youssra
Get Complete Project Material File(s) Now! »
Chapter 2 Literature survey
Introduction to nickel recovery and production
Natural sources and characteristics of nickel
Nickel comprises approximately 0.008 % of the Earth crust’s total mass and is mainly found in either sulfide or laterite ore, commonly in association with cobalt and manganese (Moskalyk and Alfantazi, 2002a; Moskalyk and Alfantazi, 2002b). Nickel has many useful metallurgical properties, including high melting point, ferromagnetic characteristics, catalytic capabilities, relative ease of electroplating, alloying properties, corrosion resistance and ductility (Kittelty, 2002; Moskalyk and Alfantazi, 2002b). These characteristics allow nickel to be employed in various industries, such as telecommunications, infrastructure, chemical production, environmental protection, energy supply, water treatment, food preparation, consumer electronics and transportation (Fornari and Abbruzzese, 1999; Moskalyk and Alfantazi, 2002a; Di Bari, 2010).
Approximately 12 % (Di Bari, 2010) of produced nickel is consumed by electroplating and electrodeposition processes, which are used for decorative, engineering and electroforming applications. Modern decorative applications are improved by the addition of organic additives to produce bright and level nickel deposits. In engineering processes, nickel deposits are used for improved corrosion or wear resistance, to enhance magnetic properties, as a preparative layer for other applications and as diffusion barriers in electronic applications. During electroforming, nickel is loosely deposited onto a mould that provides for later removal of the then formed electrodeposit. This process is used in the textile, aerospace, communication, electronic, automotive, photocopying and entertainment industries (Di Bari, 2010).
Nickel from sulfide-bearing ores
Sulfide-containing ores are the source of approximately 30 % of global base and noble metals. The main nonferrous metals recovered from these ores include nickel, copper and cobalt (Agatzini-Leonardou et al., 2009). The mining of sulfide ores normally requires underground operations. Some open pit operations are in use, but it is generally found that higher nickel and other metal contents are present at greater depths (Park et al., 2006; Crundwell et al., 2011). Approximately 55 % of mined nickel (from nickel sulfide ore) is contained in pentlandite ((Fe,Ni)9S8). Iron- and copper-containing pyrrhotite (Fe1-xS (where x ranges from 0.0 to 0.2)) and chalcopyrite (CuFeS2) are also contained in pentlandite ((Fe,Ni)9S8). Millerite (NiS) and heazlewoodite (Ni3S2) have the highest nickel contents but are only present in small quantities (Moskalyk and Alfantazi, 2002a).
Nickel from laterite sources
Mining of nickel from laterites is becoming increasingly important as nickel from sulfide sources is progressively depleted. Laterites contain approximately 70 % of the world’s nickel resources and nickel can nowadays be successfully recovered (Moskalyk and Alfantazi, 2002b). Laterite ore is found in tropical regions that were sub-tropical in past geologic epochs. These occur in close proximity to the surface (0 – 40 m) in layers that contain an iron cap consisting of goethite (FeO(OH)), an iron shot overburden, limolitic overburden, limonite (FeO(OH)∙nH2O) ore, a transition zone, saponite (Ca0.25(Mg,Fe)3((Si,Al)4O10 (OH)∙n(H2O)) and boulders, and peridotite. Nickel–magnesium silicate is present in mixtures of serpentine ((Mg,Fe,Ni,Al,Zn,Mn)₂₋₃(Si,Al,Fe)₂O₅(OH)₄), saponite (Ca0.25(Mg,Fe)3((Si,Al)4O10)(OH) n(H2O)) and deweylite. Nickelferrous components are present in limonite that also contains goethite. Laterite mines are generally open pit mines and therefore are less costly operations compared with sulfide ore processes. Other metals such as cobalt, zinc and copper are produced as by-products (Moskalyk and Alfantazi, 2002b; Agatzini-Leonardou et al., 2009; Crundwell et al., 2011).
