Highlights of the Salt Extraction Process

Transcription

1 TSpace Research Repository tspace.library.utoronto.ca Highlights of the Salt Extraction Process Aida Abbasalizadeh, Seshadri Seetharaman, Lidong Teng, Seetharaman Sridhar, Olle Grinder, Yukari Izumi and Mansoor Barati Version Post-print/Accepted Manuscript Citation (published version) Abbasalizadeh, A., Seetharaman, S., Teng, L. et al. JOM (2013) 65: Publisher s statement This is a post-peer-review, pre-copyedit version of an article published in JOM Journal of the Minerals, Metals and Materials Society. The final authenticated version is available online at: How to cite TSpace items Always cite the published version, so the author(s) will receive recognition through services that track citation counts, e.g. Scopus. If you need to cite the page number of the author manuscript from TSpace because you cannot access the published version, then cite the TSpace version in addition to the published version using the permanent URI (handle) found on the record page. This article was made openly accessible by U of T Faculty. Please tell us how this access benefits you. Your story matters.

2 HIGHLIGHTS OF THE SALT EXTRACTION PROCESS Aida Abbasalizadeh 2, Seshadri Seethasraman 1,2, Lidong Teng 2, Sridhar Seetharaman r 3, Olle Grinder 1, Yukari Izumi 4, and Mansoor Barati 5 1 Salt Extraction AB, Sweden/Japan; 2 Royal Institute of Technology, Stockholm, Sweden; 3 Univ. of Warwick, UK, 4 Kyushu Inst. Technol., Japan, 5 Univ. of Toronto. Canada I. Background: raman@kth.se; aidaa@kth.se, lidong@kth.se, s.seetharaman@warwick.ac.uk; grinder@algonet.se;hca00324@nifty.com, mansoor.barati@utoronto.ca Increasing awareness of the growing environmental concerns regarding toxic metals ending up in the biosphere and higher demand to find secondary sources for metal values necessitate that design of new processes for metal extraction is assigned high priority. The important criteria are: a) The new concepts are economically viable, both with respect to investment costs as well as running costs. b) These new processes do not lead to further contamination of the environment; but instead result in a cleaning-up of the already contaminated areas c) The processes must use, to the greatest extent possible, green energy and avoid fossil fuels. d) Raw materials are exploited that presently are regarded as waste and dumped as land-fill e.g. flue dust, slag, metal scrap and oxides. A novel process route, based on chloride metallurgy, has been developed to meet these criteria. This new process, named the Salt Extraction Process, enables the recovery of metals from a variety of sources such as metal scrap, silicate based ores (such as laterites), metallurgical slag and dust and is highly feasible in many fields of applications. The process is capable of handling raw materials with even mixtures containing different metals. II. Process concept: The Salt Extraction Process is the result of many years of R&D in the fields of chemistry of high temperature materials, chloride metallurgy and electrochemical processes. The metal values in the raw materials, especially in the secondary sources, are strongly bonded. The stabilities of some metal chlorides as well some oxides such as In2O3, Nd2O3 as well as SnO2 are presented in Fig.1. The release of the same would require a powerful agent as can be seen from a thermodynamic analysis. The authors discovered that AlCl3, now patented flux 1, is very powerful in the chlorination of the metals bound to oxide or even silicate matrices and can bring these metals into a molten salt phase comprising of salts like NaCl, LiCl and KCl. 1

3 The choice of AlCl3 as the chlorinating agent for metal values from secondary sources provides a great advantage in comparison with other chlorinating agents like Cl2 gas, FeCl3 or even CaCl2, which have been conventionally used in metallurgical processes. AlCl3 is a covalent chloride while alkali or alkaline earth chlorides are ionic. AlCl3 may release chlorine free radicals in the salt melt leading to effective chlorination. Fig. 1: The Standard Gibbs energy change of different oxides vs temperature Electrolysis of the salt melt along with the extraction would drive the reaction further so that the metals coming into the salt melt get deposited on the cathode, as shown in Figure 2. This chain process of extraction and electrolysis enables a near-complete extraction of the metal values from a given raw material source. Fe 2+ Cr 3+ Mn 2+ Fig. 2: Principle of the Salt Extraction and Electrolysis Process The extraction involves two steps, viz. (1) chlorinated dissolution of the metal and (2) the electrolysis. The chemical reaction involved in the dissolution of an oxide MO being chlorinated by AlCl3 in the temperature range K can be represented as 2

