Metal sulphides concentrates

In the classical pyrometallurgical smelting of metal sulphide concentrates most of the sulphur is driven off as sulphur dioxide. This process is becoming increasingly unacceptable from an environmental point of view. An alternative is to process the concentrates using hydrometallurgical techniques. 

W.J.S. Craigen and Canmet/MSL Staff, The CANMET Ferric Chloride Leach Process for the Treatment of Bulk Base Metal Sulphide Concentrates , MSL Division Report. MSL 89-67, June 1989. 

R.W. Adams and LG. Matthew, Leaching of Metal Sulphide Concentrates at Atmospheric Pressure using SO2/O2 Mixtures , Proc. Australasian Institute of Mining and Metallurgy. No. 280,1981,41-53. 

With the aid of a table of solubility products of metallic sulphides, we can calculate whether certain sulphides will precipitate under any given conditions of acidity and also the concentration of the metallic ions remaining in solution. Precipitation of a metallic sulphide MS will occur when [M2 + ] x [S2 ] exceeds the solubility product, and the concentration of metallic ions remaining in the solution may be calculated from the equation ... 

It must be pointed out that the above calculation is approximate only, and may be regarded merely as an illustration of the principles involved in considering the precipitation of sulphides under various experimental conditions the solubility products of most metallic sulphides are not known with any great accuracy. It is by no means certain that the sulphide ion S 2 is the most important reactant in acidified solutions it may well be that in many cases the active precipitant is the hydrogensulphide ion HS , the concentration of which is considerable, and that intermediate products are formed. Also much co-precipitation and post-precipitation occur in sulphide precipitations unless the experimental conditions are rigorously controlled. 

The Guadiana Basin occupies a total area of 66,850 km2 in Spain and Portugal, 8,350 km2 of which comprises the Iberian Pyrite Belt (IPB) one of the world s largest concentrations of base metal sulphide deposits (Leistel et al., 1997 Ribeiro et al., 1990). Three plant species (Cistus ladanifer, Lavandula luisieri and Thymus vulgaris) were utilized in this study. The soils in which these plants were sampled are developed over varied geological formations. The oldest formation (PQ Group, Upper Devonian) consists of a thick sequence of arenites and shales... 

The Ag2 S ISE has Nemstian response dE/d log a( = 0.0296 V in the sulphide concentration range 10" to 10" M and silver ions from 10 to 10 M if the solutions are prepared from pure salts, as a further concentration decrease is prevented by adsorption on the glass (see p. 76 and [87, 163]). After prolonged use, the limit of the Nemstian behaviour shifts to about 10" m [130] as a result of formation of mixed potentials on accumulation of metallic silver in the membrane surface. An analogous deterioration in the membrane function in the presence of iodine results from surface oxidation [23]. Cyanide interferes only at large concentrations the equilibrium constant of the reaction... 

The most common type of troublesome scale is that of amorphous silica and calcium carbonate. Scales of various metallic sulphides is the rule rather than the exception. By far the most abundant sulphide scale consists of iron sulphides. They include pyrite, marcasite, and pyr-rhotite (Kristmannsdottir 1989), but sulphide scale of other metals have also been observed, such as Cu, Pb, and Zn (White et al. 1963 Gallup 1989 Gallup et al. 1995 Hardardottir et al. 2001 Reyes et al. 2002). Sulphide scales are often poorly crystalline and they may be amorphous to X-rays. Moreover, the sulphidebearing scales are known to be enriched in various elements such as Ag, As, Au, Cd, and Mn. Reyes et al. (2002) observed that scales at Rotokawa, New Zealand, also contained elevated concentrations of Hg, Sb, and Se, which were incorporated in pyrite. The quantity of sulphide scale formation is generally very limited and may in fact be beneficial rather than troublesome as the scale forms a stable protective... 

It should be kept in mind that many of these decomposition reactions are equilibria. The decomposition of thiourea in the absence of a metal ion will normally be much slower than in the presence of such an ion. The metal ion removes sulphide as metal sulphide—the less soluble the sulphide, the more effective the removal at very low sulphide concentrations. This continuous removal of sulphide shifts the equilibrium to the direction of more sulphide production. The same principle holds for many other anion precursors. 

