- Email: [email protected]
Liberation study of apatite-nepheline ore comminuted by penetrating electrical discharges
International Journal of Mineral Processing, 4 (1977) 33--38
33
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
LIBERATION STUDY OF A P A T I T E - N E P H E L I N E ORE COMMINUTED BY P E N E T R A T I N G ELECTRICAL DISCHARGES
U.TS. ANDRES
Mineral Engineering Department, Technion, Israel Institute of Technology, Haifa (Israel) (Received August 8, 1975; revision accepted June 7, 1976)
ABSTRACT Andres, U.Ts., 1977. Liberation study of apatite--nepheline ore comminuted by penetrating electrical discharges. Int. J. Miner. Process., 4: 33--38. A comparative study of the comminution products of an apatite--nepheline ore was carried out in a specially designed pulse-discharge apparatus, w i t h electrical parameters and a dielectric medium as variables. Mineralogical analysis of the products confirmed the high selectivity of the process in liberation of monofractions, especially those of nepheline.
INTRODUCTION
In current practice, mineral phases in ores are liberated (with a view to separate extraction) through mechanical comminution of the aggregates. Exposure of monomineral grains is, however, a secondary consequence of the process, be it based on impact, compression or shear. Technologically speaking, all mechanical processes of this kind are multistage; and 4---5 successive passes are needed before the desired final size is achieved. The ore is reduced to a minimum grain size of the liberated mineral, irrespective of its original size distribution. Size control is maintained by means of classifiers, usually operated in closed circuit. To be effective, the technological process must preserve the monomineral grains intact and should minimize the yield of subaggregates and of fine particles. In other words, fracture should be selective and follow the boundaries of the mineral phases. Our attempt to realize an effective single-stage process along these lines is based on cutting a "channel" through the bulk of the mineral aggregate by means of high-voltage discharge. The disruptive effect of an electric discharge in water on a solid body (demonstrated in the mid-eighteenth century by Benjamin Franklin in his classical experiments with ceramic tubes with water and atmospheric electricity) received attention in the nineteen-fifties as a means of ore comminution. Yutkin's numerous pioneering experiments (1955) and the more advanced
34
studies of Maroudas (1966, 1967), Bergstrom (1961) and others dealt with comminution in pressure vessels by means of shock waves generated by rapid propagation of a spark-discharge channel, with pressures of the order of 102 kbars. Allowing for the total physical dissimilarity of the mechanical and electrohydraulic approaches, the disruptive effect in both cases is due to impact on the lump of ore, which is stressed beyond its load-bearing capacity. Of considerably greater interest, however, is the possibility of utilizing conductivity and permittivity differences between the components of the ore as the discharge is channelled through its bulk. Such a process is feasible subject to proper choice of the medium, voltage and pulse shape. (The voltage in this case is t o o low for breakdown through water alone, but sufficient if solid particles are present in its path. In addition, the energy expended in the shock wave is considerably less than the energy expended in the direct destruction of solid lumps.) In the process, disintegration may be expected along surfaces of least resistivity, which in practice often coincide with phase interfaces of the ore, thereby resulting in intergranular separation. The proposed technique dispenses with high-pressure vessels and the process can be successfully realized in plastic chambers. The main problem is the pulse shape. EXPERIMENTAL SETUP AND MATERIALS *
The continuous experimental setup (Fig.l) with capacity 10 kg/h for ores with a wide spectrum of c o m p o n e n t resistances, and 2 kg/h for ores with a narrow spectrum, consisted of the comminution chamber (equipped with a feed bin and a receiver, and filled with a dielectric medium) and a pulse generator (Babikov et al., 1955). The original material (highly resistant apatite- nephelinic syenite ore from the Khibiny massif, Kola Peninsula) had the following mineralogical composition: apatite 30%, nepheline 40%, pyroxene 10%, sphene 5%, magnetite and perovskite 3% each, and small amounts of accessory minerals. The 20-kg consignment was reduced below 20-mm size, deslimed and divided into 10 identical samples, which were checked for uniformity of mineral composition through their TiO2, Na20 and CaO contents; the results of this analysis (1.73 + 0.08% TiO2, 5.32 + 0.16% Na20 and 12.20 + 0.60% CaO) indicated sufficient uniformity for the purpose in hand. The variables in determining the optimal regime were: (1) the dielectric medium (transformer oil and tap water), (2) the air gaps of the condenser battery in the pulse generators and comminution chamber (d and D respectively, 15--25 mm each), (4) the pulse frequency (u, 5--10 min --~ ), (5) the total quantity of applied pulses per sample (N, 5 0 0 - 7 0 0 0 ) . The comminution product was separated in Thoulet solutions with sp. gr. *The e x p e r i m e n t s were carried o u t at the I n s t i t u t e o f R a r e - E l e m e n t M i n e r a l o g y a n d Geoc h e m i s t r y in Moscow. T h e e x p e r i m e n t a l s e t u p was c o n s t r u c t e d at t h e High-Voltage I n s t i t u t e at T o m s k (Russia).
