ABSTRACT
A number of different pyrites in Late Permian sulfur-containing coals and some associated rocks in Sichuan province of southw estern China have been studied system-atically by optical microscope，SEM and STM for crystal shape，distribution，morphology and atomic struc- ture. The chemical composition w ere analyzed using by EPMA，INAA techniques. The results in- dicated that there are different pyrite paragenesis and desulfurization properties in the different types of sulfur － containing coals formed on different depositional environments. Super micro- scopes reveal that there is colloidal pyrite in coal. The microanalysis show s that the several types of pyrite have difference in element composition and physical and technological property such as susceptibility etc. The crystallization of pyrite plays an important role in concentration of major elements.
INTRODUCTION
Investigation on the sulfur in coal may date back to the begining of this century ( Thissen， 1919; White，1913) in America. In recent tw enty years，more and more articles，reports and books about sulfur have been published. Key studies of sulfur in coal include Casagrande; Ber- ner; Cohen et al in America; Hunt in Austrilia; Kizilstein in Russia and so on.
Tw o-thirds of total sulfur in the high sulfur coals in China are dominated by inorganic sul- fur，at least 95% of w hich consists of iron sulfide. Therefore，researches on pyrite，a major carri- er of sulfur w hich influnces the quality of coal，have increasingly attracted coal technologists' attention. Nantong of Sichuan province is one of the important coal production area in south- w estern China，w hich is one of the high sulfur coal regions in China. Our reseach objective is to study the character of pyrites in coal and associated rock，such as their distributions，physical and physic-chemical properties etc. ，and to supply fundamental data for coal desulfurization technology，coal processing and utilizations.
SULFUR FORMS IN SICHUAN COAL
Objective coal seams of our investigated fields are Toulianzi coal of Wuyi coal mine at Anxian county，#5 and #6 seams of Nantong coal field at Chongqing，C2 seam of Muchuan county. Basically they represent for several types of coal containing different sulfur forms in Sichuan province. Table 1 show s sulfur forms of coals at different sites.
Table 1 The Sulfur Forms in Different Coals of Sichuan Province，China
Generally，w hen total sulfur content( St. d) is less than 0. 5% ，the main sulfur form in coal is organic sulfur ( Hunt，1987) . But C2 coal seam at Muchuan countuy mainly contains inorgan- ic sulfur w hich occupys 63. 08% of total sulfur. From continental to marine environment，total sulfur content in coal increases，the sulfur forms are different. The sulfur form in coal formed on tidal flat is mainly in the form of iron sulfide( Sp. d) . The Wuyi and Nantong #6 coal seams is rich in organic sulfur( So. d) . Both of them are influenced by roof rock of lime-stone or mud-lime stone of marine facies. Moreover，there is a trend that organic sulfur content increases w ith the increases of total sulfur，and pyritic sulfur is the main form of sulfur in many Sichuan coals.
THE DISTRIBUTION OF PYRITES IN COAL
Macroscopically there are rarely single crystal or film pyrite in low sufur coal formed in al- luvial sw amp at Muchuan. How ever，there are a lot of nodular，lenticular，pisolitic，chrysan- themun-like，massive，vein-like pyrites etc. in Nantong #5，#6 coal seam w here sulfur is accu- mulated on transitional facies. At the low er part of Wuyi high sulfur coal seam，there are a lot of fine disseminated pyrites show ing lamellar structure，but no pyrite can be seen by naked eye at the middle and upper part. Nodular pyrite occasionally contains marcasite and pyritized organism and biological relics.
