Immobilization of fluorides from spent carbon cathode in a copper smelting slag

: The fluorides from spent carbon cathodes could be effectively solidified in a molten copper smelting slag (FeO-Fe 3 O 4 -SiO 2 -CaO-Al 2 O 3 ) in forms of CaF 2 and Ca 4 Si 2 F 2 O 7 . The results of thermodynamic analysis, chemical analysis, and XRD and EPMA analyses showed that the F solidification efficiency increased with the CaO amount and decreased with the addition of Al 2 O 3 and SiO 2 . In addition, it was noteworthy that the F solidification efficiency decreased with an excessive CaO amount, which could be ascribed to the consumption of SiO 2 through forming CaSiO 3 and Ca 3 Si 2 O 7 . It restricted the solidification of the fluoride into Ca 4 Si 2 F 2 O 7 . Under the conditions of melting temperature of 1300°C, residence time of 60 min and N 2 flow rate of 40 ml/min, the optimum CaO and NaF amounts were found to be 20 wt.% and 6 wt.% respectively, in which the F solidification efficiency in the copper smelting slag of FeO-Fe 3 O 4 -SiO 2 -CaO-Al 2 O 3 obtained 98.35%.


Introduction
Spent carbon cathode is a hazardous solid waste that generated from the aluminium electrolysis process, which mainly contains carbon, fluorides (NaF, Na3AlF6, and CaF2 etc.) and other aluminium and cyanide compounds (Al2O3, AlN, NaCN, and Na3Fe(CN)6 etc.) [1][2][3][4]. Approximately 10 kg of spent cathode carbon are generated while produced per ton of primary aluminium, and approximately 360,000 tons of spent cathode carbon are generated annually in China in recent years [5]. The fluoride and cyanide components in it cause serious soil and water pollution if the spent carbon cathode is landfilled and exposed to air for a long time, and further threaten the growth and health of animals, plants, and human [6][7]. A harmless treatment of it is urgently needed.
Currently, massive methods for harmless and resource treatment of the spent cathode carbon have been developed, which could be divided into three methods of pyrometallurgical, hydrometallurgical, and pyro and hydro metallurgical corporation processes. The pyrometallurgical process, mainly referring to methods of combustion and vacuum evaporation [8][9][10][11], shows advantages on big processing capacity, simple operation and efficiently detoxification of fluoride and cyanide components. In the combustion process, some calcium compounds (e.g., CaO, CaCO3, and Ca(OH)2) are added to transform and solidify the soluble fluoride into CaF2, Ca4Si2F2O7 and Ca12Al14F2O22, and meanwhile the cyanide is oxidative decomposed into N2 and CO2 [12][13][14]. However, the massive consumption of high-quality graphite carbon from the spent cathode carbon causes it difficult to be carried out in an industrial application. In the vacuum evaporation process, the temperature and vacuum pressure affects the fluoride and cyanide removal rates greatly [10,11,15]. The soluble fluoride content could be reduced to 3.5 mg/L and the cyanide was completely decomposed under the conditions of vacuum pressure of 3000 Pa and temperature of 1700°C. However, the huge energy consumption restricts this vacuum evaporation process for an industrial application [15]. Through a hydrometallurgical method, the carbon and electrolyte components can be effectively recovered from the spent cathode carbon, and in additional the purity of the obtained carbon exceeds 95% [16][17][18][19]. However, massive acid or alkaline wastewater is generated, causing a secondary pollution. The carbon can also be recovered through a combined process of molten salt roasting (Na2CO3-Na2O) and water leaching [20,21]. The Na3AlF6 and CaF2 were converted to NaF in the molten salt-assisted roasting process, and then could be separated efficiently through the followed water leaching process. Meanwhile, most cyanide was converted into Na2CO3, N2, and CO. As a result, the leachable concentrations of fluoride and cyanide could be decreased to 8.17 mg/L and 0.18 mg/L respectively, and a graphite carbon with the carbon content of 95.41% was obtained [21]. But the Na2CO3 consumption is large, and the soluble fluoride in the leaching water needs a further separation for decreasing a secondary pollution.
To synergistically reutilize the carbon and fluoride components, the spent cathode carbon was used as a reductant in a dilution process of copper converter slags or smelting slags for recovering Cu and/or Co [22][23][24]. During these processes, the increase of spent cathode carbon both increased the metal recovery and the fluorides solidification efficiency in the final slag [25], and simultaneously the cyanides were transformed to N2 and CO2. The leachable Fand CNcontents in the final slag were less than 5 mg/L and 0.1 mg/L respectively. It provides a new thought for reusing the spent cathode carbon, but the transformation behaviour of fluorides in the copper slag system was focused little. Based on the thermodynamic analysis, chemical analysis, XRD and EPMA analyses, the transformation of fluorides from the spent carbon cathode in a copper smelting slag of FeO-Fe3O4-SiO2-CaO-Al2O3 was researched in this study.

