Saprolitic Ore Research Articles

Bacterial bioleaching of low grade nickel limonite and saprolite ores by mixotrophic bacteria

Utilization of indigenous bacteria should be considered to establish a successful biohydrometallurgical process. In this study, mixrotrophic iron-oxidizing bacterial consortia consisting of Comamonas testosteroni, Alicyclobacillus ferrooxydans and Pantoea septic which were isolated from Indonesian mineral ores were examined to determine their abilities to recover nickel from limonite and saprolite ores in the bioleaching experiments using stirred tank reactors. The nickel bioleaching experiments inoculated with the bacterial consortia were carried out using coarse limonite ores and weathered saprolite ores with pulp density of 10% w/v. Abiotic controls were also carried out replacing the inocula by the sterile medium. The bioleaching processes were monitored by measuring Ni and Fe contents and pH of the leaching solution as well as the total bacterial enzymatic activity measured as FDA hydrolytic activity. The effect of leaching on the mineralogy of laterite ores was investigated by the scanning electron microscope equipped with energy-dispersive spectroscopy (SEM-EDS) and X-ray powder diffraction (XRD). After 28 days of incubation, the FDA hydrolytic activity was observed in both bioleaching experiments containing limonite (17.2 μg fluorescein/mL) and saprolite ores (16.9 μg fluorescein/mL). The leached Ni and Fe in the bioleaching experiments containing limonite ores (30% Ni and 5.6% Fe) was greater than that in abiotic controls (1% Ni and 0.1% Fe) with the pH range of 2.5 to 3.5. However, the bacterial consortia were less capable of bioleaching of Ni (2.5%) with the similar leached Fe (6%) from the saprolite ores. In abiotic controls, the medium pH remained relatively constant (pH 6). It was concluded that these bacterial isolated as the consortium were capable of nickel bioleaching (precious metal) more effectively than iron (gangue metal), thus being applicable to the commercial processing of the difficult-to-process low-grade nickel laterite ores in Indonesia.

Thermodynamics of the Reduction Roasting of Nickeliferous Laterite Ores

The global nickel sulphide resources are becoming more difficult to mine and, as a result, there is increasing interest in the current and future development of the oxidic nickel laterite deposits. In comparison to the sulphide ores, the nickel laterites cannot be readily upgraded by conventional means and growing attention is being focused on the development of new methods for processing these ores. In this paper, firstly, brief overviews of laterite ore mineralogy and the conventional techniques used to extract the nickel from both the limonitic and the saprolitic nickeliferous laterites are provided. Secondly, previous research on the thermodynamic modelling of the reduction of the laterites is discussed. Thirdly, an improved thermodynamic model is used to predict the equilibrium products arising from the solid state reduction of both the limonitic and the saprolitic ores. Based on these thermodynamic predictions, the reduction behaviors of the two ore types are compared in terms of nickel recovery and grade in the ferronickel product. The effects of reduction temperature, ore composition and carbon additions were studied. Finally, the results from the simulations are compared to the experimental data available in the literature.

Leaching metals from saprolitic laterite ore using a ferric chloride solution

In this work, FeCl3 solution was used as a lixiviant to leach Ni, Co, Mg, and Mn from saprolitic laterite ore. Investigation into the leaching of these metals and the precipitation of Fe in the lixiviant, found the optimal leaching conditions to be a temperature of 150 °C, a Fe concentration in the lixiviant of 178.9 g/L, a liquid to solid ratio (L/S) of 1.0, and a leaching time of 90 min. The kinetics of the leaching process was also discussed. Saprolitic ore with and without goethite seeds was leached using FeCl3 solution under identical conditions. X-ray diffraction (XRD) analysis, infrared spectrometry (IR), and mineral liberation analysis (MLA) were used to characterize the leached residues. The leaching mechanism was analyzed based on these results.

