Neutralization leaching

Heap leaching of copper oxides and transition ores and acid/neutral leaching of zinc calcine are well-established processes to generate the feed solutions for the hydrometallurgical recovery of these metals.2... 

Figure 7 An outline of the flowsheet for zinc production by the conventional roast/neutral-leach/electro-...

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). 

In the electrolyte storage and mixing station, the neutral leach solution is mixed with spent electrolyte to adjust the composition of the electrolyte fed to the cellhouse. In the cellhouse, zinc is plated and then it is melted and cast into six-ton ingots. 

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. 

Since the start-up of the zinc plant, the leaching circuit comprised three main steps neutral leaching, hot acid leaching and jarosite precipitation, as shown in the flowsheet of Figure 1. 

An existing 80 m tank was added to low acid leaching to increase the residence time, thus assuring the extraction of the zinc oxide present because of the use of excess calcine in neutral leaching. 

The objective of this modification is to increase the residence time to maximize the oxidation of ferrous ions, contained in the jarosite solution, using pure oxygen. In doing so, this stage also helps in the neutral leaching step in removing some impurities from solution such as arsenic and antimony. 

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. 

The electrolyte fix)m acid leaching advanced through neutral leaching where the majority of the minor elements (As, bi, Ge and Al) were precipitated. A solution of ferric iron was also added to the electrolyte from acid leaching to assist minor element precipitation. A precipitate, or preconcentrate, was separated from the electrolyte in a thickener, filtered and then repulped and advanced to the indium-germanium plant. 

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. 

The above strategies (iron distribution, acid leaching and the modified neutral leaching) have collectively improved arsenic removal from the circuit, and hence improved the quality of the preconcentrate feed to the indium-germanium plant. With increasing experience in the operation of the new lead smelter, the fiime quality has also improved. This ftirther facilitates solution purification through iron precipitation as well as the production of indium and germanium. 

The solution overflowing the acid thickeners is further neutralized in the neutral leaching step with an excess of calcine to assure good quality electrolyte. The solids are separated in a thickener and are returned to acid leaching for recovery of zinc. The clear overflow electrolyte is forwarded to the purification circuit. 

Modification of the present leaching installation to different stages for neutral leaching and neutralisation... 

The existing installation will be modified to be used as the neutral leaching and neutralisation stages, with independent circuits for concentrates with high and low silver contents. 

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