J. SE Asian Appl. Geol., Jan–Jun 2011, Vol. 3(1), pp. 23-33 STUDY ON MINERALOGY AND CHEMISTRY OF THE SAPROLITIC NICKEL ORES FROM SOROAKO, SULAWESI, INDONESIA: IMPLICATION FOR THE LATERITIC ORE PROCESSING Sufriadin *1,2 , Arifudin Idrus 2 , Subagyo Pramumijoyo 2 , I Wayan Warmada 2 , and Akira Imai 3 1 Mining Engineering Study Program, Hasanuddin University, Makassar 90245, Indonesia 2 Department of Geological Engineering, Gadjah Mada University, Yogyakarta 55281, Indonesia 3 Department of Earth Science and Technology, Akita University, Akita 010-8512, Japan Abstract An investigation of mineralogy and chemistry of saprolitic nickel ores developed on ultramafic rock with different serpentinization degree from Soroako, Sulawesi has been conducted using X ray diffrac- tion, thermal analysis, FTIR, and ICP-AES. The implication for the processing of these ores under acidic media was also studied. Weathering of un- serpentinized peridotite in the Soroako west block produces saprolitic ore containing minerals such as relict olivine, goethite, quartz, talc with minor ser- pentine and smectite; whereas the weathered mate- rials overlaying serpentinized peridotite in the Pe- tea area are mainly composed of residual serpentine with lesser chlorite, maghemite, and remnant pyrox- ene and amphibole. Chemical analysis determined by ICP-AES demonstrates that west ore is higher in Si, Mg, and Ni, as compared to that Petea ore. Con- versely, Fe and Al concentrations are higher in Petea ore than in west block ore. SEM-EDX examination reveals that olivine, talc, serpentine and goethite are the Ni-bearing phases occurring in west block ore; while serpentine is the principal host for Ni in the Petea ore. Chemical leaching under sulfuric acid re- veals that olivine has highest dissolution rate in the * Corresponding author: SUFRIADIN, Mining Engi- neering Study Program, Hasanuddin University, Makas- sar 90245, Indonesia. E-mail: [email protected]west ore followed by serpentine; while talc, pyrox- ene, and iron oxides have slow dissolution rates. In contrast, serpentine in Petea ore is easily dissolved and is followed by chlorite; whereas amphibole, py- roxene, and maghemite are difficult to leach. Quartz is present in both ores and it seems to be undissolved during the chemical leaching. It is shown that Ni recovery from Petea saprolitic ore is higher than that of West Block ore. Key words: Serpentine, SEM/EDX, talc, nickel. 1 Introduction Lateritic weathering and supergene nickel en- richment developed on ultramafic rock has been studied extensively in many part of the world (Schellmann, 1989; Barros de Olievera et al., 1992; Gleeson et al., 2004; Thorne et al., 2009). Characteristic of the laterite ore resulting from chemical weathering of these rocks are in- fluenced by the nature of parent rock, climatic condition, topography, and time (Freyssinet et al., 2005). The nature of bedrock protolith particularly serpentinization degree is found to have significant role in the development of commercially Ni laterite deposit. Golightly (1981) suggested that nickel laterite ore derived from unserpentinized ultramafic rock is com- monly characterized by thinner saprolite, many 23
Transcript
J. SE Asian Appl. Geol., Jan–Jun 2011, Vol. 3(1), pp. 23-33
STUDY ON MINERALOGY AND CHEMISTRY OFTHE SAPROLITIC NICKEL ORES FROMSOROAKO, SULAWESI, INDONESIA:IMPLICATION FOR THE LATERITIC OREPROCESSING
Sufriadin∗1,2, Arifudin Idrus2, Subagyo Pramumijoyo2, I Wayan Warmada2, and AkiraImai3
1Mining Engineering Study Program, Hasanuddin University, Makassar 90245, Indonesia2Department of Geological Engineering, Gadjah Mada University, Yogyakarta 55281, Indonesia3Department of Earth Science and Technology, Akita University, Akita 010-8512, Japan
Abstract
