Recovery of cerium from glass polishing slurry

Available online at www.sciencedirect.com, JOUNALOPScienceDirectRARE EARTHSEL SEVIERJOURNAL OF RARE EARTHS, VoL 29, No. 11, Nov. 2011, P. 1075www.re-journal.com/en/Recovery of cerium from glass polishing slurryJong-Young Kim', Ung Soo Kim' , Myeong Seop Byeon', Woo-Kyu Kang', Kwang-Teak Hwang', Woo-Seok Cho'(1. Ilcheon Branch, Korea Instinute of Ceramic Engineering and Technology Gyeongchung Rd, Sindun-Myreon, lcheon-si, Kyeonggi do 467-843, Korea; 2. RINCO,Cheongjus, Chungcheongbuk do, Korea)Roceived 20 February 2011; revisod 20 June 2011Abstract: Cerium was extracted from rare earth slurry waste used for polishing a glass substrate. Iitally, glass frit and flocculant were re-moved by foth flotation and dislution. The recovered rare carth sury exhibited almost the same particle size distribution as original slurry,which could be reused as sluy for glass polishing. From the rare eath sury, the cerium solution was obtained by an oxidaive thermal treatmentand subsequent chemical leaching The cerium solution was further purified up to 94% by selective precipitation of rare cearth specics.Keywords: cerium; separation; waste; polishing slurry; rare earthsDue to its specific functional applications, cerium is the tions, the oxidation of Ce(II) to Ce(TV) may occur bymost important one of the rare earth elements. Rare earth chemical oxidation with strong oxidants, such as persulfate,oxides have been widely investigated as catalytic promoterspermanganate, bismuth, lead oxide or silver oxide, or byto improve the activity and thermal stability of catalysts'"!.electrochemical oxidation or photochemical oxidatin[12-14.Cerium plays an important role in three-way catalysis andThe separation of cerium from the rare earth elements canfluid catalytic cracking, two catalytic processes that are sigbe carried out by selective dissolution of the trivalent rarenificant because of their economic relevance and tonnagekearth hydroxides, keeping the cerium(IV) hydroxide in itsCerium and other rare earth oxides have also been studied asinsoluble form, or through its selective precipitation frompossible heterogeneous catalysts for selective oxidation of acid solution. In either case, the cerium separation is possiblehydrocarbons.due to the solubility difference between the Ce(IV) bydrox-Cerium compounds are also used as abrasive materials foride (Kp~10 54) and the RE(II) bydroxide (Ksp~10 23). How-polishing glass and silicon wafers. However, an enormousever, oxidation through permanganate solution leads to si-amount of abrasive materials is wasted after being used for multaneous precipitation of Ce(OH)4 and MnO2. After pre-polishing glass for display applicationsb. The slurry forcipitation, the solids need to be purified if a high purityglass polishing is a mineral containing a high concentrationproduct is desiredl5s. Based on this work, we showed thatof cerium and other elements such as the lanthanides (La, Nd, rare earth shurry with ~40 wt.% of cerium could be recycledPr), which are mixed with the elements such as Fe, Al, and after froth flotation, which could be used as polishing slurry.Si in the polishing process. The heterogeneous nature ofFurthermore, cerium solution of a high purity could be ob-slurry waste makes it difficult to refine cerium ftom thetained from the recycled slurry by subsequent separationwaste, but after being refined, cerium of a high purity can beprocess such as solvent extraction.obtained at a significantly reduced cost.In this work, we attempted to separate the cerium from theThe separation of a rare earth mixture into individual ele-slury waste for glass polishing by selective precipitation ofments is very difficult to achieve, due to their similar physi-rare earth element, in which tetravalent cerium remains incal and chemical propertiest 8. In an aqueous solution, rareacidic solution. As a first stage, glass frit and flocculant wereearth elements are present as trivalent cations. Cerium isseparated by froth flotation and chemical dissolution. Cemost likely to be oxidized to a tetravalent state and, givenrium was extracted from the resulting rare earth slurry bythis property, its separation is generally the easiest. In the oxidative roasting and a subsequent acid leaching process,tetravalent form, the cerium ion exhibits chemical behaviorand purified further by removing the rare earth elementsmarkedly different from other trivalent rare earth ionsheio. Inthrough selective precipitation.alkaline solutions, the trivalent cerium is readily oxidized tothe tetravalent ceric ion either by bubbling oxygen during1 Experimentalrare earth hydroxide precipitation or afterwards by drying therare earth hydroxide in the presence of air'. In acidic solu-The rare earth slurry waste used in this experiment was中国煤化工Foundation item: Projet sppored by the Einergy & Resource R&D Progam under the Ministy of Knowlcdge Eco2-P-02-0-000)Corresponding author: Jong- Young Kim (E-mail: jykim@kicet.re.kr; TeLl: +82-31-645-1423)fYHCNMHGDOI: 10.1016/51002-0721(10)60601-11076JOURNAL OF RARE EARTHS, VoL. 29, No.11, Nov. 2011supplied by Ashai glass. The slurry waste was precipitated sides glass frit, flocculants containing aluminum or iron areby flocculant and dried by filter press. Table 