4 3MO + 2AlCl3(Chloride melt) = 3MCl2(Chloride melt ) + Al2O3 (solid) (1) The ionization of MCl2 in the salt melt can be written as MCl2(Chloride melt) = M Cl - (2) The electrochemical reactions involved in the electrolysis are as follows: Anodic reaction: 2Cl - = Cl2 + 2 (e) (3) Cathodic reaction: M (e) = M (4) The total cell reaction will thus be M Cl - = M (cathode) + Cl2 (anode) (5) The above reactions can be suitably modified if the metal M is present in an alloy scrap or other combined forms. The electrode reactions presuppose that the reactions (3) and (4) are favored electrochemically and other parallel reactions are less prevalent in the electrochemical process. In this respect, it is important to check the decomposition potentials of various chlorides of interest from the view point of recovery. The decomposition voltages for different metal chlorides are presented in Figure 3. Fig. 3: The decomposition voltages of different metal chlorides during electrolysis. 3

5 Thus, simultaneous salt extraction and electrolysis of the molten salt bath enables the extraction of most of the metals including reactive metals like the rare earths even from strongly bonded matrices. The metal chlorides from the salt bath may also be separated after the extraction by leaching in the aqueous phase or fractional distillation of the vaporized chlorides. One of the important developments needed to improve the process efficiency is to overcome the loss of AlCl3 to the vapor phase in view of its high vapor pressure at the electrolysis temperature, which is in the range 973 K (700 o C) 1173 K (900 o C). This impediment could be overcome by the use of liquid aluminium anode and generating AlCl3 by the reaction between liquid Al and Cl2 deposited at the anode. This new anode design has now been patented 2. The advantage of this is that it enables the formation of AlCl3 in sufficient amounts in situ, as a function of the cell current passed. AlCl3 formed, dissolves in the chloride melt and chlorinates the metal value. This new design has eliminated the evolution of chlorine during electrolysis as also the gas phase loss of AlCl3. The cost of aluminium chloride is thus minimized due to cheaper scrap aluminium used as the anode which has a strong impact on the total cost of the process. Further the process temperature could be lowered by the use of the ternary eutectic salt melt, NaCl-KCl-LiCl. III. Important achievements: The process has led to some very important achievements, not only in the recovery of metal values from secondary sources; but also offering a more environment-friendly method for even primary sources, whereby the number of unit processes in metal extraction are minimized. This leads to an optimization of metallurgical processes from energy and environmental perspectives. III-1. Treatment of Metallurgical slags: The process was first used to recover chromium from slags from high alloy steelmaking. A number of chromium-containing slags were tried and the results were extremely promising. To illustrate this point, in the case of the experiment with a slag containing 23 wt% FeO and 3.25 wt % Cr2O3 apart from CaO, Al2O3 and SiO2, the cathode deposit obtained and the corresponding XRD pattern are shown in Fig. 4. 4

6 Fig. 4: The cathode ferrochrome deposit and the XRD patterns of the cathode products after slag electrolysis. As shown in Fig. 4, the cathode deposit was mostly ferrochrome with small amounts of entrapped salts. The recovery levels were nearly 90 % 3. The experiments could be repeated with liquid Al anode confirming the results 4. III-2. Recovery of copper from concentrates: The process was tried to recover copper from Chalcopyrite concentrate containing 26 % Cu. A high recovery of Cu as copper deposit at the cathode could be recorded 5. The deposit is shown in Figure 5. Fig. 5: Some of the SEM images of the copper deposit obtained by fused salt electrolysis of sulphide concentrate. Cyclic volumetric investigations confirmed the various cathodic reactions previously postulated, thereby reassuring that the thought process behind the design of the salt extraction process is sound. By conducting the experiments in reduced oxygen partial pressures, sulphur from the chalcopyrite ore could be recovered in the elemental form as condensate with significant positive impact on the environment. III-3: Recovery of lead from Cathode Ray Tube glass 6 : Why can t we use this process in recovering lead from the spent cathode ray tubes (CRT tubes)? The thought was initiated in Japan with the advent of LCD/LED and plasma flat screen TV sets. The old cathode ray tube TVs had to be discarded and these contain up to 23 mass % Pb. Thus, these cannot just be discarded. Export of the CRT tubes is forbidden at the receiver end and some method had to be designed in order to recover Pb from these spent CRT tubes. With the Japanese initiative, the process was applied to the recovery of Pb from spent CRT glass. Even without the electrolysis step, the recovery of lead in the salt melt was more than 97 %, which can further be improved by electrolysis. Investigations are currently under way towards a total recovery of lead from these old TV sets which pose one of the serious environmental challenges in modern technological development. 5

