Environmental impact assessment of aluminium production using the life cycle assessment tool and multi-criteria analysis

Air pollution is one of the most important problems of urban life. Since a large proportion of airborne pollutants originate from industry, it is important to address emission removal systems. One of the growing industries is the production of aluminum, which requires attention and planning since emits dangerous pollutants such as particulate matter, SO2, NOx, dioxins, furans, mercury chloride, and fl uorine compounds. The present study investigates the production life cycle of this metal and analyzes the production of gaseous pollutants and particles in different production units. Large amount of pollution is produced in the alumina production and the aluminum electrolysis units, which in the best case, for the production of one ton of fi nal aluminum, Emit 1.07, 4.73, and 1.32 kg of particulate pollutants, sulfur dioxide, and nitrogen dioxide respectively. In the next step, in the search of the optimal system for controlling particulate pollutants, SO2, NOx caused by aluminum production, by reviewing the research background and related articles and books, ranked these systems using ELECTRE, TOPSIS and SAW methods. Sedimentation chamber, internal separators, cyclones, fabric fi lters, electrostatic precipitators, and wet collectors in particle removal and condensation, absorption, adsorption, incineration, and wet washing in SO2 and NOx removal were reviewed and compared. The results show the superiority of cyclones in particle removal, wet washing system for removing SO2, and adsorption for removing NOX.


Introduction
In advanced societies, aluminum is the most widely used element after steel and its derivatives, among different industries; and after steel, aluminum is the most highly produced metal and the most produced non-ferrous metal.
Features such as low weight, corrosion resistance, and high ductility have led to a greater tendency to use aluminum in the aerospace, automotive, transportation, construction, beverage, and electrical industries [1][2][3][4]. Based on assessments in 2005, it was predicted that by the end of 2050, the demand for aluminum in various industrial applications will increase 2.6 to 3.5 times [5]. According to the statistics announced in 2013, China by 42.9% aluminum and 41.7% alumina is the largest producer in the world [6]. The production process in various industries requires a huge supply of energy. While this energy supply leads to the emission of pollutants such as CO, SO 2 , NO 2 , O 3 , PM 10, and PM 2.5 . Studies and evaluations show that most of the energy consumption of industries is related to fundamentals such as steel, cement, paper, aluminum, and plastics [5].
Among the mentioned industries, aluminum and steel have the highest energy consumption and consequently the production of carbon pollution. According to fi eld research conducted by scientifi c authorities in 2012, 66 MJ of energy was required to produce one kilogram of primary aluminum [6]. Therefore, saving energy and reducing pollutants is a Citation: Abdollahi  key element in the sustainable development of the aluminum industry [1]. Aluminum production from bauxite mines needs more energy rather than other metals, which leads to large emissions of greenhouse gases [7]. On a global scale, almost one percent of greenhouse gases is the result of aluminum production [8]. It should also be noted that during the aluminum production process, hazardous compounds such as fl uorine, sulfur dioxide, hydrogen sulfi de, and polycyclic aromatic hydrocarbons are released into the air, leading to many chronic and acute epidemiological effects on human health [9][10][11]. Two successive process chains including preparing alumina (aluminum oxide) from bauxite rock and aluminum from raw alumina are performed to produce aluminum. These processes require a lot of energy, which is a huge potential for emissions [12]. Melting steps and processes associated with primary production (including mining, purifying, and anode production for refi ning) account for 90% of all contaminants [13].
The most important source of pollution is indirect pollutants, which often occur during the process of generating electricity and account for 65% of pollution, another 18% is related to the internal production process of factories and the remaining 17% is emitted by burning fossil fuels [14]. To determine the amount of emitted pollutants in the production process of aluminum, a systematic approach and modern technologies shall be used [2]. Life Cycle Assessment (LCA) is a top-down approach to determine the consumption or production of environmental factors over the life period of a single product, which performs measurements by the time of beginning until that product is dumped [2,12]. During a study in Canada, Norgate and Rankin (2001) analyzed the emission of greenhouse gases generated by the aluminum production process using a life cycle assessment method [15]. Also, Reginald, et al. (2005) conducted research on the primary aluminum supply cycle and the resulting contaminants through the LCA method [12]. In another study, Hong et al. (2012) evaluated the production of aluminum and silicon alloys in China using an economic and environmental life cycle [16]. Ingarao, et al. (2016) studied the energy consumption and CO 2 production of Aluminum windows of high-speed trains manufacturing process using the LCA method [17]. Also, Paraskevas, et al. (2016) analyzed the environmental impacts of primary aluminum production in different countries using the life cycle assessment method [14]. In this study, due to the little attention paid to aluminum-related contaminants in industry in previous studies, it is necessary to conduct the present study with a different approach/framework. The present study in the fi rst step intends to (i) determine the amount of air pollution generated by aluminum production using the LCA method and in the next step; (ii) prioritize different treatment methods of particulate matters, nitrogen oxides, and sulfur dioxide in terms of environmental, economic and engineering using ELECTRE (ELimination Et Choix Traduisant la REalité), Entropy and SAW (Simple Additive Weighting) techniques.

