Use of spent pot linings from primary aluminium production as raw materials for the production of opal glasses

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1 Glass Technol.: Eur. J. Glass Sci. Technol. A, October 2010, 51 (5), Use of spent pot linings from primary aluminium production as raw materials for the production of opal glasses U. S. do Prado, J. R. Martinelli* & J. C. Bressiani Nuclear and Energy Research Institute, Center of Material Science and Technology, Av. Prof. Lineu Prestes, 2242, Cidade Universitaria, , Sao Paulo, SP, Brazil Manuscript received 17 July 2008 Manuscript accepted 5 June 2009 The use of the carbonaceous fraction of spent pot linings (SPL) from primary aluminium electrolytic cells as a raw material for the production of opal glasses is evaluated. The SPL was heat treated and mixed with limestone to eliminate carbon and to minimise fluorine volatilisation. This material was then mixed with other glass formers, melted, and quenched to produce a glass. The cyanide which forms part of the SPL was thermally decomposed and some of the fluorine is incorporated into the glass matrix. The final material contains small crystals of fluorspar, which are responsible for light scattering and the opacity of the material, dispersed in an amorphous phase. The crystalline phase was identified by x-ray diffraction and its morphology was observed by scanning electron microscopy. 1. Introduction Opal glass is a type of glass that reflects light and is translucent or has an intense opacity. Opal glasses are used in lighting applications, decoration, jewellery, cosmetic flasks, chinaware, and glazes. Opal glasses were originally produced in the early times by the Egyptians in the manufacturing of jewels and decorative pieces. During the Middle Age, opal glasses were extensively used, especially in the manufacturing of glass cameos. More recently, opal glasses have been used on a large scale to produce lamps, luminaries, and chinaware.(1,2) In opal glasses the opacity is usually caused by the presence of microheterogeneities uniformly spread throughout an amorphous matrix with a different refractive index. The heterogeneities act as light dispersion centres causing reflection, refraction, diffraction, and consequently light scattering. The microheterogeneities can be crystalline phases nucleated in an amorphous matrix, secondary amorphous phase due to a liquid phase separation caused by an immiscibility gap, or microscopic gas bubbles released during the melting process, and arrested in the glass matrix.() 3 When the size of the heterogeneity is larger than the wavelength of incident light, the incident light beam is partially reflected and there are successive refractions in multiple directions giving a milky appearance.(` 24 This feature is accentuated when the particle size is in the range 1-3 ptm, and the concentration is around 10' particles/ mm 3.( 5 ) Opal glasses usually contain 3-10 vol% of crystalline phases. The crystal nucleation may occur * Corresponding author. jroberto@ipen.br spontaneously during the cooling process or after a heat treatment. Traditionally fluorine is added to the glass compositions to produce opacity. In commercially available opal glasses, usually cryolite (Na 3 AIF 6 ), fluorite (CaF 2 ) or Na 2 SiF 6 are added to assist the nucleation of CaF 2 and/or NaF.(') The average amount of fluorine in the opal glass composition is in the range of 4-6%.(') Lower amounts of fluorine result in less opacity and higher amounts of fluorine accelerate mould corrosion and causes excessive volatilisation during the material processing due to the evaporation of SiF4, F 2, H 2 F 2, and BF 3.(1' 6 ) Fluorine volatilisation is in the range 20-50% and depends on the melting temperature, time and other processing conditions!') Normally CaF 2 is spontaneously nucleated, and NaF is nucleated only during a subsequent heat treatment. Fluorine can also be introduced into glass-ceramic compositions. There are commercially available glassceramics based on fluorophlogopite (KMg 3 AISi 3 O 10 F 2 ), tetrasilic fluormica (KMg 2. 5 AISi 4 O 1 of 2 ), potassic fluorrichterite (KNaCaMg 5 8 i O 22 F 2 ), fluorcanasite (K 2 _ 3 Na3_4Ca.Si 12 03GF 4 ).1 4 ) Fluorine can also help the nucleation in glass-ceramic materials normally as CaF 2. 1 The solid waste from spent linings of the electrolytic cells in aluminium metal production, named 'spent pot lining' or 'SPU is classified by international standards as hazardous waste as it contains leachable cyanide and fluoride. It has previously been reported that the carbonaceous fraction can be used as a raw material for glasses.(') The carbonaceous part of SPL, which is approximately 50 wt% of the total residue, is often referred to as 'first cut' material while the part of the lining underneath the bottom blocks is termed Glass Technology: European Journal of Glass Science and Technology Part A Volume 51 Number 5 October

