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Beneficiation of a Greek serpentinic nickeliferous ore Part II. Sulphuric acid heap and agitation leaching
Hydrometallurgy 74 (2004) 267 – 275 www.elsevier.com/locate/hydromet
Beneficiation of a Greek serpentinic nickeliferous ore Part II. Sulphuric acid heap and agitation leaching Stella Agatzini-Leonardou*, Ioannis G. Zafiratos Department of Mining and Metallurgical Engineering, Laboratory of Metallurgy National Technical University of Athens 9, Ir. Polytechniou Street, 15780 Zografos Athens, Greece Received 18 February 2004; received in revised form 24 May 2004; accepted 28 May 2004
Abstract Serpentinic laterite ore, after suitable mineral processing operations as described in Part I, was subjected to heap as well as atmospheric agitation leaching. The heap leach results showed that serpentinic laterites can be leached by this technique. The nickel and cobalt recoveries achieved were 60% and 45%, respectively, in 10 days. The sulphuric acid consumption amounted to 40 kg H2SO4/kg Ni. However, the effectiveness of leaching depended on the calcite content of the ore as this adversely affected the permeability of the heap. The agitation leach tests showed that the ore could be leached at atmospheric pressure without problems. Recoveries achieved were up to 74% for nickel and 51% for cobalt, after 2 h of leaching. The sulphuric acid consumption, depending on conditions, averaged 55 kg/kg Ni. Pulp mixing and filtration were excellent. D 2004 Elsevier B.V. All rights reserved. Keywords: Nickel laterite ores; Serpentine; Sulphuric acid; Heap leaching; Agitation leaching
1. Introduction Until recently, atmospheric pressure leaching of laterites with sulphuric acid had been studied by a relatively small number of investigators, including Apostolidis (1974), Tzouvelekis and Velouzos (1975) of LARCO S.A., Apostolidis and Distin (1978), Canterford (1978,1979), Chander (1982), the Laboratory of Metallurgy of the National Technical University of Athens (Papangelakis, 1983; Tzeferis, 1986; Panagiotopoulos, 1989; Panagiotopoulos et al., 1986; Halikia, 1987, 1991; Komnitsas, 1988) Sukla and Das (1986), * Corresponding author. Tel.: +30-210-7722216; +30-2107722234; fax: +30-210-7722218. 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.2004.05.006
Hirasawa and Horita (1987), Das et al. (1997) and Griffin et al. (2002). The bulk of the work on sulphuric acid leaching of laterites concerns pressure leaching. However, because of the problems encountered during commissioning and operation of the modern Australian plants (Murrin Murrin, Cawse and Bulong), atmospheric pressure leaching has recently received much closer consideration (Neudorf and Arroyo, 2002; Harris et al., 2003). The aim of the present work was to investigate the leachability of the ‘‘Kastoria’’ orebody by atmospheric sulphuric acid leaching, using both heap and agitation leaching techniques. The ‘‘Kastoria’’ orebody contains two different layers of laterite, a lower serpentinic with a limonitic layer above. The serpen-
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tinic component was tested during this work after an upgrading process to reject its calcite content (Part I). The limonitic component was tested without prior upgrading treatment. Both ore types were leached in columns, simulating heap leaching, by dilute sulphuric acid. The serpentinic fraction was also leached in stirred tank reactors.
2. Materials and methods 2.1. Column leach tests The chemical analyses of the limonitic and the serpentinic ores are shown in Table 1. In order to conduct a preliminary evaluation of the leaching behaviour of each type of ore, three small columns were initially used. These were of 10 cm internal diameter, 60 cm height and were filled with 6.4 kg of ore. One was loaded with limonitic ‘‘Kastoria’’, with grain size 15 mm, and the other two with upgraded serpentinic ‘‘Kastoria’’, with grain sizes 4 + 1 mm and 1 mm. The sulphuric acid leach solution was of 2N or 3N concentration. The solution was uniformly introduced onto the top surface of each column at a flow-rate of 4 L/day (500 L/ day/m2). Following the above preliminary experiments, another three columns were subsequently used. These had the same diameter (10 cm) and various heights: 140, 170 and 190 cm. They were loaded with 1 mm upgraded serpentinic ‘‘Kastoria’’ ore.
Table 1 Chemical analysis of the ‘‘Kastoria’’ ore samples Component (wt.%)
Limonitic ore
Ni (NiO) Co (CoO) Fe (Fe2O3) Al (Al2O3) Cr (Cr2O3) Ca (CaO) Mg (MgO) Mn (Mn3O4) SiO2 Loss on ignition at 1000 jC
1.05 0.05 38.69 4.34 1.45 1.24 1.52 0.57 21.65 8.73
15 mm (1.34) (0.06) (55.23) (8.20) (2.12) (1.74) (2.52) (0.79)
Upgraded serpentinic ore 1 mm 1.94 0.07 13.17 0.81 0.61 5.65 9.80 0.20 34.40 17.32
(2.47) (0.11) (18.84) (1.53) (0.90) (7.90) (16.24) (0.27)
4 + 1 mm 1.98 0.07 12.36 0.75 0.54 8.32 9.67 0.18 33.32 16.74
(2.52) (0.10) (17.68) (1.43) (0.79) (11.64) (16.04) (0.26)
Table 2 Agitation leach tests minimum and maximum levels of variables Factors
Variables
Low level
A B
Ore grain size Sulphuric acid concentration Temperature
100 mesh 3N
C
40 jC
Intermediate level
High level 1 mm 5N
60 jC
80 jC
Prior to loading into each column, the ore was homogeneously wetted with water, in a mixer, to raise total moisture from 6.5 – 7% (natural moisture) to 10%. This agglomeration was found to be beneficial for reasons explained in a previous paper (AgatziniLeonardou and Dimaki, 1994). After passing through the column, the solution was collected from the bottom in a pregnant solution tank. There, the free acid concentration was measured and adjusted to the initial predetermined value. Then, a dosometric pump recirculated it for another leach cycle to start. A number of leach cycles were thus performed until maximum nickel extraction from the ore was achieved. Finally, the exhausted ore was washed with water in the column. Samples of the pregnant solution were collected at the end of each leach cycle and analysed for nickel, cobalt, iron, calcium, magnesium, chromium, aluminium and silicon by atomic absorption spectrophotometry. For concentrations of iron higher than 0.10 g/L, volumetric analysis was used as the most reliable method. The free acidity of the pregnant solution could not be determined reliably by measuring the solution pH because of the high acidity and the relatively high ionic strength and density of the solutions encountered. Besides, titration with sodium hydroxide was not feasible due to the presence of hydrolysable cations (e.g., Fe3 +) in relatively high concentrations. Therefore, a titration method developed previously in this laboratory was used (AgatziniLeonardou and Dimaki, 1994; Agatzini-Leonardou et al., 1997). 2.2. Agitation leach tests The agitation leaching tests with sulphuric acid were carried out in mechanically stirred 500 ml spherical flasks at atmospheric pressure under various conditions (see Table 2). The feed was upgraded
S. Agatzini-Leonardou, I.G. Zafiratos / Hydrometallurgy 74 (2004) 267–275
Fig. 1. % metal recovery during heap leaching of limonitic ‘‘Kastoria’’ ore Grain size rate 4 L/day (500 L/day/m2), Ore weight 6.6 kg, Column height 60 cm).
