Low-Temperature Interaction of Sulfur Dioxide with Pacific Ferromanganese Nodules Jong H. Lee, John Gilje, and Harry Zeitlin' Department of Chemistry and Hawaii Institute of Geophysics, University of Hawaii, Honolulu, Hawaii 96822
Quintus Fernando Department of Chemistry, University of Arizona, Tucson, Ariz. 8572 1
An investigation has been carried out over the temperature range 25-95 "C of the interaction of sulfur dioxide with Pacific ferromanganese nodules. Infrared spectroscopic data show that sulfates and soluble dithionates are produced. The formatioq of dithionates, S2062-,via redox reactions between sulfur(1V) and metallic oxides such as those of iron(II1) and manganese(IV), which are present in the nodules, is affected by water, oxygen, and reaction temperature. The proportion af S2OS2- is maximized when SO2 is allowed to react with the aqueous slurry of nodules in the absence of 0 2 a t 25 "C. In the presence of 0 2 a t 25 "C, reduced but still significant amounts of &Os2- are produced along with S042-.In the absence of water, Sod2- alone could be detected irrespective of the presence or absence of 02. Valuable metals such as cobalt, nickel, and copper may be recovered effectively by extraction after reduction of ferromanganese nodules with SO2 in the presence of water. The treatment, however, leads to extensive solubilization of iron as dithionate. This is economically undesirable and complicates the recovery of other metals.
Ferromanganese nodules are of much interest as a potential source of several industrially important metals. The nodules are composed of a complex mixture of transition metal oxides, including those of manganese and iron, together with varying amounts of calcite and silicates. X-ray powder diffraction patterns indicate in previous mineralogical studies of manganese nodules ( I ) that the most commonly identified manganese oxides present are the mineral todorokite (10 A manganite) and birnessite, (Ca,Na)(Mn2+,Mn4+).XH20.The iron-bearing minerals in manganese nodules are usually amorphous to X-ray diffraction, but it is now generally accepted that the dominant mineral phase is goethite, FeO(0H). Other elements, such as Co, Ni, and Cu, are also present in small amounts (ca. 1%as their oxides) (1,2). At present, ferromanganese nodules are viewed principally as a source of the latter metals. Methods for extracting metals from the nodules are not yet fully developed, but the use of SO2 as a reducing agent during their processing has been suggested (3-5). In this context, the interaction of SO2 with ferromanganese nodules a t high temperatures (300-600 "C) has been the subject of several studies (6, 7). In the absence of oxygen the predominant reaction in this temperature range is sulfation of manganese to form MnS04 with very little reaction taking place between SO2 and other metal oxides. However, if 0 2 is introduced along with SOz, other metal oxides, but not those of iron, are also converted into their sulfates. This behavior has been interpreted in terms of a catalytic oxidation of SO2 to the trioxide with the latter compound being the active reactant toward the oxides which are unaffected by SO2. In all these studies no evidence was obtained indicating the formation of any sulfur-containing anions other than S042-. Separation processes based on the assumption that ferromanganese nodules react with SO2 in aqueous slurries a t ambient temperature to produce soluble sulfates have also been suggested. In fact, several patents have been issued for 1428
Environmental Science & Technology
such processes (3, 4 ) . However, as early as 1819 MnO2 was observed to react with aqueous SO2 solutions to produce a significant proportion of dithionate, S20s2-(8).The Manganese Ore Company process for separating manganese from low grade ores, which is based on the formation of soluble MnS04 when a slurry of the ore is treated with a gas containing SOz, is complicated by the S2OS2-formation (9).It is also known that other ions of metals such as those of Fe and Co tend to form S2OS2- a t ambient temperatures in the aqueous SO2 solution (10).Therefore, it is reasonable to expect dithionates as reaction products from the interaction of SO2 with the manganese nodules, which consist mainly of oxides of those metals. There, however, have been no reports of the details of the reaction between ferromanganese nodules and SO2 a t lower temperatures and in the presence of water. In view of the fact that simple sulfation alone may not occur and because of the possibility that low-temperature sulfation could lead to a viable process for the extraction of metals, a detailed understanding of this system appears to be required before it can be exploited in any practical fashion. In this paper we report a study of the SO2 interaction with Pacific ferromanganese nodules over the 25-95 "C temperature range.
