Bauxites are extremely dusty at moisture contents as high as 16 per cent while others become very sticky and difficult to handle at moisture contents as low as 6 per cent. In some cases bauxites, for example, at 15 per cent moisture the material can be dusty and at 18 per cent moisture the material can be extremely sticky.
Therefore, the emission factors and applicable control systems change markedly as the moisture content changes by this small amount. It is recommended at the present time that opacity be used as the main criteria for judging whether or not additional control is needed on bauxite unloading facilities and transfer points.
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Since most of the processing in the Bayer system occurs when the materials are wet, air pollution control requirements are minimal except at the beginning of the process stream (bauxite) and at the end of the process where the product (alumina) is calcined, transported for storage and/or loading for shipment.
The original new source performance standard (NSPS) require the following:
1. Total fluoride emissions must be limited to 0.9 kg of fluoride per tonne of aluminium produced from prebake potlines and 0.05 kg of fluoride per tonne of aluminium (equivalent from anode baking furnace systems).
2. Total fluoride emissions from Soderberg operations must be limited to 1.0 kg/tonne of aluminium produced.
3. Opacity from the carbon baking furnace stack must be less than 20 per cent.
4. Opacity from all other sources in the reduction plant must be less than 10 per cent.
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The guidelines document states that plant age, the cost of retrofit, the environmental conditions around the plant and future land uses should be incorporated into any decisions concerning emissions to be placed on existing aluminium plants. The guidelines specify that hooding efficiency ranges for existing prebake plants should be 90-95 per cent for fluorides and that removal efficiency for fluorides by primary control systems should be 95-98.5 per cent.
Several countries have adopted these guidelines while others have developed specific fluoride emission limitations based on the equipment efficiency criteria. Since the exact limitation is based on the assumptions of uncontrolled emission rate to which the hooding and collection efficiencies are applied, the range of allowable fluoride emission is broad.
Bauxite Handling Dust Control:
A number of ore handling and processing facilities have been sampled to determine uncontrolled emission rates. However, because of the wide range of ore types and the moisture content of these ores, it is difficult to develop a series of emission factors that will adequately describe the range of emissions that can occur.
This is particularly true of bauxite ores. Since bauxite represents a broad spectrum of materials with varying particle sizes and mineral content, the emission potential from bauxite loading and unloading systems is highly variable. The ores can be extremely dusty or they can be relatively sticky and difficult to remove from conveyor belts and control devices.
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Particle size, surface area and moisture content appear to be important characteristics that affect the emission potential. Some ores, for example, with very finely divided particles may be dusty with moisture contents in excess of 16 per cent, while ores from other sources may be quite sticky at the same moisture content. With many bauxites, the dividing line between dusty ore and sticky ore may be less than a 1 per cent change in moisture content.
This characteristic makes it difficult to design liquid spray suppression systems to reduce emissions from some dusty bauxites. The ore is also inconsistent in particle size, varying from extremely fine to large and lumpy. Thus, control equipment and handling devices must be designed to be effective through the entire range of particle sizes.
Bauxite handling facilities are usually very large systems that are used intermittently when bauxite ships arrive at a loading facility. The need for bauxite dust control should therefore be carefully evaluated on a case-by-case basis. Since some bauxites can be highly dusty, while others will generate almost no dust at all, the sources of bauxite should be established and control equipment designed accordingly. In all cases, ‘care must be taken to ensure that the collection device is compatible with the wide range of materials that exhibit quite different properties when exposed to moisture contents that may only differ by several percentage points.
Bauxite dust, when relatively dry, can be and is controlled by the use of conventional dust collection equipment such as bag houses and mechanical collectors. The dry material, although somewhat abrasive, is relatively easy to capture and the sizing of dust collection equipment is routine.
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The degree of agitation that the material receives and the distance of free-fall dictates the dust loadings that will occur when a given volume of air is removed from a transfer facility. Inlet concentrations to control equipment can range from 5 to 15 g/m3. The great difference in particle size has made it necessary and wise to size dust collection equipment on the conservative side.
