Code of Practice to reduce emissions of PM2.5 from aluminium sector: chapter 3

Official title: Code of practice to reduce emissions of fine particulate matter (PM2.5) from the aluminium sector

3. Recommended PM2.5 emission control practices

This section sets out recommended measures for mitigating emissions of particulate matter, including PM2.5, generated by the various processes associated with the primary aluminium sector. The 44 recommendations are presented in relation to four activities:

  • production of aluminium from alumina (electrolytic reduction);
  • prebaked anode production;
  • green coke calcining; and
  • alumina production.

Each recommendation is described briefly and linked to one or more of the sources of particulate matter emissions listed in Table 3-1. The recommendations outlined below should be implemented in a continuous improvement context, where required, relevant and applicable. Since emissions of PM2.5 have not been quantified in a precise manner for each of the sources identified in this Code, the order in which the recommendations are presented is subjective. The list of recommendations below is not exhaustive, and consequently aluminium smelters should consider implementing other measures to reduce PM2.5 emissions, where relevant and applicable. The appropriate frequency for the monitoring, inspection and other practices described in the recommendations is determined based on an assessment of the initial situation in a continuous improvement context. When determining the appropriate frequency, consideration should also be given to the specific characteristics of each facility, such as equipment type and age, the manufacturer’s recommendations and specifications, regulatory requirements, etc. The establishment of an inspection and monitoring schedule is recommended.

Note: In this section, the term “scrubber” means one of the following technologies: GTC, FTC, PFTC, pyroscrubber, boiler, followed by a dust collector and venturi wet scrubber.

Table 3-1: Potential sources of particulate emissions and recommendations, by activity
Activity Operating procedures Scrubbers Materials storage and handling
Production of aluminium from alumina (electrolytic reduction)

Changing of anodes (S01): R01-R05, R10-R11, R15-R18, R44

Tapping and transferring of liquid cryolite bath from one pot to another (S02): R01-R02, R04-R09, R11, R15-R18, R44

Tapping molten aluminium in a casting crucible (S03): R01-R02, R04-R06, R11, R15-R16, R18, R44

Skimming and pouring molten aluminium (S04): R02, R13, R15-R16, R44

Measuring and sampling in pots (S05): R01-R02, R04-R05, R11, R15-R16, R44

Leaks from the superstructure and fume exhaust ducts (S06): R02, R14-R15, R44

Handling and loading of anode cover material after anode change (S38): R01-R05, R12, R15-R18, R44

Post-treatment stack gas (S16): R02, R14-R15, R17, R19-R24, R25-R27, R44

GTC leaks (S17): R02, R14-R15, R19, R22, R26, R44

Alumina dust collectors (S24): R02, R14-R15, R19-R20, R22-R23, R25-R27, R44

Leaks from alumina handling systems (S25): R02, R14-R15, R19, R22, R26, R44

Losses from storage silos (S26): R14-R15, R25-R27, R44

Transport of anode butts and residual bath to prebaked anode production plant (S27): R15-R16, R28, R44

Loading and delivery of alumina to cells, in particular from overhead crane (S39): R14-R16, R22-R23, R25-R27, R44

Prebaked anode production

Cleaning of anode butts with power and blasting tools (S07): R15, R23, R25-R27, R31, R33, R44

Crushing of frozen bath recovered during cleaning of anode butts (S08): R15, R23, R25-R27, R31-R33, R44

Crushing, grinding and sieving of calcined coke (S09): R15, R23, R25-R27, R33, R44

Baking furnace operations (S10): R15, R29-R30, R33, R35, R44

Combustion gas from cast iron induction furnace (S11): R15, R33, R35, R44

Baking gas after FTC treatment (S18): R02, R15, R19-R23, R25-R27, R30, R33-R35, R44

Stack gas after PFTC treatment (S19): R02, R15, R19-R20, R22-R23, R25-R27, R33, R44

FTC and PFTC leaks (S20): R02, R15, R19, R22, R26, R33, R44

Alumina and calcined coke dust collectors (S28): R02, R15, R19-R20, R22-R23, R25-R27, R33, R44

Alumina, calcined coke and crushed frozen bath handling system leaks (S29): R02, R15, R19-R20, R22, R26, R29, R31-R33, R44

Losses from storage silos (S30): R15, R25-R27, R33, R44

Green coke calcining Losses from rotary calcining kiln (S12): R15, R36, R44

Post-treatment calcining gas (S21): R02, R15, R22-R23, R36-R38, R44

Post-treatment cooling gas (S22): R02, R15, R22-R23, R36, R39, R44

Leaks from calcining gas treatment systems (S23): R02, R15, R19, R22, R36, R44

Calcined and under-calcined coke dust collectors (S31): R02, R15, R19-R20, R22-R23, R25-R27, R36, R38, R44