Experimental Elemental Analysis. Ferromanganese nodules used in this study were collected from the Pacific Ocean a t a depth of 4350 m; longitude 121'43'W and latitude 19'49". The air-dried manganese nodules were crushed and sieved to obtain uniform representative samples of 140/200 mesh particle size. The elemental composition in these samples was determined by atomic absorption (AA) analysis using a Varian-Techtron AA-5 spectrophotometer equipped with the appropriate single element hollow cathode lamps. Five replicates of each sample were prepared for the elemental analysis employing the Teflon pressure bomb technique developed by Bernas (11).A known quantity of the ground nodules (0.5 g) mixed with aqua regia and 48%H F was digested in an oven. The dissolved material was then diluted to desired volume with 0.5 M H3B03 solution. A series of multielement standard solutions was also prepared for the AA analysis using boric acid as the solution matrix. Gas-Solid Reaction. The nodules were allowed to react with SO2 in a temperature range between 25 and 95 "C by using both a flow-through gas system and a standard all-glass high vacuum line. In the flow-through system SO2 gas a t a flow rate of 15 mL/min was introduced into a glass column containing ground nodules that were mixed with varying quantities of water until the aqueous phase was saturated. Dehydrated manganese nodules were obtained by heating overnight in an oven a t 450 "C. For the reaction carried out in the presence of oxygen, gaseous SO2 was mixed with 0 2 in a 1:l (v/v) ratio prior to introduction into the reaction system. After completion of the reaction the water was evaporated (filtered in the case of a thin slurry) and the resulting solids were dried in a vacuum desiccator prior to AA and IR analysis. In a typical experiment conducted on the vacuum line, a weighed sample (0.3 g) of air-dried manganese nodules was 0013-936X/78/0912-1428$01 .OO/O
@
1978 American Chemical Society
1 1300
I200
1100
9 00
1000
I 1300
Flgure 1. IR spectra of dithionates (A) Na2S20e;(E) dithionate. Extracted and crystallized as the Ba salt from SOp-treatedgoethite, FeO(OH),at 25 O C
I
1 1200
1100
9 00
1000
Figure 2. The effect of water on the formation of dithionates at 25 OC through treatment with SO2 (A) Dehydrated ferromanganese nodules; (B) ferromanganese nodules with 30 % w/w water; (C) ferrornanganese nodules in the presence of excess water
Table 1. Atomic Absorption Analysis of Major Elements in Pacific Ferromanganese Nodules
a
concn, % w/wa
AI Ba Ti Mg Ni co cu
1.88 0.34 0.68 1.76 1.04 0.18 0.67
element
Mn Fe Ca Si Y Na
K
900
Figure 3. The effect of temDerature on the dithionate formation Feyromanganese nodules treated with SO2 in the presence of water at: (A) 95 OC,(6) 75 O C ,
1300
(C)25 O C
I200 WAVE NUMB ER
WAVENUMBER ( c d ' )
eIement
1000
WAVE NUMB E R ( c m-1)
WAVENUMBER I c m - ' )
1300
1100
1200
concn, % w/wa
21.00 8.37 1.25 9.72 0.49 1.88 0.88
Average of five replicates.
placed in a reaction tube and slurried with about 10 mL of water. A sufficient amount of gaseous SO2 or the mixture of SO2 and 0 2 was introduced to saturate the slurry and to produce a partial pressure of about 400 Torr. The mixture was stirred frequently with a magnetic stirrer. After the reaction was completed, the tube was removed from the vacuum line and the solid residues were separated from the aqueous phase by filtration. Helium was passed through the solution to remove the excess S02. The solution was made up to 1 L using deionized and distilled water and the dissolved metal concentrations were determined by AA analysis. In some cases the aqueous phase was tested for oxidation states of the metal ions by standard qualitative analysis methods. Evaporation of a portion of solution to which saturated Ba(OH)2 was added
I100
1000
900
( E m-1)
Figure 4. The effect of temperature and oxygen on dithionate formation Ferromanganese nodules treated with a gaseous mixture of SO2 and 02 in the presence of water at (A) 95 O C , (E) 75 O C , (C) 25 OC
yielded a solid residue which was used for IR analysis. All chemicals, reagents, and gases were of analytical reagent quality. Infrared spectra were obtained by using a Beckman IR 12 spectrometer equipped with a dry-air purging system. Samples examined were pulverized, diluted to about 0.5% (w/w) with KBr, and pressed into pellets with a Carver Press. The relative sulfate-dithionate concentrations were estimated from spectra obtained from MnSO4-Na2S206 mixtures of known composition. The SOZ-treated nodule samples were also used for metal extraction; water was added to weighed portions of the treated nodules and the mixture stirred vigorously for 3 h a t 95 "C. The filtered leachate cooled to room temperature was analyzed for metals by atomic absorption.