Materials and methods
This research was conducted in two general phases including environmental assessments and prioritization of control systems. In the fi rst phase, the life cycle assessment was used to evaluate the amount of air pollution caused by aluminum production. In the second phase, different methods of controlling and treating air pollution were prioritized using the ELECTRE decision-making system and considering environmental, economic, environmental, and engineering indicators.

Life cycle assessment
Life cycle assessment is a scientifi c and systematic framework for estimating the environmental impact of a product. This is a new and underdeveloped method that was fi rst used for energy in the 1960s and then in the 1970s to prevent pollution [18]. In general, the life cycle of a single product includes the extraction and processing of raw materials, energy supply, manufacturing, use, recycling, and fi nal disposal. There are different approaches in evaluating the life cycle of a product, cradle to grave, cradle to gate, cradle to cradle, gate to gate. Also, in the cradle to grave approach, the product is under evaluation from the extraction of raw material (cradle) to its fi nal disposal (grave). In the cradle to gate approach, the evaluation starts with extraction until right before the transfer to the consumer. On the other hand, the cradle to cradle approach is a special form of cradle to grave; except that instead of the disposal stage, the recycling stage takes place. Also, in the gate-to-gate method, just parts of the production chain will be examined [18,19]. The present study intends to evaluate the life cycle of aluminum from the bauxite extraction stage (cradle) to the primary aluminum production stage (gate). The primary aluminum production process includes bauxite extraction, alumina refi ning, aluminum electrolysis, anode production, and aluminum ingot casting. It should be noted that the stages of using the fi nal product and also the recycling stage have not been considered in this study. The primary aluminum supply chain diagram is depicted in Figure 1. Furthermore, the aluminum production process and all data in this study are taken from the World Aluminum Organization data in 2010.

Gaseous pollutants separation methods
Knowing the purpose of particle removal in search of a suitable method to control pollution, different methods and techniques shall be examined and evaluated. The most common particle removal techniques are as follows.

Sedimentation chamber:
The sedimentation chamber consists essentially of a chamber, in which the velocity of the particle is reduced to such an extent that it settles due to gravity. One of the advantages of this method is that the required energy to perform this process is provided by gravity and of course the energy costs are very low. The application of this method is limited to the removal of particles with a diameter greater than 4 microns. The most common type of sedimentation chamber is a relatively long box that is placed horizontally. The gas enters the chamber from one side and leaves from the other side.

