Laterite Leach Research Articles

Sulphuric acid pressure leaching of laterites — universal kinetics of nickel dissolution for limonites and limonitic/saprolitic blends

The high-temperature acid pressure leaching of nickeliferous laterites has attracted considerable attention from the nickel industry during the recent years. The process is especially advantageous for low-grade limonitic laterites. It is also applicable to mixtures of limonites and high-grade saprolites, although at higher acid consumption due to magnesium. It is advantageous, therefore, to derive a universal kinetic equation of Ni dissolution that is equally applicable to both types of feed. In the present work, a kinetic equation was derived using the concentration of hydrogen ion “at temperature” calculated from a solution speciation approach, and the assumption that nickel associated with magnesium dissolves instantaneously. This universal kinetic equation was successfully tested against various feeds that contained from 1.2% to 1.9% of Ni and up to 3.85% of Mg at temperatures from 230°C to 270°C, acid/ore ratios from 0.15 to 0.5, and concentrations of solids in the feed (pulp density) of up to 30%. The use of acidity “at temperature” made it possible to explain an apparent dependence of the ultimate Ni extraction on the acid/ore ratio, and on the terminal stoichiometric acid concentration during leaching.

Sulphuric acid pressure leaching of laterites — speciation and prediction of metal solubilities “at temperature”

Sulphuric acid pressure leaching of nickeliferous laterites has attracted considerable attention from the nickel industry during the past 5 years. The process is especially advantageous for limonitic laterites that mostly contain goethite, because iron precipitates releasing acid and thus rendering low acid consumption. It is also applicable to mixtures of limonites and saprolites, although at a higher acid consumption due to magnesium. Effective process design requires the solubility of metals that precipitate during the process to be known. In the present work, determination of metal solubilities is based on a simple speciation program that assumes the presence of only one dominant complex for each metal. The thermodynamic data for the precipitation reactions are extracted from high-temperature experiments with monometallic systems published previously. The validity of the approach is then tested against mixed bimetallic systems, and finally applied to calculate the solubility of aluminium, iron and magnesium in laterite leaching effluents “at temperature”. In both cases of limonitic feed and limonitic/saprolitic blends, the prediction closely follows metal solubilities measured experimentally at temperatures from 230 to 270°C and at terminal-free acidities ranging from 10 to 75 g/l.

High-Temperature Conductivity Measurements for Industrial Applications. 2. H2SO4−Al2(SO4)3 Solutions

A new conductivity cell, developed previously, was employed from 15 to 250 °C, in an effort to investigate the inter-relation between the measured conductivities and the solution chemistry of high-temperature H2SO4−Al2(SO4)3 solutions. These electrolytes exist in the laterite leach slurry of the pressure acid leaching process. It was found that, at 250 °C, the effect of Al2(SO4)3 on conductivity decreases with increasing Al2(SO4)3. This is in agreement with our previous findings that aluminum mostly forms Al(SO4)+ at low molalities, whereas 85% of aluminum is associated as Al2(SO4)30 at near-saturation molalities. At 250 °C and constant H2SO4 molality, the solution conductivity drops with increasing Al2(SO4)3 molality. It is suggested that this drop is caused by a sharp decrease in ionic equivalent conductivities of H+, HSO4-, and Al(SO4)+. This drop was found to be similar in nature to the drop when H2SO4 is added at very low concentrations. A new unified correlation for ionic equivalent conductivity in terms of one-third power of individual ionic strength was found. Finally, a simple mixing rule was developed to calculate the ionic equivalent conductivities in H2SO4−Al2(SO4)3 solutions.

Purification of synthetic laterite leach solution by solvent extraction using D2EHPA

