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Today, bi - reforming of methane is considered as an emerging replacement for the generation of high-grade synthesis gas (H2:CO = 2.0), and also as an encouraging renewable energy substitute for fossil fuel resources. For achieving high conversion levels of CH4, H2O, and CO2 in this process, appropriate operation variables such as pressure, temperature and molar feed constitution are prerequisites for the high yield of synthesis gas. One of the biggest stumbling blocks for the methane reforming reaction is the sudden deactivation of catalysts, which is attributed to the sintering and coke formation on active sites. Consequently, it is worthwhile to choose promising catalysts that demonstrate excellent stability, high activity and selectivity during the production of syngas. This review describes the characterisation and synthesis of various catalysts used in the bi-reforming process, such as Ni-based catalysts with MgO, MgO–Al2O3, ZrO2, CeO2, SiO2 as catalytic supports. In summary, the addition of a Ni/SBA-15 catalyst showed greater catalytic reactivity than nickel celites; however, both samples deactivated strongly on stream. Ce-promoted catalysts were more found to more favourable than Ni/MgAl2O4 catalyst alone in the bi-reforming reaction due to their inherent capability of removing amorphous coke from the catalyst surface. Also, Lanthanum promoted catalysts exhibited greater nickel dispersion than Ni/MgAl2O4 catalyst due to enhanced interaction between the metal and support. Furthermore, La2O3 addition was found to improve the selectivity, activity, sintering and coking resistance of Ni implanted within SiO2. Non-noble metal-based carbide catalysts were considered to be active and stable catalysts for bi-reforming reactions. Interestingly, a five-fold increase in the coking resistance of the nickel catalyst with Al2O3 support was observed with incorporation of Cr, La2O3 and Ba for a continuous reaction time of 140 h. Bi-reforming for 200 h with Ni-γAl2O3 catalyst promoted 98.3% conversion of CH4 and CO2 conversion of around 82.4%. Addition of MgO to the Ni catalyst formed stable MgAl2O4 spinel phase at high temperatures and was quite effective in preventing coke formation due to enhancement in the basicity on the surface of catalyst. Additionally, the distribution of perovskite oxides over 20 wt % silicon carbide-modified with aluminium oxide supports promoted catalytic activity. NdCOO3 catalysts were found to be promising candidates for longer bi-reforming operations.

During the hydrometallurgical processing of nickel from raw materials, the leach liquors are found to be contaminated with several impurities. These impurities in the electrolytic cell affect the deposition characteristics as well as the kinetics and mechanism of nickel electrodeposition process resulting in lower current efficiency (CE) and poor nickel deposits. In order to improve the quality of the nickel deposits, it is imperative to use organic additives in the nickel plating bath to improve the structural, mechanical, and morphological properties of the deposits. Furthermore, it is usually observed that in spite of various purification techniques like cementation and solvent extraction, metals obtained at the cathode are usually contaminated with inorganic impurities. This review thus presents a comprehensive overview of some important studies and investigations performed on various inorganic and organic additives employed in nickel electrodeposition processes from various baths such as Watts, sulfate, acetate, formamide, lactate, and baths containing ionic liquids. The presence of metallic (inorganic) impurities in industrial electrolytes is very common. Most of these impurities affect the deposit’s characteristics, CE, deposition overvoltage, and cathode purity. Addition of inorganic cations such as Al3+, Mg2+, Mn2+, and Zn2+ did not have a significant effect on the CE; nevertheless, the physical appearance and crystallographic orientation of nickel deposits were significantly affected. Organic additives are usually added to the nickel electrolytic bath to counter the harmful effects of these metallic impurities entrained in the bath, where they also affect the growth and crystal building of the deposits through their adsorption onto the cathode surface. Most of these additives act as hydrogen inhibitors, crystal growth modifiers, brighteners, levelers, wetting agents, and stress reducers, and hence, their appropriate addition was important for the formation of fine-grained, smooth, and compact deposits. This review demonstrates that the quality of the nickel deposit was strongly affected if the concentration of the inorganic impurity in the nickel bath exceeded the tolerance limit. From this review article, the roles of various organic additives as a brightener, leveler, and antipitting agent, etc. in the Ni plating bath could be established, and these additives would play a significant role in the formation of bright, smooth, and coherent nickel deposits obtained during hydrometallurgical processing of laterite and sulfide ores in the metallurgical industry.

