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This historical page is being kept for research or reference
purposes and will not be updated.

This historical page is being kept for research or reference
purposes and will not be updated.

4. Recommended PM2.5 Emission Control Practices

This section outlines the measures recommended to reduce particulate matter emissions, including PM2.5, resulting from processes associated with the primary aluminum sector. The recommendations are divided into six categories, as follows:

Each recommendation followed by a brief discussion is associated with one or more sources of particulate matter listed in Table 3-1. Although PM2.5 emissions are difficult to establish with any certainty and were not precisely quantifiable when this report was drafted, the recommendations are listed in order of importance (by category) in a subjective manner with regard to the extent of potential PM2.5 emission reductions.

The constantly evolving electrolysis technologies allow each new plant to improve the energy and environmental performance of the primary aluminum industry. The effectiveness of fume extraction from cells follows this trend, thus reducing fugitive emissions of particulate matter in the potroom where the ambient air is not treated. Aside from installing new scrubbers, optimizing electrolytic cell operation methods and, to a lesser extent, the quality of raw materials (sealed prebaked anodes, cryolite bath, alumina) is the best approach to reducing the air pollution of an existing facility. This is true not only for gases, but also for fine particulate matter, which acts more like a gas and remains suspended in ambient air.

Although few studies have been conducted on particulate matter emitted through the electrolysis process, it would seem at first that fluorinated particulate matter (Na3AlF6, Na5AlF14, NaAlF4, AlF3, CaF2) discharge occurs either through vaporization and then condensation of the electrolyte through contact with gases, or through a chemical reaction in the gas.7 Approximately 50% of fluoride emissions from cells are particulate matter, of which 70% (or 35% of the total) is condensable PM2.5. It is therefore safe to say that, with the known emissions mechanisms, the operation of a smelter is responsible for PM2.5 emissions.

Recommendation A01 – Optimize work methods for opening a minimum of hoods at once and open them only once work begins. Close hoods as soon as the work is complete.

Sources Targeted – S01–S05

The operation of an alumina reduction plant requires that electrolytic cells be worked on regularly. This may involve changing the anodes, tapping the molten metal or cryolite bath, and charging the cryolite bath; each of these operations requires that the hoods be opened, which exacerbates the leakage of fine particulate matter in the potroom. It has been observed that HF emissions associated with these tasks account for approximately 60% of a plant’s HF emissions, including those from the GTC.8,9 Optimizing work methods to minimize the number of hoods open simultaneously and the length of time they remain open is the best approach to controlling these emissions. HF emissions (and therefore PM2.5) increase with the number of simultaneously open hoods,10 therefore, hoods should not be left open unnecessarily.

Recommendation A02 – Regularly diagnose the extraction efficiency of ventilators in relation to power supply, flow rate and pressure loss. Make adjustments as needed to maximize extraction rate.

Sources Targeted – S01–S06, S16–S23

The gases and particulate matter released under the pot hoods are continually ventilated and routed to the GTC through a main duct. The fume extraction efficiency depends largely on the configuration of the superstructure, with openings kept to a minimum. This maximizes the negative pressure in the pots based on the ventilation power of the back-end ventilator. When one or more hoods are open, the negative pressure decreases locally and can even be inversed, which reduces the ventilation capacity to the GTC and results in more leaks in the building. Pots located at the potroom extremities (further away from the ventilator) could be more prone to a decrease in fume extraction capacity. A reduction in the existing system’s efficiency is another aspect that lowers the extraction of particulate matter to the GTC. At this stage, it is advisable that a ventilator monitoring and maintenance program be established to maximize the extraction flow at all times according to the applicable power. This recommendation applies to all gas scrubbers that use a ventilator.

Recommendation A03 – Optimize work methods to minimize the time required to change anodes and cover them with cover material.

Sources Targeted – S01

Changing anodes leads to considerable fugitive emissions in the potroom. This generally involves opening the pot hood, breaking the crust around the anode butt, marking the height of the anode butt in the pot, removing the anode butt, cleaning the cavity with an adapted extractor (e.g., Pacman type) and placing the new anode in the cavity. This procedure generally takes more than 10 minutes and must be done diligently to prevent operational problems.11 It is therefore advisable to develop and apply an effective work method that minimizes the amount of time required to change the anodes (e.g., cleaning the cavity with the extractor immediately after removing the anode butt, and placing the tray that collects the anode butt and bath residue close to the pot).

Recommendation A04 – Put in place a program to monitor cracks in the crust by way of a visual inspection or an automated system. Ensure that the cover material is appropriate and effective as a sealant.

Recommendation A05 – Cover the taphole with cover material once the tapping or sampling work is complete.