35
1200
I
300 rRm
Fig.1. Experimental setup. 1, comminution chamber; 2, chamber discharger; 3, earthed electrode; 4, feed bin; 5, receiver; 6, pulse generator. T, set-up transformer (220/1000 V); K, kenotron rectifier; C, capacitors (0.03 uF each); RI,2, charging resistors; RD, damping resistor.
3.2, 2.9, 2.75 and 2.60 gcm -3 classified on screens with mesh size 1.0, 0.5, 0.3, 0.1, 0.07 m m and subjected to mineralogical analysis. Results were evaluated by comparing them with those of an effective mechanical process -- roll milling by stages with screening between stages. Analysis of microsections of the original ore showed that nepheline had the largest grain size. However, in spite of the relatively rectilinear boundaries, the nepheline aggregate contained considerable quantities of prisms of secondary monoclinic pyroxene, oriented parallel to the hexagonal axis and incapable of
36
monofractional extraction by the conventional methods. Apatite formed smaller grains, likewise with rectilinear boundaries and readily separable along the aggregation planes. The pyroxene grains showed peripheral flakes of biotite-substitution products of second-generation pyroxenes. The boundaries between the pyroxene and biotite phases were stable, and aggregates of these minerals could be expected. Sphene was present in the form of irregular grains containing apatite and m ~ n e t i t e inclusions, and there was likelihood of threecomponent aggregation. Magnetite and perovskite were represented by sporadic grains. R E S U L T S AND D I S C U S S I O N
The results of fractional analysis of the comminuted samples are given in Table I. The fraction of material of density greater than 3.2 gcm -3 is made up
TABLE I C o m p o s i t i o n o f c o m m i n u t e d samples. ~F
Specific gravity fraction
>3.2
3.2-2.9
2.75--2.6 <2.6
Mechanical process
Electrical c o m m i n u t i o n in t r a n s f o r m e r oil
3 stages + screening
d=25mm D = 25 m m v = 5 min-' N = 5600
4 stages + screening
d=20mm D = 15 m m v = 6 min-' N = 7000
42.3
44.1
41.6
123
1220 t
29.3 100.0
30.1 100.0
40.0 100.0
3.90) 45.40 100.0
148
122
37.70
Electrical c o m m i n u t i o n in w a t e r d=20mm D= 20mm v= 5 r a i n - ' N = 500 >3.2
37.2
3.2--2.9 2.9--2.75 2.75--2.6 <2.6
7.0 I 1.20 8.20} 46.40
~:
100.0
d=20mm D=12mm ~ = 8 min-' N = 1300 47.2
16.40
0.8 6.5 40.1 100.0
d=20mm D= 15mm v= 8 r a i n - ' N = 1800 52.1
12.70
1.3 8.1 35.8 100.0
d= 15mm D= 15ram v= 8 m i n - ' N = 2500 53.6
12.1
1.6 6.7 35.6 100.0
d= 15mm D= 17mm v= 8 rain-' N = 3100
52.6
52.3 10.8
0.7 4.9 39.7 100.0
d=25mm D=20mm v= 8 r a i n - ' N = 4000
8.0
3"81 0.6 0.6 42.4 100.0
5.0
37