Past history has shown that equipment installed to capture only large particles is incapable of handling a wide variety of particle sizes that exist when bauxite ores are loaded or unloaded. Pulse-type bag collectors are normally used at small transfer points and storage tanks.
Large shaker-type bag filter systems have been used successfully on ship unloaders where clamshell buckets remove the bauxite from sea-going vessels and deposit it in the loading hopper. Outlet concentrations of 0.05 g/m3 are achievable and can usually be guaranteed by equipment suppliers.
A major problem associated with equipment collection from bauxite handling facilities is the impact of moisture on the material to be collected. If wet dust is drawn into a duct and bag collector, the material will cake and eventually plug the duct, blind the bags and stick to belts.
When this occurs, a major effort is required to clean the entire system and make the collection workable at reasonable pressure drops. Large baghouse installations handling bauxite dust must be designed to overcome problems associated with high relative humidity situations that occur when slightly moist bauxite is handled.
Wet capture techniques generally have not been used successfully to control bauxite dust. The high energy consumption, equipment sizing and most of all, disposing of the wet material in an effective manner have steered engineers away from using the wet-type collectors on bauxite handling facilities.
Alumina Refinery Dust Control:
There are a number of situations that occur between the mining of bauxite and the shipping of aluminium ingot where dust can be generated and should be controlled. The original bauxite material is changed from a highly variable mineral ore that can be extremely dusty or wet and sticky to a dry, dusty product with an average particle size near 80-100 μm and approximately 10 per cent by weight less than 44 μm.
The major point sources that must be controlled are the alumina calciners and lime burning facilities. In addition to the major point sources, a number of small sources, including loading and unloading facilities and transfer points, should be equipped with control equipment.
Alumina Calciner Emission Control:
The major point source for alumina refining operations is the calcining system. Aluminium hydrate from the Bayer process is removed from the precipitation step by filtration and the wet cake is fed to calciners to reduce the moisture content from 45 to 2 per cent or less. Calciners consist of rotary kilns or newer fluid-bed calciner systems. Both types of systems produce off-gas streams ladened with dry alumina dust (10-25 g/m3) and approximately 40 per cent moisture.
The most effective control systems for these emissions are electrostatic precipitators. In some kiln systems, the hot end (product discharge end) is controlled by cyclones, multi-clones or electrostatic precipitators. The ‘cool end’ of the kiln is usually equipped with an electrostatic precipitator.
Fluid-bed calciners are complex systems which rely on product movement through a series of heating chambers and cyclones to maintain appropriate temperature levels during the drying process. All fluid-bed calciners are equipped with electrostatic precipitators as the final dust removal device.
A typical precipitator system on a modern flash calciner is designed to handle 2000 to 3000 m3/min at a gas temperature of 120-170°C with an inlet concentration of 11.4 to 24 g/m3. The design outlet concentration is 0.08 g/m3. The precipitator pressure drop is 1.25 cm of water and the specific collection area of the plates is 2154 m2. A typical precipitator will contain three to four separate fields, each energised and rapped separately.
The high moisture content of the gas stream, plus the presence of any sulphur emitted by the fuel burned, assists in the conditioning of the alumina particles to ensure efficient collection. Since alumina particles in high-moisture gas streams exhibit unique electrical properties, precipitator design should be based on a thorough study of the gas stream to be treated. Precipitator life is good, but a comprehensive preventive maintenance programme is required to prevent wire breakage and plate fouling.
The alumina product is conveyed either by belt conveyors or air slide conveyors from the refining process to storage facilities. It must then be transported by rail, truck or ship to reduction plants for conversion to aluminium. Each time the alumina is handled, care must be taken to control dust emissions generated by agitating the material. (The captured material is normally returned directly to the process.) Small, pulse-type dust collectors are usually placed at transfer points or on storage tanks to keep the fine alumina particles from escaping to the atmosphere.