Sources Targeted – S01–S06, S16, S17

An important factor that affects the fluoride emission rate from pots is whether the alumina and cryolite crust that covers the cryolite bath is intact, since it acts as a physical barrier against gas migration.12 It has been shown repeatedly that the intensity of emissions correlates with the extent of the holes and cracks in the crust.13 These gaps are created notably when alumina is injected with a feeder/breaker, when the metal or bath is siphoned through the taphole, and when work is being done to suppress the anode effect. Poor coverage of the new anodes is another cause. A monitoring program (visual or automated) therefore ensures that the crust contains and sustains a minimum of cracks, thus reducing the emission of particulate matter at the source. Moreover, it is recommended that the tapholes be covered with cover material as soon as sampling or tapping of the molten metal/cryolite bath is complete.

Since it is of a heterogeneous nature, the composition of the cover material may vary and have a direct impact on the quality of the crust.14 Aside from the crack monitoring plan, it is also recommended that the cover material be diligently monitored to prevent, among other things, the chronic appearance of cracks in the crust.

Recommendation A06 – During tapping activities, reroute fumes evacuated by the crucible inside the pot using flexible tubing.

Sources Targeted – S02, S03

Aluminum metal produced through electrolysis is deposited on the surface of the cathode at the bottom of the pot and is generally extracted daily to maintain optimum height. To do this, a heat-insulating crucible equipped with a siphon is used. This normally involves opening the pot hood, preparing a taphole in the crust, connecting the vent pipe and the compressed air to the crucible, placing the siphon in the taphole, tapping the metal from the crucible, removing the crucible once the tapping is complete, sealing the taphole and closing the hood. Cryolite bath tapping follows the same basic procedure and is done primarily to maintain the bath height, which is instrumental in thermal equilibrium and alumina dissolution. The extent of fugitive emissions associated with this work is closely linked to the amount of time the hoods are open (see Recommendation A01) as well as the gas expelled through the crucible’s air intake. This gas is composed of air mixed with cryolite vapours. It is therefore recommended that this gas be transferred into the pot to be captured and treated at the GTC instead of being expelled into the potroom.

Recommendation A07 – Minimize and recover cryolite bath spills and spatter on the floor.

Recommendation A08 – Pour the bath into the pot launder at an optimal speed to reduce pouring time while avoiding spatter. Avoid pouring too slowly.

Recommendation A09 – Clean the cryolite bath residue from the pot launder with a spade (or equivalent) when loading is complete.

Sources Targeted – S01, S02

After being tapped from a pot, the liquid cryolite bath may be transferred to another pot where the bath height is low. This normally consists of opening the hood, positioning the launder, positioning the crucible near the launder and opening the crucible spout, pouring the bath into the pot down the launder and through the taphole, removing the crucible and launder when the charging is done, and closing the hood. Particulate matter emissions associated with charging the cryolite bath are closely linked to the opening of the hoods (see Recommendation A01) and the exposure of the hot cryolite bath in the launder located outside the hood shaft. An increase in emissions is inevitable if the grey iron is spilled during casting or if the residual cast iron in the launder is not cleaned immediately after casting. The cryolite bath may also be spilled on the floor when changing anodes. A work method that limits these circumstances would reduce the rate of emissions into the potroom.

Recommendation A10 – Cool the hot carbon dust (dross) inside the pot. Minimize the time spent in the potroom.

Sources Targeted – S02, S03, S05

The spread of carbon dust in the electrolytic bath is associated with the quality of the anodes and the operating conditions used. Selective oxidation of tar pitch releasing coke grains, cathode wear and the addition of carbon through the cover material as well as enriched alumina are the main mechanisms leading to carbon accumulation.15 This material may cause an increase in the electric resistance in the bath, resulting in a higher temperature and reduced current efficiency, not to mention an increase in the anode consumption rate. It is therefore essential to remove carbon dust regularly. Hot, freshly collected carbon dust is imbued with cryolite, amplifying the scattering of fluorinated particulate matter in ambient air. It is therefore recommended to leave the carbon dust to cool inside the pot (deposited on top of the crust) to capture a maximum amount of PM2.5 emissions arising from it so they can be ventilated and treated at the GTC.

Recommendation A11 – Control and maintain an optimal bath level in the pot to prevent an unintended rise in bath temperature and direct contact with moist air. These two phenomena exacerbate the formation of fluorinated particles.

Sources Targeted – S01–S06, S16, S17

From an environmental perspective, it is recommended that the bath have little or no contact with the air under the crust to limit the formation of HF and fluorinated particles, depending on the mechanisms that involve moisture.16 Moreover, by controlling the bath height and the crust thickness, the thermal equilibrium can be maintained more easily, thus preventing an involuntary increase in temperature, HF and fluorinated microparticle emissions.

