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Beneficiation of a Greek serpentinic nickeliferous ore
Hydrometallurgy 74 (2004) 259 – 265 www.elsevier.com/locate/hydromet
Beneficiation of a Greek serpentinic nickeliferous ore Part I. Mineral processing Stella Agatzini-Leonardou *, Ioannis G. Zafiratos, Dionysios Spathis Laboratory of Metallurgy, Department of Mining and Metallurgical Engineering, National Technical University of Athens, 9, Ir. Polytechniou Street, Athens 15780 Zografos, Greece Received 18 February 2004; received in revised form 24 May 2004; accepted 28 May 2004
Abstract Serpentinic ore, from the ‘‘Kastoria’’ nickeliferous deposit in Northern Greece, was first processed to reject as much of its calcite content as possible. Partial separation of calcite from the ore was achieved by the use of a strong magnetic field, the extent of which depended on feed particle size. The losses of nickel in the nonmagnetic product were about 5%, while the percentage CaO removal was about 37%. Based on the experimental findings, a mineral processing scheme was devised and applied in order to prepare a suitable sulphuric acid heap or agitation leaching feed. D 2004 Elsevier B.V. All rights reserved. Keywords: Nickel laterite ores; Serpentine; Magnetic separation; Autogenous grinding; Calcite removal
1. Introduction Greece is the only EU country with extensive but low-grade nickel laterites. They mainly occur as limonitic laterites and, to a lesser extent, as serpentinic laterites. The Greek laterites are unique in the world in that they are sedimentary and have originated by transport and sedimentation of laterite-derived material, generated by weathering of ultramafic rocks (Kuhnel et al., 1974; Golightly, 1979; Manceau and Calas, 1986; Skarpelis et al., 1993; Orphanoudaki et al., 1997; Boskos et al., 2000). The Greek limonitic laterites have been exploited to produce ferronickel via a pyrometallurgical route. * Corresponding author. Tel.: +30-210-7722234; fax: +30-2107722218. E-mail address: [email protected] (S. Agatzini-Leonardou). 0304-386X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2004.05.005
This involves prereduction of the ore in rotary kilns, reduction smelting in electric furnaces, and upgrading of the raw ferronickel in a converter to the final 20 – 25% Ni grade. Because of the rising cost of energy, the method is economically marginal when ore with 0.95 – 1% nickel is treated and is uneconomic for lower grade laterites. Direct application of the above pyrometallurgical method to existing serpentinic laterites is not feasible because the slag produced is difficult to melt, resulting in higher energy requirements and poor phases (metal –slag) separation. An innovative integrated hydrometallurgical method for nickel and cobalt extraction from Greek limonitic laterites has been developed and patented (Agatzini-Leonardou and Dimaki, 1994; AgatziniLeonardou and Dimaki, 2001; Agatzini-Leonardou and Karidakis, 2000; Agatzini-Leonardou et al., 2000) as a result of many years of work at the
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Laboratory of Metallurgy of the National Technical University of Athens. The ‘‘HEap Leach LAteriteS’’ (HELLAS) method comprises heap leaching of the ore with dilute sulphuric acid at ambient temperature, purification of the leach liquor produced by chemical precipitation at atmospheric pressure, and recovery of nickel and cobalt from the purified leach liquor either by solvent extraction and electrowinning or by chemical precipitation. A research project was undertaken to study the application of ‘‘HELLAS’’ to the ‘‘Kastoria’’ serpentinic deposit in Northern Greece (Spathis, 1999). Because of its relatively high (f18%) calcium oxide content, the ore had to be first processed in order to remove as much of its calcite as possible. The only known upgrading process presently practiced in serpentinic laterite beneficiation plants around the world is rock rejection. Laterite ores often contain boulders that may be barren of nickel. These boulders are typically surrounded by very fine, loosely adhering nickeliferous material. Since the nickelbearing material is softer than the core of the boulder, mild abrasion may result in significant upgrading. This can be done in a ‘‘trommel’’ rock rejection device or in an autogenous grinding drum. In both cases, the barren cores can be washed and screened out (Queneau and Weir, 1986). On the Island of New Caledonia, Societie Le Nickel sorts out the peridotitic rock (15 – 20% of the mine output) from the serpentinic ore by means of a revolving screen, called Tritout, which is a heated trommel (Testut and Raffinot, 1985; Pelletier, 1996). The Moa Bay plant, in Cuba, uses water scrubbing in a trommel for rock rejection (Dufour, 1985). Within the framework of the research project conducted for the ‘‘Kastoria’’ deposit, a magnetic separation process, not previously evaluated for this type of ore, was applied and the results are given in the present paper.
2. Materials and methods A representative sample of 1.2 t of ‘‘Kastoria’’ ore was received from the ‘‘Kastoria’’ mine in Northern Greece, crushed to 50 mm. Its chemical analysis is given in Table 1.
