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Leaching residue of nickeliferous laterites as a source of iron concentrate
Minerals Engineering 17 (2004) 245–252 This article is also available online at: www.elsevier.com/locate/mineng
Leaching residue of nickeliferous laterites as a source of iron concentrate E. Stamboliadis a
a,*
, G. Alevizos a, J. Zafiratos
b
Department of Mineral Resources Engineering, Technical University of Crete, 73100 Chania, Greece b National Technical University of Athens, 15780 Athens, Greece Received 26 June 2003; accepted 4 August 2003
Abstract The lateritic nickeliferous iron ore deposits of Central and Northern Greece (Euboea, Locris, Boeotia and Kastoria) provide the feed material for the production of an iron-nickel alloy at a pyrometallurgical plant located in Larymna. Due to the low quality of feed ore, about 0.90–1.00% nickel, the metallurgical processing cost is high and the quantity of the slag produced and disposed amounts to millions of tonnes yearly. During the past 20 years, the Laboratory of Metallurgy of NTUA has been developing a novel method of hydrometallurgical processing of laterites, based on heap leaching of the ore with dilute sulphuric acid. Nickel is recovered from the pregnant leach liquor by solvent extraction and electrowinning. The solid leach residue is a material consisting mainly of hematite, quartz and chromite. The present research is an investigation on upgrading of this residue to a saleable iron concentrate. The upgrading, if proved successful, will potentially relieve the environment from the disposal of such a large quantity of slag, which today is produced by pyrometallurgical processing of laterites. The accomplishment of this target faces problems related mainly to the fine dissemination and interweaving of mineralogical phases and the present work is focused on addressing the associated mineralogical issues. 2003 Elsevier Ltd. All rights reserved. Keywords: Leaching; Iron ore; Tailings; Ore mineralogy; Magnetic separation
1. Introduction Nickeliferous laterites in Central Greece are secondary sedimentary deposits of the weathering products of ultra basic rocks. Surface mining of these deposits produces an ore assaying about 0.9–1.0% Ni. The main mineral constituents of the ore are: (Alevizos, 1997) • • • • •
Iron minerals: hematite and goethite Magnesium silicates: serpentine and chlorite Silicates: quartz Clay minerals: illite and kaolinite Spinels: chromite and occasionally some magnetite
Nickel is usually found as a replacement of magnesium in serpentine and chlorite. In some cases its content in these minerals can be of the order of 5–6% Ni (Boskos * Corresponding author. Tel.: +30-282-10-37601; fax: +30-282-1069554. E-mail address: [email protected] (E. Stamboliadis).
0892-6875/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2003.08.015
et al., 2000). Some calcite is also found in the particular ore coming from the limestone that makes the floor and the roof of the ore body. Upgrading of the ore is performed by heavy media and magnetic separation at the size range 1–20 mm and has a limited effect on its quality. The reason is the fine intergrowth of mineral phases. Only calcite and a small quantity of the quartz are liberated at the size of 1–20 mm and can be removed (Stamboliadis and Tzachrista, 1996). Macroscopic examination of the ore has shown that it consists of pissolites and oolites of hematite and goethite, 0.2–4 mm in diameter, which have inclusions of finely disseminated magnesium silicates, quartz and some chromite (Alevizos, 1997). These pissolites, together with some grains of quartz make an aggregate, which is glued by a binding material of the same quality as the pissolite but softer in strength. The soft binding cement is richer in nickel than the pissolites and after breakage the fine fraction )1 mm contains more nickel than the coarse fraction 1–20 mm, which is subjected to upgrading.
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The fine fraction )1 mm is pelletized and mixed with the concentrate of the 1–20 mm fraction and some quantity of lignite. The mixture is fed to a rotary kiln and subsequently is melted in an electric furnace. The molten metal is a Ni/Fe alloy assaying 12–14% Ni and is further subjected to oxidation by oxygen in a BOF furnace. The final Ni/Fe alloy assays 20% Ni and is granulated for shipment (Zevgolis et al., 1998, 1999). In the past 20 years the Laboratory of Metallurgy of NTUA has developed a new hydrometallurgical process for the recovery of nickel from lateritic ores (Agatzini and Dimaki, 1991; Agatzini-Leonardou et al., 1998). According to this process the ore crushed to )10 mm is pelletized and heap leached by 2 N sulphuric acid. The acid dissolves selectively the magnesium silicates, which are the nickel-bearing mineral while the leaching effect on hematite is limited. The nickel is subsequently recovered from the pregnant solution, while the unreacted solid leach residue is rejected. This residue consists mainly of hematite, quartz and some chromite. The aim of the present work is to examine the possibility of recovering hematite from the leach residue. It is the first approach towards this goal and as will be seen below the main difficulty is again the fine intergrowth of quartz and hematite. The main advantage for the mineral separation after leaching is the fact that magnesium silicate minerals are not present. The separation process used is magnetic separation only, but it is expected that other processes such as froth flotation will give even better results and this is something to be examined in a future work. The present work is an effort towards an ecological and perhaps wiser exploitation of mineral resources. At the present the pyro-metallurgical process used for the recovery of nickel depreciates the initial value of the hematite content in the ore. More than 90% of the hematite is transformed into an iron-silicate in the slag creating environmental problems for its rejection. On the other hand the hydro-metallurgical process recovers the nickel leaving the hematite intact. The present process is trying to recover hematite from this residue leading to a sustainable development of the ore resource.
2. Experimental
from transparent PVC material. The column was loaded with 11 kg of laterite ore. Before loading the ore is homogeneously wetted with water in a rotating drum mixer so that its total humidity is up to 10%. This pretreatment creates pellets and has a positive effect to the ore permeability, which is important to the adequate percolation of the leach solution through the column. The leach solution, 2 N sulphuric acid, is stored in the feed tank (ON tank) and pumped to the top of the ore column at a predetermined flow rate. The solution percolates through the ore mass and is collected at the bottom of the column and stored in the pregnant leach liquor tank (OFF tank). At each cycle, after the whole quantity of leach solution has percolated through the ore, the free sulphuric acid concentration of the solution is corrected by addition of acid and the solution is transferred to the ON tank to become the feed solution for the next leach cycle. The number of leach cycles conducted is determined by the measurement of the nickel concentration in the leach liquor. The last cycle is the one at the end of which the nickel concentration in the ON and OFF solution remains the same. Finally, the ore is washed by a continuous flow of water through the column, until no nickel is detected in the drainage. The leaching conditions are given in Table 1. 2.2. Classification of the leaching residue The solid residue of the leaching process is subjected to autogenous scrubbing in a rotating drum. This process breaks any soft agglomerate formed and cleans the surface of the particles that are further screened to different size fractions down to 0.25 mm. The fine fraction ()0.25 mm) is classified in a hydrocyclone at d50 about 0.010 mm. The results are presented in Table 2 and one can observe the following: • There is a small quantity of Ni remaining in the residue that was not recovered by leaching and amounts to about 0.33% Ni. • The Fe2 O3 content of the residue is almost half of its total mass. • The size fractions within 0.5–4 mm, which is the size of pissolites, are richer in Fe2 O3 .