Materials and methods 2.1 Materials 2.1.1 Basic slag
Referenced to the chemical and phase compositions of copper smelting slags (Table 1 and Fig.1(a)) obtained from a copper plant locating in Yunnan province of China, a basic slag of FeO-Fe3O4-SiO2-CaO-Al2O3 was synthesized using pure reagents of FeO, Fe3O4, SiO2, CaO and Al2O3 through a melting process in a N2 atmosphere. The chemical and phase compositions of this synthesized slag are shown in Table 2 and Fig.1(b) respectively. Figs.1(a) and (b) show that the main phases in the copper smelting slag and synthesized slag are both Fe2SiO4 and Fe3O4.

Spent cathode carbon
The spent cathode carbon, which was the first cut of the spent pot liner, was obtained from Yunnan Aluminum Co., Ltd of China. The proximate analysis result in Table 3 shows that it contains 73.24wt.% fixed carbon, 25.81wt.% ash and 0.85wt.% volatile matters. Furthermore, the ash is mainly composed of 9.49wt.% F, 8.44wt.% Na, 3.59wt.% Al2O3 and a small amount of CN -( Table 3). Fig.1(c) identifies the C, NaF and CaF2 as main phases in the spent cathode carbon. Considering the low leachability and little environmental risk of CaF2 [3], the transformation of NaF was mainly focused in this research. The NaF with a purity of 99% was used in this research instead of the spent cathode carbon, and the high-purity nitrogen used with purity of 99.999% was procured from local suppliers.

Methods
The experiments were carried out in a vertical resistance furnace (GSL-1700X-VTQ, Hefei Kejing Materials Technology Co. Ltd., China), and the experimental apparatus is presented in Fig.2. The temperature was precisely measured by a KSY intelligent temperature controller connected to a Pt-Rh thermocouple (accuracy of ±1 °C), which has been corrected by a movable Pt-Rh thermocouple before. For the experimental procedure, the dried synthesized basic slag, NaF and other additives were firstly pre-ground and sieved to below 74 μm respectively, and then mixed thoroughly at proper mass ratios. After that, the mixture was loaded into a corundum crucible and further placed in the constant-temperature zone of the furnace tube to be heated to 1300 °C in a high-purity N2 with the flow rate of 40 ml/min. Referenced to a dilution process of copper slags using waste cathode carbon in previous researches [22,24], the melting temperature, residence time and N2 flow rate were controlled at 1300°C, 60 min and 40 ml/min respectively in all experiments. Then the samples were cooled down to room temperature in the alumina tube under a high-purity N2 atmosphere with the flow rate of 40 ml/min, pulled out, weighed and ground to prepare for analysis. The F solidification efficiency (SF) in these experiments was calculated using Eq. (I).
where m1 and m2 represent the masses of the NaF and final slag respectively, g; w1 and w2 correspond to the F mass contents in the NaF and final slag (wt. %) respectively, and w3 corresponds to the leachable F mass content in the final slag (wt. %).

