Refinement of low-grade clay by microbial removal of sulfur and iron compounds using Thiobacillus ferrooxidans

JOURNALOF FERMENTATION ANDBIOENGINEERING Vol. 80, No. 1, 46-52. 1995

Refinement of Low-Grade Clay by Microbial Removal of Sulfur and Iron Compounds Using Thiobacillus ferrooxidans HEE WOOK RYU,’ KYEOUNG

SUK CH0,2 YONG KEUN CHANG,‘* AND TADAHIRO MOR13

SANG DONE KIM,’

Department of Chemical Engineering and BioProcess Engineering Research Center, Korea Advanced Institute of Science and Technology, 373-l Kusong-dong, Yusong-gu, Taejon 305-701, Korea,’ Department of Environmental Engineering, Ewha Womans University, 11-I Daehyun Dong, Seodaemun-gu, Seoul 120-750, Korea,2 and Laboratory of Environmental Biotechnology, Faculty of Agriculture, Shimane University, 1060 Nishikawatsu, Matsue, Shimane 690, Japan’ Received 2 December 1994/Accepted 8 March 1995

The refinement of low-grade clay, of which impurities are mainly sulfur and iron compounds, is required because of the recent shortage of high-grade clay for manufacturing of structural ceramics. The major impurity compound contained in the low-grade clay we treated was identified as pyrite by X-ray powder diffraction and inductively coupled plasma analyses. The well-formed crystals of pyrite had a framboidal form of 1 pm-20 pm diameter. The microbial removal of pyrite from the low-grade clay was investigated by using a sulfur and iron-oxidizing bacterium, Thiobacillus ferrooxidans. About 8290% of the pyrite was removed in 5-12 d for pulp densities up to 70% (w/v). The removal rate of pyrite ranged from 270 to 914 mg-pyritic sulfur/Z. d depending upon clay pulp density. The rate of pyrite removal (r) could be expressed as a function of pyritic sulfur concentration (S): r (mg-pyrltic sulfur/Z* h) = 1.96 x lo-* S (mg-pyrltic sulfur/l). The logarithm of the amount of oxidized pyrite per unit volume and the final pH in the reaction medium were found to have a linear relationship which could be expressed as pH = 2.43-0.55 log [Fe!%(mM)l. With the refined clay no red color due to the presence of pyrite was developed after firing, and its whiteness was similar to that of a high-grade clay. [Key words: low-grade clay, microbial refinement,

Thiobacillusferrooxiduns] their use as alternative resources to the high-grade ones. To remove sulfur and iron compounds and other impurities from raw materials, physical and chemical methods such as electrophoresis, magnetic separation, alkali and acid treatment, and Cl2 treatments are generally used (6-11). However, because of technological and economic disadvantages of the physical and chemical methods, microbial leaching should be considered as an alternative method. Microbial leaching is thought to be less energy-intensive than high-temperature chemical processes, and requires low capital and operating costs. Thiobacillus ferrooxidans , a chemoautotrophic and acidophilic (pH=2-4) bacterium can oxidize both sulfur and iron compounds. This bacterium has been widely used in the field of desulfurization of coal and leaching of ores such as copper and uranium (12-18). In this study, sulfur and iron compounds in the lowgrade clay were characterized, and the microbial removal of them was carried out using T. ferrooxidans.

