Neutral leach residue

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. 

For modelling mass flows and the link between concentrate input and neutral leach residue output, a regression analysis using non-parametric methods (in this case, neural networks) was applied. Intensive analysis and preparation of the available plant data (data reconciliation) had to be performed to produce the database used for modelling. 

De-moisturising of the ZIC is defined in three steps (Figure 3). First, the solids are filtered in two parallel drum Alters, and then the solids pass through the centrifuges. The ZIC from the centrifuges contains 20% moisture. (= dry neutral leach residue) The filtrate is filtered in presses to recover the remaining solids, which are not recovered in the centrifuges. This produces a ZIC with 30% moisture (= wet neutral leach residue). 

Plant Flowsheet of Study Ruhr-Zink Neutral Leach Residue Filtratjon... 

It is qualitatively known (6) that increased silica contents in the feed materials could increase the production of neutral leach residue. However, now it possible for Ruhr-Zink to quantitatively examine not only the influence of silica on the production of ZIC but also the synergistic influence of alumina, iron, lead, silica, and zinc on the production of ZIC. 

T.T. Chen, J.E. Dutrizac and C. Canoo, Mineralogical Characterization of Calcine, Neutral Leach Residue and Weak Acid-Leach Residue from the Vieille-Montagne Zinc Plant, Balen, Belgium , Trans. Instn. Min. Metall.. Vol. 102,1993, C19-C31. 

C. Mattich, K. Hasselwander, H. Lommert and A.N. Beyzavi, Electrolytic Zinc Manufacture with Waelz Treatment of Neutral Leach Residues , Zinc and Lead Processing. J.E. Dutrizac, J.A. Gonzalez, G.L. Bolton and P. Hancock, Eds., Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, Canada, 1998, 561-578. 

The roaster product is lcachcd with spent electrolyte 1 sulfuric acid) under near-neutral conditions to dissolve most of the zinc, copper, and cadmium, but little of the iron. The leach residue solids are releached in hot, strong add to dissolve more zinc, since it attacks the otherwise insoluble zinc femtes. The iron which is also dissolved in this second leach is then precipitated as jarosite, goethite, or hematite. The development of these iron predpitation techniques permitted the use of the hot. strong acid leach and an increase 111 zinc extraction from about 87% to greater Ilian 95%. Simultaneously, the hot acid leach frequently generates a leach residue rich enough in lead and silver to provide significant byproduct value, as well as increased recovery of cadmium and copper. 

The sulphide leaching plant (SUP) receives feed from all of the fiont-end zinc plants roasters, ZPL, and OLP. This plant treats calcine, ZPL slurry, and OLP electrolyte using a weak acid and neutral leaching process to produce impure SLP electrolyte and residue. The residue consists mainly of zinc foiites, paragoethite, jarosites, lead sulphate, as well as coprecipitated impurities. The residue slurry is fed to the lead smelter. 

Zinc residue from neutral leaching is treated in the SO2 leaching stage with spent electrolyte, and SO2 gas as the reductant for zinc ferrite. The Zn, Cd and Fe are dissolved, though Au, Ag, Pb and Cu remain in the residue. This residue is delivered to a copper smelter as a Cu and Pb raw material. The solution after SO2 leaching is neutralized in two stages with... 

The traditional oxide leach plant consisted of an acid leaching step, several neutral leaching steps and, until recently, a zinc dust purification step to remove cadmium, as shown in Figure 3. In the old scheme, most of the fume and various recycle streams were leached in spent acid in the acid leach step. The resulting acid slurry was thickened, and the lead-rich solid residues were filtered, repulped in water, combined with the residue slurry from the calcine leaching plant and pumped to the smelter. 

Ferric iron solution is now continuously added to both acid and neutral leaching. This has resulted in higher iron utilization and enhanced impurity precipitation efficiency. The addition of soluble iron to the acid leach promotes the precipitation of ferric arsenate, enhancing arsenic rejection to the residues and dramatically lowering the arsenic levels in the plant electrolytes. Previously, the arsenic concentration in the acid leach electrolyte was as high as 5 g/L. The arsenic concentration is now below one gram per liter. 

The pH range of acid leaching and the iron concentration in solution are controlled in order to maximize the precipitation of arsenic from solution. It is presumed that iron-arsenic compounds are formed, which are not soluble in acid. The increased rejection of arsenic to the residue at this stage also helped to improve the efficiency of impurity metals (including germanium) removal in the subsequent neutral leaching step. 

Changes in neutral leaching pH and the addition of iron solution were made to optimize the precipitation efficiency of impurity metals (including germanium) while minimizing the acid soluble content in the leach residue. 

In this rqrplication to recover iron from a neutral leach zinc residue, a DEHPA extractant is used in conjunction with metallic zinc as the active metal to reduce the ferric ion. Although DEHPA is an excellent solvent for the separation of ferric ion, the subsequent stripping stq> is difficult. Conversely, the stripping of ferrous ion from DEHPA is easily accomplished, even with dilute acid solutions in the range of pH 1.5 to 2.0. However, the concentration of ferrous ion that could be obtained using galvanic stripping was not known. Consequently, one of the primary objectives of this study was to determine if a relatively concentrated iron solution could be produced. 

Saprolite leach discharge is partially neutralized with recycled Secondary Neutralization Precipitation residue slurries, simultaneously recovering the metal values associated with those streams. The residual acid is then further reduced to approximately 5 g/L H2SO4 using ground limestone slurry. Temperature is maintained at 95°C. 

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. 

Jarosite containing nickel laterite leach residues are chemically stable if kept in contact with alkali (sodium) containing liquors. Jarosite breakdown remains manageable, even in the absence of the stabilizing alkali ions, particularly where excess acid neutralizing reagent has been added, as occurs when processing via the mixed hydroxide process route. 

This step produces, after dissolution is completed, a mother Kquor that consists of a neutral zinc sulfate solution. Before electrowinning, the liquor is then purified especially from interfering cations such as Fe(lll), which is removed by precipitating the hydroxide, FefOH), while other more noble cations such as Cu(II) and Ni(II) are reduced by a redox reaction or cementation by adding zinc powder to the solution. The undissolved zinc ferrite, as the major component, together with silica, gypsum, lead, and silver sulfates, forms the neutral leach ferrite residue. 

The residue is leached to give cesium sulfate solution, which can be converted to cesium chloride by ion exchange on Dowex 50 resin and elution with 10% HCl, treatment using ammonia or lime, to precipitate the alurninum, or by solvent extraction, followed by purification at neutral pH using hydrogen peroxide or ammonia. 

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