2 TabWl. Chemical composition of the SPL Na 2 O A CaO SiO 2 S03 Fe 2 O3 K20 TiQ2 M9O Sro P205 ZrO 2 C F CN- Gross residue (wt%) 204' After heat treatment (wt%) 'second cut' SPL.(9) The first cut is easily separated from the rest of the material by primary sieving because it is coarser than the second cut. It is necessary to maximise the removal of carbon, to avoid unfavorable aesthetic features, by carrying out a heat treatment. But, this treatment must be performed under conditions previously reported,( 1 ) so as to remove carbon and to thermally decompose the cyanides in the SPL, whilst retaining the fluorine in thermodynamically stable crystalline phases that will not volatilise during the glass melting process. 2. Experimental The SPL evaluated in the present work comes from an aluminium production plant located in Poqos de Caldas, Brazil managed by Alcoa. Glasses were prepared by mixing different amounts of the 'first cut' SPL with quartz. Limestone was added to help retain the fluorine. The raw material was ground and heated at 50'C/h to 870'C and kept at this temperature for 4 h.! 1 ") Table 1 presents the chemical compositions of the gross residue before and after heat treatment. Table 2 presents the glass compositions based on mixtures of SPL and quartz used in this work. The mixture was put in an alumina crucible and heated at 10'C/min in an electric furnace (Lindberg Blue-M crucible furnace) to temperatures in the range of C for 1 h. The liquid was cast in an aluminium mould 1x1x5 cm 3, annealed at 580'C for 2 h to release thermal stresses and then cooled down to room temperature insie the furnace. Table 3. Analysed OG-4 glass composition Composition SiO 2 A CaO Na 2 O F- Fe 2 03 K,O SrO S03 TiO 2 Wt % The glass with the composition OG-4 was selected for further evaluation in the current work because its aesthetic appearance was similar to the commercially available opal glasses, and it was relatively easy to cast. The density of this glass determined using the Archimedes principle was 2.57 g/cm 3. Samples were prepared for x-ray diffraction (XRD) analysis (Bruker AXS model D8 Advanced) using Cu K, radiation over the range 10-70' 20 at a scan rate of 2 /min to determine the crystalline phases, and for energy dispersive x-ray spectrometry (EDX) (Shimadzu model EDX-720) to determine the glass composition. Fluorine content was determined by the selective electrode method. Samples were also prepared for scanning electron microscopy (Philips model XL30) to observe the microstructure before and after chemical etching with concentrated H 2 SO 4 for 20 min. The dissolution rate of glass samples (lxlxl cm 3 ) was determined by measuring the weight loss after immersion in deionised water at 90'C for 28 days. The samples were previously polished to 600 grit finish SiC paper and cleaned with acetone. The samples were suspended in PTFE flasks containing water using a procedure based on the MCC-1P 'Static Leach Test Method'.(11) Differential thermal analysis (DTA) of the OG-4 glass composition was undertaken using a Setaran model Setsys 16/18. The heating rate was 10 C/min over the temperature range 'C with an air flow of 2.5 I/h. Table 2. Glass compositions based on mixtures of SPL and quartz Composition Quartz SPL! SPLb SPL' OG OG OG OG "SPL as-received, b SPL calcined at 840*C/4 h, c SPL calcined with addition of 20 wt% of limestone. 10 Zo Figure 1. XRD pattern for opal glass OG Glass Technology: European Journal of Glass Science and Technology Part A Volume 51 Number 5 October 2010

3 S12- X ,.I,.,. i "Time (days) Figure 2. Dissolution rate for glass OG-4 as a function of time 3. Results and discussion Glass OG-1 was dark due to the presence of residual carbon. Glass OG-2 was opaque but slightly greenish. Glass OG-3 was opaque and whitish and glass OG-4 was opaque with a milky like appearance similar to that of commercial opal glasses. Therefore glass OG-4 was selected for further analysis. Figure 4. Scanning electron micrograph of a glass sample after etching with concentrated H 2 S04 for 20 min Figure 3. Scanning electron micrograph of a glass sample before etching. w The XRD pattern (Figure 1) shows that fluorite was nucleated and grewin the glass during melt cooling. The composition of the OG-4 glass experimentally determined by EDX and ion selective electrode is shown in Table 3. The Ca content determined as CaO by EDX in fact includes some CaF 2. The amount of fluorine held in the matrix was 7 wt%. If there was no fluorine loss during glass processing, according to the mass balance calculation the expected amount of fluorine would be 10.7 wt%. The opal glass was achieved by spontaneous nucleation wherein crystals nucleated and grew during glass cooling, and no further heat treatment was required. Figure 2 shows the dissolution rate determined according to the MCC 1P standard,( 11 ) as a function of time. The dissolution rate is in the range (5-16)x10-` g/cm 2 d, which is close to the values reported for ordinary window glasses.( 12 ) Figures 3 and 4 show scanning electron micrographs of fluorite crystals homogeneously dispersed in the glass matrix. The crystals have a complex morphology and vary in size from 1-3 prm. The number of crystals was determined by using a metallographic method which involves counting Glass Technology: European Journal of Glass Science and Technology Part A Volume 51 Number 5 October