Leaks from green, calcined and under-calcined coke handling systems (S32): R02, R15, R19, R26, R36, R44

Losses from unloading station and storage silos (S33): R15, R25-R27, R36, R38, R40, R44

Alumina production

Hydrated alumina calcining (S13): R15, R35, R41, R43-R44

Combustion gas from steam boilers (S14): R15, R35, R41, R43-R44

Bayer liquor evaporation (S15): R15, R41, R44

 

Bauxite and metallurgical-grade alumina dust collectors (S34): R02, R15, R19-R20, R22-R23, R25-R27, R41, R44

Leaks from bauxite and metallurgical-grade alumina handling systems (S35): R02, R15, R19, R22, R26, R41, R44

Losses from storage areas and silos (S36): R15, R25-R27, R41, R44

Wind-induced transport of particulate matter from red mud disposal site (S37): R15, R42, R44

3.1 Aluminium production (electrolytic reduction)

With the constant evolution of electrolytic reduction technologies, new plants can help to improve the environmental performance of the primary aluminium industry. Gas extraction efficiency from electrolytic cells could also be improved, thereby reducing fugitive emissions of particulates in the potroom, where the ambient air is not treated. Aside from installation of scrubbers, optimization of electrolytic cell operating methods and, to a lesser extent, the quality of raw materials (sealed prebaked anodes, cryolite bath, alumina) is the best approach for reducing the air pollution situation of an existing facility. This is true not only for gas emissions, but also for fine particulate matter, which acts like a gas and remains suspended in ambient air.

3.1.1 Opening of electrolytic cell hoods

Recommendation R01 - Optimize work methods to allow a minimum number of hoods to be opened simultaneously, and to be opened only once work begins. Close hoods as soon as work is complete.

Sources targeted - S01, S02, S03, S05, S38

The operation of an aluminium production plant entails periodic interventions involving the electrolytic cells, which may include changing anodes, tapping the molten metal or cryolite bath, and loading the cryolite bath. Each of these operations requires that the hoods be opened, which results in releases of fine particulate matter within the potroom.

It has been observed that HF emissions associated with these interventions account for approximately 60% of a plant’s HF emissions, including those from the GTC.Footnote6,Footnote7 Optimizing work methods to minimize the number of hoods opened simultaneously and the length of time they remain open is the best approach for controlling these emissions. HF emissions (and therefore PM2.5) increase with the number of hoods opened simultaneously.Footnote8 Therefore, hoods should not be left open unnecessarily.

3.1.2 Gas extraction system

Recommendation R02 - Regularly assess the extraction efficiency of fans in relation to power supply, flow rate and pressure loss. Make adjustments as needed to maximize the extraction rate.

Sources targeted - S01-S06, S16-S25, S28, S29, S31, S32, S34, S35, S38

This recommendation applies to all gas scrubbers that use a fan. Gases and particulate matter released under the pot hoods are continually vented and routed to the GTC through a main duct. Gas extraction efficiency depends largely on the configuration and condition of the superstructure and the ventilation ducts, which should contain a minimum of gaps. This is important for maximizing the negative pressure in the pots based on the ventilation power of the back-end fan. When one or more of the hoods are open, the negative pressure decreases locally, leading to an increase in leaks within the building. Pots located at the potroom extremities (farthest from the fan) could be more prone to a decrease in fume extraction capacity. A decrease in the existing system’s efficiency can reduces the rate of extraction of particulates to the GTC. In this regard, it is recommended that a fan monitoring and maintenance program be established in order to maintain maximum extraction flow at all times (depending on applied power).

3.1.3 Prebaked anode changes

Recommendation R03 - Optimize work methods to minimize the time required to change anodes and cover them with anode cover material.

Sources targeted - S01, S38

Anode changing is one of the largest sources of 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 a new anode in the cavity. This procedure generally takes more than 10 minutes and must be carried out carefully to prevent operational problems.Footnote9 It is therefore advisable to develop and apply an effective work method that minimizes the amount of time it takes to change the anodes (e.g., cleaning the cavity with the extractor immediately after removing the anode butt, and placing the anode butt tray and bath residue close to the pot).

3.1.4 Crust covering the cryolite bath

Recommendation R04 - Implement a program to monitor cracks in the crust by visual inspection or using an automated system. Ensure that the anode cover material is a suitable and effective sealant.

