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Code of Practice to Reduce Emissions of Fine Particulate Matter (PM2.5) from the Aluminium Sector

2. Operations and sources of emissions

This section provides an overview of operational activities in the primary aluminium sector that are covered by the Code and may lead to releases of particulate matter to the atmosphere. The purpose of this section is to describe the nature and the scope of the activities in question, particularly those that raise concerns with regard to fine particulate matter emissions. Figures 2-1 to figure2-4 illustrate the main processes pertinent to the Code and their associated particulate matter releases. It should be noted that releases of particulate matter associated with other activities as discussed in Section 1.1 are not included.

It is important to note that this section identifies sources of total particulate matter, given the difficulty of separating out fine particulate matter, specifically PM2.5. Best practices have been developed to control emissions of total particulate matter, and have led to a complementary reduction in PM2.5 emissions. It is difficult, however, to establish with certainty the proportion of PM2.5 associated with each source of total particulate matter. Consequently, the specific PM2.5 control efficiency for each source is not discussed in this Code.

2.1 Production of aluminium (electrolytic reduction)

Aluminium is produced in special cells through the electrolytic reduction of alumina dissolved in a molten bath. Aside from electrolytic cells, an aluminium plant also includes aluminium casting centre and gas treatment centre (Figure 2-1).

Each electrolytic cell consists of a carbon cathode insulated with refractory bricks inside a rectangular steel shell in which carbon anodes are suspended. The cells are connected in series to form a potline. During operations, a continuous high-amperage current passes from the anode through the electrolytic molten bath (cryolite) to the cathode. From there, it goes through a busbar to the next cell. Alumina is added to the cells to maintain an alumina content of 2% to 4% in the molten bath. In modern plants, these additions are controlled by a computer. Fluoride compounds (e.g., aluminium fluoride) are also added to lower the electrolytic bath’s melting point, which helps to maintain an operating temperature of about 960°C in the cells. The molten aluminium is deposited at the cathode on the bottom of the pot, while the oxygen from the alumina reacts with the carbon from the anode to form carbon dioxide.

Because the anodes are gradually consumed, they must be replaced periodically. The anode butts (rods with carbon residue covered in bath crust) are then sent to a treatment facility so that the bath coating can be scraped off and reintroduced into the electrolytic cells in the form of aggregate. The carbon residue is recycled to make anode paste.

The gases from the electrolytic reduction process are vented to a gas treatment centre (GTC) and treated before being released to the atmosphere. A dry scrubber with injection of fresh and recycled alumina is used for this purpose. The alumina that absorbs fluorinated  gas  is  recovered  with  a  dust collector  that  also  captures particulate matter produced by the electrolytic reduction process. The “fluorinated” alumina that leaves the GTC is used as a raw material in the electrolytic cells. Alumina and bath handling systems are equipped with baghouses or other devices that control particulate emissions.

Molten aluminium is regularly siphoned into a crucible and then transferred to a casting centre to be transformed into ingots or other products. The molten metal typically passes through a holding furnace to control its temperature.

Sources of particulate matter

Of the various activities associated with the primary aluminium industry, the production of aluminium is the main source of air emissions. These emissions are mainly associated  with  the  electrolytic reduction of the alumina dissolved  in  the  cryolite  bath and with anode consumption. Combined, these two processes release fluorinated gases (e.g., hydrogen fluoride [HF] and perfluorocarbons [PFC]), SO2 and particulate matter. It has been shown that the particles released in the potroom originate mainly from the electrolytic bath, cover material and alumina.Footnote 3 There  is  a  complex relationship between the level of particulate emissions in the potroom, particulate-generating  mechanisms  and  the  factors  that  influence  them  (e.g., electrolytic reduction  technology, quality of raw materials, work methods, transportation and handling).

Although  few  studies  have  been  conducted  on  emissions of particulate  matter from the electrolytic reduction process, it appears that fluorinated particulates (e.g., Na3AlF6, Na5Al3F14, NaAlF4, AlF3, CaF2) are emitted through vaporization and then condensation of the electrolyte,  carried along with  gases, or  through  a  chemical  reaction  in  the  gasFootnote 4.   Approximately  30% to 50%  of  fluoride  emissions  from  the electrolytic cells consist of  particulate  matter,  a large portion of which is PM2.5. Therefore, given the known emission mechanisms, an aluminium smelter’s operations can be considered a source of PM2.5 emissions.

In an aluminium smelter, there are stationary sources of emissions (e.g., GTCs and dust collectors) and fugitive emissions resulting from the handling of solids (e.g., alumina) and the operation of electrolytic cells (e.g., replacement of anodes). The potential sources of particulate matter from the aluminium production process are shown in Figure 2.1.