The world mineral industry is experiencing an unprecedented interest in nickel–cobalt extraction from laterite ores through acid pressure leach and SX-EW processes. The recovery of cobalt and nickel from the leach solution through direct solvent extraction is of great interest as this would result in significant capital and operating cost savings. In the direct solvent extraction approach, the separation of zinc, calcium, copper and, in particular, manganese from cobalt and nickel is highly important. A series of shakeout tests was undertaken to investigate the fundamentals of the separation of the above impurities from cobalt and nickel using di-2-ethylhexyl phosphoric acid (D2EHPA) in kerosene. D2EHPA pH-extraction isotherms from solutions each containing a single element showed that the extraction order for the seven elements of interest as a function of pH 50 was Zn 2+>Ca 2+>Mn 2+>Cu 2+>Co 2+>Ni 2+>Mg 2+. This confirmed that manganese would be extracted from sulfate solution ahead of cobalt and nickel. Extraction isotherms from solutions containing Zn, Ca, Mn, Cu, Co, Ni and Mg showed that the separation of zinc and calcium from the other elements was not difficult and the separation of copper and manganese from cobalt and nickel was possible. The separation of manganese from cobalt and nickel by D2EHPA in kerosene was affected by temperature and pH. At pH 3.0, better separation of manganese from cobalt and nickel was achieved at room temperature (23°C). At pH 3.5, better separation of manganese from cobalt was achieved at room temperature (23°C). However, better separation of manganese from nickel could be obtained at elevated temperatures (40–60°C). The McCabe–Thiele diagram for the system showed that at pH 3.5 and 40°C, two theoretical extraction stages at A/O ratio 1:1 were needed to extract 99.9% manganese from the aqueous solution and to reduce the manganese concentration from 2.0 g/L to 3 ppm. Multiple stage extraction with fresh aqueous solution showed that cobalt and nickel were crowded out by zinc and manganese. Multiple stage extraction with fresh organic solution showed that manganese and copper in the aqueous solution were eliminated. Multiple stage scrubbing of the loaded organic solution with manganese solution indicated that after one stage of contact, only about 3 ppm cobalt and nickel were present in the organic solution.

The ion-association-interaction approach as applied to aqueous H2SO4-Al2(SO4)3-MgSO4 solutions at 250 °C

A hybrid ion-association-interaction approach is implemented to describe the chemistry and thermodynamics of aqueous H2SO4-Al2(SO4)3-MgSO4 solutions at 250 °C. These solutions are relevant to the sulfuric acid pressure leaching of nickeliferous laterites. Strong complexes in solution are handled via the ion-association approach. Nonidealities, including weak ion pair formations, are treated through the Pitzer ion-interaction theory. The existing complexes in solution and the Pitzer ion-interaction parameters were identified through processing solubility data in the binary (H2SO4-Al2(SO4)3 and H2SO4-MgSO4) as well as the ternary (H2SO4-Al2(SO4)3-MgSO4) electrolyte solutions at or near 250 °C. The existing aqueous aluminum-bearing species identified were Al3+, Al(SO4)+, and Al2(SO4)30, with Al2(SO4)30 as the dominant species at moderate to high H2SO4 concentrations. The existing aqueous magnesium-bearing species found were Mg2+ and MgSO40, with Mg2+ being dominant except at low concentrations of H2SO4. The dominant species identified for Al and Mg explain why a higher H2SO4 concentration in solution is required during the processing of high-magnesium laterites.

Pressure acid leaching of laterites at 250 °C: A Solution chemical model and its applications

Laterite leach solutions were simplified to a ternary electrolyte system by lumping all divalent metal sulfates into one with the chemical and thermodynamic properties of MgSO4. The simplified leach solution was assumed to contain H2SO4, Al2(SO4)3, and pseudo-MgSO4. The ion-association-interaction approach, as developed previously, along with previously identified species and interaction parameters for this ternary electrolyte system, was implemented to develop a chemical model for the laterite leach solutions at 250 °C. The model was tested against experimental solubility data for aluminum, and a very good match was obtained for pseudo-MgSO4 concentrations less than 0.25 molal. The model was also implemented to calculate true acidities “at temperature.” The need to maintain higher H2SO4 concentrations in solution during processing of high-magnesium ores was explained. Finally, a new rate equation for nickel extraction was developed. It is based on true acidity at temperature and is applicable to both limonites and limonite-saprolite blends.

Scale formation in a batch reactor under pressure acid leaching of a limonitic laterite

The formation of scale under sulphuric acid pressure leaching conditions of a limonitic laterite ore was investigated. Experiments were conducted using a batch titanium autoclave in order to study the effects of acidity (0.15–0.40 M), and temperature (250–270°C) on scale formation. It was found that scale increases with acidity reaching a maximum at 0.22 M free sulphuric acid. Further increase of acidity causes reduction of scale, but scale is not affected by temperature. The scale mainly consists of hematite with traces of hydronium alunite as revealed by SEM photographs and EDX analyses. Anionic surfactants were tested as a means of scale reduction with some success. A reduction of 37% in scale formation was achieved at an acidity of 0.22 M and 270°C.