Fig.2 Schematic diagram of the experimental system 2.3 Characterization
The chemical composition of the sample was analysed using chemical titration and atomic absorption spectroscopy, and the leachable F content in the final slag was detected via an ion chromatography method. All these measurements were conducted three times and the average value was taken as the final result. Phase transformation and microstructure of the sample were detected by X-ray diffraction analysis (XRD, Rigaku, TTR-III) and electron probe microanalysis techniques (EPMA, JXA82, JEOL). For the XRD analysis, the diffraction was measured in 10 to 90 deg 2θ using a Cu Ka radiation at 40kV and 40mA and a step size of 0.01 deg. The thermodynamic analysis was carried out using the FToxid, FTmisc, and FactPS databases in the FactSage 7.2 software.

Thermodynamic analysis
To detect the transformation of NaF in the molten synthesized basic slag of FeO-Fe3O4-SiO2-CaO-Al2O3, the FactSage 7.2 software was used to calculate equilibrium phases for the reaction system of 100 g synthesized basic slag + 8 g NaF with different addition amounts of CaO, Al2O3 and SiO2 at 1300 °C based on minimizing Gibbs free energy under isothermal, isobaric and fixed-mass conditions. The results are shown in Fig.3.
With no addition of CaO, Al2O3 and SiO2, Figs.3(a), (b) and (c) show that the NaF was mainly transformed to CaF2 (s), NaAlSiO4 (l) and Na4CaSi3O9 (l) through reactions (1) and (2) when it was melted with the synthesized basic slag. The CaF2 (s) was then converted into Ca4Si12F2O7 (s) through reaction (3) with the CaO addition amount increasing from 0 to 20 wt.% as shown in Fig.3(a). Simultaneously with it, the Fe2SiO4 (l) amount decreases and the FeO (s) amount increases. Increasing the CaO amount further, the amount of Ca4Si2F2O7 (s) decreases while the amounts of NaF (g) and Ca2SiO4 (s) increase. The reason might be that massive SiO2 in the basic slag are combined with CaO forming Ca2SiO4 (s) through reaction (4), and then the formation of Ca4Si2F2O7 (s) through reactions (1) and (3) is restricted.
In Fig.3(b), as the SiO2 addition amount increases from 0 to 15 wt.%, the reaction (5) is promoted, resulting the FeO (s) amount decreases and Fe2SiO4 (l) amount increases. Simultaneously, the NaF is mainly transformed to CaF2 (s). With the SiO2 addition amount ranging from 10 to 20 wt.%, the Ca4Si2F2O7 (s) is generated through reactions (5) and (3) and increased. Then with more SiO2 added, the 'F' is transferred from the Ca4Si2F2O7 (s) into the CaF2 (s), causing the amount of Ca4Si2F2O7 (s) decreases and that of CaF2 (s) increases.
In Fig.3(c), with the increase of Al2O3 addition amount from 0 to 4 wt.%, the amounts NaF (g) and Fe2SiO4 (l) decrease accompanied by the increase of NaAlSiO4 (l) and FeO (s) amounts, and the 'F' in the NaF is mainly transformed to CaF2 (s) and Ca4Si2F2O7 (s) through reactions (2) and (3). With the Al2O3 addition amount exceeding 8 wt.%, more CaO (s) and Fe2SiO4 (l) might be reacted with Al2O3 (s) forming CaAl2Si2O8 (l) and FeO (s) through reaction (6), causing the reaction (3) to be restricted and the Ca4Si2F2O7 (s) amount decreases.