Recommendation A12 – Prevent, control and minimize the anode effect. After manual or automatic suppression of the anode effect, cover the cracks with cover material.

Sources Targeted – S01–S06, S16, S17

The anode effect is a phenomenon that afflicts electrolysis with a sudden increase in voltage and decrease in amperage. This is due to the presence of a gas film on the surface of the anode that must be cleared either manually (with a long rod) or using an automated system. In both cases, the crust is affected in that cracks begin to form, increasing the emission rate. The operator should therefore minimize the anode effect and seal the cracks with a cover material as soon as they appear (see Recommendation A04). As a general rule, modern aluminum smelters prevent anode effects by automatically injecting alumina as soon as the voltage in the pot increases.

Recommendation A13 – Maximize casting centre output. Limit discharges.

Sources Targeted– S04

Liquid aluminum in the crucible is transported to the casting centre where it is transferred to a holding furnace and possibly alloyed with other metals. The molten metal is gradually moved to a casting machine to form ingots varying in shape, depending on the client’s specifications. Aluminum casting is a source of metal particles that are normally collected and treated by a dust collector, resulting in very low emission rates.17 From an environmental standpoint, optimizing the casting centre’s output (e.g., minimizing aluminum discharges) is recommended to reduce energy consumption and generation of air pollutants, including particles per tonne of aluminum.

Prebaked anode plants use coal tar pitch (or equivalent) as raw material, but particularly calcined coke (and possibly under-calcined coke), which is the source of particulate matter throughout the manufacturing process. The only exceptions are projections during the mechanical cleaning of anode butts and frozen bath crushing, which generate mixed alumina, cryolite and carbon particles. Most anode manufacturing stages generate particles that cannot be easily avoided or reduced at the source without affecting the process and quality of the anodes. This applies to stations for the mechanical cleaning of anode butts, crushing of frozen bath, and crushing, grinding and sieving of calcined coke. These stations must have particle ventilation systems to provide a safe work environment. The anode baking furnace is also a major source of particles both at the stack and in the building where the furnace is located.

Recommendation B01 – Run an effective baking furnace pit filling system with packing coke to limit coke loss in the building. Train operators to standardize work methods for handling packing coke.

Sources Targeted – S10

A conventional baking furnace is composed of a dozen or so sections arranged in series in two connected parallel rows, which allows the mobile fire to move continuously. Each section has several pits into which green anodes are placed and covered with packing coke, which provides good thermal exchange and protects the anodes from oxidation in the air. When the baking cycle is finished for one section, a special crane removes the baked anodes and loads new green anodes in addition to the packing coke. This stage therefore causes significant projections of particles at the work site if it is not carried out properly.18 The integration of a system or work method limits these coke projections to a recommended minimum.

Recommendation B02 – Operate and monitor negative pressure in the baking furnace.

Sources Targeted – S10

Anodes are baked in a closed environment without being completely sealed since the sections must be released regularly to load and unload the anodes. The air distributed downstream of the mobile fire causing combustion gases and particulate matter could therefore escape from the structure, especially at the access points between the sections that are normally covered with flexible barriers when in loading mode. However, since the furnace is maintained in negative pressure, this barrier notably prevents the entry of cold air into the heated section, which could cause the condensation of volatile compounds and the formation of corrosive chemical species (e.g., HF, H2SO3) in the gas lines.19 The negative pressure thus limits to a minimum particle leaks in the building from the furnace.

Recommendation B03 – Operate effective dusty air extraction and filtration systems for the anode butt cleaning process.

Sources Targeted – S07

The anode butts derived from the alumina reduction process are cooled and placed in storage. They are then placed in turn on an overhead conveyor that transfers them to the anode sealing facility where the frozen bath is extracted following successive stages of pre-cleaning (fragmentation of the frozen bath using power tools), cleaning (rotary brushing of carbon blocks) and grit blasting. These steps produce an average of 4–5 kg of fragmented cast iron per anode, which creates a dusty environment.20 At each of these stations, the butt is placed inside a sealed enclosure to reduce the environmental impact and protect the mechanical and hydraulic systems from the dust. It is therefore recommended that effective air extraction and dedusting systems (compartmentalized or not) be operated. Continuous or semi-continuous monitoring of particulate matter emissions from dust collectors could guarantee their effectiveness. When a sudden increase in emissions is observed, it would be good practice to promptly investigate the cause and make the necessary corrections as soon as possible (refer to Recommendation E01).

Recommendation B04 – Treat gases released from the crushing of frozen bath with dust collectors or in the GTC of the alumina reduction plant or in the FTC of the anode baking furnace.