serpentinic ‘‘Kastoria’’ ore, obtained as described in Part I and in the work of Spathis (1999). After 2 h of leaching, the slurry was filtered and the primary leach liquor was analysed for metal cations by atomic absorption spectrophotometry. The leach residue was then washed with acidified water and the filtrate was also analysed for metal cations.
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15 mm. (Sulphuric acid concentration 3 N, L/O 1, Flow
3. Results and discussion 3.1. Column leach tests The results of the column tests are presented in Figs. 1 –6 . It can be seen that, in the small columns, leaching of both types of ore was successful, yielding
Fig. 2. % metal recovery during heap leaching of upgraded serpentinic ‘‘Kastoria’’ ore. Grain size N, L/O 1, Flow rate 4 L/day (500 L/day/m2), Ore weight 6.6 kg, Column height 60 cm).
4 + 1 mm. (Sulphuric acid concentration 2
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Fig. 3. % metal recovery during heap leaching of upgraded serpentinic ‘‘Kastoria’’ ore. Grain size L/O 1, Flow rate 4 L/day (500 L/day/m2), Ore weight 6.6 kg, Column height 60 cm).
nickel recoveries around 60% in less than 9 days. Iron co-dissolution was not higher than 12%. Thus, the Fe/ Ni ratio, equal to 37:1 in the limonitic ore, was reduced to 5:1 in the leach liquor, while that of the serpentinic ore was reduced from 6.2– 6.8:1 to 1.0 – 1.4:1. This finding reconfirms earlier studies showing that heap
1 mm. (Sulphuric acid concentration 2 N,
leaching of the Greek nickel ores is highly selective for nickel (Agatzini-Leonardou and Dimaki, 1994; Agatzini-Leonardou et al., 1997). Cobalt extraction was lower than that of nickel, ranging from 30% to 45%. Generally, the limonitic and the serpentinic components of the deposit were
Fig. 4. % metal recovery during heap leaching of – 1 mm upgraded serpentinic ‘‘Kastoria’’ ore. Column height 140 cm (Sulphuric acid concentration 2 N, L/O 1, Flow rate 18 L/day (2300 L/day/m2), Ore weight 15.4 kg).
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Fig. 5. % metal recovery during heap leaching of 1 mm upgraded serpentinic ‘‘Kastoria’’ ore. Column height 170 cm (Sulphuric acid concentration 2 N, L/O 1, Flow rate 18 L/day (2300 L/day/m2), Ore weight 18.7 kg).
leached to the same extent and only showed a difference in the iron and magnesium content of the leach liquor due to the high magnesium and low iron content of the latter component. Unfortunately, the taller columns encountered percolation problems, which slowed down the process considerably. In particular, the 190-cm column had
to be stopped before the completion of leaching. These problems were attributed to gypsum formation due to the high remaining calcite content in the upgraded ore. Furthermore, the very high solution flow rate (2300 L/day/m2) applied may also have contributed to this problem if there was migration of fines within the column. Despite these drawbacks,
Fig. 6. % metal recovery during heap leaching of 1 mm upgraded serpentinic ‘‘Kastoria’’ ore. Column height 190 cm (Sulphuric acid concentration 2 N, L/O 1, Flow rate 18 L/day (2300 L/day/m2), Ore weight 20.9 kg).