of apatite and accessory minerals*; the fraction of material of density less than 2.6 gcm -3 is mainly nepheline; and the intermediate fractions subaggregations of the latter with the accessory minerals. This table shows that the degree of liberation of a mineral is determined b y the electric parameters used in the comminution process. The lowest yield of the intermediate fractions corresponds to the most selective break-up of the ore. The yield of the intermediate fractions obtained by the electrical discharge process was always lower compared with the mechanical process: 18.4--5.0% by electrical discharge as against 28.0--25.2% by mechanical comminution, with transformer oil as medium, the yield was 18.4--16.9% and with water it was 16.4--5.0%. In the latter case, moreover, the heavy fraction characteristically had a higher yield. All fractions of the products were analyzed in three representative samples -one obtained by mechanical comminution and the others b y the t w o versions of the electrical process. Results are illustrated (for the heavy fraction) in Table II, which shows that both the range and yield of the subaggregates in the mechanical process are considerably larger than in its electrical counterparts. Thus, out of twelve possible combinations, the mechanical process produced eleven with a total yield of 8.77%, while the electrical process produced only seven, with a total yield of 3.50% in transformer oil and 2.48% in water. The other fractions show a similar trend. T A B L E II Y i e l d s o f s u b a g g r e g a t e s in h e a v y f r a c t i o n Composition
Y i e l d (%) mechanical
electrical oil
water
A + P N + P A + M N + M S + M P + M A+N+P S+N+M S+A+M S+A+M P+S+M M÷P+A+N
0.88 0.20 1.06 1.18 0.16 1.40 0.30 0.05 -0.48 0.16 2.70
0.42 -0.90 1.05 0.03 -0.20 --0.20 -0.70
1.00 -0.80 -0.02 -0.03 -0.01 0.02 -0.60
Total
8.77
3.50
2.48
A , a p a t i t e ; M, m a g n e t i t e ; N, n e p h e l i n e ; P, p y r o x e n e , S, s p h e n e .
* T h e s p e c i f i c g r a v i t i e s o f t h e m i n e r a l s in t h e o r e in q u e s t i o n a r e as f o l l o w s : m a g n e t i t e , 5 . 1 ; p e r o v s k i t e , 4 . 0 ; p y r o x e n e , 3 . 5 ; s p h e n e , 3 . 4 ; a p a t i t e , 3 . 2 ; n e p h e l i n e , 2 . 6 g c m -3.
38 CONCLUSION
The above results indicate a tendency to selective liberation in the proposed process. For example, the yield of nepheline monofractions is higher by an absolute percentage of 7--10 and a relative one of 45--60, compared with mechanical comminution; the corresponding figures for pyroxene are 1 . 9 - 2 . 0 and 32--42%, respectively. In the mechanical process, the overall yield of subaggregates is considerably higher and their structure more complex. The apparent sensitivity of the process to changes in the electrical parameters opens the way for effective optimization.
