Recovery of Carbon and Cryolite from Spent Pot Lining of Aluminium Reduction Cells by Chemical Leaching
Total Page：16
File Type：pdf, Size：1020Kb
Trans. Nonferrous Met. Soc. China 22(2012) 222−227 Recovery of carbon and cryolite from spent pot lining of aluminium reduction cells by chemical leaching SHI Zhongning, LI Wei, HU Xianwei, REN Bijun, GAO Bingliang, WANG Zhaowen School of Materials and Metallurgy, Northeastern University, Shenyang 110004, China Received 29 November 2010; accepted 8 October 2011 Abstract: A twostep alkalineacidic leaching process was conducted to separate the cryolite from spent pot lining and to purify the carbon. The influencing factors of temperature, time, and the ratio of liquid to solid in alkaline and acidic leaching were investigated. The results show that the recovery of soluble compounds of Na3AlF6 and Al2O3 dissolving into the solution during the NaOH leaching is 65.0%，and the purity of carbon reaches 72.7%. During the next step of HCl leaching, the recovery of soluble compounds of CaF2 and NaAl11O17 dissolving into the HCl solution is 96.2%, and the carbon purity increases to 96.4%. By mixing the acidic leaching solution and the alkaline leaching solution, the cryolite precipitates under a suitable conditions of pH value 9 at 70 °C for 2 h. The cryolite precipitating rate is 95.6%, and the purity of Na3AlF6 obtained is 96.4%. Key words: spent pot lining; recovery; chemical leaching; aluminium electrolysis method separates the carbon and fluoride according to 1 Introduction the difference in solubility, density and surface properties between carbon and fluorides. For example, the froth With a rapid development of the primary aluminium flotation technology is a typically physical separation industry, more and more spent pot lining (SPL) is process for SPL treatment [3]. However, physical accumulated and needs to be dealt with. SPL is a separation method is difficult to improve the carbon hazardous solid waste from cell retrofitting. However, purity and recovery efficiency of the leachable substance. SPL contains some valuable inorganic compounds, such For thermal treatment, the common way is to burn the as Na3AlF6, NaF, CaF2, Al2O3 and NaAl11O17. The SPL [4−7]. Although the SPL is decomposed, some content of inorganic compounds in the SPL varies from waste in SPL transfers into the refractory components, 25% to 50% depending on the cell’s condition [1]. and the fluoride is recovered as a gaseous HF effluent Besides the inorganic compounds, the other composition which can be absorbed with smelter grade Al2O3 [2], in the SPL is carbon that has graphitized at elevated while the carbon is burnt for boiler heating. This thermal temperature of 800−900 °C for several years. treatment is complicated in operation and the valuable Graphitized carbon and inorganic compounds in SPL are source of carbon is not reused. In addition, the chemical valuable resources. In the past, the simplest way of leaching can leach the soluble compounds from the SPL dealing with SPL was the process of landfill which is not by using NaOH or Al(NO3)3∙9H2O and AlCl3∙6H2O environmentally acceptable since the fluoride in SPL will [8−10]. Because of the low reaction rate, the approach penetrate and contaminate the underground water, and no for treating Na3AlF6 and CaF2 using Al(NO3)3∙9H2O or o valuable resources are recovered. Therefore, carbon and AlCl3∙6H2O takes long time of 24 h at 25 C [9, 10], inorganic compounds may be recovered as a secondary which is not productive and very costly. However, the resource. NaOH leaching just takes 2−3 h and the leaching reagent Generally, there are some ways for treating SPL, is economical. including hydrometallurgical and pyrometallurgical In this work, three steps are conducted to separate processing, such as physical separation, thermal the crolyite from the carbon in SPL. The first step of treatment and chemical leaching methods [2]. Physical alkaline leaching is performed to dissolve Na3AlF6, NaF, Foundation item: Project (50804010) supported by the National Natural Science Foundation of China Corresponding author: SHI Zhongning; Tel:+862483686464; Email: [email protected] DOI: 10.1016/S10036326(11)611643 SHI Zhongning, et al/Trans. Nonferrous Met. Soc. China 22(2012) 222−227 223 and Al2O3 into the basic solution. The second step of acidic leaching is carried out to further dissolve the CaF2 and NaAl11O17 in the carbon obtained from the first step into acidic solution. In the third step, the cryolite is precipitated by mixing the basic solution and acidic solution which come from the fore two steps. 