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Nickel laterite processing technologies – where to next?
Full text linkA review has been completed of current and proposed processing technologies for nickel laterite ores. Although future supplies of nickel depend on laterite ores, their processing is inherently energy intensive and expensive as, unlike sulphide ores, they cannot be significantly upgraded, meaning the entire ore needs to be treated in the process. In addition, hydrometallurgical processes in particular have significant waste disposal problems. However, new technologies are being developed that are attempting to address the current processing issues, including some innovative processes that are in their early stages of development.
Smelting is a well known and proven technology for saprolite ores that is continually being improved. It is a high energy consumer and needs to be performed on a large scale to be cost-competitive. It remains the most popular option for large scale developments based on saprolite ores and accounts for a significant amount of nickel production from laterite sources.
High Pressure Acid Leach (HPAL) remains the process of choice for treating limonite ores, especially for large scale developments. It has the advantages of high nickel and cobalt recoveries and is applicable to a wide range of ores. However, the process does suffer from significant waste disposal issues.
Atmospheric and Heap Leaching. Smaller scale developments are being actively pursued based on Atmospheric Leaching (AL) or Heap Leaching (HL) technologies. These processes do not rely on autoclave technology and are therefore perceived to be less capital intensive and easier to operate. However, they still require significant investment in terms of infrastructure. Heap leaching is the option most investigated, whereas AL is gaining momentum for mixtures of limonitic and saprolitic ores. Waste disposal issues are again significant.
Downstream Product Recovery. The two main downstream processing technologies for nickel recovery from solution are mixed hydroxide precipitation and mixed sulphide precipitation. Refining to pure nickel and cobalt products is an option in the large scale HPAL projects.
NEW TECHNOLOGIES
Hydrochloric Acid Leaching has been investigated on a laboratory scale for some time based on the fact that the acid can be reformed from the waste liquor by pyrohydrolysis or, more recently, distillation. In order to be competitive, these processes require that acid usage is minimised. However, aggressive leach conditions with high acid consumptions are still required to give good nickel recoveries. In some processes, waste disposal can be minimised.
Bioleaching of oxide ores is a concept that is in its infancy, but does offer some advantages over conventional processes. The process uses organic matter to produce organic acids in situ that leach the ore, possibly by-passing the need for expensive mineral acids, and reducing waste disposal problems associated with current processes
Elution strategies for recovery of nickel and cobalt from laterite tails through scavenging resin-in-pulp
Full text linkIon exchange resin has been used to recover value metal in the uranium and gold industry through resin-in-pulp/leach and similar carbon-in-leach/pulp processes for decades. More recently, resin-in-pulp processes have gained attention as a potential method to improve the efficiency of nickel operations. While every nickel laterite operation is unique, many involve an acid leach, neutralization and oxidative precipitation of impurities followed by counter current decantation to separate valuable liquor from the unwanted metal residue and precipitate. Counter current decantation (CCD) of this material is challenging at best, with large CCD tanks having a large plant footprint and requiring high capital investment. Depending on the settling characteristics of the precipitate, 5% or more of the leached nickel and cobalt can be lost to the slurry underflow through solution entrainment, co-precipitation, and sorption processes on the high surface area solids. For a site producing 40,000 tonnes per annum nickel and 2,500 tonnes per annum cobalt this represents yearly losses of approximately $40 million USD, given current LME spot prices (as of March 2012).
The tremendous waste of value that these high losses of nickel and cobalt represent are the primary driving force behind the development of resin-in-pulp (RIP) scavenging from laterite tailings. RIP scavenging involves contacting ion exchange resin with nickel laterite tailings at conditions where the valuable metals load onto the resin. As the resin beads are larger than the slurry particles, they can be separated from the slurry using vibrating sieving. Following this, the resin is washed to remove residual slurry and solution, and then eluted to recover metal value. While exact values vary, typical caron process tails contain roughly 300 mg/L nickel and 50 mg/L cobalt in slurry. High pressure acid leach tailings may contain 200 mg/L nickel and 35 mg/L cobalt in slurry. With efficient resin-in-pulp contact, upwards of 90% of this otherwise lost metal value can be recovered.