Sources Targeted – S08

Bath residues removed from anode butts during the various cleaning stages are recovered from under the instruments and then transferred to a crusher to make a sufficiently granular material for recycling in the electrolytic cells as cover material. Operators should therefore treat the air released from the crusher with dust collectors or the GTC of the alumina reduction plant or the FTC of the anode baking furnace. Good GTC and FTC operation practices are applicable in this case (see Section 4.4).

Fugitive emissions of particulate matter and other contaminants occur not only during operational activities, but also as a result of gaps due to premature (or expected) wear of the structure and equipment in contact with process gases. These gaps can be reduced to a minimum provided that the equipment is properly designed, operated and maintained. It should be noted that most fugitive emissions are evacuated by roof vents in the potroom and are not subject to any particular treatment.

Recommendation C01 – Regularly inspect, according to a set schedule, the alumina reduction plant’s facilities, including fume exhaust ducts, the alumina supply system and the pot superstructure. In the event of breakdowns or malfunctions, make repairs or install appropriate replacements as soon as possible.

Recommendation C02 – Regularly inspect, according to a set schedule, the prebaked anode plant facilities, including systems for anode butt cleaning, frozen bath crushing, and calcined coke grinding and sieving as well as the baking furnace. In the event of breakdowns or malfunctions, make repairs or install appropriate replacements as soon as possible.

Recommendation C03 – Regularly inspect, according to a set schedule, the green coke calcining and cooling facilities, including sealing joints and other mechanisms that could potentially lead to a gas leak. In the event of breakdowns or malfunctions, make repairs or install appropriate replacements as soon as possible.

Recommendation C04 – Regularly inspect, according to a set schedule, alumina calcining facilities and boilers at the bauxite refining plant. In the event of breakdowns or malfunctions, make repairs or install appropriate replacements as soon as possible.

Sources Targeted – S06–S10, S12–S14

At first glance, alumina reduction pots that use prebaked anode technology are not completely airtight, which causes continuous fugitive emissions through the openings. With age, the superstructure can also lose its seal, which amplifies the problem. Therefore, expansion joints, rubber seals, gaskets and the like should be inspected regularly and promptly repaired if defective. Other trouble spots include the junction between the superstructure and the main fume duct, cracks in the alumina supply duct, and damage to the superstructure.21 This recommendation applies to all activities covered by this Code.

Recommendation C05 – Put in place an employee training plan in support of an approach to preventing premature wear and untimely breakdowns due to improper operation of the facilities.

Sources Targeted– S06–S10, S12–S14

Many defects (not all) can be caused by improper installation or operation. It would therefore be advisable to establish both an infrastructure inspection and repair program and an employee training plan on the proper approach to preventing these situations.

Recommendation C06 – Use a HEPA vacuum to clean the floor of the potroom.

Recommendation C07 – Regularly clean the ceiling of the pots.

Recommendation C08 – Regularly clear the feeder/breaker of solid residues to reduce the size of the hole in the crust after injection, thus decreasing emissions (corollary to Recommendation A05).

Sources Targeted – S01–S06, S16, S17

Cleaning the potroom floor should be included in a maintenance program. Systems that disperse dust particles from the floor into ambient air should be avoided. Using a vacuum is therefore recommended and preferable to cleaning with compressed air or with a mechanical sweeper. Regular cleaning of the pot ceilings where cryolite bath residues accumulate over time is also advisable to control their dispersion into the potroom when the hoods are opened.

The gases and particulate matter released under the pot hoods in the reduction plant are continually ventilated and routed to the GTC. A GTC typically involves collecting pot fumes through a main duct and then distributing them into vertical reactors into which fresh alumina (commonly known as primary alumina) is injected at the base. The primary objective is to intercept fluorinated compounds, which are harmful to human health and the environment. Whether by injection of alumina or by the use of a fluidized bed, the scrubbed gas must be dedusted in order to recover the “fluorinated” alumina, used as a raw material in electrolytic cells. The dust collector is a baghouse that, based on its configuration and operating parameters, captures most of the process particles (> 99%)22 while consolidating the capture of gaseous fluorine compounds on the alumina cake accumulated on the bags. This process is considered the best practice for treating gas from electrolytic cells. Still, the GTC and any other gas treatment system must be configured and operated properly to maximize performance.

Prebaked anode plants use an equivalent process for treating green anode baking gas in the FTC containing considerable particle emissions in addition to fluorinated compounds from anode butt residues. For the treatment of pitch fumes, anode plants operate a dry scrubber with calcined coke injection specially adapted to capture organic compounds. All pitch fumes are routed to the PFTC, which has facilities similar to the FTC and includes a multi-compartment venturi injection reactor followed by a baghouse.23 Most particulate matter in the PFTC stack is derived from injected coke, which has the same propensity for generating PM2.5 as alumina.