Table 1 Chemical analysis of the ‘‘Kastoria’’ ore (bulk sample) Component
Percentage (%)
Ni (NiO) Co (CoO) Fe (Fe2O3) Al (Al2O3) Cr (Cr2O3) Ca (CaO) Mg (MgO) Mn (Mn3O4) SiO2 CO2 Loss on ignition at 1000 jC
1.44 0.05 9.29 0.37 0.57 12.72 7.85 0.18 21.37 15.45 22.06
(1.83) (0.06) (13.29) (0.70) (0.83) (17.80) (13.02) (0.25)
The ‘‘Kastoria’’ laterite deposit was formed on serpentinised hartzburgite during the Eocene period. The ore appears as a discontinuous layer of sediment deposited on serpentinised ultramafic rock, which represents relics of old weathering crust (Skarpelis et al., 1993). The sample was analysed mineralogically in detail by ore microscopy and X-ray diffraction. The mineral chemistry was determined by electron microprobe analysis. The Kastoria laterite consists, predominantly, of nickeliferous serpentine [Mg3Si2O5(OH)4], nickeliferous magnesian-cronstedtite, calcite, and quartz. Goethite, hematite, saponite [(Ca, Na)0.66Mg6(Si7.34Al0.66) O20(OH)4. nH2O], chromite, talc [Mg6Si8O20(OH)4], and tremolite [Ca2Mg5Si8O22(OH)2] are present in small amounts. Serpentine is the most important mineral. In some places it is replaced by cronstedtite [(Fe2 +8Fe3 +4) (Si4Fe3 +4)O20(OH)16], an iron-rich sheet silicate mineral. Calcite appears as a vein mineral as well as in small pockets within the serpentine matrix making it very difficult to separate from the ore. Quartz aggregates, pseudo-morphically replacing serpentine, to some extent, are often associated with calcite. The quartz aggregates commonly occur as small pockets in the matrix of serpentine. The goethite content in the ore is approximately 5%. Goethite appears in veinlets or in small grains distributed in the matrix of serpentine. It is more abundant in places where cronstedtite predominates. Chromite occurs in small grains within the matrix of serpentine and cronstedtite. Its content is about 1%. Small amounts of chlorite, saponite, talc flakes, and tremolite are also present.
S. Agatzini-Leonardou et al. / Hydrometallurgy 74 (2004) 259–265 Table 2 Size analysis of the ‘‘Kastoria’’ ore
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Table 3 Size analysis of the ‘‘Kastoria’’ magnetic separation feed
Particle size (mm)
Weight on screen (g)
Percentage
Particle size (mm)
Weight on screen (kg)
Weight (%)
50 + 25 25 + 19 19 + 12.7 12.7 + 9.5 9.5 + 6.8 6.8 + 4.75 4.75 + 2 2+1 1 + 0.5 0.5 + 0.15 0.15 Total
957.7 270.4 498.3 198.1 340.9 108.3 703.2 443.9 420.9 589.3 326.8 4857.8
19.71 5.57 10.26 4.08 7.02 2.23 14.48 9.14 8.66 12.13 6.73 100.00
50 + 10 10 + 4 4+2 2+1 1 Total
9.04 5.24 4.05 2.83 10.47 31.63
28.6 16.6 12.8 8.9 33.1 100.00
The microprobe analysis showed that the minerals serpentine and cronstedtite constitute the main nickel carriers in the ‘‘Kastoria’’ ore. More specifically, the NiO content in serpentine ranges from 0.98% to 6.38%, with a mean value from 20 analyses of 3.44%. The NiO content in cronstedtite ranges from 2.78% to 11.5%, averaging 5.86%. Saponite, goethite, and hematite are also nickeliferous minerals, containing on the average 1.6%, 1.3%, and 1.8% NiO, respectively; however, their contribution to the nickel
content of the ore is limited, given their small relative amounts. As previously described, the above mineralogical analysis indicated that the main nickeliferous minerals were serpentine and cronstedtite while the main gangue minerals were quartz and calcite. Taking into consideration that nickeliferous minerals such as garnierite, limonite, and serpentine are slightly magnetic, while quartz and calcite are nonmagnetic, only a high intensity magnetic field would achieve their separation. This requires the use of high gradient magnetic separators (HGMS). Such a separation would be effective provided that an adequate degree of mineral liberation has been previously achieved by grinding. Considering that a large fraction of fines in
Fig. 1. Distribution of chemical constituents in the particle size fractions of the ‘‘Kastoria’’ ore.
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Table 4 Size analysis of the ‘‘Kastoria’’ autogenous grinding product Particle size (mm)
Weight on screen (kg)
Weight (%)
50 + 10 10 + 4 4+2 2+1 1 Total
5.15 0.75 0.21 0.11 2.82 9.04
57.0 8.3 2.3 1.2 31.2 100.00
the ore is not preferable for heap leaching, determining the coarsest size fraction of ore, for which magnetic separation is effective, was a major goal of this study. An amount of 600 kg from the initial ‘‘Kastoria’’ ore sample was homogenized in a mixing drum and divided by the ‘‘cross’’ method and homogenization to give subsamples of 150 kg and subsequently 36.5 kg. One of the 36.5-kg subsamples was further divided and a 4.9-kg portion was finally taken for dry size analysis. The results are shown in Table 2. A sample of approximately 100 g from each size fraction was taken, using a Jones riffle splitter, ground to 100 mesh, and chemically analysed in order to determine which size fractions would either be rejected, as being barren of nickel and high in calcium, or further be processed by magnetic separation.