Microscopically，there are single crystals and aggregates，framboid and its aggregates， globular，nodular，pore-nodular，biological，massive，vein，rod-like pyrites and marcasites in coals. Table 2 show s the quantitative statistics of 300 iron sulfide particles. Obviously，pyrites in Muchuan coal are mainly framboidal w hich are mostly associated w ith clay，and partly epigene- tic vein-like pyrite. In Nantong coal，there are a lot of nodular and cell-infilling pyrites. The py- rites in Wuyi coal are mainly euhedral crystal and framboid and their aggregates ( figure 1 － a) . Euhedral crystals are mostly octahedron. Wuyi coal contains more marchasites( figure 1 － b) . In general，framboidal morphology is present in all salinites. Similarly，framboidal pyrites in Sichuan coal are seen in all depositional facies. Using scanning electron microscope( SEM ) ，w e have observed pyritized Desulfovibrio，rod-chain bacteria，fine coccoides etc. . There are also a lot of pyritized algae such as Permocalculus Sinica，and Rhodophyceae. The crystal shapes of pyrite such as cube，octahedron，pentagonal dodecahedron and their combination can be easily observed by SEM. The cubic pyrite tends to distribute in continental facies，w hile the others are mostly devoloped in marine sediment，e. g. octahedron pyrite in Wuyi coal w as formed in ma- rine facies. The results are similar to that reported by Anchun Li et al( 1991) . Marcasite is usual- ly lamellar. Morever，crystal imprint，crack，grow lamination，defect，erosion relic and other mat- ter on crystal face have been seen by SEM. Especially colloid like structure，w hich devoloped in nodular pyrite，also has been observed. Coal pyrites，microscopically，contain a lot of pore pyrites，pyritized organisms and marcasites.
Table 2 Microanalysis of iron sulfide in coal
Table 2 show s that the pyrite particles in Muchuan coal are basically not liberated. In Nan- tong coal，1 ～ 5μm pyrite particles are dominant. The quantities of + 100μm pyrite particles in Nantong #6 coal are much more than those in other coals. The liberated pyrite in this size is as high as 44. 1% . In Wuyi coal，70% of pyrite particles are not liberated in 1 ～ 5μm size fraction. It results in difficulty in desulfurization. Wuyi coal also contains high organic sul- fur. therefore it is very important to study the properties of pyrites.
STM ANALYSIS ON SURFACE OF COAL PYRITES
Scanning tunneling microscope ( STM ) is an effective tool for the analysis of material sur- face. It directly reflects the image and structure of substance surface from 3μm to 1A scale. Eggleston( 1990，1992) and Fan ( 1991) successfully studied pyrite from other sources except in coal by using STM. In order to explain the difference in physical properties such as magnetic and oxidation ones of different pyrites，w e studied surface properties of pyrites w ith STM. The follow ings are our results and explanations:
Seven coal pyrites and tw o ore pyrites have been examined by CSTM -9000 scanning tunne- ling microscope for topography in constant-current mode and for atomic image in constant-hight mode. The scanning ranges are 2000X2000A to 3X3μm for topography. The granular，pellet， framboid-like，rod，fibre，bandary，milk colloidal and smooth，level，uneven etc. structure had been observed on the surface of pyrite crystal or fracture by STM. On the crystal face，oxidative corrosion pit ( relics) are often observed. Generally，on fine granular surface of pyrite，fine grains are in order w hich is gradually grow n into laminalars，w hile large granular grains are at random on coarse，and uneven surfurce of pyrite，w hich may be caused by surface corro- sion. The STM images of fracture surface of pyrites in coal show that pyrite of good crystaliza- tion is granular grain ( figure 1 － c) ，w hile nodular pyrite of poor crystalization displays more smooth and milk-colloid like face( figure 1 － d) . The above results observed by SEM indicate that there may be colloid pyrite in coal.
We have obtained some atomic images in constant-hight mode of STM ( figure 1 － e) ，they are not very nice and distinct，this is because that the pyrite is covered by an oxidational film. After the fresh surface of pyrite are covered w ith silica-oil，w e clearly observed the atomic structure of pyrite surface，and the zigzag chains in some parts. The interatomic distances reveal that the observed surface is in { 210 } direction of pyrite crystal. Eggleston and Fan observed { 100} and{ 110} direction of pyrite crystal surface.