.1 Effects of CaO addition amount
Based on the thermodynamic analysis above, the effects of CaO addition amount ranging from 0 to 24 wt.% with an interval of 4 wt.% on the F solidification efficiency were firstly assessed under the condition of melting temperature of 1300°C, NaF amount of 10 wt.%, residence time of 60 min and N2 flow rate of 40 ml/min. In Fig.4(a), the Fe2SiO4 diffraction intensity decreases with the CaO amount from 0 to 12 wt.% and disappears at CaO amount of 20 wt.%. Meanwhile, the FeO and Ca4Si2F2O7 appear at CaO addition of 12 wt.% and increase with the CaO amount. Fig.  5 also shows that the Fe and F elements are mainly distributed in the phases of Fe2SiO4 and CaF2 respectively with no addition of CaO, and the Ca4Si2F2O7 appears at 12 wt.% CaO added. These transformations accord well with the thermodynamic calculation results in Fig.3(a), indicating that the reactions (1)-(3) could be carried out in the roasting process.
More addition of CaO leads to more formation of CaF2 and Ca4Si2F2O7 through reactions (1)-(3), as a result of which the F solidification efficiency (SF) increases with the CaO amount from 0 to 20 wt.% as shown in Fig.4(b). However, increasing the CaO amount further to 24 wt.%, the SF decreases. It might be due to the consumption of SiO2 through forming Ca2SiO4 by reaction (4) restricts the occurrence of reactions (1)-(3) and some NaF (g) volatilizes into the gas phase as presented in reaction (7) and Fig.3(a). However, the Ca2SiO4 cannot be detected in the final slag according to Fig. 4 (a), the reason for which might be that it transforms to CaSiO3 and Ca3Si2O7 during the sample cooling process inferred from Fig. 6 and confirmed by Fig. 4(a). In Fig.6, during the cooling process of Ca2SiO4 from 1300°C to 50 °C, the Ca2SiO4 is decomposed and transformed to CaSiO3 ("B" and "D" points), Ca3Si2O7 ("B" and "D" points) and CaO ("C" and "E" points). To increase the F solidification efficiency, the CaO addition amount should be controlled at 20 wt.%. NaF (l) = NaF (g)

.2 Effects of NaF addition amount
Almost all the F from the NaF could be immobilized in the molten slag of FeO-Fe3O4-SiO2-CaO-Al2O3 as deduced from Fig.3, but the F solidification efficiency in Fig.4(b) is low around 75%. The equilibrium amounts of species for 100 g synthesized basic slag and 20 g CaO roasted with different NaF amount at 1300 °C were calculated and the results are shown in Fig.7. Fig.7 shows that with the NaF amount from 0 to 14 wt.%, almost all the F could be solidified in forms of Ca4Si2F2O7 (s) and CaF2 (s) thermodynamically. In details, with the increase of NaF amount from 0 to 3 wt.%, the Ca4Si2F2O7 (s), Ca3Si2O7 (l) and NaAlSiO4 (l) are formed and increased accompanied by the decrease of Ca2Al2SiO7 (l) and CaSiO3 (l), which might be due to the occurrence of reaction (8). Increasing the NaF amount over 9 wt.%, some Ca4Si2F2O7 (s) can be transformed to CaF2 (s) through reaction (9), causing the amounts of Ca4Si2F2O7 (s) and Fe2SiO4 (l) decrease and that of Na4CaSi3O9 (l), CaF2 (s) and FeO (s) increase. The EPMA results in Fig.8 accord well with this F transformation, in which the Ca4Si2F2O7 amount decreases and CaF2 amount increases with the NaF amount from 8 to 10 wt.%. While the NaF amount increases over 14 wt.%, except for solidification by the synthesized basic slag, some NaF can evaporate into the gas phase through reaction (7). The effect of NaF amount ranging from 2 to 12 wt.% on the F solidification efficiency was then experimentally researched under the conditions of melting temperature of 1300°C, CaO addition amount of 20 wt.%, residence time of 60 min and N2 flow rate of 40 ml/min, and the results are shown in Fig.9. The increase of NaF amount promotes the formation of Ca4Si2F2O7 and CaF2 through reactions (1)-(3), (8) and (9), as a result of which their intensities in the XRD analysis results increase with NaF amount as shown in Fig.9(a). However, the F solidification efficiency (SF) decreases greatly from 98.35% to 56.90% with the NaF amount from 6 to 12 wt.% as shown in Fig.9(b), which isn't consistent with the thermodynamic results in Fig.7. The reason might be that some NaF have been evaporated into the gas phase before a solidification, which could be confirmed by the more formation of gas pores in the final slag at a more NaF addition as shown in Fig.10. To decrease the environmental pollution caused by the F evaporation, the NaF amount should not be higher than 6 wt.%. Fig.7 Effects of NaF amount on the equilibrium amounts of species in the reaction system of 100 g synthesized basic slag + 20 g CaO at 1300°C Ca2Al2SiO7 (l) + 5CaSiO3 (l) + 2NaF (l) = Ca4Si2F2O7 (s) + Ca3Si2O7 (l) + 2NaAlSiO4 (l) (8) Ca4Si2F2O7 (s) + 4NaF (l) + Fe2SiO4 (l) = 3CaF2 (s) + Na4CaSi3O9 (l) + 2FeO (s) (9)

