Flotation is, arguably, the most important mineral separation technique and represents one of the more challenging applications of surface chemistry. Inefficiencies in flotation translate into both an enormous loss of revenue and an unnecessary waste of these reserves. This is particularly important in the flotation of the finer particle size ranges where flotation efficiency is often poor.
The flotation process is controlled by the thermodynamics, kinetics and hydrodynamics of the system. Fundamental aspects include:
* creation of a hydrophobic surface for a particular mineral species (thermodynamic condition);
* provision of sufficient time for bubble attachment (kinetic condition); and
* stabilisation of bubble/particle aggregates under the prevailing slurry flow (hydrodynamic condition).
The synthesis of new, more effective reagents, the perfection of existing and creation of new technological flotation processes, the elaboration of the physico-chemical models and the optimisation of flotation processes and conditioning systems of recycling waters at the plants all rely on the establishment of well founded principles of physico-chemical optimisation.
A number of various subaeration mechanical flotation machines have been operated satisfactorily for the past seven or eight decades. Part of their success is related to the processing of relatively rich, coarse mineral particles. However, with the depletion of high grade, simple ores, the hard-to-treat finely disseminated ores of low grade have to be processed. Consequently, feed preparation now requires extensive grinding to obtain adequate liberation, posing a challenge to existing technology to process the resultant fine particles, an inherently inefficient situation.
Big is better
Outokumpu flotation cells were originally designed for the effective flotation of base metal minerals at the company's own mines, but soon the excellent features of the machine also inspired concentrator operators outside of metals mining. The first installations within the minerals industry were made 25 years ago for talc flotation and they were soon followed by apatite, coal and sericite operations. Later, the list of users was extended to plants processing phosphate, kaolin, calcite, baryte, feldspar, fluoride and tar sands as well as some very rare industrial minerals in some 20 countries. The total of Outokumpu Mintec flotation cells in industrial mineral applications today exceeds 750.
Flotation has been successfully used as a stand alone method in the talc, kaolin and phosphate industries, and has also proved beneficial in combination with other methods. With the ever- increasing demands placed on today's minerals industry, obtaining a higher quality product is crucial. Outokumpu Mintec claims that the superior beneficiation obtained with its large flotation cells offers an economical solution.
Large flotation cells have been used with even greater economic benefits than smaller units - large cells require less floor space and support constructions, which saves in building costs; the capital outlay for a given flotation volume of large cells is less than that of the equivalent number of small cells; a small number of large cells is easier to install, operate and maintain; and lower power consumption of large cells further complements the savings in operation costs.
Given the complexity of the physico-chemical interactions in a flotation system, however, it is clear that not all applications can be successfully addressed by using a mechanism that has only one design. Outokumpu Mintec has developed, therefore, several versions of its proven OK mechanism, which has confirmed its reputation over the years as a most effective and troublefree tool for uniform slurry mixing and, above all, excellent air dispersion.
The heart of the mechanism is the OK-Rotor, which has a unique downward-tapering shape and different channels for air and slurry flows. Its shape allows the rotor to be started under full load, for example, after a power failure. This minimises the restart time, as there is no need to empty the cells.
According to Outokumpu Mintec, the MM (MultiMix) version creates optimum conditions for flotation of medium and fine particles (less than 150 microns). Since the slurry flows more than once through the maximum shear zone between the rotor and stator, the conditions for maximum bubble/particle contact prevail. The strong shear forces create the high energy needed for small particles to break through the water film barrier of the air bubble. A special version for superfine flotation has been developed and successfully used in slimes flotation for particle sizes from 10 microns and smaller.
For coarse particles or viscous slurries, where laminar flow is the key to successful flotation, Outokumpu Mintec has developed the FF (FreeFlow) type mechanism which keeps the slurry in semi-laminar motion and avoids excessive turbulence. Whilst ensuring a high probability of coarse particles maintain firm contacts with air bubbles under these conditions, the mechanism simultaneously keeps the slurry in homogeneous suspension at a low energy consumption.
Although the mechanism is the core of the flotation machine, the optimum engineering of other elements is the key for successful scale up of cells. New requirements for mixing, air distribution and froth transport have to be fulfilled. The cylindrical Outokumpu TankCell design is an answer.
