coarse and fine particle flotation

COARSE AND FINE PARTICLE FLOTATION H.E. Wyslouzil1, J. Kohmeunch2, L. Christodoulou3, M. Fan3 1

Canadian Process Technologies Inc. Unit 1 - 7168 Honeyman Street Delta, BC Canada, V4G 1G1 2

Eriez Manufacturing 2200 Asbury Road Erie, USA, PA 16505

3

Canadian Process Technologies Inc. 2200 Asbury Road Erie, USA, PA 16505

ABSTRACT Eriez HydroFloat separators and Canadian Process Technologies (CPT) flotation columns have been developed to overcome the limitations of traditional flotation cells in recovering coarse and fine particles, respectively. The flotation column can effectively separate 30-5 μm phosphate. Several industrial-scale HydroFloat separators have been installed in phosphate and potash beneficiation plants. Industrial-scale ultra-coarse (+850 μm) phosphate separation results suggest that a more than 40% BPL recovery increase and a 37% collector consumption reduction can be achieved. The article describes the applications of Eriez HydroFloat separators to coarse particle separation and the CPT flotation columns to fine/ultrafine particle flotation.

INTRODUCTION Mineral particle size is an important parameter in froth flotation. In 1931, Gaudin, et al. [1] showed that coarse and extremely fine particles are more difficult to recover by froth flotation as compared to intermediate size particles. This was approved by Morris [2] in 1952. Generally, the flotation recovery and efficiency deteriorate rapidly when operating in the extremely fine (250 μm). For minerals such as phosphate, the existing conventional flotation practices are optimal only for the recovery of particles in the size range of about 45 to 250 μm. The improved flotation performance of coarse and fine particles has long been a goal in the minerals processing industry. A lot of efforts have been made to overcome the inefficiencies associated with processes and equipment. To overcome the limitations of traditional flotation cells in recovering coarse particles, a new separation device, the Eriez HydroFloat separator has been developed. The HydroFloat cell separates particles based on the apparent density differences between hydrophilic particles and particle-bubble aggregates after the selective attachment of air bubbles to the hydrophobic component of the feed stream. The HydroFloat separator is different from teeter-bed separators that are commonly used in minerals industry. In teeter-bed separation, the hindered-settling velocity of particles can be described by the equation advocated by Kohmuench et al. [3]:

(1) where g is the acceleration due to gravity, d is the particle size, ρs is the density of the solid particles, ρf is the density of the fluidized suspension, η is the apparent viscosity of the fluid, Re is Reynolds number, φ is the volumetric concentration of solids, φmax is the maximum volumetric packing, and β is a constant and dependent on Reynolds number Re. This equation indicates that the settling velocity of a particle in a hindered-settling environment is primarily a function of both particle size and density. It can be seen from Equation (1) that a teeter-bed separator can be efficiently utilized as a gravity separator only when the feed particle size range is relatively narrow and the density difference between the mineral and gangue is sufficient. However, teeter-bed separators often suffer from the misplacement of low-density coarse particles to the high-density underflow, especially when the particle size range is relatively wide. Some coarse low-density particles accumulate at the top of the bed due to being too light to penetrate the teeter bed and too heavy to be carried by the rising water into the overflow launder. Therefore they are eventually forced by mass action downward to the discharge as more particles accumulate at the top of the teeter bed. The effort to elutriate the low-density solids to the overflow by increasing the teeter-water velocity will cause fine, high-density solids to be misplaced to the overflow launder. HydroFloat separators overcome the shortcomings of traditional teeter-bed separators and flotation cells by combining their advantages. Figure 1 schematically shows the full-scale HydroFloat separator, which consists of an upper separation chamber and a lower dewatering cone. The particles to be separated may be naturally hydrophobic or made hydrophobic through the addition of flotation collectors. Pulp feed enters near the top of the separation chamber. The HydroFloat separator operates like a traditional hindered-bed separator with the feed settling against an upward current of fluidization (teeter) water. The teeter water is supplied through a network of pipes that extend across the bottom of the entire cross-sectional area of the separation chamber. In addition to the teeter water, the HydroFloat separator is continuously aerated by injecting compressed air and a small amount of frothing agent into the fluidization water. The rising air bubbles will attach to the hydrophobic particles and reduce their effective density. The lighter bubble-particle aggregates rise to the top of the denser teeter bed and overflow the top of the separation chamber. Unlike flotation, the bubble-particle agglomerates do not need to have sufficient buoyancy to rise to the top of the cell because the teetering effect of the hindered bed forces the low-density agglomerates to overflow into the product launder [4]. Hydrophilic particles that do not attach to the air bubbles settle down in the teeter bed and are eventually discharged at the bottom of dewatering cone.