The use of clay minerals has become diverse with increasing industrial applications. Clay minerals are essential resources in porcelain and pottery manufacturing, production of construction materials, nuclear waste treatment, and chemical, electric, steel and petrochemical industries. In various applications of clay minerals, their quality is very important. As is the case for other natural resources, however, reserves of high-grade clays are almost exhausted due to long term mining. For example, Sekishu roofing tile (Sekishu Kawara) is a well-known structural ceramic, and it has been used in building traditional Japanese houses. The high-grade clay from the Yusunotsu deposit in Etsu City, Shimane, has been used as the raw material for manufacturing Sekishu Kawara since more than 400 years ago. The consumption of the high-grade clay is 70,000 t/year, and its reserve is expected to be exhausted in 10 years from now. A large amount of low-grade marine clay which contains sulfur and iron compounds as impurities is deposited in the Yusunotsu deposit also. The low-grade clay from the Yusunotsu deposit can be utilized as an alternative resource for manufacturing Sekishu Kawara. However, when the low-grade clay was used to manufacture the roofing tile, a number of serious problems occurred: red color or black core development in the roofing tile, reduction of refractoriness by iron oxides, air pollution, corrosion of tile kilns, and formation of cracks due to SO2 production during the firing of the roofing tile. Some researchers of other countries have also demonstrated that sulfur and iron compounds in ceramic raw materials caused similar problems (l-5). For these reasons, the refinement of low-grade clays is required for

MATERIALS

AND METHODS

Clay samples Low-grade clay samples were taken from the marine clay layers of Yusunotsu deposit in Kuromatsu, Etsu City, Shimane, Japan. High-grade clay samples were provided by the Institute of Industrial Science and Technology, Shimane. The low-grade clay samples were crushed into pieces (smaller than 5 mm) after drying at room temperature before being used for microbial refinement experiments. Microorganism and media T. ferrooxidans (ATCC 19859) was employed to refine the low-grade clay in shake flasks. Silverman’s 9K medium was used for the seed culture (19). The 9K medium consisted of mineral

* Corresponding author. 46

VOL. go, 1995

REMOVAL OF SULFUR AND IRON COMPOUNDS FROM CLAY

salt medium (MS medium) and 45 g/l of FeS04.7Hz0 as the energy source. The MS medium has the following composition (per liter): (NH&S04, 3.0 g; K2HP04, 0.5 g; KCl, 0.1 g; MgS04.7Hz0, 0.5 g; Ca(NO&, 0.01 g. The pH of the MS medium was adjusted to 2.0 with 0.1 N HzSO+ Cells, grown in a 10 1 bottle under forced aeration for 2-3 d at room temperature, were harvested by centrifugation at 8,000 x g for 15 min. The concentrated cells were adapted to pyrite prior to inoculation. In this adaptation process, 5 g/l of pyrite particles was added to 1OOml of the MS medium. The cells were assumed to be fully adapted to pyrite after incubation for 14 d. The pyrite particles were separated from the liquid medium by aseptic centrifugation at 1,500 x g for 5 min. The supernatant was again centrifuged at 8,000 x g for I5 min for harvesting cells. The harvested cells were suspended in the MS medium and were used for inoculation. Experiments Microbial experiments refinement were carried out in 250ml shake flasks. The flasks were charged with 1OOml of the MS medium and a desired pulp density of the clay. The initial pH of the medium was adjusted to 2.0 using 0.1 N H$O.,. The medium was inoculated with 5 ml of the pyrite-adapted cells (lo9 cells/ml). An uninoculated flask containing 100 ml of the MS medium and 20 g of sterilized clay was used as the control. The flasks were incubated at 28°C and 200rpm on a rotary shaker. The pH and redox potential in the reaction medium were monitored, and 1 ml sample from each flask was taken every 24 h for ferric and ferrous iron analyses. To examine the refinement of the clay by the inherent microorganisms in the clay, experiments were also carried out without inoculation. The effect of the adaptation of T. ferrooxidans to pyrite was evaluated by comparing the rate of pyrite removal by pyrite-unadapted cells to that by pyrite-adapted cells. Using distilled water instead of the MS medium, the effect of mineral salts on the refinement was examined. Shake-flask experiments were carried out in clay pulp densities ranging from 5% (w/v) to 70% (w/v). The pulp density was defined as weight of clay per unit volume of liquid medium; 70% (w/v), for example, means 700g of air-dried clay mixed with 11 of liquid medium. Analytical methods Clay powder samples were examined using X-ray powder diffraction (Rigaku goniometer, CuKa radiation, Tokyo) to identify sulfur and iron compounds. The clay samples were also observed with a scanning electron microscope (Hitachi S-2100, Ibaraki) equipped with an energy-dispersion system (Horiba EMAX 3000, Tokyo). The total elemental analyses, including sulfur and iron, of the clay samples were made using inductively coupled plasma analysis (ICP, Shimadzu ICPS-2000, Kyoto). Pyritic sulfur and iron in the clay samples were also quantified by using ICP after HN03 extraction of the samples (20). Samples from the shake flasks were filtered through Whatman no. 2 filter paper to remove clay particles from the broth. The filtrated clay particles were washed with 5 ml of 0.1 N HCl to recover ferric and ferrous iron adsorbed on them. Pyritic sulfur removal from the clay was followed by measurement of the amount of total iron (Fez+ and Fe3+) released as a result of microbial pyrite oxidation. The amount of total iron in the liquid filtrate was measured with a calorimeter; 1 ml of 1% hydroquinone was added to 1 ml of diluted filtrate to reduce ferric iron into ferrous iron and the total ferrous

