Thermodynamic analysis of the carbothermic reduction roasting of a nickeliferous limonitic laterite ore

As the sulfide ore deposits become less economically viable as a source of nickel, increasing attention is being paid to the nickeliferous laterite ores. However, in contrast to the sulfide ores, these oxide ores cannot be as easily concentrated by current technologies. Consequently, considerable research effort is being directed at developing new techniques for beneficiating the nickeliferous laterites. The pyrometallurgical production of a high-grade ferronickel alloy using a low cost carbonaceous reductant at relatively low temperatures is particularly attractive. In the current research, a thermodynamic model has been developed to aid in the understanding of the carbothermic reduction roasting process as a potential upgrading method for the nickeliferous limonitic laterite ores. The effects of process parameters such as temperature and reductant to ore ratio on the grade of the ferronickel alloy produced and the nickel recovery in the alloy have been studied. The thermodynamic predictions are shown to be in general agreement with the experimental results currently available in the literature.     

Activation pretreatment of limonitic laterite ores by alkali-roasting method using sodium carbonate

Activation pretreatment of Cr-containing limonitic laterite ores by Na₂CO₃ roasting to remove Cr and Al, as well as its effect on Ni and Co extraction in the subsequent pressure acid leaching process were investigated. X-ray diffraction (XRD), thermogravimetric (TG), and scanning electron microscopy/X-ray energy dispersive spectroscopy (SEM/XEDS) techniques were used to characterize the laterite ores and the water leaching residues of alkali roasting and found that goethite is the major Ni-bearing mineral and chromite the minor one. Alkali-roasting pretreatment breaks the mineral lattices of the laterite, exposing their Ni and Co, which leads to higher extraction of these two metals under milder operation conditions in the subsequent pressure acid leaching process. Experimental results showed that the leaching of Cr and Al were up to 99 wt% and 80 wt%, respectively, under optimal alkali roasting and water leaching conditions. Compared with the direct pressure acid leaching of the raw laterite ores, leaching of Ni and Co increased from 79.96 wt% to 97.52 wt% and 70.02 wt% to 95.33 wt%, respectively, after alkali-roasting activation pretreatment was performed. Meanwhile, the grade of acid leaching iron residues increased from 55.31 wt% to 62.92 wt%, and these residues with low Cr content could be more suitable as the raw materials for iron-making.     

Microwave carbothermic reduction roasting of a low grade nickeliferous silicate laterite ore

The known resources of nickel sulphide ores are quickly diminishing and in order to satisfy future nickel demands, nickel laterite deposits are being investigated as an alternative. Currently, expensive leaching and smelting processes are used to process the nickel laterite ores. The objective of the present research was to produce a high grade nickel concentrate via microwave carbothermic reduction roasting followed by magnetic separation. A thermodynamic model was developed for the roasting process in order to determine the optimum experimental conditions. The experimental variables investigated were: microwave energy and argon shrouding for the reduction tests and the magnetic field strength for the concentration stage. The behaviours of nickel and cobalt were studied in the reduction and magnetic separation processes. By optimizing the reducing and magnetic separation conditions, a high grade concentrate containing 9.2% nickel with a nickel recovery of 88.8% was achieved.     

褐铁型红土镍矿低温熟化—常压水浸镍钴过程研究

在较低温度下采用硫酸低温熟化—水浸工艺处理褐铁型红土镍矿,对熟化—水浸渣进行矿物学表征,并对熟化过程动力学进行了研究。结果表明,红土镍矿经过熟化—水浸处理,矿物的矿相发生转变:铁由针铁矿相转变为赤铁矿等铁的氧化物,大部分镍钴矿物转变为硫酸镍和硫酸钴,小部分转变为难溶的铁酸镍和铁酸钴造成镍和钴的损失。硫酸熟化过程动力学研究表明,镍钴硫酸化动力学过程符合典型的多相液固区域反应动力学模型,镍钴硫酸熟化反应表观活化能分别为12.4 k J/mol和3.2 k J/mol,这一过程受内扩散控制。     

