A study of atmospheric acid leaching of a South African nickel laterite

Nickel and cobalt acid leaching from a low-grade South African saprolitic laterite using sulphuric acid was studied. Ore characterisation was performed by XRD and XRF. Batch agitation leaching tests were conducted at atmospheric pressure investigating main parameters: particle size and percent solids at 25 °C and 90 °C. Ore characterisation showed that the ore is a saprolitic laterite with nickel present in lizardite. Leaching tests showed that nickel and cobalt could be leached from the ore at atmospheric pressure. Nickel was found to be more leachable from the coarser −106 + 75 μm fraction, with 98% Ni being extracted at 90 °C after 480 min. Cobalt was not favoured by variation in particle size and increased percent solids. Increasing ore percent solids improved nickel extraction at 25 °C however at 90 °C extraction decreased due to a diffusion layer build-up as a result of amorphous colloidal silica. The co-dissolution of magnesium and iron was elucidated. Nickel leaching data at increased temperature and percent solids fit the shrinking core model equation, k_dt = 1−2/3x − (1 − x)^2/3 showing that nickel leaching reaction was diffusion controlled under the set conditions.     

Innovative technology for processing saprolitic laterite ores by hydrochloric acid atmospheric pressure leaching

An innovative technology for processing saprolitic laterite ores from the Philippines by hydrochloric acid atmospheric leaching and spray hydrolysis is proposed. The factors that affect the hydrochloric acid atmospheric leaching of the laterite ores and spray hydrolysis of the atmospheric acid leach solution are investigated. Experimental results show that the leaching of Ni, Fe, and Mg is 98.9 wt%, 97.8 wt%, and 80.9 wt%, respectively, under optimal acid leaching conditions. The hydrolysis of Ni and Fe by the atmospheric acid leach solution approaches 100 wt% at the temperature range of 450–500 °C. Characterization results show that a serpentine mineral, nominally Mg₃Si₂O₅(OH)₄, is the major component and goethite, FeO(OH), is the minor one in the laterite ores. Treatment by hydrochloric acid atmospheric leaching breaks the mineral lattices of the laterite ores and makes amorphous silica the primary product in the atmospheric acid leach residue. The grade of Ni in the hydrolyzate increases to 4.55%. The hydrolyzate with high Ni content can be utilized for ferro-nickel production.     

Agglomeration and column leaching behaviour of goethitic and saprolitic nickel laterite ores

Processability of complex, low-grade nickel (Ni) laterite ores via heap leaching is very limited due to some intractable geotechnical and hydrological challenges such as poor heap porosity/permeability and structural stability. This work presents some investigations on laboratory batch drum agglomeration and continuous column leaching behaviour of saprolitic (SAP) and goethitic (G) Ni laterite ores as part of the quest for an effective ore pre-treatment process for enhanced heap leaching. As a focus, the effect of ore mineralogy/chemistry on the agglomeration and column leaching behaviour of −2 mm (crushed from −15 mm run-of-mine) G and SAP Ni laterite ores was examined. To produce ∼5–40 mm agglomerates in <15 min, the SAP ore required a higher H₂SO₄ (30 wt.%) binder dosage compared with the G ore, although both ores displayed substantially similar, coalescence-controlled agglomeration mechanism. The resulting G agglomerates were more robust than the SAP ones based upon their compressive strength and acidic solution soak test measurements. However, over 100 days of continuous column leaching, the structural stability of the SAP agglomerate bed was slightly greater than that of G agglomerates, reflecting a lesser slump of the former. The pregnant leach solution analysis revealed greater Ni/Co extraction rates from the SAP than the G agglomerates. Whilst the total mass of acid consumed per ton dry ore processed was greater for the SAP ore, the total kg acid per kg Ni extracted was markedly lower. Incongruent leaching of gangue minerals’ constituent elements (e.g., Fe, Mn, Mg, Al and Si) occurred and contributed significantly to the overall acid consumption. The findings show the relevance of agglomeration and column leaching tests for providing useful information for plant designing and optimization of Ni laterite heap leaching operations.     