Pyrometallurgical metal production results in side streams, such as dusts and slags, which are carriers of metals, though commonly containing lower metal concentrations compared to the main process stream. In order to improve the circular economy of metals, selective leaching of copper from an intermediate raw material originating from primary base metal production plant was investigated. The raw material investigated was rich in Cu (12.5%), Ni (2.6%), Zn (1.6%), and Fe (23.6%) with the particle size D80 of 124 µm. The main compounds present were nickel ferrite (NiFe2O4), fayalite (Fe2SiO4), cuprite (Cu2O), and metallic copper. Leaching was studied in 16 different solutions. The results revealed that copper phases could be dissolved with high yield (>90%) and selectivity towards nickel (Cu/Ni > 7) already at room temperature with the following solutions: 0.5 M HCl, 1.5 M HCl, 4 M NaOH, and 2 M HNO3. A concentration of 4 M NaOH provided a superior selectivity between Cu/Ni (340) and Cu/Zn (51). In addition, 1-2 M HNO3 and 0.5 M HCl solutions were shown to result in high Pb dissolution (>98%). Consequently, 0.5 M HCl leaching is suggested to provide a low temperature, low chemical consumption method for selective copper removal from the investigated side stream, resulting in PLS (pregnant leach solution) which is a rich in Cu and lead free residue, also rich in Ni and Fe.

The effect of sodiumlaurylsulphate (SLS) on the cathodic current efficiency (CE), surface morphology, crystallographic orientations and polarization behaviour of the stainless steel cathode during nickel electrowinning from acidic sulphate solutions was investigated. The results indicated that SLS did not have a significant effect on the cathodic current efficiency. However, noticeable changes in the surface morphology and deposit quality were observed. Cyclic and linear sweep voltammetric studies revealed that an increase in SLS concentration from 2 to 40 mg dm− 3 increased the cathodic potential and shifted the nucleation overpotential (NOP) towards more negative values which was also reflected in the decrease in the rate of electron transfer i.e. i0 values obtained on both the stainless steel and nickel substrates. The presence of SLS did not change the fcc structure of the electrodeposited nickel but affected the peak intensities of the various crystal planes.

The effect of thiourea on the cathodic current efficiency (CE), deposit quality, crystallographic orientations, surface morphology, and polarization behavior of the cathode was investigated during nickel electrodeposition from acidic sulfate solutions for 2 hours at 60 °C. A slight decrease of 3 to 4 pct in the CE was observed, when the concentration of thiourea was increased from 2 to 40 mg dm−3. The nickel deposit quality deteriorated significantly at higher thiourea concentrations; the surface morphology deteriorated and the contamination of the nickel deposits increased. The presence of thiourea affected the peak intensities of the crystal planes. Cyclic voltammetric studies on nickel deposition at 25 °C revealed depolarization behavior of the cathode at lower thiourea concentrations, ≤10 mg dm−3; however, a mixed behavior is observed at higher thiourea concentrations. These changes were also observed in the exchange current density (i 0) values.

The effect of pyridine, 2-picoline and 4-picoline on current efficiency (CE), deposit morphology, crystallographic orientation and polarization of the cathode during electrodeposition of nickel from acidic sulphate solutions containing boric acid were investigated. These additives were found to have no significant effect on current efficiency. However, the nickel deposits obtained were smoother, more compact and more leveled in case of picolines than with pyridine. The (200) plane was the most preferred plane and was not affected by the presence of these additives in the electrolyte. Each of the additives polarized the cathode with the order of polarization being 4-picoline>2-picoline>pyridine. The quality of the deposit was improved by these additives in the order 4-methyl pyridine>2-methyl pyridine>pyridine.

