Atmospheric leach

POSTER TITLE Atmospheric Leaching of Enargite in Chloride Media... 

Chloride leaching of copper sulfides has received a great deal of attention in past decades. The chloride processes have distinct advantages over sulfate-based processes. The chloride systems have faster kinetics, making atmospheric leaching... 

ATMOSPHERIC LEACHING OF ZINC SULPHIDE CONCENTRATES USING REGENERATED FERRIC SULPHATE SOLUTIONS... 

C.J. Perron, New Atmospheric Leach Process for Copper Sulphide Ores and Concentrates , Cooper 99 Cobre 99. Volume IV. The Minerals, Metals and Materials Society, Warrendale, PA, U.S.A., 1999,151-165. 

COMPARISON OF DIRECT PRESSURE LEACHING WITH ATMOSPHERIC LEACHING OF ZINC CONCENTRATES... 

Both pressure leaching and atmospheric leaching processes convert the sulphide sulphur in the feed zinc concentrate to elemental sulphur, rather than to sulphur dioxide, decoupling the... 

In the atmospheric leach process, a high iron concentration is required and this is provided by an appropriate recycle of iron precipitates. Upon contact of the iron oxide residue with the spent electrolyte, which contains sulphuric acid, ferric iron is dissolved as follows, depending on the species of iron oxide. 

Comparative pressure leach and atmospheric leach tests and residue treatment tests were carried out in the Dynatec laboratory in Fort Saskatchewan in 1999 and 2000. The procedures and results arc summarized in the following sections. 

The pressure and atmospheric leach tests were performed in a 3.S L titanium autoclave equipped with baffles, dual axial impellers and a water cooled condenser. The operating conditions are presented in Table III. Samples were taken throughout and at the completion of the leach tests. 

Table III - Conditions in the Pressure and Atmospheric Leach Testwork...

The particle sizes of the zinc concentrates fed to the pressure leach tests were all in the range of 90% passing 32 to 38 pm. Greater than 98% zinc extraction was achieved with all three concentrate feeds within 60 minutes. The third concentrate was very reactive under pressure leach conditions, achieving 99.1% zinc extraction after only 30 minutes. The three concentrates were also subjected to atmospheric leaching. The atmospheric leach tests results are summarized in Table VI. 

The residues from the atmospheric and pressure leach tests were floated and the flotation solids were analyzed for zinc. Based on these assays, and the weights and assays of the feed to the leaching step, the overall deportment of zinc to the flotation tailings was calculated and is presented in Table VII. The overall deportment of zinc to tailings was in the range of 0.8 to 2% in the atmospheric leach tests, and in the range of 0.1 to 0.5% in the pressure leach tests. 

Final solutions from both pressure and atmospheric leach tests were submitted for chemical analysis, and the results are presented in Table VIA. The concentrations of cobalt, nickel and selenium in the final solutions were not significantly different. Arsenic, antimony and tellurium levels were about the same or lower in the solution generated in pressure leaching (the arsenic concentration was substantially lower in the pressure leach liquor for... 

Concentrate A). Both germanium and thallium were not consistently lower or higher in the solution generated from either type of leach. The iron concentration in solution, which must be precipitated in an iron rejection circuit, is significantly higher in the atmospheric leach solution. 

The sulphidic portions of the leach residues were subjected to sulphur melting and hot filtration. The arsenic and mercury contents of the elemental sulphur byproducts originating from the pressure leach treatment of the concentrates were consistently lower than the arsenic and mercury contents of the elemental sulphur originating fi om atmospheric leaching. The selenium content was about the same or lower in the elemental sulphur originating from pressure leaching compared to atmospheric leaching. 

It should be noted that none of the concentrates tested by Dynatec yielded 95% zinc extraction in less than 24 hours in an atmospheric leach. Based on the results for the different concentrates presented in this paper, the extraction in the atmospheric leach is assumed to be 95%, compared with 99% extraction in the pressure leach. The overall zinc recoveries are assumed to be 1% lower in each case. The difference in zinc recovery between pressure and atmospheric leaching is 1,600 t/y, which must be supplied with additional zinc concentrate at a cost of 0.53 million per year. 

Further, it has been assumed that the concentrate would require grinding to 90% passing 44 pm for pressure leaching, and 90% passing 20 pm for atmospheric leaching. The additional grinding in a vertical stirred mill is estimated to cost 0.66/t concentrate, or 56,000 per year. 

The atmospheric leach circuit for Concentrate 1 is estimated to consist of five 5.6 m diameter by 7.7 m tall tanks, equipped with 75 kW motors. A 60 t/d oxygen plant is included. The overall installed capital cost for this circuit is estimated at 7.07 million. 

The atmospheric leach circuit, by virtue of the large amount of heat lost with the vent gases, requires a substantial steam addition to maintain the energy balance. The comparison of the operating costs is shown in Table XL The operating costs of the two circuits are roughly comparable, excluding the steam. 

Operating Cost Item Atmospheric Leach Pressure Leach... 

Solutions from both types of leaching were submitted for limited minor impurity analyses. There were no major differences, in terms of minor impurities, between the solutions generated from atmospheric or pressure leaching. The higher levels of iron in the atmospheric leach solution will obviously have to be precipitated at some point in the process. More efficient iron precipitation and a higher level of zinc pick-up are benefits of pressure leaching. 

The impurity content of the elemental sulphur filtrate produced from the pressure leach residues was slightly lower than that produced from the atmospheric leach residues. The elemental sulphur produced from the pressure leach residues contained less arsenic and mercury and about the same level of selenium in comparison to the elemmtal sulphur produced from the atmospheric leach residues. 

For the zinc concentrates tested by Dynalcc an expansion of an existing roast-leach-electrowin plant using pressure leaching offers economics that arc favourable in comparison to those of an atmospheric leach. 

Keywords Nickel Laterite, Atmospheric Leaching, Jarosite Precipitation, Pressure Acid... 

In the late 1990 s a series of flowsheets were proposed and tested involving simultaneous atmospheric leaching, iron precipitation and acid regeneration. These were considered particularly suited to sequential leaching of limonite and saprolite ores. A number of laterite sulfuric acid leaching processes utilising this chemistry have been put into commercial operation. 

Freeport considered atmospheric leaching for the Moa Bay project, but noted that iron dissolution was high [3] ... 

Development of the Jarosite process in the zinc industry in the 1960s led to an understanding that iron could be precipitated from nickel laterite atmospheric leach solutions, at moderate acid levels, by the addition of an alkali such as potassium, sodium, or ammonia, whilst maintaining a temperature in excess of 90°C [4]. 

Figure 1. AMAX Saprolite Atmospheric Leach Flowsheet [14],...

Amongst the options considered were heap leaching [17], agitated tank atmospheric leaching [18, 19,20, 21] and combinations of PAL and atmospheric leaching [22, 23, 24]. 

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