Furthermore, in Canadian calcining plants, hot calcining gases comprising combustion gases, unburned organic compounds and coke particles are scrubbed using a pyroscrubber or boiler followed by a baghouse. Since cooling gas is essentially a vapour flow, it is treated with a venturi-type wet scrubber adapted to wet conditions.

Note: In this section, the term “scrubber” means one of the technologies described above, in other words, GTC, FTC, PFTC, pyroscrubber, boiler, followed by a baghouse and venturi wet scrubber.

Recommendation D01 – Monitor daily gas flow for each compartment of the baghouse in the scrubber (where applicable), ensuring it is uniform. Monitor pressure loss in order to identify anomalies requiring correction.

Sources Targeted – S16, S18, S19

A gas treatment centre is composed of more than 10 parallel compartments (typically 12 to 14), each of which has an injection reactor and filtration unit. The gas flows leaving the compartments are then combined and sent to the stack, whereas the adsorbent (enriched coke or alumina) is directed to a storage silo. Particulate emissions therefore come directly from the filtration unit. The filtration capacity of the baghouse is established on the basis of an air-to-cloth ratio, thus requiring a constant flow of gas for optimal performance. A variable flow (inside a given range) between compartments due to variable pressure loss reduces the performance of the baghouse. It is therefore important to regularly monitor pressure loss and flow in the compartments to avoid premature use of the bags.

Recommendation D02 – Adjust the cleaning frequency and duration of scrubber baghouses (where applicable) to balance gas flow for each compartment and maximize collection efficiency.

Sources Targeted – S16, S18, S19

Higher pressure loss is caused primarily by flow resistance in the gas ducts, injection reactor and baghouse. Bag cleaning controls this pressure loss and maintains the flow within the design parameters of the baghouse. It is therefore important to adjust the cleaning frequency and duration to maintain balanced flow in each compartment.

Recommendation D03 – Wherever possible, limit recycling of enriched alumina in GTC and FTC injection reactors without influencing HF capture. Monitor the daily recycling rate to ensure it is optimal. Measurement and monitoring of the fluoride concentration in alumina could also be considered.

Sources Targeted – S16, S18

In the GTC and FTC, a large part of enriched alumina is recycled in the reactors with a view to controlling its production level depending on the size of the silos and the needs of the alumina reduction plant. However, alumina undergoes continuous attrition during the process, thus increasing the proportion of fine particulate matter in the GTC and FTC supply and, by extension, PM2.5 emissions in the stack.24 Moreover, the specific surface of alumina increases, which improves the HF absorption capacity. Recycling optimization is therefore required. It should be noted that this recommendation does not apply to the PFTC since the enriched coke is generally not recycled. Instead, this coke is returned to the green paste facility.

Recommendation D04 – For the FTC only, operate the cooling tower so as to capture most of the tar contained in the baking gas. Otherwise, add a prefilter (e.g., ceramic packing) to capture most particulate and condensable matter, including tar.

Sources Targeted – S18

The FTC for treating anode baking gas uses a process very similar to the GTC, except for the addition of a cooling tower upstream. This stage involves the injection of a non-saturating amount of water that evaporates entirely, reducing the gas temperature. In so doing, minimal quantities of tar residue, including a small fraction of inbound solids, are collected at the bottom of the tower. The elimination of tar is also critical for the proper operation of the alumina injection reactor and baghouse, where agglomerates may form in the presence of tar. In this case, the performance of the baghouse, where most particulate matter is captured, would be affected.

Recommendation D05 – Optimize the operating parameters of the pyroscrubber to maximize incineration of coke particles in addition to VOCs. As needed, follow up with a system designed to detect particles leaving the pyroscrubber and adjust accordingly.

Sources Targeted – S21

The pyroscrubber has a closed combustion chamber lined with refractory bricks that is maintained at a temperature above 1000°C and receives an injection of air in a number of places to optimize combustion.25 It is sized according to the gas flow to be treated, the time necessary to complete combustion (10–12 s) and the configuration of the equipment. The temperature and oxygen concentration in the pyroscrubber must also be controlled to achieve maximum performance. Moreover, strict monitoring of the pyroscrubber operating parameters is advised to reduce the effect of calciner variations on its performance. Nonetheless, a fraction of the particulate matter, most of which is PM2.5, is not incinerated.26 Clearly, optimizing and monitoring operating parameters are the best ways for reducing these emissions, aside from installing a new, more effective scrubber.

Recommendation D06 – Optimize the performance of the cyclones and baghouse based on the total particle load. As needed, replace bags with more efficient ones.