Electrogenerative leaching of galena with ferric chloride

A generative principle was applied to FeCl₃ leaching of galena for the first time in this study. It has been confirmed that a certain quantity of electrical energy, as well as the leached products, could be obtained during the leaching process. The factors affecting the power output, such as electrode structure, temperature, agitation, solution compositions and concentration, and the connections between catholyte and anolyde compartments were investigated by a dual cell system. The experiments indicated that the interior resistance of the leaching system and the activation of the electrode are of key importance. Agitation speeds have little influence over a certain value. The effects of ferric chloride concentration in the leaching are similar to the traditional leaching. The activation energy was determined to be 14.96 kJ mol⁻¹ for FeCl₃ leaching of galena under the present conditions.     

Study of thermally conditioned and weak acid-treated serpentinites for mineralisation of carbon dioxide

This contribution assesses the dissolution behaviour of serpentinite specimens, featuring distinct stages of serpentinisation, by treating the specimens with aqueous solutions of formic acid. We have observed a marked improvement in the extraction of magnesium when the samples were finely ground and thermally conditioned before treatment with formic acid. An extraction of 42% for −25 μm particles activated at 700 °C (29% residual OH) could be obtained from the forsterite–lizardite bearing specimen whereas 66% of magnesium was leached out of the fully serpentinised antigorite mineral, which was crushed to a particle size of −53 μm and baked at 720 °C (36% residual OH). Combined results derived from FTIR and XRD indicate that heat activation between 500 and 720 °C results in a reorganisation of lizardite and antigorite to amorphised material, forsterite and silica. Unreactive enstatite forms from the amorphised material and silica once the heating temperature exceeds 800 °C. Semi-quantitative XRD analysis yields an estimate of the crystalline and non-crystalline (forsterite) fractions of the activated material, permitting approximation of relative rates of dissolution of amorphous and forsterite phases. Although FTIR provides important information on forsterite and silica formation, it cannot detect the amorphous material. Forsterite and amorphous phases alike dissolve in the weak acid but the formation of skins of the amorphous silica limits the overall magnesium yield on a laboratory time scale. The material that constitutes the skins originates from two sources: (i) silica formed in forsterisation of serpentine minerals undergoing heat treatment, and (ii) silica produced during extraction of Mg by a weak acid from amorphous and forsterite phases. Heat activation also leads to the formation of andradite and modified chlorite minerals that exhibit less solubility than forsterite and amorphous phases in weakly acidic medium.     

Process development for the recovery of europium and yttrium from computer monitor screens

This work describes the development of a process for the recovery of Eu and Y from cathode ray tubes (CRTs) of discarded computer monitors with the proposition of a flow sheet for the metals dissolution. Amongst other elements, europium and yttrium are presented in the CRTs in quantities – 0.73 w/w% of Eu and 13.4 w/w% of Y – that make their recovery worthwhile. The process developed is comprised of the sample acid digestion with concentrated sulphuric acid followed by water dynamic leaching at room temperature. In the CRTs, yttrium is present as oxysulphide (Y₂O₂S) and europium is an associated element – Y₂O₂S:Eu³⁺ (red phosphor compound). During the sulphuric acid digestion, oxysulphide is converted into a trivalent Eu and Y sulphate, in solid form, with the liberation of H₂S. In the second step, metals are leached from the solid produced in the acid digestion step by dynamic leaching with water. This study indicates that a proportion of 1250 g of acid per kg of the sample is enough to convert Eu and Y oxysulphide into sulphate. After 15 min of acid digestion and 1.0 h of water leaching, a pregnant sulphuric liquor containing 17 g L⁻¹ Y and 0.71 g L⁻¹ Eu was obtained indicating yield recovery of Eu and Y of 96% and 98%, respectively. Both steps (acid digestion and water leaching) may be performed at room temperature.     