Results and Discussion The results of the analysis of the manganese nodules for the major elements are given in Table I and provide base line data for subsequent recovery studies. Representative IR spectra of water-soluble components obtained after treating Pacific ferromanganese nodules with SO2 under varying conditions are shown in Figures 1-4, and the results are summarized in Table 11. The spectra are restricted to the region between 900 and 1350 cm-1, where characteristic S-0 stretching vibrations occur (12). Infrared spectra of authentic Na2S2Os and of BaSz06,obtained from the reduction of goethite, FeO(OH), by SO2followed by precipitation with Ba2+,are also included in Figure 1for comparison purposes. Table I11 furnishes analytical data on the five metallic components in the manganese nodules that were extracted following treatment with SO2 Volume 12, Number 13, December 1978
1429
Table II. Factors Affecting the Formation of Dithionates in the Low-Temperature Reaction of Sulfur Dioxide with Ferromanganese Nodules temp, "C
estd yleld 01 dlthlonates, mol % a
25 75 95
54 22 7
25 75 95
21 8 trace
SOn without On dehydrated at 450 "C before gas absorption
25 75 95
none detected none detected none detected
SOn mixed with O2 dehydrated at 450 O C before gas absorption
25 75 95
none detected none detected none detected
sample treatment
SO2 without On aqueous slurry
SOn mixed with aqueous slurry
O2
a Percent values were obtained by assigning 100 for the sum of S042- and S2062- Comparison was made with the spectra obtained from synthetic mixtures of MnS04and Na2S2O6of known mole ratio
under varying conditions. Inspection of the data indicates that when the manganese nodules are treated with aqueous SO2 solutions a t low temperatures, substantial amounts of both sulfates and dithionates are produced, with the anion product ratios seeming to depend upon oxygen, water, and the reaction temperature. An extensive study has been reported on the reaction of aqueous SO2 solutions with the metal oxides (10) which are present as major components in the manganese nodules. Both iron and manganese are capable of oxidizing S02. In the case of MnO2, reaction seems to occur at the surface of the solid and produces a mixture of manganese sulfate and dithionate. The proportion of the two anions depends on such factors as the temperature, pH of the solution, and the mineral phase of the MnO2. At room temperature, using saturated SO2 solutions, the proportion of S2062-varies from about 30% for reactions of amorphous MnO2 to above 95% for P-Mn02. The welldefined mineral phases and distinctly crystalline MnO2 invariably produce in excess of 60% dithionate. Ferric ion also oxidizes SO2 (10).With this ion, however, reaction occurs in the solution phase, presumably between an iron-sulfur dioxide or iron-sulfite complex. The proportion of dithionate formed under conditions employed in this study a t ambient temperatures is in excess of 95% and is not dependent upon the nature of the oxide in which Fe3+ was originally present. As shown in Figure 1, FeO(0H) slurried with water (0.3 g/15 mL) produces dithionate alone, not sulfate, upon interacting with SO2 in the absence of 0 2 since the characteristic S-0 stretching modes for S042-in the region between 1100 and 1150 cm-l were absent. Similar results were obtained from hematite, a-Fe203. The cobalt(II1) ion, a minor
component of the manganese nodules, is also capable of producing dithionate, but its contribution would be minor in comparison to those from the major metallic species. The presence of oxygen drastically alters the reaction of SO2 with the nodules in ways which have not previously been appreciated ( 3 , 4 ) .As can be seen in Figures 3 and 4, the spectra obtained from the nodules that were treated with SO2 in the presence of 02 invariably contain far weaker S2062-bands than those from analogous reactions carried out with the exclusion of 0 2 . Apparently the presence of 0 2 enhances sulfate formation a t the expense of dithionate production. This is generally true when the reaction is carried out in the presence of water. Oxygen, itself, can oxidize SO:! to both Sod2- and &Os2in solution. This oxidation is affected by metal ions (10,13), the presence of which influences both rates of reaction and the proportions of sulfate and dithionate which form. Among the ions which are present in or could be obtained from those originally in nodules, manganous ion has by far the greatest