An investigation of mineralogy and chemistry ofsaprolitic nickel ores developed on ultramafic rockwith different serpentinization degree from Soroako,Sulawesi has been conducted using X ray diffrac-tion, thermal analysis, FTIR, and ICP-AES. Theimplication for the processing of these ores underacidic media was also studied. Weathering of un-serpentinized peridotite in the Soroako west blockproduces saprolitic ore containing minerals such asrelict olivine, goethite, quartz, talc with minor ser-pentine and smectite; whereas the weathered mate-rials overlaying serpentinized peridotite in the Pe-tea area are mainly composed of residual serpentinewith lesser chlorite, maghemite, and remnant pyrox-ene and amphibole. Chemical analysis determinedby ICP-AES demonstrates that west ore is higher inSi, Mg, and Ni, as compared to that Petea ore. Con-versely, Fe and Al concentrations are higher in Peteaore than in west block ore. SEM-EDX examinationreveals that olivine, talc, serpentine and goethite arethe Ni-bearing phases occurring in west block ore;while serpentine is the principal host for Ni in thePetea ore. Chemical leaching under sulfuric acid re-veals that olivine has highest dissolution rate in the
∗Corresponding author: SUFRIADIN, Mining Engi-neering Study Program, Hasanuddin University, Makas-sar 90245, Indonesia. E-mail: [email protected]
west ore followed by serpentine; while talc, pyrox-ene, and iron oxides have slow dissolution rates. Incontrast, serpentine in Petea ore is easily dissolvedand is followed by chlorite; whereas amphibole, py-roxene, and maghemite are difficult to leach. Quartzis present in both ores and it seems to be undissolvedduring the chemical leaching. It is shown that Nirecovery from Petea saprolitic ore is higher than thatof West Block ore.Key words: Serpentine, SEM/EDX, talc, nickel.
1 Introduction
Lateritic weathering and supergene nickel en-richment developed on ultramafic rock hasbeen studied extensively in many part of theworld (Schellmann, 1989; Barros de Olieveraet al., 1992; Gleeson et al., 2004; Thorne et al.,2009). Characteristic of the laterite ore resultingfrom chemical weathering of these rocks are in-fluenced by the nature of parent rock, climaticcondition, topography, and time (Freyssinetet al., 2005). The nature of bedrock protolithparticularly serpentinization degree is foundto have significant role in the development ofcommercially Ni laterite deposit. Golightly(1981) suggested that nickel laterite ore derivedfrom unserpentinized ultramafic rock is com-monly characterized by thinner saprolite, many
23
SUFRIADIN et al.
boulder, but higher in Ni; while the weatheringof serpentinized ultramafic protolith tends toproduce thicker saprolite but lower Ni content.Weathering process of ultramafic rock concen-trates Ni in the two principal mineralizationzones (Luo et al., 2009): limonitic laterite whichlies in the upper part of the profile and sapro-litic laterite which is located in lower part ofthe profile or just above of unaltered ultrabasicrock. In general, lateritic nickel ores may beclassified into three types based on the domina-tion of Ni bearing minerals (Brand et al., 1998;Elias, 2002): (1) hydrous Mg silicate ore (typeA) with garnierite is the dominant Ni host, (2)clay silicate ore (type B) with smectite (e.g. non-tronite) as principal Ni host, and (3) oxide orein which is dominated by goethite.
Currently, two processes can be employed forextracting the Ni from lateritic ores: pyromet-allurgy (smelting), producing nickel matte andferronickel and hydrometallurgy (leaching),producing both Ni and Co. The choice betweenpyrometallurgical and hydrometallurgical pro-cessing depends on mineralogical characteristicand chemical dissemination of the ore. Due tothe continuously depleting of global Ni-sulfideore and high energy requirement associatedwith the Ni extraction by smelting, leads toincrease the research interest in the recovery ofNi and (Co) from nickel laterite ore by leaching.
Mineralogical characterization of the nickellaterite ore is essential step that should be per-formed before the application of process op-tion. Soroako nickeliferous laterite deposit isthe most important ore as source for Ni in In-donesia. Despite this deposit has been pro-duced for nearly four decades, the informa-tion on its mineralogical characteristic is min-imal. The present paper discusses the miner-alogy and chemistry of saprolitic Ni ores fromSoroako, Sulawesi and its implication for lat-eritic Ni ore processing by leaching experimentunder acidic media.