2.1. Column leaching In the present work, column leaching at the Laboratory of Metallurgy of NTU Athens simulated the heap leaching process. The column reactors have been proven to be excellent simulators of heaps of similar height, as shown by the long history of industrial operation of this technique in copper, gold and uranium mines. The leaching experiment was conducted in a small column of 100 mm diameter and 1 m height, constructed
Table 1 Column leaching factor levels Factor
Units
Ore particle size Sulphuric acid concentration Leach solution volume/ore weight ratio
)10 2 0.8
mm N l/kg
Solution flow rate
8.8 1100
l/day l/day m2
E. Stamboliadis et al. / Minerals Engineering 17 (2004) 245–252
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Table 2 Size analysis of leach residue Fe2 O3 (%)
CaO (%)
SiO2 (%)
Al2 O3 (%)
MgO (%)
Ni (%)
C (%)
Cr2 O3 (%)
+4 +1 +0.25 +0.01 )0.01
Size (mm)
Weight (%) 13.9 39.0 25.2 15.4 6.4
42.86 53.14 52.43 45.11 39.86
1.23 0.55 0.51 1.03 4.42
46.00 31.10 30.00 34.20 45.10
4.36 7.00 7.99 8.51 6.14
1.05 1.01 1.13 1.46 0.35
0.19 0.30 0.37 0.46 0.36
0.03 0.03 0.04 0.05 0.03
1.55 2.09 2.42 4.28 1.24
Total
100.0
49.43
0.96
34.30
7.06
1.07
0.33
0.03
2.38
• SiO2 is the second most abundant constituent of the leach residue and accounts for about one third of the total mass. • The coarse fraction +4 mm has the highest content of SiO2 and as it can be seen macroscopically it contains a lot of liberated silica grains. • As it was expected the MgO content of the leach residue is low because it is a component of serpentine and chlorite, which reacted with sulphuric acid. • Cr2 O3 is also present and originates from chromite as will be seen by the following mineralogical analysis. Obviously, this mineral was not affected by leaching. • The Al2 O3 content is higher than what is corresponding to the presence of chromite and it must exist in a form, which is difficult to identify even by microscopy. • Calcium is mainly found in the cyclone overflow and as expected it is present in the form of gypsum (CaSO4 Æ 2H2 O), which is the reaction product of the initial calcite with sulphuric acid. 2.3. First stage of upgrading The size fractions produced by classification of the leaching residue were treated by magnetic separation, taking advantage of the difference in magnetic susceptibility between hematite and quartz, which were the main minerals present. A high intensity Perm-Roll belt magnetic separator was used for the coarse fractions 4–10 and 1–4 mm. The roll diameter is 70 mm, the magnetic field configuration is 2:6 mm and the magnetic elements are made of ironboron-neodymium alloy. The separator used for the fine fractions 0.250–1.000 and 0.010–0.250 mm was a laboratory size Induced Roll electromagnetic separator made by Carpco. Both types of separators are now available by Outokumpu Technology Inc. The cyclone overflow was not treated by magnetic separation due to its fineness. The results obtained are presented in Table 3 and the following remarks can be made. • There is an obvious difference in the content of Fe2 O3 and SiO2 between the magnetic and the non-magnetic products.
• The Fe2 O3 content of the magnetic products, although higher than in the initial residue, is still low. There is enough silica remaining in the concentrate, which is not visible by eye. • The SiO2 content of the non-magnetic products is high and quartz grains can be distinguished even by optical inspection. Obviously some of the quartz is liberated but, as will be seen below, there is enough locked in the hematite of the magnetic products. • Cr2 O3 responds to the magnetic products. • The small amount of calcium present in the form of gypsum is distributed to the non-magnetic products. • Nickel is mainly distributed in the magnetic products associated with the hematitic pissolites. 2.4. Mineralogical analysis The mineralogical analysis of the products was performed both by X-ray diffraction (XRD) and polished sections microscopy. The instrument used for XRD was a SIEMENS model D-500 with a copper lamp working at U ¼ 35 kV, I ¼ 35 mA with graphite secondary monochromator. For polished sections the model JENALAB-POLAR microscope was used. The samples selected for the mineralogical analysis are the magnetic and non-magnetic products of the size fraction 0.25– 1.00 mm. The XRD analysis suggests that the main mineral phase in the magnetic product is hematite associated with some quartz while in the non-magnetic one the prevailing phase is quartz associated with some hematite. The chemical analysis of the magnetic product, which is the concentrate, shows not only the presence of Fe2 O3 and SiO2 that are the constituents of hematite and quartz respectively but also Cr2 O3 , Al2 O3 and some MgO. However no other phases can be clearly detected by XRD. As described bellow, the microscopic analysis of the sample indicates also the existence of chromite, which is not detected by XRD. This is obviously due to the small quantity of chromite and the large quantity of hematite that is the prevailing phase in the sample. The sample was treated according to the process described by Mehra and Jackson (1960) in order to dissolve hematite selectively and make the other constituents
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Table 3 Magnetic separation of the leach residue fractions Fe2 O3 (%)
CaO (%)
SiO2 (%)
Al2 O3 (%)
MgO (%)
Ni (%)
Cr2 O3 (%)
+4
Size (mm) Mags Non-Mags Total
7.8 6.1 13.9
54.86 25.29 41.86
0.51 1.61 0.99
33.60 65.00 47.40
5.78 3.98 4.99
1.21 1.05 1.14
0.25 0.03 0.15
2.59 0.00 0.69
+1
Mags Non-Mags Total
32.1 6.9 39.0
62.14 28.14 56.10
0.44 1.23 0.58
25.20 62.10 31.76
6.39 5.09 6.16
1.05 1.08 1.06
0.24 0.01 0.20
2.37 0.78 2.09
+0.25
Mags Non-Mags Total
21.0 4.2 25.2
62.86 20.00 55.71
0.42 0.57 0.45
23.50 70.50 31.33
7.34 6.08 7.13
1.18 0.67 1.10
0.34 0.01 0.29
2.64 0.66 2.31
+0.01
Mags Non-Mags Total
12.7 2.7 15.4
51.71 11.86 44.73
0.65 1.13 0.74
28.00 77.30 36.64
8.70 6.85 8.38
1.56 0.50 1.37
0.72 0.00 0.59
5.23 0.10 4.33
)0.01
Total
6.4
39.86
4.42
45.00
6.14
0.35
0.36
1.24
100.0
51.22
0.88
35.44
6.58
1.08
0.29
2.35
Grant total
Weight (%)
detectable. XRD shows that after the dissolution of hematite, quartz becomes the prevailing phase and chromite is now detected as well. The microscopic analysis of the polished sections confirms the existence of the phases detected by XRD but it also provides useful information concerning their intergrowth. In the magnetic product most of the grains consist of hematite with inclusions of quartz and some chromite as shown in Fig. 1. The size fraction examined is the 0.25– 1.00 mm and the inclusions are of the order 0.020–0.070 mm. Only a few liberated grains of hematite appear but liberated grains of quartz and some ones of chromite are found although these minerals are the minority ones, as seen in Fig. 2. In the non-magnetic product the majority of the grains consist of liberated quartz and some non-liberated grains of hematite, like the ones found in the magnetic product (Fig. 3). Chromite grains can be very rarely detected.