Organic Acid Biogeneration by <i>Aspergillus niger </i> and its Utilization for Indirect Bioleaching of Nickel Laterite Ore

This paper discusses the results of indirect bioleaching test works of nickel laterite ore by the assistance of Aspergillus niger. Series of metabolic acid biogeneration shake-flask experiments were performed to investigate the effects of substrate type, nitrogen and phosphate dosages, the presence of magnesium and manganese salts, as well as aeration of the culture media on the effectiveness of the metabolic acid biogeneration. The investigation results showed that after 16 days of incubation, the solution pH of 1.44 was obtained from the acid biogeneration using cassava starch as carbon source in the presence of 0.1% (w/w) KH2PO4, 1% (w/w) (NH4)2SO4 and 0.25 g/L of MgSO4 under aerated condition. Leaching experiments of limonite and saprolite ore samples were carried out by using the generated metabolic acids at various ore particle size distributions, solid to liquid ratio and temperature. The highest nickel extraction percentage of 88.9% was obtained from the leaching of saprolite ore at 95°C, stirring speed of 400 rpm, particle size distribution of-80+100 mesh, solid to liquid ratio of 4.65 g/mL after 240 minutes, while that of 84.9% was obtained from the leaching of limonite ore at 95°C, stirring speed of 400 rpm, particle size distribution of-80 +100 mesh and solid-liquid ratio of 5.2 g/ml after 1440 minutes. The leaching of saprolite ore by using the biogenerated acids was selective to magnesium, with co-extracted Mg of only 1.5% after 24h of the agitation leaching test.

Overcoming the Bacteriostatic Effects of Heavy Metals on <i>A.</i> <i>thiooxidans</i> Direct Bioleaching of Saprolitic Ni Laterite Ores

In this study, the adaptation of A. thiooxidans to heavy metals leached from saprolitic Ni laterite ores was performed by gradual acclimatisation. The bacteria was cultivated in heavy metals (Ni, Co, Fe, Mg, Cr and Mn) with total concentrations of 2400 to 24000 ppm equivalent to total dissolution of 1 to 10% (w/v) pulp densities of the saprolitic Ni laterite ore. Adaptation evolution mapped from its tolerance index was found to be dependent on metal concentration, acid generation, and period of adaptation. Bio-stimulation of cell growth and acid production was promoted by heavy metal stress on the bacteria. Pre-established heavy metal tolerance of the bacteria improved the leaching rate in its early phase; 20% and 7% increase in Ni and Co metal recoveries were observed in using adapted bacteria. However heavy metal tolerance was also achieved by the bacteria during the leaching process, albeit delayed by a lag phase. These results confirm the robust nature and suitability of A. thiooxidans in direct biomining of Ni ores.

Utilization of Metabolic Citric Acid from <i>As</i><i>pergillus</i> <i>niger</i> Using Corn Starch in the Nickel Leaching of Indonesian Saprolitic Ore

Citric acid has been proved to be the most effective organic acid for nickel extraction from nickel lateritic ores. Citric acid can be produced from fungal metabolism by utilizing several types of carbon source as fungal nutrient. In the current experiment, production of metabolic citric acid from metabolism of Aspergillus niger by using corn starch as a carbon source was investigated. The application of the citric acid produced in the leaching of nickel from Indonesian saprolitic ore under atmospheric pressure was also conducted. The optimum citric acid concentration (i.e. around 0.05 M) can be produced by using 5% w/v of corn starch after 5 days incubation of A. niger, 30°C of temperature, shaker speed of 120 rpm, and 3% v/v of methanol as an additive. The metal leaching of Indonesian saprolitic ore was conducted using <75μm of ore particle size, 5% w/v of pulp density and 200 rpm of shaker speed at different leaching temperatures (30°C, 40°C, and 60°C). The results showed that the optimum nickel recovery (around 40%) can be reached after 3 days of leaching process at 40°C. It was also found that the use of metabolic citric acid was more effective for nickel leaching compared to the use of chemical citric acid at similar citric acid concentration (i.e. 0.05 M). It can be concluded that the metabolic citric acid produced from corn starch by A. niger will be an excellent leaching reagent for extracting nickel from low-grade Indonesian saprolitic ore.