Collectors of this type probably account for several per cent of the total weight of the alumina that is handled. The particle size of the collected material averages approximately 10 μm. Dust collectors installed at transfer points may be exposed to inlet concentrations on the order of 3 to 8 g/m3.
Care should be taken in designing the dust collector system to ensure that the exhaust velocity to the baghouse system is not so large that it removes dust material from the conveying system that would not normally be airborne. High-velocity off takes are less desirable than tightly hooded transfer points with more modest air removal rates.
Ship unloading of alumina is a particularly difficult control problem. Some ship unloaders are of the pneumatic variety where the alumina is removed from the ship by a vacuum system and deposited in an on-shore storage facility. However, in many cases, available vacuum unloading systems are too small or too energy intensive to adequately serve the large ocean-going vessels that deliver alumina to the ports. In these situations, clamshell unloaders on gantry cranes are utilised.
Two major problems exist, at these loaders. First, dust is generated in the hold of the ship because the alumina is usually warmer than the surrounding environment. Since alumina has excellent insulating qualities, the heat is retained from the calcining operation imparted to the alumina at the refining plant. This heat creates turbulence in the hold which allows dust to be emitted from the large open hold as the clamshell unloading operation is under way when the hold cleanup operation occurs.
The problem is exacerbated by the turbulence created when winds blow across the large open holds. To date, there is no workable solution to this problem since the configuration of holds is variable and the size of the openings must be large enough to accommodate the clamshell unloading systems.
The second dust situation occurs when the clamshell bucket dumps its load into the receiving hopper. These areas are normally controlled by large baghouse facilities that create a face velocity of approximately 50-70 m/min at the hopper entrance. With appropriate baffling and aprons around the hopper entrance to direct air flow and spilled alumina, the dusting situation on the dock facility can be adequately controlled. The baghouse systems for such facilities are quite large but effective, easily capable of maintaining outlet emissions below 0.02 g/m3.
Reduction Plant Emission Control:
Fluoride emissions from aluminium reduction plants have been of major concern. As aluminium plants became larger in production capacity, the fluoride emission rates became significant enough to cause discernible effects on sensitive vegetation and grazing animals. Initial attempts to control these emissions utilised a variety of wet scrubber techniques.
Today, improved wet scrubbers, wet electrostatic precipitators and dry scrubbers are utilised. Since 1970, all of the aluminium reduction facilities have been equipped with dry scrubbers and all existing plants retrofitted with new control systems have also used dry scrubbers, with the exception of four horizontal stud Soderberg plants. Therefore, the major emphasis of this discussion will be on dry scrubbers.
Aluminium reduction plants are large, consisting of 2 to 28 separate buildings up to 600 m in length. Plants can contain over 900 separate reduction cells, each of which must be worked individually. These cells are usually hooded to provide a mechanism for the fluoride-bearing off-gases to be collected for treatment. These gases are called primary emissions.
The level of hooding efficiency that can be achieved is finite since the cells must be opened periodically for anode work, bath adjustments, metal removal and other process operations. In addition, perfect hooding is difficult to maintain as the cell ages. Current hooding technology for modern, prebake cells consists of closely fitting side shields and hinged end skirts.
The side shields should be substantial enough to withstand the normal wear associated with daily removal and replacement in the hot environment around the electrolytic cells. Some new hooding systems are also equipped with latches that draw each shield close to the adjacent shield to minimise the possibility of gaps in the hooding system.
Latches are not needed if hooding placement is proper. The hooding systems without latches can achieve similar efficiency levels through management attention, but the latched system increases the probability of good hood placement and higher hooding efficiency.
The exhaust volumes associated with cell operations also play an important part in the hooding efficiency obtained. A number of criteria can be used to establish exhaust volume levels. Some rely on an exhaust rate based on the meters of cell perimeter or the square meters of cell surface area.