Figure 1 - Schematic illustration of a full-scale HydroFloat separator For fine particle flotation, column flotation cells were introduced to the market place as devices capable of producing concentrates that were lower in impurities than those produced by other types of flotation machines. The ability to operate columns with deep froth beds and to wash the froth was the main reasons cited for the improved metallurgical performance. In recent years, many phosphate producers have installed column flotation systems as a means of boosting production whilst reducing operating costs. The size range of recoverable apatite particles has been extended from about 30 μm down to 5 μm through the introduction of column flotation [5]. The high degree of selectivity achieved by this equipment has made it economical to treat material previously considered to be tailings. The depletion of high grade reserves coupled with increasing market pressure for improved product quality has forced iron ore producers to re-examine their process flowsheets and evaluate alternate or supplemental processing routes. The requirement for higher quality pellets demands that the silica content be lowered to levels ranging from 2.0% SiO2 to below 1.0% SiO2. Reverse flotation (silica is floated away from the iron concentrate) has proven to be an economical and effective method for reducing the concentrate silica content to very low levels. Laboratory and commercial test-work has demonstrated some significant metallurgical and economic advantages when column flotation cells are used for this application. Excellent metallurgical performance [6-8] along with low capital and operating costs [9] has made column flotation popular in the mineral processing industry. For iron ore applications, the ability to wash the froth has provided a means of obtaining low concentrate silica levels while keeping iron losses to a minimum. Recent cost comparisons [10] have shown that the cost of installing a column flotation circuit is typically 20% - 30 % less than an equivalent conventional flotation circuit but can be as much as 50% lower depending on the circuit and plant location. The Brazilian iron ore industry has led the world in adopting column flotation technology for reducing the silica content of iron pellets [11]. CPT, a wholly owned subsidiary of Eriez Manufacturing, has been instrumental on supplying flotation columns to this industry. Several companies have installed, or are in the process of installing column cells into their process flowsheets. Samarco Mineracao S.A., the first Brazilian producer to use column cells, installed columns to increase flotation capacity as part of a plant expansion program [12] in 1990. Since that time, they have installed additional columns for the recovery of fine iron from the desliming circuit and for a recent plant expansion program. The application of column flotation for silica rejection is being actively investigated by several iron ore companies in Brazil, Canada, the United States, Venezuela, and India. COARSE PARTICLE HYDROFLOAT CELL FLOTATION

Eriez Manufacturing has conducted extensive HydroFloat testing in applications including coarse phosphate [13], potash, diamonds and coal [14]. One of most elaborate and complete test programs was conducted at the Mosaic Company’s South Fort Meade Mine. This test program encompassed laboratory, pilot and full scale testing to evaluate the Eriez HydroFloat separator performance. Pilot-Scale and Full-Scale Testing During early pilot-scale testing, feed to the test unit was continuously classified, conditioned, and upgraded in a HydroFloat separator. In the HydroFloat phosphate separation process, the classified feed is suspended in a fluidized-bed and then aerated. The reagentized phosphate particles encounter and attach to the rising air bubbles. The phosphate particle-bubble aggregates report to the product launder while hydrophilic gangue particles are discharged from the bottom of the separator as a high solids content (65-70%) tailing. The test results indicate that the HydroFloat separator can produce a high-grade phosphate product in a single stage of separation. Product quality ranged between 70-72% BPL and 5-10% insols. BPL recoveries exceeded 98% [4]. The fluidized bed acts as a “resistant” layer through which no bubble/particle aggregates can penetrate. In addition, the high solids content of the teeter bed promotes bubble-particle attachment and reduces the cell volume required to achieve a given capacity. Subsequent pilot-scale tests showed similar results for both coarse and ultra-coarse material. Preliminary estimates indicated that the surfactant (i.e., frother) usage would be decreased by 22% and 40% for the coarse and ultra-coarse circuits, respectively. Furthermore, it was also predicted that the addition rate of the fatty acid and fuel oil blend would be decreased by nearly 40% for both circuits [13]. Based on the pilot-scale test results, several HydroFloat units as shown in Figure 2 were installed at South Fort Meade as retrofits [13].