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Table 3 Percent recovery of metal cations during agitation leaching of the upgraded serpentinic ‘‘Kastoria’’ ore before washing Ni (%)
Co (%)
Fe (%)
Mg (%)
I
II
Average
I
II
Average
I
II
(a0b0c0) (a1b0c0) (a0b1c0) (a1b1c0) (a0b0c1) (a1b0c1) (a0b1c1) (a1b1c1) (a0b0c2) (a1b0c2) (a0b1c2) (a1b1c2)
40.1 F 6.2 32.2 F 6.2 37.0 F 6.2 44.6 F 6.2 34.3 F 6.2 43.8 F 6.2 49.9 F 6.2 53.3 F 6.2 37.9 F 6.2 43.1 F 6.2 44.1 F 6.2 74.0 F 6.2
42.4 F 6.2 36.1 F 6.2 41.8 F 6.2 53.3 F 6.2 27.3 F 6.2 49.2 F 6.2 65.9 F 6.2 59.7 F 6.2 49.4 F 6.2 31.6 F 6.2 54.6 F 6.2 66.9 F 6.2
41.3 F 4.4 34.2 F 4.4 39.4 F 4.4 49.0 F 4.4 30.8 F 4.4 46.5 F 4.4 57.9 F 4.4 56.5 F 4.4 43.7 F 4.4 37.3 F 4.4 49.4 F 4.4 70.4 F 4.4
29.6 F 4.7 20.1 F 4.7 28.3 F 4.7 27.2 F 4.7 22.3 F 4.7 35.2 F 4.7 28.7 F 4.7 39.8 F 4.7 30.1 F 4.7 17.8 F 4.7 35.1 F 4.7 40.7 F 4.7
28.4 F 4.7 22.8 F 4.7 29.5 F 4.7 36.0 F 4.7 25.1 F 4.7 44.7 F 4.7 38.5 F 4.7 41.8 F 4.7 20.2 F 4.7 28.1 F 4.7 33.4 F 4.7 46.8 F 4.7
29.0 F 3.3 21.4 F 3.3 28.9 F 3.3 31.6 F 3.3 23.7 F 3.3 39.9 F 3.3 33.6 F 3.3 40.8 F 3.3 25.2 F 3.3 22.9 F 3.3 34.3 F 3.3 43.8 F 3.3
10.7 F 2.4 7.1 F 2.4 9.8 F 2.4 11.0 F 2.4 4.6 F 2.4 9.8 F 2.4 14.6 F 2.4 19.3 F 2.4 8.8 F 2.4 4.5 F 2.4 12.9 F 2.4 17.9 F 2.4
4.5 F 2.4 8.8 F 2.4 11.6 F 2.4 14.8 F 2.4 4.0 F 2.4 4.5 F 2.4 15.2 F 2.4 24.3 F 2.4 9.8 F 2.4 4.3 F 2.4 14.6 F 2.4 22.1 F 2.4
Average 7.6 F 1.7 8.0 F 1.7 10.4 F 1.7 12.6 F 1.7 4.3 F 1.7 7.1 F 1.7 14.9 F 1.7 21.8 F 1.7 9.3 F 1.7 4.4 F 1.7 13.7 F 1.7 20.0 F 1.7
I
II
Average
38.8 F 8.0 37.7 F 8.0 38.5 F 8.0 46.1 F 8.0 41.0 F 8.0 51.5 F 8.0 54.7 F 8.0 55.3 F 8.0 38.9 F 8.0 27.3 F 8.0 69.4 F 8.0 65.1 F 8.0
39.9 F 8.0 53.1 F 8.0 42.1 F 8.0 68.7 F 8.0 29.5 F 8.0 38.0 F 8.0 65.9 F 8.0 47.3 F 8.0 35.4 F 8.0 36.3 F 8.0 78.6 F 8.0 73.5 F 8.0
39.3 F 5.6 45.4 F 5.6 40.3 F 5.6 57.4 F 5.6 35.2 F 5.6 44.7 F 5.6 60.3 F 5.6 51.3 F 5.6 37.1 F 5.6 31.8 F 5.6 74.0 F 5.6 69.3 F 5.6
II
Average
57.3 F 9.7 66.3 F 9.7 60.2 F 9.7 78.6 F 9.7 34.4 F 9.7 47.5 F 9.7 79.0 F 9.7 58.4 F 9.7 52.3 F 9.7 38.6 F 9.7 82.6 F 9.7 85.5 F 9.7
53.2 F 6.9 61.8 F 6.9 54.2 F 6.9 70.3 F 6.9 43.6 F 6.9 54.1 F 6.9 73.3 F 6.9 65.1 F 6.9 53.1 F 6.9 35.5 F 6.9 81.0 F 6.9 79.9 F 6.9
All experimental errors have been calculated with U = 12 degrees of freedom.
Table 4 Percent recovery of metal cations during agitation leaching of the upgraded serpentinic ‘‘Kastoria’’ ore after washing Treatment Combination
Ni (%)
Co (%)
Fe (%)
I
II
Average
I
II
Average
I
II
Average
(a0b0c0) (a1b0c0) (a0b1c0) (a1b1c0) (a0b0c1) (a1b0c1) (a0b1c1) (a1b1c1) (a0b0c2) (a1b0c2) (a0b1c2) (a1b1c2)
52.8 F 8.2 42.4 F 8.2 45.2 F 8.2 47.0 F 8.2 49.8 F 8.2 52.6 F 8.2 59.9 F 8.2 65.5 F 8.2 47.1 F 8.2 45.8 F 8.2 59.8 F 8.2 78.0 F 8.2
44.6 F 8.2 43.4 F 8.2 51.6 F 8.2 65.7 F 8.2 33.4 F 8.2 59.1 F 8.2 77.3 F 8.2 76.9 F 8.2 61.6 F 8.2 37.8 F 8.2 68.1 F 8.2 69.6 F 8.2
48.7 F 5.8 42.9 F 5.8 48.4 F 5.8 56.3 F 5.8 41.6 F 5.8 55.8 F 5.8 68.6 F 5.8 71.2 F 5.8 54.4 F 5.8 41.8 F 5.8 63.9 F 5.8 73.8 F 5.8
39.0 F 6.2 26.5 F 6.2 34.6 F 6.2 42.2 F 6.2 32.5 F 6.2 42.3 F 6.2 38.3 F 6.2 44.5 F 6.2 31.9 F 6.2 21.0 F 6.2 47.6 F 6.2 42.9 F 6.2
29.9 F 6.2 27.3 F 6.2 36.4 F 6.2 44.4 F 6.2 30.8 F 6.2 50.7 F 6.2 49.9 F 6.2 58.1 F 6.2 28.1 F 6.2 36.2 F 6.2 34.8 F 6.2 48.7 F 6.2
34.4 F 4.4 26.9 F 4.4 35.5 F 4.4 43.3 F 4.4 31.6 F 4.4 46.5 F 4.4 44.1 F 4.4 51.3 F 4.4 30.0 F 4.4 28.6 F 4.4 41.2 F 4.4 45.8 F 4.4
13.8 F 2.8 9.2 F 2.8 15.3 F 2.8 14.2 F 2.8 5.9 F 2.8 15.3 F 2.8 22.7 F 2.8 25.0 F 2.8 11.4 F 2.8 5.8 F 2.8 16.7 F 2.8 23.1 F 2.8
5.8 F 2.8 13.8 F 2.8 13.5 F 2.8 17.2 F 2.8 4.7 F 2.8 8.0 F 2.8 27.5 F 2.8 28.2 F 2.8 11.4 F 2.8 5.0 F 2.8 16.9 F 2.8 25.6 F 2.8
9.8 F 2.0 11.5 F 2.0 14.4 F 2.0 15.7 F 2.0 5.3 F 2.0 11.7 F 2.0 25.1 F 2.0 26.6 F 2.0 11.4 F 2.0 5.4 F 2.0 16.8 F 2.0 24.4 F 2.0
All experimental errors have been calculated with U = 12 degrees of freedom.