Others attempt to estimate the square centimeters of cell opening that exist with all hoods in place and establish a 1 to 2 m/sec average inlet velocity through these openings. Most often, exhaust volumes are determined through smoke bomb tests and experience with other operating systems.
Many exhaust systems are equipped with dual exhaust volume capabilities. During normal operations, when all the cells controlled by a common exhaust system, usually 8 to 32 cells, have all hoods in place, each is exhausted at the same rate. When one to three of the cells are opened to be worked, the dual volume system will allow the exhaust rates of these cells to be increased by 50-200 per cent at the expense of the closed cells.
The total volume from the system is not increased but simply redistributed by opening dampers in the cells that are being worked. The slight reduction in inlet head loss causes the exhaust volume to increase significantly in those cells to overcome potential losses that would occur when side shields are removed.
Large, modern prebake cells are provided with exhaust rates of 6000-9000 am3/hr. Vertical stud Soderberg cells are exhausted at approximately one-tenth that rate because of the difficulties associated with hooding those types of cells. Horizontal stud cells are usually exhausted at rates higher than prebake cells because the hooding system consists of large enclosures which fit over the entire cell rather than individual side shields.
The hooding efficiencies of 85-90 per cent can easily be achieved with horizontal stud Soderberg cells. Vertical stud cells can achieve 75-80 per cent hooding efficiency. Modern, prebake cells can achieve 90-95 per cent hooding efficiency on existing plants while new cells with modern hooding systems included as part of the cell design can achieve 97 per cent efficiency or more.
The gases that escape the hooding systems, 20-25 per cent for vertical stud Soderbergs to 3-10 per cent for modern, prebake cells, are removed from the cell room by natural draft ventilation or in a few instances, power ventilators. These ventilation rates may be as high as 1 million m3/tonne of aluminium produced, while the primary exhaust from the hooding systems is of the order of 80000-1,25,000 m3/tonne of aluminium produced.
These ventilation gases are dilute, often containing less than 1 mg of total fluoride per cubic meter and 2 to 10 mg/m3 of total particulate matter with a mass median diameter in the 3 to 10 μm range. These dilute gases and small particle sizes discourage the use of control systems to treat these emissions. Normally control attention is focused on the hooding and primary control systems rather than the cell building ventilation air.
Primary control systems for aluminium reduction plants must have high reliability and should operate with high efficiency. It has been determined that 98.5 per cent total fluoride removal efficiency is achieved in control systems added to existing reduction plants.
Most state regulations require 95-98.5 per cent removal efficiency. New facilities often achieve removal efficiencies of 99 per cent or more. To achieve these levels of efficiency, the control systems must be effective for gaseous fluoride as well as fine particulate and fume.
Low-energy wet scrubbers have often been used. Spray towers, moving bed scrubbers and spray screens are in common use.
Calcium or sodium compounds are added to the scrubber water to enhance the gaseous fluoride removal, producing compounds that can be removed as sludges:
The major problems to be avoided are system scale-up in the scrubber water recirculation system, nozzle pluggage in the scrubber system, scrubber pluggage due to particulates in the off-gas system (and hydrocarbons in Soderberg cell off-gases) and corrosion. All wet scrubbers should have an extensive preventive maintenance programme designed and implemented at startup.
Many wet scrubber systems are preceded by cyclones, multi-clones or electrostatic precipitators to remove some of the particulate matter from the off-gases. Such systems operate with relatively low head loss and low efficiency. Since the mass median diameter of the particulates is less than 10 μm in such systems, high efficiency is impossible to achieve with mechanical collectors and difficult to achieve and maintain with simple electrostatic precipitators.
In a limited number of situations, wet electrostatic precipitators are used for both gaseous and particulate fluoride removal and particulate control. Such systems are capable of handling the off-gases from Soderberg cells with increased hydrocarbons present. Again, an effective preventive maintenance programme is needed to ensure reliable operations of these systems.
Although wet scrubber systems are still in use in India, all of the aluminium reduction facilities built since 1972 have been equipped with dry scrubber systems and most of the existing plants have been converted to dry scrubber systems during that time.