Recommendation R05 - Cover the tap hole with anode cover material once the tapping or sampling work is complete.

Sources targeted - S01-S03, S05, S38

An important factor that affects the rate of fluoride emission from pots relates to the integrity of the alumina and cryolite crust that covers the cryolite bath, since the crust acts as a physical barrier against gas migration.Footnote10 Poor coverage of new anodes can lead to gaps. It has been shown repeatedly that the intensity of emissions correlates with the extent of the holes and cracks in the crust.Footnote11 These gaps are created when alumina is injected using the feeder/breaker, when the molten metal or bath is siphoned through the tap hole, and when work is done to suppress the anode effect. A monitoring program (visual or automated) can help to ensure that the crust contains and sustains a minimum of cracks, thus reducing the emission of particulate matter at the source. Where applicable and relevant, as it may result in the tap door being open longer, it is recommended that the tap holes be covered with anode 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 anode cover material may vary and have a direct impact on the quality of the crust.Footnote12 In addition to implementing a crack monitoring program, it is also advisable to carefully monitor the anode cover material properties to prevent the chronic appearance of cracks in the crust.

3.1.5 Tapping of molten metal and cryolite bath

Recommendation R06 - During tapping operations, use flexible tubing to reroute fumes released from the crucible within the pot.

Sources targeted - S02, S03

Aluminium produced through electrolytic reduction is deposited on the surface of the cathode at the bottom of the pot and is generally extracted daily to maintain an optimum level. A heat-insulating crucible equipped with a siphon is used for this purpose. This normally involves opening the pot hood, preparing a tap hole in the crust, connecting the vent pipe and the compressed air to the crucible, placing the siphon in the tap hole, tapping the metal from the crucible, removing the crucible once the tapping is complete, sealing the tap hole and closing the hood. Tapping of the cryolite bath follows the same basic procedure and is done primarily to maintain the bath height, which is instrumental for thermal equilibrium and alumina dissolution. The extent of the fugitive emissions associated with this work is closely linked to the amount of time the hoods are open (see Recommendation R01) as well as the gas expelled through the crucible’s air exhaust. 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.

3.1.6 Cryolite bath spatter and spills

Recommendation R07 - Minimize and recover cryolite bath spills and spatter on the floor.

Recommendation R08 - Pour the bath into the pot launder at an optimal speed to reduce pouring time and avoid spatter. Avoid pouring too slowly.

Recommendation R09 - Minimize and recover cryolite bath residue in the launder when loading is complete.

Source targeted - 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 involves 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 tap hole, removing the crucible and launder when loading is done, and closing the hood. Particulate matter emissions associated with loading of the cryolite bath are closely linked to the opening of the hoods (see Recommendation R01) and the exposure of the hot cryolite bath in the launder located outside the ventilation hood perimeter. An increase in emissions is inevitable if the cryolite bath is spilled during pouring or if the residual bath in the launder is not cleaned immediately after pouring. Cryolite bath may also be spilled on the floor during anode changing. A work method that prevents these circumstances would reduce the rate of emission within the potroom.

3.1.7 Skimming of carbon dust

Recommendation R10 - Cool the hot carbon dust inside the pot or in a container with a cover. Minimize the amount of time carbon dust spends in the potroom.

Source targeted - S01

The dispersion of carbon dust in the electrolytic bath is dependent on the quality of the anodes and on operating conditions. Selective oxidation of tar pitch releasing coke grains, cathode wear and the addition of carbon from the anode cover material and the enriched alumina are the main mechanisms leading to carbon accumulation.Footnote13 This material may cause an increase in the electrical 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 while minimizing the time this operation takes. Freshly collected hot carbon dust is impregnated with cryolite, amplifying the release of fluorinated particulate matter to the ambient air. With some cell technologies, there is a metal plate just inside the tap door at floor level where the carbon dust can be left to cool. If applicable, it is recommended that the carbon dust be allowed to cool inside the pot so as to capture a maximum amount of PM2.5 emissions, which can then be vented to and treated in the GTC. Carbon dust can also be left to cool outside the pot in a container with a cover.

3.1.8 Control of operating parameters

Recommendation R11 - 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-S03, S05

From an environmental perspective, it is recommended that the liquid bath have little or no contact with the air under the crust in order to limit the formation of HF and fluorinated particles through moisture-related mechanisms.Footnote14 In addition, by controlling bath height and crust thickness allows thermal equilibrium to be maintained more easily, thus preventing an unwanted increase in temperature and in emissions of HF and fluorinated microparticles.

