Chapter- 1 INTRODUCTION

Transcription

1 18 Chapter- 1 INTRODUCTION

2 19 INTRODUCTION Gravity separation has been the best-proven and accepted technique for beneficiation of minerals for more than a century. It has gained wide acceptance in the industry as a primary form of mineral concentration technique for treating coarse size ranged material because of high separation efficiency and low cost. Concentrating minerals by gravity separation has always been the first consideration in any flow sheet development program and features in many flow sheets where there is sufficient difference between the specific gravities of valuables and gangue. In the last one-and-half decade, there has been tremendous improvement in the design of new gravity separators for their adaptation to beneficiate fine particles. Most gravity separators separate minerals of different specific gravities based on their relative movement in response to gravitational and/or other forces. The latter often being the resistance to motion offered by a viscous fluid, such as water or air. For treating the fine particles where the fluid medium is water, the specific gravity (SG) of the fluid approaches close to the SG of the light particle and hence better separation efficiency is required. Normally, the concentration criteria (CC) value exceeding 2.5 is preferred for adoption of gravity separation process. The ratio value falling between 2.5 to 1.25 needs support of centrifugal force and hence enhanced gravity separation techniques came into the market. It is important to note that the value of CC in turn is guided by both particle shape and size factors. The shape factor can be related to the relative ratios of the terminal settling velocities (V m ) of the particle in consideration to that of a spherical particle of the same size. The commonly used units for fine particle processing are spiral concentrators, wateronly cyclones, multi gravity separators, floatex density separators (FDS), reflux classifiers and Kelsey jigs etc. Significant amounts of fines are generated during mechanized mining and processing of different minerals and fines. Mechanized mining and the nature of ore body result in fines being generated in large quantities. In addition to the above, there are various sources for

3 20 the generation of fines, which needs to be, processed (Table 1.1). Further, during crushing and sizing, friable iron ores of India have a greater tendency to break down into micron sizes as compared to lumpy hard ores. About 15% of the mined ore is converted into this fraction. Handling of these fines is a challenge to the mineral industry also causing environmental issues like safe disposal, lack of space and maintaining these slime ponds etc. Also, there is a threat for closer of the mines due to non-availability of suitable place to dump. In addition to this, there is a necessity to grind different industrial minerals such as chromite, manganese, dolomite, etc. for better liberation to below 1mm particle size. These fines need to be classified at different stages prior to concentration process. There are various concentration/separation processes viz. gravity concentration, flotation, etc. which are commonly used for such particle size range. In view of the above, effective fine particle classification is becoming complicated using conventional classifiers in the processing plants designed to handle/treat intermediate size range of particles. Therefore, there is a need for development of an efficient classifying unit operation for the recovery of intermediate fine particles. Screens are used for coarser sizes while cyclones are applied to ultrafine sizes. There are few conventional classifiers such as rake classifier, spiral classifiers, hydrosizer, etc used for this particle size range. Nevertheless, the classification as well as separation efficiency of these units are very low and in turn, affecting the downstream operations. Literature cited on application of conventional hindered settling classifier reveal that, these classifiers suffer from limitations of low capacities and separation efficiencies (Elder et al, 1999; McKnight, et al, 1996; Westerfield, 2004; Venkatraman, et al., 2000). Despite significant research work in the area of hindered settling classification, the focus towards the FDS or Teeter Bed Separators has to be amplified. The detailed study of these unit operations has shown the existence of an avenue to improve the classifying performance further more. The FDS was designed and manufactured as sand classification equipment and processing to meet the majority requirements of glass, foundry and construction sand producers, in combination with certain machines for incorporation in heavy mineral gravity separation