Selective pressure leaching of Fe (II)-rich limonitic laterite ores from Indonesia using nitric acid

The selective extraction of nickel and cobalt over iron from an Indonesian limonitic laterite was investigated using nitric acid pressure leaching (NAPL). The mineralogical analysis showed that the major minerals were goethite and magnetite, and the content of the divalent iron was as high as 7.06%. Nickel and cobalt were mainly distributed in these two minerals; however, the distribution was non-uniform. A series experiments were conducted to examine the basic parameters and propose the optimal conditions for the extraction. When the ore was treated via HPAL under the optimal condition, the extracted nickel and cobalt were less than 75%, and the iron concentration in the leach liquor was over 12.5 g/L. By contrast, over 85% of nickel and cobalt were extracted and about 1.8 g/L iron was achieved using NAPL. The loss of nickel and cobalt can be mainly attributed to the undissolved magnetite and manganese minerals. The leaching process of NAPL is a dissolution–oxidation–precipitation mechanism, and in this process nitric acid acts as both a lixiviant and an oxidant. The formation of hematite results in a low iron concentration in the leach liquor without oxygen injected. Meanwhile, the oxidation and the precipitation of dissolved divalent iron results in a calculated savings in acid consumption of about 120 kg nitric acid per ton of ore can be obtained, which is equal to over 93 kg of sulfuric acid per ton of ore. Moreover, lower residual acid (20 g/L nitric acid) is a significant advantage of NAPL. The iron residues had a high iron content (>56 wt%) with no sulfur, making it suitable as raw materials for ironmaking.     

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.     

Microwave heating of ultramafic nickel ores and mineralogical effects

Although mineral carbon sequestration (MCS) has several advantages over other carbon storage techniques, an economic process for MCS has not yet been developed. To reduce costs, it is suggested that MCS be combined with other processes and that waste products be used as feedstock. A process is proposed where ultramafic nickel ores are treated with microwave radiation to convert serpentine, the primary gangue mineral, to olivine. It is hypothesized the conversion of serpentine to olivine in ultramafic nickel ores may improve the mineral processing of these ores, and increase the carbon storage capacity of the flotation tailings. The microwave heating characteristics of ultramafic nickel ores, and the mineralogical changes that occur as a result of microwave treatment, are described in this paper. Ultramafic nickel ores were found to heat well in response to microwave radiation and the temperatures achieved were sufficient to convert serpentine to olivine. Microwave treatment was also found to convert pentlandite, the valuable nickel mineral, to other Fe–Ni–S minerals.     

Leaching of limonitic laterite ore by acidic thiosulfate solution

Cobalt is usually recovered as a by-product of copper and nickel processing, and only a small amount of cobalt is derived from laterites although a vast majority of cobalt resources in them. The exploitation of limonitic laterite containing high content of cobalt is becoming increasingly important. The mineralogy of a limonitic laterite ore was characterized by environmental scanning electron microscope (ESEM) and X-ray diffraction (XRD) in this paper. The results show that nickel occurs in goethite mainly, while cobalt is predominantly associated with manganiferous minerals. Thiosulfate is found to be able to selectively leach cobalt from limonitic laterite in the presence of sulfuric acid, and 91% Co, 22% Ni, 10% Fe are leached from an ore containing 0.13% Co, 1.03% Ni within the first 5 min at 90 °C under the conditions of 10 g/L sodium thiosulfate, 8% (w/w) sulfuric acid and 10:1 L/S ratio. The leaching kinetics of Mn and Co by acidic sodium thiosulfate solution can be characterized by the Avrami equation. In acidic solution, thiosulfate readily decomposes into sulfur and sulfur dioxide as intermediary reagents to reduce pyrolusite (MnO₂) and goethite (FeOOH); therefore, nickel and cobalt associated with goethite and pyrolusite respectively are extracted due to reduction dissolution. Furthermore, cobalt is selectively leached over iron and nickel because pyrolusite is preferentially reduced by acidic thiosulfate rather than goethite. The novel process may give an alternative method to selectively recover cobalt as the primary product from limonitic laterites at atmospheric pressure.     