Sources Targeted – S21

The purpose of the boiler is to produce steam by reducing the temperature of the calcining gas to an acceptable level for the baghouse downstream (typically < 200°C). The baghouse captures the coke particles, the concentration of which is barely influenced by the boiler. PM2.5 is therefore controlled through the baghouse although it can be expected to represent over 90% of the residual particles in the stack.27 There are few options for reducing residual PM2.5 unless the baghouse is not being used to its full potential. Strict monitoring of operating parameters would therefore minimize PM2.5 emissions, depending on the system in place. Replacing filters with more efficient ones where micrometric and submicrometric particles are concerned (e.g., synthetic filters with a membranous polytetrafluoroethylene (PTFE) coating)28 could also be considered. These filters would, however, have to be adapted to the system in place.

Recommendations D01 and D02 also apply for this technology.

Recommendation D07 – Monitor daily gas flow through the scrubber based on the flow of water fed through the venturi, the ratio of which has a direct effect on pressure loss and the effective capture of particulate matter, including PM2.5. Optimize performance based on the system in place.

Sources Targeted – S22

A wet scrubber is designed to clean gas flow with a liquid. In this case, the gas is introduced into a narrow (venturi-type) pass, where it accelerates and atomizes the injected water. The gas/liquid mist is then fed to a cyclonic stripping column where the wash water is recovered and then partially recycled in the venturi neck. The effectiveness of this wet scrubber is highly dependent on the size of the particles to be recovered and the pressure loss applied to the venturi neck. Collection of fine particulate matter is normally very high (e.g., +99% for PM10) but diminishes exponentially for ultrafine particulate matter (e.g., 40–99% for PM1). An increase in pressure loss generally improves effectiveness.29 It is therefore advisable that the operation of the wet scrubber be monitored and optimized.

Recommendation D08 – Regularly inspect, according to a set schedule, the scrubber, including the superstructure, sealing joints, ventilator (corollary to Recommendation A02), alumina (or calcined coke) supply system and baghouse (if applicable). Repair any breakdowns or malfunctions as soon as they are noted.

Recommendation D09 – For the baghouses, replace the bags at the end of their service life. Do not wait until a breakdown occurs.

Sources Targeted – S16–S23

Maintenance of the scrubbers is essential for consistent performance. Regular inspections can reveal breaks in the structure that could lead to leakage of particles (e.g., break in alumina duct or gas duct). Wear of the ventilator (e.g., blower wheel) and the various sealing joints of the superstructure must also be taken into consideration when monitoring the condition (corollary to Recommendation A02). The baghouse bags also deteriorate with time and must be changed regularly before cracks appear, which would increase stack emissions.

Recommendation D10 – For the GTC and FTC only, follow a set schedule to regularly inspect ducts that are prone to accumulation of hard gray scale. Clean if too much has accumulated.

Sources Targeted – S16, S18

The formation of an amorphous material made up of alumina, bath and water (hard gray scale) on the walls of the steel ducts is a problem that can affect the performance of the GTC or FTC and the service life of the baghouse bags.30 Hard gray scale can occur in injection reactors, baghouse and enriched alumina ducts. Among other things, it can increase pressure loss, lower the quality of the gas/alumina mix and create an imbalance in the gas flow between compartments. To prevent these situations, it is highly recommended that at-risk areas be monitored and the ducts cleaned if a harmful accumulation of hard gray scale is observed.

Particulate matter emissions are generated not just by the process, but also during the handling and transportation of solids entering or exiting the process. For instance, metallurgical grade alumina, which is generally dense and powdery, must be protected from the elements when being stored and transported to feed electrolytic cell hoppers; otherwise, material losses and fugitive emissions would result. A closed conveyor handling system equipped with dust collectors is normally used.

The following raw materials, products and by-products typically pass through screw (or pneumatic) conveyors (or the equivalent) between the various transfer points (e.g., storage silo and hopper): fresh alumina, enriched alumina, fresh calcined coke, enriched calcined coke, under-calcined coke, ground frozen bath and bauxite. It is true that most gross particulate matter emissions from these materials are larger than 2.5 microns, although, with the installation of dust collectors, the fraction of PM2.5 at the outlet increases greatly at around 70%.31 Optimizing dust collectors is therefore a solution to minimizing PM2.5 emissions resulting from the handling and storage of various powdery materials.

Recommendation E01 – Monitor emissions of particulate matter from dust collectors. Investigate the causes of sudden increases in particulate matter emissions and make necessary adjustments.

Recommendation E02 – Carry out a visual check of pneumatic injection and mechanical handling systems according to a set schedule in order to detect leaks. Make repairs as soon as possible.