Dissolution kinetics of primary chalcopyrite ore in hypochlorite solution

Dissolution kinetics of primary chalcopyrite ore from Artvin-Murgul, Turkey has been studied in hypochlorite solution. The dissolution kinetics was found to be controlled by diffusion through the product layer as the rate-determining step. The activation energy (E) was calculated as 19.88 kJ mol⁻¹.     

Enhancing Mg extraction from lizardite-rich serpentine for CO2 mineral sequestration

Carbon capture and storage by mineralisation (CCSM) is a promising technology that sequesters CO₂ from flue gases into stable mineral carbonates. Although the development of indirect pH swing processes (dissolution at acid pH and carbonation at basic pH) able to recycle the chemicals used are promising, there are still limitations in reaction rate of mineral dissolution being slow in view of a large deployment of the technology. The extraction of Mg from lizardite using magnesium bisulphate has been studied as a function of temperature, reagent concentration, solid to liquid ratio, thermal and mechanical pre-activation. Although the overall highest Mg extraction (95%) was obtained after 3 h, the reduction of the dissolution time to 1 h can consistently reduce the volumes to be treated per unit time leading to low capital costs in a hypothetical mineralisation plant. About 80% of Mg was extracted from lizardite in 1 h at 140 °C, 2.8 M NH₄HSO₄, particles <250 μm and a solid to liquid ratio of 100 g/l. At 140 °C, serpentine undergoes extensive structural modifications as indicated by XRD and FTIR analyses, producing amorphous silica and accelerating the kinetics of the reaction. Particles with diameter less than 250 μm were obtained by grinding the lizardite at 925 rpm for 10 min consuming 33 kW h/t_rock.     

Bioleaching of djurleite using Acidithiobacillus ferrooxidans

The bioleaching of djurleite using Acidithiobacillus ferrooxidans (LD-1) was investigated in this paper. Experiments were carried out in shake flasks at pH 2.0, 160 r/min and 30 °C. The leaching residues were analyzed using X-ray diffraction (XRD), Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The total copper extraction of djurleite under optimal condition reached 95.12%. The XRD analysis indicated the residues mainly consisted of ammoniojarosites and S₈. It was observed by the SEM image that the djurleite was heavily etched. The XPS results confirmed the intermediate product formed during djurleite leaching was CuS. The result indicates the reaction pathway is: Cu₃₁S₁₆ → CuS → tCu²⁺ and S⁰.     

Cobalt recovery from mixed Co–Mn hydroxide precipitates by ammonia–ammonium carbonate leaching

The research work presented in this paper focused on the recovery of cobalt from mixed Co–Mn hydroxide precipitates (obtained from sulphate leach liquors of nickel oxide ore), using ammonia–ammonium carbonate leaching. The characterization of the initial mixed hydroxide precipitates, as well as the corresponding leached residue was carried out by X-ray Diffraction, TG–DTA and Scanning Electron Microscopy.Cobalt and manganese precipitation was based on the statistical design and analysis of experiments, in order to determine the main effects and interactions of the precipitation factors, which were the equilibrium pH and the temperature. Co and Mn were precipitated as hydroxides at pH = 10.5 and T = 25 °C, using 5 M NaOH as a neutralizing agent, by 99.9% and 99.5%, respectively. The main mineralogical phases were, Mn₃O₄ (Hausmannite), γ-Mn₃O₄ and CoMn₂O₄, while Co(OH)₂ and Mn(OH)₂ (Pyrochroite) were also present as minor constituents.Cobalt and manganese separation was based on selective cobalt recovery by ammonia–ammonium carbonate leaching of the produced mixed hydroxide precipitate. The factors studied were the ammonia–ammonium carbonate concentration and the solid to liquid ratio. The cobalt recovery efficiency reached 93%. Mn₃O₄ (Hausmannite) was the main mineralogical phase of the leached residue, while MnCO₃ (Rhodochrosite) and Mn₂O₃ were also present. Small quantities of cobalt were also observed in the residue as CoMn₂O₄.     