2 Location and Geological Setting
Soroako nickeliferous laterite deposit is locatedabout 40 km from Malili, a capital city ofEast Luwu regency or around 600 km from
Makassar, a provincial city of South Sulawesito the northeast (Figure 1). This deposit is re-sulted from the intensively chemical weather-ing of the Cretaceous ultramafic rocks that isincluded in the East Sulawesi Ophiolite (ESO).This ophiolite is tectonically dismembered andcropped out over than 10,000 km2 in the east-ern Sulawesi (Monnier et al., 1995). Peridotitearound Soroako is petrologically dominated byby harzburgitic-peridotite with higher in Cr#(Kadarusman et al., 2004). However, lherzoliteand locally dunite are also present. Elsewherein the Soroako area, the bedrock is essentiallyunserpentinized where serpentine being onlyrestricted to border of joints as thin rim or asfine grained matrix of tectonic breccias (Soeria-Atmadja et al., 1974).
The multiple tectonic events have occurredfrom Paleogene to Neogene (Dirk, 2001) withinthis area. These have led to the formation ofat least three regional sinistral strike-slip faultshitting this complex: Matano Fault is in thenorth followed by Lawanopo trend in the centerand Kolaka Fault in south. Matano Fault whichis run from the Tolo Gulf in the east and mergedwith Palu Koro Fault in the west, seems to havesignificant role in the development of laterite inthis area (Ahmad, 2005).
3 Methods
Samples
Two representative samples used in this studywere collected from Soroako mining district(Figure 1). Each sample represents a compositematerial from three saprolitic horizons. Sam-ple west ore (WO) was derived from lateriticore that has been formed over unserpentinizedperidotite of West Block; whereas the east ore(EO) sample was taken from Petea area whichis typically covered by serpentinized peridotite.After drying at 100 ◦C for 1 hour, samples wereground with agate mortar and vibrating mill,followed by sieving to -100# (< 0.15mm).
Analytical techniques
X-ray diffraction (XRD) analysis was performedby a Rigaku RINT 2000 and RIGAKU Multiflex
X ray diffractometer (Cu-Kα radiation, λ=1.541)with the voltage of 40 kV and current at 20mA. The patterns of diffraction were obtainedby scanning random powder mounts from 2–65◦ 2θ, scanning step at 0.02◦ and counting time2◦/minute. For clay analysis, three times scan-ning from 2 to 40◦ 2θ were employed includ-ing air-dry, ethylene glycol salvation, and heat-ing to 550 ◦C. Phase identification and semi-quantitative proportion of minerals containedin the samples were executed by MATCH! 1.10software.
For TG/DTA analysis, about 20 mg powdersample placed in platinum crucible was ana-lyzed by a simultaneous differential thermal an-alyzer (SSC/5200 SII-SEIKO Instrument). Datawere collected in air atmosphere with flowrate at 20 mL/min, temperature range of 27 –1000 ◦C, heating rate 10 ◦C/min and calcinedAl2O3 used as inert substance.
The infrared analysis of these samples wasalso carried out to interpret the molecular struc-ture of minerals present. A mixture about 2g samples and 200 g KBr was prepared forpressed disks. The disk then was scanned un-der absorbance mode within the range frequen-cies of 4000 – 400 cm−1 by means of JUSCOFTIR spectrometer. Chemical analysis was con-ducted under digestion samples using ICP-
AES, while chemical composition of individualmineral was determined under carbon coatedmaterials by SEM-EDX spectrometer.
Leaching experiment
Leaching tests were conducted in batch reac-tor using a 250 ml glass beaker. For each run,the slurry was prepared by the mixture of 10g dried ore sample with 50 ml of a 25 % sul-furic acid solution. It was then heated at thetemperature of 90 ◦C with the agitating speedat 500 rev/min using REXIM RSH-1A magneticstirrer. The leaching time was set for 30 to 180minutes with an interval of 30 minutes. Separa-tion of solid residues with leached solution wasmade by using a 0.45 μm-sized of membrane fil-ter. Residues were washed with distilled waterfor several times, dried at 100 ◦C for 2 hours,and analyzed with XRD, SEM-EDX, and FTIR.Chemical composition of leached solutions wasdetermined by ICP-AES.