Available in capacities from 50 [m.sup.3] 160 [m.sup.3], the round tank provides absolutely symmetrical and stable conditions throughout the cell. Tracer tests in various applications have proved that the TankCell operates as an ideal mixer with no short circuiting. In a copper application, two 70 [m.sup.3] TankCell-units gave the same recovery as a row of twenty conventional cells. Both circuits had the same retention time. Moreover, large Outokumpu TankCells consume typically less than 1 kW/[m.sup.3] of energy (as low as 0.7 kW/[m.sup.3] in some instances).
Outokumpu Mintec has placed a lot of emphasis on efficient froth handling in its designs. Particles remain only a certain time in the froth bed. Thus, the froth has to be kept in continuous movement in order to prevent especially the coarsest particles from dropping back into the slurry. The high capacity launder design moves the froth effectively to the launder and keeps the froth transport distance in the 100 [m.sup.3] TankCell the same as is used in the conventional 3 [m.sup.3] cells.
Also, because the mechanism is at situated at the bottom of most of Outokumpu Mintec's large cell designs,, its high turbulence mixing does not disturb the froth zone as much as is the case in smaller cells. This can be seen in improved selectivity and the largest cells, against general belief, actually perform well as cleaners as well as roughers.
With large Outokumpu TankCells, the cylindrical tank allows a flexible layout design since all directions are equal with regard to the feed and discharge points. In addition, the unit reactor principle of TankCells performs excellently with only a simple level and air control and provides a stable operating base on which to build more sophisticated process control strategies. The large unit size decreases the cost of automation dramatically and makes it economically viable even to have high level automation for every cell in the process line. Operational parameters may be changed cell-by-cell without disturbing performance up or down stream and individual cells in a bank may be bypassed for maintenance for maximum plant availability.
BQR flotation cells
The BQR range of round tank flotation cells from Bateman Process Equipment are claimed to achieve superior operation through easy air rate adjustment, robust level and froth cone control. The size and number of BQR cells are selected on the basis of required residence time distribution. The selection of the unique overhung stator and open rotor mechanism is based on the specific process requirements of each application to ensure a good balance between grade and recovery.
The open rotor produces high shear forces for better bubble-particle contact. Start-up under load is achieved as the overhung stator shields the rotor from being hindered by settling solids. The rotor and stator units are easily adjusted to optimise particle suspension and pulp-froth stability. Because of the overhung stator and external launders, a quiescent pulp-froth interface is achieved, whilst effective power input is maximised due to the 'open' inlet design of the flotation mechanism.
The overhung stator also ensures easy maintenance. The complete mechanism can be removed from the flotation cell for maintenance, thus reducing the cost of downtime. Furthermore, the rotor and stator are of simple, although technically sophisticated design, thus reducing the cost of spares.
In October, the five largest BQR flotation cells in Africa were commissioned at Amplats' Potgietersrust Platinum as part of its concentrator expansion (MM, September 1998, pp130-131). These 50 [m.sup.3] units are operating as rougher cells.
Although flotation columns have shown advantages for fine particle processing, they complement, rather than replace, conventional, highly turbulent flotation machines, upon which the majority of industrial flotation applications still rely. Nonetheless, column flotation is considered to be one of the most significant achievements in the filed of mineral processing in recent decades.
The process is highly dependent upon the froth characteristics of the system, which affect both recovery and grade of mineral concentrates. It is a particularly important process in the recovery of fine particles [TABULAR DATA OMITTED] (although the detrimental effects of the finest particles on the kinetics and separation efficiency of the process cannot be overlooked).
Most Brazilian phosphate producers have now turned to Cominco column cells for the recovery of ultra-fine apatite. CESL has been working with the Brazilians since 1994 when Arafertil S.A. began replacing its conventional flotation cells with Cominco column cells to improve plant performance and reduce operating costs. Due to the success of that programme, column cells have since been Fosfertil and Goiasfertil, whilst Serrana has just concluded a deal with CESL for the installation of column cells at its facilities.
An important prerequisite in recovering apatite by flotation is desliming; a process where ultra-fine particles are removed. Advancements in Cominco flotation cell designs has meant that the cut size for desliming could be lowered, thereby reducing the losses of fine apatite.
Attrition due to the agitators in mechanical cells results in the continuous generation of fresh slimes. Low grade concentrates result from impurities physically entrained in the froth. Consequently, finer feeds require more chemical depressants and cleaning stages to reach acceptable grades.
Besides generating fewer new slimes, Cominco flotation cells have efficient froth-washing capabilities, where the wash water partially replaces the depressants, resulting lower chemical dosage. Entrained impurities are removed without significantly reducing apatite recovery. The efficiency of the Cominco column cells is such that often a single stage of cleaning can replace three stages of cleaning in mechanically agitated cells.