Sulphuric acid pressure leaching of a limonitic laterite: chemistry and kinetics

Sulphuric acid pressure leaching of limonitic laterites is the process of choice to recover nickel and cobalt from equatorial lateritic ores, replacing the energy intensive pyrometallurgical methods. This process achieves a high nickel and cobalt extraction (more than 95%) with a high selectivity due to simultaneous iron and aluminium dissolution and precipitation. Experiments were carried out using batch pressure leaching techniques. A titanium autoclave equipped with acid injection and sample withdrawal units was employed. Conditions close to the industrial practice were tested: pulp density 30%, acid to ore ratio 0.2 and temperature ranging from 230 to 270°C. Raw limonite and the evolution of the nature of solid products during leaching were characterised using transmission electron microscopy. It was observed that limonite consists of aggregates of needle-like particles of goethite compacted together. Nickel was found to be predominately associated with this phase. During leaching, goethite dissolves continuously liberating nickel whilst iron re-precipitates as dense hematite particles in solution by ex situ precipitation. Several kinetic models for porous solids were also tested. The grain model was finally proposed to best describe nickel dissolution kinetics. The rate-controlling step was suggested to be pore diffusion of sulphuric acid.

The leaching of nickeliferous laterite with ferric chloride

Several experiments were conducted to investigate the extraction of nickel from nickeliferous laterite by ferric chloride solutions as a function of pulp density, solution composition, and temperature. Solubility relationships for goethite and nickel laterite in aqueous solution were reviewed in terms of leaching rates and reaction mechanisms. Generally, the amount of nickel extracted increased with temperature, the amount of “free acid,” and ferric chloride concentration; however, the amount was inhibited by ferrous chloride. In this investigation, as much as 96 pct of the available nickel was extracted by ferric chloride solution. Nickel extraction was found to be more dependent on ferric chloride concentration than on the concentration of hydrochloric acid. Mechanistically, nickel extraction occurred by the formation of an intermediate ferric chloride complex, which was then hydrolyzed to hematite.

Object-oriented simulation of hydrometallurgical processes Part III. Application to leaching of laterites and pyrites

The steady-state process simulator developed by Kiranoudis et al. has been used for the detailed simulation of sulfuric acid pressure leaching of laterite ores for the extraction of nickel and cobalt, and aqueous pressure oxidation of pyrites for the recovery of gold. Advanced hydrometallurgical process models for the specific unit operations involved were developed and are appropriately described. The simulation mainly focuses on studying the overall effects of certain design parameters on the entire plant efficiency. In the case of pyrites, the autothermal performance of the pressure autoclaves can be maintained by means of the oxidized recycle stream that greatly influences the fundamental heat balances of the reactor. Flashing the reactor pulp at the exit of the autoclaves results in further precipitation of solids related to ionic equilibrium reactions. The effect of grinding is important since most reactions are facilitated by small particle diameters. The ratio of feed pyrites influences the amount of precipitation of solids in the autoclave.

5328103 Process for impact crushing of rock and ore lumps and an apparatus for performing same

Two different nickel sulfide intermediates must be refined to pure nickel and cobalt products: sulfide matte from nickel sulfide smelting and converting and sulfide precipitates from nickel laterite leaching. These data indicate that hydrometallurgy is the principal method for making high-purity nickel from these mattes and sulfide intermediates. The objective of this chapter is to describe the overall hydrometallurgical processes used to refine mattes and sulfide intermediates. Sulfide precipitates are refined to nickel and cobalt at following refineries; Fort Saskatchewan, Canada (Sherritt); Murrin Murrin, Australia (Minara Resources); and, Niihama, Japan (Sumitomo). Nickel is leached from laterite ores and precipitated as a sulfide. The objectives of the leaching stage are to extract the nickel and cobalt into solution, to oxidize the cobalt so that it is present as cobaltic hexamine. The filtrate is sent to ammonia stripping while the solids are sent to the cobalt circuit. The solution from the cobaltic hexammine precipitation is treated to reduce the concentration of ammonia in two steps: stripping with air and distillation. This solution passes through four reactors known as the ‘copper boil’ where copper is removed. Elemental sulfur and sulfur dioxide are added to the second reactor, which results in the precipitation of copper as copper sulfide. The processing of nickel–copper mattes is significantly different from the processing of mixed sulfide precipitates. This difference arises from the need to separate nickel from copper in the refinery. The Nikkelverk refinery at Kristiansand, Norway, uses chloride chemistry to refine nickel–copper matte. The Norilsk refinery at Harjavalta, Finland, uses sulfate chemistry to refine mattes that are relatively low in copper.

