3. Sources of Particulate Matter

• 3.1 Alumina Reduction Plant
• 3.2 Prebaked Anode Plant
• 3.3 Green Coke Calcination Plant
• 3.4 Bauxite Refining Plant

Aside from the operational activities set out in Section 2, this section provides an overview of situations that may lead to emissions of particulate matter into the work environment or the atmosphere. Figures 2-1 to 2-4 identify these potential sources of particulate matter, which were considered during the development of the Code. The complete list is provided in Table 3-1. It should be noted that particulate matter emissions from port and/or rail services for various materials are not included.

It is important to note that this section identifies sources of total particulate matter, whereas fine particulate matter such as PM_2.5 is difficult to separate. Good practices have therefore been developed to control total particulate matter, leading to a reduction in PM_2.5 emissions as an additional result. It is, however, difficult to establish with any certainty the proportion of PM_2.5 for each source of total particulate matter. This is why the effectiveness of controlling PM_2.5 emissions is not discussed in the Code.

Of the various activities associated with the primary aluminum industry, plants that reduce alumina to aluminum metal are the primary source of air pollution.This pollution is essentially associated with the electrolysis process in the cryolite bath, releasing fluorinated gases (e.g., hydrogen fluoride (HF) and perfluorocarbons (PFC)), SO₂ and particulate matter from the anodes, alumina and cryolite bath (fluorinated particulate matter). It has been shown that the particles released from the bath originate mainly from bath fumes emissions and dust from anode covers.⁶ Particulate matter emissions are therefore inevitable. There is a complex relationship between the level of particulate matter released in the potroom, the particulate-generating mechanisms and the factors that influence them (e.g., electrolysis technology, quality of raw materials, work methods, transportation and handling).

In an aluminum smelter, there are emissions from fixed sources (e.g., GTCs and dust collectors) and fugitive emissions occurring during the handling of solids (e.g., alumina) and the operation of electrolytic cells, the casting centre and the GTC. The level of particulate matter emissions at the outlet of the GTC is highly dependent on the effectiveness of the baghouse downstream from the dry scrubber. Meticulous control and maintenance of the GTC keeps the unit’s particulate matter emissions to a minimum, within the baghouse design efficiency level. The capture of fine particulate matter is difficult, and the baghouse cannot entirely prevent the penetration of very fine particulate matter. This typically accounts for over 70% of particulate matter released into the atmosphere by the baghouse.

With regard to fugitive emissions, there are two sub-categories: intermittent emissions occurring during operating activities (when opening pots to change anodes, casting metal, etc.) and continuous emissions resulting from leaks (through the superstructure, the alumina handling system, etc.). These 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 released through roof vents in the potroom and are not subject to any particular treatment.

Particulate matter occurs at several locations, particularly during coke and bath crushing and grinding, which can result in considerable emissions. They are, however, strictly controlled by the collection and scrubbing systems. Anode baking is also a significant source of particulate matter, the emission rate of which depends on the configuration of the furnace (e.g., open with horizontal flow or closed with vertical flow) and the effectiveness of the baghouse at the fume treatment centre (FTC). The PFTC is also responsible for particulate matter emissions, due primarily to the injection of coke and not fumes. The particulate matter concentration in stack gas is not high, but due to a large off-gas volume, the mass flow of particulate matter becomes significant. Other sources of particulate matter include pneumatic and mechanical calcined coke and alumina handling systems that are normally equipped with hoods and dust collectors at various drop points.

Calcination 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 baghouse), which must treat a larger gas flow, compared to the wet scrubber, which essentially treats steam flow. Other particulate matter emissions can occur, particularly during 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.

The Bayer process operates mainly in wet conditions, which limits the potential for fugitive emissions of particulate matter into the ambient air. In fact, the main source of particulate matter is the alumina calciner and the boilers, which are fuelled by oil, natural gas or electricity depending on the market conditions at the time. Concentrations of particulate matter are more significant when oil is used. Another major source is the lime kiln for the production of quicklime (CaO), used primarily to isolate phosphorus and improve the solubility of alumina in Bayer liquor. This process is not considered in the Code since the Vaudreuil plant does not operate one. The same applies to the wet grinding of bauxite (in the case of the Vaudreuil plant), which unlike ball mills is not supposed to generate particulate matter. Lastly, the handling and storage of raw materials (bauxite, lime), calcined alumina and red mud may also lead to emissions of particulate matter, as they are all powdery materials, with the exception of bauxite.

The Bayer process involves a number of crystal washing phases (Figure 2-4) to recover caustic soda, thereby causing a gradual dilution of the Bayer liquor. The elimination of this excess water through evaporation is necessary to maintain the concentration of NaOH at an acceptable level for bauxite digestion. However, this may cause microscopic particles (aerosols) of caustic soda to be carried with the water vapour. In the absence of empirical studies on the subject, it is impossible to establish good practices in this regard. It is nonetheless clear that these aerosols would be released into the atmosphere with the vapour, should that occur.

⁶ Wong, D. S., Tjahyono, N. I., Hyland, M. M., Visualising the sources of potroom dust in aluminium smelters, Light Metals 2012 , p.833-838.