For stationary sources, the amount of particulate emissions strongly depends on the efficiency of the filter system. The optimal operation and careful maintenance of the GTC and the dust collectors, for example, make it possible to minimize releases of particulate matter.

There are two subcategories of fugitive emissions: intermittent emissions occurring during operating activities (opening of hoods to change anodes, metal casting, etc.) and continuous emissions resulting from leaks (from the superstructure, the alumina handling system, etc.). In general, operations involving exposure of hot anode butts, molten bath-related materials (cavity cleanings), exposed bath (during anode changes, bath transfers, metal tapping, etc.) have a greater impact on PM2.5 emissions than raw material handling operations (anode cover application, alumina loading, transportation of raw materials, etc.).Footnote 5

Good dust collection, along with good ventilation and a good seal on pots, is essential to minimize fugitive emissions. In addition, it is essential for equipment to be properly designed, operated  and  maintained.  Fugitive  emissions  of fine particles are released through potroom roof vents and are not subject to any particular treatment; these may be the primary source of fine particulate emissions from electrolytic reduction plants.

Figure 2–1: Schematic diagram of aluminium production (electrolytic reduction) illustrating the potential sources of  particulate matter emissions

Figure 2­1: Schematic diagram of aluminium production (electrolytic reduction) illustrating the potential sources of particulate matter emissions. (See long description below)

Description of Figure 2-1

Figure 2-1 is a schematic diagram of an alumina reduction plant showing the potential sources of particulate matter emissions. The plant involves electrolytic cells, a gas treatment centre, an aluminium casting centre and alumina silos, enriched alumina silos and cryolite and other silos. Particulate emissions potentially come from the alumina silos; the transfer of alumina to the GTC; the gas treated in the GTC; the transfer of alumina from the GTC to the enriched alumina silos; the enriched alumina silos; the transfer of fluorinated alumina to the electrolytic cells; the electrolytic cells; the casting centre; and the transfer of anode butts to the prebaked anode plant.

2.2 Prebaked anode production

The sealed prebaked anode manufacturing process involves several integrated steps that are carried out in separate facilities: facilities where green anodes are formed and baked, and where baked anodes are sealed (Figure 2-2). Air pollution control systems are installed on the production equipment used in these three steps of the process.

All Canadian aluminium smelters using prebaked anode technology recycle anode butts on-site or at a supplier’s facility to recover the rods, cast iron and carbon residue. A preliminary butt-cleaning step using power tools is integrated into the process in order to recover as much residue from the fluorine-rich bath as possible, which is then crushed and reused as a raw material in the electrolytic reduction cells. The carbon residue is sent to the green paste plant on-site (if the smelter has a prebaked anode production plant) or to an anode supplier, where it is crushed and then ground with fresh calcined coke. The ground coke is then preheated and mixed with coal tar pitch to produce hot green paste which is formed into compact blocks (green anodes) suited to the configuration of the electrolytic cells. The pitch fumes generated by the green anode manufacturing process are routed to the pitch fume treatment centre (PFTC), which uses a dry scrubber with calcined coke injection to capture the VOCs and PAHs contained in the fumes.

The green anodes are baked in a special furnace with mobile fire fuelled by natural gas or heavy oil. The furnace converts part of the pitch into elemental carbon through carbonization and eliminates the remainder through combustion, thereby producing an anode with minimum levels of aromatic hydrocarbons. The baking gas is normally treated using an alumina injection-based dry scrubber followed by a dust collector to capture particulate matter, PAHs and fluoride resulting from the recycling of carbon from the anode butts. The baked anodes are then sealed on clean rods with molten cast iron from an induction furnace.

Sources of particulate matter

Particulate matter is released from a number of operations, particularly during coke and bath crushing and grinding, which can generate significant emissions. These emissions are, however, strictly controlled through capture at the source and filtration systems. Anode baking is also a significant source of particulate matter, but the emission rate depends on the configuration of the furnace (e.g., open top with horizontal flow or closed type with vertical flow) and on the effectiveness of the dust collection system in the fume treatment centre (FTC). The green anode production process and the pitch fume treatment centre (PFTC) also responsible of  particulate matter emissions..

The concentration  of particulate matter in stack gases is low, but given the very large volume of  off-gases, particulate emissions could become significant if this equipment is not operated properly. Other sources of particulate matter include pneumatic and mechanical handling systems for calcined coke and alumina (in the FTC), which are normally equipped with hoods and dust collectors at the various drop points.