Internal separators:
In addition to gravity, these units use another way to improve separation and collection. This Cyclones: Cyclones are another powerful particle removal method, which occurs based on the centrifugation principle. Here, particles (which have a higher weight and density) are thrown towards the tank walls and then slip into a collector. Cyclones are the most widely used dust separators due to the high percentage of separation in these units. Generally, cyclones are used when the particles are large and high concentrated and also when there is no need for high effi ciency.
Fabric fi lters: Filtration is one of the most widespread and oldest suspended particle separating methods. A fi lter is a compact porous medium made of fi brous or granular material through which gas passes and suspended particles remain among the fi lter, depending on the application location and the expected effi ciency, different types of fi lters are available. In recent years, deformed fi lters called cartridges have found their place on the market, which can be cleaned in different ways such as pulse jets, etc. They range from paper to chemical and heat resistant. Fabric fi lters generally show a high effi ciency even when removing micron-sized particles. This system is often preferred when recycling valuable dry materials is of a high priority. However, the compulsion to keep the gas at a higher temperature than the dew point is one of the limitations of this method. Usually, if the gas temperature and volume are relatively low, this method is more applicable. Particle control devices should be designed in such a way that they can remove 10% higher than the load leaving the industrial unit. Various systems have been designed to remove gases and vapors (including sulfur oxides, nitrogen oxides, toxic mercury vapors, and other gaseous pollutants). The basic performance of these systems is classifi ed into four groups: Condensation: In this process, gas or steam is liquefi ed, which is done either by decreasing the temperature or increasing the pressure. Temperature drops are usually more common due to their low cost. Condensers are simple and inexpensive devices that use water or air to cool and compress air. The effi ciency of these devices in removing pollutants is very low and they are mostly used as pre-treatment. Using them before sorbents and incinerators are convenient because they reduce the gas volume and refi ning costs. There are two types of condensers The fi rst type is contactual (direct contact), in which the condenser of the cooling and condensing environment is combined. The second type is surface condensers (indirect contact) that separate the cooling medium and the condensed steam. There are various methods (e.g., using low-sulfur fuels and natural gas, injection of limestone in both dry and wet forms, catalytic oxidation with vanadium pentoxide, and washing with alkaline sodium) to control sulfur oxides. it is also possible to implement Combustion with low excess air to control nitrogen oxides as an additional solution. Two-stage combustion, fl ue gas recirculation, burner design change, using wet washers and catalysts are other ways to control nitrogen oxide pollutants.

Decision-making methods
Electre: In this method, all options are evaluated using non-ranking comparison which eliminates ineffective options.
All stages are based on a concordance and discordance set therefore it is called "inconsistency analysis". First, by Equation 1, the decision matrix becomes a scale less matrix. Later on, the concordance matrix of S kl and the discordance matrix of D kl will be calculated using Equation 2, then the coeffi cient criterion between A k and A l will be estimated based on Equation 3. Meanwhile, the higher the value of parameter I kl ; Indicates the more appropriateness of A k 's assessment concerning A l .
The discordance matrix is formed by Equation 4 and the effective concordance matrix is formed based on the minimum threshold of Boolean F and G matrices with zero and one element, which are described in Equation 5.
Finally, the general matrix h, which represents the order of relative preferences of the options, will be calculated using Equation 6.

The SAW method
The SAW method is the simplest way of weighting and prioritizing parameters. In this method, in addition to the weight of each comparative parameter, an importance coeffi cient as Equation 7 is applied which makes it a reliable and simple method.
Results and discussion

The amount of emission in different units of the aluminum production process
According to a study conducted by the International Aluminum Institute, the amount of emission during the production process of this metal in various units is presented in Table 1 that the most important of which are carbon dioxide, sulfur, sulfur dioxide, nitrogen dioxide, compounds containing mercury and its vapors, and halogenated particles containing

Prioritizing air pollution control methods
As mentioned before, controlling air pollution is inevitable, and given the signifi cance of pollutants such as particles,  Table 2 for all options.
There are fi ve general methods to remove gaseous pollutants (including condensation, absorption, adsorption, incineration, and wet washing). Thus, the value of different gas removal options for sulfur dioxide and nitrogen oxides follows in Table 3. The basic criterion for prioritizing refi ning methods in this study was the superiority of employment in the aluminum production process. The cyclones and sedimentation chamber has been preferred over other methods and also the cyclone system has obtained more votes than the sedimentation chamber for controlling particles. The best method for controlling sulfur dioxide is the air washer system, which is followed by the adsorption method. Evaluation of nitrogen oxide control methods also resulted in the superiority of absorption and adsorption methods which was associated with the relative superiority of adsorption.     Figure 8: Application prioritization of pollution control systems (1. sedimentation chamber; 2. internal separators; 3. Cyclones; 4. fabric fi lters; 5. electrostatic precipitators; and, 6. wet collectors, respectively) for particle control, derived from ELECTRE, TOPSIS, and SAW classifi cation methods..