Another, and on the face of it, rather different example, is the coprecipitation of solid solution compounds, such as CulnSi and CulnSei—semiconductors of particular interest due mainly to their applicability for photovoltaic cells. It was shown, by X-ray diffraction, that the precipitate resulting from reaction between H2S and an aqueous solution containing both Cu" and In " ions was, at least in part (depending on the concentrations of the cations), single-phase CulnSi [3]. Two factors were found to be necessary for this compound formation (1) the presence of sulphide on the surface of the initially precipitated colloidal solid metal sulphide and (2) one of the cations being acidic and the other basic. The monovalent Cu cation is relatively basic, while the trivalent In cation is relatively acidic. It is not clear what the physical reason is for this latter requirement. A difference in practice between acidic and basic cations is that, in an aqueous solution of both cations, the acidic cation is more likely to be in the form of some hydroxy species (not to be confused with hydrated cations), while the basic cation is more likely to exist as the free cation. 

These processes have been described for rapid precipitation reactions. However, they should also be valid in general for slow precipitation—i.e., for CD— with possible differences due to the very different kinetics involved. Thus, if free sulphide is involved, since it is always present in very low concentration, the lower-solubility product metal sulphide is more likely to deposit first, compared to rapid precipitation. Solid-state diffusion processes have much more time to occur in CD (although they may occur in rapid precipitation after the precipitation itself), increasing the probability of solid solution formation. 

Relations between the structure and properties have been investigated in a variety of solids such as metal oxides, chalcogenides, pnictides and halides. In addition to studying model systems for testing theoretical predictions, solid state chemists have been preparing new classes of solids as well as novel members of known types of solids. In this section, we have chosen three classes of solids, viz. metal oxides, metal sulphides and metal fluorides, to discuss structure-property relations we shall concentrate especially on their electrical and magnetic properties. 

An alternative that is rapidly emerging is to process the concentrate using hydrometallurgical techniques. Aqueous solutions of certain metal salts will chemically attack most metal sulphides, taking into solution the constituent metals and leaving behind a residue of elemental sulphur. Because of the large... 

Hydrogen trisulphide is much more easily combustible than the crude parent hydrogen polysulphide. Exposure to light tends to accelerate its decomposition. It slowly reduces concentrated sulphuric acid to sulphur dioxide, whilst on contact with dry silver oxide, cupric oxide, lead dioxide or mercuric oxide, it bursts into explosive combustion,2 a residue of the metallic sulphide being obtained. Many other metallic oxides and most salts bring about a less vigorous decomposition metals in the massive condition only react with it slowly. With potassium permanganate or dichromate the reaction is violent. 

The salts are usually prepared by the interaction of arsenious sulphide with the metallic sulphide, hydrosulphide or carbonate, taking care to exclude air to prevent the formation of thioarsenates. Thus, Nilson12 obtained the salts of the alkali and alkaline earth metals by dissolving arsenious sulphide in the aqueous solutions of the respective hydrosulphides and concentrating in vacuo. 

Precipitation of sulphides Hydrogen sulphide gas is a frequently used reagent in qualitative inorganic analysis. When hydrogen sulphide gas is passed into a solution, metal sulphides are precipitated. For this precipitation the rule mentioned above can be applied precipitation may take place only if the product of concentrations of metal ions and sulphide ions (taken at proper powers) exceed the value of the solubility product. While the concentration of metal ions usually does fall into the range of 1-10 3 mol 1, the concentration of sulphide ion may vary considerably, and can easily be selected by the adjustment of the pH of the solution to a suitable value. 

The characteristic colours and solubilities of many metallic sulphides have already been discussed in connection with the reactions of the cations in Chapter III. The sulphides of iron, manganese, zinc, and the alkali metals are decomposed by dilute hydrochloric acid with the evolution of hydrogen sulphide those of lead, cadmium, nickel, cobalt, antimony, and tin(IV) require concentrated hydrochloric acid for decomposition others, such as mercury(II) sulphide, are insoluble in concentrated hydrochloric acid, but dissolve in aqua regia with the separation of sulphur. The presence of sulphide in insoluble sulphides may be detected by reduction with nascent hydrogen (derived from zinc or tin and hydrochloric acid) to the metal and hydrogen sulphide, the latter being identified with lead acetate paper (see reaction 1 below). An alternative method is to fuse the sulphide with anhydrous sodium carbonate, extract the mass with water, and to treat the filtered solution with freshly prepared sodium nitroprusside solution, when a purple colour will be obtained the sodium carbonate solution may also be treated with lead nitrate solution when black lead sulphide is precipitated. 

Metal sulphides Sulphide produced in bacterial sulphate respiration can precipitate heavy metal ions from solution when the sulphide concentration is in excess of that demanded by the solubility product of the... 

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