The main conclusion from the microscopic analysis is that the poor Fe2 O3 quality of the concentrate is mainly due to the fact that hematite is not liberated and, secondarily, to the existence of free quartz that was not properly removed by magnetic separation. Further
Fig. 1. Grain of hematite with inclusions of quartz and chromite. Reflected light, // nicols, scale 100 lm.
Fig. 3. Liberated grains of quartz. Reflected light, // nicols, scale 100 lm.
Fig. 2. Liberated grain of chromite and non liberated grain of hematite with inclusions. Reflected light, // nicols, scale 100 lm.
E. Stamboliadis et al. / Minerals Engineering 17 (2004) 245–252
In a second approach, a new sample of the leach residue was washed and classified and the different size fractions were treated by magnetic separation as mentioned above. The combined magnetic product was further ground in a rod mill under conditions appearing in Table 4. The ground product was then wet screened to +63 lm and the )63 lm fraction was classified in a hydrocyclone at d50 above 10 lm. The +63 lm fraction was further treated using an Induced Roll Magnetic Separator. The cyclone underflow passed through a wet High Gradient Magnetic Separator (HGMS) made by CARPCO. The cyclone overflow was not treated again due to its fineness. The results obtained are presented in Tables 5–8 while the following remarks are made: The first magnetic separation removes most of free quartz in the non-magnetic product, as it was also the case of the previous attempt in Section 2.3. Similarly the magnetic concentrate still contains a lot of silica as inclusions in the hematite. After grinding the magnetic concentrate the magnetic separation of the fine liberated fractions produced does not give satisfactory results.
Table 4 Grinding conditions Rod mill diameter Rods load Solids Water Rotation frequency Grinding time
249
20 cm 8 kg 1 kg 0.6 l 0.6 of critical frequency 30 min
grinding is inevitable for better if not complete liberation of hematite. 2.5. Second stage of upgrading It is obvious from the previous stage that magnetic separation of the leach residue samples did not give a product that could be accepted as an iron concentrate. Following the findings of chemical and mineralogical analysis of the magnetic product it is obvious that the ore requires further grinding to achieve sufficient liberation of hematite. Table 5 First magnetic separation Parameters
Product
Weight (%)
Fe2 O3 (%)
SiO2 (%)
Al2 O3 (%)
MgO (%)
Ni (%)
Cr2 O3 (%)
Perm-roll and induced-roll
Mags Non-Mags Total
86.33 13.67 100.00
53.34 18.01 48.51
31.41 74.10 37.24
6.77 2.57 6.20
1.09 0.86 1.06
0.45 0.22 0.42
3.60 0.85 3.22
Table 6 Grinding of magnetic product Parameters
Size (lm)
Weight (%)
Fe2 O3 (%)
SiO2 (%)
Al2 O3 (%)
MgO (%)
Ni (%)
Cr2 O3 (%)
Rod-Mill 30 min
+63 10–63 )10 Total
16.07 47.79 22.47 86.33
57.71 53.57 49.73 53.34
30.36 31.33 32.33 31.41
5.02 6.96 7.63 6.77
0.99 1.30 0.70 1.09
0.44 0.45 0.45 0.45
2.17 3.35 5.16 3.60
Table 7 Fraction +63 lm, Induced roll magnetic separator Separation parameters
Product
Weight (%)
Fe2 O3 (%)
SiO2 (%)
Al2 O3 (%)
MgO (%)
Ni (%)
Cr2 O3 (%)
125 RPM 0.5 A
Mags Middlings Non-Mags Total
9.83 4.26 1.98 16.07
62.45 58.22 33.11 57.71
24.64 30.04 59.47 30.36
5.52 4.84 2.95 5.02
1.06 0.99 0.66 0.99
0.47 0.45 0.29 0.44
2.39 2.18 1.07 2.17
Table 8 Fraction 10–63 lm, HGMS magnetic separator Separation parameters
Product
Weight (%)
Fe2 O3 (%)
SiO2 (%)
Al2 O3 (%)
MgO (%)
Ni (%)
Cr2 O3 (%)
4A 8A 8A
Mags Mags Non-Mags Total
21.55 15.63 10.61 47.79
58.62 57.25 36.03 53.15
24.82 27.21 52.80 31.82
7.71 7.78 5.29 7.20
1.39 1.19 0.99 1.24
0.46 0.48 0.34 0.44
4.14 3.21 1.68 3.29
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E. Stamboliadis et al. / Minerals Engineering 17 (2004) 245–252
Although the magnetic products are better than the non-magnetic ones, their quality is still low in Fe2 O3 content. In order to investigate the cause of this bad performance the products were mineralogically examined. 2.6. Mineralogical analysis of the ground product At this stage the identity of the mineral phases was known from the previous investigation and XRD was not used. The emphasis was given to the polished sections microscopy of the separation products. The results of this analysis are the following: In the magnetic product +63 lm the major constituent is hematite. It appears mainly in liberated grains and in a lesser degree in grains with inclusions of other minerals. The second most abundant mineral phase is quartz, appearing only in liberated grains. There are also some liberated grains of chromite present but in small quantity. It is obvious that the reason of the bad quality of the product is the poor magnetic separation since liberated quartz grains are present in the magnetic product. In the non-magnetic product +63 lm the same minerals are present, the difference being that there is more quartz than hematite and even less chromite. For the )63 lm magnetic and non-magnetic products the remarks are the same as above. However, although the liberation of the mineral phases is better the magnetic separation is not satisfactory and obviously worse than what observed for the +63 lm fraction. 2.7. Electron microscopy It is obvious from the previous paragraphs that after grinding one can achieve the liberation required, regardless of the fact that the process used for the mineral separation was not adequate. In future work the target will be to find a more appropriate method for mineral separation at these fine size ranges. At the present situation there is still an important point that was revealed during this work. The presence of chromite in the magnetic product suggests that some of the Al2 O3 found in the chemical analysis is associated with this mineral. Previous work by Alevizos (1997) has shown that the Cr2 O3 /Al2 O3 ratio in chromites of this type of mineral deposits is in the range 2.5–4.0 with an estimated statistical average at about 3.5. Alevizos also reports that microanalysis of the hematite in similar deposits shows the appearance of SiO2 and Al2 O3 in a ratio that suggests the existence of kaolinite, although this mineral is not apparent in the polished sections. In the products of magnetic separation of the +63 lm fraction, the following calculations were made, using the data of Table 7. From the Cr2 O3 content of the sample the amount of Al2 O3 associated with chromite is calcu-
lated and the rest is assumed to be present as kaolinite. The expected amount of kaolinite is then estimated from the equivalent Al2 O3 content. From the data in Table 7 it is calculated that the Al2 O3 Æ 2SiO2 Æ 2H2 O to Fe2 O3 (kaolinite to hematite) ratio in all products is about 0.18–0.20. Based on these results, one should expect that hematite grains contain about 10–15% kaolinite. In order to confirm the existence of kaolinite some grains of hematite were tested using electron microscopy. The results of the point analysis of such a hematite grain indicates the existence of both Al2 O3 and SiO2 together with Fe2 O3 , as shown in Fig. 4. Although kaolinite did not appear in the polished sections during the optical microscopy examination and not even in XRD analysis it was detected by electron microscopy. Obviously, it must be finely disseminated in the mass of hematite as a result of the conditions under which the ore body was formed and the description of which is beyond the scope of the present work (Alevizos and Muecke, 2001).