Thus the ability to successfully treat the fines, has allowed Brazilian producers to reduce apatite losses from the desliming circuit. By retreating the desliming cyclone overflow in another stage of cyclones, significant amounts of apatite are being recovered for concentration in a separate flotation circuit. The Cominco column cells have proven extremely effective at upgrading this ultra-fine material into a saleable concentrate. Concentrate grades ranging from 32% - 33.5% [P.sub.2][O.sub.5] are being produced with flotation recoveries of 70% - 80%. Particle sizes being treated range from (d80) 15 microns to 40 microns.
Operators have found the ultrafine concentrate to be a valuable product. Since it is being produced from a tailings stream, additional concentrate production can be achieved at a fraction of the cost of treating run-of-mill ore. Furthermore, downstream processing costs for this concentrate are lower than regular concentrates as further grinding is not necessary.
In another development, CESL has now launched an automatic sparging system that automatically adjusts to maintain equal air distribution to each sparger, compensates for line pressure variations, maintains constant bubble size under varying air flow rates, and 'fails closed' on loss of air pressure.
According to Harold Wyslouzil, CESL's divisional manager and patent holder of the SlamJet, "Power failures, compressor malfunctions, fluctuating air line pressures, burst airlines and incorrect manual sparger settings are potential major headaches for some operators. These factors can result in a loss of air, unequal air distribution and performance decreases that affect the grade and recovery of valuable minerals in column flotation plants. These potential problems more than justify the attention that these new units are receive."
The benefits of the SlamJet include:
* Automatic, self-regulating flow control;
* 'Fail closed' mechanism to prevent the inflow of process slurries during power failures;
* Improved metallurgy via continuous automatic pressure adjustment;
* Adjustable 'opening' pressure for process compensation; and
* Control of bubble size based on precision air-flow and pressure control.
Now part of the integrated Baker Process enterprise, Eimco Process Equipment has made several innovative improvements to its Wemco mechanical flotation cells and Pyramid column flotation cells over the past years.
The largest Wemco cell successfully field tested is the Wemco 5650 (160 [m.sup.3] net volume). This machine is one of the generation of Wemco SmartCells first introduced to the market in early 1996. Over 350 of these units have been purchased by customers for copper, zinc, gold, platinum and other applications.
The units feature bevelled cylindrical tanks with space saving cylindrical connection boxes, a combination of radial and peripheral froth collection launders for maximum lip length to cell volume ratios, proven Wemco 1+1 self-aspirating mechanisms, top mounted rotor for the easiest restart after shutdowns on load and for minimum wear, conventional V-drives, manual or automatic air control, and ultrasonic pulp level control. Several installations include embedded expert control systems for increased overall economic performance.
Pyramid column flotation cells are redefining the role of column technology to include all separation applications achievable with flotation. The key to this success has been the column's bubble generating systems. These systems include both the external bubble generation system and the JetStream sparger. Advanced control and total system management technology allow realtime control of reagent dosage, the size and number of air bubbles (volume gas holdup), froth depth and froth washwater flow rate. Where column flotation is viable, Pyramid[R] columns offer enhanced metallurgical performance at lower capital and operating costs than both mechanical and other types of flotation columns. More than 200 Pyramid columns have been sold around the world in applications from kaolin to gold.
Turbo columns from Multotec
Multotec Process Equipment, a highly regarded manufacturer of column flotation systems, is leading the field in the development of turbo column flotation systems and their application in local conditions. Although the turbo column concept is designed to meet the needs of only about 15% of the flotation market, Multotec Process Equipment is in the process of developing other new products which should meet the needs of the remainder of the market as well.
Known as the Colmech Cell, Multotec Process Equipment's patented innovation combines the strengths of a column flotation device with those of conventional flotation cells. A key feature of this new system is the mechanical agitation that is achieved. Preliminary research carried out by Multotec Process Equipment to date has indicated that with mechanical agitation, recoveries are greatly improved for materials which are not highly floatable and for coarser size fractions.
The company is also currently developing, in conjunction with Wits University, a 'jet break-up device'. Installed inside a pre-aeration system on a turbo column, this new unit provides a superior method of aeration and produces up to 60% more bubble surface compared to current systems available. The system can be easily retrofitted to existing units through a simple modification.