4 the number of crystals per unit area Crystals were counted by putting a 80 dot mesh on a scanning electron micrograph (magnified 1000 times) taken from a previously polished and chemically etched glass sample (concentrated sulphuric acid). Different regions of the surface were taken for that purpose. The amount of crystals was estimated to be 9 vol%. This value is equivalent to 11.5 wt%, assuming that the fluorite density is 3.30 g/cm 3, and that the glass density is 2.47 g/cm 3. If all fluorine in the glass composition was restricted to the formation of CaF 2, the total amount of fluorite would be 14.4 wt%. The selective etching with concentrated H 2 SO 4 revealed the microstructure features by only attacking the CaF 2 crystals, and thus allowing a quantitative evaluation of the numbers of crystals. The exothermic peak at 930'C on the DTA curve for glass OG-4 was assigned to the maximum crystallisation temperature of CaF 2, and an endothermic peak at 1360'C was assigned to the melting temperature of CaF 2.The glass transition temperature could not be clearly determined. 4. Conclusions An alternative process to dispose the carbonaceous fraction of the SPL was developed. The hazardous waste SPL can be used to replace fluorite (CaF 2 ) and cryolite (Na 3 AIF 6 ) as raw materials in the formation of opal glasses. The resulting amount of fluorine in the glass matrix is within the concentration range of ordinary opal glasses. When the amount of SPL in the glass composition is adequate, there is nucleation and growth of CaF 2 crystals, which are spread over the glass matrix, causing the light dispersion and consequently the glass becomes opaque. In addition SPL can be considered a flux since fluorine helps to decrease the melting temperature. Acknowledgments The authors thank Alcoa Aluminio S.A. and Nuclear and Energy Research Institute (IPEN) for the support provided during this research. References 1. Commons, C. H. Past and present practice and theory of opaque glass. Am. Ceram. Soc. Bull.,1948, 27, Flannery, J. E. & Wexell, D. R. Opal glasses - commercial glasses. In: Advances in Ceramics. The American Ceramic Society, Columbus, OH, 1986, Vol. 18, Navarro, J. M. F El vidrio. Second Edition. Colecci6n Textos Universitarios, CSIC, 1991, Vol Pinckney, L. R. Phase separated glasses and glasses ceramics. In: Ceramic and Glasses in Engineered Materials Handbook. ASM International, 1986, Vol. 4, Hlavac, J. The technology of glass and ceramics - an introduction. In Glass Science and Technology. SNTL/Elsevier, Prague 1983, Vol Juma'a, Q. A. & Parker, J. M. Crystal growth in the fluoride opal glasses. In: Advances in Ceramics. American Ceramic Society, Columbus, OH, 1982, Vol. 4, Stmad, Z. Glass ceramic materials. In: Glass Science and Technology. SNTL/Elsevier, Prague, Vol Prado, U. S., Sene, F. F., Martinelli, J. R. & Bressiani, J. C.. Glasses based on waste generated in the aluminum pots (SPL). Proc. Third Int. Congr. of Aluminium, Sao Paulo, SP, Brazil, May 2007, Sorli, M. & Oye, H. A. Cathodes in Aluminium Electrolysis. Second Edition. Aluminium-Verlag, 1994, p Prado, U. S., New alternative for reuse hazardous waste from the primary aluminium production process (SPL): production of the opal glass and frits PhD Thesis, University of Sio Paulo, Brazil, MCC-1P Static Leach test Method. Materials Characterization Center, In: Nuclear Waste Materials Handbook - Waste Form Test Methods, Reis, S. T., Karabulut, M. & Day, D. E. J. Non-Cryst. Solids, 2001, 292, Van der Voort, G. F. Metallography - principles and practice. McGraw Hill Book Co, Glass Technology: European Journal of Glass Science and Technology Part A Volume 51 Number 5 October 2010

5 COPYRIGHT INFORMATION Author: Title: Use of spent pot linings from primary aluminium production as raw materials for the production of opal glasses Source: ISSN: Prado, U. S. do; Martinelli, J. R.; Bressiani, J. C. Glass Technol 51 no5 O 2010 p Publisher: Society of Glass Technology Unit 9, Twelve O'Clock Court, 21 Attercliffe Road, Sheffield S4 7WW, United The magazine publisher is the copyright holder of this article and it is reproduced with permission. Further reproduction of this article in violation of the copyright is prohibited. To contact the publisher: This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sublicensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.