Comparison of effectiveness of citric acid and other acids in leaching of low-grade Indonesian saprolitic ores

Comparison effect of chemical citric acid and other leaching reagents including inorganic acid and other organic acid on nickel extraction from low-grade Indonesian saprolitic ores was evaluated. Two saprolitic ores from two different mining areas (Sulawesi Island and Halmahera Island) in Indonesia were used to investigate the leaching performance and mineral dissolution behaviors of different ore samples using different leaching reagents. Leaching was performed using citric, sulfuric, nitric, hydrochloric, lactic, and oxalic acids. The saprolitic ore from Sulawesi Island (SS) has higher serpentine and lower goethite contents than the saprolitic ore from Halmahera Island (SH). These differences significantly affect the leaching performances and metal dissolution behaviors. Citric acid and sulfuric acid were more effective than other acid solutions for nickel extraction from both samples. Citric acid was very effective for dissolving nickel from serpentine, but did not recover nickel from goethite. In contrast, inorganic acids, namely sulfuric, nitric, and hydrochloric acids, can extract nickel from lateritic ores by dissolving goethite as well as serpentine, but the nickel recoveries achieved with sulfuric acid were higher than those achieved with other inorganic acids. A comparison of the leaching performances of the two samples shows that nickel recovery from SS was higher than that from SH when citric acid leaching was used, but the samples gave similar nickel recoveries in all inorganic acid and lactic acid leaching processes. Moreover, oxalic acid is the least effective reagent for nickel extraction from both samples, because of nickel oxalate precipitation after nickel dissolution. In addition, the effect of a sulfuric acid–citric acid mixture on the nickel dissolution rate was investigated to confirm the individual influences of citric acid and sulfuric acid on the leaching behavior of each sample. The results show that an increase in the amount of sulfuric acid affected the dissolution rate of nickel in leaching of SH much more than that in leaching of SS. In general, the effect of citric acid in the mixture of sulfuric acid and citric acid is attractive. Moreover, citric acid offers not only high nickel recovery and high selectivity of leaching but also an environmentally safe process and low acid consumption.

Study on mineralogy and chemistry of the saprolitic nickel ores from Soroako, Sulawesi, Indonesia: Implication for the lateritic ore processing

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 diffraction, thermal analysis, FTIR, and ICP-AES. The implication for the processing of these ores under acidic media was also studied. Weathering of unserpentinized peridotite in the Soroako west block produces saprolitic ore containing minerals such as relict olivine, goethite, quartz, talc with minor serpentine and smectite; whereas the weathered materials overlaying serpentinized peridotite in the Petea area are mainly composed of residual serpentine with lesser chlorite, maghemite, and remnant pyroxene 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. Conversely, 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 reveals that olivine has highest dissolution rate in the west ore followed by serpentine; while talc, pyroxene, and iron oxides have slow dissolution rates. In contrast, serpentine in Petea ore is easily dissolved and is followed by chlorite; whereas amphibole, pyroxene, 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.

Kinetics of nickel extraction from Indonesian saprolitic ore by citric acid leaching under atmospheric pressure

The kinetics of leaching of a saprolitic ore from Indonesia by citric acid solution under atmospheric pressure was investigated. An examination of the effects of leaching temperature, citric acid concentration, pulp density and ore particle size on the dissolution rate ofnickel found that they all had significant influence on the rate. The highest nickel recovery (95.6 percent) was achieved under the leaching conditions of ore particle size of 212–355 microns, citric acid concentration of 1 M, leaching time of 15 days, pulp density of 20 weight/volume percent, leaching temperature of 40°C and shaker speed of 200 rpm. The shrinking core model was found to be appropriate for describing the leaching kinetics of this ore in citric acid solutions at atmospheric pressure. The experimental data were well interpreted by this model with rate ofreaction controlled by diffusion through the solid product layer. Using the Arrhenius expression, the apparent activation energy for nickel dissolution was evaluated as 12.38 kJ/mol. Finally, on the basis of the shrinking core model, a proposed empirical kinetic model for the leaching of nickel from this Indonesian saprolitic ore was expressed as a mathematical model, which was verified as consistent with the obsenved experimental results.