A further illustration of the company's success is in the flotation of ultra fine coal (-150 micron material). Traditionally, this material has been discarded as waste because high reagent costs and poor performance of available flotation units made rendered flotation an uneconomic option. Exacerbating the problem is the need to dry efficiently the final product.
Multotec Process Equipment's turbo column as well, as a high shear conditioner, has made the processing of ultra-fine coal products a viable proposition, as acceptable fine coal drying processes have also been developed. The high shear conditioner, a joint development between Multotec Process Equipment and Ingwe Coal Corp., has been shown to reduce reagent consumption by up to 40%.
VERTI-MIX for coarse particles
Dorr Oliver's advanced flotation technology is re-engineered from the ground up, and, based on process-specific metallurgical design programs, guarantees high mineral recoveries.
The Dorr Oliver flotation cell incorporates hexagonal froth collection launders which allow cell rows to be installed in a honeycomb pattern for the most efficient use of floor space. Radial launders are included for added froth collection. The conical-bottomed round tank provides full flotation capacity of up to 200 [m.sup.3] and has been designed for total solids suspension. A two piece hollow shaft and eight piece stator design allow for ease of maintenance. The company's new VERTI-MIX stator achieves the maximum in coarse particle recovery whilst advanced process control allows for simple, efficient integration into the modern concentrator.
With an installed power rating of 150-175 hp, up to 800 t/h can be fed through the largest Dorr Oliver cells, which provide an aeration rate of 28 [m.sup.3]/min per cell, dispersible at 0.4 bar with 30% solids in the feed.
Some of the benefits claimed by Dorr Oliver include: least floor space requirement; high froth lip length to volume ratio; can be installed at basement level to utilise available headroom; minimum sanding provides maximum retention times for optimum metallurgy; low power consumption with proven energy efficient rotor-stator; superior air dispersion; fewer cells to install, operate and maintain compared with smaller cells; and the variable rotor speed options permit fine tuning of each cell or each bank of cells for specific service.
One of the major highlights for MIM Process Technologies during the past year was its breakthrough into the US coal market with four different producers purchasing the company's Jameson Cell. Four J5000/16 units were purchased by AT Massey Coal, and one J5000/16 cell by each of Vigo Coal, R&P Coal and Sedgman.
In Australia meanwhile, Jameson Cell technology continues to dominate the coal flotation cell market and is the preferred method for fine coal recovery at more than half of Australia's coal flotation plants.
One of the earliest installations in Australia was at BHP's Goonyella mine in 1994. Since then, MIM Process Technologies has spent considerable R&D time on improving the original Jameson Cell design. Significantly, the latest machines do not require any consumable maintenance for the first 5-10 years. The latest improvements include the Deep Throat Orifice and a new washwater distribution system.
The company has now commercialised the newly designed Mark III cell which incorporates and integral pump box/feed sump and tailings recycle mechanism into a single unit. Variations in feed flow are hydraulically compensated internal to the cell, with no control system, ensuring consistent operation.
Also available are designs appropriate for the coal industry. At Oaky Creek, for example, conventional cells have been replaced by six newly designed R5230/12 rectangular coal cells. The cells incorporate the standard coal downcomer, modified to accept the Deep Throat orifice plate, and have been divided into two sections so that the downcomers can be removed with the restrictions of limited overhead space. To further economise on space, the three concentrate launders of the cells have been joined so that the cells have a single concentrate outlet.
Deep vanes from Svedala
Having acquired the US flotation equipment specialist Denver in 1993, Svedala embarked on an evaluation of available designs for flotation machines during the second half of 1994. The company's researchers found that the recovery of particles in existing machines tended to fall rapidly at the coarse and ultra-fine ends of the particle size recovery range suggesting that a mechanism was required to minimise slurry turbulence within the cell and to increase the number of passes of each particle through the high energy zone of the mechanism.
The result is Svedala's RCS flotation machine, designed with a circular tank and an impeller shaped to generate a radial flow pattern with strong return flows to both the lower and upper zones of the mechanism. The units are available in cell volumes ranging from 5[m.sup.3] to 130 [m.sup.3] and have been installed in Europe, South America, North America, South Africa and Australia.
Incorporated in the RCS design is Svedala's patented DV (Deep Vane) flotation mechanism which consists of a unique arrangement of vertical vanes with shaped lower edges and dispersion shelf. This provides the following benefits:
* Creates strong bulk slurry flow patterns to ensure solids suspension with good particle- bubble contacts and to allow solids re-suspension after power outages;
* Gives effective air dispersion and distribution;
* Extends mechanism wear life: and
* Minimises absorbed power.