Particulates in hydrometallurgy: Part II. Dewatering behavior of unflocculated laterite acid leach residues

The effects of ore type and leaching conditions (e.g., temperature and solid/liquid ratio) on the settling behavior of laterite acid leach residues were investigated. Settling rate, supernatant turbidity, and filtrability of the slurry were used in evaluating the settling behavior. The slurries showed three different regimes of sedimentation, involving free, hindered, and compression settling behavior. An improvement in the dewatering characteristics of the leach residues was observed as the leaching temperature was increased. Furthermore, the leach residues showed markedly different settling behaviors depending on the temperature. Below 175°C, the settling rate for both 2.5 and 25 pct solids increased steadily with temperature. However, the results indicated clearly that at higher temperatures (>175°C), there is a steep increase in settling rate. These pronounced differences in settling behavior with temperature increase are believed to be due to the sharp increase in particle size above 175°C. The mineral content in the ore has a great effect on the settling behavior of the leach residues. It was found that the oxide-silicate material (Ore No. 2 leach residues) showed a lower settling rate than the iron-rich material (Ore No. 1 leach residues). This behavior is attributable to the presence of a gelatinous, siliceous product in the Ore No. 2 leached pulp.

Particulates in hydrometallurgy: Part III. Dewatering behavior of flocculated laterite acid leach residues

Three polyacrylamide-based polymers of different chemical properties (polymer A, 34 pct anionic, 11×106 mol wt; polymer B, 7 pct anionic, 7.5×106 mol wt; polymer C, nonionic, 13.5×106 mol wt) were used to evaluate the flocculation behavior of laterite acid leach residues. The solid-liquid separation characteristics of the leach residues were investigated with the aid of settling rate, supernatant turbidity, and slurry filtrability measurements. The polymeric flocculants were found to be effective in improving the dewatering properties of the acid leach residues. Polymer effectiveness increased with increasing polymer dosage for all the polymers, but an optimum polymer dose was only found for polymer A (34 pct anionic, 11×106 mol wt) in the studied range of polymer addition. Similarly, the dewatering behavior was improved at higher polymer molecular weight. In addition, it was found that the flocculation performance was adversely affected by an increase in the degree of polymer hydrolysis which, in turn, increases the ratio of carboxylic to amide functional groups in the polymer chain. Polymer C (nonionic ∼0 pct hydrolysis, 13.5×106 mol wt) was found to be the most efficient flocculant in terms of all the performance criteria investigated. The preceding results were rationalized in terms of bridging flocculation, the ionization and molecular configuration of the polymers, hydrogen bonding, and the solid/aqueous interfacial charge.

Leaching of a low grade hematitic laterite ore using fungi and biologically produced acid metabolites

Heterotrophic microbial leaching of Greek nickeliferous laterites was investigated. A variety of microbial screening, adaptation and leaching techniques were used to determine microorganisms, media and optimum conditions for nickel and cobalt recoveries. Two bioleaching techniques were developed: (a) leaching in the presence of microorganisms and (b) chemical leaching at high temperature (95°C) with a solution consisted of the metabolic products obtained from the cultivation of fungus strains at 30°C in glucose and sucrose carbohydrate media. It was found that up to 55–60% of the nickel was extracted using the first technique (a) with fungi strains P 2 ( Penicillium sp.) and A 3 ( Aspergillus sp.) in almost 50 days. Losses of soluble nickel in the fungal biomass were found to be 3.5% and 10.8%, respectively. The cobalt recovery achieved was around 50%. Nickel recoveries obtained by the second technique (b) were in the range of 70–72%. Effectiveness of strains used was found to depend on their ability to produce hydroxycarboxylic acids, especially citric acid, and also other metabolites. Possibilities for future investigations are discussed.

Mineral leaching of low-grade laterite ores using “bioacids” by molasses fungal metabolism

Factory beet molasses media were used as cheap sources of sucrose for microbial leaching of Greek nickeliferrous laterites using strains of Penicillium and Aspergillus sp. Nickel recoveries of up to 62% and cobalt recoveries of up to 50% were obtained by leaching in a two-phase system in which bioacids were produced by fungal metabolism in molasses ferrocyanide pretreated media. The addition of potassium ferrocyanide to the molasses substrates prior to leaching was found to have a beneficial effect by increasing the level of citric acid bioproduction, thus improving the leaching efficiency of the fermentation liquid. The metals known to interfere with citric acid production (i.e., Fe, Cu, Ca and Mn) were those most efficiently precipitated. Considering the much lower commercial price of molasses in the industrial market compared to sucrose and glucose carbohydrate media, molasses could prove to be a promising alternative, despite the cost of the required pre-treatment.