4 Results and Discussion
Mineralogical studies
The patterns of X-ray diffraction of two sapro-litic ore samples are given in Figure 2. Min-erals identified in the studied samples var-
ied from primary to secondary sheet silicatesand lastly to iron oxi-hydroxides (Table 2).Forsterite (Mg2SiO4), enstatite (MgSiO3) andtrace amount of spinel are the primary mineraloccurring in west block ore. Secondary min-erals detected in this sample are principallyquartz (3.14 Å) with lesser amounts of serpen-tine (7.32 Å) and talc (9.38 Å) (Figure 2a). Inthe sample EO, the most intense reflections thatoccurred at 7.308 Å and 3.655 Å are diagnosticpeaks of lizardite-1M [(Mg,Fe)3Si2O5(OH)4],a serpentine end-member (Figure 2b). Othersecondary minerals that coexist with lizarditeare chlorite (14.20 Å), goethite (4.20 Å), mag-netite/maghemite (2.51 Å), and quartz (3.34 Å);while the primary residual minerals such asenstatite (2.88 Å) and amphibole (8.43 Å) arealso present in small amounts. Talc in whichonly detected in sample WO is inferred to bemore residually material derived from primarytalc of the parent rocks. It is originally formedby hydrothermal alteration of enstatite and/orolivine was relatively more resistant duringchemical weathering. However, Ni2+ substi-tutions for Mg2+ within crystal structure oftalc may be possible. Other Ni-bearing phasesfound in west ore sample are olivine, serpen-tine, and iron oxide. In the case of Petea ore(EO) sample, lizardite is likely the principalNi bearing mineral but chlorite and amphibolecould also be the carrier for nickel.
DTA curve of west ore sample (Figure 3A)can be divided into two parts: (1) low temper-ature region (<400 ◦C) and (2) high tempera-ture region (400 – 800 ◦C). In low temperatureregion the curve shows endothermic reactionat about 250 ◦C. This peak corresponds to theloss of adsorbed water in two steps as illus-trated in TG curve (dashed line in Figure 3A). Inhigh temperature region, the broad endother-mic peak occurs at about 570 ◦C and it reachedmaximum at around 805 ◦C. These peaks re-late to the loss of hydroxyl water. The shortexothermic maximum occurs at about 825 ◦Cthat might be linked to crystallization of hy-drous minerals such as serpentine and talc. Therelatively elevated temperature of exothermicpeak in low temperature reactions may be dueto the higher proportion of anhydrous miner-
als are present in sample such as forsterite andquartz. In the east ore sample (EO), TG/DTAcurves show two endothermic peaks in lowertemperature region (Figure 3B). The first andsecond endothermic peak within this region oc-curs at 80 ◦C and 260 ◦C corresponding to theloss of adsorbed and bound water respectively.In the higher temperature region, the strong en-dothermic peak at about 560 ◦C may be relatedto the loss of hydroxyl water and the weightloss is completed around 800 ◦C. Above thistemperature, DTA curve shows sharp exother-mic peak, indicating a complete recrystalliza-tion.
Infrared spectra of WO and EO samples showtwo absorption bands appeared in the range of3700 – 3200 cm−1 (Figure 4). The bands at 3676and 3683 cm−1 can be attributed to stretchingvibrations of hydroxyls bonded to magnesiumatom in octahedral layer (Liu et al., 2010). Thewide absorption bands at 3427 and 3433 cm−1
were assigned to hydroxyl bonded to trivalentcations (e.g. Fe3+ or Al3+) in octahedral coor-dination (Fuchs et al., 1998). Absorption bandsin the region 1100 – 700 cm−1 can be related todifferent vibrations of Si-O bond in tetrahedron.The band at 1009 cm-1 with shoulders at 1036cm-1 and 985 cm−1 observed in EO sample maybe originated from Si-O in-plane stretching vi-bration. Likewise the strong band at 1009 withweak bands at 887 and 791 cm−1 in WO samplemay also be arisen by Si-O bonds linked withtrivalent cation. The absence of bands around800 – 700 cm−1 in the EO sample indicate thelesser amounts of free silica content comparedto the WO sample. In the 700 - 400 cm−1 re-gions, the bands at 661 cm-−1, 625 cm−1, and615 cm−1 can be attributed to deformation ofMg2+-O-H bonds (Sontevska et al., 2007). Thisdeformation may be linked to the substitutionof iron, nickel or aluminum. The strong bandsaround 550 – 400 cm−1 are due to the Si-O-Sibending vibration.