Research on the recycling of valuable metals in spent Al2O3-based catalyst

A new technology was developed to recover multiple valuable elements in the spent Al₂O₃-based catalyst by X-ray phase analysis and exploratory experiments. The experiment results showed: In the condition of roasting temperature of 750 °C and roasting time of 30 min, mol ratio of Na₂O: Al₂O₃ 1.2, the leaching rate of alumina, vanadium and molybdenum in the spent catalyst is 97.2%, 95.8% and 98.9%, respectively. Vanadium and molybdenum in sodium aluminate solution can be recovered by barium hydroxide and barium aluminate, the precipitation rate of vanadium and molybdenum is 94.8% and 92.6%. Al(OH)₃ is prepared from sodium aluminate solution with carbonation decomposition process, and the purity of Al₂O₃ is 99.9% after calcinations, the recovery of alumina can reach 90.6% in the whole process. The Ni–Co concentrate was leached by sulfuric acid, a nickel recovery of 98.2% and over 98.5% cobalt recovery was obtained respectively under the experimental condition of 30% (w/w) H₂SO₄, 80 °C, reaction time 4 h, liquid:solid ratio (8:1) by weight, stirring rate of 800 rpm.     

Atmospheric acid leaching of siliceous goethitic Ni laterite ore: Effect of solid loading and temperature

In this study, atmospheric acid leaching behaviour of siliceous goethitic nickel (Ni) laterite ore is investigated. Specifically, the effect of −200 μm feed solid loading (30 vs. 45 wt.%) and temperature (70 vs. 90 °C) on leach kinetics, acid consumption capacity and Ni and cobalt (Co) extraction was studied under isothermal, batch (4 h) leaching conditions at pH 1. Incongruent leaching was observed for constituent elements reflecting slow but steady release of value (Ni and Co) and some of gangue metals such as Fe, Mg and Al accompanied by faster and sharp release of Na and Si. Higher temperature and lower pulp solid loading, both led to a 40–50% increase in overall Ni and/or Co extraction and higher acid consumption. At 70 °C and 45 wt.% solid loading, Ni/Co extraction after 4 h was the lowest (∼14/16%) whilst the highest extraction (∼67/56%) was observed at 90 °C and 30 wt.% solid loading. Temperature appeared to have dramatic influence on Ni/Co and other impurity metals’ extractions revealing the chemical reaction controlled nature of the leaching. Higher solid loading and longer leaching time also both slowed down the leach kinetics. A two-stage chemical reactions-controlled leaching mechanism involving a faster initial leaching kinetics followed by a slower leaching at lower rate constants and higher activation energies was established for release of Ni, Co, Fe and Mg. The mechanism reflects the fast leaching of reactive host mineral phases (e.g., clays and Mg–silicates) during first 30 min followed by slow leaching of more refractory mineral phases (e.g., goethite and quartz) during the rest of leaching period. The findings provide a greater understanding for enhanced atmospheric acid leaching process of siliceous goethitic laterite ores.     

Synchrotron X-ray photoelectron spectroscopic study of the chalcopyrite leached by moderate thermophiles and mesophiles

SXPS (Synchrotron X-ray Photoelectron Spectroscopy) and NEXAFS (Near Edge X-ray Absorption Fine Structures) have been applied to study the surface chemical species of chalcopyrite leached by a moderate thermophilic consortia, Leptospirillum ferrooxidans and a mesophilic mixed culture of L. ferrooxidans, Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans. A sulfur-rich layer dominated by S_n²⁻ developed with time, which was found to control the rate of bioleaching. Fe L_2,3-edge NEXAFS and Fe 2p spectra indicate the formation of jarosite during bioleaching. Thermophiles significantly enhanced the leaching efficiency, in which 1.34 g/L copper was dissolved in 25 days, while less than 0.3 g/L copper was released in 30 °C bioleaching. This was mostly caused by the increased abiotic reaction rate. The solution copper concentration in presence of L. ferrooxidans was higher than that with mesophilic mixed culture, which suggests the synergistic effect of mixed microorganisms did not play a comparably important role as temperature under the conditions used in this study. Explicit evidence of elemental sulfur was only found in samples leached by L. ferrooxidans by Raman spectroscopy. However, the formation of elemental sulfur does not significantly hinder the leach rate.     