Figure 2 - Full-scale HydroFloat separators Figure 3 shows the results obtained from a full-scale HydroFloat separator retrofit. In a single stage, the HydroFloat separator provided BPL recoveries approaching and exceeding 90% while the existing cells struggled to achieve 80% BPL recovery. It can be clearly seen from the figure that the performance of single HydroFloat separator is close to the performance of the combined rougherscavenger pneumatic cells [13]. The single HydroFloat rougher cell provided metallurgical results superior to the pneumatic rougher flotation cell installed on a parallel coarse feed stream. The HydroFloat separator retrofits are able to treat 56% more material than the previously installed pneumatic cells. A feed rate of 20 t/h/m2 was obtainable for HydroFloat separator. A BPL recovery increase more than 40% and a 37% collector consumption reduction can be achieved by the application of this new technology to industrial-scale ultra-coarse (+0.85 mm) phosphate separation [13].

Figure 3 - Test results of full-scale HydroFloat cell and pneumatic cell T

Advantages of the HydroFloat Separator The HydroFloat separator is both a flotation device and a density separator, which combines the advantages of both froth flotation and gravity separation. For treating coarse particles, the advantages of the HydroFloat cells over traditional flotation processes includes enhanced separation recovery, higher throughput capacity, reduced reagents and air consumption etc. (1) Increased coarse particle flotation recovery or the maximum particle size limit: Two key steps in coarse particle flotation are bubble-particle attachment and detachment. The hindered settling/rise conditions within the teeter bed of the HydroFloat cell significantly reduced the bubble rising velocity and particle settling velocity. The reduced velocities will increase the particle’s sliding time on the bubble surface after the bubble-particle collision, improve the probability of bubble-particle attachment, and thus enhance the flotation recovery. The use of fluidization water in the HydroFloat cell can keep particles dispersed in suspension without the intense agitation required by mechanical flotation machines. Consequently, the reduced cell turbulence will decrease the centrifugal force that pulls the particle away from the attached bubble surface, decrease the probability of bubble-particle detachment, and so promote the flotation recovery or the maximum particle size limit. (2) Higher throughput capacity: The presence of the high-solids teeter bed reduces the turbulence commonly associated in traditional flotation units and therefore enhances the buoyancy of the particles. The teetering effect of the hindered-bed relinquishes the need for bubble-particle aggregates to have sufficient buoyancy to rise to the top of the cell. The low density agglomerates can easily overflow into the product launder. Consequently, the teetering increases the separation capacity. (3) Reduced reagents and air consumption: The teeter bed in the HydroFloat cell can act as an air distributor, which may decrease the probability of air bubble coalescence and reduce the frother consumption. The separation of the HydroFloat Cell is based on the density differences between hydrophilic particles and hydrophobic particle-bubble aggregates. The density of the bubble-particle aggregate need only be smaller than the effective density of the teeter bed to achieve a separation. The separation can be achieved even if the buoyancy of the bubble-particle aggregate is too small to lift the particle load in an open pool of liquid [14]. This means the diameter or number of air bubbles required to float a given particle can be greatly decreased. In some cases, the aeration rate may be only 10% of that employed by mechanical cells or columns. This also indicates that lower contact angle and lower collector consumption are required. In the full-scale tests, the fatty acid and fuel oil blend (FA/FO) was reduced between 10-38% [13]. The HydroFloat cell separation needs effective desliming of the feed in order to remove fines that would otherwise be carried nonselectively into the clean product. For fine or ultra-fine particle separation, flotation columns provide better performance than the HydroFloat cells.

FINE AND ULTRA-FINE PARTICLE COLUMN FLOTATION Ultra-Fine Phosphate Column Flotation Column flotation has been applied to a wide variety of phosphate ore types ranging from volcanic to sedimentary. Although benefits are seen across the entire particle size range, column cells are particularly well suited to the production of fine and ultra-fine concentrates. One representative application of the CPT flotation column to fine and ultra-fine flotation is the Barreiro carbonatite complex located in Araxá, MG. The major components of the rock formation are carbonatitic and glimmeritic rocks. The major source of phosphate is derived from apatite, which comprises approximately 30% of the minerals in the ore zones. The major impurities consist of iron oxides and silicate minerals. A simplified phosphate processing flowsheet [5] is shown in Figure 4. The ore is crushed, screened, and then fed to the concentrator, where it is subjected to grinding (rod and ball mills), classification in hydrocyclones, low intensity magnetic separation, desliming in hydrocyclones, flotation and dewatering etc.

