Jarosite precipitation processes

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

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 ... 

Today, hydrometaUurgy is well established as the principal method for extraction of many important industrml metals. Hydrometallurgy for the direct treatment of base metal sulfide concentrates, as a widely used technology, must yet prove itself. The roast-leach electrowinning of zinc is a noteworthy exception and is evolving as standard practice hi the zinc industiy worldwide. Relatively recent developments by way of jarosite and iron oxide hydrolysis and precipitation processes have improved recovery and helped secure zinc hydromemllurgy as standard in the industiy.w... 

With this Hematite process, the step of jarosite precipitation is avoided (v. Rbpenack... 

Saprolite slurry is then added. Much of the magnesium and nickel is leached, causing the free acid to drop to a level where jarosite precipitation begins, regenerating acid and furthering the leach process. Jarosite Precipitation and acid regeneration are represented by equation 3, with the saprolite leach by equation 4. 

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. 

Atmospheric Leach and Precipitation Processes. Aluminum is completely precipitated at the pH used for ferrous iron oxidation. The precipitated aluminum reports to the residue recycle stream and is releached in the Induced Jarosite Precipitation stage. There is therefore the risk of a recirculating load building up in the circuit between the Induced Jarosite Precipitation and Secondary Neutralization circuits. However, the pH of the intervening Primary Neutralization step is sufficiently high to precipitate excess aluminum and provide a safe bleed from the process. 

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. 

Sulfuric acid atmospheric leaching of laterites with jarosite precipitation has proven itself to be a versatile process, capable of treating both limonites and saprolites and variable ratios of these two ore types. 

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. 

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 ... 

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... 

The selective hydrolysis of metal ions to produce various forms of hydrated oxides is the most widely used form of precipitation. In particular, the removal of iron from hydrometallurgical process streams is a continuing problem. Iron enters the circuit as a constituent of a valuable mineral, such as chalcopyrite (CuFe2), or an impurity mineral, such as the ubiquitous pyrite or pyrrhotite. So far, effective removal of the iron has been achieved by the precipitation of iron(III) as jarosite (MFe3(S04)2(OH)6),401 goethite (FeOOH)402 or hematite (Fe203).403... 

The jarosite process separates icon(III) from zinc in acid solution by precipitation of MFe2(0H)g(S0 2 where M is an alkali metal (usuaUy sodium) or ammonium (see Fig. 2) (40,41). Other monovalent and hydronium ions also form jarosites which are found in the precipitate to some degree. Properly seeded, the relatively coarse jarosite can be separated from the zinc-bearing solution efficiently. The reaction is usuaUy carried out at 95 0 by adding ammonia or sodium hydroxide after the pH has been adjusted with calcine and the iron oxidized. The neutral leach residue is leached in hot acid (spent + makeup) with final acidity >20 g/L and essentiaUy aU the zinc, including ferrite, is solubilized. Ammonium jarosite is then precipitated in the presence of the residue or after separating it. If the residue contains appreciable lead or silver, they are first separated to avoid loss to the jarosite waste solids. Minimum use of calcine in jarosite neutralization is required for TnaxiTniiTn recovery of lead and silver as weU as zinc and other metals. 

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