Figure 22: Schematic diagram of prebaked anode production illustrating the potential sources of particulate matter emissions

Figure 2­2: Schematic diagram of prebaked anode production illustrating the potential sources of particulate matter emissions.  (See long description below)

Description of Figure 2-2

Figure 2-2 is a schematic diagram of a prebaked anode plant showing the potential sources of particulate matter emissions. The process for manufacturing sealed prebaked anodes for transfer to the alumina reduction plant consists of several steps including mixing the pitch, the coke and the anode butts; green anode formation; green anode baking and prebaked anode sealing. Potential sources of particulate emissions are the calcined coke silos; coke sieving, grinding and crushing; the transfer of crushed coke to be preheated and mixed; the gas treated in the fume treatment centre and the pitch fume treatment centre; the transfer of coke to the PFTC; the transfer of enriched coke to the calcined coke silos; the alumina silo; the transfer of alumina to the FTC; the transfer of enriched alumina to the reduction plant; anode baking; the induction furnace; anode butt clean-up; frozen bath crushing; the transfer of the crushed bath to the silo; and the crushed bath silo.

2.3 Green coke calcining

Green coke calcining plants use a rotary kiln for calcining. The process involves the following components: a station for unloading and storing green coke, the kiln, a coke cooler, a system that handles calcined coke until storage, and an air emissions scrubber system (Figure 2-3). The purpose of calcining is to remove unwanted compounds from the green coke (which generally contains 8–15% volatile matter, 5–15% humidity, and 1–5% sulphur) and thus improve its crystalline structure  and  electrical  conductivity,  an  important  aspect  during  the  electrolytic reduction process.

Green coke sent to plants by ship or train is received at an unloading station and then carried by conveyor to the storage silos. The green coke is fed into the rotary kiln which is slightly inclined, allowing the material to move slowly downward and heat gradually in contact with the combustion gases flowing counter currently through the kiln. At start-up, a natural gas burner raises the temperature to the self-ignition point for  coke’s volatile components.  Hot  calcined coke exiting the rotary kiln is transferred through a refractory channel to a slightly inclined cylindrical rotary cooler. As the coke-- (which is hotter than 1000°C) --moves downward, it is cooled by water sprayed from a series of nozzles at the front of the cylinder. The calcined coke, which has been cooled to 150–200°C, is unloaded onto heated conveyors for transfer to the storage silos.

The gas stream at the kiln outlet includes combustion gases, unburned volatile compounds and large quantities of coke particles which must be removed before the gases are released to the atmosphere. Canadian plants use a gas combustion system (pyroscrubber or boiler followed by a dust collector) with an upstream expansion chamber to recover up to 80–95% of coarse particulate matter. This residual coke, in addition to the calcined coke recovered by dust collectors (under-calcined coke), is typically used as a fuel or raw material for the production of anode paste. The wet gas from the cooler is directed to a scrubbing system (e.g., venturi wet scrubber) to control the coke particles drawn into the flow of steam.

Sources of particulate matter

Calcining gas treatment and cooling systems are designed primarily to control coke particulate matter carried by gas flows. Most of the post-treatment particulate matter comes from the pyroscrubber (or boiler plus dust collector), which must treat a larger amount of gas flow than the wet scrubber, which essentially treats the flow of steam. Other particulate emissions may occur, particularly during the handling of various forms of coke on a closed conveyor. At the various drop points, the conveyors are generally equipped with hoods and dust collectors, especially for calcined coke, which has a relatively powdery texture.

Figure 2–3: Schematic diagram of green coke calcining illustrating the potential sources of particulate matter emissions

Figure 2­3: Schematic diagram of green coke calcining illustrating the potential sources of particulate matter emissions.  (See long description below)

Description of Figure 2-3

Figure 2-3 is a schematic diagram of a green coke calcination plant showing the potential sources of particulate matter emissions. The plant mainly involves a coke unloading station, a rotary kiln, a cooler, a pyroscrubber or boiler, a wet scrubber and silos for green, subcalcined and calcined coke. Potential sources of particulate emissions are the unloading station; the green coke silo; the transfer of green coke to the rotary kiln; the rotary kiln; unloading of calcined coke into the calcined coke silo; the calcined and subcalcined coke silos; the transfer of subcalcined coke to the pyroscrubber; the pyroscrubber; and the treated gas from the pyroscrubber and the wet scrubber.

