Abstract

Highlights

  • This process solves the environmental pollution of wastewater from the aluminum electrolysis industry.
  • A process for treating a closed-circuit cycle of aluminum electrolytic industrial wastewater was proposed.
  • High-quality cryolite was synthesized at low temperature (30 °C) to reduce energy consumption in industrial applications.

The industrial wastewater of aluminum electrolysis contains a large amount of fluorine and valuable metals, and the pH value is high. Direct discharge will cause water pollution. Therefore, this paper studies a process of synthesizing cryolite from aluminum electrolysis industrial wastewater by supplementing aluminum source and adjusting pH value by adding acid, which is used as electrolyte for electrolytic aluminum. Fluoride ion electrode, XRD, Fourier transform infrared spectroscopy, laser particle size analyzer, XPS and SEM-EDS were used to characterize the fluoride ion concentration, phase composition, structure characterization, particle size distribution, chemical bond and morphology-element composition. The results showed that when the synthesis temperature was 30 °C, the pH value was 7 and the ratio of aluminum to fluorine was 1:6, the fluorine recovery rate was the highest, reaching 98.7 %. The synthesized product is cryolite (molecular ratio is 2.815) with an average particle size of about 20 ?m. When used as electrolyte, the liquidus temperature is 950 °C, the solubility of Al2O3 is 6.94 %. This process solves the environmental pollution of wastewater from the aluminum electrolysis industry. It provides an efficient, economical and green way for the recycling of aluminum electrolysis industrial wastewater.

Graphical abstract

Introduction

Aluminum metal is one of the most produced and consumed metals in the world, and China’s electrolytic aluminum industry has always maintained a relatively fast development rate, which, according to statistics, grew from 17.73 million tons in 2010 to 37.34 million tons in 2020 (Mahecha-Rivas et al., 2021; Liu et al., 2015). Currently, the cryolite-alumina molten salt electrolysis is the only method for the industrial production of aluminum, which is based on the principle of electrolysis of alumina in a high-temperature molten salt electrolyte to obtain an aluminum liquid. Due to electrolyte erosion during electrolysis, the cathode surface and the electrolyzer lining are permeated with large amounts of fluoride (Meirelles and Santos, 2014; Birry et al., 2016; Tschöpe et al., 2012). During the production of aluminum electrolysis, for every 1 ton of aluminum produced, 30–40 kg of overhaul slag will be generated, and the global annual production of overhaul slag is as high as about 2 million tons (Chang and Liu, 2007). Therefore, a large amount of hazardous waste is generated in the aluminum electrolysis generation process. In the process of long-term open storage, a large amount of fluorine-containing waste liquid will be generated. Unlike fluorine-containing wastewater from other industries, aluminum electrolysis industrial wastewater contains not only high fluorine content, but also elements such as Na (15,090 mg/L), Al (320 mg/L), K (300 mg/L), Mg (220 mg/L), and Ca (950 mg/L) (Habuda-Stani? et al., 2014). These high concentrations of fluorine-containing waste liquids can cause contamination of surface water, soil and groundwater if they are discharged directly without treatment (Lacson et al., 2021). And it will cause a large amount of fluorine resources and valuable elements to be lost. Therefore, it is of great significance to realize the removal and recovery of fluorine and other valuable elements from fluorine wastewater. It helps to promote the development of circular economy in the aluminum industry, reduce the procurement cost of enterprises, improve the efficiency of resource utilization, reduce the pollution of the environment, and conform to the concept of sustainable development. Realize the win-win situation of economic and environmental benefits.
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With the enhancement of people’s awareness of sustainable development and environmental protection, the removal of fluorine from fluorine-containing wastewater has received more and more attention, and there are a variety of fluorine removal techniques for fluorine-containing wastewater. The adsorption method is to treat fluoride in wastewater by adsorption, ion exchange and surface complexation. Xu, Wenjing et al. (Xu et al., 2018) synthesized a PAA-Al-250 adsorbent for fluoride removal and the fluorine adsorption rate was only 92.2%. Adsorption is limited to treating waste liquids with low fluoride concentrations, and adsorbents are limited as carriers for fluoride adsorption. The electrocoagulation method is to immerse the electrode in the waste liquid, dissolve the aluminum ion by applying a DC sacrificial electrode, and form different forms of hydroxide intermediates in the hydrolysis process and the condensation process to adsorb the fluoride in the waste liquid. Ya, Vinh (Ya et al., 2019) et al. crystallized Na3AlF6 from fluorine-containing waste streams by electroflocculation and obtained 93% fluorine removal under optimum conditions. However, the energy consumption in the production was high, ranging from 3.1–6.7 kW h for the removal of 1 mol F. The results showed that the energy consumption of the electroflocculation method was high. The ion-exchange method of fluoride removal uses certain anions contained in the exchanger to exchange with F? in the wastewater for the purpose of fluoride removal. Singh (Singh et al., 2020) et al. prepared HAIX-Zr resin by impregnating nano ZrO2. The resin material achieved 60% fluoride removal in 30 min with a maximum adsorption capacity of 12 mg/g. Ion exchange method is highly selective and suitable for high purity purification and separation. However, a significant disadvantage lies in the limited capacity of the ion exchange resins employed (Gong et al., 2012), and resin rupture occurs when the exchanger reaches saturation (Kumar et al., 2019). The traditional chemical precipitation method mainly involves supplementing calcium salts into the fluorine-containing waste liquid and utilizing Ca2+ to combine with F? in the waste liquid to produce CaF2 precipitation. Calcium chloride and lime coagulation and precipitation are often used by industry to treat highly concentrated fluoridated wastewater (Takaya et al., 2021). M. F. Chang (Chang et al., 2007) et al. used polymerized aluminum chloride to flocculate fluoride in semiconductor wastewater to precipitate out as CaF2. However calcium fluoride is a very fine particulate matter with low density and high viscosity. It is colloidal in the precipitation process, and the settling speed is slowed down, which will produce secondary pollution of calcium fluoride and high post-treatment cost. The existing processes mainly focus on post-pollution treatment to remove fluoride, without recovering the valuable elements in wastewater as a resource. This leads to the loss of a large amount of fluorine resources and other valuable elements in wastewater. Therefore, the development of a technology that can both reduce the fluorine content in wastewater and recycle fluoride from aluminum electrolysis industrial wastewater is imminent.
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Given that aluminum electrolysis industrial wastewater contains a large amount of fluorine, sodium and aluminum. This study proposes a process to synthesize cryolite from aluminum electrolysis industrial wastewater by supplementing the aluminum source with acid to adjust the pH value, and then return the synthesized cryolite back to the aluminum electrolysis process to be used as electrolyte. The effects of pH, aluminum-fluorine ratio, and temperature in the synthesis of cryolite from aluminum electrolysis industrial wastewater were studied. The liquid phase line temperature and Al2O3 solubility were determined when the synthesized cryolite was used as an electrolyte. The process can realize closed-circuit circulation treatment of aluminum electrolysis industrial wastewater and eliminate the environmental pollution of aluminum electrolysis industrial wastewater. It provides ideas for the resource utilization of aluminum electrolysis industrial wastewater.
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