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COAL CLEANING BY HIGH GRADIENT MAGNETIC SEPARATION
C. van Driel, J. Sikkenga, C. Kerkdijk
To cite this version:
C. van Driel, J. Sikkenga, C. Kerkdijk. COAL CLEANING BY HIGH GRADIENT MAG- NETIC SEPARATION. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-775-C1-778.
�10.1051/jphyscol:19841158�. �jpa-00223631�
JOURNAL DE PHYSIQUE
Colloque Cl, supplbment au n o 1, Tome 45, janvier 1984 page Cl-775
C O A L CLEANING BY H I G H G R A D I E N T MAGNETIC SEPARATION
C.P. van D r i e l , J. sikkengar and G.B.W. Kerkdijk
FDO Engineering Consultants, P.O. Box 379, 1000 AJ ~rnsterdam, The Netherlands ' ~ o l e c ~ e d e r l a n d B.V., P.O. Box 23, 7550 M Hengelo, The Netherlands
Resume - Les r e s u l t a t s d ' e x p e r i e n c e s de p u r i f i c a t i o n de charbon en HGMS s o n t p r e s e n t 6 s . L ' a c c e n t e s t m i s s u r l a r e l a t i o n e n t r e l e s r6- s u l t a t s d e l a p u r i f i c a t i o n e t l e s c a r a c t e r i s t i q u e s du charbon.
A b s t r a c t - R e s u l t s of experimental work on HGMS c o a l c l e a n i n g w i l l be p r e s e n t e d with emphasis on t h e r e l a t i o n between c l e a n i n g r e s u l t s and c o a l c h a r a c t e r i s t i c s .
I INTRODUCTION
High G r a d i e n t Magnetic S e p a r a t i o n (HGMS) i s a p a r t i c l e s e p a r a t i o n technique which i s based on t h e d i f f e r e n c e s i n t h e magnetic p r o p e r t i e s of t h e p a r t i c l e s . A survey of t h e p r i n c i p l e s , t h e o r y , experimental work and a p p l i c a t i o n s h a s been given by B i r r s and Parker / I / .
HGMS can be a p p l i e d t o p u l v e r i z e d c o a l i n o r d e r t o remove p y r i t i c sulphur and a s h forming m i n e r a l s , t h u s producing a c l e a n c o a l . P y r i t e and m i n e r a l m a t t e r
p a r t i c l e s a r e removed by s t r o n g magnetic f o r c e s due t o t h e s e p a r t i c l e s being paramagnetic. For p r a c t i c a l purposes, pure c o a l p a r t i c l e s can be considered t o be non-magnetic and a r e t h e r e f o r e n o t a f f e c t e d by s t r o n g magnetic f i e l d g r a d i e n t s . F l y a s h can be processed by HGMS t o o b t a i n two o r more f r a c t i o n s which d i f f e r i n p r o p e r t i e s , among o t h e r s c o l o r , p a r t i c l e s i z e , and chemical composition (mainly of Fe and t r a c e e l e m e n t s ) . The s e p a r a t i o n i s accomplished due t o t h e presence o r absence of h i g h l y paramagnetic i r o n compound i n t h e i n d i v i d u a l f l y a s h p a r t i c l e s . Extensive HGMS c o a l c l e a n i n g experiments on f o u r German c o a l t y p e s have been performed and a r e r e p o r t e d by Van D r i e l e t a l . / 2 , 3 / . Also f o u r f l y a s h e s , o r i g i n a t i n g from c o a l f i r e d e l e c t r i c u t i l i t y b o i l e r s , were used i n experimental work on HGMS f l y a s h upgrading. Experimental r e s u l t s a r e r e p o r t e d by Van D r i e l e t a l . /4/.
I n t h i s paper some r e s u l t s of t h e HGMS c o a l c l e a n i n g experiments w i l l be p r e s e n t e d , more s p e c i f i c , t h e dependence of c o a l c l e a n i n g r e s u l t s on t h e c o a l c h a r a c t e r i s t i c s w i l l be t r e a t e d . For more d e t a i l e d r e s u l t s t h e r e a d e r i s r e f e r r e d t o Ref. 2 .
I1 COAL CLEANING BY HGMS
Coal can be considered a s a mixture of pure c o a l and mineral m a t t e r (mm). Pure c o a l can be considered t o be non-magnetic f o r p r g c t i c a l purposes ( i n f a c t it i s
d i a m a g n e t i c ) , while t h e mm i s e i t h e r non-magnetic o r paramagnetic.
I n t h e i d e a l c a s e , c o a l c l e a n i n g by HGMS proceeds a s f o l l o w s : c o a l i s p u l v e r i z e d a s t o produce a mixture of p u r e c o a l and pure mm p a r t i c l e s . T h i s mixture i s s u p p l i e d t o an HGMS-system. The paramagnetic p a r t i c l e s a r e removed from t h e mixture by s t r o n g magnetic f o r c e s (which should exceed t h e o t h e r f o r c e s on t h e
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19841158
Cl-776 JOURNAL DE PHYSIQUE
particles, viz. viscous, gravity and inertial forces), while the non-magnetic particles are unaffected by the magnetic field strength gradients. Therefore, in the ideal case, coal is produced with less mm (the non-magnetic minerals end up with the non-magnetic coal). Sulphur bound in pyrite (FeS2) is removed;
pyrite is paramagnetic.
In practice, pulverization of coal will also lead to the presence of mixed particles, i.e. particle containing both coal and mineral matter. These mixed particles result in an incomplete separation of paramagnetic mm from pure coal particles: the cleaned coal contains more mm than in the ideal case. Also coal is removed by magnetic forces if a paramagnetic mm particle adheres to a coal particle: coal is lost in the refuse stream.