Mg (%) I 49.1 F 9.7 57.3 F 9.7 48.3 F 9.7 61.9 F 9.7 52.9 F 9.7 60.7 F 9.7 67.6 F 9.7 71.7 F 9.7 53.8 F 9.7 32.4 F 9.7 79.5 F 9.7 74.2 F 9.7
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Treatment combination
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the nickel recoveries obtained (40 – 55%) were moderately good and show that the nickeliferous serpentine can be leached by dilute sulphuric acid at ambient temperature. Sulphuric acid consumption was high in all columns because the magnesium contained in the nickeliferous minerals and the incomplete calcite removal. For the serpentinic ore, it ranged from 36 to 44 kg H2SO4/kg Ni, depending on the conditions, whereas that for the limonitic ore was 27 kg H2SO4/kg Ni. The highest figure observed was 1.4 times the sulphuric acid consumption reported for the Bulong and Murrin Murrin plants and 1.8 times that for the Cawse plant for pressure leaching of laterite (Mayze, 1999). Aluminium extraction values were around 12%, while silica was not dissolved during the leach process. Detailed concentration values for all metal cations in the various leach liquors, including Al, Cr, Si and Mn, are not given here, due to space limitations, but they are available on request. 3.2. Agitation leach tests A factorial experiment 2 2 3, in duplicate, was conducted in order to study the effects and interactions of the factors on agitation leaching efficiency. The factors studied and their levels are given in Table 2. Responses measured were the metal cation recoveries in the leach solution and the sulphuric acid consumption. The results of the tests are presented in Tables 3 and 4. Recoveries achieved, after leach residue washing were up to 74% for nickel and 51% for cobalt after 2 h. The corresponding sulphuric acid consumption was around 55 kg H2SO4/kg Ni, confirming earlier work in our laboratory (Papangelakis, 1983). The Fe/Ni ratio, equal to 6.8:1 in the ore, was reduced to 1.4:1 in the leach liquor, showing again the selectivity for nickel over iron during atmospheric pressure sulphuric acid leaching of the Greek laterite ores. On the contrary, the Mg/Ni ratio in the leach liquor (5.2:1) was on average almost equal to the value of this ratio in the ore (5.1:1). This indicates that both nickel and magnesium come into the leach liquor from the dissolution of serpentine and cronstedtite, the main nickel carriers in the ‘‘Kastoria’’ ore, which are leached preferentially to haematite and goethite.
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The data for the various responses were statistically analysed. As the statistical analysis for nickel recovery showed, the only statistically significant factor was the sulphuric acid concentration, at the significance level a = 0.01. Its main effect was positive, within the range of levels of factors studied. As no interactions were found, it was concluded that nickel recovery increases with sulphuric acid concentration, independently of the levels of the other factors. Therefore, agitation leaching can be conducted at temperatures slightly above ambient and without excessive ore grinding. Cobalt recovery as well as magnesium and iron co-dissolution were also found to increase with increasing the sulphuric acid concentration. The similarity in the leaching behaviour of nickel, cobalt, magnesium and iron, exposed by the statistical analysis indicates that these metals are leached from the same mineralogical phases, in particular serpentine and cronstedtite. It should be stressed that during agitation leaching, pulp mixing and filtration were excellent. This fact indicates that the mild leaching conditions used did not substantially fragment the serpentine silicate lattice to produce fine silica (silicate) particles in the liquor.
4. Conclusions 1. The nickeliferous ores from the ‘‘Kastoria’’ deposit can be leached by dilute sulphuric acid at atmospheric pressure, using both heap and agitation leach processes. 2. Calcite contained in the ore adversely affects the permeability of the heap and the sulphuric acid consumption. Its removal prior to leaching is an important pre-requisite. 3. The nickeliferous serpentine is dissolved in preference to the haematite and goethite minerals under the mild conditions for heap leaching. In the case of agitation leaching, the selectivity is lower. 4. Nickel and cobalt recoveries achieved during heap leaching were 60% and 45%, respectively, in 10 days. It is thought that these values would be higher if less calcite was present in the upgraded ore. The sulphuric acid consumption amounted to 40 kg H2SO4/kg Ni.
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5. Maximum nickel and cobalt recoveries obtained during agitation leaching, within the range of values studied, were 74% for nickel and 51% for cobalt, after 2 h. The average sulphuric acid consumption was 55 kg/kg Ni. The major acid consumers were residual calcite in the upgraded ore and magnesium in the nickeliferous phase of serpentine. Nickel and cobalt recoveries as well as magnesium and iron co-dissolution were found to depend only on sulphuric acid concentration, increasing independently of the ore grain size and the temperature. 6. The agitation leaching technique was readily applicable to the ’’Kastoria’’ ore, provided that market values of sulphuric acid and nickel are favourable. Further research is being conducted in order to improve the nickel/cobalt extractions and lower the acid consumption.
Acknowledgements The authors wish to express their gratitude to the European Commission, Directorate General XII, for financial support according to the contract no. BRE2CT94-1020 and also to G.M.M.S.A. LARCO for the laterite samples provided.
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pressure. Diploma Thesis. Department of Mining and Metallurgical Engineering, TUA, Athens (in Greek). Tzouvelekis, K., Velouzos, A., 1975. A pyro-hydrometallurgical method for the recovery of nickel and cobalt from low-grade nickeliferous oxide ores. Greek Patent No. 51250, Ministry of Commerce of Greece, 27 March 1975.