4 21 plants. It is basically a hindered settling classifier employing the principles that have been used for many years but bringing together high efficiency and high capacity. FDS have been well accepted throughout the glass sand industry where stringent size specification is being imposed to reduce its melting time by reducing the top size being fed to the furnace. Further, it is capable to remove free silts/slimes. Hindered settling can provide a small degree of attrition. Moreover, for many years, hindered settling classifiers have been used for the concentration of heavy minerals but these machines have limited capacity. With the inception of FDS, such constraints were addressed in a limited way. However, in the recent past, there have been considerable increased demands for application of FDS as a pre-concentrator and as a cleaner in conjunction with spirals to minimize the number of spiral stages. There has been demand for its increased utilization for improving the efficiency of the downstream gravity concentrating circuits. J Elder et al., 1999 demonstrated that the floatex-spiral circuit jointly could produce a higher-grade product with 99.6% recovery for heavy minerals concentration. This synergistic benefit was attained by combining the FDS with spiral concentrators. Some workers have expressed satisfaction on performance of the FDS (Galvin et al., 1999b; Kari et al., 2006; Sarkar et al., 2008a, b). Theoretical studies adopting relative velocity approaches were also taken up to predict the performance of the FDS by a few group of scientist (Das et al., 2009a,b; Kapure et al., 2007; Galvin et al., 1999a; Kim and Klima, 2004). These studies were based on simplified role of autogenous suspension generated inside the FDS. The combined effect of liquid fluidization and autogenous dense medium generation renders the problem extremely complex. FDS is an outcome of the experience gained by the working engineers over the years in the plant, essentially designed to utilize the difference in settling velocities of particles. In this separator, the separation is sought due to difference in settling rates witnessed by differences occuring in both size and density of particulate solids (which eventually means the mass of a paritcle) against a raising current of water that divides the single stream feed into two product streams. Although there have been several research papers published on floatex density separators but the exact information on the proper understanding of the

5 22 influence of process variables with the feed material characteristics is still lacking. Hence, the main objective of the present research work is to identify the key process variables affecting performance and analyze their influence on separation while treating fine particles. Further, a model will be developed to predict the performance of the FDS. Table 1.1 Fines generation factors evaluated for different mining operation (After Ghosh, 2006) S.No. Source Emission factor 1 Overburden excavation scraper loading No data Shovel excavation 1.0 to 3.0 kg/t Bucket wheel excavation 0.7 to 2.0 kg/t Loading in vehicles 0.4 to 0.7 kg/t 2 Transportation Conveyor belt 0.1 to 0.5 kg/t Each transit point Dumper 1.5 to 3.0 kg/km of tunnel earthen dry surface Total emission will be 0.1 to 3.0 kg/km of tunnel solid surface kg/vehicle/km/d 3 Unloading & piling conveyor 0.8 to 1.5 kg/t system Dumper - Bull dozer 1.5 to 4.0 kg/t 4 Mineral excavation Bucket 0.5 to 1.0 kg/t excavators Shovel 0.8 to 1.5 kg/t Loading conveyor belt 0.08 to 0.1 kg/t each travel point Loading dumper 0.07 to 0.3 kg/t average 5 Transport Conveyor belt 0.05 to 0.1 kg/t Each travel point Dumper/Truck 1.5 to 3.0 kg/km of travel dry surface Dumper/Truck 0.2 to 0.5 kg/km of travel by soiled road 6 Stock piling/loading conveyor 1.0 to 1.5 kg/t Dumper/manual 1.5 to 4.0 kg/t 7 Size reduction Jaw crusher 1.5 to 2.5 kg/t Screening 2.5 to 5.0 kg/t Loading 0.8 to 1.5 kg/t Stock piling and retrieving 1.0 to 4.0 kg/t

6 23 Important Points to Note 1. The author in the present investigation, while working on the floatex Density Separator has limited to vary the levels of the variables at two/three values only. In order avoid large number of experiments, which otherwise the thesis would become voluminous. 2. The above approach has been adopted to study different types of materials (synthetic materials) having different densities (low, medium and high) for better understanding of separation characteristics of the fine particles in FDS. 3. In most of the analysis approach of surface plots using has been adopted owing to limited number of data points obtained while plotting of the graphs. Drawing interpretations with two or three number of data points would result in ambiguity while making the interpretations and drawing conclusions. 4. The synthetic materials used for testing purpose although they are considered to be pure in state but their purity is not 100% but it is nearer to 100%. This approach has been adopted because of the constraints faced in procuring pure materials (of 100% purity) in large amount required for conducting experiments with tests using FDS. 5. However, owing to the point mentioned above (item 4) no chemical analysis on the products generated has been carried out (as they were assumed to be pure in their state). 6. Most of the text in the results & discussion chapter while discussing the effect of variables runs in descriptive form because the analysis of interactional effects considered are of qualitative nature. 7. The author has made his best possible efforts to perform mathematical analysis where ever it is possible and in which ever case the results obtained exhibit a good trend.