Recommendation R12 - Prevent, control and minimize anode effect frequency and duration. After manually suppressing anode effects, cover cracks in the crust with anode cover material.

Source targeted - S39

The anode effect is a phenomenon that adversely affects electrolytic reduction by causing a sudden increase in voltage and a decrease in amperage. This is due to the presence of a gas film on the surface of the anode which must be cleared either manually by an operator (with a long wooden rod or pole) or using an automated pot control system. Automated suppression of the anode effect avoids the need to open the pot hoods, allowing gas to be channelled to the GTC. In both cases, the crust is affected in that cracks begin to form, increasing the rate of emissions. The operator should therefore minimize the anode effect and seal cracks with anode cover material as soon as they appear (see recommendation R04). As a general rule, modern aluminium smelters prevent anode effects by automatically injecting alumina as soon as the voltage in the pot increases.

3.1.9 Casting of molten aluminium

Recommendation R13 - Minimize releases of particulate matter from the casting centre.

Source targeted - S04

Liquid aluminium 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 of various shapes depending on the client’s specifications. Aluminium casting is a very minor source of emissions of metal particles.Footnote15 From an environmental standpoint, the optimization of fuel consumption in the casting centre can help reduce the generation of air pollutants, including particulates.

3.1.10 Maintenance activities

Recommendation R14 - Regularly inspect, according to a set schedule, the aluminium production (electrolytic reduction) plant’s facilities, including the condition of the hoods, the gas exhaust ducts in each cell, the alumina supply system and the pot superstructure. In the event of breakdowns or malfunctions, make repairs or install appropriate replacement parts as soon as possible.

Sources targeted - S06, S16, S17, S24-S26, S39

Aluminium production pots based on prebaked anode technology are not completely airtight, which means that fugitive emissions can continuously escape through gaps. Fugitive emissions of particulates and other contaminants occur not only during operational activities, but also as a result of gaps caused by premature (or expected) wear of the structure and equipment in contact with process gases. With age, the superstructure can also lose its seal, exacerbating this problem. 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 through roof vents in the potroom and are not controlled by any particular treatment system.

Therefore, expansion joints, rubber seals, gaskets and the like should be inspected periodically and promptly repaired if found to be defective. Other trouble spots include the junction between the superstructure and the main gas duct, cracks in the alumina supply duct, and damage to the superstructure.Footnote16

Recommendation R15 - Implement an employee training plan in support of an approach for preventing premature wear and breakdowns caused by improper operation of the facilities.

Sources targeted - S1-S39

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

3.1.11 Electrolytic reduction plant cleaning activities

Recommendation R16 - Use a HEPA vacuum to clean the floor of the potroom and other buildings.

Sources targeted - S01-S05, S27, S38, S39

Recommendation R17 - Regularly clean the suction inlet located inside the pot superstructure (slots, ventilation hoods section, etc.)

Sources targeted - S01, S02, S16, S38

Recommendation R18 - Regularly clear solid residues from the feeder/breaker to reduce the size of the hole in the crust after injection, thus decreasing emissions (corollary to Recommendation R05).

Sources targeted - S01-S03, S38

Cleaning of the potroom floor should be included in a maintenance program. Systems that may disperse dust particles from the floor into the 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 ceiling area where cryolite bath residues accumulate over time is also advisable to control their dispersion into the potroom when the hoods are opened.

3.1.12 Monitoring of scrubber operations

The gases and particulate matter released under the pot hoods are continually vented and routed to the GTC. A typical GTC collects pot gases through a main duct and then distributes them into vertical reactors into which fresh and fluorinated alumina is injected at the base. The primary objective is to intercept fluorinated compounds, which are harmful to the environment. Whether an alumina injection of or a fluidized bed system is used, the scrubbed gas must be dedusted in order to recover the “fluorinated” alumina, which is used as a raw material in the electrolytic cells. The dust collector consists of filtration media which, depending on its configuration and operating parameters, captures most of the process particles (> 99%).Footnote17 This process is considered the best practice for treating gas from electrolytic cells. Nonetheless, the GTC and any other gas treatment system must be configured and operated properly in order to maximize performance.

Recommendation R19 - Regularly and periodically monitor the gas flow in each scrubber compartment while ensuring it is uniform. Monitor pressure loss in order to identify anomalies requiring correction.

Sources targeted - S16-S20, S23-S25, S28, S29, S31, S32, S34, S35.

