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2. Operational Activities

This section describes the main functions of each operational activity in the primary aluminum sector subject to the Code. The purpose of this section is to identify the nature and scope of the activities covered by the Code, particularly those that raise concerns with regard to fine particulate matter emissions, which are discussed in Section 3. Figures 2-1 to 2-4 illustrate the main processes relating to the Code and particulate matter emissions in the environment.

2.1 Alumina Reduction Plant

Aluminum is produced through the electrolytic reduction of alumina dissolved in a molten bath consisting of aluminum fluoride and sodium fluoride (cryolite) at a temperature of approximately 960°C. Aside from electrolytic cells, an aluminum plant also contains an aluminum casting centre and a gas treatment centre (Figure 2-1).

Each electrolytic cell is composed of a carbon cathode insulated with refractory bricks inside a rectangular steel tank in which carbon anodes are suspended from a suspension beam. The cells are connected in series to form an electric reduction line (potline). During operation, a continuous high-amperage current passes from the anode through the electrolytic molten bath 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 between 2 and 6% in the molten bath. These additions are controlled by a computer in modern plants. Fluoride compounds (e.g., aluminum fluoride) are also added to lower the electrolytic bath’s melting point, which helps maintain a lower operating temperature in the cells. The molten aluminum liquid is deposited at the cathode on the bottom of the pot, while the oxygen from the ore reacts with the carbon from the anode to form carbon dioxide.

The anodes must be replaced regularly as the carbon is gradually used up. 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 emitted through the electrolysis process are ventilated into a gas treatment centre (GTC) that captures the fluorine from the cryolite bath before being released into the atmosphere. A dry scrubber with fresh alumina injection is used for this function. The alumina that has absorbed the fluorinated gas is recovered with a baghouse that also captures particulate matter produced by the electrolysis process. The “fluorinated” alumina that leaves the GTC is used as a raw material in the electrolytic cells.

Molten aluminum metal is regularly siphoned into a crucible and then transferred to a casting centre to be transformed into ingots. The molten metal typically passes through a holding furnace to control its temperature. The liquid aluminum then passes through a series of culverts where it may undergo degassing or filtration treatments or have alloying metals added before reaching the casting machine, which produces various shapes of ingots.

Figure 2-1: Schematic Diagram of an Alumina Reduction Plant Illustrating the Potential Sources of Particulate Matter Emissions

Schematic Diagram of an Alumina Reduction Plant 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 aluminum 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 Plant

The sealed prebaked anode manufacturing process involves a number of integrated steps that are related and grouped into facilities where green anodes are formed and baked, and where baked anodes are sealed (Figure 2-2). Air pollution control systems are also found at various steps of the process.

All Canadian aluminum smelters with prebaked anodes recycle anode butts to recover the rods, cast iron and carbon residue. A preliminary step in dry cleaning the butts with power tools is integrated in order to recover as much residue from the fluorine-rich baths as possible, which is then crushed and recycled back to the electrolysis plant. The carbon residue is sent to the green paste facility to be crushed and then ground with fresh calcined coke. The ground coke is then preheated and mixed with tar pitch to produce a hot green paste that must be cast into compact blocks (green anodes) according to the configuration of the electrolytic cells. At the same time, the pitch fumes generated by the anode manufacturing process are routed to the pitch fume treatment centre (PFTC), consisting of a dry scrubber with calcined coke injection, in order to capture VOCs and PAHs contained in these fumes.

The green anodes are baked in a special furnace with mobile fire fed by natural gas or heavy oil to make them hard and electrically conductive. The objective is to convert part of the pitch into elemental carbon through carbonization, eliminate the remainder and thus obtain an anode with a minimum of aromatic hydrocarbons. The baking gas is normally treated with an alumina injection-based dry scrubber followed by a baghouse to capture particulate matter and fluorine resulting from the anode butts. The baked anodes are then sealed on clean rods with molten cast iron from an induction furnace. Each anode set includes two carbon blocks per rod. The sealed prebaked anodes are stored awaiting use in the electrolysis plant.