Chemical composition
Results of chemical analysis of two ore sam-ples are shown in Table 1. The concentrationsof SiO2 and MgO in sample WO are higherthan that in sample EO. These values are con-
sistent with the higher proportion of quartzand forsterite in sample WO as determined byXRD method. This feature is also supportedby FTIR data as shown in Figure 4. Fe2O3and Al2O3 concentrations show higher valuein sample EO as compared to that in sampleWO. Iron in sample EO is not only occurredas compounds in maghemite and goethite butit also likely appears to be incorporated intooctahedral coordination of serpentine/lizarditeand chlorite structures. Similarly, Al might alsobe present as adsorbed or substituted elementwithin goethite and serpentine structure.
Nickel content in sample EO is slightly lowerthan that in sample WO. It is expected that,Ni in sample EO is mainly hosted in lizardite,while in sample WO Ni is probably associatedwith talc serpentine, goethite, and olivine. Con-centrations of Mn, Co, and Zn are relativelysimilar for both ore samples. The content ofCr in the sample EO is higher than that in sam-ple WO. Chromium is mainly present in spinel,lesser in goethite, and trace amounts in pyrox-ene.
Dissolution of minerals in the ores
The strongest reflections at 5.12 Å, 3.72 Å, and1.75 Å observed in sample WO (Figure 2) thatbelong to forsteritic olivine, disappeared insolid residues. Disappearance of such peaks in-dicates complete dissolution of forsterite in sul-phuric acid. Serpentine also seems to be read-ily dissolved in sample WO which is shown bythe weakness of basal reflection in solid residuefor 30 minutes leaching and dissappears after150 minutes reaction. However, the existence ofstronger reflection lines of other phases such asquartz, talc, enstatite, and iron oxides in solidresidues exhibit that these phases were difficultto dissolve in sulphuric acid solution.
The strongest lines of lizardite at 7.31 Å and3.65 Å in Petea ore sample were substantiallyreduced in leach residues. The basal spac-ing at 7.308 Å in raw ore sample reduced to7.132 Å in solid residue after leaching for 150minutes. These indicated that lizardite was eas-ily reacted with sulphuric acid. Although re-flections of spinel and/or goethite seem to becollapsed in leach residue, it is prossibly dis-
solved slowly. However, the peaks of miner-als such as quartz, enstatite and amphibole re-main strong in leached residues. This revealsthat these minerals were rather difficult to leachin sulphuric acid. Table 2 shows the semi quan-titative mineral in solid residues under differentleaching time determined by XRD.
The overall results of leaching experiments(Table 2) indicate that minerals such as quartz,enstatite and talc in the west ore (WO) are themost difficult to leach in sulphuric acid. On theother hand, minerals such as quartz, enstatiteand amphibole from Petea ore are very hard todissolve in sulphuric acid. Nearly all olivineand serpentine in the west block sample dis-solved in sulphuric acid after 30 minute leach-ing, indicating congruent dissolution. How-ever, serpentine in the east ore sample seemsnot to be completely dissolved and is usuallycalled incongruent dissolution.
FTIR analysis of two solid residues afterleaching at 150 minutes (Figure 5) show disap-pearance of the bands at 3676 and 3683 cm−1
in sample WO and EO respectively indicatingdecomposition of octahedral layer. The strongbands at 1066 cm−1 with shoulder at 1109 cm−1
followed by medium band at 793 in sample WOsuggest that silica was produced from dissolu-tion of minerals such as olivine and serpentine.Likewise, the bands around 1200 – 1100 cm−1
region and medium band at 793 cm−1 also ap-pear in sample EO. This evidence is consistentwith XRD data.