A gas treatment centre is composed of many parallel compartments (typically 12 to 14), including injection reactors and filtration units. The gas leaving the compartments are combined and sent to the stack, while the adsorbent (alumina) is directed to a storage silo. The filtration capacity of the dust collector is determined by the air-to-cloth ratio, and requires a constant flow of gas and maximum speed to ensure optimal performance. A variable flow (within a given range) between compartments caused by variable pressure loss reduces filter performance.

It is important to regularly monitor pressure loss and flow in the compartments to avoid premature wear of the bags. Measurement and monitoring of the fluoride concentration in the alumina may also be considered.

Recommendation R20 - Adjust cleaning frequency and duration for the dust collectors or scrubber filtration media to balance gas flow for each compartment and maximize collection efficiency.

Sources targeted - S16, S18, S19, S24, S28, S29, S31, S34

An increase in pressure loss is caused primarily by flow resistance in the gas ducts, injection reactor and baghouse. Bag cleaning helps to control this pressure loss and maintain the flow within the design parameters of the baghouse. It is therefore important to adjust cleaning frequency and duration to maintain balanced flow in each compartment. Ideally, the flow of each compartment should be controlled by adjusting each dedicated fan or, in the case of a centralized fan, by adjusting the control valve of each compartment.

Recommendation R21 - Where possible, depending on the production sector, limit the recycling of enriched or fluorinated alumina in GTC and FTC injection reactors, without influencing HF capture. Make regular and periodic monitoring of the recycling rate, to ensure it is optimal.

Sources targeted - S16, S18

In the GTC and FTC, a large portion of the enriched alumina is recycled in the reactors for the purpose of controlling its level of production while taking into account the size of the silos and the needs of the electrolytic reduction plant. However, the alumina undergoes continuous attrition during the process, thus increasing the proportion of fine particulates in the GTC and FTC’s supplies and, therefore, PM2.5 emissions from the stack.Footnote18 Frequent optimization of the recycling rate is therefore required in order to maximize HF capture and minimize recycling of enriched alumina. The optimization of recycling should be based on the concentration of HF emissions at the GTC and FTC stacks. The recycling rate should be generally higher during the summer than in winter. This recommendation does not apply to the PFTC since the enriched coke is generally not recycled but is instead returned to the green paste facility.

3.1.13 Maintenance of scrubbers, dust collectors and related systems

Recommendation R22 - Depending on the production sector and the filtration system, Rregularly inspect, according to a set schedule, the scrubber, including the collection system, sealing joints, fan (corollary to Recommendation R02) and, alumina (or calcined coke) supply systems, and filtration system, depending on the production sector. Repair any breakdowns or malfunctions as soon as they are noted.

Sources targeted - S16-S25, S28, S29, S31, S34, S35, S39

Recommendation R23 - For the dust collector, replace the bags and other filtration media at the end of their service life. Do not wait until a breakdown occurs.

Sources targeted - S07-S09, S16, S18, S19, S21, S22, S24, S28, S31, S34, S39

Maintenance of the scrubbers is essential for consistent performance. Regular inspections can help to identify breaches in the structure that could lead to leakage of particles (e.g., break in alumina duct or gas duct). Wear of the fan (e.g., blower wheel) and of the various sealing joints of the superstructure must also be taken into consideration when monitoring is performed (corollary to Recommendation R02). The baghouse bags and other filtration media also deteriorate with time and must be changed regularly before tears appear, to avoid an increase in stack emissions.

Recommendation R24 - For the GTC, regularly inspect ducts that are prone to accumulation of hard gray scale. Clean if too much has accumulated.

Source targeted - S16

The formation of an amorphous material made up of alumina, bath residue and water (hard gray scale) on the walls of the steel ducts is a problem that can affect the performance of the GTC and the service life of the filtration media.Footnote19 Hard gray scale can form in injection reactors, dust collectors, fluidized bed reactors 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.

3.1.14 Monitoring of facilities and their outputs

Recommendation R25 - Monitor emissions of particulate matter from dust collectors. Investigate the causes of sudden increases in particulate matter emissions and make necessary adjustments.

Sources targeted -S07-S09, S16, S18, S19, S24, S26, S28, S30, S31, S33, S34, S36, S39

Recommendation R26 - 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 - S07-S09, S16-S20, S24-S26, S28-S36

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 of the coarse particulates emitted from these materials are larger than 2.5 microns; however, with the installation of dust collectors, the fraction of PM2.5 at the outlet may increase.Footnote20 The optimization of dust collectors is therefore a solution for minimizing PM2.5 emissions resulting from the handling and storage of various powdery materials.