7 III-4. Recovery of Rare earth metals from magnet scrap 7,8 : With increasing demand for rare earth metals in high power magnets for various applications, the salt extraction process is currently being tried towards the recovery of Nd from Fe-Nd-B magnets. Thermodynamic calculations of the standard Gibbs energies of reactions (6) and (7) indicate that the chlorination of Nd by AlCl3 is thermodynamically favoured while the corresponding reaction with Fe is not. AlCl3 (salt) + Nd(s) = NdCl3 + Al(liq.) (6) ΔG o 2 (T = 1073 K) = kcal (7) AlCl3 (salt) + Fe(s) = FeCl3 + Al(liq.) (8) ΔG o 3 (T = 1073 K) = kcal (9) It should be noted that the above calculations do not take into account the activities of the components involved in the reactions. It seen that, NdCl3 is likely to be extracted from Nd-Fe alloys by using AlCl3. Experiments were conducted to examine the formation of NdCl3 in the salt melt containing LiCl- NaCl-KCl-AlCl3 in contact with Fe-Nd-B magnet scrap. The SEM/EDS images of the salt bath were taken after 6 h of extraction at 1073 K (800⁰C). EDS mapping confirmed the formation of neodymium trichloride in the salt bath. It was found that neodymium and chlorine are distributed in the same areas, showing the formation of neodymium trichloride. The decomposition voltages for the chlorides in the Fe-Nd-B magnet alloy are presented in Fig. 6. 6

8 Fig. 6: Changes of decomposition voltage with temperature for all the components present in the salt bath. It is seen that NaCl, LiCl and KCl have higher decomposition potentials compared to NdCl3 which would facilitate the electrodeposition of Nd. One of the important criteria for the process is that the cathodic deposit should not contain Fe in noticeable amounts. Our repeated experiments showed successful results. The microstructures of the cathode samples were also analyzed by SEM/EDS and are presented in Fig. 7. The phase with bright contrast in these images; indicated by A was confirmed to be a metallic phase consisting mostly of neodymium. The phase with dark contrast indicated by B is oxide phase containing aluminum and oxygen. A B (a) (b) Fig. 7, SEM images of cathode deposition after electrolysis of Nd-Fe-B magnet using AlCl3 as flux. Electrolysis was done at V=3.4 (V) and T=1073 K during 6 hours. A is metalic phase and B is the Oxide phase. XRD pattern of the cathode deposit is presented in Fig. 8. 7

9 Fig. 8: XRD pattern of neodymium deposite on graphite electrode in KCl-NaCl-LiCl molten salt at 1123 K. It is seen that the deposit consits of Nd metal as the dominant phase as well as AlCl3. Peaks corresponding to Al2O3 and oxychloride phases are identified in the sample. No Fe or B could be detected in the deposit. Thus, it is reasonable to conclude that, apart from the formation of some amount of AlNd3, the major part of the cathodic deposit is metallic Nd. Similar experiments are currently being conducted for extracting Dy by the salt extraction process. The results indicate that Dy gets deposited at the cathode. Attempts are currently made for fine-tuning the cathode potentials in order to achieve the separation of Nd from Dy. IV. Future Potentials The salt extraction process flow sheet is given in Fig. 9: 8

10 Fig. 9: The salt extraction process-flow sheet Fig 10 illustrates the growth of the concept towards different applications: Salt Extraction for the future RE metals Recovery Calculations -ongoing Si extraction? Recovery of Metals from old batteries CRT glass, Pb recovery Japan collaboration Dross from secondary Al smelting Cr, Mn, Fe Recovery MISTRA project Fig. 10: The growth tree of the Salt Extraction Process 9