Sources Targeted – S24–S26, S28–S30, S31–S32, S34–S36

Screw and pneumatic conveyors are closed systems equipped with hoods and dust collectors with bags at various transfer points (e.g., loading of coke onto scales in the green paste facility).To ensure that particulate matter is captured effectively, it is advisable that a system be used to monitor particulate matter emissions at the outlet of the dust collector (e.g., visual, mechanical or electronic system equipped with an alarm). In the event of sudden high emissions, the operator could investigate the cause, make the necessary adjustments and thus limit particulate matter emissions. Moreover, a visual check of the handling systems to repair any breaks and/or leaks should be carried out according to a regular schedule.

Recommendation E03 – Periodically monitor and maintain dust collectors and replace bags when they reach the end of their service life (corollary to Recommendations D08 and D09).

Sources Targeted – S24–S26, S28–S30, S31–S32, S34–S36

Refer to Section 4.4.5.

Recommendation E04 – Use covered trays (or equivalent) to cool and transport anode butts (or crust and hot cryolite bath) to the storage room. Minimize the amount of time butts are exposed to open air in the potroom or outside. If possible, ventilate the gas from the anode butt storage room to the GTC to capture HF and possible condensable microparticles.

Sources Targeted – S27

When anode butts are removed from the pots, they are at a temperature of about 960°C. At this temperature (> 700°C), part of the bath evaporates to form, among other things, NaAlF4, which then hydrolyzes in the presence of moisture to form HF.32 The condensation of certain fluorinated species in the air also generates PM2.5. Anode butts can be covered with a closed tray, granular material or the like to cut off the air required for combustion of the butt or bath and to contain the emissions, which are more intense during initial cooling.33 When the butts are completely cooled, they can be removed from the tray and treated accordingly. The gases and PM2.5 in the tray will be released. It is therefore advisable that a ventilation system be used in the butt storage room and, if possible, to route the gas flow to the GTC.

Recommendation E05 – In the coke calcining plant, unload the green coke in a closed building. Move green coke between various transfer points using closed conveyors or similar equipment.

Sources Targeted– S33

Green coke arriving by truck from a port or train station is unloaded at a central station from which it is transported by conveyor to the storage silos that feed the coke calciner. Due to its rather granular and hydrated texture (e.g., average size of 6 mm), green coke is predisposed to generating particles when placed in open air. Coke projections can, however, occur when trucks are moving or being unloaded and at coke drop points on the conveyor. To counter these emissions, it is advisable to equip trucks with tight fitting covers and operate the unloading station as well as the conveyor in a closed building. Unlike calcined coke, the proportion of microparticles in green coke is negligible. In this case alone, it would not be necessary to use hoods equipped with dust collectors at the various drop points, although these would prevent projections of total particulate matter in ambient air nonetheless.

Recommendation E06 – Set up physical and/or chemical barriers for red mud dumping in order to limit dusting in warm, dry and windy weather conditions.

Sources Targeted – S37

The discharge of red mud at waste disposal sites adjacent to the bauxite refining plant may lead to a particle problem, especially if it is placed in warm, dry and windy areas. The industry is trying to improve mud storage conditions by dehydrating it, for instance, thus reducing the risks of infiltration into the soil while increasing the site’s capacity. However, drying the mud is faster in these conditions, resulting in a dusting effect in persistent wind (size of red mud particles less than 1 mm, 70% of which are PM10).34 In order to curb particle and PM2.5 emissions, it is advisable to erect physical or chemical dusting “barriers,” including building embankments (or another means of obstructing wind), controlling soil erosion and spreading a chemical binder over the soil.

Fossil fuels are normally required as a source of heat in a process. As they burn, they emit particulate matter, most of which is PM2.5. The type of fuel and the consumption rate therefore have a direct effect on PM2.5 emissions at this level. For the primary aluminum sector, this type of emission applies to induction furnaces, boilers, and coke and alumina calciners.

Recommendation F01 – With regard to particulate matter emissions, use hydro power instead of fossil fuels if the current system allows. Otherwise, use natural gas instead of fuel oil (or another heavy fuel).

Sources Targeted – S10, S11, S13, S14

The rate of particulate matter emissions from fuel oil or other heavy fuel combustion is known to be higher. Using natural gas instead of a light oil would reduce microparticle emissions, even if all particulate matter released from the combustion of natural gas is PM2.5.35 Speaking strictly in terms of PM2.5, if all other conditions are the same, it is preferable to use hydro power followed by natural gas, instead of liquid or solid fossil fuels.

Recommendation F02 – For boilers and alumina calciners, minimize natural gas or fuel oil consumption per tonne of alumina produced using efficient heat recovery systems.

