Leaching processes, agitators

Although the overall processes have evolved for specific metals, and leachants, there are principally only two leaching techniques that provide the pregnant leach solution for metal recovery percolation leaching and agitated tank leaching. 

The parameters that control the leaching process are as follows temperature, carbon dioxide pressure, agitation, slurry density, particle size, and leachant composition. In general the effects of these parameters on a slurry of Mg(OH)2 are as follows ... 

Three particle collision mechanisms can occur in an agitated vessel. These are (a) particle-vessel, (b) particle-impeller and (c) particle-particle. Most of the work on collisions has been related to secondary nucleation, but there are other systems where mechanical abrasion following impact may occur and may be undesirable, e.g. breakdown of friable catalysts, or in mammalian cell culture on microcarriers or desirable, e.g. removal of an impervious outer skin which forms on ore particles during some leaching processes. 

It is frequently possible to design the leaching process on a continuous co-current basis, where acid, water and ore are fed into a vessel continuously, or at regular intervals, and discharged in a similar manner. In these circumstances the agitation system is usually designed to suspend the solids sufficiently in the liquid phase so as to allow them both to overflow from... 

Finely ground solids may be leached in agitated vessels or in thickeners. The process can be unsteady-state batch or the vessels can be arranged in a series to obtain a... 

A detailed laboratory study has been presented by Dresler et al. (1998) on the Roast, Acid-Leach Process that exactly follows the process previously used by the LCA (now EMC) at their North Carolina deposit. It is reviewed here because of the additional details that it supplies on this very effective spodumene process. Their ore was from Wekusko Lake, Manitoba, which as at LCA s deposit was an unzoned and low-grade (0.79% Li) spodumene ore, but with a very small crystal size. Electron micrographs showed that each of the minerals in the ore (Table 1.18) was present as discrete crystals, but the liberation size was quite small. Consequently, they ground the ore to a —212 p,m (—65 mesh) size, and made into a 23% slurry to be agitated and conditioned with 2 kg of sodium hydroxide/mt of ore for 20 min. 

Modem manufacturing processes quench the roast by continuous discharge into the leach water held in tanks equipped with agitators. At this point the pH of the leach solution is adjusted to between 8 and 9 to precipitate aluminum and siHcon. The modem leaching operations are very rapid because no or htde lime is used. After separation of the ore residue and precipitated impurities using rotary vacuum filters, the cmde Hquid sodium chromate may need to be treated to remove vanadium, if present, in a separate operation. The ore residue and precipitants are either recycled or treated to reduce hexavalent chromium to Cr(III) before disposal. 

By way of an example, one may consider the case of hydrometallurgical reactors. Leaching is the most important of the different unit operations, and is prominently placed and assigned due emphasis in a typical hydrometallurgical process flowsheet. A representative list of the various types of reactors used for agitation leaching is given in Table 1.21. 

In liquid-solid extraction (LSE) the analyte is extracted from the solid by a liquid, which is separated by filtration. Numerous extraction processes, representing various types and levels of energy, have been described steam distillation, simultaneous steam distillation-solvent extraction (SDE), passive hot solvent extraction, forced-flow leaching, (automated) Soxh-let extraction, shake-flask method, mechanically agitated reflux extraction, ultrasound-assisted extraction, y -ray-assisted extraction, microwave-assisted extraction (MAE), microwave-enhanced extraction (Soxwave ), microwave-assisted process (MAP ), gas-phase MAE, enhanced fluidity extraction, hot (subcritical) water extraction, supercritical fluid extraction (SFE), supercritical assisted liquid extraction, pressurised hot water extraction, enhanced solvent extraction (ESE ), solu-tion/precipitation, etc. The most successful systems are described in Sections 3.3.3-3.4.6. Other, less frequently... 

Agitation, pilot plant, 18 730-731 Agitation leaching, 16 153 Agitation process, 9 165 Agitator dryers, 9 131-133 Agitators... 

Leaching has in the past been carried out mainly as a batch process although many continuous plants have also been developed. The type of equipment employed depends on the nature of the solid—whether it is granular or cellular and whether it is coarse or fine. The normal distinction between coarse and fine solids is that the former have sufficiently large settling velocities for them to be readily separable from the liquid, whereas the latter can be maintained in suspension with the aid of only a small amount of agitation. 

The use of chemical modelling to predict the formation of secondary phases and the mobility of trace elements in the CCB disposal environment requires detailed knowledge of the primary and secondary phases present in CCBs, thermodynamic and kinetic data for these phases, and the incorporation of possible adsorp-tion/desorption reactions into the model. As noted above, secondary minerals are typically difficult to identify due to their low abundance in weathered CCB materials. In many cases, appropriate thermochemical, adsorption/desorp-tion and kinetic data are lacking to quantitatively describe the processes that potentially affect the leaching behaviour of CCBs. This is particularly tme for the trace elements. Laboratory leaching studies vary in the experimental conditions used (e.g., the type and concentration of the extractant solution, the L/S ratio, and other parameters such as temperature and duration/ intensity of agitation), and therefore may not adequately simulate the weathering environment (Rai et al. 1988 Eary et al. 1990 Spears Lee, 2004). 

BioCOP [Biological COPper] A bacterial process for leaching copper from its ores. BioCOP uses an agitated tank for oxidation and leaching of copper sulfides. Developed by BUP Billiton and Codelco, which operated a demonstration plant in Chuquicamata, Chile, from 2003. Several plants have been operated commercially in Chile since then. 

Ultrasonic energy is frequently used to accelerate the dissolution of solid samples under soft conditions of temperature, pressure and chemical reagents. Similar to direct dissolution by agitation, US-assisted soft digestion is not used to the same extent as other operations of the analytical process such as leaching, derivatization or detection. The simplicity of this operation with some types of samples and the operator s lack of awareness of its error contribution are responsible for the absence of optimization studies for this process. Inappropriately conducted soft digestion can result in major errors and affect the quality of the results. 

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