Fungal leaching of nickeliferous laterites

Extraction of nickel by microbial leaching of Greek laterites is feasible by using ofAspergillus andPenicillium. The effectiveness was found to depend on the ability of the microorganism to produce hydroxycarboxylic acids, especially citric acid, as well as other metabolites. The nickel recoveries achieved were as high as 60%, in 48 d, when the ore was leached in the presence of the living fungi, in a sucrose medium, and as high as 70%, in a much shorter time, when the ore was leached by their metabolic products after pH adjustment by means of sulfuric acid. The use of much cheaper, factory grade, Greek beet molasses as a growth medium proved promising, giving the possibility of making the process more attractive in economic terms.

Development of a high speed mechanical alloying system: T. Azizawa, J. Kihara (University of Tokyo, Tokyo, Japan)

Two different nickel sulfide intermediates must be refined to pure nickel and cobalt products: sulfide matte from nickel sulfide smelting and converting and sulfide precipitates from nickel laterite leaching. These data indicate that hydrometallurgy is the principal method for making high-purity nickel from these mattes and sulfide intermediates. The objective of this chapter is to describe the overall hydrometallurgical processes used to refine mattes and sulfide intermediates. Sulfide precipitates are refined to nickel and cobalt at following refineries; Fort Saskatchewan, Canada (Sherritt); Murrin Murrin, Australia (Minara Resources); and, Niihama, Japan (Sumitomo). Nickel is leached from laterite ores and precipitated as a sulfide. The objectives of the leaching stage are to extract the nickel and cobalt into solution, to oxidize the cobalt so that it is present as cobaltic hexamine. The filtrate is sent to ammonia stripping while the solids are sent to the cobalt circuit. The solution from the cobaltic hexammine precipitation is treated to reduce the concentration of ammonia in two steps: stripping with air and distillation. This solution passes through four reactors known as the ‘copper boil’ where copper is removed. Elemental sulfur and sulfur dioxide are added to the second reactor, which results in the precipitation of copper as copper sulfide. The processing of nickel–copper mattes is significantly different from the processing of mixed sulfide precipitates. This difference arises from the need to separate nickel from copper in the refinery. The Nikkelverk refinery at Kristiansand, Norway, uses chloride chemistry to refine nickel–copper matte. The Norilsk refinery at Harjavalta, Finland, uses sulfate chemistry to refine mattes that are relatively low in copper.

Technical note a thermodynamic approach for the sulphuric acid pressure leaching of nickeliferous laterites

This work introduces a thermodynamic approach for the sulphuric acid pressure leaching of nickeliferous laterites. The results obtained are used to interpret the leaching and hydrolytic reactions, including crust formation, that may occur inside the autoclave.

Mineralogical properties and fundamental mineral processing behaviour of the chromite contained in laterite leach residue

In order to utilize chromite in the laterite residue discharged from the large-scale Ni-leaching plant in the Philippines, its state of distribution, mineralogical properties, grindability, floatability, and magnetism were investigated. Chromite particles are concentrated over a 37μm size range, and poor in liberation. There are three different groups of chromite (A, B, and C), categorized according to their polished section, chemical composition, lattice constant, and specific gravity. Some of these differences affect the mineral processing behavior to a considerable degree. The grindability of chromite in leach residue is relatively high compared with that of chromites from other regions. Flotation is controlled by interfacial electrical attraction when ionic collectors are used, and the flotation peak appears in the vicinity of the PZC when NaOl is used. The order of floatability among the three groups of chromite particles is A>B>C, which indicates that a difference in surface structure by group affects floatability. Chromite from leach residue reveals paramagnetism at room temperature. The higher the solid solubility of magnetic ions (Fe 2+, Cr 3+, and Fe 3+), the greater the susceptibility is. In other words, the susceptibilities among the three groups are in the order of C>A>B.

Precipitate flotation of nickel from acidic laterite leach solutions

Precipitate flotation has been used to preferentially concentrate nickel over iron dissolved in solutions obtained by sulfuric acid leaching of a selectively reduced laterite. Leach solutions, containing about 3 g/L nickel, 0.5 g/L ferrous, 0.5 g/L magnesium, were neutralized using sodium carbonate to give mixed precipitates of a basic nickel carbonate and mainly non-carbonate iron compounds. At the optimum pH of 8.2, stage additions of sodium oleate and Dowfroth 250 (Dow Chemical Company product) totalling 0.07 g/g total nickel and 0.08 g/L solution, respectively, gave flotation recoveries of 93% nickel and 36% iron into concentrates analyzing 41% nickel and 2.8% iron. By neutralizing with sodium hydroxide instead of sodium carbonate, non-selective flotation of nickel and iron is obtained with increased consumption of collector and frother.