Screw and pneumatic conveyors--closed systems equipped with hoods and baghouses--are used  at various transfer points (e.g., loading of coke onto scales in the green paste facility). To maintain the efficiency of particulate matter capture, it is advisable to have a system 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 a sudden surge in emissions, the operator could investigate the cause, make the necessary adjustments and thus minimize particulate emissions. In addition, visual monitoring of the handling systems should be carried out on a regular schedule in order to repair any breaks and/or leaks.

Recommendation R27 - Periodically monitor and maintain dust collectors and replace filtration media at end of service life (corollary to Recommendations R22 and R23).

Sources targeted - S07-S09, S16, S18, S19, S24, S26, S28, S30, S31, S33, S34, S36, S39

Refer to Section 3.1.13.

3.1.15 Transportation of anode butts

Recommendation R28 - Minimize air exposure time (and transport time) of anode butts within or outside the potroom. For example, make effective use of covered trays (or equivalent) to cool and transport anode butts (or crust and hot cryolite bath) to the storage room.

Source 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.Footnote21 The condensation of certain fluorinated species in the air 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 emissions, which are more intense during initial cooling.Footnote22 When the butts have completely cooled, they can be removed from the tray and treated. The gases and PM2.5 in the tray will nonetheless be released. It is therefore advisable to have a ventilation system in the anode butts storage room and, where  possible, to route the gas stream to the GTC.

3.2 Prebaked anode production

Prebaked anode production plants use raw materials such as coal tar pitch (or equivalent) and calcined coke (and possibly under-calcined coke), which releases particulate matter throughout the manufacturing process. Furthermore, mechanical cleaning of anode butts and frozen bath crushing generate a mixture of alumina, cryolite and carbon particles. Most anode manufacturing stages produce particulates that cannot be easily avoided or reduced at the source without affecting the process and the 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 require particle ventilation and scrubber systems to provide a safe work environment and environmental protection. The anode baking furnace is also a major source of stationary and fugitive emissions of particulates.

3.2.1 Baking furnace

Recommendation R29 - Maintain an effective system for filling baking furnace pits with packing coke to limit coke losses in the building. Train operators in order to standardize work methods for handling packing coke.

Sources targeted - S10, S29

A conventional baking furnace is composed of many sections, each containing several pits into which green anodes are placed and covered with packing coke, which provides good thermal exchange and protects the anodes from air oxidation. When the baking cycle is finished for one section, a special overhead crane sucks up the packing coke and stores it in a hopper and removes the baked anodes. The crane then loads new green anodes and injects packing coke from the hopper. To avoid particulate emissions during these operations, the crane should be equipped with an efficient coke dust control system.Footnote23

Recommendation R30 - Monitor negative pressure at the FTC inlet or at the exit of the baking furnace.

Sources targeted - S10, S18

Anodes are baked in a closed environment that is not completely airtight since the furnace sections must be removed regularly to load and unload the anodes. Monitoring of negative pressure at the furnace exit is essential for safety reasons but also to minimize releases of fine particulates in the furnace building. Effective sealing is required in order to achieve optimal negative pressure for operations. For example, special attention should be given to the condition of port plates, which prevent the entry of cold air into the heated section and can prevent condensation of volatile compounds, formation of corrosive chemical species (e.g., HF, H2SO3) in the gas lines and incomplete combustion.Footnote24

3.2.2 Cleaning of anode butts

Recommendation R31 - Efficiently operate the collection, extraction and filtration systems for dust resulting from the anode butt cleaning process.

Sources targeted - S07, S08, S29.

Spent anode butts from the electrolytic reduction process are cooled and placed in storage. They are subsequently placed on an overhead conveyor that transfers them to the anode sealing facility where the frozen bath material is extracted during 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 4-5 kg of fragmented bath per anode, on average, and generates a lot of dust.Footnote25

At each of these stations, the butt is placed inside a sealed enclosure to reduce environmental impacts and protect the mechanical and hydraulic systems from the dust. It is therefore recommended that effective air extraction (compartmentalized or not) and dust collection systems be operated. Continuous or semi-continuous monitoring of particulate emissions from dust collectors would help to ensure their effectiveness. When a sudden increase in emissions is observed, it is important to promptly investigate the cause and make the necessary adjustments as soon as possible (refer to Recommendation R25).

3.2.3 Crushing of frozen bath

Recommendation R32 - Filter emissions of fine particulates matter released from the frozen bath crushing process using dust collectors.