3. Discussion Leaching of nickeliferous laterites with sulphuric acid dissolves the nickel bearing minerals, which are mainly magnesium silicates such as serpentines and chlorites. The mineral phases detected by X-ray diffraction and polished section microscopy are quartz, hematite and chromite. Hematite is not liberated and contains inclusions of quartz and chromite. Some quantity of quartz is liberated and can be separated by magnetic separation at coarse size. The resulting hematite concentrate does not fulfil the specifications in order to be considered as a saleable iron concentrate. Optical microscopy has shown that this is due to poor liberation of hematite. Further grinding of the magnetic concentrate liberates hematite from quartz and chromite. However, the separation using the magnetic separators available at the laboratory was inadequate and the magnetic concentrates produced was still not satisfactory. It is suggested that one should investigate other separation processes such as froth flotation, which can be used for the separation of hematite from quartz at fine particle sizes. An interesting finding of the present work is the fact that kaolinite is found to be disseminated in hematite. This is concluded from the existence of Al and Si in point analysis of hematite grains by electron microscopy. The existence of a source of Al2 O3 other than chromite is also concluded from the chemical analysis of the samples. The use of the chemical analysis data suggests that hematite contains kaolinite at an estimated amount of about 10–15%. The existence of finely disseminated kaolinite in hematite leads to the prediction that even in the case
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251
Fig. 4. Electron microscopy analysis of a hematite pissolite. Fe originates from hematite, Si and Al indicate the presence of kaolinite.
that a satisfactory process to separate quartz from hematite is devised, the product will still contain some Al2 O3 and SiO2 impurities. It is expected that these will not be detrimental for the use of the hematite concentrate. The SiO2 is combined with Al2 O3 and will not consume Fe2 O3 in the slag of a metallurgical process in case the concentrate is used in steel making. In some cases bauxite is used as an additive to regulate the slag in steel making and it must be investigated whether the presence of kaolinite in the concentrate is acceptable (Papanastasiou and Nikolaou, 2000).
4. Conclusions Although the present work did not provide the final solution to the problem for an alternative route of utilisation of the leach residue of the hydrometallurgical processing of nickeliferous laterites, it has shown that: • The existing pyrometallurgical process produces a Ni/ Fe alloy but the Fe2 O3 present in the ore in the form of hematite is indefinitely lost in the slag.
• The deposition of the slag consists a major ecological problem. • On the other hand, the hydrometallurgical process recovers nickel from laterites leaving the existing hematite intact. • The recovery of hematite from the leach residue will improve the effectiveness of the overall hydrometallurgical process leading to the sustainable development of this kind of mineral resources. • The present work has revealed the main issues that must be overcome before the final development of the process. These issues are: 1. The liberation of hematite at fine sizes. 2. The need for an effective separation process at these sizes. 3. The existence of finely disseminated kaolinite in the mass of hematite.
References Agatzini-Leonardou, S., Cox, M., Dimaki, D., Boskos, E., 1998. A new approach to the metallurgical treatment of nickeliferous
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laterites. Proceedings of the First Annual Workshop, EUROATHEN’98, Athens, Greece. Agatzini, S., Dimaki, D., 1991. Recovery of nickel and cobalt from low-grade nickel oxide ores by sulphuric acid heap leaching at room temperature. Greek Patent No. 910100234, 31 May 1991. Alevizos, G., 1997. Mineralogy, geochemistry and genesis of the sedimentary nickeliferous iron-ores of Locris (Central Greece), Ph.D. Thesis, Technical University of Crete, p. 267 (in Greek). Alevizos, G., Muecke, A., 2001. Erzpetrographisch-mikroanalytische Untersuchung und Genese der sedimentaeren Eisennickelvorkommen von Agios Ioannis und Marmeiko (Lokris-Griechenland), N. Jb. Miner. Abh. 176, pp. 67–88. Boskos, E., Orfanoudaki, A., Perraki, Th., 2000. The Ni Distribution in the Mineral Phases of Greek Fe–Ni Laterite Deposits, 3rd Conference of Mineral Wealth, T.C.G, Athens, vol. A, pp. 107–115 (in Greek).
Mehra, O.P., Jackson, M.L., 1960. Iron oxide removal from soils and clays by a dithionite-citrate system buffered with sodium bicarbonate. Proc. 7th Nat. Conf. Clay Min. 5, pp. 317–327. Papanastasiou, D.I., Nikolaou, N.D., 2000. Bauxite as a Blast Furnace Flux––Comparative Evaluation. 3rd Conference of Mineral Wealth, T.C.G, Athens, vol. B, pp. 331–337 (in Greek). Stamboliadis, E., Tzachrista, V., 1996. Study for the Concentration of the Nickeliferous Ore of the Kopais Deposit, Technica Chronica, T.C.G., section C Athens, vol. 16 (1–2), pp. 17–27. Zevgolis, E.N., Neou-Syngouna, P., Halikia, H., Carabinis, C., 1998. Theoretical investigation of metal production from slags of nickeliferous laterites. Mineral Wealth 106, 7–20 (in Greek). Zevgolis, E.N., Gaitanos, J.F., Kontos, J., 1999. Ferronickel Slag: Uses and Environment. 35th Metallurgical Seminar of GDMB, on Slag in Metallurgy, Aachen, Germany, March.
