Reducing the Carbon Footprint: Primary Production of Aluminum and Silicon with Changing Energy Systems

The world now pushes for a low-carbon future, and international goals for greenhouse gas emission reductions have been set. Industrial processes, including metallurgical processes, make up more than a fifth of the total global emissions, and those have been rising with infrastructure development and the expansion of the middle-class worldwide. This paper focuses on two energy-intensive processes, aluminum production and metallurgical grade silicon production, and how the carbon footprints from these industrial processes have developed in recent decades. The main trend is that the increased demand for these metals has led to expanding primary production for both of them, based on energy with an increasing share of fossil-based electric power. In fact, the average carbon footprint of the energy used in aluminum and silicon production has increased by 38% and 43%, respectively, from 2000 to 2019. The change in energy mix offsets any progress in process efficiencies. This work addresses this and discusses opportunities for improvements. Graphical Abstract

Effect of Carbonaceous Reducers on Carbon Emission during Silicon Production in SAF of 8.5 MVA and 12.5 MVA

The silicon manufacturing process produces a large amount of carbon emissions, which is of deep concern to the Chinese government. Previous research has calculated the amount of carbon emissions incurred in silicon production, while research on the factors that affect carbon emissions during the silicon production process has been scarce. The effect of the carbonaceous reducers' consumption on the carbon emission during silicon production was investigated using statistical analysis of the actual production data in order to lower the carbon emissions of silicon production. The effect of different type furnaces (8.5MVA and 12.5MVA) on the carbon emission were also investigated in the study. Based on the results, the soft coal has the greatest impact on carbon emissions when using the 8.5MVA submerged arc furnace. When using the 12.5MVA furnace, petroleum coke has the greatest impact on carbon emission. The use of the 12.5MVA furnace reduces the carbon dioxide emissions of the production of one ton of silicon by approximately 74 kg compared to the 8.5MVA furnace. To obtained reduced carbon emissions in silicon production, we suggest that the silicon manufacturers should (1) use the 12.5MVA submerged arc furnace as much as possible; (2) and optimize the ratio of carbonaceous reducing agents in raw materials for the different furnace types.

Homogenised model for the electrical current distribution within a submerged arc furnace for silicon production

Silicon is produced in submerged arc furnaces which are heated by electric currents passing through the furnace. It is important to understand the distribution of heating within the furnace in order to accurately model the silicon production process, yet many existing studies neglect aspects of this current flow. In the present paper, we formulate a model that couples the electrical current to thermal, material flow and chemical processes in the furnace. We then exploit disparate timescales to homogenise the model over the timescale of the alternating current, deriving averaged equations for the slow evolution of the system. Our numerical simulations predict a minimum applied current that is required in order to obtain steady-state solutions of the homogenised model and show that for high enough applied currents, two spatially heterogeneous steady-state solutions exist, with distinct crater sizes. We show that the system evolves to the steady state with a larger crater radius and explain this behaviour in terms of the overall power balance typically found within a furnace. We find that the industrial practice of stoking furnaces increases the overall rate of material consumption in the furnace, thereby improving the efficiency of silicon production.

Effect of Silica and Carbon-reducing Agents on Ni and Ti iImpurities During Silicon Production

The Ni and Ti contents of industrial silicon has a significantly affect the of organic silicon. On the basis of large specific production data, the chemical component of silica and the carbon-reducing agent effect of the Ni and Ti contents of silicon were investigated using statistical techniques. Two furnaces were also studied—an 8.5 MVA furnace and a 12.5 MVA furnace. The effects of TiO2 and NiO impurities on the power consumption of both furnaces were also evaluated using the correlation of the TiO2 and NiO impurities in raw materials with specific power consumption. The consumption of raw materials exhibited a high negative correlation with the TiO2 and NiO impurities in industrial silicon, as determined by linear regression—that is, 82% < |r| < 99%. The influence of Ni on industrial silicon production was also stronger than that of Ti. With an increase in Ni, the power consumption of the 8.5 MWA furnace significantly decreased, whereas that of the 12.5 MWA furnace is increased. Adjusting the content of Ni content can reduce the power consumption of industrial silicon production in the large furnace.

Research into the chemical composition of refinery slag from silicon production for its efficient recycling

The aim was to investigate the chemical composition of refinery slag obtained during silicon production in order to identify approaches to its further recycling. Research samples were collected from the slag remained after oxidation refining at the JSC Silicon (AO Kremny), RUSAL (Shelekhov, Irkutsk Oblast). The methods of X-ray phase, X-ray fluorescence, metallographic and scanning electron microscopy were employed to investigate the chemical composition of the samples. It was found that the refinery slag under study includes such basic components as elemental silicon, its carbide and oxide, as well as elemental carbon. It was shown that silicon carbide is the product of incomplete reduction, resulting from melting silica-containing ores in a smelting furnace. According to the conducted X-ray fluorescent analysis, the samples also contain (wt %): Ca - 7.40; Al - 3.80; Fe - 0.30; Ba - 0.19; K - 0.14; Na - 0.09; Sr - 0.09; Mg - 0.08; Ti - 0.05; S - 0.02. Calcium and aluminium are present in the slag mostly in the form of oxides. Complex oxides of an anor-thite type were also found: CaO Al2O3 2SiO2. The refinery slag under study also features insignificant amounts of other metal oxides, which are released from the furnace slag forming during the smelting process. The slag produced by oxidation refining during crystalline silicon production is a technogenic raw material containing valuable components. Due to the significant content of silicon in the refinery slag (from 42% to 65%), the existing methods applied to recycle such an industrial material were analysed in terms of additional silicon extraction or production of commercial silicon-containing products, which are in demand in various industries.

Analysis of the Composition and Properties of the Silicon Production Wet Cleaning Sludge to Identify Sustainable Techniques for its Processing

The paper is devoted to the urgent issue of processing the dust waste of metallurgical-grade silicon production, i.e. wet cleaning sludge, which contains a significant amount of valuable silica. The paper analyzes the formation of finely dispersed techno-genic materials that are generated in significant quantities (up to 120 t/d) at the Kremniy JSC. The composition and properties of the silicon production wet cleaning product have been studied. In analytical studies of the wet cleaning sludge samples, the modern certified analysis techniques have been used: laser diffraction, X-ray diffraction, and X-ray fluorescence. According to the analysis, the L:S ratio of liquid sludge is 2.1:1; after dehydration, the sludge cake has a grain size of 150 μm, with the prevailing (90 %) grain size of 59.65 μm in the test sample. The chemical composition of the sludge is 95.86 % SiO2; therefore, the wet cleaning sludge is a valuable raw material to produce metallurgical-grade silicon. Based on the analysis of the composition and properties of the silicon production wet cleaning sludge sample, we have developed a program for its processing. Sustainable sludge processing techniques are aimed at obtaining a briquetted charge, which can be used as an additive to the main raw material.