Figure 2-2: Schematic Diagram of a Prebaked Anode Plant Illustrating the Potential Sources of Particulate Matter Emissions

Schematic Diagram of a Prebaked Anode Plant 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 Plant

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 (8–15% of volatile matter, 5–15% of humidity, a part of sulphur at 1–5%) and thus improve its crystalline structure and electrical conductivity, an important aspect during the alumina electrolysis process.

Figure 2-3: Schematic Diagram of a Green Coke Calcining Plant Illustrating the Potential Sources of Particulate Matter Emissions

Schematic Diagram of a Green Coke Calcining Plant 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.

Green coke sent to plants by ship or train is received at an unloading station and then sent by conveyor to the storage silos. Green coke is fed into the rotating kiln and slightly inclined so that it can slide slowly down the furnace, heating gradually as it makes contact with the combustion gases flowing against the current. At start-up, a natural gas burner raises the temperature to the self-combustion level for coke’s volatile components. Hot calcined coke at the end of the rotating kiln is transferred by a refractory channel to a slightly inclined rotating cylindrical cooler. As everything moves down, the coke, which is more than 1000°C, is cooled by water spraying through a series of nozzles at the front of the cylinder. The calcined coke, cooled to 150–200°C, is unloaded onto heated conveyors to be taken to the storage silos.

The gas flow at the outlet of the kiln contains combustion gases, unburned volatile compounds and large quantities of coke particles that must be removed before being released into the atmosphere. Canadian plants use a gas combustion system (pyroscrubber or boiler followed by a baghouse) with an expansion chamber further ahead to recover up to 80–95% of coarse particulate matter. This residual coke, in addition to the calcined coke recovered through dust collectors (under-calcined coke), is normally used as a fuel or raw material to make anode paste. In the meantime, 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 steam flow.

2.4 Bauxite Refining Plant

The Vaudreuil plant in Jonquière operates a process involving alkaline extraction of alumina from bauxite (Bayer process). The general flow diagram is shown in Figure 2-4. Bauxite is an ore varying in structure and containing primarily aluminum oxides (35–65%), iron (2–30%) and silicon (1–10%). The composition of bauxite depends on the deposit, found mainly in Australia, South America and west Africa. 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 there are many subtle variations, particularly in managing effluents and thermal energy.

Bauxite is generally milled to a fine powder and then mixed with a concentrated and preheated caustic soda (NaOH) solution (Bayer liquor), before being transferred to pressurized boilers. The Vaudreuil plant uses a wet mill where the liquor is mixed directly with the ore during grinding. At high temperatures (150–250°C), the alkaline mixture breaks down gibbsite or boehmite (aluminum oxides) into soluble aluminate (Al2O42-), while the other bauxite components (iron oxides, silica, etc.) remain in the form of 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 0.6 tonnes of red mud per tonne of alumina extracted from bauxite. This mud is transported to ore waste disposal sites (red mud lakes) to be dried and piled.

The liquor is cooled to a level that can induce crystallization of the aluminate into hydrated alumina flakes. Pure alumina is added to the mix to facilitate the formation of crystals. After a precipitation stage, the crystals are classified based on their dimensions. “Small” crystals are recycled to increase the volume, while “large” flakes are dried and then calcined at approximately 1000°C in a rotating kiln or a fluidized bed after being washed. Under these conditions, the hydrated alumina breaks down into metallurgical grade alumina through the elimination of water associated with the molecule. In the meantime, the sodium hydroxide liquor is concentrated through evaporation and then recycled to the initial milling stage.

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

Figure 2-4: Schematic Diagram of an Alumina Production Plant (Bayer Process) Illustrating the Potential Sources of Particulate Matter Emissions

Schematic Diagram of an Alumina Production Plant (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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