Selective sulphidation and flotation of nickel from a nickeliferous laterite ore

The sulphidation of a nickeliferous lateritic ore was studied at temperatures between 450 and 1100 °C and for sulphur additions of 25–1000 kg of sulphur per tonne of ore. The experiments demonstrated that the nickel could be selectively sulphidized to form a nickel–iron sulphide. It was found that both the grade and the sulphidation degree largely depended upon the temperature and the sulphur additions, with temperatures above 550 °C exhibiting the highest nickel sulphidation degrees and grades. A DTA/TGA with mass spectrometer was used to further elucidate the nature of the phase transformations that occurred upon heating of the ore in the presence of sulphur.It was found that at low temperatures, the Fe–Ni–S phase was submicron in nature and heating to temperatures between 1050 and 1100 °C allowed for the growth of the particles, due to the increased sulphide mobility associated with the formation of a liquid sulphide matte phase, containing dissolved oxygen. Flotation studies conducted on 60 g samples showed that the sulphides responded to flotation with maximum grades of up to 6–7 wt.% nickel being achieved. Recoveries were approximately 50% on a sulphide basis and it was determined that the low nickel grades were due to the entrainment of magnetite fines.     

Microwave heating of gold ores for enhanced grindability and cyanide amenability

In leaching processes, the mass transport of lixiviants from the bulk of the solution to the site of the mineral of interest is usually the slowest step. Diffusion becomes slower when the particles to be leached are occluded in host minerals. Access to the mineral of interest is enhanced by micro-crack formation within the host minerals as it enhances percolation and migration of lixiviants. Micro-crack formation also augments grinding and allows size reduction and liberation at a lower stress level. In this research, microwave pretreatment was used to augment the grinding of a free-milling gold ore containing quartz, silicates and iron oxides. Under microwave irradiation, selective heating of the different mineral components resulted in thermal stress cracking. Microwave processing enhanced the grindability of the ore, and crushing strength was reduced by 31.2%. The presence of micro-cracks improved leaching rate, and over 95% extraction was achieved within 12 h as against 22 h for the non-microwaved sample. Such a strategy can be used to maximize recovery and man-hours on processing plants.     

Kinetics of selective chlorination of pre-reduced limonitic nickel laterite using hydrogen chloride

The selective chlorination process of pre-reduced limonitic nickel laterite using hydrogen chloride was investigated in this work. In HCl-O₂-H₂O-N₂ atmosphere, the effects of variations in temperature, the partial pressures of hydrogen chloride, oxygen and water vapor, and total gas flow rate were all studied. It was found that the chlorination of nickel and cobalt could be described by two stages; and it was controlled by gas diffusion through product layer. Four apparent activation energies, including 20.43 kJ/mol and 12.89 kJ/mol for the first and second stage of nickel chlorination, 14.95 kJ/mol and 13.02 kJ/mol for the first and second stage of cobalt chlorination respectively, were obtained in the temperature range of 420-460 °C. And the apparent reaction order of hydrogen chloride was also obtained. During the selective chlorination, the oxidation of pre-reduced ore could take place with the selected conditions; chlorinated iron was mostly rejected as hematite and thus nickel and cobalt were selectively chlorinated by hydrogen chloride.     