Figure 4 - Typical phosphate processing flowsheet Traditionally, the desliming stage is designed to remove particles finer than about 25-30 μm. The high surface area and high impurity content associated with this particle size class make it difficult to treat by conventional flotation equipment. The removal of these slimes represents a major source of phosphate loss, which could represent as much as 10% -15% of the total reserves. The application of column flotation makes it possible to extend the size range of particles that can be treated by flotation to about 5-10 μm. Figure 5 shows a typical circuit arrangement for ultra-fine phosphate separation [5]. By re-processing the primary slimes (-30 μm) in second stage of hydrocyclones cutting at 5-10 μm, the cyclone underflow is fed to a series of conditioners where the pulp is treated with caustic soda, starch and a collector prior to introduction to the flotation columns. The flotation columns are arranged in a conventional rougher-scavenger-cleaner configuration with intermediate products recycled internally.

Figure 5 - Typical circuit arrangement for ultra-fine phosphate separation Table 1 shows a typical Brazilian phosphate mass balance for an ultra-fines flotation circuit. Assuming that the chemical analysis of the slimes is essentially the same as the feed grade, the loss of phosphate in the slimes is equal to the mass rejection. It can be seen from Table 1 that the P2O5 grade of ultra-fine phosphate can be increased from 8.1% up to 33.5%. The flotation tailing P2O5 grade is only 2.7%. By treatment in flotation columns, it is possible to obtain an ultra-fine concentrate ideally suited for the production of Single Super Phosphate (SSP) fertilizers. The fine particle size minimizes the costs of concentrate regrinding at the fertilizer plant saving additional processing costs. Table 1 - Typical phosphate mass balance for ultra-fines circuit Stream Mass P2O5 (%) (%) Primary Slimes 100.0 6.0 Slimes Reject 60.0 4.6 Ultrafine Flotation Feed 40.0 8.1 Cleaner Concentrate 7.0 33.5 Flotation Tailings 33.0 2.7 Faced with difficult ores, and low-grade deposits, the Brazilian phosphate producers have been world leaders in adapting this technology to enhance fine phosphate recovery. Most of the major producers are operating an ultra-fines recovery circuit in their concentrators. Fine Iron Ore Column Flotation The integration of column flotation into existing iron ore plants can be done in a number of different ways depending on metallurgical and economic objectives. Some of the common objectives are: • Incorporate column flotation into an existing plant to permit the production of low silica pellets. • Increase the flotation capacity of an existing plant. • Improve the overall plant iron recovery by re-treatment of the tailings. • Install a circuit capable of producing more than one product grade. Many plants require the flexibility to produce more than one product grade (eg blast furnace (2.2% SiO2) and direct reduction (0.8% SiO2)) using the same equipment. Usually when the higher silica blast furnace products are produced, the grinding requirements for liberation decrease and therefore the plants can operate at increased throughputs. The higher feed rate will affect the feed size distribution and must be taken into account in the circuit design. The following examples show some of the flowsheet configurations which have been adopted by iron ore producers.

Example I-Cleaner-Scavenger Circuit For this Brazilian flotation plant, the original circuit consisted of four parallel mechanical flotation lines containing a rougher, cleaner, and two scavenging stages. The depletion of reserves at the existing mine and the subsequent development of a new ore zone necessitated the modification of the existing flowsheet [11] since ore from the new mine is more difficult to grind and requires longer flotation time. A number of circuit alternatives were considered: • Installation of an additional line of conventional cells. • Increasing flotation capacity by installing a line of column cells. • Adding column cells to existing flotation lines to act as recleaners. Pilot plant testing results revealed that the third option provided the best combination of plant throughput and iron recovery. The flowsheet for the modified circuit [15] is shown in Figure 6.