Nickel recovery and production processes
After mineral processing of the nickel-containing ore, further processing depends on the characteristics of the specific ore, energy costs and environmental constraints. Pyrometallurgical or hydrometallurgical routes can be followed, after which electrometallurgical steps are incorporated to obtain a pure nickel product (Kittelty, 2002; Crundwell et al., 2011).
Pyrometallurgical processes
For ores that contain nickel together with magnesium and silica, pyrometallurgical processing is preferred. The pyrometallurgical processing route for nickel sulfide-type ore involves roasting to fully oxidise the iron present, with the evolution of all sulfur as SO2 gas as by-product. The oxidised iron is then removed by melting the product with a siliceous flux. The iron forms a liquid silicate that is separated from a molten sulfide phase. The remainder of the sulfur is then oxidised and removed from the melt while iron still present is oxidised and the silicate is removed in converters (Kittelty, 2002; Crundwell et al., 2011).
Nickel oxide ore is treated in an arc furnace, which produces a matte. The matte product is roasted to an oxide product, from which nickel metal is then recovered. In some cases, the product can be treated with chloride or sulfate reagents to produce water-soluble products that are further treated hydrometallurgically. The nickel can also be recovered by electrorefining processes (Fornari and Abbruzzese, 1999; Whittington and Muir, 2000). There are some environmental constraints, as well as energy cost issues, that make pyrometallurgical processes non-ideal in some cases. The SO2 gas that is emitted during these processes is monitored and controlled as it is dangerous to the environment. Costly and time-consuming procedures such as pre-concentration of the ore are also generally necessary before pyrometallurgical processing to make it effective and viable. Therefore, hydrometallurgy is also employed to process ore further as well as recover nickel successfully from nickel-bearing ore, especially in new plants all over the world (Kittelty, 2002; Crundwell et al., 2011).
Hydrometallurgical processes
Ore that contains nickel with a high iron or cobalt content is normally processed hydrometallurgically. Hydrometallurgical processes generally aim to selectively leach an ore to recover only certain specific elements. It is carried out in acid or base media at elevated temperatures and pressures to improve kinetics. A commonly used hydrometallurgical leaching procedure for sulfide ore is the Sherritt–Gordon process, which is carried out in aqueous ammonia. Impurities and unwanted metals are removed and nickel and cobalt are recovered by hydrogen reduction (Kittelty, 2002). More often than not, a combination of pyro- and hydrometallurgical processes is employed for nickel oxide ore. In some cases, specifically in laterite ore processing, the leach product contains nickel and cobalt sulfides, which are processed by the Caron process. This process consists of several steps, including: drying and grinding of high-limonitic ore, reduction roasting, leaching with ammonia–ammonium carbonate to dissolve the nickel and cobalt as amine complexes and, lastly, the recovery of the base metal from the solution as a nickel oxide product (Moskalyk and Alfantazi, 2002a). Nickel and cobalt are reduced to form alloys with iron at temperatures of around 700 °C. Calcine is then removed by leaching (selectively for nickel and cobalt) in ammonia–ammonium carbonate solution. The ammonia is then removed by boiling and the product is nickel carbonate, which is calcined at 1200 °C to form nickel oxide. This type of process is often costly, energy consumption is high and nickel recovery is only moderate while cobalt recovery is low (Kittelty, 2002). The developed pressure acid leaching (PAL) process is nowadays often used. Leaching is carried out in a sulfate- or chloride-containing medium followed by solvent extraction and then electrolytic recovery by means of electrowinning processes. The PAL system involves several centrifugal, diaphragm pumping and heat recovery stages. The leach product contains nickel, cobalt and metallic impurities. Some metallic impurities, such as iron, may then be precipitated by pH adjustment. Sulfide precipitation, re-leach, hydrogen reduction or solvent extraction procedures may then follow, before electrowinning or electrorefining. This PAL method is generally more environmentally friendly, reduces cost and has a better nickel recovery compared with the Caron process (Crundwell et al., 2011).