Beneficiation of a Greek serpentinic nickeliferous ore Part II. Sulphuric acid heap and agitation leaching Stella Agatzini-Leonardou*, Ioannis G. Zafiratos Department of Mining and Metallurgical Engineering, Laboratory of Metallurgy National Technical University of Athens 9, Ir. Polytechniou Street, 15780 Zografos Athens, Greece Received 18 February 2004; received in revised form 24 May 2004; accepted 28 May 2004
Abstract Serpentinic laterite ore, after suitable mineral processing operations as described in Part I, was subjected to heap as well as atmospheric agitation leaching. The heap leach results showed that serpentinic laterites can be leached by this technique. The nickel and cobalt recoveries achieved were 60% and 45%, respectively, in 10 days. The sulphuric acid consumption amounted to 40 kg H2SO4/kg Ni. However, the effectiveness of leaching depended on the calcite content of the ore as this adversely affected the permeability of the heap. The agitation leach tests showed that the ore could be leached at atmospheric pressure without problems. Recoveries achieved were up to 74% for nickel and 51% for cobalt, after 2 h of leaching. The sulphuric acid consumption, depending on conditions, averaged 55 kg/kg Ni. Pulp mixing and filtration were excellent. D 2004 Elsevier B.V. All rights reserved. Keywords: Nickel laterite ores; Serpentine; Sulphuric acid; Heap leaching; Agitation leaching
1. Introduction Until recently, atmospheric pressure leaching of laterites with sulphuric acid had been studied by a relatively small number of investigators, including Apostolidis (1974), Tzouvelekis and Velouzos (1975) of LARCO S.A., Apostolidis and Distin (1978), Canterford (1978,1979), Chander (1982), the Laboratory of Metallurgy of the National Technical University of Athens (Papangelakis, 1983; Tzeferis, 1986; Panagiotopoulos, 1989; Panagiotopoulos et al., 1986; Halikia, 1987, 1991; Komnitsas, 1988) Sukla and Das (1986), * Corresponding author. Tel.: +30-210-7722216; +30-2107722234; fax: +30-210-7722218. 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.2004.05.006
Hirasawa and Horita (1987), Das et al. (1997) and Griffin et al. (2002). The bulk of the work on sulphuric acid leaching of laterites concerns pressure leaching. However, because of the problems encountered during commissioning and operation of the modern Australian plants (Murrin Murrin, Cawse and Bulong), atmospheric pressure leaching has recently received much closer consideration (Neudorf and Arroyo, 2002; Harris et al., 2003). The aim of the present work was to investigate the leachability of the ‘‘Kastoria’’ orebody by atmospheric sulphuric acid leaching, using both heap and agitation leaching techniques. The ‘‘Kastoria’’ orebody contains two different layers of laterite, a lower serpentinic with a limonitic layer above. The serpen-
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tinic component was tested during this work after an upgrading process to reject its calcite content (Part I). The limonitic component was tested without prior upgrading treatment. Both ore types were leached in columns, simulating heap leaching, by dilute sulphuric acid. The serpentinic fraction was also leached in stirred tank reactors.
2. Materials and methods 2.1. Column leach tests The chemical analyses of the limonitic and the serpentinic ores are shown in Table 1. In order to conduct a preliminary evaluation of the leaching behaviour of each type of ore, three small columns were initially used. These were of 10 cm internal diameter, 60 cm height and were filled with 6.4 kg of ore. One was loaded with limonitic ‘‘Kastoria’’, with grain size 15 mm, and the other two with upgraded serpentinic ‘‘Kastoria’’, with grain sizes 4 + 1 mm and 1 mm. The sulphuric acid leach solution was of 2N or 3N concentration. The solution was uniformly introduced onto the top surface of each column at a flow-rate of 4 L/day (500 L/ day/m2). Following the above preliminary experiments, another three columns were subsequently used. These had the same diameter (10 cm) and various heights: 140, 170 and 190 cm. They were loaded with 1 mm upgraded serpentinic ‘‘Kastoria’’ ore.
Table 1 Chemical analysis of the ‘‘Kastoria’’ ore samples Component (wt.%)
Limonitic ore
Ni (NiO) Co (CoO) Fe (Fe2O3) Al (Al2O3) Cr (Cr2O3) Ca (CaO) Mg (MgO) Mn (Mn3O4) SiO2 Loss on ignition at 1000 jC
1.05 0.05 38.69 4.34 1.45 1.24 1.52 0.57 21.65 8.73
15 mm (1.34) (0.06) (55.23) (8.20) (2.12) (1.74) (2.52) (0.79)
Upgraded serpentinic ore 1 mm 1.94 0.07 13.17 0.81 0.61 5.65 9.80 0.20 34.40 17.32
(2.47) (0.11) (18.84) (1.53) (0.90) (7.90) (16.24) (0.27)
4 + 1 mm 1.98 0.07 12.36 0.75 0.54 8.32 9.67 0.18 33.32 16.74
(2.52) (0.10) (17.68) (1.43) (0.79) (11.64) (16.04) (0.26)
Table 2 Agitation leach tests minimum and maximum levels of variables Factors
Variables
Low level
A B
Ore grain size Sulphuric acid concentration Temperature
100 mesh 3N
C
40 jC
Intermediate level
High level 1 mm 5N
60 jC
80 jC
Prior to loading into each column, the ore was homogeneously wetted with water, in a mixer, to raise total moisture from 6.5 – 7% (natural moisture) to 10%. This agglomeration was found to be beneficial for reasons explained in a previous paper (AgatziniLeonardou and Dimaki, 1994). After passing through the column, the solution was collected from the bottom in a pregnant solution tank. There, the free acid concentration was measured and adjusted to the initial predetermined value. Then, a dosometric pump recirculated it for another leach cycle to start. A number of leach cycles were thus performed until maximum nickel extraction from the ore was achieved. Finally, the exhausted ore was washed with water in the column. Samples of the pregnant solution were collected at the end of each leach cycle and analysed for nickel, cobalt, iron, calcium, magnesium, chromium, aluminium and silicon by atomic absorption spectrophotometry. For concentrations of iron higher than 0.10 g/L, volumetric analysis was used as the most reliable method. The free acidity of the pregnant solution could not be determined reliably by measuring the solution pH because of the high acidity and the relatively high ionic strength and density of the solutions encountered. Besides, titration with sodium hydroxide was not feasible due to the presence of hydrolysable cations (e.g., Fe3 +) in relatively high concentrations. Therefore, a titration method developed previously in this laboratory was used (AgatziniLeonardou and Dimaki, 1994; Agatzini-Leonardou et al., 1997). 2.2. Agitation leach tests The agitation leaching tests with sulphuric acid were carried out in mechanically stirred 500 ml spherical flasks at atmospheric pressure under various conditions (see Table 2). The feed was upgraded
S. Agatzini-Leonardou, I.G. Zafiratos / Hydrometallurgy 74 (2004) 267–275
Fig. 1. % metal recovery during heap leaching of limonitic ‘‘Kastoria’’ ore Grain size rate 4 L/day (500 L/day/m2), Ore weight 6.6 kg, Column height 60 cm).