catalytic effect on SO2 oxidation. The Mn2+ catalyzes the oxidation of SO2 to but does not greatly affect the rate of formation of S2062- with the result that in the presence of Mn2+ more than 95% of any SO2 which is oxidized by 0 2 is converted to S042- (10). Clearly such an oxidation occurs when slurries of manganese nodules react with SO2 since increased amounts of S042-are produced when 0 2 is present. Water also plays a critical role in the formation of S2062-. When the nodules are dehydrated at 450 "C and then treated with SO2 a t room temperature, regardless of whether or not oxygen is present, only S042-and some unreacted starting material can be detected by IR spectroscopy as shown in Figure 2 . With air-dried ferromanganese nodules which contain approximately 30% (w/w) of water about 5 mol % of dithionate could be detected after treatment with SO2 a t room temperature. Finally, if an aqueous slurry was treated with SO2 at the same temperature in the absence of 0 2 , dithionate amounted to more than 50%of the reaction products (Table 11). The influence of temperature on the proportions of S042and S2062- is predictable. The dissolution of SO2 in water creates an acidic solution. Dithionic acid is known to be unstable a t high temperature (10) and the decomposition is significant above 50 OC: Accordingly, the amount of S2OS2-detected in reactions run a t high temperatures is less than in those conducted a t low temperatures. As mentioned above, a t room temperature about 50% of the reaction products can be attributed to the formation of dithionates. The proportion of S2062-decreases with an increase in the reaction temperature and a t 95 "C it is reduced to less than 10%. Under anaerobic conditions, the oxidation of aqueous SO2
Table 111. Metals Extracted from Pacific Ferromanganese Nodules through SOz Treatment at Low Temperatures metals extracted, % w/w temp,
0 2 flow rate,
sample treatment
OC
mL/mln
Mn
Fe
Ni
pass-through gas system reaction time: 2 h SOn flow: 15 mL/min total gas flow: 50 mL/min adjusted with He as carrier
25
0 15 0 15 0 15
77.3 69.6 84.5 72.5 80.8 65.5 75.8 61.1
73.0 90.0 80.0 83.0 90.0 90.0 81.0 79.0
54.8 5.3
52.5 51.4
vacuum system reaction time: 3 days reaction time: 1 h
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Environmental Science & Technology
95
0 15
92.6 98.2 95.4 98.7 92.9 98.2 96.2 97.4
25 25
0 0
97.3 100
50 75
co
cu
52.0 67.3 70.5 77.9 77.9 71.0 64.1 73.7
72.5 83.3 83.3 81.2 85.4 85.4 83.3 74.3
100 96
100 100
by ferromanganese nodules always produces significant - . the Pacific Ocean amounts of both S042- and S Z O ~ ~Since nodules employed in this study contain manganese and iron in about a 2:l ratio, and because the reaction of Fe3+ with aqueous SO2 should produce almost no sulfate (IO),a significant proportion of the SO2 oxidized by the manganese must have been convertdd to S042-.As mentioned above, however, only amorphous MnOz produces substantial quantities of S042- (10). Therefore, it is likely that the MnO2 phase in the manganese nodules should consist largely of a poorly crystalline oxide. The terrestrial todorokite and birnessite are generally accepted as the manganese oxide minerals in the nodules ( I ) , and these minerals do not exhibit short-range order in their X-ray patterns. Thus, they are most likely akin to the “amorphous” MnO2 mentioned in the study of the S02-Mn02 reaction (10). From the standpoint of devising a method by which various metals can be effectively separated, it is most important to note that the formation of water-soluble salts is affected by the length of reaction time and the condition under which the reaction is carried out. When the nodules are allowed to react with SO2 in an aqueous slurry for several days under anaerobic conditions, both iron and manganese can be extracted nearly quantitatively into the aqueous phase as Fe2+ and Mn2+. If O2 is present both metals can still be found in the aqueous solution, but in this case iron exists as Fe3+. If, however, the SOz reaction is carried out for a shorter period of time, the amount of soluble iron decreases, and in reactions which are allowed to proceed for less than an hour, the level of iron detected in the aqueous phase is negligible, while the manganese is still almost totally converted to a soluble form (Table 111). In fact, as shown in Table 111, all five metals, Mn, Fe, Co, Ni, and Cu, can be extracted from ferromanganese nodules by treatment with SO2 at low temperatures (