47

iron concentration was determined by the o-phenanthroline method (21). Sulfur removal was calculated using the stoichiometric relationship between the sulfur and iron contents of pyrite (SpyriJFepyrite= 1.148). The refined clay was washed with 0.1 N HCl, and filtered through Whatman no. 2 filter paper by centrifugation at 8,000 x g for 10 min. The filtrated clay was rewashed with distilled water, and then fired at 1,200”C for 2 h. After cooling to room temperature, the chromaticity of the clay was measured with a calorimeter (ND3OOA,Rigaku goniometer, Tokyo). RESULTS AND DISCUSSION Identification low-grade clay

of sulfur

and iron compounds

in the

Figure 1 shows the X-ray powder diffraction (XRD) patterns of the high-grade clay which is presently used to manufacture structural ceramics (Fig. la) and the low-grade clay mined from the marine clay layer (Fig. lb). The presence of feldspar (F) and kaolinite (K) in both samples is shown. The peaks at 14” and 28” are characteristic of feldspar and kaolinite, respectively. However, the peaks at 33”, 37”, 41”, 48”, and 57” (marked with v) were observed only in the lowgrade clay sample. These peaks were attributed to pyrite. To identify these peaks in detail, the low-grade clay was examined by XRD after treatment with HCl and HF to remove feldspar and kaolinite. It was found that the three peaks at 33”, 37”, and 57” were characteristic of pyrite (P; Fig. lc). Table I shows the elemental compositions of the lowgrade and the high-grade clay samples. The sulfur content in the low-grade clay was 17.35 mg/g-clay, which

p &lbl!L P

P

FIG. 1. X-ray diffractograms of clay samples. (a) High-grade clay, (b) low-grade clay before HCI and HF treatment, (c) low-grade clay after HCI and HF treatment, K: kaolinite, F: feldspar, P: pyrite.

48

J. FERMENT. BIOENG.,

RYU ET AL. TABLE 1.

Compositions

of the low-grade clay and the high grade clay

Low-grade clay (marine clay)

High-grade clay

Total sulfur” Sulfate sulfur Pyritic sulfur

17.35 4.61 (26.6%) 12.74 (73.4%)

0.16;2?0%) 0.48 (75.0%)

Total irona Non-pyritic iron Pyritic iron

14.8 3.66 (24.7%) 11.14 (75.3%)

0.42

Pyrite=

23.88

0.9

Proximate analysis Moisture Volatile matterb

(wt%) 10.6 1.5

Mineral analysis Al K Ti Si Na Mg N

(g/l00 g dry clay) 10.6 2.3 0.8 30.9 0.8 0.2 2.3

a mg/g-clay. b Dry basis.