2.4 Alumina production

The Vaudreuil plant in Jonquière uses the Bayer process, which involves the alkaline extraction of alumina from bauxite. The general flow diagram is shown in Figure 2-4. Bauxite is an ore of variable structure that consists mainly of aluminium oxides (35–65%), iron (2–30%) and silica (1–10%). The composition of bauxite depends on the deposit from which it is extracted, with the main deposits located in Australia, South America, West Africa and Asia. The Bayer process has five successive steps: bauxite preparation, bauxite digestion, settling of red mud, crystallization and precipitation of alumina trihydrate, and calcination into metallurgical grade alumina. It should be noted that few alumina refineries are identical, as many differences exist particularly with respect to methods used to manage effluents and thermal energy.

Bauxite is generally grinded to a fine powder and then mixed with a concentrated and preheated caustic soda (NaOH) solution (Bayer liquor), before being transferred to pressurized chemical reactors. The Vaudreuil plant uses a wet mill where the liquor is mixed directly with the ore during grinding.  At  high  temperatures  (140–150°C),  the  alkaline  mixture  breaks  down  gibbsite  or boehmite (aluminium oxides) into soluble sodium aluminate while the other bauxite constituents (iron oxides, silica, etc.) persist as solid crystals. The liquid suspension then moves to the decantation stage (and possibly filtration; Figure 2-4), where the solid residue called “red mud” is removed from the aluminate solution. The Vaudreuil plant generates roughly 0.7 tonnes of red mud per tonne of alumina extracted from bauxite. This mud is transported to waste disposal sites (red mud ponds) to be dried and piled.

The liquor is cooled to a point at which crystallization of the aluminate into hydrated alumina crystals can occur. Hydrated alumina (primer) is added to the mix to facilitate the formation of crystals. After a precipitation stage, the crystals are classified based on their size. “Small” crystals are recycled to increase  their  volume,  while  “large”  crystals  are  dried  and  then  calcined at approximately 1000°C in a rotary kiln or a fluidized bed calciner after being washed. Under these conditions, the hydrated alumina breaks down into metallurgical grade alumina through the elimination of water associated with the molecule. The sodium hydroxide liquor is recycled for use in the initial grinding stage.

The Bayer process includes a number of heating and cooling stages for the various material flows. An alumina refinery has several boilers to provide the necessary heat for bauxite digestion. The Vaudreuil plant has six boilers, in addition to three alumina calciners. The plant also has a network of heat exchangers that minimize energy consumption.

Sources of particulate matter

The Bayer process operates mainly in wet conditions, which greatly limits the potential for fugitive emissions of particulate matter to the ambient air. The main source of particulate matter is the alumina calciner and the boilers, which are fuelled by oil, natural gas or electricity at the Vaudreuil plant, depending on prevailing market conditions. Higher concentrations of particulate matter are associated with the use of fuel oil. Another major source is the lime kiln for the production of quicklime (CaO), which is used primarily to sequester phosphorus and improve the solubility of alumina in Bayer liquor. This process is not addressed in the Code since the Vaudreuil plant does not operate a lime kiln.

The same applies to the wet grinding of bauxite (in the case of the Vaudreuil plant), which generates less particulate matter than ball mills. Lastly, the handling and storage of raw materials (bauxite, lime), calcined alumina and red mud may also lead to emissions of particulate matter, as all of these materials are powdery, with the exception of bauxite.

The Bayer process includes a number of crystal washing phases (Figure 2-4) which are designed to recover caustic soda and which cause gradual dilution of the Bayer liquor. This excess water must be eliminated through evaporation in order to maintain the NaOH concentration at an acceptable level for bauxite digestion. However, this may cause microscopic particles (aerosols) of caustic soda to be entrained in the water vapour. In the absence of empirical studies on the subject, it is impossible to establish best practices in this regard. It is nonetheless clear that these aerosols would be released into the atmosphere with the vapour.

Figure 2–4: Schematic diagram of alumina production (Bayer process) illustrating the potential sources of particulate matter emissions

Figure 2­4: Schematic diagram of alumina production (Bayer process) illustrating the potential sources of particulate matter emissions.  (See long description below)

Description of Figure 2-4

Figure 2-4 is a schematic diagram of an alumina production plant (Bayer process) showing the potential sources of particulate emissions. This process for extracting alumina from bauxite involves the following steps: wet grinding, digestion, decantation, filtration, wash water disposal, red mud disposal, precipitation, evaporation, classification, washing, filtration, calcination, steam production in the boilers, as well as silos for bauxite, lime, NaOH and alumina. Potential sources of particulate emissions are the bauxite silo; the lime silo; the transfer of bauxite and lime for wet grinding; the boilers; steam from evaporation; alumina calcining and the resulting treated gas; the transfer of metallurgical alumina; the alumina silo; and disposal of red mud.

 
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