the

: Fe2+ molar ratio in the initial solution above

SO32–

SO32–

SO32–

SO32–

SO32–

SO32–

908070605040302010

(a)

2

345

5040

3025

SO2–3

(b)

908070605040302010

mol %

Fe2+

Fe2+

SO2–3

50

40

302

34

56

1.5 2.0 2.5 3.0 3.5 4.0pH

Fig. 1. FeCl2–Na2SO3–(H+ or OH–)–H2O system at

25°C: isolines of equal degree of (a) Fe2+ and (b)

precipitation Fe2+(final)/Fe2+(initial) (%) and

(final)/ (initial) (%).

SO32–

SO32–

SO32–

146

DOKLADY CHEMISTRY Vol. 420 Part 2 2008

MOTOV, VASYOKHA

2.0 : 1, a phase in which the : Fe2+ molar ratio is

1.375 : 1 forms until the : Fe2+ molar ratio in theinitial solution is 3.44 : 1 and a phase in which the

: Fe2+ molar ratio is 2.75 : 1 forms when the

: Fe2+ molar ratio in the initial solution exceeds

3.44 : 1. With a decrease in the : Fe2+ molar ratio

in the initial solution, a phase in which the : Fe2+

molar ratio is 0.75 : 1 forms when the : Fe2+ molarratio in the initial solution is below 0.625 : 1 and a

phase in which the : Fe2+ molar ratio is 0.5 : 1

forms when the : Fe2+ molar ratio in the initialsolution is below 0.35 : 1.

SO32–

SO32–

SO32–

SO32–

SO32–

SO32–

SO32–

SO32–

SO32–

The nature of these precipitates is to be studied.In transition from the superequimolar to the sub-

equimolar range, the composition of the liquid phase

abruptly changes from the : Fe2+ molar ratio 16 :1 to 0.04 : 1.

In the equimolar range, the compositions of thesolid and liquid phases coincide.

Thus, by the example of the FeCl2–Na2SO3–(H+ orOH–)–H2O system, the construction of isolines of equaldegree of cation and anion precipitation further devel-oped the information visualization, related the cationand anion constituents of phase precipitation, and opti-mized the separation of ferrous sulfite by its direct iso-lation from solution without ferrous cake precipitation,which is now a nonutilizable waste. Ferrous sulfite canbe a precursor of ferric oxide pigment.

ACKNOWLEDGMENTS

This work was supported by the Russian Foundationfor Basic Research (project no. 06–08–00449).

REFERENCES

1. Motov, D.L. and Vasekha, M.V., Zhelezistyi kek i prob-lema ego utilizatsii (Ferrous Cake and Problems of ItsUtilization), Apatity: Izd. KNTs Ross. Akad. Nauk,2007.

2. Motov, D.L. and Vasekha, M.V., Zh. Neorg. Khim., 2004,vol. 49, no. 10, pp. 1742–1745.

3. Motov, D.L. and Vasyokha, M.V., Dokl. Chem., 2004,vol. 397, part 1, pp. 141–142 [Dokl. Akad. Nauk, 2004,vol. 397, no. 1, pp. 61–63].

4. Motov, D.L., Fizikokhimiya i sul’fatnaya tekhnologiyatitano-redkometall’nogo syr’ya (Physical Chemistry andSulfate Technology of Titanium/Rare Metal Raw Mate-rial), Apatity: Izd. KNTs Ross. Akad. Nauk, 2002, vol. 1.

5. Motov, D.L. and Vasekha, M.V., Metally, 2007, no. 3,pp. 8–13.

SO32–

SO2–3

90

80

70

60

50

40

30

20

10Fe2+

mol %

10 20 30 5040 7060 80 90

Initial [SO2–3 ]/([Fe2+] + [SO2–

3 ]), %

Liquid phase

Solidphases

Fig. 2. FeCl2–Na2SO3–(H+ or OH–)–H2O system at 25°C:liquid and solid phase compositions.