Although the chelating ion exchange resins proposed for use in nickel laterite RIP are selective for nickel and cobalt over other unwanted metals, laterite tailings solutions contain a relatively small amount of these metals of interest. Depending on the composition of the original ore and the method of leaching, the neutralized slurry can contain large amounts of solution phase magnesium and manganese (in the case of acid leaching) and vast amounts of ferric iron, silica, aluminium, and chromium in the solid phase. The presence of other cations that compete with nickel and cobalt for resin loading sites complicates resin-slurry equilibria. In general, there is a trade off between recovery of nickel and cobalt and purity of loaded resin. To recover a high amount of the nickel and cobalt value, one must accept the presence of impurity metals on the resin. When resin is eluted, these impurity metals can follow value metals into the eluate.
To date, the majority of resin elution work has focused on metal recovery via acid contact (usually H2SO4). When one has produced a resin loaded with a high fraction of value metals, quantitative elution in this fashion is attractive. Using strong acid, metal is recovered in a small volume of eluent with rapid kinetics. However, as more impurities are loaded onto a resin, strong acid elution becomes less attractive as quantitative elution of a low purity resin produces a low purity eluate. In such a case, a method of selectively recovering value metal from resin is desirable.
One selective elution method involves two stages – dilute acid to remove weakly bound impurity metals, followed by strong acid to recover the remaining metals[3]. For the iminodiacetic acid resins most commonly investigated for nickel RIP, this method achieves selectivity of nickel and cobalt over magnesium, calcium, and manganese, but does not separate nickel and cobalt from ferric iron or chromium. Another option is the use of ammoniacal elution[8]. Nickel and cobalt readily form stable amine complexes and have high solubility in strong ammonia solutions, unlike the majority of other metals present in nickel laterite processing. By taking advantage of this chemistry, nickel and cobalt can be effectively eluted separate from impurity metals.
The value of selective elution depends on the purity of the loaded resin. With less impurities, the additional expense of a second stage of elution or the higher cost of ammonia reagents relative to sulphuric acid make selective elution less attractive. In order to better determine the performance of different elution methods, a resin produced through scavenging RIP of plant tails from Queensland Nickel was treated using three elution strategies – single stage strong acid, two stage weak acidstrong acid, and single stage ammoniacal elution
The development of a resin-in-pulp process for the recovery of nickel and cobalt from laterite leach slurries
Full text linkThe processing of laterite ores for the recovery of nickel and cobalt has increased as the reserves of exploitable sulphide ores have become depleted. The pressure acid leach process (PAL) has become the preferred option for the treatment of laterite ores.
Difficulties associated with the poor settling characteristics of the pulp in the counter current decantation (CCD) circuit after pressure leaching has resulted in as much as 10% of the soluble nickel and cobalt reporting to the tailings. The objective of this project is the development of an alternative processing step for the recovery of soluble nickel and cobalt from the PAL tailings using resin-in-pulp (RIP) technology.
Commercially available chelating resins with the iminodiacetate functional group have been studied for their suitability for the adsorption of nickel and other metal ions from PAL tailings pulp. The Amberlite IRC 748 resin was found to be superior despite its lower nominal loading capacity. The resin with the highest nominal capacity was observed to adsorb less nickel as a result of the adsorption of greater amounts of the impurity ions. The equilibrium loading for nickel on the preferred resin was found to be similar from the ammonium and protonated form of the resin although the kinetics of adsorption is greater when the resin is initially in the ammonium form.
A comprehensive study has been made of the equilibrium adsorption of several metal ions on the resin as a function of the equilibrium pH of the solution. A relatively simple model of the equilibrium adsorption which includes the effect of pH has been developed and the results compared with the experimental data obtained in the M2+/Na+/H+ system. The model which requires two equilibrium constants has been found to fairly well describe the experimental results.
A study of the kinetics of the loading of nickel and cobalt from both solutions and pulp has shown that the rate can be described in terms of a first-order approach to equilibrium. The kinetic and equilibrium parameters were used to simulate the performance of a multi-stage counter current resin-in-pulp operation A semiquantitative study of the elution of the adsorbed metal ions from the resin by dilute sulphuric acid solutions was also undertaken.
