Jarosites

Fig. 2. Flow sheet for modified jarosite precipitation process.

In a modification the conversion process, the jarosite residue is hydrothermaHy decomposed to hematite by autoclaving at 220—250°C. This solubilizes zinc and other metal values and the hematite has a potential for iron recovery. Hematite stockpiles are less of a problem than jarosite because hematite is denser and holds up less of the soluble metals. 

The jarosite process controls both alkaU metals and sulfate in the zinc-plant circuit and consumes Htde neutralizing agent. It is used in 16 plants worldwide, which account for ca 25% of the noncommunist world s primary zinc output. 

The goethite process precipitates crystalline aFeO-OH (goethite) as well as PFeO-OH, aFe202, and amorphous phases. The reaction is carried out at 90°C and pH 3.0, for 4—6 h in either batch or continuous fashion, and the iron(III) ion must be kept <1 g/L. Both jarosite and goethite soHds are usually lagooned. 

Jahn-Teller distortions cobalt and copper complexes, 2, 91 hydrates, 2, 308 Jahn-Teller effect, 5, 535 Jahn-Teller theorem, 1, 247 Jarosites... 

Mbssbauer spectra measured by the Opportunity rover at the Meridiani Planum landing site (see Fig. 8.35) revealed four mineralogical components in Meridiani Planum at Eagle crater jarosite- and hematite-rich outcrop (see Eig. 8.34), hematite-rich soil, olivine-bearing basaltic soil, and a variety of rock fragments such as... 

Fig. 8.34 Lefty, outcrop rocks found at the crater wall of Eagle Crater, where the rover Opportunity landed on 24 January 2004. Clearly, the sedimentary structure is seen. Right) in the spectrum, taken on sol 33 (sol = Martian day) of the mission, the mineral Jarosite, an Fe -sulfate, could be identified at the Meridiani Planum landing site. It forms only under aqueous conditions at low pH (< 3 ) and is therefore clear mineralogical evidence for aqueous processes on Mars...

Iron in the feed concentrate is rejected either as unreacted pyrite mixed with elemental sulfur or as jarosites in the leach residue. The pyrite/sulfur mixtures said to be suitable for indefinite storage, but the well known environment effects caused by pyrite weathering are likely to make storage of this material a less than straightforward problem. Besides this, there are problems associated with the disposal of the leach residues from the pressure leach process. 

In the jarosite process, the precipitation of iron occurs from acidic sulfate solutions as one of a group of basic ferric sulfates known as jarosites. The conditions for the precipitation of iron in the specific form of jarosite require a solution pH of about 1.5 and a temperature of about 95 °C. The reaction may simplifiedly be represented as ... 

The conversion process (developed by Outokumpu) is a modification of the jarosite process and involves simultaneously zinc ferrite dissolution and jarosite precipitation in the same reaction vessel. The overall reaction may be represented in simplified form as ... 

The principal differences between the goethite and the jarosite processes take place following the hot acid leaching of the zinc ferrite residues. In the goethite process, the liquor from hot acid leaching, holding (in g l-1) 100 Zn, 25-30 Fe3+ and 50-60 H2S04, is initially subjected to a reduction step, where the ferric iron is reduced to the ferrous form by reaction with unroasted zinc sulfide concentrate at 90 °C ... 

In the ultimate analysis it may be pointed that the aforesaid hydrolysis processes are no doubt technically very satisfactory and tolerable, but environmentally this is not the case. The different processes yield jarosite, goethite and hematite, all of which retain considerable amounts of other elements, especially, zinc and sulfur. The zinc originates mainly from undissolved zinc roast in the iron residues, and sulfur from sulfate, which is either embodied into the crystal lattice or adsorbed in the precipitate. As a consequence of the association of the impurities, none of these materials is suitable for iron making and therefore they must be disposed of by dumping. The extent of soluble impurities present in the iron residues means that environmentally safe disposal not an easy task, and increasing concern is being voiced about these problems. An alternative way of removing iron from... 

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