Such systems have the advantage of closed-cycle operation where no contaminated water or sludges are produced and the collected materials can be returned directly to the cells. Since the dry scrubber system returns material directly to the process, it should be designed as part of the process itself, rather than as an ‘end-of-the-pipe’ treatment process, if total success is to be achieved.
Dry scrubber systems utilise alumina as the scrubbing medium. After use the material is transported to the cells and used as the ore feed for the process. The collected fluorides and particulates are also returned to the cell and become part of the electrolytic bath. Appropriate adjustments must be made to the bath chemistry to maintain the proper balance in the cells.
The returned fluoride and alumina dust are valuable constituents which may offset the cost of operation of the dry scrubber system. However, trace contaminants such as iron and silica are also returned to the cells and ultimately reach an equilibrium concentration in the metal produced.
If high purity metal is desired, some cells may be operated on pure alumina, while the aluminium used to scrub the off-gases is directed to other cells. Such ore management is possible for a limited number of cells, but because the sorption capacity of the ore for fluoride is finite, care must be taken in designing the ore management system to ensure that the scrubber systems receive the required amount of feed.
The factors that dictate the efficiency of the ore to adsorb fluoride include surface area of the ore, sodium content of the ore, the relative humidity of the off-gas stream and the contact time between the ore and the fluoride gases.
A surface area of 45 m2/g is normally recognised as the minimum that should be utilised for dry scrubbing applications. At surface areas in this range, alumina can adsorb approximately 4 per cent of its weight in gaseous fluoride. However, to achieve this high removal a long contact time is required. There is evidence that the sodium content of the alumina develops active sites that are more efficient at sorbing fluoride.
Therefore, the sodium content of the alumina plays a role in the ability of the alumina to retain fluoride. The relative humidity of the gases in contact with the alumina plays a role in fluoride adsorption. There is an optimum relative humidity level for each situation.
However, since off-gas concentration, surface area and contact time also are important, it is difficult to quantify the impact of relative humidity. In most situations, the relative humidity is considered an uncontrolled variable that cannot be used as a design criteria to increase fluoride removal.
Perhaps the most important variable in the design of a dry scrubber system is the contact time. It appears that a contact time of 1 to 3 seconds is appropriate for good fluoride adsorption. Shorter contact times can be used if good distribution of alumina particles throughout the gas stream is accomplished.
The two major types of dry scrubber systems differ in the way that the contact time parameter is achieved. The injected alumina system relies on contact between the absorbing medium, alumina and the fluoride-bearing gases in a disbursed phase system.
Proper distribution of the alumina throughout the gas stream is critical for efficient fluoride removal. Alumina is injected into the gas stream through various types of jets and nozzles and the contact occurs as the alumina is carried along in the gas stream. Some systems utilise venturi sections to ensure good turbulent mixing and to aid in the dispersion of the alumina particles in the gas stream.
The alumina off-gas mixture enters a disengaging section where much of the scrubbing medium is removed by gravity. The gas stream is then directed to a bag filter or electrostatic precipitator system to capture the finer fraction of the ore. The scrubbing medium is returned to the cell room feed system or recycled to storage tanks where it can again be passed through the injection system.
Most injected systems rely on some recycle to ensure adequate removal of the fluoride gases. In some cases, this recycle may amount to 200 per cent of the ore feed and appears to be related to how effectively the ore is distributed throughout the gas stream during the injection step.
The injected alumina scrubber systems have the advantage of accomplishing the dry scrubbing at lower head loss requirements than fluid-bed systems. The disadvantages of the system include the fact that recirculation is often required and the attrition of the alumina and abrasion of the equipment is increased.
The other major type of dry scrubber is the fluid-bed system. In this type of system, the off-gas stream is contacted with the scrubbing ore by passing the gas through an expanded bed of alumina. The bed moves along a horizontal perforated plate and the gas stream enters through the perforations, fluidising the ore. Fresh alumina is fed into one end of the reactor system and the reacted ore is removed at the opposite end.