褐铁矿型红土镍矿硫酸高压浸出的研究

研究了褐铁矿型红土镍矿硫酸高压浸出过程中浸出温度和时间、硫酸用量对镍、钴、锰、铁、铝、镁、硅、铬浸出的影响。在浸出温度为250℃,浸出压力3.9 MPa,浸出时间为40 min,硫酸用量240kg/t干矿的优化条件下,镍,钴,锰的浸出率分别为96.8%,96.6%,98.7%,铁、铝、镁、硅的浸出率分别降低到2.6%,16.9%,61.8%,1.9%,铬的浸出率降低到0.7%,浸出液中的铬以Cr~(3+)的形式存在,浸出液中没有Cr~(6+)。静置分离试验表明,在此浸出条件下的浸出矿浆固液分离效率较高。扫描电镜分别对30、40、60 min浸出渣相微观形貌的分析表明,浸出30 min时针铁矿中Fe~(3+)绝大部分溶出并经过水解重新沉淀,延长浸出时间只是使铁相的等粒-圆粒状颗粒分布趋于均化。浸出渣相的主要成分为Fe_2O_3,(H_3O)Al_3(OH)_6(SO_4)_2,KAl_3(SO_4)_2(OH)_6及Si O_2。     

Use of oxidation roasting to control NiO reduction in Ni-bearing limonitic laterite

Use of limonitic laterite as an iron source in conventional ironmaking is restricted due to its gangue composition and small particle size. Even direct reduction cannot effectively produce direct reduced iron (DRI) because NiO would be reduced together with iron oxide to form Fe–Ni. A small amount of Ni (about 2 wt.%) in DRI degrades the physical properties of final steel products. The current study investigated how oxidation roasting of limonitic laterite ores affected NiO reduction, with the goal of producing Ni-free DRI and Ni-bearing slag. Ni-bearing slag can be a good secondary Ni resource. Oxidation roasting made NiO inert under H₂ reduction at 900 °C by forming Ni-olivine. Optimum roasting temperature was proposed by examining phase transformation of limonitic laterite ores during heating and by FactSage calculation of the equilibrium Ni fraction in Ni-bearing phases. Furthermore, the effect of Mg–silicate forming additives on the control of NiO reducibility was clarified to maximize the suppression of NiO reduction. Among various additives such as MgSiO₃, Mg₂SiO₄ and Fe–Ni smelting slag, Ni-free olivine-typed flux was found to be the most effective form of Ni-olivine because Ni–Mg ion exchange between Ni-bearing phase and Ni-free olivine occurs more readily than other Ni-olivine formation schemes. Finally, the mechanism of Ni-olivine formation during roasting was studied using a diffusion couple test. Calculated diffusivity values of Ni in Mg₂SiO₄ indicated that the two major routes of Ni-olivine formation while roasting limonitic laterite ore are (1) Ni partitioning within Mg–Ni silicate before crystallization and (2) Ni diffusion from spinel to Ni free olivine after crystallization.     

Study of phase transformation of laterite ores at high temperature

The phase transformation of laterite minerals reduced at temperatures up to 800 °C and the phase reversibility with cooling were investigated in this study. The phases were studied by in situ synchrotron radiation based X-ray powder diffraction with temperature resolved studies. The degrees of reduction were confirmed by a leaching of reduced ore with ammoniacal solution. Key features, which were observed in this study, include the role of dehydroxylation in releasing nickel and cobalt from the main gangue minerals in which these metals are incorporated. The transformation of hydrated gangue, goethite in limonite to magnetite and the serpentine minerals to the olivine phases in the saprolite and control of these phase transformations, in particular the recrystallisation of magnesium silicate to the forsterite phase were also investigated.     

Bioleaching nickel laterite ores using multi-metal tolerant Aspergillus foetidus organism

Immediate environmental conditions expose the fungi organisms to a multitude of stresses in an in situ bio-leaching process. The most significant abiotic stress is from heavy metals which are dissolved by the metabolic products of the organism. Heavy metal stress can stimulate changes in the fungi metabolism which could inactivate its cellular functions critical in maintaining bio-acid production and leaching of minerals. Gradual acclimatization of the organism to increasing concentration of single metals forms the main method of adapting strains for this process. However in a bioleaching process, several metals are dissolved which exposes the organism to various levels of cation toxicity. In this study Aspergillus foetidus was trained using multi metal (Al, Co, Cr, Cu, Fe, Mg, Mn, Ni and Zn) environment to mimic a leaching system. The effect of the metal stress on the fungi metabolism was examined. Bioleaching of weathered saprolite ore using the heavy metal tolerant organism was assessed.     