In summary，STM is a new and effective technique for probing surface topography，struc- ture，crystal grow ing，atomic structure as w ell as surface oxidation and corrosion. STM and SEM directly prove that colloid pyrite exists indeed.
EPMA ANALYSIS OF PYRITES IN COAL
Electron probe microscope analysis( EPMA) is a method for composition analysis. The pre- cision of EPMA is 1% to 0. 1% . In this study，860-EDX EPMA of Link Coporation in U. K. and WDX-2A EPMA of Microspec Coporation in U. S. A w ere used. The latter has a bet- ter precision. Some conclusions about EPMA analysis of pyrite in coal are as follow ing.
Fig. 1 Microphotograph of iron sulfides in coals and associated rocks.
1. Ordinarily，besides Fe and S，pyrite in coal often contains Si，Al，Ca，Mg，Ti，Zr，Nb. The content of those elements in pyrite are proportional to those in matrix vitrinite. The Co，Ni，Cu， Pb etc，w hich can constitute the crystal latitices of pyrite，are minor in concentration，but they occur none or little in matrix vitrinite. In general，as crystal is transformed to framboid and cell- infilling pyrite，associated elements increase gradually.
2. In same coal seam，S content and S / Fe atomic ratio: euhedral crystal > cell-infilling > circular shape > framboidal increase in the order of pyrite. This indicates that crystalization plays a role in sulfur enrichment. Either S or Fe in nodular pyrite is richer in centre than that at the edge. Apparently nucleation plays a major role in nodular pyrite formation. The framboidal pyrite in Muchuan coal has very high of S / Fe atomic ratio( > 5) . It contains a large amount of Co，Ni etc. ，and Co / Fe atomic ratio is near to 3 /4. So some parts of framboids are not pyrite. Because framboid pyrite particle is small，the determined value influenced by matrix substance is usually lower than actual value. It is only considered as referential value. But S / Fe ratio is less influ- enced.
3. High S / Fe ratio of pyrite devoloped in matrix vitrinite w as found to be 6 to 17. The S / Fe ratio of pyrite is proportional to the organic sulfur of desmocollinite. The Fe content of the desmocollinite in high sulfur coal is relatively low.
INAA ANALYSIS OF PYRITES IN COAL AND ASSOCIATED ROCK
Instrumental Neutron Activation Analysis( INAA ) has the advantages of high sensitivity， good accuracy，multielemental analysis and non-destruction. We used INAA to measure trace elements，and associated elements of pyrite for searching the genesis，physical properties of pyrite. Nine coal pyrite and tw o ore pyrite samples are analysed by INAA in Institute of High Energy Physics，Academia Sinica ( 11 ) . Table 3 is part of the INAA results，it reveals the follow ing facts:
1. The elements in pyrites of different coals are different paragenetic types，e. g. in the py- rite of Nantong #6 coal，∑REE，∑3 ( sum of lithogenic element) are very low ，w hile Ni，Zn， Na are higher. There are a large amount of associated elements such as Mn，Cr，∑2，V，Ba，Ca， Mg etc. in the matrix-like pyrite of Nantong.
2. In the pyrite from coal and associated rock，macroscopically，from crystal to chrysanthe- man，nodular，vein-like and matrix-like pyrite，Fe content decreases gradually，and Co，Mn， ∑REE and other assoociated elements increase as the crystalizational degree of pyrite becomes w orses. The w orse the pyrite crystal the higher the ∑3 lithogenic elements.
3. The difference betw een coal-and ore-pyrite is that ∑3 and ∑REE content is low in ore- pyrite，and As and∑1 is rich，especially for hydrothermal type of pyrite( No. 39) . 4. Table 3 show s some relations betw een INAA results and susceptibility of pyrite. Mn is closely proportional to susceptibility of pyrite. Mg，Co，Ca etc. in coal-pyrite have a relation w ith susceptibility，w hile ore-pyrites，w hich are rich in As，have low magnetic property. ∑2 ( Mn， Co，Ni，Cr) elements，∑3 elements have a relation w ith susceptibility of pyrites.