was about 27-fold that in the high-grade one. About 73% of the total sulfur in the low-grade clay was pyritic sulfur, and the remaining portion was water-soluble sulfate. Figure 2 shows scanning electron micrographs of pyrite crystals in the low-grade clay, and its elemental composition determined by energy-dispersion analysis. The well-formed crystals of pyrite were mainly observed in fossil diatoms (Fig. 2a). The pyrite crystals had a framboidal morphology, and their diameters ranged from 1 pm to 20 /*m. The pyrite crystals contained high concentrations of S and Fe, and traces of Al and Si. The molar ratio of Fe to S was found to be 0.5 (Fig. 2~). When HCl was added into the low-grade clay, no hydrogen sulfide (H2S) was generated (data not shown). This indicated that no amorphous sulfur or iron compounds such as FeS were present in the low-grade clay. From the results described above, the major sulfur and iron compounds in the low-grade were confirmed to be pyrite. From Fig. 2c consistent with Fig. 2a, the pyrite in the low-grade clay from the marine clay layer was considered to have been formed by biological reactions (2224). Under anaerobic conditions, sulfate-reducing bacteria utilize organic compounds as carbon sources, which are present inside of dead diatoms in sediment of the sea, and reduce sulfate to sulfide to obtain energy for growth and maintenance. Then, the biogenic sulfide is precipitated as FeS through a reaction with ferrous ion. The resulting amorphous FeS crystallizes to pyrite through a reaction with elemental sulfur. Pyrite in clay is an impurity which induces the formation of black cores and SO* in firing of ceramics, and its existence in shale causes iron staining of shale bricks (1, 4). Microbial refinement of the low-grade clay The rates of pyrite removal Effect of inoculation observed in shake-flask experiments are shown in Fig. 3. The initial iron concentration in the medium is due to the contribution of soluble iron compounds in the clay. The concentrations of leached sulfur from pyrite were

(Cl t

r I

Fe \ 5.12

0.00

Energy

(KeV)

FIG. 2. Scanning electron micrographs of (a) pyrite crystals inside a fossil diatom in the low-grade clay, (b) free pyrite crystal in the low-grade clay, and (c) its elemental composition by energy dispersion analysis. from the leached iron concentrations in the medium. In the shake flasks inoculated with T. ferrooxidans, removal of large amount of pyrite was observed. No removal of pyrite was observed in the uninoculated flask containing sterilized marine clay. In the uninoculated flask containing unsterilized clay, the removal of pyrite began to be observed after a 4 d of lag time. These results indicated the existence of inherent pyrite-oxidizing microorganisms in the marine clay. No further removal of pyritic sulfur was observed after 7 d by the pyrite-adapted cells, after 10 d by pyriteunadapted cells, and after 15 d by the inherent microorganisms in the low-grade clay. Approximately 87% of pyrite was removed by the pyrite-adapted cells, but only 76% by the pyrite-unadapted cells and the inherent microorganisms in the low-grade clay. From the slopes calculated

reaction

VOL.

REMOVAL OF SULFUR AND IRON COMPOUNDS FROM CLAY

80, 1995

4

0

8

12

16

20

Time (d‘, FIG. 3. Effect of microorganism inoculation (pulp density= 20%). (0) Uninoculated flask containing sterilized clay (control); (0) uninoculated flask containing unsterilized clay with inherent bacteria; (v) flask containing unsterilized clay inoculated with pyriteunadapted T. ferrooxidans; (7) flask containing unsterilized clay inoculated with pyrite-adapted T. ferrooxidans. in Fig. 3, the following maximum pyrite removal rates were calculated: 192 mg-S/I ‘d for the clay without inoculation, 440 mg-S/f. d for that inoculated with pyriteunadapted cells, and 637 mg-S/I.d for that inoculated with pyrite-adapted cells. These results demonstrate that the use of concentrated and preadapted cells as an inoculum enhances the rate and extent of pyrite removal. In microbial removal Effect of HCI pretreatment of pyrite from coal, the activity of microorganisms has been reported to be inhibited by heavy metals and other organic compounds contained in coal (13, 14, 23). These inhibitors can be removed from the coal by pretreatment with acid. An experiment was carried out after washing the low-grade clay with 0.1 N HCl, and the rate of pyrite removal was compared with that obtained without HCl pretreatment (data not shown). Similar maximum rates of pyrite removal were obtained: 743 mg-S/I-d and