Leaching residue of nickeliferous laterites as a source of iron concentrate E. Stamboliadis a
a,*
, G. Alevizos a, J. Zafiratos
b
Department of Mineral Resources Engineering, Technical University of Crete, 73100 Chania, Greece b National Technical University of Athens, 15780 Athens, Greece Received 26 June 2003; accepted 4 August 2003
Abstract The lateritic nickeliferous iron ore deposits of Central and Northern Greece (Euboea, Locris, Boeotia and Kastoria) provide the feed material for the production of an iron-nickel alloy at a pyrometallurgical plant located in Larymna. Due to the low quality of feed ore, about 0.90–1.00% nickel, the metallurgical processing cost is high and the quantity of the slag produced and disposed amounts to millions of tonnes yearly. During the past 20 years, the Laboratory of Metallurgy of NTUA has been developing a novel method of hydrometallurgical processing of laterites, based on heap leaching of the ore with dilute sulphuric acid. Nickel is recovered from the pregnant leach liquor by solvent extraction and electrowinning. The solid leach residue is a material consisting mainly of hematite, quartz and chromite. The present research is an investigation on upgrading of this residue to a saleable iron concentrate. The upgrading, if proved successful, will potentially relieve the environment from the disposal of such a large quantity of slag, which today is produced by pyrometallurgical processing of laterites. The accomplishment of this target faces problems related mainly to the fine dissemination and interweaving of mineralogical phases and the present work is focused on addressing the associated mineralogical issues. 2003 Elsevier Ltd. All rights reserved. Keywords: Leaching; Iron ore; Tailings; Ore mineralogy; Magnetic separation
1. Introduction Nickeliferous laterites in Central Greece are secondary sedimentary deposits of the weathering products of ultra basic rocks. Surface mining of these deposits produces an ore assaying about 0.9–1.0% Ni. The main mineral constituents of the ore are: (Alevizos, 1997) • • • • •
Iron minerals: hematite and goethite Magnesium silicates: serpentine and chlorite Silicates: quartz Clay minerals: illite and kaolinite Spinels: chromite and occasionally some magnetite
Nickel is usually found as a replacement of magnesium in serpentine and chlorite. In some cases its content in these minerals can be of the order of 5–6% Ni (Boskos * Corresponding author. Tel.: +30-282-10-37601; fax: +30-282-1069554. E-mail address: [email protected] (E. Stamboliadis).
0892-6875/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2003.08.015
et al., 2000). Some calcite is also found in the particular ore coming from the limestone that makes the floor and the roof of the ore body. Upgrading of the ore is performed by heavy media and magnetic separation at the size range 1–20 mm and has a limited effect on its quality. The reason is the fine intergrowth of mineral phases. Only calcite and a small quantity of the quartz are liberated at the size of 1–20 mm and can be removed (Stamboliadis and Tzachrista, 1996). Macroscopic examination of the ore has shown that it consists of pissolites and oolites of hematite and goethite, 0.2–4 mm in diameter, which have inclusions of finely disseminated magnesium silicates, quartz and some chromite (Alevizos, 1997). These pissolites, together with some grains of quartz make an aggregate, which is glued by a binding material of the same quality as the pissolite but softer in strength. The soft binding cement is richer in nickel than the pissolites and after breakage the fine fraction )1 mm contains more nickel than the coarse fraction 1–20 mm, which is subjected to upgrading.
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The fine fraction )1 mm is pelletized and mixed with the concentrate of the 1–20 mm fraction and some quantity of lignite. The mixture is fed to a rotary kiln and subsequently is melted in an electric furnace. The molten metal is a Ni/Fe alloy assaying 12–14% Ni and is further subjected to oxidation by oxygen in a BOF furnace. The final Ni/Fe alloy assays 20% Ni and is granulated for shipment (Zevgolis et al., 1998, 1999). In the past 20 years the Laboratory of Metallurgy of NTUA has developed a new hydrometallurgical process for the recovery of nickel from lateritic ores (Agatzini and Dimaki, 1991; Agatzini-Leonardou et al., 1998). According to this process the ore crushed to )10 mm is pelletized and heap leached by 2 N sulphuric acid. The acid dissolves selectively the magnesium silicates, which are the nickel-bearing mineral while the leaching effect on hematite is limited. The nickel is subsequently recovered from the pregnant solution, while the unreacted solid leach residue is rejected. This residue consists mainly of hematite, quartz and some chromite. The aim of the present work is to examine the possibility of recovering hematite from the leach residue. It is the first approach towards this goal and as will be seen below the main difficulty is again the fine intergrowth of quartz and hematite. The main advantage for the mineral separation after leaching is the fact that magnesium silicate minerals are not present. The separation process used is magnetic separation only, but it is expected that other processes such as froth flotation will give even better results and this is something to be examined in a future work. The present work is an effort towards an ecological and perhaps wiser exploitation of mineral resources. At the present the pyro-metallurgical process used for the recovery of nickel depreciates the initial value of the hematite content in the ore. More than 90% of the hematite is transformed into an iron-silicate in the slag creating environmental problems for its rejection. On the other hand the hydro-metallurgical process recovers the nickel leaving the hematite intact. The present process is trying to recover hematite from this residue leading to a sustainable development of the ore resource.
2. Experimental
from transparent PVC material. The column was loaded with 11 kg of laterite ore. Before loading the ore is homogeneously wetted with water in a rotating drum mixer so that its total humidity is up to 10%. This pretreatment creates pellets and has a positive effect to the ore permeability, which is important to the adequate percolation of the leach solution through the column. The leach solution, 2 N sulphuric acid, is stored in the feed tank (ON tank) and pumped to the top of the ore column at a predetermined flow rate. The solution percolates through the ore mass and is collected at the bottom of the column and stored in the pregnant leach liquor tank (OFF tank). At each cycle, after the whole quantity of leach solution has percolated through the ore, the free sulphuric acid concentration of the solution is corrected by addition of acid and the solution is transferred to the ON tank to become the feed solution for the next leach cycle. The number of leach cycles conducted is determined by the measurement of the nickel concentration in the leach liquor. The last cycle is the one at the end of which the nickel concentration in the ON and OFF solution remains the same. Finally, the ore is washed by a continuous flow of water through the column, until no nickel is detected in the drainage. The leaching conditions are given in Table 1. 2.2. Classification of the leaching residue The solid residue of the leaching process is subjected to autogenous scrubbing in a rotating drum. This process breaks any soft agglomerate formed and cleans the surface of the particles that are further screened to different size fractions down to 0.25 mm. The fine fraction ()0.25 mm) is classified in a hydrocyclone at d50 about 0.010 mm. The results are presented in Table 2 and one can observe the following: • There is a small quantity of Ni remaining in the residue that was not recovered by leaching and amounts to about 0.33% Ni. • The Fe2 O3 content of the residue is almost half of its total mass. • The size fractions within 0.5–4 mm, which is the size of pissolites, are richer in Fe2 O3 .