Sources Targeted – S13, S14

Alumina calciners and boilers in a bauxite refining plant require a large quantity of fuel to operate. Unlike coke calcining, which uses energy from VOCs in green coke, alumina calcining is a rather high energy-consuming process (3–5 GJ/t of alumina).36 Recovery of heat contained in calcined alumina and calcining gas is therefore the best way to minimize a plant’s use of energy, derived essentially from fossil fuels.

For boilers, a significant reduction in a plant’s steam use (e.g., through exhaust heat recovery) would also prevent particulate matter emissions as would regular boiler maintenance. A well-maintained boiler maintains its design efficiency at all times (thus minimizing fuel consumption) and prolongs its service life.

7 Gaertner, H. et al., Particulate Emissions from Electrolysis Cells, Light Metals 2011, p. 345.
8 Broek, S., et al., Considerations Regarding High Draft Ventilation as an Air Emission Reduction Tool, Light Metals 2011, p. 361.
9 Aljabri, N. et al., HF Emission from Dubai’s Electrolysis Cell, Light Metals 2003, p. 487.
10 Dando, N. R., Tang, R., Impact of Tending Practices of Fluoride Evolution and Emission from Aluminum Smelting Pots, Light Metals 2006, p. 203.
11 Lindsay, S. J., Effective Techniques to Control Fluoride Emissions, Light Metals 2007, p. 199.
12 Tarcy, G.P., The Affect of Pot Operation and Work Practices on Gaseous and Particulate Fluoride Evolution, Light Metals 2003, p. 193.
13 Slaugenhaupt, M. L., et al., Effect of Open Holes in the Crust on Gaseous Fluoride Evolution From Pots, Light Metals 2003, p. 199.
14 Tessier, J. et al., Image Analysis for Estimation of Anode Cover Material Composition, Light Metals 2008, p. 293.
15 Gudmundsson, H., Anode Dusting from a Potroom Perspective at Nordural and Correlation with Anode Properties, Light Metals 2011, p. 471.
16 Light Metals Research Centre, Fluoride Emissions Management Guide (FEMG), Version 4, February 2011.
17 European Commission, Integrated Pollution Prevention and Control – Draft Reference Document on Best Available Techniques for the Non-Ferrous Metals Industries, July 2009.
18 de Vasconcelos, P. D. S., Mesquita, A. L. A., Environmental Improvements During the Handling of Packing Coke at the Albras’ Bake Furnace, Light Metals 2009, p. 1049.
19 Mahieu, P. et al., High Performance Sealing for Anode Baking Furnaces, Light Metals 2011, p. 881.
20 Dupas, N., New Rodding Shop Solutions, Light Metals 2008, p. 899.
21 Lindsay, S. J., Effective Techniques to Control Fluoride Emissions, Light Metals 2007, p. 199.
22 Light Metals Research Centre, Fluoride Emissions Management Guide (FEMG), Version 4, February 2011.
23 Vendette, H., Anode Paste Plants: Innovative Solution for Optimum Emission Performances, Light Metals 2010, p. 993.
24 Iffert, M. et al., Reduction of HF Emissions from the Trimet Aluminum Smelter, Light Metals 2006, p. 195.
25 SNC-Lavalin Environment, Description des procédés de fabrication d’anodes et de coke calciné utilisés dans les alumineries canadiennes, prepared for Environment Canada, March 2011.
26 Environment Canada, NPRI Toolbox, Alumina and Aluminum, Aluminum Spreadsheet.
27 U.S. EPA Environmental Technology Verification Program, The Evolution of Improved Baghouse Filter Media as Observed in the Environmental Technology Verification Program, June 2008.
28 Idem.
29 U.S. EPA, Section 6 – Chapter 2: Wet Scrubbers for Particulate Matter in EPA Air Pollution Control Cost Manual – Sixth Edition (EPA/452/B-02-001), July 2002.
30 Dando, N. R., Lindsay, S. J., Hard Gray Scale, Light Metals 2008, p. 227.
31 Environment Canada, NPRI Toolbox, Alumina and Aluminum, Aluminum Spreadsheet.
32 Girault, G., et al., Investigation of Solutions to Reduce Fluoride Emissions from Anode Butts and Crust Cover Material, Light Metals 2011, p. 351.
33 Gagné, J.-P., et al., Update on the Evaluation of HF Emission Reduction Using Covered Anode Trays, Light Metals 2010, p. 291.
34 Beaulieu, C., Revue de la littérature portant sur les boues rouges, École polytechnique de Montréal, 2002.
35 Environment Canada, NPRI Toolbox, Alumina and Aluminum, Aluminum Spreadsheet.
36 European Commission, Integrated Pollution Prevention and Control – Draft Reference Document on Best Available Techniques for the Non-Ferrous Metals Industries, July 2009.

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