Table 5 Results of autogenous grinding tests on the fraction of the ‘‘Kastoria’’ ore Particle size (mm) wt.% Fe2O3 Ni Co SiO2 CaO MgO
50 + 10 57 7.04 0.96 0.03 37.65 23.09 9.62
10 + 4 8.3 8.04 1.05 0.04 32.52 23.65 9.79
4+2 2.3 8.46 0.54 0.04 34.23 22.31 9.62
50 + 10 mm size 2+1
1.2 9.47 0.57 0.04 32.95 21.27 10.28
Feed
31.2 100 13.47 9.19 2.00 1.28 0.07 0.04 29.09 34.42 18.39 21.63 14.10 11.04
The results of the chemical analysis of the ore size fractions are given in Fig. 1. The rest of the 36.5-kg sample (31.6 kg) was screened, using 10-, 4-, 2-, and 1-mm sieves, for processing by different magnetic separators available in the laboratory. The size fractions thereby produced are given in Table 3.
3. Results and discussion The 50 + 10 mm size fraction was fed to a rotating autogenous grinding drum in order to effect rock rejection, prior to feeding the ore to magnetic separators. The product, after 15-min grinding in the drum and water washing of the coarsest fraction, was passed through the same as above sieve sizes and the results are given in Table 4. The 50 + 10 mm fraction of the autogenous grinding product was set aside for possible rejection,
Table 6 Results of magnetic separation tests on the fraction of the ‘‘Kastoria’’ ore
Fig. 2. Variation of Ni/CaO ratio with particle size in the serpentinic run-of-mine ‘‘Kastoria’’ ore.
1
10 + 4 mm size
Material
Magnetic product
Nonmagnetic product
Feed
wt.% Ni CaO
64.9 1.70 16.09
35.1 1.07 25.47
100 1.48 19.38
Material
Ni units
Ni distribution (%)
CaO units
CaO distribution (%)
Magnetic Nonmagnetic Feed
1.10 0.38
74.32 25.68 100.00
10.44 8.94
53.87 46.13 100.00
S. Agatzini-Leonardou et al. / Hydrometallurgy 74 (2004) 259–265 Table 7 Results of magnetic separation tests on the of the ‘‘Kastoria’’ ore
4 + 2 mm size fraction
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Table 9 Results of magnetic separation tests on the of the ‘‘Kastoria’’ ore (low field intensity)
2 + 1 mm size fraction
Material
Magnetic product
Nonmagnetic product
Feed
Material
Magnetic product
Nonmagnetic product
Feed
wt.% Ni CaO
82 1.69 13.15
18 0.46 35.35
100 1.47 17.14
wt.% Ni CaO
89.8 1.74 12.26
10.2 0.40 34.70
100 1.60 14.55
Material
Ni units
Ni distribution (%)
CaO units
CaO distribution (%)
Material
Ni units
Ni distribution (%)
CaO units
CaO distribution (%)
Magnetic Nonmagnetic Feed
1.39 0.08
94.56 5.44 100.00
10.78 6.36
62.89 37.11 100.00
Magnetic Nonmagnetic Feed
1.56 0.04
97.50 2.50 100.00
11.01 3.54
75.67 24.33 100.00
depending on its nickel content, while the other fractions were added to the respective fractions of the initial sample for the magnetic separation study. The 10 + 4 and 4 + 2 mm fractions were fed to a permanent magnet HGMS machine, while the 1 and 2 + 1 mm fractions were processed in a variable intensity magnetic separation device. As seen in Fig. 2, no fraction of the run-of-mine ore can be rejected as having a sufficiently low Ni/ CaO ratio. Likewise, only the finest fraction ( 0.15 mm) can be passed on to the leaching stage as having a significantly high Ni/CaO ratio. However, it was decided that all fractions should be processed for calcite rejection prior to leaching. The results of the autogenous grinding of the ‘‘Kastoria’’ ore are shown in Table 5, while the magnetic separation results on various size fractions are presented in Tables 6 –11. A
nickel content of less than 0.5% was targeted for the material rejected, whereas the lowest possible CaO content was desirable for the upgraded material. As seen in Table 5, the coarse fraction ( 50 + 10 mm) of the autogenous grinding product contained 0.96% nickel and it could not be rejected. This fraction was subsequently ground to 10 mm and added to the magnetic separation feed material. It was concluded that a feed material of smaller particle size is necessary for an efficient separation by autogenous grinding. Tables 6 – 11 show that, in most instances, a satisfactory degree of separation of calcite from the ore can be achieved by the use of a strong magnetic field, depending on feed particle size. This incurred relatively small losses of nickel in the nonmagnetic product according to the feed material. As seen in
Table 8 Results of magnetic separation tests on the 2 + 1 mm size fraction of the ‘‘Kastoria’’ ore (high field intensity)
Table 10 Results of magnetic separation tests on the the ‘‘Kastoria’’ ore (high field intensity)
Material
Magnetic product
Nonmagnetic product
Feed
Material
Magnetic product
Nonmagnetic product
Feed
wt.% Ni CaO
84.6 1.85 10.97
15.4 0.47 32.49
100 1.64 14.28
wt.% Ni CaO
94.6 1.98 11.73
5.4 0.61 32.91
100 1.90 12.88
Material
Ni units
Ni distribution (%)
CaO units
CaO distribution (%)
Material
Ni units
Ni distribution (%)
CaO units
CaO distribution (%)
Magnetic Nonmagnetic Feed
1.57 0.07
95.73 4.27 100.00
9.28 5.00
64.99 35.01 100.00
Magnetic Nonmagnetic Feed
1.87 0.03
98.42 1.58 100.00
11.10 1.78
86.18 13.82 100.00
1 mm size fraction of
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Table 11 Results of magnetic separation tests on the the ‘‘Kastoria’’ ore (low field intensity)