Electrometallurgical recovery
The electrochemical deposition or similar electrometallurgical processes of metals such as zinc, copper and nickel is widely used in the final step of hydrometallurgical or pyrometallurgical processes to recover a desired metal product from aqueous solution or melt (Popov et al., 2002).
Approximately 45 % of the nickel produced annually in the world makes use of electrorefining and electrowinning processes (Moskalyk and Alfantazi, 2002a). Most nickel-containing solutions produced after hydrometallurgical or pyrometallurgical processes contain residual impurities even after complex solvent extraction procedures have been carried out. Some of these are organic in nature and come from the leach processes. Others are metallic impurities inherent to the ore. These impurities are often dealt with by adjustment of the parameters and additives to obtain desired plate cathodes, metal powders and mixed sulfides (Küzeci and Kammel, 1994; Wu et al., 2003; Crundwell et al., 2011). Electrometallurgy refers to the electrochemical deposition of metals during electrowinning, electroplating, electrorefining and electroforming. During electrowinning, metals are extracted from aqueous solution or molten salts by electrodeposition. Electrowinning refers to the reduction of metal compounds from the electrolyte (or solution after leaching and purification) onto a metal cathode. Electrorefining refers to processes of purification of metals by means of electrolysis. An impure metal anode is dissolved electrolytically and metal of higher purity is plated onto the cathode. The aim of both electrowinning and electrorefining is to produce compact, pure metal deposits at relatively low current densities and low energy consumption.
Electroplating is an electrolytic process where a bare surface metal is coated by cathodic metal deposition to change the surface properties of the metal while keeping the bulk properties constant (Popov et al., 2002). This application is extensively used in the electronics industry or in engineering applications to improve, for instance, corrosion properties or abrasion sensitivity.
Electroforming is used for the manufacturing of articles by electrodepositing a metal onto a mandrel or master template. The deposited metal must then be smooth and level, and easily removed from the template (Malevich et al., 2008). Electrowinning of nickel specifically can be performed in chloride- or sulfate based medium or a combination of the two. In a sulfate-based medium, oxygen is produced as by-product while in chloride medium, chlorine gas is produced. The sulfate routes are today used to electrowin nickel successfully from laterite ores after solvent–extraction procedures (Alfantazi and Shakshouki, 2002; Wu et al., 2003). The highest purity nickel commercially available produced via this route is referred to as London Metal Exchange (LME) grade nickel, which is approximately 99.80 % pure nickel. Impurities include Bi, C, Ca, Co, Fe, Mn, Pb, S, Sb, Si, Sn and Zn (Kittelty, 2002).
Acknowledgements
Publications and conference presentations
Table of contents
List of tables
List of figures
List of terms and symbols
Abstract
Chapter 1: Introduction
1.1. Introduction and problem statement
1.2. Research approach and project objectives
1.3. Presentation of the work
Chapter 2: Literature survey
2.1. Introduction to nickel recovery and production
2.1.1. Natural sources and characteristics of nickel
2.1.1.1. Nickel from sulfide-bearing ores
2.1.1.2. Nickel from laterite sources
2.1.2. Nickel recovery and production processes
2.1.2.1. Pyrometallurgical processes
2.1.2.2. Hydrometallurgical processes
2.1.2.3. Electrometallurgical recovery
2.2. Parametric effects in nickel electrowinning from sulfate electrolyte
2.2.1. Reactions at the anode and cathode
2.2.2. Effect of electrolyte parameter changes on polarisation parameters and nickel deposit
