Nickel continued sulphidation

Continued exposure of the nickel-chromium alloy to more severely sulphurising and reducing atmospheres results in local depletion of chromium to such an extent that nickel sulphide and the eutectic are formed internally. The latter constituents are not often observed in service failures, but the relative instability of nickel sulphide in the presence of chromium sulphide can result in its reduction to nickel during slow cooling on shut down. That nickel sulphide is formed is suggested by the frequent occurrence of blisters, associated with the formation of molten eutectic on the surface of sulphur-attacked specimens . 

General corrosion damage was the cause of failure of an A1 alloy welded pipe assembly in an aircraft bowser which was attacked by a deicing-fluid — water mixture at small weld defects . Selective attack has been reported in welded cupro-nickel subjected to estuarine and seawater environments . It was the consequence of the combination of alloy element segregation in the weld metal and the action of sulphate reducing bacteria (SRB). Sulphide-coated Cu-enriched areas were cathodic relative to the adjacent Ni-rich areas where, in the latter, the sulphides were being continuously removed by the turbulence. Sulphite ions seemed to act as a mild inhibitor. 

Hydrogenation studies were undertaken on the parent iron-tin treated coal (Drum 289) as well as the THF insolubles, preasphaltene, asphaltene and oil derived from a continuous reactor run as previously discussed. Studies with no additional catalyst added (case A) and with the addition of a sulphided nickel molybdate catalyst supported on alumina (case B) were performed. The results are presented in Table 1. The Ni/Mo catalyst in case B did not increase the conversion of the coal or the THF insolubles beyond that for case A because sufficient amounts of iron and tin materials were already... 

Flotation has been used for more than 100 years to separate sulphides, oxides and other salts from ores, as well as to obtain phosphates, barite, chromite and other materials. Up to 90% of copper, lead, nickel, zinc are extracted using flotation in the USA [152 - 153]. In Russia, flotation is widely used to additionally obtain apatite, barite and phosphates. Flotation of iron oxides is not used in practise yet, but the number of experiments carried out in this direction is rather large. The main physicochemical principles of flotation have been discussed above [59 -74]. Here, only some practical problems will be discussed. In [153], requirements are pointed out which apply to three-phase flotation foams, and the main components of the process are defined, i.e. surfactant - collector surfactant - frother activator, depressants, colligend, gangue. The peculiarities of flotation and foam separation in batch and continuous modes are outlined as well as the structure and properties of the main types of flotation agents described. As surfaces of the majority of mineral particles are hydrophilic in nature, hydrophobisation of particles is necessary for a selective separation. 

Sometimes, depending on the system and conditions, the duplex oxide-sulphide layer will appear as a lamellar arrangement, as in the case of iron. " With nickel, however, the sulphide forms in a more massive, less organized arrangement. In either case, the sulphide probably forms as a continuous network to account for the high ionic conductivity of the duplex layer. 

Below 600 °C the scale consists of a duplex nickel oxide and nickel sulphide intimate mixture in which the sulphide was found to form a continuous network. As in the case for iron, the continuous nickel sulphide provides a pathway for very rapid nickel-ion diffusion to the scale-gas interface. This results in a high nickel activity there and promotes the duplex formation as described previously. Such a scale is shown in Figure 7.13. 

Figure 8.12 Optical micrographs showing a comparison of nickel specimens heated 24 h at 1000 °C in a sealed quartz tube which contained Na2S04 in an AI2O3 crucible. The nickel specimens did not touch the Na2S04. (a) The quartz tube was evacuated and sealed. A layer of nickel sulphide (arrows) has formed on the metal beneath a dense continuous layer of NiO. The nickel sulphide has been etched electrolytically with NaNOs. (b) The qnartz tnbe was backfilled with oxygen to give a pressure of 0.9 atm at 1000 °C. A dense layer of NiO has been formed. No evidence of sulphide formation was observed.

Figure 8.13 Model for the Na2S04-induced accelerated oxidation of nickel, (a) An oxygen activity gradient is produced across the Na2S04 layer by the formation of NiO. (b) Sulphur enters the metal to form nickel sulphide and oxide ions react with NiO to form nickelate ions. The nickelate ions diffuse toward theNa2S04-gas interface where they decompose to NiO particles and oxide ions. NiO is not stable on the metal but forms away from the metal as a non-protective scale. Accelerated oxidation takes place, (c) Sulphur stops entering the alloy and oxide ions are no longer produced. The Na2S04 becomes saturated with nickel and NiO forms as a continuous layer on the metal surface. Accelerated oxidation no longer occurs.

The two types of high temperature fuel cell are quite different from each other (Table 6). The molten carbonate fuel cell, which operates at 650°C, has a metal anode (nickel), a conducting oxide cathode (e.g. lithiated NiO) and a mixed Li2C03/K2C03 fused salt electrolyte. Sulphur attack of the anode, to form liquid nickel sulphide, is a severe problem and it is necessary to remove H2S from the fuel gas to <1 ppm or better. However, CO is not a poison. Other materials science problems include anode sintering and degradation, corrosion of cell components and evaporation of the electrolyte. Work continues on this fuel cell in U.S.A. and there is some optimism that the problem will be solved within 10 years. 

The reduction of the C—X bond continues to be extensively studied. For example, it is well recognized that vinyl sulphides are important in organic synthesis. However, their desulphurization to afford olefins especially using Raney nickel can be accompanied by problems, such as over-reduction. A neat way of overcoming these difficulties, in a stereospecific fashion, involves the use of 2-propylmagnesium bromide in the presence of 3-8 mol.% (Ph3P)2NiCl2. ... 

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