1 mm size fraction of
Material
Magnetic product
Nonmagnetic product
Feed
wt.% Ni CaO
95.3 1.92 12.34
4.7 0.65 32.74
100 1.86 13.30
Material
Ni units
Ni distribution (%)
CaO units
CaO distribution (%)
Magnetic Nonmagnetic Feed
1.83 0.03
98.39 1.61 100.00
11.76 1.54
88.42 11.58 100.00
Tables 6– 11, nickel losses were 5%, at most, for a feed particle size less than 4 mm. The respective percentage CaO removal was about 37%, which was considered adequate for the enhancement of the permeability of this ore. On the other hand, magnetic separation of the 1-mm fraction resulted in small amounts of nonmagnetic products in the order of 5% of the feed. Therefore, processing of this fraction was considered unnecessary. The 10 + 4 mm fraction should be ground to 4 mm in order to achieve sufficient calcite liberation.
4. Conclusions Based on the above experimental findings, the following mineral processing scheme was devised and applied for the ‘‘Kastoria’’ ore in order to prepare a suitable sulphuric acid heap or agitation leaching feed: 1. Classification of the 50-mm feed material by 16, 10, 4, and 1 mm size screens. 2. Crushing of the 50 + 16 mm ore in a jaw crusher to the size of 16 mm. 3. Classification of the crusher product to the sizes of stage 1. 4. Autogenous grinding of the 16 + 4 mm fraction in a revolving drum. 5. Classification of the autogenous grinding product to the sizes of stage 1. 6. Chemical analysis of the coarse size fractions for possible rejection of those with less than 0.5% Ni.
7. Crushing to 4 mm of coarser fractions with nickel more than 0.5%. 8. Magnetic separation of the 4 + 1 mm fraction in a HGMS device. The above scheme is expected to improve the leaching behaviour of the serpentinic ore by lowering the acid consumption during both agitation and heap leaching as well as improving the permeability of the ore heap.
Acknowledgements The authors wish to express their gratitude to the European Commission, Directorate General XII, for financial support according to contract no. BRE2CT94-1020 and, also, to G.M.M.S.A. LARCO for the laterite samples provided.
References Agatzini-Leonardou, S., Dimaki, D., 1994. Recovery of nickel and cobalt from low-grade nickel oxide ores by the technique of extraction in heaps using dilute sulphuric acid at ambient temperature. Greek Patent GR1001555, 22 March 1994. Agatzini-Leonardou, S., Dimaki, D., 2001. Method for obtaining nickel and cobalt from oxidised nickel and cobalt ore by extraction in heaps using dilute sulphuric acid prepared by using sea water, at ambient temperature. Greek Patent GR1003569, 23 April 2001. Agatzini-Leonardou, S., Karidakis, T., 2000. Production of a magnesium hydroxide-containing mixture suitable for use as a filler in polymers and as an additive in cement. Greek Patent GR1003693, 18 July 2000. Agatzini-Leonardou, S., Zafiratos, I.G., Oustadakis, P.T., 2000. Process for the removal of aluminium and/or chromium from nickel and/or cobalt sulphate solutions at atmospheric pressure. Greek Patent GR1003419, 01 Sept. 2000. Boskos, E., Orphanoudaki, A., Perraki, T.H., 2000. The Ni distribution in the mineral phases of Greek Fe – Ni laterite deposits. 3rd Congress of Mineral Wealth, Athens, Greece, November 2000, Proceedings Vol. A, Technical Chamber of Greece, Athens, 107 – 115. Dufour, M.F., 1985. Processing of nickel-bearing lateritic ores— Moa Bay. In: Weiss, N.L. (Ed.), SME Mineral Processing Handbook, vol. 2. SME, New York, pp. 17 – 39. Golightly, J.P., 1979. Nickeliferous laterites: a general description. In: Evans, D.J.I., Shoemaker, R.S., and Veltman, H. (Eds.), Intnl. Laterite Symposium. AIME, New York, pp. 3 – 23. Kuhnel, R.A., Van Hilten, D., Roorda, H.J., 1974. The crystallinity of mineral in alternation profiles: an example on goethite in
S. Agatzini-Leonardou et al. / Hydrometallurgy 74 (2004) 259–265 laterite profiles. Delft Progress Report. Series E, Geosciences 1, 1 – 18. Manceau, A., Calas, G., 1986. Nickel bearing clay minerals: intracrystalline distribution of nickel: an X-ray absorption study. Clay Minerals 21, 341 – 360. Orphanoudaki, A., Boskos, E., Kastritsis, I., 1997. Mineralogical and geochemical study of the nickeliferous laterite of the Palaeochori (Grevena) area. Bulletin of the Hellenic Geological Society 31, 7 – 22. Pelletier, B., 1996. Serpentines in nickel silicate ore from New Caledonia. In: Grimsey, E.J., Neuss, I. (Eds.), Nickel 96—Mineral to Market. AusIMM, Melbourne, pp. 197 – 205. Queneau, P.B., Weir, D.R., 1986. Control of iron during hydromet-
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allurgical processing of nickeliferous laterite ores. In: Dutrizac, A.J., Monhemius, A.J. (Eds.), Iron Control in Hydrometallurgy. Ellis Horwood, Chichester, pp. 76 – 105. Skarpelis, N., Laskou, M., Alevisos, A., 1993. Mineralogy and geochemistry of the nickeliferous lateritic iron ores of Kastoria, NW Greece. Chemie der Erde 53, 331 – 339. Spathis, D. 1999. Leaching of a serpentinic laterite after removal of calcite by magnetic separation. Diploma thesis, Department of Mining and Metallurgical Engineering, NTUA, Athens (in Greek). Testut, R.J., Raffinot, P., 1985. Processing of nickel-bearing lateritic ores—Le Nickel. In: Weiss, L. (Ed.), SME Mineral Processing Handbook,vol. 2. SME, New York, pp. 25 – 31. Chapter 17.