The effect of Al3+ on the cathodic current efficiency, deposit morphology, crystallographic orientations and polarisation behaviour of the cathode during electrodeposition of nickel from acidic sulfate solutions was investigated. Higher concentration of Al3+ (>10 mg dm−3) significantly deteriorated the surface quality of the nickel deposit as well as the current efficiency. X-ray diffraction studies revealed that the (200) plane was the most preferred crystal plane and was not affected by the presence of varying concentration of Al3+ in the electrolytic bath. The presence of Al3+ caused polarisation of the cathode, which increased with increasing Al3+ concentration. The effect of Al3+ on the electrokinetic parameters: Tafel slope (b), transfer coefficient (α) and exchange current density (i0) were also investigated.

The effect of Cd2+ on the electrodeposition of nickel from a sulphate bath in the absence and presence of boric acid has been investigated. The presence of Cd2+ ion suppresses the nickel electrodeposition and shifts the deposition potential to more negative values. The cathodic polarisation is found to depend on the composition of the electrolytic bath. For the various baths investigated, the cathodic polarization order is found to decrease in the order NiSO4+H3BO3NiSO 4+H3BO3+Na2SO4NiSO 4+Na2SO4NiSO4. For the Ni 2+ electroreduction reaction on stainless steel and nickel substrates the exchange current density decreases with increase in Cd2+ concentration in the electrolytic bath. However the Tafel slope and transfer coefficient are unaffected.

The effect of Cr3+ on the cathodic current efficiency (CE), deposit morphology, crystallographic orientations and polarisation behaviour during nickel electrodeposition from acidic sulfate solutions containing boric acid was investigated. A progressive decrease in CE was seen with increase in Cr3+ concentration showing a maximum decrease in CE of ∼10-13% at concentration ∼100 mg dm-3. The presence of Cr3+ did not change the fcc structure of the electrodeposited nickel but affected the peak intensities of the crystal planes. Polarisation of the cathode was seen in the presence of Cr3+ in the electrolyte along with a shift in the electroreduction potential of nickel (II) ion towards more negative values. The effect of Cr3+ on the electrokinetic parameters: Tafel slope (b), transfer coefficient (α) and exchange current density (i0) has also been investigated.

The effect of Cd2+ on the current efficiency, surface morphology and crystallographic orientations of the electrodeposited nickel from sulfate baths of varying compositions was investigated. The results indicated that Cd2+ had no significant effect on the current efficiency but there was a noticeable change in the surface morphology and deposit quality. The X-ray diffraction studies revealed that the (200) plane was the most preferred orientation and was not generally affected by the presence of Cd2+ in the electrolytic bath.

The electrochemical reactions occurring during electrodeposition of nickel from acidic sulfate solutions were examined by cyclic and linear sweep voltammetry techniques. The effect of pyridine, 2-picoline and 4-picoline on the electrode polarization behaviour and electron transfer parameter of the cathodic reduction process was also investigated. Strongest electrode polarization was seen with 4-picoline. The order of cathodic polarization was 4-picoline > 2-picoline > pyridine. Kinetic parameters such as Tafel slope, transfer coefficient and exchange current density were determined to analyse the nature of the electrode reactions. The exchange current density for nickel deposition on nickel and stainless steel substrates was in the order 4-picoline < 2-picoline < pyridine.

The effects of pyridine and its derivatives on current efficiency, surface morphology and crystallographic orientations of electrodeposited nickel from acidic sulfate solutions were investigated. The results indicated that the presence of pyridine and picolines had no significant effect on current efficiency. The deposits obtained were smoother, more compact and uniform with picolines than with pyridine. A significant change in surface morphology of the electrodeposits was observed and picolines were found to be better additives than pyridine, 4-picoline being the best. X-ray diffraction revealed that the (200) plane was the most preferred plane and was not affected by the presence of any of these additives in the electrolyte.