Jarosites, formation

The usual jarosites formed in these situations are hydronium, potassium or ammonium forms in which A is hydrogen, potassium or ammonium ion. In leaching situations, jarosite formation can strip out essential microbial nutrients such as potassium and ammonium ions (Duncan and Walden, 1972 L.A.V. Sulligoi, 1972, personal communication). 

A flowsheet for the jarosite atmospheric leach is shown in Figure 3. Both limonite and saprolite are slurried in an alkali containing water, such as seawater, to ensure there is enough for jarosite formation. 

The combined leach slurry passes to induced jarosite precipitation (IJP) and primary neutralization (PN). Recycle streams from downstream processes are leached initially, further decreasing free acid and allowing more jarosite formation to occur. Limestone is then added to decrease iron levels further and neutralize free acid. 

AMD are often very heterogeneous and reactive systems. Therefore, to describe their properties is often an issue of analytical concern as it is difficult to obtain representative samples of the affected watercourses. The instability of AMD is related to the formation and transformations of ochre-products, such as jarosite, schwertmannite and goethite (Bigham et al. 1996). Mineralogical transformations involving these ochre-precipitates have been the subject of intensive research (Kawano Tomita 2001 Kim et al. 2002 Knorr Blodau... 

The mineral alunite, and its iron equivalent jarosite, is a special case. Alunite (KAl2(S02)2(OH)2) contains four sites where elements containing stable isotopes are found and both the sulfate and hydroxyl anionic groups may provide information on fluid source and condition of formation. 

The thermodynamic incompatibility of many of the solid phases present with each other as well as their local environment, results in formation of secondary minerals. Although the secondary materials may comprise only a small volume fraction of the waste, they (1) tend to increase in amount with time, as weathering processes proceed, (2) typically form at grain surfaces and are thus physically liable to react with percolating gas or liquids, and (3) may exhibit sites suitable for sorption or crystallo-chemical incorporation of trace elements (see Donahoe, 2004). Frequently observed secondary minerals include jarosite and ettringite the former is known to sorb ions such as Mn and As, whereas ettringite can form solid solutions, in which SO4 is replaced by Cr04 (Kumarathasan et al. 1990). 

Many mineral species are known to be selectively crystallized by the presence of bacteria. Carbonate minerals, such as calcite, aragonite, hydroxycalcite, and siderite oxide minerals, such as magnetite and todorokite oxalate minerals, such as whewellite and weddellite sulfide minerals, such as pyrite, sphalerite, wurtzite, greigite, and mackinawite and other minerals, such as jarosite, iron-jarosite, and g3q>sum, are known to precipitate in the presence of bacteria. Therefore, investigations have been developed to analyze the formation of banded iron ore by the action of bacteria, and to analyze the ancient environmental conditions of the Earth through the study of fossilized bacteria. 

Long D. T., Fegan N. E., McKee J. D., Lyons W. B., Hines M. E., Macumber P. G., et al. (1992b) Formation of alunite, jarosite and hydrous iron oxides in a hypersaUne system Lake Tyrrell, Victoria, Australia. In The Geochemistry of Acid Groundwater Systems (eds. W. E. B. Lyons et al.). Chem. Geol. 96, 183-202. 

Re-solution of precipitated ferric hydroxide consumes hydrogen ions. These reactions provide a buffering effect against rapid pH fluctuations by maintaining the pH in the range 2.0—2.2 due to precipitation and resolubilization of ferric iron (Duncan and Walden, 1972). Other acid-generating reactions which involve ferric and other ions are the formation of jarosites (eqn (3)) ... 

The fundamental chemistry associated with the thermodynamics of formation of the alkali jarosites (M = Na, K or NH4) has been studied, and it has been found that substitution of the alkali ions by Ag" " or 1 /2 Pb + produces jarosites that are often formed during the processing of lead or zinc ores, which generally contain silver as a valuable byproduct. In addition, partial or complete substitution of iron(III) by divalent metal ions can occur, resulting in the formation of an extensive range of substituted jarosites such as beaverite, PbCuFe2(S04)2(0H)e. The order of incorporation into the lead jarosite structure has been found to be Fe + > Cu " " > > Co " "... 

The main phase in the ZIC is zinc-ferrite, while other phases include ZnO, Zn2Si04, gypsum, Pb-K jarosite, PbS04, ZnS and Fe-hydroxide. Silica (gel) can be formed as described in Equations 2 to 4. The zinc ferrite is free of silica. The mineralogical assays, however, do not provide enough information to correlate the formation of ZIC with the composition of the calcine. 

Hydrolysis reactions remove iron from solution with the formation of hydrated ferric oxide or hydronium jarosite and result in the regeneration of some sulphuric acid. 

Ferric iron in solution exists in several forms. The more importam forms arc Fe(SO)J, FeS04, FEj(OH)2, and FeOH. The sulfate complexes are greatly favot over the hydroxyl complmies. To illustrate the importance of iron complex formation, a typical leach solution will be consideted in equilHnium with precipitateid hydrogen jarosite, having an approximate free-sulfste activity of 0.02. The equilibrium is represented by... 

In the induced jarosite precipitation and primary neutralization processes, this is typically done in-slurry, at temperatures in excess of 80°C. Few issues with scale formation occur, apparently due to the presence of a large surface area available from the leach residue solids, relative to equipment surfaces. It is important to provide adequate retention time for the complete reaction of the limestone, to avoid reactions continuing to occur in downstream thickeners and pipework. Such reactions can lead to both process upsets and scale formation over the longer term. 

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