Apparently，the susceptibility of coal pyrites are all higher than that of ore pyrites. So High Gradient Magnet selection ( HGMS ) technology can remove coal pyrite better than ore py- rite. The magnet analyses of coals indicate that susceptibilities of bituminous coal are all negative value; w hile anthracitous coals are all positive value w hich are approximate to those of coal py- rites. therefore HGMS is more suitable for the desulfurization of coking coals. We are continuing to undertake study on those fields. Here is only the brief conclusion.
CONCLUSIONS
The follow ing conclusions can be draw n from the above analysis:
1. The sulfur content in limnic coal is very poor w hich is the main property of framboidal pyrite. The marine-continent transitional facies coal is rich in sulfur，w hich is mainly in the form of inorganic sulfur. The nodular and cell-infilling pyrite are main types of pyrite and pyrite particles are large. The marine facies coal has high sulfur content，w hich is rich in high organic sulfur. Monocrystal，framboid pyrite and marcasite particles are small. Nodular pyrite often contains more marcasite，pyritized organisms and relics.
Table3 PartINAA Element Results and Susceptibility of Pyrites
Note: 1. INAA unit is ppm. Susceptibility unit is 10-7 eum /g.2.∑i = Cu +As +Se +Sb +Zn;∑2 = Co + Ni+ Cr+Mn;∑3 = Na + Al+Ca +Mg;∑ REE = La +Ce +Nd +Sm +Eu +Tb +Yb +Lu.3. - is that element hasn't been determined by INAA.
2. Liberation analyses show that the pyrite in transitional facies coal contains more librated particles，w hile marine coal has less amount of liberated pyrite. Nantong coal contains more nodular pyrites，in w hich pyrite particles is larger and liberated. It is predicted that they possess good behavior for physical desulfurization. But Wuyi coal contains more fine crystal，framboid and pyrites are not liberated. Moreover it has very high organic sulfur，so its desulfurization behavior must be very poor.
3. By STM ，w e clearly obtain atomic image on { 210} surface of pyrite，this is another new result follow ing the studies by Eggleston and Fan for{ 100} and{ 110} of pyrite crystal sur- face respectively. Moreover，w e have observed several pyrite structures such as granular，ban- dary，milk colloid-like pyrite，as w ell as corrosoion pit and oxidational film. STM ，SEM directly prove that colloid pyrite exists in coal. STM is a very potential technique for the study of pyrite.
4. The composition analyses show : As the pyrite crystallinity become better，Fe，S and S / Fe in coal pyrite increase，the associated elements decrease. There are Si，Al，Ca，Mg，Ti，Zr，Nb etc. in coal pyrite. The pyrite w ith high S / Fe ratio grow s in desmocollinite w hich S / Fe atomic ratio is 6 － 17. The S / Fe ratio of pyrite is proportional to the organic sulfur in desmocollinite.
5. The susceptibility of coal pyrites are much higher than that of ore pyrites. The suscepti- bility of coal pyrite has a relation w ith Mn，Co，Mg，Ca etc. elements. Bituminous coals have negative value of susceptibility，anthracite coals positive. HGMS is more suitable for the desulfu- rization of bituminous coals.
ACKNOWLEDGEMENTS
The authors express thanks to Institute of High Energy Physics of Academia Sinica，the University of Science and Technoloy of Beijing，Mr. Libing Liao，China University of Geosci- ence for their help w ith the use of facilities. We also w ish to acknow ledge financial support from National Natural Science Foundation of China and Genaral National Coal Cooperation of China.
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( 本文由唐躍剛、任德貽合著，原載 Processing and Utilization of High-sulfur Coals Ⅴ，( Coal Science and Technology 21) ，1993 年)