Sources targeted - S08, S29

Bath residues removed from anode butts during the various cleaning stages are recovered from beneath the equipment and then transferred to a crusher to produce granular material that can be recycled in the electrolytic cells as anode cover material. Operators should therefore operate dust collectors to treat the air released from the crusher.

3.2.4 Maintenance activities

Recommendation R33 - Regularly inspect, according to a set schedule, the prebaked anode production 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 replacement parts as soon as possible.

Sources targeted - S07-S11, S18-S20, S28-S30

See also recommendations R15, R22, R23, R24 and R27.

3.2.5 Monitoring of operations (FTC and PFTC)

Prebaked anode production plants use the FTC to treat green anode baking fumes which contain particles, 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 which is specially designed to capture organic compounds. Pitch fumes are routed to and treated in the PFTC. A large portion of the particulates emitted to the PFTC stack, including a small fraction of PM2.5, comes from the injected coke.

Recommendation R34 - For the FTC only, operate the cooling tower so as to condense most of the tar contained in the baking gas. If necessary, add a prefilter (e.g., ceramic packing) to capture most particulate and condensable matter, including tar.

Source targeted - S18

The FTC for treating anode baking fumes uses a process very similar to the GTC, except for the cooling tower located upstream of the reactors and dust collectors. This stage involves the injection of a non-saturating amount of water that reduces the temperature of the gas. As a result, most of the tar is condensed and is later adsorbed onto the alumina injected into the reactor. A minimal quantity of tar residue, including a small fraction of incoming solids, is collected at the bottom of the tower. Removal of the tar is also critical for the proper operation of the alumina injection reactor and the dust collector, where agglomerates may form in the presence of the tar. In such cases, the performance of the dust collector, where most particulate matter is captured, would be affected.

See also recommendations R02, R19, R20, R21, R25 and R26.

3.2.6 Type of fuel

Recommendation R35 - With regard to particulate matter emissions, use hydroelectric power in favour of fossil fuels if possible with the current system. Otherwise, use natural gas instead of fuel oil (or other heavy fuels). For facilities that do not have access to hydro power, careful consideration should be given to other electric power sources before making a decision to switch fuels.

Sources targeted - S10, S11, S13, S14, S18

Fossil fuels are normally required as a source of heat in a process. As they burn, they emit particulate matter, which consists mainly of PM2.5. The type of fuel used and the consumption rate therefore have a direct effect on PM2.5 emissions. In the primary aluminium sector, these emissions are associated with holding and homogenization furnaces, anode baking furnaces, induction furnaces, boilers, and coke and alumina calciners.

The combustion of fuel oil and other heavy fuels is known to generate high levels of particulate emissions. Using natural gas instead of light oil would reduce microparticle emissions, even if all the particulate matter released from the combustion of natural gas were PM2.5.Footnote26 Strictly from the perspective of PM2.5, all other things being equal, the use of hydro power, followed by natural gas, is preferable to liquid or solid fossil fuels.

3.3 Green coke calcining

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

3.3.1 Maintenance activities

Recommendation R36 - 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 and emissions of PM2.5. In the event of breakdowns or malfunctions, make repairs or install appropriate replacement parts as soon as possible.

Sources targeted - S12, S21-S23, S31-S33

See also recommendations R15, R22, R23 and R27.

3.3.2 Monitoring of operations (pyroscrubber)

Recommendation R37 - Optimize the operating parameters of the pyroscrubber to maximize incineration of coke particles in addition to VOCs. Where necessary, follow up with a system designed to detect particles leaving the pyroscrubber and adjust accordingly.

Source targeted - S21

The pyroscrubber has a closed combustion chamber lined with refractory brick which is maintained at a temperature above 1,000°C and receives an injection of air through several openings to optimize combustion.Footnote27 It is sized according to the gas flow to be treated, the residence 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 in order to mitigate the effect of calciner variations on its performance. Nonetheless, a fraction of the particulate matter, most of which is PM2.5, is not incinerated.Footnote28 Clearly, optimizing and monitoring the operating parameters are the best ways to reduce these emissions, aside from installing a new, more effective scrubber.

3.3.3 Monitoring of operations (boiler followed by dust collector)

Recommendation R38 - Optimize the performance of the cyclones and dust collector based on the total particle load. When necessary, replace bags with more efficient ones.