2.1. Column leaching In the present work, column leaching at the Laboratory of Metallurgy of NTU Athens simulated the heap leaching process. The column reactors have been proven to be excellent simulators of heaps of similar height, as shown by the long history of industrial operation of this technique in copper, gold and uranium mines. The leaching experiment was conducted in a small column of 100 mm diameter and 1 m height, constructed
Table 1 Column leaching factor levels Factor
Units
Ore particle size Sulphuric acid concentration Leach solution volume/ore weight ratio
)10 2 0.8
mm N l/kg
Solution flow rate
8.8 1100
l/day l/day m2
E. Stamboliadis et al. / Minerals Engineering 17 (2004) 245–252
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Table 2 Size analysis of leach residue Fe2 O3 (%)
CaO (%)
SiO2 (%)
Al2 O3 (%)
MgO (%)
Ni (%)
C (%)
Cr2 O3 (%)
+4 +1 +0.25 +0.01 )0.01
Size (mm)
Weight (%) 13.9 39.0 25.2 15.4 6.4
42.86 53.14 52.43 45.11 39.86
1.23 0.55 0.51 1.03 4.42
46.00 31.10 30.00 34.20 45.10
4.36 7.00 7.99 8.51 6.14
1.05 1.01 1.13 1.46 0.35
0.19 0.30 0.37 0.46 0.36
0.03 0.03 0.04 0.05 0.03
1.55 2.09 2.42 4.28 1.24
Total
100.0
49.43
0.96
34.30
7.06
1.07
0.33
0.03
2.38
• SiO2 is the second most abundant constituent of the leach residue and accounts for about one third of the total mass. • The coarse fraction +4 mm has the highest content of SiO2 and as it can be seen macroscopically it contains a lot of liberated silica grains. • As it was expected the MgO content of the leach residue is low because it is a component of serpentine and chlorite, which reacted with sulphuric acid. • Cr2 O3 is also present and originates from chromite as will be seen by the following mineralogical analysis. Obviously, this mineral was not affected by leaching. • The Al2 O3 content is higher than what is corresponding to the presence of chromite and it must exist in a form, which is difficult to identify even by microscopy. • Calcium is mainly found in the cyclone overflow and as expected it is present in the form of gypsum (CaSO4 Æ 2H2 O), which is the reaction product of the initial calcite with sulphuric acid. 2.3. First stage of upgrading The size fractions produced by classification of the leaching residue were treated by magnetic separation, taking advantage of the difference in magnetic susceptibility between hematite and quartz, which were the main minerals present. A high intensity Perm-Roll belt magnetic separator was used for the coarse fractions 4–10 and 1–4 mm. The roll diameter is 70 mm, the magnetic field configuration is 2:6 mm and the magnetic elements are made of ironboron-neodymium alloy. The separator used for the fine fractions 0.250–1.000 and 0.010–0.250 mm was a laboratory size Induced Roll electromagnetic separator made by Carpco. Both types of separators are now available by Outokumpu Technology Inc. The cyclone overflow was not treated by magnetic separation due to its fineness. The results obtained are presented in Table 3 and the following remarks can be made. • There is an obvious difference in the content of Fe2 O3 and SiO2 between the magnetic and the non-magnetic products.
• The Fe2 O3 content of the magnetic products, although higher than in the initial residue, is still low. There is enough silica remaining in the concentrate, which is not visible by eye. • The SiO2 content of the non-magnetic products is high and quartz grains can be distinguished even by optical inspection. Obviously some of the quartz is liberated but, as will be seen below, there is enough locked in the hematite of the magnetic products. • Cr2 O3 responds to the magnetic products. • The small amount of calcium present in the form of gypsum is distributed to the non-magnetic products. • Nickel is mainly distributed in the magnetic products associated with the hematitic pissolites. 2.4. Mineralogical analysis The mineralogical analysis of the products was performed both by X-ray diffraction (XRD) and polished sections microscopy. The instrument used for XRD was a SIEMENS model D-500 with a copper lamp working at U ¼ 35 kV, I ¼ 35 mA with graphite secondary monochromator. For polished sections the model JENALAB-POLAR microscope was used. The samples selected for the mineralogical analysis are the magnetic and non-magnetic products of the size fraction 0.25– 1.00 mm. The XRD analysis suggests that the main mineral phase in the magnetic product is hematite associated with some quartz while in the non-magnetic one the prevailing phase is quartz associated with some hematite. The chemical analysis of the magnetic product, which is the concentrate, shows not only the presence of Fe2 O3 and SiO2 that are the constituents of hematite and quartz respectively but also Cr2 O3 , Al2 O3 and some MgO. However no other phases can be clearly detected by XRD. As described bellow, the microscopic analysis of the sample indicates also the existence of chromite, which is not detected by XRD. This is obviously due to the small quantity of chromite and the large quantity of hematite that is the prevailing phase in the sample. The sample was treated according to the process described by Mehra and Jackson (1960) in order to dissolve hematite selectively and make the other constituents
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Table 3 Magnetic separation of the leach residue fractions Fe2 O3 (%)
CaO (%)
SiO2 (%)
Al2 O3 (%)
MgO (%)
Ni (%)
Cr2 O3 (%)
+4
Size (mm) Mags Non-Mags Total
7.8 6.1 13.9
54.86 25.29 41.86
0.51 1.61 0.99
33.60 65.00 47.40
5.78 3.98 4.99
1.21 1.05 1.14
0.25 0.03 0.15
2.59 0.00 0.69
+1
Mags Non-Mags Total
32.1 6.9 39.0
62.14 28.14 56.10
0.44 1.23 0.58
25.20 62.10 31.76
6.39 5.09 6.16
1.05 1.08 1.06
0.24 0.01 0.20
2.37 0.78 2.09
+0.25
Mags Non-Mags Total
21.0 4.2 25.2
62.86 20.00 55.71
0.42 0.57 0.45
23.50 70.50 31.33
7.34 6.08 7.13
1.18 0.67 1.10
0.34 0.01 0.29
2.64 0.66 2.31
+0.01
Mags Non-Mags Total
12.7 2.7 15.4
51.71 11.86 44.73
0.65 1.13 0.74
28.00 77.30 36.64
8.70 6.85 8.38
1.56 0.50 1.37
0.72 0.00 0.59
5.23 0.10 4.33
)0.01
Total
6.4
39.86
4.42
45.00
6.14
0.35
0.36
1.24
100.0
51.22
0.88
35.44
6.58
1.08
0.29
2.35
Grant total
Weight (%)
detectable. XRD shows that after the dissolution of hematite, quartz becomes the prevailing phase and chromite is now detected as well. The microscopic analysis of the polished sections confirms the existence of the phases detected by XRD but it also provides useful information concerning their intergrowth. In the magnetic product most of the grains consist of hematite with inclusions of quartz and some chromite as shown in Fig. 1. The size fraction examined is the 0.25– 1.00 mm and the inclusions are of the order 0.020–0.070 mm. Only a few liberated grains of hematite appear but liberated grains of quartz and some ones of chromite are found although these minerals are the minority ones, as seen in Fig. 2. In the non-magnetic product the majority of the grains consist of liberated quartz and some non-liberated grains of hematite, like the ones found in the magnetic product (Fig. 3). Chromite grains can be very rarely detected.