49

752 mg-S/I.d for the untreated and the treated clays, respectively. This result reflected well the negligible amount of heavy metals and low content of volatile matter in the clay as shown in Table 1. Some mineral salts, such as E#ect of mineral salts ammonium sulfate, potassium phosphate, magnesium sulfate, and calcium nitrate are essential for microbial growth. In most published reports on T. ferrooxidans, the MS medium mentioned in Materials and Methods was considered an adequate nutrient supplement for microbial oxidation of pyrite (3). Various mineral salts were contained in the low-grade clay as shown in Table 1, and to investigate whether these salts could be used as nutrients for growth of T. ferrooxidans, the removal of pyrite from the low-grade clay in distilled water instead of the MS medium was investigated (Fig. 4). At 20% (w/v) pulp density, the pyrite removal based on the amount of iron liberated in both the MS medium and distilled water was about 93-97X in 8 d. At 40% (w/v) pulp density, the pyrite removal was about 96% in 12 d in the MS medium, and about 89% in distilled water. The maximum rate of pyrite removal, which was calculated from the concentration of leached iron, was higher in the MS medium than in distilled water: 743 mg-S/I.d in the MS medium and 457 mg-S/Z.d in distilled water at

10

6

9

Time (d) 100 80 s g 60 .L-, B ” 40

1.8 g 1.6

A 20 0 !Y 0

2

4

6

8

10

12

14

Time (d) FIG. 4. Effect of nutrient salt addition on pyrite removal. No addition of nutrient salts: ( 0) 20% pulp density, ( 0 ) 40% pulp density, addition of nutrient salts, (0) 20% pulp density, ( n ) 40% pulp density.

0

3

6

9

12

15

Time (d) FIG. 5. Profiles of (a) concentrations

of leached iron and (b) pH in the culture broth at different clay pulp densities. ( 0 ) 5%; ( l ) 10%; (0 ) 20%; ( n ) 30%; (V) 50%; (V) 70%.

J. FERMENT.BIOENG.,

RYU ET AL.

50

20% (w/v) pulp density and 857 mg-S/I.d in the MS medium and 560 mg-S/I.d in distilled water at 40% (w/v) pulp density. Although the removal rates with distilled water were lower than those with the MS medium, it should be noted that pyrite was removed without the need for mineral salt supplementation. This result implies that the nutrients necessary for the growth of T. ferrooxidans were present in the marine clay itself. Generally, the rate of pyrite oxidation by T. ferrooxidans increases with increasing concentration of mineral salts. However, an insoluble sulfate compound such as jarosite is formed when the concentration of mineral salts exceeds a certain level (25). The precipitated jarosite on the clay surface inhibits the access of bacteria, O2 and COZ transfer, and adsorption of cells to reaction sites (pyrite surface). This can significantly reduce the rate and extent of pyrite removal. Therefore, a moderate mineral salts concentration is preferred to suppress jarosite precipitation and thus to maintain a high rate of pyrite oxidation. The clay pulp density Eflect of clay pulp density affects slurry mixing and the rate of pyrite removal. It has a close relationship with operating cost of the refinement process. To evaluate the effect of clay pulp density on the removal of pyrite, experiments were carried out for various pulp densities. Figure 5 shows the concentrations of leached iron and the pH values in the culture broth for pulp densities from 5% to 70%. As a representative example, the changes in pH, concentration of leached iron, and rate of pyrite removal at 70% (w/v) pulp density are shown in Fig. 6. The concentration of leached iron increased with increasing pulp density of the clay (Fig. 5a). No increase of the concentration of leached iron was observed after incubation for 5-12 d. This indicated that pyrite removal from the clay terminated within 5-12 d depending on the clay pulp density. For the range of the pulp densities tested, 82-90% of the pyrite was removed. In the case of pyrite removal from coal, the lower sulfur removal rates and extents at higher pulp densities may be due to the limited supply of gaseous nutrients (0, and COs), the agglomeration of coal particles, and inhibitory effects of some organic compounds solubilized from the coal (13). Because the extent of microbial removal of pyrite from coal can be highly obtained only for a low pulp density of below 30% (w/v), the practical applications of microbial coal 20

