Metal sulphates, dissolution

Aerobic mineral oxidation resulting in mineral degradation and product mobilization Aerobic bacterial oxidation of elemental sulphur (S°), of various mineral sulphides such as pyrite (FeS2), chalcopyrite (CuFeS2), arsenopyrite (FeAsS), sphalerite (ZnS), cobalt sulphide (CoS) and nickel sulphide (NiS) to corresponding metal sulphates, and of uraninite (UO2) to U02 are examples in which oxidizable minerals undergo dissolution of one or more of their constituents, which are thus mobilized (see Ehrlich, 2002a). 

The enthalpy of dissolution of Se02(cr) to (aq, 1 500 - 1 600) in the acids (sin) was determined to be (2.68 0.04) kJ-mol from which the review calculates an enthalpy of formation of H2Se03(sln, 1 500) of-(508.5 1.0) kJ-mol. The evaluation in the paper used - 514.6 kJ moP. The enthalpies of formation of the metal sulphates and mercury chloride in the media were not determined and the values at infinite dilution were used. The review employs enthalpies of formation at a dilution of (aq, 1 500) and assumes that HgCl is formed rather than HgCl2(aq). This introduces a correction of - 3.4 kJ mol . The results are entered in Table A-53. 

For some non-ferrous metals (copper, lead, nickel) the attack by sulphuric acid is probably direct with the formation of sulphates. Lead sulphate is barely soluble and gives good protection. Nickel and copper sulphates are deliquescent but are gradually converted (if not leached away) into insoluble basic sulphates, e.g. Cu Cu(OH)2)3SO4, and the metals are thus protected after a period of active corrosion. For zinc and cadmium the sulphur acids probably act by dissolution of the protective basic carbonate film. This reforms, consuming metal in the process, redissolves, and so on. Zinc and cadmium sulphates are formed in polluted winter conditions whereas in the purer atmospheres of the summer the corrosion products include considerable amounts of oxide and basic carbonate. ... 

Thus for non-ferrous metals, SO is consumed in the corrosion reactions whereas in the rusting of iron and steel it is believed that ferrous sulphate is hydrolysed to form oxides and that the sulphuric acid is regenerated. Sulphur dioxide thus acts as a catalyst such that one SOj" ion can catalyse the dissolution of more than 100 atoms of iron before it is removed by leaching, spalling of rust or the formation of basic sulphate. These reactions can be summarised as follows ... 

Pure tin is completely resistant to distilled water, hot or cold. Local corrosion occurs in salt solutions which do not form insoluble compounds with stannous ions (e.g. chloride, bromide, sulphate, nitrate) but is unlikely in solutions giving stable precipitates (e.g. borate, mono-hydrogen phosphate, bicarbonate, iodide) . In all solutions, oxide film growth occurs and the potential of the metal rises. Any local dissolution may not begin for several days but, once it has begun, it will continue, its presence being manifested... 

Salt solutions When a zinc sheet is immersed in a solution of a salt, such as potassium chloride or potassium sulphate, corrosion usually starts at a number of points on the surface of the metal, probably where there are defects or impurities present. From these it spreads downwards in streams, if the plate is vertical. Corrosion will start at a scratch or abrasion made on the surface but it is observed that it does not necessarily occur at all such places. In the case of potassium chloride (or sodium chloride) the corrosion spreads downwards and outwards to cover a parabolic area. Evans explains this in terms of the dissolution of the protective layer of zinc oxide by zinc chloride to form a basic zinc chloride which remains in solution. 

Even small traces of certain corrosion stimulants, notably soluble chlorides and sulphates, can maintain a continuing corrosion process under a paint film because the salts accelerate the initial dissolution of ferrous iron (and other metal ions) but are not immobilised in the hydrated oxide corrosion products. Filiform corrosion is the most spectacular example of this phenomenon, but progressive spread, preceded by blistering, is also observed from scratches or other breaks in a coating, for example during salt spray tests. 

On the other hand the succession of the processes proceeding at the anode (e. g. dissolution of metals or discharging of anions) will follow the order, in which the absolute values of their reduction potentials will increase. Thus e. g. at the copper anode, dipped into the acidified solution of copper sulphate,... 

With concentrated nitric acid a protective film of lead nitrate is formed on th surface of the metal and prevents further dissolution. Dilute hydrochloric or sulphuric acid have litde effect owing to the formation of insoluble lead chloride or sulphate on the surface. 

The materials were prepared from silica free deionised water (0.1 mol L ) to which the compounds of interest were added in the form of acidic aqueous solutions. Sulphate, phosphate and chloride were added as ammonium salts, Al, Ca, Fe, K, Mg and Na as nitrates whereas the solution of Mn was obtained by dissolution of the pure metal in nitric acid. The concentrations expected upon spiking are given in Table 8.1. 

For zinc and copper which are metals whose corrosion resistance may be ascribed to a protective layer of basic carbonates and basic sulphates the pH value of rain seems to be of significance. If the pH of rainwater falls to values close to 4 or even lower, as may be seen in a potential- pH diagram for copper in FIG. 7, this may lead to dissolution of the protective coatings. 

The resulting liquor contains titanyl sulphate (Ti0S04) and iron sulphate (FeSOa) dissolved in sulphuric acid. To ensure that all the Fe is in dissolution, the liquor is passed tlu ough scrap metal (Fe reduction step)... 

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