Sources targeted - S21, S31, S33

The purpose of the boiler is to produce steam by reducing the temperature of the calcining gas to an acceptable level for the dust collector  downstream (typically < 200°C). The dust collector captures coke particles whose concentration is barely affected by the boiler. PM2.5 is therefore controlled by the dust collector, even though these particulates can be expected to represent over 90% of the residual particles in the stack.Footnote29 There are few options for reducing residual PM2.5 except in cases where the dust collector is not being used to its full potential. Strict monitoring of operating parameters would therefore help to minimize PM2.5 emissions, depending on the system in place.

The replacement of filters with more efficient ones for capturing micrometric and submicron particles (e.g., synthetic filters with a membranous polytetrafluoroethylene (PTFE) coating) could also be considered.Footnote30 These filters would, however, have to be adapted to the existing system. Recommendations R19 and R20 also apply to this technology.

See also recommendations R02, R25 and R26.

3.3.4 Monitoring of operations (venturi wet scrubber)

Recommendation R39 - Regularly and periodically monitor the gas flow rate through the scrubber by measuring the water flow rate to the venturi inlet. The ratio of the two has a direct effect on pressure loss and the effective capture of particulate matter, including PM2.5. Optimize performance for the existing system.

Source targeted - S22

A wet scrubber uses a liquid to clean a gas stream. When the gas is fed into a narrow (venturi-type) neck, the flow accelerates and the injected water is atomized. 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 on the pressure loss applied to the venturi neck. Collection efficiency is usually very high for fine particulate matter (e.g., +99% for PM10) but an exponential decrease is observed for ultrafine particulates (e.g., 40-99% for PM1). An increase in pressure loss generally improves efficiency.Footnote31 It is therefore advisable to monitor and optimize the operation of the wet scrubber.

3.3.5 Green coke storage

Recommendation R40 - Unload the green coke in a closed building at the coke calcining plant. Move green coke between various transfer points using closed conveyors or similar equipment or any other measures that can control dust emissions.

Source targeted - S33

Green coke transported by truck from a port or train station is unloaded at a central station from which it is carried by conveyor to the storage silos for the feeding coke to the calciner. Because of its granular and hydrated texture (e.g., average size of 6 mm), green coke is not predisposed to generate particles when exposed to air. Emissions of coke fines 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 to operate the unloading station as well as the conveyor in a closed building. The proportion of microparticles in green coke is negligible, which is not the case for calcined coke.

3.4 Alumina production

3.4.1 Maintenance activities

Recommendation R41 - Regularly inspect, according to a set schedule, alumina calcining facilities and boilers at the alumina production plant. In the event of breakdowns or malfunctions, make repairs or install appropriate replacement parts as soon as possible.

Sources targeted - S13-S15, S34-S36

 See also recommendations R15 and R27.

3.4.2 Red mud disposal

Recommendation R42 - Set up physical and/or chemical barriers for red mud disposal sites in order to minimize dusting when weather conditions are conducive to the dispersion of dust.

Source targeted - S37

Discharges of red mud at the disposal site adjacent to the alumina production plant may lead to a particulate emissions problem, especially if the mud is dumped during unfavourable weather conditions (e.g., warm, dry and windy conditions). The industry is working on improving storage conditions through practices such as dewatering the mud in order to reduce the risk of infiltration into the soil and increase the site’s storage capacity. However, the mud dries faster under warm, dry and windy conditions, which can promote dusting (red mud particles smaller than 1 mm, 70% of which are PM10).Footnote32 To prevent particulate emissions, it is advisable to put in place physical or chemical dusting “barriers,” including embankments (or other means of blocking the wind), soil erosion control facilities and the application of a chemical binder to the soil.

3.4.3 Consumption rate

Recommendation R43 - For boilers and alumina calciners, minimize natural gas or fuel oil consumption per tonne of alumina produced by using efficient heat recovery systems.

Sources targeted - S13, S14

The operation of alumina calciners and boilers at a alumina production plant requires a large quantity of fuel. Unlike green coke calcining, which uses energy from the VOCs contained in green coke, alumina calcining is a process with a high energy consumption rate (3-5 GJ/t of alumina).Footnote33 The heat contained in calcined alumina and calcining gas is recovered mainly from the steam, and makes it possible to minimize the 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 help to prevent particulate emissions as would regular boiler maintenance. With proper maintenance, a boiler will continue to operate at its design efficiency at all times (thus minimizing its fuel consumption) and prolong its service life.

See also recommendations R25, R26 and R35.

3.5 Environmental management practices

Recommendation R44 - Implementation of the recommendations set out in the Code should be integrated with the facility’s environmental management plan, which should could include analysis of the initial situation situation, a training plan, auditing protocols and determination and implementation of corrective measures in a continuous improvement context.

Sources targeted - S1-S39

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