100

1

18

r=x=kS

where r is reaction rate of pyritic sulfur (mg-pyritic sulfur/l.h), k rate constant (h-l), and S concentration of pyritic sulfur in the liquid medium (mg-pyritic sulfur/[). In this study also, the rate of pyrite oxidation could be represented by Eq. 1 and the rate constant was calculated to be 1.96 x lo-* h-l. Reported values of k to date for, in most cases, coals are in the range of 2 x 10-T-6 x 10e3 hh’. We consider the reason for the high k value in this study to be that the clay we treated contained a relatively low level of inhibitory organic compounds, Relationship between the amount of oxidized pyrite As already shown in Fig. 5b, the pH values and pH

in the culture broth sharply declined when the oxidation of pyrite by the cells occurred at a high rate after 2 d, but no significant changes of pH values were observed in the stationary phase. This result suggested that there might be a quantitative relationship between the amount of pyrite oxidized and the pH. The overall reaction of pyrite oxidation is summarized as follows. 2FeS2+ 7.502 + 2H20 ---fFe2(S0J3 + HzS04 (2) Since 1 mol of pyrite is oxidized to produce 1 mol of 2soo

5.0

- 4.5

s

16

80 8

‘;;

14

T

2ooo

- 4.0

g - 3.5 z E3 a 2 - 3.0 4o ‘8 w - 2.5 PI

6 4

20 0

0

2

4

6

8

10

12

0 14

Removal of pyrite at 70% pulp

a

1500 G 3 1000 "

2.0

0

I

500

1.0

I

0

0

0

::-:I

10 20

I

I

I

1

30

40

50

60

,o”

Pulp density (% w/v)

Time (d) FIG. 6.

a

5 2

0

1.5

2

0

0

(j”

*& 12 a r” 10 8 s 8

0

desulfurization have been limited due to the necessity of large reactors (13, 23). However, in this study, high removal rates were obtained even for 70% (w/v) pulp density. This is attributed to the fact that the concentration of organic compounds in the low-grade clay was only 1.5% (w/w) as shown in Table 1. The effect of pulp density on the rate of pyrite removal is shown in Fig. 7. The maximum rates were calculated from the slopes of the curves in Fig. 5a. The maximum rate (rmax) per unit weight of clay (2.510.1 mg-Fe&/g-clay-d) linearly decreased with increasing pulp density. On the other hand, the volumetric removal rate (the maximum removal rate per unit volume of the reactor) increased almost linearly with the pulp density up to 30% w/v, and then slightly decreased. It ranged from 506 to 1,714 mg-Fe&/l-d. The following expression for rate exhibiting first-order reaction kinetics which depends only on the pyrite concentration has been proposed for pyrite oxidation (26, 27): dS

density.

FIG. 7.

Effect of pulp density on the rate of pyrite removal.

REMOVAL OF SULFUR AND IRON COMPOUNDS FROM CLAY

VOL. 80, 1995

51

(a)

l l l

I -

E t

\

l

1.2

1.0

/

t

1.4

1.6

I 1.8

2.0

PH FIG. 8. Relationship between pH and the amount of oxidized pyrite. (0) Experimental data; (-) Eq. 5.

hydrogen ion (H+), the concentration pressed as follows:

of H+ can be ex-

[H+l = W+lo+ [F&loxi~ized

(3)

where [Hflo is the initial concentration of H+, and [FeS&,xidized the amount of oxidized pyrite per unit volume. From Eq. 3, the pH value in the reaction solution can be expressed as PH= -log[H+l=

-bdW+lo+

[F&loxiccied (4)