REFERENCES Babikov, M.A., Komarov, N.S. and Sergeev, A.S., 1955. High-voltage technology. Gosenergoizdat, Moscow. Bergstrom, B.M., 1961. The electrohydraulic crusher. Eng. Min. J., 162(Feb.): 134. Maroudas, N.G., 1967. Electrohydraulic crushing, Br. Chem. Eng., 12(4). Maroudas, N.G. et al., 1966. The Mechanism of Electrohydraulic Crushing, Proc. 2nd European Comminution Symp., Amsterdam, 1966. Yutkin, L.A., 1955. Electrohydraulic Effect. Mashgiz (State Scientific Technical Press for Machine Construction Literature), Moscow. (In Russian)
33
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
LIBERATION STUDY OF A P A T I T E - N E P H E L I N E ORE COMMINUTED BY P E N E T R A T I N G ELECTRICAL DISCHARGES
U.TS. ANDRES
Mineral Engineering Department, Technion, Israel Institute of Technology, Haifa (Israel) (Received August 8, 1975; revision accepted June 7, 1976)
ABSTRACT Andres, U.Ts., 1977. Liberation study of apatite--nepheline ore comminuted by penetrating electrical discharges. Int. J. Miner. Process., 4: 33--38. A comparative study of the comminution products of an apatite--nepheline ore was carried out in a specially designed pulse-discharge apparatus, w i t h electrical parameters and a dielectric medium as variables. Mineralogical analysis of the products confirmed the high selectivity of the process in liberation of monofractions, especially those of nepheline.
INTRODUCTION
In current practice, mineral phases in ores are liberated (with a view to separate extraction) through mechanical comminution of the aggregates. Exposure of monomineral grains is, however, a secondary consequence of the process, be it based on impact, compression or shear. Technologically speaking, all mechanical processes of this kind are multistage; and 4---5 successive passes are needed before the desired final size is achieved. The ore is reduced to a minimum grain size of the liberated mineral, irrespective of its original size distribution. Size control is maintained by means of classifiers, usually operated in closed circuit. To be effective, the technological process must preserve the monomineral grains intact and should minimize the yield of subaggregates and of fine particles. In other words, fracture should be selective and follow the boundaries of the mineral phases. Our attempt to realize an effective single-stage process along these lines is based on cutting a "channel" through the bulk of the mineral aggregate by means of high-voltage discharge. The disruptive effect of an electric discharge in water on a solid body (demonstrated in the mid-eighteenth century by Benjamin Franklin in his classical experiments with ceramic tubes with water and atmospheric electricity) received attention in the nineteen-fifties as a means of ore comminution. Yutkin's numerous pioneering experiments (1955) and the more advanced
34
studies of Maroudas (1966, 1967), Bergstrom (1961) and others dealt with comminution in pressure vessels by means of shock waves generated by rapid propagation of a spark-discharge channel, with pressures of the order of 102 kbars. Allowing for the total physical dissimilarity of the mechanical and electrohydraulic approaches, the disruptive effect in both cases is due to impact on the lump of ore, which is stressed beyond its load-bearing capacity. Of considerably greater interest, however, is the possibility of utilizing conductivity and permittivity differences between the components of the ore as the discharge is channelled through its bulk. Such a process is feasible subject to proper choice of the medium, voltage and pulse shape. (The voltage in this case is t o o low for breakdown through water alone, but sufficient if solid particles are present in its path. In addition, the energy expended in the shock wave is considerably less than the energy expended in the direct destruction of solid lumps.) In the process, disintegration may be expected along surfaces of least resistivity, which in practice often coincide with phase interfaces of the ore, thereby resulting in intergranular separation. The proposed technique dispenses with high-pressure vessels and the process can be successfully realized in plastic chambers. The main problem is the pulse shape. EXPERIMENTAL SETUP AND MATERIALS *
The continuous experimental setup (Fig.l) with capacity 10 kg/h for ores with a wide spectrum of c o m p o n e n t resistances, and 2 kg/h for ores with a narrow spectrum, consisted of the comminution chamber (equipped with a feed bin and a receiver, and filled with a dielectric medium) and a pulse generator (Babikov et al., 1955). The original material (highly resistant apatite- nephelinic syenite ore from the Khibiny massif, Kola Peninsula) had the following mineralogical composition: apatite 30%, nepheline 40%, pyroxene 10%, sphene 5%, magnetite and perovskite 3% each, and small amounts of accessory minerals. The 20-kg consignment was reduced below 20-mm size, deslimed and divided into 10 identical samples, which were checked for uniformity of mineral composition through their TiO2, Na20 and CaO contents; the results of this analysis (1.73 + 0.08% TiO2, 5.32 + 0.16% Na20 and 12.20 + 0.60% CaO) indicated sufficient uniformity for the purpose in hand. The variables in determining the optimal regime were: (1) the dielectric medium (transformer oil and tap water), (2) the air gaps of the condenser battery in the pulse generators and comminution chamber (d and D respectively, 15--25 mm each), (4) the pulse frequency (u, 5--10 min --~ ), (5) the total quantity of applied pulses per sample (N, 5 0 0 - 7 0 0 0 ) . The comminution product was separated in Thoulet solutions with sp. gr. *The e x p e r i m e n t s were carried o u t at the I n s t i t u t e o f R a r e - E l e m e n t M i n e r a l o g y a n d Geoc h e m i s t r y in Moscow. T h e e x p e r i m e n t a l s e t u p was c o n s t r u c t e d at t h e High-Voltage I n s t i t u t e at T o m s k (Russia).
