Comminution process, minerals

The raw ROM (run of mine) ore is reduced in size from boulders of up to 100 cm in diameter to about 0.5 cm using jaw cmshers as weU as cone, gyratory, or roU-type equipment. The cmshed product is further pulverized using rod mills and ball mills, bringing particle sizes to finer than about 65 mesh (230 p.m). These size reduction (qv) procedures are collectively known as comminution processes. Their primary objective is to generate mineral grains that are discrete and Hberated from one another (11). Liberation is essential for the exploitation of individual mineral properties in the separation process. At the same time, particles at such fine sizes can be more readily buoyed to the top of the flotation ceU by air bubbles that adhere to them. 

The recovery of petroleum from sandstone and the release of kerogen from oil shale and tar sands both depend strongly on the microstmcture and surface properties of these porous media. The interfacial properties of complex liquid agents—mixtures of polymers and surfactants—are critical to viscosity control in tertiary oil recovery and to the comminution of minerals and coal. The corrosion and wear of mechanical parts are influenced by the composition and stmcture of metal surfaces, as well as by the interaction of lubricants with these surfaces. Microstmcture and surface properties are vitally important to both the performance of electrodes in electrochemical processes and the effectiveness of catalysts. Advances in synthetic chemistry are opening the door to the design of zeolites and layered compounds with tightly specified properties to provide the desired catalytic activity and separation selectivity. 

Once the comminution process is completed, the succeeding operations in mineral processing are taken over by what is known as separation. Regardless of the method or methods used, the aim is always the same-to take a natural aggregate of minerals (an ore) and separate it into two or more mineral products. In general, the products of separation are (i) the concentrate which contains the valuable minerals and (ii) the tailings which contain primarily materials of little or no value. It may be borne in mind that minerals have been liberated, either by grinding or by chemical means, must usually be sized prior to... 

See also Minerals recovery/processing Minerals recovery/processing, 16 595-668. See also Minerals concentration Minerals processing classification in, 16 618-622 economic aspects of, 16 606-609 environmental aspects of, 16 609—610 flow sheets in, 16 603-605 materials handling in, 16 660-663 from ocean waters, 17 695—697 ores, 16 598-603 process control in, 16 663-665 size reduction (comminution) in, 16 610-615... 

Flow Sheets. AH minerals processing operations function on the basis of a flow sheet depicting the flow of soHds and Hquids in the entire plant (6,13,14). The complexity of a flow sheet depends on the nature of the ore treated and the specifications for the final product. The basic operations in a flow sheet are size reduction (qv) (comminution) and/or size separation (see Separation, size), minerals separation, soHd—Hquid separation, and materials handling. The overaH flow sheet depends on whether the specification for the final mineral product is size, chemical composition, ie, grade, or both. Products from a quarry, for example, may have a size specification only, whereas metal concentrates have a grade specification. 

Size reduction (qv) or comminution is the first and very important step in the processing of most minerals (2,6,10,20—24). It also involves large expenditures for heavy equipment, energy, operation, and maintenance. Size reduction is necessary because the value minerals are intimately associated with gangue and need to be Hberated, and/or because most minerals processing/separation methods require the ore mass to be of certain size and/or shape. Size reduction is also required in the case of quarry products to produce material of controlled particle size (see Size measurement of particles). In some instances, hberation of valuables or impurities from the ore matrix is achieved without any apparent size reduction. Scmbbers and attritors used in the industrial minerals plants, eg, phosphate, mtile, glass sands, or clay, ate examples. 

A novel comminution device, the roll press, has achieved commercial success [Schoenert, International J. Mineral Processing, 22, 401-412 (1988)]. It is used for fine crushing, replacing the function of a coarse-baU mill or of tertiary crushers. Unlike ordinary roll crushers which crush individual particles, the roll press is choke fed and acts on a thick stream or ribbon of feed. Particles are crushed mostly against other particles, so wear is veiy low. Energy efficiency is also greater than in ball mills. 

Separation processes, as could be seen from Figure 2.1, position themselves at the back end of the sequence in operations in the mineral processing flowsheet. The front-end operations has been found virtually to terminate with the liberation or the size-reduction processes involving crushing and grinding. It is important to limit the amount of size reduction to that at which adequate liberation is accomplished. The term adequacy is related to the cost involved in comminution and to performance of the concentration methods that follows. The concentration is obtained by separation processes which rely on differences in the properties of the particles, the physical and physico-chemical characteristics of minerals. In this context, it will only be relevant to refer to Table 2.5 which presents a summary of the processes along with the properties of the minerals that are exploited. 

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. 

To use natural minerals it is necessary to grind them down to a desired particle size distribution. Grinding can be performed with the minerals dry or slurried in liquid. In most laboratories, this process is performed in a batch jar mill while on an industrial scale, continuous comminution equipment is used in conjunction with size classification equipment to recycle the coarse material. Figure 4.1 shows a typical comminution circuit with classification and recycle steps, as well as separation of the mineral from the conve3ring fluid. 

A)Flash Pulverization mills. This process for drying comminuting minerals w as introduced in 1945 by the Institute of Gas Technology. It was found that a wide variety of friable materials could be pulverized by introducing them into a stream of gas at mod pressure, and causing the stream to pass thru a nozde into a zone of lower pressure. When superheated steam was used as the motive fluid, the product was dried to less than 1% moisture content... 

---