.3 Effects of Al2O3 and SiO2 addition amounts
In the conditions of melting temperature of 1300°C, residence time of 60 min, N2 flow rate of 40 ml/min, CaO and NaF addition amounts of 20 wt.% and 6 wt.% respectively, the changes of F solidification efficiency with Al2O3 and SiO2 amounts are shown in Fig.11(a) and (b) respectively. Fig.11 Effects of Al2O3 (a) and SiO2 (b) addition amounts on the F solidification efficiency in the synthesized basic slag Fig.11(a) shows that the F solidification efficiency decreases greatly with the increase of Al2O3 addition amount, which differs greatly with the thermodynamic calculation results in Fig.3(b). The reason might be that some CaO are consumed through forming Ca2Al2SiO7 ( Fig.12(a)), and the solidification of F through forming CaF2 and Ca4Si2F2O7 is restricted deduced from reactions (1)-(3). In Fig.12(a), compared to the phase composition of the final slag without extra Al2O3 addition, the Ca2Al2SiO7 could be found at a Al2O3 addition of 8 wt.% and increases with the Al2O3 amount increased to 20 wt.%. Also due to the consumption of CaO by adding SiO2 through forming Ca2Al2SiO7 and CaSiO3 (Fig.12(b)), the F solidification efficiency decreases obviously with the SiO2 addition as shown in Fig.11(b). Fig.12(b) shows that the Ca2Al2SiO7 and CaSiO3 can be generated at a SiO2 amount of 10 wt.% and increase with the SiO2 addition amount. To increase the F solidification efficiency in the synthesized basic slag, the extra Al2O3 and SiO2 should not be added.
Moreover, the value of leachable Fin the final slag was detected via ion chromatography method and found to be 3.67 mg/L, which is far less than the national allowable emission concentration in China as shown in Table 4. It implies that the final slag might be treated as a general solid waste.  The results showed that the NaF could be effectively solidified in the molten slag of FeO-Fe3O4-SiO2-CaO-Al2O3.
The 'F' from NaF was mainly transformed to CaF2 and Ca4Si2F2O7 in the slag of FeO-Fe3O4-SiO2-CaO-Al2O3. In a certain range, more addition of CaO led to more formation of CaF2 and Ca4Si2F2O7, and the F solidification efficiency increased with it. However, with the CaO addition amount exceeding 20 wt.%, some SiO2 in the synthesized basic slag would be consumed by forming CaSiO3 and Ca3Si2O7, which restricted the solidification of NaF into Ca4Si2F2O7. It caused some NaF volatilized into the gas phase and the F solidification efficiency decreased. Similarly, the addition of Al2O3 and SiO2 also hindered the formation of CaF2 and Ca4Si2F2O7 and decreased the F solidification efficiency through theirs priority combination with CaO forming Ca2Al2SiO7 and/or CaSiO3. Under the conditions of melting temperature of 1300°C, residence time of 60 min and N2 flow rate of 40 ml/min, the optimum CaO and NaF amounts were found to be 20 wt.% and 6 wt.% respectively, in which the F solidification efficiency in the copper smelting slag of FeO-Fe3O4-SiO2-CaO-Al2O3 obtained 98.35%.