serpentinic ‘‘Kastoria’’ ore, obtained as described in Part I and in the work of Spathis (1999). After 2 h of leaching, the slurry was filtered and the primary leach liquor was analysed for metal cations by atomic absorption spectrophotometry. The leach residue was then washed with acidified water and the filtrate was also analysed for metal cations.
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15 mm. (Sulphuric acid concentration 3 N, L/O 1, Flow
3. Results and discussion 3.1. Column leach tests The results of the column tests are presented in Figs. 1 –6 . It can be seen that, in the small columns, leaching of both types of ore was successful, yielding
Fig. 2. % metal recovery during heap leaching of upgraded serpentinic ‘‘Kastoria’’ ore. Grain size N, L/O 1, Flow rate 4 L/day (500 L/day/m2), Ore weight 6.6 kg, Column height 60 cm).
4 + 1 mm. (Sulphuric acid concentration 2
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Fig. 3. % metal recovery during heap leaching of upgraded serpentinic ‘‘Kastoria’’ ore. Grain size L/O 1, Flow rate 4 L/day (500 L/day/m2), Ore weight 6.6 kg, Column height 60 cm).
nickel recoveries around 60% in less than 9 days. Iron co-dissolution was not higher than 12%. Thus, the Fe/ Ni ratio, equal to 37:1 in the limonitic ore, was reduced to 5:1 in the leach liquor, while that of the serpentinic ore was reduced from 6.2– 6.8:1 to 1.0 – 1.4:1. This finding reconfirms earlier studies showing that heap
1 mm. (Sulphuric acid concentration 2 N,
leaching of the Greek nickel ores is highly selective for nickel (Agatzini-Leonardou and Dimaki, 1994; Agatzini-Leonardou et al., 1997). Cobalt extraction was lower than that of nickel, ranging from 30% to 45%. Generally, the limonitic and the serpentinic components of the deposit were
Fig. 4. % metal recovery during heap leaching of – 1 mm upgraded serpentinic ‘‘Kastoria’’ ore. Column height 140 cm (Sulphuric acid concentration 2 N, L/O 1, Flow rate 18 L/day (2300 L/day/m2), Ore weight 15.4 kg).
S. Agatzini-Leonardou, I.G. Zafiratos / Hydrometallurgy 74 (2004) 267–275
271
Fig. 5. % metal recovery during heap leaching of 1 mm upgraded serpentinic ‘‘Kastoria’’ ore. Column height 170 cm (Sulphuric acid concentration 2 N, L/O 1, Flow rate 18 L/day (2300 L/day/m2), Ore weight 18.7 kg).
leached to the same extent and only showed a difference in the iron and magnesium content of the leach liquor due to the high magnesium and low iron content of the latter component. Unfortunately, the taller columns encountered percolation problems, which slowed down the process considerably. In particular, the 190-cm column had
to be stopped before the completion of leaching. These problems were attributed to gypsum formation due to the high remaining calcite content in the upgraded ore. Furthermore, the very high solution flow rate (2300 L/day/m2) applied may also have contributed to this problem if there was migration of fines within the column. Despite these drawbacks,
Fig. 6. % metal recovery during heap leaching of 1 mm upgraded serpentinic ‘‘Kastoria’’ ore. Column height 190 cm (Sulphuric acid concentration 2 N, L/O 1, Flow rate 18 L/day (2300 L/day/m2), Ore weight 20.9 kg).