SiO<sub>2</sub>/MgO Ratio Effect on Carbothermic Reaction of Synthetic Nickelliferrous Mixtures

The objective of this study is to understand the effect of SiO2/MgO ratio on the reducibility of Fe and Ni in the synthetic nickel laterite mixtures of similar composition to Indonesia saprolitic nickel ore. The samples were made with addition (in wt% basis ) of 12% cokes, 2 % NiO, and varied SiO2 of 0 %, 10 %, 20%, 30%, and 40 % . Two types of samples were made, with and withou 14% MgO addition. Fe2O3 was added to both types of samples as balance. The synthetic compounds were mixed homogenously and compacted, before subjected to carbothermic reduction process at temperature of 1250 °C for 120 minutes. The results showed that Ni was strongly influenced by MgO content after reduction processes. Higher content of MgO was more unfavorable for nickel reduction, while a higher content of SiO2 was unfavorable for Fe reduction. According to the characterization results of carbothermic product, nickel oxide had been reduced to form Fe-Ni phase. Nickel recovery tends to increase with increasing of SiO2 addition on sample with MgO addition.

Lateritic saprolite leaching in concentrated magnesium chloride media

Magnesium chloride brines present a number of potential advantages for the processing of lateritic saprolite ores for nickel production. Concentrated MgCl2 solutions enhance the activity of acid used, allow atmospheric leaching at elevated temperature, and inhibit magnesium dissolution, which reduces acid consumption and increases metal selectivity. However, with a chloride based leach it is economically requisite to recover hydrochloric acid, conventionally accomplished by pyrohydrolysis. This work was performed in conjunction with a novel flowsheet for the processing on saprolite ores, which recovers HCl by the precipitation and subsequent decomposition of magnesium hydroxychlorides, alleviating some of the issues with pyrohydrolysis. Leaching experiments have been conducted in concentrated MgCl2 brines, up to 4·5 m, to determine the amenable process conditions and explore the kinetics of the process. It was determined that >95% extraction of metals was possible using both aqueous and gaseous HCl, while suppressing the dissolution of Mg from the ore.Les saumures de chlorure de magnésium présentent un nombre d’avantages potentiels dans le traitement des minerais de saprolite latéritique pour la production du nickel. Les solutions concentrées de MgCl2 augmentent l’activité de l’acide utilisé, permettent la lixiviation atmosphérique à haute température, et inhibent la dissolution du magnésium, ce qui réduit la consommation d’acide et augmente la sélectivité du métal. Cependant, avec une lixiviation à base de chlorure, on doit récupérer l’acide hydrochlorique pour raison économique, ce qui est accompli conventionnellement par pyrohydrolyse. On a effectué ce travail en conjonction avec un nouveau schéma de procédé pour le traitement des minerais de saprolite, lequel récupère le HCl par précipitation et par décomposition subséquente des hydroxychlorures de magnésium, ce qui atténue certains des problèmes reliés à la pyrohydrolyse. On a effectué des expériences de lixiviation dans des saumures concentrées de MgCl2, jusqu’à 4·5 m, afin de déterminer les conditions favorables du procédé et d’explorer la cinétique du procédé. On a déterminé qu’il était possible d’extraire >95% des métaux en utilisant à la fois le HCl aqueux et gazeux, tout en supprimant la dissolution du Mg du minerai.

Effects of Several Parameters on Nickel Extraction from Laterite Ore by Direct Bioelaching Using <i>Aspergillus niger </i>and Acid Rock Drainage from Coal Mine as an Organic Substrate

