1 Fine Particle Processing Separation methods in mineral processing. Fine particles in mineral processing. Flotation Entrainment. SEPARATION METHODS IN MINERAL PROCESSING Gravity Magnetic Electric Phys1cochem1cal Concentration Separatfon Separation Separation in Dense in Water in A1r Media Figure 1. Classification of separation methods. Flotation is the most important separation method in the group of the physicochemical methods. It is based on differences in surface properties of treated minerals while the other methods are based on differences in bulk properties such as density, magnetic susceptibility, etc. Figure 2. Illustration of the flotation separation process in which hydrophobic particles(the particles which attach to bubbles)are separated from the hydrophilic particles(the particles which do not attach to bubbles)
1 Fine Particle Particle Particle Particle Processing Processing Processing Processing Separation Separation Separation Separation methods methods methods methodsin mineral mineral mineral mineral processing. processing. processing. processing. Fine particles particles particles particlesin mineral mineral mineral mineral processing. processing. processing. processing. Flotation Flotation Flotation Flotation. Entrainment Entrainment Entrainment Entrainment. Figure 1. Classification of separation methods. Flotation is the most important separation method in the group of the physicochemical methods. It is based on differences in surface properties of treated minerals while the other methods are based on differences in bulk properties such as density, magnetic susceptibility, etc. Figure 2. Illustration of the flotation separation process in which hydrophobic particles (the particles which attach to bubbles) are separated from the hydrophilic particles (the particles which do not attach to bubbles)
2The separation results strongly depend on particle size and all methods can handle onlyparticles ofagiven sizerange.Figure 3. Optimal particle size ranges for different operationsUnit operations in the mineral processing plant can broadly be divided into four distinctgroups:comminution (that also includes classification),separation (beneficiation, concentration),dewatering of the beneficiation products, and water clarification (Fig. 4).COMMINUTIONCONCENTRATIONWATERCLARIFICATIONPRODUCTDEWATERINGFig.4.Unit operations in mineral processingplant circuits
2 The separation results strongly depend on particle size and all methods can handle only particles of a given size range. Figure 3. Optimal particle size ranges for different operations. Unit operations in the mineral processing plant can broadly be divided into four distinct groups: comminution (that also includes classification), separation (beneficiation, concentration), dewatering of the beneficiation products, and water clarification (Fig. 4). Fig. 4. Unit operations in mineral processing plant circuits
3The separation processescanonlyyield satisfactoryresults if mineral particles in thefeedareliberated.This isachievedbysizereductionoftherun-of-minerockdowntotheliberationsize. In the lowgrade ores -which are now mined -the valuable minerals are disseminated in theform of very fine grains and thus to achieve liberation the rock must by crushed and grounddown to this liberation size (Fig. 5).Figure 5. Liberation size of the grains of valuable mineral.Flotation,however,cannotefficientlydeal withveryfineparticles.10090hei80700040ChalcociteTime1minFrotherPPG400O-KEXOKE0m01000AverageParticleSize(um)Figure 6.Response ofchalcociteto collector addition (potassium ethyl xanthate)(afterTrahar,1981)
3 The separation processes can only yield satisfactory results if mineral particles in the feed are liberated. This is achieved by size reduction of the run-of-mine rock down to the liberation size. In the low grade ores - which are now mined - the valuable minerals are disseminated in the form of very fine grains and thus to achieve liberation the rock must by crushed and ground down to this liberation size (Fig. 5). Figure 5. Liberation size of the grains of valuable mineral. Flotation, however, cannot efficiently deal with very fine particles. Figure 6. Response of chalcocite to collector addition (potassium ethyl xanthate) (after Trahar, 1981)
4esrr604020C51020501005002Averageparticlesize(um)Figure 7. Recovery of siderite as a function of particle size. Recovery in the presence of frotherbutwithoutcollector()isassumedtobebyentrainmentonlywhereasrecoverywithcollectorand forther () is by entrainment and true flotation."The difference curve (dotted line)estimates recovery due to the “"true flotation" only. (after Warren and Trahar, 1981)Whileparticles in the-100+20um sizerange floatmostlybya trueflotationmechanism, fine particles can also be transported to a froth product by entrainment. Therefore,flotation of very finely ground flotation feeds is not only difficult and slow it is also veryunselective(1)P:P=probability of flotation,P,=probability of particle-to-bubble collisionPa=probability of particle-to-bubble attachment, P,=probability of the formation of stableparticle-bubble aggregate.The first factor, the probability of collision, is purely hydrodynamic,it depends onparticle and bubble sizes and hydrodynamic conditions prevailing in the flotation cell. For veryfine particles the probability of collision is very small.The Ps, is entirely determined by the wettability of the mineral particles and also by their densityIt is, therefore, proportional to the contact angle, the larger is hydrophobicity (the larger is thecontact angle) the more likely it is that the formed particle-bubble aggregate will be stableenough to withstand shearing forces in the flotation cell
4 Figure 7. Recovery of siderite as a function of particle size. Recovery in the presence of frother but without collector (●) is assumed to be by entrainment only whereas recovery with collector and forther (■) is by entrainment and “true flotation.” The difference curve (dotted line) estimates recovery due to the “true flotation” only. (after Warren and Trahar, 1981). While particles in the -100 + 20 μm size range float mostly by a true flotation mechanism, fine particles can also be transported to a froth product by entrainment. Therefore, flotation of very finely ground flotation feeds is not only difficult and slow it is also very unselective. P = (1) P = probability of flotation, Pc = probability of particle-to-bubble collision, Pa = probability of particle-to-bubble attachment, Ps = probability of the formation of stable particle-bubble aggregate. The first factor, the probability of collision, is purely hydrodynamic, it depends on particle and bubble sizes and hydrodynamic conditions prevailing in the flotation cell. For very fine particles the probability of collision is very small. The Ps, is entirely determined by the wettability of the mineral particles and also by their density. It is, therefore, proportional to the contact angle, the larger is hydrophobicity (the larger is the contact angle) the more likely it is that the formed particle-bubble aggregate will be stable enough to withstand shearing forces in the flotation cell
ROTHHASEOCCPHASESLURRYHydrophobicParticles图HydrophilicParticlesFigure 8.Simplified description of flotation kinetics:(1)transfer from the slurry to the froth ofhydrophobic particles (true flotation); (2) transfer from the froth cell over the cell lip; (3) drop-back from the froth to the slurry and (4) entrainment of gangue particles (after Laplante et al.,1989),Fig.9.Schematicillustrationoffrothdrainage.Filledsymbolsrepresenthydrophobic particles, open symbolshydrophilicparticles.It is well established that the stability of the froth critically affects the grade of the frothproduct. If the froth is too stable there is no additional upgrading in the froth.Since entrainment results from the water which is carried on to the forth layer along with all fineparticle (also gangue), the upgrading must be related to the froth stability and drainage whichremoves hydrophilic particles back to the pulp
5 Figure 8. Simplified description of flotation kinetics: (1) transfer from the slurry to the froth of hydrophobic particles (true flotation); (2) transfer from the froth cell over the cell lip ; (3) dropback from the froth to the slurry and (4) entrainment of gangue particles (after Laplante et al., 1989). It is well established that the stability of the froth critically affects the grade of the froth product. If the froth is too stable there is no additional upgrading in the froth. Since entrainment results from the water which is carried on to the forth layer along with all fine particle (also gangue), the upgrading must be related to the froth stability and drainage which removes hydrophilic particles back to the pulp. Fig.9. Schematic illustration of froth drainage. Filled symbols represent hydrophobic particles, open symbols hydrophilic particles