Van Driel et al. /2,3/ reported on the removal of pyritic sulphur and ash forming minerals from four German coal types (labelled## 3, 4 (111) , 7 (111) , and
Pattberg) using HGMS. Pulverized coal samples (pulverized to 100% - 200 mesh)
were treated in the form of a 30% wt coal-water slurry at magnetic field strengths of up to 7 Tesla at slurry transport velocities up to 25 cm/s.
Grade-recovery curves at different process conditions were obtained.
Maximum pyritic sulphur reductions ranging from 40 to 95%, and ash reductions ranging from 60 to 75% were obtained at high magnetic field strengths (7 Tesla) and low slurry velocities (1.5 cm/s). Detailed results are given in Table 1.
Table 1 Best results with wet HGMS
Conditions: v = 1.7 cm/s and B = 7 T.
(percentages are on a daf basis)
Recovery (i.e. Btu-recovery, or the percentage of heating value of the feed which is collected in the cleaned fraction) ranged from 70'to 908. It should be
emphasized that the processconditions at which the best results are obtained are not necessarily processconditions from an economic point of view.
The mineral matter composition has been determined, see Table 2.
Table 2 Mineral matter composition ( % by weight)
7 ( I I I )
-
2 0 . 7 1 8 . 2 4 3 . 6
-
5.2 6 . 0
-
1 . 9
4.3
100 4 ( 1 1 1 1
6 . 8 6 . 8 6 1 . 6 6 . 3 1 0 . 6
-
2 . 5 2 . 1
- 3.2
100 quartz
i l l i t e p y r i t e
(Peco31 c a l c i t e (CaCo3) ankerite Fe-sulphate k a o l i n i t e mixed s i l i c a t e s mixed sulphates misc.+unknorm t o t a l
Pattberg
5 . 9 25.7 2 2 . 2 9 . 6 6 . 8 5 . 8 8 . 7 8 . 6
6.7
100 3
3 . 7 1 . 9 5 3 . 7 1 1 . 1 7 . 3 4.2 3 . 2
- - 4.9
100
From this mm composition, the ash reduction has been calculated assuming that HGMS only removes paramagnetic mm particles (recovery data are also included in the calculation, therefore the calculated ash reduction can be negative).
Experimentally obtained ash reductions are larger, see Table 3 for a comparison.
Table 3 Comparison between calculated and measured ash reduction
Therefore it is concluded that non-magnetic mm is removed by HGMS. This can be explained by the presence of conglomerates of non-magnetic and paramagnetic minerals.
The magnetic susceptibility of the coal samples has been obtained, and using the mineral matter composition data, the magnetic susceptibility of the natural pyrite can be calculated. This results in: 9 3 4 6 . 1 0 ~ ~ emu/gr, # 7(III) 40.10-~,
*Pattberg 40.10-6,# 4(III) < 5 . 1 0 ~ ~ . The lowest pyrite reduction (40%, see Table 1) has been obtained for coal#4(111) which shows some consistency with the magnetic susceptibility data. It should be emphasized however that
nevertheless 40% pyritic sulphur has been removed.
The HGMS theory on particle capture efficiency, which has been shown to be valid in a large number of single and multiple wire capture experiments, indicates that a nearly 100% capture efficiency can be expected for micronsize pyrite particles, even at low magnetic susceptibilities (say 1 to 5.10-~ emu/gr). This 100% pyrite removal is essentially obtained for# 7(III); pyrite particle size equals 17% wt < 5 pm, 39% < 10 pm, and 53% < 20 pm. For # 3 and Pattberg, which has a comparable pyrite particle size as *7(111), an incomplete pyrite removal is obtained. This should be attributed to an incomplete liberation of pyrite from the coal matrix. No quantitative information on the degree of liberation was obtained in the present work.
In this paper emphasis was on the relation between pyritic sulphur and mm reduction and coal characteristics. If such a relation can be established and quantified, HGMS coal cleaning performance can be predicted. This predictability is of utmost importance in a situation where an HGMS system is meant to process different and varying types of coal.
For the PETC two-stage coal-pyrite flotation process an excellent correlation between experimentally obtained and predicted pyritic sulphur reduction has been obtained /5/. The prediction is based on the following coal characteristics:
reflectance rank, fraction of liberated pyrite, pyritic sulphur as a percentage of total sulphur, percent of pyrite with size smaller than 32 pm. A similar approach should be followed for the HGMS process in order to establish and quantify the cleaning versus coal characteristics relationship.
ACKNOWLEDGEMENTS
The authors wish to thank: the staff of the High Field Magnet Laboratory of the Catholic University of Nijmegen for support during the experimental work, Dr. J. Vleeskens of the Energy Research Foundation in Petten for providing the mineral matter composition data.
JOURNAL DE PHYSIQUE
REFERENCES
1. Birrs R.R. and Parker M.R., High Intensity Magnetic Separation, published in Progress in Filtration and Separation Volume 2, Ed. Wakeman R.J., Elsevier 1981.
2. Van Driel C.P., Kerkdijk C.B.W., Segal H.R., and Sikkenga J., Coal Cleaning by HGMS,February 1983(Available from FDO, P.O. Box 379, Amsterdam, the Netherlands)
.
3. Van Driel C.P., Kerkdijk C.B.W., Segal H.R., and Sikkenga J.,Coal Cleaning by HGMS, August 1983 (Available from FDO, P.O. Box 379, Amsterdam, the Netherlands).
4. Van Driel C.P., Kerkdijk C.B.W., and Sikkenga J., Fly Ash and AFBC Ash Upgrading by HGMS, August 1983, To be published.
5. Tomich R.S., Petrographic evaluation in the products from two-stage coal-pyrite flotation, DOE/PC-30134-1, November 1981.