2 Experimental All used chemical reagents were analytical grade. Spent pot lining samples came from a 300 kA aluminium reduction cell. All the leaching experiments were run at a mechanical stirring system of 300 r/min at normal pressure. The concentration of NaOH leaching solution Fig. 1 XRD patterns of SPL sample and leaching products was 2.5 mol/L, while the concentration of HCl leaching solution was 9.7 mol/L. The experiment parameters of leaching rate. This is because increasing L/S ratio leaching and precipitation are shown in Table 1, where θ − provides a mass of OH to react with the soluble Na3AlF6 is the leaching temperature in alkaline leaching, acidic and Al2O3 in the L/S ratio range of 2−4.5. The leaching and cryolite precipitating experiments, and t is concentration of soluble Na3AlF6 and Al2O3 in the the leaching time in alkaline leaching, acidic leaching leaching solution decreases with the increase of L/S ratio, and cryolite precipitate experiments, L/S is the liquid to which makes the leaching rate decrease when the L/S solid ratio. The alkaline leaching solution after filtration ratio further increases from 4.5 to 6. Therefore, the was characterized by Raman spectroscopy (Horiba Jobin leaching rate does not increase when the L/S ratio is Yvon, HR800, France). further increased from 4.5 to 6. Moreover, as seen in Table 2, the highest purity of carbon is 73.0%, and the Table 1 Leaching and precipitating experimental parameters highest leaching rate is 65.4%. This is not a desirable (L/S)/ Step θ/°C t/min pH result for reusing the carbon, and so the second step is (cm3 ∙ g −1) needed. After washing and filtering, the resulted solid Alkaline leaching 60−100 20−240 2−6 carbon still contains insoluble compounds of CaF2 and Acidic leaching 60−100 20−240 2−6 NaAl11O17 characterized by XRD, as shown in Fig. 1(b). Cryolite precipitate 20−100 20−1440 6−12 Table 2 Effect of L/S ratio on alkaline leaching 3 Results and discussion (L/S)/ Leaching rate of Purity of carbon/% (cm3 ∙ g −1) soluble compounds/% 3.1 Alkaline leaching 6 73.0 65.4 The particle size of the SPL samples for alkaline 5.5 72.9 65.2 leaching is in the range of 100−165 µm after grinding. 5 72.9 65.2 The element contents (mass fraction) of SPL are 13.91% 4.5 73.0 65.0 Na, 13.83% F, 6.82% Al, 48.83% C, 1.19% Ca, and the 4 68.8 57.7 balance of O and other trace elements measured by chemical analysis. The phase compositions of the SPL 3 64.9 49.4 2 55.9 26.1 are Na3AlF6, NaF, CaF2, Al2O3, NaAl11O17, and carbon characterized by Xray diffraction (Panalytical B.V, PW 3040/60H, Netherlands), as shown in Fig. 1(a). The 3.1.2 Effect of temperature carbon in SPL has graphitized to 78.3% calculated by the In the solution of 2.5 mol/L NaOH with a L/S ratio Franklin Model [11]. of 5, the SPL was leached for 3 h at different 3.1.1 Effect of L/S ratio temperatures of 60, 70, 80, 90 and 100 °C. The results SPL samples were leached in 2.5 mol/L NaOH are shown in Table 3. It is observed that the temperature solution for 2 h at 100 °C, and the L/S ratio varied in the has an appreciable effect on the leaching rate. The range of 2−6 cm 3/ g. The results are shown in Table 2. It leaching rate increases from 36.8% to 65.0% when the can be seen that the leaching rate of soluble compounds temperature increases from 60 °C to 100 °C, while the increases with increasing the L/S ratio in the range of carbon purity increases from 59.6% to 72.7%. This effect 2−4.5. When the L/S goes up to 4.5, the increase of can be explained that the reaction velocity constant K L/S ratio has no more effect on the carbon purity and the increases with the increase of temperature. 224 SHI Zhongning, et al/Trans. Nonferrous Met. Soc. China 22(2012) 222−227 Table 3 Effect of temperature on alkaline leaching leaching, one solution is the product of reaction between Leaching rate of NaOH and Na3AlF6, and the other is sodium aluminate Temperature/°C Purity of carbon/% soluble compounds/% solution. 60 59.6 36.8 It can be seen from Fig. 2 that there are same peaks in spectra (a), (b) and (c) at about 625 cm −1 which are 70 61.9 42.6 assigned to Al(OH) − ion [12−14]. However, the intensity 80 67.1 54.2 4 of spectrum (a) is lower than that of spectra (b) and (c). It 90 69.2 58.4 can be explained that the presence of more ions in the 100 72.7 65.0 − leaching solution diminishes the Al(OH)4 peak intensity significantly. Moreover, there are two other peaks at 3.1.3 Effect of reaction time about 427 cm −1 and 470 cm −1 in spectra (a) and (b) in 4− The leaching tests were carried out at 100 °C in 2.5 Fig.