Aspergillus niger is a prominent fungi that has been used for bioleaching of nickel laterite ore and commercial production of citric acid. Series of shake flask bioleaching assays have been conducted to study the effects of ore type, ore particle size distribution, solid percentage, and substrate volume percentages as well as sulphur addition on the nickel extraction from Indonesian laterite ore. Acid rock drainage (ARD) from local coal mine was used as a substrate for carbon and nutrient sources of the fungus. The results show that saprolite ore exhibits a better leaching with fungal metabolic acids than limonite. The highest nickel extraction of saprolite ore was 43%, which was obtained from the shake flask bioleaching assays with ore particle size distribution of -60+80 mesh, pulp density of 2.5%, and ARD substrate volume percentage of 10%, after 24 days of incubation. For both saprolite and limonite ore samples, coarse particle size ditribution of -60+80 mesh provides a better nickel extraction than the finer ore particle size. The addition of potato dextrose agar and fresh innoculum after 16 incubation days generated a re-increase of nickel extraction, indicating the refreshment of the active fungus. Based on the results, it was found that Aspergilus niger growth can take place with an organic substrate of liquid ARD obtained from coal mining area.

Effects of Individual Use, Mixed Culture and Sulfur Addition on the Effectiveness of Nickel Laterite Ore Bioelaching with <i>Penicillium verruculosum</i> and <i>Galactomyces geotrichum</i>

The type of fungi used in fungal bioleaching influence the effectiveness of metals extraction due to the dissimilarity of each microorganism survival in environments with certain metals content. Several metals have prominent toxicities to certain type of fungi, which result in low metals extraction and leaching rate. In the present work, the influences of individual use of Penicillium verruculosum and Galactomyces geotrichum on the nickel extraction from Indonesian laterite ore are compared with the fungis mixed culture. The effect of sulfur addition in the mixed culture of Penicillium verruculosum and Galactomyces geotrichum on the sulfate ion formation was also investigated. Shake flask bioleaching assays have been performed in a medium growth of Indonesian food production waste over the course of 28 days. It was observed that mixed culture of Penicillium verruculosum and Galactomyces geotrichum has a higher effect in nickel extraction compared with the use of individual fungi for both saprolite and limonite ores. For saprolite ore, the fungi species of Galactomyces geotrichum demonstrated a greater nickel extraction than Penicillium verruculosum. In contrast, the extraction of nickel from limonite ore by Penicillium verruculosum was slightly more effective than Galactomyces geotrichum. The addition of elemental sulfur in the mixed culture was found to generate sulfate anion, having a significant increase in its concentration with time and decreasing solution pH within 28-day of incubation. The proportion of sulfur addition must be optimized in order to avoid ore surface passivation with the excess of sulfur. XRD analysis detected the presence of un-oxidized sulfur on surface of the leaching residues along with the biomass.

Direct extraction of nickel and iron from laterite ores using the carbonyl process

The carbonyl method of refining nickel and iron was invented more than 100years ago and has been used for refining of nickel commercially. CVMR® developed the process of direct extraction of nickel and iron from laterite ores as metal carbonyls which in turn produced pure nickel and iron metals.CVMR®’s carbonyl technology has been applied to several types of limonite and saprolite ores containing other metals such as copper, cobalt and PGE. The process consists of reducing the ore with hydrogen, extracting of iron and nickel in the form of volatile metal carbonyls, separating the metal carbonyls and producing high purity nickel and iron metals; and production of copper, cobalt and PGE concentrate by gravity or magnetic separation. Economic evaluation of this process shows significant increase in cash flow. The CVMR® process does not produce liquid waste and does not require tailing dumps. This makes CVMR®’s process attractive for projects in areas that are environmentally sensitive, or have a high level of rainfall.

Effect of sulphur addition on roasting reduction of saprolitic nickel ores in CO/CO2 gas mixture

The objective of the work presented here was to examine the effect of sulphur on formation of high temperature recrystallised phases and further on reducibility of saprolitic nickel ores. The ores blending with FeS2 were first heated up to 1300°C to investigate the changes that might happen upon heating. As a result of sulphur addition, the exothermic peak, which is evidently associated with crystallisation of new phases, changed in size compared to ores without sulphur addition. In addition, the dehydration and recrystallisation of minerals was also observed to begin earlier, when sulphur is added in the ores. Additionally, the ores were reduced in CO/CO2 atmosphere at temperatures 750 and 900°C, in order to determine the effect of sulphur and temperature on the reducibility. Although sulphur prevented the formation of difficult reducible olivine, as well as pyroxene phases in Colombia-2 and Mirabela, the degree of removable oxygen was not enhanced when increasing the temperature from 750 to 900°C. This is due to low reduction of iron oxides to metalic iron at 900°C. The beneficial effect of sulphur on metallisation degree of the ores was observed by comparison of these results with the experiments carried out with the pellets without sulphur addition. The results obtained in this study clearly indicate that the reduction of saprolitic nickel ores appears to be strongly dependent on the characteristics of the starting materials, amount of additive used and the process conditions.