By linear approximation of Eq. 4, Eq. 5 can be obtained and used for calculating pH with an acceptable erro range over a relatively small range of pH, e.g., 1.2-2.0 as in this study. pH = A-K log [Fe$]

(5)

where A and K are constants. Such a linear relationship can be observed on the data plotted in Fig. 8. From the intercept and slope in Fig. 8, A and K were found to be 2.43 and 0.55mM-l, respectively. The progress of pyrite oxidation could be monitored simply from the pH in the reaction solution by using Eq. 5 without measuring the concentration of leached iron. Figure 9 shows Characteristics of the refined clay X-ray powder diffractograms of the clay samples before

2

FIG. 9. X-ray powder diffractograms of low-grade clay samples. (a) Before microbial refinement; (b) after refinement. P, Pyrite.

and after microbial refinement by T. ferrooxidans. The clay samples were treated with HCI and HF before the analysis to remove kaolinite and feldspar. Only a small amount of pyrite was detected in the refined clay. The color of clay is very important in manufacturing ceramics. Generally, a high-grade clay is required to have a high whiteness after firing. Otherwise the desired color of ceramics cannot be easily obtained even after an elaborate coloring process (1, 2, 5). If clay has a strong red color, it is generally used only to produce low-grade ceramics such as red bricks. In the present study, after firing at 1,200”C for 2 h, the refined clay had not turned red, whereas the raw clay had turned red due to the presence of iron oxides produced from pyrite. Figure 10 shows the chromaticity diagram of the clay samples before and after refinement. As a control, the chromaticity of the high-grade clay which is currently used for Sekishu Kawara production is also shown in Fig. 10. The high-grade clay has a high whiteness (63.68), and a low redness (4.2), whereas the unrefined low-grade clay has a low whiteness (31.96) and a high redness (7.17). The refined clay has a similar whiteness (63.3) and lower redness (- 1.53) compared with the high-grade one. This result suggests that the quality of a low-grade clay con-

(cl Brightness

Brightness

Brightness

100

Degrees 28 CuKa

@I

(4

80

60

40

20

Whiteness .... j! 65 633.ji]j;jj;

7.5

a.3

Redness

_, m 7.5 Redness

.::q 8.6

Yellowness

Yellowness

Yellowness

FIG. 10. Chromaticity diagrams of clay samples after firing. (a) High-grade clay, (b) low-grade clay before microbial refinement, and (c) low-grade clay after microbial refinement.

52

J . FERMENT,BIOENG.,

RYU ET AL.

taining pyrite as an impurity can be improved to that of a high-grade clay by microbial refinement, and that the refined clay can be used as an alternative resource to high-grade clay. REFERENCES 1. Ford, R. W., Littler, C. D., and West, H. W. H.: Iron staining of shale bricks. Br. Ceram. Trans. J., 86, 3-29 (1987). 2. Povlov, V. F. and Meshcheryakova, V.: Reducing the coloring effects of iron oxides in porcelain bodies. Glass and Ceramics, 40, 50-152 (1983). 3. Ratzenberger, H.: The influence of the mineralogical composition of structural ceramics and heavy clay materials on kiln scumming and efflorescence. Ziegelind Int., 41, 99-105 (1988). 4. Schneider, J. W. and Schneider, K.: Indirect method for the determination of pyrite in clays and shales after selective extraction with acid solutions. Am. Ceram. Sot. Bull., 69, 107-109 (1990). 5. Stepkowska, E. T. and Jefferis, S. A.: Influence of microstructure on firing color of clays. Appl. Clay Sci., 6, 319-342 (1992). Ido, T.: Powder engineering. Asakurashoten, Tokyo (1965). Inoue, K. and Yoshida, A.: Iron leaching of Shirasu by acid treatment. J. Ceram. Sot. Japan, 92, 520-524 (1984). Japan Ceramic Society: Ceramic handbook. Giboudou, Tokyo (1969). Klmura, K. and Tateyama, H.: Refinement of the low-grade Amakusa pottery stone by hydrothermal treatment. J. Ceram. Sot. Japan, 97, 439-446 (1989). 10. Otsuka, N., Hayashi, T., Okanishi, K., and Shiraki, Y.: The removal of iron oxide from clay by sodium dithionite-sulfuric acid system (III). Nendo Kagaku, 14, 45-57 (1974). 11. Suzuki, T. and Tomizald, S.: Study on the removal of iron from ceramic materials by chloride treatment. Kyogyokagaku, 57, 29-31 (1954). 12. Andrews, G. F. and Maczuga, J.: Bacterial coal desulfurization. Biotechnol. Bioeng. Symp., 12, 337-348 (1982). 13. Andrews, G. F., Darroch, M., and Hansson, T.: Bacterial removal of pyrite from concentrated coal slurries. Biotechnol. Bioeng., 32, 813-820 (1988). 14. Chandra, D. and Mishira, A. K.: Removal of sulfur from assam coals by bacterial means, p. 632-652. In Wise, D. L. (ed.), Bioprocessing and biotreatment of coal. Marcel Dekker Inc.,