According to Wolff-Boenisch et al. (2006), dis-solution rate of silicate minerals could be re-lated to the number of Si–O bond present anddegree of polimerization. For example, crystalstructure of forsterite is formed by a number ofisolated Si–O tetrahedra which is linked by Mg–O octahedra. Forsterite has non bridging oxy-gen so that there is no Si–O polimerization existin its structure. In contrast, quartz has struc-ture with three-dimentionally polimerized, im-plying that all Si–O tetrahedra are connected toeach other. Hence, forsterite would has fasterdissolution rate than quartz.
The octahedral Mg–O bonds break morequickly than tetrahedral Si–O bonds in layersilicates (Saldi et al., 2007). This evidence re-
Figure 5: Infrared spectra of ore samples after 150 minutes leaching. (WO) sample from west blockand (EO) sample from Petea area
veals more faster of Mg released from octahe-dral stuctures than that of Si from tetrahedralcoordination. Brantley (2008) suggested thatdissolution rates of silicate minerals decreasewith an increase in the number of bridgingoxygens. This author introduces a new termwhich she called “connectedness”, that is anumber of bridging oxygens per tetrahedralcation. Connectedness of 0 is a forsterite, 2 is apyroxene, 2.5 is an amphibole, and 3 is a phyl-losilicate and 4 is a quartz. It is inferred thatdissolution rates of primary minerals would bein the reducing order: forsterite > enstatite >hornblende > quartz. For secondary mineral,the decreasing of dissolution rates would be:serpentine/lizardite > chlorite > talc.
Results of XRD and FTIR analyses also showthat solid residues from west block samplescontain significant amount of crystalline silica(quartz), while leached residues from Petea aremainly composed of amorphous silica. Theresidual or amorhous silica is easily pulverizedand very reactive compared to the crystallinesilica. Therefore, it is a good potential to pro-duce silicon carbide by using this residual silicaas a raw material (Hirasawa and Horita, 1987).
The relatively higher dissolution rates of ser-pentine (Mg/Si=3/2) in acid solution than thatof talc (Mg/Si=3/4) could be explained by con-
sidering their crystal structures. In serpen-tine structure, one octahedral layer is bound toone tetrahedral layer. The octahedral sheet ismainly formed by Mg–O bonds and tetrahedralsheet is occupied by Si–O bonds. Other cationssuch as Ni2+, Co2+, and Fe2+ may replace Mg2+
in octahedral sheets. Accordingly in order toleach valuable metals from those structures, it isrequired for breaking the octaheral sheet with-out disruption of tetrahedral sheet. This featureis known as incongruent dissolution (Lin andClemency 1981). The acid attack on octahedralsheets may takes place through both edges andgallery access mechanisms (Kaviratna and Pin-navaia 1994) (Figure 6A). In contrast, talc be-long to 2:1 layer silicates in which one octahe-dral sheet sandwiched between two tetrahedrallayers. Similarly with serpentine, octahedralsheet in talc also consists of Mg–O bonds andtetrahedral layer mainly contains Si–O bonds.Relatively slow dissolution rate of talc becausethe leach layer forms only at the edges of grain(Jurinski and Rimstidt, 2001) (Figure 6B).
It is shown that, dissolution rates of serpen-tine in the west ore is higher than in Petea sam-ple. This is likely because Al content of Peteasample is higher than in west block sample. Thepresence of aqueous Al3+ has been observedto decelerate dissolution of silicates (Brantley,
STUDY ON MINERALOGY AND CHEMISTRY OF THE SAPROLITIC NICKEL ORES FROM SOROAKO
Figure 6: Schematic displaying the illustration of proton attack on octahedral sheet during the sul-phuric acid leaching: (A) The exchange of Mg cations with 2 protons in serpentine structure maytake place either via edges or gallery attack mechanism, (B) the exchange of Mg with 2 protonsin talc structure may only occurs through edges attack mechanism (modified after Kaviratna andPinnavaia, 1994; Saldi et al., 2007)
2008). The effect of Al-inhibition was attributedto increase adsorption of trivalent cation.