Beneficiation of a Greek serpentinic nickeliferous ore Part I. Mineral processing Stella Agatzini-Leonardou *, Ioannis G. Zafiratos, Dionysios Spathis Laboratory of Metallurgy, Department of Mining and Metallurgical Engineering, National Technical University of Athens, 9, Ir. Polytechniou Street, Athens 15780 Zografos, Greece Received 18 February 2004; received in revised form 24 May 2004; accepted 28 May 2004
Abstract Serpentinic ore, from the ‘‘Kastoria’’ nickeliferous deposit in Northern Greece, was first processed to reject as much of its calcite content as possible. Partial separation of calcite from the ore was achieved by the use of a strong magnetic field, the extent of which depended on feed particle size. The losses of nickel in the nonmagnetic product were about 5%, while the percentage CaO removal was about 37%. Based on the experimental findings, a mineral processing scheme was devised and applied in order to prepare a suitable sulphuric acid heap or agitation leaching feed. D 2004 Elsevier B.V. All rights reserved. Keywords: Nickel laterite ores; Serpentine; Magnetic separation; Autogenous grinding; Calcite removal
1. Introduction Greece is the only EU country with extensive but low-grade nickel laterites. They mainly occur as limonitic laterites and, to a lesser extent, as serpentinic laterites. The Greek laterites are unique in the world in that they are sedimentary and have originated by transport and sedimentation of laterite-derived material, generated by weathering of ultramafic rocks (Kuhnel et al., 1974; Golightly, 1979; Manceau and Calas, 1986; Skarpelis et al., 1993; Orphanoudaki et al., 1997; Boskos et al., 2000). The Greek limonitic laterites have been exploited to produce ferronickel via a pyrometallurgical route. * Corresponding author. Tel.: +30-210-7722234; fax: +30-2107722218. E-mail address: [email protected] (S. Agatzini-Leonardou). 0304-386X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2004.05.005
This involves prereduction of the ore in rotary kilns, reduction smelting in electric furnaces, and upgrading of the raw ferronickel in a converter to the final 20 – 25% Ni grade. Because of the rising cost of energy, the method is economically marginal when ore with 0.95 – 1% nickel is treated and is uneconomic for lower grade laterites. Direct application of the above pyrometallurgical method to existing serpentinic laterites is not feasible because the slag produced is difficult to melt, resulting in higher energy requirements and poor phases (metal –slag) separation. An innovative integrated hydrometallurgical method for nickel and cobalt extraction from Greek limonitic laterites has been developed and patented (Agatzini-Leonardou and Dimaki, 1994; AgatziniLeonardou and Dimaki, 2001; Agatzini-Leonardou and Karidakis, 2000; Agatzini-Leonardou et al., 2000) as a result of many years of work at the
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Laboratory of Metallurgy of the National Technical University of Athens. The ‘‘HEap Leach LAteriteS’’ (HELLAS) method comprises heap leaching of the ore with dilute sulphuric acid at ambient temperature, purification of the leach liquor produced by chemical precipitation at atmospheric pressure, and recovery of nickel and cobalt from the purified leach liquor either by solvent extraction and electrowinning or by chemical precipitation. A research project was undertaken to study the application of ‘‘HELLAS’’ to the ‘‘Kastoria’’ serpentinic deposit in Northern Greece (Spathis, 1999). Because of its relatively high (f18%) calcium oxide content, the ore had to be first processed in order to remove as much of its calcite as possible. The only known upgrading process presently practiced in serpentinic laterite beneficiation plants around the world is rock rejection. Laterite ores often contain boulders that may be barren of nickel. These boulders are typically surrounded by very fine, loosely adhering nickeliferous material. Since the nickelbearing material is softer than the core of the boulder, mild abrasion may result in significant upgrading. This can be done in a ‘‘trommel’’ rock rejection device or in an autogenous grinding drum. In both cases, the barren cores can be washed and screened out (Queneau and Weir, 1986). On the Island of New Caledonia, Societie Le Nickel sorts out the peridotitic rock (15 – 20% of the mine output) from the serpentinic ore by means of a revolving screen, called Tritout, which is a heated trommel (Testut and Raffinot, 1985; Pelletier, 1996). The Moa Bay plant, in Cuba, uses water scrubbing in a trommel for rock rejection (Dufour, 1985). Within the framework of the research project conducted for the ‘‘Kastoria’’ deposit, a magnetic separation process, not previously evaluated for this type of ore, was applied and the results are given in the present paper.