morphology
2.2.2.1. pH
2.2.2.2. Temperature
2.2.3. Sulfate electrolyte composition and effect thereof on polarisation parameters and deposit morphology
2.2.3.1. Nickel concentration
2.2.3.2. Sodium sulfate concentration
2.2.3.3. Boric acid concentration
2.2.3.3.1. Citric acid buffers
2.2.3.4. Effect of specific additives to the electrolyte
2.2.3.4.1. Saccharin
2.2.3.4.2. Sodium lauryl sulfate
2.2.3.4.3. Pyridine
2.2.3.5. Effect of specific metallic impurities common to nickel electrowinning processes
2.2.3.5.1. Cobalt
2.2.3.5.2. Copper
2.2.3.5.3. Aluminium
2.2.3.5.4. Selenium
2.3. Fundamental aspects of electrocrystallisation – nucleation and growth of electrodeposits
2.3.1. Introductory overview
2.3.2. Transfer of metal ions from the bulk electrolyte to the substrate surface
2.3.3. Surface diffusion and incorporation of adatoms into the growing crystal lattice
2.3.4. Nucleation and growth
2.3.5. Inhibition intensity and morphological changes during electrocrystallisation
2.4. Practical aspects of electrocrystallisation – measurement and control of nucleation and growth
2.4.1. Introduction
2.4.2. Overpotential, nucleation overpotential and plating overpotential
2.4.3. Typical changes in polarisation behaviour during electrodeposition
2.4.4. Conventional techniques used to measure polarisation parameters
Chapter 3: Materials, methods and experimental techniques
3.1. Reagents and solution preparation
3.1.1. Impurity analysis
3.1.2. Determination of the Ni2+ concentration
3.2. Electrochemical experimental setup
3.2.1. Preparation of the working electrode
3.3. Calculation of current efficiency
3.4. Physical properties, quality and morphological evaluation of nickel deposits
3.5. Investigation of buffer characteristics of various electrolytes
3.6. Industrial application
Chapter 4: Development of repeatable galvanodynamic measurement technique
4.1. Initial experiments
4.2. Finalising the galvanodynamic technique
4.3. IR compensation
4.4. Repeatability of polarisation measurement
Chapter 5: Variation in polarisation parameters with changes in the electrolyte and effect thereof on developing morphology
5.1. Introduction
5.2. Effect of nickel concentration
5.3. Effect of sodium sulfate concentration
5.4. Effect of changes in temperature
5.5. Effect of changes in pH
5.6. Effect of boric acid
5.6.1. Replacing boric acid with citric acid
5.7. Effect of additives
5.7.1. Effect of sodium lauryl sulfate
5.7.2. Effect of saccharin
5.7.3. Effect of pyridine
5.8. Effect of impurities
5.8.1. Effect of cobalt
5.8.2. Effect of copper
5.8.3. Effect of aluminium
5.9. Effect of amphoterics
5.9.1. Effect of selenium
5.10. Investigation of buffering capabilities of various electrolytes
Chapter 6: Correlation of changes in polarisation parameters with changes in developing morphology
6.1. Introduction
6.2. First region of similar morphology – highly cathodic Ep and positive ∆E values
6.3. Second region – highly cathodic Ep and negative ∆E values
6.4. Third region – less cathodic Ep and negative ∆E values
6.5. Fourth region – less cathodic Ep and positive ∆E values
Chapter 7: Correlation of results with industrial electrolyte
7.1. Introduction
7.1.1. Introduction to RBMR tankhouse for nickel electrowinning
7.1.2. Nickel electrowinning process at RBMR
7.2. Comparison of results for electrolytes containing sodium lauryl sulfate
7.3. Comparison of results for electrolytes containing saccharin
7.4. Comparison of results for electrolytes containing cobalt
7.5. Comparison of results for electrolytes containing copper
7.6. Comparison of results for electrolytes containing selenite
7.7. Industrial applicability
Chapter 8: Conclusions and future work
8.1. Galvanodynamic measuring technique
8.2. Inhibition, nucleation and growth
8.3. Buffering characteristics of sulfate electrolytes
8.4. Industrial application
8.5. Future work
References
Appendices
Appendix A
Appendix B
Appendix C