The flow of alumina across the bed is controlled by the inlet feed rate. A baghouse system is placed above the fluid bed to collect the particulate matter that is carried from the bed to the gas stream. Pulse-jet bag systems or shaker-bag systems may be used for this application. In the case of the former system, the reactor operates continuously. With shaker-bag systems, the air flow must be shut off to each individual reactor unit for 20-30 minutes every few hours to clean the bags.
The advantage of the dry scrubber system is that even if alumina feed is lost to the dry scrubber system for a period of several hours, the fluid bed retains the capacity to continue to remove fluoride with high efficiency. Breakthrough tests have indicated that, under normal conditions, fluid-bed systems may have up to 8 hours of ‘safety factor’ built into the system should alumina feed be lost.
In the fluid- bed systems, no recirculation of the ore is required so alumina handling can be simplified. A major disadvantage of the system is that the fluid bed itself and the perforated plates required to support the bed generate higher head losses than the injected alumina systems. Therefore, there is an energy penalty associated with the operation of fluid-bed dry scrubber systems.
In both types of dry scrubber systems that utilise baghouses for final particulate collection, bag lives on the order of 15-24 months have been reported for the pulsed-air systems and 48-60 months for the shaker-bag systems. Most bag failure problems are associated with poor design of the entrance conditions that allow alumina-laden air streams to impact directly on the bag systems. Careful design of entrance systems plenums and bag supports can significantly impact the bag life achieved in dry scrubber systems.
The off-gas temperatures from reduction cells will vary as the effectiveness of the hooding systems vary. However, in most cases, the off-gas temperatures are in the range of 110 to 125°C and necessitate the use of bags that can withstand these types of temperatures. Efforts to condition the gas stream by wet or dry cooling systems have not proved effective and efforts to bleed in ambient air for cooling purposes resulted in increased power cost and reduced exhaust volumes from the cells themselves.
Since reduction cells operate continuously, dry scrubber systems must be designed to also treat the off-gases continuously. Scrubber maintenance, fan maintenance and bag cleaning requirements necessitate periodic shutdown of units, so the scrubber systems are constructed with multiple units in parallel.
Typical scrubber systems may consist of 5 to 20 units of 1,00,000 m3/hr. capacity in a bank to serve all or a portion of a potline. The removal of any single unit from the system will have little impact on the collection rate from the cells and will have no impact on the removal efficiency of the operating units.
Most dry scrubber systems are capable of achieving fluoride removal efficiencies in excess of 99 per cent. Inlet gases containing 100 mg/m3 of fluoride are exhausted at concentrations less than 1 mg/m3. Particulate loadings of 100 to 200 mg/m3 are also reduced to similar levels. Dry scrubber emissions are normally low, representing less than 10 per cent of the total plant discharge.
Carbon Baking Furnace Emission Control:
A major emission point from aluminium reduction facilities utilising the prebake anode configuration is the carbon baking furnace. In this operation, the anode blocks are formed using a mixture of coke and pitch and are subsequently baked at high temperatures to form a block with the appropriate strength and electrical resistivity characteristics required for reduction cell operations.
The baking process is accomplished in furnaces that consist of pits separated by flues in which fuel is burned to generate the heat necessary to perform the baking operation. During the baking cycle, the volatile materials in the pitch binder are driven from the anode blocks and collected in the flues. A portion of these hydrocarbons is burned in the flue system.
However, some of this material escapes along with particulate matter that may leak from the baking pits into the flue system. In addition, since anode butts removed from the cell when the anode becomes too small to effectively conduct electrical current to the process are crushed and reused in new anodes, some fluoride contamination is carried into the baking operation. During the baking cycle, this fluoride is also volatilised and escapes from the furnace system.
A variety of control devices to remove the particulate, hydrocarbons and fluorides from the baking process have been utilised. Wet scrubber systems have been used for a number of years, but the nature of the collected material makes it difficult to remove the sludge from the system. In addition, once removed, the sludge poses a significant disposal problem.