2. Materials and methods A representative sample of 1.2 t of ‘‘Kastoria’’ ore was received from the ‘‘Kastoria’’ mine in Northern Greece, crushed to 50 mm. Its chemical analysis is given in Table 1.
Table 1 Chemical analysis of the ‘‘Kastoria’’ ore (bulk sample) Component
Percentage (%)
Ni (NiO) Co (CoO) Fe (Fe2O3) Al (Al2O3) Cr (Cr2O3) Ca (CaO) Mg (MgO) Mn (Mn3O4) SiO2 CO2 Loss on ignition at 1000 jC
1.44 0.05 9.29 0.37 0.57 12.72 7.85 0.18 21.37 15.45 22.06
(1.83) (0.06) (13.29) (0.70) (0.83) (17.80) (13.02) (0.25)
The ‘‘Kastoria’’ laterite deposit was formed on serpentinised hartzburgite during the Eocene period. The ore appears as a discontinuous layer of sediment deposited on serpentinised ultramafic rock, which represents relics of old weathering crust (Skarpelis et al., 1993). The sample was analysed mineralogically in detail by ore microscopy and X-ray diffraction. The mineral chemistry was determined by electron microprobe analysis. The Kastoria laterite consists, predominantly, of nickeliferous serpentine [Mg3Si2O5(OH)4], nickeliferous magnesian-cronstedtite, calcite, and quartz. Goethite, hematite, saponite [(Ca, Na)0.66Mg6(Si7.34Al0.66) O20(OH)4. nH2O], chromite, talc [Mg6Si8O20(OH)4], and tremolite [Ca2Mg5Si8O22(OH)2] are present in small amounts. Serpentine is the most important mineral. In some places it is replaced by cronstedtite [(Fe2 +8Fe3 +4) (Si4Fe3 +4)O20(OH)16], an iron-rich sheet silicate mineral. Calcite appears as a vein mineral as well as in small pockets within the serpentine matrix making it very difficult to separate from the ore. Quartz aggregates, pseudo-morphically replacing serpentine, to some extent, are often associated with calcite. The quartz aggregates commonly occur as small pockets in the matrix of serpentine. The goethite content in the ore is approximately 5%. Goethite appears in veinlets or in small grains distributed in the matrix of serpentine. It is more abundant in places where cronstedtite predominates. Chromite occurs in small grains within the matrix of serpentine and cronstedtite. Its content is about 1%. Small amounts of chlorite, saponite, talc flakes, and tremolite are also present.
S. Agatzini-Leonardou et al. / Hydrometallurgy 74 (2004) 259–265 Table 2 Size analysis of the ‘‘Kastoria’’ ore
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Table 3 Size analysis of the ‘‘Kastoria’’ magnetic separation feed
Particle size (mm)
Weight on screen (g)
Percentage
Particle size (mm)
Weight on screen (kg)
Weight (%)
50 + 25 25 + 19 19 + 12.7 12.7 + 9.5 9.5 + 6.8 6.8 + 4.75 4.75 + 2 2+1 1 + 0.5 0.5 + 0.15 0.15 Total
957.7 270.4 498.3 198.1 340.9 108.3 703.2 443.9 420.9 589.3 326.8 4857.8
19.71 5.57 10.26 4.08 7.02 2.23 14.48 9.14 8.66 12.13 6.73 100.00
50 + 10 10 + 4 4+2 2+1 1 Total
9.04 5.24 4.05 2.83 10.47 31.63
28.6 16.6 12.8 8.9 33.1 100.00
The microprobe analysis showed that the minerals serpentine and cronstedtite constitute the main nickel carriers in the ‘‘Kastoria’’ ore. More specifically, the NiO content in serpentine ranges from 0.98% to 6.38%, with a mean value from 20 analyses of 3.44%. The NiO content in cronstedtite ranges from 2.78% to 11.5%, averaging 5.86%. Saponite, goethite, and hematite are also nickeliferous minerals, containing on the average 1.6%, 1.3%, and 1.8% NiO, respectively; however, their contribution to the nickel
content of the ore is limited, given their small relative amounts. As previously described, the above mineralogical analysis indicated that the main nickeliferous minerals were serpentine and cronstedtite while the main gangue minerals were quartz and calcite. Taking into consideration that nickeliferous minerals such as garnierite, limonite, and serpentine are slightly magnetic, while quartz and calcite are nonmagnetic, only a high intensity magnetic field would achieve their separation. This requires the use of high gradient magnetic separators (HGMS). Such a separation would be effective provided that an adequate degree of mineral liberation has been previously achieved by grinding. Considering that a large fraction of fines in
Fig. 1. Distribution of chemical constituents in the particle size fractions of the ‘‘Kastoria’’ ore.