The technical feasibility of the RIP process for the recovery of nickel and cobalt from the PAL tailings has been successfully demonstrated in both laboratory and pilot-plant studies using a five-stage adsorption process. Important parameters such as the operating pH and the residence times of pulp and resin in each stage were identified through the batch test work coupled to the simulation procedure. The optimum pH for adsorption was found to be in the range 4 to 5 as this pH is high enough to maximise the adsorption of nickel and cobalt while preventing precipitation of nickel and cobalt as hydroxides from the pulp. A method for minimizing the competition from more strongly loaded ions such as iron(II) and chromium(III) which are present in the pulp was also developed in the initial laboratory phase of the project and utilized during the pilot operation.
Problems associated with the preparation of the pulp preparation, elution of the loaded resin and control of the adsorption train were resolved during several pilot plant runs on site at a local PAL plant. These and other minor improvements and adjustments to the operating procedure culminated in a successful continuous run for several days during which the target recovery of 90% for nickel and 60% for cobalt was exceeded throughout the run
Geotechnical characterisation of a laterite soil from Youndé (Camerun) used as Nkolfoulou landfill bottom barrier
Full text linkopenIn questo elaborato viene preso in esame un terreno composto da laterite prelevato nei pressi di Youndé (Camerun), dove costituisce la barriera di fondo della discarica di Nkolfoulou, allo scopo di caratterizzarlo dal punto di vista geotecnico e mineralogico, e di verificarne le proprietà idrauliche e di resistenza.
Vengono quindi eseguite una serie di prove di compattazione e misura della permeabilità del terreno compattato all’optimum, confrontando i risultati anche con quelli ottenuti da prove effettuate in situ sul materiale allo stato naturale, riportate nella tesi di uno studente camerunense
An electrochemical investigation of the formation of CoSx and its effect on the anodic dissolution of iron in ammoniacal-carbonate solutions
Full text linkIt has been found that the co-presence of cobalt (II) and thiosulphate ions in ammoniacal-carbonate solutions promotes the passivation of iron, under conditions in which it would otherwise continue to dissolve anodically. Electrochemical experiments have shown a relationship between the immersion time required for passivation and the formation of a solid species on the iron surface, which is thought to be implicated in the mechanism of passivation, whilst not being itself the protective species. Based on a combination of electrochemical, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and grazing incidence X-ray diffraction (GIXRD) characterisation techniques, the said species has been identified as CoSx, resulting from the interaction of cobalt (II) and thiosulphate ions. It is thought to form as a product of the cathodic reactions taking place on the iron surface during its active dissolution.
These findings are particularly relevant to the Caron process, in which the ammoniacal-carbonate solutions containing dissolved cobalt and thiosulphate ions are used to leach nickel and cobalt from pre-reduced laterite ores rich in metallic iron. Both the loss of cobalt into the CoSx layer and the passivation of iron and of its alloys with nickel and cobalt, are potential contributing factors to the low cobalt and nickel recoveries, which are typical of the Caron process. This study provides a better understanding of the conditions under which the CoSx layer forms and promotes the passivation of iron, and may therefore provide useful information to help minimise the effect this may have on the extraction efficiency of the process. In particular, at the cobalt and thiosulphate ion concentrations usually encountered at a Caron plant, the passivation of iron was found to be prevented by maintaining a high enough concentration of ammonia
Geotechnical Characterization and Sedimentation Behavior of Laterite Slurries
No full textAbstract: Laterites are the main source of economic metals such as nickel and cobalt. In many parts of the globe, these metals are extracted using the pressure acid leach operation. An efficient rate of sedimentation and a high solids content of the material are two prerequisites for this process. To understand the geotechnical behavior of laterite slurries under ambient process conditions, a comprehensive laboratory characterization and sedmentation protocol was developed. Results indicate that due to changes in material characteristics, sedimentation behavior of laterite slurries is improved during the metal extraction process. The rate and concentration of sedimentation are directly related for this class of materials
