FlotationColumn-a_novel_technique_in_mineral_processing.pdf

Flotation column – a novel technique in mineral processing S.K. Biswal Regional Research Laboratory, Bhubaneswar – 751013, India. The development of flotation column is the most significant achievement in the area of mineral processing in the last few decades. This related to design, new spargers and wash water delivery systems, on-line gas hold up measurement in slurries, the role of solids on gas hold up, dynamic modelling, neural networks for improved level detection accuracy, scale-up and application of flotation columns. The industrial use of flotation columns in the mineral processing industry has experienced a remarkable growth in practical knowledge in the recent years related to column, design, construction, operation, sparging and control. Key words Flotation column, axial mixing, counter current flow, superficial flow rate, baffle, entrainment in froth, bubble generation, metallurgical perance. 1. Introduction The concept of bubble-column flotation was first developed and applied in the mid-1910s by Flinn and Towne Ghal, 1916 at plant site of Inspiration Copper Company. However, it was not continued because the gas distributor did not work satisfactorily due to settling of solids on it. Subsequently, a detailed investigation was carried out by Boutin and Wheeler in the early 1960s. The first commercial application for Mo cleaning was done at Les Mines Gaspe, Canada in 1980. Bubble columns commonly used as reactors and/or absorbers in the chemical industries, are finding, of late, new application in the mineral processing as flotation columns. A lot of work on column mixing has been carried out for bubble columns by deriving expressions of the axial dispersion coefficient as a function of column flow and geometry, and engineering application of gas/liquid contactors. However, the same is not directly applicable to flotation column practice since the typical gas flow rates employed in bubble columns are much higher. The basic principal involved in a flotation column as shown in Figure 1 is the counter current flow of air bubbles and solid particles and behavior similar to a plug flow reactor. The bubbles are generated by injecting air in the diffuser placed in the bottom of the column. Bubbles move upward in counter direction to downward flow of slurry. The attachment of hydrophobic mineral particles to the air bubbles take place in the lower enrichment section of the column between the feed point and air inlet known as flotation zone. The froth from flotation zone moves to cleaning zone between interface and top of the column. The cleaning zone is a mobile packed bubble bed that is contacted counter currently with wash water from the top of the column to remove the entrained gangue particles from the froth and send back to the flotation zone. Design and operating philosophy of flotation column are totally different in comparison with conventional flotation cell. Also, the basic concept of flotation column looks relatively simple, but the fundamental principles related to perance of flotation column are quite complex. During the last four decades, there has been significant development related to design, operation, scale-up and application of flotation column on industrial scale. Successful research efforts from many mineral processing laboratories located in Canada, Russia, USA, European countries, China and India have led to improved versions of flotation columns that have gained widespread industrial acceptance due to their improved metallurgical perance in comparison with conventional flotation cells. It has been well established that certain distinct advantages of column over the conventional mechanical cells are such as simple construction and easy fabrication without having any moving parts, material requirements for columns for equivalent capacity being significantly lower, less electrical power consumption low operating and maintenance cost, easy to operate with better computer control facilities, reduction of number of stages of operation for required grade and recovery Finch and Dobby, Biswal and Bhaumik, Tuteja et al.. 2. Column Parameters and their Effect on Metallurgical Perance The effect of various column operating as well as design parameters on metallurgical perance has been reported by many researchers including the present author. Design parameters are diameter of the column, height of the collection and clearing zones, diffuser size, type of wash water system, type of baffles, booster system etc. The perance of flotation column depends mostly on various operating parameters. The manipulated operating variables are wash water rate, tailing rate, air rate, frother dosage, collector dosage and feed rate. The controlled variables are interface level, froth depth, bias rate, percentage of solids in concentrate, gas hold up and pulp density of slurry in the column. Some typical case studies including those from RRL, Bhubaneswar are described as follows. 2.1 Effect of Various Column Operating Parameters Superficial feed rate Many researchers including those of RRL, Bhubaneswar have reported on the effect of superficial feed rate on the metallurgical perance of the column. It was observed that the recovery of concentrate increases at low superficial feed rate. The explanation given by these researchers is that lower superficial velocity of feed provides more retention time and as a result the recovery of the concentrate is more. Froth depth Floatability characteristics is also another factor which decides the height of the flotation zone. If the material is high floatability like molybdenum, graphite and coking coal, the froth height increases and does not affect much on recovery but simultaneously the concentrate grade increases as demonstrated by studies from RRL, Bhubaneswar. Further it has been seen that by increasing the froth depth, the entrained gangue minerals in the concentrate reduces significantly due to coalescence of bubbles, positive bias water and reduction of feed water in the concentrate. Reagent dosage The effect of reagent dosages such as collector, frother, depressant, dispersion, pH regulator etc. on metallurgical perance of the column is almost similar to that observed in conventional cells. The dosage of frother plays the vital role because it affects on bubble size distribution and mean bubble size due to change of surface tension of the liquid. In flotation, the bubble is the driving force and it determines the metallurgical perance. 