Metal sulphates

The decompositions of anhydrous metal sulphates are often endothermic and usually reversible 

However, unlike the carbonates discussed above, the reaction is further complicated by the dissociation of the gaseous product 

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Sulphur attack on nickel-chromium alloys and nickel-chromium-iron alloys can arise from contamination by deposits resulting from the combustion of solid fuels, notably high-sulphur coals and peat. This type of corrosion, which has been observed on components of aircraft, marine and industrial gas turbines and air heaters, has been associated with the presence of metal-sulphate and particularly sodium sulphate arising directly from the fuel or perhaps by reaction between sodium chloride from the environment with sulphur in the fuel. Since such fuels are burned with an excess of air, corrosion occurs under conditions that are nominally oxidising although the deposits themselves may produce locally reducing conditions. 

The following are suitable anions for urea precipitations of some metals sulphate for gallium, tin, and titanium formate for iron, thorium, and bismuth succinate for aluminium and zirconium. 

Hydrated metal sulphates have long been used to study water removal processes, and characteristic kinetic behaviour is conveniently illustrated by reference to these substances. Frost and co-workers [602,603] have investigated the structures, stabilities and adsorption properties of various intermediate amorphous phases, the immediate reaction products which can later undergo reorganization to yield crystalline phase. 

Kinetic data available for the reactions of other metal sulphates refer to reaction conditions closer to those of actual pyrometallurgical processes [790]. 

Selected kinetic characteristics for the decomposition of metal sulphates... 

Relatively little kinetic information is available concerning the decompositions of other metal sulphates. Decomposition of the lanthanide sulphates [800,801] proceeds to completion in two stages. 

Most mixed and complex ammonium metal sulphates (and selenates) [948,949] lose NH3, H20 and S03 (or Se03) to form the simple metal sulphate (or selenate) some of the ammonia may be oxidized [949]. The basic aluminium ammonium sulphate [950], (NH4)20 3 A1203 4 S03 xH20 (x = 6—8), loses water at 473 K. Deammination and complete dehydration commences at >673 K, and S03 evolution starts at about 873 K to yield residual A1203 which contains traces of S03. a—Time data for most of the stages obeyed the contracting volume equation [eqn. (7), n = 3] [951]. 

Early discussions, by Tammann and by Hedvall, considered the possible existence of a common characteristic (approximate) temperature for a solid at which chemical interactions with other reactants became detectable. For example [111], such a characteristic temperature for CaO, measured in various reactions with CuS04, Co3(P04)2, MgC03, and MnSi03, was found to be 788—838 K. Similarly, the onset of reaction of BaO with the sulphates of Mg, Ca, Sr, Co, Cu, and Zn occurred between 601 and 645 K. In the latter example, it has been shown that the fusion of Ba(OH)2 (an impurity not easily excluded from BaO) could contribute to the initiation of reaction. Eutectic formation during the reactions of BaCl2 with alkali metal sulphates... 

The production of sulphuric acid by the contact process, introduced in about 1875, was the first process of industrial significance to utilize heterogeneous catalysts. In this process, SO2 was oxidized on a platinum catalyst to S03, which was subsequently absorbed in aqueous sulphuric acid. Later, the platinum catalyst was superseded by a catalyst containing vanadium oxide and alkali-metal sulphates on a silica carrier, which was cheaper and less prone to poisoning. Further development of the vanadium catalysts over the last decades has led to highly optimized modem sulphuric acid catalysts, which are all based on the vanadium-alkali sulphate system. 

In this way we find that, in oxides for example, many cation arrangements are identical to the arrangements of the atoms in known (or plausible) alloy structures. (In some cases the cations and atoms are identical in the two cases.) This is particularly helpful in those oxide examples where it has not previously been possible to describe the structure in any simple terms, often because the anion array is not regular in any simple way, for instance in many metal sulphates. But it is also revealing in some cases that are describable in conventional terms, e.g. the humites. A long (but still incomplete) list of examples is given in Table 3. Sufficient specific cases are described and discussed in detail in the text in order to expose the principles and some of the advantages of this unfamiliar approach. 

Many metalliferous ore deposits and most coalfields are characterized by the presence of sulphide minerals, such as pyrite (FeS2), galena (PbS), or sphalerite (ZnS). When exposed to water and oxygen, these sulphides have a tendency to oxidize, releasing dissolved metals, sulphate and, in the case of pyrite, acid (equations I and 2). 

The specific conductances of solutions of a few representative metal sulphates are shown in Fig. 1. Metal sulphates react completely with... 

Solid metal sulphates and phosphates also exhibit acid—base properties their acid strength is lower than that of silica—alumina but they are stronger acids than some oxide catalysts [5]. Correlation of activity with electronegativity of cations has been obtained for several reactions [9, 50,51],... 

Another fine distinction among salt catalysts was obtained by following the activity and olefin/ether selectivity of metal sulphates in the dehydration of ethanol and 1-propanol. A linear correlation between the electronegativity of the metal ion and the activity has been found, but the selectivity gave a curve with a minimum [51]. 

Typical acidic catalysts are silica—alumina, transition metal sulphates or chlorides, calcium phosphate etc. They are characterised by low deuterium kinetic isotope effects and low stereoselectivity (see Tables 8,11 and 12). These results correspond to the E2cA or El mechanisms, between which a transition may be observed due to the influence of the structure of the reactants, i.e. according to the polarity of the Ca—X and Cp—H bonds. Again, the reactions of 1,2-dibromoethane and 1,1,2,2-tetrachloroethane yielded the evidence. The deuterium kinetic isotope effect on silica—alumina was 1.0 for the dibromo-derivative, which indicates a pure El mechanism, whereas for the tetrachloro-derivative, the value of 1.5 was found. 

It is not hydrolysed by water but reacts quantitatively with NaOH to yield sodium perchlorate and fluoride. It oxidizes iodides to iodine. Perchloryl fluoride reacts with ammonia to yield ammonium perchlorylamide NH4NHCIO3 [45a]. It reacts with potassium and caesium hydroxide to yield crystalline precipitates of K2NCIO3 and CS2NCIO3. It is isomorphous with metal sulphates, is explosive and very sensitive to flame, shock and friction. 

The tests for non-volatile matter, chlorides, pyridine, heavy metals, sulphates, and per cent of NIL, as described under... 

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