Mineralogical characterisation of Indonesian laterites prior to and post atmospheric leaching

Nickel laterite is of increasing importance as an economically viable source of Ni. However, practical strategies for low cost atmospheric leaching of laterites have been hampered in part by poor recovery and the formation of potentially environmentally harmful residues, for instance jarosite. It is important therefore to define the mineralogical forms of nickel present, using readily accessible laboratory-based approaches, in the ores and residues, and the form of secondary precipitates in the residues, as a means for future optimisation of leach parameters. To this end a detailed mineralogical study of an Indonesian laterite and residues resulting from atmospheric acid leaching has been undertaken. The leach process involved the sequential reactions of first, limonite (3h) and then saprolite (11h) in sulphuric acid leach liquor. The acid, dry limonite and saprolite ores were added in the weight ratio of 1.4:1:1. At 100°C, over 80% of the total Ni was extracted into solution during the whole leach process.The limonite ore was found to contain Ni (1.2wt.%) predominantly within Fe oxy/hydroxides, serpentine and phyllomanganate whilst in the residue Ni (0.4wt.%) is present in highly crystalline leach resistant minerals, such as spinels. The majority of the Ni (1.6wt.%) in the saprolite ore was present in serpentine with a small fraction also present in phyllomanganate. This association is further confirmed by the similar saprolite leach extraction curves of Ni and Mg. Goethite and jarosite are both found to be concentrated in the saprolite residue, as is consistent with the decrease in Fe extraction during the saprolite leach. For the final 11h residue of the saprolite leach, the Ni (0.3wt.%) is mainly present in undissolved lizardite and leach resistant pyroxene and spinel minerals.

Banded iron formation to high-grade iron ore: a critical review of supergene enrichment models

All the major worldwide direct-shipping iron ore deposits associated with banded iron formations (BIF) are characteristically deeply weathered. They extend to considerable depths below the water table and show well-preserved primary structures and textures, but characteristically most deposits contain no evidence of chert bands being present prior to weathering. Recent studies have found evidence of hydrothermal and/ or metamorphic influences in the development of certain ore deposits and new genesis models such as the supergene-modified hypogene model have been postulated for major high-grade iron ore deposits. Nevertheless, there are many high-grade deposits that show no evidence of hypogene alteration and for which a hypogene or metamorphic genesis is unreasonable that are automatically ascribed to supergene enrichment, commonly erroneously attributed to lateritic weathering in tropical environments. Laterite (sensu lato) is a soil formation in which primary textures are destroyed and is underlain by a pallid zone showing the preservation of chert and the depletion, not enrichment, of iron oxides and thus is totally incompatible with the formation of the high-grade ore deposits. Various theories and models that purported to explain the conditions under which such a uniquely BIF-related dissolution of quartz and residual accumulation of hematite could occur by supergene processes typically conflict with current understanding of groundwater hydrology, chemistry, weathering processes and soil formation.Supergene enrichment of ore is universal in the leaching of gangue minerals such as iron silicates, carbonates and apatite and supergene enrichment of BIF to low-grade ore is common in near surface environments above the water table such as ferrugenised BIF outcrops, detrital ore deposits, and some shallow ore deposits that have been subjected to prolonged exposure to fresh meteoric water. In all cases of supergene enrichment traces of the chert bands are visible and the dissolution or replacement processes for the removal of quartz are clear, in direct contrast to the most important deep saprolite ore deposits that show no trace of chert bands.The widespread acceptance of an inappropriate and untenable supergene enrichment model inhibits search for the true origin of the ore and our ability to predict and find concealed high-grade ore deposits.