Extraction of metals from saprolitic laterite ore through pressure hydrochloric-acid selective leaching
No full text
In this study, pressure hydrochloric-add selective leaching was used to leach Ni and Co from saprolitic laterite ore on the basis of potential-pH diagrams constructed for Fe-Si-Cl-H2O and Ni-Si-Cl-H2O systems. The leaching of Ni, Co, Mg, Mn, and Fe was investigated under various conditions. The results showed that the optimal output was obtained under the following conditions: leaching temperature of 150 degrees C, HCl concentration of 350 g/L, liquid to solid ratio (I/s) of 1.0, and leaching time of 90 min. The saprolitic laterite and leached residue were characterized via X-ray diffraction analysis, infrared spectrometry, and mineral liberation analysis. The related acid leaching mechanism was analyzed based on the results, and the kinetics of the leaching process was discussed. The results of this study suggest that pressure hydrochloric-acid selective leaching is a viable method for efficiently leaching Ni from laterite ores. (C) 2015 Elsevier B.V. All rights reserved.</p
Effects of gradation on hydraulic conductivity properties of compacted laterite soil liner: a review
No full textThis paper presents the recent review on the effects of gradation on hydraulic conductivity properties of compacted laterite soil liner. The distribution of different grain sizes affects the engineering properties of soil such as compressibility, shear strength, and hydraulic conductivity . It is observed that there are variations in hydraulic conductivity values in laterite soils from various researches around tropical countries of the world . Hydraulic conductivity varies from 4.36×10 -3 m/s to 4.7×10 -11 m/s relative to fine contents ranging from 1.3% to 69% and coarse contents ranging from 31% to 98.7%. Generally, there is no clear trend established for effects of gradation on hydraulic conduc- tivity properties of compacted laterite soil liners. This is because laterite soils with less than 50 fines content cannot be used as liner or hydra ulic barriers because their hydraulic conductivities are less than the minimum requirement of 1 × 10 -9 m/s. It is anticipated that with logical understanding of the effects of gradation on hydraulic conductivity properties of compacted laterite soil it wil l serve as a guide in the design of hydraulic barriers for engineered sanitary landfills in tropical countries around the world
The electrochemistry of the leaching reactions in the Caron process II. Cathodic processes
Full text linkThe cathodic reactions which are involved in the dissolution of iron alloys in solutions typically used in the Caron Process have been investigated by electrochemical methods. The results have confirmed that cobalt(III) ammine complexes are the main oxidising agents but also highlight the important role of thiosulfate. A solid product is deposited on platinum as well as iron electrodes below -0.75 V/SCE and oxidised at more positive potentials. The product is either Ni or Co metal in the absence of thiosulfate or a metal sulfide when thiosulfate is present. The rate of reactions in the presence of thiosulfate is significantly greater than its absence. The probable chemical reactions taking place during leaching in the Caron Process have been revised on the basis of these observations
Precipitation of alpha-Fe2O3 and recovery of Ni and Co from synthetic laterite-leaching solutions
No full textAtmospheric hydrochloric-acid leaching of nickel laterite can produce a leaching solution containing a large amount of Fe. To recover Ni and Co, Fe should be separated from the laterite-leaching solutions. In this study, hydrolytic distillation was used to remove Fe from the synthetic laterite-leaching solutions; Ni and Co remained as soluble chlorides in the solutions, while most Fe was removed as Fe2O3. The conversion of Fe, recovery of Ni and Co, recovery of HCI, and the concentration of HCI, were investigated under various conditions. The results showed that the optimal output was obtained under the following process conditions: the hydrolysis temperature was 170 degrees C, the pressure was 0.03 MPa, the water flow rate was 1.67 mL/min, and the hydrolysis duration was 2 h. The effects of impurities under the optimized conditions were also studied, showing that as the amount of metal chlorides increased, the conversion of Fe decreased. The residues were characterized by X-ray diffraction analysis and scanning electron microscopy with X-ray energy-dispersive spectroscopy. Using the results, a growth model of alpha-Fe2O3 (hematite) spheres from the synthetic laterite-leaching solutions was constructed. (C) 2015 Elsevier B.V. All rights reserved
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