35
1200
I
300 rRm
Fig.1. Experimental setup. 1, comminution chamber; 2, chamber discharger; 3, earthed electrode; 4, feed bin; 5, receiver; 6, pulse generator. T, set-up transformer (220/1000 V); K, kenotron rectifier; C, capacitors (0.03 uF each); RI,2, charging resistors; RD, damping resistor.
3.2, 2.9, 2.75 and 2.60 gcm -3 classified on screens with mesh size 1.0, 0.5, 0.3, 0.1, 0.07 m m and subjected to mineralogical analysis. Results were evaluated by comparing them with those of an effective mechanical process -- roll milling by stages with screening between stages. Analysis of microsections of the original ore showed that nepheline had the largest grain size. However, in spite of the relatively rectilinear boundaries, the nepheline aggregate contained considerable quantities of prisms of secondary monoclinic pyroxene, oriented parallel to the hexagonal axis and incapable of
36
monofractional extraction by the conventional methods. Apatite formed smaller grains, likewise with rectilinear boundaries and readily separable along the aggregation planes. The pyroxene grains showed peripheral flakes of biotite-substitution products of second-generation pyroxenes. The boundaries between the pyroxene and biotite phases were stable, and aggregates of these minerals could be expected. Sphene was present in the form of irregular grains containing apatite and m ~ n e t i t e inclusions, and there was likelihood of threecomponent aggregation. Magnetite and perovskite were represented by sporadic grains. R E S U L T S AND D I S C U S S I O N
The results of fractional analysis of the comminuted samples are given in Table I. The fraction of material of density greater than 3.2 gcm -3 is made up
TABLE I C o m p o s i t i o n o f c o m m i n u t e d samples. ~F
Specific gravity fraction
>3.2
3.2-2.9
2.75--2.6 <2.6
Mechanical process
Electrical c o m m i n u t i o n in t r a n s f o r m e r oil
3 stages + screening
d=25mm D = 25 m m v = 5 min-' N = 5600
4 stages + screening
d=20mm D = 15 m m v = 6 min-' N = 7000
42.3
44.1
41.6
123
1220 t
29.3 100.0
30.1 100.0
40.0 100.0
3.90) 45.40 100.0
148
122
37.70
Electrical c o m m i n u t i o n in w a t e r d=20mm D= 20mm v= 5 r a i n - ' N = 500 >3.2
37.2
3.2--2.9 2.9--2.75 2.75--2.6 <2.6
7.0 I 1.20 8.20} 46.40
~:
100.0
d=20mm D=12mm ~ = 8 min-' N = 1300 47.2
16.40
0.8 6.5 40.1 100.0
d=20mm D= 15mm v= 8 r a i n - ' N = 1800 52.1
12.70
1.3 8.1 35.8 100.0
d= 15mm D= 15ram v= 8 m i n - ' N = 2500 53.6
12.1
1.6 6.7 35.6 100.0
d= 15mm D= 17mm v= 8 rain-' N = 3100
52.6
52.3 10.8
0.7 4.9 39.7 100.0
d=25mm D=20mm v= 8 r a i n - ' N = 4000
8.0
3"81 0.6 0.6 42.4 100.0
5.0
37