Effect of mineralogy and reducing agent on reduction of saprolitic nickel ores

The aim of this work was to explore the effect of mineralogy and gas reductant on the reduction of nickel laterites. Three different nickel saprolitic ores reduced by carbon monoxide were examined, and the results were compared with hydrogen reduction of the same ores. The reduction experiments were carried out in a thermobalance furnace which allows continuous measurement of mass change. The ores were reduced in CO/CO2 (72%∶28%) at 750 and 900°C for 90 min. Chemical analysis was utilised to determine the degree of metallisation of the reduced pellets. It was found that the reduction rate and degree of removable oxygen at 750 and 900°C in CO/CO2 was lower compared to reduction experiments in H2/N2 (72%∶28%). The reducibility of the samples seems to depend on mineralogy and their modes of decomposition and recrystallisation. The results clearly indicate that using various reducing gases has an impact on the reduction behaviour and generation of metallics of nickel saprolitic ores.

Effect of mineralogy on reducibility of calcined nickel saprolite ore by hydrogen

The aim of the work reported here was to examine the effect of mineralogy on reducibility of three different nickel saprolite ores originating from Brazil (Mirabela deposit) and from Colombia (Cerro Matoso S.A. deposit) as well as find the optimal reduction conditions for the ores. The reduction experiments were carried out in a thermobalance furnace at constant temperatures 600, 750 and 900°C in H2/N2 (72%/28%) gas mixture, followed with scanning electron microscopy (SEM–EDS) and chemical analyses. The results clearly indicate that the reducibility of the ores depends on the initial mineralogy of the ore and on the phases formed during heating, before the reduction stage. The presence of antigorite in Colombia-1 ore results in a simultaneous dehydroxylation and transformation of magnesium silicate to olivine and pyroxene during heating. Due to the recrystallisation, the degree of removable oxygen Rt is decreased when increasing the temperature from 750 to 900°C. The low reducibility at 900°C was also confirmed by chemical analysis. Colombia-2 and Mirabela do not, however, contain antigorite and heating the samples resulted in formation of an easily reducible intermediate phase. Thus, the recrystallisation is not initiated at 750°C like in Colombia-1 and their Rt values were also enhanced at 900°C. According to SEM–EDS results, a complex series of phase and microstructure changes has been observed along with the formation of Fe–Ni alloy particles.

Nickel geochemistry of a Philippine laterite examined by bulk and microprobe synchrotron analyses

The Ni geochemistry of limonite and saprolite laterite ores from Pujada in the Philippines has been investigated using a mixture of laboratory and synchrotron techniques. Nickel laterite profiles are typically composed of complicated mineral assemblages, with Ni being distributed heterogeneously at the micron scale, and thus a high degree of spatial resolution is required for analysis. This study represents the first such analysis of Philippine laterite ores. Synchrotron bulk and microprobe X-ray absorption spectroscopy (XAS), comprising both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies, together with synchrotron microprobe X-ray fluorescence microscopy (XFM) and diffraction (XRD) have been applied to provide quantitative analysis of the mineral components and Ni speciation.Synchrotron microprobe EXAFS spectroscopy suggests that the limonite Ni is associated with phyllomanganate via adsorption onto the Mn oxide layers and substitution for Mn within these layers. Laboratory scanning electron microscopy, coupled to electron dispersive spectroscopy analyses, indicates that Ni is also associated with concentrated Fe containing particles and this is further confirmed by synchrotron bulk and microprobe investigation. Linear combination fitting of the bulk EXAFS limonite data suggests 60±15% of the Ni is associated with phyllomanganate, with the predominant fraction adsorbed above vacancies in the MnO6 layers with the remainder being substituted for Mn within these layers. The remaining 40±10% of the Ni in the limonite ore is incorporated into goethite through replacement of the Fe. In the saprolite ore, 90±23% of the Ni is associated with a serpentine mineral, most likely lizardite, as a replacement for Mg. The remaining Ni is found within phyllomanganate adsorbed above vacancies in the MnO6 layers.