Extraction of metals
Distribution of metal extractions from the oreswithin the range of leaching time are shown inFigure 7. It can be seen that there are wide vari-ation of metal recoveries throughout the exper-iment. In general, a better condition of metalextraction of sample WO could be achieved for150 minutes reaction (Figure 7A). At this con-dition, as much as 47 % Ni, 65 % Mn, 59 % Mg,60 % Fe, 70 % Co, and 79 % Zn could be recov-ered. Conversely, the extraction rates of Al andCr at this condition is low, only about 12 % and6 % respectively. In contrast, as much as 58 %Ni, 58 % Mn, 74 % Mg, 50 % Fe, 48 % Co, and52 % Zn could be extracted from sample EO for120 minutes. At this time, only around 22 %Al and 19 % Cr could be recovered (Figure 7B).From these figures, it can be inferred that leach-ing behavior of metals in both ores were slightlydifferent. In order to recover the higher concen-tration of metals from these ores, it is necessarya longer time for leaching of sample WO com-pared to that of sample EO. It is also expectedthat mostly Mg, Ni, some Mn, and Fe extractedfrom sample WO was derived by dissolution offorsterite.
The complexity of minerals that appear in
both ores might be one factor in inhibiting morevaluable metals leached from ores mainly Niand Co in sulfuric acid solution. The anoma-lous value of Mg leached from sample EO re-veal that it is easier to release from the octahe-dral structure then trivalent cations from tetra-hedral structure of serpentine. This confirmsthe study performed by Lin and Clemency(1981). On the other hand, the lower concen-tration of chromium in leach solution for bothores indicates that Cr was mainly present as ox-ide minerals such as magnetite/maghemite andchromite. These phases were relatively unre-acted in acid solution, even after long periods ofextraction (McDonald and Whittington, 2008).The lower Al content in leach solution also ex-hibit this metal has low leachability particularlywhen it occupies tetrahedral position in silicatestructure.
Alternatively, the leachabilities of metal ele-ments from the ores in sulphuric acid solutionmight also be affected by the bond strength ofmolecules present within minerals. The bondstrength of Si–O (799.6 kJ/mol) is the highestover the other moleculer functional groups insilicate structures. Al–O with bond strength511 kJ/mol and Cr–O (461 kJ/mol) are muchhigher than that of Fe–O (390.4 kJ/mol), Co–O(384.5 kJ/mol), Ni–O (382.0 kJ/mol), and Mg–O (363.2 kJ/mol) (Lide, 2004). The lower bond
Figure 7: Graphs showing the percentage of metals extraction from Soroako Ni-laterite ores underdifferent leaching time. A: West ore sample (WO), and B: East ore sample (EO)
strength of Mg–O is one reason of Mg has themuch higher solubility than other metals.
5 Conclusions
Mineralogical and chemical study with the oreprocessing implication was conducted for theSoroako saprolitic nickel ore. Based on the re-sults from the current work, some conclusionscan be drawn as follow:
1. Mineralogy of west ore (WO) is mainlycomposed of quartz, olivine, and pyrox-ene with minor talc, serpentine and spinel;whereas east ore (EO) contains principallyresidual serpentine with subordinate chlo-rite, pyroxene, amphibole, maghemite, andquartz.
2. Nickel concentration in the West Block oreis higher than that of Petea ore. Olivine,talc, serpentine and Fe/Mn-oxides are theprincipal Ni-bearing phases in the westore; whereas residual serpentine (lizardite)with subordinate amounts of chlorite andamphibole are the main host minerals forNi in the east ore.
3. Forsteritic olivine is the highest dissolutionrate among the minerals appearing in thewest ore and it indicates congruent disso-lution. Conversely, the highest dissolutionrate of minerals present in the east ore isserpentine, however, it seems to have in-congruent dissolution.
4. About 58 % Ni could be extracted fromeast ore during two hours reaction, whileonly about 47 % Ni could be recovered
from west ore and it was achieved for threehours leaching reaction.
Acknowledgements
This work is supported by the doctoral re-search grant from The Research Institute ofGadjah Mada University (grant no. LPPM-UGM/895/BID.1/2011) and SEG Foundation(H.E. McKinstry Fund). Thanks are due toJASSO for providing scholarship during thefirst author conducting short term research atDepartment of Earth Resources Engineering,Kyushu University, Japan.
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