2.2 Effect of Various Column Design Parameters on Metallurgical Perance Column diameter The flotation tests were carried out in 10, 22 and 100 mm columns for coal fines, it has been observed that the concentrate grade slightly decreases with increasing diameter at running of carrying capacity as reported by Banarjee et al. 2001 vide Table 1. It has been observed that the air hold-up commercial column is less in comparison with pilot and laboratory column under similar conditions. It happens due to larger bubble size in plant columns. The larger bubble size in plant columns may be due to differences in sparger type or superficial sparger area on both of these factors. Table 1 Comparison of laboratory and pilot column Ash, Recovery, Column diameter, Product Tail Feed Wt. Carbon 10 22 100 13.1 15.6 16.6 17.6 14.5 16.7 46.3 48.0 51.4 52.1 43.7 44.7 31.0 30.0 34.0 31.4 26.1 27.3 46.1 55.6 50.0 60.0 60.2 62.1 58.1 67.8 63.2 72.0 69.6 71.2 3. Mixing Effect in the Flotation Column The first application of mixing theory to flotation column was made by Rice and his co-workers in mid ‘70s. Later on, considerable amounts of work have been carried out by different investigators to develop a scale up ology for flotation column. They have utilized RTD measurements to determine the mixing behaviour of both liquid, gas and solid phases within a flotation column. From the RTD data, mathematical correlation are proposed by many authors including the present author 2001 based on geometrical dimension and operating variables of flotation column. A suitable correlation has to be developed considering all process variables and design parameters with dimensionless groups, so that it will help to minimize the error during scale-up of the flotation column. 3.1 Reduction of Mixing Effect in Column Mixing has been characterized by earlier researchers for both baffled and un-baffled conditions using a pulse tracer technique in a laboratory column. These studies show that baffles can reduce mixing by 50 ; however, no attempt has been made to relate this finding to the column perance. The horizontally baffled column developed by Eisele et al. 1998 and Kawatra and Eisele 1995 was designed for improvement of perance in coal flotation. It has been also applied in industrial columns to reduce the axial mixing. The baffles are divided into two zones; one above and one below the feed inlet. The benefit of the upper baffles in the column is to reduce the coalescence of air bubbles as they rise through the column, as a result it helps to reduce the axial mixing and increases the separation efficiency of the column. The lower baffles increase the retention time of the sinks to ensure that every floatable particle should get the opportunity to float. Both horizontal and vertical baffles were designed in commercial scale for cleaning of the graphite by Biswal et al. 2000. It has been observed that stability of plug flow characteristics has improved a lot. The coarse means flaky particles could be floated without any problem. One more advantage is the drastically reduction of reagent consumption during the cleaning of the concentrate. 4. Flotation Studies at Regional Research Laboratory, Bhubaneswar Regional Research Laboratory RRL, Bhubaneswar has also carried out considerable flotation work and patented the same in 1966 by Narasimhan et al. The laboratory has designed different diameter columns for beneficiation of different minerals like coking coal, graphite, non-coking coal, iron ore fines, sulphide minerals, limestone, magnesite, chromite and sillimanite. The flow sheet for beneficiation of high ash oxidized non-coking coal using modified laboratory flotation column was patented in 1994. The design aspects like arrangement of wash water system and diffuser design and location etc have optimized to float coarse and heavy minerals in a commercial column as described earlier in the text. RRL has designed and installed a pilot column at West Bokaro Coal Washery. TISCO and Sudamudih Coal Washery, CCL through CMPDIL, Ranchi, for recovery of coal fines from plant tailings and low grade coal fines, respectively, at a capacity of 3 tph of feed material. Two columns of 1.7 m diameter and 12 m height each have been installed at Rajpura-Dariba mines of Hindustan Zinc Ltd. for Zn cleaning with the technology of EIL, New Delhi and RRL Bhubaneswar. Another commercial flotation column 1 m dia x 7 m height designed by RRL, Bhubaneswar was installed in the cleaning circuit of Utkal Graphite, Orissa, at a capacity of 3 tph. Based on pilot scale study at Sudamudih, a global tender has been floated by CMPDIL, Ranchi to install commercial flotation column at the capacity of 40 tph. A pilot plant column for recovery of iron ore fines by M/s. Sesa Goa, Goa and a commercial plant for flotation of chromite fines by Orissa Mining Corporation are under consideration, at present. 5. Concluding Remarks Some typical observations made in the present study may be summarized as follows. Although the basic concept of flotation column looks relatively simple, but the fundamental principles related to perance of flotation column are quite complex. The type of the relative motion of particles and bubbles is a major factor governing the probability of bubble/particle attachment, bubble loading, flotation rate and power requirement of the processes. Counter current flow of slurry and air causes a reduction in the rising velocity of the swarm of bubbles, which increases their retention time in the slurry, reduces the compressed air requirement and increases specific throughput of the apparatus. Without stirrers and at a low turbulence of slurry flows in a column, inertia forces causing the detachment process are insignificant. Further more, in a counter current flow, the probability of bubble-particle collision is higher because of the large aerated volume of the cell and a long way the particles and bubbles have to travel along the column height. And lastly, intensity of longitudinal slurry mixing is low in a column. All this shows that the flotation rate in the counter current direction is higher than with other directions of slurry and air flows. Reference Book Mineral Processing Engineering Indian Institute of Chemical Engineers Kolkata