Comminution leaching

It has often been found that for deriving the optimum results in leaching, it becomes necessary to have the starting feed material pretreated. These pretreatments generally fall in to the following categories comminution, concentration, and pyrotreatments. 

Comminution leads to an increase in surface area and exposes the mineral grains to attack by the solvent. The mineral grains may not, however, be liberated completely from the associated gangue material by this process. As long as a direct contact between some portions of the mineral grain and the leachant is not inhibited or lost, the mineral leaching process proceeds in an uninterrupted manner. 

In the materials processing industry, size reduction or comminution is usually carried out in order to increase the surface area because, in most reactions involving solid particles, the rate of reactions is directly proportional to the area of contact with a second phase. Thus the rate of combustion of solid particles is proportional to the area presented to the gas, though a number of secondary factors may also be involved. For example, the free flow of gas may be impeded because of the higher resistance to flow of a bed of small particles. In leaching, not only is the rate of extraction increased by virtue of the increased area of contact between the solvent and the solid, but the distance the solvent has to penetrate into the particles in order to gain access to the more remote pockets of solute is also reduced. This factor is also important in the drying of porous solids, where reduction in size causes both an increase in area and a reduction in the distance... 

The term leaching as used hereafter is taken to mean the dissolving of a constituent of a comminuted material and the separation of the resultant solution from the undissolved portion or residue. Two general methods are in use first, percolation, in which dissolution and the separation of the dissolved material are done coincidently, and second, dissolution by agitation of a finely divided pulp held in suspension and separation of the solids by decantation or filtration or both. 

Unlike fracturing, this approach should cause only moderate disruption of the coal matrix and might not lead to complete comminution, but this method could be envisioned as a prelude to coal grinding. Removal of material from structural boundaries would yield a more friable, "soft" coal structure. The leaching of fine mineral particles from the coal matrix results in a more porous structure and should also improve the coal s grindability. The prior removal of some mineral matter would also reduce the coal s abrasiveness. 

After comminution, soluble impurities either inherent to the raw materials or introduced during processing can be extracted by washing (e g. with water), followed by filtration [6, 23]. Chemical leaching and magnetic separation are also used to purify raw materials. In a more specialized process, a frothing agent can be used to promote differential adsorption of impurity particles onto gas bubbles to separate out the desired product [23]. 

Alunite [Kz Al6(QH)i2 (504)4 is a potential source of potassium sulfate, alumina, and sulfur dioxide byproduct. The economics of production hinge on the value of the alumina. Processing involves ore comminution, roasting, and leaching to recover potassium sulfate solution. Filtered solids can be processed in a Bayer-type process plant for alumina recovery [111]. 

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