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Table 3 Percent recovery of metal cations during agitation leaching of the upgraded serpentinic ‘‘Kastoria’’ ore before washing Ni (%)
Co (%)
Fe (%)
Mg (%)
I
II
Average
I
II
Average
I
II
(a0b0c0) (a1b0c0) (a0b1c0) (a1b1c0) (a0b0c1) (a1b0c1) (a0b1c1) (a1b1c1) (a0b0c2) (a1b0c2) (a0b1c2) (a1b1c2)
40.1 F 6.2 32.2 F 6.2 37.0 F 6.2 44.6 F 6.2 34.3 F 6.2 43.8 F 6.2 49.9 F 6.2 53.3 F 6.2 37.9 F 6.2 43.1 F 6.2 44.1 F 6.2 74.0 F 6.2
42.4 F 6.2 36.1 F 6.2 41.8 F 6.2 53.3 F 6.2 27.3 F 6.2 49.2 F 6.2 65.9 F 6.2 59.7 F 6.2 49.4 F 6.2 31.6 F 6.2 54.6 F 6.2 66.9 F 6.2
41.3 F 4.4 34.2 F 4.4 39.4 F 4.4 49.0 F 4.4 30.8 F 4.4 46.5 F 4.4 57.9 F 4.4 56.5 F 4.4 43.7 F 4.4 37.3 F 4.4 49.4 F 4.4 70.4 F 4.4
29.6 F 4.7 20.1 F 4.7 28.3 F 4.7 27.2 F 4.7 22.3 F 4.7 35.2 F 4.7 28.7 F 4.7 39.8 F 4.7 30.1 F 4.7 17.8 F 4.7 35.1 F 4.7 40.7 F 4.7
28.4 F 4.7 22.8 F 4.7 29.5 F 4.7 36.0 F 4.7 25.1 F 4.7 44.7 F 4.7 38.5 F 4.7 41.8 F 4.7 20.2 F 4.7 28.1 F 4.7 33.4 F 4.7 46.8 F 4.7
29.0 F 3.3 21.4 F 3.3 28.9 F 3.3 31.6 F 3.3 23.7 F 3.3 39.9 F 3.3 33.6 F 3.3 40.8 F 3.3 25.2 F 3.3 22.9 F 3.3 34.3 F 3.3 43.8 F 3.3
10.7 F 2.4 7.1 F 2.4 9.8 F 2.4 11.0 F 2.4 4.6 F 2.4 9.8 F 2.4 14.6 F 2.4 19.3 F 2.4 8.8 F 2.4 4.5 F 2.4 12.9 F 2.4 17.9 F 2.4
4.5 F 2.4 8.8 F 2.4 11.6 F 2.4 14.8 F 2.4 4.0 F 2.4 4.5 F 2.4 15.2 F 2.4 24.3 F 2.4 9.8 F 2.4 4.3 F 2.4 14.6 F 2.4 22.1 F 2.4
Average 7.6 F 1.7 8.0 F 1.7 10.4 F 1.7 12.6 F 1.7 4.3 F 1.7 7.1 F 1.7 14.9 F 1.7 21.8 F 1.7 9.3 F 1.7 4.4 F 1.7 13.7 F 1.7 20.0 F 1.7
I
II
Average
38.8 F 8.0 37.7 F 8.0 38.5 F 8.0 46.1 F 8.0 41.0 F 8.0 51.5 F 8.0 54.7 F 8.0 55.3 F 8.0 38.9 F 8.0 27.3 F 8.0 69.4 F 8.0 65.1 F 8.0
39.9 F 8.0 53.1 F 8.0 42.1 F 8.0 68.7 F 8.0 29.5 F 8.0 38.0 F 8.0 65.9 F 8.0 47.3 F 8.0 35.4 F 8.0 36.3 F 8.0 78.6 F 8.0 73.5 F 8.0
39.3 F 5.6 45.4 F 5.6 40.3 F 5.6 57.4 F 5.6 35.2 F 5.6 44.7 F 5.6 60.3 F 5.6 51.3 F 5.6 37.1 F 5.6 31.8 F 5.6 74.0 F 5.6 69.3 F 5.6
II
Average
57.3 F 9.7 66.3 F 9.7 60.2 F 9.7 78.6 F 9.7 34.4 F 9.7 47.5 F 9.7 79.0 F 9.7 58.4 F 9.7 52.3 F 9.7 38.6 F 9.7 82.6 F 9.7 85.5 F 9.7
53.2 F 6.9 61.8 F 6.9 54.2 F 6.9 70.3 F 6.9 43.6 F 6.9 54.1 F 6.9 73.3 F 6.9 65.1 F 6.9 53.1 F 6.9 35.5 F 6.9 81.0 F 6.9 79.9 F 6.9
All experimental errors have been calculated with U = 12 degrees of freedom.
Table 4 Percent recovery of metal cations during agitation leaching of the upgraded serpentinic ‘‘Kastoria’’ ore after washing Treatment Combination
Ni (%)
Co (%)
Fe (%)
I
II
Average
I
II
Average
I
II
Average
(a0b0c0) (a1b0c0) (a0b1c0) (a1b1c0) (a0b0c1) (a1b0c1) (a0b1c1) (a1b1c1) (a0b0c2) (a1b0c2) (a0b1c2) (a1b1c2)
52.8 F 8.2 42.4 F 8.2 45.2 F 8.2 47.0 F 8.2 49.8 F 8.2 52.6 F 8.2 59.9 F 8.2 65.5 F 8.2 47.1 F 8.2 45.8 F 8.2 59.8 F 8.2 78.0 F 8.2
44.6 F 8.2 43.4 F 8.2 51.6 F 8.2 65.7 F 8.2 33.4 F 8.2 59.1 F 8.2 77.3 F 8.2 76.9 F 8.2 61.6 F 8.2 37.8 F 8.2 68.1 F 8.2 69.6 F 8.2
48.7 F 5.8 42.9 F 5.8 48.4 F 5.8 56.3 F 5.8 41.6 F 5.8 55.8 F 5.8 68.6 F 5.8 71.2 F 5.8 54.4 F 5.8 41.8 F 5.8 63.9 F 5.8 73.8 F 5.8
39.0 F 6.2 26.5 F 6.2 34.6 F 6.2 42.2 F 6.2 32.5 F 6.2 42.3 F 6.2 38.3 F 6.2 44.5 F 6.2 31.9 F 6.2 21.0 F 6.2 47.6 F 6.2 42.9 F 6.2
29.9 F 6.2 27.3 F 6.2 36.4 F 6.2 44.4 F 6.2 30.8 F 6.2 50.7 F 6.2 49.9 F 6.2 58.1 F 6.2 28.1 F 6.2 36.2 F 6.2 34.8 F 6.2 48.7 F 6.2
34.4 F 4.4 26.9 F 4.4 35.5 F 4.4 43.3 F 4.4 31.6 F 4.4 46.5 F 4.4 44.1 F 4.4 51.3 F 4.4 30.0 F 4.4 28.6 F 4.4 41.2 F 4.4 45.8 F 4.4
13.8 F 2.8 9.2 F 2.8 15.3 F 2.8 14.2 F 2.8 5.9 F 2.8 15.3 F 2.8 22.7 F 2.8 25.0 F 2.8 11.4 F 2.8 5.8 F 2.8 16.7 F 2.8 23.1 F 2.8
5.8 F 2.8 13.8 F 2.8 13.5 F 2.8 17.2 F 2.8 4.7 F 2.8 8.0 F 2.8 27.5 F 2.8 28.2 F 2.8 11.4 F 2.8 5.0 F 2.8 16.9 F 2.8 25.6 F 2.8
9.8 F 2.0 11.5 F 2.0 14.4 F 2.0 15.7 F 2.0 5.3 F 2.0 11.7 F 2.0 25.1 F 2.0 26.6 F 2.0 11.4 F 2.0 5.4 F 2.0 16.8 F 2.0 24.4 F 2.0
All experimental errors have been calculated with U = 12 degrees of freedom.