Atmospheric leaching characteristics of nickel and iron in limonitic laterite with sulfuric acid in the presence of sodium sulfite

Atmospheric leaching of nickel from limonitic laterite ores is regarded as a promising approach for nickel production, despite its low nickel recovery and slower leaching rate than high pressure acid leaching. Sulfur dioxide can enhance the sulfuric acid leaching of laterite, but its behavior for enhancing atmospheric sulfuric acid leaching was uncertain due to SO₂ losses and emission. In this study, sodium sulfite was used as a substitute for SO₂ gas in the leaching and the sulfuric acid leaching characteristics of Ni and Fe from a limonitic laterite in the presence of sodium sulfite were investigated. A linear correlation exists between the extraction of Ni and Fe, indicating the difficulty in selective leaching of Ni over Fe. Most nickel is isomorphically substituted within the goethite and it is difficult to dissolve in a high oxidation–reduction potential solution environment, resulting in a low nickel recovery. SO₂(aq) generated from the reaction of sodium sulfite in sulfuric acid solution, lowers the potential for the reducing reaction of FeOOH to give Fe²⁺, accelerating the iron extraction and nickel liberation from goethite.     

The effects of mineralogy on the biological leaching of nickel laterite ores

The amenability of limonite ore, the iron rich nickel laterite phase, was observed to be less than saprolite, the silicate rich phase, when subjected to biological leaching with heterotrophic microorganisms. To understand this difference, controlled leaching of the various nickel laterite minerals was conducted using citric acid to mimic the chemical action of the organisms and subsequent characterisation of the leached residues. The raw ore and the leached mineral residues were examined using high-resolution synchrotron based X-ray diffraction and optical microscopy. The possible contribution of electrosorption and solid porosity was also considered. The results suggest that although electrosorption contributes to nickel and cobalt losses, it is the structural changes induced by the acid in the minerals which effects the recovery of the metals. Citric acid was found to dehydroxylate saprolite minerals resulting in an unstable nickel oxide and thus making it susceptible to acid attack. This behaviour was not observed in the limonite ore, which was found to be poorly leached by citric acid.     

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 100 years 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.     

硅镁型红土镍矿脱羟基相变过程动力学研究

镍具有优良的物理化学性质,应用领域广泛,镍资源主要存在于硫化镍矿和红土镍矿中,目前红土镍矿正逐渐成为主要的镍提取原料,由于红土镍矿含水量大,在冶炼过程中通常要对矿石进行高温预处理,目的是为了去除矿石晶格里的羟基水以及预先还原部分金属,但这部分羟基水的脱除需要消耗大量能量,并且产生物相变化,对预还原及浸出都会产生影响,因此探究红土镍矿脱羟基过程的物相变化及动力学条件对后续加工作业具有重要的研究意义。本文利用热力学分析软件模拟计算了红土镍矿加热过程中可能发生的化学反应,并采用热重-差热分析及 XRD 分析研究了硅镁型红土镍矿加热过程的物相变化,从室温到1400°C范围内进行了5种升温速率的非等温动力学实验,在256.8°C、582.8°C和823.0°C 处有三个特征峰,分别对应褐铁矿脱羟基、蛇纹石脱羟基以及硅酸盐矿物的相变。采用非等温动力学模型求解来确定反应的表观活化能及指前因子,使用Flynn-Wall-Ozawa 法和Kissinger-Akahira-Sunose 法计算了蛇纹石脱羟基过程的活化能,并利用Satava-Sestak法对常用的30种机理函数进行拟合,保留符合条件的机理函数,确定 Avrami-Erofeev 方程的A1/3函数符合脱羟基过程,其积分形式为G(α)= [-ln(1-α)]3,求得蛇纹石脱羟基反应的活化能为258.71 kJ·mol-1, lnA为28.94 S-1,平均线性相关系数为 0.9951,蛇纹石脱羟基过程符合随机成核及随后生长模型。