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Table 4 Size analysis of the ‘‘Kastoria’’ autogenous grinding product Particle size (mm)
Weight on screen (kg)
Weight (%)
50 + 10 10 + 4 4+2 2+1 1 Total
5.15 0.75 0.21 0.11 2.82 9.04
57.0 8.3 2.3 1.2 31.2 100.00
the ore is not preferable for heap leaching, determining the coarsest size fraction of ore, for which magnetic separation is effective, was a major goal of this study. An amount of 600 kg from the initial ‘‘Kastoria’’ ore sample was homogenized in a mixing drum and divided by the ‘‘cross’’ method and homogenization to give subsamples of 150 kg and subsequently 36.5 kg. One of the 36.5-kg subsamples was further divided and a 4.9-kg portion was finally taken for dry size analysis. The results are shown in Table 2. A sample of approximately 100 g from each size fraction was taken, using a Jones riffle splitter, ground to 100 mesh, and chemically analysed in order to determine which size fractions would either be rejected, as being barren of nickel and high in calcium, or further be processed by magnetic separation.
Table 5 Results of autogenous grinding tests on the fraction of the ‘‘Kastoria’’ ore Particle size (mm) wt.% Fe2O3 Ni Co SiO2 CaO MgO
50 + 10 57 7.04 0.96 0.03 37.65 23.09 9.62
10 + 4 8.3 8.04 1.05 0.04 32.52 23.65 9.79
4+2 2.3 8.46 0.54 0.04 34.23 22.31 9.62
50 + 10 mm size 2+1
1.2 9.47 0.57 0.04 32.95 21.27 10.28
Feed
31.2 100 13.47 9.19 2.00 1.28 0.07 0.04 29.09 34.42 18.39 21.63 14.10 11.04
The results of the chemical analysis of the ore size fractions are given in Fig. 1. The rest of the 36.5-kg sample (31.6 kg) was screened, using 10-, 4-, 2-, and 1-mm sieves, for processing by different magnetic separators available in the laboratory. The size fractions thereby produced are given in Table 3.
3. Results and discussion The 50 + 10 mm size fraction was fed to a rotating autogenous grinding drum in order to effect rock rejection, prior to feeding the ore to magnetic separators. The product, after 15-min grinding in the drum and water washing of the coarsest fraction, was passed through the same as above sieve sizes and the results are given in Table 4. The 50 + 10 mm fraction of the autogenous grinding product was set aside for possible rejection,
Table 6 Results of magnetic separation tests on the fraction of the ‘‘Kastoria’’ ore
Fig. 2. Variation of Ni/CaO ratio with particle size in the serpentinic run-of-mine ‘‘Kastoria’’ ore.