1 represents the included because they are used to precipitate the solid. Polychemical composition of the slury waste. The slury wastealuminum chloride or FeCl3*xH2O can be used as a floccu-contains rare earth elements (cerium, lanthanum, praseo-lant to make the sudge cake by lowering the pH. Therefore,dymium), a glass frit component, mainly consisting of SiO2,we attempted to remove the flocculant by chemical dissolutionand flocculant such as poly- aluminium chloride. Initally, 12and then separate the glass frit from the rare earth materials bykg of the waste was mixed with 1 kg of xalic acid (or citricfroth flotation using their difference in density and surfaceacid) and 22 kg of water (pH~l .5) and stirred for 4 h at room properties even though their sizes are similar (<10 um)4temperature to remove the glass frit and flocculant. The re-Initally, inorganic flocculant was dissolved by mixingaction was carried out in a froth flotation vessel and ultra-organic acid, which does not form any salt with cerium, withsonic wave (28 MHz) was applied. After the acid treatment,the slury waste. Citric acid or oxalic acid was used as thethe remaining glass frit was dissolved by 33 kg of 3% so-organic acid. The slurry waste was reacted with organic aciddium hydroxide solution (pH~1 1.5). The resulting slury wassolution at room temperature at a pH of 1.0 6.0 to turm thesieved (1450 mesh) and washed with disilled water severalflocculant into dissolved species. With organic acid, thetimes, and then dried for 24 hat 90°C.glass frit floats in a separation vessel with air bubbles andThe dried powder was roasted at 600。C for 2 h in a rotaryultrasonication because of its hydrophobic surface and lowkiln-type fumace in air. The roasted powder was leached bydensity (d=1.4), while rare earth particles, that have a highersulfuric acid solution (6 N) with a concentration of 100 g/L.density (d=7.0), will precipitate.To eliminate the rare earth elements apart from cerium, theFor more efficient separation, ultrasonic waves were ap-resulting solution was reacted with Na2SO4 at 50。C for 4 h. plied during froth flotation to separate the adsorbed glass fritThe ratio of Na2SO4 to TREO was varied from 0.5 to 1.0.from the cerium abrasive. Furthermore, air bubbles wereThe dissolved cerium solution was analyzed by Inductivedispersed from the bottom of the vessel to remove the glassCoupled Plasma spectroscopy. Powder XRD patterms werefrit efficiently. After the flotation process, the remainingrecorded at room temperature on a Rigaku RINT 2000 dif-glass frit was dissolved by an alkaline solution at a pH valuefractometer with a Cu Ka radiation source. The X-ray tubefrom 9.0 to 13.0. The cerium abrasive was sieved (1450voltage and current were at 40 kV and 200 mA, respectively.mesh) and dried at 90 °C for 24 h. Fig. 1 shows the particlesize distribution for original polishing slurry, recycled slurry,and recycled and sonicated slurry, respectively; the figure2 Results and discussionreveals that recycled slurry exhibits almost the same particleAbrasive materials used for polishing glass, which is usedsize distribution as the original slurry. Table 2 summarizesas a substrate for LCD and PDP displays are usually miner-the characteristics for particle size distribution. Figs. 2(a), (b),als with a high concentration of rare earth elements, such asand (C) show the SEM (scanning electron microscopy) im-cerium. Consequently, the slurry waste contains glass fritages for the original polishing slurry,recycled sturry, andoriginating ftom the glass substrate, which exists as mixedrecycled and sonicated slurry, respectively. The recoveredpowder or adsorbed on rare earth abrasive materials's. Be-rare earth slury exhibits almost the same particle size dis-Table 1 Results of chemical analysis of the raw materials andtribution as the original slurry, which can be reused as slurryleaching solutionfor glass polishing'。Elements Raw materials Leaching at 60°C Leaching at 60°C Leachingat 80°CTable 1 shows the composition of the ceria abrasive waste(M.%, XRF)(roasted by Muffle (masted by rtary (roasted by rotay .before acid leaching. The cerium content is about 39% andfurmace)(mg/L) kiln)(mg/L)kiny(mg/L)_the major impurity is lanthanum (~20%). According to XRDsi0.27773195patterm after oxidation by rotary kilm (Fig. 3), cerium oxide4.308exists as CeO2, which means that cerium was oxidized to a39.4048958565701945710Original slurry20.0023905353Recycled slurry0.22072-- Recycled andsonicated slurryF0.600.300.010.400.110.040.02).1中国煤化工0.15cFig. 1 Particle size di.MHC N M H Gcyeled sury,3.68recycled and sonicated surry1078JOURNAL OF RARE EARTHS, VoL. 29, No. 11, Nov 2011when Na2SO/RE is larger than 0.625. The purity of ceriumReferences:decreased when the ratio becomes 0.625. Therefore, the[1] Trovrlli A, de Leitenburg C, Boaro M, Dolcetti G. The utili-equivalent amount of Na2SO4 lies between 0.5 and 0.625.zation of ceria in industrial catalysis. Catalysis Today, 1999,When Na2SO/RE is 0.5, the purity is highest and the yield is50: 353.~ 63%; therefore, the optimum precipitation condition is[2] Magee J, Mitchell M. Fluid Catalytic Cracking Science andNa2SOJRE -0.5. Such impurities as lanthanum, sodium canTechnology. 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