New York and Base1 (1990). 15. Guay, R. and Silver, M.: Ferrous iron oxidation and uranium extraction by Thiobacillus ferrooxidans. Biotechnol. Bioeng., 19, 727-740 (1977). 16. Huber, T. F., Ras, C., and Kossen, N. W. F.: Design and scale-up of a reactor for the microbial desulfurization: a kinetic model for bacterial growth and pyrite oxidation, p. 151-159. In Proceedings of Third European Congress on Biotechnology, Miinchen, vol. 3. Verlag Chemine, Weinheim-Deerfield Beach/ Florida-Base1 (1984). 17. Ryu, H. W., Chang, Y. K., and Kim, S. D.: Microbial coal desulfurization in an airlift bioreactor by sulfur-oxidizing bacterium Thiobacillus ferrooxidans. Fuel Process. Technol., 36, 267-275 (1993). 18. Torma, A. F.: Biohydrometallurgy as an emerging technology, p. 23-33. In Ehrlich, H. L. and Holmes, D. S. (ed.). Workshoo bn biotechnology for the mining, metal refining add fossil fuel processing industries. John Wiley & Sons, Inc., New York (1986). 19. Silverman, M. P. and Lundgren, D. G.: Studies on the chemoautotrophic iron bacterium Ferrobacillus ferrooxidans. J. Bacterial., 77, 642-647 (1959). 20. American Society of Testing Materials: Standard method for determining forms of sulfur in coal. ASTM Annual Book, D2492,05, American Society for Testing and Materials, Philadelphia (1985). 21. Furman, N. H.: Standard methods of chemical analysis, 6th edition. R. E. Krieger Publishing Co., Huntington, New York (1975). an update. 22. Berner, R. A.: Sedimentary pyrite formation: Geochim. Cosmochim. Acta, 48, 605-615 (1984). 23. Raiswell, R. and Berner, R. A.: Pyrite and organic matter in phanerozoic normal marine shales. Geochim. Cosmochim. Acta, 50, 1967-1976 (1986). 24. Suzuki, N. and Phip, R.P.: Formation of melanoidins in the presence of HIS. Org. Geochem., 15, 361-366 (1990). 25. Torma, A. F.: The role of Thiobacillus ferrooxidans in hydrometallurgical processes. Adv. Biochem. Eng., 6, l-37 (1977). 26. Detz, C. M. and Barvinchak, G.: Microbial Desulphurization of Coal. Min. Conger. J., 65, 75-86 (1979). 27. Klein, J., Beyer, M., Afferden, M. V., Hodek, W., Seewald, F. H., Wolff, H., Fischer, E., and Juotgen, H.: Coal in biotechnology, p. 497-567. In Rehm, H. J. and Reed, G. (ed.), Biotechnology, vol. 6b. VCH, Weinheim (1988).