Figure 6 - Columns added to an existing circuit One column has been added as a recleaner to each processing line to preserve circuit flexibility. The recleaner column feed grade ranges between 1% and 6% silica, therefore, one column stage is sufficient to produce either blast-furnace or direct-reduction concentrate. A scavenger column has been added to the flowsheet to maximize fine iron recovery particularly during the periods where direct reducing grade is being produced. Because these columns have been added to an existing silica flotation plant as recleaners, consideration needs be given to the effects of variations in the roughing and primary cleaning circuit. Generally the fine, fast floating silica is removed in the roughing stage and therefore the column feed tends to be enriched in coarse silica. Furthermore, any operational problems with the existing plant tend to flow through to the column circuit resulting in variable feed conditions. The flotation rate constant for the silica particles is affected by feed grade and particle size. Many iron ore concentrators use the quantity of +150 μm material in the flotation feed as a measure of the performance of the grinding circuit. This particle size is well within the normal size range for flotation and presents no particular problems. Particles larger than 300 μm however become increasingly difficult to float and to retain in the froth phase. For plants using hydrocyclones for classification, the distribution of silica is not even throughout the particle size range. The silica often tends to concentrate in the coarser size fractions. As the amount of +150 μm in the feed increases the quantity of +300 μm tends to increase exponentially thus compounding the problem. Particles in the +300 μm size range produce relatively poor response

with the maximum recoveries often below 50%. In a column the slurry falls by gravity through the collection zone. Consequently the coarser particles settle at a rate higher than the average slurry. For example, if the mean slurry residence time is 12 minutes, the residence time of the +300 μm particles could be 6 - 8 minutes depending on the slurry rheology within the collection zone. Therefore, when considering columns for this type of application it is important to have a good understanding of the degree of variation of particle sizes in the existing circuit to permit proper sizing of the column circuit. Example II-Silica Reduction from Magnetite Concentrate CPT Inc. has recently completed the basic engineering for a 1180 TPH flotation plant designed to lower the silica content of a magnetite concentrate from 7% - 9% SiO2 to 2.2% SiO2. The basic flowsheet [15] is shown in Figure 7. New feed is ground to 80% passing 150 μm and is fed to primary magnetic separators. The magnetic concentrate is reground to approximately 75% -45 μm and is retreated in secondary magnetic separators which produce a concentrate containing between 7% - 9% SiO2. The magnetic concentrate is conditioned first with caustic starch solution and then with amine prior to being fed to a single stage flotation column where the silica content is reduced to below 2.2% SiO2.

Figure 7 - Column flotation circuit for magnetite treatment The ore is highly variable in magnetite content and therefore it was necessary to design the circuit to tolerate large fluctuations in feed rates. A high level of instrumentation has been specified to provide on-stream analysis, automatic reagent metering and the ability to switch groups of columns into “hot stand-by” mode. During periods of decreasing feed rates, individual columns will be bypassed, the reagent addition stopped and the columns placed in recycle mode. When the feed increases to normal levels, the process is reversed and the columns resume normal operation. SUMMARY AND CONCLUSIONS Eriez HydroFloat cell has been developed to overcome some of the shortcomings associated with traditional flotation machines in recovering coarse particles. This novel separator is based on the density difference between the hydrophilic particles and particle-bubble aggregates after bubble attachment to the hydrophobic particles. Compared to traditional flotation equipment for treating coarse particles, the HydroFloat cell has the advantages of enhanced separation recovery, higher throughput capacity, reduced reagents and air consumption. Test data obtained from the full-scale HydroFloat separator tests indicate that the HydroFloat separator provided BPL recoveries approaching and exceeding 90% while the existing cells struggled to achieve 80% BPL recovery. The performance of single HydroFloat cell is close to the performance of the combined rougher-scavenger pneumatic cells. The feed rate to an existing HydroFloat separator can be as high as 20 t/h/m2 and treats 56% more material than the previously installed pneumatic cells.

CPT flotation columns have been successfully applied to separation of ultra-fine particles (305 μm) that are difficult to treat by conventional flotation equipment. When the P2O5 grade of the column flotation feed is 8.1%, the concentrate P2O5 grade of 33.5% and tailing grade of 2.7% can be achieved. Many iron-ore producers world-wide are considering columns as a viable alternative to conventional flotation machines for the reduction of silica in fine pellet feed. The abilities to operate with wash water and deep froth beds result in improved iron recoveries particularly when producing direct reduction grade concentrate. The circuit chosen is dependent on the range of flows the column is required to handle and the metallurgical performance expected. Depending on the metallurgical objectives and the degree of upgrading required, columns circuits may be configured as a rougher and cleaner or as a cleaner and scavenger. The Eriez, CPT combination now allows customers to successfully recover both fine and coarse fractions previously thought to be challenging to recover. REFERENCES 1.

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2.

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3.

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4.

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6.

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7.

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8.

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9.

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