Review of rate constants for thiosulphate leaching of gold from ores,concentrates and flat surfaces: Effect of host minerals and pH

A review of literature data for different types of sulphide concentrates and gold ores has been carried out to examine the impact of host minerals and pH upon gold leaching. Analysis of initial rate data over the first 30–60 min of gold leaching from sulphide concentrates or silicate ores over a range of ammonia, thiosulphate, and copper(II) concentrations, pH (9–10.5) and temperatures up to 70 °C shows the applicability of a shrinking sphere kinetic model with an apparent rate constant of the order k_ss = 10⁻⁶–10⁻³ s⁻¹. The dependence of apparent rate constant on pH and initial concentrations of copper(II) and thiosulphate is used to determine a rate constant k_Au(ρr)⁻¹ of the order 1.0 × 10⁻⁴–7.4 × 10⁻⁴ s⁻¹ for the leaching of gold over the temperature range 25–50 °C (ρ = molar density of gold, r = particle radius). These values are in reasonable agreement with rate constants based on electrochemical and chemical dissolution of flat gold surfaces: k_Au = 1.7 × 10⁻⁴–4.2 × 10⁻⁴ mol m⁻² s⁻¹ over the temperature range 25–30 °C. The discrepancies reflect differences in surface roughness, particle size and the effect of host minerals.     

Self-heating activation energy and specific heat capacity of sulphide mixtures at low temperature

Energy of activation (E_a) and specific heat capacity (C_p) for mixtures of sulphide minerals that on their own do not self-heat (SH), sphalerite/pyrite, pyrite/galena, chalcopyrite/galena and sphalerite/galena, were determined using a self-heating apparatus at temperatures below 100 °C in the presence of moisture. The mixtures all gave E_a ranging from 22.0 to 27.8 kJ mol⁻¹, similar to the range reported for Ni- and Cu-concentrates. The E_a is close to that for partial oxidation of H₂S which adds to the contention that the partial oxidation of H₂S contributes to SH of sulphides at low temperature. The C_p values ranged from 0.152 to 1.071 JK⁻¹ g⁻¹ as temperature rose from 50 °C to 80 °C, similar to the reported findings on Ni- and Cu-concentrates. The role of galvanic interaction in promoting SH is tested by examining correlations with the rest potential difference of the sulphides in the mixture.     

Leaching behaviour of natural and heat-treated brannerite-containing uranium ores in sulphate solutions with iron(III)

Uranium leaching tests were conducted on two naturally occurring, highly metamict brannerite ores from the Crockers Well and Roxby Downs deposits, South Australia. The ores were leached over a range of temperatures and Fe^(III) and H₂SO₄ concentrations. As well, samples of the ores were calcined at 1200 °C in air to investigate the effect of thermally induced recrystallisation on uranium dissolution. For the unheated samples, a maximum of ∼80% U dissolution was obtained using an Fe^(III) concentration of 12 g/L, an acid concentration of 150 g/L H₂SO₄ and a temperature of 95 °C. The heat treated samples performed poorly under identical conditions, with maximum uranium dissolution of <10% recorded. High uranium dissolution from natural brannerite can be achieved providing; (i) acid strength, oxidant strength and temperatures are maintained at elevated levels (compared to those traditionally used for uraninite leaching), and, (ii) the brannerite has not undergone any significant recrystallisation (e.g. through metamorphism).     