Available in cell volumes of 5[m.sup.3] to 200[m.sup.3], the first RCS unit was installed in 1995 at a European lead-zinc operation where high lead values (1.5-2.5% Pb) were reporting to the zinc concentrate. The RCS17 unit reduced the lead impurity, mostly in the form of ultra fine particles, to 0.7%, and recent installations of banks of RCS cells at this particular operation have resulted in improvements to both lead and zinc recoveries, and the concentrate grades.
A similar improvement in recovery was also reported at a European copper mine. The installation of RCS cells saw copper recovery in a pre-rougher flotation stage increase by 2%, with an overall 1% improvement in copper recovery in the rougher flotation section.
Denver-based Quinn Process Equipment designs and manufactures flotation machines up to 2.8 [m.sup.3]/cell (100 [ft.sup.3]). These include the 0.008 [m.sup.3]/cell (0.3 [ft.sup.3]) to 0.7 [m.sup.3]/cell (24 [ft.sup.3]) range of self-aspirating "Sub-A" cells, manufactured for both cell-to-cell and open-flow configurations, and the larger 1.4 [m.sup.3] (50 [ft.sup.3]) and 2.8 [m.sup.3] (100 [ft.sup.3]) T-BR (Top and Bottom Recirculating) machines.
Quinn Process' cell-to-cell design allows for a great deal of slurry-to-air contact in the impeller/diffuser region and recent design changes have greatly improved the control of air intake in these machines. Modifications include a slurry recycle design on all Sub-A machines and, on #15 and #18 Spl. machines, adjustable feed gates on each cell to throttle the flow of feed to the next cell. These developments allow for much greater control over the air addition to the cell, which can be critical in the efficiency and performance of the actual flotation process, and also in the ability to suspend the solids within the cell to avoid sanding.
The T-BR machines, meanwhile, require blower added air and are designed to recirculate finer pulp from the upper regions of the cell to the lower region thereby assisting in the suspension of coarser particles. They are also designed to lift and keep suspended the coarser fraction which tends to settle in the cell.
Quinn Process utilises all urethane impellers, impeller shaft sleeves, diffuser feed pipes and diffusers for exceptional wearing life. Other materials of construction are available as necessary to suit the application mild steel, stainless steels and fibreglass reinforced polyester. Various linings are available including chlorobutyl, natural rubber and neoprene.
In June this year, New Jersey-based Cytec Industries acquired the OrePrep business from Baker Petrolite Corp., thereby providing Cytec with a significantly broader dimension for serving the industry Cytec is strongly established as a leader in collector technology, whilst OrePrep is a leading supplier of speciality frothers.
This combination has expanded Cytec's opportunities to explore the synergies of frother and collector technologies and various combinations leading to the next generation of processing chemicals. Considering the company's long history of investment in mining chemicals research and development (currently at 5% of its revenues), the mining industry can expect new products that will enhance its processing efforts in both the long and short term.
One of two new series of promoters, Cytec's alkyl hydroxamate-based series of AERO promoters which has demonstrated improvements in reverse kaolin clay flotation, providing higher brightness and recoveries, and resulting in increased output of higher value, higher purity grades. Recovery increases from 6090% have been achieved. These products, such as AERO 6493 promoter, have also been used for crude clay upgrading in those instances which have traditionally proved difficult to treat.
Alkyl hydroxamates have also proven to be excellent copper oxide collectors and are opening new performance opportunities in sulphide circuit copper oxide recovery. According to Frank Cappuccitti, director of global mineral processing chemicals, these new products give operators another tool to recover oxide copper otherwise lost to tailings in a typical sulphide circuit.
Cytec's 5000-series promoters, introduced in the early 1990s, have gained solid market acceptance because of their improvement in metallurgical performance and selectivity at lower alkalinities than xanthates, allowing significant reductions in lime consumption.
Other developments include a new class of polymeric depressants that are designed for selective sulphide separation and insols depression. They are currently replacing natural polymers and inorganic depressants in copper, copper/molybdenum, base metal and precious metals circuits.
Cytec also offers a full line of collectors, frothers, flocculantts, and dewatering aids for the alumina, coal, and mineral processing industries, as well as a line of phosphine-based solvent extractants.
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|Title Annotation:||innovations of flotation technology systems utilized in mineral processing|
|Date:||Nov 1, 1998|
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