Mg (%) I 49.1 F 9.7 57.3 F 9.7 48.3 F 9.7 61.9 F 9.7 52.9 F 9.7 60.7 F 9.7 67.6 F 9.7 71.7 F 9.7 53.8 F 9.7 32.4 F 9.7 79.5 F 9.7 74.2 F 9.7
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Treatment combination
S. Agatzini-Leonardou, I.G. Zafiratos / Hydrometallurgy 74 (2004) 267–275
the nickel recoveries obtained (40 – 55%) were moderately good and show that the nickeliferous serpentine can be leached by dilute sulphuric acid at ambient temperature. Sulphuric acid consumption was high in all columns because the magnesium contained in the nickeliferous minerals and the incomplete calcite removal. For the serpentinic ore, it ranged from 36 to 44 kg H2SO4/kg Ni, depending on the conditions, whereas that for the limonitic ore was 27 kg H2SO4/kg Ni. The highest figure observed was 1.4 times the sulphuric acid consumption reported for the Bulong and Murrin Murrin plants and 1.8 times that for the Cawse plant for pressure leaching of laterite (Mayze, 1999). Aluminium extraction values were around 12%, while silica was not dissolved during the leach process. Detailed concentration values for all metal cations in the various leach liquors, including Al, Cr, Si and Mn, are not given here, due to space limitations, but they are available on request. 3.2. Agitation leach tests A factorial experiment 2 2 3, in duplicate, was conducted in order to study the effects and interactions of the factors on agitation leaching efficiency. The factors studied and their levels are given in Table 2. Responses measured were the metal cation recoveries in the leach solution and the sulphuric acid consumption. The results of the tests are presented in Tables 3 and 4. Recoveries achieved, after leach residue washing were up to 74% for nickel and 51% for cobalt after 2 h. The corresponding sulphuric acid consumption was around 55 kg H2SO4/kg Ni, confirming earlier work in our laboratory (Papangelakis, 1983). The Fe/Ni ratio, equal to 6.8:1 in the ore, was reduced to 1.4:1 in the leach liquor, showing again the selectivity for nickel over iron during atmospheric pressure sulphuric acid leaching of the Greek laterite ores. On the contrary, the Mg/Ni ratio in the leach liquor (5.2:1) was on average almost equal to the value of this ratio in the ore (5.1:1). This indicates that both nickel and magnesium come into the leach liquor from the dissolution of serpentine and cronstedtite, the main nickel carriers in the ‘‘Kastoria’’ ore, which are leached preferentially to haematite and goethite.
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The data for the various responses were statistically analysed. As the statistical analysis for nickel recovery showed, the only statistically significant factor was the sulphuric acid concentration, at the significance level a = 0.01. Its main effect was positive, within the range of levels of factors studied. As no interactions were found, it was concluded that nickel recovery increases with sulphuric acid concentration, independently of the levels of the other factors. Therefore, agitation leaching can be conducted at temperatures slightly above ambient and without excessive ore grinding. Cobalt recovery as well as magnesium and iron co-dissolution were also found to increase with increasing the sulphuric acid concentration. The similarity in the leaching behaviour of nickel, cobalt, magnesium and iron, exposed by the statistical analysis indicates that these metals are leached from the same mineralogical phases, in particular serpentine and cronstedtite. It should be stressed that during agitation leaching, pulp mixing and filtration were excellent. This fact indicates that the mild leaching conditions used did not substantially fragment the serpentine silicate lattice to produce fine silica (silicate) particles in the liquor.
4. Conclusions 1. The nickeliferous ores from the ‘‘Kastoria’’ deposit can be leached by dilute sulphuric acid at atmospheric pressure, using both heap and agitation leach processes. 2. Calcite contained in the ore adversely affects the permeability of the heap and the sulphuric acid consumption. Its removal prior to leaching is an important pre-requisite. 3. The nickeliferous serpentine is dissolved in preference to the haematite and goethite minerals under the mild conditions for heap leaching. In the case of agitation leaching, the selectivity is lower. 4. Nickel and cobalt recoveries achieved during heap leaching were 60% and 45%, respectively, in 10 days. It is thought that these values would be higher if less calcite was present in the upgraded ore. The sulphuric acid consumption amounted to 40 kg H2SO4/kg Ni.
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5. Maximum nickel and cobalt recoveries obtained during agitation leaching, within the range of values studied, were 74% for nickel and 51% for cobalt, after 2 h. The average sulphuric acid consumption was 55 kg/kg Ni. The major acid consumers were residual calcite in the upgraded ore and magnesium in the nickeliferous phase of serpentine. Nickel and cobalt recoveries as well as magnesium and iron co-dissolution were found to depend only on sulphuric acid concentration, increasing independently of the ore grain size and the temperature. 6. The agitation leaching technique was readily applicable to the ’’Kastoria’’ ore, provided that market values of sulphuric acid and nickel are favourable. Further research is being conducted in order to improve the nickel/cobalt extractions and lower the acid consumption.
Acknowledgements The authors wish to express their gratitude to the European Commission, Directorate General XII, for financial support according to the contract no. BRE2CT94-1020 and also to G.M.M.S.A. LARCO for the laterite samples provided.
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