Acid leaching and rheological behaviour of a siliceous goethitic nickel laterite ore: Influence of particle size and temperature

This study investigates the isothermal, batch, H₂SO₄ acid leaching behaviour of siliceous goethitic (SG) nickel (Ni) laterite ore and its links to pulp rheology. Specifically, the effect of feed ore particle size (−0.2 vs −2.0 mm), leaching temperature (70 vs 95 °C) and pulp rheology on Ni and pay metal, cobalt (Co) extraction kinetics and yield was studied for 4 h on 40 wt.% solid dispersions at pH 1. The leaching behaviour was distinctly incongruent, reflecting the disproportionate proliferation of major gangue mineral’s constituent elements (e.g., Fe, Al, Mg, Na, Si) alongside Ni and Co in the pregnant leach solution. At 70 °C, Ni/Co extraction rates were notably lower (<20%) in contrast with 95 °C where a significant increase in Ni/Co extraction to 78/77% and 74/77%, respectively, for the −0.2 and −2.0 mm feeds occurred. The slurries displayed a non-Newtonian, shear thinning Bingham plastic rheological behaviour of which the viscosity and shear yield stress increased markedly in the course of 4 h leaching. The pulp viscosity and shear yield stress were greater at lower temperature than at higher temperature and they were also greater in slurries with finer than coarser feed particles. The dynamic pulp rheology, however, had no marked effect on the overall Ni/Co extraction rates. Whilst the feed ore particle size had no remarkable impact on overall Ni/Co extraction, it led to noticeably higher acid consumption and enhanced slurry rheology in the finer sized ore. The mechanism of leaching the SG ore followed a two-stage, first order chemical reaction-controlled shrinking core model, the kinetics of which gave higher rate constants and lower activation energies for the release of Ni, Co, Fe and Mg in the first stage. A faster leaching process involving more reactive minerals during the first 30 min is envisaged to be followed by leaching of the more refractory minerals.     

Influence of silver ions on bioleaching of cobalt from spent lithium batteries

The influence of silver ions (Ag⁺) on the bioleaching of cobalt from spent lithium batteries using Acidithiobacillus ferrooxidans (A. ferrooxidans) bacteria was investigated. The best effect was observed at Ag⁺ concentration of 0.02 g/L, and the amount of leached cobalt was 98.4% in a period of 7 days, whereas in the absence of Ag⁺, the amount of leached cobalt was only 43.1%. Based on the results of SEM, XRD and EDX investigation, it is deduced that Ag⁺ first reacted with cobalt to form AgCoO₂ as an intermediate. A mechanism that is based on catalytic interactions is proposed to explain the enhanced leaching of Co using A. ferrooxidans.     

Calcination of low-grade laterite for concentration of Ni by magnetic separation

With the continuous depletion of high-grade nickel ores such as millerite and niccolite, nickeliferous laterites have become the major source for the production of nickel metal. However, only 42% of the world’s production of nickel comes from laterites, since the concentration of Ni is relatively low (ca. 2 wt.%). In addition, other metals, such as magnesium, iron and silicon can be found in laterite, which make the concentration of nickel even more difficult.In this study, a low-grade nickeliferous laterite ore was first calcinated and then processed by using a wet magnetic separator in order to recover nickel. Since, the ore contains both Ni and Fe, the calcination of laterite is effective in altering the crystalline structure of Fe species and therefore its magnetic properties, which in turn enable the selective concentration of nickel by magnetic separation that is an easy and environmentally-friendly technique. The experimental results have indicated the importance of carefully controlling: (1) the calcination temperature; (2) the pulp density and (3) applied magnetic field strength. The main finding of this work was that magnetic separation is effective in recovering 48% of nickel from laterite, increasing the Ni grade in the recovered product from 1.5% to 2.9%, when prior to the separation the ore was calcinated at 500 °C for 1 h.