of apatite and accessory minerals*; the fraction of material of density less than 2.6 gcm -3 is mainly nepheline; and the intermediate fractions subaggregations of the latter with the accessory minerals. This table shows that the degree of liberation of a mineral is determined b y the electric parameters used in the comminution process. The lowest yield of the intermediate fractions corresponds to the most selective break-up of the ore. The yield of the intermediate fractions obtained by the electrical discharge process was always lower compared with the mechanical process: 18.4--5.0% by electrical discharge as against 28.0--25.2% by mechanical comminution, with transformer oil as medium, the yield was 18.4--16.9% and with water it was 16.4--5.0%. In the latter case, moreover, the heavy fraction characteristically had a higher yield. All fractions of the products were analyzed in three representative samples -one obtained by mechanical comminution and the others b y the t w o versions of the electrical process. Results are illustrated (for the heavy fraction) in Table II, which shows that both the range and yield of the subaggregates in the mechanical process are considerably larger than in its electrical counterparts. Thus, out of twelve possible combinations, the mechanical process produced eleven with a total yield of 8.77%, while the electrical process produced only seven, with a total yield of 3.50% in transformer oil and 2.48% in water. The other fractions show a similar trend. T A B L E II Y i e l d s o f s u b a g g r e g a t e s in h e a v y f r a c t i o n Composition
Y i e l d (%) mechanical
electrical oil
water
A + P N + P A + M N + M S + M P + M A+N+P S+N+M S+A+M S+A+M P+S+M M÷P+A+N
0.88 0.20 1.06 1.18 0.16 1.40 0.30 0.05 -0.48 0.16 2.70
0.42 -0.90 1.05 0.03 -0.20 --0.20 -0.70
1.00 -0.80 -0.02 -0.03 -0.01 0.02 -0.60
Total
8.77
3.50
2.48
A , a p a t i t e ; M, m a g n e t i t e ; N, n e p h e l i n e ; P, p y r o x e n e , S, s p h e n e .
* T h e s p e c i f i c g r a v i t i e s o f t h e m i n e r a l s in t h e o r e in q u e s t i o n a r e as f o l l o w s : m a g n e t i t e , 5 . 1 ; p e r o v s k i t e , 4 . 0 ; p y r o x e n e , 3 . 5 ; s p h e n e , 3 . 4 ; a p a t i t e , 3 . 2 ; n e p h e l i n e , 2 . 6 g c m -3.
38 CONCLUSION
The above results indicate a tendency to selective liberation in the proposed process. For example, the yield of nepheline monofractions is higher by an absolute percentage of 7--10 and a relative one of 45--60, compared with mechanical comminution; the corresponding figures for pyroxene are 1 . 9 - 2 . 0 and 32--42%, respectively. In the mechanical process, the overall yield of subaggregates is considerably higher and their structure more complex. The apparent sensitivity of the process to changes in the electrical parameters opens the way for effective optimization.
REFERENCES Babikov, M.A., Komarov, N.S. and Sergeev, A.S., 1955. High-voltage technology. Gosenergoizdat, Moscow. Bergstrom, B.M., 1961. The electrohydraulic crusher. Eng. Min. J., 162(Feb.): 134. Maroudas, N.G., 1967. Electrohydraulic crushing, Br. Chem. Eng., 12(4). Maroudas, N.G. et al., 1966. The Mechanism of Electrohydraulic Crushing, Proc. 2nd European Comminution Symp., Amsterdam, 1966. Yutkin, L.A., 1955. Electrohydraulic Effect. Mashgiz (State Scientific Technical Press for Machine Construction Literature), Moscow. (In Russian)