The main conclusion from the microscopic analysis is that the poor Fe2 O3 quality of the concentrate is mainly due to the fact that hematite is not liberated and, secondarily, to the existence of free quartz that was not properly removed by magnetic separation. Further
Fig. 1. Grain of hematite with inclusions of quartz and chromite. Reflected light, // nicols, scale 100 lm.
Fig. 3. Liberated grains of quartz. Reflected light, // nicols, scale 100 lm.
Fig. 2. Liberated grain of chromite and non liberated grain of hematite with inclusions. Reflected light, // nicols, scale 100 lm.
E. Stamboliadis et al. / Minerals Engineering 17 (2004) 245–252
In a second approach, a new sample of the leach residue was washed and classified and the different size fractions were treated by magnetic separation as mentioned above. The combined magnetic product was further ground in a rod mill under conditions appearing in Table 4. The ground product was then wet screened to +63 lm and the )63 lm fraction was classified in a hydrocyclone at d50 above 10 lm. The +63 lm fraction was further treated using an Induced Roll Magnetic Separator. The cyclone underflow passed through a wet High Gradient Magnetic Separator (HGMS) made by CARPCO. The cyclone overflow was not treated again due to its fineness. The results obtained are presented in Tables 5–8 while the following remarks are made: The first magnetic separation removes most of free quartz in the non-magnetic product, as it was also the case of the previous attempt in Section 2.3. Similarly the magnetic concentrate still contains a lot of silica as inclusions in the hematite. After grinding the magnetic concentrate the magnetic separation of the fine liberated fractions produced does not give satisfactory results.
Table 4 Grinding conditions Rod mill diameter Rods load Solids Water Rotation frequency Grinding time
249
20 cm 8 kg 1 kg 0.6 l 0.6 of critical frequency 30 min
grinding is inevitable for better if not complete liberation of hematite. 2.5. Second stage of upgrading It is obvious from the previous stage that magnetic separation of the leach residue samples did not give a product that could be accepted as an iron concentrate. Following the findings of chemical and mineralogical analysis of the magnetic product it is obvious that the ore requires further grinding to achieve sufficient liberation of hematite. Table 5 First magnetic separation Parameters
Product
Weight (%)
Fe2 O3 (%)
SiO2 (%)
Al2 O3 (%)
MgO (%)
Ni (%)
Cr2 O3 (%)
Perm-roll and induced-roll
Mags Non-Mags Total
86.33 13.67 100.00
53.34 18.01 48.51
31.41 74.10 37.24
6.77 2.57 6.20
1.09 0.86 1.06
0.45 0.22 0.42
3.60 0.85 3.22
Table 6 Grinding of magnetic product Parameters
Size (lm)
Weight (%)
Fe2 O3 (%)
SiO2 (%)
Al2 O3 (%)
MgO (%)
Ni (%)
Cr2 O3 (%)
Rod-Mill 30 min
+63 10–63 )10 Total
16.07 47.79 22.47 86.33
57.71 53.57 49.73 53.34
30.36 31.33 32.33 31.41
5.02 6.96 7.63 6.77
0.99 1.30 0.70 1.09
0.44 0.45 0.45 0.45
2.17 3.35 5.16 3.60
Table 7 Fraction +63 lm, Induced roll magnetic separator Separation parameters
Product
Weight (%)
Fe2 O3 (%)
SiO2 (%)
Al2 O3 (%)
MgO (%)
Ni (%)
Cr2 O3 (%)
125 RPM 0.5 A
Mags Middlings Non-Mags Total
9.83 4.26 1.98 16.07
62.45 58.22 33.11 57.71
24.64 30.04 59.47 30.36
5.52 4.84 2.95 5.02
1.06 0.99 0.66 0.99
0.47 0.45 0.29 0.44
2.39 2.18 1.07 2.17
Table 8 Fraction 10–63 lm, HGMS magnetic separator Separation parameters
Product
Weight (%)
Fe2 O3 (%)
SiO2 (%)
Al2 O3 (%)
MgO (%)
Ni (%)
Cr2 O3 (%)
4A 8A 8A
Mags Mags Non-Mags Total
21.55 15.63 10.61 47.79
58.62 57.25 36.03 53.15
24.82 27.21 52.80 31.82
7.71 7.78 5.29 7.20
1.39 1.19 0.99 1.24
0.46 0.48 0.34 0.44
4.14 3.21 1.68 3.29
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Although the magnetic products are better than the non-magnetic ones, their quality is still low in Fe2 O3 content. In order to investigate the cause of this bad performance the products were mineralogically examined. 2.6. Mineralogical analysis of the ground product At this stage the identity of the mineral phases was known from the previous investigation and XRD was not used. The emphasis was given to the polished sections microscopy of the separation products. The results of this analysis are the following: In the magnetic product +63 lm the major constituent is hematite. It appears mainly in liberated grains and in a lesser degree in grains with inclusions of other minerals. The second most abundant mineral phase is quartz, appearing only in liberated grains. There are also some liberated grains of chromite present but in small quantity. It is obvious that the reason of the bad quality of the product is the poor magnetic separation since liberated quartz grains are present in the magnetic product. In the non-magnetic product +63 lm the same minerals are present, the difference being that there is more quartz than hematite and even less chromite. For the )63 lm magnetic and non-magnetic products the remarks are the same as above. However, although the liberation of the mineral phases is better the magnetic separation is not satisfactory and obviously worse than what observed for the +63 lm fraction. 2.7. Electron microscopy It is obvious from the previous paragraphs that after grinding one can achieve the liberation required, regardless of the fact that the process used for the mineral separation was not adequate. In future work the target will be to find a more appropriate method for mineral separation at these fine size ranges. At the present situation there is still an important point that was revealed during this work. The presence of chromite in the magnetic product suggests that some of the Al2 O3 found in the chemical analysis is associated with this mineral. Previous work by Alevizos (1997) has shown that the Cr2 O3 /Al2 O3 ratio in chromites of this type of mineral deposits is in the range 2.5–4.0 with an estimated statistical average at about 3.5. Alevizos also reports that microanalysis of the hematite in similar deposits shows the appearance of SiO2 and Al2 O3 in a ratio that suggests the existence of kaolinite, although this mineral is not apparent in the polished sections. In the products of magnetic separation of the +63 lm fraction, the following calculations were made, using the data of Table 7. From the Cr2 O3 content of the sample the amount of Al2 O3 associated with chromite is calcu-
lated and the rest is assumed to be present as kaolinite. The expected amount of kaolinite is then estimated from the equivalent Al2 O3 content. From the data in Table 7 it is calculated that the Al2 O3 Æ 2SiO2 Æ 2H2 O to Fe2 O3 (kaolinite to hematite) ratio in all products is about 0.18–0.20. Based on these results, one should expect that hematite grains contain about 10–15% kaolinite. In order to confirm the existence of kaolinite some grains of hematite were tested using electron microscopy. The results of the point analysis of such a hematite grain indicates the existence of both Al2 O3 and SiO2 together with Fe2 O3 , as shown in Fig. 4. Although kaolinite did not appear in the polished sections during the optical microscopy examination and not even in XRD analysis it was detected by electron microscopy. Obviously, it must be finely disseminated in the mass of hematite as a result of the conditions under which the ore body was formed and the description of which is beyond the scope of the present work (Alevizos and Muecke, 2001).