In recent years, several types of electrostatic precipitators, both wet and dry, have been used to treat the off-gases from carbon baking furnaces. In some types of bake furnace operations where oxygen conditions are low and off-gas temperatures are in the appropriate range, these systems have proved to be effective.
However, in other types of furnaces where oxygen conditions are higher and off-gases may be above 250°C, such systems can pose safety problems because of the fire potential associated with electrostatic precipitators. The materials collected by these systems are normally sold to reprocessors for conversion to fuels or other by-products. The material cannot be reused directly in the anode forming process.
Dry scrubbers have also been utilised for carbon baking furnace emission control. Both injected- alumina systems and fluid-bed systems are currently in operation. In the injected systems, precoolers may be required to condition the gas stream. The temperature is lowered, usually with water sprays, to the point that a major portion of the hydrocarbons will condense and can be adsorbed by the alumina particles.
In these systems, distribution of the alumina in the gas stream is critical in determining the efficiency of the collector for fluoride and hydrocarbon removal. Baghouse systems follow the injection device and disengaging section. Alumina recycle capabilities are designed into the system to improve collection efficiency.
In the fluid-bed-type systems used to control carbon baking furnace emissions, the gas conditioning and scrubbing normally take place simultaneously. An appropriate amount of water is added to the reactor with the off-gases to be treated to ensure that appropriate collection of hydrocarbons is achieved. Both hydrocarbons and fluoride are collected by the alumina bed. A baghouse system, usually of the pulsed type, is placed above the fluidised bed to collect alumina entrained in the exhaust gas stream.
The alumina from either type of system can be calcined to remove the hydrocarbons collected or reused directly in the reduction process. Evidence indicates that if calcining occurs at or below 600°C, the adsorbed fluoride will not be driven off the alumina. Recent experience indicates that calcining may not be necessary if hydrocarbon levels in the furnace off-gases are low.
The off-gases from carbon baking furnaces may contain several hundred milligrams per cubic meter of condensable hydrocarbons and 100 mg/m3 of fluoride. In order to meet new source performance standards, the hydrocarbon emissions from new furnaces must be reduced to the point that the opacity of the plume is less than a Ringelmann 1. This requires reduction to the 10 to 20 mg/m3 range. Fluoride emissions from new furnaces must be reduced to 0.05 kg/tonne of aluminium which is equivalent to approximately 5 mg/m3.
Attempts have been made by various manufacturers to utilise coke material as the scrubbing medium in carbon baking emission control systems. However, low fluoride capture and material transport problems have led researchers away from this concept.
Recent advances in furnace firing techniques have resulted in significant energy savings as well as improved combustion of the hydrocarbon waste products. These advances have made it possible to achieve the Ringelmann 1 opacity limits without control systems.
However, the stringent fluoride emission standards may not be achievable through process management. This area requires further study to determine the minimum fluoride levels that can be achieved through improved butt cleaning practices and process improvement.
Control system costs for primary aluminium production facilities are highly variable since there are a number of methods utilised to achieve acceptable emission levels. Dust control systems for bauxite unloading facilities represent 10-12 per cent of the total unloading system costs. Electrostatic precipitators for calciner dust control at refineries may be 16-20 per cent of the total calciner cost.
The international primary aluminium institute has determined that fluoride emission control systems, exclusive of the hooding systems, range in cost from 2 to 11 per cent with an average cost of 6.6 per cent. Carbon baking emission control systems are 8-10 per cent of the cost of a baking facility.
In all cases, there is some return of usable material to the process from well-designed and operated control equipment. The value of these returned materials is difficult to quantify, since the impacts of these returned materials on process operations and product quality are not fully understood. However, in most cases, it is generally believed that the recovered material value is enough to offset the operating costs of the control systems. In some instances, the recovered material value may also be large enough to amortize a portion of the capital required to install the systems.