1
10 + 4 mm size
Material
Magnetic product
Nonmagnetic product
Feed
wt.% Ni CaO
64.9 1.70 16.09
35.1 1.07 25.47
100 1.48 19.38
Material
Ni units
Ni distribution (%)
CaO units
CaO distribution (%)
Magnetic Nonmagnetic Feed
1.10 0.38
74.32 25.68 100.00
10.44 8.94
53.87 46.13 100.00
S. Agatzini-Leonardou et al. / Hydrometallurgy 74 (2004) 259–265 Table 7 Results of magnetic separation tests on the of the ‘‘Kastoria’’ ore
4 + 2 mm size fraction
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Table 9 Results of magnetic separation tests on the of the ‘‘Kastoria’’ ore (low field intensity)
2 + 1 mm size fraction
Material
Magnetic product
Nonmagnetic product
Feed
Material
Magnetic product
Nonmagnetic product
Feed
wt.% Ni CaO
82 1.69 13.15
18 0.46 35.35
100 1.47 17.14
wt.% Ni CaO
89.8 1.74 12.26
10.2 0.40 34.70
100 1.60 14.55
Material
Ni units
Ni distribution (%)
CaO units
CaO distribution (%)
Material
Ni units
Ni distribution (%)
CaO units
CaO distribution (%)
Magnetic Nonmagnetic Feed
1.39 0.08
94.56 5.44 100.00
10.78 6.36
62.89 37.11 100.00
Magnetic Nonmagnetic Feed
1.56 0.04
97.50 2.50 100.00
11.01 3.54
75.67 24.33 100.00
depending on its nickel content, while the other fractions were added to the respective fractions of the initial sample for the magnetic separation study. The 10 + 4 and 4 + 2 mm fractions were fed to a permanent magnet HGMS machine, while the 1 and 2 + 1 mm fractions were processed in a variable intensity magnetic separation device. As seen in Fig. 2, no fraction of the run-of-mine ore can be rejected as having a sufficiently low Ni/ CaO ratio. Likewise, only the finest fraction ( 0.15 mm) can be passed on to the leaching stage as having a significantly high Ni/CaO ratio. However, it was decided that all fractions should be processed for calcite rejection prior to leaching. The results of the autogenous grinding of the ‘‘Kastoria’’ ore are shown in Table 5, while the magnetic separation results on various size fractions are presented in Tables 6 –11. A
nickel content of less than 0.5% was targeted for the material rejected, whereas the lowest possible CaO content was desirable for the upgraded material. As seen in Table 5, the coarse fraction ( 50 + 10 mm) of the autogenous grinding product contained 0.96% nickel and it could not be rejected. This fraction was subsequently ground to 10 mm and added to the magnetic separation feed material. It was concluded that a feed material of smaller particle size is necessary for an efficient separation by autogenous grinding. Tables 6 – 11 show that, in most instances, a satisfactory degree of separation of calcite from the ore can be achieved by the use of a strong magnetic field, depending on feed particle size. This incurred relatively small losses of nickel in the nonmagnetic product according to the feed material. As seen in
Table 8 Results of magnetic separation tests on the 2 + 1 mm size fraction of the ‘‘Kastoria’’ ore (high field intensity)
Table 10 Results of magnetic separation tests on the the ‘‘Kastoria’’ ore (high field intensity)
Material
Magnetic product
Nonmagnetic product
Feed
Material
Magnetic product
Nonmagnetic product
Feed
wt.% Ni CaO
84.6 1.85 10.97
15.4 0.47 32.49
100 1.64 14.28
wt.% Ni CaO
94.6 1.98 11.73
5.4 0.61 32.91
100 1.90 12.88
Material
Ni units
Ni distribution (%)
CaO units
CaO distribution (%)
Material
Ni units
Ni distribution (%)
CaO units
CaO distribution (%)
Magnetic Nonmagnetic Feed
1.57 0.07
95.73 4.27 100.00
9.28 5.00
64.99 35.01 100.00
Magnetic Nonmagnetic Feed
1.87 0.03
98.42 1.58 100.00
11.10 1.78
86.18 13.82 100.00
1 mm size fraction of
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Table 11 Results of magnetic separation tests on the the ‘‘Kastoria’’ ore (low field intensity)
1 mm size fraction of
Material
Magnetic product
Nonmagnetic product
Feed
wt.% Ni CaO
95.3 1.92 12.34
4.7 0.65 32.74
100 1.86 13.30
Material
Ni units
Ni distribution (%)
CaO units
CaO distribution (%)
Magnetic Nonmagnetic Feed
1.83 0.03
98.39 1.61 100.00
11.76 1.54
88.42 11.58 100.00
Tables 6– 11, nickel losses were 5%, at most, for a feed particle size less than 4 mm. The respective percentage CaO removal was about 37%, which was considered adequate for the enhancement of the permeability of this ore. On the other hand, magnetic separation of the 1-mm fraction resulted in small amounts of nonmagnetic products in the order of 5% of the feed. Therefore, processing of this fraction was considered unnecessary. The 10 + 4 mm fraction should be ground to 4 mm in order to achieve sufficient calcite liberation.
4. Conclusions Based on the above experimental findings, the following mineral processing scheme was devised and applied for the ‘‘Kastoria’’ ore in order to prepare a suitable sulphuric acid heap or agitation leaching feed: 1. Classification of the 50-mm feed material by 16, 10, 4, and 1 mm size screens. 2. Crushing of the 50 + 16 mm ore in a jaw crusher to the size of 16 mm. 3. Classification of the crusher product to the sizes of stage 1. 4. Autogenous grinding of the 16 + 4 mm fraction in a revolving drum. 5. Classification of the autogenous grinding product to the sizes of stage 1. 6. Chemical analysis of the coarse size fractions for possible rejection of those with less than 0.5% Ni.
7. Crushing to 4 mm of coarser fractions with nickel more than 0.5%. 8. Magnetic separation of the 4 + 1 mm fraction in a HGMS device. The above scheme is expected to improve the leaching behaviour of the serpentinic ore by lowering the acid consumption during both agitation and heap leaching as well as improving the permeability of the ore heap.
Acknowledgements The authors wish to express their gratitude to the European Commission, Directorate General XII, for financial support according to contract no. BRE2CT94-1020 and, also, to G.M.M.S.A. LARCO for the laterite samples provided.
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