11 The process potentials of the Salt Extraction Process are quite impressive that this will most probably lead to new process strategy with respect to the extraction and refining of metal values, both in the case of base metals as well as more reactive ones. The traditional hydrometallurgical processes have been shown to create an excess amount of aqueous phase that are expensive to process further and the waste water can end up in ground water sources. One of the biggest advantages of the present innovation is the environmental friendliness of the process that enables complete recycling of all the by-products of the process. In the extraction of metal values from lean resources, as for example copper from low grade ores or metallurgical refractory wastes, the process is expected to be very efficient. In the extraction of metals from sulphide ores, as for example copper or nickel, the process could be extremely efficient in the selective extraction of the metal value while sulphur can be collected in the elemental form. Sulphur emission during copper production is a serious problem and, in our approach, this can be almost totally eliminated. In one step, electrolytic copper is produced resulting in better process economics as well as improved environmental benefits as compared to the conventional processes. In the case of strategic metals like nickel, cobalt, which are extremely important in various applications, this process has shown (in preliminary experiments) to be highly valuable, even in processing secondary resources. Titanium processing from scrap or secondary sources has been a serious problem. Today, there is no suitable solution to recycle Ti. The salt extraction process can offer a very promising route to recover and recycle Ti from primary as well as secondary sources. In the process of extracting lead from CRT glass by the salt extraction method, we observed an intense reaction between the byproduct silica and the salt melt, resulting in the formation of an emulsion. If further work can be done to confirm this process, it is highly possible to extract Si from all waste products or primary/secondary sources. We are currently examining the possibility of extracting aluminium from scrap by salt extraction process. Experiments are also planned for the separation of the rare earth metals from each other. This opens up the potential of the extraction of rare earths without going through the tedious and environmentally unfriendly hydrometallurgical route. One of the important problems today is the disposal of nuclear wastes from nuclear plants. At present, we are planning collaborations with Swedish Academy for Engineering Sciences in this area. 10

12 V. Summary of the Merits of the New Process The salt extraction process is a novel route towards the recovery of metal values from primary as well as secondary resources, The process is environment friendly. The process is adapted to the use of green energy and avoids fossil fuels, thus avoiding CO2, SO2 and NOx emissions, The process has proved efficient in the recovery of harmful metals like chromium and lead from slags, glasses and other matrices where these are strongly bonded, The process is capable of recovery and separation of rare earth metals from scrap magnetic waste materials from electrical appliances. References 1. A process for chlorinating resources containing recoverable metals, Innovators: S. Seetharaman and O. Grinder: Patent 2009 No. ; SE53674, PCT:WO 2009/ A1; Salt Extraction AB. 2. A process for Recovering Metals and an Electrolytic Apparatus for Performing the Process, Innovators: S. Seetharaman, L. D. Teng and S. Sridhar WO (A1) Patent rights owned by Jernkontoret.. 3. The salt extraction process: a novel route for metal extraction Part I Cr, Fe recovery from EAF slags and low grade chromite ores. X.L. Ge, O. Grinder, S. Seetharaman, Trans. Inst. Min. Metall. C: Mineral Processing and Extractive Metallurgy, 2010, 119 (1), p Optimization of Salt Extraction Process through cyclic production and consumption of aluminium chloride by a novel anodic reaction B. Khalaghi, L. D. Teng and Seshadri Seetharaman. Presented in the International Conference on Molten Slags, Fluxes and Salts, Molten 2012, Beijing, China, May The salt extraction process: a novel route for metal extraction Part 2 Cu/Fe extraction from copper oxide and sulphides X.L. Ge, S. Seetharaman Trans. Inst. Min. Metall. C: Mineral Processing and Extractive Metallurgy, 2010, 119 (2), p

13 6. Recovery of Lead and Indium from Glass, primarily from Electronic Waste Material, Innovators: L. D. Teng, S. Seetharaman, K. Yamaguchi and Y.Izumi, WO (A1) Patent rights owned by Jernkontoret. 7. Neodymium extraction using salt extraction process A. Abbasalizadeh, L. D. Teng, nd S. Seetharaman, Presented at the 5 th Conf. on Ionic Liquids-COIL conference, November Neodymium extraction using salt extraction process A.Abbasalizade 1, L. D. Teng, S. Sridhar and S. Seetharaman Sent for publication to Trans. Inst. Min. Metall., Sect. C, July