3. Discussion Leaching of nickeliferous laterites with sulphuric acid dissolves the nickel bearing minerals, which are mainly magnesium silicates such as serpentines and chlorites. The mineral phases detected by X-ray diffraction and polished section microscopy are quartz, hematite and chromite. Hematite is not liberated and contains inclusions of quartz and chromite. Some quantity of quartz is liberated and can be separated by magnetic separation at coarse size. The resulting hematite concentrate does not fulfil the specifications in order to be considered as a saleable iron concentrate. Optical microscopy has shown that this is due to poor liberation of hematite. Further grinding of the magnetic concentrate liberates hematite from quartz and chromite. However, the separation using the magnetic separators available at the laboratory was inadequate and the magnetic concentrates produced was still not satisfactory. It is suggested that one should investigate other separation processes such as froth flotation, which can be used for the separation of hematite from quartz at fine particle sizes. An interesting finding of the present work is the fact that kaolinite is found to be disseminated in hematite. This is concluded from the existence of Al and Si in point analysis of hematite grains by electron microscopy. The existence of a source of Al2 O3 other than chromite is also concluded from the chemical analysis of the samples. The use of the chemical analysis data suggests that hematite contains kaolinite at an estimated amount of about 10–15%. The existence of finely disseminated kaolinite in hematite leads to the prediction that even in the case
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251
Fig. 4. Electron microscopy analysis of a hematite pissolite. Fe originates from hematite, Si and Al indicate the presence of kaolinite.
that a satisfactory process to separate quartz from hematite is devised, the product will still contain some Al2 O3 and SiO2 impurities. It is expected that these will not be detrimental for the use of the hematite concentrate. The SiO2 is combined with Al2 O3 and will not consume Fe2 O3 in the slag of a metallurgical process in case the concentrate is used in steel making. In some cases bauxite is used as an additive to regulate the slag in steel making and it must be investigated whether the presence of kaolinite in the concentrate is acceptable (Papanastasiou and Nikolaou, 2000).
4. Conclusions Although the present work did not provide the final solution to the problem for an alternative route of utilisation of the leach residue of the hydrometallurgical processing of nickeliferous laterites, it has shown that: • The existing pyrometallurgical process produces a Ni/ Fe alloy but the Fe2 O3 present in the ore in the form of hematite is indefinitely lost in the slag.
• The deposition of the slag consists a major ecological problem. • On the other hand, the hydrometallurgical process recovers nickel from laterites leaving the existing hematite intact. • The recovery of hematite from the leach residue will improve the effectiveness of the overall hydrometallurgical process leading to the sustainable development of this kind of mineral resources. • The present work has revealed the main issues that must be overcome before the final development of the process. These issues are: 1. The liberation of hematite at fine sizes. 2. The need for an effective separation process at these sizes. 3. The existence of finely disseminated kaolinite in the mass of hematite.
References Agatzini-Leonardou, S., Cox, M., Dimaki, D., Boskos, E., 1998. A new approach to the metallurgical treatment of nickeliferous
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laterites. Proceedings of the First Annual Workshop, EUROATHEN’98, Athens, Greece. Agatzini, S., Dimaki, D., 1991. Recovery of nickel and cobalt from low-grade nickel oxide ores by sulphuric acid heap leaching at room temperature. Greek Patent No. 910100234, 31 May 1991. Alevizos, G., 1997. Mineralogy, geochemistry and genesis of the sedimentary nickeliferous iron-ores of Locris (Central Greece), Ph.D. Thesis, Technical University of Crete, p. 267 (in Greek). Alevizos, G., Muecke, A., 2001. Erzpetrographisch-mikroanalytische Untersuchung und Genese der sedimentaeren Eisennickelvorkommen von Agios Ioannis und Marmeiko (Lokris-Griechenland), N. Jb. Miner. Abh. 176, pp. 67–88. Boskos, E., Orfanoudaki, A., Perraki, Th., 2000. The Ni Distribution in the Mineral Phases of Greek Fe–Ni Laterite Deposits, 3rd Conference of Mineral Wealth, T.C.G, Athens, vol. A, pp. 107–115 (in Greek).
Mehra, O.P., Jackson, M.L., 1960. Iron oxide removal from soils and clays by a dithionite-citrate system buffered with sodium bicarbonate. Proc. 7th Nat. Conf. Clay Min. 5, pp. 317–327. Papanastasiou, D.I., Nikolaou, N.D., 2000. Bauxite as a Blast Furnace Flux––Comparative Evaluation. 3rd Conference of Mineral Wealth, T.C.G, Athens, vol. B, pp. 331–337 (in Greek). Stamboliadis, E., Tzachrista, V., 1996. Study for the Concentration of the Nickeliferous Ore of the Kopais Deposit, Technica Chronica, T.C.G., section C Athens, vol. 16 (1–2), pp. 17–27. Zevgolis, E.N., Neou-Syngouna, P., Halikia, H., Carabinis, C., 1998. Theoretical investigation of metal production from slags of nickeliferous laterites. Mineral Wealth 106, 7–20 (in Greek). Zevgolis, E.N., Gaitanos, J.F., Kontos, J., 1999. Ferronickel Slag: Uses and Environment. 35th Metallurgical Seminar of GDMB, on Slag in Metallurgy, Aachen, Germany, March.